Coherent-scattering Regions Research Articles

Structure and Thermoelectric Properties of Nanostructured (Bi, Sb)<sub>2</sub>Te<sub>3</sub> (Review)

The investigation of the structure and thermoelectric properties of nanostructured solid solutions (Bi, Sb)2Te3 p-type has been carried out. The samples were obtained by grinding of original compositions in a planetary ball mill and by spark plasma sintering (SPS). Initial powder has an average particle size of 10 - 12 nm according to transmission electron microscopy, and the size of the coherent scattering region (CSR) obtained by X-ray line broadening. During sintering at Ts = 250°C - 400°C, the grain size and CSR increased, which was associated with the processes of recrystallization. The maximum of size distribution of CSR shifts to larger sizes when Ts increases so that no broadening of X-ray lines at Ts = 400°C can take place. At higher Ts, the emergence of new nanograins is observed. The formation of nanograins is conditioned by reducing of quantity of the intrinsic point defects produced in the grinding of the source materials. The study of the electrical conductivity and the Hall effect in a single crystal allows to estimate the mean free path of the holes-L in the single crystal Bi0.5Sb1.5Te3 which at room temperature is 2 - 5 nm (it is much smaller than the dimensions of CSR in the samples). The method for evaluation of L in polycrystalline samples is proposed. At room temperature, L is close to the mean free path in single crystals. Scattering parameter holes in SPS samples obtained from the temperature dependence of the Seebeck coefficient are within the measurement error equal to the parameter of the scattering of holes in a single crystal. The figure of merit ZT of SPS samples as a function of composition and sintering temperature has been investigated. Maximum ZT, equal to 1.05 at room temperature, is obtained for the composition Bi0.4Sb1.6Te3 at Ts = 500°C and a pressure of 50 MPa. The causes of an apparent increase in thermoelectric efficiency are discussed.

Structural features of “Carbonado” diamond polycrystalline composite materials

A thermodynamic analysis of the graphite → diamond transformation during the synthesis of diamond polycrystals is performed. It is established that the critical nucleus size decreases as the pressure increases, being 2.3 nm for p = 12 GPa. It is established when studying the crystal structure of diamond polycrystals that their block size (coherent scattering region (CSR)) is independent of the synthesis pressure and exceeds the block size of the starting graphite. It is shown that the features for martensite transformation in metals and alloys are unobservable during the synthesis of diamond polycrystals.

The Peculiarities of Structure and Diffraction by Misfit Mixed-Layer Nanotubes

The analysis of diffraction by separate mixed-layer goffered nanotube’s lattice is offered. Two extreme cases of the large and small size of coherent scattering regions (CSR) in a radial direction are considered. The qualitative explanation of observed diffraction effects is given.

Production and studies of properties of nanopowders on the basis of CeO2

Nanopowders (NPds) of pure CeO2 of a specific surface area up to 210 m2/g and those doped with copper, carbon, and iron (≤1 wt % of dopant) of a specific surface area in the range 130–160 m2/g have been produced by evaporation with a pulsed electron beam. According to the X-ray phase analysis data, no secondary phases (aside from the cubic CeO2 phase) were found in the produced NPds. All the powders contained fine- and coarse-crystalline fractions differing in the size of their coherent scattering region (CSR) and in their amorphous component. The degree of crystallinity of the powders was not above 22%. The powders have a fractal structure and consist of agglomerates of sizes from dozens to hundreds of nanometers formed by crystalline nanoparticles (NPts) 3–5 nm in size with a very narrow particle size distribution. NPds have a high structural defectiveness degree, which was reflected in their magnetic properties. The room ferromagnetism was established in NPds of pure CeO2 − x and those doped with nonmagnetic elements (carbon and copper): here the ferromagnetic state in the CeO2-C system was established for the first time, whereas the magnetic moment on the carbon atom was 35-fold lower than the theoretical estimation. The ferromagnetic contribution to CeO2 NPd increases with a decrease in the NPt size and reaches 0.1 emu/g in CeO2-Fe NPd (x Fe = 0.54 wt %). It has been established that there is no direct dependence between magnetization and the content of iron ions in the CeO2-Fe-based NPds.

Low-temperature mechanochemical synthesis of nanosized silicon carbide

The methods of X-ray diffraction analysis, scanning electron microscopy, synchronous thermal analysis, and adsorption are used to study the mechanochemical synthesis of silicon carbide through the reaction Si + C → β-SiC. The reaction is found to take place in several stages. At the first stage, i.e., at activation doses below approximately 5 kJ/g, the powders of the components are independently ground to increase the specific surface area of the mixture to 145 m2/g, graphite is amorphized, and the sizes of the coherent-scattering regions of silicon drastically diminish. At the second stage (doses of 5–15 kJ/g), dense Si/C aggregates are formed and two fractions (coarse and fine) with different particle sizes arise in silicon crystallites. As the activation dose is enhanced, the amount of the fine fraction rises, while the sizes of coherent-scattering regions decrease to 2–3 nm. When samples are heated at 800°C, the fine fraction of silicon interacts with carbon to yield silicon carbide with crystallite sizes of 3–4 nm, whereas the coarse fraction of silicon recrystallizes. At the third stage, i.e., at doses of higher than 15 kJ/g, the mechanochemical synthesis of SiC occurs through the following scheme: fine fraction Si + C → amorphous SiC → crystallization of SiC.

Alteration in amorphicity of quartz structures during grinding of ferrous quartzites

The author discusses evaluation of coherent-scattering regions in quartz in the structure of ferrous quartzites during commercial and laboratory grinding of wet magnetic separation refuses and the correlation equation that confirms the activity versus dispersion behavior of the analyzed refuses.

Temperature dependences of YBa2Cu3Ox and La2 − xSrxCuO4 resistivity interms of the negative-U centers model

The temperature dependence of the normal-phase resistivityρ(T) ofYBa2Cu3Ox (YBCO)and La2 − xSrxCuO4 (LSCO) is analyzed by using the Boltzmann equation for holes in theτ-approximation, in which the dependence of the relaxation time on energy and temperatureτ(ε, T) is due to acoustic phonons. The temperature dependence of the chemical potentialμ(T) and effectivecarrier mass m* ∼ 10–100 free electron masses, obtained by fitting to the temperaturedependence of the Hall coefficient in terms of the negative-U centers model, areused to calculate ρ(T). The results obtained are compared with experimental data. It isshown that they are consistent over a wide range of doping levelsx and coincide inabsolute magnitude, ρ ∼ 10 − 4–10 − 3 Ω cm. In addition, the slope ratio of the dependence of the normal-phaseρ(T) on thesize of the x-ray coherent-scattering region and on the oxygen content is calculated for a set of epitaxialYBa2Cu3Ox films.

Особенности фазового превращения материала полиакрилонитрильной нити в структуру термостабилизированного волокна

The influence of stretching loading to phase composition and average size of coherent-scattering regions of polyacrylonitrile (PAN) fibers during isothermal treatment have been considered. Increase of stretching loading leads to acceleration of PAN crystal phase dispersion and formation on the new highly dispersed phase.

Changes of polyacrylonitrile fiber fine structure during thermal stabilization

In this article regularities of length changes of polyacrylonitrile (PAN) fiber, as well as its structure transformation into the structure of thermally stabilized fiber, have been studied in detail. The fiber has been manufactured with the use of dimethyl sulfoxide (PAN1) and dimethyl formamide (PAN2), with methylacrilate content of 5% (mass). Thermal stabilization has been carried out under isothermal conditions at 245, 255 and 265, 275, 285 and 290 °C in atmospheric air at constant load of 0.6 g/tex. By X-ray structural analysis it has been shown that the structure of PAN fiber crystal component is depicted by two kinds of coherent-scattering regions (CSR), differing in the average size by an order of magnitude. Phase transition of PAN into the new highly dispersed phase is accompanied by dispersion of PAN larger CSR. Qualitative and quantitative regularities of fine structure evolution have been determined at thermal treatment in the indicated temperature range. It has been shown that the transition of PAN1 fiber into the new two-phase state during thermal stabilization progressed more actively than the same process in PAN2, by ~25%. Increase of thermal treatment temperature leads to significant acceleration of PAN crystal phase dispersion and formation on the new highly dispersed phase. At that instance, average sizes of CSR of highly dispersed components of thermally stabilized fiber equal ~1 nm at the end of the process, when aromatization index of the material amounts to 51–58%.

Grain-boundary heat conductance in nanodiamond composites

Sintering of detonation nanodiamonds (DND) has been studied at a pressure of ∼ 7 GPa in the 700–2000 °C temperature range. The X-ray coherent-scattering regions in DND have been found to grow with increasing sintering temperature. It is shown that diamond crystallites grow by the oriented attachment mechanism. It is demonstrated that the increase of thermal conductivity of the composites thus obtained is initiated by variation of the heat conductance of the boundaries separating DND crystallites.

Distribution of copper- and nickel-containing modifier components in the pore space of HZSM-5 zeolite

The distribution of copper- and nickel-containing components in the pore space of HZSM-5 zeolite was quantitatively studied. It was found that the detailed distribution of a modifier in the micropore and mesopore volumes of the zeolite depends on both the chemical nature of the modifier and the conditions of supporting and the regime of M2+ polycondensation in the pore space of the zeolite. The experimental data on the low-temperature adsorption of nitrogen on Cu(n)ZSM-5 catalysts can be interpreted as the result of the partial filling of the zeolite micropore space (10 vol %) and the finest mesopores with D < 3 nm with the modifier. In the case of Ni(n)ZSM-5 catalysts, the penetration of the modifier into zeolite channels (micropores) in detectable amounts was not found, and it was arranged in mesopores on the surface of zeolite crystallites. The reason for differences between modifier distributions in the pore structure of the zeolite was explained from the standpoint of different structures of copper and nickel polyhydroxo complexes in impregnating solutions after polycondensation. It was found that, in the Cu(n)ZSM-5 and Ni(n)ZSM-5 catalysts, the modifier component contained copper and nickel only in a doubly charged state and mainly octahedral oxygen environments. In this case, three-dimensional nanoparticles or coarsely dispersed particles of CuO were not detected in the pore space of the support, whereas the presence of a small amount of sufficiently large NiO crystals with a coherent-scattering region of 80–100 nm was detected in Ni(n)ZSM-5, and these crystals occurred on the surface of zeolite crystals. It was found that the apparent density of a copper-or nickel-containing component arranged in the pore space of the zeolite was lower than the density of the bulk CuO and NiO phases by a factor of ∼3 and 4, respectively, because of the size effect.

Fabrication and characterization of nanostructured Ti6Al4V powder from machining scraps

In recent years researches on properties of nanocrystalline materials in comparison with coarse-grained materials has attracted a great deal of attention. The present investigation has been based on production of nanocrystalline Ti6Al4V powder by means of high energy mechanical milling. In this regard, Ti6Al4V powder was produced by ball milling of machining scraps of Ti6Al4V. The structural and morphological changes of powders were investigated by X-ray diffraction, scanning electron microscopy, and microhardness measurements. The results revealed that ball milling process reduced the size of the coherent-scattering region of Ti6Al4V to approximately 20 nm. Also a remarkable change in morphology and particle size was occurred during ball milling. Moreover, phase evolution during milling was taken into consideration. The as-milled Ti6Al4V powder exhibited higher microhardness comparing to the original samples.

Study of the Re0.50Rh0.50 products of thermobaric treatment

The products of thermobaric treatment of the metastable Re0.50Rh0.50 nanocrystalline phase (a= 2.733(2) A, c = 4.364(4) A, space group P63/mmc, CSR ∼5 nm) in a high-pressure chamber were studied. Storage of the phase at 2000°C and 1.5 GPa for 3 min led to the formation of a fused particle (O∼1 mm). The initial composition did not change, but the coherent scattering region (CSR) increased to 52 nm. An increase to 4 GPa in pressure also did not lead to a decomposition of the metastable Re0.50Rh0.50 phase, the unit cell parameters remained the same, and the size of the CSR increased by a factor of four.

Structural characteristics of the boron nitride sphalerite modification synthesized in a shock wave

The real structure of ultradispersed powders of sphalerite boron nitride synthesized under shock compression has been studied by X-ray diffraction. The lattice microstrain (〈e2〉1/2) and average sizes of coherent-scattering regions were determined from the shape of one diffraction line by the method of restricted moments. It is established that the investigated powders are characterized by the values 〈e2〉1/2 in the range 0.8–1.5% and sizes of coherent-scattering regions are from 3.6 to 7.2 nm; these data are consistent with the average sizes of crystalline grains found by electron microscopy.

Effect of the size of x-ray coherent-scattering regions on the electrical parameters of semiconducting SmS

The relation of the electrical properties of semiconducting samarium sulfide (SmS) to the size of coherent scattering regions for x-rays is considered. The dependence of the carrier concentration on the size of coherent scattering regions in single-crystal and polycrystalline SmS samples, as well as in thin samarium sulfide films, is established experimentally. A satisfactory agreement of the calculated curves with the experiment suggests that the size of coherent scattering regions has a dominant effect on the concentrations of charge carriers and defect samarium ions in SmS. The effect of thermal shocks on the size of coherent scattering regions in SmS single crystals and polycrystals, as well as in polycrystalline Sm0.4Eu0.6S samples, is investigated.

Effect of nucleation centers on the formation of nanostructure during decomposition of metal-organic compounds

The effect of nucleation on thermal decomposition of metal-organic compounds (MOC) is considered. It is found that introduction of nucleation centers into the starting MOC affects the structure and composition of the forming product. The structure of nucleation centers determines the structure of forming phases, and the amount of introduced nuclei determines the phase composition of the final product. It is shown that it is possible to use the decomposition products of the same MOC as the centers of nucleation. The effect of decreasing the sizes of the coherent scattering regions (CSR) of crystalline phases, which are formed on the nucleation centers with the larger CSR of the same phases, is revealed. This effect manifests itself with equality of the temperature of seed obtainment and decomposition of metal organics in its presence.

Synthesis, structure, and properties of the thermolysis products of [Os(NH3)5Cl][ReCl6

The crystal structure of [Os(NH3)5Cl][ReCl6] has been refined by X-ray powder analysis: a = 11.645(3) A, b = 8.3788(2) A, c = 15.277(4) A, β = 91.029(6)°, V = 1490(1) A3, d x = 3.163 g/cm3, space group P21/m, Z = 4. The thermolysis product of the salt in a hydrogen atmosphere is a solid substitution solution Os0.5Re0.5: a = 2.753(2) A, c = 4.366(3) A, space group P63/mmc; coherent scattering region (CSR) is ∼230 A.

Investigation of the structure of nanocrystalline refractory oxides by X-ray diffraction, electron microscopy, and atomic force microscopy

The structures of nanocrystalline fibrous powders of refractory oxides have been investigated by different methods: determination of coherent-scattering regions, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic-force microscopy (AFM). The sizes of nanograins of different crystalline phases of refractory metal oxides have been determined during the formation of these nanograins and the dynamics of their growth during heat treatment in the temperature range 600–1600°C has been studied. The data on the structure of nanocrystalline refractory oxide powders, obtained by different methods, are in good agreement. According to the data on coherent-scattering regions, the sizes of the ZrO2 (Y2O3) and Al2O3 grains formed are in the range 4–6 nm, and the particle sizes determined according to the TEM and AFM data are in the ranges 5–7 and 2–10 nm, respectively. SEM analysis made it possible to investigate the dynamics of nanoparticle growth at temperatures above 1000°C and establish the limiting temperatures of their consolidation in fibers.

Structural-phase condition, unelastic and plastic behavior and nanohardness of the TiNi surface layers modified by an ion- and electron irradiation

Structural-phase conditions, nano- and microhardness of the Ti 49.5Ni 50.5 alloy surface layers modified by the high-dose ion implantation (HDII) and the pulsed low-energy high-current electron beam (LEHCEB) were studied by Auger electron spectroscopy, grazing incidence X-ray diffraction analysis and the CSEM Nano Hardness Tester. It was found that the Ni-depleted surface layer (>160 nm) was observed for the samples treated by LEHCEB at pressure ∼10 −4 mbar. Irradiation in oil-less vacuum does not change the Ni, Ti and O concentration depth profiles. The influence by means LEHCEB and then by HDII (by Zr + or Ti + ions) leads to the formation of a Ti–Zr–(Ni < 20 at.%)–O and Ti–(Ni < 20 at.%)–O surface layer. Due to fast (∼10 9 K/s) quenching from the melt the single-phase (B2) structure in the surface layer of thickness 1–5 μm is formed. The new B2-phase has the grain size 1–5 μm, the lattice parameter a = 3.0072 ± 0.0005 Å (initial a B2 = 3.0129 ± 0.0005 Å), the coherent-scattering region size D csr ≈ 10–20 nm. Within the outer layer (∼300 nm in depth) modified by LEHCEB microhardness are twice as higher as of the initial sample.

Magnetic and high resolution TEM studies of nanographite derived from nanodiamond

The time evolution of graphitization was analyzed based on the structural and magnetic properties of nanodiamond samples annealed at 1600 °C for various time intervals. High resolution TEM and XRD show that the nanodiamond particles are converted to spherical onions for short annealing time intervals, and then they are completely transformed to polyhedral nanographite through the annealing for 120 min. The in-plane orbital susceptibility χ orb (300 K) analyzed on the basis of Kotosonov’s equation remains fairly constant in the range −5.5 × 10 −6 to −5.9 × 10 −6 emu/g, suggesting that the coherent scattering region (CSR) and the Fermi energy are independent of the annealing times. It also indicates the presence of serious defects affecting the electronic structure of nanographite. As the annealing time increases from 2 to 120 min, the Pauli paramagnetic susceptibility decreases. The broad ESR signal is associated with the spins localized on the edge states of the π-electron graphite network. The ESR linewidth becomes nine times less as the heat-treatment time rises from 2 to 120 min. The relatively large linewidth is associated with the scattering of π-electrons by the edge phonon modes and magnetic interaction between the edge-localized spins. During continuous annealing the nanoparticle structure changes from more defective to less defective and the number of edge-localized spins in a particle drops from ca. 2 to ca. 1 despite the CSR size and the Fermi energy remain practically the same.