Increase Of Critical Thickness Research Articles

Influences of Increased Pressure Foaming on the Cellular Structure and Compressive Properties of In Situ Al-4.5%Cu-xTiB2 Composite Foams with Different Particle Fraction.

Metallic foams have drawn increasing attention in applications ranging from lightweight structures to energy absorption devices. Mechanical properties of metallic foams depend on both their microstructure and cellular structure. In situ Al-4.5%Cu-xTiB2 composites were used as start materials for fabrication of closed-cell foams through liquid route under atmosphere pressure and increased pressure, aiming at simultaneously strengthening the cell wall material and optimizing the cellular structure. Macro-structural features of the foams were determined by micro X-ray computed tomography (µCT); results exhibit that increasing weight ratio of in situ TiB2 particles leads to coarsened cell structure for foams made under atmosphere pressure, due to the increase in critical thickness of cell wall rupture. Significant reduction of cell size and increase in cell circularity were observed for foams fabricated under increased pressure. Quasi static compression test results indicate that yield strength of foam samples increases with increasing particle fraction and refinement of cell structure. Microstructure observation shows that the continuous network at interdendritic regions consists of in situ TiB2 particles and intermetallic compounds are responsible for the reduced ductility of cell wall materials and the reduction in energy absorption efficiency of foams with high particle fraction. The influences of cell structure on the normalized strength and specific energy absorption were also discussed, and it was found that the improvement of yield strength and energy absorption of composite foams attributes to both the reinforcement of in situ TiB2 particles and the refinement of cellular structure.

A drastic increase in critical thickness for strained SiGe by growth on mesa-patterned Ge-on-Si

We demonstrate that the critical thickness for Ge-rich strained SiGe layers can be drastically increased by a factor of more then two by means of growth on mesa-patterned Ge-on-Si. The Si0.2Ge0.8 layer grown on sub-millimeter mesa Ge-on-Si is fully strained and free from ridge roughness, while the same Si0.2Ge0.8 layers grown on unpatterned Ge-on-Si and a Ge substrate are partially strain-relaxed with the surface covered by high-density ridge roughness. This demonstrates that the proposed patterning method can provide thick and stable strained SiGe films as promising templates for realization of strained SiGe-based optoelectronic and spintronic devices.

Dynamic model of contact of grain with the surface of wood

Grinding holds the specific place among other types of machining of wood caused by specific features of the occurring physical phenomena and features of the tool. When processing the mechanical, chemical, thermal, electric and accompanying those phenomena arise a multiple-point cutting tool of preparations from wood or wood materials, naturally. The mechanical phenomena are shown owing to repeated contact of an abrasive paper from the preparation processed by a surface, that is, the fissile sliding friction. These phenomena are followed by a pollution of a cutting tool, including consolidation of space between abrasive grains that reduces operability of an abrasive paper. On the basis of the conducted theoretical researches and the analysis of the removed equations for determination of critical sizes of front corner, thickness of cut and speed of cutting, it is possible to draw conclusion that all listed parameters have significant effect on the size of bearing of the processed material abrasive grain, therefore, and on intensity of emergence of contact collapses. Increase in critical thickness of a cut leads to increase in deformation of near-surface layers of the processed material, and, therefore, and to increase in depth and the number of contact destructions.

Cell Structure Evolution of Aluminum Foams Under Reduced Pressure Foaming

Ti-H particles are used to increase the gas content in aluminum melts for reduced pressure foaming. This paper reports on the RPF process of AlCa alloy by adding TiH2, but in smaller amounts compared to traditional process. TiH2 is completely decomposed by stirring the melt, following which reduced pressure is applied. TiH2 is not added as the blowing agent; instead, it is added for increasing the H2 concentration in the liquid AlCa melt. It is shown that pressure change induces further release of hydrogen from Ti phase. It is also found that foam collapse is caused by the fast bubble coalescing during pressure reducing procedure, and the instability of liquid film is related to the significant increase in critical thickness of film rupture. A combination of lower amounts of TiH2, coupled with reduced pressure, is another way of increasing hydrogen content in the liquid aluminum. A key benefit of this process is that it provides time to transfer the molten metal to a mold and then apply the reduced pressure to produce net shape foam parts.

A study on the critical wall thickness of the inner tube for magnetic pulse welding of tubular Al–Fe parts

Microstructure and mechanical properties of the welding zone highly depend on the thickness of inner tube for jointing two tubes using magnetic pulse welding (MPW). In this study, the critical thickness of inner 20Fe tube for the Al–Fe welding tubes is investigated by combining the theoretical calculation and experiments. Theoretical analysis demonstrates that plastic deformation zone of the inner tube expands from its inner surface to its outer surface. With equilibrium equations and modified Tresca yield criterion, the critical thicknesses of inner tube under various collision velocities are derived so as to assure the high quality of the welding. The results show that the critical thickness of inner tube increases linearly with the discharge voltage. With the discharge voltage increases by every 2kV, the critical thickness increases by 0.5mm accordingly. The theoretical predictions agree well experiments with an error of less than 2.5%. Furthermore, effects of thickness of inner tube on the mechanical properties of welding zone are also investigated.

Dependence of the critical thickness on Si doping of InGaAs on GaAs

The formation of misfit dislocations during the initial stages of relaxation of In0.04Ga0.96As epitaxial layers on (001) GaAs has been studied by in situ high-resolution double crystal x-ray topography during molecular beam epitaxy growth. Relaxation is initially anisotropic with the fast B(g) dislocations being nucleated before the slow A(g) set. On doping with Si up to a maximum concentration of 4×1018 atoms/cm3, an increase in critical thickness was observed for both dislocation sets. The data can be fitted to an extension of the Matthews–Blakeslee model that includes a lattice friction force varying linearly with the dopant concentration.

Effect of surfactants on the stability of films between two colliding small bubbles

The stability of partially mobile drainage thin liquid film formed between two slightly deformed approaching bubbles or drops is studied. The intervening film is assumed to be thermodynamically unstable. The material properties of the interfaces (surface viscosity, Gibbs elasticity, surface and bulk diffusion) are taken into account. To examine the stability of the thin film we consider the coupling between the drainage and the disturbance flows. The velocity and pressure distributions due to the drainage flow are obtained by using the lubrication approximation. The disturbance flow is examined by imposing small perturbations on the film interfaces and liquid flow. The long wave approximation is applied. We solved the linear problem for the evolution of the fluctuations in the local film thickness, interfacial velocity and pressure. The linear stability analysis of the gap region allows us to calculate the critical thickness, at which the system becomes unstable. Quantitative explanation of the following effects is proposed, (i) the increase of critical thickness with the increase of the interfacial mobility; (ii) the role of surface viscosity, compared with that of the Gibbs elasticity; (iii) the significant destabilization of the gap region with the decreasing droplet radius in the case of buoyancy driven motion. The analytical expressions for critical thickness in the case of negligible surface viscosity and tangentially immobile interfaces are presented.

In situ transmission electron microscope observations of misfit strain relaxation and coalescence stages of Si1−xGex on Si(001)

The misfit strain relaxation and the coalescence stages of three-dimensional islands during the heteroepitaxy of Si1−xGex on Si(001) have been investigated using an ultra-high vacuum transmission electron microscope combined with a conventional molecular beam epitaxy chamber. Moiré-fringes appear at depositions of 10, 25 and 100 ML for Ge compositions x=1, 0.7 and 0.3, respectively. This increase in the deposition roughly corresponds to the increase in critical thickness for Si1−xGex layers. The relaxation of misfit strain in Si1−xGex islands proceeds when the island height exceeds the critical thickness but the strain relaxation has not finished, with depositions up to 200 ML. For x=1, 86% of total misfit strain is relaxed, with deposition up to 100 ML, by the generation of misfit dislocations (70%) and stacking faults (16%). Furthermore, the generation of a threading dislocation at the interface of coalesced islands is observed.

The stability of Si1−xGex strained layers on small-area trench-isolated silicon

The combined effects of isolation stress, active area size, and SiGe misfit strain on dislocation generation in an advanced SiGe heterojunction bipolar transistor (HBT) process were studied. Eight-inch wafers were patterned with polysilicon-filled deep, and oxide-filled shallow trench isolation similar to that used in IBM's analog SiGe HBT technology. Half of the wafers were subjected to an additional stress-producing oxidation prior to SiGe growth. Si1−xGex films containing 0, 5.5, 9, and 13 at.% Ge were grown epitaxially by ultrahigh vacuum chemical vapor deposition (UHV CVD). The films were of constant thickness with an intrinsic Si cap. Some samples received an additional relaxation anneal following deposition. After the growth and anneal cycles, the dislocation density was determined by transmission electron microscopy (TEM). On nonstressed samples, no dislocations were observed in the device areas, even at Ge concentrations which are not stable to misfit dislocation generation in blanket form. This small area effect has been observed on patterned substrates that do not have functional device isolation. On the stressed-isolation wafers, the compressive stress from the oxidation of the trench sidewalls was found to intensify stress in the SiGe films, and to lower the critical strain at which misfit dislocations appeared. In large active areas on these wafers, two distinct dislocation regions were observed. Defects at the edge resembled those caused by isolation stress, while the defects in the center were more typical of the misfit dislocations associated with lattice-mismatch epitaxial films. It is clear that isolation stress must be minimized when fabricating integrated circuits using SiGe epitaxial films. It is also evident that SiGe films grown on nonstressed isolation exhibit the same increase in critical thickness with decreasing lateral dimension that has been observed on much simpler patterned substrates.

Relaxation of InGaAs layers grown on (111)B GaAs

The relaxation behavior of InGaAs layers grown by molecular beam epitaxy on (111)B GaAs is investigated and compared with simultaneously grown (100) reference samples. Surface morphology, defect microstructure, and optical quality of the layers during the relaxation process are studied by Nomarski interference contrast, transmission electron microscopy, low-temperature photoluminescence, and Raman spectroscopy. These techniques reveal an inhomogeneous and anisotropic relaxation in (111) samples. In (111) samples, the increase of critical thickness and the slower relaxation dependence on thickness, as compared with the (100) reference samples, is discussed