Supporting Information

A Double-Layered Composite for Lightning Strike Protection via Conductive

and Thermal Protection

Qianshan Xiaa, Zhichun Zhanga, Hao Meia, Yanju Liub, Jinsong Lenga,*

a National Key Laboratory of Science and Technology on Advanced Composites in

Special Environments, No. 2 YiKuang Street, Science Park of Harbin Institute of

Technology (HIT), Harbin, 150080, P.R. Chinab Department of Aerospace Science and Mechanics, No. 92 West DaZhi Street, Harbin

Institute of Technology (HIT), Harbin, 150001, P.R. China

Buckypaper (BP) was prepared by single-wall carbon nanotubes (SWCNTs, TNSR,

Chengdu Organic Chemicals Co., Ltd.) and surfactant (Triton X-100, Aladdin

Chemical Reagent Co., Ltd.) [1]. The size of the as-prepared BP was 370370 mm2

and its areal density was 4 mg/cm2. The as-prepared BP was treated at 350 oC for 2 h,

to remove the residual surfactant. One side of a piece of BP was adhered with a

plastic film completely as a cathode and a graphite plate was placed in electrophoretic

deposition (EPD) bath as an anode. Two electrodes kept parallel and their distance

was around 20 mm. To prepare the electroplating solution, 55 g silver nitrate (AgNO3,

Shanghai Shiyi Chemicals Reagent Co., Ltd.) was dissolved in 5000 mL deionized

water with stirring, then 2.5 g magnesium nitrate (Mg(NO3)2, Damao Chemical

Reagent Co., Ltd.) was added into the AgNO3 solution [2]. After mechanical stirring

for 30 min, the mixed solution was transferred into the EPD bath, and the anode was

overwhelmed by the liquid level. During the EPD process, the applied DC voltage

was 5 V and the deposition time was 240 s. Finally, the as-prepared silver modified

buckypaper (SMBP) was dried and peeled from the plastic film. The size of the

SMBP was about 370370 mm2.

The carbon fiber/phenol-formaldehyde (CF/PF) prepreg was fabricated via the resin

transfer molding (RTM) process, as shown in Figure S1. The carbon fiber woven

fabric (CF, Weihai Guangwei Composites Co., Ltd., W-1021) was cut into 370370

mm2. The CF was placed on the mold surface. Paved flow media on the CF surface

and sealed the mold as a vacuum bag. Moreover, fixed two rubber pipes on the

vacuum bag for exhausting air and importing resin. Connected one pipe to a vacuum

pump and exhausted the residual air in the vacuum bag for 5 min. To inject the PF

solution (Institute of Chemistry, Chinese Academy of Sciences, Content 75 wt%), put

another pipe into a beaker that held some PF solution and kept the pump working.

Figure S1. The schematic diagram of the fabrication process of the CF/PF prepreg

Carbon fiber reinforced polymer (CFRP), (Cu mesh/carbon fiber reinforced polymer)

Cu/CFRP, SMBP/CFRP and SMBP-CF/PF-CFRP test specimens were fabricated by

an autoclave through vacuum hot pressing. CFRP prepregs (Weihai Guangwei

Composites Co., Ltd., T300 125 g) were cut into 370370 mm2. 32 layers of CFRP

prepregs were paved together to form a CFRP laminate and the stacking sequence was

[0°/90°]16S. The CFRP composite was cured in a vacuum bag. Cu mesh (Dexmet

Corporation, 142gsm) and SMBP were placed on the outmost layers of two CFRP

laminates containing 32 layers of prepregs, respectively. In addition, one layer of the

CF/PF prepreg was firstly paved on the CFRP laminate surface which contained 31

layers of prepregs, then the SMBP was paved on the CF/PF surface. Subsequently,

Cu/CFRP, SMBP/CFRP and SMBP-CF/PF-CFRP laminates were also cured under

the same conditions as the CFRP composite. The schematic profile of preform

fabrication of SMBP-CF/PF-CFRP laminate was shown in Figure S2a). Temperature-

time and pressure-time curves used in the hot-pressing process were shown in Figure

S2b).

Figure S2. a) Schematic profile of the preform fabrication and b) temperature and pressure curves used in the curing process

The sample was positioned in the setup before the simulated lightning strike (LS) test

as shown in Figure S3a), and the simulated LS process was shown in Figure S3b) and

Video S1.

Figure S3. a) The sample in the setup before the simulated LS test and b) the composite panel during the simulated LS test

Figure S4 demonstrates the Raman spectrum of the BP for analyzing structural defects

of CNTs caused by the EPD. The ID/IG value of the BP is 0.023.

Figure S4. Raman spectrum of the BP

Silver (Ag) particles cover part of intrinsic pores of CNT networks and generate some

new pores through particles stacking on the BP surface. Thus, it is necessary to

investigate pore features of the SMBP for the preparation of the CFRP matrix

composite. The pore diameter distribution of the SMBP is displayed in Figure S5a).

Its pore volume peak appears at 31.3 nm and range of the pore diameter distribution is

1.8-52.2 nm. According to the International Union of Pure and Applied Chemistry’s

(IUPAC) classification [3], the SMCNP can be considered as a typical mesoporous

material. In addition, the spreading ability of the resin on the SMBP surface could be

influenced by Ag particles adding. Contact angles of epoxy and PF resin droplets on

two sides of SMBP were observed, for evaluating their spreading abilities on SMBP

surfaces. As shown in Figure S5b) and c), contact angles of epoxy droplets on non

EPD and EPD sides of the SMBP at room temperature are 15.9° and 25.3°,

respectively. While, contact angles of PF droplets on non EPD and EPD sides of the

SMBP are 19.8° and 28.6° at room temperature in Figure S5d) and e), which are little

larger than epoxy droplets. It illustrates that both the two kinds of resins can easily

spread on two sides of the SMBP. Furthermore, viscosity of both two resins will

reduce during the preparation processes of CFRP matrix composites, owing to

ambient temperature increasing. It means that SMBPs can be permeated by the rich

epoxy and PF resins to form the composite with stable structure, and its resin

compatibility is slightly affected by the EPD process.

Figure S5. a) The pore size distribution of the SMBP; contact angles of epoxy resin droplets on b) non EPD and c) EPD sides of the SMBP; contact angles of PF resin droplets on d) non EPD and e) EPD sides of the SMBP.

At the lightning attachment point, the huge Joule heat caused by a lightning current of

100 kA will generate the ultra-high surface temperature, which will cause pyrolysis of

the CFRP composite. Pyrolysis processes of two protective layers were studied,

including SMBP and CF/PF. Figure S6b) shows that the whole decomposition process

of the SMBP contains three main stages. The first stage starts at 200 oC and its

decomposition peak appears at 237 oC. It attributes to decomposition of the residual

AgNO3. Decomposition products include silver (Ag), oxygen (O2) and nitrogen

dioxide (NO2). The decomposition temperature of the Ag2O is 250 oC, thus the

elemental silver will not oxidize anymore during the subsequent heating process. The

onset decomposition temperature of the residual Mg(NO3)2 is 290 oC and its

decomposition rate reaches the maximum at 410 oC. The Mg(NO3)2 decomposes and

generates magnesium oxide (MgO), oxygen (O2) and nitrogen dioxide (NO2). When

the temperature reaches 450 oC, pyrolysis of CNTs starts. The temperature goes up to

600 oC and the decomposition rate reaches the maximum. When the temperature rises

to 665 oC, the mass of the remnant no longer changes. The main remnant is the

elemental silver, a few residual MgO and oxides of residual catalysts of CNTs.

Compared with Figure S6a), the decomposition temperature of CNTs of the SMBP

increases and its decomposition rate is slower. In addition, Ag content of the SMBP is

about 25.22%, according to TGA curves of BP and SMBP. It indicates that the SMBP

possesses low areal density. TGA and DTG curves of the CF/PF composite layer were

analyzed to study its pyrolysis process. The decomposition process of the CF/PF

composite can be divided into three main stages (Figure S6c). In the initial stage, the

CF/PF composite starts to degrade at 118 oC and its peak appears at 177 oC. It

corresponds to volatilization of residual curing agents and raw materials without

reaction. The mass loss of the first stage is less, which is only about 1.7 wt%. The

pyrolysis of PF resin starts, when the temperature goes up to 319 oC. The maximum

rate occurs at 594 oC that corresponds to the pyrolysis of the PF resin in the second

stage. Its pyrolysis products mainly contain water (H2O), carbon monoxide (CO), and

carbon dioxide (CO2), acetylene (C2H4) [4].When the temperature rises to 640 oC, the

PF resin decomposes completely and the CF starts to pyrolyze. The decomposition

rate of CF reaches maximum at 770 oC. When the temperature increases beyond 806 oC, there is no residual CF [5].

Figure S6. TGA and DTG curves of a) BP, b) SMBP and CF/PF composite.

Lightning strike protection (LSP) performance of the LSP structure can be directly

evaluated by the residual strength of the composite after the LS. In term of the ASTM

D7137, the residual strength values of the above-mentioned composite panels were

obtained through compressive tests [6]. The neat CFRP panel without the LS, as the

control specimen, was cut into the rectangular specimen as shown in Figure S7a).

Compressive specimens of neat CFRP, Cu/CFRP, SMBP/CFRP and SMBP-CF/PF-

CFRP panels after LS tests were displayed in Figure S7b), c), d) and e), respectively.

Compressive strength values of control specimen, neat CFRP specimen, Cu/CFRP

specimen, SMBP/CFRP specimen and SMBP-CF/PF-CFRP specimen are 292.1 MPa,

176.58 MPa, 280.68 MPa, 265.05 MPa and 284.08 MPa, respectively. In contrast with

the compressive strength of the control specimen, retention rates of compressive

strength of neat CFRP, Cu/CFRP, SMBP/CFRP and SMBP-CF/PF-CFRP composites

after LS tests are 60.45%, 96.09%, 90.74% and 97.25%, respectively. The residual

strength of the SMBP-CF/PF-CFRP composite is 60.88% higher than that of the

CFRP composite without protection and 1.21% higher than the Cu/CFRP composite.

It indicates that the SMBP-CF/PF composite LSP structure can effectively prevent

damage of the CFRP matrix caused by the LS.

Figure S7. Compressive specimens of a) control group, b) neat CFRP, c) Cu/CFRP, d) SMBP/CFRP and e) SMBP-CF/PF-CFRP composites, f) compressive strength values and retention rates of specimens.

During the LS process, fuel tank, engine and other parts of the aircraft need strictly

limiting the temperature variation of the non LS side of the composite, thus it is

significant to study the maximum temperature difference of the LSP composite.

Figure S8a), b), c) and d) present maximum temperatures of non LS sides of CFRP,

Cu/CFRP, SMBP/CFRP and SMBP-CF/PF-CFRP panels during simulated LS tests,

respectively. When lightning strikes the CFRP surface, more LS energy is conducted

along the through-thickness direction of the composite and causes serious depth

damage, owing to its poor conductivity. During the LS process, the maximum

temperature variation of the non LS face of the CFRP panel was 114.8 oC, as

displayed in Figure S8a). It illustrates that the ability of the CFRP composite to

dissipate the LS energy is too poor. Adding Cu mesh as a LPS layer can obviously

enhance the conduction ability along the composite surface for the LS energy and

reduce Joule heat conducting along the through-thickness direction of the CFRP

matrix. As shown in Figure S8b), the maximum temperature variation of the non LS

side of the Cu/CFRP panel was 51.1 oC. For the SMBP/CFRP composite, the

maximum temperature variation of its non LS face was 52.8 oC during the LS test

(Figure S8c). Because the LS energy is conducted on the SMBP surface by the plane

conduction and the ability of the SMBP to dissipate the LS energy is increased by the

re-solidified silver frame forming. It reduces the LS energy transferring into its CFRP

matrix. Figure S8d) displays that the maximum temperature variation of the non LS

face of the SMBP-CF/PF-CFRP panel was only 44.9 oC during the LS process, which

was lower than that of CFRP and Cu/CFRP panels. Besides electrical protection of the

SMBP, the PF resin decomposes rapidly and absorbs part of Joule heat under the high

temperature. The carbon layer generated by pyrolysis of the PF resin radiates residual

heat to the environment and can reduce the surface temperature obviously. It proves

that introduction of the SMBP-CF/PF LSP structure can effectively reduce the LS

energy transferring to the inner CFRP matrix.

Figure S8. Infrared thermal images of maximum temperatures of non LS sides of a) neat CFRP, b) Cu/CFRP, c) SMBP/CFRP and d) SMBP-CF/PF-CFRP composite panels during LS tests.

The surface temperature of the composite cannot be acquired by the state of the art

during the LS process, owing to ultra-short time and ultra-high temperature. The heat

transmission process of the CF/PF layer is studied through the numerical simulation

method, for verifying the structural reasonability of the SMBP-CF/PF-CFRP

composite. The heat transmission process of the CF/PF layer and the CFRP matrix as

a system is calculated with the finite element modeling (FEM) simulation by the

COMSOL Multiphysics software [7]. Thermal protection of the LS is similar to the

ablative protection, thus the numerical model of the ablative protection is used to

analyze the thermal transmission process of the CF/PF layer during the LS process

[8]. The heat transmission is only considered along the thickness-direction of the

system and the heat transferring along the other directions is ignored for simplifying

the mathematical model as a quasi one-dimensional model, because the damage depth

will affect the residual strength of CFRP matrix composite seriously. Thus, the quasi

one-dimensional model can illustrate the heat-protective function of the CF/PF layer

and analyze its LSP mechanism. The Joule heat that was generated by the LS current

causes rapid increase of the surface temperature of the CF/PF. The formation of the

ultra-high temperature difference between the top and bottom surfaces of the CF/PF

layer leads to the heat transferring from the high-temperature zone to the low-

temperature zone. The transmission mode is mainly depended on the heat conduction.

PF resin will rapidly pyrolyze through the endothermic reaction under the high

temperature. The pyrolytic gas flow will cause the heat convection, which can be

ignored resulting from small thickness of the CF/PF layer and short time of the LS

process. In addition, top surface of the CF/PF layer after the carbonization will

spontaneously radiate heat to the environment. Therefore, the heat radiation

phenomenon should be considered as the boundary of the FEM model [9].

Based on the above analysis, the mathematical model of the CF/PF layer is described

as following. During the LS, the thermal transmission process of the CF/PF layer

follows the law of energy conservation and its equation under the thermal loading

condition can be described as [9]:

ρC ∂ T∂ t

=∇ ∙ ( k∇T )+ ∂ ρ∂ t

∆ H (S1)

where , C, T, t, k are density, specific heat capacity, thermodynamic temperature,

time of the heat transmission process and thermal conductivity of the CF/PF layer in

Equation (S1), respectively. H is the energy consumption of resin per unit mass

pyrolysis.

To simplify calculation, density, specific heat capacity and thermal conductivity are

obtained by weighted averages of thermophysical properties and volume fractions of

component phases. Then, equations are described as following:

ρ=ε f ρf +εm ρm (S2)

C=ε f C f +ϵm Cm (S3)

k=ε f k f +εm km (S4)

where f and m as subscripts in Equation (S2), (S3) and (S4) stand for carbon fiber and

PF resin of the CF/PF layer, respectively.

Volume fractions of carbon fiber and PF resin of the CF/PF layer are expressed as ε f

and ε m, respectively. The relationship betweenε f and ε m can be written as:

ε f +εm=1 (S5)

When the temperature reaches the decomposition temperature, PF resin will pyrolyze.

The density variation of the PF resin can be expressed by the Arrhenius law [10]:

∂ ρm

∂ t=

−J o

ρb( ρm−ρm

∞ )exp (−EA

Rθ) (S6)

where m, Jo, b，ρm∞, EA are density of the resin, pre-exponential factor, density of

the resin phase, density of the resin after decomposition and activation energy in

Equation (S6), respectively. R stands for the gas constant, which is 8.314 J/(molK).

During the pyrolysis process, specific heat capacity of the PF resin changes with the

temperature and can be described as [10]:

C ()=❑bC b❑b( )+❑P Cp❑p()

❑bCb+❑PC p (S7)

where C(), b, Cb, b(), p, Cp and p() mean specific heat capacity of the matrix,

density of the resin phase, specific heat capacity of the resin phase, temperature of the

resin phase, density of the pyrolytic phase, specific heat capacity of the pyrolytic

phase, temperature of the pyrolytic phase in Equation (S7), respectively.

Heat flow and energy radiating to the environment should be considered on the LS

side of the CF/PF layer. The boundary condition of LS side of the CF/PF layer can be

written as:

−k ∂ T∂

=qn−σε (T w4 −T ∞

4 ) (S8)

where qn and are heat flow cause by the LS and radiation coefficient of the pyrolysis

layer, respectively. is the Stephan-Boltzman constant, which is 5.6710-8 W/m2K4

[11]。

The initial condition is:

T 0t=0=293 K (S9)

During the LS process, the mode of heat transmission of the CFRP matrix is the same

as the CF/PF layer, thus equations and boundary condition of the CFRP matrix are

also the same as the CF/PF layer.

The surface temperature of the composite cannot be acquired by the state of the art

during the LS process, owing to ultra-short time and ultra-high temperature. The heat

transmission process of the CF/PF layer is studied through the numerical simulation

method, for verifying the possible mechanism of the CF/PF protective layer. The heat

transmission process of the CF/PF layer and the CFRP matrix as a system is

calculated with the finite element modeling (FEM) simulation by the COMSOL

Multiphysics software. Thermal protection of the LS is similar to the ablative

protection, thus the numerical model of the ablative protection is used to analyze the

thermal transmission process of the CF/PF layer during the LS process. The heat

transmission is only considered along the thickness-direction of the system and the

heat transferring along the other directions is ignored for simplifying the

mathematical model as a quasi one-dimensional model, because the damage depth

will directly affect the residual strength of the CFRP composite seriously. Thus, the

quasi one-dimensional model can illustrate the heat-protective performance of the

CF/PF layer and analyze its LSP mechanism. The Joule heat that is generated by the

LS current causes rapid increase of the surface temperature of the CF/PF. The

formation of the ultra-high temperature difference between the top and bottom

surfaces of the CF/PF layer leads to the heat transferring from the high-temperature

zone to the low-temperature zone. The transmission mode is mainly depended on the

heat conduction. PF resin will rapidly pyrolyze through the endothermic reaction

under the high-temperature. The pyrolytic gas flow will cause the heat convection,

which can be ignored resulting from small thickness of the CF/PF layer and short time

of the LS process. In addition, top surface of the CF/PF layer after the carbonization

will spontaneously radiate heat to the environment. Therefore, the heat radiation

phenomenon should be considered as the boundary of the FEM model. Based on

above analysis, the mathematical model of the CF/PF layer is described as Equation

(S1)-(S9) and the schematic diagram of the FEM model is shown in Figure S9a).

Figure S9. a) Schematic diagram of geometry and boundary of the FEM model, b) temperature-time curves of top and bottom surfaces of the CF/PF-CFRP system during the LS process in 30 s; the inset is temperature-time curves of top and bottom

surfaces of the CF/PF-CFRP system during the LS process in 0.6 s, c) temperature fields of the CF/PF-CFRP system at 0 s, 0.38 s, 0.4 s and 6.26 s.

The numerical model was solved by the COMSOL solver. Figure S9b) shows the

temperature of the CF/PF layer increases rapidly during the initial LS stage. Under the

high temperature, the PF resin pyrolyzes and generates gas and charring layer. During

the pyrolysis process, the PF resin absorbs some heat with the enthalpy change and

the charring layer radiates part of heat to the environment. The CF/PF layer can

effectively dissipate most of the Joule heat, but there is still residual heat transferring

into the CFRP matrix. Temperature fields of the simulating heat transmission process

are described as Figure S9c). The top surface temperature of the CF/PF layer reaches

the maximum at 0.38 s. The heat flux stops at 0.4 s and the top surface temperature

start dropping, owing to pyrolysis of the PF resin dissipating heat. The surface

temperature of the CFRP matrix rises more slowly and reaches the maximum

temperature at 0.44 s, due to thermal protection of the CF/PF layer. Then, continuous

pyrolysis of the PF resin leads to the temperature gradually falling. When the

temperature of the system drops below 600 K at 6.26 s, the PF resin will not

decompose. The system is only depended on the radiation to transfer the heat and

cooled to the initial temperature.

Table S1. Parameters of D, B and C* waveforms

D waveform B waveform C waveform

Ipeak/avg (kA) 100 2 0.4

AI (A2s) 0.25106 - -

Q (C) - 10 21

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