Varistor Ceramics

The development of varistor ceramics is reviewed with an emphasis on ZnO-based varistors formed by liquid-phase sintering. Particular attention is given to the effects of microstructural disorder on the properties of varistors and to the different modes of failure that can occur.     

Segregation in Yttrium Aluminum Garnet: II, Theoretical Calculations

Calculations based on ionic space-charge models of doped yttrium aluminum garnet (YAG) have been compared to experimental measurements of surface segregation in crystals of various compositions. The comparison allows limits for vacancy-formation energies to be set. The range for anion:cation formation-energy ratios has been established to be 0.20-0.23, based on the reasonable assumptions that the formation energy of the yttrium ion is 75% of that of the aluminum ion and the Schottky defect formation energy is 4.2 eV. The model explains the experimental observation of calcium at the surface regardless of net acceptor excess or net donor excess. The relationship between vacancy-formation energy and dopant excess has been used to construct segregation maps for YAG, which are useful in materials design strategies.     

Thermophysical Properties of α-Tungsten Carbide

Upper and lower bounds for the thermal expansion of polycrystalline tungsten carbide (α-WC) are predicted at ultrahigh temperatures from low-temperature experimental data. The lower bound is obtained from an α _VK_TV model, where α _V is the volume thermal expansion, K_T the isothermal bulk modulus for a randomly oriented polycrystalline sample, and V the molar volume. For many materials, the α _VK_TV product approaches a constant value that is similar to the specific heat at the highest temperatures. The upper bound uses Grüneisen's rule with a constant Grüneisen parameter gamma at temperatures >1.3θ_D (where θ_D is the Debye temperature) and experimental data below that temperature. Literature data for the thermophysical properties of α-WC have been reviewed and used in our α _VK_TV model to calculate a lower bound for the thermal expansion at temperatures >2θ_D and to calculate the temperature dependence of the bulk modulus. The ultrahigh-pressure thermal expansion has been calculated from the lower bound. Model predictions of the thermophysical properties of WC are given for an extended temperature range.     

A factorial design applied to polypropylene functionalization with maleic anhydride

Maleic anhydride functionalized PP was prepared in a melt-mixer according to a factorial design. Two levels of maleic anhydride concentration (MA), dicumyl peroxide concentration (DCP), reaction time, and temperature were employed. The isolated and interaction effects of these variables on the degree of functionalization and molecular weights were analyzed. The products were characterized by Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), titration, and torque measures. The reaction time showed no appreciable effect within the studied range (10 to 20 minutes); however, the other three factors showed significant main effects. MA and temperature presented an important interaction effect on the degree of functionalization and molecular weights. The highest degrees of functionalization were found to be related to the smallest molecular weights.     

Simultaneous Disruption of Pheromone Communication in Choristoneura rosaceana and Pandemis limitata with Pheromone and Antagonist Blends

In British Columbia, trapping and wind-tunnel studies demonstrated that (Z)-9-tetradecenyl acetate (Z9–14:OAc), a minor component of the sex pheromone for Pandemis limitata, acted as a pheromone antagonist to a sympatric species, Choristoneura rosaceana. Addition of >1% Z9–14:OAc to the four-component C. rosaceana pheromone in a wind tunnel resulted in significant reductions in the proportion of male C. rosaceana that wing fanned, locked on to the plume in flight, oriented upwind, and made source contact, compared to the responses to the pheromone alone. Disruption of pheromone communication was tested in 33.3 × 33.3-m plots, at a release rate of 10 mg/ha/hr using Conrel fiber dispensers. Z9–14:OAc applied alone did not disrupt orientation to virgin-female-baited traps for either C. rosaceana or P. limitata. A 1:1 mixture of Z9–14:OAc and the four-component C. rosaceana pheromone was as effective as the pheromone alone at disrupting orientation of C. rosaceana males to virgin-female-baited traps, demonstrating that disruption apparently did not occur through false-trail following. The 1:1 mixture of Z9–14:OAc and the C. rosaceana pheromone also reduced catches of P. limitata males in virgin-female-baited traps, but not significantly more than the 83% disruption caused by the pheromone alone. Therefore, the C. rosaceana pheromone could be used alone or with Z9–14:OAc to disrupt communication and, presumably, mating in both leafrollers simultaneously.