Superconducting current stability in composite superconductors

The maximum transport current in multifilament composite superconductors has been studied. The experimentally obtained values of transport current exceed the normal phase minimum propagation current substantially. The current-voltage characteristics of the investigated samples have been studied. It is shown that the relatively high value of the maximum transport current is attributed, under the given experimental conditions, to the behaviour of the current-voltage characteristic in the region of low values of the electric field. The theory and experiment are in good agreement.     

Increasing thermomagnetic stability of composite superconductors with additives of extremely-large-heat-capacity substances

We have studied the thermomagnetic stability (with respect to magnetic flux disturbances) of composite superconductors screened by additives of rare earth compounds possessing extremely high heat capacity at low temperatures. Three tubular composite structures have been manufactured and studied with respect to screening of the central region from variations of an external magnetic field. The effect of large-heat-capacity substances (LHCSs) was evaluated by measuring a jump in the magnetic flux in response to the rate of variation (ramp) of the external magnetic field. It is established that the adiabatic criterion of stability (magnetic-flux jump field) in the sample structures containing LHCSs significantly increases—by 20% for HoCu₂ intermetallic compound and 31% for Gd₂O₂S ceramics—as compared to the control structure free of such additives.     

Self-equilibrium configurations of composite strands

We consider the problem of determining the equilibrium of composite strands consisting of a single core yarn surrounded by n-helical side yarns with pre-twists in the absence of any external load: the self-equilibrium configuration (SEC). We formulate the self-equilibrium problem on the basis of the Kirchhoff rod model in which the contact between yarns in three-dimensional space is taken into account in order to include large helical angles and geometrical constraints. We numerically determine the SECs and elucidate the effect of the following factors on them: the pre-twist of each yarn, the ratio of Young’s modulus of the core yarn to that of the side yarn, and the number of side yarns. The present results allow us to characterize the initial unloaded state of composite strands.     

Current-carrying capacity of composite superconductors

The maximum transport current Imof the composite superconductors is investigated both theoretically and experimentally. It is shown that the high values of the transport current observed in these materials is due to the non-linear part of the current-voltage characteristic in the range of low electric fields (E lsim 10^{-6}Vcm^-1). The conductors of rather different structure with Nb-Ti superconducting filaments were tested in a wide range of the external parameters. It is shown that in the external magnetic fieldsB_{a} gsim 1 Tthe ratio Im/Ic(where Icis the critical current) is the universal function of the single dimensionless parameter which depends on the sample properties and the external conditions. The theory and experiment are in a good agreement.     

The dimensionless analysis of the stability of composite superconductors with respect to thermal disturbances

The stability of superconductors with respect to thermal disturbances is determined by many factors: external conditions, properties of the composite, and nature of the heat exchange with the environment. To simulate the thermal processes mathematically, it was necessary to consider non-linear heat conduction equations which take into account the temperature dependences of the specified thermophysical parameters. The results obtained in this way depend on many parameters, which hinder establishing any general regularities. The solution of the corresponding equations in a dimensionless form reduces the number of variables and hence a generalized analysis of the phenomena can be performed     

Normal zone initiation in composite superconductors

The initiation of superconducting state instability in composite superconductors due to heat disturbances is investigated. The instability development results as a rule in the transition of the superconductor into the resistive state and normal zone propagation along the conductor. It is shown that in the case of high current density, thick conductors and poor cooling the heat power necessary to initiate the instability may be small compared to the heat power necessary to warm the superconductor to the temperature of the resistive transition.     

Propagation of normal zones in composite superconductors

This paper describes calculations of propagation velocities of normal zones in composite superconductors. Full account is taken of (1) current sharing, (2) the variation with temperature of the thermal conductivity of the copper matrix, and the specific heats of the matrix and the superconductor, and (3) the variation with temperature of the steady-state heat transfer at a copper-helium interface in the nucleate-boiling, transition, and film-boiling ranges. The theory, which contains no adjustable parameters, is compared with experiments on bare (uninsulated) conductors. Agreement is not good. It is concluded that the effects of transient heat transfer may need to be included in the theory to improve agreement with experiment.     

Flux jumps in internally cooled composite superconductors

This paper is concerned with the stability against flux jumpes of current and temperature distribution in a composite superconductor which has internal cooling channels. The stability and perturbation increments have been found.     

Superconducting and mechanical properties of YBCO-Ag composite superconductors

The measurements of electrical properties and X-ray structural analysis were made for sintered composite materials prepared by mixing the powders of the ceramic superconductor YBa₂Cu₃O_7–x (YBCO) and the powders of the pure metal element M with with M = Ni, Cu and Ag at 50 vol% proportions. The superconducting and mechanical properties were studied further on the YBCO-Ag system in the wide volume fractions 0 to 92%Ag. The onset of the superconductivity is found to occur even for samples containing more than 80 vol % Ag. The value of the critical current density is increased initially by adding silver. The mechanical strength against fracture is also improved. The X-ray structural analysis coupled with scanning electron micrographs revealed that silver and YBCO remain intact after sintering and that fine YBCO particles form continuous networks around homogeneously distributed silver particles. This explains well both superconducting and mechanical properties in the present YBCO-Ag composite system.     

The influence of nucleate to film boiling transition in liquid helium on the stability of composite superconductors under ac conditions

A model describing ac superconducting composite behaviour by taking account of the boiling transition of liquid helium, used as a refrigerant, is described. A comparison of the resultant volt-ampere characteristics with the experimental values proves the validity of the model.     

Temperature characteristics and thermal instability in composite ac superconductors

The evolution of thermal instability in a superconducting system which is operating in a normal state can lead to severe damage (burnout) of the system.On the basis of numerical calculations, the temperature characteristics of a uniform, helium-cooled, ac superconductor operating in a normal state were analyzed. The system is thermally stable as long as the current is lower than the maximum equilibrium current $\text{i}{}_{\mathrm{～}}{}^1$, or its frequency is higher than the minimum equilibrium frequency Ω¹. Near the limiting values $\text{i}{}_{\mathrm{～}}{}^1$ and Ω¹, a narrow region of parameters has been found where the conductor characteristics change drastically. According to the specific changes in the temperature characteristics in this region, the approach of dangerous values $\text{i}{}_{\mathrm{～}}{}^1$ and Ω¹ can be detected beforehand.Peculiarities of the conductor's behaviour in this region are explained and the limiting equilibrium ac cycles described. The onset of thermal instability as the superconductor overheats is also studied.The findings are well supported by experimental evidence.     

Influence of adiabatic conditions on the losses in composite superconductors

The losses in composite multifilamentary superconductors during a pulse of a transverse magnetic field under adiabatic conditions have been studied. A considerable difference in value and character of the loss dependences under adiabatic conditions, as compared to those under isothermal conditions, was found. The influence of various factors and conductor parameters on the losses was investigated.     

Dynamic analysis of composite coil springs of arbitrary shape

The dynamic behavior of composite coil springs of arbitrary shape is investigated. The Timoshenko beam theory is adopted in the derivation of the governing equation. The material of the rod is assumed to be homogeneous, linear elastic and anisotropic. The effects of the ratio maximum diameter of the cylinder/thickness (D_max/d), the number of active turns (n), the helix pitch angle (α) and the ratio of the minimum to maximum cylinder radii (R_min/R_max) on the dynamic behavior of the composite barrel and hyperboloidal springs are investigated. The free and forced vibrations of composite coil springs of arbitrary shape such as barrel and hyperboloidal springs are analyzed through various examples.     

Investigation of melt-textured superconductors on the nanoscale

For the further development of the bulk, melt-processed high-T_c superconductors it is an essential issue to control the material properties on the nanoscale, as the length scale where flux pinning takes place is of the order of 10 nm. As a consequence, we need to investigate the properties of the samples accordingly on the nanoscale. Therefore, we have performed atomic force microscopy (AFM) and scanning tunnelling microscopy (STM) scans of sample surfaces at ambient conditions which have resolved a rich variety of microstructures in the bulk samples. With the recent developments, also the (electron backscatter diffraction) EBSD technique reaches the nanometre range enabling to study the crystallographic details, especially the effect of embedded nanoparticles on the superconducting matrix. In order to obtain a direct proof of the pinning effect, the output of low-temperature STM revealing the electronic nature of the samples is studied as well. Further developments of the STM technique, e.g., employing ferromagnetic tips, may further bring informations on the flux pinning properties.     

Flux jumps in composite superconductors in a time dependent magnetic field

Experimental investigations of the stability behaviour of NbTi filamentary superconducting composites in a time-varying external magnetic field are reported. The magnetization due to the induced interfilamentary screening currents shows flux jumps. The influence of this sort of flux jump on dH/dt and on the twist rate has been measured, together with the influence exerted by the presence of a thin high-resistivity CuNi sheath surrounding the filaments.     

Possible method to improve the stability of twisted superconductors

The resistivity of the normal region between neighboring and opposite superconducting stripes/strands/filaments in twisted structures should be large enough to obtain the coupling losses at acceptable level. The stability of such structures can be then very low, as the individual superconductors are more or less electrically insulated. In such structures, any electromagnetic perturbations or spatial inhomogeneities can be detrimental for the conductor and eventually for the whole design. Concentrating mainly on helically wound striated coated conductors, we show that placing some normal sheets on the superconductor tapes without increasing the coupling losses considerably can enhance the electromagnetic coupling. The results may be extended also to similar superconducting structures with not very high coupling losses, but requiring better stability, because current sharing between superconducting parts is necessary for electromagnetic stability of longer conductors.     

Current voltage characteristics of composite superconductors with high contact resistance

An experimental study has been made of current-voltage characteristics of composite superconductors with contact resistance between superconducting filaments and normal metal with high electrical conductivity. It is shown that stable resistive states exist in such conductors over a wide range of currents. The presence of resistive states is interpreted in terms of the resistive domain concept. The minimum and maximum currents of resistive states are found to be dependent on the electrical resistance of normal metal and magnetic field.     

An experimental study of propagation of a normal zone in composite superconductors

This paper describes the technique and results of an experimental study on the propagation of normal zones under non-equilibrium conditions in composite superconductors. Bifilar windings of these superconductors were separated from the power source by a uniform magnetic field to allow the characteristic features of propagation to be studied. Normal zones were produced by heating elements. The experimental results obtained are compared graphically with theoretical results derived previously.     

Thermal stability of composite slab-structures in harsh environment

This paper presents a complete analysis of the effect of harsh environment found in the Arabian Gulf region, especially in UAE, on thermal stability of composite slab-structures with temperature-sensitive internal heat generation. These structures can include building slabs, highway bridge slabs and tunnels, cooling towers, silos, and rotundas. Depending on the application, composite slab-structures may be subjected to sudden and violent thermal effects on their surface due to different applications and various severe environmental conditions. The combination of theses conditions with the possible internal heat generation can provoke some defects that may lead to the degradation of the material properties and the structure function. Therefore, thermal instability of structures will result if the structure fails to dissipate all heat conducted into and generated from/within the structure. It is very important for a designer to select the appropriate material for an application and predict the structure behavior under different operating conditions. In this regard, it is interesting to note that the Great Coulee dam in France was poured with pre-chilled concrete just to compensate for the heat release for the exothermic reaction within the concrete long after the pouring process. An analytical model was developed for the generalized problem that includes the effect of the non-linearity and non-homogeneity of the internal heat generation. Three case studies with combinations of encountered severe boundary conditions are presented and analyzed. The results of these analyses indicate that the critical parameters for thermal stability are critically dependent on the environmental curing process, structure size, the material properties, the heat transfer coefficients, and other constants accrued from the structure environment. The critical values for thermal stability of the composite slab structures may be significant to the service-worthiness of industrial products, to the life span of the structures, as well as to public construction safety.