Modeling of Heat Transfer in an Aluminum X-Ray Anode Employing a Chemical Vapor Deposited Diamond Heat Spreader

Abstract

X-ray sources are used for both scientific instrumentation and inspection applications. In X-ray photoelectron spectroscopy (XPS), aluminum Kα X-rays are generated through electron beam irradiation of a copper-based X-ray anode incorporating a thin surface layer of aluminum. The maximum power operation of the X-ray anode is limited by the relatively low melting point of the aluminum. Hence, optimization of the materials and design of the X-ray anode to transfer heat away from the aluminum thin film is key to maximizing performance. Finite element analysis (FEA) has been employed to model the heat transfer of a water-cooled copper-based X-ray anode with and without the use of a chemical vapor deposited (CVD) diamond heat spreader. The modeling approach was to construct a representative baseline model, and then to vary different parameters systematically, solving for a steady-state thermal condition, and observing the effect on the maximum temperature attained. The model indicates that a CVD diamond heat spreader (with isotropic thermal properties) brazed into the copper body reduces the maximum temperature in the 4 μm aluminum layer from 613 °C to 301 °C. Introducing realistic anisotropy and inhomogeneity in the thermal conductivity (TC) of the CVD diamond has no significant effect on heat transfer if the aluminum film is on the CVD diamond growth face (with the highest TC). However, if the aluminum layer is on the CVD diamond nucleation face (with the lowest TC), the maximum temperature is 575°C. Implications for anode design are discussed.