The temperature dependence of effective thermal conductivity of open-celled steel alloy foams

The effective thermal conductivity of steel alloy FeCrAlY (Fe-20 wt.% Cr-5 wt.% Al-2 wt.% Y-20 wt.%) foams with a range of pore sizes and porosities was measured between 300 and 800 K, under both vacuum and atmospheric conditions. The results show that the effective thermal conductivity increases rapidly as temperature is increased, particularly in the higher temperature range (500-800 K) where the transport of heat is dominated by thermal radiation. The effective conductivity at temperature 800 K can be three times higher than that at room temperature (300 K). Results obtained under vacuum conditions reveal that the effective conductivity increases with increasing pore size or decreasing porosity. The contribution of natural convection to heat conduction was found to be significant, with the effective thermal conductivity at ambient pressure twice the value of vacuum condition. The results also show that natural convection in metal foams is strongly dependent upon porosity. © 2003 Elsevier B.V. All rights reserved.

Low-temperature (≤600 °C) semi-insulating oxygen-doped silicon films by the PECVD technique for large-area power applications

This work describes the deposition, annealing and characterisation of semi-insulating oxygen-doped silicon films at temperatures compatible with polysilicon circuitry on glass. The semi-insulating layers are deposited by the plasma enhanced chemical vapour deposition technique from silane (SiH4), nitrous oxide (N2O) and helium (He) gas mixtures at a temperature of 350 °C. The as-deposited films are then furnace annealed at 600 °C which is the maximum process temperature. Raman analysis shows the as-deposited and annealed films to be completely amorphous. The most important deposition variable is the N2O SiH4 gas ratio. By varying the N2O SiH4 ratio the conductivity of the annealed films can be accurately controlled, for the first time, down to a minimum of ≈10-7Ω-1cm-1 where they exhibit a T -1 4 temperature dependence indicative of a hopping conduction mechanism. Helium dilution of the reactant gases is shown to improve both film uniformity and reproducibility. A model for the microstructure of these semi-insulating amorphous oxygen-doped silicon films is proposed to explain the observed physical and electrical properties. © 1995.

Temperature Dependence of the Tribological Properties of Laser Re-Melted Al-Cu-Fe Quasicrystalline Plasma Sprayed Coatings

The aim of this study was to investigate the effect of temperature on tribological properties of plasma-sprayed Al-Cu-Fe quasicrystal (QC) coating after laser re-melting treatment. The laser treatment resulted in a more uniform, denser and harder microstructure than that of the as-sprayed coatings. Tribological experiments on the coatings were conducted under reciprocating motion at high frequency in the temperature range from 25 to 650 degreesC. Remarkable influence of temperature on the friction behavior of the coating was recorded and analyzed. Microstructural analysis indicated that the wear mechanisms of the re-melted QC coatings changed from abrasive wear at room temperature, to adhesive wear at 400 degreesC and severe adhesive wear at 650 degreesC owing to the material transfer of the counterpart ball. It was also observed that the ratio of the icosahedral (i)-phase to beta-Al-50(Fe,CU)(50) phase in the coating was higher after test at 400 'C than that at 650 'C. The variation of the ratio UP of coating and of the property of the counterpart ball and coating with the temperature are the two main factors influencing the wear mechanisms and value of the friction coefficient.

Effects on Dense-phase Zone Temperature of Fluidized Bed with Uneven Air Distribution

由于采用非均匀布风,内旋流流化床的移动区空气量不足,导致燃烧不充分,温度较低。当移动区未流化时,密相区内存在较明显的温度不均匀性。随着移动区流速的提高,温度差迅速减小。当移动区流速超过2.0#mu#m后,密相区温度基本均匀一致。流动区流速对密相区温度均匀有一定的影响,流速越高,温度越均匀。

Erosion-corrosion of mild steel in a temperature gradient

Thinning of heat-exchanger tubes by erosion-corrosion has been a problem in fluidized bed combustors (FBCs), particularly at lower metal temperatures where thicker, mechanically protective oxide scales are unable to form. Many laboratory-scale tests have shown a decrease in material loss at higher temperatures, in a similar manner to FBC boilers, but also show a decrease in wastage at low temperatures (e.g. 200°C) which has not been detected in boilers. It has been suggested that this difference is due to laboratory tests being carried out isothermally whereas in a FBC boiler the fluidized bed is considerably hotter than the metal heat exchanger tubing. In this laboratory study the simulation was therefore improved by internally cooling one of the two low carbon steel specimens. These were rotated in a horizontal plane within a lightly fluidized bed with relative particle velocities of 1.3-2.5 m s-1. Tests were carried out over a range of bed temperatures (200-500°C) and cooled specimen surface temperatures (115-500°C), with a maximum temperature difference between the two of 320°C. Although specimens exposed isothermally still showed maximum wastage at intermediate temperatures (about 350°C), those which were cooled showed high levels of wastage at temperatures as low as 200°C in a similar manner to FBC boilers. Cooling may modify the isothermal erosion-corrosion curve, causing it to broaden and the maximum wastage rate to shift to lower temperatures. © 1995.

Low temperature (≤ 600°C) semi-insulating oxygen-doped silicon films by the PECVD technique for large area power applications

This work describes the annealing and characterisation of semi-insulating oxygen-doped silicon films deposited by the Plasma Enhanced Chemical Vapour Deposition (PECVD) technique from silane (SiH4), nitrous oxide (N2O) and helium (He) gas mixtures. The maximum process temperature is chosen to be compatible with large area polycrystalline silicon (poly-Si) circuitry on glass. The most important deposition variable is shown to be the N2O SiH4 gas ratio. Helium dilution results in improved film uniformity and reproducibility. Raman analysis shows the 'as-deposited' and annealed films to be completely amorphous. A model for the microstructure of these Semi-Insulating Amorphous Oxygen-doped Silicon (SIAOS) films is proposed to explain the observed physical and electrical properties. © 1995.

Laboratory-scale fluidized bed rig for high-temperature tube wastage studies

As part of a study of the wear of candidate heat exchanger tube materials for use in fluidized bed combustors, two similar laboratory-scale rigs have been built and characterized. Specimens of selected alloys are carried on counter-rotating rotors immersed in a fluidized bed, and are exposed to particle impact velocities of up to approximately 3 ms-1 at temperatures up to 1000°C. The performance of this design of apparatus has been investigated in detail. The effects of several experimental variables have been studied, including angle of particle impact, specimen speed, position of the rotor within the fluidized bed, duration of exposure, bed material particle size, degradation of the bed material, degree of fluidization of the bed, and size of specimen. In many cases the results obtained with steel specimens at elevated temperatures are similar to those observed with polymeric specimens at low temperatures.

High temperature direct double side cooled inverter module for hybrid electric vehicle application

In this paper a novel approach to the design and fabrication of a high temperature inverter module for hybrid electrical vehicles is presented. Firstly, SiC power electronic devices are considered in place of the conventional Si devices. Use of SiC raises the maximum practical operating junction temperature to well over 200°C, giving much greater thermal headroom between the chips and the coolant. In the first fabrication, a SiC Schottky barrier diode (SBD) replaces the Si pin diode and is paired with a Si-IGBT. Secondly, double-sided cooling is employed, in which the semiconductor chips are sandwiched between two substrate tiles. The tiles provide electrical connections to the top and the bottom of the chips, thus replacing the conventional wire bonded interconnect. Each tile assembly supports two IGBTs and two SBDs in a half-bridge configuration. Both sides of the assembly are cooled directly using a high-performance liquid impingement system. Specific features of the design ensure that thermo-mechanical stresses are controlled so as to achieve long thermal cycling life. A prototype 10 kW inverter module is described incorporating three half-bridge sandwich assemblies, gate drives, dc-link capacitance and two heat-exchangers. This achieves a volumetric power density of 30W/cm3.

Numerical simulation of temperature field evolution of metallic alloy surface due to pulsed laser treatment

This paper performed a numerical simulation on temperature field evolution for the surface layer of a metallic alloy subjected to pulsed Nd:YAG laser treatment. The enthalpy method was adopted to solve the moving boundary problem, I.e. Stefan problem. Computational results were obtained to show the temperature field evolution. Effects of latent heat and mushy zone width on the temperature field were investigated. The results also show very high values of temperature gradient and cooling rate, which are typical characteristics during the solidification process.

Low Temperature Properties of PI/SiO_2 Nanocomposite Films

In this paper, the mechanical properties of PI/Si_O, nanocomposite hybrid films with different silica doping levels are experimentally studied at low temperature. Experimental results show that the coefficient of thermal expansion (CTE) of the PI/Si_O, nanocomposite hybrid films gradually reduces when the ambiance temperature is decreased. At the liquid nitrogen temperature (77 K), the CTE value is about five times less than that at room temperature (287 K). The measured CTEs of hybrid films greatly decrease when doped with inorganic silica, especially when the silica doping level is more than 1 wt.%. However, too high silica contents (more than 10 wt.%) can cause problem to disperse effectively and the specimens become quite opaque. Experimental results also show that the effects of the pre-applied stress levels can be neglected on the CTE testing. When the ambient temperature changes from 287 to 77 K, the measured average values of the films' ultimate tensile strength (UTS) and Young's modulus increase about 60 and 90%, respectively, while the breaking elongation decreases about 42%.

Molecular/Cluster Statistical Thermodynamics Methods To Simulate Quasi-Static Deformations At Finite Temperature

The rapid evolution of nanotechnology appeals for the understanding of global response of nanoscale systems based on atomic interactions, hence necessitates novel, sophisticated, and physically based approaches to bridge the gaps between various length and time scales. In this paper, we propose a group of statistical thermodynamics methods for the simulations of nanoscale systems under quasi-static loading at finite temperature, that is, molecular statistical thermodynamics (MST) method, cluster statistical thermodynamics (CST) method, and the hybrid molecular/cluster statistical thermodynamics (HMCST) method. These methods, by treating atoms as oscillators and particles simultaneously, as well as clusters, comprise different spatial and temporal scales in a unified framework. One appealing feature of these methods is their "seamlessness" or consistency in the same underlying atomistic model in all regions consisting of atoms and clusters, and hence can avoid the ghost force in the simulation. On the other hand, compared with conventional MD simulations, their high computational efficiency appears very attractive, as manifested by the simulations of uniaxial compression and nanoindenation. (C) 2008 Elsevier Ltd. All rights reserved.

Thermal fatigue on pistons induced by shaped high power laser. Part I: Experimental study of transient temperature field and temperature oscillation

Thermal fatigue behavior is one of the foremost considerations in the design and operation of diesel engines. It is found that thermal fatigue is closely related to the temperature field and temperature fluctuation in the structure. In this paper, spatially shaped high power laser was introduced to simulate thermal loadings on the piston. The incident Gaussian beam was transformed into concentric multi-circular beam of specific intensity distribution with the help of diffractive optical element (DOE), and the transient temperature fields in the piston similar to those under working conditions could be achieved by setting up appropriate loading cycles. Simulation tests for typical thermal loading conditions, i.e., thermal high cycle fatigue (HCF) and thermal shock (or thermal low cycle fatigue, LCF) were carried out. Several important parameters that affect the transient temperature fields and/or temperature oscillations, including controlling mode, intensity distribution of shaped laser, laser power, temporal profile of laser pulse, heating time and cooling time in one thermal cycle, etc., were investigated and discussed. The results show that as a novel method, the shaped high power laser can simulate thermal loadings on pistons efficiently, and it is helpful in the study of thermal fatigue behavior in pistons. (C) 2007 Elsevier Ltd. All rights reserved.