bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of Tensile and Compressive Creep Behavior in Silicon Nitride
Authors: Yoon Kyung Jin , Wiederhorn Sheldon M. , Luecke William E.
Published: July 2010

The creep behavior of a commercial grade of Si3N4 was studied at 1350° and 1400°C. Stresses ranged from 10 to 200 MPa in tension and from 30 to 300 MPa in compression. In tension, the creep rate increased linearly with stress at low stresses and exponentially at high stresses. By contrast, the creep rate in compression increased linearly with stress over the entire stress range. Although compressive and tensile data exhibited an Arrhenius dependence on temperature, the activation energies for creep in tension, 715.3 ± 22.9 kJ/mol, and compression, 489.2 ± 62.0 kJ/mol, were not the same. These differences in creep behavior suggests that mechanisms of creep in tension and compression are different. Creep in tension is controlled by the formation of cavities. The cavity volume fraction increased linearly with increased tensile creep strain with a slope of unity. A cavitation model of creep, developed for materials that contain a triple‐junction network of second phase, rationalizes the observed creep behavior at high and low stresses. In compression, cavitation plays a less important role in the creep process. The volume fraction of cavities in compression was ∼18% of that in tension at 1.8% axial strain and approached zero at strains <1%. The linear dependence of creep rate on applied stress is consistent with a model for compressive creep involving solution–precipitation of Si3N4. Although the tensile and compressive creep rates overlapped at the lowest stresses, cavity volume fraction measurements showed that solution–precipitation creep of Si3N4 did not contribute substantially to the tensile creep rate. Instead, cavitation creep dominated at high and low stresses.
Strength and Toughness of Tape‐Cast Yttria‐Stabilized Zirconia
Authors: Selçuk Ahmet , Atkinson Alan
Published: July 2010

The biaxial flexural strength and fracture toughness of tape‐cast yttria‐stabilized zirconia, for application as the electrolyte in solid oxide fuel cells, have been measured at room temperature and at a typical operating temperature of 900°C. The flexural strength was measured in ring‐on‐ring loading and decreased from 416 MPa at room temperature to 265 MPa at 900°C. The fracture toughness was measured using two different techniques: indentation fracture and double‐torsion loading. The latter was more reliable and gave a fracture toughness of 1.61 ± 0.12 MPa·m1/2 at room temperature and 1.02 ± 0.05 MPa·m1/2 at 900°C. The flexural strength and fracture toughness were quantitatively consistent with fracture being initiated at the observed surface defects. The lower fracture toughness at 900°C is partly due to a reduction in elastic modulus and partly due to a reduction in the work of fracture.
Ordered Structure and Dielectric Properties of Lanthanum‐Substituted Ba(Mg1/3Ta2/3)O3
Authors: Tien Lai‐Cheng , Chou Chen‐Chia , Tsai Dah‐Shyang
Published: July 2010

Both 1:2 and 1:1 ordered structures form in the perovskite solid solutions of La‐substituted BMT Ba1−xLax(Mg(1+x)/3Ta(2−x)/3)O3, sintered at 1600°C. The 1:2 ordered structure exists in the composition range 0.0 ≤x≤ 0.12, while that of 1:1 ordered structure exists in a wider composition range 0.04 ≤x≤ 1.0. Two ordered phases coexist in 0.04 ≤x≤ 0.12. High‐resolution micrographs indicate that 1:2 and 1:1 ordered domains coexist in one grain. The ordering parameter of 1:2 phase decreases withx, yet that of 1:1 phase increases withx. Both increase with soak time. Variations in ordering are discussed in terms of cation occupancy and crystal chemistry. The quality factor increases withx, reaches a maximum, then decreases withx. The dielectric constant increases withxfirst, and levels off.
Effects of Glass Frit Oxides on Crystallization and Zircon Pigment Dissolution in Whiteware Coatings
Authors: Earl David A. , Clark David E.
Published: July 2010

In experimental whiteware coatings comprised of zircon pigment and multi‐oxide silicate glass frits that incorporated ZrO2, the quantity, size, and morphology of zircon that precipitated during “fast‐firing” were mainly dependent upon the ZnO, SrO, and Al2O3:alkali levels. In these frits, little or no zircon pigment dissolution occurred, and zircon crystallization was nearly complete by 1000°C. Microstructures of the coatings were consistent over a range of peak firing temperatures from 1000° to 1100°C. Fritted ZnO in these coatings stimulated zircon crystallization and produced high opacity. Replacing ZnO with SrO in the same frits prevented zircon from precipitating and resulted in transparent coatings. In frits without ZrO2, significant zircon pigment dissolution and crystallization of calcium‐based silicates occurred during firing. In these unstable coatings, crystallization and dissolution increased from 1000° to 1100°C and produced a range of microstructures. Raising the Al2O3:alkali ratio caused frits with ZrO2 to precipitate more zircon, and frits without ZrO2 to form less of the calcium‐based silicates and dissolve less pigment.
Fabrication and Electrical Properties of Textured Sr0.53Ba0.47Nb2O6 Ceramics by Templated Grain Growth
Authors: Duran Cihangir , Trolier‐McKinstry Susan , Messing Gary L.
Published: July 2010

Textured Sr0.53Ba0.47Nb2O6 ceramics with a relative density of >95% were fabricated using templated grain growth (TGG). Acicular KSr2Nb5O15 template particles synthesized via a molten salt process were aligned by tape casting in a mixture of solid‐state‐synthesized SrNb2O6 and BaNb2O6 powders. The resulting ceramics possessed strong fiber texture along the polar axis ([001]) of the strontium barium niobate. Samples with 15.4 wt% templates attained a textured fraction of 0.82 after sintering at a temperature of 1450°C for 4 h. These materials showed peak dielectric constants of 7550 at 1 kHz, remanent polarizations of 13.2 μC/cm2, saturation polarizations of 21 μC/cm2 (60%–85% of the single‐crystal value), piezoelectric strain coefficients of 78 pC/N (70%–85% of the single‐crystal value), and room‐temperature pyroelectric coefficients of 2.9 × 10−2μC·(cm2·°C)−1 (52% of the single‐crystal value). These results show that TGG is a viable option for accessing single‐crystal properties in polycrystalline ceramics.
29Si MASNMR Spin‐Lattice Relaxation Study of α‐, β‐, and Amorphous Silicon Nitride
Authors: Fujimori Hirotaka , Sato Naoya , Ioku Koji , Goto Seishi , Yamada Tetsuo
Published: July 2010

The spin‐lattice relaxation times, T1, in α‐, β‐, and amorphous Si3N4 have been obtained for the first time, using a multiple‐pulse saturation recovery method. The saturation recovery of the 29Si magnetization follows exponential behavior under magic‐angle spinning conditions, within the limits of experimental error. A rather wide dispersion of T1 values is observed for the phases of Si3N4: 284 ± 29 min (–46.686 ppm) and 260 ± 23 min (–48.812 ppm) for the α‐phase, 36 ± 4 min for the β‐phase, and 11 ± 1 min for the amorphous phase, assuming an exponential recovery. The values obtained for the exponent in the power‐law fitting are 0.599(9) (–46.686 ppm) and 0.61(1) (–48.812 ppm) for the α‐phase, 0.52(2) for the β‐phase, and 0.53(3) for the amorphous phase.
Orientation Effects on the High‐Temperature Morphological Evolution of Pore Channels in Sapphire
Authors: Powers James D. , Glaeser Andreas M.
Published: July 2010

Arrays of semi‐infinite and of controlled‐aspect‐ratio pore channels, both of controlled orientation, were introduced into undoped basal‐plane sapphire substrates, using microfabrication techniques, ion‐beam etching, and hot pressing. The breakup of these channels via Rayleigh instabilities during a series of annealings at a temperature of 1700°C was monitored. In all cases, the channels broke up with a characteristic wavelength (λ) that was much larger than that expected for a material with isotropic surface energy, which reflected stabilization effects that were due to anisotropy of the surface energy. The break‐up wavelength also was very dependent on orientation: channels that were oriented along the [11¯00] and [112¯0] directions yielded the smallest and largest pore spacings (λ‐values), respectively, which is in qualitative agreement with prior observations. The critical (minimum) aspect ratio for the breakup of finite‐length channels into multiple pores also is dependent upon channel orientation, and the trend mirrors that observed for semi‐infinite channels. The pattern of channel evolution suggested two‐fold rotational symmetry within the basal plane, where, because of the nature of the experiment, apparent six‐fold symmetry is expected. Several factors that may contribute to or cause an apparent or real loss of symmetry have been discussed.
Rapid Formation and Growth of Bixbyite‐Type (In0.67Fe0.33)2O3 by 28 GHz Microwave Irradiation
Authors: Takizawa Hirotsugu , Uheda Kyota , Endo Tadashi
Published: July 2010

(In0.67Fe0.33)2O3 with the bixbyite structure was synthesized via 28 GHz microwave irradiation, using multimode microwave heating equipment. Indium sesquioxide strongly absorbs 28 GHz microwaves, and this strong coupling with microwave energy can be used to drive a reaction with iron sesquioxide. A mixture of In2O3 and α‐Fe2O3 powders (In:Fe ratio of 2:1) was irradiated with microwaves at a frequency of 28 GHz. The mixture was heated to 1400°C during the microwave irradiation. The formation of a solid solution was completed within a minute, which indicated a drastic enhancement of the reaction rate. Scanning electron microscopy revealed remarkable grain growth under microwave irradiation.
New Sol–Gel Route for the Preparation of Pure α‐Alumina at 950°C
Authors: Bahlawane Naoufal , Watanabe Tadahiko
Published: July 2010

An anhydrous alumina (Al2O3) sol was prepared from aluminum isopropoxide and an organic solvent, using an acetic acid stabilizer. The complete conversion of the dried sol to α‐Al2O3 was accomplished at a temperature of 950°C by a single transition via γ‐Al2O3. Al2O3 that was deposited via dip coating resulted in amorphous films, even after annealing at 1100°C, because of the silicon diffusion from the substrate. This phenomenon was avoided using a rapid thermal treatment in a flame after dip coating, which resulted in uniform thin films that are converted to α‐Al2O3 via heat treatment.
Interactions between Polymeric Dispersants and Calcium Silicate Hydrates
Authors: Popova Alla , Geoffroy Ghita , Renou‐Gonnord Marie‐France , Faucon Pascal , Gartner Ellis
Published: July 2010

To better understand the mechanism of interaction between hydrating silicate‐based cements and polymeric dispersants of the type used as “superplasticizers” in modern construction concretes, two different types of polymeric dispersant were added (at concentrations of 1 and 10 g/L) during the synthesis of calcium silicate hydrate (C‐S‐H) via the “pozzolanic reaction” in dilute slurries of lime and reactive silica, at Ca/Si ratios in the range of 0.66–1.50. Although both polymers gave degrees of adsorption of >79% in all cases studied, no significant structural modifications of the resulting C‐S‐H products were observed via X‐ray diffraction or 29Si magic angle spinning–nuclear magnetic resonance. These results differ from recent work in which it was shown that similar types of polymer could intercalate into the interlayers of C‐S‐H that was made using an alternative process. It is suggested that the process by which the C‐S‐H is formed may have a strong influence on whether C‐S‐H can intercalate polymers. This observation is relevant to understanding the fate of such polymers in concrete.
Nucleation and Inhibition Control during the Hydrothermal–Electrochemical Growth of Barium Titanate Films
Authors: Escobar Iván S. , Silva Carmen I. , Vargas Tomás , Fuenzalida Víctor M.
Published: July 2010

The control of the microstructure of BaTiO3 films grown on titanium by the hydrothermal–electrochemical method was investigated. Experiments were conducted in a three‐electrode high‐pressure electrochemical cell in a 0.1M Ba(OH)2 electrolyte at 150°C. Results showed that the spontaneous initial nucleation linked to pure hydrothermal BaTiO3 formation can be inhibited by cathodically protecting the titanium electrode from the moment it is immersed in the electrolyte. The application of initial nucleation pulses of varying cathodic potentials affected the grain size of the deposit. It is suggested that the formation of a titanium oxide layer is a necessary step previous to the nucleation of BaTiO3.
Preparation of Mullite Ceramics from Coprecipitated Aluminum Hydroxide and Kaolinite Using Hexamethylenediamine
Authors: Pascual José , Zapatero José , de Haro María C. Jiménez , del Valle Antonio J. Ramírez , Pérez‐Rodríguez José L. , Sánchez‐Soto Pedro J.
Published: July 2010

Mullite ceramics have been prepared from an aqueous suspension of kaolinite (raw or ground) and aluminum hydroxide. The precursor was coprecipitated in the mixture using hexamethylenediamine (HMDA) or ammonium hydroxide, and a solution of aluminum chloride from acid dissolution of wastes of aluminum metal. The precursor and the resulting materials were characterized and studied by X‐ray diffraction, thermal methods, and mechanical strength and porosity measurements. The feasibility of the proposed chemical processing route for mullite preparation was demonstrated, in particular using HMDA as a precipitating agent for aluminum hydroxide instead of ammonium hydroxide, which adversely affects the system reactivity. The use of HMDA, as compared with ammonium hydroxide, and ground kaolinite produces single‐phase mullite and enhances the flexural strength (maximum of 49 MPa) of the resultant ceramic porous bodies (porosity ca. 52–45 vol%) fired at 1550–1600°C for 30 min.
Piezospectroscopic Study of Residual Stresses in Al2O3–ZrO2 Directionally Solidified Eutectics
Authors: Pardo José A. , Merino Rosa I. , Orera Víctor M. , Peña José I. , González Carlos , Pastor José Y. , LLorca Javier
Published: July 2010

The residual thermoelastic stresses were studied in Al2O3–ZrO2 (monoclinic zirconia, m‐ZrO2) and Al2O3–ZrO2(Y2O3) (tetragonal zirconia, t‐ZrO2) fibrous eutectics that were produced via the laser floating zone method, using different piezospectrosocopic probes. The luminescence of the R‐lines of ruby (Cr3+ in Al2O3 phase) was used to determine the stresses in the Al2O3 matrix, assuming that the stress state in the fibers was transversally isotropic. The sapphire matrix was subjected to tensile stresses in the Al2O3–m‐ZrO2 eutectics. The hydrostatic stress component attained a value of 1.13 GPa in well‐ordered regions. In contrast, sapphire was in compression in the Al2O3–ZrO2(Y2O3) fibers, and the hydrostatic stress was −0.37 GPa in both ordered and disordered regions. The influence of the microstructure in the residual stresses was explained through thermoelastic analyses, based on a self‐consistent method. In addition, the Raman spectra of m‐ZrO2 and the 417 cm−1 Raman mode of Al2O3 were measured in samples that showed different microstructures and thermoelastic stresses. An approximate linear dependence was observed in the tension–compression range between frequency shifts of the Al2O3R‐lines and those of the 417 cm−1 Raman mode.
Thermal Expansion Kinetics: Method to Measure Permeability of Cementitious Materials: I, Theory
Authors: Scherer George W.
Published: July 2010

When a porous material that is saturated with liquid is heated, the liquid typically expands much more than the solid phase; if the permeability is high, then the liquid flows out of the pores as it expands. However, if the permeability is low, then the liquid may not be able to escape, so it expands within the pores and contributes to the dilatation of the body; as the liquid drains from the pores, the body contracts to the dimension dictated by the thermal expansion coefficient of the solid phase. In the low‐permeability case, by analyzing the kinetics of dilatation during a change in temperature, it is possible to extract the permeability. This technique, which previously has been successfully applied to gels, is also applicable to conventional ceramics, such as porous glass, sintered ceramics, cement, and concrete. However, for these rigid materials, the analysis must be modified to take account of the finite compressibility of the liquid. In this article, the thermal expansion kinetics are analyzed for cylindrical and platelike samples. The results are applied to cement paste in a companion paper.
Grain Boundary Evolution in Hot‐Pressed ABC‐SiC
Authors: Zhang Xiao Feng , Sixta Mark E. , De Jonghe Lutgard C.
Published: July 2010

Silicon carbide with aluminum, boron, and carbon additions (ABC‐SiC) was hot‐pressed to full density. The samples were examined by transmission electron microscopy (TEM), with an emphasis on high‐resolution electron microscopy (HREM). Amorphous grain boundary interlayers, typically less than 2 nm wide, were formed between SiC grains. Heat‐treating the ABC‐SiC at temperatures as low as 1100°C in Ar crystallized the grain boundary interlayers completely without significantly changing the dominant chemical constituents. Chemical microanalyses demonstrated Al and O enrichment for all examined grain boundaries in both as‐prepared and annealed samples. Quantitative EDS analyses revealed Al2OC‐ and Al2O3‐related species (with Si, C, B, or S substitutions) as two of the most likely grain boundary interlayer materials, both before and after heat treatment. Al2O3, and (Al1−xSix)2OC with a 2H‐type wurtzite structure, were identified as grain boundary films by HREM images. The structural evolution in the grain boundary phases during the hot pressing and postannealing is discussed.
Necessary Conditions for the Formation of {111} Twins in Barium Titanate
Authors: Lee Byoung‐Ki , Chung Sung‐Yoon , Kang Suk‐Joong L.
Published: July 2010

The experimental conditions for {111} twin formation in BaTiO3 were investigated. When BaTiO3 compacts without excess TiO2 were sintered either in an oxidizing atmosphere (air) or in a reducing atmosphere (95N2–5H2), no {111} twins formed within the BaTiO3 grains and no abnormal grain growth occurred. In contrast, many {111} twins were present within the abnormally grown grains in the excess‐TiO2‐containing BaTiO3 samples sintered in air, while no twins were observed in the excess‐TiO2‐containing samples sintered in 95N2–5H2. X‐ray diffraction analysis showed that excess TiO2 forms a Ba6Ti17O40 phase during sintering with the space group A2/a in air and a Ba6Ti17O40−x phase with the space group C in 95N2–5H2. It appears therefore that excess TiO2 and an oxidizing atmosphere are necessary for {111} twin formation in BaTiO3. These results may also indicate that the interface structure between BaTiO3 and Ba6Ti17O40 influences the twin formation.
Preparation of Nanometer Zinc Oxide Powders by Plasma Pyrolysis Technology and Their Applications
Authors: Lin Yuanhua , Tang Zilong , Zhang Zhongtai , Yuan Fangli , Ling Yuanbin , Lee Jinlin , Huang Shulan
Published: July 2010

ZnSO4 and NH4HCO3 were used as raw materials for the preparation of a Zn5CO3(OH)6 precursor. Nanometer ZnO powders were synthesized from the precursor by controlled plasma pyrolysis. The average particle size of the ZnO powders was about 20 nm as observed by transmission electron microscopy (TEM). The content of ZnO reached 99.8 wt% as determined by induction coupled plasma (ICP) analysis, and the specific surface area was about 77 m2/g. When compared with conventional ZnO powders, one half of the amount of nanometer ZnO had the equivalent effect on the properties of rubber.
Microstructure and Bending Strength of 3Y‐TZP/12Ce‐TZP Ceramics Fabricated by Liquid‐Phase Sintering at Low Temperature
Authors: Qin Haixia , Huang Xiaoxian
Published: July 2010

With the addition of 1 wt% of MgO–Al2O3–SiO2 glass as a sintering aid, 3Y‐TZP/12Ce‐TZP ceramics (composed from a mixture of 3Y‐TZP and 12Ce‐TZP powder) have been fabricated via liquid‐phase sintering at 1250°–1400°C. In the sintered bodies, the grain growth of Y‐TZP is almost unaffected, whereas that of Ce‐TZP is inhibited. MgO·Al2O3 spinel and an amorphous phase that contains Al2O3 and SiO2 (from the sintering aid) fully fill the grain junctions. The bending strength of 3Y‐TZP/12Ce‐TZP, when sintered at 1250°–1300°C, is ∼800–900 MPa, which is greater than that of 3Y‐TZP ceramics without Ce‐TZP particles. Ce‐TZP grains and MgO·Al2O3 spinel in 3Y‐TZP/12Ce‐TZP ceramics may impede crack growth, and the bending strength is enhanced.
Preparation of Ba6−3xNd8+2xTi18O54 via Ethylenediaminetetraacetic Acid Precursor
Authors: Xu Yebin , Chen Xiang Ming , Wu Yong Jun
Published: July 2010

Ba6−3xNd8+2xTi18O54 ceramic powders were synthesized by the modified Pechini method using ethylenediaminetetraacetic acid (EDTA) as a chelating agent. A purplish red, molecular‐level, homogeneously mixed gel was prepared, and transferred into a porous resin intermediate through charring. Single‐phase and well‐crystallized Ba6−3xNd8+2xTi18O54 powders were obtained from pulverized resin at a temperature of 900°C for 3 h, without formation of any intermediate phases. Meanwhile, the molar ratio of EDTA to total metal cation concentration had a significant influence on the crystallization behavior of Ba6−3xNd8+2xTi18O54. The Ba6−3xNd8+2xTi18O54 (x= 2/3) ceramics prepared via EDTA precursor have excellent microwave dielectric characteristics: ɛ= 87, Qf= 8710 GHz.
Organic Distributions in Dried Alumina Green Tape
Authors: Jean Jau‐Ho , Wang Hong‐Ren
Published: July 2010

Distributions of organic binder (poly(vinyl butyral) (PVB)) and plasticizer (dibutyl phthalate (DBP)) in alumina green tape dried at different temperatures are studied. More PVB and DBP are observed on the bottom (Mylar side) than on the top surface (air side) of the green tape. Inside the green tape, however, PVB distribution, which remains relatively unchanged with drying temperature in the range of 30°–80°C, increases with distance from the bottom to the top. In contrast, the DBP concentration remains relatively unchanged with the depth of green tape when the drying temperature is <50°C. At 80°C, however, a significant drop in DBP concentration near the top surface of green tape is found. Mathematical analysis using the finite difference method is completed to describe the PVB distribution in alumina green tape, and the results show reasonable agreement with experimental observations.
Phase Stability and Ferroelectric Properties of Lead Strontium Zirconate Titanate Ceramics
Authors: Yu Yongjian , Tu Jinbiao , Singh Raj N.
Published: July 2010

The effect of compositional modifications on the field‐induced phase‐transition behavior and dielectric properties of strontium‐doped lead zirconate titanate (PZT) ceramics was studied. PZT compositions with different strontium and titanium contents, within the general formula Pb1–xSrx(Zr1–yTiy)O3 and located in the tetragonal antiferroelectric (AFE) and rhombohedral ferroelectric (FE) phase fields were prepared by tape casting and sintering. X‐ray diffraction and polarization measurements were used to locate compositions suitable for investigation of the field‐induced AFE–FE phase transition. The results indicated that a higher Sr2+ content decreased the polarization and hysteresis and increased the switching field; a lower Ti4+ content decreased the polarization and increased the switching field and hysteresis. A high room‐temperature dielectric constant was obtained for compositions near the phase boundary. These results suggest that a combination of both A‐site and B‐site modifications can be used to tailor ferroelectric properties, such as the switching field and hysteresis, of these strontium‐doped PZTs displaying a field‐induced AFE–FE phase transition.
Thermal Conductivity of β‐Si3N4: III, Effect of Rare‐Earth (RE = La, Nd, Gd, Y, Yb, and Sc) Oxide Additives
Authors: Kitayama Mikito , Hirao Kiyoshi , Watari Koji , Toriyama Motohiro , Kanzaki Shuzo
Published: July 2010

β‐Si3N4 ceramics sintered with a series of rare‐earth (RE = La, Nd, Gd, Y, Yb and Sc) oxide additives were fabricated by hot pressing and subsequent annealing. Their microstructures, lattice oxygen contents, and thermal conductivities were evaluated. Mean grain size increased, while lattice oxygen content decreased, and hence, thermal conductivity increased with decreasing ionic radius of the rare‐earth element. In all cases, a marked change was observed in the order of ionic radius from La to Nd to Gd, and a little change was observed below them. Rare‐earth oxide additives significantly influenced the thermal conductivity of β‐Si3N4, unlike in the case of AlN.
Thermal Expansion Kinetics: Method to Measure Permeability of Cementitious Materials: II, Application to Hardened Cement Pastes
Authors: Ai Hua , Young J. Francis , Scherer George W.
Published: July 2010

A new approach to determine the permeability of cementitious materials is presented in this paper. The method involves the measurement of the thermal dilatation of thin slabs of saturated cement paste. On heating and subsequent isothermal holding, the sample first expands, then gradually contracts over time. The paste is treated as a porous solid with low permeability in which the initial expansion is partly due to the liquid expanding within the pores, putting both solid and liquid under stress. The time‐dependent contraction is due to the liquid flowing out of the pores to restore the pressure to equilibrium. By analyzing the kinetics of the thermal expansion and contraction, the liquid permeability of the porous solid can be calculated. The theoretical analysis (presented in a previous paper) is applied to determine the water permeability coefficients of selected cement pastes modified with silica fume and made with different water/solid ratios.
High‐Temperature Mechanical Properties of SiC–Mo5(Si,Al)3C Composites
Authors: Zhu Qingshan , Shobu Kazuhisa
Published: July 2010

SiC–Mo5(Si,Al)3C composites were fabricated by the melt infiltration process, and the infiltration characteristics were studied in detail. Fracture strength and toughness were measured up to 1600°C using a three‐point bending test and indentation strength method, respectively. Both fracture strength and toughness significantly increased at 1400°C with respect to the values at room temperature. These increases were mainly attributed to plastic deformation of the infiltrated Mo5(Si,Al)3C phases at elevated temperatures, which acted as ductile toughening inclusions. Compressive creep tests were used to study the creep behavior of the composite in the range of 1550°–1650°C and 150–200 MPa. The stress exponent and activation energy were 1.3 and 277 kJ/mol, respectively. Preliminary oxidation tests showed that the composites exhibited good oxidation resistance at 1500°C because of the formation of a dense oxide scale.
Effect of Magnesium Oxide on Faceted Vicinal (0001) Surface of Aluminum Oxide
Authors: Park Chan Woo , Yoon Duk Yong
Published: July 2010

Atomic force microscopy has been used to observe the vicinal (0001) surface of Al2O3. The surface develops a hill‐and‐valley structure when heat‐treated in air and becomes defaceted and therefore atomically rough when heat‐treated in an MgO atmosphere at 1600°C. The results are consistent with the previously observed polyhedral equilibrium shape of Al2O3 crystal and the surface roughening effect of MgO.
Crystal Structure Refinement of La0.683(Ti0.95Al0.05)O3 Perovskite by the Rietveld Method
Authors: Ali Roushown , Yashima Masatomo , Yoshimura Masahiro , Yoshioka Hideki
Published: July 2010

The structural parameters of orthorhombic and tetragonal phases (space group P/mmm and P4/mmm) of A‐site deficient La0.683(Ti0.95Al0.05)O3 perovskite have been refined by Rietveld analysis through the X‐ray powder diffraction patterns measured in the temperature range from 15° to 500°C. With an increase in temperature the unit‐cell parameters a, b, and c increased, while the b/a ratio decreased and became unity between 200° and 400°C. No significant changes were observed for atomic coordinates throughout the temperature studied. These results strongly suggest that the phase transition is induced by lattice distortion.
Bimodal Ceramic Ink for Continuous Ink‐Jet Printer Plasma Spraying
Authors: Blazdell Philip , Kuroda Seiji
Published: July 2010

The issue of the low ceramic volume loading currently used in ceramic inks for direct jet‐printing applications, such as plasma spraying, was addressed. Judicious additions of a second, finer zirconia powder were made to the submicrometer zirconia suspensions that are conventionally used in direct ceramic jet‐printing applications. Various amounts of oligomeric dispersant were added to the ceramic suspensions. The sediment packing densities and the viscosities of the suspensions were recorded. The highest gravitational sediment‐packing densities occurred when 2.5–3 wt% of dispersant was used. This corresponded well with the minimum ink viscosity of 8 mPa·s that occurs at the same level of dispersant. Bimodal ceramic ink was formulated based on these observations. The ceramic volume loading of the ink was increased by 250% (i.e., from 2 to 5 vol%) using a bimodal size distribution. This ink was injected into a plasma flame using a continuous ink‐jet printer. The splats produced were comparable, although significantly smaller, to those produced by conventional plasma spraying.
Pressure‐Induced Phase Transformation of Controlled‐Porosity Pb(Zr0.95Ti0.05)O3 Ceramics
Authors: Tuttle Bruce A. , Yang Pin , Gieske John H. , Voigt James A. , Scofield Timothy W. , Zeuch David H. , Olson Walter R.
Published: July 2010

Chemically prepared Pb(Zr0.95Ti0.05)O3 (PZT 95/5) ceramics were fabricated with a range of different porosity levels, while grain size was held constant, by systematic additions of added organic pore former (Avicel). Use of Avicel in amounts ranging from 0 to 4.0 wt% resulted in fired ceramic densities that ranged from 97.3% to 82.3%. Hydrostatic‐pressure‐induced ferroelectric (FE) to antiferroelectric (AFE) phase transformations were substantially more diffuse and occurred at lower hydrostatic pressures with increasing porosity. An ∼12 MPa decrease in hydrostatic transformation pressure per volume percent added porosity was observed. The decrease in transformation pressure with decreasing density was quantitatively consistent with the calculated macroscopic stress required to achieve a specific volumetric macrostrain (0.40%). This strain was equivalent to experimentally measured macrostrain for FE‐to‐AFE transformation. The macroscopic stress levels were calculated using measured bulk modulus values that decreased from 84 to 46 GPa as density decreased from 97.3% to 82.3%. Good agreement between calculated and measured values of FE‐to‐AFE transformation stress was obtained for ceramics fired at 1275° and 1345°C.
Thickness Alteration of Grain‐Boundary Amorphous Films during Creep of a Multiphase Silicon Nitride Ceramic
Authors: Jin Qiang , Wilkinson David S. , Weatherly George C. , Luecke William E. , Wiederhorn Sheldon M.
Published: July 2010

Experimental observations of the creep response of a commercial sintered silicon nitride ceramic are presented. The stable microstructure of this material at high temperature contains secondary crystalline phases which result from partial devitrification of the intergranular phase. The widths of amorphous films along grain boundaries (between silicon nitride grains) and phase boundaries (between silicon nitride and secondary phase grains) are characterized by transmission electron microscopy. The thickness distributions of grain‐boundary films before and after creep are analyzed by a statistical method. While the film widths are highly uniform before creep, a bimodal distribution is observed after creep. The results suggest that viscous flow of the boundary amorphous films occurs during creep deformation.
Development of Surface Compressive Stresses in Zirconia–Alumina Composites by an Ion‐Exchange Process
Authors: Lin Guang‐Yong , Virkar Anil V.
Published: July 2010

A two‐step ion‐exchange technique was developed for introducing compressive stresses on the surface of ZrO2–Al2O3 composites. In the first step, a thin layer (∼250 μm) of Na‐β″‐Al2O3 was formed on the surface of the composite by a vapor‐phase process at ∼1400°C. In the second step, Na+ ions were replaced by K+ ions by a heat treatment at ∼385°C for 2 h in a molten KNO3 bath. Replacement of sodium by potassium led to the creation of surface compressive stresses. The flexural strength and Weibull modulus of ZrO2–Al2O3 composite were ∼915 MPa and 10, respectively, for the as‐sintered samples. By contrast, the flexural strength and Weibull modulus were ∼1140 MPa and 26, respectively, for the ion‐exchanged samples. A residual surface compressive stress of ∼480 MPa was measured by a strain‐gauge technique in K+‐ion‐exchanged samples. The presence of surface compressive stresses also was confirmed using an indentation technique. The technique developed here can be used to introduce compressive stresses on components of virtually any shape.
Three‐Dimensional Simulation of Grain Growth in the Presence of Mobile Pores
Authors: Tikare Veena , Miodownik Mark A. , Holm Elizabeth A.
Published: July 2010

A kinetic, three‐dimensional Monte Carlo model for simulating grain growth in the presence of mobile pores is presented. The model was used to study grain growth and pore migration by surface diffusion in an idealized geometry that ensures constant driving force for grain growth. The driving forces, pore size, and pore mobilities were varied to study their effects on grain‐boundary mobility and grain growth. The simulations captured a variety of complex behaviors, including reduced grain‐boundary velocity due to pore drag that has been predicted by analytical theories. The model is capable of treating far more complex geometries, including polycrystals. We present the capabilities of this model and discuss its limitations.
Synthesis of Cerium α‐SiAlON with Nuclei Addition
Authors: Wang Chang‐Meng , Mitomo Mamoru , Xu Fan‐Fang , Hirosaki Naoto , Bando Yoshio
Published: July 2010

Cerium α‐SiAlON ceramics were made from a powder mixture of Si3N4‐AlN‐CeO2 that contained 1 wt% yttrium α‐SiAlON powder. Plasma‐activated sintering was used to examine the effect of the cooling rate on the formation of α‐SiAlON. The formation of cerium α‐SiAlON was suggested to be controlled by the nucleation at the surface of α‐SiAlON nuclei, because α‐phase formation could not occur without the addition of SiAlON powder. The solubility of cerium in the α‐SiAlON was shown to be less than a previously predicted critical value.
In Situ Reaction Synthesis of Silicon Carbide–Boron Nitride Composites
Authors: Zhang Guo‐Jun , Ohji Tatsuki
Published: July 2010

SiC–BN composites were prepared via the proposed in situ reaction, which used Si3N4, B4C, and carbon as reactants. Adding SiC powder to the reactants controlled the BN content in the composite. For comparison, SiC–BN composites with the same phase compositions were produced via conventional processing. The in situ process was advantageous for obtaining better composites with fine grain size and homogeneous microstructures. The in situ composite that had a BN content of 53.71 vol% exhibited considerably high strength (342 MPa) and a very low elastic modulus (107 GPa). The SiC–25‐vol%‐BN in situ composite had a peak strength of 588 MPa, which was 95% of that of monolithic SiC; however, the elastic modulus was as low as half that of monolithic SiC. These in situ SiC–BN composites can be expected to have excellent thermal shock resistance and mechanical strain tolerance.
Effect of Liquid Content on the Abnormal Grain Growth of Alumina
Authors: Hong Seong‐Hyeon , Kim Doh‐Yeon
Published: July 2010

Alumina specimens with small amounts of CaO and TiO2 were prepared and their microstructural evolution during sintering was investigated. Because of the appearance of a liquid phase during sintering, a duplex microstructure of a few abnormal grains and fine matrix grains was obtained when the CaO + TiO2 content was small (≤0.04 wt%). When the CaO + TiO2 content was relatively high (≥0.1 wt%), many grains grew and impinged upon each other. As a result, a rather uniform and homogeneous microstructure was observed.
Mechanism of the Cubic‐to‐Tetragonal Phase Transition in Zirconia and Yttria‐Stabilized Zirconia by Molecular‐Dynamics Simulation
Authors: Schelling Patrick K. , Phillpot Simon R. , Wolf Dieter
Published: July 2010

We present a new interatomic potential capable of describing the cubic and tetragonal phases of zirconia. From an analysis of molecular‐dynamics simulations of the structural fluctuations in the cubic phase close to the transition temperature, we show that the cubic‐to‐tetragonal transition is displacive. In addition, the discontinuous change in volume and the latent heat at the transition demonstrate clearly that the transition is first order. The model correctly predicts that doping with yttria tends to stabilize the cubic phase at lower temperatures. A description of the first‐order nature of dopant‐driven transitions is given.
Hydrothermal Synthesis of Nanosized Titania Powders: Influence of Tetraalkyl Ammonium Hydroxides on Particle Characteristics
Authors: Yang Juan , Mei Sen , Ferreira José M. F.
Published: July 2010

Three types of tetraalkyl ammonium hydroxides (TANOHs)—namely, tetramethyl ammonium hydroxide (TMNOH), tetraethyl ammonium hydroxide (TENOH), and tetrabutyl ammonium hydroxide (TBNOH)—were used as peptizing agents for the hydrothermal synthesis of nanocrystalline TiO2powders. X‐ray diffractometry, Brunauer‐Emmett‐Teller surface‐area analysis, differential thermal analysis‐thermogravimetry, scanning electron microscopy, and transmission electron microscopy were used to characterize the powders. The results showed that the carbon‐chain length of TANOHs had a great influence on the particle size, particle shape, and the phase transformation of the hydrothermally derived TiO2particles. Anatase phase was obtained in all the samples, regardless of the peptizer used, and the particle size increased as the peptizer cation size decreased. In the presence of TMNOH, the particle shape changed from spherical at low concentration to rodlike with increasing TMNOH concentration, whereas a transition from a spherical morphology to an asterisk‐like structure was observed in the TENOH peptized samples. However, spherical particles were formed in all the TBNOH peptized samples. The anatase‐rutile transition occurred at a lower temperature for the TENOH‐derived powders, relative to the other two peptizers.
Limitations of the Augis and Bennett Method for Kinetic Analysis of the Crystallization of Glasses and Conditions for Correct Use
Authors: Gotor Francisco J. , Criado José M. , Málek Jiri
Published: July 2010

The equation proposed by Augis and Bennett for determining the kinetic exponent of the Johnson–Mehl–Avrami (JMA) model is thoroughly analyzed; a new expression, calculated accurately with no assumptions introduced, is proposed. This new method of calculation has been extended to the different kinetic models more commonly used in the literature for describing solid‐state reactions. However, determining the JMA exponent from the Augis and Bennett method can lead to an incorrect interpretation of the reaction mechanism unless an additional, independent test is used. A testing method for verifying the applicability of the Augis and Bennett method is proposed. The kinetic analysis of the crystallization of Ge0.3Sb1.4S2.7 has been used for checking this method.
Grain Growth Phenomena in the Interface Region of α‐Alumina Bilayer Composites
Authors: Zhao Hong , Hu Xiao‐Zhi , Bush Mark B.
Published: July 2010

Microstructural development in the interface region of α‐Al2O3 bilayer composites has been systematically investigated in terms of the sintering additive CaO–SiO2, residual impurity level in the starting powders (particularly MgO), and sintering conditions. The interfacial microstructure is strongly related to relative CaO–SiO2 doping levels in the two constituting layers and to residual impurities in the starting powders. The presence of high levels of impurities in the starting powder can substantially modify the features of CaO–SiO2‐Al2O3 liquid at the interface region, thereby strongly influencing α‐Al2O3 grain growth across the interface. Three grain growth modes in the interface region thus have been identified for different combinations of impurity level and CaO–SiO2 dopant in the α‐Al2O3 bilayer. This provides an important mechanism for controlling two‐dimensional structures in coatings, films, and layered ceramic materials for various engineering applications.
Thermodynamic Modeling of the Isomorphous Phase Diagrams in the Al2O3–Cr2O3 and V2O3–Cr2O3 Systems
Authors: Kim Sung S. , Jr. Sanders Thomas H.
Published: July 2010

The phase diagrams in the Al2O3–Cr2O3 and V2O3–Cr2O3 systems have been assessed by thermodynamic modeling with existing data from the literature. While the regular and subregular solution models were used in the Al2O3–Cr2O3 system to represent the Gibbs free energies of the liquid and solid phases, respectively, the regular solution model was applied to both phases in the V2O3–Cr2O3 system. By using the liquidus, solidus, and/or miscibility gap data, the interaction parameters of the liquid and solid phases were optimized through a multiple linear regression method. The phase diagrams calculated from these models are in good agreement with experimental data. Also, the solid miscibility gap and chemical spinodal in the V2O3–Cr2O3 system were estimated.
Preferred Grain Orientation Relationships in Sintered Perovskite Ceramics
Authors: Ernst Frank , Mulvihill Maureen L. , Kienzle Oliver , Rühle Manfred
Published: July 2010

Sintered BaTiO3, SrTiO3, and Pb(ZrxTi1−x)O3 ceramics were examined using orientation imaging microscopy (OIMTM), which is a system for the automated measurement of grain orientations by evaluating electron backscatter diffraction patterns in scanning electron microscopy. As expected for sintered materials, none of the three materials exhibited a pronounced texture. However, analysis of the orientation relationships (ORs) between adjacent grains revealed a statistically significant preference of Σ= 3 ORs. Accordingly, the boundaries between grains with a Σ= 3 OR were particularly important for the macroscopic properties of the materials in question.
Yttrium Aluminum Garnet (YAG) Films through a Precursor Plasma Spraying Technique
Authors: Parukuttyamma Sujatha D. , Margolis Joshua , Liu Haiming , Grey Clare P. , Sampath Sanjay , Herman Herbert , Parise John B.
Published: July 2010

Coatings of yttrium aluminum garnet (Y3Al5O12, YAG), which is a promising high‐temperature ceramic, were developed for the first time using a novel precursor plasma spraying (PPS) technique. The precursor sol was sprayed using a radio‐frequency induction plasma technique. X‐ray diffraction analysis of the as‐sprayed coatings confirmed that a metastable hexagonal yttrium aluminate (H‐YAlO3) was the major phase. The above‐described specimen, on further treatment with plasma, was converted to cubic garnet (YAG) as the major phase, with a minor amount of orthorhombic YAlO3 (O‐YAP) phase. 27Al magic‐angle spinning nuclear magnetic resonance of the YAG coating corroborated the X‐ray results and confirmed the presence of YAG and O‐YAP phases. Formation of the garnet phase through the PPS technique is proof that the chemistry can be controlled in the plasma. This finding opens up new avenues for developing complex functional oxide deposits.
Electrical Conductivity and Lattice Defects in Nanocrystalline Cerium Oxide Thin Films
Authors: Suzuki Toshio , Kosacki Igor , Anderson Harlan U. , Colomban Philippe
Published: July 2010

The results of the electrical conductivity and Raman scattering measurements of CeO2 thin films obtained by a polymeric precursor spin‐coating technique are presented. The electrical conductivity has been studied as a function of temperature and oxygen activity and correlated with the grain size. When compared with microcrystalline samples, nanocrystalline materials show enhanced electronic conductivity. The transition from extrinsic to intrinsic type of conductivity has been observed as the grain size decreases to <100 nm, which appears to be related to a decrease in the enthalpy of oxygen vacancy formation in CeO2. Raman spectroscopy has been used to analyze the crystalline quality as a function of grain size. A direct comparison has been made between the defect concentration calculated from coherence length and nonstoichiometry determined from electrical measurements.
Heat‐Resistant Silicon Carbide with Aluminum Nitride and Erbium Oxide
Authors: Kim Young‐Wook , Mitomo Mamoru , Nishimura Toshiyuki
Published: July 2010

Fully dense SiC ceramics with high strength at high temperature were obtained by hot‐pressing and subsequent annealing under pressure, with AlN and Er2O3 as sintering additives. The ceramics had a self‐reinforced microstructure consisting of elongated SiC grains and a grain‐boundary glassy phase. The strength of these ceramics was ∼550 MPa at 1600°C, and the fracture toughness was ∼6 MPa·m1/2 at room temperature. The beneficial effect of the new additive composition on high‐temperature strength might be attributable to the introduction of aluminum from the liquid composition into the SiC lattice, resulting in a refractive grain‐boundary glassy phase.
Interaction of Gamma Rays with Calcium Aluminoborate Glasses Containing Holmium or Erbium
Authors: El‐Batal Hatem A. , Azooz Moenis A. , Ezz‐El‐Din Fathy M. , El‐Alaily Nagia A.
Published: July 2010

The optical absorption spectra of nominally pure and rare‐earth‐ion‐doped (Ho3+ or Er3+) calcium aluminoborate (cabal) glasses were measured from 190 to 900 nm before and after γ‐ray irradiation. The induced absorption spectra, calculated as the difference between the spectra of the irradiated and the unirradiated glasses, exhibit the characteristic absorption bands caused by the respective rare‐earth ions. The intensity of the induced bands depends on the rare‐earth oxide content. The response of the cabal glass to γ‐ray irradiation is related to the formation of color centers associated with the intrinsic defects in the lattice structure of the cabal glass. The rates of formation and annihilation of the color centers are believed to approach saturation or equilibrium with prolonged irradiation.
Electron‐Beam‐Induced Phase Transformations from Metakaolinite to Mullite Investigated by EF‐TEM and HRTEM
Authors: Lee Sujeong , Kim Youn Joong , Lee Hwack Joo , Moon Hi‐Soo
Published: July 2010

Both energy‐filtered electron diffraction and EDS data clearly indicate extraction of amorphous silica from the parent metakaolinite by electron beam heating. Compared with the extracted silica through furnace heating, a larger population with inhomogeneous distribution and higher structural disorder of extracted silica by beam heating was noticed. There is a possibility that the spinel‐type phase has lower symmetry than the suggested cubic symmetry, while the 〈111〉 orientation of the spinel‐type phase is not perpendicular to the (001) plane of metakaolinite. HRTEM images of mullite crystals apparently demonstrate their random orientations with respect to the parent metakaolinite.