bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrothermal Synthesis of Nanometer‐Sized Barium Titanate Powders: Control of Barium/Titanium Ratio, Sintering, and Dielectric Properties
Authors: Pinceloup Pascal , Courtois Christian , Leriche Anne , Thierry Bernard
Published: July 2010

Nanometer‐sized BaTiO3 powders have been synthesized hydrothermally from Ba(OH)2 and titanium alkoxide at 150°C for 2 h, and the Ba/Ti ratio has been measured with an accuracy of ±0.003. Stoichiometric powders can be obtained by adjusting the Ba/Ti ratio of the reactants to a value of 1.018. At a lower Ba/Ti ratio, the solubility of Ba(OH)2 prevents full incorporation of barium, and barium‐deficient powders result. A higher Ba/Ti ratio leads to the incorporation of excess barium in the powder. Ks(BaTiO3,‐25°C) = 7 × 10‐8 has been calculated for the equilibrium reaction. From this result, two reproducible processes for the synthesis of stoichiometric BaTiO3 are proposed. The processes rely only on very accurate control of the chemical composition (Ba/Ti ratio) of the precursor suspension. The sintering behavior of powders having Ba/Ti ratio values between 0.965 and 1.011 is described from results of dilatometric measurements and isothermal sintering. Room‐temperature dielectric constants as high as 5600 and losses as low as 0.009 have been obtained for a stoichiometry slightly less than 1.000. It is expected that optimum sintering behavior and electrical properties are obtained in the stoichiometry range 0.995‐1.000.
Effect of Carbides on the Microstructure and Properties of Ti(C,N)‐Based Ceramics
Authors: Park Dong‐Soo , Lee Yang‐Doo , Kang Shinhoo
Published: July 2010

Ti(C,N)‐based ceramics that contained various carbides were prepared via pressureless sintering, to study the effects of the carbides on the microstructure and properties of the ceramics. All the systems showed good densification behavior and formed solid solutions. Impurities introduced from the milling media were sufficient to promote the densification of the ceramics. The grain structures of the systems were a function of carbide crystal structure and their affinity with nitrogen. Microhardness, three‐point flexural strength, and fracture toughness of the Ti(C,N)‐based ceramics were measured and determined to be 16‐18 GPa, 0.7‐1.1 GPa, and 3.5‐5.5 MPa·m1/2, respectively.
Mechanical Properties of Tape‐Cast Alumina‐Glass Dental Composites
Authors: Kim Dae‐Joon , Lee Myung‐Hyun , Kim Chang‐Eun
Published: July 2010

Alumina‐glass dental composites were prepared by tape casting and sintering at 1120°C, followed by glass infiltration at 1100°C. Flexural strength and fracture toughness of the composites were investigated in terms of the influence of tape constituents, namely, alumina powder, binder, and plasticizer, on the mechanical properties. Both strength and toughness increased with increasing alumina fraction in tapes and decreased with increasing binder content in binder/plasticizer mixtures. These observations were consistent with the influence of the constituents on mean alumina particle distance in tapes, suggesting that the high strength of glass‐infiltrated alumina composites is related to toughening by crack bowing. The strength and fracture toughness of the tape‐cast composites, optimized for forming dental crowns, were 508 MPa and 3.1 MPa·m1/2, respectively, obtained from biaxial tests. Shrinkage of the composites decreased with increasing thermocompression pressure, applied to the tapes prior to sintering, and heating rate to the sintering temperature.
Surface Nitridation of Yttria‐Doped Tetragonal Zirconia Polycrystals (Y‐TZP): Microstructural Evolution and Kinetics
Authors: Chung Tai‐Joo , Lee Jong‐Sook , Kim Doh‐Yeon , Song Huesup
Published: July 2010

Sintered tetragonal‐zirconia polycrystals (TZP) were embedded in a zirconium nitride powder bed and heat‐treated at various temperatures. Surface layers of the TZP specimens were transformed to a stabilized cubic‐zirconia by nitrogen incorporation, and their thickness was observed to increase by a parabolic rate law. The nitrogen diffusivity was evaluated from the temperature dependence of the nitridation rate. The microstructure of the nitrided layer was composed of different zones of equiaxed and columnar grains. The columnar grains were developed along the nitrogen flux lines. The observed microstructural evolution was explained in terms of nucleation and growth kinetics of nitridation.
Silicon/Silicon Carbide Composites Fabricated by Infiltration of a Silicon Melt into Charcoal
Authors: Shin Dong‐Woo , Park Sam Shik , Choa Yong‐Ho , Niihara Koichi
Published: July 2010

Dense Si/SiC composites were fabricated via a conventional reaction‐bonding process, using oak charcoal that exhibited a honeycomb structure. The silicon melt was infiltrated into the porous oak charcoal (density of ~0.6 g/cm3) while the sample was heated to 1700°C under vacuum (10‐3 torr (~0.133 Pa)), which resulted in in situ silicon‐fiber/SiC composites. The reaction product had an average density of 2.8 g/cm3 and showed three‐point flexural strengths of 330 MPa at room temperature and 280 MPa at 1300°C. Good oxidation resistance also was observed at temperatures up to 1300°C in flowing air. This process provided excellent shape‐making capability, because the charcoal that was used as a preform was readily machinable.
Bimaterial Composites via Colloidal Rolling Techniques: III, Mechanical Properties
Authors: Menon Mohan , Chen I‐Wei
Published: July 2010

The mechanical properties of layered Ce‐TZP/Al2O3 composites with laminate and cellular morphologies have been investigated. The strength and toughness increase as the layer thickness decreases, and the amount of transformation in the Ce‐TZP layer increases discontinuously at the laminate/cellular transition. Very high strengths (1.1 GPa) and toughnesses (16 MPa·m1/2) have been obtained in some cases. These results indicate that the progressive refinement of layer microstructure and the disruption of planar connectivity of phases are beneficial to the mechanical performance, because they provide more stress concentrators to trigger stress‐assisted transformation for toughening functions. The composites of finer microstructure, with a layer thickness of lessthan equal to20 µm, have a homogeneous hardness of 11.5 GPa, which is a considerable improvement over that of Ce‐TZP alone.
Fabrication and Testing of Oxide/Oxide Microcomposites with Monazite and Hibonite as Interlayers
Authors: Parthasarathy Triplicane A. , Boakye Emmanuel , Cinibulk Michael K. , Petry Melvin D.
Published: July 2010

Oxide/oxide microcomposites were fabricated and tested to evaluate the effectiveness of monazite (LaPO4) and hibonite (CaAl12O19) as interlayers in sapphire‐reinforced Al2O3‐matrix composites. For interlayer thicknesses of 0.3‐0.5 µm, both interlayers showed evidence of crack deflection; however, debond lengths in hibonite‐coated specimens were limited to just a small fraction of the fiber diameter. Monazite‐coated specimens showed multiple matrix cracks and extensive debonding at the coating/matrix interface. Composite strengths were relatively high for both coatings, considering the fiber strength degradation during processing. The strengths were greater than the calculated matrix cracking stresses. However, the mean strengths were not significantly different from those of the control specimens, although coated composites had higher Weibull moduli. The lack of difference in strength is attributed to porosity in the matrix. The results imply that matrix density needs to be >85% to evaluate novel interface strategies reliably.
Experimental Phase‐Diagram Study and Thermodynamic Optimization of the Silver‐Strontium‐Copper‐Oxygen (Ag‐Sr‐Cu‐O) and Silver‐Calcium‐Copper‐Oxygen (Ag‐Ca‐Cu‐O) Systems
Authors: Assal Jérôme , Hallstedt Bengt , Gauckler Ludwig J.
Published: July 2010

The phase relations in the silver‐strontium‐copper‐oxygen (Ag‐Sr‐Cu‐O) and silver‐calcium‐copper‐oxygen (Ag‐Ca‐Cu‐O) systems were studied in oxygen, air, and nitrogen environments, using differential thermal analysis and energy‐dispersive X‐ray spectrometry. The presence of silver reduces the melting temperature of the oxide liquid to a monotectic in the strontium‐ and calcium‐containing systems by 62 and 82 K, respectively (to 1222 and 1244 K, respectively) in oxygen. The oxide liquid dissolves silver (up to a metallic ratio of 0.17) in the Ag‐Sr‐Cu‐O system in oxygen at a temperature slightly higher than that required for monotectic reaction. In the Ag‐Ca‐Cu‐O system, the silver content has been measured to be 0.29 (metallic ratio). The oxide systems have been optimized using the experimental data from monotectics and those from lower‐order systems that have been published previously. This work is part of a project to evaluate the phase relations within the silver‐bismuth‐strontium‐calcium‐copper‐oxygen (Ag‐Bi‐Sr‐Ca‐Cu‐O) system and, in particular, to focus on the influence of silver on the phase equilibria around the superconducting phases during partial‐melt processing.
Co2+‐Substitution Effect in Ce‐TZP/La M‐type Hexaferrite Composites
Authors: Kojima Takashi , Sakamoto Wataru , Yogo Toshinobu , Fujii Takashi , Hirano Shin‐ichi
Published: July 2010

A Ce‐TZP/platelike La(Co(Fe0.9Al0.1)11)O19 composite was synthesized in situ while sintering from a mixture of Ce‐TZP, La(Fe0.9Al0.1)O3, Fe2O3, Al2O3, and CoO powders. Platelike La(Co(Fe0.9Al0.1)11)O19 crystals were grown in a dense Ce‐TZP matrix after sintering at temperatures of 1200°–1350°C. The temperature range for sintering Ce‐TZP/La(Fe,Al)12O19 composites was expanded widely by substituting Co2+ ions for Fe2+ ions in its structure. The highest value of the bending strength of the Ce‐TZP/La(Co(Fe0.9Al0.1)11)O19 composites was 880 MPa, which was higher than that of the Ce‐TZP/La(Fe,Al)12O19 composite (780 MPa) and Ce‐TZP (513 MPa). The saturation magnetization of the Ce‐TZP/La(Co(Fe0.9Al0.1)11)O19 composite was a constant value of 7.7 emu/g after the composite was sintered at 1200°–1350°C.
Modeling of Grain‐Boundary Segregation Behavior in Aluminum Oxide
Authors: Cho Junghyun , Rickman Jeffrey M. , Chan Helen M. , Harmer Martin P.
Published: July 2010

It is believed that the segregation of oversized dopant ions to grain boundaries in Al2O3 hinders grain‐boundary diffusion, thereby reducing the tensile creep rate in this system by ∼2–3 orders of magnitude. In order to explain this improvement in creep behavior, it is helpful to characterize both the effective cation and interstitial volumes at grain boundaries, because the relative openness of some boundary structures suggests a great accommodation of oversized ions. In this study, the boundary volume is determined by a spatially local Voronoi construction, which highlights cation (Al3+) substitutional sites as well as large interstitial voids. In particular, we examine the spatial distribution of free volume near grain boundaries and, in addition, the dependence of the driving force for segregation on misfit strain in doped Al2O3. We interpret our results in light of recent evidence that selective codoping can provide a more efficient means of filling available space near boundaries, thereby further enhancing creep resistance.
R‐Curve Behavior and Crack‐Closure Stresses in Barium Titanate and (Mg,Y)‐PSZ Ceramics
Authors: Meschke Frank , Raddatz Oliver , Kolleck Arnd , Schneider Gerold A.
Published: July 2010

R‐curves, process zones, and shielding stresses of barium titanate (BaTiO3) and partially stabilized zirconia (PSZ) have been studied using compact‐tension (CT) specimens. BaTiO3 and PSZ exhibited pronounced R‐curves that rose over similar crack lengths and showed steady‐state toughnesses of 0.7 and 6.4 MPa·m1/2, respectively. Both steady‐state toughnesses were ∼80% larger than the initial fracture toughnesses. Ferroelastic domain switching was the main toughening mechanism in BaTiO3, whereas, in PSZ, transformation toughening was the main toughening mechanism. The crack process zone and crack‐opening‐displacement (COD) profile of each material was studied in detail using atomic force microscopy. Crack‐closure‐stress distributions were extracted from the COD profiles, using weight‐function methods. The resulting stress profiles indicated that compressive residual stresses of 40 MPa in BaTiO3 and 400 MPa in PSZ acted in a limited region behind the crack tip. In the PSZ, crack bridging seemed to be a competing mechanism to transformation toughening.
HRTEM and EELS Studies on the Amorphization of Hexagonal Boron Nitride Induced by Ball Milling
Authors: Huang Jianyu Y. , Yasuda Hidehiro , Mori Hirotaro
Published: July 2010

We present a new approach, namely ball milling, to synthesize amorphous boron nitride (a‐BN). The amorphization kinetics are revealed by X‐ray diffractometry (XRD), high‐resolution transmission electron microscopy (HRTEM), and electron energy loss spectroscopy (EELS). HRTEM investigations indicate that, in the early stage of milling, the thick sp2 layers are sliced into many thinner sheets because of cleavage along the basal planes. In the intermediate stage of milling, deformation is accommodated primarily by simultaneous shearing along the basal planes. As a result of sustained shearing, a number of defects, such as stacking faults, (0002) [11¯20] twinning, simultaneous shearing of lattice planes, and half Frank loops with Burgers vectors of 1/2[0001], are introduced in the hexagonal BN (h‐BN) grains. Simultaneous shearing also causes significant change in the lattice symmetry of most grains. In the final stage of milling, the fiberlike tightly bonded sp2 sheets are broken and refined further, until a nanocrystalline and amorphous mixture is formed. XRD of the sample milled for 180 h exhibits an amorphous halo pattern; nevertheless, HRTEM demonstrates that the end product is essentially a nanocrystalline and amorphous mixture. The grain sizes of the nano‐crystals are <3 nm, and their stacking is turbostratic. EELS investigations of the a‐BN indicate that bonding is primarily sp2, but a small fraction of sp3a‐BN is also found, which is assumed to be the nuclei of the cubic phase (c‐BN) in the high‐pressure and high‐temperature experiments and thus facilitates the hexagonal to cubic transition. The present a‐BN fabrication method can provide an effective way to facilitate the synthesis of sintered bulk c‐BN materials.
Functionally Graded Zircon–Molybdenum Materials without Residual Stresses
Authors: Torrecillas Ramón , Espino Ana M. , Bartolomé José F. , Moya José S.
Published: July 2010

A layered, zircon–molybdenum functionally graded material was obtained by starting from commercial powders of molybdenum (median particle size (d50) of ∼3 μm) and zircon (d50∼ 0.8 μm). Conventional processing led to a material that was free of internal residual stress. The sintering behavior of green compacts with compositions that corresponded to the different layers was studied via dynamic sintering. The thermal expansion coefficients of each layer were measured using conventional dilatometry. The presence of residual stresses was determined using Vickers indentations. Both thermal expansion mismatch and differential shrinkage between the layers were negligible.
Further Improvement in Mechanical Properties of Highly Anisotropic Silicon Nitride Ceramics
Authors: Imamura Hisayuki , Hirao Kiyoshi , Brito Manuel E. , Toriyama Motohiro , Kanzaki Shuzo
Published: July 2010

Si3N4ceramics were fabricated by tape casting of a raw‐powder slurry seeded with three types of rodlike β‐Si3N4particles. The effects of seed size on the microstructure and mechanical properties of the sintered specimens were investigated. All the seeded and tape‐cast silicon nitrides presented an anisotropic microstructure, where the elongated grains grown from seeds were preferentially oriented parallel to the casting direction. The orientation degree of these grains,f0, was affected by seed size, and small‐seed addition led to the highestf0value. This material exhibited high bending strength (∼1.4 GPa) and high fracture toughness (∼12 MPa.m1/2) in the direction normal to the grain alignment, which were attributed to the highly anisotropic and fine microstructure.
Grain‐Boundary Viscosity of BaO‐Doped SiC
Authors: Pezzotti Giuseppe , Nishimura Hitoshi , Ota Ken'ichi , Kleebe Hans‐Joachim
Published: July 2010

Internal friction characterization of the viscosity of a residual SiO2/BaO glass, segregated to grain boundaries of polycrystalline SiC, is presented. The anelastic relaxation peak of internal friction, arising from viscous slip along grain boundaries wetted by a glass phase, is analyzed. Two SiC polycrystals, containing SiO2/BaO glasses with different compositions, are studied and compared with a SiC polycrystal containing only pure SiO2. The internal friction peak is first analyzed with respect to its shift upon frequency change. This analysis allows quantitative assessment of both the intrinsic viscosity and the activation energy for viscous flow of the grain‐boundary phase. Both parameters markedly decrease with increasing amounts of BaO dopant, which is consistent with data reported in the literature on SiO2 and SiO2/BaO bulk glasses with the same nominal composition. Analysis of the peak morphology is also attempted, considering the evolution of peak width while varying the grain‐boundary glass composition. Moreover, the role of microstructural parameters, such as the distributions of grain size and grain‐boundary angles, on the broadening of the internal friction peak is addressed, and a procedure is proposed that allows quantitative evaluation of the activation energy for viscous flow of intergranular glass merely from the width of the internal friction peak.
Oxidation of Ti(C,N)‐Based Ceramics Exposed at 1373 K in Air
Authors: Park Dong‐Soo , Park Chan , Lee Yang‐Doo
Published: July 2010

Ti(C,N)‐based ceramics that contained NbC, TaC, and WC were exposed at 1373 K in air for 10 h to study the effect of these metal carbides on the oxidation resistance. The addition of 10 mol% of NbC and 10 mol% of TaC to Ti(C,N) ceramics improved the oxidation resistance, whereas additions of 10 mol% of WC worsened the oxidation resistance. Weight gain due to the oxidation of Ti(C,N) ceramics with added WC varied in a complicated way, relative to the amount of WC that was involved. There was a competition between weight gain due to the oxidation and weight loss due to the volatile tungsten oxide.
Preparation Method of Submicrometer‐Sized α‐Alumina by Surface Modification of γ‐Alumina with Alumina Sol
Authors: Han Kyoung R. , Lim Chang Sup , Hong Mee‐Jeong , Jang Jin Wook , Hong Kug Sun
Published: July 2010

A method is introduced to prepare almost‐spherical submicrometer‐sized α‐alumina via surface modification of γ‐alumina with an alumina sol. Milled γ‐alumina, in the presence of 3 wt% of α‐alumina with a median particle size (d50) of 0.32 μm (AKP‐30), produced irregularly shaped α‐alumina with d50∼0.3 μm after heat treatment at 1100°C for 1 h. γ‐alumina that had been surface‐modified by milling in the presence of 3 wt% of the alumina sol resulted in almost‐monosized, spherical α‐alumina ∼0.3 μm in size after heat treatment at 1100°C for 1 h. Furthermore, almost‐spherical α‐alumina 0.1—0.2 μm in size was obtained by milling γ‐alumina with 3 wt% of AKP‐30 alumina in the presence of 3 wt% of the alumina sol, followed by heat treatment at 1100°C for 1 h. The alumina sol that has been introduced in this work seems to act as a dispersant, in addition to helping to form a spherical shape.
Electronic Structure and Bonding of β‐SiAlON
Authors: Ching Wai‐Yim , Huang Ming‐Zhu , Mo Shang‐Di
Published: July 2010

The α‐ and β‐SiAlONs are ceramic solid solutions with charge‐neutral substitutions in α‐ and β‐Si3N4. They have high potential for applications as structural materials. We have calculated the electronic structure and bonding of β‐Si6—zAlzOzN8—zforz= 0, 1, 2, 3, 4 using a first‐principles method. Total energy calculations show that the bulk modulus of β‐Si6—zAlzOzN8—zdecreases aszincreases, in general agreement with experimental trends. Simultaneous substitution of the (Si,N) pair by (Al,O) results in impurity‐like states in the upper portion of the bandgap of β‐Si3N4. Aszincreases, more and more states are introduced into the gap, forming a new conduction band (CB) edge for SiAlON. Atz= 4, the calculated bandgap is ∼1.3 eV. Density of states (DOS) calculations show the top of the valence band remains steep for allz, and the bottom of the CB is formed predominately by Si—O antibonding states. Orbitally resolved partial DOS calculations in the CB region are used to predict the trends of the electron‐energy‐loss near‐edge spectra (ELNES) of Si‐L2,3, Al‐L2,3, Si‐K, Al‐K, O‐K, and N‐Kedges in β‐SiAlON. The impurity‐like states near the CB edge result in pre‐edge structures in all ELNES spectra. Effective charge and bond order calculations show that the overall bond strength in β‐SiAlON decreases only slightly aszincreases. Although the stronger Si—N bonds are replaced by weaker Al—O bonds, the remaining Si—N and Al—O bonds actually strengthen aszincreases because of the effective charge redistribution after substitution. This is a very interesting finding that may partly explain the superior mechanical properties of the SiAlON system that render them suitable for structural applications.
Metastability of the Fluorite, Pyrochlore, and Perovskite Structures in the PbO—ZrO2—TiO2 System
Authors: Polli Andrew D. , Lange Fred F. , Levi Carlos G.
Published: July 2010

The crystallization, transformation, and partitioning of amorphous PbO—ZrO2—TiO2 powders produced by pyrolytic decomposition of partially hydrolyzed, mixed alkoxide precursors were investigated. Materials have the general formulation Pb1+χTi[1/(1+φ)]Zr[φ/(1+φ)]O3+χ, where −0.2 ≤χ≤ 0.2 is the fraction of PbO nominal excess/deficiency and 0 ≤φ≤ 1 is the Zr/Ti molar ratio. Most compositions first crystallized as a metastable fluorite structure with varying degrees of pyrochlore‐like cation ordering, which transformed to a single perovskite phase upon additional heat treatment. Higher Zr/Ti ratios enhanced the retention of fluorite and reduced the incidence of cation ordering. Compositions with off‐stoichiometric amounts of PbO often yielded extended solid solutions prior to partitioning. For example, metastable perovskites with as much as 20% PbO deficiency (χ=−0.2) could be prepared for 0 ≤φ≤ 1, but only ∼10% PbO excess could be incorporated in solid solution for 0.33 ≤φ≤ 1. Increasing PbO content was found to promote crystallization, suggesting that this oxide acts as a network modifier enhancing mobility within the initial amorphous precursor powder. Higher PbO was also noted to favor cation ordering in the metastable phase and to accelerate the transformation to perovskite, as well as to promote partitioning for hyperstoichiometric compositions. The findings are discussed in light of structural relationships between the fluorite, pyrochlore, and perovskite phases, as well as current understanding of the thermodynamics of the system.
Fabrication and High‐Temperature Phase Stability of Mo(Al,Si)2—MoSi2 Intermetallics
Authors: Ramberg C. Eric , Worrell Wayne L.
Published: July 2010

A two‐step processing technique was used to make dense, homogeneous intermetallics in the Mo(Al,Si)2—MoSi2 system. A variation of self‐propagating high‐temperature synthesis was used, in which starting pellets were nucleated at room temperature to make intermetallic powders from metallic precursors, followed by uniaxial hot pressing at 1600°—1800°C to achieve densification. The samples were held at the hot‐pressing temperatures for several hours; therefore, this study also provided qualitative phase‐stability information. The solubility limit of Al for Si in MoSi2 was <5% at 1800°C. Samples that had 10% Al substituted for Si yielded approximately equal amounts of MoSi2 and Mo(Al,Si)2.
Hollow Beads Composed of Nanosize Ca α‐SiAlON Grains
Authors: Komeya Katsutoshi , Zhang Cheng , Hotta Mikinori , Tatami Junichi , Meguro Takeshi , Cheng Yi‐Bing
Published: July 2010

Carbothermal reduction—nitridation (CRN) of SiO2 is an attractive method to manufacture Si3N4 powders with controlled grain morphology. Moreover, β‐SiAlON powders could also be synthesized from either pure powder mixture or some inexpensive raw minerals by CRN and the resulting powders favored the sintering of SiAlON product. However, there have been few works on preparing α‐SiAlON powders so far. In this work, Ca α‐SiAlON powder was synthesized by CRN of a SiO2—Al2O3—CaCO3 mixture. An unusual morphology of hollow beads 200 to 500 nm in diameter with a great deal of nanosize α‐SiAlON particles around 10 to 30 nm in diameter was observed from the resultant Ca α‐SiAlON powders, which has not been reported for SiAlON ceramics before.
Low‐Temperature Synthesis of Bismuth Titanate Niobate (Bi7Ti4NbO21) Nanoparticles from a Metal‐organic Polymeric Precursor
Authors: Durán Pedro , Moure Carlos , Villegas Marina , Tartaj Jesús , Caballero Amador C. , Fernández José F.
Published: July 2010

This paper describes the preparation of homogeneous Bi7Ti4NbO21 single‐phase ceramic powders of ∼55 nm crystallite size, at temperatures as low as 400°–500°C using a metal citrate complex method based on the Pechini‐type reaction route. The thermal decomposition/oxidation of the polymerized resin, as investigated by TG/DTA, XRD, and SEM, led to the formation of a well‐defined orthorhombic Bi7Ti4NbO21 compound with lattice parameters a= 0.544, b= 0.540, and c= 2.905 ± 0.0005 nm. Reaction takes place through an intermediate binary phase with a stoichiometry close to Bi20TiO32 which forms between 300° and 375°C. The metal‐organic precursor synthesis method, where Bi, Ti, and Nb ions are first chelated to form metal complexes and then polymerized to give a gel, allows control of the Bi/Ti/Nb stoichiometric ratio leading to the rapid formation of nanosized bismuth titanate niobate (Bi7Ti4NbO21) ceramic powders, at temperatures much lower than usually needed by conventional processing of mixed‐oxide powders.
Aging of Lime Putty: Effects on Traditional Lime Mortar Carbonation
Authors: Cazalla Olga , Rodriguez‐Navarro Carlos , Sebastian Eduardo , Cultrone Giuseppe , De la Torre Maria Jose
Published: July 2010

The influence of storing slaked lime under water for extended periods of time (i.e., aging) on Ca(OH)2 crystal morphology, texture, and carbonation evolution of various lime mortars has been studied by the combined use of X‐ray diffractometry, phenolphthalein tests, porosity measurements, electron microscopy, and ultrasonic wave propagation analyses. Mortars prepared using traditional aged lime putties (up to 14 years storage under water) show rapid, extensive carbonation, resulting in porosity reduction and ultrasonic speed increase. The aged hydrated lime mortar carbonation reaction (i.e., Ca(OH)2+ CO2= CaCO3+ H2O) follows a complex diffusive path, resulting in periodic calcite precipitation as Liesegang rings. In this case, binder:aggregate ratios >1:4 result in crack development. Nonaged commercial hydrated lime mortars show slower carbonation and need a higher binder:aggregate ratio (1:3). The carbonation of nonaged lime mortars follows a normal diffusion‐limited continuous path progressing from the mortar sample surface toward the core. Differences between aged and nonaged lime mortar carbonation evolution are explained considering Ca(OH)2 crystal shape changes (from prisms to platelike crystals) and size reduction that occurs on aging of lime putty. Implications of these results on historic building conservation using traditional lime mortars are discussed.
ZrO2 Nanopowders Prepared by Low‐Temperature Vapor‐Phase Hydrolysis
Authors: Xia Bin , Duan Lianyun , Xie Youchang
Published: July 2010

ZrO2 powder is prepared by low‐temperature vapor‐phase hydrolysis of ZrCl4. TG‐DTA, XRD, Raman, BET, and TEM methods are used to investigate the particle size, phase composition, and agglomeration before and after heat treatment. The results show that the as‐prepared ZrO2 powder is characterized by large surface area (150 m2/g), fine grain size (5.8 nm), and weak agglomeration. Additionally, the as‐prepared ZrO2 powder shows predominantly tetragonal phase attributed to a grain size effect. This route is free of powder drying and calcination processes that are essential for wet chemical preparation, contributing to less agglomeration.
Mullitization from a Multicomponent Oxide System in the Temperature Range 1200°–1500°C
Authors: Shin Hyunho , Kim Cheol‐Soo , Chang Soon‐Nam
Published: July 2010

Mullitization from a multicomponent oxide system (alumina–kaolin–quartz–feldspar–talc) was analyzed as a function of firing temperature from 1200° to 1500°C based on quantitative XRD and SEM. In the present study, whisker forms of mullite grew in three characteristic stages. In the first stage (1255°–1295°C), mullitization (nucleation) took place from glass via alumina dissolution into glass under the condition of no apparent change in glass content. The reaction in this stage was rate‐limited by alumina dissolution into glass. Extensive mullitization occurred in the 1295°–1335°C range (second stage) directly from glass. Unlike in the sol–gel‐based binary system, alumina dissolution into glass was not shown to be the rate‐controlling mechanism during this extensive mullitization stage. Finally (>1335°C, third stage), the reaction was saturated, accompanied by an apparent decrease in glass consumption rate. The impingement of mullite whiskers by other whiskers and crystals was speculated to cause mullite to grow in the transverse direction, yielding a diminished reaction rate in the final stage. Mullitization stages in this work were compared with those of the alumina–silica binary system shown in the literature.
Superstructure in a Triclinic Phase of Tricalcium Silicate
Authors: Urabe Kazuyori , Shirakami Tatsuya , Iwashima Minori
Published: July 2010

A triclinic phase of tricalcium silicate (TI) was investigated using transmission electron microscopy (TEM). Electron diffraction patterns were analyzed by introducing a subcell with the cell parameters of a= 7.081 Å, b= 7.043 Å, c= 25.230 Å, α= 89.97°, β= 90.37°, and γ= 119.44°. It was proven that the coordinates of all the reflections can be expressed to be ha*+kb*+lc*±m/6(a*+ 2b*+ 2c*), where m= 0, 1, 2, and 3. The result indicates that the structure modulation in TI is a one‐dimensional type with a structural modulation normal to (122). The modulated structure could be observed in a high‐resolution TEM image as wavy contrast streaking parallel to the plane with an interval of six times the (122) spacing.
Formation of Convoluted Silica Precipitates during Amorphous Phase Separation in the Ca3(PO4)2–SiO2–MgO System
Authors: Oliveira José M. , Correia Rui N. , Fernandes Maria H. V.
Published: July 2010

Dispersed aggregates of peculiar morphology have been obtained in phase‐separated glasses of the system Ca3(PO4)2–SiO2–MgO, where the separated phase is amorphous silica. The formation of such convoluted aggregates is tentatively explained in terms of a fast coalescence process of initial isolated quasi‐spherical droplets which behave as a dispersed phase in an emulsion‐like system.
Scanning Electron‐Beam Dielectric Microscopy for the Investigation of the Temperature Coefficient Distribution of Dielectric Ceramics
Authors: Cho Yasuo , Jintsugawa Osamu , Yamanouchi Kazuhiko
Published: July 2010

Studies on scanning electron‐beam dielectric microscopy are reported. This microscopy technique is used for determining the temperature coefficient distribution of dielectric materials using an electron beam as a heat source instead of a light beam as in photothermal dielectric microscopy. This microscopy technique, which has the ability to simultaneously observe SEM images and the material composition by EPMA, has a resolution better than that of photothermal dielectric microscopy. To demonstrate the usefulness of this technique, the two‐dimensional image of a two‐phase composite ceramic composed of TiO2 and Bi2Ti4O11 is measured.
Influence of Apparent Pressure on Wear Behavior of Self‐Mated Alumina
Authors: Ravikiran A.
Published: July 2010

Dry sliding wear experiments were conducted on self‐mated polycrystalline alumina using a ball‐on‐disk configuration. Instantaneous nominal pressures and corresponding wear rates were calculated from the measured contact diameter during sliding. Nominal pressure decreased with sliding and the wear rate decreased with decreasing pressure. These results show a good correlation between the nominal pressure and the wear rate, implying the importance of nominal pressure on the wear behavior of alumina. Although such a correlation is not new for metals, it is for ceramics.
Toughening and Strengthening of NiAl with Al2O3 by the Addition of ZrO2(3Y)
Authors: Kitaoka Atsushi , Hirota Ken , Yoshinaka Masaru , Miyamoto Yoshinari , Yamaguchi Osamu
Published: July 2010

NiAl/10‐mol%‐ZrO2(3Y) composites of almost full density have been fabricated via spark plasma sintering (SPS) for 10 min at 1300°C and 30 MPa. The former intermetallic compound, which contains a trace amount of Al2O3, has been prepared via self‐propagating high‐temperature synthesis. The composite microstructures are such that tetragonal ZrO2 (∼0.2 μm) and Al2O3 (∼0.5 μm) particles are located at the grain boundaries of the NiAl (∼46 μm) matrix. Improved mechanical properties are obtained: the fracture toughness and bending strength are 8.8 MPa·m1/2 and 1045 MPa, respectively, and high strength (>800 MPa) can be retained up to 800°C.
Effect of Annealing Conditions on Microstructural Development and Phase Transformation in Silicon Carbide
Authors: Zhan Guo‐Dong , Mitomo Mamoru , Tanaka Hidehiko , Kim Young‐Wook
Published: July 2010

The effect of annealing with and without applied pressure on the microstructural development and phase transformation was investigated in fine‐grained β‐SiC ceramics containing α‐SiC seeds. Materials annealed without pressure had a microstructure consisting of elongated grains, while materials annealed with pressure showed a duplex microstructure consisting of small matrix grains and some of elongated grains. However, annealing with pressure (25 MPa) was found to greatly retard phase transformation from β→α polytypes and inhibit grain growth. This change in lattice parameter suggests that the retardation of phase transformation and grain growth might be attributed to a reduced mass transport rate, which is the result of Al being introduced into the SiC by the annealing pressure.
Chemical Interactions Promoting the ZrO2 Tetragonal Stabilization in ZrO2–SiO2 Binary Oxides
Authors: Del Monte Francisco , Larsen Willa , Mackenzie John D.
Published: July 2010

Metastable tetragonal ZrO2 phase has been observed in ZrO2–SiO2 binary oxides prepared by the sol–gel method. There are many studies concerning the causes of ZrO2 tetragonal stabilization in binary oxides such as Y3O2–ZrO2, MgO–ZrO2, or CaO–ZrO2. In these binary oxides, oxygen vacancies cause changes or defects in the ZrO2 lattice parameters, which are responsible for tetragonal stabilization. Since oxygen vacancies are not expected in ZrO2–SiO2 binary oxides, tetragonal stabilization should just be due to the difficulty of zirconia particles growing in the silica matrix. Furthermore, changes in the tetragonal ZrO2 crystalline lattice parameters of these binary oxides have recently been reported in a previous paper. The changes of the zirconia crystalline lattice parameters must result from the chemical interactions at the silica–zirconia interface (e.g., formation of Si–O–Zr bonds or Si–O− groups). In this paper, FT‐IR and 29Si NMR spectroscopy have been used to elucidate whether the presence of Si–O–Zr or Si–O− is responsible for tetragonal phase stabilization. Moreover, X‐ray diffraction, Raman spectroscopy, and transmission electron microscopy have also been used to study the crystalline characteristics of the samples.
Novel Powder‐Processing Methods for Advanced Ceramics
Authors: Sigmund Wolfgang M. , Bell Nelson S. , Bergström Lennart
Published: July 2010

Novel approaches for optimized powder processing of advanced ceramics are reviewed with an emphasis on direct‐casting methods and solid freeform fabrication techniques. The fundamental principles for green body formation are discussed; a distinction is made between physical and chemical gels based on the nature of the induced gelation. An overview of the properties of dense suspensions is presented, with a focus on the factors controlling the maximum solids loading. Recent work on direct measurements of interparticle forces in ceramic systems is presented and related to rheological properties.
Preparation of Porous Ca10(PO4)6(OH)2 and β‐Ca3(PO4)2 Bioceramics
Authors: Engin N. Özgür , Taş A. Cüneyt
Published: July 2010

Submicrometer‐sized, pure calcium hydroxyapatite (HA, (Ca10(PO4)6(OH)2)) and β‐tricalcium phosphate (β‐TCP, Ca3(PO4)2) bioceramic powders, that have been synthesized via chemical precipitation techniques, were used in the preparation of aqueous slurries that contained methyl cellulose to manufacture porous (70%–95% porosity) HA or β‐TCP ceramics. The pore sizes in HA bioceramics of this study were 200–400 μm, whereas those of β‐TCP bioceramics were 100–300 μm. The pore morphology and total porosity of the HA and β‐TCP samples were investigated via scanning electron microscopy, water absorption, and computerized tomography.
Alumina–Aluminide Alloys (3A) Technology: II, Modeling of TixAly–Al2O3 Composites Formation
Authors: Gaus Shaun P. , Harmer Martin P. , Chan Helen M. , Caram Hugo S. , Bruhn Jörn , Claussen Nils
Published: July 2010

The reaction sintering of TixAly–Al2O3 composites from TiO2/Al starting powder mixtures has been characterized by thermogravimetry and differential thermal analysis (TG/DTA), in situ temperature measurements, and predictions via a continuum model. In order to model the TiO2/Al reaction system, it was necessary to first determine the postmill reactant concentrations and the dominant reaction. The postmill reactant concentrations were obtained from TG/DTA measurements in air, while X‐ray diffraction (XRD) was used to gain insight into the reaction mechanisms. A continuum model of the process was fitted to in situ temperature measurements by adjusting two parameters. The model was then used to investigate the effects of various processing conditions on the reaction behavior.
A New Vapor Deposition Method to Form Composite Anodes for Solid Oxide Fuel Cells
Authors: Tang Eric Z. , Etsell Thomas H. , Ivey Douglas G.
Published: July 2010

A solid oxide fuel cell (SOFC) is a complete solid‐state energy conversion device with the potential advantages of high efficiency, silent operation, and low emissions. However, the current performance of SOFCs is still limited by a number of problems. Investigations in this field have indicated that it is imperative to fabricate efficient and compatible anodes for SOFCs to minimize polarization loss and to concurrently achieve long‐term stability. In this paper, a critical review of previous studies is given and several criteria for the theoretically ideal anode are summarized. Accordingly, a newly developed vapor deposition technique, polarized electrochemical vapor deposition (PEVD), is applied to fabricate composite anodes to meet these criteria. The initial experimental results in the present study show that PEVD is capable of depositing a thin layer of yttria‐stabilized zirconia on a porous metallic electrode to form a composite anode. This will not only provide continuous ionic and electronic conducting paths in the anode to reduce the overpotential loss and resistance, but also protect the metallic electrode from further sintering, vapor loss, and poisoning in the harsh SOFC operating conditions.
Wick Debinding of Molybdenum‐Silicon‐Boron Extrudates
Authors: Summers Eric M. , Akinc Mufit
Published: July 2010

Wick debinding was investigated as possible means of binder removal for the Mo‐Si‐B extrudates. With increased temperature the amount of binder wicked out of the extrudate increased because of a decrease in viscosity, 2513 mPa·s at 75°C down to 7 mPa·s at 250°C. The permeability of the binder in the 1 μm wicking powder was higher than that in the 0.05 μm powder, 3.1 × 10−13 m2 compared to 6.024 × 10−15 m2. The amount of binder removed at a given temperature was considerably lower, which could be attributed to the larger capillary pressure difference between the 0.05 μm wick and the extrudate (1.55 × 105 Pa compared to 1.46 × 104 Pa for the 1 μm powder). Wicking removed approximately 80% of the binder in <10 h at 250°C with no defect formation.
Influence of Ca2PbO4 on Phase Formation and Electrical Properties of (Bi,Pb)2Sr2Ca2Cu3O10/Ag Superconducting Composites
Authors: Guo Yuan Chang , Liu Hua Kun , Dou Shi Xue
Published: July 2010

(Bi,Pb)2Sr2Ca2Cu3O10 (Bi2223) precursor powders with large and small amounts of Ca2PbO4 phase were prepared and used to make superconductor/silver composite tapes. The melting behavior of the powders and tapes was examined by differential thermal analysis (DTA). The influence of Ca2PbO4 on the formation and microstructure of Bi2223, and electrical properties of the tapes, was investigated by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and critical current measurements. It was found that the melting onset temperature (Tm,onset) of precursor powders and composite tapes was strongly dependent on the amount of Ca2PbO4. Tapes with a small amount of Ca2PbO4 had a higher Tm,onset and a higher optimum sintering temperature compared with tapes with a large amount of Ca2PbO4. Also because of the higher sintering temperature, the total sintering time required for the former tapes was drastically shortened compared with the latter ones (250 vs 110 h). Furthermore, the microstructure and the current‐carrying capacity of the tapes were significantly improved by reducing the Ca2PbO4 content of the precursor powders. These results are of practical significance for the commercialization of Bi‐based high‐temperature superconductors.
Ionic Mobilities and Association Energies from an Analysis of Electrical Impedance of ZrO2–Y2O3 Alloys
Authors: Luo Jie , Almond Darryl P. , Stevens Ron
Published: July 2010

The dc conductivities of ZrO2–Y2O3 ceramic alloys (in the range 3–12 mol% of Y2O3) have been obtained from ac impedance measurements at temperatures between 250° and 370°C. The Almond–West ac conductivity model has been applied to evaluate hopping rates in this system. The migration enthalpies were evaluated and shown to increase with yttria concentration, but all values determined were shown to be lower than the corresponding activation enthalpies for conductivity. The association enthalpies thus calculated were shown to be very small in 3 mol% Y2O3–ZrO3 and to increase with yttria concentration until the yttria contents were high enough to form fully stabilized cubic zirconia. For these samples the association enthalpies are about 0.19 eV, and no longer sensitive to yttria content. The low hopping rate at high yttria concentration might be attributed to low entropy in the system, which might be attributed to the formation of vacancy clusters and/or an ordering of the structure.
Direct‐Current Field Dependence of Dielectric Properties in Alumina‐Doped Barium Strontium Titanate
Authors: Wu Long , Chen Yih‐Chien , Huang Cheng‐Liang , Chou Yi‐Ping , Tsai Yuan‐Tu
Published: July 2010

The dielectric properties of (Ba0.6Sr0.4)TiO3and Al2O3‐doped (Ba0.6Sr0.4)TiO3have been characterized. The grain size of the specimen is maximum for (Ba0.6Sr0.4)TiO3that has been doped with 1 wt% Al2O3. The density and the real part of the relative dielectric constant each decrease as the Al2O3content increases. The loss factor is minimum for (Ba0.6Sr0.4)TiO3that has been doped with 2 wt% Al2O3. The dielectric constant of the specimens decreases as the applied dc field increases. The influence of the dc field on the loss factor is much less than that on the dielectric constant. The tunability is ∼24% for (Ba0.6Sr0.4)TiO3that has been doped with 1 wt% Al2O3.
Preparation and Characterization of Barium Titanate Suspensions for Stereolithography
Authors: Jang Jae Hyuk , Wang Shuhai , Pilgrim Steven M. , Schulze Walter A.
Published: July 2010

Ferroelectric photoactive suspensions for stereolithography have been developed by dispersing a high volume fraction of barium titanate powder in hexanediol diacrylate (HDDA) with the aid of effective photoinitiators and dispersants. Rheological properties showed a shear thinning behavior and a low viscosity at a shear rate adequate for the recoating process. The barium titanate–HDDA suspension showed poor curing behavior due to the large refractive index difference between the ceramic and the resin. The coarse barium titanate–HDDA suspension showed a smaller surface reflectance and a larger cure depth than the fine barium titanate–HDDA suspension.
Bimodal Microstructure in Silicon Nitride–Barium Aluminum Silicate Ceramic‐Matrix Composites by Pressureless Sintering
Authors: Fang Yi , Yu Feng , White Kenneth W.
Published: July 2010

A distinct bimodal microstructure has been obtained in a Si3N4–BAS (barium aluminum silicate) ceramic‐matrix composite by pressureless sintering. It is shown that the addition of coarse β‐Si3N4 seeds causes abnormal grain growth in this composite, and hence encourages the formation of a bimodal microstructure. This abnormal grain growth is due to the nature of the heterogeneous nucleation mechanism in Si3N4α‐to‐β phase transformation, and is promoted by the transformation. After complete phase transformation, further abnormal grain growth is comparably slow and governed by the Ostwald ripening mechanism. Therefore, a stable bimodal microstructure can be easily achieved by pressureless sintering.
Effect of Interfacial Layers on Wear Behavior of a Dental Glass‐Ceramic
Authors: Ravikiran Attota , Jahanmir Said
Published: July 2010

Wear studies on a glass‐ceramic dental restorative material were performed under two types of lubrication conditions with distilled water. In one set of experiments, the contact interface was continuously flushed with fresh water to remove the wear debris. In a second set of experiments, the water was not replaced during the testing period. Flushing resulted in a lower friction coefficient and more than twice the wear volume compared with the nonflushing condition. Examination of the worn surfaces in SEM confirmed the presence of an interfacial layer at the sliding interface for the nonflushing experiments. These results suggest that the specific conditions at the sliding interfaces and particularly the nature of interfacial layers must be considered when evaluating the tribological performance of ceramics and especially when the wear data from different studies are compared.
Resistance Degradation of Iron‐Doped Strontium Titanate Investigated by Spatially Resolved Conductivity Measurements
Authors: Rodewald Stefan , Fleig Jürgen , Maier Joachim
Published: July 2010

Resistance degradation in perovskites generally is assumed to be caused by stoichiometry variations that occur during high field stress. According to this assumption, distinct conductivity profiles should develop during degradation. In this study, microcontact impedance spectroscopy is used to determine the conductivity profiles in iron‐doped strontium titanate after dc stress with a spatial resolution of ∼20 μm. All the features of the characteristic conductivity distributions experimentally observed in single crystals agree with the theoretically predicted distributions and confirm the validity of the stoichiometry polarization model. The degraded samples are concluded to exhibit an oxygen‐vacancy enhancement at the cathode and a vacancy depletion at the anode, which leads to a hole–ionic–electron conduction (p–v–n) transition of the conduction mechanism. Conductivity measurements at polycrystals revealed “subprofiles” within single grains, which can be explained by the blocking character of grain boundaries and the resulting accumulation of vacancies on one side of the grain boundaries and a depletion of vacancies on the other side.