Volume 57, Issue 1

A model is suggested that describes enhanced strain rate sensitivity of nanocrystalline and ultrafine-grained metals. Within the model, plastic deformation of such metals incorporates dislocation transmission across grain boundaries (GBs) in the stress fields of dislocation pileups, the emission of individual dislocations from GBs as well as GB sliding accommodated by GB dislocation climb and/or Coble creep. The model predicts a strong increase in the strain rate sensitivity and a decrease in the activation volume with decreasing grain size, in accord with experimental data.We also considered the effect of GB sliding and Coble creep on the anomalous dependence of the activation volume on temperature observed in nanocrystalline Ni. It is demonstrated that although an account for GB sliding and Coble creep leads to the appearance of cusps in the temperature dependence of the activation volume, these mechanisms alone cannot be responsible for the observed anomalous dependence of the activation volume on temperature.

ZnO nanoparticles doped with transition metal ions are intensively studied nanomaterials, due to their charges and the spins of electrons that provides new magnetic, optical and transport properties. They find a vast range of applications, ranging from optoelectronics to spintronics. In this context especially important is the room temperature ferromagnetism observed for ZnO doped nanomaterials, although this phenomenon is still a controversial and open topic in material science, mostly due to low reproducibility of results from samples prepared by different techniques. In the first part of this article a short review of papers using magnetometric methods to determine the magnetic characteristics of Co-doped ZnO nanomaterials is presented. Different models introduced to explain room temperature ferromagnetism (carrier mediated ferromagnetism, Co 2+ -oxygen vacancy pairs, blocked superparamagnetic clusters, Co 2+ -Zn interstitial pairs, heterogeneous distribution of magnetic ions) are examined and discussed. In the second part, magnetisation study of a new series of nCoO/(1-n)ZnO nanocomposites synthesized by hydrothermal method under higher than previously applied pressure will be described. The obtained experimental results will be analysed and information on magnetic systems responsible for the observed characteristics and the involved magnetic interactions will be deduced.

Molecular dynamics is applied to study the effect of crystallographic orientation on the plastic deformation mechanisms and mechanical properties of NiAl intermetallic nanofilms subjected to uniaxial tension. It is observed that the deformation mechanisms qualitatively depend on the crystallographic orientation of the nanofilms with respect to the loading direction. Plastic deformation of the nanofilms along [557] crystallographic direction is associated with the edge dislocation sliding in the slip system [001](110). As for the nanofilms stretched along [554] and [111] directions, their deformation occurs first through the dislocation sliding followed by the formation of (112)[11 1] twins. Uniaxial tension of the nanofilms along [559] and [55 11] leads to the nucleation and growth of a martensitic phase followed by their rupture along an interface. The maximum (minimum) strength of 9.9 (7.0) GPa is observed for the nanofilms stretched along the [559] ([554]) crystallographic direction, while the largest (smallest) strain to failure of 27 (15)% is for [559] ([55 11]). Various deformation mechanisms of the nanofilms are explained through computing the Schmid factor for the operational slip system. The results indicate that the crystallographic orientation is among the key parameters controlling the deformation mechanisms and mechanical properties of intermetallic nanofilms.

Magnetic susceptibility and EPR of iron-containing solid solutions with layered perovskite-like structure Bi2BaNb 2-2x Fe 2x O 9-δ and Bi 5 Nb 3-3x Fe 3x O 15-δ have been studied. The solid solutions Bi 2 BaNb 2-2x Fe 2x O 9-δ as well as iron oxides FeO, Fe 2 O 3 , and Fe 3 O 4 were studied by the NEXAFS spectroscopy. The formation of exchange-bound aggregates of Fe(III) atoms with antiferroand ferromagnetic exchange types has been found in the solid solutions. In the ESR spectra of the solid solution samples, the lines with g ≈ 2.0 and g = 4.27 having a weak shoulder at g ~ 8 were attributed to Fe(III) atoms in the octahedral field with strong rhombic distortion.

A laser beam heats a mixture of ethanol and anthocyanins producing the effect of a reciprocating machine. This behaviour is analyzed in terms of the stable, metastable and unstable states of Landau and Lifschitz. Trying first with the van der Waals equation of state, with negative results because the system state is normally far from the critical state, a modified version, with van der Waals coefficients depending linearly on the temperature is proposed, based in the thermodynamic properties of ethanol at room and critical temperatures. The resulting theory allows us to understand the discontinuous results concerning the absorption of the laser beam in terms of a thermodynamic cycle and its frequency dependence on the laser light focus position. Slight changes in the experimental setup may enable the measurement of thermodynamic properties in metastable states.

A model is proposed describing the effect of crack bridging on the fracture toughness of ceramic/graphene composites. The dependences of the fracture toughness on the graphene content and the sizes of the graphene platelets are calculated in the exemplary case of yttria stabilized zirconia (YSZ)/graphene composites. The calculations predict that if crack bridging prevails over crack deflection during crack growth, the maximum toughening can be achieved in the case of long graphene platelets provided that the latter do not rupture and adhere well to the matrix. The model shows good correlation with the experimental data at low graphene concentrations.

The stress concentration and distribution around a triple-junction pore of three-fold symmetry in a polycrystalline ceramic material is considered. The perturbation method in the theory of plane elasticity is used to solve the problem of a nearly circular pore of three-fold symmetry under remote loading in the first approximation. The solution was specified to the uniaxial tension of convex and concave rounded triangular pores. The stress concentration on the pore surface and the stress distribution in vicinity of the pore along its symmetry axes are studied and discussed in detail. The numerical results, issued from the first-order approximation analytical solution, are compared with those of finite-element calculations.

Results on structural, compositional, optical and electrical characterization of MOVPE grown AlGaN/GaN heterostructures with focus on understanding how the AlN buffer synthesis affects the top films are reported. The study demonstrates very good correlation between different methods providing a platform for reliable estimation of crystalline quality of the AlGaN/GaN structures and related to that electrical performance which is found to be significantly affected by threading dislocations (TD): higher TD density reduces the electron mobility while the charge carrier concentration is found to be largely unchanged. The attempt to vary the ammonia flow during the AlN synthesis is found not to affect the film composition and dislocation densities in the following heterostructures. An unusual phenomenon of considerable diffusion of Ga from the GaN film into the AlN buffer is found in all samples under the study. The obtained results are an important step in optimization of AlGaN/GaN growth towards the formation of good quality HEMT structures on sapphire and transfer of technology to Si substrates by providing clear understanding of the role of synthesis parameter on structure and composition of the films.

A fundamentally new method for synthesis of thick (200-400 nm) epitaxial films of silicon carbide on single-crystal Al 2 O 3 substrates is proposed. The method develops the previously discovered method of topochemical substitution of atoms during the transformation of Si into SiC. In the proposed method, a layer of epitaxial Si of orientation (100) or (111) is being preliminarily deposited on the sapphire via CVD or other technique. Then, using a topochemical reaction between the CO gas and the deposited epitaxial silicon film, a part of Si atoms is being substituted by carbon atoms. As a result, the silicon film transforms into SiC film. As experimental studies have shown, in the process of this transformation a high-quality epitaxial SiC layer can be formed on the sapphire surface under certain synthesis conditions. A detailed investigation of grown SiC layers using the methods of electron diffraction, ellipsometry, Raman spectroscopy, X-ray diffractometry and scanning microscopy is conducted and an analysis of experimental data is given. Formation of both cubic 3C-SiC and hexagonal SiC polytypes during the transformation of Si into SiC was revealed by the XRD method. A detailed analysis of the current state of the problems of growth of epitaxial SiC layers on Al 2 O 3 using various methods is provided. The method developed in this study opens up completely new perspectives not only for synthesis of semiconductors, but also for creation of new classes of composite, heat-resistant and other solid coatings.

Results of the studies of the properties of single-crystalline bulk β-Ga 2 O 3 grown from the melt are presented. High chemical purity and phase uniformity of the grown material are demonstrated. Raman spectroscopy studies confirmed low energies of optical phonons in β-Ga 2 O 3 , which makes it a promising material for laser optics applications.

Commercially pure (Grade 4) titanium specimens with two types of microstructure (coarse-grained and nanostructured by means of equal channel angular pressing) were joined by linear friction welding (LFW). Microhardness measurements, tensile and compression tests as well as fracture surface inspection by scanning electron microscopy revealed similarity in tensile properties and significant discrepancy in compression behavior of two types of Ti specimens. Tensile test shows that fracture occurs in both cases in a region of heat affected zone. Fracture surface analysis displayed mostly ductile mode. In case of compression test coarse-grained titanium linear friction welded demonstrates a pronounced strain hardening region without fracture but nanostructured LFW titanium shows typical plastic behavior with mixing of brittle-ductile mode of rupture in the welding zone.

The electron work function (EWF) of ultrafine grained (UFG) aluminum structured by high pressure torsion (HPT) has been investigated. For the first time, the dependence of the EWF on the specific length of grain boundaries (or the grain size) for UFG Al has been obtained. The change of average grain size was achieved by short term annealing of HPT-processed aluminum at different temperatures from the range 90-400 °C. It has been shown that the state of grain boundaries (GBs) affects the magnitude of the EWF. It has been found that the transformation of GBs due to annealing at 90 °C from a nonequilibrium to more equilibrium state while maintaining the specific length of GBs and their average misorientation is accompanied by a decrease in average GB specific energy by 0.3 J m -2 . This transition provides a sharp increase in the EWF of the UFGAl by 0.25 eV.

The paper presents the investigation of lithium-potassium-alumoborate glasses (0- 25 mol.% Li 2 O) doped by copper ions. The effect of glass annealing on the Raman scattering patterns is discussed. It is shown that such an annealing results in drastic changes in the Raman scattering patterns for the glass with 10 mol.%of Li 2 O: the band at 481 cm -1 conventionally attributed to boron-oxygen bonds in metaborate rings disappears, while a set of narrow bands at 1236, 1500, and 1783 cm -1 is observed. We consider one of these bands, 1783 cm -1 , as being the identifier of the formed Li(Al 7 B 4 O 17 ) nanophase, XRD analysis supported the above suggestion. The effect of newly formed nanophase on the luminescence spectra is discussed.

The article begins with analytical review of the works performed from 1940 up to nowadays, concerning the phenomenon of autohesion in the powders of inorganic compounds. The issues of terminology are concerned, the difference is discussed between the phenomena of autohesion, adhesion, and cohesion. Three groups of the components of autohesion forces are considered: van der Waals and electrical forces, cohesion and capillary forces and mechanical cohesion, as well as internal and external factors affecting these forces. The second part of the article is devoted to the results of experiments performed to study the influence of the autohesion forces on the properties of friable inorganic powder materials. In particular, the rheological properties of powders were studied by static and dynamic methods on four kinds of electrocorundum powder with similar particle size and particle size distribution corresponding to the class of grinding powders F1200 according to FEPA standard. Special attention is paid to assessing the effect of humidity on technological properties of the powder, obtained by spray drying of ceramic slurry. This powder is widely used as a component of building mixes. The authors came to conclusion that detailed study is necessary of the autohesion effect on the parameters of friable systems for the rational choice of technological modes. The work can be useful for researchers and practitioners associated with the problems of processing, storage and transportation of powder materials.

A theoretical model is suggested which describes micromechanism of the plastic deformation in nanotwinned metallic film formed by electrical deposition onto metallic substrate. In the framework of the model, the micromechanism of the plastic deformation is widening of nanoscale twins due to migration of twin boundaries. The energy and stress characteristics of the twin widening are calculated.