| AVS 56th International Symposium & Exhibition | |
| Vacuum Technology | Wednesday Sessions |
| Session VT-WeM |
| Session: | Partical and Theoretical Aspects of Gas Dynamics |
| Presenter: | H. Yoshida, National Metrology Institute of Japan / AIST, Japan |
| Authors: | H. Yoshida, National Metrology Institute of Japan / AIST, Japan M. Shiro, National Metrology Institute of Japan / AIST, Japan K. Arai, National Metrology Institute of Japan / AIST, Japan M. Hirata, National Metrology Institute of Japan / AIST, Japan H. Akimichi, National Metrology Institute of Japan / AIST, Japan |
| Correspondent: | Click to Email |
For precise calculations of conductance and pressure distribution in vacuum chamber at molecular flow, it is important to know a degree of realization of diffuse reflection (also called cosine law) at surface. The conductance of an experimental channel changing the surface material and roughness was measured and compared with the results using Monte Carlo calculation assuming diffuse reflection.
The experimental channel consisted of two parallel disks was equipped with the vacuum chamber with an inner volume of 8.42x10-2 m3.The lower disk made from polished stainless steel (SS) has a diameter of 40 mm and a hole of 10 mm in diamter at the center. The upper disk with 51 mm in diameter is located as facing parallel to the lower one. The space between the upper and the lower disks was determined from 0.3 mm to 0.7 mm using gauge blocks as a spacer. After the vacuum chamber was filled with N2, Ar, or He gas at approximately 100 Pa, it was evacuated from the hole of lower disk by turbo molecular pump (0.22 m3/s) through the space between two parallel disks. The conductance of the channel was obtained from the pressure decrease rate in the vacuum chamber.
Eleven upper disks with different material and surface were prepared: polished SS, unpolished SS, quartz, Ti, Cu, Al, alumina with smooth surface, alumina with rough surface, SS with Au coating, SS with Pt coating, and SS with DLC coating. The effect of surface material and roughness on conductance was estimated from the measurement of the conductance of the channel by replacing the upper disk.
The conductance for N2 using polished SS with 0.5 mm in space was 3.68x10-4 m3/s (±2.7%), which was comparable to the calculated value of 3.67x10-4 m3/s (±1.3%). Similarly, the experimental values for N2 and Ar using polished SS, quartz and SS with DLC coating showed good agreement with the calculated ones within the measurement uncertainty. On the other hand, the conductance using SS with Pt coating was about 7% smaller than the calculated one. Their surfaces morphologies were analyzed by optical microscope and atomic force microscope (AFM). The microscopic surfaces of polished SS, quartz and SS with DLC coating were very smooth with the roughness less than 1 nm and the specific area less than 1.01. In the case of SS with Pt coating, however, the microscopic surface was rough with the specific area of 1.10. Judging from the results of all disks, the conductance seemed to be influenced by surface roughness rather than surface material. In the case of He, the experimental value was about 4% larger than the calculated one. This reason should be the influence of the specular reflection and/or the lobular scattering.