Deposition mechanism and microstructure of pyrocarbon prepared by chemical vapor infiltration with kerosene as precursor

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Deposition mechanism and microstructure of pyrocarbon prepared by chemical vapor infiltration with kerosene as precursor Xiaowen Wu, Ruiying Luo*, Jincao Zhang, Qiao Xiang, Yongfeng Ni Center of Materials Physics and Chemistry, School of Science, Beihang University, 37# Xueyuan Road, Beijing 100191, PR China

A R T I C L E I N F O

A B S T R A C T

Article history:

Large-size carbon/carbon composites (U 450 · U 230 · 15 mm3) have been produced by

Received 2 July 2008

chemical vapor infiltration with kerosene as precursor. The microstructure of pyrocarbon

Accepted 21 January 2009

was examined by polarized light microscopy and scanning electron microscopy. The infil-

Available online 31 January 2009

tration kinetics was analyzed to investigate the infiltration rate limitation by parameters such as temperature. The results show that rough laminar carbon constitutes the majority of the matrix at a medium temperature (about 1100 C), while smooth laminar and isotropic structures occur at temperatures lower than 1000 C and higher than about 1200 C, respectively. The apparent activation energy of kerosene decomposition in the temperature range 900–1200 C is about 125.6 kJ/mol.  2009 Elsevier Ltd. All rights reserved.

1.

Introduction

During the last 10 years, an effective method, chemical vapor infiltration (CVI), has been developed to efficiently fabricate carbon/carbon (C/C) composites [1,2]. A comparison of this method with the classical isothermal technique found that the strong thermal gradients are involved in cold wall reactors and a mobile densification front appears in the porous preform. The precursor can be either liquid reagent in which the preform is immersed or its vaporized gas. To date, many studies [1–8] have been conducted to investigate the potential influences of the deposition parameters (temperature, precursor, etc.), substrate types, and the state of the precursor (gas or liquid) on the densification rate and the formation process of different types of pyrocarbon. To the best of our knowledge, however, these studies were mainly performed in small experimental reactors, and the C/ C composites reported in the previous publications are usually of thin-walled tubular shape or disc shape with the typical outer diameter about 100 mm. Generally, only one piece of preform is expected to obtain in each run. Besides, the forma-

tion process of pyrocarbon by CVI is very complex and there have been some uncertain relations between processing parameters and texture of pyrocarbon. Benzinger et al. [9], Benzinger and Huttinger [10], and Feron et al. [11] have pointed out that pyrocarbon is mainly produced from the competition between homogeneous reaction in the gas phase and heterogeneous one on the substrate surface. Regarding the matrix precursors to fabricate C/C composites, kerosene is an effective candidate, which had been proved by Ji et al. [12]. In this paper, by choosing kerosene as precursor, C/C composites were successfully fabricated by CVI with an improved device developed by us as shown in Section 2. The size of composites was much larger than that of samples reported by Wang et al. [6] and Ji et al. [12], which is important to extend their engineering applications especially in civil field such as brake discs for racing cars and fast trains. Moreover, several pieces of samples can be densified simultaneously. The texture and morphology of the samples were investigated by using the polarized light microscopy (PLM) and scanning electron microscopy (SEM).

* Corresponding author: Fax: +86 10 51736729. E-mail address: [email protected] (R. Luo). 0008-6223/$ - see front matter  2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.01.035

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2.

Experimental procedures

2.1.

Materials and apparatus

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Commercial kerosene was used as the matrix precursor. Its components and characteristics are shown in Table 1. A carbon felt from Lanzhou Carbon Plant in China, made from polyacrylonitrile based carbon fiber, has been used as preform in this work. The typical size, bulk density and fiber volume fraction of the preforms were U 450 · U 230 · 15 mm3, 0.2 g/cm3 and 11%, respectively. A schematic of the experimental device used is shown in Fig. 1. The key components of the reactor, two graphite susceptors sandwiched by the preforms, were located in a cylindrical quartz reactor and further supported by several quartz sticks. To heat the reactor, inductive Joule effect was applied and in situ processing temperature measured by K type thermocouples. The by-products, as well as the precursor, were recycled by the water-cooled condenser located at the top of the reactor.

2.2.

Fig. 1 – Sketch of the CVI system.

Fabrication of the C/C composites

The reactor was firstly evacuated and then fed with N2 for safety before the inductive power supply was on. As the temperature of kerosene went up to its boiling point, the precursor began to vaporize and infiltrated into the preforms simultaneously. Then, the pyrolysis reaction took place at the deposition front zone because its temperature reached the decomposition point of the hydrocarbon gases. During the process of densification, the reactor was always kept under atmospheric pressure and kerosene was fed in to balance the consumption of infiltration. The processing temperature could be controlled by adjusting the output power of the generator. In this way, several groups of samples could be fabricated with different deposition temperature (Td). The processes were interrupted several times to obtain the data of mass gains. The infiltration parameters and characteristics of these composite samples are schematically shown in Table 2.

2.3.

Measurement of density and porosity

The bulk density of samples was calculated by dividing specimen mass by its volume. Their mass was measured by a microbalance. The porosity was calculated by P ¼ 1  mf 

q  m f qf qp

ð1Þ

where q is the bulk density of the composite samples, mf is the volume fraction of carbon fibers, qf and qp are the density of

carbon fiber and pyrocarbon, and their value are 1.76 and 2.07 g/cm3, respectively.

3.

Results and discussion

3.1.

Analysis of infiltration kinetics

3.1.1.

Densification rate

The samples in each group are labeled as S1–S4 along their location from the top down and their density deviation is within 5% during the process. So, one sample S2 was chosen from each group to investigate the infiltration rate and kinetics. It can be seen from Table 2 that the initial densification rate of sample S2 increases from 1.76 to 16.34 g/min when Td rises from 900 to 1200 C. These values of densification rate are much larger than that of other CVI as reported previously [13]. The processing temperature curves of sample S2 in each group, recorded by thermocouples at different positions, is presented in Fig. 2. It shows that a sharp thermal gradient exists in the porous preform. Since the chemical reactions take place only at the place where the temperature reached the decomposition point of precursor gases, the densification front are thus formed at the inner part of the preform initially, and then extend to the outer surface of the preform during the infiltration. Consequently, no crust is formed on the outer surface of preform. It means that one cycle is sufficient to fabricate the composites. With the help of strong thermal gradient, the mass exchange and diffusion of precursor gases become faster than that of isothermal chemical vapor infiltra-

Table 1 – Properties of the liquid precursor kerosene. Components (vol.%) n-decane

n-dodecane

m-xylene

n-butylbenzene

ps-cumene

55

25

11

6

3

Boiling point (C)

Density (g/cm3)

180–230

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Table 2 – Infiltration parameters and characteristics of composite samples. Group

A B C D

Td (C)

900 1000 1100 1200

Total infiltration time (h) 30 30 30 30

Mass increase of samples (kg) S1

S2

S3

S4

2.72 2.78 2.75 2.59

2.73 2.79 2.76 2.60

2.73 2.78 2.76 2.60

2.72 2.77 2.77 2.60

The density of sample S2 (g/cm3)

The initial densification rate of sample S2 (g/min)a

1.71 1.75 1.74 1.64

1.76 3.51 7.78 16.34

a Calculated by the data from the first densification cycle as shown in Figs. 3 and 4.

next groups show good linearity only at the initial infiltration stage and its slope decreases drastically at the latter stage. This result could be explained by the theory of infiltration kinetics.

3.1.2.

Fig. 2 – Time evolution of the temperature in two axial locations of sample S2 for different Td.

Infiltration kinetics

To analyze infiltration kinetics, the important factors to be discussed are Td, the ratio of substrate surface area to volume, ½A=V , and the Knudsen number, K n . The densification rates as a function of infiltration time for different Td is shown in Fig. 4. It can be seen that the densification rate decreases with the increase of the infiltration time during the whole process. Moreover, higher Td leads to higher initial densification rate and more sharp decrease. The relationship between temperature and the reaction rate during the initial stage follows the Arrhenius law given in Ea

k ¼ k0 eRT

ð2Þ

tion (ICVI). As a result, the deposition front can be described as in a situation of high temperature, high gas pressure, and high gas concentration. This is the reason why the densification rate of this method is more rapid than that of ICVI. The mass gains of sample S2 as a function of infiltration time for different Td are plotted in Fig. 3. The densification rate becomes larger with the increase of Td, and it can be understood from the fact that higher Td can result in larger decomposition rate of the precursor. Nevertheless, there are some differences among these curves. We can see that the mass gain curve of group A for Td = 900 C shows good linearity during a long time, whereas the mass gain curves of the

However, the value of ½A=V  has a decisive influence on the interactions of homogeneous gas phase and heterogeneous surface/deposition reactions [14]. The ½A=V  ratio was determined by the method reported by Zhang et al. [15]. From the curves of relationship between ½A=V  ratio and the infiltration time, shown in Fig. 5a, we can see that the value of surface area/volume increased with increasing the infiltration time, whereas their slope decreased. The increase of ½A=V  ratio results from a dominating carbon deposition on the fiber substrate. However, with the increase of infiltration time, formation of larger species in the gas phase is limited because reactive intermediates formed in gas phase can be

Fig. 3 – The mass gains as a function of infiltration time for different Td.

Fig. 4 – The densification rates as a function of infiltration time for different Td.

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Fig. 5 – Surface area/volume ratio (a) and porosity (b) as a function of infiltration time for different Td.

chemisorbed at the larger surface to a much higher extent. It restrains the formation of pyrocarbon and reduces its deposition rate. Furthermore, the competition between reaction and diffusion can also affect the mass increases of these samples. Fig. 5b shows the porosity of samples varied with the infiltration time for different Td. It can be seen that the porosity decreased drastically with increasing the infiltration time. So, the Knudsen number K n increased and the diffusion mechanism transformed from bulk diffusion to Knudsen diffusion regime. As a result, the rate of mass gain decreased with the increase of time during the whole process. Besides, we can see from Fig. 4 that the sharpest decrease of the densification rate with the increase of time occurs on sample S2 of group D for Td = 1200 C. Because the temperature of the deposition front in preform is hyperthermal and some unwanted reactions take place at the position away from the front. It frustrates the infiltration in the latter stage and its rate decreases more severely. As a result, the density of sample S2 of group D is the lowest one among all the investigated samples (see Table 2). Bruneton et al. [1] revealed that the densification rate variation is practically exponential with Td. However, the data in this study is not strictly in accordance with this law because the method is not pure film boiling CVI which lets the preform immerse directly in the liquid hydrocarbon. Fig. 6 shows the Arrhenius plot of densification rate at Td ranging from 900 to 1200 C with apparent activation energy about 125.6 kJ/mol. This value is lower than the apparent activation energy of cyclohexane decomposition, 222 kJ/mol [1], and methane decomposition, 445 ± 10 kJ/mol [16], owing to many reactions taking place simultaneously.

3.2.

The microstructure of pyrocarbon

3.2.1.

Texture of pyrocarbon

It is known that the performances of C/C composites, such as density, mechanical, electrical or thermal properties, are determined by the microstructure of pyrocarbon [17]. According to the classification method proposed by Lieberman and Pierson [18], Pierson and Lieberman [19], and Reznik and Huttinger [20], textures of the pyrocarbon are classified into three main types: rough laminar (RL), smooth laminar (SL) and isotropic (ISO). The RL is optically anisotropic with a high phase

Fig. 6 – Arrhenius plot obtained from the results present in Table 2 showing exponential relationship between the densification rate and the reciprocal of Td in the range from 900 to 1200 C.

shift, characterized by various amounts of growth cones issued from the substrate or from the deposit itself. The SL is also optically anisotropic, but with a lower phase shift, and is devoid of cones and corresponds also to a concentric deposit. The ISO is optically ISO, the phase shift is nil. The PLM micrographs of the polished surfaces of the samples prepared at different Td, measured by PLM apparatus (NEOPHOT 21) with a magnification of 1000 times, were shown in Fig. 7. We can see clearly that the fibers are surrounded concentrically by a ring of pyrocarbon and RL carbon constitutes the majority of the matrix. However, SL occurs at Td lower than 1000 C (see Fig. 7a and b) and ISO occurs at a higher Td about 1200 C (see Fig. 7d). The phase shifts for the PLM images from Fig. 7a–d are 21.3, 23.5, 29.8 and 6.5, respectively. The similar result of relationship between texture and temperature has been reported in the previous literatures [21,22] with methane as a carbon source. These phenomena would be explained by the formation mechanism of the pyrocarbon at different temperatures studied by the combination of several models including the particle-filler model [23]. The precursor in this experiment, kerosene, is a mixture of hydrocarbons without constant melting and boiling point.

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Fig. 7 – PLM micrographs of samples prepared at different Td, (a) 900 C; (b) 1000 C; (c) 1100 C and (d) 1200 C.

The thermodynamics, evaporation, decomposition, and the saturated vapor pressure of the reagent gas will vary with the temperature, which would result in variable reaction pathways and rates. As a consequence, different textured pyrocarbons were obtained. At a lower Td, the heat energy supplied to reagent is low and the saturated vapor pressure improves relatively to some extent. Therefore, a variety of large pieces of intermediates are preferentially formed, which tend to bring SL at a low transformation rate through dehydrocyclization [5]. Besides, the formation of sufficient amounts of aromatic hydrocarbons is limited, and the aromatic growth mainly depends on ethine addition reactions. According to the existence of an excess of small linear hydrocarbons, ethine addition to three carbon bays becomes significant, which leads to fivemembered rings and thus to medium-textured carbon of SL [23]. On the contrary, at a much higher Td, the heat energy supplied to reagent is greatly strong and the saturated vapor pressure decreased, the reagent pressure and concentration are rather high correspondingly. As a result, large polycyclic aromatic hydrocarbons (PAHs) are preferentially formed through gaseous action among reagents. Hydrocarbons are usually shaped through the densification of these large PAHs on the basal layers of the substrate. Because of much high carbonization rate, transition for crystallization of the deposition species is retarded. Thus, the low-textured pyrocarbon of ISO were formed at 1200 C. At a medium Td, a great amount of small intermediates may be formed owing to the stronger activation function and lower saturated vapor pressure compared with the lower Td. These species favor the regular arrangement in local range, but chaos in the whole area because of much high den-

sification rate. Hence, the RL is obtained preferentially. Simultaneously, the growth process of aromatic hydrocarbons is mainly decided by the comprehensive function of the aryl– aryl combination followed by intramolecular dehydrocyclization and ethine addition reactions. When an optimum ratio of cyclic aromatic hydrocarbons to small linear hydrocarbons is formed, high-textured pyrocarbon would be obtained [24]. In addition, some cones can be seen in Fig. 7b and these cones originate at the fiber surface because of their roughness. We can see from Fig. 7c that most of the texture is the RL. As a result, 1100 C is an optimum Td resulting in high-textured pyrocarbon with kerosene as precursor. According to the particle-filler model [23], it can also be deduced that an optimum ratio of aromatic hydrocarbons to ethane is obtained at Td of about 1100 C.

3.2.2.

Morphology of pyrocarbon

The SEM micrographs of fracture planes of samples prepared at different Td, obtained by SEM instrument (JEOL JSM-5800) were shown in Fig. 8. We can see that some similar characteristics exist in all samples. For instance, residual holes originate in fibers which were pulled out from the matrix. Moreover, we can see that the fracture planes in Fig. 8b and c are not simply smooth, but scalar in different extent. These phenomena indicate that the bonding at the interface between fibers and pyrocarbons are moderate. These efficient interfaces not only exhibit sufficient bonding strength, but can also be detached under applied stress. Hence, a desirable pseudoplastics failure mode is obtained in virtue of additional surface energy, pulling-out work and friction work. If the bonding strength is too great or weak, a brittle behavior may occur when cracks propagate. Fig. 8a and d show flatter fracture planes compared with Fig. 8b and c on account of

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Fig. 8 – SEM micrographs of samples prepared at different Td, (a) 900 C; (b) 1000 C; (c) 1100 C and (d) 1200 C.

the existence of parts of SL or ISO. Because the bonding strength between fibers and medium-textured of SL is so weak that the interfaces rupture directly under loads. We can also see from the morphology of composites that the fibers were surrounded by pyrocarbon with several layers. Furthermore, the surfaces of pyrocarbon formed at 900 C (see Fig. 8a) are smoother than that of pyrocarbon formed at 1200 C (see Fig. 8d). It is revealed that the pyrocarbon grows laminar by laminar in the pattern of the curved surface from Fig. 8d. The figure also indicates that the initial nucleation takes place in the gaseous condition. When the nuclei grow up to small spheres or semi-global grains, they drop randomly to the curved surfaces of the substrate. Then, the grains or densification units develop in the mode of the slice. Because there are many nuclei on the substrate surfaces, the laminae of the adjacent cones touch each other. The growth laminae propagate, resulting in the final cones of the deposited pyrocarbon [11], which can also be found in Fig. 8c. It seems that the initial nucleation in the gaseous condition tends to take place at higher Td. The initial nucleation happens on the surface of substrate at lower Td and the pyrocarbon grows laminar by laminar. Hence, the surface of pyrocarbons prepared at Td of 900 and 1000 C (see Fig. 8a and b) shows a smooth appearance. Besides, it can be seen that the texture of sample densified at 1200 C (see Fig. 8d) is less compact than the next samples (see Fig. 8a–c) owing to its lower bulk density.

4.

Conclusions

Large-size C/C composite samples were fabricated by CVI with multi-disc and single-cycle using kerosene as precursor. The initial densification rate increases from 1.76 to 16.34 g/ min with Td increasing from 900 to 1200 C. Sample of group D for Td = 1200 C shows the sharpest decrease of the densifi-

cation rate with the increase of time. The apparent activation energy of kerosene decomposition is 125.6 kJ/mol. Although, RL carbon constitutes the majority of the matrix, SL carbon occurs at Td lower than 1000 C and ISO structure occurs at a high Td of about 1200 C. The surfaces of pyrocarbon formed at low Td are smoother than that of pyrocarbon formed at high Td.

Acknowledgement The work was supported by Program for New Century Excellent Talents in University NCET 05-0195.

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