Dielectric losses in glass

The dielectric dissipation factor tan 5 is frequency-and temperature-dependent. Owing to the diverse mechanisms which cause dielectric losses in glasses, there is a minimum of tan 5 in the region of 10 -10 Hz, and increasing values at lower and higher frequencies (Fig. 3.4-21). 

Fig. 3.4-21 Schematic representation of the frequency spectrum of dielectric losses in glasses at room temperature. The solid curve gives the total losses, made up of (1) conduction loss, (2) relaxation loss, (3) vibration loss, and (4) deformation loss...

Dielectric Loss. The total dielectric loss in glass is the sum offour different loss mechanisms, namely, conduction, dipole relaxation, vibration, and deformation losses. As... 

Barium Carbonate. BaC03, decomposes at 1450°C sp. gr. 4.4. Occurs naturally in Durham, England, as witherite. The principal use in the ceramic industry of the raw material is for the prevention of efflorescence on brickwork for this purpose it is added to brick-clays containing soluble sulphates. The pure material is used in the manufacture of barium ferrite permanent magnets, and in some ceramic dielectrics to give lower dielectric loss. In the glass industry this compound is used in optical glass and television tubes it is also used in some enamel batches. 

These relationships are known as the Debye formulae. The Debye process has a relaxation time distribution, which is symmetrical around /niax= niax/2n and has a full width at half-maximum of 1.14 decades in frequency for the dielectric loss. In most cases, the half width of measured loss peaks is much broader than the predicted by eqn [26] and in addition, their shapes are asymmetric and with a high-frequency tail. This is the non-Debye (or nonideal) relaxation behavior found in many glass formers. In the literature, several empirical model funaions, mostly generalization of the Debye function, have been developed and tested which are able to describe broadened and/or asymmetric loss peaks. Among these empirical model functions, the most important are the Kohlrausch-Williams-Watts (KWW), Cole-Cole (CC), Cole-Davidson (CD), and the Havriliak-Negami (HN) function. The HN function, with two shape parameters, is the most commonly used funaion in the frequency domain. 

Electrical. Glasses are used in the electrical and electronic industries as insulators, lamp envelopes, cathode ray tubes, and encapsulators and protectors for microcircuit components, etc. Besides their abiUty to seal to metals and other glasses and to hold a vacuum and resist chemical attack, their electrical properties can be tailored to meet a wide range of needs. Generally, a glass has a high electrical resistivity, a high resistance to dielectric breakdown, and a low power factor and dielectric loss. 

Interference filters consist of several evaporated dielectric layers on a glass or quartz substrate. Their transmittance can be tailored by choosing appropriate layers. A problem is their limited bandwidth of transmission which is usually above AT. = 30 nm. Also, a substantial loss in sensitivity has to be accepted since the maximum transmission is limited to less than 40%. 

An ESR spectrometer (Varian model E-3) was used to observe and quantify Mn2+ species at a field strength of 3155 50 G and a frequency of 9.5 GHz. A flat fused silica ribbon cell (Wilmad Glass No. WG-812) was used at very low concentrations to optimize the signal-to-noise ratio by minimizing dielectric losses. Microwave power was set routinely to 4 mW, but was occasionally raised to optimize sensitivity at very low concentrations. Quantitation was based on the height of the lowest-field peak in the first derivative of the absorption spectrum. As reported by others (63), this technique is characterized by precision and accuracy of about 1% relative standard deviation over a linear range from CIO"6 to If)"4 M (<0.05-5 mg/L). 

The most noticeable difference between syntactic foams with the same filler but different binders is seen in the tangent of the dielectric loss angle (Table 25)11(. If glass microspheres replace organosilicon ones for the same binder, not only tan 8, but also e decrease 1). But also the dielectric properties and the concentration of the binder affect the final foam s e (Fig. 18) n). 

Perhaps the best comparison is that of Alford and Dole (1955) who measured the specific heat of a sample of polyvinyl chloride from the same source as the polyvinyl chloride used by Fuoss (1941) in his extensive dielectric loss studies. The comparison between c and e", the dielectric loss factor measured at 60 cps, is shown in Fig. 15, and covers the glass transition range. Note that the peak of the dielectric loss curve is at 100° C, about 20 degrees higher than the inflection point of the c — T curve. 

In forsterite ceramics the mineral forsterite (Mg2Si04) crystallizes. They have excellent low-dielectric-loss characteristics but a high thermal expansion coefficient which imparts poor thermal shock resistance. During the 1960s they were manufactured for parts of rather specialized high-power devices constructed from titanium and forsterite and for which the operating temperature precluded the use of a glass-metal construction. The close match between the thermal expansion coefficients of titanium and forsterite made this possible. Today alumina-metal constructions have completely replaced those based on titanium-forsterite and the ceramic is now manufactured only to meet the occasional special request. 

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