Thermal conductivity, dislocation density and GaN device design

The performance of high power transistor devices is intimately connected to the substrate thermal conductivity. In this study, the relationship between thermal conductivity and dislocation density is examined using the 3 omega technique and free standing HVPE GaN substrates. Dislocation density is measured using imaging cathodoluminescence. In a low dislocation density regime below 10⁵ cm⁻², the thermal conductivity appears to plateau out near 230 W/K m and can be altered by the presence of isotopic defects and point defects. For high dislocation densities the thermal conductivity is severely degraded due to phonon scattering from dislocations. These results are applied to the design of homoepitaxially and heteroepitaxially grown HEMT devices and the efficiency of heat extraction and the influence of lateral heat spreading on device performance are compared.     

Mössbauer and infrared spectroscopic studies of novel mixed valence states in cobaltous ferrocyanides and ferricyanides

Novel mixed valence states have been obtained by the treatment of cobaltous ferrocyanides (Co⁺²Fe^II) and ferricyanides (Co⁺²Fe^III) in an ozone flow. The CN stretching bands occur at 2085 cm^–1 for Co⁺²Fe^II and at 2160 cm^–1 for Co⁺²Fe^III. After the ozonization process of Co⁺²Fe^II, an intense band approximately at 2125 cm^–1 is detected. This intermediate band must correspond to a mixed valence state of the type: Fe^II–CN–Co²⁺–NC–Fe^III Mössbauer spectra recorded in situ during the ozonization of Co⁺²Fe^II show the presence of two components: a doublet with isomer shift and quadrupole splitting values close to the cobalti ferricyanide and a very broad line for the mixed valence state. From the Mössbauer and infrared spectra of the aged samples of the Co⁺²Fe^II after ozonization, a relaxation process to the initial state of the samples is observed but the mixed valence state is stable.     

Electrokinetic investigations of charged porous membranes

Asymmetric ultrafiltration membranes were prepared from fully aromatic polyamides differing in the diamine monomers of the polymeric backbone and from polysulfone. Nanofiltration membranes were made from polysulfone and polyethersulfone. The polysulfone as well as the polyethersulfone were chemically modified to change the surface charges of the membranes that were made from these polymers. This means neutral, positively as well as negatively charged membranes could be employed for the measurements. The surface properties of the membranes as a function of pH were determined by measuring the streaming potential in a perpendicular and horizontal mode. Applying proteins the values of the streaming potential changed depending on the original charges of the membranes as well as on the pH of the solution. The values shifted to either higher or lower absolute values. Thus, characterization of unused and used membranes can be carried out by electrokinetic measurements. This was also demonstrated using a membrane fitted out with invertase. The zeta potential of nanofiltration membranes, however, was only evaluated from the results obtained with the horizontally run cell.     

Hydroxide-promoted redox reactions in water of α-Phenyl-4-nitro-benzenemethanol, α-(p-Nitrophenyl)-4-pyridinemethanol,and α-(p-Nitrophenyl)-4-Pyridinemethanol N-Oxide steric inhibition of resonance

α-Phenyl-4-nitrobenzenemethanol ( 3 ) reacted with 1 M sodium hydroxide to yield 4, 4′-dibenzoyl-azoybenzene ( 5 ) (51%), 4-hydroxy-4′-benzoylazobenzene ( 6 ) and benzoic acid (12% each), and smaller amounts of 4-aminobenzophenone and 4-nitrobenzophenone. Both α-phenyl-2-nitrobenzenemethanol ( 9 ) and 3, 5-dimethyl-4-nitrobenzenemethanol ( 10a ) did not react with 1 M sodium hydroxide, presumably due to steric hindrance. α-(p-Nitrophenyl)-4-pyridinemethanol ( 14 ) and its N-oxide 11 with 1 M sodium hydroxide yielded 4,4′-diaroylazoxybenzenes 15a and 12a , respectively, 4,4′-diaroylazobenzenes 15b and 12b , respectively, as well as 4-hydroxy-4′-aroylazobenzenes 16 and 13 , respectively. The relative reaction rates were 11 > 14 > 3 . Studies with 11 showed that the nitro group is involved in the redox reaction in preference to the N-oxide group.