bluuebunny/crossref_metadata_2025 huggingface dataset:
Sintering and Crystallization of a Glass Powder in the Li2O–ZrO2–SiO2 System
Authors: de Oliveira Antonio Pedro Novaes , Manfredini Tiziano , Barbieri Luisa , Leonelli Cristina , Pellacani Gian Carlo
Published: July 2010

Sintering and crystallization of a 23.12 mol% Li2O, 11.10 mol% ZrO2, 65.78 mol% SiO2 glass powder was investigated. By means of thermal shrinkage measurements, sintering was found to start at about 650°C and completed in a very short temperature interval (ΔT similar/congruent 100°C) in less than 30 min. Crystallization took place just after completion of sintering and was almost complete at about 900°C in 20 min. Secondary porosity prevailed over the primary porosity during the crystallization stage. The glass powder compacts first crystallized into lithium metasilicate (Li2SiO3), which transformed into lithium disilicate (Li2Si2O5), zircon (ZrSiO4), and tridymite (SiO2) after the crystallization process was essentially complete. The microstructure was characterized by fine crystals uniformly distributed and arbitrarily oriented throughout the residual glass phase.
Oxygen Diffusion in Single‐Crystal Zinc Oxide
Authors: Tomlins Gregory W. , Routbort Jules L. , Mason Thomas O.
Published: July 2010

The models used to describe zinc oxide‐based varistors rely on diffusion and defect data obtained using techniques that have been replaced by more modern methods such as secondary ion mass spectrometry. We report values for oxygen diffusivities in undoped, single‐crystal ZnO as a function of temperature and orientation. Evaporation was taken into consideration when analyzing experimental results. Within experimental error, the energetics of diffusion are isotropic, but are slightly faster in the c‐direction. In comparing diffusion data from the literature, the intrinsic activation energy was estimated to be between 3.6 and 4.2 eV. Comparisons with recent theoretical calculations of relevant defect energies are discussed.
Restricted Diffusion of Chromium Nitrate Salt Solutions into Porous Sol‐Gel‐Silica Monoliths
Authors: Kunetz James , Hench Larry
Published: July 2010

The non‐steady‐state diffusion of chromium nitrate salt solutions into porous gel‐silica monoliths has been studied by optical absorption for a range of pore morphologies. The effective diffusion coefficient for the hexa‐aqua species decreases by 50 times the bulk diffusion coefficient for the smallest pore size investigated. These results indicate that pores smaller than the average pore size might be controlling the effective diffusion of chromium into the porous silica network or that additional forces, due to the smaller size of the pores, are contributing to the reduction of the ion's movement.
Hot Hydrogen Exposure Degradation of the Strength of Mullite
Authors: Herbell Thomas P. , Hull David R. , Garg Anita
Published: July 2010

Near‐stoichiometric mullite (3Al2O32SiO2) that contained small amounts of calcium and magnesium was exposed to pure dry hydrogen gas at elevated temperatures. Exposure temperatures were 1050° and 1250°C, and exposure times were up to 500 h. Preferential attack of the aluminosilicate glass that was present in the grain boundaries of the mullite occurred after 125 h at 1250°C. Hydrogen scrubbing of the silica from the glassy grain boundaries and the mullite grains yielded a porous alumina‐rich surface. The room‐temperature strength increased after short exposure times at 1250°C (up to 125 h) and then decreased by 53% after exposure for 500 h. At 1050°C, all exposure times (25‐500 h) decreased the strength. The room‐temperature strength of mullite decreased 22% after 500 h in hydrogen at 1050°C. We also observed a rapid 25% strength loss after short exposure times at 1050°C, which was attributed to the calcium/hydrogen‐assisted crystallization of the glassy grain‐boundary phase.
Intrinsic Size Effects in a Barium Titanate Glass‐Ceramic
Authors: McCauley Daniel , Newnham Robert E. , Randall Clive A.
Published: July 2010

A series of glass ceramics have been synthesized to produce bulk materials with nanometer‐sized barium titanate (BaTiO3) crystals grown in a residue glass matrix. Structure‐property relations have been made to determine the size distribution and the dielectric temperature dependence of the ceramics. Through dielectric and density mixing laws, it has been inferred that depolarization fields limit the dielectric polarizability of the particles and influence the transition temperature. The transition temperature, dielectric anomaly broadening, and peak dielectric constant all scale systematically with the mean size of the BaTiO3 crystals, which is consistent with an intrinsic size effect. In addition, scaling the transition temperature with the Ishikawa relation predicts a critical size of 17 nm, for which BaTiO3 cannot support a ferroelectric transition. These results are discussed in relation to other size studies on ferroelectric materials.
Characterization of Lead Oxide Thin Films Produced by Chemical Vapor Deposition
Authors: Madsen Lynnette D. , Weaver Louise
Published: July 2010

As a step toward creating a chemical vapor deposition (CVD) process for PbTiO3 thin films, lead oxide films were deposited and then examined. The reaction was oxidation controlled, with an apparent activation energy of 97 kJ/mol in this low‐temperature, low‐pressure metalorganic CVD (MOCVD) process. Across the deposition parameters examined, several distinct types of morphology were observed. Growth occurred as a combination of layer‐on‐layer and island formation. The structural and chemical properties of the lead oxide were examined by Auger electron spectroscopy, Raman spectroscopy, X‐ray diffraction, scanning electron microscopy, and electron diffraction. Various forms of lead oxide were produced (litharge, massicot, and scrutinyite, singly or in combination with each other). The deposition parameters used in this work showed a tendency to maintain the same crystalline form from the initial nucleation stages through post‐deposition annealing. Lead oxide formed readily on SiO2 surfaces (contrary to studies by other researchers) and, indeed, reacted with the underlying SiO2 layer.
Elastic Properties of Mullite
Authors: Ledbetter Hassel , Kim Sudook , Balzar Davor , Crudele Scott , Kriven Waltraud
Published: July 2010

Using ultrasonic methods, we determined the ambient‐temperature elastic constants of dense (99.7%) hot‐pressed polycrystalline 3:2 mullite (3Al2O32SiO2). We report the usual polycrystal elastic constants: Young's, shear, and bulk moduli, and the Poisson's ratio (nu). The Poisson's ratio (nu = 0.280) suggests interatomic bonding that is very different from that of either alumina or silica. Temperature dependences of the elastic constants show a strong irregularity; for example, nu decreases as the temperature increases. This irregularity suggests an internal‐state change that is perhaps related to the incommensurate structural modulation.
Development of Ceramic Inks for Direct Continuous Jet Printing
Authors: Teng Wan D. , Edirisinghe Mohan J.
Published: July 2010

A ceramic ink that contains 30 vol% of zirconia in butyl acetate and 2 wt% of a commercial oligomeric dispersant is considered. The dispersion and viscosity of this ink was optimized in previous work; however, its conductivity is too low for continuous ink‐jet printing. The conductivity has been improved by the addition of ethanol and ammonium nitrate, but this addition detrimentally affects the dispersion, viscosity, and surface tension of the ink. This problem has been largely overcome by reducing the amount of ceramic in the ink. The ceramic inks that have been developed are tested in a pressurized, continuous ink‐jet printer, and, depending on their performance, the ink composition is modified further.
Fracture Toughness and Interfacial Design of a Biological Fiber‐Matrix Ceramic Composite in Sea Urchin Teeth
Authors: Wang Rizhi
Published: July 2010

The working zone of a sea urchin tooth is a ceramic‐fiber‐reinforced ceramic‐matrix composite. It is composed of reinforcing calcitic fibers, a matrix of high‐magnesium‐containing calcite crystals, and organic material. It is much harder and tougher than most other calcitic materials. Fracture toughness and microhardness have been studied using microindentation. A comparison of fracture morphologies between intact teeth and teeth from which the organic component has been removed reveals that the thin organic sheath at the fiber/matrix boundary has an important contribution to the fiber pull‐out fracture behavior and, hence, to the toughness of the teeth.
Fourier Transform Infrared Studies of Propane Pyrolysis over Calcium Aluminate Melts
Authors: Fondeur Fernando , Mitchell Brian S.
Published: July 2010

Propane pyrolysis over calcium aluminate melts was carried out to evaluate carbon interactions with calcium, aluminum, and oxygen atoms. Infrared emission and attenuated total reflectance spectroscopies indicate that aluminum carbides and carbide oxides were formed early in the reaction as a result of carbon interactions with the melt. Moreover, time step emission spectra indicate that carbide oxides were formed before carbides, and that the carbon‐melt reaction product carbon monoxide accompanied carbide formation. Calcium carbide formation was not observed. It is proposed that the initial appearance of carbides and carbide oxides is due to the preferential reaction between carbon and pentahedrally coordinated aluminum atoms. These results suggest that polymerization of alumina polyhedra rather than particle filler effects is responsible for the viscosity buildup and stream stabilization in the formation of calcium aluminate fibers by inviscid melt spinning.
Effects of an Applied Electric Field on the Modulus of Rupture of Poled Lead Zirconate Titanate Ceramics
Authors: Fu Ran , Zhang Tong‐Yi
Published: July 2010

The effects of applied dc electric fields of ±10 kV/cm on the modulus of rupture of poled PZT‐841 ceramics were studied using three‐point bending tests. At each level of the applied electric field, 54 or 55 samples were tested and the data were statistically analyzed. The results showed that the measured moduli of rupture followed a Weibull distribution with a Weibull modulus of 10.6 when no electric field was applied. When the applied electric field was either parallel or antiparallel to the poling direction, the distribution of the modulus of rupture revealed two peaks and was fitted by a two‐peak Weibull distribution. One peak occurred at about 95 MPa, approximately the same as that without the presence of the applied electric field, while the other peak occurred around 50 MPa under either a positive or a negative electric field. Obviously, either a positive or a negative electric field assisted the applied mechanical loading to fracture the samples.
Solid‐State Immiscibility and Thermodynamics of the Calcium Oxide‐Strontium Oxide System
Authors: Jacob K. Thomas , Waseda Yoshio
Published: July 2010

Solid‐state miscibility gap in the pseudo‐binary calcium oxide‐strontium oxide (CaO‐SrO) system is delineated by X‐ray diffractometry studies on samples equilibrated either under vacuum or in a flowing inert‐gas atmosphere at temperatures of 973‐1273 K. For the CaxSr1−xO solid solution, an asymmetric phase boundary that is characterized by a critical temperature of 1173 (±3) K and a composition of x= 0.53 (±0.01) is obtained. The excess Gibbs energy of mixing of the solid solution, which is derived from the experimental phase‐boundary compositions and temperatures, can be represented by the expression ΔGE=x (1−x)[(27040 − 5.82 T) x+ (24915 − 5.34 T)(1 −x)] (in units of J/mol). It is necessary to include excess entropy terms to obtain a good fit to the experimental data. The chemical spinodal curve is computed from the thermodynamic parameters. The results obtained in this study are compared with phase‐diagram and calorimetric information that is available in the literature.
Gold‐Particle‐Enhanced Crystallite Growth of Thin Films of Barium Titanate Prepared by the Sol‐Gel Process
Authors: Masaki Yuichi , Koutzarov Ivoil P. , Ruda Harry E. , Farrell Mark
Published: July 2010

The effects of gold‐particle incorporation on the structure of barium titanate (BaTiO3) thin films was investigated. BaTiO3 thin films (110 nm thick) with different gold concentrations were prepared via sol‐gel processing and analyzed using X‐ray diffractometry. The diffraction results showed that the crystal structure of the thin films changed from predominantly the cubic phase to the tetragonal phase, and the crystallite size increased as the gold concentration increased. Possible mechanisms for the gold‐enhanced crystallization have been discussed.
Silicon Nitride/Silicon Carbide Nanocomposite Materials: II, Hot Strength, Creep, and Oxidation Resistance
Authors: Rendtel Andreas , Hübner Heinz , Herrmann Mathias , Schubert Christian
Published: July 2010

The mechanical properties of Si3N4/SiC nanocomposite materials that contained nanosized intercrystalline SiC dispersions that originated from different starting powders and were made via different fabrication routes were studied in the temperature range of 1400°‐1550°C. The strength retained at 1400°C was between 70% and ∼100% of the room‐temperature strength. Both creep and oxidation resistance were very high and were comparable to or better than those of the best Si3N4‐based materials published previously. The effect of SiC particles on the creep properties can be understood in terms of a recent model of dilatational creep; however, the model invokes a series of microstructural, micromechanical, and chemical modifications.
Toughening Behavior of a Two‐Dimensional SiC/SiC Woven Composite at Ambient Temperature: I, Damage Initiation and R‐Curve Behavior
Authors: Nair Shanti V. , Wang Yu‐Lin
Published: July 2010

The damage initiation and R‐curve behavior for a two‐dimensional (2‐D) SiC/SiC woven composite are characterized at ambient temperature and related to in situ microscopic observations of damage accumulation and crack advance. Matrix cracking and crack deflection/branching are observed and dominate fracture behavior in the early loading stage such that primary crack extension occurs at apparent stress intensity values as high as 12 MPam1/2. Linear elastic fracture mechanics (LEFM), though questionable, was assumed to be valid in the early stages of damage initiation prior to primary crack advance, but was clearly invalid once primary crack extension had occurred. Such a high primary crack extension toughness value is confirmed by a renotch technique whereby the crack wake is removed and the fracture resistance drops close to the initial value. Based on microstructural observations, multiple matrix cracks are found to be arrested at fiber bundles. The key to toughening appears to be associated with the mechanics of crack arrest at fiber bundles in the woven architecture. Toughening mechanisms include multiple matrix cracking (similar to microcracking), crack branching, and crack deflection in the crack frontal zone. Application of models to evaluate toughening based on these mechanisms results in values comparable to experimental data. In the regime of primary crack extension, a J‐integral technique was applied to investigate the R‐curve behavior and results showed a rising JR‐curve which started at 1500 J/m2 and reached 6150 J/m2 after about 13 mm of primary crack extension. There was evidence of substantial crack bridging by fiber tows and fibrous pull‐out in this regime of crack advance.
Effect of Annealing Environment on the Crack Healing and Mechanical Behavior of Silicon Carbide‐Reinforced Alumina Nanocomposites
Authors: Chou Irene A. , Chan Helen M. , Harmer Martin P.
Published: July 2010

The crack healing and strength behavior of an alumina‐silicon carbide (Al2O3‐SiC) nanocomposite (Al2O3+ 5 vol% 0.2 μm SiC particles) has been studied, as a function of the crack size and the annealing environment. Results show that annealing treatments can significantly increase the indentation strength. The annealing atmosphere has a profound influence on the extent of crack healing and the degree of strength recovery. Annealing in argon results in a strength increase of 50%, whereas annealing in air yields a three‐fold improvement in the indentation strength. Scanning electron microscopic observation has shown that healing of indentation cracks occurs in both environments, with the greater degree of healing occurring during annealing in air. Implications of the findings to the strengthening mechanism in Al2O3 (SiC) nanocomposites will be discussed.
Nanocrystalline Lead Titanate and Lead Titanate/Vinylidene Fluoride‐Trifluoroethylene 0‐3 Nanocomposites
Authors: Chen Yan , Chan Helen L. W. , Choy Chung Loong
Published: July 2010

Lead titanate (PT) powders of nanometer particle sizes were prepared using a sol‐gel process. The PT gel was annealed at various temperatures to produce powders with different particle sizes. X‐ray diffractometry (XRD), thermogravimetric analysis (TGA), and differential thermal analysis (DTA) revealed that the PT gel initially crystallized at an annealing temperature of 500°C. The crystallite diameter measured by XRD increased from 27 to 54 nm, while the size of PT particles increased from 30 to 270 nm as the annealing temperature increased from 500° to 900°C. Lead titanate/vinylidene fluoride‐trifluoroethylene (PT/P(VDF‐TrFE)) 0‐3 nanocomposites with PT volume fractions of 0.18 and 0.41 were prepared using PT powders annealed at various temperatures. Because the pyroelectric coefficients of the ceramic and polymer phases had like signs and the piezoelectric coefficients had opposite signs, the poled composite had high pyroelectric but very low piezoelectric activity, thereby reducing vibration‐induced electrical noise in pyroelectric sensor applications.
Cation‐Aided Joining of Surfaces of β‐Silicon Nitride: Structural and Electronic Aspects
Authors: Dudesek Pavel , Benco L'ubomír
Published: July 2010

An atomic‐scale approach has been applied to the examination of both the physical and electronic structures of stable surfaces of β‐Si3N4. Sterical constraints prevent the (001) surface from effective chemical reaction with the interface. The theoretical surface‐to‐surface bonding is investigated by using a periodical tight‐binding approach. Based on the interpretation of the density of states, the balance of the number of states and electrons is performed for stoichiometric Si3N4, ideal N‐terminated (110) surfaces, oxygen‐overlayered (110) slabs, and the metal monolayer with which the slabs are brought into contact. The stable electronic configuration, which is attained when the cation binds to the interface, represents the electronic driving force behind the diffusion of the additive and/or impurity atoms toward grain boundaries. The different bonding propensities of the (001) and (110) surfaces imply that effective bonding of the planes parallel to the c‐direction to the interphase restrains the crystal from growth in the lateral direction. Conversely, geometry‐constrained bonding of the (001) planes allows the crystal growth that produces the rod‐shaped β‐grains.
Anisotropic Grain Growth in Diphasic‐Gel‐Derived Titania‐Doped Mullite
Authors: Hong Seong‐Hyeon , Messing Gary L.
Published: July 2010

Densification and anisotropic grain growth in diphasic‐gel‐derived, titania‐doped mullite were studied. Titania enhanced initial and intermediate stage densification in diphasic mullite gels by reducing the glass viscosity. Rodlike anisotropic mullite grains started to grow in titania‐doped diphasic mullite gels once a dense, equiaxed microstructure was achieved. The onset temperature for anisotropic grain growth decreased with increasing titania concentration because the sintering temperature for final‐stage densification decreased. The lowest onset temperature for anisotropic grain growth was ∼1500°C in 5 wt% titania‐doped mullite. The aspect ratio and area fraction of anisotropic mullite grains increased with higher titania concentration and were strongly dependent on the initial titania particle size. Kinetic studies demonstrated that anisotropic grain growth in titania‐doped diphasic mullite gels followed the empirical equation Gn ‐ G0n=Kt, with growth exponents of 3 and 6 for the length [001] and thickness [110] directions, respectively. The activation energies for grain growth were 690 kJ/mol for the length and 790 kJ/mol for the thickness directions.
Measurement of Gas Transport through Fiber Preforms and Densified Composites for Chemical Vapor Infiltration
Authors: Starr Thomas L. , Hablutzel Nicole
Published: July 2010

Gas transport via pressure‐driven permeation or via concentration‐driven diffusion is a key step in the chemical vapor infiltration (CVI) process. This paper describes methods for the measurement of these properties for CVI preforms and partially infiltrated composites. Results are presented for Nicalon‐fiber cloth layup preforms and composites, Nextel‐fiber braid preforms and composites, and a Nicalon‐fiber three‐dimensional (3‐D) weave composite. The permeability of Nicalon cloth layup preforms is strongly dependent on the packing density over the range of 29‐40 vol% but is only weakly dependent on the orientation of the alternating cloth layers. The specimen‐to‐specimen variation is high (standard deviation of 20%), which reflects the statistical nature of the interlayer separation and alignment. This variability also affects the results for the partially densified specimens, where the permeability values for specimens of similar density can be very different. Trends in the diffusion factor results are similar to those for permeability, which reflects the fact that both transport parameters are dependent on the structure of the same pore network, although in somewhat different ways. The permeability of Nextel braid preforms is dependent on the thread count and the weight for cloths with similar construction and packing density. The gas permeability of the finer wave (6.3 tows/cm (16 tows/in.)) is approximately one‐half that of the coarser weave (3.5 tows/cm (9 tows/in.)). Results are reported for a small number of infiltrated composites with Nextel fiber reinforcement. Attempts to mount a Nicalon‐fiber 3‐D weave preform specimen have been unsuccessful. Results for a small number of composite specimens with 3‐D weave reinforcement are reported.
Structural Investigation of 95(0.6Li2S0.4SiS2)5Li4SiO4 Oxysulfide Glass by Using X‐ray Photoelectron Spectroscopy
Authors: Hayashi Akitoshi , Tatsumisago Masahiro , Minami Tsutomu , Miura Yoshinari
Published: July 2010

S2p and O1s photoelectron spectra were obtained for the 95(0.6Li2S0.4SiS2)5Li4SiO4 oxysulfide glass prepared by twin‐roller quenching. A four‐peak deconvolution technique was used to separate the S2p peak of the glass into the components of bridging and nonbridging sulfur atoms. As a result of the deconvolution of the S2p peak, we found that ∼92% of the sulfur atoms were present as nonbridging atoms. The O1s peak of the glass was separated into two components: bridging and nonbridging oxygen atoms. This separation of the O1s peak indicated that ∼85% of the oxygen atoms were present as bridging oxygen atoms.
Post‐Hot‐Pressing and High‐Temperature Bending Strength of Reaction‐Bonded Silicon Nitride‐Molybdenum Disilicide and Silicon Nitride‐Tungsten Silicide Composites
Authors: Zhang Bing‐Rong , Marino Francesco , Sglavo Vincenzo M.
Published: July 2010

A hot‐pressing technique was used for the further densification of reaction‐bonded silicon nitride‐molybdenum disilicide and silicon nitride‐tungsten silicide (Si3N4‐MoSi2 and Si3N4‐WSi2, respectively) compacts that were prepared via a presintering step and a nitriding process from silicon‐molybdenum or silicon‐tungsten powders. After hot pressing was performed at 1650°C (25 MPa for 1 h), most of the alpha‐Si3N4 that formed during the reaction‐bonding process was transformed to β‐Si3N4 and, moreover, a very small amount of Mo5Si3 (W5Si3) was formed in addition to MoSi2 (WSi2). Three‐ and four‐point bend tests were performed at room temperature (25°C), 1000°C, 1200°C, and 1400°C. The bend strength of the Si3N4‐WSi2 composite increased slightly from room temperature up to 1000°C, whereas the Si3N4‐MoSi2 composite showed a more‐pronounced increase up to 1200°C. Microstructural analysis was performed on the fracture surfaces of both composites that were tested at different temperatures.
Evolution of in Situ Refractories in the 20th Century
Authors: Lee William E. , Moore Robert E.
Published: July 2010

A common theme over the past 100 years in refractories science and technology has been to generate a protective refractories layer in a high‐temperature container, often by reaction of the refractory materials with the contents (glass, slag, or atmosphere). The history of refractories during the 20th century is used in this review to illustrate how techniques‐such as slag splashing, in situ spinel generation in castables, magnesia dense layer formation in magnesia‐carbon brick in steelmaking, clinker adhesion in cement kilns, and viscous boundary layer generation in glass tanks‐have evolved to their present status.
Dense Perovskite, La1‐xA′xFe1‐yCoyO3‐δ (A′= Ba, Sr, Ca), Membrane Synthesis, Applications, and Characterization
Authors: Tsai Chung‐Yi , Dixon Anthony G. , Ma Yi Hua , Moser William R. , Pascucci Marina R.
Published: July 2010

La1‐xA′xFe0.8Co0.2O3‐δ (A′= Ca, Sr, Ba) perovskite powders were synthesized to attain the desired properties of high O2 flux and stability under reducing conditions. Steady‐state oxygen permeation rates for La1‐xA′xFe0.8‐Co0.2O3‐δ perovskite membranes in nonreacting experiments with air on one side and helium on the other side of the membrane were in the order A′x= Ba0.8 > Ba0.6 > Ca0.6 > Sr0.6. Partial oxidation of methane to syngas (CO + H2) was performed in a dense La0.2Ba0.8Fe0.8Co0.2O3‐δ membrane reactor at 850°C in which oxygen was separated from air and simultaneously fed into the methane stream. The reducing atmosphere affected the membrane reaction‐side surface while barium enrichment occurred on the air‐side surface. Oxygen continuously transported from the air side appeared to stabilize the membrane interior, and the reactor was operated for up to 850 h.
Strengthening and Toughening Mechanisms of Ceramic Nanocomposites
Authors: Ohji Tatsuki , Jeong Young‐Keun , Choa Yong‐Ho , Niihara Koichi
Published: July 2010

Crack‐tip bridging by particles is considered to be one of the primary strengthening mechanisms of ceramic nanocomposites. Small, brittle particulate inclusions have been shown to cause crack‐tip bridging at short distances behind the crack tip. This mechanism leads to modest toughness but a very steep R‐curve, and it is the latter that produces the very high fracture strength of the ceramic nanocomposite. Localized high residual stress around the particles (particularly in the case of silicon carbide‐alumina material) causes the strengthening mechanism to operate effectively, even at a small volume fraction of 5%. The present study predicts the magnitude of the toughness increase and the extent of R‐curve behavior for the nanocomposite.
Stability and Oxidation Properties of RE‐α‐Sialon Ceramics (RE = Y, Nd, Sm, Yb)
Authors: Nordberg Lars‐Olov , Nygren Mats , Käll Per‐Olov , Shen Zhijian
Published: July 2010

Oxidation studies of hot‐pressed RE‐α‐sialons, REx‐Si12‐4.5xAl4.5xO1.5xN16‐1.5x (with x= 0.40 for RE = Nd, Sm, Yb; and x= 0.48 for RE = Y) were carried out in oxygen in a TG apparatus for ca. 20 h. Very good oxidation resistance was found for the Yb‐doped samples, with parabolic rate constants Kp similar/congruent 0.09 10‐6‐3 10‐6 mg2cm‐4s‐1 in the temperature range 1250‐1350°C. The promising performance of this material was corroborated by long‐term oxidation experiments (5 days) in air at 1350°C. Although the oxidation kinetics can be described by simple equations related to the parabolic rate law (e.g., the arctan equation, ΔW/A0=α arctan bt+ct), the oxidation process in these materials is likely to be complex. The significantly lower oxidation resistance of the RE = Nd, Sm doped α‐sialons, especially at higher temperatures, is related to the formation of melilite, RE2Si3−yAlyO3+yN4−y (y∼ 1), in these systems. The melilite phase is also responsible for the thermal instability of the Nd‐ and Sm‐α‐sialons.
Microstructure of Molybdenum Disilicide‐Silicon Carbide Nanocomposite Thin Films
Authors: Aldrich Darin J. , Jones Kim M. , Govindarajan Shrinivas , Moore John J. , Ohno Tim R.
Published: July 2010

Composite thin films of molybdenum disilicide‐silicon carbide (MoSi2‐SiC) have been deposited via rf magnetron sputtering onto molybdenum substrates. An intermediate layer was deposited in the presence of nitrogen gas and evaluated as a potential diffusion barrier layer. The composite films have been characterized using X‐ray diffractometry, scanning electron microscopy, transmission electron microscopy, and Auger electron spectroscopy. The as‐deposited films were amorphous but crystallized into nanometer‐sized grains after annealing under vacuum at 1000°C for 30 min. There was a significant amount of interdiffusion between the film and substrate, which resulted in the formation of subsilicides such as Mo5Si3 and MoSi3, as well as Mo2C. The films that were deposited via reactive sputtering in a nitrogen ambient were amorphous in both the as‐deposited and annealed conditions. Significantly fewer second phases were detected with the presence of the intermediate layer, which suggests the potential use of the nitrided (MoSixNyCz) layer as a high‐temperature diffusion barrier layer for the silicon and carbon.
Comparative Study of Microwave Sintering of Zinc Oxide at 2.45, 30, and 83 GHz
Authors: Birnboim Amikam , Gershon David , Calame Jeffery , Birman Amnon , Carmel Yuval , Rodgers John , Levush Baruch , Bykov Yurii V. , Eremeev A. G. , Holoptsev V. V. , Semenov Vladimir E. , Dadon David , Martin Peter L. , Rosen Moshe , Hutcheon Ron
Published: July 2010

Temperature gradients that develop in ceramic materials during microwave heating are known to be strongly dependent on the applied microwave frequency. To gain a better understanding of this dependence, identical samples of ZnO powder compacts were microwave heated at three distinct widely separated frequencies of 2.45, 30, and 83 GHz and the core and surface temperatures were simultaneously monitored. At 2.45 GHz, the approximately uniform “volumetric” heating tends to raise the temperature of the sample as a whole, but the interior becomes hotter than the exterior because of heat loss from the surface. At 30 and 83 GHz, this interior to exterior temperature difference was found to be reversed, especially for high heating rates. This reversal resulted from increased energy deposition close to the sample's surface associated with reduced skin depth. A model for solving Maxwell's equations was incorporated into a newly developed two‐dimensional (2‐D) heat transport simulation code. The numerical simulations are in agreement with the experimental results. Simultaneous application of two or more widely separated frequencies is expected to allow electronic tailoring of the temperature profile during sintering.
Residual Strains in an Al2O3‐Ni Joint Bonded with a Composite Interlayer: Experimental Measurements and FEM Analyses
Authors: Rabin Barry H. , Williamson Richard L. , Bruck Hugh A. , Wang Xun‐Li , Watkins Thomas R. , Feng Yue‐Zhong , Clarke David R.
Published: July 2010

A cylindrical Al2O3‐Ni joint bonded with a 4.0 mm thick composite interlayer of 40 vol% Al2O3‐60 vol% Ni was fabricated by powder processing, and the residual strains in the specimen were studied experimentally using neutron diffraction, X‐ray diffraction, and optical fluorescence spectroscopy. Experimental measurements were compared with finite element modeling results obtained using a variety of different constitutive assumptions. The predicted residual strain distribution within the Al2O3 along the center of the specimen was in reasonable agreement with neutron diffraction measurements, although the magnitude of the error in the experimental measurements did not allow a distinction to be made between the various modeling assumptions examined. In contrast, the predicted peak strains within the Al2O3 along the specimen surface were significantly higher than those measured by X‐ray diffraction or optical fluorescence spectroscopy, suggesting that strain relief occurred near the free edge during cooling. The mechanism of strain relief is believed to involve damage accumulation within the composite interlayer that was not accounted for in the constitutive models used.
Effect of Water Content on the Structure and Mechanical Properties of Magnesia‐Phosphate Cement Mortar
Authors: Hall David A. , Stevens Ronald , Jazairi Bayhass El
Published: July 2010

Magnesia‐phosphate cement mortars, based on magnesia and ammonium dihydrogen orthophosphate, were prepared with water contents (measured as a weight fraction of the dry material) in the range 5% to 12%. It was shown by XRD that the major reaction product was magnesium ammonium phosphate hexahydrate (MgNH4PO46H2O), also known as the mineral struvite. Evidence was also found to suggest the presence of an amorphous or poorly crystalline phase with a lower degree of hydration in materials prepared with water contents less than 8%. For higher water contents, the excess water gradually evaporated during aging to leave an interconnected fine porous structure which exhibited poor mechanical properties. An increase in the water content from 5% to 12% also led to a significant increase in the crystallite size, from around 0.5 µm to 10 µm.
Dielectric Properties of Complex Perovskite Lead Scandium Tantalate under dc Bias
Authors: Chu Fan , Fox Glen R. , Setter Nava
Published: July 2010

The weak‐field dielectric properties of ordered, disordered, and lead‐deficient disordered Pb(Sc0.5Ta0.5)O3 (PST) ceramics (PST‐O, PST‐D, and PST‐DV, respectively) were investigated under various dc‐bias conditions. The dc‐bias field influences the paraelectric‐to‐ferroelectric transition in PST‐O and the relaxor‐to‐normal‐ferroelectric transition in disordered PST‐D. Phase diagrams, with respect to dc bias and temperature, were proposed for ordered and disordered PST materials. A field‐induced ferroelectric state that originated from the residual non‐ordered regions in PST‐O was revealed by analyzing the Curie‐Weiss behavior of the materials under bias. It is shown that, in PST‐DV, a dc bias cannot induce an abrupt ferroelectric‐to‐relaxor transition. A dielectric anomaly observed at low temperatures in all three types of PST materials is attributed to a poling process.
Zinc Oxide Varistor Gas Sensors: II, Effect of Chromium(III) Oxide and Yttrium Oxide Additives on the Hydrogen‐Sensing Properties
Authors: Shimizu Yasuhiro , Lin Feng‐Cang , Takao Yuji , Egashira Makoto
Published: July 2010

The effect of Y2O3 addition on the hydrogen‐sensing properties of two series of porous ZnO‐based varistors has been investigated; the two varistor series are with and without 5.0 mol% Cr2O3, but both series contain 1.0 mol% Bi2O3 and small amounts of other additives essential for realizing nonlinear current‐voltage characteristics. The variation in H2 sensitivity, which is defined as the shift in breakdown voltage upon exposure to H2 balanced with air, is discussed in terms of microstructural and crystallographical changes of the varistors. The addition of Y2O3 to the base ZnO varistors without Cr2O3 results in a decrease in ZnO grain size and an enhancement in H2 sensitivity, with the highest sensitivity being achieved with an addition of 0.25 mol% of Y2O3. The addition of 5.0 mol% of Cr2O3 to the base ZnO varistor also has been confirmed to be effective for enhancing the H2 sensitivity, which is accompanied by stabilization of the δ‐Bi2O3 phase to room temperature and a decrease in ZnO grain size. The H2 sensitivity is further enhanced by the simultaneous addition of Y2O3 to the ZnO‐based varistors that contain 5.0 mol% Cr2O3, whereas the ZnO grain size remains almost unchanged. This series of varistors also exhibits the highest sensitivity with 0.25 mol% of Y2O3. The enhanced sensitivity has been anticipated to be related to the decrease in ZnO grain size, i.e., the increase in the number of H2‐sensitive grain boundaries, and the sensitization of grain boundaries themselves. The sensitization of grain boundaries has been suggested to occur not only via the decrease in ZnO grain size but also via stabilization of the δ‐Bi2O3 phase that exhibits high oxygen‐ion conductivity. Thus, the present study confirms that the mobility of excess oxygen ions and the reactivity of excess oxygen ions and oxygen adsorbates determine the H2 sensitivity of porous varistors; this study also shows that the existence of such oxygen species is essential for the formation of a double Schottky barrier of the varistors.
Texture and Fracture Toughness Anisotropy in Silicon Carbide
Authors: Kim Wonjoong , Kim Young‐Wook , Cho Duk‐Ho
Published: July 2010

Quantitative texture analysis, which included calculation of the orientation distribution function, was used to demonstrate textures in hot‐pressed and subsequently annealed silicon carbide (SiC). The results indicated that the hot pressing and annealing could produce strong textures in SiC. Grain rotation during hot pressing and preferred grain growth of plate‐shaped α‐SiC grains during annealing both apparently contributed to texture development in the SiC materials. The {111} pole figure in hot‐pressed material (mostly ß‐SiC) and the (004) pole figure in annealed material (mostly α‐SiC) were consistent with the microstructural observations. The fracture toughness of hot‐pressed and annealed material measured parallel to the hot‐pressing direction (5.7 MPam1/2) was higher than that measured perpendicular to the hot‐pressing direction (4.4 MPam1/2), because of the texture and the microstructural anisotropy.
Fabrication and Mechanical Properties of Al2O3‐WC‐Co Composites by Vacuum Hot Pressing
Authors: Tai Weon‐Pil , Watanabe Tadahiko
Published: July 2010

Al2O3‐WC‐Co composites were fabricated by vacuum hot‐pressing mixtures of Al2O3, WC, and cobalt powders. The phases formed with WC additions of up to 40 wt% were α‐Al2O3, WC, Co3W3C, and small amounts of f‐Co (face‐centered cubic cobalt) and carbon (graphite); no cobalt or carbon phases formed at >40 wt% WC. A more‐uniformly distributed and connected WC matrix formed as the WC content increased. The 10Al2O3‐80WC‐10Co (in wt%) composite exhibited high bending strength (1250 MPa), fracture toughness (9 MPam1/2), and hardness (20.6 ± 0.5 GPa) simultaneously. The high bending strength was mainly attributed to fewer fracture origins due to the uniformly distributed and connected WC matrix together with a lower porosity. Increased fracture toughness was caused mainly by crack deflection and crack bridging in a uniformly connected WC matrix. High hardness resulted from finer WC metallic compounds and Co3W3C precipitation in almost all ranges.
Dislocations and Stacking Faults in Ti3SiC2
Authors: Farber Leonid , Barsoum Michel W. , Zavaliangos Antonios , El‐Raghy Tamer , Levin Igor
Published: July 2010

The ternary carbide Ti3SIC2 fabricated by a reactive hot‐press route is investigated by transmission electron microscopy. The material consists mainly of large elongated grains with planar boundaries, and is characterized by a low defect density. Dislocations are observed in the grains and at grain boundaries. Perfect dislocations with b = 1/3<1120> lying in (0001) basal planes are present. These basal plane dislocations are mobile and multiply as a result of room‐temperature deformation. All of the stacking faults observed lie in the basal planes.
Origin of a Shrinkage Anomaly in Anatase
Authors: Kim Dong‐Wan , Kim Tae‐Gyun , Hong Kug Sun
Published: July 2010

In our present study, a shrinkage anomaly present during sintering in an anatase powder compact was examined. It was observed that the shrinkage stagnates during sintering. This anomaly has been investigated using dilatometry, X‐ray powder diffraction, and scanning electron microscopy. Quantitative phase analysis using X‐ray diffraction revealed that the shrinkage anomaly is correlated to the phase transition from anatase to rutile. A calculation based on specific volumes of anatase and rutile showed that the phase transition generated 8.6% additional porosity in the anatase powder compact and this retards the densification as well as the shrinkage in the anatase sample. Microstructural observation of anatase and rutile samples having the same shrinkage confirmed this result.
Characterization of Lithium Niobate Thin Films Derived from Aqueous Solution
Authors: Ono Satomi , Böse Olaf , Unger Wolfgang , Takeichi Yoshikuni , Hirano Shin‐ichi
Published: July 2010

Lithium niobate (LiNbO3, LN) films prepared from aqueous precursor solutions were characterized, in terms of the hydroxyl contents on the surface layers through crystallization and optical properties. Approximately 98% of the hydroxyl residues on the surface layers of crystalline films were polymerized after heat treatment at 350° or 500°C, whereas 50% of the hydroxyl residues remained on the surface layers of amorphous films heat‐treated at 200°C. Highly preferred oriented LN films had a tendency to consist of in‐plane rotation by 60° during nucleation and crystallization. Optical losses (at 632.8 nm) were 2 dB/cm in the TE0 mode and 4 dB/cm in the TM0 mode of the LN films (thickness of 670 nm) on sapphire (001) substrates prepared from a neutralized precursor solution after annealing in oxygen at a temperature of 500°C (where TE and TM are the transverse electric and magnetic modes, respectively).
Sintering of Nanosized MnZn Ferrite Powders
Authors: Rozman Marko , Drofenik Miha
Published: July 2010

The sintering and microstructural evolution of nanosized MnZn ferrite powders prepared by a hydrothermal method were investigated. The microstructure of sintered ferrite compacts depends strongly on the strength of the agglomerates formed during the compacting of nanosized ferrite powders. It was found that at 700°C the theoretical density of sintered compacts can almost be reached, while above 900°C an increase of porosity was identified. The formation of extra porosity at higher sintering temperatures is caused mainly by the oxygen release which accompanies the dissolution of relatively large grains of residual alpha‐Fe2O3 in the spinel lattice.
Nickel/Yttria‐Stabilized Zirconia Cermets from Combustion Synthesis: Effect of Process Parameters on Product Microstructure
Authors: Anselmi‐Tamburini Umberto , Arimondi Marco , Maglia Filippo , Spinolo Giorgio , Munir Zuhair A.
Published: July 2010

The synthesis of a nickel/yttria‐stabilized zirconia (Ni/YSZ) cermet with tailored microstructural characteristics has much interest for many solid‐state electrochemical applications. These materials are generally obtained through a route based on the reduction, via hydrogen gas, of poorly sintered mixtures of YSZ with nickel oxide. The present paper proposes an alternative route based on a thermite reaction performed in a combustion regime. Such an approach indicates the application of combustion synthesis or a self‐propagating high‐temperature synthesis (SHS) technique. The more‐interesting features of this route include the absence of any external source of energy during the reaction, the possibility to synthesize a porous cermet in a single step, its sintering, and the doping of the ceramic component with yttria. The synthesis of Ni/YSZ cermets through this route is analyzed in some detail, and this analysis shows the influence of the process parameters on the phase composition and microstructure of the products.
Flexure Strength of Melt‐Infiltration‐Processed Titanium Carbide/Nickel Aluminide Composites
Authors: Plucknett Kevin P. , Becher Paul F. , Waters Shirley B.
Published: July 2010

TiC/Ni3Al composites were prepared using a simple melt‐infiltration process, performed at either 1300° or 1400°C, with the Ni3Al content varied over the range of 8–25 vol%. Densities >96% of theoretical were obtained for all composites. Four‐point flexure strengths at 22°C increased as the Ni3Al content increased (i.e., ∼1100 MPa at 20 vol% Ni3Al), with the highest strengths being observed for composites processed at 1300°C, because of reduced TiC grain size. Strengths at elevated temperatures increased with test temperature, up to ∼1000°C. As with the yielding behavior of the Ni3Al alloy used, a maximum in composite strength (∼1350 MPa) versus temperature was observed; this occurred at 950°C, which is ∼300°C above the yield maximum for the alloy. Extensive plastic strain was achieved in the composites even at high loading rates at 1135°C, and the yield stress was dependent on the applied loading rate.
Effect of Internal Stress on Physical Temperature Characteristics of Cerium‐Doped and Gadolinium‐Doped Barium Titanate Ceramics
Authors: Park Yung , Cho Kurn , Kim Ho‐Gi
Published: July 2010

The effect of internal stress on the physical properties (the temperature dependence of the dielectric constant and the lattice constant, and the diffuseness (gamma)) of solid solutions of 0.97BaTiO3‐0.03Gd2O3:1.5TiO2 and 0.97BaTiO3‐0.03CeO2:1.5TiO2 was investigated using various particle sizes of the starting BaTiO3, pressure‐dielectric spectroscopy, and high‐pressure X‐ray analysis. The grain size was almost proportional to the particle size of the starting BaTiO3 and was inversely proportional to the internal stress. A sudden change in internal stress occurred at grain sizes of ∼2 µm for cerium‐doped BaTiO3 and ∼3 µm for gadolinium‐doped BaTiO3. With an increase in external pressure and a decrease in grain size, the lattice anomalies of the cubic‐to‐tetragonal transformation and two other factors (the tetragonal‐to‐orthorhombic and orthorhombic‐to‐rhombohedral transformations) shifted to lower and higher temperatures, respectively. The temperature dependencies of the dielectric constant and the lattice constant were less sensitive to external pressure in cerium‐doped core‐shell‐grained BaTiO3 than in gadolinium‐doped homogeneous‐grained BaTiO3; this difference was attributable to internal stress. In cerium‐doped BaTiO3, gamma was related to internal stress.
Fracture Origin and Strength in Advanced Pressureless‐Sintered Alumina
Authors: Krell Andreas
Published: July 2010

Advanced raw materials and shaping approaches enable the production of pressureless‐sintered alumina parts where, in bending, the average maximum stress at the fracture origin is as high as 800 MPa. In individual specimens that fracture at lower stresses (450–600 MPa), failure often originates at volume flaws, as known for hot‐pressed alumina with a similar strength. Also, transgranular and intergranular fracture modes along the crack path are the same as those observed in hot‐pressed alumina. If the size and the frequency of volume flaws are reduced, fracture initiates at smaller defects in the ground surfaces and bodies with a bending strength of >800 MPa are produced without hot pressing. The grain‐size dependence of grinding‐induced surface damage contributes to a grain‐size effect for the strength.
Formation of an α‐SiAlON Layer on ß‐SiAlON and Its Effect on Mechanical Properties
Authors: Jiang Xin , Baek Yong‐Kee , Lee Sung‐Min , Kang Suk‐Joong L.
Published: July 2010

We have developed a technique for in situformation of an alpha‐SiAlON layer on sintered ß‐SiAlON. The technique consists of packing a powder compact of ß‐SiAlON composition with alpha‐SiAlON composition powder, presintering, and sintering. Using this technique, it is possible to control the thickness of the layer by changing the presintering conditions during heating to a sintering temperature. The introduction of an alpha‐SiAlON layer increases the surface hardness and improves the wear and oxidation resistance, with a moderate reduction of flexural strength. The surface modification of ß‐SiAlON may provide opportunities of wider application of ß‐SiAlON to wear‐ and oxidation‐resistant components.