Thermal diffusivity of some crystalline rocks

Thermal diffusivity data at room temperature and uniaxial pressure of 1 MPa are reported for five sets of crystalline rocks—granite, granodiorite, gabbro, basalt and gneiss. Diffusivity ranges between approximately 0.6 and 1.9 mm²/s, the lower end of the range being appropriate for basic rocks and the upper end for quartz-bearing acidic rocks. The scatter in diffusivity for each data set is significantly more than that of thermal conductivity, because the diffusivity of water is typically less than 10% of the diffusivity of most common minerals, whereas water conductivity is 25–30% of the conductivity of the minerals. For a sample set of uniform mineralogy in which porosity varies, a greater variation of diffusivity than of conductivity is therefore expected. For three of the sets sufficient mineralogical data were available to permit the assessment of methods of estimating thermal diffusivity from mineral content. All models tested yielded higher mean values of diffusivity than the means of the measured values. No model was found to be able to predict diffusivity to better than approximately 20%, but if that accuracy is sufficient, a simple geometrical model, for which only quartz content must be known, is adequate. The diffusivity data have been combined with measurements of thermal conductivity and density to provide estimates of specific heat. These all tend to be higher than those reported in the literature. For some rocks, such as the basalts, this can be explained in terms of relatively high water content and the very high specific heat of water compared with that of most common minerals. For the granites and granodiorites, the new specific heat data redefine the previously published means and ranges, by increasing the data base by approximately an order of magnitude.     

Determination of the thermal transport properties of ammonia borane and its thermolysis product (polyiminoborane) using the transient plane source technique

Reliable thermal property data are necessary to improve the fidelity of chemical hydride thermal decomposition models. The thermal diffusivity and conductivity of ammonia borane (NH₃BH₃) and its partial thermolysis product (polyiminoborane) were measured at various packing densities using a transient plane source technique under ambient conditions. The particle size of the ammonia borane powder was between 200 and 600 μm, while the particle size of the polyiminoborane powder was between 10 and 30 μm. The thermal diffusivity and conductivity of the ammonia borane increased from 0.17 to 0.24 mm²/s and 0.19 to 0.44 W/m K (±10%), respectively, when its packing density was increased from 0.37 to 0.58 g/cm³. The increase in thermal conductivity is due to the increase in contact area between particles and the increase in the thermal diffusivity is related to an increase in density and volumetric heat capacity caused by compaction. The thermal conductivity of the polyiminoborane powder was approximately three times lower, likely due to its higher porosity. The thermal diffusivity and conductivity of this product changed from 0.21 to 0.12 mm²/s and 0.068 to 0.23 W/m K (±10%), respectively, when its packing density was increased from 0.13 to 0.96 g/cm³.     

Direct measurement and modeling relative gas diffusivity of PEMFC catalyst layers: The effect of ionomer to carbon ratio,operating temperature,porosity, and pore size distribution

A modified Loschmidt cell was used to measure the relative gas diffusivity (D¹) of the porous catalyst layers (CLs) of polymer electrolyte membrane fuel cells as a function of CL ionomer/carbon weight ratio (I/C) (0.5, 1.1, and 1.5) and operating temperature (20, 40, and 72 °C). D^* decreased by 80% when I/C was increased from 0.5 to 1.5. While the effective gas diffusivity of CL increased with temperature, D^* decreased because binary diffusion increases more rapidly than Knudsen diffusivity with temperature. The structure of CL was modeled through considering a packed-sphere model for carbon particles within agglomerates, and a network of overlapped spherical agglomerates forming the CL. The gas diffusion problem was solved analytically for the CL structure considering both Knudsen and molecular mechanisms, and, results were validated. Using the model, the effect of porosity, pore size distribution and ionomer coverage on gas diffusivity was evaluated.     

Measurement of thermal conductivity, thermal diffusivity and heat capacity of highly porous building materials using transient plane source technique

This paper presents both experimental and theoretical works concerning evaluation of the thermal conductivity, thermal diffusivity and heat capacity of wood composites. Moreover, the aim of this study is to show that the transient plane source technique originally used for measuring thermal properties of isotropic materials can be spread worthy of heat capacity, thermal conductivity and thermal diffusivity measurements of highly porous materials. Measurements of the thermal conductivity, thermal diffusivity and heat capacity have been performed at room temperature (20 ± 0.5°C) and normal pressure. An attempt has been made to predict the thermal diffusivity of wood composites from the predicted values of thermal conductivity using a Verma et al's model based on Ohms law and the calculated values of heat capacity using the enthalpy concept. The predicted values by the proposed model are compared with the values of the thermal diffusivity measured using the TPS method. A comparison shows a good agreement.     

Measurements of the thermal diffusivity of linear-medium-density-polyethylene/aluminium composites using a transient comparative method

Metal-powder/polymer composites are an interesting class of material because their physical properties may, within limits, be selected to match a particular application. It is therefore important to be able to measure and model the physical properties of these composites. The effective diffusivity of linear-medium-density-polyethylene/aluminium composites was measured for a range of volume fractions using a simple, transient comparative method. Effective thermal conductivity data were calculated from the effective thermal diffusivity data. The effective thermal conductivity data were modelled well by the EMT equation.     

Hydrogen diffusivity in different microstructural components in martensite matrix with retained austenite

We elucidate the hydrogen diffusivity in martensite matrix with retained austenite (RA). Two aspects are focused: effect of microstructure on hydrogen diffusion behavior; hydrogen diffusivity calculation for different microstructural components. Quenched martensite (QM) had the highest effective hydrogen diffusion coefficient because of high dislocation density. Effective hydrogen diffusion coefficient decreased with the increase of intercritical annealing temperature because of decrease in dislocation density and increase of RA. According to the principle of Maxwell-Garnett equation, the hydrogen diffusion coefficient for grain boundary (GB) was 7.99 × 10^?8 m²/s and hydrogen diffusion coefficient of tempered martensite (TM) was 7.84 × 10^?11 m²/s.     

Measurements of thermal conductivity and thermal diffusivity of alternative refrigerants in liquid phase with a transient short‐hot‐wire method

A transient short‐hot‐wire method has been proposed to simultaneously measure the thermal conductivity and thermal diffusivity of liquids. The method has been applied to the refrigerant HCFC‐22, alternative refrigerants HFC‐32, HFC‐125, HFC‐134a and refrigerant mixtures HFC‐32/125 [35/65, 50/50 (HFC‐410A), 68/32 wt%], HFC‐32/125/134a [23/25/52 (HFC‐407C) wt%]. From the present study, it is shown that the measured results agree well with the literature values on thermal conductivity and those on thermal diffusivity obtained from NIST's thermophysical property database, REFPROP Ver. 6.0. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(8): 540–552, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20032     

Thermal diffusivity study of aged Li-ion batteries using flash method

Advanced Li-ion batteries with high energy and power density are fast approaching compatibility with automotive demands. While the mechanism of operation of these batteries is well understood, the aging mechanisms are still under investigation. Investigation of aging mechanisms in Li-ion batteries becomes very challenging, as aging does not occur due to a single process, but because of multiple physical processes occurring at the same time in a cascading manner. As the current characterization techniques such as Raman spectroscopy, X-ray diffraction, and atomic force microscopy are used independent of each other they do not provide a comprehensive understanding of material degradation at different length (nm² to m²) scales. Thus to relate the damage mechanisms of the cathode at mm length scale to micro/nanoscale, data at an intermediate length scale is needed. As such, we demonstrate here the use of thermal diffusivity analysis by flash method to bridge the gap between different length scales. In this paper we present the thermal diffusivity analysis of an unaged and aged cell. Thermal diffusivity analysis maps the damage to the cathode samples at millimeter scale lengths. Based on these maps we also propose a mechanism leading to the increase of the thermal diffusivity as the cells are aged.     

Temperature dependence of thermal conductivity and thermal diffusivity of some composites using the transient plane source technique

The recently developed transient plane source (TPS) technique has been applied for the simultaneous measurement of thermal conductivity and thermal diffusivity of two composite materials namely, marble and magnesium oxychloride cement in the range of temperatures from 30 to 150°C. The experimental results of these samples show that there is very slight variation in thermal conductivity and thermal diffusivity of these materials in this range of temperature. An effort has been made to express this variation of thermal conductivity and diffusivity with temperature by a linear relation, in these materials.     

High‐temperature stability in yttria‐stabilized zirconia

Yttria‐stabilized zirconia has been studied as a candidate standard reference material for the determination of thermal properties. This study evaluated the high‐temperature stability, which is important for yttria‐stabilized zirconia to be used for a standard reference material, using a common material of fine ceramics, Referceram ZR1 (zirconia). The high‐temperature stability was evaluated by measuring the change in the thermal diffusivity before and after heat treatment at temperatures between 200 ^°C and 1500 ^°C. No change in the thermal diffusivity was observed when the samples were treated at temperatures equal to or below 900 ^°C. However, it was revealed that the changes in the thermal diffusivity were caused by the transformation and separation of the crystal phase and the cracks that occur at grain boundaries when the samples were treated at temperatures equal to or above 1000 ^°C. From these results, we confirmed that Referceram ZR1 is sufficiently stable for use as a reference material at temperatures equal to or below 900 ^°C. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(2): 57–67, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20193     

Biodiesel production from Nerium oleander (Thevetia peruviana) oil through conventional and ultrasonic irradiation methods

In the present research work, Nerium oleander oil has been used as raw material for producing biodiesel using both ultrasonic transesterification and a magnetic stirrer method. A two-step transesterification process was carried out for optimum condition of 0.40% V/V methanol to oil ratio, 1% V/V H₂SO₄ catalyst, 55°C temperature, and 60 min reaction time followed by treatment with 0.2% V/V methanol to oil ratio, 1% V/W KOH alkaline catalyst, 55°C temperature, and 60 min reaction time. The process is repeated with an ultrasonic method at the frequency of 28 kHz using ultrasonic horn type reactor (50 W) for about 10–15 min. Biodiesel obtained from ultrasonic method and magnetic stirrer was then compared for their percentage yield and physiochemical properties. Ultrasonic transesterification process gave a maximum yield of 97% by weight of oleander biodiesel along with improved physiochemical characteristics. Therefore, it is concluded that ultrasonic method is the most effective method for converting crude oleander oil into biodiesel.     

Thin layer electrolyte impregnation into porous anode-supported fuel cell by ultrasonic spray pyrolysis

An ultrasonic spray pyrolyzed thin layer electrolyte impregnates into a porous anode and extends the depth of triple phase boundaries (TPBs) to several times that of the dense thin-layer thickness. This process, without an anode functional layer, improves the conventional repeated impregnation that results in anode porosity blocking and difficult extending to the anode/electrolyte interface. A (La_0.75Sr_0.2Ba_0.05)_0.175Ce_0.825O_1.981 (LSBC) electrolyte thin layer (iLSBC) is deposited on a porous La_0.3Sr_0.7TiO₃ (LST) anode by ultrasonic spray pyrolysis using 0.1 M LSBC precursor at a substrate temperature of 450 °C, while controlling the solution flow rate and spray volume. The spray pyrolyzed iLSBC/LST half-cells co-fired at 1350 °C/6 h exhibit no evident second phase in the X-ray diffraction analysis. The depth profile of iLSBC/LST indicates an approximately 2.75 μm-thick layer with the Ce element line scanning showing a significantly higher impregnation depth. Slight lattice distortion among the impregnated region may be due to the iLSBC diffusion, resulting in the extension of the TPB area and length. The half-cell reaches a low impedance and achieves a power density of 377 mW/cm² at 750 °C.     

基于连续超声波测量气泡参数的实验研究

研究了一种基于声阻原理的测量液体中毫米级气泡粒度、速度并能同时计数的方法。超声波在含气泡液体中传播时,由于气泡和液体间声阻抗差极大,致使其在气泡表面发生强烈反射/散射,阻碍了声波通过。利用频率为200 kHz的超声连续波,采用一发一收式测量,对某润滑油中的气泡进行测量,实验中气泡大小为2～6 mm,速度在0.10～0.30 m·s~(-1)之间,气泡通过测量区的频率为5～10 Hz。通过分析实验数据的波形幅值与气泡粒度、波形转变时间与气泡速度、通过数之间的对应关系,并利用图像法进行了标定和校验。实验结果表明,利用连续超声波可以测得油中连续通过的气泡,其原理和测量装置简单,测量结果稳定。     

Analysis of thermal diffusivity by parameter estimation in converging thermal-wave technique

The in-plane thermal diffusivity of several kinds of metal foils was measured by the converging thermal-wave technique. This is a typical technique which can obtain the thermal diffusivity by taking the temperature evolution at the center of pulsed annular laser beam when the beam irradiates the surface of the samples. The thermal diffusivity is normally calculated using the maximum time t_m or half-maximum time t_1/2 when the temperature evolution is half and maximum, respectively, however, the rapid temperature increase and the nonlinearity of the infrared detector in the earlier part, convection heat loss from the sample surface, and some times the low signal to noise ratio can produce errors. In this study, a nonlinear least-square regression was performed to estimate the optimal values of several separate parameters by fitting the data to the theoretical equation. The thermal diffusivity of the samples obtained from the optimal values of the estimated parameters was compared with the values from t_m and t_1/2, and the reference values, and it shows that the thermal diffusivity obtained by this parameter estimation technique agrees quite well with the reference data within 3.4% at maximum.     

The investigation of Ag & LaCo0.6Ni0.4O3−δ composites as cathode contact material for intermediate temperature solid oxide fuel cells

The high interface resistance between cathodes and interconnects is a major cause for performance degradation of solid oxide fuel cells (SOFCs). Ag particles were mixed to LaCo_0.6Ni_0.4O_3?δ (LCN) matrix which prevented the silver densification and demonstrated porosity microstructure. The composites with different Ag content were evaluated as cathode contact materials with SUS430 alloy as interconnects. The area specific resistance (ASR) of SUS430/10%Ag & LCN/SUS430 showed the optimal performance in which the ASR was 73 mΩ cm² after 50 h at 750 °C and showed stable property in 10 thermal cycles from 200 °C to 750 °C. The excellent performance of 10%Ag & LCN is attributed to the high conductivity of silver, the stable microstructure of LCN and its good interface adhesion with the interconnect alloy. With 10%Ag & LCN as cathode contact materials, the power density of a single cell reached 0.623 W/cm² at 750 °C and the average degradation is lower than 1% in 3 thermal cycles.     

Estimating the thermal contact conductance of metals by ultrasonic waves

Experiments and numerical simulations were performed to study heat transfer through a contact interface between two metallic solids in order to clarify the effects of roughness and waviness of the contact surface on the mean thermal contact conductance h_m. In the experiments, the characteristics of an ultrasonic transmission were also investigated so as to obtain a relation between h_m and the transmitted sound energy ratio, with and without attenuation, at the contact interface E_r. Ten pairs of brass specimens were tested in an atmospheric environment at room temperature. Each specimen was a cylindrical block 40 mm in diameter and 45 mm long. The contact surface was finished by polishing, grinding, milling, or turning. Mean nominal contact pressures p_m from 0.08 to 1.67 MPa were applied to the test columns. Our analysis takes account of the radial distribution of thermal contact conductance, using numerical solutions of the two-dimensional cylindrical heat conduction equation to simulate the heat transfer experiments. From the simulations, the effects of roughness and waviness on the temperature fields around the contact surfaces and on h_m were clarified. Finally, for flat rough surfaces, an empirical correlation expression between h_m and E_r is proposed. Using this expression, h_m can be predicted to within about ±50%. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(2): 130–141, 1998     

Performance evaluation of machined and powder metallurgically fabricated Crofer®22 APU interconnects for SOFC applications

This study aimed to evaluate the performances of solid oxide fuel cell (SOFC) interconnects made of commercially available bulk Crofer®22 APU alloy through machining, and powder metallurgy approach (P/M) from Crofer®22 APU powders. To this goal, interconnects with 40 × 40 mm² active area were fabricated via both methods. Porosity, coefficient of thermal expansion (CTE) measurements, thermal shock, and single-cell performance tests were carried out. While no porosity was available for the machined interconnect, P/M interconnect was found to have 9% porosity so the increased surface area. CTE values of interconnects, one of the critical parameters in fuel cell stack design, were found to be in close agreement. The maximum power values of 3.12 and 2.97 W were obtained for machined, and P/M interconnects, respectively, at operating temperature condition of 800 °C. Even though P/M interconnect exhibited slightly lower performance in terms of power density, it was concluded that P/M interconnect can be considered a reliable alternative to the conventionally produced machined one due to ease of production, lower scrap rate, and potential to have a better cell performance as it has an increased surface area.     

Approach to calculation time-dependent moisture diffusivity for thin layered biological materials

A method is presented to determine the effective diffusivity from experimental drying kinetics as a time-dependent parameter. The method combines an analytical solution of Fick's equation in which the Fourier number is approximated using the empirical coefficients a and b with a semi-theoretical equation derived for quasi-stationary conditions. The resulting equation has been applied to calculate the effective diffusivity from literature data on the drying of tobacco lamina and sliced celery. The applicability of the method was confirmed by good agreement of calculated and experimental data. It was found that the effective diffusivity for tobacco lamina rises sharply from practically zero at the beginning of drying to a maximum of 9·10^{− 10} m²/s at 89 s, and then gradually decays with time of drying. The same trend was found for sliced celery, but the maximum of the effective diffusivity (1.6·10^{− 7} m²/min) was attained at 56 s. The exact definition of the effective diffusivity vs. drying time identifies two phases of drying: the first phase was characterized by the rising intensity of drying with the maximum at the end of this phase, followed by the phase of slow decrease in the intensity of moisture removal. The rising intensity of drying observed during the initial phase of the process can be explained by warming up of the dried material during the initial phase of drying.     

Effect of low thermal conductivity wick on the performance of compact loop heat pipe

The existing work deals with the evaluation of compact loop heat pipe by means of a low thermal conductivity sintered chrysotile wick to avoid large heat leaks as of the evaporator to the compensation chamber. Accordingly, a wick with low thermal conductivity (0.068–0.098 W/mK) chrysotile powder of a mean particle diameter of 3.4 μm is fabricated through sintering. Nine chrysotile wicks are sintered with different compositions of binders (bentonite and dextrin) and pore-forming agent NaCl at sintering temperatures of 500°C, 600°C, and 700°C with a sintering time of 30 min. The wick properties, for instance, porosity, permeability, wettability, and capillary rise are studied owing to sintering temperature. Consequently, it is observed that a pure chrysotile powdered wick at a sintering temperature of 600°C exhibits a high porosity of 61.8% with permeability 1.04 × 10⁻¹³ m² and a capillary rise of 4.5 cm in 30 s and is considered optimal. This optimal wick is used for performance evaluation in compact loop heat pipe and a decrease of 36.1% in thermal resistance is found when compared with copper mesh wick in a loop heat pipe. The lowermost thermal resistance originates to be 0.147 K/W at 120 W with wall temperature 57.7°C. This indicates that loop heat pipe with sintered chrysotile wick can operate at lower heat loads efficiently when compared with copper mesh wick and as heat load increases a chance of dry out condition occurs. The highest evaporative heat transfer coefficient obtained is 65.7 kW/m² K at a minimum heat load.     

Development of pentadecane/diatomite and pentadecane/sepiolite nanocomposites fabricated by different compounding methods for thermal energy storage

Global warming is one of the most important consequences of excess energy consumption. Phase change materials (PCMs) have prominent advantages in thermal energy storage owing to their high latent heat capacities and small temperature variations during the phase change process. However, leakage is a major problem that limits the use of PCMs. Leakage may occur in encapsulated PCMs or in composites where the PCM is attached to the surface of a supporting material or within the pores of that material. In this study, pentadecane/diatomite and pentadecane/sepiolite nanocomposites were fabricated by using unmodified and microwave‐irradiated diatomite and sepiolite samples and by using different compounding processes, such as direct impregnation, vacuum impregnation, and ultrasonic‐assisted impregnation methods. The microstructures and the chemical and thermal properties of the composites were characterized by scanning electron microscopy, Fourier‐transform infrared spectroscopy, and differential scanning calorimetry. Subsequently, the thermal reliability and stability and the thermal conductivity of the PCM composites were also investigated. A melting temperature of 9.25°C and a latent heat capacity of 58.73 J/g were determined for the pentadecane/diatomite composite that was prepared with the direct impregnation method using a microwave‐treated diatomite sample. The pentadecane/sepiolite composite prepared in the melting temperature range 7.98°C to 8.53°C and latent heat capacity range 41.05 to 46.02 J/g. The results of the thermal analysis indicate that fabricated diatomite‐based or sepiolite‐based PCM composites have good potential as thermal energy storage materials.