Systematic Study of Graphite Encapsulated Nickel Nanocrystal Synthesis with Formation Mechanism Implications
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Systematic study of graphite encapsulated nickel nanocrystal synthesis with formation mechanism implications Jonathon J. Host and Vinayak P. Dravida) Department of Materials Science and Engineering & Materials Research Center, Northwestern University, Evanston, Illinois 60208
Mao-Hua Teng Department of Geology, National Taiwan University, 245 Chou-Shan Road, Taipei, Taiwan, Republic of China (Received 10 January 1997; accepted 2 September 1997)
By systematically varying the carbon content, chamber pressure, arc current, and blowing gas velocity in a tungsten-arc encapsulation setup, the effects of each of these variables on the encapsulation of nickel in graphite layers were observed. The data from these optimally designed experiments revealed that the properties of the arc translate into changes in the encapsulated product. Specifically, a larger, hotter arc results in more encapsulation in the final sample. These findings, along with evidence of graphite layers which have formed on precrystallized particles, indicate that the graphite layers may form by two sequential formation steps. The first step is the simple phase segregation of carbon from a cooling liquid particle, resulting in surface graphite. The second step is the growth of carbon on a crystallized nickel particle, regardless of the temperature at which this occurs. The proposed formation mechanism has significant implications for both a scientific understanding of the encapsulation phenomena, and possible commercial applications.
I. INTRODUCTION 1,2
Graphite encapsulated magnetic nanocrystals are expected to have interesting properties and commercially valuable applications, ranging from magnetic data storage and ferrofluids to biomedical applications3 , due to their magnetic properties4 and immunity to most environments as a result of the protective graphite coating.5 Graphite encapsulated metal nanocrystals of Fe,6 Co,7 Ni,8 Cu,9 Sm-Co,10 and FeNdB11 have been produced in small quantities, but because the basic encapsulation process is not well understood, improvements in the amount produced and the quality of the product have been rare. Direct observation of the encapsulation mechanism has proven difficult due to the small size of the nanocrystals, and the fact that the normal production of these materials occurs in the aggressive environment of an electric arc. This has resulted in uncertainty concerning the encapsulation process. The lack of information on the encapsulation mechanism has slowed process optimization for the large scale production of these materials. To alleviate this problem and satisfy scientific curiosity, possible formation mechanisms have been proposed by various authors. These include a catalytic mechanism,12 a surface segregation mechanism,12 the two-step mechanism,13,14 and others. a)
Address correspondence to this author. J. Mater. Res., Vol. 13, No. 9, Sep 1998
Though the formation mechanism has been the subject of significant speculation,12,15–25 few quantitative studies
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