Low-Temperature Fabrication of Porous β-SiC Ceramics in Sodium Vapor

β-silicon carbide (SiC) porous ceramics were synthesized from pelletized powder mixtures of silicon (Si) and fullerene or Si and amorphous carbon (carbon black) at 1000 K for 24 h in sodium (Na) vapor. The relative density of the ceramics was 29%–34% of the theoretical density of SiC. Scanning electron microscopic observation of the fracture surface showed that the ceramics prepared with fullerene were agglomerates of submicrometer-sized SiC particles and open spaces. The samples prepared with carbon black had a smooth fracture surface with cavities and voids. Using transmission electron microscopy, grains of over 250 nm and a diffuse electron diffraction ring pattern of β-SiC were observed for the sample prepared with fullerene, and grains of 10–20 nm with a β-SiC spot ring pattern for the sample prepared with carbon black. A surface area of 11–17 m²/g and a mesopore size distribution in the range of 2–10 nm were shown by a nitrogen adsorption technique. Energy-dispersive X-ray analysis detected 1–5 at.% of Na against Si on the fracture surface of the ceramics.     

Ferroelectric and Photostrictive Properties of Fine-Grained PLZT Ceramics Derived from Mechanical Alloying

Single-phase lead lanthanum zirconate titanate (PLZT) solid solution powder was synthesized from the constituent oxides at ambient temperature through a mechanical alloying (MA) process and was then densified to fine-grained ceramics by sintering and hot-pressing. The anomalous photovoltaic effect (APV) and photoinduced strain of the resultant PLZT ceramics were investigated and analyzed in association with the influence of grain size. It was found that a photoinduced voltage up to 6000 V·cm⁻¹ can be obtained as the grain size is reduced to 0.42 μm. This is extremely high and about three times that achievable in normal micrometer-grained PLZT ceramics. The maximum photoinduced strain of the PLZT ceramics with an average grain diameter of 0.54 μm reached 0.01%, which is equivalent to electric-field-induced strain of common piezoelectric materials.     

Synthesis of Dense Lead Titanate Ceramics with Submicrometer Grains by Spark Plasma Sintering

Dense PbTiO₃ ceramics consisting of submicrometer-sized grains were prepared using the spark-plasma-sintering (SPS) method. Hydrothermally prepared PbTiO₃ (0.1 μm) was used as a starting powder. The powder was densified to ≳98% of the theoretical X-ray density by the SPS process. The average grain size of the spark-plasma-sintered ceramics (SPS ceramics) was ≲1 μm, even after sintering at 900°–1100°C, because of the short sintering period (1–3 min). The measured permittivity of the SPS ceramics showed almost no frequency dependence over the range 10¹–10⁶ Hz, mainly because pores were absent from the ceramics. The coercive field of the SPS ceramics was somewhat higher than that of conventionally sintered ceramics, which could be attributed to the small-grained microstructures of the SPS ceramics.     

Fabrication of Grain-Oriented Bi2WO6 Ceramics

Grain-oriented Bi₂WO₆ ceramics were fabricated by normal sintering techniques. Platelike crystallites were initially synthesized by a fused salt process using an NaCl-KCI melt. When calcined at <800°C, the Bi₂WO₆ crystallites are 3∼5 μ m in size and, at >850°C, =100 μm. After dissolving away the salt matrix, the Bi₂WO₆ particles were mixed with an organic binder and tapecast to align the platelike crystallites. Large particles were easily oriented by tapecasting but the sinterability of the tape was poor. Preferred orientation of small particles was increased by tapecasting and grain growth during sintering further improves the degree of orientation. Sintering above the 950°C phase transition, however, results in discontinuous grain growth and low densities. Optimum conditions for obtaining highly oriented ceramics with high density occur at sintering temperatures of 900°C using fine-grained powders which yield orientation factors of =0.88 and densities of 94% theoretical.     

Dielectric Properties of the "Twinned" 8H-Hexagonal Perovskite Ba8Nb4Ti3O24

The dielectric properties of dense ceramics of the "twinned" 8H-hexagonal perovskite Ba₈Nb₄Ti₃O₂₄ are reported. Single-phase powders were obtained from the mixed-oxide route at 1325°C and ceramics (>92% of the theoretical X-ray density) by sintering in air or flowing O₂ at 1400°–1450°C. The ceramics are dc insulators with a band gap >3.4 eV that resonate at microwave frequencies with relative permittivity, ɛ_r∼44–48, quality factor, Q × f _r∼21 000–23 500 GHz (at f _r∼5.5 GHz) and temperature coefficient of resonant frequency, TC _f,∼+115 ppm/K.     

Enhancing Electrical Properties in NBT–KBT Lead-Free Piezoelectric Ceramics by Optimizing Sintering Temperature

Conventional sintering of (Na_{1− x} K _x )_0.5Bi_0.5TiO₃ (abbreviated as NKBT x , x =18–22 mol%) lead-free piezoelectric ceramics was investigated to clarify the optimal sintering temperature for densification and electrical properties. Both sintered density and electrical properties were sensitive to sintering temperature; particularly, the piezoelectric properties deteriorated when the ceramics were sintered above the optimum temperature. The NKBT20 and NKBT22 ceramics synthesized at 1110°–1170°C showed a phase transition from tetragonal to rhombohedral symmetry, which was similar to the morphotropic phase boundary (MPB). Because of such MPB-like behavior, the highest piezoelectric constant ( d ₃₃) of about 192 pC/N with a high electromechanical coupling factor ( k _p) of about 32% were obtained in the NKBT22 ceramics sintered at 1150°C.     

Characterization of Fine-Grained Bismuth Vanadate Ceramics Obtained Using Nanosized Powders

Nanocrystalline bismuth vanadate (n-BiV) powders with a crystallite size of <50 nm have been prepared, at room temperature, by subjecting a stoichiometric mixture of bismuth oxide and vanadium pentoxide to mechanical activation. The n-BiV powders show enhanced sinterability, in comparison to the conventionally prepared micrometer-sized bismuth vanadate (m-BiV) powders and yield ceramics with a uniform microstructure. High-density (∼98% of the theoretical value), fine-grained (average grain size of ∼2 μm) ceramics, obtained using n-BiV have a high dielectric constant and a high pyroelectric coefficient and are associated with low dielectric loss, both at room temperature and at the transition temperature. These fine-grained ceramics show diffused phase transition and relaxor behavior, which are attributed to the irregular distribution of defects and/or compositional inhomogeneities in these ceramics. The fine-grained ceramics exhibit ferroelectric hysteresis loops with higher remanent polarization and lower coercive field values than the coarse-grained ceramics.     

Preparation of High-Strength Calcium Phosphate Ceramics with Low Modulus of Elasticity Containing β-Ca(PO3)2 Fibers

Novel calcium phosphate ceramics were fabricated by hot-pressing fibrous products extracted from crystallized products of calcium ultraphosphate glasses by aqueous leaching. The ceramics were dense materials with a relative density of >95%; these ceramics were composite materials that consisted of β-Ca(PO₃)₂ fibrous crystals with CaO–P₂O₅ glass, which was formed during hot pressing, as the matrix phase. These ceramics showed a high bending strength of 150–220 MPa and a low Young's modulus of 30–60 GPa. The high toughness contributed to the high strength, with fiber pull-out and crack deflection observed as the primary toughening mechanism.     

Processing and Electromechanical Properties of (Bi0.5Na0.5)(1−1.5x)LaxTiO3 Ceramics

Bismuth sodium titanate (Bi_0.5Na_0.5TiO₃, BNT) with 0–6 at.% lanthanum was prepared by the conventional mixed oxide method. Each composition was calcined at 800–900°C for 2–5 h to form a pure perovskite phase. Green pellets were sintered at 1050–1150°C for 1–4 h to obtain dense ceramics with at least 95% of theoretical density. X–ray diffraction (XRD) showed phase distortion as lanthanum was added to this system. Meanwhile, a small amount of La was found to affect the grain size and had an influence on the poling conditions and electrical properties. The BNT–based composition with 1 at.% La doping provided a dielectric constant ( K ) of 560, a piezoelectric charge constant ( d ₃₃) of 92 pC/N, and a hydrostatic piezoelectric coefficient ( d _h) of 72 pC/N.     

Porous Pb(Zr0.95Ti0.05)O3 Ceramics from Chemically Prepared Powders Without Pore Formers

Porous Zr-rich lead zirconate titanate (PZT 95/5) ceramics having 77.9–92.2% of relative densities were prepared without additional pore formers. PZT 95/5 powders were obtained by a one-step pyrolysis process at 450°C for 10 h using the Pechini method. Although the X-ray diffractometry results showed an incomplete perovskite phase for the powders, phase-pure PZT 95/5 ceramics could be obtained after sintering at 1100°–1150°C for 2 h. The density and diameter variations of the PZT 95/5 ceramics as a function of sintering temperature were investigated.     

Dense ceramics of BaTiO3 produced from powders prepared by a chemical process

A sintering optimization of barium titanate ceramics from fine-grained and homogeneous reproducible powders obtained by the citric process is presented. Different sintering parameters are studied: heating rates, final temperature, dwelling times at this final temperature, and influence of the powder deagglomeration step. The sintering is followed by dilatometric measurements. The ceramics obtained by sintering at 1230 or 1300 °C are free of barium carbonate, the residual carbon content being estimated at about 400 ppm in the surface layer. They exhibit a grain size close to 1 μm, a structure in which the cubic and tetragonal phases coexist, and a density of about 96% of the theoretical density. Their permittivity and loss factor are respectively about 5000 and 2.5 × 10⁻² at 25 °C.     

Properties of Thorium Oxide Ceramics

Included in this report are data concerning the strength, density, and porosity that may be expected of thoria ceramics formed by dry-pressing and isostatic pressing and fired at 1400°, 1600°, and 1800°C. By dry-pressing and firing at 1400°C., a density of 60% of theoretical (10.05) was obtained; at 1800°C. the density was increased to 80% of theoretical. By isostatic pressing a density of 87% of theoretical was obtained at 1800°C. A study of the densification of thoria ceramics by small additions of other inorganic materials showed the very marked effect of CaO (or CaF₂) in proportions of 0.5 to 3%. In proportions of 0.5 to 1%, CaO additions produced densities of 97% of theoretical for ThO₂ when fired at 1800°C. Experimental results intended to explain this phenomenon tended to substantiate the theory that it is due to formation of holes by substitutional solid solution and migration of ions in the altered structure. Thoria ceramics in the form of 12-in. rods, 1.5 in. in diameter, and small (0.1-in.) pellets with CaO additions were made having densities up to 97% of theoretical; evaluation of the abrasion resistance and strength is given. Porous thoria ceramics with densities of only 50 to 60% of theoretical were produced by addition of volatile material. Tests of the solubility of thoria ceramics showed that water at 250°C. had no effect. Ready dissolution was obtained with nitric acid and hydrofluoric acid as a catalyst.     

Influence of process parameters in gel casting of a pure yttria nanopowder to fabricate transparent ceramics

The goal of this research is to fabricate pure transparent yttria ceramics through gel casting and vacuum sintering. A specific processing method has been used and optimized for this purpose. A pure yttria nanopowder was synthesized as the starting material to produce pure transparent ceramics through a low-temperature sintering process. It was attempted to minimize the undesirable nanopowder hydration by using the as-synthesized yttria nanopowder and a rapid deagglomeration and slurry preparation process. The synthesized nanopowders were deagglomerated to enhance the efficiency of both powder shaping and sintering stages. Carrageenan was used as the gelling agent because it is a low-cost and abundant material, and because the temperature is the only catalyst needed for its gelation; therefore, it is possible to control its gelation to obtain high-density and pure optical ceramics. The effect of the deagglomeration method and the processing parameters, including the amounts of dispersant, gelling agent, solid loading, pH, and deagglomeration time, on the rheology of slurry, density, and microstructure of the obtained green yttria ceramics was examined and optimized in order to obtain high solid loading nanoyttria suspensions of 38 vol%, which is more than those obtained in many of the previous investigations. The precise gelling temperature and time were measured, and green gel cast ceramics with a density of 63 % of the theoretical density were produced. A rapid deagglomeration and slurry preparation method was used instead of using a conventional planetary ball-milling approach to minimize the risk of the hydrolysis of yttria nanopowder. No sintering aid was necessary, and transparent yttria ceramics with 99 % of the theoretical density were produced after vacuum-furnace sintering at 10^-2 mbar and 1715 °C.     

High-Permittivity Double Rare-Earth-Doped Barium Titanate Ceramics with Diffuse Phase Transition

On the basis of Ti-vacancy defect compensation mode, high-permittivity La- and Ce-doped barium titanate ceramics (BLTC) with perovskite structure, i.e., (Ba_{1− x} La _x )(Ti_{1− x /4− y} Ce _y )O₃, where x =0.01–0.05 and y =0.05, were prepared by conventional ceramic processing techniques. Dielectric characteristics, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and hysteresis loops were measured. Defect chemistry and diffuse phase transition (DPT) in BLTC are discussed. Co-doping with the relatively smaller La³⁺ ions at Ba sites and the relatively larger Ce⁴⁺ ions at Ti sites in the BaTiO₃ host lattice resulted in a fine-grained microstructure (0.8–1.1 μm), marked raising, and broadening of the Curie peak characteristic of DPT. The Curie temperature ( T _C) at 800 Hz decreased rapidly at a dramatic rate of −30°C/at.% La when y =0.05. By means of co-doping with La and Ce, this is the first time that high k "Y5V" ceramics (BLTC: 0.03≤ x ≤0.04, y =0.05) with ɛ'_RT>10 000 over a frequency range of 1–100 kHz have been achieved in rare-earth-doped BaTiO₃ ceramics.     

(Na0.8K0.2)0.5Bi0.5TiO3 Nanowires: Low-Temperature Sol–Gel–Hydrothermal Synthesis and Densification

The sol–gel–hydrothermal processing of (Na_0.8K_0.2)_0.5Bi_0.5TiO₃ (NKBT) nanowires as well as their densification behavior were investigated. The morphology and structure analyses indicated that the sol–gel–hydrothermal route led to the formation of phase-pure perovskite NKBT nanowires with diameters of 50–80 nm and lengths of 1.5–2 μm, and the processing temperature was as low as 160°C, but the conventional sol–gel route tended to lead to the formation of NKBT agglomerated porous structured nanopowders, and the processing temperature was higher than 650°C. It is believed that the gel precursor and hydrothermal environment play an important role in the formation of the nanowires at a low temperature. Owing to the better packing efficiency and therefore a good sinterability of the freestanding nanowhiskers, the pressed pellets made by NKBT nanowires showed >98% theoretical density at 1100°C for 2 h. The sol–gel–hydrothermal-derived ceramics have typical characteristics of relaxor ferroelectrics, and the piezoelectric properties were better than the ceramics prepared by the conventional sol–gel and solid-state reaction.     

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.     

Low-Temperature Reactive Sintering of 0.65PMN·0.35PT

Dense, fine-grained 0.65PMN·0.35PT ceramics were fabricated by reactive sintering of a mixture of metal oxides at 1000°C. The fine nature of the precursor powder resulted in rapid, direct transformation to perovskite PMN–PT at 600°C. Because of the fine structure of the perovskite, no excess PbO was required for sintering.     

Electrical Conduction in Nano-Structured La0.9Sr0.1Al0.85Co0.05Mg0.1O3 Perovskite Oxide

Nanocrystalline La_0.9Sr_0.1Al_0.85Co_0.05Mg_0.1O₃ oxide powder was synthesized by a citrate–nitrate auto-ignition process and characterized by thermal analysis, X-ray diffraction, and impedance spectroscopy measurements. Nanocrystalline (50–100 nm) powder with perovskite structure could be produced at 900°C by this process. The powder could be sintered to a density more than 96% of the theoretical density at 1550°C. Impedance measurements on the sintered samples unequivocally established the potential of this process in developing nanostructured lanthanum aluminate-based oxides. The sintered La_0.9Sr_0.1Al_0.85Co_0.05Mg_0.1O₃ sample exhibited a conductivity of 2.40 × 10⁻² S/cm in air at 1000°C compared with 4.9 × 10⁻³ S/cm exhibited by La_0.9Sr_0.1Al_0.85Mg_0.15O₃.     

Densification, Structure, and Thermophysical Properties of Ytterbium–Gadolinium Zirconate Ceramics

(Yb _x Gd_{1− x} )₂Zr₂O₇ (0≤ x ≤1.0) ceramic powders synthesized with the chemical-coprecipitation and calcination method were pressureless-sintered at 1550–1700°C to develop new thermal barrier oxides with a lower thermal conductivity than yttria-stabilized zirconia ceramics. (Yb _x Gd_{1− x} )₂Zr₂O₇ ceramics exhibit a defective fluorite-type structure. The linear thermal expansion coefficients of (Yb _x Gd_{1− x} )₂Zr₂O₇ ceramics increase with increasing temperature from room temperature to 1400°C. The measured thermal conductivity of (Yb _x Gd_{1− x} )₂Zr₂O₇ ceramics first gradually decrease with increasing temperature and then slightly increase above 800°C because of the increased radiation contribution. YbGdZr₂O₇ ceramics have the lowest thermal conductivity among all the composition combinations studied.     

Effects of Silver Doping on the Sol–Gel-Derived Ba4(Nd0.7Sm0.3)9.33Ti18O54 Microwave Dielectric Ceramics

Tungstenbronze-type Ba₄(Nd_0.7Sm_0.3)_9.33Ti₁₈O₅₄ (BNST) microwave dielectric ceramics doped with 0–10 wt% silver (Ag) particles were successfully fabricated by a citrate sol–gel method. The influence of Ag doping on the sinterability, microstructure, bulk conductivity, and dielectric properties of BNST was investigated. The desired tungstenbronze-type phase was obtained at 900°–950°C. The sintering temperature of BNST decreased to 1100°C with the aid of a small amount of Ag addition (1 wt%). No chemical reaction between the tungsenbronze phase and Ag was detected. The particle size of the powders decreased with increasing Ag content up to 1 wt% and it then increased with a further increase in the Ag content. The dense fine-grained ceramics with submicrometer grains (∼300 nm) were obtained with 1 wt% Ag addition. The submicrometer-grained ceramics had excellent dielectric properties of ɛ_r∼81 and Q × f ∼11 000 GHz. Both the dielectric constant and dielectric loss significantly increased with large additions (>3 wt%) of Ag due to the percolation effect.