Experience with IEC 1109 1000 h salt fog ageing test for compositeinsulators

This paper covers some examples of the STRI and NGK experience with the standard IEC 1109 salt fog test and some modifications of this test for different types of test objects     

Behavior of insoluble materials in artificial contamination tests

It was reported that the dc flashover voltage under an artificial contamination test sometimes was affected considerably by the type of insoluble materials in the contaminant. To clarify the cause of this phenomenon, the physical characteristics of some contamination slurries and the conductivity of the artificially contaminated insulator surfaces are investigated experimentally. The test results lead to the conclusion that the variation in the flashover voltage is caused mainly by the characteristics of conductive films formed on the insulator surface in the clean fog test     

Electrical characterization of crystal defects and oxygen in czochralski silicon using a gate-controlled diode

Defect generation in Czochralski (CZ) silicon crystal during heat treatment and effect of the defects on generation currents, measured by gate controlled diodes, were investigated. Sample wafers were obtained from an as-grown CZ silicon ingot which has a wide range of oxygen concentration but keeps other characteristics nearly constant. In the diode fabrication process, a two-step heat treatment method was utilized to control defect generation. It was found that stacking faults and dislocation loops, whose densities depend on oxygen concentration, increase the reverse current of the diode and the reverse current increases caused by defects vary with heat treatment condition. The most noticeable result was that the reverse current is enhanced by increasing oxygen concentration, even if no defect is observed in the device active region because of low defect density induced by heat treatment or of denuded zone formation. This result suggests the existence of some kind of electrically active defect caused by oxygen atoms in the crystal. Surface generation current is independent of crystal quality and two-step heat treatment conditions.     

Delayed retardation phenomena of fatigue crack growth in various steels and alloys

The delayed retardation phenomena of fatigue crack growth resulting from a single application of overload were investigated for five steels, two aluminium alloys and a titanium alloy. As long as the small scale yielding condition was satisfied at the overloaded crack tips, the retardation behaviour of these materials was expressed consistently by four parameters; the peak/baseline stress ratio, r, the exponent in the Paris equation, m, the overload-affected zone size, _D, and the crack distance at the minimum rate of crack growth, _B. Then the parameters, _B and _D, characterizing the retardation phenomena for these materials were determined. The retardation of aluminium alloys was stronger than that of the other materials. This is attributed to the lower value of _B/ _D in aluminium alloys than in the other materials. In the case of r=2, the overload-affected zone sizes, _D, were nearly equal to 1.5 ₀ in HT80 steel and aluminium alloys, slightly lower than 1.5 ₀ in SNCM8 steel and much larger than 1.5 ₀ in A553 steel and the titanium alloy, where ₀ is the monotonic plastic zone size calculated according to the Dugdale model. The dependence of retardation on baseline stress intensity, K ₁, appeared somewhat complicated. In the cases of A553 steel and A2017 aluminium alloy the amount of retardation increased with increasing K ₁ value, while in the cases of HT80 steel and Ti-6A1-4V titanium alloy the tendency appeared in the reverse direction. The former behaviour was related to the change in the stress state from plane strain to plane stress at the overloaded crack tips and the latter was related to the threshold of stress intensity.     

Free volume,excess entropy and mechanical behavior of polymeric glasses

Free volume fraction of the Williams-Landel-Ferry(WLF)-Doolittle type has been applied to the analysis of the nonequilibrium state for the glassy polymers. The free volume fraction is a two parameter variable since it depends on the free volume and the occupied volume. The excess entropy is derived from the mixing of vacant and occupied sites, and for polymers a factor is added which corresponds to disordering the molecular segments. A relationship between the excess entropy and enthalpy is derived. For a given level of fractional free volume, there is a unique rate constant associated with changing that level of the fractional free volume. The reciprocal of this rate constant, which depends on the exponential order of the fractional free volume, can be considered as a time constant for changing the molecular conformational probability, and its value is in the order of the average dielectric relaxation time in the corresponding state. When mechanical deformation is. imposed with a rate which is too fast as compared to this time constant, the deformation without a change in the conformational probability, i.e., the reversible elastic deformation, will ensue. In contrast, a sufficiently slow deformation will be accompanied by a change in fractional free volume and the excess entropy, and the above-mentioned time constant will change with deformation. Since dilation will tend to shorten this time constant, tensile deformation will result in reducing the modulus accompanied by the increase in entropy. Shear deformation is considered as a mixture of compression and tension, where only tension contributes to a change in entropy, and the net result is the strain softening which can be predicted from the tensile behavior. Uniaxial compression is controlled by the shear behavior, and the excess entropy and the fractional free volume increase, while the occupied volume decreases with strain. A constitutive relationship has been proposed which accounts for the effects of temperature, pressure, strain rates and thermal history on nonlinear viscoelastic behavior of polymeric glass.     

Entropy,Free Volume,and Cooperative Relaxation

The high frequency end of the relaxation spectrum for polymer molecules involves the rotation of the segmental bonds. This fast relaxation process, however, cannot take place easily in the condensed state crowded by the densely packed conformers, necessitating the slower cooperatively synchronous relaxation. As the temperature is lowered, the domain of cooperativity grows towards the infinite size at the Kauzmann zero entropy temperature, though actually the system deviates from the equilibrium as the glass transition intervenes typically at 50 K above that temperature. The excess enthalpy and entropy drop faster than predicted by the rotational isomeric states which would reach zero only at 0 K. The real ΔC_P is greater than that of the RIS value. The actual volume in excess of the crystalline lattice volume, however, points towards zero at 0 K. Thus, a polymer with higher T_g typically exhibits a lower density and modulus in the glassy state. Since the configurational entropy associated with the free volume is proportional to the logarithm of the latter, the Kauzmann temperature can be scaled by ln M, where M is the algebraic average of the conformer molecular weight. The temperature dependence of the most dominant, i.e., the largest equilibrium domain size will result in the Adam-Gibbs and Vogel equations for the characteristic relaxation time. The cooperative domain distribution leads to the relaxation spectrum that follows a power law. The relationship between the characteristic relaxation time and the rate of physical aging is derived.     

Sidelobeless multiple-object discriminant filters recorded as discrete-type computer-generated holograms

The computer generation of sidelobeless multiple-object discriminant correlation filters has been stressed. We propose to synthesize the filter functions by use of the simulated-annealing algorithm. By this method the filters can be obtained as discrete-type computer-generated holograms. The filters can suppress the sidelobes and provide sharp correlation peaks. A computer simulation and an optical experiment were performed, and the expected correlation responses were obtained.