A method for preparation of a high-quality high temperature superconducting ceramics

Transcription

1 Bull. Mater. Sci., Vl. 14, N. 2, April 1991, pp ,E; Printed in India. A methd fr preparatin f a high-quality high temperature supercnducting ceramics L A KOTOK, L A KVICHKO, V T ZAGOSKIN, Yu G LITVINENKO and E T MOGHILKO Institute fr Single Crystals, Lenin a~.60, Kharkv , USSR Abstract. An imprved methd fr ht pressing f high temperature supercnducting (HTSC) pwders prepared by cnventinal slid phase synthesis f the initial BaCO3, CuO and Ln20.~ allws ne t btain HTSC targets and magnetic shields pssessing a high degree f hmgeneity. By sputtering such targets HTSC films with critical current density f ~ A/cm 2 have been depsited. Shields, prepared accrding t this methd, shw a shielding cefficient f l05 and tt,. f 79 (3e in the cnstant magnetic field and in the alternating magnetic field the amplitude is 90 Oe in the frequency range f Hz. Keywrds. Ht pressing; high temperature supercnducting ceramics; magnetic shields; slid phase synthesis. I. Intrductin In an attempt t wrk ut a methd fr preparatin f HTSC ceramics the physicchemical regularities f the frmatin prcess, the regin f temperature stability and the decmpsitin kinetics f the supercnducting phase in the cmpunds f the cmpsitin LnBazCu30%,, where Ln = La, Y, Sm, Eu, Gd, Dy, Er, Tm, H, Yb have been studied. Differential thermal analysis (DTA), thermgravimetry (TG), X-ray diffractin analysis including high temperature, dilatmetry, electrn micrscpy and electrn-prbe micranalysis have been used fr this study. Determinatin f regularities f phase frmatin has made it pssible t ptimize cnditins fr the synthesis and develp a methd fr preparatin f HTSC materials with a reprducible cmplex f the set prperties by a ceramic technique using raw materials with varius prehistries. 2. Results and discussin The scaled-up prductin technique fr materials with preset prperties is based n a well-knwn prcedure f slid-phase interactin f BaCO3, CuO and Ln2 03 initial cmpunds during heat treatment. The electrphysical prperties f HTSC cmpunds strngly depend n their phase cmpsitin defined by synthesis cnditins. We have studied the dependence f supercnducting prperties n the phase cmpsitin, xygen partial pressure and synthesis temperatures f LnBazCu30 v_7,- type cmpunds (where Ln = La, Y, Sm, Eu, Gd, Dy, Er, Tm, H, Yb). By means f thermgravimetry, dilatmetry, X-ray diffractin and electrn-prbe micranalysis it has been established that the supercnducting phase frmed via the intermediate cmpunds LnzBaCuO 5 (I) and BaCuO2 {II) is stable in a narrw Tf... -Td... p temperature interval and is decmpsed int the same cmpunds I, II and CuO (IIIj. The values f T {i.e. the temperature f cmpund frmatin) fr varius 167

2 168 L A Ktk et al rare-earth elements are in the range f C. The values f decmpsitin temperature (Td... p) increase with increase f rare-earth element radius and xygen partial pressure (Seminzhenk et al 1988; Blank et al 1988). The duratin f the initial material heat treatment in the prcess f synthesis is limited by the interactin rate f barium carbnate with cpper xides and rare-earth elements at a preset temperature. T ptimize the use f pwders required fr preparatin f ceramics, the kinetics f phase frmatin has been investigated by means f heat treatment f Y203, CuO and BaCO 3 stichimetric cmpund. The interactin degree f the cmpnents has been determined with respect t the residual carbnate in cntent in the samples which have undergne isthermal treatment f varying duratins. The kinetics f the prcess calculated with respect t the interactin degree is in gd agreement with the results f phase cntent determinatin using X-ray phase analysis. At varius stages f the synthesis pressed pwder samples have been made, their phase cmpsitin and Tc being determined. The best quality samples have been btained when the phase cntent is nt less than 95% (figure 1). The prductin technique cnsists f preparatin f the initial cmpnents, their mixing in stichimetric rati and heat treatment with a gradual temperature rising up t T. Heating is repeated several times, the tablets being grund and hmgenized befre every heating. The residue mainly cnsists f particles up t 5-6 #m. It has been fund (Antnva et al 1988) that the tw-stage prcess f ceramic sample preparatin including pressing at rm temperature and subsequent heating prevents essential densificatin f HTSC ceramics. Hwever, simultaneus pressure and heating effects prvide a cnsiderable increase f density and prmte the preparatin prcess. Synthesized HTSC pwder is briquetted in dies made f Cr-Ni ally and pressed at a temperature f C and a pressure up t 0.6 GPa t btain ceramics. Heating, cling, pressure lading and unlading rates are chsen by experiments 100 t 3 ~ I I I n, cycles 12hr durtin eech Figure phase cntent (A) and transfrmatin degree (O) in pwder and tablets vs heat treatment time.

3 Preparatin f high quality H TSC ceramics 169 t prevent the samples frm heat cracking and t prvide the cmplete transitin f tetragnal phase int rthrhmbic mdificatin (Zagskin et al 1988). By pressure and ht-pressing temperature variatins the ceramic density may be mdified ver a wide range, as shwn in figure 2. Special attentin shuld be paid t the hmgeneity f temperature distributin ver the samples, since the tablet density is inhmgeneus alng the sample diameter wing t the temperature gradient n the plane perpendicular t cmpressing pressure. This inhmgeneity may be eliminated by chsing prper heating cnditins fr the die and prper arrangement fr heating. The reductin f the temperature difference between the tablet centre and its side surface favurs preparatin f density-unifrm ceramics (figure 3) OO*C 5.6 ~ Figure 2. pressing. l MP YBa2Cu]O1-7, ceramics density dependence vs pressure and temperature f ht 5-6 I _ 1 I- I_ AT : 20"C r, ~ (rnm) Figure 3. Radial distributin f the material density f the ceramic HTSC target YBa2Cu307 r, dia68mm.(a) AT- 80 C, (b) AT= 20 C.

4 170 L A Ktk et al X - Oxygen index in YB z Cu 3 07-,, , I 0 I 1 I I 1 I I 1 I l 5 10 (ram) Figure 4. Oxygen cntent (index,c) in the material f the YBa2Cu3OT_ ~. target at varius distances frm its surface. (Q) befre heating. (A) after additinal heating I 6kber 2 k bar..- II..u 5,2 (.9 5.Q 4.8 I ] I Time (h) Figure 5. YBazCu307,., ceramics density vs pressure f ht pressing and the time f additinal heating ~96 0,-~ II -" ~, 1.2 ~ ~-u94 92 Figure 6. treatment. I I L ] I i Time (h) Characteristics f the transitin int the supercnducting state vs time f heat

5 Preparatin ~[ h iyh quality H TSC ceramics 171 Additinal heating f ht-pressed ceramics at C allws eliminatin f xygen deficit n the surface (figure 41 t increase ceramic density (figure 5) and imprve its supercnducting prperties (figure 6). The latter effect is prbably cnnected with the cncentratin decrease f impurities lcated at grain bundaries and cmpensatin f xygen nn-stichimetry. On the basis f these studies the preparatin technique fr LnBa2Cu3OT_y (where Ln = La, Y, Yb, H, Er, Eu, Gd, Dy, Sm, Tm} targets has been develped. The targets may be made in the frm f disks (10 210mm in diameter and 2 20mm thick} r cylinders (with uter and inner diameters f and 6 8 mm respectively and 10-30mm high). YBazCu30~-v films btained using these targets have the fllwing characteristics: ~/~,=90 92K, AT,.~1.5 2K, l,.~3.3x 10 /~/cm 2 at 77.4K. A technlgy fr manufacturing magnetic shields frm the YBazCu30 7_ v pwder by an isstatic cmpressin has als been elabrated. Density f the shield material varied frm 78 t 93",, with respect t the theretical value, depending n the particular cnditins f pressing. Size f mdel samples f shields is as fllws: uter diameter 15 20mm, inner diameter 6 12mm and well depth 40 60mm. The shielding cefficient, n lwering the temperature frm 93 t 90 K, varied frm 1 t nt less than 2 x 104 fr the lngitudinal external magnetic field. In the transverse magnetic field the shielding cefficient in the frequency range f Hz was nt less than 105 up t the field H,l = 79 Oe. Unfrtunately the sensitivity f the measuring path prevented estima'tin f shielding cefficient with higher accuracy. The absence f frequency dependence f the shielding cefficient reveals that the true ne is 1 2 rders higher. 3. Cnclusins A methd has been wrked ut fr the preparatin f high-quality HTSC ceramics with cmpsitin and which pssesses high supercnducting characteristics, structural and phase unifrmity. This methd enables manufacture f varius ceramic articles, including targets fr magnetrn and laser sputtering, their size reaching 210mm. Films btained frm these targets have Ic f A/cm2(77 K) with T = K and ATc = K. Shields prepared accrding t this methd have a shielding cefficient f 105 with Hc = 79 Oe in the cnstant magnetic field and in the alternating magnetic field the amplitude is 90 Oe in the frequency range f Hz. References Antnva V V et a11988 Study f the cnditins fr the preparatin, cmpsitin and prperties f dense supercnducting ceramics Y-Ba-Cu-O, Preprint Inst. Metal-lfi~iki Akad. Nauk UkrSSR. Kiev. N IMF-13-88, p. 5 Blank A B, Zagskin V T, Kvichk L A, Ktk LA, Litvinenk Yu G and Seminzhenk V P 1988 Physica pp Seminzhenk V Pet al 1988 Dkl. Akad. Nauk. Ukr.SSR, ser. A, p, 50 Zagskin V T, Kuznetsv V P, Lakin E E, Litvinenk Yu G, Puzikv V M, Mghilk E T and Seminzhenk V P Eurpean Crystall,qr. Meetin(4, Pr,qramme chan~4es and late abstracts, Austria, p. 9