Microstructure,elastic, and inelastic properties of biomorphic carbons carbonized using a Fe-containing catalyst

The microstructure and amplitude dependences of the Young’s modulus E and internal friction (logarithmic decrement δ), and microplastic properties of biocarbon matrices BE-C(Fe) obtained by beech tree carbonization at temperatures T _carb = 850–1600°C in the presence of an iron-containing catalyst are studied. By X-ray diffraction analysis and transmission electron microscopy, it is shown that the use of Fe-catalyst during carbonization with T _carb ≥ 1000°C leads to the appearance of a bulk graphite phase in the form of nanoscale bulk graphite inclusions in a quasi-amorphous matrix, whose volume fraction and size increase with T _carb. The correlation of the obtained dependences E(Т _carb) and δ(T _carb) with microstructure evolution with increasing Т _carb is revealed. It is found that E is mainly defined by a crystalline phase fraction in the amorphous matrix, i.e., a nanocrystalline phase at Т _carb < 1150°C and a bulk graphite phase at T _carb > 1300°C. Maximum values E = 10–12 GPa are achieved for samples with Т _carb ≈ 1150 and 1600°C. It is shown that the microplasticity manifest itself only in biocarbons with T _carb ≥ 1300°C (upon reaching a significant volume of the graphite phase); in this case, the conditional microyield stress decreases with increasing total volume of introduced mesoporosity (free surface area).     

Excitonic representation: Collective excitation spectra in the quantized Hall regime and spin biexciton

The excitonic representation method for describing collective excitations in the quantized Hall regime makes it possible to simplify analysis of the spectra and to obtain new results in the strong magnetic field limit, when E _C ??ω_c (ω_c is the cyclotron frequency and E_C is the characteristic Coulomb energy). For an integer odd filling factor ν greater than unity (i.e., for ν = 3, 5, 7,...), the spectra of one-cyclotron magneto-plasma excitations are calculated. For unit filling factor, the existence of a spin biexciton (bound state of two spin waves) corresponding to excitation with a spin change (δ_S = δ_Sz = ?2) is proved. The exact equation determining the ground state of the biexciton is derived in the thermodynamic limit N_Φ → ∞ (_N? is the system degeneracy). The exchange energy of this state is lower than for a single spin wave (with δ_S = δ_Sz = ?1) for the same value of the 2D wavevector q. In the limit q → ∞ corresponding to the decay of a biexciton into a pair of quasiparticles one of which is a trion with a spin of ?3/2, the energy is found to be lower than the energy (e²/εl _B )√π/2 required for exciting an electron-hole pair in the strictly 2D case (l_B is the magnetic length and ε is the dielectric constant), although this energy is higher than another “classical” result (e²/εl _B )√π/2, corresponding to the excitation of a skyrmion-antiskyrmion pair (|δS|=|δS _z |?1). The solution of the exact equation gives the trion binding energy and the activation gap for quasiparticles whose excitation corresponds to a change in the total spin by δS = δ S_z =?3. The energy of a spin biexciton is calculated for values of the wavevector such that ql _B ?1.     

Study on ionic conductivity and dielectric properties of PEO-based solid nanocomposite polymer electrolytes

The ionic conductivity and dielectric properties of the solid nanocomposite polymer electrolytes formed by dispersing a low particle-sized TiO₂ ceramic filler in a poly (ethylene oxide) (PEO)-AgNO₃ matrix are presented and discussed. The solid nanocomposite polymer electrolytes are prepared by hot press method. The optimum conducting solid polymer electrolyte of polymer PEO and salt AgNO₃ is used as host matrix and TiO₂ as filler. From the filler concentration-dependent conductivity study, the maximum ionic conductivity at room temperature is obtained for 10 wt% of TiO₂. The real part of impedance (Z′) and imaginary part of impedance (Z″) are analyzed using an LCR meter. The dielectric properties of the highest conducting solid polymer electrolyte are analyzed using dielectric permittivity (ε′), dielectric loss (ε″), loss tangent (tan δ), real part of the electric modulus (M′), and imaginary part of the electric modulus (M″). It is observed that the dielectric constant (ε′) increases sharply towards the lower frequencies due to the electrode polarization effect. The maxima of the loss tangent (tan δ) shift towards higher frequencies with increasing temperature. The peaks observed in the imaginary part of the electric modulus (M″) due to conductivity relaxation shows that the material is ionic conductor. The enhancement in ionic conductivity is observed when nanosized TiO₂ is added into the solid polymer electrolyte.     

Electron interaction effects on the thermoelectric power of a quantum dot at T &gt; T<Subscript>K</Subscript>

In the absence of phonon thermal conductivity, we theoretically investigate the output power of an interacting quantum dot thermoelectric setup that is moderately coupled to two electronic reservoirs in the regime T ? T _K . In the noninteracting case, the output power is maximized when the energy level of the dot is around a critical value ε _c . We find that when the energy level of the dot is lower than ε _c , Coulomb interaction can enhance the maximum thermoelectric power that can be achieved by tuning the bias and a wider operating region is also observed. However, when the energy level of the dot is higher than ε _c , Coulomb interaction suppresses the maximum power. Finally when the dot level is around ε _c , Coulomb interaction has minimal effects on the maximum power.     

On the theory of the resonant interaction of electrons with a high-frequency electric field in one-dimensional two-barrier nanostructures with symmetric barriers of finite height and width

The theory of the interaction of electrons with a high-frequency electric field in one-dimensional two-barrier nanostructures with symmetric barriers of finite height and widths was developed. An exact solution to the Schrödinger equation was found for electrons in this nanostructure in the absence of high-frequency electric field. An analytical expression for the direct current I ₀ induced in this structure by an incident electron flux with energy ε differing slightly from the resonant level energy ε _r (|ε ? ε _r | << ε _r ) was derived. In the small-signal approximation, the active (field-phased) component I _c of the alternating electric current was calculated. At ε > ε _r , the current I _c is negative in the entire frequency range, which suggests the possibility of ac electric field amplification and generation in the two-barrier resonant-tunneling structure with the barriers of finite height and width. Within the applicability of the theory (?ω << ε _r ), the frequency at which amplification and generation of the ac electric field are possible reaches ω ? 10¹³ s ^?1; the power transferred by electrons to the field is ～1 W/cm².     

Size effects of the strength and elastic properties of individual phases and interphase boundaries of polycrystalline materials

Size effects of hardness are studied and numeric values of Young’s modulus E, hardness H, and fracture toughness coefficient K_c of individual phases and interphase boundaries of polycrystalline samples of ferruginous quartzite are determined by means of micro- and nanoindentation methods. It is found that the interphase boundary of magnetite and hematite is the one most strengthened, while the boundary of the hematite and quartz is the one least durable.     

Multistage radiation-stimulated changes in the microhardness of silicon single crystals exposed to low-intensity β irradiation

Radiation-stimulated and postradiation changes in the microhardness of silicon single crystals exposed to irradiation with a low-intensity flux of β particles (I = 9 × 10⁵ cm^?2 s^?1, W = 0.20 + 0.93 MeV) are studied. It is established that the inversion of the radiation-induced plastic effect occurs at a characteristic irradiation time τ_c = 75 min; i.e., irradiation of silicon single crystals for a time τ < τ_c leads to nonmonotonic reversible hardening, whereas nonmonotonic reversible softening is observed under irradiation for a time τ > τ_c. It is demonstrated that there exists a correlation between the nonmonotonic dependences of the microhardness and the concentration of electrically active defects at acceptor levels with energies E _c ? 0.11 eV, E _c ? 0.13 eV, and E _c ? 0.18 eV on the irradiation time.     

Photoelectron Energy Loss in Al(002) Revisited: Retrieval of the Single Plasmon Loss Energy Distribution by a Fourier Transform Method

A Fourier transform (FT) algorithm is proposed to retrieve the energy loss function (ELF) of solid surfaces from experimental X-ray photoelectron spectra. The intensity measured over a broad energy range towards lower kinetic energies results from convolution of four spectral distributions: photoemission line shape, multiple plasmon loss probability, X-ray source line structure and Gaussian broadening of the photoelectron analyzer. The FT of the measured XPS spectrum, including the zero-loss peak and all inelastic scattering mechanisms, being a mathematical function of the respective FT of X-ray source, photoemission line shape, multiple plasmon loss function, and Gaussian broadening of the photoelectron analyzer, the proposed algorithm gives straightforward access to the bulk ELF and effective dielectric function of the solid, assuming identical ELF for intrinsic and extrinsic plasmon excitations. This method is applied to aluminum single crystal Al(002) where the photoemission line shape has been computed accurately beyond the Doniach–Sunjic approximation using the Mahan–Wertheim–Citrin approach which takes into account the density of states near the Fermi level; the only adjustable parameters are the singularity index and the broadening energy Г (inverse hole lifetime). After correction for surface plasmon excitations, the q-averaged bulk loss function, <Im[??1 / ε(E, q)]> _q , of Al(002) differs from the optical value Im[??1 / ε(E, q?=?0)] and is well described by the Lindhard–Mermin dispersion relation. A quality criterion of the inversion algorithm is given by the capability of observing weak interband transitions close to the zero-loss peak, namely at 0.65 and 1.65 eV in ε(E, q) as found in optical spectra and ab initio calculations of aluminum.     

Treatment of pairing in small systems by Green functions

A finite system of fermions with pairing interaction is treated by the Green function method. It is shown that a finite number of “bound pairs” must be assumed to get the correct properties of the system in that region of the interaction strength where the BCS-solution is incorrect. Also the difference betweenE ₀(N+2)?E ₀(N) andE ₀(N)?E ₀(N?2),E ₀(N) being the ground state energy of theN-particle system, has to be considered. The formulae derived give an interpolation between the region where perturbation theory applies and the region of validity of the BCS-equations.     

Investigation of the local mechanical properties of potassium chloride single crystals by atomic force microscopy

Using an atomic force microscope (AFM), the of hardness H and Young’s modulus E are measured in near-surface layers of KCl single crystals to a depth of 300 nm at loads of 5–100 µN. The values of H and E are estimated indirectly by analyzing P(h) curves (load vs. indentation depth curves). The value of H is also estimated directly by measuring the area of an indentation with the help of an AFM with a nanoscale resolution. The effect of structural features of the surface around an indentation on the accuracy of the H and E estimates is revealed. The sharp dependence of H on the load (the nanoscale effect) is revealed. The experimental results agree qualitatively with the predictions of the geometrically necessary dislocation model developed by Nix and Gao. However, in order to quantitatively estimate mass transfer from a nanoindenter, a structural analysis is required with allowance for plastic deformation in crystals.     

Thermal annealing effect on structural and optical properties of 2,9-Bis [2-(4-chlorophenyl)ethyl] anthrax [2,1,9-def:6,5,10-d′e′f′] diisoquinoline-1,3,8,10 (2H,9H) tetrone (Ch-diisoQ) thin films

Thin films of 2,9-Bis [2-(4-chlorophenyl)ethyl] anthrax [2,1,9-def:6,5,10-d′e′f′] diisoquinoline-1,3,8,10 (2H,9H) tetrone (Ch-diisoQ) were prepared by thermal evaporation technique. Structural properties of these (as-prepared and annealed at 373, 423, 473 and 523 K) films were determined by X-ray diffraction and scanning electron microscopy, which showed that the grain sizes increasing by the annealing effect. The transmittance and reflectance of all Ch-diisoQ thin films were measured in the range 200–2500 nm. Some optical constants such as optical band gap (E _g ), dispersion energy (E _d ), single oscillator energy (E _o ) and optical dielectric constant at a higher frequency (ε _∞ ) were calculated at different annealing temperatures. The optical band gap of the samples is decreased with the increase of annealing temperatures due to the increasing of the π-dislocation. Finally, the values of the optical susceptibility, χ⁽³⁾, were found to be annealing dependence.     

Investigation of the initial stages of defect formationin carbon nanotubes under irradiation with argon ions

The formation of defects in carbon nanotubes under irradiation with argon ions is investigated. The π plasmons generated in single-walled and multiwalled carbon nanotubes are examined using electron energy-loss spectroscopy. In the course of experiments, the supramolecular structure of nanotubes is stepwise modified by an argon ion beam (the maximum irradiation dose is 360 μC/cm²). The content of argon ions implanted into a nanotube structure is controlled using Auger electron spectroscopy. The effect of ion irradiation on the π-plasmon energy E_π and on the half-width at half-maximum δE of the π-plasmon spectrum is determined experimentally. An expression relating the above quantities and the concentration of implanted argon is derived. It is shown that the formation of defects under ion irradiation is a discontinuous process occurring in a stepwise manner. A qualitative phenomenological interpretation is proposed for the experimentally revealed decrease in the π-plasmon energy E_π and for its attendant broadening of the π-plasmon spectrum. The assumption is made that the microscopic mechanism of the observed phenomena is associated with the narrowing of the energy π subbands in the electric field of charged defects generated by ions.     

On resonance and nonresonance interactions between electrons and spatially periodic clusters

The effects of resonance and nonresonance interactions between electrons and spherical structures with spatial periodicity in the radial direction (clusters) were studied. It was shown analytically and by numerical calculations that the δ _l phase shift of the wave function, which arises in resonance electron scattering by such a periodic structure of a fairly large radius r₀, was not small even at a small ratio between the U₀ amplitude of the periodic potential and scattered electron energy E(ε₀=U₀/E?1) and equaled |δ _l |=π/4 (modulo π). This phase shift corresponded to the limiting case of a large Born parameter for the cluster, ξ₀=r₀U₀/?v?1, where v is the characteristic velocity of the electron. The effect of nonresonance electron scattering by a periodic potential whose spatial period was incommensurate with the Brillouin wavelength of the scattered electron was considered analytically. The effect of nonresonance scattering was shown to be of a higher order in the ε₀?1 parameter than resonance scattering. The cross section of electron scattering by a cluster was calculated, which allowed the conductivity of a medium containing clusters to be estimated.     

Electronic structure and energies of interatomic bonds in the TiSi<Subscript>2</Subscript> compound with a <Emphasis Type="Italic">C</Emphasis>49 crystal structure

Full-electron calculations of the electronic structure of the TiSi₂ compound in the structural modification C49 are performed using the augmented-plane-wave method. The total energy, the electronic band structure, and the density of states are calculated for an extended translational unit cell Ti₄Si₈, which is formed during the growth of a silicon nanowire on a p-Si substrate. Calculations are also carried out for two orthorhombic unit cells of the nonstoichiometric compositions Ti₃Si₉ and Ti₅Si₇. The energies of the interatomic bonds are determined to be E _Si-Si = 1.8 eV, E _Ti-Ti = 2.29 eV, and E _Ti-Si = 4.47 eV. The dependence of the total energy of the unit cell E _tot(V) on the unit cell volume V is obtained by optimizing the unit cell volume. The bulk modulus B ₀ = 132 GPa is determined from the Murnaghan equation of state for solids and the dependence E _tot (V). This value of the bulk modulus is used to estimate the activation energy for interstitial diffusion of silicon atoms Q _i(Si) ≈ 0.8 eV.     

Intergroup azimuthal correlations of secondary particles in the interaction between <Superscript>56</Superscript>Fe nuclei of momentum 2.5 GeV/<Emphasis Type="Italic">c</Emphasis> per nucleon and photoemulsion nuclei

Angular correlations between secondary particles emitted in the interaction between ⁵⁶Fe nuclei of momentum 2.5 GeV/c per nucleon and photoemulsion nuclei were measured. These secondaries are partitioned into four groups: s, g, b, and f. For particles from each pair of the above groups, the average values of the collinearity factor and of the asymmetry parameter, as well as the parameter η of the function α(1+η cos ε) approximating the distribution of azimuthal-angle differences ε, were calculated.     

“Negative” gap in the spectrum of localized states of (In<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>0.9</Subscript>(SrO)<Subscript>0.1</Subscript>

The reflection R(?ω), transmission t(?ω), absorption α(?ω), and refraction n(?ω) spectra of polycrystalline In₂O₃–SrO samples with low optical transparency, which contain In₂O₃ and In₂SrO₄ crystallites with In₄SrO_{6 + δ} interlayers, are examined. In the region of small ?ω values, the reflection coefficient decreases as the resistance of samples saturated with oxygen increases. Spectral dependences n(?ω) and α(?ω) are calculated using the classical electrodynamics relations. The results are compared to the data based on the t(?ω) spectra. The calculated absorption spectra are interpreted within the model with an overlap of tails of the density of states in the valence band and in the conduction band. A “negative” gap E _gn in the density of states with a width from–0.12 to–0.47 eV is formed in highly disordered samples in this model. It is demonstrated that the high density of defects and the band of deep acceptor states of strontium in the major matrix In₂O₃ phase are crucial to tailing of the absorption edge and its shift toward lower energies. The direct gap E _gd = 1.3 eV corresponding to the In₂SrO₄ phase is determined. The energy band diagram and the contribution of tunneling, which reduces the threshold energy for interband optical transitions, are discussed.     

Anomalous temperature dependence of magnetic relaxation in YBa<Subscript>2</Subscript>Cu<Subscript>2</Subscript>O<Subscript>7 − δ</Subscript> oxygen-deficient single crystals

The dependence of the magnetization relaxation rate S = ?d lnM/dlnt on temperature T is measured in YBa₂Cu₃O_{7 ? δ} samples with various oxygen concentrations. It is found that the S(T) curve changes qualitatively when oxygen deficit δ exceeds the threshold value δ_th = 0.37. For δ < δ_th (T _c > 60 K, where T _c is the superconducting transition temperature), function S(T) has the well-known peak at T/T _c = 0.4. For δ > δ_th (at T _c < 51 K), this peak transforms into a plateau and a new sharp peak appears at T/T _c = 0.1. The threshold value δ_th of the oxygen deficit corresponds to the transition of the sample from the disordered state into the ordered state of oxygen vacancies. We consider the change in the shape of the S(T) curve as a macroscopic manifestation of this transition.     

Morphological transformations of diblock copolymers in binary solvents: A simulation study

Morphological transformations of amphiphilic AB diblock copolymers in mixtures of a common solvent (S1) and a selective solvent (S2) for the B block are studied using the simulated annealing method. We focus on the morphological transformation depending on the fraction of the selective solvent C _S2, the concentration of the polymer C _p , and the polymer–solvent interactions ε _ij (i = A, B; j = S1, S2). Morphology diagrams are constructed as functions of C _p , C _S2, and/or ε _AS2. The copolymer morphological sequence from dissolved → sphere → rod → ring/cage → vesicle is obtained upon increasing C _S2 at a fixed C _p . This morphology sequence is consistent with previous experimental observations. It is found that the selectivity of the selective solvent affects the self-assembled microstructure significantly. In particular, when the interaction ε _BS2 is negative, aggregates of stacked lamellae dominate the diagram. The mechanisms of aggregate transformation and the formation of stacked lamellar aggregates are discussed by analyzing variations of the average contact numbers of the A or B monomers with monomers and with molecules of the two types of solvent, as well as the mean square end-to-end distances of chains. It is found that the basic morphological sequence of spheres to rods to vesicles and the stacked lamellar aggregates result from competition between the interfacial energy and the chain conformational entropy. Analysis of the vesicle structure reveals that the vesicle size increases with increasing C _p or with decreasing C _S2, but remains almost unchanged with variations in ε _AS2.     

Photoconduction and photocontrolled collective effects in the Peierls conductor TaS<Subscript>3</Subscript>

Light illumination of thin crystals of CDW conductor TaS₃ is found to result in dramatic changes of both linear (G) and nonlinear conduction. The increase of G is accompanied by suppression of the collective conduction, growth of the threshold field E _T , and appearance of the switching and hysteretic behavior in the nonlinear conduction. The effects in the nonlinear conduction are associated with increase of CDW elasticity due to illumination that leads, in particular, to the appearance of a relation E _T ∝ G^1/3 expected for the one-dimensional pinning.     

Ab initio calculations of B<Subscript>2</Subscript> type RHg (R = Ce,Pr, Eu and Gd) intermetallic compounds

Spin polarized ab initio calculations have been carried out to study the structural, electronic, elastic and thermal properties of RHg (R = Ce, Pr, Eu and Gd) intermetallic compounds in B₂ structure. The calculations have been performed by using both generalized gradient approximation (GGA) and local spin density approximation (LSDA). The calculated value of lattice constant (a ₀) for these compounds with GGA is in better agreement with the experimental data than those with LSDA. Bulk modulus (B), first-order pressure derivative of bulk modulus and magnetic moment (μ _B ) are also presented. The energy band structure and electron density of states show the occupancy of 4f states for light as well as heavy rare earth atom. The elastic constants are predicted from which all the related mechanical properties like Poisson’s ratio (σ), Young’s modulus (E), shear modulus (G _H ) and anisotropy factor (A) are calculated. The ductility or brittleness of these compounds is predicted from Pugh’s rule (B/G _H ) and Cauchy pressure (C ₁₂ ? C ₄₄). The Debye temperature (θ _D ) is estimated from the average sound velocity, which have not been calculated and measured yet.