Growth characteristics of β-SiC by chemical vapour deposition

A twin-plane re-entrant corner effect (TPRE) in growth of chemical vapour deposited (CVD) -SiC is described by the film and particles of gas-phase homogeneous nucleation. The structural morphology has been characterized by scanning electron microscopy and transmission electron microscopy. Morphological characteristics of the deposited crystals, such as triangularity, hexagons or facets have been explained in terms of the re-entrant corner effect at twin junctions, which were proposed as preferential growth sites for perfect crystals. For real deposits, screw dislocations and/or the re-entrant corner effect are not expected to be compatible. The majority of chemical vapour deposited SiC crystals have a high defect density comprised of {111} twins and dislocations associated with the process variables. Infrared transmission spectra and electron spectroscopy of chemical analysis indicated that the major chemical bonds of CVD -SiC were Si-C and C-H bonds. The positions of the 1s or 2p corelevel peaks for deposits are described.     

Infrared absorption ofβ-SiC particles prepared by chemical vapour deposition

Infrared absorption characteristics of-SiC particles prepared by chemical vapour deposition were studied. These particles were either solid or had a silicon core and/or were hollow. The solid particles exhibited a single absorption peak between the transverse optical frequency (_TO = 794 cm^–1) and the longitudinal optical frequency (_LO = 976 cm^–1) of-SiC. This absorption peak shifted to a lower frequency with increasing lattice parameter of-SiC and increasing free silicon content. The particles containing a silicon core and/or were hollow exhibited double absorption peaks close to _TO and _LO. The peak at the LO side shifted to a lower frequency and that at the TO side to a higher frequency with decreasing silicon core size and increasing hollow size. Using the calculations based on the effective medium theory assuming surface phonon mode, the relationship between the infrared absorption characteristics and microstructures of the-SiC particles are explained.     

Bone–titanium oxide interface in humans revealed by transmission electron microscopy and electron tomography

Osseointegration, the direct contact between an implant surface and bone tissue, plays a critical role in interfacial stability and implant success. Analysis of interfacial zones at the micro- and nano-levels is essential to determine the extent of osseointegration. In this paper, a series of state-of-the-art microscopy techniques are used on laser-modified implants retrieved from humans. Partially laser-modified implants were retrieved after two and a half months'' healing and processed for light and electron microscopy. Light microscopy showed osseointegration, with bone tissue growing both towards and away from the implant surface. Transmission electron microscopy revealed an intimate contact between mineralized bone and the laser-modified surface, including bone growth into the nano-structured oxide. This novel observation was verified by three-dimensional Z-contrast electron tomography, enabling visualization of an apatite layer, with different crystal direction compared with the apatite in the bone tissue, encompassing the nano-structured oxide. In conclusion, the present study demonstrates the nano-scale osseointegration and bonding between apatite and surface-textured titanium oxide. These observations provide novel data in human specimens on the ultrastructure of the titanium–bone interface.     

Chemical thinning of III–V compound semiconductors for transmission electron microscopy

A vapour jet etching method and suitable equipment are tested in order to thin monocrystalline and polycrystalline material of GaAs, InP and InAs for transmission electron microscopy. The reactive vapour jet consists of air, bromine and methanol. The thinning procedure is monitored by an optical microscope. Large thin areas could be obtained using a bubbler temperature of 60° C, 20% bromine in methanol, a distance between nozzle and sample surface of 1.5 mm and a nozzle mouth diameter of 0.7 mm. During pre-etching the vapour flow rate was 0.71 min^–1, and 0.31 min^–1 was realized during final etching up to perforation. The etching times ranged between 1 to 2 min with an initial sample thickness of 350 to 365 m. Large thin areas have only been obtained using an excentric position of the nozzle with respect to the sample centre in combination with a manual rotation of the specimen holder.     

Structural studies on γ-MnO2 by transmission electron microscopy

Single-crystal electron diffraction patterns were obtained on five specimens of -MnO₂: one natural, two electrolytical (EMD) and two chemical (CMD) samples. EMDs are best described by the orthorhombic structure proposed by De Wolff which is derived from the ramsdellite structure. A CMD prepared from MnCO₃ fits the hexagonal cell of -MnO₂. Flaky grains from the natural sample and fibres from a CMD prepared from Mn(NO₃)₂ are hexagonal with a new cell:a 0.494,c 0.539 nm. No simple relation between chemical composition, morphology and structure could be found.     

Aging behavior of Cu–Ti–Al alloy observed by transmission electron microscopy

Aging behavior of Cu–3 at.%Ti–4 at.%Al alloy at 723 K has been examined from mechanical, electrical, and microstructural points of view. Compared with binary Cu–3 at.%Ti alloy, the electrical conductivity improved six times to about 6%IACS (international annealed copper standard); whereas the peak hardness decreased from 280 to 180 Hv. The major strengthening phase is the tetragonal α-Cu₄Ti, which forms not via spinodal decomposition but based on the nucleation and growth mechanism. The precipitates grow in the c direction of the tetragonal phase, which lies along one of the axes of the matrix fcc Cu phase. This growth mode minimizes the strain energy arising from the lattice mismatch of about 2% between the matrix and precipitate; and results in a square rod shape, which reaches about 50 nm in length after 100 h anneal. Another precipitating phase is AlCu₂Ti (D0₃, Strukturbericht notation), with the major habit plane close to {110} of the fcc Cu matrix. The orientation relationship was not definitely determined, but it was found that the angle between the 100 and 110 poles of the matrix and precipitates, respectively, is about 5°, while the angle between the two 001 axes being about 7°. It was suggested that the formation of this ternary phase reduced the solute Ti concentration, leading to the decrease in the resistivity.     

Scanning tunnelling microscopy and transmission electron microscopy study of ultrafine structure and surface relief of bainite in a Cu–Zn–Al alloy

The fine structure of bainite (1 plate) in a Cu–25.9 wt% Zn–4.0 wt% Al–0.1 wt% Re alloy has been studied by scanning tunnelling microscopy (STM) and transmission electron microscopy (TEM). The experimental results demonstrated that bainite plates are composed of subplates, and the subplates are composed of subunits. STM investigation shows that the surface relief with a bainite plate is composed of groups of small reliefs which correspond to subplates and subunits. The relief arising from the formation of bainite is tent like, which is different from that with martensite. Ledges exist on the broad faces of the subunit, indicating that it grows by a ledgewise mechanism. Three types of nucleation of the subunits were observed under TEM: face to face, edge to edge and edge to face. Based on the experimental results concerning the ultrafine structure and surface relief accompanying bainite, the sympathetic nucleation–ledgewise growth mechanism of bainite is proposed.     

Influence of surface oxidation on transmission electron microscopy characterization of Fe–Ga alloys

Fe–Ga alloys are rapidly oxidized when exposed in air, forming both amorphous and crystalline surface oxides. These oxides hinder the observation of the ordered phases of B2 and D0₃ in Fe–Ga alloys by dark-field imaging and high-resolution imaging of transmission electron microscopy (TEM) techniques. Proper imaging techniques and reduction of surface oxides are necessary to obtain representative microstructural features to the bulk alloys by TEM.     

The nanometre-scale physiology of bone: steric modelling and scanning transmission electron microscopy of collagen�Cmineral structure

The nanometre-scale structure of collagen and bioapatite within bone establishes bone''s physical properties, including strength and toughness. However, the nanostructural organization within bone is not well known and is debated. Widely accepted models hypothesize that apatite mineral (‘bioapatite’) is present predominantly inside collagen fibrils: in ‘gap channels’ between abutting collagen molecules, and in ‘intermolecular spaces’ between adjacent collagen molecules. However, recent studies report evidence of substantial extrafibrillar bioapatite, challenging this hypothesis. We studied the nanostructure of bioapatite and collagen in mouse bones by scanning transmission electron microscopy (STEM) using electron energy loss spectroscopy and high-angle annular dark-field imaging. Additionally, we developed a steric model to estimate the packing density of bioapatite within gap channels. Our steric model and STEM results constrain the fraction of total bioapatite in bone that is distributed within fibrils at less than or equal to 0.42 inside gap channels and less than or equal to 0.28 inside intermolecular overlap regions. Therefore, a significant fraction of bone''s bioapatite (greater than or equal to 0.3) must be external to the fibrils. Furthermore, we observe extrafibrillar bioapatite between non-mineralized collagen fibrils, suggesting that initial bioapatite nucleation and growth are not confined to the gap channels as hypothesized in some models. These results have important implications for the mechanics of partially mineralized and developing tissues.     

Preparation and Dielectric Properties of Nonstoichiometric β-SiC Powder by Combustion Synthesis

The nonstoichiometric β-SiC powders were synthesized via combustion reaction of Si and C system in a 0.1 MPa nitrogen atmosphere, using Teflon as the chemical activator. The prepared powders were invistigated by XRD and Raman spectra. The results indicates that the cell parameters of all the prepared β-SiC powder are smaller than the standard value of β-SiC because of generation of CSi defects. The complex permittivity of prepared products was carried out in the frequency range of 8.2-12.4 GHz. It shows tha...     

Microstructure and fracture toughness of liquid-phase-sintered β-SiC containing β-SiC whiskers as seeds

The effect of seeding on microstructural development and fracture toughness of -SiC with an oxynitride glass was investigated by the use of morphologically rodlike -SiC whiskers. A self reinforced microstructure consisting of rodlike -SiC grains and equiaxed -SiC matrix grains was obtained by seeding 1–10 wt% SiC whiskers, owing to the epitaxial growth of -SiC from the seed whiskers. Further addition of seeds (20 wt%) or further annealing at higher temperatures led to a unimodal microstructure, owing to the impingement of growing seed grains. By seeding -SiC whiskers, fracture toughness of fine-grained materials was improved from 2.8 to 3.9–6.7 MPa · m^1/2, depending on the seed content.     

Nanofabrication by advanced electron microscopy using intense and focused beam∗

Abstract

The nanogrowth and nanofabrication of solid substances using an intense and focused electron beam are reviewed in terms of the application of scanning and transmission electron microscopy (SEM, TEM and STEM) to control the size, position and structure of nanomaterials. The first example discussed is the growth of freestanding nanotrees on insulator substrates by TEM. The growth process of the nanotrees was observed in situ and analyzed by high-resolution TEM (HRTEM) and was mainly controlled by the intensity of the electron beam. The second example is position- and size-controlled nanofabrication by STEM using a focused electron beam. The diameters of the nanostructures grown ranged from 4 to 20 nm depending on the size of the electron beam. Magnetic nanostructures were also obtained using an iron-containing precursor gas, Fe(CO)₅. The freestanding iron nanoantennas were examined by electron holography. The magnetic field was observed to leak from the nanostructure body which appeared to act as a ‘nanomagnet’. The third example described is the effect of a vacuum on the size and growth process of fabricated nanodots containing W in an ultrahigh-vacuum field-emission TEM (UHV-FE-TEM). The size of the dots can be controlled by changing the dose of electrons and the partial pressure of the precursor. The smallest particle size obtained was about 1.5 nm in diameter, which is the smallest size reported using this method. Finally, the importance of a smaller probe and a higher electron-beam current with atomic resolution is emphasized and an attempt to develop an ultrahigh-vacuum spherical aberration corrected STEM (Cs-corrected STEM) at NIMS is reported.     

High resolution transmission electron microscopic observation of β-Sn thin films

Thin films of β-Sn were studied by high resolution transmission electron microscopy (HRTEM). Diffraction patterns and experimental images agreed with those calculated on the basis of the known crystal structure of β-Sn. By matching the observed images to those calculated, the thicknesses of the films were estimated. Defects were observed as illustrated by a high resolution image of a [100] tilt grain boundary (Σ = 13, θ = 27°). The 20 nm thick films were found to be covered by a carbonaceous deposit. The presence of small clusters of β-Sn in the amorphous contamination layers covering the films was also revealed by HRTEM and by their conversion into SnO₂ particles in the electron beam.     

Mechanical properties and microstructure of β-SiAION-β-SiC composites by pressureless sintering

-SiAION--SiC composites containing up to 12 wt% -SiC were prepared by pressureless sintering. The strength of composites at room temperature remained relatively unchanged, whereas strength at 1200 °C increased for composites. The fracture toughness (K _IC) for composites was higher than that for -SiAION ceramics. The maximum value was 5.4 MPa m^1/2 for 6 wt% -SiC, and this was an improvement of 15% in comparison with -SiAION ceramics. From SEM observations, an improvement inK _IC values was attributed to crack deflections and branching-off of cracks. Intra-granular fractures were frequently observed in -SiAION. From TEM observations, -SiAION crystals were nanocomposites, within which existed the fine crystals in -SiAION crystal. For composite, -SiAION and -SiC crystals were directly in contact. The mismatching zone was observed in -SiC.     

A Density Functional Theory study of the chemical surface modification of β-SiC nanopores

The dependence of the electronic band structure and density of states on the chemical surface passivation of cubic porous silicon carbide (PSiC) is investigated by means of the ab-initio Density Functional Theory and the supercell method in which pores with different sizes and morphologies were created. The porous structures were modeled by removing atoms in the [0 0 1] direction producing two different surface chemistries; one with both Silicon (Si) and Carbon (C) atoms and the other with only Si or C atoms. The changes in the electronic band gap due to a Si-rich and C-rich phase in the porous surfaces are studied with two kind of surface passivation, one with hydrogen atoms and other with a combination between hydrogen and oxygen atoms. The calculations show that for the hydrogenated case, the band gap is larger for the C-rich than for the Si-rich case. For the partial oxygenation the tendency is contrary, by decreasing and increasing the band gap for the C-rich and Si-rich configuration, respectively, according to the percentage of oxygen in the pore surface.     

Synthesis of β-SiC nanowhiskers by high temperature evaporation of solid reactants

β-SiC nanowhiskers were synthesized in large scale by evaporating the solid mixtures of silicon and silicon dioxide in a graphite crucible heated by a high-frequency induction system. Carbon source used for formation of the nanowhiskers came from the cheap common high-purity graphite at 1600 °C. XRD and TEM show that the nanowhiskers are crystalline β-SiC, and have diameters ranging from 15 to 50 nm and length up to several micrometers. Most of the nanowhiskers were wirelike and some nanowhiskers have high density stacking faults in the structure. The normal direction of the stacking layers ([111]) tilts by 12° with respect to the growth orientation ([223]). The growth mechanism of nanowhiskers is based on the reaction between silicon monoxide and carbon monoxide.     

In situ transmission electron microscopy observation of microstructure and phase evolution in a SnO₂ nanowire during lithium intercalation

Recently we have reported structural transformation features of SnO(2) upon initial charging using a configuration that leads to the sequential lithiation of SnO(2) nanowire from one end to the other (Huang et al. Science2010, 330, 1515). A key question to be addressed is the lithiation behavior of the nanowire when it is fully soaked into the electrolyte (Chiang Science2010, 330, 1485). This Letter documents the structural characteristics of SnO(2) upon initial charging based on a battery assembled with a single nanowire anode, which is fully soaked (immersed) into an ionic liquid based electrolyte using in situ transmission electron microscopy. It has been observed that following the initial charging the nanowire retained a wire shape, although highly distorted. The originally straight wire is characterized by a zigzag structure following the phase transformation, indicating that during the phase transformation of SnO(2) + Li ? Li(x)Sn + Li(y)O, the nanowire was subjected to severe deformation, as similarly observed for the case when the SnO(2) was charged sequentially from one end to the other. Transmission electron microscopy imaging revealed that the Li(x)Sn phase possesses a spherical morphology and is embedded into the amorphous Li(y)O matrix, indicating a simultaneous partitioning and coarsening of Li(x)Sn through Sn and Li diffusion in the amorphous matrix accompanied the phase transformation. The presently observed composite configuration gives detailed information on the structural change and how this change takes place on nanometer scale.     

Development of physical vapour deposited Mg–Zr alloys Part 3 – Comparison of alloying and corrosion behaviour in Mg–V and Mg–Zr physical vapour deposited alloys

Abstract

The microstructures and corrosion products of Mg–V and Mg–Zr physical vapour deposited alloys were compared. Factors affecting the alloying behaviour of Mg with V or Zr as well as with other alloying additions such as Ti, Cr, Mn, and Si were considered. The greater thermodynamic stability of the Zr oxide in air and Zr hydroxide in 3 wt-%NaCl were the main reasons for the enhanced corrosion resistance of Mg–Zr alloys compared to Mg–V alloys.     

Preparation,characterization and microwave absorption properties of bamboo-like β-SiC nanowhiskers by molten-salt synthesis

Bamboo-like and cubic single-crystalline silicon carbide nanowhiskers (SiC_NWs) were synthesized using multiwalled carbon nanotube via a process of calcination in the molten-salt circumstance. The system was heated to 1,250 °C and maintained for 6 h in argon atmosphere, and obtained the sample. The as-prepared sample was characterized by a series of techniques. Especially, the microwave absorption properties of SiC_NWs/paraffin composites (30 wt%) were investigated over 2–14 GHz. The result shows the optimal reflection loss can reach ?48.1 dB at 13.52 GHz when the thickness of the match is only 1.9 mm. The excellent microwave absorption properties of the SiC_NWs/paraffin composites due to the dielectric loss would make it as a promising candidate for the application of absorbing materials. In addition, a possible growth mechanism of SiC_NWs was also discussed.