bluuebunny/crossref_metadata_2025 huggingface dataset:
Knoop Microhardness Anisotropy and the Indentation Size Effect on the Basal Plane of Single‐Crystal Alumina (Sapphire)
Authors: Kaji Masaki , Stevenson Michael E. , Bradt Richard C.
Published: July 2010

The Knoop microhardness anisotropy profile was determined for the basal plane of a Czochralski grown alumina single crystal for indentation test loads from 100 through 1000 g. Microhardness maxima occur at low indentation test loads for the long axis of the Knoop indenter parallel to the 〈2[Onemacr][Onemacr]0〉. Minima exist for the long axis parallel to the 〈10[Onemacr]0〉. This low indentation test load profile is attributed to slip on the primary slip system, the (0001)〈[Onemacr][Onemacr]20〉, as previously noted by Brookes and co‐workers. The degree of the microhardness anisotropy decreases for higher indentation test loads. This results from the activation of multiple slip systems to accommodate the greater amounts of plastic flow required by the larger indentation sizes. The microhardness profile becomes more uniform with increasing indentation test load until the Knoop microhardness approaches a test‐load‐independent, orientation‐independent microhardness of 1167 ± 34 kg/mm2. The indentation size effect (ISE) was further investigated through lubricated indentation hardness measurements. Lubrication of the test specimen surface significantly reduces the ISE. Results indicate that friction between the test specimen surface and the indenter facets is a major portion of the ISE.
Characterization of Nanocrystalline Forsterite Fiber Synthesized via the Sol–Gel Process
Authors: Tsai Mu‐Tsun
Published: July 2010

An alkoxide sol–gel route was developed to prepare forsterite fibers. Transparent precursor gel fibers were converted to the crystalline phase of forsterite by heating above 550°C. Fibers heated at 1100°C for 2 h showed a porous microstructure with nanocrystalline sizes of about 50–100 nm. The structural evolution of fibrous gel was characterized. Effects of water and ethanol content on microstructures and strengths of fired fibers are also discussed. Some properties of the fired fibers were investigated.
A Novel Way to Synthesize Yttrium Aluminum Garnet from Metal–Inorganic Precursors
Authors: Lu Qingmei , Dong Wensheng , Wang Haojing , Wang Xinkui
Published: July 2010

Yttrium aluminum garnet (YAG, Y3Al5O12) was synthesized by sol–gel processing from the stoichiometric amounts of aluminum pellets, Y(NO3)3·6H2O, and Al(NO3)3·9H2O or AlCl3·6H2O, with suitable kinds of acid (citric acid, acetic acid, etc.) as catalysts. Polycrystalline YAG powder was obtained by drying the YAG precursor followed by calcination at temperatures above 900°C. Thermogravimetry/differential thermal analysis and Fourier transform infrared specotrscopic analyses in air showed an exothermic peak at ∼900°C, attributed to the formation of a polycrystalline YAG phase and weight loss of 60% at 1000°C, caused by the decomposition of hydroxyl and NO3−, etc. X‐ray diffraction analysis showed that YAG can be formed at 900°C, and no other intermediate was observed. In particular, the YAG sol can be used for dry‐spinning fibers with the aid of some organic polymer.
Self‐Mending of Microcracks in Barium Titanate Glass‐Ceramic Thin Films with High Dielectric Constant
Authors: Yao Kui , Tay Francis Eng Hock , Zhu Weiguang
Published: July 2010

BaTiO3 glass‐ceramic thin films were deposited on silicon substrates by the sol–gel method. The films exhibited a mixed structure of glass phase and BaTiO3 grains tens of nanometers in diameter, and had a high dielectric constant, low leakage current, and dielectric loss. An interesting self‐mending phenomenon for microcracks due to the fluidity of the glass phase during the annealing process was observed in the films. The effects of the existence of the self‐mended microcracks on electrical properties were evaluated. Our experimental results indicate the potential value of BaTiO3 glass‐ceramic thin films for integrated high‐dielectric‐constant media.
Preparation of BaTi4O9 from Oxalates
Authors: Cernea Marin , Chirtop Elena , Neacsu Dana , Pasuk Iuliana , Iordanescu Sergiu
Published: July 2010

A barium titanate precursor with a barium:titanium ratio of 1:4 was prepared by controlled coprecipitation of mixed barium and titanium species with an ammonium oxalate aqueous solution at pH 7. The results of thermal analysis and IR measurement show that the obtained precursor is a mixture of BaC2O4·0.5H2O and TiO(OH)2·1.5H2O in a molar ratio of 1:4. Crystallized BaTi4O9 was obtained by the thermal decomposition of a precipitate precursor at 1300°C for 2 h in air. The dimensions of the powder calcined at 1000°C are between 100 and 300 nm. The grain dimensions of the sintered sample for 2 h at 1300°C are of the order of 10 to 30 μm. Dielectric properties of disk‐shaped sintered specimens in the microwave frequency region were measured using the TE011 mode. Excellent microwave characteristics for BaTi4O9—ɛ= 38 ± 0.5, Q= 3800–4000 at 6–7 GHz and τf= 11 ± 0.7 ppm/°C—were found.
Single‐Step Synthesis of Chemically Cross‐Linked Polysilastyrene and Its Conversion to β‐Silicon Carbide
Authors: Krishnan Venkatasubbaiah , Bindu Ravindranathan , Chandrasekhar Vadapalli , Murthy Varanasi S. R.
Published: July 2010

A new method for chemically cross‐linking polysilastyrene using divinylbenzene as the cross‐linking agent is reported. The procedure involves a single‐step synthesis using the alkali‐metal sodium to promote the polymerization of dimethyldichlorsilane in the presence of the comonomers phenylmethyldichlorosilane and divinylbenzene. The cross‐linked polymer can be readily converted to β‐SiC on pyrolysis at 1500°C. The β‐SiC obtained by this procedure is nanocrystalline and has a grain‐size distribution of 8–20 nm.
Electrostatic Stabilization of Ultrafine Titania in Ethanol
Authors: Widegren Johnny , Bergström Lennart
Published: July 2010

The colloidal stability of a suspension of very fine titania (TiO2) powder in ethanol has been investigated. The electrokinetic behavior and stability ratios were determined as a function of operational pH and salt concentration. We show that the stability is strongly dependent on ionic strength; the TiO2suspensions will aggregate relatively rapidly at ionic strengths >10−3M, while suspensions exhibiting long‐term stability can be made at ≤10−4M, providing that the zeta potential of the TiO2particles is sufficiently high, i.e., |ζ|= 40–50 mV. Theoretical estimates of the stability based on electrostatic repulsion show a relatively good correlation with the experimental results, in particular in the acidic operational pH (pH*) range.
Reducing the Sintering Temperature for MgO–Al2O3Mixtures by Addition of Cryolite (Na3AlF6)
Authors: Chen Swe‐Kai , Cheng Min‐Yuan , Lin Su‐Jien
Published: July 2010

The preparation of near stoichiometric spinel and alumina‐rich spinel composites from Al2O3and MgO powders with the addition of Na3AlF6up to 4 wt% in the temperature range 700°–1600°C was studied; 98 wt% spinel containing 72 wt% Al2O3can be produced from the mixture of 72 wt% (50 at.%) Al2O3+ 28 wt% (50 at.%) MgO powders with the addition of 1 wt% Na3AlF6fired at 1300°C for 1 h. Spinels containing 81–85 wt% Al2O3can be produced from either the mixture of 90 wt% (78 at.%) Al2O3+ 10 wt% (22 at.%) MgO or the mixture of 95 wt% (88 at.%) Al2O3+ 5 wt% (12 at.%) MgO powders with the addition of 4 wt% Na3AlF6in the temperature range 1300°–1600°C by using a torch‐flame firing for 3 min, followed by quenching in water, while the same system under slow cooling in a furnace results in spinel containing 74–76 wt% Al2O3. Microscopic studies indicate that the alumina‐rich spinel composites consist of a continuous majority spinel phase and an isolated minority corundum phase, regardless of slow cooling in a furnace or quenching in water.
Effect of Ammonia Treatment on the Crystallization of Amorphous Silicon–Carbon–Nitrogen Ceramics Derived from Polymer Precursor Pyrolysis
Authors: Wan Julin , Gasch Matthew J. , Mukherjee Amiya K.
Published: July 2010

An effort was made toward modifying the Si3N4‐SiC phase ratio in bulk nanocomposites obtained from polymer precursors. While pyrolyzing the polymer, flowing ammonia was introduced, to facilitate a chemical exchange, resulting in a different C/N ratio in the ceramic pyrolysis product. A pre‐pyrolysis/binding/pyrolysis approach was used for sample consolidation. Comparison was made between the crystallization behavior of the pyrolysis‐derived ceramic powders and consolidated bulk samples. A profound enhancement in crystallization tendency was observed in the consolidated samples whose nitrogen content was increased by ammonia treatment. A mechanism based on the particle/binder interface energy was proposed to account for this observation.
Fracture Strength of Plate and Tubular Forms of Monolithic Silicon Carbide Produced by Chemical Vapor Deposition
Authors: Cockeram Brian Vern
Published: July 2010

The fracture strength of silicon carbide (SiC) plate deposits produced by chemical vapor deposition (CVD) was determined from room temperature to 1500°C using a standard 4‐point flexural test method (ASTM C1161). CVD SiC materials produced by two different manufacturers are shown to have only slightly different flexural strength values, which appear to result from differences in microstructure. Although CVD deposition of SiC results in a textured grain structure, the flexural strength was shown to be independent of the CVD growth direction. The orientation of machining marks was shown to have the most significant influence on flexural strength, as expected. The fracture strength of tubular forms of SiC produced by CVD deposition directly onto a mandrel was comparable to flexural bars machined from a plate deposit. The tubular (O‐ring) specimens were much smaller in volume than the flexural bars, and higher strength values are predicted based on Weibull statistical theory for the O‐ring specimens. Differences in microstructure between the plate deposits and deposits made on a mandrel result in different flaw distributions and comparable strength values for the flexural bar and O‐ring specimens. These results indicate that compression testing of O‐rings provides a more accurate strength measurement for tubular product forms of SiC due to more representative flaw distributions.
Temperature Coefficient of Microwave Resonance Frequency of a Low‐Temperature Cofired Ceramic (LTCC) System
Authors: Jantunen Heli , Uusimäki Antti , Rautioaho Risto , Leppävuori Seppo
Published: July 2010

A family of low‐temperature cofired ceramics (LTCC) based on mixtures of a commercial dielectric, MgTiO3–CaTiO3(designated MMT‐20) and ZnO, SiO2, and B2O3, has been investigated for microwave applications. The main objective was to optimize the three key properties—relative permittivity (ɛr), dissipation factor (DF), and the temperature dependence of the microwave resonance frequency (τf)—through adjustment of the composition. A further objective was to estimate the limits on compositional variability while maintaining acceptable properties. The developed microstructures, after firing at 900°C, were studied using X‐ray diffractometry and scanning electron microscopy/energy dispersive spectrometry techniques and compared with the dielectric parameters. The optimum composition (wt%) was found to lie in the ranges 45.8–44.9, ZnO; 17.25–17.55, B2O3; 6.95–7.05, SiO2; and 30–30.5, MMT‐20, yielding values of ɛr= 8.5–9.5, DF < 0.93 × 10−3 ppm/K, and τf< ±10 ppm/K.
Formation of Zinc–Antimony‐Based Spinel Phases
Authors: Kumari K. G. Vasantha , Vasu P. Divya , Kumar Viswanathan , Asokan Thangavelu
Published: July 2010

Formation of spinel phases in ZnO–Sb2O3and ZnO–Sb2O3–Bi2O3systems is studied by the use of X‐ray diffraction. The formation of nonstoichiometric Zn2.33Sb0.67O4phase is observed in both the systems at ∼900°C. However, in these systems, at higher temperatures (T≥ 1100°C), formation of the inverse spinel phase Zn7Sb2O12is observed. The study has been extended to understand the effect of CrO3doping on the stability of the different spinel phases in the previously mentioned systems. Interestingly, in both the systems, samples doped with CrO3, displayed the presence of Zn2.33Sb0.67O4phase <1200°C, indicating the stabilization of the spinel phase by CrO3.
Estimating Bounds on Fracture Stresses Determined from Mirror Size Measurements
Authors: Salem Jonathan A. , Jenkins Michael G.
Published: July 2010

Statistical techniques are used to derive closed‐form, approximate solutions for estimating a standard deviation on the fracture stress determined from mirror size measurements. The estimated coefficient of variation of fracture stress is noted to be twice as sensitive to the variation of the mirror constant,Aj, as to the variation in the measured boundary radius,rj. This allows the hackle radius to be reasonably estimated and bounded from the mirror and branch radii when it cannot be clearly delineated.
Direct Observation of Crack Tip Stress Field Using the Mechanoluminescence of SrAl2O4:(Eu,Dy,Nd)
Authors: Sohn Kee‐Sun , Seo Soo Yeon , Kwon Yong Nam , Park Hee Dong
Published: July 2010

The present investigation aims at visualizing the crack tip stress field using a mechanoluminescence material. The well‐known compound SrAl2O4:Eu2+ was adopted as a mechanoluminescence material. Two more trivalent rare‐earth elements such as Dy and Nd were taken into consideration as codopants to provide the appropriate trap levels. Samples of a variety of compositions were prepared by varying Eu2+, Dy3+, and Nd3+ doping contents, for which the combinatorial chemistry method was used. In order to search for the optimum composition for the highest mechanoluminescence, the luminescence induced by a compressive loading was monitored by using a specially designed pressing device including a CCD camera. In parallel, a compact tension specimen was prepared by mixing the luminescence powders of optimum composition and epoxy resin. Crack initiation from the mechanically machined sharp notch tip and its growth during loading were found to be associated with the extent of light emission from SrAl2O4.
Influence of a Piezoelectric Secondary Phase on Thermal Shock Resistance of Alumina‐Matrix Ceramics
Authors: Wang Lin , Shi Jian‐Lin , Gao Jian‐Hua , Yan Dong‐Sheng
Published: July 2010

Using monolithic alumina ceramics as the reference, the thermal shock behavior of the hot‐pressed alumina matrix ceramics with 3 mol% neodymium titanate was investigated. The thermal shock resistance of the materials was evaluated by water quenching and a subsequent three‐point bending test to determine the flexural strength degradation. The hot‐pressed composite exhibited a temperature differential of the thermal shock resistance 120°C higher than the monolith, mainly because of significantly improved fracture toughness.
Crystal Structure of Zirconia Prepared with Alumina by Coprecipitation
Authors: Kikkawa Shinichi , Kijima Akio , Hirota Ken , Yamamoto Osamu
Published: July 2010

Zirconia was prepared by firing the coprecipitate from ZrOCl2and AlCl3mixed aqueous solution with ammonia. When fired above 600°C, the products were fine crystalline tetragonal zirconia of crystallite size <10 nm. In previous studies, the tetragonal phase had been assumed to be a (Zr1−x4+Alx3+)O2−x/2solid solution, where x≤ 0.25. However, X‐ray diffraction pattern simulation and Al K‐edge XANES spectroscopy confirmed the present product to be a mixture of t‐ZrO2fine powder with a small amount of δ‐Al2O3of very low crystallinity, even below the expected compositional range of x≤ 0.25 in the (Zr1−x4+Alx3+)O2−x/2solid solution.
Novel Method to Prepare Electroconductive Titanium Nitride–Aluminum Oxide Nanocomposites
Authors: Li Jingguo , Gao Lian , Guo Jingkun , Yan Dongsheng
Published: July 2010

A novel method for the preparation of TiN–Al2O3nanocomposites was developed. TiN–Al2O3nanocomposite powders were prepared by the direct nitridation of TiO2–Al2O3nanocomposite powders that were derived from the simultaneous hydrolysis of tetra‐butyl titanate and precipitation of aluminum nitrate. Dense sintered bodies of these TiN–Al2O3nanocomposite powders were obtained by hot pressing at 1450°–1650°C and 30 MPa for 60 min. The resistivity of nanocomposite reaches a minimum (1.5 × 10−3Ω·cm) at 25 vol% TiN additions. The percolation concentration of nanocomposite is ∼10 vol% TiN.
Optimal Composition of Zircon–Fused Silica Ceramic Cores for Casting Superalloys
Authors: Chao Chin‐Hsiao , Lu Hong‐Yang
Published: July 2010

The dependence of flexural strength on crystalline phase content, residual porosity, and sintering temperature has been investigated for ceramic cores based on a mixture of fused silica and zircon. The rule of binary particle mixture, adopted for three particle sizes of fused silica, predicts a critical weight fraction of fine (zircon) particles of 46.9 wt% for the best packing. However, the optimal composition of a 37.8 wt% zircon–fused silica mixture for the flexural strength of ceramic cores found experimentally is lower in zircon content. The content of α‐cristobalite +α‐quartz crystallized from infilling silica sols exceeds ∼3 wt% when the strength decreases from ∼10 MPa to ∼6 MPa in samples sintered at 899–927°C. The degradation is ascribed to microcracking and the loss of coherency between the fused silica grains and the infilled silica due to β‐ →α‐phase transformation on cooling and differential densification during sintering.
Determination of Binder Decomposition Kinetics for Specifying Heating Parameters in Binder Burnout Cycles
Authors: Shende Rajesh V. , Lombardo Stephen J.
Published: July 2010

The decomposition kinetics of poly(vinyl butyral) binder from barium titanate multilayer ceramic capacitors with platinum metal electrodes were analyzed by thermogravimetric analysis as a function of the heating rate. The activation energy and pre‐exponential factor for the decomposition kinetics were determined from two types of integral equations, from the Redhead method, and from the variation in heating rate method. The accuracy of the kinetic parameters determined from these methods was then evaluated for describing the observed rate of binder decomposition. Although the individual models yielded very different kinetic parameters, all were capable of describing the experimental data within ±15% accuracy. The kinetic parameters were then used in a coupled transport and kinetic model for describing the buildup of pressure within the ceramic green body as a function of the heating cycle. A methodology based on calculus of variations was also developed to predict the minimum duration for the binder burnout cycle.
Structural Evolution and Vanadium Distribution in the Preparation of V4+‐ZrSiO4 Solid Solutions from Gels
Authors: Torres Francisco Jose , Folgado Jose Vicente , Alarcón Javier
Published: July 2010

Vanadium‐containing ZrSiO4‐gel precursors with nominal compositions Vx‐ZrSiO4 with x= 0.0, 0.002, 0.004, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.1, and 0.2 were prepared using a previously reported procedure and thermally treated over a range of temperature up to the formation of the V4+‐ZrSiO4 solid solution. The structural evolution and the V4+ location and its homogenous distribution were followed using powder X‐ray diffractometry and electron spin resonance spectroscopy (ESR). Our experiments showed that a tetragonal form of V4+‐ZrO2 was the first crystalline phase obtained on heating the gels. On further heating, a phase transformation to the monoclinic form of V4+‐ZrO2 took place. Finally, the monoclinic form reacted with amorphous SiO2 to form the V4+‐ZrSiO4 solid solution. A semiquantitative determination of the vanadium solubility in the ZrSiO4 phase using ESR confirmed previously reported results obtained by lattice parameter variation over the solid‐solution series.
Effects of Yttria‐Stabilized Zirconia on Plasma‐Sprayed Hydroxyapatite/Yttria‐Stabilized Zirconia Composite Coatings
Authors: Fu Lei , Khor Khiam Aik , Lim Joo Peng
Published: July 2010

The low bonding strength between hydroxyapatite (HA) and the metal substrate interface of plasma‐sprayed HA coating has been a point of potential weakness in its application as a biomedical prosthesis. In the present study, yttria‐stabilized (8 wt%) zirconia (YSZ) has been used to enhance the mechanical properties of HA coatings. The effects of YSZ additions (in the range 10–50 wt%) on the phase composition, microstructure, bond strength, elastic modulus, and fracture toughness of plasma‐sprayed HA/YSZ composite coatings have been studied. The results indicated that decomposition of HA during plasma spraying was reduced significantly with the addition of zirconia. The higher the zirconia content, the lower the amount of calcium oxide, tricalcium phosphate, and tetracalcium phosphate formed in the coatings. In addition, there was a trace of calcium zirconate formed when less than 30 wt% zirconia was present. A solid solution of HA mixed with YSZ formed during plasma spraying; however, the amount of unmelted particles increased as the zirconia increased. The mechanical properties of the HA/YSZ composite coatings, such as bond strength, elastic modulus, and fracture toughness, increased significantly as the contents of zirconia increased.
Thermal Conductivity of Platelet‐Filled Polymer Composites
Authors: Hill Richard F. , Supancic Peter H.
Published: July 2010

Platelet‐shaped particles of similar size and shape were investigated as fillers for improving the thermal conductivity of polymer–ceramic composite materials. The conductivities of composites filled with hard, stiff ceramic particles exceeded 3.5 W·(m·K)−1, or >20 times the conductivity of the polymer matrix, and were shown to be almost independent of the intrinsic filler conductivity range of 33–300 W·(m·K)−1. In contrast, the thermal conductivity of composites filled with soft, platelet‐shaped BN fillers reached over 13 W·(m·K)−1. A mechanism is proposed whereby deformation of the soft filler particles provides improved particle‐to‐particle connectivity and allows greater packing density, resulting in the ability to achieve much higher conductivity than is possible for hard and stiff particles of similar initial morphology. Experimental results are discussed in light of various thermal conductivity prediction models in the literature.
Thermally Induced Delamination of Symmetrically Graded Multilayers
Authors: Sørensen Bent F.
Published: July 2010

A fracture mechanics model is used for calculating the steady‐state energy release rate of symmetrically graded ceramic multilayers. Results from the present study and previous results on asymmetric multilayers are generalized to simple design rules for avoidance of delamination of graded multilayers: (A) For asymmetric multilayers, delamination can be suppressed by inserting a few additional interlayers. (B) For symmetric multilayers, multilaying is not efficient; instead materials should be selected which have a sufficiently low thermal expansion mismatch. The later can be judged by making bilayers of identical thickness as the desired symmetric multilayers. If the bilayer delaminates it is unlikely that symmetric multilayers can be processed, and other materials (having a smaller thermal expansion coefficient mismatch) should be tested.
Effect of Lateral Cracks on Fracture Toughness Determined by the Surface‐Crack‐in‐Flexure Method
Authors: Quinn George D. , Salem Jonathan A.
Published: July 2010

The surface‐crack‐in‐flexure (SCF) method uses a Knoop indenter to create small, semielliptical surface precracks in beam specimens. Lateral cracks may interfere with the primary median crack and cause errors of up to 10% in determination of fracture toughness, particularly for materials for which the fracture toughness is ∼3 MPa·m1/2 or less. Although the residual‐stress‐damage zone is ground or polished away by hand by removing 4.5–5 times the indentation depth, this amount may not be sufficient to completely remove the lateral cracks in low‐fracture‐toughness materials. A series of tests were conducted on sintered alpha silicon carbide with different amounts of material removed after indentation. Once the lateral cracks were fully removed, the SCF results concurred with single‐edged‐precracked‐beam and chevron‐notched‐beam data collected in accordance with ASTM Designation C1421. A simple remedy for the SCF method is to examine the outer ground surface for remnants of lateral cracks before fracture and to remove more material if necessary.
Effect of Grain Coalescence on the Abnormal Grain Growth of Pb(Mg1/3Nb2/3)O3‐35 mol% PbTiO3 Ceramics
Authors: Chung Ui‐Jin , Park Jong‐Keuk , Hwang Nong‐Moon , Lee Ho‐Yong , Kim Doh‐Yeon
Published: July 2010

Abnormal grain growth (AGG), which occurred during the heat treatment of Pb(Mg1/3Nb2/3)O3‐35 mol% PbTiO3 (PMN‐35PT) with excess PbO, was investigated. AGG has been suggested to be the consequence of grain coalescence that results in the formation of Σ3 coincidence site lattice and low angle grain boundaries. Because of reentrant edges appearing at the ends of these boundaries, the coarsening rate of grains was significantly enhanced and AGG occurred.
Microstructure Development in Unsupported Thin Films
Authors: Gorman Brian P. , Anderson Harlan U.
Published: July 2010

To better understand the role of the substrate in the microstructural evolution of thin films, unsupported nanocrystalline yttrium‐stabilized zirconia (ZrO2:16%Y or YSZ) films were examined as a function of temperature and annealing time. Grain growth, texturing, and pinhole formation were measured using transmission electron microscopy (TEM) and electron diffraction. Films were produced and subsequently annealed on metallic grids using a previously developed technique that results in near full density films at low annealing temperatures. Microstructural evolution in these films was unique compared with constrained films. Grains were found to spheroidize much more readily, ultimately resulting in the formation of porosity and pinholes. Grain growth was found to stagnate at a size particular to each annealing temperature, presumably due to the effects of Zener pinning. It is proposed that the lack of substrate strain and confinement effects allows for the dominance of surface energetics with respect to microstructural evolution.
Fretting Fatigue in Alumina Tested under Oscillating Normal Load
Authors: Huq M. Zahedul , Celis Jean‐Pierre
Published: July 2010

The effect of an oscillating normal force (fretting Mode II) on the degradation mechanism in an alumina versus a WC‐Co ball‐on‐flat contact is presented. A sinusoidal oscillation of the normal force at a fixed frequency and normal force amplitude was applied for different numbers of cycles. The evolution of the surface damage with an increasing number of cycles at a constant force amplitude is reported. In general, wear is taking place in the slip region on alumina balls and on WC‐Co flats. The oscillating normal force generates fatigue cracks on the surface of the alumina balls, but not on the WC‐Co flats. Fatigue crack initiation life under fretting Mode II test conditions is evaluated for alumina in a plot of maximum normal load versus number of fretting cycles.
Hydrothermal Synthesis of Weakly Agglomerated Nanocrystalline Scandia‐Stabilized Zirconia
Authors: Xu Gang , Zhang Yawen , Liao Chunsheng , Yan Chunhua
Published: July 2010

A pure cubic phase of weakly agglomerated, nanocrystalline, 8‐mol%‐Sc2O3‐stabilized zirconia (8ScSZ) was synthesized by a two‐step hydrothermal treatment in the presence of urea: a stock solution of metal nitrates and urea was heated at 80°C for 48 h and then at 180°C for 72 h. The omission of the first low‐temperature treatment resulted in more monoclinic phase. The effects of urea concentration and calcining temperature on the crystallization of the as‐synthesized nanocrystalline 8ScSZ are also discussed.
Synthesis and Characterization of (Ce0.67Tb0.33)MnxMg1−xAl11O19 Phosphors Derived by Sol–Gel Processing
Authors: Zhang Junying , Zhang Zhongtai , Tang Zilong , Lin Yuanhua
Published: July 2010

A (Ce0.67Tb0.33)MnxMg1−xAl11O19 phosphor powder was synthesized, using a simple sol–gel process, by mixing citric acid with CeO2, Tb4O7, Al(NO3)3·9H2O, Mg(OH)2·4MgCO3·6H2O, and Mn(CH3COO)2. The phosphor crystallized completely at 1200°C, and the phosphor particle size was between 1 and 5 μm. The excitation spectrum was characteristic of Ce3+, while the emission spectrum was composed of lines from Tb3+ and Mn2+. The Mn2+ gave a green fluorescence band, and concentration quenching occurred when x > 0.10. The luminescent properties of the phosphor were explained by a configurational coordinate model.
High‐Temperature Strength of Liquid‐Phase‐Sintered SiC with AlN and Re2O3(RE = Y, Yb)
Authors: Kim Young‐Wook , Mitomo Mamoru , Nishimura Toshiyuki
Published: July 2010

Using AlN and RE2O3(RE = Y, Yb) as sintering additives, two different SiC ceramics with high strength at 1500°C were fabricated by hot‐pressing and subsequent annealing under pressure. The ceramics had a self‐reinforced microstructure consisting of elongated α‐SiC grains and a grain‐boundary glassy phase. High‐temperature strength up to 1600°C was measured and compared with that of the SiC ceramics fabricated with AlN and Er2O3. SiC ceramics with AlN and Y2O3showed the best strength (∼630 MPa) at 1500°C, while SiC ceramics with AlN and Er2O3the best strength (∼550 MPa) at 1600°C.
Mechanochemical Synthesis and Pressureless Sintering of TiB2–AlN Composites
Authors: Kim Hyung‐Jong , Choi Heon‐Jin , Lee June‐Gunn
Published: July 2010

TiB2‐AlN composites have been fabricated by the pressureless sintering of a mechanochemically processed Ti, Al, and BN powder mixture. TiB2‐AlN powder was obtained from the mixture of Ti, Al, and BN, which had a composition corresponding to 45.7 wt% TiB2‐54.3 wt% AlN, after mechanochemical processing for longer than 24 h. X‐ray diffraction and transmission electron microscopy analysis showed that the powder subjected to mechanochemical processing for 60 h consisted of crystallites less than 300 nm in size with a disordered crystal structure. TiB2‐AlN composites with 95% relative density, a flexural strength of 172 MPa, a fracture toughness of 4.6 MPa·m1/2, a hardness of 12.0 GPa, and an electrical resistivity of 1488 μΩ·cm were obtained by pressureless sintering at 1700°C for 2 h of the powder subjected to mechanochemical processing for 60 h.
Nonradiative Energy Losses and Radiation Trapping in Neodymium‐Doped Phosphate Laser Glasses
Authors: Ehrmann Paul R. , Campbell John H.
Published: July 2010

Fluorescence radiation trapping and nonradiative energy losses from the Nd3+4F3/2 state are reported for two widely used commercial phosphate laser glasses (LHG‐8 and LG‐770). The effects of hydroxyl‐group, transition‐metal (Cu, Fe, V, Co, Ni, Cr, Mn, and Pt), and rare‐earth (Dy, Pr, Sm, and Ce) impurities on the 4F3/2 nonradiative decay rate in these glasses are quantified. Nd concentration quenching effects are reported for doping levels ranging from about 0.5 × 1020 to 8.0 × 1020 ions/cm3. The results are analyzed using the Förster–Dexter theory for dipolar energy transfer. Quenching rates for transition‐metal ions correlate with the magnitude of spectral overlap for Nd emission (donor) and the metal ion absorption (acceptor). The nonradiative decay rates due to hydroxyl groups follow Förster–Dexter theory except at low Nd‐doping levels (≲2 × 1020 ions/cm3) where the quenching rate becomes independent of the Nd concentration. The data suggest a possible correlation of OH sites with Nd ions in this doping region. The effects of radiation trapping on the fluorescence decay are reported as a function of sample size, shape, and doping level. The results agree well with the theory except for samples with small doping‐length products; in these cases, multiple internal reflections from the sample surfaces enhance the trapping effect.
NMR Studies of Fluorine in Aluminosilicate–Lanthanum Fluoride Glasses and Glass‐Ceramics
Authors: Youngman Randall E. , Dejneka Matthew J.
Published: July 2010

NMR determinations of fluorine environments in transparent oxyfluoride glass‐ceramics were made to learn about the crystallization of LaF3, as well as to ascertain the structural role of fluorine in the surrounding glassy matrix. The fraction of fluorine in LaF3 was measured as a function of heat treatments, demonstrating significant differences between glasses modified with barium and those containing sodium. The results of these measurements showed that not all of the fluorine formed LaF3 in these glass‐ceramics, with resolution of additional fluoride sites at −135 and −185 ppm, due primarily to Si–F and Al–F bonding, respectively. Not only is the evidence of Si–F bonding unexpected, given the presence of aluminum, but the amount of Si–F bonding is sensitive to the type of modifier in the glass. Samples containing barium oxide as the modifier showed a higher fraction of Si–F bonding than those modified with sodium oxide.
Correlation between Macro‐ and Microstructural Changes in Ge:SiO2 and SiO2 Glasses under Intense Ultraviolet Irradiation
Authors: Takahashi Masahide , Uchino Takashi , Yoko Toshinobu
Published: July 2010

The photochemical transformation mechanism of defects in germanosilicate and silica glasses under ultraviolet (UV) laser irradiation has been investigated based on the changes in Raman spectra before and after irradiation. Two types of silica glasses, fused silica (type I) and dry synthetic silica (type IV), and germanosilicate optical fiber preforms were irradiated by intense UV photons from excimer lasers. Spectral changes in optical absorption and Raman spectra were examined to clarify a correlation between the microscopic defect formation and the macroscopic structural changes causing a photorefractive effect. Successive generation of E′ centers through divalent centers is closely correlated with changes in Raman spectra, indicating that large structural changes in the glass network involved in this process would be the origin of photon‐induced densification of the glasses. In addition, it has been proposed that the successive generation of E′ centers is mediated by transient divalent centers converted from relaxed cation homobondings.
Evolution of Mullite Texture on Firing Tape‐Cast Kaolin Bodies
Authors: Chen Chin‐Yi , Tuan Wei‐Hsing
Published: July 2010

Kaolin undergoes a series of phase changes on heating to elevated temperature, proceeding through kaolin, metakaolin, γ‐Al2O3 spinel, and mullite. The morphologic evolution of the kaolin–mullite reaction series was investigated in the present study. A highly textured kaolin green body was prepared by a tape‐casting technique, and the morphology evolution from kaolin flakes to mullite aciculars on firing from 450° to 1600°C was then monitored using X‐ray diffractometry and scanning electron microscopy. Equiaxed mullite nuclei first appeared at 1000°C; the aspect ratio of the mullite grains then increased with increased firing temperature. The mullite aciculars rearranged their orientation in a glassy phase until they impinged on each other. A highly textured mullite specimen was prepared by firing the kaolin tape at a temperature >1500°C.
Preparation of Titanium Nitride/Alumina Laminate Composites
Authors: Zhang Jing‐Xian , Huang Zheng‐Ren , Jiang Dong‐Liang , Tan Shouhong H. , Shen Zhijian , Nygren Mats
Published: July 2010

A preparation route for TiN/Al2O3 laminate composites has been described. A water‐based process using Al2O3 and TiN slurries with solids contents of 40 and 35 vol%, respectively, was used to make TiN and Al2O3 tapes. The removal of the binder was monitored by weight‐loss measurements in a thermogravimetry unit. Bodies composed of Al2O3 and TiN tapes were densified at temperatures of 1400° and 1500°C using the Spark Plasma Sintering® (SPS) technique. Densities of >98% of the theoretical densities were approached. Crack‐free and almost fully densified TiN/Al2O3 compacts were prepared by heating the burned‐out green bodies to the final sintering temperature (1500°C) at a rate of 100°C/min, and with a holding time of 5–10 min, under a pressure of 75 MPa. The microstructures of the obtained compacts were studied using scanning electron microscopy. Grain sizes in the sintered Al2O3 and TiN compacts were similar to those of the precursor powders. Hardness and indentation fracture toughness were measured at room temperature, and the monolithic compacts as well as the laminate composites exhibited anisotropic mechanical behavior; i.e., the cracks propagated much more easily in a direction parallel to the laminas than perpendicular to them.
Thermoreversible Gelcasting: A Novel Ceramic Processing Technique
Authors: Montgomery John K. , Drzal Peter L. , Shull Kenneth R. , Faber K. T.
Published: July 2010

A new ceramic processing method, thermoreversible gelcasting (TRG), has been developed. The method uses a gelation process that can be reversed multiple times. Two Al2O3 systems have been explored with the use of a specific triblock copolymer to form a binding gel network. The mixture becomes a free‐flowing liquid on heating above 60°C. However, on cooling below 60°C, the slurry instantaneously transforms to a physical gel. This process can be reversed easily (essentially with an infinite processing time window) while the solvent is present in the system, which is advantageous when trying to produce high‐quality dense pieces if initial casting irregularities occur. Near‐theoretically dense specimens have been produced with properties consistent with reported values of high‐density Al2O3.
Origin of Microwave Dielectric Loss in ZnNb2O6‐TiO2
Authors: Kim Dong‐Wan , Ko Kyung Hyun , Kwon Do‐Kyun , Hong Kug Sun
Published: July 2010

(1−x)ZnNb2O6·xTiO2 ceramics were prepared using both anatase and rutile forms of TiO2. At a composition of x= 0.58, a mixture region of ixiolite (ZnTiNb2O8) and rutile was observed and the temperature coefficient of resonant frequency (τf) was ∼0 ppm/°C. We found that although ɛr and τf were comparable, the quality factor (Q×f, Q≈ 1/ tan δ, f= resonant frequency) of 0.42ZnNb2O6·0.58TiO2 prepared from anatase and rutile was 6000 and 29 000, respectively. The origin of the difference in Q×f of both samples was investigated by measuring electrical conductivity and by analysis of the anatase–rutile phase transition. The anatase‐derived sample had higher conductivity, which was related to the reduction of Ti4+. It is suggested that the increase of dielectric loss originates from an increase in Ti3+ and oxygen vacancies due to an anatase–rutile phase transition.
Low‐Temperature Sintering and Electromagnetic Properties of Copper‐Modified Z‐type Hexaferrite
Authors: Zhang Hongguo , Li Longtu , Wang Yongli , Zhou Ji , Yue Zhenxing , Gui Zhilun
Published: July 2010

The influence of the substitution of copper for cobalt on the low‐sintering densification, microstructure, lattice parameters, and electromagnetic properties of planar Z‐type hexaferrites, which have a stoichiometric composition of Ba3Co2(10.8−x)‐Zn0.4Cu2xFe24O41, were investigated. The experimental results show that densification of the copper‐substituted hexaferrite is dependent on the amount of copper substitution and sintering temperatures. XRD analysis reveals that the solid solubility of copper in Z‐type hexaferrite is limited to x≤ 0.30. At the same time, the lattice parameters (a and c) of Z‐type hexaferrites with copper substitution showed a small change, that is, decrease of a parameter in the basal plane and increase of the c axial. Moreover, the microstructure becomes more homogeneous and intragranular pores decrease considerably, but the temperature‐stable range of the Z‐type phase formation is narrow. The copper‐substituted hexaferrites, with x= 0.20 and sintered at ∼1125°C for 4 h, resulted in a high density sample of ∼4.68 g/cm3 as the optimal value, good magnetic properties, and the enormous potentiality for fabrication of multilayer chip inductors used in hyper‐frequencies.
New Red‐Shade Ceramic Pigments Based on Y2Sn2−xCrxO7−δ Pyrochlore Solid Solutions
Authors: Pavlov Radostin Stefanov , Castelló Juan B. Carda , Marzá Vicente Blasco , Hohembergerger J. M.
Published: July 2010

New high‐stability red‐shade ceramic pigments based on pyrochlore solid solutions Y2Sn2−xCrxO7−δ (x= 0–1, 0 < δ= 0–1/2x) were developed employing conventional solid‐state reaction synthesis. The relationship between their optical properties and microstructure was analyzed by X‐ray diffractometry and ultraviolet–visible‐light spectroscopy. Decomposition of the structure with formation of the perovskite YCrO3 observed for chromium substitutions x≥ 5 was attributed mainly to high concentration of defects. The structural and optical parameters of the materials were found to correlate with the concentration of a species with higher oxidation state than Cr(III), such as Cr(IV) or Cr(V).
Experimental Method for a Dynamic Biaxial Flexural Strength Test of Thin Ceramic Substrates
Authors: Cheng Ming , Chen Weinong , Sridhar K. R.
Published: July 2010

A dynamic piston‐on‐three‐ball experimental technique has been developed for a biaxial flexural strength test of thin ceramic substrates at high loading rates. Analytical modeling of the technique guided the experimental design and was used to judge the validity of experimental results. Thin ceramic substrates made of 1‐mol%‐Al2O3‐doped 8‐mol%‐yttria‐stabilized zirconia (8YSZ) were tested using this experimental technique, as well as its standard counterpart—the quasi‐static piston‐on‐three‐ball technique. Results show that the mean biaxial flexural strength increased from ∼322 to ∼465 MPa as the average loading rate was increased from quasi‐static to dynamic (830 GPa/s) level.
Determination of the Pcmn/Ibm2 Phase Boundary at High Temperatures in the System Ca2Fe2O5‐Ca2Al2O5
Authors: Fukuda Koichiro , Ando Hisashi
Published: July 2010

The crystals of Ca2(AlxFe1−x)2O5 with 0 ≤x≤ 0.4 were examined using powder XRD at temperatures between 25° and 1000°C. The Pcmn to Ibm2 phase transition was readily detected by the disappearance of the 131 and 151 reflections (h+k+l odd). Thus, the phase relationship has been determined as combined functions of the temperature and x‐value. At 25°C, the space group changed from Pcmn (0 ≤x≤ 0.23) to Ibm2 (0.24 ≤x≤ 0.40) across x= 0.235. With increased x‐value, the cell dimensions of both phases steadily decreased, showing the nearly complete continuity at that phase boundary. For the crystal with x= 0 (Ca2Fe2O5), the cell dimensions steadily increased during heating to 685°C, at which temperature the phase transition occurred. There was a slight discontinuous decrease in cell dimensions just after the transition. Subsequent heating to 1000°C of the Ibm2 phase led to a steady thermal expansion along the b‐axis and c‐axis, while a slight contraction occurred along the a‐axis up to 800°C.
Suppression of Rhombohedral‐Phase Appearance and Low‐Temperature Sintering of Scandia‐Doped Cubic‐Zirconia
Authors: Hirano Masanori , Oda Takayuki , Ukai Kenji , Mizutani Yasunobu
Published: July 2010

Inhibition of cubic‐rhombohedral phase transformation and low‐temperature sintering at 1000°C were achieved for 10‐mol%‐Sc2O3‐doped cubic‐ZrO2 by the presence of 1 mol% Bi2O3. The powders of 1‐mol%‐Bi2O3–10‐mol%‐Sc2O3‐doped ZrO2 were prepared using a hydrolysis and homogeneous precipitation technique. No trace of rhombohedral‐ZrO2 phase could be detected, even after sintering at 1000°–1400°C. The average grain size of the ZrO2 sintered at 1200°C was >2 μm because of grain growth in the presence of Bi3+. Cubic, stabilized Bi‐Sc‐doped ZrO2 sintered at 1200°C had sufficient conductivity at 1000°C (0.33 S/cm) to be used as an electrolyte for a solid‐oxide fuel cell (SOFC) and at 800°C (0.12 S/cm) for an intermediate‐temperature SOFC.
Direct Coagulation Casting of Alumina Suspensions Using Jack Bean as a Urease Source
Authors: Studart André R. , Pandolfelli Victor C. , Tervoort Elena , Gauckler Ludwig J.
Published: July 2010

This paper reports on the use of Jack bean meal as an alternative source of urease for direct coagulation casting (DCC) processes. Al2O3 suspensions were prepared to compare the effect of Jack bean meal and pure urease on the hydrolysis of urea, the slurry coagulation rate, and the mechanical properties of consolidated bodies. Results indicate that the Jack bean meal is a promising economic alternative for the consolidation of ceramic parts, mainly because of its lower susceptibility to enzyme deactivation when exposed to room temperature and relatively humid environments.
Densification of Precursor‐Derived Si‐C‐N Ceramics by High‐Pressure Hot Isostatic Pressing
Authors: Ishihara Satoru , Gu Hui , Bill Joachim , Aldinger Fritz , Wakai Fumihiro
Published: July 2010

Densification behavior of precursor‐derived Si‐C‐N ceramics by hot isostatic pressing (HIP) has been investigated to obtain dense ceramics derived from polymer precursor. An as‐pyrolyzed ceramic monolith, which had a porosity of about 17%, could be deformed up to a strain of 8% in preliminary uniaxial compression tests. The flow stress of the material was much higher than 200 MPa at 1600°C; thus high stress was necessary for densification by HIP. The density of the monolith increased from 1.9 to 2.4 g/cm3 by HIP at 1600°C and 980 MPa. Although the number of pores decreased, large pores were formed in the hot isostatically pressed monolith. On the other hand, denser ceramics, in which pores were not observed by optical microscopy, were obtained by hot isostatically pressing the pyrolyzed powder compact.
Electrophoretic Deposition of Lead Zirconate Titanate Ceramics
Authors: Ma Jan , Cheng Wen
Published: July 2010

Electrophoretic deposition (EPD) of submicrometer lead zirconate titanate (PZT) powders in ethanol was conducted to form thick green PZT films up to 160 μm thick. The PZT colloid stability, as a function of pH, was studied by zeta‐potential measurement. The electrical condition of the suspension was quantified by conductivity measurement at various pH values. The effect of different applied current densities on the deposition mass was investigated. A kinetic model for the constant‐current‐density EPD process, taking into account the particle concentration variation, was also examined, based on the experimental results obtained in the present work. An empirical relationship between the kinetic constant and the applied current density has been proposed.
Formation and Properties of Hydroxyapatite–Calcium Poly(vinyl phosphonate) Composites
Authors: Greish Yaser E. , Brown Paul W.
Published: July 2010

Ceramic–polymer composites composed of hydroxyapatite (Ca10(PO4)6(OH)2, HAp) and calcium poly(vinyl phosphonate) salt were prepared by warm‐pressing powder mixtures of tetracalcium phosphate (Ca4(PO4)2O, TetCP) and poly(vinyl phosphonic acid) (PVPA) at a weight ratio of 3.5:1. The effects of temperature (to 300°C), pressure (to 690 kpsi), or compaction time (to 1 h) on the extent of conversion were studied using X‐ray diffraction, Fourier transform infrared spectroscopy, differential scanning calorimetry, and scanning electron microscopy coupled with energy dispersive spectroscopy. The conversion of TetCP to HAp and formation of the calcium poly(vinyl phosphonate) salt was enhanced at higher temperature, pressure, and/or longer compaction time. Mechanical property determinations showed both the tensile strengths and elastic moduli continuously increase with increasing temperature, pressure, and compaction time. However, the glass transition temperature values of the composites were only minimally higher than that of the unreacted polymer.