Development of SAW filters based on GaPO<inf>4</inf>

Abstract

The results of research into Surface Acoustic Wave -SAW -devices have been recognized for their efficiency and versatility in the electrical signals processing [1], [3]. Actual progress in the industrial application of piezoelectric materials such as Lithium Niobate (LiNbO 3 ), Langasite (LGS), Lanthanum-Gallium Silicate La 3 Ga 5 SiO 14 and Gallium Orthophosphate (GaPO 4 ), allows the manufacturing of devices with performances, which overcome the limits obtained with quartz crystals [2]. The single crystal materials have a long term high stability -near to infinite -and moreover, some of these have an excellent behavior with temperature variation, as is the case of GaPO 4 [4]. Today, GaPO 4 with its properties is by far the best suited piezoelectric material to be used in sensor applications for machine monitoring and pressure measurements, at high temperatures. SAW microdevices based on GaPO 4 operate at temperatures of up to 800 0 C. We have developed five SAW filters based on GaPO 4 . The paper describes the steps to solve the problems regarding the use of GaPO 4 substrates for the development of SAW filters. In the 3rd chapter of the paper are given some experimental results obtained with Network Analyzer.

Rare-Earth Calcium Oxyborate Piezoelectric Crystals ReCa4O(BO3)3: Growth and Piezoelectric Characterizations

“…However, the maximum operating temperature of LN-based piezoelectric devices, restricted by their low electrical resistivity (a requirement of >10 6 Ohm· cm was proposed for comparison, where the materials with low resistivity yet can be used for high frequency applications [1]) at elevated temperature is limited to <600 °C, though the Curie temperature is above 1150 °C [5]. Other important trigonal piezoelectric crystals include the langasite family with the general formula of A 3 BC 3 D 2 O 14 [6][7][8][9][10][11][12] and gallium orthophosphate GaPO 4 , in the point group of 32 [13][14][15][16][17][18][19], these crystals were reported to show modest piezoelectric coefficients (5-7 pC/N) and high melting points (1300-1500 °C for langasite family crystals and ~1670 °C for GaPO 4 ), prior to which, there are no phase transitions observed for langasite family crystals (the phase transition for GaPO 4 is about 970 °C). However, the costly component Ga 2 O 3 restricted their further implements.…”

“…and fresnoite crystals (point group 4mm, such as Ba 2 TiSi 2 O 8 ) were investigated for piezoelectric applications. These crystals show the merits of high melting points (1400-1700 °C) and high effective piezoelectric coefficients d eff (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18) pC/N) [20][21][22][23][24][25][26]; the evaluations of the temperature dependence of dielectric, piezoelectric and electromechanical properties, however, are limited. Of particular significance is that the monoclinic rare-earth calcium oxyborate crystals (ReCa 4 O(BO 3 ) 3 , ReCOB, Re: rare earth), which have been extensively investigated for nonlinear optical applications in the last two decades [27][28][29][30][31][32][33], were reported to exhibit good piezoelectric properties and high electrical resistivity at an elevated temperature of 1000 °C, with no phase transition prior to their melting points (~1400-1520 °C) [1][2][3][34][35][36][37][38].…”