Activation Energy of Decomposition of Y 2 bacuo 5 Compound in Wet Co 2 at Elevated Temperatures
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ACTIVATION ENERGY OF DECOMPOSITION OF Y 2BaCuO5 COMPOUND IN WET CO 2 AT ELEVATED TEMPERATURES
John J. Simmins, Michael J. Hanagan, Gary S. Fischman and Robert L. Snyder Institute for Ceramic Superconductivity, New York State College of Ceramics at Alfred University, Alfred, NY 14802-1296
Abstract Decomposition of the Y2BaCuO 5 in C02 was studied using simultaneous thermal analysis (STA) and X-ray diffraction (XRD). The time derivative to the thermogravimetric analysis data (DTG) was used to calculate the activation energy. The activation energy of decomposition by C02 into barium carbonate, copper oxide and yttrium oxide was found to be 23.4 KJ mol
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INTRODUCTION
The rate of a reaction, da/dt, is a complicated function of time and temperature dependent variables da/dt
=
E f(t) x Aexp tf.
(1)
The function, f(t) is difficult to obtain. Fortunately if the natural log is taken of both sides this time dependent function may be subtracted out and a plot of the log of the heating rate, 8 vs. l/T gives a line whose slope is Ea/R. The slope of the line is calculated from the fractional area as a function of temperature at several heating rates (after Flynn[1]). This method is applicapable to both the differential thermal analysis (DTA) data as well as the derivative of the thermogravimetric analysis data with respect to time (DTG). The DTG data was used for several reasons: 1. DTG generally gives a flatter baseline. 2. DTG peaks are unaffected by changes in heating segment which often caue dramatic artifacts in the DTA. 3. Complicated DTG data can be treated mathematically (i.e. deconvoluted) [4] to give activation energy, rate constants and masses of individual reactions. This last property of the DTG curve is a useful check on the validity of the calculation.
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EXPERIMENTAL PROCEDURE
A nitrate preparation method was used to synthesize the 211 powder. A stoichiometric mixture of research grade Y(N0 3 ) 3 , Ba(N0 3 )2 and Cu(N0 3 ) 2 was calcined in
Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
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an alumina crucible at 850'C for 2 hours. The sample powder was then ground and calcined at 940'C for 12 hours. X-ray diffraction of the resulting powder indicated a phase pure 211 diffraction pattern. Thermal analysis data were obtained using a simultaneous thermal analyser *(STA). 0 Data were collected at 2°C/min, 4 C/min, and 10°C/min. The atmosphere was 50% argon with the balance carbon dioxide bubbled through water at room temperature to give an approximately 1% water vapor atmosphere. X-ray analysis was performed
using a powder diffractometert with Cu Ka radiation and a diffracted beam graphite monochrometer. An off axis cut quartz single crystal was used as a sample holder to minimize background and obtain data from the small samples used in the STA. The step size used in data collection was 0.050' 20 with count time of 2 seconds at each step. Data reduction and plotting of thermal analysis data was accomplished using
the program STAPLOT[2] The program STAPLOT transforms the data f
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