CAPÍTULO II: REVISIÓN DE LITERATURA
2.1. Marco Teórico
2.1.16. De la fórmula empleada por la Municipalidad Provincial de Puno para
Y=0.
300 -
40m /800Z 200 -
60m/800 OnVSOO 1---1--- r25
35
45
55
65
75
85
TEMPERATURE (K)
95
F igure 6.5.1. D C resistance versus tem perature o f
Pb^Sr^(YQ j ^ C a Q ^ ^ )2Cu4 0^ sam ples synthesised w ithout p recu rso r
technique and annealed fo r different periods.
Table 6.5.2. Superconducting transition tem peratures o f PbgSr^(Y]_
x ^ ^ x ) 2 ^ ^ 4 ^ z ( x=0.35, 0.5 a n d 0.65) annealed fo r 50 m inutes in nitrogen at 800°C. Calcium content --- > X =0.35 X =0.5 X =0.65 O btained stoichiom etry
Pc. ons. Pc. zer Pc. ons. Pc. zer. Pc.ons. Pc. zer.
Pb2.5Sr3(Y i_x E ^x)2Eu3 J Oy
80K 59K 80K 74K 72K 54.5K
F igure 6.5.3 exhibits the diam agnetic behaviour o f two different com positions,
P b 3S r3 (Y o,5C ao.5)2Cu4 0 2 and Pb3Sr3(Y o.35Cao 65)2Cu4 0 2 . The sam ples w ere
characterised by rf-inductive m easurem ent at f= 1040.0kHz, w here dc voltage com es dow n at the transition tem perature. The transition for sam ple x=0.5 starts around 80K but rem ains w eak till the tem perature reaches to 75K w here a sharp fall is present. T his shows that the diam agnetic volum e fraction is quite large at 75K and confirm s the zero resistance transition tem perature. The sample w ith calcium content, x=0.65, shows a very small variation from 70K dow n to 55K and broadness rem ains to 46K. This is the case for m ost o f the sam ples in each com position w hen the resistive transition is broader and sem iconducting behaviour is present in norm al state. This can be interpreted in term s of small num ber o f Cooper pairs density that also relates the volum e fraction of diam agnetism of superconducting sample.
Bulk Pb-3324: different Calcium
200
£ JZ O£ 150 -
UJz 100 -
I,:
C::90
100
80
60
70
40
50
TEMPERATURE (K)
F ig u re 6.5.2. D C resisîcuîce versus teuiperciiure o f FhgSrj(Yj.yCciy^)2C u ^ O - ( x=0.35, 0.5 a n d 0.65) sam ples anne a le d f o r 50 minute in yV?-
The com positions without Ca or Y remained highly resistive and were not characterised to show any transition at low temperature. These results again suggest that the variations in Y;Ca ratio can easily lead to an increased charge carrier density and the superconducting transition temperature, but higher transition temperature (T^ zero >ÜOK) is difficult in lead-based material. These results further demonstrate a different mode o f achieving high superconducting transition temperature that depends on air or pure nitrogen annealing.
2.08 -
> >85
45
65
75
35
TEMPERATURE (K)
F ig u re 6.5.3. D C voltage versus tem perature in rf-inductive m easurem ents show ing the dia m a g n etic behaviour o f the sam ples containing calcium
15
Table 6.5.3. Characteristic X R D d-spacings a n d relative intensities f o r
d(A ) EI(%) d(Â) !/!(%) d(A) I/I(%)
5.0083 25.7 2.4924 6.4 1.6593 36.9 3.9545 2 . 0 2.3878 7.2 1.5928 3.3 3.8235 1.1 2.3263 5.0 1.5729 3.0 3.7248 3.6 2.2258 2 . 2 1.5468 1.8 3.6029 0.7 2.1642 2.1 1.4962 11.2 3.5092 1.5 2.0578 3.0 1.4385 6.4 3.2671 1.2 2.0311 29.2 1.4170 1.2 3.1581 3.5 1.9692 2 . 2 1.3803 2.3 3.0901 2.5 1.9126 4.6 1.2853 8.4 2.8757 1 0 0 1.8826 6 . 2 1.2819 7.2 2.7458 18.3 1.8656 3.1 1.2051 5.5 2.6927 6 . 2 1.7520 5.4 1.1733 3.9 2.5284 5.5 1.7261 2 . 0
6000
^ 3000
.1 4000 -
<3000 -
L: 2000 -
Z m 1 0 0 0 -z
0
- P b-3324 bulk CuK XR D Y =0.7 Y = L 3 Y=1 ^Ar>—A^I »I .k.,10
20
30
40
50
60
70
SO
90
ANGLE (20)
F ig u r e 6.5.4. X R D pattern o f F h jS r g ( Y i _yCay)2Cii4();r ( Y=().7, 1 and l .3} material.Figure 6.5.4 and table 6.5.3 show the XRD pattern oi' the data obtained in Imperial C ollege, at Materials Department, by Dr.G.C.Tyrrell, using (CuKQ^:l= 1.54060 radiation, step size 2 0 = 0 .0 4 and time per ste p = lse c) diffractometer type: P W 1710, 45K V and 40m A generator current. In figure 6.5.4, all the intensity peaks have been
norm alised w ith the peak intensity at 0= 17 .69 5° (d=5.0083, I/I(% )=25.7) and showing variations in FW H M w ith the change in Y concentration in the sam ples, suggesting different oxidation levels. W hen com paring XRD o f these com positions w ith Pb-2213 phase m aterial, m ost o f the I/I(%) values and peak positions are not m atching w ith one another, required again a detailed XRD analysis.
In sum m ary, a variation effect o f the calcium level and period for reduction o f oxygen in
P-3324 com position have been studied system atically. A t a level o f x=0.5 and 50
m inutes time scale for reduction in nitrogen at 800°C provide the optim um
superconducting transition tem perature o f 80K. However, the structure o f this
com position has to be determ ined by any expert in Crystallography. The processing technique used to synthesise does not believes in the percentage o f oxygen in nitrogen and provides some new results regarding lead-based m aterials.
6.6 Summary
A detailed study has been made to produce im provem ent in superconducting properties through various com positions and nitrogen annealing. It is observed that lead-based m aterials take up excess oxygen from air even at high tem perature (>870°C) and can be
converted into superconducting material through precise oxygen adjustm ent. A
m odification and m inim um tim e scale for the synthesis o f lead-based m aterial was reported during processing.
Initially, the ceram ic precursor technique was used for the synthesis o f Pb-2213 phase that believes in tw o-step precursor technique, and can evolve out C O 2 after reacting above 950°C a m ixture o f all the carbonates and oxides except PbO for a period o f 5-16hours w ith interm ediate grindings. This m ethod can induce som e loss o f precursor and inhom ogeneity in second step. The second main problem was to control oxygen stoichiom etry in lead cuprates using 1% O 2 in N 2 for oxidation during synthesis. A m odification in m ethod was developed to fabricate m aterial w ithout precursor to evolve out C O 2 that saves several hours o f fabrication time, and the obtained results through m odification in synthesis are better than the other published reports. The Pb-3314 com position was fabricated and characterised using ED X and X RD methods. It is observed that both Pb-2213 and Pb-3314
153
but the reducing period o f this com position in nitrogen atm osphere was found sm aller com pared with Pb-2213 phase.
A different approach was developed to rem ove C O 2 from a m ixture o f carbonates to fabricate Pb-1212 phase material. A shortest fabrication period was dem onstrated w ith better superconducting transition tem perature w ithout high pressure oxygen anneal. In sum m ary, a system atic study o f the effect o f variation o f the calcium level, x, in the Pb- 1212 phase from x=0.4 to 0.8 was performed. A t a level o f x=0.7, a transition tem perature o f 82K (onset) and 76K (zero) was obtained, the highest so far achieved in (Pb, Cu)-1212 phase. Bi-cations w ere substituted in the PbO -layer enhances the onset superconducting
transition tem perature (85K), but suppresses the ^ero to 34K, and this seem s due to
sticking and poor-diffusive ability com pared w ith lead.
A m inim um tim e scale for the synthesis o f lead-based m aterial was reported during
processing o f various com positions. Polycrystalline sam ples o f Pb-1213 com positions
w ere synthesised w ithout using a precursor technique. The obtained results again suggest that the electrical properties and superconducting transition tem perature are sensitive to
oxidation state and calcium content helps the charge transfer to C u 0 2 sheets. The
m axim um obtained transition tem perature for zero resistance is 60K for Ca=0.55 value. The full w idth at h alf m axim um (FW HM ) and intensities increase with calcium content and
oxidation state, w here bigger Ca+^ ions replacing ions and oxidation can induce
m odulation in the (Y, Ca) plane.
It was interesting to know the effect of excess strontium in lead cuprates and their superconducting properties. The com position PbSr3(Y o,55Cao.45)C u3 0 g, (Pb-1313) was synthesised that exhibited a small transition w idth (A T-4K ) after N 2-annealing for an hour at 800°C provided Tg onset o f 34K and Tq zero 30K. The X RD pattern o f Pb-1313 com pared with Pb-2213 phase shows a deviation from relative intensities o f published X RD data for Pb-2213 phase and small variations in d-spacings suggest that the m aterial is different and needs X RD refinem ent and electron m icroscopic analysis.
A com position P b3S r3(Y i_ xC ax)C u n+ i0 2n + 4+ x was readily achieved in polycrystalline bulk form as a result o f an attem pt to induce higher superconducting 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 :C a ratio. The X RD o f Pb-3324 phase m aterial w as com pared
w ith Pb-2213 phase material. It is found that m ost o f the IxlOO/I„ values and peak
positions are not m atching w ith one another, required again a detailed X RD refinem ent analysis. A variation in calcium level for reduction o f oxygen in Pb-3324 com position has
superconducting transition tem perature o f 80K was achieved ju st only for 50 m inutes annealing period for reduction in nitrogen at 800°C.
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