Nanoscale U-Type Hexaferrites as Microwave Absorbers INVITED

Abstract

With the unprecedented surge in wireless communication systems, microwave (MW) absorbers are becoming increasingly important for civil as well as military applications. When a MW signal is incident on a surface, the signal energy is divided into three parts: (i) reflected, (ii) transmitted and (iii) absorbed energies. For the usual metal-backed MW absorbing coating, transmitted energy=0; thus, minimizing reflection is the key for good absorbers. The reflection of EM energy can be curtailed by two methods: (i) an increase in absorption, and (ii) the quarter wavelength cancellation [1,2]. The loss of EM energy may originate from a number of processes within the absorber such as conduction, resonances, and relaxations etc. MW signal, on passing through an absorber dissipates its energy in the form of heat through interactions with the dielectric and magnetic structures of a material. Hence, in terms of two characteristic material parameters: relative permeability (μ*) and relative permittivity (ε*) in the absorbing medium are to be tailored for attaining a strong and broadband MW absorption. For MW absorbing applications, U-type hexaferrite phase with chemical formula Ba4Me2Fe36O60 (Me2U, Me: divalent transition-metal cation) has most complicated crystal structure and is least studied [3-11]. In present work, we investigate the preparation and MW absorbing properties of the nanoscale U-type hexaferrites (NANO-U) for development of a broadband MW absorber. The nano-powders are prepared using two different methods: (1) mechanical jet-milling of bulk U-type hexaferrite prepared by the standard solid-state reaction route at ∼1250 °C, designated here as (NANO-U)JM and (2) a novel one-pot chemical route, designated here as (NANO-U)C. In the first method, average particle size ∼20–50 nm powders were achieved; whereas, the (NANO-U)C powders of controlled particle sizes (∼65 to ∼170 nm) were prepared by changing the holding time (4, 6, 8 and 10 h) at a constant optimized working temperature of ∼900 °C. Four samples of (NANO-U)C were prepared that are designated as per their holding time as- (NANO-U)C4, (NANO-U)C6, (NANO-U)C8, (NANO-U)C10. A blend of all powders in equi-proportion, is designated as (NANO-U)CMIX, The advantage of one-pot chemical method is attainability of comparable particles sizes of a few tens of nm, at much lower working temperatures. We compare the EM parameters and MW absorbing properties of (NANO-U)JM , (NANO-U)C and bulk U-type hexaferrite powders. It is interesting to note that the (NANO-U)JM demonstrates excellent reflection loss (RL) of ∼ -43.8 dB (99.99% MW absorption) at ∼11.3 GHz frequency with a broad bandwidth of 8.6 GHz (8.9 ≤ f ≤ 17.5 GHz) for 90% or more MW absorption. The values for (NANO-U)C too are comparable, with RL of -44.2 dB (99.99% MW absorption) at 11.6 GHz frequency for (NANO-U)C8 and a broadband MW absorption with an excellent bandwidth of 8.4 GHz (9.3 ≤ f ≤ 17.7 GHz) for (NANO-U)CMIX. Significantly improved MW absorption performance are demonstrated in NANO-U hexaferrites, in comparison to the previously reported nanoscale spinel ferrites (-38.9 dB or 99.8% MW absorption along with a bandwidth of ∼3.7 GHz only). We believe that nanoscale particle sizes in U-type hexaferrites result in synergistic enhancements of the dielectric and magnetic losses. A self-explanatory schematic representation of the various possible loss mechanisms in NANO-U absorbers is shown in Fig. 1. Easy tunability, large magnetic anisotropy, presence of disordered surface spin states, defect polarization centres, greater magnetic and dielectric losses and large hopping conduction are responsible for large and broad-band MW absorption in NANO-U hexaferrites. These economical and easily processed U-type hexaferrite ultrafine particles can be used in the paints and sheets for wide-band and strong MW absorbing coating applications.