Nanoengineering of Flux Pinning Sites in High-Tc Superconductors

Volume pinning forces were determined for a variety of bulk high-Tc superconductors of the 123-type from magnetization measurements. By means of scaling of the pinning forces, the acting pinning mechanisms in various temperature ranges were identified. The Nd-based superconductors and some YBCO crystals exhibited a dominating pinning of the δTc-type (i.e. , small, superconducting pinning sites). In contrast to this, the addition of insulating 211 particles provided pinning of the δ/-type; providing effective pinning in the entire temperature range acting as a "background" pinning mechanism for the peak effect. Due to the small coherence lengths of the high-Tc compounds, effective pinning sites are defects or particles of nanometer size relative to ζ3. Integral magnetic measurements of the magnetization as a function of temperature in large applied magnetic fields (up to 7 T) revealed that practically all high-Tc compounds were spatially inhomogeneous, which could be caused by oxygen deficiency (YBCO)     

Sample Size Dependence of Second Magnetization Peak in Type－II Superconductors

We show that the second magnetization peak (SMP), i. e., an increase in the magnetization hysteresis loop width in type-II superconductors,vanishes for samples smaller than a critical size. We argue that the SMP is not related to the critical current enhancement but can be well explained within a framework of the thermomagnetic flux-jump instability theory, where flux jumps reduce the absolute irreversible magnetization relative to the isothermal critical state value at low enough magnetic fields. The recovering of the isothermal critical state with increasing field leads to the SMP. The low-field SMP takes place in both Iow-Tc conventional and high-Tc unconventional superconductors. Our results show that the restoration of the isothermal critical state is responsible for the SMP occurrence in both cases.     

Modulating the magnetic relaxation of lanthanide-based single-molecule magnets

In the area of molecular magnetism,single molecule magnets (SMMs) containing lanthanide elements are of immense scientific and technological interest because of their large energy barriers and high measured hysteresis temperature.Although there has been significant progress in the synthesis and characterization of lanthanide-based SMMs,there are still challenges,for instance,how single-ion anisotropy of lanthanide elements can be exploited,and how zero-field tunneling of magnetization can be suppressed.This article is devoted to the progress in various methodologies for modulating magnetic relaxation,especially in terms of crystal field and magnetic interactions.The crystal field plays a dominant role in creating single-molecule magnets with largely anisotropic f-elements,while the strong coupling between magnetic centers is able to suppress quantum tunneling of magnetization efficiently.     

Application of high magnetic fields in advanced materials processing

Recently,steady magnetic fields avail-able from cryogen-free superconducting magnets open up new ways to process materials. In this paper,the main results obtained by using a high magnetic field to process several advanced materials are re-viewed. These processed objects primarily include superconducting,magnetic,metallic and nanome-ter-scaled materials. It has been found that a high magnetic field can effectively align grains when fab-ricating the magnetic and non-magnetic materials and make inclusions migrate in a molten metal. The mechanism is discussed from the theoretical view-point of magnetization energy.     

Strong Localization Effect of Oxygen Deficiency on Carriers in T1 Based Superconductors

Investigation on the effect of Fe-doped T1-1223 superconductors has been carried out by the simultaneous measurements of the spectra of positron annihilation lifetime and Doppler broadening of position annihilation,together with the measurement of Hall coefficient. The results of samples with different doping level show that the occupation of Fe atoms on Cu sites results in a linear decrement of superconducting transition temperature. The electron concentration in Cu-O layer has been enhanced by Fe doping. The difference in valence between Fe3+ and Cu2+ induces extra oxygen into the lattice and forms the extra oxygen defects. This Fe dopant leads to a strong localization of the electrons in the Cu-O layer. So the decrement of the concentration of the itinerant electrons results in a decline of the superconducting transition temperature.     

Measurements and analysis of force and moment of caudal fin model in C-start

The unsteady hydrodynamic forces and moments acting on caudal-fin models of fish with different shapes and different swing durations were experimentally measured to simulate the fish C-starts. The motion of models was characterized by rotating the model to a maximum deflection angle in an excursion time T_u and back to the initial position in a return time T_d around its root-axis. Studies show that the caudal-fin plays an important role in fish C-starts and the caudal-fins with different shapes and different swing durations generate different forces and moments. In addition, the hydrodynamic forces and moments acting on the models with different shapes can be normalized by the 2nd and 3rd moments of area, respectively. The forces and moments acting on the models with different swing durations, but the same ratio of T_u to Td can also be scaled.     

Synthesis,characterization and magnetic properties of KFeO2 nanoparticles prepared by a simple egg white solution route

Nanoparticles of potassium ferrite(KFeO₂)in this work were synthesized by a simple egg white solution method upon calcination in air at 773,873,and 973 K for 2 h.The effects of calcination temperature on the structural and magnetic properties of the synthesized KFeO₂ nanoparticles were investigated.By varying the calcination temperature,X-ray diffraction and transmission electron microscopy results indicated the changes in crystallinity and morphology including particle size,respectively.Notably,the reduction in particle size of the synthesized KFeO₂ was found to have a remarkable influence on the magnetic properties.At room temperature,the synthesized KFeO₂ nanoparticles prepared at 873 K exhibited the highest saturation magnetization(M_S)of 2.07×10⁴ A·m^?1.In addition,the coercivity(H_C)increased from 3.51 to 16.89 kA·m^?1 as the calcination temperature increased to 973 K.The zero-field cooled(ZFC)results showed that the blocking temperatures(T_B)of about 125 and 85 K were observed in the samples calcined at 773 and 873 K,respectively.Therefore,this work showed that the egg white solution method is simple,cost effective,and environmentally friendly for the preparation of KFeO₂ nanoparticles.     

Josephson TunnelLing in YbCo-Nb Bulk Junctions

After the discovery of very high transition temperature in the metallicoxide superconductors many efforts have been devoted to the determinationof the relevant physical parameters of these materials such as the energygap value, the coherence lenght, the magnetic penetration depth, etc. Tothis purpose bulk junctions based on YbCO pelliets employing a film of a     

Analysis of magnetic mechanisms of 3d-doped ZnO diluted magnetic semiconductors by an abnormal peak on M-T curve

The crystallographic structures and magnetic properties of a Zn0.95Co0.05O thin film deposited on a C-sapphire substrate using a dual-beam pulsed laser deposition method were characterized. It was shown from crystallographic analysis that the film belongs to the wurtzite structure with the C-axis aligned with that of the substrate. Magnetic hysteresis loops were observed till up to room temperature. A small peak around 55 K was noticed on the magnetization vs. temperature curve. The corresponding temperature of the small peak is close to that of ‘the abnormal peak’ reported by X.M. Zhang et al. From the results obtained, no correlation was found between the abnormal peak and the quantum effects. The magnetic behaviors in the Zn0.95Co0.05O film cannot be explained by the ferromagnetism in diluted magnetic semiconductors. The magnetic mechanisms in ZnO-based diluted magnetic semiconductors are also discussed.     

Influence of structure and atom sites on Sn-based anode materials for lithium ion batteries: a first-principle study

To understand the influence of structure and atom sites on the electrochemical properties of Sn-based anode materials,the lithium intercalation–deintercalation mechanisms into SnNi2Cu and SnNiCu2phases were studied using the first-principle plane wave pseudo-potential method.Calculation results showed that both SnNi2Cu and SnNiCu2were unsuitable anode materials for lithium ion batteries.The Sn-based anode structure related to the number of interstitial sites,theoretical specific capacity,and volume expansion ratio.Different atom sites led to different forces at interstitial sites,resulting in variations in formation energy,density of states,and hybrid orbital types.In order to validate the calculated model,the SnNi2Cu alloy anode material was synthesized through a chemical reduction-codeposition approach.Experimental results proved that the theoretical design was reasonable.Consequently,when selecting Snbased alloy anodes,attention should be paid to maximizing the number of interstitial sites and distributing atoms reasonably to minimize forces at these sites and facilitate the intercalation and deintercalation of lithium ion.     

Micro-mechanism of metal magnetic memory signal variation during fatigue

Tensile fatigue tests were designed to study the relation between the tangential magnetic memory signal and dislocations. According to experimental results, in the early stage of fatigue, the magnetic signal and the dislocation density rapidly increase; while in the middle stage, the magnetic signal gradually increases, the dislocation density remains steady, and only the dislocation structure develops. On the other hand, in the later stage, the magnetic signal once again increases rapidly, the dislocation structure continues to develop, and microscopic cracks are formed. Analysis reveals that the dislocations block the movement of the domain wall, and the area of dislocation accumulation thus becomes an internal magnetic source and scatters a field outward. In addition, the magnetic memory field strengthens with increasing dislocation density and complexity of the dislocation structure. Accordingly, the dislocation pinning factor related with the dislocation density and the dislocation structure has been proposed to characterize the effect of dislocations on the magnetic memory signal. The magnetic signal strengthens with an increase in the dislocation pinning factor.     

Effect of reduction roasting by using bio-char derived from empty fruit bunch on the magnetic properties of Malaysian iron ore

Beneficiation of Malaysian iron ore is becoming necessary as iron resources are depleting. However, the upgrading process is challenging because of the weak magnetic properties of Malaysian iron ore. In this study, bio-char derived from oil palm empty fruit bunch (EFB) was utilized as an energy source for reduction roasting. Mixtures of Malaysian iron ore and the bio-char were pressed into briquettes and subjected to reduction roasting processes at 873–1173 K. The extent of reduction was estimated on the basis of mass loss, and the magnetization of samples was measured using a vibrating sample magnetometer (VSM). When reduced at 873 K, the original goethite-rich ore was converted into hematite. An increase in temperature to 1073 K caused a significant conversion of hematite into magnetite and enhanced the magnetic susceptibility and saturation magnetization of samples. The magnetic properties diminished at 1173 K as the iron ore was partially reduced to wustite. This reduction roasting by using the bio-char can assist in upgrading the iron ore by improving its magnetic properties.     

Sample Size Dependence of Second Magnetizaion Peak in Type-ⅡSuperconductors

We show that the second magnetization peak(SMP),i.e.,an increase in the magnetization hysteresis loop width in type-11 superconductors,vanishes for samples smaller than a critical size,We argue that the SMP is not related to the critical current enhancement but can be well explainde within a frameword of the thermomagenetic flux-jump in stability theory,where flux jumps reduce the absolute irreversible magnetization relative to the isothermal critical state value at low enough magnetic fields.The recovering of the isothermal critical sate with increasing field leads to the SMP.The low-field SMP takes place in both low-T.conventional and high*t.unconventional superconductors.Our results show that the rsstoration of the isothermal critical state is rsponsible for the SMP occurrence in both cases.     

Doping effects of ZrC and ZrB2 in the powder-in-tube processed MgB2 tapes

We have investigated the effects of ZrC and ZrB2 doping on the superconducting properties of the powder-in-tube processed MgB2/Fe tapes. Sam- ples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM/EDX), transport and magnetic measurements. We confirmed the fol- lowing quite different roles of ZrC and ZrB2 in MgB2. ZrC doping was found to decrease the transport critical current density (Jc) at 4.2 K, while the critical temperature (Tc) kept constant. In contrast, the Jc values in magnetic fields were enhanced greatly by the ZrB2 addition, which resulted in a decrease in Tc by only 0.5 K. The reason for different effects of two dopants is also discussed.     

Thermal stability and magnetic properties of Fe–Co–M–Zr–Nb–Ge–B(M=Mo,Cr) bulk metallic glasses

Fe62Co8 xMxZr6Nb4Ge1B19(M=Mo, Cr) bulk metallic glasses were synthesized in the diameter range up to 2 mm by copper mold casting,which exhibit high thermal stability and large glass-forming ability. The super-cooled liquid region diminishes by the dissolution of Mo. The addition of 2 at% Cr leads to the broading of the liquid region remarkably, resulting in the improvement of thermal stability. The crystallization takes place through a single exothermic reaction, accompanying the precipitation of more than three kinds of crystallized phases such as α-Fe,Fe2Zr and ZrB2. The Fe-based alloys show soft ferromagnetic properties. The saturation magnetization(ss) decreases with increasing Mo or Cr content while the saturated magnetostriction increases with raising Mo or Cr content. There is no evident change in the ssand coercive force(Hc)with annealing temperature below the crystallization temperature, which suggests a more relaxed atomic configuration the glasses have. The crystallization causes a substantial enhancement in both ssand Hc. Each soft magnetic property of the glasses containing Cr with higher thermal stability is superior to that of the alloys containing Mo.     

Infield superconducting properties of nano-sized Ag added Cu_(0.5)Tl_(0.5)Ba_2Ca_2Cu_3O_(10-δ)

Infield superconducting properties of {(Ag)_x/CUTl-1223};(x = 0, 0.5,1.0.2.0 and 4.0 wt %) nanoparticlessuperconductor composites were investigated under different applied magnetic field up to H = 8 T. The increase in critical temperature {T_c(0)} and decrease in normal state resistivity(p_o) were observed with increasing contents of Ag nanoparticles in CuTl-1223 superconducting matrix. The value of T_c~(onset)(K) remained almost unaffected by applying external magnetic field, but a decreasing trend in T_c(0) was observed by increasing the value of external applied magnetic field. The transition region below T_c~(onset)(K) showed Arrhenius behavior due to thermally activated flow of magnetic vortices. Enhancement in flux flow activation energy(U_o) and suppression in transition width(ΔT) of CuTl-1223 phase with addition of Ag nanoparticles showed the improvement in flux pinning strength of superconductor.     

Superconductivity in MgB2

In January of 2001 the superconductivity of the compound MgB2 with a critical temperature Tc of up to 39 K was discovered. This Tc is the highest in all intermetallic compound and alloy superconductors. MgB2 has a simple structure and its manufacturing capital cost is lower, therefore it could become a practical superconductor in the future. The recent progress is reviewed here which covers the progress in electronic structure, high Tc mechanism, superconducting parameters (Debye temperature, specific heat coefficient of electron, critical fields, coherent length, penetration depth, energy gap, critical current and relaxation rate of flux). Moreover the issue on power transmission is discussed.     

On the structural features of fiber suspensions in converging channel flow

The structural features of fiber suspensions are dependent on the fiber alignment in the flows. In this work the orientation distribution function and orientation tensors for semi-concentrated fiber suspensions in converging channel flow were calculated, and the evolutions of the fiber alignment and the bulk effective vis-cosity were analyzed. The results showed that the bulk stress and the effective viscosity were functions of therate-of-strain tensor and the fiber orientation state ; and that the fiber suspensions evolved to steady alignment and tended to concentrate to some preferred directions close to but not same as the directions of local stream-lines. The bulk effective viscosity depended on the product of Reynolds number and time. The decrease of ef-fective viscosity near the boundary benefited the increase of the rate of flow. Finally when the fiber alignment went into steady state, the structural features of fiber suspensions were not dependent on the Reynolds numberbut on the converging channel angle.