Effect of Calcination Conditions and Excess Alkali Carbonate on the Phase Formation and Particle Morphology of Na0.5K0.5NbO3 Powders

Sodium-potassium niobate [Na_0.5K_0.5NbO₃] powders were prepared following the conventional mixed oxide method. An orthorhombic XRD pattern, consistent with single-phase Na_0.5K_0.5NbO₃, was obtained after calcination at 900°C for 6 h. Introducing 5 mol% excess Na₂CO₃ and K₂CO₃ into the starting mixture allowed milder calcination conditions to be used, for example 800°C for 2 h. Primary particles in 5 mol% excess samples were cuboid, with maximum sizes of ∼2.5 μm. Equiaxed 0.3–0.4-μm particles were formed for non-excess powders, and also for powders prepared with 1 and 3 mol% excess alkali carbonates. The results suggest liquid formation during calcination of the excess 5-mol% starting powders.     

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.     

Synthesis of Nd:YVO4 Thin Films by a Sol-Gel Method

Nd: YVO₄ powders and thin films were successfuly synthesized by the sol–gel method using metal alkoxides. A homogeneous and stable solution was prepared by the reaction of Y(OEt)₃, VO(O i Pr)₃, and Nd(OEt)₃ in 2-methoxyethanol. The precursor was a mixture of vanadium and yttrium double alkoxide. Precursor films were prepared by dip coating and crystallization to single-phase YVO₄ at 500°C. Nd:YVO₄ films were crystallized with (200) preferred orientation on glass substrates, which showed the characteristic optical absorption of neodymium.     

Sintering of Ceria-Doped Tetragonal Zirconia Crystallized in Organic Solvents, Water, and Air

Amorphous CeO₂–ZrO₂ gels were prepared by coprecipitation in ammonia solutions. The onset of crystallization of the gels, from calcining in air, was 420°C, while 200° to 250°C in the presence of water and organic solvents such as methanol and ethanol. The sintering behaviors of CeO₂–ZrO₂ powders were sensitive to the crystallizing conditions, since hard agglomerates formed when the precipitated gels were crystallized by normal calcination in air, whereas soft agglomerates formed when they were crystallized in water or organic solvents. CeO₂–ZrO₂ powders crystallized in methanol and water at 250°C were sintered to full theoretical density at 1150° and 1400°C, respectively, whereas that crystallized by calcination in air at 450°C was sintered to only 95.2% of theoretical density, even at 1500°C.     

Hollow Al2O3 Microspheres Derived from Al/AlOOH·nH2O Core-Shell Particles

Hollow Al₂O₃ spheres with ∼2 μm in diameter and ∼200 nm in wall thickness were prepared successfully via the calcination of the Al/AlOOH· n H₂O core-shell particles, prepared by wet-chemical method using commercial microscale aluminum powders as raw materials, at 900°–1100°C in air. X-ray powder diffraction, differential scanning calorimetric analysis, scanning electron microscopy, and transmission electron microscopy were used to characterize the obtained samples, and the possible formation mechanism was discussed. These hollow Al₂O₃ microspheres may have promising applications in insulators, lightweight fillers, and catalyst carriers because of their unique hollow structure.     

Hydrothermal Synthesis for Large Barium Titanate Powders at a Low Temperature: Effect of Titania Aging in an Alkaline Solution

Monodisperse and spherical barium titanate (BaTiO₃) powders with diameters of 200–470 nm were directly prepared by a low-temperature hydrothermal method at 90°C. Spherical titania (TiO₂) powders, ranging in size from 150 to 420 nm, were initially prepared by a controlled hydrolysis and condensation reaction, aged in a highly alkaline solution for 12 h, and then hydrothermally reacted with barium hydroxide to be converted to BaTiO₃ without a morphological change. The aging step of the TiO₂, where the surface of TiO₂ was highly densified through elimination of the pores, was indispensable to retain the sizes and shapes of TiO₂ in the resulting BaTiO₃. This was due to the fact that the formation of BaTiO₃ proceeded by an in situ reaction mechanism. The resulting BaTiO₃ powders exhibited dense and nonporous structures even after calcination at 1000°C.     

Molten-Salt Synthesis of Ba1–xPbxTiO3 in the System BaTiO3–PbCl2

Ba_{1– x} Pb _x TiO₃ powder with a fixed composition was prepared by the reaction of BaTiO₃ powders with molten PbCl₂at various PbCl₂/BaTiO₃ molar ratios at 600° and 800°C in a nitrogen atmosphere. When 0.1 μm powder was used, the reaction was finished when x = 0.9. Two phases of BaTiO₃and a solid solution of Ba_{1– x} Pb _x TiO₃ coexisted, but the final phase gave a solid solution of Ba_{1– x} Pb _x TiO₃ at 800°C. When 0.5 μm powder was used, the two phases coexisted in the products at 600°C at PbCl₂/BaTiO₃= 1.0. A sintered compact of Ba_{1– x} Pb _x TiO₃ powders solid solution was prepared by hot isostatic pressing, and its dielectric constant was measured in the temperature range 20°–550°C.     

Thermal Decomposition of Titanium Alkoxide and Silicate Ester in Organic Solvent: A New Method for Synthesizing Large-Surface-Area, Silica-Modified Titanium(IV) Oxide of High Thermal Stability

Silica-modified titanium oxide (S-TiO₂) powders that have an anatase structure were synthesized via the thermal decomposition of mixtures of titanium(IV) isopropoxide (TIP) and tetramethyl orthosilicate (TMOS) in toluene at 300°C. These S-TiO₂ materials had high rutile-transformation temperatures and maintained large surface areas at elevated temperatures (550°–1000°C). For example, the product that was prepared from a 9:1 TIP:TMOS mixture transformed to rutile at ∼1100°C and possessed a surface area of 160 m²/g, even after calcination at 800°C for 1 h.     

Solid-State Reactive Sintering of Transparent Polycrystalline Nd:YAG Ceramics

Transparent polycrystalline Nd:YAG ceramics were fabricated by solid-state reactive sintering a mixture of commercial Al₂O₃, Y₂O₃, and Nd₂O₃ powders. The powders were mixed in methanol and doped with 0.5 wt% tetraethoxysilane (TEOS), dried, and pressed. Pressed samples were sintered from 1700° to 1850°C in vacuum without calcination. Transparent fully dense samples with average grain sizes of ∼50 μm were obtained at 1800°C for all Nd₂O₃ levels studied (0, 1, 3, and 5 at.%). The sintering temperature was little affected by Nd concentration, but SiO₂ doping lowered the sintering temperature by ∼100°C. Abnormal grain growth was frequently observed in samples sintered at 1850°C. The Nd concentration was determined by energy-dispersive spectroscopy to be uniform throughout the samples. The in-line transmittance was >80% in the 350–900 nm range regardless of the Nd concentration. The best 1 at.% Nd:YAG ceramics (2 mm thick) achieved 84% transmittance, which is equivalent to 0.9 at.% Nd:YAG single crystals grown by the Czochralski method.     

Low-Temperature Synthesis of YVO4 Nanoparticles and their Photocatalytic Activity

Yttrium orthovanadate (YVO₄) nanoparticles with nearly spherical shape were successfully synthesized via a molten salt method at a low temperature of 200°C. The as-prepared powders were characterized by X-ray diffraction, transmission electron microscopy and UV-Vis spectroscopy, respectively. The results show that increasing salt amount and/or elevating calcining temperature can greatly promote the crystallization and growth of YVO₄ phase. UV-Vis absorption spectra suggested that YVO₄ nanoparticles with the smaller particle size have the stronger UV absorption, and the sequent photocatalytic degradation data also confirmed their higher photocatalytic activity.     

Synthesis of Monodispersed Fine Hafnium Diboride Powders Using Carbo/Borothermal Reduction of Hafnium Dioxide

Fine hafnium diboride (HfB₂) powders have been prepared by modified carbothermal/borothermal reduction of hafnium dioxide (HfO₂) at relatively low temperatures (1500°–1600°C) for 1–2 h. The XRD patterns could be indexed as hexagonal HfB₂ and no evidence of HfC, HfO₂, or other impurities was observed. Glow discharge mass spectrometer analysis indicates that the synthesized HfB₂ powders had high purity. The synthesized HfB₂ powders had small average crystallite size (around 1 μm) and low oxygen content (<0.30 wt%). Scanning electron microscopy observation of the as-prepared powders demonstrated quasi-column morphology and laser particle size analysis showed monodispersity (polydispersity 0.005).     

Rapid Formation of Active Mesoporous TiO2 Photocatalysts via Micelle in a Microwave Hydrothermal Process

Rapid formation of active, mesoporous, and crystalline TiO₂ photocatalysts via a novel microwave hydrothermal process is presented. Crystalline anatase mesoporous nanopowders 100–300 nm in size with worm hole-like pore sizes of 3–5 nm were prepared by a modified sol–gel of titanium tetra-isopropoxide, accelerated by a microwave hydrothermal process. The organic surfactant, tetradecylamine, which is used as a self-assembly micelle in the sol–gel and microwave hydrothermal process, enables to harvest crystallized mesoporous anatase nanoparticles with a high-surface area. Mesoporous worm hole-like and crystalline powders with surface areas of 243–622 m²/g are obtained. X-ray diffraction, N₂-adsorption isotherms (Barrett–Joyner–Halenda and Brunauer–Emmet–Teller method), scanning electron microscope, and transmission electron microscope are used to identify the characteristics and morphologies of the powders. It is shown that crystallization by calcination at 400°C/3 h inevitably reduced the surface area, while the microwave hydrothermal process demonstrated a rapid formation of crystalline mesoporous TiO₂ nanopowders with a high-surface area and excellent photocatalytic effects.     

Preparation of Strontium- and Zinc-Doped LaGaO3 Powders via Precipitation in the Presence of Urea and/or Enzyme Urease

Solid oxide fuel cell powders having a composition of La_0.8Sr_0.2Ga_0.8Zn_0.2O_2.8 (LSGZ) were prepared by aqueous chemical precipitation in the presence of decomposing urea, followed by single-step calcination in air. In some synthesis experiments the decomposition of urea was catalyzed by the enzyme urease. The calcination behavior of the precursor powders was studied over the temperature range of 90–1300°C, in an air atmosphere. Characterization of the samples was performed by XRD, TG/DTA, FTIR, FESEM, EDS, and carbon analyses. Two tentative XRD patterns have been created for the hydroxycarbonate precursors and the product LSGZ ceramics, respectively.     

Spark Plasma Sintering of Nano-Sized TiN Prepared from TiO2 by Controlled Hydrolysis of TiCl4 and Ti(O-i-C3H7)4 Solution

Nano-sized TiO₂ powders were prepared by controlled hydrolysis of TiCl₄ and Ti(O-i-C₃H₇)₄ solutions and nitrided in flowing NH₃ gas at 700°–1000°C to form TiN. Nano-sized TiN was densified by spark plasma sintering at 1300°–1600°C to produce TiN ceramics with a relative density of 98% at 1600°C. The microstructure of the etched ceramic surface was observed by SEM, which revealed the formation of uniformly sized 1–2 μm grains in the TiCl₄-derived product and 10–20 μm in the Ti(O-i-C₃H₇)₄-derived TiN. The electric resisitivity and Vickers micro-hardness of the TiN ceramics was also measured.     

Chemical Preparation of Pure and Strontium- and/or Magnesium-Doped Lanthanum Gallate Powders

Powder compositions of LaGaO₃, La_0.9Sr_0.1GaO_2.95, and La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 were prepared via a Pechini-type process that uses citric acid and ethylene glycol. The calcination behavior of the precursor powders of the above-mentioned phases was studied in the temperature range of 200°–1400°C in an air atmosphere. Characterization of the powder samples were performed using several processes, including X-ray diffractometry, thermogravimetry/differential thermal analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, inductively coupled plasma–atomic emission spectroscopy, and carbon and nitrogen analyses.     

A Novel Hybrid Method of Sol–Gel and Ultrasonic Atomization Synthesis and Piezoelectric Properties of SrBi4Ti4O15 Ceramics

SrBi₄Ti₄O₁₅(SBTi) powders were synthesized by a novel hybrid method of sol–gel and ultrasonic atomization. TiO₂ particle was used as a starting material to replace other expensive soluble titanium salts. X-ray diffraction results showed that the pure-phase SBTi powders were obtained at 700°C for 2 h, which is much lower than the calcination temperature (800°–850°C) required in solid-state reactions. The ceramics sintered at 1100°C for 1 h exhibited 94.5% of relative density and a piezoelectric coefficient of 21 pC/N. The results showed that this hybrid method could lead to an attractive method for the industrial fabrication of SBTi materials.     

Alkoxide–Hydroxide Route for the Preparation of γ-LiAlO2-Based Ceramics

An alkoxide-hydroxide route has been developed to prepare Li_{4 + x} Al_{4 − 3 x} Si_{2 x} O₈ (0 ≤ x ≤ 0.25) powders by taking into account fundamental aspects of the sol-gel process. This technique allows one to prepare powders which exhibit the β-LiAlO₂ type of structure after drying at 150°C. The β→γ-LiAlO₂ topotactic transformation spreads over a large temperature range (746–839°C for x = 0.125) with no significant dilatometric and enthalpic change. Stoichiometric γ-LiAlO₂-based ceramics with a large variety of uniform microstructures are fabricated by a direct sintering of β-LiAlO₂ powders in the temperature range of 900–1100°C.     

La2Ti2O7 Ceramics

La₂Ti₂O₇ powders were prepared using three different techniques. Single-phase material was obtained at 1150°C by calcination of mixed oxides, at 1000°C by molten salt synthesis, and at 850°C by evaporative decomposition of solutions. Particle sizes and morphologies of the powders differed substantially, as did the sintered microstructures and dielectric properties. Very dense (99%), translucent, grain-oriented lanthanum titanate was fabricated by hot-forging at 1300°C under a 200-kg load. Anisotropy was demonstrated by X-ray diffraction, scanning electron microscopy, thermal expansion, and dielectric measurements.     

Synthesis and Processing Characteristics of Ba0.65Sr0.35TiO3 Powders from Catecholate Precursors

Powders of composition Ba_0.65Sr_0.35TiO₃ were prepared from catecholate precursor phases, BaTi(C₆H₄O₂)₃ and SrTi (C₆H₄O₂)₃. The physical and chemical properties of the base powders, and those doped with 0.2 wt% manganese, are reported in detail. The dimensions of the primary particles in the starting powders were of the order of 20–50 nm, but the occurrence of abnormal grain growth during sintering promoted grain sizes in the ceramic of up to ∼100 μm. In some microstructures, coarse grains coexisted with a ∼1-μm fraction to produce a characteristic bimodal grain size distribution. By contrast, under comparable sintering conditions, namely 1350° or 1400°C for 1 h, grain growth in Mn-doped samples was suppressed, leading to uniform microstructures with a grain size of only a few micrometers. The pellet densities were nevertheless similar, 97% of theoretical in both doped and undoped samples. No significant difference was observed in the dielectric permittivity of the two compositions: the peak relative permittivity occurred at ∼20°C, with a maximum value of ∼22 000.     

Characterization of Photoluminescent (Y1–xEux)2O3 Thin Films Prepared by Metallorganic Chemical Vapor Deposition

The purpose of this study was to identify and correlate the microstructural and luminescence properties of europium-doped Y₂O₃ (Y_{1– x} Eu _x )₂O₃ thin films deposited by metallorganic chemical vapor deposition (MOCVD), as a function of deposition time and temperature. The influence of deposition parameters on the crystallite size and microstructural morphology were examined, as well as the influence of these parameters on the photoluminescence emission spectra. (Y_{1– x} Eu _x )₂O₃ thin films were deposited onto (111) silicon and (001) sapphire substrates by MOCVD. The films were grown by reacting yttrium and europium tris(2,2,6,6-tetramethyl–3,5-heptanedionate) precursors with an oxygen atmosphere at low pressures (5 torr (1.7 × 10³ Pa)) and low substrate temperatures (500°–700°C). The films deposited at 500°C were smooth and composed of nanocrystalline regions of cubic Y₂O₃, grown in a textured [100] or [110] orientation to the substrate surface. Films deposited at 600°C developed, with increasing deposition time, from a flat, nanocrystalline morphology into a platelike growth morphology with [111] orientation. Monoclinic (Y_{1– x} Eu _x )₂O₃ was observed in the photoluminescence emission spectra for all deposition temperatures. The increase in photoluminescence emission intensity with increasing postdeposition annealing temperature was attributed to the surface/grain boundary area-reduction effect.