Tatiana Prikhna

Superconducting joining of melt-textured Y-Ba-Cu-O bulk material

Abstract The Tm–Ba–Cu–O solder can be successfully used to produce a superconductive joint between melt-textured YBa 2 Cu 3 O 7− δ (MT-YBCO) parts. The peculiarities of solidification, phase formation, structure transformations and electromagnetic properties of MT-YBCO soldered with TmBa 2 Cu 3 O 7− δ are discussed.

Studies of the oxidation stability, mechanical characteristics of materials based on max phases of the Ti-Al-(C, N) systems, and of the possibility of their use as tool bonds and materials for polishing

Thermogravimetry and differential thermal analysis have been used to study the resistance to the air oxidation of high-density samples of Ti3AlC2, Ti2AlC and Ti2Al(C1−xNx) solid solutions. It has been shown that the Ti3AlC2 samples are more stable than Ti2AlC and Ti2Al(C1−xNx) solid solutions and as the nitrogen content of the solid solution increases to x = 0.75, the oxidation resistance decreases. The following characteristics have been exhibited by the material containing 89 wt % Ti3AlC2 (the rest being Al2O3 and TiC) having density 99% of theoretical: bending strength Rbm = 500 MPa, compressive strength Rcm = 700 MPa, fracture toughness KIc = 10.2 MPa·m0.5, hardness HRA = 70 GPa, HV = 4.6 GPa, Young modulus = 149.4 ± 28.7 GPa. After sintering with diamonds or cBN (50 wt %) at 5.5–7.7 GPa and 1350–1960°C for 0.07–1.0 h the Ti3AlC2 MAX phase decomposes to form TiC and TiAl or TiB2 and a thin layer of Al4C3 forms at the interface with diamond. The Al4C3 decomposition in a composite material due to the interaction with the air moisture results in the crack initiation along the diamond perimeter, which brings about the material fracture in 1–2 weeks. It has been found that the Ti3AlC2 powder is efficient for polishing natural and synthetic jewelry crystals and competitive in polishing efficiency and quality with ACM 2/1 grade diamond.

Formation of Higher Borides During High-Pressure Synthesis and Sintering of Magnesium Diboride and Their Positive Effect on Pinning and Critical Current Density

Critical current density (j_c) of high-pressure (2 GPa) manufactured MgB₂-based superconductors depends on the amount and distribution of higher borides (MgB₁₂) in MgB₂ matrix, which in turn are determined by the nature of the initial components first of all B or MgB₂ and the temperature of sintering or synthesis. Ti and Ta additions can improve j_c by promoting the higher boride formation via impurity hydrogen absorption, thus preventing MgH₂ detrimental for j_c being formed, which possibly increases the MgB₁₂ nucleation barrier. SiC (0.2-0.8 mum) addition increases j_c of MgB₂, allowing us to get j_c = 10⁶ A/cm² at 20 K in the 1 T field: pinning is increased by SiC and higher boride grains and there is no notable interaction between SiC and MgB₂ . As the synthesis temperature increases from 800 to 1050degC, Ti and SiC additions may affect the oxygen segregation and formation of Mg-B-O inclusions enriched with oxygen as compared to the amount of oxygen in the MgB₂ matrix, which can also promote an increase in pinning. Materials high-pressure synthesized from Mg and B taken in 1:4, 1:6, 1:7, 1:8, 1:10, 1:12, 1:20 ratios were superconductive with T_c of about 37 K. High j_c (7middot10⁴ - 2middot10⁴ A/cm² in zero field at 10-30 K, respectively) showed materials with the matrix composition near MgB₁₂ stoichiometry, they have doubled microhardness of MgB_2.

Spark plasma synthesis and sintering of superconducting MgB 2-based Materials

Superconducting (SC) and mechanical properties of spark plasma (or SPS) produced MgB2 –based materials allow their efficient applications in fault current limiters, superconducting electromotors, pumps, generators, magnetic bearings, etc. The synthesized from Mg and B at 50 MPa, 1050 °C for 30 min material has a density of 2.52 g/cm3, critical current density, jc = 7.1•105 A/cm2 at 10 K , 5.4 •105 A/cm2 at 20 K, and 9•104 A/cm2 at 35 K in zero magnetic field; at 20 K its field of irreversibility Birr(20)=7 T and upper critical field Bc2(20)=11 T; microhardness HV=10.5 GPa and fracture toughness K1C =1.7 MPa•m1/2 at 4.9 N-load. SPS-manufactured in- situ MgB2-based materials usually have somewhat higher jc than sintered ex-situ. The pressure variations from 16 to 96 MPa during the SPS-process did not affect material SC characteristics significantly; the jc at 10-20 K was slightly higher and the material density was higher by 11%, when pressures of 50-96 MPa were used. The structure of SPS-produced MgB2 material contains Mg-B-O inclusions and inclusions of higher borides (of compositions near MgB4, MgB7, MgB12, MgB17, MgB20), which can be pinning centers. The presence of higher borides in the MgB2 structure can be revealed by the SEM and Raman spectroscopy.

High-Pressure Synthesized Nanostructural

A variety of samples made via different routes were investigated. Samples are nanostructured (average grain sizes are about 20 nm). The advantage of high-pressure (HP)-manufactured (2 GPa, 800-1050°C, 1 h) MgB₂ bulk is the possibility to get almost theoretically dense (1-2% porosity) material with very high critical current densities reaching at 20 K, in 0-1 T j_c = 1.2 1.0 · 10⁶ A/cm² (with 10% SiC doping) and j_c = 9.2 - 7.3 10⁵ A/cm² (without doping). Mechanical properties are also very high: fracture toughness up to 4.4 ± 0.04 MPa · m^0.5 and 7.6 ± 2.0 MPa · m^0.5 at 148.8 N load for MgB₂ undoped and doped with 10% Ta, respectively. The HP-synthesized material at moderate temperature (2 GPa, 600°C, 1 h) from B with high amount of impurity C (3.15%) and H (0.87%) has j_c = 103 A/cm² in 8 T fleld at 20 K, highest irreversibility fields (at 18.4 K H_irr = 15 T) and upper critical fields (at 22 K H_C2 = 15 T) but 17% porosity. HP materials with stoichiometry near MgB₁₂ can have T_c = 37 K and j_c = 6 · 10⁴ A/cm² at 0 T and H_irr = 5 T at 20 K. The spark plasma synthesized (SPS) material (50 MPa, 600-1050°C 1.3 h, without additions), demonstrated at 20 K, in 0-1 T j_c = 4.5-4 10⁵ A/cm². Dispersed inclusions of higher magnesium borides, which are usually present in MgB₂ structure and obviously create new pinning centers can be revealed by Raman spectroscopy (for the first time a spectrum of MgB₂ was obtained). Tests of quench behavior, losses on MgB₂ rings and material thermal conductivity show promising properties for fault current limiters. Due to high critical fields, the material can be used for magnets.

{\hbox {MgB}}_{2}

We have studied thin-film superconductor - semiconductor (with quantum dots)-superconductor MoRe-Si(W)-MoRe junctions, where electrons are tunneling through a single or several quantum dots within the Si(W) barrier. Current-voltage characteristics (CVCs) of the samples have been measured in a wide voltage range from -900 to 900 mV at temperatures from 4.2 to 8 K. At relatively high tungsten content in the barrier, we have observed emergence of the Josephson effect. Characteristic voltages IcRN, the product of the critical Josephson supercurrent Ic and the normal-state resistance RN, of the samples were unusually high. Simultaneously, we have observed large excess quasiparticle currents Iexc in the dissipative part of CVCs, which is a strong evidence of intensive electron-to-hole Andreev reflections in the junctions studied. When the W content in the barrier was decreased, the Josephson current disappeared, and we have observed resonant current peaks in the CVCs at bias voltages from 40 to 300 mV, which were symmetrical for positive and negative voltages. In the studied heterostructures, metal clusters inside the barrier behave as quasi-one-dimensional quantum dots; hence, the charge transport can be adequately described by scattering matrices within the quantum model of one-dimensional charge transport.

Materials With High Performance of Superconductivity, Suitable for Fault Current Limitation and Other Applications

Bulk MgB₂- and YBaCuO-based materials are competitive candidates for applications. The properties of both compounds can be significantly improved by high temperature-high pressure preparation methods. The transformation of grain boundary pinning to point pinning in MgB₂-based materials with increasing manufacturing temperature from 800 to 1050^°C under pressures from 0.1 MPa to 2 GPa correlates well with an increase in critical current density in low and intermediate magnetic fields and with the redistribution of boron and oxygen in the material structure. As the manufacturing temperature increases (to 2 GPa), the discontinuous oxygen-enriched layers transform into distinct Mg-B-O inclusions, and the size and amount of inclusions of higher borides MgB_X (X>;2) are reduced. The effect of oxygen and boron redistribution can be enhanced by Ti or SiC addition. The oxygenation of melt-textured YBa₂Cu₃O_{7 - δ} (MT-YBaCuO) under oxygen pressure (16 MPa) allows one to increase the oxygenation temperature from 440°C to 700-800°C, which leads to an increase of the twin density in the Y123 matrix and to a decrease of dislocations, stacking faults, and the density of microcracks, and as a result, to an increase of the critical current density, <i>J</i>_c, and the trapped magnetic field. In MT-YBaCuO, practically free form dislocations and stacking faults and with a twin density of 22-35 μm^-1, <i>J</i>_c of 100 kA/cm² (at 77 K, 0 T) has been achieved, and the importance of twins in Y123 for pinning was demonstrated experimentally.

Charge Transport in Hybrid Tunnel Superconductor—Quantum Dot—Superconductor Junctions

A MgB2 superconductor was prepared from Mg flakes or powders and B powders using the powder-in-tube technique with explosive consolidation. Compaction of Mg and B powders resulted in a two-phase alloy, due to the relatively low temperature developed during compaction as well as the very short duration of the process. Formation of MgB2 was obtained after subsequent heat treatment in argon atmosphere at a maximum temperature of 950degC.

Pinning in

The mechanical properties and temperature stability in air and hydrogen of the highly dense (ρ=4.27 g/cm3, porosity 1 %) material based on nanolaminated MAX phase Ti3AlC2 (89 % Ti3AlC2, 6 % TiC, 5 % Al2O3) manufactured by hot pressing (at 30 MPa) have been investigated. At room temperature the samplesexhibited microhardness HV = 4.6 GPa (at 5 N), hardness HV50 = 630 MPa (at 50 N ) and HRA=70 (at 600 N), Young modulus was 140 ± 29 GPa, fracture toughness K1C=10.2 MPa·m0.5compression strength 700 MPa and bending strength 500 MPa. After 1000 hours of exposition at 600 °C the oxide film (containing mainly Al2O3 and TiO2) formed on the surface and material demonstrated a higher oxidation resistance than chromium ferrite steels. Due to the surface oxidation the defects self-healing took place and the bending strength of the porous Ti3AlC2 (22% porosity) after exposition for 3 h at 600 oC in air slightly (for 3%) increased as compared to that at 20 oC. Besides, the porous Ti3AlC2 material resisted to high-temperature creep and after being kept in H2 at 600 °C for 3h its bending strength reduced by 5 %.

\hbox{MgB}_{2}

Addition of Ti and Zr to high-pressure (HP) synthesized MgB2 results in an increase of critical current density of the material due to the absorption of impurity hydrogen coming most likely from the materials of a high-pressure cell. The results of the studies of structure, critical current density, trapped field and mechanical characteristics are discussed. High-pressure synthesized MgB2 (with Ti additions) blocks were for the first time used in a SC electromotor at 20 K and demonstrated the efficiency similar to that of MT-YBCO bulk (at the same working temperature).