Composite electromagnetic interference shielding materials for aerospace applications

Composite Structures 91 (2009) 467–472

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Composite electromagnetic interference shielding materials for aerospace applications Christopher J. von Klemperer a,*, Denver Maharaj b a b

Department of Mechanical Engineering, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa School of Mechanical Engineering, University of KwaZulu-Natal, Howard College Campus, Durban 4041, South Africa

a r t i c l e

i n f o

Article history: Available online 8 April 2009 Keywords: Electromagnetic interference Electrical conductivity Carbon fibre laminates Shielding effectiveness

a b s t r a c t Electromagnetic interference (EMI) occurs when electronic devices are subject to electromagnetic radiation from unwanted sources at the same frequency ranges that these devices operate. Metals typically serve as excellent EMI shielding agents, but their heavy weight, high cost and susceptibility to forms of environmental degradation make them an undesired choice for many current electronic devices. Conversely fibre reinforced polymeric (FRP) composite materials are normally light weight, and can be cheaper to produce, but typically lack the inherent EMI shielding capabilities that may be required. This research work addresses the viability FRP composite materials for use as EMI shielding structures, specifically for aerospace applications. It was found that carbon fibre could suffice this purpose, but likely required filler materials to enhance electrical conductivity and shielding effectiveness (SE). Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Aircraft of today are essentially ‘fly-by-wire’ systems, which refers to the use of digital control systems opposed to the use of analogue control systems of older aircraft. Additional to this, modern aircraft contain cockpit automation and digital navigation systems [1]. Due to the high amount of on-board digital equipment, the concern regarding prevention of electromagnetic disturbances is not unfounded. Despite the aerospace industry continually striving for higherperformance vehicles, comparatively little research has been undertaken regarding new electromagnetic shielding options in the past ten years [2]. Fibre composite materials have, however, been identified in recent years as being the desired choice for the replacement of orthodox metallic aluminium and ferrous alloys in this regard. One of the chief obstacles preventing a substitution is the inherent lack of electromagnetic shielding capabilities possessed in fibrous composite materials. EMI in aircraft may be classified into three sub-classes, which are: on-board systems, passenger carry-on devices, and externally generated EMI [1]. When on-board systems interfere with each other, it is referred to as electromagnetic compatibility [1]. Passenger carry-on devices include portable electronic devices that can transmit and receive frequencies, compact disc players and computers. Anyone who has flown on-board an aircraft has been cautioned against use of these devices during flight. Externallygenerated EMI refers to disturbances caused by external sources * Corresponding author. Tel.: +27 21 650 5762; fax: +27 21 650 3240. E-mail address: [email protected] (C.J. von Klemperer). 0263-8223/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2009.04.013

such as lightning strike, high-intensity radiated fields (HIRF) and electromagnetic pulses (EMPs) [1]. There are three shielding mechanisms that could result in attenuation of EMI viz. reflection (R), absorption (A), and multiple reflections (B). The primary mechanism for shielding in highly electrically conductive structures, such as metals, is reflection. Reflection relies on mobile charge carriers, such as electrons, being present within the material. Therefore, the shielding material tends to be electrically conductive, although this is not an essential requirement for shielding [3]. Electrical conduction requires connectivity in the conduction path, whereas shielding does not [3]. Thus, high electrical conductivity is not typically a requirement for shielding, but shielding was found to be enhanced by connectivity [4]. The materials could likely be used on exterior surfaces of aircraft and could be exposed to lightning strikes. It was therefore essential that materials with high conductivity be investigated as well. Reflection loss is a function of rr/lr, where rr refers to the conductivity of a material relative to copper, and lr refers to the permeability of a material relative to copper [3,4]. The secondary mechanism for shielding in these structures is absorption. Significant absorption of the waves by the shield requires electric and/or magnetic dipoles within the shield material. Absorption loss is proportional to shield thickness and is a function of rr  lr. Clearly for aerospace applications increasing the thickness is not a preferred approach. Absorption increases with increasing frequency, whereas reflection tends to decrease with an increase in the frequency [3,4]. The third mechanism for shielding is multiple reflections. Multiple wave reflections take place at surfaces or interfaces within the shield. This mechanism requires the presence of large surface areas

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or interfaces within the shield [3,4]. Thus, it is primarily a property of the part geometry rather than the material. When defining the performance of a shielding material, the term often quoted is shielding effectiveness (SE). The value, obtained in decibels, provides an indication of the quality of shielding a material possesses. The frequency range in this work ranged from 800 MHz to 5 GHz. This is referred to as being ‘‘far-field”, or ‘‘plane waves”, measurements [5]. The criteria for these measurements are ZW = 377 X and r > k=2p, where ZW is the free-space impedance, r = distance from the source (m), k ¼ wavelength (m). A common approach to finding total SE can be obtained by summing the individual contributions of reflection (R), absorption (A) and multiple reflections (B) in dB [5].

SEdB ¼ RdB þ AdB þ BdB

ð1Þ

where : RdB ¼ 108:1  10log10 ðlr fMHz =rr Þ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi AdB ¼ 1314:3tcm fMHz rr lr  pffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffi  BdB ¼ 20log10 1  e2t pf lr ej2t pf lr

ð2Þ ð3Þ ð4Þ

Eqs. (1)–(4) were originally used for computations of metal shielding materials, but researchers have used them for composite materials as well. However, inspection of Eq. (3) shows that a high absorption (AdB) value will result, as it is dependent on the frequency. This is found to be doubtful, as highly conductive materials such as aluminium sheet material should display a very low AdB value. However, Das et al. have used a form of Eq. (3), but the result was reduced by a factor of 103, which appears to be more likely [6]. Thus the preferred equation for AdB is:

AdB ¼ 1:32t

qffiffiffiffiffiffiffiffiffiffiffiffi f rr lr

ð5Þ

Researchers have used other approximations for composite material SE in recent years. Colaneri and Shaklette have developed an equation for electrically thin samples (thickness  skin depth) for far-field calculations [7]:

  Z 0 rd SE ¼ 20 log 1 þ ; 2

ð6Þ

where Z0 = 377 X, r = electrical conductivity (X/m), d = shielding material thickness (m). Yang et al. [3] have used the ‘‘Simon Formalism”, and the ‘‘Classical Good Conductor Approximation” for their SE calculations. The respective formulae are: 1

SE ¼ 50 þ 10log10 ðqf Þ1 þ 1:7tðf =qÞ2

ð7Þ

RA where : q ¼ volume resistivity ðX=cmÞ l

ð8Þ

R = volume resistance (X), A = area (cm2), l = length (cm), f = frequency (MHz), t = shielding material thickness (cm) and,

 SE ¼ 10 log



r d þ 20 logðeÞ d 32pf e0

ð9Þ

where r = bulk conductivity (X/cm), f = frequency (MHz), e0 ¼ 8:854  1014 F/cm, d = shielding material thickness (cm), d = skin qffiffiffiffiffiffiffiffiffiffi depth l 1pf r (cm), l ¼ 4p  109 {H}/{cm}, e = 2.718281828. 0

However, it was found that there are large discrepancies between the results obtained from these various analytical equations. This has also been well-documented by previous researchers [8–10]. These discrepancies have been reported as being quite substantial, and in some cases have been greater than 100 dB, especially around jointed surfaces [8]. The accurate computation of SE by use of theoretical equations is thus regarded as being a problem.

2. Experimental work 2.1. Materials Unidirectional carbon; 12 K woven carbon; ±45° stitched carbon; and 80/20 unidirectional E-glass fabrics were obtained for use as structural materials. LR20 resin, with LH281 hardener, and Prime 27 resin, with Prime 20 slow hardener, was obtained for use as matrix materials. Aluminium and copper powders, and Alumesh 401 metal fabrics was obtained for use as filler materials. 2.2. Composite manufacture Initial tests were performed on 80/20 unidirectional E-glass fabric. This material was not considered as an option for actual shielding material, but was used to test repeatable manufacturing processes for the carbon fibre fabrics. Hand lay-up and vacuum-assisted resin transfer moulding (VARTM) were used as the manufacturing processes in this work. Tests were later performed on the carbon fabrics as well, and it was found that hand lay-up manufacture using the materials with LR20, with LH281 hardener, yielded the most consistent results. This manufacturing process and matrix material was thus used for the production of the shielding materials tested in this work. It was decided that composite laminates with increasing quantities of metallic filler powders, and increasing quantities of Alumesh should be fabricated and investigated to determine the influence that the metal has on shielding. The focus was not to determine the exact percolation threshold of the powder in the composite, but rather to ascertain if a general trend with increasing filler loading was observed in mechanical, electrical and SE properties in the composites. It was decided that the aluminium powder; copper powder; and a mixture of both powders in equal quantities by weight; should be introduced into the matrix materials in 7.5% and 15% filler loadings. It was chosen that 15% by weight would be the maximum upper limit for these discontinuous fillers. It was decided that one and two layers of metal fabrics would be investigated, to determine if increasing the metal content in this way has an influence on SE. The samples with the metal mesh contained a layer of unidirectional E-glass between the Aluminium mesh and the carbon fabric to prevent possible galvanic corrosion. The composites were manufactured in sheets with an approximate thickness of 2.5 mm, post-cured for 16hrs at 65  C, and then machined to the dimensions of the test specimens. 2.3. Testing 2.3.1. Mechanical testing Tensile and flexural testing was undertaken using an Instron 5500R universal testing machine. The tests were performed in accordance with ASTM 638-02a and ASTM 69272-02, respectively. Compressive testing was outsourced and performed using a Lloyds universal testing machine. That test was performed in accordance with ASTM 695-02a. 2.3.2. Electrical resistance testing Bulk electrical resistance measurements were made using a Philips PM6303 RCL universal bridge circuit. The method was in accordance with ASTM B 193-02, and similar work has been done by Xu et al. [11].The value obtained was in X, and Eq. (8) was used to obtained bulk resistivity. The electrical conductivities were obtained by use of:

r¼

1

q

ð10Þ

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Fig. 1. Schematic diagram of S-parameters of a two-ports network system [12].

A copper plate of the same size was measured on the instrument and relative conductivity was obtained by use of:

rr ¼

r rC

ð11Þ

where rC = electrical conductivity of copper (X/cm). 2.3.3. Shielding effectiveness testing Far-field SE measurements were performed at the University of Pretoria using a Scientific Atlanta compact range, which contains a Hewlett Packer 8510C network analyzer. Testing equipment of this type is referred to as a ‘‘two-port network system”, and variables referred to as ‘‘scattering parameters” (S-parameters) are obtained from testing [12]. The test method was suggested by Odendaal [13], as the frequency range in question in this work was above that described in ASTM 4935-99. The sample sizes were 300 mm squares and were measured relative to a 2 mm aluminium plate (see Fig. 1). The S12(S21) and S11(S22) parameters refer to the transmission and reflection coefficients, respectively [12]. The analysis of S-parameters yield:

  ET  T ¼   ¼ jS12 j2 ¼ jS21 j2 EI  0 2 EI  R ¼   ¼ jS11 j2 ¼ jS22 j2 EI AþRþT ¼1

ð12Þ ð13Þ ð14Þ

The test for S12(S21) parameters yield a transmission value in dB, which can be used to determine EMSE by use of the following equations [12]: T dB

T R ¼ 10 20 1 SE ¼ log TR

ð15Þ ð16Þ

where TR = linear transmission value. Percentage penetration through the shielding material is given by [13]: T dB

% Penetration ¼ 10 20  100

ð17Þ

3. Results and discussion 3.1. Mechanical results Tensile testing showed that the stitched carbon fibre laminates possessed surprisingly low strength and elongation properties. The range of strength was between 54 MPa for samples with 7.5% of aluminium and copper powder to 82 MPa for samples two metal

469

mesh layers. Failure was characterised by separation of the fibres at the midsection of the gauge length. All of these samples deflected excessively during flexural testing, and thus the bending strength was indeterminate. The samples had elastic moduli between 7 GPa for samples with 15% copper powder to 14 GPa for samples without filler, which is unsurprising given the level of flexibility they displayed. The compressive strength ranged from 63–96 MPa for samples with 7.5% aluminium and copper powder to that which had 7.5% copper powder. Failure was characterised by separation of the fibres at the midsection of the gauge length. The poor properties displayed by these composites were thus indicative of the stitched carbon fabric itself, as the results were all in accordance. The results for the woven and unidirectional samples were far more promising. The tensile strength of the woven samples ranged between 820 MPa for samples without filler to 595 MPa for samples with 15% aluminium and copper powder. Failure was characterised by complete fracture of the material in the gauge section. The bending strength of the woven samples was found to increase with increasing filler loading. The samples without filler thus displayed the lowest strength. However, the elastic modulus was found to decrease quite significantly, from 73 GPa without filler to as low as 45 GPa with 7.5% copper powder. Failure was characterised by snapping of the outer fibres at the midsection of the gauge length. The compressive strength results were dubious, as there was evidence of crushing on the end surfaces. There was no visible failure on the gauge length of the specimens, and the results indicate far lower compressive strengths than tensile strengths. The tensile strength displayed by the unidirectional samples was the highest, with the sample without filler having a strength of 841 MPa. However, tensile strength increased with increasing loadings of copper and aluminium powder mixture, and was 904 MPa at 15% loading. Strength generally decreased with inclusion of the other fillers. Failure was generally characterised by snapping of the fibres across the outer width of gauge length. Bending strength did not significantly change in the samples with discontinuous fillers, and was in the range of 1 GPa. However, bending stress decreased greatly with increasing mesh content, and was 326 MPa in the samples with two metal mesh layers. The samples without filler displayed a high elastic modulus of 180 GPa, and it generally decreased with increasing filler loading. Failure in these samples was characterised by a loud noise, but only slight fracture across the width of the gauge length was visible. Compressive strength was also dubious in these samples, as deformation of the end surfaces was visible, and there was no visible damage on the specimen length. Again, these values indicate compressive strength was far less than tensile strength. 3.2. Electrical resistance results The results from electrical resistance testing are shown in Table 1. It can be seen that electrical resistance increased with inclusion of fillers in some cases. The electrical properties of composite materials, such as unidirectional carbon laminates are highly dependent on their fibre orientation. The electrical resistances of these laminates were measured perpendicular to the fibre direction, and the results unsurprisingly showed that resistance was higher in the order of 10. Thus, the disordered arrangement of conductive particles, which were lower in resistance to the fibres themselves, could have led to discrepancies in the bulk resistance measurements. However, the unidirectional carbon laminates were found to display the best results from the carbon materials tested in this work. From the results obtained, it can be seen that these materials

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Table 1 Electrical properties of the materials. Material*

Measured resistance (X)

Resistivity (X/cm)

Conductivity (X/cm)

Relative conductivity (to copper plate)

Copper plate Aluminium plate Stitched CF NF Stitched CF 7.5%Al Stitched CF 15%Al Stitched CF 7.5%Cu Stitched CF 15%Cu Stitched CF 7.5%H Stitched CF 15%H Stitched CF 1ply Stitched CF 2ply Woven CF NF Woven CF 7.5%Al Woven CF 15%Al Woven CF 7.5%Cu Woven CF 15%Cu Woven CF 7.5%H Woven CF 15%H Woven CF 1ply Woven CF 2ply UD CF NF UD CF 7.5%Al UD CF 15%Al UD CF 7.5%Cu UD CF 15%Cu UD CF 7.5%H UD CF 15%H UD CF 1ply UD CF 2ply

0.09 0.10 8.10 7.20 13.90 21.03 23.20 26.30 42.20 0.59 0.75 23.20 50.00 16.64 28.00 64.00 64.00 46.00 0.48 0.69 5.93 10.74 11.02 11.32 14.37 25.80 31.00 0.73 0.29

0.01 0.02 1.85 1.45 2.79 4.29 4.88 6.17 9.70 0.14 0.20 4.80 13.42 4.48 7.18 16.49 16.21 10.90 0.10 0.16 1.24 2.72 2.76 2.64 3.25 6.67 7.13 0.19 0.07

141.10 61.16 0.54 0.69 0.36 0.23 0.21 0.16 0.10 7.36 5.06 0.21 0.07 0.22 0.14 0.06 0.06 0.09 9.96 6.43 0.81 0.37 0.36 0.38 0.31 0.15 0.14 5.38 13.65

1.00000 0.43345 0.00383 0.00490 0.00254 0.00165 0.00145 0.00115 0.00073 0.05216 0.03588 0.00147 0.00053 0.00158 0.00099 0.00043 0.00044 0.00065 0.07055 0.04557 0.00571 0.00261 0.00257 0.00268 0.00218 0.00106 0.00099 0.03814 0.09674

*

Note: CF refers to carbon fibre, UD refers to unidirectional, NF refers to materials with no filler, Al refers to aluminium, Cu refers to copper, and H refers to the mixture of aluminium and copper power.

will suffice shielding application in practice, as it has been suggested that good shielding materials require resistivity’s of 10 X cm and less [14]. Dhawan et al. [15] also report that materials with resistivity’s less than 102 X cm will adequately suffice for the purpose of electromagnetic shielding. From the obtained electrical data, SE was analytically obtained using SE Eqs. (1), (6), (7), and (9) presented earlier. The SE results obtained by use of Eqs. (6), (7), and (9) are similar, but the results obtained by use of Eq. (1) do not correlate with the other SE calculations. It has been mentioned that results obtained via use of the-

oretical equations are known to contain discrepancies when compared with actual results, and thus use of the equations is suitable for approximation only (see Fig. 2). 3.3. Shielding effectiveness results The results obtained from actual SE testing show that these carbon fibre laminates are suitable for shielding purposes. The S12(S21) were obtained by testing, and SE was obtained by use of Eqs. (15) and (16). The laminates without filler displayed inherent shielding

EMSE (analytical) vs. Frequency for unidirectional carbon fibre specimens containing 7.5% aluminium and copper powder 55 50 45

SE -White

EMSE (dB)

SE -Colaneri et al (Far field)

40

SE -Simon Formalism SE -Good conductor approx

35 30 25 20

Frequency (MHz) Fig. 2. EMSE (analytical) vs. frequency for unidirectional carbon fibre specimens containing 7.5% aluminium and copper powder.

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capabilities, and inclusion of fillers enhanced this further. In some tests, the composite samples were found to display better shielding quality than the aluminium plate itself. However, this could be due to diffraction at some frequencies. There were no significant improvements in SE when two metal layers were used instead of one. The minor increase in SE, was not significant enough to incur the costs of using two layers (see Figs. 3 and 4). 3.4. Discussion There is initially a higher cost when using carbon fibre laminates rather than orthodox metals in aircraft. Such costs are further compounded when the recent restrictions placed on sale of carbon fibre is taken into account. However, the long-term cost saving can in actuality provide a far greater return when increasing fuel cost and material life-cycles are considered. Metals are far heavier, and many of the laminates produced in this work were 50% lighter. The stitched carbon samples, however, will be of no use for structural support. The woven material displayed high strength and good shielding properties, but were also the heaviest of the lami-

nates, being 30% lighter than aluminium on average. The unidirectional carbon fibre was on average 45% lighter than aluminium, and displayed far superior strength properties as well. The cost of including metal powders in matrix materials is less than US$0.01/g, and even with 15% powder included by weight, only about 30 g is required to enhance SE in laminates of the size fabricated in this work. The cost of including metal mesh is significantly greater, at about $140.00/m2, but may required in regions prone to lightning strike for the dissipation of higher currents. Samples with discontinuous fillers were viewed under a microscope and it was seen that connectivity between particles is greatly improved between 7.5% filler loading and 15% filler loading. However, there were fewer particles, and thus lesser connectivity between the copper particles opposed to aluminium particles, due to the comparatively heavy weight and small size of them. Greater connectivity was found in aluminium powder and the mixture of aluminium and copper powders in all of the samples. However, this did not adversely affect SE of the copper samples, and many of the samples with copper powder displayed greater SE than the aluminium plate in the frequency range tested.

Shielding Effectiveness relative to Al Plate vs. frequency. 1.7 0% Filler 7.5% Al and Cu

1.6

EMSE Relative to Al plate

15% Al Cu

1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 750.00

1,281.25

1,812.50

2,343.75

2,875.00

3,406.25

3,937.50

4,468.75

5,000.00

Frequency (Hz) Fig. 3. EMSE relative to the aluminium plate vs. frequency for unidirectional carbon fibre specimens with copper and aluminium powder.

Shielding Effectiveness with Al mesh relative to Al Plate vs. frequency. 1.6 0% Filler 1 Mesh layer 2 mesh layer

EMSE Relative to Al plate

1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 750.00

1,281.25

1,812.50

2,343.75

2,875.00

3,406.25

3,937.50

4,468.75

5,000.00

Frequency (Hz) Fig. 4. EMSE relative to the aluminium plate vs. frequency for unidirectional carbon fibre specimens with metal mesh layers.

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4. Conclusion The principal of adding metal powders to carbon fibre reinforced polymer composites as a cost effective method of improving the shielding effectiveness has been demonstrated. Although some analytical models exist to model the shielding effectiveness of composite materials, the high variation between the various models and also the lack of correlation with physical testing means that currently they cannot be used with any confidence. It has also been shown in this work that there was a slight improvement in SE when metal mesh was used compared with materials with the discontinuous fillers, however, not significantly as to justify the use of metal meshes in carbon fibre composites unless the surfaces are exposed to high current such as lightening strikes. Ongoing work is focussing on the impact of these filler materials on the processing of the composite materials. References [1] Shooman ML. A study of occurrence rates of electromagnetic interference (EMI) to aircraft with a focus on HIRF (external) high intensity radiated fields. NASA contractor report 194895, National Aeronautics and Space Administration Langley Research Center, Hampton, Virginia; 1994. [2] Schlechter M. EMI: Materials and technologies – abstract, .

[3] Yang S et al. Electromagnetic interference shielding effectiveness of carbon nanofiber/LCP composites. Compos Part A: Appl Sci Manuf 2005;36:691–7. [4] Chung DDL. Electromagnetic interference shielding effectiveness of carbon materials. Carbon 2001;vol. 39:279–85. [5] White DRJ. Electromagnetic shielding materials and performance. USA: Don White Consultants, Inc.; 1980. [6] Das NC et al. Electromagnetic interference shielding effectiveness of carbon black and carbon filled EVA and NR based composites. Compos Part A: Appl Sci Manuf 2000;31:1069–81. [7] Pomposo JA et al. Polypyrrole-based conducting hot melt adhesives for EMI shielding applications. Synth Metal 1999;104:107–11. [8] Kunkel G. EMI shielding theory; equipment protection magazine fall 2003. USA: Webcom Publishing; 2003. [9] Wojkiewicz JL et al. Electromagnetic shielding properties of polyaniline composites. Synth Metal 2003;135–136:127–8. [10] Colaneri NF, Shacklette LW. IEEE Trans Instr Meas 1992;41:2. [11] Xu J et al. Preparation, electrical and mechanical properties of vapor grown carbon fiber (VGCF)/vinyl ester composites. Compos Part A: Appl Sci Manuf 2004;35:693–701. [12] Hong YK et al. Electromagnetic interference shielding characteristics of fabric complexes coated with conductive polypyrrole and thermally evaporated Ag. Curr Appl Phys 2001;1:439–42. [13] Private Communication – Prof. J.W. Odendaal, University of Pretoria, SA; September 2006. [14] Satheesh Kumar KK et al. Freestanding polyaniline film for the control of electromagnetic radiations. Curr Appl Phys 2005;5:603–8. [15] Dhawan SK et al. Shielding behaviour of conducting polymer-coated fabrics in X-band, W-band and radio frequency range. Synth Metal 2002;129: 261–7.