However, ex-situ mixing leads to poor dispersion of filler particles and failure to overcome their agglomeration tendencies that results in inferior and non-reproducible electrical and electromagnetic attributes.
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In contrast, the electronic properties of such synthetic metals can be strictly controlled by following in-situ incorporation Fig. In a typical reaction, monomer s , filler and dopant or catalyst are charged into a suitably designed reactor to maintain required temperatre T , pressure P and agitation stirring conditions.
During such pre-polymerization process, monomers are generally adsorbed over dispersed nano-filler particles. As already mentioned and shown in Fig. But the organic conjugated polymers are insulators in their undoped forms e. However, controlled doping leads to enhancement of conductivity due to formation of charge carriers Fig.
Therefore, increasing dopant concentration leads to increase in concentration and mobility of proto-generated charge carriers resulting in enhancement of conductivity.
Furthermore, such a conductivity enhancement in conductivity is strongly dependent on nature and concentration of dopant and in some case conductivity well exceeds the required limit Olmedo et al, ; Saini et al, for exhibiting good shielding effectiveness. At percolation threshold, ICP particles form a 3D conductive network within host matrix, which can be easily estimated by plotting the electrical conductivity as a function of the reduced volume fraction of filler Fig. The values of scaling law parameters i.
When the densities of the host polymer and the filler are similar, mass fraction m becomes same as volume fraction v and can be used in above calculations. However, it has been observed that formation of such networks and percolation thresholds minimum loading level at which first continuous network of conducting particles is formed critically depend on nature of ICP, its intrinsic conductivity, particle shape, morphology, aspect ratio, its concentration, degree of dispersion and extent of compatibility with host matrix.
Interestingly, when ICPs are combined with other conducting fillers e. Polyaniline with MWCNT, Saini et al, significant reduction in percolation threshold, higher conductivity and better shielding performance is observed as compared to pristine unfilled ICPs. Protonic acid doping of polyaniline leading to formation of charge carriers polarons radical cations and bipolarons dications. Inset shows the percolation and scaling details.
In many cases conjugated polymers are used as matrix instead of conventional insulating polymers. When conducting fillers e. In contrast, the loading of above conducting fillers within microwave non-transparent doped intrinsically conducting ICP matrices lead to further enhancement Fig. Therefore, above improvement in conductivity can be attributed to bridging of these metallic islands Saini et al, a by the metallic filler particles facilitating enhanced inter-particle transport.
The increase in conductivity is strongly dependent on nature, concentration and aspect ratio of filler particles as well as type and morphology of host ICP matrix.
Nevertheless, the establishment and enhancement of electrical conductivity is of paramount importance because it leads to parallel enhancement of reflection and absorption loss components Fig. Interestingly, absorption loss SE A increases by much larger magnitude with conductivity compared to corresponding reflection loss SE R component. For a non-magnetic material, this can be explained on the basis of logarithmic [i.
A secondary mechanism of shielding is absorption for which shield should possess electric or magnetic dipoles. These dipoles can interact with transverse electric E and magnetic H vectors of the incident EM waves to introduce losses into the system. It is interesting to note that pure without any external filler loading conjugated polymers in their undoped base forms possess poor dielectric and magnetic properties. However, controlled doping leads to marked improvement Fig. Therefore, observed improvement of dielectric properties with doping level can be attributed formation and increase in concentration of above localized carriers.
The correlation between dielectric properties and shielding response for various ICPs is presented in Fig. Furthermore, the complex dielectric constant dependence of absorption loss SE A component inset of Fig. Solid lines are guides to the eye.
Reprinted with permission from [J. Joo and A.
Epstein, Appl. Copyright , American Institute of Physics. Interestingly, for ICPs, besides doping induced polarization filler induced interfacial polarization may also contribute towards dielectric properties. For example, when conducting fillers like metal particles, graphite or carbon nanotubes are introduced into ICP matrices; further improvement of dielectric properties was observed. Such polarization and related relaxation phenomenon contribute towards energy storage and losses.
The actual losses can be computed by normalization of these losses with storage terms [i. In case of in-situ formed MWCNT-polyaniline nanocomposites, improvement of dielectric properties leads to high value of loss tangent Fig. However, though for a given thickness, total shielding is dominated by absorption, reflection loss component becomes too high form the viewpoint stealth technology.
Nevertheless, despite good dielectric properties, magnetic properties of ICPs remained poor to extend any significant contribution towards EMI regulation. In contrast, for moderately conducting materials e. Therefore, it is expected that any improvement in magnetic properties will lead to definite improvement of absorption loss alongwith parallel reduction of reflection loss. Consequently, in recent years, lot of work has been carried out to formulate composites of polyaniline with the dielectric or magnetic filled inclusions, either by in-situ polymerization or by ex-situ physical mixing processes Abbas et al, , , , Furthermore, most insulating polymer matrices possess poor electrical, dielectric or magnetic properties and are transparent to radio frequency RF or microwave MW electromagnetic radiations EMRs.
Therefore, only fillers contribute towards shielding and leakage of radiation from EMR transparent regions tends to degrade shielding effectiveness. However, microwave non-transparency Olmedo et al, , ; Saini et al, of ICPs compared to conventional polymers is an added advantage as both filler and matrix contribute towards shielding. Moreover, the dominant shielding characteristic of absorption for above nanocomposites materials other than that of reflection for metals render ICPs more useful in applications requiring not only high EMI SE but also shielding by absorption, such as in stealth technology.
When ICPs are exploited as microwave non-transparent matrices, the added dielectric or magnetic filler particles result in establishment of properties e. The incorporation of magnetic fillers e. Such a combination is expected to display additional magnetic loss leading to enhanced absorption. The magnetization plots Fig. The saturation magnetization M s value of of these particles was found to be The SPM character imparts fast relaxation behaviour and originates due to small size of the ferrite particles i.
However, PANI possesses weak ferromagnetic behaviour and with increase in ferrite content, enhancement of M s was observed with as parallel reduction of coercivity H c. The initial permeability i of ferromagnetic materials can be expressed as Stonier, :. The above equation shows that permeability can be enhanced either by enhancing M s or by reducing H C. The results show that M s value increases 5. Therefore, It can be seen from the eqn.
In many cases, highly doped ICP particles are used as conducting fillers in place of metal or carbon based materials for various insulating polymer host matrices. Again the magnetic properties remained poor due to non-magnetic nature of most ICPs. However, when magnetic filler loaded ICPs is use hybrid filler, improvement in magnetic properties has also been observed besides regular improvement of dielectric attributes.
Most importantly, losses due to reflection SE R and absorption SE A follows the permittivity and permeability trends and exhibit corresponding increase. However, SE A was more sensitive towards electromagnetic attributes compared to SE R which may be attributed to their square root and logarithmic dependences.
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For a good conductor i. Now shallower is the skin depth, higher will be absorption loss for a given thickness of material. This section is devoted to measurement and interpretation of shielding effectiveness alongwith detailed analysis of reflection and absorption sub-components. Frequency and dopant concentration dependence of total shielding effectiveness SET value of samples prepared by doping of emeraldine base EB with different concentrations of acrylic acid AA viz.
Nevertheless, though based on microwave dielectric constant and electrical conductivity values many speculations were made about the shielding properties of ICPs, the first direct evidence of shielding response of ICP based composites alongwith actual shielding effectiveness values was presented by Taka A PHM has no intentional holes or slits but lacks homogeneity. The shielding efficiency depends strongly on the amount of conducting polymer mixed in the blends due to regulation of conductivity.
EMI shielding effectiveness at 1. The results are graphically presented in Fig. Far field SE of dB was obtained for the melt blend of polyaniline again at higher loading level of 30 wt. Pomposo et al have prepared PPY based conducting hot melt adhesives by melt mixing appropriate amounts of ethylene-co-vinyl acetate EVA copolymer and PPY, which was synthesized with oxidant of FeCl 3.
Propagation of Electromagnetic Waves through a Stratified Medium. I Bernard Salzberg J. Plane electromagnetic wave propagating parallel to the gradient of the refractive index Bernard Jancewicz J. Matrix formalism for calculation of electric field intensity of light in stratified multilayered films Koji Ohta and Hatsuo Ishida Appl. More Recommended Articles.
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