1 Introduction

Blanket-design studies revealed that electrically insulating duct walls will be required to
reduce the magnetohydrodynamic (MHD) pressure drop in liquid-metal-cooled blankets used in
high-magnetic-field fusion devices, and development of insulator coatings was recommended as
the most appropriate approach to achieving the desired goal [1]. The major requirements for a
viable insulator coating are
" Chemical compatibility in liquid metal
- Chemical compatibility with structural metal
" Adequate electrical insulating characteristics
" Stability under irradiation environment
" Long-term stability, including self-healing, under thermal cycling conditions.
Based on a review of available information on electrical resistivity and Li compatibility,
CaO has been considered a viable insulator coating for the V-4Cr-4Ti alloy in fusion first-wall
applications [2]. Figure 1 illustrates the thermodynamic stability of CaO in an Li environment.
At temperatures in the range of 200-700 C, of interest in the fusion application, the CaO coating
will be thermodynamically stable over a wide range of O concentrations in Li, from a low value
of 38 wppm in Li that is cold trapped at 200 C, to values as high as 1000 wppm. Figure 2 shows
the temperature dependence of the electrical resistivity of CaO, indicating that the values at 200-
700 C are at least several orders of magnitude greater than needed for the first-wall application,
based on the criterion that the product of the coating resistivity times the coating thickness
should be >100 Q-cm2. This paper addresses the development and characterization of a CaO
coating that was applied on V-4Cr-4Ti alloy by a vapor transport (or thermal and chemical vapor
deposition) process. The results will be used to assess the applicability of these coatings in a V-
Li self-cooled fusion reactor blanket.
2 Experimental procedure
The nominal composition of the alloy used in this study was V-4 wt.%Cr-4 wt.%Ti. The
samples were either 1-mm-thick tabs or 5-mm-diam rods. All of the specimens were annealed
for 1 h at 10000C in a 1.3 x 10-5 Pa vacuum before use. The Ca metal was redistilled <6-mesh
granules with a purity of 99.5% on the metals basis. A stainless steel wire screen with 1-mm
square openings was used to fabricate the ~20 mm-diam cylindrical inner specimen chamber.
The outer chamber, ~35 mm in diam, was a quartz tube for the vacuum process, and a stainless
steel tube for the He-flow process. Stainless steel wire and bolts/nuts were used to hold the