147 task to induce superconductivity in these materials One can expect any possible

increase in superconducting transition tem perature w ith an increase in the num ber o f

C u 0 2 layers, sim ilar to other materials, such as Tl2Ba2Can_ iC u ^ 0 2 n + 4 - or

B i2Sr2C an_iC un02n+4~system s where n = l,2 ,3 and 4.

Synthesis using excess cationic ratios has been used to grow single crystals o f Pb- 2213 phase. These crystals were synthesised using two or three-steps precursor and non-stoichiom etric m elts with an excess o f PbO, CuO, SrO and N aC l co-fluxes [2, 26, 27, 28]. C ava et al.[2] and M arezio et al.[26] used m elts o f excess PbO, CuO and tw o-step's precursor m ethod but transition tem perature w as around 5OK. Xue et al. used three-step precursor method, and the m olar ratios o f SY C C O :PbO :N aC l for starting com positions were 2:9:60. A long duration high-tem perature processing then annealing around 600°C /15hrs in N 2 was required for the crystals to produce a sharp transition to zero resistance at 80K[3]. W hereas, Lukina et al.[28] used excess fluxes o f PbO , SrO, CuO, high temperature range from 1030-800°C over a period o f m ore than 50 hrs heat treatment. The cubic shaped single crystals extracted from m elts o f

dim ensions 0.6x0.6x0.5 mm possessed a T^ onset o f 80K and T^ %ero 75K.

This attem pt is also belonging to induce higher transition tem perature in lead-based m aterial through either variations in cationic ratios or keeping constant ratio o f Pb, Sr, Cu and changing Y:Ca ratio. Following five different com positions were synthesised and first three com positions were checked by resistive transition or rf-inductive m easurem ent (M eissner effect) and XRD.

l)P b 3S r3(Y o.5C ao.5)2C u40z 2) Pb3Sr3(Y o.65C ao.35)2C u40z

3) Pb3S r3(Y o.35C ao.65)2C u40z 4) P b3S r3Y 2C u40z 5) P b3S r3 C a2 C u4 0z

6.5.2

of Pb3Sr3(Yi.xCax)n-lCun+i02n+6+x

Stoichiom etric am ounts o f high purity pow ders were m ixed to synthesise bulk superconductors [Pb3Sr3(Yo.5Cao.5)2Cu4 0 z, Pb3Sr3(Y o.65C ao .35)2Cu4 0 z, and Pt»3Sr3(Yo.35Cao.65)2Cu40z].

T he first step was the same as described in section 6.1.2. In the second step, the tem perature o f reform ed pellets was raised to 880°C at the rate o f 6°C /m in for half an hour, cooled down to 875°C for 15 hours in air then cooled down to room tem perature at the rate o f 10°C /min. The air annealed pellets w ere cut into pieces o f equal dim ensions and placed into a furnace at 300°C and the tem perature was raised to 800°C. Later the nitrogen flow was started to reduce the samples. The

resistances o f each sample which were checked by a resistance meter. W hen approxim ately m inim um resistance across the sam ple was obtained, other characterisation m ethod w ere used.

6.5.3 Results and discussion

F igure 6.5.1 and table 6.5.1 show the resistances o f sam ples w ith a

P b 3S r3 (Y o.35Ca() 55)2Cu4 0 2 com position, annealed in a pure nitrogen for different periods at constant tem perature (800°C). The constant calcium content in the com position show s the effect o f oxygen reduction and its sensitivity tow ards dc electrical current of 2.0m A . The resistance o f the sample annealed only 40 m inutes shows higher value and sem iconducting behaviour w ith m inute negative slope. However, zero resistance w as not achieved dow n to 25K but the Tg onset was around 74K. W hen the annealing period increases by 10 m inutes the negative slope disappears but resistance rem ains constant up to To onset ^2K and zero resistance is at 54K. Further increase in annealing period by 10

m inutes, the resistance com es dow n but onset is 7 IK . A positive curvature around

55K is present that can be due to oxygen inhom ogeniety in the sam ple and Tg^ zero o f 42K . Figure 6.5.2 and table 6.5.2 show the tem perature dependence o f the resistance o f the P b 3Sr3(Y i_ x C ax )n -lC u n + iO z samples w ith nom inal values o f calcium from x=0.35 to 0.65, after annealing in N 2 for 50 minutes at 800°C. The x=0.35 and 0.5 sam ples exhibit nearly constant resistances and m inim um for x=0.5 in their norm al state w ith T^ onset o f 80K and T^ zero o f 59K and 74K respectively. Increasing the calcium content to x=0.65 value, norm al state resistance increases and a small sem iconducting slope appears, show ing that the charge carrier density is sm aller than for the sam ples w ith x=0.5 and 0.35 values. The T^ onset is also shifted to 72K and T^ zero o f 54.5K.

Table 6.5.1: Superconducting transition tem peratures o f

Pb^Srg(YQ j ^ C a Q ^ ^ )2Cu4 0^ samples annealed f o r different p erio d in nitrogen

A nnealed in N 2 for the p e rio d --- >

1 40 min. 50 min. 1 60 min.

O btained _{^ c . ons.}T _{Tc. Tc. ons Tc.zer Tc. Tc. zer.}

stoichiom etry _{7,er ons.}

Pb2.5Sr3(Y 35Ca 65)2Cu3 74 72K 54K 71 42K

149