bluuebunny/crossref_metadata_2025 huggingface dataset:
Pseudopotential Periodic Hartree‐Fock Study of the Cristobalite Phases of Silica and Germanium Dioxide
Authors: Silvi Bernard , Allavena Marcel , Hannachi Yacine , D'Arco Philippe
Published: March 2005

Periodic pseudopotential Hartree–Fock calculations have been performed on the cristobalite strucure of silica and germanium dioxide in order to investigate the topology of the ground‐state Born–Oppenheimer energy surface. For both oxides the averaged X–O bond lengths, the two unequivalent O–X–O bond angles, and the tilt angle have been optimized. The tilt angle, which measures the rotation of the XO4 tetrahedra around their local C2 axes, is directly related to the O–X–O bond angle and connects the low‐temperature phase twin structures; its zero corresponds to the ideal β‐phase. For SiO2 the calculated values of these coordinates agree within 1% with 10 K experimental data whereas for GeO2 the agreement between calculation and experiment is much less satisfactory. It is shown that the tilt angle is the most important structural parameter. The structure of β‐cristobalite is rationalized on the basis of the energy dependence upon tilt angle and the absence of β‐structure in GeO2 is explained. The one‐electron properties, i.e., band structure and density of states, have been calculated and are compared with the results of previous calculations carried out by other methods. As expected in Hartree‐Fock calculations, the band gap is overestimated. The analysis of the calculated electron density and of density of states projected onto basis orbitals indicates that these oxides are mostly ionic. It is also shown that the GeO bond is more covalent than the SiO one, which explains the different behavior of these two oxides.
Effect of Carbon Reduction on the Properties of 13 mol% Tio2─3 mol% Y2O3─84 mol% ZrO2
Authors: Haberko Krzysztof , Pyda Waldemar , Piekarczyk Jan , Bucko Miroslaw M.
Published: March 2005

Fully tetragonal and sintered 13 mol% TiO2─3 mol% Y2O3─84 mol% ZrO2 was thermally treated at 1300°C for 1 h in argon in the presence of carbon. No phase changes occurred on the as‐received surface and in the bulk of the material, but t→m transformation occurred on polished surfaces under reducing conditions, and it resulted in increased fracture toughness, Young's modulus, and modulus of rigidity. Deoxidation of the system occurred and 0.174 wt% of carbon was found in the sample. This seemed to stabilize the tetragonal phase.
Decomposition of Carbon Dioxide to Carbon with Active Wustite at 300°C
Authors: Kodama Tatsuya , Tominaga Ken‐ichi , Tabata Masahiro , Yoshida Takashi , Tamaura Yutaka
Published: March 2005

Active wustite (FeδO, with a δ value of 0.98) was prepared by keeping normal wustite (δ value of 0.94) in a N2 atmosphere at 300°C for 10 min. This reaction is given by (4δ2–3)Feδ1O→(4δ1–3)Feδ2O + (δ2–δ1)Fe3O4 where δ1= 0.94 and δ2= 0.98. The equation indicates that the normal wustite undergoes eutectoid decomposition into active wustite and stoichiometric magnetite (Fe3O4). Carbon dioxide (1.013 × 105 Pa) was almost completely (100%) decomposed into carbon (zero valence) by the active wustite at the low temperature of 300°C, which was associated with the transformation of the active wustite into the stoichiometric magnetite. The internal pressure of the reaction cell eventually became a vacuum.
Effect of Preparation Methods and Condition of Precursors on the Phase Composition of Yttria‐Stabilized Zirconia Powders
Authors: Gongyi Guo , Yuli Chen
Published: March 2005

An yttria‐stabilized zirconia powder, free of monoclinic phase, may be prepared by an oxalate method in an ethanol solution at strong acidity. This study demonstrates that the control of pH in the preparation of precursors has a significant effect on the ability of precursors to crystallize and hence plays an important role in determining the formation and fraction of various crystalline phases in the resulting yttria‐stabilized zirconia powder. With the increase of pH, a precursor with a certain crystalline form may be transformed into an amorphous precursor, and a monoclinic phase appers in the phase composition of the resulting powder. The results of XRD analysis and Raman spectroscopy are discussed.
Creep Due to a Non‐Newtonian Grain Boundary Phase
Authors: Chadwick Margaret M. , Wilkinson David S. , Dryden John R.
Published: March 2005

A model has been developed for creep due to viscous flow in a material containing a non‐Newtonian, amorphous grain boundary phase. The model describes the process whereby glass is squeezed out from between grain boundary layers in compression, and flows toward grain boundary layers under tension. A pseudo steady state, in which the creep rate is constant, is seen at small strains. However, beyond a critical strain, the strain rate starts to decrease rapidly, and no creep is possible once adjacent grains come into contact. The maximum possible strain is twice as large in tension as in compression. This leads to a form of anisotropy which may explain the shift in neutral axis which occurs during flexural creep.
Tensile Ductility of Superplastic Al2O3–Y2O3–Si3N4/SiC Composites
Authors: Rouxel Tanguy , Wakai Fumihiro , Izaki Kansei
Published: March 2005

Si3N4/SiC composites are ceramic materials that exhibit excellent performance for high‐temperature applications. Prepared from an ultrafine amorphous Si‐C‐N powder, sintered materials are constituted mainly of a β‐Si3N4 matrix with SiC inclusions and have a very small grain size (less than 1 μm). Such a microstructure is propitious for superplastic forming. Superplasticity has been studied in tension, from 1550° to 1650°C, under nitrogen atmosphere. Elongations over 100% have been achieved. In many cases, at the highest temperatures and slowest strain rates, materials are damaged by different processes, including microcracking, cavitation, and chemical decomposition. A map of the most suitable (strain‐rate/temperature) domain has been established. It allows the prevention of any structural alteration by selecting carefully the testing conditions. Since specimens suffered considerable strain‐induced hardening, sources for this phenomenon are examined. Although the experiments have involved high temperature and extensive strain, neither static nor dynamic grain growth has occurred. Crystallization of the amorphous grain‐boundary phase, which is reported in most cases, may be invoked. However, based on microstructural observations, it is not the unique origin for flow hardening.
Calculation of Ultraslow Dielectric Relaxation of Doped BaTiO3 Ceramics
Authors: Sun Hongtao , Zhang Liangying , Yao Xi
Published: March 2005

Using a linear voltage ramp method, ultraslow dielectric relaxation processes of La‐doped BaTiO3 boundary layer (BL) capacitors are measured at room temperature. For the electric polarization model of a double‐layer dielectric, a procedure to calculate the electrical parameters of grain and grain boundary has been developed. Calculated results suggest that there is a shell on the surface of the grain. TEM images of the BL capacitor also show a lot of microregions with minor structural modifications. Relaxation times are estimated from the equivalent circuit and are in agreement with fitted results.
Initial Morphology and Grain Growth in Donor‐Doped Barium Titanate
Authors: Drofenik Miha
Published: March 2005

The influence of the initial grain morphology on the anomalous grain growth in donor‐doped barium titanate has been studied. The results show that powders with aspherically shaped grains exhibit a much lower critical donor‐dopant concentration, inhibiting anomalous grain growth during sintering, than powders with equisized spherically shaped grains and a similar, or even a lower, initial specific surface area.
Nuclear Waste Glass Durability: I, Predicting Environmental Response from Thermodynamic (Pourbaix) Diagrams
Authors: Jantzen Carol M.
Published: March 2005

Pourbaix diagrams are shown to conceptually explain dissolution of glasses in terms of their thermodynamic stability and surface layer formation. To predict the long‐term effects of ground‐water contact on the durability of nuclear waste glass in a geologic repository, the nature of the glass dissolution process has been extensively examined. Hydration thermodynamics has been used to quantify the role of glass composition and the effect of solution pH. The glass compositions examined vary widely in composition and include natural, lunar, medieval, and nuclear waste glasses, as well as some glass‐ceramics. The known effects of solution pH and oxidation potential (Eh) on glass dissolution are empirically described by thermodynamically calculated Pourbaix diagrams. Statistical analysis of over 300 glass durability tests demonstrates that the Pourbaix diagram can be quantified because of (1) the colinearity of the hydration free energy with solution pH defined by the Nernst equation and (2) the colinearity of the solution pH with the concentration of dissolved silicon and boron in the solution defined by the pH dependence of the ion activities. Construction of Pourbaix diagrams to describe glass dissolution serves to unify and categorize various existing experimental glass dissolution data.
Evaluation of the Strength and Creep–Fatigue Behavior of Hot Isostatically Pressed Silicon Nitride
Authors: Ferber Mattison K. , Jenkins Michael G.
Published: March 2005

The strength of a commericially available hot isostatically pressed silicon nitride was measured as a function of temperature. To evaluate long‐term mechanical reliability of this material, the tensile creep and fatigue behavior was measured at 1150°, 1260°, and 1370°C. The stress and temperature sensitivities of the secondary (or minimum) creep strain rate were used to estimate the stress exponent and activation energy associated with the dominant creep mechanism. The fatigue characteristics were evaluated by allowing individual creep tests to continue until specimen failure. The applicability of the four‐point load geometry to the study of strength and creep behavior was also determined by conducting a limited number of flexural creep tests. The tensile fatigue data revealed two distinct failure mechanisms. At 1150°C, failure was controlled by a slow crack growth mechanism. At 1260° and 1370°C, the accumulation of creep damage in the form of grain boundary cavities and cracks dominated the fatigue behavior. In this temperature regime, the fatigue life was controlled by the secondary (or minimum) creep strain rate in accordance with the Monkman–Grant relation.
Sintering of in‐Situ Synthesized Sic–TiB2 Composites with Improved Fracture Toughness
Authors: Ohya Yutaka , Hoffmann Michael J. , Petzow Günter
Published: March 2005

TiB2‐particle reinforcement is one of the most successful methods for improving the fracture toughness of SiC ceramics.1–3 Commercially available TiB2 powders, however, have a large particle size and/or are highly reactive so that they are not favorable as a starting powder. In the present work, TiB2 particles are formed by an in situ reaction between TiC and boron. The reaction takes place during sintering between 1000° and 1600°C and is accompanied by a large volume expansion. Under optimum conditions, dense composites (> 98% of theoretical) can be obtained by pressureless sintering using B and C as sintering additives. The in situ reaction method enables, for the first time, a complete densification of SiC‐particulate composites by pressureless sintering. The fracture toughness of the composites was approximately 30% higher than that of the monolithic SiC ceramic.
Joining SiAlON Ceramics Using Composite β‐SiAlON–Glass Adhesives
Authors: Walls Philip A. , Ueki Masanori
Published: March 2005

Commercial β‐SiAlON ceramics were joined using mixed Si3N4, Y2O3, Al2O3, and SiO2 powders. At a joining temperature of 1600°C and a hold time in excess of 10 min, the adhesive was converted to an approximate 60:40 vol% composite of β‐SiAlON–glass‐ceramic. The grain size of the acicular β‐SiAlON grains precipitated in the joint (submicrometer diameter, average aspect ratio of 10) was significantly smaller than those in the adherend ceramic (1–5 μm diameter). Intergrowth of β‐SiAlON grains at the joint interface resulted in high bond strengths. The chemistry and microstructure of the ceramic adhesives used are described.
Strengthening of Silicon Carbide Ceramics by Surface Nitridation during Hot Isostatic Pressing
Authors: Dongliang Jiang , Jihong She , Shouhong Tan , Greil Peter
Published: March 2005

Surface strengthening of SiC by in situ surface nitridation during post‐hot isostatic pressing (post‐HIPing) in N2 was investigated. The formation of a thin (5–15 μm) layer of submicrometer β‐Si3N4 on the surface of SiC was obtained at 1850°C and 200 MPa. While SiC HIPed in Ar attained a mean bending strength of 660 MPa, a significant increase in strength (with a maximum fracture stress above 1000 MPa) was observed for the SiC/Si3N4‐layer composite material. Generation of residual compressive stresses on the surface layer caused by the differences in thermal expansion may account for the observed strengthening. Thus, in situ surface nitridation by post‐HIPing in N2 may offer an attractive way to improve surface‐sensitive mechanical properties of complex‐shaped SiC components.
Frequency of Pore Location in Sintered Al2O3
Authors: Liu Yixiong , Patterson Burton R.
Published: March 2005

The relative frequencies of pore location on grain corners, edges, and surfaces in final stage sintering have been measured in both pure and MgO‐doped Al2O3. Both materials showed identical location frequencies, with an average of 58% of the pores on grain corners, 29% on grain edges, and 13% on grain surfaces, over a density range from 92% to > 99% of full density.
Unique Opportunities for Microstructural Engineering with Duplex and Laminar Ceramic Composites
Authors: Harmer Martin P. , Chan Helen M. , Miller Gary A.
Published: March 2005

Duplex (two‐phase) microstructures and laminar composites offer some unique opportunities for improving the room‐temperature mechanical reliability (e.g., flaw tolerance) and the high‐temperature microstructural stability (e.g., resistance to grain growth and creep damage) of structural ceramic materials. Examples illustrating the approach to designing novel multiphase microstructures and laminar composites with enhanced structural reliability are given. These are based on current work on various alumina‐based ceramics as well as zirconia‐, silicon nitride‐, and silicon carbide‐containing ceramics. Critical issues and areas for future work are discussed.
Texture and Anisotropy in Silicon Nitride
Authors: Lee Farnjeng , Bowman Keith J.
Published: March 2005

In this investigation quantitative texture analysis, including calculation of the orientation distribution function, is used to demonstrate the degree of preferred orientation in β‐Si3N4 which has been hot‐pressed or hot‐worked. The results indicate that plane strain compression can produce strong textures. The texture is decided by the processing parameters including temperature, sintering additives, and stress state. Grain rotation and preferred grain growth apparently both contribute to texture development in β‐Si3N4. Basal (00l) pole figures obtained from the orientation distribution function are consistent with microstructural observations and are reflected in indentation fracture toughness anisotropy. In plane strain the ratio of maximum to minimum fracture toughness is greater than 2.
Crystal Structure and Microwave dielectric Properties of Ba(Zn1/3Ta2/3)O3–(Sr,Ba)(Ga1/2Ta1/2)O3 Ceramics
Authors: Kageyama Keisuke
Published: March 2005

The microwave dielectric properties and crystal structure of Ba(Zn1/3Ta2/3)O3– (Sr,Ba)(Ga1/2Ta1/2)O3 ceramics were investigated in the present study. The Q value of Ba(Zn1/3Ta2/3)O3 was improved by adding 5 mol% Sr(Ga1/2Ta1/2)O3. The maximum Q value of Q×f= 162000 GHz was obtained at 0.95Ba(Zn1/3Ta2/3)O3. 0.05Sr(Ga1/2Ta1/2)O3. For this composition, a lattice super structure caused by hexagonal ordering was observed. A further improvement in the Q value was attained when some Sr was replaced with Ba, and 0.95Ba(Zn1/3Ta2/3)O3· 0.05(Sr0.25Ba0.75)(Ga1/2Ta1/2)O3 exhibited a maximum Q value such that Q×f= 210000 GHz. Despite the increased Q value with the replacement of Sr by Ba, the c/a value, which indicates the degree of lattice distortion, remained constant near

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3/2. The Q value thus improved without lattice distortion in the system Ba(Zn1/3Ta2/3)O3‐(Sr,Ba)(Ga1/2Ta1/2)O3, whereas the improvement of Q value increased with lattice distortion in the solid solution system with Ba(Zn1/3Ta2/3)O3 as an end member.
Stress Distributions in Ceramic Composites Containing Faceted Inclusions
Authors: Rao Shyam S. , Tsakalakos Thomas , Cannon W. Roger
Published: March 2005

Using a Fourier transform approach, the micromechanical stress distributions in and around inclusions in ceramic composites were calculated and were found to be in excellent agreement with the Eshelby approach when applied to an ellipsoidal geometry. Both inhomogeneous modulus and crystalline anisotropy were incorporated into the technique. The calculations were then extended to a more realistic inclusion shape, namely, the octahedron, and the effects of faceted geometries and stress concentration sites were delineated. Calculations show that the approximation of ellipsoidal inclusion shape for a faceted inclusion in ceramic composites yields the general features but can be misleading in predicting the micromechanical properties. Our model is applied to predict nucleation of cracks at faces, edges, and corners of octahedrally shaped SiC inclusions in Al2O3 and nucleation sites of the room‐temperature phase transformation of octahedrally shaped particles of zirconia (from tetragonal to monoclinic) in an alumina matrix, where the martensitic nucleation is governed by strain fields.
Electromechanical Determination of the High‐Field Phase Transition of Pb(Mg1/3Nb2/3)O3–PbTiO3–(Ba,Sr)TiO3 Relaxor Ferroelectrics
Authors: Pilgrim Steven M. , Massuda Mona , Sutherland Audrey E.
Published: March 2005

Ceramics in the (1 –x)[(1 –y)Pb(Mg1/3Nb2/3)O3·yPbTiO3] ·xMeTiO3 system, where Me is Sr or Ba, exhibit very large electrostrictive strains at reasonable drive fields. However, the optimum use temperature and frequency vary with the particular composition used. As relaxor ceramics, each composition has a broad transition from electrostrictive to partially piezoelectric behavior. The transition temperature (Tt) can be roughly determined from strain or polarization properties; however, it can be more quantitatively determined from the effective electro‐mechanical Q. A plot of induced transverse strain/induced polarization squared (effective Q12) as a function of temperature shows a sharp and unmistakable change in slope—this defines Tt. The slope of induced transverse strain/polarization (effective g31) also shows a change in slope at Tt, although this is more gradual than that of effective Q. The indicated Tt correlates with those found from measurement of strain and polarization.
Thermal Analysis of Reactions in Soda–Lime Silicate Glass Batches Containing Melting Accelerants: II, Multicomponent Systems
Authors: Hong Kug Sun , Lee Sang Won , Speyer Robert F.
Published: March 2005

The glass melting reactions in a multicomponent system (sand–soda ash–calcite–dolomite–feldspar) were studied using data from DTA, TGA, and XRD interactively. The first‐formed liquid phase occurred at 700°C from eutectic melting among CaCO3, Na2CO3, and MgO. Further liquid phase formed at the CaCO3–Na2CO3, eutectic at 785°C and a fusion reaction among SiO2, CaO, and the molten phase at 812°C. Reactions between molten soda ash and silica grains to form a sodium disilicate coating also occurred in this temperature range. The effects of reaction accelerant additions (Na2SO4, NaNO3, NaCI) on batch fusion were analyzed. Sodium chloride was found to be the most effective melting accelerant due to the formation of a NaCI–Na2CO3 eutectic liquid phase at ∼636°C, which effectively attacked the silica relic. CO2 gas release terminated ∼80°C earlier with 1 wt% NaCI additions to the base glass.
High‐Temperature Electrical Properties of the Bi2.1Sr1.9(Ca1–xYx)Cu2Oy Solid Solution
Authors: Hong Byungsun , Mason Thomas O.
Published: March 2005

By a combination of conventional physical property measurements and high‐temperature electrical property studies, the solid solution limit, transport parameters, and potential defect regimes of the Bi2.1Sr1.9(Ca1 ‐ xYx)Cu2Oy solid solution were established. A continuous solid solution extends to x= 0.7 or 0.8. The electrical properties indicate that the product of the hole density‐of‐states and mobility for semiconducting compositions is approximately an order of magnitude smaller than for the other p‐type superconducting cuprates. A pronounced drop in hole concentration accompanies the tetragonal‐to‐orthorhombic transition at x= 0.5, whereafter superconductivity disappears. The electrical properties also indicate that a composition x change occurred reversibly below 1100°C.
Model for the Effect of Powder Packing on the Driving Force for Liquid‐Phase Sintering
Authors: Shaw Thomas M.
Published: March 2005

A model for liquid‐phase sintering is presented that explicitly considers the effect that the pore size distribution of the sintering compact has on the capillary forces that drive densification. In particular, the effect that liquid redistribution in the pore structure has on the driving force for sintering is considered under the assumption that the liquid can easily move to find a low‐energy configuration in the pore structure. It is shown that, for a powder compact that has a narrow pore size distribution, densification exhibits approximately the same time dependencies as those predicted by the Kingery model for liquid‐phase sintering. However, systematic changes in the absolute densification rate with the volume fraction of liquid, and the mean and breadth of the pore size distribution, are predicted. With more extreme pore size distributions, such as a bimodal distribution, behavior significantly different from that predicted by Kingery is found. In particular, it is predicted that, without there being a change in sintering mechanism, abrupt changes in densification rate may occur if the peaks in the bimodal distribution are well separated. The model provides a rational basis for interpreting how powder packing and processing steps can influence densification by liquid‐phase sintering.
Cracking due to Localized Hot Shock
Authors: Tvergaard Viggo , Xia Z. Cedric , Hutchinson John W.
Published: March 2005

Results are presented for the cracking to be expected when a sudden temperature increase is applied over a localized region on a surface of a brittle solid. A localized temperature increase is applied to the surface of a body whose interior is initially at uniform temperature. A three‐part analysis is conducted: (i) for the evolving temperature distribution, (ii) for the evolving thermal stresses induced by the nonuniform temperature field, and (iii) for stress intensity factors of cracks oriented either parallel to or perpendicular to the surface and initiated at times when the stresses are critical. Plane‐strain and axisymmetric versions of the problems are considered. For the plane‐strain problem, the complete trajectory of the crack is determined under the assumption that its tip advances maintaining a pure mode I field at all times. Conditions for excluding cracking due to localized hot shock are given.
In Situ Observation of Zirconia Particles at 1200°C by High‐Resolution Electron Microscopy
Authors: Kusunoki Michiko , Yonemitsu Kyoko , Sasaki Yukichi , Kubo Yukio
Published: March 2005

Zirconia (ZrO2) particles (average diameter, 30 nm) were observed in an in situ heating experiment up to 1200°C using a 400‐kV high‐resolution electron microscope. Thermal vibration of atoms on a (001) surface plane was observed at 1100°C. At 1200°C, grain growth and sintering phenomena were recorded on a videotape, showing (100) lattice planes migrating on a surface of a particle. Direct observation of the sintering process on a lattice level was accomplished for the first time.
P–T Boundary of α‐PbO2 Type and Baddeleyite Type High‐Pressure Phases of Titanium Dioxide
Authors: Tang Jie , Endo Shoichi
Published: March 2005

In situ X‐ray diffraction experiments have been carried out under high pressures up to 18 GPa and temperatures up to 950°C to determine the stable regions of the α‐PbO2 type and baddeleyite type high‐pressure phases of TiO2. Observation of the transitions among the three phases, including the rutile phase, along various paths on the P–T diagram results in the phase boundary equation, T(°C) = 188.7P(GPa) ‐ 2192.5, separating the α‐PbO2 and baddeleyite phases.
Model for the Formation of Silicon Carbide from the Pyrolysis of Dichlorodimethylsilane in Hydrogen: II, Silicon Carbide Formation from Silicon and Methane
Authors: Cagliostro Domenick E. , Riccitiello Salvatore R.
Published: March 2005

A model is developed for the deposition of silicon carbide from the pyrolysis of dichlorodimethylsiane in hydrogen, in a tubular reactor at temperatures from 700° to 1100°C and 1.013 × 105 Pa (1 atm) pressure. Concentration of dichlorodimethylsilane varied from 2 to 8 vol%. Gas chromatography was used to determine the volatile products of reaction, and gravimetric analysis was used to determine the total silicon and silicon carbide deposition on the tube. The model developed based on the experimental data that assumes the following chemical reactions:

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The rate constants derived from a nonlinear regression analysis are reported.
Development of Functional Microstructure during Sintering of a ZnO Varistor Material
Authors: Olsson Eva , Dunlop Gordon , Österlund Ragnar
Published: March 2005

The effects of sintering time and rate of cooling on the microstructure and current/voltage characteristics of a ZnO varistor material have been investigated. The results provide information about the development of microstructure during sintering and have been interpreted in terms of the role which microstructure plays in the determination of electrical properties. A continuous network of intergranular Bi2O3, which lies mainly along the triple junctions between ZnO grains, can give a substantial contribution to the leakage current. The conductivity of this skeleton depends upon its internal microstructure.
A Methodology for Estimating the Mechanical Properties of Oxides at High Temperatures
Authors: Hillig William B.
Published: March 2005

There are few data regarding the temperature dependence of elastic and related properties of materials. This paper describes a semiquantitative basis for estimating such properties in oxide materials to temperatures even approaching their melting points. The various elastic constants are shown to correlate with the field strength of the cationic species, and on Vo, the apparent atomic volume of oxygen. The value of Vo is readily determined from crystallographic data. High‐temperature diamond pyramid hardness measurements are shown to correlate with Vo and to provide additional information regarding the structural stability of materials at high temperature. The various refractory oxides are analyzed using these approaches, allowing identification of oxides that appear to be exceptionally strong, stiff, and creep resistant at high temperature.
Creep Behavior of Potassium Chloride–Rubidium Chloride Solid Solution Alloys
Authors: Duong H. , Beeman M. , Wolfenstine J.
Published: March 2005

The creep behavior of three KCl–RbCl solid solution alloys and pure KCl and pure RbCl single crystals compressed along the 〈100〉 direction was investigated at 600°C at stresses between 0.5 and 15 MPa. The reduced primary creep stage, a value of the stress exponent of about 3 for the KCl–20 mol% RbCl and KCl–30 mol% RbCl alloys versus about 5 for pure KCl and pure RbCl and the results of the stress reduction tests for these alloys are in good agreement with creep behavior observed in class I metallic solid solution alloys, where the creep rate is controlled by a dislocation glide process. The dislocation substructure developed in the KCl–RbCl alloys consisted of well‐developed subgrains.
Preparation and Characterization of Platinum–Ceramic Composite Powders and Thin Films
Authors: Kohno Kazushige , Takeda Yasuo , Imanishi Nobuyuki , Sakaki Yoshinori , Sakamoto Katsuyuki , Yamamoto Osamu
Published: March 2005

Pt–ceramaic (TiO2, ZrO2, and ZrO2–5 mol% Y2O3 (PSZ)) composite powders were prepared by liquid‐phase reaction. These composite powders show highly functional properties such as the following. The sintered films of these powders are good metallic conductors. The resistivity of the Pt–PSZ film at room temperature is about 1.5 × 10−5Ω·cm, which is almost comparable to that of pure platinum (1.05 × 10−5Ω·cm). This film shows excellent cathodic oxygen reduction. The overpotential for the oxygen reduction at 750°C is less than 20 mV even at a high current density of 0.1 A/cm2. The Pt–TiO2 composite powders have highly catalytic activity for the reduction of NO gas.
Measurement of Small Concentrations of Chromium and Iron in Rutile (TiO2) Using Electron Spin Resonance
Authors: de Biasi Ronaldo S. , Fernandes Antonio A. R. , Grillo Maria L. N.
Published: March 2005

Electron spin resonance linewidths of Cr3+ and Fe3+ in rutile were measured in samples doped with Cr and Fe. The linewidth was found to change in a predictable way with Cr and Fe concentration. The experimental results show that the technique can be used to measure, rapidly and nondestructively, small concentrations of Cr and Fe in commercial TiO2.
Electrical Nonuniformity of Grain Boundaries within ZnO Varistors
Authors: Sun Hong‐Tao , Zhang Liang‐Ying , Yao Xi
Published: March 2005

ZnO varistors with and without ZnO crystalline seeds have been prepared through conventional ceramic processing. Their electrical nonuniformity has been carefully examined using microcontact measurements of single grain boundaries, current‐voltage (I‐V) characteristics, and dielectric temperature spectra of bulk samples. Three types of grain boundaries with barrier heights of 0.2, 0.5, and 0.6 eV are identified in a ZnO varistor with the seeds, while only one with 0.4 eV barrier height has been found in a varistor without the seeds. Using a computerized electric circuit simulation, the influence of boundary thickness and grain size on I‐V characteristics of varistor is investigated extensively. The simulated results show that currents passing across various grain boundaries are quite different. For a model varistor (1 × 1 × 1 mm3) with different grain size, a good agreement between measured data and simulated curves can be achieved.
Plasma Spray Deposition of Fine SiC Powder
Authors: Pan Wen Xia , Okamoto Taira , Miyamoto Yoshinari , Ning Xiao Shan , Matsumoto Hiroshi , Yoshida Toyonobu
Published: March 2005

Plasma powder deposition of SiC has been investigated to develop a suitable process for making a dense body of covalent bond ceramics without additives. Several kinds of β‐and α‐SiC powders under 10 μm were fed into Ar + H2 dc plasma jet generated at 1 × 104 Pa to 1.3 × 104 Pa and were deposited on a heated graphite substrate. Dense SiC specimens of 5 mm × 5 mm × 2 mm or 16‐mm dia × 0.5 mm could be obtained within a deposition time of 120 s. They bonded to the substrate well, and the hardness reached about 24–30 GPa. Surface activation effects caused by the interaction between plasma and particles are considered to play an important role in enhancing the dense deposition.
Atomic‐Level Characterization of Cubic Silicon Carbide Surfaces—A Review
Authors: Tsong Ignatius S. T.
Published: March 2005

Scanning tunneling microscopy (STM) images of the cubic β‐SiC (100) and β‐SiC (111) surfaces are taken after annealing to 1200°C to eliminate the surface oxide. Low‐energy electron diffraction (LEED) patterns of the β‐SiC (111) surface show a 6 √ 3 × 6 √ 3 geometry, while STM images show a 6 × 6 geometry. Contrast reversal is observed as tunneling voltage bias is reversed. Spectroscopic I/V measurements indicate the presence of a graphite layer on the top surface. A model of the surface is proposed where an incommensurate graphite monolayer is grown over a (1 × 1) Si‐terminated β‐SiC (111) surface. This model helps to explain the discrepancy between the 6 √ 3 × 6 √ 3 LEED pattern and the 6 × 6 geometry observed in STM images. Charge transfer between certain carbon atoms and silicon atoms gives rise to the 6 × 6 geometry and the contrast reversal.
Creep Behavior of a Sintered Silicon Nitride
Authors: Chadwick Margaret M. , Jupp Robert S. , Wilkinson David S.
Published: March 2005

A commercial sintered silicon nitride has been crept in bending and compression at temperatures of 1100°C to 1400°C. In the as‐sintered condition the material contains an amorphous intergranular phase. This phase undergoes partial devitrification as a result of high temperature exposure. Preannealing the material to a stable microstructure has very little effect on the creep properties. Deformation behavior compares well with that predicted from a model for creep due to viscous flow of a non‐Newtonian grain boundary phase. In bending, the model predicts an initial constant strain rate at low strains as the intergranular phase is squeezed out from between grains under compression. Samples crept in compression are not expected to have this same initial constant strain rate regime. The model also predicts a strong initial strain rate dependence (in bending) on the initial thickness of the amorphous grain boundary layer. Experimentally this strain rate is not affected by partial grain boundary crystallization, suggesting that partial devitrification does not alter the intergranular film thickness or viscosity. This is supported by transmission electron microscopy, which has shown that crystallization of the intergranular phase occurs largely in the pockets between grains, leaving amorphous films between grains.
High‐Temperature Dynamic Mechanical Thermal Analysis of a Lithium Zinc Silicate Glass‐Ceramic
Authors: Hill Robert , Gilbert Paul
Published: March 2005

High‐temperature dynamic mechanical thermal analysis (DMTA) has been used to study a lithium zinc silicate glassceramic. The DMTA technique was capable of measuring the glass transition temperature of the original starting glass composition, as well as being able to follow crystallization and structural changes within the glass. The greater sensitivity of DMTA compared to differential thermal analysis, or differential scanning calorimetry, enabled the detection and measurement of the glass transition temperature of the residual glass phase in a largely crystalline glassceramic. An activation energy of 542 kJ·mol‐1 for this glass transition was obtained from a multifrequency experiment. A second transition was found in the crystallized glassceramics, and this is thought to be due to a crystal phase transition. Some evidence was also found supporting the view that in these glasses nucleation occurs by an amorphous phase separation mechanism.
Effect of Milling Treatment on the Particle Size in the Preparation of AIN Powder from Aluminum Polynuclear Complexes
Authors: Hashimoto Noboru , Yoden Hiroyoshi , Deki Shigehito
Published: March 2005

Aluminum nitride powder was synthesized from aluminum polynuclear complexes. Basic aluminum chloride (BAC) provided the aluminum polynuclear complexes. The effect of the milling treatment for the precursor made from basic aluminum chloride and glucose on the particle size was investigated. BAC and glucose were dissolved in water and mixed homogeneously. AIN powder was obtained by calcining under a nitrogen gas flow after drying, milling by vibration mill, and precalcining at 800°C. Then excess carbon was removed by firing in air. It was found that the milling treatment affected the particle size of AIN powder and nitridation mechanism. Without the milling treatment, AIN powder was synthesized directly from the γ‐alumina of an intermediate product. In using a milled precursor, however, α‐alumina was formed during the calcining, and the particle size of the AIN powder obtained was about five times larger than that of AIN powder synthesized from a raw precursor. The formation of α‐alumina phase began at the rather low temperature of 800°C. These results suggest that the mechanochemical effects added by the milling promotes the formation of α‐alumina during calcining, and the α‐alumina phase formed accelerates the particle growth. AIN powders obtained were very uniform. The oxygen content and the surface area of AIN powders synthesized from the raw and milled precursors were 2.9 wt% and 17.5 m2/g, and 1.1 wt% and 3.6 m2/g, respectively.
Computer Simulation of the Elastic and Fracture Behavior of Sodium Chloride Crystal
Authors: Iwagoshi Thawi , Gupta Prabhat K. , Dregia Suliman A.
Published: March 2005

Molecular statics calculations of mechanical properties have been carried out for NaCI using the Born–Mayer–Huggins pair potential and periodic boundary conditions. Three truncation techniques are utilized. For each, calculations have been performed as a function of the truncation distance. The stress–strain curve, the fracture strength, and the fracture strain are found to be much more sensitive to the truncation technique and to variations in the truncation distance than other properties such as the elastic constants, the cohesive energy, or the lattice parameter.
Interaction of γ‐rays with Some Alkali‐Alkaline‐Earth Borate Glasses Containing Chromium
Authors: El‐Batal Hatem Abdel Rahman , Ezz‐El‐Din Fathy Mahmoud
Published: March 2005

The change in the visible spectra of some irradiated ternary borate glasses containing chromium has been studied by increasing the γ‐ray doses with changing alkali content or increasing chromium concentration. In the glass without chromium, the γ‐rays induce two absorption bands which are attributed to the formation of positive hole centers, whereas in the glass containing chromium at least three absorption bands are observed. The results showed that the visible absorption bands before and after γ‐ray irradiation exhibited changes with the radiation dose and/or the chemical composition of the glass. The response of the glasses to γ irradiation is related to the creation of defect color centers, the approach of a saturation condition, and the possible photochemical effect on the transition metal.
Mixed Alkali Effect in Noncoventional Alkali Gallotitanate Glasses
Authors: Miyaji Fumiaki , Hasegawa Shinya , Yoko Toshinobu , Sakka Sumio
Published: March 2005

The mixed alkali effect on electrical conductivity, that is, the reduction of conductivity due to alkali mixing, was observed in Na2O–K2O–Ga2O3–TiO2 glasses, which are nonconventional in the sense that glass‐forming oxides defined by Zachariasen are not involved. The magnitude of the reduction in conductivity of the present glasses due to alkali mixing was similar to that of corresponding mixed alkali silicate and phosphate glasses. The activation energy for electrical conduction showed a maximum around the composition Na/(Na + K) = 0.5, where the conductivity was at a minimum.
Liquid Permeability of Packed Particles: Why Perpetuate the Carmen—Kozeny Model?
Authors: Reed James S.
Published: March 2005

Liquid transport through the interstices of packed particles is commonly described using the Carmen–Kozeny mean hydraulic radius model, which calibrates the effective pore dimension from mean macroscopic parameters. However, the experimental aqueous permeability of sets of porous powder compacts varying widely in porosity and pore structure was shown to be much better described in terms of the linear mean pore size determined from mercury penetration porosimetry. Here it is shown that the latter model is supported by studies of the permeability of porous rock and percolation theory.
Diffusion of 57Co in Surface Layers of Nickel Oxide and Alumina
Authors: Macey Edward P. , Stubican Vladimir S.
Published: March 2005

Diffusion of 57Co isotope on NiO (110) and AI2O3 (0001) surfaces was investigated by using the edge source method. The surface diffusion parameter αD,Δ, where α is the segregation factor, D, the surface diffusion coefficient, and Δ the thickness of the high‐diffusivity layer, was determined for the temperature range 750°–1200°C. For calculation of experimental results, the Whipple solution was used. The Arrhenius plot shows a break at ∽900°C for the surface diffusion of 57Co isotope on AI2O3 (0001) plane. Above this temperature, vapor transport seems to be the overriding diffusion mechanism. Below this temperature, ionic transport predominates. The apparent activation energy for the ionic transport was calculated to be 120 ± 12 kj/mol.

Ionic transport predominated in the surface diffusion of 57Co on NiO over the entire investigated temperature range. This can be explained by the weak bond between Co‐vapor species and the NiO surface. The results obtained suggest that the surface diffusion of Co2+ ion on NiO at 750°C is ∼7 orders and at 1200°C ∼5 orders of magnitude faster than volume diffusion. Activation energies are 139 and 227 kJ/mol, for surface and volume diffusion, respectively.
Reactive Cerium(IV) Oxide Powders by the Homogeneous Precipitation Method
Authors: Chen Pei‐Lin , Chen I‐Wei
Published: March 2005

CeO2 powders have been prepared by aging a cerium(III) nitrate solution in the presence of hexamethylenetetramine. Oxidation of Ce3+ occurs in the precipitate and the wet precipitate is identified as crystallized CeO2 before any heat treatment. The cold‐pressed powders can be sintered to full density at temperatures as low as 1250°C in just 6 min. Moreover, the sinterability of the powders is insensitive to the calcination temperatures, particle size, or green density. The powders calcined at 850°C with a crystallite size of 600 Å have a sinterability as good as the powders calcined at 450°C with a crystallite size of 145 Å. The mechanisms for direct CeO2 precipitation and its relation to the excellent sinterability are discussed.
Semiconducting–Insulating Transition for Highly Donor‐Dopod Barium Titanate Ceramics
Authors: Kurata Natsuko , Kuwabara Makoto
Published: March 2005

BaTiO3 ceramics doped with La (0.01–0.84 at.%) were prepared only with the addition of La and stoichiometric TiO2. As a result, even when BaTiO3 was doped with 0.53 at.% La, it could be converted to a semiconductor by sintering at 1540°C for 2 h in air and cooled slowly in the furnace. Differential thermal analysis data clearly demonstrated that the Curie point in the materials shifted toward lower temperatures with increased content of La substituted at the Ba site up to a critical concentration that varied with the sintering temperature. The obtained results suggest that the semiconducting–insulating transition for highly donor‐doped BaTiO3 was closely related to the incorporation of donor into the grains and to the resultant grain size, which were significantly affected by the sinterability of the BaTiO3 starting powders and sintering conditions used.
Oxidation of Silicon, Silicon Carbide, and Silicon Nitride in Gases Containing Oxygen and Chlorine
Authors: Hsu PeiPei , Ip Sikyin , Park Chan , McNallan Michael J.
Published: March 2005

Chlorine contamination accelerates the oxidation of silicon‐based ceramics through the formation of volatile silicon chloride or silicon oxychloride species which degrade the protective character of the SiO2 film. Accelerated attack may occur by active corrosion or formation of bubbles in the oxide layer. Si3N4 is much more resistant to this attack than either silicon or SiC. This resistance may be related to the presence of a thin silicon oxynitride layer below the SiO2 scale which forms on Si3N4.
Processing of Molybdenum Disilicide Using a New Reactive Vapor Infiltration Technique
Authors: Patibandla Nag , Hillig William B.
Published: March 2005

Processing of MoSi2 via a lower‐temperature reactive vapor infiltration route is explored. In this process a loosely compacted molybdenum powder is exposed to a gaseous silicon precursor. Initially, a surface MoSi2 layer forms and, as the process proceeds, a distinct reaction front moves progressively inward. At 1200°C, the silicide layer grows at a rate of about 20 μm/h. The MoSi2 layer is of the high‐temperature tetragonal phase. No excess silicon or molybdenum is present, unlike in chemically vapor deposited molybdenum silicide. Irrespective to the volumetric increases involved in siliciding molybdenum to MoSi2, none of the samples, silicided between 1100° and 1300°C, show swelling or surface cracks.
Suspension Dry Spinning and Rheological Behavior of Ceramic‐Powder‐Loaded Polymer Solutions
Authors: Nunn S. D. , Popovic D. , Baskaran S. , Halloran J. W. , Subramanian G. , Bike S. G.
Published: March 2005

Green ceramic fibers of Al2O3, Si3N4, Ce‐ZrO2, and SiC were prepared by suspension dry spinning of powderloaded polymer solutions. The fiber‐forming polymer was ethyl methacrylate dissolved in methyl ethyl ketone. The experimental conditions for fiber formation were defined for the alumina system. Thread lengths measured as a function of extrusion rate showed a transition from capillary‐limited fracture to continuous fibers in agreement with Ziabicki's theory of thread formation. The conditions for spinnability were correlated to rheological parameters. Spinnable Al2O3 dopes were elastic fluids with a high viscosity (60 Pa·s at 0.46 s−1) under the conditions of this study.
Preparation of Barium Titanate Films at 55°C by an Electrochemical Method
Authors: Bendale Priya , Venigalla Sridhar , Ambrose John R. , Verink Ellis D. , Adair James H.
Published: March 2005

Work by previous investigators has shown that BaTiO3 films can be synthesized from solution over temperature ranges from 80°C to greater than 200°C. In the present work, electrically insulating crystalline films of BaTiO3 have been electrochemically deposited on titanium substrates at temperatures as low as 55°C. Auger spectroscopic analyses with depth profiling indicate that a titanium oxide layer whose thickness is governed by current density acts as a precursor to BaTiO3. Formation of BaTiO3 is found to be favored only in highly alkaline solutions. This is consistent with the phase stability reported for the Ba─Ti─CO2─H2O system at 25°C. Lower processing temperatures (55°C) favor the formation of thick, electrically resistive, and wellcrystallized BaTiO3 films, apparently due to increased oxygen solubility in the electrolyte solution. Films produced at 100°C are much thinner and are electrically conductive due to fissures and pores in their microstructure. Initial studies on the effect of current density indicate the formation of thinner and porous films with thicker titanium oxide intermediate layers.
Vanadate Hot Corrosion Behavior of India, Yttria‐Stabilized Zirconia
Authors: Jones Robert L. , Reidy Richard F. , Mess Derek
Published: March 2005

Powders of In2O3–Y2O3“dual stabilized”ZrO2 (IYSZ) were prepared using sol‐gel procedures and tested for resistance to destabilization by molten NaVO3 at 700° and 900°C. IYSZ powders with stabilizer oxide compositions ranging from 100% down to about 50% In2O3 were superior to 100%‐Y2O3‐stabilized ZrO2 (YSZ) in resistance to destabilization, especially at 700°C. Small additions of Y2O3 were speculated to reduce the acidity of the ZrO2 oxide anion lattice, and, therefore, improve bonding of In2O3 (more acidic than Y2O3) into the ZrO2 lattice.
Equilibrium Phase Diagrams in the Systems PbO–Ag and CuO–Ag
Authors: Shao Zhong B. , Liu Ke R. , Liu Ling Q. , Liu Hua K. , Dou Shi‐xue
Published: March 2005

Silver has played a critical role in the fabrication of metal/ high‐Tc superconductor composites. Phase equilibrium in the binary PbO–Ag and CuO–Ag systems has been investigated using differential thermal analysis (DTA), thermogravimetry (TG), and X‐ray diffractometry (XRD) techniques. Composition versus temperature diagrams have been established for these systems in air. Eutectic reactions CuO + Ag = L at a composition of Ag = 98.6 mol% and 932°± 3°C for the CuO–Ag system and PbO + Ag = L at a composition of Ag = 8.3 mol% and 825°± 3°C for the PbO–Ag system have been detected. The CuO–Ag system has two immiscible liquid phases in the composition regime of Ag = 30.6–98 mol%, and there are two immiscible liquid phases at the composition range of Ag = 11.7–95.7 mol% in the PbO–Ag system.