bluuebunny/crossref_metadata_2025 huggingface dataset:
New High‐Permittivity AgNb1‐xTaxO3 Microwave Ceramics: Part I, Crystal Structures and Phase‐Decomposition Relations
Authors: Valant Matjaz , Suvorov Danilo , Meden Anton
Published: July 2010

The crystal structure of monophasic AgNb1/2Ta1/2O3 was investigated via Rietveld refinement using powder X‐ray diffractometry (XRD) data. The study revealed a monoclinic unit cell of the P2/m space group with the following unit‐cell parameters: a= 3.9286(3) Å, b= 3.9259(2) Å, c= 3.9302(3) Å, and β= 90.49(1)°. High‐temperature XRD studies of the decomposition of AgNb1/2Ta1/2O3 showed that the kinetics of the decomposition are influenced by the volatilization of silver. Suppression of silver volatilization by enclosing the sample in a corundum tube and applying an oxygen atmosphere yielded sintered, almost‐monophasic AgNb1/2Ta1/2O3 ceramics with >97% of the theoretical density. The Ag8(Nb,Ta)26O69 phase, which regularly appeared as a result of partial decomposition of Ag(Nb1−xTax)O3, was studied by energy‐dispersive spectroscopy analysis and XRD. The Ag8(Nb,Ta)26O69 phase exhibited a bronzelike orthorhombic structure with the following unit‐cell parameters: a= 37.116(5) Å, b= 12.432(3) Å, and c= 7.826(2) Å. Indexed powder diffraction data for Ag8(Nb,Ta)26O69 have been reported.
New High‐Permittivity AgNb1‐xTaxO3 Microwave Ceramics: Part II, Dielectric Characteristics
Authors: Valant Matjaz , Suvorov Danilo
Published: July 2010

Ceramic samples with the AgNb1−xTaxO3 (0.46 < x < 0.54) composition were synthesized and analyzed, with respect to their dielectric properties, in a microwave‐frequency range and at radio frequencies. The permittivity over the compositional range 0.46 lessthan equal to x lessthan equal to 0.52 was >410, whereas the Q×f value, measured at 2 GHz, reaches values of 640–900 GHz. Dependence of the resonant frequency on temperature was not linear but did display a minimum at a particular temperature, which was dependent on the composition of the ceramic. Samples that were measured at radio frequencies displayed low dissipation factors (tgδ= 7 ×10−4−18 × 10−4), a low aging rate (0.2% per time decade), and good dielectric strength (8 V/μm at a thickness of 0.6 mm). Microstructural investigations revealed the presence of precipitates and transformation twins that should be eliminated during additional processing optimization.
Creep and Fatigue Behavior in Hi‐Nicalon‐Fiber‐Reinforced Silicon Carbide Composites at High Temperatures
Authors: Zhu Shijie , Mizuno Mineo , Kagawa Yutaka , Cao Jianwu , Nagano Yasuo , Kaya Hiroshi
Published: July 2010

Monotonic tension, creep, and fatigue tests of a composite where a silicon carbide (SiC) matrix that contains glass‐forming, boron‐based particulates has been reinforced with Hi‐Nicalon™ fiber (Hi‐Nicalon™/SiC) were conducted in air at 1300°C, and creep tests also were conducted in argon at 1300°C. The ultimate tensile strength (UTS) of the Hi‐Nicalon™/SiC composite was similar to that of a SiC composite where a pure SiC matrix is reinforced with Nicalon fiber (standard SiC/SiC) and a SiC composite where a matrix of glass‐forming, boron‐based particulates is reinforced with Nicalon™ fiber (enhanced SiC/SiC); however, the strains at UTS of the Hi‐Nicalon™/SiC composite and the enhanced SiC/SiC composite were much larger than that of the standard SiC/SiC composite. The Young's modulus of the Hi‐Nicalon™/SiC composite was ∼140 GPa, which is higher than that of the enhanced SiC/SiC composite (90 GPa) and lower than that of the standard SiC/SiC composite (200 GPa) at a temperature of 1300°C. The minimum strain rates of cyclic creep (fatigue) were lower than those of static creep. The time to rupture under creep loads was slightly shorter than that under fatigue loads at a given maximum stress. The creep strain rates of the Hi‐Nicalon™/SiC composite in air were lower than those in argon. Consequently, the time to rupture at a given stress in air was longer than in argon. The creep and fatigue resistance of the Hi‐Nicalon™/SiC composite both were similar to that of the enhanced SiC/SiC composite but were much better than that of the standard SiC/SiC composite in air. However, in argon, the standard SiC/SiC composite had the lowest creep rate, whereas the enhanced SiC/SiC composite had the highest creep rate. The time to rupture of the standard SiC/SiC composite was the shortest and the Hi‐Nicalon™/SiC composite had the longest life.
Thermophysical Properties of α‐Tungsten Carbide
Authors: Reeber Robert R. , Wang Kai
Published: July 2010

Upper and lower bounds for the thermal expansion of polycrystalline tungsten carbide (α‐WC) are predicted at ultrahigh temperatures from low‐temperature experimental data. The lower bound is obtained from an αVKTV model, where αV is the volume thermal expansion, KT the isothermal bulk modulus for a randomly oriented polycrystalline sample, and V the molar volume. For many materials, the αVKTV product approaches a constant value that is similar to the specific heat at the highest temperatures. The upper bound uses Grüneisen's rule with a constant Grüneisen parameter gamma at temperatures >1.3θD (where θD is the Debye temperature) and experimental data below that temperature. Literature data for the thermophysical properties of α‐WC have been reviewed and used in our αVKTV model to calculate a lower bound for the thermal expansion at temperatures >2θD and to calculate the temperature dependence of the bulk modulus. The ultrahigh‐pressure thermal expansion has been calculated from the lower bound. Model predictions of the thermophysical properties of WC are given for an extended temperature range.
Elastic Moduli of Transparent Yttria
Authors: Yeheskel Ori , Tevet Ofer
Published: July 2010

The elastic moduli of yttria (Y2O3) samples that were made from powders with various particle morphologies were studied by means of ultrasonic measurements. The soundwave velocities in the longitudinal and transverse modes were measured. The elastic moduli were calculated from the sound velocities and density. For the high‐purity, high‐density (>5000 kg/m3) Y2O3 that was prepared in the present study, the average density and elastic moduli (and their standard deviations) were as follows: density (ρ) of 5020 ± 18 kg/m3, Young's modulus (E) of 179.8 ± 4.8 GPa, shear modulus (G) of 69.2 ± 2.0 GPa, bulk modulus (B) of 148.9 ± 3.0 GPa, and Poisson's ratio (ν) of 0.299 ± 0.004. The average longitudinal and transverse soundwave velocities (Vl and Vt, respectively) were 6931 ± 65 and 3712 ± 49 m/s, respectively. The elastic moduli of lanthana‐strengthened yttria (LSY) were ∼6% lower than those of high‐purity Y2O3, and the nu value for LSY was ∼0.304. It has been argued that soundwave velocity is better than density, in regard to predicting the elastic moduli of fully dense and slightly porous materials. A linear equation that describes the change of the elastic moduli with soundwave velocity alone has been suggested. This equation was applicable to a relative elastic moduli range of 0.75–1.02.
Fiber Effects on Minicomposite Mechanical Properties for Several Silicon Carbide Fiber‐Chemically Vapor‐Infiltrated Silicon Carbide Matrix Systems
Authors: Morscher Gregory N. , Martinez‐Fernandez Julian
Published: July 2010

Several different types of SiC fiber tows were coated with BN and composited using chemically vapor‐infiltrated SiC to form single‐tow minicomposites. The types of SiC fiber included Nicalon™, Hi‐Nicalon™, and the new Sylramic™ polycrystalline SiC fiber. The interfacial shear stresses were determined from unload–reload tensile hysteresis‐loop tests. The ultimate stress and strain properties also were determined for the minicomposites. The ultimate strengths of the newer Hi‐Nicalon and Sylramic fibers were superior to that of Nicalon minicomposites with similar fiber volume fractions. The Sylramic minicomposites had the lowest strain to failure and highest interfacial shear strength, respectively, because of the high modulus of the fiber and the rough surface of this fiber type. The apparent interfacial shear strength increased as the stress increased for the Sylramic minicomposites, which also was attributed to the surface roughness of this fiber.
Stresses Due to Temperature Gradients in Ceramic‐Matrix‐Composite Aerospace Components
Authors: John Reji , Zawada Larry P. , Kroupa Joseph L.
Published: July 2010

Ceramic‐matrix composites (CMCs) are being considered as replacement materials for divergent flaps and seals in advanced aerospace turbine engines. During service, these components are subjected to severe temperature gradients across the width of the flaps. This paper discusses an analytical procedure to estimate the stresses generated in the CMC flaps due to the temperature gradients. The analytical procedure can be used for a material with nonlinear temperature‐dependent stress–strain behavior. This procedure was used to predict the thermal stresses in four candidate CMC systems due to temperature gradients. The thermal stresses along the edges typically exceed the proportional limit, and sometimes the fatigue limit of the CMC.
Observation of Vacancies by Positron Trapping in Cerium‐Doped Zirconolites
Authors: Hadley Joseph H. , Hsu Frank H. , Hu Yong , Eric R. Vance , Bruce D. Begg
Published: July 2010

Positron annihilation lifetime measurements on cerium‐doped zirconolites have given strong indications of positron trapping in samples that have previously been deduced to contain cation vacancies, mainly from X‐ray absorption spectroscopy, thermogravimetry, and microanalysis. The increases in lifetime of the majority lifetime component are ∼20% for samples deduced to contain a few percent of vacant cation sites. Lanthanum‐doped SrTiO3 samples, in which it is generally agreed that A‐site vacancies are produced via the lanthanum doping, have been used as a model system and similar increases in lifetime have been observed for samples that contain more than ∼0.1 formula units of lanthanum.
Novel Room‐Temperature Synthesis of Microcrystalline Zirconia
Authors: Bandyopadhyay Krisanu , Sainkar Sudhakar R. , Vijayamohanan Kunjukrishnapillai
Published: July 2010

A novel room‐temperature method is described for the preparation of microcrystalline zirconia (ZrO2) in monoclinic form via a simple electrochemical technique. Almost‐epitaxial growth is observed, with crystal sizes in the range of 5–15 μm. A tentative reaction scheme also is suggested, to elucidate the underlying process that leads to the formation of the ZrO2 phase.
Gas‐Sensing Properties of Spinodally Decomposed (Ti,Sn)O2 Thin Films
Authors: Arakawa Shuichi , Mogi Kenji , Kikuta Ko‐ichi , Yogo Toshinobu , Hirano Shin‐ichi , Seki Yoji
Published: July 2010

The gas‐sensing properties of spinodally decomposed (Ti,Sn)O2 thin films on sapphire substrates were investigated for CO, C3H8, and C2H5OH, and hydrogen gases at a temperature of 500°C. The variation in the d‐spacing of the (101) plane of (Ti0.5Sn0.5)O2 films showed behavior that was typical of spinodal decomposition during annealing at a temperature of 900°C. Transmission electron micrographs of the spinodally decomposed (Ti,Sn)O2 films on sapphire (0112) substrates revealed the characteristic modulated structure. The modulated lamella microstructure consisted of TiO2‐ and SnO2‐rich regions at intervals of ∼10 nm. The films were very sensitive to hydrogen gas and revealed anisotropic electrical conduction that was influenced by the modulated microstructure, which is characteristic of spinodal decomposition.
Near‐Net‐Shaped (Ba,Sr)Al2Si2O8 Bodies by the Oxidation of Machinable Metal‐Bearing Precursors
Authors: Viers David S. , Sandhage Kenneth H.
Published: July 2010

A novel process has been used to fabricate refractory aluminosilicate bodies of near net shape: the oxidation of machinable, alkaline‐earth‐metal‐bearing precursors. Ba–Sr–Al–Al2O3–SiO2 precursors of similar molar volume as strontium‐doped celsian, (Ba,Sr)Al2Si2O8, were prepared by powder metallurgical processing. Precursor powder mixtures, obtained by high‐energy vibratory ball milling, were compacted and formed into bars or disks by uniaxial pressing. The bar‐shaped precursors were machined on a metal‐working lathe using hardened steel tooling. The shaped precursors were oxidized and converted into celsian. The resulting celsian bodies retained the shapes, and fine machined features, of the precursors. The measured changes in dimensions were small and in good agreement with calculated values.
Aqueous Sol‐Gel Route to Barium Aluminosilicate Glass‐Ceramics
Authors: Zhang Bo , Zhang Litong , Zhou Wancheng
Published: July 2010

The rapid hydrolysis of some alkoxides creates problems in alcohol‐based sol–gel processes. This paper describes a simple aqueous sol–gel process for the synthesis of barium aluminosilicate gels and gel‐derived glasses, in which the problem of the rapid hydrolysis of aluminum isopropoxide (the source of aluminum) is solved by a precipitation–peptization process that is cost effective. Tetraethylorthosilicate, aluminum isopropoxide, and barium acetate are used as the starting materials, and water is the solvent. The molecular network, crystallization behavior, weight‐loss, and thermal characteristics of the gel and the gel‐derived glass have been detected by Raman spectroscopy, XRD, DTA, and TGA.
Chemorheology of Aqueous‐Based Alumina‐Poly(vinyl alcohol) Gelcasting Suspensions
Authors: Morissette Sherry L. , Lewis Jennifer A.
Published: July 2010

A new gelcasting system based on aqueous‐based alumina‐poly(vinyl alcohol) (PVA) suspensions cross‐linked by an organotitanate coupling agent has been developed. The chemorheological properties of this system exhibited a strong compositional dependence. A sol‐gel phase diagram was established, which yielded the critical titanium concentration ([Ti]c) required for gelation at a given PVA volume fraction as well as the minimum PVA volume fraction (φminPVA= 0.0245) and titanium concentration ([Ti]min= 9.984 × 10‐4 g of Ti/mL) below which gelation was not observed irrespective of solution composition. The gelation time of suspensions of constant PVA volume fraction (φsolnPVA) decreased with increased cross‐linking agent concentration, temperature, and solids volume fraction. The steady‐state viscosity and elastic modulus of polymer solutions (φsolnPVA= 0.05) of varying [Ti] were well described by the percolation model, giving scaling exponents of 0.84 and 1.79, respectively. The steady‐state elastic modulus of gelcasting suspensions, which provided a measure of their handling strength in the as‐gelled state, increased with increased solids volume fraction.
Processing and Characterization of Microemulsion‐Derived Lead Magnesium Niobate
Authors: Ng Wei Beng , Wang John , Ng Ser Choon , Gan Leong Ming
Published: July 2010

Lead magnesium niobate, Pb(Mg1/3Nb2/3)O3 (PMN), with 3 wt% excess PbO content has been successfully prepared via a microemulsion processing technique. By stepwise hydrolysis using aqueous ammonia as the precipitant, hydroxide precursor was obtained from nitrate solutions dispersed in the nanosized aqueous domains of a microemulsion consisting of cyclohexane, mixed poly(oxyethylene)5 nonyl phenol ether (NP5) and poly(oxyethylene)9 nonyl phenol ether (NP9), and an aqueous phase. Upon calcination of the microemulsion‐derived precursor at 780°C, an ultrafine perovskite PMN powder with less than 5% pyrochlore phase was obtained. The percentage of perovskite phase increases with increasing calcination temperature, reaching ∼98% at 900°C. The resulting PMN powder exhibits a near‐spherical particle morphology although particle agglomerates of ∼0.3 µm in average diameter occur when they are calcined at 850°C. When sintered at 1150°C, the microemulsion‐derived PMN showed a maximum relative density of ∼95.6% theoretical density, which gives a maximum dielectric constant of ∼11 000 at 1 kHz and a Curie temperature of ‐6°C.
Sinter‐Forging of Nanocrystalline Zirconia: II, Simulation
Authors: Hague Douglas C. , Mayo Merrilea J.
Published: July 2010

A quantitative computer simulation of densification, pore‐size evolution, and grain growth during sinter‐forging has been developed and applied to the sinter‐forging of nanocrystalline yttria (30 mol%)‐stabilized zirconia (3Y‐TZP) at 1050° and 1100°C. Densification is simulated as a superposition of stress‐assisted and plastic‐strain‐controlled pore‐shrinkage mechanisms. Grain growth is simulated as a pore‐controlled process during intermediate‐stage densification and as a combination of normal (static) grain growth and dynamic grain growth during final‐stage densification. The densification portion of the model offers very good agreement with the experiment under a wide variety of imposed forging conditions, despite the absence of adjustable variables. Grain‐growth predictions qualitatively illustrate a key feature in the sinter‐forging of 3Y‐TZP: i.e., the minimization of grain size, as a function of density, under forging conditions that promote high strain rates. This particular effect seems to be due to the quick elimination of large pores by plastic strain while small pores (which shrink by diffusion) are still available to control grain growth.
Variation of the Oxidation Rate of Silicon Carbide with Water‐Vapor Pressure
Authors: Opila Elizabeth J.
Published: July 2010

Chemically vapor deposited silicon carbide (CVD SiC) was oxidized at temperatures of 1000°‐1400°C in H2O/O2 gas mixtures with compositions of 10‐90 vol% water vapor at a total pressure of 1 atm. Additional experiments were conducted in H2O/argon mixtures at a temperature of 1100°C. Experiments were designed to minimize impurity and volatility effects, so that only intrinsic water‐vapor effects were observed. The oxidation kinetics increased as the water‐vapor content increased. The parabolic oxidation rates in the range of 10‐90 vol% water vapor (the balance being oxygen) were approximately one order of magnitude higher than the rates that were observed in dry oxygen for temperatures of 1200°‐1400°C. The power‐law dependence of the parabolic oxidation rate on the partial pressure of water vapor at all temperatures of the study indicated that the molecular species was not the sole rate‐limiting oxidant. The determination of an activation energy for diffusion was complicated by variations in the oxidation mechanism and oxide‐scale morphology with the partial pressure of water vapor and the temperature.
Creep‐Induced Residual Stress Strengthening in a Nicalon‐Fiber‐Reinforced BMAS‐Glass‐Ceramic‐Matrix Composite
Authors: Widjaja Sujanto , Jakus Karl , Ritter John E. , Lara‐Curzio Edgar , Watkins Thomas R. , Sun Ellen Y. , Brennan John J.
Published: July 2010

The feasibility of inducing a compressive residual stress in the matrix of a Nicalon‐fiber‐reinforced BMAS‐glass‐ceramic‐matrix composite through a creep‐load transfer treatment was studied. Specimens were crept at 1100°C under constant tensile load to cause load transfer from the matrix to the fibers, then cooled under load. Upon removal of the load at room temperature, the matrix was put into compression by the elastic recovery of the fibers. This compressive residual stress in the matrix increased the room‐temperature proportional limit stress of the composite. The increase in the proportional limit stress was found to be dependent upon the applied creep stress, with an increase in creep stress resulting in an increase in the proportional limit stress. Acoustic emission results showed that the onset of significant matrix cracking correlated closely to the proportional limit stress. Changes in the state of residual stress in the matrix were supported by X‐ray diffraction results. Fracture surfaces of all specimens exhibited fiber pullout behavior, indicating that the creep‐load transfer process did not embrittle the fiber/matrix interface.
Ambient‐ and High‐Temperature Properties of Titanium Carbide–Titanium Boride Composites Fabricated by Transient Plastic Phase Processing
Authors: Brodkin Dmitri , Zavaliangos Antonios , Kalidindi Surya R. , Barsoum Michel W.
Published: July 2010

Ambient‐ and high‐temperature properties of a class of titanium carbide‐titanium boride composites that have been produced by transient plastic phase processing are presented. The composites produced are comprised of Ti3B4, TiB2, and TiC0.65 at their equilibrium composition (34.5, 30.5, and 34.9 vol%, respectively), and the Ti3B4 phase in these composites occurs either as equiaxed grains or as platelets, depending on the starting mixture composition. Measurements of the ambient‐ and high‐temperature flexure strength and fracture toughness, thermal shock susceptibility, oxidation resistance, and wear resistance of this class of composites are presented. The role of various microstructural parameters—such as the morphology of the Ti3B4 phase, the length scale of the microstructure, and the volume fraction of borides—on these properties has been identified.
Thermal Conductivity of Plasma‐Sprayed Monolithic and Multilayer Coatings of Alumina and Yttria‐Stabilized Zirconia
Authors: Ravichandran Kakkaveri S. , An K. , Dutton Rollie E. , Semiatin S. L.
Published: July 2010

The temperature dependence of the thermal conductivity of plasma‐spray‐deposited monolithic coatings, as well as multilayer coatings that consisted of Al2O3 and ZrO2 that was stabilized by 8% Y2O3 (YSZ), was investigated. The coatings exhibited a large reduction in thermal conductivity at all temperatures, when compared to the bulk monolithic Al2O3 and YSZ. This reduction was due to porosity as well as thermal resistance that was caused by interfaces in the coatings. The largest decrease in the thermal conductivity of the coatings, relative to monolithic fully dense materials, was due to splat interfaces within each layer, as well as the coating/substrate interface. On the other hand, the multilayer coatings showed little variation in the thermal conductivity, relative to the number of layers, which suggests that the influence of interlayer interfaces on heat transfer is relatively small. A one‐dimensional analysis of steady‐state heat transfer has been presented to illustrate the significance of porosity, splat interfaces, and interlayer interfaces, with respect to the overall thermal conductivity of multilayer coatings.
Experimental Phase Diagram Study and Thermodynamic Optimization of the Ag‐Bi‐O System
Authors: Assal Jérôme , Hallstedt Bengt , Gauckler Ludwig J.
Published: July 2010

The Ag‐Bi‐O system has been experimentally studied using differential thermal analysis (DTA) and scanning electron microscopy (SEM) with energy dispersive X‐ray spectroscopy (EDX), and thermodynamically optimized. The temperatures of the eutectic, monotectic, and Bi2O3 allotropic transformations have been measured in N2, in air, and in O2 by DTA. There are no ternary phases stable at ambient pressure. Presently measured transformation temperatures have been combined with existing oxygen activity measurements in the metal liquid to optimize thermodynamic parameters describing the liquid phase. The resulting fit is excellent. EDX measurements of the composition in the oxide liquid have a rather low precision but confirm the thermodynamic optimization. However, some uncertainties remain concerning the liquid composition at the eutectic transformation and the shape of the miscibility gap at higher temperatures.
Structure of Calcium Silicate Hydrate (C‐S‐H): Near‐, Mid‐, and Far‐Infrared Spectroscopy
Authors: Yu Ping , Kirkpatrick R. James , Poe Brent , McMillan Paul F. , Cong Xiandong
Published: July 2010

The mid‐, near‐, and far‐infrared (IR) spectra of synthetic, single‐phase calcium silicate hydrates (C‐S‐H) with Ca/Si ratios (C/S) of 0.41–1.85, 1.4 nm tobermorite, 1.1 nm tobermorite, and jennite confirm the similarity of the structure of these phases and provide important new insight into their H2O and OH environments. The main mid‐IR bands occur at 950–1100, 810–830, 660–670, and 440–450 cm−1, consistent with single silicate chain structures. For the C‐S‐H samples, the mid‐IR bands change systematically with increasing C/S ratio, consistent with decreasing silicate polymerization and with an increasing content of jennite‐like structural environments of C/S ratios >1.2. The 950–1100 cm−1group of bands due to Si‐O stretching shifts first to lower wave number due to decreasing polymerization and then to higher wave numbers, possibly reflecting an increase in jennite‐like structural environments. Because IR spectroscopy is a local structural probe, the spatial distribution of the jennite‐like domains cannot be determined from these data. A shoulder at ∼1200 cm−1due to Si‐O stretching vibrations inQ3sites occurs only at C/S lessthan equal to 0.7. The 660–670 cm−1band due to Si‐O‐Si bending broadens and decreases in intensity for samples with C/S > 0.88, consistent with depolymerization and decreased structural order. In the near‐IR region, the combination band at 4567 cm−1due to Si‐OH stretching plus O‐H stretching decreases in intensity and is absent at C/S greater than ∼1.2, indicating the absence of Si‐OH linkages at C/S ratios greater than this. The primary Si‐OH band at 3740 cm‐1decreases in a similar way. In the far‐IR region, C‐S‐H samples with C/S ratio greater than ∼1.3 have increased absorption intensity at ∼300 cm−1, indicating the presence of CaOH environments, even though portlandite cannot be detected by X‐ray diffraction for C/S ratios <1.5. These results, in combination with our previous NMR and Raman spectroscopic studies of the same samples, provide the basis for a more complete structural model for this type of C‐S‐H, which is described.
Fabrication of Nanograined Silicon Carbide by Ultrahigh‐Pressure Hot Isostatic Pressing
Authors: Shinoda Yutaka , Nagano Takayuki , Wakai Fumihiro
Published: July 2010

Dense nanograined SiC ceramics were obtained by using hot isostatic pressing (HIP). The starting powder was ultrafine β‐SiC powder, which had a mean particle size of 30 nm and contained 3.5 wt% free carbon. SiC powders‐both boron‐doped and undoped‐were densified via HIP under an ultrahigh pressure of 980 MPa at a temperature of 1600°C. Both doped and undoped SiC attained the same density (3.12 g/cm3) (relative density of 97.1%). The average grain sizes of boron‐doped and undoped SiC were 200 and 30 nm, respectively. The compressive flow stress of undoped SiC was 3 times higher than that of boron‐doped SiC at temperatures of 1800° and 1700°C; however, the flow stresses of both materials were almost the same at 1600°C. The HIPed SiC that was doped with boron could be deformed at a stress that was one‐third lower than that of hot‐pressed boron‐ and carbon‐doped SiC with a grain size of 0.8 µm.
Resistance of Alumina‐Spodumene Ceramics to Thermal Shock
Authors: Bayuseno Athanasius P. , Latella Bruno A. , O'Connor Brian H.
Published: July 2010

A processing strategy is presented for obtaining alumina‐spodumene ceramics with controlled microstructures from a new, low‐cost alumina powder and spodumene (Li2OAl2O3·4SiO2) mineral, both being produced in Western Australia. The addition of 15 wt%β‐spodumene to alumina is used to produce ceramics with (i) high thermal expansion mismatch and (ii) a glassy phase to aid in liquid‐phase sintering. Specifically, the effects of spodumene addition and grain size on tolerance to thermal shock are addressed. The thermal shock resistance of the alumina‐spodumene ceramics is evaluated by water quenching and subsequent three‐point bend testing of strength diminution. Comparisons are made with results from parallel experiments conducted using a pure monolithic alumina ceramic. The reference alumina shows the expected substantial strength losses when thermally quenched from ∼200°C above room temperature. By contrast, the alumina‐spodumene ceramics, while displaying reduced strength relative to the reference aluminas, exhibit minimal strength degradation under severe thermal shock conditions.
Defect Reactions and the Controlling Mechanism in the Sintering of Magnesium Aluminate Spinel
Authors: Ting Ching‐Jui , Lu Hong‐Yang
Published: July 2010

Pressureless sintering studies have been conducted for excess Al2O3, stoichiometric, and excess MgO compositions of MgAl2O4 at 1500‐1625°C. Initial powders of various compositions are prepared by solid‐state reaction of MgO and Al2O3. A Brouwer defect equilibrium diagram is constructed that assumes intrinsic defects of the Schottky type. The densification rate derived from sintering kinetics is compared with the compositions investigated when the concentration is converted to the activity of the two oxide components in MgAl2O4. The grain‐size exponent of p similar/congruent 3 suggests that densification takes place by a lattice‐diffusion mechanism in the solid state. Determined activation enthalpies of 489‐505 kJmol‐1 are close to those obtained from oxygen self‐diffusion derived in previous sintering studies. It is, therefore, proposed that oxygen lattice diffusion through vacancies is the rate‐controlling mechanism for the sintering of nonstoichiometric MgAl2O4 compositions. The discrepancy between densification‐rate ratios in experimental results and oxygen vacancy concentration in the Brouwer diagram is accounted for by the defect associates formed in the nonstoichiometric compositions.
Ultrafine Barium Titanate Powders via Microemulsion Processing Routes
Authors: Wang John , Fang Jiye , Ng Ser‐Choon , Gan Leong‐Ming , Chew Chwee‐Har , Wang Xianbin , Shen Zexiang
Published: July 2010

Three processing routes have been used to prepare barium titanate powders, namely conventional coprecipitation, single‐microemulsion coprecipitation using diether oxalate as the precipitant, and double‐microemulsion coprecipitation using oxalic acid as the precipitant. A single‐phase perovskite barium titanate was obtained when the double‐microemulsion‐derived oxalate precursor was calcined for 2 h at a temperature of as low as 550°C, compared to 600°C required by the single‐microemulsion‐derived precursor. A calcination for 2 h at >700°C was required for the conventionally coprecipitated precursor in order to develop a predominant barium titanate phase. It was, however, impossible to eliminate the residual TiO2 impurity phase by raising the calcination temperature, up to 1000°C. The microemulsion‐derived barium titanate powders also demonstrated much better powder characteristics, such as more refined crystallite and particle sizes and a much lower degree of particle agglomeration, than those of the conventionally coprecipitated powder, although they contained ∼0.2 wt% BaCO3 as the impurity phase.
Surface Structure and Particulate Generation in Pb0.97Nd0.02(Zr0.55Ti0.45)O3 Ablation Targets During Pulsed Laser Deposition
Authors: Lappalainen Jyrki , Frantti Johannes , Lantto Vilho
Published: July 2010

A pulsed XeCl excimer laser was used to deposit amorphous Nd‐modified lead zirconate titanate thin films with thicknesses between 25 and 500 nm. Atomic force microscopy was used to study the thin‐film surface morphology and to determine the particulate number density (PND) on film surfaces. The dependence of the PND and the growth rate of the film on the laser‐beam fluence and the density of Pb0.97Nd0.02(Zr0.55Ti0.45)O3 targets was also studied. It was found that the PND of particulates with diameters larger than 150 nm increased with increasing laser‐beam fluence and decreased with increasing target density. The surface morphology and quality of the target after ablation were studied by scanning electron microscopy. The generation of laser‐cone structures and particulates at the target surface was also studied. X‐ray diffraction measurements and element analyses by the energy dispersive spectroscopy of X‐rays revealed the formation of extra compounds and phases at the target surface during ablation processes. Raman spectroscopy was also used to study the structure of the topmost surface layer of the target after ablation.
Controlled Firing of Reaction‐Bonded Aluminum Oxide (RBAO) Ceramics: Part II, Experimental Results
Authors: Gaus Shaun P. , Sheedy Paul M. , Caram Hugo S. , Chan Helen M. , Harmer Martin P.
Published: July 2010

The reaction‐bonded aluminum oxide (RBAO) process is an attractive alternative to conventional processing of ceramics, because of advantages such as lower costs, enhanced green machinability, near‐net‐shape forming, and broad microstructural variability. However, various problems are still encountered in the production of RBAO ceramics. Part I of the paper presented model predictions that may allow for the controlled firing of RBAO ceramics. The current work investigates the reaction behavior of RBAO ceramics under the model‐predicted conditions (i.e., for varying oxygen content, heat loss, heating cycles, and scale) via thermogravimetry, differential thermal analysis, and analysis of samples that have been fired in a box furnace. By controlling the reaction, one can produce large, crack‐free RBAO ceramics.
Templated Grain Growth of Textured Bismuth Titanate
Authors: Horn Jeffrey A. , Zhang S. C. , Selvaraj U. , Messing Gary L. , Trolier‐McKinstry Susan
Published: July 2010

Textured bismuth titanate, Bi4Ti3O12(BiT), was produced by templated grain growth (TGG). Molten‐salt‐synthesized BiT platelets were dispersed in a matrix of 200 nm BiT powder and aligned by tape casting. Highly textured BiT was obtained with the use of only 5 vol% template particles by sintering at 1000°C for 1 h. The uniformity of the through‐thickness texture is much higher than reported in the literature for BiT tapes cast with 100% platelets. Initial platelet alignment is shown to increase because of frequent interaction with the fine powder particles during tape casting. By avoiding pressure densification techniques and using only a small portion of anisometric particles, TGG is a low‐cost option for fabricating textured ceramics.
Packing of Quaternary Mixtures of Fibrous Particles
Authors: Zou Rui‐Ping , Yu Ai‐Bing , Lin Xin‐Yu
Published: July 2010

This paper presents an experimental investigation of packing of quaternary mixtures of fibrous particles of the same diameter but different lengths. The results indicate that the packing density is heavily dependent on the fractional solid volumes and hence the size and shape distributions involved. The packing of fibrous particles appears to be dominated by the shape effect rather than the size effect, and can be satisfactorily predicted by the modified linear packing model.
Preparation of Dense BaTiO3 Ceramics with Submicrometer Grains by Spark Plasma Sintering
Authors: Takeuchi Tomonari , Tabuchi Mitsuharu , Kageyama Hiroyuki , Suyama Yoko
Published: July 2010

Dense BaTiO3 ceramics consisting of submicrometer grains were prepared using the spark plasma sintering (SPS) method. Hydrothermally prepared BaTiO3 (0.1 and 0.5 µm) was used as starting powders. The powders were densified to more than similar/congruent95% of the theoretical X‐ray density by the SPS process. The average grain size of the SPS pellets was less than similar/congruent1 µm, even by sintering at 1000‐1200°C, because of the short sintering period (5 min). Cubic‐phase BaTiO3 coexisted with tetragonal BaTiO3 at room temperature in the SPS pellets, even when well‐defined tetragonal‐phase BaTiO3 powder was sintered at 1100° and 1200°C and annealed at 1000°C, signifying that the SPS process is effective for stabilizing metastable cubic phase. The measured permittivity was similar/congruent7000 at 1 kHz at room temperature for samples sintered at 1100°C and showed almost no dependence on frequency within similar/congruent100‐106 Hz; the permittivity at 1 MHz was 95% of that at 1 kHz.
Tribooxidational Effects on Friction and Wear Behavior of Silicon Nitride/Tool Steel and Silicon Nitride/Gray Cast Iron Contacts
Authors: Gomes José R. , Miranda António S. , Silva Rui F. , Vieira Joaquim M.
Published: July 2010

Unlubricated pin‐on‐disk wear tests of Si3N4 against tool steel and gray cast iron were performed at 5 N of normal load, 0.5 m/s of sliding speed, and environmental temperature in the range 22°‐600°C. The friction coefficient of Si3N4 sliding against tool steel and gray cast iron had maximum values of 0.88‐0.98 for tests at 100°C. The friction coefficient of Si3N4 sliding against gray cast iron couples had minimum values of 0.48‐0.57 at 400°C. Because of the increased third‐body protection, the wear coefficient of the Si3N4 pins of the Si3N4/gray cast iron couples decreased by 1 order of magnitude from 1.6 10‐5 mm3/(Nm) at room temperature to 1.3 10‐6 mm3/(Nm) at 600°C. Fe2O3 and Fe3O4 resulting from tribooxidation of the metallic disks were the main constituents of the wear debris and adherent tribolayers. Activation energy values (6.3‐13.7 kJ/mol) were comparable to those of oxidation wear of steel (7.3‐11.8 kJ/mol) but were much lower than the activation energy for oxidation of iron alloys in static conditions. Calculations of the activation energy of the oxidation wear corroborate the morphological observations of a sacrificial action of the metallic surface protecting the ceramic material.
Effect of Grain Thermal Expansion Mismatch on Thermal Conductivity of Porous Ceramics
Authors: Litovsky Efim , Gambaryan‐Roisman Tatiana , Shapiro Michael , Shavit Artur
Published: July 2010

Many ceramics contain microcracks, which are often situated between sintered grains. These microcracks constitute thermal resistances, which may affect heat transfer through the material and its effective thermophysical properties. The thicknesses and the contact areas of the microcracks change with temperature as a result of the thermal expansion mismatch between the grains on opposite sides of the microcracks. This physical mechanism affects changes of the material's thermal conductivity, k, with temperature. The above mechanism usually plays a minor role at atmospheric pressure, where heat may flow via the gas filling the cracks. Hence, the temperature‐induced changes of the crack geometry have little effect on heat transfer. However, at low gas pressures, where the heat flow between the grains occurs mainly via the contact areas, the grains' thermal expansion mismatch causes unusual temperature behavior of the material's thermal conductivity observed for several industrial refractories. In this paper, the influence of the above physical mechanism is discussed relative to other heat transfer mechanisms described in the literature. A simple physical model of the thermal expansion of grains bonded by an agent, having different thermal expansion coefficients, is developed. This model allows calculation of the contact area and the average microcrack opening between the grains as functions of the temperature, the characteristic grains sizes and their thermal expansion coefficients, and the permanent crack area. These parameters are evaluated and used to calculate the effective thermal conductivity of ceramic materials containing microcracks that appear as a result of thermal contraction of grains. The calculated thermal conductivity satisfactorily correlates with the experimental data collected for several chrome‐magnesite refractories over a wide range of temperatures and gas pressures.
Use of Blended Precursors of Poly(vinylsilane) in Polycarbosilane for Silicon Carbide Fiber Synthesis with Radiation Curing
Authors: Narisawa Masaki , Idesaki Akira , Kitano Shuhei , Okamura Kiyohito , Sugimoto Masaki , Seguchi Tadao , Itoh Masayoshi
Published: July 2010

Blended organosilicon precursors containing 10 or 20% of poly(vinylsilane) (PVS) in polycarbosilane (PCS) were prepared and shaped into fiber form by melt‐spinning. The influence of PVS addition on the spinning, radiation‐curing, and pyrolysis processes was investigated. The addition of PVS increased the spinability of the precursor melt and increased the oxidation sensitivity of the precursor. By adjusting the precursor compositions and the radiation conditions, highly heat‐resistant silicon carbide fibers were obtained.
Solubility of Bismuth Oxide in Barium Titanate
Authors: Zhou Liqin , Vilarinho P. M. , Baptista J. L.
Published: July 2010

The solubility of bismuth oxide in barium titanate was investigated by X‐ray diffraction and scanning electron microscopy. Bismuth was added in two schemes: Ba1‐xBixTiO3, where only site balance was considered, and Ba1‐1.5yBiyTiO3, where both site balance and charge balance were considered. The solubility limits of bismuth in BaTiO3sintered samples for these two schemes are slightly different, 3 and 5 at.%, respectively, at 1250°C.
Superplastic Sinter‐Forging of Silicon Nitride with Anisotropic Microstructure Formation
Authors: Kondo Naoki , Suzuki Yoshikazu , Ohji Tatsuki
Published: July 2010

The microstructure and mechanical properties of silicon nitride, produced by a superplastic sinter‐forging technique, were investigated. The obtained silicon nitride exhibited a highly anisotropic microstructure, where rod‐shaped grains tended to be aligned perpendicular to the forging direction. A very high bending strength of 2108 MPa as well as a high fracture toughness of 8.3 MPam1/2 were achieved when a stress was applied perpendicularly to the pressing direction. This very high strength was considered to be due to the reduced flaw size by the superplastic sinter‐forging process and the steep R‐curve behavior caused by the grain alignment.
Effective Length Scale for Predicting Solvent‐Debinding Times of Components Produced by Powder Injection Molding
Authors: Shivashankar Tirumani S. , German Randall M.
Published: July 2010

The debinding of powder‐injection‐molded components is critical and time consuming, especially when the sizes of the powders used become relatively small (<5 µm). Mathematical relationships have been derived for predicting the polymer removal times for solvent‐, thermal‐, and wick‐debinding processes by considering one‐dimensional mass flow. However, the debinding of a component produced by powder injection molding occurs through all of its surfaces, and, hence, mass flow along all three dimensions needs to be considered. An effective length scale is introduced that considers the entire bulk of the component to compute the solvant‐debinding time and the wick‐debinding processes. The effective length scale is defined as the volume to surface area ratio of the compact. Introduction of this length scale in the existing equations successfully explains the debinding behavior of various geometries. Furthermore, the length scale helps us to understand the effect of the component's shape and dimensions on the critical heating rates for polymer burnout.
Cold Compaction and Solid‐State Sintering of WC‐Co‐Based Structures: Experiments and Modeling
Authors: Favrot Nathalie , Besson Jacques , Colin Christophe , Delannay Francis
Published: July 2010

Cold compaction (200‐1900 MPa) and sintering (1250°‐1350°C) of cermets based on WC‐Co were experimentally studied using die compaction, cold isostatic pressing, sintering, and creep tests. Two different‐sized WC powders were used. The cobalt content varied over a range of 10‐30 wt%. Cold‐compaction behavior has been described by using a Cam‐Clay model. Die‐wall friction was measured by using green powder compacts that had different aspect ratios. Friction coefficients were 0.28‐0.85, depending on the WC particle size and cobalt content. Simple constitutive equations have been used to model the high‐temperature behavior (sintering and creep). The constitutive equations were implemented in a finite‐element program to model the compaction, ejection, and sintering of bilayer structures that had different cobalt contents. The model can represent the effect of die‐wall friction on the average density, as well as deformation inside the green compact. Density gradients were generated; they were revealed during sintering, because the compact does not deform homogeneously. Simulation also can be used to evaluate deformations that are induced by sintering.
Low‐Temperature Hydrothermal Synthesis of Yttrium‐Doped Zirconia Powders
Authors: Tsukada Takayuki , Venigalla Sridhar , Morrone Augusto A. , Adair James H.
Published: July 2010

The feasibility of low‐temperature synthesis of yttrium‐doped zirconia (Y‐ZrO2) crystalline powders in aqueous solutions at lessthan equal to100°C has been evaluated, and the hydrothermal crystallization mechanism for Y‐ZrO2 powders also has been investigated. Coprecipitated (Y,Zr) hydroxide gel, mechanical mixtures of Y(OH)3 and Zr(OH)4 gel, and Y(OH)3 gel have been reacted in boiling alkaline solutions. Coprecipitated (Y,Zr) hydroxide gel crystallized to cubic or tetragonal Y‐ZrO2 at pH 13.9. The yttrium content in the powder synthesized from coprecipitated (Y,Zr) hydroxide is consistent with the initial precursor solution composition, as expected from the similarity in solubility of Zr(OH)‐5 and Y(OH)‐4. A diffusionless mechanism for the transformation of the (Y,Zr) hydroxide gel to Y‐ZrO2 is proposed, and the phase stability in aqueous solution is discussed in terms of an in situcrystallization model. It is also demonstrated through thermodynamic arguments with experimental verification that the stable form of the Y‐ZrO2 at 25°C is the anhydrous phase, not the metal hydroxide as previously thought.
Influence of Isothermal Chemical Vapor Deposition and Chemical Vapor Infiltration Conditions on the Deposition Kinetics and Structure of Boron Nitride
Authors: Leparoux Marc , Vandenbulcke Lionel , Clinard Christian
Published: July 2010

An experimental study has been performed to gain some insight into the correlations between the deposition conditions and the structure of boron nitride (BN) coatings that are used in ceramic‐matrix composites. BN has been deposited at 700°C from BCl3‐NH3‐H2 mixtures on various substrates, by using chemical vapor deposition (CVD) and isothermal‐isobaric chemical vapor infiltration (ICVI) processes, simultaneously in the same reactor. A kinetic study has shown that the CVD process is governed either by a combination of mass transfer with chemical kinetics at low flow rates or by the heterogeneous kinetics only at high flow velocities. In contrast, the limiting contribution of mass transfer always is observed for the ICVI process. The influence of diffusion cages that are positioned around the fibrous preforms is reported. The structure of BN deposits has been studied as a function of the various deposition conditions via transmission electron microscopy. The chosen CVD conditions lead to a poor organization of the BN deposits. Fairly well‐organized BN coatings are deposited on all fibers of a fibrous preform via ICVI. The results are discussed in terms of supersaturation and deposition yields. The use of diffusion cages and the adjustment of the inlet composition and mass flow rate seem to be very important to obtain the best BN organization and thickness uniformity.
Grain‐Boundary Migration and Dielectric Properties of Semiconducting SrTiO3 in the SrTiO3‐BaTiO3‐CaTiO3 System
Authors: Kim Joo‐Seon , Kang Suk‐Joong L.
Published: July 2010

Chemically induced grain‐boundary migration and its effects on the interface and dielectric properties of semiconducting SrTiO3 have been investigated. Strontium titanate specimens that had been doped with 0.2 mol% of Nb2O5 were sintered in 5H2/95N2. The sintered specimens were diffusion annealed at 1400°C in 5H2/95N2 with BaTiO3 or 0.5BaTiO3‐0.5CaTiO3 (mole fraction) packing powder. The grain boundaries of the annealed specimens were oxidized in air. In the case of BaTiO3 packing, grain‐boundary migration occurred with the diffusion of BaTiO3 along the grain boundary. The effective dielectric constant of the specimen decreased gradually as the temperature increased but showed two peaks, possibly because of barium enrichment at the grain boundary and an oxidized Sr(Ba)TiO3 layer. In the case of 0.5BaTiO3‐0.5CaTiO3 packing, although barium and calcium were present at the grain boundary of the specimen, no boundary migration occurred, as in a previous investigation. With the diffusion of barium and calcium, the resistivity of the specimen increased and the variation of the effective dielectric constant with temperature was much reduced, in comparison to those without solute diffusion. These enhanced properties were attributed to the solute enrichment and the formation of a thin diffusional Sr(Ba,Ca)TiO3 layer at the grain boundary.