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PiezoStar®
Crystals
A New Dimension inSensor Technology
Dr. Claudio Cavalloni,Dr. Roland Sommer,Kistler Instrumente AG,Winterthur
PiezoStar® Crystals
A New Dimension in Sensor Technology
For more than 40 years, Kistler has been developing and manufacturing piezo-electric sensors that can be used to measure pressure, force and acceleration,even under extreme conditions. Nowadays, the sensor elements are increasinglybeing made from new types of crystals.
Market trends towards miniaturizationand higher operating temperaturesresulted in a need for new types ofcrystals capable of satisfying the demandfor greater sensitivity and higher tempe-rature. To this end, research was con-ducted for over ten years in cooperationwith universities and institutes through-out the world to investigate new crystalcompounds and develop growingprocesses. The fruit of this research is awhole family of crystals with outstandingproperties for piezoelectric sensors. Since1998, crystals measuring up to 65 mm indiameter and weighing up to 2,5 kg,
have been grown and processed byKistler to form sensor elements, thuscreating a new group of pressure, forceand acceleration sensors with superiorproperties.Kistler has optimized the PiezoStar crystalelements for use in sensors for deman-ding applications, thus strengthening itstechnological edge in sensor technology.Crystals KI85 and KI91 from thePiezoStar group are currently being usedin sensors.
Milestones in the History of PiezoStarCrystals1995 1995 First miniature pressure sen-
sor (20 pC/bar) with a KI85 crys-tal
1995 First high-temperature force sen-sor with KI85 crystal
1998 First KI85 crystal grown at Kistler1999 First miniature pressure sensor
(temperature-compensated)with a KI91 crystal
2002 First KI91 crystal grown at Kistler
3
Single crystal
Measuring element
High-temperature pressure sensor
PiezoStar
20 pC/bar
400 °C
The Outstanding Properties of Kistler’sPiezoStar Crystals Include:• High piezoelectric sensitivity (up to
5x higher than quartz)• Low temperature dependence• High stability of the properties• Can be used at temperatures of up to
more than 600 °C• No phase transition up to the melting
point (above 1300 °C)• No twin formation• Growing process can be reproduced
on an industrial scale• Tested and successfully used in high-
quality piezoelectric sensors
Piezoelectric Characteristics
Piezoelectric sensitivity of various high-temperature crystals. KI85 and KI91 arecrystals from the PiezoStar group.
Sensitivity shift as a function of the tem-perature of various crystals
ProductsExamples of proven and tested sensorswith PiezoStar crystal elements:
Miniature M5 high-temperature pres-sure sensors Type 6052B and pressureprobes Type 6055/6057 for use ininternal combustion engines
Measuring spark plug M14 Type 6117Band M12 Type 6115A for use in internalcombustion engines
High-sensitivity 3-component forcesensors (11 pC/N) Type 9017A/9018Ain the ForceLink Type 9317A/9318A
Miniature 5mm HighSens force sensorType 9215 (100 pC/N)
HighSens strain sensor Type 9232A andstrain transmitter Type 9234A
Miniature 3-component dynamometerMiniDyn Type 9256A for micromachi-ning
4
KI85 KI91 Quartz Tourmaline GaPO40
1
2
3
4
5
6
7
8
9
10
Long
itudi
nal S
ensi
tivity
[pC
/N] 9.1
6.7
2.31.8
4.6
0 100 200 300 400 500-4
-2
0
2
4
6
8
10
Sens
itivi
ty S
hift
[%
]
Temperature [C]
PiezoStar KI85 PiezoStar KI91 Polystable Quartz Tourmaline GaPO
4
Long
itudi
nal S
ensi
tivity
[pC
/N]
KI85 KI91 Quartz Tourmaline GaPO4
Sens
itivi
ty S
hift
[%
]
Temperature [°C]
PiezoStar KI85PiezoStar KI91Polystable QuartzTourmalineGaPO4
Crystal Properties PiezoStar crystals belong to the familyof calcium gallogermanates quartz iso-types, such as langasite (La3Ga5SiO14).The first crystal of this family(Ca3Ga2Ge4O14) was discovered atMoscow State University as long ago as1979. Since then over 100 differentcompounds from this family have beengrown. Only for few of them the piezo-electric properties have been measured.As the crystals belong to the samecrystallographic class (32) as quartz, it ispossible to obtain the same crystal cuts(longitudinal, transversal and shear).However, unlike quartz or galliumorthophosphate, crystals from the lan-gasite family do not have a phase tran-sition point below melting point andthis explains their excellent stability upto extreme temperatures.
Bibliography: B.V. Mill and Yu.V. Pisa-revsky, Proc. IEEE International FrequencyControl Symposium, 2000, p. 133-144.
Apart from their use in sensor techno-logy, these crystals can be applied inresonators and surface acoustic wavedevices for oscillators, frequency filtersand delay circuits for mobile communi-cations and television sets.
Advantages of PiezoStar CrystalsEasy and economical to grow, goodyield, good stability, high sensitivity, notwin formation, no phase transitionbelow the melting point, usable atextremely high temperatures, no pyro-electric effect.
DisadvantagesLower mechanical strength and highercost than quartz.
Disadvantages of Tourmaline Only natural crystals can be obtained(varying quality, uncertain availability),low sensitivity.
Disadvantages of GaPO4Very expensive to grow, only small crys-tals can be obtained, low yield, twinformation, phase transition at 970 °C.
Disadvantages of QuartzLow sensitivity, twin formation, phasetransition at 573 °C.
5
Peculiarity Pyroelectric Tmax* Production
effect Process Volume CostsQuartz High mechanical strength No 573 °C Hydrothermal Large LowKI85 High sensitivity No Tm Czochralsky Average HighKI91 Low temperature No Tm Czochralsky Average High
coefficientGaPO4 Low temperature No 970 °C Hydrothermal Small High
coefficientTourmaline High temperature Yes >900 °C Natural Small High
stabilityPiezoceramic High sensitivity Yes 250 °C Sintering Large Low(PZT)
* Phase transition temperature. Tm = no transition below the melting point.
Comparison of Various Piezoelectric Materials
KI85 crystal, Ø 55 mm, 1,8 kg
Czochralsky Growth (or Crystal Pulling)
The Czochralsky process has beenknown since 1917. Kistler uses it togrow large, high quality crystals withina relatively short period of time (forexample, our KI85 crystal (Ø55 mm,1.8 kg) is grown within one week). Theindividual components (mostly oxidepowder) are mixed in a crucible andheated up to the melting point (induc-tion heating). As this temperature isapproximately 1400 to 1500 °C, thecrucible must be made of a noble metal(platinum or iridium). A seed crystal (i.e. a piece of crystal of the same com-pound) is dipped into the melt. Thetemperature of the melt has to becontrolled very precisely: if it is too hot,the seed crystal will melt down, while ifit is too cold, there is a risk that the meltwill quickly solidify. Once the crystalgrowth process has started, the seedcrystal is pulled very slowly (approxima-tely 1 mm/h). The size (diameter) of thecrystal is automatically controlled bymeans of the heating power and thepulling speed.
Though the process is simple in theory,its practical application is complicatedby complex and chaotic phenomenaoccurring in 3 dimensions. In particular,the temperature must be controlledvery precisely. If the crystal grows tooquickly, there is a risk that cracks orimperfections will form in the crystalstructure. Thus, growing a perfectcrystal requires precise control of theheating power and considerable expe-rience on the part of the operator.
6
Schematic representation of the Czochralsky
process
KI91 crystal Ø 65 mm, 2,5 kg
Crystal growing system according to the Czochralsky process (photo by Kistler)
After-Heater
Seed
Insulator
Melt
Crystal
Coil
Crucible
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Headquarters
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www.kistler.com
920-
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-07.
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