Effect of Small Amounts of Porosity on Grain Growth in Hot-Pressed Magnesium Oxide and Magnesiowustite

Grain growth was studied at 1300°, 1400°, and 1500°C in nearly theoretically dense (0.1 to 0.8% porosity) hot-pressed magnesium oxide (99.99+%) and magnesiowustite (0.10 and 0.48 wt% Fe₂O₃). Small amounts of porosity had large effects on the kinetics of grain growth. Grain growth is probably porosity-controlled in MgO at all temperatures and in magnesiowustite at 1400° and 1500°C if pores remain on grain boundaries or at grain intersections. The tendency for the entrapment of porosity is enhanced as the temperature is increased and as the dopant (Fe₂O₈) concentration is decreased. Small amounts of porosity (<1%) can cause limiting growth situations at grain sizes well below 100 μm. The Fe₂O₃ dopant stabilizes squared grain growth kinetics at 1300°C and decreases the rate of grain growth at all three temperatures.     

Sintering Behavior of 0.8 mol%-CuO-Doped 3Y-TZP Ceramics

In recent years, 3 mol% yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) doped with copper oxide has obtained increasing interest due to its enhanced superplastisity and good potential in tribological applications. In this work, the effect of addition of small amounts (0.8 mol%) of copper oxide on the sintering behavior of 3Y-TZP was studied using a dilatometer and high-temperature X-ray diffraction (XRD). A qualitative sintering model was established based on several reactions during sintering as indicated by thermal analysis and XRD. Some of these reactions remarkably retard densification and consequently result in low final density (86%) of the sample sintered at 1400°C in air. The reaction between molten Cu₂O and yttria as segregated to the Y-TZP grain boundaries at around 1180°C leads to the depletion of yttria from Y-TZP grains, which results in the formation of monoclinic phase during cooling. A relatively higher oxygen partial pressure can inhibit the dissociation of CuO to Cu₂O. This inhibition in dissociation is one of the reasons why a dense (>96%) 0.8 mol% CuO-doped 3Y-TZP ceramic can be obtained after sintering at 1400°C in flowing oxygen.     

Properties of Yttrium Oxide Ceramics

The cubic structure of yttrium oxide is stable to 1800°C. in air as indicated by petrographic, X-ray, and differential thermal analyses. A change in lattice parameter of less than ±0.007 a.u. was observed on heating the oxide to 1800°C. The mean specific heat of Y₂O₃ to 1600°C. was 0.13 cal. per gm. per °C. The coefficient of linear expansion to 1400°C. was 9.3 × 10⁻⁶ in. per in. per °C. Compacts of Y₂O₃ required a temperature of 1800°C. for vitrification. In equimolecular binary mixtures heated in the powdered state at 1500°C., Y₂O₃ formed compounds with Al₂O₃ and Fe₂O₃ and solid solutions with ZrO₂ and HfO₂. Y₂O₃ did not react with CaO, MgO, or ThO₂. Crystal types and unit-cell sizes of the reaction products are included.     

Fabrication and Sintering of Fine Yttria-Doped Ceria Powder

Yttria–doped ceria powder was prepared from oxalate precursors. The oxalate coprecipitation bath parameters were closely monitored and found to influence the sintering behavior of the subsequently obtained oxide powders strongly. The use of concentrated (Ce,Y) metal nitrate solutions and dilute neutralized oxalic acid for coprecipitation were identified as the most–important parameters. Following calcination at 700°C, compacts of such powders were sintered to high density (98%) at 1400°C (4 h). Ball milling of the powder further reduced the sintering temperature. Dry milling, for tape–casting applications of the powder in particular, was more effective than wet milling. Tape–cast membranes were fired at 1400°C (2 h), with resulting densities of 98%.     

Oxidation Behavior of NBD 200 Silicon Nitride Ceramics

The oxidation behavior of NBD 200 Si₃N₄ containing 1 wt% MgO sintering aid was investigated in oxygen at 900°-1300°C. The oxide growth followed a parabolic rate law with an apparent activation energy of 260 kJ/mol. The oxide layers were enriched with sodium and magnesium because of outward diffusion of intergranular Na⁺ and Mg²⁺ cations in the ceramics. The 2-4 orders of magnitude higher oxidation rate for NBD 200 Si₃N₄ than for other Si₃N₄ ceramics with a similar amount of MgO could be attributed to the presence of sodium. The oxidation process was most likely rate limited by grain-boundary diffusion of Mg²⁺.     

Effects of Titanium Oxide on Equilibria Among Refractory Phases in the System CaO-MgO-Iron Oxide

The role of titanium oxide in some important refractory systems was elucidated by studying selected equilibria in the system CaO-MgO-iron oxide-titanium oxide at O₂ pressures of 0.21 atm (air) and 10⁻⁹ atm and under the extreme reducing conditions imposed by the presence of metallic Ti in contact with the oxide phases. Solidus relations were determined for the system CaO-MgO-TiO₂ in air; 6 composition triangles were delineated, within each of which 3 crystalline phases coexist in equilibrium with liquid at a constant solidus temperature. The solidus temperatures range from 1407° to 1670°C. There is also a composition area within which MgO coexists with a Ca₄Ti₃O₁₀-Ca₃Ti₂O₇ solid solution, with solidus temperatures varying continuously from 1659° to 1670°C. Studies of reactions between MgO and titanium oxide in contact with metallic Ti in a closed system indicate that the mutual solubility between MgO and TiO at 1400°C is very small. Addition of 5 wt% TiO₂ to the system CaO-MgO-iron oxide at 1500°C in air and in 10⁻⁹ atm O₂ decreases the amount of iron oxide which can be absorbed by a CaO-MgO body without formation of a liquid phase; hence, titanium oxide has a strong deleterious effect on the refractoriness of such bodies.     

Polycrystalline infrared-transparent MgO fabricated by spark plasma sintering

In the present research, polycrystalline magnesium oxide (MgO) bodies were fabricated using spark plasma sintering (SPS) at different temperatures and times from MgO nanopowder. Microstructural development, densification, and optical properties were investigated during SPS. The critical pressure of plastic deformation of the MgO compacts during sintering was also analyzed. The results showed that the plastic deformation phenomenon had a profound effect on the grain size and optical properties. In addition, the optical properties and microstructure of MgO bodies were strongly dependent on sintering temperature and time. Full-dense infrared-transparent magnesium oxide with a relative density of 99.99% was prepared at 1200 °C for 5 min under the pressure of 80 MPa. The spark plasma sintered MgO demonstrated the highest infrared transmittance of 72% in the 3–7 μm wavelength range, which was comparable with the values reported for MgO single crystal.     

Mechanical Behavior of Lithium Fluoride at High Temperatures

Single crystals of lithium fluoride were pulled in tension at high temperatures; the results of these tests are compared with data for magnesium oxide. Interpenetration of {110}〈110〉 slip systems does not occur in LiF as readily as in MgO. Because of this lack of interpenetrability, plastic instability and completely ductile fracture do not occur in LiF below 700°C (0.87 T _mp); the high-temperature tensile strength of LiF decreased very little from 300° to 700°C. This lack of interpenetrability of slip systems in LiF at high temperatures also had a profound effect on the deformation processes, the development of substructure, and the strain-hardening and fracture characteristics of the material. This work emphasizes the importance of interpenetration of slip not only in the high-temperature ductility and strain-hardening processes but also in the maximum tensile strength which ionic crystals develop before fracture.     

Effect of Nickel Oxide/Yttria-Stabilized Zirconia Anode Precursor Sintering Temperature on the Properties of Solid Oxide Fuel Cells

An NiO/yttria-stabilized zirconia (YSZ) layer sintered at temperatures between 1100° and 1500°C onto dense YSZ electrolyte foils forms the precursor structure for a porous Ni/YSZ cermet anode for solid oxide fuel cells. Conflicting requirements for the electrochemical performance and mechanical strength of such cells are investigated. A minimum polarization resistance of 0.09 Ω^.cm²at 1000°C in moist hydrogen is obtained for sintering temperatures of 1300°–1400°C. The mechanical strength of the cells decreases with increased sintering temperature because of the formation of channel cracks in the electrode layers, originating in a thermal expansion coefficient mismatch between the layers.     

Steam Sintering of Uranium Dioxide

The following variables were considered in an investigation of the sintering of uranium dioxide: (1) sintering aids, i.e., small additions of substances that might aid the densification process, (2) soaking temperature, (3) soaking time, (4) furnace atmosphere, and (5) particle size of the uranium dioxide. The incipient sintering temperature, the effect of binders, and the effect of fabricating pressure also were determined. The data obtained in the investigation led to the following optimum conditions for obtaining dense UO₂ by cold pressing and sintering: (1) preparing the UO₂ as a submicron powder but not fine enough to be pyrophoric, (2) cold pressing at 20 to 30 tons per sq. in. without the use of organic binders, (3) using the following sintering schedule: ( a ) in H₂ to 1400°C., (b) in steam or a wet inert gas from 1400° to 1500°C., ( c ) in steam or a wet inert gas at 1500°C. for a 1-hour soak, and (d) cooling in H₂ from 1500°C. to room temperature. By following this procedure, pellets of UO₂ (3/8 in. in diameter by 1/4 in. high) were obtained that were 97 to 98% of theoretical density.     

Oxidation Behavior of Zirconium Diboride Silicon Carbide Produced by the Spark Plasma Sintering Method

Dense samples of ZrB₂–20 vol% SiC were successfully fabricated by spark plasma sintering without the use of sintering aids. Oxidation behavior of these samples was characterized by exposing them to 1400°, 1500°, and 1600°C in an ambient atmosphere for 150 min, and by measuring the weight gains of the sample and crucible, as well as the thickness of the oxide scale and the glassy outer layer. The effects of gravity on the viscous outer layer are shown to result in significant heterogeneity within a sample. The oxidation scales were characterized by scanning electron microscopy and transmission electron microscopy with energy dispersive spectroscopy analysis. The oxide scale was found to be composed of three layers: (1) a SiO₂-rich glassy outer layer, (2) an intermediate layer of a ZrO₂ matrix with interpenetrating SiO₂, and (3) a layer containing a ZrO₂ matrix enclosing partially oxidized ZrB₂ with Si–C–B–O glass inclusions.     

Phase Equilibria in the Ga2O3In2O3 System

Subsolidus phase relationships in the Ga₂O₃–In₂O₃ system were studied by X-ray diffraction and electron probe microanalysis (EPMA) for the temperature range of 800°–1400°C. The solubility limit of In₂O₃ in the β-gallia structure decreases with increasing temperature from 44.1 ± 0.5 mol% at 1000°C to 41.4 ± 0.5 mol% at 1400°C. The solubility limit of Ga₂O₃ in cubic In₂O₃ increases with temperature from 4.X ± 0.5 mol% at 1000°C to 10.0 ± 0.5 mol% at 1400°C. The previously reported transparent conducting oxide phase in the Ga-In-O system cannot be GaInO₃, which is not stable, but is likely the In-doped β-Ga₂O₃ solid solution.     

Freezing Dilute Ceramic/Camphene Slurry for Ultra-High Porosity Ceramics with Completely Interconnected Pore Networks

Highly porous alumina ceramics with completely interconnected pore channels were fabricated by freezing dilute alumina/camphene slurries with solid loadings ranging from 5 to 20 vol%. This method fundamentally made full use of the three-dimensional camphene dendritic network for producing interconnected pore channels and the concentrated alumina powder network for achieving dense alumina walls. Firstly, alumina/camphene slurries were prepared at 60°C using ball milling and then cast into molds at 20°C. After subliming the frozen camphene, the samples were sintered at 1400°C for 5 h. This method enabled us to freeze very dilute ceramic slurries with a low solid loading of ≤20 vol% without the collapse of the sample after sintering. As the initial solid loading decreased from 20 to 5 vol%, the porosity linearly increased from 66% to 90% with an increase in the pore size, while completely interconnected pore networks were obtained in all cases. In addition, the free surfaces of the alumina walls showed full densification after sintering even at a low temperature of 1400°C, while some pores were present in the inner regions of the alumina walls.     

New Uniformly Porous CaZrO3/MgO Composites with Three-Dimensional Network Structure from Natural Dolomite

Porous CaZrO₃/MgO composites with a uniform three-dimensional (3-D) network structure have been successfully synthesized using reactive sintering of highly pure mixtures of natural dolomite (CaMg(CO₃)₂) and synthesized zirconia powders with LiF additive. Equimolar dolomite and zirconia powders doped with 0.5 wt% LiF were cold isostatically pressed at 200 MPa and sintered at 1100–1400°C for 2 h in air. Through the liquid formation via LiF doping, strong necks were formed between constituent particles before completion of the pyrolysis of dolomite, resulting in the formation of a 3-D network structure. During and after the formation of the network structure, CO₂ was given off to form a homogeneous open-pore structure. The pore-size distribution was very narrow (with pore size ∼ 1 μm), and the porosity was controllable (e.g., ∼30%–50%) by changing the sintering temperature. The porous composites can be applied as filter materials with good structural stability at high temperatures.     

Sintering of Alumina at Temperatures of 1400°C. and Below

Previous investigators have indicated that both small particles and the addition of certain oxides promote the sintering of alumina at temperatures below 1700°C. By utilizing combinations of oxides and small particle size, the sintering temperature of 96% alumina bodies was reduced in this investigation to the 1300° to 1400°C. range. It is proposed that this low-temperature sintering is aided by the formation of a liquid phase. Thin sections of the alumina sintered at low temperatures revealed bodies with small grain size whose bulk densities were above 3.80 gm. per cc.     

Influence of precursors of Li2O and MgO on surface and catalytic properties of Li‐promoted MgO in oxidative coupling of methane

The influence of the catalyst precursors (for Li₂O and MgO) used in the preparation of Li‐doped MgO (Li/Mg = 0.1) on its surface properties (viz basicity, CO₂ content and surface area) and activity/selectivity in the oxidative coupling of methane (OCM) process at 650–750 °C (CH₄/O₂ feed ratio = 3.0–8.0 and space velocity = 5140–20550 cm³ g⁻¹ h⁻¹) has been investigated. The surface and catalytic properties are found to be strongly affected by the precursor for Li₂O (viz lithium nitrate, lithium ethanoate and lithium carbonate) and MgO (viz magnesium nitrate, magnesium hydroxide prepared by different methods, magnesium carbonate, magnesium oxide and magnesium ethanoate). Among the Li–MgO (Li/MgO = 0.1) catalysts, the Li–MgO catalyst prepared using lithium carbonate and magnesium hydroxide (prepared by the precipitation from magnesium sulfate by ammonia solution) and lithium ethanoate and magnesium acetate shows high surface area and basicity, respectively. The catalysts prepared using lithium ethanoate and magnesium ethanoate, and lithium nitrate and magnesium nitrate have very high and almost no CO₂ contents, respectively. The catalysts prepared using lithium ethanoate or carbonate as precursor for Li₂O, and magnesium carbonate or ethanoate, as precursor for MgO, showed a good and comparable performance in the OCM process. The performance of the other catalysts was inferior. No direct relationship between the basicity of Li‐doped MgO or surface area and its catalytic activity/selectivity in the OCM process was, however, observed. © 2000 Society of Chemical Industry     

The Role of Excess Magnesium Oxide or Lead Oxide in Determining the Microstructure and Properties of Lead Magnesium Niobate

Near-phase pure perovskite lead magnesium niobate (PMN) with MgO or PbO additives was produced by reacting PbO with MgNb₂O₆ at 800°C and sintering at 1200°C. Dense ceramics were characterized by scanning electron microscopy, X-ray diffraction, and dielectric measurements. The microstructural studies showed that excess MgO exists as micrometer spherical particles either in the grain boundary as a discrete particle or in the perovskite grain as an inclusion. The pyrochlore phase exists in large isolated grains in the microstructure. The 10 mol% MgO excess composition had a peak dielectric constant of 19 500 at 100 Hz, which suggests very "clean" or uninhibiting grain boundaries. The excess addition of PbO did not improve the yield of perovskite PMN phase and decreased the dielectric constant. PMN grain boundaries are the dominant path of fracture. This paper, to a certain degree, explores the chemistry and characteristics of these grain boundaries.     

Processing of a Silicon-Carbide-Whisker-Reinforced Glass-Ceramic Composite by Microwave Heating

A calcium magnesium aluminosilicate-based glass that contained 10 wt% of silicon carbide whiskers (SiC _w ) as reinforcement was prepared by tape casting, followed by sintering either in a conventional furnace or in a microwave oven. The results were consistent with retardation of glass sintering through whisker bridging. The glass, by itself, was sintered to almost-full density at 750°C for 4 h by conventional furnace sintering; the best sintered composite, with an estimated density of ∼90%, was obtained at 800°C with a dwell time of 4 h. Sintering at a temperature of >800°C did not improve the densification but rather resulted in severe whisker oxidation. A reduced densification rate was observed for the samples that were sintered in nitrogen. By contrast, in the microwave oven, almost-full density for the glass and ∼95% of the theoretical density for the composite were obtainable at 850°C for 15 min, which represented a reduction of ∼10 h of the total processing time and a reduced SiC _w oxidation.     

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.     

Electrical Conductivity in Fe1-xO-MgO Solid Solutions

Solid solutions of magnesium oxide containing iron oxide in concentrations varying from a few percent to 100% were fabricated in polycrystalline form for studying the mechanism of electrical conduction. These semiconductors were ohmic in the range 0 to 10 v/cm, conducted by hole transfer, and showed reproducible conductivity data from room temperature to about 300°C. The conductivity of these solid solutions increases with increasing Fe³⁺ concentration. The electrical conductivity is also dependent on the total iron oxide content, decreasing rapidly (nonlinear dependence) as Fe_{1- x} O is diluted with MgO. The activation energy for electrical conduction varies from 0.1 to about 0.5 ev for the dilute solid solutions (less than about 15 cation % iron).