Microwave shielding effectiveness (SEA) for 10- and 20-dB bandwidth (BW) in near and far field in BaCoxTixFe(12−2x)O19 (x = 0.1, 0.3, 0.5, 0.7).
Ferrites are a wide class of materials that are still a very rich field of scientific interest and under the scope of recent research. The polycrystalline Co2+-Ti4+ substituted Ba hexagonal ferrite has been synthesized by the standard ceramic method. The vector network analyzer has been incorporated to measure different microwave parameters at X-band (8.2–12.4 GHz) frequencies. The microwave shielding effectiveness is evaluated by S-parameters for near field and AC conductivity as well as skin depth for far field. The doping of Co2+ and Ti4+ ions causes absorption in composite x = 0.5 to exhibit good shielding effectiveness and it exhibits large 20-dB bandwidth of 4.70 GHz in the near field and 3.60 GHz for far field respectively. The AC conductivity increases with frequency in composites x = 0.1, 0.3, and 0.5 and skin depth decreases with frequency in all composites. The shielding effectiveness, AC conductivity, and skin depth are correlated to each other.
- microwave shielding
- AC conductivity
Ferrites are a wide class of materials containing iron. These materials are formed in different crystalline symmetries. A simple form of ferrites is the spinel AB2O4 of cubic structure [1, 2, 3, 4, 5, 6, 7, 8, 9]. The orthoferrites ABO3 are another important form with an orthorhombic perovskite crystal system [10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. The third class of ferrites are garnets of form A3B5O12 [20, 21, 22, 23, 24, 25, 26, 27, 28, 29]. The fourth class, termed as hexaferrites, may be divided into five main groups: M-type (AB12O19), W-type (AMe2B16O27), X-type A2Me2B28O46, Y-type A2Me2B12O22, and Z-type or A3Me2B24O41 [30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49]. The preparation of these materials and their characterization are very rich topics because of the wide range of applications and the cheap materials obtained. Ferrites are a very interesting class of materials whose wide range of applications are related to electromagnetic interference suppression as well as their use in radar absorbing material (RAM) coatings.  From this point of view, great scientific interests are devoted to use these materials as RAM devices [51, 52, 53, 54]. In this work the intensive highlights is devoted to the microwave applications and which class is the best candidate for this application.
The tremendous rise in speed of electronic devices and widespread incorporation of information technology for various technological applications have pumped up electromagnetic pollution to dangerous levels. The high-speed electronic gadgets emit spurious wireless signals rendering the electromagnetic disturbance/interference (EMI) to the electrical and/or electronic circuits in the vicinity.
A microwave absorber reduces unwanted radiation emitted from high-speed electronic devices such as radar, oscillators, and supercomputers. The ferrimagnetic materials ferrites have the potential ability to reduce electromagnetic interference (EMI) in contrast with conventional dielectrics owing to their magneto/dielectric properties [55, 56, 57, 58, 59, 60]. Electronic devices constitute integrated circuits (ICs) wherein numerous components are embedded and such components are encapsulated with ferrite films to mitigate EMI. The frequency range of application of extensively used spinel ferrites is limited by Snoek’s limit and they are not effective at GHz range. M-type hexagonal ferrites are tailored for EMI diminution in the higher end of microwave region, that is, X-band, Ku-band, K-band, etc. [61, 62, 63, 64, 65]: these ferrites allow to tune in the frequency region through doping accompanied by anisotropy field. Both the electric and magnetic properties define the capabilities of these materials to store energy and are described by analyzing the real parts of complex permittivity (ε′) and permeability (μ′), respectively. On the other side, imaginary parts (ε′′, μ″) are very important parameters that describe the loss of electric and magnetic energy.
Different researches have been devoted to electromagnetic interference (EMI) shielding effectiveness (SE) and EMI shielding mechanisms [66, 67, 68] of high structure carbon black (HS-CB)/polypropylene (PP) composites and multiwalled carbon nanotubes-polymethyl methacrylate (MWCNT-PMMA) in the X-band frequency range. They studied different thickness of composite plates electrical conductivities. Their results showed that the absorption loss contribution to the overall attenuation is more than the contribution of the reflection loss for HS-CB/PP composites. Moreover, EMI SE up to 40 dB in the frequency range 8.2–12.4 GHz (X-band) was achieved in Ref.  by stacking seven layers of 0.3-mm-thick MWCNT-PMMA composite films compared with 30 dB achieved by stacking two layers of 1.1-mm-thick MWCNT-PMMA bulk composite.
Recently, graphene composites have been found to be one of the most promising candidates for high-performance porous microwave absorbers in ref.  because of their 3D conductive network and multiple scattering. A qualified frequency bandwidth (reflection loss <−10 dB) reaches 5.28 GHz covering almost the entire Ku band at 2 mm thickness. These results might open the door for a new design of lightweight coating absorber. This may allow us to say that the performance of microwave devices is mainly based on the properties of the used materials. Knowledge of the frequency dependence of such material is a prerequisite to select suitable materials for various microwave applications and vice versa [71, 72, 73]. Novel nanocomposite systems are prepared for microwave applications such as para-toluene sulfonic acid (p-TSA)-doped polyaniline (PANI)-graphene nanoplatelet (GRNP) composite films. The addition of GRNPs in the PANI matrix allows to improve the conductivity and dielectric properties of the composites due to the formation of 3D conducting networks. Shielding effectiveness of the PANI-GRNP composite films doped with p-TSA was examined by using S-parameters obtained from vector network analyzer in the X-band microwave frequencies. The efficiency of shielding for these composites depends on GRNP’s content in the PANI matrix . Electrical and mechanical properties of carboxylic (▬COOH) functionalized multiwall carbon nanotube (MWNTs)/epoxy composites at low wt.% (0.5, 0.75, and 1 wt.%) are studied in Ref. . Microwave shielding effectiveness (SE) for X-band (8–12 GHz) and the flexural properties showed that the total SE of the nanocomposites was increased with the positive gradient of MWNT contents. Great efforts have been made to improve the requirements for microwave applications in X-band and new materials are being tested [70, 76, 77, 78]. Promising results were found and the search for new materials continues.
In the present chapter, we have explored EMI shielding effectiveness characteristics of M-type Ba-Co-Ti hexagonal ferrites.
2. Experimental details
The M-type BaCoxTixFe(12−2x)O19 hexaferrites, with x = 0.1, 0.3, 0.5, and 0.7, were prepared by ceramic method. The powder chemicals were mixed thoroughly, ground, and sintered in an electric furnace at 900°C for 7 h. The pellets were made of the powder with the hydraulic press at uniform pressure of 75 kN/m2 and final sintering was done at 1100°C for 9 h. The crystal structure was measured using Bruker D8Diffractometer of Cu X-ray radiation.
The microwave properties have been studied by the vector network analyzer, Agilent model N5225A. Before performing the measurements, permittivity and permeability of air were measured with an analyzer for calibration purposes. The DC resistivity (ρdc) was investigated using Keithley Electrometer, model 6514. The selected thickness of composites for optimized characteristics are x = 0.1–3.3 mm, x = 0.3–3.8 mm, x = 0.5–3.4 mm, and x = 0.7–3.2 mm.
3. Results and discussion
Figure 1 shows patterns obtained from X-ray diffraction of BaCoxTixFe(12−2x)O19 hexaferrite composites. The observed XRD peaks confirm M-type phase of hexaferrite with space group P63/mmc. The change of intensity in the peaks shows that the substituted Co2+ and Ti4+ ions have occupied crystallographic sites.
3.1 Shielding in near field
The shielding effectiveness (SE) is accompanied by reflection or absorption of unwanted microwave signal (EMI) and can be represented as SE = SER + SEA with SER due to reflection and SEA as absorption. When the microwave signal passes through the material, part of the signal is reflected and remaining transmitted or absorbed. The reflected power (Pr) and transmitted power (Pt) are derived from measured S-parameters: Pr = |S11|2 and Pt = |S22|2, SEA and SER can be calculated as:
Figure 2 shows plots of EMI shielding effectiveness (SEA) versus frequency for doping of Co2+ and Ti4+ ions. Composites x = 0.5 and 0.7 exhibit highest (38.9 dB) and lowest (7.9 dB) values at 10.26 and 12.03 GHz respectively and these composites stay at maximum and minimum values in the frequency regime.
All composites exhibit nonlinear decrease in SEA with frequency and composites x = 0.1, 0.3, and 0.5 show more dispersion in SEA with frequency: x = 0.1, 0.3, and 0.7 displaying maxima at 9.27 GHz and x = 0.5 at 10.26 GHz. All composites stay at SEA > 10 dB or 90% absorption, encompassing the entire frequency region.
Figure 3 depicts the response of shielding effectiveness (SER) of BaCoxTixFe(12−2x)O19 ferrite versus frequency for doping of Co2+ and Ti4+ ions. All composites exhibit: (i) minimum SER in comparison to SEA encompassing the entire frequency region, (ii) nearly the same trend of SER in the investigated frequency regime, and (iii) maxima in the mid-frequency region. The shielding effectiveness (SER) due to reflection is very small and SER owe excursion between 0.2 and 2.5 dB. The low SER implies no composite can act as a microwave reflector shield. The composite x = 0.5 has largest SER = 2.48 dB at 10.39 GHz.
3.2 Shielding in far field
Shielding effectiveness for far field can be evaluated by classical electromagnetic field theory with the following relation :
where σac is the AC conductivity, ω is the angular frequency, ε0 is the absolute permittivity, d is the thickness of the shield, δ is the skin depth, and μr is the relative permeability.
Furthermore, σac = ωε0ε″ and δ = (2/μoωσac)1/2, where μo and ε″ are dielectric loss and absolute permeability respectively. The first term, 10 log(σac/16ωε0), in Eq. (3) is the shielding effectiveness due to reflection and second term, 20(d/δ) log e, relates to the absorption of the microwave signal. The second term is effective at high frequencies and Eq. (3) can be rewritten as:
Figure 4 depicts the graph of AC conductivity (σac) as a function of frequency for doping of Co2+ and Ti4+. It increases with doping from x = 0.1, 0.3 and x = 0.5 followed by prevalent fall in x = 0.7: composite x = 0.5 observes more dispersion with frequency and large value of σac in comparison to other composites. The rise in σac is seen with frequency in composite x = 0.1, 0.3, and 0.5; however, it remains nearly independent of frequency in x = 0.7. This increase in σac is ascribed to Koops-Wagner model, which explains ferrite comprising of heterogeneous structure : ferrites owe layers of good conducting grains, effective at high frequencies, are separated by poor conducting grain boundaries that are effective at low frequencies.
The composites x = 0.1, 0.3, 0.5, and 0.7 have DC resistivity (ρdc) of 693.6 MΩ cm, 2.8 kΩ cm, 0.5 kΩ cm, and 33.8 MΩ cm, respectively. The composite x = 0.1 has the highest resistivity but still a large σac attributed to the presence of more strength of Fe3+: electron hopping between Fe3+–Fe2+ ions is responsible for conduction in ferrites . Among all composites, composite x = 0.5 (i) owe maximum σac besides with diminution in the number of Fe3+ ions and (ii) has the lowest DC resistivity. The competition between these factors altogether increases σac in this composite. Similarly, steep fall of σac in x = 0.7 is associated with the least number of Fe3+ ions available for electron hopping and large DC resistivity.
The dependence of skin depth (δ) on frequency for a different level of substitution is shown in Figure 5. The decrement trend in δ is observed with frequency, and x = 0.7 and 0.5 exhibit large and small δ respectively among the composites in the frequency regime. The large conduction loss, as shown in σac (Figure 4), causes minimum δ, which attenuates the propagating microwave signal in the composite and vice versa; thus further penetration of signal is not possible inside the thickness of composite: the signal is attenuated more in x = 0.5 due to highest σac depicted in Figure 4, thereby causing lowest δ.
The dependence of shielding effectiveness (SEA) on AC conductivity (σac 0.5) for different levels of doping is shown in Figure 6: it increases with doping from x = 0.1 to x = 0.5 and steep decrement is seen thereafter in x = 0.7. All composites display a monotonic trend of increase in SEA with σac 0.5 and x = 0.5 owe maximum value while x = 0.7 stay at lowest one.
Table 1 shows bandwidth (10 dB and 20 dB) of SEA for both near and far field versus doping: 10 and 20 dB means 90% and 99% absorption respectively. For near field, x = 0.1, 0.3, and 0.7 exhibit 10-dB bandwidth of 2.23, 2.34, and 2.12 GHz respectively whereas 20-dB bandwidth of 1.54, 0.89, and 3.60 GHz is observed in x = 0.1, 0.3 and 0.5 respectively. For far field, x = 0.1, 0.3, and 0.5 show 10 dB-bandwidth of 3.20, 3.70, and 0.50 GHz respectively, and 20-dB bandwidth of 4.70 GHz is seen in x = 0.5 only.
|x||Near field||Far field|
|Freq. band (GHz)||10 dB bandwidth (GHz)||Freq. band (GHz)||20 dB bandwidth (GHz)||Freq. band (GHz)||10 dB bandwidth (GHz)||Freq. band (GHz)||20 dB bandwidth (GHz)|
For near and far field, microwave shielding effectiveness in BaCoxTixFe(12−2x)O19 ferrite is governed by absorption and doping of Co2+ and Ti4+ ion increases SEA from x = 0.1, 0.3, and 0.5. Composite x = 0.5 owes the highest SEA of 38.9 dB at 10.26 GHz and 3.4 mm thickness; σac 0.5, ρdc and δ are the contributing factors and same composite carries with highest SEA of 44.6 dB at σac 0.5 of 4.5 (Ohm.cm)−0.5 for far field; s-parameter is the deciding factor. Furthermore, SEA increases monotonically with frequency and it can be tuned by varying intrinsic and extrinsic parameters. Composite x = 0.5 has far field and near field wideband of 4.70 and 3.60 GHz respectively for 20 dB SEA. The studied composites have the potential for practical absorber applications. The applications of these composite materials or other composite materials are very an important subject and more research is needed to find the optimum properties and optimum materials for X-band microwave applications.
The author IA Abdel-Latif, is thankful to the Deanship of Scientific Research in Najran University for their financial support NU/ESCI/16/063 in the frame of the local scientific research program support.