Calorimetric measurement of very low dielectric loss at low temperatures

Abstract

A calorimetric method is described for measuring very low dielectric loss angles at and near liquid helium temperatures and audio frequencies. The accuracy in loss angle is about 3 × 10−7 for thermoplastics. Changes of δ with frequency and temperature are resolved to an even smaller limit. The method is believed to be more sensitive than bridge methods at the same temperature, and is probably capable of considerable improvement. Preliminary measurements on samples of polypropylene, polyethylene, PTFE, poly(ethylene terephthalate) and a glass microscope cover slip give loss angles (at 1 kHz) of 3·7 × 10−6, 2·7 × 10−6, 1·2 × 10−6, 1·2 × 10−4 and 1·4 × 10−3 respectively. Polyethylene shows an interesting single-relaxation-time Debye peak, which may be connected with phonon-assisted tunnelling; at the lowest temperatures its height decreases with increasing voltage. A review is given of previous dielectric loss measurements on reasonably good insulators at helium temperatures and audio frequencies.

Low temperature dielectric loss spectra of polyolefins

“…The chief disadvantage of bridge methods is that the 'unknown' capacitance is compared with a 'standard' which may also have an unknown loss angle of the order of lOjurad, so that absolute accuracy is rather poor. The calorimetric measurement of power dissipation [8] avoids this difficulty and also allows upward extension of the frequency range, but is slower and generally less versatile. We have used, and recommend using both methods alternately [11].…”

“…7 shows how two of them evolve during the course of an oxidation treatment consisting of high-temperature exposure to air in darkness. The Vincett-Phillips relaxation [8,28] which reaches maximum strength early on, is due to a chemically and thermally unstable group whose occurrence and properties are recognisably those of a hydroperoxide group [16,34]. A peak similar to Carson's [29] resulted from adding a long-chain secondary alcohol to unoxidised polyethylene [35].…”

“…This means that a suitable material with 5<20jurad at 10 MV/m will probably have 5 not exceeding 10/nrad when tested at low voltage. Vincett [8] showed that polyethylene and polypropylene both intrinsically meet this requirement but subsequent investigators have found higher 5 values for both [3,7,9 ] . Some audiofrequency isochronal plots are reproduced in Fig.…”

Low-temperature dielectric study of fused nitrates and other glasses

“…Examples are the silicates, nitrates and borates (Stevels 1980) on the one hand; PMMA, polycarbonate, polypropylene glycol, polyimide (Frossati 1978) and polyethyleneterephthalate (Vincett 1969) on the other. The oxide glasses tend to have dielectric permittivity in the range 5-7, and the organic glasses 2-2.5.…”

Relaxations diélectriques dans les polyéthylènes à très basses températures

“…Elle est caractérisée par une relaxation dite y (à T = 170 ± 5 K pour f = 1 kHz) et une zone (10 K T 160 K) où d'éventuels pics de rela-xation sont peu intenses. Dans les cas [1, 2, 3, 4] où les courbes sont tracées à partir des températures de l'hélium liquide un phénomène est parfois décelé [3,4] que nous désignons sous le nom de relaxation de Vincett ou pic V. Vincett [5] a mis au point un montage expérimental dont le principe est de mesurer, à l'aide de résistances de carbone, l'élévation de température d'une capacité à diélectrique solide lorsqu'elle est alimentée par une tension alternative connue. Il a ainsi trouvé sur un polyéthylène haute densité polymérisé par le procédé Phillips (appelons ce matériau HDP), des courbes isothermes b( f ) dont l'allure est celle de la relaxation de Debye.…”