The electrical conductivities of two specimens of clear fused quartz were measured in vacuo in the temperature range of 600 to 1400°C. The method of potential probes was used to evaluate the platinum contacts and eliminate effects of high contact resistance. One specimen obeyed Ohm's law as evidenced by the linear dependence of the current on the potential between probes and the uniform potential distribution. This behavior established the specimen as an electronic conductor. The value of specific conductivity was ∼10−4 mho/cm over the interval from 1000 to 1400°C. Ohm's law was not found applicable for the other specimen, which displayed many dielectric anomalies and polarization effects such as polarization voltages, anomalous potential distributions, reverse currents, and current decays immediately after application of a voltage. The phenomena indicated the presence of space-charge polarization and here the conductivity appeared to be substantially ionic. Heat treatment of this specimen stabilized the electrical behavior and caused increased or decreased resistance, depending on voltage, polarity, and temperature of measurement. The heating was also found to produce partial crystallization. It is suspected that the marked contrasts in the behavior of the two specimens are due to differences in the impurity contents.
The direct piezoelectric effect has been observed in roll-elongated films of polyvinylchloride and polyvinylfluoride. The effect was produced by applying tensile stress to a clamped specimen at a fixed frequency of 20 Hz. The piezoelectric modulus g31 was determined to be 0.2 to 0.7 V m−1/N m−2 in polyvinylchloride; in polyvinylfluoride g31 ≃ 0.2 V m−1/N m−2 and g32 ≃ 1 V m−1/N m−2. The piezoelectric effect in these films is believed to be due to mechanical distortion of oriented dipoles, resulting from the tensile stress, and the orientation appears to differ in the two materials.
This paper presents an elementary review of thermal imaging systems, with emphasis on the application of thermography to nondestructive testing.Topics discussed include heat radiation theory; early and contemporary thermal imaging systems; performance characteristics; effects of emissivity, background temperature, atmosphere, and field of view.Examples of various applications of thermography to nondestructive testing are given. A bibliography is included.
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