Synthesis of the Bi-2222 Compounds of Bi–Sr–Ln(Zr)–Cu–O (Ln = Sm,Eu, Gd,and Dy)

Bi-based superconducting compounds with the 2222 structure has been already synthesized in the Bi₂Sr₂- (Ln_1−x Ce _x )₂Cu₂O_10+y (Ln = Sm, Eu, and Gd) systems. One of the characteristics of these compounds is the existence of the fluorite-like (Ln_1−x Ce _x )₂O₂ block between two CuO₅ pyramids in the crystal structure. The tetravalent ions of Ce⁴⁺ are reported to be necessary to stabilize the 2222 structure. Recently, we have discovered that the Bi-2222 phase could be composed in the Bi₂Sr₂(Ln_2−x Zr _x )Cu₂O _z (Ln = Sm, Eu, Gd, and Dy) systems, where Zr⁴⁺ is used as a new tetravalent ion stabilizing the 2222 structure in stead of Ce⁴⁺. In the new system, nearly single 2222 phase samples have been obtained at the nominal composition of x=0.5 (Ln = Sm, Eu, and Gd) and in the range of 0.1≤x≤0.3 (Ln = Dy). Among them, the sample with Ln = Gd has the smallest resistivity at 273 K. But it is a semiconductor, and the conduction process at low temperatures is assumed to be followed by a two-dimensional VRH. The experimental results for the Gd samples with a partial substitution of Pb for Bi in the Bi₂Sr₂(Gd_2−x Zr _x )Cu₂O _z system are also reported.     

Influence of Buoyancy Force on Thermocapillary Convection Instability in the Differentially Heated Annular Pools of Silicon Melt

The influence of buoyancy force on the thermocapillary convection instability in the annular pools (R _i = 20 mm, R _o = 40 mm, and depth d ranging from 1 to 10 mm) of silicon melt (Pr = 0.011), differentially heated at the outer wall and cooled at the inner wall, is investigated numerically. The critical Marangoni numbers (Ma _c) for the incipience of oscillatory flow are determined by linear stability analysis (LSA) under both microgravity and normal gravity conditions. The results indicate that the buoyancy force destabilizes the thermocapillary convection under different liquid layer depths from 3 to 10 mm. With increasing the layer depth, the critical Ma number, critical azimuthal wave number and critical phase velocity decrease. Some of 3-D simulation results are compared with those of LSA. 3-D results are found consistent with the LSA results except for a case of D = 0.05 where 3-D simulation gives a stationary 3-D flow under a large Ma.     

Methane-to-hydrogen conversion in the filtration-combustion wave of rich methane-air mixtures

The superadiabatic-effect-based process of noncatalytic conversion of methane to a hydrogenous synthesis gas in a cocurrent filtration-combustion wave has been considered. The influence of the composition and specific flow rate of the working mixture on the basic parameters of conversion — the maximum temperature in the filtration-combustion wave, the wave’s propagation velocity, and the composition of the reaction products — has been investigated. Filtration-combustion regimes optimum from the viewpoint of the most efficient conversion of methane to a hydrogenous synthesis gas have been found from the experimental data obtained. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 2, pp. 90–95, March–April, 2007.     

Very Low Temperature Electronic Transport in Al-Pd-Re Quasicrystalline Alloys

Electrical resistivities of two icosahedral (I) Al-Pd-Re alloys have been measured between room temperature and mK temperatures. One quasicrystalline (QC) polygrain Al-Pd-Re sample exhibited insulating behavior in its resistivities, increasing by a factor of r=R(4 K)/R(300 K)=7.76; its room temperature resistivity was 9,890 μΩ cm. A “phenomenological” expression fitted the conductivity data well between 300 K to 0.5 K. Below 0.4 K a crossover to an activated variable-range hopping law was observed. Low temperature magnetoresistance ratio data and fits using the wave function shrinkage theory are presented. A second QC Al-Pd-Re sample had a small resistance temperature ratio r=2.12. The room temperature resistivity was extremely large, ρ(300 K)≈40,980 μΩ cm. Its conductivity could be described well using a simple temperature power law between 300 K to 20 K. Below 20 K there was a crossover to a new behavior. Below 1 K, the conductivity could be fitted using a very weakly insulating power law where σ(T)≈11.37T ^0.032 in (Ω cm)⁻¹, suggesting that this sample is located just below the metal-insulator transition. The magnetoconductivity data could not be fitted successfully using the 3D weak localization (WL) theory and inserting into it physical and realistic fitting magnitudes for the inelastic magnetic field B _in.      

Effect of Firing Temperature on the Structure and Superconducting Properties of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> Films

YBa₂Cu₃O_7−x (YBCO) films were prepared on LaAlO₃ single crystal substrate under various firing temperatures (750–800 °C) in the crystallization process by metalorganic deposition (MOD) method. The coating solution was made by mixing the fluorine-free precursor solution containing Y and Cu with Ba–fluorine precursor solution (Ba-TFA). The effect of firing temperature on the structure and superconducting properties of YBCO films was systematically investigated. The results indicated that YBCO-films were smooth, crack-free, exhibited good textures and retain high oxygen content according to the XRD and SEM images. Sample of YBCO-film fired at 780 °C showed highest superconducting properties including high critical transition temperature T _c=89 K, sharp transition temperature ΔT _c<1 K, and critical current density J _c=2.8 MA cm⁻², which are attributable to excellent in-plane textures and dense microstructures with good connectivity between the grains.     

High-Temperature Thermoelectric Power of The Metal Oxides: La2?xSrxCuO4 and Bi1?xSrxMnO3

We have investigated the high-temperature thermoelectric power (TEP) of La_{2− x} Sr _x CuO₄ (0.05 ≤ x ≤ 0.35) and Bi_{1− x} Sr _x MnO₃ (0.5 ≤ x ≤ 0.8) up to 700 K. Based on the TEP results we have discussed the phase transitions on each case. In the case of high-T _C cuprates, La_{2− x} Sr _x CuO₄ (0.05 ≤ x ≤ 0.35), the TEP shows different temperature dependences in three temperature regions. At low temperature, the positive TEP rises showing a broad peak at temperature T _P, which shifts to lower temperature upon Sr doping. Right above T _P, the TEP decreases linearly as temperature increases. At high temperature, TEP deviates from the linear-T dependence at a certain temperature, T _H, showing a saturation behavior. The systematic change of the TEP behavior is discussed in terms of the two-fluids model, which is an intrinsically inhomogeneous state, consisted of bound pairs and independent carriers in the normal state of the high-T _C superconductors. For Bi_{1− x} Sr _x MnO₃ (0.5 ≤ x ≤ 0.8), the negative TEP is almost temperature-independent in the high temperature regime (T _CO < T < 700 K). Near the charge ordering temperature (T _CO), however, TEP suddenly decreases with decrease of temperature, indicating the suppression of carrier mobility with charge ordering transition. As Bi concentration decreases, T _CO shifts to lower temperature from T _CO ∼ 520 K for x = 0.5 to T _CO ∼ 435 K for x = 0.8, which suggests that charge ordering is related to the local lattice distortion due to highly polarizable 6s² character of Bi³⁺ ion. In comparison with the resistivity data, the TEP results have been discussed in terms of the carrier localization accompanied by local lattice distortion.     

Fatigue properties of notched specimens made of FeP04 steel

Fatigue properties of specimens with different notches made of FeP04 steel are presented. The specimens are characterized by double symmetric lateral notches with radius of the notch root ranging from ρ = 0.2 to 10 mm. An MTS 809 servohydraulic device was used for the tests. All fatigue tests were performed under force control by imposing a constant value of the nominal load ratio (R = 0) and a load amplitude P _a  = 6 kN for the notch root ρ = 0.2 mm and 7 kN for the notch roots ρ = 1.25, 2.5 and 10 mm. The test frequency varied from 13 to 15 Hz. During the tests performed under constant loading, we observe the fatigue weakening of the material and an increase in strain.     

Effect of cobalt substitution on magnetic and transport properties of Nd0·5Sr0·5Mn1?xCoxO3 (x = 0·1, 0·3 and 0·5)

The magnetic and transport properties of the compounds Nd_0·5Sr_0·5Mn_1-x_{{\rm 1}-{x}}Co_x_{{x}}O₃ (x = 0·1, 0·3 and 0·5), synthesized by citrate–gel route have been investigated. The spin transition in cobaltates at low temperatures affects the magnetic as well as transport properties. The irreversibility behaviour between the zero-field cooled (ZFC) and field cooled (FC) magnetization as a function of temperature becomes stronger with increasing Co content. This is understood on the basis of glassy behaviour, which becomes more robust with increasing Co substitution. The non-saturating M–H behaviour indicates strong magnetic inhomogeneities which may cause the magnetic phase separation at the nanoscopic length scale. The double exchange interaction is stronger between Mn^3 +–O^2 −–Mn^4 + as compared to Co^3 +–O^2 −–Co^4 + pairs. Co-substitution suppresses the double exchange which will lead to cluster/spin glass like behaviour as well as semiconducting features due to localization of charge carriers (mobile e_g{e}_{\rm g} electrons).     

Stability of Thermocapillary Convection in Rotating Shallow Annular Pool of Silicon Melt

In order to understand the effect of pool rotation on stability of thermocapillary convection, the critical conditions for the incipience of oscillatory flow in rotating shallow annular pools of silicon melt (Pr = 0.011) are investigated by means of linear stability analysis (LSA) under different pool rotation rates ranging from Ta = 0 to 1,513.9 and under microgravity condition. The results indicate that with increase of Ta numbers, the critical Marangoni for the incipience of hydrothermal wave (HTW) increases, i.e., pool rotation stabilizes the steady axisymmetric thermocapillary convection of silicon melt. The critical azimuthal wave number m _c decreases linearly with increase of Ta number when Ta ≤ 378.5. However, when Ta > 378.5, m _c almost keeps constant with m _c = 34. When Ta < 550 the HTW propagates in the same direction as the pool rotation direction, while it is reversed when Ta > 550.     

Cryptanalysis of ‘Less Short’ RSA Secret Exponents

 In some applications of RSA, it is desirable to have a short secret exponent d. Wiener [6], describes a technique to use continued fractions (CF) in a cryptanalytic attack on an RSA cryptosystem having a ‘short’ secret exponent. Let n=p ⋅ q be the modulus of the system. In the typical case that G=gcd(p−1, q−1) is small. Wiener’s method will give the secret exponent d when d does not exceed (approximately) n ^1/4. Here, we describe a general method to compute the CF-convergents of the continued fraction expansion of the same number as in Wiener (which has denominator d ⋅ G) up to the point where the denominator of the CF-convergent exceeds approximately n ^1/4. When d<n ^1/4 this technique determines d, p, and q as does Wiener’s method. For larger values of d there is still information available on the secret key. An estimate is made of the remaining workload to determine d, p and q. Roughly speaking this workload corresponds to an exhaustive search for about 2r+8 bit, where r=ln₂ d/n ^1/4. Received: September 30, 1996; revised version: March 7, 1997     

Influence of Heat Treatment on Structure and Superconducting Properties of YBCO Film by Chemical Solution Deposition Method

YBCO films have been fabricated on a (00l) LaAlO₃ single-crystal substrate via self-developed fluorine-free polymer-assisted metal organic deposition (PA-MOD) method. The influence of heat treatment on texture, microstructure and superconducting properties of YBCO films has been investigated. After a pyrolysis process ranging from 145 °C to 500 °C with different heating rates, the samples were fired at 760–780 °C in Ar and O₂ mixture gas followed by annealing at 450 °C in pure O₂. The results indicate the film fired at 770 °C after decomposition at the rate of 0.5 °C/min showed the highest T _c of 90.4 K and J _c (77 K, 0 T) over 2 MA/cm². According to the XRD patterns, phi-scan and omega-scan curves as well as SEM images, the good properties may be attributed to better biaxial texture and purer YBCO phase as well as better grain connectivity.     

Experimental Investigation of a Bubbly Two-Phase Flow in an Open Capillary Channel under Microgravity Conditions

The study of a bubbly laminar two-phase flow in an open capillary channel under microgravity conditions was conducted aboard the sounding rocket, Texus-45. The channel geometry, the liquid (FC-72) and the experimental conditions were chosen based on an analysis for application toward liquid management in space. The channel consists of two parallel plates that were b = 25 mm wide and separated by a distance of a = 10 mm. The flow along the length l = 80 mm is bounded by a free surface on one side and a plate on the opposite side. Bubbles are injected at the inlet of the capillary channel via six capillary tubes. The features of the gas injection were chosen with regard to the required bubble size, the injection frequency, and the gas and liquid flow rates. Different liquid and gas flow rates were tested leading to a volumetric quality ranging between 0.07 and 0.11. The experimental results show the interaction among bubbles and with the liquid free surface.     

Fracture and Fatigue Behaviour of Aluminium Matrix Composite Automotive Pistons

The fracture and fatigue behaviour of prototype automotive pistons produced in an aluminium alloy matrix composite in industrial conditions has been studied. Fracture toughness increased when the testing temperature rose from 20° to 75°C and kept near constant up to 250°C, when a significantly lower value was recorded. A change in the failure operating mechanism, which can explain this trend, was observed by analysing the fracture surfaces in the scanning electron microscope. Room temperature fatigue tests performed with R = 0.1 stress ratio led to an average value of the Paris law exponent higher than those reported in aluminium alloys but low for an industrially produced brittle composite. A higher exponent and a much larger scattering were observed in those fatigue tests carried out under R = 0.5 stress ratio.     

Application of non-linear k–ω model to a turbulent flow inside a sharp U-bend

Simulation of the complex flow inside a sharp U-bend needs both refined turbulence models and higher order numerical discretization schemes. In the present study, a non-linear low-Reynolds number (low-Re) k–ω model including the cubic terms was employed to predict the turbulent flow through a square cross-sectioned U-bend with a sharp curvature, R _C/D = 0.65. In the turbulence model employed for the present study, the cubic terms are incorporated to represent the effect of extra strain-rates such as streamline curvature and three-dimensionality on both turbulence normal and shear stresses. In order to accurately predict such complex flowfields, a higher-order bounded interpolation scheme (Song et al., 1999) has been used to discretize all the transport equations. The calculated results by using both the non-linear k–ω model and the linear low-Reynolds number k–ɛ model (Launder and Sharma, 1974) have been compared with experimental data. It is shown that the present model produces satisfactory predictions of the flow development inside the sharp U-bend and well captures the characteristics of the turbulence anisotropy within the duct core region and wall sub-layer.     

The Effect of Sintering Temperature Variation on the Superconducting Properties of ErBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Superconductor Prepared via Coprecipitation Method

The effect of heat treatment on the superconducting properties of ErBa₂Cu₃O_7−δ (ErBCO) ceramic materials has been studied. The nano-metal oxalate precursor was prepared using coprecipitation (COP) method. The prepared materials were subjected to calcination process at 900 °C for 12 h and then sintered under oxygen environment for 15 h at 920 °C, 930 °C, 940 °C, and 950 °C, respectively. All samples showed a metallic behavior and single-step transition in the R–T curves. The best zero critical current, T _C(R=0)=91.4 K, was for the sample sintered at 920 °C. XRD data showed single phase of an orthorhombic structure. As the sintering temperature increases, the formation of nonsuperconducting phases (impurities) was observed when the samples sintered above 920 °C. The formation of nano-oxalate powders via COP method is a very efficient procedure to produce high-quality superconductors with less processing temperature required.     

Dynamic Remanent Vortices in Superfluid 3He-B

We investigate the decay of vortices in a rotating cylindrical sample of ³He-B, after rotation has been stopped. With decreasing temperature vortex annihilation slows down as the damping in vortex motion, the mutual friction dissipation α(T), decreases almost exponentially. Remanent vortices then survive for increasingly long periods, while they move towards annihilation in zero applied flow. After a waiting period Δt at zero flow, rotation is reapplied and the remnants evolve to rectilinear vortices. By counting these lines, we measure at temperatures above the transition to turbulence ∼0.6 T _c the number of remnants as a function of α(T) and Δt. At temperatures below the transition to turbulence T≲0.55 T _c, remnants expanding in applied flow become unstable and generate in a turbulent burst the equilibrium number of vortices. Here we measure the onset temperature T _on of turbulence as a function of Δt, applied flow velocity v=v _n−v _s, and length of sample L.     

Comparison of the dimensions of electrical resistance units, supported on the basis of the hall quantum effect, in the All-Russia Research Institute of Metrological Service and the Czech Metrological Institute

VNIIMS (Russia) and CMI (Czech Republic) quantum Hall resistance standards are compared using a VNIIMS portable resistance standard of 1 and 10 kΩ. Conformity is established for the dimensions of units within the limits of relative expanded (k = 2) uncertainty of 10⁻⁷. Translated from Izmeritel’naya Tekhnika, No. 12. pp. 58–61, December, 2008.     

Specific contact resistance and metallurgical process of the silver-based paste for making ohmic contact structure on the porous silicon/p-Si surface of the silicon solar cell

Porous silicon has been considered as a promising optoelectronic material for developing a variety of optoelectronic devices and sensors. In the present study, the electrical properties and metallurgical process of the screen-printed Ag metallization formed on the porous silicon surface of the silicon solar cell have been investigated. The contact structure consists of thick-film Ag metal contact patterned on the top of the porous silicon surface. The sintering process consists of a rapid firing step at 750–825 °C in air ambient. It results in the formation of a nearly perfect contact structure between the Ag metal and porous silicon/p-Si structure that forms the top metalization for the screen-printed silicon solar cells. The SEM picture shows that Ag metal firmly coalesces with the silicon surface with a relatively smooth interfacial morphology. This implies that high temperature fire-through step has not introduced any signs of adverse effect of junction puncture or excessive Ag indiffusion, etc. The three-point probe (TPP) method was applied to estimate the specific contact resistance, ρ _c (Ω-cm²) of the contact structure. The TPP measurement shows that contact structure has excellent ohmic properties with ρ _c = 1.2 × 10⁻⁶ Ω-cm² when the metal contact sintered at 825 °C. This value of the specific contact resistance is almost three orders of magnitude lower than the corresponding value of the ρ _c = 7.35 × 10⁻³ Ω-cm² obtained for the contact structure sintered at 750 °C. This improvement in the specific contact resistance indicates that with increase in the sintering temperature, the barrier properties of the contact structure at the interface of the Ag metal and porous silicon structure improved which in turn results a lower specific contact resistance of the contact structure.     

Photoresponse of n -ZnO/ p -Si heterojunction towards ultraviolet/visible lights: thickness dependent behavior

A series of n-ZnO/p-Si thin film heterojunctions have been fabricated by a low cost sol–gel technique for different ZnO film thicknesses and the dark as well as photo current–voltage (I–V) characteristics have been investigated in details. The heterojunction with ZnO thickness of 0.46 μm shows the best diode characteristics in terms of rectification ratio, I _F/I _R = 5.7 × 10³ at 5 V and reverse leakage current density, J _R = 7.6 × 10⁻⁵ A cm⁻² at −5 V. From the photo I–V curves and wavelength dependent photocurrent of the heterojunctions, it is found that the junction with 0.46 μm ZnO thickness shows the highest sensitivity towards both UV and visible lights.     

Determination of the size of the fatigue prefracture zone by the method of phase-shifting interferometry

We consider a new method for determining the size of the fatigue prefracture zone (a new structural-mechanical parameter d* of constructional materials) by the method of phase-shifting interferometry, based on the results of a comparative analysis of the surface profile of metallic specimens in the neighborhood of a constructional stress concentrator prior to and after a certain quantity of loading cycles. To obtain interferograms in laser light, we applied a Twyman–Green interferometer. Using these results, we determined the 3-D distribution of plastic strains near the concentrator vertex, which enabled us to establish that d* = 256–268 μm for low-carbon 08kp steel at a load ratio R = 0.1. The obtained value of this parameter is in good agreement with data obtained by other methods.