Longitudinal dielectric function and collective surface oscillations in superconductors

The electrodynamic approach to the problem of collective oscillations in superconductors is considered. The kinetic equation for quasiparticles is used to find the longitudinal dielectric function ₁ (, k) of a pure superconductor at ( is the collision frequency). With the help of the evaluated ₁ (, k) the existence of collective oscillations localized near the boundary of the superconductor is investigated.     

High-field electrical conduction in m Hgl2

The electrical and dielectric properties of illuminated Hgl₂ were studied at room temperature under various a.c.-signal amplitudes in the frequency range 1 Hz to 10 kHz. Below 40 H, We real part of the dielectric constant, ,was found to vag slightly with voltege for low electric fields (E < 10³V cm^–1, above which it showed a steady increase with the applied voltage. At higher frequencies, no voltage dependence of (or the geometrical capacitance) of the crystal was observed. On the other hand, the imaginary part of the dielectric constant, or the a.c. conductivity, (=_o) was found to decrease considerably with the applied voltage forE < 10³V cm^–1 at all frequencies. For higher fields (E > 10³V cm^–1), exp [C(E/)^1/2], whereC is e constant. Above 40 Hz, this variation was in good agreement with the behaviour of the bulk, resistance of the crystal. Such behaviour is discussed in the view of Richardnon-Schottky and Poole-Frenkel conduction mechanisms, which seem to be operative in Hgl₂ at room temperature.     

Thermal transport properties in helium near the superfluid transition. I.4He in the normal phase

The thermal conductivity and the associated relaxation time to reach steady-state conditions are reported for the normal phase of several very dilute mixtures of³He in⁴He (X<4 × 10^–6) at saturated vapor pressure near T. The measurements were made over the reduced temperature range 2.5 × 10^–6<<2×10^–1, where (T–T)/T, and are representative for pure⁴He. The spacing between the cell plates was 0.147 cm. The systematic uncertainty in the conductivity data is estimated to increase from 2% for =0.2 to 4% for =3 × 10^–6. The random scatter due to finite temperature resolution increases to 7% at the smallest . The data are in agreement within the combined uncertainty with recent ones by Tam and Ahlers (cell F, spacing 0.20 cm) and with previous ones in this laboratory taken with a different plate spacing. The thermal diffusivity coefficientD _T = / C _p obtained from is found to agree within better than 15% with the calculated one using data for , the density , and the specific heatC _p . Measurements of the effective boundary resistivityR _b in the superfluid phase are described.R _b is found to depend on the thermal history of the cell when cycled up to 77 K and above. Also,R _b shows the beginning of an anomalous increase for ¦¦10^–4. The possible reasons for this anomaly are discussed, and their impact on the analysis of conductivity data in the normal phase is appraised.     

Theory of motional inhibition of interlayer quantum tunneling in thin 3He films

An attempt is made to interpret NMR data on ³He films for coverages just over one monolayer in terms of motion due to the quantum exchange of particles between layers. A summary of the relevant data and of various possible relaxation mechanisms is given and it is found that a portion of the data seems amenable to an interlayer exchange interpretation. The detailed theory of this process requires the use of the exchange operator concept and a Kubo-theory treatment of the effect of second-layer motion on the exchange process. It is shown that the bare interlayer exchange process characterized by constant J ₁₂ is slowed by second-layer translational motion so that the effective exchange parameter becomes \~J ₁₂ J ₁₂ ²/₂, where ₂ is a second-layer single-particle translational energy. In order to fit the NMR data it is found that ₂ must be evaluated in the classical limit rather than the degenerate Fermi limit, and that \~J ₁₂ J ₁₁, the exchange energy within the first-layer solid. These conditions require a helium second-layer effective mass of m ^* 5m and J ₁₂600J ₁₁, which are anomously large values for these parameters. The relation of this analysis to the problem of surface ferromagnetization in bulk ³He is also discussed briefly.     

Environmental stress cracking of glassy polymers and solubility parameters

The use of solubility parameters to predict critical stress ( _c ^* ) or strains (_c) for environmental cracking/crazing in several glassy polymers (e.g. PMMA, PPO, PS, PVC, PSF and PC) is re-examined. It is shown that the enthalpic component ( _H ) of the Flory-Huggins semi-interaction parameter () does not always give a good correlation between _c and _H even though solvent molar volume and polymer-solvent molecular interactions have already been considered. Re-analysis of available experimental data using Gent's theory shows that there is a general trend for _c ^* (or _c) to increase with . These results, therefore, support Gent's proposed mechanism of environmental stress crazing/cracking. It is finally concluded that unless a definite relationship can be established between _c or _c ^* with _H it is not possible to predicta priori _c or _c ^* , given the empirical solubility parameters of a solvent. Unfortunately, there are not many such relationships as discovered in this paper.     

Thermal boundary resistance in superfluid4He nearT λ

New measurements of the boundary resistivity in superfluid⁴He (2ppb³He) nearT are reported as a function of ¦¦ ( =T/T (Q) — 1) and of heat flux Q in a cell with parallel polished copper surfaces. Here we call T(Q) the temperature where the superfluid state abruptly disappears. In this design, the sidewall gaps between the copper pieces and the stainless steel spacer were eliminated. In contrast to several previous experiments but in agreement with those of Li and Lip a, no largeQ-dependent boundary resistivity anomaly was detected. However, as ¦¦ 0 the small weakly divergent resistivity was observed and its dependence onQ over the experimental range 1 <Q < 80 W/cm² was found to be very small. These new results are compared with previous experiments and predictions. An explanation of the previously observed anomalous transport phenomena is presented in terms of a heat flow through the sidewall gaps in these cells, and its limitation by a critical flow value _c. This phenomenological model can be fit satisfactorily to the observations. In the appendix we calculate _c from mutual friction.     

The surface of liquid 4He,based on the idea that π i<j f(r ij) describes a droplet

We argue that the wave function f(r _ij) describes the ground state of a droplet of liquid ⁴ He. With this wave function, expressions for the surface energy and the surface tension of liquid ⁴ He at T = 0 are derived. Choosing particular f(r) and density profile, and the simplest pair correlation function, we plot the variation of and with surface thickness t. For slow variation of density at the surface, becomes proportional to t. The surface thickness is found to be about 4 ». The inclusion of phonon zero-point motion correlations in the wave function leads (at T = 0) to a –R ² log R term in the energy of a droplet of radius R, implying a logarithmic divergence in both and . At T > 0 the phonon correlations give a log T dependence of and and a negative bulk specific heat. Suggestions as to the reason for these problems are explored, but no definite conclusions are reached.     

Low endurance fatigue in metals and polymers

The fatigue behaviour of commercially pure aluminium and of nylon under sequentially varying strain amplitudes is compared with a damage law of the type suggested by Miner. Aluminium obeys such a law for both cyclic and uniaxial prestrains but the behaviour of nylon is significantly affected by microcracking, which produces a marked effect of loading sequence.Appendix N Number of strain cycles at a given time - N _f Value of N at failure - True tensile stress - True stress range for a strain cycled specimen - _h Value of at half the life of the specimen - True tensile strain - Total true strain range - _p True plastic strain range (= the breadth of the hysteresis loop at = 0) - _d True diametral strain range - E Young's modulus - Linear strain hardening rate when tested at a particular value of _p - D Damage due to cycling - D _p Damage due to prestrain - _p Prestrain. C, K, K₁, , are constants     

Mass transfer of a single bubble in a minimally fluidized granular bed

The transfer of a gas admixture through the boundary of a cloud of closed circulation of gas is examined with a view to both molecular and convective dispersion.Notation a particle radius - c concentration of the admixture - D, D _c tensors of effective diffusion coefficients and of coefficients of convective dispersion, respectively - D parameter in (14) - D₀, D_m coefficients of molecular diffusion, respectively not taking and taking compressibility into account - F, f functions in (17) and (18), respectively - f fraction of the volume of the sphere with radius R_B occupied by the wake of the bubble - g acceleration of gravity - K mass-transfer coefficient determined in (22) - k coefficient in (7) - L coefficient in (22) - p, s functions determined in (13) - Q, q flow of admixture for the entire bubble and local flow, respectively - r_B, R_c radii of bubble and of cloud of closed circulation, respectively - r radial coordinate - U lift velocity of the bubble - u=v-w; u₀=u_*/u_* minimum velocity of fluidization - V volume of bubble - v, w mean gas velocities in the gaps between particles and of the particles, respectively - parameter determined in (8) - , parameters from (14) - porosity of the dense phase and of the cloud - polar angle - independent variable introduced in (12) - p = 1 - flow function Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 2, pp. 323–333, August, 1980.     

Polypyrrole Based Microwave Absorbers

Reflection of microwave radiations from single layer and two-layer materials is calculated. Microwave absorbing materials are formulated by mixing a commercially available paint or rubber with the conducting polypyrrole (PPy) powder. The reflection loss strongly depends on thickness and complex permittivity of the material. For a single layer material, optimum values of the real part, , and imaginary part, , of the complex permittivity are found by calculations which lead to a minimum reflectivity at a given sample thickness. The ability to readily tailor the conductivity of the PPy powder enables the design of microwave absorbers according to theoretical desired values of and . A paint panel containing 2 wt% of PPy powder with a thickness of 2.5 mm exhibits a reflectivity < – 10 dB (i.e. at least 90% absorption of the incident radiation) over 12 to 18 GHz. Blending and milling during the manufacturing process can destroy the original fibrous shape of PPy aggregates leading to low radiation absorption. In an attempt to achieve a broadband absorber, a two-layer system consisting of a first layer containing PPy powder and a second layer containing carbonyl iron has been fabricated.     

Problem of measuring the thermophysical characteristics of thin resistive and dielectric films

We consider the theoretical and experimental possibilities of determining a complex of thermophysical characteristics of thin films on a substrate, on the basis of a nonstationary method with boundary conditions of the fourth kind.Notation time - x coordinate perpendicular to the film surface - ₂, ₀, ₁ coefficients of thermal activity (=/a) of the substrate, the film, and the secpnd body - T₀₀ initial temperature of the film-substrate system - T₁₀ initial temperature of the second body - =T₁₀ – T₀₀ T ₀ ⁰ temperature at the boundary after contact as 0 - T ₀ ^t8 temperature at the boundary after contact as Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 42, No. 1, pp. 98–101, January, 1982.     

Dynamic scaling and ultrasonic attenuation in 4He near the superfluid transition point

Attenuation of first sound has been measured in ⁴He under saturated vapor pressure near the lambda temperature T at frequencies /2 ranging from 10.2 to 271 MHz. The frequency dependence of the critical part of the attenuation is determined and the dynamic scaling hypothesis is examined. Above the lambda point, it is found that the critical attenuation is described by a scaling function (, ) = ^1+y F(), where = ₀^–x and = T/T – 1, with the results x = 1.02±0.05 and y = 0.33±0.03. The characteristic frequency of the order-parameter fluctuation with the wave number k equal to the inverse correlation length is then proportional to ^x , which is in an excellent agreement with the prediction of dynamic scaling. Below the lambda point, a characteristic relaxation time or times shorter than previously expected at lower frequencies appears to exist in the present frequency range.Based on a Ph.D dissertation submitted by K. Tozaki to the University of Tokyo (1977).     

Analysis of the effect of the decomposition of char-forming heat-insulating materials on the free discharge of a gas mixture

The article presents results of a numerical solution of a nonsteady problem on the free discharge of a mixture of gases from a hemispherical volume with allowance for thermal decomposition of heat-insulating materials.Notation V volume - S area - t - P p - T - u v - Q q, dimensional and dimensionless time, pressure, temperature, TIM decomposition rate, and heat flux - adiabatic exponent - R gas constant - density - H specific enthalpy - c specific heat - thermal conductivity - , , _s dimensionless complexes - coefficient expressing the radiative properties of the gas medium and the heat-transfer surface - Stefan-Boltzmann constant Indices 0 initial state and scale factors - s surface - coke - M TIM material - P pyrolysis front - A ablation front - v volatile degradation products - adiabatic conditions - c completion of discharge Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 54, No. 5, pp. 787–793, May, 1988.     

Rejuvenation procedures to recover creep properties of nickel-base superalloys by heat treatment and hot isostatic pressing techniques

The rejuvenation procedures to recover the creep properties of nickel-base superalloys by atmospheric pressure heat treatment and hot isostatic pressing techniques have been reviewed in detail. It is very important that such treatments be applied at an optimum stage in the service life of a turbine blade. In other words, the rejuvenation procedures must be applied early enough to prevent catastrophic failures or irreparable damage and late enough to give a cost-effective benefit. The optimum stage at which to undertake a rejuvenation procedure to extend the creep lives of superalloys is immediately prior to the tertiary stage. By using these techniques it is not possible to extend the creep lives of superalloys indefinitely because of the accumulation of some permanent damage incurred during service conditions.Nomenclature ERF Economic repair factor - P _r Price of repaired and rejuvenated part - P _n Price of new part - L _n Potential operational life of new part - L _r Potential operational life of repaired/rejuvenated part - N Cavity density or number of cavities per unit area (mm^–2) - n _v Number of cavities per unit volume (mm^–3) - Creep strain - ₁ Maximum principal stress (MPa) - ¯ von Mises effective shear stress (MPa) - t _f Time to failure - t _t Time to commencement of tertiary creep - Creep damage tolerance parameter - _f Strain at fracture (or failure) - T _m Absolute melting temperature - ₀ Friction stress - r Spherical radius of cavities - 2x Intercavity spacing - Grain boundary width - P _I Cavity gas pressure - P _H External hydrostatic pressure - Atomic volume - k Boltzmann constant - T Absolute temperature - Surface energy of the cavity - D _b Grain boundary diffusion coefficient - _d Ductility recovery parameter - Strain to reach the same acceleration after recovery annealing - ₀ Strain necessary for standard material to reach a given acceleration of the secondary-creep rate in the tertiary region - _t Strain needed to have produced the reduced cavity volume after rejuvenation annealing - Creep rate - Secondary or minimum creep rate - ₁ Strain previous to the regenerative annealing period - n Total number of strain/regenerative anneal cycles - _v Recovery parameter for cavity volume - V ₀ Original total cavity volume at the start of the recovery - V _t Cavity volume after recovery annealing for a timet     

Dielectric properties of polyarylate blends

Dielectric properties and molecular motion were studied by use ofdielectric spectroscopy and differential scanning calorimetry for twoblends, fully transesterificated polyarylate of bisphenol A withterephthalate/isophthalate (50/50) (PA)/polycarbonate of bisphenol A(PC) blends and PA/poly(ethylene terephthalate) (PET) blends. All thequenched PA/PC and PA/PET blends were amorphous and the glasstransition temperature (Tg) was varied with the blends ratioaccording to Gordon-Taylor equation. The PA/PET blends with more than60% of PET crystallized above the crystallization temperature. ThePA/PC and PA/PET blends showed two dielectric relaxations, above Tg and below Tg, which are related to a glasstransition and a local motion of short segment, respectively. The relaxation moved to lower temperatures as PC or PET contentincreases, reflecting the lowering Tg faithfully. In the PA/PETblends, the static (₀) and the limiting dielectricconstant (), and the increment () for the relaxation decreases with increasingtemperature and the decrease falled on one curve, independent of theblend ratio. Any ferro- and piezoelectricity were not observed fortwo blends.     

The dielectric properties of aluminium nitride substrates for microelectronics packaging

The dielectric behaviour of sintered polycrystalline aluminium nitride substrates has been examined over the frequency range 500 Hz to 10 MHz and correlated with composition and microstructure. For pure, white AlN at 20 ° C both the permittivity () and dielectric loss () are frequency independent giving = 9.2±0.05 and tan = (2.1±0.1) × 10^–3. The permittivity is less than for pure alumina substrates ( = 10.2) but tan compares favourably, with that (1.4 × 10^–3) of alumina, which though used more widely has a thermal conductivity some eight times less than that of AlN. The addition of impurities, particularly iron, to give opaque black AlN causes large, frequency dependent increases in ; at 500 Hz the loss is seven times that of pure white AlN and is two times greater above 100 kHz. The temperature coefficient of permittivity [( – 1)( + 2)]^–1 [/T]_p between –180 and +180 ° C for pure white AlN is 1.05×10^–5 K^–1 which is similar to the value of 9×10^–6 K^–1 for pure Al₂O₃. For impure black AlN the coefficient below 20 ° C is the same but above 20 ° C there is a rapid, non-linear increase of with temperature. Below 180 ° C for pure white AlN and 20 ° C for impure black AlN the values of temperature coefficient are frequency independent at least up to 200 kHz.     

Investigation of microstructure of molybdenum—copper black electrodeposited coatings with reference to solar selectivity

Optical and microstructural properties of electrodeposited molybdenum-copper (Mo-Cu) black coatings have been studied with reference to their selectivity in absorption of solar radiation. Such coatings were found to have a solar absorptance, , about 0.87 and low thermal emittance, , such that the selectivity, /, was 3.6. Electrodeposited molybdenum-black coatings generally have selectivity /3. The oxidation state of molybdenum in (Mo-Cu) black coatings as determined by X-ray photoelectron spectroscopy is about + 5 (which is fairly close to that of Mo₄O₁₁). Large numbers of irregular particles were found on the surface of molybdenum-copper black coatings. There is evidence that the particles contain copper oxide.     

Dielectric properties of thin solid films formed on silicon

We report on the measurement of the frequency-dependent complex permittivity, ()=()-i(), over the frequency range, 30 MHz to 6 GHz, of silicon wafers and of thin dielectric films formed on silicon. Measurements, as a function of temperature and time treatments, were obtained by means of an HP Network Analyzer and dielectric probe and the resulting ()and()plots for the silicon wafers are shown to have a Debye-type [1] profile, thereby indicating that the associated polarization mechanism is of the orientational variety.     

Calculation of mixed-mode stress field at a sharp notch tip using <Emphasis Type="Italic">M</Emphasis><Subscript>1ɛ</Subscript>-integral

Direct computation of the mixed-mode stress field at a sharp notch tip appears to be difficult in that the mode I and mode II asymptotic stresses are in general governed by different orders of singularity. In this paper, we first present a path-independent integral termed M₁. The relation between M₁ and the generalized stress intensity factors is then derived and expressed as function of the notch angle. Once the M₁-integrals are accurately computed, the generalized SIF's and, consequently, the asymptotic mixed-mode stress field can thus be properly determined. No extra complementary solutions are required in the formulation. Further, no particular singular elements are required when the integration is performed by using finite elements. This work has been partially supported by National Science Council Grant No. NSC90-2211-E-008-040 to National Central University.     

Synthesis of the mineral geikielite MgTiO3

The preparation of pure geikielite, MgTiO₃, is carried out by precipitation with NaOH solution at pH 12 from solutions Ti⁴⁺/Mg²⁺ in molar ratio 1/1 M and calcination of the precipitate at 600°C in a dynamic atmosphere: heating rate 300°Ch^–1.At room temperature and a 1 KHz of frequency the dielectric constant () and the electric resistivity () were measured. The values were = 1 5.0 and = 1.1×10¹⁰ cm^–1.