Chapter 4 Permittivity, permeability, and EMI shielding effectiveness of LFO-based
4.1 Introduction
Day by day, special gadgets and electronic devices are becoming a necessary part and part of our lives. There are advantages and disadvantages associated with these devices, such as EMI, which are of deep concern for the smooth functioning of the devices. The EMI created at microwave frequencies causes severe threats not only to military equipment and commercial communication systems but also to normal human health.
[174][175][99][176] With emerging 5G technology, the complications associated with the EMI are expected to enhance. The EMI shield can be achieved by attenuating electromagnetic radiation by absorption or reflection. The EM reflection from various metallic bodies such as ships, aircraft, and the walls of the anechoic chamber can be reduced by tailoring the dielectric and magnetic loss. Absorption, reflection, transmission, and multiple reflections occur when EM waves strike the shielding materials. The absorption depends on electric and magnetic dipoles of the shielded materials, whereas mobile charge carriers are required in order to have efficient reflections. Reflection from various interfaces is termed as multiple reflections, which is neglected in most cases since the re-reflected waves used to be absorbed in the materials. Metal sheets used to be considered an efficient material for EMI shielding; however, oxidation prone nature, processing difficulty, and
heavyweight restrict their use in various practical applications [44]. Again, carbon-based polymer composite emerged as an alternative to metals due to its lightweight, large surface area, and high electrical conductivity. Carbon-based materials such as carbon nanotubes (CNT), carbon nanowires, and reduced graphene are extensively studied as filler components. [177][178][179][180] The unstable nature of polymer with temperature limits its use in EMI applications. Further, the ceramic-based composite attracted attention in the last few years due to its better temperature stability, hardness, and easy processing methodology.[181][182] Among carbon-based materials, carbon black (CB) is a well- explored and suitable material due to its low cost, easily available, stable and easy processing criteria. Also, it helps to enhance the electrical conductivity of the material.
Sang et al.[183] synthesized the carbon nanotubes/alumina@ nickel ceramic composites by hot press sintering. The EMI SE and electrical conductivity of 9CNTs/Al2O3 ceramic composites were found to be 33.6 dB and 103.1 S/m, respectively. Huang et. al. [52]
developed reduced graphene oxide/silica ceramics that are mechanically reliable and lightweight for EMI application. They mainly analyzed the toughness and flexural strength that enhance the balanced interfacial interaction and hierarchical structure. They achieved 33 dB SE in the X-band regime. Yuchang et. al.[184] prepared BaTiO3/graphene nanosheets (GN) ceramics, and they reported the total SE was achieved greater than 40 dB for this composite. They also reported that the complex permittivity was enhanced with GN content. Qing reported the complex permittivity, permeability, and SE of BaTiO3/NiFe2O4. With the two ceramics (one soft magnetic component), they achieved an SE of 34 dB in X- band.[54] BiFeO3 nanowire-rGO having absorption bandwidth of 2.2GHz and RLmax of - 28.68 dB were observed by Ghosh et al.[185] The exceptional absorbing performance is ascribed to the unique microstructure of the absorber and impedance matching. The ferrite- based materials such as Fe3O4, CoFe2O4, and NiFe2O4 for EMI shielding and microwave
ceramics composites absorber have been reported. [186][187][188] The spinel ferrites exhibit high saturation magnetization, high Curie temperature, low resonance line width, moderate dielectric, and magnetic loss. A combination of carbon-based filler with magnetic material as the matrix can be a better replacement for polymer-based composites as it can result in high magnetic and dielectric loss, better temperature stability, high eddy current loss, and interfacial polarization. Various rare earth substitutions like Y, Pr,[189] Nd[190], Er[191] and Dy[192]
in the spinel ferrites are also reported recently to enhance the EMI efficiency.
However, the ideal material for EMI shielding cannot be obtained using single materials like metal, polymer, and carbon-based compounds. Looking forward to a better perspective of stability, low cost, easy processing, and temperature stability, ceramic composite with carbon-based material is the best option. In the contest to the situation mentioned above, It is realized that the EMI shielding response of a material mainly originates from the synergistic effect of conduction loss, magnetic loss, and dielectric loss.
Spinel ferrite, like lithium ferrite, can be a better alternative to be used as a ceramic. LFO possesses high saturation magnetization, high magnetic loss, moderate permeability, and high curie temperature. Further, It is a known fact that rare (RE) ions contain unpaired 4f electrons, which promotes spin-orbit solid couplings. Incorporating RE ion in spinel ferrite leads to 4f ‒ 3d coupling, which may also modify magnetic and electrical properties. For significant EMI shielding, the shielded material should have magnetic and electric dipoles interacting with the electromagnetic field. Most of the researchers focused either on the magnetic or dielectric perspective. But the cumulative effect of electrical, magnetic, and dielectric loss mechanisms is essential to understand the performance of the material for this type of application which is lacking in the literature and hence this study. In this work, It is intended to achieve high EMI shielding efficiency by incorporating CB into lithium
ferrite and Dy substituted lithium ferrite matrix. The Dy substitution is expected to promote polarization, magnetic loss, and microstructure, enhancing the EMI efficiency.