Textural evolution of zirconia and yttria-doped zirconia on thermal treatment under different conditions

Colloids and Surfaces A: Physicochemical and Engineering Aspects 113 (1996) 165-174

ELSEVIER

iikolDS A SURFACES

Textural evolution of zirconia and yttria-doped zirconia on thermal treatment under different conditions M.R. Alvarez *, M.J. Torralvo Dpto. Quimica Inorgbnica

I, Facultad

de Quimicas,

Received 5 November

Universidad

1995; accepted

Complutense,

28040 Madrid,

Spain

26 March 1996

Abstract Pure zirconia and yttria-doped zirconia samples have been prepared from the corresponding microporous gels under different thermal treatment conditions. Doped materials show a delay in the high symmetry (cubic or tetragonal) to low symmetry (monoclinic) phase transformation, compared to pure ZrO,. However, as in pure zirconia, thermal treatment leads to the textural degradation of doped samples. Ageing of gels at 723 K in air yields mesoporous powders. On increasing the yttria content in the sample, adsorption

capacity and mesopore surface area decrease and the most probable pore radii are slightly smaller. Differences observed in the porous texture of pure and doped zirconia could be justified in terms of the different packing of the primary particles in the gels. In both pure and doped gels, the evolution of the porous structure is highly affected by the conditions at which thermal dehydration is carried out. Under vacuum, the development of a mesoporous texture is hindered. Condensation and elimination of water produce the most important reduction of the micropore volume. Keywords:

Argon and nitrogen adsorption; Crystalline zirconias; Microporous-mesoporous terization; Thermal ageing; Yttria-doped zirconia

1. Introduction Thermal treatment produces the textural degradation of precipitated zirconia gels Cl-61 and, in some cases, the evolution of the porous structure and surface area of crystalline samples has been related to structural changes [6-81. Crystal growth, phase transformation and agglomeration have been identified as the main processes controlling the detrimental textural evolution [8]. Therefore, and in order to improve the textural stability, structural stabilizers such as La,03 [9] or Y,O, [lo] have been used to prepare non-

* Corresponding author. 0927-7757/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved PII SO927-7757(96)03644-X

zirconias; Textural charac-

porous or mesoporous samples [7,8]. Both YZO, and La,O, are good structural and textural stabilizers. They retard the textural degradation by avoiding the surface diffusion and consequently the crystal growth [ 7-101. These results show that textural and structural transformations must be closely related. However, the texture of pure and doped zirconia is highly affected by the preparation conditions and in general the precipitated gels are porous (frequently microporous) materials. In addition to intraparticle pores, the size distribution of primary particles and the degree of packing and joining together determine the porous texture in the gels [2]. Moreover, when the porous gels are heated at relatively low temperature, condensation of water and the elimination of volatile species

166

M. R. Alvarez,M. J. TorralvolColloids Surfaces A: Physicochem.Eng. Aspects113 (1996) 165-174

takes place. These processes also cause detrimental changes in the gels [2-5, 11-131. We have shown [ 141 that microporous zirconia and yttria-doped zirconia gels become mesoporous when heated at 723 K in air. However, in doped samples, in which phase transformations are delayed, the porous texture is more heavily affected by the thermal treatment. In this paper, we analyze the results obtained when zirconia and yttria-doped zirconia gels are treated under different conditions. The ‘samples have been characterized by X-ray powder diffraction, scanning electron microscopy and nitrogen and argon adsorption.

2. Experimental 2.1. Samples The materials used in this work were prepared from microporous gels of Zr02, ZrO, -2 mol% Y,O, and ZrO, - 5 mol% Y,OJ [ 141 (samples denoted Z, Z2Y and Z5Y). The gels were: (i) heated in air at 723 K for several periods ranging from 2 h to 30 days; (ii) heated under vacuum (z 10-l Pa) up to 623 and 723 K (at a heating rate of 2 K min-‘); and (iii) treated under controlled reaction rate thermal analysis (CRTA)’ [ 151 up to 623 and 723 K. In the latter case, the experiments were carried out with a controlled residual water pressure of c 8 Pa and a controlled rate of dehydration of 3 mg h-l. Samples were labelled as follows: (i) X(723/treatment time A); (ii) X( temperature/V); and (iii) X(temperature/CRTA) where X = Z, Z2Y, Z5Y; treatment time=2 h, 20 h, 7 days, 30 days; A: air, V: vacuum. 2.2. Experimental

techniques

and methods

Scanning electron micrographs were taken with a Jeol JSM-6400 electron microscope. X-ray powder diffraction patterns were recorded with a Siemens D5000 diffractometer. Diagrams

1The CRTA experiments CNRS (Marseille, France).

were carried

out at the CTM

of

were scanned in steps of 0.02” (20) and with a 3 s per step counting time. The adsorption-desorption isotherms of nitrogen and argon at 77 K were determined by the conventional static procedure and prior to the adsorption the samples were degassed at 393 K down to about 10-l Pa. The isotherm analysis was made by the BET equation; the BET area was calculated taking values of 0.138 and 0.162 nm2 as the molecular cross-sectional areas for adsorbed argon and nitrogen respectively [ 161. Pore size distributions were obtained from the adsorption branch of the isotherms, taking into account the type of hysteresis loop [ 171, using the Kelvin equation assuming a cilindrical pore model [ 181. The upper and lower limits of calculation were P/P,= 0.93-0.9 (r 6 15 nm) and 0.3

3. Results X-ray powder diffraction data reveal that pure and doped zirconia treated up to 623 K under CRTA conditions are amorphous. When the three gels are treated under vacuum to the same temperature they crystallize in the cubic form of ZrO,, although the cubic (200) diffraction peak of pure zirconia shows a slight asymmetry. Doped samples treated up to 723 K under CRTA and under vacuum, and kept at 723 K in air for 2 h, also correspond to the cubic form. However, patterns recorded for pure zirconia treated under vacuum and in air at the same temperature, show a slight splitting of the reflections (200) and (220) of the cubic phase. This split indicates the onset of the cubic-tetragonal transformation and at this point

M. R Alvarez, M. J. TorralvolColloids Surfaces A: Physicochem. Eng. Aspects 113 (1996) 165-I 74

both phases must be present. In the case of pure zirconia treated up to 723 K under CRTA, the material shows a very poor crystallinity. On lengthening of the thermal treatment in air, the metastable cubic and tetragonal phases progressively transform into the thermodynamically stable monoclinic phase, but these transformations are more delayed as the yttria content in the sample increases [ 141. Fig 1 shows representative scanning electron micrographs of several samples. In all cases they consist of irregular aggregates formed of agglomerates of primary particles. We can see that the

Fig. 1. Scanning ZSY(723/CRTA);

electron micrographs (f) Z5Y(723/V).

of samples:

(a) 2(723/2hA);

161

treatment under vacuum produces larger agglomerates and more compact aggregates. This effect seems to be more important for doped samples. After longer periods of treatment at 723 K in air, the individual agglomerates become less connected, this effect being more evident as the yttria content in the sample increases [ 141. In Fig. 2 we have plotted the adsorptiondesorption isotherms of nitrogen and argon for samples treated under different conditions. Some characteristic textural parameters are summarized in Table 1. They include the BET areas (SBET(NZ) and &&AT)), the ratio of the monolayer capacities

(b)

2(723/CRTA);

(c) 2(723/V);

(d)

ZW(723/2hA);

(e)

168

M. R Alvarez, M. J. Torralvo/Colloids

Surfaces A: Physicochem.

Eng. Aspects 113 (1996) 165-l 74

a

Vads cm3g -l

I-

40

30

20

10

1

I

I

I

t

0.2

0.4

0.6

0.8

1

1.0

d.2

I

I

I

0.4

0.6

0.8

P/p,

Vads cm3g-l

PIP,

Vads cm3g-l

C

60-

60

50-

50

.’

1 1.0

d

I 40

30

-HLI.r___-___.__

__.- -

20

‘“Li57i7Vo 10

I

0.2

PIP,

.

Fig. 2. Adsorption-desorption isotherms of N, (--) and AK (---) (a) 2 h and (b) 30 days and (c) at 623 K under CRTA; (d) sample (0) 723/CRTA; (a) 723/V.

I

0.4

I

0.6

I

1

1.0

0.8

PIP,

at 77 K: Z, Z2Y and Z5Y samples treated at 723 K in air for Z treated at 723 K under different conditions: (0) 723/2hA;

M.K Alvarez, M.J. Torralvo/Colloids Surfaces A: Physicochem. Eng. Aspects 113 (1996) 165-174

169

Table 1 Textural parameters" obtained from nitrogen and argon adsorption for the sample treated under different conditions

Sample

SBET(N2)

SBET(Ar)

Vm(N2)/Vm(Ar) N2

Ar

Spb

Vpb

K

Vr

Spb

Vpb

V,

Vf

Z(623/CRTA) Z2Y(623/CRTA) Z5Y(623/CRTA)

151.7 105.1 66.3

146.2 95.7 64.6

0.88 0.93 0.87

12.4 2.6 1.0

0.017 0.004 0.002

0.082 0.048 0.028

0.066 0.043 0.026

8.2 2.3 1.5

0 . 0 1 0 0.071 0.003 0.041 0 . 0 0 2 0.028

0.063 0.038 0.026

Z(723/2hA) Z2Y(723/2hA) Z5Y(723/2hA)

110.0 72.1 48.4

108.3 69.4 40.3

0.86 0.89 1.02

28.0 10.0 3.5

0.034 0.011 0.005

0.074 0.041 0.026

0.056 0.036 0.023

19.6 7.7 3.7

0 . 0 2 3 0.065 0 . 0 0 9 0.037 0 . 0 0 5 0.020

0.056 0.036 0.018

Z(723/30dA) Z2Y(723/30dA)

23.1 16.2

20.2 12.5

0.97 1.10

25.0 18.0

0.057 0.035

0.053 0.033

15.1 9.7

0.042 0.045 0 . 0 2 3 0.025

Z(723/2hA) Z(723/CRTA) Z(723/V)

110.0 98.4 65.0

108.3 92.4 65.0

0.86 0.91 0.85

28.0 10.4 9.0

0.034 0.016 0.014

0.074 0.056 0.042

19.6 6.0 7.5

0 . 0 2 3 0.065 0 . 0 0 7 0.045 0 . 0 1 0 0.036

,

0.056 0.041 0.030

0.056 0.041 0.030

a SBETand Sp are expressed in m 2 g - i and Vp, Vt and Vf in cm3 (liq.) g-1. b Cumulative surface areas and pore volumes in pores with 1.7-~
for adsorbed nitrogen and argon (Vr~(N2)/Vm(Ar)) and the cumulative surface areas (Sp(N2),Sp(Ar)) and pore volumes (Vp(N2),Vp(Ar)). The values of total volumes (Vt(N2),Vt(Ar)) read from the isotherms, and the amounts of nitrogen and argon (Vf(N2), ~(Ar)) obtained from the intercept of the linear part of each isotherm with the adsorbed volume axis, are also given. The filling of the pores with r ~< 15 nm is included in the total volumes (Vt). Fig. 3 shows pore size distributions obtained for each sample from nitrogen and argon adsorption; broken lines correspond to the results obtained when the calculations were made up to the low pressure region. We can observe (Fig. 2) that in the low pressure region, the amount adsorbed at a given P/Po value is higher for nitrogen than for argon. Moreover, although in most of the cases there is a crossing of the isotherms, the ratio of the monolayer capacities for adsorbed nitrogen and argon (Table 1) is always higher than the ratio of the molar volumes of the adsorptives in the three-dimensional liquid state (0.82). This fact indicates that at the monolayer the concentration of the nitrogen is higher than that for argon, this being a consequence of the stronger interaction of the nitrogen molecule

with the polar sites of the surface [19,20]. In Table 1 we can also observe that the values of total volume (Vt) are higher for nitrogen than for argon but the amounts Vf(N2) and Vf(Ar), which correspond to the filling of most of the pores (Figs. 2 and 3), are similar. Therefore, differences in Vt(N2) and Vt(Ar) must be due to the adsorption in the external surface, which is included in the total volume. From the results of both nitrogen and argon adsorption it can be said that: • Pure zirconia treated under vacuum or under CRTA up to 623 and 723 K gives adsorptiondesorption isotherms indicative of micro- and mesopores (Figs 2c and 2d). These results are similar to those obtained with the treatment in air at 723 K for 2h [14]. However, treatment in air produces the most developed mesoporous texture (Table 1) and slightly larger most probable pore radii (Figs. 3b and 3c). Nevertheless, the maxima in pore size distributions are placed, in all cases, at the lower end of the mesopore range. • The adsorption isotherms of doped samples treated under the same conditions as pure zirconia (Figs 2a and 2c), show a more pronounced type I character on increasing the yttria content and,

M. P~ Alvarez, M.J. Torralvo/Colloids Surfaces A." Physic©chem. Eng. Aspects 113 (1996) 165-174

70

AVp/A [p

A V p / A rp 100-

40-

c m 3 (liq) xlo -4 n m -1 o ;I Ii

80-

a

cm3(liq) x 10-4 n m -1 0'0"°'%

20-

!;io

Z

/f~\\ /Z2Y oo' ~ k / / / Z5Y

Ar

; ,o.O~ ,',z/

o-

"% o / y o

'

u I.

-b,~

/

~o

z

Z2Y

/ J_~._..

Oi'

60-

I oI

Z

I

I

l

I

I

I

1

2

3

4

5

6

Fp

I ,/ / / z s y

/!,,/o/

nm

ool/

40-

N2 20-

o o

Z / Z2Y

o.

I

\

/

o

~+

I

I

I

I

I

I

2

3

4

5

6

7

A V p / A rp c m 3 diq)x i0-4 n m -I 120-

+

~+~+'-+~

, ~

~p nm

b

60-

AVp/AFp 100-

II I !

60-

I

,i l

40-

,hi =1| |1o

100~

t.~/

20-

40-

Z5Y

80-

o II

Ii I

!

!

|

1

2

3

fp rim

Z

40-

60-

40-

/Z2Y i,,// Z5Y

Ar

II |1

illl

|111 III

I!,

C

cm3lliq ix10"4 nm-1

l,; Z f%.~ '/ Z2Y /Ill/ /

lOll

80-

Ar

I'1,

20-

! o i

!

I

II

l®

ko

e

|

,

,

,

,

,

,

I

1

2

3

4

5

~

o

r© n- -m

i

20-

N2

Y/

20-

N2

I I

o

~.,~ I

I

I

I

I

I

2

3

4

5

6

7

~p nm

'

1

'

2

'

rp

nm

Fig. 3. Pore size distributions of Z, Z2Y and Z5Y samples: (a) treated at 723 K in air for (©) 2 h and ( + ) 30 days; (b) treated at 623 K under CRTA; (c) sample Z treated at 723 K under different conditions; (©) 723/2hA; (O) 723/CRTA; (01) 723/V.

M.K Alvarez, M.J. Torralvo/Colloids Surfaces A: Physicochem. Eng. Aspects 113 (1996) 165-174

at the same time, a diminution of the amount adsorbed at each relative pressure. Doped samples have lower BET areas and lower cumulative surface areas and pore volumes (Table 1). The BET areas of samples Z2Y and Z5Y treated up to 723 K under vacuum and under CRTA are smaller than 5 m 2 g-1. Pore size distributions (Figs 3a and 3b) show maxima in the range narrow mesoporeslarge micropores, which shift slightly to the smaller pore radii when the yttria content increases. • After longer periods of treatment at 723 K in air, pure and doped zirconia samples give type IV isotherms (Fig. 2b) with hysteresis loops reveal-

ing heterogeneous mesopore size distributions (Fig. 3a) 1-14]. Fig. 4 shows the variation of characteristic textural parameters (obtained from nitrogen adsorption) with the yttria content. They include: the surface area (S), the contribution of pores with r ~< 1.7nm to the BET area (Sip) and the most frequent pore radii (rp(max)) obtained from the pore size distributions. Surface area refers to the BET area (SBET) and the cumulative area (Sp). The values of Sip have been calculated from the corresponding BET and cumulative areas. The results for samples treated in air at 723 K for 20 h and 7

S

SLp

m 2 g-1

160-

171

100-

%

75

50

25

0

÷ - - 4 -

rp(max)

50-

6-

rim

Sp t

I

+--

_+

I

30-

X

10-

0 •

, 0

}

~

2

5

Y203 mo---~o

~ ....

0 __ •

0 --@

Y203

mo~

Fig. 4. Variation of textural parameters obtained from nitrogen adsorption for the three gels treated under different conditions, as a function of the yttria content: (0) 623/CRTA; (©) 723/2hA; (*) 723/20hA; ( x ) 723/7dA; ( + ) 723/30d; (•) 723/CRTA; ( ~ ) 723/V).

172

M.IE Alvarez, M.J. Torralvo/ColloidsSurfaces A: Physicochem. Eng. Aspects 113 (1996) 165-174

days have been reported in a previous work [ 14]. We can see in Fig. 4 that under any of the investigated treatment conditions, doped samples have lower adsorption capacity and lower mesopore surface area. We can also observe that doped samples have a higher contribution of the finest pores (r ~< 1.7nm) to the total area (with the exception of the samples treated in air for 7 and 30 days). Similar results are obtained from argon adsorption data.

4. Discussion

Precipitated zirconia gels consist of tridimensional aggregates of primary particles which are formed by hydrolysis and polymerization of the tetramer units [Zr4(OH)8"16 H20] 8+ present in zirconium oxychloride solutions [21]. The polymerization rate depends on the concentration of OH groups (i.e. pH and base addition rate) and it controls the size and degree of order in the polymer, as well as the amount of coordinated water and O H - ions closely associated with the tetramer units [22-26]. For the gels precipitated at a high pH value (,~ 10) and in a rather rapid way, polymerization occurs rapidly in many directions at once, leading to the formation of large particles. These present numerous but small ordered regions, random assemblages and empty spaces (intraparticle pores). In the aggregates, the primary particles are linked together through molecular water hydrogen-bonded to surface hydroxyl groups, forming a network of interparticle pores. Thermal treatment of the gels provokes the contraction of micropores [ 11,27] and the enlargement of larger pores [2,6] and, therefore, the progressive reduction of surface area and pore volume. The widening of the pores seems to be more important as the temperature increases and during the thermal treatment in air [2,4,6]. However, the most important effects occurring when the dehydration is carried out under vacuum (at relatively low temperature) are the loss of micropore volume and the reduction in the average pore size [5,27]. Our results for pure zirconia agree with the reported results and are consistent with a dehydra-

tion process recently proposed [28]. According to this, dehydration seems to occur in three stages: (i) elimination of terminal H20/OH groups; (ii) loss of inner H20/non-bridging OH groups; and (iii) oxolation of double hydroxy bonds, elimination of any residual H20/OH groups and crystallization. Although the three stages probably overlap, the water removed in the first stage must be mainly physisorbed water and water hydrogen-bonded to the surface hydroxyl groups. This process causes the emptying of the pores and the formation of hydrogen bonds between terminal hydroxyl groups of different particles. At higher temperatures, the elimination of the inner water (coordinated water) increases the microporosity, and the condensation of terminal OH groups provokes the closing up of pores and/or narrowing of the pore widths. The departure of water can also open the small constrictions to force a way through. This effect will be more pronounced as the vapour pressure of the water inside the particles and/or aggregates is higher. This is probably the reason why enlargement of pores is more important when the dehydration process takes place in air. Final oxolation of the bridged hydroxyl groups results in the crystallization of the gel and hard agglomerates are formed due to the strong Zr-O-Zr bonds between adjacent particles [29]. With crystallization, the microporosity decreases because of the structural rearrangement [5] and, when the temperature increases, the main textural variations (widening of the pores and reduction of surface area) are due to the crystallite growth and phase transformation [2,6]. According to this, thermal treatment of doped samples must result in a more stable texture provided that crystal structure is stabilized and the crystallite growth inhibited. This is the case with non-porous or mesoporous doped samples [7,8,30]. However, the question is more complicated when we compare pure and doped materials obtained from microporous gels. Our results show that under any of the investigated treatment conditions, textural degradation of doped samples occurs more easily, even though the crystallite size is smaller than for pure zirconia treated under the same conditions (as shown by X-ray and electron diffraction and electron microscopy [31]). In

M.K Alvarez, M.J. Torralvo/Colloids Surfaces A: Physicochem. Eng. Aspects 113 (1996) 165-174

doped samples, intraparticle sintering and the packing of the particles into the agglomerates are probably different than in pure zirconia, in such a way that they favour the collapse of the porous structure. Although it is difficult to establish clearly the factors that determine the packing behaviour of the particles, they must be related to the superficial charge. ZrO2, Y203 and yttria-doped zirconia are characterized by different isoelectric points depending on the preparation conditions. For the same conditions, the isoelectric point of hydrated Y2Oa (more basic character) is higher than that of hydrated ZrO21-32,33]. In doped materials, Y(III) must be distributed between the bulk and surface of the particles [34], and their isoelectric points will be between the values for Y203 and ZrO2. If this is so, the final pH of precipitation (~10.5) must be higher than the isoelectric points of both pure and yttria-doped zirconia but closer to the values of the doped samples. In this case, the particles having a minor superficial charge would pack together with a higher average coordination number, forming weaker and denser aggregates. During the drying process, denser agglomerates with narrower interparticle pores are formed. In this way sintering is promoted and the development of a mesoporous texture is prevented.

Acknowledgments The authors are grateful to Dr. J. Rouquerol, Dr. F. Rouquerol and Dr. Y. Grillet from the Centre de Thermodynamique et de Microcalorim&rie du CNRS (Marseille, France) for providing the opportunity to use the CRTA equipment and for their valuable assistance. We also thank Centro de Microscopia Electr6nica de la UCM for the use of facilities.

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