K. Gangadharan

On the preparation and structure of the 81 K single phase superconductir in the BiCaSrCuO system

Abstract A new and elegant method to synthesize a single phase 2122 compound in the Biue5f8Caue5f8Srue5f8Cuue5f8O system is described. Electrical resistivity measurements show the superconducting transition temperature ( R = 0) at 81 K. X-ray and neutron diffraction analysis indicate that the single phase compound has cell parameters a = 5.362(6) A , b = 5.443(6) A and c = 30.77(2) A and space group Fmmm. Results of neutron diffraction profile analysis show that the orthorhombic distortion is significantly larger than hitherto reported and also indicate possible substitution of Bi for Sr/Ca.

Synthesis and properties of a 125 K superconductor in the Tl-Ca-Ba-Cu-O system

A method for the synthesis of a 125 K superconducting phase having a nominal composition TlCaBaCu2 Ox is described. It is shown that by sintering at 1200 K in flowing oxygen containing a controlled amount of water vapor, a zero electrical resistance state (Tcf) at temperatures as high as 125 K can be obtained. A transport critical current density (Jc ) of ∼240 A/cm2 at 77 K was obtained in this 125 K material.

Superconducting transition temperature of single-phase Tl-2223: Crucial role of Ca-vacancies and Tl-content

Abstract Detailed investigations on the synthesis and phase-indentification of a number of Tl-Ca-Ba-Cu oxides show that single-phase Tl-2223 superconductor with the best transition temperature ( R = 0 at 130 K) can be prepared from a thallium-deficient starting composition, namely Tl ≈1.0 Ca 2 Ba 2 Cu 3 O x . A number of other starting compositions containing Tl: Ca T c -values between 115 K and 130 K. It is argued that the existence of vacancies at Ca-sites is crucial to the production of pure 2223-phase and the highest T c (130 K) is obtained when the number of such vacancies reaches an optimum. Starting compositions deficient in Tl lead to such a possibility by aiding the migration of some Ca-ions to vacant Tl-sites. Under standardised preparation conditions single-phase Tl-2223 structure is obtained for a combined nominal stoichiometry 3 ⩽ (Tl + Ca) ⩽ 4.

Insensitivity of Tc to high K substitution in the Tl-Ba-Ca-Cu-O system

Abstract We report that substitution of potassium for thallium upto as high as 70% in the Tl-Ba-Ca-Cu-O superconducting system does not adversely affect the T c(zero) which can be as high as 123 K for 50% K and 118 K for 70% K containing compounds. The major lines in the X-ray diffraction patterns of the 50% K substituted compounds are quitte similar to those in the pure Tl containing compound. Possibility of complete substitution of Tl by K was also explored and the results discussed.

Luminescence properties of Ti-doped gem-grade zirconia powders

A detailed investigation on luminescence properties of gem-grade zirconia (NFC) as a function of Ti doping is presented. The effect of various parameters such as Ti concentration, environment of heat treatment and temperature was studied in detail and the optimum conditions for producing zirconia with luminescence properties comparable to standard material determined.

On the synthesis of high-temperature superconducting compounds in the BiSrCaCuO system

Abstract Thermal treatment of BiSrCaCu x O y ( x =2, 3) in the temperature range 1125–1175 K in air, oxygen and argon and its effects on superconducting transition temperature ( T c ) is investigated. Zero electrical resistance state at 84 K is obtained on samples sintered at 1175 K and cooled slowly in air.

Synthesis of single phase Tl-2223 superconductors: How much thallium do we really need?

This paper describes the synthesis and microwave absorption studies of single phase Tl-2223 compound, withTc=(129 ± 1) K. The single phase compound has been synthesized using a precursor matrix method. It has been found that Tl-deficient starting composition gives the best results.

High temperature superconductivity in bismuth-alkaline earth-copper-oxygen system

Electrical resistivity measurements of a series of compounds in the bismuth-alkaline earth-copper-oxygen system have been carried out. Several compositions (BiSrCaCu2Ox, BiSrCaCu3Ox, Bi2Sr2CaCu4Ox and Bi4Sr3Ca3Cu8Ox) showed superconducting onset temperatures in the range 90–130 K, though none of them showedTcf(R=0) above 77 K. ExtrapolatedTcf of three compounds are in the range 72–75 K. All compounds are multiphasic.

Positive role of potassium in neutralising the non-stoichiometry effects in T1-2212: Synthesis and microwave investigations

Abstract T1 2 Ba 2 CaCu 2 O n (designated as T1-2212) has been synthesized using a precursor matrix reaction method. The superconducting phase with T c = 101 K was confirmed by microwave methods. A series of compounds T1 2 Ba 2 − y CaCu 2 O n with barium deficiency ( y = 0.0–0.75) were also prepared by the same route. The value of T c was found to monotonically decrease with deficiency of barium. Typically the value of T c reduced by about 20 K when barium concentration per formula unit reduced from 2.0 to 1.25 (keeping the concentration of all other compounds constant). The whole series of compounds were found to exhibit a single superconducting transition by microwave investigations. It was found that potassium substitution in barium deficient compounds enhanced the T c nearly to its original value. Further it was shown that even in thallium deficient compounds the lowering of T c was arrested and restored to its original value by substituting it with potassium. Thus it was shown that presence of potassium in T1-2212 has unique role of stabilizing the T c against the deteriorating effects due to possible T1/Ba nonstoichiometry. X-ray diffraction data suggests that thallium substitution occurs at barium site in the former case, whereas potassium substitutes at barium and thallium sites and the extent of substitution is dependent on the relative concentrations of thallium and barium in the starting composition.

Evidence for superconductivity in fluorinated La2CuO4 at 35 K: Microwave investigations

In the fluorinated La2CuO4−x prepared using a solid state reaction with NH4HF2 as a fluorinating agent at 550 K at ambient pressure, superconductivity was detected by microwave and EPR techniques with aTc of 35 K.