This EArly Grant for Exploratory Reserach (EAGER) award provides funding for exploration preparation of novel giant magnetoresistance (GMR) magnetic carbon nanocomposites by combining the stability of carbon and strong magnetization of metal nanoparticles, potentially providing promising alternatives to the traditional metal GMR materials. This project will involve the surface functionalization of commercial magnetic nanoparticles and will evaluate the feasibility of the large scale production of carbon nanocomposites. The stabilization, carbonization and graphitization conditions needed to treat these polymer nanocomposites will be determined to allow the large scale manufacturing of GMR magnetic carbon nanocomposites. The feasibility of using graphitic carbon to protect magnetic nanoparticles against oxidation will be tested and process-structure-property relationships will be established.
If successful, the research will provide novel magnetic carbon nanocomposites for magnetic field sensing, and also will unveil the nature of the electron magneto-transport in these novel GMR materials. This will advance the knowledge required to manufacture next-generation GMR materials and provide transformative sensing nanotechnology. As compared to the easy oxidation of conductive metals, magnetic carbon nanocomposites may potentially be applied in telecoms, electronics, sensors, anode catalysts and the aerospace industry, due to their easy manufacturing, better mechanical properties, high stability in acids, and isotropic properties. They also have potential for device miniaturization and light weight. The success of this EAGER project will provide initial evaluations on these GMR magnetic carbon nanocomposites for magnetic field sensing in harsh environments and make their deployment more attractive to both industrial researchers and academic scientists.
Part I: GMR performance dependence on the nanofillers Magnetic Carbon-Composite Fibers (MF) The preparation of MFs took place three steps as illustrated in Scheme 1: (1) thermal stabilization of PAN fibers at 250 oC for 2 hours in air atmosphere with a heating rate of 2 oC/min; (2) dipping the stabilized PAN fibers with different inorganic salt solutions for 12 hours and then drying the fibers at 80 oC; (3) carbonization at 800 oC for 2 hours in nitrogen with a heating rate of 5 oC/min; (4) nanoparticle oxidation at 250 oC for 2 hours in air with a heating rate of 5 oC/min and then cooling down naturally. The carbon fibers produced from different heat treatment are designated as CF, MF1, and MF2, respectively. CF1 represents carbon fibers made from pure PAN fibers. The other two samples, MF1 and MF2, are the magnetic composite fibers synthesized from stabilized PAN fibers after treatment by 0.3 M Co(NO3)2 and 0.1 M Fe(NO3)3/0.2 M Co(NO3)2, respectively. Figure 1 shows the magnetoresistance of CF and nanocomposite fibers at 290 K. The MR results demonstrate different magnetic field dependent behaviors that can be divided into two groups, one is positive MR obtained from CF and MF2, the other is very large negative MR from MF1. This unique phenomenon reveals that the electrical conductivity of the 1D fiber can be tuned via this facile nanostructure coating process. Positive MR observed in MF and MF2 was attributed to the shrinkage in the overlap of the electron wave function after applying a magnetic field and thus reduced the average hopping length. To reveal the behind physics, a few models have been proposed to explain the negative MR phenomena relating to the conduction mechanism. For example, strong localization in the highly disordered materials made the conduction mechanism follow a variable range hopping (VRH) model, which has been realized as the major reason for obtaining negative MR. Due to the unique core-shell structure of the decorated nanoparticles and their strong interfacial interaction with carbon fibers in MF1, the negative MR is probably caused by spin-dependent scattering. The cobalt core serves as magnetic scattering center. In zero magnetic field, the orientation of the magnetization of each cobalt core is random, resulting in a spin-disordered state. The applied magnetic field aligns all the cobalt core moments and reduces the spin disorder, which reduces the spin dependent scattering and leads to a reduced resistance, thus a negative MR is observed. The unique MR switching phenomena observed in these nanocomposite fibers offer a promising approach to achieve desired MR property. However, to control the switching phenomena, further studies need get involved to achieve an atomic level control of nanocomposite and then tune their electronic structure and magnetic property effectively. Part II: Anisotropic properties of the GMR performance in one-dimensional project In order to study the construction of the nanofibers, the properties dependent on the geometry need be addressed. Here, we used commercial carbon fibers for demonstration. We also considered minimizing the inter-fiber distance by using conductive polymer polyaniline as an adhesive. Figure 2 shows the morphology of the as-received CFs and the PANI/CFs with different PANI loadings. The as-received CFs have uniform pores on the smooth surface (Figure 2(a)), while the surface of PANI/CFs became much rougher (Figure 21(b-d)). It was noted that PANI was distributed homogeneously on the surface of CFs. The average diameter of 11.8 μm is determined for the as-received CFs by the SEM images (Figure 2(a)). The MR results of the as-received CFs and PANI/CFs with 30 wt% PANI loading at 130 and 290 K were shown in Figure 3. The small negative MR (~ 0.1%) is observed for the as-receive CFs in the vertical direction. However, the positive MR is observed for the PANI/CFs with a 5.0 wt% PANI loading at 290 K. It is noted that the PANI/CFs with a 30.0 wt% PANI loading exhibit a large negative MR (- 7.5%) at 290 K (Figure 3E). The obvious difference in the MR behavior indicates different electron transport models at 130 and 290 K. When the temperature is 130 K, the negative MR is often interpreted in terms of the quantum interference effect among many possible paths in the magnetic field. Different MR is observed for the CFs and PANI/CFs in the diagonal direction at 130 and 290 K. Figure 3D&F. In summary, we have exploratory demonstration of the feasibility to use electrospun fibers to serve as a media for magnetic field dependent resistivity sensing. Further studies regarding the nanofiller size, loading and interface effects on the GMR sensing are required. The sensitivity of the GMR sensors should be tested and explained from different models as reported in our recent review paper (selected as a feature article).
