Conductive-dielectric loss

Additions of BN powder to epoxies, urethanes, silicones, and other polymers are ideal for potting compounds. BN increases the thermal conductivity and reduces thermal expansion and makes the composites electrically insulating while not abrading delicate electronic parts and interconnections. BN additions reduce surface and dynamic friction of rubber parts. In epoxy resins, or generally resins, it is used to adjust the electrical conductivity, dielectric loss behavior, and thermal conductivity, to create ideal thermal and electrical behavior of the materials [146]. 

KEY WORDS Si3N4-AlN composites, bending strength, thermal conductivity, dielectric loss... 

The changes in mechanical properties caused by absorption are more or less reversible on drying out after short periods of contact with the environment, but more prolonged immersion tends to make full recovery of initial properties much more difficult. The mechanical property changes are accompanied by corresponding alterations to electrical conductivity, dielectric loss and other physical properties. 

The effect of the electric field upon a polymer could be to cause ionic or electronic conductance, dielectric loss or breakdown. Common polymers are good insulators, some values of volume resistivity appear in Table 2.3, but problems can arise with hydrophilic polymers such as the polyamides. Nylon 66 has a specific resistance of about 10 ft cm when dry, but on equilibration with saturated air at room temperature this substance absorbs about 8% of water and its specific resistance falls to about lO ft cm. When a polymer is placed in an electric field, the effect is to displace the centres of gravity of electronic and nuclear charges so that the material becomes dielectrically polarised. Further... 

Diffusion is the underlying basis of mass transfer properties such as permeation, electrical conduction, dielectric loss, viscosity, and chemical durability. 

In contrast to triaxial porcelains, packaging materials such as 99% AI2O2 prepared by a soHd-state sintering process, display significantly lower dielectric loss. In these materials, there is no residual glassy phase with its associated mobile ion content, and therefore, conduction losses are minimized. 

Fig. 4.9. The data of Fig. 4.8 are represented in the electric-field, stress plane to show that the anomalous response occurs above a critical stress and critical electric field. The response is found to be due to dielectric loss or shock-induced conduction (after Davison and Graham [79D01]).

Typically, large-scale gas filling makes the main characteristics of foam plastics — coefficients of heat and temperature conductivity, dielectric permeability, and the tangent of the dielectric loss angle — totally independent of the chemical structure of the original polymer [1],... 

Because conduction losses are high for carbon black powder it can be used as lossy impurities or additives to induce losses within solids for which dielectric losses are too small. 

Microwave energy is not transferred primarily by conduction or convection as with conventional heating, but by dielectric loss [28]. The dielectric loss factor (loss factor, e") and the dielectric constant (e ) of a material are two determinants of the efficiency of heat transfer to the sample. Their quotient is the dissipation factor (tan 8),... 

Differences in sample size, shape and composition can also affect heating rates. The last case particularly applies when ionic conduction becomes possible through the addition or formation of salts. For compounds of low molecular weight, the dielectric loss contributed by dipole rotation decreases with rising temperature, but that due to ionic conduction increases. Therefore as an ionic sample is microwave-irradiated, the heating results predominantly from dielectric loss by dipole rotation initially, but the contribution from ionic conduction becomes more significant with temperature rise. 

Method involves placing a specimen between parallel plate capacitors and applying a sinusoidal voltage (frequencies ranging from 1 mHz to 1 MHz) to one of the plates to establish an electric field in the specimen. In response to this field, a specimen becomes electrically polarized and can conduct a small charge from one plate to the other. Through measurement of the resultant current, the dielectric constant and dielectric loss constant for a specimen can be measured. The sharp increases in both the dielectric constant and the dielectric loss constant during a temperature scan are correlated with the occurrence of Tg... 

The statistical mechanics of such impurity systems has been treated by Lidiard61-53 and his method has been widely employed in the interpretation of experimental data, e.g. ionic conductivity,6 51 dielectric loss,8 thermoelectric power,16-36 diffusion,31... 

Some important dielectric behavior properties are dielectric loss, loss factor, dielectric constant, direct current (DC) conductivity, alternating current (AC) conductivity, and electric breakdown strength. The term dielectric behavior usually refers to the variation of these properties as a function of frequency, composition, voltage, pressure, and temperature. 

The electrical properties of materials are important for many of the higher technology applications. Measurements can be made using AC and/or DC. The electrical properties are dependent on voltage and frequency. Important electrical properties include dielectric loss, loss factor, dielectric constant, conductivity, relaxation time, induced dipole moment, electrical resistance, power loss, dissipation factor, and electrical breakdown. Electrical properties are related to polymer structure. Most organic polymers are nonconductors, but some are conductors. 

In order to quantify diffiisional effects on curing reactions, kinetic models are proposed in the literature [7,54,88,95,99,127-133]. Special techniques, such as dielectric permittivity, dielectric loss factor, ionic conductivity, and dipole relaxation time, are employed because spectroscopic techniques (e.g., FT i.r. or n.m.r.) are ineffective because of the insolubility of the reaction mixture at high conversions. A simple model, Equation 2.23, is presented by Chem and Poehlein [3], where a diffiisional factor,//, is introduced in the phenomenological equation, Equation 2.1. 

We wanted to be able to correct measurements of dielectric loss (conductance) and dielectric constant of polymerizing styrene solutions for whatever contribution arose from the dead polystyrene present in the solutions. What better way to make polystyrene that was free of all catalyst fragments and polar groups than to irradiate pure, dry styrene Using the same exhaustive drying technique that we were developing for our a-methylstyrene studies, we prepared a batch of pure, dry styrene. This was then to be irradiated under such conditions that approximately 15% conversion to polymer would occur. 

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