Temperature Dependence Of Material Parameters Research Articles

Finite Element Simulations on Scaling Effects of 3D SiGe Thermoelectric Generators

ABSTRACTWe report 3D finite element simulations analyzing scaling effects on the performance of single Silicon Germanium thermoelectric generator with 170 μm tall metal contacts. Temperature dependent material parameters are included to accurately model device performance. Power density was extracted for a range of widths, heights, and operating temperature. Depending upon cross sectional area of the SiGe leg and operating temperature, height can be optimized for maximum power density.

Modeling of Set and Reset Operations of Phase-Change Memory Cells

Phase-change memory elements with 25-nm Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> thickness and 25-nm heater diameter with ±2-nm protrusion/recess of the heater are studied using 2-D finite-element simulations with rotational symmetry. Temperature-dependent material parameters are used to solve current continuity and heat equations self-consistently. Melting is accounted for by including latent heat of fusion in heat capacity at melting temperature. Electrical breakdown is modeled using additional field-dependent conductivity terms to enable set simulations. Analyses on current, voltage, energy, power, and minimum pitch requirements are summarized for reset/set operations with 1-ns/20-ns voltage pulses leading to ~500× difference between the reset and set resistance states.

Modeling of Temperature-Sensitive Polyelectrolyte Gels by the Use of the Coupled Chemo-Electro-Mechanical Formulation

Polyelectrolyte gels show adaptive viscoelastic characteristics. In water-based solutions they have enormous swelling capabilities under the influence of different possible stimulation types, such as chemical, electrical, or thermal stimulation. Possible applications for these intelligent materials can be either actuators, e.g., when used as artificial muscles or chemo-electric energy converters, or sensors, e.g., for measuring ion concentrations or pH-values of the solutions. In order to represent this active behavior, a coupled multi-field model is applied. The chemo-electro-mechanical multi-field formulation considers changes of concentrations, electric potential, or strains due to varying (initial) conditions. In the present work, a fully coupled 3-field formulation for polyelectrolyte gels using the Finite Element Method is applied. This formulation consists of a chemical, electrical, and mechanical field equation. The chemical field is described by a partial differential equation (PDE) first order in time and second order in space and includes diffusional, migrational, and convectional effects. The electrical field is represented by the Poisson equation, an elliptical PDE of second order in space. Finally, the mechanical field is described by a PDE of first order derived from the conservation of momentum. Due to the slow processes in time, inertia terms are neglected. The mechanical field is coupled to the chemo-electrical field by a prescribed strain stemming from an osmotic pressure term. A large dependency between the applied temperature and the actual swelling degree of the gel has been proven in experiments. In the present research, the thermal stimulation is investigated using two alternative approaches: The first is a straight forward modeling by modifying the actual temperature in the osmotic pressure term. As this approach does not lead to promising results, as a second alternative temperature-dependent material parameters obtained from experimental measurements are applied. The calibration of the derived simulation results is performed with experimental results from the literature. The Finite Elements introduced for the coupled multi-field formulation contain degrees of freedom for concentrations, electric potential, and mechanical displacements. They are adopted and applied in the commercial software package ABAQUS as a user defined element. For the numerical solution, the Newton-Raphson method in conjunction with the implicit backward Euler time integration scheme is applied.

A Study of Temperature Dependence for Capacitive Pressure Sensor

In order to reveal the temperature dependence of a touch mode capacitive pressure sensor, temperature dependence of material parameters of the sense have been studied. Using Finite Element Method (FEM) to simulate and solve capacitance, the results show that the relation of capacitance and temperature is almost linear in touch state of the sensor. At the same time, temperature sensitivities under different pressure are slight difference, which are 0.0056pF/K and 0.0040pF/K, respectively. Therefore, the high-temperature performance of the sensor is greatly outstanding.

Simulation of temperature-dependent material parameters and device performances for GaInAsSb thermophotovoltaic cell

Temperature-dependent material parameters and device performances of Ga x In 1− x As 1− y Sb y TPV cells applied in low temperature (800–1200 °C) radiators are simulated using the PC-1D. As is well known, the optimum bandgap ( E g ) decreases towards lower radiator temperatures. So far, the lowest achievable E g of Ga x In 1− x As 1− y Sb y at 300 K is 0.5 eV. We mainly considering the Ga 0.8In 0.2As 0.18Sb 0.82 ( E g = 0.5 eV) TPV cell. The effects of doping concentration and recombination mechanisms of the emitter layer on photovoltaic conversion efficiencies ( η cel ) are analyzed in detail, and η cel can be improved by optimizing doping concentration and suppressing carrier recombination. The effects of GaSb window layer on η cel are also presented. It shows the type-II energy-band alignment GaSb(window)/GaInAsSb(emitter) heterostructure affect η cel mainly through V oc . For the first time, the effects of operating temperatures on device performances are analyzed based on temperature-dependent material parameters, and the temperature coefficients of the device performances are presented.

BUCKLING OF AN AXIALLY RESTRAINED STEEL COLUMN UNDER FIRE LOADING

Analytical procedure, based on the linearized stability analysis, is presented for the determination of the buckling load and the buckling temperature of a straight, geometrically perfect, axially loaded steel column subjected to an increasing temperature simulating fire conditions. The nonlinear kinematical equations and the nonlinearity of material are considered. The stress–strain relation for steel at the elevated temperature and the rules for reduction of material parameters due to increased temperature are taken from European standard EC 3. Theoretical findings are applied in the parametric analysis of a series of Euler's columns subjected to two parametric fires. It is found how the slenderness of the column, the material nonlinearity, the temperature dependence of material parameters, and the stiffness of restraints at supports effect the critical temperature. While these parameters have major influence on the critical temperature, they have no effect on the shape of the buckling mode.

Electrothermomechanical analysis of partially insulated field-effect transistors using hybrid nonlinear finite element method

Using the hybrid nonlinear finite element method (FEM), electrothermomechanical analysis of partially insulated field-effect transistors (PiFETs) is performed in this paper, with their temperature-dependent material parameters treated rigorously. Two types of PiFETs, namely partially insulating oxides (PiOX) under the drain/source (PUSD) and channel (PUC), are studied and compared. The impact of self-heating effect (SHE) on their I– V characteristic, current degradation, temperature and thermal stress distribution are investigated, with some comparisons also made among normal MOSFET, PUSD PiFET, PUC PiFET and SOI FET. The influences of PiOX length, PiOX thickness, and electrode material on their maximum temperature and thermal stress are predicted, which can provide some pragmatic criterion for the development of PiFETs with good reliability.

Influences of Variable Friction Coefficient on the Hot Formability of Ultra High Strength Steel Sheet

Friction coefficient is an important parameter in sheet metal forming especially in hot forming. Friction condition not only influences material flow but also affects the thermal conductivity between blank and tools. In this study, varied friction coefficient is introduced to the hot forming simulation of B-pillar made of ultra high strength steel sheet 22MnB5. Three curves of friction coefficient vs. temperature are investigated. All of the heat transferred by conductivity, radiation and convection are considered in the simulation. And the temperature-dependent material and process parameters are supplied. It is demonstrated that the coupling effect among the strength and hardness of the metals, the properties of the oxide film covering blank surface and viscosity of the lubrication oil leads to the fact that the friction coefficient changes with temperature instead of constant during hot forming. The friction coefficient curve characterized by increasing first then decreasing gives the best simulation results and then is followed by the one which is characterized by decreasing first then increasing. The constant friction coefficient is the last.

Fatigue assessment of thermal cyclic loading conditions based on a short crack approach

Thermal loading conditions of nuclear power plant components cause local stress-strain hystereses. For the fatigue life prediction of nuclear power plant components under thermal cyclic and structural loading a new method based on the local strain approach is to be presented. This method involves finite-element simulations as well as the experience gathered from lifetime assessment methods based on short crack models. The local stresses and strains are obtained from coupled-field FE-analyses. The calculation of the hysteresis-loops relies on appropriate material models and experimentally verified temperature-dependent material parameters in order to describe the elasto-plastic behavior of the material as realistically as necessary. Due to the temperature dependence of the material parameters the resulting hysteresis loops are of non-conventional shapes and similar to those observed under multiaxial non-proportional structural loading. Hence, fatigue methodologies developed for non-proportional loading conditions during the past years bear good prospects for successful application under non-isothermal loading conditions

Accounting for porosity, time and temperature dependency in fracture mechanics concepts on polyvinylidene fluoride material

The polyvinylidene fluoride (PVDF) under study is a semi-crystalline polymer that exhibits sensitivity of mechanical properties to both strain rate and temperature. Furthermore, this material is subjected to a significant cavitation during deformation. A comprehensive experimental database was built in order to analyze the fracture behaviour in the ductile to brittle transition domain. Tensile tests were carried out on smooth and notched specimens at temperatures ranging from −50 °C to 20 °C. The results were used to determine temperature-dependent material parameters by using the mechanics of porous media. The obtained set of parameters was validated on two kinds of pre-cracked specimens, by using the local approach of fracture mechanics. With the help of a finite element code, both global and local approaches of fracture mechanics were shown to complement one another: whereas classical formulae of J-integral fail to characterize crack initiation for this PVDF, the present methodology allowed the plot of J 1C values with respect to temperature.

RF performance assessment of AlGaN/GaN MISHFET at high temperatures for improved power and pinch‐off characteristics

AbstractAn investigation of temperature model for an AlGaN/GaN MISHFET is presented and a relative comparison is done with conventional HFET structures. The proposed analytical model demonstrates its inherent ability to operate at higher temperature. The contributions from various temperature dependent material parameters are taken into account alongwith the effect of temperature on the sheet carrier concentration and threshold voltage of the device. The model is further extended to predict the temperature dependence of transconductance, cut off frequency and pinch‐off characteristics. The investigated temperature range is from 25°C–300°C. Non linear Fermi potential (Ef) variation with the sheet carrier concentration (ns), and the highly dominant effects of spontaneous and piezoelectric polarization at the AlGaN/GaN heterointerface are also considered. AlGaN/GaN MISHFETs demonstrate larger drain currents, cut‐off frequency and better pinch‐off characteristics at high temperatures. The model is based on closed form expression and does not involve elaborate computation. The analytical results on the transport characteristics of both the devices are compared with available experimental data and are in good agreement. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 1942–1949, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24461

Numerical analysis of quenching – Heat conduction in metallic materials

Metallic materials present a complex behavior during heat treatment processes. In a certain temperature range, change of temperature induces a phase transformation of metallic structure, which alters physical properties of the material. Indeed, measurements of specific heat and conductivity show strong temperature-dependence during processes such as quenching of steel. Several mathematical models, as solid mixtures and thermal–mechanical coupling, for problems of heat conduction in metallic materials, have been proposed. In this work, we take a simpler approach without thermal–mechanical coupling of deformation, by considering the nonlinear temperature-dependence of thermal parameters as the sole effect due to those complex behaviors. The above discussion of phase transformation of metallic materials serves only as a motivation for the strong temperature-dependence as material properties. In general, thermal properties of materials do depend on the temperature, and the present formulation of heat conduction problem may be served as a mathematical model when the temperature-dependence of material parameters becomes important. For this mathematical model we present the error estimate using the finite element method for the continuous-time case.

Modeling Friction Stir Welding Process of Aluminum Alloys

The friction stir welding (FSW) process of aluminum alloys has been modeled using a two-dimensional Eulerian formulation. Velocity field and temperature distribution are strongly coupled and solved together using a standard finite element scheme. A scalar state variable for hardening is also integrated using a streamline integration method along streamlines. A viscoplastic constitutive equation to consider plastic flow and strength variations was implemented for the process modeling. Precipitates inside AA6061 alloys are sensitive to elevated temperatures and affect strength evolution with temperature. The overall effects of the precipitate variations with temperature on strength were reflected using temperature-dependent material parameters. The material parameters of constitutive equations were obtained from isothermal compression tests of various temperatures and strain rates. The effects of FSW process conditions on heating and hardening were investigated mainly near the tool pin. The microhardness distribution of the weld zone was compared with the prediction of strength. In addition, crystallographic texture evolutions were also predicted and compared with the experimental results.

Electrothermal simulation of the self-heating effects in 4H-SiC MESFETs

A thermal model of 4H-SiC MESFET is developed based on the temperature dependences of material parameters and three-region I - V model. The static current characteristics of 4H-SiC MESFET have been obtained with the consideration of the self-heating effect on related parameters including electron mobility, saturation velocity and thermal conductivity. High voltage performances are analysed using equivalent thermal conductivity model. Using the physical-based simulations, we studied the dependence of self-heating temperature on the thickness and doping of substrate. The obtained results can be used for optimization of the thermal design of the SiC-based high-power field effect transistors.

Temperature dependent analytical model of sub-micron GaN MESFETs for microwave frequency applications

In this paper, a temperature dependent analytical model of sub-micron GaN MESFETs for microwave frequency applications is presented. The contributions from various temperature dependent material parameters are taken into account in order to develop an accurate I– V model along with the effect of gate leakage current on the threshold voltage of the device. The model is further extended to predict the temperature dependence of transconductance, output conductance, cut off frequency and maximum frequency of oscillation. The analytical results show excellent agreement with the experimental data at temperatures up to 200 °C for a 0.25 μm device.

Existence and Uniqueness of Solutions of a Nonlinear Heat Equation

A nonlinear partial differential equation of the following form is considered: u′ − div a(u)∇u + b(u) |∇u|2 = 0, which arises from the heat conduction problems with strong temperature-dependent material parameters, such as mass density, specific heat and heat conductivity. Existence, uniqueness and asymptotic behavior of initial boundary value problems under appropriate assumptions on the material parameters are established.

Comparison of isotropic hardening and kinematic hardening in thermoplasticity

A coupled thermo-mechanical problem is presented in this paper. The constitutive model is based on thermoplastic model for large strains where both kinematic and isotropic hardening are included. It is shown that a non-associated plasticity formulation enables thermodynamic consistent heat generation to be modeled, which can be fitted accurately to experimental data. In the numerical examples the effect of heat generation is investigated and both thermal softening and temperature-dependent thermal material parameters are considered. The constitutive model is formulated such that pure isotropic and pure kinematic hardening yield identical uniaxial mechanical response and mechanical dissipation. Thus, differences in response due to hardening during non-proportional loading can be studied. Thermally triggered necking is studied, as well as cyclic loading of Cook’s membrane. The numerical examples are solved using the finite element method, and the coupled problem that arises is solved using a staggered method where an isothermal split is adopted.

Associative coupled thermoplasticity at finite strain with temperature-dependent material parameters

The paper deals with associative coupled thermoplasticity at finite strains. J2 plasticity model is based on hyperelastic formulation with multiplicative decomposition of deformation gradient into elastic and plastic parts. As a novel aspect, temperature dependence of all material parameters is introduced. For such model, variational procedure is carried out and discretization by the mixed finite element method is performed. Consistent linearization is carried out and algorithmic elasto-plastic tangent moduli are given. Verification of algorithm is provided by two examples.

Probing the temperature during switching of YBCO films

The switching of YBCO thin films under high current load is fundamental for fault current limiters and active power switches. Its mechanism is still under debate, however, with thermal and nonthermal models being discussed. To clarify the situation, we have placed an array of thermometers directly on top of a YBCO strip and have measured the quench propagation with high spacio-temporal resolution. We compare the results with a numerical model of heat diffusion in 3D with temperature dependent material parameters and find nearly quantitative agreement. This confirms thermal runaway as the mechanism of switching.

Transient 3D heat flow analysis for integrated circuit devices using the transmission line matrix method on a quad tree mesh

This paper presents a 3D transmission line matrix (TLM) implementation for the solution of transient heat flow in integrated semiconductor devices. The implementation uses a rectangular discontinuous mesh to allow for local mesh refinement. This approach is based on a quad tree meshing technique which can have a complex geometry using blocks of varying sizes. Each such block can have a maximum of two adjacent blocks on any vertical side and a maximum of four blocks on the top or bottom.The TLM implementation is based on a physical extraction of a resistance and capacitance network and then the creation of the appropriate TLM matrix. The formulation allows for temperature-dependent material parameters and a non-uniform time stepping.The simulator is first tested using a 2D example of a heat source in a rectangular region. Using this example the numerical error is determined and found to be less than 0.4%. Next, non-linearities are included, and a number of non-uniform time stepping algorithms are tested. Then, a 3D problem is also compared to an analytical solution and again the error is very small. Finally, an example of a full solution of heat flow in a realistic Si trench device is presented.