bluuebunny/crossref_metadata_2025 huggingface dataset:
Disorientations in dislocation structures: Formation and spatial correlation
Authors: Pantleon Wolfgang
Published: March 2008

During plastic deformation, dislocation boundaries are formed and orientation differences across them arise. Two different causes lead to the formation of two kinds of deformation-induced boundaries: a statistical trapping of dislocations in incidental dislocation boundaries and a difference in the activation of slip systems on both sides of geometrically necessary boundaries. On the basis of these mechanisms, the occurrence of disorientations across both types of dislocation boundaries is modeled by dislocation dynamics. The resulting evolution of the disorientation angles with strain is in good agreement with experimental observations. The theoretically obtained distribution functions for the disorientation angles describe the experimental findings well and explain their scaling behavior. The model also predicts correlations between disorientations in neighboring boundaries, and evidence for their existence is presented.
Effect of electron exposure on optical properties of aluminized polyimide film
Authors: Li Chundong , Yang Dezhuang , He Shiyu , Mikhailov M. M.
Published: March 2008

The effect of the irradiation of electrons with 10–70 keV on optical properties, including spectral reflectance ρ and solar absorptance as, of aluminized polyimide films was investigated. The spectral reflectance was measured in situ before and after electron exposure. Experimental results showed that the reflective properties of aluminized polyimide film were apparently degraded in the 500–1200 nm wavelength range of the solar spectrum. Under the exposure of electrons, no charging effects were found on the aluminized polyimide film serving as an ion-conductive polymer. After the exposure, an “annealing” or “bleaching” effect occurred. At a given irradiation fluence, the change in solar absorptance (Δas) of the aluminized polyimide film was increased with electron energy. There is a threshold value of electron flux φcr which affects the change in Δas of the aluminized polyimide film, approximately φcr = 6 × 1012 electrons/cm2s. When φ < φcr, the change in Δas is independent from the flux; when φ > φcr, Δas increases with the flux monotonically. The change in Δas with electron fluence Φ can be expressed in the form of a power function: Δas = αΦβ. The factors α and β are related to electron energy and show a maximum and a minimum value at 50 keV, respectively.
Synthesis of aligned carbon nanotubes in organic liquids
Authors: Zhang Yafei , N.-Gamo Mikka , Nakagawa Kiyoharu , Ando Toshihiro
Published: March 2008

A simple and novel method was developed for efficient synthesis of aligned multiwalled carbon nanotubes (CNTs) in methanol and ethanol under normal pressure. The CNTs' alignment and structures were investigated using Raman scattering and x-ray diffraction spectroscopy. A unique kind of coupled CNT was synthesized in which one rotated to the left and one rotated to the right. Chains periodically bridged the coupled CNTs. The growth mechanism of the CNTs within organic liquid is proposed to be a catalytic process at the Fe film surface in a dynamic and thermal nonequilibrium condition in organic liquids.
Microstructure and microstructural evolution in BaTiO3 films fabricated using the precursor method
Authors: Bendersky L. A. , Lu C. J. , Scott J. H. , Chang K. , Takeuchi I.
Published: March 2008

Pulsed laser deposition of TiO2 and BaF2 layers at room temperature and subsequent annealing in flowing oxygen were used to form homogeneous epitaxial BaTiO3 films on LaAlO3. This oxide film synthesis method, known as the precursor technique, is frequently used for making combinatorial libraries. In this paper, we investigated the microstructures of the films at different stages of annealing using cross-sectional transmission electron microscopy, high-resolution imaging, and electron energy loss spectroscopy. It was shown that epitaxial BaTiO3 thin films with large grains could be formed on a LaAlO3 substrate. Their formation process consists of the following stages: At 200 °C, the BaF2 layer is partially oxidized. At 400 °C, the amorphous TiO2 layer crystallizes, further transformation of BaF2 into BaO takes place, and interdiffusion begins. At 700 °C, the formation of a polycrystalline structure with different Ba–Ti oxides occurs, epitaxial BaTiO3 grains nucleate on the film/substrate interface, and significant interdiffusion takes place. Finally, at 900 °C, the interdiffusion is completed, and the epitaxial BaTiO3 grains coalesce and grow. The presence of nonepitaxial polycrystalline regions in fully annealed films can be explained as the following: (i) stoichiometric transient regions not yet consumed by recrystallization of BaTiO3; (ii) nonstoichiometric regions resulting from inhomogeneous deposition of BaF2.
Processing of highly oriented lithium tantalate films by chemical solution deposition
Authors: Ono Satomi , Hirano Shin-ichi
Published: March 2008

The synthesis of lithium tantalate films by a chemical solution deposition method was studied. A precursor solution was prepared by dissolving lithium ethoxide and tantalum pentaethoxide in ethanol. The addition of formic acid to this precursor solution was very effective in the preparation of homogeneous and transparent precursor films on substrates by spin coating. Lithium tantalate films crystallized on sapphire (001) substrates with a highly preferred orientation along the c axis with heat-treating at temperatures above 450 °C. The refractive index of the film prepared at 550 °C was 2.049, which is close to the value for single crystals of lithium tantalate (2.176).
Quantification and control of the sulfur c(2 × 2) superstructure on {100}〈100〉 Ni for optimization of YSZ, CeO2, and SrTiO3 seed layer texture
Authors: Cantoni C. , Christen D. K. , Heatherly L. , Kowalewski M. M. , List F. A. , Goyal A. , Ownby G. W. , Zehner D. M. , Kang B. W. , Kroeger D. M.
Published: March 2008

We investigated the influence of a chemisorbed S template with c(2 × 2) structure on the epitaxial growth of different oxide buffer layers on {100}〈100〉 Ni. The sulfur superstructure spontaneously forms on the Ni surface during the texturing anneal as a consequence of segregation. However, depending on the initial S bulk concentration and/or specific annealing conditions, the S layer can cover less than the entire substrate's surface. We show that an incomplete c(2 × 2) coverage causes degradation of the seed buffer layer texture as compared to the substrate texture. A simple step consisting of an H2S predeposition anneal can be used to control the superstructure coverage and optimize the seed layer texture.
Fabrication, characterization, and leach testing of hollandite, (Ba,Cs)(Al,Ti)2Ti6O16
Authors: Carter M. L. , Vance E. R. , Mitchell D. R. G. , Hanna J. V. , Zhang Z. , Loi E.
Published: March 2008

The dissolution in de-ionized water (DIW) at 90 and 150 °C of Cs and Ba from mechanically polished Cs-doped Ba hollandite samples is essentially congruent. The normalized Ba and Cs release rates were <0.001 g/m2/day after 56 days in DIW at 90 °C, and the Ba normalized release rate of a Cs-free sample was 0.01 g/m2/day after 56 days in DIW at 150 °C. Varying the pH between approximately 2.5 and 12.9 affected only the Ba dissolution rates of hollandite by half an order of magnitude. The dissolution rates of all species decrease with increasing leaching time due to the formation of partly impervious surface coatings of Al- and Ti-rich species. These surface coatings were investigated by scanning electron microscopy, and in some cases by cross-sectional transmission electron microscopy and x-ray photoelectron spectroscopy.
Effect of compositional modification on ductility of B2 phase in Ti–Al–Nb intermetallic alloy
Authors: Tang Feng , Hagiwara Masuo
Published: March 2008

Room-temperature ductility of two Ti–Al–Nb intermetallic alloys with close chemical compositions was investigated by tensile testing. The two alloys' ductilities in the B2 single-phase state were significantly different, which indicated that the ductility of B2 phase state was sensitive to the chemical composition. The alloy with more ductile B2 phase exhibited higher ductility in O + B2 two-phase state.
Novel titanium foam for bone tissue engineering
Authors: Wen C. E. , Yamada Y. , Shimojima K. , Chino Y. , Hosokawa H. , Mabuchi M.
Published: March 2008

Titanium foams fabricated by a new powder metallurgical process have bimodal pore distribution architecture (i.e., macropores and micropores), mimicking natural bone. The mechanical properties of the titanium foam with low relative densities of approximately 0.20–0.30 are close to those of human cancellous bone. Also, mechanical properties of the titanium foams with high relative densities of approximately 0.50–0.65 are close to those of human cortical bone. Furthermore, titanium foams exhibit good ability to form a bonelike apatite layer throughout the foams after pretreatment with a simple thermochemical process and then immersion in a simulated body fluid. The present study illustrates the feasibility of using the titanium foams as implant materials in bone tissue engineering applications, highlighting their excellent biomechanical properties and bioactivity.
Epitaxial growth of antiperovskite GaCMn3 film on perovskite LaAlO3 substrate
Authors: Choi H. S. , Kim W. S. , Kim J. C. , Hur N. H.
Published: March 2008

We report on the magnetic and transport properties of the antiperovskite GaCMn3 film epitaxially grown on LaAlO3 using the pulsed laser deposition technique. Upon cooling from room temperature, the GaCMn3 film undergoes magnetic transitions from paramagnetic to ferromagnetic to antiferromagnetic. The Curie and Neél temperatures of the film shift to lower and higher temperatures, respectively, by comparison with those of the bulk sample. This discrepancy is mainly ascribed to the compressive strain effect induced by the lattice-mismatch between film and substrate. Negative magnetoresistance, which is about 20% at 0.5 T, is observed near the Neél temperature.
Understanding nanoindentation unloading curves
Authors: Pharr G. M. , Bolshakov A.
Published: March 2008

Experiments have shown that nanoindentation unloading curves obtained with Berkovich triangular pyramidal indenters are usually welldescribed by the power-law relation P = α(h − hf)m, where hf is the final depth after complete unloading and α and m are material constants. However, the power-law exponent is not fixed at an integral value, as would be the case for elastic contact by a conical indenter (m = 2) or a flat circular punch (m = 1), but varies from material to material in the range m = 1.2–1.6. A simple model is developed based on observations from finite element simulations of indentation of elastic–plastic materials by a rigid cone that provides a physical explanation for the behavior. The model, which is based on the concept of an indenter with an “effective shape” whose geometry is determined by the shape of the plastic hardness impression formed during indentation, provides a means by which the material constants in the power law relation can be related to more fundamental material properties such as the elastic modulus and hardness. Simple arguments are presented from which the effective indenter shape can be derived from the pressure distribution under the indenter.
Depth-sensing indentation at macroscopic dimensions
Authors: Thurn Jeremy , Morris Dylan J. , Cook Robert F.
Published: March 2008

A macroscopic-scale depth-sensing indentation apparatus with the ability to be mounted on an inverted microscope for in situ observation of contact events was calibrated using the Oliver and Pharr [J. Mater. Res. 7, 1564 (1992)] procedure with a two-parameter area function. The calibrated Vickers tip was used to determine the projected contact area at peak load and the modulus and hardness of a variety of non-metallic materials through deconvolution of the measured load-displacement traces. The predicted contact area was found to be identical to the measured area of residual contact impressions. Furthermore, for transparent ceramic materials the projected contact area during loading was found to be the same as the area measured from the diagonal of post-indentation residual contact impressions. The modulus and hardness values deconvoluted from the load–displacement traces were compared with independent measurements. The effects of sample clamping, column compliance, and tip radius on the load–displacement data and inferred materials properties were also examined. It is suggested that the simplicity of instrumentation and operation, combined with the ability to observe indentations optically, even in situ, makes macroscopic-scale depth-sensing indentation ideal for fundamental studies of contact mechanics.
Kinetics of subsurface hydrogen adsorbed on niobium: Thermal desorption studies
Authors: Cabrera A. L. , Espinosa-Gangas J. , Jonsson-Akerman Johan , Schuller Ivan K.
Published: March 2008

The adsorption/absorption of hydrogen and the adsorption of carbon monoxide by niobium foils, at room temperature, was studied using thermal desorption spectroscopy. Two hydrogen desorption peaks were observed with a maximum at 404 and 471 K. The first hydrogen desorption peak is regarded as hydrogen desorbing from surface sites while the second peak, which represents desorption from surface sites stronger bound to the surface, also has a component—due to its tailing to higher temperatures—of hydrogen diffusing from subsurface sites. Carbon monoxide adsorption was used to determine the number of surface sites, since it does not penetrate below the surface. Two carbon monoxide desorption peaks are observed in these experiments: at 425 and 608 K. The first peak is regarded as the adsorption of molecular carbon monoxide, and the second, as carbon monoxide dissociated on the niobium surface. The crystallographic orientation of the foils was determined by x-ray diffraction and showed a preferential (110) orientation of the untreated foil due to the effect of cold rolling. This preferential orientation decreased after hydrogen/heat treatment, appearing strong also in the (200) and (211) orientations. This change in texture of the foils is mainly due to the effect of heat treatment and not to hydrogen adsorption/desorption cycling. The kinetics of hydrogen and CO desorption is compared with that of Pd and Pd alloys.
Synchrotron radiography and x-ray topography studies of hexagonal habitus SiC bulk crystals
Authors: Argunova T. S. , Gutkin M. Yu. , Je Jung Ho , Kang H. S. , Hwu Y. , Tsai W-L. , Margaritondo G.
Published: March 2008

Phase-sensitive synchrotron radiation (SR) radiography was combined with x-ray diffraction topography to study structural defects of SiC crystals. The particular bulk SiC crystals examined had a low micropipe density and a hexagonal habitus composed of prismatic, pyramidal, and basal faces well developed. X-ray diffraction topography images of the sliced (0001) wafers, which were formed due to the complex lattice distortions associated with defective boundaries, demonstrated the existence of two-dimensional defective boundaries in the radial direction, normal to the (0001) planes. In particular, those parallel to the 〈1120〉 directions extended rather far from the seed. On the other hand, by phase-sensitive SR radiography the effect of micropipe collection was detected. Micropipes grouped mostly in the vicinities of the defective boundaries but rarely appeared between groups. Some general remarks about possible reasons for the development of such peculiar defect structures were made.
Chemical mechanical polishing of thermal oxide films using silica particles coated with ceria
Authors: Lee Seung-Ho , Lu Zhenyu , Babu S. V. , Matijević Egon
Published: March 2008

Thermal oxide covered silicon wafers were polished with slurries containing either nano-sized ceria (CeO2) or newly prepared uniform colloidal silica particles coated with ceria. The polish rate of the latter was significantly higher than that of pure ceria. The experiments were carried out using different concentrations of the abrasives at pH 4 and 10. Little effect on the polishing rates was noted when the conditions of the slurries were varied, which was explained by the compensation of two opposite polishing mechanisms.
Cold isostatic compaction of nano-size powders: Surface densification and dimensional asymmetry
Authors: Li Wenxia , Lannutti John J.
Published: March 2008

Cold isostatic pressing (CIP) is often used in the compaction of nano-sized powders. For technological reasons, however, uniaxial pressing prior to CIP takes place. This paper reveals the first quantitative measurements of density gradients within and the asymmetric sintering response of nanoscale zirconia compacts formed by (i) simple uniaxial compaction and (ii) specific ratios of uniaxial and CIP pressure. We find that CIP forms an exterior “skin” of higher but variable surface density and decreases the width of the density distribution. It does not eliminate density gradients; nonuniform shrinkage still occurs during sintering. The high- and low-density zones (the moving and fixed ram ends, respectively) that form during uniaxial compaction are reversed during CIP. Considering both density distribution width and spring-back cracking, the “best” uniaxial-CIP pressure combination is 1–20 ksi for this particular powder and an L/D of 1.0. The greater final compaction of the low-density zone during CIP causes relatively large variations in final dimensions (nearly 400 microns) in spite of the smaller density distribution width. The usually neglected uniaxial pressing step has definite technological impacts on the production of nanostructured components via compaction.
Hydrothermal epitaxy of KtaO3 thin films
Authors: Goh Gregory K. L. , Levi Carlos G. , Lange Fred F.
Published: March 2008

Heteroepitaxial KTaO3 thin films were synthesized by the hydrothermal method below 175 °C in highly alkaline aqueous KOH solutions. KTaO3 films synthesized in 7 M KOH solutions contained a defect pyrochlore phase that had (400) and (111) out-of-plane orientations. The (400) oriented pyrochlore islands were epitaxially related to the (100) SrTiO3 substrate while the (111) oriented pyrochlore islands had four in-plane variants. Pyrochlore-free KTaO3 films could be synthesized in a 15 M KOH solution. Pyrochlore-free films were also grown at 7 M KOH by delaying the introduction of the (100) SrTiO3 substrate into the synthesis solution until the conditions favoring the formation of the pyrochlore phase had dissipated.
Self-propagating high-temperature synthesis of nano-sized titanium carbide powder
Authors: Nersisyan H. H. , Lee J. H. , Won C. W.
Published: March 2008

The combustion process of a titanium–carbon system with sodium chloride as an inert diluent was investigated. The combustion laws and microstructure of final products according to diluent content were obtained. It was shown that sodium chloride not only decreases combustion temperature but also makes effective protective shells around primary carbide crystals and keeps this ultrafine structure up to the end of combustion. As a result, nano-sized titanium carbide powders were successfully obtained.
Relationships between the thermal stability, friction, and wear properties of reactively sputtered Si–aC:H thin films
Authors: Evans Ryan D. , Doll Gary L. , Glass Jeffrey T.
Published: March 2008

The friction and wear performance were correlated with the thermal stability of reactively sputtered Si–aC:H thin films containing various Si and H concentrations. The average steady-state friction coefficients as measured by dry sliding pin-on-disk tests decreased with increasing Si and H content. Furthermore, the films with high Si and H formed thick transfer films as compared to the films with little or no Si and H content. Minimums in average ball abrasion rate and average film wear rate were observed at the Si/C = 0.10 film composition. The most intense and distinct “graphitic” Raman peaks were collected from the Si/C = 0.10 transfer film debris. In addition, the Si/C = 0.10 film also had the most distinguishable graphitic Raman signature after annealing in air at 500 °C compared to the other Si–aC:H films, suggesting a possible relationship between the nature of transfer films resulting from dry sliding in air and the bulk films that were annealed in air.
Creep behavior of eutectic Sn–Ag lead-free solder alloy
Authors: Huang M. L. , Wang L. , Wu C. M. L.
Published: March 2008

Precipitation-strengthened tin-based eutectic Sn–3.5 Ag alloy was investigated for its creep behavior at three temperatures ranging from 303 to 393 K, under the tensile stress range of σ/E = 10−4 to 10−3. The steady-state creep rates cover seven orders of magnitude (10−3 to 10−9 s−1). The initial microstructure was found to have Ag3Sn intermetallic compound finely dispersed in the matrix of β–Sn. By incorporation of a threshold stress, σth, into the analysis, the creep data of eutectic Sn–Ag at all temperatures can be fitted by a single straight line with a slope of seven after normalizing the steady-state creep rate and the effective stress, indicating that the creep rates are controlled by the dislocation-pipe diffusion in the Sn matrix. The steady-state creep rate, , can then be expressed as , where QC is the creep activation energy, G is the temperature-dependent shear modulus, b is Burger's vector, R is the universal gas constant, T is the absolute temperature, σ is the applied stress, A is a material-dependent constant, and , in which σOB is the Orowan bowing stress and kR is the relaxation factor.
Residual stresses in functionally graded plates
Authors: Neubrand Achim , Chung Tai-Joo , Rödel Jürgen , Steffler Eric D. , Fett Theo
Published: March 2008

Macroscopic residual stresses in Al2O3/Al plates of graded composition were determined experimentally and by the finite element method (FEM). Experimental stress data were determined by sawing a notch in the plates and measuring displacements by Moiré interferometry. Residual stresses were calculated from the displacements using the weight function method. Experimentally determined stresses agreed with FEM data only if measured thermomechanical properties for the different compositions were used for finite element calculations.
Interactions between toughening mechanisms: Transformation toughening versus plastic deformation
Authors: Tuan Wei-Hsing , Chen Rong-Zhi
Published: March 2008

In this study, the interactions between transformation toughening and plastic stretching were investigated experimentally. Zirconia and metals, nickel or silver, were incorporated simultaneously into an alumina matrix. The extent of phase transformation of zirconia particles was enhanced due to the coexistence of soft metals. The ductility of nickel was also enhanced in the Al2O3–Ni–ZrO2 composites. However, the presence of zirconia particles at the alumina/silver interface reduced the ability of silver to deform plastically. Due to the interactions, the ratio of composite toughness to matrix toughness for the Al2O3–Ni–ZrO2 composite was higher than the product of the ratio of the composites containing only nickel and only zirconia.
Synthesis and thermoelectric properties ofp-type barium-filled skutterudite BayFexCo4−xSb12
Authors: Tang X. F. , Chen L. D. , Goto T. , Hirai T. , Yuan R. Z.
Published: March 2008

Single-phase barium-filled skutterudite compounds, BayFexCo4−xSb12(x= 0 to 3.0, y = 0 to 0.7), were synthesized by a two-step solid-state reaction method. The maximum filling fraction of Ba (ymax) in BayFexCo4–xSb12increased with increasing Fe content and was found to be rather greater than that of CeyFexCo4–xSb12. Theymaxvaried from 0.35 to near 1.0 when Fe content changed from 0 to 4.0. BayFexCo4–xSb12showedp-type conduction at a composition range ofx= 0 to 3.0,y= 0 to 0.7. Carrier concentration and electrical conductivity increased with increasing Fe content and decreased with increasing Ba filling fraction. The Seebeck coefficient increased with increasing Ba filling fraction and with decreasing Fe content. Lattice thermal conductivity decreased with increasing Ba filling fraction and reached a minimum at a certain Ba filling fraction (y= 0.3 to 0.4). The greatestZTvalue of 0.9 was obtained at 750 K forp-type Ba0.27Fe0.98Co3.02Sb12. It is expected that further investigation on the optimization of filling fraction would result in a higherZTvalue at the moderately low Fe content region.
Magnetic properties of undoped and Co-doped n-type β–FeSi2.5 single crystals
Authors: Arushanov E. , Ivanenko L. , Eckert D. , Behr G. , Rößler U. K. , Müller K-H. , Schneider C. , Schumann J.
Published: March 2008

Results of magnetization and magnetic susceptibility measurements on undoped and Co-doped FeSi2.5 single crystals are presented. The temperature dependence of the magnetic susceptibility of the Co-doped sample in the range of 5–300 K can be explained by temperature-dependent contributions due to paramagnetic centers and the carriers excited thermally in the extrinsic conductivity region. The values of the paramagnetic Curie temperature and activation energy of the donor levels were estimated. It is also shown that the magnetic susceptibility of Co-doped samples cooled in zero external field and in a field are different. This resembles the properties of spin-glasses and indicates the presence of coupling between magnetic centers.
In situ laser recrystallization of Si layers during low-pressure chemical vapor deposition: Recrystallization dynamics and influence of the seed layer
Authors: Sala D. Della , Santoni A. , Fornarini L. , Lancok J. , Loreti S. , Menicucci I. , Minarini C.
Published: March 2008

The growth of polycrystalline silicon on glass by low-pressure chemical vapor deposition and in situ laser induced recrystallization was investigated with the aim to study the influence of the seed layer and the mechanism of the recrystallization dynamics on the structural and morphological properties of the grown film. A seed layer was used to trigger the solidification process of many additional in situ laser-crystallized overlayers. One-dimensional calculations of the thermal flow produced by laser irradiation were used to clarify the complex interaction between the substrate and the molten silicon surface layer during nucleation and growth. The experiments show the relevant role played by the seed layer and the peculiar shaping of the film surface due to the preferential aggregation of molten silicon. Compact polysilicon films with thicknesses up to 4 μm with almost monocrystalline grains of 1–2-μm size were obtained.
Nanocrystalline grain structures developed in commercial purity Cu by low-temperature cold rolling
Authors: Wang Y. M. , Chen M. W. , Sheng H. W. , Ma E.
Published: March 2008

Nanocrystalline pure copper was obtained by cold rolling a commercial bulk Cu to very large extensions at subambient temperatures. The eventual formation of nanocrystalline grain structures is attributed to dynamic grain refinement (recrystallization) mechanisms activated by the low-temperature continuous plastic deformation that leads to ultrahigh densities of dislocations.
Influence of electropulsing on nucleation during phase transformation
Authors: Zhou Yizhou , Guo Jingdong , Zhang Wei , He Guanhu
Published: March 2008

Phase transformation about precipitation in a Cu–Zn alloy was studied. It was found that with an electropulsing treatment the number of nuclei during phase transformation could be dramatically enhanced and nucleation of precipitates was more homogeneous. The phenomena did not result from the effect of rapid heating or rapid cooling during electropulsing but resulted from the electric current itself. The results were in good agreement with the theoretical model that electric current can increase nucleation by decreasing the thermodynamic barrier during phase transformation.
Enhanced plasticity in a Ti-based bulk metallic glass-forming alloy by in situ formation of a composite microstructure
Authors: He G. , Löser W. , Eckert J. , Schultz L.
Published: March 2008

A bulk metallic glass-forming Ti–Cu–Ni–Sn alloy with in situ formed composite microstructure prepared by both centrifugal and injection casting presents more than 6% plastic strain under compressive stress at room temperature. The in situ formed composite contains dendritic hexagonal-close-packed-Ti solid solution precipitates and a few Ti3Sn, β –(Cu, Sn) grains dispersed in a glassy matrix. The composite microstructure can avoid the development of the highly localized shear bands typical for the room-temperature deformation of monolithic glasses. Instead, highly developed shear bands with evident protuberance are observed, resulting in significant yielding and homogeneous plastic deformation over the entire sample.
Porous calcium aluminate ceramics for bone-graft applications
Authors: Kalita S. J. , Bose S. , Bandyopadhyay A. , Hosick H. L.
Published: March 2008

Calcium aluminate scaffolds with controlled porosity were processed for bone-graft applications. Indirect fused deposition process was used to fabricate these structures. Phase analyses were done using x-ray diffraction technique on powdered samples of calcium aluminates at different compositions. Hg porosimetry was used to determine the pore sizes and the pore volumes present in these controlled porosity structures at different calcium aluminate compositions. Cylindrical samples were tested under uniaxial compressive loading as a function of composition and volume fraction porosity (VFP). Samples of 29% and 44% VFP (designed) with average pore size of 300 μm showed compressive strength between 2 and 24 MPa. Cytotoxicity and cell proliferation studies were conducted with a modified human osteoblast cell line (HOB). These materials showed good cell attachment and a steady cell growth behavior with HOB cells during the first three weeks of in vitro analyses.
Microwave dielectric properties of MO–La2O3–TiO2 (M = Ca, Sr, Ba) ceramics
Authors: Jawahar Isuhak Naseemabeevi , Santha Narayana Iyer , Sebastian Mailadil Thomas , Mohanan Pezholil
Published: March 2008

Single-phase polycrystalline ceramics in the MO–La2O3–TiO2 (M = Ca, Sr, Ba) system, such as cation-deficient hexagonal perovskites CaLa4Ti4O15, SrLa4Ti4O15, BaLa4Ti4O15, and Ca2La4Ti5O18 and the orthorhombic phases CaLa4Ti5O17 and CaLa8Ti9O31, were prepared through the solid-state ceramic route. The phases and structure of the ceramics were analyzed through x-ray diffraction and scanning electron microscopy. The microwave dielectric properties of the ceramics were studied using a network analyzer. The investigated ceramics show high εr in the range 42 to 54, high quality factors with Q ×f in the range 16,222 to 50,215 GHz, and low Tf in the range –25 to +6 ppm / °C. These high dielectric constant materials with high Q × f up to 50,215 GHz are suitable for applications where narrow bandwidth and extremely low insertion loss is necessary, especially at frequencies around 1.9 GHz.
Second harmonic generation from CsLiB6O10-containing glass-ceramics
Authors: Yamamoto Yoshitsugu , Hashimoto Tomohiro , Nasu Hiroyuki , Hashimoto Tadanori , Kamiya Kanichi
Published: March 2008

Glass-ceramics containing ferroelectric CsLiB6O10 crystals were prepared by heat treating Cs2O–Li2O–B2O3 glasses. Second harmonic generation (SHG) was observed from samples prepared by two-step heat treatment (nucleation and growth); however, the intensity was rather weak. On the other hand, one-step heat treatment by the addition of a small amount of TiO2 to the glass resulted in the glass-ceramics containing small single crystal-like CsLiB6O10 particles. The strongest SHG observed was comparable with that of Y-cut quartz. The preferred orientation of precipitated CsLiB6O10 crystals was attributed to the occurrence of SHG from the glass-ceramics.
Superhard B–C–N materials synthesized in nanostructured bulks
Authors: Zhao Y. , He D. W. , Daemen L. L. , Shen T. D. , Schwarz R. B. , Zhu Y. , Bish D. L. , Huang J. , Zhang J. , Shen G. , Qian J. , Zerda T. W.
Published: March 2008

We report here the high-pressure synthesis of well-sintered millimeter-sized bulks of superhard BC2N and BC4N materials in the form of a nanocrystalline composite with diamond-like amorphous carbon grain boundaries. The nanostructured superhard B–C–N material bulks were synthesized under high P–T conditions from amorphous phases of the ball-milled molar mixtures. The synthetic B–C–N samples were characterized by synchrotron x-ray diffraction, high-resolution transmission electron microscope, electron energy-loss spectra, and indentation hardness measurements. These new high-pressure phases of B–C–N compound have extreme hardnesses, second only to diamond. Comparative studies of the high P–T synthetic products of BC2N, BC4N, and segregated phases of diamond + cBN composite confirm the existence of the single B–C–N ternary phases.
Thermal conductivity of ceramics in the ZrO2-GdO1.5system
Authors: Wu Jie , Padture Nitin P. , Klemens Paul G. , Gell Maurice , García Eugenio , Miranzo Pilar , Osendi Maria I.
Published: March 2008

Low thermal conductivity ceramics in the ZrO2–GdO1.5 system have potential in structural (refractories, thermal barrier coatings, thermal protection) and nuclear applications. To that end, the thermal conductivities of hot-pressed xGdO1.5 ·(1 – x)ZrO2 (where x = 0.05, 0.15, 0.31, 0.50, 0.62, 0.75, 0.89, and 1.00) solid solutions were measured, for the first time, as a function of temperature in the range 25 to 700 °C. On the ZrO2-rich side, the thermal conductivity first decreased rapidly with increasing concentration of GdO1.5 and then reached a plateau. On the GdO1.5-rich side, the decrease in the thermal conductivity with increasing concentration of ZrO2 was less pronounced. The thermal conductivity was less sensitive to the composition with increasing temperature. The thermal conductivity of pyrochlore Gd2Zr2O7 (x = 0.5) was higher than that of surrounding compositions at all temperatures. A semiempirical phonon-scattering theory was used to analyze the experimental thermal conductivity data. In the case of pure ZrO2 and GdO1.5, the dependence of the thermal conductivity to the absolute temperature (T) was less than 1/T. Therefore, the minimum thermal conductivity theory was applied, which better described the temperature dependence of the thermal conductivity of pure ZrO2 and GdO1.5. In the case of solid solutions, phonon scattering by cation mass fluctuations and additional scattering by oxygen vacancies on the ZrO2-rich side and by gadolinium vacancies on the GdO1.5-side seemed to account for the composition and temperature dependence of the thermal conductivity.
Investigation of propagation modes and temperature/velocity variation on unstable combustion synthesis
Authors: Li H. P.
Published: March 2008

Combustion synthesis/micropyretic synthesis is a technique in which material synthesis is accomplished by the propagation of a combustion front across the sample. In some cases, the combustion front may propagate in an unstable mode where the propagation velocity and combustion temperature of the combustion front are altered periodically. In this study, the processing conditions leading to unstable combustion reaction were first studied theoretically. The boundary temperatures separating stable and unstable reactions were then determined. The numerical analysis showed that the combustion temperature and the propagation velocity changed periodically during unstable combustion. As the combustion reaction became unstable, the average propagation velocity and the oscillatory frequency of front propagation decreased. The products of unstable combustion synthesis possessed the banded structures, implying the occurrence of the unstable oscillatory propagation, as demonstrated experimentally. In this study, high activation energy combustion (Ti + 2B reaction) and low activation energy combustion (Ni + Al reaction) were both chosen to illustrate the effect of unstable combustion. It is the first time the experimental and numerical results were combined to investigate the temperature and propagation velocity variations during unstable combustion.
Effect of total particle surface area on the light transmittance of glass particle-dispersed epoxy matrix optical composites
Authors: Naganuma Tamaki , Kagawa Yutaka
Published: March 2008

The effect of particle volume fraction fp on the light transmittance of glass particle-dispersed epoxy matrix composites with a particle volume fraction of fp = 0.0001 to 0.4 was studied. The particle size used was much larger than the wavelength of light in a visible wavelength. The transmitted laser beam scattering pattern and light transmittance of the composites were obtained, and the transmitted laser beam pattern showed a broadening, which increased with an increase in particle volume fraction. This behavior appeared more remarkable in particles with smaller diameters. The light transmittance of the composite was affected by the particle volume fraction fp and was divided into two characteristic regions according to fp. For fp < 0.01, the light transmittance was slightly affected by the incorporation of the particles, while the light transmittance of the composite with fp > 0.01 was strongly affected by fp and size of the glass particle dp. The effects of fp and dp on the light transmittance are explained by the introduction of a normalized total surface area 〈 S〉 A guideline to obtain higher light transmittance of the composite is discussed based on the parameter 〈S 〉.
Domain variance and superstructure across the antiferroelectric/ferroelectric phase boundary in Pb1−1.5xLax(Zr0.9TiM0.1)O3
Authors: Knudsen Jesper , Woodward D.I. , Reaney Ian M.
Published: March 2008

Transmission electron microscopy, x-ray diffraction, relative permittivity as a function of temperature, and polarization versus field loops were used to study the antiferroelectric/ferroelectric (AFE/FE) phase boundary in Pb1–1.5xLaxZr0.9Ti0.1O3 (PLZT, 100x/90/10) ceramics. X-ray diffraction and electrical measurements indicated a FE rhombohedral (R) to AFE tetragonal (T) phase transition between PLZT 2/90/10 and 4/90/10. Both phases exhibited superstructure reflections in electron-diffraction patterns at ½{hkl} positions consistent with rotations of the octahedra in antiphase. Previously, neutron diffraction suggested that the FER has an a−a−a− tilt system (Glazer notation), in agreement with its macroscopic symmetry. By analogy, it is proposed that the AFET phase has an a0a0c− tilt system. The AFE phase was also characterized by incommensurate superstructure along pseudocubic 〈110〉p directions, whereas the FE phase had extra commensurate superlattice reflections at 1/2{hk0}p positions. 1/2{hk0}p reflections are forbidden in both tilt systems, but their presence is explained by Pb ion displacements averaged along 〈111〉 but with short coherence antiparallel components along 〈110〉 directions. The antiparallel Pb displacements are coupled to an a−b−b− (a ≈ b) monoclinic tilt system in the vicinity of the AFE/FE boundary.
Using electro-deoxidation to synthesize niobium sponge from solid Nb2O5 in alkali–alkaline-earth metal chloride melts
Authors: Yan X.Y. , Fray D.J.
Published: March 2008

The method of electro-deoxidation was applied to reduce solid Nb2O5 directly to niobium metal, in the form of sponge, in the eutectic CaCl2–NaCl and BaCl2–NaCl systems, respectively. The direct electrochemical reduction of solid Nb2O5 was achieved by electrolysis in the chloride melts at temperatures between 850 and 950 °C and at a controlled potential of 3.1 V, well below the decomposition potentials of the melts used. The obtained results demonstrated that the method is applicable for preparing niobium sponges directly from solid Nb2O5 under the present experimental conditions. The niobium sponge prepared was proven to possess as remarkable superconducting properties. The parameters affecting the rate and the extent of electro-deoxidation were investigated experimentally, including initial particle sizes of the starting Nb2O5 powders, compaction pressure, sintering temperature and times, electrolysis temperature and duration, and electrolyte systems. The mechanism by which the Nb2O5 pellets were electro-deoxidized to the niobium sponges was also discussed on the basis of the present experimental observations.
Synthesis and characterization of sulfur-incorporated microcrystalline diamond and nanocrystalline carbon thin films by hot filament chemical vapor deposition
Authors: Gupta S. , Weiner B.R. , Morell G.
Published: March 2008

The synthesis of microcrystalline and nanocrystalline carbon thin films using sulfur as an impurity addition to chemical vapor deposition (CVD) was investigated. Sulfur-incorporated microcrystalline diamond (μc-D:S) and nanocrystalline carbon (n-C:S) thin films were deposited on Mo substrates using methane (CH4), hydrogen (H2), and hydrogen sulfide (H2S) gas feedstocks by hot-filament CVD. These films were grown under systematically varied process parameters, while the methane concentration was fixed at 0.3% and 2% for μc-D:S and n-C:S, respectively, to study the corresponding variations of the films’ microstructure. Through these studies we obtained an integral understanding of the materials grown and learned how to control key material properties. The nanocrystalline nature of the material was proposed to be due to the change in the growth mechanisms in the gas phase (continuous secondary nucleation). The growth rate (G) was found to increase with increasing TS and [H2S] in gas phase, thus following the chemisorption model that describes the surface reactions. One of the propositions for the increase was that H2S increases the production rates of methane and consequent methyl radicals without much of its own consumption, which is almost negligible and increases the carbon-containing species. This is analogous to the increase of G with increasing methane concentration, but for the relatively high S/C ratio used here, there is a possibility of its incorporation in the material, however small. This particular conjecture was verified. In this context, the results are discussed in terms of the decomposition of reactant gases (CH4/H2/H2S) that yield ionized species. The inferences drawn are compared to those grown without sulfur to study the influence of sulfur addition to the CVD.
Strain-rate sensitivity of hardness of nanocrystalline Ni75at.%Al25at.% alloy film
Authors: Ngan A.H.W. , Pethica J.B. , Ng H.P.
Published: March 2008

Room-temperature indentation experiments carried out on nanocrystalline Ni75at.%Al25at.% alloy films with a range of grain sizes revealed that the strain-rate sensitivity of hardness is nearly zero and that the hardness increases as grain size decreases. The strain-rate insensitivity of hardness indicates that the room-temperature strength of these alloy films is dominated by an athermal, strain-rate-insensitive component. The hardness of the films was found to be in the range of 2.4 to 3.3 GPa, depending on grain size.
Influence of nanoparticle seeding on the phase formation kinetics of sol-gel-derived Sr0.7Bi2.4Ta2O9thin films
Authors: Sung Yun-Mo , Anilkumar Gopinathan M. , Hwang Seung-Joon
Published: March 2008

Sr0.7Bi2.4Ta2O9(SBT) thin films were deposited on unseeded and SBT nanoparticle (approximately 60–80 nm) seeded Pt/Ti/SiO2Si substrates via sol-gel and spin-coating techniques. The SBT thin films were heated at 600 °C for 1 h to form the fluorite phase, and these fluorite films were further heated at 730–760 °C for fluorite-to-Aurivillius phase transformation. The volume fractions of Aurivillius phase formation obtained through quantitative x-ray diffraction analyses showed highly enhanced kinetics in seeded SBT thin films. Johnson–Mehl–Avrami isothermal kinetic analyses were performed for the characterization of Aurivillius phase formation in unseeded and seeded SBT thin films using the volume fraction values. The Avrami exponents were determined as approximately 1.4 and approximately 0.9 for unseeded and seeded SBT films, respectively, which reveals different nucleation modes. By using Arrhenius-type plots, the activation energy values for the phase transformation of unseeded and seeded SBT thin films were determined to be approximately 264 and approximately 168 kJ/mol, respectively. This gives a key reason for the enhanced kinetics in seeded films. Microstructural analyses on unseeded SBT thin films showed formation of randomly oriented needlelike crystals, while those on seeded ones showed formation of domains comprising directionally grown wormlike crystals. On the basis of the phase formation kinetics and microstructural development, a model representing different nucleation and crystal growth mechanisms for the unseeded and seeded SBT thin films was proposed.
Evolution of crystallographic phases in the system (Pb1−xCax)TiO3: A Rietveld study
Authors: Chandra Amreesh , Pandey Dhananjai
Published: March 2008

X-ray powder diffraction studies on (Pb1−xCax)TiO3 ceramic powders revealed the presence of superlattice reflections due to antiphase and inphase tilts of oxygen octahedra for x ≥ 0.421. Rietveld analysis of the powder x-ray diffraction data confirmed that the structure of (Pb1-xCax)TiO3 is orthorhombic with Pbnm space group and a−a−c+ tilt system for x ≥ 0.421. For compositions with 0 < x ≤ 0.416, the structure was tetragonal, and the tetragonality decreased with increasing Ca2+ content.
Tensile and fatigue fracture mechanisms of a Zr-based bulk metallic glass
Authors: Zhang Z.F. , Eckert J. , Schultz L.
Published: March 2008

The tensile and fatigue fracture behavior of Zr59Cu20Al10Ni8Ti3 bulk metallic glass was investigated. It was found that under tensile load the metallic glass always displays brittle shear fracture and the shear fracture plane makes an angle of θT (=54°) with respect to the stress axis, which obviously deviates from the maximum shear stress plane (45°). Under cyclic tension-tension loading, fatigue cracks first initiate along the localized shear bands on the specimen surface, then propagate along a plane basically perpendicular to the stress axis. Tensile fracture surface observations reveal that fracture first originates from some cores, then propagates in a radiate mode, leading to the formation of a veinlike structure and final failure. The fatigue fracture processes of the specimens undergo a propagation stage of fatigue cracks followed by catastrophic failure. Based on these results, a tensile fracture criterion for bulk metallic glasses is proposed by taking the effect of normal stress into account. It is suggested that both normal and shear stresses affect the fracture process of metallic glasses and cause the deviation of the fracture angle away from 45°