Grinding-Induced Texture in Ferroelastic Tetragonal Zirconia

Ceria- and yttria-doped tetragonal polycrystalline zirconia ceramics were ground at temperatures as high as 1100°C. X-ray diffraction revealed that the intensity ratio I ₍₀₀₂₎/ I ₍₂₀₀₎ increased (to as high as ∼4.5) compared with that from the as-sintered surfaces (∼0.55). The enhancement in I ₍₀₀₂₎/ I ₍₂₀₀₎ at temperatures well above the m → t transition temperature shows that it is not related to transformation, reversible or otherwise, but can be explained by ferroelastic domain switching.     

Effect of Trace A12O3 on Transformation of Quartz to Cristobalite

The effect of additions of 0.22, 0.44, 0.88, and 1.76% A1₂O₃ (Si⁴⁺/A1³⁺ ratio of 200:1, 100:1, 50:1, and 25:1) on the transformation of Brazilian quartz to cristobalite was studied at 1500°, 1530°, and 1570°C. The smaller percentages of A1₂O₃ (0.22 and 0.44%) catalyzed the transformation of quartz and the formation of cristobalite considerably. The rates of transformation of quartz with 0.88 and 1.76% A1₂O₃ were slower than with 0.22 or 0.44%, indicating a critical A1³⁺/Si⁴⁺ ratio where the catalytic effect was found to be maximum. This appeared to occur at about 0.5% A1₂O₃. The transformation rate of quartz indicated that the reaction was first order. Cristobalite, however, showed two different rates; the initial rapid growth was followed by a slower rate. The point of changeover was found to be at about 30 ± 5% cristobalite. The critical nature of the A1³⁻/Si⁴⁺ ratio at about 0.01 (or A1₂O₃/SiO₂± 0.005) may have some bearing on the properties of silica refractories with more or less than 0.5% A1₂O₃.     

Low-Temperature Fabrication of Cordierite Ceramics from Kaolinite and Magnesium Hydroxide Mixtures with Boron Oxide Additions

Thermal reactions of mixtures of ultrafine particles of magnesium hydroxide (Mg(OH)₂) and kaolinite in a composition of MgO:Al₂O₃:2SiO₂ were investigated to obtain dense cordierite ceramics at temperatures <1000°C. While heating the mixture of kaolinite and Mg(OH)₂ with the equivalent of 2 mass% of boron oxide (B₂O₃) (in the form of magnesium borate, 2MgOB₂O₃), an amorphous phase formed at a temperature of ∼850°C after thermal decomposition. Firing the mixture at a temperature of 900°C yielded dense ceramics with an apparent porosity of almost zero. The addition of B₂O₃ promoted the densification at 850°-900°C and accelerated the crystallization of alpha-cordierite. The specimen with 3 mass% of B₂O₃ that was fired at a temperature of 950°C showed a linear thermal expansion coefficient of ∼3 × 10⁻⁶ K⁻¹, a bending strength of >200 MPa, and a relative dielectric constant of 5.5 at 1 MHz. These cordierite ceramics may be used as substrate materials for semiconductor interconnection applications.     

Static Fatigue Limit for Sintered Silicon Carbide at Elevated Temperatures

The modified static loading technique for estimating static fatigue limits was used to study the effects of oxidation and temperature on the static fatigue limit, K ₁₀ for crack growth in sintered silicon carbide. For as-machined, unoxidized sintered silicon carbide with a static load time of 4 h, K ₁₀× 2.25 MPa * m^1/2 at 1200° and ∼1.75 at 1400°C. On oxidation for 10 h at 1200°C, K ₁₀ drops to ∼1.75 MPam^1/2 at 1200° and ∼1.25 at 1400°C when tested in a nonoxidizing ambient. Similar results were obtained at 1200°C for tests performed in air. A tendency for strengthening below the static fatigue limit appears to result from plastic relaxation of stress in the crack-tip region by viscous deformation involving an oxide grain-boundary phase for oxidized material and, possibly, diffusive creep deformation in the case of unoxidized material.     

Mullite Crystallization from SiO2-Al2O3 Melts

SiO₂-Al₂O₃ melts containing 42 and 60 wt% A1₂O₃ were homogenized at 2090°C (∼10°) and crystallized by various heat treatment schedules in sealed molybdenum crucibles. Mullite containing ∼78 wt% A1₂O₃ precipitated from the 60 wt% A1₂O₃ melts at ∼1325°± 20°C, which is the boundary of a previously calculated liquid miscibility gap. When the homogenized melts were heat-treated within this gap, the A1₂O₃ in the mullite decreased with a corresponding increase in the Al₂O₃ content of the glass. A similar decrease of Al₂O₃ in mullite was observed when crystallized melts were reheated at 1725°± 10°C; the lowest A1₂O₃ content (∼73.5 wt%) was in melts that were reheated for 110 h. All melts indicated that the composition of the precipitating mullite was sensitive to the heat treatment of the melts.     

Low-Temperature Electrochemical Synthesis of ZrO2 Films on Zirconium Substrates: Deposition of Thick Amorphous Films and in situ Crystallization on Zirconium Anode

The feasibility of synthesizing crystalline ZrO₂ films at low temperatures was evaluated using an electrochemical method. Anodization of zirconium-metal substrates in tetraethyl ammonium hydroxide (TEAOH) solutions under constant applied voltage conditions at ∼25° and ∼100°C was investigated. The chemistry and microstructure of the anodic oxide films deposited on the zirconium-metal substrates under the above conditions were characterized using X-ray diffractometry and scanning electron microscopy. The results indicated that, with sufficiently high applied voltages (in the range of 300 V) at pH ∼9.5, the initial dissolution of the zirconium anode resulted in the local saturation of the electrolyte solution with Zr⁴⁺, forming Zr(OH)⁻₅, which deposited electrophoretically on the anode as a thick, gelatinous film at 25°C. Similar treatments at 100°C resulted in an in situ crystallization of Zr(OH)₄ gel to monoclinic ZrO₂.     

Sc2O3 Nanopowders via Hydroxyl Precipitation: Effects of Sulfate Ions on Powder Properties

Hydroxyl-type Sc₂O₃ precursors have been synthesized via precipitation at 80°C with hexamethylenetetramine as the precipitant. The effects of starting salts (scandium nitrate and sulfate) on powder properties are investigated. Characterizations of the powders are achieved by elemental analysis, X-ray diffractometry (XRD), differential thermal analysis/thermogravimetry (DTA/TG), high-resolution scanning electron microscopy (HRSEM), and Brunauer-Emmett-Teller (BET) analysis. Hard-aggregated precursors (γ-ScOOH·0.6H₂O) are formed with scandium nitrate, which convert to Sc₂O₃ at temperatures ≥400°C, yielding nanocrystalline oxides of low surface area. The use of sulfate leads to a loosely agglomerated basic sulfate powder having an approximate composition of Sc(OH)_2.6(SO₄)_0.2·H₂O. The powder transforms to Sc₂O₃ via dehydroxylization and desulfurization at temperatures up to 1000°C. Well-dispersed Sc₂O₃ nanopowders (∼64.3 nm) of high purity have been obtained by calcining the basic sulfate at 1000°C for 4 h. The effects of SO₄²⁻ on powder properties are discussed.     

Structure Evolution in Hydrothermally Processed (<100°C) BaTiO3 Films

Thin films of cubic BaTiO₃ were processed hydrothermally at 40°–80°C by reacting thin layers of titanium organo metallic liquid precursors in aqueous solutions of either Ba(OH)₂ or a mixture of NaOH and BaCl₂. All films (thickness ∼1 μm) were polycrystalline with grain sizes ranging from nano- to micrometer dimensions. BaTiO₃ formation was facilitated by increasing [OH^-], [Ba²⁺], and the temperature. The film structure was related to the nucleation and growth behavior of the BaTiO₃ particles. Films processed at relatively low [OH^-], [Ba²⁺], and temperatures were coarse grain and opaque, but increasing [OH^-], [Ba²⁺], and temperature caused the grain size to decrease, resulting in transparent films.     

Pressureless Densification of Zirconium Diboride with Boron Carbide Additions

Zirconium diboride (ZrB₂) was densified (>98% relative density) at temperatures as low as 1850°C by pressureless sintering. Sintering was activated by removing oxide impurities (B₂O₃ and ZrO₂) from particle surfaces. Boron oxide had a high vapor pressure and was removed during heating under a mild vacuum (∼150 mTorr). Zirconia was more persistent and had to be removed by chemical reaction. Both WC and B₄C were evaluated as additives to facilitate the removal of ZrO₂. Reactions were proposed based on thermodynamic analysis and then confirmed by X-ray diffraction analysis of reacted powder mixtures. After the preliminary powder studies, densification was studied using either as-received ZrB₂ (surface area ∼1 m²/g) or attrition-milled ZrB₂ (surface area ∼7.5 m²/g) with WC and/or B₄C as a sintering aid. ZrB₂ containing only WC could be sintered to ∼95% relative density in 4 h at 2050°C under vacuum. In contrast, the addition of B₄C allowed for sintering to >98% relative density in 1 h at 1850°C under vacuum.     

Glass-Free KxBa1−xGa2−xGe2+xO8 Ceramics for Low-Temperature Cofired Ceramics Technology: Synthesis, Phase Transitions, Sintering, and Microwave Dielectric Properties

K _x Ba_{1− x} Ga_{2− x} Ge_{2+ x} O₈ (0.6≤ x ≤1) polycrystalline ceramics are potential materials for glass-free low-temperature cofired ceramics (LTCC) substrates. We have made a comprehensive study of the kinetics of the monoclinic-to-monoclinic P 2₁/ a ⇔ C 2/ m phase transition. The low-temperature-stable P 2₁/ a phase with a high Q × f value was synthesized using a subsolidus method and was well sintered at the LTCC temperature with a H₃BO₃ additive. A good combination of low sintering temperature (910°–920°C), high Q × f values (96 700–104 500 GHz), low permittivities (5.6–6.0), and a small temperature coefficient of resonant frequency (∼−20 ppm/°C) was obtained for ceramics with x =0.67 and 0.9 and with 0.1 wt% of H₃BO₃.     

High-Temperature Defect Structure of Acceptor-Doped Strontium Titanate

The defect structure of acceptor-doped (Fe, Al, Cr) polycrystalline strontium titanate was investigated by measuring the equilibrium electrical conductivity as a function of oxygen activity (10⁻²¹≤a_o2≤1) and temperature (85°C≤ T ≤ 1050°C). The electrical conductivity was n type with ∼−1/4 dependence on a ₀₂ for a₀₂<10⁻¹⁰. For a₀₂>10⁻⁸, the observed data for Fe- or Al-doped samples were proportional to ∼1/4.5 power of a₀₂. In this region for Cr-doped samples, the value of m in.σp αa₀₂^+1/m varies from ∼4.80 to ∼9.00 as the concentration of Cr is increased from 460 to 10 000 ppm. The onset of p -type conductivity depends on the amount. of acceptor impurity added to the sample. The absolute values of the conductivity in the acceptor-doped samples were lower in the n-type region than those for undoped SrTiO₃. The conductivity minima shift toward lower oxygen activity with increasing acceptor concentration. For the entire oxygen activity rangae used in this study, the defect structure of SrTiO₃ is dominated by the added acceptor impurities.     

Solid Solubility of Holmium, Yttrium, and Dysprosium in BaTiO3

The solid solubility of R ions (R = Ho³⁺, Dy³⁺, and Y³⁺) in the BaTiO₃ perovskite structure was studied by quantitative electron-probe microanalysis (EPMA) using wavelength-dispersive spectroscopy (WDS), scanning electron microscopy (SEM), and X-ray diffractometry (XRD). Highly doped BaTiO₃ samples were prepared using mixed-oxide technology including equilibration at 1400° and 1500°C in ambient air. The solubility was found to depend mainly on the starting composition. In the TiO₂-rich samples a relatively low concentration of R incorporated preferentially at the Ba²⁺ lattice sites (solubility limit ∼Ba_0.986R_0.014Ti_0.9965(V^"_Ti^")_0.0035O₃at 1400°C). In BaO-rich samples a high concentration of R entered the BaTiO₃ structure at the Ti⁴⁺ lattice sites (solubility limit ∼BaTi_0.85R_0.15O_2.925(V_O^••)_0.075at 1500°C). Ho³⁺, Dy³⁺, and Y³⁺incorporated preferentially at the Ti⁴⁺ lattice sites stabilize the hexagonal polymorph of BaTiO₃. The phase equilibria of the Ho³⁺–BaTiO₃ solid solutions were presented in a BaO–Ho₂O₃–TiO₂phase diagram.     

Low-Temperature Synthesis of Praseodymium-Doped Ceria Nanopowders

Praseodymium-doped ceria (CeO₂) nanopowders have been synthesized via a simple but effective carbonate-coprecipitation method, using nitrates as the starting salts and ammonium carbonate as the precipitant. The precursors produced in this work are ammonium rare-earth double carbonates, with a general formula of (NH₄)_0.16Ce_{1− x} Pr _x (CO₃)_1.58·H₂O (0 < x ≤ 0.20), which directly yield oxide solid solutions on thermal decomposition at a very low temperature of ∼400°C. Praseodymium doping causes a gradual contraction of the CeO₂ lattice, because of the oxidation of Pr³⁺ to smaller Pr⁴⁺, and suppresses crystallite coarsening of the oxides during calcination. Dense ceramics have been fabricated from the thus-prepared nanopowders via pressureless sintering for 4 h at a low temperature of 1200°C.     

Conversion of a Precursor Derived from Cage-Type and Cyclic Molecular Building Blocks into Al-Si-N-C Ceramic Composites

A precursor derived from (HAlN ⁱ Pr) _m and [MeSi(H)NH] _n , which mainly consisted of cage-type compounds and cyclic compounds, respectively, was converted into Al-Si-N-C ceramic composites via pyrolysis. A dehydrocoupling reaction between AlH groups and NH groups occurred at low temperatures (≤∼250°C), which mainly accounts for its high ceramic yield (69% up to 900°C). At high temperatures (≥∼250°C), the organic groups were decomposed. The product pyrolyzed at 1350° and 1500°C under Ar contained a 2H wurtzite-type compound and a β-Si₃N₄-type compound, while β-SiC was clearly detected in addition to these compounds in the product pyrolyzed at 1600°C under Ar. On the other hand, the product pyrolyzed at 800°C under NH₃ and subsequently at 1350°C under N₂ consisted of AlN and β-Sialon.     

Sol–Gel Route to Nanocrystalline Lithium Metasilicate Particles

Crystalline lithium metasilicate (Li₂SiO₃) nanoparticles have been synthesized using a sol–gel process with tetraethylorthosilicate and lithium ethoxide as precursors. The particle size examined by using transmission electron microscopy and BET-specific surface area techniques is in the range 5–50 nm, depending on the temperature at which the material is calcined. The crystalline Li₂SiO₃ forms at ambient temperature (∼40°C), and it remains in this phase after calcination at temperatures up to 850°C. The BET-specific surface area is ∼110 m²/g for material calcined at temperatures below 500°C, decreasing to ∼29 and ∼0.7 m²/g following calcination at 700° and 850°C, respectively. Solid-state ²⁹Si NMR spectroscopy shows the presence of only Q ² structural units in the material. The lithium metasilicate is further characterized using differential scanning calorimetry and thermogravimetric analysis, and Fourier transform infrared spectroscopy.     

The Study on Carbon Nanofiber (CNF)-Dispersed B4C Composites

Carbon nanofiber (CNF)-dispersed B₄C composites have been synthesized and consolidated directly from mixtures of elemental raw powders by pulsed electric current pressure sintering (1800°C/10 min/30 MPa). A 15 vol% CNF/B₄C composite with ∼99% of dense homogeneous microstructures (∼0.40 μm grains) revealed excellent mechanical properties at room temperature and high temperatures: a high bending strength (σ_b) of ∼710 MPa, a Vickers hardness ( H _v) of ∼36 GPa, a fracture toughness ( K _{I C} ) of ∼7.9 MPa m^1/2, and high-temperature σ_b of 590 MPa at 1600°C in N₂. Interfaces between the CNF and the B₄C matrix were investigated using high-resolution transmission electron microscopy, EDS, and electron energy-loss spectroscopy.     

Influence of Non-Stoichiometry on the Structure and Properties of Ba(Zn1/3Nb2/3)O3 Microwave Dielectrics: I. Substitution of Ba3W2O9

A narrow region of Zn-vacancy-containing cubic perovskites was formed in the (1− x )Ba₃(ZnNb₂)O₉−( x )Ba₃W₂O₉ system up to 2 mol% substitution ( x =0.02). The introduction of cation vacancies enhanced the stability of the 1:2 B-site ordered form of the structure, Ba(Zn_{1− x} □ _x )_1/3(Nb_{1− x} W _x )_2/3O₃, which underwent an order–disorder transition at 1410°C, ∼35° higher than pure Ba(Zn_1/3Nb_2/3)O₃. The Zn vacancies also accelerated the kinetics of the ordering reaction, and samples with x =0.006 comprised large ordered domains with a high lattice distortion ( c/a =1.226) after a 12 h anneal at 1300°C. The tungstate-containing solid solutions can be sintered to a high density at 1390°C, and the resultant ordered ceramics exhibit some of the highest microwave dielectric Q factors ( Q × f =1 18 000 at 8 GHz) reported for a niobate-based perovskite.     

Solid-State Synthesis of LaCoO3 Perovskite Nanocrystals

A solid-state reaction process has been developed to synthesize perovskite-type LaCoO₃ nanocrystals with grain diameters of 15–40 nm. In the first step of the preparation, ∼5 nm composite hydroxide nanoparticles are synthesized by the solid-state reaction of La(NO₃)₃· n H₂O and Co(NO₃)₂·6H₂O with KOH at ambient temperature. A perovskite-type rhombohedral LaCoO₃ phase appears at 550°C, after the hydroxide has been calcined at various temperatures. The phase transformation process is complete at ∼800°C, yielding a single-phase binary oxide. The results indicate that the new process is convenient, inexpensive, and effective for obtaining LaCoO₃ nanocrystals with high yield.     

Synthesis and Electrical Properties of Stabilized Manganese Dioxide (α-MnO2) Thin-Film Electrodes

Manganese dioxide (α-MnO₂) thin films have been explored as a cathode material for reliable glass capacitors. Conducting α-MnO₂ thin films were deposited on a borosilicate glass substrate by a chemical solution deposition technique. High carbon activities originated from manganese acetate precursor, (Mn(C₂H₃O₂)₂·4H₂O) and acetic acid solvent (C₂H₄O₂), which substantially reduced MnO₂ phase stability, and resulted in Mn₂O₃ formation at pyrolysis temperature in air. The α-MnO₂ structure was stabilized by Ba²⁺ insertion into a (2 × 2) oxygen tunnel frame to form a hollandite structure. With 15–20 mol% Ba addition, a conducting α-MnO₂ thin film was obtained after annealing at 600–650°C, exhibiting low electrical resistivity (∼1 Ω·cm), which enables application as a cathode material for capacitors. The hollandite α-MnO₂ phase was stable at 850°C, and thermally reduced to the insulating bixbyte (Mn₂O₃) phase after annealing at 900°C. The phase transition temperature of Ba containing α-MnO₂ was substantially higher than the reported transition temperature for pure MnO₂ (∼500°C).     

Structure and Properties of Hot-Pressed Clay Mineral/Acrifavine Complex Disk

Saponite Na_{( a-b )}⁺[(Mg_{6- b} Al _b ) (Si_{8- a} Al _a )O₂₀(OH)₄]⁻, a kind of clay mineral, was intercalated with acriflavine to form their complex (SAC). The resulting SAC powder was hot-pressed into small disks (II mm in diameter, ∼10 mm long) under ∼20 MPa and temperatures up to 1300°C. Saponite powder (Sap) and saponite/carbon black mixture (SKM) reference samples were also hot-pressed according to the same procedure. H₂, CO, and CO₂ gases evolved more abundantly, and at lower temperature, from the SAC than from the two reference samples. Fine carbon particles dispersed homogeneously throughout the SAC after the sample was heated. Enstatite and forsterite crystals formed in the SAC at higher temperatures and grew less favorably than in the reference samples. The electrical resistivity of the SAC disk sharply decreased with increased heat treatment temperature. At 1300°C, the SAC disk with only 3 wt% of carbon content exhibited a resistivity of 2 × 10⁻¹Ω/cm, about 1/20 that of the SKM with a 5.5 wt% carbon after heating to 1300°C. The compressive strength of SAC after heating to 1300°C was near that of Sap and twice that of SKM.