Christian Reiter

Material properties of GaPO4 and their relevance for applications

Abstract A set of physical constants of gallium(ortho-)phosphate GaPO 4 has been determined in recent years. These constants allow to calculate precisely the properties of the most important applications of GaPO 4 such as BAW and SAW devices.

GaPO/sub 4/ resonators with Q factors of some millions in the fundamental mode

Gallium orthophosphate (GaPO/sub 4/) is a novel, synthetic piezoelectric crystal which is homeotypic to quartz. Most physical properties are stable until 970/spl deg/C. New applications are thickness shear resonators (fundamental mode) with a cut angle near Y-84/spl deg/ and very low coupling coefficients which have shown very high Q factors (Reiter et al, Proc. 15th Euro. Freq. and Time Forum, pp. 50-54, 2001), and thus it is called the HiQ cut. The temperature behavior and the electrical equivalent circuits of some new HiQ resonators are investigated with a HP network analyzer and an oscillator. The oscillator had a special design because of the high motional resistances of the HiQ resonators (several k/spl Omega/ at normal pressure conditions which decrease to several hundreds of Q under vacuum conditions). The highest Q factors (up to 13/spl times/10/sup 6/) are obtained with the network analyzer at a pressure of about 0.04 mbar. The next measurements are focused on short term stability to obtain oscillators with very low noise.

Temperature sensors based on GaPO/sub 4/

Results of investigations of temperature sensors using singly rotated Gallium Orthophosphate (GaPO/sub 4/) Y-cut resonators are presented in this paper. Depending on rotating angle the most important parameters - sensitivity, resolution and linearity differ in a wide range. Theoretical calculations leads to cuts owning nearly a linear response with sensitivity between 44.5 ppm//spl deg/C and 46.5 ppm//spl deg/C within a temperature range from room temperature to 600/spl deg/C with a high resolution of about 10/sup -6//spl deg/C. First measurements confirm the theoretically predicted sensitivity. The broad thermal stability range of GaPO/sub 4/ can also be used for temperature sensors based on surface acoustic wave devices. By theoretical calculations, suitable orientations with high temperature coefficients are identified.

The temperature-stable piezoelectric material GaPO 4 and its sensor applications

Gallium (ortho) phosphate (GaPO4) has a similar structure as quartz, but the high thermal stability up to 970 °C and the high sensitivity make it a very attractive choice for a wide range of uncooled high temperature applications. Furthermore it shows no pyroelectricity, no outgassing and a high electric resistance to guarantee high precision piezoelectric measurements. GaPO4 offers good temperature compensation for a new generation of crystal microbalances operating at temperatures up to at least 700 °C as shown in thermogravimetric and film thickness measurements. It also allows the production of SAW sensors with a very high thermal stability.