Carbon-bonded carbon fiber composites containing uniformly distributed silicon carbide

Abstract

Carbon-bonded carbon fiber composites (CBCFs) containing SiC particles with uniform microstructure were produced using an innovative dispersion and flocculent approach. The mixed phenolic (P_f), SiC particles and chopped carbon fibers (C_f) were blended uniformly using polyethyleneimine (PEI) as a dispersant and polyacrylamide (PAM) as a flocculating agent. A homogeneous distribution of SiC particles coated CBCFs was obtained with the addition of 0.6 wt% PEI and 0.8 wt% PAM. The effect of PEI and PAM content on the microstructure, slurry dispersibility and stability of SiC-modified CBCFs was investigated. The compressive strength and compression modulus of the SiC-modified CBCFs were improved compared with unmodified CBCFs. This simple and versatile approach can be used to prepare large-scale and modified CBCFs.

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1. Introduction

Carbon-bonded carbon fiber composites (CBCFs) have attracted great attention due to their high porosity, low thermal conductivity and high temperature capability.¹ The unique properties of CBCFs suggest many potential applications, e.g. high-temperature heat insulation, lightweight structure, and thermal protection system (TPS) materials matrix.² CBCFs were developed in the late 1960s and improved in the 1980s as insulating materials to protect specific equipment launched in NASA space missions.^3,4 They were also used as adsorbent materials and gas separation materials.⁵ In recently years, many ceramic matrix composites (CMCs) have been fabricated using CBCFs as a green matrix by chemical vapor infiltration (CVI),^6,7 chemical vapor deposition (CVD)^8,9 and precursor impregnation and pyrolysis (PIP).¹⁰ As a thermal protection materials matrix, the interface bonding between CBCFs and other reinforcing phases, as well as oxidation resistance, should be enhanced.^11–13 CBCFs modified with SiC will increase the high-temperature tolerance and oxidation resistance.¹⁴ However, it is susceptible to delaminating and accumulation during slurry molding and shaping. During the fabrication of modified CBCFs with multi-phase systems, one of the key issues to ensure the uniform structures of CBCFs is to achieve the effective dispersion of chopped carbon fibers (C_f), phenolic (P_f) and SiC particles.

Flocculants play an indispensable role in many industrial sectors, such as wastewater treatment,¹⁵ mineral processing,^16,17 and sludge dewatering.¹⁸ The dispersion mechanism of organic flocculants is usually attributed to a bridging effect between solid particles and flocculating substance with a three-dimensional network structure.¹⁹ However, during CBCFs processing, water-based slurry contains C_f and solid particles with different density, and it is necessary to use a suitable flocculant to bind the fine solid particles to the C_f without agglomeration of solid powder particles. Polyethyleneimine (PEI) is positively charged and provides a steric repulsion force by its branch and high chemical activity due to the presence of many amines and can form a covalent bonding with other molecules.^20,21 Many reported examples have shown the effectiveness of such dispersing agent in the system of ceramic colloidal process and surface adsorbent modification.^22–24

At present, most CBCFs composites reported in previous investigations were manufactured by Calcarb Ltd.^25–28 The fabrication procedures of the CBCFs composites are by vacuum filtration and pressure filtration technique.²⁹ These technique consists of dispersing the fibers and powdered resin in water by mechanical dispersion methods. However, the fibers are inclined to generate fiber agglomerations which reduce the mechanical and thermal property anisotropies²⁷ and this processing is time-consuming during the process of filtration. Recently, our research group³⁰ has successfully fabricated CBCFs with uniform chopped C_f distribution using the PAM as the dispersant. However, it seems to be difficult to prepare modified CBCFs containing other solid ceramic particles (i.e., SiC). It will engender delamination (Fig. 1) along the z direction and uplifts along the xy direction in local region during curing and carbonization due to the agglomeration and un-uniform distribution of the introduced SiC powder particles.

Fig. 1 Optical picture of a cured CBCFs with the size of 220 × 220 × 20 mm³. (a) The z axis direction with and without the dispersion and flocculent approach; (b) high magnification without the dispersion and flocculent approach; (c) high magnification with the dispersion and flocculent approach.

Here we report a novel and efficient dispersion and flocculent method, not only to disperse P_f and SiC powder particles uniformly in CBCFs, but also to separate the SiC particles from water easily. The merits of this method are twofold: (i) uniform dispersion of mixtures in deionized water is prepared with the effect of PEI; (ii) solid particles are evenly adsorbed at C_f surface with the synergetic action among PEI, PAM and water.

2. Experimental

2.1. Materials

C_f (Jilin Jiyan High-tech Fibers Co., Ltd. Jilin, China), SiC (1 μm, purity >99.5%, Weifang Kaihua Micropowder Co., Ltd., Weifang, China) and P_f (PF4090, fineness >95%/200 mesh, Holy spring chemical co., Ltd., Juxian, China) were used as raw materials. PEI (Aladdin Co. Ltd., Shanghai, China), a cationic polymer with an average molecular weight of 10 000 was used as the dispersant. PAM (Fuchen Chemical Reagent Co. Ltd., Tianjin, China), a nonionic polymer with an average molecular weight of 5 000 000 was used as the flocculating agent.

2.2. Preparation of modified CBCFs

Firstly, the required amount of PEI (20 mg mL⁻¹) was added to 200 mL deionized water with mechanical stirring for 10 min. Then the C_f with 1 mm lengths were added to disperse in the deionized water for 20 min, and homogeneous black dispersed slurry was obtained. P_f and SiC particles were subsequently added to the slurry. The slurry color changed from black to light yellow due to the addition of P_f and SiC particles. The above mixed slurry was still stirred, ensuring PEI sufficiently grafted onto the surface of the added solid particles and fibers. After stirring for 2 h, the stirred speed was increased to 400 rpm, and a PAM solution with 10 mg mL⁻¹ concentration was added to the slurry and further stirred for 15 min. The PAM flocculant should be added slowly in case of local concentration.

The mixed slurry was quickly poured into a cylindrical steel mould³⁰ to remove the water from the slurry under an appropriate pressure to obtain a desired density. The acquired wet billets were cured at 80 °C for 2 h and 150 °C for 3 h to remove the residual water. Then the cured composites were carbonized at 1000 °C in Ar (heating rate: 10 °C min⁻¹; soaking time: 1 h). The composites were formed into cylinders of Φ 30 × 20 mm with a density of 0.26 g cm⁻³.

2.3. Characterization

The zeta potential of the dilute suspensions was subjected to zeta-potential measurement (Zeta sizer Nano ZS, Malvern, UK) to evaluate their surface properties. The tested liquid samples were diluted to 0.05 mg mL⁻¹ before measurements. The ionic strength was maintained at 10⁻³ m using NaCl. The purpose of adding this indifferent electrolyte was to ensure a fairly constant ionic strength and activity coefficient for the slurries.³¹

The surface functional groups were investigated by transmission infrared spectra (FTIR) obtained from a FTIR spectrometry (Spectrum One, PerkinEmer, US). The samples were mixed with KBr at a ratio of 5 : 95. FTIR were recorded within a range of 400–4000 cm⁻¹.

Particle size distributions were measured by a laser particle size analyzer (Hylology Instruments Co. Ltd., Dan-dong, China) using 100 g of solutions with 0.6 wt% PEI and with different quantities of PAM in deionized water. For comparison, the blank samples containing SiC with 0.6 wt% PEI, C_f with 0.6 wt% PEI, P_f with 0.6 wt% PEI and the water-based slurry contains SiC, C_f and P_f with 0.6 wt% PEI were separately determined and were denoted as SiC-0.6 wt% PEI, C_f-0.6 wt% PEI, P_f-0.6 wt% PEI and C_f–P_f–SiC-0.6 wt% PEI, respectively.

The microstructure was observed by scanning electron microscopy (SEM, FEI Sirion, Holland) at 20 kV. The Raman spectra were collected on a Raman Station (B&WTEK, BWS435-532SY) with a near infrared laser operating at 532 nm. Compression testing tests were performed on a Suns Co. CMT-5304 electronic universal testing machine to determine the mechanical property of the composites. The effective size of the compressed samples was 10 × 10 × 12 mm and at a crosshead speed of 2 mm min⁻¹.

3. Results and discussion

3.1. Effects of PEI on slurry dispersibility and stability

Fig. 2 shows the relationship of the zeta potential as a function of PEI. The zeta potential for pristine C_f, SiC and P_f are −31.4, −15.2 and −23.5 mV, respectively. It is evident that they are negatively charged. With the addition of the PEI, an increase of the measured zeta potential revealed that PEI was not only absorbed onto the SiC surface,³² but also onto the P_f and C_f surface. The absolute maximum zeta potential for C_f, SiC and P_f, which are 35.0, 40.2 and 57.5 mV, respectively, were obtained when 0.6 wt% PEI was used as dispersant in deionized water. The effect of PEI was attributed to the electrostatic repulsion provided by adsorbed surfactants, which stabilize the particles against van der Waals attraction.³³ Zeta potential is usually used as a criterion for surface charge, because it is close to the Stern potential and it can be measured. The high relative zeta potential implied the better dispersion of the SiC in the medium, since the zeta potential reflects the electrostatic dispersion effect.³⁴ When the PEI content increased to 0.8 wt%, the zeta potential of SiC, P_f and C_f decreases slightly. It is conceivable that adsorption of ionized polymer chains is dominated by electrostatic interactions between the ionized sites on the polymer and the surface-charged sites on the solid.³⁵ This result showed that C_f, P_f and SiC powders could be well dispersed in deionized water with 0.6 wt% PEI.

Fig. 2 Zeta potential of suspension vs. PEI content.

The FTIR upon C_f, SiC, P_f, C_f-0.6 wt% PEI, SiC-0.6 wt% PEI and P_f-0.6 wt% PEI are shown in Fig. 3. Fig. 3(a) shows the FTIR spectra of C_f and C_f-0.6 wt% PEI. The characteristic absorption peaks of C_f as shown in Fig. 3(a) are presented at 3458, 2050, 1640, 1400, 1098 and 672 cm⁻¹. The band of O–H stretching vibrations at 3454 cm⁻¹ due to surface hydroxylic groups and chemisorbed water.³⁶ The 2050 cm⁻¹ is C–C absorption peaks. The presence of absorption peaks at 1640 cm⁻¹ can be attributed to the stretching vibrations of C=O moieties. The peaks at 1400 and 1098 cm⁻¹ were hydroxyl group absorption bands. In contrast, the C_f-0.6 wt% PEI spectra has increased relatively in absorbance of 2360 and 843 cm⁻¹ due to the effect of N–H. The broad peak at 2050 and 1098 cm⁻¹ both disappear in the system of C_f-0.6 wt% PEI. This means that the PEI was grafted onto C_f by hydrogen bonding interactions. In addition, due to the lower PEI content, three characteristic peaks of amide group at 1672, 1610 and 1425 cm⁻¹ are covered by the characteristic absorption peaks of carbon fibers.³⁷

Fig. 3 FTIR of different sample. (a) Curve-1, C_f; curve-2, C_f-0.6 wt% PEI; (b) curve-1, SiC; curve-2; SiC-0.6 wt% PEI; (c) curve-1, P_f; curve-2, P_f-0.6 wt% PEI.

The curve 1 and 2 in Fig. 3(b) represent SiC and SiC-0.6 wt% PEI, respectively. A new absorption peak is observed at 2372 cm⁻¹ in SiC-0.6 wt% PEI. The absorption at 1622 cm⁻¹ increases to 1652 cm⁻¹ was due to the effect of amide groups.³⁸ It suggested that the dendrite PEI could adsorb onto the surface of SiC particles with a higher positive charge density.

In the FTIR spectra of P_f-0.6 wt% PEI (Fig. 3(c)), the absorption peaks at 3000–3500 cm⁻¹ are broadened which can be attributed to the overlap peak of phenol hydroxyl and –NH₂. Meanwhile the absorption at 672 cm⁻¹ and the characteristic bands of the 1,3,5-trisubstituted aromatic ring at 1008 cm⁻¹ disappeared after the PEI treatment. Furthermore, it was also seen that a strong increase of the absorption peaks at 874 cm⁻¹ appeared, corresponding to the formation of N–H from PEI.

3.2. Effects of PAM on slurry dispersibility and stability

Here, the stability of the mixed slurries with 0.6 wt% PEI and different amounts of PAM were studied through particle size distribution measurement. The particle size distribution of C_f-0.6 wt% PEI, P_f-0.6 wt% PEI, SiC-0.6 wt% PEI and C_f–P_f–SiC-0.6 wt% PEI (as in Fig. 4) were measured to explain the effect of PAM. As shown in Fig. 4 (curve-4), the particle size distribution peak of slurry (C_f–P_f–SiC-0.6 wt% PEI) was formed by the stacking of C_f, P_f and SiC. It can be divided into four main regions: (i) the region of 0.25–6.0 μm mainly was determined by SiC and P_f particles, as shown in Fig. 4 (curve-1) and Fig. 4 (curve-2); (ii) the region of 6.0–48.0 μm was mainly determined by SiC, P_f particles and C_f; (iii) the region of 48.0–230.6 μm was mainly determined by P_f particles and C_f; (iv) above 230.6 μm is mainly determined by C_f. In general, Fig. 4 shows that both SiC and P_f were bimodal distribution and C_f was trimodal distribution. The slurry was multimodal distribution due to their superposition effect.

Fig. 4 Particle size distributions of samples and slurry in the presence of PEI with content of 0.6 wt%.

Noticeably, once solid particles (i.e. SiC and P_f) are absorbed onto the C_f surfaces, the flocculation of C_f will take place and the distributed particles change to be larger aggregates. Fig. 5 indicate that particle content in area (i) and (ii) gradually decreases with increasing PAM content when the PAM flocculant content is less than 0.8 wt%. When the PAM content is 0.4–0.8 wt%, the particle content in area (i) is zero, in area (iii) gradually decreases and in area (iv) gradually increases. This can be explained as follows: after the flocculant is added to slurry, SiC and P_f particles are adsorbed onto the C_f surface by the effect of the flocculant. Flocculation among the C_f can also make the particle content increase in area (iv). In addition, with higher PAM content (1.0 wt%), the particle content (in area (iii) and (iv)) decreased while it increased in area (ii) and reappeared in area (i), corroborating thus the expected deflocculating action of these excess flocculant.

Fig. 5 Effect of the increasing PAM content on the particle size distribution in slurry.

Fig. 6(a)–(e) showed the SEM images of mixtures with different PAM content. It can be found that the particles can be uniformly adsorbed onto the surface of C_f with 0.8 wt% PAM additive, as shown in Fig. 6(c)–(e). As depicted in Fig. 6(a) and (b), some agglomerations with the diameters of 50–100 μm were detected due to the excess flocculant with 1.0 wt% PAM additive. This indicated that the excess flocculant would lower the absorbability of C_f and lead to the agglomeration of solid particles.

Fig. 6 Morphology of mixtures with different PAM content. (a and b) 1.0 wt% PAM and high magnification; (c–e) 0.8 wt% PAM and high magnification.

Fig. 7 showed the Raman spectra of pristine SiC particles, C_f and C_f coated by SiC particles. The Raman spectra of pristine C_f (Fig. 7(a)) showed two characteristic peak of C at 1335 cm⁻¹ (D peak) and 1557 cm⁻¹ (G peak). Pristine SiC particles consisted of one transverse optical (TO) peak at 780 cm⁻¹ and one longitudinal optical (LO) phonon peak at 960 cm⁻¹.³⁹ As expected, the corresponding curves of C_f coated by SiC particles were comparable to pristine SiC particles and C_f as show in Fig. 7(c).

Fig. 7 Raman spectra of (a) pristine SiC particles, (b) pristine C_f and (c) C_f coated by SiC particles.

Table. 1 shows the mechanical properties of the unmodified CBCFs and CBCFs modified by SiC. Compared to the unmodified CBCFs, the compressive strength and compression modulus of the SiC-modified CBCFs have been improved both in xy and z direction. Due to the introduction of both P_f and SiC, the load-bearing capacity of chopped carbon framework was enhanced, and the movement and deformation of the fibers was restricted during the compression process especially at the junctions of fibers.

Table 1 Mechanical properties of CBCFs and SiC-modified CBCFs

Specimen	x/y direction		z direction
	Compressive strength (MPa)	Modulus of compression (MPa)	Compressive strength (MPa)	Modulus of compression (MPa)
CBCFs	1.17 ± 0.05	15.52 ± 0.08	0.52 ± 0.04	8.98 ± 0.08
SiC-modified CBCFs	1.36 ± 0.06	17.33 ± 0.07	0.61 ± 0.03	11.36 ± 0.12

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3.3. Dispersion and flocculation mechanisms of modified CBCFs

The dispersion and flocculation mechanisms of water-based slurry are shown in Fig. 8. PEI with a low molecular weight can be absorbed onto the solids (C_f, SiC and P_f) surface by its branch and high chemical activity, which enables them have positive charge and steric repulsion force. So the modified solid particles (i.e., SiC) can achieve well dispersion in water-based slurry, which is similar to the previous reports by Zhang.^40,41 When PAM is added to the system, PAM macromolecular chains work with the PEI and are absorbed onto the surfaces of P_f, SiC and C_f by effect of winding and bridging, making solid particles preferentially absorbed onto the C_f surface. The flocculation process leads to the absorbing of solid particles on the C_f surface with a good capacity for water-resistance⁴² and separate from water, so the color of slurry changes from light yellow to black as show in Fig. 8(c) and (d).

Fig. 8 Dispersion and flocculation mechanisms of water-based slurry (a) the initial raw materials; (b) schematic showing the dispersion mechanisms of water-based slurry; (c) photographs of dispersion progress of water-based slurry; (d) schematic showing the flocculation mechanisms of water-based slurry; (e) photographs of flocculation progress of water-based slurry.

4. Conclusions

This investigation has revealed that CBCFs containing uniformly distributed SiC can be prepared by an effective dispersion and flocculent approach. PEI was identified to be absorbed onto the surface of C_f, P_f and SiC, which imparts the positive charge to the solid surface and promotes a good dispersion effect in water. By introducing PAM flocculant into the water-based slurry, PAM with long-chain molecules can work with branched PEI on the surface of C_f, SiC and P_f. With the synergistic action between PEI and PAM, the SiC and P_f can be absorbed onto the surface of C_f, thus avoiding the gradient sedimentation and achieving uniform microstructure of CBCFs. The excessive PEI and PAM can cause the aggregation of P_f and SiC. The optimum amounts to obtain a stable and homogeneous SiC dispersion for preparing CBCFs are 0.6 wt% PEI and 0.8 wt% PAM as demonstrated in the present work.

Acknowledgements

This research was financially supported by the National Natural Science Foundation of China (no. 51272056, 11121061 and 91216301). FXQ is supported under JSPS Fellowship.

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