Effect of dopant atoms on the roughness of III-V semiconductor cleavage surfaces

We demonstrate that the presence of dopant atoms influences the roughness, morphology, and optical mirror properties of III-V semiconductor (110) cleavage surfaces. High concentrations of Te dopant atoms in GaAs lead to macroscopically curvatured (110) cleavage surfaces with high step concentrations. This "glass-like" fracture behavior is explained by the "lattice superdilation phenomenon" induced by high concentrations of Te dopant atoms in GaAs. (C) 2000 American Institute of Physics. [S0003-6951(00)02303-2]

New multifunctional lightweight materials based on cellular metals - manufacturing, properties and applications

Cellular metallic materials are a new class of materials which have been the focus of numerous scientific studies over the past few years. The increasing interest in cellular metals is due to the fact that the introduction of pores into the materials significantly lowers the density. These highly porous materials also possess combinations of properties which are not possible to achieve with other materials. Besides the drastic weight and material savings that arise from the cell structure, there are also other application-specific benefits such as noise and energy absorption, heat insulation, mechanical damping, filtration effects and also catalytic properties. Cellular metallic materials are hence multi-functional lightweight materials

Atmosphere monitoring in a continuous sintering belt furnace

The atmosphere composition in a continuous belt furnace during sintering of powder metallurgical aluminium components with ethylenbisstearamide (EBS) pressing agent was determined by Fourier Transform Infrared Spectroscopy (FTIR). The spatial distribution of the decomposition products was measured at four positions in the dewaxing zone. It is shown that the major decomposition products of the dewaxing process are CH groups, which mainly occur between 310 and 410 degrees C. As the powder compacts pass the dewaxing zone, each part drags along its own bell-shaped gas atmosphere. In the vertical direction, a flow profile with two separated concentration zones is observed, leading to large quantities of unused process gas passing the furnace. The amount of unused gas is mainly controlled by the gas flow and the belt speed of the furnace. In particular, high belt speeds and low gas flow parameters may lead to the introduction of harmful carbon into the sintering zone

Identification of surface anion antisite defects in (110) surfaces of III-V semiconductors

We identify surface anion antisite defects in (110) surfaces of GaAs, GaP, and InP using scanning tunneling microscopy combined with density-functional theory calculations. In contrast to subsurface arsenic antisite defects, surface antisite defects are electrically inactive and have a very localized defect state which gives rise to a distinct feature in scanning tunneling microscopy images. (C) 2001 American Institute of Physics

Molybdenum - A biodegradable implant material for structural applications?

Molybdenum as a potentially new biodegradable material was investigated. Degradation behavior of commercially high purity molybdenum was observed in simulated physiological salt solutions (Kokubo's SBF with/without TRIS-HCl, Cu2+ addition and 0.9% NaCl solution). Potentiodynamic polarization, immersion mass loss and ion concentration measurements paired with REM/EDX analysis reveal gradual dissolution of molybdenum in the proper order of magnitude for stent application, associated with formation of thin, non-passivating corrosion products. The underlying corrosion mechanism is discussed as well as a comparison to literature data. However, formation of calcium phosphates (CaP) in SBF significantly decreases corrosion rates. In-situ polarization was found to be a potential way for overcoming this problem and simultaneously enhancing corrosion above the benchmark for a degradable stent material

Synthesis and properties of open-celled metal foams

Open-celled metal foams were synthesized using a replication technique. Therefore a reticulated polyurethane template was coated by a slurry and removed thermally, followed by a sintering step. Since the process is feasible for a multiplicity of metals the expts. were performed on the example of stainless steel 316L. Highly porous components were obtained showing adjustable densities between 0.3 and 2.0 g/cm3. The cell structure is exceedingly homogeneous and the cell sizes may be chosen in the range of 10 - 80 ppi. In order to characterize the properties, compression tests and acoustical tests were carried out. A significant influence of the d. and the cell size on the acoustical and mech. properties was noticed

High - Strength steel hollow spheres

Metal hollow sphere structures (MHS) feature excellent properties for functional applications [1]. But, since their mechanical properties are still too weak for an use in lightweight constructions. Automotive industry demands a significant increase in strength and a decrease of density. In the present work, a study on the increase in the strength of carbon steel hollow spheres by additional heat treatment was conducted. Such additional heat treatments for improving the steel properties are quite usual. But, in contrast to the hardening of bulk material, metal hollow spheres show an extreme low thermal conductivity and high internal surfaces. Thus, the additional heat treatment of carbon steel hollow spheres gives rise to three main challenges, which are closely related. First the carbon content must be adjusted at an amount of 0.6 -0.8 wt.-percent and this amount must be held without a loss during austenitisation. This carbon content is optimal for high strength combined with high ductility. The second challenge is how to avoid the oxidation. Metal Hollow spheres show high internal surfaces. Hence, oxidation is promoted. Since massive oxidation of the shells of the MHS will harm the mechanical properties, it can hide the effect of the increasing strength of the residual shell due to hardening. Oxidation can occur during austenitisation by the atmosphere, during cooling by water or air and after the process, due to remaining water between the MHS. The third challenge is the fast cooling of MHS. Low alloyed and unalloyed carbon steel requires fast cooling in order to obtain phase changes from austenite to martensite. On the one hand water as cooling fluid allows the maximal cooling rate due to its high heat capacity. On the other hand, cooling in water is challenging owing to the corrosiveness of remaining water. Furthermore, packages and structures of MHS are closed-cell foams with low thermal conductivity. This lowers the cooling rale in the centre of structure or packages. Consequently, incomplete phase transition may occur, causing a gradient in the strength of such structures. Entnommen aus TEMA</a

Atmosphere study on the thermal decomposition behavior of delubrication aiding additives

In the present study, the effect of delubrication aiding additives on the thermal decomposition behavior was investigated. Such additives have been introduced in order to reduce stain formation caused by soot particles. Therefore, the atmosphere during the heat treatment of FeCuC components with Kenolube lubricant and a clean burn additive has been monitored by in-situ FTIR atmosphere analysis directly in the furnace. It was found, that delubrication aids reduce the decomposition into stable species like aromatic compounds. Furthermore, the formation of CO, CO2 and CH4 was increased considerably. In comparison to compacts without delubrication aid, the maximum peak temperatures of the gas evolution does not change considerably. However, with such an additive further peaks are visible. In particular, the bands caused by CO2 and alkene show additional peaks at lower temperatures. As a conclusion it is shown, that delubrication aids lead to improved lubricant decomposition as well as intensified soot burn-off