Young’s modulus and internal friction in porous biocarbon white pine wood precursors

This paper reports on a study performed in the temperature range 100–293 K, in air and in vacuum, for the amplitude and time dependences of the Young’s modulus and the internal friction (ultrasound damping) of biocarbon precursors prepared from white pine wood at two pyrolysis (carbonization) temperatures of 1000 and 2400°C. The measurements have been conducted by the resonance technique with a composite vibrator on samples cut along and across the tree growth direction. The desorption of molecules of the external medium at low amplitudes of ultrasonic vibrations has been found to produce the pronounced influence on the effective elastic modulus and elastic vibration decrement. The data obtained from acoustic measurements of the amplitude dependences of the elastic modulus have been used to estimate the microplastic properties of the samples. It has been shown that increasing the carbonization temperature gives rise to noticeable changes in the Young’s modulus and internal friction, as well as to reduction of the microplastic stress σ _y of the biomaterial studied. The stress σ _y of the samples cut across the growth direction has been found to be substantially smaller than that of the “longitudinal” samples. The elastic and microplastic properties of precursors prepared from white pine wood have been compared with those of the white eucalyptus wood.     

The superconducting properties of YBa2(Cu1−xMx)3O7 for M=Zn and Ni

Transverse and zero-field μSR measurements were made on YBa₂(Cu_1−xNi_x)₃O_7−y withx=0.1 and 0.2, and YBa₂(Cu_1−x Zn _x )₃O_7−y withx=0.03, 0.06, 0.1, and 0.16, wherey≈0.1. Since doping may lead to magnetic ordering this was searched for with both zero and transverse field μSR, but no evidence was found over the temperature range studied: 10–100 K. However, depolarization rates as functions of temperature were obtained, and the low temperature values of these are σ=3.2 μs⁻¹.1.6μs⁻¹, and 1 μs⁻¹ forx=0.01, and 0.2 Ni, respectively, and σ=0.8 μs⁻¹, 0.75 μs⁻¹, 0.65 μs⁻¹, and 0.4 μs⁻¹ forx=0.03, 0.06, 0.1, and 0.16 Zn, respectively. Estimates for the effect of decreasing electron concentration for Zn are made, but these alone do not account for the drop in σ. Estimates for the effect of scattering on λ and hence σ are made. The reduction in σ for Ni dopant is in surprisingly good agreement with these estimates. For Zn the order of magnitude is correct, but the relative lack of further change in σ after the effect of the first 0.03 addition seems to imply a saturation of the effect of scattering.     

Diffusion and transport of a magnetic field in a turbulent medium with helicity

An analysis is made of the relation between accurate formulas for the coefficients of turbulent diffusion D _T and the alpha effect α _T for a magnetic field in the Lagrange and Euler representations. It is shown that the quadratic term with respect to α _T in the diffusion coefficient derived by Moffatt and Kraichnan is incorrect and should be dropped. First, a numerical solution of the nonlinear equation (DIA equation) for the Green function is presented, describing the transport of a magnetic field for the case of incompressible, uniform, isotropic, steady-state turbulence possessing helicity. These solutions are used to calculate the steady-state coefficients D _T and α _T for various values of the parameters ξ ₀=u ₀ σ ₀/R ₀, a=H ₀/u ₀ ² p ₀, σ ₀/σ ₁, and R ₀/R ₁, where u ₀, σ ₀, and R ₀ are the characteristic velocity, lifetime, and scale of the turbulent pulsations, and H ₀, σ ₁, and R ₁ are similar values describing the helicity of the medium h(1,2)=〈u(1)· (∇×u(2))〉, and the parameter α characterizes the degree of helicity. The DIA values of D _T and α _T and the self-consistent values of these quantities calculated using the Green tensor in the diffusion approximation are in qualitative agreement. It is shown that the coefficient of turbulent diffusion is always positive for all the types of turbulence studied. Nonsteady-state values of D _T(t) and α _T(t) calculated by a self-consistent method are given. Zh. éksp. Teor. Fiz. 112, 1312–1331 (October 1997)     

Essentially different distributions of current in the inverse problem for the Grad-Shafranov equation

The paper answers a question debated by physicists for many years. It is proved that, for almost equal gradients of the magnetic flux u at its zero-level curve ∂ω, which is the piecewise smooth boundary of a simply-connected domain ω ⋐ ℝ², the inverse problem for the Grad-Shafranov equation of plasma equilibrium in a tokamak (in the cylindrical approximation) admits essentially different profiles of distributions f _u : ω ∋ (x, y) ↦ f(u(x, y)) = u _xx (x, y) + u _yy (x, y) ⩾ 0 in the class of third-order polynomials f(u) = Σ _m=0³ a _m u ^m .     

Scalar σ meson at a finite temperature in a nonlocal quark model

The properties and temperature behavior of the π and σ bound states are studied in the framework of the nonlocal model with a separable interaction kernel, based on the quark Dyson-Schwinger and meson Bethe-Salpeter equations. M _π(T), f _π(T), M _σ(T), and Γ_{σ → ππ}(T) are considered above and below the deconfinement and chiral restoration transitions. Talk at the Round-Table Discussion “Searching for a Mixed Phase of Strongly Interacting Matter at the JINR Nuclotron,” Dubna, July 7–9, 2005. The text was submitted by the authors in English.     

Temperature dependence of the heat capacity and saturation magnetization of dilute reentrant ferrimagnetic spinels

Using model objects consisting of dilute reentrant spinels Li_0.5Fe_2.5−x Ga_xO₄ with x=1.0–1.2, this paper describes the temperature dependence of the magnetic contribution to the heat capacity, C _m (T), at H=0 and temperatures exceeding the freezing temperature (T _f ∼10 K) and, for T⩾4.2 K, the low-field magnetization σ _H (T) and the saturation magnetization σ _S (T), as well as the magnetization isotherms σ _T (T) in fields of up to 10 kOe. The specific features of the behavior of the overall characteristics of C _m (T) and σ _S (T) are found for the states of a frustrated ferrimagnetic that occur in the conditions considered here (H=0 and T>T _f or T⩾4.2 K and strong magnetic fields) and are discussed in connection with changes in the magnetic excitation spectrum caused by local breakdowns of collinear spin ordering and frustrations. Zh. éksp. Teor. Fiz. 113, 1339–1349 (April 1998)     

Temperature dependences of the high-field magnetization of dilute frustrated ferrimagnetic spinels

Results are presented of an investigation of the magnetic properties of dilute frustrated ferrimagnetic spinels Li_0.5Fe_2.5−x Ga_xO₄ (x=0.8–1.2), which characterize the main parameters of the ferrimagnetic state and provide evidence of local violation of collinear spin ordering and frustrations. In particular, measurements were made of the concentration dependences of the magnetic moment n ₀(x) and the Curie point T _c (x), the magnetization isotherms σ _T (H) at T=4.2 K and H⩽10 kOe, and also the low-and high-field magnetization polytherms σ _H (T). It was established that for x⩾0.8 in fields exceeding the technical saturation field H _s ∼2 kOe, the temperature dependences of the high-field magnetization σ _H (T) between 4.2 and 230 K cannot be described by the Bloch T ^3/2 law whereas this law is satisfied for undiluted Li spinel (x=0). Over the entire temperature range (4.2–230 K) the experimental curves σ _H (T) may be approximated by σ _H (T)=σ ₀(1−AT ^3/2 − BT ^5/2) for x=0.8–1.0 and σ _H (T)=σ ₀[1−CT ^3/2exp(μ(H−H ₀)/k _B T)] for x=1.1, 1.2, where μH ₀∼15 K is the internal field produced by competition between exchange interactions and frustrations. Fiz. Tverd. Tela (St. Petersburg) 40, 1075–1079 (June 1998)     

Detection of (<Emphasis Type="Italic">C</Emphasis> + <Emphasis Type="Italic">A</Emphasis>)-type dislocation self-blocking in magnesium

The self-blocking effect predicted theoretically and then observed in intermetallides is established for a pure metal, namely, for magnesium. This effect can be observed only for magnesium single crystals whose axis is parallel to the c axis and whose yield stress behavior σ _y (T) has a temperature anomaly. For such single crystals, the self-blocking of the (c + a)-type edge dislocations is established during pyramidal slip of type II. The self-blocking is proved by dislocation extension along the preferred direction without external stress. In this case, the á 1[`1]00 ñ \left\langle {1\bar{1}00} \right\rangle directions appear preferred. TEM images of (c + a) dislocations extended along the preferred directions are presented. It is demonstrated that two effects – temperature anomaly of σ _y (T) and dislocation self-blocking – have the common nature: a two-valley potential relief of the dislocation. A model of two-valley relief of the (c + a) dislocations in Mg is proposed.     

Dielectric properties and phase transitions in crystals of TlInS x Se2 − x solid solutions

This paper reports on a study of the dielectric permittivity and electrical conductivity of single-crystal TlInS _x Se_{2 − x} solid solutions as functions of temperature and composition. It has been found that the dielectric permittivity ɛ and electrical conductivity σ decrease with increasing x and increase with growing temperature. The temperature dependences ɛ = f(T) and σ = f(T) for TlInS _x Se_{2 − x} crystals have been demonstrated to reveal anomalies in the form of maxima, which suggests that these crystals undergo phase transitions. The temperatures of the phase transitions increase with increasing x.     

Overall values of conventional discrepancy indices for crystals with heavy atoms. Part II. Results for monoclinic and orthorhombic crystals when the heavy-atom part alone constitutes the model

Theoretical expressions of 〈y _N〉, 〈|y _N − σ₁ y _P ^c |〉 and 〈|y _N ² −σ ₁ ² (y _P ^c )²|〉 (wherey _N andy _P ^c are the normalized structure amplitudes of the structure and the model respectively) are derived in terms of the heavy atom contributionσ ₁ ² for monoclinic and orthorhombic crystals containing a few (i.e., 1 or 2) heavy atoms of the same kind per asymmetric unit by taking the heavy atom part alone as the model. Results are obtained for both the related and unrelated cases. The local values of 〈y _N〉 and 〈|y _N ⁿ − σ ₁ ⁿ (y _P ^c ) ⁿ |〉, (n=1, 2) calculated from these expressions can be used to calculate the overall values of the conventionalR-indicesR(F) andR(I) for the related and unrelated cases. These overall values could be used to check the correctness of heavy atoms located in the structure. Contribution No. 550     

Influence of γ irradiation on the electrical conductivity and dielectric properties of β TlInS2 in the temperature range of the incommensurate phase

The influence of γ irradiation on the electrical conductivity σ, the dielectric permittivity ɛ, and the dielectric loss tangent tanδ of β-TlInS₂ crystals is investigated in the temperature range of the incommensurate phase. It is established that γ irradiation lowers the values of ɛ and tanδ over the entire investigated temperature range and decreases the maxima of the σ(T) and tanδ(T) curves in the vicinity of the transition temperature T _c, but does not alter the transition temperature. Fiz. Tverd. Tela (St. Petersburg) 39, 1088–1090 (May 1997)     

New line of thermodynamic phase transitions on the x-T diagram of a dilute Heisenberg system with short-range interactions Li0.5Fe2.5−x GaxO4

It is established on the basis of an investigation of the temperature dependences of the saturation magnetization σ_S(T) and the magnetic contribution to the specific heat C _m(T) that in the reentrant region of the x-T diagram (0.9⩽x⩽1.5) of dilute iron spinels Li_0.5Fe_{2.5− x} Ga_xO₄ there exists between the line of Curie points T _C(x) and the line of freezing temperatures T _f(x) another line of first-order phase transitions T ₁(x) to a noncollinear ferrimagnetic phase: T _C(x)⩽T ₁(x)⩽T _f(x). Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 329–333 (10 March 1998)     

Second-order moving average and scaling of stochastic time series

Long-range correlation properties of stochastic time series y(i) have been investigated by introducing the function σ² _MA = [y(i) - (i)]², where (i) is the moving average of y(i), defined as 1/n y(i - k), n the moving average window and N_max is the dimension of the stochastic series. It is shown that, using an appropriate computational procedure, the function σ _MA varies as n^H where H is the Hurst exponent of the series. A comparison of the power-law exponents obtained using respectively the function σ _MA and the Detrended Fluctuation Analysis has been also carried out. Interesting features denoting the existence of a relationship between the scaling properties of the noisy process and the moving average filtering technique have been evidenced. Received 31 December 2001     

Magnetic,electrical, magnetoelectrical,and magnetoelastic properties of La<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>MnO<Subscript>3 − <Emphasis Type="Italic">y</Emphasis></Subscript> manganites

This paper reports on a study of the influence of oxygen deficiency on the magnetization, paramagnetic susceptibility, electrical resistivity, magnetoresistance, and volume magnetostriction of the La_0.9Sr_0.1MnO_{3 − y} manganite with y = 0.03, 0.10, and 0.15. The magnetization M(T) behaves in a complex way with temperature; for T < 80 K, it only weakly depends on T, and at 80 ≤ T ≤ 300 K, the M(T) curve shows a falloff. Within the interval 240 K ≤ T ≤ 300 K, the long-range magnetic order breaks up into superparamagnetic clusters. For T < 80 K, the magnetic moment per formula unit is about one-fourth that which should be expected for complete ferromagnetic alignment of Mn ion moments. Although the composition with y = 0.03, in which part of acceptor centers is compensated by donors (oxygen vacancies), the negative magnetoresistance Δρ/ρ and volume magnetostriction ω are observed to pass through maxima near the Curie point, their values are one to two orders of magnitude smaller than those for the y = 0 composition. In compositions with y = 0.10 and 0.15 with electronic doping, the values of Δρ/ρ and ω are smaller by one to two orders of magnitude than those observed for the y = 0.03 composition. They do not display giant magnetoresistance and volume magnetostriction effects, which evidences the absence of ferrons near unionized oxygen vacancies. This allows the conclusion that the part played by both compensated and uncompensated doubly charged donors consists in forming dangling Mn-O-Mn bonds, which lead to a decrease in the Curie temperature with increasing y and to the formation above it of superparamagnetic clusters of the nonferron type.     

On the mechanism of phase transformation in Ag2Se

A differential thermal analysis ΔT _y (T) in vacuum has been performed, and the temperature gradient ΔT _x (T) along the Ag₂Se sample during the transition α → β has been studied. It has been shown that the transitions α → α′ and β′ → β are displacive transitions and that the transition α′ → β′ is a reconstructive transition. It has been found that the temperature gradient along the sample during the transition α′ → β′ passes through a deep minimum due to a strong increase in the specific heat capacity.     

(T1u+T1g)⊗(hg+τ1u)vibronic interaction and superconductivity in C 60 n− fullerides

The closeness of low-lying T_1u and T_1g levels of C ₆₀ ⁻ could enable their mixing under an odd parity vibration of (T_{1 u} + T_{1 g} ⊗ (h_g + τ_{1 u})type. In addition, the two levels are susceptible to Jahn-Teller interaction due to five-fold degenerate h_g vibrations. This complex problem of (T_1u+T_1g)⊗(h_g+τ_1u) vibronic interaction is transformed to a form similar to T_2g ⊗ (ε_g + τ_2g) vibronic problem of octahedral symmetry. The problem is analysed in an infinite coupling model and compared with the experimental spectroscopic results for the C ₆₀ ⁻ radical. The resulting parameters are used to calculate the pair-binding energy and superconducting transition temperature in C ₆₀ ⁿ⁻ fullerides. Vibronic mixing with the T_1g level is found to be responsible for maximising the pair-binding energy at the doping level n=3. It is also found to be an important source of T_c enhancement.     

Effect of ionizing radiation on the dielectric characteristics of TlInSe<Subscript>2</Subscript> and TlGaTe<Subscript>2</Subscript> single crystals

The temperature dependences of the electrical conductivity and the permittivity of TlInSe₂ and TlGaTe₂ crystals unirradiated and irradiated with 4-MeV electrons at a doze of 10¹⁶ cm⁻² have been investigated. It has been established that electron irradiation leads to a decrease in the electrical conductivity σ and the permittivity ɛ over the entire temperature range under study (90–320 K). It has been revealed that the TlInSe₂ and TlGaTe₂ single crystals undergo a sequence of phase transitions characteristic of crystals of this type, which manifest themselves as anomalies in the temperature dependences σ = f(T) and ɛ = f(T). Electron irradiation at a doze of 10¹⁶ cm⁻² does not affect the phase transition temperatures of the crystals under investigation.     

Speed and shape of large-amplitude solitary waves in ion-beam plasma system

Large-amplitude solitary waves are investigated in ion-beam plasma system. The Sagdeev’s pseudopotential is determined in terms of the ion speedu. It is found that there exists a critical value ofu ₀, the value ofu at (u′)² = 0, beyond which the solitary waves cease to exist. The critical value also depends on σ (the ion temperature) or σ_b (the ion beam temperature). One of the author (PC) is grateful to UGC, India for the financial support under SAP(No F.510/8/DRS/2004(SAP-1)).     

Photoconductivity of poly-<Emphasis Type="Italic">p</Emphasis>-xylylene + CdS nanocomposite films over a wide temperature range

For poly-p-xylylene + CdS (PPX + CdS) nanocomposite films, the dependences of the photo-conductivity σ _ph (T) on the concentration C of CdS nanoparticles, intensity and wavelength of exciting light, and temperature T within 15–300 K are examined. An appreciable photocurrent appears at C ≥ 10 vol %, when a large percolation cluster of CdS nanoparticles is formed. The photocurrent spectrum is compared to the absorption spectrum of the film. The photocurrent I _ph (P) increases with the intensity of light flux P in a wavelength range near 435 nm according to the I _ph (P) ∼ P ⁿ power law, where n < 1. At 15 K, the photoconductivity of films with C ≈ 11.5 and 13.5 vol % is higher than that of a pure CdS film (C = 100 vol %) by factors of ≈100 and ≈30, respectively. For films with C > 11.5 vol %, the σ _ph (T) dependence at low T exhibits a metal-like character (σ _ph (T) decreases with increasing temperature). Atomic force microscopy is used to examine the surface topography of PPX + CdS films, which is found to be strongly dependent on the concentration of nanoparticles. The dark conductivity and photoconductivity of nanocomposite films arise due to the thermo- and photoexcitation transfer of electrons from the CdS nanoparticles to the PPX matrix with the formation of an electronic double layer at the PPX matrix-large percolation CdS cluster interface, a process that populates the phenyl rings of the adjacent PPX layer with excess electrons. As a result, various mechanisms of electron transfer in the polymer matrix can be realized: Mott’s hopping conduction mechanism with variable-range hopping in the matrix between CdS clusters and the metal-like behavior of the conductivity in the polymer shell of the large cluster at low temperatures. The polymer shell contains excess electrons on the phenyl rings -C₆H₄- in the composition of anion-resonances -C₆H₄⁻-.     

Thermal stability of submicrocrystalline copper strengthened with HfO2 nanoparticles in the temperature range 20–500 °C

The thermal stability of the yield stress and Young’s modulus was investigated in ultrafinegrained copper (99.98%) and a Cu-HfO₂ composite obtained by intensive plastic deformation using the method of equichannel angular pressing. It is shown that both the pure copper and the composite strengthened with HfO₂ nanoparticles demonstrate in this state a high yield stress (σ _0.2≈400 MPa). When the two-hour annealing temperature T _a is increased above 200 °C, the yield stress in pure copper decreases to 40 MPa at T _a=400 °C, whereas in the Cu-HfO₂ composite, high yield stresses are conserved up to T _a=500 °C. A recovery stage of Young’s modulus is found at around 200 °C both in pure copper and in the Cu-HfO₂ composite. It is concluded that this stage reflects the transition of the grain boundaries from a nonequilibrium to an equilibrium state, and the high-strength properties of the materials are determined mainly by the grain size and depend weakly on the grain-boundary structure. Fiz. Tverd. Tela (St. Petersburg) 40, 1268–1270 (July 1998)