Crystallization behaviors of the hydrogenated Zr-rich amorphous ZrxNi1−x:H alloys and the effects of titanium substitution

In order to investigate the effect of hydrogen on the thermal stability of the amorphous Zr_xNi_1−x alloys, the crystallization of the hydrogenated alloys is examined using differential scanning calorimetry. By hydrogenation, the crystallization reaction is accelerated and the cubic ZrH₂ phase is formed at first. At the next step, cubic ZrH₂ is further transformed to the tetragonal structure. The corresponding mechanism is different from that of the as-received state and it is proposed to be related to the motion of nickel atoms, these having smaller atomic radius. When partially substituting titanium, the crystallization temperature and activation energy are greatly increased for both the as-received and the hydrogenated systems. This is thought to be caused by the retardation of the nickel diffusion and by hardening of the amorphous structure elastically, due to the presence of titanium atoms.     

Stress analysis of solid oxide fuel cell anode microstructure reconstructed from focused ion beam tomography

The degradation and ultimately lifetime of solid oxide fuel cells (SOFCs) is determined in part by the stresses generated within the different layers of the device. For fully dense materials such as the electrolyte, when modelling these stresses on a macro-scale the material properties can be considered to be homogeneous (evenly distributed) allowing the prediction of volume average stresses due to differential thermal expansion in the layer. However, detailed stress analysis of real, multiphase porous layers such as those found in SOFC electrodes, on the micron and sub-micron scale has not been possible to date as detailed geometry and convenient methods to generate a finite element model have not been available.In this paper we present work that combines microstructural characterisation of a porous solid oxide fuel cell anode with three dimensional stress analysis to inspect the stresses within the individual phases of the anode, and at phase boundaries. The electrode microstructure has been characterised using focused ion beam (FIB) tomography and the resulting microstructure used to generate a solid mesh of three dimensional tetrahedral elements. A temperature field was applied to simulate the heating of the sample from room temperature (298 K) to operating temperature (1073 K). The maximum principal stress in the nickel phase was found to exceed the yield strength, while the minimum principal stress in the yttria-stabilized zirconia (YSZ) phase was found to exceed the characteristic strength of that volume of YSZ, indicating that the probability of failure of the YSZ matrix is significant.     

Methodology on sizing and selecting thermoelectric cooler from different TEC manufacturers in cooling system design

The search and selection for a suitable thermoelectric cooler (TEC) to optimize a cooling system design can be a tedious task as there are many product ranges from several TEC manufacturers. Although the manufacturers do provide proprietary manuals or electronic search facilities for their products, the process is still cumbersome as these facilities are incompatible. The electronic facilities often have different user interfaces and functionalities, while the manual facilities have different presentations of the performance characteristics. This paper presents a methodology to assist the designer to size and select the TECs from different manufacturers. The approach will allow designers to find quickly and to evaluate the devices from different TEC manufacturers. Based on the approach, the article introduces a new operational framework for an Internet based thermoelectric cooling system design process that would promote the interaction and collaboration between the designers and TEC manufacturers. It is hoped that this work would be useful for the advancement of future tools to assist designers to develop, analyze and optimize thermoelectric cooling system design in minimal time using the latest TECs available on the market.     

A new general model for phase-change heat transfer of waxy crude oil during the ambient-induced cooling process

The thermal process during shutdown (a stoppage state of the pipeline), of which the essence is an irregular phase-change process accompanied by natural convection, non-Newtonian behavior, and sometimes turbulence, is a critical problem in crude oil transportation engineering. An accurate calculation of the thermal process during shutdown is more than necessary for the safety of crude oil pipeline; however, it faces some challenges due to the complexity of the phase change. In this study, the phase change of waxy crude oil during the cooling process is divided into four stages, which includes a pure liquid natural convection, solid/liquid dispersion natural convection, coexistence of dispersion system natural convection and porous media natural convection, and pure porous media convection, according to different heat transfer mechanisms on different stages. Based on this division, a general phase-change heat transfer model is proposed for the thermal calculation of waxy crude oil during shutdown. Compared with the previous research, this model appropriately includes the influences of non-Newtonian behavior, phase evolution as well as turbulence. With the proposed model, the temperature drop characteristic of a sample pipeline is analyzed and the influencing factors are investigated.     

Synthesis and characterization of nanocrystalline ScSZ electrolyte for SOFCs

Sc₂O₃ stabilized zirconia (ScSZ) was processed by combustion synthesis. X-ray diffraction was carried for analyzing the phase and crystallinity of the processed powder. The results indicate the effectiveness of the combustion synthesis process to produce nanocrystalline ScSZ from the precursor salts without any intermediate calcination step. Comparison with the X-ray diffraction pattern of a commercial 8YSZ sample showed that the process is also effective in producing the desired cubic fluorite phase. The powder was compacted and sintered to produce a dense electrolyte pellet. The sintering temperature was found to be considerably lower compared to conventional microcrystalline zirconia ceramics. The electrical conductivity of the ScSZ electrolyte was found to be higher compared to 8YSZ processed under same conditions. The Arrhenius plot of conductivity yielded two activation energies corresponding to low and high temperature regions. The activation energy of the 10ScSz electrolyte was found to be considerably lower than the 8YSZ sample.     

Thermal and crystallization kinetics of yttrium and lanthanum calcium silicate glass sealants for solid oxide fuel cells

The crystallization kinetics of glass sealants is an essential parameter to check the suitability of glass as a sealant. The crystallization kinetic behavior of calcium borosilicate glasses were studied by Differential thermal analysis (DTA), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). These glasses were exposed for different heat treatment durations in oxidizing atmosphere at 800 and 900 °C. XRD results indicate the presence of La₂SiO₅ crystalline phase which increases the thermal expansion of the glasses. However, yttria doped calcium borosilicate glass (CaY) sample could not form any crystalline phase even after 10 h heat treatment at 900 °C. Moreover, the thermal expansion coefficient (TEC), viscosity values and fragility index of the glasses indicate that lanthanum doped calcium borosilicate glass (CaLa) might be a better glass sealant for solid oxide fuel cell as compared to CaY glass.     

Oxidation induced cubic-tetragonal phase transformation in titanium hydride powders

We studied the effect of oxidation temperature and initial hydrogen concentration on the occurrence of cubic (δ) to tetragonal (ε) phase transformation induced by thermal oxidation of titanium hydride powders in air. X-ray diffraction analysis indicates that δ → ε transformation occurs during oxidation at 400 °C, but not at lower (300 °C) or higher (500 °C) oxidation temperatures. The relative amounts of hydrogen loss during oxidation were determined by temperature-programmed desorption mass spectrometry (TPD-MS). The findings suggest that oxygen stabilizes the tetragonal phase (ε), and that the δ/ε phase composition following oxidation depends on both oxygen and hydrogen concentrations. For high oxidation temperatures (500 °C), some degree of hydride decomposition leads to a decrease in H concentration and to destabilization of the tetragonal phase after cooling down to room temperature.     

Geometric optimization of thermoelectric coolers in a confined volume using genetic algorithms

The demand for thermoelectric coolers (TEC) has grown significantly because of the need for a steady, low-temperature operating environment for various electronic devices such as laser diodes, semiconductor equipment, infrared detectors and others. The cooling capacity and its coefficient of performance (COP) are both extremely important in considering applications. Optimizing the dimensions of the TEC legs provides the advantage of increasing the cooling capacity, while simultaneously considering its minimum COP. This study proposed a method of optimizing the dimensions of the TEC legs using genetic algorithms (GAs), to maximize the cooling capacity. A confined volume in which the TEC can be placed and the technological limitation in manufacturing a TEC leg were considered, and three parameters––leg length, leg area and the number of legs––were taken as the variables to be optimized. The constraints of minimum COP and maximum cost of the material were set, and a genetic search was performed to determine the optimal dimensions of the TEC legs. This work reveals that optimizing the dimensions of the TEC can increase its cooling capacity. The results also show that GAs can determine the optimal dimensions according to various input currents and various cold-side operating temperatures.     

Crack propagation of planar and corrugated solid oxide fuel cells during cooling process

The corrugated solid oxide fuel cell (SOFC) can effectively improve energy density and transformation efficiency compared with conventional planar SOFC, but its stability and durability have not been systematically analyzed. The residual stress of SOFC may lead to crack initiation and propagation during cooling process, so stress distributions of planar and corrugated SOFCs are simulated to analyze the location of crack initiation. The materials of electrolyte, anode, and cathode in this paper are yttria‐stabilization zirconia (YSZ), Ni‐YSZ, and strontium‐doped lanthanum manganite (LSM), respectively. The result shows that the edge of cell is more prone to cracking. Therefore, precracks including edge crack and middle crack are introduced into anode‐electrolyte interfaces to investigate crack propagation of two types of SOFCs during cooling process. For corrugated SOFC, the cracks propagate more slowly, and the cell is less prone to interfacial delamination compared with planar SOFC. In addition, the interface energy release rates are obtained to further analyze crack propagation of two types of SOFCs, and the corrugated SOFC has lower energy release rate. The research in this paper provides guidance for stability analysis and lays a foundation for future mechanical analysis of corrugated SOFC.     

Thermo-mechanical stress analyses of solid oxide fuel cell anode based on three-dimensional microstructure reconstruction

During high-temperature operation of a solid oxide fuel cell, the stresses caused by mismatch of thermal expansion coefficients between different materials and external mechanical loads may cause the rising of damage risk of nickel-yttria-stabilized zirconia anode. It is quite difficult to quantify the mechanical characteristics of a composite anode without investigating on the stress distribution in its real microstructure. However, the high operating temperature and extremely complex microstructure in micro-scale determine the high difficulty in in-situ measurement of thermo-mechanical stress distribution. In this work, the microstructures of six different anode samples, fabricated by using identical nickel oxide-yttria-stabilized zirconia powder mixture, are reconstructed in three-dimension based on the dual-beam focused-ion-beam-scanning-electron-microscopy. The three-dimensional thermo-mechanical stress distributions of different microstructures are conducted at operating temperature based on the finite element method. The effects of both thermal expansion coefficients mismatch between nickel and yttria-stabilized zirconia and external mechanical loads are analyzed. The mechanical failure probabilities of yttria-stabilized zirconia phase in different reconstructions are estimated based on the obtained stress distributions to investigate the influence of microstructure characterizations on nickel-yttria-stabilized zirconia anode strength.     

Effective modulus and coefficient of thermal expansion of Ni–YSZ porous cermets

Nickel zirconia, Ni–YSZ, porous cermets used in solid oxide fuel cells can be considered as composite materials consisting of three phases, namely nickel (Ni), ytrria-stabilized zirconia (YSZ), and voids. Based on Ni–YSZ's microstructural features, such as the volume fractions, average particle sizes and their correlations of a physical sample, a three-dimensional stochastic reconstruction method is used to recreate the three-phase composite. The digitally reconstructed microstructure is then transferred into finite element models to obtain the material's corresponding effective elastic modulus and effective coefficient of thermal expansion at various temperatures. Predictions from such a numerical methodology agree well with experimental results.     

Structural and electrical properties of Ni-YSZ cermet materials

Ceramic-metal composites (cermets) containing yttria-stabilized zirconia, YSZ, and Ni particles are commonly used as anode materials in solid oxide fuel cells. The long-term performance of fuel cells is strictly related to both the structural and electrical properties of anode materials. In order to achieve high mixed electrical conductivity and high activity of electrochemical reactions and hydrocarbon fuel reforming, it is necessary to select an appropriate chemical composition and a suitable method of preparation. Materials containing 8 mol% yttria-stabilized zirconia and Ni were prepared by means of two methods: co-precipitation and impregnation. The structure of the materials was characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and porosity studies. The thermal expansion coefficient (TEC) was determined using the dilathometric method. Electrochemical impedance spectroscopy (EIS) and the Wagner polarization method were used to determine electrical conductivity and the electron transference numbers, respectively.     

Heat transfer enhancement for thermal energy storage using metal foams embedded within phase change materials (PCMs)

In this paper the experimental investigation on the solid/liquid phase change (melting and solidification) processes have been carried out. Paraffin wax RT58 is used as phase change material (PCM), in which metal foams are embedded to enhance the heat transfer. During the melting process, the test samples are electrically heated on the bottom surface with a constant heat flux. The PCM with metal foams has been heated from the solid state to the pure liquid phase. The temperature differences between the heated wall and PCM have been analysed to examine the effects of heat flux and metal foam structure (pore size and relative density). Compared to the results of the pure PCM sample, the effect of metal foam on solid/liquid phase change heat transfer is very significant, particularly at the solid zone of PCMs. When the PCM starts melting, natural convection can improve the heat transfer performance, thereby reducing the temperature difference between the wall and PCM. The addition of metal foam can increase the overall heat transfer rate by 3-10 times (depending on the metal foam structures and materials) during the melting process (two-phase zone) and the pure liquid zone. The tests for investigating the solidification process under different cooling conditions (e.g. natural convection and forced convection) have been carried out. The results show that the use of metal foams can make the sample solidified much faster than pure PCM samples, evidenced by the solidification time being reduced by more than half. In addition, a two-dimensional numerical analysis has been carried out for heat transfer enhancement in PCMs by using metal foams, and the prediction results agree reasonably well with the experimental data.     

Effect of nickel-phosphorus interactions on structural integrity of anode-supported solid oxide fuel cells

An integrated experimental/modeling approach was utilized to assess the structural integrity of Ni-yttria-stabilized zirconia (YSZ) porous anode supports during the solid oxide fuel cell (SOFC) operation on coal gas containing trace amounts of phosphorus impurities. Phosphorus was chosen as a typical impurity exhibiting strong interactions with the nickel followed by second phase formation. Tests were performed using Ni-YSZ anode-supported button cells exposed to 0.5-10 ppm of phosphine in synthetic coal gas at 700-800 °C. The extent of Ni-P interactions was determined by a post-test scanning electron microscopy (SEM) analysis. Severe damage to the anode support due to nickel phosphide phase formation and extensive crystal coalescence was revealed, resulting in electric percolation loss. The subsequent finite element stress analyses were conducted using the actual anode support microstructures to assist in degradation mechanism explanation. Volume expansion induced by the Ni phase alteration was found to produce high stress levels such that local failure of the Ni-YSZ anode became possible under the operating conditions.     

Investigation of electrical and mechanical properties of tetragonal/cubic zirconia composite electrolytes prepared through stabilizer coating method

In this study tetragonal/cubic zirconia based composite electrolytes were prepared through stabilizer coating technique. Microstructures were characterized by XRD and SEM. The electrical properties were studied by impedance spectroscopy as a function of temperature. Results of electrical conductivity measurement proved that by increasing the 3YSZ content of the composite samples, the electrical conductivity at high temperatures (T > 550 °C) decreased while at low temperatures (T < 550 °C) the conductivity increased. It was shown that stabilizer coating is an attractive technique to prepare high performance solid electrolytes with improved electrical and mechanical properties with low level of consumed stabilizer.     

Effect of buckling on the cooling performance of free-standing planar thermoelectric coolers

This article investigates the effect of buckling on the cooling performance of planar thermoelectric (TE) coolers (TECs). The TEC is made up of n-type and p-type TE elements with large length-to-thickness ratio. Each TE element is modeled as a fixed–fixed thin plate. Theoretical model for the solutions of temperature and electric potential fields of the TE element after buckling is established. The corresponding coefficient of performance (COP) that indicates the cooling performance of TEC is also given. Influence of Seebeck coefficient, thermal conductivity, temperature difference, and the ratio of length-to-thickness on the cooling performance are discussed. It is found that buckling of TEC will reduce its cooling performance. A bigger Seebeck coefficient and smaller thermal conductivity can both improve the value of COP. It is also found that there is no maximum COP when the temperature difference across the TEC is zero. However, the effect of buckling on the cooling performance of TEC can be ignored if the TEC achieves the maximum COP. The peak value of COP is independent of the ratio of length-to-thickness of the TEC. An optimized value of the electric current corresponding to the maximum COP of the TEC is obtained.     

Functionally-graded composite cathodes for durable and high performance solid oxide fuel cells

A double-layer dual-composite cathode is fabricated and has an ideal cathode microstructure with large electrochemical active sites and enhanced the durability in solid oxide fuel cells (SOFCs). The insertion of a yttria-stabilized zirconia (YSZ)-rich functional layer between the electrolyte and the electrode allows for a graded transition of the YSZ phase, which enhances ionic percolation and minimizes the thermal expansion coefficient mismatch. Electrochemical measurements reveal that the double-layer composite cathode exhibits improved cathodic performance and long-term stability compared with a single-layer composite cathode. A cell with a well-controlled cathode maintains nearly constant interfacial polarization resistance during an 80 h accelerated lifetime test.     

A numerical study on the performance of miniature thermoelectric cooler affected by Thomson effect

Miniature thermoelectric cooler (TEC) has been considered as a promising device to achieve effective cooling in microprocessors and other small-scale equipments. To understand the performances of miniature thermoelectric coolers, three different thermoelectric cooling modules are analyzed through a three-dimensional numerical simulation. Particular attention is paid to the influence of scaling effect and Thomson effect on the cooling performance. Two different temperature differences of 0 and 10 K between the top and the bottom copper interconnectors are taken into account. In addition, three different modules of TEC, consisting of 8, 20 and 40 pairs of TEC, are investigated where a single TEC length decreases from 500 to 100 μm with the condition of fixed ratio of cross-sectional area to length. It is observed that when the number of pairs of TEC in a module is increased from 8 to 40, the cooling power of the module grows drastically, revealing that the miniature TEC is a desirable route to achieve thermoelectric cooling with high performance. The obtained results also suggest that the cooling power of a thermoelectric cooling module with Thomson effect can be improved by a factor of 5-7%, and the higher the number of pairs of TEC, the better the improvement of the Thomson effect on the cooling power.     

Effect of alumina on the curvature, Young's modulus, thermal expansion coefficient and residual stress of planar solid oxide fuel cells

Planar solid oxide fuel cells (SOFCs) are composites consisting of porous and dense functional layers as electrodes and electrolytes, respectively. Because of the thermo-elastic mismatch between the individual layers, residual stresses develop during manufacturing and cause unconstrained cells to warp. The addition of alumina decreases the thermal expansion coefficient (TEC) of the NiO and yttria-stabilized zirconia (YSZ) anode-support material. Correspondingly, the lower TECs have flattened the half cells during fabrication. In addition, the residual stress at room temperature (RT) for samples with more than 4 wt% alumina is only 20% of the residual stress of the samples without alumina, at approximately 100 MPa. The effects of Al₂O₃ on the curvature, Young's modulus, TEC and residual stress of the SOFC with (NiO-YSZ)_1−x(Al₂O₃)_x (x = 1-5 wt%) anode support are discussed in this work.     

Characteristics of solidification and melting in the water‐saturated porous medium cooled from the top (response of solid–liquid interface due to time‐varying cooling temperature)

This paper describes the response of a solid–liquid interface in a water‐saturated porous box to a time‐varying cooling temperature. Spherical soda glass beads with an average diameter of 5 mm constitute a porous matrix. The lower boundary of the matrix is kept at 8°C at all times during the experiments, while the upper plate is set at a temperature, lower than the liquids 0°C. After a steady state is reached, the cooling temperature is varied periodically with a fixed amplitude of 4°C. The solid–liquid interface positions are measured and the characteristic amplitudes and the phase delays are determined for different periods ranging from one hour to ten hours at four different cooling temperatures. It has been found that the amplitude of the interface is proportional to the cooling temperature period length, and that a thicker solid layer causes larger phase delays. The proposed one‐dimensional model has been found appropriate for predicting the response of the horizontally averaged position of the solid–liquid interface. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(5): 330–341, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20015