Effects of aluminium on the electrical and mechanical properties of PTCR BaTiO3 ceramics as a function of the sintering temperature

The effect of AI additions on the electrical behaviour of positive temperature coefficient of resistance (PTCR) BaTiO₃ ceramic sintered in air at temperatures ranging between 1220 and 1400° C have been investigated. Two batches of material, both showing a PTCR effect, were prepared identically except that additions of AI₂O₃ (0.55 mol %) were made to one of them. It has been confirmed that the presence of aluminium results in an increase in the temperature at which the maximum resistivity, _max, occurs as well as reducing the sintering temperature, in the presence of silicon, to 1240° C. Additionally, direct comparisons between the two materials have demonstrated that such additions result in an increase of 100% in the minimum resistivity, _min, at sintering temperatures beyond 1280° C. A similar increase in _max for sintering temperatures below 1360° C and a five-fold reduction in the ratio of _max/_min in samples sintered above 1320° C have also been attributed to the presence of aluminium. It was further found that aluminium increases the average grain size by 30% and promotes the formation of a liquid phase.     

The structure and electrical conductivity of Mn-Cd ferrite

Polycrystalline samples of mixed ferrite Mn_{l –x} Cd _x Fe₂O₄, wherex = 0, 0.2, 0.4, 0.6 and 0.8, were studied by X-ray analysis and d.c. electrical conductivity. lt was found that the calculated lattice parameter (a ₀) increases from 0155 to 0.861 nm according to increases inx from 0 to 0.8. The apparent density and theoretical density exhibit the same behaviour. lt was also found that the activation energy of the conduction mechanism and log _{27° C} increase with increasing cadmium content, which affect the exchange interaction mechanism in tetrahedral (A) sites and octahedral (B) sites.     

The effect of a magnetic field on fourth sound in 3He

The influence of a magnetic field on the propagation of fourth sound in superfluid ³He is studied. The field and temperature dependences of the average superfluid density \- _s / and fourth sound Q are measured. The field dependence of \- _s / is very different in a porous medium than predicted by Ginzburg-Landau theory applied to bulk liquid. In particular, a magnetic suppression of \- _s / is observed in the temperature and pressure ranges corresponding to the A phase in bulk liquid. There is strong evidence of a magnetic suppression of T _c itself. The measured \- _s / has a slight history dependence in a magnetic field, but none in zero field. The fourth-sound Q values are compared to the theoretical work of Smith, Jensen, and Wolfle. Quantitative confirmation of their work is problematic.     

Anomalous electrical resistive behaviour of Al-O-Pb sputtered alloys

Application of the sputtering technique to (Al-O) _x Pb_100-x alloys containing an immiscible lead element has been found to result in the formation of a duplex material consisting of f cc lead particles dispersed in an amorphous Al _x O _y oxide matrix. The particle size and interparticle distance of the lead phase were about 10 to 50 nm and 10 to 40 nm, respectively. The Al-O-Pb alloys have been found to exhibit an extremely high electrical resistivity (), e.g., 4.06 x 10⁶ cm for (Al-O)_87.5 Pb_12.5 at 273 K, as well as a large positive temperature-dependent resistivity reaching 92% Of 273. The peculiar resistivity behaviour was interpreted by assuming the mechanism that only the lead phase embedded in Al_xO_y matrix contributes to electrical conductivity and the mobility of lead electrons is greatly reduced in the intervening oxide region among lead phases. It has thus been demonstrated that the composite material exhibiting peculiar characteristics, which cannot be achieved in metallic composite materials, is obtained by simultaneously sputtering oxide and an insoluble metallic element.     

The microhardness of injection moulded polystyrene and polyethylene

Injection moulded plaques of polystyrene and of two types of high density polyethylene have been produced under different moulding conditions. The mean density of each plaque together with its surface microhardnessV _H have been measured. Polystyrene shows only a slight increase in with increasing mould temperatureT _m together with a reduced spread in andV _H values. In polyethylene, however, andV _H both increase linearly withT _m, giving a usefulV _H against relation, which is attributed to increased percentage crystallinity.     

Pressure effects of resistivity along thec axis in La2−x Sr x CuO4

Out-of-plane resistivity p_c(T) of optimally-doped high-T_c superconductor La_2–xSr_xCuO₄ has been investigated under hydrostatic pressure up to 8GPa. We found a drastic decrease in _c which reaches 50% at 8GPa. The large change of _c is understood as due to a small variation of the lattice parameter c. In comparison with a change of p_c(T) estimated from a change of c due to thermal contraction, we conclude that the well known temperature-linear dependence of _c is apparent metallic one; essentially _c is nonmetallic over the whole temperature range, and can be described in terms of tunneling or quantum hopping conduction between CuO₂ layers. Such tunneling conduction is enhanced by the scattering in the CuO₂ plane at the oxygen defects which affect the in-plane resistivity only very weakly.     

Shear viscosity measurements of liquid 4He and 3He-4He mixtures near the tricritical point

A torsional oscillator cell is described, by means of which simultaneous precision measurements of () and of the molar volume can be made in liquid ⁴He-⁴He mixtures over the temperature range between 0.5 and 3 K. Here is the mass density, the shear viscosity and in the superfluid phase they become the contributions _n and _n of the normal component. The results of for ⁴He near the superfluid transition are compared with the predictions by Schloms, Pankert and Dohm, and by Ferrell. Measurements of () are reported for mixtures with 0.64X0.74, where X is the ³He mole fraction. Those for X = 0.67 and 0.70 are compared with data by Lai and Kitchens. The viscosity experiments show no evidence of a weak singularity at the tricritical point.     

Soft magnetic properties of NiFeTa thin films for biasing film applications in magnetoresistive elements

The effects of Ta addition on the magnetic properties of permalloy thin films have been investigated. The alloy compositions on a weight basis are (Ni₈₁Fe₁₉)_1-x Ta _x with 0 x 0.105, and the films are sputtered onto a glass substrate at between room temperature and 300 C. The saturation magnetization and anisotropic magnetic field decrease with increasing Ta content. The saturation magnetization is 0.75 T at 5 wt % Ta. The coercivity remains constant at 125 Am^-1. The electrical resistivity increases linearly with increasing Ta content, then saturates at approximately 7.5 wt% Ta. The saturation resistivity is approximately 1.00 m. The magnetoresistivity ratio (/) decreases with increasing Ta content, mainly due to increased electrical resistivity (). The magnetostriction changes from negative to positive with increasing Ta content and reaches nearly zero at 2 wt% Ta. The NiFeTa films containing 5–6 wt% Ta have potential for use as the soft-biasing film in magnetoresistive elements.     

Ohm resistivity of electroless copper layers as a function of their thicknesses

The electric Ohm resistivity of electroless Cu depositions on dielectric substrates as a function of their thicknesses is studied. Substantial deviations (up to 10–20 times) from the standard resistivity ( = 1.7 cm^–1) below 0.5 (m thicknesses are observed. The experimental data show for the entire region of thicknesses (d 0.07–5 m) a power function between the relative resistivity changes (/) and the inverse thickness of depositions (d )–(/ (1/d )^0.8. This empirical relation is discussed as an effect of the porous structure of the metallic layers deposited on the substrate. A scanning electron micrography was applied in order to visualize the morphology of the depositions. The micrographs clearly show the evolution of the deposition profile: starting from separate islands at the very beginning of the process, and gradually covering the entire area with continuous but porous metal layers.     

Transport and localization in Nd1.82Ce0.18CuO4−δ film

Temperature- and magnetic-field — dependences of the resistivity and Hall effect R_H .     

Penetration depth and related properties of Al films with enhanced superconductivity

Using combined resistivity, susceptibility, critical field, and NMR measurements it was found that Al films with enhanced superconductivity can be divided into two separate categories depending on their residual resistivities ₀. Films with resistivities smaller than 10 ^–3 ·cm are metallic (_(300K)/ ₀ is larger than but very close to unity); the experimentally measured penetration depth, the upper critical field H, and the penetration field H_p are in good agreement with theoretically calculated values using an experimental value of the mean free path1 _eff. The density of states for such films as inferred from NMR measurements is very close to that of bulk. On the other hand, films with ₀ larger than 10 ^–3 ·cm have a semiconducting behavior ( ₃₀₀ K/ ₀ 1) and are grossly inhomogeneous. The experimental values of H no longer agree with the theoretical estimates using1 _eff. In such films ₀ is no longer a meaningful parameter that can be related to1 _eff, for it is to a large extent a tunneling resistivity as shown by the fact that films not superconducting down to 0.95 K, when measured resistively, are superconducting at 1.2 K, as shown by a susceptibility measurement. These experiments seem to indicate that in a homogeneous film1 _eff has a lower limit of approximately 1 Å.     

Flux motion in anisotropic type-II superconductors nearH c 2 with arbitrary vortex orientation

Flux motion in anisotropic type-II superconductors near H_{c 2} is studied in the framework of the time-dependent Ginzburg-Landau (TDGL) theory for the case that the average flux densityB is oriented in an arbitrary direction relative to the principal axes of the sample. The linearized TDGL equation for a uniformly translating order parameter is solved and expressions for all elements of the flux-flow resistivity tensor _ij (including the off-diagonal Hall elements) are obtained. The diagonal elements _ii show the angular scaling property that _ii(B) = _ii(B/H_{c 2}(, ø)), whereas the Hall elements _ij (i j) have additional angular dependences that are not contained in H_c2. For the case that the normal state Hall elements _ij ⁽ⁿ⁾ B _k with i j k, the ratios _ij/ _ij ⁽ⁿ⁾ are functions of B/H_c2 (, ) only.     

Critical behavior of liquid4He at negative pressures

We examine the equation of state of liquid⁴He at negative pressures close to the spinodal density _s where the hydrodynamic speed of sound vanishes. The non-analytic behavior of the equation of state and the speed of sound in the vicinity of the spinodal density are calculated in two and in three dimensions; we find for the speed of sound the non-analytic behavior mc _s ² ( — _s)^2/5 in three dimensions and mc _s ² [(-_s)/¦ln(-_s)¦]^1/2 in two dimensions.We then examine the low density regime numerically, using a semi-analytic microscopic theory. It is found that non-analytic exponents are visible only in a negligible density regime around the spinodal point. Estimates for the spinodal densities, and the range of critical fluctuations are provided.     

Isochoricp-ϱ-T measurements on Difluoromethane (R32) from 142 to 396 K and pentafluoroethane (R125) from 178 to 398 K at pressures to 35 MPa

Thep--T-relationships were measured for difluoromethane (R32) and pentafluoroethane (R125) by an isochoric method with gravimetric determinations of the amount of substance. Temperatures ranged from 142 to 396 K for R32 and from 178 to 398 K for R125, while pressures were up to 35 MPa. Measurements were conducted on compressed liquid samples. Determinations of vapor pressures were made for each substance. I have used vapor pressure data and thep--T data to estimate saturated liquid densities by extrapolating each isochore to the vapor pressure, and determining the temperature and density at the intersection. Publishedp--T data are in good agreement with this study. For thep T apparatus. the uncertainty of the temperature is ±0.03 K. and for pressure it is ±0.01%, atp > 3 MPa and ±0.05% atp < 3 MPa. The principal source of uncertainty is the cell volume (28.5193 cm³ at 0 K and 0 M Pa), which has a standard uncertainty of ±0.003 cm³. When all components of experimental uncertainty are considered. the expanded uncertainty (at the two-sigma level) of the density measurements is estimated to be 0.05%.     

C-Axis resistivities of single- and double-layered Bi-system cuprates: A comparative study

The measurements of anisotropic resistivities of single- and double-layered Bi-system cuprates at different doping levels are presented. The two types of materials exhibit qualitatively common features in both doping and temperature dependences of their c-axis resistivities. The c-axis conduction is incoherent in nature because the _c data are deeply on the insulating side of the Mott limit, even though positive slopes (d_c/dT>0) are observed below room temperature for overdoped samples. Some differences are revealed by quantifying the trend of _c(T) data, which may reflect differences in the charge excitation spectra.     

Electrical resistivity of polycrystalline vanadium films

The electrical resistivity of polycrystalline vanadium films (20 to 400 nm thick) deposited on to amorphous and crystalline substrates heated to 473 K during deposition was studied. The data were interpreted in terms of the classical size effect for thick films. The mean free path,I ₀, the average resistivity, ₀, and the density of conduction electrons,N, were calculated. These values are compared with those found in earlier works.     

Decay rate for supercurrent in thin wire

We present a new evaluation of the fluctuations triggering the decay of supercurrents. Contrary to the existing treatment available in the literature, our result emerges in a simple and closed form. This is due to the fact that, in a polar decomposition = e ⁱ of the order parameter, we sum over all azimuthal paths explicitly, thereby arriving at a fluctuation determinant for the variable alone which can be evaluated exactly.Alexander von Humboldt Fellow. On leave of absence from the University of Ankara, Ankara, Turkey.Supported by Scientific and Technical Research Council of Turkey.     

Compositional range,thermal stability,hardness and electrical resistivity of amorphous alloys in Al-Si (or Ge)-transition metal systems

An amorphous single phase was found to be formed in wide compositional ranges in rapidly solidified Al-Si-transition metal (M) and Al-Ge-M alloys. The compositional ranges are in the range from 12 to 53 at. % Si or Ge and 8 to 23% M and Al-Si-Co and Al-Ge-Fe alloys have the widest glass-formation ranges. Because the interaction between aluminium and silicon or germanium atoms is thought to be repulsive from the immiscible equilibrium phase diagrams, the glass formation is probably due to an attractive interaction of M-Si (or Ge) and Al-M pairs. Hardness, H _v, and crystallization temperature, T _x, increase with increasing M content and the highest values reach 1120 DPN and 715 K, while the change with silicon or germanium content is much smaller for H _v and is hardly seen for T _x. Additionally, the H _v and T _x have maximum values for Al-Si (or Ge)-M (M=Cr, Mn or Fe), decrease with the decrease and increase in the group number of M element and are the lowest for Al-Si (or Ge)-Ni alloys. The compositional dependence is interpreted under the assumption that T _x and H _v of the aluminium-based amorphous alloys are mainly dominated by the attractive interaction of M-(Si or Ge) and Al-M pairs. Room-temperature resistivity, _RT, increases in the range of 220 to 1940 cm with increasing silicon or germanium and M contents. The change in _RT with the group number of M elements shows a maximum phenomenon for manganese. It has thus been clarified that the characteristics of the Al-Si-M and Al-Ge-M amorphous alloys have the different compositional dependence as compared with those for conventional metalmetalloid amorphous alloys, probably because of the unusual interaction among the constituent elements.     

Nonlinear asymptotic theory of hypersonic flow past a circular cone

Summary The hypersonic small-disturbance theory is reexamined in this study. A systematic and rigorous approach is proposed to obtain the nonlinear asymptotic equation from the Taylor-Maccoll equation for hypersonic flow past a circular cone. Using this approach, consideration is made of a general asymptotic expansion of the unified supersonic-hypersonic similarity parameter together with the stretched coordinate. Moreover, the successive approximate solutions of the nonlinear hypersonic smalldisturbance equation are solved by iteration. Both of these approximations provide a closed-form solution, which is suitable for the analysis of various related flow problems. Besides the velocity components, the shock location and other thermodynamic properties are presented. Comparisons are also made of the zeroth-order with first-order approximations for shock location and pressure coefficient on the cone surface, respectively. The latter (including the nonlinear effects) demonstrates better correlation with exact solution than the zeroth-order approximation. This approach offers further insight into the fundamental features of hypersonic small-disturbance theory.Notation a speed of sound - H unified supersonic-hypersonic similarity parameter, - K hypersonic similarity parameter, M - M freestream Mach number - P pressure - T temperature - S entropy - u, v radial, polar velocities - V freestream velocity - shock angle - cone angle - density - density ratio, /() - ratio of specific heats - polar angle - stretched polar angle, / - (), (), () gage functions     

Van der Pauw measurement of metal fibre orientation in a plastic-metal composite

Stainless steel fibres in ABS plastic form a composite with an anisotropic resistivity. Samples are rectangular shapes with uniform thickness. By assuming two principal resistivities and by using van der Pauw's technique, we find ² = _{x y} . For rectangular samples, field theory determines _y / _x and hence fibre direction. Results for three sample geometries agree with the theoretical predictions of the fibre patterns and with X-ray data. Samples formed by a centre-sprue feed are the best for fabricating large, uniform samples, while samples with a large length-to-width ratio have the most uniform metal density and fibre orientation. Resistivity was also measured by the more-common two-probe technique. Results correlate well to van der Pauw data, with 95% confidence.