Stress and dislocations in diamond–SiC composites sintered at high pressure,high temperature conditions

Diamond–silicon carbide composites were sintered at 10 GPa and three different temperatures: 1600, 1800, and 2000 °C. Distributions of residual surface stresses in diamond crystals were obtained by the analysis of Raman band shifts and splitting. It was noted that stresses concentrate around points of contacts between diamond crystals. Average stress increase with increasing sintering temperature. Complementary information on average sizes of crystallites, concentration of stacking faults, and population of dislocations in both diamond and SiC were obtained from X-ray diffraction profile analysis. It was observed that for both diamond and silicon carbide phases the average crystallite sizes decrease. The population of dislocations in the diamond phase increases with increasing sintering temperature and the population fluctuates in the SiC phase. Concentration of stacking faults was significant only in SiC.     

Field emission performance of SiC nanowires directly grown on graphite substrate

Lawn-like SiC nanowire arrays were successfully synthesized on graphite substrates by thermal evaporation of silicon powders at high temperature. The morphology, microstructure and composition of the nanowires were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. The product grown on graphite substrates was hexagonal prism-shaped single-crystal 3C-SiC nanowires with high aspect ratio. Planar defects, such as microtwins and stacking faults were observed in SiC nanowires. Field emission measurements of the SiC nanowires grown on graphite substrate showed a very low threshold field of 2.1 V μm⁻¹, high brightness and stable field emission performance.     

Flexible Transparent and Free‐Standing SiC Nanowires Fabric: Stretchable UV Absorber and Fast‐Response UV‐A Detector

Transparent and flexible materials are desired for the construction of photoelectric multifunctional integrated devices and portable electronics. Herein, 2H‐SiC nanowires are assembled into a flexible, transparent, self‐standing nanowire fabric (FTS‐NWsF). The as‐synthesized ultralong nanowires form high‐quality crystals with a few stacking faults. The optical transmission spectra reveal that FTS‐NWsF absorbs most incident 200–400 nm light, but remains transparent to visible light. A polydimethylsiloxane (PDMS)–SiC fabric–PDMS sandwich film device exhibits stable electrical output even when repeatedly stretched by up to 50%. Unlike previous SiC nanowires in which stacking faults are prevalent, the transparent, stretchable SiC fabric shows considerable photoelectric activity and exhibits a rapid photoresponse (rise and decay time < 30 ms) to 340–400 nm light, covering most of the UV‐A spectral region. These advances represent significant progress in the design of functional optoelectronic SiC nanowires and transparent and stretchable optoelectronic systems.     

Role of stacking faults in solid state transformations

This article illustrates the two different roles played by stacking faults in solid state transformations viz. (i) in accommodating part of the transformation strains as observed in the noble metal-based alloys undergoing martensitic transformations, and (ii) in providing a mechanism for changing the stacking sequence of layers in a variety of materials like SiC, ZnS, Co and its alloys, and certain steels. Diffraction patterns taken from the martensitic phases of noble-metal-based alloys as well as from SiC and ZnS crystals undergoing transformation from one close-packed modification to another reveal the presence of characteristic diffuse streaks. It is shown that from a theoretical analysis of the observed intensity distribution along streaked reciprocal lattice rows in terms of physically plausible models for the geometry and distribution of faults, one can make a choice between various possible routes for transformation. From simple computer simulation studies, it is shown that the observed arrest of transformations in SiC is essentially due to the insertion of stacking faults in a random space and time sequence leading to an irregular distribution of solitons.     

TEM study of silicon carbide whisker microstructures

-SiC whiskers produced by a number of manufacturers have been examined in the transmission and scanning electron microscopes. In all cases defective microstructures were found with high densities of planar defects such as stacking faults and microtwins. Two distinct types of defective whisker can be identified. The first contains regions of very closely spaced twins on {111} planes arranged perpendicular to the whisker axis [111], these were sometimes separated by defect-free regions. In these whiskers a rough surface profile was normal with the roughness closely associated with the highly defective regions of the whisker. The second type of whisker contained stacking faults spaced relatively widely also on {111} planes but now on the planes inclined to the [111] axis of the whisker. This leads to a characteristic chevron contrast in the TEM. This second type of whisker had a much smoother surface profile than the first type with perpendicular defects. No whisker contained both defect types but some batches of whiskers contained populations of both types of whisker. The first type of whisker is shown to have defects similar to those reported as common during vapour-liquid-solid whisker growth. This is also consistent with the higher impurity content and the presence of voids found in these whiskers. The second type may be indicative of a different growth mechanism possible under certain conditions of SiC whisker synthesis.     

哑铃形碳化硅晶须的微观结构分析

利用透射电子显微镜观察和分析了仿生哑铃形碳化硅晶须的微观结构。这种碳化硅仿生晶须由成分和结构均不相同的两部分构成：中间是直杆状碳化硅晶须。在晶须上包裹着非晶态的念珠小球。由于成分组成的不同,念珠状小球具有均匀的和不均匀的两种微观结构;由于在其中存在大量的微孪晶、堆垛层错等缺陷,直杆状晶具有一维无序、竹节状等复杂的微观结构。在具有不同微观结构和形貌的直杆状晶须上,念珠状小球的开关和分布也有所不同。念球状小球中单质Ｓｉ的含量是影响仿生晶颜色的主要因素,Ｓｉ含量越高则仿生晶须的颜色越深。仿生晶须的生成过程是：先由气相反应生成直杆状碳化硅晶须,然后在直杆状碳化硅晶须上沉积生成非晶态念珠状小球。     

Synthesis of β-silicon carbide nanofiber from an exfoliated graphite and amorphous silica

Silicon carbide (SiC) nanofibers were synthesized from exfoliated graphite containing silica particles at 1425 °C in a 25% H₂/Ar atmosphere. Two types of SiC nanofibers with different morphologies were formed depending on the silica content. A higher silica content led to straight nanofibers with a regular diameter size. The SiC nanofibers derived from the exfoliated graphite/40 wt% SiO₂ powder mixture contained a large number of stacking faults and grew along the [1 1 1] direction. A gas–gas reaction mechanism was proposed to explain the formation of SiC nanofibers.     

Analysis of Microstructure of Silicon Carbide Fiber by Raman Spectroscopy

The SiC fiber was prepared by chemical vapour depostion, which consists of tungsten core, SiC layer and carbon coating. The microstructure of the fiber was investigated using Raman spectroscopy, illustrating SiC variation in different region of the fiber. The result shows that the SiC layer can be subdivided into two parts in the morphologies of SiC grains; their sizes increase and their orientations become order with increasing distance from the fiber center. It is demonstrated that the mount of free carbon in the fiber is responsible for the variation of SiC grains in sizes and morphologies. The analysis of Raman spectra shows that the predominant β-SiC has extensive stacking faults within the crystallites and mixes other polytypes and amorphous SiC into the structure in the fiber.     

Morphology and structure of airborne β-SiC fibres produced during the industrial production of non-fibrous silicon carbide

Clusters of airborne SiC fibres from the SiC industry have been studied by scanning and transmission electron microscopy (SEM, TEM). These fibres are produced as an unwanted side product during the industrial manufacturing of non-fibrous silicon carbide. It was found that the complex morphology seen for clusters of SiC fibres can be explained on the basis of the crystal structure of the fibres. By use of electron diffraction (ED) and high resolution electron microscopy (HREM) it was found that the SiC fibres can be described as needles based on face centred cubic structure (space group F 3m and a = 0.4385 nm). The needles are covered by a thin amorphous layer of carbon. The needles contain a high degree of stacking faults and twinned areas. No long-range order of a hexagonal structure as a result of stacking faults was found. The SiC needles grow along the (111) directions. The existence of equivalent (111) directions in the cubic structure, separated by an angle of 70.52°, explains the most common angle found between branches and the parent needle. Single and double twinned areas in the cubic structure have been found to produce a second and third set of angles between branches and a parent needle. The second set of angles is 38.94°, 43.31°, 55.53°, 56.25° and 66.16°. One example from the third set is the angle ∼83° (∼97°. These parent needles with branches may be significant to the toxicological effects of the airborne SiC fibres.     

A simple route to ultra long SiC nanowires

SiC nanowires are prepared by pyrolysis of hexamethyldisilane (HMDS), at 1200 degrees C in a flowing Ar atmosphere. The length of the nanowires is in millimeter scale. Transmission electron microscopy observations indicate that the diameters of the SiC nanowires are in the range of about 8 to 120 nm, and that most of the nanowires have numerous stacking faults. The formation mechanism of the nanowires is proposed.     

Inner potential fluctuation in SiC nanowires with modulated interior structure

SiC nanowires with modulation in diameter and interior structure along the growth direction have been fabricated via a self-organized process. The SiC nanowires, the basic structure of which is zincblende-type, contain many bunching stacking faults along the growth direction inhomogeneously. In other words, the SiC nanowires consist of alternate stacks of the perfect crystal and the defective regions. We have found the difference in the values of mean inner potential between the perfect crystal and the defective regions by means of electron holography.     

哑铃形碳化硅晶须生长的机理

研究了仿生哑铃形碳化硅晶须的生长机理，发现组成仿生晶须的念珠状小球与直杆状碳化硅晶须的生成过程是相对独立的，而且念珠状小球在直杆状晶须上的生长位置是一定的，首先，直杆状碳化硅晶须在反应空间中生成，然后由Si,SiO,SiO2等组成的非晶态物质的直杆状晶须上的缺陷位置沉积长大，形成包裹在晶须上的念珠涉球，念珠状小球不仅可以在制备碳化硅晶须的过程中生成，而且能够在已有的碳化硅和钛酸钾等晶须上生成。     

Ion assistance in epitaxial growth as a strategy to suppress twinning

Homoepitaxy on Ir(111) at 350 K through physical vapor deposition without and with ion assistance is compared in a scanning tunneling microscopy study. During growth without ion assistance thin Ir films on Ir(111) rapidly develop stacking faults such that for films of more than 50 atomic layers thickness the majority of the film surface displays twins. Ion assistance with 100 eV Ar⁺ at normal incidence as well as with 500 eV Ar⁺ at grazing incidence both effectively suppress stacking fault formation and twinning in the growing film. The mechanisms of twin suppression are identified.     

Transmission electron microscopy study of plastic deformation in NiZr2

Plastic deformation mechanisms in the body centred tetragonal (C16-structure) NiZr₂ compound have been studied using transmission electron microscopy. Dislocations, stacking faults and microtwins have been characterized. Four families of dislocations have been identified: screw-type (b=[0 0 1]), edge-type ([0 0 1](1 1 0) and [110] (1¯10) systems) and mixed-type (1/2[1¯11] (110) system). Partial dislocations associated with stacking faults have also been detected and analysed. The plane of the faults is shown to be (1¯10) and the fault vectors 1/4 [1 1 1] and 1/4 [11¯1]. A hard-sphere model illustrating the formation mechanism of these stacking faults is proposed. The microtwins are found to be 60° rotation twins around the [1¯10]^* direction and can be described by the stacking faults scheme.     

Synthesis of SiO2 covered SiC nanowires with milled Si,C nanopowders

SiO₂ covered β-SiC nanowires were directly synthesized with a novel method, annealing the milled Si,C nanopowders at 900–1100 °C on Si wafer or Al₂O₃ substrate, and there is no any metal catalysts used. The diameters of the nanowires are range of 20 to 50 nm, and the lengths of the nanowires are up to several hundreds of microns. There is a uniform SiO₂ amorphous layer on the surface of SiC nanowires. The axial direction of SiC nanowires is < 111>, and there are stacking faults and twin lamellae in the SiC nanowires. The synthetic mechanism of SiC nanowires includes two solid–solid reactions and one gas–solid reaction between SiO, Si, C and CO.     

Synthesis of B4C nanobelts in porous SiC bodies

B4C nanobelts were synthesized in porous SiC bodies, which had a sponge structure. The interconnected pore size of the SiC bodies was around 600 microm. The raw materials used for the B4C nanobelts were B2O3 for boron and phenolic resin and carbon black for carbon. The nanobelts grew fully inside the porous SiC when heat treated at 1400 degrees C for 1 h using LiCl as a volatilizing agent and cobalt as a catalyst. The thickness of the rhombohedral B4C nanobelts ranged from 0.1 to 1 microm, and their width was 0.5 to 10 microm. The length of the grown B4C belts was up to several hundreds of micrometers, and their growth direction was [110]. These single crystal nanobelts did not show any structural defects such as stacking faults, steps and twins. The low temperature synthesis in this study is attributed for the clean surface. It is suggested that the nanobelts were nucleated by the VLS mechanism, and then grew by the VS mechanism.     

Transmission electron microscope observations on GaP electroluminescent diode materials

Transmission electron microscope observations were made on GaP material grown by liquid epitaxy, vapour phase epitaxy and Czochralski pulling from the melt. The vapour phase material was found to contain many intrinsic stacking faults. Most were linear but a few were tetrahedral. Both Frank and Shockley partial dislocations were found. The fault density increased with increase in the Te dopant concentration in the vapour phase material. The Czochralski material also contained intrinsic stacking faults, and its defect content was higher than that of layers grown by liquid phase epitaxy. In all specimens with free carrier concentrations greater than 10¹⁸ cm^–3 evidence was found to suggest that precipitation of impurities had taken place.This work was carried out at the Department of Metallurgy, Imperial College, London, SW7, UK.     

SiCp/AZ91D镁基纳米复合材料的室温拉伸行为及塑性变形机理

为得到高强度和高塑性的镁基复合材料,通过高能超声分散法和金属型重力铸造工艺制备了SiC纳米颗粒分散均匀的SiCp/AZ91D镁基纳米复合材料,并进行T4固溶热处理和室温拉伸。利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)对试样拉伸后的显微组织和塑性变形机理进行观察与研究。结果表明:T4态SiCp/AZ91D镁基纳米复合材料室温下抗拉强度达到296 MPa,伸长率达到17.3%。经室温拉伸变形后复合材料基体微观组织中出现了大量的孪晶和滑移,孪生和滑移是复合材料塑形变形的主要机制。在室温拉伸过程中,α-Mg基体中SiC纳米颗粒周围形成高应变场,高应变场内形成大量位错和堆垛层错,这些位错和堆垛层错在拉伸应变的作用下演变成大量的滑移带和孪晶,这是SiCp/AZ91D镁基纳米复合材料在室温下具有高塑性的微观塑性变形机理。     

Hetero-epitaxy and structure characterization of Si films on 6H-SiC substrates

In order to realize the non-ultraviolet application of SiC optoelectronic devices, Si/6H-SiC heterojunctions were prepared by the low-pressure chemical vapour deposition at 850 °C. The X-ray diffraction (XRD) and the selected area electron diffraction (SAED) results indicate that Si thin films have a monocrystalline structure and were grown along the (111) crystal plane. The rationality of the (111) growth plane was also analyzed by the theoretical calculation. High-density structural defects such as stacking faults and twins were observed on Si films by the high-resolution transmission electron microscopy. This phenomenon was also validated by the SAED patterns of defect-rich regions on Si films.     

An X-ray Fourier line shape analysis of hexagonal tellurium films vacuum-evaporated under normal incidence

Detailed X-ray line profile studies have been made of thin films of hexagonal tellurium (an elemental semiconductor) in the thickness range 600–55 000 Å deposited by the vacuum evaporation method with normal incidence of the vapour beam. The analysis shows a considerable size effect (about 155–326 Å) and appreciable microstrain (about2.98(?1.47)×10^?3) in these tellurium films, as well as a near-isotropy in the size and strain parameters. The domain sizes for the fault-affected reflections (H ? K = 3N ± 1, L₀ odd or even) indicate that the occurence of both intrinsic (α) and growth (β) stacking faults is negligible throughout the range considered here. A high density of dislocations (about 10¹¹ cm^?2) has been observed in this work. Tellurium films grown on glass substrates were found to cleave along the prismatic plane instead of the basal plane, indicating the presence of c axis texture. A line shift analysis showed that the intrinsic stresses as well as the lattice parameter changes are small. Post-deposition annelaing affects the dislocation density slightly whereas all other microstructural parameters (except stacking faults, orientation and the intrinsic stresses) are significantly influenced for films deposited on pre-heated substrates.