Abstract
This article highlights the microwave absorption and dielectric attributes of synthesized polyaniline (PAni)-yttrium trioxide (Y2O3) composites. Temperature-dependent conductivity measurements were carried out in the temperature range of 300–473 K, which indicates the presence of hopping conduction. The PAni-Y2O3 composites showed semiconductor behavior with the exponential variation of inverse temperature dependence of electrical conductivity. Microwave measurements were carried out in the X-band (8–12 GHz) frequency; the composites exhibit absorption-dominated shielding effectiveness (SEA) in the range -33 to -35 dB (>99% microwave attenuation) and minimum electromagnetic reflection. The ε′ and ε″ values of the composites are in the range of 81.5–97.5 and 80–118.5, respectively. The PAni-Y2O3 composites showed significant improvement in microwave SE and complex permittivity of the composites, indicating the influence of Y2O3 in PAni.
1 Introduction
The proliferation of electronics and the ever-developing wireless communication industry demand the requirement of microwave-absorbing and electromagnetic interference (EMI) shielding materials to safeguard electrical/electronic circuitry and devices (1, 2). This electromagnetic environmental pollution not only affects proper functioning of electrical and electronic devices but also creates health threats (3–5). Microwave- or radar absorbing materials are also finding increasing importance in military applications in order to reduce the radar signatures of strategic targets (stealth technology) (6, 7). EMI shielding materials at higher gigahertz range are in high demand as microwave ranges are increasingly being exploited by circuitry engineers in wireless telecommunication systems, radar, local area network, medical equipment, etc., owing to saturation in the lower frequency bands. Especially, microwave shielding in the X-band (8–12 GHz) is more important for commercial and military applications. Telephone microwave relay systems, radar, Doppler, weather and TV picture transmission lie in the X-band (8). There are a number of studies on the X-band EMI shielding properties of various composite materials reported in the literature (9–16). Most of these composites were more reflective than absorptive to incident electromagnetic energy with lower EMI shielding effectiveness (SE) values. Although these results are encouraging, very few studies have been reported on both microwave shielding and dielectric response in the broad microwave frequency range (8–12 GHz). A shield is conceptually a barrier to the transmission of electromagnetic energy. Shielding materials attenuate the electromagnetic waves through the mechanisms of reflection, absorption, and multiple internal reflections, with the remaining energy being transmitted for interference. Among these three mechanisms, reflection and absorption suppress most of the electromagnetic fields. Shielding by absorption is important for various practical applications as compared to reflection.
Conducting polymers have emerged as an important class of electronic materials because of their potential applications in optoelectronic devices (17), as corrosion inhibitors for iron and mild steel (18, 19), and in EMI shielding (20, 21). These synthetic metals have widespread technological applications where lightweight, flexible, and high-conductivity materials are required. Among the various conducting polymers, polyaniline and its composites have been widely investigated owing to its ease of protonic acid doping in the emeraldine form and to its environmental stability in both doped and undoped forms (22–24). In addition, polyanilines are relatively lightweight and corrosion-resistant conducting materials, which may be easily produced in large scale at a low cost (25). Conducting polyaniline is known for its electromagnetic shielding and microwave absorption properties (26–28), but the studies on broadband microwave shielding properties of polyaniline-inorganic oxide composites are very limited. The present study is inspired by the fact that the electromagnetic properties of polyaniline can be modified by the addition of inorganic dispersants (29, 30). That is, inclusion of stable inorganic oxide with better dielectric properties provides materials exhibiting novel functionalities and improved dielectric properties (31, 32). One of the most cost-effective means of shielding electromagnetic interference and dissipating electrostatic charge is to use conducting polymer composites having dielectric loss and stable dielectric attributes. Yttrium oxide is a subject of technological interest owing to its stable dielectric properties (33–35), and PAni possesses high dielectric losses owing to intrinsic conductivity and charge localization (36). Consequently, by incorporation of Y2O3 particles, the electromagnetic properties of polyaniline can be improved to obtain a maximum absorption of electromagnetic energy. In this study, we have focused on correlating broadband EMI shielding with the dielectric properties in PAni-Y2O3 composites compared to our previous work on the transport properties and low-frequency EMI SE of PAni-Y2O3 composites (30).
In this paper, we report the microwave absorption and dielectric properties of PAni-Y2O3 composites in the 8- to 12-GHz frequency range. The PAni-Y2O3 composites were prepared with different amounts of Y2O3 dispersant. The influence of the Y2O3 content with respect to the electromagnetic properties, the EMI SE and the complex permittivity of the composites was systematically investigated by waveguide transmission line technique by using a vector network analyzer.
2 Experimental
2.1 Materials
Aniline monomer was distilled under reduced pressure and stored below 0°C. Y2O3, ammonium peroxydisulfate [(NH4)2S2O8, APS], and hydrochloric acid (HCl) were all of research grade and used as received. All reagents were purchased from Sigma-Aldrich (India).
2.2 Preparation of PAni-Y2O3 composites
Synthesis of PAni-Y2O3 composites was carried out by a single-step, in situ chemical oxidative polymerization technique with ammonium persulfate (APS) as the oxidant (37). Fine-grade Y2O3 powder at weight percentages (wt%) of 10, 20, 30, 40 and 50 was added to a freshly prepared reaction mixture (0.2 mol/l aniline and 0.25 mol/l APS in 1 mol/l HCl) at 5°C. The mixture was stirred during the polymerization of aniline for 10 h. The resultant solution was filtered and washed with deionized water and acetone. The recovered composite was dried at 60°C in a hot air oven for 8 h to achieve constant weight. These synthesized PAni composites with various weight percentages of Y2O3 (PY1, PAni with Y2O3 of 10 wt%; PY2, 20 wt%; PY3, 30 wt%; PY4, 40 wt%; and PY5, 50 wt%) were compressed into rectangular pellets under a pressure of 9 tons in a table-top hydraulic pellet press. These rectangular pellets of standard X-band (WR-90) dimensions of thickness of up to 0.0025 m fit exactly between the X-band waveguide adapters of a vector network analyzer.
2.3 Instrumental analysis
Scanning electron microscopy (SEM; XL30 ESEM, Philips, Amsterdam, Netherlands) and transmission electron microscopy (TEM; JEM 2100, JOEL, Akishima, Tokyo, Japan) were used to investigate the morphology of samples. The temperature-dependent electrical conductivity of the PAni-Y2O3 composites was measured by a two-probe method using a laboratory-made setup with a Keithley 224 constant current source and a Keithley 617 digital electrometer (Keithley Instruments, Solon, OH, USA) attached to a Lake Shore 331 temperature controller (Lake Shore Cryotronics, Westerville, OH, USA). For the conductivity measurements, compressed pellets of composite powder sample were prepared at a pressure of 9 tons in a circular die (10 mm in diameter) and contacts were made on each end using a silver paste. The measurements were recorded during the cooling cycle. Microwave shielding and dielectric measurements were carried out on an HP vector network analyzer (model 8510, 45 MHz to 26.56 GHz; Hewlett-Packard Development Company, USA) using the waveguide transmission line technique (22) in the frequency range of 8–12 GHz (X-band). The rectangular pellets of thecomposites were placed into the X-band waveguide adapter in transverse cross section, and EM waves of frequency 8–12 GHz were made to incident (38). The electromagnetic reflection and transmission scattering parameters (S11, S21) were measured and used for the analysis of the electromagnetic properties. A full two-port calibration was performed using quarter-wavelength offset and terminations to remove errors due to source match, load match, directivity, isolation, and frequency response in the measurements.
3 Results and discussion
3.1 Morphological features
SEM and TEM were used to determine the morphology and distribution of Y2O3 in polyaniline. It has been reported that the morphology of the dispersant has an important impact on the electrical properties of the composites (39, 40). Figure 1A–E shows the scanning electron micrographs and transmission electron micrographs of PAni, Y2O3, and PAni-Y2O3 composites. PAni (Figure 1A) exhibits an aggregated globular morphology, which is in agreement with those of the literature (41–43). Pure Y2O3 is made up of polyhedral flaky aggregates about 200–400 nm in size (Figure 1B). It has been reported that higher absorption losses at microwave frequencies were obtained with the inclusion of flake-like dispersants in the composites (7). The scanning electron micrograph of the PAni-Y2O3 composite (Figure 1C) shows the polymerization of aniline over the Y2O3 particle, which forms the interconnected chain-like structure. This is confirmed by the transmission electron micrograph. The TEM of the composite (Figure 1D) shows that the dispersant Y2O3 (dark shaded) is covered by the PAni chains (light shaded). That is, Y2O3 particles are encapsulated by the polyaniline chains. The homogeneous dispersion of Y2O3 particles in the PAni matrix is confirmed by the high-magnification, high-resolution TEM image (Figure 1E). These morphological characteristics are expected to play an important role in utilizing the PAni-Y2O3 composites for effective microwave attenuation, as these heterogeneous hybrid polymer composites are strongly polarizable. Electrical properties and polarization mechanisms contribute to the dielectric losses in materials (44, 45). In conducting polymer composite materials, the relaxation of electric oscillations is strongly affected by structural disorder and is important in determining their high-frequency characteristics. The composite materials in the present study, in which yttrium oxide particles are embedded in PAni, introduce a disorder in the form of matrix-dispersant interfaces. This disorder may bring about changes in the internal electric fields by dipole interactions and complex electrical relaxation behavior.
Scanning electron micrographs of (A) PAni, (B) Y2O3, (C) PAni-Y2O3 composite (20 wt%); (D and E) TEM and high-magnification, high-resolution TEM images of the PAni-Y2O3 composite.
3.2 Electrical conductivity
Temperature-dependent conductivity measurements in the temperature range of 300–473 K revealed that the conductivity increases exponentially with increasing temperatures for all samples. Figure 2 shows the variations in conductivity of the PAni-Y2O3 composite samples as a function of inverse temperature. The samples exhibit a semiconducting behavior in the specified temperature range. A similar behavior in conductivity has been reported in previous works (46, 47).
Variations in conductivity with inverse temperature for the pellets of PAni-Y2O3 composites. The inset illustrates the ln(ρ)-T-1/4 variation for the composites.
The conductivity variations of conducting polymers are best explained by Mott’s variable-range hopping (VRH) model. In this model, the temperature (T) dependence of conductivity (σ) follows the relation (48)
where T0 is the Mott characteristic temperature and σ0 is a pre-exponential constant. T0 and σ0 are determined by the localization length, the density of state, and the hopping distance in the material. The r (r=2, 3 and 4) is connected with the effective dimensionality of the researching system. In a three-dimensional VRH model, Equation 1 can be expressed as
A linear variation was observed in the plot of ln ρ(T) with T-1/4 (inset of Figure 2). In the measured temperature range, it shows the three-dimensional hopping transport. A decrease in conductivity observed for higher concentrations (wt%) of Y2O3 in the polyaniline matrix (i.e., for PY3, PY4, and PY5) indicates that increased Y2O3 content increases the structural disorder in the composites. The observed conductivity for these composite samples in the range of 0.51×10-2 to 0.283 S/cmis in the prescribed range (0.001–1 S/cm) for acceptable microwave attenuation/shielding (49).
3.3 Microwave absorption analysis
The EMI SE of a material is defined as the attenuation of the propagating electromagnetic (EM) waves produced by the shielding material. SE can be expressed as (50–52)
where Pi and Pt are the power of the incident and transmitted electromagnetic waves, respectively. For a shielding material, the total SE is the sum of the contribution due to reflection (SER), absorption (SEA) and multiple reflections (SEM). With the use of the scattering parameters S11 and S21 of the vector network analyzer (obtained by the waveguide transmission line technique), the reflection coefficient (R) and transmission coefficient (T) are given as R=|S11|2 and T=|S21|2. The absorption coefficient (A) can be calculated from the simple relation A+R+T=1 (53–55). If the effect of multiple reflections between both interfaces of the material is negligible, the relative intensity of the effectively incident EM wave inside the material after reflection is based on the factor (1-R), and with respect to (1-R), the effective absorbance can be expressed as Aeff=(1-R-T)/(1-R). The reflectance and effective absorbance expressed in decibel as SER and SEA, respectively, are given by (56)
Figure 3A and B shows the variation of the SEA and SER in the X-band frequency. The result shows that the conducting PAni-Y2O3 composites had a SE higher than that of pure PAni, mainly due to absorption. The SE of the composites due to absorption (SEA) was in the range -33 to -35 dB. The minimum SE due to reflection (SER) compared to that of pure PAni in the X-band indicates the microwave absorbing nature of the composites in the measured broadband microwave frequency range. The SE of the composites increased by up to 20 wt% of Y2O3 content in the PAni matrix and decreased marginally for higher weight percentage loadings of Y2O3. This indicates the presence of percolation threshold at 20 wt% of Y2O3 in PAni. Although classical percolating systems (with a percolation threshold around 16% by volume fraction for globular conducting objects dispersed in an insulating medium in three dimensions) have been studied in detail for many years (57), the percolation characteristics of conducting polymer blends and composites are always unique. The formation of polyaniline-inorganic dispersant composites provides a new class of percolating systems (37, 58). All the PAni-Y2O3 composites exhibit stabilized values for all the frequencies in the X-band. The observed increase in microwave absorption characteristics of the PAni-Y2O3 composites may be attributed to the higher dielectric losses observed in the composites. It is reported that, for most of the industrial applications, -30 dB SE is sufficient to attenuate the interference, as it can suppress 99.9% of electromagnetic energy (51, 59, 60). As the PAni-Y2O3 composites in the present study have a SE >-30 dB, these materials will be a potential candidate for broadband microwave absorption and EMI shielding with better results than those available in the literature (11–16, 45, 61–65). Thus a combination of PAni and Y2O3 exhibited a synergetic effect in improving microwave shielding.
(A and B) Dependence of shielding effectiveness (SEA and SER) as a function of X-band frequency.
3.4 Dielectric attributes
The microwave absorption behavior of materials depends on complex permittivity (εr=ε′-jε″). The real part (ε′, dielectric constant) is mainly associated with the amount of polarization occurring in the material, whereas the imaginary part (ε″) is a measure of dielectric losses. The complex permittivity of the composites was calculated from the scattering parameters (S11 and S21) using the Nicholson-Ross-Weir method (66, 67) and is shown in Figure 4A and B for the X-band. The dispersant Y2O3 in the PAni matrix resulted in the formation of a heterogeneous composite system with multiple interfaces. This caused the space charge accumulation at these interfaces and contributed towards the observed higher microwave absorption of the composites in the frequency range 8–12 GHz. The polarization and modified relaxation effects occurred owing to the presence of polaron/bipolaron and other bound charges (dipoles) (68) in the PAni-Y2O3 composites, which resulted in high values of ε′ and ε″ compared to the permittivity of pure PAni. The electromagnetic waves can be absorbed in materials that have high permittivity (2, 24, 69, 70); PAni-coated Y2O3 composites worked effectively for attenuating the electromagnetic interference. The Y2O3 particles that act as an interconnecting bridge between the conducting chains of PAni increased the inter-chain transport, and more mobile charges interacted with the incident electromagnetic fields in the microwave radiation. This increase in long-range charge transport and in the number of possible relaxation modes led to enhanced absorption losses. Also, the effect of accumulation of more space charge and strong orientation polarization was stimulated owing to the relative dielectric constant of Y2O3,and the PAni matrix reflected the improved values of SE due to microwave absorption. It was observed that both the dielectric constant (ε′) and the dielectric loss (ε″) were decreased marginally for higher concentrations (wt%) of Y2O3 content in the PAni matrix, indicating the percolation characteristics of the composite. Therefore, the moderate content of Y2O3(20 wt% of Y2O3) is more effective for increased microwave absorption of the PAni-Y2O3 composite sample.The observed absorption-dominated shielding characteristics and dielectric properties of the composites indicate that these composites can be utilized effectively for shielding applications in the X-band.
(A and B) Variations in the real and the imaginary part of permittivity of PAni-Y2O3 composites as a function of X-band frequency.
4 Conclusions
PAni-Y2O3 composites have been synthesized by in situ chemical oxidative polymerization. From the analysis of SEM and TEM images, it was found that Y2O3 particles are well incorporated into the PAni matrix with the formation of polyaniline over the Y2O3 particles. The dielectric and microwave shielding properties of synthesized PAni-Y2O3 composites have been studied in the broad microwave spectrum that spans over practically relevant X-band frequency. The incorporation of Y2O3 particles into the PAni matrix increased the complex permittivity and microwave attenuation of the composites. The main advantage of the synthesized composite is the absorption-dominated shielding ability in the broadband (8–12 GHz) that can be used in the design of microwave absorbers.
Acknowledgment
The authors would like to acknowledge the management of PES Institute of Technology-Bangalore South Campus, for their support and encouragement towards carrying out this work.
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