Lower Jc Values Research Articles

A clinical and time savings evaluation of a deep learning automatic contouring algorithm.

Automatic contouring algorithms may streamline clinical workflows by reducing normal organ-at-risk (OAR) contouring time. Here we report the first comprehensive quantitative and qualitative evaluation, along with time savings assessment for a prototype deep learning segmentation algorithm from Siemens Healthineers. The accuracy of contours generated by the prototype were evaluated quantitatively using the Sorensen-Dice coefficient (Dice), Jaccard index (JC), and Hausdorff distance (Haus). Normal pelvic and head and neck OAR contours were evaluated retrospectively comparing the automatic and manual clinical contours in 100 patient cases. Contouring performance outliers were investigated. To quantify the time savings, a certified medical dosimetrist manually contoured de novo and, separately, edited the generated OARs for 10 head and neck and 10 pelvic patients. The automatic, edited, and manually generated contours were visually evaluated and scored by a practicing radiation oncologist on a scale of 1-4, where a higher score indicated better performance. The quantitative comparison revealed high (> 0.8) Dice and JC performance for relatively large organs such as the lungs, brain, femurs, and kidneys. Smaller elongated structures that had relatively low Dice and JC values tended to have low Hausdorff distances. Poor performing outlier cases revealed common anatomical inconsistencies including overestimation of the bladder and incorrect superior-inferior truncation of the spinal cord and femur contours. In all cases, editing contours was faster than manual contouring with an average time saving of 43.4% or 11.8 minutes per patient. The physician scored 240 structures with > 95% of structures receiving a score of 3 or 4. Of the structures reviewed, only 11 structures needed major revision or to be redone entirely. Our results indicate the evaluated auto-contouring solution has the potential to reduce clinical contouring time. The algorithm's performance is promising, but human review and some editing is required prior to clinical use.

Low temperature properties of spin filter NbN/GdN/NbN Josephson junctions

A ferromagnetic Josephson junction (JJ) represents a special class of hybrid system where different ordered phases meet and generate novel physics. In this work we report on the transport measurements of underdamped ferromagnetic NbN/GdN/NbN JJs at low temperatures. In these junctions the ferromagnetic insulator gadolinium nitride barrier generates spin-filtering properties and a dominant second harmonic component in the current-phase relation. These features make spin filter junctions quite interesting also in terms of fundamental studies on phase dynamics and dissipation. We discuss the fingerprints of spin filter JJs, through complementary transport measurements, and their implications on the phase dynamics, through standard measurements of switching current distributions. NbN/GdN/NbN JJs, where spin filter properties can be controllably tuned along with the critical current density (Jc), turn to be a very relevant term of reference to understand phase dynamics and dissipation in an enlarged class of JJs, not necessarily falling in the standard tunnel limit characterized by low Jc values.

Nanostructural Defects and Critical Current Densities in High-Quality <inline-formula> <tex-math notation="TeX">$\hbox{Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{8+{\rm X}} $</tex-math></inline-formula> Epitaxial Thin Films Prepared by MOCVD

Nanostructural defects were investigated by transmission electron microscopy in c-axis-oriented epitaxial Bi 2 Sr 2 CaCu 2 O 8+X (Bi2212) thin films with high T c (R = 0) ≥ 83 K and high transport critical current density ofJ c >10 10 A/m 2 at 10 K. We observed misfit dislocations at twin boundaries, dislocations associated with stacking faults parallel to the a-b plane, and antiphase boundaries. We compared the magnetic-field dependent J c values at various temperatures in two Bi2212 films. About 110-nm-thick film A showed lower J c than ~280-nm-thick film B at low temperatures (≤ 20 K) or in low magnetic fields (≤ 0.1 T). However, the J c of film B decreased more rapidly in high magnetic fields, leading to a crossover of the two J c -B curves at temperatures of 30 K and above. This is probably because film B contained a higher density of antiphase boundaries, which caused stronger pinning and higher J c values at low temperatures but may have caused enhanced thermally activated motion of pancake vortices that eventually resulted in lower Jc values in high magnetic fields at moderate temperatures.

Strongly enhanced current densities in Sr0.6K0.4Fe2As2 + Sn superconducting tapes

Improving transport current has been the primary topic for practical application of superconducting wires and tapes. However, the porous nature of powder-in-tube (PIT) processed iron-based tapes is one of the important reasons for low critical current density (Jc) values. In this work, the superconducting core density of ex-situ Sr0.6K0.4Fe2As2 + Sn tapes, prepared from optimized precursors, was significantly improved by employing a simple hot pressing as an alternative route for final sintering. The resulting samples exhibited optimal critical temperature (Tc), sharp resistive transition, small resistivity and high Vickers hardness (Hv) value. Consequently, the transport Jc reached excellent values of 5.1 × 104 A/cm2 in 10 T and 4.3 × 104 A/cm2 in 14 T at 4.2 K, respectively. Our tapes also exhibited high upper critical field Hc2 and almost field-independent Jc. These results clearly demonstrate that PIT pnictide wire conductors are very promising for high-field magnet applications.

Influence of Fe buffer thickness on the crystalline quality and the transport properties of Fe/Ba(Fe1−xCox)2As2 bilayers

The implementation of an Fe buffer layer is a promising way to obtain epitaxial growth of Co-doped BaFe2As2 (Ba-122). However, the crystalline quality and the superconducting properties of Co-doped Ba-122 are influenced by the Fe buffer layer thickness, dFe. The well-textured growth of the Fe/Ba-122 bilayer with dFe = 15 nm results in a high Jc of 0.45 MAcm$^{-2}$ at 12 K in self-field, whereas a low Jc value of 61000 Acm$^{-2}$ is recorded for the bilayer with dFe = 4 nm at the corresponding reduced temperature due to the presence of grain boundaries.

Comparative study of mono- and multi-filamentMgB2 wires with different boron powders and malic acid addition

We evaluated the effects of different boron (B) powders and of malic acid(C4H6O5) addition on the criticalcurrent density (Jc) ofmono- and multi-filament MgB2 wires. We found that Jc of the wires made from the amorphous B showed the best performance. Lowsintering processing keeps the grain size small, which leads to strong fluxpinning. From transmission electron microscopy (TEM) observations, weconfirmed that the crystalline B powders supplied by SMI and Tangshan had aβ-rhombohedral structure as a main phase. These two powders also had numerousdefects, such as twin and stacking faults inside the powder particles. However, thesedefects are not enough to promote the full reaction between magnesium (Mg) andB crystalline powder when sintered at low temperatures, and brings about lowJc values. Even though the morphologies of the two crystalline B powders appeared somewhat different,Jc for the two wires had similar quantitative values. For the malic acid doped wires,Jc of the wire made from the amorphous B was estimated to be above 100 000 A cm − 2 at6 T and 4.2 K. This value is comparable to the low temperature superconductor Nb–Ti. In addition, theJc was25 000 A cm − 2 at 10 T and 4.2 K. This performance is the bestJc reported so far in in situprocessed MgB2 wires.Interestingly, the low-field Jc performance of malic acid doped wire fabricated with crystalline B is higher than that ofthe pure wire fabricated with amorphous B. At 20 K, enhanced performance ofJc by malic acid doping is much higher than that of the pureMgB2 wire. TheJc at 5 T and 20 K wasestimated to be 13 000 A cm − 2.

拡散法による 7 芯 MgB2 線材の作製とその超伝導特性

Powder-In-Tube(PIT) method is the most popular method to fabricate MgB2 wires and tapes. However, PIT processed tapes and wires show relatively low Jc values due to the low density and, hence, low connectivity of MgB2 cores. The internal Mg diffusion (IMD) process is one of the effective methods to obtain high density MgB2. In this paper we report the heat treatment temperature dependence of superconducting properties of IMD processed 7-core MgB2 wires. A pure Mg rod with a diameter of 2 mm was placed at the center of a Ta tube with an outer diameter of 6 mm and inner diameter of 3.5 mm, and space between the Mg rod and the Ta tube was filled with B powder or B-SiC mixed powder. The composite was successfully cold worked into 1.3 mm wire at room temperature without any breakage. Seven pieces of the mono-core wires were bundled and inserted into a Cu-Ni tube and then, cold worked into 1.3 mm wire. The wires were heat treated at 590~800°C for 1 h under an Ar gas atmosphere. At the temperature of 640°C B layer was almost completely reacted with Mg to form MgB2. X-ray diffraction analysis indicated that the major phase in the reacted layer is MgB2. SEM analysis of the heat treated wire clearly indicated that the density of MgB2 layer in the wire was higher than that of a PIT processed wire. Transport Jc values (calculated for the reacted layer) reached 9.9×102 A/mm2 in 10 T at 4.2 K and 1.3×103 A/mm2 in 3 T at 20 K for the SiC added wire heat treated at 640°C. These Jc values are much higher than those of usual PIT processed wires. These high Jc values can be attributed to the high density MgB2 layer obtained by this diffusion method.

MOD-Processed SmBCO Films on ${\rm LaAlO}_{3}$ (100) Substrates

SmBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-delta</sub> (SmBCO) thin films were successfully prepared on the LaAlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (100) substrate by the TFA-MOD method in an extremely reduced oxygen atmosphere. The precursor solution was prepared dissolving SmBCO powder into the trifluoroacetic acid solution. The precursor solution was spin-coated on the LaAlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (100) single crystalline substrates, calcined at the temperature up to 400degC, and fired at various high temperatures for 2 h in reduced oxygen atmospheres. Optimally processed SmBCO thin film exhibited T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C,zero</sub> value of 90.2 K and J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> value of 0.8 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 77 K, respectively. Relatively lower J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> values in SmBCO films compared with MOD-processed YBCO films are attributed to the existence of a-axis oriented grains.

Effects of substrates on the NdBa2Cu3O7−δ films fabricated by the PLD process

NdBa2Cu3O7−δ (NdBCO) films were grown by pulsed-laser deposition (PLD) on SrTiO3 (STO)(100), MgO(100), and STO-buffered MgO(100) substrates for a comparative study on the effect of substrate on the textures, microstructures, and superconducting properties of NdBCO films. All NdBCO films were deposited at the same substrate temperature of 800°C under the same total pressure of 800mTorr with 1%O2/Ar gas. The transport Jc value of 3.5MA/cm2 at 77K in self-field was obtained from the NdBCO/STO with Tc value of 91.2K, while the NdBCO/MgO with Tc of 86.8K exhibited the low Jc value of 0.25MA/cm2. On the other hand, Jc value of 1.55MA/cm2 was obtained from NdBCO film on STO-buffered MgO(100) substrate with Tc of 91.5K. The higher Jc value of the NdBCO/STO than NdBCO/STO/MgO resulted from much better in-plane and out-of-plane textures of NdBCO/STO, while the low Jc value of the NdBCO/MgO can be attributed to both depressed Tc value and the existence of 45°-rotated grains with [001]NdBCO∥[110]MgO.

The Effect of Oxygen Partial Pressure on Melt Growth Processing for Filamentary Dy-Ba-Cu-O Superconductors

Precursor filaments of Dy-Ba-Cu-O super-conductor were prepared by a solution spinning method and partially melting in an atmosphere gas with various oxygen concentrations. It was found that the transport Jc value at 77 K and 0 T of the filamentary sample was strongly dependent on the partial-melting temperature and gas atmosphere. The samples partially melted under flowing 1%O/sub 2/+Ar gas exhibited Jc values larger than 10/sup 4/ A/cm/sup 2/ over a wide temperature range of 980-1040/spl deg/C and a dense microstructure. The filamentary sample with the starting composition of Dy:Ba:Cu=1.18:2.12:3.09 treated at 1020/spl deg/C under flowing 1%O/sub 2/+Ar showed a transport Jc value greater than 4.3/spl times/10/sup 4/ A/cm/sup 2/ at 77 K and 0 T, and the Jc value of 2.0/spl times/10/sup 3/ A/cm/sup 2/ was maintained up to the magnetic fields of 14 T. The samples partially melted in 20%O/sub 2/+Ar gas and 100%O/sub 2/ gas showed relatively low Jc value of 6.0/spl times/10/sup 3/ A/cm/sup 2/ and 1.2/spl times/10/sup 2/ A/cm/sup 2/ at most.

Growth model and the effect of CuO nanocrystallites on the properties of chemically derived epitaxial thin films of YBa2Cu3O7−x

In metalorganic deposition using trifluoroacetates (TFA-MOD), CuO nanocrystallites in calcined film, which influence the critical current density (Jc) of the resulting film, have seldom been considered before. CuO nanocrystallites ripen and grow during the calcining process at 200–250 °C and grow into large CuO grains in the YBa2Cu3O7-x (YBCO) film. The final diameter of the grains is about 0.1 μm. When highly purified coating solution is used, suppressing the creation of nanocrystallites in the precursor film is an effective way to obtain high-Jc YBCO film because the nanocrystallites consist only of CuO. We obtained the highest-Jc YBCO film on LaAlO3 single crystal with a calcining process of 9h43m at 200–250 °C. It was 141 nm thick and had a Jc of 7.5 MA/cm2 (77 K, 0 T), as measured by the four-probe method. The firing process of TFA-MOD still has several unsolved problems; why can H2O and HF gas diffuse quickly within the film? These phenomena can be explained by a model with a quasiliquid consisting of Y, Ba, Cu, O, H, and F. The H2O and HF can move quickly in the quasiliquid network within the film during the firing process. According to the model, we can get more conversion of CuO nanocrystallites into quasiliquid by increasing the amount of water vapor. Jc of the film was increased from 3.3 to 4.5 MA/cm2 (77 K, 0 T) by increasing the humidity from 4.2% to 12.1% during the long calcining profile of 66h40m at 200–250 °C. These results confirm the above model. CuO nanocrystallites in precursor films induce nonstoichiometric metal contents in the quasiliquid. Isolated CuO grains in YBCO film indirectly cause nonstoichiometric quasiliquid in other areas. Such nonstoichiometric quasiliquid leads to non-YBCO materials such as BaO, Y2O3, and Y2Cu2O5. Thus, films derived from non-highly purified coating solution and ones prepared with an inappropriate calcining process both have lower Jc values. Micrographs obtained by transmission electron microscopy and Jc results support this hypothesis.

The effects of variable oxygen partial pressures during Bi-2223 tape processing

The effect of changes in the oxygen partial pressure during the heat treatment of Bi-2223 tapes has been investigated. Distinct differences were observed in the phase assemblages, compositions, and critical current densities. Of particular importance in the correlation of transport properties and microstructure was the redistribution of lead in both the secondary phases and the Bi-2223 matrix. The highest Jc values were associated with tapes that contained Ca2PbO4 as part of the phase assemblage coupled with lower concentrations of lead in the superconducting phase. The high Jc of 30.4 kA/cm2 was obtained in tapes processed at 820 °C where the oxygen partial pressure was increased from 10% O2/Ar to 20% O2/Ar after 25 h of each 50-h sinter cycle. Samples processed only in 10% O2/Ar at 820 °C did not contain Ca2PbO4, had slightly higher concentrations of lead in the superconducting phase, and had significantly lower Jc values.

Current-induced displacements and precession of a Bloch wall in Ni-Fe thin films

Rectangular or exponential current pulses of duration ≂0.1 μs traversing a Bloch wall are found to induce wall displacements Δx≤10 μm/pulse in a Ni81Fe19 film of thickness ≂263 nm. The critical current density for wall displacement is jc≂1.35×106 A/cm2, about 20–200 times lower than for Néel or cross-tie walls investigated earlier. Wall motion arises probably from the precession of wall spins under the influence of the ‘‘s-d exchange torque’’ exerted by conduction electrons crossing the wall. The low jc value and easy precession of spins in a Bloch wall reflect the very small value ≂300 μT of the wall demagnetizing field which opposes spin orientation out of the wall plane in films of this thickness. A dc in-plane hard-axis field causes a linear decrease of jc. A dc easy-axis field smaller than Hc produces only a very slow decrease of jc.

ＮｂＴｉ極細多芯超伝導線の臨界電流密度とピンニング特性 Ｉ． 熱処理線

Critical current density Jc and its temperature dependence in multifilamentary NbTi wires were measured. Further their pinning behavior was examined. Used samples which have been fabricated by a double stacking method are divided into two groups. One group was heat-treated in the fabrication process in order to produce the precipitates of α-Ti and the filament diameter df of NbTi wires ranges from 0.39 to 1.53μm. For the other group, such a heat-treatment was not done and the df of some samples was reduced to the level of below 0.1μm. In this paper, the experimental results on the former group, namely heat-treated (HT) samples with Cu-CuNi mixed matrix or CuNi matrix, are given. The Jc of samples is affected by df. In case of 4.2K, these samples show a slight difference in Jc at high fields. As the temperature rises, the effect of df on Jc becomes remarkable and the Jc-B curves are very different among samples. Normally, samples with small df show low Jc values and reveal a rapid decrease in Jc with increasing magnetic field. An anomalous change in Jc at low fields of below 0.1T was seen at temperatures close to Tc for samples with sub-micron sized filaments. The pinning force Fp vs. reduced field b(=B/Bc2) curves at 4.2K for different samples coincide with each other except for the sample with df=0.39μm. The reduced pinning curves (Fp/Fp, max vs. b) obtained at various temperatures have similar shapes for each sample. The bm where the maximum of Fp takes place is around 0.35 except for the sample with df=0.39μm. The pinning properties of these samples are discussed in terms of their upper critical field Bc2 and transition temperature Tc.

Thin YBCO films prepared by low-temperature spray pyrolysis

Thin YBCO films were prepared from aerosol by a low-temperature deposition process consisting of two steps-the deposition at atmospheric pressure and firing in vacuum degraded by partial pressure of oxygen-both at temperatures not exceeding 600 degrees C. A stoichiometric 1-2-3 aqueous nitrate solution of Y, Ba and Cu constituents was used as a source of aerosol. MgO and Al2O3 substrates were heated to 140 to 160 degrees C during deposition. The obtained films were 1 to 10 mu m thick with Tc>80 K and Jc=103 to 104 A cm-2. However, detailed TG, DTG and DTA studies performed by the authors and others showed that thermal decomposition of the nitrates used only starts at temperatures higher than 200 degrees C. The use of low substrate temperatures during deposition (140 to 160 degrees C in this case) is apparently the reason for rather low Jc values. Optimization of the preparation process is suggested, which could lead to higher Jc values at processing temperatures not exceeding 600 to 700 degrees C. Such temperatures are still interesting for substrates with elevated diffusivity into prepared films.

Temperature dependence of the critical current and pinning behaviour in superfine filamentary NbTi wires

Field dependence of critical current density Jc and its temperature dependence in superfine filamentary NbTi wires were measured to investigate the effect of filament diameter df on Jc. Samples which are in the df range of 0.061-1.53 mu m have CuNi matrix or Cu-CuNi mixed matrix. Field dependences of Jc at 4.2 K are very different among samples with different values of df. As df is decreased, Jc values at 2 T reach a maximum, 1*106 A/cm2, around df=0.072 mu m. In high fields, the Jc of these samples depends on df and lower Jc values are obtained in samples with finer filaments. As the sample temperature rises, the Jc is highly reduced, especially, for finer filamentary samples.

Review of Chemical Solution Processes for Fabrication of Oxide Superconductors

The situation of research and development of chemical solution routes to superconductors is reviewed, Chemical solution processes were compared with the processes using solid state-derived powders in terms of the processing factors affecting the critical current densities (Jc) of superconductors. Though powders derived by chemical solution methods have better characteristics than those derived by solid state processes, these improved characteristics have not been reflected in Jc, the level of which remains much too low for practical use. The low Jc value is due to high porosity and weak links at grain boundaries; this problem has not yet been solved. It is necessary to pursue two approaches in the development of chemical solution processes. One is to combine with processes that have already been developed to fabricate high Jc superconductors, applying the characteristics of chemical solution-derived powders (fine and uniform particles). Another is to fabricate high Jc superconductors by the solid state sintering method using ultra-fine and highly sinterable powders derived from chemical solution methods. The former is expected to be in practical use within several years and the latter will be necessary in long-range research and development.

Auger study of grain boundaries in large-grained YBa2Cu3Ox

A scanning Auger microscopy study has been carried out on fractured surfaces of YBa2Cu3Ox samples prepared by standard sintering procedures from powders. It is concluded that in the large-grained samples examined, a majority of the grain boundaries are sufficiently ‘‘open’’ that standard metallographic preparation causes them to become contaminated with C. This suggests that an actual loss of contact at the grain boundaries during sample preparation is responsible for the low Jc values observed in bulk-sized large-grained YBa2Cu3Ox.

Effect of Temperature Cycles on the Critical Current Density of YBa2Cu3O7-δ

ABSTRACTThe transport Jc of the polycrystalline YBa2Cu3O7-δ superconductor seems to be dominated by weak links between high Jc regions as evidenced by low Jc values and their strong field dependence. The possible effects of thermal expansion-contraction and the tetragonal-orthorhombic transformation on the weak links and the Jc values were investigated by repeated thermal cycling of sintered pellets between -196°C and various high temperatures (600–850°C) using a furnace heating and cooling in an oxygen atmosphere. While more than a five-fold decrease in Jc from -400 to -70 A/cm2 (at 77K in zero field) is observed after 5 temperature cycles between -196'C and 850°C, only a slight decrease (to -370 Atm2) is noticed after 5 cycles between -196*C to 600°C, the temperature span of which is not all that much smaller than the former cycles. Transmission electron microscopy analysis clearly indicates that the drastic deterioration in Jc by repeated phase transformation is caused by increased amount of microcracks on (001) basal planes near the grain boundaries. The results will be discussed in terms of the large thermal expansion anisotropy of this layer-structured compound.