Improvements in Atmosphere Sintering of Transparent PLZT Ceramics

An improved atmosphere sintering process was developed for fabricating large transparent PLZT plates for electrooptic applications. Cold-pressed 9/85/35 PLZT slugs were sintered in O₂ in Pt crucibles for ∼45 min at 1180°C and were then heat-treated in air for 60 h at 1200°C in Al₂O₃ crucibles containing PbZrO₃ atmosphere powder. Transparent plates as large as 8.4 cm in diameter and I cm thick were thus fabricated. A mechanism is proposed which qualitatively accounts for the success of this process.     

Effect of Manganese Ion Diffusion on the Microstructural Evolution of Lead Lanthanum Zirconate Titanate Ceramic

Microstructural evolution of lead lanthanum zirconate titanate (PLZT) ceramics caused by diffusion of the Mn ion was observed. Specimens with layered structures were fabricated by copressing a PLZT powder (9/65/35) doped with Mn and same PLZT powder without the dopant. When the copressed specimen was sintered at 1200°C in air, the Mn ion diffused out of the doped region. The region originally containing the Mn ions was totally free of pores while all other regions remained porous. The formation of lattice vacancies, as a result of Mn diffusion, was attributed to the enhanced material transport and the resultant rapid densification.     

Effects of Preparation Conditions on the Structural and Optical Properties of Spark Plasma-Sintered PLZT (8/65/35) Ceramics

Transparent lanthanum-doped lead zirconate titanate (PLZT) ceramics with high density were fabricated using spark plasma sintering (SPS), a recently developed hot-pressing method. A wet–dry combination method was used to prepare the fine PLZT powders. The average grain size of the PLZT ceramics was less than 1 μm, because of a relatively low sintering temperature and a very short sintering time. The transmittance of PLZT ceramics increased with an increase of calcination temperature up to 700°C and then it slightly decreased with further increase of calcination temperature. The transmittance strongly depended on the SPS temperature and heat-treatment temperature. The pellet sintered at 900°C for 10 min and heat treated at 800°C for 1 h with a thickness of 0.5 mm showed a transmittance of 31% at a wavelength of 700 nm. The relationships between the transmittance and the microstructure were investigated.     

Fabrication of Optically Transparent Lead Lanthanum Zirconate Titanate ((Pb,La)(Zr,Ti)O3) Ceramics by a Three-Stage-Atmosphere-Sintering Technique

An easy technique has been developed to fabricate optically transparent lanthanum-modified lead zirconate titanate (PLZT) ceramics. This technique consists of three stages: (1) sintering in an oxygen atmosphere, (2) elimination of pores in a carbon dioxide atmosphere, and (3) elimination of oxygen vacancies in an oxygen atmosphere. The carbon dioxide atmosphere enhances the diffusion of oxygen from the pores to outside the sintered body. The experimental results reveal that use of a carbon dioxide atmosphere effectively decreases residual pores and improves optical transmittance. From commercially available raw powders, an optical transmittance of 51% (wavelength of 550 nm) can be achieved for 0.7 mm thick polished PLZT9/65/35 ceramics using a carbon dioxide atmosphere, whereas the value is only 34% without a carbon dioxide atmosphere. The advantage of this technique is that PLZT ceramics having high optical quality can be obtained using conventional sintering tools.     

Fabrication of Transparent Electrooptic PLZT Ceramics by Atmosphere Sintering

PLZT electrooptic ceramics were fabricated by a unique atmosphere sintering technique. The ceramics produced were significantly more transparent than those obtained by ordinary O₂ sintering methods; in fact, they were comparable in transparency to those obtained by hot-pressing. To achieve good transparency, the powder must be batched with excess PbO, which at the sintering temperature is present as a liquid phase at the grain boundaries. Sintering was conducted at 1200°C for 60 h in an atmosphere containing O₂ and a relatively high partial pressure of lead oxide.     

Sintering of lunar regolith structures fabricated via digital light processing

Architectural and functional structures composed of lunar regolith-simulant CLRS-2 were fabricated via digital light processing and sintered at 1100 °C and 1150 °C under an air or argon atmosphere. This work is to investigate effects of atmosphere and temperature on mechanical properties, microstructure, and chemical composition of lunar regolith products. Samples sintered at 1150 °C in air underwent the highest sintering shrinkage and showed the best mechanical properties, likely due to the formation of glassy phase and dense structure following sintering. Conversely, argon-sintered samples exhibited lower density resulting from the lack of glassy phase. Phase analysis revealed varying chemical composition and therefore different underlying reaction mechanisms under two sintering atmospheres, indicating that sintering atmosphere significantly influences the microstructure and macroscopic properties of lunar regolith products.     

Synthesis and Microwave Dielectric Properties of MgSiO3 Ceramics

MgSiO₃ ceramics were synthesized and their microwave dielectric properties were investigated. The Mg₂SiO₄ phase was formed at temperatures lower than 1200°C, while the orthorhombic MgSiO₃ phase started to form by the reaction of SiO₂ and Mg₂SiO₄ in the specimen fired at 1200°C. The structure of the MgSiO₃ ceramics was transformed from orthorhombic to monoclinic when the sintering temperature exceeded 1400°C. A dense microstructure was developed for the specimens sintered at above 1350°C. The excellent microwave dielectric properties of ɛ_r=6.7, Q × f =121 200 GHz, and τ_f=−17 ppm/°C were obtained from the MgSiO₃ ceramics sintered at 1380°C for 13 h.     

Sinterability and Decomposition of Pb0.9175La0.055Zr0.975Ti0.025O3: Influence of Calcination and Sintering Temperature

The sinterability and decomposition of PLZT, (Pb,La)(Zr,Ti)O₃, depend on the temperature and soaking time of both the calcination and sintering temperature. They were determined from the density, linear shrinkage, weight loss, and appearance of extra phases. At moderate calcination temperatures and times, there is no escape of PbO from the PLZT. At calcination temperatures higher than 1050°C and soaking times above 3 h, PbO escapes, and ZrO₂ and La₂Zr₂O₇ can be detected. Even when sintered in a PbO-rich atmosphere, some PbO evaporates during sintering either from free PbO or from the PbO bound in the PLZT in regions in the outer surfaces of the sintered body. An aggressive depletion of PbO during sintering can result in a complete disappearance of the grain boundary phase, giving an intragranular fracture.     

Spark Plasma Sintering of Nanocrystalline Niobium Nitride Powders

Nanocrystalline niobium nitride (NbN) powders were sintered by spark plasma sintering under a nitrogen atmosphere at temperatures from 1040° to 1230°C. Fully dense bulk NbN ceramic with grain sizes of 0.5–1.0 μm was obtained at 1130°C. The effects of sintering temperature on the density, phase content, electrical conductivity, Vickers hardness, and microstructure of the NbN ceramic were discussed.     

Novel Design of Microchanneled Tubular Solid Oxide Fuel Cells and Synthesis Using a Multipass Extrusion Process

A novel, microchanneled tubular solid oxide fuel cell was fabricated using a multipass extrusion process, with an outside diameter of 2.7 mm that contained 61 cells. Cell materials used in this work were 8 mol% yttria-stabilized zirconia (8YSZ), La_0.8Sr_0.2MnO₃ (LSM), and NiO–8YSZ (50:50 vol%) as electrolyte, cathode, and anode, respectively. Three stages of heat-treatment processes were applied, at 700°C in N₂ condition, at 1000°C in air, and then sintered at 1300°C for 2 h, respectively. The X-ray diffraction analysis confirmed that no reaction phases appeared after sintering. The microstructures of anode and cathode were fairly porous while the electrolyte had a dense microstructure (relative density >96%). The thickness of electrolyte, anode, and cathode were 20, 30, and 40 μm, respectively, and the diameter of the continuous channels was 150 μm.     

Fabrication and Characterization of Ce0.8Sm0.2O1.9 Microtubular Dual-Structured Electrolyte Membranes for Application in Solid Oxide Fuel Cell Technology

Samaria-doped ceria (SDC, Ce_0.8Sm_0.2O_1.9) ceramic powders of submicrometer size were synthesized by a sol–gel auto-combustion method. From these powders microtubes with a dual structure comprising of a dense layer and a porous substrate layer were fabricated in a single step through a phase inversion/sintering technique. A sintering temperature in excess of 1450°C is required for SDC to achieve gastight microtubes. The mechanical strength of the SDC microtubes increases with increasing sintering temperature and may attain up to 208 MPa when sintered at 1500°C. Electrical impedance spectroscopy studies indicate that the SDC microtubes have electrical conductivities of 4.46 × 10⁻⁴–0.072 S/cm and corresponding activation energy of 81.9 kJ/mol at temperatures between 400° and 800°C. Full fuel cells were fabricated by coating Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) on to the inner surface and a Ni-SDC cermet on to the outer surface of the gastight microtubes to act as the cathode and the anode, respectively. The resultant BSCF|SDC|Ni-SDC microcells have a stable output maximum of 106 mW/cm² at 750°C when hydrogen and air were used as fuel and oxidant gas, respectively.     

Oxidation Behavior of Zirconium Diboride Silicon Carbide Produced by the Spark Plasma Sintering Method

Dense samples of ZrB₂–20 vol% SiC were successfully fabricated by spark plasma sintering without the use of sintering aids. Oxidation behavior of these samples was characterized by exposing them to 1400°, 1500°, and 1600°C in an ambient atmosphere for 150 min, and by measuring the weight gains of the sample and crucible, as well as the thickness of the oxide scale and the glassy outer layer. The effects of gravity on the viscous outer layer are shown to result in significant heterogeneity within a sample. The oxidation scales were characterized by scanning electron microscopy and transmission electron microscopy with energy dispersive spectroscopy analysis. The oxide scale was found to be composed of three layers: (1) a SiO₂-rich glassy outer layer, (2) an intermediate layer of a ZrO₂ matrix with interpenetrating SiO₂, and (3) a layer containing a ZrO₂ matrix enclosing partially oxidized ZrB₂ with Si–C–B–O glass inclusions.     

Effects of sintering atmosphere on the densification and microstructure of yttrium aluminum garnet fibers prepared by sol-gel process

Yttrium aluminum garnet (YAG) fibers were prepared by a sol-gel method, and then sintered in air or nitrogen atmosphere, respectively. The effects of sintering atmosphere on the densification process and microstructure of YAG fibers were investigated. No obvious difference can be found in the fibers sintered below 800 °C. At 1100 °C, the grain size of YAG fibers sintered in nitrogen is much smaller than in air. This difference is much clearer at the higher temperature of 1200 °C. The fine grains are explained by the existence of residual carbon in YAG fibers, which can be trapped at the grain boundaries to hinder the movement of grain boundary. Meanwhile, the densification degree of fibers sintered in nitrogen is higher than in air at 1200 °C, due to the smaller grain size and higher oxygen vacancy concentration generated in the nitrogen atmosphere, which leads to a higher fiber densification rate.     

Sintering Behavior of Doped Lanthanum and Yttrium Manganite

The sintering behavior of doped manganite powders was found to be highly dependent on changes in calcination conditions and A/B cation ratio. Coarsening of combustion synthesized powders by calcination allowed for higher green densities in dry-pressed compacts, which resulted in higher sintered densities for powders calcined in the temperature range 800°-1200°C. Sintered densities decreased for calcination temperatures greater than 1200°C. Preparation of manganites with a deficiency of A-site cations improved the densification behavior substantially. This effect was attributed to an increased concentration of A-site vacancies which enhanced the diffusion of A-site cations during sintering. Modification of doped manganites by alteration of composition and calcination conditions allowed their sintering shrinkage to be "tailored" to more closely match the shrinkage of yttria-stabilized zirconia.     

Optical and mechanical characterization of transparent α-alumina fabricated by spark plasma sintering

The aim of this work was the analysis of the experimental results of a transparent alumina (BMA15) ceramic which was fabricated by Spark Plasma Sintering (SPS) from nanopowder (BMA15, Baikowski Chimie, France), at different temperatures (1200°C, 1250°C, 1300°C). With the application of a maximum uniaxial pressure of 73 MPa during all the fabrication-cycle (more than 3 hours). We sought an optimal sintering temperature combining better optical and mechanical properties of our pellets. The sintered alumina (BMA15) has a crystalline and dense microstructure. The samples sintered at 1200°C exhibit the best optical properties, in particular: good real inline transmission (RIT) and an optical gap greater than those of the samples sintered at 1250°C and 1300°C. Due to their low density, the Young modulus of alumina sintered at 1200 °C, deduced by ultrasound, has a low value which is about 385 GPa. Similarly, its small grain size gives it a better Vickers hardness ~ 21 GPa. Therefore, the value of the coefficient of friction μ stabilizes around the mean value of 0.21.     

Influence of Chemical Reactions in Magnesia-Graphite Refractories: II, Effects of Aluminum and Graphite Contents in Generic Products

A group of magnesia-graphite and magnesia-graphite-aluminum materials, the compositions of which represent a wide range of graphite contents (~10-16.4 wt%), aluminum contents (0-5.2 wt%), and MgO and graphite qualities, were fabricated, using standard commercial practices. Chemical analysis and determination of room-temperature modulus of rupture (MOR) and Young's modulus, as well as a complete microstructural characterization of the as-received materials, were performed. Mechanical characterization at high temperature (1000°, 1200°, and 1450°C) was done in terms of Young's modulus and MOR in an argon atmosphere (<1000 ppm oxygen at 1000°C). Modulus-of-elasticity values ranged from 4 to 16 GPa, and their evolution with temperature was determined by the evolution of the microstructure in the bulk of the specimens. A strong effect of aluminum-metal concentration on Young's modulus overrode other microstructural differences among the materials. MOR values ranged from 6 to 20 MPa, and their evolution with temperature was determined by the evolution of the microstructure in the bulk of the specimens at the lower testing temperatures ( T lessthan equal to 1200°C) and by phase assemblages in the surface regions of the specimens-essentially by the presence of the dense MgO zone-at 1450°C. The thickness of the dense MgO zone in the aluminum-containing materials was determined by the amount of aluminum and the MgO aggregate size.     

Electrochemical Synthesis and Sintering of Nanocrystalline Cerium(IV) Oxide Powders

Nanocrystalline CeO₂ powders were prepared electrochemically by the cathodic electrogeneration of base, and their sintering behavior was investigated. X-ray diffraction and transmission electron microscopy revealed that the as-prepared powders were crystalline cerium(IV) oxide with the cubic fluorite structure. The lattice parameter of the electrogenerated material was 0.5419 nm. The powders consisted of nonaggregated, faceted particles. The average crystallite size was a function of the solution temperature. It increased from 10 nm at 29°C to 14 nm at 80°C. Consolidated powders were sintered in air at both a constant heating rate of 10°C/min and under isothermal conditions. The temperature at which sintering started (750°C) for nanocrystalline CeO₂ powders was only about 100°C lower than that of coarser-grained powders (850°C). However, the sintering rate was enhanced. The temperature at which shrinkage stopped was 200°-300°C lower with the nanoscale powder than with micrometer-sized powders. A sintered specimen with 99.8% of theoretical density and a grain size of about 350 nm was obtained by sintering at 1300°C for 2 h.     

Effect of Silicon Substitution on the Sintering and Microstructure of Hydroxyapatite

The substitution of between 0 and 1.6 wt% silicon (Si-HA) in hydroxyapatite (HA) inhibited densification at low temperatures (1000°–1150°C), with these effects being more significant as the level of silicon substitution was increased. For higher sintering temperatures (1200°–1300°C), the sintered densities of HA and Si-HA compositions were comparable. Examination of the ceramic microstructures by scanning electron microscopy (SEM) showed that silicon substitution also inhibited grain growth at higher sintering temperatures (1200°–1300°C). The negative effect of silicon substitution on the sintering of HA at low temperatures (1000°–1150°C) was reflected in the hardness values of the ceramics. However, for higher sintering temperatures, e.g., 1300°C, where sintered densities were comparable, the hardness values of Si-HA compositions were equal to or greater than that of HA, reflecting the smaller grain sizes observed for the former.     

Effect of Silver Doping on the Physical and Electrical Properties of PLZT Ceramics

Additions of small amounts of silver to a PLZT dielectric with emphasis on the 88/12/70/30, Pb/La/Zr/Ti, composition were investigated. It was found that a few mole percent of Ag¹⁺ could be incorporated into the PLZT lattice as a large acceptor cation and that it tended to reduce the lead vacancies, which are normally generated by the substitution of La³⁺ in the PLZT dielectric. The addition, up to 2 mol% of silver, decreased the 25°C dielectric constant from 2300 to 1700. However, the temperature coefficient of capacitance was improved to ±5% between -55° and +125°C, and the dissipation factor was reduced from 1.5 to 0.5%. Although the gravimetrically measured lead loss appeared to depend on the relative vapor pressure of lead oxide during sintering, the compensation mechanism of Ag¹⁺ was not affected.     

Effect of PO2 on Bulk and Grain Boundary Resistance of n-Type BaTiO3 at Cryogenic Temperatures

Complex impedance analysis at cryogenic temperatures has revealed that the bulk and grain boundary properties of BaTiO₃ polycrystals are very sensitive to the oxygen partial pressure during sintering. Polycrystals sintered at P _O2 as low as 10⁻¹⁵ atm were already electrically heterogeneous. The activation energy of the bulk conductivity in the rhombohedral phase was found to be close to that of the reduced undoped single crystal (i.e., 0.093 eV). The activation energy of the grain boundary conductivity increases with the temperature of the postsinter oxidation treatment from 0.064 to 0.113 eV. Analysis of polycrystalline BaTiO₃ sintered in reducing atmosphere and then annealed at P _O2= 0.2 atm has shown that the onset of the PTCR effect occurs at much higher temperatures than expected in the framework of the oxygen chemisorption model. The EPR intensity of barium and titanium vacancies increases after oxidation at T > 1000°C. A substantial PTCR effect is achieved only after prolonged annealing of the ceramic in air at temperatures as high as 1200–1250°C. This result suggests that the PTCR effect in polycrystalline BaTiO₃ is associated with interfacial segregation of cation vacancies during oxidation of the grain boundaries.