Planar faults in MoSi2 single crystals deformed at high temperatures

Abstract

Dislocation structure and planar faults have been examined in MoSi₂ single crystals deformed at high temperatures. Pure stacking faults were found in a crystal deformed at 900°C. The formation of the stacking fault is closely related to the phase stability of the C11_b and C40 ordered structures. Profuse stacking faults with increasing deformation temperature assist the ductility improvement of the MoSi₂ above about 1200°C. The critical resolved shear stress for {110}(331) and {013}(331) slip is determined in the temperature range 1000 to 1500°C.     

Investigation of {111} stacking faults and nanotwins in epitaxial BaTiO3 thin films by high-resolution transmission electron microscopy

{111} stacking faults and nanotwins in epitaxial BaTiO₃ thin films on MgO substrates have been investigated by high-resolution transmission electron microscopy. In many cases, the stacking faults and nanotwins were found to be accompanied by partial dislocations. These partial dislocations can be classified as two different types, analogous to the situation in the fcc structure. One is of the Shockley type with the Burgers vector (a/3)<112>. The other is of the Frank type with the Burgers vector (a/3)<111>. The movements of both types of partial can lead to the {111} stacking faults and the {111} twins observed in these films.     

Epitaxial Ni-Al thin films on NaCl using a Ag buffer layer

Epitaxial nanoscale [001] films of Ni _x Al_100-x (x = 62.5) have been prepared by physical vapour deposition on to a thin film of Ag [001] on NaCl (001) faces with occasional hillocks. The Ag film contains numerous dislocations and stacking faults and has a rms surface roughness of 2 nm. The Ni-Al film is ordered in the B2 structure and reveals many dislocations as well as antiphase boundaries between ordered domains. The formation of subgrains in the Ni-Al film results in severe height variations up to 30 nm across the surface. A cross-sectional model for the growth of both films is presented.     

Sol-gel-hydrothermal growth of oxide thin films at low temperatures

Lead zirconate titanate (PZT) (Pb(Zr_0.52Ti_0.48)O₃) and barium titanate (BaTiO₃) thin films have been successfully prepared using a novel sol-gel- hydrothermal (SG-HT) technique at low temperatures, which involves a combination of the conventional sol-gel process and a hydrothermal method. Highly (111)-oriented PZT thin films with a single perovskite phase and polycrystalline BaTiO thin films with well developed crystallites were obtained at a processing temperature as low as 160⁰C. The microstructural characteristics demonstrate that the SG-HT-derived PZT and BaTiO₃ thin films with good crystallinity and surface morphology are converted from the amorphous phase to the desired perovskite phase on platinum-coated and bare silicon substrates at a low processing temperature of 100-200 C. These results suggest that the SGHT technique, which is of great significance because of its low processing temperature, will become a potential and promising process for fabricating PZT, BaTiO₃ and other oxide thin films.     

Epitaxial decagonal thin films on crystalline substrates

Al-Cu-Fe-Cr quasicrystalline thin films were grown on atomically flat Al 2 O 3 sapphire (0001) substrates by single-target magnetron sputtering followed by annealing. A decagonal phase with the tenfold axis A 10 parallel to the substrate surface normal was observed. The epitaxial decagonal film had two different unique orientations: a twofold P axis A 2P and a twofold D axis A 2D parallel to of the substrate. These two configurations were explained using a coincidence reciprocal lattice planes model for the interface energy. We show that this classic approach for crystal-crystal epitaxy can be applied to quasicrystal-crystal systems.     

Polarity determination of zinc oxide nanorods by defocused convergent-beam electron diffraction

The crystal polarity of wurtzite zinc oxide nanorods grown hydrothermally parallel to the c?=?[0001] direction has been determined by convergent-beam electron diffraction using a defocused probe. The method enables a simple comparison of the diffracted intensities in ±0002 reflections across side-on nanorods, demonstrating growth in this case is in the c?=?[0001] direction. The method is shown to be viable for nanorods down to about 11?nm in diameter.     

Mobility of c dislocations in Ti3, Al

Abstract

The rôle of dislocations with Burgers vectors, b, given by b = [0001] during deformation of samples of the intermetallic compound Ti₃Al has been assessed. At room temperature, the experimental evidence is consistent with these dislocations being sessile, their density and morphology being similar to that in undeformed samples. In samples deformed at 650°C and above, it is concluded that motion of these dislocations is effected by dislocation climb. The line directions of the various segments of dislocations with b= [0001] are shown to be perpendicular to planes that contain sheets of Ti atoms, with an expected tendency to exhibit a high Peierls stress.     

Stacking-fault formation in [001] small-angle symmetric tilt grain boundaries in cubic zirconia bicrystals

Small-angle symmetric [001] tilt grain boundaries in cubic zirconia bicrystals with misorientation angles 2θ =1.0° and 2θ =5.0° have been fabricated by diffusion bonding. High-resolution electron microscopy observations revealed that the 1.0° boundary consists of a periodic array of mixed dislocations with Burgers vector b =( a /2)[101] or b = ( a /2)[101], while the 5.0° boundary consists of a periodic array of edge dislocations with Burgers vector b = ( a /2)[100], associated with stacking faults at alternate intervals. This suggests that there is a critical angle for structural transitions in the series of the [001] small-angle tilt grain boundaries.     

Structural characterization of double stacking faults induced by cantilever bending in nitrogen-doped 4H-SiC

Defects in highly nitrogen-doped 4H-SiC deformed by cantilever bending at 550°C have been identified by weak-beam and high-resolution transmission electron microscopy techniques. The induced-defects consist of double stacking faults (DSFs) whose expansion produces a local 4H?→?3C phase transformation. Each DSF is bound by two identical 30° Si(g) partial dislocations which glide on two adjacent basal planes. The DSFs belong to three different populations which differ by their extension as a function of the applied-stress and the 30° Si(g) characteristics (line direction L , Burgers vector b , glide planes and glide direction). The external mechanical stresses are the main driving forces involved in the DSF expansion. However, extra driving forces such as thermodynamic or electronic forces are also likely to be involved.     

Nano-undulations of nickel thin films on a substrate under compressive stress

Nickel thin films on a substrate have been studied in situ by atomic force microscopy during deformation. Undulations have been observed on the debonding regions, of very low amplitude and of periodicity of the order of 1mum. Attempts are made to compare these experimental results with previous models of buckling instabilities of thin plates.     

Cascade effects on the irradiation stability of amorphous SiOC

We studied the heavy ion radiation tolerance of amorphous silicon oxycarbide (SiOC) alloys by in situ Kr ion irradiation within a transmission electron microscopy. The amorphous SiOC thin films were grown via co-sputtering from SiO₂ and SiC targets on a surface-oxidized Si (100) substrate. These films were irradiated by 1 MeV Kr ions at both room temperature and 300 °C with damage levels up to 5 displacements per atom (dpa). TEM characterization shows no sign of crystallization, void formation or segregation in all irradiated samples. Our findings suggest that SiOC alloys are a class of promising radiation tolerant materials.     

Effects of annealing temperature on structural,optical, and electrical properties of antimony-doped tin oxide thin films

Antimony-doped tin oxide (ATO) films, approximately 320 nm in thickness, have been prepared by electron beam evaporation onto glass substrates. The films were annealed at temperatures between 400°C and 550°C in air and their structure and surface morphologies were observed by X-ray diffraction (XRD) and atomic force microscopy (AFM) after the different annealing treatments. XRD patterns of the ATO thin films as-deposited and annealed at 400°C showed that they were amorphous, but annealing beyond 400°C caused the films to become polycrystalline with tetragonal structure and orientated in the (1 1 0) direction. The grain size in the annealed films, obtained from the XRD analysis, was in the range 146–256 Å and this increased with the annealing temperature. The dislocation density, cell volume and strain were found to decrease gradually with increasing annealing temperature. Photoluminescence spectra revealed an intensive blue/violet peak at 420 nm, which increased gradually in height with annealing. It is suggested that an increase in the population of Sb⁺⁵ ions might be the reason for the enhancement of the blue/violet emission. The optical properties of the films were also investigated in the UV-visible-NIR region (300–1000 nm). The optical constants, namely the refractive index n and the extinction coefficient k in the visible region were calculated. The optical energy band gap, as determined by the dependence of the absorption coefficient on the photon energy at short wavelengths, was found to increase from 3.59 to 3.76 eV with annealing temperature.     

Interfacial defects in YBa2Cu3O7-δ/SrTiO3(0 0 1) heterostructures studied by aberration-corrected ultrahigh-resolution electron microscopy

The microstructure of interfacial defects in YBa₂Cu₃O_7-δ/SrTiO₃(0?0?1) heterostructures has been investigated by aberration-corrected ultrahigh-resolution electron microscopy. We determine that c-axis-oriented YBa₂Cu₃O_7-δ thin films epitaxially grow on SrTiO₃(0?0?1) with two types of interface structure. The coalescence of nucleation sites with different types of interface structure leads to the formation of antiphase domain boundaries in YBa₂Cu₃O_7-δ thin films, which terminate at planar faults with different configurations near the interface. Stand-off misfit dislocations are observed and the dislocation core structure is explored. Based on the interface structure and interfacial defects, the initial growth mode of YBa₂Cu₃O_7-δ thin films on SrTiO₃(0?0?1) is discussed.     

Synthesis and characterization of nanocrystalline MoBi2Te5 thin films for photoelectrode applications

Molybdenum bismuth telluride thin films have been prepared on clean glass substrate using arrested precipitation technique which is based on self-organized growth process. As deposited MoBi₂Te₅ thin films were dried in constant temperature oven at 110°C and further characterized for their optical, structural, morphological, compositional, and electrical analysis. Optical absorption spectra recorded in the wavelength range 300–800?nm showed band gap (E _g) 1.44?eV. X-ray diffraction pattern and scanning electron microscopic images showed that MoBi₂Te₅ thin films are granular, nanocrystalline having rhombohedral structure. The compositional analysis showed close agreements in theoretical and experimental atomic percentages of Mo⁴⁺, Bi³⁺, and Te^2? suggest that chemical formula MoBi₂Te₅ assigned to as deposited molybdenum bismuth telluride new material is confirmed. The electrical conductivity and thermoelectric power measurement showed that the films are semiconducting with n-type conduction. The fill factor and conversion efficiency was characterized by photoelectrochemical (PEC) technique. In this article, we report the optostructural, morphological, compositional, and electrical characteristics of nanocrystalline MoBi₂Te₅ thin films to check its suitability as photoelectrode in PEC cell.     

ω-phase formation in a rapidly solidified Cr-40 at.% Al alloy

The diffuse ω structure has been identified by electron diffraction for the first time in a C11_b matrix of the melt-spun Cr-40 at.% Al alloy ribbon. The C11_b matrix consists of nanometre-scale ordered domains produced by a long-period chemical ordering in the precursor A2-B2 structure. Each C11 _b unit cell in the Cr-40 at.% Al alloy is based on three cubic B2 cells along the c axis and contains two antiphase boundaries. The structure of the diffuse ω is consistent with that observed by De Fontaine. The relative stability of the crystalline ω structure has been predicted with respect to bcc-type precursor phases as a function of the displacement parameter z from calculation of the cohesive energy of the ω phase using the full-potential linear muffin-tin orbital method. The results show that the three-dimensional crystalline ω structure is stable with respect to bcc-type precursor phases in the Cr-Al system and reveals the physical background as to why the ω structure in Cr-40 at% Al is diffuse in nature.     

An explanation for the low-temperature H evolution peak in hydrogenated amorphous silicon films deposited 'on the edge of crystallinity'

The puzzling existence of a sharp low-temperature (T = 400°C) H evolution peak in compact hydrogenated amorphous silicon (a-Si : H) films deposited 'on the edge of crystallinity' is examined. From infrared absorption and X-ray diffraction (XRD) measurements, we show that none of the standard methods used to explain the existence of this peak in a-Si : H materials is applicable to the present films. From the Si-H wag-mode peak frequency, we postulate the existence of very small Si crystallites contained within the amorphous matrix. While the crystallite volume fraction is too small to be detected by XRD in the as-grown films, crystallization is observed for this material at anneal temperatures as low as 500°C. It is proposed that these crystallites catalyse the crystallization of the remainder of the amorphous matrix upon moderate annealing, enabling H surface desorption and H₂ out-diffusion to the sample surface along newly formed grain boundaries at low anneal temperatures.     

Single stacking faults in high-stress deformed semi-insulating GaAs

Abstract

Single intrinsic stacking faults in semi-insulating undoped GaAs have been studied by transmission electron microscopy. The crystals were deformed at room temperature by uniaxial compression and under hydrostatic pressure. It is shown that the stacking faults are produced by the dissociation of 60°(β) dislocations under very high stress (τ?0·75 GPa). The partial dislocations bounding the stacking faults are systematically 30°(β) in character. These observations are consistent with the classification of mobilities of partial dislocations that had been previously established when studying deformation microtwins in the same material: 30°(β)<30°(α) <90°(α or β).     

Effect of grain orientation and local strain on the quality of polycrystalline YBa 2 Cu 3 O 7 superconductive films

The critical current densities of superconducting thin films and their dependence on the film structural characteristics has been a major research interest for more than a decade. Controlling this relationship is crucial if large-scale high-quality YBa 2 Cu 3 O 7 (YBCO) tapes are to be produced. Two major keystones of information have been established in this field. Firstly, there is a direct relationship between the critical current density and the grain-boundary angle in polycrystalline YBCO films. Grain boundaries with a mismatch angle higher than 5° usually result in reduced critical current densities. This detrimental effect of large-angle grain boundaries to the quality of YBCO films has been attributed to strain fields resulting from such grain boundaries. Secondly, the quality of the YBCO film can be enhanced by straining its lattice in specific direction. Here, we report, for the first time, direct experimental results coupling local grain orientation and local strain maps of thin YBCO films deposited on a (001) biaxially textured nickel substrate. These results were correlated to the quality of the film and showed how grain structure in the nickel substrate affects the grain structure in the YBCO films even in the presence of several buffer layers. More importantly, the data show that highquality films with high critical current densities can be produced, in spite of large-angle grain boundaries, if the film is compressed in the range of 0.5% strain normal to the a axis.     

Effect of film thickness and grain size on fatigue-induced dislocation structures in Cu thin films

This letter presents systematic experimental observations of fatigue damage and corresponding dislocation structures in thin Cu films as a function of film thickness made using transmission electron microscopy and focused-ion-beam microscopy. It is found that, in thick films and grains of at least 3.0 μm diameter, coarse surface extrusions and dislocation wall and cell structures occur whereas, in thin films or in small-diameter grains, finer extrusions occur but no clearly defined dislocation structures are present. This minimum required dimension of 3.0 μm for fatigue damage formation may be caused by constrained dislocation motion in small dimensions.