bluuebunny/crossref_metadata_2025 huggingface dataset:
Shear faults and dislocation core structures in B2 CoAl
Authors: Vailhé C. , Farkas D.
Published: March 2008

Interatomic potentials of the embedded atom and embedded defect type were derived for the Co–Al system by empirical fitting to the properties of the B2 CoAl phase. The embedded atom potentials reproduced most of the properties needed, except that, in using this method, the elastic constants cannot be fitted exactly because CoAl has a negative Cauchy pressure. In order to overcome this limitation and fit the elastic constants correctly, angular forces were added using the embedded defect technique. The effects of angular forces to the embedded atom potentials were seen in the elastic constants, particularly C44. Planar fault energies changed up to 30% in the {110} and {112} γ surfaces and the vacancy formation energies were also very sensitive to the non-central forces. Dislocation core structures and Peierls stress values were computed for the 〈100〉 and 〈111〉 dislocations without angular forces. As a general result, the dislocations with a planar core moved for critical stress values below 250 MPa in contrast with the nonplanar cores for which the critical stress values were above 1500 MPa. The easiest dislocations to move were the 1/2〈111〉 edge superpartials, and the overall preferred slip plane was {110}. These results were compared with experimental observations in CoAl and previously simulated dislocations in NiAl.
Spinel solid solutions in the systems MgAl2O4–ZnAl2O4 and MgAl2O4–Mg2TiO4
Authors: Petrova M. A. , Mikirticheva G. A. , Novikova A. S. , Popova V. F.
Published: March 2008

Phase relations in two binary systems MgAl2O4–ZnAl2O4 and MgAl2O4–Mg2TiO4 have been studied and phase diagrams for them have been constructed. Based on the data of x-ray phase and crystal-optical analyses, the formation of a continuous series of solid solutions with spinel structure between the terminal members of the systems studied has been established. In the MgAl2O4–ZnAl2O4 system the solid solution is stable in the range from room temperature to melting temperature. In the MgAl2O4–Mg2TiO4 system the solid solution decomposes below 1380 °C, yielding the formation of limited regions of homogeneity on the basis of MgAlM2O4 and Mg2+2δ Ti1–δO4. Decomposition of the solid solution is accompanied by crystallization of MgTiO3.
Phase structure and thermal evolution in coating films and powders obtained by sol-gel process: Part II. ZrO2–2.5 mole% Y2O3
Authors: Caruso R. , Benavídez E. , de Sanctis O. , Caracoche M. C. , Rivas P. C. , Cervera M. , Caneiro A. , Serquis A.
Published: March 2008

Powders and coatings of zirconia doped with 2.5 mole% yttria have been produced via the sol-gel route. The phase structure and subsequent thermal evolution in heating and cooling cycles have been investigated using mainly perturbed angular correlations spectroscopy. Thermal analyses and XRD as a function of temperature have also been performed to obtain complementary information. Upon heating, the amorphous gels crystallized into the tetragonal structure and showed the same hyperfine pattern and thermal behavior as observed in tetragonal zirconia obtained by the ceramic route: the two configurations of vacancies around zirconium ions denoted as t1 and t2 forms and their mutual t1 → t2 transformation. While the powder sample exhibited an incipient thermal instability above 1000 °C and underwent completely the t2 form to m–ZrO2 transition during subsequent, gradual cooling below 500 °C, the coating retained the tetragonal phase within the whole temperature range investigated. Hyperfine results suggest that the tetragonal phase stabilization is favored by the highly defective nature of the t1 form and consequently hardened by the availability of oxygen. The PAC derived activation energy for the fast diffusion of the oxygen vacancies inherent to the t2 form was determined as 0.54 ± 0.14 eV.
Elastic properties of tetragonal PbTiO3 single crystals by Brillouin scattering
Authors: Kalinichev A. G. , Bass J. D. , Sun B. N. , Payne D. A.
Published: March 2008

Brillouin light scattering was used to obtain the elastic and piezoelectric constants for tetragonal PbTiO3 single crystals at room temperature. Approximately 170 measurements of longitudinal and transverse acoustic wave velocities were inverted using literature dielectric constant values to obtain the full set of elastic and piezoelectric constants. Our data indicate a greater electromechanical anisotropy in the polar c direction than previously reported, but are otherwise in general agreement with previous studies. We discuss briefly the degree to which individual elastic and piezoelectric constants are resolved.
Impedance spectroscopy of SrBi2Ta2O9 and SrBi2Nb2O9 ceramics correlation with fatigue behavior
Authors: Chen Tze-Chiun , Thio Chai-Liang , Desu Seshu B.
Published: March 2008

In this research, a fatigue model for ferroelectric materials is proposed. The reasons for the electrical fatigue resistance of SrBi2Ta2O9 (SBT), SrBi2Nb2O9 (SBN), and PbZr1−xTixTixO3 (PZT) are discussed in terms of the bulk ionic conductivities of the compounds. To obtain the bulk ionic conductivity of SBT and SBN, we have used impedance spectroscopy which provides an effective method that allows us to separate the individual contributions of bulk, grain boundaries, and electrode-ferroelectric interfaces from the total capacitor impedance. The bulk ionic conductivities of SBT and SBN (∼10−7 S/cm) are much higher than those of the perovskite ferroelectrics, e.g., PZT (∼10−11–10−10 S/cm). The high ionic conductivities led us to conclude that the good fatigue resistance of SrBi2Ta2O9 and SrBi2Nb2O9 is due to easy recovery of defects. Specifically, oxygen vacancies entrapped within the capacitors are easily released, resulting in limited space charge buildup and domain wall pinning during the polarization reversal process. However, the oxygen vacancies in PZT are trapped at trap sites to become space charges, resulting in capacitor fatigue.
Strontium and calcium zirconyl citrates as precursors for the low-temperature synthesis of SrZrO3 and CaZrO3 fine powders
Authors: Rajendran M. , Rao M. Subba
Published: March 2008

Synthesis and the thermal decomposition behavior of new molecular precursors, strontium, and calcium zirconyl citrates are presented. The pathway to the metazirconate formation has been found to proceed through a multistep process. The precursors yield SrZrO3 and CaZrO3 fine powders at temperatures as low as 650 °C. Physico-chemical, spectroscopic, thermoanalytical, and microscopic techniques have enabled the identification of the sequence of events leading to the perovskite formation and proposition of a thermolysis scheme. Retention of the molecular level mixing of the metal ions during the course of the precursor decomposition is supported by these techniques. Prior to the formation of MZrO3 (M = Sr and Ca) an ionic oxycarbonate, M2Zr2O5CO3 (M = Sr and Ca), intermediate is produced by the thermal decomposition of the citrate precursors.
Pulsed laser deposition of KNbO3 thin films
Authors: Martín M. J. , Alfonso J. E. , Mendiola J. , Zaldo C. , Gill D. S. , Eason R. W. , Chandler P. J.
Published: March 2008

The laser ablation of stationary KNbO3 single crystal targets induces a Nb enrichment of the target surface. In rotated targets this effect is observed only in those areas irradiated with low laser fluence. The composition of the plasma formed close to the target surface is congruent with the target composition; however, at further distances K-deficient films are formed due to the preferential backscattering of K in the plasma. This loss may be compensated for by using K-rich ceramic targets. Best results so far have been obtained with [K]/[Nb] = 2.85 target composition, and crystalline KNbO3 films are formed when heating the substrates to 650 °C. Films formed on (100)MgO single crystals are usually single phase and oriented with the (110) film plane parallel to the (100) substrate surface. (100)NbO may coexist with KNbO3 on (100)MgO. At substrate temperatures higher than 650 °C, niobium diffuses into MgO forming Mg4Nb2O9 and NbO, leading to K evaporation from the film. Films formed on (001) α–Al2O3 (sapphire) show the coexistence of (111), (110), and (001) orientations of KNbO3, and the presence of NbO2 is also observed. KNbO3 films deposited on (001)LiNbO3 crystallize with the (111) plane of the film parallel to the substrate surface. For the latter two substrates the Nb diffusion into the substrate is lower than in MgO and consequently the K concentration retained in the film is comparatively larger.
Schwertmannite: A unique mineral, contains a replaceable ligand, transforms to jarosites, hematites, and/or basic iron sulfate
Authors: Barham R. Joseph
Published: March 2008

An infrared study is made of the chemical changes of synthetic schwertmannite, which normally is a coal oxidation by-product formed by thiobacillus ferrooxidan bacteria. Schwertmannite, a class of minerals, is a metastable compound that transforms to jarosite, hematite, and/or basic iron sulfate at relatively low temperatures, and is analogous in structure to basic iron sulfate, but with a –Fe–O– cage. A proposed formula for schwertmannite is Fe4O4(OH)2·AN(2/e)*nH2O, where AN is the anion andeis its charge, where sulfate can be replaced with other anions, a possible catalyst. The facile conversion of schwertmannite to a black or yellow material might make a useful raw material for the pigment industry.
Iron-silica and nickel-silica nanocomposites prepared by high energy ball milling
Authors: Corrias Anna , Ennas Guido , Musinu Anna , Paschina Giorgio , Zedda Daniela
Published: March 2008

Metal-silica nanocomposites with different metal volume fractions have been prepared via solid state exchange reactions induced by ball milling followed by a reduction treatment in H2flux. In nickel-containing mixtures oxygen transfers directly from NiO to Si while NiO is reduced to Ni. When NiO is present in a large ratio, its excess can be reduced by a thermal treatment in H2flux. Nickel crystallites are obtained with nanometer size in the milling process and there is no significant growth during thermal treatment. Similar process conditions applied to Fe-containing mixtures give rise to a more complex reaction path which prevents the complete conversion of Fe(III) to Fe. Nickel-silica and iron-silica nanocomposites are also produced by ball milling mixtures of either nickel or iron with amorphous silica.
Controlled processing and properties of large Pt-doped Y–Ba–Cu–O pseudocrystals for electromagnetic applications
Authors: Lo Wai , Cardwell D. A. , Dewhurst C. D. , Leung H-T. , Chow J. C. L. , Shi Y. H.
Published: March 2008

The quality and preparation of Sm–Ba–Cu–O (SmBCO) seeds, the level of Pt-doping, and the temperature distribution in the thermal processing system have been investigated in detail and identified as key processing factors in the fabrication of large melt processed Pt dope Y–Ba–Cu–O pseudocrystals. Optimization of these process variables and features has been performed to enable controlled processing of these materials up to several centimeters in diameter. Inhomogeneities and second phases present in the YBCO-Pt pseudocrystal microstructure have been analyzed and correlated with a variation of superconducting properties within the bulk of the sample. These observations have clear implications for the controlled processing of large YBCO-Pt pseudocrystals for electromagnetic applications.
The thermopower of Nd1+xBa2−xCu3Oy in a comparative study of effects of Ba-site doping versus chain-Cu-site doping
Authors: Fisher B. , Genossar J. , Patlagan L. , Reisner G. M. , Knizhnik A.
Published: March 2008

We report the results of measurements of resistivity up to 300 K and of thermoelectric power up to 400 K on ceramic samples of Nd1+xBa2−xCu3Oy with 0 ≤ x ≤ 0.65. The samples were fully oxygenated; they were characterized by x-ray diffraction and iodometric titration. The results are compared with data from literature reporting on effects of substitutions on the Ba site in 1–2–3 compounds and with our earlier experiments on substitution of Co for chain Cu. The focus is on the metal-nonmetal transition and on the nonmetallic regime in all these systems. Tc and the temperature dependence of S close to the metal-nonmetal transition and in the nonmetallic state seem to be determined by a single parameter, irrespective of the nature of the dopant. This result and its implications on the electronic structure of Rba2Cu3Oy (R = Y, or lanthanide ion) will be discussed.
The effect of ion milling on the morphology of ramp-type Josephson junctions
Authors: Blank Dave H. A. , Rogalla Horst
Published: March 2008

Artificial barriers in Josephson junctions make it possible to change the height and width of the barrier independently. This technique can be realized in high-Tc Josephson junctions using the ramp technique. In this article the fabrication of ramp-type junctions is discussed and the importance of the morphology of the ramp is pointed out. Detailed investigations are described which address the surface roughness and the damage due to ion-beam structuring of ramps. It is shown that hard masks can significantly improve the ramp quality by reducing the ion impact angle. Furthermore, annealing of the so structured ramps leads to unit cell steps enforcing a step-flow growth mode.
Development of a reliable materials base for superconducting electronics
Authors: Zhou JiPing , Lo Rung-Kuang , McDevitt John T. , Talvacchio John , Forrester Martin G. , Hunt Brian D. , Jia Q. X. , Reagor D.
Published: March 2008

Careful studies of the corrosion, redox, galvanic, and oxygen evolution/uptake reactions associated with YBa2Cu3O7–δ and related compounds have been completed. These studies have led to an understanding of the many factors that contribute to the poor material characteristics exhibited by these popular high-Tc phases. With knowledge of the structure-reactivity relationships, a powerful crystal engineering approach has been developed that is capable of producing cation substituted versions of YBa2Cu3O7–δ; the resulting compounds therefrom produced exhibit markedly improved processability, oxygen stability, and durability characteristics. These materials have been combined in thin film structures so as to make prototype SNS junctions and SQUID sensors which exhibit promising device performance characteristics.
From meandering to straight grain boundaries: Improving the structures of artificially induced grain boundaries in superconducting YBa2Cu3Oy bicrystals
Authors: Zhang Xiao-Feng , Todt Volk R. , Miller Dean J.
Published: March 2008

This paper presents several key aspects of our approach to preparing artificially induced [001] tilt grain boundaries (GB's) with uniform, well-defined structures in YBa2Cu3Oy (YBCO) superconductors. GB structures formed in thin film and bulk bicrystals, respectively, will be compared. In YBCO thin film bicrystals, meandering rather than planar GB's are formed. Using a low film deposition rate has been demonstrated to reduce the magnitude of meander significantly, but complete elimination of the meander has not yet been accomplished. Thus, we have developed a dual-seeded-melt-texture process to produce uniform, planar GB's with controllable misorientation angles in YBCO bulk bicrystals. Transmission electron microscopy (TEM) studies reveal a remarkably planar and simple configuration on different length scales. Such a simple structure allows for an insightful interpretation of transport behavior across individual GB's.
Kinetics of the alignment and the formation of the Bi(2223) platelets in the powder-in-tube processed Bi(2223)/Ag composite tapes
Authors: Wu Li-jun , Wang Y-L. , Bian Weimin , Zhu Yimei , Thurston T. R. , Sabatini R. L. , Haldar P. , Suenaga M.
Published: March 2008

The alignment and the formation of (Bi, Pb)2Sr2Ca2Cu3O10 inside an Ag sheath were investigated for tapes from four different precursor powders. Microstructural characterization along with in situ transmission x-ray diffraction experiments revealed that the kinetics depended strongly on the processing technique and the phase assemblage of the precursors. The alignment process was governed by the preferential grain growth of the Bi-cuprates along the a-b plane and the constraint applied by the sheath. The formation mechanism of (Bi, Pb)2Sr2Ca2Cu3O10 was either the intercalation or the nucleation and growth, depending on whether excessive liquid is adjacent to the platelets.
Vortex dynamics during N–S and S–N transitions of Y–Ba–Cu–O superconductors under the effect of temperature gradient and thermal cycling
Authors: Kirschner I. , Bódi A. C. , Laiho R. , Lähderanta L.
Published: March 2008

AC susceptibility of ac has been measured simultaneously in three different ranges of Y–Ba–Cu –O ceramic samples in the presence of a large and variable temperature gradient. The results obtained for normal-superconducting or superconducting-normal transitions under the effect of the one-dimensional nonequilibrium temperature distribution reveal the vortex motion to consist of not only conventional flux expulsion (or flux penetration), but flux exchange too, appearing between different ranges of samples and between samples and their close physical environment. The thermal cycles are shown to represent a supplementary heat treatment, increasing the homogeneity of the sample and decreasing the pinning, which accelerate the process of vortex motion.
Pulsed electrodeposition of diamond-like carbon films
Authors: Wang Hao , Shen Ming-Rong , Ning Zhao-Yuan , Ye Chao , Zhu He-Sun
Published: March 2008

Diamond-like carbon (DLC) films have been prepared by electrolysis of methanol solution using a pulse-modulated source. The deposition rate of the films is enhanced significantly compared to that of dc value. That the films do not contain bonded hydrogen is confirmed by infrared spectra. The structures of the films are characterized by Raman spectroscopy. These films show chemical inertness and hardness values in the range 12.5–19 GPa. Current-voltage characteristics of the films are measured, indicating that the resistivity is in the 107 Ω cm range and the breakdown field is larger than 1 MV cm−1.
Nano-sclerometry measurements of superhard materials and diamond hardness using scanning force microscope with the ultrahard fullerite C60 tip
Authors: Blank V. , Popov M. , Lvova N. , Gogolinsky K. , Reshetov V.
Published: March 2008

The new procedure for the hardness measurements of superhard materials including diamond using the scanning force microscope with the ultrahard fullerite C60 tip was developed. It is shown that diamond is plastically deformed under the indentation by the ultrahard fullerite indenter at room temperature. Now the correct measurements of diamond hardness have become possible. The hardness values measured are 137 ± 6 and 167 ± 5 GPa for the diamond faces (100) and (111), respectively.
X7R type lead complex perovskite ferroelectric ceramics with high dielectric constant
Authors: Ruan Lijian , Li Longtu , Gui Zhilun
Published: March 2008

A new X7R type dielectric ceramics for the PMN-BT-PT system with a high dielectric constant (4832) and a low firing temperature (1100–1130 °C) was prepared by the mixed-sintering method. The results of XRD, SEM, EDAX, and dielectric measurements showed that the dielectric ceramics is a two-phase composite with the high dielectric constant originating from the ferroelectric relaxor, and the temperature stability of the dielectric properties from two-phase coexistence.
Study of microstructures of Ag-sheathed (BiPbSrCaCuO) multifilamentary tapes in various stages of processing
Authors: Wang R. K. , Wang X. H. , Bhasale R. , Liu H. K. , Dou S. X.
Published: March 2008

Microstructures of 27-filament (Bi, Pb)2Sr2Ca2Cu3O10+x (BPSCCO 2223) tape at various stages of repetitive rolling and sintering have been investigated using TEM and SEM. It was found that the dislocation density increases with increasing sintering time with the maximum dislocation density of 1012/cm2 achieved for tapes sintered for 220 h. The interface between Ag-sheath and oxide core was observed to be wavelike. Small irregular 2223 colonies and cracks in the oxide cores were often observed near the Ag-sheath/oxide core interface. Repetitively rolled and sintered specimen with a total sintering time of 220 h was observed to have optimum phase purity of 2223 phase. Prolonged sintering results in recrystallization of the 2223 grains, degrading the texture of the oxide core.
First-principles investigation on environmental embrittlement of TiAl
Authors: Liu Y. , Y. K. , Zhang J. H. , Lu G. , Hu Z. Q.
Published: March 2008

To investigate the hydrogen embrittlement and Mn ductilization effects in TiAl, the electronic structures of pure, H-doped, Mn-doped, and Mn, H-codoped TiAl have been studied by the first-principles discrete variationalXacalculations. Local environmental total bond order (LTBO), which is developed for the description of the cohesive properties in a local atom environment involving impurities, should be regarded as a new microscopic criterion for embrittlement. The larger LTBO presents the stronger cohesion and the better ductility of the system. Our results show that H obviously decreases LTBO while Mn increases it, which suggests H as an embrittler while Mn as a ductilizer. It is of key importance to understand hydrogen embrittlement in which hydrogen causes the weakening of its surrounding metal-metal bonds.
Microstructural control of pitch matrix carbon-carbon composite by iodine treatment
Authors: Kajiura H. , Tanabe Y. , Yasuda E. , Kaiho A. , Shiota I. , Yamada S.
Published: March 2008

Matrix microstructure of a pitch-based carbon-carbon composite was controlled by an iodine treatment. Coal-tar pitch having the softening point of 101 °C was used as a matrix precursor. The iodine treatment was carried out on a pitch-impregnated specimen at 90 °C for 3–20 h. The specimen was carbonized at 800 °C and graphitized at 2000–3000 °C. The carbon yield increased from 73% to 93% by the iodine treatment. Microstructures of carbonized specimens changed from a flow type texture to a mosaic type one by the iodine treatment. The microstructural development to graphitic structure was suppressed by the iodine treatment.
The initiation of spontaneous infiltration of alloys into carbon preforms in air
Authors: Yang X. F. , Xi X. M. , Xiao L. M.
Published: March 2008

It was found in a previous work that the Al–Si alloy could spontaneously infiltrate into carbon preforms in air. In this study, the initiation stage of the infiltration process was investigated in detail through two different infiltration experiments. In one experiment, carbon preforms were fully dipped into an alloy bath that was exposed to air, and in the other experiment a carbon preform was only partially dipped into an alloy bath that was protected with a flowing Ar or N2 gas. Experimental results have suggested that the initiation of infiltration is controlled by the pressure of oxidizing gases such as O2 or CO at the infiltration front and is not affected by the presence or absence of N2 gas. The critical pressure of oxidizing gases is estimated to be on the order of 10−4 atm for systems investigated in our experiments. An effective way to reduce the O2 or CO pressure is to flush a preform with nonoxidizing gases during or before infiltration, or to use an active metal to reduce the O2 pressure and thus the corresponding CO pressure.
Metallurgical reactions controlling the brazing of Al2O3 with Ag–Cu–Ti filler alloys
Authors: Paulasto M. , Kivilahti J.
Published: March 2008

Metallurgical reactions controlling the Ti activation in brazing of Al2O3 have been studied by means of microstructural and thermodynamic analysis. The reactions of titanium with oxygen and copper are shown to be decisive in active brazing. Themiscibility gap in the Ag–Cu–Ti system divides the liquid braze into Ag-rich (L1) and TiCu-rich (L2) liquids. The liquid L2 reacts with alumina forming the mixed oxide (Ti, Al)4Cu2O. Besides with alumina, Ti reacts with oxygen that the filler alloys usually contain and forms a brittle ribbon composed of TixO and Ti–Cu–O phases in the braze. The formation of Ti oxides next to the alumina is possible only in the filler alloys of highest Ag content.
Influence of the microstructure of Pt/Si substrates on textured growth of barium titanate thin films prepared by pulsed laser deposition
Authors: Hwang Cheol Seong , Vaudin Mark D. , Schenck Peter K.
Published: March 2008

Pt-coated silicon substrates with strong (111) Pt texture were annealed in an oxidizing atmosphere at temperatures from 500 °C to 750 °C. BaTiO3 thin films were deposited by pulsed laser ablation on the substrates. Observation by transmission electron microscopy showed that the substrate anneal caused the formation of TiO2 in the Pt layer, accompanied by the formation of a high density of faceted protrusions on the Pt surface, particularly at the higher anneal temperatures. The Pt protrusions had (111) facets, parallel to the substrate surface, on which (100)-oriented BaTiO3 grains were observed. BaTiO3 grains with an epitaxial relationship to the Pt lattice were observed on inclined facets of the Pt protrusions [which were not (111) planes], and also on the nonplanar regions of the Pt surface. These epitaxial BaTiO3 grains had (111) preferred orientation relative to the substrate surface. Thus, the BaTiO3 films displayed bimodal growth behavior, with both (100) texture and (111) epitaxy. We propose a model for this behavior based on surface energy considerations.
Transmission electron microscopy of worn zirconia surfaces
Authors: Rainforth W. M. , Stevens R.
Published: March 2008

The dry sliding wear behavior of 3 mol% tetragonal zirconia polycrystals (3Y-TZP) and a composite containing 20 vol.% SiC whiskers have been examined by transmission electron microscopy. High wear rates for the TZP were associated with dramatic microstructural changes. The extreme outer ∼ 400 nm consisted of an amorphous surface layer containing both alumina and zirconia. Below this, the t-ZrO2 grain size was an order of magnitude smaller than in the starting material. At a depth of 1–2 μm the tetragonal grains had become elongated, with a maximum aspect ratio of 30 : 1. The first monoclinic zirconia was found at a depth of 5 μm. In contrast, the composite exhibited a wear rate 5 orders of magnitude lower, associated with minor microstructural changes.
Origin of color in aerosol-derived vanadium-doped zirconia pigments
Authors: Tartaj P. , Serna C. J. , Soria J. , Ocaña M.
Published: March 2008

The preparation of vanadium-zirconia yellow pigments from amorphous spherical V–ZrO2 particles obtained by the hydrolysis of liquid aerosols consisting of a mixture of vanadium (V) oxychloride and zirconium n-propoxide is reported. The composition of the amorphous precursors (V/Zr ratio) and the heating temperature were systematically varied to determine their influence on the optical properties of the pigments. The origin of the yellow color of the pigments was found to be mainly due to the presence of V2O5 in the outer layers of the zirconia grains. A small fraction (∼15%) of the total vanadium content was found as vanadium (IV) forming a solid solution with the monoclinic zirconia lattice.
Elastic loading and elastoplastic unloading from nanometer level indentations for modulus determinations
Authors: Gerberich W. W. , Yu W. , Kramer D. , Strojny A. , Bahr D. , Lilleodden E. , Nelson J.
Published: March 2008

A new method for evaluating modulus and hardness from nanoindentation load/ displacement curves is presented. As a spherical indenter penetrates an elastoplastic half-space, the elastic displacement above the contact line is presumed to diminish in proportion to the total elastic displacement under the indenter. Applying boundary conditions on the elastic and plastic displacements for elastic and rigid plastic contacts leads to an expression that can be best fit to the entire unloading curve to determine E*, the reduced modulus. Justification of the formulation is presented, followed by the results of a preliminary survey conducted on three predominantly isotropic materials: fused quartz, polycrystalline Al, and single crystal W. Diamond tips with radii ranging from 130 nm to 5 μm were used in combination with three different nanoindentation devices. Results indicate that the method gives property values consistent with accepted values for modulus and hardness. The importance of surface roughness and indentation depth are also considered.
Preparation of LaFeO3 particles by sol-gel technology
Authors: Vázquez-Vázquez C. , Kögerler P. , López-Quintela M. A. , Sánchez R. D. , Rivas J.
Published: March 2008

The study of submicroscopic particles in already known systems has resulted in a renewed interest due to the large differences found in their properties when the particle size is reduced, and because of possible new technological applications. In this work we report the preparation of LaFeO3 particles by the sol-gel route, starting from a solution of the corresponding metallic nitrates and using urea as gelificant agent. Gels were decomposed at 200 °C and calcined 3 h at several temperatures, T, in the range 250–1000 °C. The samples were structurally characterized by x-ray diffraction (XRD) showing that the orthoferrite crystallizes at T as low as 315 °C. From the x-ray diffraction peak broadening, the particle size was determined. The size increases from 60 to 300 nm as the calcination T increases. Infrared spectroscopy was used to characterize gels and calcined samples. From these studies a mechanism for the gel formation is proposed. Study of the magnetic properties of LaFeO3 particles shows the presence of a ferromagnetic component which diminishes as the calcination temperature increases, vanishing at T = 1000 °C.
Practical aspects in the drawing of an optical fiber
Authors: Roy Choudhury S. , Jaluria Y.
Published: March 2008

The transport processes in the furnace for the continuous drawing of optical fibers have been studied numerically and analytically. Practical circumstances and operating conditions are considered. A peripheral gas flow configuration has been modeled, along with irises at the ends, as employed in practical furnaces. The neck-down profile of the fiber is not chosen, but has been generated on the basis of a surface force balance. The results obtained are validated by comparisons with earlier experimental results. A detailed analysis has been carried out to determine the relative contributions of different forces during the drawing process. Even though the internal viscous stress is shown to be the major contributor to the draw tension, it is found that under certain operating conditions, the force due to gravity is significant, especially at the beginning of the neck-down region. For a peripheral flow configuration, the effect of flow entrance is found to be very important in determining the necking shape. However, the effect of the iris size on the fiber temperature field is found to be negligible. It is found that for a given furnace temperature and fiber radius, there is an upper limit for draw-down speed at which a fiber can be drawn without rupture. Practical ranges of draw speeds and furnace temperature conditions are identified for the process to be feasible.
Crystallization of ternary Zr-based glasses—Kinetics and microstructure
Authors: Dey G. K. , Savalia R. T. , Baburaj E. G. , Banerjee S.
Published: March 2008

The effect of ternary addition on the thermal stability and the sequence and the kinetics of crystallization of metallic glasses Zr76Fe(24−x)Nix (x = 0, 4, 8, 12, 16, 20, 24) have been examined. It has been found that the surface crystallization occurs in the composition range 16 < x < 20, leading to the formation of an ordered Fe-rich (Fe, Ni)3Zr cubic phase, followed by the transformation of the bulk to a mixture of α−Zr and Zr2Ni. Crystallization of alloys containing 12 to 20% Fe occurs at lower temperatures by primary crystallization of Zr3(Fe, Ni), followed by decomposition of the remaining amorphous matrix by eutectic crystallization giving rise to α−Zr + Zr2Ni. At higher temperatures these alloys transform polymorphically to Zr3(Fe, Ni) in which Ni partially substitutes Fe in the Zr3Fe lattice. Copious nucleation of Zr3(Fe, Ni) phase in these alloys, leading to the formation of a nanophase structure, has been observed. This is consistent with the prediction of increasing nucleation rate for Fe-rich compositions. The crystal nucleation and growth kinetics have been examined for primary, eutectic, and polymorphic crystallization processes. The observed nucleation and growth behaviors have been rationalized by considering the role of the quenched in nuclei and the activation energies of nucleation and growth.
Synthesis of functional ceramic materials from aqueous solutions
Authors: Yao Takeshi
Published: March 2008

Methods for synthesizing ceramic materials from aqueous solutions at ordinary temperature and pressure are advantageous because of the applicability to making films with wide areas and/or complicated shapes with no requirement of vacuum or high temperature, and because of lower cost. Powder of ZrO2 or LnMeO3 (Ln = La, Nd; Me = Cr, Mn, Fe, Co) perovskite was dissolved in hydrofluoric acid and a solution of fluoro-complex ions was obtained. Boric acid was added to the solution, the fluoride ions were consumed by the formation of BF4-, and then the fluoro-complex ions were hydrolyzed to ZrO2 or LnMeO3 in order to increase the amount of fluoride ions. A number of synthesized particles of ZrO2 or LnMeO3 were observed on the substrates in scanning electron microscope images.
Size dependent dewetting and sideband reaction of eutectic SnPb on Au/Cu/Cr multilayered thin film
Authors: Zheng D. W. , Jia Z. Y. , Liu C. Y. , Wen Weijia , Tu K. N.
Published: March 2008

Dewetting of eutectic SnPb on blank Au(500 Å)/Cu(1 μm)/Cr(800 Å) layered structure was found to have a solder size dependence. At 250 °C, if the solder weight fell below 4 mg, dewetting occurred from the center of the solder cap; if the solder weight went beyond 6 mg, dewetting happened from the cap edge. In the latter case, a smaller cap with a higher wetting angle was formed at the center and a ring of solder was left around the edge. Large voids were left in the solder cap after dewetting in both cases. In contrast, all solder caps were found to dewet from the edge on a patterned film at 250 °C if the solder ball was large enough to wet the whole film initially, irrespective of the solder size. For comparison, pure Sn, eutectic SnAg, and eutectic SnBi caps also dewetted from the edge of the Au/Cu/Cr thin film, irrespective of the solder size or whether the substrate was patterned. Since eutectic SnPb on blank Au/Cu/Cr is the only case in which a large sideband growth was found and the dewetting occurred from the center, we postulated sideband to be the main factor which controls the unusual dewetting. The link between them is discussed.
Formation of intermetallic nanocomposites in the Ti–Al–Si system by mechanical alloying and subsequent heat treatment
Authors: Liu K. W. , Zhang J. S. , Wang J. G. , Chen G. L.
Published: March 2008

Formation of nanocrystalline Al3Ti, TiAl, Ti3Al, and Ti5Si3 composites by mechanically alloying (MA) in the Ti–Al–Si system and subsequent annealing treatment are investigated. Microstructure development was monitored by x-ray diffraction, differential thermal analysis, and transmission electron microscopy. An amorphous phase could be generated through milling for 100 h. The results of annealing at different temperatures on this amorphous phase show that the formation of titanium aluminides (Al3Ti, TiAl, and Ti3Al, according to the initial relative amount of Ti and Al) and Ti5Si3 (the only silicide produced by the crystallization reaction) take place. Annealing produces nanocrystalline composites of Al3Ti, TiAl, Ti3Al, and Ti5Si3 with a grain size less than 20 nm. With increasing annealing temperature, the crystalline sizes of the phases increased.
Two critical thicknesses in the preferred orientation of TiN thin film
Authors: Oh U. C. , Je Jung Ho , Lee Jeong Y.
Published: March 2008

The preferred orientation of the TiN film grown by sputter-deposition was studied by the cross-sectional TEM. The preferred orientation was changed from the (200) through the (110), and then finally to the (111) with the film thickness. The cross-sectional microstructure also shows that the film consists of three layers which are all columnar structure. The (111) preferred orientation was observed in the top layer, and the (110) in the middle layer, and finally the (200) in the bottom layer. It is very surprising that the (110) preferred orientation could be observed in a medium thickness region and there are two kinds of critical thicknesses. These results surely show the strong dependence of the change in the preferred orientation on the strain energy in TiN thin films.
Influence of atmosphere on crystallization of zirconia from a zirconium alkoxide
Authors: Collins David E. , Bowman Keith J.
Published: March 2008

Dibutoxybis (acetylacetonato) zirconium, a difunctional zirconium alkoxide, was polymerized at 130 °C for 5 hin vacuoto produce oligomers that could be pyrolyzed to form a tetragonal zirconia (t-ZrO2), metastable at room temperature. This metastable phase was retained considerably below the equilibrium transformation temperature (∼1200 °C) without the use of dopants. Comparative pyrolysis of the oligomers between 600 and 900 °C in either flowing O2or N2for processing times under 12 h indicatedt-ZrO2nucleated first. Pyrolysis in oxygen facilitated transformation to the monoclinic symmetry, whereas pyrolysis in nitrogen demonstrated retention of the tetragonal phase. The formation of oxygen vacancies during pyrolysis, their role in stabilizing the metastable tetragonal phase, and contributions of O2and crystallite size in the polymorphic transformation are discussed.
Aging of iron manganite negative temperature coefficient thermistors
Authors: Battault T. , Legros R. , Rousset A.
Published: March 2008

“Aging,” defined as the drift of resistance with temperature after 1000 h, was investigated for iron manganite temperature coefficient thermistors. For these devices, aging is relatively large, about 10%. The cationic distributions before and after aging were determined by Mössbauer spectroscopy. These distributions explain all the x-ray diffraction and correlated electrical data. The origin of the aging observed on iron manganites thermistors has been identified. It is due to the migration of Fe3+ ions from tetrahedral to octahedral sites of the spinel structure in order to reach a structural equilibrium.
Ba0.7Sr0.3TiO3 ferroelectric film prepared with the sol-gel process and its dielectric performance in planar capacitor structure
Authors: Wang Fan , Uusimäki Antti , Leppävuori Seppo , Karmanenko S. F. , Dedyk A. I. , Sakharov V. I. , Serenkov I. T.
Published: March 2008

Ferroelectric Ba0.7Sr0.3TiO3 thin films were successfully deposited on sapphire (r-cut) substrates by the sol-gel process, and the deposited films were annealed at various temperatures and for various soaking times. The compositional and structural characteristics of the films were systematically examined with the aid of x-ray diffraction, scanning electron microscopy, and medium energy ion scattering techniques. Their dependence on thermal processes was investigated. A planar capacitor structure based on the BSTO films was fabricated to evaluate the electrical and dielectric performance. These results, together with the microstructure characteristics, were analyzed and an optimal process was finally established.
Characterization of sputtered iridium dioxide thin films
Authors: Liao P. C. , Ho W. S. , Huang Y. S. , Tiong K. K.
Published: March 2008

Iridium dioxide (IrO2) thin films, deposited on Si substrates by reactive rf sputtering method under various conditions, were characterized by atomic force microscopy (AFM), x-ray diffraction (XRD), electrical-conductivity, spectrophotometry, ellipsometry and Raman scattering measurements. The average grain sizes of the films were estimated by AFM. A grain boundary scattering model was used to fit the relation between the average grain size and electrical resistivity. The optical and dielectric constants were determined by the ellipsometry measurements. The results of the electrical and optical studies show a metallic character of the films deposited at higher temperatures. The results of XRD and Raman scattering indicate that the IrO2 films deposited at temperatures higher than 300 °C show the presence of (200) texture.
Phase equilibria in the BaTiO3–La2TiO5–TiO2 system
Authors: Škapin Srečo , Kolar Drago , Suvorov Danilo , Samardžija Zoran
Published: March 2008

Subsolidus phase relations in the BaTiO3–La2TiO5–TiO2 part of the ternary BaO–La2O3–TiO2 system at 1300 °C in air were determined. The phases were characterized by x-ray diffraction, scanning electron microscopy, and electron probe wavelength dispersive spectroscopic microanalysis. A combination of techniques was employed because of insensitivity in detecting secondary phases by x-ray diffraction. The location and extent of Ba6−xLa8+2x/3Ti18O54 ternary solid solution 0.2(1) ⩽ x ⩽ 2.3(1) and Ba1−yLayTi1−y/4 (VTi)y/4O3 binary solid solution 0 ⩽ y ⩽ 0.3 at 1300 °C was established. Tie lines between various barium polytitanates with a sequence of Ba6−xLa8+2x/3Ti18O54 solid solution regions were determined.
Thermal evolution of the microstructure of nanosized LaFeO3 powders from the thermal decomposition of a heteronuclear complex, La[Fe(CN)6] · 5H2O
Authors: Traversa Enrico , Nunziante Patrizia , Sakamoto Masatomi , Sadaoka Yoshihiko , Carotta Maria Cristina , Martinelli Giuliano
Published: March 2008

The thermal decomposition of a heteronuclear complex, La[Fe(CN)6] · 5H2O, leads to the preparation of nanosized single-phase perovskite-type LaFeO3 powders. The microstructural evolution of LaFeO3 with the temperature has been studied by x-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The product of the decomposition at 500 °C consists of nanoporous grains which have the morphology of the complex, but diffracting as a monocrystal of LaFeO3. At the higher temperatures, the nanosized particles start to separate from each other, still keeping the shape of the complex grains and forming soft agglomerates. The formation of LaFeO3 from the complex at low temperatures is facilitated by the formation of an orthorhombic transition phase.
Crystallization behavior and cordierite formation in rapidly quenched MgAl2O4–SiO2 glasses of various chemical compositions
Authors: Okada Kiyoshi , Kawashim Hiroshi , Hayashi Shigeo , Sugai Mikio , MacKenzie Kenneth J. D.
Published: March 2008

Crystallization behavior of various compositions of MgAl2O4 –SiO2 glasses was investigated. Glasses with chemical compositions from MgAl2O4/SiO2 = 1/1 to 1/8, spanning the range of cordierite composition (1:2.5) were prepared by a rapid quenching method using an arc image furnace and twin roller. During thermal treatment, all the glasses first crystallized to form high-quartz solid solution (HQss), then transformed to high-cordierite at higher temperature. Transformation from high-cordierite to low-cordierite required prolonged firing times even at high temperature. The crystallization temperature of HQss and the transformation temperature from HQss to high-cordierite changed only slightly in the glasses with MgAl2O4/SiO2 ratios greater than cordierite composition, whereas large increases were found for glasses with MgAl2O4/SiO2 ratios lower than cordierite. The HQss phase appeared in the samples spanning a wide MgAl2O4/SiO2 range and showed superlattice reflections which doubled the fundamental lattice parameters. The cause for HQss formation prior to the appearance of cordierite in these glass samples is discussed from a structural viewpoint involving ordering-disordering of SiO4 and AlO4 tetrahedra deduced from 29Si and 27Al MAS-NMR spectra.
Optical anisotropy of flexible polyimide thin films
Authors: Li Baozhong , He Tianbai , Ding Mengxian , Zhang Ping , Gao Fubin , Jing Feng
Published: March 2008

Optical anisotropy of thin films of an organo-soluble flexible polyimide based on 1,4-bis(3,4-dicarboxyphenoxy) benzene dianhydride (HQDPA) and 2,2-dimethyl-4,4′-methylene dianiline (DMMDA) was detected by a prism-coupler technique. A mechanism is proposed, based on the model of gel film collapse. The degrees of optical anisotropy of the thin films were evaluated via the level of negative birefringence. The residual solvent in the films lessens the levels of negative birefringence so that the residual solvent must be evacuated. The levels of negative birefringence are independent on the solid content of the initial solution, but dependent on the thickness of the films. For a film of 16 μm thick, zero birefringence was achieved, postulated from the dependence of negative birefringence on the thickness of thin films. The relationship between the optical anisotropy and solution properties shows that the degrees of optical anisotropy of thin films on the same scale of thickness depend on macromolecular sizes in their dilute solutions.
Continuous measurements of load-penetration curves with spherical microindenters and the estimation of mechanical properties
Authors: Alcalá J. , Giannakopoulos A. E. , Suresh S.
Published: March 2008

Elastic and plastic properties of metals and Young's modulus of ceramics are determined in the microindentation regime by continuous measurements of load versus depth of penetration with spherical indenters. Calibration procedures, usually applied in nanoindentation experiments, are not needed in the microregime where spherical indenters (rather than sharp indenters with microscopical spherical tips) can be manufactured. As indenters of larger diameters are used, the elastic response of the specimen can be probed during the loading stage of the indentation tests (and not only during unloading, as is the case with nanoindenters). Hence, an accurate determination of Young's modulus can be achieved without a prior knowledge of possible “piling up” or “sinking in” which may occur at the perimeter of the contact area. The contact response of materials is shown to undergo four distinct regions: (i) pre-Hertzian regime, (ii) Hertzian regime, (iii) small-scale plasticity, and (iv) large-scale plasticity. A general methodology for estimation of yield strength and hardening exponent of metals is proposed in the last regime.
Local Electronic Structure in Ordered Aggregates of Carbon Nanotubes: Scanning Tunneling Microscopy/scanning Tunneling Spectroscopy Study
Authors: Carroll D. L. , Ajayan P. M. , Curran S.
Published: March 2008

The recent application of tunneling probes in electronic structure studies of carbon nanotubes has proven both powerful and challenging. Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), local electronic properties in ordered aggregates of carbon nanotubes (multiwalled nanotubes and ropes of single walled nanotubes) have been probed. In this report, we present evidence for interlayer (concentric tube) interactions in multiwalled tubes and tube-tube interactions in singlewalled nanotube ropes. The spatially resolved, local electronic structure, as determined by the local density of electronic states, is shown to clearly reflect tube-tube interactions in both of these aggregate forms.
Evaluation of Young's Modulus of Carbon Nanotubes by Micro-Raman Spectroscopy
Authors: Lourie O. , Wagner H. D.
Published: March 2008

Micro-Raman spectroscopy is used to monitor the cooling-induced compressive deformation of carbon nanotubes embedded in an epoxy matrix. Young's modulus of single- and multiwall nanotubes may then be derived from a concentric cylinder model for thermal stresses, using the D*-band shift for each tube type. The resulting values of the elastic moduli are in very good agreement with predicted theoretical values, and with the published experimental data set of Treacy et al., Nature (London) 381, 678 (1996).