Low self-affine exponents of fractured glass ceramics surfaces

The geometry of postmortem rough fracture surfaces of porous glass ceramics made of sintered glass beads is shown experimentally to be self-affine with an exponent zeta=0.40+/-0.04, remarkably lower than the "universal" value zeta=0.8 frequently measured for many materials. This low value of zeta is similar to that found for sandstone samples of similar microstructure and is also practically independent on the porosity phi in the range investigated (3%< or =phi< or =26%) as well as on the bead diameter d and of the crack growth velocity. In contrast, the roughness amplitude normalized by d increases linearly with phi while it is still independent, within experimental error, of d and of the crack propagation velocity. An interpretation of this variation is suggested in terms of a transition from transgranular to intergranular fracture propagation with no influence, however, on the exponent zeta.     

Crack motion in viscoelastic solids: The role of the flash temperature

We present a simple theory of crack propagation in viscoelastic solids. We calculate the energy per unit area, G(v), to propagate a crack, as a function of the crack tip velocity v. Our study includes the non-uniform temperature distribution (flash temperature) in the vicinity of the crack tip, which has a profound influence on G(v). At very low crack tip velocities, the heat produced at the crack tip can diffuse away, resulting in very small temperature increase: in this “low-speed” regime the flash temperature effect is unimportant. However, because of the low heat conductivity of rubber-like materials, already at moderate crack tip velocities a very large temperature increase (of order of 1000 K) can occur close to the crack tip. We show that this will drastically affect the viscoelastic energy dissipation close to the crack tip, resulting in a “hot-crack” propagation regime. The transition between the low-speed regime and the hot-crack regime is very abrupt, which may result in unstable crack motion, e.g. stick-slip motion or catastrophic failure, as observed in some experiments. In addition, the high crack tip temperature may result in significant thermal decomposition within the heated region, resulting in a liquid-like region in the vicinity of the crack tip. This may explain the change in surface morphology (from rough to smooth surfaces) which is observed as the crack tip velocity is increased above the instability threshold.     

Hot cracks in rubber: origin of the giant toughness of rubberlike materials

We study crack propagation in rubberlike materials and show that the nonuniform temperature distribution which occurs in the vicinity of the crack tip has a profound influence on the crack propagation, and may strongly enhance the crack propagation energy G(v) for high crack velocities v. At very low crack-tip velocities, the heat produced at the crack tip can diffuse away, but already at moderate crack-tip velocities a very large temperature increase occurs close to the crack tip resulting in a "hot-crack" propagation regime. The transition between the low-speed regime and the hot-crack regime is very abrupt and may result in unstable crack motion, e.g., stick-slip motion or catastrophic failure.     

Time growth rate optimisation of terahertz electromagnetic wave generation by converting occupied plasma region from annular plasma to filled plasma in the core,in an elliptical Cherenkov maser with two energy sources

A theoretical model is described here for studying the effect of temperature on nanomaterials. The thermodynamic equation of state (EoS) proposed by Goyal and Gupta in High Temp.-High Press. 45, 163 (2016); Oriental J. Chem. 32(4), 2193 (2016), is extended in the present study using Qi and Wang model [Mater. Chem. Phys. 88, 280 (2004)]. The thermal expansion coefficient is expressed in terms of shape and size and used to obtain the isobaric EoS of nanomaterials for the change in volume \(V/{V_0}\). The variation in \(V/{V_0}\) with temperature is estimated for spherical nanoparticles, nanowires and nanofilms. It is found that the volume thermal expansivity decreases as size of the nanomaterial increases, whereas \(V/{V_0}\) increases with temperature across nanomaterials of different sizes. The lattice parameter variation with temperature is studied in Zn nanowires, Se and Ag nanoparticles. It is found that lattice constant increases with increase in temperature. Also, bulk modulus is found to increase with temperature in nanomaterials. The results obtained from the present model are compared with the available experimental data. A good consistency between the compared results confirms the suitability of the present model for studying thermal properties of the nanomaterials.     

Symmetry and lattice parameters of SrTiO<Subscript>3</Subscript>-PbTiO<Subscript>3</Subscript> solid solutions

The lattice parameters of ceramic samples of (1 ? x)SrTiO₃-xPbTiO₃ solid solutions are measured at room temperature. It is found that the samples have cubic symmetry in the concentration range x = 0?0.3 and tetragonal symmetry for x > 0.3. The lattice parameter a is virtually independent of temperature for x < 0.8 and slightly decreases in the range x = 0.8?1.0, while the lattice parameter c increases with increasing x. The reduced cubic parameter varies nonlinearly and deviates from Vegard’s linear law as the concentration x increases.     

Nonequilibrium energy of Rayleigh waves in half-limited crystals with hot electrons close to the defect surface

Nonequilibrium energies of surface (Rayleigh) lattice oscillations in half-limited crystals with static defects and a two-dimensional layer of hot Fermi and Boltzmann electrons close to the stress free surface were calculated. Substances with electrons heated by an external field retaining their intrinsic temperature for a certain time, T _e ? T, where T is the temperature of the lattice, were considered. As shown earlier, the thermodynamic characteristics of thin films can then be determined by nonequilibrium energy of Rayleigh waves (R-phonons) caused by their interaction with hot electrons. This energy decreases as the widths of the energy spectrum of R-phonons increase. In this work, the earlier calculated spectrum widths are used. These widths are caused by the scattering of R-phonons by electrons and static defects close to the surface (point and extended surface defects, edge dislocations perpendicular to the surface and emerging to it, and random lattice grooves in the lattice plane). In all the calculations, the Keldysh diagram technique transformed for half-limited media was used.     

Cluster-Type Structure of Amorphous Smooth Hydrocarbon CD<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Films (<Emphasis Type="Italic">x</Emphasis> ~ 0.5) from T-10 Tokamak

Structure of smooth hydrocarbon CD _x films with a high deuterium ratio x ~ 0.5 redeposited from T-10 tokamak D-plasma discharges (NRC Kurchatov Institute, Moscow) has been studied. For the first time, small and wide angle X-ray scattering technique using synchrotron radiation and neutron diffraction have been employed. A fractal structure of CD _x films is found to consist of mass-fractals with rough border, surface fractals (with rough surface), plane scatterers and linear chains forming a branched and highly cross-linked 3D carbon network. The found fractals, including sp² clusters, are of typical size ~1.60 nm. They include a C₁₃ fragment consisting of three interconnected aromatic rings forming a minimal fractal sp² aggregate 9 × C₁₃. These graphene-like sp² clusters are interconnected and form a 3D lattice which can be alternatively interpreted as a highly defective graphene layer with a large concentration of vacancies. The unsaturated chemical bonds are filled with D, H atoms, linear sp² C=C, C=O, and sp³ structural elements like C-C, C-H(D), C-D_2,3, C-O, O-H, COOH, C _x D(H) _y found earlier from the infrared spectra of CD _x films, which are binding linear elements of a carbon network. The amorphous structure of CD _x films has been confirmed by the results of earlier fractal structure modeling, as well as by researches with X-ray photoelectron spectroscopy which allow finding a definite similarity with the electron structure of their model analogues — polymeric a-C:H and a-C:D films with a disordered carbon network consisting of atoms in sp³ + sp² states.     

On the Hall effect in a two-dimensional doped antiferromagnet

The conductivity and the Hall coefficient of a doped 2D antiferromagnet in the normal state are considered using the Kondo lattice model in the multimoment approximation. The anomalous temperature dependence of the kinetic coefficients is explained by the strong anisotropic charge-carrier scattering from the spin subsystem and found to be in qualitative agreement with the experimental data for the normal state of high-T _c superconductors.     

Instability of the crystal lattice on the verge of structural phase transitions

Conclusion Thus, the data of the present review lead to the conclusion that the pretransition phenomena observed on the verge of SPTs have the character of an intraphase transformation usually of the second or higher order and are observed both before low-temperature martensite transformations and in other temperature intervals. These intraphase transformations are associated with the harmonic softening of the crystal lattice, which may be both isotropic and anisotropic. The directions of the lattice softening coincide with the expected directions of its rearrangement. At the martensite transformation temperature the residual phase, too, undergoes a phase transformation, while the lattice softening observed during intraphase transformations increases.The conclusions reached refer as yet to a relatively small number of alloys and are interpreted within the framework of a simple model of the crystal lattice. The problem of farther investigations is to expand the number of alloys investigated and determine how general the character of both the phenomena of intraphase transitions and of phase transformations preceding and accompanying the SPTs and the regular changes in the properties of the lattice described here are.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 5, pp. 118–126, May, 1985.     

Plasmon resonances in a two-dimensional lattice of metal particles in a dielectric layer: Structural and polarization properties

The results of experimental and theoretical investigation of planar two-dimensional (2D) samples of plasmon structures are presented. The samples represent a 2D lattice of gold nanoparticles embedded in a thin dielectric layer and are studied by atomic force microscopy (AFM) and optical methods. Absorption bands associated with the excitation of various surface plasmon resonances (SPR) are interpreted. It is found that the choice of the mutual orientation of the polarization plane and the edge of the unit cell of the 2D lattice determines the spectral position of the lattice surface plasmon resonance (LSPR) related to the lattice period. It is shown that the interaction of p- and s-polarized light with a 2D lattice of nanoparticles is described by the dipole–dipole interaction between nanoparticles embedded in a medium with effective permittivity. Analysis of the spectra of ellipsometric parameters allows one to determine the amplitude and phase anisotropy of transmission, which is a consequence of the imperfection of the 2D lattice of samples.     

Calculations of Infrared Laser Beam Scattering from Rough Dielectric Objects

The coherent and incoherent scattering cross sections of Infrared Laser Gaussian beam scattering from arbitrarily shaped convex dielectric objects with rough surfaces are investigated by using plane wave spectrum method and physical optics approximation. In the paper, the infrared laser scattering cross sections of rough sphere are calculated at 10.6 m , and the influence of the beam size, permittivity, and polarization as well as roughness parameters is analyzed numerically. When the beam size is much larger than the size of object, the results in the paper can reduce to those of an incident plane wave. On the other hand, for the case of roughness statistical parameter close to zero, only the forward scattering has a parent difference compared with the result of gaussian beam scattering from smooth sphere.     

Lattice models of hydrogen-bonded solvents. II

We consider a lattice model of a binary mixture in which each molecule of one component can form zero, one, or two bonds to molecules of the same species on neighboring vertices of the square lattice. We allow the energy of molecules with two bonds to depend on the valence angle, thus generalizing the first paper in this series. If straight polymeric configurations are favored over all others, then a phase transition occurs for low enough temperature. On the other hand, if bent configurations are favored, there is no phase transition. Analogous results are obtained for the hexagonal lattice, where we distinguish energetically betweencis andtrans isomerism of four bonded molecules.Supported by the Danish Science Foundation under grant 511-3635.     

Different-dimensional structures of antimony formed selectively on graphite

When antimony (mostly Sb₄) is deposited on highly oriented pyrolytic graphite (HOPG), in situ scanning tunneling microscopy images reveal that three-dimensional (3D) spherical islands, quasi-2D films and 1D nanowires (NWs) are formed. The spherical islands develop into faceted crystallites in the later growth stage. The lattice parameters of the 2D and 3D structures are close to those of α-Sb bulk, whereas the NWs appear in a compressed state. The Laplace pressure, which can reach the GPa range in a nanostructure, is considered the driving force for the compressive lattice structures of Sb NWs. We found conditions of controlling the dimensionality of Sb nanostructures in their self-assembly on HOPG to a certain extent. At room temperature and with a low Sb flux, 3D islands grow exclusively. At a substrate temperature of 100 °C, 2D and 1D structures are dominant when a high deposition flux is used, whereas only NWs are formed initially when a low flux is used. These results are explained in terms of different activation energies for Sb₄ diffusion and conversion to a chemisorption or dissociation state on HOPG. As the temperature increases, the rate of conversion to the chemisorption or dissociation state increases more rapidly than that of diffusion since the chemisorption activation energy is much higher than the diffusion barrier of physisorbed Sb₄, resulting in enhanced 2D and 1D structural nucleation and growth, which are further favored with the increase in deposition flux. The bonding nature of various Sb structures with a graphite surface and the conditions for growing aligned Sb NWs exclusively are discussed. PACS 61.46.+w; 64.70.Nd; 68.37.Ef; 68.65.-k; 81.07.-b     

Effect of the block-spin configuration on the location of βc in two-dimensional Ising modelsin two-dimensional Ising models

We consider the nearest neighbor Ising model on the 2D square lattice and divide the lattice into 2 by 2 blocks. Each block is assigned one spin value (1 or –1) and these block spin values are kept fixed. We then impose the majority rule and look at the effect on the phase transition that was present in the original unconstrained spin system. We find that for the checkerboard block-spin configuration, Monte Carlo simulations show that _c is close to 1, which, compared to the original nearest neighbor Ising _c = 0.44..., shows that the critical temperature has been reduced by more than one half. For none of the other 11 block-spin configurations that we have considered is there any indication of a phase transition in the constrained system of original spins.     

Study of electrical and thermoelecrical properties of sulfides Tm x Mn1–x S

Variable-valence Tm _x Mn_1–x S (0 ? x ? 0.15) compounds have been synthesized and their structural, electrical, and thermoelectrical properties have been studied in the temperature range of 80–1100 K. The regions of existence of solid solutions of sulfides Tm _x Mn_1–x S with the NaCl-type fcc lattice have been determined. It has been found that, as thulium ions are substituted for manganese cations, the electrical resistivity increases, and the lattice parameter increases more sharply than that corresponding to the Vegard’s law. The study of the temperature dependences of the thermopower coefficient has revealed that the current carrier sign is retained to 500 K for all the substitution concentrations, and the charge carrier type changes from the hole type to the electron type with variations in the temperature. The experimental data have been explained in terms of the exciton model.     

Magnetization of cobalt nanodiscs

We have studied the magnetization and AC susceptibility of a dilute colloidal dispersion of ε$\epsilon$-Co nanodiscs. The temperature dependence of the magnetization implies that the nanodisc moments become largely static below a blocking temperature which exceeds room temperature. The related anisotropy energy is significantly larger than the magnetocrystalline anisotropy for bulk ε$\epsilon$-Co, which may result either from surface modifications to the crystalline anisotropy, or perhaps from shape anisotropy. The saturation moment is found to be temperature independent, over a wide range of temperatures and also close to the blocking temperature, although the field at which saturation occurs varies considerably with temperature. A coercive field of 1700 Oe is found at low temperature, indicating that magnetization reversal involves the coordinated rotation of the moments in individual nanodiscs. Aging effects are observed in the AC susceptibility, implying that the spatial coherence of the room temperature dispersions increases over time, leading to static short-range order.     

The influence of annealing temperature on the slip plane activity and optical properties of nanostructured ZnO films

The zinc oxide films were prepared by the sol-gel method on the ordinary glass substrates. The activity of slip systems were evaluated by X-ray diffraction line broadening analysis using convolution multiple whole profile (CMWP) fitting procedures. It was found that in all temperatures the 〈a〉 type dislocations is dominating and its fraction increases with the rise of annealing temperature in the range of 350-600 °C. The investigation on the optical properties of films showed that the optical band gap energy increases linearly with the annealing temperature and crystallite size but decreases with the lattice strain.     

On the behaviour of the new Kondo lattice CeCuAl3

The temperature dependence of the electrical resistivity, thermopower, specific heat, susceptibility and magnetization of CeCuAl₃ are presented. CeCuAl₃ behaves as a Kondo lattice system with antiferromagnetic ground state properties (T _N 2.8 K). The valency of Ce in this tetragonal compound is close to 3 and the overall crystal field splitting found from our results is about 150 K. The Kondo temperatureT _K in the crystal field ground state, estimated from the magnetic susceptibility and the specific heat, is of the order of 8 K.     

Theoretical diagnostic and prediction of physical properties of quaternary InGaAsP compound using artificial neural networks optimized by the Levenberg Maquardt algorithm

The quaternaries \(In_{1 - x} Ga_{x} As_{y} P_{1 - y}\) are the main promising elements for the fabrication of optoelectronic devices. The adjustment of their physical parameters is assumed by the change of the molar fraction \(x\) and \(y\). These parameters can be affected by the variation of temperature and pressure. To make the theoretical diagnosis of these materials, it is fundamental to know the energy gap ‘\(\varvec{E}_{\varvec{g}}\)’ and the lattice parameter ‘\(a\)’, over a wide range of chemical compositions \(0 \le x \le 0.47\) and \(0 \le y \le 1\), at different temperatures and pressures. We show that by using the Artificial Neural Network method optimized by the Levenberg Maquardt algorithm ANN-LM, it is possible to obtain results very close to the experiment. The scatter plot and error calculation show that the ANN-LM model provides more accurate values of the lattice parameter than those calculated by Vegard’s law. On the other hand, the energy gap values \(Eg (x, y, T)\) estimated, using the ANN-LM model, proved to be close to the experimental values that those calculated by the empirical equations. In addition, the ANN-LM method allowed us to estimate with great accuracy the values of the energy gap at different temperatures and pressures \(Eg (P, T)\). Our work provides crucial information on the physical properties of the quaternary without the use of approximations, and without taking into account the hypothesis of a perfect agreement between \(InGaAsP\) and \(InP\) substrate.