Modeling Temperature in Coupled Electrical and Thermal Simulations of the Electroconsolidation® Process

Electroconsolidation® is a process for densifying complex-shaped parts by using electrically conductive particulate solids as a pressure-transmitting medium. The part is immersed in a bed of the particulate medium contained in a die chamber. Sintering temperature is achieved by resistive heating of the medium while applying compaction pressure. The process is capable of ultrahigh temperatures and short cycle times and offers the potential for low processing costs. Control of the process and selection of process conditions require knowledge of the temperatures within the die. Temperature gradients exist because of the high heating rate and because of variations of density and electrical resistivity of the medium due to the presence of the part. Direct measurement of temperature with thermocouples or other conventional means is impractical because of the high temperatures, high currents, and high pressures that are involved. Therefore, a computer model was developed to predict temperature as a function of time and applied voltage for any location in the die. The computer model is composed of three parts: a geometrical model to approximate the density and resistivity variations in the medium, a finite-element model to calculate the rate of resistive heating within each element, and a finite-difference model to calculate the temperature distribution based on solution of the heat-transfer equations. Predicted temperatures have been shown to be in excellent agreement with measurements, and numerical simulation provided encouraging consistency and reasonably accurate predictions of temperature profiles within the die. The model demonstrated the feasibility of a new process to achieve simultaneous application of pressure and heat to powder densification in Electroconsolidation.     

Surface characteristics of antimony-doped tin oxide films

The surface composition of antimony-doped tin oxide films and its variation after electron bombardment and resistive heating were investigated. It was found that impurities such as carbon, potassium, sodium, calcium and chlorine segregate at the surface and that the carbon and chlorine are removed easily by electron bombardment. It is proposed that resistive heating caused the antimony to migrate rapidly to the surface at about 780°C, resulting in an antimony-rich phase which quickly reached a temperature high enough for sublimation. All substances except tin and oxygen and a trace amount of chlorine were removed. This process was accompanied by a resistivity change of the sample from 50 to 15000ω. Both electron bombardment and resistive heating above 780°C caused an increase in the tin-to- oxygen ratio as determined by Auger spectroscopy and electron spectroscopy for chemical analysis.     

High-temperature effects on the electrical properties and macrostructure of carbon composites

The electrical conductivity of MPG-6 and MPG-7 carbon composites has been measured before and after 1.4-MeV electron irradiation and ac resistive heating up to degradation temperatures (above 2500°C). The results demonstrate that both heating and electron irradiation reduce the resistivity of the materials and increase the defect density at the macrostructural level, while x-ray diffraction analysis reveals no significant structural changes in the temperature range studied. Detailed characterization of the composites suggests that their strength is limited by crystallite or grain boundaries.     

Direct observation of resistive barriers in a BaTiO3 based thermistor

The remote electron beam induced current technique has been used to form resistive contrast images of the resistive barriers developed in 80 °C thermistor materials. Linescans extracted from the images have been used to calculate the temperature variation of resistivity of selected interfaces within the material. It is seen that the major resistive changes are associated with only a few of the grain boundaries in this material. This revised version was published online in November 2006 with corrections to the Cover Date.     

Electronic processes in molecular dynamics simulations of nanoscale metal tips under electric fields

Electronic effects play a crucial role in the temperature evolution of metal parts which have electric currents running through them. The increase in temperature due to resistive heating can cause the melting of metal nanoscale wires creating damage in electric circuits. Likewise, electric currents are also present in sharp features on metal surfaces exposed to high electric fields. The destruction of such tips can lead to vacuum arcs, supplying the neutral species to build up plasma over the surface. To follow the temperature evolution caused by electric currents in such a tip, we developed a new model, based on an existing molecular dynamics code, to include resistive heating and electronic thermal conduction. The results given by the new simulation model are in good agreement with analytical predictions.     

Investigation of the Temperature Limits of Polymorphous Transformation in Zr-1%Nb Alloy under Different Thermal Conditions

The method of pulsed resistive heating is used to investigate the structural transformation in low-alloy zirconium. The dependence of the transformation temperature on the direction of the process is revealed: in the cooling stage, the transformation occurs at lower temperatures than in the case of heating the sample. Experiments are performed which involve the interruption of heating in the region of two-phase state. A delay of return to the low-temperature phase is revealed, which leads to a temperature drop immediately after the heating current is switched off, irrespective of the completion of the process of structural transformation.     

Thermophysical properties of liquid platinum

Material properties of liquid metals are inherently difficult to measure. Static measurements are difficult to make on most metals because of the typically high values of critical temperature and pressure, problems with sample-container contamination, and physical strength limits of high-pressure vessels. Data on thermophysical properties of metals are needed for a variety of applications, and measurements on most liquid metals are performed using dynamic techniques. Dynamic pulse heating experiments are typically performed on nanosecond to millisecond timescales, providing data that would not otherwise be obtainable. We use a resistive pulse heating method to reach high-temperature expanded liquid-metal states at a constant pressure. This technique can be used for a variety of metals and allows accurate data to be obtained over a wide range of temperature. Metallic wire-shaped samples (1×25 mm) are resistively heated in an inert gas atmosphere for a period of about 10^–4 s by an almost-square current pulse (15×l0³ A). Samples expand along an isobaric path, with remote diagnostics providing data on current, voltage, temperature, volume, and sound speed. These basic quantities are then used to calculate several derivative quantities. We report measurements of enthalpy, temperature, volume, electrical resistivity, and sound velocity of liquid platinum for temperatures from the melting point up to 5100 K.Paper presented at the Third Workshop on Subsecond Thermophysics, September 17–18, 1992, Graz, Austria.     

Calculation of the temperature distribution at the HPHT growing of diamond single crystals in cells with two growth layers

Finite element method was used to calculate the distribution of temperatures in growth cells of the toroid TC-40 -type HPA. The experimental investigations of the process of growing type IIa diamond single crystals were performed in high-pressure cells with two growth layers. It is shown that in using the cell materials having appropriate properties and defined configuration of the system of the resistive heating the temperature gradients are 5.4–5.6°C/mm and the growth rate is 2.46 mg/h. The total weight of obtained structurally perfect type IIa diamond single crystals in the upper and lower growth layers is 1.18 and 1.13 carats, respectively, the nitrogen content in all grown crystals is 1–3 ppt.     

In situ epitaxial MgB2 thin films for superconducting electronics

The newly discovered 39-K superconductor MgB2 holds great promise for superconducting electronics. Like the conventional superconductor Nb, MgB2 is a phonon-mediated superconductor, with a relatively long coherence length. These properties make the prospect of fabricating reproducible uniform Josephson junctions, the fundamental element of superconducting circuits, much more favourable for MgB2 than for high-temperature superconductors. The higher transition temperature and larger energy gap of MgB2 promise higher operating temperatures and potentially higher speeds than Nb-based integrated circuits. However, success in MgB2 Josephson junctions has been limited because of the lack of an adequate thin-film technology. Because a superconducting integrated circuit uses a multilayer of superconducting, insulating and resistive films, an in situ process in which MgB2 is formed directly on the substrate is desirable. Here we show that this can be achieved by hybrid physical-chemical vapour deposition. The epitaxially grown MgB2 films show a high transition temperature and low resistivity, comparable to the best bulk samples, and their surfaces are smooth. This advance removes a major barrier for superconducting electronics using MgB2.     

A model for calculating a.c. losses in multistage superconducting cables

Superconducting magnets in tokamaks for fusion experiments are subjected to fast variations in magnetic field. As the high current conductors used in these magnets are made of multistage cables, these variations induce interstrand coupling currents that create losses. These losses are usually characterized by the so-called time constant of the conductor. A model is given to calculate this time constant. Working formulas are also proposed to calculate the current induced in the different cabling stages. This model takes into account the strand characteristics and the detailed cabling pattern. Using it, a method is also given to deduce the time constant from resistive measurements. The influence of the resistive barrier (chrome plating, CuNi shell, outer bronze matrix) is pointed out. Finally, this model is applied to a conductor that is foreseen for the toroidal coils of the International Thermonuclear Experimental Reactor (ITER).     

A Study of the Paramagnetic to Ferromagnetic Transition in the Optimally Doped CMR Compound La0.65Ca0.35MnO3 and Its Dependence on Oxygen Mass

We present a study of the paramagnetic to ferromagnetic transition in the CMR compound La_0.65Ca_0.35MnO₃ and its dependence on magnetic field and oxygen mass. The transition is characterized by two temperatures, the thermodynamic transition temperature at T _c, obtained from specific heat and thermal expansion data, and the resistive transition obtained from the resistivity maximum. The resistive transition occurs well within the paramagnetic range. The magnetic susceptibility in the paramagnetic range is isotope dependent up to 400 K. The magnitude of the Curie-Weiss constant indicates the presence of small clusters of about 4–5 unit cells. The resistive transition occurs when the percolation limit for these clusters is reached.     

Designing metal matrix composites to meet their target: particulate reinforced aluminium alloys for missile applications

Abstract

Aluminium alloys have traditionally been used for the manufacture of missile structural parts. As the performance of missiles improves, kinetic heating will increase and the continued use of conventional monolithic aluminium alloys for the manufacture of structural parts for short to medium range missiles will be limited by their elevated temperature performance. Steel or titanium alloys could be used but these may add weight or be more expensive. The alternative is to exploit the potential benefits of the high specific properties of aluminium based metal matrix composites (MMCs) which may be substantially retained during short term exposure to elevated temperatures. The improvements in strength and stiffness could enable weight savings to be made by reductions in wall thickness of the missile structure or alternatively, the improved stiffness of components such as wings and fins may prove beneficial for improved missile accuracy. The aim of the present paper is to highlight the results of demonstrator programmes which have been designed to assess the potential benefits of both particulate and fibre reinforced aluminium based MMCs for missile applications.     

A Study of the Paramagnetic to Ferromagnetic Transition in the Optimally Doped CMR Compound La0.65Ca0.35MnO3 and Its Dependence on Oxygen Mass

We present a study of the paramagnetic to ferromagnetic transition in the CMR compound La_0.65Ca_0.35MnO₃ and its dependence on magnetic field and oxygen mass. The transition is characterized by two temperatures, the thermodynamic transition temperature at T _c, obtained from specific heat and thermal expansion data, and the resistive transition obtained from the resistivity maximum. The resistive transition occurs well within the paramagnetic range. The magnetic susceptibility in the paramagnetic range is isotope dependent up to 400 K. The magnitude of the Curie-Weiss constant indicates the presence of small clusters of about 4–5 unit cells. The resistive transition occurs when the percolation limit for these clusters is reached.     

Cure simulation with resistively in situ heated CFRP molds: Implementation and validation

In composite processing of parts with varying cross-sections, homogeneous cure is sought but poses a significant challenge. Electrically heated molds for resin transfer molding (RTM) processes offer the potential to locally introduce heat and, thus, achieve more homogeneous cure and enhanced part quality. However, low conductivity of CFRP poses a risk of uncontrolled exothermic reactions. To target this potential, an appropriate and efficient numerical method is presented in this study to simulate part cure governed by resistive heated CFRP molds. A numerical control algorithm for 3D finite element cure simulations is developed, which uses the reaction flux of a temperature boundary condition to calculate the arising tool temperature field. The capability of this method to predict non-uniform tool temperatures of self-heated CFRP molds with close to thermocouple accuracy during the cure process is shown by means of numerical verification and experimental validation on a self-heated CFRP plate.     

焦耳热处理降低石墨烯/环氧树脂发热涂层的体积电阻率

石墨烯基发热涂层一项重要的用途是降低其体积电阻率。文中结合发热涂层本身可以产热的特点,对涂层施加电压让其产热来对涂层进行焦耳热处理以降低涂层的体积电阻率。首先研究了通电焦耳热处理后的涂层体积电阻率随处理温度、时间和周期的变化,然后用X射线衍射和扫描电镜说明了涂层体积电阻率降低的原因以及该过程与外热处理的区别,讨论了石墨烯含量、片径大小对处理后的涂层体积电阻率降低的影响,最后对处理前后涂层的发热温度进行了比较:对涂层进行130℃,30 min的处理后,涂层在60 V电压下,表面发热温度由67.6℃升高到120.4℃     

High temperature resistivity determination of high-melting point metals and alloys

A method of determination of temperature-dependent electrical resistivity for metals and alloys of high-melting points at high temperatures is presented. It is based on the computer simulation of wire heating by direct current and basic electrical measurements. The authors present new results for 25Re75W and 47.5Re52.5Mo alloys along with the results of test measurements for tungsten. Electrical resistivity for tungsten obtained with presented method is in good agreement with well known data for the metal.     

Thermophysical properties of solid and liquid beryllium

A submillisecond resistive heating technique under high pressure (0.12 GPa) has been used to measure selected thermophysical properties of both solid and liquid beryllium. Data have been obtained between room temperature and 2900 K. Results on enthalpy, volume expansion, electrical resistivity, and sound velocity measurements are presented.Paper presented at the Third Workshop on Subsecond Thermophysics, September 17–18, 1992, Graz, Austria.     

Properties of indium tin oxide films deposited using High Target Utilisation Sputtering

Indium tin oxide (ITO) films were deposited on soda lime glass and polyimide substrates using an innovative process known as High Target Utilisation Sputtering (HiTUS). The influence of the oxygen flow rate, substrate temperature and sputtering pressure, on the electrical, optical and thermal stability properties of the films was investigated. High substrate temperature, medium oxygen flow rate and moderate pressure gave the best compromise of low resistivity and high transmittance. The lowest resistivity was 1.6 × 10^− 4 Ω cm on glass while that on the polyimide was 1.9 × 10^− 4 Ω cm. Substrate temperatures above 100 °C were required to obtain visible light transmittance exceeding 85% for ITO films on glass. The thermal stability of the films was mainly influenced by the oxygen flow rate and thus the initial degree of oxidation. The film resistivity was either unaffected or reduced after heating in vacuum but generally increased for oxygen deficient films when heated in air. The greatest increase in transmittance of oxygen deficient films occurred for heat treatment in air while that of the highly oxidised films was largely unaffected by heating in both media. This study has demonstrated the potential of HiTUS as a favourable deposition method for high quality ITO suitable for use in thin film solar cells.     

Influence of heat treatment on electrical properties of thermally evaporated tin diselenide (SnSe2) thin films

SnSe₂ thin films were prepared on glass substrate by thermal evaporation method. The resistivity of the films were measured in the temperature range 30–200 °C. It is observed that the as grown films are highly resistive and on heating the resistance abruptly changes. The heating and cooling curves of log of resistance versus temperature are not reproducible till the third cycle and this reproducible curve is similar to that for the films annealed at 200 °C for 3 hours.     

Chains of conducting particles that determine the resistivity of thick resistive films

A statistical model of the chain-like structure of thick resistive films is presented. The theoretical relation between the resistivity and the volume fraction of conductive phase is compared with the relations obtained using two groups of data for ruthenate films. An approximation procedure which utilizes computer techniques was used in order to fit the experimental dependences. The routine uses only two variable model parameters, i.e. the resistivity of the conductive phase and a parameter which we call the “order function”. The results of the approximation were found to be satisfactory and allowed us to describe some of the characteristics of the behaviour of the conductive phase in resistive structures.