A.c. conductivity for amorphous TeO2-P2O5 glass system

The a.c. conductivity for the TeO₂-P₂O₅ glassy system was measured in the temperature range 300–573 K and in the frequency range 100 Hz to 10 kHz. The a.c. conductivity () increased with frequency according to the relation ()^s. The frequency exponent s was found to decrease with increasing temperature. The composition dependence of the conductivity was also investigated. The density of states was also calculated using the Elliott model. The a.c. conductivity increased over the studied temperature range. The obtained experimental data have been analysed with reference to various theoretical models. The analysis shows that the correlated barrier hopping (CBH) model is the most appropriate mechanism for conduction in the TeO₂-P₂O₅ glass system.     

Electrical conductivity,TEP and dielectric studies on mol% 66.6 Agl-22.2 Ag2O-11.1 (0.8 V2O5-0.2 P2O5) electrolyte material for solid state batteries

In the superionic conducting quarternary system Agl-Ag₂O-V₂O₅-P₂O₅, the best ionic conductivity was obtained for the composition 66.6% Agl-33.3% (2Ag₂O-1 (V₂O₅-P₂O₅)), when the GF/GM ratio was varied from 0.20 to 5.0. Then fixing the GF/GM ratio at 0.50, the ratio of the glass formers V₂O₅ and P₂O₅ were varied and the highest conducting composition was obtained as 66.6% Agl-22.2 Ag₂O-11.1% (0.8 V₂O₅-0.2 P₂O₅). A preliminary investigation using this material in the form of an electrolyte in a solid state electrochemical cell is reported. The polycrystalline and amorphous compounds were prepared from the same melt, by open air crucible melting and the rapid quenching technique. The ionic conductivity for the best conducting polycrystalline (hence referred as 66VP82P) and amorphous (66VP82G) samples was obtained as 8.3 × 10^–3 and 4.2 × 10^{–2 –1} cm^–1 respectively. The electronic conductivity of the order 10^{–10 –1} cm^–1 was observed for 66VP82G and 10^{–8 –1} cm^–1 for 66VP82P samples. Thermoelectric power studies revealed that the charge carriers are the Ag⁺ ions, with an activation energy of 0.288eV for 66VP82G, which correlated well with the activation energy obtained from the conductivity measurements. The dielectric constant, dielectric loss and the loss tangent were calculated for both polycrystalline and glassy 66VP82 material. It was observed that the dielectric loss is more for the glassy material than the polycrystalline material. Solid state galvanic cells with 66.6% Agl-22.2% Ag₂O-11.1% V₂O₅, 66.6% Agl-22.2%-Ag₂O-11.1% P₂O₅ and 66.6% Agl-22.2% Ag₂O-11.1% (0.8 V₂O₅-0.2 P₂O₅) (coded as 66V, 66P and 66VP82 respectively) electrolytes were constructed. Both polycrystalline and amorphous electrolyte cells were fabricated for a comparative study and the polarization effects were observed to be negligible in amorphous cells. The variation of open circuit voltage with temperature was reported and the current discharge curves indicate that the 66VP82 material has higher current capacity with high current drain when compared to 66V and 66P cells.     

Electrical conductivity in hot-pressed nitrogen ceramics

The a.c. and d.c. electrical conductivities of some hot-pressed polycrystalline nitrogen ceramics have been measured between 400 and 1000° C. The materials examined were Si₃N₄, 5.0% MgO/Si₃N₄ and two sialons, Si_(6–z) · Al _z · O _z · N_(8–z) having z 3.2 and z 4.0 respectively. The electrical behaviour of all the materials showed similar general features. The d.c. conductivities were about 10^{–10 –1} cm^–1 at 400° C and rose to between 10^–6 and 10^{–5 –1} cm^–1 at 1000° C. The a.c. Data, taken over the frequency range 15 Hz to 5 kHz showed that below about 500° C the a.c. conductivity ( _a.c.) varied with frequency as _a.c. ^s where 0.7 _d.c.) agreed well with the relation _d.c. = A exp(–B/T ^1/4). Above 700° C both the a.c. and d.c. conductivities followed log T ^–1. Hall effect and thermoelectric power measurements enabled the Hall mobility to be estimated as less than 10^–4 cm² V^–1 sec^–1 at 400° C and showed that the materials were all p-type below 900° C and n-type above 900° C. The electrical properties of all four materials are consistent with the presence of a glassy phase.     

A study of the crystallization process of Na1.6Zn0.8Si1.2O4 glass and the ionic conductivity properties of partially crystallized samples

The crystallization process of Na_1.6Zn_0.8Si_1.2O₄ glass was studied by means of differential scanning calorimetry, X-ray powder diffraction and the platinum/carbon replication technique. Partially crystallized samples were made by rapidly cooling samples from elevated temperatures using the DSC apparatus, and the ionic conductivity of the materials was determined by means of impedance measurements conducted at lower temperatures where the crystallization rate was negligible. The glass was found to crystallize at 830 K by precipitation and three-dimensional grain-growth of a crystobalite-type phase with the same composition as the glass. The overall activation energy for the crystallization process was determined from isothermal DSC measurements to be 340 kJ mol^–1. The bulk ionic conductivity for partially crystallized samples increases smoothly from 9.3 x 10^–5( cm)^–1 at 600 K for the glass to 2.4 x 10^–3( cm)^–1 for the crystallized material.     

Varistor characteristics in PTCR-type (Ba,Sr)TiO3 ceramics prepared by single-step firing in air

This paper deals with donor, acceptor-codoped (Ba_0.4Sr_.6)TiO₃ ceramics with distinct varistor characteristics at room temperature, which were prepared by single-step firing in air. The materials, with the Curie point at around –90 °C, exhibited a large PTCR (positive temperature coefficient of resistivity) effect of more than seven orders of magnitude in the temperature range –90 °C (the resistivity 10³ · cm) to room temperature ( > 10¹⁰ · cm). An apparent dielectric constant of >20000 and tan < 0.05 (at 100 kHz) were observed for the present materials at room temperature, and moreover, the materials exhibited nonlinear current-voltage characteristics with the nonlinear coefficient, , in the range 7–12 and the varistor field, E_v, in the range 0.3–1.0 kV/cm. The value of in the present materials increased systematically with increasing in their PTCR temperature range. It has been found that there exists a close correlation between and the grain-boundary potential barrier height, e, obtained from the -T characteristic of the materials. An almost linear relationship was also found to exist between and log E_v for the present materials.     

Thermal transport properties of helium near the superfluid transition. II. Dilute3He-4He mixtures in the superfluid phase

Measurements of the average thermal conductivity _exp hQ/T and of the thermal relaxation time to reach steady-state equilibrium conditions are reported in the superfluid phase for dilute mixtures of³He in⁴He. Hereh is the cell height,Q is the heat flux, andT is the temperature difference across the fluid layer. The measurements were made over the impurity range 2×10^–9<X(³He)<3×10^–2 and with heat fluxes 0.3<Q<160 µW/cm². Assuming the boundary resistanceR _b , measured forX<10^–5, to be independent ofX over the whole range ofX, a calculation is given for _exp. ForQ smaller than a well-defined critical heat fluxQ _c (X) X ^0.9, _exp is independent of Q and can be identified with the local conductivity _eff, which is found to be independent of the reduced temperature = (T–T)/T for –10^–2. Its extrapolated value at T is found to depart forX10^–3 from the prediction X ^–1 , tending instead to a weaker divergence X ^–a witha0.08. A finite conductivity asX tends to zero is not excluded by the data, however. ForQ >Q _c (X), a nonlinear regime is entered. ForX10^–6, the measurements with the available temperature resolution are limited to the nonlinear conditions, but can be extrapolated into the linear regime forX2×10^–7. The results for _exp(Q),Q _c (X), and _eff(XX) are found to be internally consistent, as shown by comparison with a theory by Behringer based on Khalatnikov's transport equations. Furthermore, the observed relaxation times (X) in the linear regime are found to be consistent forX>10^–5 with the hydrodynamic calculations using the measured _eff(X). ForX<10^–5, a faster relaxation mechanism than predicted seems to dominate. The transport properties in the nonlinear regimes are presented and unexplained observations are discussed.     

Anomalous Conductivity in Random Magnetic (Ga,Cr) As

The electrical conductivity of molecular beam epitaxy grown zinc blende Ga_1–x Cr _x As, x = 0.1, exhibits anomalous behavior below room-temperature. The room-temperature resistivity is small, = 0.1 cm, and comparable to (Ga,Mn)As. Near room temperature, the conductivity is activated, following = ₀ exp(–E _A/kT ), with an activation energy of E _A = 63 meV. In this activated region, Hall measurements also show activated behavior in the hole concentration where p 10²⁰ cm^–3, indicating that Cr can also act as an acceptor similar to Mn. For decreasing temperature, the resistivity increases rapidly because of hopping or tunneling conduction, and becomes strongly insulating at low temperatures. From T = 20 to 200 K the conductivity follows exp[–(T ₁/T)^1/2] over eight orders of magnitude range of conductivity, possible evidence of tunneling between metallic-like polarons.     

Surface tension of helium at the superfluid critical end-point

The singularity in the vapor-liquid interfacial tension, (T), of helium at the transition to superfluidity is analyzed theoretically. The universal amplitude ratio R ⁺ =K₊( ₀ ⁺ )^d–1/k _B T , where K₊ and K_– are the amplitudes of the |T–T|^µ singularity in , with =1.34 ₃ , is known from recent work to first order in =4–d for the general n-vector model in d dimensions. Extrapolation to d=3 for n=2 indicates R ⁺ =0.05–0.08, which is shown to be consistent with the experimental data. Further analysis of the experiments establishes that the universal ratio Q=K₊/K _– exceeds 0.35, and is consistent with the recent prediction Q0.9; this demonstrates the inadequacies of earlier theoretical treatments. The existence in the observed surface tension of an anomalous, negative contribution of unknown origin at a few millikelvin beneath T is stressed.     

Magnetic field dependence of zero-sound attenuation close to the pair-breaking edge in3He-B due toJ=1−,J z =±1 collective modes

Our previous theory yielded for the Zeeman splitting of the imaginaryJ=1 collective mode in³He-B the result =2+0.25J _z ( is the effective Larmor frequency). In this paper we take into account the downward shift of the pair-breaking edge from 2 to 2₂– (₂ and ₁ are the longitudinal and transverse gap parameters). This leads to a complex Landé factor: the frequencies of theJ _z =±1 components become =2+0.39J _z , and the linewidths of these resonances become finite: =0.18. The coupling amplitudes of theJ _z =±1 components to density are found to be proportional to gap distortion, (₁–₂/(/)². Our results for the ultrasonic attenuation due to theJ _z =±1,J=1 modes are capable of explaining the field dependence of the attenuation close to the pair-breaking edge as observed by Dobbs, Saunders, et al. The observed peak is caused by theJ _z =–1 component: its height increases due to gap distortion as the field is increased, and the peak shifts downward in temperature and its width increases with the field due to the complex Landé factor. TheJ _z =+1 component gives rise to a corresponding dip relative to the continuum attenuation.     

Cu2+ ions in sodium fluoride-sodium borate glasses studied by EPR and optical absorption techniques

Electron paramagnetic resonance (EPR) and optical absorption spectra of Cu²⁺ ions in 80Na₂B₄O₇-(20 – x)NaF – xCuO (NFNB) glass system with 0 x 6 mol% have been studied. EPR spectra of all the glass samples exhibit resonance signals characteristic of Cu²⁺ ions. The values of spin-Hamiltonian parameters indicate that the Cu²⁺ ions in sodium fluoride-sodium borate (NFNB) glasses were present in octahedral sites with tetragonal distortion. The number of spins (N) participating in resonance was calculated as a function of temperature for NFNB glass sample containing 1 mol% of Cu²⁺ ions and the activation energy was calculated. From the EPR data, the paramagnetic susceptibility () was calculated at various temperatures and the Curie constant was calculated from the 1/ – T graph. The optical absorption spectra of these samples show a broad absorption band centered at 13280 cm^–1 which is assigned to the ² B _1g ² B _2g transition of Cu²⁺ ions in distorted octahedral sites. The optical band gap energy (E _opt) and Urbach energy (E) are calculated from their ultraviolet edges. It is observed that as the copper ion concentration increases, E _opt decreases while E increases. This has been explained as due to the creation of additional localized states by CuO, which overlap and extend in the mobility gap of the matrix. By correlating the EPR and optical data, the molecular orbital coefficients have been evaluated.     

Preparation and Physical Properties of Ag<Subscript>5</Subscript>Pb<Subscript>2</Subscript>O<Subscript>6</Subscript>-Ag Ceramics

A new process for Ag₅Pb₂O₆ synthesis is described. Ag₅Pb₂O₆-Ag ceramics are prepared and characterized using x-ray diffraction, thermal analysis, and scanning electron microscopy. The room-temperature resistivity of Ag₅Pb₂O₆ ceramics is 0.35–0.37 m cm, and that of Ag₅Pb₂O₆-Ag ceramics is 0.03–0.04 µ cm.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 5, 2005, pp. 628–634.Original Russian Text Copyright © 2005 by Akimov, Savchuk.     

Critical scattering in the electrical resistance of GdB6 and DyB6

Detailed investigations of the electrical resistance of GdB₆ and DyB₆ near T _N, their antiferromagnetic transition temperatures, and of nonmagnetic LaB₆ are reported. In GdB₆ a recently reported magnetic phase transition around 6 K has been found to be sample dependent. The resistance of this compound shows a very rapid increase at about T _N = 15.15 K and a small maximum near 15.5 K. Analysis of the data using r = r₀ + Bt + A_±t^1–, where t = (T–T _N )/T _N , for –4 × 10^–3 t –7 × 10^–2, yields 0.36. This is in agreement with a renormalization group theory prediction for a system with a three-dimensional lattice (d = 3) that is magnetically isotropic and has six spin degrees of freedom (n = 6). The resistance of DyB₆ shows features of a typical antiferromagnetic transition with electron scattering mainly dominated by fluctuations in short-range spin correlations.Using the above expression for –1.8 × 10^–2 t 4.8 × 10^–2 with the A ₊term when t is positive and A _–when t is negative, we obtain T _N = 20.32 K and = –0.16, which is close to the theoretical value ( = –0.17) for d = 3 and n = 4. The results are discussed in terms of the present theories for critical scattering.This research was supported in part by a grant from the Natural Sciences and Engineering Research Council of Canada.     

Examining the temperature dependence of the mechanical properties for materials in large-diameter tubes

Measurements have been made on the mechanical properties of largediameter tubes made of 09G2S, 14G2SAF, and 17G1S steels and on welded joints in them at strain rates of 200 sec^–1 and 113–293 K. Fivefold microspecimens 1.2 mm in diameter have been employed. The parameters have been determined in the temperature-rate dependence of the Yaroshevich yield point, lower brittleness temperature, and fracture resistance as affected by the ferrite diameter. A method is proposed for predicting the brittleness range from the dependence of the notch bottom narrowing B on the generalized parameter = S_f/_y — _t/_i for a Menaget specimen.Translated from Problemy Prochnosti, No. 4, pp. 43–50, April, 1994.     

Amplifier based on a field-effect triode with negative current feedback

Conclusions A field-effect triode amplifier with series negative current feedback allows a voltage gain of the order of 200–300 to be obtained for a load resistance R_s1 M. The coefficient K_u begins to decrease noticeably only for a feedback resistance above 500 .The current gain reaches (8–10)·10³. Increasing the resistances R_s and R_L to hundreds of ohms has practically no effect on K_i. For a further increase of R_s and R_L the coefficient K_i decreases.The power gain reaches its maximum value (of the order of 10⁴ or more) for R_s100 and R_L=10–100 k. An increase in R_s leads to a reduction of K_pmax and to a shift of the extremum of the function K_p=f(R_L) into the range of higher values of R_L.A large input resistance of the amplifier (tens of megohms and higher) is obtained when R_s increases to 10–100 M. The maximum input resistance is obtained for R_L and R_s and may exceed values of from hundreds of megohms to several gigaohms. The minimum input resistance is hundreds of kilohms for R_L and R_s0.The minimum input resistance (5–10 k or less) is ensured for R_g and R_L0. An increase of the output resistance to hundreds of megohms or higher occurs for R_g and R_s.Translated from Izmeritel'naya Tekhnika, No. 9, pp. 67–70, September, 1971.     

Electrical conductivity study of oxidation process in manganese-substituted magnetites

During oxidation in air of finely-grained manganese-substituted magnetites (Mn _0.8x ²⁺ Fe _1–0.8x ³⁺ )_A– (Fe _1+0.6x ³⁺ Fe _1–0.8x ²⁺ Mn _0.2x ³⁺ )_BO ₄ ^2– (A=tetrahedral, B=octahedral) the temperature dependence of the electrical conductivity over a temperature range of 100 to 700° C was investigated. Below 500° C the evolution of electrical conductivity might be closely associated with the position and nature of cations in the spinel lattice. The profile of the =f(t) curves show that the mechanism of electrical conduction in the temperature range 150 to 300° C can be explained in terms of the oxidation of Fe²⁺ to Fe³⁺ ions at octahedral sites. For the temperature range 300 to 400° C the conductivity involves the hopping of electrons from tetrahedral-site Mn²⁺ ions to tetrahedral-site Mn³⁺ ions. Above 500° C the oxidation of Mn²⁺ ions leads to an increase in conductivity with the generation of new phases of -Fe₂O₃, Mn₂O₃ and -(MnFe)₂O₃.     

Well-characterized sintered submicrometer copper powder for low temperature heat exchangers

The results are presented of a systematic study of the influence of temperature, time, pressure, and atmosphere during pretreatment and sintering on the surface area, packing factor, structure, hardness, mechanical contact to a foil, and electrical conductivity of sinter produced from 900-Å Cu powder. Conditions for the reproducible production of sinter with high surface area (2–3 m²/g) and good conductivity (₃₀₀=10–15 µ-cm andR ₃₀₀/R_4.23) are determined.On leave from Institute for Low Temperature and Structure Research, Polish Academy of Sciences, Wroclaw, Poland.     

The observation of resistance oscillations in metals at high magnetic fields using a SQUID

Quantum oscillatory effects provide particularly unambiguous information about the properties of quasiparticles in metals, and have consequently been used for the purpose of understanding the low temperature behaviour of a wide variety of metallic elements and compounds, including the recently discovered heavy fermion materials. Unfortunately, the signals which are produced by these oscillations are often so weak as to be undetectable, because of the high effective masses of the quasiparticles, or the presence of imperfections in the samples, or otherwise. In order to improve our ability to measure these effects, the author has developed a system for the detection of resistance oscillations which uses a SQUID as the amplifying element and which fits inside a toploading dilution refrigerator probe. It has proven possible to use this instrument to investigate very elusive phenomena in samples which have been cooled to 0.02 K in magnetic fields of 13.7 tesla. For example, although no resistance oscillations have previously been seen in aluminum, except under conditions of magnetic breakdown, the new system has made it possible to detect resistance oscillations in this material under relatively arbitrary conditions. In the case of the heavy fermion compound CeRu₂Si₂, oscillations of 10^–9 have been detected against a background resistance of about 10^–5 , using excitation currents of around 10^–2 A.     

The thermal conductivity of propane

This paper presents thermal conductivity measurements of propane over the temperature range of 192–320 K, at pressures to 70 MPa, and densities to 15 mol · L^–1, using a transient line-source instrument. The precision and reproducibility of the instrument are within ±0.5%. The measurements are estimated to be accurate to ±1.5%. A correlation of the present data, together with other available data in the range 110–580 K up to 70 MPa, including the anomalous critical region, is presented. This correlation of the over 800 data points is estimated to be accurate within ±7.5%.Nomenclature a _n, b_ij, b_n, c_n Parameters of regression model - C Euler's constant (=1.781) - P Pressure, MPa (kPa) - P _cr Critical pressure, MPa - Q ₁ Heat flux per unit length, W · m^–1 - t time, s - T Temperature, K - T _cr Critical temperature, K - T ₀ Equilibrium temperature, K - T _re Reference temperature, K - T _r Reduced temperature = T/T _cr - T _TP Triple-point temperature, K Greek symbols Thermal diffusivity, m² · s^–1 - T _i Temperature corrections, K - T Temperature difference, K - T _w Temperature rise of wire between time t ₁ and time t ₂, K - T ^* Reduced temperature difference (T–T _cr)/T_cr - _corr Thermal conductivity value from correlation, W · m^–1 · K^–1 - _cr Thermal conductivity anomaly, W · m^–1 · K^–1 - _e Excess thermal conductivity, W · m^–1 · K^–1 - ^* Reduced density difference - Thermal conductivity, W^–1 · m^–1 · K^–1, mW · m^–1 · K^–1 - _bg Background thermal conductivity, W · m^–1 · K^–1 - ₀ Zero-density thermal conductivity, W · m^–1 · K^–1 - Density, mol · L^–1 - _cr Critical density, mol · L^–1 - _re Reference density, mol · L^–1 - _r Reduced density Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.