Thin Film Non-Noble Transition Metal Thermophysical Properties

The transient thermoreflectance (TTR) technique coupled with a pump-probe experimental setup enables the observation of thermal transport phenomena on a sub-picosecond time scale. The reflectance from non-noble transition metals (at least one unoccupied d-orbital in the conduction band) can be shown to have a linear dependence when compared to small changes in the electron and lattice temperatures. This thermal dependence can be combined with the parabolic two step (PTS) model to enable measurement of the electron-phonon coupling factor and thermal conductivity of thin film materials. Experimental results are presented for thin film samples of the non-noble transition metals platinum and nickel. Results are presented using laser wavelengths ranging from 740 nm to 805 nm and using a range of laser fluences (ranging from ～0.35 to 2 J/m²). Over this range of wavelengths and fluences the material properties are shown to be independent of the measurement conditions.     

Effect of impurities on intergranular fracture of cadmium during thermocycling and creep

Abstract

The paper is devoted to an analysis of experimental data obtained as a result of investigation of an influence of the impurity nature on the failure of a model material of a high purity under condition of its thermocyclic treatment and high temperature creep. The fact that impurities promote the process of cavity formation was known long ago. However, since investigations were conducted with commercial materials, which had a variety of impurities, it was impossible to investigate the influence of each chemical element separately on the phenomenon in the presence of other additives. Experimental studies are described that have clarified what sort of impurities are responsible for the phenomenon of cavity formation, in what way they act in combination, over which temperature interval, whether neutralisation of them is possible and how this neutralisation can be achieved.     

Kinetics and evolution of WRe platelets in irradiated W–Re alloys in fusion power plant

Abstract

A recent theory of WRe platelet formation in irradiated W–Re alloys is applied to the plasma facing surfaces (composed of W) in a conceptual fusion power plant, where the 14 MeV neutron irradiation transforms some of the W atoms to Re (and Os). In the first wall material, numerous small platelets of multiple WRe dumbbells are expected to form and to strongly harden and severely embrittle the metal, e.g. increase its ductile–brittle transition temperature by ～350 K. At the higher temperature of the divertor material, the thermal instability of small multiples leads to a nucleation difficulty so that only an almost negligible number of platelets is expected to form and thus, give almost no hardening and embrittling effects.     

MICROTHERMOPHOTOVOLTAICS POWER SYSTEM FOR PORTABLE MEMS DEVICES

Abstract

A novel concept of microthermophotovoltaics (MTPV) systems is proposed for powering MEMS devices. The system uses hydrogen or hydrocarbon as fuel and does not involve any moving parts. Its fabrication and assembly are relatively simple. In this article, energy conversion efficiencies of a GaSb MTPV system incorporating broadband SiC and selective emitter material were first analyzed. Numerical and experimental studies on microcombustion processes in the MTPV system were carried out. The results show that uniform temperature distribution above 1000 K along the wall of microcylindrical combustors with a unique backward-facing step can be achieved. Finally, a prototype MTPV power system using SiC as the material for combustor and emitter, and a hexagonal GaSb cell array for energy conversion, was fabricated and tested. Electric power output ranging from 0.07 W to 0.74 W was measured. The potentials and further approaches of MTPV system were discussed. It is believed that MTPV would be a very attractive and competitive system among other power MEMS developments.     

TEMPERATURE AT A PROPAGATING CRACK TIP IN A VISCOPLASTIC MATERIAL

Abstract

Rupture processes at a propagating crack tip and the dissipation of mechanical energy in the vicinity of a crack tip can give rise to substantial temperature increases. In this paper we consider the (;transient) time-dependent temperature field. It is shown that under certain conditions. which are discussed in some detail, the adiabatic approximation gives a very acceptable approximation for the near-tip temperature. The adiabatic approximation has been used to calculate the crack tip temperature for a crack that propagates in a viscoplastic material. The mechanical behavior of the material is represented by a model proposed by Bodner and Partom. The Bodner-Partom model accounts for flux of energy into the crack tip as well as for dissipation of mechanical energy in the near-tip region. The temperatures due to both effects have been analyzed. and estimates are presented for the maximum crack tip temperature that may be reached during the crack propagation process.     

NUMERICAL ALGORITHM FOR ESTIMATING TEMPERATURE-DEPENDENT THERMAL CONDUCTIVITY

Abstract

The hybrid scheme of the Laplace transform technique and the central difference approximation is applied to estimate the temperature-dependent thermal conductivity by utilizing temperature measurements inside the material at an arbitrary specified time. In the present study the functional form of the thermal conductivity is not known a priori. Thus, this problem can be regarded as the functional estimation in inverse calculation. The accuracy of the predicted results is examined from various illustrated cases using simulated exact and inexact temperature measurements obtained within the medium. Results show that a good estimation on the thermal conductivity can be obtained with any arbitrary initial guesses of the thermal conductivity. The advantage of the present method in the inverse analysis is that, for most types of boundary conditions, the relation between the thermal conductivity and temperature at any specified time can be determined without measuring the early temperature data.     

TRANSIENT THERMAL CHANGE ON A SOLID SURFACE: COUPLED DIFFUSION OF HEAT AND MOISTURE

Abstract

The sudden change of moisture and/or temperature on a solid can significantly disturb the resulting stresses and strains and cause material damage. This investigation is concerned with the sudden rise (or drop) of temperature over a finite segment of a half-space that is free of tractions. The mutual dependence of moisture and temperature is examined by coupled equations such that the hygrothermal stresses and strains are transient in character. The moisture, temperature, and stress are computed by a time-dependent finite-element method and are found to peak and oscillate as a function of time in the vicinity where the surface temperature is altered. These effects are displayed for the 5208 epoxy resin material commonly used in fiber-reinforced composities.

The locations of possible failure sites are also determined by application of the strain-energy density criterion. This assumes that the peaks and valleys of the strain-energy density function dW/dV within the solid are associated with material damage. Depending on the characteristic value of dW/dV, the failure sites may alter in location, a result which was not known previously.     

Influence of surface radiation on the transition to unsteadiness for a natural convection flow in a differentially heated cavity

Abstract

The influence of surface radiation on the transition to the unsteady state in natural convection is studied numerically. The configuration of the differentially heated square cavity with adiabatic horizontal walls is chosen to generate an internal natural convection flow. It is known that radiative transfers reduce the temperature difference between the adiabatic walls, which consequently reduces the thermal stratification of the central zone and increases the velocity flow. Many studies have focused on the stationary regime, but few of them have investigated the transition to unsteady flow. For this purpose, the effect of the wall emissivity on the critical Rayleigh number and the associated critical frequency was studied for a given cavity length. The cavity length and mean temperature of isothermal walls are set for the whole study. The results show that all these values are between the values obtained without radiation and those obtained for perfectly conducting horizontal walls. The critical Rayleigh number decreases with emissivity while the associated frequency increases. Moreover, the symmetry of fluctuating properties of the flow is changed when the radiation is taken into account.     

Peat Pyrolysis and the Analytical Semi-empirical Model

Abstract

Pyrolysis of peat could convert this material into useful fuels and valuable hydrocarbons. A study of peat pyrolysis can also serve as a useful bridge between studies of coal pyrolysis and biomass pyrolysis. Using an analytical model of pyrolysis that has previously been applied to biomass and to coal, we present here the results of applications of this model to a representative peat. The analysis suggests means of organizing and processing rate and yield data that should be useful in applications of pyrolysis for the production of fuels and chemicals.     

Phase transition of doped LaFeO3 anode in reducing atmosphere and their power generation property in intermediate temperature solid oxide fuel cell

In general, transition metal-doped La_0.6Sr_0.4FeO₃ (LSF) has been used as a cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs) because of its high mixed electronic−ionic conductivity and catalytic properties. Recently, some research groups have been investigating the doped LSF as an anode material. In this study, we evaluated the influence of dopant in LSF on anodic properties of LSF in SOFCs. Whereas Mn-doped LSF showed typical perovskite oxide structure even after reduction in hydrogen at high temperature, the LSF and Co-doped LSF exhibited phase transition partially to LaSrFeO₄ and exsolution of metal particles after reduction. The phase transition and metal exsolution occurred at temperature higher than 1008 K in a reducing atmosphere. Despite the partial phase transition, the cell using Co-doped LSF anode exhibited fairly high power density of 1.33 W/cm² at 1173 K with the lowest polarization resistance. These results may originate from the high oxygen-ion conductivity of LaSrFeO₄–La(Sr)Fe(Co)O₃ and the high hydrogen oxidation property of the Co–Fe particles on ceramic anode surface.     

Experimental Investigation on Thermal Protection of High Temperature and High Velocity Jet Impinging a Cross-Shaped plate

Abstract

This paper investigated the heat transfer of high temperature jet impinging a cross-shaped plate. An experimental mockup was designed and used to produce high temperature (～500?°C) impinging gas jet with stable high speed (～56?m/s) at the nozzle exit. A one-dimensional theoretical model was proposed to predict the temperature increment of the back-side surface(s) that opposed to the impinging surface, by simplifying and assuming the heat transfer process can be governed by convection. Three materials were selected due to its potential high heat transfer resistance, speedy installation and long duration under severe impinging conditions. For each material, four types of thermal insulation structures were designed and subjected for testing. The following conclusions from comparison of the results of the model predictions with experimental data can be drawn. The proposed simplified one-dimensional model can be used to predict heat transfer at the back-side under high temperature and high-speed impinging. The optimal case among the 12 thermal protecting designs was obtained, majorly due to low back temperature and less material consumption.     

Citrate sol–gel synthesised Li4SiO4: conductivity and dielectric behaviour

Abstract

Pure Li₄SiO₄ ceramic material is obtained using citrate sol–gel method followed by sintering at 750°C. The crystallographic phase of the material is investigated by X-ray diffraction. The conductivity of the ceramic material is determined at different temperatures. Meanwhile, the dielectric properties are observed in order to obtain further information on ion dynamics in the material. The X-ray diffraction result shows the formation of a pure monoclinic Li₄SiO₄ crystal structure with lattice parameters a?=?5·140 Å, b?=?6·094 Å, c?=?5·293 Å and β?=?90°. The conductivity of the material increases linearly with the increase in temperature. The conductivity of the sample is 1·16×10^?4 S cm^?1 at 100°C. The frequency dependence of conductivity follows the universal power law variation σ_ac (ω)?=?σ_o+Aω^s. The plot of pre-exponent s versus temperature suggests that the conduction mechanism in the system can be described using correlated barrier hopping model. The increase in dielectric constant and dielectric loss and the peak shift of tan?δ to higher frequencies with temperature indicate that the increase in conductivity with temperature is due to the increase in number and hopping rate of charge carriers with temperature.     

Wood as prospective materials for sustainable development—Evaluation of tactile warmth by heat transfer analysis

ABSTRACT

Wood can be a sustainable resource with the ability to fix the carbon dioxide by photosynthesis if the cycle of the felling, planting, and growing of trees would be controlled. The good tactile warmth of wood is treated by heat transfer analysis to sustain the cycle with increasing the demand for felled wood. The heat transfer analysis shows that the governing material property on the tactile warmth is the thermal effusivity. The contact surface temperature is proposed as the quantitative scale of the tactile warmth defined uniquely from physical quantities.     

Mechanistic model for stresses in the oxide layer formed on zirconium alloys

Zirconium alloys are widely used as the cladding material in pressurized-water-reactors. The oxide formed is subjected to compressive stresses, which relax over time. This may affect the protectiveness of the oxide layer by allowing crack formation. We present a mechanistic model to predict these stresses as a function of temperature and oxidation kinetics. Material parameters for elastic deformation, creep, and thermal expansion are taken from appropriate experimental studies and the resulting predictions for the evolution of the stress distributions are compared with other experimental data. Dislocation glide in the oxide is found to be the dominant mechanism of stress relaxation for temperatures below 900?K.     

RESIDUAL THERMAL STRESSES IN A SYMMETRIC,DOUBLE-LAP JOINT

Abstract

This investigation presents an evaluation of the thermal stresses in a symmetric, double-lap joint which result from cooling from an elevated cure-temperature down to room temperature. The analysis is based upon variational principles and considers viscoelastic response for the adhesive material. The stresses within the adhesive are computed and compared with the elastic results, it is observed that a significant viscoelastic relaxation takes place within the adhesive.     

Study of the Reaction Mechanisms of Hydrocarbons with Calcium Sulfate

In this article, thermal simulation experiments on the reactions in TSR process (CH₄-CaSO₄, C₂H₆-CaSO₄, and C₃H₈-CaSO₄) were carried out using autoclave at high temperature and high pressure. The gaseous and solid products were characterized by some advanced analytical methods. On the basis of the experimental results, the reaction mechanisms were tentatively investigated. It was found that three reactions can proceed to produce H₂S, H₂O, and CaCO₃ as the main products at the temperature range of 450–700°C. The high simulation temperatures also resulted in the production of undesired material. The results obtained in this article can provide important information for the investigation on the natural gas destruction in gas reservoirs.     

Service induced fcc→hcp martensitic transformation in a Co-based superalloy

Co-based superalloy, ECY768, applied on gas turbine vanes, shows the presence of cracks after service. EBSD studies revealed hcp transformation in the base material near cracked regions. This phase arises from a martensitic transformation of fcc matrix and bestows high fragility. The phase transformation is related to temperature and loading distribution that characterises components in service. However, at the service temperatures, the hcp transformation is not expected for ECY768. An in-house thermodynamic database was developed using the Calphad approach and thermodynamic calculations were applied to the complicated alloy composition for phase stability range evaluation. Moreover, a testing campaign was planned to artificially create this martensitic transformation and to comprehend the influence of plastic strain on fcc-hcp transformation. The transformation mechanisms were understood and some methods were developed for hcp-phase removal through refurbishment heat treatment.

This paper is part of a thematic issue on the 9th International Charles Parsons Turbine and Generator Conference. All papers have been revised and extended before publication in Materials Science and Technology.     

Influence of thermal ageing on the stability of polymer bulk heterojunction solar cells

A new approach is presented in order to improve the thermal stability of polymer: [6-6]-phenyl C₆₁ butyric acid methyl ester (PCBM) bulk heterojunction solar cells. The central idea in this approach is the use of a polymer with high glass transition temperature (T_g), well above the normal operating temperatures of the devices. In this paper, a PPV-derivative with a T_g of 150 °C was used as an electron donor and the thermal stability of the obtained solar cells was compared with solar cells based on the reference material poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) with a T_g of 45 °C. The use of the material with higher glass transition temperature resulted in a significant improvement of the thermal stability of the photovoltaic parameters. Furthermore, a systematic transmission electron microscope (TEM) study demonstrates that the better thermal stability of performance coincides with a more stable active layer morphology. Both improvements are attributed to the reduced free movement of the electron donor material (PCBM) within the active layer of the solar cell.     

Circumferentially Cracked Hollow Cylinder Under Thermal Shock—Revisited

Abstract

The axisymmetric problem of a circumferentially cracked transversely isotropic hollow cylinder under thermal shock is considered. It is demonstrated that appropriately normalized stress intensity factors (SIFs) depend only on three material parameters and the transient temperature distribution. It is also found that only one of these parameters has a significant effect on the normalized SIFs. Reduction in the number of material parameters, from seven to practically one, makes it possible to propose simple approximate expressions for the calculation of normalized SIFs in terms of geometric parameters. Both inner and outer surface cracks are addressed. Approximate expressions giving SIFs are presented.     

Effect of heat transfer on the instabilities and transitions of supercritical CO2 flow in a natural circulation loop

System stabilities are of critical importance in natural circulation applications. Investigations show that for supercritical CO₂ flow new behaviors can be seen in natural convection systems considering its temperature-sensitive physical properties. In the present study, numerical simulations on a supercritical CO₂ natural circulation loop have been carried out to investigate the flow transitions and instabilities of such systems. In the present model heat sink temperature is kept at 298 K while heat source temperature varies in the range of 310–1023 K as a controlling parameter. It is found for the first time that for the present supercritical CO₂ model there exists a transition heat source temperature at which the system changes from unstable repetitive-reversal flow into stable one-direction flow with the increase of temperature, which is fundamentally different from previous studies for normal fluid. In particular, the critical transition fluid temperature is found to be near the second “pseudo-critical temperature” at around 375 K where the fluid properties experience major transitions with the increase of temperature. In addition, characteristics of flow stability behaviors are also analyzed in detail.