Properties of piezoelectric crystals and ceramics are reviewed as they affect use of such materials in electroacoustic transducers. Extensive data on lead titanate-zirconate ceramics are presented to help in selection of the appropriate material for a variety of applications in the lower and medium frequency range, up to several megacycles per second. Quartz and several recently discovered piezoelectric crystals will share the higher megacycle range.

Piezoelectric transducer materials

Core-level and valence-band spectra have been obtained by means of x-ray photoemission spectroscopy for the group IV, V, and VI elements Ge, Sn, Pb, As, Sb, Bi, S, Se, and Te and the group IV-VI compounds GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, and PbTe. These results, taken under ultrahigh-vacuum conditions with unmonochromatized x rays, are presented and discussed in terms of the bonding in materials with an average valence of 5. The effects of relaxation on the chemical shifts are found to be relatively small. The chemical shifts are found to vary in a manner similar to that expected from the magnitude of the elemental electronegativities except for the ordering of the shifts of GeS and GeSe. Relative charge transfers are calculated from the chemical shifts and are found to be in general agreement with ionicities calculated using the Phillips-Van Vechten theory although there is some disagreement as to their magnitudes. A consideration of the structures of the compounds relative to the charge transfers demonstrates the importance of metallic as well as covalent and ionic bonding in determining the most stable structure. The value of critical ionicity carried over from the average-valence-4 materials does not apply to the average-valence-5 materials and this concept does not appear useful in understanding their bonding because of the increased importance of nondirectional metallic bonding.

Bond ionicity and structural stability of some average-valence-five materials studied by x-ray photoemission

Piezoelectric crystals and ceramics are used as detectors and radiators of acoustic power from very low frequencies to above 109 Hz. The lower frequency range is now covered almost exclusively by the piezoelectric ceramics which date from the late 1940’s, and the upper frequency range is covered by new piezoelectric crystals discovered in the 1960’s. The very highest frequency range has been opened up by oriented deposited films of several of the new crystals.

Piezoelectric Crystals and Ceramics

Publisher Summary This chapter discusses the theory of mobility of electrons in macroscopically homogeneous solids on mechanisms that inhibit the flow of electrons; that is, scattering by imperfections of various kinds. There is very little fundamental difference between the transport of electrons in metals and semiconductors. The most outstanding distinction between the two is that in the former, the electron distribution is highly degenerate at all temperatures of interest, whereas in the latter, Boltzmann statistics is frequency applicable. The chapter also presents a resume of the statistics obeyed by electrons and derivation of the equilibrium probability distribution of an assembly of electrons, and the formulation of the Boltzmann transport equation. The solution of the Boltzmann transport equation helps in obtaining formulas for various transport phenomena such as electrical and thermal conductivity, the thermoelectric effects, and effects which appear in the presence of a magnetic field. The chapter also discusses the diverse scattering mechanisms which permit a distorted distribution function to relax to equilibrium together with the “phonon-drag” effect, particularly its influence on thermoelectric properties and the “hot electron” problem in semiconductors.

Theory of Mobility of Electrons in Solids

Publisher Summary This chapter illustrates the spectra resulting from vibrational transitions in solids It describes the vibrational part of the internal energy The transitions between different vibrational energy levels give rise to the near infrared emission or absorption spectra and are stimulated provided there is a change of dipole moment connected with such a transition The factor group analysis and the normal modes of oscillation of brucite are discussed in the chapter The chapter describes the vibrational spectra of a number of crystals of varying composition and structure The vibrational and rotational transitions in a molecule may also be induced in another way by the scattering of light Under certain conditions, the electric field of an incident radiation may induce a dipole moment and the molecule can, thus, interchange energy with the photon This exchange produces the Raman spectrum that in its structure resembles the infrared spectrum, although the two originate from two different processes—namely light scattering and absorption or emission of radiation

Vibration Spectra of Solids

Publisher Summary This chapter presents the physics of semiconductors. It discusses chemical binding and energy band structures of semiconductors, lifetime of electron-hole pairs, and impurity and lattice defect center. Sufficient information is now available about the semiconductor properties of a wide variety of materials to understand qualitatively the relations between the semiconductor properties and chemical binding and to indicate trends in the fundamental properties of semiconductor materials. In discussing these relations and trends, however, the chapter is restricted to homologous series of semiconductors with simple structures. These include (1) the group IV-B elements with diamond structures, (2) the M III- B - N V-B , M II-B - N VI-B , and M I-B - N VII-B compounds having the zinc blende or wurtzite structures, (3) the M IV-B - N VI-B , M III-B - N VI-B and the M I-B - N VII-B compounds having the sodium chloride or cesium chloride structures, and (4) the M 2 II-A - N IV-B compounds having the fluorite structure. The chapter discusses forbidden energy gaps and mobilities of materials in the various homologous series of semiconductors with other pertinent physical properties.

The Physics of Semiconductor Materials

A survey of inorganic piezoelectric materials

Of the several processes by which heat is directly transformed to useful electric power thermoelectricity offers unique advantages for several areas of application. The absence of moving parts and the simplicity promise high reliability, silence, and long-lived, maintenance-free operation both as a power source and as a heat pump for refrigeration and temperature control. A substantial effort devoted to materials development has produced new semiconductors with an integrated theoretical efficiency of about 20 per cent. Further progress in materials development is needed to achieve major goals in both military and commercial applications, but important uses can already be visualized. Small power plants for remote areas and auxiliary power supplies for space vehicles are awaiting engineering improvements, some of which again require improved materials for use as electrical contacts and electrical and thermal insulators. Similarly in refrigeration, important military applications and special commercial devices can be produced with existing thermoelectric materials when engineering problems and the auxiliary materials problems are solved.

Paul H. Egli

Papers

The Physics of Semiconductor Materials

A survey of inorganic piezoelectric materials

Progress in Thermoelectricity

3. Direct Energy Conversion