A Simple and Reliable Chemical Preparation of Y B ~ ~ C U Superconductors ~O~-~ An Experiment in High Temperature Superconductivity for an Advanced Undergraduate Laboratory Peter I. Djurovich and Richard J. watts' University of CaliforniaSanta Barbara, Santa Barbara, CA 93106 The discovery of high temperature copper oxide superconductors (1) (specifically the compound YBazCu30,), oRen refered to as 1-2-3 superconductors, created a n extensive burst in the study of materials science and superconductivity theory that continues unabated even today.' The resulting inteiest in chemical aspects of superconductivity and ready availablity of a refrigerant, liquid NZ(bp = 77 K).. ,necessak to demonstrate su~erconductineheno omena created a secondary increase in the incorporation of these topics in chemical education (2).Classroom kits are now commercially available that allow one to demonstrate the Meissner effect bv levitatine a maenet over a suoerconductor, thereby sho&ig the e g c t s orzero electrical resist a n ~ eThe . ~ synthesis of high T, superconductors has appeared even in teaching laboratories because their preparation is relatively straightforward (1, 3). However, most of these synthetic procedures consist of the traditional method of ceramic synthesis that involves the grinding of the metal powder oxides in the pmper stoichiometry followed heating in a furnace. These "shake and bake" methods have several disadvantages.
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The repeated mixing and grinding of the a i d e powders needs to be conducted in a fume hood to minimize student emosure to dust. The mixing itself might be incomplete due to the relatively large gram ires of commercially available oxides, resulhng in failure to achieve the intimate mixing necessary create a homogeneous composition. These "shake and bake" methods do not reflect the various ways ceramic materials synthesis has been transformed through the application of chemical procedures (4). In fad. much research has been camed out in the ~ a s t to improve the synthesis of these compound's in five order to achieve a greater uniformity in their composition. We have adapted one of these methods, homogeneneous co~reci~itation. for use in the freshman l a b o r a t o ~and have used it s u c e s s f ~ lin l ~order to introduce the tipic of solid-state chemical synthesis to students. The synthetic method produces a repmducibly consistent ceramic precursor powder which can processed into a reliable superconductor. &
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Synthesis of YBazCuaOl-x YBazCu3 oxide preceramic powder is prepared by the method of homogeneous coprecipitation (5).An aqueous urealoxalic acid solution of the metal salts in the proper 1-2-3 stoichiometry is prepared and heated. At a tempera'Author to whom correspondence should be addressed. 2The 1990fall meeting of the Materials Research Society lists 315 abstracts relating to high T, oxide superconductors while the 1992 spring meeting lists 249 abstracts on the topic. 3~vailablefrom Edmunds ScientificCo. 101 E. Gloucester Pike, Barrington. NJ 08007-1380.
ture between 80 and 100 'C hydrolysis of urea takes place with the simultaneous evolution of COz and N& according to the following equation: CO(NH.& + HzO + 2 NH3 + C02
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(1)
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As the urea hvdrolvzes. eraduallv . . the oH of the solution .. rises. Increasing tho pH causes the metal ions to precipitate out as their hydroxide, oxalate, or carbonate salts. The final stoichoimetry of the powder is determined by the final pH of tho solution which in turn is determined by the initial molarity ofthe urea solution. An initially high urea concentration favors the proper 1-2-3stoichiometry in the prereramic powder. The precipitate is then separated, washed, and dried. It is then heated at 900 'C for at least 16 h in air ta bum out all the residual carbon. The oowder obtained a t this stage is pressed into a pellet and s'intered a t 900 'C for 4 h, followed by annealing in oxygen a t 500 'C for another 16 h. This results in a material with a composition of YBa2Cu3O7, (Ocra RecrLplr. 1987.5A. 1 6 7 h J,.. ." . 2. la. On. lay. I) W , Narhthb. N. II F m m m W A Ckmrarr) S l u n r p of Change;
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Savndcn Collcw Pub8xhmy Phdadclphna. 1990. pp 1b11-1012 h Jurv-, E I1 . E l l u . A 8 .lherkmann.(i.iI.Pcrhn*.R I J Chrm 6dur 1981.M 6 5 1 4-5 . 1(ci~aki:er R.;ihompson,J. C. J. ~&m.'Educ. 1887, 64, 853.(d)Jamb, A. T.; Pechmann.C.I.:Ellia.A.B.J. Chem. Edue. 19W.ES. 1094-1095. 3. Grant, P M.Nm SCL.1987.115(JYly30). 38-39, 4. Pao1.R. Science 1992,255.1011-1078. 5. Liu,R. S.; Cheng, C. T.; Wu, P. T. Inorg. Chem. 1889.28, 16C156.
6. Ellia. A B.J. C h Ed=. 1887.64.836441. 1.Kini,A M.; Greiser,U.; Kao, HE.I.: Carlson. K.D.; Wang, H. H.;Monaghan,M.R.; Williams,J.M. 1-8. C k m . 1987,26,18361836.
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