Arnold H. Scott

Precise measurement of dielectric constant by the two-fluid technique

Recently developed transformer bridges have made it possible to easily make measurements of capacitance to a few ppm (parts per million), particularly at audio frequencies. The question naturally arises: can dielectric constant and phase angle measurements be made with comparable accuracy?

Insulation resistance measurements

I am not saying anything new when I say that d-c electrical conductance or conductivity (reciprocal resistance or resistivity) of electrical insulating materials is not generally a property of major concern to the user of these materials. In fact, the conductance of an insulating unit is usually several orders of magnitude smaller than is required for efficient use. Only in those cases where extremely small currents are involved do the actual values of the conductances become important as such.

Dielectric absorption in low loss materials

In 1949 a paper was presented before this Conference on Electrical Insulation giving the results of measurements of volume and surface conductivities made on Polystyrene, Polymethylmethacrylate and Polyvinylbutyral under standard room conditions. The use of a vibrating reed electrometer in place of a more sluggish quadrant electrometer has permitted a more detailed study of the current flow through dielectric specimens having very low conductivities, Dr. Paul Ehrlich has recently published a paper(1) on results obtained with the vibrating reed electrometer in which the change of volume conductivity with time was followed from a few seconds up to 10 4 seconds, for Polystyrene and Polymethylmethacrylate. In the work now being described measurements were made of both volume and surface conductivities at various relative humidities. The materials on which measurements were made are Diallyl Phthalate, Polymethyl Methacrylate, Polystyrene and Teflon.

Measurement of dc dielectric conductance (reciprocal resistance) at elevated temperature

The general principles given in ASTM D 257(1) apply to measurements of dielectric conductance at elevated temperatures. However, special precautions must be taken in the design of the cell or holder and the measuring circuit to insure that the supporting insulator conductances are Iow enough so that they do not appreciably affect the instrument indication when the conductance of a specimen is being measured. This includes conductance between the measuring electrode and the guard ring when volume conductance is being measured. This generally requires that the supporting insulators for the measuring electrode be Iocated outside the heated area. lt is possible to have the supporting insulators in the heated area if special insulating materials are used and special care is taken to have long lengths of insulators with relatively small cross section. Surface conductance usually has little meaning at elevated temperatures.

Techniques for using the air-gap method for the precise determination of the dielectric constant and loss angle of solid disk specimens

The air-gap method of making dielectric measurements is an attractive one because it is not necessary to apply electrodes to the specimen or to measure its area. The disadvantages of the air-gap method are (1) that it can be used effectively for precise measurements only with a guard-ring type of cell (or holder) and (2) that the thickness of the specimen must be measured more precisely than when contact electrodes are used. The second requires that the specimen must be very carefully prepared to a uniform thickness. This means that a three-terminal measurement must be made and such measurements can in general only be made at frequencies of 100 KHz and below.

Long-time effects of humidity change on the dielectric properties of certain polymers

The study of the long-time effects of humidity change on the dielectric properties of dielectric materials was for the most part made on disk specimens from 2 to 5 mm thick which were cut down to a diameter of about 4 cm for measurement in micrometer electrode holders and on which evaporated metal (usually gold) electrodes were applied. Most of the results described in this paper were obtained on specimens of this type using a frequency of 1000 c/s at 23°C.

Residual losses in a guard-ring micrometer-electrode holder for solid-disk dielectric specimens

Residual losses occurring in the specimen holder and in the bridge standard capacitor must be taken into account in order to make accurate bridge measurements of the dielectric losses of materials that have very low losses. Also, knowledge of the exact spacing of the electrodes is required for accurate determination of the dielectric constant of materials. To better meet these requirements, a new specimen holder has been designed and constructed. By a technique using ball reference gages, the electrodes of the holder can be adjusted to be parallel to about one micron, and the zero correction of the micrometer can be determined to ±1 micron. Using a modification of a technique used by Astin1 and others, the residual loss angles of the holder and its connecting leads and of the bridge standard capacitor were determined to ±1 or 2 microradians.

A study of materials for their suitability as Standard dielectric reference materials

The Standards Group within the Dielectrics Section of the National Bureau of Standards has undertaken an investigation with the object of producing a set of Standard Specimens with well-defined dielectric constant and loss index. An important attribute of such a group of specimens is that their electrical properties should not be subject to unpredictable change as a function of environment or time. Furthermore, both high-loss and low-loss specimens are needed.

The effect of crystallinity on the dielectric properties of polychlorotrifluoroethylene

Polychlorotrifluoroethylene is a linear straight-chain polymeric material with the chemical structure (Cl -C F — F C- F) n . It is well known that this material can be crystallized and that the melting point of the crystals is close to 215°C(1,2). Above this temperature, the polymer is in the liquid or “rubbery” state. On quenching the material very rapidly from the liquid state, crystallization can be largely prevented, and the system is then for the most part in the supercooled liquid, or at lower temperatures, in the glassy state.

Improved specimen holder for use at temperatures up to 500°C

A specimen holder and associated equipment were required which would permit the accurate measurement of the dielectric constant and dissipation factor of dielectric materials at temperatures up to 500°C and over as wide a frequency range as possible. Holders have been designed for use at elevated temperatures (l, 2, 3), hut none were found which would cover the temperature and frequency ranges required.