나노단위 재료 구조설계를 통한 고기능 가스센서 제작

Abstract

The gas sensor have attracted a lot of interest during the last decade, because of severe air pollution, caused death of 7 million people in 2012, and use of hydrogen energy, highly dangerous energy, for alternative fuel. For this reason, considerable effort has been devoted to improving the sensitivity and reducing price of gas sensor. It is important to introduce high specific surface area structure of gas sensing materials to improve the gas sensitivity of gas sensor. However, control of pore size is as much as important to improve sensitivity of gas sensor. If gas sensing materials had high specific surface area and don't form appropriate pore size, the material centers do not work effectively in gas sensing reactions because there are no gas flow channels. As a result, gas diffusion to the inside of the materials is difficult. The higher reliability and cost effective manufacturing process is essential for commercialization of gas sensor. Almost all resistive-type gas sensor use interdigitated electrode (IDE) to improve stability of sensing properties and surface area and enhance the current response. However, most IDE are fabricated by lithography technique that has a lot of disadvantages such as complex process, expensive equipment, a lot of material loss and waste. For this reason, it is difficult to commercialize low-cost gas sensor. In addition, reliability of IDE fabricated by lithography technique is not quite because electrodes can easily exfoliated when they use long time or exposure at high temperature. For commercialization of gas sensor, it is necessary to develop novel electrode fabrication method to overcome disadvantages of lithography technique and make reliability. The first objective of this dissertation is to enhance the gas diffusivity by formation of macroporous structured sensing materials and improve the sensitivity of gas sensor. The macro-mesoporous Pt/Al2O3 was fabricated with the assistance of polymer bead template and the effect of macropores on exothermic properties induced by hydrogen oxidation was investigated. In addition, diffusion mechanism depending on pore size were characterized and diffusion coefficients were theoretically calculated. Development of novel fabrication method of IDE type electrode to ensure low cost and reliability of gas sensor and fabrication of various gas sensor on the novel electrode were second objective of this dissertation. We applied multilayer ceramic capacitor process (MLCP) to fabrication of electrode, and fabricated and characterized gas sensor by deposition and alignment of metal oxide semiconductor film, carbon nanotube and chalcogenide nanorod on the MLCP electrode. MLCP is cost effective manufacturing process because of very simple process and almost waste and material loss free. And we expected that possibility of commercialization of MLCP based gas sensor is easy because multilayer ceramic capacitor is already widely used in electronic device. In addition, the electrodes fabricated by MLCP have good reliability because exfoliation is not occurred due to the electrodes are embedded between ceramic. We fabricated (1) metal oxide semiconductor based gas sensor using spray pyrolysis deposition method, (2) noble metal decorated carbon nanotube based gas sensor using dielectrophoresis alignment and hydrogen plasma and (3) chalcogenide nanorod based gas sensor using galvanic displacement on the MLCP electrodes. These represented good sensing properties comparable with literature reports that fabricated by lithography technique. |최근 심각한 대기오염으로 인해 한 해에 700 만명의 사람이 목숨을 잃고 있으며, 수소에너지 활용 증가로 인해 수소 폭발 피해가 늘어나고 있다. 따라서 수소 및 인체 유해가스를 감지할 수 있는 가스센서에 대한 연구개발 필요성이 높아지고 있다. 우리는 본 연구에서 가스센서의 민감도 향상 및 비용절감을 통한 상용화에 적합한 고기능성 가스센서 제작에 대한 연구를 진행하였다. 가스센서의 민감도 향상을 위해 높은 비표면적을 갖는 가스센서 물질이 주로 이용된다. 하지만 높은 비표면적을 갖는 것만큼 기공의 크기를 제어하는 것 또한 중요하다. 아무리 높은 비표면적을 갖는 센싱 물질이라 하여도 적절한 기공의 크기를 갖지 않으면 가스의 확산이 제한되어 물질 중심부에서는 반응이 진행되지 못하기 때문이다. 따라서 기체분자가 기공 내에서 빠르게 확산 할 수 있도록 기공의 크기를 제어하는 기술 개발이 필요하다. 또한, 가스센서의 상용화를 위해서 비용 절감 및 신뢰성 확보가 필수적이다. 저항변화식 가스센서의 경우 센싱 물질의 높은 비표면적 확보와 신호의 안정성 및 높은 저항변화 확보를 위해 lithography 기술을 통해 제작된 interdigitated electrode(IDE)를 주로 이용하고 있다. 하지만 lithography 기술의 경우 복잡한 공정, 고가의 공정 장비, 유해화학물질사용, 물질의 손실 및 폐기물이 많이 발생하는 단점이 있어 경제적인 가스센서 제작 및 상용화에 어려움을 겪고 있다. 또한 고온 및 장시간 사용에 의해 전극이 기판에서 박리될 가능성이 높아 신뢰성이 떨어진다. 따라서 가스센서의 상용화를 위해 lithography 공정의 단점을 극복하고 안정성을 확보할 수 있는 전극 형성 방법에 대한 기술 개발이 필요하다. 본 학위 논문의 첫 번째 목적은 센싱 물질의 기공크기 제어를 통한 기체 확산 속도의 제어 및 민감도 향상에 있다. 폴리머 비드를 템플릿으로 이용하여 마크로-메조 기공을 갖는 Pt/Al2O3 나노복합체를 형성하였으며, 마크로 기공의 유무에 따른 수소산화반응에 의한 발열특성을 비교하였다. 또한 기공 내의 기체분자 확산거동을 고찰하고 이를 통해 기공 크기에 따른 기공 내 기체분자의 확산속도를 예측하였다. 경제적이고 안정적인 가스센서 제작을 위해 새로운 IDE형태의 전극 형성방법 개발 및 이를 이용한 다양한 센서의 개발이 본 연구의 두 번째 목적이다. 이를 위해 적층형 세라믹 케페시터(Multi-Layer Ceramic Capacitor, MLCC) 제작 방법을 이용하여 가스센서의 전극을 형성하였고, 금속산화물 반도체(Metal oxide semiconductor), 탄소나노튜브(Carbon nanotube), 칼코게나이드 나노로드(Chalcogenide nanorod) 등의 센싱 물질을 전극에 배열 및 증착하여 센서 특성을 고찰하였다. 적층형 세라믹 케페시터의 경우 제작공정이 간단하며 폐기물 발생이 적어 제품 가격이 저렴하고 이미 전자기기에 널리 이용되고 있어 상용화 가능성도 높다. 또한 전극이 기판 표면에 증착된 형태가 아닌 세라믹 사이에 삽입된 형태를 하고 있어 고온 및 장시간 사용에도 전극의 박리현상에 의한 신뢰성 저하 문제가 나타나지 않는다. 적층형 세라믹 케페시터 제작 방법을 통해 만들어진 IDE 형태의 가스센서 전극에 (1) 분무열분해증착공정(Spray pyrolysis deposition)을 이용하여 금속산화물 반도체를 증착하였으며, (2) 전기영동(Dielectrophoresis) 및 수소플라즈마(H2 plasma)를 이용해 귀금속 나노입자가 분산된 탄소나노튜브를 정렬하였다. 또한 (3) 갈바닉치환법(Galvanic displacement)을 이용하여 칼코게나이드 나노로드를 성장시켜 가스센서를 제작하였으며, 제작된 센서들의 센싱 특성을 고찰하였다. 이는 기존에 보고된 고가의 반도체 공정을 이용해 제작된 센서 이상의 센싱 특성을 나태고 있음을 확인하였다.; The gas sensor have attracted a lot of interest during the last decade, because of severe air pollution, caused death of 7 million people in 2012, and use of hydrogen energy, highly dangerous energy, for alternative fuel. For this reason, considerable effort has been devoted to improving the sensitivity and reducing price of gas sensor. It is important to introduce high specific surface area structure of gas sensing materials to improve the gas sensitivity of gas sensor. However, control of pore size is as much as important to improve sensitivity of gas sensor. If gas sensing materials had high specific surface area and don't form appropriate pore size, the material centers do not work effectively in gas sensing reactions because there are no gas flow channels. As a result, gas diffusion to the inside of the materials is difficult. The higher reliability and cost effective manufacturing process is essential for commercialization of gas sensor. Almost all resistive-type gas sensor use interdigitated electrode (IDE) to improve stability of sensing properties and surface area and enhance the current response. However, most IDE are fabricated by lithography technique that has a lot of disadvantages such as complex process, expensive equipment, a lot of material loss and waste. For this reason, it is difficult to commercialize low-cost gas sensor. In addition, reliability of IDE fabricated by lithography technique is not quite because electrodes can easily exfoliated when they use long time or exposure at high temperature. For commercialization of gas sensor, it is necessary to develop novel electrode fabrication method to overcome disadvantages of lithography technique and make reliability. The first objective of this dissertation is to enhance the gas diffusivity by formation of macroporous structured sensing materials and improve the sensitivity of gas sensor. The macro-mesoporous Pt/Al2O3 was fabricated with the assistance of polymer bead template and the effect of macropores on exothermic properties induced by hydrogen oxidation was investigated. In addition, diffusion mechanism depending on pore size were characterized and diffusion coefficients were theoretically calculated. Development of novel fabrication method of IDE type electrode to ensure low cost and reliability of gas sensor and fabrication of various gas sensor on the novel electrode were second objective of this dissertation. We applied multilayer ceramic capacitor process (MLCP) to fabrication of electrode, and fabricated and characterized gas sensor by deposition and alignment of metal oxide semiconductor film, carbon nanotube and chalcogenide nanorod on the MLCP electrode. MLCP is cost effective manufacturing process because of very simple process and almost waste and material loss free. And we expected that possibility of commercialization of MLCP based gas sensor is easy because multilayer ceramic capacitor is already widely used in electronic device. In addition, the electrodes fabricated by MLCP have good reliability because exfoliation is not occurred due to the electrodes are embedded between ceramic. We fabricated (1) metal oxide semiconductor based gas sensor using spray pyrolysis deposition method, (2) noble metal decorated carbon nanotube based gas sensor using dielectrophoresis alignment and hydrogen plasma and (3) chalcogenide nanorod based gas sensor using galvanic displacement on the MLCP electrodes. These represented good sensing properties comparable with literature reports that fabricated by lithography technique.