Synthesis and Characterization of Fe3O4@CNT & its Transport Properties

Abstract

Magnetite is a well-known ferrimagnetic material with a metallic cubic inverse-spinel structure at room temperature and a high Curie temperature of 858K. Magnetite also possesses a low-temperature phase transition known as the Verway transition, at 120K. Below the Verwey transition, magnetite adopts a monoclinic structure and exhibits changes in its electrical magnetic and thermal properties. Fe3O4 is a ferrimagnetic oxide of iron, which has numerous practical applications. The present work is about encapsulation of magnetite inside multiwalled carbon nanotubes.First part of the project synthesis and scaling up of Fe@CNTs was done using the Chemical vapourization method (CVD). The produced CNTs were characterized using X-ray diffraction (XRD), Raman Spectroscopy, Field-Emission Scanning Electron Microscopy (FESEM) and High-Resolution Transmission Electron Microscopy (HR-TEM) for structure, composition and morphology of the Carbon nanotubes and the encapsulate. The CVD-synthesized and characterized sample was used to make a pellet of about 8mm diameter and 2mm thickness and oxidised in a CO2 environment to produce Fe3O4@CNT, which were again characterized using the above-mentioned techniques. Rietveld Refinement, which is a structural refinement process was carried out on the XRD profile of the Fe3O4@CNTs to confirm the phases present and composition of the sample. Fe3O4@CNT pellet was used like a pencil to write between two gold contacts on a Printed Circuit Board (PCB). This writing creates a clean electrical path of Fe3O4@CNTs between the gold contacts. This can be seen as Fe3O4@CNT pellet is made up of CNT ink, which can be used to write between two gold contacts to make an electrical path which allows for electrical conductivity between them. A Closed cycle Refrigerator (CCR) was used for obtaining temperature variation of resistance in the temperature range of 5 - 300 K in the interest of probing the Verway transition and other transport properties of Fe3O4@CNT. Studies on Fe3O4 thin films of 85nm thickness have shown a change in resistivity of about 3 to 4 orders of magnitude over the range of 50 - 300 K. However, the bare nano-particles of Fe3O4 in ultra small (<15 nm) range have shown suppression of Verwey transition. Multiwalled CNTs, on the other hand, typically show resistance decreasing with temperature in the range 5-300 K. In our case, where Fe3O4 nanoparticles are encapsulated inside carbon nanotubes, we find that the resistance still decreases with increase in temperature in the range of 5–300 K. Thus we still find semi-conducting-like behaviour in Fe3O4@CNT. We do not find any anomaly corresponding to the Verway transition down to 5K. Further, Variable-Hoping Range (VRH) conduction model fitting was done to the R vs. T data obtained, and we found that the 3-D VRH model explains the R vs. T behaviour in a limited range at low temperatures (54 K < T < 100 K). The deviation below 54 K may be due to the interface effects existing between the graphitic shells of CNTs and the Fe3O4 encapsulate.