Specific features of low-temperature phonon scattering in materials with tilted disclination loops

Phonon scattering by static stress fields of circular wedge disclination loops is investigated in the framework of the deformation potential approach. Numerical calculations of the mean free path l and thermal conductivity κ demonstrate that the temperature dependence of κ exhibits a minimum at a certain temperature T* in the low-temperature range. The thermal conductivity κ sharply increases as T ^?3 with a decrease in temperature (T<T*) and exhibits a dislocation behavior (κ ～ T ²) with an increase in temperature (T>T*). The results obtained for the wedge disclination loop are compared with the available data for uniaxial disclination dipoles. It is shown that the properties of uniaxial disclination dipoles serving as sources of phonon scattering are similar to those of wedge disclination loops.     

To the Question of Forming Geometrically Necessary Disclinations in Triple Junctions of Grain Boundaries in Metals

The Bollman and King models are tested by means of molecular dynamics simulation for the formation of geometrically necessary disclinations in triple junctions of grain boundaries in metals. It is shown that the stresses arising in a triple junction due to the non-multiple lengths of low-angle tilt boundaries to the distance between grain boundary dislocations is not compensated for mainly by the formation of an additional disclination in the junction (the King model) but by the bending of one or several grain boundaries, accompanied by the displacement of grain boundary dislocations. A triple junction of the Bollman U-type (containing a geometrically necessary disclination) is not formed at the conjugation of tilt boundaries with common misorientation along the junction or at the conjugation of mixed-type boundaries.     

Misfit disclination dipoles in nanocrystalline films and coatings

A theoretical model is proposed for describing the special physical micromechanism of misfit stress relaxation in nanocrystalline (NC) films and coatings. According to this model, under certain conditions, grain boundary sliding occurs in NC films and coatings, which is accompanied by the formation of an ensemble of disclination dipoles (rotational defects). These dipoles produce elastic stress fields, which partially compensate misfit stresses in NC films and coatings. Using the proposed model, it is shown that the nucleation of disclination dipoles in a film (coating) can significantly decrease the total energy of the film/substrate composite for the AlN/6H-SiC and GaN/6H-SiC systems over a wide range of structural parameter values.     

Point defects and ring defects in a nematic liquid crystal in a cylindrical capillary

The bidirectional escape into the third dimension of a linear disclination of strength m=1 (L ₊₁ ^p in a cylindrical capillary with normal boundary conditions is investigated. It is shown that in this case two types of defects arise in the capillary: point defects and ring defects, each of which can be of the radial or hyperbolic type. Exact solutions are obtained for the equation of equilibrium of the elastic field. The free energy of the point and ring defects is calculated approximately in a narrow, long capillary. New scenarios are proposed for the escape of the disclination L ₊₁ ^p . Zh. Tekh. Fiz. 69, 29–32 (July 1999)     

Ion-beam-induced collective rotation of nanocrystals

Vapor-deposited nanocrystalline titanium layers have been irradiated at room temperature with 350-MeV-Au ions up to 4x10;{15} Au/cm;{2}. Bombardment-induced texture changes were determined at the BESSY synchrotron light source. During off-normal irradiation, the nanocrystals undergo grain alignment and rotation up to approximately 90 degrees at the highest ion fluence. At the same time, the whole layer exhibits shear flow very similar to that observed previously in amorphous materials. Below 1x10;{15} Au/cm;{2}, a reversal of the ion incidence angle leads to a back rotation of the grains. These effects are absent or immeasurably small in coarse-grained titanium but have also been found in nanocrystalline TiN and NiO. The observations can be modeled by assuming that grain boundaries behave during ion bombardment like amorphous matter or by assuming a generation of disclination dipoles moving along grain boundaries.     

Effect of screening of the elastic field of the disclination located at the interface of two half-spaces by a dislocation ensemble

The self-consisted dynamics of a dislocation ensemble in the elastic field of the disclination located at the interface of two half-spaces has been considered for two cases, namely, for half-spaces with different densities of mobile dislocations and for a bicrystal where dislocations are absent in one half-space. The elastic energy W of the disclination screened by the dislocation ensemble has been calculated for the rectangular zone centered relative to the disclination. It has been shown that W increases as ～ $\sqrt R $ (R is the transverse size of the zone in the plastically deformed half-space).     

First-principles calculations of BC4N nanostructures: stability and electronic structure

In this work, we apply first-principles methods to investigate the stability and electronic structure of BC₄N nanostructures which were constructed from hexagonal graphite layers where substitutional nitrogen and boron atoms are placed at specific sites. These layers were rolled up to form zigzag and armchair nanotubes, with diameters varying from 7 to 12 Å, or cut and bent to form nanocones, with 60^° and 120^° disclination angles. The calculation results indicate that the most stable structures are the ones which maximize the number of B–N and C–C bonds. It is found that the zigzag nanotubes are more stable than the armchair ones, where the strain energy decreases with increasing tube diameter D, following a 1/D ² law. The results show that the 60^° disclination nanocones are the most stable ones. Additionally, the calculated electronic properties indicate a semiconducting behavior for all calculated structures, which is intermediate to the typical behaviors found for hexagonal boron nitride and graphene.     

Investigation of the thermophysical properties of oxide ceramic materials at liquid-helium temperatures

The main regularities in the transport of thermal phonons in oxide ceramic materials are investigated at liquid-helium temperatures. The dependences of the thermophysical characteristics of ceramic materials on their structural parameters (such as the grain size R, the grain boundary thickness d, and the structure of grain boundaries) are analyzed. It is demonstrated that, in dense coarse-grained ceramic materials with qR?1 (where q is the phonon wave vector), the grain boundaries and the grain size are the main factors responsible for the thermophysical characteristics of the material at liquid-helium temperatures. A comparative analysis of the thermophysical characteristics of optically transparent ceramic materials based on the Y₃Al₅O₁₂ (YAG) and Y₂O₃ cubic oxides synthesized under different technological conditions is performed using the proposed criterion.     

Influence of the grain size and structural state of grain boundaries on the parameter of low-temperature and high-rate superplasticity of nanocrystalline and microcrystalline alloys

A model has been proposed for calculating the grain size optimum for the deformation of nanocrystalline and microcrystalline materials under superplasticity conditions. The model is based on the concepts of the theory of nonequilibrium grain boundaries in metals. It has been demonstrated that the optimum grain size d _opt can be calculated as the size at which a high level of nonequilibrium of grain boundaries is combined with a high intensity of the accommodation of grain boundary sliding. The dependences of the quantity d _opt on the rate and temperature of the strain and the thermodynamic parameters of the material have been derived. The results obtained have been compared with the experimental data on the superplasticity of nanocrystalline and microcrystalline aluminum and magnesium alloys.     

The solution of a wedge disclination dipole interacting with an annular inclusion and the force acting on the disclination dipole

The interaction between a wedge disclination dipole and an elastic annular inclusion is investigated. Utilizing the Muskhelishvili complex variable method, the explicit series form solutions of the complex potentials in the matrix and the inclusion region are derived. The image force acting on the disclination dipole centre is also calculated. The influence of the location of the disclination dipole and the thickness of the annular inclusion as well as the elastic dissimilarity of materials upon the equilibrium position of the disclination dipole is discussed in detail. The results show that a stable equilibrium point of the disclination dipole near the inclusion is found for certain combinations of material constant. Moreover, the force on the disclination dipole is strongly affected by the position of the disclination dipole and the thickness of annular inclusion. The repulsion force increases （or the attraction force reduces） with the increase of the thickness of the annular inclusion. An appropriate critical value of the thickness of the annular inclusion may be found to change the direction of the force on the disclination dipole. The present solutions include previous results as special eases.     

Influence of grain boundary sliding on fracture toughness of nanocrystalline ceramics

A theoretical model is proposed describing a new physical microscopic mechanism of increased fracture toughness of nanocrystalline ceramics. According to this model, when a ceramic with a microcrack is deformed, intensive grain boundary sliding occurs near the crack tip under certain conditions. This sliding is accompanied by the formation of an array of disclination dipoles (rotational defects) producing elastic stresses. These stresses partially compensate the high local stresses concentrated near the microcrack tip and thereby hamper the microcrack growth. The proposed model is used to theoretically estimate the increase in the critical microcrack length (the length above which the catastrophic growth of microcracks occurs) caused by the formation of disclination dipoles during grain boundary sliding in nanoceramics. The increase in the critical microcrack length is a quantitative characteristic of the increased fracture toughness of nanoceramics.     

Continuous description of a grain boundary in forsterite from atomic scale simulations: the role of disclinations

We present continuous modelling at inter-atomic scale of a high-angle symmetric tilt boundary in forsterite. The model is grounded in periodic arrays of dislocation and disclination dipoles built on information gathered from discrete atomistic configurations generated by molecular dynamics simulations. The displacement, distortion (strain and rotation), curvature, dislocation and disclination density fields are determined in the boundary area using finite difference and interpolation techniques between atomic sites. The distortion fields of the O, Si and Mg sub-lattices are detailed to compare their roles in the accommodation of lattice incompatibility along the boundary. It is shown that the strain and curvature fields associated with the dislocation and disclination fields in the ‘skeleton’ O and Si sub-lattices accommodate the tilt incompatibility, whereas the elastic strain and rotation fields of the Mg sub-lattice are essentially compatible and induce stresses balancing the incompatibility stresses in the overall equilibrium.     

Acceleration of magnetic dipoles by a sequence of current-carrying turns

Magnetic dipoles are accelerated by a running gradient of the magnetic field that is produced by sequentially energizing current-carrying turns. Magnetic dipoles d _sh = 60 mm in diameter and l _tot = 1 m in length are gasdynamically preaccelerated to velocity V _in = 1 km/s, with which they are injected into the main accelerator. The turnover of the dipoles in the field of an accelerating pulse is prevented and focusing of dipoles is provided by directing the dipoles into a titanium tube. The weight of the dipoles is m = 2 kg, and they acquire final velocity V _fin = 5 km/s over acceleration length L _acc = 300 m.     

Grain size refinement due to relaxation of disclination junction configurations in the course of plastic deformation of polycrystals

A model is proposed for the formation of the substructure in polycrystals during plastic deformation. According to this model, fragmentation of a grain occurs through the formation of a system of diagonal low-angle boundaries, which originate at the edges of a rectangular grain. Misorientation boundaries form through relaxation of a nonsymmetric junction quadrupole disclination configuration accumulated at the grain corners under severe deformation when the disclination strength reaches a certain critical value. The energetics of this process is analyzed. A general case is considered where the disclinations at the junctions of the chosen grain differ in strength. The energetic approach used makes it possible to determine the misorientation angle ω_x of the resulting boundaries corresponding to the maximum energy gain and to find the dependence of this angle on the degree of asymmetry of the quadrupole configuration of junction disclinations. According to the proposed model, the splitting of a grain with a short edge greater than 0.5 μm is energetically favorable and decreases the latent energy of the grain for any ratio between the junction disclination strengths if the grain length-to-width ratio is less than 30. It is shown that the minimum possible grain size in the proposed model does not exceed 0.1 μm.     

Plastic flow and fracture of amorphous intercrystalline layers in ceramic nanocomposites

A theoretical model has been proposed for describing the plastic flow and fracture of amorphous intercrystalline layers in ceramic nanocomposites. The mechanism of plastic deformation has been considered as homogeneous nucleation and growth of liquidlike phase inclusions subjected to plastic shear. It has been demonstrated using a nanoceramic material consisting of TiN nanocrystallites and Si₃N₄ amorphous layers as an example that, when the length of the amorphous layer is reached and a considerable dislocation charge is accumulated, these inclusions induce the formation and growth of Mode I–II cracks in neighboring amorphous layers. In this case, the possibility of opening and growing the crack depends very strongly on the test temperature, the layer orientation, and the size of nanoceramic grains. An increase in the temperature and the angle of orientation and a decrease in the size of nanoceramic grains favor an increase in the crack resistance.     

Element analysis of complex materials by calibration-free laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is a promising method for fast and quantitative element analysis of complex materials. We report on LIBS measurements of multi-component oxide materials and the compositional analysis of materials by a calibration-free (CF) method. This CF-LIBS method relies on modeling of the optical emission of laser-induced plasma assuming local thermodynamic equilibrium. Various materials are investigated and the calculated concentration values (C _CF) of oxides CaO, Al₂O₃, MgO, SiO₂, FeO, and MnO are in agreement with nominal concentration values (C _N) from reference analysis. The relative error in oxide concentration e _r=|C _CF?C _N|/C _N decreases with increasing concentration. The quantification is limited to major oxides (C _N≥1 wt%). Slag samples from industrial steel production are analyzed on site by means of a mobile measurement system. LIBS measurements are performed at different sample temperatures. The results obtained show that CF-LIBS is applicable to fast compositional analysis of complex materials in harsh environments.     

Theoretical and experimental investigations of the properties of Ge2Sb2Te5 and indium-doped Ge2Sb2Te5 phase change material

We have carried out comprehensive computational and experimental study on the face-centered cubic Ge₂Sb₂Te₅ (GST) and indium (In)-doped GST phase change materials. Structural calculations, total density of states and crystal orbital Hamilton population have been calculated using first-principle calculation. 5 at.% doping of In weakens the Ge–Te, Sb–Te and Te–Te bond lengths. In element substitutes Sb to form In–Te-like structure in the GST system. In–Te has a weaker bond strength compared with the Sb–Te bond. However, both GST and doped alloy remain in rock salt structure. It is more favorable to replace Sb with In than with any other atomic position. X-ray diffraction (XRD) analysis has been carried out on thin film of In-doped GST phase change materials. XRD graph reveals that In-doped phase change materials have rock salt structure with the formation of In₂Te₃ crystallites in the material. Temperature dependence of impedance spectra has been calculated for thin films of GST and doped material. Thickness of the as-deposited films is calculated from Swanepoel method. Absorption coefficient (α) has been calculated for amorphous and crystalline thin films of the alloys. The optical gap (indirect band gap) energy of the amorphous and crystalline thin films has also been calculated by the equation \( \alpha h\nu = \beta (h\nu - E_{\text{g }} )^{2} \) . Optical contrast (C) of pure and doped phase change materials have also been calculated. Sufficient optical contrast has been found for pure and doped phase change materials.     

Defect microstructure in titanium nitride submicrocrystals

Transmission electron microscopy has been applied to the defect structure of titanium nitride in the submicrocrystalline (SMC) state produced under conditions of deviation from equilibrium in ion-plasma synthesis. The submicrocrystals contain a new type of defect substructure having a continuum disclination density up to 2.5 rad/μm². Direct structure methods give evidence for a high density of partial disclinations at the SMC grain boundaries in the nitride phase. A novel method has been used to examine substructures having a high defect density, which has been used to estimate the partial disclination density at the submicrocrystal boundaries. The origin of this highly defective state and the effects of it on the properties of SMC materials is discussed. Kuznetsov Siberian Technical Physics Institute, Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 3–12, July, 1998.