bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetics of the Internal Oxidation of (Mg, Fe)O Solid Solutions
Authors: LUECKE WILLIAM , KOHLSTEDT D.L.
Published: March 2005

Single crystals of Mg1‐xFexO,x= 0.01, have been oxidized in air at temperatures between 1146 and 1389 K to determine the kinetics of the internal oxidation reaction. A two‐phase region composed of MgFe2O4 precipitates in a (Mg, Fe)O matrix grows inward from the surface as the reaction progresses. The thickness of this layer as a function of time has been determined by optical microscopy; the growth kinetics are parabolic. In addition, an external oxide layer of (Mg, Fe)O grows outward from the original surface. The thickness of this external oxide layer as a function of time was determined by following the displacement of inert markers with Rutherford backscattering spectrometry; the kinetics of the growth of this external layer are also parabolic in time. The parabolic reaction rate constant determined for the growth of the external layer accurately predicts that for the growth of the internally oxidized region.
Decomposition of Wustite in the System Ca‐Fe‐O
Authors: Bergman Bill , Song Chengyi
Published: March 2005

The influence of cooling rate upon the decomposition of wustite in the system Ca‐Fe‐O has been studied. By SEM and EDS measurements the eutectic‐like decomposition product is shown to be a mixture of wustite, Ca2Fe2O5, and Fe formed by a eutectoid decomposition of wustite. The CaO‐FeO section in the Ca‐Fe‐O system is shown not to be eutectic, as previously suggested. The phase diagram suggested by Abbatista is regarded as the correct alternative.
Kinetics of Spinodal Decomposition in the TiO2‐SnO2 System: The Effect of Aliovalent Dopants
Authors: YUAN THOMAS C. , VIRKAR ANIL V.
Published: March 2005

Dense samples in the TiO2‐SnO2 system were fabricated by pressureless sintering in air using commercial powders as well as powders obtained by coprecipitation. Phase equilibria in the system were examined using X‐ray diffraction on samples that were annealed for long periods of time. Samples of near equi‐molar composition made with controlled amounts of Al and Ta (separately) as dopants were annealed inside the coherent spinodal. X‐ray diffraction and electron microscopy were the principal characterization tools used to follow the kinetics of phase transformation. The composition modulations occurred along the [001] direction consistent with prior work. During the early stages of the decomposition process, interlamellar surfaces were coherent. At later stages, strain mismatch was accommodated by interface dislocations. The kinetics of decomposition were strongly influenced by the type and the amount of dopant. Specifically, trivalent aluminum enhanced the decomposition while pentavalent tantalum suppressed it. The kinetics in a sample doped with 0.5 mol% Al2O3 were more than 3 orders of magnitude more rapid than a sample doped with 1.0 mol% Ta2O5. These observations are rationalized on the premise that cation interstitial mobility is greater than cation vacancy mobility.
Phase Transformation and Thermal Expansion of Zirconium and Hafnium Titanates and Their Solid Solutions
Authors: IKAWA HIROYUKI , IWAI AKIHITO , HIRUTA KAZUYUKI , SHIMOJIMA HIROMASA , URABE KAZUYORI , UDAGAWA SHIGEKAZU
Published: March 2005

Phase transformations and thermal expansions were studied for compounds having the α‐PbO2 type of structure in the ZrO2‐HfO2‐TiO2‐SnO2 system. Continuous phase transformation of ZrTiO4 accompanied by a marked shrinkage of the b parameter was confirmed. All the compounds including HfTiO4 existed as dimorphs, although the difference in the b parameters between the high‐ and low‐temperature forms decreased with the content of Hf and Sn in the Zr1‐xHfxTiO4 system and the ZrxTiySnzO4 (x + y + z= 2) system, respectively. The striking contrast in thermal expansions between ZrTiO4 and HfTiO4 was confirmed for furnace‐cooled specimens at a rate of 10°C/min. The high‐temperature ZrTiO4 quenched from 1500°C exhibited thermal expansion significantly smaller than that of the furnace‐cooled ZrTiO4. The low‐temperature ZrTiO4 heated at 1000°C for 720 h exhibited thermal expansion larger than that of the furnace‐cooled ZrTiO4. The thermal expansion of furnace‐cooled ZrxTiySnzO4 (x + y + z= 2) decreased with the Sn content, and that of ZrTi0.4Sn0.6O4 was found to be almost identical with that of HfTiO4. A number of anomalies exhibited by these mixed oxides are noticed. And a hypothesis based on the interaction of orbitals by using d electrons of highly charged cations has been proposed to explain the anomalies.
Effect of ZrO2 Inclusions on the Sinterability of Al2O3
Authors: LANGE FRED F. , YAMAGUCHI TAKASHI , DAVIS BURTON I. , MORGAN PETER E.D.
Published: March 2005

The sinterability of Al2O3/ZrO2 composite powder compacts containing 2 and 10 vol% ZrO2 was compared to the sinterability of their single‐phase constituents through constant‐heating‐rate experiments. The ZrO2 inclusion phase delayed the initiation of bulk shrinkage and the temperature of maximum strain rate by 100°C. The ZrO2 inclusion phase also significantly inhibited grain growth. These results, discussed with regard to the thermodynamics of pore disappearance, suggest that phenomena inhibiting grain growth may also inhibit densification.
Mechanical and Microstructural Characterization of Mullite and Mullite‐SiC‐Whisker and ZrO2‐Toughened‐Mullite—SiC‐Whisker Composites
Authors: RUH ROBERT , MAZDIYASNI K. S. , MENDIRATTA M. G.
Published: March 2005

High‐purity mullite‐SiC‐whisker composites and mullite‐ZrO2‐SiC‐whisker composites were fabricated in situ by hot‐pressing using a matrix prepared by the alkoxide process. Varying degrees of ZrO2 stabilization were achieved by varying amounts of Y2O3 or MgO addition. Microstructural characterization was accomplished using SEM and energy dispersive analysis. Room‐temperature flexural strength and fracture toughness were determined as a function of SiC‐whisker content (0% to 30%) and ZrO2‐stabilizer content. The flexural strength of mullite varied with composition and was increased ∼50% by the addition of ∼30% ZrO2 phase. The flexural strength of mullite and mullite + 30% ZrO2 was increased ∼50% for 30% SiC‐whisker additions. The fracture toughness of mullite + 30% ZrO2 was nearly twice that of mullite. For a 30% SiC‐whisker addition, the fracture toughness of mullite was doubled, and the fracture toughness of mullite + 30% ZrO2 was increased 25% to 50%.
Blunt or Sharp Indenters: A Size Transition Analysis
Authors: MOUGINOT RÉMI
Published: March 2005

The first event occurring in a glass or a ceramic material beneath a blunt punch is conical crack initiation and beneath a sharp punch is plastic deformation. These behaviors are here analyzed as punch size transition. The transition radius of the punch is determined by using the method first proposed by Kendall (1978), based on both fracture (Griffith) criterion and yield (Von Mises) criterion analyses. The effect of the punch geometry or of the punch sliding is computed. The transition radius appears then as a quantitative brittleness indicator, which also provides a mechanical definition for the sharpness or the bluntness of the punch.
Effect of Sintering Atmosphere on Densification of MgO‐Doped Al2O3
Authors: Paek Y.‐K. , Eun K.‐Y. , Kang S.‐J.L.
Published: March 2005

The densification behavior of Al2O3‐MgO (0.1 wt%) has been studied in O2 and N2 atmospheres. Powder compacts have been sintered at 1600°C for 0.5 to 8 h. For some specimens the sintering atmosphere has been changed after 30 min of sintering. Irrespective of sintering atmosphere, sintered densities are approximately the same up to 99% relative density, implying that the capillary pressure effect dominates the atmosphere effect for most of the densification stage. During extended sintering treatment the density of specimens sintered in O2 becomes higher than that in N2. When the sintering atmosphere is changed from O2 to N2, enhanced densification results, and vice versa. Such an effect of sintering atmosphere is explained by the diffusiveness of gases entrapped in pores, as well as by oxygen potential differences inside and outside of the specimen. Differences in grain growth rate in various atmospheres are discussed on the basis of different densification rates.
Strength and Toughness of Continuous‐Alumina‐Fiber‐Reinforced Glass‐Matrix Composites
Authors: MICHALSKE TERRY A. , HELLMANN JOHN R.
Published: March 2005

Unidirectional, continuous‐fiber composites were fabricated using polycrystalline alumina fibers and four different silicate glass matrices of differing thermal expansion. Fracture toughness measurements, strength measurements, and fractographic analysis of failed specimens are used to identify the failure mechanism. Results show that the elastic modulus mismatch between the matrix and the fibers shields the reinforcing fibers from matrix crack extension, thereby increasing composite toughness without fiber pullout. Fractographic analysis shows that fiber shielding leads to fiber failure ahead of matrix crack. Composite toughness increases linearly with increases in the residual compressive stress in the matrix phase. Ultimate composite strengths are dependent upon thermal‐expansion‐induced residual stresses and fiber strength.
Aluminum‐27 and Silicon‐29 Magic‐Angle Spinning Nuclear Magnetic Resonance Study of the Kaolinite‐Mullite Transformation
Authors: Sanz J. , Madani A. , Serratosa J. M. , Moya J. S. , Aza S.
Published: March 2005

The 27Al and 29Si magic‐angle spinning nuclear magnetic resonance (MAS‐NMR) study of the kaolinite‐mullite transformation has shown the presence of Al in tetra‐ and pentacoordination in dehydroxylated kaolinite. The 29Si NMR signal analysis of samples heated above 400°C demonstrates that the tetrahedral sheet of kaolinite begins to break down near 600°C and continues to do so to 900°C. From the 27Al NMR signal evolution, it can be deduced that the exothermic peak at 980°C in DTA curves is associated with the modification of the coordination of Al, which changes from the tetra‐ or pentacoordination to the more stable octahedral coordination. Heating the sample at 880°C for 36 h produces the same transformation in the coordination of Al ions and the elimination of the exothermic peak at 980°C in the DTA diagram. After this transformation, all spectra show two tetrahedral lines characteristic of mullite, indicating that nuclei of mullite with low crystallinity are generated during the exothermic process which are not detected by XRD. At higher temperatures tetrahedral NMR peaks increase in intensity, yielding, at 1200°C, the 3:2 mullite NMR spectrum.
Phase Transformation Kinetics in the Doped System LiAl5O8—LiFe5O8
Authors: Kim Seong J. , Chen Zhien C. , Virkar Anil V.
Published: March 2005

Solid solutions of various compositions in the system LiAl5O8–LiFe5O8 were prepared by calcining mixtures of Li2CO3, Al2O3, and Fe2O3. Lattice parameters of the solid solutions were determined by X‐ray diffraction (XRD). Samples of equimolar composition were also prepared with titanium and magnesium as dopants. The doped and undoped samples were annealed at 1050°C for up to 192 h in air. Annealing at this temperature, which is inside the miscibility gap for an equimolar composition, led to the decomposition of the originally single‐phase solid solution. The extent of phase transformation was followed by XRD. The fraction transformed was estimated using integrated peak intensities. The kinetic data were analyzed in light of the conventional kinetic equation and the previously derived equation which takes into account the diffusive and the interface transfer processes. It was observed that titanium enhances and magnesium suppresses the kinetics of phase separation. These results are rationalized on the premise that the predominant point defects are Schottky defects in this spinel system.
Synthesis and Phase Stability of Cotunnite‐Type Zirconia
Authors: Ohtaka Osamu , Kumi Shoichi , Ito Eiji
Published: March 2005

The phase boundary between two high‐pressure phases (ortho I and ortho II) of ZrO2 was obtained by synthesis experiments using a 6–8 anvil‐type device. The phase boundary was linearly approximated, its slope being +400 K/GPa. The phase with cotunnite (PbCl2) structure (ortho II) appeared above 15 GPa. A single phase of Y‐doped ZrO2 with the cotunnite structure was also synthesized, but the stabilized tetragonal phase remained unchanged after the treatments at conditions where the ortho I phase forms in pure ZrO2.
Structural Aspects of Phase Equilibria in the Strontium‐Titanium‐Oxygen System
Authors: Udayakumar K. R. , Cormack Alastair N.
Published: March 2005

Energies of formation of the members of the homologous series SrO · nSrTiO3, through a Ruddlesden‐Popper type of superlattice ordering, have been examined by employing atomistic computer simulation techniques. Results of the calculations have been shown to provide a theoretical basis for the observation that a limited number of superstructures form single phases in the Sr‐Ti‐O system.
Influence of Sintering Temperature on Intrinsic Trapping in Zinc Oxide‐Based Varistors
Authors: SLETSON LISA C. , POTTER MIKE E. , ALIM MOHAMMAD A.
Published: March 2005

The ac electrical data of the ZnO‐Bi2O3 varistor system, in the frequency range 5 Hz to 13 MHz, when analyzed in the complex capacitance (C*) plane, yields a distinct depressed semicircular relaxation having an average time constant on the order of 10−6 s. This trapping is attributed to the possible formation of ionized intrinsic/native defects. The decrease in the depression angle and average relaxation time, monitored as a function of the increasing sintering temperature (≥1100°C), indicated an improvement in the degree of uniformity in the loss‐conductance and rapidity of response of the intrinsic trapping. The dependence of the ac parameters related to this trapping response obtained from the C* plane on the sintering temperature has been presented.
Specific Permeability of Porous Compacts as Described by a Capillary Model
Authors: LUKASIEWICZ STANLEY J. , REED JAMES S.
Published: March 2005

The specific liquid permeabilities of a wide variety of phosphatebonded, porous alumina compacts prepared from alumina powders of differing size distribution, and ranging in porosity from 32 to 50 vol%, were not described well by the Carmen‐Kozeny model but were described by a capillary model based on mean entry pore radius. The specific permeability varied directly with the square of the mean entry pore radius determined by mercury intrusion. The compacts containing the lowest phosphate bond phase obeyed Poiseuille's law.
Two‐Mode Behavior in Complex Perovskite Materials
Authors: Tochi Kunio , Takeuchi Nozomu , Emura Shūichi
Published: March 2005

Infrared reflection and Raman spectra have been measured for two complex perovskite ceramic materials, Ba(Mn1/3Ta2/3)O3 and Ba(Ni1/3Ta2/3)O3. Two‐mode behavior was observed for both of the compounds in the inner vibrations of the BO6‐type octahedrons, where B indicates Mn, Ta, and Ni ions. This behavior indicates that the local symmetry about Mn, Ni, and Ta ions is octahedral (Oh), although the superlattice structure of the compounds (D3d) is observed by XRD analysis.
Reaction Sequencing During Processing of the 123 Superconductor
Authors: Thomson William J. , Wang Hong , Parkman David B. , Li Dong X. , Strasik Michael , Luhman Thomas S. , Han Chan , Aksay Ilhan A.
Published: March 2005

Reaction sequencing studies were conducted for two precursors to the 123 superconductor in air and helium environments. In air, the reactants in both precursor systems produce an intermediate 123 phase at 1000 K without going through the BaCuO2 and Y2BaCuO5 intermediate phases, observed in earlier work with mixed powder precursors. In helium, the fully developed 123 tetragonal phase is formed at temperatures as low as 880 K, but it also decomposes to Y2BaCuO5, BaCu2O2, and BaCuO2; the extent of decomposition depends on the temperature. The 123 tetragonal phase completely decomposes by 1075 K, but it can be reformed if the atmosphere is switched back to air at a temperature above 950 K.
Formation of Grain‐Boundary Carbon‐Containing Phase During Annealing of YBa2Cu3O6+x
Authors: Zhang Lijie , Chen Jie , Chan Helen M. , Harmer Martin P.
Published: March 2005

In situ annealing studies of YBa2Cu3O6+x performed in an optical hot stage revealed that, at temperatures ∧450°, localized melting occurred. On subsequent cooling, a discrete second phase was observed at the YBa2Cu3O6+x grain boundaries. Quantitative chemical analysis using X‐ray wavelength dispersive spectroscopy indicated that the second phase was composed of a barium oxycarbonate. The source of the carbon in the second phase was identified to be CO2 in the atmosphere.
Measurement of Interfacial Shear Strength in SiC‐Fiber/Si3N4 Composites
Authors: Laughner James W. , Bhatt Rham T.
Published: March 2005

An indentation method for measuring shar strength in brittle matrix composites was applied to SiC‐fiber/Si3N4‐matrix samples. Three methods were used to manufacture the composites: reaction bonding of a Si/SiC preform, hot‐pressing, and nitrogen‐overpressure sintering. An indentation technique developed by Marshall for thin specimens was used to measure the shear strength of the interface and the interfacial friction stresses. This was done by inverting the sample after the initial push through and retesting the pushed fibers. SEM observations showed that the shear strength was determined by the degree of reaction between the fiber and the matrix unless the fiber was pushed out of its (well‐bonded) sheath.
Thermodynamic and Kinetic Effects of Oxygen Removal on the Thermal Conductivity of Aluminum Nitride
Authors: Virkar Anil V. , Jackson T. Barrett , Cutler Raymond A.
Published: March 2005

High thermal conductivity, low dielectric constant, high electrical resistivity, low density, and a thermal expansion coefficient that matches well with that of silicon are the principal attributes of AIN that have attracted much attention over the past decade. It is also now well established that oxygen as an impurity lowers the thermal conductivity of AIN. Processing techniques have been developed which not only facilitate pressureless densification of AIN but also enhance its thermal conductivity. The present work explores the thermodynamics and the kinetics of oxygen removal and the resultant enhancement of thermal conductivity. Polycrystalline AIN ceramics were fabricated with Y2O3, Dy2O3, Yb2O3, CaO, BaO, or MgO as additives. Samples were sinter/annealed at 1850°C for up to 1000 min. The AIN grain size of sintered samples ranged between 2 and 9 μm. The samples typically contained two or three phases with the predominant phase being AIN. Secondary phases in Y2O3‐doped AIN consisted of yttrium aluminates which were along three grain junctions and along grain facets. The presence of Y3Al5O12, YAIO3, and Y4Al2O9, as well as Y2O3, depending upon the Y2O3/Al2O3 ratio, was revealed by X‐ray diffraction. Thermal conductivity increased with the amount of additive and annealing time. Thermal conductivity also depended on the type of additive. Samples with thermal conductivity up to 200 W/(m · K) were fabricated. The variation in thermal conductivity with the type and the amount of the additive is explained on the basis of the thermodynamics of oxygen removal. In particular, the higher thermal conductivity of CaO‐doped, in comparison with MgO‐doped, samples is rationalized on the basis that the free energy of formation, ΔG°, of CaAl2O4 is less than that of MgAl2O4. It is proposed that the higher the |ΔG°|, with ΔG° < 0, the higher is the resultant thermal conductivity. An increase in the thermal conductivity with annealing time is attributed to the kinetics of oxygen removal from AIN grains.
Cation Interdiffusion in Polycrystalline Cubic C‐Type Yttria–Zirconia–Hafnia Solid Solutions
Authors: Sakka Yoshio , Oishi Yasumichi , Ando Ken , Masuda Masaaki
Published: March 2005

Cation inter diffusion in cubic C‐type solid solutions was investigated for the polycrystalline systems Y2O3—ZrxHf1‐x‐1O2 and Y2O3–ZrO2 in the small‐grain, deep‐penetration condition at 1615° to 1822°C. Lattice and grain‐boundary diffusion parameters were calculated from cation concentration distributions by using Oishi and Ichimura's grain‐boundary diffusion equation. The results indicated that Zr–Y and Zr–Hf interdiffusion coefficients decreased with an increase in Y content. The cation diffusivities were lower than anion diffusivities, and the interdiffusion parameters were lower in the C‐type cubic than in fluorite‐type cubic solid solutions. The results were compared with diffusivities in other C‐type cubic oxides.
Processing and Characterization of BaTi4O9
Authors: Mhaisalkar S. G. , Lee W. E. , Readey D. W.
Published: March 2005

BaTi4O9 powder prepared by calcining BaCO3 and TiO2 powders was sintered to over 97% of theoretical density. Less than 5% Ba2Ti9O20 occurred as a second phase in “pure” BaTi4O9, and Al2O3 impurities from processing formed isolated hollandite (∼BaAl2Ti6O16) grains, which were identified by fringes in bright‐field TEM images. For pure BaTi4O9 at 1 MHz, a dielectric loss (tan δ) of 5 × 10−4 and dielectric constant of 39 were recorded. Hollandite impurities were found to increase tan δ by 2 orders of magnitude, whereas firing in oxygen decreased tan δ by an order of magnitude.
Kinetics of Combustion Synthesis in the Ti‐C and Ti‐C‐Ni Systems
Authors: Dunmead Stephen D. , Readey Dennis W. , Semler Charles E. , Hol J. Birch
Published: March 2005

The kinetics and mechanisms of combustion reactions in the Ti‐C and Ti‐C‐Ni systems were studied. Samples were produced by igniting compacts of elemental Ti, C, and Ni powders with a tungsten heating coil under an inert argon atmosphere. Using an elementary model of the process, the “apparent” activation energies of these highly exothermic reactions were determined by measuring combustion wave velocities and combustion temperatures. For the Ti + C ⇒ TiC reaction, two different combustion regimes were found. The first, for combustion temperatures greater than 2711 K, was postulated to be controlled by the dissolution of carbon into a titanium melt with an apparent activation energy of 124 ± 31 kJ · mol−1. The second, for combustion temperatures less than 2711 K, was postulated to be controlled by the diffusion of carbon through a solid TiC layer with an apparent activation energy of 364 ± 25 kJ · mol−1. For the reaction Ti + C + 25 wt% Ni ⇒ TiC + 25 wt% Ni the apparent activation energy was measured to be 133 ± 50 kJ · mol−1, corresponding to the dissolution of carbon into a Ti‐Ni melt. Temperature profile and microstructural information are also presented.
Pressure Effects on the Thermal Stability of Silicon Carbide Fibers
Authors: Jaskowiak Martha H. , DiCarlo James A.
Published: March 2005

Commerically available polymer‐derived SiC fibers were treated at temperatures from 1000° to 2200°C under vacuum and at argon gas pressures of 0.1 and 138 MPa. Effects of increasing inert gas pressure on the thermal stability of the fibers were determined through studies of the fiber microstructure, weight loss, grain growth, and tensile strength. The 138‐MPa argon gas treatment was found to shift the onset of fiber weight loss from 1200° to above 1500°C. Grain growth and tensile strength degradation were correlated with weight loss and were thus also inhibited by high‐pressure treatments. Retreatment in 0.1 MPa of argon of the fibers initially treated in 138 MPa of argon caused further weight loss and tensile strength degradation, thus indicating that high‐pressure inert gas conditions were effective only in delaying fiber strength degradation and that no permanent microstructural changes were induced.
Particle Size Distribution and Rate of Strength Development of Portland Cement
Authors: Osbaeck Bjarne , Johansen Vagn
Published: March 2005

The fineness of a portland cement is a key factor in determining its strength development characteristics. A simple one‐parameter fineness characterization such as a Blaine value or a sieve residue is often inadequate for a precise evaluation of the influence of fineness on the strength development of cement, especially when comparing cements ground with different grinding equipment. Information on the whole particle size distribution (PSD) of the cement must be used. A mathematical model relating PSD to strength, based on comparison of cements made from the same basic materials (clinker and gypsum) but ground to different PSD, is presented. The model assumes that at a given time of hydration all particles have reacted to the same depth from the original surface. Integrating over the total PSD will give the degree of hydration for a specific size distribution. From a correlation between strength and degree of hydration, differences in degree of hydration can be converted into relative strengths. The applicability of the model is demonstrated by two examples. The first example compares two cements which are ground to different finenesses in the same mill. The second example compares two cements which are ground to the same specific surface area in two different mill systems, bringing about cements with PSDs of different shape. The depth of reaction values found to give the best description seems to be proportional to the square root of time of reaction, indicating a diffusion‐controlled hydration reaction.
Electrical Conduction in Magnesium Phosphate Glasses Containing Heavy Water
Authors: Hosono Hideo , Kamae Takayuki , Abe Yoshihiro
Published: March 2005

The relation between deconductivity and the concentration of protons or deuterons was investigated in 45MgO · 55P2O5 (in mol%) glasses containing small amounts of “water.” The conductivity was shown to increase in proportion to the square of the proton concentration as reported in calcium phosphate glasses. The same relation was found to be valid for the deuterated specimens, although their conductivity was somewhat lower than that in the protonated glasses when compared at the same concentration of OD or OH groups. These results strongly suggest that protons are the charge carriers in these glasses. The conductivities were measured also for specimens containing both protons and deuterons in different ratios. No distinct deviation was observed from additivity within the concentration range examined.
Microstructure Development of Zinc Oxide in Hydrogen
Authors: Quadir Tariq , Readey Dennis W.
Published: March 2005

Zinc oxide powder compacts with green densities of 60% of theoretical sintered in hydrogen do not densify but exhibit significant grain coarsening. The temperature dependence of the coarsening was greater than that expected from vapor‐phase coarsening models and is close to that obtained for grain growth in dense ZnO. This suggests that the rate of particle coarsening is controlled by grain‐boundary mobility. This possibility was reinforced by the reduction in coarsening with ZnAl2O4 present as a second phase. The results emphasize the importance of controlling grain growth throughout all stages of densification.
Phase Equilibria and Crystal Chemistry in the Quaternary System Ba‐Sr‐Y‐Cu‐O in Air
Authors: Roth Robert S. , Rawn Claudia J. , Whitler John D. , Chiang C. K. , Wong‐Ng Winnie K.
Published: March 2005

Studies in the Sr‐Y‐Cu‐O and Ba‐Sr‐Y‐Cu‐O systems have revealed that Sr will substitute for Ba in (Ba,Sr)2YCu3O6+x up to about 60%. There are no ternary compounds in the Sr‐Y‐Cu‐O system equivalent to the three ternary phases in the Ba system. A new binary phase, “Sr14Cu24O41”(CuO ∼ 63.158 mol%), was found which forms a solid solution with Y2O3 to a Sr:Y ratio of approximately 2:1. This phase can also incorporate considerable amounts of Ba and Ca and many other large ions.
Vibrational Spectroscopic Study of Structural Evolution in the Coprecipitated Precursors to La2Sn2O7 and La2Ti2O7
Authors: Takahashi Junichi , Ohtsuka Toshiaki
Published: March 2005

Structural evolution in the X‐ray amorphous precursors to La2Sn2O7 and La2Ti2O7 is examined using IR and Raman spectroscopy. These precursors are prepared by rapid coprecipitation from mixed aqueous solutions of the corresponding metal chlorides. Rapid coprecipitation from an SnCl2−6 and La3+‐containing aqueous solution yields microcrystalline particles of SnO2·nH2O and La(OH)3, which instantaneously interconnect to form an ultimate, complex colloid particle. The Ti(OH)2+2 and La3+ in the other solution system coprecipitate into a different, complex colloid (an unidentified phase), which is definitely not a mixed dispersion of single‐component colloids. A comparative examination of the vibrational spectra of the coprecipitates heated to various temperatures indicates that the SnO2 and anatase phases develop in the respective precursors before crystallization of the desired double oxides. Crystallization itself can be attributed to a solid‐state reaction among the various microcrystallites of each single‐metal oxide in a gel particle of the precursor.
Humidity‐Sensitive Characteristics of Porous La‐Ti‐V‐O Glass‐Ceramics
Authors: Shimizu Yasuhiro , Okada Hideaki , Arai Hiromichi
Published: March 2005

The humidity‐sensitive characteristics of La2O3–TiO2–V2O5 glass‐ceramics were investigated as a function of additive amount of V2O5 to the precursor glass and the heating temperature of the glass to iduce phase separation. The microstructure of each glass‐ceramic was strongly dependent upon the heating temperature. The specific impedance was lowered by increasing the amount of V2O5 additive. Among the elements studied, the LTV 2–2 element, which was prepared from as‐cast glass, consisted of Na2O:B2O3:La2O3: TiO2: V2O5= 8.3:32.9:7.2:31.6:20.0 (molar ratio) after heating at 450°C for 12 h and subsequent leaching at 85°C for 24 h, and was found to be the most suitable material from the standpoint of humidity sensitivity, measurability, and response time.
Low‐Temperature Sintering of Lead‐Based Piezoelectric Ceramics
Authors: Zhilun Gui , Longtu Li , Suhua Gao , Xiaowen Zhang
Published: March 2005

The low‐temperature sintering of lead‐based piezoelectric ceramics has been studied. The sintering temperature of lead zirconate titanate (PZT) ceramics could be reduced from ∼ 1250° to ∼960°C by the addition of a small amount of the lower‐melting frit, B2O3–Bi2O3—CdO. It exhibited the following dielectric and piezoelectric properties: Kp= 0.52 to 0.58, Qm= 1000, εT33/ε0= 800 to 1000, tan δ= 50 × 10−4, ρ= 7.56 to 7.64 g/cm3. Ceramics with the aid of suitable dopants (CdO, SiO2, and excess PbO) in the Pb‐(Ni1/3Nb2/3)O3—PZT family could be sintered at 860° to 900°C. For these materials, Kp= 0.56 to 0.61, Qm= 1000, εT33/ε0= 1500 to 2000, tan δ≤ 50 × 10−4, ρ= 7.80 to 8.03 g/cm3. The microstructure, sintering mechanism, and the effects of various impure additions have been analyzed by means of scanning electron microscopy, scanning transmission electron microscopy, electron probe microanalysis, and X‐ray photoelectron spectroscopy.
Silicon Carbide Monofilament‐Reinforced Silicon Nitride or Silicon Carbide Matrix Composites
Authors: Kodama Hironori , Sakamoto Hiroshi , Miyoshi Tadahiko
Published: March 2005

SiC‐monofilament‐reinforced SiC or Si3N4 matrix composites were fabricated by hot‐pressing, and their mechanical properties and effects of filaments and filament coating layers were studied. Relationships between frictional stress of filament/matrix interface and fracture toughness of SiC monofilament/Si3N4 matrix composites were also investigated. As a result, it was confirmed experimentally that in the case of composites fractured with filament pullout, the fracture toughness increased as the frictional stress increased. On the other hand, when frictional stress was too large (>about 80 MPa) for the filament to be pulled out, fracture toughnesses of the composites were almost the same and not so much improved over that of Si3N4 monolithic ceramics. The filament coating layers were found to have a significant effect on the frictional stress of the SiC monofilament/Si3N4 matrix interface and consequently the fracture toughness of the composites. Also the crack propagation behavior in the SiC monofilament/Si3N4 matrix composites was observed during flexural loading and cyclic loading tests by an in situ observation apparatus consisting of an SEM and a bending machine. The filament effect which obstructed crack propagation was clearly observed. Fatigue crack growth was not detected after 300 cyclic load applications.
Formation of Yttrium Barium Cuprate Powder at Low Temperatures
Authors: Lay Kenneth W.
Published: March 2005

Yttrium barium cuprate can be formed at temperatures below 800°C from mixed oxides and carbonate if a low‐oxygen‐pressure atmosphere is used. This eliminates the formation of a high‐oxygen‐content nonsuperconducting phase which has been previously shown to form at lower temperatures. Reaction of barium carbonate and the oxides of copper and yttrium at temperatures as low as 750°C in reducing atmospheres is shown to form tetragonal YBa2Cu3Ox with x about 6. This compound can then be oxidized by cooling in an oxidizing atmosphere to obtain the desired x value of about 7.
Effects of Particle Packing Characteristics on Solid‐State Sintering
Authors: Zheng Jingmin , Reed James S.
Published: March 2005

Alumina compacts fabricated with different green densities and different pore size distributions were characterized and the changes of the pore characteristics during solid‐state sintering were studied. A critical ratio of pore size to mean particle size for pore shrinkage was determined. Porosity in the compact could be classified into two classes: the first class contains pores smaller than the critical ratio, and the second class contains pores larger than the critical ratio. Pores belonging to a different class of porosity behaved differently during sintering. Pores larger than the critical ratio were not totally eliminated during sintering. The first class of porosity controlled the ultimate sintering shrinkage, and the second class of porosity controlled the final sintered density.
Dielectric Properties of Microporous Glass in the Microwave Region
Authors: Yamamoto Joyce K. , Lanagan Michael T. , Bhalla Amar S. , Newnham Robert E. , Cross L. Eric
Published: March 2005

The dielectric properties of a high‐silica microporous glass were determined at 5.5 and 11.4 GHz. Two measurement methods were used to measure these properties, the resonant cavity and resonant post (Hakki–Coleman) techniques. The accuracy of the two measurement techniques was evaluated and the limiting conditions of measurement were determined. The glass, consisting of pore sizes on the order of tens of angstroms, was subjected to thermal treatments in an oxidizing atmosphere at temperatures from 300° to 1100°C, and chemical treatments in 2% and 10% HF for varying amounts of time. Heat treatment of the glass at 800°C produced a minimum dielectric constant of 2.8 and loss of 0.001. The acid treatment had little effect on the dielectric properties.
Flaw Generation During Constrained Sintering of Metal‐Ceramic and Metal–Glass Multilayer Films
Authors: Cheng Tsungnan , Raj Rishi
Published: March 2005

Sintering of symmetrical multilayer films has been studied theoretically and experimentally. Experiments were conducted on ceramic/metal/ceramic and glass/metal/glass films. The sintering rate of free films was compared to the sintering rate of multilayers. The origin of processing flaws was examined. The following results were obtained: (1) Differential sintering rates of the components in the multilayer give rise to in‐plane tensile and compressive stresses. The film that is stressed in tension in the early stage of sintering is most susceptible to fracture. Experiments with ceramic/metal/ceramic multilayer are in agreement with this prediction. (2) A theoretical prediction that the glass/metal/glass multilayer will not develop defects because of a high value of the shear relaxation factor in glass is confirmed by experiments. (3) The likelihood of developing a tensile stress in the multilayer depends only on geometry, the green density, and the ratio of the intrinsic sintering pressures. (4) The in‐plane shrinkage of the multilayer depends on the difference in the free‐sintering rates and the shear relaxation factors, and is reasonably well predicted by the analysis. (5) We have evidence that the metal layer deforms plastically when it is placed in tension by differential sintering.
High‐Temperature Behavior of the Aluminum Oxycarbide Al2OC in the System Al2O3–Al4C3 and with Additions of Aluminum Nitride
Authors: Lihrmann Jean Marc , Zambetakis Theofilos , Daire Marc
Published: March 2005

A large number of high‐temperature experiments involving binary mixtures of alumina and aluminum carbide (Al4C3) were performed to clarify the behavior of the aluminum oxycarbide Al2OC. It is first shown from heat treatments on the molar composition 55Al2O3·45 Al4 C3 that, in conditions of stable equilibrium, Al2OC decomposes to Al4O4 C and Al4 C3 at 1715°C according to the reaction 4Al2 OC → Al4 O4 C+Al4 C3. However, Al2 OC can be obtained at room temperature from compositions of lower carbide contents (<20 mol%) and by rapid cooling. In that case Al2 OC is solidified in a metastable state, and, when subsequently annealed to temperatures above 1200°C, its lattice reorganizes and progressively transforms into the lattice of Al4 O4 C according to the reaction Al2 O3+xAl2 OC → (1—x)Al2 O3+xAl4 O4 C. This transformation is described in terms of TTT curves and is accompanied by a decrease in hardness and wear resistance. Additions of AlN to Al2 O3 and Al4 C3 have been found to create a solid‐solution Al2 OC—AlN in an unexpected region of the ternary system Al2 O3—Al4 C3—AlN. As a result the quasi‐binary section Al2 OC—AIN of the ternary diagram Al2 O3—Al4 C3—AlN was extensively investigated. We report the solubility limit of AlN in Al2OC, the improved high‐temperature stability of the Al2OC—AlN solid solution compared with pure Al2OC, and eventually the existence of a possible intermediate ternary compound of formula Al10O3C3N4.
Crystal Structure of Orthorhombic Zirconia in Partially Stabilized Zirconia
Authors: Kisi Erich H. , Howard Christopher J. , Hill Roderick J.
Published: March 2005

A neutron powder diffraction investigation confirms that, in tough magnesiapartially‐stabilized zirconia cooled to 30 K, most of the tetragonal zirconia transforms to an orthorhombic phase. This phase is retained on heating to room temperature; the lattice parameters at 295 K are a = 0.5068, b = 0.5260, and c = 0.5077 nm. The room‐temperature crystal structure (space group Pbc21) is determined by multiphase Rietveld refinement from the neutron diffraction pattern. This orthorhombic structure is compared with the parent tetragonal structure and with the structure of monoclinic zirconia, which it closely resembles.
Hot Isostatically Pressed Alumina‐Silicon Carbide‐Whisker Composites
Authors: Björk Lennart , Hermansson Leif A. G.
Published: March 2005

Alumina‐silicon carbide‐whisker composites were hot isostatically pressed at 1550°C and 200 MPa for 1 h. The silicon carbide whiskers were treated in different acid and gas environments before they were pressed. All samples exhibited linear elastic behavior with no ductility tendency. Improved strength and fracture toughness were obtained compared with unreinforced alumina. Mechanisms for the improved mechanical properties are discussed. These include grain growth control, whisker encapsulation of defects, and related stress relief at the defect.
Dynamic Grain Growth During Superplastic Deformation of Yttria‐Stabilized Tetragonal Zirconia Polycrystals
Authors: Nieh Tai‐Gang , Wadsworth Jeffrey
Published: March 2005

Both static and dynamic grain growth were studied during superplastic deformation of fine‐grained yttria‐stabilized tetragonal zirconia. It was found that significant grain growth does not take place below 1300°C. Both static and dynamic growth were found to obey a similar equation of the form D3−D30=kt, where D and D0 are the instantaneous and initial grain sizes, respectively, t is the annealing time, and k is the kinetic constant for either static or dynamic grain growth. The activation energies were approximately 580 and 520 kJ/mol for static and dynamic grain growth, respectively.
Melting of Silica up to 7 GPa
Authors: Kanzaki Masami
Published: March 2005

Melting relations of SiO2 were determined up to 7 GPa. The triple point of quartz‐coesite‐liquid is located at 4.5 GPa and 2450°C. The melting curve of coesite has a steep initial slope (150°C/GPa) and reaches about 2700°C at 7 GPa. Densities of the quenched glasses are also reported.
Effect of Ta2O5, Nb2O5, and HfO2Alloying on the Transformability of Y2O3‐Stabilized Tetragonal ZrO2
Authors: Kim Dae‐Joon
Published: March 2005

The addition of Ta2O5, Nb2O5, and HfO2enhanced the transformability of Y2O3‐stabilized tetragonal ZrO2polycrystal (Y‐TZP), which was indicated by an increase in phase transformation temperatures and fracture toughness of Y‐TZP. Comparison of the alloying effects of these oxides on the transformability and crystal structure of Y‐TZP suggested that an alloying oxide which increases thec/aaxial ratio (tetragonality) of TZP also increases the transformability. Empirical equations to predict the tetragonality are proposed. Calculated tetragonalities showed good agreement with measured values in the systems ZrO2‐Y2O3‐Ta2O5, ‐Nb2O5, and ‐HfO2.
Microstructure Development and Dielectric Properties of Potassium Strontium Niobate Ceramics
Authors: Kimura Toshio , Miyamoto Seiji , Yamaguchi Takashi
Published: March 2005

Sintering of a KSr2Nb5O15 powder compact at 1350°C resulted in a duplex structure. Prefiring of the compact between 1200° and 1300°C inhibited the abnormal grain growth responsible for the duplex structure. The Curie temperature and dielectric constant were dependent on the microstructure.
Rapid Sintering of Stoichiometric Zinc‐Modified Lead Magnesium Niobate
Authors: Landin Steven M. , Schulze Walter A.
Published: March 2005

A rapid thermal‐processing technique was used to densify zinc‐modified lead magnesium niobate (PMZN30) of the form Pb(Mg0.7Zn0.3)1/3Nb2/3O3. This dielectric composition showed relaxor behavior with a maximum permittivity near room temperature. Rapid densification was observed at temperature. Rapid densification was observed at temperatures of 1100°C and above, whereas lower temperatures showed a significant reduction of densification and electrical properties. A sintering schedule of 4 min at 1100°C with no prevention of PbO volatilization resulted in densification on the order of 90% of theoretical density and a weak field dielectric constant in excess of 14 000. Perovskite grain size was consistently in the range of 1 μm or less. The formation of pyrochlore on sample surfaces showed a direct relation to time and temperature.
Grain Boundaries in Silicon from Zero Temperature through Melting
Authors: Phillpot Simon R. , Wolf Dieter
Published: March 2005

The results of atomistic simulations of twist grain boundaries in covalent silicon are presented and compared with similar studies in metals. Three aspects are discussed in detail: (i) the zero‐temperature structure‐energy correlation, (ii) the elastic anomalies near a twist boundary at zero temperature, and (iii) the high‐temperature stability of a boundary and its role in thermodynamic melting. In each case striking similarities with studies on metals are found, which are attributed to the important role played by atoms in close proximity. By contrast the covalent nature of bonding in silicon appears to play only a minor role.
Microstructure‐Mechanical Property Relationships in Hot Isostatically Pressed Alumina and Zirconia‐Toughened Alumina
Authors: Shin Dong‐Woo , Orr K. K. , Schubert Helmut
Published: March 2005

The rates of densification and the mechanical properties of pure Al2O3 and ZrO2‐toughened Al2O3 (ZTA) have been investigated as a function of the temperatures and time schedules used for hot isostatic pressing (HIP) as a postsintering heat treatment for samples which had already been pressureless sintered in air at 1460°C for 45 min. ZTA hot isostatically presed at 1400°C had a finer grain size and a narrower grain size distribution than ZTA hot isostatically pressed at 1600°C. At both HIP conditions, the density which could be obtained was almost the maximum theoretical density. The amount of grinding‐induced and fracture‐induced monoclinic ZrO2 formed as a result of the tetragonal → monoclinic martensitic transformation in ZTA was higher in the samples hot isostatically pressed at 1400°C. ZTA hot isostatically pressed at 1600°C and 100 MPa had fewer flaws and higher strengths than ZTA hot isostatically pressed at 1400°C for the same time, with a gradual improvement in mechanical properties with increasing HIP time at each of these two temperatures. The best mechanical properties were obtained from ZTA hot isostatically pressed at 100 MPa and 1600°C for 1 h: these specimens had a four‐point bend strength of 940 ± 15 MPa at room temperature and 540 ± 15 MPa at 1000°C and an indentation fracture toughness at room temperature of 9.4 ± 0.2 MPa·m1/2.
Microstructural Properties of Combustion‐Synthesized and Dynamically Consolidated Titanium Boride and Titanium Carbide
Authors: Kecskes Laszlo J. , Kottke Thomas , Niiler Andrus
Published: March 2005

Full‐density TiB2 and TiC have been fabricated by combustion synthesis reactions followed by dynamic compaction of the still hot, porous ceramic body. The relationship between the morphologies and purities of the precursor powders used and the ceramic product structures is presented. Intergrain bonding and residual porosity of the dynamically consolidated products are found to depend strongly on the impurity levels of the precursor powders. Analysis of the TiC indicates that density and microhardness increase as a function of the C/Ti ratio, with maximum values at the ratio of 1.0.
Thermal Expansion of Laminated, Woven, Continuous Ceramic Fiber/Chemical‐Vapor‐Infiltrated Silicon Carbide Matrix Composites
Authors: Eckel Andrew J. , Bradt Richard C.
Published: March 2005

Thermal expansions of three two‐dimensional laminate, continuous fiber/chemical‐vapor‐infiltrated silicon carbide matrix composites reinforced with either FP‐Alumina (alumina), Nextel (mullite), or Nicalon (Si‐C‐O‐N) fibers are reported. Experimental thermal expansion coefficients parallel to a primary fiber orientation were comparable to values calculated by the conventional rule‐of‐mixtures formula, except for the alumina fiber composite. Hysteriesis effects were also observed during repeated thermal cycling of that composite. Those features were attributed to reoccurring fiber/matrix separation related to the micromechanical stresses generated during temperature changes and caused by the large thermal expansion mismatch between the alumina fibers and the silicon carbide matrix.
Knoop Microhardness Anisotropy of Single‐Crystal Rutile
Authors: Li Hong , Bradt Richard C.
Published: March 2005

Knoop microhardness profiles were determined for singlecrystal rutile on the (100), (001), (110), and (111) for test loads from 50 to 300 g. The profiles were related to the slip systems as they determine the corresponding effective resolved shear stress (ERSS) diagrams according to the concepts advanced by Brookes and co‐workers. Several previously reported slip systems were confirmed while slip on the {111̄} <101> was also suggested. Cleavage of rutile on the {110} and {100} was substantiated. The load dependence of the microhardness was addressed in terms of the classical Meyer's law, P=Adn. The n values were between 1.53 and 1.77, varying with the crystallographic planes and directions. However, no obvious systematic variation was evident.