bluuebunny/crossref_metadata_2025 huggingface dataset:
Propagation of Acoustic Waves in Periodic and Random Two-dimensional Composite Media
Authors: Vasseur J. O. , Deymier P. A.
Published: March 2008

Transmission of acoustic waves in two-dimensional composite media composed of arrays of Duralumin cylindrical inclusions embedded in a poly vinyl chloride (PVC) matrix is studied. Experimental and theoretical results for the transmission spectrum of a periodic array of cylinders organized on a square lattice are reported. Local gaps in the first two-dimensional Brillouin zone are predicted and observed. The experimental measurements of power spectra for a random array of inclusions show a weak correlation between disorder and acoustic absorptions up to a high degree of disorder. Only in the case of highly random arrangements does the transmission spectrum diverge from the one obtained with a periodic array of inclusions.
Effects of gravity on processing heavy metal fluoride fibers
Authors: Tucker Dennis S. , Workman Gary L. , Smith Guy A.
Published: March 2008

The effects of gravity on the crystal nucleation of heavy metal fluoride fibers have been studied in preliminary experiments utilizing NASA's KC-135 reduced gravity aircraft and a microgravity sounding rocket flight. Commercially produced fibers were heated to the crystallization temperature in normal and reduced gravity. The fibers processed in normal gravity showed complete crystallization while the fibers processed in reduced gravity did not show signs of crystallization.
The growth of decagonal Al–Co–Ni single crystals as a function of chemical composition
Authors: Zhang B. , Estermann M. , Steurer W.
Published: March 2008

Decaprismatic single crystals taken from a series of alloys of nominal compositions within Al65–77Co3–22Ni3–22have been studied by means of x-ray diffraction techniques. The substitution of Co by Ni in increasing amounts changes the (pseudo)decagonal diffraction patterns drastically and indicates structural changes which range from a single-crystalline approximant via orientationally ordered nanodomain structures and quasiperiodic phases with different types of ordering phenomena, to a basic decagonal phase. A quantum phase diagram analysis shows a clear separation of the stability regions of the ternary systems described in this study and other decagonal phases.
Correlation between energy transfers and solid state reactions induced by mechanical alloying on the Mo33Si66system
Authors: Liu L. , Magini M.
Published: March 2008

Phase transformations of the Mo33Si66powder mixture under different milling conditions have been systematically investigated by x-ray diffraction, and scanning and transmission electron microscopy. The effect of the milling conditions on the Mo/Si solid state reactions (SSR) has been examined in detail. The energy transfer from the milling tools to the powder under processing has been quantified by an already assessed collision model. It has been found that the higher energetic input favors the formation of the room temperature stable phase αMoSi2, while the lower energetic input promotes the formation of the metastable phase βMoSi2. In addition, if the energy transfer is high enough, the Mo/Si reaction proceeds in a form of self-propagating high temperature synthesis (SHS). Thermodynamics and kinetics aspects related to the different SSR's are discussed.
Thermal cycling effects in high temperature Cu–Al–Ni–Mn–B shape memory alloys
Authors: Font J. , Muntasell J. , Pons J. , Cesari E.
Published: March 2008

The effects of thermal cycling through the martensitic transformation have been studied in three Cu–Al–Ni–Mn–B high temperature shape memory alloys. An increase of the martensitic transformation temperatures with the number of cycles (up to ∼7 K after 60 cycles) has been generally observed by DSC measurements. The microstructure of these alloys is rather complicated, with the presence of big manganese or aluminum boride particles and small boron precipitates, as well as the formation of dislocations during thermal cycling. By means of aging experiments, it has been shown that the evolution of transformation temperatures during cycling is mainly due to the step-by-step aging in parent phase accompanying the thermal cycling, and that the dislocations formed during cycling have only a very small effect, at least up to 60 cycles.
Aging of the Inconel 718 alloy between 500 and 750 °C
Authors: Slama C. , Servant C. , Cizeron G.
Published: March 2008

The aging of the NC 19 Fe Nb alloy (Inconel 718), previously quenched from 990 °C, is characterized by a hardness peak at 650 °C, then a maximum in hardness at about 750 °C. Over this temperature, the hardness progressively decreases. In the 550–650 °C temperature range, TEM observations have revealed that β (Ni3Nb) precipitates are formed as long platelets parallel between them within the same grain, as well as extremely fine γ′[Ni3(Ti, Al)] particles responsible for the observed improvement in hardness. For a tempering temperature higher than 650 °C, a first hardening occurs after a 4 h treatment, which has been associated with the γ′ phase precipitation, with a more or less spherical shape. Beyond this time, a second hardening takes place linked to the γ″ phase precipitation (Ni3Nb, bct D022 structure), as thin platelet shaped, perfectly coherent with the matrix. The misfit between the γ and γ″ phases is about 3% in the 〈001〉γ″ direction and lower than 1% in the 〈100〉γ″ and 〈010〉γ″ directions. During a longer aging at 750 °C, the γ″ platelets progressively dissolve while β precipitates grow.
Critical assessments of accommodation process by liquid phase for superplastic flow in Si3N4/Al–Mg–Si metal matrix composites
Authors: Mabuchi Mamoru , Iwasaki Hajime , Jeong Ha-Guk , Hiraga Kenji , Higashi Kenji
Published: March 2008

A liquid phase serves to relax stress concentrations caused by sliding at interfaces and grain boundaries in high-strain-rate superplasticity for aluminum matrix composites. However, the presence of a liquid phase does not always lead to high-strain-rate superplasticity because too much liquid causes decohesion at a liquid phase. The critical conditions of the optimum distribution, thickness, and volume in a liquid phase are discussed based on the observation results by differential scanning calorimetry and transmission electron microscopy. As a result, a very thin and discontinuous liquid phase is required both to assist relaxation of the stress concentrations and to limit decohesion at a liquid phase.
Fabrication of multilaminated Si3N4–Si3N4/TiN composites and its anisotropic fracture behavior
Authors: Huang Jow-Lay , Chang Yen-Lon , Lu Horng-Hwa
Published: March 2008

Laminated composites containing alternate layers of Si3N4 and TiN/Si3N4 materials were used as model material for investigating the crack behaviors and mechanical properties. Results indicated that both strength and toughness in laminated composites were higher than that of monolithic silicon nitride. The failure profiles were affected by the stored strain energy prior to failure and the stress gradient in each layer. Cracks deviated successively from one layer to the other due to periodic stress distribution. Samples with better strength and toughness also had a longer crack propagation path and higher amplitude of crack deviation. The periodic stress distribution in laminated composites was confirmed by the measurements of indentation crack length. Results also suggested a tensile stress in the Si3N4 layer and compressive stress in the TiN/Si3N4 layer, in directions normal to the free sample interface.
Investigation of Si3N4–TiN/Si3N4–Si3N4trilayer composites with residual surface compression
Authors: Huang Jow-Lay , Chou Feng-Chi , Lu Horng-Hwa
Published: March 2008

The present study involved the fabrication of three-layered composites consisting of outer layers that contained Si3N4and an inner layer that contained TiN in a Si3N4matrix. Surface compressive stresses were developed upon cooling due to the relatively higher thermal expansion coefficient (CTE) in the inner layer. The flexural strength of layered Si3N4composites was substantially greater than that of monolithic Si3N4. This was attributed to the surface compressive stress. The effects of TiN composition and inner layer thickness on the mechanical properties were investigated. Layered samples containing 20 vol.% TiN had lower flexural strength than Si3N4–10% TiN/Si3N4–Si3N4due to the formation of microcracks in the inner layer. Crack behaviors in layered samples were affected by the residual stress, interface, and free sample surface. Both theoretical and experimental results indicated that the strength and toughness of layered composites were substantially greater than those of monolithic materials. The determination of fracture toughness in three-layered materials by the surface indentation technique should be done carefully due to the influence of residual stress.
Domain pattern formation in ferroelastic Pb3(PO4)2 by computer simulation
Authors: Parlinski K. , Kawazoe Y.
Published: March 2008

A model of lead phosphate, which describes the rhombohedral-monoclinic phase transition, is used to form domain patterns in the annealing process. The obtained domain structures show W and W′ types of domain walls in agreement with the stress-free laws proposed in Sapriel's theory. The observed W domain walls are parallel to the ternary symmetry axis, while the W′ ones are tilted with respect to the same axis. The antiphase domain walls take no preferential orientations, and remain parallel to the ternary axis. The calculated density of the potential energy of the domain wall of type W is estimated to be Edw = 49 K/Å2 at T = 300 K.
Electrical properties of ultrafine-grained yttria-stabilized zirconia ceramics
Authors: Jiang Shusheng , Schulze Walter A. , Amarakoon Vasantha R. W. , Stangle Gregory C.
Published: March 2008

Nanoparticles of yttria-doped tetragonal zirconia polycrystalline ceramics (Y-TZP) with an average crystallite size of less than 9 nm were prepared by a combustion synthesis process. Dense and fine-grained (<200 nm) Y-TZP ceramics were obtained by fast-firing using temperatures lower than 1400 °C and dwell times of less than 2 min. Impedance spectroscopy was employed to measure conductivities of oxygen vacancies in the grain and the grain boundary of the fine-grained Y-TZP. The relationships between the concentration of the oxygen vacancies in the grain boundary and measurable physical parameters were determined semiquantitatively. The oxygen vacancy concentrations and activation energies for the oxygen-ion conduction in the grain and the grain boundary of the fine-grained Y-TZP were found to be independent of the average grain size in the average grain-size range of 90–200 nm. These experimental results suggest that, in order to retain the abnormally high oxygen vacancy concentrations of the Y-TZP nanoparticles and thus enhance the oxygen-ion conductivity, it may be necessary to decrease the average grain size to approximately 10 nm.
Deposition of mullite and mullite-like coatings on silicon carbide by dual-source metal plasma immersion
Authors: Monteiro Othon R. , Wang Zhi , Brown Ian G.
Published: March 2008

Mullite and mullite-like coatings on silicon carbide have been produced by a metal plasma immersion ion implantation and deposition (Mepiiid) technique based on two cathodic vacuum arc sources and concurrent pulse biasing of the substrate in an oxygen atmosphere. The deposition was carried out at oxygen partial pressures of between 0.66 and 3.33 Pa. The Al : Si ratio in the films varied from 1 : 1 to 8 : 1 and was controlled by varying the pulse duration of the separate plasma guns. High bias voltage was used early in the deposition process in order to produce atomic mixing at the film-substrate interface, while lower bias voltage was used later in the deposition; low ion energy allows control of the physical properties of the film as well as faster deposition rates. The as-deposited films were amorphous, and crystalline mullite was formed by subsequent annealing at 1100 °C for 2 h in air. Strong adhesion between the mullite and the SiC was achieved, in some cases exceeding the 70 MPa instrumental limit of our pull-tester.
Carbothermal synthesis of TaC whiskers via a vapor-liquid-solid growth mechanism
Authors: Johnsson M. , Nygren M.
Published: March 2008

Tantalum carbide whiskers have been synthesized via a vapor-liquid-solid (VLS) growth mechanism in the temperature region 1200–1300 °C in nitrogen or argon. The starting materials consisted of Ta2O5, C, Ni, and NaCl. Carbon was added to reduce tantalum pentoxide, via a carbothermal reduction process, and Ni was used to catalyze the whisker growth. Thermodynamic calculations showed that tantalum is transported in the vapor phase as an oxochloride rather than as a chloride. An alkali metal chloride such as NaCl can be used as a source of Cl. The formation of TaC whiskers was found to be strongly dependent on the processing conditions used, on the choice of precursor materials, e.g., their particle sizes, and on the mixing procedure. So far we have obtained TaC whisker in a yield of 75–90 vol %. These whiskers are 0.1–0.6 μm in diameter and 10–30 μm in length, and they are straight and exhibit smooth surfaces. The main impurities are TaC particles, minor amounts of unreacted carbon, and remnants of the Ni catalyst.
Determination of elastic modulus of thin layers using nanoindentation
Authors: Menčík J. , Munz D. , Quandt E. , Weppelmann E. R. , Swain M. V.
Published: March 2008

Elastic modulus of thin homogeneous films can be determined by indenting the specimen to various depths and extrapolating the measured (apparent) E-values to zero penetration. The paper shows the application of five approximation functions for this purpose: linear, exponential, reciprocal exponential, Gao's, and the Doerner and Nix functions. Comparison of the results for 26 film/substrate combinations has shown that the indentation response of film/substrate composites can, in general, be described by the Gao analytical function. In determining the thin film modulus from experimental data, satisfactory results can also be obtained with the exponential function, while linear function may be used only for thick films where the relative depths of penetration are small. The article explains the pertinent procedures and gives practical recommendations for the testing.
Thermal stresses in carbon-coated optical fibers at low temperature
Authors: Shiue Sham-Tsong , Lee Wen-Hao
Published: March 2008

The thermal stresses in carbon-coated optical fibers at low temperature have been analyzed. The thermally induced lateral pressure in the glass fiber would produce microbending loss. In order to minimize such a microbending loss, the thickness, Young's modulus, and Poisson's ratio of the carbon coating should be decreased. On the other hand, the maximum thermal stress is the tangential stress in the carbon coating that occurs at the interface of the carbon coating and glass fiber. It was experimentally observed that if the maximum thermal stress is larger than the tensile strength of the carbon coating, the carbon coating will be broken along the axial direction. In order to minimize such a maximum thermal stress, the thickness of the carbon coating should be increased, but Young's modulus, thermal expansion coefficient, and Poisson's ratio of the carbon coating should be decreased. Finally, an optimal selection of the carbon coating for optical fiber is discussed.
Directional solidification by appropriate chemically active single crystal seed: An alternative way of generating large superconducting 123 single domain
Authors: Cloots R. , Auguste Fr. , Rulmont A. , Vandewalle N. , Ausloos M.
Published: March 2008

A Dy2O3 single crystal has been used as a seed for the growth of isothermally melt-textured Dy-123 material. The nucleation-controlled step has been observed to be related to the heterogeneous nucleation of 211 particles at the surface of the dysprosium oxide single crystal. The subsequent growth mode seems to be controlled by a high concentration gradient of dysprosium in the liquid phase. This leads to a directional solidification process of the 123 phase. The size of the 211 particles seems to decrease as the distance from the dysprosium oxide single crystal increases.
A time-resolved x-ray diffraction study of Ti5Si3 product formation during combustion synthesis
Authors: Kachelmyer C. R. , Khomenko I. O. , Rogachev A. S. , Varma A.
Published: March 2008

Time-resolved x-ray diffraction (TRXRD) was performed during Ti5Si3 synthesis by the self-propagating high-temperature synthesis mode for different Ti size fractions. It was determined that the time for product formation (ca. 15 s) was independent of Ti particle size. However, the formation of Ti5Si4 phase occurred when relatively large titanium particles were used. A simultaneous measurement of the temperature and TRXRD allowed us to attribute the shifting of XRD peaks at high temperature to thermal expansion of the Ti5Si3 product. The thermal expansion coefficients differ for different crystal planes, and their numerical values compare well with those reported previously in the literature.
Ultrasound driven aggregation and surface silanol modification in amorphous silica microspheres
Authors: Ramesh Sivarajan , Koltypin Yuri , Gedanken Aharon
Published: March 2008

Post formed, silica submicrospheres synthesized by Stober's method have been subjected to a high intensity ultrasound radiation (20 kHz, 100 W/cm2) and their size, morphology, and surface silanol structure modified in situ. The processed silica powders have been characterized by a variety of techniques, such as powder x-ray diffraction (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS), BET nitrogen adsorption, and FT-IR spectroscopy. The silica microspheres formed through an irreversible sol-gel transition have been shown to aggregate by the condensation of interparticle silanols to larger particles under the influence of the shock waves emanating from an imploding cavity. The particle size as a function of sonication time passes through a maximum, suggesting the disintegration of the aggregates on longer exposure to ultrasound radiation. The sonication of dried silica microspheres in an inert dispersant decalin also led to the aggregation of microspheres to a lesser degree, suggesting the deactivation of surface silanols. Infrared spectroscopic investigations suggest a disruption of the hydrogen bonded network of surface silanols. The observed morphological changes have been discussed in terms of direct effect of cavitation on well-formed spheres rather than changes in growth mechanism and capture of primary particles.
Preparation and characterization of nanosized zirconium (hydrous) oxide particles
Authors: Pérez-Maqueda Luis A. , Matijević Egon
Published: March 2008

A method for the preparation of nanosized zirconium (hydrous) oxide particles of narrow size distribution is described. The procedure yields stable dispersions at low temperatures and short reaction times in the absence of surfactants, using inorganic zirconium salts. Crystal structure, particle size distribution, electrokinetic properties, stability, and thermal behavior of the prepared particles were investigated. Colloidal dispersions were treated with ultrasound to study their effect on the crystal structure of the calcined samples.
Ag : Sb and Sb : Ag implantations into high purity silica
Authors: Anderson T. S. , Magruder R. H. , Kinser D. L. , Wittig J. E. , Zuhr R. A. , Thomas D. K.
Published: March 2008

Silica composites containing nanometer dimension colloids have been fabricated by implantation of Ag ions followed by Sb ions, and by implantation of Sb ions followed by Ag ions. Doses for the sequential element implantations were in ratios of 9 : 3 Ag : Sb and 3 : 9 Sb : Ag with the total dose held constant at 12 × 1016 ions/cm2. Energies of implantation were 305 keV for the Ag ions and 320 keV for the Sb ions. Single element colloids were also fabricated by implantation of Ag or Sb using the same nominal dose and implantation energy of the sequential implantations. Approximately spherical particles were formed in all implanted samples. Microstructures of the nanoclusters in the various samples were markedly different. Selected area diffraction techniques revealed that alloyed phases of Ag–Sb were formed in some of the sequential implantations. The microstructure and the optical response of the nanocluster glass composites were found to be strongly dependent upon the order of the ion species implanted. The optical spectra of the 3 : 9 Sb : Ag sample displays two resonance peaks indicative of a Ag resonance peak and a resonance peak of an alloyed phase of Ag–Sb. Optical spectra for the 9 : 3 Ag : Sb sample displays two broad absorption peaks indicative of coated particles.
Insights into the ion-assisted nucleation of diamond on silicon
Authors: McGinnis Sean P. , Kelly Michael A. , Hagström Stig B.
Published: March 2008

The ion-assisted nucleation of diamond was studied in a microwave plasma chemical vapor deposition system to gain insights into the processes controlling this phenomenon. The dependence of the nucleation density on bias voltage and temperature, as well as experiments with an electrically isolated substrate, are consistent with an ion bombardment mechanism for diamond nucleation. However, the growth of these nuclei is dominated by neutral species rather than ions. Measurements of the bias current under various conditions also provide details on the roles of the incident ion flux and substrate electron emission during this process. Furthermore, Monte Carlo simulations of the ion energy distribution at the substrate are compared to experimental measurements. Preferential sputtering, thermal spike, and carbon subplantation nucleation mechanisms are assessed based on the experimental and modeling results.
Interdiffusions and reactions in Cu/TiN/Ti/Si and Cu/TiN/Ti/SiO2/Si multilayer structures
Authors: Rha Sa-Kyun , Lee Won-Jun , Lee Seung-Yun , Kim Dong-Won , Park Chong-Ook , Chun Soung-Soon
Published: March 2008

Sputtered TiN (30–120 nm thick)/Ti (30 nm thick) films were studied as a diffusion barrier between silicon substrate and copper films. The effects of TiN thickness and the existence of a SiO2layer between Ti and silicon substrate on the diffusion barrier property were investigated using various characterization methods. The copper diffusion barrier property of TiN/Ti was found to be affected not only by the TiN thickness, that is diffusion distance, but also by the microstructure of the TiN, which changes with the thickness of TiN film. The existence of the SiO2layer enhanced the diffusion barrier property of TiN/Ti. This is because the SiO2layer between Ti and Si inhibited the formation of titanium silicides, so the Ti layer was available to be used as the sacrificial diffusion barrier for copper.
Diamond nucleation on smooth Si3N4-coated substrate
Authors: Xu Ning , Zheng Zhihao , Du Yuancheng
Published: March 2008

Diamond was deposited on smooth Si3N4-coated Si substrates using hot-filament chemical vapor deposition (CVD). Prior to diamond deposition, the substrates were pretreated under high temperature (700–900 °C) by decomposed pure hydrogen. The experimental results show that these pregrowth processes at a substrate temperature of 850 °C enhanced the diamond nucleation on Si3N4. This indicated that decomposed hydrogen acted on the substrate to produce active centers (<100 nm), which promoted diamond nucleation. It has also been suggested that Si3N4 coating can inhibit carbon transport into the substrate, and it is beneficial to the diamond nucleation.
Pulsed laser deposition of carbon nitride thin films in nitrogen gas ambient
Authors: Okoshi Masayuki , Kumagai Hiroshi , Toyoda Koichi
Published: March 2008

Carbon nitride thin films have been successfully deposited by ablating a graphite target (99.999%) in nitrogen gas ambient using the second (532 nm) or third (355 nm) harmonic of a Q-switched Nd : YAG laser. Carbon nitride films consisting of approximately 40% nitrogen were obtained at 7.5 × 10−3 Pa of nitrogen gas pressure using the third harmonic laser. The C–N chemical bond in the films was observed by x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Ellipsometric studies revealed that the refractive index of the fabricated films decreased with increasing nitrogen concentration.
Influence of nanostructure size on the luminescence behavior of silicon nanoparticle thin films
Authors: Seraphin A. A. , Werwa E. , Kolenbrander K. D.
Published: March 2008

We demonstrate the effect of particle size and quantum confinement on the luminescence properties of nanoscale silicon thin films. Thin films of agglomerated silicon nanoparticles are synthesized using pulsed laser ablation supersonic expansion. Following deposition, standard semiconductor processing techniques are employed to reduce the nanoparticle size. Films are oxidized both in air and chemically to reduce the silicon core dimensions, resulting in a shift of the luminescence emission peak to shorter wavelengths. Removal of the oxide using hydrofluoric acid (HF) results in further blueshifting of the luminescence, as does subsequent reoxidation in air and using nitric acid. The luminescence properties of samples are also studied as a function of excitation intensity. For room temperature excitation with a pulsed 355 nm source, a saturation of the photoluminescence intensity at high excitation intensity is observed, along with a blueshift of the peak PL wavelength. This behavior is found to persist at reduced temperature. A saturation of PL intensity, but no blueshift, is observed for high excitation intensity using a cw 488 nm source at room temperature. At reduced temperatures, no saturation of emission intensity occurs for high intensity 488 nm cw excitation. Both the irreversible shifting of the peak PL wavelength with size reducing treatments and the PL behavior at high excitation intensities indicate that quantum confinement determines the luminescence wavelength.
A novel polymer electrolyte based on oligo(ethylene glycol) 600, K2PdCl4, and K3Fe(CN)6
Authors: Di Noto Vito
Published: March 2008

New electrolytic systems were prepared by reacting K3Fe(CN)6 and K2PdCl4 in a mixture of water and poly(ethylene glycol) 600 (PEG). The reaction occurs in two steps: first a gel is formed, which then shrinks, releasing the solvent. The product thus obtained has the consistency of a smooth, solid plastic paste and is very stable. The influence of the reaction mixture on the structure, morphology, and conductivity of the products was investigated carrying out three preparations (I, II, III) at increasing ratio PEG 600/H2O. By FT-IR studies and analytical data it was concluded that these materials are inorganic-organic networks containing CN bridges between Fe and Pd atoms and PEG 600 bridges between Pd atoms. Scanning electron microscopy studies revealed that the morphology of polymers I, II, and III is significantly influenced by the conditions of the synthesis. Conductivity measurements made at different temperatures showed that polymers I, II, and III conduct ionically. The conductivity of polymer I, which was synthesized with the highest water/PEG 600 ratio, is on the order of 1.4 · 10−3 Sycm at 25 °C.
A novel approach to paint sludge recycling: Reclaiming of paint sludge components as ceramic composites and their applications in reinforcement of metals and polymers,
Authors: Nakouzi S. , Mielewski D. , Ball J. C. , Kim B. R. , Salemeen I. T. , Bauer D. , Narula C. K.
Published: March 2008

About 15 × 106 lbs of paint sludge, produced every year at Ford plants, is disposed of in landfills. An economical alternative to this disposal method, which reclaims or recycles components of paint sludge, is highly desirable to preserve valuable natural resources. Here, we describe an alternative to landfill disposal whereby paint sludge is converted into ceramic composites that can be used as reinforcing materials. The conversion of paint sludge to ceramic composite, I/N2/600, is achieved by pyrolysis under a nitrogen atmosphere. Two additional composites, labeled I/N2/1000 and I/NH3/1000, respectively, are prepared by sintering I/600 at 1000 °C under N2 and NH3. All three composites contain crystalline CaTiO3, BaTiO3, TiO2, amorphous alumina, and carbon. I/NH3/1000 contains an additional crystalline phase of titanium nitride. The application of these composites as reinforcing materials is demonstrated in the fabrication of representative metal matrix composites (MMC's) and reinforced plastic components.
Alternative reagent to mercuric nitrate catalyst for dissolution of aluminum-clad nuclear fuels in nitric acid
Authors: Anderson Philip A. , Christian Jerry D.
Published: March 2008

A substitute additive, HBF4, has been discovered that will replace Hg(NO3)2catalyst for dissolving aluminum spent nuclear fuels in nitric acid for recovery of usable materials. The catalyst or substitute is necessary to penetrate a protective oxide film that continuously forms on the Al surface in the oxidizing acid. A penetration rate of alloy Al-6061 T6 of 40 mg/cm2-h can be achieved in a continuous dissolution process at 100 °C using a dissolvent of 0.15 M HBF4in 7 M HNO3that achieves a steady-state composition of 1.0 M Al3+and 3.3 M HNO3, while maintaining a corrosion rate of a type 304L stainless steel dissolver vessel of 0.015 mm/mo. The penetration rate of aluminum is correlated with the equilibrium concentration of HF in the system. The postulated mechanism involves dissolution of the alumina film by approximately 0.006 M HF in equilibrium with the HBF4and complexed aluminum fluoride species in nitric acid, which provide a large semi-buffered supply of HF. This allows the HNO3to attack the aluminum metal. The small concentration of HF does not compete favorably with HNO3for reaction with and consumption by Al.
Polystyrene foams with dispersed catalyst for a design of recyclable plastics
Authors: Hirose Tamaki , Takai Yoshifumi , Azuma Naoto , Morioka Yoshio , Ueno Akifumi , Okada Mitsunori
Published: March 2008

The present authors have already reported that barium oxide is the most effective catalyst for the conversion of waste polystyrene into styrene including both monomer and dimer. Around 85 wt.% of waste polystyrene was converted into styrene when it was degraded at 623 K in the presence of barium oxide as catalyst. In this paper, the preparation and the thermal degradation of polystyrene films with dispersed catalyst are described for a design of recyclable plastics for better environment. More than 85 wt.% of the films was degraded into styrene at 623 K without addition of other catalytic compounds. Techniques for foaming the films using either nitrogen or carbon dioxide are discussed. It was found that the polystyrene foams with dispersed catalyst are well converted into styrene when they are simply degraded at 673 K.
Processing and properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants
Authors: Suchanek Wojciech , Yoshimura Masahiro
Published: March 2008

This paper reviews the past, present, and future of the hydroxyapatite (HAp)-based biomaterials from the point of view of preparation of hard tissue replacement implants. Properties of the hard tissues are also described. The mechanical reliability of the pure HAp ceramics is low, therefore it cannot be used as artificial teeth or bones. For these reasons, various HAp-based composites have been fabricated, but only the HAp-coated titanium alloys have found wide application. Among the others, the microstructurally controlled HAp ceramics such as fibers/whiskers-reinforced HAp, fibrous HAp-reinforced polymers, or biomimetically fabricated HAp/collagen composites seem to be the most suitable ceramic materials for the future hard tissue replacement implants.
A micromechanistic model of the combustion synthesis process: Mechanism of ignition
Authors: He Cheng , Stangle Gregory C.
Published: March 2008

A micromechanistic model of the combustion synthesis has been extended to study the detailed mechanism and influential parameters of a combustion synthesis process, as well as the development of ignition criteria in the Nb–C system. The case of constant heat-flux ignition conditions has been used to illustrate the details of the ignition process, in order to elucidate the various physical and chemical processes that take place during the initial stages of the combustion synthesis process; however, the results of this study can be generally extended to the other modes of the ignition process. The results showed that the ignition criteria for the Nb–C system corresponded to the establishment of a proper balance between the rates of enthalpy redistribution within the sample, and to the establishment of a kind of positive feedback loop during the ignition process that is necessary for self-propagation to occur. If the heat supplied from an external source to initiate the combustion synthesis process is less than a certain critical value, the combustion wave stops at a certain short distance from the ignition surface. Otherwise, the reaction proceeds in a self-propagating manner.
An attempt to prepare carbon nitride by thermal plasma chemical vapor deposition from graphite and nitrogen
Authors: Matsumoto S. , Chattopadhyay K. K. , Mieno M. , Ando T.
Published: March 2008

RF induction thermal plasma was applied to the chemical vapor deposition of carbon nitride from graphite powders and Ar–N2 gas at about 1 atm. Low-density and fragile amorphous powder-like bulk deposits whose color is light yellow were obtained. Elementary analysis by a combustion method and x-ray photoelectron spectroscopy showed that the N/C ratio is higher than that of stoichiometric C3N4. Also, a large amount of hydrogen and oxygen are included, which seems to be due to the absorption of moisture and oxygen after exposure to air. Infrared absorption spectra suggest the presence of sp CN and sp2 CN bonds, and nitrogen-containing polycondensed ring structures. Thermogravimetric analysis with mass spectroscopy shows that the deposits decompose almost completely at 800 °C, suggesting that the polycondensed rings are not large and not well cross-linked.
Structural investigation of Ba6−3xLn8+2xTi18O54 (x = 0.27, Ln = Sm) by single crystal x-ray diffraction in space group Pnma(No. 62)
Authors: Rawn C. J. , Birnie D. P. , Bruck M. A. , Enemark J. H. , Roth R. S.
Published: March 2008

Single crystals of barium samarium titanium oxide Ba6−3xSm8+2xTi18O54 (x = 0.27) have been synthesized and studied using x-ray diffraction. Superstructure reflections, which cause a doubling of the cell along the short axis, were taken into account and the refinement was conducted in the orthorhombic space group Pnma. Unit cell parameters from single crystal x-ray diffraction were a = 22.289(1), b = 7.642(1), and c = 12.133(1) Å. Refinement on F resulted in R1 = 5.37% for 1410 Fo > 4σ with the thermal parameters of the Sm and Ba atoms refined anisotropically and the thermal parameters of the Ti and O atoms refined isotropically. The structure is made up of a network of corner sharing TiO6−2 octahedra creating rhombic (perovskite-like) and pentagonal channels. The two pentagonal channels are fully occupied by Ba atoms. The refinement suggests that one rhombic channel is fully occupied by Sm atoms (Sm3/Sm4), one rhombic channel is partially occupied by Sm atoms (100% Sm1/86.25% Sm5), and one rhombic channel is shared by BaySm atoms (59.25% Ba3/40.75% Sm2), resulting in a formula of Ba10.38Sm17.08Ti36O108 with Z = 1. The above site occupancies differ from the site occupancies previously reported in the literature for refinements conducted with the short axis approximately equal to 3.8 Å.
Dielectric properties of dielectrophoretically assembled particulate-polymer composites
Authors: Bowen C. P. , Newnham R. E. , Randall C. A.
Published: March 2008

The dielectrophoretic effect is a phenomenon in which dipole-dipole interactions are induced between particles in a suspension by an electric field.1−5 This dipole interaction leads to the formation of chains or fibrils parallel to the applied electric field. Recently, the dielectrophoretic effect has been shown to be a possible composite assembly technique permitting property changes to be induced with the appropriate electric fields.6,7 The results presented in this paper show that the dielectrophoretic assembly process can be used to engineer anisotropy into composite materials. Various filler materials are aligned in a thermoset polyurethane matrix and the dielectric properties are measured. Comparisons are drawn between the dielectrophoretically assembled composites and those processed conventionally in the absence of an electric field. Dielectric properties are modeled with modified mixing laws and discussed in relation to the composite microstructure and the alpha relaxations of the polymer phase.
The preparation of metal-polymer composite materials using ultrasound radiation
Authors: Wizel Shlomit , Prozorov Ruslan , Cohen Yair , Aurbach Doron , Margel Shlomo , Gedanken Aharon
Published: March 2008

Ultrasound radiation is used to prepare a composite material made of polymethylacrylate and amorphous iron nanoparticles. Two preparation methods are described, in which the monomer, methylacrylate, is the starting material. The magnetic properties of the composite material are measured and reveal a superparamagnetic behavior.
Nanometric powders and sintered ceramics studied by atomic force microscopy
Authors: Dias A. , Moreira R. L. , Mohallem N. D. S. , Vilela J. M. C. , Andrade M. S.
Published: March 2008

Atomic force microscopy, as well as the Brunauer, Emmett, and Teller technique and x-ray diffraction, was used to analyze ultrafine NiZn ferrite powders hydrothermally synthesized at 200 °C, for 5 h. The particle sizes, measured through AFM images acquired from the surface of pressed powders, were 52 ± 6 nm, which were higher than those obtained by the other techniques. The particles were monodispersed in size and approximately spherical, meeting the requirements for the production of high density sintered components. The observations performed on ceramic bodies sintered at different conditions (1100–1400 °C, 5 to 240 min) showed necks characteristic of the early stages of sintering (1100 °C) and the expected pore curvature evolution (1400 °C) with sintering time in the final stage of the sintering process. Using a straightforward sample preparation technique, AFM proved to be a powerful tool for direct analysis of ceramic powder particles on the nanometric scale and sintered ceramics at different sintering stages.
Preparation and properties of potassium-vermiculite films
Authors: Minker-Villemin Caroline , Bowen Paul , Lemaître Jacques , Ring Terry A.
Published: March 2008

Flexible films of vermiculite have been prepared from aqueous suspensions after swelling by cation exchange and mechanical delamination. Two different swelling cations, lithium and butylammonium, have been investigated. The degree of swelling and delamination during the suspension preparation was characterized by the percentage of water reabsorbed by dried clays and the adsorption of methylene blue. The vermiculite saturated with lithium ions is more easily delaminated but contains more water than those saturated with butylammonium. Good quality coherent flexible films could be prepared from both the lithium and butylammonium exchanged vermiculites but the high percentage of water found in the films has a detrimental effect on their dielectric properties. To reduce the amount of water in the exchanged vermiculites a second ion exchange with potassium, a less hydratable cation, was investigated. Films prepared after exchange with potassium showed significant improvements in their dielectric properties, with a dielectric constant ∈ around 10 and a dissipation factor tan δ around 0.06 at 25 °C and a frequency of 1 kHz.
Surface precipitates on single crystal LiNbO3 after dry-etching by CHF3 plasma
Authors: Shima Kaori , Mitsugi Naoki , Nagata Hirotoshi
Published: March 2008

The CHF3 electron cyclotron resonance (ECR) plasma etched LiNbO3 (LN) surface was analyzed chemically and crystallographically to investigate the dry-etch machining process for LN crystal, which was recently needed to obtain broader-band optical modulators. The etched surface was entirely covered with amorphous-like precipitates having ~70 nm diameter. These precipitates (or a part of them) were thought to be LiF from Auger electron and x-ray photoelectron spectroscopy. The results indicated that the LiF was formed and remained on the etched surface while the Nb was almost completely removed.
Stresses during thermoset cure
Authors: Adolf Douglas B. , Martin James E. , Chambers Robert S. , Burchett Steven N. , Guess Thomas R.
Published: March 2008

Production problems attributed to excessive stresses generated during the cure of epoxies led us to develop a formalism to predict these stresses. In our first studies, we developed a fundamental understanding of the complex evolution of viscoelasticity as the cure progresses. We then incorporated these results into a proper tensorial constitutive equation that was integrated into our finite element codes and validated using more complicated geometries, thermal histories, and strain profiles. The formalism was then applied to the original production problem to determine cure schedules that would minimize stress generation during cure. During the pursuit of these activities, several interesting and puzzling phenomena were discovered that have stimulated further investigation.
High-temperature x-ray diffraction studies of a precursor mixture for Pb-substituted Bi-2223 superconducting wires
Authors: Drews A. R. , Cline J. P. , Vanderah T. A. , Salazar K. V.
Published: March 2008

High-temperature x-ray diffraction measurements of a (Bi, Pb)-2223 precursor mixture used to produce high-Jcsuperconducting tapes were conducted on silver and ZrO2substrates. The precursor mixture consisted primarily of 2212, Ca2PbO4, and CuO. Phase evolution was markedly sensitive to oxygen partial pressure: In 10% O2growth of the 2223 phase on silver was rapid, proceeded at the expense of the 2212 phase, and was preceded by the disappearance of the Ca2PbO4phase. When slowly heated on a silver substrate in 7.5% O2the 2212 phase melted near 800 °C and subsequently recrystallized near 820 °C in a highly textured form, but with no detectable 2223 formation. Under similar conditions on a ZrO2substrate, the mixture exhibited no marked changes in the XRD patterns up to 850 °C. The dramatic reactivity on silver was also highly dependent on heating rate; rapid heating in 7.5% O2to 825 °C did not result in melting of the 2212 phase or appearance of the 2223 phase. In experiments leading to formation of 2223, thec-lattice parameter of the 2212 phase contracted just prior to the onset of formation of 2223. This result is consistent with the formation of an intermediate Pb-doped phase of 2212. A transient amorphous phase appeared briefly at the onset of formation of 2223. No evidence for intergrowth conversion of 2212 to 2223 was observed.
Formation mechanism of Y2BaCuO5 pattern in growing YBa2Cu3Oxgrains during melt-infiltration process
Authors: Jee Young A. , Kang Suk-Joong L. , Chung Hyungsik
Published: March 2008

When a melt of BaO and CuO mixture was infiltrated into sintered Y2BaCuO5(211) compact to form YBa2Cu3Ox(123) superconductor, butterfly-like plane patterns of 211-free regions were observed to form within growing 123 grains. In a 123 grain, the 211-free region was found to be a pair of vertex-shared pyramids and 211 entrapped region to be the rest of the bulk of the grain. An observation of patterns and cracks formed within 123 grains revealed the base of the pyramids to be (001) plane. The difference in entrapment, which depends on crystallographic planes and results in the formation of the pattern, was explained by the dihedral angles between 123 and 211. The dihedral angle between a - (or b-) plane and 211, which is believed to be greater than zero degree, might cause the entrapment of 211 particles in a [100] (or [010]) direction. In contrast, the dihedral angle of most probably zero degree between c-plane and 211 inhibited the entrapment. The observed shape of 211 particles in front of a-(or b-) and c-planes supports the above explanation of 211 entrapment to form the butterfly-like patterns. was explained by the dihedral angles between 123 and 211. The dihedral angle between a- (or b-) plane and 211, which is believed to be greater than zero degree, might cause the entrapment of 211 particles in a [100] (or [010[) direction. In contrast, the dihedral angle of most probably zero degree between c-plane and 211 inhibited the entrapment. The observed shape of 211 particles in front of a-(or b-) and c-planes supports the above explanation of 211 entrapment to form the butterfly-like patterns.
Optical properties of nanometer-sized CdO organosol
Authors: Xiaochun Wu , Rongyao Wang , Bingsuo Zou , Li Wang , Shaomei Liu , Jiren Xu , Wei Huang
Published: March 2008

In this paper, nanometer-sized CdO organosol was prepared by using microemulsion methods. Its electronic structure and optical properties are characterized through UV-visible light absorption, photoluminescence, and Z-scan techniques. Some newfeatures were observed. Fluorescence and nonlinear optical responses are mainly related to surface trapped states.
Quantitative analysis of microstructures produced by creep of Ti–48Al–2Cr–2Nb–1B: Thermal and athermal mechanisms
Authors: Morris M. A. , Leboeuf M.
Published: March 2008

A γ-based TiAl alloy with equiaxed microstructure and fine grain size has been studied to analyze the deformation mechanisms responsible for the creep behavior. The microstructures produced by creep and high temperature deformation have been examined by TEM to obtain information about the different aspects characterizing the primary and secondary stages of creep. Mechanical twinning has been confirmed to occur in a fraction of the grains that never exceeds 50% while 1/2 ‹110› dislocations are active within all the γ grains. The twins are only responsible for a small amount of strain, but they lead to a subdivision of the microstructure and determine (directly or indirectly) the hardening process observed during the primary stage of creep. We have proposed that during the secondary stage the creep rate is determined by the unblocking of pinned dislocations by processes such as a pipe diffusion or cross slip that allow thermally activated glide of 1/2‹110› dislocations on (001) planes.
Microstructural development of BaTiO3 heteroepitaxial thin films by hydrothermal synthesis
Authors: Chien A. T. , Zhao L. , Colic M. , Speck J. S. , Lange F. F.
Published: March 2008

The hydrothermal growth of epitaxial BaTiO3 thin films on single-crystal SrTiO3 substrates occurs by the island growth mode. The aqueous solution chemistry is found to control interfacial characteristics and plays an important role in controlling film formation and faceting. Island faceting can be changed by the introduction of additional cations during synthesis. Electrophoretic data, confirmed by adsorption measurements, show that barium is a potential determining counterion and adsorbs on SrTiO3 surfaces. Initial electrical measurements show that the BaTiO3 films have a dielectric constant of 141 with a loss tangent of 0.9 that decreases with heat treatment.
The influence of direct current bias on the initial aging of a doped lead magnesium niobate ceramic
Authors: Wang Y. , Chan Y. C. , Gui Z. L. , Li L. T.
Published: March 2008

The initial dielectric aging behaviors of a Mg and Mn doped lead magnesium niobate ceramic were investigated over a wide range of direct current (dc) bias. Both the dielectric constant-log(time) and the loss tangent-log(time) were regressed in terms of a linear relationship. The dc bias is found to have a strong influence on the dielectric parameters at the start of aging and to suppress the aging of dielectric constant and loss tangent. The frequency dependence of the dielectric aging is also evidently affected by the dc bias.