Optical Properties of Zinc Aluminate, Zinc Gallate, and Zinc Aluminogallate Spinels

The optical spectra of zinc aluminate (ZnAl₂O₄), zinc gallate (ZnGa₂O₄), and zinc aluminogallate (ZnAlGaO₄) spinel powders were studied at wavelengths in the range of 250-900 nm using reflectance spectroscopy. The ZnAl₂O₄ and ZnGa₂O₄ powders were synthesized by using conventional ceramic processing techniques and had systematic variations in the molar ratio of ZnO to M₂O₃ (M = Al or Ga). The cubic spinel crystal structure of each composition was confirmed via powder X-ray diffractometry. The ZnAl₂O₄ powders showed optical properties in the ultraviolet wavelength region and had combined characteristics that were similar to that of ZnO (wurtzite structure) and Al₂O₃ (corundum structure), which result from the similar local environments of the zinc and aluminum cations within the cubic spinel crystal structure. A mechanically induced optical absorption (optomechanical effect) in the ultraviolet wavelength region was also observed in ZnAl₂O₄. The ZnGa₂O₄ powder followed a similar behavior, with the exception that the optomechanical effect did not occur in the gallate. The ZnAlGaO₄ showed optical spectra that were intermediate to that of the endpoint compositions.     

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.     

Porous, Biphasic CaCO3-Calcium Phosphate Biomedical Cement Scaffolds from Calcite (CaCO3) Powder

Calcite (CaCO₃) is a geologically abundant material, which can be used as a starting material in producing biomedical scaffolds for clinical dental and orthopedic applications. Bone-filling applications require porous, biocompatible, and resorbable materials. Commercially available CaCO₃ powders were physically mixed, for 80–90 s, with an orthophosphoric acid (H₃PO₄) solution, which was partially neutralized to pH 3.2 by adding NaOH, to form biphasic, micro-, and macroporous calcite-apatitic calcium phosphate (Ap-CaP) cement scaffolds of low strength. The resultant carbonated and Na-doped Ap-CaP phase in these scaffolds crystallographically and spectroscopically resembled calcium hydroxyapatite. Upon mixing CaCO₃ powders and the setting solution, carbon dioxide gas was in situ generated and formed the pores. Thus formed scaffolds contained pores over the range of 20–750 μm. Scaffolds were also converted to single-phase Ap-CaP, without altering their porosity, by soaking them in 0.5  M phosphate buffer solutions at 80°C for 36 h in glass bottles. Soerensen's buffer solution was also shown to be able to convert the calcite powders into single-phase Ap-CaP powders upon soaking at 60–80°C. This robust procedure of synthesizing Ap-CaP bioceramics is simple and economical.     

Particle Size Comparison of Soft-Chemically Prepared Transition Metal (Co, Ni, Cu, Zn) Aluminate Spinels

Nanocrystalline MAl₂O₄ (M=Co, Ni, Cu, Zn) spinel powders are synthesized by pyrolysis of complex compounds of aluminum and transition metal (cobalt, nickel, copper, and zinc) with triethanolamine (TEA). The precursor materials are formed on complete dehydration of the soluble complexes of aluminum–TEA and transition metal–TEA (Co, Ni, Cu, Zn), maintaining the resulting pH of each of the solutions at 4–5. The precursors are heat treated at different temperatures to provide phase formations. Single-phase MAl₂O₄ spinel powders were obtained after heat treatment of the precursor material at 600°C, but for CuAl₂O₄, the precursor passed through an intermediate-phase CuO at a lower treatment temperature. The precursor and the heat-treated powders were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectrometry, thermal gravimetric and differential thermal analysis, and transmission electron microscopy (TEM); the surface area was measured by a Brunauer–Emmet–Teller (BET) adsorption isotherm. The average particle sizes that were obtained from XRD, TEM, and BET adsorption isotherm were compared.     

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.     

Zirconium Aluminum Carbides: New Precursors for Synthesizing ZrO2–Al2O3 Composites

ZrO₂–Al₂O₃ nanocrystalline powders have been synthesized by oxidizing ternary Zr₂Al₃C₄ powders. The simultaneous oxidation of Al and Zr in Zr₂Al₃C₄ results in homogeneous mixture of ZrO₂ and Al₂O₃ at nanoscale. Bulk nano- and submicro-composites were prepared by hot-pressing as-oxidized powders at 1100°–1500°C. The composition and microstructure evolution during sintering was investigated by XRD, Raman spectroscopy, SEM, and TEM. The crystallite size of ZrO₂ in the composites increased from 7.5 nm for as-oxidized powders to about 0.5 μm at 1500°C, while the tetragonal polymorph gradually converted to monolithic one with increasing crystallite size. The Al₂O₃ in the composites transformed from an amorphous phase in as oxidized powders to θ phase at 1100°C and α phase at higher temperatures. The hardness of the composite increased from 2.0 GPa at 1100°C to 13.5 GPa at 1400°C due to the increase of density.     

Fabrication and Microstructure Characterization of Ti3SiC2 Synthesized from Ti/Si/2TiC Powders Using the Pulse Discharge Sintering (PDS) Technique

Ti/Si/2TiC powders were prepared using a mixture method (M) and a mechanical alloying (MA) method to fabricate Ti₃SiC₂ at 1200°–1400°C using a pulse discharge sintering (PDS) technique. The results showed that the Ti₃SiC₂ samples with <5 wt% TiC could be rapidly synthesized from the M powders; however, the TiC content was always >18 wt% in the MA samples. Further sintering of the M powder showed that the purity of Ti₃SiC₂ could be improved to >97 wt% at 1250°–1300°C, which is ∼200°–300°C lower than that of sintered Ti/Si/C and Ti/SiC/C powders using the hot isostatic pressing (HIPing) technique. The microstructure of Ti₃SiC₂ also could be controlled using three types of powders, i.e., fine, coarse, or duplex-grained, within the sintering temperature range. In comparison with Ti/Si/C and Ti/SiC/C mixture powders, it has been suggested that high-purity Ti₃SiC₂ could be rapidly synthesized by sintering the Ti/Si/TiC powder mixture at relatively lower temperature using the PDS technique.     

Titanium Diboride–Tungsten Diboride Solid Solutions Formed by Induction-Field-Activated Combustion Synthesis

Solid solutions of titanium diboride–tungsten diboride (TiB₂–WB₂) were synthesized by induction-field-activated combustion synthesis (IFACS) using elemental reactants. In sharp contrast to conventional methods, solid solutions could be formed by the IFACS method within a very short time, ∼2 min. Solutions with compositions ranging from 40–60 mol% WB₂ were synthesized with a stoichiometric ratio (Ti + W)/B =½; however, samples with excess boron were also made to counter the loss of boron by evaporation. The dependence of the lattice constants of the resulting solid solutions on composition was determined. The "a" parameter decreased only slightly with an increase in the WB₂ content, whereas the "c" parameter exhibited a significant decrease over the range 40–60 mol% WB₂. Solid-solution powders formed by the IFACS method were subsequently sintered in a spark plasma sintering (SPS) apparatus. After 10 min at 1800°C, the samples densified to relative density 86%. XRD analysis showed the presence of only the solid-solution phase.     

Hydrothermal Synthesis of Cerium(IV) Oxide

CeO₂ powders have been prepared from cerium(III) nitrate, cerium(IV) sulfate, and cerium(IV) ammonium sulfate under hydrothermal conditions at 120° to 200°C for 5 to 40 h. The effects of the starting cerium compounds, hydrothermal treatment temperature, and the concentration of the solutions on the crystal growth of CeO₂ were investigated. CeO₂ powders hydrothermally synthesized at 180°C for 5 h from cerium(IV) salts had very fine particle sizes (30 Å); on the other hand, the powder from the cerium(III) salt had a relatively coarse particle size (160 Å). Although the crystallite size of the powder synthesized from the cerium(IV) compounds depended on the treatment temperature, that from the cerium(III) compound was insensitive to the treatment temperature. The mechanisms for the growth of CeO₂ particles under hydrothermal conditions are discussed.     

Effect of Sodium on Crystallite Size and Surface Area of Zirconia Powders at Elevated Temperatures

Fine ZrO₂ powders were synthesized by an aqueous precipitation method using zirconyl nitrate. By adding the precursor salt to NaOH, single-phase ZrO₂ powders were formed, and the monoclinic phase did not appear upon heat treatment up to 1000°C. The samples were digested in NaOH for different amounts of time. Different levels of washing of digested samples produced surface area at 900°C for 4 h ranging from 8 to 100 m²/g. It was found that the properties of the powders at elevated temperatures were sensitive to the sodium content. The surface area decreased while the crystallite size and pore size of the samples increased with increased sodium content. Our results indicated that sodium is detrimental to the stabilization of surface area at elevated temperatures.     

Orthophosphates and Diphosphates Containing Zinc and Copper and Zinc and Cobalt

Solid solutions of diphosphates of zinc and copper and of zinc and cobalt were synthesized from mixtures of pure diphosphates at temperatures up to 1000°C. Their X-ray diffractometry patterns varied continuously from one end member to the other. Solid solutions of orthophosphates of composition Zn_3−xCox(PO₄)2, with x = 0.4–1.6, were formed at temperatures up to 950°C; all exhibited the structure of γ-Zn₃(PO₄)2. Solid solutions of orthophosphates of composition Zn_3−xCu_x(PO₄)2 exhibited more-complex behavior. At 1000°C and copper contents of 20–80 mol%, a phase that is related to Cu₃(PO₄)2, termed here the "ε-phase," predominated. At 850°–950°C and in the region from 20 mol% to ∼33 mol% of copper, the solid solutions (the "η-phase") adopted the structure of graftonite. At 800°–900°C and 10–15 mol% of copper, the solid solutions exhibited a new structure (the "δ-phase"), which we found to be related to the mineral sarcopside. At temperatures 950°C, the solutions that contained 5–15 mol% of copper (the "β-phase") had the structure of β-Zn₃(PO₄)2, whereas at 800°–850°C, solutions with 5 mol% of copper (the "-phase") exhibited the structure of γ-Zn₃(PO₄)2. Attempts to synthesize Cu⁺ZnPO₄ and Cu⁺Cu²⁺Zn₃(PO₄)3 were unsuccessful.     

Single-Source Sol-Gel Synthesis of Nanocrystalline ZnAl2O4: Structural and Optical Properties

Nanometer-sized zinc aluminate (ZnAl₂O₄) particles were synthesized from heterometal alkoxides, [ZnAl₂(OR)₈], possessing an ideal cation stoichiometry for the ZnAl₂O₄ spinel. ZnAl₂O₄ is formed at 400°C, which is the lowest temperature reported for the formation of monophasic ZnAl₂O₄. ²⁷Al magic-angle spinning nuclear magnetic resonance spectroscopy revealed that ZnAl₂O₄ possesses an inverse structure at <900°C, while the normal spinel phase is observed at higher temperatures. The homogeneity of the in-depth composition and Zn:Al stoichiometry (1:2) was confirmed by electron spectroscopy for chemical analysis. Evaluation of the valence-band spectra of ZnAl₂O₄ and ZnS suggested that the hybridization of O 2 p and Zn 3 d orbitals is responsible for lowering the bandgap in the latter. The average crystallite size showed an exponential relationship to the calcination temperature (X-ray diffractometry and transmission electron microscopy data). The optical spectra of different spinel powders (average particle sizes, 20–250 nm) showed that the absorption edge exhibits a blue shift as particle size decreases.     

Morphology of Zirconia Synthesized Hydrothermally from Zirconium Oxychloride

Monoclinic zirconia has been synthesized hydrothermally from zirconium oxychloride in the range 20–100 MPa, 250–650°C, for run duration from 20 to 100 h and in the presence of NaOH, Na₂CO₃, Na₂SO₄, NH₄F, HNO₃, and H₂SO₄ additives. Isometric, platelike, and elongated crystal morphologies were obtained depending on the additive. Both spherulitic and isolated textures were encountered with H₂SO₄. Growth of isometric crystals follows a general dissolution–precipitation mechanism. For H₂SO₄, two growth steps were identified: an early spherulitic step followed by an isolated crystals step resulting from Ostwald ripening of preexisting spherulites. The formation of spherulites is consistent with the specific properties of the sulfuric hydrothermal medium.     

Stoichiometry Control and Phase Selection in Hydrothermally Derived BaxSr1–xTiO3 Powders

Ba _x Sr_{1- x} TiO₃ (BST) powders were processed at temperatures <100°C by reacting nanosized TiO₂ powders in alkaline, aqueous solutions of BaCl₂, SrCl₂, and NaOH. The effects of processing variables (NaOH concentration, time, temperature, and the ratios of barium, strontium, and titanium initially in solution) on the resultant BST powder stoichiometry and solid solubility were examined. In all cases, strontium was more readily incorporated into the BST powders than barium, and the extent varied systematically with the processing variables. BST powders that were processed in solutions with a large initial excess of barium and strontium, relative to titanium, consisted of a single-phase solid solution. In contrast, BST powders that were processed in solutions with a small initial excess of barium and strontium, relative to titanium, contained a biphasic solid solution which corresponded to separate barium-rich and strontium-rich phases.     

Effects of Processing Parameters on Alumina Coatings Deposited on Nickel Substrates by Reacting Aluminum Chloride and Hydrogen/Carbon Dioxide Gas Mixtures

Pure nickel coupons were used as substrates in the deposition of alumina (Al₂O₃) from the reaction of aluminum chloride (AlCl₃) with hydrogen/carbon dioxide gas mixtures in the temperature range of 954°–1100°C and system pressures of 2.7–13.3 kPa. The apparent activation energy estimated from the coating growth rate averaged 320 kJ/mol at 13.3 kPa. At temperatures <1000°C, transition theta, kappa, and delta modifications were codeposited with alpha-Al₂O₃, whereas single-phase alpha-Al₂O₃ was deposited at higher temperatures. At high AlCl₃ partial pressures, nickel aluminide phases were sometimes codeposited with Al₂O₃, which was attributed to the reaction of AlCl₃ with the nickel substrate in the presence of hydrogen gas.     

Synthesis of Ultrafine Ceria Powders by Mechanochemical Processing

The synthesis of ultrafine cerium dioxide (CeO₂) powders via mechanochemical reaction and subsequent calcination was studied. Anhydrous CeCl₃ and NaOH powders, along with NaCl diluent, were mechanically milled. A solid-state displacement reaction—CeCl₃+ 3NaOH → Ce(OH)₃+ 3NaCl—was induced during milling in a steady-state manner. Calcination of the as-milled powder in air at 500°C resulted in the formation of CeO₂ nanoparticles in the NaCl matrix. A simple washing process to remove the NaCl yielded CeO₂ particles ∼10 nm in size. The particle size was controlled in the range of ∼10–500 nm by changing the calcination temperature.     

Reactive Synthesis of a Porous Calcium Zirconate/Spinel Composite with Idiomorphic Spinel Grains

Porous CaZrO₃/MgAl₂O₄ composites were synthesized in air by pressureless reactive sintering of an equimolar mixture of dolomite (CaMg(CO₃)₂), monoclinic zirconia ( m -ZrO₂), and α-alumina powders, with a 0.5 wt% lithium fluoride additive. The reaction behavior of the mixed powders (with/without LiF additive) was studied using high-temperature X-ray diffraction. A bulk porous composite resulted from sintering at 1300°C for 2 h (in a nearly closed container, so as to increase the LiF-doping effect), which consisted of fine grains (CaZrO₃ and MgAl₂O₄, ∼0.5–1 μm) and well-grown idiomorphic ones (MgAl₂O₄ octahedra ∼ 2–4 μm). The idiomorphic spinel grains were located around the inner walls of relatively large pores. The composite showed appreciably high bending strength (σ_f= 110 ± 8 MPa for a porosity of 31%). The porous CaZrO₃/MgAl₂O₄ composites can be applied as high-temperature filters and lightweight structural components.     

Synthesis of Zinc Aluminate Spinel Film through the Solid-Phase Reaction between Zinc Oxide Film and α-Alumina Substrate

We synthesized spinel ZnAl₂O₄ film on α-Al₂O₃ substrate using a solid-phase reaction between the pulsed-laser-deposited ZnO film and α-Al₂O₃ substrate. Auger electron spectroscopy showed that the atomic distribution in the spinel ZnAl₂O₄ was inhomogeneous, which indicated that the reaction was diffusion controlled. Based on X-ray fluorescence measurements, the apparent growth activation energy of ZnAl₂O₄ was determined as 504 kJ/mol. X-ray diffractometry spectra showed that, as the growth temperature increased, the ZnAl₂O₄ film became disoriented from the single (111) orientation. The ZnAl₂O₄ (333) diffraction peak shifted toward a small angle, and its full-width at half-maximum decreased from 1.30° to 0.37°. At the growth temperature of 1100°C, the morphology of the ZnAl₂O₄ was initially transformed from islands to stick structures, then to bulgy-line structures with increased growth time. X-ray diffractometry spectra showed that these transformations were correlated with changes of ZnAl₂O₄ orientation.     

Fabrication of Translucent Magnesium Aluminum Spinel Ceramics

A precursor for magnesium aluminum spinel powder, composed of crystalline ammonium dawsonite hydrate (NH₄Al(OH)₂CO₃·H₂O) and hydrotalcite (Mg₆Al₂(CO₃)(OH)₁₆·4H₂O) phases, was synthesized via precipitation, using ammonium bicarbonate as the precipitant. The precursor was characterized by differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. Reactive spinel powder, which could be densified to translucency under vacuum at 1750°C in 2 h without additives, was obtained by calcining the precursor at 1100°C for 2 h.     

Sol–Gel Synthesis and Characterization of Ytterbium Silicate Powders

Ytterbium silicate (Yb₂SiO₅) is a promising environmental barrier coating material for SiC-based ceramic matrix composites. Yb₂SiO₅ powders were prepared by the sol–gel process using tetraethyl orthosilicate and ytterbium nitrate as starting materials. The Yb₂SiO₅ gel was characterized by Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis–differential scanning calorimetry (TGA–DSC). After calcining the gel at 1200°C for 1 h, Yb₂SiO₅ powders were obtained, which were examined by X-ray diffraction, FTIR, transmission electron microscope, and scanning electron microscope. The results showed that single-phase Yb₂SiO₅ powders of 200–300 nm in size were successfully synthesized.