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.     

Mineralizing Effect of Li2B4O7 and Na2B4O7 on Magnesium Aluminate Spinel Formation

Lithium borate (Li₂B₄O₇) and sodium borate (Na₂B₄O₇) mineralize spinel formation from stoichiometric MgO and Al₂O₃ between 1000° and 1100°C. Mineralization with both compounds is shown to be mediated by B-containing liquids which form glass on cooling. However, the liquid compositions depend on the type of mineralizer and temperature, suggesting that templated grain growth or dissolution–precipitation mechanisms are operating, one dominating over the other under certain conditions. Na₂B₄O₇-mineralized compositions show predominantly templated grain growth at 1000°C, which changes to dissolution–precipitation at 1100°C, whereas Li₂B₄O₇-mineralized compositions show dissolution–precipitation from 1000°C. Li₂B₄O₇ is a stronger mineralizer as spinel formation is complete with 3 wt% Li₂B₄O₇ at 1000°C and with ≥1.5 wt% addition at 1100°C, whereas Na₂B₄O₇-mineralized compositions are found to retain some unreacted corundum even at 1100°C.     

Phase Equilibrium Studies in the System Iron Oxide-Al2O3-Cr2O3

Subsolidus phase relations in the system iron oride-Al₂O₂-Cr₂O₃ in air and at 1 atm. O₂ pressure have been studied in the. temperature interval 1250° to 1500°C. At temperatures below 1318° C. only sesquioxides with hexagonal corundum structure are present as equilibrium phases. In the temperature interval 1318° to 1410°C. in air and 1318° to 1495° C. at 1 atm. O₂, pressure the monoclinic phase Fe₂O₃. Al₂O₃ with some Cr₂O₃ in solid solution is present in the phase assemblage of certain mixtures. At temperatures above 1380°C. in air and above 1445°C. at 1 atm. O₂ pressure a complex spinel solid solution is one of the phases present in appropriate composition areas of the system. X-ray data relating d- spacing to composition of solid solution phases are given.     

Stability of Mullite as Derived from Equilibria in the System CoO-Al2O3-SiO2

The free energy of reaction for the formation of mullite from its oxide components was derived from equilibrium studies in the system CoO-Al₂O₃-SiO₂. Within this system there appears, at solidus temperature in a certain composition area, the phase assemblage mullite + silica + spinel (= cobalt aluminate) + liquid. Determination of the oxygen pressure of a gas phase at which metallic cobalt precipitates from this phase assemblage and from the phase assemblage spinel (= cobalt aluminate) + corundum in the system CoO-Al₂O₃ permits calculation of ΔG° for the reaction 3Al₂O₃+ 2SiO₂= Al₆Si₂O₁₃. The value obtained at 1422°C is -5.8 kcal.     

Coherent Precipitation in the System MgO-Al2O3-Cr2O3

An isothermal section of the ternary system MgO–Al₂O₃-Cr₂O₃ was determined at 1700°± 15°C to delineate the stability field for spinel crystalline solutions (cs). Crystalline solutions were found between the pseudobinary joins MgAl₂O₄–Cr₂O₃ and MgCr₂O₄-Al₂O₃, and the binary join MgAl₂O₄-MgO. The first two crystalline solutions exhibit cation vacancy models while the latter can probably be designated as a cation interstitial model. Precipitation from spinel cs may proceed directly to an equilibrium phase, (Al_1-xCr_x)₂O₃, with the corundum structure or through a metastable phase of the probable composition Mg(Al_1-xC_r)₂₆O₄₀. The composition and temperature limits were defined where the precipitation occurs via metastable monoclinic phases. The coherency of the metastable monoclinic phase with the spinel cs matrix can be understood by considering volume changes with equivalent numbers of oxygens and known crystallographic orientation relations. Electron probe and metallographic microscope investigations showed no preferential grain boundary precipitation.     

Character of Hydration of 3CaO.Al2O3

The C₃A compacts were hydrated and the reaction was studied by DTA, X-ray diffraction, mercury porosimetry, and volume change analysis. The hexagonal hydroaluminates C₂AH₈ and C₄AH₁₉ formed at 2°, 12°, and 23°C by a direct mechanism between C₃A and H₂O. The hydration reaction at 52° and 80°C was stopped by formation of C₃AH₆ around the C₃A grains. The rate of conversion of the hexagonal hydrates to cubic C₃AH₆ increased with temperature. Volume change analysis confirmed that C₃AH₆ grows epitaxially on the surface of the C₃A grain. The reaction at this surface and the passage of water through the layer of hexagonal hydroaluminates control the overall reaction rate. The conversion of the hexagonal hydrates to C₃AH₆ accelerates the reaction by removing the layer of products from around the C₃A grain by a solution mechanism. At 52° and 80°C, C₃AH₆ may form without the intermediate formation of the hexagonal hydrate.     

Mechanical Activation of Heterogeneous Sol–Gel Precursors for Synthesis of MgAl2O4 Spinel

MgAl₂O₄ spinel precursor was prepared using a heterogeneous sol–gel process. The effect of high-energy milling on the precursor decomposition and spinel formation was investigated. The milling decreased the Al(OH)₃ dehydroxylation temperature from 190° to about 130°C. The activation energy for spinel formation decreased from 688 kJ/mol for the as-prepared precursors to 468 kJ/mol for the precursors milled for 5 h. Milling of the precursor lowered the incipient temperature of spinel formation from 900° to 800°C, and the temperature of complete MgAl₂O₄ spinel formation from >1280° to ∼900°C.     

Direct Electroplating of Lithium Cobalt Oxide Film on Platinum Substrate in 100°–200°C Aqueous Solution

Well crystallized, shape-formed, and electrochemically active lithium cobalt oxide (LiCoO₂) films are electroplated directly on an electron-conducting substrate in an aqueous solution using a soft solution processing that is economical, consumes less energy consuming, and is environmentally friendly. Although LiCoO₂ films are easily and economically prepared without any post-synthesis heat treatment, the estimated film properties show a possibility of using the deposited films as a cathode film for lithium rechargeable microbatteries. In addition, the soft solution processing reveals that an exact understanding of chemical reactions and the proper combination of the chemical reactions can create an advanced synthetic procedure.     

Reducing the Sintering Temperature for MgO–Al2O3Mixtures by Addition of Cryolite (Na3AlF6)

The preparation of near stoichiometric spinel and alumina-rich spinel composites from Al₂O₃and MgO powders with the addition of Na₃AlF₆up to 4 wt% in the temperature range 700°–1600°C was studied; 98 wt% spinel containing 72 wt% Al₂O₃can be produced from the mixture of 72 wt% (50 at.%) Al₂O₃+ 28 wt% (50 at.%) MgO powders with the addition of 1 wt% Na₃AlF₆fired at 1300°C for 1 h. Spinels containing 81–85 wt% Al₂O₃can be produced from either the mixture of 90 wt% (78 at.%) Al₂O₃+ 10 wt% (22 at.%) MgO or the mixture of 95 wt% (88 at.%) Al₂O₃+ 5 wt% (12 at.%) MgO powders with the addition of 4 wt% Na₃AlF₆in the temperature range 1300°–1600°C by using a torch-flame firing for 3 min, followed by quenching in water, while the same system under slow cooling in a furnace results in spinel containing 74–76 wt% Al₂O₃. Microscopic studies indicate that the alumina-rich spinel composites consist of a continuous majority spinel phase and an isolated minority corundum phase, regardless of slow cooling in a furnace or quenching in water.     

Crystal Chemistry and Phase Relations in the System Li2O-Fe2O3-TiO2

Above 755°C, compounds along the spinel join LiFe₅O₈-Li₄Ti₅O₁₂ form a complete solid solution and below that temperature a two-phase region separates the ordered LiFe₅O₈ and the disordered spinel phase. At 800° and 900°C, cubic LiFeO₂ ( ss ) and monoclinic Li_zTi0₃ ( ss ) exist on the monoxide join LiFeO₂-Li₂TiO₃. The distributions of cations in both the spinel and monoxide structures were calculated as a function of equilibrium temperature and composition. Sub-solidus equilibria in the system Li₂O-Fe₂O₃-TiO₂ at 800° and 900°C were determined for compositions containing ∼50 mol% Li₂O.     

TEM Characterization of Nanostructured MgAl2O4 Synthesized by a Direct Conversion Process from γ-Al2O3

Nanostructured MgAl₂O₄ spinel was synthesized by a direct conversion process from cubic γ-Al₂O₃. The effect of post-annealing temperature (300°, 500°, and 800°C) on MgAl₂O₄ phase formation was investigated using transmission electron microscopy, selected area electron diffraction (SAED), electron energy loss spectroscopy (EELS), and energy-dispersive spectroscopy (EDS). Relative diffraction intensities as well as lattice parameter measurements from SAED revealed that MgAl₂O₄ spinel structure starts forming at temperatures as low as 300°C. EELS and EDS spectrum images also revealed an increase in elemental homogeneity with increasing annealing temperature. The degree of ordering of Mg and Al between octahedral and tetrahedral sites has been determined from relative diffraction intensities. Results show that annealing to 800°C leads to a spinel phase with an order parameter of 0.78.     

Activity–Composition Relations in MnCr2O4–CoCr2O4 Solid Solutions and Stabilities of MnCr2O4 and CoCr2O4 at 1300°C

Phase equilibria in the system MnO–CoO–Cr₂O₃ were investigated at 1300°C under controlled oxygen partial pressures by using the gas equilibration technique. The CoO activities in various phase assemblages of the system were measured by determining the partial pressures of oxygen in the gas phase for coexistence with metallic cobalt. The activity data revealed that at 1300°C, MnO–CoO and MnCr₂O₄–CoCr₂O₄ solid solutions exhibit mild positive departures from ideal behavior. The activities in the stoichiometric spinel solutions were found to be in good agreement with those predicted from a model based on cation distribution equilibria. The standard free energy of formation of the compound CoCr₂O₄ from its oxide components at 1300°C was determined as −37 636 J/mol, while that for MnCr₂O₄ was found as −44 316 J/mol.     

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.     

Activity–Composition Relations in NiAl2O4─MnAl2O4 Solid Solutions and Stabilities of NiAl2O4 and MnAl2O4 at 1300° and 1400°C

Activities of NiO were measured in the oxide and spinel solutions of the system MnO–NiO–Al₂O₃ at 1300° and 1400° C with the aim of deriving information on the thermodynamic properties of the spinel phases. Synthetic samples in selected phase assemblages of the system were equilibrated with metallic nickel and a gas phase of known oxygen partial pressures at a total pressure of 1 atm. The data on NiO activities and directions of conjugation lines between coexisting oxide and spinel phases were used to establish the activity–composition relations in spinel solid solutions at 1300° and 1400°C. The MnAl₂O₄–NiAl₂O₄ solid solutions exhibit considerable negative deviations from ideality at these temperatures. The free energy of formation of MnAl₂O₄ from its oxide components (MnO + Al₂O₃) at 1300° and 1400°C is calculated to be −24.97 and −26.56 kJ. mol⁻¹, respectively. The activities determined in the stoichiometric spinel solid solutions are more negative as compared with those predicted from cation distribution models.     

Grain Growth of ZnO in ZnO-Bl2O3 Ceramics with Ai2O3 Additions

Grain growth of ZnO during liquid-phase sintering of a ZnO-6 wt% Bi₂O₃ ceramic was investigated for A1₂O₃ additions from 0.10 to 0.80 wt%. Sintering in air for 0.5 to 4 h at 900° to 1400°C was studied. The AI₂O₃ reacted with the ZnO to form ZnAl₂O₄ spinel, which reduced the rate of ZnO grain growth. The ZnO grain-growth exponent was determined to be 4 and the activation energy for ZnO grain growth was estimated to be 400 kJ/mol. These values were compared with the activation parameters for ZnO grain growth in other ceramic systems. It was confirmed that the reduced ZnO grain growth was a result of ZnAl₂O₄ spinel particles pinning the ZnO grain boundaries and reducing their mobility, which explained the grain-growth exponent of 4. It was concluded that the 400 kJ/mol activation energy was related to the transport of the ZnAl₂O₄ spinel particles, most probably controlled by the diffusion of O^2- in the ZnAl₂O₄ spinel structure.     

Growth of Single-Crystal and Polycrystalline Thin Films of MgAl2O4 and MgFe2O4

Single-crystal and polycrystalline films of Mg-Al₂O₄ and MgFe₂O₄ were formed by two methods on cleavage surfaces of MgO single crystals. In one procedure, aluminum was deposited on MgO by vacuum evaporation. Subsequent heating in air at about 510°C formed a polycrystalline γ-Al₂O₈ film. Above 540°C, the γ-Al₂O, and MgO reacted to form a single-crystal MgAl₂O₄ film with {001} MgAl₂O₄‖{001} MgO. Above 590°C, an additional layer of MgAl₂O₄, which is polycrystalline, formed between the γ-Al₂O₃ and the single-crystal spinel. Polycrystalline Mg-Al₂O₄ formed only when diffusion of Mg²⁺ ions proceeded into the polycrystalline γ-Al₂O₃ region. Corresponding results were obtained for Mg-Fe₂O₄. MgAl₂O₄ films were also formed on cleaved MgO single-crystal substrates by direct evaporation, using an Al₂O₃ crucible as a source. Very slow deposition rates were used with source temperatures of ∼1350°C and substrate temperatures of ∼800°C. Departures from single-crystal character in the films may arise through temperature gradients in the substrate.     

Synthesis of MgAl2O4 Spinel Powder by Combination of Sol–Gel and Precipitation Processes

MgAl₂O₄ spinel precursor was prepared by a novel method combining a sol–gel process with the "traditional" precipitation process. The thermal decomposition and phase development of the precursor were analyzed, and the degree of agglomeration of the calcined powder was assessed by determining its particle size and crystal size. The optimum calcination temperature was determined based on the variation of specific surface areas, crystal size, and particle size. Completely crystallized ultrafine spinel powder ( d ₅₀=600 nm, specific surface area=105 m²/g) was obtained after calcination at 900°C.     

Sol–Gel Synthesis and Characterization of Ag and Au Nanoparticles in SiO2, TiO2, and ZrO2 Thin Films

Silver and gold nanoparticles were synthesized by the sol–gel process in SiO₂, TiO₂, and ZrO₂ thin films. A versatile method, based on the use of coordination chemistry, is presented for stabilizing Ag⁺ and Au³⁺ ions in sol–gel systems. Various ligands of the metal ions were tested, and for each system it was possible to find a suitable ligand capable of stabilizing the metal ions and preventing gold precipitation onto the film surface. Thin films were prepared by spin-coating onto glass or fused silica substrates and then heat-treated at various temperatures in air or H₂ atmosphere for nucleating the metal nanoparticles. The Ag particle size was about 10 nm after heating the SiO₂ film at 600°C and the TiO₂ and ZrO₂ films at 500°C. After heat treatment at 500°C, the Au particle size was 13 and 17 nm in the TiO₂ and ZrO₂ films, respectively. The films were characterized by UV–vis optical absorption spectroscopy and X-ray diffraction, for studying the nucleation and the growth of the metal nanoparticles. The results are discussed with regard to the embedding matrix, the temperature, and the atmosphere of the heat treatment, and it is concluded that crystallization of TiO₂ and ZrO₂ films may hinder the growth of Ag and Au particles.     

Synthesis of Celsian (BaAl2Si2O8) from Solid Ba-Al-Al2O3-SiO2 Precursors: I, XRD and SEM/EDX Analyses of Phase Evolution

An intimate Ba-Al-Al₂O₃-SiO₂ powder mixture, produced by high-energy milling, was pressed to 3 mm thick cylinders (10 mm diameter) and hexagonal plates (6 mm edge-to-edge width). Heat treatments conducted from 300° to 1650°C in pure oxygen or air were used to transform these solid-metal/oxide precursors into BaAl₂Si₂O₈. Barium oxidation was completed, and a binary silicate compound, Ba₂SiO₄, had formed within 24 h at 300°C. After 72 h at 650°C, aluminum oxidation was completed, and an appreciable amount of BaAl₂O₄ had formed. Diffraction peaks consistent with hexagonal BaAl₂Si₂O₈, BaAl₂O₄, β-BaSiO₃, and possibly β-BaSi₂O₅ were detected after 24 h at 900°C. Diffraction peaks for BaAl₂O₄ and BaAl₂Si₂O₈ were observed after 35 h at 1200°C, although SEM analyses also revealed fine silicate particles. Further reaction of this silicate with BaAl₂O₄ at 1350° to 1650°C yielded a mixture of hexagonal and monoclinic BaAl₂Si₂O₈. The observed reaction path was compared to prior work with other inorganic precursors to BaAl₂Si₂O₈.     

Formation of NiAl2O4 by Solid State Reaction

The growth of nickel-aluminum spinel, NiAl₂O₄, in diffusion couples of polycrystalline Al₂O₃ and NiO was investigated between 1200° and 1500°C. The growth kinetics for the spinel layer obeyed a parabolic rate law in this temperature range. Marker experiments showed that the spinel layer formed by counterdiffusion of nickel and aluminum ions. Comparison of experimental and theoretical values of the parabolic rate constants suggests that the diffusion of aluminum ions through the spinel layer is rate controlling.