Dielectric properties of spark-plasma-sintered BaTiO3

Spark-plasma-sintering (SPS) has been applied to BaTiO₃ to prepare dense ceramics consisting of submicrometre-sized powder. Relatively dense (typically 97% of the theoretical X-ray density) pellets with an average grain size remaining similar to that of the starting powder, approximately 0.6 m, were obtained by the SPS process. Fixed frequency (1 kHz) measurements show the room temperature permittivity of SPS ceramics to be relatively high, approximately 3500, and at least double the value of conventionally sintered ceramics, approximately 1500. Alternating current (a.c.) impedance spectroscopy measurements show that SPS is an effective process to reduce the influence of intergranular (grain boundary) effects on the permittivity and direct current (d.c.) resistance characteristics of BaTiO₃ ceramics substantially.     

Experimental Investigation on Noise Suppression in Supersonic Jets from Convergent-Divergent Nozzles with Baffles

The acoustic properties of supersonic jet noise from a convergent-divergent nozzle with a baffle have been studied experimentally over the range of nozzle pressure ratios from 2.0 to 8.0. Acoustic measurements were conducted in a carefully designed anechoic room providing a free-field environment. A new approach for screech noise suppression by a cross-wire is proposed. Schlieren photographs were taken to visualize the shock wave patterns in the supersonic jet with and without the cross-wire. The effects of the baffle and the cross-wire on acoustic properties are discussed. It is shown that the baffle has little effect on the screech frequency for the underexpanded supersonic jet without the cross-wire. Also, the cross-wire introduced in supersonic jets is found to lead to a significant reduction in overall sound pressure level.