Preparation of High-Purity T1 2 Ca n Ba 2 Cu n+1 O 6+2n (n=1,2) Powders from Stoichiometric Reactant Mixtures
- PDF / 200,366 Bytes
- 4 Pages / 420.48 x 639 pts Page_size
- 71 Downloads / 165 Views
PREPARATION OF HIGH-PURITY T12CanBa 2 Cun+ 1O6 + 2n (n=1,2) POWDERS FROM STOICHIONETRIC REACTANT MIXTURES. NAE-LIH WUAND YEONG DER YAO* National Taiwan University, Department of Chemical Engineering, Taipei, Taiwan, R.O.C. *Institute of Physics, Academia Sinica, Taipei, Taiwan, R.O.C.
ABSTRACT
New powder synthesis methods have been developed for preparing the single-phase Tl 2 CaBa 2 Cu2 O8 (the 2122 compound) and Tl 2 Ca2 Ba2 Cu3 O1 o (the 2223 compound) powders from stoichiometric reactant mixtures. The 2122 compound was prepared from a stoichiometric mixture of T120 3 , CuO, and CaBa 2 CuO4 , while the 2223 compound was prepared from the same mixture but with additional CaO and CuO to match the correct ( TI:Ca:Ba:Cu=2:2:2 :3 ) stoichiometry. The single-phase 2122 samples with Tc above 110 K were obtained by using one-step calcination at 830°C, while the 2223 samples with Tc ranging between 115 and 120 K were obtained by employing a first calcination at 830 0 C for 5 hrs and a second calcination at 870 0 C. Powder melting, which is strongly associated with the conventional methods, was significantly suppressed in the new methods. INTRODUCTION
High-Tc superconducting TI-Ca-Ba-Cu-O compounds, including TI 2 CaBa 2 Cu2 0 8 (the 2122 compound) and Tl 2 Ca 2 Ba2 Cu 3 O1 0 (the 2223 compound), have commonly been prepared by directly calcining mixtures containing binary oxides of their constituent elements [1-61 at temperatures above 890 °C. Calcination conducted at such high temperatures resulted in incongruent melting of the powder. As a result, when the 2122 and 2223 compounds are prepared by this method, it is often necessary to use a starting mixture with a stoichiometry different from that of the compound to be prepared in order to obtain an appreciable amount of that compound [1-51. The difference in the stoichiometry between the reactant mixture and the desired compound will certainly lead to fairly large amounts of impurity phases in the calcined powder. In this paper, we describe new solid-state reaction methods which allow us to synthesize the Tl 2 CanBa 2 Cun+1 O6 + 2 n compounds at temperatures lower than those in the conventional method. Consequently, by using these new methods, the extent of powder melting was significantly suppressed, and the single-phase 2122 and 2223 powders with high-Tc were prepared from stoichiometric reactant mixtures.
Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
406
EXPERIMENTAL
The 2122 and 2223 compounds were prepared, following calcination processes: > Tl2CaBa2Cu208 CaBa 2 CuO4 + T1203 + CuO 2 22 23 830 C CaBa 2 CuO 4 + T1 2 0 3 + CaO + 2 CuO -.......-...-830°C, 5 hrs > (Tl
+ Ca-Cu-O mixtures)
--- 5--> T12 Ca2 Ba2 Cu3Ol0
respectively,
(1),
by the
and
2 CaBa 2 Cu 2 O8
(2).
The CaBa 2CuO 4 powder [7] was prepared by calcining a stoichiometric 0 mixture of CaCO3 , BaCO3 , and CuO at 830-930 C for 60 hrs. Reactant mixtures involved in the processes (1) and (2) were uniaxially compressed to form pellets of 1-inch diameter. Bar-shaped samples
Data Loading...
