EMI shielding of MWCNT/ABS nanocomposites in contrast to graphite/ABS composites and MWCNT/PS nanocomposites

Abstract

Acrylonitrile–butadiene–styrene (ABS) nanocomposites filled with multiwall carbon nanotubes (MWCNTs) are prepared through localized conductive patterning, using dry powder tumble mixing succeeded by hot compression technique. Electrical conductivity and complex permittivity together with electromagnetic interference shielding effectiveness (SE) are investigated in 8–12 GHz frequency range. Abrupt increase in dc conductivity by number of orders of magnitude on addition of 5 × 10⁻⁴ volume fractions of MWCNTs marked as percolation threshold, has adequately described the conductivity behavior with statistical parameters of percolation and GEM models. The SE of these nanocomposites increases with increase of MWCNTs content. SE ∼ 1 dB at 12 GHz produced with 0.05 wt% MWCNTs grows to ∼40 dB on addition of 5 wt% MWCNTs. SE exhibit marginal dependence or even independence on frequency for most of the compositions with 0–5 wt% MWCNTs. SE reflection (SE_ref) and absorption (SE_abs) both are increasing with the rise of MWCNTs content. However increase in SE_abs is faster than SE_ref in addition to its dominance. Complex dielectric constants in 8–12 GHz frequency range have been calculated by means of S11/S12 parameters using Nicolson Ross Weir algorithm. The variation of the SE as a function of dc conductivity which has rarely been examined in literature, investigated and compared with other polymer composites here.

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Introduction

Electromagnetic interference (EMI) is a disruption that disturbs, blocks or otherwise destroys or confines the operational performance of an electronic circuit due to either electromagnetic induction or electromagnetic (EM) radiation emitted from an external source. The presence of radio frequency EM in surroundings is progressively increasing due to rapid conspicuous use of mobile phones and other devices. EMI particularly at radio-frequency is one of the concerns which can be problematic even up to GHz range. In this scenario the top priority is to protect sensitive electronics from the ill effects of these interruptions. The best solution is to design electronic circuits either immune to EMI or shielded from external EM radiations. Shielding is used on a range of gadgets in many marketplaces, consist of communications and telecommunications; computers and data processing; aviation and aerospace; military/defense; medical and health; automotive; office equipment; electronics; instrumentation and control systems; home electronics, audio and video; appliances; radio, TV, cable and recording.

Presently lightweight and effective EMI shielding materials are sought to screen workspaces, equipment on board, computers, telecommunication paraphernalia besides circuits of defense installation and diagnostic equipment in hospitals. Polymer-based conducting systems are considered to be versatile EMI shielding materials because of easy synthesis, lower mass, cost effective in large-scale production and simple processing.^2,3 Conducting composites with polymer matrix surpass their metallic counter parts due to added range of mechanical properties like strength, flexibility, and environmental resistance. For making of conductive polymer composites, thermoplastic and thermoset matrices filled with carbon or metallic fillers (powder/fiber) have normally been used. Electrical and dielectric properties of these composites determined by the filler amount, particles size, shape, material defects, connectivity and the processing techniques.^4–7 An intrinsically conducting polymer^8–10 can be used as material for EMI shielding, but disadvantageous because of poor processability, environmental instability, high cost, and lack of mechanical properties. Extrinsically conducting polymer composites are lighter in weight, easy to develop, cost effective and seams can either be reduced or completely eliminated by molding.¹¹ For an efficient shielding, the conducting filler in polymer composites should have large aspect ratio in addition to small size and high conductivity.¹² A special skill is needed for appropriate control of EM behavior of such composites, since these cannot be made efficiently or easily into the requisite forms with a minimum amount of filler. For a convincing SE, extremely low amount of nanosize fillers have been identified.^13–15 With the advent of nanotechnology radar absorbing materials are gaining importance due to prospective increase of shielding and microwave absorbing properties besides decrease in mass and volume of structures. In defense and space applications low mass materials are significant because of high costs of launch and launcher payload mass. Among nanosize fillers, carbon nanotube (CNT) has offered to improve the properties of the host polymer matrix due to distinctive features inclusive high strength and low density.¹⁶ The percolation threshold is noticeable with a very small content of nanofiller which is produced as a result of creation of network of conductive paths. The threshold is recognized as inverse relation to the aspect ratio of CNT.¹⁷ Several researchers^18–20 described very low percolation threshold which depends on dispersal of conducting filler in the polymer matrix^21,22 besides processing technique.⁶ For realization of lower percolation threshold good dispersion of CNT is essential²³ for reason of CNTs agglomeration tendency.^24,25 Functionalized CNT shows higher conductivity of composites equated with the un-functionalized. Studies revealed the functionalization made improvement in dispersion of CNT in natural rubber [NR] matrix leading to decrease in electrical resistivity of CNT–NR composites,²⁶ attributed to preferential dispersion of filler in composites.

In this research work focus is on analysis of acrylonitrile butadiene styrene (ABS) composites filled with minimum amount of conducting filler with a good-sized EMI shielding capability. ABS offers a comfortable processability besides low cost, and dependable notch impact resistance. Generally, ABS plastics are employed for mechanical works in addition to several combinations with other materials for varied range of features and uses. Recently, the EMI shielding capabilities of polymeric materials filled with multiwall carbon nanotubes (MWCNTs)^27–35 have been investigated using solution processing technique together with sonication and compression molding. MWCNTs possess tubular geometry offers huge surface area besides high aspect ratio is expected to allow property enhancement at lower concentration compared to conventional fillers. SE obtained was ∼30 dB with 5 wt% MWCNTs in ABS matrix,²⁷ whereas in the present work 40 dB of SE is achieved by addition of 5 wt% MWCNTs to ABS, using simpler method of tumble mixing of dry powders followed by hot compression.^1,7,15 Earlier this group has produced graphite-filled ABS composites⁷ through the same method of tumble mixing of dry powders followed by compression molding at 90 °C. The shape of graphite particles is essentially flake. During mixing graphite flakes coat on to the surface of ABS particles. Gaps are obvious in coats among graphite flakes due to their planer geometry. These gaps are filler depleted areas containing ABS only which is transparent to EM waves and serve as windows for EM waves escape. The dimension of gaps would be larger for lower packing of graphite, decreases with higher loading. The SE was <30 dB for 5 wt% graphite in ABS in the X-band frequency range. Thus besides fabrication method and processing conditions the shape of filler is also important to achieve better results.

Experimental

Materials

MWCNTs are picked up from Sigma Aldrich Ltd. having descriptions: >99% carbon basis; inner diameter, outer diameter, length and density are ∼2–6 nm, ∼10–15 nm, 0.1–10 mm and 1.7–2.1 g mL⁻¹ at 25 °C correspondingly as indicated in manufacturer catalogue number: 677248-5G. ABS-92 bought from Lanxess ABS Ltd., Baroda, India was in granules form and cryogenically fragmented to particle size of ∼1–5 micron.¹ X band waveguide size pellets of thickness ∼2.8 mm are prepared in a specifically designed tempered piston cylinder mold. ABS powder in mold was first heated at 115 °C for some time at atmospheric pressure, brought back to 90 °C and compacted at 75 MPa pressure for 15 min. Measurements of density and dc electrical conductivity of ABS pellet at normal pressure are 0.9583 g cm⁻³ and 1.15 × 10⁻¹⁴ S cm⁻¹ respectively.¹

Sample preparation

The preparation methods of mixing of CNTs into molten polymer matrix/polymer solution succeeded by machine-driven mixing and passing through chemical reaction between modified CNTs and polymer matrix suffer with the disadvantages like poor dispersion caused by viscosity of polymer melts and re-clustering of the CNT during the drying process has been recognized.¹ For that reason, ABS nanocomposites are developed here by way of uniform dispersion of MWCNT in ABS powder under well-controlled processing conditions using earlier approving.^7,15 Tumble mixing of MWCNTs and ABS powders for 200 min initially thru low speed succeeded by high speed, opens the entangled MWCNT agglomerates and subsequently disperses uniformly in the ABS matrix. Agglomerates of MWCNTs disintegrate due to collisions with the stirrer and the tumbler walls. Implanting/coating of the MWCNTs onto the ABS particles by high clang forces and the frictional heat are logical. Hot compaction of resultant mixture in solid state at 90 °C and 75 Mpa for 15 min localize the conductive MWCNTs at interfacial places between ABS particles in absence of any shear create a network pattern of conductive paths. A series nanocomposite pellets of X band waveguide shape possessing 0–5 wt% of MWCNT in ABS are prepared. Three pellets of each composition are made ready. The thicknesses of the pellets are between 2 and 3 mm. Measurements of dc conductivity, hardness and density of pellets are performed using Kelvin's four wires connection system, D shore hardness tester and by considering ratio of specimen's mass to volume respectively. Insignificant deviation in the values of conductivity of three pellets of each composition supports the uniform dispersion of MWCNTs into the ABS matrix.

Results and discussion

MWCNTs/ABS nanocomposites for various compositions are characterized for density, hardness and dc conductivity.¹ The conductivity of a polymer nanocomposite depends on various parameters like type of host polymer matrix, nanofiller properties such as aspect-ratio, specific surface area, surface conductivity besides nanofiller dispersion and interactions with the host matrix. During tumble mixing, dispersed MWCNTs obviously get coated on to ABS particles has been identified with field emission scanning electron microscopy (FESEM). Micrograph in Fig. 1(a) illustrates a lump consisted of powder ABS particles evenly coated with MWCNTs. van der Waals interactions between MWCNTs coated ABS particles has made lumps. Hot compaction deforms ABS particles and results in a formation of compacted continuous polymer phase, where conductive paths of MWCNTs are located at the interfacial places on the boundaries between ABS particles, accountable for network connectivity. Fig. 1(b)–(d) near percolation threshold demonstrates the FESEM images of ABS/MWCNT pellets samples, prepared by fracture in liquid nitrogen. Fig. 1(b) inset, illustrates the particle structure of ABS matrix covered by MWCNTs. The presence of MWCNTs with broken links in Fig. 1(b) describes the pattern prior to percolation. MWCNTs continuity in Fig. 1(c) gives an evidence for abrupt increase in dc conductivity (σ_dc) by ten orders of level from 1.15 × 10⁻¹⁴ S cm⁻¹ to 1.24 × 10⁻⁴ S cm⁻¹ and the identified main increase of seven orders in σ_dc (2.29 × 10⁻⁷ S cm⁻¹) take place on addition of 0.1 wt% MWCNTs under the semiconducting range is decisive for percolation threshold.¹ Such a small value of the percolation threshold can be endorsed to good dispersion of high aspect ratio MWCNTs in the nanocomposites in addition to creation of a meshwork of conductive passages and quantum mechanical tunneling of electrons therein.

Fig. 1 FESEM micrographs (a) lump of powder ABS particles coated with MWCNTs, scale bar 2 μm; MWCNTs in ABS interfacial places (b) 0.05 wt%, (c) 0.1 wt%, (d) 0.2 wt%, scale bar is 500 nm; inset (b) scale bar 2 μm for broader view.

Composites with pre-localized networked of MWCNT are acknowledged better to dispersed MWCNTs (solution polymer mixing) in relation to lower electrical conductivity and higher compressive strength.¹⁵ The locations of conductive MWCNTs in the insulating ABS matrix would obviously add toward improvement of electrical and dielectric properties of the composites. Thus the consideration of complex permittivity (ε*) would be of great importance and relevance.

ε* = ε′ − jε′′ (1)

ε′ and ε′′ are the real and imaginary parts. The values for dielectric constant (ε′), dissipation factor (tan δ) and loss factor (ε′′) for MWCNT/ABS pellets are evaluated using Nicholson–Ross–Weir method³⁶ from S-parameters measurements using vector network analyzer.

Fig. 2(a)–(c) are displaying ε′, tan δ and ε′′ variations with frequency for various ABS nanocomposites pellets with 0.0–5.0 wt% MWCNTs. Up to 1.0 wt% MWCNTs in ABS, ε′ increases with increasing MWCNTs, yet close to matrix ABS and almost independent of frequency. The value of ε′ is primarily based upon the effects of interfacial orientation, atomic and electronic polarizations in the nanocomposite. The orientation and interfacial polarizations depend on the content of filler. Probably, the MWCNTs have brought about an increment in ε′ value by increasing the MWCNTs loading. It is observed that for >1 wt% MWCNTs loading, the value of ε′ gives maximum values at higher frequencies which can be explained by bearing in mind that MWCNT/ABS nanocomposites are made of number of micro-capacitors formed by nanofillers/aggregates and the polarization centers created from the defects in the nanofiller structure²⁷ contribute towards rise in ε′ with increasing frequency. This is anticipated to be as a result of effect of orientation and polarization in network pattern of conductive paths.

Fig. 2 Frequency dependencies for the MWCNT/ABS nanocomposites with different MWNT content for dielectric constant (ε′); (b) dissipation factor (tan δ); (c) loss factor (ε′′).

The dissipation factor tan δ as a function of frequency at various MWCNTs loadings is shown in Fig. 2(c), tan δ registers decrease by increasing the frequency for >1 wt% MWCNTs loading. It has been noticed that increasing the MWCNTs in the low frequency region represents a significant variation, it is minimum for ABS and maximum for 5 wt% MWCNTs nanocomposite. In the higher frequency range, tan δ curves come nearer even crossover the lower value curve. Increasing MWCNTs content produce an increase in orientation polarization that results in an improvement in the dissipater. Dissipation factor tan δ is described as tan δ = ε′′/ε, where ε′′ is the dielectric loss. Loss factor ε′′ is the average power factor over a given period of time describes the losses in transmission and distribution. The variation of ε′′ for the composites is shown in Fig. 2(c). It can be observed that ε′′ is low in lower frequency region and is found maximum for the composite with MWCNTs 5 wt%. By increasing frequency, the value of ε′′ decreases.

It is interesting to note, ac conductivity (σ_ac) for all MWCNTs/ABS nanocomposites is almost independent of frequency. σ_ac as function of frequency for these nanocomposites have been calculated through ε′′ using relation.

σ_ac = ωε₀ε′′ (2)

ω and ε₀ are stand for angular frequency and permittivity of free space (8.85 × 10⁻¹² F m⁻¹). The behavior of σ_ac in X band is displayed in Fig. 3.

Fig. 3 σ_ac variation with frequency in X band for various MWCNTs contents in ABS.

At a certain frequency σ_ac rises with the increase of MWCNTs content. Variation of σ_ac with frequency for pristine ABS and other MWCNT/ABS nanocomposites have displayed curves similarities.

At 10 GHz representative frequency, behavior of σ_ac for MWCNTs/ABS nanocomposites is made known in Fig. 4. Qualitative variation σ_ac is similar to σ_dc.¹ However σ_ac values are all the way higher than σ_dc. This can be explained yet again by above consideration that MWCNTs/ABS nanocomposites are made of number of micro-capacitors formed by nanofillers/aggregates and the polarization centers created from the defects in the nanofiller structure.²⁷ Thus, MWCNTs/ABS nanocomposite is a heterogeneous system consists of microstructure resistor-capacitor networks that contain both dielectric (capacitor) and conductive (resistor) regions. At higher frequency (10 GHz) the σ_ac of the micro-capacitors admittance would be more than the resistors conductivity for all compositions of MWCNT/ABS. For ABS nanocomposites with small content of MWCNTs, hopping/tunneling of electrons contributes towards the σ_dc. Nevertheless, at higher contents of MWCNTs, some extra conductive networks are put up within the ABS matrix and the σ_dc proceeds to σ_ac with increasing MWCNTs.

Fig. 4 Conductivity behavior of MWCNT/ABS nanocomposites with rise in MWCNTs content at 10 GHz.

Theory

The σ_ac behavior MWCNT/ABS nanocomposites above percolation can be explained by the power-law model. The equation is analogous to the σ_dc used earlier.¹ Inset Fig. 4 recognizes a best linear fit meant for ac value of critical percolation for MWCNTs 0.00051 vol frac with critical index 1.50 ± 0.06. This is as good as dc critical percolation value of 0.00045 vol frac with critical index 1.93 ± 0.05.¹ The theoretical value of critical index obtained by Kirkpatrick³⁷ is in excellent agreement to the σ_ac. Above critical volume fraction, a good quantitative consensus between the ac evaluates and power law theory is witnessed. General effective media (GEM) theory³⁸ can likewise be explained in terms of σ_ac for MWCNTs/ABS nanocomposites, illustrated in Fig. 4. The added advantage of GEM equation is capable of being applied for all volume fractions of MWCNTs inclusive of near insulator–conductor changeover as in case of percolation theory. Assessed parameters show that GEM theory can satisfactorily define the σ_ac behavior. Qualitatively and quantitatively decent accord between investigation and theory has been observed for all MWCNT/ABS nanocomposites under consideration.

Shielding effectiveness [SE]

Shielding effectiveness [SE] is an effective value of shielding barrier put up to attenuate radiated or conducted electromagnetic energy. SE and Return Loss (RL) are the straightforward basics for analysis of EM shielding which can be achieved through S11/S22 and S12/S21 scattering parameters using two ports of vector network analyzer. Fig. 5 demonstrates the total SE and RL values of ABS nanocomposites determined for 0.0–5.0 wt% MWCNTs in 8–12 GHz frequency range.

Fig. 5 SE and RL of ABS nanocomposites as a function of frequency at different MWCNT contents.

EM radiations can pass through neat ABS. Its total SE is almost zero. SE increases with increase of MWCNTs content in ABS nanocomposites. Nanocomposite containing a 0.05 wt% of MWCNTs produces SE ∼ 1 dB at 12 GHz which raises to ∼40 dB on addition 5 wt% MWCNTs. Most of the compositions showed minimal dependency of SE on frequency. For nanocomposite with 5 wt% MWCNTs the highest and smallest SE values are 40.2 dB at 12 GHz and 38.2 dB at 8.5 GHz respectively. The uncertainties in SE measurements are ±0.07 to 1 dB using VNA uncertainty calculator. The RL values are observed to be cumulative with frequency for all compositions of MWCNT/ABS. It is noticeable that the nanocomposites having a high value of SE showed lower value of RL. This implies that lower the value of RL higher is the reflection owing to larger value of conductivity. For higher loading of MWCNTs, RL is smaller. For MWCNT/ABS nanocomposites with 5 wt% and 0.05 wt% MWCNTs the RL values are ∼1 dB and ∼6 dB respectively at 12 GHz.

Analysis of SE

SE is a measure of attenuation of EM signal predominantly through reflections and absorption after a shield is introduced. Consequently, SE = SE_ref + SE_abs. SE_ref can be evaluated³⁹ using equation SE_ref = −10 log₁₀(1 − R). Reflectance R is the ratio of reflected power density (P_r) to incident power density (P_i). In case of normal incidence, R = P_r/P_i = Anti log₁₀(−RL/10). Fig. 6 show the input of reflection (SE_ref) and absorption (SE_abs) to the total SE of ABS nanocomposites for various contents of MWCNTs. Both SE_ref and SE_abs remain almost independent of frequency in case of samples with less than 1 wt% MWCNTs, while for the samples with more than 1 wt% MWCNTs, the SE_ref decreases and SE_abs increases with increase in frequency in accordance with the theoretical considerations. Furthermore this can be explained by considering the speculation and behavior of ε′ and ε′′ with frequency in Fig. 2(a) and (c). At a certain frequency both SE_ref and SE_abs are growing with rise in MWCNT contents. However increase in SE_abs is faster than SE_ref.

Fig. 6 SE_ref and SE_abs for MWCNT/ABS nanocomposites as a function of frequency.

Evaluated values for R, and A (absorbance) for various compositions of MWCNT/ABS nanocomposites in X-band frequency range using above known relations are summarized in Table 1. R is rising with the increase of MWCNTs since more EM waves meet with more number of MWCNTs. The increase in R with MWCNTs is due to the increase in dielectric constant.⁴⁰ The positions of MWCNTs within the ABS matrix are thought to be responsible for the magnitude of R and A. Reflectance R takes place before absorbance A. Thus there is a lower amount of the radiation left for A. The proportion of the power truly absorbed by the materials reaches a maximum before decreasing at high electrical conductivity. It actually never surpasses 45.7% for sample with a thickness of 1.28 mm at 12 GHz. Although the absorption capability is improved, there is a lesser part of the radiation left for absorption after the extensive reflection linked to the high conductivity.

Table 1 Evaluated values for R, and A for several compositions of MWCNT/ABS nanocomposites in 8–12 GHz frequency range

MWCNT wt%	0.1%			0.2%			0.5%			1%			3%			5%
Freq (GHz)	R%	A%	Aeff%	R%	A%	Aeff%	R%	A%	Aeff%	R%	A%	Aeff%	R%	A%	Aeff%	R%	A%	Aeff%
8.0	27.0	2.3	3.2	27.9	11.2	15.5	28.2	29.5	41.0	45.6	41.5	76.3	75.0	24.7	98.84	81.3	18.7	99.92
8.5	25.1	2.1	2.9	25.8	10.6	14.3	26.6	28.7	39.0	43.1	42.4	74.7	72.1	27.5	98.87	79.4	20.6	99.93
9.0	23.9	2.1	2.7	24.5	10.3	13.6	25.5	28.1	37.7	41.3	43.2	73.5	69.6	30.0	98.92	77.7	22.3	99.93
9.5	23.0	2.1	2.9	23.4	10.1	13.2	24.8	27.5	36.6	39.7	43.8	72.7	67.5	32.2	98.98	76.3	23.7	99.94
10.0	22.3	2.3	2.9	22.5	10.0	12.9	24.3	27.1	35.8	38.5	44.3	72.0	65.7	34.0	99.03	75.1	24.9	99.95
10.5	21.9	2.1	2.7	21.8	9.6	12.3	24.0	26.6	35.0	37.6	44.6	71.4	64.3	35.4	99.09	74.2	25.7	99.95
11.0	21.4	2.1	2.7	21.1	9.7	12.3	23.7	26.4	34.7	36.7	44.9	71.0	63.0	36.7	99.14	73.3	26.6	99.96
11.5	20.8	2.2	2.8	20.1	9.7	12.1	23.4	26.3	34.3	35.9	45.4	70.8	61.7	38.0	99.20	72.6	27.4	99.96
12.0	20.3	2.3	2.9	19.1	9.3	11.5	23.2	25.8	33.6	35.1	45.7	70.5	60.7	39.0	99.24	71.8	28.2	99.97
Thickness (mm)	2.57			2.09			2.38			1.28			2.52			2.80

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SE_abs can be expressed³⁹ by a relation, SE_abs = −10 log₁₀(1 − A_eff) = −10 log₁₀{T/(1 − R)}. A_eff and T are the effective absorption and transmittance respectively. A_eff can be evaluated using this relation. Table 2 displays the values of A_eff at 9 GHz demonstrative frequency for MWCNT/ABS (this work) which rises with increasing content of MWCNT together with dominance in contrast to R. This trend is identical to behavior reported earlier for MWCNT/polystyrene (PS)¹⁵ (Table 2). A_eff is 99.9% compared to 77.7% of R on 5 wt% of MWCNTs in ABS. MWCNT/ABS is more absorptive than the MWCNT/PS for all equivalent contents of MWCNTs, comparison of A_eff reveals.

Table 2 Calculated values of ‘R’, ‘A’, ‘A_eff’ and ‘T’ at 9 GHz illustrative frequency

MWCNT wt%	MWCNT/ABS				MWCNT/PS (ref. 15)
	R%	A%	Aeff%	T%	R%	A%	Aeff%	T%
0	21.6	0.5	0.68	77.9	—	—	—	—
0.05	23.1	1.2	1.6	75.6	—	—	—	—
0.1	23.9	2.1	2.7	74.0	22.9	18.1	23.4	59
0.2	24.5	10.3	13.6	65.2	29.9	26.4	37.6	43.7
0.5	25.5	28.1	37.7	46.4	47.0	30.7	57.9	22.3
0.8	39.0	41.6	68.3	19.3	—	—	—	—
1	41.2	43.2	73.5	15.5	39.1	31.9	52.3	29.0
1.2	47.2	39.2	74.3	13.6	—	—	—	—
2	63.9	34.0	94.2	2.1	—	—	—	—
3	69.6	30.0	98.9	0.3	70.8	26.5	90.6	2.8
5	77.7	22.3	99.9	0.01	78.1	21.3	97.4	0.6

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SE performance of MWCNT/ABS nanocomposites

Genuine comparison of EMI SE of polymer composites and evaluation of their performance based on data available in literature would not be precise because of dissimilar processing and material parameters. Graphite (Gr)/ABS composite⁷ and MWCNT/PS nanocomposite¹⁵ are selected here for this purpose since these were prepared under exact identical conditions. Same tumble mixing process and hot compression molding with identical parameters were used for fabrication of specimens. Complete statistics about the exact characteristics of the fillers and polymer used in the study can be found in their Experimental section. Fig. 7 presents a depiction related to influence of filler (Gr) and nanofiller (MWCNT) contents on SE of ABS and PS polymers at 9 GHz.

Fig. 7 Influence of filler Gr and nanofiller MWCNT contents on SE of ABS and PS at 9 GHz.

At lower contents of MWCNTs, SE of PS nanocomposites are more than the ABS nanocomposites. With more addition of MWCNTs, there is a net rise of SE in both. MWCNT/ABS demonstrates progressively improved SE and excels over the MWCNT/PS. At 5 wt% MWCNTs, SE for ABS nanocomposite grows up to ∼38 dB compared 22 dB of PS nanocomposite. Effect of use of filler Gr in place of MWCNT⁷ on SE can also be noticed in Gr/ABS composites; SE is all the way less than PS and ABS nanocomposites.

Conductivity debate on SE

The ability of the fillers to deliver electrical conductivities to the polymer is of vital importance. For reason of analysis of SE properties of polymer composites in relation to conductivity would decidedly be enlightening. The variation of the SE as a function of the conductivity is being considered here for polymer composites. The conductivity behavior has made known that increasing concentration of MWCNTs reduce the distances between MWCNTs in ABS matrix. As a result, more EM waves come across with additional MWCNTs, and hence MWCNT rich parts attenuate more radiation as compared with ABS occupied areas, generating an upsurge in SE with increasing MWCNTs content. The growing MWCNTs concentration results an increase in σ_dc of MWCNT/ABS nanocomposites. Fig. 8 displays the behavior of SE of MWCNT/ABS (this work), MWCNT/PS¹⁵ and graphite/PS⁷ nanocomposites as a function of σ_dc in X-band at 9 GHz representative frequency.

Fig. 8 SE of MWCNT/ABS, MWCNT/PS and Gr/ABS composites as a function of σ_dc at 9 GHz.

As shown in Fig. 8, SE increases continually with the dc conductivity for MWCNT/ABS. There are two regimes: first, just lower than the percolation threshold, the SE rises faintly with increase in MWCNTs since in this arrangement a small change in filler concentration abruptly raises the σ_dc while the influence on the SE rests minimal. On inclusion of 0.2 wt% MWCNTs, the σ_dc of ABS nanocomposite increases suddenly by ten orders of magnitude in the semiconducting range. The major rise of seven orders in σ_dc on addition of 0.1 wt% MWCNTs yields 1.3 dB SE only. Further increase of 0.5 wt% MWCNTs to ABS enhances σ_dc to 0.001 S cm⁻¹, and builds SE to 3.3 dB. In the second regime after the percolation threshold, though σ_dc does not increase significantly with the addition of MWCNTs, however SE value boosts to 38.3 dB. Additional MWCNTs from 0.5 wt% to 5 wt% flatten the value of σ_dc from 1 × 10⁻³ S cm⁻¹ to 44 × 10⁻³ S cm⁻¹, yet a small change in σ_dc causing significant increase in SE whereas in first regime a major increase in σ_dc results a small improvement in the value of SE. Almost similar SE trend has been witnessed for MWCNT/PS nanocomposite¹⁵ and graphite/ABS⁷ composite at 9 GHz shown in Fig. 8. Thus the behavior of σ_dc is not the only conclusive aspect for controlling the degree of SE. Some additional factors like low percolation threshold conceivably significant. For achievement of filler's low percolation threshold, large aspect ratio is desirable. Development of close packed conductive linkages in the polymer composites⁸ which increase with the rise in filler content has also been stated for improvement of SE. The conductive network formed owing to adding of conductive filler that interacts with incident radiation likewise be an origin for SE. Addition of filler narrowly packs the conductive network. Therefore their capability to attenuate more EM radiation increases the SE. In this work although the accumulation of MWCNTs from 0.5 to 5 wt% in ABS nanocomposites provide a negligible rise in σ_dc, but the increase in MWCNTs provide a close packed conductive network. It is recognized earlier¹ in such nanocomposites with high MWCNTs content (above percolation) there is making of mesh of conductive network that provide an additional attenuation of EM radiation, like Faraday cage consists of mesh of conducting material which has the capability to stop external electric fields. Meaningful to mention that the conducting mechanism for conductivity needs connectivity between conducting paths, whereas shielding does not.¹²

SE of metals is because of reflection for the reason of conductivity. Insulating polymers are almost transparent to incident radiation. SE of conductive polymer composite is essentially attribute to reflection and absorption both. Therefore it is necessary to probe the behavior of SE_ref and SE_abs of polymer composites as well to evaluate SE_abs overriding contribution to SE.

Dominance of SE_abs

Fig. 9(a)–(d) illustrate performance of evaluated SE_ref and SE_abs in total SE as function of frequency for each loading of MWCNTs from 0.05 wt% to 0.2 wt% in MWCNT/ABS nanocomposites. SE and SE_ref both are decreasing with increasing frequency in Fig. 9(a)–(c) while SE_abs remains almost unchanged. Total SE is growing with increasing content of MWCNTs and SE_ref > SE_abs. For MWCNTs 0.5 wt%, SE_abs starts dominating with a crossover to SE_ref [Fig. 9(d)].

Fig. 9 SE_ref and SE_abs in SE for MWCNTs (a) 0.05 wt%; (b) 0.1 wt%; (c) 0.2 wt%; (d) 0.5 wt%.

After SE_abs > SE_ref, Fig. 10(e)–(h) illustrate the shielding performance of MWCNTs (e) 1.0 wt%; (f) 2.0 wt%; (g) 3.0 wt%; (h) 5.0 wt% in ABS nanocomposites. Progress in value of SE_abs is much faster than the SE_ref. In specimen with MWCNTs 0.05 wt%, the value of SE_abs and SE_ref at 12 GHz are 0.11 dB and 0.96 dB improve to 34.69 dB and 5.5 dB at 5 wt% through 1.78 dB and 1.15 dB at 0.5 wt% with crossover (SE_abs > SE_ref).

Fig. 10 Performance after crossover of SE_abs in SE in X band frequency range for MWCNTs (a) 1.0 wt%; (b) 2.0 wt%; (c) 3.0 wt%; (d) 5.0 wt%.

Studies have discovered that both the contributions increase with the conductivity by way of dominance of SE_abs over SE_ref. The increase in SE_abs is larger than SE_ref, which suggests that the input of absorption to the SE increases with the conductivity, since the reflection increases with the filler content along with increase in dielectric constant.⁴⁰ Placement of MWCNTs within the interfacial places surrounded by insulating ABS matrix environment is anticipated to induce dielectric property by causing space charge polarization at the interfaces.⁴¹ MWCNT/ABS nanocomposites exhibited an increase in the ε′ with rise in MWCNT loading shown in Fig. 2(a). For small content of MWCNTs near to percolation threshold, the ε′ have observant values that are basically nearby to host matrix ABS, do not vary with frequency. ε′ exhibits increase with frequency above 1 wt% of MWCNTs. However these trends are more prominent at higher content of MWCNTs. The ε′ of polymer nanocomposites is stated to be a measurement of number of micro-capacitors formed by nanofillers/aggregates and the polarization centers created from the defects in the nanofiller structure.²⁷ These micro-capacitors created by MWCNTs aggregates are anticipated to work as electrodes filled with insulating polymeric material ABS. Therefore, the rise in MWCNT loading in ABS nanocomposites increases ε′ for the reason of increase in the number of these micro-capacitors and structural defects. Moreover, the decrease of gap owing of increasing MWCNTs content thus increases the polarization of the polymeric material by filling the gap between the nanoparticles, thus enhances the absorption loss. A_eff values obtained at 9 GHz for MWCNT/ABS and MWCNT/PS¹⁵ nanocomposites in Table 2 are in accordance to the conjecture.

Conclusion

MWCNT/ABS nanocomposites aimed at localized conductive MWCNTs patterning are fabricated using dry powders tumble mixing and subsequent hot compression technique. Achieved SE ∼40 dB for ABS nanocomposite with 5 wt% MWCNTs (this work) confirm the superiority of networked nanocomposites compared to ∼28 dB of randomly distributed MWCNTs for an equivalent content of 5 wt% in ABS nanocomposite prepared via solution technique.³¹ 0.1 wt% MWCNTs percolation threshold obtained in this work is reasonably smaller compare to documented 0.5 wt% MWCNTs.²⁷ The electrical and dielectric privileged circumstances are in relevance to higher SE. σ_ac in the X-band frequency range has been evaluated thru complex permittivity valuations. Increase in MWCNTs close packs the conductive network and dissipate more mobile charge carriers leading to higher ε′′, result higher σ_ac and consequently the higher EM wave absorption. σ_ac > σ_dc for samples with low MWCNTs where hopping/tunneling of electrons make contribution to the conductivity. σ_dc approaches to σ_ac on additional MWCNTs concentration for reason of more number of conduction pathways raised for reason of close packed network.

Acknowledgements

Scholarly discussion with Prof. Pratap Sigh (HES) DGC, Gurgaon is gratefully acknowledged.

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Footnote

† Miranda House, University of Delhi, Delhi.

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