Fabrication of CaO Insulator Coatings by MOCVD for Application in Fusion Reactor Blankets
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Fabrication of CaO Insulator Coatings by MOCVD for Application in Fusion Reactor Blankets Z. Zeng and K. Natesan Argonne National Laboratory, Energy Technology Division, Argonne, IL 60439 ABSTRACT The liquid lithium blanket for fusion reactors requires an electrically insulating coating on the duct to minimize the magnetohydrodynamic pressure drop that occurs during the flow of liquid metal in a magnetic field. Calcium oxide (CaO) is a good candidate for the coating material because it is an excellent electrical insulator and it is stable in a liquid lithium environment. In this paper, details are presented on metalorganic chemical-vapor deposition method that was used to fabricate the CaO coating. Composition and phase analyses of the coating were performed by energy dispersive X-ray analysis and X-ray diffraction. Scanning electron microscopy images show that the coating did not crack after several thermal cycles from room temperature to 715°C. The resistance of the coating is high enough for an insulating coating on the liquid lithium blanket of fusion reactors. INTRODUCTION In fusion reactor designs, liquid metals are used as a coolant in fusion reactor blankets. The main challenge in the design of self-cooled blankets is to accommodate the strong influence of the magnetic field on the liquid-metal flow. The magnetic field can cause a large electrical current if the duct wall is a conductor. An electrical current flowing perpendicular to a magnetic field results in a mechanical force that leads to a magnetohydrodanamic (MHD) pressure drop. This pressure drop could be as high as 8.6 MPa in a poloidal duct of an inboard blanket segment [1]. Therefore, insulator coatings on the duct wall are needed to reduce the MHD pressure drop. Lithium-containing liquid metals are attractive materials in fusion reactor blankets. The insulator coatings should be compatible with liquid lithium, which is a very strong reduce reagent. CaO is a good candidate as a coating material since it is a good insulator and its free energy (-128 kcal/mol at 700°C) is lower than that of Li2O (-114 kcal/mol at 700°C). Experiments show that CaO is stable in liquid lithium [2,3]. Metalloragnic chemical vapor deposition (MOCVD) provides a promisingmethod to prepare a CaO coating on a complex channel geometry for the fusion reactor blanket. In this paper, we report the preparation of CaO coatings by the MOCVD method. EXPERIMENTAL The CaO coatings were fabricated in a low pressure, cold wall MOCVD system. The MOCVD apparatus used in this study is shown in Fig. 1. Calcium bis(2,2,6,6-tetramethyl-3,5heptanedionato) [Ca(TMHD)2] is used as a precursor for the CaO coating. The vapor pressure of this chemical is low. To achieve a precursor pressure high enough for the MOCVD coating, the chemical has to be heated over 200°C. However, this precursor starts to decompose at 240°C.
P5.3.1
Quartz reactor chamber
Isothermal tube heater
Substrate heater
Precursor vaporizer
Valve
Ar O2
Pump
Figure 1. Schematic diagram of MOCVD apparatus for CaO coating Sin
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