Reflection Loss Properties Research Articles

Synthesis, characterization and microwave absorbing properties of the novel ferrite nanocomposites

In this study, the synthesis of magnetic nanocomposites with the special stoichiometries of Ba0.34Sr0.34Cd0.16Zn0.16Fe12O19 (S1) and Ba0.69Sr0.17Cd0.07Zn0.07Fe12O19 (S2), having microwave absorbing properties were successfully carried out and reported. These nanoparticles were synthesized using self-assembly reflux reaction followed by heating treatment. The scanning electron microscopy (SEM) images depicted the uniform morphology of particles with the average sizes of 43 and 38nm for the samples (S1) and (S2), respectively. The saturation magnetizations of 65.09 and 66.59emu/g were analyzed using a VSM technique at room temperature for these uniform nanoparticles. The X-ray diffraction (XRD) patterns revealed that the matrices of both produced samples have been made from M-type hexagonal structure. The microwave absorbing characteristics were investigated via measuring the absorption of electromagnetic waves by these products with various thicknesses in the frequency range of 8–12GHz (X-band). These nanocomposites revealed the enhanced reflection loss (R.L.) properties due to calcination process at different temperatures and improvement of the crystalline structures. The reflection losses of −35 and −23dB were simply obtained for the resulting (S1) and (S2) nanocomposites with the optimum thickness of 2.1mm. These results suggest that the synthesized magnetic composites can be employed as effective microwave absorbers in various devices.

Magnetic and Reflection Loss Characteristics of Terbium Substituted Cobalt Ferrite Nanoparticles/Functionalized Multi-Walled Carbon Nanotube

Terbium substituted cobalt ferrite dot array was formed onto surface of multi-walled carbon nanotube by hetero-coagulation. X-ray diffraction (XRD), and scanning electron microscope (SEM) were used to demonstrate the successful attachment of ferrite nanoparticles to M WCNTs. Mossbauer spectroscopy results confirm the preference of terbium for tetrahedral sites. Vibrating sample magnetometer (VSM) confirms the relatively strong dependence of saturation of magnetization and coercivity with the volume percentage of MWCNTs. The complex permittivity, permeability, and reflection loss properties of the ferrite and nanocomposites were studied separately. Reflection loss of nanocomposites is abruptly enhanced in compare to that of ferrite and carbon nanotubes. Reflection loss evaluations indicated that the nanocomposites display a great potential application as wide-band electromagnetic wave absorbers.

γ-MnO 2/polyaniline composites: Preparation, characterization, and applications in microwave absorption

MnO 2/doped polyaniline (PANI) is prepared by an in situ polymerization method using γ-MnO 2 as the addition agent and hydrochloric acid as the doping agent. Products are characterized by FT-IR, UV–vis, XRD, and TEM. Conductivity, electromagnetic properties, and microwave absorption properties are first discussed on the basis of structural characterization. The as-prepared products of MnO 2/PANI are partially crystalline in nature and spherical in pattern with grain sizes of 50–70 nm. MnO 2 particles are successfully decorated with doped PANI. MnO 2/PANI displays moderate electric conduction, excellent dielectric losses, and microwave absorption capabilities. Compared to pure MnO 2, the dielectric and reflection loss properties of MnO 2/PANI composites exhibit significant improvements, with an effective absorption band at 5 GHz under −10 dB and maximum reflection loss of −21 dB at 13.56 GHz. Pure MnO 2 shows an effective absorption band of 3 GHz under −10 dB and a maximum reflection loss of −14.20 dB at 11.5 GHz. Thus, MnO 2/PANI composites are found to be a promising microwave absorption material.

Doping Effect of Single-Wall Carbon Nanotubes on the Microwave Absorption Properties of Nanocrystalline Barium Ferrite

The microwave absorption properties of single-wall carbon nanotubes (SWNCTs) and barium ferrite nanocrystalline (SWCNT/BaFe12O19) composites with different doping ratios are investigated in the frequency region of 2–18 GHz. The transmission line theory is used to calculate the reflection loss properties, and microwave absorptive mechanism of the SWCNT/BaFe12O19 composites is discussed. The experiment results reveal that the microwave absorption properties of composite are very sensitive to the volume percentage of SWCNTs. Owing to the multiple absorptive mechanisms, the microwave absorption properties of composite are evidently improved. When SWCNTs are doped with 6 vol % of the sample volume, the maximum reflection loss of the SWCNT/BaFe12O19 composite with a 3 mm thickness reaches 30.79 dB at 10.5 GHz, and the range of resonance absorption peak below -10 dB is about 6 GHz.

Surfactant-Assisted Solvothermal Synthesis of Ba(CoTi)xFe12−2xO19 Nanoparticles and Enhancement in Microwave Absorption Properties of Polyaniline

Barium ferrite (BaFe12O19) nanoparticles have been successfully synthesized from solvothermal route with the assistance of surfactant P123 (EO20PO70EO20), which has been proven to be a better method for barium-ferrite synthesis as compared to conventional hydrothermal technique, and the resultant product shows high-purity crystalline phase, small particle size, excellent magnetic properties, and characteristic of single magnetic domains. A series of Co−Ti-doped barium-ferrite nanoparticles [Ba(CoTi)xFe12−2xO19, 0.25 ≤ x ≤ 1.0] were also prepared by this method, indicating that this route is versatile for barium-ferrite nanoparticles with different chemical compositions. With increasing heteroatoms content, saturation magnetization and coercivity drastically decreased because of the selective substitution of Fe3+ sites by Co2+ and Ti4+ in the magnetoplumbite structure. By supplying these inorganic nanoparticles as nucleation sites, polyaniline/ferrite composites were prepared through in situ polymerization technique. Compared with pure polyaniline, these composites exhibited essentially enhanced reflection loss properties because of the synergetic behavior between organic and inorganic materials, and proper heteroatom substitution can even improve the reflection loss. More importantly, well reflection loss over a wide frequency range can be simply achieved by manipulating the absorber thickness, suggesting these composites may be used as lightweight and highly effective microwave absorbers.

The role of cations distribution on magnetic and reflection loss properties of ferrimagnetic SrFe12−x(Sn0.5Zn0.5)xO19

The site preference and magnetic properties of Zn–Sn substituted strontium ferrite SrFe12−x(Sn0.5Zn0.5)xO19 particles with x=0–5 were studied using both F57e and S119n Mössbauer spectroscopies and magnetic measurements. The results show that the magnetization and magnetocrystalline anisotropy are closely related to the distributions of Zn–Sn ions on the five inequivalent crystal sites provided by the hexagonal structure of the ferrite. Mössbauer spectra show that the Zn–Sn ions preferentially occupy the 2b and 4f2 sites. The preference for these sites is responsible for the anomalous increase in the magnetization at low Zn–Sn substitutions. Based on microwave measurements of reflectivity, SrFe12−x(Sn0.5Zn0.5)xO19 is a good candidate for electromagnetic compatibility and other practical applications at high frequencies.

Microwave Absorption and Magnetic Studies of Strontium Hexaferrites Nanoparticles Synthesized by Modified Flux Method

M-type strontium hexaferrite nanocrystals of radar absorbing material (NRAM) i.e. SrFe12O19 were synthesized by modified flux method that combine the controlled chemical co-precipitation process for nucleation and complete uniform growth during annealing with NaCl flux. Uniform structural morphological transformation of nanocrystals from needle to hexagonal prism faces were noticed after annealing with increasing of heat treatment (HT) temperature from 800 to 1200°C for 4h. X-ray diffraction (XRD) results show the formation of various phases with increase in annealing temperature. The crystallinity and crystallite size are found to increase with increase in heat treatment temperature (15-40nm). The superparamagnetic behavior of strontium hexaferrite is confirmed by vibrating sample magnetometer (VSM) wherein it was noticed that magnetic field (10000 gauss max) did not have any effect on these materials. The transformation of magnetic properties i.e. from superparamagnetic to ferromagnetic behaviour after heating at various HT temperatures have been revealed by hysteresis loops under VSM study. The increase in saturation magnetization from 2.44 to 75.03 emu/gm is observed. Formation of ultrafine particles has been confirmed through field emission scanning electron microscope (FESEM). About 5 to10% of the needle like crystals in the ‘as synthesized’ condition were transformed to hexagonal pyramidal structure and most of the crystals are found to have plate like hexagonal structures with increase in heat treatment temperatures. The effect of such systematic morphological transformation of nanocrystals was observed in reflection loss properties in X band (8-12 GHz). The maximum reflection loss of -20.05, -24.31, -24.16, -25.22, -25.12, -24.01 dB at 8.1, 8.6, 9.2, 9.6, 10.7, and 12 GHz respectively are observed for the material heat treated at 1200°C. A significant increment from - 6.5 to -25.22 dB at 9.6 GHz in reflection loss (RL) is noticed due to symmetric morphological growth of RAM nanocrystal during annealing.

Preparation and electromagnetic properties of multiwalled carbon nanotubes/Ni composites by γ-irradiation technique

Multiwalled carbon nanotubes (MWCNTs) were coated with Ni nanoparticles prepared from reduction of Ni 2+ by a γ-irradiation technique, producing MWCNTs/Ni composite. It was determined that surface modification of MWCNTs and generation of hydrophilic environment by γ-irradiation was crucial to the dispersion of metal nanoparticles on MWCNTs. The prepared MWCNTs/Ni composite was demonstrated to be a ferromagnetic material, and the relative complex permeability and permittivity were studied. It was calculated that after coating with Ni nanoparticles, the reflection loss properties of MWCNTs were greatly improved in the frequency range between 3.8 GHz and 18 GHz, with a maximum absorption of −7.2 dB at 6.4 GHz. Furthermore, the microwave absorbing properties could be modulated simply by manipulating the thickness of the prepared MWCNTs/Ni composite for application in different frequency bands.

A Comparative Study on Process-Properties Correlation of Nano Radar Absorbing Heat Treated Materials

Single phase M-type barium hexaferrite nano radar absorbing material (NRAM) i.e., BaFe12O19 were synthesized by modified flux method that combines the controlled chemical co-precipitation process for nucleation and complete uniform growth during annealing with NaCl flux under microwave annealing (MWA) and vacuum annealing (VA). Uniform morphological transformation of nano crystals from spherical (~ 10 nm) to prism faces (~ 35 nm) were observed under TEM during annealing. The effect of such systematic nano morphological transformation of NRAM was observed on magnetic and reflection loss (RL) properties. Maximum reflection loss (RL) was improved to 37.15 dB at 16.00 GHz for MWA at 760 watt and 27.56 dB at 15.75 GHz for VA at 1200 oC with continuous increasing absorption range under −10 dB for 2 mm thick coating layer in the Ku Band (12.4-18.0 GHz).. Excellent microwave absorption properties are a consequence of accurate EM match in the nano morphological planes, a strong natural resonance, as well as multipolarization. This process of crystal growth, morphology evolution and RL enhancement with respect to the heat treatments were also explained in terms of Ostwald ripening and quantum size effect.

Influence of matching thickness on the absorption properties of doped barium ferrites at microwave frequencies

AbstractThe development and characterization of hexagonal ferrite powders of BaFe9Mn1.5Ti1.5O19, BaFe9Mn1.5Co1.5O19, and BaFe9Ti1.5Co1.5O19 as the microwave absorbers have been investigated. The ferrites were fabricated by conventional ceramic technology. The developed ferrite powders 80% by weight were mixed with polyvinylchloride plasticizer and fired to form rubber‐ferrite with the thickness of 1.5 mm, 2 mm and 2.5 mm. XRD was used to identify structures of the samples. The magnetoplumbite structures for all the samples have been formed. Vibrating sample magnetometer was used to determine the hysteresis loops of barium ferrite and BaFe9Mn1.5Ti1.5O19 at room temperature. The vector network analyzers in the frequency range of 12 to 20 GHz have measured the reflection loss properties of rubber‐ferrite. It was concluded that BaFe9Mn1.5Ti1.5O19 with thickness of 2 mm could be designed as a wide‐band microwave absorber. By SEM, the size and morphology of grains in BaFe9Mn1.5Ti1.5O19 ferrite were examined. The results showed that ferrite with grain size of 5 µm do not have any impurities. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Estimation of seabed reflection loss properties from direct blast pulse shape (L)

The decay rate of a smeared multipath arrival received many water depths from a broadband source has been shown to be closely related to bottom reflection loss properties [Smith, J. Acoust. Soc. Am. 50, 332–336 (1971)]; [Harrison, J. Acoust. Soc. Am. 114, 2744–2756 (2003)]. This suggests a simple way of measuring bottom properties with active sonars. The method is demonstrated with experimental data measured in a shallow water region of the Mediterranean Sea. Seabed properties are deduced directly by the method in the form of the rate of change of reflection loss with respect to angle close to grazing. Agreement is shown between the results of the method and the results of matched field inversions carried out in the same area. Good agreement is also shown between measurements and predictions of propagation loss made using seabed properties deduced via the method.