Standardization of strength evaluation methods using critical distance stress

The stress and displacement fields near the bonding edge, sharp notch, and contact edge show singularity behaviors, so methods of evaluating the strength of these points using maximum stresses calculated by a numerical stress analysis, such as the finite element method, are generally not valid. We have previously presented a new method of evaluating the strength of these singular points using two stress singularity parameters H and λ. The difficulty with this method was in obtaining the critical value of the intensity of the stress singularity parameter H_c for each order of stress singularity λ. Then we have developed a method of formularizing critical value of stress singularity parameter H_c for each order of stress singularity λ by utilizing critical distance stress theory. Firstly we apply this method to the delamination strength evaluation. The estimated delamination criteria H_c(λ) agrees well with the experimental results. Then we apply this method to general sharp notch corner and contact edge fatigue problems. In these cases the fatigue-crack initiation criteria can be derived from two typical strength parameters, namely, fatigue limit σ_w0 and threshold stress intensity factor range ΔK_th. These estimated critical H_th(λ) value agrees well with the experimentally measured value. Using this simple critical distance stress approach we estimated the fretting fatigue-crack initiation criteria for any contact edge angle and optimized the contact edge geometry. Moreover, we can apply this new strength criterion to stress singularity fields for general stress-concentration structures.     

Interplay Between Superconductivity and Antiferromagnetism in HoNi2B2C

A microscopic theory of interplay between superconductivity and antiferromagnetism in rare-earth nickel boride, HoNi₂B₂C is developed from first principles. Self-consistent equations for the superconducting order parameter Δ and magnetic order parameter Γ are derived using a Green’s function technique and an equation of motion method. The theory is applied to explain the experimental results in the antiferromagnetic superconductor HoNi₂B₂C. The present model explains the true coexistence of superconductivity and antiferromagnetism in this system. The behavior of the superconducting order parameter (Δ), the magnetic order parameter (Γ), the specific heat, the density of states, the free energy and critical field (H _c) is also studied for the system HoNi₂B₂C. Distinct features of the coexistence region are discussed. There is the convincing evidence that the theory is fully compatible with the key experiments.     

A correlation of critical points

Critical points were correlated for fluids which obey the three-parameter equation of state according to which the compressibility factor, $\text{Z=}\text{P}\text{dRT}$, is given by Z=1 + k_Bδκ (θ, δ), in which κ (θ, δ) is a universal function of reduced temperature, $\text{θ =}\text{T}\text{T}{}_{\mathrm{B}}$, and reduced density, $\text{δ =}\text{d}\text{d}{}_{\mathrm{<mtext>O</mtext>}}$. The three parameters which distinguish one fluid from another are k_B, T_B, and d_O. It is shown that experimental inflection points of the pressure versus density isotherms of gases obeying this equation of state all fall on the same reduced curve of θ versus δ. Critical points also must lie on this curve, at a location dependent on k_B. Critical densities are too uncertain experimentally to permit a definite correlation, but the critical temperatures of sixteen fluids are shown to fall on a common θ_c versus k_B curve with an average deviation of 0.4 degrees. The critical pressures of these same fluids lie within a few tenths of an atmosphere on a common π_c ($\text{π =}\text{P}\text{d}{}_{\mathrm{<mtext>O</mtext>}}\text{RT}{}_{\mathrm{<mtext>B</mtext>}}$) versus k_B line. This correlation of critical points is accomplished using values of the three parameters that had previously been determined by fitting supercritical PVT data to the equation of state.     

Temperature dependence ofH c2 andк 1 in strong coupling superconductors

According to the law of corresponding states, the properly scaled critical fields of a BCS superconductor are universal functions of the reduced temperatureT/T _c . In realistic superconductors strong-coupling effects can cause deviations from this universal behavior. We present numerical results for the strong-coupling anomalies in the temperature dependence of the upper critical fieldH _c2 and the Ginzburg-Landau parameter κ₁. The calculations are based on the Eliashberg theory for strong-coupling superconductors and are performed for several realistic superconductors as well as for a series of model superconductors with increasing coupling strength. It is demonstrated that the magnitude of the deviations can be estimated from the magnitude of the ratioT _c /〈ω〉, where〈ω〉 is an average phonon frequency. This rule does not apply to amorphous superconductors. The reason for this is shown to be due to the large weight ofα ² F(ω) at low frequencies which has been observed in these materials.     

Reaction-diffusion in a simple pooled chemical system

The reaction scheme based on the cubic autocatalator A + 2B→ 3B B → C is considered in a closed vessel with the reactant Abeing replenished by the slow decay of a precursor Pvia the simple step P→A The concentration of Pis assumed to be very much larger than that of the intermediates Aand Bso that the pooled chemical approximation can be made. Spatial variations are allowed for within the vessel and it is assumed that initially the vessel contains only the reactant Pand that at time ? = 0 a quantity of B,characterized by the parameter β_o; is introduced into some localized region. The long-time behaviour is discussed and it is shown that this depends on the parameters β_oand μ(where μis a non-dimensional parameter representing the reaction rates) with the value of the diffusion parameter λ effectively controlling the form of the transient path leading to the final behaviour. Analytic solutions are derived for the cases when β_ois small and λ is large. These are complemented by numerical solutions for general values of the parameters. It is found that there are three possibilities: a finite equilibrium may be reached with Aand Bhaving constant concentrations, the concentrations of Aand Bmay oscillate, or Bmay die away leaving just the conversion of P to AIn the first two cases it is found that this behaviour is left behind a propagating reaction-diffusion front. The nature of this front is analysed in detail.     

Superconductivity of Nb1−x Mg x B2: Impact of Stretched c-Parameter

We have performed a systematic study on the occurrence of superconductivity in Nb_1?x Mg _x B₂ (0.0≤x≤0.40). X-ray diffraction and magnetization measurements are carried out to determine the changes in lattice parameters, superconducting transition temperature (T _c ) and critical field (H _c1). The substitution of Mg at Nb site results in considerable stretching of c-parameter with only a slight change in a parameter. Rietveld analysis on X-ray diffraction patterns gives a=3.11 Å and c=3.26 Å for pure NbB₂ while a=3.10 Å and c=3.32 Å for Nb_0.60Mg_0.40B₂. This increased c-parameter introduces superconductivity in niobium diboride. Magnetization measurements though indicate the absence of superconductivity in NbB₂, the same shows a clear diamagnetic signal at about 10 K for Nb_0.60Mg_0.40B₂ sample. The magnetization M(H) plots exhibit weak superconductivity like hysteresis loops. The stretching of c-parameter from around 3.26 to 3.32 i.e. by 0.06 cannot be explained solely by substitution of Nb by Mg in the lattice. It seems that some Nb deficiencies are introduced in the Nb_1?x Mg _x B₂ as Mg is not substituted completely at the vacant Nb sites. This could be seen from XRD results, where one can clearly notice the presence of small amount of MgO in Nb_1?x Mg _x B₂ samples.     

The critical properties of binary mixtures containing carbon dioxide: Krichevskii parameter and related thermodynamic properties

The values of Krichevskii parameter are given, which are calculated using the data on critical lines for binary CO₂ + solute mixtures (n-alkanes, alcohols, refrigerants, inert gases, water, diatomic gases, and other organic compounds), and detailed comparison is made with the results of other researchers obtained using different measuring techniques. In this review, special attention is given to the investigation of the thermodynamic (partial molar properties, Henry’s constant, distribution coefficient, solubility, and others) and structural (direct and total correlation integrals, size of clusters) properties of infinitely dilute solutions (IDS) of CO₂ + solute in the vicinity of the critical point (CP) of pure solvent (CO₂), for which purpose the values of Krichevskii parameter and the properties of pure solvent are used. The procedure is given of estimating the effect of small impurities on the accuracy of determination of the critical parameters of pure substances. The importance of critical lines (pattern and qualitative form) and of the Krichevskii parameter in the investigation of the scaling behavior (renormalization of the critical behavior) of thermodynamic functions of IDS in the vicinity of the CP of pure CO₂ is discussed in detail. The boundaries are determined of regions in the T-x and Δρ-x planes, in which the renormalization is observed of the critical behavior of weakly divergent (isochoric heat capacity) and highly divergent (isobaric heat capacity and isothermal compressibility) properties in CO₂ + solute mixtures.     

Analytic Expressions for a 2D Permanent Magnetic Lattice with a 3D Bias Magnetic Field for Ultracold Atoms

In this paper, we discuss a permanent magnetic lattice for trapping and manipulating ultracold atoms and quantum degenerate gases. Considering three components of an external bias magnetic field, B_1x, B_1y and B_1z which are applied for controlling the magnetic lattice parameters, we obtain analytic expressions for the magnetic field minima locations as well as their absolute values, curvatures of the absolute value of the magnetic field, trap frequencies and barrier heights between minima in all directions. B_1z had previously been considered in numerical works, but not in analytic calculations, due to difficulty in obtaining the analytic expressions for the physical parameters and sufficiency of considering only B_1x and B_1y. Including the third component in this study provides further control over the magnetic lattice parameters. Also, in the presence of unavoidable bias magnetic fields with a z-component such as the Earth’s magnetic field, our analytic calculations could be useful. In numerical calculations, we have considered up to n _s  = 1001 square magnetic slabs in each direction of the 2D lattice.     

Comparison of Different Critical Current Density Models for?Undoped Monofilamentary Ti-Sheathed MgB2 Tapes

Two J _c dependency on B models with physical backgrounds and two semiempirical models are fitted in fields B>1.5 T to critical current measurements. The measurements were previously performed on various undoped titanium sheathed monocore MgB₂ tapes. The models are fitted to magnetic measurements performed at temperatures ranging from 4.2 to 15 K and to transport measurements in both parallel and perpendicular directions. These parameters of the introduced models have clear physical interpretations, such as, the zero field critical current density, the zero resistivity field, and the steepness factor. The parameters are solved through numerical optimization. The goal is to find a model with simple parameters that produce computed values which match those observed through direct measurements. In the future, these parameters can be used to gather information about pinning, to compare the performance of the conductors, and to develop these further. The results indicate that many kind of models are sufficient to describe the J _c(B) behavior of different kinds of tapes and measurement methods in various temperatures. They also enable the reconstruction of the J _c(B) dependence even from a limited number of data points or a narrow measurement range. In addition, the F _p behaviors of the samples are compared to the characteristic grain boundary pinning in MgB₂.     

The upper critical field of layered superconductors

The results of a theoretical study of the upper critical fields of layered superconducting compounds are presented. It is found that for sufficiently decoupled layers the parallel critical fieldsH _c2,‖ can be quite large compared to those expected with ordinary type II superconductors and essentially determined by the destruction of the superconductivity in the individual superconducting layers. It is found thatH _c2,‖ can be calculated from standard pair breaking theory in terms of a pair breaking parameter appropriate for layered compounds. The pair breaking parameter includes both orbital effects and the effects of Pauli paramagnetism and spin-orbit scattering.     

Superconducting Properties of In Situ Mg1.05(B1−xCx)2 Doped with Melanin (C16H2O3N2) via Ultrasonication in Ethanol

In this paper, we have reported melanin (C₁₆H₂O₃N₂) as a dopant of MgB₂ for the first time. Here, the effects of melanin doping to the microstructures and superconducting properties of bulk MgB₂ are thoroughly studied from XRD, SEM, TEM, magnetization, and resistivity data. We have analyzed the critical current density (J _c), irreversibility field (H _irr), flux pinning, resistivity, lattice parameters, grain sizes, critical temperatures (T _c), and other microstructures of all the samples. We have varied the doping percentage according to the nominal atomic ratio of Mg_1.05(B_1?x C _x )₂, x=0,0.02,0.06,0.08,0.1. The J _c of all the melanin-doped samples are improved as compared to that of the undoped sample in high-field region (above 6 Tesla) at low temperature. The 8 and 10 % doped samples give the best results. The 8 % doped sample registers an enhancement of J _c by a factor of 3.6 at 7 T and 5 K as compared to that of the undoped one. But, in the low-field region, melanin doping reduces J _c. The H _irr shows remarkable enhancement at low temperatures below 20 K. The best value of H _irr was found for the 8 % doped sample. However, H _irr reduces at high temperatures above 20 K in all the melanin-doped samples. The volume pinning strength of all the doped samples is enhanced over the entire field range. Further improvement in superconducting properties can be achieved by further reducing the size of the melanin particles, increasing density, and improving the homogeneity of doping.     

The effects of annealing temperature on the in-field Jc and surface pinning in silicone oil doped MgB2 bulks and wires

The effects of sintering temperature on the lattice parameters, full width at half maximum (FWHM), strain, critical temperature (T_c), critical current density (J_c), irreversibility field (H_irr), upper critical field (H_c2), and resistivity (ρ) of 10 wt.% silicone oil doped MgB₂ bulk and wire samples are investigated in state of the art by this article. The a-lattice parameter of the silicone oil doped samples which were sintered at different temperatures was drastically reduced from 3.0864 Å to 3.0745 Å, compared to the un-doped samples, which indicates the substitution of the carbon (C) into the boron sites. It was found that sintered samples at the low temperature of 600 °C shows more lattice distortion by more C-substitution and higher strain, lower T_c, higher impurity scattering, and enhancement of both magnetic J_c and H_c2, compared to those sintered samples at high temperatures. The flux pinning mechanism has been analyzed based on the extended normalized pinning force density f_p = F_p/F_p,max scaled with b = B/B_max. Results show that surface pinning is the dominant pinning mechanism for the doped sample sintered at the low temperature of 600 °C, while point pinning is dominant for the un-doped sample. The powder in tube (PIT) MgB₂ wire was also fabricated by using of this liquid doping and found that both transport J_c and n-factor increased which proves this cheap and abundant silicone oil doping can be a good candidate for industrial application.     

Superconductivity and magnetism in the pseudoternary system Dy(Rux Rh 1−x)4B4

The pseudoternary system Dy(Ru_xRh_1?x)₄B₄ has been studied by means of ac magnetic susceptibility, electrical resistance, heat capacity, static magnetic susceptibility, and magnetization measurements. A low-temperature phase diagram was determined for the range of concentrations 0.1<x≤1. Superconductivity occurs forx?0.35, with a sharp depression of the superconducting critical temperature nearx～0.3. Long-range magnetic order exists for all concentrationsx; the rhodium-rich compounds exhibit antiferromagnetism, while the ruthenium-rich compounds are ferromagnetic. Pseudoternary compounds near the critical limit possibly exhibit reentrant antiferromagnetic superconductivity, a hitherto unobserved phenomenon. The competition between antiferromagnetism and ferromagnetism apparently leads to a mixed phase in a comparatively wide range of concentrations.     

On the coupling current losses in a multifilamentary superconducting composite

The coupling current losses in a superconducting multifilamentary composite exposed to trapezoidally varying external magnetic field and carrying a small transport current are investigated for the volume pinning force density described with the dependence: F_v = α B^1?γ. Such a model allows estimation of the deviations from the solution based on Bean's (γ = 0) critical state model. Results indicate that there exists a region of small magnetic field amplitudes for which discrepancies are largest. They are also very sensitive to the rate of magnetic field change.     

Behavior of alkali element uranosilicates and uranogermanates in saturated aqueous solutions

The behavior of uranosilicates and uranogermanates of the series A^IHB^IVUO₆·nH₂O (A^I = Li, Na, K, Rb, Cs, NH₄; B^IV = Si, Ge) in aqueous solutions at pH from 0 to 14 was studied. The acid-base intervals of the existence of these compounds were determined, their conversion products were identified, and the solubility of A^IHB^IVUO₆·nH₂O was measured. The physicochemical analysis of the heterogeneous systems A^IHB^IVUO₆·nH₂O-aqueous solution was performed. The equilibrium constants of the dissolution reactions and the Gibbs functions of formation of A^IHB^IVUO₆·nH₂O were calculated. The solubility curves and the speciation diagrams of U(VI) and element B^IV in aqueous solutions and of the equilibrium solid phases were constructed.     

Concentration,mobility and 1/f noise of electrons and holes in thin bismuth films

We have studied bismuth films evaporated to a thickness t in the range of 300– 14000 Å. The resistivity ?, Hall coefficient R_H, magnetoresistance Δ?/(?B²) and the 1/f noise were measured as functions of thickness at room temperature. With the help of ?, R_H, Δ?/(?B²) and literature data for the thermoelectric power P of bismuth, the electron and hole concentrations n and p, and the mobilities μ_n and μ_p were calculated as functions of the thickness. The results are compared with values from the literature. The sensitivity of calculated values of n, p, μ_n and μ_p to measurement errors have been investigated. The magnitude of the 1/f noise can be characterized by the parameter α. It has been found experimentally that α decreases with thickness.     

Forms of crystal field Hamiltonians - A critical review

Our survey reveals that various disparate forms, both compact and expanded ones, of crystal field (CF) Hamiltonians, H_CF, expressed in the Wybourne notation have been used in the literature. It turns out that the disparities in the symbolic explicit forms of H_CF are especially important for monoclinic and triclinic site symmetry. The extent of the inconsistencies identified in selected papers has prompted us to embark on a systematic critical review of the H_CF forms employed in optical spectroscopy and related areas. Most crucial results of this survey are presented here. Comparative analysis has been carried out to establish the interrelations between CF parameters (CFPs) expressed in disparate forms. The usage of inconsistent or confusing H_CF forms has implications also for CFP conversions between the Stevens and Wybourne notations as well as for theoretical modeling of CFPs. This review reveals that comparison of CFP data taken from various sources should be carried out with special care, especially for low symmetry cases.