Microstructural Evolution of Transparent PLZT Ceramics Sintered in Air and Oxygen Atmospheres

Transparent lanthanum-doped lead zirconate titanate (PLZT) ceramics were fabricated by air-pressure sintering. When the PLZT (9/65/35) specimens were sintered in air, the microstructure was not uniform throughout the body; the outer region near the surface was completely dense, while the inner region of the body was porous. The thickness of the outer dense layer increased parabolically with sintering time. When the specimen was sintered in air at 1200°C for 8 h, the thickness of the dense layer was ∼0.25 mm. Therefore, when the specimen had a thickness of <0.5 mm, it was dense and transparent. This difference in microstructure was attributed to the formation of lattice vacancies as a result of PbO evaporation from the surface. The sintering atmosphere also was important in determining the thickness of the dense layer. The thickness was strongly dependent on the oxygen partial pressure of the atmosphere. The oxygen-gas trapped in pores was deemed to migrate easily through the lattice vacancies. By sintering in an oxygen-gas atmosphere at 1200°C for 8 h, a transparent PLZT with thickness up to 2 mm was fabricated.     

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 Co-Precipitation on the Low-Temperature Sintering of Biphasic Calcium Phosphate

Three types of biphasic calcium phosphate (BCP) powders were prepared and their sintering behavior was investigated. The specific surface area and HA/TCP ratio were similar in all three specimens. Most of the densification in the co-precipitated s-BCP occurred before the β- to α-TCP phase transformation, and a maximum density of ∼95% was obtained at 1100°C. The mixture of separately precipitated and calcined hydroxyapatite (HA) and tricalcium phosphate (TCP) (m-BCP) showed a poor sintering behavior, and the apparent density was below 70% at temperatures up to 1200°C. In the commercial HA and TCP mixture (c-BCP), the low temperature sintering was poor, but densification continued without the phase transformation due to the presence of MgO, achieving almost full densification at 1200°C.     

Sintering of Lanthanum Zirconate

Lanthanum zirconate (La₂Zr₂O₇) was prepared by coprecipitating lanthanum nitrate and zirconyl oxychloride at pH 10, followed by ethanol washing. The initial high surface area of ∼304 m²·g⁻¹ decreased very rapidly with increased sintering temperature and decreased to an immeasurably small value after heating at 1200°C for 15 h. The major parameters studied were phase evolution, crystallite size, porosity, surface area reduction, and shrinkage during sintering. Three temperature regions were identified based on these studies: below the crystallization temperature, between the crystallization temperature and ∼1100°C, and above 1100°C. The main contribution of surface area reduction in the region 800°–1100°C was due to surface diffusion; the main contribution above 1100°C was due to grain-boundary diffusion coupled with surface diffusion.     

Intermediate Phase Development in Phosphorus-Doped Barium Titanate

In the present work, the phase formation and thermal evolution in phosphorus-doped BaTiO₃ have been studied using differential thermal analysis, X-ray diffractometry, scanning electron microscopy coupled with energy-dispersive spectroscopy, transmission electron microscopy, and high-temperature nuclear magnetic resonance. Phosphorus cations that are incorporated from ester phosphate form a surface layer that covers the BaTiO₃ particles. This layer acts as a reactive coating during sintering. Phosphorus-doped BaTiO₃ samples that have been treated at temperatures of 650°–900°C show the presence of crystalline Ba₂TiP₂O₉ and/or Ba₃(PO₄)₂ phases. The appearance of secondary phases is dependent on the cooling rate. Higher temperatures (900°–1200°C) result in the presence of a phosphorus–BaO-rich phase that covers the BaTiO₃ particles. As a consequence, the remaining titanium-rich BaTiO₃ drives the formation of a liquid phase at temperatures >1200°C. In regard to the reported sintering behavior of P⁵⁺-doped BaTiO₃, the formation of a phosphorus–BaO-rich phase that covers the BaTiO₃ particles could be the origin of the improved porosity coalescence and removal that is observed at the earlier stages of sintering.     

High-Temperature Proton-Conducting Lanthanum Ortho-Niobate-Based Materials. Part II: Sintering Properties and Solubility of Alkaline Earth Oxides

The sintering properties and microstructure of La_{1− x} A _x NbO₄ powders ( x =0, 0.005, and 0.02 and A=Ca, Sr, and Ba), prepared by spray pyrolysis have been investigated. Dense materials (>97%) were obtained by conventional sintering at 1200°C and by hot pressing (25 MPa) at 1050°C, respectively. Homogeneous materials were obtained and the average grain size obtained by the two densification methods was ∼2.0 and ∼0.4 μm, respectively, for the 2% doped materials. Pure lanthanum ortho-niobate (LaNbO₄) showed a higher degree of grain growth. In the acceptor-doped materials, secondary phases were observed to inhibit grain growth at 1200°C. At 1400°C or higher, molten secondary phases in the Ba-doped materials resulted in severe grain growth, causing microcracking during cooling due to crystallographic anisotropy. A low solubility of AO (A=Ca, Sr, and Ba) in LaNbO₄ is inferred from the presence of secondary phases, and 1 mol% solubility of SrO in LaNbO₄ was found by electron microprobe analysis. The electrical conductivity in wet hydrogen of the materials demonstrated that the main charge carrier was protons up to 1000°C and reached a maximum value of ∼8·10⁻⁴ S/cm at 900°C.     

Evaporation of Silver during Cofiring with Barium Titanate

Silver and Ag-Pd alloy are cofired with BaTiO₃-based dielectrics during the manufacturing of capacitors. The diffusion of silver into BaTiO₃ during sintering is very slow. However, the vapor pressure of silver is high at temperatures greater than the melting point of silver (960°C). The effect of the evaporation of silver during the sintering with BaTiO₃ at 1200°, 1250°, and 1290°C is investigated in the present study. The silver vapor can transport through the pore channel of the dielectrics to a distance of a few hundred micrometers. The melting point of Ag-Pd alloy is higher than that of silver; therefore, the transportation of silver vapor from Ag-Pd alloy is hardly observed at temperatures >1200°C.     

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.     

Segregation of Mg to the (0001) Surface of Doped Sapphire

Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED) were used to study the segregation of Mg to the surface of a sapphire crystal doped with 40 ppm of Mg. In situ vacuum annealing and AES measurements at 900° to =1800°C failed to reveal Mg segregation on the (0001) plane above the detectability limit of the cylindrical mirror analyzer. However, when identical crystals were placed together to simulate a "closed system" and annealed in air, surface segregation of Me was detected above 1200°C. The variation of the equilibrium surface concentration of Mg with temperature in the range 1300° to =1500°C, gave an effective Langmuir heat Of segregation of Mg of =–146 kJ/mol. Evidence for the formation of a two-dimensional ordered structure on the sapphire surface was also obtained through LEED studies. The implications of these segregation results for our understanding of the sintering behavior of magnesia-doped alumina are discussed.     

Effect of Minor Additions on Sintering of MgO

Fourteen selected metal ions were added to magnesium basic carbonate, and the MgO obtained after calcining was fired at various temperatures to determine the effect of the additives on the sintering behavior. The magnesia without additives showed rapid increase in density with firing temperature near 1200° C. The majority of the additives caused increased density, at a given firing treatment, but some were without effect and one (Cr) dramatically inhibited sintering in certain percentages. It is believed that most of the additives which aided sintering did so by entering into the magnesia lattice and creating defects, although at least one (V) clearly promoted sintering by liquid formation.     

Strengthening of Porous Mullite and Zirconia CMC Matrices by Evaporation/Condensation

A processing method using evaporation/condensation sintering in an HCl atmosphere was developed for strengthening porous materials without shrinkage. Strengthening without shrinkage is useful in preventing voids and cracks that might be formed during constrained densification, e.g., a porous matrix in a continuous fiber reinforced ceramic composite. Mixtures of mullite and zirconia (monoclinic, tetragonal (3 mol% Y₂O₃), and cubic (8 mol% Y₂O₃)) were studied and exposed to HCl vapor at temperatures up to 1300°C. It was observed that the evaporation–condensation mass transport process produced a porous material with minimal shrinkage. As the crystal structure of the starting tetragonal and cubic zirconia powders did not change after extensive coarsening, it appeared that zirconium and yttrium were transported in the same proportion via evaporation/condensation. The process produced significant coarsening of the zirconia grains, which made the material resistant to densification when heated to 1200°C in air. Because the sintering produced coarsening without shrinkage, the pores also coarsened and a porous microstructure was retained. Mixtures of mullite and zirconia were used because mullite does not densify under the processing conditions used here, namely, heat treatments up to 1300°C. The mullite particles acted as a non-densifying second phase to further inhibit shrinkage when the mullite/zirconia composite was heated up to 1200°C in air. The coarsened cubic zirconia plus mullite mixture had the least densification after heat treatments in air of 100 h at 1200°C.     

High-Temperature Young's Modulus of Alumina During Sintering

High-temperature Young's modulus of a partially sintered alumina ceramic has been studied dynamically during the sintering process. Comparative, room-temperature Young's modulus data were obtained for a suite of partially sintered alumina compacts with different porosities. The dynamic Young's modulus of a 1200°C partially sintered material was observed to decrease linearly with temperature, but then above 1200°C it increased sharply as sintering and densification of the alumina became dominant. The evolution of the Young's modulus due purely to sintering exhibited an exponential relationship with porosity in excellent agreement with room-temperature measurements of equivalent porous alumina ceramics.     

Grain-Boundary Grooving of Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier Coatings

The focus of this study was to determine the mechanisms responsible for the microstructural changes of plasma-sprayed 7 wt% Y₂O₃–ZrO₂ thermal barrier coatings with annealing from 800° to 1400°C. Mullins's thermal grooving theories have been applied to plasma-sprayed TBCs to determine the dominant mass transport mechanism at various temperatures. Grain-boundary groove widths were measured as a function of annealing time and temperature using atomic force microscopy (AFM). The same collection of grains was analyzed after progressive heat treatments. Surface diffusion was found to be the dominant diffusion mechanism at 1000°C, corresponding to the disappearance of intralamellar cracks at that temperature. At 1100°C, both surface and volume diffusion were active. Volume diffusion, found to be the dominant diffusion mechanism at 1200°C and above, was responsible for the sintering of interlamellar pores observed from AFM analysis of a single, progressively heat-treated interlamellar boundary. Surface roughening was observed to coarsen with increased annealing time and disappear with increased annealing temperature.     

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.     

Microstructural and Mechanical Properties of Hydroxyapatite–Zirconia Composites

This work focuses on the improvement of the mechanical properties of hydroxyapatite (HA) through the addition of 3 mol% yttria partially stabilized zirconia (PSZ). Enamel-derived HA (EHA) from freshly extracted human teeth and commercial HA (CHA) were chosen as the matrix. The effects of addition up to 10 wt% of PSZ and of sintering temperature (1000°–1300°C) on the density, microhardness, and compression strength were evaluated. For EHA–PSZ composites, the density and mechanical properties were generally enhanced by adding 5 wt% PSZ, especially after sintering at 1200°C, whereas CHA–PSZ composites showed lower strength values at sintering temperatures of 1200° and 1300°C with respect to EHA–PSZ composites. This may be due to the lower stability of CHA–PSZ composites with higher amounts of calcium zirconate formed over 1100°C when compared with EHA–PSZ composites.     

Preparation of Dense Lead Magnesium Niobate–Lead Titanate (Pb(Mg1/3Nb2/3)O3–PbTiO3) Ceramics by Spark Plasma Sintering

The effect of spark plasma sintering (SPS) on the densification behavior of Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ ceramics has been investigated. Specimens with a density of >99% of the theoretical density (TD) were obtained using SPS treatment at 900°C. Through normal sintering at 1200°C, however, the density of the specimen was only ∼92% of TD.     

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.     

Mechanical Behavior at 20° and 1200°C of Nicalon-Silicon-Carbide-Fiber-Reinforced Alumina-Matrix Composites

Tensile and fracture tests were conducted at 20° and 1200°C on a ceramic-matrix composite that was composed of an alumina (Al₂O₃) matrix that was bidirectionally reinforced with 37 vol% silicon carbide (SiC) Nicalon fibers. The composite presented nonlinear behavior at both temperatures; however, the strength and toughness were significantly reduced at 1200°C. In accordance with this behavior, matrix cracks were usually stopped or deflected at the fiber/matrix interface, and fiber pullout was observed on the fracture surfaces at 20° and 1200°C. The interfacial sliding resistance at ambient and elevated temperatures was estimated from quantitative microscopy analyses of the saturation crack spacing in the matrix. The in situ fiber strength was determined both from the defect morphology on the fibers and from the size of the mirror region on the fiber fracture surfaces. It was shown that composite degradation at elevated temperature was due to the growth of defects on the fiber surface during high-temperature exposure.     

Oxidation Behavior of Titanium Boride at Elevated Temperatures

The oxidation behavior of dense TiB₂ specimens was investigated. Hot-pressed TiB₂ with 2.5 wt% Si₃N₄ as a sintering aid was exposed to air at temperatures between 800° and 1200°C for up to 10 h. The TiB₂ exhibited two distinct oxidation behaviors depending on the temperature. At temperatures below 1000°C, parabolic weight gains were observed as a result of the formation of TiO₂( s ) and B₂O₃( l ) on the surface. The oxidation layer comprised two layers: an inner layer of crystalline TiO₂ and an outer layer mainly composed of B₂O₃. When the oxidation temperatures were higher than 1000°C, gaseous B₂O₃ was formed along with crystalline TiO₂ by the oxidation process. In this case, the surface was covered with large TiO₂ grains imbedded in a highly textured small TiO₂ matrix.     

Synthesis of Pyrochlore-Free 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 Ceramics via a Soft Mechanochemical Route

Single-phase perovskite 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (0.9PMN–0.1 PT) from a stoichiometric mixture of starting materials was synthesized by applying a mechanochemical technique to the stage of a precursor. A stoichiometric mixture of PbO, TiO₂, Mg(OH)₂, and Nb₂O₅ was milled for 60 min and heated at temperatures as low as 850°C for 4 h to obtain a single phase. The maximum dielectric constant of the samples from the milled mixture increased as the sintering temperature increased, with the remarkable grain growth, and attained 24600 at 1200°C. In contrast, poor densification and coexistence of the pyrochlore phase were observed on the samples from the nonmilled mixture. Further observation suggested that the pyrochlore phase concentrated near the surface during sintering and then migrated into the PbZrO₃ packing powder, leading to a pyrochlore–free phase at 1250°C. The dielectric constant of the latter ceramics was explained by the series mixing rule for the dielectric constant of a diphasic solid.