Assistant Professor

School of Applied Science

Suresh Gyanvihar University

 Jaipur, Rajasthan, India 

Introduction

Every communication and wide-casting device produces electromagnetic radiation at different frequencies, which ranges from big wavelength, low energy radio waves to too tiny wavelength, higher energy gamma rays. Electromagnetic interference is the term for a problem that arises when one device's electromagnetic waves interfere with another's ability to function, causing disturbances [1-2]. Even in daily life, the impacts are evident.

Here are a few instances of it:

i. Intermittent disruptions in TV and radio reception caused by passing cars, mixer-grinder electric shavers, etc.

ii. Malfunctioning of flight controlling system owing to use of laptop by passenger.

iii. A patient's pacemaker malfunctioned as a result of using a walkie talkie.

iv. It may result in the reset of computers and a change in the status of control equipment, which could cause data loss.

v. Living systems and the environment are also impacted.

Therefore, in addition to interfering with the proper operation of electronic devices, it may also lead to a loss of income, energy, time, or even human life, posing a hazard as a particular form of environmental pollution. The quick expansion of electromagnetic noise in the atmosphere is made possible by the widespread availability of electronic devices. Exposure to such an atmosphere has an impact on human health. When electromagnetic radiation is released into the atmosphere, the majority of it is absorbed by the body and can result in health problems such muscle discomfort and skin rashes, as well as occasionally infertility. Because of the increasing demand for using electronic devices in a variety of applications and the improvements in the electronic sector, electromagnetic interference (EMI) has become a more serious issue as technology advances to make devices lighter, quicker, and smaller [2]. Due to the development of wireless electronic gadgets, electromagnetic radiation has spread across the environment and is currently a major hazard. As EMI problems have gotten worse, scientists have focused their efforts on creating materials and techniques that can act as a blocking mechanism to manage the phenomenon.

There are two possible outcomes in an EMI case:

(i) The electromagnetic interference caused by cables and circuitry radiating out. This is resolved by creating devices that filter electromagnetic radiation and

(ii) Electromagnetic interference (EMI) from radiated emissions that seep into vulnerable components. Encasing the device in an electromagnetic shielding or absorption material allows for their inspection.

Shields are used to isolate an area, stop external interference, and stop harmful radiation from internal sources from leaking out. These are greatly wanted for safeguarding delicate circuits and shielding the work area and surroundings from deepening radiation from computers and telecommunications equipment. Shielding can be achieved in two ways: either the device is covered with material that will absorb electromagnetic waves that are incident upon it, or it is placed inside an enclosure that will reflect all electromagnetic waves, shielding it from external electromagnetic interference.

According to the Faraday's cage principle, no electrical field exists inside an enclosed conductor and no electromagnetic field may leave from it. Since conducting materials are widely available from mobile charge carriers, they appear to be the greatest option for shielding by reflection. A significant portion of the incident field is impacted by the impedance mismatch created by these mobile charge carriers between the necessary shield impedance and the free space wave impedance. Thus, covering the interior of devices with a shroud or cage or using a coating of metallic ink is a quick and straightforward method of shielding. Another option is to use metal shields or wire mesh made of conductive metals like copper or aluminum. Although this works well, the only way to get enough shielding is to use thick metal sheets, which not only makes the device heavier but also has the disadvantage of being corrodible and inflexible. This limits how compact the device can be made. We anticipate that smart devices will get smaller and offer more features on a daily basis because technology is developing so quickly. This entails placing more electronic components in a single device and having more devices all around us. We need shields that are thin, light, and simple to put to devices of all shapes and sizes in order to ensure that all these electronic components function without interfering with one another. Because of this, the use of thick metal sheets is limited by size, weight, shape and hindrances offered during manufacturing process. 

Another method involves covering apertures with metal foil, which acts as a transmitting slot antenna and exacerbates the interface issue. Shielding behaviors are also enabled by conducting polymers. Although general polymers are naturally very insulating, their flexibility, light weight, lack of corrosion, affordability, ease of mass manufacture, and ability to be processed thinly in a variety of forms make them appear to be a potential alternative. Additionally, we have seen that electronic gadget housing made of plastic is becoming more common. This increases the importance of EMI shielding since most modern housing polymers are insulators, allowing electromagnetic waves to travel through them readily. To stop the waves, conductive barriers must be used as shields. One way to offer shielding is to apply a conductive coating to the plastic housing or modify the housing to become conducting. According to the research, fillers made of nanomaterials in polymer matrices may be able to better block and confine electromagnetic interference while also having the advantage of being incredibly thin and simple to apply on any surface.

Materials that act as electromagnetic wave absorbers can also act as a shield by absorbing undesired electromagnetic signals. Materials that insulate electromagnetic radiation by absorbing it have also garnered a lot of attention recently. It is necessary for the electromagnetic absorber material to have a broad absorption frequency, excellent thermal stability, and low weight. By introducing filler components into polymeric materials, such as carbon nanotubes, metal oxide nanoparticles, or inorganic magnetic particles, these desirable features can be precisely manufactured [3–11]. These days, a lot of work is going into creating metal oxide-filled polymers as they have both the conventional compositional qualities and additional special thermo physical characteristics. Depending greatly on the kind of filler (metal oxide) used, a property's uniqueness we have concentrated on the metal oxide since it establishes the framework for the polymer's conduction mechanism. 

Apart from the type of filler the quantity and structural topography of the injected filler too also has effect on the absorption of e.m. radiation. Literature analysis reveals that a lot of work has been reported with CNTs, iron and zinc oxide etc.., but as the attenuation process is mostly based on losses whether electric loss or magnetic, these parameters must be examined attentively [3,4,7,8,10]. At very high frequency as the permeability of materials tend to be near one. Therefore, we need a thorough grasp of the constants of the materials, such as permittivity, dissipation factor, skin depth, extinction coefficient, etc., in order to create an effective e.m. radiation absorber material. To properly design the material for impedance matching, we must raise these constants. Consequently, in order to create polymeric composites that can shield and absorb electromagnetic waves, we often search for filler elements that have electrical conductivity. Therefore, in order to create and develop such materials, we must first evaluate the dielectric constant, ac conductivity, and optical constant values by studying the electric and optical properties of these materials.

For military and other commercial uses, electromagnetic interference shielding in the 8.2 to 12.4 GHz (X band) is crucial since this spectrum is used for critical applications such as weather radar, TV picture transmission, Doppler, and telephone microwave relay systems. One of the main areas of possible application is the addition of metal oxide to polymers as EMI shielding materials [10, 12–14]. Such composites have higher shielding efficiency as the filler loading is increased. However, the amount of filler that can be loaded into a polymer has a limit, and as the filler loading increases, the mechanical properties of the composite system deteriorate [7]. Research indicates that in order to achieve a respectable level of shielding efficiency, filler-loaded polymer composites must be at least 1 mm thick [10, 14]. Some researchers have attempted to build thin composite film layers to achieve good shielding efficacy. They came to the conclusion that the composite stacking process is a better fabrication technique for this purpose after comparing the effectiveness of their stacked layer composites with bulk composites of the same thickness [3, 15–16]. Following their lead, we too chose to achieve significant EMI SE by stacking layers of our produced composite thin films.           

The use of shielding to address the EMI issue:

With the development and improvement of contemporary electronic equipment, electromagnetic radiations have grown extremely widespread, or pervasive. The classic circuit devices for releasing electromagnetic radiations were radios and televisions, but the introduction of wireless gadgets has heated this up to create a crowded atmosphere of these radiations. This crowded environment will become nearly saturated due to the increase in the speed, frequencies, and activities of these devices. This establishes the basis for the requirement for electromagnetic shielding and absorbing devices. Given their smaller size, lighter weight, and lower cost, as well as their ability to perform in a variety of ways and mold into a variety of desirable forms, it seems that polymers are promising candidates for use as potential materials to create objects with the desired qualities. Fortunately, a large range of fillers have made precise modification of polymers possible, but unfortunately, polymeric materials are not suitable for shielding against electromagnetic radiation absorbance.

“The ability of a material to attenuate electromagnetic waves is measured by its shielding effectiveness (SE), which is defined as the ratio of the e.m. field strength in the presence and absence of shielding material.

 SE in dB = 20 log E₁/E₂

                  = 20 log H₁/H₂                              …. (1)

Since E and H have their usual meaning.

Reflection, absorption, and multiple reflections are the shielding mechanisms [17]. Any conductive material (containing free charge carriers electrons) becomes a very promising material for shielding due to reflection. This can be explained as the result of reflection, which happens when the wave impedance of free space and the shield's essential impedance mismatch in terms of impedance. Because of reflection, expanded copper or aluminum alloys with high electrical conductivity (107 S/m) offer good shielding at a decibel level of about 100 dB. However, as aperture size increases and above 1 GHz, their performance declines. They also have a high oxidation susceptibility. When there is multiple reflections shielding, these reflections happen at the shield-to-shield border. It is possible to ignore numerous reflections if absorption is higher than 15 dB [2, 12, 17]. "EMI-absorbing materials are employed to reduce the EMI since the performance of typical conductive materials degrades at high frequencies. [12-13]. Electric or magnetic dipoles produce shielding in the absorption mechanism-related shielding [12]. There are two categories of materials that can absorb electromagnetic interference (EMI):(i) Free space absorbers: these absorb over a specific or limited frequency range; (ii) absorbers of the cavity type: these have high permeability and permittivity [14]. The conductivity, dielectric loss, and magnetic loss parameters of the absorber material determine its ability to absorb. The metal particles can be employed as an absorber shield for composites. These particles can be applied uniformly to fibers incorporated in a composite or utilized as paint by dispersing them with a polymer. In the industry, metallic mesh, electroless metal coating, vacuum metalizing, or the use of conductive paints are the usual techniques for shielding against electromagnetic interference. In order to effectively shield electromagnetic interference, metal fillers or carbon fibers can be inserted into polymeric materials, as their electrical conductivity is relatively low in these materials [2, 4–10]. 

Thus, it is evident that in order to evaluate the produced material's performance ability, it is also important to examine the dielectric and optical properties of the literature on electromagnetic interference shielding. Therefore, in order to discuss a few key research papers in this area in the articles that follow, we conducted an extensive and meticulous literature survey on the synthesis of these materials as well as on the characterization techniques for the structural, dielectric, and optical properties of such materials done by other researchers. 

Brief account of literature about EMI SE

Global material researchers are working to develop these affordable, highly functional materials for EMI shielding because metal complex filled polymer composites offer a wide range of applications in the electronics, automotive, agricultural, aviation, cosmetic, and biotechnology industries. “According to Jonathan Smuga's research [13], electrically conductive fillers made of nickel, carbon, copper, aluminum, and silver are made and then distributed throughout PMMA to create a coating. For these coatings, the SE was measured up to 70 dB. Additionally, the researcher has talked about how expanded graphite powder can be electrolessly nickel plated to create filler materials. For these samples, a 34 dB SE was attained. Using COMSOL MULTIPHYSICS software, they have examined the EM shielding behavior of their designed materials in addition to conducting traditional tests.”. “The disadvantages and restrictions of traditional materials for EMI SE have been covered by Jalali [12] in his research description. He has conducted comparative analysis on several metallic particles packed polymers (cobalt, iron, nickel, iron oxide). His work was focused on creating composites with high magnetic loss and high absorption capacity. They found that the polymer metal composite, which was created by covering polymers with iron particles that were 50 m in size, produced outstanding SE of 20 dB in the X band range.”. “By stacking seven layers of thin MWCNT-doped PMMA sheets, S. Pande et al. [15] have created a material that is both robust and lightweight for EMI absorption. The highest recorded value in the X band frequency range is 40 dB, which was achieved for their composites at 10% volume of multiwall carbon nanotube in PMMA. According to Yuen et al. [3], ten layers of 100 m thin stacked MWCNT–PMMA composite films yielded a total shielding effectiveness of almost 25 dB. Additionally, a 4.76% concentration of MWCNT in PMMA mono bulk composite with a 1 mm thickness produced a SE of roughly 10 dB.” “R. Jan et. al. [6] have researched the EMI SE for composite consisting of graphene sheets in PMMA in the frequency range (25 kHz-5MHz). Their research indicates that attenuation in the lower frequency range is mostly caused by reflection, but as frequency rises approaching 5 MHz, attenuation caused by absorption phenomena becomes extremely prominent. “Y. li et. al. [8] has prepared the graphene sheets stacked polyacrylate latex composites by a solvent free latex technology for ultra-efficient electromagnetic shielding. These composites' electrical, dielectric, structural, and electromagnetic shielding properties have all been studied. A. Joshi et al. [9] have presented a review on the current developments in carbon-based composite materials for electromagnetic interference (EMI) shielding. They report that the EMI SE of these composites got enhanced on increasing the graphene sheet content with composite containing 6 wt% graphene sheet exhibiting SE of ~66dB in X-band range due to pronounced conduction loss, dielectric relaxation, and multi scattering. The paper focuses on graphene, graphene oxide, carbon nanotubes, and several other cutting-edge carbon-based composites. The work on this that has been published in the literature, according to the authors, is restricted to a low-frequency range. The creation of composite materials that protect radiations at various frequencies is necessary in the quest to increase shielding over a wide frequency range. “S. Geeta has given a broad and exhaustive review on various methods and materials for EMI shielding. [10]. She has discussed the theory of electromagnetic interference shielding and shed light on the numerous techniques for measuring the effectiveness of shielding, including the open field or free space approach, the shielded box method, the coaxial transmission line method, and the shielded room method.  Her analysis describes the many materials used for EMI shielding for example, usage of metals, integrated metal foils in plastic materials, conductive coatings on plastics, combining the polymers with conductive fillers, inherently conducting polymers, stainless steel fibers etc”.  According to N. Li et al. [17], for a 2 mm thick SWCNT epoxy composite with 15% SWCNT doping in epoxy matrix, a SE of roughly 40 dB was obtained in the 10 MHz–1.5 GHz frequency range. With this 15% SWCNT, their greatest EMI SE was 49 dB at 10 MHz. They have shown how EMI SE and d.c. conductivity are correlated. Additionally, they presented research on how aspect ratio and wall defect in SWCNTs affect shielding efficacy. The electromagnetic interference shielding of flexible and lightweight composites constructed of graphene has been extensively studied by Chen et al. [4] and Hsiao et al. [5]. Carbon nano tube composites as electromagnetic shielding material in GHz range has been discussed by M. Gonzalez et.al. too [18 ].   An other noteworthy observation is that reinforcements hold great potential for generating new phenomena and endowing these materials with exclusive characteristics. This un-derlines the reason why polymer matrix composites are promising materials of the future and why they excite considerable attention in the study sector. The remarkable qualities open up intriguing applications in the fields of microwave absorbers, sensors, optoelectronics, electronic devices, EMI shielding, and rechargeable batteries, among others.

It has been noted that the concentration of the metal oxide within the polymer as well as the degree of metal oxide dispersion in the host polymer have a significant impact on the properties of metal oxide polymer nanocomposites, which are created by introducing metal compounds into polymers. The conductivity of these composites is influenced by a number of variables, including the size, loading concentration, compactness, and interfacial contact of the filler. Because metal particles are present in a material, their ability to absorb electromagnetic radiation depends on the size, homogeneity, and crystallographic structure of the particles within the polymeric material. Apart from thickness of absorber the suitable mix of intrinsic qualities like conductivity, permittivity, and attenuation constant for the incident wavelength decides the applicability of a material for e.m. shielding purpose. The kind and concentration of filler used in polymeric materials have a significant impact on these fundamental qualities. 

Effectiveness of EMI shielding: A theoretical framework

The purpose of thin-film magnetic shielding is to reduce or prevent the coupling of unwanted radiated energy in the path between the emitter and the recipient of electromagnetic radiations. A portion of an electromagnetic wave incident on a shielding surface is reflected, while the remaining wave penetrates the barrier and, following partial absorption, causes consecutive internal reflections at the shielding layer interfaces. The Maxwell’s equations for a uniform plane wave comprising of electric component E and magnetic component H are   

dE /dx=  - jwmH             and                dH/dx =-(s+jwe)E        …. (2)

     where m=m₀m_r is the permeability of the material;

m₀ is absolute permeability of air (m_{0 =}4p× 10^-7 henry/meter);

m_r is the permeability of the material to air;

s is the conductivity of material in mho/meter;

e = e₀e_r is the permittivity of the material;

e₀ = 1/ (36p × 10^-9) farad/meter is the absolute permittivity of air;

e_r is the permittivity of the material to air;

w= 2pf where f is frequency in Hz.

All homogenous materials are characterized by the intrinsic impedance

                        h= √(jwm/(s+jwe)                           …. (3)

where for dielectric materials s  << we so that the impedance is h= √ m/s ) and conversely, for a conductor defined by s  >> we, the impedance  h= √(jwm/s) .

The propagation constant for a electromagnetic wave is given by 

                      g = √ [jwm/(s+jwe)]                   …. (4)

For a medium to be a good conductor as s/jwe>>1  so g =√(jwm/s) = (1+j)√(pmf/s

The impedance of an electromagnetic wave is defined by the tangential component of E-field (electric) and H-field (magnetic), Z = |E|/|H|. For a homogenous shield (layer) of thickness t, the impedance [17] is

                           Z=  h [Z(t)coshgt+hsinhgt] / [hcoshgt+Z(t) sinhgt]        …. (5)

Reflection occurs at the boundary when Z(t) ≠ h.

Representing Eⁱ and Hⁱ as incident electric and magnetic fields, E^r and H^r as the reflected fields, and E^t and H^t the transmitted fields and considering the continuity of the tangential field components at the boundary, Eⁱ≠E^r= E^t and Hⁱ≠H^r =H^t, the reflection coefficients are defined [16] by

R_E          = E^r/Eⁱ            =   [Z(t)-h]/[Z(t)+h]    and

R_H      = H^r/Hⁱ       =    [h-Z(t)]/[ h+Z(t)]             …. (6)

and the corresponding transmission coefficients are

T_E = 1+R_E     and   T_h = 1+R_H             …. (7)                

For re-reflection effect, the transmission coefficients across the layer is defined [16] as

              T = T_E e^-gt / [1-R_Ee^-2gt]                           …. (8)

 The total shielding effectiveness (SE) is defined as 

             SE =   20log₁₀|T|                    …. (9)

It is the sum of the absorption a_A=20 log₁₀|e^-gt|, the reflection a_R=20 log₁₀|T_E| and the re-reflection a_B=20 log₁₀(1-R_Ee^-2gt). This theoretical framework is explained in detail in reference [91].

Figure-2: An illustration of multilayer shielding

Numerous factors, including frequency, the separation between the interference source and the shielding layers, the polarization of the fields, and the discontinuities in the shield, affect practical shielding. As seen in figure 2, multilayer shielding consists of n layers with ti thicknesses and n+1 interfaces. The continuity of the electric and magnetic fields at each contact (border) is necessary for the transmission line theory, and the impedance of a homogenous thin film with a thickness of ti can be expressed [16] as

Zi = h_i(Z(t_i-1)coshg_it_i + h_isinhg_it_i)/ h_icoshg_it_i+Z(t_i-1)sinhg_it_i                     …. (10)

i = 1...n, where h_i, g_i, and t_i are the intrinsic impedance, propagation constant, and thickness of the ith layer, respectively. h₀ and g₀ is that of the substrate, respectively. Z₀= 377 W and Z_i is the impedance at interface t_i looking into the right of the plane. If Z_i≠h_i, reflection occurs at the interface.

The transmission coefficient in eq. (11) for multilayer becomes

T =p[(1-q₀e^-2g0t0)(1-q₁e^-2g1t1)…..(1-q_ne^-2gntn)]^-1*e-^g0t0-^g1t1...gntn         ….  (11)

Where p = 2.Z_w2h₀.2h₁.2h_2.......2h_{n /}(Z_{w +}h) (h_{0 +}h₁)(h_{1 +}h₂)........(h_n + Z_w)     …. (12)

q_{i =} (h_i -h_i -1)(h_i -Z_i+1) / h_i +h_i -1)(h_i + Z_i+1)               …. (13)

The total shielding effectiveness is composed of the absorption (a_A), reflection (a_R), and successive internal re-reflection (a_B). The absorption of the n layers is the attenuation

So for a material

                                                             A+T+R=1                       …. (14)

Where A is absorbance, T is transmittance and R reflectance of the material for incident e.m radiations.

For purpose of measurement using vector network analyzer, the transmittance T of the material is defined as the ratio of transmitted power P_T to the incident power P_I

i.e                                                         T = P_T /P_I                                               …. (15) 

This ratio of transmitted power P_T to incident power P_I when expressed in decibels is referred as Shielding Effectiveness (SE) of the material

Thus as defined in equation (9) SE_T = -20 logT                                                           

                                                       SE (dB) = -20 log P_T / P_I                     ….(16)     

Similarly the shielding effectiveness due to reflectance is written as

                                                      SE_R = -20 log(1-R)                  …. (17)

And that due to effective absorbance is given by

                                                      SE_A = -20 log (1-A)                                                 

                                                      = - 20 log [T/(1-R)]                 …. (18)

Thus the total shielding effectiveness SE_total of the material is

                                                  SE_total = SE_A +SE_R = SE_T       …. (19)

Potential for further work

It is clear from this work that further research is needed to develop these composites into potentially useful materials for electromagnetic shielding. Increasing the thickness of these materials or casting them into thick sheets or pellets will help achieve the necessary effects in terms of shielding effectiveness. Another approach is to create nanoparticles of these oxides through synthesis, since research suggests that inserting nano-filers is a more promising method. Additional application of conducting filler, such as carbon nanotubes or carbon black, and conducting polymers, such as polypyrrole or PANI, may also be beneficial.  

In this instance, the potential of polymer composites as a shielding material has only been investigated in relation to two metal oxide fillers and PMMA as the host matrix. I anticipate that a range of inorganic nanomaterials combined with a range of polymers will produce innovative composites with distinctive characteristics. Additionally, by carefully choosing synthesis methods and processes and by understanding the underlying physical phenomena, innovative composite materials with intriguing applications in the optoelectronic, thermo physical, or magneto optical domains can be created.

References

1. A. K, Gupta, M. Bafna, S Shrivastava, R. K. Khanna, Y. K. Vijay “Study of Elecrtomagnetic Shielding Effectiveness of metal oxide Polymer Composite in their Bulk and Layered forms” Environment Science and pollution Research, vol. 28 pp. 3880-3887, 2021.

2. M. Bafna, A. K, Gupta, R. K. Khanna, Y. K. Vijay “Development of potassium permanganate (KMnO4) doped polymethyl methaacrylate (PMMA) composite using layered structure for electromagnetic shielding purpose” Materials today procedding, vol, 30,  pp. 11-16, 2020.

3. S. M. Yuen, C. C. Ma, C. Y. Chuang, K. C. Yu, S. Y. Wu, C. C. Yang, and M. H. Win, “Effect of processing method on the shielding effectiveness of electromagnetic interference of MWCNT-PMMA Composites,” Science and Technology, vol. 68, no. 3, pp. 963-968, 2008.

4. P. Z. Chen, C. Xu, Q. C. Ma, C. W. Ren, and M. H. Cheng, “ Lightweight and flexible grapheme foam composites for high-performance electromagnetic interference shielding,” Adv. Mater, vol. 25, pp. 1296-1300, 2013.

5. T. Hsiao, M. C. C. Ma, W. H. Tien, H. W. Liao, M. S. Li, Y. C. Yang, C. S. Lin, and B. R. Yang “Effect of covalent modification of grapheme nano sheets on the electrical property and electromagnetic interference shielding performance of a water-borne polyurethane composite,” Acs. Appl. Mater. Inter, vol. 7, pp. 2817-2826, 2013.

6. R. Jan, A. Saboor, A. N. Khan, and I. Ahmad, “Estimating EMI effectiveness of graphene polymer composite at elevated temperatures,” Materials Research express, vol. 4, no. 18, pp. 34-39, 2017.

7. G. Sanchez, and Marta, “Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range,” Intech doi.org/10.5772/62508, pp. 296-321, 2016.

8. Y. Li, S.Zhang and Y. Ni “graphene sheets stacked polyacrylate latex composite for ultra-efficient electromagnetic shielding,” Materials research express, vol. 3, pp. 97-103, 2016.

9. A. Joshi, and S. Datar, “Carbon nanostructure for electromagnetic interference shielding,” PRAMANA- Journal of Physics, vol. 84, no. 6, pp. 1099-1116, 2015.

10. S. Geetha, K. K. Satheesh Kumar, Chepuri R. K. Rao, M. Vijayan, and D. C. Trivedi “EMI Shielding: Methods and Materials-A Review,” Journal of Applied Polymer Science, vol. 112, pp. 2073–2086, 2009.

11.  C. X. Tong, “Advanced Materials and Design for Electromagnetic Interference Shielding,” CRC Press, pp. 1–35, 2008.

12. M. Jalali, “Improving Electromagnetic Shielding with Metallic Nanoparticles”, A Thesis In the Department of Mechanical and Industrial Engineering Presented in Partial Fulfillment of the Requirements For the Degree of Doctor of Philosophy (Mechanical and Industrial Engineering) at Concordia University, Canada, June 2013.

13. J. R. Smuga, “Conductively Filled Poly(methyl methacrylate) Composites; Manufacture and Testing Processes for EMI Shielding Effectiveness”, A thesis submitted in partial fulfilment of the requirements of Edinburgh, Napier University, for the award of Doctor of Philosophy, June 2012.

14. J. C. Huang, “EMI shielding plastics: A review,” Adv. Polymer Technology, vol. 14, pp. 137-150, 1995.

15. S. Pande, B. P. Singh R. B. Mathur T. L. Dhami P. Saini and S. K. Dhawan “Improved Electromagnetic Interference Shielding Properties of MWCNT–PMMA Composites Using Layered Structures,” Nanoscale Res Lett, vol. 4, pp. 327–334, 2009.

16. S. M. Yang, Y. Y. Chang, Y. C. Hsieh, and Y. J. Lee, “Electromagmetic Shielding Effectiveness of Multilayer metallic Thin Film on plastic Substractes” Journal of Applied Polymer Science, Vol. 110, pp. 1403–1410, 2008.

17. Ninghi, Y. Huang, F. Du, X. H. Xiaolin, H. Gao, Y. Ma, F. Li, Y. Chen, and P. C. Eklind “Electromagnetic interference (EMI) shielding of single walled carbon nanotube epoxy composites,” Nanotubes, vol. 6, pp. 1141-1145, 2006.

18. M. González, G. Mokry, M. Nicolás, J. Baselga and J. Pozuelo “Carbon Nanotube Composites as Electromagnetic Shielding Materials in GHz Range” Carbon Nanotubes - Current Progress of their Polymer Composites, pp. 297-321, 2016.