Showing papers in "Journal of the American Ceramic Society in 1996"
TL;DR: In this paper, a polycrystalline bulk sample of Ti sub 3, SiC sub 2 was fabricated by reactively hot-pressing Ti, graphite, and SiC powders at 40 MPa and 1,600 C for 4 h.
Abstract: Polycrystalline bulk samples of Ti{sub 3}SiC{sub 2} were fabricated by reactively hot-pressing Ti, graphite, and SiC powders at 40 MPa and 1,600 C for 4 h. This compound has remarkable properties. Its compressive strength, measured at room temperature, was 600 MPa, and dropped to 260 MPa at 1,300 C in air. Although the room-temperature failure was brittle, the high-temperature load-displacement curve shows significant plastic behavior. The oxidation is parabolic and at 1,000 and 1,400 C the parabolic rate constants were, respectively, 2 {times} 10{sup {minus}8} and 2 {times} 10{sup {minus}5} kg{sup 2}{center_dot}m{sup {minus}4}{center_dot}s{sup {minus}1}. The activation energy for oxidation is thus {approx}300 kJ/mol. The room-temperature electrical conductivity is 4.5 {times} 10{sup 6} {Omega}{sup {minus}1}{center_dot}m{sup {minus}1}, roughly twice that of pure Ti. The thermal expansion coefficient in the temperature range 25 to 1,000 C, the room-temperature thermal conductivity, and the heat capacity are respectively, 10 {times} 10{sup {minus}6} C{sup {minus}1}, 43 W/(m{center_dot}K), and 588 J/(kg{center_dot}K). With a hardness of 4 GPa and a Young`s modulus of 320 GPa, it is relatively soft, but reasonably stiff. Furthermore, Ti{sub 3}SiC{sub 2} does not appear to be susceptible to thermal shock; quenching from 1,400 C into water does not affect the postquench bend strength.more » As significantly, this compound is as readily machinable as graphite. Scanning electron microscopy of polished and fractured surfaces leaves little doubt as to its layered nature.« less
1,491 citations
TL;DR: In this article, a method of determining the Hamaker constant of suspended particles is developed by modeling the relationship between the particle inter-action energy and the suspension stability, and a three-probe dc technique is used to map the voltage profile around the depositing electrode and the results are used to explain discrepancies between the calculated and experimentally observed voltage drops during deposition.
Abstract: The mechanisms of electrophoretic deposition (EPD) are discussed and their shortcomings identified. The kinetics of the processes involved are analyzed for constant-current and constant-voltage conditions. A method of determining the Hamaker constant of suspended particles is developed by modeling the relationship between the particle inter-action energy and the suspension stability. A three-probe dc technique is used to map the voltage profile around the depositing electrode, and the results are used to explain discrepancies between the calculated and experimentally observed voltage drops during deposition. A mechanism of deposition is proposed based on DLVO theory and particle double-layer distortion/thinning on application of a dc field to the suspension. Kinetic equations are developed for constant-current and constant-voltage EPD using mass balance conditions; these are verified by experiments. After the phenomenon is introduced and discussed, a critique of the application of EPD to the synthesis of ceramic shapes and coatings is given.
1,162 citations
TL;DR: The present study shows that even pure titanium metal and its alloys can bond to living bone, if their surfaces are pre-treated with alkali hydroxide solutions.
Abstract: Generally, artificial materials implanted into bone defects are encapsulated by a fibrous tissue isolating them from the surrounding bone. Only limited kinds of ceramics are known to bond to living bone without forming the fibrous tissue, and already they are being used clinically as important artificial bones. However, they cannot be used under highly loaded conditions, since their fracture toughnesses are not so high as that of human cortical bone. The present study shows that even pure titanium metal and its alloys can bond to living bone, if their surfaces are pre-treated with alkali hydroxide solutions. Thus-treated metals are believed to be useful as artificial bones even under highly loaded conditions because of their high bone-bonding ability as well as high fracture toughness.
669 citations
TL;DR: In this article, the control of grain boundary segregation through purity, microstructure, and thermal history is discussed from the objective of engineering the grain-boundary impedance of polycrystalline ionic conductors.
Abstract: Solute segregation at grain boundaries has been correlated with grain-boundary conductivity in high-purity 15-mol%-CaO-stabilized ZrO2. STEM measurements of solute coverage show that the segregation of impurity silicon (present at bulk levels 103 at 500°C. At the lowest levels of segregation achieved, <0.1 monolayer, σspgb remains ∼102 less, and possibly represents an “intrinsic” limiting value for the grain boundary. Comparison with Y2O3-doped ZrO2 suggests similar behavior in this system. The control of grain-boundary segregation through purity, microstructure, and thermal history is discussed from the objective of engineering the grain-boundary impedance of polycrystalline ionic conductors.
415 citations
TL;DR: In this article, the concept of a master sintering curve has been developed that characterizes the Sintering behavior for a given powder and green-body process regardless of the heating profiles.
Abstract: One of the ultimate objectives for sintering studies is to be able to predict densification results under different thermal histories for a given processing method. It has been reported that the geometric parameters related to sintering often are functions only of density for a given powder and green-body process, provided that one diffusion mechanism dominates in the sintering process. Based on this report, the concept of a master sintering curve has been developed that characterizes the sintering behavior for a given powder and green-body process regardless of the heating profiles. The formulation and construction of the master sintering curve are given in this paper. A model experiment on sintering of alumina is used and analyzed to demonstrate this new concept. Examples of the master sintering curves obtained from other powder systems (ZnO, nickel, Al 2 O 3 (5 vol% TiO 2 ), and Al 2 O 3 (5 vol% ZrO 2 )) are presented. When this new method is used, densification behavior can be predicted under arbitrary temperature-time excursions following a minimal set of preliminary experiments, and these predictions can be used in planning sintering strategies. Moreover, deviations from the assumption of a single mechanism can be observed readily.
405 citations
TL;DR: In this paper, the Johnson-Mehl-Avrami equation was used to evaluate reaction mechanisms for the hydrothermal synthesis of barium titanate and showed that dissolution-precipitation was the dominant reaction mechanism in the early stages of the process.
Abstract: Reaction mechanisms for the hydrothermal synthesis of barium titanate are evaluated. Feedstocks of barium hydroxide octahydrate and anatase titania are reacted for varying durations (1–72 h) to provide intermediate-stage samples for characterization by transmission electron microscopy/ energy-dispersive spectroscopy (TEM/EDS), X-ray diffractometry (XRD), and inductively coupled plasma spectroscopy (ICP). Quantitative evaluation of the extent of reaction by ICP and XRD methods permits the analysis of data with the Johnson-Mehl-Avrami equation. This analysis reveals two reaction-rate regimes. Kinetic analysis, based on reaction progress, yields insight into the first reaction-rate regime but is inconclusive in the analysis of the second reaction-rate regime. In the first regime, at the early stage of barium titanate formation, a dissolution-precipitation mechanism dominates. In contrast, in the second regime, at longer reaction times, an in-situ transformation mechanism is probably dominant. However, multiple reaction mechanisms (e.g., in-situ transformation and dissolution-precipitation) may be competing for rate control. Alternatively, dissolution-precipitation may be the dominant mechanism throughout the barium titanate synthesis, with nucleation and growth controlling the first regime and dissolution rate controlling the second regime.
351 citations
TL;DR: In this paper, the effect of citric acid on the viscosity of alumina is investigated using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory.
Abstract: The interaction between citric acid and alumina in aqueous solution is characterized. Adsorption isotherms of the dispersant on the alumina surface, electrophoretic mobility of the alumina particles as a function of the citric acid concentration, and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy of the citratealumina surface complex have been used. The adsorption behavior of citric acid is dependent on the pH of the suspension and the concentration of the citric acid. The maximum amount of citric acid adsorbed on the alumina surface, 2.17 μ.mol/m2 at pH 3, decreases to 1.17 μmol/m2 at pH 8. The adsorption of citrate causes a highly negatively charged powder surface and a shift of the isoelectric point (IEP) to lower pH values. The IEP of alumina can be fixed at any pH value between 9 and 3 by proper adjustment of the citric acid concentration. In situ ATR-FTIR spectroscopy of the citrate-alumina surface complex gives evidence for a direct interaction between the carboxylate groups of the citrate and the surface aluminum(III) atoms. The rheological properties of alumina suspensions are studied as a function of the citric acid concentration. The data obtained from the viscosity and dynamic electrophoretic measurements correlate well and allow the construction of a stability map of alumina suspensions stabilized with citric acid. The influence of citric acid on the viscosity is discussed using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The interaction potential between the particles is determined by the citrate adsorbed on the surface, leading to a negative particle charge, and the citrate anions remaining in the solution, resulting in an increase of the ionic strength. The adsorption of citric acid also creates a steric barrier that inhibits the complete mutual approach of the individual alumina particles.
348 citations
TL;DR: In this paper, preliminary leaching of rice husks with a solution of hydrochloric acid before their combustion at 600°C is shown to be required to obtain relatively pure silica (∼99.5%) with a high specific surface area that is maintained even after heating at 800°C.
Abstract: Preliminary leaching of rice husks with a solution of hydrochloric acid before their combustion at 600°C is shown to be required to obtain relatively pure silica (∼99.5%) with a high specific surface area (∼260 m2/g) that is maintained even after heating at 800°C. Transmission electron microscopy observations indicate that this material has a homogeneous size distribution of nanometric particles. However, if the leaching with HCl is performed on the white ashes obtained by combustion of the rice husks at 600°C, an amorphous silica with the same purity also is obtained, but its specific surface area decreases to 1 m2/g. This behavior is due to a strong interaction between the silica and the potassium contained in the rice husks, which leads to a dramatic decrease of the specific surface area if K+ cations are not removed prior to the heat treatment at 600°C. This finding leads to a better understanding of the effect of potassium on the morphology of silica.
312 citations
TL;DR: In this article, the effects of microstructure, atmosphere, and several dopants on the electrical conductivity of bismuth titanate (Bi4Ti3O12, BIT) were investigated.
Abstract: A study was conducted on the effects of microstructure, atmosphere, and several dopants on the electrical conductivity of bismuth titanate (Bi4Ti3O12, BIT), Increased grain size increased the conductivity in undoped BIT as did acceptor dopants that substituted for either Bi (Ca and Sr) or Ti (Fe), A donor dopant (Nb) decreased the conductivity in BIT by as much as 3 orders of magnitude, The increased resistivity of Nb-doped BIT improved the polarization in an electric field, A piezoelectric coefficient, d(33), of 20.0 pC/N was achieved with a Nb-doped BIT composition corresponding to Bi4Ti2.86Nb0.14O12.
281 citations
TL;DR: Using electron paramagnetic resonance, the motion of oxygen vacancies within the oxygen octahedron in perovskite BaTiO{sub 3} is observed via the alignment of oxygen vacancy-related defect dipoles induced by bias/heat combinations.
Abstract: Using electron paramagnetic resonance, the motion of oxygen vacancies within the oxygen octahedron in perovskite BaTiO{sub 3} is observed via the alignment of oxygen vacancy-related defect dipoles induced by bias/heat combinations. The vacancy motion is found to have an activation energy of 0.91 eV, in excellent agreement with the predicted. It is found that the onset of resistance degradation is also concurrent with oxygen vacancy motion. This result spectroscopically demonstrates that oxygen vacancy migration in the lattice is likely responsible for the observed degradation.
272 citations
TL;DR: In this paper, a novel ceramic synthesis technique, combustion synthesis, was explored to produce red-emitting Cr{sup 3+}-doped Y{sub 3}Al{sub 5}O{sub 12} and Eu{sub 2}Oµsub 3µ phosphors with improved physical and luminescent properties.
Abstract: A novel ceramic synthesis technique, combustion synthesis, was explored to produce red-emitting Cr{sup 3+}-doped Y{sub 3}Al{sub 5}O{sub 12} and Eu{sup 3+}-doped Y{sub 2}O{sub 3} phosphors with improved physical and luminescent properties. This technique involves the reaction of metal nitrates (oxidizers) and an organic fuel (urea, carbohydrazide, glycine) at 500 C. Resulting powders are well-crystallized, with a large surface area and small particle size. The spectral energy distribution was observed using photoluminescence measurements. The effects of processing parameters such as type of fuel, fuel-to-oxidizer ratio, furnace temperature, and batch water content were studied. An increase in phosphor brightness with increasing reaction temperature was observed. Postreaction heat treatments (1000, 1300, and 1600 C) increased the luminous intensity of as-synthesized powders. Residual carbon content and chromium site symmetry were investigated as possible explanations for the increase in brightness with increasing heat treatment temperature. By tailoring the reaction chemistry, the optimal conditions for producing the most luminescent phosphors have been identified.
TL;DR: In this paper, the electrical properties of ceramic BaTiO3 were investigated by ac impedance spectroscopy over the ranges 25°-330°C and 0.03 Hz-1 MHz.
Abstract: The electrical properties of ceramic BaTiO3 were investigated by ac impedance spectroscopy over the ranges 25°-330°C and 0.03 Hz-1 MHz. Results are compared with those obtained from fixed-frequency measurements, at 1 kHz and 100 kHz. Fixed-frequency Curie-Weiss plots show deviations from linearity at temperatures well above tc. The ac measurements show that grain boundary impedances influence Curie-Weiss plots in two ways: at high temperatures, they increasingly dominate the fixed-frequency permittivities; at lower temperatures, closer to Tc, the high-frequency permittivity contains a contribution from grain boundary effects. Methods for extraction of bulk and grain boundary capacitances from permittivity and electric modulus complex plane plots are discussed. The importance of selecting the appropriate equivalent circuit to model the impedance response is stressed. A constriction impedance model for the grain boundary in BaTiO3 ceramics is proposed: the grain boundary capacitance is neither temperature-independent, nor shows Curie-Weiss behavior. The grain boundary is ferroelectric, similar to the grains, but its impedance is modified by either air gaps or high-impedance electrical inhomogeneity in the region of the necks between grains; the activation energy of the constriction grain boundary impedance differs from that of the bulk, suggesting differences in defect states or impurity levels.
TL;DR: In this paper, the effects of the dopants, Mg 2+, Ca 2+, Sr 2+, Sc 3+, Yb 3+, Y 3+, Gd 3+, La 3+, Ti 4+, Zr 4+, and Nb 5+, on the grain boundary mobility of dense CeO 2 have been investigated from 1270° to 1420°C.
Abstract: The effects of the dopants, Mg 2+ , Ca 2+ , Sr 2+ , Sc 3+ , Yb 3+ , Y 3+ , Gd 3+ , La 3+ , Ti 4+ , Zr 4+ , and Nb 5+ , on the grain boundary mobility of dense CeO 2 have been investigated from 1270° to 1420°C. Parabolic grain growth has been observed in all instances. Together with atmospheric effects, the results support the mechanism of cation interstitial transport being the rate-limiting step. A strong solute drag effect has been demonstrated for diffusion-enhancing dopants such as Mg 2+ and Ca 2+ , which, at high concentrations, can nevertheless suppress grain boundary mobility. Severely undersized dopants (Mg, Sc, Ti, and Nb) have a tendency to markedly enhance grain boundary mobility, probably due to the large distortion of the surrounding lattice that apparently facilitates defect migration. Overall, the most effective grain growth inhibitor at 1.0% doping is Y 3+ , while the most potent grain growth promoter is either Mg 2+ (e.g., 0.1%) or Sc 3+ at high concentration (greater than 1.0%).
TL;DR: In this paper, homologous compounds of (ZnO){sub m}In{sub 2}O{sub 3} (m = integer) with layered structures were synthesized by reaction-sintering a mixed powder of ZnO and In {sub 2]O{Sub 3} at 1,823 K for 2 h in air, and their thermoelectric properties were measured at 500 to 1,100 K.
Abstract: Homologous compounds of (ZnO){sub m}In{sub 2}O{sub 3} (m = integer) with layered structures were synthesized by reaction-sintering a mixed powder of ZnO and In{sub 2}O{sub 3} at 1,823 K for 2 h in air, and their thermoelectric properties, i.e., electrical conductivity, Seebeck coefficient, and thermal conductivity, were measured at 500 to 1,100 K. Their thermoelectric figure of merit depended on the composition, and an optimum value of m apparently existed giving the largest figure of merit.
TL;DR: In this paper, the microstructures, secondary phases, and grain boundaries were characterized using a range of analytical techniques including TEM, SEM, AES, and XRD, and the fracture toughness was derived either from bend tests of beam-shaped samples with a controlled surface flaw or from standard disk-shaped compact-tension specimens precracked in cyclic fatigue.
Abstract: “In situ toughened” silicon carbides, containing Al, B, and C additives, were prepared by hot pressing. Densification, phase transformations, and microstructural development were described. The microstructures, secondary phases, and grain boundaries were characterized using a range of analytical techniques including TEM, SEM, AES, and XRD. The modulus of rupture was determined from fourpoint bend tests, while the fracture toughness was derived either from bend tests of beam-shaped samples with a controlled surface flaw, or from standard disk-shaped compact-tension specimens precracked in cyclic fatigue. The R-curve behavior of an in situ toughened SiC was also examined. A steady-state toughness over 9 MPa·m1/2 was recorded for the silicon carbide prepared with minimal additives under optimum processing conditions. This increase in fracture toughness, more than a factor of three compared to that of a commercial SiC, was achieved while maintaining a bend strength of 650 MPa. The mechanical properties were found to be related to a microstructure in which platelike grain development had been promoted and where crack bridging by intact grains was a principal source of toughening.
TL;DR: In this article, the authors used viscous sintering of the scale to close pores that formed during the initial transient oxidation period, due to volatilization of molybdenum oxide.
Abstract: Mo{sub 5}Si{sub 3} shows promise as a high-temperature creep-resistant material. The high-temperature oxidation resistance of Mo{sub 5}Si{sub 3} has been found to be poor, however, limiting its use in oxidizing atmospheres. Undoped Mo{sub 5}Si{sub 3} exhibits pest oxidation at 800 C. Mass loss occurs in the temperature range 900--1,200 C due to volatilization of molybdenum oxide, indicating that the silica scale that forms does not provide a passivating layer. The addition of boron results in protective scale formation and parabolic oxidation kinetics in the temperature range of 1,050--1,300 C. The oxidation rate of Mo{sub 5}Si{sub 3} was decreased by 5 orders of magnitude at 1,200 C by doping with less than 2 wt% boron. Boron doping eliminates catastrophic pest oxidation at 800 C. The mechanism for improved oxidation resistance of boron-doped Mo{sub 5}Si{sub 3} is viscous sintering of the scale to close pores that form during the initial transient oxidation period, due to volatilization of molybdenum oxide.
TL;DR: In this article, the concept of creating functionally graded metal-ceramic composite microstructures for thermal barrier coatings used in gas-turbine applications was explored, which offers the possibility of significantly improving the life and reliability of thermal barrier coating.
Abstract: This feature article explores the concept of creating functionally graded metal-ceramic composite microstructures for thermal barrier coatings used in gas-turbine applications. From a thermomechanical perspective, this concept offers the possibility of significantly improving the life and reliability of thermal barrier coatings. However, prior research reveals that progress has been somewhat limited because of the oxidative instability exhibited by some metal-ceramic composite microstructures. The present study addresses some of the materials criteria and research issues associated with preparing chemically stable, yet mechanically durable, graded metal-ceramic microstructures for realistic application environments.
TL;DR: In this paper, the effects of the dopants, Mg2+, Sr2+, Sc3+, Yb3+, Gd3+, La3+, Ti4+, Zr4+, Ce4+, and Nb5+, on the grain boundary mobility of dense Y2O3 have been investigated from 1500° to 1650°C.
Abstract: The effects of the dopants, Mg2+, Sr2+, Sc3+, Yb3+, Gd3+, La3+, Ti4+, Zr4+, Ce4+, and Nb5+, on the grain boundary mobility of dense Y2O3 have been investigated from 1500° to 1650°C. Parabolic grain growth has been observed in all cases over a grain size from 0.31 to 12.5 μm. Together with atmospheric effects, the results suggest that interstitial transport is the rate-limiting step for diffusive processes in Y2O3, which is also the case in CeO2. The effect of solute drag cannot be ascertained but the anomalous effect of undersized dopants (Ti and Nb) on diffusion enhancement, previously reported in CeO2, is again confirmed. Indications of very large binding energies between aliovalent dopants and oxygen defects are also observed. Overall, the most effective grain growth inhibitor is Zr4+, while the most potent grain growth promoter is Sr2+, both at 1.0% concentration.
TL;DR: In this paper, the effect of heat treatment on the grain size, phase assemblage, and mechanical properties of a 3 mol% Y-TZP ceramic was investigated.
Abstract: The effect of heat treatment on the grain size, phase assemblage, and mechanical properties of a 3 mol% Y-TZP ceramic was investigated. Specimens were initially sintered for 2 h at 1450 C to near theoretical density; some specimens were then heat-treated at 1550, 1650, 1750, or 1850 C to coarsen the microstructure. The average grain size increased with heat treatment from 1750 C. The maximum fraction of tetragonal phase that transformed during fracture corresponded with the largest tetragonal grain size of {approximately}5--6 {micro}m. Strength was on the order of 1 GPa, and was surprisingly insensitive to heat-treatment temperature and grain size, contrary to previous studies. The fracture toughness increased from 4 to 10 MPa{center_dot}m{sup 1/2} with increasing grain size, owing to an increasing transformation zone size. Grain sizes larger than 5--6 {micro}m spontaneously transformed to monoclinic phase during cooling. Such critical grain sizes are much larger than those found in past investigations, and may be due to the greater fraction of cubic phase present which decreases the strain energy arising from more » crystallographic thermal expansion anisotropy of the tetragonal phase. « less
Abstract: Sr- and Mg-doped LaGaO{sub 3} powders were prepared from a salt acetate solution. The stable solution was peptized by reacting ammonium hydroxide with the precursor solution. Thermal analysis (DTA/TGA) was used to characterize first the dehydration and then the thermal decomposition of the organic ligands of the dried gel. The transformation from amorphous powders into a crystallized, homogeneous oxide phase corresponds to two endothermic peaks in the DTA curve; the first one at 150 C is related to the removal of water and is followed by a shoulder at 250 C. The second peak at 300 C corresponds to a superposition of two decomposition reactions: acetate salt into its oxycarbonate and this oxycarbonate into its oxide. Two subsequent exothermic peaks correspond to oxidation of evolved gases such as methane, hydrogen, and carbon monoxide. TEM observations show an average 10 nm particle size of the LaGaO{sub 3} powder after annealing at 600 C. X-ray diffraction patterns indicate a pure primitive-cubic phase is formed by 1,300 C without formation of any SrLaGaO{sub 3} impurity. The impedance spectroscopy on a 93%-dense sample exhibits no grain-boundary contribution and an ac conductivity {sigma} = 0.11 {Omega}{sup {minus}1} {center_dot} cm{sup {minus}1} at 800 C.
TL;DR: In this paper, the elastic modulus (E), flexural strength (MOR), and Vickers hardness (H v ) increase with the amount of carbon in the oxycarbide glasses reaching the maximum values (E≃115 GPa, MOR≃550 MPa, and H v ≃9 GPa).
Abstract: Silicon oxycarbide glasses have been fabricated, in the shape of thin rods suitable for flexural test experiments, by pyrolysis in an inert atmosphere at 1000 and 1200°C of sol-gel precursors containing Si-CH 3 and Si-H bonds The amount of carbon in the silicon oxycarbide network has been controlled by varying the carbon load in the precursor gel Density and surface area analysis revealed that all of the samples pyrolyzed at 1200°C were well-densified silicon oxycarbide glasses whereas for the glasses treated at 1000°C, compositions with low carbon loads showed the presence of a residual fine porous phase The elastic modulus (E), flexural strength (MOR), and Vickers hardness (H v ) increase markedly with the amount of carbon in the oxycarbide glasses reaching the maximum values (E≃115 GPa, MOR≃550 MPa, and H v ≃9 GPa) for samples with the highest carbon content The experimental elastic modulus values of the silicon oxycarbide glasses compare well with the theoretical estimations obtained using the Voigt and Reuss models assuming the disordered network formed by the corresponding thermodynamic compositions
TL;DR: An electrophoretic deposition (EPD) method was applied for the preparation of yttria-stabilized zirconia (YSZ) films for solid oxide fuel cell (SOFC) applications as discussed by the authors.
Abstract: An electrophoretic deposition (EPD) method was applied for the preparation of yttria-stabilized zirconia (YSZ) films for solid oxide fuel cell (SOFC) applications. Dense YSZ films with uniform thickness can be readily prepared with the EPD method by using acetylacetone or acetone as a solvent. The open-circuit voltages of SOFC, for which the YSZ films were prepared by the EPD method, increased with increasing repetitions of deposition and sintering. It was found that the open-circuit voltage exceeded 1.0 V after five repetitions. When the planar SOFC was fabricated using La{sub 0.6}Sr{sub 0.4}MnO{sub 3} as a cathode, and electroless plating Pt as an anode, the open-circuit voltage and the maximum power density attained were 1.03 V and 1.84 W{center_dot}cm{sup {minus}2}, respectively. Consequently, it became evident that the electrophoretic deposition was a suitable processing route for the formation of gas-tight YSZ films with thickness less than 10 {micro}m.
TL;DR: Barium strontium titanate, (Ba x,Sr 1-x )TiO 3, thin films of various compositions were prepared by a sol-gel method as mentioned in this paper.
Abstract: Barium strontium titanate, (Ba x ,Sr 1-x )TiO 3 , thin films of various compositions were prepared by a sol-gel method. Solutions consisting of acetate powders and titanium IV isopropoxide in a mixture of acetic acid and ethylene glycol were spin-coated onto silicon and platinum-coated silicon substrates. Processing parameters were optimized to develop stable solutions which yielded films with relatively low crystallization temperatures. It was determined that ethylene glycol was a necessary component of the solution to increase stability to precipitation and to decrease the crystallization temperature of the films. The grain size of the films varied with annealing temperature and atmosphere and directly affected the dielectric properties. A dielectric constant of 400 and a dissipation factor of 0.04 were measured at 1 kHz for (Ba 0.8 Sr 0.2 )TiO 3 films heated to 700°C for 1 h with a thickness of approximately 400 nm. Films of this composition maintained low leakage current densities for extended time periods when measured at an applied field of 75 kV/cm.
TL;DR: In this article, a CMC that exhibits damage-tolerant behavior without weak interactions has been demonstrated, which relies on the heterogeneous distribution of fiber bundles within a porous matrix having homogeneous, fine porosity.
Abstract: A ceramic-matrix composite (CMC) that exhibits damage-tolerant behavior without weak interactions has been demonstrated. The concept relies on the heterogeneous distribution of fiber bundles within a porous matrix having homogeneous, fine porosity. It also depends on the development of residual stress from thermal expansion mismatch. The present demonstration uses Al{sub 2}O{sub 3} fibers with either a silicon nitride or a mullite matrix. The latter is inherently oxidation resistant.
TL;DR: In this article, a flame spray pyrolysis of alcoholic solutions of a double alkoxide precursor was used to produce ultrafine crystalline spinel powder with particle formation by rapid oxidation of the organic ligands followed by nucleation and growth from oxide species.
Abstract: Ultrafine crystalline spinel powder has been prepared using flame spray pyrolysis of alcoholic solutions of a novel double alkoxide precursor. The particles produced are spherical, dense, single crystals with diameters of 10--100 nm and specific surface areas ranging from 40 to 60 m{sup 2}/g. Powder production rates of 50--100 g/h are achieved using a bench-top apparatus. Particle formation appears to occur by rapid oxidation of the organic ligands followed by nucleation and growth from oxide species.
TL;DR: In this paper, the synthesis of high-purity, single-phase gallium nitride powder has been achieved in a hot-wall tube furnace via the reaction of gallium with ammonia (NH{sub 3}) and the conversion of Ga{sub 2}O(sub 3}.
Abstract: Synthesis of high-purity, single-phase gallium nitride powder has been achieved in a hot-wall tube furnace via (i) the reaction of gallium with ammonia (NH{sub 3}) and (ii) the conversion of gallium oxide (Ga{sub 2}O{sub 3}). For complete reaction, the optimum temperatures, NH{sub 3} flow rates, and boat positions relative to the NH{sub 3} inlet were 975 C, 400 standard cubic centimeters per minute (sccm), and 50 cm, respectively, for the gallium-NH{sub 3} reaction, and 1,050 C, 500 sccm, and 50 cm, respectively, for the Ga{sub 2}O{sub 3} conversion. Polyhedra of various shapes were obtained from both processes; some rod-shaped crystals also were observed in the material derived from Ga{sub 2}O{sub 3}.
TL;DR: In this paper, heat generation in various types of multilayer PZT-based actuators was studied and it was shown that heat generation is mainly caused by ferroelectric hysteresis loss in the stress-free state.
Abstract: Multilayer piezoelectric actuators when driven under high frequency, generate significant heat, which influences the reliability and other piezoelectric properties. In this paper, heat generation in various types of multilayer PZT-based actuators was studied. Experimental results showed that heat generation is mainly caused by ferroelectric hysteresis loss in the stress-free state. A simplified analytic method was established to evaluate the temperature rise, which is useful for the design of multilayer and other high-power actuators.
TL;DR: The relationship between ferroelectric domain growth and the size of crystallites in ultrafine BaTiO 3 powders has been investigated in this paper, where the authors showed that the presence of Ferroelectric domains depends on the constraining force introduced by hard agglomeration of the crystallites.
Abstract: The relationship between ferroelectric domain growth and the size of crystallites in ultrafine BaTiO 3 powders has been investigated. It was observed that the critical size for the c → t-BaTiO 3 phase transformation was about 30 nm and that BaTiO 3 crystallites probably transformed from single-domain to multidomain when the particle size was larger than 100 nm. X-ray diffraction patterns of BaTiO 3 /PVDF composites before and after poling indicated that the degree of poling efficiency for composites with fine BaTiO 3 powders is much smaller than for coarser powders larger than 100 nm. The present work indicates that the presence of ferroelectric domains in BaTiO 3 powder depends on the constraining force introduced by hard agglomeration of the crystallites. The particles obtained by calcining at a higher temperature (larger than 100 nm and partially sintered), composed of several crystallites, may behave similar to grains in fine-grained BaTiO 3 ceramics where ferroelectric domains are generated due to mechanical clamping. On the other hand, particles of small size composed of single crystallite and without clamping force, may deform similar to a lattice cell and avoid formation of twins.
TL;DR: In this article, a universal pore size distribution, normalized by particle size, has been determined and found to be a function of density only, with or without densification, signifying homogenization at all stages.
Abstract: Microstructural evolution during sintering has been investigated using fine powders of CeO2 and Y2O3 with excellent sinterability. A universal pore size distribution, normalized by particle size, has been determined and found to be a function of density only. Microstructure evolves toward the universal distribution, with or without densification, signifying homogenization at all stages. This may even involve the elimination of supercritical pores, at low densities, which are otherwise thermodynamically not sinterable. Theoretical justification for these observations is made by using a network model with a random, but spatially homogeneous, distribution of spherical particles. Final microstructure after full density is reached is also found to evolve toward a universal steady state of grain shape/grain size distribution regardless of initial state.
TL;DR: In this paper, the authors show that reactive metal penetration produces a fine-grained alumina skeleton with an interspersed metal phase with a Young's modulus and hardness similar to that of Al2O3.
Abstract: Ceramic-metal composites have been made to near net-shape by reactive penetration of dense ceramic preforms by molten Al. In contrast to processes involving physical infiltration of a porous medium, this process works with dense ceramic preforms. Ceramic-metal composite formation by reactive metal penetration is driven by a strongly negative Gibbs energy for reaction. For Al, the general form of the reaction is (x + 2)Al + (3/y)MOy, → Al2O3+ M3Alx, where MOy, is an oxide that is wet by molten Al. In low Po2 atmospheres and at temperatures above about 900°C, molten Al reduces mullite to produce Al2O3 and silicon. The Al/mullite reaction has a ΔGr°(1200 K) of −1014 kJ/mol and, if the mullite is fully dense, the theoretical volume change on reaction is less than 1%. Experiments with commercial mullite containing a silicate grain boundary phase average less than 2% volume change on reaction. In the Al/mullite system, reactive metal penetration produces a fine-grained alumina skeleton with an interspersed metal phase. With enough excess aluminum, mutually interpenetrating ceramic-metal composites are produced. Properties measurements show that ceramic-metal composites produced by reactive metal penetration of mullite by Al have a Young's modulus and hardness similar to that of Al2O3, with improved fracture toughness ranging from 5 to 9 Mpa·m1/2. For penetration times less than 1 h, reaction layer thickness varies as the square root of time, which allows ceramic-metal composite coatings to be fabricated by controlling the penetration time. Thermodynamic calculations indicate that other compositions also are candidates for in situ reaction synthesis, which suggests that reactive metal penetration may be a general route to composite synthesis with the prospect for near net-shape processing.