Prefabrication of SiC whiskers through induction of carbon fiber

HUANG Feng-ping(黄凤萍), LI He-jun (李贺军), LI Ke-zhi(李克智)，WANG Chuan-hui (王传辉)

C/C Composites Technology Research Center, Northwestern Polytechnical University, Xi’an 710072, China

Received 10 April 2006; accepted 25 April 2006

Abstract: A new way to prepare SiC whiskers through the induction of carbon fiber was suggested. With the processing steps of opening furnace firstly and then shutting it, rice hall, as the only raw material, was carbonified to get excess quantity of Si in rice hull.  After a certain catalyzer was added, SiC whiskers were prepared by means of the induction of carbon fiber. The component and morphology of the whisker were analyzed by XRD, SEM and TEM. The results reveal that the diameters of the whiskers range in 0.5-2 ?m and their lengths in 100-500 ?m. The whiskers are straight β-SiC crystals with smooth surface. The whisker is homogeneous and its productivity is 100%.  Two kinds of formation mechanisms, both VLS mechanism and vapor formation mechanism, are involved during the growth of the whiskers. But the vapor formation mechanism relatively plays a key role.

Key words: silicon carbide whisker; carbon fiber; carbonizing rice hall; induction

1 Introduction

SiC whiskers have attracted extensive attention due to their excellent properties, such as high elastic modulus, high strength to weight ratio, excellent thermal stability and chemical stability. The whiskers are one of the most popular reinforcing materials for both metal matrix and ceramic matrix composites because they possess satisfied chemical compatibility as well as wettability with aluminium, magnesium, and titanium. The high-performance composites with SiC whiskers as reinforcing material have been extensively used in such areas as national defense, power source, machinery, and chemical engineering[1-3].

There are two methods to prepare SiC whiskers according to the state of precursors: gas phase method and solid phase method. Gas phase method is to synthesize SiC whiskers by making use of the gas with C and Si, or organosilicone gas with C and Si. By contrast, solid phase method uses different raw materials, with appropriate activators, to obtain different SiC whiskers in argon atmosphere. Solid phase method is more popular in industry and preparing SiC whiskers from rice hall is attributed to this method.

Rice halls contain the necessary carbon and silica, intimately dispersed or carbided, provide a nearly ideal source material for production of SiC. Cutler had produced SiC whiskers from rice halls in 1976[4]. Using rice hall as raw material, the processing is simple and, accordingly, well-suited to be industrialized. But obtained SiC whiskers usually contain carbonaceous residue[5]. In order to get high quality SiC whiskers, it is necessary to remove the carbonaceous residue from the product. For this reason, a new process to prepare SiC whiskers through the induction of carbon fiber by carbonizing of rice hall was investigated. SiC whiskers are kept separate from the residue spontaneously in this processing, therefore, the SiC purity is obviously increased.

2 Experimental

Put the washed rice hall in the furnace at 650-700 ℃ (open the furnace door 2-10 min firstly and then shut it to maintain 3-5 h).  After heat treatment, carbonized rice hall and water were grinded 10 min in rapid grinder, and then the sample was dried. The catalyzer, 4% NaF, was added into dried sample and the mixture was made. The mixture was layered into graphite crucible, inside which was brushed with boron nitride as isolator in advance. The reaction material filled in such a height as three-quarters height of graphite crucible. Carbon fiber was arranged on the top of the mixture. Finally, the crucible was covered and set into vacuum furnace with graphite heater. In argon atmosphere, the sample was heated to 1 400 ℃ and then kept at this temperature for 4 h to form the SiC whiskers.

Both NaF and boron nitride are chemically pure. Carbon fiber is PAN-Ⅰfrom Jilin Carbon Factory.

The component, morphology and microstructure of the whiskers were analyzed by X’Pert PRO MPD(PANalytical) X-ray diffractometer(XRD), JEOLJXA-840 scanning electron microscope(SEM), energy dispersive spectroscope(EDS) and JEM-3010 transmission electron microscope(TEM).

3 Results and discussion

3.1. Microstructure of SiC whiskers

Fig.1 shows the SEM micrograph of the surface of SiC whiskers.  The diameters of the whiskers range 0.5-2 ?m and their lengths in 100-500 ?m. The whiskers are homogeneous with smooth surface and its productivity is 100%. There exist no inclusion and spherical crystal at all within the whiskers.

Fig. 1 SEM morphology of SiC_w heated to 1 400 ℃ and kept for 4 h

Fig.2 shows the X-ray diffraction patterns of the SiC whiskers. No impurity element diffraction appears in the XRD pattern, which indicates that high pure β-SiC whisker was prepared by this method.

Fig.3 shows the TEM morphology of the SiC_w whiskers. Integral multidimensional diffraction spots can be viewed in the Fig.3(a). It reveals that the whiskers are homogeneous crystallization. Lentoid submicrostruc-

ture can also be observed in Fig.3(a), and they are twinned. Upright diffraction stripes are obviously shown in the Fig.3(b). It reveals that the whiskers are integral with little crystal defect [6-9].

As indicated above, high quality SiC whiskers can

Fig.2 XRD pattern of the SiC_w

Fig.3 TEM morphologies of SiC_w

be prepared though induction of carbon fiber. The whiskers are pure and have integral crystalline state. Carbonaceous residue has been separated from the whiskers spontaneously and anticipated result was attained.

High quality SiC whiskers can be prepared by means of induction of carbon fiber owing to that the whiskers grow in a vapor phase reaction. SiO₂ and C in rice hall turn into amorphous SiO₂ and amorphous C with high chemical activity after carbonization. SiO₂ is decomposed firstly at a certainty temperature [10].

                     (1)

At the same time,

SiO₂+C→SiO(g)+CO(g)                       (2)

                           (3)

SiO(g)+2C→SiC+CO(g)                       (4)

SiO(g)+3CO(g)→SiC(g)+2CO₂(g)               (5)

Herein SiC grain is produced through reaction (4) by vapor-solid (VS) mechanism. SiC gas is produced though reaction (5) by vapor phase reaction. At the same time SiO gas diffuses into catalytic ball of rich carbon in powder. SiO reacts with carbon to form Si as

SiO(g)→Si+2CO                            (6)

Si+C→SiC                                 (7)                                           

SiC crystal nucleus is formed from reaction material. With the processing steps of opening furnace first and then shutting it, rice hall was carbonified to get more quantity of Si than C in carbonized rice hall. 

Fig.4 shows the area EDS pattern of the carbonized rice hall.

Fig.4 EDS spectrum of carbonized rice hall

In this way metastable SiC nucleus initially formed from the reaction material will, very possibly, be decomposed as follows:

2SiO₂+SiC→3SiO+CO                        (8)

When SiO and CO escape from the reaction material and react according to reaction (5), SiC gas can also be obtained.

SiC gas escapes from the reaction material and adsorbs inside the graphite crucible or the surface of carbon fiber which has been arranged on the top of graphite crucible. Inside graphite crucible has been covered with isolated layer in the experiment, enhancing the structure barrier between inside the crucible and SiC.  Relatively the structure barrier between carbon fiber and SiC weakens. Thus SiC gas adsorbs more easily on carbon fiber which has been arranged on the top of the crucible. It facilitates the induction of carbon fiber and achieves ideal prefabrication of SiC whiskers. More and more SiC gas is adsorbed on the carbon fiber along with reaction temperature rise and keeping-temperatur time, and supersaturation degree of SiC increases continually.  SiC is eventually crystallized when the supersaturation of SiC gas reaches a critical value.  Atoms adsorbed by crystal surface transfer along with one-dimensional direction by surface diffusion.  SiC whiskers gradually form.  Fig.5 shows the SEM micrographs of SiC whisker tip.

Fig.5 SEM micrographs of SiC whisker tip

Most of the whiskers are tip flat and have not catalytic ball observed at the tips (Fig 5(a)). These whiskers probably grow in a vapor phase reaction [3]. At the same time, a very few whiskers are observed to have a catalytic ball at the tips of the whiskers (Fig 5(b)), which suggests that these whiskers might grow in the vapor-liquid-solid(VLS) mechanism[11]. The results reveal that two kinds of formation mechanisms, both VLS mechanism and vapor formation mechanism, are involved during the growth of the whiskers. The reason is that catalyzer reacts with impurity (such as iron) and become low temperature volatiles in the reaction material.  The volatiles can evaporate from reaction residue and further form the catalytic ball with rich carbon on the carbon fibers which have arranged on the top of the mixture.  SiC crystal nuclei are formed according to reactions (6) and (7) when SiO gas diffuses into catalytic ball.  SiC crystal nuclei grow along with certain direction to produce the whiskers in the presence of catalyst. The catalytic balls are lifted and remained continually on the tips of whiskers.  But the diffraction peak of impurity has not been found in XRD (Fig.2).  It reveals that few whiskers grow in the vapor-liquid-solid (VLS) mechanism. The vapor formation mechanism relatively plays a key role.

4 Conclusions

1) SiC whiskers were prepared by means of the induction of carbon fiber. Carbonaceous residue has been separated from the whiskers and expected purpose was attained.

2) The diameters of the whiskers range between 0.5–2 ?m and their lengths in 100–500 μm.  The whiskers are straight β-SiC crystals with smooth surface. The whisker is homogeneous and its productivity is 100%. Both inclusion and spherical crystal were not found among whiskers.

3) Two kinds of formation mechanisms, both VLS mechanism and vapor formation mechanism, are involved during the growth of the whisker.  But the vapor formation mechanism relatively plays a key role.

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(Edited by LONG Huai-zhong)

Foundation item: Project(50225210) supported by the National Natural Science Foundation of China for Distinguished Young Scholars; Project(03H53044) supported by the Aeronautic Science of China

Corresponding author: LI He-jun, Tel: +86-29-88495004; Fax: +86-29-88491982; E-mail: Lihejun@nwpu..edu.cn