Hot Isostatic Pressing and Characterization of Sol-Gel-Derived Chromium(III) Oxide

Chromium (III) oxide (Cr₂O₃) crystallizes at low temperatures from an amorphous material prepared by adding hydrazine monohydrate to an aqueous solution of Cr(NO₃)₃-9H₂O. Individual particles of Cr₂O₃ tend toward a hexagonal morphology above 800°C. Well-densified Cr₂O₃ pellets (98.8% of theoretical density) have been fabricated by hot isostatic pressing for 2 h at 1100°C and 196 MPa. Their fracture toughness is 4.4 MPa.m^1/2. The sample annealed in air for 12 h at 1300°C exhibits a high electrical conductivity of 3.6 Ω^-1.m^-1at 700°C.     

Characterization and Sintering of Reactive Cerium(IV) Oxide Powders Prepared by the Hydrazine Method

Nanocrystalline cerium(IV) oxide (CeO₂) powders have been prepared by adding hydrazine monohydrate to an aqueous solution of hydrous cerium nitrate (Ce(NO₃)₃·6H₂O), followed by washing and drying. The lattice parameter of the as-prepared powder is a = 0.5415 nm. The powder characteristics and sinterability of reactive CeO₂ have been studied. The surface areas of powders that have been heated at low temperatures are high, and these surface areas do not decrease to 10 m²/g until the temperature is >1200°C. Crystallite size and particle size are strongly dependent on the heating temperature. Optimum sintered densities are obtained by calcining in the temperature range of 700°–800°C. Ceramics with almost-full density can be fabricated at a temperature as low as 1150°C.     

Formation of TiO2(B) Nanocrystallites in Sol-Gel-Derived SiO2-TiO2 Film

TiO₂(B), one of the polymorphs of TiO₂, has been formed by annealing a sol-gel-derived SiO₂-TiO₂ amorphous film on a silicon wafer at 900°C in air. Transmission electron microscopy (TEM) revealed that nanocrystallites with a size of 5-10 nm were dispersed in the amorphous SiO₂ matrix in the film. The X-ray diffraction pattern and lattice fringe spacing in high-resolution TEM images corresponded to those of TiO₂(B). These TiO₂(B) nanocrystallites are probably stable with the presence of surrounding SiO₂ in the film at 900°C, because previous works reported that this phase should be converted to anatase at temperatures higher than 550-700°C.     

Preparation of Beryllium Fluoride Glasses Containing Xenon(VI) and/or Thallium (I)

The dissolution of CsXeF₇ in molten alkali fluoroberyllate glasses between 250° and 500°C is described. The apparent small solubility of Xe(VI) in these ionic glass-forming melts (about 1 to 2 wt% or less) may be related to retention of a molecular configuration by XeF₆ that is caused by its strong fluoride ion donor properties and/or to the nonmelting behavior of numerous MF-XeF₆ salts when they are heated. Rather low-softening (about 100°C) glasses were also prepared in the TIHF₂-NaHF₂-BeF₂ system, whereas TlHF₂ has a relatively low melting point (about 85°C). Contrary to expectations, Xe(VI) appears to oxidize TI⁺ in these molten glass environments at 250°C.     

Thermal Expansion Behavior of Titanium-Doped La(Sr)CrO3 Solid Oxide Fuel Cell Interconnects

La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ perovskite has been designed as an interconnect material in high-temperature solid oxide fuel cells (SOFCs) because of its thermal expansion compatibility in both oxidizing and reducing atmospheres. La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ shows a single phase with a hexagonal unit cell of a = 5.459(1) Å, c = 13.507(2) Å, Z = 6 and a space group of R -3 C . Average linear thermal expansion coefficients of this material in the temperature range from 50° to 1000°C were 10.4 × 10⁻⁶/°C in air, 10.5 × 10⁻⁶/°C under a He–H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁵ atm at 1000°C), and 10.9 × 10⁻⁶/°C in a H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁹ atm at 1000°C). La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ perovskite with a linear thermal expansion in both oxidizing and reducing environments is a promising candidate material for an SOFC interconnect. However, there still remains an air-sintering problem to be solved in using this material as an SOFC interconnect.     

Decomposition of Al2TiO5 and Al2(1-x)MgxTi(1+x)O5 Ceramics

The decomposition of Al_{2(1- x )}Mg _x Ti_{(1+ x )}O₅ ceramics in air has been studied between 900° and 1175°C for 0 lessthan equal to x lessthan equal to 0.6. The decomposition temperature versus composition x predicted using a thermodynamic model based on the regular solution approach is in satisfactory agreement with the experimental results. The decomposition kinetics has been studied at 1100°C for x = 0, 0.1, and 0.2 and follows a nucleation and growth mechanism. Random nucleation of the reaction products is hindered by the high elastic stresses that result from the molar volume change related to decomposition because of the small chemical driving force available. Decomposition occurs only at a limited number of sites, probably associated with the presence of impurities and/or glassy phase. The decomposition products grow as nodules formed by an Al₂O₃ (+ MgAl₂O₄ for x > 0) core and a TiO₂ shell. The growth is parabolic for x = 0 and linear for x = 0.1 and 0.2. The rate-controlling step in the decomposition mechanism of pure Al₂TiO₅ ( x = 0) is the transport of Al³⁺ ions through the TiO₂-rutile phase.     

Fabrication and Microstructure Characterization of Ti3SiC2 Synthesized from Ti/Si/2TiC Powders Using the Pulse Discharge Sintering (PDS) Technique

Ti/Si/2TiC powders were prepared using a mixture method (M) and a mechanical alloying (MA) method to fabricate Ti₃SiC₂ at 1200°–1400°C using a pulse discharge sintering (PDS) technique. The results showed that the Ti₃SiC₂ samples with <5 wt% TiC could be rapidly synthesized from the M powders; however, the TiC content was always >18 wt% in the MA samples. Further sintering of the M powder showed that the purity of Ti₃SiC₂ could be improved to >97 wt% at 1250°–1300°C, which is ∼200°–300°C lower than that of sintered Ti/Si/C and Ti/SiC/C powders using the hot isostatic pressing (HIPing) technique. The microstructure of Ti₃SiC₂ also could be controlled using three types of powders, i.e., fine, coarse, or duplex-grained, within the sintering temperature range. In comparison with Ti/Si/C and Ti/SiC/C mixture powders, it has been suggested that high-purity Ti₃SiC₂ could be rapidly synthesized by sintering the Ti/Si/TiC powder mixture at relatively lower temperature using the PDS technique.     

Enhanced Thermal Stability of Zinc-Based Titanate (Dielectric) Ceramics Prepared by the Modified Sol–gel Route

A multi-component substitution of Co and Ni was incorporated into ZnTiO₃ to form pure hexagonal Zn_{1− x} (Co_1/2Ni_1/2) _x TiO₃ ( x =0,0.8,0.9,1.0) dielectric ceramic powders by a modified sol–gel route, following heat treatments at 600°C for 3 h and at 800°C for 6 h. Differential scanning calorimetry measurements revealed that the order of increasing thermal stability of solid solution compound Zn_{1− x} (Co_1/2Ni_1/2) _x TiO₃ was ZnTiO₃ (945°C), Zn_0.1Ni_0.9TiO₃ (1346°C), Zn_0.1(Co_1/2Ni_1/2)_0.9TiO₃ (1390°C), and Zn_0.1Co_0.9TiO₃ (>1400°C). Both the dielectric constant and loss tangent reached a maximum at x =0.8 and then decreased with solubility, x , and measurement frequency.     

Thermal Expansion of the Hexagonal (6H) Polytype of Silicon Carbide

The thermal expansion of the hexagonal (6H) polytype of α-SiC was measured from 20° to 1000°C by the X-ray diffraction technique. The principal axial coefficients of thermal expansion were determined and can be expressed for that temperature range by second-order polynomials: α¹¹= 3.27 × 10^–6+ 3.25 × 10^–9T – 1.36 × 10^–12 T ² (1/°C), and ş₃₃= 3.18 × 10^–6+ 2.48 × 10^–9 T – 8.51 × 10^–13 T ² (1/°C). The σ₁₁ is larger than α₃₃ over the entire temperature range while the thermal expansion anisotropy, the δş value, increases continuously with increasing temperature from about 0.1 × 10^–6/°C at room temperature to 0.4 × 10^–6/°C at 1000°C. The thermal expansion and thermal expansion anisotropy are compared with previously published results for the (6H) polytype and are discussed relative to the structure.     

Crystal Growth of Rare-Earth Orthoarsenates

Single crystals of rare-earth arsenate compounds (of the type RAsO₄) were synthesized and grown by the flux technique, using lead pyroarsenate as the solvent. Rare-earth oxides in concentrations of 2 mole % were dissolved in molten lead pyroarsenate (Pb₂As₂O₇) at 1300°C. After 6 hr at temperature the solution was cooled at a rate of 4°C/hr or less for recrystallization of RAsO₄. Nucleation and growth occur from 1300° to 925°C. Details for preparation of Pb₂As₂O₇ are given. Experiments on the growth of mixed rare-earth orthoarsenates and mixed phosphate-arsenate compositions are discussed together with the anomalous behavior of cerium and neodymium arsenates.     

Electrochemical Chromium(III) Oxide Coatings on Non-oxide Ceramic Substrates

SiC and TiB₂ were electrochemically coated with Cr₂O₃ from a 0.1 M aqueous solution of chromium nitrate hydrate with ethanol additives. On both substrate materials poly-crystalline Cr₂O₃ was formed at current densities from 5 to 50 mA/cm² and deposition durations of 5 to 30 min. The coating weight increased with current density and with deposition time. The as-deposited coatings contained microcracks due to drying shrinkage. Microstructural observations indicate that sintering of the Cr₂O₃ coatings on TiB₂ at 1100°C for 1 n in a reducing atmosphere in a closed graphite crucible causes the densification of the coating via a liquid phase, which forms by oxidation of TiB₂. Under similar conditions, the Cr₂O₃ coatings on SiC may be sintered via an evaporation–condensation mechanism.     

Oxidation Kinetics and Composite Scale Formation in the System Mo(Al,Si)2

The oxidation kinetics of hot-pressed Mo(Al_0.01Si_0.99)₂ and Mo(Al_0.1Si_0.9)₂ were measured at 480°C, and between 1200° and 1600°C. The qualitative oxidation of arc-melted Mo(Al_0.1Si_0.9)₂, Mo(Al_0.3Si_0.7)₂, Mo(Al_0.5Si_0.5)₂, and Mo₃Al₈ was examined after 600°C for 1000 h in air. At all temperatures, the compositional difference between the materials yielded very different oxidation rates and scale microstructures. At 1400° and 1500°C, microstructural evolution of the oxide scales resulted in improved oxidation resistance at long times (>400 h). At these temperatures, a significant reduction in the long-time oxidation kinetics was correlated with the in situ formation of an inner mullite scale. At 480° and 600°C, oxidation resistance improved significantly with increasing aluminum concentration. Contrary to the behavior of MoSi₂, samples of Mo(Al_0.01Si_0.99)₂ did not demonstrate catastrophic oxidation, and samples of Mo(Al_0.1Si_0.9)₂ were very oxidation resistant.     

Low-Temperature Fabrication of Transparent Yttrium Aluminum Garnet (YAG) Ceramics without Additives

A carbonate precursor of yttrium aluminum garnet (YAG) with an approximate composition of NH₄AlY_0.6(CO₃)_1.9(OH)₂·0.9H₂O was synthesized via a coprecipitation method from a mixed solution of ammonium aluminum sulfate and yttrium nitrate, using ammonium hydrogen carbonate as the precipitant. The precursor precipitate was characterized using chemical analysis, differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. The sinterability of the YAG powders was evaluated by sintering at a constant rate of heating in air and vacuum sintering. The results showed that the precursor completely transforms to YAG at ∼1000°C via the formation of a yttrium aluminate perovskite (YAP) phase. YAG powders obtained by calcining the precursor at temperatures of ≤1200°C were highly sinterable and could be densified to transparency under vacuum at 1700°C in 1 h without additives.     

Activity of Nickel(II) Oxide in Silicate Melts

Activity–composition relations of NiO have been determined at 1435°C in melts of the system CaO–NiO–SiO₂ and at 1400°C in melts of the systems CaO–MgO–NiO–SiO₂, CaO–MgO–NiO–Al₂O₃–SiO₂, and CaO–MgO–NiO–K₂O–SiO₂. In the CaO–NiO–SiO₂ and CaO–MgO–NiO–SiO₂ systems the activity coefficient of NiO (γ_NiO) decreases as the polymerization of the melt increases (basicity decrease). γ_NiO stays contant up to several weight percent NiO for melts with similar basicity, an indication of Henry's law behavior. Minima in the NiO activity coefficient are observed in melts of the CaO–MgO–NiO–Al₂O₃–SiO₂ and CaO–MgO–NiO–K₂O–SiO₂ systems at NBO/Si ratios between 1.0 and 1.5; i.e., γ_NiO decreases with decreasing basicity for NBO/Si ratios >1.5 and increases with decreasing basicity for melts with NBO/Si ratios <1.0 (NBO/Si; nonbridging oxygens per silicon). The addition of Al₂O₃ and K₂O to the CaO–MgO–NiO–SiO₂ system results in an increase in the activity coefficient of NiO.     

Reducing the Sintering Temperature for MgO–Al2O3Mixtures by Addition of Cryolite (Na3AlF6)

The preparation of near stoichiometric spinel and alumina-rich spinel composites from Al₂O₃and MgO powders with the addition of Na₃AlF₆up to 4 wt% in the temperature range 700°–1600°C was studied; 98 wt% spinel containing 72 wt% Al₂O₃can be produced from the mixture of 72 wt% (50 at.%) Al₂O₃+ 28 wt% (50 at.%) MgO powders with the addition of 1 wt% Na₃AlF₆fired at 1300°C for 1 h. Spinels containing 81–85 wt% Al₂O₃can be produced from either the mixture of 90 wt% (78 at.%) Al₂O₃+ 10 wt% (22 at.%) MgO or the mixture of 95 wt% (88 at.%) Al₂O₃+ 5 wt% (12 at.%) MgO powders with the addition of 4 wt% Na₃AlF₆in the temperature range 1300°–1600°C by using a torch-flame firing for 3 min, followed by quenching in water, while the same system under slow cooling in a furnace results in spinel containing 74–76 wt% Al₂O₃. Microscopic studies indicate that the alumina-rich spinel composites consist of a continuous majority spinel phase and an isolated minority corundum phase, regardless of slow cooling in a furnace or quenching in water.     

Single-Calcination Synthesis of Pyrochlore Free Pb(Mg1/3Nb2/3)O3 Powders Using Particle-Coating Method

Using two-step particle-coating method, pyrochlore-free Pb(Mg_1/3Nb_2/3)O₃ (PMN) powders have been successfully synthesized by a single calcination step at a relatively lower calcined temperature of 850°C. The XRD and EDS results confirmed that the Mg–citric acid polymeric complex coatings effectively prevent direct contact between PbO and Nb₂O₅ and thus avoid the formation of pyrochlore phase. The coated powders were calcined directly without the ball-milling procedure at 850°C. The pyrochlore-free PMN powders obtained showed uniform and even grain size. The results showed that this method is an attractive method for the synthesis of PMN-based composite powders.     

Use of Boron Phosphate in the Solid-State Preparation of Crystalline Iron(III) Phosphate and Manganese(II) Diphosphate

The possibility of using BPO₄ for the solid-state preparation of crystalline phosphates of trivalent iron, manganese, and chromium has been investigated. If BPO₄ and Fe₂O₃ are heated at 900°C, a complete conversion of BPO₄ to the quartz form of FePO₄ results after 9 h. If FeCl₃.6H₂O is used instead of Fe₂O₃, a multiphase crystalline product is obtained in which the dominant phase is the quartz form of FePO₄. The reaction of BPO₄ with Mn₂O₃ or MnO₂ at 830°C yields Mn₂P₂O₇; the amount of conversion of BPO₄, to Mn₂P₂O₇ after 4 h is 80% (if Mn₂O₃ is used) and 75% (if MnO₂ is used). However, the method is not successful in the preparation of phosphates of chromium: the reaction of BPO₄ and some chromium compounds yields α-Cr₂O₃ as the only chromium-containing product.     

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.     

High-Temperature Stability of Lanthanum Orthophosphate (Monazite) on Silicon Carbide at Low Oxygen Partial Pressures

The stability of lanthanum orthophosphate (LaPO₄) on SiC was investigated using a LaPO₄-coated SiC fiber at 1200°–1400°C at low oxygen partial pressures. A critical oxygen partial pressure exists below which LaPO₄ is reduced in the presence of SiC and reacts to form La₂O₃ or La₂Si₂O₇ and SiO₂ as the solid reaction products. The critical oxygen partial pressure increases from ∼0.5 Pa at 1200°C to ∼50 Pa at 1400°C. Above the critical oxygen partial pressure, a thin SiO₂ film, which acts as a reaction barrier, exists between the SiC fiber and the LaPO₄ coating. Continuous LaPO₄ coatings and high strengths were obtained for coated fibers that were heated at or below 1300°C and just above the critical oxygen partial pressure for each temperature. At temperatures above 1300°C, the thin LaPO₄ coating becomes morphologically unstable due to free-energy minimization as the grain size reaches the coating thickness, which allows the SiO₂ oxidation product to penetrate the coating.