bluuebunny/crossref_metadata_2025 huggingface dataset:
Amorphization mechanisms of NiZr2 by ball-milling
Authors: Galy D. , Chaffron L. , Martin G.
Published: March 2008

The microstructure of NiZr2 in the course of amorphization by ball-milling is studied by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The evolution from the initial fully crystalline alloy to a fully amorphized material is described. It is shown that prior to amorphization, the powder aggregates achieve a 100% nanocrystalline structure; the amorphous phase then appears and develops to the expense of the nanocrystalline phase. No massive chemical disordering is observed, but a small amount cannot be ruled out. It is proposed that amorphization occurs by chemical disordering at interfaces, induced by the scattering of shear waves.
Plastic deformation of oxide scales at elevated temperatures
Authors: Zhang Yifan , Gerberich William W. , Shores David A.
Published: March 2008

The atomic force microscope (AFM) has been used to observe and characterize for the first time surface steps and grooves on the faces of Cr2O3 grains formed as an oxide scale on Ni−30Cr and Ni−30Cr−0.5Y alloys during high temperature oxidation. The very high spatial resolution of the AFM is required to characterize these features. We propose that these surface features, whose dimensions are in the range of nanometers and tens of nanometers, may be interpreted as evidence of highly localized plastic deformation of the oxide scale. The size and spacing of the steps and grooves are consistent with models of plastic deformation based on slip bands derived from dislocation climb or dislocation glide. Mechanical twinning and the models for stress-driven surface instability are also possibly responsible for some surface features. The addition of yttrium to the alloy seemed to enable enhanced plastic deformation of the scale. The strain corresponding to the observed features, estimated by simple models, could relax a significant part of oxide growth and thermal stresses.
Chemical vapor deposited SiC (SCS-0) fiber-reinforced strontium aluminosilicate glass-ceramic composites
Authors: Bansal Narottam P.
Published: March 2008

Unidirectional SrO · Al2O3 · 2SiO2 glass-ceramic matrix composites reinforced with uncoated chemical vapor deposited (CVD) SiC (SCS-0) fibers have been fabricated by hot-pressing under appropriate conditions using the glass-ceramic approach. Almost fully dense composites having a fiber volume fraction of 0.24 have been obtained. Monoclinic celsian, SrAl2Si2O8, was the only crystalline phase observed in the matrix by x-ray diffraction. No chemical reaction was observed between the fiber and the matrix after high temperature processing. In three-point flexure, the composite exhibited a first matrix cracking stress of ∼231 ± 20 MPa and an ultimate strength of 265 ± 17 MPa. Examination of fracture surfaces revealed limited short length fiber pull-out. From fiber push-out, the fiber/matrix interfacial debonding and frictional strengths were evaluated to be ∼17.5 ± 2.7 MPa and 11.3 ± 1.6 MPa, respectively. Some fibers were strongly bonded to the matrix and could not be pushed out. The micromechanical models were not useful in predicting values of the first matrix cracking stress as well as the ultimate strength of the composites.
Synthesis of aluminum oxide-based ceramics by laser photoinduced reactions from gaseous precursors
Authors: Alexandrescu R. , Borsella E. , Botti S. , Cesile M. P. , Martelli S. , Giorgi R. , Turtù S. , Zappa G.
Published: March 2008

Laser-driven synthesis of Al2O3based ceramic powders from gaseous precursors has been accurately investigated. Different concentrations of the reactant gaseous precursors are shown to influence both the process yield and the synthesized powder composition. Depending on the relative concentration of TMA: Al(CH3)3and N2O, the process leads either to the formation of nanocrystallineγ−Al2O3with large free carbon contamination and traces of the Al3O3Nphase or to the formation of a mixedγ−Al2O3, Al2OC compound. The different reaction paths have been attributed to the intermediate formation of aluminum carbide. Particular attention has been paid to the gaseous reaction products to correctly interpret the source of carbon contamination observed in the formed powders. Calcining at moderate (900 °C) and high (1400 °C) temperatures induces nanosizedγ−Al2O3powder and theγ→α-Al2O3transition with particle coalescence and growth.
Microstructural characterization of a titanium-tungsten oxide gas sensor
Authors: Ferroni Matteo , Guidi Vincenzo , Martinelli Giuliano , Sberveglieri Giorgio
Published: March 2008

Thin films of Ti–W–O were prepared from a W–Ti alloy target by rf magnetron sputtering in reactive atmosphere. Analysis devoted to investigate the microstructural properties of this material was carried out in order to explain the origin for the high sensing performance of a W–Ti-oxide gas sensor. Scanning and transmission electron microscopy techniques showed that after annealing the film consists of a polycrystalline layer, isostructural to tetragonal WO3, over which crystallites of pure WO3 are dispersed. The WO3 crystallites are insulated from each other and do not enter into the process of conduction of the layer. It was shown that Ti is soluted in the tetragonal WO3 lattice of the underlying layer. This layer exhibits fine granularity, which is an optimal feature for materials suited to gas sensing.
Combustion synthesis of Si3N4 powder
Authors: Lee Wei-Chang , Chung Shyan-Lung
Published: March 2008

A combustion synthesis (SHS) process has been developed for the synthesis of Si3N4 powder under low nitrogen pressures. Si and NaN3 powders were used as the reactants, and NH4Cl powder was added as a catalytic agent. These powders were mixed and pressed into a cylindrical compact. The compact was wrapped up with an igniting agent (i.e., Ti + C), and the synthesis reaction was triggered by the combustion of the igniting agent. Addition of NH4Cl was found necessary for the combustion synthesis reaction under low nitrogen pressures (< 1.2 MPa). The product as synthesized is mostly in the form of agglomerated fine particles (0.1–1 μm in diameter) and is composed mainly of α-phase and a minor amount of β-phase. Effects of various experimental parameters (N2 pressure, NaN3, NH4Cl, and Si3N4 contents) on the product conversion and the combustion temperature were investigated. A possible reaction mechanism was proposed that explains the effects of the experimental parameters on the synthesis reaction.
Hydrothermal synthesis of PbTiO3 films
Authors: Cho Woo-Seok , Yoshimura Masahiro
Published: March 2008

Well-crystallized polycrystalline films of tetragonal PbTiO3 have been synthesized on Ti metal substrates in concentrated alkaline solutions of Pb(OH)2 containing KOH by the hydrothermal method under saturated vapor pressure (2.0 MPa) at 200 °C. The film microstructure depended strongly on the Pb(OH)2 and KOH concentration. The grain size became larger from approximately 0.6 to 6 μm with an increase of KOH concentration from 1 to 4 M at 0.07 M Pb(OH)2, whereas the grains decreased in size with an increase of Pb(OH)2 concentration at 1 M KOH concentration. PbTiO3 film prepared in the aqueous solution of 0.07 M Pb(OH)2 containing 2 M KOH showed a relative dielectric constant of about 215 and loss of about 5% at 1 kHz.
A four-point bending technique for studying subcritical crack growth in thin films and at interfaces
Authors: Ma Qing
Published: March 2008

A technique was developed to obtain the subcritical crack growth velocity in a 4-point bending sample by analyzing the load-displacement curve. This was based on the observation that the compliance of a beam increases as the crack grows. Beam theory was used to analyze the general configuration where two cracks propagated in the opposite directions. A simple equation relating the crack velocity to the load and displacement was established, taking advantage of the fact that the compliance was linearly proportional to the crack lengths; thus the absolute crack length was not important. Two methods of obtaining crack velocity as a function of load were demonstrated. First, by analyzing a load-displacement curve, a corresponding velocity curve was obtained. Second, by changing the displacement rate and measuring the corresponding plateau load, a velocity value was calculated for each plateau load. While the former was capable of obtaining the dependence of crack velocity versus load from a single test, the latter was found to be simpler and more consistent. Applications were made to a CVD SiO2 system. In both cases of crack propagation either inside the SiO2 layer or along its interface with a TiN layer, the crack growth velocity changed with the stress intensity at the crack tip exponentially. As a result, a small crack will grow larger under essentially any tensile stresses typically existing in devices, provided that chemical agents facilitating stress corrosion mechanisms are also present.
Microstructure and strength of Al-sapphire interface by means of the surface activated bonding method
Authors: Akatsu T. , Sasaki G. , Hosoda N. , Suga T.
Published: March 2008

Sapphire (α–Al2O3) and Al were joined by means of the surface activated bonding (SAB) method in an ultrahigh vacuum at room temperature. Tensile tests have shown that failure occurred not along the interface but inside the Al bulk near the interface. High resolution transmission electron microscopy has revealed the formation of a direct interface between Al and sapphire, indicating the possibility to artificially fabricate an atomically direct interface of dissimilar materials at room temperature. However, an intermediate layer was partially observed, which might be attributed to the effect of fast atom beam irradiation of the sapphire surface.
Grain Refinement Mechanism in Undercooled Cu30Ni70
Authors: Xiao J. Z. , Leung K. K. , Kui H. W.
Published: March 2008

When undercooled molten Cu30Ni70 crystallizes at an undercooling ΔT≅ 145 K (ΔT is defined as Tl – Tk where Tl is the liquidus of Cu30Ni70 and Tk is the kinetic crystallization temperature), its grain size undergoes a rapid decrease by as much as two orders of magnitude in a narrow temperature range. This phenomenon is termed grain refinement. It was found that grain refinement is brought about by multiplication of dendrites. Composition analysis of the dendrite indicates that it has the least Ni concentration at its axis. The Ni content then increases radially from the central axis. Therefore, the dendrite is unstable against melting since the melting temperature of Cu–Ni increases with Ni content. The origin of grain refinement is attributed to the remelting of these dendrites.
Seeded Crystal Growth of Yba2Cu3O6.5 in Semisolid Melts
Authors: Honjo S. , Cima M. J. , Flemings M. C. , Ohkuma T. , Shen H. , Rigby K. , Sung T. H.
Published: March 2008

Melt textured single crystals of YBa2Cu3O6.5 (123) superconductors were produced by isothermal solidification from a semisolid melt using single crystal NdBa2Cu3O6.5 or SmBa2Cu3O6.5 seeds. The microstructure within the single crystals shows an inhomogeneous segregation of Y2BaCO5 (211) particles trapped in the 123 crystals during solidification. The concentration of 211 particles varies with the crystal axes in 123 crystals produced from precursors with compositions of 80 wt.% 123 powder and 20 wt.% excess 211. The 211 particle concentration along the c-axis in the crystal is much lower than that along the a- or b-axes during initial crystallization. This concentration increases in both directions as the crystal grows larger. The 211 concentration along the c-axis increases more quickly than the concentration along the other axes during solidification, which allows the 211 concentration to approach that on the other axes as the solidification continues. 211 particle segregation in stoichiometric 123 samples formed “X”-shaped tracks instead of the variations in 211 concentration described above. A single crystal growth model of 123 is proposed and employed to interpret these experimental observations. Quenched samples were prepared to investigate the volume fraction of 211 particles in the liquid phase. A constant distribution of 211 particles was observed in the liquid, except very near the crystal interface, where the 211 concentration decreased rapidly. Copper oxide content in the liquid was also measured. It is found that the copper content is lower at the (001) interface compared with (100) or (010) interfaces.
Transmission Electron Miscroscopy Observation of the Decomposition of YBa2Cu4O8 into YBa2Cu3O7-δ and CuO
Authors: Reder M. , Krelaus J. , Müller D. , Heinemann K. , Freyhardt H. C.
Published: March 2008

The decomposition of Yba2Cu4O8 (Y-124) into Yba2Cu3O7-δ (Y-123) and CuO at high temperatures has been expected to create Y-123 with finely dispersed CuO precipitates suitable for flux pinning. In fact, samples of thermally decomposed Y-124 exhibit a critical current density, Jc, which is enhanced with respect to the starting material as well as to pure Y-123. Transmission electron microscopy (TEM) studies of furnace annealed Y-124 were not suitable to clarify the reason for this Jc enhancement. Nevertheless, the formation and growth of CuO precipitates have been observed by in situ decomposition of the Y-124 starting material due to electron beam heating within the TEM.
The Thermal Stability of Thin Copper Films Deposited on TiO2(110) Studied by Scanning Tunneling Microscopy
Authors: Carroll D. L. , Wagner M. , Rühle M. , Bonnell D. A.
Published: March 2008

The morphology of Cu thin films is strongly dependent on the temperature of the substrate during and after deposition. Films grown at temperatures between 300 °C and 400 °C form clusters distributed in close packed domains and isolated clusters across the surface. Increased substrate temperatures result in cluster shape evolution indicative of mass flow and sintering. Deposition of Cu at substrate temperatures higher than 500 °C results in a completely different morphology of the film and the suppression of cluster formation. Annealing these Cu films to temperatures of 700 °C allows the system to relax into an equilibrium state characterized by large facets in the film and large areas of exposed surface. These observations are discussed in terms of basic thermodynamic data for bulk Cu oxidation and surface tensions for this system.
Aging Characteristics of a Hybrid Sol-gel Pb(Zr, Ti)O3 Precursor Solution
Authors: Boyle Timothy J. , Dimos Duane , Schwartz Robert W. , Alam Todd M. , Sinclair Michael B. , Buchheit Catherine D.
Published: March 2008

The “aging” characteristics of an acetic acid/methanol solvent-based lead zirconate titanate (PZT) precursor solution, prepared by the Inverted Mixing Order (IMO) process, have been studied for an extended period of time. The changes in film properties were characterized using optical microscopy, optical scattering, and ferroelectric testing. Films generated from the IMO process exhibit an increase in thickness as a function of solution age due to chemical “aging” (esterification) of the precursor solution. This increased thickness results in a decrease in the microstructural uniformity, which affects the electrical and optical properties. In order to understand and eventually control this phenomenon, we have quantified the “aging” of this solution using a variety of analytical methods, including 1H NMR spectroscopy, pH measurements, and Fourier transform infrared (FTIR) spectroscopy. It is of note that we have discovered a method that circumvents this “aging” problem by removal of the volatile material, forming an IMO powder which can be redissolved to produce high quality PZT thin films whenever desired.
Joining of AlN to Cu Using In-base Active Brazing Fillers
Authors: Huh Dae , Kim Dae-Hun
Published: March 2008

Brazing of aluminum nitride (AlN), which is a good ceramic substrate in high power electronic applications, to copper was investigated using In-base active fillers. Compositions of brazing fillers were chosen as In–1 wt.% Ti (IT1), In–19 wt.% Ag–2 wt.% Ti (IAT2), In–15 wt.% Ti (IT15), and In–52 wt.% Ag–20 wt.% Cu–3 wt.% Ti (ACIT3). Brazing operation was performed in vacuum at temperatures of 650–900 °C. The brazing fillers showed good wetting on AlN and led to a strong bond between AlN and braze alloy. From the microstructural analysis, no evidence of reaction layer was clearly found at the interface under the experimental brazing conditions. The composition of brazing alloy layer changed into Cu9In4 phase due to the extensive dissolving of Cu from base metal. Bond strength, measured by 4-point bend test, was obtained as high as 23–30 kgf for the Cu/AlN/Cu joint brazed with IT15 and ACIT3 fillers, and shown to be nearly constant even when the temperature was varied within 700–800 °C. Most of the fracture appeared to proceed through the interior of the AlN ceramic. Based on the experimental results, it is believed that a strong bonding between AlN and braze alloy can be achieved without the apparent forming of a Ti-rich reaction layer at the interface.
Momentum and Thermal Boundary-layer Thickness in a Stagnation Flow Chemical Vapor Deposition Reactor
Authors: Dandy David S. , Yun Jungheum
Published: March 2008

Explicit expressions have been derived for momentum and thermal boundary-layer thickness of the laminar, uniform stagnation flows characteristic of highly convective chemical vapor deposition pedestal reactors. Expressions for the velocity and temperature profiles within the boundary layers have also been obtained. The results indicate that, to leading order, the momentum boundary-layer thickness is inversely proportional to the square root of the Reynolds number, while the thermal boundary-layer thickness is inversely proportional to the square root of the Peclet number. Values computed using the approximate expressions are compared directly with numerical solutions of the equations of motion and thermal energy equation, for a specific set of conditions typical of diamond chemical vapor deposition. Because values of the Lewis number do not vary significantly from unity for many different chemical vapor deposition systems, the expression derived here for thermal boundary-layer thickness may be used directly as an approximate concentration boundary-layer thickness.
Synthesis and Characterization of the Inorganic Ion Exchanger Based on Titanium 2-carboxyethylphosphonate
Authors: Bortun Anatoly I. , Bortun Lyudmila , Clearfield Abraham , Jaimez Enrique , Villa-García María A. , García José R. , Rodríguez Julio
Published: March 2008

An inorganic ion exchanger based on titanium 2-carboxyethylphosphonate (TiPC) has been synthesized by reaction between solutions of TiCl3 and 2-carboxyethylphosphonic acid at elevated temperature. The solid was characterized by chemical analysis, 31P MAS NMR, x-ray powder diffraction, IR spectroscopy, and TG analysis. It was found that TiPC is a highly crystalline layered solid with the interlayer distance 13.1 Å, and exhibits a high thermal stability. The intercalation of n-alkylamines and the ion exchange properties of TiPC toward alkali, alkaline earth, and some transition metal cations have been studied. The exchanger shows high affinity to alkaline earth metal cations and some di- and trivalent cations.
Preparation of Epitaxial BaTiO3 Thin Films by the Dipping-pyrolysis Process
Authors: Kim S. , Manabe T. , Yamaguchi I. , Kumagai T. , Mizuta S.
Published: March 2008

Epitaxial BaTiO3 thin films were prepared on SrTiO3 (100) substrates by the dipping-pyrolysis process using metal naphthenates as starting materials. Highly oriented BaTiO3 thin films were crystallized by heat treatment at 800 °C and higher, from amorphous precursor films pyrolyzed at 470 °C. XRD pole-figure and reciprocal-space mapping analyses showed that the films were epitaxially grown on SrTiO3substrates and were pseudocubic phase with an a║/aτ ratio of 1.003, smaller than the c0/a0 ratio (=1.011) of bulk tetragonal BaTiO3.
In situ Growth of Fatigue-free SrBi2Ta2O9 Films by Pulsed Laser Ablation
Authors: Yang Hung-Ming , Luo Jian-Shing , Lin Wen-Tai
Published: March 2008

In situ growth of SrBi2Ta2O9 (SBT) films as a function of Bi concentration in the target, substrate temperature, oxygen pressure, and the thickness of bottom Pt electrode by pulsed laser deposition was studied. The SBT phase initially formed at a temperature of 500–520 °C. The SBT films grown from the stoichiometric target generally showed Bi deficiency. A well-crystallized and stoichiometric SBT film could be grown at a temperature of 550–580 °C in 300 mTorr of O2 from the surplus Bi targets, which showed c-axis preferred orientation. The formation temperature of SrTa4O11 (ST) phase was above 600 °C, depending on the Bi concentration in the target. Higher oxygen pressure raised the formation temperatures of the SBT and ST phases and concomitantly enriched the Bi concentration of the SBT films. For the bottom Pt electrode 1200 Å thick, the voids were not observed in the SBT overlayer until the deposition temperatures were above 590 °C. Annealing at temperatures above 700 °C in an atmosphere of O2 was required to improve the contact between Pt electrode and the SBT film and hence the ferroelectric properties of the SBT film. In the present study, a smooth, stoichiometric, and c-axis oriented SBT film, about 350 nm thick, could be grown on Pt (1200 Å)/Ti/SiO2/Si at a temperature of 550–580 °C in 300 mTorr of O2 from the Bi surplus targets, which showed remnant polarization (Pr) of 3.0–3.5 μC/cm2 and coercive field (Ec) of 30–40 kV/cm at 4 V. No fatigue was observed up to 109. switching cycles
Preparation and Characterization of BaTiO3 Thin Films on MgO-buffered Si(100) Substrates by RF Sputtering
Authors: Kim Sangsub , Hishita Shunichi
Published: March 2008

We report the results of a study on the deposition and characterization of partially oriented BaTiO3 thin films on MgO-buffered Si(100) by radio-frequency magnetron sputtering. The structural and morphological characteristics of the MgO buffer layer were investigated as a function of substrate temperature. The x-ray θ-2θ, φ scans, and observation of surface morphology revealed that MgO grew with a tendency of (001) orientation. Partially (00l) or (h00) textured BaTiO3 thin films were obtained on Si(100) with the MgO buffer layer while randomly oriented BaTiO3 thin films with large-scale cracks on the surface were made without the MgO layer. Pt/BaTiO3/Pt multistructures were formed on Si(100), MgO/Si(100), and MgO(100) single crystal substrates to conduct preliminary electrical measurements for metal-insulator-metal type capacitor. Comparison of the crystallographic orientation, morphology, and electrical properties between the BaTiO3 films on Si(100) with and without the MgO buffer layer supported the favorable role of the MgO layer as a buffer for the growth of BaTiO3 films on Si(100).
Boron diffusion into diamond under electric bias
Authors: Sung T. , Popovici G. , Prelas M. A. , Wilson R. G. , Loyalka S. K.
Published: March 2008

Three natural type IIa diamond crystals were used for forced diffusion of boron. The diffusion was performed under bias otherwise with the same conditions. The boron diffusion coefficient in diamond was found to be 8.4 × 10−15 and 4 × 10−14 cm2/s at 1000 °C, depending on the direction of the electric field. The drift velocity of boron in diamond under 850 V at 1000 °C was found to be about 1.2 × 10−8 cm/s.
The synthesis of NbSi2 by mechanical alloying
Authors: Lou Taiping , Fan Guojiang , Ding Bingzhe , Hu Zhuangqi
Published: March 2008

The stoichiometric intermetallic compound NbSi2 has been synthesized by mechanical alloying (MA) elemental Nb and Si powders. The alloying process has been investigated by means of x-ray diffraction (XRD) and differential scanning calorimetry (DSC). It was found that the formation of the Nb2Si intermetallic compound occurs abruptly after 65 min of milling without any interruptions during the alloying process. However, short interruptions at a 5 min interval during ball milling result in a gradual reaction for the formation of the NbSi2 compound as well as a new metastable bcc structured solid solution. We conclude that the temperature rise during mechanical alloying plays an important role in initiating the abrupt reaction after an incubation milling time.
Hydrothermal synthesis of heteroepitaxial Pb(ZrxTi1−x)O3 thin films at 90–150 °C
Authors: Chien A. T. , Speck J. S. , Lange F. F.
Published: March 2008

Pb(ZrxTi1−x)O3 and PbZrO3 heteroepitaxial thin films were produced in an aqueous solution (10 M KOH) at ambient pressure and low temperatures (90–150 °C) on (001) SrTiO3 and LaAlO3 single crystal substrates. Growth of the Pb(ZrxTi1−x)O3 and PbZrO3 thin films initiates by the formation of {100} faceted islands. Energy dispersive spectroscopy (EDS) analysis of the Pb(ZrxTi1−x)O3 thin film shows that the Zr: Ti ratio is 45: 56, nearly identical to the molar ratio of the precursors. This route might provide a viable low temperature alternative for the formation of high dielectric constant thin films for applications such as dynamic random access memory (DRAM).
Raman microscopy examination of phase evolution in Bi(Pb)–Sr–Ca–Cu–O superconducting ceramics
Authors: Wu K. T. , Fischer A. K. , Maroni V. A. , Rupich M. W.
Published: March 2008

Raman microspectroscopy (RMS) and imaging Raman microscopy (IRM) were used to probe the composition and spatial distribution of chemical phases in Bi(Pb)–Sr–Ca–Cu–O (BSCCO) ceramic superconductor powders and silver-BSCCO composites. The Raman techniques were used to identify various phases, including alkaline earth cuprates, CuO, Bi-2212, Bi-2223, and Pb-containing phases. Changes in the Ca/Sr ratios in (Ca, Sr)2CuO3 phases were distinguished by differences in orientation with respect to polarization of the exciting radiation. Variations were observed in the content and distribution of lead in various phases formed during intermediate stages of the thermal processing of composite conductors. The spatial distribution of the various phases detected in powder and composite conductors was established to a resolution of a few microns by collecting images of the Raman scattering at wavelengths corresponding to the signature peaks of the observed phases. Reference Raman spectra of the major phases observed in the BSCCO system are also reported. The Raman techniques, when combined with complementary techniques, such as x-ray diffraction and electron microscopy, can provide valuable information about the reaction paths and mechanisms of the high temperature BSCCO superconducting ceramics.
Thin films for superconducting electronics: Precursor performance issues, deposition mechanisms, and superconducting phase formation-processing strategies in the growth of Tl2Ba2CaCu2O8films by metal-organic chemical vapor deposition
Authors: Hinds Bruce J. , McNeely Richard J. , Studebaker Daniel B. , Marks Tobin J. , Hogan Timothy P. , Schindler Jon L. , Kannewurf Carl R. , Zhang Xiao Feng , Miller Dean J.
Published: March 2008

Epitaxial Tl2Ba2CaCu2O8thin films with excellent electrical transport characteristics are grown in a two-step process involving metal-organic chemical vapor deposition (MOCVD) of a BaCaCuO(F) thin film followed by a postanneal in the presence of Tl2O vapor. Vapor pressure characteristics of the recently developed liquid metal-organic precursors Ba(hfa)2• mep (hfa = hexafluoroacetylacetonate, mep = methylethylpentaglyme), Ca(hfa)2• tet (tet = tetraglyme), and the solid precursor Cu(dpm)2(dpm = dipivaloylmethanate) are characterized by low pressure thermogravimetric analysis. Under typical film growth conditions, transport is shown to be diffusion limited. The transport rate of Ba(hfa)2• mep is demonstrated to be stable for over 85 h at typical MOCVD temperatures (120 °C). In contrast, the vapor pressure stability of the commonly used Ba precursor, Ba(dpm)2, deteriorates rapidly at typical growth temperatures, and the decrease in vapor pressure is approximately exponential with a half-life of ∼9.4 h. These precursors are employed in a low pressure (5 Torr) horizontal, hot-wall, film growth reactor for growth of BaCaCuO(F) thin films on (110) LaAlO3substrates. From the dependence of film deposition rate on substrate temperature and precursor partial pressure, the kinetics of deposition are shown to be mass-transport limited over the temperature range 350–650 °C at a 20 nm/min deposition rate. A ligand exchange process which yields volatile Cu(hfa)2and Cu(hfa) (dpm) is also observed under film growth conditions. The MOCVD-derived BaCaCuO(F) films are postannealed in the presence of bulk Tl2Ba2CaCu2O8at temperatures of 720–890 °C in flowing atmospheres ranging from 0–100% O2. The resulting Tl2Ba2CaCu2O8films are shown to be epitaxial by x-ray diffraction and transmission electron microscopic (TEM) analysis with thec-axis normal to the substrate surface, with in-plane alignment, and with abrupt film-substrate interfaces. The best films exhibit aTc= 105 K, transport-measuredJc= 1.2 × 105 A/cm2at 77 K, and surface resistances as low as 0.4 mΩ (40 K, 10 GHz).
Evaluation of a porous fiber coating in SiC–Si3N4minicomposite
Authors: Ogbuji Linus U. J. T.
Published: March 2008

A porous and discontinuous Si3N4-based fiber coating was evaluated in SiC/Si3N4minicomposites by fiber push-out and fractography for debond capabilities following high temperature exposures. The exposures consisted of 500 h annealing at 1350 °C, and 25–100 h oxidation in wafer form at 1200–1400 °C. In spite of considerable microstructural coarsening and/or oxidation of the coating, fiber debond and sliding were achieved in all cases, and at stress levels comparable to those reported for tough, as-fabricated SiC/Si3N4composites with a carbon or boron nitride interphase. Therefore, the porous coating is expected to perform better than C or BN in applications where oxidation is inevitable. The role of porosity in accommodating and mitigating the effects of oxidation is thought to be important.
Heteroepitaxial growth of lanthanum aluminate films derived from mixed metal nitrates
Authors: Ng Man Fai , Cima Michael J.
Published: March 2008

Epitaxial lanthanum aluminate (LaAlO3) thin films were deposited on single-crystal substrates by pyrolysis of spin-on mixed nitrate precursors. The films are epitaxial without any second phase. TEM micrographs show that all of these films have pores with sizes ranging from 5 to 30 nm. Grain boundaries are not observed. Selected area diffraction shows that the films are single-crystal-like, despite the porosity. All the films are smooth and crack-free. The precursors first decompose into an amorphous mixture. Heterogeneous nucleation occurs on the lattice-matched, single-crystal substrate surface. The epitaxial films grow upward and consume the amorphous regions. The crystallization temperature of LaAlO3 is lower for thin films than for bulk samples due to nucleation on the substrate. The crystallization of LaAlO3 does not exhibit linear growth kinetics. The Johnson–Mehl–Avrami exponent of growth is between 1.4 and 1.5. This deviation from the linear growth model (n = 1) can be attributed to continuous nucleation on the substrate/film interface.
The ballistic failure mechanisms and sequence in semi-infinite. supported alumina tiles
Authors: Sherman Dov , Brandon D. G.
Published: March 2008

The basic ballistic failure mechanisms and their sequence occurring in dense alumina tiles during projectile penetration were investigated. The alumina tiles were supported by semi-infinite support blocks made of three different materials. Initially, a drop-weight test (DWT) was used to gain an insight into the damage mechanisms and sequence during quasi-static impact conditions. The quasi-static damage mechanisms were compared with the damage obtained in 0.3 cal. armor-piercing tests (APT). The DWT's results suggested the following sequence of quasi-static failure mechanisms: Radial tensile cracks, associated with the low tensile strength of the ceramic formed initially, as a result of the bending induced by local deformation at the opposing surface. Subsequently, a shear-dominated cone crack propagated from the edge of the contact zone. If sufficient energy was available, crushing of the material beneath the contact zone developed during the final stages of failure. It is shown that these so-called “quasi-static” damage mechanisms, identified from the DWT's, also corresponded to the damage mechanisms and sequence during APT's.
Spectroscopic study of Na–TCNQ in a poly(ethylene oxide) matrix (TCNQ = 7, 7′, 8, and 8′ tetracyanoquinodimethane)
Authors: Arena A. , Marco G. Di , Lanza M. , Patané S. , Saitta G.
Published: March 2008

X-ray Photoelectron Spectroscopy (XPS) and optical studies have been performed on P(EO)n –NaI–TCNQ (n = 1, 10) systems, obtained by means of a solid state reaction technique. The electron spectroscopy, the absorption in the ultraviolet visible near infrared range, and the infrared transmission findings allow us to conclude that the interaction among the polymer, the iodide, and the electron acceptor molecule results in the formation of the ion-radical salt Na–TCNQ and in the oxidation of I− to the metallic state. The presence within the examined specimens of crystalline and amorphous phases has been confirmed by the results of Differential Scanning Calorimetry (DSC) measurements. The optical, electronic, and thermal properties of the P(EO)n–NaI–TCNQ mixtures have then been correlated to the results of preliminary ac electrical transport investigations.
Formation kinetics of nanocrystalline Fe–4 wt.% Al solid solution during ball milling
Authors: Jiang H. G. , Perez R. J. , Lau M. L. , Lavernia E. J.
Published: March 2008

Formation of nanocrystalline Fe–4 wt.% Al solid solution has been achieved through SPEX ball milling of blended elemental Fe and Al powders. Differential scanning calorimetry (DSC) and x-ray diffraction (XRD) have been employed to follow the structural evolution during the ball-milling process. Exothermic peaks exhibited in DSC diagrams of the powders milled for 10 to 60 min yielded thermal enthalpies corresponding to the formation of Fe–4 wt.% Al solid solution, from which the kinetics of formation were found to follow the Johnson–Mehl–Avrami equation. Assessment of the kinetic parameter n reveals a diffusion controlled mechanism, in which grain and interphase boundaries may play a crucial role, during the solid solution formation of Fe–4 wt.% Al.
Pulsed laser deposition and characterization of conductive RuO2 thin films
Authors: Iembo A. , Fuso F. , Arimondo E. , Ciofi C. , Pennelli G. , Curró G. M. , Neri F. , Allegrini M.
Published: March 2008

RuO2 thin films have been produced on silicon-based substrates by in situ pulsed laser deposition for the first time. The electrical properties, the surface characteristics, the crystalline structure, and the film-substrate interface of deposited samples have been investigated by 4-probe resistance versus temperature technique, scanning electron microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, and transmission electron microscopy, respectively. The films show good electrical properties. The RuO2-substrate interface is very thin (≈3 nm), since it is not degraded by any annealing process. These two characteristics render our films suitable to be used as electrodes in PZT-based capacitors.
Phase coexistence and critical temperatures of the (Bi, Pb)2Sr2Ca2Cu3Ox phase under partial pressures of oxygen between 10−3 and 0.21 bar with and without additions of silver
Authors: Däumling M. , Maad R. , Jeremie A. , Flükiger R.
Published: March 2008

We have investigated the stability of the (Bi, Pb)2Sr2Ca2Cu3Ox phase for the stoichiometry (Bi: Pb: Sr:Ca: Cu = 1.72: 0.34: 1.83: 1.97: 3.13), subjecting it to temperatures between 700 and 850 °C under various oxygen partial pressures. A narrow region was found in which Bi, Pb(2223) was the only superconducting phase. This region follows closely the thermal decomposition line. X-ray pure Bi, Pb(2223) will partially decompose if treated outside of the stability region. For a given oxygen partial pressure, the Bi, Pb(2223) phase tends to coexist with the 2201 phase for temperatures above, and the 2212 phase for temperatures below this region. At even lower temperatures an additional lead-rich phase appears. Critical temperatures Tc vary little with treatment and range between 108.5 K and 110.8 K. If 10% silver is added to the starting powder, the phase coexistence regions shift. Silver does not seem to have a significant effect on the absolute values of the critical temperature. The Bi, Pb(2223) thermal decomposition temperature for a given oxygen pressure is lowered by at least 10 K by the presence of Ag.
Microstructures of two-phase Ti–Cr alloys containing the TiCr2 Laves phase intermetallic
Authors: Chen Katherine C. , Allen Samuel M. , Livingston James D.
Published: March 2008

Microstructures of two-phase Ti–Cr alloys (Ti-rich bcc + TiCr2 and Cr-rich bcc + TiCr2) are analyzed. A variety of TiCr2 precipitate morphologies is encountered with different nominal alloy compositions and annealing temperatures. Lattice constants and crystal structures are determined by x-ray diffraction (XRD) and transmission electron microscopy (TEM). Orientation relationships between the beta bcc solid solution and C15 TiCr2 Laves phase are understood in terms of geometrical packing, and are consistent with a Laves phase growth mechanism involving twinning.
Superparamagnetic particles in ZSM-5–type ferrisilicates
Authors: López A. , Lázaro F. J. , García-Palacios J. L. , Larrea A. , Pankhurst Q. A. , Martínez C. , Corma A.
Published: March 2008

As-synthesized, low iron content, ferrisilicates of ZSM-5-type contain well-separated Fe(III) ions in a tetrahedral environment and display paramagnetic behavior. After hydrothermal treatment, the iron ions are partially extracted from the framework, generating nanosize iron oxide or oxyhydroxide ferrimagnetic particles. This process has been studied by transmission electron microscopy (TEM), Mössbauer spectroscopy, magnetic ac susceptibility (χac), and field dependent magnetization, on samples containing up to 6.7 wt.% Fe. The experiments evidence the growth of nonaggregated particles, with a typical size around 3 nm, presumably located at the surface of the ferrisilicate crystallites. From a thorough granulometric analysis involving TEM and χacdata, it is concluded that, in the range from 1.5 to 4.6 wt.% Fe, the particle size distributions are significantly independent of the iron content.
Growth and structures of C60 shells
Authors: Sakuma H. , Tachibana M. , Sugiura H. , Kojima K. , Ito S. , Sekiguchi T. , Achiba Y.
Published: March 2008

The growth of the shells of C60 crystals was carried out under various conditions. The detailed structures of the grown shells were investigated by transmission electron microscopy and Raman spectroscopy. The shells were formed during thermal sublimation of the C60 crystals, which were irradiated with white light in air. The shells were mainly composed of a kind of amorphous carbon. From these results, it is suggested that the oxygen-induced disintegration of C60 cages is responsible for the shell formation.
Determination of local high-frequency dielectric function during the cubic-to-tetragonal phase transformation in barium titanate
Authors: Katti Kalpana S. , Qian Maoxu , Sarikaya Mehmet , Miyayama Masuru
Published: March 2008

Transmission electron energy loss spectroscopy was used to obtain local dielectric properties in barium titanate. The high frequency dielectric function of the material was studied dynamically during the cubic-to-tetragonal (c → t) phase transformation in conjunction with the effect of a small amount (0.9%) of donor dopant (niobium). In order to obtain the local dielectric function during the phase transformation, Kramers–Kronig relations were applied to the energy loss measurements. The optical excitations in the energy loss spectra were consistent with band structure results from the literature. The Re (1/∈), real part of the inverse dielectric function, obtained from the energy loss data indicated a change at the phase transformation. Specifically, a broadening of the valence plasmon excitation is observed which is attributed to the order-disorder nature of the t → c transformation. A 0.4 eV shift in the volume plasmon was observed in the Nb-doped sample in all regions (within grains as well as at grain boundaries), indicating a uniform incorporation of the dopant in the lattice. In this paper, the changes in the dielectric function, such as shifts in collective excitations, are attributed to a large contribution from loosely bound Nb electrons. Furthermore, it is demonstrated that it is possible to obtain local (≈10 nm) physical property of a complex material dynamically at relatively high temperature.
Local Cross-sectional Profiling of Multilayer Thin Films with an Atomic Force Microscope for Layer Thickness Determination
Authors: LaGraff J. R. , Murduck J. M.
Published: March 2008

A new essentially nondestructive cross-sectional method is described for measuring the individual thicknesses of multilayer YBa2Cu3O7 (YBCO) and SrTiO3 (STO) thin films using off-axis ion milling and the atomic force microscope (AFM). Since the ion-milling is done during routine patterning of a thin-film device and the AFM requires only a small area for imaging, no additional sample preparation is required. This is a significant improvement over traditional cross-sectional techniques which often require lengthy and destructive sample preparation. Also, there is no a priori reason that this technique would not be amenable to other multilayer thin-film systems.
Scanning Tunneling Microscopy and Atomic Force Microscopy Investigation of Organic Tetracyanoquinodimethane Thin Films
Authors: Gao H. J. , Zhang H. X. , Xue Z. Q. , Pang S. J.
Published: March 2008

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) investigation of tetracyanoquinodimethane (TCNQ) and the related C60-TCNQ thin films is presented. Periodic molecular chains of the TCNQ on highly oriented pyrolytic graphite (HOPG) substrates were imaged, which demonstrated that the crystalline (001) plane was parallel to the substrate. For the C60-TCNQ thin films, we found that there were grains on the film surface. STM images within the grain revealed that the well-ordered rows and terraces, and the parallel rows in different grains were generally not in the same orientation. Moreover, the grain boundary was also observed. In addition, AFM was employed to modify the organic TCNQ film surface for the application of this type of materials to information recording and storage at the nanometer scale. The nanometer holes were successfully created on the TCNQ thin film by the AFM.
Unidirectional Partial Melting and Solidification of SmBCO Superconductor
Authors: Sumida M. , Nakamura Y. , Shiohara Y. , Umeda T.
Published: March 2008

Microstructure control of the SmBCO superconductor was carried out using the floating zone partial melting and solidification method. It is generally recognized that finely and uniformly dispersed nonsuperconductive high temperature stable phase (Sm211) particles included in the superconductive Sm123 matrix act as effective pinning centers. Microstructure formation of the partial molten mixture (Sm211 particles and BaO–CuO liquid) by decomposition of the precursor Sm123 on melting and solidification of Sm123 from the mixture have to be controlled concurrently to fabricate the 123/211 composite fiber with the optimum microstructure. During unidirectional solidification, planar crystal growth which provides the single crystal growth of Sm123 becomes unstable with increased growth rate. During unidirectional melting, the mean diameter of aligned Sm211 particles behind the melting interface decreases with increased growth rate and with decreased temperature gradient at the melting interface. Initial composition of the precursor significantly affects the formation behavior of Sm211 particles. The contribution of process parameters to the microstructure formation is also briefly discussed.
Void Nucleation on a Contaminated Patch
Authors: Clemens B. M. , Nix W. D. , Gleixner R. J.
Published: March 2008

The energetics of a simple model of void nucleation on a contaminated patch between the sidewall and metal in an integrated circuit interconnect are examined to determine void nucleation behavior. The conditions under which there is no void nucleation barrier are represented by a simple relationship between the volume driving force, the equilibrium contact angle, the surface energy of the metal, and the contaminated patch radius. The void nucleation barrier, when it exists, is a strong function of these same parameters, and increases sharply as the driving force decreases, and under some conditions, increases with increasing equilibrium contact angle.
Void Nucleation in Passivated Interconnect Lines: Effects of Site Geometries, Interfaces, and Interface Flaws
Authors: Gleixner R. J. , Clemens B. M. , Nix W. D.
Published: March 2008

Stress driven nucleation of voids in passivated aluminum interconnect lines is analyzed within the context of classical nucleation theory. A discussion of sources of tensile stress in such lines leads to an upper limit of 2 GPa. Calculations suggest that even at this high stress, nucleation rates are far too low to account for observed rates of voiding. Void formation at a circular defect at the line/passivation interface is then considered. In this case, a flaw on the order of nanometers in size may develop into a void under the imposed stress. These results strongly suggest that void nucleation in aluminum interconnect lines can be controlled by eliminating defects in the line/passivation interface.
In situ X-ray Investigation of Hydrogen Charging in Thin Film Bimetallic Electrodes
Authors: Jisrawi Najeh M. , Wiesmann Harold , Ruckman M. W. , Thurston T. R. , Reisfeld G. , Ocko B. M. , Strongin Myron
Published: March 2008

Hydrogen uptake and discharge by thin metallic films under potentiostatic control was studied using x-ray diffraction at the National Synchroton Light Source (NSLS). The formation of metal-hydrogen phases in Pd, Pd-capped Nb, and Pd/Nb multilayer electrode structures was deduced from x-ray diffraction data and correlated with the cyclic voltammetry (CV) peaks. The x-ray data were also used to construct a plot of the hydrogen concentration as a function of cell potential for a multilayered thin film.
Resistance of Plasma-deposited a-C: H/fluorocarbon Films to Anodic Breakdown in Aqueous Electrolytes
Authors: Srividya C. , Sunkara M. , Babu S. V.
Published: March 2008

The corrosion characteristics of a-C:H/fluorocarbon composite films deposited on type 301 stainless steel substrates were investigated using potentiometry and electrochemical impedance spectroscopy. The films were deposited by radio frequency (13.56 MHz) plasma deposition from different mixtures of hexafluoroethane, acetylene, and argon. A 10 nm thick polysilicon film was plasma-deposited prior to a-C:H film deposition to improve adhesion. The anodic current densities recorded with all the coated samples, in an electrolyte consisting of 0.1 M NaCl and 0.1 M Na2SO4 at 1.5 V (Standard Calomel Electrode), were at least three orders of magnitude less than that for a bare steel sample. They were also at least 20 times less than that obtained with diamond-like carbon film-coated substrates. EIS spectra obtained for these samples, while exposed to 0.6 M NaCl solution over an extended period of time (30–40 days), confirm their barrier properties. All the coatings showed near capacitive behavior in the frequency range 10 mHz–4 KHz.
Mullite Precursor Part I. Characterization of Mullite Precursor Formed by a Reaction of Monosilicic Acid on Aluminum Hydroxide
Authors: Yokoyama Takushi , Nishu Keisuki , Torii Shunsuke , Ikeda Yoshio , Watanabe Tokuko
Published: March 2008

By adsorption of monosilicic acid on aluminum hydroxide, an amorphous aluminosilicate (AAS) of Si/Al ratio of 1/3 was formed. When the AAS was heated to 1000 °C, mullite (3Al2O3 · 2SiO2) crystallized directly without the formation of a spinel phase. It was clearly evidenced that correct adjustment of three factors, the Si/Al ratio, the proportion of 4- and 6-coordinated aluminums, and the single environment of a unit (no Si–O–Si bond), is important for the direct mullitization from AAS.