Effect of oxidation treatment on Li capacity in disordered carbons - The surfing cards model

Small angle neutron scattering and X-ray diffraction were used to study the effect of oxidation treatment on the structure and Li capacity of disordered carbons. The oxidation treatment increase the pore density and reduce the degree of parallel arrangement of graphene layers, all with no detectable change in pore size. Scattering from Li-doped samples indicates that the Lithium is most likely coating the pore surfaces rather than creating clusters inside the pores. The results are analyzed in terms of the newly proposed potato-chips (edge-connected graphene fragments, or cards) model of amorphous carbons, from which we infer translation, or surfing, of connected fragments during heat treatment.     

An asymptotic solution for large Prandtl number free convection

Summary The set of ordinary differential equations governing free convection boundary layer flow past an isothermal semi-infinite vertical flat plate is solved for large Prandtl numbers by means of the method of matched asymptotic expansions. The analysis leads to an expression for heat transfer which contains the Prandtl number explicitly and which is very accurate for sufficiently large values of the Prandtl number. On the other hand the analysis also has qualitative assets. Before choosing the mathematical method of solution, the physical aspects of the large Prandtl number free convection boundary layer are investigated. The mathematical solution serves to enlarge our understanding of the physical implications of a free convection boundary layer in a large Prandtl number fluid.Nomenclature a_ij coefficient defined by - b_ij coefficient defined by F_j()=b_0j+b_1j +b_2j ²+.... - c coefficient defined by equation (3) - c_p specific heat - f non-dimensional stream function of inner expansion (7) - f_n n-th perturbation of f - F non-dimensional stream function of outer expansion (15) - g non-dimensional stream function (1) - ¯g acceleration due to gravity - Gr_x local Grashof number:g(T_w–T)x³/v² - h non-dimensional temperature (2) - k coefficient of heat conduction - Nu_x local Nusselt number: - T temperature - T_w wall-temperature - T ambient temperature - u longitudinal velocity - x co-ordinate measuring distance from the leading edge - y co-ordinate measuring distance normal to the plate Greek symbols coefficient of thermal expansion - _i expansion parameter (21) - expansion parameter (22) - _i expansion parameter (33) - expansion parameter (34) - expansion parameter: ^–1/2 - inner similarity co-ordinate (9) - non-dimensional temperature of inner expansion (8) - _n n-th perturbation of - non-dimensional temperature of outer expansion (16) - _n n-th perturbation of - similarity co-ordinate (3) - kinematic viscosity - outer similarity co-ordinate (17) - density - Prandtl number:c_p/k - stream function     

Variation of the Kondo temperature with the nature and concentration of the solute metal

From the available data on the physical properties of dilute magnetic alloys, Daybell and Steyert obtain V-shaped semilog plots ofT _K vs. N (whereT _K is the Kondo temperature andN is the serial number of the magnetic solute in the 3d series). This requires a large variation in the rather small exchange interaction parameterJ, while the Kondos–d exchange model is based on a smallJE _F (whereE _F is the Fermi energy). Taking the ordinary scattering into account,T _K =T _F exp((1– cos ²)/(JD cos²)), whereJ<0, 1, is the phase shift for ordinary scattering, andD is the density of states of one spin direction per atom. It has been shown by us recently, from the concentration dependences of the Kondo slopes and the extremum values of the thermoelectric power of dilute magnetic alloys, that 0<d shells, and 3/4<< for those having less-than-half filled 3d shells. We show, on this basis, that the range of variation ofJ is now reduced to more acceptable values, as the solute metal traverses the 3d series. Again, based on our inference that increases with the increase of the solute concentrationc, we show that an increase of the more-than-half filled solute concentration decreasesT _K , while the increase of the less-than-half filled solute concentration increasesT _K . The fact that the increase of cobalt content in copper or gold favors magnetism while the increase of vanadium content in gold favors nonmagnetism is understandable on this basis.     

The Interplay of Electronic and Nuclear Magnetism in PtFe x at Milli-, Micro-, and Nanokelvin Temperatures

We have measured ac susceptibility, nuclear magnetic resonance, and nuclear heat capacity of two PtFe _x samples with concentrations of magnetic impurities x = 11 ppm and 41 ppm at magnetic fields (0 ± 0.05) mTB248 mT. The susceptibility data have been measured at temperatures of 0.3 KT100 mK, no hint for nuclear magnetic ordering could be detected to a temperature of 0.3 K. The nuclear heat capacity data taken at 1.4 KT10 mK show enhanced values which scale with x at low polarization. This effect is described by a model assuming an internal magnetic field caused by the impurities. No indication for nuclear magnetic ordering could be detected to 1.4 K. The nuclear magnetic resonance experiments have been performed on these samples at 0.8 KT0.5 mK and 2.5 mTB22.8 mT as well as on three other samples with x = 5, 10, 31 ppm in a different setup at 40 KT0.5 mK and at 5.4 mTB200 mT. Spin-lattice and effective spin-spin relaxation times ₁and ₂ ^* of ¹⁹⁵ Pt strongly depend on x and on the external magnetic field. No temperature dependence of ₁and ₂ ^* could be detected and the NMR data, too, give no hint for nuclear magnetic ordering to 0.8 K.     

Phase decomposition of liquid-quenched β-type Ti-Cr alloys

Phase decomposition behaviour of liquid-quenched (bcc) type Ti-Cr alloys was investigated by means of transmission electron microscopy and hardness measurements. It was found that decomposition of to ₁ (Ti-rich, bcc) + ₂ (Ti-lean, bcc) takes place in the intermediate composition range of the Ti-Cr system. This experimental result proves the theoretical prediction made by Menon and Aaronson, but the observed ₁ + ₂ two-phase field expands towards higher temperatures than the predicted binodal line. The coherent ₁ + ₂ two-phase state exhibits the so-called 100 modulated structure and it was concluded that the formation of such a structure is a result of spinodal decomposition of the -phase. We obtained time-temperature-transformation (TTT) diagrams of -type Ti-30, 40 and 50 at % Cr alloys. A typical sequence of structural change is coherent ₁ + ₂ incoherent ₁ + ₂ incoherent ₁ + ₂ + grain boundary precipitates stable state of + TiCr₂ or + TiCr₂. Not all the states in the above sequence appear, depending on alloy composition, liquid-quenching rate and ageing temperature.     

The effect of Si,Zr, Al and Mo on the structure and strength of Ti martensite

The structure and strength of martensite in near titanium alloys have been studied in the composition range (wt %) up to 10% Zr, 6%, Al, 1/2% Mo, 2.4% Si. [0001], 1/3 11¯20 dislocations, 1/3 10¯10 stacking faults and approximately {10¯11} twin related martensite plates are found to be common features of the martensite. Martensite midribs consist either of finely transformed material between martensite plates, or regions of low dislocation density within martensite plates.The martensite morphology is related to the alloy composition, changing from massive to plate-like with increasing solute content. The strength of the martensite is controlled largely by solid solution strengthening.     

Simulation of the Process of Water Freezing in a Round Pipe

The problem of freezing of pure water in a round pipe is treated with due regard for convection under asymmetric thermal boundary conditions in the absence of motion along the pipe. The problem is solved numerically using the control volume approach, SIMPLER algorithm, and the enthalpy method. Results are obtained for three Grashof (Gr) and six Biot (Bi) numbers: Gr = 1.55 × 10⁶, Bi = 0.305 (0 < ), Bi = 0.044 ( < 2); Gr = 1.24 × 10⁷, Bi = 0.610 (0 < ), Bi = 0.087 ( < 2); Gr = 9.89 × 10⁷, Bi = 1.220 (0 < ), Bi = 0.174 ( < 2). The correctness of calculation of the problem disregarding free-convection flows is analyzed.     

Solution of Mixed Contact Problems in the Theory of Nonstationary Heat Conduction by the Method of Summation‐Integral Equations

The laws governing the development of spatial nonstationary temperature fields in a bounded cylinder and a halfspace where one of the end surfaces of the cylinder touches the surface of the halfspace in a circular region are determined. A solution of a mixed axisymmetric nonstationary problem of heat conduction is obtained in the region of Laplace transforms. In solution of this problem, there appear summationintegral equations with the parameter of the integral Laplace transform (Lparameter) and the parameter of the finite integral Hankel transform (Hparameter).     

On the proximity effect in a superconductive slab bordered by metal

The first Ginzburg-Landau equation for the order parameter in the absence of magnetic fields is solved analytically for a superconducting slab of thickness 2d bordered by semi-infinite regions of normal metal at each face. The real-valued normalized wave function f=/ depends only on the transversal spatial coordinate x, normalized with respect to the coherence length of the superconductor, provided the de Gennes boundary condition df/dx=f/b is used. The closed-form solution expresses x as an elliptic integral of f, depending on the normalized parameters d and b. It is predicted theoretically that, for b< and dd_c=arctan(1/b), the proximity effect is so strong that the superconductivity is completely suppressed. In fact, in this case, the first Ginzburg-Landau equation possesses only the trivial solution f0.     

Nanoindentation experiments on some amorphous hydrogenated carbon (a-C:H) thin films on silicon

Ultra-low-load indentation (nanoindentation) experiments have been used to investigate the load-displacement characteristics of two types of hydrogenated carbon films (a hard and a soft version 230 and 210 nm thick, respectively) deposited from methane on to (1 0 0) single-crystal silicon wafers by a radio frequency plasma-assisted chemical vapour deposition process. Further, the technique was used to explore the changes in the properties of the films with heat treatment in vacuum at temperatures of up to 650°C. In all cases, the elastic and plastic properties of the films were compared at indentation loads in the range 0–60 mN, the higher loads causing indentor displacements greater than the thickness of the films. For the harder, stiffer coating, penetration resistance was found to decrease with increasing indenter displacement, reflecting the greater load-carrying role taken by the softer silicon with increasing applied load. However, for the softer coating, penetration resistance generally increased with displacement, perhaps reflecting progressive compaction of the coating in addition to the increasing role of the silicon. In both cases, heat treatment severely degraded the mechanical properties of the films due to thermally induced chemical changes and, in the case of the hard coating, relaxation of residual stresses. Scanning electron microscopy of both nanoindentations and low-load microhardness indentations clearly reveals the deformation mechanisms associated with contact stresses to include flow and fracture of the film and interfacial decohesion.     

The age hardening effect in Ti-6 Al-4 V due to ω and α precipitation in the β grains

The structure at room temperature of a quenched TA6V titanium alloy has been investigated. This structure is + or + according to the treatment temperature; it is always metastable. During ageing the grains decomposed by the reaction + + +; this decomposition was accompanied by a large increase of the 0.2% yield stress. No structural modification was observed in. The and phase of TA6V were separately investigated in the form of single-phase alloys. The hardness of was insensitive to ageing, while was considerably hardened by and; we deduced that the strengthening of the minor phase during ageing is mainly responsible for the hardening of TA6V.     

Synthesis of silicon nitride powder through nitrogen gas atomization

The feasibility of synthesizing silicon nitride powder utilizing reactive atomization processing was analysed. The range of times required for the flight time of particles, the cooling rate of the silicon melt, the reaction time of silicon and nitrogen, and the diffusion of nitrogen through silicon nitride layers were obtained and compared. The results of this study indicated that the production of silicon nitride powder through the reactive atomization process would be limited by diffusion of nitrogen through the nitride (ash) layer, assuming the nitride layer was coherent and the unreacted core model was a valid representation of the liquid silicon-silicon nitride system.Nomenclature k(T) reaction rate constant at temperature, T(s^–1) - k ₀ Arrhenius coefficient - E activation energy (kJ mol^–1) - R gas constant - T temperature (K) - fraction of normalized conversion of -phase in time t - fraction of normalized conversion of -phase in time t - k reaction rate constant for -phase (s^–1) - k reaction rate constant for -phase (s^–1) - k ⁱ intrinsic first-order rate constant for -phase (s^–1) - x conversion fraction of -phase in time t - x conversion fraction of -phase in time t - n reaction order for -phase = 1 - n reaction order for -phase = 0.5 - J diffusion flux (mol m^–2 s^–1) - D diffusivity, or diffusion coefficient (m² s^–1 or cm² s^–1) - dC change in concentration (mol m^–3) - dl change in distance, l (m) - A(_g) gaseous reactant A - B reactant B (may be solid or liquid) - P solid product P - b stoichiometric coefficient of reactant B - p stoichiometric coefficient of product P - t time of reaction passed (s) - time for complete reaction of a particle (s) - X _B conversion fraction - r _c core radius (m) - R _p particle radius (m) - _B molar density of reactant B (mol m^–3) - k _g mass transfer coefficient between fluid and particle (m s^–1) - C _Ag concentration of gaseous reactant A (mol m^–3) - D _e effective diffusion coefficient of gaseous reactant in ash layer (m² s^–1)     

X-ray diffraction characterisation of low temperature plasma nitrided austenitic stainless steels

The nitrided layers produced by low temperature (400–500 °C) plasma nitriding on austenitic stainless steels, AISI 316, 304 and 321, have been characterised by X-ray diffraction, in conjunction with metallographic and chemical composition profile analysis. The thin, hard and corrosion resistant layers exhibited similar X-ray diffraction patterns, but the positions of the major diffraction peaks varied with nitriding temperature and nitrogen concentration profile. The low temperature nitrided layers are predominantly composed of a phase with a face centred cubic (fcc) structure, which is named S phase. However, the positions of the diffraction peaks from the S phase deviated in a systematic way from those for an ideal fcc lattice. Detailed analysis of the deviation suggested that very high compressive residual stresses and stacking faults were formed in the layers, resulting in a highly distorted and disordered fcc structure. The lattice parameter of the distorted and disordered S phase was found to increase with increasing nitrogen concentration.     

Theoretical estimation of the effect of interfacial energy on the mechanical strength of spinodally decomposed alloys

New interfaces are produced on the slip plane when a crystal with continuous composition fluctuation arising from spinodal decomposition is deformed by slip. In this work, the energy of such interfaces is evaluated for both modulated and mottled structures, and their effects on slip behaviour are discussed. It is concluded that the contribution of this interfacial energy is large enough to account for the age-hardening concomitant with spinodal decomposition.     

Switching and Hysteresis on the I-V Curves of Submicron BSCCO (2212) Bridges

No Heading In thin BSCCO (2212) whisker-based superconducting bridges with width and length 1 m we observe random telegraph noise at discrete values of current. We show that the noise is associated with spontaneous processes of addition and removal of discrete vortex trains and reflects the regular structure of steps on the I–V curves. The average lifetime of the trains falls down with increasing temperature T in a complex way: intervals of steep drop ( an order of magnitude per Kelvin) are separated with a plateau of (T) with 10^–2 s. The 70 GHz irradiation with 100 W power results in the growth of the average switching frequency by 5 orders of magnitude; thus the HTSC bridges could be efficient detectors of microwave radiation. We discuss the peculiar features of (T).PACS Numbers: 74.25.Fy, 74.40.+k, 74.60.Ge, 74.72.Hs     

Recovery of a creep-deformed Type 316 stainless steel

The recovery of the dislocation structures produced in a Type 316 steel during creep has been examined by annealing over a range of temperatures and times, both in the presence and in the absence of stress. The influence of dislocation recovery on subsequent reloading behaviour has also been examined.Initial dislocation recovery occurs rapidly but the rate of recovery subsequently decreases as precipitate effects become more important. Dislocation recovery in the early, rapid stage appears to be controlled by vacancy diffusion between the dislocation links. The application of stress during recovery leads to an enhancement of the recovery rate in agreement with the network coarsening model whilst the incremental strains observed on reloading after recovery correlate well with the changes in dislocation structure produced during the recovery periods.List of symbols and appropriate values l dislocation link length - D _s self diffusion coefficient - b Burgers vector (2.5×10^–1 m) - C _j equilibrium jog concentration - dislocation link tension - k Boltzman's constant (1.38×10^–23 J atom^–1 K^–1) - T absolute temperature - t recovery time - M mobility term - Z frictional term associated with particles - _d dislocation density determined from micrographs - N _d number of dislocation intersections on test line - p length of test line - S foil thickness - ¯l mean dislocation link length - _c mean intragranular particle (carbide) spacing - r ₀ mean intragranular particle radius at timet=0 - r _t mean intragranular particle radius at timet - D solute diffusion coefficient - B solubility of M₂₃C₆ in austenite - particle-matrix interface energy - atomic volume (10^–29m³) - change in dislocation density during recovery period - incremental strain associated with reloading after recovery period - K constant - dislocation density - ₀ dislocation density at timet=0 - _t dislocation density at timet - ₀ friction stress associated with particles - constant (1) - shear modulus - angle between dislocation segments as dislocation breaks through a particle - A 1 cos (/2) - E constant - creep rate - F Taylor factor - L mean slip distance of dislocations - rate of dislocation recovery - stress - _y yield stress - J strength coefficient - _p plastic strain     

Behaviour of amorphous poly(ethylene terephthalate) annealed at T<T g by thermally stimulated currents

Thermally stimulated current (TSC) discharges in open circuit of amorphous poly(ethylene terephthalate) (PET) corona-charged electrets show a heteropolar relaxation at 87 °C, ₁ between and peaks. This relaxation tends to become homopolar when the sample is annealed at temperatures below the glass transition temperature. This is due to the formation of a trapped charge density on the surface of the material that originates, during the TSC discharge, a current that counteracts the one that results in ₁ This trapping effect, which initially is null, increases with annealing due to the rise in resistivity. On the other hand, TSC discharges in short-circuited annealed samples result in a heteropolar peak, ^*, that corresponds to ₁ The area of ^* increases with the annealing time in a bounded way. This peak is related to the formation of thermal nuclei (embryos) in the bulk of the material that act as heterogeneities. This suggests that ₁ is associated with a barrier-type polarization. If the annealed sample is heated to temperatures above the glass transition temperature, the tendency to the inversion of ₁ vanishes and ^* disappears, whereas and are modified. This suggests that relaxation is related to a Maxwell-Wagner-Sillars effect.     

A four-parameter corresponding-states method for prediction of Newtonian,pure-component viscosity

The extended Lee-Kesler (ELK) method, introduced for calculating thermodynamic properties of polar as well as nonpolar fluids and their mixtures, has been adapted to the calculation of Newtonian, pure-fluid viscosity. The method is a four-parameter, corresponding-states technique requiring as input the critical temperature, critical pressure, a size/shape parameter , and a polar interaction parameter . Because and have been previously tabulated for many fluids (for calculation of thermodynamic properties) and may also be obtained directly from the radius of gyration and a single liquid density, respectively, the method contains no adjustable parameters and is predictive in nature. ELK viscosity predictions were compared to experimental data for nonpolar and polar fluids. For 36 different nonpolar fluids and a total of 5748 different points, the comparison yielded an absolute average deviation (AAD) of 7.88% with a bias of –4.45%. Similarly, the AAD was 10.62% with a bias of –5.34% for a comparison of 15 different polar fluids involving 1500 different points. With this method, viscosities can be calculated within the range 0.55 T _r2.00 and 0<P _r10.     

On the mechanism of feathery crystallisation of aluminium

The growth of twinned crystals of aluminium on solidification is discussed with reference to the structure of the growing tip. It is shown that one must differentiate between two types of 112 growth directions in the twin boundary. Normally, all the tips grow in the same type of 112 growth direction and this particular type may give the tip a favourable shape, possibly by producing a sharp edge. If a tip is growing in a less favourable 112 direction, the favourable type can be established by the twins changing sides. This is observed to occur regularly in the continuous casting method according to Hunter. The re-entrant edge mechanism is not supported by the present results.     

Two shift theorems leading to lower bounds to eigenvalues

Summary Two theorems are proved by which the other bound can be obtained from the Rayleigh quotient, i.e., the bound which cannot be obtained by the Rayleigh-Ritz method.The method presented here utilizes a shift operation which is the redistribution of weight functions in the Rayleigh quotient.