bluuebunny/crossref_metadata_2025 huggingface dataset:
Microstructural Evolution in Liquid‐Phase‐Sintered SiC: Part II, Effects of Planar Defects and Seeds in the Starting Powder
Authors: Deshpande Swarnima A. , Bhatia Tania , Xu Huiwen , Padture Nitin P. , Ortiz Angel L. , Cumbrera Francisco L.
Published: July 2010

The effects of planar‐defect density in a β‐SiC starting powder and the addition of α‐SiC seeds to that powder on microstructural evolution in liquid‐phase‐sintered (LPS) SiC have been studied separately. Planar‐defect density is altered by appropriate heat treatment of an as‐received β‐SiC starting powder. It was found that a decrease in the planar‐defect density in the powder retards the β→α phase transformation rate. It is proposed that, because nucleation of α‐SiC occurs on the planar defects present in the β‐SiC starting powders, the nucleation rate and the attendant rate of transformation decrease with a reduction in planar‐defect density. Consequently, this reduces the frequency of formation of elongated β/α composite grains, resulting in lower average aspect ratios, as the initial untransformed β‐SiC grains coarsen in an equiaxed manner. In contrast, addition of external α‐SiC seeds has no effect on the β→α phase transformation rate, although a significant reduction in the average aspect ratio occurs. It is proposed that preferential equiaxed coarsening of the α‐SiC seeds over elongated coarsening of β/α composite grains occurs, resulting in a reduction of overall coarsening anisotropy.
Role of Structural Fe(III) and Iron Oxide Nanophases in Mullite Coloration
Authors: Djemai Amédée , Calas Georges , Muller Jean Pierre
Published: July 2010

Six mullite samples, derived from heat‐treated natural kaolinites with various iron content, were investigated and compared to synthetic monophasic mullite. They were analyzed by X‐ray diffraction, diffuse reflectance spectroscopy, and electron paramagnetic resonance. To quantify mullite coloration, the CIE colorimetric system was used. In contrast to synthetic mullite, samples showed charge transfer bands involving O2− and Fe3+ ions as well as ferric crystal‐field transitions due to Fe3+ ions in iron oxide nanoparticles. Absorption edges showed red shifts. The resulting yellowness, saturation of which increased with the content of iron oxide nanoparticles, is direct evidence for the coloring effect of Fe3+ ions.
Fabrication of Low‐Shrinkage, Porous Silicon Nitride Ceramics by Addition of a Small Amount of Carbon
Authors: Yang Jian‐Feng , Zhang Guo‐Jun , Ohji Tatsuki
Published: July 2010

Successful net‐shape sintering offers a significant advantage for producing large or complicated products. Porous Si3N4 ceramics with very low shrinkage were developed, in the present investigation, by the addition of a small amount of carbon. Carbon powders (1–5 vol%) of two types, with different mean particle sizes (13 nm and 5 μm), were added to α‐Si3N4−5 wt% Y2O3 powders. SiC nanoparticles formed through reaction of the added carbon with SiO2 on the Si3N4 surface or with the Si3N4 particles themselves. Such reaction‐formed SiC nanoparticles apparently had an effective reinforcing effect, as in nanocomposites. Sintered Si3N4 porous ceramics with a high porosity of 50%–60%, a very small linear shrinkage of ∼2%–3%, and a strength of ∼100 MPa were obtained.
Grain‐Size Effect on Compressive Creep of Silicon‐Carbide‐Whisker‐Reinforced Aluminum Oxide
Authors: de Arellano‐López Antonio R. , Meléndez‐Martínez Juan J. , Domínguez‐Rodríguez Arturo , Routbort Jules L. , Lin Hua‐Tay , Becher Paul F.
Published: July 2010

The steady‐state compressive creep of Al2O3 with 10 vol% SiC whiskers having grain sizes of 1.2, 2.3, and 4.0 μm has been measured at 1400°C in argon. The creep rate is related to the free volume within the whisker network, not the nominal grain size. The results are consistent with diffusional‐controlled creep with different contributions from grain‐boundary sliding. Under low stresses, only Liftshitz sliding is possible and the diffusional process controls deformation, while at stresses over a threshold, Rachinger sliding is the mechanism controlling deformation. The evolution between Liftshitz and Rachinger sliding is marked by a significant increase in the value of the stress exponent.
Strontium Zirconate and Strontium Titanate Ceramics for High‐Voltage Applications: Synthesis, Processing, and Dielectric Properties
Authors: Shende Rajesh V. , Krueger Daniel S. , Rossetti George A. , Lombardo Stephen J.
Published: July 2010

The synthesis, processing, and electrical properties of SrZrO3 and SrTiO3 materials have been examined. Phase‐pure powders of SrZrO3 and SrTiO3 materials were synthesized using the Pechini method. Powder processing routes that used water and 2‐propanol as carrier fluids were developed to achieve high green densities, which resulted in sintered densities of >99% of the theoretical density. The relative permittivity and average breakdown strength for carefully processed SrZrO3 were 60 and 40 V/μm; the corresponding values for SrTiO3 were 400 and 35 V/μm. The higher breakdown strengths suggest that these materials can be used in high‐voltage capacitor applications.
Improvement in the Strut Thickness of Reticulated Porous Ceramics
Authors: Zhu Xinwen , Jiang Dongliang , Tan Shouhong , Zhang Zhaoquan
Published: July 2010

An improved technique has been developed for the successful fabrication of reticulate porous ceramics (RPCs) with strong struts, using polyurethane sponge as the template. In this technique, the coating process is composed of two stages. In the first stage, a thicker slurry is used to coat the sponge substrate uniformly. The green body is preheated to produce a reticulated preform with sufficient handling strength after the sponge has been burned out. In the second stage, a thinner slurry is used to coat the preform repeatedly, so that that struts are greatly thickened. Furthermore, large flaws, such as longitudinal strut cracks, are eliminated by the recoating process. The mechanical properties of RPCs should be improved significantly by this technique, which will make this material conducive for use in new technological applications.
Sintering Behavior and Electrical Properties of Nanosized Doped‐ZnO Powders Produced by Metallorganic Polymeric Processing
Authors: Durán Pedro , Capel Francisco , Tartaj Jesús , Moure Carlos
Published: July 2010

Homogeneous and nanosized (28 nm crystallite size) doped‐ZnO ceramic powders were obtained by a metallorganic polymeric method. Calcining and granulating resulted in green compacts with uniform powder packing and a narrow pore‐size distribution (pore size 19 nm). Dense ceramic bodies (>99% of theoretical) were fabricated by normal liquid‐phase sintering at 850° and 940°C for 1–5 h. Apparently, the low pore‐coordination number allowed a uniform filling of the small pores by the liquid formed in the early stages of sintering, and, consequently, high shrinkage and rapid densification occurred in a short temperature interval (825°–850°C). At these sintering temperatures, limited grain growth occurred, and the grain size was maintained at <1 μm. Ceramics so‐fabricated showed a nonlinear coefficient, α, of ≥70, and a breakdown voltage, Vb (1 mA/cm2), of ≥1500 V/mm. The high electrical performance of the doped‐ZnO dense ceramics was attributed to liquid‐phase recession on cooling, which enhanced the ZnO‐ZnO direct contacts and the potential barrier effect.
Catalytic Effects of Metals on Direct Nitridation of Silicon
Authors: Pavarajarn Varong , Kimura Shoichi
Published: July 2010

Catalytic effects were investigated on the direct nitridation of silicon granules, impregnated with 0.125–2.0% by mass of calcium, yttrium, iron, copper, silver, chromium, or tungsten, in a stream of nitrogen with 10% hydrogen, using a tubular flow reactor operated at temperatures ranging from 1200° to 1390°C. Calcium and yttrium suppressed the formation of β‐silicon nitride while iron enhanced the formation of β‐silicon nitride over the temperature range investigated. An addition of 0.125% calcium resulted in about 99% overall conversion with 100%α‐phase and a 2.0% yttrium addition yielded an overall conversion over 98% with an α‐phase content above 97%. Copper promoted not only the nitridation but the formation of α‐silicon nitride at 1200°C, but enhanced the β‐phase formation at higher temperatures. The role of liquid phases on the formation of α‐/β‐silicon nitride was also discussed based on the nitridation of silicon impregnated with copper, calcium, silver, chromium, and tungsten.
Mechanical Properties of Coagulated Wet Particle Networks with Alkali‐Swellable Thickeners
Authors: Hesselbarth Daniels , Tervoort Elena , Urban C. , Gauckler Ludwig J.
Published: July 2010

Alkali‐swellable thickeners (ASTs) such as Acusol 820 and Acusol 830, as well as poly(acrylic acid) homopolymers of various molar mass, have been used as additives in aqueous electrostatically stabilized alumina suspensions. These suspensions have been destabilized by internal enzyme‐catalyzed reactions (a direct coagulation casting process) to form viscoelastic solids. The ASTs increase the strength and modulus of the wet green bodies on coagulation. The effect of their molecular architecture on the mechanical properties of wet particulate networks has been studied. At low pH (pH 4.5), ASTs are small insoluble polymer particles that have only minor influence on the low viscosity of the high‐solids‐loading suspensions. After shifting the pH toward the isoelectric point of α‐Al2O3, the suspension coagulates and the AST polymer particles swell, thereby increasing the compressive strength and modulus of the alumina‐particulate wet green bodies. The presence of small amounts of ASTs (0.4 wt%, based on the solids loading) results in a 10‐fold increase in the strength of the wet green bodies. The compressive strength of the wet green bodies that contain ASTs correlates with the size of the expanded AST molecules at pH 9. A possible explanation is that swelling of the AST particles locally decreases the interparticle distance, which leads to increased van der Waals forces between the ceramic particles.
In Situ Rheological Investigation of the Coagulation in Aqueous Alumina Suspensions
Authors: Balzer Beate , Hruschka Martin K. M. , Gauckler Ludwig J.
Published: July 2010

Electrostatically stabilized alumina suspensions can be destabilized by the enzyme‐catalyzed decomposition of urea (direct coagulation casting). Depending on the conditions, this reaction can shift the pH of a suspension to the buffer pH of the reaction products or increase the ionic strength at the buffer pH. The coagulation for both mechanisms was investigated using in situ rheological measurements. Using a vane tool in oscillation mode, the measuring conditions were optimized to find a reasonable method for time‐dependent measurements. Constant parameters (stress or strain) proved to be unsuitable, because the linear viscoelastic region shifted considerably during the coagulation. Furthermore, the gel structure produced on coagulation via increase of ionic strength (ΔI) was very sensitive to the oscillation. Therefore, for long‐time experiments, a short continuous measurement with a low strain was followed by amplitude sweeps with increased intervals to determine the linear values of G′ and G″. In this way, the increase of the moduli G′ and G″ could be followed for longer times, and it was possible to demonstrate two results. First, the final G′ of the network was about 10 times higher for ΔI‐coagulated material than for suspensions coagulated via pH shift (ΔpH). Second, particle rearrangement processes took place in ΔI‐coagulated networks even after the chemical changes were finished, whereas ΔpH‐coagulated samples were “frozen‐in” when approaching the isoelectric point and showed no further physical changes afterward.
Ferroelastic and Plastic Deformation of t′‐Zirconia Single Crystals
Authors: Baither Dietmar , Bartsch Martin , Baufeld Bernd , Tikhonovsky Aleksander , Foitzik Andreas , Rühle Manfred , Messerschmidt Ulrich
Published: July 2010

Polydomain tetragonal (t′) zirconia was deformed in compression along a 〈100〉 orientation at various temperatures between 500° and 1400°C. The stress‐strain curves showed a plateau corresponding to ferroelastic deformation, followed by plastic deformation at a higher yield stress level. In both ranges, the strain‐rate sensitivity of the stress was measured by stress‐relaxation tests. The microstructure of the tetragonal domains after ferroelastic deformation and the dislocation substructure were studied by transmission electron microscopy in a high‐voltage electron microscope. As expected, ferroelastic deformation suppressed the tetragonal variant with its c‐axis parallel to the loading direction. The dislocation structure consisted of intersecting dislocations on different slip systems with strongly bowed‐out segments. The microprocesses of deformation are discussed here by comparing the deformation data with those of cubic zirconia deformed in the same orientation and based on the observed microstructure. The particular microstructure of t′ zirconia seems to prevent recovery, so that the high flow stress of ∼700 MPa is preserved up to 1400°C.
Chromium Oxycarbide Thin Films Prepared by Inductively Coupled Radio‐Frequency Plasma‐Assisted Magnetron Sputtering
Authors: Kado Tetsuo , Fan Qiulin
Published: July 2010

Chromium oxycarbide films were prepared on SUS 304 stainless steel at temperatures of ∼673 K using the inductively coupled radio‐frequency plasma‐assisted magnetron‐sputtering technique. An inductively coupled 13.56 MHz electromagnetic field with a power of 40 W was supplied to excite the glow discharge, and a direct‐current power of 700 W was supplied to sputter a 99.9% chromium target in an Ar/CO2/CH4 mixture and in Ar/CO mixture. Electron probe microanalysis revealed that the film contained chromium, oxygen, and carbon. Vickers microhardness of the film was determined to be >1.96 × 1010 Pa. X‐ray diffractometry measurements revealed that the film structure was cubic, and the lattice constant was a0= 0.42 nm. This structure was almost the same as that of chromium oxycarbide, Cr2CO, (rock‐salt structure, lattice constant of 0.413 nm). The corrosion resistance of the SUS 304 stainless steel to 1 kmol/m3 H2SO4 at 293 K was improved by coating it with a Cr(C,O) film to a thickness of 5.92 μm.
Wet Erosive Wear of Alumina Densified with Magnesium Silicate Additions
Authors: Galusek Dušan , Brydson Rik , Twigg Peter C. , Riley Frank L. , Atkinson Alan , Zhang Yan‐Hui
Published: July 2010

A study was made of the wet erosive wear of polycrystalline alumina of mean grain size >1 μm, containing up to 10 wt% of magnesium silicate sintering aid. For pure polycrystalline alumina, the dominant wear mechanism was grain‐boundary microfracture, leading to partial or complete grain removal. In the case of the liquid‐phase‐sintered materials, wear rates could be as low as 25% of those of pure alumina of the same mean grain size, and the main material removal mechanism was transgranular fracture combined with tribochemical wear. The use of Cr3+ photoluminescence line broadening showed much higher levels of local stress in the magnesium silicate‐sintered materials (∼450 MPa) than in the pure‐alumina materials (∼200 MPa). Grain‐boundary compressive hoop stresses, caused by the thermal expansion mismatch between a continuous magnesium silicate film and the alumina grains, provided an explanation for the improved wear resistance of the alumina sintered with magnesium silicate.
High‐Temperature Fracture Energy of Superplastically Forged Silicon Nitride
Authors: Kondo Naoki , Inagaki Yoshiaki , Suzuki Yoshikazu , Ohji Tatsuki
Published: July 2010

The fracture energy of superplastically forged silicon nitride, where rodlike silicon nitride grains are aligned in one direction, was investigated at high temperatures from 1100° to 1300°C and at room temperature. Bending tests using chevron‐notched beams were conducted at two displacement rates, 0.05 and 0.005 mm/min. The superplastically forged silicon nitride showed remarkably high fracture energies, 200–630 J/m2. The fracture energy was largely dependent on the temperature and the displacement rate. The high fracture energy was attributed to grain pullout enhanced by the softened grain‐boundary glassy phase and the aligned rodlike grains.
Compressive Creep Performance of SrFeO3
Authors: Kleveland Kjersti , Wereszczak Andrew , Kirkland Timothy P. , Einarsrud Mari‐Ann , Grande Tor
Published: July 2010

Compressive creep performance of strontium‐deficient and strontium‐excess SrFeO3‐δ materials has been investigated in the temperature range 800°–1000°C and in the stress range 2.5–25 MPa. The absolute densities of the strontium‐deficient and strontium‐excess materials are 4.99 and 5.25 g/cm3, respectively, which corresponds to porosities of ∼2% and 5%, respectively. Both materials contain secondary phases because of the cation nonstoichiometry. The creep rate is faster for the strontium‐deficient material than the strontium‐excess material. The stress exponent is approximately unity, and the activation energy is 260± 30 kJ/mol for both materials. The results can be explained by a cation diffusion mechanism. The present findings are discussed in relation to previous sintering data and the possible application of these materials as oxygen‐permeable membranes.
Micromechanics of Deformation in Porous Liquid‐Phase‐Sintered Alumina under Hertzian Contact
Authors: DiGiovanni Anthony A. , Chan Helen M. , Harmer Martin P. , Nied Herman F.
Published: July 2010

A series of fine‐grained porous alumina samples, with and without a liquid phase, were fabricated in compositions matched closely to commercially available alumina used as microelectronic substrates. Hertzian indentation on monolithic specimens of the glass‐containing samples produced a greater quasi‐ductile stress–strain response compared with that observed in the pure alumina. Maximum residual indentation depths, determined from surface profilometry, correlated with the stress–strain results. Moreover, microstructural observations from bonded interface specimens revealed significantly more damage in the form of microcracking and under extreme loading, pore collapse, in the glass‐containing specimens. The absence of the typical twin faulting mechanism observed for larger‐grained alumina suggests that the damage mechanism for quasi‐ductility in these fine‐grained porous aluminas was derived from the pores acting as a stress concentrator and the grain boundary glass phase providing a weak path for short crack propagation.
Nucleation and Crystallization of New Glasses from Fly Ash Originating from Thermal Power Plants
Authors: Barbieri Luisa , Lancellotti Isabella , Manfredini Tiziano , Pellacani Gian Carlo , Rincòn Jesus Ma. , Romero Maximina
Published: July 2010

The nucleation and crystallization kinetics of new glasses obtained by melting mixtures of a Spanish carbon fly ash with glass cullet and dolomite slag at 1500°C has been evaluated by a calculation method. These glasses, whose microstructure was examined by TEM carbon replica, were susceptible to controlled crystallization in the 800°–1100°C range. The resulting glass‐ceramics developed acicular and branched wollastonite crystals or a network of dendritic pyroxene mixed with anorthite feldspar (SEM and EDX analysis). The time–temperature–transformation curves (processing of the XRD data) showed the crystallization kinetics and the critical cooling rate to be in the 12°–42°C/min range.
Effect of Rapid Solidification on Microstructural Evolution in MgO–MgAl2O4
Authors: Bhatia Tania , Chattopadhyay Kamanio , Jayaram Vikram
Published: July 2010

The response of the MgO–20 and 30 mol% Al2O3 compositions to rapid solidification has been studied. The oxides were twin‐roller quenched and the resulting flakes were characterized by X‐ray diffraction and transmission electron microscopy. The results indicate that metastable extensions of spinel and periclase occurred and the microstructural pathway was determined from the final microstructure. The flakes having MgO–20 mol% Al2O3 show a dendritic structure consisting of periclase and spinel. In the MgO–30 mol% Al2O3 composition, the liquid transforms to spinel partitionlessly. The spinel is believed to undergo decomposition by a modulation in composition, and the resulting microstructure consists of spinel and periclase. Kinetic and thermodynamic aspects of phase selection have been rationalized based on the metastable extensions of the different phase fields in the phase diagram. It has been proposed that composition fluctuations in spinel are stabilized because of the formation of disordered phases with a continuous range of order parameter on the tetrahedral sublattice.
High‐Resolution Transmission Electron Microscopy Observations on Antiphase Boundaries in Barium Lanthanum Magnesium Niobate
Authors: Lee Hwack Joo , Park Hyun Min , Song Yong Won , Cho Yang Koo , Nahm Sahn
Published: July 2010

Atomic structural observations on the antiphase boundaries (APBs) in the complex barium lanthanum magnesium niobate perovskite compound Ba0.7La0.3(Mg0.43Nb0.57)O3 (BLMN), which has a 1:1 chemical ordering of B‐site cations, were conducted using high‐resolution transmission electron microscopy. Using APB contrast, the curved APB was determined to have a ledged structure, with a terrace that was composed of the (111) plane at an atomic level. In APBs with finite widths, microfacets on the (111) planes also were observed.
Spheroidization of Titanium Carbide Powders by Induction Thermal Plasma Processing
Authors: Li Ya‐Li , Ishigaki Takamasa
Published: July 2010

Highly spherical particles of titanium carbide (TiC) have been produced by in‐flight heat processing of irregularly shaped TiC powders in an aerosol reactor under argon‐hydrogen and argon‐helium induction thermal plasma. The spherical powders obtained by the plasma treatment consist of unagglomerated and uniform particles with mean diameters between 25 and 28.5 μm, which is smaller than the original TiC particle mean diameters (29.5 μm) because of partial evaporation of the particles during the plasma treatment. The spheroidization ratio of the treated TiC powders increases with the increase of hydrogen flow rate in plasma gases and the reduction of powder feeding carrier gas flow rate. Under certain processing conditions, the TiC powders have been completely spheroidized. The morphology and structure of individual spherical particles were examined and their formation mechanism was discussed based on calculation of heat transfer kinetics of the particles in the thermal plasma.
Influence of Acidity on the Stability and Rheological Properties of Ionically Stabilized Alumina Suspensions in Ethanol
Authors: Wang Gonghou , Nicholson Patrick S.
Published: July 2010

The ionic stability of alumina particles in moderately concentrated ethanol suspensions is studied. Surface chemistry and interparticle forces are manipulated by controlling the acidity of the suspensions without dispersants. The acidity of ethanol solution is determined using ion transfer functions, wherein the relationships between acidity, alumina particle surface charge, zeta‐potential, stability, and suspension rheological behavior are established. Positive isoelectric point (IEP) shift is observed for alumina in ethanol on increasing the solids concentration. However, dilute and concentrated aqueous suspensions of alumina give the same IEP. The viscosity and flow curves for alumina/ethanol suspensions are acidity dependent. The flow curves of the suspensions follow the Casson model, and the Casson yield value is used to evaluate suspension stability.
Effects of Matrix Cracks on the Thermal Diffusivity of a Fiber‐Reinforced Ceramic Composite
Authors: McDonald Kathleen R. , Dryden John R. , Zok Frank W.
Published: July 2010

Effects of matrix cracks and the attendant interface debonding and sliding on both the longitudinal and the transverse thermal diffusivities of a unidirectional Nicalon/MAS composite are investigated. The diffusivity measurements are made in situ during tensile testing using a phase‐sensitive photothermal technique. The contribution to the longitudinal thermal resistance from each of the cracks is determined from the longitudinal diffusivity along with measurements of crack density. By combining the transverse measurements with the predictions of an effective medium model, the thermal conductance of the interface (characterized by a Biot number) is determined and found to decrease with increasing crack opening displacement, from an initial value of ∼1 to ∼0.3. This degradation is attributed to the deleterious effects of interface sliding on the thermal conductance. Corroborating evidence of degradation in the interface conductance is obtained from the inferred crack conductances coupled with a unit cell model for a fiber composite containing a periodic array of matrix cracks. Additional notable features of the material behavior include: (i) reductions of ∼20% in both the longitudinal and the transverse diffusivities at stresses near the ultimate strength, (ii) almost complete recovery of the longitudinal diffusivity following unloading, and (iii) essentially no change in the transverse diffusivity following unloading. The recovery of the longitudinal diffusivity is attributed to closure of the matrix cracks. By contrast, the degradation in the interface conductance is permanent, as manifest in the lack of recovery of the transverse diffusivity.
Effect of Dynamic Microstructural Change on Deformation Behavior in Liquid‐Phase‐Sintered Silicon Carbide with Al2O3–Y2O3–CaO Additions
Authors: Nagano Takayuki , Gu Hui , Kaneko Kenji , Zhan Guo‐Dong , Mitomo Mamoru
Published: July 2010

Nanocrystalline β‐SiC with additions of 7 wt% Al2O3, 2 wt% Y2O3, and 1 wt% CaO was subjected to tensile deformation to study its microstructural behavior under the dynamic process. The liquid‐phase‐sintered body had a relative density of >97% and an average grain size of 170 nm. Tension tests were conducted at initial strain rates ranging from 2 × 10−5 to 5 × 10−4 s−1, in the temperature range 1973–2023 K, in both argon and N2 atmospheres. Although grain‐boundary liquids formed by the additions vaporized concurrently with the decomposition of SiC and extensive grain growth, the maximum tensile elongation of 48% was achieved in argon. Annealing experiments under the same conditions revealed that vaporization and grain growth were both dependent on experimental time. Therefore, high strain rates suffered less from the hardening effect when cavitation damage was more severe. Testing in an N2 atmosphere brought about crystallization of the grain‐boundary phase and prevented severe vaporization; however, fracture occurred at only 8% elongation. Grain‐boundary sliding was still the dominant mechanism for deformation.
Crack‐Healing Behavior Under Stress of Mullite/Silicon Carbide Ceramics and the Resultant Fatigue Strength
Authors: Ando Kotoji , Furusawa Kotokaze , Chu Min Cheol , Hanagata Tsuyoshi , Tuji Kiichi , Sato Shigemi
Published: July 2010

Mullite/SiC composite ceramics were sintered and subjected to three‐point bending of specimens made according to the appropriate JIS standard. A semicircular surface crack 100 to 250 μm in diameter was made on each specimen. We systematically studied crack‐healing behavior and cyclic‐ and static‐fatigue strengths at room temperature and 1000°C (crack‐healing temperature) by using three types of specimens (smooth, cracked, and crack‐healed). The main conclusions are as follows: (i) mullite/SiC composite ceramics have the ability to heal after cracking; (ii) crack‐healed specimens exhibited higher static and fatigue strengths than those of smooth specimens, which was caused by crack‐healing; (iii) a sample crack‐healed at 1000°C had a high fatigue strength at 1000°C; and (iv) mullite/SiC ceramics can heal a crack under stress at 1000°C, and this behavior was considered using crack‐driving force and crack‐healing force, qualitatively.
Direct Compositional Analysis of Ordered Domains in Pure and La‐Doped Pb(Mg1/3Nb2/3)O3 by Analytical Electron Microscopy Techniques
Authors: Miao Shu , Zhang Xiaowen , Zhu Jing
Published: July 2010

The ordered domain structures in Pb(Mg1/3Nb2/3)O3(PMN) and Pb1–xLax(Mg1/3Nb2/3)O3 are identified using high‐resolution transmission electron microscopy (HRTEM) and nanobeam diffractometry (NBD). The chemical compositions in the ordered domains and in the disordered matrices are also acquired using energy‐dispersive spectroscopy (EDS). The best matching computer‐simulated HRTEM image has a Mg2+/Nb2+ ratio of return ½. There is no obvious chemical composition difference between the ordered domains and the disordered matrices. The number of the normalized total positive valence electrons remains almost constant in the ordered domains and in the disordered matrices for all the samples. The reason for the growth of the ordered domains in La‐doped PMN also is discussed.
Microstructure and Dielectric Properties of Barium Strontium Magnesium Niobate
Authors: Lee Hwack Joo , Park Hyun Min , Song Yong Won , Cho Yang Koo , Nahm Sahn , Byun Jae‐Dong
Published: July 2010

Dielectric properties and their related microstructural characteristics in solid solutions of (1 –x)Ba(Mg1/3Nb2/3)O3–xSr(Mg1/3Nb2/3)O3 (BMN–SMN, or BSMN) were investigated by measuring the relative permittivity (ɛr), Q values, and temperature coefficient of resonator frequency (τf), and by observing microstructure using transmission electron microscopy. When the tolerance factor (t) was >0.99 in BSMN with composition 0 < x < 0.5, where the tilting of oxygen octahedra was not involved, the microstructure included only 1:2 ordered phase. In the region where 0.99 > t > 0.97 with 0.7 < x < 1.0, the phase due to the antiphase tilting of oxygen octahedral, the disordered phase, and the 1:2 ordered phase were also present. In a few of the grains, core–shell‐type structures, whose main components were dislocations and stacking faults, were found in the solid solution of BSMN.
Dielectric Properties of Lead Magnesium Niobate and Lead Iron Niobate Prepared by the Semiwet Hydroxide Route
Authors: Mohan Dhirendra , Prasad Ram , Banerjee Srikumar
Published: July 2010

Lead magnesium niobate (PMN) and lead iron niobate (PFN) were prepared by the semiwet hydroxide route, and their dielectric properties were measured in temperature ranges around their peak dielectric constants. The dielectric constant of PFN was much larger as compared with that of PMN sintered at the same temperature. The dielectric properties of PMN and PFN are compared and explained on the basis of their structure development.
Colloidal Processing and Ionic Conductivity of Fine‐Grained Cupric‐Oxide‐Doped Tetragonal Zirconia
Authors: Sakka Yoshio , Ozawa Kiyoshi , Uchikoshi Tetsuo , Hiraga Keijiro
Published: July 2010

CuO‐doped tetragonal ZrO2 (3‐mol%‐Y2O3‐doped tetragonal zirconia, 3Y‐TZ) green bodies were consolidated from zirconia slurries with Cu2+ by a pressure filtration method. The slurries were prepared by dispersing 3Y‐TZ powder in a solution of [NH4OH + NH3NO3] = 0.1M at pH 11 and adding an appropriate amount of Cu(NO3)·3H2O solution. Green bodies with narrow pore‐size distribution were obtained after cold isostatically pressing the pressure‐filtrated bodies. Small amounts of CuO‐doped samples were densified fully at 1200°C. The size of a grain of 0.16‐mol%‐CuO‐doped 3Y‐TZ sintered at 1200°C was 84 nm. Bulk and grain‐boundary conductivities are measured by a complex impedance method. The bulk conductivity of the CuO‐doped 3Y‐TZ was almost equal to the undoped one, but the grain‐boundary conductivity decreased with CuO addition.
Hydroxyapatite Coating on Thermally Oxidized Titanium Substrates
Authors: Vatanatham Minkmas , Kimura Shoichi
Published: July 2010

Titanium substrates were oxidized in oxygen or air at temperatures of 600°–800°C, then immersed in solutions of 2.0mM– 20.7mM CaCl2 and 1.2mM–12.4mM KH2PO4 for aging periods of 0.5–10 d. The titanium surface was successfully coated with hydroxyapatite (HAP) when the substrates were oxidized in oxygen gas at 610°C for 1 h and then aged in a solution of 2.00mM Ca2+ and 1.20mM PO43−. The Ca/P ratio of the surface coating increased toward its stoichiometric HAP value (return 10/6) as the aging time increased; the Ca/P ratio attained a value of 1.66 after 10 d.
Sol‐Gel‐Derived Silicon‐Boron Oxycarbide Glasses Containing Mixed Silicon Oxycarbide (SiCxO4−x) and Boron Oxycarbide (BCyO3−y) Units
Authors: Gervais Christel , Babonneau Florence , Dallabonna Nicola , Sorarù GianDomenico
Published: July 2010

The introduction of B atoms in SiOC glass networks has been achieved through the pyrolysis of sol‐gel‐derived polyborosiloxanes under an inert atmosphere. The starting gels were obtained from hydrolysis‐condensation reactions of triethylborate (B(OEt)3) and an organically modified trialkoxysilane (EtSi(OEt)3). The resulting hybrid EtSiO1.5‐B2O3 gels showed a homogeneous dispersion of the B atoms in the siloxane network via ≡Si—O—B≦ bonds. The presence of such borosiloxane bridges prevents the formation of cyclic or cage siloxane entities and leads to relatively high ceramic yields (∼80%). The transformation of the polyborosiloxanes into amorphous SiBOC glasses was followed using Fourier transform infrared spectroscopy and multinuclear magic‐angle spinning‐nuclear magnetic resonance (MAS‐NMR) (11B, 13C, and 29Si). An important change in the carbon, silicon, and boron environments occurs during pyrolysis. Interestingly, the 11B MAS‐NMR spectra suggest a progressive replacement of the B—O bonds by B—C bonds, which leads to a distribution of trigonal BCxO3−x sites in the glass that was pyrolyzed at 1000°C, with a residual amount of B(OSi)3 sites. The resulting glasses can thus be described as silicon‐boron oxycarbide networks that are based on SiCxO4−x and BCyO3−y mixed environments.
Novel Silicon‐Boron‐Carbon‐Nitrogen Materials Thermally Stable up to 2200°C
Authors: Wang Zhi‐Chang , Aldinger Fritz , Riedel Ralf
Published: July 2010

Three novel Si‐C‐B‐N ceramic compositions, namely Si2.9B1.0C14N2.9, Si3.9B1.0C11N3.2 and Si5.3B1.0C19N3.4, were synthesized using the polymer‐to‐ceramic transformation of the polyorganoborosilazanes [B(C2H4Si(Ph)NH)3]n, [B(C2H4Si(CH3)NH)2–(C2H4Si(CH3)N(SiH2Ph))]n, and [B(C2H4Si(CH3)–N(SiH2Ph))3]n, where Ph is phenyl (C6H5), at 1050°C in argon. The Si‐B‐C‐N ceramics exhibited significant stability with respect to composition and mass change in the temperature range between 1000° and 2200°C, including isothermal annealing of the samples at the final temperature for 30 min in argon. The mass loss rate at 2200°C was as low as 1.4 wt%·h−1 for Si5.3B1.0C19N3.4, 1.7 wt%·h−1 for Si2.9B1.0C14N2.9, and 2.4 wt%·h−1 for Si3.9B1.0C11N3.2. The measured amount of mass loss rate was comparable to that of pure SiC materials. As crystalline phases, β‐Si3N4 and β‐SiC were found exclusively in the samples annealed at 2200°C at 0.1 MPa in argon. For thermodynamic reasons, β‐Si3N4 should have decomposed into the elements silicon and nitrogen at that particular temperature and gas pressure. However, the presence of β‐Si3N4 in our materials indicated that carbon and boron kinetically stabilized the Si3N4‐based composition.
Molecular Dynamics Study of Atomic Structure and Diffusion Behavior in Amorphous Silicon Nitride Containing Boron
Authors: Matsunaga Katsuyuki , Iwamoto Yuji
Published: July 2010

We have performed molecular dynamics simulations of amorphous Si3N4 containing boron (Si‐B‐N). We have examined short‐range atomic arrangements and self‐diffusion constants of amorphous Si‐B‐N systems with various boron contents. Our simulations show that boron atoms are threefold coordinated by nitrogen atoms and that nitrogen atoms are bonded to both silicon and boron atoms in the amorphous network of Si‐B‐N. Also, the self‐diffusion constant of nitrogen in Si‐B‐N is much decreased compared with that in amorphous Si3N4. This suggests that boron is important in decreasing the mobility of atoms in amorphous Si‐B‐N, which may explain the improved thermal stability of amorphous Si‐B‐N relative to amorphous Si3N4 observed experimentally.
Surface Damage Resistance of Gel‐Derived Oxycarbide Glasses: Hardness, Toughness, and Scratchability
Authors: Rouxel Tanguy , Sanglebœuf Jean‐Christophe , Guin Jean‐Pierre , Keryvin Vincent , Soraru Gian‐Domenico
Published: July 2010

Gel‐derived oxycarbide glasses have atomic network structures similar to that of vitreous silica glass but with carbon‐rich regions consisting of CSi4 tetrahedra and C–Si–O bonds finely dispersed in the glass. Therefore, oxycarbide glasses exhibit the so‐called “anomalous” hardness behavior, similar to silica‐rich glasses, with a substantial densification–strain component beneath the indenter. However, the role of carbon is twofold: on the one hand, the covalently bonded carbon atoms slightly affect the behavior, similar to the way network modifiers affect the behavior of silicate glasses, and favor a “normal” indentation behavior; and on the other hand, the free carbon, forming turbostratic graphite domains, provides easy crack initiation sites and low‐energy fracture paths. Almost concentric shear steps and microcracks, which follow the turbostratic graphite domains, are observed after indentation. The ultimate coalescence of the microcracks produces Hertzian‐type cone cracks.
Stresses Occurring during Joining of Ceramics Using Preceramic Polymers
Authors: Lewinsohn Charles A. , Colombo Paolo , Reimanis Ivar , Ünal Özer
Published: July 2010

This paper reviews the current literature and describes issues related to using preceramic polymer precursors for joining ceramics for high‐temperature applications. In particular, the effects of precursor yield, temperature, and heating rate on the maximum stress due to shrinkage associated with conversion of preceramic precursors to covalently bonded network structures are discussed. Conditions where the joint material viscosity and the shrinkage rates are high must be avoided. Fillers and prepyrolysis of the precursor can be used to decrease the stresses that occur during processing of joints.
In Situ Neutron Powder Diffraction Study of Ti3SiC2 Synthesis
Authors: Wu Erdong , Kisi Erich H. , Kennedy Shane J. , Studer Andrew J.
Published: July 2010

The synthesis of Ti3SiC2 by pressureless reactive sintering of Ti/SiC/C mixtures under an Ar atmosphere has been studied using in situ neutron diffraction. The intermediate phases TiCx and Ti5Si3Cx (x≤ 1) form first at ∼800–1400°C. These phases are consumed in the formation of Ti3SiC2, at ∼1500°C. After sintering, Ti5Si3Cx disappears but an amount of TiCx remains in the sample primarily as a surface layer. The studies appear to support a suggestion that the intermediate phases react to form Ti3SiC2 through a diffusion‐controlled process. Prolonged stepwise heating under argon in some experiments resulted in decomposition of Ti3SiC2 above ∼1400°C and significant disproportionation of the sample.
Catalytic and Electrochemical Properties of Doped Lanthanum Chromites as New Anode Materials for Solid Oxide Fuel Cells
Authors: Vernoux Philippe , Djurado Elisabeth , Guillodo Michael
Published: July 2010

Defective perovskites contained in the general formula La(Sr)Cr(Ru,Mn)O3–δ are successfully synthesized by spray pyrolysis. Powders of high phase purity are obtained after annealing, and they are used to prepare homogeneous films by spray printing. From a catalytic point of view for the methane steam reforming, these powdered perovskites do not generate a carbon deposit. Catalytic results, focused on doped ruthenium perovskites, confirm that the insertion of ruthenium in the structure of the lanthanum chromite presents a real benefit for the methane steam reforming. The electrochemical properties of La(Sr)CrO3–δ, either pure and doped with manganese, show that the perovskite films can be potential electrodes, depending on their doping, for hydrogen anodic oxidation in solid oxide fuel cells.
Fabrication of Lanthanum Manganese Oxide Thin Films on Yttria‐Stabilized Zirconia Substrates by a Chemically Modified Alkoxide Method
Authors: Hwang Hae Jin , Towata Atsuya , Awano Masanobu
Published: July 2010

Perovskite‐type thin films of lanthanum manganese oxide (LaMnO3) were prepared on yttria (8%) stabilized zirconia substrate by the sol–gel process from an alkoxide solution of lanthanum isopropoxide (La(O‐i‐C3H7)3) and manganese isopropoxide (Mn(O‐i‐C3H7)2). The alkoxide solution was chelated with 2‐ethyacetoacetate, and further modified with polyethylene glycol (PEG). The obtained LaMnO3 thin film was transparent and macroscopically crackless. X‐ray diffraction, differential thermal analysis–thermogravimetry analysis, and scanning electron microscope observations indicated that single‐phase LaMnO3 thin films with a grain size of 80 to 100 nm are formed when a spin‐coated LaMnO3 gelled film is heated at 600°C for 1 h. The porous and homogeneous grain structure with a grain size of <100 nm can be obtained when the LaMnO3 gelled film is heated at 600° and 800°C. It was considered that PEG might accelerate the crystallization of the perovskite phase, which indicates that PEG assists the formation of the La‐O‐Mn frame network during partial hydrolysis and condensation reactions in sol–gel processes.
Volume Fraction and Whisker Orientation Dependence of the Electrical Properties of SiC‐Whisker‐Reinforced Mullite Composites
Authors: Gerhardt Rosario A. , Ruh Robert
Published: July 2010

The electrical behavior of hot‐pressed mullite composites reinforced with 10, 20, and 30% SiC whiskers (SiCw) was evaluated in the frequency range 100 Hz–10 MHz and compared with 2 GHz data as well as dc resistivity measurements. It is found that the addition of SiCw has a dramatic effect on the dielectric properties as well as the resistivity of these materials. The ac properties (dielectric constant, dielectric loss, and impedance) show a strong dependence on the volume fraction of SiC and on the orientation of the electric field with respect to the hot‐pressing direction and hence the whisker orientation. The dc resistivity measurements are sensitive to the whisker volume fraction but cannot easily discriminate between those samples that were measured parallel or perpendicular to the hot‐pressing direction as was possible with the ac measurements.
Rheological Properties of Cement‐Based Extrudates with Fibers
Authors: Li Zongjin , Mu Bin
Published: July 2010

To facilitate the application of the extrusion technique in the manufacture of short‐fiber‐reinforced cement‐based building products, the rheological behavior of the fresh cementitious extrudate in an extruder has been studied. In this study, the fresh cementitious extrudate was modeled as a viscoelastic material moving in a three‐dimensional extruder channel. The viscosity‐related and elasticity‐related material parameters were obtained using a homemade rheometer. The governing equations were linearized and solved numerically by the finite‐difference method, using the suitable velocity and stress boundary conditions. The velocity profiles of the fresh cementitious extrudates were obtained and discussed. These results were useful in guiding both the die design of an extruder and the mixing proportion design of cementitious composites.
Morphology of Vickers Indent Flaws in Soda–Lime–Silica Glass
Authors: Whittle Ben R. , Hand Russell J.
Published: July 2010

The subsurface structure of Vickers indents in soda–lime–silica glass was investigated using confocal microscopy and conventional microscopy. It was determined that the lateral cracks that form beneath the indentation site propagate away from the surface. The median/radial (MR) crack system was found to be semielliptical in shape. The growth of the lateral and MR cracks was found to be codependent such that the depth of the lateral crack limited the depth of the MR crack, and the presence of the MR crack caused deflections in the direction and increased the extent of lateral crack growth.
Grain‐Boundary Viscosity of Preoxidized and Nitrogen‐Annealed Silicon Carbides
Authors: Pezzotti Giuseppe , Kleebe Hans‐Joachim , Nishimura Hitoshi , Ota Ken'ichi
Published: July 2010

Internal friction experiments were conducted on a model SiC polycrystal prepared from preoxidized (high‐purity) SiC powder. This material contained high‐purity SiO2 glass at grain boundaries in addition to a free‐carbon phase, which was completely removed upon powder preoxidation. Comparative tests were conducted on a SiC polycrystal, obtained from the as‐received SiC powder with the addition of 2.5 vol% of high‐purity SiO2. This latter SiC material was also investigated after annealing at 1900°C for 3 h in a nitrogen atmosphere. Electron microscopy observations revealed a glass‐wetted interface structure in SiC polycrystals prepared from both as‐received and preoxidized powders. However, the former material also showed a large fraction of interfaces coated by turbostratic graphite. Upon high‐temperature annealing in nitrogen, partial glass dewetting occurred, and voids were systematically observed at multigrain junctions. The actual presence of nitrogen could only be detected in a limited number of wetted interfaces. A common feature in the internal friction behavior of the preoxidized, SiO2‐added and nitrogen‐annealed SiC was a relaxation peak that resulted from grain‐boundary sliding. Frequency‐shift analysis revealed markedly different characteristics for this peak: both the magnitude of the intergranular glass viscosity and the activation energy for grain‐boundary viscous flow were much higher in the nitrogen‐annealed material. Results of torsional creep tests were consistent with these findings, with nitrogen‐annealed SiC being the most creep resistant among the tested materials.
Tensile Creep Behavior of a Gas‐Pressure‐Sintered Silicon Nitride Containing Silicon Carbide
Authors: Jr. Krause Ralph F. , Wiederhorn Sheldon M. , Li Chien‐Wei
Published: July 2010

The tensile creep behavior of a gas‐pressure‐sintered silicon nitride containing silicon carbide was characterized at temperatures between 1375° and 1450°C with applied stresses between 50 and 250 MPa. Individual specimens were tested at fixed temperatures and applied loads. Each specimen was pin‐loaded within the hot zone of a split‐tube furnace through silicon carbide rods connected outside the furnace to a pneumatic cylinder. The gauge length was measured by laser extensometry, using gauge markers attached to the specimen. Secondary creep rates ranged from 0.54 to 270 Gs−1, and the creep tests lasted from 6.7 to 1005 h. Exponential functions of stress and temperature were fitted to represent the secondary creep rate and the creep lifetime. This material was found to be more creep resistant than two other silicon nitride ceramics that had been characterized earlier by the same method of measurement as viable candidates for high‐temperature service.
Unusual Elastic and Mechanical Behaviors of Copper Phosphate Glasses with Different Copper Valence States
Authors: Miura Tadashi , Watanabe Toshio , Benino Yasuhiko , Komatsu Takayuki , Sato Ryuji
Published: July 2010

Elastic and mechanical properties such as Young's modulus E, Poisson's ratio ν, Debye temperature θD, Vickers hardness Hv, fracture toughness Kc, and fracture surface energies γf of yCuOx·(100−y)P2O5 glasses (y= 45, 50, 55) with different copper valence states, i.e., R(Cu+) = Cu+/(Cu++ Cu2+), at room temperature (humidity 64%) have been examined. The following features have been found: (1) the glass transition temperature (218–434°C), Hv (2.7–4.4 GPa), E (50.6–78.2 GPa), and θD (358–434 K) decrease largely with increasing R(Cu+); (2) the mean atomic volume, Kc (0.56–1.14 MPa·m1/2), and γf (1.9–11.2 J·m−2) tend to increase with increasing R(Cu+); (3) 50CuOx·50P2O5 glasses with R(Cu+) = 0.42 and 0.55 have a high resistance against crack formation in Vickers indentation tests and no crack is observed in the 45CuOx·55P2O5 glass with R(Cu+) = 0.57 under an applied load of about 98 N. The results demonstrate that elastic and mechanical properties of yCuOx·(100−y)P2O5 glasses depend strongly on the copper valence state and the CuOx/P2O5 ratio. The unusal mechanical and elastic properties of copper phosphate glasses are well explained qualitatively by considering unique oxygen coordination and bonding states of Cu+ ions, i.e., lower coordination number and more covalent bonding compared with Cu2+ ions.