Formation of thermal decomposition cavities in physical vapor transport of silicon carbide

The relationship between seed mounting and the formation of thermal decomposition cavities in physical vapor transport grown silicon carbide was investigated. Scanning electron microscopy, energy dispersive x-ray spectroscopy, Auger electron spectroscopy, and optical microscopy were used to characterize thermal decomposition cavities at various stages of their development. The observations indicate that the attachment layer that holds the seed to the graphite crucible lid frequently contains voids. The seed locally decomposes at void locations and Si-bearing species are transported through the void. The decomposition produces a cavity in the seed; the silicon is deposited on and diffuses into the graphite lid. The formation of thermal decomposition cavities can be suppressed by the application of a diffusion barrier on the seed crystal backside.     

Development of a Field Deployable Firebrand Flux and Condition Measurement System

Fire Technology - A new instrument to quantify firebrand dynamics during fires with particular focus on those associated with the Wildland-Urban Interface (WUI) has been developed. During WUI...     

Growth of GaBN ternary solutions by organometallic vapor phase epitaxy

Layers of Ga_1-xB_xN with compositions from x = 0 to x = 0.07 were grown by organometallic vapor phase epitaxy on sapphire substrates using trimethylgallium, triethylboron (TEB) and NH₃ as precursors. Growth was done in the temperature range from 450 to 1000°C. The presence of boron was detected by the shift in the (0002) peak position in x-ray diffraction, by x-ray photoelectron spectroscopy, secondary ion mass spectrometry measurements, and by the changes in the band gap as measured by optical transmission. It was found that for the studied range of compositions the band gap varied from 3.4 eV for x = 0 to 3.63 eVfor x = 0.05. At certain TEB concentrations in the gas phase, the growth rate decreased abruptly, most likely because of a growth poisoning by the onset of growth of a very slow growing B-rich phase. The threshold TEB concentration for this growth poisoning decreased with increasing growth temperature; and at 1000°C, less than 1% of B could be incorporated as a result. GaBN alloys with about 7% substitutional boron were also produced by implantation of 5 × 10¹⁶ cm⁻² B ions at 60 keV into GaN, as evidenced by the shift of the band edge emission in cathodoluminescence spectra from 3.4 eV for GaN to 3.85 eV for GaBN.     

Studies on photodegradation of poly(vinyl chloride) accelerated by α-hydroperoxy-tetrahydrofuran

A study of the role of THF, THFO₂ charge-transfer complex and α-hydroperoxy-tetrahydrofuran (HOOTHF) in the photodecomposition of poly(vinyl chloride) (PVC) film cast from THF is presented. Detailed e.s.r. studies show that during u.v. irradiation (254 nm), THF and/or HOOTHF, oxyTHF free radicals are formed, which participate in the dehydrochlorination of PVC. The final products of photooxidation of THF are α-hydroxy-tetrahydrofuran (HOTHF) and butyrolactone, which give strong i.r. absorption bands at 1735 and 1789 cm^?1, respectively. The presence of THF residues in PVC films cast from this solvent should always be considered when thermal or photochemical studies are made.     

Effect of thermal stabilizers on the photo-oxidative degradation of solid poly(vinyl chloride)

It has been found that metal (Ba, Ca, Cd and Pb) stearates and pure stearic acid have stabilizing effects on the photodehydrochlorination of poly(vinyl chloride) (PVC) and the formation of polyene structures. On the other hand the same compounds accelerate photo-oxidation of PVC. From oxygen uptake measurements at different temperatures we calculated the energy of activation of photo-oxidation. It was found that metal stearates decrease the energy of activation of photo-oxidation processes of PVC, whereas stearic acid increases it. A synergistic effect of an equimolecular mixture of Ba and Cd stearates has also been observed.     

Photodegradation of some poly(vinyl chloride) (PVC) blends: PVC/ethylene-vinyl acetate (EVA) copolymers and PVC/butadiene-acrylonitrile (NBR) copolymers

Poly(vinyl chloride) (PVC) and its blends with polybutadiene-acrylonitrile (NBR) (containing 21.7 weight-percent acrylonitrile (AN), a heterogeneous two-phase system; and containing 41.6 weight-percent of AN, a homogeneous one-phase system) and with polyethylene-vinyl acetate (EVA) (containing 45 weight-percent of vinyl acetate (VA), a heterogenous two-phase system; and containing 65 weight-percent VA, a homogeneous one-phase system) were UV-irradiated (at 3500 Å UV-light (solar spectrum)). After UV irradiation the kinetics measurements were made of the formation of hydroperoxy (OOH) and carbonyl (CO) groups and the changes of mechanical properties: tensile strength, elongation to break, and impact energy. As a result of the photooxidative degradation of PVC blends, decreases of mechanical properties were observed. The effects are more severe in PVC/NBR blends, which contain unsaturated bonds (polybutadiene segments) than in the case of PVC/EVA. The phase structure plays an evident role on the UV degradation only of PVC/NBR blends. The photostability of PVC blends can be slightly improved by introducing Tinuvin P or Ni-chelates photostabilizers.     

Estimating gradients for statistical tolerance synthesis

Current statistical tolerance analyses calculate the distributions of critical dimensions or design functions for a given set of tolerances. However, the analyses cannot determine the effect of small changes in tolerance values without a complete reanalysis. This inability to furnish gradient information restricts the use of optimization algorithms in tolerance synthesis to those which do not require gradients. This paper describes a technique for statistical tolerance analysis. In the technique, the continuous distribution of manufacturing variations is replaced by an equivalent discrete distribution. During the process of computing the discrete probabilities, gradient information is also computed, so that more efficient optimization algorithms can be used.     

A dileucine motif in HIV-1 Nef is essential for sorting into clathrin-coated pits and for downregulation of CD4

Nef, a approximately 200 residue multifunctional regulatory protein of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV), interacts with components of host cell signal transduction and clathrin-dependent protein sorting pathways. The downregulation of surface CD4 molecules and major histocompatibility complex (MHC) class I antigens by Nef is believed to be important in AIDS pathogenesis [1-7]. Nef contains a globular core domain and two disordered segments--a myristylated arm at the amino terminus and a carboxy-terminal loop projecting from the globular core [8,9]. Here, we aimed to determine the sorting signals in HIV-1 Nef that were responsible for its involvement in the clathrin-mediated pathway. We found that a sequence in the carboxy-terminal disordered loop of Nef is essential for downregulation of CD4. This sequence resembles the dileucine motif, one of two well-characterized sorting signals that target membrane proteins to clathrin-coated vesicles. The dileucine-motif-containing segment of Nef bound directly and specifically to the beta-adaptin subunit of the clathrin adaptor complexes AP-1 and AP-2, which are responsible for recruiting sorted proteins into coated pits. Unlike wild-type Nef, a mutant form of Nef that lacked the dileucine motif did not localize to clathrin-coated pits and did not downregulate CD4 expression, although it could downregulate MHC class I surface expression. Thus, the dileucine motif in HIV-1 is required for CD4 downregulation and for interaction with clathrin adaptor complexes.