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

Many of the recently discovered characteristics and applications to electronics of surface elastic waves are discussed. First the propagation of various elastic waves at the surfaces of solids is considered, followed by descriptions of the many ways which have been demonstrated for transduction between surface elastic waves and electromagnetic waves. Surface-wave amplification, primarily in semiconductors, and wave guiding, focusing, and reflection are examined. The properties of these waves suit them for use in a number of applications, which are discussed, ranging from realization of electronic amplifiers, frequency and analog time-domain filters, and coding devices, to the modulation of light beams and the measurement of surface properties of solids. Many references to the recent surface-wave literature are included.

Surface elastic waves

Piezoelectric materials are used in numerous applications requiring a coupling between electrical fields and mechanical strain. Despite the technological importance of this class of materials, for only a small fraction of all inorganic compounds which display compatible crystallographic symmetry, has piezoelectricity been characterized experimentally or computationally. In this work we employ first-principles calculations based on density functional perturbation theory to compute the piezoelectric tensors for nearly a thousand compounds, thereby increasing the available data for this property by more than an order of magnitude. The results are compared to select experimental data to establish the accuracy of the calculated properties. The details of the calculations are also presented, along with a description of the format of the database developed to make these computational results publicly available. In addition, the ways in which the database can be accessed and applied in materials development efforts are described.

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A database to enable discovery and design of piezoelectric materials

Measurements of nitrous oxide (N 2 O) at 16 stations in the Arabian Sea reveal high degrees of surface saturation (186 ± 37%) and consequently large atmospheric fluxes of N 2 O (4.46 ± 2.60 μ mol m −2 day −1 ). Unusually high for a non-upweling period, these values reflect the large vertical gradients of N 2 O just below the mixed layer as the vertical exchange coefficients (0.55 ± 0.32 cm 2 s −1 ) required to sustain the observed fluxes are not very high. Vertical distribution of N 2 O in the Arabian Sea conforms to the trends seen in other oxygen-deficient environments, with minimal values occurring within the secondary nitrite maximum and the maxima in the N 2 O profiles located at the peripheries of this layer. Outside the denitrification zone, only one broad maximum is observed at mid-depth. The mechanisms proposed for the production of N 2 O are re-evaluated in the light of recent data on the nitrogen isotopic composition of N 2 O from the western North Pacific Ocean. It is proposed that a nitrification-denitrification couple represented by the pathway NH 4 + → NO → N 2 O may be a potentially dominant mechanism for N 2 O production in the ocean. This can explain the observed enrichment of 15 N in N 2 O as compared to NO 3 − while nitrification or denitrification alone cannot. Appreciable horizontal (east-west) gradients in N 2 O concentrations are observed at intermediate depths close to the continental margin off the west coast of India suggesting a possible sink of N 2 O within the marginal sediments. The diffusive losses of N 2 O to the denitrifying waters and marginal sediments are estimated as >2 Tg N y −1 . The Arabian Sea as a whole appears to be a net source of N 2 O, suggesting that the gross production of oceanic N 2 O h as been underestimated.

Nitrous oxide in the Arabian Sea

The oxygen minimum zone off Peru is the site of intense nitrogen cycling, driven by the large input of organic material from highly productive overlying waters. Using three independent experimental methods, we assessed chemoautotrophic activity and nitrification rates in depth profiles through the oxygen gradient and minimum regions. Nitrifying bacteria (both ammonium and nitrite oxidizers) were present throughout the oxygen minimum zone. Even in very low oxygen waters, in situ chemoautotrophic activity and nitrogen oxidation were commonly detected. Highest abundance of nitrifying bacteria and highest activities were found in the gradient region at the upper boundary of the oxygen minimum. Due to the occurrence of strong minima and maxima in the distributions of many parameters, activities were not linearly correlated with environmental and chemical variables. The data are consistent with close coupling between nitrification and denitrification in the oxygen minimum zone, and rapid cycling of nitrogen cycle intermediates including nitrate, nitrite and nitrous oxide.

Chemoautotrophic activity and nitrification in the oxygen minimum zone off Peru

This paper is a review of the properties of the more important piezoelectric materials. Section I discusses the basic principles of the phenomena of piezoelectricity and elasticity from the standpoint of crystal physics. Section II is a brief account of the electrical measurements of piezoelectric vibrators which are required to determine the basic parameters defining the piezoelectric, elastic and dielectric properties of the material. Sections III and IV, respectively, give the properties of a range of piezoelectric crystals and ceramics including, in the most important cases, some of the derived properties which affect the use of the materials in practical devices. It is concluded that it is unlikely that new piezoelectric crystals will be found to replace those in use at present for frequency control and selection but that there is greater scope for new piezoelectric ceramics for use as electromechanical transducers or as frequency selective elements where the highest stability is not required.

Piezoelectric Materials, A Review of Progress

HITHERTO the most successful methods1,2 of growing large quartz crystals have employed a hydrothermal technique in which melting-quality quartz is used as the source material for maintaining the supersaturation of the liquid with respect to silica in the vicinity of the growing crystals. We have now found means whereby the melting-quality quartz can be replaced satisfactorily by other mineral substances rich in silica, such as flint and quartzite. These materials have the advantage of ready availability in the United Kingdom.

Influence of Impurities on the Growth of Quartz Crystals from Flint and Quartzite

DURING recent years there has been considerable interest both in Europe and the United States in the synthesis of quartz crystals large enough for use as piezo-electric oscillators in telecommunication equipment. An isothermal process for growing such crystals was described by Nora and W. A. Wooster1 in 1946 and a similar method was developed independently in Germany during the Second World War by Nacken2. The isothermal process depends on the high solubility of silica glass relative to that of α-quartz under hydrothermal conditions. In our most successful experiments we were able to deposit a total of nearly 1 mm. of good-quality quartz on to a seed crystal, cut parallel to a rhombohedral face, in an 18-hr, growth-cycle. However, difficulties were incurred in attempting to prolong the growth-cycle beyond a day, due to the devitrification of the silica glass3. We therefore re-examined alternative methods employing a temperature gradient, such as Spezia4 used in his pioneer experiments.

R. C. Kell

Papers

Piezoelectric Materials, A Review of Progress

Influence of Impurities on the Growth of Quartz Crystals from Flint and Quartzite

Growth of large quartz crystals.

Piezo-electric materials