アルギン酸アンモニウム及びクエン酸を用いた
YBa2Cu3O7−δ，Bi−Sr−Ca−Cu−O系
超伝導酸化物の合成
Synthesis of YBa2CuO7-δ Cuprates and Bi-Sr-Ca-Cu-O System
Superconducting Oxide by Ammonium Alginate
and Citric Acid Routes
管野善則 西野純一
YoshinoriKANNO JunichiNISHINO
YBa2Cu307一δ（YBCO）having high Tc above 90K and Bi−Sr−Ca−Cu−O system supercon− ductor with high Tc of l10K were fabricated through a kind of sol−gel route using ammo− nium alginate and／or citric acid． The usefulness of ammonium alginate addition was proved from the XRD measurement and the temperature variation of the resistivity of sintered tablets， and the amount of optimum addition was determined as 5−15wt％in both superconductor systems． Key words：Superconductor， YBCO， Bi−Sr−Ca−Cu−0， Ammonium alginate， Citric acidIntroduction
Extensive research has been carried out in various fields since the discovery of high Tc superconductive oxides． The superconductor for a composition of YBa2Cu307．δ（YBCO）having Tc above 90K and a high upper critical magnetic field of more than lOOT at OK was realized． Maeda et al．［1］also discovered a bismuth−based high−temperature superconductor without any rare− earth elements． The Bi−Sr−Ca−Cu−O system compounds contain a number of superconducting phases possessing Tc values in 20K−120K［2，3］． Bulk high temperature superconductor is generally made by powder metallurgical techniques， associated with solid−solid reactions largely from Bi 203， CaCO 3， SrCO 3， and CuO． The several calcining−regrinding cycles are required to obtain the basic perovskite superconductor of the composition of Bi2Sr2Can−ICunOy（nニ1，2，3）with layered structure． Whereas we hardly achieve a homogeneity in solid−state reactions for the conventional method． While it is also well known that the oxalate coprecipitation method happens to occur the composition fluctua− tion， because of the differences of solul）llity（incomplete precipitation）of each metal oxalates precipitat− ed in each mother liquor． Whereupon sol−gel process possesses an unique feature being a perfect homo− geneity of the powder before calcination， keeping of rigid stoichiometry and avoidance the problems of filtration of coprecipitated products．・ In order to overcome some of the difficulties（including the elevation of Jc）， the chemical process of pyrolysis technique of organic acid salts particularly 2−ethylhexanoates［4］， ethoxide solutions［5］and other organic acid salts［6］have been developed． This paper demonstrates the easier and low−priced preparation method of YBCO and Bi−Sr−Ca−Cu−O system superconductors with high Tc by via of a kind of sol−gel route（ammonium alginate and citric acid）without using any high−priced organometallic compounds． ＊Department of Chemistry ＊＊at present， graduate student of Nagaoka University of Technology山梨大学教育学部附属教育実践研究指導センター研究紀要1，1993
Experimental
In the system of YBCO， Y203， Ba（NO3）2and Cu（NO3）2・2H20 were dissolved in stoichiometric quanti− ties（1：2：3）in an excess of weak nitric acid water containing ammonium alginate plus glycerin solution， yielding a gel−like products under agitation on a hot plate． For Bi−Sr−Ca−Cu−O system， the molar ratio of starting powders Bi（NO3）3・5H20， Sr（NO3）2， Ca（OH）2 and Cu（NO3）2・2H20 was 2：2：2：3， and the similar preparation manner was adopted through the pro− cess using ammonium alginate and／or citric acid． The resultant products were calcined in flowing clean air without containing moisture and CO2 at a desired temperature for 2h． The powder samples were prepared， after pressing into a disk at 14．7（MPa）under an appropriate sintering process in flowing air． The calcined powders and sintered bodies were characterized by Cu−Kαradiation of XRD． The dc conductivity of the ceramics was measured from liquid nitrogen temperature to a room tempe− rature by a standard four−probe method with a current density of O．05∼0．lA／cm2 using indium contacts treated by an ultrasonic soludering technique． The temperature of the sample was measured by a plati− num resistance thermometer．Results
Fig．1shows the representative evolutiojn of XRD patterns of YBCO without containing ammonium alginate， which was caused by the increase of calcination temperature（723−1223 K）．The similar evolu− tion patterns were recognized in the case of other additions amount of ammonium alginate． The gel， when heated at 723K， has undergone conspicuous changes and contains peaks for Ba（OH）2， Y203 and CuO． All these peaks are seen to be reduced as the temperature is increased from 723K to 973K， at lO73K the single phase YBCO l−2−3 compositions is formed， and at l123K their crystallinity is improved． The characteristic most prominent peak for the superconducting phase at 2θ＝32．8°has started forming at 973K although the intensity is much less． Fig．2 shows the XRD of sintered tablets（YBCO）without containing ammonium alginate after calcin− ing at lO73K for 2h；（a）sintered at l123K for 99h and annealed at 730K for 2．4h，（b）sintered at 1179K for 10h and annealed at 730K for 10h． XRD patterns were identical to the orthorhombic phase with a space group Pbnm and the lattice parameters of a＝0．3825， b＝0．3891， c＝1．1680 nm at a room temperature［7］． The crystallinity was slightly improved for the sample sintered at higher temperatures． Fig．3shows the temperature variations of the resistance of YBCO tablets sintered at ll23K for 99h and annealed at 730K for 2．4h；the amount of ammonium alginate additions；（a）0％（b）15％． It is clear that the specimen without containing ammonium alginate is lacking in sintering temperature． The resistivity starts to sharply decrease at about 96K（Tc on）and reaches a zero−state at 86K（Tc°）．The resistivity at 273K is higher in the specimen without additions， wherefore it is clear that the addition of ammonium alginate during sol−gel process is useful for the successful formation of the orthorhombic phase having the superconductivity of 90K class． Fig．4shows the temperature variation of the resistivity of four kinds of tablets（YBCO）；sintered at ll79K for 10h and annealed at 730K for lOh， the amount of ammonium alginate addition；（a）0％，（b）15 ％，（c）30％，（d）5％． All samples show the metallic behavior above Tc， and the value of Tc is slightly elevated with increas− ing the sintering temperature． The order of their resistivity at 273K was 30％（Tc°＝88K）＞0％（Tc°＝88K）＞15％（Tc°＝91K）＞5％（Tc°＝92K） depending on the value of Tc°．The order can be also regarded as a measure of the quality of grain con− tact． The amount of ammonium alg輌nate addition is optimlzed in the range of 5−15wt％． It is well known that the superconducting behavior of YBCO is related to an exygen−deficient perovskite CryStal StrUCtUre．∩o
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d C a 20 30 40 50 60 2θ（deg） Fig．1 Evolution of the XRD patterns of YBCO without containing am．monium alginate heated at various calcination temperatures． calcination temperature；a723K， b973K， c1073K， dll23K． YBCO are strongly affected by the crystal structure， which is an orthorhombic corresponding to the high−Tc phase and／or a tetragonal structure． The Cu−O chain has been thought to play a role in increas− ing the transition temperature Tc． The Tc of YBCO is qulte sensitive to exygen stoichiometry and specifi− cally to the position of deficient oxygen atoms． In the range of O≧δ≧0．7， the crystal symmetry remains orthorhombic， and both 90K and 60K classes of superconductors were teported for O≧δ≧0．2， and O．3＞δ＞0．7， respectively． Whereas the superconduc− tivity disappeares in the tetragonal symmetry for O．8＞δ≧1．0， suggesting that the superconductivity in山梨大学教育学部附属教育実践研究指導センター研究紀要1，1993
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b 20 30 40 50 60 2θ（deg） Fig．2 XRD patterns of YBCO sintered tablets． a；sintered at 1123K for 99h and anneal− ed at 730K for 2．4h， b；sintered at ll79K for 10h and annealed at 730K for lOh． YBCO is controlled by the average Cu valencc． The fabrication of YBCO through liquid process possesses many techniques such as pyrolytic decom− position of Y， Ba， and Cu citrates sol［8］， Schiffis base polymeric method by reacting nitrates of Y， Ba， Cu in water and methanol with 2，6−diacetylpyridine and l，2−diaminoethane［10］， and pyrolysis of meta1− EDTA solutions［11］． It is therefore concluded that the polymeric system using ammonium alginate is of benefit to produc− ing the YBCO superconductors． Fig．5shows the XRD patterns of the sintered bodies for Bi−Sr−Ca−Cu−O system；（a）ammonium algi− nate 5 wt％addition，（b）、15 wt％add．，（c）30 wt％add．，（d）citric acid 25 wt％plus glycerin 25 wt％add．． All samples showed the pattern assigned to a low Tc phase， high Tc phase having c−cell dimensions of 3．6nm［12］， and additional unreacted CuO peaks， where the assignment was performed by considering the literatures［13，14，15］． The peak intensities varied subtly by the kind and concentration of additives・ It is clear that the ammonium alginate addition is favorable for the preferencial growth of high Tc phase than citric acid addition， which is also supported in the system of YBCO． This preference of alginic acid compound would be caused by possessing pyranose−ring having the ability of ion change． In the present experiment（（a），（b），（c）），15％addition of ammonium alginate is comparatively convenient for the5｝
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0r 1 0 100 200 300 TEMPERATURE（K） Fig．3 The temperature dependence of the resistivity of YBCO， sintered at l 123K for 99h and annealed at 730K for 2．4h；the amount of ammonium alginate addition；aO％， b 15％． 2 言 ．910
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（b） （a） 20 30 40 50 60 2θ（deg） Fig．5 XRD patterns of sintered Bi−Sr−Ca−Cu−O superconductor．（a）ammonium alginate 5％ add，（b）ammonium alginate 15％add，（c）ammonium alginate 30％add，（d）citric acid 25％＋ glycerin 25％add．（△）high Tc phase，（▲）low Tc phase． growth of high Tc phase， simultaneously the growth diminishes residuary CuO． The unreacted CuO dis− appears in the YBCO at a lower temperature， whereas exists in the Bi−Sr−Ca−Cu−O system． We formely reported that Bi−Sr−Ca−Cu−O crystal growth（perhaps cellular growth）is associated with the formation of single whisker−like grains in powder and thin plate−like or disk−like in sintering［16］． We can there− fore estimate that the cellular growth hinders the homogeneous solid−solid reactions． Fig．6shows the temperature variation of the resistivity of sintered tablets for Bi−Sr−Ca−Cu−O system （sample（a），（b），（c））． All samples are characteristic of a metal having ohmic behavior about Tc l lOK， following the linear decrease in resistivity with an decrease of sample temperature． The resistivity first deviates from the linear behavior at about l 20K， and a sharp drop in resistivity was appeared with10
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Fig．6 The temperature dependence of the resistivity of Bi−Sr−Ca−Cu−O super− conductor．（a）；ammonium alginate 5wt％add．，（b）；ammonium alginate 15wt ％add．，（d）；citric acid 25wt％＋glycerin 25wt％add．． further cooling． This drop is responsible for the high Tc phase， in sample（b）， the zero resistance associ− ated with high Tc phase was obtained at 110K． In the sample（a）and（d），we could not get the zero resis− tance， because of the measuring limitation of experimental apparatus．（The zero resistivity would be attained at the temperature below 80K with relating to the low Tc phase．） The room temperature resistivity， mainly considered as a measure of the quality of grain contacts， was about in the region of 3∼7．5×10’5Ωm． We could assume that the differences of the room tempera− ture resistivity among samples（（a），（b），（d））is caused by not the change in the Cu average valence on the hole concentration in the CuO22−dimensional conduction planes， but the quality of grain contacts． The large grain size， perhaps happened by the addition of citric acid， may also have an unfavorable influence on the resistivity． These phenomena seem to be caused by the difference of bonding cleavage of pyranose ring and citric acid in the process of the evolution of various anion groups． We believe that the sol−gel method using ammonium alginate provides the homogeneous precursor of YBCO and Bi−Sr−Ca−Cu−O superconducting materials from the view of the high ordering of homogeneity of the phases as well as the lowering of sintering temperature and shortening of sintering time．References
l）H．MAEDA， Y． TANAKA，M． FUKUTOMI and T． ASANO， Jpn． J．App． Phys，27， L209．（1987） 2）C．MICHEL， M． HERVIERT， M． M． BOREL， A． GRANDIN， F． DESLANDEV， J．PROVOST and B． RAVEAV， Z． Phys．，B68，421（1987） 3）K．KUGIMIYA， S． KAWASHIMA，0． INOUE and S． ADACHI， App． Phys， Lett．，52，1985（1988） 4）S．MAKIDA， H． NASU， A． MIYAMOTO， T． IMURA， Y． OSAKA， T． SHINO and T． NAKAMOTO，山梨大学教育学部附属教育実践研究指導センター・一一研究紀要1，1993 Chem． Lett．，1988，2053． 5）T．KOBAYASHI， K． NOMURA， F． UCHIKAWA， T． MASUMI and Y． UEHARA， Jpn． J．App． Phys， 27，L1880（1988） 6）H．NASU， S． MAKIDA， Y． INABA， T． KATO， T． IMURA， Y． OSAKA， ibid，27， L536（1988）
