bluuebunny/crossref_metadata_2025 huggingface dataset:
Nanocrystalline α‐Al2O3 Using Sucrose
Authors: Das Rabindra N. , Bandyopadhyay Amit , Bose Susmita
Published: July 2010

Nanocrystalline α‐Al2O3 ceramic powders have been prepared from an aqueous solution of aluminum nitrate and sucrose. Soluble Al ion‐sucrose solution forms the precursor material once it is completely dehydrated. Heat treatment of the dehydrated precursors at low temperature (600°C) results in the formation of porous single‐phase α‐Al2O3. The precursor and heat‐treated powders have been characterized by X‐ray diffraction (XRD), transmission electron microscopy (TEM), and BET surface area analysis. The phase‐pure nanocrystalline α‐Al2O3 particles had an average specific surface area of >190 m2/g, with an average pore size between 18 and 25 nm.
Effect of Neodymium:Yttrium Aluminum Garnet Laser Irradiation on Crystallization in Li2O–Al2O3–SiO2 Glass
Authors: Lee Yong Su , Kang Won Ho , Kim Byeoung Il , Song Sun Dal
Published: July 2010

A 355‐nm neodymium:yttrium aluminum garnet laser, produced by a harmonic generator, was used for the nucleation process in photosensitive glass containing Ag+ and Ce3+ ions. The pulse width and frequency of the laser were 8 ns and 10 Hz, respectively. Heat treatment was conducted at 570°C for 1 h, following laser irradiation, to produce crystalline growth, after which a LiAlSi3O8 crystal phase appeared in the laser‐irradiated Li2O–Al2O3–SiO2 glass. The present study compares the effect of laser‐induced nucleation on glass crystallization with that of spontaneous nucleation by heat treatment.
Separate Growth of α‐ and β‐Si3N4 Whiskers on or near a Carbon Substrate by Carbothermal Reduction
Authors: Shimada Shiro , Kataoka Tomoyuki
Published: July 2010

Whiskers of α‐ and β‐Si3N4 were grown on or near a carbon black substrate, respectively, 10 mm downstream from a mixed starting powder of low‐grade silica and carbon, in flowing nitrogen gas at 1400°C. The parameters (flowing nitrogen gas rate, growth time, grade of silica, and type of carbon) that promoted growth of the whiskers were examined in view of increasing the whisker yield. The shapes and sizes of both types of whiskers were observed by scanning electron microscopy (SEM). The separate growth of the whiskers is discussed here, based on X‐ray diffraction analysis and SEM observation with energy‐dispersive X‐ray analysis.
Experimental Observations of High Fracture Resistance in Silicon Carbide/Alumina Laminated Composites
Authors: Krishnamurthy Ramanthan , Sheldon Brian W.
Published: July 2010

Model laminated composites were fabricated with porous‐Al2O3 interfaces between SiC bars. The porous Al2O3 was deposited using an aerosol spray deposition technique, and the sandwich specimen was fabricated by hot pressing. Residual thermal stresses were present in the interface because of the difference in the coefficients of thermal expansion of SiC and Al2O3. Crack deflection was observed with measured interfacial fracture resistances that were considerably higher than the deflection threshold predicted by the He–Hutchinson criterion. Examination of the fracture surface revealed a tortuous crack path and significant crack–flaw interaction.
Direct Evidence for Low‐Density Regions in Compacted Spray‐Dried Powders
Authors: Saito Yutaka , Tanaka Satoshi , Uchida Nozomu , Uematsu Keizo
Published: July 2010

Pore structure in a ceramic green body was directly observed with a novel characterization tool to examine the detrimental effect of a binder on the microstructure of ceramics for powder compaction processing. The binder segregated on the surface of granules and formed a low‐density region at the boundaries of granules in the compact after binder removal. A network of pores with round shapes was produced in the sintering process from the low‐density region. This paper also explains a new characterization technique, confocal laser scanning fluorescent microscopy.
Stability of Silicon Carbonitride Phases
Authors: Badzian Andrzej
Published: July 2010

Important hard phases are included in the quaternary compositional system Si‐N‐C‐B. This paper reviews ternary amorphous and crystalline phases in the system Si‐N‐C and deliberates on the issue of stability of the binary C3N4, a hypothetical phase harder than diamond, and instability of nitrides in general. There is a tendency for nitrogen atoms to agglomerate and be released as nitrogen molecules. Stabilization of CN radicals can be achieved through ternary phases: carbonitrides metal‐C‐N. Ternary Si‐N‐C phases have been synthesized by pyrolysis of polyorganosilazanes, physical vapor deposition, and chemical vapor deposition. The crystalline α‐Si3N4:C phase can incorporate about 6 at.% C and yields enhancement of hardness and wear resistance. Other crystalline phases contain more carbon, for example, Si2CN4.
Solubility of Si3N4 in Liquid SiO2
Authors: Gu Hui , Cannon Rowland M. , Seifert Hans J. , Hoffmann Michael J. , Tanaka Isao
Published: July 2010

The nitrogen solubility in the SiO2‐rich liquid in the metastable binary SiO2‐Si3N4 system has been determined by analytical TEM to be 1%–4% of N/(O + N) at 1973–2223 K. Analysis of the near edge structure of the electron energy loss peak indicates that nitrogen is incorporated into the silicate network rather than being present as molecular N2. A regular solution model with a positive enthalpy of mixing for the liquid was used to match the data for the metastable solubility of N in the presence of crystalline Si3N4 and to adjust the computed phase diagram. The solubility of Si3N4 in fused SiO2 is far less than reported in liquid silicates also containing Al, Mg, and/or Y. Apparently, these cations act as modifiers that break anion bridges in the silicate network and, thereby, allow further incorporation of Si3N4 without prohibitive amounts of network cross‐linking. Finally, indications emerged regarding the diffuse nature of the Si3N4‐SiO2 interface that leads to amorphous regions of higher N content.
Progress Toward Making Gallium Nitride Seed Crystals Using Hydride Vapor‐Phase Epitaxy
Authors: Tavernier Philip R. , Clarke David R.
Published: July 2010

The essential steps required to create GaN seed crystals for bulk crystal growth from thick films are described. These steps include the growth of low‐dislocation‐density GaN films using a hydride vapor‐phase epitaxy route, the separation of the films from the sapphire substrate on which they are grown, and the subsequent growth of GaN on both sides of the free‐standing films. These steps are illustrated with examples that emphasize the growth of material with low threading dislocations.
Mechanisms of Dopant‐Induced Changes in Intergranular SiO2 Viscosity in Polycrystalline Silicon Nitride
Authors: Pezzotti Giuseppe , Painter Gayle S.
Published: July 2010

Mechanical spectroscopic methods and first‐principles density functional calculations were applied to attempt a quantitative analysis of both atomic structure and viscous behavior of Si3N4 grain boundaries. In particular, the effect on the intergranular structure/viscosity of small fractions of selected anion/cation dopants was examined in comparison with the undoped polycrystal. From the point of view of mechanical spectroscopy, emphasis was placed on the morphologic analysis, as a function of frequency of oscillation, of a relaxation peak that originates from grain‐boundary sliding. The morphologic characteristics of the grain‐boundary peak clearly revealed the presence of significant chemical gradients among different grain boundaries for particular dopants (e.g., Cl and Ba). On dopant addition, a reduction in activation energy for viscous intergranular flow was observed which broadened the grain‐boundary peak. Chemical inhomogeneities also broadened the peak shape by generating a spectrum of activation energies. First‐principles density functional calculations were conducted for cluster fragment models representative of the amorphous SiO2 intergranular film. The results explicitly showed the mechanism by which the respective dopants break bonds in the host, an action that directly reduces the viscosity of the SiO2 film. These complementary theoretical studies assist understanding and atomic‐scale rationalization of the differences in segregation behavior of different dopants incorporated into the SiO2 film.
Direct‐Write Fabrication of Zinc Oxide Varistors
Authors: Tohver Valeria , Morissette Sherry L. , Lewis Jennifer A. , Tuttle Bruce A. , Voigt James A. , Dimos Duane B.
Published: July 2010

Zinc oxide (ZnO)‐based pastes with tailored rheological properties have been developed for direct‐write fabrication of thick‐film varistor elements in highly integrated, multifunctional electroceramic devices. Such pastes exhibited pseudoplastic behavior with a low shear apparent viscosity of roughly 1 × 104Pa·s. Upon aging, the pastes attained printable, steady‐state viscosities of approximately 3 × 102Pa·s at 10 s−1. Square and rectangular elements were patterned on dense alumina substrates and sintered at varying temperatures between 800° and 1250°C. Varistor elements fired at 900°C exhibited nonlinearity coefficients (α= 30) that were equivalent to high‐density (>95%) varistors formed by cold isostatic pressing at 100 MPa (15 ksi) of a similar chemically derived powder heat‐treated under analogous conditions.
Effect of Internal Lubricants on Defects in Compacts Made from Spray‐Dried Powders
Authors: Balasubramanian Sreeram , Shanefield Daniel J. , Niesz Dale E.
Published: July 2010

The role of internal lubricants in the closure of large intergranular pores during dry‐pressing was investigated. Alumina was spray‐dried with and without an internal lubricant to yield granules with similar characteristics other than lubricant effects. Green and sintered microstructures were evaluated at different compaction pressures. The defects were quantified by evaluating the fracture surface of ∼90% dense sintered compacts. The samples that contained an internal lubricant had higher green densities and fewer defects at comparable compaction pressures. The internal lubricant did not cause any significant reduction in green strength or increase in springback.
Microhardness and Indentation Fracture of Potassium Dihydrogen Phosphate (KDP)
Authors: Fang Tong , Lambropoulos John C.
Published: July 2010

Potassium dihydrogen phosphate (KDP) is an important electrooptic tetragonal crystal, often used in third harmonic generation in laser systems. We have used microindentation to measure the Vickers and Knoop hardness of KDP and the resulting cracking on (100) and (001) faces. Hardness anisotropy on the (001) face, or among the (100) and (001) faces, was small (∼20%). We observed an indentation size effect for both Vickers and Knoop hardness, for indenting loads in the range 0.24–1.96 N. The large‐load Vickers hardness was estimated as 1.4 ± 0.1 GPa. We observed anisotropy in the crack sizes on (100) and (001) faces. Cracks were longer on (100) faces than on (001) faces. Assuming elastic and plastic isotropy, crack sizes were analyzed, and fracture toughness, Kc, was extracted. We present here an approximate model for analyzing crack‐load microindentation data in tetragonal crystals. The model uses the minimum elastic modulus of the material. The effect of the isotropic assumption on the extracted fracture toughness is estimated at ∼33%, with a 23% contribution resulting from elastic anisotropy and 10% from the slip‐system plastic anisotropy.
Analysis of Surface Tension from Aerodynamic Levitation of Liquids
Authors: Millot Francis , Rifflet Jean Claude , Wille Guillaume , Sarou‐Kanian Vincent , Glorieux Benoit
Published: July 2010

Aerodynamic levitation allows contactless diagnostics of very high‐temperature liquid properties. Liquids can be metals or inorganic compounds, and the levitation gas may have any composition. Temperature limitation is dependent on the heating power and radiative properties of the samples, with 3000°C possible for oxides. Surface‐tension measurement is based on previous theories, which relate it to specific vibrations of the drop. Rotation and precession of liquid drops are specific aspects of aerodynamic levitation, and their effects are quantified here. Experimental examples for nickel and alumina are also shown and compared with results from previous experiments, when available.
Effect of Microstructure on the Thermal Conductivity of Hot‐Pressed Silicon Nitride Materials
Authors: De Pablos Angel , Osendi María Isabel , Miranzo Pilar
Published: July 2010

Si3N4 materials with distinct microstructures were prepared by hot‐pressing, varying the holding time at the maximum temperature, and using different types and amounts of sintering additives. Materials with thermal conductivities of 15–82 W·(m·K)−1 were obtained by changing the processing variables. The highest conductivity was measured for the material with the coarsest microstructure. The effect of microstructural parameters, such as percentage of secondary phases, grain size, and texture on thermal properties of Si3N4 ceramics, were studied. Hot‐pressed Si3N4 ceramics were modeled as a two‐phase composite made of large grains of high conductivity and a small‐grained phase of low conductivity.
CaO‐As2O5‐H2O System at 23°± 1°C
Authors: Jr. Bothe James V. , Brown Paul W.
Published: July 2010

The phase diagram for the ternary system CaO‐As2O5‐H2O at 23°± 1°C is plotted. The calcium arsenates represented are Ca(H2AsO4)2, CaHAsO4·H2O, Ca5H2(AsO4)4·9H2O, Ca3(AsO4)2·xH2O (x= 3⅔ and 4¼), Ca5(AsO4)3OH (apatite), and Ca4(OH)2(AsO4)2·4H2O. The solubility data indicate the following invariant points: Ca(H2AsO4)2–CaHAsO4·H2O–solution, CaHAsO4·H2O–Ca5H2(AsO4)4·9H2O–solution, Ca5H2(AsO4)4·9H2O–Ca3(AsO4)2·4¼H2O–solution, Ca3(AsO4)2·3⅔H2O–Ca4(OH)2(AsO4)2·4H2O–solution, and Ca4(OH)2(AsO4)2·4H2O–Ca(OH)2–solution. The tricalcium arsenate Ca3(AsO4)2·3⅔H2O is shown to be the only hydrate to dissolve congruently.
Hydrothermal Synthesis of Xenotime‐Type Gadolinium Orthophosphate
Authors: Celebi Ahmet S. , Kolis Joseph W.
Published: July 2010

Xenotime‐type GdPO4 (tetragonal, I41/amd) has been synthesized hydrothermally at 400°C. X‐ray powder data have been presented, and the lattice parameters have been calculated as a= 0.6969(2) nm and c= 0.6095(3) nm.
Orange‐ and Violet‐Emitting Long‐Lasting Phosphors
Authors: Fu Jie
Published: July 2010

Orange phosphorescence from Eu3+‐activated CaO and SrO and violet phosphorescence from Pb2+‐activated SrO were clearly observed after the materials had been excited with ultraviolet light. The orange phosphorescence resulted from 5D0 to 7F1 transitions of the Eu3+ ions and the violet from 3P0 to 1S0 transitions of the Pb2+ ions. The phosphorescence lasted >1 h. Trapped‐electron centers and hole centers introduced into the CaO and SrO hosts by the ultraviolet light appear to have been responsible for generating the long‐lasting phosphorescence.
Inorganic Wastes in the Manufacture of Glass and Glass‐Ceramics: Quartz‐Feldspar Waste of Ore Refining, Metallurgical Slag, Limestone Dust, and Phosphorus Slurry
Authors: Gorokhovsky Alexander , Escalante‐Garcia* Jose Ivan , Gorokhovsky Vladilen , Mescheryakov Dmitrii
Published: July 2010

This work presents the results of an investigation on producing diopside glass‐ceramic materials based on various combinations of quartz‐feldspar waste from ore refining and limestone dust. Other industrial wastes were added to contribute as nucleation agents; when the concentration of the latter was insufficient, they were added in the pure state. Additionally, relatively high concentrations of alkalis were used to accelerate glass melting. Low‐cost materials with promising exploitation properties were produced. A two‐stage regime of crystallization of the basic glass composition was determined, including nucleation at 720°C and crystal growth at 950°C. Local environmental regulations must be considered for the selection of the type of admixtures and the combination of wastes.
Nonepitaxial Orientation in Sol–Gel Bismuth Titanate Films
Authors: Fuierer Paul , Li Bo
Published: July 2010

Control over crystallographic orientation in ceramic thin films is important for highly anisotropic structures. Layered perovskites, like Bi4Ti3O12, have interesting properties associated with their ferroelectric nature, which may be fully exploited only when films are highly textured. Textured films of this titanate were fabricated via a sol–gel technique without using epitaxial growth. Orientation in the film is confirmed by XRD and SEM, and supported by refractive index and dielectric measurements. In an attempt to explain the orienting effect, light scattering experiments were conducted to yield information about the molecular size, shape, and conformation of macromolecules as the sol–gel solution ages and condensation reactions proceed. These experiments clearly show an increase in the size of molecular clusters with time. We believe that it is the organization of these large clusters during spin coating, and the relationship of the backbone chemistry to the crystal structure of Bi4Ti3O12, that are responsible for the observed orientation.
Effect of High‐Temperature Treatment on n–p Transition in Titania
Authors: Radecka Marta , Rekas Mieczyslaw
Published: July 2010

This paper reports the electrical conductivity and Seebeck coefficient (thermopower) values derived for TiO2 (rutile) at a temperature range of 923–1353 K and pO2 of 5 Pa–100 kPa. The effect of high‐temperature treatment on the electrical properties of polycrystalline samples is discussed in terms of defect models. Samples sintered at ≥1573 K, as well as the single crystals, show n‐ to p‐type transition, whereas samples sintered at 1473 K remain n‐type throughout testing. No effect of grain size on thermopower is observed. These results are consistent with the defect disorder model assuming that titanium vacancies formed during high‐temperature treatment (1473–1673 K) are frozen at the measuring temperature (923–1323 K).
Does a True Fatigue Limit Exist for Continuous Fiber‐Reinforced Ceramic Matrix Composites?
Authors: Sørensen Bent F. , Holmes John W. , Vanswijgenhoven Eddy L.
Published: July 2010

The high‐cycle high‐frequency fatigue behavior of a Nicalon‐fiber‐reinforced calcium aluminosilicate ceramic composite was investigated. A key goal of the room‐temperature fatigue experiments was to determine if a true fatigue limit or endurance limit existed for this ceramic matrix composite. Although no fatigue failures occurred beyond 107 cycles, the stress–strain hysteresis modulus and frictional heating continued to change up to 108 cycles, at which point the 200 Hz experiments were terminated. This suggests that fatigue damage continued to evolve and that a true fatigue limit may not exist in ceramic matrix composites that have undergone interfacial frictional sliding.
Structural Modulations in Monoclinic Tricalcium Silicate Solid Solutions Doped with Zinc Oxide, M(I), M(II), and M(III)
Authors: Urabe Kazuyori , Nakano Hiromi , Morita Hiroyasu
Published: July 2010

Structural modulations in solid solutions of C3S doped with various amounts of ZnO were investigated via the selected‐area electron diffraction method. Three monoclinic modifications (M(I), M(II), and M(III)) were identified in the solid solutions. The modifications had pseudohexagonal subcells that were modulated to form supercells. Coordinates of the reflections attributable to the supercells could be expressed by the following linear combinations of vectors of the reciprocal lattices: m1(−2a*/5.4 + 2b*/5.4 − 7c*/5.4) for M(I), m2(−a*/5 +b*/5 −c*) for M(II), and m3(−a*/6 +b*/6 + 7c*/6) for M(III), m1 and m2=±1 and ±2, and m3=±1, ±2, and ± 3. All the monoclinic modifications possessed structural modulations of a one‐dimensional type. The modulation waves could be detected as wavy contrasts by high‐resolution transmission electron microscopy.
Microstructural Analysis of Liquid‐Phase‐Sintered β‐Silicon Carbide
Authors: Zhan Guo‐Dong , Ikuhara Yuichi , Mitomo Mamoru , Xie Rong‐Jun , Sakuma Taketo , Mukherjee Amiya K.
Published: July 2010

The microstructures of fine‐grained β‐SiC materials with α‐SiC seeds annealed either with or without uniaxial pressure at 1900°C for 4 h in an argon atmosphere were investigated using analytical electron microscopy and high‐resolution electron microscopy (HREM). An applied annealing pressure can greatly retard phase transformation and grain growth. The material annealed with pressure consisted of fine grains with β‐SiC as a major phase. In contrast, the microstructure in the material annealed without pressure consisted of elongated grains with half α‐SiC. Energy‐dispersive X‐ray analysis showed no differences in the amount of segregation of aluminum and oxygen atoms at grain boundaries, but did show a significant difference in the segregation of yttrium atoms at grain boundaries along SiC grains for the two materials. The increased segregation of yttrium ions at grain boundaries caused by the applied pressure might be the reason for the retarded phase transformation and grain growth. HREM showed a thin secondary phase of 1 nm at the grain boundary interface for both materials. The development of a composite grain consisting of a mixture of β/α polytypes during annealing was a feature common to both materials. The possible mechanisms for grain growth and phase transformation are discussed.
Characterization of Products Obtained during Formation of Barium Monoaluminate through Hydrothermal Precipitation–Calcination Route
Authors: Mishra Devabrata , Anand Shashi , Panda Rama Krushna , Das Radhanath Prasad
Published: July 2010

Nanosized (10–30 nm particle size) hexagonal barium monoaluminate, having a high surface area of ∼30 m2/g, was synthesized by calcination of hydrothermally prepared precursors. The precursors were obtained by hydrolytic precipitation, using a mixed solution of barium and aluminum nitrates in the presence of aqueous urea at 180°C. Based on the results of XRD, FTIR, SEM/EDS, TEM, and TG‐DTA studies, the most probable sequence of reactions leading to the formation of barium monoaluminate was (i) conversion of aqueous barium and aluminum nitrates in the presence of urea by hydrothermal precipitation to crystalline orthorhombic barium carbonate and boehmite, (ii) formation of an interim form consisting of amorphous fibrillar aluminum oxide(s) interlaced with crystalline barium carbonate in the calcination temperature range of ∼500°–800°C, (iii) initiation of formation of barium monoaluminate at 1000°C, and (iv) formation of a near monophase nanosized barium monoaluminate at 1200°C.
Phase Transformation and Texture in Hot‐Forged or Annealed Liquid‐Phase‐Sintered Silicon Carbide Ceramics
Authors: Xie Rong‐Jun , Mitomo Mamoru , Kim Wonjoong , Kim Young‐Wook , Zhan Guo‐Dong , Akimune Yoshio
Published: July 2010

Starting from three powder mixtures of 80 vol% SiC (100α, 50α/50β, 100β) and 20 vol% YAG, liquid‐phase‐sintered silicon carbide ceramics were prepared by hot pressing at 1800°C for 1 h under 25 MPa, and then by hot forging or annealing at 1900°C for 4 h under an applied stress of 25 MPa in argon. The phase transformation and texture development in the as‐hot‐pressed, hot‐forged, and annealed SiC ceramics were investigated via X‐ray diffraction (XRD) and the pole figure measurements. The 6H → 4H polytypic transformation was observed in samples consisting of both α‐ and β‐SiC phases when subjected to compressive deformation but absent in the case of annealing, suggesting the deformation‐enhanced solubility of aluminum in SiC. Deformation was also found to enhance the 3C → 4H transformation in the sample containing entirely β‐phase, which is due to the accelerated solution‐precipitation process assisted by grain boundary sliding. The current study showed that the β‐ →α‐phase transformation had little effect on texture development in SiC. Hot forging generally produced the strongest texture, with the calculated maximum of 2.2 times random in samples started with pure α‐SiC phase. The mechanism for texture development was explained based on the microstructural observations.
Fabrication of Optically Transparent Lead Lanthanum Zirconate Titanate ((Pb,La)(Zr,Ti)O3) Ceramics by a Three‐Stage‐Atmosphere‐Sintering Technique
Authors: Abe Yoshiyuki , Kakegawa Kazuyuki , Ushijima Hiroshi , Watanabe Yoshikazu , Sasaki Yoshinori
Published: July 2010

An easy technique has been developed to fabricate optically transparent lanthanum‐modified lead zirconate titanate (PLZT) ceramics. This technique consists of three stages: (1) sintering in an oxygen atmosphere, (2) elimination of pores in a carbon dioxide atmosphere, and (3) elimination of oxygen vacancies in an oxygen atmosphere. The carbon dioxide atmosphere enhances the diffusion of oxygen from the pores to outside the sintered body. The experimental results reveal that use of a carbon dioxide atmosphere effectively decreases residual pores and improves optical transmittance. From commercially available raw powders, an optical transmittance of 51% (wavelength of 550 nm) can be achieved for 0.7 mm thick polished PLZT9/65/35 ceramics using a carbon dioxide atmosphere, whereas the value is only 34% without a carbon dioxide atmosphere. The advantage of this technique is that PLZT ceramics having high optical quality can be obtained using conventional sintering tools.
Synthesis of Mullite Whiskers
Authors: Choi Heon‐Jin , Lee June‐Gunn
Published: July 2010

Mullite whiskers were synthesized by heating a mixture of SiO2 and silicon in an alumina tube reactor under a flow of H2 and CF4. The length and diameter of the whiskers were several hundred micrometers and >15 μm, respectively. It was postulated that a vapor phase reaction between SiF4 and AlF3 made possible the synthesis of the large mullite whiskers.
Reactivity of Aluminum Nitride Powder in Aqueous Silicon Nitride and Silicon Carbide Slurries
Authors: Krnel Kristoffer , Kosma[c‐breve] Tomaž
Published: July 2010

The reactivity of AlN powder with water in supernatants obtained from centrifuged Si3N4 and SiC slurries was studied by monitoring the pH versus time. Various Si3N4 and SiC powders were used, which were fabricated by different production routes and had surfaces oxidized to different degrees. The reactivity of the AlN powder in the supernatants was found to depend strongly on the concentration of dissolved silica in these slurries relative to the surface area of the AlN powder in the slurry. The hydrolysis of AlN did not occur if the concentration of dissolved silica, with respect to the AlN powder surface, was high enough (1 mg SiO2/(m2 AlN powder)) to form a layer of aluminosilicates on the AlN powder surface. This assumption was verified by measuring the pH of more concentrated (31 vol%) Si3N4 and SiC suspensions also including 5 wt% of AlN powder (with respect to the solids).
In Situ Surface Modification of Silicon Carbide Particles Using Al3+ Complexes and Polyelectrolytes in Aqueous Suspensions
Authors: Zhang Yongheng , Binner Jon
Published: July 2010

Silicon carbide (SiC) particles were modified in situ, using Al3+ complexes in aqueous media, via control of Al(NO3)3concentration and pH. The Al3+ formed hydrolyzed complexes that adsorbed onto the charged sites on the SiC particle surfaces. As a result, the surface‐modified SiC particles behaved in an alumina‐like manner in the approximate pH range 5–8. The modified SiC particles were further treated with two types of polyelectrolytes that were sequentially adsorbed onto the particle surface, to give a maximum surface charge. As a result of this surface‐modification process, the SiC could be dispersed with Al2O3in aqueous media without heteroaggregation.
Humidity Influence on the CO2Response of Potentiometric Sensors Based on NASICON Pellets with New Compositions, Na3Zr2−(x/4)Si2−xP1+xO12(x= 1.333)
Authors: Aono Hiromichi , Sadaoka Yoshihiko , Montanaro Laura , Di Bartolomeo Elisabetta , Traversa Enrico
Published: July 2010

CO2‐gas sensors were fabricated using NASICON‐type dense ceramics with new compositions in the Na3Zr2−(x/4)Si2−xP1+xO12system. The bulk ceramics were prepared from sol‐gel‐processed powders. Dense electrolytes were obtained for compositions with x= 0.667 and x= 1.333. An improved sinterability was observed with respect to NASICON with conventional composition, which was attributed to liquid‐phase sintering. The CO2‐gas sensors using these dense samples showed a stable electromotive force (emf) response in dry atmosphere that was similar to the theoretical Nernstian value for a two‐electron electrochemical reaction. This was observed also for the NASICON sample with composition x= 1.333 that showed a conductivity far lower than that of the NASICON with conventional composition. The emf changed quickly with changes of the CO2‐gas concentration, and steady‐state values were observed. The response time, in adsorption and desorption of CO2, was very fast, especially at high CO2concentrations. The influence of humidity on the CO2‐sensing performance was investigated. A lower sensitivity and slower response were obtained in humid CO2gas, especially at low CO2concentrations. CO2‐sensing measurements at various gas‐flow rates were performed to evaluate the reactions occurring at the measuring electrode. The occurrence of a side reaction on the measuring electrode was observed, i.e., the formation of sodium oxides. In dry gas, the reaction took place at low CO2concentrations and small flow rates, whereas the reaction was strongly enhanced in humid environments and occurred over the entire CO2concentration range. However, the sensor performance recovered after switching from humid gas to dry gas. This demonstrated that the humidity affects the emf because of the Na2Oxformation at the electrode, and, thus, the solid electrolyte itself was not degraded by humidity.
The Wulff Shape of Alumina: III, Undoped Alumina
Authors: Kitayama Mikito , Glaeser Andreas M.
Published: July 2010

Controlled‐geometry cavities were introduced into the m{100} plane of undoped sapphire substrates using photolithographic methods, and subsequently internalized by diffusion bonding the etched sapphire to an undoped high‐purity polycrystalline alumina. Pore‐boundary separation during growth of the sapphire seed into the polycrystal entrapped the pores within the single crystal. Pores with an equivalent spherical radius of ≈1 μm reached a quasi‐equilibrium shape after prolonged anneals at 1600° and 1800°C. The introduction of mechanically induced surface defects accelerated pore shape equilibration. The Wulff shape of undoped alumina was determined by characterizing the shape and facet structure of these equilibrated internal pores using optical microscopy, scanning electron microscopy, and atomic force microscopy. The observed planes in the Wulff shape of undoped alumina, c(0001), r{012}, s{101}, a{110}, and p{113} planes, were consistent with those reported by Choi et al.; however, a different energy sequence is inferred. The absence of the m‐plane in the Wulff shape is consistent with other experimental studies, but inconsistent with those lattice simulations that predict the m‐plane to be one of the lowest energy planes in pure alumina. A comparison of Wulff shapes at 1600° and 1800°C suggests that the surface energy of undoped alumina becomes more isotropic as temperature increases.
Synchrotron Radiation Diffraction Study of the Microstructure Changes in Cement Paste due to Accelerated Leaching by Application of Electrical Fields
Authors: Castellote Marta , Andrade Carmen , Alonso Cruz , Turrillas Xavier , Kvick Åke , Terry Ann , Vaughan Gavin , Campo Javier
Published: July 2010

An external dc voltage was applied to cured cement pastes to simulate their natural degradation over time; i.e. migration of ions and dissolution of solid phases. The presence of crystalline phases before and after applying a voltage was quantified by powder diffraction using synchrotron X‐ray radiation. The results were refined by the Rietveld method. The specimens were also analysed by mercury intrusion porosimetry. Results indicated that a portion of the crystalline phases dissolved and later precipitated as amorphous phases in smaller pores. This accelerated treatment led to the formation of a larger proportion of amorphous phases in the specimens studied.
X‐ray Diffraction and 151Eu Mössbauer Spectroscopic Study of the Hf1−xEuxO2−x/2(0 ≤x≤ 1.0) System
Authors: Harada Daijitsu , Hinatsu Yukio , Masaki Nobuyuki , Nakamura Akio
Published: July 2010

Powder X‐ray diffractometry (XRD) and 151Eu Mössbauer spectroscopy were performed for samples prepared in the temperature range 1500–1500°C for the hafnia–europia (HfO2–Eu2O3) system EuxHf1−xO2−x/2(0 ≤x≤ 1.0). The XRD results showed that two types of solid solution phases formed in the composition range 0.25 ≤x≤ 0.725: an ordered pyrochlore‐type phase in the middle‐composition range (0.45 < x < 0.575) and a disordered fluorite‐type phase, enveloping the pyrochlore‐type phase on both sides, in the composition ranges 0.25 ≤x≤ 0.40 and 0.60 ≤x≤ 0.725. 151Eu Mössbauer spectroscopy sensitively probes local environmental changes around trivalent europium (Eu3+) caused by the formation of these solid solution phases. In addition to the broad, single Mössbauer spectra observed in this study for the Eu3+, the values of isomer shift (IS) exhibited a pronounced minimum in the neighborhood of the stoichiometric pyrochlore phase (x≈ 0.5). Such IS behavior of Eu3+ was interpreted based on the XRD crystallographic information that the ordered pyrochlore phase has a longer (in fact, the longest) average Eu–O bond length than those of the disordered fluorite phases on both sides or the monoclinic (and C‐type) Eu2O3at x= 1.0.
Phase Transformation during Plasma Spraying of Hydroxyapatite–10‐wt%‐Zirconia Composite Coating
Authors: Chou Bang‐Yen , Chang Edward , Yao Shyue Yen , Chen Jen Min
Published: July 2010

This study aims to explore phase transformation in plasma‐sprayed hydroxyapatite (HA) + 10 wt% ZrO2–8‐mol%‐Y2O3composite coating, using separately prepared HA and ZrO2–8‐mol%‐Y2O3coatings as a control. Changes in the phase and chemistry of the coatings are characterized by X‐ray diffractometry, with lattice‐constant measurement (Cohen's method), and by transmission electron microscopy. Experimental results show evidence of diffusion, in the liquid state, of calcium ions from the HA matrix into the ZrO2. This behavior causes the formation of the following structural features in the composite coating: (i) a CaO‐doped ZrO2solid solution (ZrO2–7.7 mol% Y2O3–4.4 mol% CaO); (ii) a mixture of ZrO2and CaZrO3having a crystal‐orientation relationship; (iii) an amorphous phase containing elements of calcium, phosphorus, zirconium, and yttrium; and (iv) a remaining CaO‐poor HA matrix (Ca10−x(HPO4)x(PO4)6−x(OH)2−x; x= 0.06). Rationales for the greatly decreased impurity phases of CaO and Ca4P2O9found in the composite coating are discussed.
Coarsening of Faceted Crystals
Authors: Rohrer Gregory S. , Rohrer C. Lane , Mullins William W.
Published: July 2010

The influence of the nucleation energy barrier on the capillary‐driven coarsening of faceted crystals that exchange material by diffusion is quantified. Our calculations are based on the assumption that the transport of material between particles must happen in series with the nucleation of partial layers on flat facets. Using a numerical model based on this idea, we simulate the time evolution of distributions of crystals that are made up of perfect faceted crystals (without step‐producing defects), crystals containing step‐producing defects, and mixtures of the two types. We find that the coarsening of a distribution containing only perfect faceted crystals is arrested at a size where the nucleation energy barrier becomes prohibitive. This critical size ranges from a few nanometers to several hundred nanometers, depending on material parameters and experimental conditions. When a small fraction of the crystals have step‐producing defects (for these crystals the nucleation energy barrier vanishes), they can grow to large sizes at the expense of the perfect crystals and a bimodal grain size distribution is created. Based on these results, we hypothesize that when abnormal coarsening is observed in nature, it results from the presence of a small number of crystals with step‐producing defects.
Mirror Constant for Tyranno® Silicon‐Titanium‐Carbon‐Oxygen Fibers Measured in Situ in a Three‐Dimensional Woven Silicon Carbide/Silicon Carbide Composite
Authors: Davies Ian J. , Ishikawa Takashi
Published: July 2010

The strength, S, of ceramic and glass fibers often can be estimated from fractographic investigation using the fracture mirror radius, rm, and the relationship S=Am/(rm)1/2, where Amis the “mirror constant.” The present work estimates the value of Amfor Tyranno® Si‐Ti‐C‐O fibers in situ in a three‐dimensional woven SiC/SiC‐based composite to be 2.50 ± 0.09 MPa·m1/2. This value is within the range of 2–2.51 MPa·m1/2 previously obtained for nominally similar Nicalon® Si‐C‐O fibers.
Transmutation of Actinides
Authors: Fernández Asunción , Haas Didier , Konings Rudy J. M. , Somers Joseph
Published: July 2010

An advanced process has been developed on a laboratory scale at the Institute for Transuranium Elements (ITU) for the fabrication of transmutation fuels and targets. The process consists of the fabrication of highly porous beads using a sol–gel method, their infiltration by a nitrate solution of the relevant actinide, and calcination, compaction, and sintering to give the final product pellets. This work presents the results obtained in the qualification of this process whereby dense partially‐yttria‐stabilized zirconia pellets with high contents of cerium and plutonium have been fabricated.
Aluminum Nitride Fibers Synthesized from Alumina Fibers Using Gas‐Reduction–Nitridation Method
Authors: Suehiro Takayuki , Tatami Junichi , Meguro Takeshi , Komeya Katsutoshi , Matsuo Shuitsu
Published: July 2010

Aluminum nitride fibers were successfully synthesized from alumina fibers using an NH3–C3H8gas mixture as a reduction–nitridation agent. Observation using SEM clearly demonstrated that the morphology of the nitrided fibers was exactly the same as that of the raw alumina fibers, retaining the original regular shape and smooth surface. Up to 95% of the starting alumina was converted to aluminum nitride at 1400°C within 0.5 h via a single‐step synthesis process.
Hydrothermal Synthesis of Tetragonal Barium Titanate from Barium Chloride and Titanium Tetrachloride under Moderate Conditions
Authors: Xu Huarui , Gao Lian , Guo Jingkun
Published: July 2010

Tetragonal BaTiO3powders were prepared hydrothermally at 240°C, in only 12 h, using BaCl2·2H2O and TiCl4, which are rather easy to manipulate. Characterization via X‐ray diffractometry, scanning electron microscopy, Brunauer–Emmett–Teller analysis, and differential scanning calorimetry confirmed that increasing the NaOH excess concentration (from 0.5M to 2.0M) and decreasing the initial TiCl4concentration (from 0.625M to 0.15M) promotes the formation of tetragonal BaTiO3powders. After reaction, the powders were proved to be phase‐pure BaTiO3, with no impurities, such as Cl− and CO32−.
Dense, Near Net‐Shaped, Carbide/Refractory Metal Composites at Modest Temperatures by the Displacive Compensation of Porosity (DCP) Method
Authors: Dickerson Matthew B. , Snyder Robert L. , Sandhage Ken H.
Published: July 2010

Dense, near‐net‐shaped, carbide‐rich, carbide/tungsten composites have been produced at modest temperatures by the displacive compensation of porosity (DCP) method. Porous, freestanding WC preforms were infiltrated and reacted with a Zr2Cu liquid at 1300°C and ambient pressure. The carbon in the WC was displaced by zirconium in the melt to yield ZrCxand tungsten. The increase in solid volume associated with this displacement reaction was compensated by the prior pore volume of the preform. Dense ZrCx/WC/W composites that possessed <5 vol% residual copper‐bearing phase, and retained the shape and dimensions of the preform, were produced.
Effect of Externally Applied Plasticizer on Compaction Behavior of Spray‐Dried Powders
Authors: Balasubramanian Sreeram , Shanefield Daniel J. , Niesz Dale E.
Published: July 2010

The effects of an externally applied plasticizer on compaction behavior and green microstructure quality of spray‐dried powders was investigated. The plasticizer was applied to the external surfaces of already spray‐dried powders by spraying it on tumbling spray‐dried granules. The apparent yield point of the spray‐dried powder was reduced when the plasticizer was added. Microstructures of compacts made from these granules (with and without the externally applied plasticizer) were compared at different compaction pressures. Better knitting across granule interfaces and fewer defects were obtained for the granules with the externally applied plasticizer.
Effects of Additives on the Interfacial Microstructure of Cofired Electrode‐Ceramic Multilayer Systems
Authors: Zuo Ruzhong , Li Longtu , Gui Zhilun , Hu Xiaobing , Ji Chengxiang
Published: July 2010

An investigation of the cofired behavior between silver/palladium electrode paste and lead‐based relaxor ferroelectric ceramics demonstrated that mismatched sintering shrinkage was caused by differential sintering kinetics. Improvement of the cofired characteristics was obtained by adding different ceramic powders into the electrode paste. The sintering kinetics analyses show that the roles of ceramic additives lie in changing the densification mechanism of the silver/palladium electrode paste. Scanning electron microscopy observations indicate that ceramic additives apparently improve the interfacial bonding by the formation of chemical reaction layers. Compared to BaTiO3‐based ceramic additives, which are caused by palladium‐lead‐based reactions analyzed from the X‐ray diffraction spectrum, lead‐based perovskite additives will degrade the electronic properties and decrease the melting point of the silver/palladium electrode to a certain extent. Criteria for choosing ceramic additives have been proposed in this paper.
Synthesis of Nanostructured Silicon Carbide through an Integrated Mechanical and Thermal Activation Process
Authors: Ren Ruiming , Yang Zhenguo , Shaw Leon L.
Published: July 2010

Changes of crystal structures and microstructures of SiO2 and graphite powder mixtures induced by high‐energy milling, the effect of these changes on the reactivity of reactants, and the mechanism of enhanced SiC formation have been studied using a variety of analytical instruments, including X‐ray diffractometry, scanning electron microscopy, transmission electron microscopy, solid‐state 29Si nuclear magnetic resonance, and nitrogen adsorption (i.e., the BET method). High‐energy milling before carbothermic reduction leads to substantial changes in the structural and energy states of the reactants, which in turn increases the reactivity of the reactants and enhances the formation of nanostructured SiC particles. Furthermore, the structural and energy‐state changes contribute to the enhanced SiC formation through the increased reaction kinetics as well as the increased reaction driving force.
Use of Titanates to Achieve a Temperature‐Stable Low‐Temperature Cofired Ceramic Dielectric for Wireless Applications
Authors: Dai Steve Xunhu , Huang Rong‐Fong , Wilcox David L.
Published: July 2010

A low‐loss and near‐zero temperature coefficient of resonant frequency (Tf) low‐temperature cofired ceramic (LTCC) host dielectric was developed for portable consumer wireless device applications. The low Tf was realized by compensating the Al2O3‐filled‐glass dielectric with admixtures of TiO2 (negative temperature coefficient of dielectric constant (Tɛ)) in the starting formulation. XRD data indicated a portion of the TiO2 in the starting formulation dissolved into the glass, and extensive formation of crystalline titanium compounds was observed via a nucleation and growth mechanism. The dissolution of TiO2 in the glass and subsequent formation of titanium compounds was believed to result in the relatively small amount of TiO2 required to achieve a near‐zero Tf in the final sintered structure.
Gibbs Energy Change of Carbothermal Nitridation Reaction of Al2O3 to Form AlN and Reassessment of Thermochemical Properties of AlN
Authors: Nakao Wataru , Fukuyama Hiroyuki , Nagata Kazuhiro
Published: July 2010

The experimental method for the high‐temperature reaction equilibria in the AlN‐Al2O3 system has been established. The equilibrium N2‐CO gas compositions coexisting with AlN‐ Al2O3‐graphite have been successfully measured by quadrupole mass spectrometry and gas chromatography. From the obtained results, the standard Gibbs energy change of the forming reaction of AlN by carbothermal nitridation is determined at temperatures ranging from 1723 to 1899 K:

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From the obtained result, the standard Gibbs energy of formation of AlN and the third‐law enthalpy of formation of AlN at 298.15 K are derived as

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The disagreement between the present results and values in the NIST–JANAF thermodynamic table is discussed.
Physical Properties and Structure of rf‐Sputtered Amorphous Films in the System Al2O3–Y2O3
Authors: Fukuda Kentaro , Hanada Teiichi , Tanabe Setsuhisa , Yao Takeshi
Published: July 2010

Amorphous films in the system Al2O3–Y2O3 were prepared by the rf sputtering method in the range of 0–76 mol% Y2O3, and their density, refractive index, and elastic constants were measured. All of the physical properties of the amorphous Al2O3–Y2O3 films had a similar compositional dependence; that is, they increased continuously, but not linearly with increasing Y2O3 content. To confirm the coordination states of aluminum and yttrium ions in the amorphous Al2O3–Y2O3 films, the AlKα X‐ray emission spectra and the X‐ray absorption near edge structures (XANES) were measured. The average coordination number of aluminum ions in the amorphous films containing up to about 40 mol% Y2O3 content was 5, that is a mixture of 4‐fold‐ and 6‐fold‐coordinated states. In the region of more than about 50 mol% Y2O3, the fraction of the 6‐fold‐coordinated aluminum ions increased with increasing Y2O3 content, while the results led to the conclusion that the coordination number of yttrium ions was always 6, regardless of composition. These results indicate that, in amorphous films in the system Al2O3–Y2O3, the change of the coordination state of aluminum ions has an important effect on physical properties.
Stoichiometry of Slag Hydration with Calcium Hydroxide
Authors: Richardson John M. , Biernacki Joseph J. , Stutzman Paul E. , Bentz Dale P.
Published: July 2010

The stoichiometry of the reaction between ground granulated blast furnace slag (GGBFS) having an empirical formula of C7.88S7.39M3A and calcium hydroxide (CH) was investigated. Scanning electron microscopy (SEM) was used to determine the slag consumption as well as the Ca/Si ratio in calcium silicate hydrate gel (C‐S‐H) products. A tentative stoichiometric ratio of 2.6 mol of CH consumed per mole of slag reacted was determined using two methods. By combining consumption data determined separately for slag and CH a molar stoichiometry of 2.79 was found. Similarly, by directly determining the Ca/Si ratio in the C‐S‐H gel product, a range for the molar stoichiometry between 1.65 and 3.42 was found. Finally, a comparison of the stoichiometry of the slag/CH reaction was made with slag/portland cement hydration. The basic features of both appear similar. In the C‐S‐H gel around slag grains, a calcium‐to‐silica ratio of 1.3 to 1.4 was found for both slag/CH and slag/cement systems.