Showing papers by "Masato Murakami published in 2008"

Record flux pinning in melt-textured NEG-123 doped by Mo and Nb nanoparticles

Abstract: We report on a highly improved pinning performance in the melt-textured (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy+35mol% Gd2BaCuO5 (70nm in size) composite doped by tiny amounts of nanometer-sized MoO3 or NbO3 particles. The doping led to a spontaneous formation of clouds with extremely small (⩽10nm) (Nd,Eu,Gd)2BaCuXO5 particles, X=Mo,Nb. With an optimum content of MoO3 and NbO3, the self-field supercurrent at 77K was twofold and threefold higher than before, respectively. At 65K the supercurrent reached 700kA∕cm2 with Mo and 925kA∕cm2 with Nb at both 0 and 4.5T, and the self-field current at 90K was 100A∕cm2 in the Nb-doped sample, which is a value high enough for a safe levitation.

The effects of thermomechanical training treatment on the deformation characteristics of Fe–Mn–Si–Al alloys

Abstract: We have studied the effects of the thermomechanical training treatment on the deformation mode for Fe–30Mn–(6 − x )Si– x Al ( x = 0, 1, 2 and 3 mass%) alloys, which exhibit complicated deformation processes due to coexistence of the stress-induced fcc to hcp martensitic transformation and the mechanical twinning of the fcc matrix in addition to slip deformation. The stress–strain curves at each training cycle were analyzed for the alloys, and the deformation microstructures were observed with atomic force microscopy on a Fe–30Mn–5Si–1Al alloy before and after the training treatment. It was found that the deformation mode shifts from the mechanical twinning to the stress-induced martensitic transformation, as the training proceeds.

A study on levitation force and its time relaxation behavior for a bulk superconductor-magnet system

Abstract: We have studied the levitation force of a Gd–Ba–Cu–O bulk superconductor 60 mm in diameter and 20 mm in thickness with a newly developed superconducting magnet system that can provide high field gradient. The magnet system was composed of two superconducting coils that are co-axially placed, and currents are passed in the opposite direction. To study the effect of radial and vertical magnetic field components on the levitation force and its time relaxation, the superconductor was placed at several locations. It was confirmed that the levitation force is strongly dependent on the radial component of magnetic fields. It was also found that the time relaxation was reduced without sacrificing the levitation force by increasing the radial component of the external field.

Force Density of Magnetic Bearings Using Superconducting Coils and Bulk Superconductors

Abstract: Stable levitation or suspension of a heavy object in mid-air can be realized using a combination of a permanent magnet and a bulk superconductor with high critical current density, in that the force density has reached 100 kN/m2. The superconducting flywheel system for energy storage is attractive due to a great reduction in the rotational loss of the bearings So long as a permanent magnet is used as a magnetic source, however, the electromagnetic force (EMF) is essentially limited by its field strength. In order to overcome such a limitation, we employed a superconducting coil/magnet as a magnetic source and studied the EMF characteristics of bulk superconductors in high magnetic fields. We also measured the EMF between a bulk superconductor and a specially designed superconducting coil having a high field gradient, and confirmed that the EMF reached 9,000 N (force density: 1,190 k N/m2).

AFM observation of microstructural changes in Fe-Mn-Si-Al shape memory alloy

Abstract: We analyzed the surface relief caused by stress-induced hcp martensitic transformation in Fe-30Mn-5Si-lAl shape memory alloy by atomic force microscopy. The alloy exhibits a good shape memory effect and an improved ductility due to a small addition of Al to a conventional Fe-30Mn-6Si shape memory alloy. The orientation of an austenite matrix was determined with surface traces of four {111} f planes, which enabled us to determine the surface tilt angles for all twelve variants of hcp martensites and deformation twins. On the basis of these values, stress-induced martensite and deformation twin coexisting in the same grain were identified by studying the surface tilt angles. The surface relieves caused by the stress-induced martensite recovered after heating above the reversed transformation temperature, however some relieves originating from the deformation twin remained.

Observation of record flux pinning in melt-textured NEG-123 superconductor doped by Nb, Mo, and Ti nano particles

Abstract: Flux pinning in melt-processed (Nd 0.33 Eu 0.33 Gd 0.33 )Ba 2 Cu 3 O y "NEG-123" + 35 mol% Gd 2 BaCuO 5 "NEG-211" (70 nm in size) composite doped by Ti0 3 , Mo0 3 and Nb 2 O 5 achieved record values. The optimum values of all three dopands were found to be around 0.1 mol%. Transmission electron microscope (TEM) analysis found clouds of <10 nm sized particles in the NEG-123 matrix, shifting the pinning particle size distribution to significantly lower values. TEM by energy dispersive X-ray spectroscopy (EDX) analysis clarified that these nanoparticles contained a significant amount of Nb, Mo, and Ti. Appearance of nanometer-sized defects correlated with a significantly improved flux pining at low and medium magnetic fields, which was particularly significant at high temperatures. In the Nb-doped sample, a record J c value of 925 kA/cm 2 at the secondary peak field (4.5 T) was achieved at 65 K, 640 kA/cm 2 at zero field at 77 K, and 100 kA/cm 2 at 90.2 K, the last value having been up to now considered as a good standard for REBa 2 Cu 3 O y "RE-123" materials at 77 K. The greatly improved J c ―B performance in Nb/Mo/Ti doped samples can be easily translated to large-scale LRE-123 (LRE = light rare earths, Nd, Eu, Gd, Sm) blocks intended for real superconducting super-magnets applications.

Single domain YBCO/Ag bulk superconductors fabricated by seeded infiltration and growth

Abstract: We have applied the seeded infiltration and growth (IG) technique to the processing of samples containing Ag in an attempt to fabricate Ag-doped Y-Ba-Cu-O (YBCO) bulk superconductors with enhanced mechanical properties. The IG technique has been used successfully to grow bulk Ag-doped YBCO superconductors of up to 25 mm in diameter in the form of single grains. The distribution of Ag in the parent Y-123 matrix fabricated by the IG technique is observed to be at least as uniform as that in samples grown by conventional top seeded melt growth (TSMG). Fine Y-211 particles were observed to be embedded within the Y-123 matrix for the IG processed samples, leading to a high critical current density, Jc, of over 70 kA/cm2 at 77.3 K in self-field. The distribution of Y-211 in the IG sample microstructure, however, is inhomogeneous, which leads to a variation in the spatial distribution of Jc throughout the bulk matrix. A maximum-trapped field of around 0.43 T at 1.2 mm above the sample surface (i.e. including 0.7 mm for the sensor mould thickness) is observed at liquid nitrogen temperature, despite the relatively small grain size of the sample (20 mm diameter × 7 mm thickness).

The microstructure and properties of single grain bulk Ag-doped Y–Ba–Cu–O fabricated by seeded infiltration and growth

Abstract: Single grain, Ag-doped Y–Ba–Cu–O bulk superconductors of up to 30 mm in diameter have been fabricated successfully by a seeded infiltration and growth (IG) technique. These samples exhibit low porosity compared with samples grown by conventional top seeded melt growth (TSMG). The density of voids within as-processed IG samples can be reduced further by adding metallic Ag, rather than Ag2O, to the Y2BaCuO5 (Y-211) pre-form. An inhomogeneous distribution of Y-211 has been observed in these samples. Finally, a maximum trapped magnetic field of 0.5 T was observed for Ag-doped Y–Ba–Cu–O of diameter 30 mm at liquid nitrogen temperature (77 K).

Current progress in ternary LREBa2Cu3Oy materials and their application

Abstract: The J c and H irr values at 77 K of ternary light-rare-earth compounds, LREBa 2 Cu 3 O y “LRE-123”, are usually high enough to serve in various applications. Several sources of vortex pinning can be in these composites tailored to fit the needs of the particular application. The list comprises LRE/Ba solid solution, oxygen vacancies, large particles of secondary phases, twin planes, nanoscale lamellas, etc. By means of the latter defects one can achieve a very high irreversibility field. Refinement of secondary phase particles and the optimal choice of their amount enhance the electromagnetic performance in a broad temperature range, up vicinity of T c , allowing levitation at liquid oxygen, 90.2 K. An optimum content of MoO 3 doubles the self-field super-current at 77 K, H||c -axis. Altogether, the pinning tailoring in ternary LRE-123 materials provides a flexible and reliable way to fit the electromagnetic performance with the needs of sophisticated high-temperature and high-magnetic-field applications.

Optimum processing conditions for the fabrication of large, single grain Ag-doped YBCO bulk superconductors

Abstract: A pseudo Time–Temperature-Transformation (TTT) diagram has been constructed for the optimum melt processing conditions for the fabrication of large, single grain Ag-doped Y–Ba–Cu–O (YBCO/Ag) bulk superconductors. The thermal conditions for the formation of homogeneous nuclei have been investigated and the boundary between this region and that for no grain nucleation has been mapped as a function of holding time. YBCO/Ag bulk samples grow typically in the form of single grains when the processing temperature profile lies within the ‘no nucleation’ region of the pseudo TTT diagram. However, by studying the YBCO/Ag growth process, some samples have been grown in the form of single grains by employing a temperature profile within the homogeneous grain nucleation region of the phase diagram. Such growth may be achieved when the YBCO/Ag phase constitutes the majority of the volume of the pellet before the growth temperature reaches that at the nucleation boundary. A large, single YBCO/Ag grain of diameter 30 mm has been fabricated successfully based on these studies by both cold-seeding and seeded infiltration growth techniques.

Materials processing, pulsed field magnetization and field-pole application to propulsion motors on Gd123 bulk superconductors

Abstract: Gd123 bulk superconductor is one of the promising magnet materials. We studied the materials processing to grow high performance magnet with a doping of nano-sized metal oxides such as ZrO2 as a candidature of pinning centre. The enhancement of the critical current density was obtained. Growth of nano-sized particles of Gd211 in addition to BaZrO3 were observed by TEM. The formation of nano-sized particles appears a key to improve the integrated flux trapped inside the bulks and the TEM reveals an intriguing effect of the addition to the microstructure of bulk materials. Magnetization process is crucial especially for an extended machinery. Pulsed field magnetization was applied to the field-pole bulk on the rotor disk of the tested synchronous motor. The trapped flux density of 1.3 T for Gd123 bulk sample and of 60 mm diameter was reached in the limited dimension of the tested motor by a step cooling method down to 38 K with a closed-cycle condensed neon. The pulsed magnetic field was applied with a new type of split-armature coil. A large bulk of 140 mm diameter has also shown a potential flux trapping superior to other smaller specimens. The bulk magnet provides a strong magnetic field around the bulk body itself with high current density relative to a coil winding. A comparative drawing of a torque density of a variety of motors which is defined as the torque divided by the volume of the motor indicates a potential advantage of bulk motor as a super permanent magnet motor.

Quenching characteristics of electrical contacts using YBaCuO bulk superconductor

Abstract: A persistent current switch (PCS) is used for various superconducting applications, such as superconducting magnetic energy storage (SMES) system. The authors have proposed a mechanical switch of Y–Ba–Cu–O (YBCO) bulk as a PCS. In previous study, in order to investigate the contact characteristics of the switch, 50 A current test was performed and the contact resistance between two contacts was measured by four terminal method. As a result, it became clear that polishing and metal depositing on YBCO surface was effective to reduce the residual resistance, and the authors achieved to reduce this resistance to 0.27 μΩ, which was less than our target value of 1 μΩ. At the same time, the greater current test (above 50 A) was performed in order to examine the transition to normal conductive state in the YBCO contacts. As a result, it could be found that the current value when a local quench occurred (local quenching current: LQC) had a strong relationship with the layer thickness of deposited silver on YBCO surface. This paper focused on the relationship between the transition current to the normal conductive state in the YBCO (quenching current) and the layer thickness of deposited silver on YBCO surface. And it was also observed whether the number of current cycles affected the value of quenching current.

Homogenizing field-trapping capabilities of bulk Y-Ba-Cu-O superconductors using Ba-Cu-O oxide substrate for practical applications

Abstract: We employed Ba-Cu-O substrates for fabrication of bulk Y-Ba-Cu-O superconductors in the top-seeded melt-growth method. There were several advantages for a use of a Ba-Cu-O substrate compared to conventional substrate materials such as MgO, ZrO2, Al2O3, RE123 and RE211 (RE = rare earth). The Ba-Cu-O substrate avoided crystallization from a substrate, suppressed a liquid loss and then scarcely reacted with a precursor. Furthermore, the introduction of large-sized cracks into a grown bulk greatly was suppressed by propagating along the interface between a grown bulk and a substrate. We could obtain bulk Y-Ba-Cu-O superconductors with uniform trapped magnetic filed distributions and equal maximum trapped filed values on both top and bottom surfaces, which indicates that the field-trapping capabilities were homogenized along the c-axes of the bulks fabricated on the Ba-Cu-O substrates.

Homogenizing field-trapping capabilities of bulk Y?Ba?Cu?O superconductors fabricated using Ba?Cu?O oxide substrates for practical applications

Abstract: We employed Ba–Cu–O substrates for fabrication of bulk Y–Ba–Cu–O superconductors using the top-seeded melt-growth method. There were several advantages of using a Ba–Cu–O substrate as compared to conventional substrate materials such as MgO, ZrO2, Al2O3, RE123 (RE = rare earth) and RE211. The Ba–Cu–O substrate avoided crystallization from the substrate, suppressed liquid loss and scarcely reacted with the precursor. Furthermore, the introduction of large-sized cracks into a grown bulk was greatly suppressed by propagation along the interface between the grown bulk and the substrate. We were able to obtain bulk Y–Ba–Cu–O superconductors with uniform trapped magnetic field distributions and equal maximum trapped field values on both top and bottom surfaces, which indicates that the field-trapping capabilities were homogenized along the c-axes of the bulks fabricated on the Ba–Cu–O substrates.

Synthesis, Characterization and melt processing of Y1-x(Yb0,9,Nd0,1)xBa2Cu3Oz superconductors

Abstract: Abstract Effects of a combined substitute of Yb and Nd on Y site on the superconducting properties of YBa 2 Cu 3 O y have been studied. We synthesized Y 1− x (Yb 0.9 Nd 0.1 ) x Ba 2 Cu 3 O z compound with x = 0.2, 0.4, 0.6, 0.8 and 1.0. Here, the ratio of Yb–Nd was fixed to be 9:1 for obtaining 123 phase without secondary phases. The melt processing thermal profiles for Y 1− x (Yb 0.9 Nd 0.1 ) x Ba 2 Cu 3 O z with x = 0.2 and 0.4 and the addition of 40 mol% {Y 1− x (Yb 0.9 Nd 0.1 ) x } 2 BaCuO 5 and 0.5 wt% Pt in air were determined on the basis of the thermal analysis results. All samples showed a low grain growth rate, particularly for high x values, which may be partially ascribed to un-optimized thermal schedules. Although almost all the samples exhibited low J c values, the sample with x = 0.2 exhibited T c of 88.8 K and a relatively high J c value of 16,000 A/cm 2 at 77 K for H//c -axis.

Thermal conductivity of Er–Ba–Cu–O and Ho–Ba–Cu–O superconducting bulks

Abstract: We have measured the temperature dependence of the in-plane thermal conductivity, κab(T), for both Er–Ba–Cu–O and Ho–Ba–Cu–O superconducting bulks. The absolute values of κab(T) for both bulks are smaller than that of Y–Ba–Cu–O bulk, but are larger than that of Dy–Ba–Cu–O bulk, which is known to show an anomalous small κab. κab(T) for Ho–Ba–Cu–O shows a positive slope (dκab/dT>0) in the normal state, similarly to that for Dy–Ba–Cu–O. On the other hand, a slightly negative slope (dκab/dT<0) is found for Er–Ba–Cu–O. The out-of-plane thermal conductivity, κc, was also measured for Er–Ba–Cu–O bulk, and the anisotropy in the thermal conductivity, (κab/κc), was estimated to be about 2.

Quenching Characteristics of YBCO Bulk Contacts

Abstract: A persistent current switch (PCS) is used for superconducting applications, such as superconducting magnetic energy storage (SMES) system. The authors have proposed a mechanical switch of Y-Ba-Cu-O (YBCO) bulk as a mechanical PCS. In previous study, in order to investigate the contact characteristics of the switch, 50A current tests were performed and the contact resistance between two YBCO contacts was measured by four terminal method. As a result, it became clear that polishing and metal depositing on contact surfaces was effective to reduce the residual resistance, and the authors achieved to reduce this resistance to 0.27 muOmega, which was less than our target value of 1 muOmega. This paper focused on the transfer from superconductive to normal conductive state in the YBCO contacts (quenching characteristics), which was not obtained by former 50 A current test. In order to perform this experiment, 150 A DC power supply and two kinds of curvature silver contacts (R15 and R7.5) as an anode were introduced. The authors investigated that the relationship between the quench current and the layer thickness of deposited silver on the YBCO surface. And it was also observed whether the curvature radius for silver contact affected the value of quench current. Additionally, the contact area of the sample was analyzed with FEM. The analytical results may lead to the elucidation of a contact mechanism and a quench mechanism.

Anisotropic thermal conductivity of Er–Ba–Cu–O bulk superconductors

Abstract: We have measured the temperature dependences of the thermal conductivity in the ab -plane, κ ab ( T ), and along the c -axis, κ c ( T ), respectively, for the Er–Ba–Cu–O bulk superconductors, which were fabricated by a method of the melt texture growth. Above the critical temperature T c , the κ ab decreases quite moderately as increasing temperature or is almost independent of the temperature. Below the T c , the well-known enhancement is observed but is relatively small. The κ c ( T ) shows the small absolute values and we found that the anisotropy of the thermal conductivity ( κ ab / κ c ) for Er–Ba–Cu–O bulks is of about 2–6.

Influence of al concentration on deformation behavior and fracture mode of fe-30mn-6(SI, AL) alloys

Abstract: Microstructural and fractographic observations were systematically done on Fe-30Mn(6-x)Si-xAl (x=0, 1, 2 and 4 mass %) alloys. Optical and transmission electron microscopic observations and X-ray diffractions revealed that the deformation mode continuously shifts from the stress induced fcc/hcp martensitic transformation to the mechanical twinning of the fcc austenite as the Al content increases. It was also clarified by the scanning electron microscopic observations that the microstructural change depending on the Al content is accompanied by the change in the fracture mode from the quasi-cleavage fracture to the ductile fracture. INTRODUCTION Fe-Mn-Si-based shape memory alloys (SMAs) exhibit the shape memory effect (SME) associated with fcc (γ-austenite) / hcp (e-martensite) martensitic transformation [1]. A recoverable strain obtained in a typical Fe-Mn-Si SMA: e. g. Fe-30Mn-6Si (hereinafter compositions are shown in mass%), was reported about 2% in the solution treated condition [2]. This value can be increased to about 4% by so-called the training treatment [3, 4] and fine dispersion of precipitates such as NbC carbides [5-7], etc. One drawback of the alloy was its poor ductility of about 30%. In contrast to this, it was recently reported that the Fe-30Mn-3Si-3Al TWIP (Twinning Induced Plasticity) steel exhibits the ultra-high ductility as much as about 90% [8], but this alloy shows no significant SME. The composition of the Fe-30Mn-3Si-3Al TWIP steel is such that a part of Si in the Fe30Mn-6Si SMA is replaced by Al. In order to systematically investigate the effect of the Al content on the SME and TWIP effect, the present authors prepared four kinds of FeMn-Si-Al alloys by gradually varying the amount of Al substituting Si: i. e. Fe-30Mn(6-x)Si-xAl (x=0, 1, 2 and 4). The following two conclusions were drawn as a result [9]: i) the alloys with x=0 and 1 exhibited similar SME, but no recognizable SME was observed for the alloys with x>2, ii) the ductility linearly increased with increasing the amount of Al. The above-mentioned changing tendencies may originate from the continuous change in the deformation mode from the stress-induced γ → e martensitic transformation to the mechanical γ twinning. However, there has been no systematic study on the effect of the Al content on the deformation and fracture modes between the SMA and the TWIP. In the present paper, microstructural observations using optical microscopy (OPM) and transmission electron microscopy (TEM), phase identification using X-ray diffraction (XRD) and the fractographic observations using scanning electron microscopy (SEM) were carried out to clarify the effect of the Al content on the deformation mode and the corresponding fracture mode of the alloys. EXPERIMENTAL In this paper, hereafter the Fe-Mn-(6-x)Si-xAl (x=0, 1, 2 and 4) alloys are referred to as Al-0,Al-1,Al-2,Al-3 using mass % of Al. The specimens were prepared by vacuum induction melting. After hot forging and rolling at 1270K, the specimens were subjected to solution treatment at 1270K for 3h followed by water quenching. The OPM observations were performed on the samples, which were mechanically and electrolytically polished to obtain smooth surfaces and then extended by about 3%, using a differential interference microscope. The phase constitutions and internal microstructures in the deformed specimens were investigated with a RINT 2500 X-ray diffractometer and with a JEOL 2000FX II transmission electron microscope, respectively. The specimens for TEM observation were carefully prepared to avoid the formation of stress-induced martensite and reverse transformation on heating, using a chemical polishing solution of hydrogen peroxide and hydrofluoric acid mixed in the ratio of 10: 1. The specimens were finally subjected to electropolishing using acetic acid and perchloric acid mixed in the ratio of 20:1 at room temperature, to obtain the TEM foils. Fracture surfaces were examined on the specimens fractured at room temperature with a Hitachi S-3100 scanning electron microscope. RESULTS AND DISCUSSION DEFORMATION BEHAVIOR Figures 1 (a) to (d) show the OPM images observed on the specimens of Al-0 to Al-3, respectively, deformed by tensile strain to 3%. Some grains seen in the figures involves anneal twins. Anneal twin boundaries on {111}γ planes are indicated by arrows in the OPM photos. In each of parent and twin crystals, there are surface striations. It should be noted in Figs. 1(a) to (d) that the width and interval of the striations inside crystals becomes smaller with increase in the Al content. It has been widely accepted that the striations appeared in the Fe-Mn-Si SMAs are formed by the stress-induced γ → e martensitic transformation, while those in the FeMn-Si-Al TWIP steels are due to the mechanical γ twinning. It is inferred from the variation in the microstructures from Figs. 1(a) to (d) that the deformation mode should continuously change from the stress-induced e martensite to the mechanical γ twins, when the Al content is increased from 0 mass % to 3 mass %. Our previous result [9] showing the linear change in the ductility depending on the Al content also supports this speculation. However, it is difficult to distinguish these two deformation products by OPM observations, because both have plate shapes on the {111}γ habit. Figure 1: Deformation microstructures observed by optical microscopy on the specimens of (a) Al-0, (b) Al-1, (c) Al-2, and (d) Al-3. The observations were performed at the tensile strain of about 3%. The phase identification by means of the XRD was, therefore, performed to investigate semi-quantitatively the dependence of the amount of the e phase on the Al content. It was revealed that the intensity of peaks from the e phase relative to that of peaks from the γ phase gradually decreases with increasing the Al content, though not presented here. However, it is impossible to investigate the amount of the mechanical γ twins by the XRD. In order to confirm the existence of the e phase and the γ twins, the electron diffraction pattern analysis using TEM was employed. Figure 2(a) shows the bright field image taken in the Al-0. The plates observed in Fig. 2(a) were identified as the e phase by the corresponding electron diffraction pattern shown in Fig. 2(b). The incident beam is parallel to [011]γ // [21 1 0]e. The diffraction pattern in Fig. 2(b) clearly shows the well known features of the γ → e transformations: i) the S-N orientational relationship between the γ and e crystals, ii) the streaks along γ directions due to small thickness of the e plates. The streaks run in two directions: i. e. and . The (11 1) and (111 ) traces nominal to the corresponding streaks are seen in Fig. 2(a). Figure 2(c) and (d) show an example of the mechanical γ twins (γTM) observation in the Al-3. The zone axes of the diffraction pattern are [011]γ // [011]γTM. A lamella structure consisted of nano-sized twins and the retained austenite is formed in the specimen, being consistent with the previous results in the literature [10]. After a number of careful observations, a very small amount of the e plates were also found even in the Al-3, although it was undetectable in the XRD profile. 200 111 111 MT 200 MT