bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct and Indirect Dissolution of Sapphire in Calcia–Magnesia–Alumina–Silica Melts: Dissolution Kinetics
Authors: Sandhage Kenneth H. , Yurek Gregory J.
Published: March 2005

The dissolution rate of sapphire into CaO‐MgO‐Al2O3‐SiO2 (CMAS) melts was investigated at 1450° and 1550°C. The effects of immersion time, specimen rotation rate, and magnesia concentration in the bulk melt on the sapphire dissolution rate and on the growth rate of magnesium aluminate spinel at the sapphire/melt interface were determined. The processes of direct and indirect sapphire dissolution into CMAS melts are discussed, with the support of kinetic and microprobe data.1 A model of indirect sapphire dissolution1,2 is used along with the equations of Tedmon3 to describe the kinetics of indirect dissolution and spinel layer growth during forced‐convective and static sapphire dissolution.
Indirect Dissolution of Sapphire into Calcia‐Magnesia‐Alumina‐Silica Melts: Electron Microprobe Analysis of the Dissolution Process
Authors: Sandhage Kenneth H. , Yurek Gregory J.
Published: March 2005

Spinel, MgAl2O4, has been observed to form on sapphire during sapphire dissolution into CaO‐MgO‐Al2O3‐SiO2 (CMAS) melts at 1450°. and 1550°C. Electron microprobe analysis was used to characterize the sapphire/melt interface for cases in which spinel did (indirect dissolution) or did not (direct dissolution) form on the sapphire during dissolution into CMAS melts. The concentrations of Al2O3, MgO, CaO, and SiO2 were determined as a function of position within the spinel reaction product and in the adjacent melt. The rate‐limiting steps for direct and indirect sapphire dissolution into CMAS melts are discussed.
Sodium Self‐Diffusion in Sodium Disilicate Single Crystals
Authors: Heinemann Irmhild , Frischat Günther H.
Published: March 2005

α‐Na2Si2O5 single crystals were prepared in suitable sizes and 22Na diffusion was studied at 250° to 600°C. At 400°C, D⋆Na∽ 3 × 10−9 cm2· S−1 in the direction of the c‐axis. The diffusion in the direction of the a‐axis is about 2 orders of magnitude lower, and in the direction of the b‐axis it is not detectable at all under the conditions used. Na diffusion in a glass of Na2Si2O5 composition is about 1 order of magnitude faster than that in the direction of the c‐axis of the crystal.
Crystallization of Fine, Chemically Prepared Lead Lanthanum Zirconate Titanate Powders at Low Temperature
Authors: Yoshikawa Yoshio , Tsuzuki Kaoru
Published: March 2005

Fine, homogeneous lead lanthanum zirconate titanate (PLZT) powder prepared by the hydrolysis of an aqueous nitrate solution with ammonia gas exhibits high reactivity when calcined. The minimum temperature for crystal formation in PLZT is 400°C. The kinetics of crystallization between 400° and 600°C are studied isothermally by XRD. The isothermal crystallization process is best fitted to a first‐order equation derived for a random, nucleation‐controlled system on a large number of small particles with an activation energy of 297 kJ/mol. Characterization of the powder by XRD line‐broadening and BET surface‐area examination in the range 400° to 500°C shows a microstructure consisting of ∼20‐nm subunits agglomerated into ∼60‐nm particles. Each subunit evidently generates a single nucleus, which grows within that subunit.
Effect of Grain Size and Poling on the Fracture Mode of Lead Zirconate Titanate Ceramics
Authors: Kim Sang‐Beom , Kim Doh‐Yeon , Kim Jeong‐Joo , Cho Sang‐Hee
Published: March 2005

The fracture mode of lead zirconate titanate (PZT) ceramics at room temperature is investigated as a function of grain size. The fracture mode is almost intergranular at small grain size and changes to predominantly transgranular at large grain size. Application of an electric field (poling) causes a change in the fracture mode from transgranular to intergranular. The observed fracture behavior of PZT ceramics is explained in relation to a possible domain structure.
Reaction of Iradium with Metal Carbides in the Temperature Range of 1923 to 2400 K
Authors: Strife James R. , Smeggil John G. , Worrell Wayne L.
Published: March 2005

The reactions of titanium carbide and hafnium carbide with iridium have been studied in thin film couples fabricated by vapor deposition processes. The reaction product layers after exposure in the temperature range of 1923 to 2400 K are dependent on the stoichiometry of the metal carbide layers and range from simple solid solutions to MIrxcompounds. The observed microstructures are predictable from available thermochemical data. The morphology of residual carbon in the reacted metal carbide‐iridium product layer varies from interfacial deposits to uniform carbon dispersion and depends upon exposure temperature and metal carbide stoichiometry.
Formation Kinetics of Calcium Aluminates
Authors: Singh Vipin K. , Ali Mohammad M. , Mandal Upendra K.
Published: March 2005

The kinetics of formation of calcium aluminates was studied by firing the reaction mixes in the temperature range 12000° to 1460°C for reaction times from 15 to 360 min. Phases formed were determined by taking X‐ray diffractograms of the samples. It was observed that all stable calcium aluminates were formed and that monocalcium aluminate (CA) grew with calcium dialuminate (CA2) in a 1:2 reaction mix of CaO and Al2O3. CA reacted further with Al2O3 to form CA2. The formation of CA2 obeyed the rate law equation 1 ‐ (1 ‐ x)1/3=Kt/r2. The activation energy for the system (140 kJ·mol−1 (33.4 kcal · mol−1)) was determined by the Arrhenius equation.
Plasma Nitriding of Titanium Particles in a Fluidized Bed Reactor at a Reduced Pressure
Authors: Okubo Tatsuya , Kawamura Hiroyuki , Kusakabe Katsuki , Morooka Shigeharu
Published: March 2005

A fluidized bed reactor operated at a reduced pressure is used to modify the surface property of particles. Titanium particles of 46‐ to 74‐μm diameter are fluidized by a nitrogen gas flow of 0.35 cm3‐std/s at a pressure of about 100 Pa, and a nitride layer is formed with the assistance of a radio‐frequency plasma discharge. Modified particles are characterized with elementary analysis and XPS. The nitriding rate is enhanced by the presence of the nitrogen plasma, and the kinetics obey a parabolic law.
Effect of Particle Dispersion on the Mechanism of Combustion Synthesis of Titanium Silicide
Authors: Trambukis Joanne , Munir Zuhair A.
Published: March 2005

The combustion synthesis (SHS) of titanium silicide (Ti5Si3) was investigated. The main combustion reaction (thermal explosion) occurs in the presence of a liquid phase and is usually preceded by a relatively small reaction in the solid state. The influence of Ti particle size on the dominance of each of these types of reactions was investigated. SEM observations and X‐ray diffraction analyses indicate that the combustion synthesis of Ti5Si3 proceeds according to the following sequence: TiSi2→ TiSi → Ti5Si4→ Ti5Si3.
Source of Water and Its Effect on Tape Casting Barium Titanate
Authors: Becker Robert E. , Cannon W. Roger
Published: March 2005

The source of water and its effect on rheological and green tape properties of tape cast barium titanate were studied. The total water content, as well as the source, affect the slip rheology, green density, tensile strength, and strain to failure in the unsintered body. Sources of the water were broken down into contributions from the barium titanate powder surface versus from the solvent or ceramic additives. Water adsorbed on the powder surface was generally beneficial compared to water entering through the solvent. For carefully dried systems, below 0.15 wt% total water, properties change dramatically with small changes in water content. Thus, less stringent processing requirements can be used to achieve reproducibility at water contents above 0.15 wt%.
Size Effect in Fracture of Ceramics and Its Use To Determine Fracture Energy and Effective Process Zone Length
Authors: Bažant Zdeněk P. , Kazemi Mohammad T.
Published: March 2005

The paper shows that a previously proposed size effect law can be used to identify nonlinear fracture properties solely from measured maximum loads of geometrically similar ceramic fracture specimens of sufficiently different sizes. This law represents a first‐order global approximation of the deviations from linear elastic fracture mechanics, independent of the type of the toughening mechanism in the fracture process zone. It provides a simple and unambiguous way to determine the size‐ and shape‐independent values of the fracture energy the effective length of the process zone, and the effective crack‐tip opening displacement. It also yields the R curve, which is geometry (shape) dependent. The proximity of response to linear elastic fracture mechanics is characterized by a brittleness number, which is shape independent.
Mechanism of Material Removal from Silicon Carbide by Carbon Dioxide Laser Heating
Authors: DeBastiani Duane L. , Modest Michael F. , Stubican Vladimir S.
Published: March 2005

The mechanism of material removal from SiC by CO2 laser heating was studied using sintered and single‐crystal α‐SiC. Removal rate and width of the groove showed maxima when plotted as a function of translation speeds. Groove depth decreased as the translation speed of samples increased. Similar results were obtained if argon or air was used as gas assist, which indicated that the material removal mechanism is induced dissociation of SiC. Microstructure of the material deposited in and outside of the groove was studied by SEM. At low scanning speeds, columnar grains 10 to 50 μm long appeared. As the scanning speed increased, columnar grains became smaller and finally only irregular polycrystalline particles were observed. By using Raman spectroscopy, Auger analysis, and X‐ray diffraction, phases inside and outside the groove were identified as Si, β‐SiC, C, and SiO2. Columnar grains were identified as β‐SiC covered with thin layers of C, Si, and SiO2. Slow scanning speeds enhanced the growth of β‐SiC. At slow scanning speed, free silicon was always found in the grooves of lased single crystals but not in the grooves of lased sintered SiC. It can be concluded that the mechanism of material removal from silicon carbide by CO2 laser heating is a vaporization process, and material found in the groove and on the surface near the groove is formed by condensation from the vapor.
Hydration Reactivity of Crystalline and Vitrified Diopside under Hydrothermal Conditions
Authors: Grzeszczyk Stefania , Szuba Jadwiga
Published: March 2005

Hydration reactivity of diopside in both the crystalline and amorphous (glassy) phase was studied under hydrothermal conditions. Samples were treated in an autoclave at 200°C in saturated vapor for 24 and 72 h. The progress of hydration was determined by X‐ray powder diffractometry and IR spectroscopy. Results indicated that crystalline diopside possessed poor hydraulic activity. However, once vitrified it proved to be much more reactive. The principal hydration products found for the glassy diopside after 24 and 72 h of treatment were calcium silicate hydrate (xonotlite) and magnesium silicate hydrates (chrysotile and tremolite).
Synthesis of Monodispersed Spherical β‐Silicon Carbide Powder by a Sol‐Gel Process
Authors: Hatakeyama Fumio , Kanzaki Shuzo
Published: March 2005

Monodispersed spherical β‐SiC powder was synthesized by heating spherical gel powder derived from the hydrolysis of a mixture of phenyltriethoxysilane and tetraethyl orthosilicate. The solution was prepared in a beaker and hydrolyzed by NH4OH without stirring. The monodispersed spherical gel powder of submicrometer size was obtained when the added amount of NH4OH was greater than 16 moles per mole of silane plus alkoxide, and it became monodispersed spherical β‐SiC powder by heat‐treating at 1500°C for 4 h in an Ar atmosphere. The SiC content of the powder was 92.6 wt%.
Crack Spacing in Brittle Films on Elastic Substrates
Authors: Thouless M. D.
Published: March 2005

An approximate, analytical solution is derived for the minimum spacing that can exist between a series of parallel cracks propagating in a thin film. The analysis is pertinent to the specific case of a film and substrate having identical elastic properties. In the absence of plasticity, only three parameters govern the minimum crack spacing. The cracks are predicted to be more closely spaced if the film stress or the film thickness is increased, or if the fracture toughness of the film is decreased.
Influence of Batch Moisture and Atmosphere on Melting Behavior of Lead Oxide‐Containing Glass
Authors: de Kever Ebba , Frischat Guenther H.
Published: March 2005

Melting behavior in the sand‐potash‐lead oxide system was found to depend strongly on the amount of H2O added to the batch (between 0.1 and 9.0 wt%) and also on the melting atmosphere (air, N2, and air loaded with 30 vol% H2O). Pb3O4 and PbO (red), which were used as raw materials, behave similarly. The newly formed compounds are Pb3O4 (formed only when PbO (red) was used as a raw material), PbO (yellow), and K2Pb2Si2O7. The addition of 2 wt% As2O3 to the batch influences the processes of formation and dissolution of compounds only in some cases.
Grain Size Control of Tetragonal Zirconia Polycrystals Using the Space Charge Concept
Authors: Hwang Shyh‐Lung , Chen I‐Wei
Published: March 2005

Grain growth kinetics and grain‐boundary segregation of 12Ce‐TZP and 2Y‐TZP, containing divalent to pentavalent cationic dopants, were studied. In all cases, normal grain growth follwing the parabolic growth relation was observed at higher temperatures. The mobility of grain boundaries was suppressed by the addition of divalent and trivalent cations, unchanged or enhanced by the addition of tetravalent and pentavalent cations. Larger cations have a stronger effect in suppressing grain growth. From ESCA, AES, and STEM analysis of the near grain‐boundary regions, it is further concluded that only divalent and trivalent cations segregate. These observations can be satisfactorily rationalized using the space charge concept and the model of impurity drag.
Thermodynamic Theory of Single‐Crystal Lead Titanate with Consideration of Elastic Boundary Conditions
Authors: Rossett George A. , Udayakumar K. R. , Haun Michael J. , Cross L. Eric
Published: March 2005

A phenomenological free‐energy function including the effects of elastic boundary conditions was presented and used to investigate the single‐domain, single‐crystal properties of the ferroelectric perovskite, PbTiO3. In particular, the effects of tensile and compressive hydrostatic stress on the spontaneous polarization, Curie point, dielectric susceptibility, and piezoelectric property coefficients were examined. The calculated shift of the Curie point with hydrostatic stress, along with the entropy and enthalpy of the ferroelectric‐paraelectric phase transition, was found to be in good agreement with experimental measurements. The isothermal variation of the relative dielectric susceptibility and piezoelectric coefficients with hydrostatic stress exhibited the expected behavior near the ferroelectric‐paraelectric phase transition.
Preparation of High‐Purity Vitreous B2S3
Authors: Martin Steve W. , Bloyer Donald R.
Published: March 2005

Preparation schemes for high‐purity vitreous B2S3 (v‐B2S3) with a S:B ratio of 1.478 ± 0.032 are described. Preparation schemes reported in the literature are shown by very sensitive FT‐IR measurements to yield v‐B2S3 with significant oxygen contamination. Through extreme care taken in carbonizing silica reaction tubes and through the use of very high purity starting materials, oxygen contamination of less than 1% has been achieved. The IR spectrum for very pure v‐B2S3 contains only two absorption bands.
Traveling Reaction Zone Method for Preparation of Textured Ceramic Superconductor Thick Films
Authors: Richardson Thomas J. , De Jonghe Lutgard C.
Published: March 2005

Textured thick films of superconducting YBa2Cu3O7−x (YBCO) have been prepared on ceramic substrates using a traveling reaction zone method. The technique utilizes the rapid reaction between Y2Cu2O5 and BaCuO2 to form YBCO as the film passes through a steep temperature gradient furnance. The films consist of a single continuous superconducting phase with strong c‐axis orientation normal to the translation direction of the film.
Preparation and Use of a Boron Nitride Precursor as Binder for the Densification of Boron Nitride
Authors: Maya Leon , Angelini Peter
Published: March 2005

A precursor of boron nitride was prepared through the partial condensation of 2,4,6‐trichloroborazine and bis‐(trimethylsilyl)acetylene. This reaction was conducted at 100°C and is catalyzed by AlCl3. The condensation product pyrolyzed at 800°C, producing trimethylsilyl chloride as a volatile product and a boron nitride rich residue containing 54 wt% of the initial weight. Mixtures of the precursor and commercial boron nitride were made and hot‐pressed at 800°C and 27.6 MPa. A maximum density of 1.84 g/cm3 is reached at a loading corresponding to the deposition of 13 wt% residue derived from the precursor. Examination by analytical electron microscopy, including X‐ray energy dispersive spectroscopy and electron energy loss spectroscopy analyses, revealed the location of material derived from the precursor in BN‐binder composites through the presence of residual aluminum, silicon, and carbon. Crystallization of boron nitride from the precursor appears to have taken place, as deduced from the morphology of the phases observed and association with residual elements present in the binder.
Role of the Interfacial Thermal Barrier in the Effective Thermal Diffusivity/Conductivity of SiC‐Fiber‐Reinforced Reaction‐Bonded Silicon Nitride
Authors: Bhatt Hemanshu , Donaldson Kimberly Y. , Hasselman D.P.H. , Bhatt R. T.
Published: March 2005

Experimental thermal diffusivity data transverse to the fiber direction for composites composed of a reaction bonded silicon nitride matrix reinforced with uniaxially aligned carbon‐coated silicon carbide fibers indicate the existence of a significant thermal barrier at the matrix‐fiber interface. Calculations of the interfacial thermal conductances indicate that at 300°C and 1‐atm N2, more than 90% of the heat conduction across the interface occurs by gaseous conduction. The magnitude of the interfacial conductance is decreased significantly under vacuum or by removal of the carbon surface layer from the fibers by selective oxidation. Good agreement is obtained between thermal conductance values for the oxidized composite at 1 atm calculated from the thermal conductivity of the N2 gas and those inferred from the data for the effective composite thermal conductivity.
Structure and Reduction and Mixed Copper‐Aluminum Oxide
Authors: Patrick Valerle , Gavalas George
Published: March 2005

Mixed oxides of copper and aluminum were prepared in porous form by the citrate process under various calcination conditions. The oxide samples were characterized by several techniques to determine chemical structure and texture. Atomic absorption spectroscopy provided the fractions of copper soluble and insoluble in hot nitric acid, which closely corresponded to CuO and CuAl2O4, respectively. X‐ray diffraction (XRD) provided complementary information about the content of the pure and compound oxides. X‐ray line broadening, scanning electron microscopy, and transmission electron microscopy were used to estimate the size of crystallites or phase domains. The reduction by hydrogen of samples containing different amounts of CuO and CuAl2O4 was studied by thermogravimetric analysis and XRD. The compound oxide CuAl2O4 and part of CuO closely associated with Al2O3 were reduced much more slowly than bulk CuO. Oxides of 1+ oxidation state, Cu2O and CuAlO2, were identified as reduction intermediates.
Incorporation of Transuranic Elements in Titanate Nuclear Waste Ceramics
Authors: Matzke Hans‐joachim , Ray Ian L. F. , Seatonberry Brian W. , Thiele Hartmut , Trisoglio Claudio , Walker Clive T. , White Timothy J.
Published: March 2005

The incorporation of actinide elements and their rare‐earth element analogues in titanate nuclear waste forms is reviewed. New partitioning data are presented for three waste forms containing Purex waste simulant in combination with either NpO2, PuO2, or Am2O3. The greater proportion of transuranics partition between perovskite and zirconolite, while some americium may enter loveringite. Autoradiography revealed clusters of plutonium atoms which have been interpreted as unreacted dioxide or sesquioxide. It is concluded that the solid‐state behavior of transuranic elements in titanate waste forms is poorly understood, certainly not well enough to tailor a ceramic for the incorporation of waste from reprocessing of fast breeder reactor fuel in which transuranic species are more abundant than in Purex waste.
Dynamic Compaction of Combustion‐Synthesized Hafnium Carbide
Authors: Kecskes L. J. , Benck R. F. , Netherwood P. H.
Published: March 2005

The explosive‐consolidation technique developed to fabricate combustion‐synthesized titanium carbide and titanium diboride has been applied to hafnium carbide. Presently, the application of this technique results in a moderately dense HfC product. Sample confinement and configuration, explosive loading, and total heat available for the combustion‐synthesis process were found to influence product characteristics. The key features of the process as well as some of the properties of the HfC ceramics produced are described.
Method for Processing Metal‐Reinforced Ceramic Composites
Authors: Lange Fred F. , Velamakanni Bhaskar V. , Evans Anthony G.
Published: March 2005

A new process is developed to form a ceramic containing a three‐dimensional network of metal reinforcement. The process involves four steps: (1) forming a powder compact containing a continuous network of either organic or carbon material by pressure filtration, (2) pyrolyzing the network to form channels within the powder compact, (3) densifying the powder while retaining the channel network, and (4) intruding metal into the channel network by squeeze casting. Pressure filtration is used to form the powder compact containing the pyrolyzable network either by mixing slurries of powder with chopped fiber or by packing powder within pyrolyzable preforms. When pressure is removed after filtration, the differential strain recovery of the powder matrix relative to the organic material can cause damage. Such damage is prevalent for a powder matrix formed from flocced slurries. However, this problem is avoided by using dispersed slurries which produce consolidated bodies that alleviate stresses arising from differential strain recovery by viscous flow. Metal‐reinforced ceramic composites with different reinforcement architectures, volume fractions, and sizes can be produced with this technique.
Ionic Conductivity in Fused Silica: II, Steady‐State Behavior
Authors: Jain Vijay , Varshneya Arun K. , Bihuniak Peter P.
Published: March 2005

Direct‐current ionic conductivity measurements on type‐I doped fused silica glasses were carried out in blocking and nonblocking modes between 300° and 800°C. The analysis of conductivity in steady state showed that the behavior was non‐Arrhenius in nature and that In (conductivity) followed a second‐degree polynomial of 1/T. The normalized conductivity for Li+ and Na+ ions at a particular temperature increased with the alumina/alkali ratio up to about 8 and appeared to level thereafter. This trend was similar to that observed for viscosity in the same glasses published by Bihuniak et al.1 For K+ electrolysis, no significant trends in the normalized conductivity versus alumina/alkali ratio could be observed. The presence of nonlinearity in Arrhenius behavior could be ascribed to the combined effect of two types of sites present in fused silica: SiO− and AIO4− sites, where the AIO4− sites presumably have a lower binding energy for the Li+ Na+, and K+ ions than do the SiO− sites. It is argued that, unlike alkali aluminosilicates, the SiO− sites in fused silica do not disappear at an alumina/alkali ratio of 1, and only reach a sturation level at a ratio of about 8, a conclusion also reached by Bihuniak et al.2 using randomwalk simulation.
Effect of Powder Characteristics on Microstructures and Dielectric Properties of (Ba,Nb)‐Doped Titania Ceramics
Authors: Wu Jenn‐Ming , Chen Chi‐Jen
Published: March 2005

The effect of powder characteristics on the microstructures and dieletric properties of (Ba,Nb)‐doped TiO2 ceramics was investigated. Two powders were prepared: one by coprecipitation and the other from mixing of reagent oxides. The powder characteristics were determined by TEM, X‐ray powder diffraction, SEM, and centrifugal particle analysis. Extensive abnormal grain growth is observed in samples sintered from coprecipitated powder, while it is not found in samples sintered from reagent oxide powder. The effects of crystallite size, agglomerate distribution, and chemical homogeneity on abnormal grain growth are discussed. A mechanism of abnormal grain growth is given, and chemical homogeneity is determined to be the main factor which caused abnormal grain growth. The effect of powder characteristics on dielectric properties is also described.
Influence of Post Hot Isostatic Pressing on the Microstructural Development and Critical Current Density of YBa2Cu3O7‐x
Authors: Aslan Mesut , Jaeger Herbert , Schulze Klaus , Frisch Arnulf , Petzow Günter
Published: March 2005

Samples of nearly theoretical density are produced by post hot isostatic pressing (post‐HIP) of presintered YBa2Cu3O7‐x using argon as the pressure medium. The hot isostatically pressed samples show improved values for critical current density at 77 K. Experimental aspects of post‐HIP are described, and microstructural development of YBa2Cu3O7‐x during sintering and post‐HIP is discussed.
Fracture Mechanisms in Ferroelectric‐Ferroelastic Lead Zirconate Titanate (Zr: Ti=0.54:0.46) Ceramics
Authors: Mehta Karun , Virkar Anil V.
Published: March 2005

Fracture toughness, KIC, of a single‐phase commercial lead zirconate titanate (PZT) ceramic (Zr/Ti=0.54/0.46) of tetragonal structure (c/a=1.019) was measured using the single edge notched beam method above and below the Curie temperature. Domain switching (poling) under electrical and mechanical loading was examined using X‐ray diffraction. Surface grinding, electrical poling, and mechanical poling caused crystallographic texture. Similar texture, indicative of domain switching, was also observed on fracture surfaces of some saples fractured at room temperature. At room temperature, the highest KIC measured was 1.85 MPa·m1/2, while above the Curie temperature it was about 1.0 MPa·m1/2. Cracks emanating from Vickers indents in poled samples were different in the poling and the transverse directions. The difference in crack sizes is explained on the basis of domain switching during crack growth. These results indicate that ferroelastic domain switching (twinning) is a viable toughening mechanism in the PZT materials tested.
High‐Temperature Healing of Lithographically Introduced Cracks in Sapphire
Authors: Rödel Jürgen , Glaeser Andreas M.
Published: March 2005

A new method for producing controlled‐geometry, controlled‐crystallography, cracklike defects using photolithography has been developed. The method has been applied to sapphire, and used to study crack healing behavior at 1800°C. Effects of crack face and crack perimeter crystallography, crack face microstructure, and impurities on healing behavior have been identified.
Oxidation Behavior of Lanthanide Aluminum Oxynitrides with Magnetoplumbite‐Like Structure
Authors: Wang Xiao Hong , Lejus Anne‐Marie , Vivien Daniel
Published: March 2005

Oxidation in air of lanthanide aluminum oxynitrides LnAl12O18N (Ln=La, Ce, Nd. Pr, Sm, and Gd) of magnetoplumbite‐like structure has been studied by X‐ray diffraction, infrared transmission, and X‐ray photoelectron spectroscopy. Simultaneous recording curves of thermogravimetry and differential thermal analysis have also been performed. These experiments show that oxidation begins at 700° to 800°C, with an increasing rate in the 1000° to 1100°C temperature range. In this process, nitride ions (N3‐) are oxidized. The reaction proceeds up to 1500°C. The oxidation process is accompanied by lattice modifications, which are mainly related to the trapping of neutral atoms (N°) in the structure. The final oxidation products depend on the particular lanthanide element of the pahse. They consist mainly of LnAl11O18, LnAlO3, and α‐Al2O3. An interpretation of the oxidation behavior of magnetoplumbite oxynitrides, which are related to the spinel AION oxynitride, is proposed.
Influence of Interfacial Shear Stress on First‐Matrix Cracking Stress in Ceramic‐Matrix Composites
Authors: Singh Raj N.
Published: March 2005

The first‐matrix cracking stress and fiber‐matrix interfacial shear stress were measured in zircon‐matrix composites uniaxially reinforced with either uncoated or BN‐coated silicon carbide filaments to study the role of intentional changes in interfacial shear stress on first‐matrix cracking stress. The first‐matrix cracking stress was measured by mechanical tests performed in either tension or flexure, and the filament‐matrix interfacial shear stress was measured by a fiber pushout test. The first‐matrix cracking stress was independent of the measured interfacial shear stress and did not conform to the predictions of a number of energy‐based micromechanics models. In contrast, the first‐matrix cracking stress showed a good correlation with the first‐matrix cracking strain, which is hypothesized to be a more realistic criterion for first‐matrix cracking in this class of filament‐reinforced ceramic‐matrix composites.
Surface Stress Effects on Indentation Fracture Sequences
Authors: Tandon Rajan , Green David J. , Cook Robert F.
Published: March 2005

Ion exchange was used to introduce surface compression in soda‐lime‐silica glass (SLS) and aluminosilicate glass (ALS). In situ observations of the indentation cracking sequences on the modified surfaces were made and compared with those on the base glasses. The initiation point for each crack type formed during the loading and unloading cycle was monitored on the indenter load‐displacement traces. On the base glasses, median (in the SLS) and cone (in the ALS) cracking was observed during the loading cycle. “Modified” median, radial, and lateral cracking was observed in both glasses during unloading. The introduction of the residual compressive surface stress was found to completely suppress all cracking during the loading cycle (up to 100 N) in both the SLS and ALS. In addition, the initiation of the cracks formed during the unloading cycle was shifted to further down in the cycle, i.e., to lower values of the initiation load. A pronounced tendency for a shift in the cracking from median to radial was also observed on the residually stressed surfaces. A similar shift was observed on stress‐free surfaces during unloading as the value of the initiation load decreased for a given peak load.
Influence of Several Metal lons on the Gelation Activation Energy of Silicon Tetraethoxide
Authors: Bansal Narottam P.
Published: March 2005

The effects of nine metal cations (Li+, Na+, Mg2+, Ca2+, Sr2+, Cu2+, Al3+, La3+, and Y3+) on silica gel formation have been investigated by studying the hydrolysis and polycondensation of silicon tetraethoxide (TEOS) in the presence of metal nitrates. The influence of the water: TEOS mole ratio, metal ion concentration, and the reaction temperature has been investigated. The overall activation energy for gel formation has been determined from the temperature dependence of the time of gelation for each system. The activation energy for network formation is 54.5 kJ/mol. The gel formation time as well as the activation energy sharply increases in the presence of Cu2+, Al3+, La3+, and Y3+. In contrast, the presence of Li+, Na+, Mg2+, Ca2+, or Sr2+ lowers the gelation time but has no appreciable effect on the activation energy. This difference may be attributed to the participation or nonparticipation of the metal ions in the formation of the three‐dimensional polymeric network during polycondensation. The concentration of metal ion (Mg2+, Ca2+, Y3+) or the water: TEOS mole ratio had no appreciable effect on the gelation activation energy. A simple test has been proposed to determine whether a metal ion would act as a network intermediate or modifier in silica and other glassy networks.
Viscosity of Porous Glasses
Authors: Sura Vivek M. , Panda Prakash C.
Published: March 2005

Viscosity of porous glasses has been derived from the elastic stress analysis, using the viscous analogy. Viscosity as a function of porosity has been estimated for spherical as well as for arbitrary pore geometry. Since the pore geometry changes during sintering, a shape factor that varies with pore geometry has been considered to predict the viscosity–porosity relationship. Viscosity as a function of porosity was measured on cordierite‐type glass by isothermal sinter‐forging experiments and data showed good agreement with the analysis. Experimental data from literature on viscosity as a function of porosity on two other glasses also show good agreement with the analysis.
Effect of Nitrogen Pressure and Diluent Content on the Combustion Synthesis of Titanium Nitride
Authors: Eslamloo‐Grami Maryam , Munir Zuhair A.
Published: March 2005

The effects of gas pressure and diluent content on the combustion synthesis of titanium nitride was investigated. Combustion of titanium powder samples containing TiN as a diluent in the concentration range 0 to 60 wt% was carried out in a nitrogen atmosphere at pressures in the range 0.1 to 1.4 MPa. Analysis of the dependence of combustion wave velocity on temperature gave an activation energy of 342 ± 50 kJ. mol−1, which is in good agreement with reported values for the diffusion of nitrogen in titanium nitride.
Property‐Porosity Relationships for Polymer‐Impregnated Superconducting Ceramic Composite
Authors: Salib Sherif , Vipulanandan Cumaraswamy
Published: March 2005

A thermoplastic polymer, poly(methyl methacrylate) (PMMA), was used to improve the flexural properties of the high‐temperature superconducting ceramic (YBa2Cu3O7‐δ). Ceramic specimens with different porosities were prepared by dry compacting 12.5‐mm‐diameter disk specimens at various uniaxial pressures. Density‐pressure relationships have been developed for before‐ and after‐sintering conditions. The PMMA polymer was impregnated into the porous ceramic at room temperature. The mechanical properties were evaluated by concentrically loading simply supported disk specimens. The load‐displacement responses were analyzed using the finite‐element method. Impregnation of PMMA polymer at room temperature increased the flexural strength and modulus of the superconducting ceramic without affecting its electrical propeties. The flexural properties depended on the porosity of the ceramics, and, hence, linear and nonlinear property‐porosity relationships have been used to characterize the behavior of superconducting ceramic with and without the polymer.
Stability of YBa2Cu3O7‐x in Molten Chloride Salts
Authors: Raeder Christopher H. , Knorr David B.
Published: March 2005

The decomposition products of YBa2Cu3O7‐x depend on the composition of the molten chloride salt for exposure at 1173 K in air. The presence of dichloride salts such as CuCl2, CaCl2, or MgCl2 promote formation of CuO, Cu2Y2O5, and loss of barium to the chloride salt as BaCl2. Salts based on BaCl2 or containing LiCl result in YBa2Cu3O7‐x decomposition products of Y2BaCuO5, CuO, and BaCl2. High barium activity in the salt supports formation of the Y2BaCuO5 phase and reaction of CO2 with the salt producing BaCO3. Decomposition is most sluggish in binary NaCl‐KCl salts where minimal amounts of reaction or decomposition products are observed.
Temperature Dependence of the Thermal Diffusivity/Conductivity of Aluminum Nitride
Authors: Jackson T. Barrett , Donaldson Kimberly Y. , Hasselman D. P. H.
Published: March 2005

The temperature dependence of the thermal conductivity of a polycrystalline AIN was measured using the flash‐diffusivity technique over a range of experimental conditions. Thermal conductivity data from room temperature to 300°C, obtained with attenuated laser pulse energies to minimize the specimen temperature increase, were found to be inversely proportional to the absolute temperature, as expected from theory. For high pulse energies, the experimental data deviated significantly from expected behavior. This latter effect is offered as an explanation for the anomalously low temperature dependence for the thermal conductivity reported in the literature.
Effect of Green Density on Densification and Creep During Sintering
Authors: Rahaman Mohamed N. , De Jonghe Lutgard C. , Chu May‐Ying
Published: March 2005

The effect of green density on both the densification rate and the creep rate was measured simultaneously during sintering by loading dilatometry. The experiments were performed on zinc oxide powder compacts with five different green densities covering a range of 0.39 to 0.73 of theoretical. The samples were heated at a constant rate of 4°C/min up to 1100°C in air. The densification rate at any temperature increases significantly with decreasing green density. The data for the densification rate and creep rate as a function of density show two quite distinct regimes of behavior; the rates were strongly dependent on density below 0.80, while above this value they were weakly dependent on density. The ratio of the densification rate to the creep rate was almost independent of temperature but increased almost linearly with increasing green density. The representation of the data in terms of models for sintering and creep is discussed.
Gelcasting of Alumina
Authors: Young Albert C. , Omatete Ogbemi O. , Janney Mark A. , Menchhofer Paul A.
Published: March 2005

Gelcasting is a novel method for molding ceramic powder based on a synthesis of concepts derived from traditional ceramics and polymer chemistry. Gelcasting of alumina is described in this paper. The process is based on the in situ polymerization of acrylamide monomers as the setting mechanism for forming the green body. It has the following features: slurries with high solids loading and low viscosity (1.8 Pa·s at 62 vol% solids), dried bodies containing less than 4 wt% binder, and the ability to fabricate complex‐shaped bodies.
Preparation and Properties of Glass‐Ceramics Derived from Nitrogen‐Containing β‐Spodumene Glasses
Authors: Unuma Hidero , Miura Koh , Furusaki Tsuyoshi , Kodaira Kohei
Published: March 2005

Crystallization of Li‐Al‐Si‐O‐N oxynitride glasses based on the β‐spodumene composition and the properties of the resultant glass‐ceramics have been studied. The onset of the precipitation of metastable high‐quartz solid solution and its transformation to β‐spodumene shift to higher temperatures with increasing nitrogen content of the oxynitride glasses. Nitrided glass‐ceramics crystallized at 1200°C have negative thermal expansion coefficients, since high‐quartz structure is maintained up to 1000° and 1200°C. Knoop hardness and density of the glass‐ceramics increase with increasing nitrogen content. There was evidence that part of the nitrogen atoms were incorporated into the high‐quartz solid‐solution structure and that a small amount of the minor phase of Si2N2O was precipitated in highly nitrided glass‐ceramics.
Properties of Modified Anodic‐Spark‐Deposited Alumina Porous Ceramic Films as Humidity Sensors
Authors: Chen Zhi , Jin Mao‐Chang , Zhen Chao , Chen Guan‐Hua
Published: March 2005

α‐Alumina films formed by anodic spark deposition in the melt of NaHSO4‐KHSO4 mixture (200°C) exhibited a porous structure. It has been found that the porosity and amount of α‐phase varied with anodic current density. By electrical measurements, films were found to exhibit a continuous open‐pore structure, causing failure of humidity sensors. This problem can be overcome by reanodization of the anodicspark‐deposited alumina films in either borax aqueous solution or sulfuric acid solution for a short time to form an effective barrier layer at the pore base. The modified anodic‐spark‐deposited alumina films showed highly sensitive response to water vapor change in the range of −76° to +20°C dew point with much improved long‐term stability. The humidity characteristics of the α‐alumina films were qualitatively explained in terms of the BET theory and condensation theory.
Room‐Temperature Mechanosynthesis of Carbides by Grinding of Elemental Powders
Authors: Matteazzi Paolo , Le Caër Gerard
Published: March 2005

The direct room‐temperature synthesis (mechanochemical synthesis or mechanosynthesis—MS) of most metal carbides by milling metal—carbon powders (size some tens of micrometers) mixtures is described. The particle size of the carbides obtained is of the order of 20 nm. Stable, metastable, mixed, and new carbides can be formed. Moreover, metal—carbon alloys can be obtained, such as in the Fe—C system. The synthesized compounds have been characterized by X‐ray diffraction and Mössbauer spectroscopy for iron‐containing systems. Carbides of the following elements were obtained: Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, Hf, Ta, W, Re, Al and Si.
Effects of Annealing on the Microstructure and Phase Chemistry of Directionally Solidified Bi2Sr2CaCu2O8
Authors: Chow Hui Meng , Jiang Xiao Ping , Cima Michael J. , Haggerty John S. , Brody Harold D. , Flemings Merton C.
Published: March 2005

The effects of annealing on the microstructure and phase chemistry of directionally solidified Bi2Sr2CaCu2O8 (2212) were studied. Boules grown at fast growth rates are multiphase, contain no 2212, and exhibit some preferred orientation; Sr‐deficient 2201 exhibits a (220) texture. These samples, however, do show a significant amount of 2212 after annealing, with (220) texture similar to the texture of the preexisting 2201 phase in the as‐grown boules. In contrast, 2212 grown directly from the melt at slow growth rates has (200) texture. Boules grown at moderate growth rates are multiphase and contain 2212 as one of the phases; these boules show an increase in 2212 volume fraction after annealing. Microstructural observations in both cases indicate solidstate formation of 2212 occurs topotactically on or within the preexisting 2201 phase. Boules grown at fast growth rates are oxygen‐deficient, since they show an increase in mass with annealing, whereas boules grown at moderate growth rates show no appreciable mass increase with annealing. An estimate of the Cu(I) concentration in the liquid zone during crystal growth at fast growth rates was obtained from gravimetric analysis.
Thermal Conductivity and Electrical Resistivity of Cemented Transition‐Metal Carbides at Low Temperatures
Authors: Frandsen Marvin V. , Williams Wendell S.
Published: March 2005

The thermal conductivity, K, and electrical resistivity, ρ, of cemented transition‐metal carbides—a class of composites of high toughness—were measured for the first time at cryogenic temperatures. The dependence of these quantities on composition, grain size, and binder content was explored. The major qualitative findings are the following: (1) K for WC/Co grades has a gentle maximum ranging from 80 to 160 W/(m · K) around 150 K; (2) WC/Co has higher values of K and lower values of ρ than TiCx/NiMo, because of vacancy scattering of electrons and phonons in the latter; (3) grades with larger grains have higher values of K; (4) grades with higher percentages of binder phase have lower values of K; (5) the lattice and electronic components of K are comparable for TiCx/NiMo; (6) multicarbide grades have lower values of K than WC/Co; (7) a Callaway analysis of the lattice component of K identifies scattering of phonons by grain boundaries, point defects, electrons, and other phonons as significant for these materials, with defect scattering dominating in the liquid‐nitrogen range.
Knoop Microhardness Anisotropy of Single‐Crystal Aragonite
Authors: Han Young H. , Li Hong , Wong Thomas Y. , Bradt Richard C.
Published: March 2005

The Knoop microhardness was measured for indentation test loads from 25 to 200 g on the (100), (010), and (001) planes of single‐crystal aragonite (nonbiogenic). Although the microhardness of this aragonite is quite anisotropic on the (100) and (010), it is nearly isotropic on the (001) plane. The microhardness profiles are discussed relative to the structure of aragonite, and the hardnesses are compared with those which have been determined for calcite on its cleavage plane. The indentation load/size effect (ISE) is addressed for aragonite and compared with that of calcite on its cleavage plane.
In Situ Observations of Toughening Processes in Alumina Reinforced with Silicon Carbide Whiskers
Authors: Rödel Jürgen , Fuller Edwin R. , Lawn Brian R.
Published: March 2005

An in situ study is made of crack interfaces in composites of alumina reinforced with silicon carbide whiskers. Both qualitative observations of the whisker‐bridging micro‐mechanisms and quantitative measurements of the crack profile are made to assess the specific role of the whiskers on the toughness curve (T‐curve or R‐curve). At small crackwall separations the whiskers act as elastic restraints to the point of rupture. In some cases the whiskers remain in frictional contact with the alumina matrix over large pullout distances (more than 1 μm) corresponding to a bridging zone approaching 1 mm. The results are discussed in relation to existing models of whisker reinforcement and published long‐crack T‐curve data.
Ceramic and Glass‐Ceramic Packaging in the 1990s
Authors: Tummala Rao R.
Published: March 2005

A broad overview of packaging involving interconnecting, powering, protecting, and cooling semiconductor chips to meet a variety of computer system needs is presented. The general requirements for ceramics in terms of their thermal, mechanical, electrical, and dimensional control requirements are presented, both for high‐performance and low‐performance applications. Glass‐ceramics are identified as the best candidates for high‐performance systems, and aluminum nitride, alumina, or mullite are identified for low‐performance systems. Glass‐ceramic/copper substrate technology is discussed as an example of high‐performance ceramic packaging for use in 1990s. Lower‐dielectric‐constant ceramics such as composites of silica, borosilicate, and cordierite, with or without polymers and porosity, are projected as potential ceramic substrate materials by the year 2000.
Factors Controlling the Fracture Resistance of Brittle Cellular Materials
Authors: Brezny Rasto , Green David J.
Published: March 2005

Previous work on brittle cellular solids has indicated that the fracture toughness may be independent of cell size. The results showed agreement with two theoretical approaches proposed by Maiti et al. to predict the toughness of porous cellular solids. One approach indicates a strut strength and cell size dependence of the bulk material toughness, whereas the other depends only on the toughness of the solid portion. The present work attempted to experimentally discern between the two approaches and identify the controlling parameters governing the mechanical behavior. Reticulated vitreous carbon samples were oxidized to introduce surface flaws into the struts. The toughness and compressive strength of the bulk material were measured before and after oxidation. Fractographic analysis of the fracture surface of individual struts was essential in being able to estimate the strut strength. It was concluded that it is the strut strength which controls the strength and toughness of the bulk cellular solid.