SUPERCONDUCTIVITY AND CRYOGENICS

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1 ANNUAL REPORT

2 fmohtu dk eq[; dk;z MksIM,ethch 2 (Mg ) es a ewy vuqla/kku dk yxkrkj fd;k tkuk gs A fofhkuu MksisUV tsls,,y] usuks&lh] usuks,l vkbz lh (SiC) rfkk usuks Mk;e.M okys,ethch 2 (Mg ) ds cyd uewuks a dks la ysf kr djds Fkeks Z os qr ikwoj,l ¼Vh½ pqecdh; izfrjks/kdrk [(P(T,H)] rfkk mpp {ks=ks a es a pa qcdu,e ¼,p½ }kjk vfhkyf{kr fd;k x;k gs A,d vuwbs ifj?kvukred vuros Z ku Qkew Zys dks Mg 1-x es a,,y (Al) ladsunz.k ds,d dk;z ds :i es a voyksfdr /kukred rfkk _.kkred,l Vh fuhkzjrk nksuks a ds vuqdwyu ds fy, fodflr fd;k x;k Fkk usuks&lh] usuks&,l vkbz lh (SiC) rfkk usuks Mk;e.M dh Mksfiax dks Åijh egÿoiw.kz QhYM Hc(T), vuqrøe.kh;rk QhYM H irr (T) rfkk Mg cyd lkexzh es a egÿoiw.kz ekstwnk?kuro Jc(B,T) dh myys[kuh; o f) ds fy, n kkz;k x;k gs A 90 ANNUAL REPORT

3 A major activity of the division has been continuation of basic research in doped Mg superconductors. Bulk samples of Mg with different dopants like Al, nano-c, nano-sic and nano-diamonds were synthesized and characterized by thermoelectric power S(T), magneto-resistivity [r(t,h)] and magnetization M(H) in high fields. A unique phenomenological interpolation formula was evolved to fit both positive and negative S(T) dependence observed as a function of Al concentration in Mg 1-x. The doping of nano-c, nano-sic and nano-diamonds were shown to significantly enhance the upper critical field H c (T), the irreversibility field H irr (T) and the critical current density J c (B,T) in the Mg bulk material. ANNUAL REPORT

4 (I) Anomalous thermoelectric power of Mg 1-x system with x = 0.0 to 1.0 Thermoelectric power, S(T) of the Mg 1- Al B system has been measured for x = 0.0, 0.1, x x 2 0.2, 0.4, 0.6, 0.8 and 1.0. XRD, resistivity and magnetization measurements are also presented. It has been found that the thermoelectric power is positive for x 0.4 and is negative for x 0.6 over the entire temperature range studied up to 300 K, see Fig.7.1. The thermoelectric power of x 0.4 samples vanishes discontinuously below a certain temperature, implying existence of superconductivity. In general, the magnitude of the thermoelectric power, we have used a model in which both diffusion and phonon drag processes are combined by using a phenomenological interpolation between the low and high temperature behaviours of the thermoelectric power. The considered model provides an excellent fit to the observed data. It is further found that Al doping enhances the Debye temperature. (II) Significant improvement of flux pinning and irreversibility field in nano-carbon doped Mg superconductor Synthesis and study was done on the variation of superconductivity parameters such as transition temperature T c, upper critical field H c, critical current density J c, irreversibility field H irr Fig. 7.1 : Thermopower vs temperature plots in the temperature range 0 to 300 K for all samples of series Mg 1-x (x = 0.0 to 1.0). The experimental data points are shown by different symbol and the theoretical fits are shown by the solid lines. thermoelectric power increases with temperature up to a certain temperature, and then it starts to decrease towards zero base line. In order to explain the observed behaviour of the and flux pinning parameter (F p ) for the Mg-x system with nano-carbon doping up to x=0.20. Carbon substitutes successfully on boron site and results in significant enhancement of H irr and J c (H). Resistivity measurements reveal a continuous decrease in T c under zero applied field, while the same improves remarkably at higher fields with an increase in nano-c content for Mg-x system. The irreversibility field value is 7.6 & 6.6 Tesla at 5 and 10K respectively for the pristine sample, which is enhanced to 13.4 and 11.0 Tesla for x =.08 sample at same temperatures, see Fig.7.2. Compared to undoped sample, critical current density for the x=0.08 nano-carbon doped sample is increased by a factor of 24 at 10K at 6 Tesla field. 92 ANNUAL REPORT

5 for the 5-wt % n-sic added sample is increased by a factor of 35 at 10K and 6.5 Tesla field and by a factor 20 at 20K and 4.2 Tesla field. These results are understood on the basis of superconducting condensate (sigma band) disorder and ensuing intrinsic pining due to B site C substitution clubbed with further external pinning due to available n-sic/ Mg 2 Si pins in the composite system. Fig. 7.2 : Irreversibility field H irr versus Carbon content plots at 5, 10 & 20K for Mg-x samples. The inset shows the upper critical field (H c ) vs Normalized temperature plots for Mg-x samples (x=0.0, 0.08, 0.10 & 0.20) (III) Superconductivity of bulk Mg +nano(n)-sic composite system: A high field magnetization study A study was conducted on the effect of n-sic addition on the crystal structure, critical temperature, critical current density and flux pinning in Mg superconductor. X-ray diffraction patterns show that all the samples have Mg as the main phase with very small amount of MgO, further with n-sic addition the presence of Mg 2 Si is also noted and confirmed by SEM & EDS. The T c value for the pure Mg is 18.9K under 8 Tesla applied field, while is 20.8K for the 10-wt % n-sic doped sample under the same field. This points towards the increment in upper-critical field value with n-sic addition. The irreversibility field for the 5% n-sic added sample reached 11.3, 10 and 5.8 Tesla, compared to 7.5, 6.5, and 4.2 Tesla for the pure Mg at 5, 10 and 20K respectively (Fig.7.3). The critical current density Fig. 7.3 : Magnetization loop M(H) for Mg +n-sicx (x=0%, 3%, 5%, 7% & 10%) up to 13 Tesla field at 5, 10 & 20K (IV) High field performance of nano-diamond doped Mg superconductor Polycrystalline Mg -nd x (x= 0 to 0.1) samples are synthesized by solid-state route with ingredients of Mg, B and n-diamond. The results from magneto-transport and magnetization of nano- ANNUAL REPORT

6 diamond doped Mg -nd x are reported. Superconducting transition temperature is not affected significantly by x up to x = 0.05 and latter decreases slightly for higher x > R(T) vs H measurements show higher T c values under same applied magnetic fields for the nano-diamond added samples, resulting in higher estimated H c2 values. From the magnetization measurements it was found that irreversibility field value for the pristine sample is 7.5 Tesla at 4 K and the same is increased to 13.5 Tesla for 3-wt% nd added sample at the same temperature. The J c (H) plots at all temperatures show that J c value is lowest at all applied fields for pristine Mg and the sample doped with 3-wt% nd gives the best J c values at all fields. For the pure sample the value of J c is of the order of 10 5 A/cm 2 at lower fields but it decreases very fast as the magnetic field is applied and becomes negligible above 7 Tesla. The J c is 40 times higher than pure Mg at 10 K at 6 Tesla field in case of 3%nD doped sample and its value is still of the order of 10 3 A/cm 2 at 10 Tesla for the same sample. On the other hand at 20K the 5%nD sample shows the best performance at higher fields (Fig.7.4). These results are discussed in terms of extrinsic pinning due to dispersed n-diamond in the host Mg matrix along with the intrinsic pinning due to possible substitution of C at Boron site and increased interband scattering for highly doped samples resulting in extraordinary performance of the doped system. Fig. 7.4 : Critical current density (J c ) variation with respect to applied magnetic field (H) at 10K for nano-diamond added Mg superconductor. 94 ANNUAL REPORT