Improved Energy Storage Performance and Fatigue Endurance of Sr-Doped PbZrO3 Antiferroelectric Thin Films

Sr-doped PbZrO₃ antiferroelectric (AFE) thin films have been fabricated on the platinum-buffered silicon substrates via the sol–gel technique. The temperature-dependent dielectric properties results indicated that the AFE phase was stabilized for the Sr-modified PbZrO₃ thin films with a Curie temperature of 251°C. The recoverable energy density and energy efficiency of the Sr-doped PbZrO₃ thin films were enhanced by the doping of strontium. Compared with the pure PbZrO₃ AFE thin films, the performance against fatigue of the Sr-doped PbZrO₃ thin films were also improved greatly.     

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.     

Reaction Between Strontium-Doped Lanthanum Cuprate and Yittria-Stabilized Zirconia

Sr-doped lanthanum cuprate was recently investigated as a potential cathode material for the intermediate temperature solid oxide fuel cell using yittria-stabilized zirconia as an electrolyte. The material thermal stability of cathode material against an electrolyte at an operating temperature plays an important role in the fuel cell's performance. In this study, the structural stability between Sr-doped lanthanum cuprate and 8 mol% yittria-stabilized zirconia was investigated by using the powder mixture and diffusion couple. The chemical reaction between these two materials in the temperature ranging from 800° to 1000°C was examined by X-ray diffraction analyses. The interfacial reaction behavior between electrode and electrolyte pellets of the reaction couple was examined by a grazing X-ray diffractometer, wavelength-dispersive X-ray analysis, and an electron probe microanalyzer. No reaction products were observed as these two materials co-fired at 800°C. The reaction products of pyrochlore La₂Zr₂O₇ and perovskite SrZrO₃ were formed when the specimens were heat treated at over 900°C. Because of the formation of these reaction products, the CuO was precipitated. Furthermore, an excess of SrZrO₃ formation leads to the phase decomposition of perovskite Sr-doped lanthanum cuprate to La_{2− x} Sr _x CuO₄ and CuO due to a decrease in strontium concentration in Sr-doped lanthanum cuprate.     

Post-Hot-Pressing and High-Temperature Bending Strength of Reaction-Bonded Silicon Nitride-Molybdenum Disilicide and Silicon Nitride-Tungsten Silicide Composites

A hot-pressing technique was used for the further densification of reaction-bonded silicon nitride-molybdenum disilicide and silicon nitride-tungsten silicide (Si₃N₄-MoSi₂ and Si₃N₄-WSi₂, respectively) compacts that were prepared via a presintering step and a nitriding process from silicon-molybdenum or silicon-tungsten powders. After hot pressing was performed at 1650°C (25 MPa for 1 h), most of the alpha-Si₃N₄ that formed during the reaction-bonding process was transformed to β-Si₃N₄ and, moreover, a very small amount of Mo₅Si₃ (W₅Si₃) was formed in addition to MoSi₂ (WSi₂). Three- and four-point bend tests were performed at room temperature (25°C), 1000°C, 1200°C, and 1400°C. The bend strength of the Si₃N₄-WSi₂ composite increased slightly from room temperature up to 1000°C, whereas the Si₃N₄-MoSi₂ composite showed a more-pronounced increase up to 1200°C. Microstructural analysis was performed on the fracture surfaces of both composites that were tested at different temperatures.     

Preparation of Dense BaTiO3 Ceramics with Submicrometer Grains by Spark Plasma Sintering

Dense BaTiO₃ ceramics consisting of submicrometer grains were prepared using the spark plasma sintering (SPS) method. Hydrothermally prepared BaTiO₃ (0.1 and 0.5 µm) was used as starting powders. The powders were densified to more than similar/congruent95% of the theoretical X-ray density by the SPS process. The average grain size of the SPS pellets was less than similar/congruent1 µm, even by sintering at 1000-1200°C, because of the short sintering period (5 min). Cubic-phase BaTiO₃ coexisted with tetragonal BaTiO₃ at room temperature in the SPS pellets, even when well-defined tetragonal-phase BaTiO₃ powder was sintered at 1100° and 1200°C and annealed at 1000°C, signifying that the SPS process is effective for stabilizing metastable cubic phase. The measured permittivity was similar/congruent7000 at 1 kHz at room temperature for samples sintered at 1100°C and showed almost no dependence on frequency within similar/congruent10⁰-10⁶ Hz; the permittivity at 1 MHz was 95% of that at 1 kHz.     

Synthesis of Red-Emitting, Small Particle Size Luminescent Oxides Using an Optimized Combustion Process

A novel ceramic synthesis technique, combustion synthesis, was explored to produce red-emitting Cr³⁺-doped Y₃A1₅O₁₂ and Eu³⁺-doped Y₂O₃ phosphors with improved physical and luminescent properties. This technique involves the reaction of metal nitrates (oxidizers) and an organic fuel (urea, carbohydrazide, glycine) at 500°C. Resulting powders are well-crystallized, with a large surface area and small particle size. The spectral energy distribution was observed using photoluminescence measurements. The effects of processing parameters such as type of fuel, fuelto-oxidizer ratio, furnace temperature, and batch water content were studied. An increase in phosphor brightness with increasing reaction temperature was observed. Postreaction heat treatments (1000°, 1300°, and 1600°C) increased the luminous intensity of as-synthesized powders. Residual carbon content and chromium site symmetry were investigated as possible explanations for the increase in brightness with increasing heat treatment temperature. By tailoring the reaction chemistry, the optimal conditions for producing the most luminescent phosphors have been identified.     

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.     

Oxygen-Enhanced Crystallization of Solution-Derived 12CaO·7Al2O3

12CaO·7Al₂O₃ (C12A7) composed of nanosize cage structure can clathrate oxygen radicals (O⁻) and has a high potential to application of strong oxidizing catalysis. In the present report, we demonstrate a fabrication route to C12A7 fine powders by Chemical Solution Deposition method in order to enhance the catalytic reactivity. Aluminum sec-butoxide, calcium nitrate tetrahydrate, acetylacetone, 2-methoxyethanol, and nitric acid were used as raw materials. Precursor solution was dried and annealed at 800°–900°C in air or O₂ atmosphere. Crystalline C12A7 powders were obtained by annealing at 900°C in O₂ atmosphere. Scanning electron microscope and transmission electron microscope images of the obtained powders revealed C12A7 particles were sintered and formed several micrometer particles with many pores. BET specific surface area of the powders was 4.2 m²/g. Possibility for synthesizing C12A7 powder with higher specific surface area by the solution process was indicated.     

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.     

Synthesis and Preparation of Lanthanum Aluminate Target for Radio-Frequency Magnetron Sputtering

A polycrystalline LaAlO₃ target for the radio-frequency (rf) magnetron sputterings of LaAlO₃ thin films has been prepared. These films serve as a buffer layer for high- T _c YBa₂Cu₃O_{7– x} superconducting thin films on Si. Synthesis of lanthanum aluminate powder from a mixture of La₂O₃ and Al₂O₃ powders was performed by calcining from 1000° to 1600°C in air. Characterization of the calcined powders by X-ray diffraction indicates that full development of LaAlO₃ phase was evident in the sample calcined for 3 h at 1600°C in air. A polycrystalline LaAlO₃ target was prepared by heat treatment at 1500°C for 2 h in air after pressureless sintering at 1750°C for 3 h in Ar. Thin films of the LaAlO₃ on Si (100) were obtained by rf magnetron sputtering using the target and oxygen-annealing the as-deposited films.     

Ultra-Fine Ba2Ti9O20 Powders Synthesized by Inverse Microemulsion Processing and their Microwave Dielectric Properties

A double–inverse microemulsion (IME) process is used for synthesizing nano-sized Ba₂Ti₉O₂₀ powders. The crystallization of powders thus obtained and the microwave dielectric properties of the sintered materials were examined. The IME-derived powders are of nano-size (∼21.5 nm) and possess high activity. The BaTi₅O₁₁, intermediate phase resulted when the IME-derived powders were calcined at 800°C (4 h) in air. However, high-density Ba₂Ti₉O₂₀ materials with a pure triclinic phase (Hollandite like) can still be obtained by sintering such a BaTi₅O₁₁ dominated powders at 1250°C/4 h. The phase transformation kinetics for the IME-derived powders were markedly enhanced when air was replaced by O₂ during the calcinations and sintering processes. Both the calcination and densification temperatures were reduced by around 50°C compared with the process undertaken in air. The microwave dielectric properties of sintered materials increase with the density of the samples, resulting in a large dielectric constant ( K ≅39) and high-quality factor ( Q × f ≅28 000 GHz) for samples possessing a density higher than 95% theoretical density, regardless of the sintering atmosphere. Overfiring dissociates Ba₂Ti₉O₂₀ materials and results in a poor-quality factor.     

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.     

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.     

Some Properties of Aluminum Carbide Powder Prepared by the Pyrolysis of Alkylaluminu

Ultrafine aluminum carbide (Al₄C₃) powders with crystallite sizes of <40 nm were prepared by the pyrolyses of alkylaluminums, i.e., trimethylaluminum (Al(CH₃)₃: TMAL), triethylaluminum (Al(C₂H₅)₃: TEAL), triisobutylaluminum (Al(i-C₄H₉)₃: TIBAL) at a temperature between 950° and 1100°C. Although the pyrolysis of TMAL produced Al₄C₃ at 950°C, the pyrolysis temperature of TEAL to produce Al₄C₃ was raised up to 1100°C. The pyrolysis of TIBAL at 1100°C produced not only crystalline Al₄C₃ but also amorphous oxycarbide. The TEAL-derived powder had the highest true density (2.89 g^.cm⁻³ or 97% of the theoretical density) among the three kinds of powders.     

Reaction and Formation of Crystalline Silicon Oxynitride in Si–O–N Systems under Solid High Pressure

Oxidized amorphous Si₃N₄ and SiO₂ powders were pressed alone or as a mixture under high pressure (1.0–5.0 GPa) at high temperatures (800–1700°C). Formation of crystalline silicon oxynitride (Si₂ON₂) was observed from amorphous silicon nitride (Si₃N₄) powders containing 5.8 wt% oxygen at 1.0 GPa and 1400°C. The Si₂ON₂ coexisted with β-Si₃N₄ with a weight fraction of 40 wt%, suggesting that all oxygen in the powders participated in the reaction to form Si₂ON₂. Pressing a mixture of amorphous Si₃N₄ of lower oxygen (1.5 wt%) and SiO₂ under 1.0–5.0 GPa between 1000° and 1350°C did not give Si₂ON₂ phase, but yielded a mixture of α,β-Si₃N₄, quartz, and coesite (a high-pressure form of SiO₂). The formation of Si₂ON₂ from oxidized amorphous Si₃N₄ seemed to be assisted by formation of a Si–O–N melt in the system that was enhanced under the high pressure.     

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.     

Preparation of Superconducting Bi(Pb)-Sr-Ca-Cu-O Thick Films on Magnesia Substrate

Superconducting Bi(Pb)-Sr-Ca-Cu-O powders were prepared by the emulsion-drying method The powders consisting of the low- T _c phase (Bi(Pb)₂Sr₂Ca₁Cu₂O_y) and small amount of Ca₂PbO₄ were prepared by calcining under low oxygen partial pressure at 750°, 800°C. These calcined powders were used to prepare thick films on single-crystal MgO(100) substrates using a screen-printing technique to study the effect of the calcinations temperature and annealing procedure on the high-T_c phase (Bi(Pb)₂Sr₂Ca₂Cu₃O_y)formation. The formation of high-T_c phase was observed to be dependent on the calcinations conditions. The high- T_c phase formed very quickly (sintering at 840°C for 4 h), and the best films showed a sharp superconducting transition at about 105 K.     

Enhanced Thermal Stability of Zinc-Based Titanate (Dielectric) Ceramics Prepared by the Modified Sol–gel Route

A multi-component substitution of Co and Ni was incorporated into ZnTiO₃ to form pure hexagonal Zn_{1− x} (Co_1/2Ni_1/2) _x TiO₃ ( x =0,0.8,0.9,1.0) dielectric ceramic powders by a modified sol–gel route, following heat treatments at 600°C for 3 h and at 800°C for 6 h. Differential scanning calorimetry measurements revealed that the order of increasing thermal stability of solid solution compound Zn_{1− x} (Co_1/2Ni_1/2) _x TiO₃ was ZnTiO₃ (945°C), Zn_0.1Ni_0.9TiO₃ (1346°C), Zn_0.1(Co_1/2Ni_1/2)_0.9TiO₃ (1390°C), and Zn_0.1Co_0.9TiO₃ (>1400°C). Both the dielectric constant and loss tangent reached a maximum at x =0.8 and then decreased with solubility, x , and measurement frequency.     

Nanopowders of Na0.5K0.5NbO3 Prepared by the Pechini Method

A Pechini-based chemical synthesis route was used to produce powders of Na_0.5K_0.5NbO₃ (NKN). The thermochemistry of the gel was investigated using thermogravimetric analysis-fourier transform infrared (TGA-FTIR) evolved gas analysis; in addition, powder FTIR was used to analyze the gel residues after different heat treatments. The final decomposition of the organic components occurred at ∼650°C. However, hydrated–carbonated secondary phase(s) were detected by FTIR in powders that had been heated at 700°C, indicating that the NKN nanopowders are susceptible to a reaction with atmospheric moisture and carbon dioxide. The NKN particle sizes were in the range 50–150 nm after decomposition at 700°C.     

Preparation and Dielectric Properties of Low-Temperature-Sinterable (Zr0.8Sn0.2)TiO4 Powder

Low-temperature-sinterable (Zr_0.8Sn_0.2)TiO₄ powders were prepared using 3 mol% Zn(NO₃)₂ additive and then compared with powders prepared using 3 mol% ZnO additive and no additives. Sintering at 1200°C for 2 h produced a dielectric ceramic with ρ= 98.6% of theoretical density (TD), ɛ_r= 38.4, Q × f (GHz) = 42000, and τ _f =−1 ppm/°C. Sintering at 1250°C resulted in an excellent dielectric with ρ= 99% of TD, epsilon_r= 40.9, Q × f (GHz) = 49000, and τ _f =−2 ppm/°C. Scanning electron microscopy showed a microstructure with grains measuring 0.5 to 1 μm, and transmission electron microscopy revealed secondary phase in the grain boundaries.