Synthesis and thermal behavior of Cu/Y2W3O12 composite

Cu metal matrix composite with Y₂W₃O₁₂ as a thermal expansion compensator was fabricated by high energy ball milling followed by compaction and sintering, and its thermal properties were explored for the potential applications as heat sinks in electronic industries, high precision optics, and space structures. The volume fraction of reinforcement was varied from 40% to 70% in order to tailor the composite for the simultaneous accomplishment of low thermal expansion and high thermal conductivity. The synthesis technique was optimized by varying the parameters like milling time from 1 to 20 h and sintering temperature from 600 to 1000 °C in order to achieve densified composites. The relative density of the composites is found to be around 90% for the 10 h milled powders followed by compaction at a pressure of 700 MPa and sintering at a temperature of 1000 °C. The thermal expansion of the composites exhibits linear behavior in the temperature range 200 to 800 °C and the low coefficient of thermal expansion (CTE) is found to be for Cu–70%Y₂W₃O₁₂ composite whose value, 4.32±0.75×10⁻⁶/°C, matches with that of Si substrate. The thermal conductivities are found to increase with a decrease in the volume fraction of the reinforcement and decrease with an increase in the temperature for all the samples. The experimentally determined CTE and thermal conductivity values are found to be comparable to those predicted by the thermal expansion based Kerner and Turner model and the thermal conductivity based Maxwell model, respectively.     

Graphite blocks with high thermal conductivity derived from natural graphite flake

In this work, natural graphite flake (NG) and mesophase pitch were used as precursor carbons to prepare the graphite blocks, which were doped with Si and Ti powders. After hot-pressed at 2700 °C, we investigated the effect of mean size of NG on properties and microstructure of the graphite blocks. Results showed that both thermal conductivity and flexural strength of the graphite blocks were improved as mean size of NG in raw material increased from 50 to 246 μm. However, a decrease of thermal conductivity was observed when mean size of NG was higher than 246 μm. The density and open porosity were respectively 2.26 g/cm³ and 5.82% when mean size of NG in raw material was 246 μm. The thermal conductivity was enhanced, however, the flexural strength was reduced as hot-pressing temperature increased from 2300 to 3000 °C. The thermal conductivity and flexural strength of the graphite block were respectively 704 W/m K and 21.1 MPa when hot-pressing temperature was 3000 °C. Phase analysis demonstrated there were diffraction peaks of graphite, TiC and α-SiC crystals in the graphite block as the hot-pressing temperature was less than 2500 °C. No SiC crystals were evident when the hot-pressing temperature was 2700 °C or above.     

Thermal diffusivity of 3D C/SiC composites from room temperature to 1400 °C

A 3D C/SiC composite and a bulk CVD SiC material were prepared. The effects of the CVD SiC coating and the heat treatment on the longitudinal and transverse thermal diffusivity of the C/SiC composites were investigated. The thermal diffusivity of the C/SiC composites could be well fitted by a multinomial function from room temperature to 1400 °C which includes a power term, an exponential term and a constant term. The exponential term affected the thermal diffusivity and led to its increase above 1200 °C with activation energy of 77 kcal/mol. The microstructure change in the composites was the reason that the thermal diffusivity was increased above 1200 °C. The longitudinal thermal diffusivity of the composite was twice or more than the transverse one and increased more rapidly by the exponential term. The former was decreased by the CVD SiC coating, but the latter was increased by it. The heat treatment could increase the thermal diffusivity and make the exponential term disappeared in the functions. The functional curve before the treatment intersected that after the treatment at the treatment temperature.     

Measurement of through-plane effective thermal conductivity and contact resistance in PEM fuel cell diffusion media

An experimental study was performed to determine the through-plane thermal conductivity of various gas diffusion layer materials and thermal contact resistance between the gas diffusion layer (GDL) materials and an electrolytic iron surface as a function of compression load and PTFE content at 70 °C. The effective thermal conductivity of commercially available SpectraCarb untreated GDL was found to vary from 0.26 to 0.7 W/(m °C) as the compression load was increased from 0.7 to 13.8 bar. The contact resistance was reduced from 2.4×10⁻⁴ m²°C/W at 0.7 bar to 0.6×10⁻⁴ m²°C/W at 13.8 bar. The PTFE coating seemed to enhance the effective thermal conductivity at low compression loads and degrade effective thermal conductivity at higher compression loads. The presence of microporous layer and PTFE on SolviCore diffusion material reduced the effective thermal conductivity and increased thermal contact resistance as compared with the pure carbon fibers. The effective thermal conductivity was measured to be 0.25 W/(m °C) and 0.52 W/(m °C) at 70 °C, respectively at 0.7 and 13.8 bar for 30%-coated SolviCore GDL with microporous layer. The corresponding thermal contact resistance reduced from 3.6×10⁻⁴ m²°C/W at 0.7 bar to 0.9×10⁻⁴ m²°C/W at 13.8 bar. All GDL materials studied showed non-linear deformation under compression loads. The thermal properties characterized should be useful to help modelers accurately predict the temperature distribution in a fuel cell.     

Facile in situ template synthesis of sulfonated polyimide/mesoporous silica hybrid proton exchange membrane for direct methanol fuel cells

Highly conductive and hydration retentive organic-inorganic hybrid proton exchange membranes for direct methanol fuel cells were synthesized by in situ sol-gel generation of mesoporous silica (mSiO₂) in sulfonated polyimide (SPI) via self-assembly route of organic surfactant templates for the tuning of the architecture of silica. The microstructure and properties of the resulting hybrid membranes were extensively characterized. The mesopores of about 3 nm in silica dispersion phase were formed in the SPI matrix. The existence of the mesoporous structure of silica improved the thermal stability, water-uptake and proton conductivity as well as methanol resistance of the hybrid membranes. The hybrid membrane with 30 wt.% mSiO₂ exhibited the water-uptake of 44.8% at 25 °C, and proton conductivity of 0.214 S cm⁻¹ at 80 °C at RH = 100%, while pure SPI exhibited the values of 40.6% and 0.179 S cm⁻¹ in the same test conditions, respectively. The results suggested that the highly hydrophilic character of Si-OH groups and the large surface area of mSiO₂ should contribute to the improvement of the water-uptake, meanwhile the mesoporous channels may supply the continuous proton conductive pathway in the hybrid membranes.     

Polysulfone ionomers for proton-conducting fuel cell membranes: 2. Sulfophenylated polysulfones and polyphenylsulfones

Polysulfones and polyphenylsulfones having pendant phenyl groups with sulfonic acid units have been prepared by lithiation of the respective polymer, followed by reaction with 2-sulfobenzoic acid cyclic anhydride. The resulting ionomers were cast into membranes and properties such as thermal stability, ion-exchange capacity, water sorption and proton conductivity were evaluated. These membranes proved to have a high thermal stability, with a decomposition temperature between 300 and 350 °C, and a high proton conductivity, 60 mS/cm at 70 °C for a polyphenylsulfone with 0.9 sulfonic acid group per repeating unit measured at 100% relative humidity. Moreover, some of the membranes endured immersion in water at temperatures ranging from 20 to 150 °C without swelling extensively, and therefore kept their mechanical stability under these conditions. It was also shown that these membranes retained a high conductivity up to 150 °C under humidifying conditions. The combination of properties make these membranes potential candidates for fuel cells operating at temperatures above 100 °C.     

Compressive strength degradation in ZrB2-based ultra-high temperature ceramic composites

The high temperature compressive strength behavior of zirconium diboride (ZrB₂)-silicon carbide (SiC) particulate composites containing either carbon powder or SCS-9a silicon carbide fibers was evaluated in air. Constant strain rate compression tests have been performed on these materials at room temperature, 1400, and 1550 °C. The degradation of the mechanical properties as a result of atmospheric air exposure at high temperatures has also been studied as a function of exposure time. The ZrB₂-SiC material shows excellent strength of 3.1 ± 0.2 GPa at room temperature and 0.9 ± 0.1 GPa at 1400 °C when external defects are eliminated by surface finishing. The presence of C is detrimental to the compressive strength of the ZrB₂-SiC-C material, as carbon burns out at high temperatures in air. As-fabricated SCS-9a SiC fiber reinforced ZrB₂-SiC composites contain significant matrix microcracking due to residual thermal stresses, and show poor mechanical properties and oxidation resistance. After exposure to air at high temperatures an external SiO₂ layer is formed, beneath which ZrB₂ oxidizes to ZrO₂. A significant reduction in room temperature strength occurs after 16-24 h of exposure to air at 1400 °C for the ZrB₂-SiC material, while for the ZrB₂-SiC-C composition this reduction is observed after less than 16 h. The thickness of the oxide layer was measured as a function of exposure time and temperatures and the details of oxidation process has been discussed.     

Influence of plasticizer contents on the properties of HEC-based solid polymeric electrolytes

Solid polymeric electrolytes were obtained by the plasticization process of hydroxyethylcellulose (HEC) with different quantities of glycerol and addition of lithium trifluoromethane sulfonate (LiCF₃SO₃) salt. The samples were prepared in the form of transparent films with very good adhesion properties. These films were characterized by X-ray diffraction, thermal analysis (DSC) and UV-NIR spectroscopy. The ionic conductivity measurements were obtained by impedance complex spectroscopy as a function of both salt contents and temperature. The best conductivity values of 1.07 × 10⁻⁵ S/cm at 30 °C and 1.06 × 10⁻⁴ S/cm at 83 °C were obtained for the samples of HEC plasticized with 48% of glycerol and containing [O]/[Li] = 6. These results show that plasticized HEC is a very good material to be used for the preparation of new solid polymeric electrolytes.     

Thermophysical properties of densified pitch based carbon/carbon materials—II. Bidirectional composites

Bidirectional carbon/carbon composites were developed using high-pressure impregnation/carbonization technique with PAN based carbon fabric as reinforcement and coal tar and synthetic pitches as matrix precursors. Microstructure of these composites has been evaluated using scanning electron microscope and polarized light optical microscope. Thermophysical properties i.e. thermal conductivity and specific heat have been evaluated both at room temperature and between 40 and 300 °C. The temperature dependence of thermal diffusion, specific heat and thermal conductivity has been studied and correlated with microstructure of carbon/carbon composites. It is found that the specific heat of carbon/carbon composites shows increase with temperature with an inverse slope in the temperature range of 150-200 °C. Accordingly, though the thermal conductivity decrease with temperature due to increased phonon interactions, it shows reversible action between 150 and 200 °C.     

Irradiation effects on graphite foam

The solid state reactor is an advanced reactor concept that takes advantage of newly developed materials with enhanced heat transfer characteristics to provide an inherently safe, self-regulated heat source. High conductivity graphite foam, developed and produced at Oak Ridge National Laboratory, is being evaluated as a candidate material for the core of basic heat source modules.Irradiation studies at the Oak Ridge National Laboratory High Flux Isotope Reactor were conducted to obtain preliminary data on the effects of neutron damage on the thermal properties and volume change behavior of the graphite foam as a function of neutron dose up to 2.6 displacements per atom at an irradiation temperature of ∼740 °C. Samples were characterized for dimensional and structural changes, and thermal transport as a function of dose. Following the initial effects of the irradiation, the samples were annealed at 1000 and 1200 °C and the thermal diffusivity measured as a function of temperature. A simple microstructural model was developed for graphite foam and, by coupling this model to the known single crystal and polycrystalline irradiation behavior of graphite; a mechanism by which the irradiation-induced volume and dimensional changes in graphite foam may be explained is postulated.     

Thermal properties of carbon fibers at very high temperature

Thermal properties such as specific heat C_p, thermal diffusivity a, and thermal conductivity λ of carbon fibers are important parameters in the behaviour of the carbon/carbon composites. In this study, the specific heat and the thermal diffusivity are measured at very high temperatures (up to 2500 K). The experimental thermal conductivity estimated by the indirect relation λ = aρC_p is presented as a function of the temperature. Validations are carried out on isotropic metallic (tungsten) and ceramic (Al₂O₃) fibers. Measurements were obtained on three carbon fibers (rayon-based, PAN-based and pitch-based). Thermal conductivity results allow us to classify fibers from the most insulated to most conductive. The main result is that insulated carbon fibers have an increasing thermal conductivity when the temperature and the heat treatment temperature rise. Relationships between thermal conductivity and the structural properties (L_c and d₀₀₂) of such carbon fibers are studied. We also describe the influence of heat treatment on the thermal conductivity of carbon fibers.     

Microstructure and ionic conductivity properties of gadolinia doped ceria (GdxCe1−xO2−x/2) electrolytes for intermediate temperature SOFCs prepared by the polyol method

Gd_0.1Ce_0.9O_1.95 and Gd_0.2Ce_0.8O_1.9 powders were prepared through the polyol process without using any protective agent. Microstructural and physical properties of the samples were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry (TG) and impedance analysis methods. The results of the thermogravimetry/differential thermal analysis (TG/DTA) and XRD indicated that a single-phase fluorite structure formed at the relatively low calcination temperature of 500 °C. The XRD patterns of the samples revealed that the crystallite size of the samples increased as calcination temperatures increased. The sintering behavior and ionic conductivity of pellets prepared from gadolinia doped ceria (GDC) powders, which were calcined at 500 °C, were also investigated. The relative densities of the pellets, which were sintered at temperatures above 1300 °C, were higher than 95%. The results of the impedance spectroscopy revealed that the GDC-20 sample that was sintered at 1400 °C exhibited an ionic conductivity of 3.25×10⁻² S cm⁻¹ at 800 °C in air. This result clearly indicates that GDC powder with adequate ionic conductivity can be prepared through the polyol process at low temperatures.     

Chemical stability and electrochemical properties of CaMoO3−δ for SOFC anode

In an effort to develop alternative anode materials based on mixed conducting ceramics capable of offering high mixed ionic-electronic conductivity, stability to redox cycles, and limited activity for carbon formation to Ni/YSZ cermets, CaMoO₃ ceramics for application as a solid oxide fuel cell (SOFC) anode material were synthesized as a function of temperature and oxygen partial pressure (pO₂). CaMoO₃ perovskite-dominant powders were obtained by reducing the CaMoO₄ showing a structure of orthorhombic unit cells with the following lattice parameters: a = 5.45 Å, b = 5.58 Å, and c = 7.78 Å. The equilibrium total conductivity of CaMoO₃, measured by DC 4-probe method in 5% H₂/balance N₂ condition (pO₂ ≈ 10⁻²² atm) at various temperatures, decreased with increasing temperature below 400 °C, indicating metallic properties with an activation energy of 0.028 eV. Between 400 °C and 600 °C, the equilibrium total conductivity slightly increased, and finally sharply decreased at 800 °C. The Mo metal precipitation during measurement was thermodynamically proved by the predominance diagram for CaMoO₃. Finally, a fuel cell with CaMoO₃ anode exhibited poor performance with a maximum power density of only 14 mW/cm² at 900 °C, suggesting that further research is needed to enhance the ionic conductivity and thus improve the catalytic properties.     

Physicochemical properties of ZnCl2-NaCl-KCl eutectic melt

Physicochemical properties of ZnCl₂-NaCl-KCl eutectic melt were studied at 200-300 °C for the first time. Firstly, it was reconfirmed that the eutectic composition is ZnCl₂:NaCl:KCl = 0.6:0.2:0.2 in mole fraction, and that the eutectic temperature is 203 °C. Then, the density, viscosity, and ionic conductivity of the ZnCl₂-NaCl-KCl eutectic melt were measured at 200-300 °C. At 250 °C, the density was 2.43 g cm⁻³, the viscosity was 42.0 cP and the ionic conductivity was 8.53 S m⁻¹. The temperature dependencies of density and ionic conductivity were well fitted by the VTF equations with the same ideal glass transition temperature of 283 K (10 °C). It was found that the melt obeys the fractional Walden's rule which is explained by the decoupling effect. The electrochemical window of the melt was determined to be 1.7 V at 250 °C with the cathode limit being zinc metal deposition and the anode limit being chlorine gas evolution.     

Experimental investigation of thermo-physical properties of platelet mesoporous SBA-15 silica particles dispersed in ethylene glycol and water mixture

This paper presents an experimental investigation of thermophysical properties of platelet mesoporous SBA-15 particles dispersed in 60:40 (v/v) ethylene glycol:water mixture. The effect of weight fraction of particles and temperature is studied on density, viscosity and thermal conductivity of nanofluids. The maximum measured thermal conductivity enhancement reaches up to 22% for the nanofluids containing 5 wt% of SBA-15 at 60 °C. The SBA-15 nanofluids show Newtonian behavior in the tested temperature range. Also, the relative density increases between 0.4% and 2.2% when the weight percent of the nanoparticles varies between 1 and 5 at 60 °C. Structural and morphological characterization of synthesized SBA-15 have been carried out using Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and N₂ adsorption–desorption isotherms methods.     

Thermophysical properties of composite materials in the Si<Subscript>3</Subscript>N<Subscript>4</Subscript> - BN system

The results of a study of the thermophysical properties (thermal diffusivity, heat capacity, heat conductivity, and coefficient of linear thermal expansion) of Si₃N₄ - BN hot-pressed composite (with BN concentration varying from 10 to 60 wt.%) in the temperature range of 20 – 900°C are reported.__________Translated from Novye Ogneupory, No. 10, pp. 47 – 49, October, 2004.     

Thermal diffusivity of three-dimensional needled C/SiC-TaC composites

Three-dimensional needled carbon fiber reinforced SiC-TaC composites were prepared by combination of slurry infiltration and reactive melt infiltration. The thermal diffusivity of the composites with different TaC contents was characterized in the temperature range from room temperature to 1400 °C. The results revealed that the temperature dependent thermal diffusivity behavior was independent on TaC content, while the thermal diffusivity values of the composites were influenced by their microstructures. A model based on lattice vibration and microstructure was used to discuss the thermal diffusivity behavior of the C/SiC-TaC composites.     

Low temperature performance of graphite electrode in Li-ion cells

We studied low temperature performance of Li/graphite cell. Results show that capacity of the graphite electrode falls significantly in the temperature range of 0 to −20 °C. When lithiation and delithiation are both carried out at −20 °C, graphite only retains 12% of the room temperature capacity. However, delithiation capacity of graphite increases to 92% of the room temperature value if the lithiation is carried out at room temperature. We believe that the poor low temperature performance of the cell is due to slow kinetics of lithium ion diffusion in graphite rather than low ionic conductivity of electrolyte and solid electrolyte interface (SEI) on the graphite surface. During lithiation and delithiation processes, lithium ion has the similar apparent chemical diffusion coefficient of 10⁻⁹-10⁻¹⁰ cm²/s at 20 °C, depending on the state of lithiation of graphite. We observed a dramatic decrease in lithium ion diffusivity in the temperature range of 0 to −20 °C, and that at low temperatures of <−20 °C, lithium ion has higher diffusivity in the delithiated graphite than in the lithiated one. We also observed that temperature dependence of cycling behavior of the Li/graphite cell follows the change of lithium ion diffusivity.     

LiNi1/3Mn1/3Co1/3O2 synthesized by the Pechini method for the positive electrode in Li-ion batteries: Material characteristics and electrochemical behaviour

The feasibility of the Pechini method for the preparation of LiNi_1/3Mn_1/3Co_1/3O₂ with characteristics appropriate for Li-ion battery positive electrodes was investigated. The method involves formation of one single chemically homogenous precursor material and thus permits reduced calcination times and minimal lithium evaporation. The physical and electrochemical properties of the materials were investigated with variation in final calcination temperature. Chemical analysis showed that the materials could be prepared with high crystallinity and yet little or no loss of lithium. The material calcined at 1000 °C showed the highest specific capacity—180 mAh g⁻¹ when cycled between 4.5 and 3 V, and it maintained its capacity over 50 cycles. The advantage of this material over those prepared at 800 and 900 °C can probably be attributed to the fact that the degree of crystallinity, crystallite size and size of the primary particles increase with calcination temperature, and that the powder attains a more suitable morphology which promotes electronic connectivity to all of the oxide material. A temperature above 1000 °C should however not be used as indicated by an abrupt change in lattice parameters and decrease in electronic conductivity when going from 1000 to 1050 °C. The Pechini method presents an attractive option for the preparation of LiNi_1/3Mn_1/3Co_1/3O₂ positive electrode material.     

Hybrid proton-conducting membranes for polymer electrolyte fuel cells: Phosphomolybdic acid doped poly(2,5-benzimidazole)—(ABPBI-H3PMo12O40)

The synthesis and characterization of a novel hybrid organic-inorganic material formed by phosphomolybdic acid H₃PMo₁₂O₄₀ (PMo₁₂) and poly(2,5-benzimidazole) (ABPBI) is reported. This material, composed of two proton-conducting components, can be cast in the form of membranes from methanesulfonic acid (MSA) solutions. Upon impregnation with phosphoric acid, the hybrid membranes present higher conductivity than the best ABPBI polymer membranes impregnated in the same conditions. These electrolyte membranes are stable up to 200 °C, and have a proton conductivity of 3 × 10⁻² S cm⁻¹ at 185 °C without humidification. These properties make them very good candidates as membranes for polymer electrolyte membrane fuel cells (PEMFC) at temperatures of 100-200 °C.