Grain boundary diffusion of liquid metal coolants in optical materials for use with high power synchrotron radiation

Abstract

A major concern in the design of high-flux X-ray beam lines is heat dissipation in the first optical element, especially if that element must operate in ultrahigh vacuum. In considering methods of cooling, it is generally desirable to achieve good thermal contact over a large area, without introducing stress or vibration into the optical element, or hindering its motion for optical alignment. A method which has been considered by several groups employs a low vapor pressure liquid metal to provide stress-free thermal coupling of the optical element to a heat sink. The eutectic alloy of Ga-In seems attractive for this purpose in many respects. It is liquid at room temperature, yet has an extraordinary low vapor pressure, essentially unmeasurable at room temperature and less than 10 −12 Torr at 300°C. However, the liquid metal reacts quickly and destructively with aluminium and its alloys, materials extremely useful in the manufacture of optical elements. The reaction propagates via grain boundary diffusion, and subsequent intermetallic reactions and embrittlement destroy the aluminum. The Ga-In eutectic reacts to a lesser degree with other materials such as copper and silicon, while materials such as silicon carbide, nickel, and 304 stainless steel appear relatively resistant to attack in the absence of stress.