Study of dielectric and electrical properties of mortar in the early hydration period at microwave frequencies

The dielectric constant, , and electrical conductivity, , of mortars with various sand-cement ratios,s/c, were measured for the first 30 h hydration using microwave techniques in the frequency range 8.2–12.4 GHz. The and of the mortars were found to increase linearly with increasing water-solid ratiow/(s + c), but decrease with increasings/c. It was found that as long as thes/c values were the same, the rate of changes in and of the mortars were the same. It appears that thes/c is the key factor controlling the rates of changes in dielectric and electrical parameters of cement hydration in mortar. The relationship between compressive strength and dielectric and electrical properties of mortars was also discussed.     

Effective permittivity of layered dielectric sphere composites

A straightforward model is presented for analysing the effective permittivities of layered dielectric sphere composites. Using the present model, the effective permittivity, _eff, of layered dielectric sphere composites can be deduced using classical two-phase dielectric mixture formulae in two steps: first, the effective permittivity, _incl, of the inclusions is calculated by taking the layered dielectric sphere inclusions as sub-composites; and second, the effective permittivity, eff, of the composites is found by substituting the layered dielectric sphere inclusions with homogeneous spheres whose permittivity is equal to _incl. The present model is applicable to multi-layer sphere composites. Experiments on resin-based hollow bead composites show that the present model accurately predicts the effective permittivity of layered dielectric sphere composites.     

High-frequency dielectric behaviour of polycrystalline zinc substituted cobalt ferrites

The dielectric constant () and complex dielectric constant () of zinc substituted cobalt ferrites have been measured at room temperature in the high frequency range 100 kHz to 1 MHz. The values of dielectric loss tangent (tan ) have been computed from and . Plots of dielectric constant () versus frequency show a normal dielectric behaviour of the spinel ferrites. The frequency dependence of dielectric loss tangent (tan ) is found to be abnormal, giving a peak at certain frequency for all the ferrites under investigation. A qualitative explanation is given for the composition and frequency dependence of the dielectric constant and dielectric loss tangent. The dielectric constant for these mixed ferrites is approximately inversely proportional to the square root of the resistivity. A plot of dielectric constant versus temperature shows a transition near the Curie temperature. An attempt is made to explain the possible mechanism for this observation.     

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.     

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.     

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.     

The anomalous thermal boundary resistance in superfluid4He near Tλ

The anomalous thermal boundary resistance R of superfluid⁴He near the lambda point T was studied in a cell with parallel copper faces and with various fluid layer thicknesses. The study was made as a function of the heat current Q and reduced temperature (T — T)/T = . In all cases, R tends to a maximum value R_itmax at T = T. This value is independent of Q, and is reproducible for various experiments. This contrasts with the regular boundary resistance which can vary considerably between successive experiments. Near T, the limiting slope dR/d¦¦ is found to be proportional to Q^–1, and this leads to a scaled representation of the data. This analysis is extended to data of similar experiments with gold plated copper surfaces by Duncan and Ahlers and by Zhong et al. The measurements of R over the whole range of¦¦ where it is observable are discussed and compared with previous experiments.     

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.     

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.     

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     

Causes of the occurrence of an “effective” porosity in granular media

The cause of the difference between the total (geometric) and effective _ef (hydrodynamic) porosity of granular media is established.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 21, No. 6, pp. 1017–1024, December, 1971.     

Free convection in h horizontal porous layer with a partly heated or cooled wall

The natural convective flow in a fluid-saturated porous medium is considered for an infinite horizontal channel with the bottom wall being partially heated or cooled. The flow and heat transfer are analysed for a range of values of the two non-dimensional parameters which define the problem, namely the Rayleigh number Ra and aspect ratio . Numerical solutions are obtained for = 1 and = 0.1 for both heated and cooled cases and for a range of values of Ra. In the heated case, the nature of the flow is seen to change from unicellular for smaller values of Ra to multicellular for large Ra, with the value of Ra at this changeover being decreased as is decreased. Also, for this case, a range of values of Ra is found over which both unicellular and multicellular flows are possible. For the cooled case, a boundary layer is seen to develop on the bottom wall as Ra is increased for both the values of taken. Finally, a solution for 1 is obtained and is compared with the numerical solutions for = 0.1.     

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).     

The effect of ionizing radiation on dielectric properties of bovine achilles tendon collagen in the temperature range of thermal denaturation

The effect of -irradiation, with doses from 10²–2×10³ kGy, on the dielectric properties of solid-state collagen was studied. The temperature dependence of the constants and ' revealed a decrease in the denaturation temperature with increasing dose of irradiation. Dielectric dispersion observed in the frequency range 10 Hz to 10 kHz was suggested to be due to Maxwell-Wagner-Sillars polarization. In addition, an increase in the irradiation dose resulted in increasing activation energy of bovine achilles tendon collagen.     

Theory of dielectric polarization of a substance. Calculation of dielectric permittivity of water,ammonia, and chlorine

A theory of dielectric polarization of a substance is developed. The theory is verified by experiment and by phenomenological relations that follow from the determination of polarization, molar polarization, and dielectric permittivity.Notation _s static dielectric permittivity - high-frequency dielectric permittivity - _s permittivity perpendicular to the acisC - _s permittivity in the direction of the axisC - a average molecular polarizability - dielectric susceptibility - _i molecular hyperpolarizability - p ⁰ constant dipole moment of a molecule - p dipole moment of a molecule in condensed state - p _add additional dipole moment of a molecule - P polarization of a substance - P _m molar polarization - k Boltzmann constant - T Kelvin temperature - t Celsius temperature - angle between the vectors and - F internal electric field strength - Û internal interaction energy, J·mol^–1 - û internal interaction energy per molecule - N ₀ Avogacro number - V ₀ molar volume - a _t total molecular polarizability - H ⁰ (H) enthalpy as a function of temperature - l(x) Langevin function - n molecular concentration Murmansk State Academy of Fish Fleet. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 68, No. 5, pp. 767–773, September–October, 1995.     

Effect of intermittent polishing on low-cycle fatigue of pure aluminium

Low-cycle fatigue tests of aluminium were interrupted in order to remove surface roughening and superficial cracks produced by prior loading. Depending on the frequency of intermittent polishing and the strain level considerable increases in endurance life were observed.Nomenclature _t total strain range - _p plastic strain range - _e elastic strain range - N _p the number of cycles at which the intermittent polishing is carried out (single-step tests) - N _mp the number of cycles at which the last intermittent polishing action is performed in addition to preceding ones in every 50 cycles (multi-step tests) - N _pf the endurance life in the case of intermittent polishing (single- or multi-step tests) - N _f the endurance life without intermittent polishing     

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.     

Optimization of the designs of small-size helium cryostats

Using a computer program that simulates thermophysical processes occurring in small helium cryostats, the surface emissivities of actual cryostats are verified. The efficiency of the heat transfer from the walls of the reservoir suspension tube to the helium vapors is determined. The program makes it possible to optimize the designs of cryostats according to the specific conditions of their use.Notation Stefan-Boltzmann, constant - _bg, _ho, _sh, _res, _det emissivities of the background surrounding the cryostat, housing surface of the cryostat, radiation shield, helium reservoir, and radiation detector - T _bg,T _ho,T _sh,T _res temperatures of the background, cryostat housing, shield, and reservoir - _re1, _re2, _re3 reduced emissivities of the shield, housing, and reservoir - F _sh,F _ho,F _res surface areas of shield, housing, and reservoir - F ₁,F ₂,F ₃ areas of the entrance window in the cryostat housing, of the filter on the radiation shield, and of the inlet hole of the matching optics - ₁, ₂ emissivities of the filter and entrance window - ₁ solid angle at which the background radiation is absorbed by optical filter - ₂ solid angle at which radiation is transferred by matching optics to the radiation detector - ₁, ₂, ₃ transmission of radiation by way of the entrance window, filter, and matching optics - S cross-sectional area of the helium reservoir suspension tube - (T) temperature dependence of the thermal conductivity of the tube material - n ₁,n ₂,n ₃ number of mechanical couplings between the housing and the shield, electric wires, and mechanical couplings between the shield and reservoir - S ₁,S ₂,S ₃ cross-sectional areas of the mechanical coupling between the housing and shield, of the electric wire, and of the mechanical coupling between the shield and reservoir - l ₁,l ₂,l ₃,l ₄ lengths of mechanical couplings and electric wires over the body-shield and the shield-reservoir portions - ₁(T), ₂(T), ₃(T) temperature dependences of the thermal conductivities of the materials of mechanical couplings between the housing and shield, of electric wires, and mechanical couplings between the shield and reservoir - specific resistance of the material of electric wires - I working electric current - d inner diameter of the helium reservoir suspension tube - L ₁,L ₂,L ₃ lengths of the portions of the tube in the first, second, and third zones participating in heat exchange with helium vapors - U ₁,U ₂,U ₃ coefficients of heat conduction from the tube walls to helium vapors in the first, second, and third zones - T ₁, T ₂, T ₃ temperature differences in the corresponding zones of the tube - G ₁ weighted quantity of nitrogen evaporated per unit time - r ₁ latent heat of nitrogen vapor generation - G weighted quantity of helium evaporated per unit time - r latent heat of helium vapor generation - T _h temperature of helium vapors - c _p heat capacity of gaseous helium - G weighted quantity of helium evaporated per unit time due to heat supply only along the helium reservoir suspension tube All-Russian Scientific Center S. I. Vavilov State Optical Institute, St. Petersburg, Russia. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 68, No. 5, pp. 760–766, September–October, 1995.     

Anomalous normal state and high temperature superconductivity in the cuprates

It is generally believed that a high electronic density-of-states n(E_F), and therefore a low Fermi velocity v_F, are required to obtain a high transition temperature, since T_{c ph} exp(–1/) and = n(E_F)V. However, V = < I² >/M _ph ² , and Bardeen showed that I E_Fk_F = (1/2)k _F ² v_F. Thus one may expect that should increase with v_F. While it may not be feasable to increase the one-electron velocity v_F signifcantly, the velocity may increase greatly as a result of renormalization by electron-electron interactions. Such a renormalization exists in Hartree-Fock theory for an unscreened electron-gas. We found that for a medium-density electron-gas (r_s 10–20) imbedded in a background with a dielectric constant () such that / > 10, there is a significant increase of v_F by renormalization, even when screening is taken into consideration. The peak of v(k) at k = k_F is very narrow, the half width being somewhat less than the frequency ₀ at which () falls by a factor of 2. When () is due to ionic polarization, _o is a typical phonon frequency. The height of the peak v_F/v _F ⁰ is of order E_F/₀, and the width in units of momentum is: k/k_F (₀/E_F)². This velocity peak is associated with a minimum in the screening constant at E_p. Its sharpness causes the normal state properties to be highly anomalous; namely the conductivity is exceptionally high, with an anomalous temperature dependence; the conductivity anisotropy, thermoelectric power, and Hall constant are anomalous. Direct determination of v(k) in YBCO by several methods indeed suggests a large, sharp peak, the width being of order 10 me V. This peak manifests itself in the tunneling and point-contact spectroscopy I–V curves. We suggest that the reduced screening at the Fermi level, associated with this velocity peak, is responsible for the high T_c of the cuprates and several other exotic superconductors. Thus, the high T_c is a reflection of the anomalous normal-state properties.