Heat cycle treatment for improving the performance of piezoelectric ceramics

A process for improving the electromechanical coupling factor (Kp) of a polarized piezoelectric ceramics by thermal cycle treatment. The process comprises the steps of: PA0 (a) keeping the polarized piezoelectric ceramics at a temperature of from 50.degree. to 130.degree. C. for from 10 to 60 minutes; PA0 (b) cooling the polarized piezoelectric ceramics to room temperature rapidly; PA0 (a) repeating steps (a) and (b) until the electromechanical coupling factor of the polarized piezoelectric ceramics saturates. The process of the present invention is effective in elevating the electromechanical coupling factor (Kp) of a polarized piezoelectric ceramics by an degree of from 3 to 8 percent. The useful life of the polarized piezoelectric ceramics is therefore elongated.

BACKGROUND OF THE INVENTION 
Piezoelectric ceramic materials are ceramic materials capable of generating 
a voltage when mechanical force is applied or producing a mechanical force 
when a voltage is applied. 
The performance of a piezoelectric ceramic material is measured mainly by 
its electromechanical coupling factor (Kp), mechanical quality factor 
(Qm), ferroelectric constant (K) and dissipation factor (tan .delta.). 
In the production of piezoelectric ceramics, the raw materials are mixed, 
ball-milled, fired, further ball-milled and then sintered. The sintered 
body is then polished on both sides, adapted with electrodes and then 
polarized in a silicon oil bath of about 130.degree. C. by applying a high 
voltage direct current field (usually from 2.5 to 3.5 kV/mm) for from 
about 10 to about 30 minutes. 
The properties of a piezoelectric ceramic is determined primarily by the 
composition of the raw materials, the process parameters of the production 
and the completeness of polarization. The performance of a piezoelectric 
ceramic will unavoidably deteriorate after a certain period of usage. For 
example, the Kp value of 0.50 right after polarization will spontaneously 
decrease to 0.48-0.49. To improve the useful life and the efficiency of 
the piezoelectric ceramics, it has been tried to adopt more critical 
process or use higher polarization voltage for the production of the 
piezoelectric ceramics to increase the initial Kp value. However, critical 
process is quite costly and higher polarization voltage will adversely 
affect the structure of the final product. It is desirable to improve the 
performance of piezoelectric ceramics by other simpler and non-destructive 
methods. 
Japanese Laid-Open Patent Application No. JP63-131407 discloses a process 
to increase the dielectric constant, resonance frequency and resonance 
impedance of polarized piezoelectric ceramics by applying a constant 
temperature heat-treatment at a temperature lower than half of the Curie 
temperature for about 5-15 hours. The Kp value, however, is decreased. 
Deutsche Patent No. DE 1796226 discloses a process to decrease the aging 
rate of piezoelectric ceramic materials by a heat treatment of repeated 
heating and cooling cycle. The heating and cooling rate is lower than 
30.degree./hr. Although the frequency constant of the piezoelectric 
ceramics is stabilized, the Kp value is decreased. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a process to 
improve the Kp value of piezoelectric ceramics by simple heat treatment 
and subsequently improve the useful life of the piezoelectric ceramics. 
The present invention relates to a process for improving the 
electromechanical coupling factor (Kp) of a polarized piezoelectric 
ceramics by thermal cycle treatment, said process comprising the steps of: 
(a) keeping said polarized piezoelectric ceramics at a temperature of from 
50.degree. to 130.degree. C. for from 10 to 60 minutes; 
(b) cooling said polarized piezoelectric ceramics to room temperature 
rapidly; 
(a) repeating steps (a) and (b) until the electromechanical coupling factor 
of said polarized piezoelectric ceramics saturates. 
The process of the present invention is effective in elevating the 
electromechanical coupling factor (Kp) of a polarized piezoelectric 
ceramics by an degree of from 3 to 8 percent. The useful life of the 
polarized piezoelectric ceramics is therefore elongated.

DETAILED DESCRIPTION OF THE INVENTION 
The individual elements of the present invention will be described in 
detail below. 
Barium titanate (BaTiO.sub.3) is the firstly developed piezoelectric 
ceramic material. In 1954, new piezoelectric ceramic materials based on a 
binary system of lead zirconate-titanate (Pb(Zr,Ti)O.sub.3, commonly 
called PZT) was discovered. Due to its superior piezoelectric property, 
lead zirconate-titanate has become the predominant piezoelectric material 
in the market since its discovery. 
The piezoelectric ceramics contemplated by the present invention is 
preferably a piezoelectric ceramics of the lead series, more preferably a 
piezoelectric ceramics of the PZT type. 
Conventional PZT piezoelectric elements are produced first by mixing powers 
of lead oxide, zirconium oxide, titanium oxide and necessary additives. 
The mixture is then ball-milled, dried and subsequently fired and crushed 
to give the base ceramic powder. The ceramic powder is then sieved and 
pressed into the desired green body. The green body is then sintered. The 
sintered body is then polished, adapted with electrodes and polarized. In 
conventional process, the polarized piezoelectric ceramic element is 
directly utilized. The average Kp value of conventional piezoelectric 
ceramic elements are from about 0.44 to about 0.56. 
According to the present invention, the polarized piezoelectric ceramic 
elements are further subjected to a specific thermal cycle treatment. As a 
result, the Kp values are increased by from about 3% to about 8%, reaching 
from 0.48 to 0.56. 
Aging rate of the processed piezoelectric ceramic elements are the same as 
those not processed by the present invention. However, since Kp value of 
the processed elements are elevated, the useful life of the processed 
elements are elongated since the elements age from a higher Kp value. 
Furthermore, the piezoelectric will have better performance with higher Kp 
value. 
EXAMPLE 
Five groups of Sr-containing PZT piezoelectric ceramic test plates were 
plated with electrodes. Each group of test plates includes three test 
plates of the same composition. The compositions of the Sr-containing PZT 
piezoelectric ceramic test plates may be represented by the general 
formula: 
EQU (Pb.sub.1-x Sr.sub.x)[(Zr.sub.0.52 Ti.sub.0.48).sub.0.97 Nb.sub.0.03 
]O.sub.3. 
The x values for each of the five groups of test plates were 0, 0.02, 0.05, 
0.10 and 0.15 respectively. Test plates of same composition were rapidly 
moved individually into an oven set at 50.degree. C., 90.degree. C. and 
130.degree. C. respectively for 20 minutes. The samples were then removed 
from the oven and was allowed to cool down to room temperature. The Kp 
value of the samples were then evaluated. The heating-cooling cycles were 
repeated more than 10 times. The results were depicted in FIGS. 1(a)-1(e). 
It was found that the Kp values of the samples reached a saturation level 
after 4-6 such thermal cycles. The Kp values did not increase after they 
reach their respective saturation level even though further thermal cycles 
were applied. The drawings show that the Kp values of these polarized 
piezoelectric PZT ceramics were significantly raised. The Kp values of the 
samples were raised by an degree from about 3 to about 8%. The highest 
increase in Kp value was found in the sample which contained 0.15 mole% of 
Sr and was heated under 90.degree. C. With the increase in initial Kp 
value, the efficiency of the piezoelectric ceramics is improved and it 
useful life were also elongated.