Sol-Gel Synthesis of a New Oxide-Ion Conductor Sr- and Mg-Doped LaGaO3 Perovskite

Sr- and Mg-doped LaGaO₃ powders were prepared from a salt acetate solution. The stable solution was peptized by reacting ammonium hydroxide with the precursor solution. Thermal analysis (DTA/TGA) was used to characterize first the dehydration and then the thermal decomposition of the organic ligands of the dried gel. The transformation from amorphous powders into a crystallized, homogeneous oxide phase corresponds to two endothermic peaks in the DTA curve; the first one at 150°C is related to the removal of water and is followed by a shoulder at 250°C. The second peak at 300°C corresponds to a superposition of two decomposition reactions: acetate salt into its oxycarbonate and this oxycarbonate into its oxide. Two subsequent exothermic peaks correspond to oxidation of evolved gases such as methane, hydrogen, and carbon monoxide. TEM observations show an average 10 nm particle size of the LaGaO₃powder after annealing at 600°C. X-ray diffraction patterns indicate a pure primitive-cubic phase is formed by 1300°C without formation of any SrLaGaO₃ impurity. The impedance spectroscopy on a 93%-dense sample exhibits no grain-boundary contribution and an ac conductivity σ= 0.11 Ω⁻¹·cm⁻¹ at 800°C.     

Metastable Crystal Structure of Strontium- and Magnesium-Substituted LaGaO3

The metastable crystal structure of strontium- and magnesium-substituted LaGaO₃ (LSGM) was studied at room and intermediate temperatures using powder X-ray diffractometry and Rietveld refinement analysis. With increased strontium and magnesium content, phase transitions were found to occur from orthorhombic (space group Pbnm ) to rhombohedral (space group R [Threemacr] c ) at the composition La_0.825Sr_0.175Ga_0.825Mg_0.175O_2.825 and, eventually, to cubic (space group Pm [Threemacr] m ) at the composition La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8. At 500°C in air and at constant strontium and magnesium content, a phase transformation from orthorhombic (space group Pbnm ) to cubic (space group Pm [Threemacr] m ) was observed. For the orthorhombic modification, thermal expansion coefficients were determined to be α _a ,ortho = 10.81 × 10⁻⁶ K⁻¹, α _b ,ortho = 9.77 × 10⁻⁶ K⁻¹, and α _c ,ortho = 9.83 × 10⁻⁶ K⁻¹ (25°–400°C), and for the cubic modification to be α_cubic= 13.67 × 10⁻⁶ K⁻¹ (500°–1000°C).     

Synthesis and Characterization of Nanocrystalline SrTiO3

The synthesis and densification of fairly dense nanocrystalline SrTiO₃ and its characterization are described in this paper in detail. Significant grain growth was avoided by the application of a two-stage sintering process using hot pressing. High-resolution transmission electron microscopy and electron energy loss spectroscopy characterizations indicate pure material with no detectable chemical inhomogeneities. The electrical measurements indicate the disappearance of bulk contribution to the electrical conduction due to the overlap of depleted space charge layers if the grain size is below 100 nm. Owing to the overlap, the capacitance appears as bulk-like rather than due to space charge polarization. Non-contact sub-millimeter optical spectroscopy measurements reveal strong suppression of the dielectric constant values at low temperature.     

Defect Structure of Mg-Doped LaCrO3 Model and Thermogravimetric Measurements

Chemical stability and cation stoichiometry determine the applicability of LaCrO₃ as a high-temperature oxide electrode. A model for the behavior of acceptor-doped LaCrO₃, as a function of oxygen activity is proposed. The model is in agreement with experimental data on Mg-doped LaCrO₃. Stability regimes and compensation mechanisms at various oxygen activities and temperature are presented.     

Microstructure and Photoluminescence Properties of Mg-Doped BaTiO3:Pr3+ Phosphor

The photoluminescence of Mg-doped BaTiO₃:Pr³⁺ (Pr³⁺: 0.1 mol%) ceramics was investigated by changing the doping concentration of Mg and the sintering temperature. The results indicated that the intensity of red emission due to the ¹ D ₂→³ H ₄ transition of Pr³⁺ exhibited significant dependence on both the Mg doping content and the sintering temperature; the strongest red emission intensity was observed for 2.0 mol% Mg-doped ceramics sintered at 1050°C. An interpretation of the results obtained was made in terms of the changes in the crystal structure and microstructure of the ceramics.     

Preparation and Characterization of Alkoxy-Derived SrZrO3 and SrTiO3

High-purity strontium, zirconium, and titanium alkoxides were synthesized and characterized as precursors for complex oxides. Simultaneous hydrolytic decomposition either of strontium and zirconium alkoxides or of strontium and titanium alkoxides was used to obtain nearly stoichiometric, ideally mixed SrZrO₃ or SrTiO₃ powders of high surface activity. As-prepared helium-dried SrTiO₃ is crystalline before calcination. An ultraviolet radiation technique demonstrates the nucleation and growth of SrZrO₃ crystallites in the calcination temperature range to 350°C. The experimental results are supported by ir, TGA, and X-ray diffraction data. The high degree of control over purity, mixing uniformity, and crystallite size demonstrates the value of the alkoxide precursor approach for the solution of reproducibility problems encountered in the synthesis of electrical-quality ceramics.     

Synthesis and Characterization of Bulk Zr2Al3C4 Ceramic

Polycrystalline Zr₂Al₃C₄ was fabricated by an in situ reactive hot-pressing process using zirconium (zirconium hydrides), aluminum, and graphite as starting materials. The investigation on reaction path revealed that the liquid Al played an important role in triggering the formation of ternary zirconium aluminum carbides. The mechanical properties of Zr₂Al₃C₄ at room temperature were measured (Vickers hardness of 10.1 GPa, Young's modulus of 362 GPa, flexural strength of 405 MPa, and fracture toughness of 4.2 MPa·m^1/2). The electrical resistivity and thermal expansion coefficient were determined as 1.10 μΩ·m and 8.1 × 10⁻⁶ K⁻¹, respectively.     

Mineralizer-Assisted Hydrothermal Synthesis and Characterization of BiFeO3 Nanoparticles

BiFeO₃ (BFO) nanoparticles were successfully synthesized by a hydrothermal method assisted by a mineralizer at as low a temperature as 200°C. The products were characterized by X-ray powder diffraction, transmission electron microscope, and physical property measurement system. It was found that KNO₃ as a mineralizer played an important role in the formation of BFO nanoparticles, and the as-synthesized BFO nanoparticles showed an enhanced ferromagnetism at room temperature.     

Characterization of Freeze-Dried Al2O3 and Fe2O3

Oxide crystallite formation and growth from freeze-dried sulfates were studied for the representative materials Al₂O₃ and Fe₂O₃. Transmission and scanning electron micrographs showed the formation and growth of chainlike aggregates of crystallites. Aggregation occurred as part of the nucleation and growth of the oxide, and discrete oxide particles were never present. Orientation of the chain aggregates was related to the ice structure formed during freezing. X-ray line broadening data showed that crystallite size is a function of the 1/5 to 1/7 power of time for isothermal treatments. A qualitative analysis of material transport favored the surface diffusion mechanism.     

Synthesis and Characterization of a Remarkable Ceramic: Ti3SiC2

Polycrystalline bulk samples of Ti₃SiC₂ were fabricated by reactively hot-pressing Ti, graphite, and SiC powders at 40 MPa and 1600°C for 4 h. This compound has remarkable properties. Its compressive strength, measured at room temperature, was 600 MPa, and dropped to 260 MPa at 1300°C in air. Although the room-temperature failure was brittle, the high-temperature load-displacement curve shows significant plastic behavior. The oxidation is parabolic and at 1000° and 1400°C the parabolic rate constants were, respectively, 2 × 10⁻⁸ and 2 × 10⁻⁵ kg²-m⁻⁴.s⁻¹. The activation energy for oxidation is thus =300 kJ/mol. The room-temperature electrical conductivity is 4.5 × 10⁶Ω⁻¹.m⁻¹, roughly twice that of pure Ti. The thermal expansion coefficient in the temperature range 25° to 1000°C, the room-temperature thermal conductivity, and the heat capacity are respectively, 10 × 10⁻⁶°C⁻¹, 43 W/(m.K), and 588 J/(kgK). With a hardness of 4 GPa and a Young's modulus of 320 GPa, it is relatively soft, but reasonably stiff. Furthermore, Ti₃SiC₂ does not appear to be susceptible to thermal shock; quenching from 1400°C into water does not affect the postquench bend strength. As significantly, this compound is as readily machinable as graphite. Scanning electron microscopy of polished and fractured surfaces leaves little doubt as to its layered nature.     

Synthesis and Characterization of Thin Ferroelectric PbZr0.52Ti0.48O3 Fibrils

Thin fibrils (usually 20–100 nm in diameter) of ferroelectric PbZr_0.52Ti_0.48O₃ have a preferred nucleation and growth in the [001] direction in small particles. X-ray diffraction with the X-ray beam facing the fibril surfaces, which lie along the [001] direction, thus involves reflections of manifested intensity from (100) and (010) planes. No reflection occurs from the perpendicular planes to the [001] fibrils. A transmission electron micrograph with the corresponding electron diffraction from fibrils confers the results of the preferred growth. The fibrils have a modified orthorhombic crystal structure of lattice parameters a =0.4038 nm, b =0.4017 nm, and c =04148 nm, with an enhanced 8.033 g/cm³ density over 8.006 g/cm³ in the usual tetragonal structure of a =0.4036 nm and c =04146 nm. Surface anisotropy in a fibril that imposes (and drives) a shear stress over growing crystallites (within the fibril) along the [001] surface favors this specific crystal structure in them according to the precursor structure. A single-phase sample is obtained in 2 h by heating of an amorphous template of precursor (of the metal cations with a polymer of polyvinyl alcohol and sucrose) at 500°–800°C in air.     

Dynamic Fracture Characterization of Al2O3 and SiCw/Al2O3

The dynamic stress intensity factors, which were determined with newly developed bar impact facilities and a new data reduction procedure, for an Al₂O₃ ceramic and 29 vol% SiC_w/Al₂O₃ composite were virtually identical, thus indicating that the short SiC whiskers were ineffective under dynamic fracture. SEM studies revealed five distinct fracture morphologies with increased percentage area of transgranular fracture in both materials with rapid crack propagation. Also, the high dynamic stress intensity factor caused multiple microscopic crack planes to form and then join as the crack advanced.     

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.     

Solid-State Synthesis of Ultrafine BaTiO3 Powders from Nanocrystalline BaCO3 and TiO2

Barium titanate has been prepared by solid-state reaction of nanocrystalline TiO₂ (70 nm) with BaCO₃ of different particle size (650, 140, and 50 nm). The results give evidence of a strong effect of the size of BaCO₃ in the solid-state synthesis of barium titanate. The use of nanocrystalline BaCO₃ already leads to formation of the single-phase BaTiO₃ after calcination for 8 h at 800°C. The final powder consists of primary particles of ≈100 nm, has a narrow particle size distribution with d ₅₀=270 nm, and no agglomerates larger than 800 nm. For the coarser carbonate, 4 h calcination at 1000°C are required and the final powder is much coarser. Solid-state reaction of nanocrystalline BaCO₃ and TiO₂ represents an alternative to chemical preparation routes for the production of barium titanate ultrafine powders.     

Critical Current Densities in Thin Ceramic Tapes of Superconducting Ba2YCu3O7

Tape-cast slurries of Ba₂YCu₃O₇ powders offer a convenient means of preparing sintered ceramic samples for critical current density (J_c) measurements where the transport cross section is small and the current electrode areas are large. Samples were sintered from 900° to 1000°C and characterized for bulk density, grain size, phase composition, T_c, and J_c. Bulk density and grain size both increase with sintering temperature while all samples were single-phase perovskite except for those sintered at 900°C. The onset temperature for superconductivity is constant at about 93 K while the transition sharpens to R=0 at about 92 K for the densest samples. J_c rises with sintering temperature to a maximum of ∼10³ A/cm². A linear relationship between J_c and bulk density is predicted from microstructural considerations.     

High-pressure Synthesis of PbCrO3

A new compound with the composition PbCrO₃, with Cr in the valence state of 4, was synthesized at high pressures above a pressure-temperature line extending from about 50 kbars at 750°C to 60 kbars at 1450°C. PbCrO₃ can be quenched and retained at 1 atm but decomposes on heating above 275° C at the same pressure. PbCrO₃ is considered to be an equilibrium phase at high pressures because it was synthesized from mixtures of PbO: CrO₂ as well as from several other mixtures of compounds in the Pb-Cr-0 system. The new phase has the cubic perovskite structure and is the only known compound with Cr⁴⁺ in an octahedral site. PbCrO₃ crystallizes primarily as black cubes which are often twinned on (111).     

Characterization of Mixed Uranium Antimony Oxides USbO5 and USb3O10

The system U-Sb-O was examined in air up to 1000°C for ternary compounds. The products were characterized by chemical analysis, density determinations, TGA, X-ray diffraction techniques, and magnetic measurements. The ternary compounds found were USbO₅ and USb₃O₁₀. All cations are pentavalent in these new compounds, which are crystallographically related to α-UO₃ and U₃O₈.     

High-Pressure Synthesis of Perovskites Pb(Li1/4Nb3/4)O3 and Pb(Li1/4Ta3/4)O3

Complex perovskite-type compounds with the general formula Pb(B⁺_1/4B⁵⁺_3/4)O₃, where B⁺= Li⁺ and B⁵⁺= Nb⁵⁺ or Ta⁵⁺, were synthesized using a high-pressure technique and studied by X-ray powder diffraction. The X-ray patterns were indexed on the basis of a cubic cell with a ₀= 4.071 Å for Pb(Li_1/4 Nb_3/4)O₃ and a ₀= 4.052 Å for Pb(Li_1/4Ta_3/4)O₃. Electrical properties of the new perovskites were also studied.     

Preparation and Characterization of KNbO3 Ceramics

The processing and characterization of electrical properties of potassium niobate ceramic, KNbO₃ (KN) have been studied. The difficulty of obtaining dense samples by conventional methods limits the knowledge on electrical properties of this material. In this paper, a complete route for processing of KN with density over 94% is described. Piezoelectric and dielectric data are presented. It is observed that the major problem concerning the density is related to incorrect stoichiometry, which is believed to be a more critical issue for this system compared with other electro-ceramics.