Two-Stage Sintering of Alumina with Submicrometer Grain Size

This work verifies the applicability of two-stage sintering as a means of suppressing the final stage grain growth of submicrometer alumina. The first heating step should be short at a relatively high-temperature (1400°–1450°C) in order to close porosity without significant grain growth. The second step at temperatures around 1150°C facilitates further densification with limited grain growth. Fine-grained alumina with a relative density of 98.8% and a grain size of 0.9 μm was prepared by two-stage sintering. A standard sintering process resulted in ceramics with identical relative density and a grain size of 1.6 μm.     

Influence of Grain Growth on Dielectric Properties of Nb-Doped BaTiO3

Effects of grain size and grain growth in Nb-doped BaTiO₃ on temperature and frequency dependencies of the dielectric constant were investigated. When 0.65 μm powder is sintered to an average grain size of 1 μm, two dielectric constant peaks indicate the presence of Nb-free BaTiO₃ and of Nb-containing material. Single peaks are observed above room temperature after additional grain growth or when 0.07 μm powder is sintered to an average grain size of 1 μm. The Curie point of pure BaTiO₃ with 1 μm grains is 4 to 6°C lower than that of material with grains >10 μm. Thermodynamically, this behavior is accounted for by a phase inversion stress ∼ the room-temperature stress.     

Reformulation of an Aqueous Alumina Slip Based on Modification of Particle-Size Distribution and Particle Packing

Six alumina casting slips with particle-size distributions varying from 44 to 0.1 μm were examined. Particle packing was calculated using the approach of Andreasen. Viscosity, green density, and pore-size distribution were measured. It was found that contouring the intermediate size distribution for particles finer than 15 μm provided the most desirable viscosity for slips composed of wide size distributions. For slips containing 50 vol% solids, the lowest viscosity obtained was 196 × 10⁻³ N · s/m² (with a two-component size distribution), and a green density of 2.52 g/cm³ (65% of theoretical) was achieved with a ternary system. These casts had bimodal pore-size distributions centered around approximately 1 and 0.1μm.     

Low-Temperature Sintering of Aluminum Oxide

A fine-sized (∼0.1 μm), agglomerate-free Al₂O₃ dispersion was used to prepare homogeneous green bodies with ∼69% relative density and ∼10-nm median pore radius. Samples could be sintered at 1150°C to a relative density >99.5% and an average grain size of 0.25 μm.     

Two-Step Sintering of Nanocrystalline ZnO Compacts: Effect of Temperature on Densification and Grain Growth

Two-step sintering (TSS) was applied on nanocrystalline zinc oxide (ZnO) to control the accelerated grain growth occurring during the final stage of sintering. The grain size of a high-density (>98%) ZnO compact produced by the TSS was smaller than 1 μm, while the grain size of those formed by the conventional sintering method was ∼4 μm. The results showed that the temperature of both sintering steps plays a significant role in densification and grain growth of the nanocrystalline ZnO compacts. Several TSS regimes were analyzed. Based on the results obtained, the optimum regime consisted of heating at 800°C (step 1) and 750°C (step 2), resulting in the formation of a structure containing submicrometer grains (0.68 μm). Heating at 850°C (step 1) and then at 750°C (step 2) resulted in densification and grain growth similar to the conventional sintering process. Lower temperatures, e.g., 800°C (step 1) and 700°C (step 2), resulted in exhaustion of the densification at a relative density of 86%, above which the grains continued to grow. Thermogravimetric analysis results were used to propose a mechanism for sintering of the samples with transmission electron micrographs showing the junctions that pin the boundaries of growing grains and the triple-point drags that result in the grain-boundary curvature.     

Fabrication of Transparent Hydroxyapatite Sintered Body with High Crystal Orientation by Pulse Electric Current Sintering

Transparent hydroxyapatite (HAp) sintered body with a high crystal orientation and a relative density of 99.7% was fabricated by the pulse electric current sintering method; the c -axis of HAp crystals with a hexagonal column morphology and ca. 200 μm × 25 μm in size was oriented perpendicular to the pressure direction. The sintered temperature increased to 1200°C at a heating rate of 50°C/min for 10 min and a pressure of 50 MPa was applied. The orientation indexes were calculated from X-ray diffraction patterns of the planes perpendicular and parallel to the pressure direction. The optical transmittance was greater than 70% over the wavelength of 700 nm.     

Synthesis of Cellular Silica Structure Under Microchannel Confinement

A silica cellular structure was synthesized as a novel means of enhancing the geometrical surface area of a silicon microchannel with cell diameter of ∼10 μm and cell interconnectivity of ∼0.4. Surface-selective infiltration, assembly, and partial sintering of polystyrene microspheres in the microchannel were used as mechanisms to create a sacrificial template. The polymer template was infiltrated with a silica precursor, and the infiltrated structure was dried and calcined at 500°C to remove the polymer phase and subsequently sintered at 1100°C to form dense silica skeleton. Volume shrinkage and crack formation during calcining and sintering of the infiltrated silica structure were strongly influenced by silica particle size in the precursor. In comparison with free-standing cellular specimens prepared by similar template methods, the shrinkage and cracking issues offered an interesting challenge for synthesizing the cellular structure which could be net-shaped into the spatial confinement of the microchannel geometry.     

Sintering and Properties of Monazite-Type CePO4

Monazite-type CePO₄ powder (average grain size 0.3 μm) was dry-pressed to disks or bars. The green compacts began to sinter above 950°C. Relative density ≧ 99% and apparent porosity <1% were achieved when the specimens were sintered at 1500°C for 1 h in air. The linear thermal expansion coefficient and thermal conductivity of the CePO₄ ceramics were 9 × 10⁻⁶/°C to 11 × 10⁻⁶/°C (200° to 1300°C) and 1.81 W/(m · K) (500°C), respectively. Bending strength of the ceramics (average grain size 4 μm) was 174 ± 28 MPa (room temperature). The CePO₄ ceramics were cracked or decomposed by acidic or alkaline aqueous solutions at high temperatures.     

Influences of Particle Size of Alumina Filler in an LTCC System

A low temperature co-fired ceramics system consisting of a typical calcium aluminoborosilicate glass and alumina filler was used to investigate the effects of four different sizes, 13 nm, 0.5, 3, and 39 μm, of a commercially available alumina filler on the resultant densification, crystallization, and dielectric properties. There was definitely a proper range of alumina particle size, which leads to desirable densification and enhanced dielectric properties. The onset temperatures of densification and crystallization depended strongly on the filler particle size. The 3 μm sample as an optimum filler size exhibited a promising performance of k ∼8.1 and Q ∼160 at a resonant frequency of 14.8 GHz, which results from early densification and intensive crystallization of the anorthite CaAl₂Si₂O₈ phase. Particularly, the use of nano-sized alumina (13 nm) retarded both densification by ∼200°C and crystallization by ∼80°C compared with the results of the 3 μm alumina case. The dependence of the filler particle size was postulated as being related to the wetting and connectivity behavior of glass through consequent inter-reactions between glass and ceramic.     

Microwave Sintering of Alumina at 2.45 GHz

The sintering kinetics and microstructural evolution of alumina tubes (∼17 mm length, ∼9 mm inner diameter, and ∼11 mm outer diameter) were studied by conventional and microwave heating at 2.45 GHz. Temperature during microwave heating was measured with an infrared pyrometer and was calibrated to ±10°C. With no hold at sintering temperature, microwave-sintered samples reached 95% density at 1350°C versus 1600°C for conventionally heated samples. The activation energy for microwave sintering was 85 ± 10 kJ/mol, whereas the activation energy for conventionally sintered samples was 520 ± 14 kJ/mol. Despite the difference in temperature, grains grew from ∼1.0 μm at 86% density to ∼2.6 μm at 98% density for both conventionally sintered and microwave-sintered samples. The grain size/density trajectory was independent of the heating source. It is concluded that the enhanced densification with microwave heating is not a consequence of fast-firing and therefore is not a result in the change in the relative rates of surface and grain boundary diffusion in the presence of microwave energy.     

Effect of Heat Treatment on Grain Size, Phase Assemblage, and Mechanical Properties of 3 mol% Y-TZP

The effect of heat treatment on the grain size, phase assemblage, and mechanical properties of a 3 mol% Y-TZP ceramic was investigated. Specimens were initially sintered for 2 h at 1450°C to near theoretical density; some specimens were then heat-treated at 1550°, 1650°, 1750°, or 1850°C to coarsen the microstructure. The average grain size increased with heat treatment from <0.5 to ∼10 μ-m. Phase analyses revealed predominantly tetragonal and cubic phases below 1750°C, with a significant decrease in tetragonal content and increase in monoclinic content for temperatures >1750°C. The maximum fraction of tetragonal phase that transformed during fracture corresponded with the largest tetragonal grain size of ∼5–6 μm. Strength was on the order of 1 GPa, and was surprisingly insensitive to heat-treatment temperature and grain size, contrary to previous studies. The fracture toughness increased from 4 to 10 MPa.m^1/2 with increasing grain size, owing to an increasing transformation zone size. Grain sizes larger than 5–6 μm spontaneously transformed to monoclinic phase during cooling. Such critical grain sizes are much larger than those found in past investigations, and may be due to the greater fraction of cubic phase present which decreases the strain energy arising from crystallographic thermal expansion anisotropy of the tetragonal phase.     

An Aqueous Gelcasting Process for Sintered Silicon Carbide Ceramics

An aqueous gelcasting process for the preparation of dense as well as porous-sintered SiC ceramics has been described in this paper. A commercial silicon carbide powder coated with phenolic resin was used in this investigation. For the purpose of comparison, a pure SiC powder was also studied. ς potential and viscosity studies revealed that the pure SiC powder requires an electro-steric stabilization, whereas the phenolic resin-coated powder requires an electrostatic stabilization in order to produce their corresponding aqueous slurries with high solids content. Thermogravimetry and differential thermal analysis techniques have been used to study the decomposition behavior of phenolic resin. Aqueous slurries containing 25–50 vol% SiC powder were gelcast and sintered at 2150°C for 1 h. The sinterability of gelcast SiC samples was found to be highly influenced by the SiO₂ formed on the surface of SiC during aqueous processing, as confirmed by the Fourier transform infrared spectroscopy study. The results obtained from various characterization techniques suggest that in order to make dense SiC parts with >3.13 g/mL bulk density (a theoretical density of 97.5%) by an aqueous gelcasting process, the starting phenolic resin (∼5%)-coated SiC powder should possess a median particle size of <11.0 μm, surface area of >3.2 m²/g, a compact (green) density of >1.67 g/mL, and a B content of >0.5%. Further, by using polyethylene granules and organic foaming agents, sintered SiC foam with a porosity of >80%, a compressive strength of >16 MPa and a coefficient of thermal expansion of 4.574 × 10⁻⁶/°C between 30° and 700°C can be prepared by an aqueous gelcasting process, followed by sintering at 2150°C for 1 h.     

Dielectric Properties of Fluxed Barium Titanate Ceramics with Zirconia Additions

BaTiO₃ compacts, when fluxed with <2 vol% of a complex borate glass phase, were sintered to near theoretical density at temperatures <1175°C in 2 h. Microstructural analysis showed a uniform grain size <1.0 μm with 0.75 wt% ZrO₂ added to the flux phase as a grain growth inhibitor. TEM analysis revealed a microcrystalline grain-boundary phase with the ZrO₂ resident along the grain boundaries. These samples displayed an essentially flat dielectric profile, low dissipation factors (<2%) over the range 25° to 125°C, a near linear dependence (≅±15%) between 25° and −55°C, and significantly increased voltage stability. X-ray diffraction analyss of these small-grained materials indicated a suppression of the tetragonal structure toward a more cubic modification.     

Rapid Sintering of Stoichiometric Zinc-Modified Lead Magnesium Niobate

A rapid thermal-processing technique was used to densify zinc-modified lead magnesium niobate (PMZN30) of the form Pb(Mg_0.7Zn_0.3)_1/3Nb_2/3O₃. This dielectric composition showed relaxor behavior with a maximum permittivity near room temperature. Rapid densification was observed at temperature. Rapid densification was observed at temperatures of 1100°C and above, whereas lower temperatures showed a significant reduction of densification and electrical properties. A sintering schedule of 4 min at 1100°C with no prevention of PbO volatilization resulted in densification on the order of 90% of theoretical density and a weak field dielectric constant in excess of 14 000. Perovskite grain size was consistently in the range of 1 μm or less. The formation of pyrochlore on sample surfaces showed a direct relation to time and temperature.     

Pressureless Sintering of Zirconium Diboride: Particle Size and Additive Effects

Zirconium diboride (ZrB₂) was densified by pressureless sintering using <4-wt% boron carbide and/or carbon as sintering aids. As-received ZrB₂ with an average particle size of ∼2 μm could be sintered to ∼100% density at 1900°C using a combination of boron carbide and carbon to react with and remove the surface oxide impurities. Even though particle size reduction increased the oxygen content of the powders from ∼0.9 wt% for the as-received powder to ∼2.0 wt%, the reduction in particle size enhanced the sinterability of the powder. Attrition-milled ZrB₂ with an average particle size of <0.5 μm was sintered to nearly full density at 1850°C using either boron carbide or a combination of boride carbide and carbon. Regardless of the starting particle size, densification of ZrB₂ was not possible without the removal of oxygen-based impurities on the particle surfaces by a chemical reaction.     

Electrophoretic Deposition of Lead Zirconate Titanate Films on Metal Foils for Embedded Components

Lead zirconate titanate (PZT) thick films in the thickness range of 5–200 μm on 20 μm copper and 25 μm platinum foils were prepared by electrophoretic deposition (EPD) for application as embedded passive components. The EPD process was conducted in glacial acetic acid medium, and the effects of deposition parameters, such as dc voltage values, processing times, and suspension aging on the film thickness and composition stoichiometry were evaluated. The dependence of the film thickness on the current and aging of the suspension was established and correlated to the chemical modifications occurring in the suspension. Films sintered at 1150°C for 30 min exhibited uniform and dense microstructures with an average grain size of 1.5 μm. A dielectric permittivity of around ∼1330 and loss tangent of 0.05 were measured for films with 8 μm of thickness. The films showed remanent polarization and a maximum polarization value of 24 and 38 μC/cm², respectively. These properties, comparable with those of bulk PZT ceramics, suggested potential applications of the EPD process for the deposition of ferroelectric thick films on metal foils for embedded component applications.     

Fabrication of Mullite Ceramics With Ultrahigh Porosity by Gel Freeze Drying

Porous mullite (3Al₂O₃·2SiO₂) ceramics with an open porosity up to 92.9% were fabricated by a gel freeze-drying process. An alumina (Al₂O₃) gel mixed with ultrafine silica (SiO₂) was frozen and sublimation of ice crystals was carried out by drying the frozen body under a low pressure. Porous mullite ceramics were prepared in air at 1400°–1600°C due to the mullitization between Al₂O₃ and SiO₂. A complex and porous microstructure was formed, where large dentritic pores with a pore size of ∼100 μm contained small cellular pores of 1–10 μm on their internal walls. Owing to the complete mullitization, a relatively high-compressive strength of 1.52 MPa was obtained at an open porosity of 88.6%.     

Preparation of Dense BaTiO3 Ceramics from Sol-Gel-Derived Monolithic Gels

Dense, small-grained BaTiO₃ ceramics, with a grain size around 1 μm and a relative sintered density >98%, were obtained at 1100°C from sol-gel-derived gel monoliths without using any sintering additives. The monolithic gels asprepared had a relative density of about 50% and consisted of ultrafine pseudo-cubic BaTiO₃ particles (<50 nm). These gels, with a significantly high density compared with that of previous ones (∼30%), have been synthesized at room temperature from a sol solution with a concentration of equimolar mixture of titanium isopropoxide and barium ethoxide (0.8 mol/L), using the methanol/2-methoxyethanol mixed-solvent system. Microstructural development of the gel monoliths with increasing sintering temperature and the dielectric properties of the obtained dense BaTiO₃ ceramic have been investigated.     

Synthesis of Spherical β-Silicon Carbide Particles by Ultrasonic Spray Pyrolysis

Fine agglomerate-free spherical β-SiC powder was synthesized from a dispersion of colloidal silica, saccharose, and boric acid, by means of an ultrasonic spray pyrolysis method. Droplets of 2.2 μm were formed with an aerosol generator, operated at 2.5 MHz, and carried into a reaction furnace at 900°C with argon. Spherical X-ray amorphous gel particles of 1.1 μm were obtained. β-SiC particles with a mean diameter of 0.79 μm and spherical shape resulted when the SiC gel precursor particles were heated at 1500°C in argon.     

Preparation of Highly Dense PZN–PZT Thick Films by the Aerosol Deposition Method Using Excess-PbO Powder

Lead zinc niobate–lead zirconate titanate thick films with a thickness of 50–100 μm were deposited on silicon and alumina substrates using the aerosol deposition method. The effects of excess lead oxide (PbO) on stress relaxation during postannealing were studied. Excess PbO content was varied from 0 to 5 mol%. The as-deposited film had a fairly dense microstructure with nanosized grains. The films deposited on silicon were annealed at temperatures of 700°C, and the films deposited on sapphire were annealed at 900°C in an electrical furnace. The annealed film was detached and cracks were generated due to the high residual compressive stress and thermal stress induced by thermal expansion coefficient mismatch. However, the film deposited using powder containing 2% of excess PbO showed no cracking or detachment from the substrate after the postannealing process. The PbO evaporation at elevated temperature during the postannealing process seemed to have reduced the residual compressive stress. The remanent polarization and relative dielectric constant of the 50 μm thick films annealed at 900°C were 43.1 μC/cm² and 1400, respectively, which were comparable with the values of a bulk specimen prepared by a powder sintering process.