Synthesis of Transparent Nd-doped HfO2-Y2O3 Ceramics Using HIP

A Nd-doped HfO₂-Y₂O₃ ceramic having excellent transmittance was synthesized by HIPing, using high-purity powders (>99.99 wt%) of Y₂O₃, Nd₂O₃, and HfO₂. The mixed powder compacts of these powders were sintered at 1650°C for 1 h under vacuum and HIPed at 1700°C for 3 h under 196 MPa of Ar. The specimen after HIPing consisted of uniform grains measuring about 30 μm and having pore-free structure. The optical transmittance of 1 at.% Nd-doped 2.6 mol% HfO₂-Y₂O₃ ceramics ranging between visible and infrared wavelength was almost equivalent or superior to that of a Nd:Y₂O₃ single crystal grown by the Verneuil method.     

Effect of SiC on Interfacial Reaction and Sintering Mechanism of TiB2

Compacts of TiB₂ with densities approaching 100% are difficult to obtain using pressureless sintering. The addition of SiC was very effective in improving the sinterability of TiB₂. The oxygen content of the raw TiB₂ powder used in this research was 1.5 wt%. X-ray photoelectron spectroscopy showed that the powder surface consisted mainly of TiO₂ and B₂O₃. Using vacuum sintering at 1700°C under 13–0.013 Pa, TiB₂ samples containing 2.5 wt% SiC achieved 96% of their theoretical density, and a density of 99% was achieved by HIPing. TEM observations revealed that SiC reacts to form an amorphous phase. TEM-EELS analysis indicated that the amorphous phase includes Si, O, and Ti, and X-ray diffraction showed the reaction to be TiO₂+ SiC → SiO₂+ TiC. Therefore, the improved sinterability of TiB₂ resulted from the SiO₂ liquid phase that was formed during sintering when the raw TiB₂ powder had 1.5 wt% oxygen.     

Grain Growth, Microstructure, and Superconducting Properties of Pure and Y2BaCuO5-Doped YBa2Cu3O7–x Ceramics

Superconducting properties of YBa₂Cu₃O_7–r ceramics have been investigated with the aim of avoiding the weak links that are the principal reasons for low critical current densities in this compound. An efficient grain growth process should be useful in this respect. Superconducting ceramics were prepared from a commercial precursor powder (Hoechst) with or without addition of Y₂BaCuO₅ and from a gel precursor powder. Grain growth kinetics and densification are first derived under such conditions that control the liquid-phase contribution. Grain growth in the solid-state regime is anisotropic, possibly due to anisotropic grain boundary energies. Grains in the fine powder (gel precursor) grow 10 times faster than in the commercial reference but grain size saturates because of steric hindrance. Addition of Y₂BaCuO₅ particles inhibits grain growth in the solid-state regime. 211 particles are incorporated into grains when a transient liquid phase is present. Superconducting properties were characterized by 50-Hz magnetization hysteresis cycles at 77 K under H = 0.2 to 12 mT to probe weak-link behavior. Additional studies were performed at 5 K using higher fields. Weak-link contribution is decreased when samples from the gel route are processed in the solid-state regime (below the peritectic temperature). This may be attributed to the propensity of gel powder to develop, naturally, preferential grain boundaries due to its intrinsic structural anisotropy.     

Nd2O3-Doped Sialons with ZrO2/ZrN Additions Formed by Sintering and Hot Isostatic Pressing

β-sialon and Nd₂O₃-doped α-sialon materials of varying composition were prepared by sintering at 1775° and 1825°C and by glass-encapsulated hot isostatic pressing at 1700°C. Composites were also prepared by adding 2–20 wt% ZrO₂ (3 mol% Nd₂O₃) or 2–20 wt% ZrN to the β-sialon and α-sialon matrix, respectively. Neodymium was found to be a fairly poor α-sialon stabilizer even within the α-phase solid solution area, and addition of ZrN further inhibited the formation of the α-sialon phase. A decrease in Vickers hardness and an increase in toughness with increasing content of ZrO₂(Nd₂O₃) or ZrN were seen in both the HIPed β-sialon/ZrO₂(Nd₂O₃) composites and the HIPed Nd₂O₃-stabiIized α-sialons with ZrN additions.     

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.     

Preparation of the High-Quality Superconductor La1Ba2Cu3Oy

The relationship between the preparation procedure and superconducting properties of La₁Ba₂Cu₃O_y was studied. A series of samples was sintered in an N₂-gas atmosphere for various lengths of time ranging from 1 to 40 h, followed by a fixed annealing procedure in O₂. It was found that the shorter the sintering period, the higher was the oxygen content. The samples sintered for a period of less than 15 h contained excess oxygen compared with La₁Ba₂Cu₃O₇ and exhibited poor superconducting properties. The sample sintered for 40 h had an oxygen content y equal to 6.95, and had excellent superconducting properties. The mechanism for preparing high-quality La₁Ba₂Cu₃O_y is discussed.     

Preparation of MgAl2O4 Spinel Powders via Freeze-Drying of Alkoxide Precursors

High-sinterability MgAl₂O₄ powder has been produced from alkoxide precursors via a freeze-drying method. Clear alumina sol and magnesium methoxide were used as starting materials in the process. The spinel powders were characterized by various techniques, such as thermal analysis, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The tap density and sinterability of the spinel power are affected by the ball-milling techniques. Highly dense, transparent, polycrystalline MgAl₂O₄ has been obtained from these powders by sintering and hot isostatic pressing. Bimodal grain-size microstructure is observed in a HIPed sample.     

Phase Diagram of the System BaO-Fe2O3

The phase diagram of the system BaO-Fe₂0₃ was determined by X-ray diffraction, melting-point measurement, and microscopic methods. Since the reduction of Fe³⁺ to Fe²⁺ was observed by chemical analysis in the samples heated at high temperature, especially in molten samples, the samples were heated at 1 atmosphere pressure of oxygen in the temperature region in which the liquid was in equilibrium; 1 atmosphere pressure of oxygen was su5cient to restrain the reduction of Fe³⁺. In the temperature region of solid-solid equilibrium, the dissociation was not observed even when the samples were heated in air. BaO - 6Fe₂O₃ formed a solid solution with BaO.Fe₂0₃. The BaO:Fe₂0₃ ratio of the solid solution was BaO.4.5Fe₂0₃ at 1350°C. and BaO. 5.0Fe₂0_a at 800°C. The precipitation micro-structures of each primary solid solution were observed.     

Thermal Oxidation Kinetics of MoSi2-Based Powders

The oxidation process of MoSi₂ is very complex, and controversial results have been reported, especially for the early-stage oxidation before the formation of passive SiO₂ film. Most oxidation studies have been carried out on bulk consolidated samples, and the early stage of oxidation has not been studied. In this investigation, very fine MoSi₂ powder with an average particle size of 1.6 μm was used. Such a fine particle size makes it easier to study the early stages of oxidation since a significant portion of the powder is oxidized before the formation of passive SiO₂ film. The oxidation kinetics of commercial MoSi₂-SiC and MoSi₂-Si₃N₄ powder mixtures were also studied for comparison. Weight changes were measured at discrete time intervals at 500° to 1100°C in 0.14 atm of oxygen. X-ray diffraction was used to identify the phases formed during oxidation. Our results show the formation of MoO₃ phase and an associated weight gain at low temperatures (500° and 600°C). At temperatures higher than 900°C, Mo₅Si₃ phase formed first and was subsequently oxidized to solid SiO₂ and volatile MoO₃, resulting in an initial weight gain followed by subsequent weight loss. A model based on the assumption that oxidation kinetics of both MoSi₂ and Mo₅Si₃ are proportional to their fractions in the system describes the experimental data well.     

Cubic-to-Tetragonal Displacive Transformation in Gd2O3–Bi2O3 Ceramics

Gd₂O₃-doped Bi₂O₃ polycrystalline ceramics containing between 2 and 7 mol% Gd₂O₃ were fabricated by pressureless sintering powder compacts. The as-sintered samples were tetragonal at room temperature. Hightemperature X-ray diffraction (XRD) traces showed that the samples were cubic at elevated temperatures and transformed into the tetragonal polymorph during cooling. On the basis of conductivity measurements as a function of temperature and differential scanning calorimetry (DSC), the cubic → tetragonal as well as tetragonal → cubic → teansition temperatures were determined as a function of Gd₂O₃ concentration. The cubic → tetragonal transformation appears to be a displacive transformation. It was observed that additions of ZrO₂ as a dopant, which is known to suppress cation interdiffusion in rare-earth oxide–Bi₂O₃ systems, did not suppress the transition, consistent with it being a displacive transition. Annealing of samples at temperatures 660°C for several hundred hours led to decomposition into a mixture of monoclinic and rhombohedral phases. This shows that the tetragonal polymorph is a metastable phase.     

Hot Isostatic Pressing of Si3N4 Powder Compacts and Reaction-Bonded Si3N4

Hot isostatically pressed silicon nitride was produced by densifying Si₃N₄ powder compacts and reaction-bonded Si₃N₄ (RBSN) parts with yttria as a sintering additive. The microstructure was analyzed using scanning electron microscopy, X-ray diffraction, and density measurements. The influence of the microstructure on fracture strength, creep, and oxidation behavior was investigated. It is assumed that the higher amount of oxygen in the Si₃N₄ starting powder compared with the RBSN starting material leads to an increased amount of liquid phase during densification. This results in grain growth and in a larger amount of grain boundary phase in the hot isostatically pressed material. Compared with the hot isostatically pressed RBSN samples therefore, strength decreases whereas the creep rate and the weight gain during oxidation increase.     

Processing and Characterization of BaTi4O9

BaTi₄O₉ powder prepared by calcining BaCO₃ and TiO₂ powders was sintered to over 97% of theoretical density. Less than 5% Ba₂Ti₉O₂₀ occurred as a second phase in "pure" BaTi₄O₉, and Al₂O₃ impurities from processing formed isolated hollandite (∼BaAl₂Ti₆O₁₆) grains, which were identified by fringes in bright-field TEM images. For pure BaTi₄O₉ at 1 MHz, a dielectric loss (tan δ) of 5 × 10⁻⁴ and dielectric constant of 39 were recorded. Hollandite impurities were found to increase tan δ by 2 orders of magnitude, whereas firing in oxygen decreased tan δ by an order of magnitude.     

New Preparation Method of Low-Temperature-Sinterable Perovskite 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 Powder and Its Dielectric Properties

A relaxor ferroelectric material, 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (0.9PMN-0.1PT) with a pyrochlore-free phase, was prepared by using one-step calcination in the present study. The 0.9PMN-0.1PT powder with the pure perovskite phase was prepared successfully from a mixture of the PMN precursor and the crystalline PT by heating for 2 h at temperatures greaterthan equal to750°C. The PMN precursor was synthesized by adding an aqueous Mg(NO₃)₂ solution, rather than MgO, to the alcoholic slurry of PbO and Nb₂O₅. The 0.9PMN-0.1PT powder sintered to >96% relative density via heat treatment for 2 h at temperatures of 900°-1200°C. The highest room-temperature dielectric constant (epsilon_rt) was 24700 at 1 kHz for the samples that were sintered at 1100°C; however, the samples that were sintered at 900°C still had epsilon_rt values of 22600 at 1 kHz.     

Effect of Grain Size on the Thermal Conductivity of Si3N4

Polycrystalline Si₃N₄ samples with different grain-size distributions and a nearly constant volume content of grain-boundary phase (6.3 vol%) were fabricated by hot-pressing at 1800°C and subsequent HIP sintering at 2400°C. The HIP treatment of hot-pressed Si₃N₄ resulted in the formation of a large amount of ß-Si₃N₄ grains ∼10 µm in diameter and ∼50 µm long, and the elimination of smaller matrix grains. The room-temperature thermal conductivities of the HIPed Si₃N₄ materials were 80 and 102 Wm⁻¹K⁻¹, respectively, in the directions parallel and perpendicular to the hot-pressing axis. These values are slightly higher than those obtained for hot-pressed samples (78 and 93 Wm⁻¹K⁻¹). The calculated phonon mean free path of sintered Si₃N₄ was ∼20 nm at room temperature, which is very small as compared to the grain size. Experimental observations and theoretical calculations showed that the thermal conductivity of Si₃N₄ at room temperature is independent of grain size, but is controlled by the internal defect structure of the grains such as point defects and dislocations.     

Effect of Physical Nature of Powders and Firing Atmosphere on ZnAI2O4 Formation

The rate of ZnA1₂O₄ formation for binary powder mixtures of ZnO and α-Al₂O₃ (dense coarse particles and weak agglomerates of fine powder) fired in air or O₂ atmospheres was measured and the microstructures of those systems were observed by scanning electron microscopy. With dispersed dense particles of α-Al₂O₃, the Al₂O₃ surfaces were covered with ZnO and the spinel grew into the particles maintaining essentially a constant reaction interface area. Calculations based on geometric measurements and use of Jander's equation gave a similar high activation energy, 354 kJ/mol, which corresponds to the activation energy of volume diffusion of Zn²⁺ in ZnAl₂O₄. An oxygen atmosphere had no effect. With a matrix of fine α-Al₂O₃ powder and dispersed granules of ZnO, a higher reaction rate occurred because of an increase in reaction interface area due to penetration of the powder compact matrix by ZnO vapor, which was enhanced by an O₂ atmosphere. The reaction layer grew into the alumina matrix adjoining the ZnO granules with a parabolic rate law. Apparent activation energies below ∼200 kJ/mol were calculated.     

Reaction-Bonding Preparation of Si3N4/MoSi2 and Si3N4/WSi2 Composites from Elemental Powders

Si₃N₄/MoSi₂ and Si₃N₄/WSi₂ composites were prepared by reaction-bonding processes using as starting materials powder mixtures of Si-Mo and Si-W, respectively. A presintering step in an At-base atmosphere was used before nitriding for the formation of MoSi₂ and WSi₂; the nitridation in a N₂-base atmosphere was followed after presintering with the total stepwise cycle of 1350°C × 20 h +1400°C × 20 h +1450°C × 2 h. The final phases obtained in the two different composites were Si₃N₄ and MoSi₂ or WSi₂; no free elemental Si and Mo or W were detected by X-ray diffraction.     

Phase Stability, Phase Transformation Kinetics, and Conductivity of Y2O3—Bi2O3 Solid Electrolytes Containing Aliovalent Dopants

Single-phase, cubic solid solutions of baseline composition 25% Y₂O₃—75% Bi₂O₃ with and without aliovalent dopants were fabricated by pressureless sintering of powder compacts. CaO, SrO, ZrO₂, or ThO₂ was added as an aliovalent dopant. Sintered samples were annealed between 600° and 650°C for up to 4000 h. Samples doped with ZrO₂ or ThO₂ remained cubic, depending upon the dopant concentration, even after long-term annealing. By contrast, undoped, CaO-doped, and SrO-doped samples transformed to the low-temperature, rhombohedral phase within ∼ 200 h. Conductivity measurements showed no degradation of conductivity in samples that did not undergo the transformation. In samples that underwent the transformation, a substantial decrease in conductivity occurred. The enhanced stability of the ZrO₂- and ThO₂-doped samples is rationalized on the basis of suppressed interdiffusion on the cation sublattice.     

Size Control of α-Al2O3 Platelets Synthesized in Molten Na2SO4 Flux

α-Al₂O₃ platelet powders were synthesized in molten Na₂SO₄ flux. The size of α-Al₂O₃ platelets was significantly reduced when partially decomposed rather than pure Al₂(SO₄)₃ was used as the source of Al₂O₃; a further reduction in the platelet size was realized through additional seeding with nanosized α-Al₂O₃ seeds. The addition of microsized α-Al₂O₃ platelet seeds significantly influenced the platelet morphology of the final powder, as well. The platelet size of the final powder was in direct proportion to the size of the platelet seeds, and was in reverse proportion to the cube root of the platelet seed content.     

Electrical Conduction in Nano-Structured La0.9Sr0.1Al0.85Co0.05Mg0.1O3 Perovskite Oxide

Nanocrystalline La_0.9Sr_0.1Al_0.85Co_0.05Mg_0.1O₃ oxide powder was synthesized by a citrate–nitrate auto-ignition process and characterized by thermal analysis, X-ray diffraction, and impedance spectroscopy measurements. Nanocrystalline (50–100 nm) powder with perovskite structure could be produced at 900°C by this process. The powder could be sintered to a density more than 96% of the theoretical density at 1550°C. Impedance measurements on the sintered samples unequivocally established the potential of this process in developing nanostructured lanthanum aluminate-based oxides. The sintered La_0.9Sr_0.1Al_0.85Co_0.05Mg_0.1O₃ sample exhibited a conductivity of 2.40 × 10⁻² S/cm in air at 1000°C compared with 4.9 × 10⁻³ S/cm exhibited by La_0.9Sr_0.1Al_0.85Mg_0.15O₃.     

Sintering of Si3N4-Y2O3 Under Nitrogen Pressure

This study shows that the amount ofAl₂O₃ needed to form high density Si₃N₄-15Y₂-O₃ samples can be reduced by using high surface area Si₃N₄ powder and high N₂ overpressure (high sintering temperatures) during the sintering process. The reduction in AI₂O₃ content results in improved oxidation resistance of the sintered samples.