Graded finite element simulation of thermal stress in inhomogeneous high-Tc superconductor

YBa₂Cu₃O_y is an orthotropic material with different material properties in a, b and c directions, such as Young’s modulus, coefficient of thermal expansion (CTE), and thermal conductivity. It is assumed that the material properties of inhomogeneous high temperature superconductor (HTS) vary with different height coordinate and temperature. A model is presented in this paper to calculate the thermal stress of inhomogeneous HTS when temperature decreases from ambient to operating conditions (cryogenic temperatures). By fitting a second order polynomial to the experimental data, value of the material properties of inhomogeneous HTS can be obtained. Then, through the proposed graded finite element method, the coupled thermo-mechanical equations were solved numerically. The numerical results show that the temperature profiles distribute the function of time after soaking. It is notable that the temperature profile reaches steady in a very short period of time, so the thermal stress suddenly increases to a very high level for a bulk superconductor. It is also shown that the closer to the sample internal region it is, the larger the heat fluxes are. Besides, the maximum tensile stresses, i.e. the peeling stresses, occur near bottom corner of inhomogeneous HTS. It is intended that the model presented in this paper could be useful to researchers who are interested in mechanical properties of inhomogeneous HTS.     

Vibrational Raman scattering temperature measurements

Experimental data and analyses are presented for the determination of gas temperature by measurements of vibrational Raman scattering intensity ratios of Stokes Q-branch fundamental bands. The method is demonstrated for two thermal equilibrium experiments: (1) CO₂ (a gas well-suited for use in multi-component mixtures near ambient temperatures) in a test cell, and (2) N₂ (a gas well-suited for use at elevated temperatures) in a flame. This method of temperature measurement is of particular value for non-thermal equilibrium conditions, for which vibrational excitation temperatures can be assigned to each pair of vibrational level corresponding to observable Raman bands.     

Vibrations of bonded beams with a single lap adhesive joint

The natural frequencies and loss factors of the coupled longitudinal and flexural vibrations of a system consisting of a pair of parallel and identical elastic cantilevers which are lap-jointed by viscoelastic material over a length a_c from their free ends have been investigated. A complete set of equations of motion and boundary conditions governing the vibration of the system are derived. The solution of these equations, subject to satisfying the boundary conditions, yields the desired natural frequencies and associated composite loss factors. The numerical results have been compared with those from two other approximate methods.     

In situ X-ray diffraction study of germanium at pressures up to 11 GPa and temperatures up to 950 K

In situ X-ray diffraction measurements on germanium were conducted in the pressure range of 5-11 GPa and temperatures up to 950 K. Using our data a better defined P-T diagram for germanium is presented. The coordinates of the triple point between Ge_I-Ge_II-Ge_L have been determined to a better degree of precision. The onsets of the Ge_I-Ge_II transition were found both under hydrostatic and non-hydrostatic conditions. Anisotropy of thermal expansion coefficient for the Ge_II is characterized from the c/a ratios in the temperature interval 473-823 K. Phases Ge_III and Ge_IV are shown to be metastable forms of germanium.     

Surface roughness analysis and improvement of PMMA-based microfluidic chip chambers by CO2 laser cutting

For the microfluidic chip, the surface roughness of the chamber sidewall is an important parameter in estimating its quality. In this work, the chambers of the polymethyl methacrylate (PMMA)-based microfluidic chip were fabricated by CO₂ laser cutting, and then the surface roughness of the sections cut using different laser parameters and ambient temperature was studied by a non-contact 3D surface profiler. Our observation shows that the surface roughness results primarily from the residues on the laser-cut edge, which are produced by the bubbles bursting. To reduce the surface roughness of the cut section, a new approach is proposed, that is preheating the PMMA sheet to a suitable ambient temperature during laser processing. The results indicate that at a preheat temperature of 70-90 °C, the surface roughness resulting from the cut would be reduced. In our experiment, the best result was that the arithmetical mean roughness is R_a = 100.86 nm when the PMMA sheet was heated to 85 °C.     

The Ettingshausen-Nernst coefficient and transport properties of alumel from 200 to 473 K

The adiabatic Ettingshausen-Nernst coefficient, Q_a, and the Righi-Leduc coefficient, R_a, have been measured on Alumel and Nickel. The results show that Q_a of Alumel increases with temperature and has a precipitous drop near its ferromagnetic-paramagnetic transition temperature (T_c =425.6 K). The experimental Q_a relates well with temperature measurements errors experienced using Chromel-P vs Alumel thermocouples in crossed thermal and magnetic fields.     

Determination of the thermal desorption kinetic parameters for samples with a temperature gradient

An application of the thermal desorption technique to the study of desorption from the samples with a temperature gradient is discussed. The kinetics of first- and second-order desorption from linearly and exponentially heated samples with a parabolic temperature profile is considered. It is shown that the low-temperature part of the thermal desorption curve is described by the same equations as those for the desorption from the nongradient surface with the less effective area and with the temperature equal to that at the center of the nonuniformly heated sample. The approximate analytical expressions for the amount of adsorbed surface species as a function of time are derived. These expressions enable to find the kinetics order, the activation energy E and the preexponential factor k₀ for the desorption process from thermal desorption spectra. In a first approximation the corrections for the nonuniformity of the sample temperature do not substantially change the value of E but slightly increase the value of k₀. The correction procedure for k₀ is described in detail. The possible application of the proposed method to various experimental conditions is discussed.     

Effects of radiation heat loss on laminar premixed ammonia/air flames

Ammonia (NH₃) direct combustion is attracting attention for energy utilization without CO₂ emissions, but fundamental knowledge related to ammonia combustion is still insufficient. This study was designed to examine effects of radiation heat loss on laminar ammonia/air premixed flames because of their very low flame speeds. After numerical simulations for 1-D planar flames with and without radiation heat loss modeled by the optically thin model were conducted, effects of radiation heat loss on flame speeds, flame structure and emissions were investigated. Simulations were also conducted for methane/air mixtures as a reference. Effects of radiation heat loss on flame speeds were strong only near the flammability limits for methane, but were strong over widely diverse equivalence ratios for ammonia. The lower radiative flame temperature suppressed the thermal decomposition of unburned ammonia to hydrogen (H₂) at rich conditions. The equivalence ratio for a low emission window of ammonia and nitric oxide (NO) in the radiative condition shifted to a lower value than that in the adiabatic condition.     

Conductivité thermique des feutres de carbone,isolants à forte anisotropie: modèle de conduction par la phase solide

Uses of carbon felts for high temperature insulation are reviewed. Effects of raw material and fabrication method on the felt properties are discussed. The connection between the thermal conductivity λ and the other felt properties are analysed. Evidence is given for the existence of three hierarchized anisotropy levels of λ: crystal lattice, fiber and felt. The two directional felt conductivities — the longitudinal λ_L and the transversal λ_T ones — are modelled using a definite geometry of the felt texture and a previously introduced expression for the constriction resistance of a fiber/fiber contact. λ_T depends on both components of fiber conductivity : axial λ_A (assumed to be equal to the longitudinal) and radial λ_R. The geometrical parameters considered are: felt porosity, fiber tortuosity, aspect ratio (fiber width to heigth), fiber mean size, contact spot size a and relative frequency of direct interlayer crossing, U. The experimental validation is currently only approximate, since measurements of λ_R, a and U are not accurate enough yet.     

Phase Transition and vibration properties of MnCO3 at high pressure and high-temperature by Raman spectroscopy

The high pressure and high-temperature behavior of MnCO₃ was investigated up to 55?GPa at ambient temperature and up to 573?K at ambient pressure by Raman spectroscopy, respectively. Some new modes were detected at ～16 and ～32?GPa, which were assigned to MnCO₃-I below 16?GPa and to MnCO₃-II above 32?GPa, and to a coexisting phase of them in between. The high pressure vibration properties of all Raman modes, especially high frequency modes, were systematically reported. The coexisting phase of MnCO₃-I and MnCO₃-II had much easier compressibility than the MnCO₃-II phase. The thermal stability of MnCO₃ was at least to 573?K and its thermal expansion along the c axis was easier than a and b axes.     

Thermal expansion of dysprosium tetraboride

The temperature variations of the interplanar spacings a(T) and c(T) in the crystal lattice of dysprosium tetraboride have been studied using X-ray diffraction in the temperature range 5?C300 K. Anomalous variations of a(T) and c(T) in the temperature range of magnetic transformations, anisotropy of the thermal expansion of DyB₄, and the monoclinic distortion of the crystal structure at low temperatures have been revealed. The magnitudes of the spontaneous magnetostriction, the thermal expansion coefficients ??a and ??c, and the exchange integrals Y _a and Y _c have been determined.     

High pressure X-ray diffraction and electrical resistance study of the quasi-one-dimensional sulfide InV6S8

The ambient structural details and the results of room temperature high pressure angle dispersive X-ray diffraction and electrical resistance measurements on the quasi-one-dimensional sulfide, InV₆S₈, to a pressure of 25 GPa are reported. The material does not undergo a phase transition in this pressure range, though an anomaly in the c/a ratio has been observed around 10 Gpa. A fit of the Murnaghan equation of state to the V/V₀ versus pressure data, with the value of the derivative of B₀ with respect to pressure, B′₀, fixed at 4 has yielded a value of the bulk modulus, B₀, of 110 GPa. We also present data of the pressure dependence of the lattice constants, a and c, the ratio c/a, and the resistance at room temperature.     

All-optical soliton switching for the asymmetric fiber couplers

Coupled nonlinear Schrödinger (CNLS) equations for the fiber couplers with asymmetric self-phase modulation (SPM) and cross-phase modulation (XPM) are studied. With symbolic computation, one- and two-soliton solutions are obtained for the constant- and variable-coefficient CNLS equations. Switching dynamics of the solitons is discussed, and effects of the second-order group-velocity dispersion β ₂, SPM coefficient σ ₁, XPM coefficient σ ₂ and Kerr nonlinear intensity γ on the all-optical switching properties are studied, while other coefficients in those equations are seen not to affect the all-optical switching properties. For the constant-coefficient CNLS equations, we find that |β ₂| is proportional to the optical switching speed, and the optical extinction ratios increase with the decrease of σ ₁/σ ₂ and increase of |β ₂| and γ. A numerical simulation by the split-step Fourier and Runge-Kutta methods is presented on the constant-coefficient CNLS equations to analyse the stability of the one- and two-solitons with the random initial perturbations. For the variable-coefficient CNLS equations, effects of σ ₁/σ ₂, β ₂(z) = a ₂ e ^bz and γ(z) = a ₃ e ^bz on the optical switching are analyzed (where a ₂, a ₃ and b are all constants, and z gives the direction of propagation in the fiber couplers): optical switching speed increases with the increase of |a ₂| and decrease of |b|, and optical extinction ratios increase with the increase of |a ₂| and decrease of σ ₁/σ ₂ and |a ₃|.     

Correlation between the high temperature anomalies in V2O3 and the lattice parameters

A single phase corundum type structure was observed in V₂O₃ from 165 to 1100K. An anomalous behaviour of the lattice parameters of the rhombohedral unit cell was found in the high temperature region where V₂O₃ undergoes a broad electrical transition. The unit cell edge a_R contracts above 165 K, reaching a minimum at 533 K, after which there is a sharp increase. Above 595 Ka_R expands continuously and then almost linearly above 775 K. The temperature dependence of the rhombohedral angle shows two distinct regions, and the volume expansion follows the variation of angle; the change of the rate of expansion coincides with the minimum of a_R. The contraction of a_R corresponds to a reduction in certain second-neighbor VV distances.Our results show that a gradual transition is taking place in V₂O₃ from an α type structure to a β type characteristic of the Cr-doped V₂O₃ compounds. In addition, the sharp dip of the a_R curve occurs at nearly the same temperature as the peak previously observed in small angle scattering of neutrons. It is presumed that the two effects are related, and that the critical-like behaviour of a_R is due to some dynamic magnetoelastic effect involving second neighbor VV interactions.     

Exciton condensation in intermediate valent Sm0.90La0.10S

Sm_1−xLa_xS for x more than a few percents are metals at ambient conditions. At low temperature and high pressure they develop a small gap in the order of some meV and become semiconductors or insulators. This has been interpreted as a manifestation of the excitonic insulator. In this Letter we will concentrate on Sm_0.90La_0.10S, which is the only composition showing a first order transition. Measurements of the volume change with pressure at ambient temperature show this first order volume collapse at 5 kbar with hysteresis. The resistivity is measured in function of temperature and pressure and exhibits also at 5 kbar and ambient temperature a first order phase transition to a more metallic state. At low temperatures and in function of pressure the resistivity exhibits a peak. The optical reflectivity at 300 K has been measured at low and high pressure and transforms with pressure above 5 kbar into the golden metallic phase.     

Theoretical analysis on the exergy destruction mechanisms and reduction under LTC relevant conditions

Low temperature combustion (LTC) is a potential thermodynamic pathway to maximize the thermal efficiency of internal combustion (IC) engines. However, high exergy loss is also observed within this combustion concept. The present study focuses on the homogeneous combustion process and examines the detailed exergy destruction mechanisms under representative LTC engine conditions. By varying both equivalence ratios (φ) and temperatures (T) at initial pressure of 50?bar, it is found that the decreased total exergy destruction fraction (f_ED) with increasing initial temperature mainly results from the decreased exergy destruction in the high temperature heat release stage, while using rich mixture can significantly reduce the f_ED in the ignition delay stage, which is dominated by the reactions involving large molecules (C7 species). Reaction pathway analysis reveals that the detailed exergy destruction sources are significantly affected by the reaction pathways. Furthermore, a qualitative exergy loss φ-T map was created to illustrate the exergy loss reduction potential. It is concluded that the combustion pathway that reforming the rich fuel/air mixtures before ignition followed by the low temperature combustion of lean reforming products offers the potential to simultaneously reduce exergy destruction and avoid soot and NOx formation. However, the potential advantages of this exergy reduction combustion concept still require further work.     

Structural, high pressure and elastic properties of transition metal monocarbides: A FP-LAPW study

The structural, elastic and thermal properties of four transition metal monocarbides ScC, YC (group III), VC and NbC (group V) have been investigated using full potential linearized augmented plane wave (FP-LAPW) method within generalized gradient approximation (GGA) both at ambient and high pressure. We predict a B₁ to B₂ structural phase transition at 127.8 and 80.4 GPa for ScC and YC along with the volume collapse percentage of 7.6 and 8.4%, respectively. No phase transition is observed in case of VC and NbC up to pressure 400 and 360 GPa, respectively. The ground state properties such as equilibrium lattice constant (a₀), bulk modulus (B) and its pressure derivative (B′) are determined and compared with available data. We have computed the elastic moduli and Debye temperature and report their variation as a function of pressure.     

Study of the P-T phase diagram of pyridinium perchlorate by X-ray diffraction and Raman spectroscopy

The crystal structure and vibrational spectra of deuterated pyridinium perchlorate (d ₅PyH)ClO₄ (C₅D₅NHClO₄) are studied by means of neutron diffraction in ambient conditions, X-ray diffraction at high pressures up to 3.5 GPa in the temperature range 297–420 K, and Raman spectroscopy at high pressures up to 5.7 GPa. Deuterated pyridinium perchlorate at ambient conditions has rhombohedral structure with the R3m symmetry (paraelectric phase I). Over the pressure range of 0.5–1.2 GPa, the phase II with monoclinic symmetry Cm exists. At pressure P ～ 1.2 GPa, the phase transition to monoclinic phase III with the Pm symmetry is observed at ambient temperature. The lattice parameters, unit cell volume, and frequencies of internal vibrational modes as functions of pressure are obtained for different phases of deuterated pyridinium perchlorate. The P-T phase diagram of (d ₅PyH)ClO₄ over the extended pressure and temperature range is discussed.     

Low-temperature x-ray diffraction studies of the crystallographic parameters and thermal expansion of (CH3)2NH2Al(SO4)2 · 6H2O crystals

The a, b, c, and β crystallographic parameters of the (CH₃)₂NH₂Al(SO₄)₂ · 6H₂O crystal (DMAAS) have been measured by x-ray diffraction in the 90–300-K temperature range. The thermal expansion coefficients along the principal crystallographic axes α_a, α_b, and α_c have been determined. It was shown that, as the temperature is increased, the parameter α decreases and b increases, whereas c decreases for T<T _c (where T _c is the transition temperature) and increases for T>T _c, so that one observes a minimum in the c=f(T) curve in the region of the phase transition (PT) temperature T _c ～ 152 K. The thermal expansion coefficients α_a, α_b, and α_c vary in a complicated manner with increasing temperature, more specifically, α_a and α_c assume negative values at low temperatures, and the α_a=f(T), α_b=f(T), and α_c=f(T) curves exhibit anomalies at the PT point. The crystal has been found to be substantially anisotropic in thermal expansion.