직접 마이크로파 조사를 통한 금속 산화물의 계층적 구조 생성

Abstract

Microwave heating is known to be beneficial in reaction time and energy saving because it delivers heat by radiation, directly to the target material. However, it is well-known that metals cannot be placed inside the microwave oven because it would cause spark when irradiated by microwave. Metals have low skin depth (tens of micrometer) so it is known to be hard to be heated by microwave. However, plasma occurs when metal is irradiated by microwave under vacuum atmosphere. In this research, hierarchical structures were fabricated by direct microwave irradiation to metal substrates under low vacuum oxygen and carbon dioxide atmosphere. SUS304 stainless steel and Ni plates were used as metal substrate. With SUS304 substrate, hill-like hierarchical structures were fabricated after microwave irradiation. Hills were in 10 micro-scale size and the rods were in 50-200 nm size. Different structures were fabricated under different microwave condition; microwave power and reaction time were changed. Higher surface roughness was obtained from higher power and longer reaction, which corresponds to higher hill-like structures. Fabricated hierarchical structures were coated with hydrophobic octadecyltrichlorosilane(ODTS) to obtain superhydrophobic surface. Superhydrophobic surfaces with a water contact angle up to 169 were obtained by chemical modification of the hierarchical structures. The samples maintained superhydrophobicity under NaCl solution up to 2 weeks. Ni plates were also used as the substrate afterwards. Coral-like hierarchical structures were created after microwave irradiation. Structures of several tens of nanometers are assembled to form a structure of 2 to 5 μm in size. Size of the structures were able to be controlled by changing microwave parameter and atmosphere. Oxygen and carbon dioxide gas were used as carrier gas, and the size of the structures were differed from the different atmosphere. From the different result from different atmosphere, new heating mechanism was suggested. Previous studies could not explain the difference from the atmosphere. Irradiated microwave energy is first absorbed to the plasma to heat the plasma species, and the left energy is absorbed to heat the metal sample by eddy current. This study would help further research of plasma synthesis.