A Convenient, One-Step Synthesis An Undergraduate InorganicIMaterials Laboratory Experiment Connie D. Cogdell, Darcey G.Wayment, and Dominick J. Casadonte, ~ r ' Texas Tech University, Lubbock, TX 79409 Kim A. Kubat-Martin Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545
A current theme in chemistry pedagogy involves the incorporation of material science into the first-year curriculum a s a means of illustrating concepts of bonding and structure ( I ) . I n order to build upon the principles developed within this context students should be exposed to lahoratory exercises that reinforce the theoretical underpinnings developed in the classroom. To this end, we have recently begun the transformation of our one-semester, senior-level inorganic chemistry laboratory into a materials-oriented lab in order to provide our students with the skills necessary for the fahrication of modern materials. The Traditional Approach We use a classic experiment to illustrate modern materials in our senior-level teaching lab; it involves the synthesis of the copper oxide superconductor YBazCuO?, (Y123). The conventional approach to the preparation has entailed the high-temperature (950 'C) reaction of binary oxides, carbonates, or nitrates (2).Amajor problem in this fahrication scheme is the possible formation of several intermediate phases. Principal among these are BaCuOz andYzBaCuOs (Y-211). The latter phase is hlue-green and insulating in nature (3). I n the presence of oxygen these side products undergo eutectic reactions that lead to the inhibition of superconductivity or the formation of superconductors characterized by large grain size, low densification, and amorphous grain boundaries (41, with a resulting decrease in Jc, the critical current. In order to minimize eutectic formation, a series of repetitive steps involving grinding followed by sintering a t 950 "C in air are usually undertaken; the final sintering occurs under oxygen flow. This makes the synthesis ofY-123 time-consuming and somewhat irreproducihle. (We had a success rate of approximately 80% in the generation of superconducting pellets by students following standard procedures during the past three years.) The additional time involved makes the preparation of the superconductor material sometimes inappropriate within a lahoratory that meets for 3 h each week.
Figure 1. Ternary phase diagram for the Y20~Ba0-Cu0 system (reproduced from ref 6).
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Formation of Y-123 Using Ternary Oxides Shortly after the discovery of the Y B a ~ C u 0 7supercon~~ ductor material it was determined that Y-123 could be prepared without the generation of low-temperature eutectic phases by the reaction of ternary rather than binary oxides. I t was reported (4, 5 ) that the reaction of YzCuzOs and BaCuOz generate Y-123. Both of these phases have high melting temperatures (1135 "C and 1000 "C (4)), so the formation of liquid regions that lead to undesirable 'Author to whom correspondenceshould be addressed 840
Journal of Chemical Education
Mole % BsCuo~
BaCoOz
Figure 2. YzCuz05-BaCuOz polythermal section (reproduced from ref 7).
g r a i n boundary characteristics is suppressed. The pseudoternary phase diagram shown in Figure 1 for the Yz03-Ba0-Cu0 system a t 950 "C indicates t h a t the YzCuzOs compound is not in direct equilibrium with either YBazCuO-i, (Y-123) or BaCuOz (6). Hence, reaction of the two solids will lead to Y-123 formation a t higher temperatures without the eeneration of insulating eutectic comoositions. The polyt~ermalsection (7) shown in Figure 2 indicates that a t a 4 : l BaCu0z:YzCuzOo ratio (80 mol% BaCuOz) a solidus line exists for the formation of only Y123 up to 1000 'C. Impurity and Stability Considerations
Impurities ofY-211, CuO, or unreacted BaCuOz may result if the stoichiometrv -is ~.e r t u r b e ddue to Door mixine or grinding of the precursors or due to rapid fi&g times (less than 24 h (4)). If care is taken to insure a 4:l molar ratio and adequate sintering times, then the small amount of impurity that may result does not affect the observation of the Meisner effect a t 77 K, which is our principal criterion for determining the production of a superconducting phase. The ternarv oxides can be prepared bv the instructor in advance. (In nur lab we prepa.redthe pre>ursorsone month befbre use; a maximum shelf lire was not determined. t The experiment as performed allows the student to prepare a high-T, superconductor within a 24-h period, and can be used to expose the student to concepts involving phase equilibria (e.g., ternary phase diagrams, polythermal sections) that might not be covered in the usual Dhvsical ." chemistry or inorganic chemistry lecture courses but are nonetheless crucial in understanding the fabrication of modern solid-state materials.
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Experimental Caution:YzO,, BaO,, and CuO and all of the metal-containing products are harmful or toxic if ingested. Care should be taken when weighing out or manipulating samples to avoid breathing We dust. Disposable particle masks will help prevent airborne exposure to the solids before sintering.
The experimental procedure involves a modification of established literature methods (4). Ball milling was used in order to prepare a very fine powder for the synthesis of high-density superconductor material. This is not necessary for the generation of material adequate to display the Meisner effect a t 77 K. In order to k e e ~the ex~eriment time reasonable, we used an intermediite firinng'time (24 h) that leads to essentiallv 100%Y-123. Loneer sinterine times can be used if greater density or smaller grain size is required. All of the binary oxides used in the preparation of the ternary phases were of the highest purity available (Yz03. Aldrich, 99.999%; CuO, Aldrich, 99.99+%; BaOz, Fisher, 94.5%).A tube furnace (Lindberg Sola Basic) with built-in thermostatic control was used t%r sintering. The powder samdes were daced in a n alumina boat (10 x 13 x 75 mm: c o o k ~ e r a m k s and ) were then inserted into a 90-c&
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quartz tube with the same diameter as the tube furnace for firing. The ends of the quartz tube contained rubber stoppers for gas inletloutlet that were wrapped with graphite tape (Wilt Industries) to prevent melting. Oxygen flow was regulated a t approximately 50 cdmin. Preparation of YzCwOs
In a typical experiment, 7.90 g (0.035 moll of Yz03 and 5.57 g (0.070 moll of CuO are ground to a fine powder with a mortar and pestle. The powder is placed in an alumina boat and heated under oxygen flow at 950 "C for 12 h. The sample is allowed to cool in the tube furnace under a continued flow of oxygen. The resulting YzCuzOs is bluegreen. Yields are greater than 90%based on initial amount ofYzOs added. Preparation of BaCu@
A 1:l molar ratio of BaOz (11.86 g, 0.07 moll and CuO (5.57 g, 0.07 moll is ground to a fine powder using a mortar and pestle. The sample is placed in an alumina boat and heated under oxygen flow again at 950 "C for 12 h. The sample is also allowed to cool under a flowing oxygen atmosphere. The BaCuOz produced (again in virtually quantitative yield) is jet-black. Preparation of Y-123
The precursor ternary oxides YzCuzOs a n d BaCuOz (used without further purification) are mixed in a 1:4 molar ratio (5.773 g of YzCuz05 (0.015 moll and 8.24 g BaCuOz (0.06 mol)) and ground into a fine powder using a mortar and pestle. The resulting material is then sintered under oxygen for 24 h a t 950 "C. (We have also heated the powder to 700 "C for 24 h and achieved similar results.) The sample is allowed to cool under an oxygen flow. The superconductor is then cold-pressed at 7000 lb into 3-mmthick pellets and tested for superconductivity at 77 K. All of the samples prepared displayed a positive Meisner effed. Cleanup
Any fired material that sticks to the alumina boats may be removed bv soakinein concentrated nitric or hvdrochloric acid overGght. Thi waste should be treated as heavymetal waste and disposed of appropriately, Literature Cited 1. Ellia.A.B.;Geselbracht,M.J.;Johnson,B.J.:Lisensky,O.C.;Robinson, W R .Teaching G m m l Chemistry: A Materials Science Companion:ACS: Washington, DC, 1999 -~ ~
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2. (a1 Hamis, D. C.; Hills, M. E.; Hewston,T.A. J Chem. Educ 1987, 10,84;(b)Juergens. F H.; Ellis, A. B.: Oieckmann, D. H.: Perkins, R. I. J Chem. Edu. 1987.10, 851: i d Elday, C. Chrm. Matters 1987.5.22. 3. Roa. C.N.R. J Sol. StoteChem. 1388.74. 147. 4. Peterson, D. E.:KubatMartm, K A,: George. T. G.; zoeea, T. G.;Thompson,J. D. J. Mofer Ros. I981,6, 11. 5. Chunlin, J.; Chanmeng, C.; Kuihan, W.; Sulan, L.; Guiyi. 2.; Guofan. 2.; Cuenfu. Q.: Weiming, B.; Zhanguo,E;Qian, X. Solid State Commun. 1988.65.859. 6. Abbattista, F.; Vallh~o,M.: Mazra, 0. Matm Chem. Phys 1989,21,521. 7. Meeentseva, L. P; Degtyareva, v. Ya;Bondar, I. A ; Bokova, E. Y;Domnma, L,A: Usaakovrkaya. N. K. Rusa J. lnarg Chem. (Engl. kians1.J 1990.35.258,
Volume 72 Number 9 .September 1995
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