Processing and Sintering of Ultrafine MgO-ZrO2 and (MgO, Y2O3)-ZrO2 Powders

Chemical coprecipitation was used to produce ultrafine and easily sinterable MgO-stabilized and (MgO, Y₂O₃) stabilized ZrO₂ powders. The sintering behavior is very sensitive to post-precipitation washing because "hard" agglomerates form when the precipitated gels are washed with water, whereas "soft" agglomerates form when they are washed with ethanol. The soft agglomerates pack uniformly, resulting in homogeneous shrinkage of powder compacts to near-theoretical density. The hard agglomerates result in compacts which have regions of localized densification and a signifiint fraction of residual porosity.     

Dehydration of Hydrous Zirconia with Methanol

The washing of hydrous zirconia with alcohols to reduce the incidence of hard agglomerates on subsequent drying is well known. The results of methanol dehydration of hydrous zirconia (zirconium hydroxide), [Zr₄(μ-OH)₈(OH)₈(H₂O)₈]˙ xH₂O, show that only μ-OH groups are unaffected. This suggests two things: First, the removal of nonbridging hydrooxo groups and water with alcohols such as methanol leads to a reduction/elimination of hard agglomerates. Second, hard agglomerate formation is associated with condensation reactions involving nonbridging hydroxo groups (Zr-OH+HO-Zr→Zr-O-Zr+H₂O).     

Preparation of Porous Ca10(PO4)6(OH)2 and β-Ca3(PO4)2 Bioceramics

Submicrometer-sized, pure calcium hydroxyapatite (HA, (Ca₁₀(PO₄)₆(OH)₂)) and β-tricalcium phosphate (β-TCP, Ca₃(PO₄)₂) bioceramic powders, that have been synthesized via chemical precipitation techniques, were used in the preparation of aqueous slurries that contained methyl cellulose to manufacture porous (70%–95% porosity) HA or β-TCP ceramics. The pore sizes in HA bioceramics of this study were 200–400 μm, whereas those of β-TCP bioceramics were 100–300 μm. The pore morphology and total porosity of the HA and β-TCP samples were investigated via scanning electron microscopy, water absorption, and computerized tomography.     

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.     

Effect of Calcination on the Sintering of Gel-Derived, Zirconia-Toughened Alumina

The densification behavior of ZrO₂ (+ 3 mol% Y₂O₃)/85 wt% Al₂O₃ powder compacts, prepared by the hydrolysis of metal chlorides, can be characterized by a transition- and an α-alumina densification stage. The sintering behavior is strongly determined by the densification of the transition alumina aggregates. Intra-aggregate porosity, resulting from calcination at 800°C, partly persists during sintering and alumina phase transformation and negatively influences further macroscopic densification. Calcination at 1200°C, however, densifies the transition alumina aggregates prior to sintering and enables densification to almost full density (96%) within 2 h at 1450°C, thus obtaining a microstructure with an alumina and a zirconia grain size of 1 μm and 0.3–0.4 μm, respectively.     

Sintering of Nanosized MnZn Ferrite Powders

The sintering and microstructural evolution of nanosized MnZn ferrite powders prepared by a hydrothermal method were investigated. The microstructure of sintered ferrite compacts depends strongly on the strength of the agglomerates formed during the compacting of nanosized ferrite powders. It was found that at 700°C the theoretical density of sintered compacts can almost be reached, while above 900°C an increase of porosity was identified. The formation of extra porosity at higher sintering temperatures is caused mainly by the oxygen release which accompanies the dissolution of relatively large grains of residual alpha-Fe₂O₃ in the spinel lattice.     

Rapid Hot-Pressing of Ultrafine PSZ Powders

The process of compaction and densification of ultrafine (40- to 60-nm grain size) powder of partially stabilized zirconia with 3 mol% of Y₂O₃ (Y3-PSZ) during rapid hot-pressing was investigated. A special apparatus was designed to allow rapid application of 1.6 GPa of quasi-isostatic pressure at temperatures of 1100° to 1300°C to powder compacts encapsulated in glass under vacuum. Pressure was applied for 10 s, then the samples were rapidly cooled to room temperature, removed from the encapsulating glass, and characterized using SEM, TEM, and X-ray diffraction. Density and mechanical properties of the prepared materials were measured and compared with those of similar materials fabricated using conventional hot-pressing. SEM and TEM observations revealed that the ultrafine grains of the starting powder coarsened rapidly during the initial heating, and the compacts developed large (> 10 μm) and small (< 1 μm) pores. The process of densification under pressure consisted of closing of the large pores, whereas the small pores were relatively unaffected by the application of pressure at all investigated temperatures. The major mechanism of densification during the rapid pressing appears to be rearrangement and sliding of grains around the large pores. The material prepared by rapid pressing at 1300°C had higher hardness ( H _v= 1400 versus 1300 kg/mm²) but somewhat lower fracture toughness ( K _{I C} = 5.5 versus 6.0 MPa · m^1/2) compared with the conventionally hot-pressed Y3-PSZ. Density of the material pressed at 1300°C was 97% of theoretical density.     

Synthesis of Submicrometer Mullite Powder via High-Temperature Aerosol Decomposition

High-purity mullite powders (3Al₂O₃.2SiO₂) have been prepared using a high-temperature aerosol decomposition technique yielding submicrometer (0.6 μm average) particles of spherical morphology with no hard agglomerates using aluminum nitrate and fumed silica as precursors. Depending upon the reaction conditions used, the powders range from amorphous to crystalline with no evidence of secondary-phase formation. This mullite synthesis approach has the advantages of not requiring postsynthesis milling, the ability to use a wide range of precursor systems, and enhanced control over chemical homogeneity and particle size/shape.     

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%.     

Submicrometer Transparent Alumina by Sinter Forging Seeded γ-Al2O3 Powders

Seeding boehmite with α-Al₂O₂, followed by calcination at 600°C, results in an agglomerated alumina powder (<53 μm) that can be sinter forged to full density at 1250°C. Compressive strains as high as ɛ_x=−0.9, and radial flow (ɛ_x= 1.0) during sinter forging remove large, interagglomerate pores. The fully dense alumina has a grain size of 0.4 pm and is visually transparent. It is proposed that deformation of dense agglomerates is the primary mecha- nism responsible for large pore elimination and compact densification. The sinter forging of sol-gel-derived alumina powders offers a new technology to prepare highly transparent, optical ceramics at lower temperatures than conventional routes.     

Mechanical Properties and Failure Analysis of Alumina-Glass Dental Composites

Strength measurements and fractography were used to investigate the failure of alumina-glass dental composites containing 75 vol% alumina and 25 vol% glass. Alumina compacts were prepared by slip casting and sintering at 1100°C for 2 h. Dense composites were made by infiltrating partially sintered alumina with glass at 1150°C for 8 h. Young's modulus and the hardness of the composites were 270 GPa and 12 GPa, respectively. The mean strength (460 MPa) and fracture toughness (4.0 MPa·m^1/2) of the composites were insensitive to the glass thermal expansion coefficient (α_glass= 5.9 × 10⁻⁶ to 7.8 × 10⁻⁶°C⁻¹). Typical flaws were pores and cracklike voids formed by poor particle packing and differential sintering near agglomerates of alumina in the composite. Crack deflection and crack bridging were observed in indentation cracks. Fracture toughness was single-valued because the alumina particle size was small (∼3 μm). Alumina-glass composites are promising new ceramics for dental crown and bridge applications, because their strength and fracture toughness are ∼2 times greater than those of current dental ceramics.     

Sintering of TiO2–Al2O3 Composites: A Model Experimental Investigation

In recent theoretical work we have shown that the sintering of a composite is strongly affected by the shear deformation of the continuous phase. This phenomenon was studied experimentally in a model system consisting of a TiO₂ matrix dispersed with nondensifying agglomerates of Al₂O₃. The results of this study are reported here. Several interesting results were obtained: (a) The experimentally obtained sintering rate of the composite could be successfully predicted by measuring the concomitant shear and densification rate of the matrix phase in a sinter-forging experiment and using the developed analysis; (b) the densification rate of the composite changed with the volume fraction of the dispersed phase, but was unaffected by the size of the dispersed phase; and (c) processing flaws appeared to form only when the size of the dispersed phase was greater than about 10 μm. A technique for measuring the parameter β, which was used to correlate theory and experiments, is described. The procedures used for preparation of powders (from alkoxides) and the green compacts are also described in detail.     

Microindentation-Induced Transformation in 3.5-mol%-Yttria-Partially-Stabilized Zirconia Single Crystals

The temperature dependence of the Vickers microhardness was studied in 3.4-mol%-Y₂O₃-partially-stabilized ZrO₂ (Y-PSZ) single crystals up to 1000°C; the samples had previously been annealed at 1600°C for 150 h to develop "colony" precipitates of tetragonal ZrO₂ in the cubic ZrO₂ matrix. Indentation caused extensive stress-induced martensitic transformation of the colony precipitates to monoclinic symmetry in zones which extended in extreme cases up to several hundred micrometers from the indent. For indents made at 500°C and above, the M _d and M _f temperatures are 450° and 310°C, respectively; A _s is ∼600°C ( M _d is the temperature of initial transformation (the "martensite start temperature") in deformed samples; M _f is the temperature at which the final transformation occurs; A _s is the temperature at which the reverse (monoclinic → tetragonal) transformation begins). However, extensive transformation zones are also found for indents made at 200°, 300°, and 400°C. The dislocation density introduced during indentation is responsible for nucleating the transformation in a zone adjacent to the indent. However, the transformation zone extends further than the plastic zone around the indent, indicating extensive autocatalytic transformation. Transformation within the zone appeared to occur in individual plates with {110} habit planes. The plate dimensions (∼100 μm ×∼175 μm ×∼10 μm) are large compared to the size of the colony precipitates (∼2 μm in maximum dimension).     

Processing of a Novel Multilayered Silicon Nitride

A new type of silicon nitride with a layered structure of alternating dense and porous layers was obtained by addition of β-Si₃N₄ whiskers to the porous layers. The materials consisted of dense layers 60 μm thick and porous layers 40 μm thick with a final porosity of about 30%. Highly anisotropic shrinkage behavior was observed during sintering. A large addition of whiskers to the porous layers resulted in layers with well-oriented and tightly tangled elongated grains, where porosity is represented by anisotropic shaped pores.     

Sinterable Ceramic Powders from Laser-Driven Reactions: II, Powder Characteristics and Process Variables

Si, Si₃N₄, and SiC powders which possess a unique set of characteristics were produced by a laser-driven gas-phase synthesis process. The powders have a fine particle size (<0.1 μm), are spherical, have a narrow range of particle sizes, are free of hard agglomerates, have a high degree of phase purity, and have a high absolute purity (<0.1% including oxygen). A detailed analysis of the physical, chemical, and crystalline characteristics of Si and Si₃N₄ is presented. A brief discussion of our initial work with SiC is also included. The dependence of particle characteristics on the various process parameters (laser power, cell pressure, gas composition) is discussed and related to a model of the powder-synthesis process.     

Preparation and Ferroelectric Properties of SBN:50 Ceramics

Stoichiometric quantities of AR-grade chemicals, e.g., Ba(NO₃)₂, Sr(NO₃)₂, and Nb₂O₅, were used to synthesize Sr_0.5Ba_0.5Nb₂O₆ (SBN:50) by the solid-state reaction route. The reaction mixture was characterized by DTA/TG/DTG to locate the formation temperature of SBN:50. X-ray diffrction was used to identify the major and any additional phases formed in the calcined as well as sintered materials. The materials were further characterized by electrical measurements, the temperature dependence of the dielectric constant (ɛ) and the loss (tan δ), FE hysteresis-loop parameters ( P _s and E _c), and room-temperature dc resistivity. SBN:50 exhibited a broad maximum of ɛ in the temperature range 65° to 95°C. Moreover, the FE hysteresis loop persisted even at 130°C, which is much higher than the Curie range determined from an ɛ vs T plot. Microstructural photographs revealed varied grain-size distribution between 5 and 20 μm. The presence of residual porosity (∼ 95% dense) resulted in opaque compacts.     

Effect of Particle Size Distribution on the Sintering of Alumina

The effect of particle size distribution on the sintering of Al₂O₃ was investigated. Samples could be sintered to high relative density (∼99%), small average grain size (1 μm), and no growth of exaggerated grains using powders with either broad or narrow particle size distribution. However, the broad particle size distribution provided the advantage that powder compacts could be prepared with higher green density and, therefore, samples could be densified with less total shrinkage.     

Morphology of Highly Dispersing Precipitated Silica: Impact of Drying and Sonication

Light and X-ray scattering are used to examine the structure of two commercial precipitated silicas (Zeosil 1165 and Ultrasil 7005) and one developmental precipitated silica, Dimosil 288. All three products have a four-level hierarchical structure consisting of primary particles, aggregates, hard agglomerates and soft agglomerates, with Dimosil 288 showing the clearest evidence of the four structural levels. The impact of sonication and drying protocol was explored by light scattering for Dimosil 288. With the exception of the large-scale soft agglomerates, all the structural levels are robust under sonication of aqueous suspensions. Sonication breaks down soft agglomerates leaving hard structures approximately 11 μm in radius-of-gyration. Drying plays a critical role in hardening the soft agglomerates. If the product is never dried, sonication reduces the agglomerate size to 3.5 μm, which is identified as the size of the hard agglomerate. Although agglomerates larger than 3.5 μm are present in the never-dried product, they are friable. Large-scale agglomerates are marginally more robust when dried at 150 °C compared to room temperature drying. Given the similarity of mechanical properties of rubbers filled with these silicas, it appears that the four-level structure with friable large-scale soft agglomerates is a characteristic of highly dispersing silica products.     

Microstructure Development in Reactive-Templated Grain Growth of Bi1/2Na1/2TiO3-Based Ceramics: Template and Formulation Effects

"Reactive-templated grain growth" (RTGG) processing of Bi_1/2Na_1/2TiO₃ (BNT)-based ceramics is reported. Molten salt synthesis was used to prepare platelike (∼0.2 μm × 5 μm × 5 μm) Ruddlesden–Popper (Sr₃Ti₂O₇ (ST)) and Aurivillius (BaBi₂Nb₂O₉ (BBN)) phases which were used as "templates" in studies of RTGG with BNT-based matrixes. A "citrate-gel" route was designed to produce intimately mixed, fine-grain matrixes for these studies. The analytical techniques used were powder X-ray diffraction and microstructural examination of dry-pressed and fired compacts. For mixtures templated with BBN, single-phase perovskite readily formed, and an initially heterogeneous microstructure evolved toward a dense assemblage of anisometric, micrometer-scale grains. Perovskite formation was more sluggish in the mixtures templated with ST, and the final sintered microstructure featured larger, porous grains in an equiaxed, micrometer-scale matrix. A qualitative model, which examined the excess constituents in the matrix after formation of stoichiometric ABO₃ perovskite, is proposed to explain the observations. The model predicted an excess of Na₂O and TiO₂ in the matrix in the case of BBN templates and only excess TiO₂ in the case of ST templates. The results indicate that careful examination of matrix and template chemistry could be important in the selection of systems for RTGG processing.     

Microstructure of KTaxNb1−xO3 Thin Films on MgO (100) Single Crystals

Thin KTa_xNb_1−xO₃ (KTN) films were prepared by deposition of sol–gel precursor solutions on MgO (100) single crystals. Crystal structure and microstructure of the films as a function of processing parameters, such as rate, duration, and temperature of postdeposition heat treatment, were studied. Several techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were employed to analyze the films. It was observed that slow heating of KTN films promotes pyrochlore formation while fast-firing of the films results in predominant formation of the perovskioe phase. In slow-heated samples, TEM showed randomly oriented pyrochlore crystallites with a vermicular nanostructure of 10–30 nm with an interpenetrating porosity of the same range. In fast-fired samples, large perovskite pockets with pyrochlore crystallites scattered among them were seen. The large perovskite grains were on the order of 0.1–0.5 μm, irregular in shape and porous. Transmission electron diffraction indicated these were single crystals, and ferroelectric domains were observed in them. Films of up to 1 μm thick were obtained by multiple deposition of the sol–gel KTN. Dense films were achieved when each layer was densified at 750°C for 2 h before the next layer was deposited.