Rajendra Nath Basu

Correlation between slurry rheology, green density and sintered density of tape cast yttria stabilised zirconia

An attempt has been made to investigate the role of dispersant and powder dimension on the slurry rheology and the corresponding effect on green as well as sintered densities of tape cast yttria stabilized zirconia (YSZ). Two commonly used dispersants, viz. menhaden fish oil (MFO) and phosphate ester (PE) have been used in conjunction with YSZ powders of different particle sizes. It has been found that for this system, PE is a much better dispersant than MFO and the best dispersion is obtained with finer YSZ powders. The slurry rheology has a marked effect on the green as well as subsequent sintered densities of the tapes.

Low temperature sintering of La(Ca) CrO3 prepared by an autoignition process

Abstract Ultrafine powders of calcium substituted lanthanum chromite with average particle size of 0.57 μm, synthesized by a simple process of autoignition of citrate–nitrate gels, can be sintered at relatively low temperatures (1200–1250°C) compared to those normally reported. The sintered samples show adequate electrical conductivity and thermal expansion behavior at 1000°C for using it as an interconnect material for solid oxide fuel cell.

Improved Electrochemical Performance of Li2MnSiO4 / C Composite Synthesized by Combustion Technique

Li(2)MnSiO(4) cathode powders were synthesized by a simple combustion technique using citric acid as a chelating agent. The as-synthesized powder was ballmilled with acetylene black (0-20 wt %) and heated at 700 degrees C in argon atmosphere to form a Li(2)MnSiO(4)/C composite. X-ray powder diffraction indicated the formation of Li(2)MnSiO(4) possessing an orthorhombic crystal structure along with manganese oxide as a minor impurity phase. Field-emission-scanning electron microscopy showed that pristine Li(2)MnSiO(4) consists of large agglomerates of similar to 500 to 800 nm. The addition of acetylene black resulted in a drastic change in morphology for Li(2)MnSiO(4)/C composites consisting of uniform grains of similar to 50 nm. The electrochemical discharge capacity as well as the rate capability of Li(2)MnSiO(4) also improved dramatically with an increasing amount of conducting carbon (acetylene black) in the matrix, and a value as high as 164 mAh g(-1) was obtained at a current density of 0.01 mA/cm(2). Impedance spectroscopy showed that the addition of acetylene black decreases the charge-transfer impedance and checks the growth of cell impedance during cycling. Cyclic voltammetry showed two oxidation/reduction couples at 3.6/2.9 and 4.5/4.3 V with good reversibility.

Glass-Ceramic Sealants for Planar IT-SOFC: A Bilayered Approach for Joining Electrolyte and Metallic Interconnect

To develop suitable sealants for planar anode-supported intermediate temperature solid oxide fuel cells (IT-SOFC) operating at around 800 degrees C, several glass-ceramic compositions based on the RO-Al2O3-SiO2 (R=Ba,Ca) system have been prepared and investigated. Thermal, chemical, and electrical properties of the glass ceramics along with crystallization behavior and bonding characteristics of parent glasses with yttria-stabilized zirconia (YSZ) electrolyte and ferritic steel-based interconnect Crofer22APU are studied. The glass transition temperatures (T-g) of the developed glasses are within 600-635 degrees C. By varying the alkaline-earth metal content, the coefficient of thermal expansion (CTE) values can be tailored between that of YSZ and Crofer22APU. Based on the crystallization behavior, a single glass seal having matching CTE with Crofer22APU and a bilayer glass seal having different CTE values within the range of 10-13x10(-6)/K are found to be effective for metal-metal and metal-electrolyte sealing, respectively. The electrical resistivities of the sealants are quite high (rho(800 degrees C)> 10(5) Omega cm), and under sandwiched condition between two metals, the developed glasses are found to maintain this high resistivity even after 100 h of operation. A low helium leak rate value (< 10(-6) Pa m(2) s(-1)) has been observed for most of the developed glasses. (c) 2008 The Electrochemical Society.

Filter paper templated interconnected nanocrystalline LiMn2O4 with high coulombic efficiency and rate capability

A novel filter paper templating method has been developed to prepare nanocrystalline LiMn2O4. Whatman 42 filter paper impregnated with the precursor gel produces ultrafine powder (80–100 nm) when calcined at 800 °C. The structure and morphology of the powder and the template have been studied in detail by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). A single phase cubic spinel with interconnected powder morphology is found. The electrochemical properties have been studied using 2032 coin type cells in the potential window of 3.4 to 4.5 V versusLi/Li+. Sharp oxidation–reduction peaks at 4.09/3.90V and 4.22/4.04V in the cyclic voltagram indicate high crystallinity and good reversibility of the cathode. A typical cell shows an initial discharge capacity of 97 mAh/g with ∼98% columbic efficiency. Rate capability study of the synthesized cathode has been performed by cycling the cell for 130 cycles at different current densities (0.1–1.5 mA/cm2) and it is observed that 80–100% retention of initial capacity is possible when cycled at 0.1–0.6 mA/cm2. A capacity fading of only 0.03 mAh/g per cycle is observed even at 1.5 mA/cm2 (8C). Thus, we have shown that simple filter paper templating technique can produce nanocrystalline LiMn2O4 with high coulombic efficiency (100–95%) and rate capability.

Materials for Solid Oxide Fuel Cells

Over the past one decade, several cell component materials and their combinations have been attempted to match with the appropriate requirements of solid oxide fuel cells (SOFCs). A large number of cell component materials with superior properties have been developed. The general observation is that most of the technological challenges associated with the development of SOFCs are related to materials science. For example, development of superior oxide-ion conductor electrolyte as well as cost-effective fabrication processes involves tremendous materials challenges. The improvements of the materials properties mostly include electrical conductivity, catalytic activity, stability and thermal expansion coefficient. Of late, significant improvements have also been made in the area of fast oxide-ion conductors. These oxide-ion conductors show extraordinarily high electrical conductivity compared to traditional zirconia-electrolyte. This helps SOFC not only to operate at lower temperature but also minimizes the polarization losses which is the key factor for a high performance cell (high power density or power per unit area). The differences between the operating cell voltage and the expected reversible voltage is termed as polarization or overpotential. More clear understanding of the fundamentals of these materials has been published by several groups through numerous articles (Steele 1993, 2000, Mogensen et al. 2000, Goodenough 2003, Singhal et al. 2003, Stover et al. 2003, Kilner 2005). Reduction of electrolyte thickness also has tremendous advantages particularly from the technological point of view. Lower the electrolyte thickness lower is the internal resistance of the electrolyte, which in turn helps the cell to operate at a considerably lower temperature. The current research trend undoubtedly is more focused towards the development of high performance SOFC at low temperature (650°C and below). For making such high performance cell, inter facial contacts between two adjacent cell components is very critical. Therefore, an excellent compatibility (connectivity) between electrolyte and electrodes, and also with the interconnect (while stacking) is absolutely necessary.

Preparation of Sr-substituted LaMnO3 thick films as cathode for solid oxide fuel cell

Thick films of Sr-substituted LaMnO3 (LSM) were deposited on impervious YSZ electrolyte by the screen printing technique. The LSM powders used in this investigation were synthesized by autoignition of carboxylate-nitrate gel. Multiple coatings were applied to increase the thickness of LSM layer. Effects of coating thickness and sintering temperature on the microstructure and electrical properties of the films were studied. The properties of the films obtained from the present study, fulfill the requirements of the cathode of a solid oxide fuel cell.

Influence of ZnO on the crystallization kinetics and properties of diopside-Ca-Tschermak based glasses and glass-ceramics

We report on the influence of ZnO on the structural, thermal, and crystallization behavior of the diopside-Ca-Tschermak based glasses, and on the processing, microstructure, and the properties of the sintered glass ceramics. Four glasses with nominal compositions of CaMg0.8Al0.4Si1.8O6, CaMg0.75Zn0.05Al0.4Si1.8O6, CaMg0.70Zn0.10Al0.4Si1.8O6, and CaMg0.60Zn0.20Al0.4Si1.8O6 were obtained by melting at 1580 °C for 1 h. Structural and thermal behavior of the glasses was investigated by Fourier-transform infrared spectroscopy, density measurements, dilatometry, and differential thermal analysis. Nonisothermal crystallization kinetics has been employed to study the crystallization mechanism in the glasses. Sintering, crystallization, microstructure, and properties of the glass ceramics were investigated under nonisothermal heating conditions in the temperature range of 850–1000 °C.

Pyrolytic Carbon Deposition on Porous Cathode Tubes and Its Use as an Interlayer for Solid Oxide Fuel Cell Zirconia Electrolyte Fabrication

A high-quality fugitive pyrolytic carbon (PyC) film is used as an interlayer material in fabricating solid oxide fuel cells (SOFCs) by electrophoretic deposition (EPD). The carbon must both be highly conducting and burn away cleanly: hence, a graphitic or pyrolytic carbon is required. In this investigation, optimum conditions for PyC deposition from propylene pyrolysis (i.e., CVD) were ascertained. Propylene was decomposed at temperatures of 725, 775, and 825°C and at 1 atm pressure with different residence times (10-20 s) onto a porous LaMnO 3 tube surface. Depending on the temperature and residence time, two different ranges of carbon deposit reactivities were obtained, corresponding to disordered and ordered carbon states, respectively. Temperature-programmed oxidation, scanning electron microscopy, and micro-Raman spectroscopy analyses indicate that the creation of uniform and sufficiently thick films with the highest degree of atomic ordering occurs at temperatures between 725 and 775°C and a gas residence of about 14 s. Notably, the same carbon deposition conditions produced crack-free and gastight YSZ films (15-20 μm) during the fabrication of SOFCs by EPD and subsequent sintering.

Critical current in electrophoretically deposited thick films of YBCO superconductor

Abstract Thick films of YB 2 Cu 3 O 7 − δ (YBCO) superconductor have been prepared by the electrophoretic deposition technique using flexible silver strip as the substrate. Effects of several deposition parameters, e.g., time of deposition, particle size of the powder, number of layers deposited, etc., on the various properties of the films have been studied in detail. Microstructural investigation of the films indicates extensive grain growth and recrystallization phenomena particularly in films of large thickness. The microstructural morphology changes significantly as the thickness decreases. The transport J c (at 77 K, B =0) increases from ≈450 A/cm 2 for a film thickness of ∼ 65 μ m to > 4000 A/cm 2 for a reduced thickness of ∼ 3 μ m. Possible reasons for this dimensional dependence are discussed.