Piezoelectric Materials

crystalline substances with strongly pronounced piezoelectric properties used in the manufacture of electromechanical devices, such as piezoelectric resonators, piezoelectric transducers, and sound emitters and receivers. The basic characteristics of piezoelectric materials are (1) the electromechanical coupling factor , where d is the piezoelectric strain coefficient, E is the elastic modulus, and ∈ is the dielectric constant (for anisotropic piezoelectric materials, all values are tensor values); (2) the value k²/tan δ, which determines the efficiency of the conversion (δ is the dielectric loss angle); (3) the ratio of the mechanical power of a piezoelectric element at the resonance frequency to the square of the intensity of the electric field in the element, a ratio defined by_the value (dE)²; and (4) the values and , which determine the sensitivity of the sound receiver, respectively, in the resonance range and at low frequencies (c_s is the speed of sound in piezoelectric materials). Table 1 lists the characteristics of some of the most widely used materials.

Special requirements for piezoelectric materials depend on the intended usage. Among the required special properties are high electrical and mechanical strength, weak dependence upon temperature of the characteristics, high quality faction, and moisture resistance.

Piezoelectric materials can be categorized either as single crystals, which occur as natural minerals or are grown artificially (quartz, ammonium and potassium dihydrogen phosphate, Rochelle salt, lithium niobate, silicon selenite, germanium selenite), or as polycrystalline ferroelectric solid solutions, known as piezoelectric ceramics. These ceramics, after synthesis, are subjected to polarization in an electric field.

Few of the single crystals are actually used as piezoelectric elements. Quartz is widely used because of its mechanical strength, small dielectric loss, resistance to moisture, and the stability of its properties in the face of temperature change. However, its piezoelectric effect is comparatively weak; furthermore, the dimensions of quartz crystals are small, and quartz is difficult to work with. Quartz is used mainly in piezoelectric filters and frequency stabilizers. Laboratory applications of quartz include their use in emitters and receivers of ultrasonic waves. Ammonium dihydrogen phosphate is an artificially grown ferroelectric crystal. It is chemically stable up to its melting point (130°C), exhibits a strongly pronounced piezoelectric effect, and has a low density; it does not, however, pos-

Table 1. Basic characteristics of the most widely used piezoelectric materials at temperatures of 16°-20°C
		Density p × 103(kg/m3)	Speed of sound Cs ((103m/sec)	Dielectric constant ∈	Piezoelectric modulus d × 10-12coulombs/newton	Loss tangent tan × 102	Electromechanical coupling factor k	k2/tan δ	Remarks
Note: Numbers in parentheses for the single crystals determine the indexes of the corresponding tensor characteristics. For example, (36)/2 means 1/2d36. For the piezoelectric ceramics, the values of the constants on top have indexes (11) or (31); those on the bottom have the index (33), with values d31 < 0 and d33 > 0. The tg δ values for the crystals are given for a field E < 0.05 kilovolt/cm; for the piezoelectric ceramics, tan δ is given in the interval 0.05 kilovolt/cm ≤ E < 2 kilovolts/cm. Data for the piezoelectric ceramics made in the USSR are based on GOST (USSR Standard) 18927–68.
Quartz ..............		2.6	5.47(11)	4.5(11)	2.31(11)	<0.5	0.095	>2.0	x-cut
Ammonium dihydrogen phosphate (ADP)......		1.8	3.25(33)	15.3	24.0(36)/2	<1	0.28	>8	cut at 45° to z-axis
Lithium sulfate.........		2.05	4.7(33)	10.3(22)	16.3(22)	<1	0.30	>10	y-cut
Rochellesalt..........		1.77	3.1(22)	350(11)	275	>5	0.65	>8	cut at 45° to x-axis; substance decomposes at T>55°C
Antimony sulfoiodide.....		5.2	1.5(33)	2,200(33)		5–10	0.8(33)	6.4
Piezoelectric ceramics Barium titanate (TB-1).		5.3		1,500		2–3
	Calcium barium titanate (TBK-3)	5.4		1,200		1.3–2.5
	Group of lead titanate zirconates
	TsTs-23.........	7.4		1,100		0.75–2.0
	TsTBS-3 ........	7.2		2,300		1.2–2.0
	TsTSNV-1 .......	7.3		2,200		1.9–9.0
	PZT-5H.........	7.5		3,400		2.0–9.0			data from Clevite Corporation (USA)
	PZT-8..........	7.6		1,000		0.4–0.7			data from Clevite Corporation (USA)

sess sufficient mechanical strength. Crystals of Rochelle salt, which can be grown to large dimensions, possess high values of those characteristics that determine the sensitivity of a sound receiver. The use of Rochelle salt, however, is limited by the salt’s low resistance to moisture and low mechanical strength. Another drawback is the strong dependence of its properties on both the temperature (the salt has a low Curie point and a melting point of only 55°C) and the intensity of the electric field. Lithium niobate, silicon selenite, and germanium selenite have a strongly pronounced piezoelectric effect, high mechanical strength, and high acoustic quality. These materials are utilized in the ultrasonic frequency region. Materials such as tourmaline, potassium hydrogen phosphate, and lithium sulfate are not used. The most widely used industrial piezoelectric materials are the piezoelectric ceramics.