Characterization of Nanocrystalline Forsterite Fiber Synthesized via the Sol–Gel Process

An alkoxide sol–gel route was developed to prepare forsterite fibers. Transparent precursor gel fibers were converted to the crystalline phase of forsterite by heating above 550°C. Fibers heated at 1100°C for 2 h showed a porous microstructure with nanocrystalline sizes of about 50–100 nm. The structural evolution of fibrous gel was characterized. Effects of water and ethanol content on microstructures and strengths of fired fibers are also discussed. Some properties of the fired fibers were investigated.     

Structure Development of NiO–YSZ Oxide Mixtures in Simulated Citrate–Nitrate Combustion Synthesis

Yttria-stabilized zirconia (NiO/YSZ) composites were prepared by the modified citrate–nitrate combustion synthesis. The citrate–nitrate combustion proceeded through several consecutive steps. Evolution of structure in the system and its changes were followed up by thermogravimetry-differential scanning calorimetry, X-ray powder diffraction, extended X-ray absorption fine structure, and high resolution transmission electron microscopy analyses of intermediate products prepared at distinct temperatures that correspond to different stages of the combustion process. It was shown that the crystalline structure developed gradually, first with crystallization of nano-sized NiO particles (400°–700°C), which was followed by crystallization of YSZ (800°–900°C). The final composite material after heat treatment at 1100°C comprised of nano-crystals with an average size of 6.5±2 nm.     

Microstructure of Nanocrystalline Yttria-Doped Zirconia Thin Films Obtained by Sol–Gel Processing

Nano- and microcrystalline yttria-stabilized zirconia (YSZ) thin films with a dopant concentration of 8.3±0.3 mol% Y₂O₃ were prepared with a variation in grain size by two orders of magnitude. A sol–gel-based method with consecutive rapid thermal annealing was applied to fabricate YSZ films, resulting in about 400 nm YSZ on sapphire substrates. The average grain sizes were varied between 5 nm and 0.5 μm by heat treatment in the temperature range of 650°–1350°C for 24 h. High-resolution (HRTEM) and conventional transmission electron microscopy analyses confirmed specimens—irrespective of the thermal treatment—consisting of cubic ( c -)ZrO₂ grains with nanoscaled tetragonal precipitates coherently embedded in the cubic matrix. Energy-dispersive X-ray spectroscopy and HRTEM on a large number of specimens yielded a homogeneous yttria concentration within the grains and at the grain boundaries with the absence of impurities, i.e. silica at the grain boundaries.     

Preparation and Characterization of Nanocrystalline Misch-Metal-Substituted Yttrium Iron Garnet Powder by the Sol–Gel Combustion Process

Nanocrystalline Y_{3− x} MM _x Fe₅O₁₂ powders (MM denotes Misch-metal, x =0.0, 0.25, 0.5, 0.75, and 1.0) were synthesized by a sol–gel combustion method. Magnetic properties and crystalline structures were investigated using X-ray diffraction (XRD), a vibrating sample magnetometer (VSM), and a scanning electron microscope. The XRD patterns showed that the single-phase garnet of Y_{3− x} MM _x Fe₅O₁₂ was formed at x values ≤1.0. The saturation magnetization of powders increased with decreasing MM content and reached the maximum value at Y₃Fe₅O₁₂. The crystallite size of powders calcined at 800°C for 3 h was in the range of 38–53 nm.     

不同工艺制备的水合氧化锆凝胶之特性

用水解沉淀法和中和沉淀法两种工艺制备了水合氧化锆凝胶，并与经乙醇脱水处理后的水合氧化锆凝胶的形貌、热性能、相态以及红外吸收光谱作了比较。     

Sol–Gel Combustion Synthesis of Nanocrystalline YAG Powder from Metal-Organic Precursors

Yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) nanocrystalline powders were synthesized by a novel sol–gel combustion process. Yttrium acetate and aluminum sec-butoxide were used as the precursors and triethanolamine was used as the chelating agent and fuel. Thermal and crystallization behaviors of the YAG precursor powders were investigated by thermal gravimetric differential thermal analysis (DTA), Fourier transform infrared spectrum, and X-ray diffraction. The combustion-synthesized powders are amorphous and transform to a pure YAG crystalline phase at 900°C. The crystallization activation energy of amorphous YAG precursor was investigated by variable heating rate DTA. The calculated activation energy is 58.9 KJ/mol. The average crystalline size of heat-treated YAG powders at 900°C is ∼20 nm.     

Synthesis of Nanocrystalline Lanthanum Strontium Manganite Powder by the Urea–Formaldehyde Polymer Gel Combustion Route

Nanocrystalline lanthanum strontium manganite (LSM) powder has been synthesized by combustion of a transparent gel obtained by the polymerization of methylol urea and urea in a solution containing La³⁺, Sr²⁺, and Mn²⁺ (LSM ions). Chemistry of the transparent urea–formaldehyde (UF) polymer gel formation and structure of the gel have been proposed such that the LSM ions act in between the growing UF polymer chains by interacting through NH, OH, and CO groups by co-ordination and prevent polymer self-assembly through inter-chain hydrogen bonding as evidenced from infrared spectrum. Thermally stable structures formed by the decomposition of UF polymer below 300°C undergo combustion in the presence of nitrate oxidant in a temperature range from 350°–450°C. A perovskite LSM phase has been formed by self-sustained combustion of the dried gel initiated with little kerosene. The powder obtained after deagglomeration and calcination at 600°C for 2 h has a D ₅₀ value of 0.19 μm, and the particles are aggregates of crystallites 10–25 nm in size.     

Synthesis of Nanocrystalline Enstatite Fiber Via Alkoxide Sol–Gel Process

An alkoxide sol–gel route to precursors to transparent enstatite fibers is described. The fibrous gels were drawn from acetic acid-modified viscous solutions. The amount of acetic acid was found to be the crucial factor in the spinnability. Infrared studies indicated that the acetate anion behaved as a ligand and modified the molecular structure of the enstatite gel. The fibrous gel was amorphous after drying by X-ray, and began to convert to enstatite at temperature as low as ∼600°C. The enstatite fibers were nanocrystalline up to 1100°C. At 1300°C, transparent and dense enstatite fibers were produced, with the room temperature dielectric constants of around 5.2–5.8 (at 1 MHz).     

Enhanced Permeability and Dielectric Constant of NiZn Ferrite Synthesized in Nanocrystalline Form by a Combustion Method

The performance parameters of Ni_0.5Zn_0.5Fe₂O₄, synthesized in the nanocrystalline form by an autocombustion method, have been investigated. The sample sintered at 1200°C, with Bi₂O₃ as additive shows a very high value of initial permeability μ' _i of >400 at 1 MHz, with low loss. Similarly, a very high dielectric constant is obtained at lower frequencies. The results show that optimum magnetic and electrical properties can be achieved for the NiZn ferrite nanocrystalline powders synthesized by the present autocombustion method and sintered at a relatively lower temperature of 1200°C when compared with a temperature of 1400°C required for the materials synthesized by the conventional ceramic method.     

Ion-Beam-Induced Densification of Zirconia Sol–Gel Thin Films

Densification of sol–gel zirconia films was achieved using ion implantation instead of conventional heat treatment. Densification was observed with Xe⁺-ion doses as low as 10¹⁴ ions/cm². Ion implantation resulted in hydrogen and carbon losses as measured by forward recoil elastic spectrometry (FRES) and Rutherford backscattering spectrometry (RBS) using ¹²C(α,α)¹²C resonance, respectively. The density and chemical composition of the highest-dose implanted sol–gel films were comparable with those obtained by high-temperature sintering. The chemical and microstructural modifications in the films were attributed to both electronic interactions and nuclear collisions between the Xe⁺ ions and the target atoms.     

Sintering Behavior of Nanocrystalline Zirconia Prepared by Chemical Vapor Synthesis

Powder synthesis and ceramic processing methods have to be improved to take full advantages of new, improved properties of nanocrystalline ceramics. Sintered nanocrystalline ceramics of pure, undoped zirconia are formed from nanocrystalline powder of optimized quality obtained by the chemical vapor synthesis (CVS) method. The as-synthesized CVS ZrO₂ powder is nonagglomerated with a crystallite size of about 5 nm, narrow size distribution, and high crystallinity. On uniaxial compaction a transparent green body of ultrafine, uniform microstructure and narrow pore size distribution corresponding to the grain size distribution is formed, which is sintered under vacuum at 950°C into a transparent, fully dense ZrO₂ ceramic with a grain size of 60 nm.     

Consolidation of Alumina—Zirconia Mixtures by a Colloidal Process

The relationship between the dispersion of colloidal powder particles in Al₂O₃–ZrO₂ suspensions and the microstructures of consolidated compacts was examined. Suspensions were prepared from Al₂O₃ powder and ZrO₂ sol with average particle sizes of 390 and 62 nm, respectively. The dispersion was controlled by pH and salt concentration adjustments. The compacts composed of completely separated Al₂O₃ and ZrO₂ layers were obtained from well-dispersed suspensions with pH values below about 4 and salt concentration of 0.0527 M. An increase in pH or salt concentration resulted in macroscopically uniform compacts. The compacts made from suspensions with pH values above about 7, however, were composed of a mixture of Al₂O₃ and ZrO₂ agglomerates, with one acting as a matrix and the other a dispersed phase. Suspensions with a pH value of 4.5 and optimum salt concentrations resulted in compacts with microscopically uniform microstructure. Above or below these salt concentrations, ZrO₂ agglomerates were distributed in an Al₂O₃ matrix. The optimum concentration was dependent on solid content. In addition, the dispersion of mixed suspensions was compared with those of single-component suspensions. The ZrO₂ particles formed three-dimensional networks during agglomeration, which reduced the component separation in suspensions and during consolidation.     

Hot Pressing of Infrared‐Transparent Y2O3–MgO Nanocomposites Using Sol–Gel Combustion Synthesized Powders

A sol–gel combustion method has been used to synthesize Y₂O₃–50 vol%MgO composite nanopowders. Solutions of the precursor nitrates were mixed with citric acid and ethylene glycol, heated from 200°C to a predetermined temperature gradually, giving nanocrystalline ceramic powders. The influence of the ratio of yttrium nitrate to the whole precursor mixture and the holding temperature on the properties of the composite nanopowder was investigated using a combination of thermal analysis, X‐ray diffraction, specific surface area analysis, and scanning electron microscopy techniques. When the ratio of yttrium nitrate to the whole precursor mixture reaches 22.5 mol%, the average particle size of synthesized composite nanopowder is 13 nm and the specific surface area is 45.9 m²/g. Then the synthesized Y₂O₃–MgO composite nanopowder was consolidated by the hot‐pressing technique at 1200°C with different dwell time. As a result, the nanocomposite ceramic prepared with a dwell time of 60 min got the highest transmittance of 75% at 5 μm wavelength. The cut‐off wavelength of Y₂O₃–MgO nanocomposite ceramic reaches 9.8 μm, which is superior to other mid‐IR transparent materials. In addition, the fabricated sample is more or less transparent in visible wavelengths and the transmittance at 0.8 μm is as high as 14.5%.     

溶胶-凝胶法合成BaAl_2Si_2O_8超细粉末的研究

以Ｂａ（ＯＨ）２·８Ｈ２Ｏ，Ａｌ（ＮＯ３）３·９Ｈ２Ｏ，ＴＥＯＳ为原料，采用溶胶凝胶法制备ＢａＯＡｌ２Ｏ３ＳｉＯ２三元系粉末，研究了不同因素对生成稳定溶胶和胶化时间的影响，用ＴＧＤＴＡ、ＸＲＤ研究了溶胶凝胶的热处理过程，用ＴＥＭ观察了粉体的粒径。     

Tetragonal-to-Monoclinic Phase Transitions in Nanocrystalline Rare-Earth-Stabilized Zirconia Prepared by a Mild Hydrothermal Method

Nanocrystalline 4-mol%-Sc₂O₃-stabilized zirconia (4ScSZ) and 4-mol%-Y₂O₃-stabilized zirconia (4YSZ) powders were prepared by a mild urea-based hydrothermal method. The as-prepared 4ScSZ and 4YSZ powders behaved with different tetragonal–monoclinic ( t – m ) transitions on calcination at temperatures between 400° and 1400°C. For the as-calcined 4ScSZ samples, the monoclinic phase fraction varies discontinuously with increasing temperature, i.e., first increases, then decreases, and finally increases again; whereas the monoclinic phase content reduces monotonously for the as-calcined 4YSZ samples, and only tetragonal phase is present over 1000°C. Such interesting results can be explained satisfactorily by considering the combined influences of crystallite size effect, microstrain, and the stabilization effect of the dopant. The microstrain relaxation is mainly responsible for the unusual phase transition in the 4ScSZ samples, while for the 4YSZ samples, the microstrain effect and crystallite size effect can be masked by the stabilization effect of the Y₂O₃ dopant due to its stronger stabilization capability.     

Nanostructured Energetic Composites of CL‐20 and Binders Synthesized by Sol Gel Methods

Monolithic energetic gels were prepared in acetone by separately cross‐linking the single precursors, glycidyl azide polyol (GAP polyol polyol), nitrocellulose (NC, 12% N), and tris(hydroxymethyl)nitromethane (THMNM) and the mixed precursors (GAP polyol+NC) and (GAP polyol+THMNM) with hexamethylene diisocyanate (HDI). THMNM functions as a chain extender. The synthesis conditions were optimized according to precursor mass ratio, cross‐linking agent, solvent, catalyst concentration, and containers with various surface‐to‐volume ratios. The concentrations of reactants and cure catalyst are the most important factors. The composite energetic materials with a high degree of homogeneity were synthesized by trapping hexanitrohexazaisowurtzitane (CL‐20) on the nano scale in the energetic polymer gels using sol gel processing with a modified freeze‐drying procedure. Loadings up to 85%, 93%, and 90% by weight of CL‐20 yielded, respectively, monolithic gels for GAP/HDI, NC/HDI, and THMNM/HDI. 90% CL‐20 can be loaded into gels of the mixed precursors of (GAP polyol+NC) and (GAP polyol+THMNM). The energetic gels and composites were characterized using FT‐IR spectroscopy, DSC, SEM, and sensitivity to drop weight impact. The sensitivity of CL‐20 is reduced in the energetic nanocomposites.     

柠檬酸盐凝胶自燃烧法制备纳米磷酸氧钛钾

采用柠檬酸盐凝胶自燃烧法成功制备出了磷酸氧钦钾(KTP)纳米晶粉末.XRD结果显示,通过凝胶自燃烧可直接合成具有正交晶系的KTP粉末,采用TEM观赛KTP晶粒大小为40～50nm.     

氢化燃烧合成镁基贮氢合金的研究进展

系统综述了氢化燃烧法制备镁基贮氢合金Mg2Ni的研究进展，包括其制备原理、与其它制备方法的比较、影响氢化燃烧的因素以及材料的贮氢性能。同时简要介绍了氢化燃烧合成Mg-Fe、Mg-Co和Mg-Ni-Cu等贮氢合金的一些研究成果。     

Structural Aspects of SBA-1 Cubic Mesoporous Silica Synthesized via a Sol–Gel Process Using a Silatrane Precursor

Silatrane prepared from fumed silica and triethanolamine was used as a precursor for SBA-1 synthesis at room temperature using cationic surfactants, derived from alkyltrimethylammonium bromides, C _n TMAB ( n =14, 16, and 18), as templates in dilute solutions. The influence of acidity, alkyl chain length of the surfactant, and synthesis temperature was studied. The shape of the SBA-1 crystals was dependent on the alkyl chain length of the surfactant. At a high surfactant concentration and an elevated reaction temperature (50°C), three-dimensionally ordered mesopores were invariably produced. Both X-ray diffraction and transmission electron microscopic results showed characteristics of the three-dimensional cubic structure. Scanning electron microscopic images of SBA-1 indicated the crystalline-like appearance of an octadecahedron (18-hedron) consistent with six square {100} and 12 hexagonal {110} planes with a cubic symmetry. The surface area of the product was as high as 1000–1500 m²/g with an adsorption volume of 0.6–1.0 cm³/g.     

Emission of Pollutants from Glycine–Nitrate Combustion Synthesis Processes

Four ceramic powders were produced using the glycine–nitrate process: lanthanum-doped barium cobaltite, ceria, magnesia, and strontium-doped lanthanum chromite (LSC). Glycine-to-nitrate ratios from 0.25 to 1 were investigated. During the combustion synthesis process, careful collection of process off-gas was followed by detailed gas analyses to determine product gas composition. All of the synthesis processes produced pure phase ceramic powders, but also produced criteria pollutant emissions levels that were significant enough (up to 4500 ppm of NO _x and 9000 ppm of carbon monoxide) to warrant consideration. Equilibrium and chemical kinetic computations are used to determine the implications of the current findings.