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Nonstoichiometry and related properties of ceramic interfaces
Science of Ceramic Interfaces II J. Nowotny (Editor) 9 1994 Elsevier Science B.V. All rights reserved.
NONSTOICHIOMETRY
AND
RELATED
PROPERTIES
OF
CERAMIC
INTERFACES
Janusz Nowotny
Australian Nuclear Science and Technology Organisation, Advanced Materials Program, Lucas Heights Research Laboratories, Menai, NSW 2234, Australia
ABSTRACT The paper considers several aspects of nonstoichiometry and related charge neutrality conditions in the bulk phase and in the interface region of metal oxides. The effects of segregation on local properties of the interface layer of ceramic materials are discussed involving the development of concentration gradients, related electric fields and structural deformations. Applied aspects of ceramic interfaces are also briefly discussed. Several questions have been formulated with respect to the effect of interfaces on processing and properties of ceramic materials.
[. INTRODUCTION Bulk properties of compounds are different to those of the interface layer. The difference is mainly caused by segregation m d adsorption. Segregation results in the formation of composition [radients within the interface region. These gradients and 'elated electric fields have a strong impact on local properties uch as transport and, consequently, on reactivity. The ompositional changes within the interface region result in ocal structural deformations. Thus formed low dimensional nterface structures exhibit outstanding properties which are ot displayed in the bulk phase [i].
Polycrystalline materials are composed of individual gra ~ or crystallites. Consideration of their properties requires t! different concepts are applied to the bulk phase and I boundary layer (Figure i).
Figure I. Schematic illustration of a polycrystalline materi involving individual grains composed of the bulk phase and t boundary layer (BL)
The properties of the bulk phase are relatively we described in the literature for most compounds. It has be realized, however, that boundary regions (Figure i) ha entirely different properties in respect to both composition a structure. Figure 2 illustrates the effect of Cr on bu electrical properties determined by thermopower and the surfa E F [3]. Therefore, the characterisation of bulk properties not sufficient to explain the properties of polycrystalli materials involving interfaces. In other words full character sation of polycrystalline materials requires that chemic composition, structure and nonstoichiometry be determined f the interface layer in addition to the bulk phase. This paper will consider nonstoichiometry and relat defect disorders for nonstoichiometric oxides independently f the bulk phase and for the boundary layer. The bulk vs. bounda layer defect disorders will be discussed in terms of properti of ceramic oxide materials such as electrical properties.
Many properties of ceramic materials, such as dielectric properties, nonlinear characteristics of resistance and catalytical properties are determined by the interfacial properties rather than by bulk properties. This strong impact of interfaces on the functions of ceramic materials has resulted in an increasing interest in the determination of the properties of interfaces such as grain boundaries and gas/solid interfaces. The purpose of this paper is to consider basic aspects of nonstoichiometry of ceramic oxides as well as related appl~ed aspects. The effects of defect segregation on nonstoichiometry and related defect chemistry of the interface layer will be discussed in more detail.
2. ASPECTS OF MATERIALS
CHARACTERISATION
Routine studies of solids, based on nonstoichiometric compounds, involve the determination of the following properties: (i) chemical composition, (2) structure, and (3) nonstoichiometry. 2.1. Chemical Composition The basic chemical composition essentially involves the host lattice elements in both cation and anion sublattices. The properties of compounds also depend on the concentrations of foreign ions which are present either in a form of intentional solutes (dopants) or un-intentional additions (impurities). The effect of the impurities on properties may be substantial even if they are present at the ppb level [i, 2]. 2.2.
Structure Structural and phase relation studies aim at the determination of the stability range of the materials within a given range of temperature and gas phase composition. 2.3. Nonstoichiometry The studies on nonstoichiometry involve the determination of the extent of the deviation from stoichiometry in all sublattices and related defect disorders. In the case of metal oxides oxygen nonstoichiometry plays an important role. This nonstoichiometry is determined by oxygen activity of the gas phase surrounding the specimen during annealing at elevated temperatures.
3. NONSTOICHIOMETRY AND DEFECT STRUCTURE 3.1.
Bulk
Phase
Defect chemistry has been widely applied to explain properties of non-stoichiometric compounds such as semiconducting and transport properties.
1.1
Ni0- CrzO3
'/
1.0 >
1.1. ILl
,-fi,, o.9 ,,>, J
1.[CFN~] =IVan] 2"[CrNi] = 2[VN'i ] 3.[Cr'Ni] + [h" ] :2[VN' i ] + [VNi] 4.EXPERIMENTAL RESULTS
0.8 Z
I
I
-5
-4 ln[Cr]
I
I
-3
-2
I ! I
I
-1
[Cr in at.%]
Figure 2. Fermi energy level, EF, of Cr-doped NiO as a function of Cr concentration. Curves I, 2 and 3 correspond to the theoretical dependencies determined using the mass action law and different charge neutrality conditions. Black points (curve 4) correspond to experimental data determined by thermopower [3 ]
The concentrations of the predominant lattice defects in most of the binary metal oxides have been determined as functions of temperature and oxygen partial pressure [4].
The most reliable data on the defect structure of the bulk phase are based on properties of single crystals. In considering p o l y c r y s t a l l i n e materials one should realize that a m e a s u r e d property always involves the bulk component and the interface component [i]. Evaluation of these two components will be a major task of future studies on correct c h a r a c t e r i s a t i o n of ceramic materials. In the following section the basic rules for deriving defect disorder models will be considered for both binary and ternary metal oxides. 3.1.1. Binary Metal Oxides 3.1.1.1. Effect of p(O2) on Composition Bulk defect disorders for binary metal oxides will be considered for metal deficient oxides such as CoO and NiO. The p r e d o m i n a n t defects in these oxides are metal vacancies and electron holes which are formed as a result of oxygen interaction with the oxide crystal. The formation of these defects may be r e p r e s e n t e d by the following equilibria: i/2 0 2
~
V~'
x
+ 2h'+ O O
(1)
where V'' denotes a doubly ionised cation vacancy, h" is an electron hole and OX0 denotes oxygen in its lattice site. The lattice charge neutrality condition requires that Ch'] = 2CV~']
(2)
where the brackets denote concentrations. Eq. (2) represents a local charge neutrality requirement within the entire bulk phase, when the crystal is in thermodynamic equilibrium. Based on condition (2) one may derive simple relationships between the c o n c e n t r a t i o n of defects and oxygen partial pressure, p(O2): [h'] =
P(O2 )I/n exp {-AHf/RT}
(3)
where AHf is the change of the formation enthalpy of metal vacancies, and n is a parameter related to valency of the defects. Changes in nonstoichiometry, corresponding to shifts in equilibrium (i), may be monitored by measurements of electrical properties such as electrical conductivity:
a = q [h'] ~h
(4)
where q is the elementary charge and #h is the mobility of electron holes. According to Eq. (i)-(3) the deviation from stoichiometry in equilibrium is determined by temperature and oxygen activity in the gas phase. Therefore: y = f{T, p(O2)}
(5)
where x + [v~] + [vM]
y = z [v~,]
- 1 0 [ C~
'/~
1473 K
-10
Co0
/,
//
//Vc,o
-30
/
9
- 3.0
f
o
"
-40
-
-5.0 -
-60
-6.0 I/
I
-5 -3 -1
I
I
I
I
I
1 3 5 7 9
log po[ po:~n Pcl ]
Figure 3. The stoichiometry, concentration undoped C00 as p(O2) at 1473 K
deviation from y, and related of defects in a function of [5]
-"
- [D']
-
-50
11
/7/
1473 K
,.y
2~ o-40
(6)
-
i
I
I
-5 -3 -1
I
1
1
I
I
1 3 5 7 9
log po[ Po2in Pcl ]
Figure 4. The deviation from stoichiometry and the concentration of defects in CoO doped with a donor as a function of P(O2) at 1473 K [5]
-1.0
-2.0
CoO 1473K _ 0.1%
,#~
[A']
-3.0
~ o
-4.0
tvc~
i
-5.0
/ -6.0 ,
,/,
-5-3-I
,
,
,
,
1
3
5
79
,
[og Po2[Po~nPo.] Figure 5. The deviation from stoichiometry and the concentration of defects in CoO doped with an acceptor as a function of p(Oz) at 1473 K [5]
where [V'.] and [VX,] denote the concentration of singly ionised and neutral cation vacancies. Figure 3-5 illustrate the total nonstoichiometry, y, as well as the c o n c e n t r a t i o n of particular defects for donorand acceptor-doped CoO [5]. As seen both donors and acceptors have a substantial effect on the defect structure already at the level of 0.i at %. In both cases the effect of p(O2) on y is observed above about 105 Pa while below this value the nonstoichiometry is determined by the concentration of the addition. In equilibrium the activity of defects is the same within the entire crystal (except the boundary layer). Then Eq. (2) represents the local bulk lattice electroneutrality requirement.
3.1.1.2. Effect of p(O2) on the Mobility of Electronic Charge Carriers A c c o r d i n g to Eq. (4) the electronic component of the electrical conductivity depends on both the c o n c e n t r a t i o n of charge carriers, such as [h'] and [e'], and their mobility, ~h" It has been a general assumption that a change of oxide n o n s t o i c h i o m e t r y results in a change of its electrical conductivity via the concentration of electron holes while the mobility term, ~h, remains constant [4]. Recent study of electrical conductivity and thermopower of undoped CoO performed by Kowalski et al. [6] has shown that change in P(O2) results in changes of not only the c o n c e n t r a t i o n term, [h'], but also the mobility term, #h, of Eq. (4).
'_.~
CoO 1273 K
("4 ,.--,,
o
9 SINI3LECRYSTAL
~0./,
/
o POLYCRYSTAL
oi"
_.1 0 -r Z
o0.3 u
o
w !1 0
. . ~ 6"-
0
>- 0.2
l.--
I
2
10g p(02)
m
3 [p(02)in Pal
/+
5
O Z
,.---,,
C00
,,-/
> 0.5 15
1373 K 9 SIN6LECRYSTAL o POLYCRYSTAL
,--.., i-.
:<,
/
~
I/+73 K
I./3 l.J.l
" 0.~
a SIN6LE CRYSTAL
/
/
o
O nZ O
i~I
~
L_J uJ
03 u_ 0
o
>I.-__1 rn 0
log p(02) [p(02) in Pal
Figure 6. The mobility of electron holes in CoO, ~h, as a function of equilibrium p(O2), at 1273, 1373 and 1473 K, for both single crystal and polycrystalline specimens [6]
In other words it has been shown that change in oxide nonstoichiometry results in change of the mobility of electronic charge carriers. This effect was observed for both single crystals and polycrystalline materials but it is still not clear what is the effect of grain boundaries on the character of this dependence (Figure 6).
3.1.1.3. Effect of Aliovalent Ions Incorporation of aliovalent ions into the lattice positions of a metal deficient oxide, M1.yO, forming acceptors (A) and donors (D), may be represented by the following equilibria for mono- and tri-valent ions, respectively: x
(7)
A20 + 1/2 02 - 2A~ + h" + 200 x
(8)
D203 - 2D M + V~' + 300
The following requires that
[AI~I] + [ e ' ]
general
lattice
electroneutrality
+ 2 [ V ~ ' ] = [DM] + [ h ' ]
condition
(9)
According to the condition (9) one may control the concentration of the electronic charge carriers [e' ] and [h'] by doping the crystal with aliovalent ions. 3.1.2. Ternary Metal Oxides The predominant defects in model ternary oxides, such as BaTiO3.y , are oxygen vacancies (V0"), cation vacancies (VTi'''' and Vsa'' ) electronic charge carriers and impurities [7]. The full lattice electroneutrality condition in the crystalline bulk requires that:
[h']+2[V O " ]+[DTi ] = [e']+4[ V'''' Ti ]+2[ V'Ba' ]+[A~i ]
(10)
where [D'Ti] and [A'Ti ] denote the concentration of singly ionised donors and acceptors, respectively, in the Ti sublattice. 3.2. Interface Layer 3.2.1. Surface Charge Neutrality Requirements Construction of any defect model for the bulk phase must be based on the local lattice electroneutrality condition which requires the balance of charge at any point within the crystal. In contrast to this requirement, local charge neutrality for the surface layer is not required. Then an excess of the surface charge may be compensated by the space charge in the boundary layer as is schematically illustrated in Figure 7.
]0
+++ +++ +
+
w L9 ~r nu
I I
Z U.l l--
DISTANCE FROM THE SURFACE
~P
DISTIANCE FROMTHE SURFACE
Figure 7. Surface models of nonstoichiometric compounds involving both potential and charge distribution within the surface and space charge layers
3.2.2. Binary Metal Oxides At elevated temperature the lattice defects have a tendency to segregate resulting in an enrichment (or impoverishment) of the boundary layer in certain elements. Segregation of defects results from their different formation energies in the bulk and in the boundary layer. In consequence, segregation leads to the formation of strong concentration gradients within the boundary layer. Accordingly, the concentration of defects within the boundary layer is a function of both T and p(O2) as well as position (the distance from the interface {):
y = {T, p(O2), {}
(11)
11
"•VER /~-i(T, Po2,
LAYER STRUCTURE
~)
y- f ( T, Po2)
Ml_+yO
SUBLAYER BULK
MOl-y
Figure 8. Illustration of the nonstoichiometry the surface layer
gradient within
In the case that segregation results in an enrichment in defects their concentration increases with the decrease of the distance, ~. Then strong interactions between the defects result in the formation of a surface low dimensional structures (Figure
8). Assuming that in metal-deficient oxides metal vacancies segregate to interfaces then the interface layer is enriched with these defects resulting in the formation of a negative charge at the interface. The surface electroneutrality requires that the segregation-induced negative surface charge within the outer layer is compensated by a positive charge within the space charge layer (Figure 7). This model becomes more complicated when aliovalent cations are present in the lattice (see section 3.3.). Figures 9 and I0 illustrate the surface model of undoped and Cr-doped CoO. As seen introduction of tri-valent ions into the lattice of CoO (and also NiO [3]) results in a change of the surface model involving an inversion of the surface charge and the space charge. 3.2.3. Ternary Oxides The studies of Zhang et al. [8] have shown that segregation leads to an enrichment of the BaTiO 3 surface with both Ti and Ba vacancies, however, different formation energies of these two defects result in a Ti/Ba ratio greater than unity within the surface layer. It has been observed that the surface enrichment in BaTiO 3, expressed by the Ti/Ba ratio, depends on oxygen nonstoichiometry as well as the rate of cooling (Figures ii and 12) [8]. The results obtained are consistent with a surface model involving a negative surface charge and a positive charge in the space charge layer (Figure 13).
12
i i I
~
"~
I I I
i
"'-
SPACE CHARGE
I
I,,
I
I --
-
i
I
I
.
|
.
.
.
.
.
.
|
~_J 0_
,~ ~ 9
,
,
~
'
,
,,',
,
|
i
DISTANCE FROM THE SURFACE
Figure
9.
Surface
model
of u n d o p e d
CoO
0
__I ,.--,.
o~ . ~o >
U 0
SPACE CHARGE
Z
l..ul
_3 4:
l---
= I = ! !
l-§
|
|
|
|
§
§ §
l.ul _3 Lul
~
!
,,<~,
, I !
ugln
....
J
DISTANCE FROM THE SURFACE
Figure
I0.
Surface
model
of C r - d o p e d
CoO
~
13
Estimated 2.0 i
1.5
1.o5-1
,-
2.5 9 i
,
Probed
Depth
3.0 i
,
3.5 i
--
].oo
(nm) 4.0 i
9
Angular-Dependent Oxidised
9~
4.5 I
,
,
i.o5
XPS BaTiO 3
1.00
0.95
0.95
0.90
0.90
0.85
0.85 Rapidly
0.80
-
,
0.3
Cooled
,
Samples
,' 0.5
'
0.4
".
, 0.6
.
, 0.7
~
~-0.8
. -
, 0.9
,-
0.80 1.0
sin0
Figure ii. The Ti/Ba XPS intensity ratio vs. probed depth rapidly cooled BaTiO 3 [8]
Estimated 1.o5
.5
2.0
~ ~ , , ,
Probed
2.5
I
I
Depth
3.0
I
' ....
(nm)
3.5
l
'
--I
4.0 --'
l
4.5 '
"I
"
,- 1.05
XPS
Angular-Dependent
1 .oo
for
1.00
. ~ 0.95
0.95
0.90
0.90
b-, 0.85
0.85 Slowly
0.80 i 0.3
,
Cooled ,
o14
~ 0.5
Samples ,.
, o16
' " ' 0.7
~ 0.8
"'
,0.9
r
0.80 1.0
sin0
Figure 12. The Ti/Ba XPS intensity ratio vs. probed depth slowly cooled BaTiO 3 [8]
for
14
,.., ~. ~ ] ~ "~
Space Charge Layer
Bulk
Vo", h"
o N
-
+
-:-+ \ \
+
+++
Distance from the Surface
Figure
13. Surface model of undoped BaTiO 3 [8]
It has g e n e r a l l y been assumed that barium titanate involves only oxygen v a c a n c i e s (BaTiO3.z). The studies of Zhang et al. [8] have shown that a deficit in both cation sublattices of BaTiO 3 must also be c o n s i d e r e d within the surface layer. Accordingly, a correct formula for the boundary layer of barium titanate should be w r i t t e n assuming deficit in all sublattices: Bal_xTil_yO3_ z
(12)
where x, y and z are a function of p o s i t i o n w i t h i n the b o u n d a r y layer, ~, as well as of t e m p e r a t u r e and P(O2). In the boundary layer all of the q u a n t i t i e s x, y and z assume substantial values while in the bulk phase both x and y is very small because their t r a n s p o r t from the surface into the bulk is e x t r e m e l y slow. One may expect that the observed s u r f a c e - i n d u c e d e n r i c h m e n t of the surface layer of BaTiO 3 in B a - v a c a n c i e s has an effect on the electrical p r o p e r t i e s such as t h e r m o p o w e r (S). In fact S assumes d i f f e r e n t values for a single crystal, Ssc , and a polyc r y s t a l l i n e material, Sp.
15
It was documented that the S,c/Sp ratio assumes 1.72. This value, which is independent of temperature, coincides with the ratio of the theoretically determined thermopower value using the band model, St, to Sp [9]. This agreement suggests that the value of the ratio, Ssc/Sp=l.72 , is determined by interfaces. 3.3. Bidimensional Surface Structures Segregation results in a much higher concentration of defects in the interface layer than in the bulk phase. It was shown that the interface layer may accommodate defects which are not displayed in the bulk phase such as Co interstitials in CoO [i]. The enrichment of the interface layer in defects results in substantial interactions between the defects within the interface layer resulting in the formation of defect complexes and clusters which are not present in the bulk. When the segregation-induced concentration of defects in the boundary layer exceeds a certain critical value then a local structural reordering takes place resulting in the formation of bidimensional interface structures. Presence of these structures are limited to the outermost layer. These structures have outstanding properties not displayed in the bulk phase. Their identification is of strategic importance for the preparation of materials with enhanced properties. 3.4. Conclusions Correct understanding of the properties of the interface layer requires determination of the local nonstoichiometry and related defect disorders involving (i) the host lattice defects, (ii) intentional solutes, and (iii) impurities (unintentionally added solutes). Thus urgent studies are needed to accumulate an empirical data base on the effect of segregation on defect chemistry of the interface region and related properties. Enrichment of the interface layer of ionic crystals in defects results in local structural deformations which lead to the formation of bidimensional interface structures with outstanding properties. Studies are needed to characterise these structures. 4. EFFECT OF INTERFACES ON TRANSPORT In considerations of gas/solid kinetics it has been generally assumed that reactions taking place at the gas/solid interface are very fast and that bulk transport is rate controlling. This assumption has resulted in appropriate interpretation of the equilibration kinetic data which, according to the above assumption, have been considered as corresponding to the bulk transport kinetics.
16
It was shown that the above assumption is not always valid and that the gas/solid equilibration kinetics may be controlled by a slow process at the gas/solid interface. It was shown that the gas/solid interface may have a substantial effect on the local transport kinetics within the interface layer [i, i0]. This effect results either from the segregation-induced electric fields or structural deformation of the interface layer. In the following sections several examples of interface reactions, which are rate controlling for the gas/solid kinetics will be considered. 4.1. Effect of Seqreqation-Induced Electric Fields The electric field, which is formed along segregationinduced concentration gradients within the interface layer, may have a strong effect (either retarding or accelerating) on transport of defects across these fields [i0]. In the first case segregation results in the formation of a local surface diffusive resistance which is rate controlling the gas/solid kinetics. Therefore, diffusion data obtained by using solutions of the diffusion equation for bulk-controlled kinetics result in apparent diffusion coefficients which involve two components: the bulk diffusion component and the component corresponding to the surface diffusion resistance. Verification of the available diffusion data from the viewpoint of the surface component is required for obtaining real values of the transport numbers in the bulk phase and in the interface layer. However, application of appropriate analytical solutions of the diffusion equation, involving the segregation-induced diffusion resistance, requires knowledge of the segregation-induced concentration profiles within the surface layer [I0]. 4.2. Effect of the Local Dopinq of the Interface Layer It was shown that equilibration of the oxygen/BaTiO 3 system is very fast. As seen from Figure 14 the isothermal changes of electrical conductivity for an undoped BaTiO 3 single crystal at 1310 K during a re-equilibration is limited to about 30 min. After this time the electrical conductivity assumes a new constant value which corresponds to a new equilibrium state. Incorporation of a donor into a surface layer of BaTiO 3 results in a substantial change of the transport within this layer and, consequently, in the gas/solid kinetics. As seen from Figure 15 the incorporation of Nb into a surface layer results in an increase of the equilibration time from about 30 min for a single crystal of undoped BaTiO 3 to several months for a polycrystalline material of Nb-doped specimen.
17
0.019 ,'T
l.ul Z
t--o-.o-<)-..o-,,o p02= 3.9" 1012 Pa
0.016 0.013
w_< l_j
,.o
=
Q Z O
u
0.003
[~~
p02= 3.9.1012 Pa I
I
TIME [h] Figure 14. Change of the conductance of undoped BaTiO 3 single crystal during equilibration (reduction run) at 1310 K [9] Figure 16 illustrates the model of Nb-doped BaTiO 3 which involves a strong gradient of Nb within the surface layer of BaTiO 3 (about 0.1-0.2 ~m thick). The donor doping results in a substantial reduction of the concentration of the mobile oxygen vacancies and the formation of cation vacancies which exhibit very low mobility. Concordantly, the formation of the surface layer doped with Nb results in blocking the transport of oxygen and, consequently, in retarding the gas/solid kinetics. 4.3.
Surface Equilibration Kinetics Equilibration kinetics of the surface layer may be monitored by measurements of work function. Figure 17 illustrates changes in the contact potential difference (CPD) between yttria-doped zirconia and a Pt electrode (these CPD changes are determined by work function changes of the zirconia specimen) during consecutive runs of oxidation and reduction at 1054 K [2]. The absolute CPD values as well as their slow changes within the time of annealing, ACPD, are determined by segregation kinetics of impurities such as Ca and A1 [2]. 4.4. Conclusions Nonstoichiometry within the interface layer of undoped oxides may be considered in terms of segregation of intrinsic lattice defects such as cation and oxygen vacancies, cation interstitials and defect complexes.
18
70
60
Z
~ 50 rr
30 ~
P02 = 220 Pa
I
1
I
i
30
60
90
1
120
TIME [ DAYS ]
Figure 15. Changes of resistance of a polycrystalline of Nb-doped BaTiO 3 during isobaric equilibration [9]
I .,,-,. --.i:
Z
>
CK I-Z
w "la.
o
~
o,%
DISTANCE FROM THE SURFACE
Figure 16. Surface
layer model of Nb-doped BaTiO 3
specimen
19
I-\
~' eR
R
04 >
~ J
C3 13..
o
03
02 -
I ~
( 1.74-10 Pa ) (4.3 9lOZ'Po )
"--.2
I
i
I
i
9
20
40
60
80
100
0.1 - R-REDUCTION O-OXIDATION
- -
2
TIME
"L'~--~ Z_./
f ft
0
,00
120
[h ]
Figure 17. Changes in CPD between y t t r i a - d o p e d zirconia and Pt during p r o l o n g e d annealing at 780~ (increase of CPD c o r r e s p o n d s to increase in work function of zirconia) [2] However, assuming that all m a t e r i a l s contain less or more impurities we have to realize that the picture of the segregat i o n - i n d u c e d c o m p o s i t i o n gradients should be c o n s i d e r e d as a s u p e r i m p o s i t i o n of intrinsic defect c o n c e n t r a t i o n s and extrinsic defect c o n c e n t r a t i o n s (impurities). Lateral interactions between all these defects results in the formation of low d i m e n s i o n a l interface structures. It is e x t r e m e l y difficult to determine e x p e r i m e n t a l l y the s e g r e g a t i o n of intrinsic defects resulting from n o n s t o i c h i o m e t r y because most of the available surface t e c h n i q u e s are not sensitive enough for d e t e c t i o n of changes in crystal stoichiometry. In contrast to these intrinsic defects most of the reports on s e g r e g a t i o n in oxide m a t e r i a l s concern solutes. Impurities are also termed as u n i n t e n t i o n a l dopants present at a very low concentration. Despite this low c o n c e n t r a t i o n their effect on m a t e r i a l s properties can be significant. Traces of a l i o v a l e n t impurities added to an insulating crystal, such as Li in NiO, result in its t r a n s f o r m a t i o n into a s e m i c o n d u c t o r or even a good conductor. The impressive effect of traces of MgO on s i n t e r i n g of alumina may serve as an example of a substantial impact of additions on p r o c e s s i n g and resulting p r o p e r t i e s of the materials.
20
A c c o r d i n g to the d i s c u s s i o n presented above s e g r e g a t i o n of impurities in n o n s t o i c h i o m e t r i c compounds must be c o n s i d e r e d along with intrinsic defects and their mutual interactions in the b o u n d a r y layer. Therefore, the picture of s e g r e g a t i o n of an impurity is well defined when the spectrum of all the impurities and the conditions d e t e r m i n i n g the equilibrium, i.e. t e m p e r a t u r e and the gas phase composition, are well defined. The observed substantial effect of e q u i l i b r i u m p(O2) on the depth p r o f i l e of solute ions, such as Cr in NiO [ii], confirms a strong effect of oxide n o n s t o i c h i o m e t r y within the interface layer on the segregation p r o f i l e of the ions. C o m p e t i t i v e segregation of several elements of d i f f e r e n t driving forces may result in the formation of a s a n d w i c h - t y p e surface layer as has been reported for y t t r i a - d o p e d ZrO 2 [2]. In this case the outer surface layer is p r e d o m i n a n t l y e n r i c h e d with one elements and the sub-surface layer is enriched with another one. The enrichment coefficient (surface/bulk concentration) of defects may assume several orders of m a g n i t u d e (104 - l0 s) [i, 2]. Therefore, the s e g r e g a t i o n - i n d u c e d c o n c e n t r a t i o n s in the interface layer may assume very high values even if the segregating elements are present in the bulk phase at a very low level.
5. S E M I C O N D U C T I N G 5.1.
PROPERTIES OF INTERFACES
Effect of A l i o v a l e n t Ions Fermi energy is the basic quantity of s e m i c o n d u c t o r s which is sensitive to the density of states (donors and acceptors) and their p o s i t i o n in the band model. Thus m e a s u r e m e n t s of the Fermi energy are very sensitive to the defect structure and related s e m i c o n d u c t i n g properties. It was d o c u m e n t e d that the m e c h a n i s m of i n c o r p o r a t i o n of a solute in the bulk phase may be different to that corresponding to the interface layer [i, 3]. Parallel m e a s u r e m e n t s of thermopower, which is a bulk sensitive property, and work function, w h i c h is a surface sensitive property, have shown that Cr acts as a donor when incorporated into the bulk of NiO while in the bulk Cr forms acceptor centres (above 0.2 at%). These studies also indicate that tri-valent ions, such as Cr, result in changes of the surface polarity from negative for u n d o p e d NiO to p o s i t i v e for Cr-doped NiO. Similar model is valid for CoO [12].
2]
5.2. Thin Films Electrical conductivity and thermopower are essentially bulk sensitive properties. However, decreasing the thickness of thin films result in increasing the interface component of the measured property. Figure 18 illustrates the reciprocal of the oxygen pressure exponent of electrical conductivity for undoped CoO as a function of film thickness [12].
4,2 4,1
4.0 0 "~
3,9
O')
o .. D o., O r,.,..
'
3,8 3,7 3,6
II
3,5
. - ~
3,4 3,3
.... 900
, .... 950
, .... 1000
, .... 1050
TEMPERATURE
Figure 18. Reciprocal thin films [12]
the P(O2)
o []
7.95 I~m 1.05 pm
A
0.375 p.m
, .... 1100
. 1150
.... 1200
[~
conductivity exponent
for CoO
As seen this quantity decreases with decrease of the film thickness to values much below four which is a critical value for ideal defect model of CoO [4]. This effect indicates that the interactions between defects in the interface region of CoO are much stronger than those in the bulk phase. These interactions may be considered in terms of Co interstitials, which form preferentially in the boundary layer and, along with Co vacancies. Both these defects result in the formation of the spinel-type overlayer structure. These structural changes within the interface layer lead, in consequence, to the formation of a Schottky-type barrier within this layer (Figure 19).
22
Figure 19. A Schottky-type
barrier at the CoO surface
5.3. Conclusions Defect disorder of the boundary layer and related semiconducting properties may be entirely different to those of the bulk phase. Also the mechanism of incorporation of solute ions may be different as well. Accordingly the mechanism of doping which has been established for the bulk phase of compounds [4] is not valid for the interface layer. The determination of the effect of doping on the local defect chemistry of the interface layer requires an independent experimental approach either by studying surface sensitive properties, such as work function, or (2) by measuring bulk properties for specimens of different grain size or (3) by measuring properties of thin films.
6. APPLIED ASPECTS 6.1.
Effect on Sintering
The effect of MgO on sintering of alumina has been known since 1956 [13]. Since then intensive studies have been carried out on the determination of surface and grain boundary properties of Mg-doped alumina [14-16]. The main difficulties in the determination of Mg-segregation in alumina were produced by a very strong segregation of Ca.
23
Only very recently it has been found that Ca selectively segregates only to the prism crystallographic plane (i010) resulting in enrichment by a factor of 103 while the enrichment of the basal (0001) plane in Ca is only 50 [16]. On the other hand Mg segregates effectively to both planes, however, its presence at the external surface is limited because of substantial evaporation. 6.2. Ceramic Gas Sensors The sensing signal, which usually involves change of an electrical property of the sensor material, is generated at the gas/solid interface (Figure 20). Therefore, in the preparation of gas sensors with desired sensitivity and selectivity the main attention should be focussed on surface technology.
0--o CO C02 o-0-o H20> COATING m
BULK MATERIAL
Q
~
Figure 20. Schematic illustration of a gas sensor based on the determination of electrical conductivity In chemical gas sensors the sensing signal is generated during the charge transfer between the adsorbed (or absorbed) gas molecules and the surface of a semiconducting sensor material. Accordingly, the desired sensitivity of sensors towards a particular gas phase component may be achieved by preparation of surface which exhibits possibly a high reactivity with this component. The selectivity may also be increased by decreasing the surface reactivity with other gas phase components. Modification of the surface reactivity may be achieved by changes of surface chemical composition e.g. via local dGping and coating (Fig. 16).
24
Segregation of defects, mainly of aliovalent ions, plays an important role in the modification of surface properties. One of the most sensitive ways of measuring the sensing signal at the gas/solid interface is based on the measurement of surface potential using a high temperature Kelvin probe [17]. It appears that this probe is extremely sensitive to changes in surface composition and, therefore, may serve as a sensor for the determination of traces of gases.
Figure 21. Illustration of the PTCR effect within a single grain and cross two grains [18] 6.3. Non-linear Effects Awareness is growing that interfaces have a substantial effect on properties. This effect can be well illustrated by the positive temperature coefficient of resistance (the PTCR effect) which has been reported for BaTiO 3. This effect, however, is restricted to polycrystalline material and is not displayed by a single crystal (Figure 21 [18]). Studies of Mizutani et al. [18] have shown that at least one grain boundary is required for the PTCR effect to be observed. Attempts to understand the PTCR effect and also other properties of ceramics have resulted in an increasing interest in studies of interfaces of ceramic materials.
25
6.4. Catalysts Catalytic properties of solids, such as activity and selectivity towards the formation of desired reaction products, are determined by the properties of the surface and related active centres which are formed during the preparation of the catalyst and its subsequent activation process. 6.5. High T c Superconductors There have been several reports indicating that segregation-induced changes of the local grain boundary composition result in the formation of weak links in conducting properties of oxide cuprates [19, 20]. One of the reasons for the weak links is the decrease of the local T c within the grain boundary region. Alternative reports have shown that the local T c in grain boundary regions may assume a much higher value than in the bulk phase [19]. 6.6. Metallization of Ceramics Metallization resulting in strong adhesion at the metal/ceramic interface is an important issue in micro-electronics. The adhesion is determined by properties of individual surfaces such as chemical composition and microstructure. One may expect that segregation of lattice defects and its effect on the local interactions at the metal/ceramic interface may be an important aspect of interface engineering. 6.7. Conclusions Interfaces have a controlling effect on the properties of industrial ceramic materials such as sensors [21, 22], dielectrics [23] and non-linear resistors [18]. So far, however, the preparation of these materials is empirically based rather than based on knowledge of the local properties of interfaces. Therefore, there is an urgent need to increase our understanding of interface properties and the relationship between the local properties of interfaces and the properties of bulk materials.
7. INTERFACE
ENGINEERING
Awareness is growing that in order to meet the tough requirements of new technologies, development of advanced materials of enhanced properties will be required. In the case of functional ceramic materials the main effort should be focussed on engineering of interfaces, such as surfaces and grain boundaries, which control the properties of functional
26
ceramic materials. The engineering of interfaces should be based on through understanding of their local properties and better understanding of the relationship between the interface properties and conditions of processing such as gas phase composition, temperature, time of annealing and rate of cooling. Appropriate engineering also involves appropriate post preparative treatment of materials such as local doping of the interface layer, coating and etching. An important strategy in the development of interface engineering involves better understanding of the properties of low dimensional systems such as thin films, multilayer systems, fine grain ceramics and heterogeneously dispersed systems. An important area involves the effect of impurities on interface composition and related properties. Correct characterisation of materials for their impurity level, especially at low levels, is very difficult. It has been realised that even traces of impurities in the bulk phase may segregate during a high temperature treatment, resulting in very high enrichment at interfaces. In developing basis of interface engineering composition we have to understand the principles of segregation for compounds. Better knowledge of the segregation phenomenon may help in removing undesired impurities from the interfaces and to bring up the defects which result in the desired composition and, consequently, desired properties. We are still a long way from being able to control the interface composition via appropriate processing and, concordantly, most industrial materials are still fabricated on an empirical basis rather than through understanding of interface phenomena such as segregation and interface transport kinetics. Development of a basis of interface engineering requires the following items to be addressed: i.
There is a need for better understanding of interface nonstoichiometry and related local properties of compounds at elevated temperatures and under controlled gas phase composition.
2.
Basic studies are needed to evaluate the driving forces of segregation and to determine segregation equilibria in order to predict the effect of both un-intentional solutes (impurities) and intentional solutes (dopants) on interface chemistry. Again the segregation equilibria may be modified in a wide range by adjusting the appropriate gas phase composition during all steps of processing.
27
3.
It becomes increasingly important to control nonstoichiometry of the interface region. As in the bulk phase this may be performed either by change of partial pressure of one of the lattice component in the gas phase during annealing or by doping with aliovalent ions. In the case of doping one should realize that the bulk concentration of the dopant should be lower than its desired concentration in the boundary layer by the factor which is equal to the enrichment coefficient. When the segregation enrichment is very high then introduction of traces of elements may be sufficient to modify entirely the interface composition. Also modification of nonstoichiometry in the boundary layer in a controlled way requires knowledge of segregation equilibria.
4.
One should evaluate the relationship between interface chemistry and materials properties. In order to address this issue there is a need to accumulate a data base on chemical composition of interfaces and related properties such as electric properties and dielectric properties.
5.
The low dimensional interface structures, which are formed as a result of defect segregation, must be characterised.
6.
There is a need to develop a strategy of modification of interface properties in a desired way e.g. by forming interface structures in a controlled manner. We also need to develop processing techniques which result in the preparation of reproducible interface structures.
7.
Item #5 & 6 may be effectively addressed using more sophisticated surface sensitive techniques for 'in situ' monitoring of local interface properties of compounds during processing at elevated temperatures and under controlled gas phase composition. Therefore, there is a need to develop new surface techniques which will meet the above requirements.
8.
It is very important that characterisation of ceramic specimens involves routine determination of the concentration of impurities at the level of several ppm or even at lower level. High temperature processing may result in substantial enrichment of these impurities at interfaces. It is impossible to control interface chemistry without knowledge of the level of this un-intentional doping especially with aliovalent ions.
28
9.
Awareness is growing that cooling results in substantial changes in properties of the interface layer. One source of these changes is the formation of concentration gradients in the interface layer as a result of the incorporation of elements coming from the gas phase, such as oxygen. Another effect results from local changes in segregation equilibria. Superposition of these effects may result in a complicated picture of the interface layer. By varying the cooling rate and the gas phase composition one can impose the concentration gradients in a controlled way. So far, these changes in industrial materials have been imposed in an empirical manner rather than based on knowledge of segregation and gas/solid reactivity. Therefore, there is a needed to evaluate the changes that occur at interfaces during cooling and to develop an understanding of the kinetics of interface phenomena that allow one to impose desired changes during cooling.
8. QUESTIONS TO BE ANSWERED The purpose of this section is to formulate some questions which may arise concerning the effect of interfaces and interface segregation on the properties of ceramic materials. i.
What is the segregation-induced enrichment of the interface region in both intrinsic and extrinsic defects? Knowledge of the bulk concentration and the enrichment factor allows one to evaluate the surface composition. In this regard it is important to evaluate the predominant driving force(s) of segregation and the resulting enrichment factor which may reach several orders of magnitude.
2.
How does the segregation-induced enrichment of the boundary layer depend on bulk composition? One may expect that multisegregation results in either attractive or repulsive lateral interactions within the boundary layer. Concordantly, the extent of segregation of ions which are undesired to be present at the interface may be decreased by dissolution of secondary ions which results in repulsive interactions thus leading to decrease of the enrichment in the undesired ions.
3.
How does the segregation enrichment depend on the composition of the gas phase?
29
.
.
.
What is the depth of the segregation profile and how does it depend on crystal properties? What is the effect of segregation on the defect chemistry of the interface region and how may the defect structure be represented? What is the difference between the bulk and the interface miscibility limit and what is the impact of this upon phase relations?
7.
What is the effect of segregation on ionic reconstructions within the interface region and the formation of low-dimensional interface structures? What are the properties of these structures?
8.
What is the effect of the interface structures on material properties?
9.
What is the effect of segregation on the formation of fast diffusion pathways both along and across the interface?
i0.
What is the impact of the interface region on the reactivity and processing of ceramic materials?
Ii.
What is the effect of interface properties on the electrical signal which is generated at the gas/solid interface for sensor-type materials?
12.
What is the impact of segregation on catalytically active surface centers?
13.
What is the role of segregation on the formation of the electrical grain boundary barrier in nonlinear resistors?
14.
How may the interface layer be engineered, using the effect of segregation in order to achieve the desired properties and functions of ceramic materials?
the
formation
of
9. SUMMARY Chemical composition and nonstoichiometry of the interface layer may differ substantially from those of the bulk phase as a result of segregation and related phenomena.
30
The segregation-induced compositional gradients may lead to structural deformations of the outermost interface layer resulting in the formation of bidimensional structures with outstanding properties 9 These properties may be modified, by changes of grain size or by varying the thickness of thin films9 The effect of impurity segregation, even if the impurities are present at very low concentration in the bulk phase, may have a substantial effect on the entire picture of segregation 9 Therefore, the data concerning equilibrium segregation should correspond to well characterized materials with respect to their impurity concentrations 9 Studies are needed to engineer interface properties by imposing the desired chemical composition through an appropriate combination of processes such as segregation of desired elements, annealing and programmed cooling as well as other processes such as coating, implantation and CVD. The gas phase composition is an important processing parameter since the gas phase components interact with the ceramic body during all stages of the process 9 The reactivity depends on temperature 9 However, even at low temperatures the gas/solid reactivity may have a substantial effect on chemical composition of the interface layer and, consequently, on processing and properties of the ceramic material 9
ACKNOWLEDGEMENTS Support of the Commonwealth of Australia through the Department of Industry, Science and Technology (Grant # C91/02826) is gratefully acknowledged. This paper was kindly reviewed by Dr. Cliff Ball. His comments are sincerely appreciated.
REFERENCES
1 2.
J Nowotny, in: 'Science of Ceramic Interfaces' J Nowotny, Ed., Elsevier, Amsterdam, 1991, p. 79 J. Nowotny, M. Sloma and W. Weppner, Solid State Ionics 283o
3. 4.
5.
(1988)
144s
J. Nowotny and M. Rekas, Solid State Ionics 12 (1984) 253 P. Kofstad, 'Nonstoichiometry, Diffusion and Electrical Conductivity in Binary Metal Oxides', Wiley-Interscience, New York, 1972 J. Nowotny and M. Rekas, J. Am. Ceram. S.c., 72 (1989) 1199
31
6. 7. 8. 9. i0. ii. 12. 13. 14. 15. 16. 17. 18. 19.
20. 21. 22. 23.
K. Kowalski, Thesis, U n i v e r s i t y of Nancy I, Faculty of Sciences, 1994 J. N o w o t n y and M. Rekas, Solid State Ionics 49 (1991) 135 Z. Zhang, P.J. Pigram and J. Nowotny, Intern. C e r m . M o n o graphs 1 (1994) 455 J. N o w o t n y and M. Rekas, Ceram. Intern., 20 (1994) 265 Z. A d a m c z y k and J. Nowotny, J. Phys. Chem. Solids 47 (1986) ii W. Hirschwald, I. Sikora and F. Stolze, Surf. Interface A n a l y s i s 3 (1985) 157 A. Bernasik, K. Kowalski and J. Nowotny, unpublished results R.L. Coble, J.Appl.Phys., 32 (1961) 787 H.L. M a r c u s and M.F. Fine, J.Am. Ceram. Soc., 55 (1972) 568 W.C. J o h n s o n and D.C. Stein, J.Am. Ceram. Soc., 58 (1975) 485 S. Baik, J.Am. Ceram. Soc., 69 (1986) CI01 J. Nowotny, M. Sloma and W. Weppner, A d v a n c e d in Ceramics, 23 (198v) is9 N. Mizutani, u n p u b l i s h e d results J. Nowotny, M. Rekas, D.D. Sarma and W. Weppner, in: 'Surface and N e a r - S u r f a c e Chemistry of Oxide Materials', J. N o w o t n y and L.C. Dufour, Eds., Elsevier, Amsterdam, 1988, pp. 669 S.X. Dou, this volume, p. 239 D.Y. Wang, this volume, p. 645 K. Yamana, M. Miyamoto, K. Doi, T. Arahori and J. Nowotny, this volume, p. 571 H. Kishi and N. Yamaoka, this volume, p. 613
NONSTOICHIOMETRY
AND
RELATED
PROPERTIES
OF
CERAMIC
INTERFACES
Janusz Nowotny
Australian Nuclear Science and Technology Organisation, Advanced Materials Program, Lucas Heights Research Laboratories, Menai, NSW 2234, Australia
ABSTRACT The paper considers several aspects of nonstoichiometry and related charge neutrality conditions in the bulk phase and in the interface region of metal oxides. The effects of segregation on local properties of the interface layer of ceramic materials are discussed involving the development of concentration gradients, related electric fields and structural deformations. Applied aspects of ceramic interfaces are also briefly discussed. Several questions have been formulated with respect to the effect of interfaces on processing and properties of ceramic materials.
[. INTRODUCTION Bulk properties of compounds are different to those of the interface layer. The difference is mainly caused by segregation m d adsorption. Segregation results in the formation of composition [radients within the interface region. These gradients and 'elated electric fields have a strong impact on local properties uch as transport and, consequently, on reactivity. The ompositional changes within the interface region result in ocal structural deformations. Thus formed low dimensional nterface structures exhibit outstanding properties which are ot displayed in the bulk phase [i].
Polycrystalline materials are composed of individual gra ~ or crystallites. Consideration of their properties requires t! different concepts are applied to the bulk phase and I boundary layer (Figure i).
Figure I. Schematic illustration of a polycrystalline materi involving individual grains composed of the bulk phase and t boundary layer (BL)
The properties of the bulk phase are relatively we described in the literature for most compounds. It has be realized, however, that boundary regions (Figure i) ha entirely different properties in respect to both composition a structure. Figure 2 illustrates the effect of Cr on bu electrical properties determined by thermopower and the surfa E F [3]. Therefore, the characterisation of bulk properties not sufficient to explain the properties of polycrystalli materials involving interfaces. In other words full character sation of polycrystalline materials requires that chemic composition, structure and nonstoichiometry be determined f the interface layer in addition to the bulk phase. This paper will consider nonstoichiometry and relat defect disorders for nonstoichiometric oxides independently f the bulk phase and for the boundary layer. The bulk vs. bounda layer defect disorders will be discussed in terms of properti of ceramic oxide materials such as electrical properties.
Many properties of ceramic materials, such as dielectric properties, nonlinear characteristics of resistance and catalytical properties are determined by the interfacial properties rather than by bulk properties. This strong impact of interfaces on the functions of ceramic materials has resulted in an increasing interest in the determination of the properties of interfaces such as grain boundaries and gas/solid interfaces. The purpose of this paper is to consider basic aspects of nonstoichiometry of ceramic oxides as well as related appl~ed aspects. The effects of defect segregation on nonstoichiometry and related defect chemistry of the interface layer will be discussed in more detail.
2. ASPECTS OF MATERIALS
CHARACTERISATION
Routine studies of solids, based on nonstoichiometric compounds, involve the determination of the following properties: (i) chemical composition, (2) structure, and (3) nonstoichiometry. 2.1. Chemical Composition The basic chemical composition essentially involves the host lattice elements in both cation and anion sublattices. The properties of compounds also depend on the concentrations of foreign ions which are present either in a form of intentional solutes (dopants) or un-intentional additions (impurities). The effect of the impurities on properties may be substantial even if they are present at the ppb level [i, 2]. 2.2.
Structure Structural and phase relation studies aim at the determination of the stability range of the materials within a given range of temperature and gas phase composition. 2.3. Nonstoichiometry The studies on nonstoichiometry involve the determination of the extent of the deviation from stoichiometry in all sublattices and related defect disorders. In the case of metal oxides oxygen nonstoichiometry plays an important role. This nonstoichiometry is determined by oxygen activity of the gas phase surrounding the specimen during annealing at elevated temperatures.
3. NONSTOICHIOMETRY AND DEFECT STRUCTURE 3.1.
Bulk
Phase
Defect chemistry has been widely applied to explain properties of non-stoichiometric compounds such as semiconducting and transport properties.
1.1
Ni0- CrzO3
'/
1.0 >
1.1. ILl
,-fi,, o.9 ,,>, J
1.[CFN~] =IVan] 2"[CrNi] = 2[VN'i ] 3.[Cr'Ni] + [h" ] :2[VN' i ] + [VNi] 4.EXPERIMENTAL RESULTS
0.8 Z
I
I
-5
-4 ln[Cr]
I
I
-3
-2
I ! I
I
-1
[Cr in at.%]
Figure 2. Fermi energy level, EF, of Cr-doped NiO as a function of Cr concentration. Curves I, 2 and 3 correspond to the theoretical dependencies determined using the mass action law and different charge neutrality conditions. Black points (curve 4) correspond to experimental data determined by thermopower [3 ]
The concentrations of the predominant lattice defects in most of the binary metal oxides have been determined as functions of temperature and oxygen partial pressure [4].
The most reliable data on the defect structure of the bulk phase are based on properties of single crystals. In considering p o l y c r y s t a l l i n e materials one should realize that a m e a s u r e d property always involves the bulk component and the interface component [i]. Evaluation of these two components will be a major task of future studies on correct c h a r a c t e r i s a t i o n of ceramic materials. In the following section the basic rules for deriving defect disorder models will be considered for both binary and ternary metal oxides. 3.1.1. Binary Metal Oxides 3.1.1.1. Effect of p(O2) on Composition Bulk defect disorders for binary metal oxides will be considered for metal deficient oxides such as CoO and NiO. The p r e d o m i n a n t defects in these oxides are metal vacancies and electron holes which are formed as a result of oxygen interaction with the oxide crystal. The formation of these defects may be r e p r e s e n t e d by the following equilibria: i/2 0 2
~
V~'
x
+ 2h'+ O O
(1)
where V'' denotes a doubly ionised cation vacancy, h" is an electron hole and OX0 denotes oxygen in its lattice site. The lattice charge neutrality condition requires that Ch'] = 2CV~']
(2)
where the brackets denote concentrations. Eq. (2) represents a local charge neutrality requirement within the entire bulk phase, when the crystal is in thermodynamic equilibrium. Based on condition (2) one may derive simple relationships between the c o n c e n t r a t i o n of defects and oxygen partial pressure, p(O2): [h'] =
P(O2 )I/n exp {-AHf/RT}
(3)
where AHf is the change of the formation enthalpy of metal vacancies, and n is a parameter related to valency of the defects. Changes in nonstoichiometry, corresponding to shifts in equilibrium (i), may be monitored by measurements of electrical properties such as electrical conductivity:
a = q [h'] ~h
(4)
where q is the elementary charge and #h is the mobility of electron holes. According to Eq. (i)-(3) the deviation from stoichiometry in equilibrium is determined by temperature and oxygen activity in the gas phase. Therefore: y = f{T, p(O2)}
(5)
where x + [v~] + [vM]
y = z [v~,]
- 1 0 [ C~
'/~
1473 K
-10
Co0
/,
//
//Vc,o
-30
/
9
- 3.0
f
o
"
-40
-
-5.0 -
-60
-6.0 I/
I
-5 -3 -1
I
I
I
I
I
1 3 5 7 9
log po[ po:~n Pcl ]
Figure 3. The stoichiometry, concentration undoped C00 as p(O2) at 1473 K
deviation from y, and related of defects in a function of [5]
-"
- [D']
-
-50
11
/7/
1473 K
,.y
2~ o-40
(6)
-
i
I
I
-5 -3 -1
I
1
1
I
I
1 3 5 7 9
log po[ Po2in Pcl ]
Figure 4. The deviation from stoichiometry and the concentration of defects in CoO doped with a donor as a function of P(O2) at 1473 K [5]
-1.0
-2.0
CoO 1473K _ 0.1%
,#~
[A']
-3.0
~ o
-4.0
tvc~
i
-5.0
/ -6.0 ,
,/,
-5-3-I
,
,
,
,
1
3
5
79
,
[og Po2[Po~nPo.] Figure 5. The deviation from stoichiometry and the concentration of defects in CoO doped with an acceptor as a function of p(Oz) at 1473 K [5]
where [V'.] and [VX,] denote the concentration of singly ionised and neutral cation vacancies. Figure 3-5 illustrate the total nonstoichiometry, y, as well as the c o n c e n t r a t i o n of particular defects for donorand acceptor-doped CoO [5]. As seen both donors and acceptors have a substantial effect on the defect structure already at the level of 0.i at %. In both cases the effect of p(O2) on y is observed above about 105 Pa while below this value the nonstoichiometry is determined by the concentration of the addition. In equilibrium the activity of defects is the same within the entire crystal (except the boundary layer). Then Eq. (2) represents the local bulk lattice electroneutrality requirement.
3.1.1.2. Effect of p(O2) on the Mobility of Electronic Charge Carriers A c c o r d i n g to Eq. (4) the electronic component of the electrical conductivity depends on both the c o n c e n t r a t i o n of charge carriers, such as [h'] and [e'], and their mobility, ~h" It has been a general assumption that a change of oxide n o n s t o i c h i o m e t r y results in a change of its electrical conductivity via the concentration of electron holes while the mobility term, ~h, remains constant [4]. Recent study of electrical conductivity and thermopower of undoped CoO performed by Kowalski et al. [6] has shown that change in P(O2) results in changes of not only the c o n c e n t r a t i o n term, [h'], but also the mobility term, #h, of Eq. (4).
'_.~
CoO 1273 K
("4 ,.--,,
o
9 SINI3LECRYSTAL
~0./,
/
o POLYCRYSTAL
oi"
_.1 0 -r Z
o0.3 u
o
w !1 0
. . ~ 6"-
0
>- 0.2
l.--
I
2
10g p(02)
m
3 [p(02)in Pal
/+
5
O Z
,.---,,
C00
,,-/
> 0.5 15
1373 K 9 SIN6LECRYSTAL o POLYCRYSTAL
,--.., i-.
:<,
/
~
I/+73 K
I./3 l.J.l
" 0.~
a SIN6LE CRYSTAL
/
/
o
O nZ O
i~I
~
L_J uJ
03 u_ 0
o
>I.-__1 rn 0
log p(02) [p(02) in Pal
Figure 6. The mobility of electron holes in CoO, ~h, as a function of equilibrium p(O2), at 1273, 1373 and 1473 K, for both single crystal and polycrystalline specimens [6]
In other words it has been shown that change in oxide nonstoichiometry results in change of the mobility of electronic charge carriers. This effect was observed for both single crystals and polycrystalline materials but it is still not clear what is the effect of grain boundaries on the character of this dependence (Figure 6).
3.1.1.3. Effect of Aliovalent Ions Incorporation of aliovalent ions into the lattice positions of a metal deficient oxide, M1.yO, forming acceptors (A) and donors (D), may be represented by the following equilibria for mono- and tri-valent ions, respectively: x
(7)
A20 + 1/2 02 - 2A~ + h" + 200 x
(8)
D203 - 2D M + V~' + 300
The following requires that
[AI~I] + [ e ' ]
general
lattice
electroneutrality
+ 2 [ V ~ ' ] = [DM] + [ h ' ]
condition
(9)
According to the condition (9) one may control the concentration of the electronic charge carriers [e' ] and [h'] by doping the crystal with aliovalent ions. 3.1.2. Ternary Metal Oxides The predominant defects in model ternary oxides, such as BaTiO3.y , are oxygen vacancies (V0"), cation vacancies (VTi'''' and Vsa'' ) electronic charge carriers and impurities [7]. The full lattice electroneutrality condition in the crystalline bulk requires that:
[h']+2[V O " ]+[DTi ] = [e']+4[ V'''' Ti ]+2[ V'Ba' ]+[A~i ]
(10)
where [D'Ti] and [A'Ti ] denote the concentration of singly ionised donors and acceptors, respectively, in the Ti sublattice. 3.2. Interface Layer 3.2.1. Surface Charge Neutrality Requirements Construction of any defect model for the bulk phase must be based on the local lattice electroneutrality condition which requires the balance of charge at any point within the crystal. In contrast to this requirement, local charge neutrality for the surface layer is not required. Then an excess of the surface charge may be compensated by the space charge in the boundary layer as is schematically illustrated in Figure 7.
]0
+++ +++ +
+
w L9 ~r nu
I I
Z U.l l--
DISTANCE FROM THE SURFACE
~P
DISTIANCE FROMTHE SURFACE
Figure 7. Surface models of nonstoichiometric compounds involving both potential and charge distribution within the surface and space charge layers
3.2.2. Binary Metal Oxides At elevated temperature the lattice defects have a tendency to segregate resulting in an enrichment (or impoverishment) of the boundary layer in certain elements. Segregation of defects results from their different formation energies in the bulk and in the boundary layer. In consequence, segregation leads to the formation of strong concentration gradients within the boundary layer. Accordingly, the concentration of defects within the boundary layer is a function of both T and p(O2) as well as position (the distance from the interface {):
y = {T, p(O2), {}
(11)
11
"•VER /~-i(T, Po2,
LAYER STRUCTURE
~)
y- f ( T, Po2)
Ml_+yO
SUBLAYER BULK
MOl-y
Figure 8. Illustration of the nonstoichiometry the surface layer
gradient within
In the case that segregation results in an enrichment in defects their concentration increases with the decrease of the distance, ~. Then strong interactions between the defects result in the formation of a surface low dimensional structures (Figure
8). Assuming that in metal-deficient oxides metal vacancies segregate to interfaces then the interface layer is enriched with these defects resulting in the formation of a negative charge at the interface. The surface electroneutrality requires that the segregation-induced negative surface charge within the outer layer is compensated by a positive charge within the space charge layer (Figure 7). This model becomes more complicated when aliovalent cations are present in the lattice (see section 3.3.). Figures 9 and I0 illustrate the surface model of undoped and Cr-doped CoO. As seen introduction of tri-valent ions into the lattice of CoO (and also NiO [3]) results in a change of the surface model involving an inversion of the surface charge and the space charge. 3.2.3. Ternary Oxides The studies of Zhang et al. [8] have shown that segregation leads to an enrichment of the BaTiO 3 surface with both Ti and Ba vacancies, however, different formation energies of these two defects result in a Ti/Ba ratio greater than unity within the surface layer. It has been observed that the surface enrichment in BaTiO 3, expressed by the Ti/Ba ratio, depends on oxygen nonstoichiometry as well as the rate of cooling (Figures ii and 12) [8]. The results obtained are consistent with a surface model involving a negative surface charge and a positive charge in the space charge layer (Figure 13).
12
i i I
~
"~
I I I
i
"'-
SPACE CHARGE
I
I,,
I
I --
-
i
I
I
.
|
.
.
.
.
.
.
|
~_J 0_
,~ ~ 9
,
,
~
'
,
,,',
,
|
i
DISTANCE FROM THE SURFACE
Figure
9.
Surface
model
of u n d o p e d
CoO
0
__I ,.--,.
o~ . ~o >
U 0
SPACE CHARGE
Z
l..ul
_3 4:
l---
= I = ! !
l-§
|
|
|
|
§
§ §
l.ul _3 Lul
~
!
,,<~,
, I !
ugln
....
J
DISTANCE FROM THE SURFACE
Figure
I0.
Surface
model
of C r - d o p e d
CoO
~
13
Estimated 2.0 i
1.5
1.o5-1
,-
2.5 9 i
,
Probed
Depth
3.0 i
,
3.5 i
--
].oo
(nm) 4.0 i
9
Angular-Dependent Oxidised
9~
4.5 I
,
,
i.o5
XPS BaTiO 3
1.00
0.95
0.95
0.90
0.90
0.85
0.85 Rapidly
0.80
-
,
0.3
Cooled
,
Samples
,' 0.5
'
0.4
".
, 0.6
.
, 0.7
~
~-0.8
. -
, 0.9
,-
0.80 1.0
sin0
Figure ii. The Ti/Ba XPS intensity ratio vs. probed depth rapidly cooled BaTiO 3 [8]
Estimated 1.o5
.5
2.0
~ ~ , , ,
Probed
2.5
I
I
Depth
3.0
I
' ....
(nm)
3.5
l
'
--I
4.0 --'
l
4.5 '
"I
"
,- 1.05
XPS
Angular-Dependent
1 .oo
for
1.00
. ~ 0.95
0.95
0.90
0.90
b-, 0.85
0.85 Slowly
0.80 i 0.3
,
Cooled ,
o14
~ 0.5
Samples ,.
, o16
' " ' 0.7
~ 0.8
"'
,0.9
r
0.80 1.0
sin0
Figure 12. The Ti/Ba XPS intensity ratio vs. probed depth slowly cooled BaTiO 3 [8]
for
14
,.., ~. ~ ] ~ "~
Space Charge Layer
Bulk
Vo", h"
o N
-
+
-:-+ \ \
+
+++
Distance from the Surface
Figure
13. Surface model of undoped BaTiO 3 [8]
It has g e n e r a l l y been assumed that barium titanate involves only oxygen v a c a n c i e s (BaTiO3.z). The studies of Zhang et al. [8] have shown that a deficit in both cation sublattices of BaTiO 3 must also be c o n s i d e r e d within the surface layer. Accordingly, a correct formula for the boundary layer of barium titanate should be w r i t t e n assuming deficit in all sublattices: Bal_xTil_yO3_ z
(12)
where x, y and z are a function of p o s i t i o n w i t h i n the b o u n d a r y layer, ~, as well as of t e m p e r a t u r e and P(O2). In the boundary layer all of the q u a n t i t i e s x, y and z assume substantial values while in the bulk phase both x and y is very small because their t r a n s p o r t from the surface into the bulk is e x t r e m e l y slow. One may expect that the observed s u r f a c e - i n d u c e d e n r i c h m e n t of the surface layer of BaTiO 3 in B a - v a c a n c i e s has an effect on the electrical p r o p e r t i e s such as t h e r m o p o w e r (S). In fact S assumes d i f f e r e n t values for a single crystal, Ssc , and a polyc r y s t a l l i n e material, Sp.
15
It was documented that the S,c/Sp ratio assumes 1.72. This value, which is independent of temperature, coincides with the ratio of the theoretically determined thermopower value using the band model, St, to Sp [9]. This agreement suggests that the value of the ratio, Ssc/Sp=l.72 , is determined by interfaces. 3.3. Bidimensional Surface Structures Segregation results in a much higher concentration of defects in the interface layer than in the bulk phase. It was shown that the interface layer may accommodate defects which are not displayed in the bulk phase such as Co interstitials in CoO [i]. The enrichment of the interface layer in defects results in substantial interactions between the defects within the interface layer resulting in the formation of defect complexes and clusters which are not present in the bulk. When the segregation-induced concentration of defects in the boundary layer exceeds a certain critical value then a local structural reordering takes place resulting in the formation of bidimensional interface structures. Presence of these structures are limited to the outermost layer. These structures have outstanding properties not displayed in the bulk phase. Their identification is of strategic importance for the preparation of materials with enhanced properties. 3.4. Conclusions Correct understanding of the properties of the interface layer requires determination of the local nonstoichiometry and related defect disorders involving (i) the host lattice defects, (ii) intentional solutes, and (iii) impurities (unintentionally added solutes). Thus urgent studies are needed to accumulate an empirical data base on the effect of segregation on defect chemistry of the interface region and related properties. Enrichment of the interface layer of ionic crystals in defects results in local structural deformations which lead to the formation of bidimensional interface structures with outstanding properties. Studies are needed to characterise these structures. 4. EFFECT OF INTERFACES ON TRANSPORT In considerations of gas/solid kinetics it has been generally assumed that reactions taking place at the gas/solid interface are very fast and that bulk transport is rate controlling. This assumption has resulted in appropriate interpretation of the equilibration kinetic data which, according to the above assumption, have been considered as corresponding to the bulk transport kinetics.
16
It was shown that the above assumption is not always valid and that the gas/solid equilibration kinetics may be controlled by a slow process at the gas/solid interface. It was shown that the gas/solid interface may have a substantial effect on the local transport kinetics within the interface layer [i, i0]. This effect results either from the segregation-induced electric fields or structural deformation of the interface layer. In the following sections several examples of interface reactions, which are rate controlling for the gas/solid kinetics will be considered. 4.1. Effect of Seqreqation-Induced Electric Fields The electric field, which is formed along segregationinduced concentration gradients within the interface layer, may have a strong effect (either retarding or accelerating) on transport of defects across these fields [i0]. In the first case segregation results in the formation of a local surface diffusive resistance which is rate controlling the gas/solid kinetics. Therefore, diffusion data obtained by using solutions of the diffusion equation for bulk-controlled kinetics result in apparent diffusion coefficients which involve two components: the bulk diffusion component and the component corresponding to the surface diffusion resistance. Verification of the available diffusion data from the viewpoint of the surface component is required for obtaining real values of the transport numbers in the bulk phase and in the interface layer. However, application of appropriate analytical solutions of the diffusion equation, involving the segregation-induced diffusion resistance, requires knowledge of the segregation-induced concentration profiles within the surface layer [I0]. 4.2. Effect of the Local Dopinq of the Interface Layer It was shown that equilibration of the oxygen/BaTiO 3 system is very fast. As seen from Figure 14 the isothermal changes of electrical conductivity for an undoped BaTiO 3 single crystal at 1310 K during a re-equilibration is limited to about 30 min. After this time the electrical conductivity assumes a new constant value which corresponds to a new equilibrium state. Incorporation of a donor into a surface layer of BaTiO 3 results in a substantial change of the transport within this layer and, consequently, in the gas/solid kinetics. As seen from Figure 15 the incorporation of Nb into a surface layer results in an increase of the equilibration time from about 30 min for a single crystal of undoped BaTiO 3 to several months for a polycrystalline material of Nb-doped specimen.
17
0.019 ,'T
l.ul Z
t--o-.o-<)-..o-,,o p02= 3.9" 1012 Pa
0.016 0.013
w_< l_j
,.o
=
Q Z O
u
0.003
[~~
p02= 3.9.1012 Pa I
I
TIME [h] Figure 14. Change of the conductance of undoped BaTiO 3 single crystal during equilibration (reduction run) at 1310 K [9] Figure 16 illustrates the model of Nb-doped BaTiO 3 which involves a strong gradient of Nb within the surface layer of BaTiO 3 (about 0.1-0.2 ~m thick). The donor doping results in a substantial reduction of the concentration of the mobile oxygen vacancies and the formation of cation vacancies which exhibit very low mobility. Concordantly, the formation of the surface layer doped with Nb results in blocking the transport of oxygen and, consequently, in retarding the gas/solid kinetics. 4.3.
Surface Equilibration Kinetics Equilibration kinetics of the surface layer may be monitored by measurements of work function. Figure 17 illustrates changes in the contact potential difference (CPD) between yttria-doped zirconia and a Pt electrode (these CPD changes are determined by work function changes of the zirconia specimen) during consecutive runs of oxidation and reduction at 1054 K [2]. The absolute CPD values as well as their slow changes within the time of annealing, ACPD, are determined by segregation kinetics of impurities such as Ca and A1 [2]. 4.4. Conclusions Nonstoichiometry within the interface layer of undoped oxides may be considered in terms of segregation of intrinsic lattice defects such as cation and oxygen vacancies, cation interstitials and defect complexes.
18
70
60
Z
~ 50 rr
30 ~
P02 = 220 Pa
I
1
I
i
30
60
90
1
120
TIME [ DAYS ]
Figure 15. Changes of resistance of a polycrystalline of Nb-doped BaTiO 3 during isobaric equilibration [9]
I .,,-,. --.i:
Z
>
CK I-Z
w "la.
o
~
o,%
DISTANCE FROM THE SURFACE
Figure 16. Surface
layer model of Nb-doped BaTiO 3
specimen
19
I-\
~' eR
R
04 >
~ J
C3 13..
o
03
02 -
I ~
( 1.74-10 Pa ) (4.3 9lOZ'Po )
"--.2
I
i
I
i
9
20
40
60
80
100
0.1 - R-REDUCTION O-OXIDATION
- -
2
TIME
"L'~--~ Z_./
f ft
0
,00
120
[h ]
Figure 17. Changes in CPD between y t t r i a - d o p e d zirconia and Pt during p r o l o n g e d annealing at 780~ (increase of CPD c o r r e s p o n d s to increase in work function of zirconia) [2] However, assuming that all m a t e r i a l s contain less or more impurities we have to realize that the picture of the segregat i o n - i n d u c e d c o m p o s i t i o n gradients should be c o n s i d e r e d as a s u p e r i m p o s i t i o n of intrinsic defect c o n c e n t r a t i o n s and extrinsic defect c o n c e n t r a t i o n s (impurities). Lateral interactions between all these defects results in the formation of low d i m e n s i o n a l interface structures. It is e x t r e m e l y difficult to determine e x p e r i m e n t a l l y the s e g r e g a t i o n of intrinsic defects resulting from n o n s t o i c h i o m e t r y because most of the available surface t e c h n i q u e s are not sensitive enough for d e t e c t i o n of changes in crystal stoichiometry. In contrast to these intrinsic defects most of the reports on s e g r e g a t i o n in oxide m a t e r i a l s concern solutes. Impurities are also termed as u n i n t e n t i o n a l dopants present at a very low concentration. Despite this low c o n c e n t r a t i o n their effect on m a t e r i a l s properties can be significant. Traces of a l i o v a l e n t impurities added to an insulating crystal, such as Li in NiO, result in its t r a n s f o r m a t i o n into a s e m i c o n d u c t o r or even a good conductor. The impressive effect of traces of MgO on s i n t e r i n g of alumina may serve as an example of a substantial impact of additions on p r o c e s s i n g and resulting p r o p e r t i e s of the materials.
20
A c c o r d i n g to the d i s c u s s i o n presented above s e g r e g a t i o n of impurities in n o n s t o i c h i o m e t r i c compounds must be c o n s i d e r e d along with intrinsic defects and their mutual interactions in the b o u n d a r y layer. Therefore, the picture of s e g r e g a t i o n of an impurity is well defined when the spectrum of all the impurities and the conditions d e t e r m i n i n g the equilibrium, i.e. t e m p e r a t u r e and the gas phase composition, are well defined. The observed substantial effect of e q u i l i b r i u m p(O2) on the depth p r o f i l e of solute ions, such as Cr in NiO [ii], confirms a strong effect of oxide n o n s t o i c h i o m e t r y within the interface layer on the segregation p r o f i l e of the ions. C o m p e t i t i v e segregation of several elements of d i f f e r e n t driving forces may result in the formation of a s a n d w i c h - t y p e surface layer as has been reported for y t t r i a - d o p e d ZrO 2 [2]. In this case the outer surface layer is p r e d o m i n a n t l y e n r i c h e d with one elements and the sub-surface layer is enriched with another one. The enrichment coefficient (surface/bulk concentration) of defects may assume several orders of m a g n i t u d e (104 - l0 s) [i, 2]. Therefore, the s e g r e g a t i o n - i n d u c e d c o n c e n t r a t i o n s in the interface layer may assume very high values even if the segregating elements are present in the bulk phase at a very low level.
5. S E M I C O N D U C T I N G 5.1.
PROPERTIES OF INTERFACES
Effect of A l i o v a l e n t Ions Fermi energy is the basic quantity of s e m i c o n d u c t o r s which is sensitive to the density of states (donors and acceptors) and their p o s i t i o n in the band model. Thus m e a s u r e m e n t s of the Fermi energy are very sensitive to the defect structure and related s e m i c o n d u c t i n g properties. It was d o c u m e n t e d that the m e c h a n i s m of i n c o r p o r a t i o n of a solute in the bulk phase may be different to that corresponding to the interface layer [i, 3]. Parallel m e a s u r e m e n t s of thermopower, which is a bulk sensitive property, and work function, w h i c h is a surface sensitive property, have shown that Cr acts as a donor when incorporated into the bulk of NiO while in the bulk Cr forms acceptor centres (above 0.2 at%). These studies also indicate that tri-valent ions, such as Cr, result in changes of the surface polarity from negative for u n d o p e d NiO to p o s i t i v e for Cr-doped NiO. Similar model is valid for CoO [12].
2]
5.2. Thin Films Electrical conductivity and thermopower are essentially bulk sensitive properties. However, decreasing the thickness of thin films result in increasing the interface component of the measured property. Figure 18 illustrates the reciprocal of the oxygen pressure exponent of electrical conductivity for undoped CoO as a function of film thickness [12].
4,2 4,1
4.0 0 "~
3,9
O')
o .. D o., O r,.,..
'
3,8 3,7 3,6
II
3,5
. - ~
3,4 3,3
.... 900
, .... 950
, .... 1000
, .... 1050
TEMPERATURE
Figure 18. Reciprocal thin films [12]
the P(O2)
o []
7.95 I~m 1.05 pm
A
0.375 p.m
, .... 1100
. 1150
.... 1200
[~
conductivity exponent
for CoO
As seen this quantity decreases with decrease of the film thickness to values much below four which is a critical value for ideal defect model of CoO [4]. This effect indicates that the interactions between defects in the interface region of CoO are much stronger than those in the bulk phase. These interactions may be considered in terms of Co interstitials, which form preferentially in the boundary layer and, along with Co vacancies. Both these defects result in the formation of the spinel-type overlayer structure. These structural changes within the interface layer lead, in consequence, to the formation of a Schottky-type barrier within this layer (Figure 19).
22
Figure 19. A Schottky-type
barrier at the CoO surface
5.3. Conclusions Defect disorder of the boundary layer and related semiconducting properties may be entirely different to those of the bulk phase. Also the mechanism of incorporation of solute ions may be different as well. Accordingly the mechanism of doping which has been established for the bulk phase of compounds [4] is not valid for the interface layer. The determination of the effect of doping on the local defect chemistry of the interface layer requires an independent experimental approach either by studying surface sensitive properties, such as work function, or (2) by measuring bulk properties for specimens of different grain size or (3) by measuring properties of thin films.
6. APPLIED ASPECTS 6.1.
Effect on Sintering
The effect of MgO on sintering of alumina has been known since 1956 [13]. Since then intensive studies have been carried out on the determination of surface and grain boundary properties of Mg-doped alumina [14-16]. The main difficulties in the determination of Mg-segregation in alumina were produced by a very strong segregation of Ca.
23
Only very recently it has been found that Ca selectively segregates only to the prism crystallographic plane (i010) resulting in enrichment by a factor of 103 while the enrichment of the basal (0001) plane in Ca is only 50 [16]. On the other hand Mg segregates effectively to both planes, however, its presence at the external surface is limited because of substantial evaporation. 6.2. Ceramic Gas Sensors The sensing signal, which usually involves change of an electrical property of the sensor material, is generated at the gas/solid interface (Figure 20). Therefore, in the preparation of gas sensors with desired sensitivity and selectivity the main attention should be focussed on surface technology.
0--o CO C02 o-0-o H20> COATING m
BULK MATERIAL
Q
~
Figure 20. Schematic illustration of a gas sensor based on the determination of electrical conductivity In chemical gas sensors the sensing signal is generated during the charge transfer between the adsorbed (or absorbed) gas molecules and the surface of a semiconducting sensor material. Accordingly, the desired sensitivity of sensors towards a particular gas phase component may be achieved by preparation of surface which exhibits possibly a high reactivity with this component. The selectivity may also be increased by decreasing the surface reactivity with other gas phase components. Modification of the surface reactivity may be achieved by changes of surface chemical composition e.g. via local dGping and coating (Fig. 16).
24
Segregation of defects, mainly of aliovalent ions, plays an important role in the modification of surface properties. One of the most sensitive ways of measuring the sensing signal at the gas/solid interface is based on the measurement of surface potential using a high temperature Kelvin probe [17]. It appears that this probe is extremely sensitive to changes in surface composition and, therefore, may serve as a sensor for the determination of traces of gases.
Figure 21. Illustration of the PTCR effect within a single grain and cross two grains [18] 6.3. Non-linear Effects Awareness is growing that interfaces have a substantial effect on properties. This effect can be well illustrated by the positive temperature coefficient of resistance (the PTCR effect) which has been reported for BaTiO 3. This effect, however, is restricted to polycrystalline material and is not displayed by a single crystal (Figure 21 [18]). Studies of Mizutani et al. [18] have shown that at least one grain boundary is required for the PTCR effect to be observed. Attempts to understand the PTCR effect and also other properties of ceramics have resulted in an increasing interest in studies of interfaces of ceramic materials.
25
6.4. Catalysts Catalytic properties of solids, such as activity and selectivity towards the formation of desired reaction products, are determined by the properties of the surface and related active centres which are formed during the preparation of the catalyst and its subsequent activation process. 6.5. High T c Superconductors There have been several reports indicating that segregation-induced changes of the local grain boundary composition result in the formation of weak links in conducting properties of oxide cuprates [19, 20]. One of the reasons for the weak links is the decrease of the local T c within the grain boundary region. Alternative reports have shown that the local T c in grain boundary regions may assume a much higher value than in the bulk phase [19]. 6.6. Metallization of Ceramics Metallization resulting in strong adhesion at the metal/ceramic interface is an important issue in micro-electronics. The adhesion is determined by properties of individual surfaces such as chemical composition and microstructure. One may expect that segregation of lattice defects and its effect on the local interactions at the metal/ceramic interface may be an important aspect of interface engineering. 6.7. Conclusions Interfaces have a controlling effect on the properties of industrial ceramic materials such as sensors [21, 22], dielectrics [23] and non-linear resistors [18]. So far, however, the preparation of these materials is empirically based rather than based on knowledge of the local properties of interfaces. Therefore, there is an urgent need to increase our understanding of interface properties and the relationship between the local properties of interfaces and the properties of bulk materials.
7. INTERFACE
ENGINEERING
Awareness is growing that in order to meet the tough requirements of new technologies, development of advanced materials of enhanced properties will be required. In the case of functional ceramic materials the main effort should be focussed on engineering of interfaces, such as surfaces and grain boundaries, which control the properties of functional
26
ceramic materials. The engineering of interfaces should be based on through understanding of their local properties and better understanding of the relationship between the interface properties and conditions of processing such as gas phase composition, temperature, time of annealing and rate of cooling. Appropriate engineering also involves appropriate post preparative treatment of materials such as local doping of the interface layer, coating and etching. An important strategy in the development of interface engineering involves better understanding of the properties of low dimensional systems such as thin films, multilayer systems, fine grain ceramics and heterogeneously dispersed systems. An important area involves the effect of impurities on interface composition and related properties. Correct characterisation of materials for their impurity level, especially at low levels, is very difficult. It has been realised that even traces of impurities in the bulk phase may segregate during a high temperature treatment, resulting in very high enrichment at interfaces. In developing basis of interface engineering composition we have to understand the principles of segregation for compounds. Better knowledge of the segregation phenomenon may help in removing undesired impurities from the interfaces and to bring up the defects which result in the desired composition and, consequently, desired properties. We are still a long way from being able to control the interface composition via appropriate processing and, concordantly, most industrial materials are still fabricated on an empirical basis rather than through understanding of interface phenomena such as segregation and interface transport kinetics. Development of a basis of interface engineering requires the following items to be addressed: i.
There is a need for better understanding of interface nonstoichiometry and related local properties of compounds at elevated temperatures and under controlled gas phase composition.
2.
Basic studies are needed to evaluate the driving forces of segregation and to determine segregation equilibria in order to predict the effect of both un-intentional solutes (impurities) and intentional solutes (dopants) on interface chemistry. Again the segregation equilibria may be modified in a wide range by adjusting the appropriate gas phase composition during all steps of processing.
27
3.
It becomes increasingly important to control nonstoichiometry of the interface region. As in the bulk phase this may be performed either by change of partial pressure of one of the lattice component in the gas phase during annealing or by doping with aliovalent ions. In the case of doping one should realize that the bulk concentration of the dopant should be lower than its desired concentration in the boundary layer by the factor which is equal to the enrichment coefficient. When the segregation enrichment is very high then introduction of traces of elements may be sufficient to modify entirely the interface composition. Also modification of nonstoichiometry in the boundary layer in a controlled way requires knowledge of segregation equilibria.
4.
One should evaluate the relationship between interface chemistry and materials properties. In order to address this issue there is a need to accumulate a data base on chemical composition of interfaces and related properties such as electric properties and dielectric properties.
5.
The low dimensional interface structures, which are formed as a result of defect segregation, must be characterised.
6.
There is a need to develop a strategy of modification of interface properties in a desired way e.g. by forming interface structures in a controlled manner. We also need to develop processing techniques which result in the preparation of reproducible interface structures.
7.
Item #5 & 6 may be effectively addressed using more sophisticated surface sensitive techniques for 'in situ' monitoring of local interface properties of compounds during processing at elevated temperatures and under controlled gas phase composition. Therefore, there is a need to develop new surface techniques which will meet the above requirements.
8.
It is very important that characterisation of ceramic specimens involves routine determination of the concentration of impurities at the level of several ppm or even at lower level. High temperature processing may result in substantial enrichment of these impurities at interfaces. It is impossible to control interface chemistry without knowledge of the level of this un-intentional doping especially with aliovalent ions.
28
9.
Awareness is growing that cooling results in substantial changes in properties of the interface layer. One source of these changes is the formation of concentration gradients in the interface layer as a result of the incorporation of elements coming from the gas phase, such as oxygen. Another effect results from local changes in segregation equilibria. Superposition of these effects may result in a complicated picture of the interface layer. By varying the cooling rate and the gas phase composition one can impose the concentration gradients in a controlled way. So far, these changes in industrial materials have been imposed in an empirical manner rather than based on knowledge of segregation and gas/solid reactivity. Therefore, there is a needed to evaluate the changes that occur at interfaces during cooling and to develop an understanding of the kinetics of interface phenomena that allow one to impose desired changes during cooling.
8. QUESTIONS TO BE ANSWERED The purpose of this section is to formulate some questions which may arise concerning the effect of interfaces and interface segregation on the properties of ceramic materials. i.
What is the segregation-induced enrichment of the interface region in both intrinsic and extrinsic defects? Knowledge of the bulk concentration and the enrichment factor allows one to evaluate the surface composition. In this regard it is important to evaluate the predominant driving force(s) of segregation and the resulting enrichment factor which may reach several orders of magnitude.
2.
How does the segregation-induced enrichment of the boundary layer depend on bulk composition? One may expect that multisegregation results in either attractive or repulsive lateral interactions within the boundary layer. Concordantly, the extent of segregation of ions which are undesired to be present at the interface may be decreased by dissolution of secondary ions which results in repulsive interactions thus leading to decrease of the enrichment in the undesired ions.
3.
How does the segregation enrichment depend on the composition of the gas phase?
29
.
.
.
What is the depth of the segregation profile and how does it depend on crystal properties? What is the effect of segregation on the defect chemistry of the interface region and how may the defect structure be represented? What is the difference between the bulk and the interface miscibility limit and what is the impact of this upon phase relations?
7.
What is the effect of segregation on ionic reconstructions within the interface region and the formation of low-dimensional interface structures? What are the properties of these structures?
8.
What is the effect of the interface structures on material properties?
9.
What is the effect of segregation on the formation of fast diffusion pathways both along and across the interface?
i0.
What is the impact of the interface region on the reactivity and processing of ceramic materials?
Ii.
What is the effect of interface properties on the electrical signal which is generated at the gas/solid interface for sensor-type materials?
12.
What is the impact of segregation on catalytically active surface centers?
13.
What is the role of segregation on the formation of the electrical grain boundary barrier in nonlinear resistors?
14.
How may the interface layer be engineered, using the effect of segregation in order to achieve the desired properties and functions of ceramic materials?
the
formation
of
9. SUMMARY Chemical composition and nonstoichiometry of the interface layer may differ substantially from those of the bulk phase as a result of segregation and related phenomena.
30
The segregation-induced compositional gradients may lead to structural deformations of the outermost interface layer resulting in the formation of bidimensional structures with outstanding properties 9 These properties may be modified, by changes of grain size or by varying the thickness of thin films9 The effect of impurity segregation, even if the impurities are present at very low concentration in the bulk phase, may have a substantial effect on the entire picture of segregation 9 Therefore, the data concerning equilibrium segregation should correspond to well characterized materials with respect to their impurity concentrations 9 Studies are needed to engineer interface properties by imposing the desired chemical composition through an appropriate combination of processes such as segregation of desired elements, annealing and programmed cooling as well as other processes such as coating, implantation and CVD. The gas phase composition is an important processing parameter since the gas phase components interact with the ceramic body during all stages of the process 9 The reactivity depends on temperature 9 However, even at low temperatures the gas/solid reactivity may have a substantial effect on chemical composition of the interface layer and, consequently, on processing and properties of the ceramic material 9
ACKNOWLEDGEMENTS Support of the Commonwealth of Australia through the Department of Industry, Science and Technology (Grant # C91/02826) is gratefully acknowledged. This paper was kindly reviewed by Dr. Cliff Ball. His comments are sincerely appreciated.
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