Polycrystalline MgB2 Research Articles

Field-induced suppression of theπ-band superconductivity and magnetic hysteresis in the microwave surface resistance ofMgB2 at temperaturesnear Tc

We report on the magnetic-field-induced variations of the microwave surface resistance,Rs, ina polycrystalline MgB2 sample, at different values of temperature. We have detected a magnetic hysteresis inRs, which exhibits an unexpected plateau on decreasing the DC magneticfield below a certain value. In particular, at temperatures nearTc the hysteresis manifests itself only through the presence of the plateau.Although we do not quantitatively justify the anomalous shape of the magnetichysteresis, we show that the results obtained in the reversible region of theRs(H) curve can be quite well accounted for by supposing that, in this range of magnetic field, theπ gap is almost suppressed by the applied field and, consequently, all theπ-band charge carriers are quasiparticles. On this hypothesis, we have calculatedRs(H) supposing that fluxons assume a conventional (single core) structure and theflux dynamics can be described in the framework of conventional models.From the fitting of the experimental results, we determine the values ofHc2π(T) at temperaturesnear Tc. In our opinion, the most important result of our investigation is that, at least at temperatures nearTc, the value of the applied field that separates the reversible and irreversible regions of theRs(H) curvesis just Hc2π(T); a qualitative discussion of the possible reasons for this finding is given.

Modified scaling law of the critical current density in poly crystalline MgB2 specimens

Magnetic hysteresis loops have been measured for polycrystalline MgB2 specimens at temperatures 5 to 30K and under magnetic field up to 1T. Resultant hysteresis width is very narrow, which means the flux pinning is very weak. To such case, a flux creep – flux flow model proposed by Matsushita et al.[1] may be valid. But, the temperature dependence of the irreversibility field was far from their prediction. Direct comparison between the scaling law and the experimental results successfully explains temperature dependence of the Jc, but field dependence of the Jc is only partly reproduced. It is necessary to introduce an exponential decay with increasing magnetic field proposed by Blatter et al.[2] in order to explain the experimental results in high field region.

Enhancing the transport properties of polycrystalline MgB2 superconductors

MgB2 polycrystalline superconducting specimens were irradiated with 10 MR and 20 MR doses of γ-rays. An increase in the normal state resistance and a broadening of the resistive transition to the superconducting state were observed with increasing γ-irradiation dose within the temperature range of 28 –31 K. The change in the critical temperature, Tc was found to be insignificant after irradiation. The critical current density, Jc was found to be enhanced almost at all temperatures after irradiation. The enhancement of Jc was found to be linear with the irradiation dose at all temperatures. These results are explained by the effects of γ-rays and temperature on the migration of MgO impurities into the grains and the oxygen segregation in the grain boundaries.

Enhancing the pinning strengths in polycrystalline MgB2

AbstractThe dependence of the critical current density, Jc, on the gamma irradiation dose in MgB2 polycrystalline superconducting specimens has been studied. The MgB2 samples were irradiated with 10 MR and 20 MR doses of γ‐rays. The critical current density, Jc was found to be enhanced almost at all temperatures after irradiation. The enhancement of Jc was found to be linear with the irradiation dose at all temperatures. We also report on the correlation between the superconducting transition temperature and the residual resistivity ratio. Most of our results agree with the Testardi and Mooij rules which suggest that the dominant pairing mechanism of MgB2 is the phonon‐mediated interaction. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Modeling studies of an impinging jet reactor design for hybrid physical–chemical vapor deposition of superconducting MgB 2 films

An impinging jet reactor was developed for the deposition of superconducting MgB 2 thin films by hybrid physical–chemical vapor deposition, a technique that combines Mg evaporation with the thermal decomposition of B 2H 6 gas. A transport and chemistry model for boron film deposition from B 2H 6 was initially used to investigate the effect of carrier gas, Mg crucible temperature and gas flow rates on boron film growth rate and uniformity. The modeling studies, which were validated experimentally, demonstrated a reduction in B 2H 6 gas-phase depletion and an increased boron film growth rate using an argon carrier gas compared to hydrogen. The results were used to identify a suitable set of process conditions for MgB 2 deposition in the impinging jet reactor. The deposition of polycrystalline MgB 2 thin films that exhibited a transition temperature of 39.5 K was demonstrated at growth rates up to ∼50 μm/h.

Effect of nano-NiO doping on the magnetic susceptibility and flux pinning in MgB<SUB align=right>2 superconductor

Improving transport properties of polycrystalline MgB2 by nanoparticle doping has been widely studied by researchers. We have investigated the effect of nano-nickel (as nickel oxide) doping on the critical current density (Jc) of bulk MgB2 by measuring the ac susceptibility using locally made susceptometer. Samples were doped with nickel at 1%, 2%, 4%, 8% and 10% to investigate for the different quantities on the compound. We have studied the susceptibility as function of the amplitude of ac magnetic field and temperature. The uniformly distributed nano-Ni dopant replaced Mg ions only up to 2% and the rest was distributed uniformly throughout the material and served as pinning centres. Jc for the nano-Ni doped samples has been calculated from susceptibility data and was found to increase as we increased the Ni but saturates after 2%. Jc was enhanced by more than three times with doping. The little change in the XRD peak positions for MgB2 shown by XRD results indicated that Ni doping did not strongly affect the lattice parameters of MgB2. Susceptibility measurements reveals that Tc decreased from 37.7 for the pure sample down to 35.6 K for doped samples.

Revealing the Anisotropy in MgB2 Superconductor and Investigation of Critical Current near Critical Temperature by Using Hall Probe Susceptibility Technique

We report the results of an investigation on critical current measurements in bulk MgB2 superconductor by using Hall probe ac susceptibility. The temperature versus ac susceptibility has been measured as a function of external ac magnetic field with no dc part and magnitude in the range 240–1200 A/m by using the Hall probe method. The real part of the Hall probe susceptibility showed two-step transitions near critical temperature. We attributed this behavior to the anisotropic nature of the MgB2 compound. Close to the critical temperature the estimated anisotropy parameter is γ≈2.5, a value that is in fair agreement to the ones reported in the literature for polycrystalline and single crystals MgB2. We have also analyzed the susceptibility data within the framework of a critical state model (using the real part of the susceptibility) and an anisotropic nature of MgB2 compound. The inter-grain critical current is found to be 4×106 A/m2 at 39.59 K by using the critical state model. The temperature dependence of the critical current density was found to be in the form of J c (T)∝(1−T/T c ) β , where β was calculated as 2.30 by using critical state model, respectively. Scanning electron microscopy was used for grain size estimation.

Magnetic instability in irradiated MgB2 dense samples

High density magnesium diboride samples were irradiated with low dosages of γ-rays, protons, and electrons. They were investigated by magnetization and thermal studies in order to determine if the irradiation increases the flux pinning and consequently the critical current density Jc. Zero field cooled magnetic susceptibility and specific heat measurements confirm the bulk transition temperature (Tc) with diamagnetic signal at ∼38.5 K. Magnetic instabilities were observed in the superconducting hysteresis loop at temperatures between 2 and 23 K in all studied polycrystalline MgB2 dense samples. The occurrence of flux jumps depended of the type of irradiation and the number of jumps decreases as temperature increases. The critical current density Jc, estimated from the magnetization hysteresis using the Bean’s model, is improved for gamma irradiated sample at H=0 and T=2 K. At low temperature, the Jc decreases and several steep drops in Jc are observed as a function of applied magnetic field. Furthermore, it is observed that the influence of crystalline defects plus local disorder, induced by hot isostatic pressure and irradiation with energetic atoms, increase the Jc but at the same time the magnetothermal instabilities increase in a broad range of temperature.

Estimation of critical current density and grain connectivity in superconducting MgB 2 bulk using Campbell’s method

Many recent reports on the critical current density ( J c) in superconducting MgB 2 bulks indicated that improving the grain connectivity is important, since the obtained J c values were generally much lower than those in other metallic superconductors and it was ascribed to the poor connectivity between grains in polycrystalline MgB 2. In this study, we focused on the estimation of the global critical current density, super-current path, grain connectivity and their relationships with the faults volume fraction in the MgB 2 bulks prepared by a modified PIT (powder in tube) method. Campbell’s method was applied for the purpose of obtaining the penetrating AC flux profile and the characteristic of AC magnetic field vs. penetration depth from the sample’s surface. A computer simulation on the penetrating AC flux profile in MgB 2 bulks with randomly distributed voids, oxidized grains and other faults was also carried out. J c obtained by Campbell’s method turned out to be smaller than that obtained from the SQUID measurement, implying that the global super-current was reduced by the existence of various faults and the lack of the electrical connectivity. It was verified that the relationship between the global critical current characteristics and the faults contained in MgB 2 samples can be quantitatively clarified by comparing the simulated critical current densities and other factors with the experimental results.

Magnetic flux noise in MgB2 superconductor

Magnetic flux noise in MgB2 polycrystalline sample is measured using a high-TC rf-superconducting quantum interference device in the temperature range of 6–40K. A small magnetic field (∼200mG) was applied while cooling the sample. The flux noise exhibits 1∕fα type of behavior with α∼1.0–1.3 and shows enhanced noise around 24 and 37K. The flux noise seems to originate from thermally activated vortex hopping. The large magnetic noise at 24K indicates the presence of larger density of pinning sites with energies ∼0.061eV leading to enhanced magnetic fluctuations at temperatures much below TC.

Enhanced superconducting properties of bulk MgB2 prepared by in situ Powder-In-Sealed-Tube method

A systematic study on the superconducting properties of polycrystalline MgB 2 synthesized by in situ Powder-In-Sealed-Tube technique is carried out at different temperatures (750–900 °C). Both XRD and SEM results show well-crystallized MgB 2 grains in all the samples and grain size is found to be increasing with the sintering temperature. Sharp superconducting transitions are observed for all samples, irrespective of sintering temperatures, which implies the high degree phase purity and homogeneity of MgB 2 formed, while J C ( H ) plot gives sample dependent critical current density. The samples heat treated at relatively low temperatures show enhanced flux pinning and hence improved J C ( H ) performance. The reduced grain size and hence increased density of grain boundary pinning centers of MgB 2 bulks synthesized at low temperature are mainly responsible for the enhanced flux pinning and J C .

Physical property characterization of bulk MgB2 superconductor

We report synthesis, structure/micro-structure, resistivity under magnetic field [R(T)H], Raman spectra, thermoelectric power S(T), thermal conductivity K(T), and magnetization of ambient pressure argon annealed polycrystalline bulk samples of MgB2, processed under identical conditions. The compound crystallizes in hexagonal structure with space group P6/mmm. Transmission electron microscopy (TEM) reveals electron micrographs showing various types of defect features along with the presence of 3-4nm thick amorphous layers forming the grain boundaries of otherwise crystalline MgB2. Raman spectra of the compound at room temperature exhibited characteristic phonon peak at 600 cm-1. Superconductivity is observed at 37.2K by magnetic susceptibility C(T), resistivity R(T), thermoelectric power S(T), and thermal conductivity K(T) measurements. The power law fitting of R(T) give rise to Debye temperature at 1400K which is found consistent with the theoretical fitting of S(T), exhibiting ThetaD of 1410K and carrier density of 3.81x 1028/m3. Thermal conductivity K(T) shows a jump at 38K, i.e., at Tc, which was missing in some earlier reports. Critical current density (Jc) of up to 105 A/cm2 in 1-2T (Tesla) fields at temperatures (T) of up to 10K is seen from magnetization measurements. The irreversibility field, defined as the field related to merging of M(H) loops is found to be 78, 68 and 42 kOe at 4, 10 and 20K respectively. The superconducting performance parameters viz. irreversibility field (Hirr) and critical current density Jc(H) of the studied MgB2 are improved profoundly with addition of nano-SiC and nano-Diamond. The physical property parameters measured for polycrystalline MgB2 are compared with earlier reports and a consolidated insight of various physical properties is presented.

Doped MgB2 prepared by field assisted sintering technique

Field-assisted-sintering technique (FAST) has been employed to obtain polycrystalline MgB2 samples, pristine and doped with 5 mol % SiC and B4C, using commercial MgB2 powder. The mass density of the samples is around 90% of the theoretical value. The response in magnetic field shows that the upper critical field and the irreversibility field are depressed by comparison with pristine MgB2. The latter has an upper critical field Hc2=14.37 T close to the values reported for the single crystals. Scaling analysis of the field dependence of the pinning force suggests that both addition of SiC and B4C stabilizes two well defined pinning regimes in FAST-processed MgB2. At low temperatures, T< 24 K (H≤5T), the pinning occurs at the grain boundaries whereas at higher temperatures there is a mixed pinning. These effects are supposed to be the combined result of relatively low level of C diffusion into the lattice of MgB2, specific features in formation of the grain boundaries during FAST processing and the influence of the added materials on morphology. At high fields (above 8.5T) and 4.2K, doped samples are superior to the pristine one from the critical current density viewpoint.

ＭｇＢ２超伝導体の臨界電流特性および材料化の現状と展望 ＭｇＢ２多結晶体の常伝導状態における電流制限機構―パーコレーションモデルによる電気的結合度の解析―

The critical current density and electrical resistivity of polycrystalline MgB2 samples vary largely among samples. In the present paper, the limiting mechanism of normal-state conductivity in polycrystalline MgB2 was studied by measuring the resistivity of a series of MgB2 bulk samples with systematically varied packing factors. The packing factor dependence of phonon term resistivity was found to be well explained using a three-dimensional site-percolation model. The porosities and the wetting oxide phases at grainboundaries are suggested to be the main causes of poor electrical connectivity in polycrystalline MgB2 samples. Our model provides quantitative evaluations of the influences of such secondary phases and anisotropy in randomly oriented polycrystalline samples, as well as the fraction of the active grains that can carry current. We argue that the suppressed critical current density in rather weak-link-free MgB2 can be understood under the scenario of a seriously percolating current path.

Formation of MgO whiskers on the surface of bulk MgB2 superconductors during in situ sintering

Magnesium oxide (MgO) whiskers (with diameters of about 60–80 nm) formed on the surface of bulk polycrystalline MgB2 superconductor at a relative low temperature (720 °C) during in situ sintering process. The reaction between Mg and B powders begins at a temperature below melting point of Mg and maintains till about 750 °C. The residual Mg powders evaporate and react with trace oxygen to form MgO vapor as the temperature exceeds the melting temperature of Mg and a low supersaturation is required for the growth of MgO whiskers. The preformed MgB2 and MgO crystals act as substrates and the melted Mg powders on the surface of them serve as catalysts during the growth process of MgO whiskers. The growth process of MgO whiskers is dominated by a self-catalytic vapor–liquid–solid (VLS) mechanism.

Positron annihilation spectroscopy and specific heat study of Neon ion irradiated MgB2

Abstract Specific heat studies under magnetic field and positron annihilation spectroscopy were carried out on 160 MeV Ne ion irradiated polycrystalline MgB 2 samples. There is an unusual decrease in positron lifetime in the irradiated sample which may be due to neon ion implantation. This was also indicated by change in cell volume. Coincidence Doppler Broadening Spectra of Mg, B, irradiated and unirradiated MgB 2 show that positrons primarily annihilate in boron sublattice in the unirradiated sample whereas there is some similarity of the spectrum of the irradiated sample with that of Mg. There is Mg deficiency in the unirradiated sample whereas predominantly boron vacancies exist in Ne ion irradiated MgB 2 sample. Specific heat measurements show that there is a small increase in electronic part of the specific heat and electron–phonon coupling constant.

The superconducting properties of co-doped polycrystallineMgB2

In this study we compare the critical current density, the irreversibility line and the upper critical field offour MgB2 polycrystalline samples, which are either undoped or have 5% carbon or 5% carbonplus either 1% aluminium or 2% zirconium. We discuss how care must be takenfor the extraction of the irreversibility line in such samples. We also show howac susceptibility and Hall probe imaging can be used to examine whether thesamples remain fully connected to the highest available fields. Compared to simple5% carbon doping we find that co-doping provides modest improvement in thepinning properties at intermediate fields in the carbon plus zirconium dopedsample.

Polycrystalline ${\rm MgB}_{2}$ Films on Flexible YSZ Substrates Grown by Hybrid Physical-Chemical Vapor Deposition

We report properties of polycrystalline MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films deposited on thin flexible yttium-stabilized zirconia (YSZ) substrates by hybrid physical-chemical vapor deposition. The MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films show a transition temperature of 38.9 K with a narrow superconducting transition (0.1 K). The self-field critical current density in the films reaches 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at low temperatures. These properties do not change after repeated bending over a radius of 10 mm. Low microwave surface resistance comparable to those obtained in epitaxial MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films on single-crystalline sapphire substrate was observed. The result shows promise of polycrystalline MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films for applications in superconducting digital circuits as well as in coated conductor wires.

Magneto-Optical Studies on Polycrystalline ${\rm MgB}_{2}$ Bulks Manufactured by Different Processes

Magneto optical imaging (MOI) was performed for polycrystalline MgB2 bulks manufactured by various fabrication processes to investigate the relationship between critical current properties, microstructures and current obstacles. In dense samples prepared by the diffusion method, steep field gradients corresponding to high Jc were shown by MOI, even though some larger scale current obstacles such as cracks and local porous regions were found. For a crack-free, dense sample, high Jc over 0.4 MA/cm2 at 20 K at 150 mT was found both by MOI and magnetization hysteresis measurements. On the other hand, relatively uniform field distribution without large-scale current obstacles was observed by MOI for a porous in-situ processed sample, in spite of its much lower Jc. This suggests that uniformly dispersed current obstacles on scales finer than the MOI resolution (~ few mum) exist in porous bulks. More homogenous microstructure without such fine-scale obstructions and elimination of voids and cracks are all important for improving Jc of MgB2.

Specific Heat and Magnetic Relaxation Analysis of ${\rm MgB}_{2}$ Bulk Samples With and Without Additives

The effects of SiC and Carbon doping on the superconducting properties of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> polycrystalline samples have been analysed by means of specific heat and magnetic relaxation measurements. It is known that the addition of nanometric powders of SiC and C leads to the enhancement of B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">irr</sub> and J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . However, the underlying physical mechanism is not completely understood. Magnetic relaxation measurements did not show detectable effects of both the additions on the pinning properties of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . It follows that doping acts mainly introducing disorder into the superconductor and thus raising B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</sub> . In the case of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.9</sub> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> , specific heat measurements show that the C substitution on the B sites modifies the low temperature shoulder related to the second gap. This effect is not visible in the sample doped with SiC. From the distribution of T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> determined from the deconvolution of the calorimetric data, we argue that SiC leads to an inhomogeneous distribution of C.