All the pyrophyllite gaskets used in the measurements had an o.d. of 12.5 mm and i.d ol 5 mm. The first pyrophyllite gasket (0.4 mm thick) with a talc disc (5 mm ...
Rcccnt Trcnds ln Ed.
XIII
lligh
Pncsrurc Rccearch,
Anil K. Singtr, Oxford IBH, New Delhi, AIRAPT
-
1992. (ISBN
8l-204-0713,X)
International Conference on H igh Pressure Sciente and Technology, 1991
THERMOELECTRIC POWER OF YBa.Cu4Os UNDER PRESSURE Sheela K. Ramasesha and AnilK. Singh Materials Science Division, National Aeronautical Laboralory, Bangalore 5600'1 7, lndia
YBazCulOa is synthesised at ambient pressure ol oxygen and its thermoeleclric power
(TEP) is measured as a function of pressure up to
I
GPa. The TEP decreases
monotonically with increasing pressure.
Keywords: High
Tg
superconductor, thermoelectric power, high prsssure.
INTRODUCTlON The 80K superconduclor YBa2Cu4Oe has been studied widely only after techniques were developed lor synthesising this compound at ambient oxygen pressures [1-3]. The Tc ol YBaeCuaOe is strongly pressure dependent, increasing at a rate ol 5 KGpa-l [4]. ln this communication we presenl the thermoelectric power (TEP) data of YBaeCuaoe up to 8 Gpa.
EXPERIMENTAL DETAILS YBaeCulOs was synthesised by taking YzOs, Ba (NOs)z and CuO in required proportion and about 0.3 mole fraction of NaNOg. The mixture was mixed well and prereacted in air lor 40 mins. al 700C. The prereacted mixture was ground, pelletised and heated in flowing oxygen lor 4 days with an intermittent grinding and pelletising. The phase purity was contirmed lrom
x-ray difiraction patlern (Figure
t). Tc was determined from four lead dc-
measuremenls. The Tc (mid-point) was lound lo be 74 K (Figure
resistivity
21.
For the high pressure experiments a tungsten carbide opposed anvil set-up with 12.5 mm face was used. A stack ol three pyrophyllite gaskets was used to conlain the specimen, thermocouple junctions and the heater wire. All the pyrophyllite gaskets used in the measurements had an o.d. of 12.5 mm and i.d ol 5 mm. The first pyrophyllite gasket (0.4 mm thick) with a talc disc (5 mm diameter and 0.4 mm thick) at the center was placed on the lower anvil. Two pairs of chromel-alumel (0.1 mm dia) thermocouples were placed 1 mm aparl, situated symmetrically about the center ol the cell. A nichrome heater wire was placed
419
'?30 ;
2zo
o ot o
ot
N
rt
@ ot ol
F
a 6 t o
AI r\
CD
o
H10
50
2g
Fig.
1
(DEGREES)
T (K)
Resistivity in arbilrary units as a t unction of temPerature.
Ftg.2
X-ray diffradion Panern of
YBaCqQ.
the wires in placb' A at one end ol the specimen. Grooves were made on the gasket to hold mm thick was placed 0.4 and piece lrom the YBazCulOe pellet ot approximately 4 x 1.5 mmz was lilled with specimen gap the gasket. around The al the center of the second pyrophylliie
gasket-specimen talc-powder and epoxy mixture. The epoxy was allowed lo set and the gasket on a 600 ol the sides both on wofting by mm 0.25 to down was thinned assembly
gradeemerypaper.Thegasket-specimenassemblywasplacedabovethelirstsuchthatthe and the heater which was insulated lrom witn tne tnernroin ipecimen was
contact
iuncl'ons
talc disc at the the specimen with a thin mica sheet. The third gasket (0.25 mm thick), with lrom the top sample lhe insulated gasket which the second of placed top on centre, was
by passing current anvil. The temperature gradient was set up along the lengrth ol the sample a Leeds & voltages' lor smaller through the heater wire. A Keithley 196 microvoltmeter, and
the two chromel ruortnrup nanovoltmeler were used to measure the vollage drops across relation' the using calculated was then TEP The (Eaa) wires. (Ecc) and the two alumel S=
(*
- rSa)/ ('l -
(1)
r)
EccJ Eaa. Sa and Sc are the standard TEPs temperature, resPectivelY.
where
r=
ol alumel artd chromel al
room
BESULTS AND DISCUSSION pVK-1 TEp as a function ot pressure is shown in Figure 3. TEP has a value ol 16
"t."t?P.ll 4 pvK ' pressure and decreases monotonically with increase in pressure, reaching a value ol type polynomial ol the around I GPa. The data has been litted to a second degree S = A + BP + CP2 with A=16.2, 8=-2.66 and C=0'133' 420
a
Y10 4
L tu F
Flg.3
TEP vs pressure data.
From x-ray investigation ol YBazCulOe under pressure, it is lound that the cell parameters a,b and c decrease monotonically upto 20 GPa [5]. Volurn€ also changes srnoothly with pressure. Time ol llight neutron ditlraction experiments under pressure [6] indicate that the distance 01-Cu2 (the apical oxygen and the in-plane copper atom) ard the distance Ba-O4 (barium atom and the chain oxygen) decrease with increasing pressure, while the other bond lengrths remain almost constant wilh pressure up to 1 GPa. Thus the interplanar distances become shorter upon the application of pressure, increasing the charge transler between the planes. ln olher words, the compound becomes rnore rnetallic. The decreasing TEP with increasing pressure rellects the increasing metallic character ol the specimen under pressure. REFERENCES
1. S.Jin, H.M.O'Bryan, 2. 3. 4. 5. 6.
P.K.Gallagher, T.H.Tielel, R.J.Cava, R.A.Fastnacht and
G.W.Kammlott, Physica C 165,415 (1990) D.M.Pooke, R.G.Buckley, M.R.Presland ard J.L.Tallon, Phys. Rev. B 41, 66f6 (1990)
R.J.Cava, J.J.Krajewski, W.F.Peck
Jr., B.Batlogg, L.W.Rupp Jr.,
R.M.Fleming,
A.C.W.P.James and P.Marsh, Nature 338, 328 (1989) E.N.Van Eenige, R.Griessen, R.J.Wijngaarden, J.Karpinski, E.KaHis, S.Rusiecki and E.Jilek, Physica C 168, ,l82 (1990) H.A.LudwQ, W.H.Fietz, M.R.Dietrich, H.Wuhl, J.Karpinski, E.Kaldis and S.Rusiecki, Physica C 167, 335 (1990) Y.Yamada, J.D.Jorgensen, S.Pei, P.Lightloot, Y.Kodarna, T.Matsurnoto and F'lzumi' Physica C 173, 185 (1991)
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