Ultrasonic attenuation of longitudinal and shear waves in the normal and superconducting states of niobium

Ultrasonic attenuation measurements of both longitudinal and shear waves in the normal and superconducting states were made on niobium single crystals. Frequencies between 50 and 450 M H z were employed. Independent of the mode, frequency, and crystal studied, a deviation of the attenuation ratio _s/_n from BCS behavior was observed. The dependence of _s/_n on frequency and crystal purity are interpreted in terms of a model which assumes a difference in electron mean-free path due to phonon scattering in the normal and superconducting states.Research sponsored in part by NASA Grant 36-006-003 and by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under ASOSR Grant No. 68-1479B.     

Behavior and modeling of flow softening and ductile damage evolution in hot forming of TA15 alloy sheets

Flow softening and ductile damage behavior of TA15 titanium alloy with initial bimodal microstructure were studied using uniaxial hot tensile tests at different temperatures (750–850 °C) and strain rates (0.001–0.1 s^− 1). SEM examination of deformed specimens shows that deformation mainly occurs in β and secondary α phase (α_s). Globularization of α_s is also observed. According to the SEM observations of the cracked specimens, the mechanism of ductile damage is attributed to the breakdown of the compatibility requirements at the α/β interface. Based on the experimental results, a set of mechanism-based unified elastic-viscoplastic constitutive equations have been formulated to model the flow behavior and the damage evolution of TA15 alloy in hot forming conditions. Dislocation density, ductile damage evolution, deformation heat, phase transformation and globularization of the α_s have been modeled. The model constants have been determined by using a Genetic Algorithm (GA)-based optimization method. Furthermore, the proposed constitutive equations were evaluated in terms of correlation coefficient (R), average absolute relative error (AARE), and root mean square error (RMSE). The results indicate that the calibrated predictions, including flow stress, volume fraction of each phase, and fracture strain, are in good agreement with experimental results.     

Dislocation damping and the ultrasonic attenuation in the normal and superconducting states of pure tin

Ultrasonic attenuation studies have been carried out both in the normal and superconducting states in pure (99.9999%) single crystals of tin containing various amounts of dislocations. At low pulse amplitude the normal state attenuation is effectively independent of the dislocation density. The dislocations have been found to introduce significant deviations in the superconducting state attenuations from the BCS values. The extra attenuation Δα_s in the superconducting state due to dislocations has been found to show an inverse fourth power dependence on the temperature. Δα_s has also been shown to exhibit resonance type of behaviour over the frequency range 3 to 50 MHz. This resonance frequency depends on the dislocation density with different power laws in different ranges of the dislocation density.     

Crossover from dirty to clean superconducting limit in dc magnetron-sputtered thin Nb films

High-quality Nb (110) thin films with residual resistance ratios up to 60 and critical temperatures T_c ≈ 9.27 K have been prepared by conventional dc-magnetron sputtering on α-Al₂O₃ by careful selection of the sputtering conditions. This allowed for a systematic study of the influence of the growth rate on the structural quality and the superconducting properties of the films. The optimized growth conditions were revealed at the substrate temperature T_s = 850 °C, Ar pressure P_s = 0.4 Pa, and the growth rate g ? 0.5 nm/s. The results of the films' structural characterization by X-ray diffraction, reflection high-energy electron diffraction, and atomic force microscopy are presented. In terms of the electron mean free path l and the superconducting coherence length ξ, deduced from the magneto-resistivity data, the clean superconducting limit (l > ξ) is realized in the high-purity films. For comparison, in impure Nb films sputtered at room temperature while keeping the rest of the sputtering parameters unvaried, the opposite dirty limit (ξ ? l) ensues. The merits of these findings are discussed in the context of the demands of present-day fluxonics devices regarding the normal-state and flux-flow properties of superconducting films they are made of.     

Influence of doping of TiC and NbC on structure and superconducting properties of Bi-containing compounds

In the present paper the structure, phase composition, superconducting properties and transport current for (BiPb)₂Sr₂Ca₂Cu₃O_y doped by TiC and NbC are investigated and their characteristics are presented. Concentration of impurities-was varied from 0.5 to 4.0 wt %. It was found that TiC impurity doesn 't affect on superconducting properties of specimens when its concentration less than 2 wt %. It was observed that superconducting properties of Bicontaining compounds are significantly suppressed with introducing of the same quantity of NbC impurity.     

Engineering assessment of ductile tearing in uniaxial and biaxial tension

To decide between clearly different approaches for engineering assessment of plane stress tearing, we performed uniaxial and biaxial tensile tests on middle-cracked specimens at various in-plane constraint states. Attention was focused on determination of the crack tip opening spacing δ_n, crack tip opening angle ψ_c, crack mouth opening angle α_s, energy dissipation rate R, and specific work of fracture A_s. Our experimental results demonstrate that the values of δ_n, ψ_c, α_s, R, and A_s for a high-strength low-hardening aluminium alloy all depend on the specimen geometry, its size, and the load biaxiality ratio. However, assessment of ductile tearing by interconnected characteristic quantities α_s and A_s is more preferable over the use of δ_n, ψ_c, and R values for a number of basically important reasons discussed in this paper.     

An attempt on triple doping in ZnO by Pulsed Laser Deposition

An attempt has been made on triple doping of B, Ga and N into ZnO films using B₂O₃ and GaN doped ZnO targets by KrF excimer laser. From XRD, it has been found that, the films doped with equal percentage of B₂O₃ and GaN, show better structural properties. The suppression of the deep levels observed from PL studies reveals their good optical properties. The Hall measurement shows that films have n-type conductivities due to lack of nitrogen to compensate the donors. The low resistivity and high mobility have been observed for the film having better structural properties.     

Microwave and Terahertz Surface Resistance of MgB2 Thin Films

The knowledge of the surface resistance R _s of superconducting thin film at microwave and terahertz (THz) regions is significant to design, make and assess superconducting microwave and THz electronic devices. In this paper we reported the R _s of MgB₂ films at microwave and THz measured with sapphire resonator technique and the time-domain THz spectroscopy, respectively. Some interesting results are revealed in the following: (1) A clear correlation is found between R _s and normal-state resistivity right above T _c, ρ₀, i.e., R _s decreases almost linearly with the decrease of ρ₀. (2) A low residual R _s, less than 50 μΩ at 18 GHz is achieved by different deposition techniques. In addition, between 10 and 14 K, MgB₂ has the lowest R _s compared with two other superconductors Nb₃Sn and the high-temperature superconductor YBa₂Cu₃O_7−δ(YBCO). (3) From THz measurement it is found that the R _s of MgB₂ up to around 1 THz is lower than that of copper and YBCO at the temperature below 25 K. (4) The frequency dependence of R _s follows ω ⁿ , where ω is angular frequency, and n is power index. However, n changes from 1.9 at microwave to 1.5 at THz. The above results clearly give the evidences that MgB₂ thin film, compared with other superconductors, is of advantage to make superconducting circuits working in the microwave and THz regions.     

Thermal diffusivity measurement of Zn, Ba, V, Y and Sn doped Bi-Pb-Sr-Ca-Cu-O ceramics superconductors by photoacoustic technique

The simple open photoacoustic cell technique is demonstrated for measuring the thermal diffusivity of the Zn, Ba, V, Y and Sn doped Bi-Pb-Sr-Ca-Cu-O superconducting ceramic samples. It is based upon the measurement of the photoacoustic signal as a function of the modulation frequency in the region where the sample thickness, l_s, equal to the thermal diffusion length of the sample, _s. The obtained thermal diffusivity values of Ba, V, Y and Sn doped in Bi-Pb-Sr-Ca-Cu-O system increase with the increasing dopant concentration at Ca side. However, the thermal diffusivity values of Zn doped sample decrease with the increasing of dopant concentration in the system. The measured thermal diffusivity value was found to be very dependent on the dopant atom and dopant concentration.     

A new family of superconducting silicides: The ThMxSi2−x compounds (M = Rh or Ir with 0 < x < 1) of α-ThSi2-type structure

New superconducting materials have been prepared by substitution of silicon by Rh or Ir atoms in the α-ThSi₂ disilicide. The ThM_xSi_2?x compounds crystallize in the tetragonal α-ThSi₂-type structure with a large homogeneity range: 0? x ? 0.96 for Rh and 0? x ? 1 for Ir. A sharp increase of the superconducting transition temperature T_cr is observed for x > 0.75 in both cases. Among the superconducting compounds with an α-ThSi₂-type structure, ThRh_0.96Si_1.04 and ThIrSi have the highest T_cr values with respectively 6.45 and 6.5 K about twice that of α-ThSi₂ (T_cr = 3.16 K). Superconductivity enhancement has been attributed to the formation of Rh-Rh or Ir-Ir metallic type bonding.     

Measurement of ac losses in multifilament super-conducting wires at frequencies between 1 and 100 Hz

Calorimetric measurements of the losses of a superconducting coil wound from a moderately twisted multifilamentary superconducting wire have been made with a coil current consisting of a direct current and a superimposed ac component at frequencies between 1 and 100 Hz. There exists a frequency range where the loss per cycle decreases with increasing frequency. Magnetization measurements on a sample of the multi-filament wire in an applied inductance B = B_s + B₀sinω t confirmed the results of the calorimetric measurements and indicated that with increasing frequency the flow of magnetic flux into and out of the composite during a cycle becomes more and more diminished.     

Effect of doping on the ultrasonic attenuation in the intermediate state of superconductors

The attenuation of longitudinal ultrasonic waves has been measured in single crystals of indium (99.999%) and of indium doped with 0.003 at % of tin in the intermediate state over the frequency range 10–30 MHz. Two phases have been identified in the superconducting layers. The fundamental frequency for one phase is approximately constant in both the specimens for the same field, but increases appreciably with the increase in the magnetic field from 0.7 H _cto 0.9 H _c; the ₀ value for the other phase, on the other hand, increases with increase in the concentration of the dopant and is independent of the magnetic field. The origin of the two phases is discussed.     

Effect of Ce substitution on structural and superconducting properties of Bi-2212 system

In this study, Bi₂Sr₂Ca_1?xCe_xCu₂O_y, where x = 0, 0.01, 0.05, 0.1 and 0.25, superconducting samples were prepared by solid state method and subsequently used as feed in a laser induced directional solidification (LFZ) process. The physical properties of the samples were investigated by powder X-ray diffraction, scanning electron microscopy, dc-electrical resistance, magnetization, and magnetic-hysteresis loops measurements. It has been found that no significant difference has been observed in the critical transition temperatures of samples except for the sample with the highest Ce additions, which shows the lowest T _C compared with the other doped samples. Magnetic hysteresis measurements have shown that the hysteresis loop is greater than the doped samples. In addition, critical current density values obtained from the hysteresis loops measurements by using Bean’s critical state model show a decrease with Ce-addition. All the results indicate that Cerium substitution for Ca produces the deterioration on the superconducting properties, compared with the undoped sample.     

Metastable phase-assisted nucleation in a near-alpha titanium matrix composite with network architecture

ABSTRACT

The secondary alpha (α_s) phase is an important component of a novel TiBw/TA15 composite with network architecture. In order to investigate the precipitation mechanism of the α_s phase, the microstructure after multi-directional forging was characterised by EPMA, EBSD, and TEM. The results showed that several differently sized α_s phases separated out from the (Mo and V)-enriched phase, which was composed of a nano-sized metastable omega phase along with some retained beta phase. It could be also found that the metastable omega phase would transform to the alpha phase when adequate activation energy was provided. The precipitation mechanism of the α_s phase in this composite has been discussed in detail and it can be inferred as an omega-assisted heterogeneous nucleation process.

This paper is part of a thematic issue on Titanium.     

Investigation on the flank surface durability of gears with increased pressure angle

Within a funded research project (reference number 0325244C, BMWi – Federal Ministry for Economic Affairs and Energy of Germany), the load-carrying capacity of alternative involute gears was investigated. To find qualified variations for use in gear drives, increased pressure angles (α_n > 20°) and/or asymmetric tooth shapes (α_n < 20° and α_n > 20°) have been examined experimentally. Optimization goals were the power to weight ratio of the transmission and the power density in the drive train.In this report gears with reference tooth shape (α_n = 20°) as well as gears with modified tooth shape (α_n = 28°) are discussed with focus on the load carrying capacity of the tooth flank with special regard to the damage mechanisms pitting, micro-pitting and scuffing. The results of experimental and test accompanying studies are shown. They allow a direct comparison between standard and special gears as well as a classification in context of the actual state of knowledge.     

Flux Dynamics in NdO1−x F x FeAs Bulk Sample

We present data of multiharmonic magneto-dynamic experiments. In particular, we performed ac magnetic susceptibility experiments on layered pnictide-oxide quaternary compound NdOFeAs doped with fluorine. The experiments allow one to measure the critical temperature and probe the flux dynamic behavior using the third harmonic component of the ac susceptibility of an NdF_0.16FeAsO_0.84 bulk sample as a function of temperature and frequency of the applied ac magnetic fields. Measured signals are connected with the nonlinear superconducting flux dynamic behavior and are characterized by a “flux critical states” sustaining a superconducting critical current. In this framework the irreversibility line that describes the stable superconducting state has been extracted from the onset of the third harmonic signal vs. frequency. Finally we present also the analysis of the flux dynamic dimensionality in the investigated sample.     

Microwave Resonant Transmission in a Superconducting Fabry–Perot Bilayer

Microwave resonant transmission properties in a superconducting Fabry–Perot bilayer made of a high-temperature superconductor (YBa₂Cu₃O_7−x ) and a nearly ferroelectric superconductor (n-SrTiO₃) are theoretically investigated. The effect of high-temperature superconducting layer on the unusual resonant transmittance existing in a nearly ferroelectric superconductor is investigated. It is shown that a frequency-agile two-layer coating can be obtained with the addition of the high-temperature superconducting coating. Resonant frequencies can be shifted by varying the thickness of this coating. In addition, the ultra-narrow filtering feature makes such a bilayer resonator useful in the superconducting microwave electronics, such as a frequency sampler or spectrum analyzer in the signal processing.     

Effects of microstructure on fatigue crack propagation behavior in a bi-modal TC11 titanium alloy fabricated via laser additive manufacturing

In this study, the crack propagation behaviors in the equiaxed and equiaxed-columnar grain regions of a heat-treated laser additive manufacturing (LAM) TC11 alloy with a special bi-modal microstructure are investigated. The results indicate that the alloy presents a special bi-modal microstructure that comprises a fork-like primary α (α_p) phase surrounded by a secondary α colony (α_s) in the β phase matrix after the heat treatment is completed. The samples demonstrate a fast crack growth rate with larger da/dN values through the equiaxed grain sample versus across the equiaxed-columnar grain sample at low ΔK values (<13.8). The differences that are observed between the crack propagation behaviors (in the crack initiation stage) of the samples can be mostly attributed to the different size and morphology of the α_p lamellae and α_s colony within the grains in the equiaxed and columnar grain regions rather than the grain boundaries. The cracks prefer to grow along the α/β boundary with a smooth propagation route and a fast propagation rate in the equiaxed grain region, where the α_p and α clusters have a large size. However, in the columnar grain region, small and randomly distributed α_p lamellae generate a zigzag-shaped propagation path with a reduction in the da/dN value. Additionally, the change in the size of the α_p lamellae in the equiaxed grains (heat affected bands, HAB) is also observed to influence the propagation behavior of the crack during the crack initiation stage.     

Structural and magnetization studies on nanoparticles of Nd doped α-Fe2O3

It has been observed that, compared to bulk form, the nanocrystalline α-Fe₂O₃ is finding application in various areas. Magnetic properties of α-Fe₂O₃ are found to be influenced by the size of particles and are also sensitive to synthesis method employed for sample preparation. In the present work we have prepared a series of Nd doped α-Fe_2−xO₃ samples (x = 0.0–0.5) by combustion method, without using any fuel. The analysis of room temperature neutron diffraction patterns shows that all the compounds of the series form in the hematite (α-Fe₂O₃) structure, space group R−3c. Magnetization measurements show that there is a broad distribution of particle size in the samples. We find that the increase in the Nd content results in the dilution of magnetism of α-Fe₂O₃. From results we believe that inclusion of Nd in α-Fe₂O₃ drastically modifies the magnetic properties.     

Anisotropic Ultrafast Dynamics of Quasiparticles on CuO2 Planes of Y0.7Ca0.3Ba2Cu3O7?δ

The orientation-resolved femtosecond spectroscopy, combined with the well-textured (110)- and (001)-Y_0.7Ca_0.3Ba₂Cu₃O_7−δ thin films, serves as an effective probe to quasiparticle relaxation dynamics on the ab planes and along the diagonal orientations. The significant divergences in the temperature-dependent relaxation time (τ) associated with the opening of superconducting gap were observed along the nodal directions and on the CuO₂ planes which are dominated by the a axis and b axis in the overdoped region. Moreover, the divergence in the temperature-dependent τ along the nodal direction disappears around optimal doped region. This implies that the superconducting gap evolves from the dominant s-wave symmetry in overdoped region into the dominant d-wave symmetry in optimal doped region.