Complex impedance study of annealing effects on polycrystalline KDP conductivity

Solid State lonics 3 1 ( 1988) 49-54 North-Holland, Amsterdam l

COMPLEX IMPEDANCE STUDY OF ANNEALING POLYCRYSTALLINE KDP CON

EFFECTS ON

0. de OLIVEIRA DAMASCENO, J. de OLIVEIRA and A.L. de OLIVEIRA Departamentode Fisica, WniversidadeFederal de Minas Cerais, C.P. 702, Pampulha Al. 161, Belo Horixmte, MG, Brazil Received 3 1 May 1988; accepted for publication

1 July 1988

Annealing effects on the conductivity of KDP (KH2P~4) samples prepared either by melting under slight pressure ( 5.7 kgf/ cm’) or by powder compression ( 1.3X 103 kgf/cm’) were studied in air by complex impedance spectroscopy. In both cases. annealing at 423 K reduces the conductivities to constant values: from 6.2 x 1O-’ to 1.6 X IO-'ST- ’ cm- ’ for samples prepared by melting and from 1.9x lOA7 to 6.1 x 10e8 Q-’ cm-’ for samples prepared by compression. Heating KDP at about 500 K significantly modifies its electric properties. Two relaxation processes are observed after this treatment. One of them is associated with a fairly strong dielectric polarizability. A small conductivity jump is observed close to 440 K.

1. Introduction The KDP (KH,P04) system, whose preliminary study by complex impedance was presente where [ 11, has been extensively investigated mainly at low temperature near its ferro-paraelectric transition ( T,= 121 I(). Only few literature data deal with higher temperature [ 2-51. Many controversial issues remain unsolved about its protonic conductivity. It is not our goal, in the present work, to discuss the contradictory interpretations but merely to stress the importance of annealing effects. Data concerning three samples are presented and were prepared by melting d one was formed by compression (designated D). Complex impedance spectroscopy was utilized as the main characterization method.

P pellets were formed by both melting and The comprcs?ion techniques. For the former commercial * The present work was partially financed by CNPq (Conselho National de Desenvolvimento Cientifico e Technologico) and FINEP 1Financiadora de Estudos e Projetos).

03.50 0 Elsevier Science Publishers 0 167-2738/Q/$ olland Physics Publishing Division ) (No

powder of high purity (Merk A 487 3) Mas placed in a stainless steel mould and ted slowly until it A mod pressoftened, typically at about into the r dye sure of 5.7 &f/cm’ was ap was cooled down. In the compression w&r was placed in a stainless steel uld and submitt to a pressure of I .3 x 10’ kgf/ 2 at room tern ature. In both (mrlair;g and compression ) the fmai dimensions of the pellets were typically 19.8 mm in diameter and 2.2 mm in thickness. The conductivity was determi impedance spectroscopy using a ante Meter in the freauency 4800A Vector Im s , Measurements were made in range 5 Hz-550 air between 373 a ntical electrodes A) were coated of platinum paste ( anao the opposite flat surfaces ofthe cy~i~d~ca~ samples. The current was collected by means ~pf thir! massive platinum cylinders pressed by fo passing th~e,ugil the up er and lowcs t (fig.

1).

0. de 0. Damasceno

et al. /Annealing

Sample 2. Pt coating I-

3, itiusslve Pt 4-Tef Ien cover 5-Cu

wares

6. Thef mocourVe T-Screw Fig. I. Detail of the cell arrangement.

3. Results and discussion of ac menrsuremnts

3.1. Main observations In all cases the annealing temperature was 423 IL This temperature is about 2/3 of the melting temperature and, in this respect, is a typical annealing tempemture. Results presented in the following give an additional reason for the selection of this tempm:ure.

Tht main observations are the following: (i) Afier annealing, whatever the sample and the temperature, the impedance diagram shows a well defined semicircle (fig. 2), The usual frequency distribution Z=Z,/l

-t-(ju/wO)(‘-(y)

was verified. The depression parameter 01was found almost canstant with a value close to 0.1 which indicated a rather goad homogeneity af the measured

FQ. 2. Typical impedance spcetrum samples annealed for 47.6 h.

of the Pt I KW: PO, I Pt cell:

eflecis on polycrystalline

KDP

properties. The dielelectric constant caiculated from w. was found to be c!ose to 30 for the pre-molten samples and 32 for the pressed ones. These values compare favorably with literature data for manocrystals which are 44.3 in a direction perpendicular to the optical axe and 20.5 in the parallel direction [6]. This result shows that the semicircle characterizes the specific dielectric and conductivity properties of the material and therefore 2, provides a measurement of its resistance. (ii} After annealing, 2, exhibits only a small hysteresis on temperature cycling in the range 369- 469 IL (iii) Before annealing, the impedance diagrams depend markedly on the way the samples were preepared. With a pressed pellet the diagrams show only a simple semicircle of the type described above. With pre-molten samples the semi-circles are significantly displaced and no longer cross the origin (fig. 3). A more careful analysis indicates that the observed semicircles describe another phenomena. The main argument which supports this statement refers to the value of the associated dielectric constant which is in the 250-450 range. The specific dielectric-conductivity circle would be located between the observed semicircle and the origin. The frequency distribution of the few experimental points in this range is compactible with a dielectric constant equal to 30. Frequently, such an additionai semicricle is ascriiSed to a grain-boundary blocking effect. Here, it would not be reasonable :o adopt the same intcr-

Fig. 3. ‘Typical impedance spectrum of the Pt 1KHz PO, 1Pt cell: sampleri pre-molten and anncalcd for I .O h.

0. de 0. Dammceno

et al. /Annealing

pretation because the feature is absent with the pressed pellet which should exhibit an enhanced effect if grain-boundary blocking were effective. At this s&e no interpretation can be put forwalci for this additional semicircle probably associated with a remanent of the material enhanced dipole contribution resulting from the material melting. 3,.?. Annealing esfiect.s In the Arrhenius diagram of fig. 4, we can see that three experimental points are not in line with the others obtained under steady state condition. They were determined before submitting the sample (KJDPB) to annealing. This illustrates the need for rationalizing the measurement conditions. Figs. Sa,b and da,b show the influence of annealing on the microstructure of the sample. Whatever the initial elaboration process, annealing homogenizes the samples in terms of grain size aid pore size distribution and, surpringly enough, reduces the average size of the grains. The annealing of the pressed pellet is rather simple. Its characteristic diagram always is a semicircle

KDP

@ctcn on pol.vc~w.dline

51

from wh ,h one can deduce a dielectric constant which indeed is constant and a conductivity which varies as shown in fig. ‘7.It takes about 20 h to reach a clear1 state. With ihr: p~t~-~rrtiiie~ ~rlpir;. ailuneaillrg is more complex. A first ‘“incubation” served which lasts about 20 h. During this stage two semicricles, partly described in the investigated frequency range, are observed as reported above. The corresponding conductivities do not vary much with time. After this stage an annealing evolution, similar to that observed with the pressed sample occurs. It also lasts about 20 to 30 h and is also characterized by a rapid initial variation. 3.3. Conductivity memurernents As mentioned above, after annealing, the sample resistance is stable and the measurements performed in the 369-423 K interval show only a slight hysteresis on temperature cycling. The corresponding conductivity data (figs. 8 and 9) obey an Arrhenius law of the form: L7,=U(jiexp( -E,/kT)

.

(2)

Fitting of this equation gives, for K ff,,~=7,3X10%-

cm-’

and E? E0.S %ev (369-433

cr,, -2.1 x 10’ Q-l cm-’ and Ej ZZo.73eV (385-437 K) . The fit af the data s own in fig. 4 (KDP

gives values very close to eq. (3). Around 440 K a conducti\*ity jump OCCUTS 8 and 9 ). On heating it was observed at 445

3

Fig. 4. Arrhcnius diagram of the bulk conductivity of the KDPB sample.

52

0. de 0. Damasceno et 01. /Annealing eJcfectson polycrystallineKDP

Fig, 5. Electronic scanningpicture for KDPC sample (prepared by melting): (a) before and

Fig. 6. Electronic scanning picture for KDPD

sample (prepared

(b) afier annealing ( l~Q00xmagnificationh

by compression):

(a)

before and (‘>) after annealing

0. de 0. Dntnasceno et al. /Anneahg

efects on polycrystalline

KDP

-6

KOPD Pellet prepared

Pellet

o”o I

4

8

I2

16

20

26

24

32 time

36

40

44

(hours)

.\

Fig. 7. Conductivity of the KDPC sample at 423 K as a function of the annealing time.

Q L

23

24

Y

25 l/T

5

x 103(K?

of the an-

KDPC

*

.

22

Fig. 9. Arrhenius diagram of the bulk conducitvity ( x ) cooling. nealed KDPC sampe: ( ) heating.

I

E

by compressIon

by compressmn

Annealing

L

prepared

Pellet

prepared

by Fusion

an essential role on the magnitude of the measured conductivities. Slightly pre sample when it softens at a ut 500 K gives go contacts and the overall c of a single crystal. Ch th and the powder at room te eraturs ic ~~~~~~~~~~~~ c~~st~~ct~~~of t&e rent lanes at the COntaCt arke reduce the overall cnr?ductivity. 00 K ~~~~~~~~~t~~ modi-

lo”:

are obse*ed a is associated with

lb”..

F

P+----J i2 23

24

25

26

27

2.6

30

Fig. 8. Arrhenius diagram of the bulk conductivity ( X ) cooling. nealed KDBC sample: ( ) hrating,

of the

an-

54

0. de 0. Damasceno et al. /Annealing eflectcrson polycrystalline KDP

( I 1A.L. de Oliveira. 0. de : Damasceno. J. de Otiveira and E.J.L. Sctrouter. Mater. Res. Bull. 2 I ( 1986) 877. [ 21 M. O’Keefe and C.T. Perrino, J. Phys. Chem. Solids 28 (1967)211.

[ 3 1R.S. Bradley. D.C. Murro and S.L. A’i, J. Inorg. Nucl. Chem. 32 (1970) 2513. [4 ] L.B. Harrisand C.J. Vella, J. Chem. Phys. 58 (1973) 4550. [ 5 ] L. Glasser, Chem. Rev. 75 ( 1975) 2 I. [6] AR. Von Hippel, Dielectric materials and applications (Wiley, New York, 1954).