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Methods for studying ferroelectrics (bulk samples and films) in the microwave range

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Based on a method first proposed by Barlow, the paper discusses how the complex permittivity of a sheet material can be determined by measuring the Q and frequency change produced by a sample when it is placed in a circular evanescent waveguide which is coupled to a resonant cavity. The method appears to be satisfactory for dielectric materials with loss tangents above about 0.01. Measurements on two mixtures of polystyrene and carbon, taken at 9 GHz, are reported. Copyright © 1976 by The Institute of Electrical and Electronics Engineers, Inc.
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Small-signal dielectric constant measurements were made on single domain crystals of BaTiO3 from 25°C to 170°C at 24 kMc/sec. A typical Curie behavior was observed with a dielectric constant ε11 of about half the dc value below the upper Curie point. Measurements of ε11 were also made with a dc field (E3) applied along the c axis in order to obtain information concerning the distortion of the potential well perpendicular to the c axis when the ion is displaced along the c axis. The change Δε11 observed was about 50 for an applied field of 104 volts/cm.
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The permittivity of thin ferroelectric layers may be represented in the form of a diagonal tensor. It is shown that the influence of the “normal” component of the tensor at microwaves is negligible.
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The work considers the physical properties of ferroelectric materials, the fabrication of ferroelectric films, ferroelectric lumped elements for use in microwave technology, and ferroelectric-film microwave transmission lines. Also examined are ferroelectric microwave parametric amplifiers and methods for measuring the properties of ferroelectric devices.
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The paper gives the results of room-temperature measurements on ceramic (polycrystalline) specimens of magnesium, calcium, strontium and barium titanates at frequencies of 1.5 and 9450 Mc/s (3.18 cm), and on barium titanate at 24000 Mc/s (1.25 cm). It is found that the latter material is unique among this series in showing a considerable fall in permittivity, and a large increase in the loss tangent, at the higher frequencies, compared with the behaviour at 1.5 Mc/s. Variable temperature measurements on barium titanate show that the crystallo-graphic change which is known to occur around 120°C, the Curie point of the material, affects the permittivity at 9450 and 1.5 Mc/s in an analogous manner. Room-temperature measurements at the same frequencies on a range of barium-strontium titanate compositions are also described, and attention is drawn to the composition 56% BaTiO3: 44% SrTiO3, having a permittivity of 760 and loss tangent of 0.02 at 9450 Mc/s and 20°C, as likely to be of value in microwave work. The difficulties associated with the measurement of high-permittivity values at centimetre wavelengths are discussed, and a full description is given of the three methods developed for different parts of the investigation.
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The dielectric constant and loss tangent of single crystals of BaTiO 3 were measured as a function of temperature in the frequency range of 8.2 to 12.4 kMc. The technique consisted of looking for transmission resonances through the crystal whenever its thickness became λ/2 (where λ is the wavelength in the material). From the Curie‐Weiss behavior of the dielectric constant in the paraelectric region, the A constant was determined to be 3.77×10<sup>-5</sup>°C<sup>-1</sup>.
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The simultaneous, complex, transcendental equations linking the dielectric constant of a slab partially filling a rectangular waveguide, and the propagation constant of the composite guide have been solved by iteration. Typical results of ε = f(γ) are presented.
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New tests by the author to determine the, dielectric properties of high-loss nonmagnetic microwave materials used for waveguide absorbers and anechoic chamber linings reveal two significant results: dielectric parameters may be calculated from simple formulas without solving transcendental equations, and accuracy is limited only by the homogeneity of the sample and the precision of its fit in the waveguide. The technique used is based upon prior work by other researchers but is refined for materials with loss tangents exceeding 0.1. These previously have received little attention in the literature. Results of tests in the S-band yield loss tangents between 0.4 and 0.8, and dielectric constants from 1.5 to 2.0 for samples completely filling the cross section of WR284 waveguide. The author reviews other methods, analyzes error sources, and describes his laboratory work.
Birks, in: Physical Phenomena in Ferroelectrics
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