Synthesis of MgB 2 from magnesium rich powders

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 44, June 2006, pp Synthesis of MgB 2 from magnesium rich powders Suchitra Rajput, Sujeet Chaudhary & Subhash C Kashyap Department of Physics, Indian Institute of Technology Delhi, New Delhi Received 27 January 2006; accepted 7 April 2006 Superconducting bulk MgB 2 samples have been synthesized by employing a modified heat treatment without using any additional process steps generally undertaken in view of the substantial loss of Mg during sintering owing to its high vapour pressure at the processing temperature. Starting with Mg rich powder mixtures having different atomic ratios of Mg:B (as against the nominally required 1:2 ratio), we have obtained superconducting MgB 2 samples showing a transition temperature (T c ) in the range K when synthesized at sintering temperatures in the range C. Typically, a MgB 2 sample (T c = 40.5K) obtained from heat treating Mg and B in the ratio of 2:2 at 870 C for 1h duration exhibited critical current density in excess of A/m 2 at 38K. Our results further show that MgO is not detrimental to superconductivity. Keywords: MgB 2, Transition Temperature, Susceptibility, Superconductivity IPC Code: H01L 39/00 1 Introduction The period between 1950 and mid-1980s was the hey-day of research on the intermetallic superconductors. Till 1985, only 25 elements and thousands of alloys and compounds were shown to be superconducting with the transition temperatures 1 (T c ) 23K in Nb 3 Ge. With the discovery of high T c (~90K) superconductivity in Y 1 Ba 2 Cu 3 O 7-x (YBCO) in early 1987 (Ref. 2), the 23K ceiling for T c was surpassed. Since then many other cuprate superconductors viz., Bi-Sr-Ca-Cu-O, Tl-Ba-Ca-Cu-O, Hg-Ba-Ca-Cu-O with higher T c s (>100K) have been discovered, and the record is 164K (Ref. 3). In January 2001, the observation of a remarkably high value of T c 39K (which is nearly twice the highest T c recorded for any intermetallic compound) in a non-oxide material-mgb 2 had sparked 4 renewed interest in the connection between superconducting properties and the electronic and structural features of this non-oxide material. Speculation about the potential large scale applications of MgB 2 was then motivated primarily due to its relatively high transition temperature, better mechanical properties, weak-link free grain-boundary, high transport criticalcurrent density (J c ), large coherence-lengths and simple crystal structure 5,6. Despite the chemical and structural simplicity, the synthesis of high quality MgB 2 has proved to be very difficult, due to high volatility of Mg. The arrest of Mg vaporization, therefore, requires additional steps like covering the material with tantalum foil or sealing it in a quartz tube or use of Nb-lined stainless steel containers Also, it is reported that for obtaining good quality bulk MgB 2 samples, either high pressure sintering 11 or high pressure cells are required 12,13. For optimizing, a simpler and economic method (utilizing excess Mg) has been attempted to prepare superconducting MgB 2 in bulk morphology. In this paper, we present the results of the modified heat treatment conditions resulting into bulk superconducting MgB 2 via a single step process without requiring either the sealing of the sample or the additional process steps related with high-pressure synthesis condition. The superconducting samples thus prepared will then be utilized to establish a structure property relationship and to prepare tapes for various applications. 2 Experimental Details The superconducting MgB 2 has been synthesized by heat treating a mixture of Mg (99.8%, STREM CHEMICALS) and B (99.9%, CERAC) powders. Various ratios of Mg:B (each having Mg in excess of stoichiometric requirement) were taken in the starting mixture. Powder mixtures of each composition were thoroughly ground, and then pelletized using a die of 5 mm diameter (pressure 750 MPa). For heat treatment, a single pellet was taken at a time and kept inside the heap of Mg powder formed over alumina plate. Given the higher reactivity of Mg, the inert

2 462 INDIAN J PURE & APPL PHYS, VOL. 44, JUNE 2006 ambient environment near the sample stage was ensured by flowing Ar gas. The heating rate was kept around 5 C/min using a programmable furnace (Model-K1252, Heraeus) to attain a desirable sintering temperature, which was then maintained for 1hr (hold time). At the end of this hold time, the pellets were quenched to room temperature in air. The magnetic characterization of the samples was performed using a highly sensitive ac-susceptometer. The ac-susceptibility (χ ac ) versus temperature (T) measurements are carried out in an ac-field of 260mOe (unless otherwise stated) at 540Hz in a closed cycle He-cryostat using a lock-in amplifier to separate the in-phase (proportional to χ ac ) and the out-of-phase (proportional to χ ac ) signals (χ ac =χ ac - iχ ac ). Resistance (R) versus temperature (T) measurement on as-synthesized samples has also been performed. Both, the R versus T as well as χ ac versus T data were recorded while warming the sample from 15 to 300K. Close to transition temperature, slow rate of heating (=.25K/min) was maintained. The phase purity of the samples was checked using X-ray diffraction (12kW rotating anode, Rigaku). 3 Results and Discussion In view of the problem associated with the high volatility of Mg during the synthesis of MgB 2 and the fact that among the known compounds of the Mg-B system (viz., MgB 2, MgB 4, MgB 6 and MgB 12 ), MgB 2 is the most Mg-rich binary compound 14,15, it was decided to take excess Mg in the initial powder mixture consisting of Mg and B, as this would compensate for the loss of Mg and may lead to the formation of the stoichiometric phase-mgb 2. In order to study the effect of initial relative composition of Mg and B on the phase-formation and superconducting properties of the resulting MgB 2, a series of Mg-rich powders, designated as Mg x B 2 (with x=1.25, 1.5, 1.75 and 2.0) was heat treated. (Henceforth, samples from these batches would be referred to as MB1, MB2, MB3 and MB4, respectively). First, we looked out for the optimum value of the initial relative composition of Mg:B from the measurement of χ ac versus T on a few samples (from each of the four batches), which were heat treated for 1hr at sintering temperature (T S ) lying in the range C. Subsequently, the optimum sintering temperature for preparing MgB 2 sample with best superconducting characteristics is identified by the comparative analysis of the transition temperatures in various samples synthesized from the optimum initial relative composition, but with different T S. We now define various critical temperatures which would be used for comparing the superconducting behaviour of various samples. These are: (a) T on C,mag the temperature at which the normal state χ ac (T) is restored (b) T P the temperature at which χ ac (T) exhibits a peak and (c) T on C,RT the temperature at which the normal state resistance is restored in the R(T)-data. For example, T on C,mag is inferred from the intersection of the two extrapolated portions obtained by drawing tangents just below and above the onset of the superconducting transition of the χ ac (T) versus T-plot. Figure 1 shows the variation of the T on C,mag with the T S for a few samples from the two batches MB3 and MB4. (The samples from the other two batches, i.e., MB1 and MB2 did not show any sign of superconductivity down to the lowest investigated temperature of 15K). It is seen from Fig. 1 that for both cases of MB3 and MB4, the T on C,mag decreases when T S is varied away from 730 to 750 C, but there is a remarkable difference in the rate of decrease in the T on C,mag for the two batches. It is observed that there is a sharp fall in T on C,mag for samples synthesized from the MB3 batch as T S is decreased from about 750 C. The superconductivity seems to get severely affected in samples of MB3-batch, as the sample synthesized at 770 C did not show any superconducting signature. This implies that a Fig.1 Variation of T C,mag on with sintering temperature, T S for the two indicated initial precursor compositions of Mg:B. (A hold time, t S of 1 h duration was kept for all the samples)

3 RAJPUT et al: SYNTHESIS OF MgB 2 FROM MAGNESIUM RICH POWDERS 463 reaction temperature of 770 C is too high for the synthesis of superconducting MgB 2 with initial composition of 1.75:2 Mg:B, and at the same time excess Mg (i.e., x=1.75 as against the usual value of 1) appears inadequate to make up the loss of Mg at such an elevated sintering temperature. These observed changes could be associated with increase in the vapour pressure of Mg with increase in T S leading to excessive loss of Mg from the pellets. The melting point (mp) of Mg is only 650 C as compared to the very high value of 2300 C for B. Having obtained the superconducting MgB 2 samples from the initial relative atomic-composition of Mg:B of 2:2, without needing either high pressure synthesis or additional process steps, we then decided to see, in detail, the effect of T S on the superconducting parameters as well as on the phase purity in series of samples synthesized from MB4 batch. For this purpose, a large number of samples have been synthesized from MB4 batch with different T S ranging between 500 and 950 C. The effect of T S on superconducting characteristics can be seen by comparing T P and T on C,mag among the various samples. It may be mentioned here that in contrast to the percolative nature of the R-T measurement, the global nature of the response in the χ ac (T)-measurement is generally more conclusive, and provides sample details which are contributed from the whole volume of the sample. Hence, T S -dependence of the inferred critical temperatures T P and T on C,mag, would reflect the true effects of varying the sintering temperature for synthesizing the MgB 2 samples. Within the accuracy of ±0.2K (below 35K) and ±0.5K (35-450K) of the employed Si-diode sensor (resolution=0.01k), the MgB 2 samples synthesized from MB4 batch and sintered in the range T s = C, exhibited a on gradual increase in both the T P and T C,mag as compared to those synthesized at higher T S (see Fig.2). For the samples sintered in the range of C, the observed T P s are quite high viz., K (±0.50K). The overall decrease of T on C,mag and T P when T S is changed from 900 to 570 C could be attributed to the reduced rate of diffusion of the atomic species. A higher value of bulk critical transition temperature (T P ) of 40.74K (±0.50K) is observed for MgB 2 sample synthesized at 830 C. To our knowledge, this value competes excellently with the highest T C of 40.2 K (Ref. 16) and 40.3K (Ref. 17) reported so far for the best quality MgB 2 bulk Fig.2 Effect of sintering temperature, T S on (a) T P and (b) T on C,mag as inferred from the T- dependence of imaginary and realparts of the ac susceptibility data for samples sintered in the range C. The inset to Fig. 2(b) shows the procedure adopted on for inferring T C,mag samples. For synthesis a temperature beyond 900 C, the samples did not show any superconducting behaviour, and they broke easily into powder. This could possibly be due to the enhancement of the Mg out-diffusion at such a high temperature (above 900 C) leaving behind unreacted B and/or some other (possibly Mg deficient) impurity phase(s). The other possibility could be the formation of MgO (the source of oxygen could be the limited purity of precursor materials employed and/or the commercial grade of Ar-gas itself). This calls for a detailed phase purity investigation in the present samples. It is concluded that the values of T S >900 C are too high to form a superconducting phase of MgB 2 by the solid-state reaction method in flowing Ar ambient, and that even the excessive Mg is not able to cope up for the loss of Mg. On the other hand, as the T S is varied near the mp of Mg, transition temperature changes drastically (see Fig. 2). These samples are also investigated for their normal-state resistivity behaviour. Figure 3 shows R

4 464 INDIAN J PURE & APPL PHYS, VOL. 44, JUNE 2006 Fig.3 Normalized resistivity versus T plots of the MgB 2 samples sintered at 870 and 650 o C, showing the metallic behaviour in their normal state regime. The inset shows the variation of T C,RT on with sintering temperature, T S. (T C,RT on values shown in the inset were obtained from the R-T measurements using a constant current of 100mA through the MgB 2 samples) versus T plots of samples synthesized at 870 and 650 C (from the MB4-batch). Note that to allow for a meaningful comparison, the R(T)-curve of each of the sample is being normalized with respect to its normalstate resistance value at room temperature. Both samples can be seen to obey metal like electrical conduction mechanism in their normal state. The critical onset temperature, T C,RT on was rather low (see inset of Fig. 3) and their T S -dependence did not agree with the variation of T C,mag on with T S [see Fig. 2(b)]. This is due to the fact that the R-T measurements have been performed at rather high constant current (=100mA) compared to a current of 800μA, through the primary coil which resulted in an ac-field of 260mOe in the ac susceptibility measurements. The increase in the transition temperature with the reduction in the applied current can be seen from Fig. 4, which presents the normalized resistance versus temperature data taken for various values of the applied current. Further, from the ratio of the resistance value at room temperature to a value just above the superconducting transition (i.e., at 42K), the residual resistivity ratios (RRR) of these samples are found to be 5.5 (T S =650 C), and 1.9 (T S =870 C). The higher RRR-value for samples synthesized at 650 C is consistent with those reported previously 18,19. The samples 18,19 that have unreacted Mg tend to give RRR values to the tune of In the present case, we thus tend to believe that the samples with higher RRR values have significant amount of unreacted Mg as Fig.4 Effect of varying the applied constant current on the resistively observed superconducting transition temperature in the MgB 2 sample (synthesis parameters: T S =870 o C;t S =1hr, MB4 batch) they are synthesized from Mg-rich precursor at relatively lower synthesis temperature. Let us now see how the temperature affects the phase purity of the MgB 2 samples. In Table 1, we show various d and I/I o values of the observed XRD-peaks for a few samples from the MB4 batch synthesized at the indicated T S. In order to have some idea of the relative fraction of the superconducting MgB 2 with respect to the observed impurity phase, various standard d and I/I o values of the identified phase are also presented in Table 1. Proper care has been taken to associate a particular peak to the identified phase by taking into consideration the ratio of relative intensities of the various observed peaks (with respect to the most intense peak) for particular phase vis-à-vis the ratio in the standard data for that phase. In the investigated range of T S, the presence of MgO is always found in all the samples. All the samples prepared at lower T S values (i.e., T S =640, 650, and 750 C) contained unreacted Mg as an additional impurity phase. This could possibly be due to the slower rate of diffusion of atomic species for these lower values of T S, together with the excess Mg taken in the starting mixture. At the intermediate value of T S =790 C, only impurity phase found was MgO. On the other hand, at the other extreme, i.e., at T S =870 C, the Mg-deficient MgB 4 and unreacted B were detected in sizable quantity together with MgO impurity. Similar XRD findings were previously reported on the MgB 2 samples synthesized using Ta-tube from the off-

5 RAJPUT et al: SYNTHESIS OF MgB 2 FROM MAGNESIUM RICH POWDERS 465 T S ( C) Table 1 XRD-based phase identification of a few characteristic MgB 2 samples (from MB4 batch), sintered at the indicated T S Observed d and I/I 0 Matched with the following phases indicating their standard d(å) / I/I o values of the XRDpeaks d(å) / I/I o MgB 2 MgB 4 Mg MgO other (100), 2.116(41), 1.548(33), 2.534(27), 2.681(25), 1.496(21), 2.366(20), 1.251(19), 1.489(17), 2.325(16), 2.163(16), 1.474(16), 2.215(14), 1.971(12) 1.875(10) (100), 2.655(25),.538(16), 1.485(13), 1.756(9), 1.466(5), 1.409(3) (100), 1.496(35), 1.548(25), 2.696(24), 1.770(16), 1.164(14), 1.478(12), 2.453(11) (100), 2.14(49), 2.116(30), 2.466(24), 2.681(18), 1.493(13), 1.548(13), 1.476(13), 2.415(5), 1.909(5), (100), 1.545(29), 1.493(28), 2.688(26), 2.466(19), 1.474(16) 2.127(100), 2.671(35), 1.542(25), 1.761(14), (14), 1.470(14), 2.127(100), 2.671(35), 1.542(25), 1.761(14), 1.470(14), 1.412(4) 2.127(100), 1.542(25), 2.671(35), 1.761(14), 1.160(14), 1.470(14) 2.127(100), 2.671(35), 1.542(25), (14) 2.127(100), 1.542(25), 2.671(35), 1.470(14) 2.53(100), 2.2(50), 1.96(50) 2.106(100), 1.489(52) 1.489(52) 2.453(100) 1.489(52) 2.453(100), 2.606(41), 1.901(20) 2.106(100), 1.489(52), 2.421(10) 2.453(100) 1.489(52) B:2.36(8), B:2.32(6) B 2 O 3 :2.165(10), 1.872(5) stoichiometric precursors 20 containing, however, the isotopically pure 11 Mg x B 2 (0.8 x 1.2). We believe that at such higher T S, the overall content of Mg would be quite small due to the enhanced outdiffusion of Mg leading to dominant presence of B in addition to MgO impurity. This could also be the reason behind the very brittle nature of sample synthesized at T S 900 C. The samples synthesized at the intermediate value of T S =790 C did not contain any traces of unreacted B or Mg or any other known compounds from the Mg-B system, and exhibited optimum superconducting properties (Fig.2). Figure 5 shows the X-Ray diffractogram for the MgB 2 for one of the sample synthesized in optimum synthesisconditions (i.e., T S =790 C, Mg:B::2:2, t S =1.0h). Further, the presence of MgO does not seem to be detrimental to the superconductivity in MgB 2 as the observed T C values compete excellently with the highest values reported so far in the literature 16,17. To see if the impurity phases detected in XRD are helpful in providing sufficient pinning centers to the vortices, we have estimated the T-dependence of the critical current density (J mag C (T)) in the present MgB 2 samples. Using modified critical state models 21-24, the Fig.5 X-ray diffractogram of the MgB 2 sample (MB4 batch), synthesized at T S =790 o C for t S =1hr, showing the presence of the main MgB 2, and the only observed impurity phase MgO (Table1) J C mag (T) is determined from the shift of the peak in χ ac (T) with the strength of the applied ac-field for two samples of MB4 batch (viz., for T S =650 and 870 C) and one sample of MB3 batch (viz., for T S =650 C). Figure 6(a) shows this shift in T P for one of the samples from the MB4-batch synthesized at T s =870 C. Figure 6(b) shows the Jc mag (T) behaviour in these samples. Quite clearly, the sample synthesized at 870 C sustained higher J C mag till 40K. The J C mag (38K) value for this sample was found in excess of 10 7 A/m 2, and is in agreement with the

6 466 INDIAN J PURE & APPL PHYS, VOL. 44, JUNE 2006 Fig. 6 (a) Shift in the peak in χ ac (T) plot to lower T with the increase in the applied ac-magnetic field for a MgB 2 sample synthesized at 870 C from MB4 batch. (b) Semi-log plot of the estimated J C mag (T) behaviour for a few indicated MgB 2 samples previous report 9,25 on the polycrystalline bulk MgB 2 samples (with MgO as impurity phase, as in our case) synthesized by solid state reaction using a Mg rich (quantity unspecified) precursor. We further tend to draw a straightforward conclusion that presence of finite amount of impurity phases (e.g., MgO) in this class of newly discovered superconductors does not affect the superconducting characteristics. Rather, in view of the reported transparency of the J C with respect to the grain boundaries 26, 27, it appears that these phases, for sizes coherence length, might be providing useful pinning action to the vortices against the Lorentz force when field is applied to the superconductors, resulting in higher J C as observed in the present MgB 2 samples. 4 Conclusions Polycrystalline bulk samples of superconducting MgB 2 have been synthesized via a simple, economic and less time consuming method, without using any additional steps. Starting with Mg-rich powders, optimum conditions for the synthesis of stoichiometric MgB 2 have been identified. This was made possible on the basis of analyses of magnetic ac-susceptibility, electrical-transport and phase purity investigations on these MgB 2 samples. The samples synthesized from the Mg-rich powders having atomic-composition ratio of Mg:B=2:2 and synthesized in the temperature range o C (sintering duration=1h) possessed optimum superconducting parameters. These bulk MgB 2 samples exhibited bulk superconductivity with critical transition temperature lying in the range K (±0.5K). References 1 Nunez-Regueiro M, Tholence J L, Antipov E V, Capponi J J & Marezio M, Science, 262 (1993) Wu M K, Ashburn J R, Torng C J, Hor P H, et al., Phys Rev Lett, 58 (1987) Gavaler J, Appl Phys Lett, 23 (1973) Nagamatsu J, Nakagawa N, Muranaka T, Zenitani Y & Akimitsu J, Nature, 410 (2001) Yildirim T, Materials Today, April (2002) Cava R J, Zandbergen H W & Inumaru K, Physica C, 385 (2003) 8. 7 Cardwell D A, Hari Babu N, Kambara M & Campbell A M, Physica C, 312 (2002) Botta D, Chiodoni A, Gerbaldo R, Giunchi G, et al.,physica C, 369 (2002) Kumakura H, Takano Y, Fujii H, Togano K, Kito H & Ihara H, Physica C,363 (2001) Ya-Bin Zhu, Jia-Di Xu, Shu-Fang Wang, Yue-liang Zhou, et al., Physica C, 371 (2002) Jung C U, Park M S, Kang W N, Kim M -S, Lee S Y & Lee S -I, Physica C, 353 (2001) Prikhna T A, Gawalek W, Surzhenko A B, Moshchil V E, et al., Physica C, 372 (2002) Maple M B, Taylor B J, Frederick N A, Li S, Nesterenko V F, Indrakanti S S & Maley M P, Physica C, 382 (2002) Massalski T, Binary alloy phase diagrams, (ASM International, Materials Park, OH) 1990, 2 nd edition. 15 Canfield P C, Finnemore D K, Bud ko S L, Ostenson J E, et al., Phys Rev Lett, 86 (2001) Finnemore D K, Ostenson J E, Bud ko S L, Lapertot G & Canfield P C, Phys Rev Lett, 86 (2001) Feng Q R, Wang X, Wang X Y & Xiong G -C, Solid State Commun, 122 (2002) Jung C U, Kim H J, Park M S, Kim M S, Kim J Y, Du Z & Lee S I, Physica C, 377 (2002) 21.

7 RAJPUT et al: SYNTHESIS OF MgB 2 FROM MAGNESIUM RICH POWDERS Prozorov R, Giannetta R W, Bud ko S L & Canfield P C, Phys Rev B, 64 (2001) Ribeiro R A, Bud ko S L, Petrovic C & Canfield P C, Physica C, 385 (2003) Beans C P, Phys Rev Lett, 8 (1962) Chen D X, Nogues J & Rao K V, Cryogenics, 29 (1989) Marohnic Z & Babic E, Critical current densities from ac susceptibility data in magnetic susceptibility of superconductors and other spin systems, edited by Hein R A, Francavilla T L, and Liebenberg (D H Plenum Press, New York) 1992, p Clem J R, Phys Rev B, 50 (1994) Kusevic I, Marohnic Z, Babic E, Drobac D, Wang X L & Dou S X, Solid State Commun, 122 (2002) BuZae C & Yamashita T, Supercond Sci Technol, 14 (2001) R Jin S, Mavoori H, Bower C & Dove R B van, Nature, 411 (2001) 563.