pyrocarbon composites prepared by electrophoresis and thermal gradient chemical vapor infiltration

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available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/carbon

Densification kinetics and matrix microstructure of carbon fiber/carbon nanofiber/pyrocarbon composites prepared by electrophoresis and thermal gradient chemical vapor infiltration Jinsong Li a b

a,b

, Ruiying Luo

b,*

, Ying Yan

a

School of Aeronautical Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191, China School of Physics and Nuclear Energy Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191, China

A R T I C L E I N F O

A B S T R A C T

Article history:

Preforms were fabricated by the application of direct current fields for the alignment and

Received 11 May 2010

network formation of carbon nanofibres in the needle-punched carbon fiber felts, and infil-

Accepted 3 September 2010

trated by using the thermal gradient chemical vapor infiltration at the temperature of

Available online 9 September 2010

1000 C under the total pressure of 5 kPa. The voltage had a strong influence on the carbon nanofiber weight obtained in the preform. With the increase of the voltage, the carbon nanofiber content increased. The carbon nanofibers formed networks on the carbon fibers. When the voltage remained at 30 V, the carbon nanofibers were dispersed uniformly on the carbon fibers. However, when the voltage was larger than 60 V, the carbon nanofibers agglomerated themselves and coated the carbon fibers. The carbon nanofiber content has a strong influence on the temperature distribution and on the densification front existence, velocity and width. The achievable degree of pore filling in the carbon nanofiberadded preform at 30 V was the highest, while the carbon nanofiber-added preforms at 60 and 90 V could not be densified efficiently. The microstructure of pyrocarbon at different positions is discussed.  2010 Elsevier Ltd. All rights reserved.

1.

Introduction

Single and multiwall carbon nanotubes (CNTs) as well as carbon nanofibers(CNFs) are deemed to be the key to novel composite materials with enhanced mechanical and electrical properties [1–3]. The academic knowledge about how to beneficially use these materials in practice is already quite mature and well described in the scientific literature [4,5]. CNFs, though not as perfect in structure or as good in properties, would be a good alternative to CNTs as an additive in composites [6], because the industrial production of the large quantity of CNFs required in the plastics and composite industry is definitely more feasible [7]. Many efforts have

focused on incorporating CNTs/CNFs into pyrocarbon matrices [8–13]. In spite of the enormous interest CNTs/CNFs have attracted as potential reinforcements for carbon/carbon(C/C) composites, their performance so far has been inadequate. This is attributed by many experts to three issues: (1) the difficulty of dispersing the CNTs/CNFs in the matrix due to the fact that they tend to stick together; (2) insufficient bonding at the nanotubes/matrix interface since CNT/CNF composites have been observed to have failed by either fracture at CNT/ matrix interface; (3) the difficulty of aligning the tubes within the matrix [14]. Although such issues have not yet been resolved, extensive efforts are under way to overcome them using in situ technology. Xintao Li et al. [11] used ferrocene

* Corresponding author: Fax: +86 86 10 5173 6729. E-mail address: [email protected] (R. Luo). 0008-6223/$ - see front matter  2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.09.011

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as catalyst and toluene as the liquid precursor preparing C/C composites by chemical liquid vapor infiltration. Qiao Juan Gong et al. [12] used S and Ni powder as catalysts in a chemical vapor deposition system to grow CNTs directly on carbon cloth by catalytic pyrolysis of natural gas. However, in the in situ technology, the catalysts are retained inevitably in the C/C composites, and have a negative influence on the properties of the C/C composites. In addition, since the pore space geometry of the CNF/carbon fiber preform is quite different, i.e. different surface-to-volume ratio, different pore distribution, the pyrocarbon deposition mechanism may be different with respect to carbon fiber preform without CNFs. In this paper, an electric field is used to promote CNF alignment on the carbon fibers. Here, the thermal gradient chemical vapor infiltration (CVI) is the final step for the preparation of CNF-reinforced C/C composites. The influence of electric fields on dispersion of CNFs in the carbon fiber preforms is investigated. The influences of CNFs on the temperature distribution, pyrocarbon deposition rate, pyrocarbon microstructure at different positions are studied by varying the CNF content.

2.

Experimental

2.1.

Preparation of preforms

The CNFs (PR-19-PS of Polygraf III) obtained from Applied Sciences Inc., of Cedarville, Ohio, were used as a secondary reinforcement. They were treated in a 3:1 mixture of concentrated sulfuric acid and nitric acid, thus, more functional groups such as carboxylic acid and hydroxyl groups were formed on the CNF surface. The CNFs have an average diameter of 100–200 nm and lengths from several microns to over 200 lm. The CNFs were percolated, washed and dried; next, 5wt.% (CNF weight /methanol weight) CNFs were dispersed in methanol by vigorous mechanical stirring enhanced by the impact of ultrasound, and produced an electrostatically stabilized CNF solution. Needle-punched, 30vol.% carbon fiber felts, with dimensions 400 · 400 · 25 mm3 were fixed in the graphite electrode and then immersed in the solution. A direct current (DC) was applied along the fiber direction for 5 h. Voltage values of 0, 30, 60 and 90 V were applied respectively throughout the full curing cycle. CNF-added preforms with different CNF contents were obtained. Then, they were cut off to the dimensions of 360 mm outer diameter · 200 mm inner diameter · 25 mm thickness. The CNF-added preforms obtained at 0, 30, 60 and 90 V were named as PV0, PV30, PV60 and PV90, respectively. The preform with no CNFs was named as P0.

2.2.

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Densification of preforms

Densification of porous carbon fiber preforms was carried out in a thermal gradient CVI furnace which was cooled by water jacket and high gas flow rates. The schematic diagram of the thermal gradient CVI system is shown in Fig. 1. A temperature gradient along the radial direction of preforms was formed. The temperature was measured and controlled by two Pt–Rh type thermocouples, one of which was inserted into the preforms and was able to move along the radial direction,

Exhaust

Water cooler wall Water cooler wall Graphite heater Thermocouple

Thermocouple

Carbon felt insulation

Preform

Graphite Carbon felt insulation Electrode

Natural gas

Fig. 1 – Schematic of the thermal gradient CVI furnace.

while the other was fixed at the outer side of the preform to drive power regulation. Preforms were assembled as shown in Fig. 1 and were clamped between graphite disks. The furnace was evacuated and ensured no leakage. Natural gas was introduced into the reaction chamber. The flow rate of natural gas was controlled at about 4 m3/h. The outer side temperature of the preforms, i.e. deposition temperature, was 1000 C, and the pressure of natural gas was 5 kPa.

2.3.

Measurement of the bulk density of the samples

A piece of prefabricated sample has been taken and cut it uniformly into five pieces from the inside to the outside along the radial direction of preforms. The size of the samples was 10 · 10 · 4 mm3. Then they were weighed in an analytical balance. The bulk density of the samples can be calculated. In each density measurement, we have not performed any surface machining for crust removal. To determine the open pore volume and size distribution of the preforms, Poremaster 33, a mercury porosimeter produced by Quantachrome Instruments Corporation, Florida, USA, was used. The experimental error in the determination of the porosity values is less than 1%. The results are shown in Table 1.

2.4.

Analytical methods

A scanning electron microscope (SEM) JEOL JSM-5800, and a Polarized light microscope (PLM) NEOPHOT 21, have been used in the investigations.

3.

Results and discussion

3.1.

Effect of voltage on the CNF weight obtained

Fig. 2 shows the curves of the CNF weight obtained with different voltage. The CNF weight obtained increases with voltage increasing. As shown in Fig. 2, compared to the voltage at 0 V, the CNF weight obtained at 30 V increases by 162.7%,

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Table 1 – Results measured by mercury porosimetry. Preform

CNF wt.%

dmicropore (lm)

dmacropore (lm)

0 2.25 5.91 7.62 8.66

4.35–7.18 0.25–1.76 0.25–1.64 0.25–1.44 0.25–1.40

120–200 70–135 66–115 61–95 60–92

P0 PV0 PV30 PV60 PV90

10 9

CNF weight obtained(wt.%)

8 7 6 5 4 3 2 1 0 0

10

20

30

40

50

60

70

80

90

100

Voltage(V)

Fig. 2 – CNF weight obtained-voltage curve of the perform.

60 V by 238.67% and 90 V by 284.98%. In the CNF solution, the direction current electric field results in direct forces acting on excess charges on the CNFs and induce electric dipole moments, which cause additional CNF–CNF interactions [15]. The CNFs are free to move and thereby can form a conducting network at much lower particle concentrations in the carbon fiber preforms. The strength of the electric field, which affects the speed of the CNFs, is determined by the value of the voltage. Therefore, in the same time, the CNF weight obtained increases with the voltage rising. As shown in Fig. 3, the values of the resistivity of all the four CNF-added preforms reduce

Resistivity of preform(

-5 10 Ω .m)

16 0V 30V 60V 90V

14

12

10

8

6 0

30

60

90

120

150

180

Time(min)

Fig. 3 – The variations of the resistivity of the preforms with adsorption time in the CNF solution.

Vmicropore (%) 44 50 52 53 54

Vmacropore (%) 25 15 13 12 11

rm,0 · 104(cm1) 1.66 2.49 3.83 4.46 4.85

greatly at the beginning of the adsorption, i.e. the CNF networks are formed on the carbon fibers, and the slope of the curves increases with increasing voltage. This reveals that the voltage has a strong influence on the CNF incorporation rate. Fig. 4 shows the scanning electron microscopy images of the preforms with different CNF content. The CNFs form networks on the carbon fibers. As shown in Fig. 4a, they are not well dispersed on the carbon fibers at 0 V. However, when the voltage is 30 V, the CNFs their dispersion is uniform (Fig. 4b). For voltages 60 V and 90 V, they are not well dispersed: agglomerates of CNFs coat the carbon fibers (Figs. 4c and d). During the application of a DC electric field, a fraction of the CNFs is observed to move towards the carbon fibers, under electrophoresis, verifying the presence of negative surface charges. As soon as these CNFs are close enough to the carbon fibers to allow charge transfer, the CNFs discharge and adsorb onto the carbon fibers. Tips of CNFs connected to the carbon fibers then become sources of very high field strengths and the location for adsorption of further filler particles. As a result, ramified CNF network structures extend away from the carbon fibers. When increasing the electric field strength (higher voltage), more CNFs are incorporated into the existing oriented agglomerates on the carbon fibers, as shown in Figs. 4c and d.

3.2.

Effect of the CNFs on the infiltration processing

Measurements are conducted to analyze the temperature gradient along the radial direction of the CNF-added preforms. The original high temperature region is near the surface of the graphite heater, and the low temperature region is near the inner jacket that is cooled by circular water. As shown in Fig. 5, the temperatures of all preforms vary sharply with different positions, and the temperature gradient is marked. The temperature at different positions increases with the increase of the infiltration time. At the same position, the temperatures of the CNF-added preforms are higher than that of the preform without CNFs, because the CNF network increases the thermal conductivity. Since the rate of the CVI reaction increases exponentially with temperature, natural gas is not pyrolyzed until it reaches the deposition zone. To analyse the effects of CNFs on the chemical vapor infiltration of the CNF-added preforms using thermal gradient method. A model of the lower bound for the heat flux is suggested by Vignoles et al. [16] as the following expression: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kRT2 rm;0 KðTmax Þ max ð1Þ qJ  Ea D Here q – heat flux, Wm2; R – perfect gas constant, J mol1 K1; k – average thermal conductivity, Wm1 K1; Ea

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245

Fig. 4 – SEM micrographs of the CNF-added preforms (a: PV0; b: PV30; c: PV60; d: PV90). – activation energy, J mol1; k – Chemical deposition rate constant, ms1; rm;0 – initial internal surface area, m1; D – mass diffusion coefficient, m2 s1. If criterion (1) is satisfied, a front exists and propagates towards the cold side [17]. As shown in Fig. 6, the preform is considered to be divided into three zones: a hot zone, with parameters associated to the densified material, a front zone, the characteristic properties of which are given by the results of the local front study, and a cold zone, with parameters associated to the raw preform. As shown in Fig. 5a, after 40 h infiltration, the thermal gradient at the cold side is roughly twice as high as the gradient on the hot side, thus, the temperature gradient is strong enough to create a densification front and moves from the hot face towards the cold face. The hot side temperature of the densification front increases with the CNF content rising. The initial internal surface area of the CNF-added preforms increase with the CNF content rising (Table 1), which accelerates the homogeneous reactions. The front-average conductivity k increase with the increase of the CNF content because of the CNF-network formed in the preform. Therefore, the front velocity and width would increase with the CNF content rising at the same infiltration time, however, the front existence is more difficult to obtain with the increase of the CNF content, a fact that can be deduced from Eq. (1) [18]. Indeed, as the infiltration time increases, as shown in Fig. 5b, the front of the CNF-added preforms propagates more quickly towards the cold side than that of the preform without CNFs. During the process, the heat flux increases and the hotside temperature decreases,

this has the effect of continuously lowering the front velocity and width. The thermal gradient at the hot side is decreased more rapidly with the increase of the CNF content, which would be not sufficient to give birth to a front. As shown in Fig. 7, a non-optimal density zone is left inside the CNF-reinforced C/C composites. This is explained by the fact that this criterion is not met at the first stages of infiltration.

3.3.

Infiltration kinetics

To consider the effect of the CNFs on infiltration, the competition between diffusion and reaction is discussed. A measure of this competition is the Thiele modulus: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2Þ U ¼ 2L kS =Deff dP (U >> 1 means diffusional limitations) Here L – the length of pores, m; kS – heterogeneous reaction constant, s1; Deff – the effective diffusion coefficient, m2/s; dP – mean pore diameter, m. In this thermal gradient CVI, at the first 2 hours of the infiltration, we found that the temperature at the different position almost does not change. Therefore, the Arrhenius plot of densification rate of the CNF-added preforms at infiltration temperature ranging from 850 to 1000 C is shown in Fig. 8. We can see that the deposition rate increases with the increase of the CNF content at the same temperature because of the increased total surface area (Table 1), but the Thiele modulus also increases, i.e. the effective diffusion coefficient

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a

1000

1.76 3

800

Density(g/cm )

900

Temperature(°C)

P0 PV0 PV30 PV60 PV90

1.78

700 600

P0 Pv0 Pv30 Pv60 Pv90

500 400 90

100

110

120

130

140

150

160

1.74

1.72

1.70

1.68

170

0

Distance from the center(mm) 1100

20

40

60

80

Distance from the heater surface(mm)

Fig. 7 – The radial direction distribution profile of the density in the thermal gradient CVI process.

b

1000

P0 PV0 PV30 PV60 PV90

1.5 1.0

800

0.5 0.0

700

P0 PV0 PV30 PV60 PV90

600

90

100

110

120

130

140

lnv(mg/min)

Temperature(°C)

2.0 900

-0.5 -1.0 -1.5 -2.0

150

-2.5

Distance from the center(mm)

-3.0

Fig. 5 – The temperature variation with the position at different times(a: 40 h; b: 80 h).

-3.5 7.8

8.0

8.2

8.4

8.6

8.8

9.0

-4

1/T x 10 (1/K)

Fig. 8 – Logarithm of initial deposition rates of carbon as a function of reciprocal temperature.

Fig. 6 – Diagram of the deposition process.

decreases in the deposition zone. When the Thiele modulus is high enough (probably after 5wt.% CNFs) [13], then the efficiency of the porous medium decreases because of incomplete pore filling. From Fig. 8, the values of the apparent activation energy of P0, PV0, PV30, PV60 and PV90 are about 337.3, 322.7, 308.9, 294.1 and 291.8 kJ/mol, respectively. These

values are lower than the apparent activation energy of methane decomposition, 431.5 kJ/mol [19], owing to many reactions taking place simultaneously and the influence of the CNFs. From Table 1, in the CNF-added preforms, the average micropore diameter dmicropore is about 0.25–1.76 lm, so that Kn P 10 and the gas transport should be mainly governed by Knudsen diffusion within a fiber bundle or CNF-web layer. On the other hand, in the macropores, with an average diameter dmacropore about 60–135 lm, one has Kn  0.1 and the transport should depend on both Fick and Knudsen diffusion between the tows. So the pore size may be so small that Knudsen diffusion plays a role. As already shown in previous papers [20,21], the effective diffusion coefficient Deff can be calculated with Eq.(3): Deff ¼

ep DF=K s

ð3Þ

Where ep = the pore volume fraction; s = tortuosity factor; Dk = Knudsen diffusion coefficient, m2/s; DF = Fick diffusion coefficient, m2/s.

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Fig. 9 – The polarized microstructure of pyrocarbon at the radial cross-section of the composite P0 (a: 175 mm; b: 145 mm; c: 85 mm).

Fig. 10 – The polarized microstructure of pyrocarbon at the radial cross-section of the composite PV0 (a: 175 mm; b: 145 mm; c: 85 mm).

Within a fiber bundle or CNF-web layer, Deff  dp =sðep Þ, the pore diameter of the CNF-added preforms is less than that of the no CNF preform. As infiltration progresses, the effective diffusion coefficient decreases due to the pore diameter decrease, an overgrowing of the pore entrances is accelerated and an incomplete filling of the pores is possible with the additive content increasing, resulting in the Thiele modulus increasing, i.e. the deposition rate in the pores decreasing. When more CNFs are added in, the pore diameter reduces more quickly and the pore entrances are sealed more rapidly, as shown in Fig. 7.

3.4. The microstructure of pyrocarbon along the radial direction of preform The microstructure of CNF-reinforced composites with different CNF content is different at positions along the radial direction shown in Fig. 9. For the composite without CNFs, rough laminar (RL) pyrocarbon can be seen at the inner surface (Fig. 9a), a combination of smooth laminar and isotropic pyrocarbon microstructures can be observed at the middle position along the radial direction (Fig. 9b), and smooth laminar pyrocarbon can be seen at the outside. But comparatively, for composite PV0, as shown in Fig. 10, RL pyrocarbon can be seen at inner and middle positions (Fig. 10a), while a combination of RL and isotropic(ISO) pyrocarbons can be observed at the outside. So, CNFs might favor the formation of high-textured pyrocarbon. This is probably due to the increase in internal surface area, which is known to favor

highly anisotropic pyrocarbon deposition [22,23]. In addition, as shown in Fig. 11, when the voltage is larger than 60 V, i.e. a larger CNF content, larger cavities remain within the composites and CNFs appear as interwoven bundles and aggregates of up to several tens of microns in diameter covered with pyrocarbon. Likewise, in composite PV0, even though the CNF content is only 3.25wt.%, CNFs also appear as interwoven bundles and many pores remain between the fiber tows (Fig. 10). In contrast, the composite prepared with 30 V has a less developed porous structure (Fig. 11a). The results of the microstructure completely agree with the kinetic analysis of the process based on the diffusion/reaction competition and the effective diffusion coefficient. Therefore, the addition of CNFs at 5.91wt.%, i.e. the voltage at 30 V could improve the rates of chemical vapor infiltration combined with a maximum possible degree of pore filling.

4.

Conclusions

Preforms were fabricated by DC electrophoresis of CNFs in needle-punched carbon fiber felts, and infiltrated by thermal gradient CVI of methane. The electrical current favored the alignment of the CNFs in the structure and the formation of a network. The voltage has a strong influence on the CNF weight obtained in the preform. With the increase of the voltage, the CNF content increases. At 30 V, the CNFs (5.91wt.%) are dispersed uniformly on the carbon fibers. Above 60 V, they tended to agglomerate themselves and coat the carbon fibers. The CNF content has a strong influence on the temperature

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Fig. 11 – The polarized microstructure of pyrocarbon at the radial cross-section of the CNF-reinforced composites (a: composite PV30; b: composite PV60; c: composite PV90).

gradient, densification front existence, velocity and width. The front velocity width increase with an increase of the CNF content. When the CNF content remains at 5.91wt.%, the infiltration is optimal, i.e. it has a faster rate combined with a better maximum degree of pore filling. Because the pore network structure and local physicochemical conditions for deposition are changed as infiltration goes on, different microstructures of pyrocarbon along the radial direction of the preform are obtained.

Acknowledgements This work was supported by China Postdoctoral Science Foundation 20090450274 and The National Natural Science Foundation of China (no. 21071011) and The Beijing Municipal Science and Technology Program (Z09010300840902).

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