Showing papers on "Texture (crystalline) published in 1998"

Bimodal Character of Stress Transmission in Granular Packings

Abstract: The correlation between contact forces and the texture of a packing of rigid particles subject to biaxial compression is analyzed by means of numerical simulations. Four different aspects are investigated: stress tensor, dissipation due to friction, angular distribution of forces, and fabric tensor characterizing the anisotropy of the texture. All of them provide evidence that the contact network can be decomposed unambiguously into two subnetworks with complementary mechanical properties.

Texture control of PbTiO3 and Pb(Zr,Ti)O3 thin films with TiO2 seeding

Abstract: The nature and the role of 1 to 5 nm thick TiO2 seed layers for the growth of textured PbTiO3 and Pb(Zr, Ti)O-3 thin films on textured Pt(111) thin film substrates have been studied. Under otherwise identical in situ sputter deposition process conditions, the PbTiO3 texture could be turned from (100) to (111) orientation by adding the seed layer. This is demonstrated by patterning the TiO2 film. Auger electron spectroscopy and x-ray photoemission spectroscopy showed that the seed layer was a continuous TiO2 film. X-ray photoelectron diffraction measurements revealed epitaxial ordering in the seed layer. As there is no azimutal order among the Pt grains, the reduced information of azimutally averaged polar cuts is obtained. These give strong evidence for a strained rutile (110) structure. Various deposition experiments indicated that the TiO2 is effective only when it is ordered before the PbTiO3 nucleation starts. The epitaxial relationship between PbTiO3(111) and Pt(111) is thus mediated by the intermediate, epitaxial TiO2 film, which is dissolved of transformed to PbTiO3 afterwards. The observed growth behavior is discussed in terms of surface and interface energies. (C) 1998 American Institute of Physics. [S0021-8979(98)03607-X].

Growth of poly- and single-crystal ScN on MgO(001): Role of low-energy N2+ irradiation in determining texture, microstructure evolution, and mechanical properties

Abstract: ScN layers, 345 nm thick, were grown on MgO(001) substrates at 750 °C by ultrahigh-vacuum reactive magnetron sputter deposition in pure N2 discharges at 5 mTorr. The N2+ to Sc ratio incident at the substrate and growing film was maintained constant at 14, while the ion energy EN2+ was varied from 13 to 50 eV. All films were stoichiometric with N/Sc ratios of 1.00±0.02. However, microstructural and surface morphological evolution were found to depend strongly on EN2+. The nucleation and initial growth stages of ScN films deposited with EN2+=13 eV are dominated by the formation of 111- and 002-oriented islands, but preferred orientation rapidly evolves toward a purely 111 texture by a film thickness of ≃50 nm as 002 grains grow out of existence in a kinetically limited competitive growth mode. In distinct contrast, films deposited with EN2+=20 eV grow in a cube-on-cube epitaxial relationship with the substrate and exhibit no indication of 111-oriented grains, even in the earliest stages. Increasing EN2+ to ...

Melting point depression of Al clusters generated during the early stages of film growth: Nanocalorimetry measurements

Abstract: This work investigates the thermodynamic properties of small structures of Al using an ultrasensitive thin-film differential scanning calorimeter. Al thin films were deposited onto a Si3N4 surface via thermal evaporation over a range of thicknesses from 6 to 50 A. The Al films were discontinuous and formed nanometer-sized clusters. Calorimetry measurements demonstrated that the melting point of the clusters is lower than the value for bulk Al. We show that the melting point of the clusters is size dependent, decreasing by as much as 140 °C for 2 nm clusters. The results have relevance in several key areas for Al metallization in micro-electronics including the early stages of film growth and texture formation, the Al reflow process, and the dimensional stability of high aspect ratio Al lines.

Synthesis and characterization of oriented MFI membranes prepared by secondary growth

Abstract: MFI membranes were prepared as free-standing and supported films on porous alumina disks and nonporous substrates. They were synthesized using secondary growth of precursor layers. For self-supported films the first step was to prepare an alumina–silicalite composite film, while for supported films the substrate was first coated with layers of the nanocrystalline silicalite particles. During the following hydrothermal treatment, silicalite particles acting as seed crystals form a dense film, which consists of 0.5–100-μm-thick columnar, intergrown, preferentially oriented grains. The crystal orientation of the MFI films was examined using X-ray diffraction pole-figure texture analysis. The crystals were preferentially oriented with both sinusoidal and straight channel networks along directions nearly parallel to the membrane surface. The degree of orientation increased with increasing membrane thickness. Single gas permeances through thin, oriented membranes were measured. Apparent actiuation energies for permeation were 16, 24, 30, 22 and 26 kj/mol for H2, N2, O2, CH4, and CO2, respectively. Ideal selectivities for H2/N2, CO2/CH4, and O2/N2 were as high as 30, 10, and 3.5, respectively. Binary permeation measurements for the gas pairs CO2/CH4, O2/N2, and CO2/N2 revealed trends similar to those of single gas permeation and the properties are attributed to the membrane microstructure.

Effect of high pressure processing on the texture of selected fruits and vegetables

Abstract: Prepared fruits and vegetables were subjected to high pressure processing at 100–400 MPa for 5–60 min in an isostatic press and their influence on product texture was evaluated Pressure had a dual effect on product texture characterized by an initial loss in texture, ascribed to the instantaneous pulse action of pressure, followed by a more gradual change as a result of pressure-hold The extent of the initial loss and the subsequent partial recovery were pressure dependent with the former more prominent at higher pressures and the latter at lower pressures The pressure treated samples were generally brighter in color somewhat resembling the appearance of mildly heat treated samples For all vegetables pressure treated at 100 MPa for 60 min, the initial loss in texture was totally recovered during the pressure hold yielding an overall texture firmer than that of the raw product There was no recovery of texture during the standing period at atmospheric pressure after the pressure treatment

Microstructure and electronic properties of the refractory semiconductor ScN grown on MgO(001) by ultra-high-vacuum reactive magnetron sputter deposition

Abstract: ScN layers, 180 nm thick, were grown on MgO(001) substrates at 750 °C by ultra-high-vacuum reactive magnetron sputter deposition in pure N2 discharges. N/Sc ratios, determined by Rutherford backscattering spectroscopy, were 0.98±0.02. X-ray diffraction θ–2θ scans and pole figures combined with plan-view and cross-sectional transmission electron microscopy showed that the films are strongly textured, both in plane and along the growth direction, and have a columnar microstructure with an average column width near the film surface of 30±5 nm. During nucleation and the early stages of film growth, the layers consist of approximately equal volume fractions of 002- and 111-oriented grains. However, preferred orientation evolves toward a purely 111 texture within ≃40 nm as the 002 grains grow out of existence in a kinetically limited competitive growth mode. 002 grains exhibit local cube-on-cube epitaxy with an orientation relationship (001)ScN∥(001)MgO and [010]ScN∥[010]MgO while 111 grains have a complex four...

Orientation-dependent mechanical properties and deformation morphologies for uniaxially melt-extruded high-density polyethylene films having an initial stacked lamellar texture

Abstract: The mechanical properties and the associated plastically deformed morphologies of high density polyethylene films were investigated by tensile testing, wide-angle X-ray scattering and transmission electron microscopy. Uniaxially oriented films having a well-defined stacked lamellar morphology, both with and without row-nucleated structure were deformed at three angles, 0°, 45° and 90°, with respect to the original machine (extrusion) direction. A distinct orientation dependence of the mechanical properties was observed and this dependence has been related to the different morphologies developed during the plastic deformation processes. It was shown that lamellar separation, lamellar shear and lamellar break-up were the dominant initial deformation mechanisms for the respective 0°, 45° and 90° deformations. As a result, the 45° and 90° deformations generated a final microfibril morphology oriented along the stretch direction, while the 0° deformation resulted in broken blocks of crystalline lamellae. The presence of distinct row-nucleated crystalline fibrils in the initial structure stiffens the material in the 0° deformation; however, it significantly limits the ability of the materials to cold draw at the 90° deformation. Morphological models were proposed to explain the plastic deformation process for the different deformation angles, as well as for the deformation behaviour of semicrystalline polymers with an isotropic spherulitic morphology.

Texture development mechanisms in ion beam assisted deposition

Abstract: Three-dimensional molecular dynamics simulations of ion beam assisted deposition (IBAD) are performed to determine the mechanisms of crystallographic texture selection during the IBAD of polycrystalline films A face centered cubic bicrystal consisting of [111] and [110] oriented grains is grown while an ion beam bombards the growing film at normal incidence As the film grows, the grain boundaries delimiting the [111] and [110] grains move towards each other, eventually pinching off the [111] grain such that the film texture changes from equal densities of [111] and [110] to purely [110] Examination of single crystals grown in the presence of ion beams shows two important effects: ion beam induced atomic sputtering from the surface and ion beam induced damage are significantly reduced when the ion beam is oriented along channeling directions of the crystals The first observation suggests that grains with channeling directions aligned parallel to the ion beam grow more quickly than those where they are

Structure and mechanical properties of polycrystalline CrN/TiN superlattices

Abstract: Polycrystalline CrN/TiN superlattice films were deposited on M1 tool steel using unbalanced reactive magnetron sputtering with opposed cathodes. The Cr and Ti targets were sputtered in Ar–N2 mixtures with partial pressure control of the N2. As the N2 partial pressure was increased from 0.1 to 1.1 mTorr, TiNx films went from stoichiometric B1-cubic TiN to slightly overstoichiometric TiN, while CrNx films went from cubic Cr–N solid solutions to hexagonal Cr2N to B1-cubic CrN. Since the N2 partial pressure required to form stoichiometric CrN was ≈10 times that required to form stoichiometric TiN, nitrogen was inlet at the Cr target position to maximize the difference in N2 partial pressures. Two series of CrN/TiN superlattices, with TiN fractions of 0.4 and 0.6, were deposited with periods ranging from 2 to 60 nm. X-ray diffraction showed a very strong (111) texture with first-order satellite peaks around the (111) Bragg peak. Kinematical diffraction simulations of the superlattice x-ray patterns indicated a...

Microstructure of columnar-grained srtio3 and batio3 thin films prepared by chemical solution deposition

Abstract: The microstructure and lattice defects of SrTiO3 and BaTiO3 thin films with columnar grains prepared by a chemical solution deposition technique have been investigated by means of transmission electron microscopy. The columnar grains in the SrTiO3 films exhibit a preferential orientation with a crystallographic 〈111〉 direction parallel to the normal of film, which, in turn, follows the orientation texture of the substrate Pt layer. Cubic-to-cubic relationships have been found for the two materials. For the BaTiO3 films the columnar grains are oriented in a random way without any preferential relationship to the substrate Pt layer. Pores, lattice defects, and grain boundaries occur in either type of film, however in different configurations. This reflects the individual nature of the materials and the different formation and growth mechanisms of the films under the preparation conditions.

Texture control of freeze-dried resorcinol–formaldehyde gels

Abstract: Resorcinol–formaldehyde gels synthesized in water were freeze-dried to a monolithic and highly porous material The influence of dilution and aging time on the porous texture after drying were studied using nitrogen adsorption–desorption, mercury porosimetry and small angle X-ray scattering The drying method produces very large pores by ice crystal growth, and changes the porous texture of the original wet gel Freeze-drying does not substantially modify microporosity and mesoporosity, which are mainly fixed by reactant concentration and aging time

Switchable Columnar Metallomesogens. New Helical Self-Assembling Systems

Abstract: Chiral oxovanadium(IV), copper(II), and palladium(II) ‚-diketonates show a room-temperature columnar mesophase which undergoes ferroelectric switching. All the compounds were obtained as liquid crystals at room temperature, and crystallization or melting processes were not detected by differential scanning calorimetry carried out to -20 °C. The mesophase was investigated by optical microscopy, DSC and X-ray diffraction, and identified as a rectangular columnar ( P21). The flowerlike texture observed for all the compounds led us to deduce a high tilt angle (ca. 40°) of the molecules with respect to the column axis. Circular dichroism has confirmed the existence of a helical arrangement within the column. This result is in accordance with the so-called columnar mode found at low frequencies (ca. 10 -3 Hz) in dielectric spectroscopy studies. The electrooptical response of these materials has been examined by means of a photomultiplier. The results obtained can be explained by considering a strong influence of the high tilt angle found in the mesophase.

Effects of preferentially aligned precipitates on plastic anisotropy in Al-Cu-Mg-Ag and Al-Cu alloys

Abstract: The influence of a preferential alignment of plate shaped precipitates on the yield strength anisotropy in aluminum alloys was investigated. Stress-aging in tension, i.e. externally applied tensile stresses during aging, was utilized to produce preferential nucleation of precipitates on those 100 and 111 habit plane variants that formed the smallest angle with the load. In-plane yield anisotropy was investigated in tension for various heat treatment conditions. The data was evaluated using the Taylor/ Bishop-Hill model for texture-induced anisotropy as well as the plastic and elastic inclusion models proposed by W.F. Hosford, R.H. Zeisloft, Metall. Trans 3 (1972) 113–121 and P. Bate, W.T. Roberts, D.V. Wilson, Acta Metall. 29 (1981) 1797–1262; 30 (1982) 725–737, which incorporate anisotropic particle strengthening. In a cube textured Al-Cu alloy containing θ′ on only two out of the three possible 100 variants the maximum deviations in yield strength reached 14% when compared to conventionally aged material. In an Al-Cu-Mg-Ag alloy containing the Ω phase on 111 and having a strong brass type deformation texture, less pronounced effects were found after partial removal of one of the Ω variants. Qualitative predictions of the plastic and elastic inclusion models were in good agreement with the findings for anisotropic particle strengthening by randomly distributed precipitates. Effects of aligned precipitates coincided somewhat better with predicted trends by the elastic inclusion model, however, additional verification for either model is required. Stress-aging provides a tool to control anisotropy in high strength aluminum alloys.

Highly oriented magnetic thin films, recording media, transducers, devices made therefrom and methods of making

Abstract: The present invention provides for magnetic and magneto-optic recording media, transducers and data storage devices constructed therefrom that have highly oriented films having long range order in the crystal structure of the film. The recording medium includes a magnetic recording layer comprised of Co-based material, such as Co or one or more Co alloys having a (1010) crystal texture, a substrate, a first underlayer having an fcc structure and a (110) crystal texture disposed between the substrate and the magnetic recording layer. A second underlayer having a bcc structure and a (112) crystal texture is also disposed between the magnetic recording layer and the first underlayer. In particular, if a (110) Si single crystal substrate is non-oxidized certain metals having fcc structures, such as Ag, Cu, Al, and Au and fcc derivative structures, such L10 and L12 structures, can be epitaxially grown on the Si surface.

Structure and properties of TiB2 based coatings prepared by unbalanced DC magnetron sputtering

Abstract: TiB2 coatings with thicknesses of 3 to 4 μm were deposited by means of non-reactive unbalanced direct current (DC) magnetron sputtering using a TiB2 target. The intensity as well as the energy of ion bombardment of the growing film was varied systematically by varying the external magnetic field supplied via a Helmholtz coil system and the bias potential. Deposition conditions with less intense ion bombardment resulted in the formation of fine-grained coating structures consisting of the TiB2 phase with (100) texture and hardness values of 2200 HV0.01. Increases in the energy and intensity of ion bombardment caused grain refinement and a shift of the preferred orientation to (001). Maximum hardness values of up to 6900 HV0.01 were observed for intense bombardment with energetic argon ions. The friction coefficient during dry sliding against carbon steel was determined to vary between 0.50 and 0.65 as a result of the formation of coating chips acting as abrasive particles.

Large positive magnetoresistance in thin films of silver telluride

Abstract: Thin films of Ag2Te were obtained by vapor transport and electron beam evaporation methods. The film phase and chemical compositions were analyzed by x-ray diffraction and x-ray photoelectron spectroscopy analyses, respectively. Film composition was close to stoichiometric with a slight excess of tellurium. Strong (121) texture was found in the films obtained by the electron beam evaporation. Magnetoresistance (MR) measurements show different temperature dependencies for oriented and nonoriented films. MR of the oriented films has a strong peak up to 390% at 90 K and 5 T, whereas MR of the nonoriented films is almost temperature independent in the 10–100 K range. Some analogies can be found between these materials and colossal MR and nonmagnetic multilayer MR materials.

Evaluation of Residual Stresses in the Bulk of Materials by High Energy Synchrotron Diffraction

Abstract: High energy synchrotron diffraction is introduced as a new method for residual stress analysis in the bulk of materials. It is shown that energy dispersive measurements are sufficiently precise so that strains as small 10−4 can be determined reliably. Due to the high intensity and the high parallelism of the high energy synchrotron radiation the sample gauge volume can be reduced to approximately 50 μm×1 mm×1 mm compared to gauge volume of one mm3 up to several mm3 achievable by neutron diffraction. The benefits of the high penetration depth and the small gauge volume are demonstrated by the results of stress studies performed on a fiber reinforced ceramic, a functional gradient material and a metal-ceramic compound. Furthermore, it is shown that in case of a cold extruded metal specimen the energy dispersive measurement technique yields simultaneous information about texture and residual stresses and thus allows a detailed investigation of elastic and plastic deformation gradients.

Thickness control and defects in oriented mesoporous silica films

Abstract: By using a surfactant-based synthesis strategy, we have earlier demonstrated that the polymerization and growth of silicate micellar assemblies at the air–water interface, under quiescent and dilute acidic aqueous conditions, yields free-standing sub-micron thickness hexagonal mesoporous silica films in which the channels are oriented parallel to the film surface. TEM imaging studies of these thin films showed that microscopic defects pervade the channel structure with topologies resembling those found in lyotropic liquid crystals. This suggested that the mesoporous silica film evolved from silicification of a surface lyotropic silicate mesophase. Herein it is demonstrated that film growth, defect structure, extent of polymerization, and mesoporosity sensitively depend on the choice of synthesis acidity, temperature and mixing, and in the case of supported films, on the choice of substrate. In particular, a ten-fold increase in the thickness of the film can be obtained by simply lowering the acidity and moving to ambient temperature conditions whilst an alteration in mixing conditions can change the film from a discrete to a continuous morphology. Combined PXRD, TEM and nitrogen adsorption studies show that the silica films are hexagonal, oriented and mesoporous. Furthermore, the observation of a focal conic fan-type texture in the free-standing films shows that defect controlled director fields, that exist in a precursor hexagonal lyotropic silicate mesophase, are preserved in the channel structure of the mesoporous silica phase. Proof-of-existence of liquid crystalline texture in such free-standing mesoporous silica films, provides direct evidence that film growth evolves from the cooperative assembly and organization of silicate micellar species at the air–water interface.