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.     

Preparation and Cathodoluminescence of Mg-Doped and Zn-Doped GaN Powders

In this paper, undoped, Mg- and Zn-doped gallium nitride powders were prepared by direct nitridation of Ga₂O₃ under a flowing NH₃ gas. The phase purity, morphology and cathodoluminescence spectra were presented. The Ga₂O₃ powders can be completely nitridized to GaN at 1000°C. The resultant GaN powders agglomerated together with submicron-sized polyhedral crystals. At room temperature, the Mg- and Zn-doped powders exhibit bright blue-violet emission at around 3.05 and 2.81 eV, respectively. This provides clear evidence that magnesium or zinc is incorporated into the GaN powders as an acceptor and suggests that the luminescent materials are promising candidates for optoelectronic applications.     

The System CaO-Ga2O3

CaO and Ga₂O₃ form three compounds: 3CaO-Ga₂O₃, CaO-Ga₂O₃, and CaO-2Ga₂O₃. 3CaO-Ga₂O₃ melts incongruently to CaO plus liquid at 1263°C.; CaO Ga₂O₃ and CaO 2Ga₂O₃ melt congruently at 1369° and 1504°C. respectively. Eutectics are located at the following temperatures and compositions (in mole% Ga₂O₃): between 3CaO Ga₂O₃ and CaO Ga₂O₃, 1245°C. and 37.5%; between CaO Ga₂O₃ and CaO-2Ga₂O₃,1323^oC. and 57.0%; and between CaO -2Ga₂O₃ and β-Ga₂O₃,1457°C. and 68.0%. There is a peritectic at 1263°C. and 36.0%. Three polymorphs of CaO Ga₂O₃ are described. Compositions from approximately 35 to 70 mole% Ga₂O₃ can be quenched to yield homogeneous glasses.     

Subsolidus Phase Relations in the Ga2O3-In2O3-SnO2 System

Subsolidus phase relationships in the Ga₂O₃–In₂O₃–SnO₂ system were studied by X-ray diffraction over the temperature range 1250–1400°C. At 1250°C, several phases are stable in the ternary system, including Ga₂O₃( ss ), In₂O₃( ss ), SnO₂, Ga_{3− x} In_{5+ x} Sn₂O₁₆, and several intergrowth phases that can be expressed as Ga_{4−4 x} In_{4 x} Sn _{n −4}O_{2 n −2} where n is an integer. An In₂O₃–SnO₂ phase and Ga₄SnO₈ form at 1375°C but are not stable at 1250°C. GaInO₃ did not form over the temperature range 1000–1400°C.     

Preparation of Gadolinium Gallium Garnet [Gd3Ga5O12] by Solid-State Reaction of the Oxides

We have prepared dense polycrystalline gadolinium gallium garnet (GGG) by solid-state reaction of the oxides. The oxides were prereacted at 1350°C, ground, pressed, and sintered at 1650°C, yielding 97% dense samples. Ga₂O₃ evaporated from the sample surface leaving Gd₄Ga₂O₉ that could spall off the sample. For the short times needed to sinter samples, the bulk composition of the material remained essentially constant. The microhardness of the GGG was 11.8 ± 1.2 GN · m⁻².     

Phase Equilibria in the Ga2O3In2O3 System

Subsolidus phase relationships in the Ga₂O₃–In₂O₃ system were studied by X-ray diffraction and electron probe microanalysis (EPMA) for the temperature range of 800°–1400°C. The solubility limit of In₂O₃ in the β-gallia structure decreases with increasing temperature from 44.1 ± 0.5 mol% at 1000°C to 41.4 ± 0.5 mol% at 1400°C. The solubility limit of Ga₂O₃ in cubic In₂O₃ increases with temperature from 4.X ± 0.5 mol% at 1000°C to 10.0 ± 0.5 mol% at 1400°C. The previously reported transparent conducting oxide phase in the Ga-In-O system cannot be GaInO₃, which is not stable, but is likely the In-doped β-Ga₂O₃ solid solution.     

Subsolidus Phase Relationships in the Ga2O3–Al2O3–TiO2 System

Subsolidus phase relationships in the Ga₂O₃–Al₂O₃–TiO₂ system at 1400°C were studied using X-ray diffraction. Phases present in the pseudoternary system include TiO₂ (rutile), Ga_{2−2 x} Al_{2 x} O₃ ( x ≤0.78 β-gallia structure), Al_{2−2 y} Ga_{2 y} O₃ ( y ≤0.12 corundum structure), Ga_{2−2 x} Al_{2 x} TiO₅ (0≤ x ≤1 pseudobrookite structure), and several β-gallia rutile intergrowths that can be expressed as Ga_{4−4 x} Al_{4 x} Ti _{n −4}O_{2 n −2} ( x ≤0.3, 15≤ n ≤33). This study showed no evidence to confirm that aluminum substitution of gallium stabilizes the n =7 β-gallia–rutile intergrowth as has been mentioned in previous work.     

Synthesis and Mechanism of Nanosized AlN from an Aluminum Oleic Emulsion Using Carbothermal Reduction at Low Temperatures

Nanosized Al₂O₃ particles homogeneously dispersed in a matrix of amorphous carbon (a-C) were prepared by decomposition of an aluminum oleic emulsion at 600°C in Ar. Nanosized aluminum nitride (AlN) grains were prepared by carbothermal reduction and nitridation (CRN) of this Al₂O₃–a-C mixture in NH₃ using graphite, BN, and alumina crucibles or boats. The phases formed by CRN were identified by X-ray diffraction analysis. The morphology and grain size of the AlN were determined by transmission electron microscopy. The formation of single-phase AlN was achieved at temperatures as low as 1150°–1200°C in NH₃ using a cylindrical graphite crucible with holes in its two flat faces. Mass spectroscopy (MS) showed that a significant amount of HCN and a minor amount of C₂H₂ are formed at 500°C by reaction of NH₃ with carbon at the decomposition temperature of NH₃. A most probable formation mechanism of the AlN from nanosized Al₂O₃ and a-C in NH₃ is discussed on the basis of MS results and thermodynamic considerations.     

Phase Relations and Lattice Constants in the MgO-Al2O3-Ga2O3 System at 1550°C

Phase relations and lattice constants in the MgO–Al₂O₃–Ga₂O₃ system at 1550°C have been determined experimentally. In a large part of this system, only a nonstoichiometric spinel is stable. Compositions as extreme as 12.5 mol% MgO–20.5 mol% Ga₂O₃–67 mol% Al₂O₃ for a homogeneous spinel are possible. In the bordering phase diagrams of MgO–Al₂O₃ and MgO–Ga₂O₃, the composition of the spinel is as high as 63 mol% Al₂O₃ or Ga₂O₃, respectively. The contributions of all simple ionic exchange reactions on the lattice constant of the spinel have been deduced from X-ray diffractometry data.     

Calcium Phosphate Cements Prepared by Acid–Base Reaction

We investigated the characteristics of calcium phosphate cements (CPC) prepared by an exothermic acid–base reaction between NH₄H₂PO₄-based fertilizer (Poly-N) and calcium aluminate compounds (CAC), such as 3CaO · Al₂O₃ (C₃A), CaO · Al₂O₃ (CA), and CaO · 2Al₂O₃ (CA₂), in a series of integrated studies of reaction kinetics, interfacial reactions, in-situ phase transformations, and microstructure development. Two groups were compared: untreated and hydrothermally treated CPC specimens. The extent of reactivity of CAC with Poly-N at 25°C was in the following order: CA > C₃A ≫ CA₂. The formation of a NH₄CaPO₄· x H₂O salt during this reaction was responsible for the development of strength in the CPC specimens. The in-situ phase transformation of amorphous NH₄CaPO₄· x H₂O into crystalline Ca₅(PO₄)₃(OH) and the conversion of hydrous Al₂O₃ gel →γ-AIOOH occur in cement bodies during exposure in an autoclave to temperatures up to 300°C. This phase transformation significantly improved mechanical strength.     

The System Alumina-Gallia-Water

A study of the system Al₂O₃–Ga₂O₃–H₂O has resulted in the determination of equilibrium diagrams for the systems Al₂O₃–Ga₂O₃ and Al₂O₃.-H₂O–Ga₂O₃.H₂O. Extensive solid solution characterizes the α -Al₂O₃ and β -Ga₂O₃ structures at high temperatures, but it is shown that below 810°C. a compound, GaAlO₃, and a new series of (Al, Ga)₂O₃ structures are stable. Among the hydrates, a complete series of diaspore solid solutions extends from Al₂O₃.H₂O to Ga₂O₃.H₂O. Boehmite solid solutions extend to approximately the composition 70Al₂O₃.H₂O, 30Ga₂O₃.H₂O.     

β-Gallium Oxide as Oxygen Gas Sensors at a High Temperature

Resistive oxygen sensors based on gallium oxide were fabricated in order to analyze their sensing performances (as sensitivity, response, and recovery time) in an oxygen atmosphere at 1000°C. We prepared three types of sensors using a β-Ga₂O₃ single crystal in a sandwich structure with Pt pad electrodes and β-Ga₂O₃ polycrystalline thin films deposited by using both the sputtering technique and the chemical solution deposition method. For thin-film sensors, Pt interdigital electrodes were deposited on the surface of the films using the lift-off method. X-ray diffraction and atomic force microscopy investigations were performed to compare the structure and surface morphology of the samples. We achieved a response time of 10 s at 1000°C, while the sensitivity was 1.03 for the single crystal and 1.35–1.45 for thin films. The sensing properties depend on the preparation condition of Ga₂O₃ devices.     

Effects of Palladium Dispersions on Gas-Sensitive Conductivity of Semiconducting Ga2O3 Thin-Film Ceramics

The gas sensitivity of Ga₂O₃ thin-film n -type conductors was investigated at temperatures of 500–1000°C. Palladium dispersions whose particle sizes are dependent on the preceding annealing processes were deposited by a wet-chemical technique onto Ga₂O₃ thin-film ceramics. The palladium clusters and their temperature-dependent growth were detected using scanning electron microscopy micrographs and X-ray photoemission spectroscopy measurements. The effect of the palladium dispersions on the gas-sensitive behavior of the Ga₂O₃ ceramics was investigated in various O₂/H₂ mixtures in the N₂ carrier gas at 700°C. The conductivity of the ceramics treated in this way was dependent on the O₂ partial pressure, as well as on the H₂ partial pressure of the surrounding gas atmosphere. The ceramic conductivity can be described as a function of the O₂:H₂ ratio, in accordance with the relation σ( p O₂/ p H₂/)^−1/3.     

Synthesis of AlN Nanopowder from γ-Al2O3 by Reduction–Nitridation in a Mixture of NH3–C3H8

Aluminum nitride (AlN) powders were synthesized by gas reduction–nitridation of γ-Al₂O₃ using NH₃ and C₃H₈ as the reactant gases. AlN was identified in the products synthesized at 1100°–1400°C for 120 min in the NH₃–C₃H₈ gas flow confirming that AlN can be formed by the gas reduction–nitridation of γ-Al₂O₃. The products synthesized at 1100°C for 120 min contained unreacted γ-Al₂O₃. The ²⁷A1 MAS NMR spectra show that Al–N bonding in the product increases with increasing reaction temperature, the tetrahedral AlO₄ resonance decreasing prior to the disappearance of the octahedral AlO₆ resonance. This suggests that the tetrahedral AlO₄ sites of the γ-Al₂O₃ are preferentially nitrided than the AlO₆ sites. AlN nanoparticles were directly formed from γ-Al₂O₃ at low temperature because of this preferred nitridation of AlO₄ sites in the reactant. AlN nanoparticles are formed by gas reduction–nitridation of γ-Al₂O₃ not only because the reaction temperature is sufficiently low to restrict grain growth, but also because γ-Al₂O₃ contains both AlO₄ and AlO₆ sites, by contrast with α-Al₂O₃ which contains only AlO₆.     

Evidence for a Phase Transition of β-Gallium Oxide at Very Low Oxygen Pressures

Depending on the operating temperature, gas sensors that are based on n-type-semiconductor, polycrystalline gallium oxide (Ga₂O₃) thin films are used to detect oxygen (at temperatures, T, of 850°C) or reducing gases (T 900°C). At high temperatures (T 900°C), beta-Ga₂O₃has an oxygen deficiency in the crystal lattice that is in dynamic equilibrium with the oxygen in the surrounding atmosphere. Variations in the conductivity of the sensor are caused by variations of the concentration of ionized oxygen vacancies. Therefore, a reduction in the proportion of oxygen or an increase in the concentration of reducing gases in the atmosphere in which the sensor is located leads to an increasing number of conducting electrons and, hence, an increasing conductivity. During a research project to investigate the long-term stability of thin beta-Ga₂O₃ films in a variety of strongly reducing atmospheres at T > 600°C, a previously unknown phenomenon has been observed when measurements on low oxygen partial pressures (pO₂10_-10 Pa (10^-15bar)) have been made. A sharp decrease in sensor conductivity, by several orders of magnitude, is observed each time when pO₂ is reduced to a value of <10^-15bar at temperatures in the range of ˜750°-1000°C. The reason for this may be a phase transition in the β-Ga₂O₃ layer. However, attempts to freeze the new state with subsequent identification by X-ray diffractometry have not succeeded in identifying the new phase.     

Exsolution of β-Ga2O3 Crystalline Solutions in the System MgAl2O3-Ga2O3

The subsolidus phase equilibrium diagram for the pseudobinary join MgAl₂O₄-Ga₂O₃ was determined. The shape of the exsolution boundary was obtained by heat-treating samples pre- equilibrated at 1600°C. Crystalline solubility of Ga₂O₃ in MgAl₂O₄ decreased from 73 mole % at 1600°C to 55 mole % at 1200°C. The crystalline solution was formed by the replacement of Mg²⁺ions by Ga³⁺ ions to produce a cation defect spinel. The phase precipitated was the mono-clinic δ-Ga₂O₃ (=δ-Al₂O₃ structure). Changes in the ratios of relative X-ray diffraction intensities indicated that the crystalline solutions also disorder with temperature.     

Ionic Conductivity Enhancement of La2Mo2−xWxO9 Nanocrystalline Films Deposited on Alumina Substrates by the Sol–Gel Method

Dense, crack-free, and uniform La₂Mo_{2− x} W _x O₉ ( x =0, 0.1, and 0.2) nanocrystalline films were successfully synthesized on poly-alumina substrates via a modified sol–gel method, with inorganic salt of La(NO₃)₃·6H₂O, (NH₄)₆Mo₇O₂₄·4H₂O, and (NH₄)₆H₂W₁₂O₂₄ as precursors. Pure La₂Mo₂O₉ phase was confirmed by X-ray diffractometer when the annealing temperature was >500°C. The average grain size of the La₂Mo_{2− x} W _x O₉ films is in the range of 90–400 nm, depending upon the conditions of thermal treatment, and the thickness of films can reach 1 μm by repetitive spin-coating. The electrical conductivity increases with decreasing grain size and reaches 0.074 S/cm at 600°C in the film with a grain size of 90 nm, which is one order of magnitude higher than that in the corresponding bulk materials. W-doping can suppress the phase transition that occurs at 580°C in pure La₂Mo₂O₉ and enhance the low-temperature ionic conductivity. Furthermore, the activation energy of conductivity in the nanocrystalline La₂Mo₂O₉ films decreases to about 0.6 eV in comparison with 1.0 eV in the bulk ones, which implies that the grain resistance prevails in the total resistance, when grain size reduces to nanometer domain.     

Synthesis of Nanocrystalline Aluminum Nitride by Nitridation of δ-Al2O3 Nanoparticles in Flowing Ammonia

Nanocrystalline aluminum nitride (AlN) with surface area more than 30 m²/g was synthesized by nitridation of nanosized δ-Al₂O₃ particles using NH₃ as a reacting gas. The resulting powders were characterized by CHN elemental analysis, X-ray diffraction (XRD), Fourier transform infrared spectra, X-ray photoelectron spectra, field-emission scanning electron microscopy, transmission electron microscopy, and Brunauer–Emmett–Teller surface area techniques. It was found that nanocrystalline δ-Al₂O₃ was converted into AlN completely (by XRD) at 1350°–1400°C within 5.0 h in a single-step synthesis process. The complete nitridation of nanosized alumina at relatively lower temperatures was attributed to the lack of coarsening of the initial δ-Al₂O₃ powder. The effect of precursor powder types on the conversion was also investigated, and it was found that α-Al₂O₃ was hard to convert to AlN under the same conditions.     

Fabrication and Photoluminescence Characteristics of Eu3+-Doped Lu2O3 Transparent Ceramics

A novel co-precipitation process was adopted for the preparation of highly sinterable europium-doped lutetia powders using ammonium hydroxide (NH₃·H₂O) and ammonium hydrogen carbonate (NH₄HCO₃) as the mixed precipitant. The resultant powders calcined at 1000°C for 2 h showed good dispersity and excellent sinterability. Highly transparent polycrystalline lutetia ceramics with a relative density of ∼99.9% were fabricated by pressureless sintering in flowing H₂ atmosphere at 1850°C for 6 h without any additives. The average grain sizes of the transparent material were estimated to be 50–60 μm. Optical in-line transmittance in the visible wavelength region for Lu₂O₃ ceramics (1 mm in thickness) reached 80%. The luminescence and decay behavior of the obtained transparent plate and the corresponding nanophosphors were also investigated.     

Suppression of Rhombohedral-Phase Appearance and Low-Temperature Sintering of Scandia-Doped Cubic-Zirconia

Inhibition of cubic-rhombohedral phase transformation and low-temperature sintering at 1000°C were achieved for 10-mol%-Sc₂O₃-doped cubic-ZrO₂ by the presence of 1 mol% Bi₂O₃. The powders of 1-mol%-Bi₂O₃–10-mol%-Sc₂O₃-doped ZrO₂ were prepared using a hydrolysis and homogeneous precipitation technique. No trace of rhombohedral-ZrO₂ phase could be detected, even after sintering at 1000°–1400°C. The average grain size of the ZrO₂ sintered at 1200°C was >2 μm because of grain growth in the presence of Bi³⁺. Cubic, stabilized Bi-Sc-doped ZrO₂ sintered at 1200°C had sufficient conductivity at 1000°C (0.33 S/cm) to be used as an electrolyte for a solid-oxide fuel cell (SOFC) and at 800°C (0.12 S/cm) for an intermediate-temperature SOFC.