Formation of Silicone Carbide Membrane by Radiation Curing of Polycarbosilane and Polyvinylsilane and its Gas Separation up to 250°C

Silicon carbide (SiC) ceramic coating was developed from precursor polymer blend of polycarbosilane and polyvinylsilane on porous alumina substrate by radiation curing. The polymers were crosslinked with oxygen present in the atmosphere during irradiation and pyrolyzed at 850°C in order to convert the polymer into SiC ceramics. Fabricated SiC film was used as a membrane for gas separation, achieving high separation ratios of 206 for H₂ and 241 for He over the nitrogen at 250°C.     

Fabrication and Characterization of Ce0.8Sm0.2O1.9 Microtubular Dual-Structured Electrolyte Membranes for Application in Solid Oxide Fuel Cell Technology

Samaria-doped ceria (SDC, Ce_0.8Sm_0.2O_1.9) ceramic powders of submicrometer size were synthesized by a sol–gel auto-combustion method. From these powders microtubes with a dual structure comprising of a dense layer and a porous substrate layer were fabricated in a single step through a phase inversion/sintering technique. A sintering temperature in excess of 1450°C is required for SDC to achieve gastight microtubes. The mechanical strength of the SDC microtubes increases with increasing sintering temperature and may attain up to 208 MPa when sintered at 1500°C. Electrical impedance spectroscopy studies indicate that the SDC microtubes have electrical conductivities of 4.46 × 10⁻⁴–0.072 S/cm and corresponding activation energy of 81.9 kJ/mol at temperatures between 400° and 800°C. Full fuel cells were fabricated by coating Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) on to the inner surface and a Ni-SDC cermet on to the outer surface of the gastight microtubes to act as the cathode and the anode, respectively. The resultant BSCF|SDC|Ni-SDC microcells have a stable output maximum of 106 mW/cm² at 750°C when hydrogen and air were used as fuel and oxidant gas, respectively.     

Microstructure analysis with TEM for zeolite layer formed in pore of porous alumina substrate

A zeolite membrane for CO₂ gas separation was synthesized on a porous Al₂O₃ substrate by hydrothermal synthesis. Observations using transmission electron microscopy (TEM) showed that zeolite had formed in the pores of the substrate (in the “composite layer”). High-resolution TEM observations showed that the zeolite in the pores was MFI and that the crystal grains of the zeolite were connected directly without any grain boundary phases. This suggests that the composite layer can be pinhole free, so the zeolite membrane could function as an effective gas filter. EDS analysis showed that Al/Si ratio of a zeolite framework was larger in the composite layer. This will be a primary factor in densification of zeolite grains at the composite layer.     

Electrophoretic Deposition of Dense La0.8Sr0.2Ga0.8Mg0.115Co0.085O3−δ Electrolyte Films from Single-Phase Powders for Intermediate Temperature Solid Oxide Fuel Cells

La_0.8Sr_0.2Ga_0.8Mg_0.115Co_0.085O_3−δ (LSGMC) powders were prepared by polymeric precursor synthesis, using either polyvinyl alcohol (PVA) or citric acid (CA) as complexing agents. The powders were synthesized using different ratios between the complexing agent and the cations dissolved in solution. The obtained polymer gel precursors were dried and calcined at temperatures between 1000° and 1450°C. Single-phase LSGMC powders were obtained at a firing temperature of 1450°C, using PVA and a molar ratio between the hydroxylic groups and the total cations of 3:1. Phase-pure LSGMC powders were used to sinter (1490°C, 2 h) thick pellets. The functional properties of LSGMC pellets were assessed by electrochemical impedance spectroscopy. The electrical conductivity values and the apparent activation energies in different transport regimes were in agreement with literature data. The same LSGMC powders were deposited by electrophoretic deposition (EPD) on a green membrane containing lanthanum-doped ceria (La_0.4Ce_0.6O_{2− x} , LDC), a binder, and carbon powders. The LSGMC/LDC bi-layer obtained by EPD was cofired at 1490°C for 2 h. A dense and crack-free 8-μm-thick LSGMC film supported on a porous skeleton of LDC was obtained. The combined use of proper powder synthesis and film processing routes has thus proven to be a viable way for manufacturing anode-supported LSGMC films.     

Hierarchically Porous Ceramics from Diatomite Powders by Pulsed Current Processing

Hierarchically porous ceramic monoliths have been fabricated by pulsed current processing (PCP) of diatomite powders. The partial sintering behavior of the porous diatomite powders during PCP or spark plasma sintering was evaluated at temperatures between 600° and 850°C. Scanning electron microscopy and mercury porosimetry measurements showed that the PCP method was able to bond the diatomite powder together into relatively strong monoliths without significantly destroying the internal pores of the diatomite powder at a temperature range of 700°–750°C. Little fusion at the particle contact points occurred at temperatures below 650°C while the powder showed partial melting and collapse of both the interparticle pores and the internal structure at temperatures above 800°C.     

A Simple and Cost-Effective Approach to Fabrication of Dense Ceramic Membranes on Porous Substrates

A simple and elegant approach to fabrication of dense ceramic membranes on porous substrates, a traditional dry pressing of foam powders, has been developed to reduce the cost of fabrication. Gd-doped ceria (GDC, Gd_0.1Ce_0.9O_1.95) electrolyte membranes as thin as 8 μm are obtained by dry-pressing highly porous GDC powders. The membrane thickness can be readily controlled by the amount of powder. The electrolyte membranes are studied in a solid-oxide fuel cell (SOFC) with air as oxidant and humidified hydrogen (3% H₂O) as fuel. Open-circuit voltages of about 1.0 V are observed, implying that the permeability of the membranes to molecular gases is insignificant. Power densities of 140 and 380 mW/cm² are demonstrated at 500° and 600°C, respectively, representing a significant progress in developing low-temperature SOFCs.     

Synthesis and Characterization of Zinc Sulfide/Gallium Phosphide Nanocomposite Powders

Nanocomposite powders between zinc sulfide and gallium phosphide were synthesized. Monodisperse, submicrometer, spherical, and porous clusters of ZnS nanocrystallites were used as a host material, into which a solution containing a phosphinogallane, [( t -Bu)₂GaPR₂] _x wherein R = i -Pr, x = 2 or R = t -Bu, x = 1, was impregnated. The pure ZnS powders and the composite powders obtained after flash pyrolysis at 600°C were subsequently heat-treated at various temperatures ranging from 500° to 900°C and characterized using a variety of techniques. While pure ZnS powders undergo significant grain growth and morphology change at temperatures as low as 600°C, the composite Powders maintain their integrity up to 800°C.     

Synthesis of Oxynitride Powders via Fluidized-Bed Ammonolysis, Part I: Large, Porous, Silica Particles

Reaction of silica (SiO₂) with triethanolamine (TEA, N(CH₂CH₂OH)₃) and ethylene glycol (EG) under conditions (∼200°C) where byproduct water is removed resulted in the formation of the neutral silatrane glycolate complex, N(CH₂CH₂O)₃SiOCH₂CH₂OH (or TEASiOCH₂CH₂OH) in essentially quantitative yield. Solutions of this neutral precursor in EG, when rapidly pyrolyzed and then oxidized at 500°C, formed porous ceramic powders with high specific surface areas (>500 m²/g). These powders were nitrided via ammonolysis in a fluidized-bed reactor at temperatures of 700°-1000°C. The resulting nitrided powders were characterized by thermal and chemical analyses, diffuse reflectance infrared spectroscopy, gas sorption, and X-ray photoelectron spectroscopy. The apparent activation energy for the nitridation process was determined to be 54 kJ/mol. Following nitridation, the powders were amorphous and had nitrogen contents as high as 21 wt% with retained surface areas >300 m²/g at 1000°C. Under the nitridation conditions used, the extent of nitrogen incorporation correlated linearly with increases in material density. This linearity suggested that the change in density occurred primarily because of changes in coordination that occurred as trivalent nitrogen replaced divalent oxygen in the glass structure and nominally because of viscous flow. The linear density increase also suggested that pore trapping did not occur under these processing conditions. This work serves as a model for ongoing studies on the nitridation of high-surface-area ceramic powders produced by the rapid pyrolysis of mixed-metal TEA alkoxides.     

Strengthening of Porous Mullite and Zirconia CMC Matrices by Evaporation/Condensation

A processing method using evaporation/condensation sintering in an HCl atmosphere was developed for strengthening porous materials without shrinkage. Strengthening without shrinkage is useful in preventing voids and cracks that might be formed during constrained densification, e.g., a porous matrix in a continuous fiber reinforced ceramic composite. Mixtures of mullite and zirconia (monoclinic, tetragonal (3 mol% Y₂O₃), and cubic (8 mol% Y₂O₃)) were studied and exposed to HCl vapor at temperatures up to 1300°C. It was observed that the evaporation–condensation mass transport process produced a porous material with minimal shrinkage. As the crystal structure of the starting tetragonal and cubic zirconia powders did not change after extensive coarsening, it appeared that zirconium and yttrium were transported in the same proportion via evaporation/condensation. The process produced significant coarsening of the zirconia grains, which made the material resistant to densification when heated to 1200°C in air. Because the sintering produced coarsening without shrinkage, the pores also coarsened and a porous microstructure was retained. Mixtures of mullite and zirconia were used because mullite does not densify under the processing conditions used here, namely, heat treatments up to 1300°C. The mullite particles acted as a non-densifying second phase to further inhibit shrinkage when the mullite/zirconia composite was heated up to 1200°C in air. The coarsened cubic zirconia plus mullite mixture had the least densification after heat treatments in air of 100 h at 1200°C.     

Gas permeation properties of a metallic ion-cross-linked PIM-1 thin-film composite membrane supported on a UV-cross-linked porous substrate

Metallic ion-cross-linked polymer of intrinsic microporosity (PIM-1) thin-film composite (TFC) membranes supported on an ultraviolet (UV)-cross-linked porous substrate were fabricated. The UV-cross-linked porous substrate was prepared via polymerization-induced phase separation. The PIM-1 TFC membranes were fabricated via a dip-coating procedure. Metallic ion-cross-linked PIM-1 TFC membranes were fabricated by hydrolyzing the PIM-1 TFC membrane in an alkali solution and then cross-linking it in a multivalent metallic ion solution. The pore size and porous structures were evaluated by low-temperature N₂ adsorption–desorption analysis. The membrane structure was investigated by field-emission scanning electron microscopy. The effects of heat treatment and pore-forming additives on the gas permeance of the UV-cross-linked porous substrate are reported. The effects of different pre-coating treatments on the gas permeance of the metallic ion-cross-linked PIM-1 TFC membrane are also discussed. The metallic ion-cross-linked PIM-1 TFC membrane displayed high CO₂/N₂ selectivity (23) and good CO₂ permeance (1058 GPU).     

Porous, Biphasic CaCO3-Calcium Phosphate Biomedical Cement Scaffolds from Calcite (CaCO3) Powder

Calcite (CaCO₃) is a geologically abundant material, which can be used as a starting material in producing biomedical scaffolds for clinical dental and orthopedic applications. Bone-filling applications require porous, biocompatible, and resorbable materials. Commercially available CaCO₃ powders were physically mixed, for 80–90 s, with an orthophosphoric acid (H₃PO₄) solution, which was partially neutralized to pH 3.2 by adding NaOH, to form biphasic, micro-, and macroporous calcite-apatitic calcium phosphate (Ap-CaP) cement scaffolds of low strength. The resultant carbonated and Na-doped Ap-CaP phase in these scaffolds crystallographically and spectroscopically resembled calcium hydroxyapatite. Upon mixing CaCO₃ powders and the setting solution, carbon dioxide gas was in situ generated and formed the pores. Thus formed scaffolds contained pores over the range of 20–750 μm. Scaffolds were also converted to single-phase Ap-CaP, without altering their porosity, by soaking them in 0.5  M phosphate buffer solutions at 80°C for 36 h in glass bottles. Soerensen's buffer solution was also shown to be able to convert the calcite powders into single-phase Ap-CaP powders upon soaking at 60–80°C. This robust procedure of synthesizing Ap-CaP bioceramics is simple and economical.     

Oxygen Vacancies in Pure Tetragonal Zirconia Powders: Dependence on the Presence of Chlorine during Processing

We have used several experimental methods to study how a large extrinsic oxygen vacancy density in pure tetragonal ZrO₂ powders depends on details of how those powders are made. Samples were made from oxychloride and nitrate precursor solutions. We used perturbed angular correlation spectroscopy to determine in situ phase structure and the density of oxygen vacancies at 1200°C, XRD and SEM to determine the grain size and morphology of samples annealed at temperatures ranging from 200°–1200°C, and neutron activation analysis (NAA) to investigate purity of samples. NAA results showed that samples contain cation impurities at levels <<100 ppm. The XRD and SEM measurements showed that grains were nanometer-size, had a broad distribution, and grew from ∼10 nm at 200°C to ∼1 μm at 1200°C. The most striking process dependence is on presence of chlorine during processing. The grain size and phase above 600°C, and both the morphology and the density of oxygen vacancies at 1200°C were strongly affected by presence of chlorine-containing vapor during annealing. Samples processed in a chlorine-free atmosphere had large well-sintered grains and large (>500 ppm) oxygen vacancy concentrations at 1200°C, whereas samples processed in flowing H₂O/HCl vapor had smaller grains, porous morphology, and small (<100 ppm) vacancy density. All samples were loose powders consisting of single grain particles at <1000°C and multiple-grain particles at 1200°C.     

Microstructure and Humidity-Sensitive Characteristics of α-Fe2O3 Ceramic Sensor

The microstructure and humidity-sensitive characteristics of α -Fe₂O₃ porous ceramic were investigated. Microporous α -Fe₂O₃ powders were obtained by controlling the topotactic decomposition reaction of α -FeOOH. Water vapor adsorption thermogravimetrical experiments were carried out in the relative humidity (rh) range 0% to 95% on the α -Fe₂O₃ powder and the 900°C sintered compact. The microstructure was investigated by SEM, TEM, Hg intrusion, and N₂ adsorption porosimetry techniques. The humidity sensitivity was investigated by the impedance measurements technique in 0% to 95% rh on the compacts sintered at 50°C steps in the 850° to 1100°C range. Humidity response was found to be affected by the microstructure, i.e., the characteristics of the precursor powders and sintering temperatures.     

Joining of Calcium Phosphate Invert Glass-Ceramics on a β-Type Titanium Alloy

A bioactive calcium phosphate invert glass-ceramic containing β-Ca₃(PO₄)₂ crystals could be joined strongly with a Ti–29Nb–13Ta–4.6Zr alloy consisting of a β-titanium phase by heating the metal on which the mother glass powders with a composition 60CaO·30P₂O₅·7Na₂O·3TiO₂ (mol%) were placed, at 800°C for 1 h in air; the tensile joining strength was estimated to be ∼26 MPa on average. A compositionally gradient layer was developed on the metallic substrate during the heating. When the metal with glass powders on it was heated at 850°C in air, the phosphate glassy phase flowed viscously, permeating the oxide layer formed around the surface of the metal, which was thicker than that formed by heating at 800°C; a compositionally gradient layer was not developed, and a strong joining was not realized. The joining between the glass-ceramic and the metal is suggested to be controlled by viscous flow of the glassy phase in the glass-ceramic and by reaction of the glassy phase with the oxide phase formed around the surface layer of the metal.     

Effect of Tungsten and Molybdenum Doping on the Semiconductor–Metallic Transition in Vanadium Dioxide Produced by Evaporative Decomposition of Solutions and Hydrogen Reduction

V_{1– x} W _x O₂ and V_{1– x} Mo _x O₂, 0 < x < 0.03, powders have been produced by the evaporative decomposition of solutions of vanadyl sulfate hydrate with tungsten dioxide dichloride or molybdenum dioxide dichloride in a hydrogen/nitrogen atmosphere. The powders consist primarily of hollow, porous, spherical shells. Differential scanning calorimetric analysis of the powders indicates that the 67°C mono-clinic-tetragonal phase transition in VO₂ shifts to lower temperatures by 23°C/(at.% tungsten) and 6.3°C/(at.% molybdenum).     

Nanopowders of Na0.5K0.5NbO3 Prepared by the Pechini Method

A Pechini-based chemical synthesis route was used to produce powders of Na_0.5K_0.5NbO₃ (NKN). The thermochemistry of the gel was investigated using thermogravimetric analysis-fourier transform infrared (TGA-FTIR) evolved gas analysis; in addition, powder FTIR was used to analyze the gel residues after different heat treatments. The final decomposition of the organic components occurred at ∼650°C. However, hydrated–carbonated secondary phase(s) were detected by FTIR in powders that had been heated at 700°C, indicating that the NKN nanopowders are susceptible to a reaction with atmospheric moisture and carbon dioxide. The NKN particle sizes were in the range 50–150 nm after decomposition at 700°C.     

Electrochemical Synthesis and Sintering of Nanocrystalline Cerium(IV) Oxide Powders

Nanocrystalline CeO₂ powders were prepared electrochemically by the cathodic electrogeneration of base, and their sintering behavior was investigated. X-ray diffraction and transmission electron microscopy revealed that the as-prepared powders were crystalline cerium(IV) oxide with the cubic fluorite structure. The lattice parameter of the electrogenerated material was 0.5419 nm. The powders consisted of nonaggregated, faceted particles. The average crystallite size was a function of the solution temperature. It increased from 10 nm at 29°C to 14 nm at 80°C. Consolidated powders were sintered in air at both a constant heating rate of 10°C/min and under isothermal conditions. The temperature at which sintering started (750°C) for nanocrystalline CeO₂ powders was only about 100°C lower than that of coarser-grained powders (850°C). However, the sintering rate was enhanced. The temperature at which shrinkage stopped was 200°-300°C lower with the nanoscale powder than with micrometer-sized powders. A sintered specimen with 99.8% of theoretical density and a grain size of about 350 nm was obtained by sintering at 1300°C for 2 h.     

Processing and Properties of a Porous Oxide Matrix Composite Reinforced with Continuous Oxide Fibers

A process to manufacture porous oxide matrix/polycrystalline oxide fiber composites was developed and evaluated. The method uses infiltration of fiber cloths with an aqueous slurry of mullite/alumina powders to make prepregs. By careful manipulation of the interparticle pair potential in the slurry, a consolidated slurry with a high particle density is produced with a sufficiently low viscosity to allow efficient infiltration of the fiber tows. Vibration-assisted infiltration of stacked, cloth prepregs in combination with a simple vacuum bag technique produced composites with homogeneous microstructures. The method has the additional advantage of allowing complex shapes to be made. Subsequent infiltration of the powder mixture with an alumina precursor was made to strengthen the matrix. The porous matrix, without fibers, possessed good thermal stability and showed linear shrinkage of 0.9% on heat treatment at 1200°C. Mechanical properties were evaluated in flexural testing in a manner that precluded interlaminar shear failure before failure via the tensile stresses. It was shown that the composite produced by this method was comparable to porous oxide matrix composites manufactured by other processes using the same fibers (N610 and N720). The ratio of notch strength to unnotch strength for a crack to width ratio of 0.5 was 0.7–0.9, indicating moderate notch sensitivity. Interlaminar shear strength, which is dominated by matrix strength, changed from 7 to 12 MPa for matrix porosity ranging from 38% to 43%, respectively. The porous microstructure did not change after aging at 1200°C for 100 h. Heat treatment at 1300°C for 100 h reduced the strength for the N610 and N720 composites by 35% and 20%, respectively, and increased their brittle nature.     

New Uniformly Porous CaZrO3/MgO Composites with Three-Dimensional Network Structure from Natural Dolomite

Porous CaZrO₃/MgO composites with a uniform three-dimensional (3-D) network structure have been successfully synthesized using reactive sintering of highly pure mixtures of natural dolomite (CaMg(CO₃)₂) and synthesized zirconia powders with LiF additive. Equimolar dolomite and zirconia powders doped with 0.5 wt% LiF were cold isostatically pressed at 200 MPa and sintered at 1100–1400°C for 2 h in air. Through the liquid formation via LiF doping, strong necks were formed between constituent particles before completion of the pyrolysis of dolomite, resulting in the formation of a 3-D network structure. During and after the formation of the network structure, CO₂ was given off to form a homogeneous open-pore structure. The pore-size distribution was very narrow (with pore size ∼ 1 μm), and the porosity was controllable (e.g., ∼30%–50%) by changing the sintering temperature. The porous composites can be applied as filter materials with good structural stability at high temperatures.     

Spheroidization of SiC powders and their improvement on the properties of SiC porous ceramics

Spherical SiC powders were prepared at high temperature using commercial SiC powders (4.52 µm) with irregular morphology. The influence of spherical SiC powders on the properties of SiC porous ceramics was investigated. In comparison with the as-received powders, the spheroidized SiC powders exhibited a relatively narrow particle size distribution and better flowability. The spheroidization mechanism of irregular SiC powder is surface diffusion. SiC porous ceramics prepared from spheroidized SiC powders showed more uniform pore size distribution and higher bending strength than that from as-received SiC powders. The improvement in the performance of SiC porous ceramics from spheroidized powder was attributed to tighter stacking of spherical SiC particles. After sintering at 1800 °C, the open porosity, average pore diameter, and bending strength of SiC porous ceramics prepared from spheroidized SiC powder were 39%, 2803.4 nm, and 66.89 MPa, respectively. Hence, SiC porous ceramics prepared from spheroidized SiC powder could be used as membrane for micro-filtration or as support of membrane for ultra/nano-filtration.