William P. Harris

Piezo- and pyro-electricity in polymer electrets

In 1960 Gubkin1 published an electrostatic theory of piezoelectricity which did not involve the microscopic structure of the electret. This approach would seem to be a good description of the phenomenon of piezoelectricity in polymer materials, many of which are partially or completely amorphous. We applied Gubkins model to a specific set of experimental conditions, which resulted in some simplifications. These conditions are: 1. We measure changes in surface charge at nearly zero potential. 2. We use thin evaporated metal electrodes so that the electrodes change area in accord with changes in specimen area, A. 3. We consider only amorphous specimens which we assume to be elastically isotropic. 4. We assume the free charges to be frozen in the solid and to move in proportion to macroscopic strains, and the polarization charges to be rigid molecular dipoles whose positions move in proportion to macroscopic strains but whose effective total moment remains constant as long as the strains are isotropic.

A new ultralow-frequency bridge for dielectric measurements

A new bridge, operating in the frequency range 0.001–10,000 Hz, and featuring improved convenience of operation and high accuracy, has been built at the National Bureau of Standards, under NASA sponsorship. Though designed chiefly to measure dielectric properties, it is also capable of measuring the conductance of resistors in the range 108–1014 Ω. Large loss angles are no great problem. In the frequency range 0.01–15 Hz, capacitance resolution of 5 ppm and loss-angle resolution of 5 μrad is easily achieved. It is not yet known what resolution and accuracy will characterize the frequencies between 70 and 10, 000 Hz, but it is expected to diminish. Existing bridges cover this range better.

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?

Precision in dielectric measurements

In this paper we shall survey measurements of dielectric properties of materials with respect to precision. We shall consider both optimum and extreme conditions, and try to indicate the source and magnitude of errors. To better acquaint conferees (and readers) with the National Bureau of Standards, we shall choose a disproportionate share of the examples from work done or in progress at NBS.

A wide-range bridge for dielectric measurements

Several years ago, an ultra-low-frequency bridge employing operational amplifiers and a two-phase source was described.1 Since that time, improved operational amplifiers and signal sources have become commercially available and permit the extension of this same technique to higher frequencies.

Three-terminal cell for thin film dielectric measurements

A cell has been designed and built for measurement of the dielectric constant and dissipation factor of thin films. The principle employed is the two-fluid, three-terminal scheme, 1,2, 3 adapted to thin film measurements. Measurements of dielectric constant, dissipation factor and thickness can be obtained on films ranging from about 250 μm (10 mil) down to 1 μm (0. 04 mil). Agreement is in general better than 1%, except for the dissipation factor of very low loss films where bridge sensitivity is the limiting factor.

Low frequency dielectric behavior

Electrical engineers, as well as polymer physicists and materials researchers, can gain useful information from a study of the behavior of dielectrics, and of high-megohm resistors, at very low frequencies. This is illustrated by examples. There are two main methods of obtaining ultra-low-frequency data. One is to apply a d.c. voltage producing a time-dependent current which corresponds to an inverse frequency plot. The other is to use an alternating voltage, usually sinusoidal, and apply this to a bridge circuit including the material under study. Recent developments in the latter methods are presented, along with some typical results.

Precise determination of the area of guarded electrodes for accurate dielectric measurements on solid-disk specimens

Accurate measurements of dielectric properties of materials by the three-terminal air-gap method require accurate knowledge of the area of the electrodes. The expression giving the dielectric constant can be put in the form equation (1) where equation Equation (1) can be solved for A, giving equation The two fluid method enables us to measure the dielectric constant of a specimen without knowledge of the area of the cell electrodes, or of the thickness of the specimen, if the electrode spacing and area remain constant during the set of four measurements required1. But it is now known2 that the effective area is a function of the dielectric constant of the specimen being measured by this technique. The problem now is to determine the magnitude of this change of area, and evaluate its effect on the dielectric constants measured.

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.