Synthesis of Ba2NaNb5O15 Powders and Thin Films Using Metal Alkoxides

Transparent and highly oriented Ba₂NaNb₅O₁₅ (BNN) thin films have been prepared by using metal alkoxides. A homogeneous precursor solution was prepared by the controlled reaction of NaOC₂H₅, Nb(OC₂H₅)₅, and barium metal. The BNN precursor included a molecular-level mixture of NaNb(OC₂H₅)₆ and Ba[Nb(OC₂H₅)₆]₂ in ethanol. The alkoxy-derived powder crystallized to a low-temperature phase, and then transformed to orthorhombic BNN (tungsten bronze) at 600°C. BNN precursor films on substrates crystallized to orthorhombic BNN at 800°C via the low-temperature phase. Highly (002) oriented BNN films of tungsten bronze structure were successfully prepared on MgO (100) substrates at 700°C by using BNN underlayer.     

Dehydration of Gels and Glasses in the Systems B2 O3-SiO2 and ZrO2-SiO2 Prepared by the Sol-Gel Process from Metal Alkoxides

Dehydration of gels prepared by the sol-gel process from metal alkoxides in the systems B₂O₃-SiO₂ and ZrO₂-SiO₂ was determined by measuring the shrinkage of the gel on heating. Dehydration was enhanced with increased ZrO₂ content, whereas it decreased with B₂O₃ content. Diffusion of water was also measured in the nonporous glasses obtained by heating the gels. The diffusion rate was independent of the composition of the glass.     

Mechanical-Alloying-Assisted Synthesis of Ti3SiC2 Powder

Mechanical alloying (MA) has been used to synthesize Ti₃SiC₂ powder from the elemental Ti, Si, and C powders. The MA formation conditions of Ti₃SiC₂ were strongly affected by the ball size for the conditions used. MA using large balls (20.6 mm in diameter) enhanced the formation of Ti₃SiC₂, probably via an MA-triggered combustion reaction, but the Ti₃SiC₂ phase was not synthesized only by the MA process using small balls (12.7 mm in diameter). Fine powders containing 95.8 vol% Ti₃SiC₂ can be obtained by annealing the mechanically alloyed powder at relatively low temperatures.     

Preparation of SiO2-TiO2-ZrO2 Glasses from Alkoxides

Relatively large compact glass samples (∼5 mm in diameter) of 65SiO₂·20TiO₂·15ZrO₂ and other binary and ternary compositions were prepared by the alkoxide gel method. Prolonged storage of the gels in water before they were dried reduced the residual carbon content of the resulting glasses considerably. Optimal preparation conditions are given, and the microstructure of the gels and glasses obtained is discussed.     

Synthesis of V2O3 Powder by Evaporative Decomposition of Solutions and H2 Reduction

The evaporative decomposition of solutions method was used to form V₂O₅. Spraying above the congruent melting temperature of V₂O₅ (690°C) resulted in dense spherical particles with a smooth surface. Spraying below the V₂O₅ melting temperature yielded porous V₂O₅ powder with a rough surface. Reduction of the V₂O₅ to V₂O₃ was done in a H₂ atmosphere. Spherical V₂O₃ powder was attained when the reduction temperature was low enough to reduce the V₂O₅ surface before partial sintering (necking) between V₂O₅ particles occurred. The resulting V₂O₃ particle size was smaller than the precursor V₂O₅ powder as expected by the differences in densities between V₂O₅ ( p = 3.36 g/cm³) and V₂O₃ ( p = 4.87 g/cm³).     

Synthesis of Metal Phosphates Using SiP2O7

A new, alternative, route to the synthesis of metal phosphates, using SiP₂O₇ as a reagent, was studied. The reaction of SiP₂O₇ with metal oxides yields a solid mixture of silica and metal phosphates; its reaction with metal fluorides yields solid metal phosphates and gaseous mixtures of SiF₄ and POF₃ .     

Effects of Fast Neutron Irradiation on Thermal Conductivity of Li2O and LiAlO2

Li₂O and LiAlO₂ are two candidates for solid breeder materials in the United States' Fusion Power Program. Critical to breeder design efforts are thermophysical data, the bulk of which have only recently become available, for un-irradiated lithium ceramics. This paper expands the current limited data base by presenting thermal conductivity data between 373 and 1173 K for both materials following fast neutron irradiation. Samples were irradiated at 773 to 1173 K to lithium burnups ≤11.5 × 10²⁰ captures/cm³. Comparisons are made between these data and those from unirradiated archive samples of these same materials.     

Coprecipitation Synthesis of MgO-Doped ZrO2 Nano Powder

Nanosized 10 mol% MgO-doped ZrO₂ powder was prepared via coprecipitation, followed by a washing–drying–calcining treatment. Water, ethanol, and a mixture of water and ethanol (volume ratio 1:1) were, respectively, used as solvents for precipitating and washing. These solvents had great influence on the dispersion of the nanopowders, and hence the sintering behavior in the corresponding ceramics. When ethanol was used, uniformly dispersed MgO-doped ZrO₂ with a good sintering property was obtained. The function of ethanol was to weaken the hydrogen bonds between the adjacent nanoparticles that lead to formation of "hard agglomerates" at the first step when the hydroxide precipitate had just formed.     

ZrO2-Transformation-Toughened Glass-Ceramics Prepared by the Sol-Gel Process from Metal Alkoxides

Glasses of composition 3ZrO₂O · 2SiO₂ were prepared by the sol-gel process from metal alkoxides. Tetragonal ZrO₂ was precipitated by appropriate heat treatment at 1000° to 1200°C. The fracture toughness of these glass-ceramics increased with increasing crystallite size of the tetragonal ZrO₂, reaching ∼5.0 MN/m^3/2 at a size of ∼40 nm. The higher fracture toughness was attributed to tetragonal → monoclinic ZrO₂ transformation toughening.     

金属醇盐水解法制备纳米氧化物LiAlO2

向100mL乙醇和1mL乙酰丙酮混合溶液中加入0.18g锂片,采用金属阳极溶解法控制电流为0.2A电解铝片6h,制得纳米LiAlO2前驱体.将电解液控制pH为9.0直接水解形成凝胶,经洗涤、干燥后在550℃煅烧2h,制得纳米LiAlO2粉体.产物通过傅立叶变换红外光谱(FT-IR)、电子透射显微镜(TEM)、差热-热重分析(TG-DTA)、X射线衍射(XRD)进行表征.结果表明,所得前驱体结构为LiAl(OEt)(4-n)(acac)n(acac为乙酰丙酮基);干凝胶粒径20～30nm,适宜煅烧温度为550℃;煅烧后所得纳米LiAlO2粉体粒径为30nm.     

Low-Temperature Chemical Synthesis of Nanocrystalline MgAl2O4 Spinel Powder

Nanocrystalline MgAl₂O₄ spinel powder was synthesized by pyrolysis of complex compounds of aluminum and magnesium with triethanolamine (TEA). The soluble metal ion–TEA complexes formed the precursor material on complete dehydration of the complexes of aluminum–TEA and magnesium–TEA. Single-phase MgAl₂O₄ spinel powder resulted after heat treatment of the precursor material at 675°C. The precursor and the heat-treated powders were characterized by X-ray diffractometry (XRD), differential thermal and thermogravimetric analysis, and transmission electron microscopy (TEM). The average crystallite size as measured from the X-ray line broadening was around 14 nm and the average particle size from TEM studies was around 20 nm.     

ZrO2 Nanopowders Prepared by Low-Temperature Vapor-Phase Hydrolysis

ZrO₂ powder is prepared by low-temperature vapor-phase hydrolysis of ZrCl₄. TG-DTA, XRD, Raman, BET, and TEM methods are used to investigate the particle size, phase composition, and agglomeration before and after heat treatment. The results show that the as-prepared ZrO₂ powder is characterized by large surface area (150 m²/g), fine grain size (5.8 nm), and weak agglomeration. Additionally, the as-prepared ZrO₂ powder shows predominantly tetragonal phase attributed to a grain size effect. This route is free of powder drying and calcination processes that are essential for wet chemical preparation, contributing to less agglomeration.     

Synthesis of Glass-like Cordierite from Metal Alkoxides and Characterization by 27Al and 29Si MASNMR

A sol-gel process for the preparation of glass-like cordierite and ceramic oxide powder is described. Metal alkoxide precursors were dissolved in 2-methoxyethanol and the metal cations were complexed using 2,4-pentanedione to overcome different hydrolysis rates of metal alkoxides which lead to microscopic inhomogeneities during gelation. Heating the gel in an aerated furnace resulted in ultrafine cordierite powder of stoichiometric composition at a relatively low temperature. Cordierite gel and glass were also prepared by other methods and compared with the above gel. The environments of aluminum and silicon in the glass and gels heated at various temperatures were studied using ²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. Most of the heated gels and the glass showed resonances due to pentacoordinated aluminum along with 4- and 6-coordinated aluminum. The homogeneities of the resulting gels and glass are compared using the MASNMR data.     

Formation of Monodisperse Spherical TiO2 Powders by Thermal Hydrolysis of Ti(SO4)2

Titania powders have been prepared by the thermal hydrolysis of titanium sulfate at 80°C in a mixed solvent of n -propyl alcohol ( n -PrOH) and water. The morphology of the powders was greatly influenced by the volume ratio in n -PrOH to water (RH ratio) of the solvent. An RH ratio of 1.0 was essential for the formation of spherical TiO₂ powders. When hydroxypropyl cellulose (HPC) was used as a steric dispersant, monodisperse spherical powders 0.7 (μm in diameter were produced. As the RH ratio was increased, the zeta potential of the powders was negatively increased and the dielectric constant of the solvent was decreased. However, powders obtained at an RH ratio of 1.0 were observed to have the highest energy barrier. The colloidal stability of the powders in a mixed solvent of n -PrOH and water is discussed. As-precipitated powders were amorphous hydrates of titania and were crystallized by calcination into anatase (>600°C) and rutile (>800°C).     

Mechanism of ZrTiO4 Synthesis by Mechanochemical Processing of TiO2 and ZrO2

High-energy ball milling initiates a solid-state reaction in an equimolar mixture of TiO₂ and ZrO₂. The first stage of ball milling induced the transformation of anatase TiO₂ to high-pressure phase TiO₂ (II), isostructural with ZrTiO₄. The formation of solid solutions monoclinic ZrO₂/TiO₂ and TiO₂ (II)/ZrO₂ was observed in the intermediate stage. Afterward, a nanosized ZrTiO₄ phase was formed in the milled product from the TiO₂ (II)/ZrO₂ solid solution. The sintering of the milled product at a temperature <1100°C was examined in situ by Raman spectroscopy. The full solid-state reaction toward ZrTiO₄ ceramic is completed at a temperature considerably lower than reported in the literature.     

Evolution of the Formation of Inorganic Polymers in the CaO—SiO2—P2O5 System Using Metal Alkoxides

Inorganic polymers provide structural integrity to cements. The CaO—SiO₂—P₂O₅ system has not been studied extensively, but it is important to both cementitious and biological systems. The hydrolytic decomposition of homogeneous mixtures of tetraethyl orthosilicate and calcium isopropoxide with additions of anhydrous phosphoric acid in isopropyl alcohol was used to evaluate the formation of inorganic polymers. XRD, DTA, and TGA were used to characterize the decomposition products. The formation of inorganic polymers at room temperature and pressure was used as a criterion to establish cement-forming compositions within the CaO—SiO₂—P₂O₅ system.     

Synthesis of Nanocrystalline KTiOPO4 Powder by Chemical Method

Nanocrystalline potassium titanyl phosphate (KTiOPO₄, KTP) powder with average particle size of 25–50 nm has been synthesized from aqueous solution of titanyl chloride through coprecipitation method, using aqueous solution of potassium dihydrogen phosphate and potassium carbonate in the presence of triethanolamine (TEA) as a capping agent. In the presence of tetrapotassium pyrophosphate as a capping agent it produces the particles having average size around 70–80 nm. The same sample with average particle size ∼250 nm has also been synthesized without any capping agent. Initial amorphous phase after precipitation requires heat treatment at 500–550°C for 2 h to generate nanocrystalline powders of KTP. Formation of the KTP phase in the heat-treated samples has been confirmed through X-ray powder diffraction, thermogravimetric and differential thermal analysis, ³¹P Magic angle spinning-nuclear magnetic resonance and Fourier transform infrared spectroscopy studies, and surface area measurements.     

Crystallization of Fluorite-Type Solid Solution from Alkoxides in the System Y2O3-TiO2

Fluorite-type solid solutions crystallize directly at lower temperatures from amorphous materials prepared by the simultaneous hydrolysis of yttrium and titanium alkoxides. The lattice constant of the solid solution changes continuously as a function of composition. The kinetics of crystallization of the fluorite phase have been studied by X-ray measurements. The initial stage at each temperature proceeds rapidly in a short time. The final stage can be expressed in terms of the contracting cube equation 1—(1 —f)¹¹³= kt, the activation energy being 184 mol⁻¹. A pyrochlore-type solid solution corresponding to the composition Y₂Ti₂O₇ crystallizes at 8l5° to 900°C.     

Powder Characteristics of Pb(Mg1/3Nb2/3)O3 Prepared by Molten Salt Synthesis

Lead magnesium niobate (Pb(Mg_1/3Nb_2/3)O₃, PMN) was prepared by a molten salt synthesis method. Powder characteristics were greatly influenced by the processing parameters, such as amount of salt, species of salt, and the presence of excess PbO. Grown in sulfate salts, PMN crystallites showed faceted morphology with a bimodal particle-size distribution in which the smaller particles were in the submicrometer range. The presence of excess PbO in the sulfate salts greatly influenced the faceted morphology and resulted in rounded PMN crystallites. The possible origin of particle morphology changes due to the processing parameters is discussed.