Hydrothermal Reaction-Sintering of Monoclinic HfO2

Sintered monoclinic HfO₂ bodies were fabricated below the transformation temperature from Hf metal and water by hydrothermal reaction-sintering. Sintering was observed above 900°C under 100 MPa for 3 h. Generally, both the bulk density and the crystallite size of the sintered bodies increased with increasing temperature. Bodies with the maximum relative density (0.98) were obtained by treatment above 1000°C.     

Effects of Calcination on the Microstructures and Photocatalytic Properties of Nanosized Titanium Dioxide Powders Prepared by Vapor Hydrolysis

Ultrafine titanium dioxide powders are produced in an aerosol reactor using vapor hydrolysis of titanium tetraisopropoxide (TTIP) at 260°C and higher temperatures (600°, 700°, 800°, and 900°C). The effect of calcination on the microstructure characteristics and the photoactivity is studied. The powders are characterized using Brunauer-Emmett-Teller (BET) surface area, X-ray diffraction (XRD), and transmission electron microscopy (TEM) analyses. The photocatalytic activity of the powders is also studied using degradation of phenol in water as a test reaction. The powder produced at 260°C is calcined at 500° to 900°C while those produced at higher temperatures are calcined at 600°C for 3 h. Raw powder produced at 260°C is amorphous but becomes crystalline after calcination. As the calcination temperature increases, the surface area decreases but the rutile-to-anatase ratio and the anatase and rutile crystallite sizes increase. The photoactivity increases as calcination temperature increases to 900°C, when the powder becomes densified and the surface area drops significantly because of sintering. Powders produced at higher temperatures are predominantly anatase and are generally more photoactive. Calcination of the powders at 600°C for 3 h results in little loss of surface areas and enhances the photoactivity. Among the factors examined, large surface area and good dispersion of the powders in the reaction mixture are favorable to photoactivity. Conversely, prolonged calcination at high temperatures is detrimental to photoactivity. However, surface area, crystallite size, anatase-to-rutile ratio, and dispersity of the powders alone cannot account for the observed trend of photoactivity. The role of crystallinity needs to be investigated.     

Characterization of Sinterable Oxide Powders: I, BeO

The discovery of readily sinterable BeO powder facilitated the production of dense, high-strength BeO ceramics and created interest in the properties and preparation of sinterable BeO. BeO powders prepared by the thermal decomposition of Be(OH)₂ were studied in an effort to relate sinterability with other more basic characteristics of the oxide powder. Different BeO powders made from analyzed hydroxides showed a wide range of sinterability. The temperatures required for sintering the powder compacts to theoretical density ranged from 2300° to above 3200°F. Lattice parameters, thermal decomposition, surface areas, and refractive indices were determined on these powders after calcination in air at 750°, 1470°, 1830°, and 2190°F. Although unexpected variations in these properties were observed, no simple relation with sinterability was found. Occluded and surface impurities appeared to have a critical effect on sintering behavior.     

Mechanochemical Activation-Assisted Low-Temperature Synthesis of CaZrO3

Calcium zirconate (CaZrO₃, CZ) was prepared using a solid-state reaction with mechanochemical activation through vibro-milling, aiming at completing the reaction CaO+ZrO₂=CaZrO₃ at relatively low calcination temperatures. Changes in the crystallite size and homogeneity of the mixed components CaO and ZrO₂ in the starting mixtures were observed with different milling times. The influence of milling on the incipient temperature of CZ formation and completion of CZ formation was investigated. It is concluded that milling of the reactants for 20 h lowered the incipient temperature of CZ formation from 800° to 600°C, and the temperature of complete CZ formation from above 1100° to 800°C.     

Coprecipitation and Hydrothermal Synthesis of Ultrafine 5.5 mol% CeO2-2 mol% YO1.5ZrO2 Powders

Ultrafine 5.5 mol% CeO₂—2 mol% YO_1.5ZrO₂ powders with controllable crystallite size were synthesized by two kinds of coprecipitation methods and subsequent crystallization treatment. The amorphous gel produced by ammonia coprecipitation and hydrothermal treatment at 200°C for 3.5 h results in an ultrafine powder with a surface area of 206 m²/g and a crystallite size of 4.8 nm. The powder produced by urea hydrolysis and calcination exhibits a purely tetragonal phase. In addition, the powders crystallized by hydrothermal treatment exhibit high packing density and can be sintered at lower temperature (,1400°C) with nearly 100% tetragonal phase achieved.     

Preparation of Manganese Zinc Ferrite Powders by Alcoholic Dehydration of Citrate/Formate Solution

Chemically homogeneous manganese zinc ferrite powders of submicrometer size and desired stoichiometry were achieved using alcoholic dehydration and subsequent calcination of citrate/formate solution. The successful powder preparation was dependent on the water content in the starting solution, the pH of the solution, the amount of alcohol used, and the washing and drying step. Calcination temperature was also critical since the evaporation of ZnO took place above 1000°C. A two-step calcination process was utilized to develop submicrometer ferrite powders without any Zn loss.     

PbTiO3-PbO-B2O3 Glass-Ceramics by a Sol-Gel Process

PbTiO₃ (PT)-PbO-B₂O₃ glass-ceramics were produced by a sol-gel process. Achievement of high PT content at levels unattainable by conventional glass-ceramics preparation methods was semiquantitatively shown. PT content was similar to the designed PT volume fraction and seemed rather unaffected by the composition of the glassforming component at a calcination temperature of 700°C. A higher ratio of PbO to B₂O₃ in the glassforming component resulted in a low PT crystallization temperature, <500°C. PT crystals 1-2 μm in size were obtained at calcination temperatures >600°C.     

Thermal Expansion of Al2O3, BeO, MgO, B4C, SiC, and TiC Above 1000°C.

Thermal-expansion data on Al₂O₃, BeO, MgO, B₄C, SiC, and TiC were obtained to the temperatures where permanent deformation begins, due to sintering or other causes. The thermal expansion for these materials was found to be approximately linear over the measured temperature range. But as a linear extrapolation to room temperature was not possible, the coefficient of thermal expansion is not a constant over this temperature range. The results are compared with the latest published data for each material. The average coefficients of linear thermal expansion are given as follows:

All BeO specimens which were heated to above 2050°C. had a very large expansion. Visual examination of the cooled specimens revealed that they had bent and cracked, the physical dimensions had enlarged, and the color had changed to a bright milky white. A brief discussion of the probable reasons for these changes is given. In the attempt to extend the expansion measurements, the melting point of BeO was obtained. A specimen of hot-pressed BeO melted at 2450°± 30°C., whereas a slip-cast BeO specimen melted at 2470°± 20° C.     

Tetragonal-to-Monoclinic Phase Transitions in Nanocrystalline Rare-Earth-Stabilized Zirconia Prepared by a Mild Hydrothermal Method

Nanocrystalline 4-mol%-Sc₂O₃-stabilized zirconia (4ScSZ) and 4-mol%-Y₂O₃-stabilized zirconia (4YSZ) powders were prepared by a mild urea-based hydrothermal method. The as-prepared 4ScSZ and 4YSZ powders behaved with different tetragonal–monoclinic ( t – m ) transitions on calcination at temperatures between 400° and 1400°C. For the as-calcined 4ScSZ samples, the monoclinic phase fraction varies discontinuously with increasing temperature, i.e., first increases, then decreases, and finally increases again; whereas the monoclinic phase content reduces monotonously for the as-calcined 4YSZ samples, and only tetragonal phase is present over 1000°C. Such interesting results can be explained satisfactorily by considering the combined influences of crystallite size effect, microstrain, and the stabilization effect of the dopant. The microstrain relaxation is mainly responsible for the unusual phase transition in the 4ScSZ samples, while for the 4YSZ samples, the microstrain effect and crystallite size effect can be masked by the stabilization effect of the Y₂O₃ dopant due to its stronger stabilization capability.     

Calcination and Sintering Study of Thoria

Data on the effect of calcination temperature on surface area, apparent crystallite size, compactibility, sintered density, and volume shrinkage are presented for thoria prepared from the oxy-carbonate, chloride, nitrate, and oxalate. Surface area and volume shrinkage decreased with rising calcination temperature. Thoria obtained from the oxycarbonate exhibited the greatest sinterability; material derived from the nitrate showed the least. Maximum bulk densities were achieved using material resulting from the calcination of the oxycarbonate between 600° and 100°C. Densities of 95 to 98% of the theoretical value of thoria were attained by compacting these powders at pressures above 20 tsi and firing at 1500°C for 24 hours. Urania-thoria solid solutions incorporating thoria obtained by calcination of the chloride exhibited the highest fired densities.     

Effects of Preparation Conditions on the Structural and Optical Properties of Spark Plasma-Sintered PLZT (8/65/35) Ceramics

Transparent lanthanum-doped lead zirconate titanate (PLZT) ceramics with high density were fabricated using spark plasma sintering (SPS), a recently developed hot-pressing method. A wet–dry combination method was used to prepare the fine PLZT powders. The average grain size of the PLZT ceramics was less than 1 μm, because of a relatively low sintering temperature and a very short sintering time. The transmittance of PLZT ceramics increased with an increase of calcination temperature up to 700°C and then it slightly decreased with further increase of calcination temperature. The transmittance strongly depended on the SPS temperature and heat-treatment temperature. The pellet sintered at 900°C for 10 min and heat treated at 800°C for 1 h with a thickness of 0.5 mm showed a transmittance of 31% at a wavelength of 700 nm. The relationships between the transmittance and the microstructure were investigated.     

Equilibria of SiF4 with SiO2, Be2SiO4, and BeO in Molten Li2BeF4

Equilibrium partial pressures of SiF₄ were measured for the reactions 2SiO₂( c )+2BeF₂( d )⇋SiF₄( g )+Be₂SiO₄( c ) (log P _siF4(mm) = [8.790 - 7620/ T ] ±0.06(500°–640°C)) and Be₂SiO₄( c ) +2BeF₂( d )⇋SiF₄( g ) +4BeO( c )(log P _siF4(mm) = [9.530–9400/T] ±0.04 (700°–780°C)), wherein BeF₂ was present in solution with LiF as molten Li₂BeF₄. The solubility of SiF₄ was low (∼0.04 mol kg^-1 atm^-1) in the melt. The results for the first equilibrium were combined with available thermochemical data to calculate improved Δ H_f and Δ G_f values for phenacite (–497.57 ±2.2 and –470.22±2.2 kcal, respectively, at 298°K). The few measurements above 700°C for the second equilibrium are consistent with the temperature of the subsolidus decomposition of phenacite to BeO and SiO₂ and with the heat of this decomposition as determined by Holm and Kleppa. Below 700°C, the pressures of SiF₄ generated showed an increasing positive deviation from the expression given for the equilibrium involving Be₂SiO₄ and BeO. This deviation might have been caused by the formation of an unidentified phase below 700°C which replaced the BeO; it more likely resulted from a metastable equilibrium involving BeO and SiO₂.     

Mechanochemical Synthesis of 0.9[0.6Pb(Zn1/3Nb2/3)O3·0.4Pb(Mg1/3Nb2/3)O3]·0.1PbTiO3

Lead zinc niobate–lead magnesium niobate–lead titanate (PZN–PMN–PT) ceramic powders of perovskite structure have been prepared via a mechanochemical processing route. A single-phase perovskite powder of ultrafine particles in the nanometer range was successfully synthesized when a MZN powder (columbite precursor) was mechanically activated for 10 h together with mixed lead and titanium oxides. The following steps are involved when the ternary oxide mixture is subjected to an increasing degree of mechanical activation. First, the starting materials are significantly refined in particle size as a result of the continuous deformation, fragmentation and then partially amorphized at the initial stage of mechanical activation. This is followed by the formation of perovskite nuclei and subsequent growth of these nuclei in the activated oxide matrix with increasing activation time. When calcined at various temperatures in the range of 500–800°C, pyrochlore phase was not detected by XRD phase analysis in the mechanochemically synthesized powder. Only a minor amount (∼2%) of pyrochlore phase was observed when the calcination temperature was raised to 850°C. The PZN–PMN–PT derived from the mechanochemically synthesized powder can be sintered to ∼98% relative density at a sintering temperature of 950°C. The PZN–PMN–PT sintered at 1100°C for 1 h exhibits a dielectric constant of ∼18 600 and a dielectric loss of 0.015 at the Curie temperature of 112°C when measured at a frequency of 0.1 kHz, together with a d ₃₃ value of 323 ×10⁻¹² pC/N.     

Effects of alumina incorporation in coprecipitated NiO–Al2O3 catalysts

The effect of Al₂O₃ levels on the properties of NiO in coprecipitated NiO–Al₂O₃ samples were investigated, using samples with up to 60.7 wt.% Al₂O₃ that had been calcined in the range 300–700°C. Characterization techniques included BET surface area of fresh and reduced catalysts, X-ray diffraction analysis of structure and crystallite size, magnetic susceptibility measurements, oxidizing power, and reducibility in H₂. Only NiO was detected in samples with up to 4.1 wt.% Al₂O₃ for all sample calcination temperatures. Surface areas were similar for all fresh samples but decreased rapidly after calcination at high temperatures. The surface area loss was less for the higher Al₂O₃-containing samples. Nickel oxide crystallite sizes increased at higher calcination temperatures, but remained approximately the same for each Al₂O₃ level.

The NiO was nonstoichiometric (NiO_1+x), with x decreasing at higher calcination temperatures and increasing with small amounts of added Al₂O₃ through a maximum at about 3 wt.% Al₂O₃. However, this did not correlate well with microstrain in the NiO crystallites nor with reducibility, which decreased with Al₂O₃ addition. At higher levels of Al₂O₃ (13.6 wt.% and above), surface areas increased with higher Al₂O₃ loadings, but NiO crystallite sizes remained approximately the same, independent of both Al₂O₃ content and calcination temperature. X-ray diffraction patterns were very diffuse, and it was not possible to rule out the presence of pseudo-spinel combinations of NiO and Al₂O₃. Reducibility was more difficult than with low Al₂O₃ levels, and nonstoichiometry was low and independent of Al₂O₃ content.

Reducibilities of all samples calcined at 300°C correlated well with the final BET surface area of the reduced samples, indicating that more dispersed NiO crystallites are more difficult to reduce, a conclusion that supports a model for reduction proposed previously.     

Cation Diffusion in Single-Crystal and Polycrystalline BeO

The diffusion of Be was measured in single crystals of BeO in directions normal and parallel to the hexagonal c axis. The diffusion coefficients, D , in the two directions were nearly equal to each other. The values of D , measured between 1500° and 2000°C were quite similar to values reported previously for high-density, high-purity, polycrystalline BeO. The single-crystal data, taken in conjunction with polycrystalline data, show that the observed diffusion in polycrystalline BeO proceeds entirely by volume diffusion with no significant diffusion along grain boundaries, although there is some evidence of rapid surface diffusion along microcracks in the surface. The data are interpreted in terms of extrinsic impurity-dependent diffusion via mobile vacancies.     

Phase Studies in BeO–Metal Oxide Systems Using a Porous-Collector Technique

The phase relations in high-temperature BeO–metal oxide systems were investigated by means of a porous-collector technique. In this technique, which was found to be applicable to simple eutectic oxide systems with fluid melts, the liquid formed during heating of a water-shaped specimen was absorbed in a porous collector. The lowest temperature at which the liquid formed and the material balance were used to establish the eutectic temperature and composition. The eutectics in four binary systems were determined as (1) ZrO₂–BeO, 2045°× 10°C, 59 ± 2 mole % BeO, CeO₂–BeO, 1890°× 20°C, 63 ± 3 mole % BeO, (3) MgO–BeO, 1860°× 10°C, 69 ± 2 mole % BeO, and (4) MgO–ZrO₂, 2080°× 10°C, 65 ± 3 mole % MgO. Experiments in the ternary system MgO–ZrO₂–BeO indicated an extensive region of solid solubility in the ZrO₂-rich portion of the system and a eutectic plane at 1720°× 10°C.     

Synthesis and sintering of Y2O3-doped ZrO2 powders using two Pechini-type gel routes

Yttria-tetragonal zirconia polycrystal (ZrO₂ + 4.5 mol% Y₂O₃) nanocrystalline powder was synthesized by two Pechini-type gel routes, the in situ polymerized complex (IPC) method and the PEG/AF method. FTIR spectra confirmed coordination of metal ions with the polymer by different routes, depending on the method used. The crystallite size of the powder increased from 5 nm to 8 nm when the temperature was increased from 450 °C to 600 °C and calcination times increased from 2 h to 24 h. The morphology of the powders differed only when the organic impurities were not completely eliminated. After calcination, the morphology of the powders produced by the two methods showed porous agglomerates composed of smaller particles. All the resulting microstructures were very similar, regardless of the method employed to obtain the powder or the calcination times and temperatures.     

Metal-Ceramic Interactions: II, Metal-Oxide Interfacial Reactions at Elevated Temperatures

Interfacial reactions of various metals (Be, Mo, Nb, Ni, Si, Ti, and Zr) with dense oxide specimens (Al₂O₃, BeO, MgO, ThO₂, and TiO₂) were investigated at temperatures up to 1800°C. The reactions in general occurred in the same order as thermodynamic stabilities indicated. Four general types of behavior were observed. There was little or no reaction at 1400°C., but considerable reaction took place in many systems at 1800°C.     

Decomposition and Crystallization of a Sol–Gel-Derived PbTiO3 Precursor

A lead titanate (PbTiO₃) precursor, prepared by the Pechini method, has been heat treated to study the transformation from amorphous to crystalline PbTiO₃. Nucleation of PbTiO₃ in the temperature interval 400°–475°C occurred before completion of the thermal decomposition of the polymeric precursor, resulting in nanocrystalline PbTiO₃ with an unexpectedly high tetragonality ( c/a ratio). Annealing and crystallite growth at 600°C resulted in an increasing c/a ratio with annealing time in line with the expected finite size effect of PbTiO₃. The unusually high c/a ratios observed in PbTiO₃ nucleated at 400°–475°C are discussed in relation to partial reduction and point defects in PbTiO₃.     

Oxidation Behavior from Room Temperature to 1500°C of 3D-C/SiC Composites with Different Coatings

A carbon-fiber-reinforced silicon carbide composite (3D-C/SiC) was prepared by chemical vapor infiltration. A SiC and SiC/Si-Zr coating were deposited on the composite to investigate the effect of different coatings on the oxidation behavior of 3D-C/SiC composites. The 3D-C/SiC(SiC/Si-Zr) composite decreased in weight below 1000°C and increased in weight above 1000°C. With an increasing oxidation time, the weight loss increased greatly and the weight gain increased little. The 3D-C/SiC(SiC) composite always decreased in weight over the full temperature range. With an increasing oxidation time, the weight loss increased rapidly below 1000°C and reached its minimum value at 1400°C. The 3D-C/SiC(SiC/Si-Zr) composite had a higher oxidation resistance above 1000°C, and the 3D-C/SiC(SiC) composite had a higher oxidation resistance below 1000°C. The wider the coating cracks, the larger the maximum weight loss and the lower the temperature corresponding to the maximum weight loss. With an increasing oxidation time, the activation energy of the 3D-C/SiC(SiC/Si-Zr) composite increased from 96 to 138 kJ/mol, and the 3D-C/SiC(SiC) composite increased from 130 to 180 kJ/mol.