Dielectric Materials Low-Frequency Properties

Dielectrics and Electrooptics I i. .i Dielectric Materials Low-Frequency Properties 823... 

Electrical Properties. AH polyolefins have low dielectric constants and can be used as insulators in particular, PMP has the lowest dielectric constant among all synthetic resins. As a result, PMP has excellent dielectric properties and alow dielectric loss factor, surpassing those of other polyolefin resins and polytetrafluoroethylene (Teflon). These properties remain nearly constant over a wide temperature range. The dielectric characteristics of poly(vinylcyclohexane) are especially attractive its dielectric loss remains constant between —180 and 160°C, which makes it a prospective high frequency dielectric material of high thermal stabiUty. 

Because of very high dielectric constants k > 20, 000), lead-based relaxor ferroelectrics, Pb(B, B2)02, where B is typically a low valence cation and B2 is a high valence cation, have been iavestigated for multilayer capacitor appHcations. Relaxor ferroelectrics are dielectric materials that display frequency dependent dielectric constant versus temperature behavior near the Curie transition. Dielectric properties result from the compositional disorder ia the B and B2 cation distribution and the associated dipolar and ferroelectric polarization mechanisms. Close control of the processiag conditions is requited for property optimization. Capacitor compositions are often based on lead magnesium niobate (PMN), Pb(Mg2 3Nb2 3)02, and lead ziac niobate (PZN), Pb(Zn 3Nb2 3)03. 

The van der Waals interaction depends on the dielectric properties of the materials that interact and that of the medium that separates them. ("Dielectric" designates the response of material to an electric field across it Greek Si- or Si a- means "across.") The dielectric function e can be measured experimentally by use of the reflection and transmission properties of light as functions of frequency. At low frequencies, the dielectric function e for nonconducting materials approaches a limit that is the familiar dielectric constant. The dielectric function actually has two parts, one that measures the polarization properties and the other that measures the absorption properties of the material. 

Measurements of dielectric properties have been used to monitor chemical reactions in organic materials for more than fifty years. In 1934, Kienle and Race 11 reported the use of dielectric measurements to study polyesterification reactions. Remarkably, many of the major issues that are the subject of this review were identified in that early paper the fact that ionic conductivity often dominates the observed dielectric properties the equivalence between the conductivity measured with both DC and AC methods the correlation between viscosity and conductivity early in cure the fact that conductivity does not show an abrupt change at gelation the possible contribution of orientable dipoles and sample heterogeneities to measured dielectric properties and the importance of electrode polarization at low frequencies. 

Since the late nineteenth century, dielectric spectroscopy has been used to monitor dynamical properties of solid and liquid materials. At that time, dielectric measurements were performed either at a single frequency or in a very limited frequency range now, however, measurement technique and instrumentation have developed to such an extent that dielectric spectroscopy is today a well-established method to probe molecular dynamics over a broad range in frequency or time (cf. reviews by Johari [1], Bottcher and Bordewijk [34], Williams [35,36], and Kremer and Schonhals [37]), even with commercially available equipment. Including the latest developments, one can even say that nowadays dielectric spectroscopy is the only method that is fully able to realize the idea of 0- to 1-THz spectroscopy. In data sets that cover the range of up to 10 6—1013 Hz—that is, from ultra-low frequencies up to the far infrared—the full range of reorientational dynamics in... 

The dielectric properties of materials are defined by two different parameters, namely the dielectric constant and the dielectric loss. The dielectric constant, e, describes the ability of a molecule to be polarized by the electric field. At low frequencies, e reaches a maximum as the maximum amount of energy can be stored in the material. The dielectric loss, s, measures the efficiency with which the energy of the electromagnetic radiation can be converted into heat. The dielectric loss goes through a maximum as the dielectric constant falls [16]. The dissipation factor (tan d) is the ratio of the dielectric loss of the sample, also called loss factor , to its dielectric constant tan 8 = e /s. 

Electrical properties of glass ceramics are determined by the properties of both the crystalline phases and the residual glass. Electrical conductivity and dielectric loss (at low frequencies) are dominated by the concentration and mobility of alkali ions in the glass phase. The dielectric constant is dominated by the crystalline phase, especially when that phase consists of high dielectric constant materials such as ferroelectric crystals. The... 

Two basic contributions are expected to the variation of dielectric properties of a hydrated material with respect to those of a dry one that of the polar water molecules themselves and the second one due to the modification of the various polarization and relaxation mechanisms of the matrix material itself by water [37]. In the low frequency region of measurements, there is a third contribution, often ignored in works dealing with high frequency measurements, which arises from the influence of moisture on conductivity and conductivity effects. The increase of electrical conductivity of the sample is the major effect present in wet samples dielectric response is often masked by conductivity, and it superposes the dielectric processes in the loss spectra and demands a conductivity correction of the dielectric loss spectra [9]. This dc conductivity strongly affects the modifled loss factor, e". In this case, it can be expressed as shown in the following equation ... 

Wu et al. [73] prepared Ba Sri Ti03 (BST) nanoparticles via solid-state techniques by heating stoichiometric amounts of the raw materials BaCOs, SrCOg, and Ti02. The BST ceramics showed good dielectric properties at both low frequencies and microwave frequency (2 GHz). 

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