An energy efficient and scalable process has been developed for the preparation of intermetallic Fe–Si–Al alloy particles. The preparation process involves initial homogenization & mechanical activation of elemental metal powders followed by vacuum annealing at elevated temperature ~ 650 °C. The physical properties of the as synthesized powders were investigated by means of X-ray Diffraction (XRD), Energy Dispersive Xray Flourence (EDXRF),Vibration Sample Magnetometer (VSM), Scanning Electron Microscope (SEM) and Particle Size Analyser. Composite coatings have been prepared by dispersing 30–60 wt% of Fe–Si–Al alloy in polyurethane (PU) resin. The effect of loading of Fe–Si–Al alloy on dielectric and electromagnetic (EM) properties of composites are studied in 6–18 GHz. A suitable formulation (50 wt% Fe–Si–Al) has been identified, wherein, proper combination of dielectric and magnetic loss of coating composite showed optimal microwave (MW) absorption in 6–18 GHz. The identified composite coating of thickness ~ 1.6 mm showed maximum Return Loss (RL) value ~ − 43 dB at 11.9 GHz and the loss values can exceed more than 10 dB within 6.5 to 16 GHz while varying thickness of coating from 1.2 to 2.2 mm. In the present study, microwave absorption properties of coatings are mainly credited to dielectric and magnetic loss values of the composites. The Fe-Si-Al filled resin composite with optimal loading of alloy particles can be a promising candidate for microwave absorbing coating applications.
Iron based intermetallic alloy such as FeNi , FeCo [2, 3], FeCr  and Fe–Si–Al [5,6,7] etc. have received extensive attention as EM absorbing functional filler materials due to their advantage of high permeability in GHz region and good thermal stability at higher temperature. Among these material systems, ternary Fe–Si–Al alloy shows unique properties like high saturation magnetization, high temperature resistance, water resistance and corrosion resistance in comparison to other alloy systems [8, 9]. For EM absorbing applications, Fe–Si–Al alloy required to be dispersed in suitable insulating matrix in appropriate ratio for making composites, wherein, each filler material acts as EM wave absorbing entity. Further, filler-host matrix interaction is also important for multiple scattering of wave within composite to obtain enhance microwave absorbing properties. Depending on the area of applications, different host matrix i.e. paraffin wax , resin , rubber , etc. have been used for making EM absorbing composites. Particularly, polymer resin matrix is attractive for their light weight, corrosion resistance, flexibility and processing advantages. Further, the resin matrix also offers making composite in the form of coating, which can be applied on any surface and shape of the objects.
There are few reports on Fe–Si–Al based composite coatings, wherein, Fe–Si–Al has been used as functional filler material to achieve EM absorbing properties. For example, W. Yang et al. prepared composite coating by taking micron size Fe–Si–Al and nano-size BaTiO3 powders in epoxy resin matrix. The authors have studied the effect of these fillers on dielectric and microwave absorption properties of the composites . Similarly, Zhou et al. prepared different composite coatings by varying the ratio of Fe–Si–Al and ZnO in epoxy resin. The composite coating filled with 35 wt% Fe–Si–Al and 20 wt% of ZnO showed reflection loss more than 10 dB over 8.6 to 12 GHz at coating thickness ~ 2.2 mm . In another report, discrete absorption coatings have been fabricated using flake shape Fe–Si–Al alloy as well as carbon black and their EM absorption properties have been investigated in 2–18 GHz. The effect of discrete structures on the absorption performance has also been studied . The authors observed RL more than 8 dB having 5.1 GHz bandwidth with minimum loss value ~ − 28 dB at 11.5 GHz. In most of the reported literatures, Fe-Si-Al alloy has been used along with other additives i.e. carbon black, BaTiO3 and ZnO etc. and dispersed in either PU or epoxy resin to obtain enhanced microwave absorption properties in GHz region. Although, few experiments have been carried out, however, EM absorbing capability of Fe–Si–Al alone in resin matrix has not been explored much. Therefore, it will be interesting to study the Fe–Si–Al–resin composites to arrive suitable formulation for EM absorbing coating for practical application.
On the preparation side, it is always being a challenge to produce intermetallic Fe–Si–Al alloy from multicomponent systems due to large differences of melting point among each elements and high reactivity of Aluminium resulting segregation within alloy system . To obtain pure phase Fe–Si–Al alloy, different techniques have been used i.e. Ultrahigh energy mechanical alloying, , Mechanical Milling (MM), Spark plasma sintering,  Self-propagation high temperature synthesis  etc. Among these techniques, mechanical milling is most used technique for the preparation of Fe–Si–Al alloy system, wherein, very fine mix between initial elemental powders can be produced resulting homogeneous compound through solid state diffusion. However, during this process, there is a possibility of local welding of particles by plastic deformation resulting formation of solid solution leading to unwanted chemical compounds in the final powder. Therefore, preparation of Fe–Si–Al alloy using simple, cost effective, scalable technique is highly anticipated to obtain reproducible properties.
In the present work, a simple, scalable and energy efficient process has been established for the preparation of intermetallic Fe–Si–Al alloy. In this process, initial dry milling has been carried out, followed by mechanical activation of elemental powders and thereafter the powder mixture was annealed under vacuum for formation of pure phase intermetallic alloy. The as prepared powder was investigated for structural, morphological, and magnetic properties by using XRD, SEM and VSM techniques respectively. The EM absorbing characteristics of the powder was investigated by dispersing them in polyurethane (PU) resin matrix. Different resin–alloy composites have been prepared and the effect of loading percentage of alloy on EM absorbing properties are studied. The choice of taking PU resin as host matrix in this work is that, it offers advantages of flexibility, abrasion resistance and weather resistance of coating specially for aircraft application.
Preparation of Fe–Si–Al alloy
Analytical reagent raw materials i.e. Fe, Al and Si powders (200–300 mesh size) were procured from M/s SRL, India. These elemental powders were used for synthesis work without any further purification. The PU resin (Shivapol 315) supplied from M/s Shivathane Lenopack, India was used for making composite coatings. For the preparation of intermetallic alloy system, initially elemental powders of Fe, Si and Al were taken by weight in the ratio of 80:10:10 respectively. To obtain homogeneous mixture, all these powders were dry milled along with Stainless Steel (SS) for 5 min at 100 rpm in Planetary ball mill (Fritsch, Pulverisette 5), while taking Ball-to-powder mass ratio 10:1. After this process, the powder mixture was again milled in spirit media for 10 min at 400 rpm for mechanical activation of the surface of the elemental powders. The slurry of mixture was taken out from the jar and dried in ambient condition. Thereafter, the powder was separated from the steel balls for further reaction. The activated powder mixture was transferred to alumina crucible for annealing under vacuum conditions. The powder loaded crucible was kept in vacuum furnace and annealed under vacuum condition ~ 5 mbar at 550 °C and 650 °C for 3 h for phase formation and grain growth. After completion of reaction, the powder was crushed by mortar-pestle to obtain final powder. The schematic for preparation of Fe–Si–Al alloy is shown in Fig. 1.
Preparation of coating composites
Resin–alloy composites coatings were prepared by dispersing alloy powder in PU-resin matrix. For this dispersion, predetermined quantity of resin and alloy was added in to 1:l capacity bead mill. A small quantity of xylene solvent was added into mixture for better dispersion. The resin–alloy mixture was continuously stirred for 2 h in bead mill to get final paint composites. The prepared paint composite was mixed with predetermined quantity of hardener (N75) before spraying on to substrate. For investigation of EM properties, air spray coating of composites was carried out on PVA coated metal substrate. During spraying the air pressure was maintained ~ 5 bar. A layer by layer coating was carried out to obtained thickness of coating ~ 2 mm. The coating was cured for 24 h in ambient condition and lifted out from the substrate followed by removal of PVA by using hot water. The composite samples were then cut into desired dimensions for evaluation EM parameters. Different composite coatings were prepared by loading 30, 40, 50 and 60 wt% of Fe–Si–Al alloy in PU resin and named as A1, A2, A3 and A4 thereafter. The process of preparation of composite coating samples are illustrated in Fig. 2.
The crystal structure and phase composition of the synthesized powder was characterized by powder X-ray diffraction performed on a Philips X’Pert Pro system using Cu Kα1 (λ = 1.540 Å) radiation. To identify the actual composition of alloy in final prepared powder, energy dispersive X-ray flourecence (EDXRF) has been carried out by using XENEMATRIX, EX-2600 X-Calibur system. The morphology of powder was investigated by Carl Zeiss EVOMA-15 scanning electronic microscopy (SEM). Particle size distribution was measured by means of laser diffraction scattering method using Particle Size Analyser (Mictotrac S3500). For particle size distribution the synthesized powder was ultra-sonicated in distilled water for 1 h before measurement. The magnetic properties of the powder were investigated by using ADE Model EV5 Vibrating Sample Magnetometer (VSM) at room temperature. The magnetization (emu/g) was calculated from the data collected and the mass of the sample. The EM properties measurement of the composites has been carried out by wave-guide technique using R&S ZVM Vector Network Analyser (VNA).
Result and discussion
For the preparation of pure phase intermetallic alloys, the powder mixture was annealed at two different temperatures and corresponding XRD spectra are shown in Fig. 3. The powder annealed at 550 °C showed the formation of Fe–Si–Al alloy phase along with other elemental peaks of Al, Si and Fe. However, on heating at 650 °C, we observed formation of pure phase of alloy. The appearance of diffraction peaks can be indexed to (110) and (200) planes of bcc structure of α-Fe–Si–Al alloy (JCPDS card no 65–4899). From this study, it indicates that annealing temperature ~ 650 °C is found suitable for the preparation of pure phase of Fe–Si–Al alloy.
To obtain quantitative composition of as prepared Fe–Si–Al alloy powder, XRF spectra has been recorded as shown in Fig. 4. The spectra shows presence of Al kα, Si kα and Fe kα at 1.4 eV, 1.7 eV and 6.3 eV respectively indicating the presence of elements in prepared alloy. The actual atomic weight percentage of Fe, Si and Al elements present in the alloy system are found to be 63.4%, 16.1% and 18.9% respectively. This result indicated that the chemical omposition of the prepared alloy is comparable to that of the elemental powders taken during synthesis work.
The typical morphology of the synthesized alloy powder is shown in Fig. 5. From the micrographs the particles are found irregular shape with particles size in the range of ~ 5 to 50 μm.
The particle size distribution (Fig. 6) estimated through laser diffraction scattering method is found to be 3–60 μm with mean particle size ~ 15 μm. The observed size distribution is good agreement with particle size obtained from SEM studies.
The variation of magnetization of Fe–Si–Al alloy particles with magnetic field at room temperature was investigated and shown in Fig. 7. The powder shows ferromagnetic behaviour with a saturation magnetization (Ms) ~ 88.5 emu/g and coercive field value ~ 12.1 Oe (inset Fig. 7).
To obtain microwave absorption properties of composite coatings, the complex permittivity and permeability values have been calculated by Nicolson and Ross algorithm using S-parameters measured through waveguide transmission line technique over MW frequency range (6–18 GHz). The experimental set up for EM parameter measurement is shown in Fig. 8. The imaginary values of permittivity and permeability (ε″, μ″) exemplify the MW attenuation/absorption characteristics of materials. Figure 9a–d shows the frequency dependence of the complex permittivity and permeability for the composite coatings made with 30, 40, 50 and 60 wt% of Fe–Si–Al powder in the frequency range 6 to 18 GHz.
The real (εʹ) permittivity of coating composite samples (A1, A2, A3 and A4) are shown in Fig. 9a. The real permittivity values are found increased with increasing loading fraction up to 50 wt% loading of alloy in resin matrix without any variation over 6–18 GHz. The permittivity values are observed ~ 6.9, 10.7 and 18 for A1, A2 and A3 composite samples respectively over 6–18 GHz. However, the real permittivity values for higher loading composite (60 wt%) is found decreased from 18 to 8.3.
Similarly, imaginary permittivity (εʹʹ) value for the composite samples A1, A2 and A3 are found to be nearly constant ~ − 0.18, − 1.5 and 2.0 respectively (Fig. 9b) in the frequency range 6–18 GHz. The imaginary permittivity values are increased with increase of loading fraction of alloy from 30 to 50 wt% in resin matrix. However, permittivity values decreased for higher loading alloy composite sample (A4) and values are found to be ~ − 0.45 over 6–18 GHz. The increasing trend of real permittivity values up to 50 wt% loading of Fe-Si-Al alloy suggest that, higher loading of alloy particles makes composites more conducting in nature. In low loading of alloy, the Fe-Si-Al particles are separated by the PU resin matrix resulting less conducting of composite and thereby reduction of permittivity values are observed.
Figures 8d and 9c shows frequency dependence of real and imaginary permeability of the A1, A2, A3 and A4 composites samples. The measured real permeability values for all the composite samples showed higher values at 6 GHz frequency, however, the values decreased with increasing frequency from 6 to 18 GHz. The real part of permeability (μʹ) value decreased from 1.08 to 0.97, 1.08 to 0.96, 1.50 to 0.95, 1.12 to 0.99 and 1.50 to 0.95 with increase in frequency from 6 to 18 GHz for A1, A2, A3 and A4 samples respectively. There is no significant change of real permeability values are observed for all samples in entire frequency range. Further, the imaginary permeability values are also increased with increasing the loading percentage of alloy from 30 to 60 wt%. The values of imaginary permeability (μʺ) are found decreased from –0.13 to –0.059 and –0.17 to –0.096 with increase of frequency from 6 to 18 GHz for samples A1 and A2 respectively. However, slight decrease of imaginary permeability values from − 0.21 to − 0.16 and − 0.24 to − 0.179 are observed for A3 and A4 samples respectively.
The dielectric loss tangent values (tan δe) were calculated from complex permittivity values are plotted in Fig. 10a. The significant increase of loss values from ~ 0.026 to ~ 0.11 are observed while increasing loading of alloy from 30 to 40 wt%. On further increasing of loading, slight increase of loss value ~ 0.12 is observed in the composite sample A3. The loss value decreased drastically (~ 0.04) for higher loading of alloy (A4). As the loss tangent values for A1 and A4 samples are found negligible, which is not expected to contribute any loss of EM energy within composites. The magnetic loss tangent values are found increased with increasing in loading of alloy particles in composites (Fig. 10b). For composite samples A1 and A2, the values are found in the range of ~ − 0.12 to − 0.06 and − 0.12 to − 0.09 with decreasing trend towards higher frequency. In case of higher loading, the values are found in the range of ~ − 0.18 to − 0.17 and − 0.21 to − 0.19 for samples A3 and A4 respectively. The higher value of magnetic loss resulting from more numbers of alloy particles present in the composite as each particle absorbs energy due to spin rotation.
Although, we have observed high magnetic loss in higher loading composite (A4), however, reverse trend is observed in dielectric loss values. The above studies indicate that decrease of dielectric behaviour of the composite and the same is not observed in magnetic behaviour of the composite having high content of Fe–Si–Al alloy particles. The similar decrease behaviour of dielectric in higher content of filler is also observed in earlier study, wherein, BaTiO3 and Fe–Si–Al alloy has been taken as fillers in epoxy resin (10). The effective permittivity of the PU resin–Fe–Si–Al composite strongly influenced by the interfacial polarization at the interface between PU resin and Fe–Si–Al particles. Further, polarization associated with PU-resin and Fe–Si–Al particles also equally important for contributing permittivity of the composites. At higher loading, there may be possibility of restriction of motion of polymer chains due to interfacial adhesion between Fe–Si–Al alloy and PU resin. As a result, the polymer chains lose their freedom to relax and unable to contribute electrical polarization. This behaviour is observed by several authors for the composites containing various types of filler materials (18, 19). In the similar line, the observed less dielectric loss in composite sample A4 may be due to the decrease of interfacial polarization at higher loading of Fe–Si–Al alloy particles.
To obtain low reflection loss in wide frequency range, it is necessary for composites having suitable complex permittivity and permeability. In order to investigate microwave absorption properties of Fe–Si–Al filled resin composite samples, the MW reflection losses (RL) have been calculated on the basis on these measured complex permittivity and permeability values at a given thickness and frequency. According to transmission line theory, the input impedance at the air interface is given by following equations:
where Zin is the input impedance d is the thickness of absorber f is the frequency of electromagnetic wave μr is the complex relative permeability, εr is the complex relative permittivity.
The RL value of 10 dB is comparable to the 90% microwave absorption according to above equations, which is considered as adequate microwave absorption. The calculated RL characteristics for A1, A2, A3 and A4 composite samples at coating thickness ~ 1.8 mm are shown in Fig. 11. The sample A1 with 30 wt% Fe-Si-Al shows RL values less than 5 dB in 6–18 GHz. The RL values are enhanced with increase of Fe–Si–Al content from 30 to 50 wt% and then further decreased in higher content (60 wt%) of Fe–Si–Al as shown in Fig. 11. The sample A3 shows significant microwave absorption performance more than 10 dB over 8.2 to 11 GHz with maximum loss value ~ − 28 dB at 9.5 GHz. Further, RL for A3 composite with varying thickness was calculated for 6–18 GHz (Fig. 12) The effective reflection loss of more than 10 dB could be achieved over a frequency range 6.5–16 GHz while varying thickness from 1.2 to 2.2 mm.
From these figures (Figs. 11 and 12), reflection loss values are shifted towards lower frequency with increasing the filler loading and thickness of coating. The observed shift can be explained by using quarter wavelength theory, wherein, thickness of absorber and matching frequency is inversely relating each other . The matching frequency shifted lower side with increase of thickness as well as real permittivity (εʹ) and permeability (μʹ) values. In the present study, the real permittivity and permeability values are enhanced with higher content of Fe-Si-Al (sample A3) and therefore, absorption peak observed at lower frequency in comparison to other composite samples. The absorption peak can be tuned by varying filler loading and coating thickness of composites.
Further, for better understanding the absorption properties among these coating composites, we have calculated normalized input impedance (Zin/Z0 = (μ/ε)1/2) of composites at thickness ~ 1.8 mm in 6–18 GHz (Fig. 13). For effective absorption of microwave, two important factors need to be considered i.e. (a) impedance match between absorber and free space at the surface to enter microwave inside the absorbing layer for its further attenuation (b) lossy nature of the absorber for attenuation of microwave energy inside the absorbing layer. Both these factors are equally important to get effective absorption of microwave energy for an absorber. Ideally, normalized impedance (Zin/Z0) value should be closer to ~ 1 for better entering of EM wave at the surface of absorber. In the present study, normalized input impedance for A1 composite is found closer to ~ 1 in wide frequency range in comparison to other composites, whereas, less impedance matching is observed for A3 composite. Although, less impedance matching observed, however, the absorption performance of A3 is found higher among these composites. This observation indicates that both dielectric and magnetic loss values of A3 composite are dominated over impedance matching to obtain better microwave absorption properties. Similarly, for A4 composite, higher magnetic loss values are observed however, inferior absorption is observed due to less dielectric loss values of the composite. From above discussions, it reveals that 50wt% loading of Fe-Si-Al powder in PU resin (A3) resulted proper combination of dielectric and magnetic loss values of the composite to obtain better microwave absorption in 6–18 GHz.
Fe–Si–Al alloy particles were prepared as functional filler and their physical properties have been investigated by using different techniques. The developed process has capability to scaling up the powder in large quantity with reproducible properties. The coating composites have been formulated by dispersing the alloy in different concentrations in PU resin matrix. The prepared composites showed both dielectric and magnetic loss in the frequency range in 6–18 GHz. The identified composite (50 wt%) coating showed maximum absorption value ~ − 43 dB at 11.9 GHz for thickness ~ 1.6 mm. More than 90% microwave absorption can be tuned within 6.5 to 16 GHz with variation of thickness of coating from 1.2 to 2.2 mm.
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Janu, Y., Chaudhary, D., Chauhan, V. et al. Preparation of Fe–Si–Al intermetallic alloy and their composite coating for EM absorbing application in 6–18 GHz. SN Appl. Sci. 2, 874 (2020). https://doi.org/10.1007/s42452-020-2664-9
- Microwave absorbing coating