The System HfO2-TiO2

The system HfO₂-TiO₂ was studied in the 0 to 50 mol% TiO₂ region using X-ray diffraction and thermal analysis. The monoclinic ( M ) ⇌ tetragonal ( T ) phase transition of HfO₂ was found at 1750°± 20°C. The definite compound HfTiO₄ melts incongruently at 1980°± 10°C, 53 mol% TiO₂. A metatectic at 2300°± 20°C, 35 mol% TiO₂ was observed. The eutectoid decomposition of HfO_2,ss) ( T ) → HfO_2,ss ( M ) + HfTiO_34,ssss occurred at 1570°± 20°C and 22.5 mol% TiO₂. The maximum solubility of TiO₂ in HfO_2,ss,( M ) is 10 mol% at 1570°± 20°C and in HfO_2,ss ( T ) is 30 mol% at 1980°± 10°C. On the HfO₂-rich side and in the 10 to 30 mol% TiO₂ range a second monoclinic phase M of HfO₂( M ) type was observed for samples cooled after a melting or an annealing above 1600°C. The phase relations of the complete phase diagram are given, using the data of Schevchenko et al. for the 50% to 100% TiO₂ region, which are based on thermal analysis techniques.     

Thermal Expansion, Compressibility, and Polymorphism in Hafnium and Zirconium Titanates

Using X-ray diffraction techniques, thermal expansion and compressibility were measured on the orthorhombic compounds HfTiO₄, Hf_1.26Ti_0.74O₄, and ZrTiO₄ (both quenched and cooled slowly from 1300°C). The thermal expansion of HfTiO₄ is highly anisotropic; the thermal expansion coefficients along the crystallographic axes are α _a =+(8.7±0.5)×10⁻⁶°C⁻¹, α _b =−(5.2±0.5)×10⁻⁶°C⁻¹, and α _c =+ (5.3±0.5)×10⁻⁶°C⁻¹. The thermal expansion of Hf_1.26Ti_0.74O₄ was similar to that of HfTiO₄ but that of ZrTiO₄ was markedly less anisotropic. The compressibilities of HfTiO₄ and ZrTiO₄ also differed markedly. All compounds investigated, however, behaved similarly in exhibiting a polymorphic transition to a high-pressure phase having the monoclinic baddeleyite (ZrO₂) structure. The polymorphism can be explained qualitatively on the basis of crystal structure.     

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).     

Alkoxide Route to La0.5Sr0.5CoO3 Films and Powders

An all-alkoxide route to films and nano-phase powders of the La_0.5Sr_0.5CoO₃ perovskite is described. To our knowledge, this is the first purely alkoxide-based route to (La_{1− x} Sr _x )CoO₃, and it yields phase-pure and elementally homogeneous perovskite at 700°C by heating at 2°C/min. At 700°C, a cubic unit cell was obtained with a _c=3.853Å, and after further heating to 1000°C, a rhombohedral cell could be indexed: a _r=5.417 Å, α_r=59.94°. Ninety to 130 nm thick films of La_0.5Sr_0.5CoO₃ were obtained by spin coating. The gel-to-oxide conversion was studied in some detail, using thermo-gravimetric analysis, differential scanning calorimetry, powder X-ray diffraction, IR spectroscopy, and transmission electron microscope equipped with an energy-dispersive X-ray spectrometer.     

Preparation of Lead Zirconate Titanate (Pb(Zr0.52Ti0.48)O3) by Homogeneous Precipitation and Calcination

Preparation of phase-pure PZT (Pb(Zr_0.52Ti_0.48)O₃) powders was achieved, in the presence of urea (CH₄N₂O), by homogeneous precipitation. Aqueous solutions of PbCl₂, ZrOCl₂·8H₂O, and TiCl₄ were used as the starting materials in the synthesis of phase-pure PZT powders. Phase evolution behavior of precursor powders was studied by powder X-ray diffraction (XRD) in air, over the temperature range of 90° to 750°C. The morphology of the formed powders was studied by scanning electron microscopy (SEM). Semiquantitative chemical analyses of the samples were performed by energy-dispersive X-ray spectroscopy (EDXS).     

Low-Temperature Synthesis of Nanocrystalline α-Si3N4 Powders by the Reaction of Mg2Si with NH4Cl

Nanocrystalline α-Si₃N₄ powders have been prepared with a yield of 93% by the reaction of Mg₂Si with NH₄Cl in the temperature range of 450° to 600°C in an autoclave. X-ray diffraction patterns of the products can be indexed as the α-Si₃N₄ with the lattice constants a = 7.770 and c = 5.627 Å. X-ray photoelectron spectroscopy analysis indicates that the composition of the α-Si₃N₄ samples has a Si:N ratio of 0.756. Transmission electron microscopy images show that the α-Si₃N₄ crystallites prepared at 450°, 500°, and 550°C are particles of about 20, 40, and 70 nm in average, respectively.     

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.     

Thermal Expansion of Solid Solutions in the ZrTiO4—In2O3—M2O5 (M = Sb, Ta) System

Axial and dilatometric thermal expansions and phase transformations were studied for solid solutions having the α-PbO₂ structure in the ZrTiO₄—In₂O₃—M₂O₅ (M = Sb, Ta) system with nominal formulas of Zr _x Ti _y In _z Sb _z O₄ and Zr _x Ti _y In _z Ta _z O₄ where x + y + 2 z = 2. With increased substitution of z , the cell volume increased, the difference in the b parameters at room temperature between those quenched from 1400° and 1000°C decreased, and the thermal expansion decreased. The axial thermal expansion of ZrTi _y In _z · Ta _z O₄ with z = 0.3 was almost identical with that of HfTiO₄, and those with z = 0.4 and z = 0.45 were smaller than that of HfTiO₄. Unit-cell volumes of these compound were compared with those of single oxides to make it clear that the unit-cell volume of ZrTiO₄ was small anomalously and to distinguish the normal and abnormal substitution systems. These results were explained by the working hypothesis proposed for these compounds.     

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.     

Reverse Micellar Route to Nanocrystalline Titanates (SrTiO3, Sr2TiO4, and PbTiO3): Structural Aspects and Dielectric Properties

Nanoparticles of strontium titanates (SrTiO₃, Sr₂TiO₄) and lead titanate (PbTiO₃) have been obtained using reverse micelles as nanoreactors. Powder X-ray diffraction studies of the powders after calcining at 800°C show monophasic SrTiO₃, Sr₂TiO₄, and PbTiO₃. X-ray line broadening studies and transmission electron microscopic studies show spherical grains of 30–40 nm size for strontium titanates, while PbTiO₃ is obtained in the form of nanorods. The dielectric constant of SrTiO₃ and Sr₂TiO₄ is found to be 90 and 30, respectively, (at 100 kHz) for samples sintered at 1000°C. PbTiO₃ shows a dielectric constant of 160 (at 100 kHz) after sintering at 900°C. The dielectric constant of Sr₂TiO₄ (with temperature) is highly stable. The temperature variation studies of the dielectric constant of PbTiO₃ show a ferroelectric phase transition at 490°C (1 kHz). The T _c varies with frequency and is found to decrease to 470°C at 100 kHz.     

Solid-State Phase Equilibria in the ZnS-Ga2S3 System

The ZnS-Ga₂S₃ equilibrium phase diagram has been determined to 50 mol% over the temperature range 700° to 900°C. Samples of various compositions were prepared via solid-state diffusion starting from powders of the pure components. The identification of the phases was determined by X-ray diffraction methods. The principal feature of the phase equilibria is the eutectoid transformation at 818 ± 5°C of hexagonal wurtzite containing 16 ± 1 mol% Ga₂S₃ to cubic ZnS and tetragonal ZnGa₂S₄. ZnGa₂S₄ is the equilibrium compound at 50 mol% Ga_zS₃, but it exists over a considerable range of stoichiometry. The solubility of Ga₂S₃ in ZnS increases with increasing temperature to a maximum of 9 ± 1 mol% at the eutectoid temperature.     

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.     

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.     

Magnesium Aluminate (MgAl2O4) Spinel Powders from Water-Based Sols

Magnesium aluminate (MgAl₂O₄) spinel powders of irregular and spherical morphologies were obtained from the bi-component water-based sols following the sol–gel and sol–emulsion–gel methods, respectively. For the synthesis of the oxide microspheres, the surfactant concentration and viscosity of the sols were found to affect the characteristics of the derived microspheres. The gel and calcined powders were investigated by using thermogravimetry analysis, differential thermal analysis, X-ray diffraction (XRD), optical and scanning electron microscopy (SEM), Fourier transformed infrared spectroscopy, and particle size analysis. XRD results indicated crystallization of the only phase MgAl₂O₄ spinel from 200° to 1000°C. Formation of hollow microspheres with a single cavity was identified by SEM.     

Single-Phase MnFe2O4 Powders Obtained by the Polymerized Complex Method

The present investigation reports on a simple technique for the preparation of single-phase MnFe₂O₄ nanocrystalline powders using the polymerized complex method, starting from manganese carbonate and iron nitrate. A mixed aqueous solution with citric acid, ethylene glycol, Fe, and Mn ions was polymerized. The phases formation, the homogeneity, and the structure of the obtained powders have been investigated by thermogravimetry, X-ray diffraction (XRD), scanning and transmission electron microscopy, and Mössbauer spectroscopy measurements. The magnetic hysteresis loop behavior was measured at 5 K in a vibrating sample magnetometer. XRD results demonstrated that thermally induced crystallization of cubic MnFe₂O₄ from the Mn–Fe polymeric precursor occurred at temperatures as low as 400°C. Pure single-phase MnFe₂O₄ nanocrystallites without any impurity or amorphous phases were obtained when the precursor was treated at 600°C for 1 h.     

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.     

Sonochemical Preparation of PbTiO3 Fine Powders

The sonochemical preparation of PbTiO₃ fine powders with lead acetate trihydrate and tetrabutyl titanate as precursors is reported in this paper. Narrow size distribution (40–60 nm) of gel-derived PbTiO₃ nanocrystallites at 520°C was revealed by transmission electron microscopy. The preparation processings with ultrasonic irradiation and a control test without ultrasonic irradiation for comparison were investigated. Results of phase evolution, microstructure, and thermal analysis of gels were characterized and discussed between the control test and the ultrasonic preparations. Acoustic cavitation exerted an important influence on nucleation, gelation, and homogenization of as-prepared gel, which contributed to the evolution of the resulting gel on further heat treatment. The sonochemical gel was detected to crystallize at temperature, as low as about 410°C, which was confirmed by X-ray powder diffraction and thermogravimetric analysis and differential thermal analysis (TGA–DTA).     

Preparation of Strontium Ferrite Particles by Spray Pyrolysis

Crystalline, submicrometer strontium ferrite powders, including SrFeO_2.97, SrFe₂O₄, Sr₂FeO₄, Sr₃Fe₂O_6.16, and SrFe₁₂O₁₉, were prepared by spray pyrolysis of an aqueous solution of mixed metal nitrates. The Sr:Fe mole ratio in the precursor solution was retained in the final products. Phase-pure materials were typically obtained only at the highest temperatures investigated (>1100°C) and powders prepared at lower temperatures frequently contained crystalline Fe₂O₃. The as-prepared particles were unagglomerated, polycrystalline, and hollow at lower temperatures, but densified in the gas phase at higher temperatures to give solid particles. The strontium ferrite (SrFe₁₂O₁₉) system was studied in detail as a representative example of the Sr-Fe-O system. At temperatures of 1200°C, dense, phase-pure magnetoplumbite-structure material, SrFe₁₂O₁₉, was obtained, while at lower temperatures, small amounts of Fe₂O₃ were observed. The particles prepared at 800° and 1100°C were 0.1-1.0 μm in diameter, and consisted of crystallites <100 nm, and were nearly solid. The difficulty in forming phase-pure SrFe₁₂O₁₉ was the different thermal decomposition temperatures of Sr(NO₃)₂ (725°C) and Fe(NO₃)₃9H₂O (125°C) as demonstrated by thermogravimetric analysis in the SrFe₁₂O₁₉ system.     

Oxygen Evolution During MnO-Mn3O4 Dissolution in a Borosilicate Melt

Foam evolution during dissolution of MnO-Mn₃O₄ pellets and powders in borosilicate glass was recorded photographically. The pellets were placed horizontally in transparent crucibles, covered with molten glass, and held at 1150°C. If the Mn₃O₄ content in pellets was more than 31 wt%, they developed foam after an initial foamless period. The length of the foamless period decreased and the duration of foaming increased as the Mn₃O₄ content increased. Batches prepared from MnO-Mn₃O₄ powders and frit, and soaked at 1150°C, foamed without an initial foamless period. The foam developed and collapsed before the set temperature was established within the melt and rose to a higher level than foam produced by pellets. Thermogravimetry of batches heated in 1 atm (∼10⁵ Pa) of O₂ shows oxidation at 400° to 600°C followed by mass loss due to volatilization and oxygen evolution.     

Novel Low-Temperature Synthesis of Ferroelectric Neodymium-Doped Bismuth Titanate Nanoparticles

In this study, we report on the synthesis of nanopowders of ferroelectric Bi_3.5Nd_0.5Ti₃O₁₂ ceramic at temperatures below 500°C via a simple chemical method using citric acid as a solvent. The calcined powders were characterized using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). Heating the as-dried powders in air first leads to crystallization of the Bi₂Ti₂O₇ phase at ∼310°C, followed by crystallization of the perovskite Nd-doped Bi₄Ti₃O₁₂ phase at ∼490°C as suggested by the peaks in the DSC analysis and confirmed by the evolution of phases in XRD patterns of the powders calcined at various temperatures. TEM of particles calcined at 550°C for 1 h in air showed an average particle size of 50–60 nm. The temperature dependence of capacitance of nanopowders calcined at 700°C for 1 h in air showed a Curie temperature of ∼615°C evincing a ferroelectric transition.