Reactive Hot Pressing of Alumina-Molybdenum Disilicide Composites

Al₂O₃-MoSi₂ composites were prepared by reactive hot pressing using molybdenum, aluminum, and mullite powders as precursors. The Gibbs free energy was highly negative for the composite-forming reaction, which indicated that the products were stable relative to the reactants. After the reaction, the composites had high relative density, ∼96%. Based on the composite-forming reaction, the composites should have contained 18 vol% MoSi₂ in an Al₂O₃ matrix. Scanning electron microscopy revealed that the MoSi₂ inclusions were elongated, with an average thickness of ∼5 μm and inclusion lengths that ranged from 5 to 50 μm. Average composite strength was 467 MPa, and toughness was 3.7 MPa·m^1/2.     

Low-Temperature Sintering of Seeded Sol—Gel-Derived, ZrO2-Toughened Al2O3 Composites

Seeding a mixture of boehmite (AIOOH) and colloidal ZrO₂ with α-alumina particles and sintering at 1400°C for 100 min results in 98% density. The low sintering temperature, relative to conventional powder processing, is a result of the small alumina particle size (∼0.3 μm) obtained during the θ-to α-alumina transformation, homogeneous mixing, and the uniform structure of the sol-gel system. Complete retention of pure ZrO₂ in the tetragonal phase was obtained to 14 vol% ZTA because of the low-temperature sintering. The critical grain size for tetragonal ZrO₂ was determined to be ∼0.4 μm for the 14 vol% ZrO₂—Al₂O₃ composite. From these results it is proposed that seeded boehmite gels offer significant advantages for process control and alumina matrix composite fabrication.     

In Situ TiC/TiB2 Particulate Locally Reinforced Steel Matrix Composites Fabricated Via the SHS Reaction of Ni–Ti–B4C System

The composites synthesized with three kinds of B₄C particles mainly consist of TiC, TiB₂, and the alloy austenite containing Ni element. Ceramic particulate sizes in the composites synthesized with ∼3.5 and ∼45 μm B₄C particles are larger than that synthesized with ∼140 μm B₄C particle. No pores are found between the reinforcing region and matrix in the composites synthesized with ∼3.5 and ∼45 μm B₄C particles, while some large pores exist in the composites synthesized with ∼140 μm B₄C particle. With the decrease of B₄C particle size, the pores in the composites become fewer and the hardness and wear resistance of the composites increase.     

Toughening and Strengthening of NiAl with Al2O3 by the Addition of ZrO2(3Y)

NiAl/10-mol%-ZrO₂(3Y) composites of almost full density have been fabricated via spark plasma sintering (SPS) for 10 min at 1300°C and 30 MPa. The former intermetallic compound, which contains a trace amount of Al₂O₃, has been prepared via self-propagating high-temperature synthesis. The composite microstructures are such that tetragonal ZrO₂ (∼0.2 μm) and Al₂O₃ (∼0.5 μm) particles are located at the grain boundaries of the NiAl (∼46 μm) matrix. Improved mechanical properties are obtained: the fracture toughness and bending strength are 8.8 MPa·m^1/2 and 1045 MPa, respectively, and high strength (>800 MPa) can be retained up to 800°C.     

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.     

Synthesis and Electrical Characterization of Thin Films of PT and PZT Made from a Diol-Based Sol-Gel Route

Lead titanate (PT) sols were prepared using propanediol, butanediol, or pentanediol solutions of lead acetate trihydrate and titanium diisopropoxide bis(acetylacetonate). Precursor sols for PbZr_0.53Ti_0.47O₃ (PZT) films were prepared from propanediol solutions, with zirconium tetrapropoxide being used as the zirconium source. Films were formed by spin-coating the sols onto silicon and platinized silicon substrates; the resulting gel layers were converted to ceramic films by adopting a two-stage heating schedule with final firing temperatures of 600–700°C. Information on film crystallization, microstructure development, and electrical properties is presented for both compositions. The limiting thickness of surface-smooth crack-free single-layer films was ∼1 μm. The PT films exhibited a "linear" polarization-electric field ( P-E ) response, while the PZT films gave rise to characteristic ferroelectric P-E hysteresis loops. A 0.5 μm thick single-layer PZT film exhibited remanent polarization (P_r) values of ∼34 μC·Cm⁻², with a coercive field ( E _c) of ∼45 kV·Cm⁻²; the relative permittivity (ɛ_r) and the dissipation factor ( D ) were ∼1250 and 0.07. For a 1 μm single-layer PZT film, the respective values were P _r∼19 μC∼Cm⁻², E _c∼40 kV∼Cm⁻², ɛ_r∼750, and D = 0.03.     

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.     

Fabrication of Textured Bi3NbTiO9 Ceramics

Highly textured Bi₃NbTiO₉ ceramics are fabricated by normal sintering from molten salt-synthesized plate-like crystallites. Fine Bi₃NbTiO₉ plate-like crystallites (∼1 μm) not only facilitate the densification, but also enhance texture in Bi₃NbTiO₉ ceramics. Weak-agglomerated platelets exhibit higher sinterability and can be densified at a temperature as low as 1000°C, which is about 100°C lower than that of equiaxed powders prepared by directly calcining Bi₃NbTiO₉ precursor. Meanwhile, the orientation degree of textured Bi₃NbTiO₉ ceramics increases with sintering temperature. Highly oriented Bi₃NbTiO₉ (orientation degree of ∼0.91) ceramic with a relative density of ∼92% is obtained at 1150°C. Because of the oriented grain microstructure, textured Bi₃NbTiO₉ ceramic exhibits anisotropic electrical properties.     

Preparation of Magnesium Nitride Powder by Low-Pressure Chemical Vapor Deposition

Submcrometer-scale magnesium nitride (Mg₃N₂) powder was prepared by low-pressure chemical vapor deposition, i.e., the reaction of Mg vapor with mixed NH₃–N₂ gases at 800°C under a pressure of ∼1 kPa. The mixing ratios of NH₃–N₂ gases were 0% NH₃–100% N₂ (pure N₂), 20% NH₃–80% N₂, 40% NH₃–60% N₂, 60% NH₃–40% N₂, 80% NH₃–20% N₂, and 100% NH₃–0% N₂ (pure NH₃). The reactions between Mg vapor and NH₃–N₂ gases produced platy Mg₃N₂ particles <0.2 μm and acicular particles with long-axis length of ∼0.2 μm, whereas the reaction of Mg vapor with pure NH₃ gas produced spherical Mg₃N₂ particles with diameters of ∼0.1 μm.     

Pressureless Sintering of SiC-Whisker-Reinforced Al2O3 Composites: I, Effect of Matrix Powder Surface Area

Pressureless sintering of SiC-whisker-reinforced Al₂O₃ composites was investigated. In Part I of the study, the effect of the matrix (Al₂O₃) powder surface area on densification behavior and microstructure development is reported. Compacts prepared with higher surface area Al₂O₃ powder showed enhanced densification at lower whisker concentrations (5 and 15 vol%). Samples with 15 vol% whiskers could be pressureless sintered to ∼97% relative density with zero open porosity and ∼1.6-μm matrix average grain intercept size.     

Translucent Sintered MgAl2O4

A translucent polycrystalline MgAl₂O₄ ceramic was prepared from finely divided coprecipitated spinel in which a small amount of CaO added as a sintering aid was uniformly distributed. The CaO promotes densification through the formation of a liquid phase at the sintering temperatures. Depending on the sintering treatment, the relative density of the sintered spinel was 99.7 to ∼100% of theoretical. The in-line optical transmission was > 10% from 0.3 to 6.5 μm. Total transmission in the visible region was between 67 and 78%.     

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.     

High Strain, 〈001〉 Textured 0.675Pb(Mg1/3Nb2/3)O3–0.325PbTiO3 Ceramics: Templated Grain Growth and Piezoelectric Properties

Lead magnesium niobate–lead titanate, 0.675Pb(Mg_1/3Nb_2/3)O₃–0.325PbTiO₃ (PMN–32.5PT) ceramics were textured (grain-oriented) in the 〈001〉-crystallographic direction by the templated grain growth process. The textured PMN–32.5PT ceramics were produced by orienting {001}-SrTiO₃ (ST) platelets (∼10 μm in diameter and ∼2-μm thickness) in a submicron PMN–32.5PT matrix. The templated growth of 〈001〉-oriented PMN–32.5PT grains on the ST platelets resulted in textured ceramics with ∼70% Lotgering factor and >98% theoretical density. Unlike most lead-based ceramics, excess PbO was not needed for sintering or grain growth. Based on unipolar stain-field measurements at 0.2 Hz, the textured samples displayed >0.3% strain at 50 kV/cm. Low-field d ₃₃-coefficients of >1600 pC/N (<5 kV/cm) were measured directly from unipolar measurements. The low drive field d ₃₃-piezoelectric coefficient of the highly textured samples is two times greater than polycrystalline PMN–32.5PT.     

Crystal Structure and Dielectric Properties of LaYbO3

The crystal structure and dielectric properties of LaYbO₃ ceramics prepared by the mixed-oxide route have been investigated. Rietveld refinements performed on X-ray and neutron diffraction data show the room-temperature structure to be best described by the orthorhombic Pnma space group [ a =6.02628(9) Å, b =8.39857(11) Å, and c =5.82717(7) Å; Z =4, and theoretical density, D _x =8.1 g/cm³] in agreement with electron diffraction experiments. LaYbO₃ ceramics fired at 1600°C for 4 h attain ∼97% of D _x and their microstructures consist of randomly distributed equiaxed grains with an average size of ∼8 μm. Conventional transmission electron microscopy shows densification to occur in the absence of a liquid phase and reveals domain-free grains. The relative permittivity, ɛ_r, of LaYbO₃ ceramics at radio frequencies is ∼26 in the range ∼10–300 K; however, a small dielectric anomaly is detected at ∼15 K. At room temperature and microwave frequencies, LaYbO₃ ceramics exhibit ɛ_r∼26, Q × f _r∼20 613 GHz (at 7 GHz), and τ_f∼−22 ppm/K. Q × f _r show complex subambient behavior, decreasing from a plateau value of ∼20 000 GHz between ∼300 and 200 K to a second plateau value of ∼6000 GHz at ∼90 K before decreasing to <1000 GHz at ∼10 K. The large decrease in Q × f _r at low temperature may be related to the onset of antiferromagnetism at ∼2.7 K. ¹     

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).     

Kinetics and Microstructural Evolution of Heterogeneous Transformation of θ-Alumina to α-Alumina

Ultrafine (<0.1 μm) high-purity θ-Al₂O₃ powder containing 3–17.5 mol%α-Al₂O₃ seeds was used to investigate the kinetics and microstructural evolution of the θ-Al₂O₃ to α-Al₂O₃ transformation. The transformation and densification of the powder that occurred in sequence from 960° to 1100°C were characterized by quantitative X-ray diffractometry, dilatometry, mercury intrusion porosimetry, and transmission and scanning electron microscopy. The relative bulk density and the fraction of α phase increased with annealing temperature and holding time, but the crystal size of the α phase remained ∼50 nm in all cases at the transformation stage (≤1020°C). The activation energy and the time exponent of the θ to α transformation were 650 ± 50 kJ/mol and 1.5, respectively. The results implied the transformation occurred at the interface via structure rearrangement caused by the diffusion of oxygen ions in the Al₂O₃ lattice. A completely transformed α matrix of uniform porosity was the result of appropriate annealing processes (1020°C for 10 h) that considerably enhanced densification and reduced grain growth in the sintering stage. The Al₂O₃ sample sintered at 1490°C for 1 h had a density of 99.4% of the theoretical density and average grain size of 1.67 μm.     

Microwave Dielectric Properties of Hexagonal 12R-Ba3LaNb3O12 Ceramics

The dielectric properties of dense ceramics of the n =0 member of a newly identified homologous series Ba_{3+ n} LaNb₃Ti _n O_{12+3 n} , where n =0, 1, and 2, are reported. Single-phase powders can be obtained from the mixed-oxide route at 1350°C and dense ceramics (>97% of the theoretical X-ray density) with uniform microstructures (3–5 μm) can be obtained by sintering in air at 1500°C. The ceramics are excellent dc insulators with a band gap >2.6 eV that resonate at microwave frequencies with a relative permittivity, ɛ_r∼44, a quality factor, Q × f _r, of ∼9000 at f _r∼5.5 GHz and a temperature coefficient of resonant frequency, TC_f,∼−100 ppm/K.     

Isothermal Grain Growth of Pressure-Sintered PLZT Ceramics

PLZT specimens 99.99% of theoretical density containing 7 at.% La with a 65/35 Zr/Ti atom ratio were fabricated with average grain diameters of ∼1 μm by pressure sintering. The dense, single-phase specimens were isothermally heat-treated in a low PbO activity atmosphere. For grain diameters from 1 to 5 μm, grain growth obeyed the D³-D₀³=k't relation, with an apparent activation energy of 86 kcal/mol. The mechanism controlling the grain-growth rate is consistent with a solid-solution impurity drag caused by an La ion concentration gradient near the grain boundaries.     

Hot Forging Characteristics of Fine-Grained ZrO2 and Al2O3/ZrO2 Ceramics

Preliminary results indicate that large strains (∼80%) and strain rates (0.001 s⁻¹) can be obtained without tearing (or cracking) in fine-grain ZrO₂ (0.3 μm) and Al₂O₃/ZrO₂ (1 μm) ceramics. Alumina develops crystallographic and morphological texture as previously reported by Heuer et al.¹     

Grain Growth in Very Porous Al2O3 Compacts

Extensive grain growth was observed by scanning electron microscopy in very porous Al₂O₃ compacts, even at densities <40% of theoretical. After ∼7% shrinkage at 1700°C, the grain size increased from ∼0.3 to 0.51 μm in a compact having a relative green density of 0.31. During grain growth in highly porous compacts, the grains appear initially to be chainlike, then to be oblong, and finally to be equiaxed. The proposed mechanism of initial grain growth involves the filling of necks between adjacent grains followed by the movement of the grain boundary through the smaller grain. Although grain growth in very porous compacts is quite different from coalescence and ordinary grain growth, the kinetics are similar.