High Degree Of Preferred Orientation Research Articles

THIN-FILM RECHARGEABLE LITHIUM BATTERIES FOR IMPLANTABLE DEVICES

Thin films of LiCoO2 have been synthesized in which the strongest x-ray reflection is either weak or missing, indicating a high degree of preferred orientation. Thin film solid state batteries with these textured cathode films can deliver practical capacities at high current densities. For example, for one of the cells, 70% of the maximum capacity between 4.2 and 3 V (approximately 0.2 mAh/cm2) was delivered at a current of 2 mA/cm2. When cycled at rates of 0.1 mA/cm2, the capacity loss was < or = 0.001%/cycle. The reliability and performance of Li-LiCoO2 thin film batteries make them attractive for application in implantable devices such as neural stimulators, pacemakers, and defibrillators.

Effect of SiO2 buffer layers on the structure of SrTiO3 films grown on silicon by pulsed laser deposition

Thin films of SrTiO3 were grown by pulsed laser deposition on Si and SiO2/Si at 35 and 650 °C in a 50 mTorr oxygen discharge (300 V). The effect of introducing a SiO2 buffer layer between the Si substrate and the complex oxide on the crystallinity and microstructure of the SrTiO3 films was investigated at both deposition temperatures. All films grown at 35 °C were amorphous. Surface morphology examination by scanning electron microscopy (SEM) showed that these films were continuous and homogeneous when grown on Si, but were porous and had low-density noninterconnecting lines when grown on SiO2/Si. Films prepared at 650 °C were polycrystalline and their x-ray-diffraction patterns exhibited peaks corresponding to the (001), (110), (111), and (002) reflections of the SrTiO3 cubic phase (a=3.904 Å). The films deposited on SiO2/Si were found to grow with a high degree of preferred orientation along the (110) direction. SEM studies on the surface morphology of the films grown at high temperature showed the presence of a ‘‘rosette’’ structure. The mean size of the rosettes was ∼80 nm in 40-nm-thick films grown on Si and ∼100 nm in films of similar thickness grown on SiO2/Si. Additional atomic force microscopy studies on the topography of these samples indicated that the rosettes were constituted by ∼35-nm-diam grains. Typical peak-to-valley surface roughness of these films was 0.5–2 nm.

TaSi2-Si composites as wide-bandpass optical elements for synchrotron radiation (abstract)

The wide matrix rocking curves of the in situ eutectic composite TaSi2-Si make it attractive as a wide-bandpass monochromator for synchrotron radiation. Wafers with Si[111], Si[110], or Si[100] orientation were studied to determine the origin of the wide rocking curves. The high degree of preferred orientation of the TaSi2 rods relative to the Si matrix was examined using synchrotron Laue patterns and the TaSi2 [100], TaSi2 [003], and TaSi2 [102] reflections. Double and triple axis diffractometry were used to show that the large widths were due to strain and mosaic and not long-range bending; copper radiation (for some double axis results) and 120 and 160 keV synchrotron radiation were used. At 8 keV, rocking curve widths were about twenty times broader than those from perfect Si, and peak reflectivities approached 20%. Rocking curves from Si[333] and Si[444] (120 and 160 keV, respectively) had identical profiles and reflectivities of about 25%. The triple axis results show compressive strains in the Si matrix along Si[111] (i.e., parallel to the rods) and dilational strains orthogonal to the rods. These results confirm the promise of TaSi2-Si as a wide-bandpass optical element for synchrotron radiation.

The effect of Ag diffusion on properties of BiPbSrCaCuO thin films

Superconducting BiPbSrCaCuO films have been evaporated on Ag/MgO and MgO substrates (with and without Ag buffer layer) by the electron-beam technique. The influence of annealing temperature on the crystalline structure and superconducting properties of BiPbSrCaCuO films was investigated by X-ray diffraction, X-ray fluorescence spectroscopy, scanning electron microscopy, critical temperature, critical current density and room temperature resistivity measurements. It was shown that the annealing of both types of films at 835°C resulted in formation of the mixed Bi-2223 and Bi-2212 phases with a high degree of preferential orientation with the c-axis perpendicular to the substrates. Annealing of films on Ag/MgO substrates is additionally accompanied by Ag diffusion from the buffer layer into BiPbSrCaCuO films. The higher rate of crystallization of the Bi-2223 and Bi-2212 phases, the higher degree of c-axis orientation, the higher dense surface morphology, the reduced lattice parameter c (by 0.6–0.8%) the reduced room temperature resistivity (2–3 times), the significantly enhanced critical temperature ( T c=106 K at R=0) and the critical current density ( J c = 1.1 × 10 4 A/cm 2 at 77 K) were observed for the Ag-doped films (on Ag/MgO substrates), in comparison with those for the undoped films (on MgO substrates). The temperature dependence of the Ag diffusion coefficient in Bi-2212 films in the range 650–800°C was described by the relation D=2.2×10 −5 exp(−1.2/ kT).

Growth of PbS and CdS thin films by low-pressure chemical vapour deposition using dithiocarbamates

Thin films of cadmium and lead sulphides grown by chemical vapour deposition (CVD) and remote plasma enhanced chemical vapour deposition (RPECVD) using dithiocarbamates as precursors were prepared on fused silica, sapphire, (111)Si and (111)InP substrates. These films were deposited in the temperature range 473–873 K. It was established that the activation energy of the CVD process is 191.5±1.5 kJ mol −1. The structure of polycrystalline films was halenide for PbS and wurtzite for CdS. It was also found that r.f.-plasma activation of the gas phase decreases remarkably the growth temperature and orders the film structure. RPECVD sulphide films had a high degree of preferred orientation.

Processing of BSCCO superconducting rods for current leads

Abstract Current leads made of ceramic superconductors promise to be one of the first applications of these materials. The high critical temperature and low thermal conductivity of ceramic superconductors make them suitable for current leads in low T c devices. Rods and tubes of BSCCO were fabricated using Cold Isostatic Pressing (CIP). The platelet morphology of BSCCO was utilized to process rods with a high degree of preferred orientation using suitably designed cans for cold isostatic pressing. The effect of starting powder, processing parameters, and loading configuration were studied and an optimum processing cycle was developed. Rods with consistent current capacities of several hundred amperes over 15 cm have been fabricated using this technique. In addition, the critical currents of these rods were measured at various temperatures and magnetic fields and found to be very satisfactory for current lead applications.

Characterization of TaSi2–Si composites for use as wide-bandpass optical elements for synchrotron radiation

The wide rocking curves of matrix reflections of the in situ eutectic composite TaSi2–Si make wafers of this material attractive for use as wide-bandpass monochromators for synchrotron radiation, and characterization of wafers of TaSi2–Si for use with energies normally accessible at storage rings (i.e., 5–40 keV) is the focus of the present report. A wafer with [111]Si orientation and a wafer with [110]Si orientation are studied. The high degree of preferred orientation of the TaSi2 rods relative to the Si matrix is examined using synchrotron Laue patterns, and the 100TaSi2, 003TaSi2, 101TaSi2, and 102TaSi2 reflections are used to establish the orientation relationship and to determine that the spread of rod orientations is at least 5° and probably no greater than 6°. Double-axis diffractometry with Cu Kα radiation reveals matrix reflections with rocking curve widths that are about 20 times broader than those from perfect Si and with peak reflectivities approaching 20%. The rocking curves widths are found to be relatively insensitive to irradiated area, thus indicating that most of the observed width is not due to long-range bending. Triple-axis diffractometry with Cu Kα radiation reveals that considerable compressive strain exists in the matrix and that much of the width of the diffraction peak is due to mosaicity. The performance of the [111]Si TaSi2–Si wafer and a perfect [111] Si wafer as monochromators for microradiography are compared, and a gain of an order of magnitude in x-ray intensity delivered to the sample is demonstrated with the composite crystal.

Epitaxial quality of thin Ag films on GaAs(100) surfaces cleaned with various wet etching techniques

2θ and pole figure x-ray analysis have been used to examine the crystal structure and orientation of Ag films deposited on GaAs(100) substrates cleaned by a variety of wet etches. Where epitaxy was observed, it was of the type Ag(110)/GaAs(100). The H3PO4/HCl sequential etch yielded the film with the highest degree of preferred orientation, with the H2SO4/HCl, NH4OH, and HF etches producing films of decreasing quality in the order named. The epitaxial quality is thought to scale with elemental As concentration on the GaAs(100) surface, and have an inverse relationship to the amount of surface oxides present before deposition.

PZT, PLZT, LSC and platinum thin films produced in the open atmosphere using combustion Chemical Vapor deposition

Abstract PZT, PLZT, LSC, and Pt thin film coatings have been deposited onto crystalline and amorphous substrates using the open atmosphere Combustion Chemical Vapor Deposition (CCVD(SM) technique. X-ray diffraction (XRD) scans have indicated a high degree of preferred orientation for all of these coatings. Furthermore, XRD pole figure analysis has shown that coatings deposited onto single crystal substrates are single phase, while depositions onto amorphous substrates yield coatings with a fiber axis. Entrapment of airborne particles into the flame caused enough film inhomogeneity that electrical measurements could not be performed at this stage.

Thermal effects on structural characterization of evaporated CdTe films during and after deposition

CdTe films were thermally deposited onto amorphous substrates at different temperatures (25–250°C). Post-annealing under vacuum at 300°C for 2 h has been also carried out. Using X-ray diffraction, the structural characteristics (preferential orientation, stoichiometry, microstructural properties) have been studied. Due to the high degree of preferred orientation, Voigt analysis of single reflection was used to determine the microstructural properties (crystallite size and microstrain). Raising the substrate temperature was observed to lead to a decrease in both integrated intensity and degree of preferred orientation as well as an increase in crystallite size and internal microstrain associated with improving the film stoichiometry. Post-annealing was found to increase the integrated intensity, the crystallite size and the degree of preferred orientation. On the other hand, it resulted in a decrease of FWHM and microstrain. The relative change in such parameters decreases as the film thickness increased with a pronounced change in thinner films.

Large current high T/sub c/ superconducting rods for current lead applications

The high critical temperature and low thermal conductivity of ceramic superconductors make them suitable for current leads in low T/sub c/ magnets. The plastic extrusion process was applied for the fabrication of YBaCuO and BiSrCaCuO rods for this application. The platelet morphology of BiSrCaCuO was utilized to process rods with a high degree of preferred orientation, through a combination of plastic extrusion and cold isostatic pressing. Rods with consistent current capacities of few hundred amperes over 15 cm have been fabricated. The critical currents of these rods were measured at various temperatures and magnetic fields. Other related aspects such as contact resistance, mechanical properties and degradation due to thermal cycling have also been evaluated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Indentation-induced deformation and microcracking of highly textured superconducting (Bi,Pb)2Sr2Ca2Cu3Ox ceramic

A high Tc ceramic superconductor has been prepared having a high degree of preferred orientation, this having been generated by means of a large magnetic field during the early stages of processing. The texture of the material has been characterized using XRD textural analysis and the microstructure by high resolution electron microscopy. Pyramidal indentation (Vickers indent), deformation, and cracking have been investigated using high resolution SEM. Semispherical deformation zones were formed under indents, a result of slipping of adjacent ab/ab (basal) plane grain boundaries and some microcrack development, together with lattice deformation on the basal plane. Densification of the green compact was also observed in the deformation zones, where lateral cracking has been seen to take place along the ab/ab plane grain boundaries. Radial cracking has been seen to take place along the low angle grain boundaries of the lattice ab/ab plane when indentations were made on the surface perpendicular to the preferred texture surface of the materials and also at the highly deformed corners of indent diagonals. The stresses generated resulted in the fracture of the flake-shaped grains and initiated small radial cracks. The Vickers microhardness of the textured ceramics was found to decrease with increasing load.

Phyllosilicate preferred orientation in relation to strain path determination in the lower Paleozoic Stavelot-Venn Massif (Ardennes, Belgium)

Phyllosilicate preferred orientation, measured by means of an X-ray pole figure goniometer, allows a quantitative exploration of the slaty cleavage development. The relationship between phyllosilicate preferred orientation and strain, as demonstrated in the March model, furthermore enables the application of phyllosilicate pole figures in a strain determination in slate belts. Both applications are illustrated in this paper, in which the results are presented of a crystallographic fabric analysis of argillites in the southern domain of the lower Paleozoic Stavelot-Venn Massif (Belgium). In these argillites the development of a phyllosilicate preferred orientation is primarily defined by the low-grade metamorphism and the deformation history. The metamorphism, causing recrystallisation to be active during cleavage development, assures the development of a high degree of preferred orientation. On the other hand, the deformation history defines the type of phyllosilicate pole figure pattern. Based on the distinction of different pole figure patterns, a possible strain path for the argillites in the southern domain of the Stavelot-Venn Massif is proposed. An axially symmetric pole figure pattern indicates a Caledonian cleavage development in a pure flattening regime. The Variscan cleavage development, on the other hand, is characterised by an additional extensional component in the cleavage plane. If the primary phyllosilicate fabric, due to a pre-deformational compaction or a Caledonian cleavage development, underwent a complete reorientation caused by syntectonic recrystallisation, a typical orthorhombic pole figure pattern is formed. Otherwise a transition pole figure pattern develops, reflecting the superposition of the Variscan cleavage development on a pre-existing Caledonian cleavage fabric. The general deformation history of the southern domain of the Stavelot-Venn Massif differs substantially from that described for the northern domain. Contrary to the northern domain the southern domain seems to have undergone considerable Variscan deformation.

Control of Y 2O 3-stabilized ZrO 2 thin film orientation by modified bias sputtering

An attempt was made to control the crystal orientation of yttria-stabilized zirconia (YSZ) thin films using a modified bias sputtering technique. Two specially devised electrodes installed in a sputtering system were found to play an important role in obtaining films with a high degree of preferred orientation. An argon ion flux extracted from the discharge space formed by these electrodes appeared to impinge on the growing film to form films with this strong preferred orientation. The pole figure obtained by texture analysis revealed the clear evidence for alignment of the in-plane crystal axes of YSZ buffer layers for the growth of YBa 2Cu 3O y onto polycrystalline metal substrates. An apparent in-plane texturing was also confirmed in subsequently laser-deposited YBa 2Cu 3O y films. As a result, the current-carrying capacity of YBa 2Cu 3O y films was markedly improved.

Substrate Effects on The Growth of Oriented MgO Thin Films

ABSTRACTMgO thin films were deposited onto SrTiO3 and LaAlO3 single crystal substrates using off-axis rf-Magnetron sputtering. Films from 1000–3000Å thick were grown in an Ar - 20%O2 atmosphere at temperatures between 200 and 700°C. X-ray measurements indicate MgO grows epitaxially on SrTiO3 at temperatures above 400°C. The Microstructure of these films was smooth and dense. At deposition temperatures below 400°C, orientation is Maintained, but a rough plate-like microstructure begins to take over. MgO films deposited on LaAlO3 grow with a high degree of preferred orientation. Film Microstructure resembled MgO deposited on SrTiO3 at temperatures below 400°C. Variations in the quality of MgO films grown on LaAlO3 were also observed which may be related to variations in steps on the substrate surfaces.

Spinning characteristics of mesophase pitches derived from naphthalene and methylnaphthalene with HF/BF 3

Transverse shape, texture, and the degree of preferred orientation along the fiber axis in mesophase pitch fibers were studied by varying spinning temperature and spinning nozzle to clarify the influences of the structure of mesophase pitch. The mesophase pitch derived from methylnaphthalene (mNP) showed much larger Lc(002) and smaller d 002 than that from naphthalene (NP), indicating larger and better ordered stacking of the aromatic planes in the former pitch, although their softening points, melt viscosities, spinning temperatures, and molecular weight distributions were much the same. MNP provided a higher degree of preferred orientation in the round shaped fiber, which led to an open wedge in the radial texture in a particular spinning temperature range, and larger width/ thickness ( W T ) ratio of tape spun through the slit-shaped spinning nozzle. Addition of NP to mNP in their melt states decreased Lc(002), lowering the degree of orientation and decreasing W T ratio according to the added amount. Such structural factors of fiber and tape are related to the stacking of aromatic molecules in the mesophase pitches. The higher stacking may emphasize the dimensional anisotropy of molecular assembly, influencing the viscoelastic flow of the mesophase pitch in the spinning nozzle. Larger anisotropy may also contribute to the better alignment of molecular assembly along the spinning nozzle wall.

Use of oriented polycrystalline films in x-ray optics

X-ray diffractors having doubly curved crystal planes can have a high collection efficiency for radiation from a point source and can provide point-to-point focusing of monochromatic x rays as shown by some of the authors. These kinds of diffractors can be used to form x-ray microprobes that can give lower detection limits than electron microprobes or to make wavelength dispersive spectrometers with better signal and signal/background ratio than conventional ones employing the Johann or Johansson geometry. However, fabrication of diffractors of the desired geometry by the use of bent crystals has been difficult. In this paper, we explore the use of oriented polycrystalline films to fabricate doubly curved diffractors for x rays of short wavelength.Oriented polycrystalline films (OPF) have been of interest for many applications, e.g., magnetic recording media, non-linear optical device components, inexpensive large-area substrates for semiconductor device fabrication, etc. These films consist of small monocrystalline grains that have a high degree of preferential orientation of one crystallographic axis relative to the substrate surface.

Size-dependent structural characteristics of thin bismuth films

Bismuth thin films in the thickness range from 10 to 200 nm were deposited on glass substrates by thermal evaporation. The films were investigated by electron microscopy and X-ray diffraction. No anomalous dependence of the structural features on the film thickness was observed but only a normal monotonic variation. All the samples were polycrystalline with a high degree of preferred orientation. The [0001] H or [111] R fibre texture was against heat treatment. The residual strain decreased abruptly with increasing thickness up to 30 nm while the grain size increased up to a thickness of 100 nm and then exhibits almost no further change. The substructure within the individual grain, such as twining and/or stacking faults, was considered on the basis of the large values of grain size revealed by direct observation with transmission electron microscopy compared with those obtained from X-ray diffraction line profile analysis. A structure-properties (electrical) intercorrelation was proved.

Effect of constraint during imidization of polyamic acid films on graphitizability of resultant carbon films

Three kinds of polyimide film specimens were prepared by imidizing polyamic acid under different constraint conditions (viz. as fixed on a glass substrate), in free space, and with 20% stretching in length, respectively. X-ray diffraction and magnetoresistance parameters were measured after their graphitization at 3000°C. The films prepared under constraint, on a glass and with stretching, were found to be highly graphitizable, showing small interlayer spacing, large crystallite size along the c-axis, high magnetoresistance, and high degree of preferred orientation of graphite layers, as compared with that prepared in free space. These results imply that the orientation of molecules in the polyimide film is very important in order to get graphite films with high crystallinity and high degree of orientation.

LPE growth of high T c Bi 2Sr 2Ca 1Cu 2O x films

X-ray diffraction patterns of the LPE-grown BSCCO films displayed a high degree of preferred orientation with their c-axes perpendicular to the a–b planes. The resistivity versus temperature data were plotted by the transport technique and a.c. mutual induction method. A series of critical temperatures, T c at zero resistance varying from 63 k to 80 k were obtained. This variation in T c is interpreted in terms of the solidification phenomena of melt-compositions along the liquidus of a pseudo-binary phase diagram constructed analytically from the (Sr,Ca)O-CuO-Bi 2O 3 ternary system.