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Formation of ice in Earth's atmosphere at temperatures above approximately -20 °C is one of the outstanding problems in cloud physics. Contact nucleation has been suggested as a possible mechanism for freezing at relatively high temperatures; some laboratory experiments have shown contact freezing activity at temperatures as high as -4 °C. We have investigated Arizona Test Dust and kaolinite as contact nuclei as a function of size and temperature and find that the fraction of submicron particles that are active as contact ice nuclei is less than 10-3 for -18 °C and greater. We also find that the different dusts are quite distinct in their effectiveness as contact nuclei; Arizona Test Dust catalyzed freezing in the contact mode at all mobility diameters we tested a -18 °C whereas kaolinite triggered freezing only for mobility diameters of 1000 and 500 nm at that temperature. [ABSTRACT FROM AUTHOR]

A detailed evaluation of 122 air samples collected in and around Libby, Montana and previously analyzed by EPA has been performed. Photographic records of each analyzed particle were collected and archived in a searchable database. The results of the study indicate that the EPA's failure to follow generally accepted procedures overestimated the concentration of regulated asbestos fibers by 10 times and by 3 times for all asbestiform fibers. These results are applicable to the analysis of naturally occurring asbestos in all parts of the country. The results demonstrate the need for detailed morphological and chemical analyses to reliably determine potential exposures and associated risks when evaluating mixed asbestos/non-asbestos environments. [ABSTRACT FROM AUTHOR]

Scanning electron microscopy (SEM) cathodoluminescence (CL) experiments were used to determine the existence and direction of piezoelectric fields in a commercial InGaN multiple-quantum-well (MQW) light-emitting diode (LED). The CL emission peak showed a blueshift with increasing reverse bias due to the cancellation of the piezoelectric field. A full compensation of the piezoelectric field was observed followed by a redshift with a further increase of reverse bias, indicating that flat-band conditions had been reached. We determined the piezoelectric field points in the [000-1] direction and estimated the magnitude to be approximately 1 MV/cm. SEM-CL carrier generation density variation and electroluminescence experiments were also used to confirm the existence of a piezoelectric field in the InGaN MQW LED. [ABSTRACT FROM AUTHOR]

Minority-carrier diffusion lengths of electrons and holes were measured in a GaN-based light-emitting diode using the electron-beam-induced current technique in the line-scan configuration. A theoretical model with an extended generation source and a nonzero surface recombination velocity was used to accurately extract the diffusion length of the p- and n-type layers. A minority-carrier diffusion length of L[sub n]=(80±6) nm for electrons in the p-type GaN layer, L[sub p]=(70±4) nm for holes in the n-type GaN:Si,Zn active layer, and L[sub n]=(55±4) nm for electrons in the p-type Al[sub 0.1]Ga[sub 0.9]N layer were determined. The results from this model are compared with two simpler and widely used theoretical models. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]