K.P. Gupta

Structural and electrical resistivity studies on Bi1.6Pb0.4Sr2Ca4Cu5O y composition synthesized by different routes

Since the discovery of superconductivity in the Bi-Sr-Ca-Cu-O system [1], several experimental investigators have verified that Bi-Sr-Ca-Cu-O systems synthesized by different routes possess phases with Tc of 20, 80 and 110 K. Studies of disproportionation reaction by varying the doping concentration of Pb [2-4] in this system have also been reported. In this investigation we varied the synthesis conditions in Bi2_xPbxSr2Ca4CusOy composition in order to investigate the formation of superconducting phases, their disproportionation reaction and possible textured growth. We attempted synthesis of the above composition by three different routes: matrix ceramic (MC), glass ceramic (GC) and melt texture (MT). The resultant samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAD) and resistivity [p(T)] measurements and the results are reported.

Electrical resistivity in polycrystalline La1.8Sr0.2CuO4 and YBa2Cu3O7−δ - temperature dependence and critical fluctuations

Abstract High resolution electrical resistivity data have been taken from 300 K down to zero-resistance temperatures in polycrystalline La 1.8 Sr 0.2 CuO 4 anf YBa 2 Cu 3 O 7−δ samples. Although there are abundant similar studies in YBaCuO, there is hardly any report in a relatively low T c system like LaSrCuO. The resistivity data of both systems fit better to the metallic formula than the Anderson-Zou (A-Z) relation. Using Aslamazov and Larkins theory of excess conductivity, we find strong evidence of three-dimensional critical fluctuations in both the systems. In the case of YBaCuO, it is possible to go beyond the mean-field region to the critical one through a crossover regime because of its higher T c . In log e -log e plots of excess conductivity vs reduced temperature, we observe continuous curvature for both systems. However, the local slopes found in different regimes are consistent with the theory of Lobb for 3D fluctuations. Such plots are shown to be rather sensitive to the choice of T c .

Ag2O as nucleating agent in the crystallization of superconducting phase in Bi2Sr2Ca4Cu5γ glass composition

Abstract The nucleating action of silver oxide (1 mol% Ag 2 O) in the crystallization of superconducting phase in Bi 2 Sr 2 Ca 4 Cu 5 O γ system is quite remarkable. The characteristics of these glass ceramics are found to depend chiefly upon the formation conditions of the initial glass composition and a subsequent phase separation. We observe from the SEM studies the evidence of highly textured growth. The presence of long platelet and square-like platelet-type particles could be explained on the basis of an orientation effect. Besides being influencing the structural properties markedly, the nucleating agent also affects the physical properties, in particular, the normal state resistivity (ϱ). The above effects result in broadening of X-ray diffraction lines and a change in slope, in the normal state region (190–220 K) of ϱ ( T ) plots. Out of the three procedures used to fit the ϱ ( T ) data, the usual linear fit ( ϱχT ) does not work out at all over the temperature range 140–300 K. On the other hand non-linear fits, ϱχT n ( n =0.5) and the piece-wise linear fits (140–200 K and 220–300 K) work out much better. These results are interpreted on the basis of a possible intergrowth of two superconducting phases.

Phase separated glass and resultant superconducting glass ceramics in BiSrCaCuO (2245) composition

The crystallisation of superconducting phases in a glass of nominal composition Bi2Sr2Ca4Cu5Oy has been found to depend on phase separation within the glass. The most interesting result is the attainment of zero resistance even though the volume fraction of superconducting phase, estimated from Xac measurements, is smaller than 0.1%. These results have been discussed on the basis of exsolution of different phases, partial melting and rearrangement of ions at around grain boundaries. Presumably, a narrow superconducting path is formed along the flow line of the partially melted state.

Effect of optimum thermal treatment on BiSrCaCu2Oy

BiSrCaCu2Oy compositions have been subjected to different atmosphere treatments and cooling schedules after initial calcination and firing in air. The large variation in properties has been explained on the basis of oxygen disorder effects and possible insertion of oxygen in different planes.

Phase equilibria in the REFeB system at 1000 °C

Abstract Phase equilibria in the REFeB system (where RE represents the total rare earth content of Indian Misch metal) at 1000 °C has been investigated at the ironrich side, up to about 40 at.% RE and about 45 at.% B, using metallography, X-ray diffraction and thermomagnetic analysis techniques. The isothermal section shows the presence of two ternary intermediate phases which have been identified as the RE2Fe14B (Γ) and the REFe4B4 (Δ) phases. At the iron end, a b.c.c. α phase has been found. The Fe2B (β) phase has practically no solubility of RE. The Γ phase appears to have a very limited solid solubility. Both the Γ and the Δ phases are tetragonal with lattice parameters a = 8.784 A , c = 12.177 A and a = 7.106 A , c = 17.011 A respectively. The Γ phase has a Curie temperature of about 245 °C and Ms ≈ 120 e.m.u. g−1. The Δ phase also shows a magnetic transition between 240 and 440 °C.

Superconducting glass ceramics in BiPbSrCaCuO system

Abstract Glass ceramic based superconductors in four different compositions in BiPbSrCaCuO system have been synthesised. The amorphous flakes were initially produced by crucibleless melting between twin rollers. The as prepared amorphous flakes were semiconductors. The different heat-treatment schedules were estimated from thermal analysis measurements. Characterisation of differently heated samples show that the growth of superconducting phase in the amorphous matrix is dependent upon the heat-treatment schedule. The results of X-ray diffraction data is consistent with the results of resistivity, AC susceptibility and optical microstructure.

Phase equilibria in the mischmetal-cobalt-iron system on the transition-metal-rich side

Abstract A systematic study of phase equilibria in the commercially important mischmetal(MM)-Co-Fe system was made on the transition-metal-rich side. This investigation shows that most of the phases of the MM-Co system with high cobalt contents extend far into the ternary system; the phase of stoichiometry 2:17 possibly extends from RE-Co to binary RE-Fe where RE denotes the rare earth elements present in mischmetal. The RE 2 (Co,Fe) 17 and RE(Co,Fe) 5 phases extend away from their respective stoichiometric lines towards the transition-metal-rich side. The RE 2 (Co,Fe) 7 phase appears to be a new type of 2:7 phase. Three new ternary phases, designated as the S, T and A phases, were found to exist in the region of the ternary system investigated and the S phase was found to occur very close to and to run almost parallel to the 2:17 stoichiometric line. The diffraction patterns of all the new phases resemble the diffraction pattern of the 2:17 phase; the S phase structure is possibly a super lattice based on the 2:17 phase structure.

Phase relations in the mischmetall-cobalt system

Abstract While phase relations in binary systems of pure rare earths with cobalt have been studied extensively, only limited information is available on the MM-Co system, where the mischmetall (MM) contains cerium, lanthanum, neodymium and praseodymium as primary constituents. In the mischmetallcobalt system, six phases MMCo 3 , MM 2 Co 7 , MM 5 Co 19 , MMCo 5 , MM 2 Co 17 and β-Co were identified in the composition range 55–84 wt.% Co. The MMCo 3 , MM 2 Co 7 and MM 5 Co 19 phases have very similar X-ray patterns and hence they were identified by electron probe microanalysis and thermomagnetic analysis. The MM 2 Co 7 phase exists in both hexagonal and rhombohedral modifications. Of the various phases, only the MMCo 5 phase has an observable homogeneity region extending over 2 wt.% Co at 900 °C. The MMCo 5 phase is unstable at temperatures below 700 °C and the addition of iron increases its instability, leading for example to a nearly complete disappearance in a sample with 12 wt.% Fe in 30 days at 700 °C. The decomposition products of MMCo 5 occur at grain boundaries and inside the MMCo 5 grains of alloys with low iron contents but they occur as separate grains in alloys with high iron contents.

The misch metal-iron system

Abstract The misch metal-iron (MM-Fe) system has been investigated in the temperature range from 700 to 1150 °C at the iron-rich end (up to 33.3% RE, where RE is the total rare earth content of the MM). Two intermediate phases, Fe 17 RE 2 and Fe 2 RE, and two broad three-phase fields, α - Fe + Fe 17 RE 2 + liquid and Fe 17 RE 2 + Fe 2 RE + liquid , exist in the investigated region of the MM-Fe system. The γ-Fe phase cannot be retained by quenching the alloys from high temperature. The solid solubility of RE in iron is less than 0.5 at.%.