Growth of Sb2S3 single crystals by chemical vapour transport

Single crystals of antimony trisulfide have been grown by a chemical vapour transport technique using iodine as the transporting agent. Single crystals were obtained at a much lower temperature when antimony and sulfur in the stoichiometric ratio were taken as the source along with iodine. However when polycrystalline antimony trisulfide was taken as the source with iodine, single crystals were not obtained even at the melting point of Sb₂S₃. This observation has been explained on the basis of bond energy values.     

Deformation mechanisms in polymer crystals

The deformation of polydiacetylene single crystals has been investigated by scanning electron microscopy. Three types of deformation twins have been identified. The twinning planes are (0 1 2), (¯2 1 2), and (2 1 2), types not previously reported in polymer crystals. Deformation features in polydiacetylene single crystals observed by other workers are interpreted in terms of these three twins. The possibility of similar types of twins occurring in other polymer crystals is discussed. The twins in the polydiacetylene single crystals and those found in other non-polymeric materials are compared.     

Twinning of bismuth single crystals bombarded by boron ions

Twinning of ion-bombarded single crystals has been investigated for the first time. It has been established that bombardment of bismuth single crystals with boron ions stimulates mobility of twinning dislocations and quenches their sources. This result is explained using the dislocation model of a wedge-shaped twin. Pis’ma Zh. Tekh. Fiz. 24, 1–9 (April 26, 1998)     

Effect of hydrogen on orientation dependence of critical shear stress and mechanism of straining in single crystals of stable stainless steel

The effect of hydrogen on the critical shear stress (τ_cr), dislocation structure, and mechanisms of straining (slip vs. twinning) in single crystals of Fe-26Cr-32Ni-3Mo (wt %) austenite stainless steel has been studied. It is shown for the first time that hydrogenation leads to the appearance of the orientation dependence of τ_cr, which is absent in the initial hydrogen-free crystals. This dependence of τ_cr is related to a decrease in the stacking fault energy, which determines a change of the mechanism of straining from slip to twinning.     

The elastic constants of antimony-25.5 at. % arsenic alloy single crystals

The six independent elastic constants of single crystals of the homogeneous, minimum melting point alloy of composition antimony 74.5 at. % and arsenic 25.5 at. % have been measured. The elastic behaviour of the alloy is compared with that of the rhombohedral A7 structure elements arsenic and antimony and is found to show characteristics consistent with those expected for layer-type crystals. Elastic wave velocity surface cross-sections, particle displacement and energy flux vectors and Young's modulus are presented and discussed. The Debye temperature is 219 K.     

Crystal growth of chemical Co-P and electrodeposited Co on Ni and Cu single crystals

Experiments on the chemical deposition of Co P alloys on (0 1 1) and (1 1 1) faces of Cu and Ni single crystals have shown that the crystal growth occurs corresponding to the growth on (0 0 1) faces. Three ranges of crystal growth may be identified. For thin deposits, no pseudomorphism has been detected, moreover martensitically transformed hcp Co is observed from the first layers on (1 1 1) faces. For medium thick deposits, twinning on {1 0.1} composition planes has been confirmed; this anomalous twinning has been attributed to the high density of growth faults and is also observed in the case of electrodeposition. At high thickness, oriented regions are observed and the bath orientation predominates. The occurrence of these orientations has been related to the competing role in the adsorbed layer of Co-hydrolized species and hypophosphite ions.     

New channels for mechanical twinning

The creation of twins at twinning wedge boundaries in bismuth single crystals whose surface is deformed by a concentrated load, has been observed and explained for the first time. Pis’ma Zh. Tekh. Fiz. 24, 43–49 (May 12, 1998)     

Czochralski growth of antimony single crystals

The conditions for the Czochralski growth of antimony crystals have been optimized, and single crystals 20 × 30 and 40 × 50 mm² in cross section and ?250 and ?800 g in weight have been grown. Their thermoelectric power, resistivity, and thermal conductivity were measured at room temperature in different crystallographic directions.     

Features of plastic strain localization at the yield plateau in Hadfield steel single crystals

Spatiotemporal distributions of local components of the plastic distortion tensor in Hadfield steel single crystals oriented for single twinning have been studied under active tensile straining conditions using the double-exposure speckle photography technique. Features of the macroscopically inhomogeneous strain localization at the yield plateau are considered. Relations between local components of the plastic distortion tensor in the zone of strain localization are analyzed.     

Structural,physical and damping properties of melt-spun Ni–Mn–Ga wire-epoxy composites

Ni–Mn–Ga is a ferromagnetic shape memory alloy that exhibits large, magnetic-field- and stress-induced strains via energy dissipating twinning when processed into single crystals. Grain boundaries suppress twinning and render polycrystalline Ni–Mn–Ga brittle. Ni–Mn–Ga/polymer composites overcome the drawbacks of polycrystals and could thus provide a less expensive and easier to handle alternative to Ni–Mn–Ga single crystals for damping applications. Ni–Mn–Ga wires were produced by melt-spinning and were polycrystalline in the as-spun state. Annealed wires were ferromagnetic at room temperature with non-modulated martensite and a bamboo microstructure. The annealed wires displayed a hysteretic stress–strain behavior typical for twinning. Ni–Mn–Ga wire-epoxy matrix composites were fabricated with as-spun and annealed wires. The damping behavior of annealed Ni–Mn–Ga wire-epoxy matrix composites was higher than that of as-spun Ni–Mn–Ga wire-epoxy matrix composites and of pure epoxy.     

Twins,disorders and other demons

Dramatic improvements in both the hardware and software used to perform X-ray crystallographic studies on single crystals have led to the publication of thousands of new structures each year. These improvements have also made it possible to successfully resolve a larger percentage of structural problems such as disorder and complicated twinning. Examples are included of twins that were first thought to be disorders structures with both twinning and disorder, and four-fold twinning.     

Stress induced twinning of polydiacetylene single crystal fibres in composites

Scanning electron microscopy has been used to show that twins induced by compression of DCHD-polydiacetylene fibres occur on a single crystallographic plane. Other twinning modes appear to be prohibited by the nature of the sidegroup packing. Under the optical microscope identical twinning patterns have been observed on fibres in composite specimens where matrix shrinkage had been induced by post-curing the epoxy at 100°C. Resonance Raman spectroscopy has been used to monitor the strain in a single fibre embedded in an epoxy matrix which was subjected to uniaxial compressive stress. The initiation of twinning was found to occur when the axial strain in the fibre was approximately 0.2%.     

Electron imaging of twins in polyethylene crystals

Twins have been identified in the electron image of polyethylene crystals by means of their contrast behaviour on tilting the specimen and using different diffracted beams for dark field microscopy. Both {310} and {110} twinning has been found. The effect of twinning on Moiré fringes has been investigated and experimental observations confirm the twin features to be correctly interpreted.     

Lattice thermal conductivity of bismuth-antimony alloy single crystals at low temperatures

Thermal conductivity measurements in a number of bismuth-antimony alloy single crystals, with antimony concentrations from 1.72 at.% to 20 at.% are reported. Analysis of the results shows that in this alloy system, below about 25 K, the heat carriers are predominantly phonons. Callaway's theoretical model for lattice thermal conductivity has been utilized for the analysis of the results, using various phonon-scattering processes, and this model has been found to give a good fit to the experimental data, particularly below 25 K, where lattice conduction predominates. The Casimir model for finding the characteristic length for boundary scattering fails to explain the experimental results. This indicates the existence of microscale fluctuations in the composition of the alloy.     

Growth and some properties of barium-cadmium formate single crystals: (I) Crystal growth and morphology

The growth of barium-cadmium formate BaCd(HCO₂)₄·2H₂O single crystals by slow cooling method and their characterization by selective etching are reported. It was found that BaCd(HCO₂)₄·2H₂O crystallizes from aqueous solution in 2/m class of the monoclinic system. Crystals grown during a period of 1 month have dimensions of about 2 × 1.5 × 10 cm³. The typical twinning for these crystals has been observed and investigated by the selective etching. The dislocation density has been estimated to be 3·10² – 2·10³ cm^?2.     

Orientation dependence of deformation twinning in Cu single crystals

Cu single crystals were subjected to dynamic compression plastic deformation to investigate orientationdependent twinning.The experimental results showed that twinning is closely related to the ratio of the maximum Schmid factor for twinning partial(m_T)to the maximum Schmid factor for perfect dislocation(m_S),i.e.,m_T/m_S,rather than m_T.The twin volume fraction V_Tincreases with the m_T/m_Svalue and the most favorable orientation for twinning has the maximum m_T/m_Svalue(1.15).The relationships of m_T/m_Swith both effective stacking fault energy γ_(eff)and threshold stress for twinning τ_Twere established for understanding orientation-dependent twinning.Further insights into the orientation-dependent twinning and guidance for developing bulk high density nanotwinned materials are provided.     

Optical properties of undoped and chromium-doped VA-VIA-VIIA single crystals

SbSl∶Cr, SbSel∶Cr, BiSl∶Cr, and BiSel∶Cr single crystals were grown by the vertical Bridgman technique using the ingots synthesized from the high-purity (99.9999%) elements antimony, bismuth, sulphur, selenium, iodine and chromium. The grown single crystals crystallized in the orthorhombic structure and had an indirect energy-band structure. The temperature dependence of the optical energy gap was well fitted by the Varshni equation. When chromium was doped into the single crystals, impurities in the optical absorption peaks appeared, and were attributed to the electron transitions between the energy levels of Cr²⁺ ions sited at the T_d symmetry point in the host lattice.     

Formation and characterization of diamond crystals synthesized by microwave plasma CVD system

Abstract

Diamond crystals have been successfully synthesized on (100) Si wafer using microwave plasma CVD. The growth was conditioned in a flowing system in which the parameters, such as CH₄/H₂ ratio, pressure, temperature and microwave power were varied. Cubo‐octahedra or tetrakaidecahedra are the equilibrium shape of diamond single crystals obtained under all conditions and are therefore the basic unit for the formation of polycrystalline diamond films, mostly through repetitive twinning and secondary growth of diamond crystals on {100} habit planes of cubo‐octahedra. Both X‐ray diffraction and Raman spectroscopy were used to facilitate the analysis of the diamond crystallinity and purity. These qualities are similar to those of natural diamonds.     

Key Properties of Ni?Mn?Ga Based Single Crystals Grown with the SLARE Technique

Magnetic shape memory alloys (MSMAs), exhibit large strains and hence are materials, which could substitute giant magnetostrictive and piezoelectrical materials in actuating devices. The actuation stress needed to induce the strain is much lower than in other actuator materials. Since the strain can be induced without phase transformation by a magnetic field, the development of actuators with high working frequencies is possible. However, for reasonable applications, large strains have to be induced with small magnetic fields. Up to now repeatable magnetically induced strains of 5–10% in magnetic fields of less than 500 mT have been achieved only in single crystals. The production of Ni? Mn? Ga based single crystals is difficult and time consuming. The crystal quality is affected by porosity and impurities. With the Bridgeman based method called Slag Remelting and Encapsulation (SLARE) single crystalline ingots of Ni? Mn? Ga, Ni? Mn? Ga? Fe, and Ni? Mn? Ga? Co of high quality were grown and characterized. The results show that MSMA properties depend on the position within the single crystalline rods due to a composition gradient. The influence of surface treatment demonstrates that the decrease of surface roughness leads to a decrease of twinning stress. MSMAs with twinning stresses above 1 MPa show a magnetic field induced strain (MFIS) when tilting is not restricted by constraints. Softer samples can adapt to constraints much better and show large MFIS. Substituting Ni by Fe and Co, shifted the phase transitions successfully to higher temperatures. Ni? Mn? Ga alloyed with up to 6 at.% Co showed three different martensite structures: a non‐modulated tetragonal structure, a modulated tetragonal structure, showing the same behavior as Ni? Mn? Ga with identical structures and a non‐modulated orthorhombic structure with a stress–strain‐behavior explainable by the double twinning mechanism.     

A study of twinning in polyethylene

Both {1 1 0} and {3 1 0} deformation twinning modes have been predicted for polyethylene. The existence of the former twinning mode has been confirmed in this work by non-photographic X-ray methods. When comparing and explaining bulk and single crystal twinning behaviour in terms of the influence of fold surface geometry, certain discrepancies arise. These may be resolved by noting that {1 1 0} twins occur at relatively low stresses and that {3 1 0} twins are formed under conditions of high strain during which the crystalline morphology is changed from lamellar to microfibrillar.