Improved filling rate and enhanced magnetic properties of Fe-filled carbon nanotubes by annealing and magnetic separation Xuchun Gui a,b , Jinquan Wei a,b , Kunlin Wang a,b, * , Wenxiang Wang a,b , Ruitao Lv c , Jianguo Chang a,b , Feiyu Kang c , Jialin Gu c , Dehai Wu a,b a Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, China b Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China c Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Received 24 April 2007; received in revised form 30 January 2008; accepted 31 January 2008 Available online 23 February 2008 Abstract The filling rate of Fe nanowires in carbon nanotubes (CNTs) was improved by a post-treatment which involved annealing and magnetic separation. The annealing process transformed g-Fe to ferromagnetic a-Fe, resulting in a-Fe-filled CNTs with improved magnetic properties. The Fe-filled CNTs were then separated from those unfilled CNTs due to the different attractive forces under magnetic field. Thermal gravimetric analysis (TGA) revealed that the purity of CNTs was improved after magnetic separation and the filling rate can be up to 40.0 wt.%, which is increased about 8.1% comparing with annealed CNTs. The saturation magnetization of CNTs reached 31.45 emu/g after magnetic separation. # 2008 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructures; C. Thermogravimetric analysis (TGA); C. X-ray diffraction; D. Magnetic properties 1. Introduction Carbon nanotubes (CNTs), filled with ferromagnetic materials (Fe, Ni, Co, etc.), have attracted much scientific and technological attention by the virtue of their unique structures and significant magnetic properties [1–8]. To date, many potential applications have been proposed for the Fe-filled CNTs, such as in microwave adsorptions [1], biomedicines [2], sensing probe tips for magnetic force microscopy [3], high-density magnetic recoding media [9], etc. It is well known that a body-centered-cubic (bcc, a-Fe) Fe is ferromagnetic and it is thermodynamically stable at room temperature, whilst the face-centered-cubic (fcc, g-Fe) Fe exhibits anti- ferromagnetic behavior. Therefore, the generation of homogenous and highly filled a-Fe nanowires inside CNTs is of significantly important. As-grown Fe-filled CNTs usually contain a lot of non-ferromagnetic g-Fe due to the compression on the Fe nanowires by the CNTs wall and its severe thermal stress caused by the rapid cooling [4]. As a result, the properties and applications of the Fe-filled CNTs are limited due to low and non-homogenous filled of Fe nanowires [5,10]. The rapid cooling however is inevitable during CNTs production, thus, post- www.elsevier.com/locate/matresbu Materials Research Bulletin 43 (2008) 3441–3446 * Corresponding author at: Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China. Tel.: +86 10 62792844; fax: +86 10 62770190. E-mail address: wangkl@mail.tsinghua.edu.cn (K. Wang). 0025-5408/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2008.01.028