tailieunhanh - Conductive Polymers and Plastics in Industrial Applications Part 8

Tham khảo tài liệu 'conductive polymers and plastics in industrial applications part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Conductive Thermoplastic Compounds 145 measurements for both radiated and conducted emissions are taken in an open field. Such testing requires sophisticated equipment and must be performed by accredited test labs. ENGINEERING EVALUATION The objective of engineering evaluation is to determine suitability of materials to perform shielding. Less precise than compliance testing engineering evaluation still requires a controlled environment for accuracy. Generally an open field test site is used for greater accuracy. Equipment is less sophisticated than required for agency compliance. Common techniques used include dual chamber shielded box as used in ASTM ES-7-83 discontinued transverse electromagnetic cell TEM-cell and coaxial transmission line as in ASTM D 4935-89. AUDIT SCREENING This testing methodology is primarily a screening process providing a quick look at shielding characteristics of a conductive thermoplastic composite. Volume and surface resistivity microwave reflectance and attenuation tests on a spectrum analyzer are screening methods that can be used. Results obtained are useful for ranking and screening of candidate shielding composites. Further validation of test results is required through engineering evaluation and compliance levels. VOLUME AND SURFACE RESISTIVITY Volume resistivity is electrical resistance through a unit mass of material and is expressed as ohm-cm. There is a good correlation between volume resistivity and shielding effectiveness of conductive thermoplastic composites. Volume resistivity of 100 to 10-3 ohm-cm is generally thought to be acceptable for effective shielding composites. Surface resistivity is electrical resistance over a unit area of material s surface and is expressed as ohms square. Surface resistivity and shielding effectiveness can be correlated where electrical conductivity is a surface property as in applied conductive films or coatings. In some applications coating thickness is a variable in shielding .