A Systems Approach to Materials Processing Rentian Xiong1, and Martha Gallivan2 1,2 Georgia Institute of Technology, School of Chemical & Biomolecular Engineering, Atlanta, GA 30309, USA. E-mail:
[email protected]
ABSTRACT: With the IREE supplement grant the authors collaborated with the research group headed by Professor Yukio Yamaguchi at the University of Tokyo, Japan from December 13, 2006 to March 7, 2007. The project is titled “Investigation of catalyst by a combinatorial method and synthesis of single-walled carbon nanotube.” A combinatorial method was used to obtain different nominal thickness of Cobalt/Aluminum binary catalyst on one substrate and to study their effect on carbon nanotube (CNT) growth. An in-situ video camera was used to monitor CNT growth. It was observed that CNT growth rate on Co/Al binary catalyst is much faster than on Co catalyst alone. The growth rate is also dependent on the location on the Co/Al catalyst library. Various amount of Al was sputtered to study the effect of Al. When twice of regular amount of Al was sputtered, a novel columnar structure of CNT was obtained. This is the first time to observe such phenomena of self-organization of CNT. Further increase of Al thickness, however, did not yield any CNT. This project provided additional context and examples for the systematic approach to material processing proposed in the original project. The design of the CVD system in the host group also helped to solve a nonuniformity problem in the authors’ own CVD system. Through this project new material characterization techniques such as Raman spectroscopy and TEM were also learned. In addition to research activities, the travel to Japan also increased Mr. Xiong’s familiarity with Japanese language, culture, and technological trends and business practice in nanotechnology. He attended a Japanese Language class for Foreign Students and Researchers offered by the University of Tokyo, visited several museums in Tokyo, and visited other ancient and modern cities such as Kamakura, Yokohama and Nikko. Mr. Xiong also attended Nano Tech 2007 International Nanotechnology Exhibition & Conference.
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INTRODUCTION The awardee for current NSF award is Dr. Martha Gallivan from the School of Chemical & Biomolecular Engineering at Georgia Institute of Technology. The funded project is titled “CAREER: A Systems Approach to Materials Processing.” The goal of this project is to apply the systematic approach that is well established for the engineering of macroscopic systems such as oil refineries to material processing at small scale. Compared to the conventional empirical approach, the systems approach aims to understand the underlying physics and the “process-structure” relationship, and to apply systematic tools of optimization, control and design to material processing. The model system under study is metalorganic chemical vapor deposition (MOCVD) of yttriastabilized zirconia on silicon substrates. This model system was selected because MOCVD is a dynamic process involving material structure evolution at multiple scales, and also is a process with a wide range of applications in industry. A low-pressure MOCVD reactor and its data acquisition and instrument control software have been built. Currently there are two sub-projects that are being carried out under the award. One is to design experiments (DOE) to develop and validate the process models and to evaluate the design and control of the process. The other one is to develop an in-situ sensor for the MOCVD process, which is necessary for feedback process control. The in-situ sensor under study is a software-based sensor, in which only a limited number of optical sensors are required. The sensor data are combined with models in an observer to generate an estimate of material properties that are consistent with both the sensor data and the physical model. Various estimation techniques such as Kalman filter, particle filter, and moving horizon estimation are under investigation. With the IREE supplement grant the authors collaborated with the research group headed by Professor Yukio Yamaguchi at the University of Tokyo, Japan. The cooperated project is titled “Investigation of catalyst by a combinatorial method and synthesis of singlewalled carbon nanotube.” The combinatorial method used in this project represents another effective systematic approach to investigate process-structure relationships, with a goal to significantly reduce the amount of experimental work. This is consistent with the concept of a systems approach in the original grant proposal and its methodology is shared by an ongoing DOE project in Dr. Gallivan’s group. The consideration of a second model system allows us to better explore the general paradigm presented in the proposal, and to look for common problems and issues in both model systems. Furthermore, this interaction provided access to other activities in the Yamaguchi group, including one project to structure knowledge in nanomaterials design. Professor Yamaguchi’s group is conducting research on “bottom-up” nanotechnology based on self-organization, to develop new routes for the structure control and the function design of materials. Processing methods under investigation include both wet and dry methods for assembly, with material systems including carbon nanotubes, metal nanoparticle catalysts, and colloids. Applications include magnetic recording, solar cells, and medical diagnostics. Under this IREE award, Dr. Martha Gallivan and her graduate student, Mr. Rentian Xiong, traveled to Japan. Mr. Xiong stayed from December 13, 2006 to March 7, 2007 and Dr. Gallivan visited from February 24 to March 1, 2007.
RESEARCH ACTIVITIES AND ACCOMPLISHMENTS OF THE INTERNATIONAL COOPERATION Carbon nanotubes (CNT) have a wide range of potential applications due to their excellent mechanical, electrical, and chemical properties [1-6]. These applications require controlled growth of CNT on various substrates. Currently CVD is the most commonly used method to grow CNT. CVD requires the appropriate amount of metal nanoparticles as catalysts. However, the empirical approach to find the optimum amount of catalyst is time-consuming. To solve this problem a combinatorial method (CMD) was developed in Professor Yamaguchi’s group [7]. CMD uses a mask to control the flux of catalyst at different locations on the substrate during sputtering. With CMD various amounts of catalysts can be obtained on one substrate in one sputtering experiment. In this IREE project CMD was used to study the binary catalyst of Cobalt/Aluminum. With the help of a graduate student, Hisashi Sugime, Co/Al library was prepared using a standard recipe. The schematic of the prepared catalyst library is shown in Figure 1. An in-situ video camera was used to monitor the CNT growth. Some snapshots are shown in Figure 2. The arrow indicates the direction of decreasing thickness. As shown, CNT growth on some regions occurred earlier while other regions have incubation period. This indicates a location-dependent reactivity. Compared to previous data by Sugime, CNT growth rate on Co/Al is much faster than on Co alone.
Co
Co (0.2 ~ 24.6 nm)
Al (1.5 ~ 87.8 nm)
Al
0 sec
30 sec
1 min
3 min
5 min
7 min
Figure 1 Co/Al catalyst library
Figure 2 Snapshots of CNT growth process
To study the effect of Al, various amounts of Al was sputtered while Co amount was kept the same as regular recipe. When twice of regular amount of Al was sputtered, a novel columnar structure of CNT was obtained. The SEM image of the columnar structure is shown in Fig. 3. This is the first time to observe such phenomena of self-organization of CNT. The well aligned CNT has the potential to be used as field emission source. Continued efforts are now underway in Professor Yamaguchi’s group to study the field emission property of the novel structures deposited by Mr. Xiong. Throughout the research the author received tremendous amount of help from graduate students in Professor Yamaguchi’s group. These include the operation of the sputtering machine, CVD reactor, Raman spectroscopy, SEM and TEM characterization of the CNT. Once a week there was a group meeting with a focus on CNT research. Progress on
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various sub-projects of CNT research such as field emission CNT, supergrowth of CNT, were reported in the meeting. The discussions were held in both Japanese and English. The author also attended a larger seminar every week. The seminar highlighted progress on other research topics currently being carried out in Professor Yamaguchi’s group such as optical and magnetic storage and colloidal self-assembly. From the operation of the CVD reactor in Prof. Yamagchui’s group, some better ideas and design were borrowed to improve the author’s own CVD reactor at Georgia Tech. For example, a thermal diffuse plate was made based on Prof. Yamaguchi’s design and it successfully solved a problem of film nonuniformity that persisted in author’s CVD system.
Co = 3.8 nm
Co = 1.8 nm
Co = 0.9 nm
Co = 0.5 nm
Figure 3 SEM image of columnar structure of CNT
BROADER IMPACTS OF THE INTERNATIONAL TRAVEL The scope of the currently funded NSF program was expanded through this second application in catalysis and CNT growth. In the original proposal, film microstructure is the design objective, and this is now expanded to include nanostructured materials, in the catalyst nanoparticles and the carbon nanotubes. Mr. Xiong also learned new characterization methods for these nanostructures, including TEM and Raman, which can be applied for the MOCVD metal oxide films at Georgia Tech. Additionally, the technology of combinatorial experiments will be an important component of rational design of materials structure, and is an important and complementary technology to the modeling, experimental design, and in-situ sensing methods that are part of the original NSF grant. In addition to research activities, the travel to Japan also increased Mr. Xiong’s familiarity with Japanese language, culture, and technological trends and business practice in nanotechnology. The author attended Japanese Language class three nights a week. The class was offered by the University of Tokyo for foreign students and researchers. During the class not only the Japanese language itself but also the Japanese culture and history were taught. The class greatly improved the author’s Japanese
language skills. The author also gained more knowledge of Japanese history and culture by visiting many local museums. The Tokyo National Museum is the oldest and largest museum in Japan. The museum collects, houses, and preserves a comprehensive collection of art works and archaeological objects of Asia, focusing on Japan. The EdoTokyo Museum is a museum of the history of Tokyo. On the other hand, the National Science Museum and the National Museum of Emerging Science and Innovation are great places to get hands-on exploration of the latest developments in cutting-edge science and technology, including interactions with robots, virtual-reality rides, a planetarium, and displays that suggest future applications such as non-invasive medical procedures and an environmentally friendly home. Besides Tokyo the authors also visited other ancient and modern cities such as Kamakura, Yokohama and Nikko. Through these trips the author had the opportunity to experience both historical and contemporary aspects of Japanese society. Finally the author attended Nano Tech 2007 International Nanotechnology Exhibition & Conference held in Tokyo on February 2007. The conference was the world's largest international nanotechnology exhibition and conference program offering unparalleled business opportunities and showcasing the state-of-the-art in nanotechnology. There are more than 400 exhibiting enterprises and organizations from around the world responsible for the rapid R&D progress in nanotechnology. Through the exhibition the author had direct sense of technological trends and business practice in the field of nanotechnology.
DISCUSSION AND SUMMARY In this IREE research project, a combinatorial method was used to study the effect of Co/Al binary catalyst on the growth of carbon nanotubes in chemical vapor deposition. An in-situ video camera was used to monitor CNT growth for the first time. It was observed that much faster growth rate can be obtained on Co/Al binary catalyst than on Co alone. While studying the role of Al, a novel columnar structure of CNT was obtained. This is the first time to observe such phenomena of self-organization of CNT. The columnar structure of CNT has the potential to be used as field emission source. The interaction with other members in the host group through group meetings and seminars enabled the author to gain understanding on a wide range of nanotechnology applications and research. The better ideas and design of CVD system in the host group were borrowed to improve the author’s own CVD system and the film nonuniformity problem was solved. In addition to research activities, Mr. Xiong had the opportunity to improve his Japanese language skills by taking Japanese language class. The author gained better knowledge of Japanese culture and history by visiting various museums and sightseeing spots. He learned current technological trends and business practice in nanotechnology by attending Nano Tech 2007 International Nanotechnology Exhibition & Conference. In both authors’ views, the IREE program is a great program. It offers an invaluable opportunity for early-career researchers to gain international research experience. This research experience, together with the familiarity with foreign languages and culture, are becoming increasingly important in the context of current economic globalization. The IREE program definitely should be continued. From the discussion with other graduate
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students and young faculty in the authors’ institution, many exhibited strong interests in participating in the IREE program. The authors believe the IREE program will definitely become more popular and successful in the future. The only recommendation for the future IREE program is to establish a guideline to help applicants to estimate the cost of living in the host institution. Additionally, we note that Mr. Xiong is not a U.S. citizen, and we encourage NSF to continue to open this program to all early career researchers.
ACKNOWLEDGEMENTS The authors wish to thank Professor Yukio Yamaguchi and Dr. Suguru Noda for their host. They also thank group member Hisashi Sugime, Kazunori Kakehi and Yosuke Shiratori for technical assistance. This work was supported by the National Science Foundation under grant No. 0348397.
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BRIEF BIOGRAPHIES OF RESEARCHERS Rentian Xiong received his B.S. degree in Chemical Engineering from Zhejiang University, Hangzhou, China in 1998. He then worked as a research assistant in the National Laboratory of Secondary Resources Chemical Engineering in China, with a particular focus on supercritical fluid chromatography and its application to prepare high purity Omega-3 fatty acids. In 2001 Mr. Xiong attended the University of Cincinnati and received the M.S. degree in Chemical Engineering in 2003. His M.S. research under the supervision of Dr. Jerry Lin
focused on the development of novel inorganic adsorbent for high temperature carbon dioxide separation. In 2003 Mr. Xiong joined Dr. Martha Gallivan’s group at Georgia Institute of Technology to pursue his doctoral degree. His PhD research is focused on in situ sensing for chemical vapor deposition based on estimation theory. Martha A. Gallivan is an assistant professor in the School of Chemical & Biomolecular Engineering at the Georgia Institute of Technology. She earned her B.S. in Mechanical Engineering from the University of Illinois at UrbanaChampaign in 1996, and her M.S. in Mechanical Engineering from the California Institute of Technology in 1997. Prof. Gallivan’s Ph.D. thesis was on modeling and control of thin film deposition, with advisors Richard Murray, David Goodwin, and Harry Atwater. She earned her Ph.D. in Mechanical Engineering in 2003, with a minor in Control and Dynamical Systems, and she joined the faculty at Georgia Tech after completing her Ph.D. Her overall research program is on optimization and control of molecular-scale structure using self assembly with applications in inorganic crystalline films, polymer structure, and nanowire assembly.
