bluuebunny/crossref_metadata_2025 huggingface dataset:
Fabrication of Transparent Lead Lanthanum Zirconate Titanate Ceramics from Fine Powders by Two‐Stage Sintering
Authors: Yoshikawa Yoshio , Tsuzuki Kaoru
Published: March 2005

Transparent lead lanthanum zirconate titanate (PLZT) ceramics were fabricated from fine powders using an inexpensive two‐stage sintering technique. The powders were prepared by hydrolysis from low‐cost inorganic precursors. In the two‐stage sintering method, uniaxially pressed green pellets were densified to nearly theoretical values in an oxygen gas atmosphere during the first‐stage sintering, at 1000°C for 1 h, and then residual, free lead oxide in the pellets was removed by second‐stage sintering at 1100°C for 12 h. Transparent ceramic with an average grain size of 1.6 μm and a porosity of 1.3% was obtained. The transparency and dielectric characteristics of the present samples were compared with those of hot‐pressed samples: The study of the polarization–field hysteresis loops of the present samples yielded a remanent polarization of 6.8 μC/m2 and a coercive field of 1.6 kV/cm. The low coercive field of PLZT ceramics could potentially reduce the driving voltage of electrooptic devices in many applications.
Major Influences on the Particle‐Size–Transformation‐Temperature Relation in Al2O3–ZrO2 Composites
Authors: Yuan Li‐jian , Yen Tung‐sheng
Published: March 2005

The energetics of martensitic transformation in ZrO2 is studied using a thermodynamic approach, with particular reference to Al2O3–ZrO2 composites. The different characters of three types of transformation‐toughened ceramics are analyzed, and several factors affecting the t→m transformation in Al2O3–ZrO2 composites are discussed. The expression of transformation temperature dependence on particle size is derived and has good agreement with experimental results. The energetics concerned with nucleation of martensitic transformation is also discussed.
Rapid and Continuous Hydrothermal Synthesis of Boehmite Particles in Subcritical and Supercritical Water
Authors: Adschiri Tadafumi , Kanazawa Katsuhito , Arai Kunio
Published: March 2005

Hydrothermal synthesis of AlOOH particles from an Al(NO3)3 aqueous solution was examined in subcritical and supercritical water. Continuous and rapid production of AlOOH fine particles was achieved by mixing a metal salt aqueous solution with preheated water fed from another line. The effects of temperature, pressure, and initial concentration of solution on the particle size, morphology, and crystal structure of the metal (hydrous) oxides were examined. These properties are strongly affected by slight changes in pressure and temperature. This strong effect is attributed to the large changes in the properties of water near its critical point resulting from the slight changes in pressure and temperature. The results suggest the simultaneous control of size, crystal structure, and morphology of the particles by hydrothermal synthesis in supercritical water.
Harper–Dorn and Power‐Law Creep in Single‐Crystalline Magnesium Oxide
Authors: Ruano Oscar A. , Wolfenstine Jeffrey , Wadsworth Jeffrey , Sherby Oleg D.
Published: March 2005

The creep behavior of single‐crystalline MgO tested in the 〈100〉 direction is reviewed in the temperature range 1300° to 1800°C. At low stresses, the stress exponent is equal to about unity, and the deformation process is attributed to Harper–Dorn creep. At high stresses, the stress exponent is equal to approximately 5 and the deformation process is attributed to dislocation glide controlled by climb. The creep behavior in both regions is successfully predicted by an internal stress model for Harper–Dorn creep.
R‐Curve Behavior and Strength for In‐Situ Reinforced Silicon Nitrides with Different Microstructures
Authors: Li Chien‐Wei , Lee Dong‐Joo , Lui Siu‐Ching
Published: March 2005

R‐curves for two in‐situ reinforced silicon nitrides A and B of nominally the same composition are characterized using the Griffith equation and indentation fracture mechanics. These R‐curves are calibrated against fine‐grained silicon nitrides which have a known chevron‐notch (long‐crack) toughness and with a nearly flat R‐curve behavior. Silicon nitride A, with its coarser microstructure and higher chevron‐notch toughness, shows lower resitance to crack growth than silicon nitride B if the crack size is less than ∼200 μm. These results are consistent with the indentation–Strength measurements which show a crossover of strength between the two materials at an indentation load between 49 and 98 N, and below the crossover A has a lower strength. The toughening behavior is explained using an elastic‐bridging model for the short crack, and a pullout model for the long crack. The effects of R‐curve properties on design are discussed.
Diffusion‐Accommodated Grain Boundary Sliding and Dislocation Glide in the Creep of Sintered Alpha Silicon Carbide
Authors: Nixon Robert D. , Davis Robert F.
Published: March 2005

Constant compressive stress creep experiments in the temperature and stress ranges of 1770 to 2020 K and 35 to 373 MPa have been performed on polycrystalline alpha SiC. Specimens having average grain sizes of 3.5, 4.9, and 7.5 μm have been studied in order to determine the grain size dependence of steady‐state creep in this material and to distinguish the relative contributions of diffusion‐accommodated grain boundary sliding and dislocation glide. Analysis of steady‐state creep rate data revealed different kinetics of creep below and above 1920 K as follows: activation energies of 387–541 and 838–877 kJ/mol, stress exponents of 1.0–1.4 and 1.4–2.5, and inverse grain size exponents of 2.5 and 4. The m value of 2.5 indicated Coble‐controlled creep with a contribution of Nabarro–Herring creep in the low‐temperature range. The m value was less reliable for the high‐temperature range because of the large differences in the microstructures of the three materials. The large number of high aspect ratio grains with increasing average grain size resulted in different contributions of diffusion‐accommodated grain boundary sliding and dislocation glide between thsee materials.
Superconducting Properties of Barium‐Doped (Bi,Pb)2Sr2Ca2Cu3Ox Glass‐Ceramics
Authors: Komatsu Takayuki , Ueta Michiko , Ohki Tomohiro , Sato Ryuji , Matusita Kazumasa
Published: March 2005

The Ba‐doped superconducting (Bi,Pb)2Sr2‐xBaxCa2Cu3Oy and (Bi,Pb)2Sr2Ca2‐xBaxCu3Oy (0 ≦x≦ 1.0) were prepared by using a melt‐quenching method, and the effect of Ba additions on the glass‐forming ability and the crystalline phase was examined. The glass‐forming ability was not improved by substitution of Ba for Sr or Ca, and particularly BaPbO3 as well as CaO was observed in the melt‐quenched sample of (Bi,Pb)2SrBaCa2Cu3Oy. BaPbO3 crystals were precipitated in all glass‐ceramics with Ba substituted for Sr or Ca. The partial substitution of Ba substituted for Sr was effective for the formation of the high‐Tc phase, and (Bi,Pb)2Sr1.4Ba0.6Ca2Cu3Oy glass‐ceramics obtained by annealing at 830°C for 100 h exhibited superconductivity with a Tc of 103 K, although BaPbO3 and the low‐Tc phase were still largely present.
Plasma‐Enhanced Chemical Vapor Deposition of Boron Nitride Using Polymeric Cyanoborane as Source
Authors: Maya Leon
Published: March 2005

The plasma‐enhanced chemical vapor deposition (PECVD) of boron nitride films was studied using polymeric cyanoborane, (CNBH2)n, a material previously examined by thermally activated CVD. The PECVD procedure yields boron nitride coatings containing ≅20 wt% paracyanogen as a contaminant. This impurity can be removed by heat treatment under vacuum or in an ammonia atmosphere. The boron nitride coatings are hexagonal and appear to be boron deficient. The PECVD process takes place at 300°C, measured at the backside of the substrate, as compared with 600°C in the thermally activated CVD process.
Yttrium Substitution in CaS
Authors: Vanderah Terrell A. , Lowe‐Ma Charlotte K.
Published: March 2005

Substitution of Y3+ in CaS results in the cubic rock‐salt‐type solid‐solution Ca1−xY2x/3□x/3S, with x 0.37 and where □ denotes cation vacancies that form for charge compensation. The substitution range determined from polycrystalline samples prepared at 1025°C corresponds to a solubility limit of 16 mol% Y2S3 in CaS. Evolution of the unit cell edge with x indicates that the CaS lattice expands about the cation vacancies. Single‐crystal X‐ray diffraction studies of samples grown from a eutectic CaCl2–KCl flux revealed no indication of superstructure formation, indicating that Ca2+, Y3+, and vacancies are disordered across the cation sites. Thermogravimetric analysis curves for Ca0.7Y0.2S, CaS, and the ternary compound CaY2S4 indicate oxidative stability up to 430°, 410°, and 490°C, respectively.
Thermal Analysis of Reactions in Soda–Lime Silicate Glass Batches Containing Melting Accelerants: I, One‐ and Two‐Component Systems
Authors: Hong Kug Sun , Speyer Robert F
Published: March 2005

To identify each glass melting reaction in a multicomponent system, one‐component and two‐component reaction processes were studied using DTA, TGA, and XRD. Two‐component mixtures were prepared by choosing pairs in the same ratio as in a commercial container glass batch composition (sand‐soda ash‐calcite‐dolomite‐feldspar). The presence of silica in the silica‐calcite system decreased the termination temperature of the decomposition of calcite, but did not alter the onset of decomposition. Similar behavior was found in the dolomite‐silica system. A double carbonate (Na2Ca(CO3)2) formed via solid‐state reaction in the calcite‐soda ash system, and metasilicate/disilicate phases were detected during the fusion process in the soda ash‐silica system. The effects of reaction accelerant additions in the soda ash‐silica system were investigated using 1 wt% additions of sodium sulfate, sodium nitrate, and sodium chloride. Sodium chloride was the most effective melting accelerant, lowering the termination temperature of CO2 release by ∼80°C compared with the soda ash‐silica system with no additives. NaCl additions caused complete reaction and/or fusion of Na2CO3 prior to its melting temperature. Sodium sulfate additions acted to suppress metasilicate/ disilicate formation by coating quartz grains and shifted consequent CO2 release to higher temperature.
Reaction Mechanisms in the Formation of Lead Zirconate Titanate Solid Solutions under Hydrothermal Conditions
Authors: Cheng Humin , Ma Jiming , Zhu Bin , Cui Yuhong
Published: March 2005

Reaction mechanisms in the formation of PZT solid solution were studied under hydrothermal conditions (Pb/(Zr+Ti) = 1.0 to 1.9, Zr/Ti = 0/10 to 10/0, 1M to 5M KOH, 100° to 220°C, 2 h). A yellow tabular crystallite with tetragonal symmetry and Pb/Ti ∼ 2 was formed at 100° to 130°C. A PZT crystallite was formed just above 150°C. The crystallite was a mixture of Ti‐rich PZT and Zr‐rich PZT phases. When the temperature and KOH concentration were increased, the composition of the PZT product tended to be homogeneous. The PZT in the morphotropic phase boundary zone was formed at Zr/Ti = 5/5, 5M KOH, 220°C, 2 h. Neither PbTiO3 nor PbZrO3 was detected as a separate phase under the above hydrothermal conditions.
Synthesis of a Polyborosilazane and Its Conversion into Inorganic Compounds
Authors: Funayama Osamu , Kato Tomohiro , Tashiro Yuji , Isoda Takeshi
Published: March 2005

A novel polyborosilazane was synthesized by reacting per‐hydropolysilazane with trimethyl borate. The chemical structure of this polymer was investigated by the techniques of infrared spectroscopy and nuclear magnetic resonance measurements. Amorphous silicon nitride was obtained by pyrolysis of this polymer in a stream of anhydrous ammonia to 1000°C with high ceramic yield. The pyrolysis product remained amorphous after additional heating to 1700°C under N2. Crystallization to α‐Si3N4 and β‐Si3N4 proceeded with heat treatment at 1800°C under N2. These results indicate that polyborosilazane is a good precursor for amorphous silicon nitride based materials.
Humidity Response Characteristics of Barium Titanate
Authors: Hwang Tae Jin , Choi Gyeong Man
Published: March 2005

Humidity response characteristics of BaTiO3 doped with lanthanum were examined using complex impedance measurements. A sample with relative density of 71% showed a nearly log‐linear increase of conductivity with humidity at 118 Hz. The average capacitance of bulk changed little with humidity; however, the resistance showed a gradual decrease. The equivalent circuit explaining such an observation was presented.
Transformation Textures in Zirconia
Authors: Bowman Keith J. , Chen I‐W.
Published: March 2005

The large shear component of the tetragonal‐to‐monoclinic transformation in zirconia causes a stress‐induced preferred orientation of the tetragonal and monoclinic variants. The resultant texture, which is dependent on the loading condition, has been analyzed in terms of stress assistance to transformation and experimentally verified in simple tension and compression. Such a preferred orientation is a clear indication of the shear contribution to transformation plasticity and to fracture. Other implications of this analysis are also explored. For example, while the monoclinic texture is obviously most relevant to generating strains and plastic work, it is the tetragonal texture which might be advantageously tailored to enhance transformability and toughness. Transformation texture causes the ratio of {111} monoclinic and tetragonal X‐ray peaks in tensile fracture to be higher than the actual ratio of monoclinic‐to‐tetragonal phase fractions, making it generally unsuitable for estimating phase fraction in stress‐induced transformation.
Preparation of Silicon Carbonitrides from an Organosilicon Polymer: II, Thermal Behavior at High Temperatures under Argon
Authors: Bahloul Djamila , Pereira Mario , Goursat Paul
Published: March 2005

The preparation and the conversion in ceramics of the precursor cross‐linked copolysilazane (CPSZ) of formula (ViSiHNH)x(MeSiHNH)y have been summarized in Part I. Part II will examine the evolution and the thermal behavior of CPSZ at high temperatures under argon atmosphere. Gaseous phase, graphite or alumina crucible environments, and the activity of free carbon in ceramic residues have shown decisive effects on reaction products. The changes in crystallinity, powder morphology, XRD spectra, SEM observations, and oxidation behavior for samples annealed from 1200° to 1450°C are consistent with an amorphous‐to‐crystalline transformation. The influence of nitrogen atmosphere on the structural rearrangement will be shown in Part III.
Mass Transport in the Presence of Internal Defect Reactions—Concept of Conservative Ensembles: I, Chemical Diffusion in Pure Compounds
Authors: Maier Joachim
Published: March 2005

Chemical diffusion is considered for the case in which the diffusing species take part in internal defect chemical reactions giving rise to nonvanishing source terms. As long as local equilibrium is maintained, the concept of conservative ensembles allows one to derive generalized expressions for such cases, e.g., when ionic defect states can change their redox state (variable ionization degrees). In the same way the formulation of boundary conditions is analyzed.
Adherent Zirconia Films by Reactive Ion Implantation
Authors: Bunker Stephen N. , Armini Anthony J.
Published: March 2005

Conventional methods of forming ceramic coatings on metal substrates, such as CVD or plasma spray, typically retain a sharp interface and may have adhesion problems. In order to produce a completely mixed interface for better adhesion, a method using reactive ion implantation was used which can grow a thick stoichiometric film of an oxide ceramic starting from inside the substrate. Zirconium oxide ceramic films have been produced by this technique using a high‐energy zirconium ion beam in an oxygen gas ambient. Compositional data are shown based on Auger electron spectroscopy of the film. Tribological properties of the layer were determined from wear and friction measurements using a pin‐on‐disk test apparatus. The adhesion was measured both by a scratch technique as well as by thermal shock. Results show an extremely adherent ZrO2 film with good tribological properties.
Effect of Precursor Particle Size on the Densification and Crystallization Behavior of Mullite
Authors: Fahrenholtz William G. , Smith Douglas M. , III Joseph Cesarano
Published: March 2005

Stoichiometric mullite, 3Al2O3·2SiO2, has been prepared from a variety of colloidal and solution precursors. In order to change the level of mixing, the size of the silica particles has been varied while keeping the alumina (boehmite) particle size constant. The effect of varying particle size on the crystallization and densification behavior has been studied. Densification behavior was characterized by measuring the bulk density of pellets as a function of heat treatment temperature and by dilatometry. Phase development was examined by thermal analysis and X‐ray diffraction. Results showed that the formation of crystalline phases including mullite inhibited densification. The formation of mullite was controlled by the initial segregation of alumina and silica in the gel. A heat treatment time‐temperature profile designed to optimize the densification of the colloidal powders was investigated in order to compare the ability of the different gels to densify.
Glass‐Forming Ability in Calcium Aluminate‐Based Systems
Authors: Uhlmann Eugenie V. , Weinberg Michael C. , Kreidl Norbert J. , Goktas Aydin A.
Published: March 2005

Glass formation and stability of calcium aluminate‐based glasses were assessed, qualitatively, by changing melt volumes and cooling rates. Stable glasses, obtained through the addition of sodium, barium, and strontium to the binary system, were investigated in detail. Glass stability was evaluated via DTA analysis. The crystallization products were determined by X‐ray diffraction techniques. In addition, IR spectroscopic measurements were performed to gauge the relative water contents of the glasses and to assess the influence of composition upon the IR cutoff. It was determined that such additions greatly improve the glass‐forming ability and do not appear to negatively impact the optical properties of the glass.
Microstructure and Fracture Toughness of Hot‐Pressed Zirconia‐Toughened Sialon
Authors: Cain Markys G. , Lewis Mike H.
Published: March 2005

Zirconia‐toughened sialon composites have been fabricated using conventional hot‐pressing techniques. The fracture toughness and microstructure were determined for CeO2‐and Y2O3‐stabilized ZrO2 additives and also as a function of volume percent ZrO2. The yttria system showed a linear increase in fracture toughness with increasing volume fraction zirconia content while the ceria‐stabilized system exhibited a peak in fracture toughness at 20 vol % ZrO2 content. The fracture toughness at 800°C was measured and correlated with the microstructure. High‐temperature stability was determined and it was found that the deleterious nitride phases of zirconium could be precluded from the microstructure.
Plastic Deformation Mechanisms in SiC‐Whisker‐Reinforced Alumina
Authors: De Arellano‐López Antonio R. , Domínguez‐Rodríguez Arturo , Goretta Kenneth C. , Routbort Jules L.
Published: March 2005

SiC‐whisker‐reinforced Al2O3 samples (SiCw/Al2O3), obtained from three different manufacturers, containing 0–30 vol% SiC have been crept under compression at 1400°C in flowing argon. Compressive creep tests and microstructural observations have been used to characterize the plastic deformation mechanisms. The presence of whiskers decreased the creep rate by reducing grain‐boundary sliding. Damage formation was increased, however, because the whiskers acted as stress concentration sites. For specimens with whisker loadings greater than 15%, the absolute creep rate was not strongly dependent on whisker concentration, and the formation of cavitation damage was negligible below a critical stress that depended on the fabrication procedure of the specimen. This creep regime was characterized by a stress exponent of approximately 1, in which deformation occurred primarily by diffusional flow. For the materials with less SiC, the deformation occurred primarily by grain‐boundary sliding.
Sol—Gel Fabrication of Pb(Zr0.52 Ti0.48)O3 Thin Films Using Lead Acetylacetonate as the Lead Source
Authors: Selvaraj Ulagaraj , Brooks Keith , Prasadarao Alamanda V. , Komarneni Sridhar , Roy Rustum , Cross L. Eric
Published: March 2005

Lead zirconium titanate (PZT) thin films of the morphotropic phase boundary composition [Pb(Zr0.52Ti0.43)O3] were deposited on platinum‐coated silicon by a modified sol‐gel process using lead acetylacetonate as the lead source. The precursor solution for spin coating was prepared from lead acetylacetonate, zirconium n‐butoxide, and titanium isopropoxide. The use of lead acetylacetonate instead of the widely used lead acetate trihydrate provided more stability to the PZT precursor solution. Films annealed at 700°C for 12 min formed well‐crystallized perovskite phase of Pb(Zr0.52Ti0.48)O3. Microstructures of these films indicated the presence of submicrometer grains (0.1 to 0.2 μm). The dielectric constant and loss values of these films measured at 10 kHz were approximately 1200 and 0.04, respectively, while the remanent polarization and coercive field were ∼ 13 μC/cm2 and ∼ 35 kV/cm. Aging of the solution had almost no effect on the dielectric and ferroelectric properties of these films.
Mössbauer Spectroscopy Studies on Zn‐Bearing Ferrite
Authors: Kanzaki Tadao , Kitayama Kenzo , Shimokoshi Kazuo
Published: March 2005

Ion distribution in Zn‐bearing ferrite ZnxFe3−xO4 was examined using Mössbauer spectra at room temperature. Ion distribution models I and II have been proposed for the samples prepared by a solid‐state reaction and from an aqueous Zn(II)–Fe(II) hydroxides suspension, respectively.

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Properties of Zirconia–Ceria–Hafnia Alloys
Authors: Funkenbusch Eric F. , Plovnick Ross H. , Becher Paul F.
Published: March 2005

Tetragonal zirconia alloys containing up to 18 mol% ceria and 30 mol% hafnia were prepared by several wet‐cnemical techniques, sintered to high density, and their microstructural and physical properties studied. Toughening in such alloys depends on the retention of the metastable tetragonal phase and the stress required for its transformation to the monoclinic phase. The degree of metastability of the tetragonal phase in these ternary zirconia alloys was a function of the relative amounts of ceria and hafnia present. Such alloying, along with control of grain size, provides an ability to modify the fracture toughness and the temperature range over which the toughness is optimized.
Microstructural Characterization of Structural Ceramics Using Image Processing and Analysis
Authors: Hefter Jesse , Hecker Arlene , Mahoney F. Michael , Harris Jody E.
Published: March 2005

Digital imaging and analysis methods are applied to the quantitative study of microstructural changes which occur during hot‐pressing of yttria‐doped silicon nitride. Effects of processing changes upon the grain growth and microstructural anisotropy are described. Relationships between grain cross‐sectional area and mechanical properties are established. It was found that, over the range of processing conditions used, increases in grain size correlated strongly with an increase in fracture toughness. The grain size distribution broadened significantly with hot‐press time, resulting in reduced flexural strength. Although no significant change in the mean grain shape factor was observed, the variance in shape factor decreased as the hot‐press time was extended.
Various Scale Levels of the Gel‐to‐Glass Transformation
Authors: Chmel Alexandre , Krivda Alexy , Mazurina Ekatherina , Shashkin Victor , Zhizhenkov Valery
Published: March 2005

The gel‐to‐glass transition in SiO2 xerogels prepared by inorganic sol‐gel synthesis was studied. The evolution of the molecular structure is traced using the infrared and lowfrequency Raman spectroscopy methods. The elastic moduli of the samples as well as the pore wall moduli at various stages of heat treatment are determined from the data on Brillouin scattering. The formation of monolithic glass on the macroscopic level manifests itself within a narrow temperature range by the dramatic increase of Young's modulus to the accepted value for fused silica. This phenomenon coincides with structural transformations on the molecular scale: (i) the definite correlation radius (the long‐range order sphere) appears; (ii) local distortions of the silica network relax. The presence of structural defects influences the kinetics of vitreous SiO2 formation during xerogel heat treatment.
Indentation‐Cycling Tests on Soda–Lime Glass
Authors: Thiruvengadaswamy Ramesh , Scattergood Ronald O.
Published: March 2005

Repeated indentation loading produces lateral‐crack‐controlled chipping in brittle materials. This has been investigated in soda–lime glass for a load‐cycling range Pmin < Pmax≤P, where P is the indentation load. Extensive chipping damage occurs after a critical number of cycles for Pmax=P and Pmin= 0, whereas subcritical cycling with Pmin > 0 suppresses the failures. This is consistent with an environmental interaction. Postcycling aging can produce delayed chipping, and the response depends upon the load‐cycling history.
Resistance of Tin Dioxide to Chemical Reaction with Vanadate—Sulfate Melts
Authors: Jones Robert L.
Published: March 2005

A thermogravimetric analysis/SO3 equilibrium technique has been used to show that tin dioxide (SnO2) is inert to chemical reaction with NaVO3 at 700° and 800°C under SO3 partial pressures as high as 5 × 10−2 bar (5 kPa). The results suggest that SnO2 may be potentially useful as a material for protection against molten vanadate–sulfate hot corrosion at moderate temperatures (<800°C).
Characterization of Interlaminar Shear Strength of Ceramic Matrix Composites
Authors: Fang Nancy J.‐J. , Chou Tsu‐Wei
Published: March 2005

The interlaminar shear strengths of three ceramic matrix composites have been characterized using a double‐notch shear (DNS) test. The material systems investigated are plain woven C/SiC, plain woven SiC/SiC, and cross‐plied SiC/calcium aluminosilicate‐II. The use of the double‐notch shear test for measuring the interlaminar shear strength of ceramic matrix composites is evaluated first. Numerical stress analyses are performed to investigate the effect of DNS specimen length, notch distance, and specimen supporting jig on the stress distribution in the expected fracture plane and the interlaminar shear strength. The numerical findings are then compared with an analytical model proposed elsewhere and correlated with the experimental results. The validity of this test technique has been established.
Structural Evolution of Alkoxide Silica Gels to Glass: Effect of Catalyst pH
Authors: Ying Jackie Y. , Benziger Jay B. , Navrotsky Alexandra
Published: March 2005

Structural and energetic changes of alkoxide silica during densification were followed in situ using photoacoustic Fourier‐transform infrared spectroscopy (PAS‐FTIR), transposed temperature drop calorimetry, and drop solution calorimetry. Acid‐catalyzed silica gels contained higher concentrations of adsorbed water, silanol groups, and unreacted alkoxy groups than did the base‐catalyzed gels/precipitates. The surface species in acid‐catalyzed gels were removed easily, allowing this material to be densified at 800°C. The densified material was both structurally and energetically equivalent to fused silica glass. Base‐catalyzed gels had fewer silanol groups in the green state, but free silanol groups emerged during drying at 500°C. The base‐catalyzed gels did not densify below 1000°C, maintaining a high surface area of 100 m2/g. Base‐catalyzed alkoxide gels comprised strained ring structures; their IR spectra were similar to those of fumed silica, but they were energetically much less stable.
Cyclic Fatigue Crack Propagation Behavior of 9Ce‐TZP Ceramics with Different Grain Size
Authors: Liu Tianshun , Mai Yiu‐Wing , Grathwohl Georg
Published: March 2005

Cyclic fatigue crack growth behavior has been investigated in 9 mol% Ce‐TZP ceramics with grain sizes varying from 1.1 to 3.0 μm. To ascertain the interaction between crack resistance curve behavior and cyclic fatigue crack growth, cyclic fatigue tests were conducted with short double‐cantilever‐beam specimens in two conditions: (a) with a sharp precrack without preexisting t‐m transformation and (b) with a sharp crack after R‐curve measurements, i.e., with preformed t‐m transformation in the crack region. Fatigue crack propagation occurs at applied stress intensity factor values as low as about 40% of the KI,∞ values measured in the R‐curves. The size and shape of the t‐m transformation zones are found to be different for specimens obtained in monotonic loading R‐curve measurements and in cyclic fatigue tests. For the specimens without preexisting t‐m transformation the overall crack growth behavior can be described by the Paris power law relation: da/dN=AδKmI with m values of 15 for the 1.1‐μm grain size and between 8 and 9 for the material with larger grain sizes. For the specimens with the preformed transformation zone, a “V”shape da/dN versus ΔKI relation is obtained. Explanations for these different results in the two conditions are discussed in terms of crack tip shielding effects.
Residual Stresses in Fiber‐Reinforced Ceramics due to Thermal Expansion Mismatch
Authors: Kuntz Meinhard , Meier Bernhard , Grathwohl Georg
Published: March 2005

Principles for the calculation of residual stresses due to thermal expansion mismatch and previous solutions are reviewed. A general model for this subject is proposed. Considering the effective thermoelastic properties of a unidirectional composite in axial and transverse directions allows a more comprehensive treatment of the residual stress situation in fiber‐reinforced composites. Parameter variations show the important influence of the thermomechanical properties of an interfacial layer, which is disregarded in most of the conventional calculations.
Tensile Creep and Creep‐Strain Recovery Behavior of Silicon Carbide Fiber/Calcium Aluminosilicate Matrix Ceramic Composites
Authors: Wu Xin , Holmes John W.
Published: March 2005

The tensile creep and creep strain recovery behavior of 0° and 0°/90° Nicalon‐fiber/calcium aluminosilicate matrix composites was investigated at 1200°C in high‐purity argon. For the 0° composite, the 100‐h creep rate ranged from approximately 4.6 × 10−9 s−1 at 60 MPa to 2.2 × 10−8 s−1 at 200 MPa. At 60 MPa, the creep rate of the 0°/90° composite was approximately the same as that found for the 0° composite, even though the 0°/90° composite had only one‐half the number of fibers in the loading direction. Upon unloading, the composites exhibited viscous strain recovery. For a loading history involving 100 h of creep at 60 MPa, followed by 100 h of recovery at 2 MPa, approximately 27% of the prior creep strain was recovered for the 0° composite and 49% for the 0°/90° composite. At low stresses (60 and 120 MPa), cavities formed in the matrix, but there was no significant fiber or matrix damage. For moderate stresses (200 MPa), periodic fiber rupture occurred. At high stresses (250 MPa), matrix fracture and rupture of the highly stressed bridging fibers limited the creep life to under 70 min.
Grain Growth Studies of Silicon Nitride Dispersed in an Oxynitride Glass
Authors: Krämer Martin , Hoffmann Michael J. , Petzow Günter
Published: March 2005

Isothermal growth of β‐Si3N4 crystals dispersed in an oxynitride glass (Y‐Si‐Al‐O‐N) was studied by electron microscopy after heat treatment at temperatures between 1550° and 1640°C for 1 to 18 h. The β‐crystals exhibited growth striations introduced by intermediate coolings and these striations were used for developing a sophisticated technique for analysis of growth. It was determined that α/β‐transformation and Ostwald ripening can be treated as different kinetic stages of grain growth, while β‐nucleation was found to be insignificant. The mean diameter of the needlelike β‐grains was almost constant during phase transformation, indicating negligible growth of the β‐prism plane; growth was mainly one‐dimensional with the maximum mean length and aspect ratio just at the end of the phase transformation. The growth rate of the β‐basal plane was independent of diameter, indicating interface‐controlled growth. During Ostwald ripening, the length distribution broadened and the aspect ratio of smaller grains decreased. Dissolution of small grains caused an increase in the mean diameter, while the mean aspect ratio decreased.
Stabilized Zirconias Prepared by Mechanical Alloying
Authors: Michel Daniel , Faudot Françoise , Gaffet Eric , Mazerolles Léo
Published: March 2005

Cubic zirconias stabilized by various additive oxides have been obtained by mechanical alloying using high‐energy ball‐milling. Starting materials are powders of monoclinic zirconia mixed with magnesia, calcia, or yttria. Solid‐state reaction is induced by ball‐milling and, under given experimental conditions, a single phase consisting of cubic‐zirconia nanocrystals is prepared from constituent oxides. Energy dispersive analyses in electron microscopy on ball‐milled powders confirm that cubic zirconias have the composition corresponding to that of starting powders.
Preventing Fatal Fractures in Carbon‐Fiber–Glass‐Fiber‐Reinforced Plastic Composites by Monitoring Change in Electrical Resistance
Authors: Muto Norio , Yanagida Hiroaki , Nakatsuji Teruyuki , Sugita Minoru , Ohtsuka Yasushi
Published: March 2005

A method which monitors the changes in electrical resistance in CFGFRP (carbon‐fiber–glass‐fiber‐reinforced plastics) composites was found to be a promising technique for foreseeing fractures and preventing fatal ones. CFGFRP composites containing two types of carbon fibers along with 31.6 vol% glass fibers were used. U‐shaped copper electrodes were attached to both ends of the CFGFRP composites, using a conductive adhesive agent, and electrical resistance was measured via a two‐thermal dc method. A strain gauge was attached onto a specimen to measure strain. On loading, the changes in electrical resistance increased linearly with increasing strain, and the value reached 38% for the CFGFRP composites containing 0.36 vol% PAN‐based high‐strength carbon fibers. A tremendous change in electrical resistance was seen for the CFGFRP composites containing 0.38 vol% pitch‐based high‐property carbon fibers, and the composites could maintain their shape owing to a hybrid effect after the carbon fibers fractured. The change in electrical resistance can be controlled through suitable selection of the type of carbon fibers according to their values of ultimate elongation. Permanent, residual electrical resistance was found to remain, and the changes in resistance were dependent on the maximum previous strain.
Synthesis of Aluminum Nitride/Boron Nitride Composite Materials
Authors: Xiao Tongsan D. , Gonsalves Kenneth E. , Strutt Peter R.
Published: March 2005

Aluminum nitride/boron nitride composite was synthesized by using boric acid, urea, and aluminum chloride (or aluminum lactate) as the starting compounds. The starting materials were dissolved in water and mixed homogeneously. Ammonolysis of this aqueous solution resulted in the formation of a precomposite gel, which converted into the aluminum nitride/boron nitride composite on further heat treatment. Characterization of both the precomposite and the composite powders included powder X‐ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. Analysis of the composite revealed that the aluminum nitride phase had a hexagonal structure, and the boron nitride phase a turbostratic structures.
Effect of pH of the Solution on the Deposition of Zinc Oxide Films by Spray Pyrolysis
Authors: Caillaud F́rédéric , Smith Agnès , Baumard Jean‐François
Published: March 2005

A spray pyrolysis method has been used to deposit ZnO films on polycrystalline alumina substrates. The precursor is dissolved in aqueous solutions with different pH values. Depending on the pH, the growth rate either is low (less than 0.2 μm·h‐1) for pH ≤3.1 and pH ≥4.7 or has significant values (>1.5 μm·h‐1) for 3.5 ≤ pH ≥ 4.3. These experimental data are discussed in terms of a model which describes the chemical nature and concentrations of the Zn precursors which are potential candidates for the film formation.
Fracture Behavior of Chemically‐Vapor‐Deposited SiC‐Coated Graphite: I, Experimental Results
Authors: Watkins Thomas R. , Green David J.
Published: March 2005

CVD SiC coatings on polycrystalline graphite substrates exhibited a crack arrest/multiple cracking phenomenon when fractured in four‐point flexure. This fracture behavior appeared to be controlled by the magnitude of the coating thickness. When the coating thickness exceeded ∼50 μm, catastrophic failure of the sample was observed. However, when the coating thickness was less than ∼50 μm, crack arrest and multiple cracking were observed in the tensile coating surface before catastrophic failure of the composite. In order to understand the fracture behavior of the layered composites, it was necessary to characterize coating thickness and strength as well as the fracture toughness of the substrate and the state of residual stress in the system.
Viscosities and Sintering Rates of a Two‐Dimensional Granular Composite
Authors: Jagota Anand , Scherer George W.
Published: March 2005

The effective sintering rates and viscosities of two‐dimensional granular composites are studied using discrete computational models. The composites consist of randomly mixed soft and hard spheres on a triangular lattice. The numerical formulation is based on a requirement of quasistatic equilibrium for each particle in the packing, Two distinct models are employed: the truss model in which only force equilibrium is enforced for each particle, and the beam model in which force and moment equilibrium are enforced for each particle. The differences between the two models are illustrated by the specific problem studied. The effective composite properties display a transition from soft to hard behavior at a well‐defined fraction of hard particles. If contacts between hard particles resist interpenetration, shear, and bending (bonded case), the transition coincides with the site percolation threshold. If contacts between hard particles can slide and bend but resist inter‐penetration (sliding case), the transition coincides with percolation of triangular units. Thresholds and scaling of effective viscosities and sintering rates are computed. Because of the mathematical analogy between linear viscous and elastic deformations, these results can also be used to predict the effective moduli and coefficients of thermal expansion of such composites.
Transformation Driving Force for Indentation Cracking in Zirconia Ceramics
Authors: Liang Kaiming , Gu Koufen
Published: March 2005

The transformation driving force for indentation cracking in phase‐transformation‐toughened ZrO2 ceramics is confirmed with the direct crack measurement method. This driving force is produced by expansion of the plastic zone beneath the indenter due to stress‐induced transformation, and it promotes the extension of the indentation cracks. The driving force cannot be neglected when evaluating the indentation fracture toughness of the materials.
Influence of Amorphous Grain Boundary Phases on the Superplastic Behavior of 3‐mol%‐Yttria‐Stabilized Tetragonal Zirconia Polycrystals (3Y‐TZP)
Authors: Gust Maria , Goo Grace , Wolfenstine Jeff , Mecartney Martha L.
Published: March 2005

Amorphous silicate grain boundary phases of varying chemistry and amounts were added to 3Y‐TZP in order to determine their influence on the superplastic behavior between 1200° and 1300°C and on the room‐temperature mechanical properties. Strain rate enhancement at high temperatures was observed in 3Y‐TZP containing a glassy grain boundary phase, even with as little as 0.1 wt% glass. Strain rate enhancement was greatest in 3Y‐TZP with 5 wt% glass, but the room‐temperature hardness, elastic modulus, and fracture toughness were degraded. The addition of glassy grain boundary phases did not significantly affect the stress exponent of 3Y‐TZP, but did lower the activation energy for superplastic flow. Strain rate enhancement was highest in samples containing the grain boundary phase with the highest solubility for Y2O3 and ZrO2, but the strain rate did not scale inversely with the viscosity of the silicate phases. Grain boundary sliding accommodated by diffusional creep controlled by an interface reaction is proposed as the mechanism for superplastic deformation in 3Y‐TZP with and without glassy grain boundary phases.
Toughness Anisotropy in Textured Ceramic Composites
Authors: Lee Farnjeng , Sandlin Michael S. , Bowman Keith J.
Published: March 2005

In this investigation, a model predicting toughness anisotropy in textured ceramics containing elongated grains and in composites reinforced with rod‐shaped particles is presented. The model predictions are based on the assumption that crack deflection is the only toughening mechanism. In the model, toughness anisotropy is calculated as a function of texture degree. For composite materials, the volume fraction of the reinforcement phase is also an input parameter. Correspondence between model and experiment was established by comparing measured toughness anisotropies in β‐Si3N4 and Al2O3/SiC whisker composites to model predictions. In these model predictions, measured orientation distributions from hot‐pressed and hot‐forged specimens were employed. The potential for relating other toughening mechanisms in a similar format is also addressed, since the model and experimental measurements give different results. The crack deflection model simultaneously overpredicts the toughening enhancement and underpredicts the toughening anisotropy observed in the experiments.
Structure of Aluminum Nitride/Aluminum and Aluminum Oxide/Aluminum Composites Produced by the Directed Oxidation of Aluminum
Authors: Breval Else , Aghajanian Michael K. , Biel John P. , Antolin Stanislav
Published: March 2005

Unreinforced AlN/Al ceramic/metal composites were produced by the directed oxidation of molten Al alloys in nitrogen. The microstructures of these composites were compared with those of previously studied Al2O3/Al composites. In both composite systems the ceramic phase was interconnected and oriented in a columnar structure. The metallic phase showed no significant crystallographic orientation and appeared as both interconnected channels and isolated inclusions. The columns in the nitride system were found to be of micrometer size and to contain subgrains weakly defined by lattice defects, unlike the oxide system, where the columnar structure was shown to be of millimeter size and to contain well‐defined micrometer‐sized subgrains. Finer structures were obtained in both systems via the addition of Ni to the parent alloy.
Phase Development and Luminescence in Chromium‐Doped Yttrium Aluminum Garnet (YAG:Cr) Phosphors
Authors: Kinsman Karin M. , McKittrick Joanna , Sluzky Esther , Hesse Kenneth
Published: March 2005

The phase development and luminescence of chromium‐doped yttrium aluminum garnet (Y3Al5O12:Cr or YAG:Cr) phosphors, prepared by both a chemical precipitation technique and a solid‐state reaction, were studied. The YAG structure formed at a much lower temperature and by a different phase development sequence when the chemical method was employed. The light output of the chemically synthesized powders, measured by laser excitation, was discovered to increase with increasing heat treatment temperature and was found to be brightest when the YAG:Cr phosphor had excess aluminum.
Chemical States of Implanted Aluminum Ions in Silica and Silicon Ions in Alumina
Authors: Fnkumi Kohei , Chayahara Akiyoshi , Makihara Masaki , Fujii Kanenaga , Hayakawa Junji , Satou Mamoru
Published: March 2005

Aluminum and silicon ions have been implanted in silica glass and α‐alumina single crystal, respectively, to doses ranging from 1 × 1015 to 1 × 1017 ions·cm‐2. The chemical states of these implanted ions have been studied by X‐ray fluorescence spectroscopy. It is found that the implanted aluminum atoms are coordinated only by oxygen atoms, irrespective of implantation dose. On the other hand, the implanted silicon atoms are coordinated only by oxygen atoms at low doses and by both oxygen and silicon atoms at high doses. Although the chemical state of the aluminum atoms is unchanged by heat treatment, that of the silicon atoms is changed toward a less positively charged state. It is inferred that the chemical states of the implanted atoms are controlled by the transport process, although these tend to obey the thermodynamic stability.
High‐Temperature Fracture Toughness of Sapphire
Authors: Newcomb Stephen A. , Tressler Richard E.
Published: March 2005

The fracture toughness of sapphire with crack propagation parallel to the basal plane was measured from 1200° to 1500°C. Fracture surfaces of near c‐axis fibers tested in tension were used to determine the values. The toughness was constant and equal to 1.4 ± 0.1 Mpa.m1/2 over the entire temperature range.
Experimental Evidence for Harper–Dorn Creep in Mn–Zn Ferrite
Authors: Wang Jian N.
Published: March 2005

Creep data on polycrystalline Mn–Zn ferrite are presented to show the operation of Harper–Dorn creep at low stress and large grain size. The main evidence for this suggestion is that under these conditions Newtonian creep does not depend on the grain size of specimens, and the observed strain rates are a few orders of magnitude faster than those predicted by diffusional creep. Deformation mechanism maps constructed for Mn–Zn ferrite show that the deformation of this material may be controlled by Harper–Dorn creep over a wide range of temperature and grain size at low stresses.
Phase Equilibria of the Bismuth Oxide‐Copper Oxide System in Oxygen at 1 atm
Authors: Tsang Chi‐Fo , Meen James K. , Elthon Don
Published: March 2005

Phase equilibria of the Bi2O3–CuO system have been determined from 700° to 1040°C in oxygen at 1 atm pressure. Bi2O3 is stable in the Bi‐rich portions of the system and undergoes a phase transition from α‐Bi2O3 to δ‐Bi2O3 at 730°C. Bi2CuO4 is the only binary compound found to be stable; it melts incongruently to CuO + liquid at 849°C. CuO is the stable copper oxide at temperatures below 1040°C. The liquidus is composed of three segments. The Bi2O3+ liquid segment intersects the Bi2CuO4+ liquid segment at a eutectic at 784°C with 92% Bi2O3. The Bi2CuO4+ liquid segment and the CuO + liquid segment meet at a peritectic at 849°C at 66% Bi2O3. Superliquidus liquids with Bi:Cu = 2 were formed at 950°C at values of oxygen fugacity between 0.05 and 1. Glasses quenched from these liquids were analyzed for Cu+ and total Cu. All contained both Cu+ and Cu2+. The value of Cu2+/Cu+ increases as the oxygen fugacity increases but, even in pure oxygen, more than half the copper in the glasses is monovalent. The variation in Cu+:Cu2+ in liquids with Bi:Cu = 4 at an oxygen fugacity of 1 is analyzed as a function of temperature. At the lowest temperatures (<900°C), the copper is almost all divalent; at >∼1100°C, most of the copper is univalent. There is more univalent copper relative to divalent in liquids at lower oxygen fugacity and higher temperature. The value of Bi:Cu in the glass also affects the ratio of Cu+ to Cu2+. At T < ∼900°C (andfo2= 1), copper in liquids with Bi:Cu = 4 is dominantly divalent, whereas copper in liquids with Bi:Cu = 2 is composed of subequal amounts of Cu2+ and Cu+. The system Bi2O3–CuO is a pseudobinary as at least some liquids in the system are actually in the Bi2O3–Cu2O–CuO system.