STABILIZATION EFFECTS AT THE PHASE TRANSITION TETRAGONAL : RHOMBOHEDRAL IN LEAD ZIRCONATE TITANATE CERAMICS

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STABILIZATION EFFECTS AT THE PHASE TRANSITION TETRAGONAL : RHOMBOHEDRAL

IN LEAD ZIRCONATE TITANATE CERAMICS

H. Thomann

To cite this version:

H. Thomann. STABILIZATION EFFECTS AT THE PHASE TRANSITION TETRAGONAL :

RHOMBOHEDRAL IN LEAD ZIRCONATE TITANATE CERAMICS. Journal de Physique Col-

loques, 1972, 33 (C2), pp.C2-281-C2-283. �10.1051/jphyscol:1972298�. �jpa-00215034�

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JOURNAL DE PHYSIQUE Colloque C2, supplkment au no 4, Tome 33, Avril 1972, page

C2-281

STABILIZATION EFFECTS AT THE PHASE TRANSITION TETRAGONAL : RHOMBOHEDRAL IN LEAD ZIRCONATE TITANATE CERAMICS

H. THOMANN

Siemens Research Laboratories, D 8000 Miinchen 80, Balanstrasse 73, Germany

Resume.

-

La variation de la stabilitk en frequence des ceramiques pi6zo6lectriques Pb(ZrTi)03 en fonction de la temptrature se linkarise en presence de certains oxydes tels que CrzO, et MnOz.

Ceci est la conskquence de I'inhibition de la transformation de phase par des charges superficielles.

Les porteurs sont piegts au cours de rkactions chimiques par les atomes des metaux polyvalents.

Abstract. -

The temperature dependence of the frequency constant of piezoelectric Pb(ZrTi)Os- ceramics becomes more linear in the presence of some oxides, e.

^g.

Cr203 or Mn02. This is

a

consequence of inhibition of phase transformation by grain surface charges. The charge car- riers are trapped

by

chemical reactions of the multivalent metal atoms.

Lead zirconate titanate ceramics have been proved useful for a lot of applications. In many cases, the control of temperature dependence of the frequency constant is important. Three different effects are rele- vant for this

:

the influence of the morphotropic phase boundary, aging connected with the formation of optimal domain configuration and stabilization of phases and domains in the presence of certain addi- tives.

As a rule, with ferroelectric materials a minimum of the frequency constant is found when a phase transformation takes place. This is shown for the transition from the rhombohedral to the tetragonal phase in the solid solution system PbTi0,-PbZrO, in figure 1. The phase boundary is very steep, and there- fore the minimum is very flat. These curves are modi- fied by aging in the way that a peak appears at the aging temperature (Fig. 2). The formation of this peak goes ahead with the time constant of domain aging after Bradt and Ansell [I]

;

thus we conclude that this mechanism of formation of 90°-domains which dimi- nishes elastic stresses is responsible for this tempera- ture memory effect. The minimum of the frequency constant at the phase boundary is usually interpreted as a consequence of fluctuating phase transformation.

But a study of Pb(ZrTi)O,-ceramics with an addition of certain oxides, e. g. C r 2 0 3 or MnO,, leads to another conclusion. With these additions, tetragonal compositions show a positive slope and rhombohedral compositions a negative one, as they do without addi- tion

;

compositions corresponding to the phase boun- dary, however, show a flat line over a wide tempera- ture range, and the minimum is hardly distinguishable.

The slope of the curve near room temperature is stabilized by these additions, and no

cr

aging peak

^)>

occurs, though aging is detected also in these ceramics.

For an explanation, we may assume that the phase

phase boundary

I

FIG. 1. -

Influence of

phase boundary

on temperature dependence

^of

frequency constant.

transformation is inhibited. For confirmation, the temperature response may be varied by strong electric fields as well as by elevated temperatures. For example, in figure 3 the behavior of a sample after poling is

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972298

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H. THOMANN

117'3-

1172- kHz

1173

-

117,O

-

116,9

-

shown with the negative slope of rhombohedral mate-

FIG. 3. - Temperature dependence of Pb(Zr.505Ti.495)03

+

¹

weight % MnOz : a) 3 days after poling ; b) after annealing 30 min at 150 OC.

Afr

3-

-

FIG, 4.

-

Coupling coefficient (radial mode) of Pb(Zr.505Ti.495)03

after annealing near the Curie temperature, plotted against the annealing temperature. The dotted line indicates the dielectric

constant at the annealing temperature.

rials. After a heat treatment, the sample shows a posi- tive slope. It has become tetragonal and so it remains as long as it is not treated with strong electric fields.

If the sample is heated and a strong d. c. field is applied

fr 20

during cooling, the initial state is regenerated. So we conclude that the rhombohedral phase is formed in the electric field and is stabilized at room temperature, but transition to the tetragonal phase takes place if the stabilization effect is eliminated by heat.

Similar effects have already been reported by Banno and Tsunooka [2]. Another result relevant here was found by Takahashi [3]. From the asymmetric coercive

i60 O C

field of stabilized ceramics he concluded the existence of a space charge. We attempted to detect such space

-1-

charges by measurements of thermocurrents, but the

-2-

results were not conclusive. But another effect was detected

:

if the poled samples were heated above the Curie point for some minutes and then cooled to room temperature, a piezoelectric coupling was found. In figure 4 this coupling is plotted against the annealing

FIG. 2.

-

Temperature response of frequency constant of

temperature. From this result we may conclude the

aged Pb(Ti.53Zr.47)03.

existence of a memory in the ceramic even above the

Curie point. This memory relaxes with a time constant

of several minutes. From the resistivity of about

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RHOMBOHEDRAL IN LEAD ZIRCONATE TITANATE CERAMICS C2-283

lo7 Qcm and the relative dielectric constant of about effect is found only with metal oxides of variable lo4 near the Curie point one calculates a dielectric valency. The charges apparently are trapped by a relaxation time in the order of 10 ms. Thus, the stabi- change of valency of these atoms. Thus, the long life lization effect cannot be caused by free electric charges. time of the traps is connected with the kinetics of With this respect it is noticeable that the stabilization chemical redox reactions of multivalent atoms.

References

[I]

BRADT

(R. C.)

and ANSELL

(G. S.), J. Amer. Ceram. [2]

BANNO

(H.)

and TSUNOOKA (T.),

Japan. J. Appl. Phys.,

SOC., 1969, 52, 192. 1967, 6, 954.

[3]

TAKAHASHI

(M.), Japan. J. Appl. Phys., 1970, 9 , 1236.