Shape-Tailored Hydrothermal Synthesis of CdMoO4 Crystallites on Varying pH Conditions

CdMoO₄ powders with well-controlled shapes, including sphere, cake, cubic sugar, flake, and floccule have been successfully synthesized by a low-temperature (≤90°C) hydrothermal method. The effects of pH, reaction time, and temperature on the crystalline size and shape were explored by using X-ray powder diffraction and scanning electron microscopy. Room-temperature photoluminescence spectra of the as-synthesized CdMoO₄ crystallites were also investigated, which showed that their optical properties obviously relied on the particle size and morphology of CdMoO₄ powders.     

Hydrothermal Synthesis and Characterization of CdWO4 Nanorods

Cadmium tungstate (CdWO₄) nanorods were successfully synthesized by a hydrothermal process at as low a temperature as 70°C. The products were characterized by X-ray powder diffraction, transmission electron microscopy, and photoluminescent spectra techniques. The results showed that the morphology of nanocrystallites significantly varied with the reaction temperature, and CdWO₄ nanorods exhibited a better luminescent property than nanofibers.     

Hydrothermal Synthesis of Nanosized CoAl2O4 on ZnAl2O4 Seed Crystallites

ZnAl₂O₄-seeded CoAl₂O₄, with a core-shell structure, has been prepared under hydrothermal conditions when the Co²⁺ salt solution is substituted by 10% Zn²⁺ as a precursor. The ZnAl₂O₄ seed is generated during the synthesis process. The seeding process can decrease the synthesis temperature from 245° to 230°C and the particle size from 67 to 20 nm. The process can economize the consumption of Co²⁺ and control the particle size effectively.     

Low-Temperature Synthesis of NiFe2O4 by a Hydrothermal Method

Nickel ferrite (NiFe₂O₄) nanoparticles were successfully synthesized via a hydrothermal process and characterized by X-ray diffraction and transmission electron microscope techniques. The effects of reaction temperature, holding time, and RH ratio (isopropyl alcohol/water) were discussed. The NiFe₂O₄ nanoparticles could be obtained at 60°C within 3 h. The crystallization of the spinel ferrites was promoted by the increase in reaction temperature, holding time, and RH ratio.     

Synthesis of Al4SiC4

The conditions necessary for synthesizing Al₄SiC₄ from mixtures of aluminum, silicon, and carbon and kaolin, aluminum, and carbon, as starting materials, were examined in the present study. The standard Gibbs energy of formation for the thermodynamic reaction SiC( s ) + Al₄C₃( s ) = Al₄SiC₄( s ) changed from positive to negative at 1106°C. SiC and Al₄C₃ formed as intermediate products when the mixture of aluminum, silicon, and carbon was heated in argon gas, and Al₄SiC₄ then formed by reaction of the SiC and Al₄C₃ at >1200°C. Al₄C₃, SiO₂, Al₂O₃, SiC, and Al₄O₄C formed as intermediate products when the mixture of kaolin, aluminum, and carbon was heated under vacuum, and Al₄SiC₄ formed from a reaction of those intermediate products at >1600°C.     

DADD-Assisted Hydrothermal Synthesis of t-ZrO2 Nanoparticles

Tetragonal ZrO₂ (t-ZrO₂) nanoparticles (diameter: ca. 10 nm; BET-specific surface area: 100 m²/g) are hydrothermally prepared via crystallization of 1,12-diaminododecane (DADD)-hydrous zirconia gels at 230°C for 3 days. The volume content of monoclinic phase is ca. 0.1. The surface-adsorbed hydrocarbon layer (ca. 3 wt%) is observed by FTIR and TGA. They account for the formation of our t-ZrO₂ nanoparticles with high-phase purity.     

Mineralizer-Assisted Hydrothermal Synthesis and Characterization of BiFeO3 Nanoparticles

BiFeO₃ (BFO) nanoparticles were successfully synthesized by a hydrothermal method assisted by a mineralizer at as low a temperature as 200°C. The products were characterized by X-ray powder diffraction, transmission electron microscope, and physical property measurement system. It was found that KNO₃ as a mineralizer played an important role in the formation of BFO nanoparticles, and the as-synthesized BFO nanoparticles showed an enhanced ferromagnetism at room temperature.     

High-Pressure Synthesis of Perovskites Pb(Li1/4Nb3/4)O3 and Pb(Li1/4Ta3/4)O3

Complex perovskite-type compounds with the general formula Pb(B⁺_1/4B⁵⁺_3/4)O₃, where B⁺= Li⁺ and B⁵⁺= Nb⁵⁺ or Ta⁵⁺, were synthesized using a high-pressure technique and studied by X-ray powder diffraction. The X-ray patterns were indexed on the basis of a cubic cell with a ₀= 4.071 Å for Pb(Li_1/4 Nb_3/4)O₃ and a ₀= 4.052 Å for Pb(Li_1/4Ta_3/4)O₃. Electrical properties of the new perovskites were also studied.     

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.     

Synthesis of Li4SiO4 by a Modified Combustion Method

We report for the first time the synthesis of Li₄SiO₄ by the modified combustion method, a rapid chemical process that takes 5 min for completion. This method uses nonoxidizer compounds instead of nitrate mixtures, which are not always commercially available.
The effects of the following parameters on the production of Li₄SiO₄ were studied: (1) different lithium hydroxide:silicic acid:urea (LiOH:H₂SiO₃:CH₄N₂O) molar ratios; (2) the presence of air flow in the furnace chamber; and (3) the furnace heating temperature. It was found that LiOH:H₂SiO₃:CH₄N₂O molar ratios 6:1:3 heated at 1100°C in the presence of additional air in the muffle chamber formed the best precursors to produce Li₄SiO₄.     

Preparation of ZnGa2O4 Spinel Fine Particles by the Hydrothermal Method

ZnGa₂O₄ fine particles with a single phase of spinel were synthesized from a mixed solution of gallium sulfate and zinc sulfate in the presence of aqueous ammonia under hydrothermal conditions above 180°C. The effects of treatment temperature and ZnO/Ga₂O₃ molar ratio in the starting solution on the crystallite size, morphology, lattice parameter, and chemical composition of the ZnGa₂O₄ spinel particles were examined. Spinel with different morphologies, cubic nanoparticles, and elongated rodlike particles were thought to be formed based on the structure of amorphous gallium hydroxide and needlelike GaO(OH) particles, respectively. By treatment at a higher temperature, these particles with nonstoichiometric composition grew large and thick, and their composition approached ZnO/Ga₂O₃= 1. With an increase in the starting ZnO/Ga₂O₃ molar ratio, the lattice parameter of the synthesized ZnGa₂O₄ spinel approached the reported value for the stoichiometric composition and reached a = 0.8335 nm at ZnO/Ga₂O₃= 1.95 by treatment at 240°C for 50 h.     

Equilibria of SiF4 with SiO2, Be2SiO4, and BeO in Molten Li2BeF4

Equilibrium partial pressures of SiF₄ were measured for the reactions 2SiO₂( c )+2BeF₂( d )⇋SiF₄( g )+Be₂SiO₄( c ) (log P _siF4(mm) = [8.790 - 7620/ T ] ±0.06(500°–640°C)) and Be₂SiO₄( c ) +2BeF₂( d )⇋SiF₄( g ) +4BeO( c )(log P _siF4(mm) = [9.530–9400/T] ±0.04 (700°–780°C)), wherein BeF₂ was present in solution with LiF as molten Li₂BeF₄. The solubility of SiF₄ was low (∼0.04 mol kg^-1 atm^-1) in the melt. The results for the first equilibrium were combined with available thermochemical data to calculate improved Δ H_f and Δ G_f values for phenacite (–497.57 ±2.2 and –470.22±2.2 kcal, respectively, at 298°K). The few measurements above 700°C for the second equilibrium are consistent with the temperature of the subsolidus decomposition of phenacite to BeO and SiO₂ and with the heat of this decomposition as determined by Holm and Kleppa. Below 700°C, the pressures of SiF₄ generated showed an increasing positive deviation from the expression given for the equilibrium involving Be₂SiO₄ and BeO. This deviation might have been caused by the formation of an unidentified phase below 700°C which replaced the BeO; it more likely resulted from a metastable equilibrium involving BeO and SiO₂.     

Lead-Free SrBi4Ti4O15 and Bi4Ti3O12 Material Fabrication Using the Microwave-Assisted Molten Salt Synthesis Method

In this paper, the microwave-assisted molten salt method (MAMSS) and molten salt method (MSS) were used to synthesize SrBi₄Ti₄O₁₅ (SBT). The phase constitution was determined by powder X-ray diffraction and the microstructure of powder was examined by scanning electron microscopy. In contrast to the conventional MSS method, MAMSS produces more distinct plate-like grains and synthesizes both SBT and Bi₄Ti₃O₁₂ (BTO) at 600°C with a 30-min soaking time. The increase of temperature and soaking time can make the plate-like grains of BTO more distinct.     

Synthesis, Characterization, and Consolidation of Si3N4 Obtained from Ammonolysis of SiCl4

Thermal decomposition of silicon diimide, Si(NH)₂, in vacuum resulted in very-high-purity, fine-particle-size, amorphous Si₃N₄ powders. The amorphous powder was isothermally aged at 50° to 100° intervals from 1000° to 1500°C for phase identification. Examination of ir spectra and X-ray diffraction patterns indicated a slow and gradual transition from an amorphous material to a crystalline α-phase occurring at 1200°C for >4 h and/or 1300° to 1400°C for 2 h. As the temperature was increased to ≥1450°C for 2 h, the crystalline β-phase was observed. Phase nucleation and crystallite morphology in this system were studied by electron microscopy and electron diffraction combined with TG as functions of temperature for the inorganic polymer starting materials. Powders prepared in this manner with 4 wt% Mg₃N₂ added as a sintering aid were hot-pressed to high-density fine-grained bodies with uniform microstructures. The optimum hot-pressing condition was 1650°C for 1 h. Silicon concentration steadily increased as the hot-pressing temperature or time was increased. A method for chemical etching for high-density fine-grained Si₃N₄ is described. Electrical measurements between room temperature and ∼500°C indicated dielectric constant and tan δ values of 8.3±0.03 and 0.65±0.05×10⁻², respectively.     

Phase Equilibria in the Systems UF4-ThF4 and Li F-UF4-ThF4

As part of a study of materials potentially useful as fluid fuels for high-temperature reactors, equilibrium diagrams for the condensed systems UF₄–ThF₄ and LiF-UF₄–ThF₄ have been determined. Both thermal analysis and quenching techniques were used with phase identification accomplished by microscopic and X-ray dsraction analyses. A complete series of solid solutions without maximum or minimum is formed by UF₄ and ThF₄. Within the system LiF-UF₄–ThF, there occur three invariant points and seven primary phases of which four are ternary solid solutions. Optical properties for all the solid solutions, fractionation paths for the ternary solid solutions, and compatibility triangles for the ternary invariant points have been determined.     

Hydrothermal Reaction-Sintering of Monoclinic HfO2

Sintered monoclinic HfO₂ bodies were fabricated below the transformation temperature from Hf metal and water by hydrothermal reaction-sintering. Sintering was observed above 900°C under 100 MPa for 3 h. Generally, both the bulk density and the crystallite size of the sintered bodies increased with increasing temperature. Bodies with the maximum relative density (0.98) were obtained by treatment above 1000°C.