Synthesis and characterization of excess magnesium MgB2 superconductor under inert carbon environment

The structural, transport and magnetic properties of MgB₂ superconductor heavily blended with Mg is studied. The samples are synthesized with a new approach in both, pressed carbon environment and in flowing argon. The excess magnesium used is observed to play dual role: one being the prevention of Mg losses during the synthesis process and hence maintaining the stoichiometry of MgB₂ phase, and second being the formation of Mg milieu probably all around the MgB₂ grains to give a dense structure. Excess Mg also improves the grain connectivity by going into the pores and there by minimizing the insulating junctions. The residual resistivity of the sample is observed to decrease from 57.02 μΩ cm to 10.042 μΩ cm as it is progressively filled with superconductor–normal–superconductor (SNS) type junctions amongst the grains by the virtue of increased magnesium content. The synthesized samples devoid of porosity show the superconducting transition, T_c in the range of 39–34 K as of clean MgB₂ samples, though overloaded with Mg. The excess Mg resulted in enhanced critical current density, J_c from 6.8 × 10³ A cm^?2 to 5.9 × 10⁴ A cm^?2 at 20 K and 10 kOe, with reasonable decrease in the superconducting transition. Thus our samples being overloaded with Mg impart mechanical strength and competitive superconducting properties, which forms a part of interest.     

MgB2 coated conductors directly grown on flexible metallic Hastelloy tapes by hybrid physical–chemical vapor deposition

MgB₂ coated conductors (CCs), which can avoid the low packing density problem of powder-in-tube (PIT) processed wires, can be a realistic solution for practical engineering applications. Here we report on the superior superconducting properties of MgB₂ CCs grown directly on the flexible metallic Hastelloy tapes without any buffer layer at various deposition temperatures from 520 to 600 °C by using hybrid physical–chemical vapor deposition (HPCVD) technique. The superconducting transition temperatures (T_c) are in the range of 38.5–39.4 K, comparable to bulk samples and high quality thin films. Clear (101) and (002) reflection peaks of MgB₂ are observed in the X-ray diffraction patterns without any indication of chemical reaction between MgB₂ and Hastelloy tapes. From scanning electron microscopy, it was found that connection between MgB₂ grains and voids strongly depend on the growth temperature. A systematic increase in the flux pinning force density and thereby the critical current density with decreasing growth temperature was observed for the MgB₂ CCs. The critical current density (J_c) of J_c(5 K, 0 T) ～10⁷ A/cm² and J_c(5 K, 2.5 T) ～10⁵ A/cm² has been obtained for the sample fabricated at a low growth temperature of 520 °C. The enhanced J_c (H) behavior can be understood on the basis of the variation in the microstructure of MgB₂ CCs with growth temperature.     

Sputtered magnesium diboride thin films: Growth conditions and surface morphology

Magnesium diboride (MgB₂) thin films were deposited on C-plane sapphire substrates by sputtering pure B and Mg targets at different substrate temperatures, and were followed by in situ annealing. A systematic study about the effects of the various growth and annealing parameters on the physical properties of MgB₂ thin films showed that the substrate temperature is the most critical factor that determines the superconducting transition temperature (T_c), while annealing plays a minor role. There was no superconducting transition in the thin films grown at room temperature without post-annealing. The highest T_c of the samples grown at room temperature after the optimized annealing was 22 K. As the temperature of the substrate (T_s) increased, T_c rose. However, the maximum T_s was limited due to the low magnesium sticking coefficient and thus the T_c value was limited as well. The highest T_c, 29 K, was obtained for the sample deposited at 180 °C, annealed at 620 °C, and was subsequently annealed a second time at 800 °C. Three-dimensional (3D) AFM images clearly demonstrated that the thin films with no transition, or very low T_c, did not have the well-developed MgB₂ grains while the films with higher T_c displayed the well-developed grains and smooth surface. Although the T_c of sputtered MgB₂ films in the current work is lower than that for the bulk and ex situ annealed thin films, this work presents an important step towards the fabrication of MgB₂ heterostructures using rather simple physical vapor deposition method such as sputtering.     

Flux pinning in columnar-structured and single crystalline MgB2 films

We have investigated the flux pinning effect of columnar grain boundary in columnar-structured and single crystalline MgB₂ films. The MgB₂ films with columnar structure showed much higher J_c than that of single crystalline thin film, and sample having smaller grain size had a higher J_c in high magnetic fields. At 5 K, the MgB₂ film with grain size of 460 nm showed an abnormal double-peak behavior in pining force density, F_p(B), caused by competition of different types of pinning sites, such as planar defects and point defects. Field dependences of F_p in columnar-structured films suggest that the columnar grain boundary is a strong pinning source in the MgB₂ film and it plays a crucial role in enhancing J_c over a wide range of magnetic fields and temperatures.     

Effects of surface damage on critical current density in MgB2 thin films

We investigated the influence of surface damage on the critical current density (J_c) of MgB₂ thin films via 140-keV Co-ion irradiation. The J_c(H) of the surface-damaged MgB₂ films was remarkably improved in comparison with that of pristine films. The strong enhancement of J_c(H) caused by a surface damage in MgB₂ films can be ascribed to additional point defects along with an atomic lattice displacement introduced through low-energy Co-ion irradiation, which is consistent with the change in the pinning mechanism, from weak collective pinning to strong plastic pinning. The irreversible magnetic field (H_irr) at 5 K for surface-damaged MgB₂ films with a thickness of 850 and 1300 nm was increased by a factor of approximately 2 compared with that of a pristine film. These results show that the surface damage produced by low energy ion irradiation can serve as an effective pinning source to improve J_c(H) in a MgB₂ superconductor.     

Effect of W-addition on pinning property of MgB2

The 5d transition metal W was added into the MgB₂ superconductor. The Mg, B and W were sintered at 1173 K for 30 min under H₂/Ar atmosphere in the electric furnace. The W_x(MgB₂)_1?x samples were prepared in the W concentration range of 0 ? x ? 0.05. Temperature and field dependences of magnetization were measured by the SQUID magneto-meter. The field and x dependences of J_c at 20 K were analyzed by the extended critical state model. The enhancement of J_c became maximum for the x = 0.02 sample.     

MgB2 fabrication by diffusion-controlled three layered (B-Mg-B) technique

MgB₂ was successfully fabricated through diffusion-controlled three-layered (B-Mg-B) technique under high pressure. Due to melting temperature of Mg, the material was pre-heat treated at 600 °C between 1 and 48 h. Optimum pre-heat treatment condition was found to be 600 °C for 48 h. Then, the compacted material was grinded and pelletized under pressure of 2 ton. The pellets were heat treated at 600-900 °C for 1-48 h. Optimum heat treatment condition was determined to be 800 °C for 1 h for formation of almost pure MgB₂. Diffusion coefficient was determined with Fick's law and EDX data. Diffusion coefficient value for B in Mg matrix and Mg in B matrix was determined to be 1.66×10⁻⁷ and 3.14×10⁻⁸ cm²/sn, respectively. Best T_c value (39.4 K) was obtained for material heat treated at 800 °C for 1 h. A symmetric hysteresis was obtained for the best MgB₂ material and magnetization decreased with increase in the temperature and the applied magnetic field.     

Effect of film thickness on the transport properties of MgB2 synthesized by spray pyrolysis

Polycrystalline MgB₂ films of different thickness have been prepared by employing spray pyrolysis technique on MgO (1 0 0) substrate. The MgB₂ and other phases have been confirmed using X-ray diffraction technique and no trace of impurities phases have been found. The resistivity behavior shows that the superconducting transition temperature lies in the range of 37–39 K with narrow transition width. The transport critical current density vary with films thickness and achieved highest value ～1.2 × 10⁶ A/cm² at 20 K for 2.0 μm thick film and its values increase as thickness increases.     

MgB2 coated superconducting tapes with high critical current densities fabricated by hybrid physical-chemical vapor deposition

The MgB₂ coated superconducting tapes have been fabricated on textured Cu (0 0 1) and polycrystalline Hastelloy tapes using coated conductor technique, which has been developed for the second generation high temperature superconducting wires. The MgB₂/Cu tapes were fabricated over a wide temperature range of 460-520 °C by using hybrid physical-chemical vapor deposition (HPCVD) technique. The tapes exhibited the critical temperatures (T_c) ranging between 36 and 38 K with superconducting transition width (ΔT_c) of about 0.3-0.6 K. The highest critical current density (J_c) of 1.34 × 10⁵ A/cm² at 5 K under 3 T is obtained for the MgB₂/Cu tape grown at 460 °C. To further improve the flux pinning property of MgB₂ tapes, SiC is coated as an impurity layer on the Cu tape. In contrast to pure MgB₂/Cu tapes, the MgB₂ on SiC-coated Cu tapes exhibited opposite trend in the dependence of J_c with growth temperature. The improved flux pinning by the additional defects created by SiC-impurity layer along with the MgB₂ grain boundaries lead to strong improvement in J_c for the MgB₂/SiC/Cu tapes. The MgB₂/Hastelloy superconducting tapes fabricated at a temperature of 520 °C showed the critical temperatures ranging between 38.5 and 39.6 K. We obtained much higher J_c values over the wide field range for MgB₂/Hastelloy tapes than the previously reported data on other metallic substrates, such as Cu, SS, and Nb. The J_c values of J_c(20 K, 0 T) ∼5.8 × 10⁶ A/cm² and J_c(20 K, 1.5 T) ∼2.4 × 10⁵ A/cm² is obtained for the 2-μm-thick MgB₂/Hastelloy tape. This paper will review the merits of coated conductor approach along with the HPCVD technique to fabricate MgB₂ conductors with high T_c and J_c values which are useful for large scale applications.     

Pulsed-laser deposition of MgB2 and B thin films

Thin films of the novel superconductor MgB₂ were deposited from an Mg-enriched MgB₂ target or by alternating ablation from Mg and B targets, depositing multilayers. The superconducting films were achieved in situ by a two-step process: deposition at low temperatures ranging from room temperature to 200 °C and subsequently heating to 600 °C. The color of the plasma originating from Mg or Mg-enriched MgB₂ targets during the deposition is an indicator of the constituents of the plasma and can be used to adjust the plasma parameters like pressure and energy density. The films showed a reduced critical temperature (T_c) compared to the bulk value (39 K), which is attributed to the small grain sizes and the relatively high base pressure of the system (10^-7 mbar) causing impurities (oxygen, carbon...). To investigate B oxidation and to determine the suitable deposition conditions for B, films made by pulsed-laser deposition (PLD) from B target were analyzed by XPS. The films are very sensitive to the ambient gas purity and the base pressure. We anticipate an improvement of T_c and the crystallinity of MgB₂ thin films by using PLD in high vacuum and with a high purity Ar and H₂ gas mixture. PACS 74.70.Ad     

On the superconductivity in in situ synthesized MgB2 tapes

Single core stainless steel (SS) sheathed MgB₂ tapes have been made by the powder-in-tube (PIT) method using commercial Mg and B powders in two series, one with nominal composition and the other with excess Mg. The electrical resistivity and susceptibility measurements have been carried out to evaluate residual resistivity ratio (RRR), the coherence length ξ(0) and critical current density J_C(T) in these tapes. Detailed structural analysis of the core material has been carried out to correlate the superconducting properties with the crystallinity. In the optimized growth condition the MgB₂ tapes exhibited an estimated J_C of ∼1.4×10⁷ A/m² at 39.45 K in zero field and the zero temperature coherence length is found to be ∼68 Å. MgB₂ tapes fabricated from starting powders having nominal Mg-composition have been shown to exhibit higher J_C than those fabricated from excess magnesium composition of the starting powders. The strained lattice together with the presence of nanosized MgO inclusion having size smaller than the coherence length, are shown to be responsible for the observed higher J_C.     

Superconductivity and x—ray photoemission study of MgB2 thin films

Highly c-axis oriented MgB2 thin films with Tc^onset of 39.6K were fabricated by magnesium diffusing into pulsedlaser-deposited boron precursors.The estimation of critical current density Jc,using hysteresis loops and the Bean model,has given the value of 10^7A/cm^2(15K,0T),which is one of the highest values ever reported.The x-ray photoemission study of the MgB2 thin films has revealed that the binding energies of Mg 2p and B 1s are at 49.4eV and 186.9eV,which are close to those of metallic Mg and transition-metal diborides,respectively.     

X-ray photoemission study of MgB<Subscript>2</Subscript> films synthesized from in-situ annealed MgB<Subscript>2</Subscript>/Mg multilayers

Superconducting MgB₂ films were obtained by in-situ annealing of precursor multilayers deposited at low substrate temperature by sputtering from a MgB₂ stoichiometric target and by thermal evaporation of pure Mg. After an in-situ annealing at 500–600 °C, the films showed a zero resistance critical temperature up to 31 K. The as-obtained MgB₂ films were investigated by X-ray photoelectron spectroscopy (XPS) and X-ray auger electron spectroscopy (XAES). The electronic structure was studied by monitoring the B 1s, Mg 2p, O 1s core-levels and the Mg KL₂L₃ Auger line. For comparison, the electronic structure of an MgB₂ commercial superconducting sputtering target, of a not-annealed precursor film and of a sample obtained by direct sputtering from the MgB₂ target have also been investigated. Electron spectroscopy showed that in the superconducting systems the Mg KL₂L₃ Auger line kinetic energy position is always higher by about 0.9 eV with respect to the energy position of the same Auger line measured in the non-superconducting samples. PACS 74.25Jb; 74.78.Bz; 74.70.Ad     

利用电泳法在金属基底上制备MgB2超导厚膜

利用电泳技术在高熔点金属基底Ta，Mo和W上制备MgB₂超导厚膜.厚膜中的MgB2晶粒结合紧密，粒度小于1μm，呈随机取向生长.电阻测量表明沉积在Ta，Mo，W上的MgB₂厚膜的超导起始转变温度分别为36.5K，34.8K，33.4K，对应的转变宽度为0.3K，1.5K和2.0K.三种基底上制备的MgB₂厚膜的临界电流密度在不同温度下随外磁场的变化情况 基本相同，MgB₂/Mo厚膜的临界电流密 关键词： 2超导厚膜')" href="#">MgB₂超导厚膜 电泳 金属基底     

Influence of different mixing ratios of Mg and B powder on structural and superconducting properties in MgB2

We fabricated MgB₂ bulks by an in situ Mg diffusive reaction method from compacted B and compacted mixtures of Mg and B respectively. All samples were sintered at 1100 °C for 2 h. MgO impurity phase, density, and critical current density (J_c) were found to be dependent of the starting amount of mixing Mg powder. We also found that the MgO formation in MgB₂ matrix was not mainly attributed to the starting amount of additional Mg powder for Mg diffusive reaction. This indicates that MgO presented in the starting Mg powder is hardly diffused into compacted B.     

MgB2 thick films with remarkable ductility on stainless steel substrate

We fabricated several superconducting MgB₂ thick films on stainless steel (SS) substrates by using hybrid physical-chemical vapor deposition (HPCVD) technique. The thickness was in the 10μm to 20μm range, and the onset critical transition temperature T _c (onset) and the width of the superconducting transition (ΔT) were about 37.8 and 1.2 K. They were dense and textured along (101) direction with high tenacity, despite the existence of a little amount of MgO and Mg. We bent the films at different degrees and studied the ductility and transport properties of these MgB₂ thick films under applied force. The results demonstrated that the superconducting properties of these thick films, prepared by HPCVD, stay almost unaffected even with the films bent to a large degree with a curvature of 0.5 mm. This indicated that the superconducting wires or tapes of MgB₂ with a core of SS had the advantages of avoiding rigidity and brittleness in industrial handling. The technique of HPCVD has, therefore, a high application potential. __________ Translated from Chinese Journal of Low Temperature Physics, 2005, 27(2) (in Chinese)     

Effects of copper doping in MgB2 superconductor

Magnesium diboride, recently discovered as superconductor, is extremely interesting in respect to pairing mechanisms and phonon-mediated superconductivity, but its transition temperature (39 K) is decidedly too low for practical applications. A rush for substitutions started, intended to raise T_c and possibly maintain acceptable critical current density. The difficult introduction of Cu was attempted, through a complex procedure employing CuB₂₄ instead of metallic Cu: owing to the tendency of Cu to form inter-metallic compounds with Mg, only a maximum of 3 mol% of Cu was safely substituted to Mg in MgB₂ lattice and a small parameters contraction proportional to Cu content was recorded. The results are a decrease in T_c of about 0.5 K, not far from that found with Al addition, whose reasons are discussed and interpreted in terms of holes doping.     

Superconducting MgB2 films on MgO substrates

We report the preparation of homogeneous, polycrystalline MgB₂ films on single-crystalline MgO substrates by screen printing and airbrush methods. The superconducting material was obtained by the reaction of Mg vapor with boron precursor layers in special steel casings under argon/hydrogen atmosphere at 800 °C and ambient pressure. After polishing of the surface, films with a thickness of about 2 m were obtained. Electrolytic polishing improved the surface smoothness of the samples. Transition temperatures of about 33 K were measured with irreversibility fields of up to 6 T at 20 K. Transport critical current densities of 2×10⁹ Am^-2 were obtained at 4.2 K in self-field for a MgB₂ film on single-crystalline MgO. PACS 74.72.Yg; 74.76.Db; 74.60.Jg     

Superconducting films with <Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript>=39 K prepared from stoichiometric MgB<Subscript>2</Subscript> targets

Oriented films of MgB₂ high-T_c superconductor are synthesized by pulsed laser sputtering of stoichiometric MgB₂ targets with subsequent annealing of the amorphous Mg-B material deposited onto MgO(111) substrates. The critical temperature of the films depends on the purity of the targets sputtered. The purification of boron powder in a vacuum makes it possible to minimize the content of impurities in the targets and to prepare MgB₂ films exhibiting a critical temperature above 39 K and a sharp inductive transition. A high room-temperature to residual resistivity ratio (more than 3) indicates the good quality of the films.     

Effect of processing temperature on the superconducting properties of acetone doped MgB2 tapes

Correlation of phase formation, critical transition temperature T_c, microstructure, and critical current density J_c with sintering temperature has been studied for acetone doped MgB₂/Fe tapes. Sintering was performed at 600–850 °C for 1 h in a flowing Ar atmosphere. High boron substitution by carbon was obtained with increasing the sintering temperature; however, the acetone doped samples synthesized at 800 °C contain large size MgB₂ grains and more MgO impurities. Incomplete reaction for the acetone doped samples heated at 600 °C result in bad intergrain connectivity. At 4.2 K, the best J_c value was achieved in the acetone doped sample sintered at 700 °C, which reached 24,000 A/cm² at 10 T and 10,000 A/cm² at 12 T, respectively. Our results indicate that the small grain size and less impurity were also important for the improvement of J_c–B properties besides the substitutions of B by C.