PICA Stack Reliability Investigation
Introduction
PI Ceramic (PIC) has been investigating the reliability of PICA actuators for many years.
This paper contains some typical test results that demonstrate the reliability of PICA
Stack actuators under AC- and DC-operation conditions.

1. AC operation
The stacks for these tests were built from pellets with 10 mm diameter and 0.75 mm
height. The applied voltage was a unipolar sine wave 0…1000 V. The resulting electricfield E at 1000 V is 1.5 kV/mm.
Test conditions: normal laboratory environment; room temperature 20 to 25°C, humidity 30
to 60% rh.

Tab. 1: Tests with AC operation
Material

Stack
Height[mm]Stack
Diameter[mm]

PelletHeight[mm]ElectricField[kV/mm]Frequency[Hz]

No. ofFailedStacks

Reached No.
of Cycles

PIC151

21

100.751.51200 of 4

1.2 ·1010

PIC15121100.75

1.530

0 of 19

1.7 ·109

PIC15140100.75

1.530

0 of 11

1.7 ·109

A lifetime could not be calculated for these samples because none of them failed. So, withthis drive signal the meantime to failure (MTTF) of all stacks is much better than 2 ·109cycles, in the first case even better than 1010 cycles.

2. DC operation
The DC-lifetime of an actuator is limited by a decrease of its insulation resistance. In thetests a sample failure is defined as the decrease of the resistance to 0.1 % of its initialvalue. Please consider that this value is no specification because the tolerable leakage
current depends also on the power of the amplifier which is used. The MTTF wascalculated when at least two thirds of the samples had failed, otherwise the testing time
was considered to be the lower limit of the MTTF.
Tests in a normal laboratory environment showed that it is nearly impossible to find adependency of the MTTF from the electric field in a short time (see section 2a). Therefore
tests under severe conditions were made (see section 2b).

1
PI Ceramic GmbH, Lindenstr., 07589 Lederhose, Germany, Tel. +49 (0) 36604 / 882 0, Fax +49 (0) 36604 / 882 25, www.piceramic.com

a) long-term lifetime tests under lab conditions
For these tests, the sample stacks were built from different materials and feature differentpellet heights. Therefore the applied DC-voltage of 1000 V results in different electricfields.
Remark: a standard PICA Stack actuator has a pellet height of 0.5 mm. Hence thenominal voltage of 1000 V induces a nominal electric field of 2 kV/mm.
Test conditions: normal laboratory environment; room temperature 20 to 25 °C, humidity
30 to 65% rh. Test duration: 17,900 hours (2 years).

Tab. 2a: Tests with DC operation in a normal laboratory environment
Material

Stack Height[mm]StackDiameter[mm]

Pellet Height[mm]ElectricField[kV/mm]No. of FailedStacks

MTTF
[hrs]

PIC25572

102.000.5

0 of 3much better
than 2 years

PIC25536101.00

1.00 of 3

much better
than 2 years

PIC25528

100.751.50 of 3

much better
than 2 years

PIC153

34

25

0.75

1.5

1 of 3much better
than 2 years

PIC255

3425

0.50

2.0

2 of 313150

PIC15119100.50

2.0

3 of 313110b) accelerated lifetime test in a severe climatic environment
In this test PICA actuators with 10 mm diameter that consist of pellets of different heights
were tested with an applied DC-voltage of 1000 V. The environment was chosen tosimulate severe climatic conditions especially high humidity.
Test conditions: temperature 40°C, humidity 70% rh.

2PI Ceramic GmbH, Lindenstr., 07589 Lederhose, Germany, Tel. +49 (0) 36604 / 882 0, Fax +49 (0) 36604 / 882 25, www.piceramic.com

Tab. 2b: Tests with DC operation in a severe humid environment
Material

Stack Height
[mm]

Stack
Diameter[mm]

Pellet Height[mm]ElectricField[kV/mm]No. of failedstacks

MTTF
[hrs]

PIC 151

16100.4

2.50

3 of 376

PIC 151

19100.5

2.00

3 of 3117

PIC 151

26

100.7

1.43

3 of 3296

PIC 151

30100.8

1.25

3 of 3290

PIC 151

36101.01.00

3 of 3803

Comparing these results with the results of the test under normal climatic conditions at
2 kV/mm (see section 2a) one recognizes that under dry conditions the lifetime is about
100 times better.
With the results from the accelerated test the following dependency of the MTTF from theelectric field can be observed:

Fig. 2b: Lifetime in dependency of the electric field
MTTF vs . electric field
1,0E+06
values at 40°C and 70% rhvalues at normal laboratoryenvironment

MTTF in h

1,0E+05

1,0E+04

1,0E+03

1,0E+02

1,0E+01
0,11

10Electric Field in kV/mm

The linear fit-curve on the log-log-plot shows the characteristics of the relation between
electric field E and the MTTF (circles in Fig. 2b). The MTTF is proportional to (1/E)n, where
an exponent n = 2.45 can be calculated.
3PI Ceramic GmbH, Lindenstr., 07589 Lederhose, Germany, Tel. +49 (0) 36604 / 882 0, Fax +49 (0) 36604 / 882 25, www.piceramic.com

This exponent can be used to estimate the increase in lifetime when lowering the voltage.
In addition it is also possible to extrapolate the MTTF for the values in section 2a with thegiven values at 2 kV/mm (see dashed line in Fig. 2b).

3. Conclusion
PICA Stack actuators show an excellent behavior at AC-signal driving. Lifetimes far
above 109 cycles are proven in several tests.

More care has to be spent at DC-signals especially under humid conditions.
A two-year DC-lifetime test under lab conditions on PICA Stack actuators made ofdifferent materials and geometries showed very good results at electric fields whichcorrespond to reduced DC-driving voltages.
An accelerated lifetime test was conducted at elevated humidity which lowers themeasured DC-lifetime considerably. Therefore, PI Ceramic recommends to use PICA
Stack actuators at elevated humidity just at significantly reduced DC-voltages. The results
showed an exponential law for the MTTF-DC-electric-field-relation where the exponent is2.45. This relation can be used to extrapolate the measured MTTF of the tests in thisreport to lower voltage ranges.
Remark: humidity effects on the lifetime are just relevant for DC-signals because the self
heating effect of the stack during AC-operation results in a dramatically lower local
humidity.

October 11th, 2010 PIC-BBr, Rev. cPPe, BSc

4
PI Ceramic GmbH, Lindenstr., 07589 Lederhose, Germany, Tel. +49 (0) 36604 / 882 0, Fax +49 (0) 36604 / 882 25, www.piceramic.com