Constant Temperature Field Research Articles

Determination of pinning potential energy for TlBaCaCuO superconductors

A new method designed to determine flux pinning potential U o is described. The method is simply a measurement of flux jump at either constant temperature or constant field. At 4.3 K, we recorded the large hysteresis loop with several flux jumps by sweeping fields up to 8 Tesla for a Tl 2Ba 2Ca 2Cu 3O x sample. At 0.03 Tesla, we recorded also a temperature dependence of magnetization in temperatures between 60 K and 130 K in a cooling approach described in text and found flux jump as well. Based on the GL theory, these flux jumps are analyzed and U o of 1 eV at the both cases have been obtained for the polycrystalline Tl 2Ba 2Ca 2Cu 3O x sample. These are larger than the reported thermal activation energy for cuprate superconductors remarkably. Some relevant issues are discussed.

Elastic and thermoelastic characteristics of composites reinforced with unidirectional fibre layers

Composites reinforced with infinite, unidirectional cylindrical fibres are considered. It is assumed that the thermoelastic characteristics of every fibre are functions of the distance from its axis of symmetry. The material of the medium and the fibres is transversally iostropic, with the axis of isotropy coinciding with the direction of the reinforcing fibres. The problem of thermoelasticity for a medium containing an isolated, cylindrically inhomogeneous inclusion in constant external stress and temperature fields is considered first. The problem is solved using the method developed in /1/. An efficient numerical algorithm for constructing the solution is given for the case of a cylindrically layered fibre. The interaction between the fibres in the composite is described using the effective (selfconsistent) field method /2, 3/. Tensors of effective moduli of elasticity and coefficients of linear expansion of the composities reinforced with cylindrically layered fibres are constructed. Formulas for estimating the concentrations of microstresses in the fibres within the composite are obtained, and some numerical results are given.

Model of formation of a hydraulic-pulse densified aluminum coating

Calculations of the temperature fields characterizing the formation of a hydraulic-pulse densified aluminum coating have enabled the temperature distribution after the passage of the unloading wave (during the formation of the pulse densified coating) to be determined with greater accuracy: The temperature of the inner zone of the coating falls to a constant value, while that of the outer zone rises to that level. A constant temperature field is maintained for a period of time sufficient for the formation and annealing of the coating.

Phase-locked yttrium iron garnet magnetometer for remote measurement of small field changes in a fluctuating background

The ferromagnetic resonance frequency in small yttrium iron garnet (YIG) spheres placed in a rf filter structure is used to measure small changes in large magnetic fields in the presence of high-level random fluctuations of the background field. (Such measurements are needed for fusion plasma diagnostics.) This YIG magnetometer probe is driven by a microwave network analyzer whose working frequency is phase locked to the probe resonance, yielding an output frequency that is linearly related to the magnetic field magnitude. The YIG filter’s phase response is used to control the system frequency, allowing measurement resolutions almost two orders of magnitude better than the YIG resonance 3 dB linewidth. Noise is effectively removed by the phase lock circuitry’s low cut-off frequency and by time averaging using a frequency counter. Accuracy is essentially limited by the uncertainty in determining the constants in the linear field-frequency equation and by maintenance of constant temperature and magnetic field direction during incremental measurements.

Natural convection between concentric spheres in a slightly-thermally stratified medium

In the present paper, analytical solutions are obtained using perturbation expansion in powers of Grashof number for steady, axisymmetric flow of a viscous fluid contained between two concentric spheres. A uniform gravity field acts vertically downward. The outer sphere is assumed to be maintained at a variable temperature such that conditions for vertical stratification are satisfied. Analysis is presented for two cases: when a constant-heat-flux condition on the inner sphere surface is imposed or when its surface temperature is kept constant. Streamlines, isotherms and velocity components are shown graphically in an axial plane for each case. For the case of isothermal inner sphere, a dimensionless stratification parameter S governs the flow. Solutions for S = 0 correspond to the unstratified case. When S tends to infinity, the flow pattern has both vertical and horizontal symmetry. But when the inner sphere surface is kept at constant heat flux, the flow and temperature fields are governed by another dimensionless parameter Q. The case Q = 0 corresponds to thermally insulated inner sphere. For this case, flow is similar to that occurring when S tends to infinity, but the directions of the streamlines are reversed.

A model for gas phase chemistry in interstellar clouds. I - The basic model, library of chemical reactions, and chemistry among C, N, and O compounds

A time-dependent gas phase chemical model of the chemistry in interstellar clouds is presented which uses a comprehensive library consisting of over 1400 reactions for 137 species. Specified constant density, temperature, and radiation field intensity are used to generate the evolution with time of these species for 10/sup 7/ years in four model clouds to simulate the outer and inner components of zeta Oph and of Orion. No a priori assumptions are made concerning production and loss mechanisms, and the algorithm decides which are the major processes from an extensive list of potential production and loss mechanisms for each species. The C, N, and O families of chemical species are discussed in detail with particular emphasis on production and loss processes, and on the interrelationships among the chemistries of these families. A number of diagrams are used to illustrate the chemistry in interstellar clouds resulting from the models. The utility of the model is that it allows for easy testing of schemes for the chemistry and chemical evolution in interstellar clouds.

Two-dimensional magnetic flux shell model for magnetohydrodynamic simulations

A magnetic flux shell model is described which is suitable for numerical simulations of cylindrically symmetric plasmas. The magnetohydrodynamic equations in magnetic field line coordinates are derived including the effects of finite electrical conductivity and anisotropic heat conductivity. Subsequently, the equations of motion of coaxial plasma shells of constant density, temperature, axial velocity, and axial magnetic field are derived including (a) full radial dynamics and (b) radial pressure balance. Sample simulations of a laser heated solenoid plasma and a ϑ-pinch reactor are described.

Ultrasonic study of terbium in a magnetic field

A model is presented which describes longitudinal ultrasonic propagation characteristics in magnetic materials. The model specifically incorporates magnetoelastic and exchange interactions when an external magnetic field is applied. Spin fluctuations and spin polarization are seen to play a dominant role, and particular attention is focused on the effect of critical fluctuations near the magnetic-phase-transition temperatures. The model is applied to single-crystal terbium. Longitudinal ultrasonic attenuation data are presented under conditions of constant temperature and constant magnetic field. The constant-temperature data reveal a very large attenuation enhancement by a magnetic field in the paramagnetic region. The constant-field data show anomalous attenuation maxima even when the applied field is sufficient to quench the spin-spiral state; a magnetic phase diagram for terbium is deduced from the constant-field data. A quantitative comparison between the experimental results and theory is presented for the constant-temperature data, and a qualitative comparison between experimental results and theory is presented for the constant-field data. Good agreement is achieved.

A thermal-hereditary theory for linear viscoelasticity

A constitutive law is presented that predicts the mechanical response of a linear viscoelastic material in a time varying temperature field. The development is based upon a modification of the superposition theory and includes the hereditary effects of temperature; that is, the temperature at a previous time can influence the response at the present time. The mathematical structure of the two resulting constitutive equations is relatively uncomplicated and has the advantage of predicting the response of the material in a time varying temperature environment from data obtained in experiments run in time and spatially constant temperature fields.

Paramagnetic Effect in Superconductors Part I. Quasi-Paramagnetism and Resistance of Indium at the Superconducting Transition

As a continuation of the previous investigation on the paramagnetic effect in tin we have investigated the same effect in indium. The minimum of current I 0 necessary for the occurrence of the paramagnetic effect has been represented by the same functions of the temperature T and the maximum of external magnetic field H 0 , beyond which we cannot observe the quasi-paramagnetism at that temperature, as in the case of tin i.e. I 0 =ξγ d ( T c - T ) and H 0 =ξ( T c - T )- I g /γ d . Here I g , ξ and T c are characteristic constants of the superconductor and have values 0.6 amp., 94.4 oersted/deg. and 3.422°K respectively for the case of indium: γ is 0.27 amp./mm. oe. The maximum of external magnetic field H 0 and the specimen diameter d are measured in oersted and in mm. respectively. These formulas are understood in a good approximation as those for the intersection of the plane I = I g +γ d H with the transition surface in the ( I – H – T ) space. In parallel with the measurement of the magnetic flux, plotted as a function of the current at constant temperature and external magnetic field, a measurement of the resistance of the same specimen has been performed. The results obtained are in good accordance with an expression derived in a modified form from the theory proposed by H. Meissner.