Sinter-Forging of Nanocrystalline Zirconia: I, Experimental

Nanocrystalline (15 nm) yttria (3 mol%)-stabilized zirconia (3Y-TZP) was sinter-forged under conditions of varying temperature (1050–1200°C), plastic strain rate (5 × 10⁻⁵ to 2 × 10⁻³s⁻¹), and green density (33–48%), using constant-crosshead-speed tests, constant-load (i.e., load-and-hold) tests, and constant-loading-rate tests. The densification and pore size evolution results indicate that plastic strain is largely responsible for elimination of large pores, while diffusional mechanisms control the elimination of small pores. Grain growth during sinter-forging is observed to be dependent solely on porosity during intermediate-stage sintering. Once the powder compact enters final-stage sintering, however, both static (time- and temperature-dependent) and dynamic (plastic-strain-dependent) grain growth take place, greatly accelerating the overall rate of grain growth. The use of fast strain rates to impose plastic strain before the onset of dynamic grain growth is proposed as a method of preserving small grain sizes during sinter-forging.     

Effect of Oxygen Partial Pressure During Liquid-Phase Sintering on the Dielectric Properties of 0.9MgTiO3–0.1CaTiO3

Microstructural evolution and microwave dielectric properties of liquid-phase-sintered 0.9MgTiO₃–0.1CaTiO₃ dielectric ceramic material have been investigated as a function of oxygen partial pressure (     ) during sintering. Sintering in a weakly reducing atmosphere (     =10⁻¹⁴ atm) generally increased the density, permittivity, quality factor ( Q × f ), and the temperature coefficient of resonance frequency (τ_f), but further reducing atmosphere down to     of 10⁻¹⁴ atm generally decreased Q × f and τ_f. When the 5 wt% lithium borosilicate glass-added specimen was sintered at 950°C and     =10⁻¹⁴ atm, it demonstrated a permittivity of 18.8, Q × f of 19 000 GHz, and τ_f of 10 × 10⁻⁶ K⁻¹.     

Effect of Hydrogen-Water Atmospheres on Corrosion and Flexural Strength of Sintered α-Silicon Carbide

Sintered α-SiC was exposed for 10 h to H₂ containing various partial pressures of H₂O ( P _H2O from 5×10⁻⁶ to 2×10⁻² atm; 1 atm≅10⁵ Pa) at 1300° and 1400°C. Weight loss, surface morphology, and room-temperature flexural strength were strongly dependent on P _H2O. The strength of the SiC was not significantly affected by exposure to dry H₂ at a P _H2O of 5×10⁻⁶ atm; and following exposure at P _H2O >5×10⁻³ atm, the strength was even higher than that of the as-received material. The increase in strength is thought to be the result of crack blunting associated with SiO₂ formation at crack tips. However, after exposure in an intermediate range of water vapor pressures (1×10⁻⁵< P _H2O <1×10⁻³ atm), significant decreases in strength were observed. At a P _H2O of about 1×10⁻⁴ atm, the flexural strength decreased approximately 30% and 50% after exposure at 1300° and 1400°C, respectively. The decrease in strength is attributed to surface defects caused by corrosion in the form of grain-boundary attack and the formation of pits. The rates of weight loss and microstructural changes on the exposed surfaces correlated well with the observed strength changes.     

Correlation Between the Microstructure and the Electrical Properties of ZrTiO4 Ceramics

A series of ZrTiO₄ (ZT) ceramics were prepared by sintering ball-milled precursor powders. Ceramics with homogenously dispersed micropores containing both low-temperature (ordered) and high-temperature (disordered) ZrTiO₄ phases were achieved by sintering at 1400°C. The porosity and the morphology were modified by changing the sintering time. The impedance spectra followed the observed changes in the ceramics' microstructure. The grain boundaries' shape, rather than the grain size, has the major influence on the conductivity. The lowest value for electrical conductivity, 8.1 × 10⁻¹⁰ (Ω·m)⁻¹, was obtained in the ceramics sintered at 1400°C for 4 h. This confirms the blocking effect of the imperfect grain boundaries; i.e., the defects of the grain boundaries, which were formed during the grain growth, were followed by an almost complete loss of pores.     

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.     

Densification and Grain Growth of Fe-Doped and Fe/Y Codoped Alumina: Effect of Fe Valency

The effect of iron and iron/yttrium codoping on the densification and grain growth of ultra high-purity (99.995%) fine-grained alumina has been studied. The experiments were carried out under both oxidizing (flowing air) and reducing conditions (N₂/H₂ mixture, p O₂∼5.1 × 10⁻¹⁴). For studies carried out in air, relative to undoped alumina, the addition of 1000 ppm Fe was found to reduce the densification rate by a factor of 5 and also retard the grain growth rate. This result, which was consistent with tensile creep data obtained in a separate study, was attributed to the retardation of grain-boundary diffusive processes by segregating Fe(III) ions. In contrast, under reducing conditions the 1000 ppm Fe- doped samples exhibited an increase in the densification rate of 2.5 orders of magnitude over that of the undoped samples. In the case of the codoped compositions (1000 ppm Fe/1000 ppm Y), for heat treatment in air, the densification behavior did not differ significantly from that of samples singly doped with Y (1000 ppm). However, under reducing conditions, the presence of the Fe²⁺ in the samples appeared to compensate for the retarding effect of the yttrium, such that the densification rate of the codoped samples was comparable with that of the undoped material. A mechanism based on compensating point defects is invoked to rationalize the more rapid kinetics under reducing conditions.     

Oxidation of Thin Sheet Reaction-Sintered Silicon Nitride

Oxidation of reaction-sintered silicon nitride was studied in damp air. The formation of "passive" silica films was investigated at 1 atm and 700 to 1100°C and some limited work on weight loss behavior was performed in vacuo of 10⁻⁸ to 10⁻⁵ atm at 1050 to 1200°C. Passive behavior was dominated by reaction in the pore network. Oxidation was extensive at 900 to 1000° but slight at 700 to 800°C. At 1100°C a protective skin limited reaction. Weight loss in vacuo was slight at 1050°C. The vacuum pressure required to suppress the weight loss increased from 4 to 5 × 10⁻⁷ atm at 1050° to 1.5 to 2.5 × 10⁻⁵ atm at 1200°C.     

Electrical Properties and Defect Structure of HfO2

The defect structure of high-purity, polycrystalline HfO₂ was investigated by measuring the oxygen partial pressure dependence of the electrical conductivity and the sample weight. From 1000° to 1500°C and above oxygen partial pressures of 10 ⁻⁶, the conductivity is electronic and proportional to p o₂^1/5. The predominant defect is completely ionized hafnium vacancies. At lower oxygen partial pressures a broad shallow minimum in the lower temperature conductivity isotherms indicates the presence of an oxygen pressure independent source of electronic charge carriers. By combining the weight change and conductivity data, mobility values were found to vary from 1.6 × 10⁻³ to 3 × 10⁻⁴ cm²/V-sec. The activation energies for the hole mobilities were calculated to be 0.2 ev above 1300° C and 0.7 ev below this temperature.     

Sintering of Zinc Sulfide

The mechanisms of the sintering of ZnS were determined by measurement of the rate of growth of the necks between polycrystalline spheres. In vacuum (10⁻⁶ mm Hg) at temperatures higher than 600° C the mechanism of sintering is that of volume diffusion with coefficient D_v, = 1.06 × 10⁻² exp (-26,400/RT); below 600°C surface diffusion predominates, with coefficient D₃, = 9.14 × 10_-3 exp (-5700/RT). In Zn vapor (10⁻² mm Hg) between 550° and 650°C the predominating mechanism of sintering is surf ace diffusion, D₃, = 7.06 × 10⁻² exp (-8600/RT). It has been found that in an argon atmosphere the diffusion coefficient is much lower.     

Rapid Densification and Deformation of Li-Doped Sialon Ceramics

Two lithium-doped sialon ceramics were densified and superplastically deformed by spark plasma sintering (SPS). Rapid densification with linear shrinkage rates of approximately 5 × 10⁻³ s⁻¹ were observed for samples heated at a rate of 100°C/min up to ∼1400°C under a uniaxial pressure of 40 MPa. Isothermal deformation by SPS-preprepared, fully densified ceramics performed at T ≥ 1450°C yielded strain rates in the order of 10⁻² s⁻². It is suggested that a high heating rate promotes material transport via formation of a nonequilibrated oxygen-rich liquid of low viscosity. This finding most likely holds true for other liquid-phase sintered ceramics as well and has implications for cost-effective manufacturing of ceramic components.     

Electrical Conduction in Single-Crystal and Polycrystalline Al2O3 at High Temperatures

The electrical conductivity and ion/electron transference numbers in Al₃O₃ were determined in a sample configuration designed to eliminate influences of surface and gas-phase conduction on the bulk behavior. With decreasing O₂ partial pressure over single-crystal Al₂O₃ at 1000° to 1650°C, the conductivity decreased, then remained constant, and finally increased when strongly reducing atmospheres were attained. The intermediate flat region became dominant at the lower temperatures. The emf measurements showed predominantly ionic conduction in the flat region; the electronic conduction state is exhibited in the branches of both ends. In pure O₂ (1 atm) the conductivity above 1400°C was σ≃3×10³ exp (–80 kcal/ RT ) Ω⁻¹ cm⁻¹, which corresponds to electronic conductivity. Below 1400°C, the activation energy was <57 kcal, corresponding to an extrinsic ionic condition. Polycrystalline samples of both undoped hot-pressed Al₂O₃ and MgO-doped Al₂O₃ showed significantly higher conductivity because of additional electronic conduction in the grain boundaries. The gas-phase conduction above 1200°C increased drastically with decreasing O₂ partial pressure (below 10⁻¹⁰ atm).     

Control of the Thermal Expansion of Strontium-Doped Lanthanum Chromite Perovskites by B-site Doping for High-Temperature Solid Oxide Fuel Cell Separators

The thermal expansion of La_0.9Sr_0.1Cr_{1- x} M _x O₃(M = Mg, Al, Ti, Mn, Fe, Co, Ni; 0 ≤ x ≤ 0.1) perovskites has been studied in oxidizing and reducing atmospheres in the temperature range from 50° to 1000°C. Cobalt doping of La_0.9Sr_0.1CrO₃was an effective way of increasing the average linear thermal expansion coefficient (TEC), whereas titanium doping showed a negative effect. No effect on the TECs was observed for the B-site dopants in perovskites with the remaining dopants. Linear thermal expansion behavior was observed in the La_0.9Sr_0.1Cr_{1- x} M _x O₃ perovskites with doping of ≥1 mol% aluminum or 10 mol% cobalt. TECs of La_0.9Sr_0.1Cr_0.96Co_0.02Al_0.02O₃ were 10.5 × 10⁻⁶/°C in air, 10.7 × 10⁻⁶/°C under He–H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁵ atm at 1000°C), and 11.8 × 10⁻⁶/°C in H₂ atmosphere.     

Fabrication of Optically Transparent and Electrooptic Strontium Barium Niobate Ceramics

The tetragonal tungsten bronze (TTB) type of Sr _x Ba_{1− x} ^m Nb₂O₆ (SBN) (0.5 ≤ x ≤ 0.75) was synthesized by a mixed-oxide route. Using the two-step densification process of pressureless sintering followed by oxygen HIPing, transparent ceramics were successfully fabricated. Microstructural, dielectric, optical, and electrooptic properties were investigated. All single-phase TTB-type SBN showed the characteristics of relaxor ferroelectrics. The extinction coefficients of the sample with the highest transparency were calculated from transmission spectra to be 30.4, 5.8, and 2.4 cm⁻¹ at 600, 1450, and 2000 nm, respectively. The linear electrooptic coefficients for SBN55, r ₃₃ and r ₁₃, were 46 × 10⁻¹² and 21 × 10⁻¹² m/V at 633 nm, respectively.     

Extrinsic OH− Absorption in Transparent Polycrystalline Lanthana-Doped Yttria

Transparent lanthana-doped yttria fabricated by transient solid second-phase sintering under wet hydrogen typically has a broad absorption band with a peak at 3.08 μm. The absorption band shift observed in samples treated in wet deuterium indicated that the 3.08-μm absorption was due to OH⁻ ions. The diffusion rates of hydrogen defects in lanthana-doped yttria were determined in the temperature range from 1000° to 1400°C. The changes in the concentrations of OH⁻ ions upon anneals were determined by measuring infrared absorbance at 3.08 μm. The diffusion coefficient is 1.3 × 10⁻⁷, 9.9 × 10⁻⁷, and 4.1 × 10⁻⁶ cm²/s at 1000°, 1200°, and 1400°C, respectively, with an activation energy of 140 kJ/mol. Annealing in a controlled oxygen partial-pressure environment can remove the OH⁻ absorption band and bring the total absorption in the 3- to 5-μm range closer to the intrinsic values.     

Microstructure and High-Temperature Corrosion Behavior of a Cr–Al–C Composite

A Cr–Al–C composite was successfully synthesized by a hot-pressing method using Cr, Al, and graphite as starting materials. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses revealed that the composite contained Cr₂AlC, AlCr₂, Al₈Cr₅, and Cr₇C₃. The orientation relationships and atomic-scale interfacial microstructures among Cr₂AlC, AlCr₂, and Al₈Cr₅ are presented. This composite displays both excellent high-temperature oxidation resistance in air and hot-corrosion resistance against molten Na₂SO₄ salt. The parabolic rate constants for the oxidation in air at 1000°, 1100°, and 1200°C are 3.0 × 10⁻¹², 6.2 × 10⁻¹¹, and 6.2 × 10⁻¹⁰ kg² (m⁴·s)⁻¹, respectively, while the linear weight gain rates for the hot corrosion of Na₂SO₄-coated samples at 900° and 1000°C are, respectively, 1.2 × 10⁻³ and 4.4 × 10⁻³ mg (cm²·h)⁻¹. The mechanism of the excellent high-temperature corrosion resistance can be attributed to the formation of a protectively alumina-rich scale.     

Microstructural and Thermoelectric Characteristics of Zinc Oxide-Based Thermoelectric Materials Fabricated Using a Spark Plasma Sintering Process

M-doped zinc oxide (ZnO) (M=Al and/or Ni) thermoelectric materials were fully densified at a temperature lower than 1000°C using a spark plasma sintering technique and their microstructural evolution and thermoelectric characteristics were investigated. The addition of Al₂O₃ reduced the surface evaporation of pure ZnO and suppressed grain growth by the formation of a secondary phase. The addition of NiO promoted the formation of a solid solution with the ZnO crystal structure and caused severe grain growth. The co-addition of Al₂O₃ and NiO produced a homogeneous microstructure with a good grain boundary distribution. The microstructural characteristics induced by the co-addition of Al₂O₃ and NiO have a major role in increasing the electrical conductivity and decreasing the thermal conductivity, resulting from an increase in carrier concentration and the phonon scattering effect, respectively, and therefore improving the thermoelectric properties. The ZnO specimen, which was sintered at 1000°C with the co-addition of Al₂O₃ and NiO, exhibited a ZT value of 0.6 × 10⁻³ K⁻¹, electrical conductivity of 1.7 × 10⁻⁴Ω⁻¹·m⁻¹, the thermal conductivity of 5.16 W·(m·K)⁻¹, and Seebeck coefficient of −425.4 μV/K at 900°C. The ZT value obtained respects the 30% increase compared with the previously reported value, 0.4 × 10⁻³ K⁻¹, in the literature.     

Effect of Oxygen on the Reaction Between Copper and Sapphire

The effect of oxygen potential on the wetting behavior and interfacial energy between Cu and sapphire was studied using the sessile drop technique in a CO-CO₂ atmosphere. A linear relation was found between γ_SL and log p O₂ (atm) from 10⁻¹⁶ to 10⁻⁵. Beyond 10⁻⁵ atm γ_SL approached a constant value asymptotically. A barrier surface layer was proposed to explain this change. The Gibbs adsorption equation was used to evaluate the characteristics of the interfaces. Formation of a Cu₂O film at the liquid-vapor interface and a CuAlO₂ film at the solid-liquid interface is suggested. The work of adhesion reached a maximum at ∼ 0.01 at.% oxygen, corresponding to p O₂∼ 10⁻⁹atm. Measurements of the basal radius as a function of oxygen content were used to evaluate the role of oxygen in promoting spreading. Spreading on sapphire is directly proportional to the logarithm of oxygen present in the molten Cu drops.     

Strength of Hot Isostatically Pressed Si3N4 After Heat Treatment in Ar-O2 and H2-H2O

The effects of heat treatment in Ar-O₂ and H₂-H₂O atmospheres on the flexural strength of hot isostatically pressed Si₃N₄ were investigated. Increases in room-temperature strength, to values significantly above that of the aspolished material, were observed when the Si₃N₄ was exposed at 1400°C to (1) H₂ with water vapor pressure ( P _H2O) greater than 1 × 10⁻⁴ MPa or (2) Ar with oxygen partial pressure ( P _O2) of between 7 × 10⁻⁶ and 1.5 × 10⁻⁵ MPa. However, the strength of the material was degraded when the P _H2O in H₂ was lower than 1 × 10⁻⁴ MPa, and essentially unaffected when the P _O2 in Ar was higher than 1.5 × 10⁻⁵ MPa. We suggest that the observed strength increases are the result of strength-limiting surface flaws being healed by a Y₂Si₂O₇ layer formed during exposure.     

Superplastic Alumina Ceramics with Grain Growth Inhibitors

Superplastic deformation of alumina ceramics was studied at 1400° to 1450°C and at a strain rate of 4 × 10⁻⁵ to 5 × 10⁻⁴ s⁻¹. MgO and ZrO₂ were introduced to suppress dynamic grain growth. The latter was especially effective; grain growth was minimal in 10-vol%-ZrO₂-containing material. Both materials were superplastically stretched under biaxial tension to 100% engineering strain with good surface finishing, demonstrating the feasibility of superplastic forming for alumina ceramics.     

Effect of Oxygen Partial Pressure on the Dielectric Properties and Microstructures of Cofired Base-Metal-Electrode Multilayer Ceramic Capacitors

The dielectric properties, dopant distributions, and microstructures of BaTiO₃-based multilayer ceramic capacitors (MLCCs) sintered in H₂–N₂–H₂O atmospheres with     =10^−7.5 Pa (BMX-7.5) and     =10^−9.5 Pa (BMX-9.5) were studied, and the effects of oxygen partial pressures were analyzed. Dielectric measurements showed that BMX-7.5 had a lower dielectric constant at temperatures above 20°C, but a higher dielectric constant at temperatures below 10°C when compared with BMX-9.5. The coexistence of core–shell and core grains was observed in bright field (BF) transmission electron microscopy images in both types of capacitors. Triple-point and grain boundary phases were observed more frequently in BMX-9.5 than in BMX-7.5, and energy-dispersive X-ray spectrometer point-by-point analysis revealed that these second phases contained high concentrations of dopants such as Si, Y, and Ca. The dopant concentration in the shell regions in BMX-7.5 was higher than that in similar regions in BMX-9.5. Smeared and twisted grain boundaries with fringes observed in both types of MLCCs indicated that the shell regions in both samples were formed either by diffusion of foreign ions into BaTiO₃ or by crystallization of grain boundary and triple-point liquid phases. It was deduced that the partial pressure of oxygen in the sintering atmosphere influenced the microstructures, dopant distributions, and core–shell ratios of the grains in these materials.