Research on silicon microchannel array oxidation insulation technology and stress issues

Optimization design by evolutionary computation for minimizing thermal stress of a thermoelectric generator with varied numbers of square pin fins

The oxidation behaviour of binary Ni-(3, 5)Al and ternary Ni-(3, 4, 5)Al-(15, 20)Cr alloys (all in wt%) was investigated in both Ar-20%O2 (dry oxygen) and Ar-20%O2-20%H2O (wet oxygen) at 750 °C. For Ni-3Al alloy, an external NiO and an internal oxidation zone (IOZ) were formed with an enhanced thickening kinetics in wet oxygen. Increasing Al to 5 wt% led to a partial protection of the surface by a thin alumina scale, together with non-protective NiO and IOZ as that for Ni-3Al. Adding Cr into Ni–Al alloys significantly increased oxidation protection. In dry oxygen, all ternary alloys formed mainly a thin protective alumina scale. This protective effect was significantly reduced in the presence of water vapour. For Ni-(3, 4)Al-15Cr, a complex multi-layered oxide structure was observed, with an external NiO, an inner oxide layer with a chromia band at the bottom, and an IOZ with alumina precipitates. Further increasing Cr to 20 wt% led to a dominant chromia band formation in the inner oxide region. For high Al (5 wt%) ternary alloys, a protective alumina scale was formed in most of the surface area. Oxide characterisation revealed that this thin layer of alumina was α-Al2O3. The effects of water vapour and alloy composition on oxide formation were discussed based on classic diffusion theory and oxidation kinetic analysis.

Silicon dioxide (SiO2) thin films are most commonly used insulating films in the fabrication of silicon-based integrated circuits (ICs) and microelectromechanical systems (MEMS). Several techniques with different processing environments have been investigated to deposit silicon dioxide films at temperatures down to room temperature. Anodic oxidation of silicon is one of the low temperature processes to grow oxide films even below room temperature. In the present work, uniform silicon dioxide thin films are grown at room temperature by using anodic oxidation technique. Oxide films are synthesized in potentiostatic and potentiodynamic regimes at large applied voltages in order to investigate the effect of voltage, mechanical stirring of electrolyte, current density and the water percentage on growth rate, and the different properties of as-grown oxide films. Ellipsometry, FTIR, and SEM are employed to investigate various properties of the oxide films. A 5.25 Å/V growth rate is achieved in potentiostatic mode. In the case of potentiodynamic mode, 160 nm thickness is attained at 300 V. The oxide films developed in both modes are slightly silicon rich, uniform, and less porous. The present study is intended to inspect various properties which are considered for applications in MEMS and Microelectronics.

ABSTRACTA comprehensive dynamic finite-element simulation method was proposed to study the wheel–rail impact response induced by a single wheel flat based on a 3-D rolling contact model, where the influences of the structural inertia, strain rate effect of wheel–rail materials and thermal stress due to the wheel–rail sliding friction were considered. Four different initial conditions (i.e. pure mechanical loading plus rate-independent, pure mechanical loading plus rate-dependent, thermo-mechanical loading plus rate-independent, and thermo-mechanical loading plus rate-dependent) were involved into explore the corresponding impact responses in term of the vertical impact force, von-Mises equivalent stress, equivalent plastic strain and shear stress. Influences of train speed, flat length and axle load on the flat-induced wheel–rail impact response were discussed, respectively. The results indicate that the maximum thermal stresses are occurred on the tread of the wheel and on the top surface of the middle rail; the strain rate hardening effect contributes to elevate the von-Mises equivalent stress and restrain the plastic deformation; and the initial thermal stress due to the sliding friction will aggravate the plastic deformation of wheel and rail. Besides, the wheel–rail impact responses (i.e. impact force, von-Mises equivalent stress, equivalent plastic strain, and XY shear stress) induced by a flat are sensitive to the train speed, flat length and axle load.

Experimental study on cool down characteristics and thermal stress of cryogenic tank during LN2 filling process

In the piercing process of large-diameter seamless steel pipe blanks after heating, severe lateral cracks easily occur on the surface of pierced pipe blanks owing to the effects of the large temperature difference and thermal stress during the heating phase, influencing the finished product ratio and the safety and stability of use. Therefore, obtaining pipe blanks with uniform temperature distributions and low thermal stress peaks is the basis for the quality assurance of seamless pipes. In this paper, a study was conducted with large-diameter TP321 seamless pipe blanks with an example. The heating process of the pipe blank was investigated from two perspectives, temperature fields and thermal stress fields. Moreover, the effects of heating rate, initial furnace temperature, and hot-charging temperature were quantitatively analyzed. It was found that the peaks of the temperature difference and thermal stress both occurred at the early stage of heating. The temperature field and thermal stress distribution of the pipe blank gradually changed during heating. At the initial stage of heating, the temperature at the outer diameter edge was the highest, and the maximum thermal stress zones were concentrated on the inner hole edge and external surface. At the late stage of heating, the highest temperature zone and the maximum thermal stress zone were both focused on the inner hole surface. Lower initial furnace temperature and higher hot-charging temperature were more conducive to decreasing the maximum temperature difference and peak thermal stress, while the changes in heating rate had insignificant effects.

Currently, the most widely used method to calculate boiler efficiency is based on wet basis oxygen which is measured by online instrument, rather than on dry oxygen. Consequently, it often results in the deviation of calculation results, as well as bad adaptability to different kinds of coal. In this study, an online calculation method of boiler efficiency as deducted based on the wet oxygen, and validated by typical soft coal and lignite. It is revealed that the results of this calculation model is more close to that of standard calculation method by elementary analysis. Moreover, the online calculation method can exhibit a better performance of coal adaption, which can be used as a method to guide for the better optimization of utility boiler combustion.

In this paper, finite element analyses were carried out on thermal stresses at the tubesheet of floating-head or U-tube heat exchangers with square layout of tubes based on parametric finite element models of the tubesheet established with ANSYS parametric design language (APDL). It is found that for thick tubesheet with large temperature difference between two the sides, the maximum thermal stress can be comparable with the mechanical stress induced by pressures and should not be neglected in some circumstances. When hot fluid flows on the tube-side, both the shell-side and tube-side surfaces of the tubesheet are in tensile stress state while the region in the middle of the tubesheet is in compressive. Conversely, if the cold fluid flows on the tube-side, compressive thermal stresses are produced at the two tubesheet surfaces, which is helpful to prevent the tubesheet from cracking. By changing design parameters in APDL commands, parametric analysis was completed and the results show that the thermal stresses at the surface center of the tubesheet are sensitive to the thickness and the perforated region size of the tubesheet. Based on sufficient numerical results, a regressed formula for evaluating the thermal stress at the surface center of the tubesheet with squarely arranged tubes was obtained, this can facilitate the strength design of the tubesheet in floating-head or U-tube heat exchangers when considering the thermal stress.

Numerical investigation on chill-down and thermal stress characteristics of a LH2 tank during ground filling

Surface sputtering and structural modifications induced in silicon dioxide thin films (SiO 2 /Si) deposited on silicon substrates and irradiated by swift (10–40 MeV) heavy Au q+ (q = +4, +6, +7, and +9) ions were investigated by grazing‐incidence X‐ray diffraction (GIXRD) spectroscopy, Rutherford backscattering (RBS) spectrometry and time‐of‐flight elastic recoil detection (ToF‐ERDA) technique. The GIXRD analysis of the as‐deposited and irradiated samples revealed increasing structural modifications of the SiO 2 thin films under Au q+ ion impacts with increasing ion‐beam energy. The changes consisted of decreased grain sizes with increased strain accompanied by a phase transformation from crystalline to amorphous films. RBS analysis showed a decrease in the mean stoichiometric (O/Si) ratio from (2.2 ± 0.1) to (1.7 ± 0.1), due to preferential sputtering of oxygen, as the incident ion energy increased. The obtained RBS‐results were then completed by those of ToF‐ERDA analysis technique using a 40 MeV Au 9+ heavy ion beam. The preferential sputtering yield ratios (Y Si /Y O ) were determined experimentally both versus electronic stopping power and ion fluence. The obtained results were then compared to numerical values derived from the inelastic thermal spike (i‐TS) model, Sigmund's analytical formula and SRIM simulation code. A good agreement was observed between the measured preferential sputtering data and the i‐TS calculated values, when considering both nuclear elastic and electronic inelastic collision mechanisms. Besides, a close correlation is observed between the electronic stopping power dependent measured sputtering yields and the XRD peak intensity degradation per unit fluence. These observations suggest that the same mechanism of MeV heavy ion‐irradiation induced extended atomic disordering, occurs both in the case of structural modifications and surface sputtering. Finally, the obtained experimental results are discussed on the basis of the i‐TS model.

Graphite was bonded to Ni under a compressive stress range from 3 to 33 MPa in a vacuum over a temperature range from 973 to 1273 K using a RF-induction furnace. The influence of joining conditions on the bending strength of the graphite/Ni joint, and changes of microstructure and hardness near the joining interface of Ni, were investigated. Thermal stress induced in the joint was estimated with a finite element method. On the basis of these results the influence of thermal stress on the bending strength of the joint as examined.Completion of the graphite/Ni joint depends on both the compressive stress and the joining temperature. Good solid state bonding becomes feasible under low compressive stress in case of higher joining temperatures. Axisymmetric thermoelastic finite element analysis suggests that maximum tensile thermal stress is induced at a distance of 0.64 mm from the joining interface on the surface of the graphite and is increased with increasing joining temperatures. The position of fracture in a bending test corresponds approximately to that of the maximum tensile thermal stress. A part of the thermal stress in practical joints is relaxed and less than that calculated by finite element method. The bending strength of the joint increases with decreasing residual tensile stress on the surface of graphite. Relaxation of maximum tensile thermal stress depends on the amount of carbon atoms which diffuse into Ni. This may be related to the changes in plasticity of Ni and graphite, that is, the changes in the amounts of C super-saturately dissolved in Ni and the point defects introduces in graphite.

Numerical investigation on fatigue life of aluminum brazing structure with fin-plate-side bar under the low temperature thermal-structure cyclic stress

In order to investigate the phenomenon of elongation occurred in heated P. E. T. fibers under a constant length after hot drawing, the effects of draw ratio and drawing temperature on longitudial and thermal stress changes for successive heating and cooling at a constant rate were discussed. And also the fine structure of specimens, quenched at 0°C from transformation point of longitudial change-temperature curves were studied by X-ray, density and birefringence. The results obtained are as follows:1) The low stretched specimens at 70° or 80°C in water and subsequently passed the thermal history of heating and cooling show a-axis orientation in X-ray diffraction patterns, and the length of fiber may becomes slightly longer than that of the original, because of superposing thermal creep effects on the spontaneous elongation beyond a temperature range of maximum shrinkage. On the other hand, the specimens high stretched at 70°C (draw ratio: 3.8) shows a fiber-like orientation in X-ray pattern as well as the one at 60°C (draw ratio: 2.0). Both of these show a larger thermal contractive stress, The shrinkage at a range of 85_??_90°C as well as the abnormal elongation in the vicinity of 115°C become smaller. And the specimen stretched at 70°C (draw ratio: 2.5) shows an intermediate orientation in X-ray pattern, and the orientation of its crystal may have preferentially a fiber-like orientation, On the other hand, the molecular chains in the amorphous phase do not orient so much and accordingly the maximum thermal contractive stress becomes smaller. The thermal shrinkage becomes larger, while the abnormal elongation smaller due to the unstable, structure.2) The drawn specimen at 90°C (draw ratio: 2.0) shows negative birefringence and a-axis orientation in X-ray pattern. While both the hot shrinkage and contractive thermo-stress of this sample become smaller, the, spontaneous elongation does not occur so much.3) The contribution of crystalline birefringence Δnc•X for a-axis oriented fiber on the temperature range of cold crystallization shows negative, and for c-axis oriented fiber strongly positive. On next heating and cooling processes, absolute values of Δnc•X may increase with closed regularity of crystals. On the other hand, the amorphous birefringence Δna shows the minimum values on the temperature range of maximum shrinkage, and on the heating and cooling processes values of Δna only increase in spite of rising and lowering temperature.

In-situ investigation of coal particle fragmentation induced by thermal stress and numerical analysis of the main influencing factors

The oxidation of single crystal, n‐type silicon has been investigated in the temperature range 900°–1200°C. These studies involved the use of three different oxidizing atmospheres, dry oxygen, wet oxygen, and steam, all at atmospheric pressure. Kinetic data, including thickness and weight rate constants, activation energies, and oxidation efficiencies, were compared among the three systems. The silicon oxides produced by the different methods were characterized by certain properties, such as density, dielectric strength, masking ability, and number of imperfections. It was demonstrated that the wet oxygen process produces oxides exhibiting the best protective properties relative to semiconductor device applications. No effects on oxidation characteristics were found due to impurity type or concentration, but a surface orientation effect was observed.