| Abstract |
Silica accumulation in small bore tubes of heat exchangers in geothermal binary power plants leads to reduced power output. Diamond-like carbon coatings (DLC) can suppress silica accumulation by shifting the adhesion mechanism from chemical to physical, depending on the hydrogen content in the DLC structure. However, conventional DLC deposition methods are unsuitable for the inner surfaces of pipes due to the difficulty of introducing plasma within the narrow pipes. In this study, we developed a novel deposition system that uses the pipe itself as the vacuum chamber to deposit DLC on its inner surface. A gas mixture of acetylene and argon was introduced into 0.5 m-long, 16 mm-inner diameter pipes under low pressure, and a bias voltage was applied to generate argon plasma. This plasma decomposed the acetylene, enabling DLC deposition. Initially, plasma was concentrated at the pipe tip, it caused non-uniform deposition of DLC. To solve this issue, plasma was transported along the pipe using gas flow, resulting in more uniform plasma distribution and hydrogen content. The hydrogen content, which is critical for reducing silica adhesion, was analyzed using Elastic Recoil Detection Analysis (ERDA), revealing the deposited DLC had almost same hydrogen content through 0.5m length pipe (hydrogen content in bulk of DLC were 31.6-32.3at% from inlet to outlet of pipe). These results suggest that the proposed system could enable uniform deposition of low-adhesion DLC coatings on entire length of pipes by plasma transport with gas flow. |