Bulk growth of Hg0.8Cd0.2Te single crystals by a combined CVT-seeding technique

The growth of high quality Hg_0.8Cd_0.2Te bulk single crystals by CVT, combined with an in-situ seeding technique, is reported here for the first time. For this purpose, a temperature difference of 590° → 540° C with a gradient of 40°-50° C/cm at the solid-vapor interface, and about 0.1 atm of HgI₂ as a transport agent, were employed. The bulk crystals have the expected stoichiometry and compositional homogeneity. Etch pit densities of 10⁴-10⁵ cm⁻² on the (111) face and hitherto unreported etch pits on the (100) face were observed in this work. Possible origins of the sub-grain structure are discussed.     

A time series approach to fatigue crack propagation

The inherently nonlinear phenomenon of fatigue crack propagation is modeled as a linear random process. To a first approximation, simple, nonstationary time series models are introduced and standard techniques for determining the parameters of autoregressive integrated moving-average processes are applied. Multiplicative time series models are next utilised for the representation of a group of crack history curves. Implementation of the models on the Virkler experimental data set yields satisfactory results. Reliable Gaussian approximations to the distribution of the time required by a crack to reach a specified critical length are obtained, and the usefulness of the approach is demonstrated when updating lifetime predictions after periodic inspections.     

Prediction of foam growth and its nucleation in free and limited expansion

The influence volume approach (IVA) is often utilized for modeling the mass transfer process dictating bubble growth dynamics in physical foaming. However, the assumed concentration profile in the IVA method is only valid when the changes in dissolved gas concentration are small (less than 5%). In addition, the validity of the IVA method is difficult to justify in chemical foaming applications because of the difficulties involved in defining the dissolved gas concentration profile.In the present work, we define two distinct stages of bubble growth for physical foaming. These two stages are termed as free and limited expansion and are controlled by the bubble nucleation rate. Bubble nucleation is assumed to occur only in the free expansion stage. In this stage, the bubble pressure drops substantially from an initially high pressure in the supersaturated state while the dissolved gas concentration changes very little. The second stage of our two-stage mass transfer model is termed the limited expansion stage and accounts for bubble growth in the late stages of foam evolution, when the pressure changes become small. However, in the limited stage of bubble growth the dissolved gas concentration drops significantly, as the available dissolved gas is depleted. To summarize our two-stage mass transfer model of foam expansion, the pressure difference between the bubble phase and the liquid phase is the primary mechanism for driving mass transfer in the early (free) stages of foam growth and the concentration difference is the driver for bubble growth in the late (limited) stages of growth. The first stage can be regarded as the nucleation stage and it is relatively short; while the second stage can be regarded as the bubble growth stage and is much longer. Most of the bubble volume expansion takes place in the second stage.The concentration gradient at the bubble edge, which is often ignored in other models, is analyzed in detail in this paper. The details of our novel mass transfer model are also presented.     

Liquid phase epitaxial growth of pure and doped GaSb layers: morphological evolution and native defects

Undoped and Te-doped gallium antimonide (GaSb) layers have been grown on GaSb bulk substrates by the liquid phase epitaxial technique from Ga-rich and Sb-rich melts. The nucleation morphology of the grown layers has been studied as a function of growth temperature and substrate orientation. MOS structures have been fabricated on the epilayers to evaluate the native defect content in the grown layers from theC-V characteristics. Layers grown from antimony rich melts always exhibitp-type conductivity. In contrast, a type conversion fromp- ton- was observed in layers grown from gallium rich melts below 400 C. The electron mobility of undopedn-type layers grown from Ga-rich melts and tellurium doped layers grown from Sb- and Ga-rich solutions has been evaluated. Paper presented at the poster session of MRSI AGM VI. Kharagpur. 1995     

Influence of Gaseous Atmosphere on Morphology and Cellular Fatty Acid Composition of Campylobacter jejuni

ABSTRACT: We evaluated the effect of growth environment on morphology and fatty acid (FA) profiles of 2 strains of C ampylobacter jejuni (ATCC 29428 and 33560) grown under various gaseous conditions. Viable counts were determined by plate count and percentages of coccoid cells. FA profiles were measured by gas chromatography. Plate counts were lowest when cultures were grown in air and highest in CO₂ (10%), O₂ (5%), and N₂ (85%). For 29428, percentages of coccoid cells did not differ among treatments. For 33560, percentages of coccoid cells were greater than for 29428 and varied among treatments (13% to 87%). There were no significant ( P > 0.05) relationships between percentages of coccoid cells and amounts of individual or combinations of FA either between or within strains.     

Influence of third monomer on the crystal phase transition behavior of ethylene-tetrafluoroethylene copolymer

Crystal phase transition between the low- and high-temperature phases has been investigated for ethylene (E)-tetrafluoroethylene (TFE) alternating copolymer (ETFE) containing the third monomeric species by the temperature dependent measurements of wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) and differential scanning calorimetry. Nonafluoro-1-hexene (NFH) and hexafluoropropylene (HFP) were chosen as the third monomers, where they are different in the side-branch length, -(CF₂)₃CF₃ and -CF₃, respectively. In the case of E/TFE/NFH copolymer (ET-C4F9), the crystal phase transition temperature of the original ETFE two-components copolymer was not very much affected by the existence of NFH in the range of NFH content from 0.7 to 3 mol%. Contrarily, the crystal phase transition temperature of E/TFE/HFP copolymer (ET-CF3) was found to decrease drastically with increasing HFP content. The melting temperature and the higher-order structure were also affected sensitively depending on the HFP content. This difference in phase transition behavior between ET-C4F9 and ET-CF3 copolymers is reasonably interpreted as follows: the short side groups (-CF₃) of HFP monomeric unit are included in the crystal lattice of E/TFE chains and the unit cell is expanded gradually with an increment of the HFP content, resulting in the decrease in phase transition point because of easier thermal motion of the chains. On the other hand, the long side groups [-(CF₂)₃CF₃] of NFH monomeric units are excluded out of the crystal lattice and located on the lamellar surfaces or in the amorphous region and do not affect very much the phase transition temperature even when the NFH content is increased. In association with such a change in crystal structure, the long period of stacked lamellar structure was found to decrease remarkably in the case of NFH, whereas it does not change very much for HFP, consistent with the interpretation of the above-mentioned WAXD data.     

Heavy arsenic doping of silicon grown by atmospheric pressure selective epitaxial chemical vapor deposition

Selective epitaxial Si with a high arsenic concentration of 2.2×10¹⁹ atoms/cm³ was deposited at a high growth rate of 3.3 nm/min under atmospheric pressure. It was confirmed that this method had excellent selectivity and produced films having good crystalline quality, abrupt dopant profiles at the interfaces, and smooth surfaces. The growth mechanism is discussed in terms of the relationship between the effects of arsenic surface segregation and etching by hydrogen chloride.     

Formation reactions of chalcopyrite compounds and the role of sodium doping

The selenisation of the thin film precursors indium, Cu-In and Cu-In-Ga has been studied by in-situ X-ray diffraction. The influence of sodium doping on the kinetics of the solid-state reactions observed agrees with the predictions derived from a phenomenological model presented earlier. In this model a layer of sodium polyselenide is assumed to form at the surface of the precursor, or around each crystal grain. This layer might be responsible for impeded ion exchange in certain solid-state reactions.     

Factors Affecting the Squareness of Hysteresis Loops of Sintered NdFeB Magnets

Investigation into the magnets with different squareness of hysteresis loop (SHL) reveals that the microstructure of sintered NdFeB magnets has great effects on the SHL of the magnets. The abnormal grain growth deteriorates the SHL seriously. The shape of the grain and the grain boundary affect the intensity of demagnetization field, and consequently on the SHL. The added elements have effects on the phase structures and distributions in the magnets, which influences the uniform of demagnetization field.