Structural and magnetic properties of type-M barium ferrite (BaFe12O19) nanoparticles (~ 20 nm) embedded in nonmagnetic thermoplastic natural rubber (TPNR) matrices were investigated. The TPNR matrices were prepared from high density polyethylene (HDPE) and natural rubber (NR) in the weight ratios of 80:20 and 60:40, respectively, with 10 wt% of NR in the form of liquid natural rubber (LNR) which act as a compatibiliser. BaFe12O19 – filled nanocomposites with 2 – 12 wt% BaFe12O19 ferrite were prepared using a melt- blending technique. Magnetic hysteresis was measured using a vibrating sample magnetometer (VSM) in a maximum field of 10 kOe at room temperature (25oC). The saturation magnetisation (MS), remanence (MR) and coercivity (Hc) were derived from the hysteresis loops. The results show that the structural and magnetic properties of nanocomposites depend on both the ferrite content and the composition of the natural rubber or plastic in the nanocomposites. All the nanocomposites exhibit an exchange bias-like phenomenon
resulting from the exchange coupling of spins at the interface between the core ferrimagnetic region and the disordered surface region of the nanoparticles.
This paper reports on the preparation of magnetic lumen loaded handsheets from bleached and unbleached mixed tropical hardwood kraft pulps. The lumen coating technique is a physical approach whereby fillers were deposited inside the fibre lumen. In order to produce magnetically responsive fibres, magnetic fillers were loaded into the fibre lumen. The magnetic filler chosen was magnetite which is usually used to make mylar as found in a diskette. Low and high molecular weights of polyacrylamide (PAM) were used as retention aids. The effect of different molecular weight of PAM on filler content in the bleached and unbleached handsheets were studied. The results showed that the amount of fillers deposited in the pulp fibres increased with increasing molecular weight of PAM using both pulps. However the bleached pulps gave better lumen loading than the unbleached when using high molecular weight of PAM.
The mechanical strength of magnetic lumen loaded handsheets was reported to be lower than the unloaded handsheets. This effect is due to the deposition of filler inside the fiber lumen and some on the fibre surface which interfere with the fibre to fibre bonding. Hence, in order to improve the handsheets strength, cationic starch is used as a dry strength additive. In this study, mixed tropical hardwood pulps were used throughout the experiment. The magnetite particles were deposited in the fibre lumen via the lumen loading technique. The addition of cationic starch was found to increase the handsheet strength. However, it disturbed and influenced the location and distribution of the magnetic fillers. Some of the magnetite particles were observed to be displaced from the fiber lumen and pit apertures. The charges of the filler particles and cationic starch played an important role in producing charge repulsion and pulling effect which lead to filler dislocation.
Magnetic paper were prepared via the in situ synthesis method with ferrites in the presence of polyethylenimine (PEI). In this work, the thermomechanical pulp (TMP) fibers were used due to low percentage of collapse lumen and the large lumen size for optimum loading degree. Four cycles of the reaction were performed on the TMP fibers with pH values of 4-10. It was found that variation of pH value played an important role in the loading degree of pulp during synthesis process. The magnetic, morphological and structural properties of the magnetic paper obtained were reported. At the
optimum pH of 6.0, saturation magnetization was found to be 3.08 emu/g, remainance magnetization was 0.11 emu/g and coercive force was 12.64 Oe. The optimum loading degree was found to be 23.25%.
The complex scattering parameters (S11* and S21*), relative dielectric permittivity (er* = er' - jer"), relative magnetic permeability (mr* = mr' - jmr") and absorption characteristics of some thermoplastic natural rubber (TPNR) – Fe3O4/YIG composites were investigated by means of a microwave vector network analyser and specular absorber method. The measurements were performed in the frequency range of 1 – 13 GHz with and without the presence of a transverse external magnetic field with magnitude of 1 T. The specular absorber method is used in determining the reflection loss (RL) of the composites and its dependence on material properties, thickness and the external magnetic field. In toroidal form, the composites under study seem to transmit more (Pt > 60%) but absorb and reflect less (Pa + Pr < 30%) microwave power in the frequency range used in this study and in both unmagnetised and magnetised states. The external magnetic field is seen to have the effects of reducing S11*, S21*, er', er", mr' and mr". The suppression of the relaxation and resonance behaviours on the mr' and mr" plots for all samples are observed for measurements performed in the presence of the external field. The composites, as revealed by the specular absorber method, show conditions of minimal reflection on RL versus frequency plot for all sample thickness where more than 95% of the microwave power is not reflected back. The location of the dips of minimal reflection on the frequency domain depend on the thickness, the dielectric and magnetic properties of the materials. The conditions of minimal reflection was found to occur when the thickness (t) of the sample equals the odd number multiple of a quarter wavelength in the material (lm), t = nlm/4 (n = 1, 3, 5, 7 …), where a geometrical cancellation took place at the surface of the absorber between the reflected waves, resembling a thin filem inteference phenomenon. The dips are suppressed and shifted to a higher frequency in the presence of the external magnetic field. With the knowledge of the dielectric permittivity and magnetic permeability of a material, the specular absorber method may provides a simple theoretical graphic aids in determining the absorption characteristics and location of the matching condition in the frequency domain.
Magnetite (Fe3O4) nanoparticles have been synthesized using the chemical coprecipitation method. The Fe3O4 nanoparticles were likely formed via dissolution-recrystallization process. During the precipitation process, ferrihydrite and Fe(OH)2 particles formed aggregates and followed by the formation of spherical Fe3O4 particles. The synthesized Fe3O4 nanoparticles exhibited superparamagnetic behavior and in single crystal form. The synthesis temperature and the degree of agitation during the precipitation were found to be decisive in controlling the crystallite and particle size of the produced Fe3O4 nanoparticles. Lower temperature and higher degree of agitation were the favorable conditions for producing smaller particle. The magnetic properties (saturation magnetization and coercivity) of the Fe3O4 nanoparticles increased with the particle size.