Short-range Order Characteristic Research Articles

Effect of silicon carbide incorporation and heat treatment on tribological properties of electroless Ni–B–W alloy coating

Concentration of reinforcing agents govern various characteristics of electroless composite coatings. In this study, silicon carbide (SiC) particles are incorporated into matrices of electroless ternary Ni–B–W alloy coatings. Addition of variable amounts of SiC into electroless bath solution used for the deposition helped in the fabrication of Ni–B–W-based electroless composite coatings of different compositions. Higher hardness value is noticed in composite coatings fabricated with the 1 g/l concentration of SiC particles in the plating bath. The resultant Ni–B–W–SiC composite coating is then subjected to heat treatment at three different temperatures (250 °C, 450 °C, and 650 °C) to study its effects on microstructural, mechanical and tribological characteristics. Alongside the composite coating, characteristics of Ni–B–W alloy coating are also observed in the same route for comparison. Matrices of as-deposited coatings show short-range order characteristics. Heat treatment at different temperatures affects magnitude of crystallization, degree of dispersion, sizes of precipitates within coatings' matrices differently. Development of various borides, such as Ni3B and Ni2B and several silicides, namely Ni3Si, Ni5Si2, and NiSi within coatings' matrices as a function of heating temperature governs the mechanical and tribological properties of coatings. Cumulative effects of composite coatings fabricated with the 1 g/l concentration of SiC particles in plating solution and heat treated at 450 °C evolved superiority in terms of mechanical and tribological properties as compared to other coated specimens in this study. Heat treatment at a higher temperature (650 °C) causes degradation in coatings’ properties due to the coarsening and coalescence of nickel boride precipitates and grain growth. Moreover, at this temperature, nickel reacts with the SiC particles and produces several nickel silicide phases, namely Ni3Si, Ni5Si2, and NiSi, negatively affecting properties of the composite coatings.

PREDICTION OF THE THERMOPHYSICAL BEHAVIOR OF AMORPHOUS OF ALLOYS Ni<sub>0.333</sub>Zr<sub>0.667</sub> AND La<sub>80</sub>Al<sub>20</sub> ACCORDING TO PROPERTIES OF METALS

Amorphous alloys (metallic glasses) are obtained by ultrafast cooling of melts. As a result of this process, the chaotic distribution of atoms in space is “frozen” while maintaining the short-range order characteristic of liquids. The high homogeneity of the solid state leads to unique mechanical, magnetic and other physical properties. This has led to the fact that cheaper metallic glasses have replaced a number of traditional materials in some industries. On the other hand, significant disadvantages of disordered media are temperature and time instabilities. The solution to this problem is possible with knowledge of the thermophysical properties of amorphous alloys: heat capacity, thermal expansion coefficient, thermal conductivity and thermal diffusivity. Even estimating, for example, their temperature dependences is a complex and urgent task. Therefore, in this paper, it is proposed to use the mixing rule for components whose thermophysical properties are known. This makes it possible to predict the temperature dependences of heat capacity, thermal expansion coefficient and thermal diffusivity at known values of thermal conductivity for different temperatures. Note that the obtained curves for the Ni0.333Zr0.667 and La80Al20 amorphous alloys are in good agreement with the experimental data.

FORMATION OF COMPOSITE COATINGS WITH MAGNETIC PROPERTIES IN AN ELECTROMAGNETIC FIELD

The article presents and analyzes the results of the study of the influence of a constant magnetic field on the electrodeposition processes of the ferrum family metals and their alloys. The studies performed indicate the effect of accelerating deposition, improving the surface morphology and hardness of the obtained coatings. Composite coatings applied under the influence of a constant magnetic field can have higher corrosion resistance, improved photocatalytic and magnetic properties than those obtained without it. It has been established that the presence of a constant magnetic field changes the chemical composition of the studied material. There is an increase in the ferromagnetic and a decrease in the diamagnetic component. To assess the magnetic properties, samples of Fe-Co-W and Fe-Co-Mo coatings deposited by unipolar pulsed current at different electrolysis durations were selected, which is due to the difference in the thickness and distribution of the components. The results obtained suggest that in the nonequilibrium process of electrodeposition in three-component alloys, clusters with a short-range order characteristic of a number of intermetallic nonmagnetic compounds are formed, which leads to a decrease in the saturation magnetization of the alloy. The Fe-Co-W coatings are characterized by higher values of the coercive force compared to amorphous alloys, in which the elements P, B, Si act as a non-magnetic component. Probably, for the Fe-Co-W alloy, large clusters with a compositional order similar to the arrangement of atoms in the paramagnetic intermetallic phase with tungsten, along with surface roughness and free volume, play a significant role in magnetization reversal. The results obtained make it possible to classify the obtained Fe-Co-W galvanic alloys as magnetically hard, and Fe-Co-Mo as magnetically soft materials, which, in combination with high microhardness, opens up prospects for the use of such systems in the production of magnetic elements for recording and reproducing information and microelectromechanical systems respectively. With properly designed field structures, new magnetic nanostructures and a new level of control over industrial catalytic and electroplating processes can be created.

Implementation Variants of the Lineal Path Function Applied to Hardened Cement Paste Microstructure

The microstructure of hardened cement pastes comprises of a heterogeneous agglomeration of distinct quasihomogeneous domains with variable physical, chemical, and morphological features, denoted here as material phases. Accurate material characterization rests on a precise description and quantification of underlying principal phases, focusing, in particular, on their volumetric proportions and spatial configuration within the microstructure, as these affect, to a large extent, the macroscopic properties of the composite material. A realistic cement paste microstructure used in this study was obtained from micro-computed X-ray tomography, following the application of suitable segmentation filters, highlighting and isolating the sought phase – anhydrous cement grains – for statistical analysis. The present paper then compares and assesses several implementations of a lineal path function, all applied to quantifying the phase connectedness and short-range order characteristics of the grains. The main emphasis rests on assessing the accuracy and evaluation speed of the implemented algorithms.

Х-ray Study and Computer Simulation of the Structure of Amorphous-Crystalline Titanite

The structure of amorphous-crystalline titanite obtained by mechanical activation was studied by X-ray diffraction and simulation methods. The short-range order characteristics were calculated using Finbak-Warren's method. It was found that the coordination numbers of metal atoms decreased as the result of titanite grinding. The atomic configurations of short-range order of ground titanite were constructed by translation of titanite unit cell. The theoretical X-ray patterns were calculated using Debye's method and were compared with the experimental curves. The structure of ground titanite in the mill with centrifugal factor 40 g was described satisfactorily by the model of mechanical mixture of clusters containing 2016 atoms, disordered during the molecular dynamics with clusters containing 12096 atoms. The increase of grinding intensity led to the sharp decrease of sizes of small cluster.

X-ray Study and Computer Simulation of the Structure of Amorphous-Crystalline Titanite

The structure of amorphous-crystalline titanite obtained by mechanical activation was studied by X-ray diffraction and simulation methods. The short-range order characteristics were calculated using Finbak-Warren's method. It was found that the coordination numbers of metal atoms decreased as the result of titanite grinding. The atomic configurations of short-range order of ground titanite were constructed by translation of titanite unit cell. The theoretical X-ray patterns were calculated using Debye's method and were compared with the experimental curves. The structure of ground titanite in the mill with centrifugal factor 40 g was described satisfactorily by the model of mechanical mixture of clusters containing 2016 atoms, disordered during the molecular dynamics with clusters containing 12096 atoms. The increase of grinding intensity led to the sharp decrease of sizes of small cluster. Keywords: amorphous-crystalline titanite, X-ray diffraction, computer simulation, mechanical activation, Debye's method.

Simple quasicrystal model of liquid and its applications: calculations of enthalpy of vaporisation and adsorption energy in slit nanopore

ABSTRACTPhysical properties of liquids are important to both theoretical study and engineering designs. These properties are determined by the spatial configuration of liquids. Based on the main characteristics of short-range order of liquid configuration, a simple quasicrystal model of liquid with close-packed body-centred cubic structure is proposed to simulate the spatial structure of liquids. Then the bonding potential energy of liquid molecule is analytically expressed with Lennard-Jones intermolecular potential. Enthalpy of evaporation can be calculated with this model, which has obvious physical meaning and the calculated values agree with experimental data. Based on the fact that liquid in a confined nano-space are distributed in layers, the characteristics of liquids in slit nanopores can also be described with this simple model. The potential profiles of liquid molecules in slit nanopores and the heat of adsorption can be well reproduced with this quasicrystal model of liquid. This model can capture the main characteristics of liquid. It provides a simple method to describe the physical mechanism and directly estimate the characteristic parameters of simple liquid systems.

Pulsed laser micromachining of Mg–Cu–Gd bulk metallic glass

Micromachining of Mg-based bulk metallic glasses (BMGs) is performed using two kinds of pulsed nanosecond lasers: a 355nm ultraviolet (UV) laser and a 1064nm infrared (IR) laser. Precision machining on the micrometer scale and the preservation of amorphous or short-range order characteristics are important for the application of BMGs in micro-electro-mechanical systems. A higher micromachining rate is achieved using the UV laser than using the IR laser due to a better absorption rate of the former by Mg-based BMGs and a higher photon energy. The cutting depth of Mg-based BMGs ranges from 1 to 80μm depending on the laser parameters. By appropriate adjustment of the laser power and scan speed, successful machining of the Mg-based BMG with preservation of the amorphous phase is achieved after the laser irradiation process. Short-pulse laser cutting represents a suitable alternative for machining of micro components.

Modified Eyring viscosity equation and calculation of activation energy based on the liquid quasi-lattice model

Viscosity is an important physical parameter of fluid, and the Eyring viscosity equation is a popular viscosity theory. Based on the Eyring reaction rate equation and Boltzmann statistical theory, and including the probabilities of creating a hole in liquid and the transition to the neighboring hole, a modified Eyring viscosity equation was proposed. According to the structural characteristics of short-range order, liquid is treated as a quasi-lattice structure in a small region. The activation energy, which is the minimum energy needed for the molecule to jump to its neighboring hole because of the restriction of other molecules around it, was analytically calculated from an intermolecular Lennard-Jones potential function and a Stockmayer potential function. The viscosity values of 37 kinds of typical liquids at 25°C and the dependence of viscosity of three kinds of liquids on temperatures were calculated with this modified viscosity equation, and the calculated results agree with the experimental values to some extent. This work not only enriches the understanding of the mechanism of liquid viscosity, but also could provide some theoretical guides for the relevant studies and applications.

Modification of atomic structure of thin amorphous V2O5 films under UV laser irradiation

Influence of ultra-violet radiation of the KrF laser (wave length 248 nm, pulse duration 20 ns) on atomic structure of amorphous vanadium pentoxide thin films, prepared by the pulsed laser deposition method, is studied. Calculations of the short-range order characteristics (radii and diffusiveness of coordination spheres, coordination numbers) were performed by the Finbak -Warren method. It is established that minimal structure unit of amorphous V2O5 film before and after irradiation is a strongly deformed oxygen octahedron. Distortions of tetragonal pyramids in the initial and modified film are different. Also, oxygen deficiency in a tetragonal pyramid is observed.

ModelingIn−Seamorphous alloys

The structure of amorphous ${\mathrm{In}}_{x}{\mathrm{Se}}_{y}$ $(a\text{\ensuremath{-}}{\mathrm{In}}_{x}{\mathrm{Se}}_{y})$ alloys has been studied by a first principles tight-binding molecular dynamics technique. The three-dimensional amorphous structures with different densities at different compositions were prepared by quick quenching from the liquid phase. The characteristics of short-range order, namely radial distribution functions, coordination numbers, bond angle distribution functions, and the electronic structure have been analyzed. The local bonding environments of different ${\mathrm{In}}_{x}{\mathrm{Se}}_{y}$ crystals (in particular, ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$, InSe, and ${\mathrm{In}}_{4}{\mathrm{Se}}_{3}$) were found to be present in the amorphous phase. The average coordination number of indium is mainly four, whereas selenium is mostly two- or threefold coordinated. The majority of the bonds are heteropolar, but homopolar bonds are also present in $a\text{\ensuremath{-}}{\mathrm{In}}_{x}{\mathrm{Se}}_{y}$ so that they cannot be excluded from a realistic description of the amorphous structure. Larger content of indium in $a\text{\ensuremath{-}}{\mathrm{In}}_{x}{\mathrm{Se}}_{y}$ leads to an increased number of $\mathrm{In}\ensuremath{-}\mathrm{In}$ bonds, as expected.

Calorimetric and Magnetic Studies on a Ferromagnetic Phase Transition of the Metal-Assembled Complex MnCu(obbz)·H2O

Abstract Heat capacities of the metal-assembled ferromagnet MnCu(obbz)·H2O (obbz = oxamidobis(benzoato)) have been measured in the temperature region between 0.1 and 301 K by adiabatic calorimetry. A broad phase transition due to the three-dimensional ferromagnetic ordering was observed around 17 K. Above the transition temperature, a heat capacity tail arising from the short-range order characteristic of a low-dimensional magnet was found. The enthalpy and entropy gained by the phase transition are 227 J mol-1 and 12.7 J K-1 mol-1, respectively. This entropy acquisition is very close to the value Rln 5 ( = 13.4 J K-1 mol-1) expected for the spin manifold of the spin quantum number S = 2 ground state, where R stands for the gas constant. The thermal anomaly above the transition temperature can be accounted for in terms of the S = 2 two-dimensional ferromagnetic Heisenberg model rather than the one- or three-dimensional models. This suggests that this complex can be regarded as a two-dimensional ferrimagnet, in which strong antiferromagnetic interaction exists between a pair of Mn(II) (S = 5/2) and Cu(II) (S = 1/2) but the magnetic coupling between the pairs is very weak. Magnetic measurements for the complex revealed a bend of the hysteresis loop above the transition temperature and a magnetic relaxation below the transition temperature. These results lead to the conclusion that the broadness of the phase transition may be attributed to superparamagnetism caused by the fact that the present complex is prepared only in a fine powder.

Low-Temperature Heat Capacities and Ferromagnetic Phase Transition of the Organic Free Radical Ferromagnet, 4-(4-Chlorobenzylideneamino)-2,2,6,6-tetramethylpiperidin-1-oxyl (CATMP)

Abstract Heat capacities of the organic free radical ferromagnet, 4-(4-chlorobenzylideneamino)-2,2,6,6-tetramethylpiperidin-1-oxyl (CATMP) have been measured in the temperature range between 0.1 and 311 K. A ferromagnetic phase transition was observed at 0.28 K and a heat capacity hump was found above the transition temperature. This hump is due to the short-range order characteristic of low-dimensional magnetic spin systems. The enthalpy and entropy gains due to the ferromagnetic phase transition were estimated to be 4.06 J mol-1 and 5.71 J K-1 mol-1, respectively. This entropy agrees well with the theoretical value Rln 2 ( = 5.76 J K-1 mol-1) expected for the magnetic entropy of a spin system with the spin quantum number S = 1/2, where R is the gas constant. The heat capacity hump above the transition temperature is well accounted for in terms of the S = 1/2 two-dimensional ferromagnetic Heisenberg model of square lattice with the intralayer exchange interaction J/kB = 0.42 K, where the spin Hamiltonian H = -2JSi·Sj is adopted and kB stands for the Boltzmann constant. The analysis of the magnetic heat capacities below the transition temperature on the basis of the spin wave theory clarified that the magnetic system settles down in three-dimensional ferromagnetic order below the transition temperature with the interlayer exchange interaction J′/kB = 0.024 K.

Short-range order in Fe-Rh solid solutions

We have used diffuse scattering of x-rays (DSX) and a model potential to show short-range order exists in iron-rhodium alloys. After the alloy Fe-25 at. % Rh was quenched following annealing at 900°C for 1 h, a clearly visible diffuse maximum (characteristic of short-range order) appeared in the diffraction pattern, in addition to reflections from the α- and γ-phases. For the single-phase, dilute solid solution Fe-2.5 at. % Rh, starting with the effects of static displacements, we have calculated the short-range order parameters (α1 = −0.02, α2 = −0.09, α3 = 0.02, α4 = 0.05). The elastic constants of Fe-2.5 at. % Rh needed to compute the short-range order parameters from the DSX data were calculated using a model potential. Theoretical calculations of the order energy and the short-range order parameters, performed using a model potential for these alloys, agree with the experimental data.

Intermediate phase between deuteron-glass and antiferroelectric order in the D-RADA system

This paper reports the results of a dielectric investigation of deuterated glass D-RADA, x = 0.39. The shape of the temperature dependence of the imaginary part of the permitivity ε″ was analyzed. Analysis of the shape of the temperature dependence of ε″( T, v) proved the existence of two phases of different mechanisms of relaxation, forming among the antiferroelectric domains. One of these mechanisms can be described by the Vogel-Fulcher equation with the parameters T 0 = 25.7 K, E c = 105.6 K (9.1 meV) and v 0 = 1.16 × 10 8 Hz, and is typical for clusters with short-range order characteristic of proton (deuteron) glass. The other relaxational mechanism is the thermally activated Arrhenius dipolar reorientation with activation energy E c = 1105 K (95.2 meV) and frequency v o = 1.44 × 10 12 Hz. It is related to free dipoles which have been released from the melting long-range ordering but have not managed to form a cluster yet.

The Effect of Synthesis Conditions on the Short-Range Order Characteristics in Silicon Nitride Layers

By means of X-ray investigations structural differences in silicon nitride amorphous films are studied in accordance with the synthesis conditions. It is shown that plasmochemical silicon nitride layers are either alloyed excess atoms of hydrogeneous Si (with high ammonia contents), or it is a heterogeneous system consisting of amorphous silicon nitride clusters, ordered according to phase type α-Si3N4, and amorphous heterogeneous silicon clusters. [Russian Text Ignored].

X-ray and neutron diffraction studies of liquid TlSe system

The composition dependence of the structure for liquid Tl xSe 1−x (x=0.4, 0.5 and 0.667) was investigated by means of X-ray and TOF neutron diffractions. The difference in experimental total structure factors from both radiations was used to infer the feature of partial structural functions for each composition. A significant structural change was found between these compositions. The nearest neighbour peak at r=2.4 A ̊ in the SeSe distribution function observed at x=0.4 and 0.5 , agrees well with that in pure liquid Se, indicating the existence of SeSe chains at these compositions. On the other hand, this SeSe peak disappears at x=0.667. The liquid structure at this composition may be considered to have a short-range order characteristic of the crystalline Tl 2Se.

Short-range local order of the Co/Si(111) interface studied by the extended Auger fine-structure technique.

Extended Auger fine-structure (EXFAS) spectra have been successfully recorded on ultrathin films of cobalt deposited on silicon (111) surface. Thanks to a detailed comparison of the static disorder of the interface, the short-range order characteristics of EXFAS and its surface-structural sensitivity have been clearly demonstrated.

Spin correlation studies of FeF2 and Fe0.6Zn0.4F2 by optical exciton-magnon transitions

The fine structure and the temperature dependence of the moments M0, M1 and M2 of a sharp and isolated exciton-magnon absorption line near lambda =465 nm are studied on FeF2 and Fe0.6Zn0.4F2. The results compare well with theoretical predictions of Tanabe and co-workers, since overlap with other magnon or phonon sidebands is nearly negligible. All moments show cusp-like behaviour at TN owing to singularities of the spin correlation functions involved. In Fe0.6Zn0.4F2 additional precursor peaks arise at T approximately=1.2 TN. They are attributed to enhanced short-range order characteristic of reduced dimensionality.

Anderson Localization in Ferromagnetic Semiconductors Due to Spin Disorder I. Narrow Conduction Band

AbstractTheoretical investigation is made of the temperature induced metal‐insulator transition in doped ferromagnetic semiconductors, described by s‐d exchange model. The transition is a result of the mobility edge movement, the disorder being due to magnetic ions spin density fluctuations. The electrons are described in tight binding approximation. Using ideas and methods of Anderson localization theory simple formulas are obtained, which connect the mobility edge with shortrange order characteristics of the magnetic subsystem‐static spin correlators. The theory developed is applied to the description of the resistivity of CdCr2Se4. The theoretical results are in agreement with experiment in a wide temperature range.