Work Package 02 - Materials

Selecting the materials according to the specifications for developing optimal sensors and adapted electrode to operate on the whole temperature range.

 

Piezoelectric materials are required in SAW devices to allow transduction of electrical signal applied to inter-digitated electrodes into mechanical waves. A benchmark of piezoelectric materials has been achieved regarding operating conditions specified for the targeted applications. A comparison of the presented piezoelectric materials, evaluating their properties regarding the given application requirements, commercial availability, piezoelectric properties and current knowledge about the materials is given in the following table:

While GaPO4 (Gallium Ortho-Phosphate) exhibits exceptional low losses at high temperatures (above 500°C), the availability and the current status of research regarding SAW applications prevents the use of this materials for commercial scale applications. AlN is suffering from the same issue but thin films may allow for alternative solutions in that matter. While the few existing high temperature measurements indicate a good high temperature stability, there are currently no AlN bulk crystals suitable for SAW applications available on the market.
Compared to conventional LiNbO3, which exhibits only poor high temperature properties, stoichiometric LiNbO3 at temperature above 500°C is expected to exhibit a significantly higher stability at high temperatures. Although stoichometric LiNbO3 is currently not available in large amounts and wafers (2”), a growing demand for this material may change the situation. Existing technologies and processes can be used to process those wafers. However, the knowledge about the behavior of stoichiometric LiNbO3 at temperatures above 500°C is currently limited and should be therefore subject of further research, before a commercial application of this material should be considered.
Despite the increasing availability of high temperature stable piezoelectric ceramics, their applicability for SAW devices is strongly limited by their poly-crystal structure.
Considering all these properties, the langasite-type crystals (langasite and langatate) are currently the most promising materials for high temperature SAW applications. Although they show higher losses than for example GaPO4, they are in general usable at very high temperatures due to the absence of phase transformations up to their melting point. An advantage compared to the other presented materials is the commercial availability of high quality wafers, especially of langasite, which can be obtained by different manufacturers also in larger scales. Since the piezoelectric properties of langasite, especially for SAW applications, are studied more in detail than those of langatate, langasite should be the preferred material to achieve the goals of this project.

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