Recent developments of smart electromagnetic absorbers based polymer-composites at gigahertz frequencies

The rapid increase in electromagnetic interference has received a serious attention from researchers who responded by producing a variety of radar absorbing materials especially at high gigahertz frequencies. Ongoing investigation is being carried out in order to find the best absorbing materials which can fulfill the requirements for smart absorbing materials which are lightweight, broad bandwidth absorption, stronger absorption etc. Thus, to improve the absorbing capability, several important parameters need to be taken into consideration such as filler type, loading level, type of polymer matrix, physical thickness, grain sizes, layers and bandwidth. Therefore, this article introduces the electromagnetic wave absorption mechanisms and then reveals and reviews those parameters that enhance the absorption performance

Carbon nanofibers catalyzed by iron oxide extracted from mill scale waste for microwave absorbing materials

Currently a wide range of materials are used for the design and development of microwave absorbing materials or radar absorbing materials (RAM). However, none of the materials use low-cost mill scale as a potential absorbing material. Materials from steel waste (mill scale) have been introduced in order to be used as low-cost filler. As-grown carbon nanofibers (CNF) from low-cost-mill scale with different milling time (4, 20, 40 h) was synthesized using chemical vapor deposition (CVD) method. As-grown CNF with lightweight characteristics were used as fillers showed a significant enhancement of reflection loss which can reaching -26 dB at 10.5 GHz. As the thickness increased from 1 mm to 3 mm, the reflection loss peak shifted towards lower frequency. Results showed that mill scale waste having nanometer starting particle size which was used as catalyst to grow CNF could enhanced the absorption capability

Compositional and frequency dependent-magnetic and microwave characteristics of indium substituted yttrium iron garnet

Effect of In3+ ion substitutions on yttrium iron garnet samples’ morphology, magnetic properties and dc electrical resistivity was examined closely and reported in this study. A series of polycrystalline garnet ferrites with composition of Y3Fe5−xInxO12 (0 ≤ x ≤ 0.5) were prepared by using the mechanical alloying technique. The morphological properties of the samples was analysed by using a TEM, XRD, FESEM and EDX. The electrical dc resistivity of the samples was investigated as a function of temperature and composition by using a Picoammeter, the complex permeability was analysed by using an Impedance Materials Analyzer meanwhile microwave properties was measured by using VNA. The lattice constant increases as In3+ content increases which can be understood by the difference in ionic radii of In3+ ions replacing the smaller Fe3+ ions. The grain size also increased with In3+ content, indicating that the In3+ ion acts as a grain growth promoter. Both complex permeability components, µ′ and µ″ reaching about 92.75 and 85.03 respectively at x = 0.3; later decreased with further In substitution. This result is synchronized with FMR linewidth which manifests magnetic loss of the samples. By correlating the phase analysis, morphology, electrical resistivity and complex permeability results, it is believed that there was an increase in number of crystalline-growth regions with increasing In3+ content, which together increased a total mass of ferromagnetic grains with the latter starting to dominate the samples. The results also showed that In3+ ions increase the dc resistivity of the system

Enhancing Microwave Absorbing Properties of Nickel-Zinc-Ferrite with Multi-walled Carbon Nanotubes (MWCNT) Loading at Higher Gigahertz Frequency

The rapid growth of electronic systems and devices operating within the gigahertz (GHz) frequency range has increased electromagnetic interference. In order to eliminate or reduce the spurious electromagnetic radiation levels more closely in different applications, there is strong research interest in electromagnetic absorber technology. Moreover, there is still a lack of ability to absorb electromagnetic radiation in a broad frequency range using thin thickness. Thus, this study examined the effect of incorporating magnetic and dielectric materials into the polymer matrix for the processing of radar absorbing materials. The experiment evaluated the sample preparation with different weight percentages of multi-walled carbon nanotubes (MWCNT) mixed with Ni0.5Zn0.5Fe2O4 (Nickel-Zinc-Ferrite) loaded into epoxy (P) as a matrix. The prepared samples were analysed by examining the reflectivity measurements in the 8 – 18 GHz frequency range and conducting a morphological study using scanning electron microscopy analyses. The correlation of the results showed that different amounts of MWCNT influenced the performance of the microwave absorber. As the amount of MWCNTs increased, the reflection loss (RL) peak shifted towards a lower frequency range and the trend was similar for all thicknesses. The highest RL was achieved when the content of MWCNTs was 2 wt% with a thickness of 2 mm with an RL of – 14 dB at 16 GHz. The 2.5 GHz bandwidth corresponded to the RL below -10 dB (90% absorption) in the range of 14.5 – 17 GHz. This study showed that the proposed experimental route provided flexible absorbers with suitable absorption values by mixing only 2 wt% of MWCNTs

Influence of microstructural evolution on the magnetically group dominance in polycrystalline Y3Fe5O12 multi-samples

In present work, the effect of changing microstructure on magnetic properties which evolves in parallel, in particular from amorphous-to-crystalline development, in yttrium iron garnet was investigated. 9 toroidal samples of polycrystalline yttrium iron garnets were prepared by using the mechanical alloying technique and sintered at low to high sintering temperature for microstructure-dependent-magnetic evolutions. A brief, yet revealing characterization of the samples were carried out by using an X-ray Diffraction, Field Emission Scanning Electron Microscopy, Impedance Material Analyzer, LCR-meter and, Picoammeter. It is believed that microstructural features such as amorphous phase, grain boundary, secondary phase and intergranular pores contribute significant additional magnetic anisotropy and demagnetizing fields, thus affecting the initial permeability accordingly. A scrutinizing observation of the permeability component results show that they tend to fall into three groups of magnetic permeability according to degree of magnetic behaviour dominance. The Curie temperature remained relatively stable and unaffected by the evolution, thus confirming its intrinsic character of being dependent only on the crystal structure and compositional stoichiometry. The increased electrical resistivity while the microstructure was evolving is believed to strongly indicate improved phase purity and compositional stoichiometry

Development of magnetic B-H hysteresis loops through stages of microstructure evolution of bulk BaFe12O19

A series of polycrystalline BaFe12O19 bulk samples produced have been used to investigate the parallel evolving relationship between the microstructural and magnetic properties of the magnetic material. The raw material starting powders were prepared via the mechanical alloying method and subsequently moulded and pressed by using a hydraulic pressing machine. The nanosized samples were sintered from 700 to 1300 ∘C with 100 ∘C increments in static air conditions. The sintered samples were characterized and analysed with increasing sintering temperature to study their crystallinity, microstructural and magnetic properties. The result shows the magnetic B-H hysteresis loop varies with grain size, which was attributed to the increasing crystallinity and changing magnetic parameters. The microstructural properties like grain size were measured by using FeSEM, the phase analysis of the material was analysed with X-ray diffractometry (XRD) and density was determined by a densimeter, while the hysteresis loop was measured by a B-H tracer and Curie temperature was measured with an impedance analyser. The characterized samples were found to be divided into three groups which consisted of grain size varied from 0.30 to 0.39 μm (paramagnetic state), 0.46 to 0.94 μm (intermediate ferromagnetic state) and ≥1.78 μm (strong ferromagnetic state). For the latter two groups, slanting sigmoid hysteresis loops were observed. Well-defined sigmoid-shaped B-H hysteresis loops were obtained when high crystallinity was attained, allowing strong exchange interaction between neighbouring atomic magnetic moments. This indicated that the ferromagnetic state had been achieved. The observed three B-H hysteresis groups exhibited different magnetic properties, and these behaviours are useful for industrial applications

Electromagnetic wave absorbing characteristics of C/Co-Mn and C/Co-Zn doped barium hexaferrite sandwiched nanocomposites

The development of high reflection loss and broad frequency bandwidth for microwave absorbing materials (MAMs) has been intensified in recent years. Carbon nanocomposites have stimulated attention as the EM-wave absorbing materials due to their high conductivity with high dissipating capability of electrostatic charges and EM-wave radiation shielding. In this study, we report on the analysis of microwave absorption through absorption spectrum of carbon/barium hexaferrite nanocomposites with Co-Mn and Co-Zn as dopants. The nanoparticles of Co-Mn-doped and Co-Zn-doped barium hexaferrites were synthesised using the mechanical alloying technique. The resultant particles were then mixed thoroughly with epoxy resin with ratio of 8:92 wt.% prior to sandwich with carbon black layer at different thicknesses of 1, 2 and 3 mm to form nanocomposites. The samples were characterised on their morphological, magnetic and microwave properties using field emission scanning electron microscope (FESEM), vibrating sample magnetometer (VSM) and vector network analyser (VNA) respectively; there follow rational discussions on the effect of dopant substitutions and microstructure on the absorption/reflection loss and bandwidth broadening of MAM. The results show that the saturation magnetisation of BaCoZnFe16O27 is higher as compared to that of BaCoMnFe16O27 and pure BaFe12O19 in which Zn behaves as an ion modifier that compensates the magnetic moment in hexaferrite sublattices, thus translated into an increase of net magnetic moment per unit volume of the sample. By using carbon black as a matching layer and BaCoMnFe16O27 as an absorbing layer with total sandwich thickness of 2 mm, the highest reflection loss of 39.83 dB was obtained. The capability to tune the absorption and bandwidth of all the samples at various frequencies indicates that these materials would be a superior MAM. Through scrutinisation of relationship between absorption/reflection loss with different dopants and thickness variation, further discussion on mechanism of adsorption/reflection characteristics for research optimisation would be provided

Materials’ characterization and properties of multiwalled carbon nanotubes from industrial waste as electromagnetic wave absorber

The development of high-frequency devices has attracted more research interest in electromagnetic wave–absorbing materials having lightweight, low filler content, thin thickness, minimum reflection loss and broad absorption bandwidth. Nevertheless, none of the materials uses steel waste (mill scale) as a potential low-cost catalyst to synthesize carbon nanotubes (CNT) as an electromagnetic (EM) wave absorber. Hence, multiwalled carbon nanotubes loaded in epoxy resin with an increasing polymer composite thickness of 1 mm, 2 mm, and 3 mm were introduced in this study. With varying milling times of mill scale (4 h, 20 h and 40 h) as catalyst, as-synthesized carbon nanotubes were produced using the chemical vapour deposition (CVD) method. Two main phases (carbon and iron carbide) were obtained from the synthesized carbon nanotubes. The samples’ morphology was mostly straight like, spiral, twisted carbon and spring pasta-like structures. The two-dimensional (2D) network structure of as-synthesized CNT loaded into epoxy resin, extends the transmission route of EM wave being absorbed. Moreover, the ratio of ID/IG is consistent at around 1.0 attributed to defective structure or a lower graphitization degree. In addition, higher electrical resistivity in the sample indicates wider separation between CNTs allowing for better EM wave absorption. The as-synthesized carbon nanotubes that are utilized as filler with lightweight properties, improved the reflection loss approach to − 25 dB (10.5 GHz) for growth CNT catalyzed by mill scale milled for 20 h loaded into polymer matrix (GM20h/P) at thickness of 3 mm. As the thickness of the polymer composites increased from 1 to 3 mm, all composite samples reflected a loss peak closer to a lower frequency range. The results demonstrated that the EM wave absorption ability was improved to 99.9% by using nanometer size mill scale waste as a catalyst to grow carbon nanotubes and further used as an EM wave absorber

A comparative study of different sintering routes effects on evolving microstructure and B–H magnetic hysteresis in mechanically-alloyed Ni–Zn ferrite, Ni0.3Zn0.7Fe2O4

A comparative study between two sintering routes has been carried out to reveal the parallel evolutions of microstructure and B–H hysteresis in a mechanically-alloyed Ni–Zn ferrite. The starting powders were mixed and crushed via the mechanical alloying technique. Subsequently two portions of the resulting powder were subjected to two respective sintering routes: multi-sample and single-sample. In the multi-sample sintering, the samples were sintered from 600 to 1400 °C with any one sample being subjected to only one sintering temperature. For the single-sample sintering, only a single sample was subjected to repeat sintering from 600 to 1400 °C. In B–H hysteresis measurement, the same trends but with different values were observed for both sintering routes. Saturation induction, Bs, values range from 23.9 to 1076.0 G for multi-sample sintering and from 23.4 to 930.7 G for single-sample sintering. Three distinct behaviour groups could be distinguished which correspond to a particular range of grain sizes and domain state of the samples. The activation energies of grain growth for multi-sample and single-sample sintering show three different ranges of values for each route which are 6.80, 99.31 and 143.39 kJ/mol and 14.60, 29.42 and 162.83 kJ/mol respectively. These different ranges of values characterized the different diffusion mechanisms

Materials’ characterization and properties of multiwalled carbon nanotubes from industrial waste as electromagnetic wave absorber

The development of high-frequency devices has attracted more research interest in electromagnetic wave–absorbing materials having lightweight, low fller content, thin thickness, minimum refection loss and broad absorption bandwidth. Nevertheless, none of the materials uses steel waste (mill scale) as a potential lowcost catalyst to synthesize carbon nanotubes (CNT) as an electromagnetic (EM) wave absorber. Hence, multiwalled carbon nanotubes loaded in epoxy resin with an increasing polymer composite thickness of 1 mm, 2 mm, and 3 mm were introduced in this study