Self-healing and Oxidation Resistance of B-Si Doped Carbon Materials Derived from Modification Coal Tar Pitch

Preparation of B-Si-doped carbon materials

Coal tar pitch came from Wuhan Iron and steel (Group) Company. Pyridine borane was purchased from Henan Wanxiang Chemical Trade Co. Ltd. Polycarbosilane was bought from Suzhou Caifei Co. Ltd.

Coal tar pitch was grinded and extracted in toluene solution. The soluble components were removed from the toluene solution by distillation to obtain toluene soluble fraction of coal tar pitch. The softening point and pyrolysis yield of toluene soluble fraction of coal tar pitch was 52.4 °C and 39.5 wt. %, respectively. The reaction among coal tar pitch toluene soluble fractions (44.6 wt. %), polycarbosilane (29.8 wt. %) and pyridine borane (29.8 wt. %) had taken place in toluene solution through condenser pipe, accompany by a temperature of 180 °C for 6 h. The toluene was removed by distillation and the forming residue was a soluble brown solid that was modification coal tar pitch. The residue was then heated to 450 °C for 2 h at 2 MPa in autoclave. The semi-carbonization materials were obtained in a tube furnace under argon to rise from 800 °C to 1,600 °C for 1 h. The B-Si-doped carbon materials were obtained and used as experimental and characterization.

Characterization of B-Si-doped carbon materials

Phase compositions of B-Si-doped carbon materials were analyzed by X-ray diffraction (Philips X’PERT PRO MPD, made in Netherlands). The X-ray wavelength of copper target (Cu Kα) as a radiation source was 0.15406 nm. The working tube voltage and current was 40 kV and 30 mA, respectively. The thermal gravimetric and differential scanning calorimetry (TG-DSC) analyses of samples were measured by a comprehensive thermal analyzer (NETZSCH STA499C type, produced by Germany). In each measurement, 50–90 mg of sample was used. The sample was heated from ambient to 1,500 °C at a heating rate of 5 °C·min^–1 in dry air with a flow rate of 30 mL·min^–1. A scanning electron microscope (TESCAN VEGA3, provided by Chech) was used to observe the morphology of B-Si-doped carbon materials before and after oxidation.

XRD analysis of the B-Si-doped carbon materials

The effects of different temperatures on oxidation resistance of B-Si-doped carbon materials are investigated by the physical phase compositions of the materials. XRD patterns of the B-Si-doped carbon materials which were obtained by carbonization treatment in the temperature range of 800–1,600 °C are shown in Figure 1. The broad and weak diffraction peaks at about 2θ=25° can be attributed to the (002) reflection of the pseudo-graphite structure carbon, which expresses the formation of carbon with a low degree of graphitization during carbonization treatment. The diffraction peak of the materials obtained by carbonization treatment at 1,600 °C is stronger. The intensity of carbon peak of the samples for the temperatures of 800 and 1,000 °C is lowered deliberately. Because the higher the carbonization temperatures are, the more the graphitization of pitch carbon is [10]. When carbonization temperatures are 800–1,200 °C, the diffraction peaks at 2θ=14.6°, 27.8° correspond to the cubic B₂O₃ phase（JCPDS Card No. 00-006-0297）. B₂O₃ is produced by chemical reaction between small amount of oxygen contained in polycarbosilane and borane which are produced by the decomposition of pyridine borane during heat treatment. The diffraction peaks of B₂O₃ phase disappears when B-Si-doped carbon materials are obtained by carbonization treatment in the temperature range of 1,400–1,600 °C, which owes to the intense vaporization of B₂O₃ or reacts between B₂O₃ and carbon at high temperatures [11]. At the same time, the diffraction peaks at 2θ=33.6 °, 35.6 °, 41.4 °, 60.0 °, 71.8 ° in the materials obtained by carbonization at 1,200–1,600 °C are assigned to hexagonal β-SiC phase. The SiC present in the carbon matrix formed by carbonization treatment at 1,200 °C and 1,400 °C appears to be nano-crystalline [1]. However, as the samples obtained by carbonization treatment at 800 °C and 1,000 °C, there is no obvious characteristic peak of SiC phase, which shows that SiC phase is not formed in the carbon materials or SiC is an amorphous structure. With the carbonization temperatures increased, the relative intensity of the diffraction peaks increases and the full width at half maximum of the characteristic peaks of SiC decreases, which may result in the rapid growth of SiC crystal. In all cases, the B₄C phase does not appear in the B-Si-doped carbon materials obtained by carbonization treatment at 800–1,600 °C, which may be attributed to the non-crystalline structure of B₄C [9].

Figure 1:

XRD patterns of the B-Si-doped carbon materials obtained by carbonization treatment at different temperatures for 1 h.

SEM analysis of the B-Si-doped carbon materials

The morphology and structure of B-Si carbon materials are analyzed by SEM in order to investigate oxidation resistance of the carbon materials obtained by carbonization treatment at different temperatures (as shown in Figure 2). The B-Si carbon materials obtained by carbonization at 800 °C are loose structure and there are some micro-holes on its surface in Figure 2(a). The EDS spectrum of the carbon materials shows that it is mainly composed of carbon, boron, oxygen and silicon in Figure 2(b). The EDS spectrum and the above XRD analysis indicate that the materials isn’t formed crystalline SiC, which is better to prevent oxygen internal the materials. As can be seen from Figure 2(c), the SiC grain sizes of the carbon materials obtained by carbonization treatment at 1,200 °C are small. The materials have a relatively dense structure and the surface has few micro-cracks (as shown by the arrow). The materials also contain carbon, boron, oxygen and silicon (see Figure 2(d)) consistent with the XRD pattern of the carbon materials shown in Figure 1. The formation of dense structure may benefit from sintering action of B₂O₃ in the materials (see Figure 1), which is commonly used as a flux former and reduce the sintering temperature to get dense structure [12]. The SiC grain sizes of the carbon materials obtained by carbonization treatment at 1,600 °C are larger in Figure 2(e). There are some micro-cracks and micro-holes on the surface of the materials obtained by carbonization at 1,600 °C. The structure may result in the intense volatilization and escaping of B₂O₃, reaction between B₂O₃ and carbon during carbonization treatment at the temperature, whose boiling point is 1,500 °C at atmospheric pressure. Figure 2(f) shows few oxygen element, which infers to contain other oxide. The work should be the next further research. The SiC grain size of the materials obtained by carbonization at 1,600 °C becomes larger, which can be confirmed in Figure 1.

Figure 2:

Typical SEM images and EDS spectra of the B-Si carbon materials obtained by carbonization treatment at different temperatures (a, b) 800 °C, (c, d) 1,200 °C and (e, f) 1,600 °C.

Oxidation behavior of the B-Si-doped carbon materials

To investigate the influence of carbonization temperatures on the oxidation resistance of the resulting B-Si-doped carbon materials obtained by carbonization treatment at 800–1,600 °C, the materials were put into a tube furnace, and heated from room temperature to 1,000 °C for 1–10 h in air. Oxidation weight loss curves as a function of time are shown in Figure 3. The weight loss of the materials obtained by carbonization treatment at different temperatures is more for less than 3 h during oxidation in air. The oxidation weight loss of the materials tends toward stability when the oxidation time exceeds 3 h. It can be seen from Figure 3, in the same oxidation condition, the weight loss of carbon materials obtained by carbonization treatment at 1,200 °C is less than that of others. The oxidation weight loss of the materials obtained by carbonization treatment at 1,200 °C for 3 h in air is ca. 15 wt. %. However, the weight loss for the materials obtained by carbonization treatment at 1,600 °C is ca. 29 wt. %, which are much higher than those of the corresponding other materials. Obviously, the oxidation resistance of the materials obtained by carbonization treatment at 1,200 °C is better than that of others. The weight loss of B-Si-doped carbon materials owes to decreasing its mass for carbon oxidation and increasing its mass for SiC and B₄C oxidation. The carbon materials obtained by carbonization treatment at 800 °C and 1,000 °C do not form SiC crystal grains. Oxygen in air diffuses into the interior through loose structure and the micro-holes and reacts with carbon in the materials during oxidation, leading to start a continuous weight loss of the materials. After oxidation for a while, the silicon and boron elements are oxidized to generate B₂O₃ and SiO₂ that can form glassy protective film to prevent oxygen into the materials. So weight loss of the materials tends stability after 3 h in air. The SiC crystal in the materials obtained by carbonization at 1,200 °C has begun to form. To a certain extent, the structure of the materials prevents oxygen into the interior. The grain sizes of SiC of the carbon materials obtained by carbonization at 1,400 °C and 1,600 °C increase significantly. It is difficult to form continuous protective film on the surface of carbon materials because oxidation rate of the SiC crystal is slow. The above results of oxidation resistance indicated that choosing the appropriate carbonization temperatures in the following carbonization stage plays an important role in improving the oxidation resistance of the obtained B-Si carbon materials.

Figure 3:

Oxidation weight loss curves at 1,000 °C in air as a function of time for the B-Si carbon materials obtained by carbonization treatment at different temperatures.

In order to further understand the influence of carbonization temperatures on the oxidation behavior of the B-Si carbon materials, the morphology and structure of the materials after oxidation in air at 1,000 °C for 3 h have been observed by SEM. As shown in Figure 4, the micro-holes on the surface of the materials obtained by carbonization treatment at 800 °C disappear and form a smooth glassy film. But much pitch carbon is consumed because of reaction with diffusion oxygen before the micro-holes is sealed by the glassy film. The materials obtained by carbonization treatment at 1,200 °C have a relatively dense structure (see Figure 2(b)). The presence of B₂O₃ is beneficial for retarding of the oxidation of the carbon in the carbon materials in the temperature range of 450–850 °C, owing to that B₂O₃ has low viscosity and good mobility and these characters endow it with the function of sealing the micro-cracks [13]. When the oxidation temperature reaches 1,000 °C, B₂O₃ cannot protect the carbon materials owing to forming gasification to escape. However, the surface of the materials has a smooth continuous protective film which seals the micro-crack to prevent oxygen to diffuse (see Figure 4(b)). The protective film contains SiO₂ and B₂O₃ and prevents oxygen to diffuse into the internal. The surface of the materials obtained by carbonization treatment at 1,600 °C has some closed micro-holes, shown in Figure 4(c). The higher the carbonization temperatures are, the larger the SiC grain sizes of the materials become. Large SiC grains are difficult to be oxidized, and as a result there is no continuous protective oxide films formed on the surface of the carbon materials during air oxidation at 1,000 °C. Oxygen in air diffuses into the interior of the carbon materials through the micro-cracks and micro-holes and then reacts with carbon, which leads to more weigh loss of the materials (see Figure 3). After a while, the B₂O₃ and SiO₂ form a continuous protective oxide film sealing the micro-cracks and micro-holes in the materials, which retards the diffusion of oxygen and decreases the oxidation rate of carbon.

Figure 4:

Typical SEM images of the B-Si-doped carbon materials obtained by carbonization treatment at different temperatures after oxidation at 1,000 °C in air for 3 h (a) 800 °C，(b) 1,200 °C，(c) 1,600 °C.

TG-DSC curves of the B-Si-doped carbon materials obtained by carbonization treatment at 1,200 °C are investigated in air flow at a heating rate of 5 °C/min in order to analyze dynamic oxidation behavior of the materials in Figure 5. The mass of the carbon materials decreases gradually when the oxidation temperature is from room temperature to 350 °C. The weight loss of the carbon materials in air flow at 350 °C is about 1.3 wt. %, which may result in the volatilization of the moisture absorbed and easily volatile components in the materials. Weight loss of the materials is not obvious mass change in the temperature range of 350–840 °C. Weight loss of the carbon materials is 4.0 wt. % when the oxidation temperature is in the temperature range of 840–1,500 °C. The weight loss in this temperature range obviously increases. With the oxidation temperature increasing, the protective glassy film composed of SiO₂ and B₂O₃ forms gasification to escape at high temperature in the flowing air. The mass of the materials decreases obviously. In addition, the DSC curve showed that the endothermic peak appeared near 220 °C, which is the result of the volatilization of the moisture absorbed and easily volatile components. There are exothermic peaks at 1,100 °C, 1,330 °C and 1,440 °C. It is indicated that the materials occurs aggravate oxidation reaction with oxygen. It is consistent with the TG results.

Figure 5:

TG-DSC curves of the carbon materials obtained by carbonization treatment at 1,200 °C in air flow at a heating rate of 5 °C/min.

Oxidation resistance mechanism of B-Si-doped carbon materials

It was complex to occur many kinds of chemical reactions with B-Si-doped carbon materials and oxygen at a certain condition. The possibility reaction and phase transition were analyzed by changing results of the standard Gibbs free energy. The results showed that it was easy to produce SiO₂, B₂O₃ and CO₂ in reaction process of B-Si-doped carbon material and oxygen. It played an important role that the SiO₂ and B₂O₃ influenced on oxidation resistance properties of B-Si-doped carbon materials. The B₂O₃ has a low melting point (460 °C) and good fluidity to seal the micro-holes in the temperature range of 450–850 °C. It can reduce the sintering temperature to obtain dense ceramics. At the same time, B₂O₃ was beneficial for the retarding of the oxidation of the carbon in the materials in the temperatures. However, the SiO₂ formed by the oxidation of SiC did not possess the function of sealing the micro-holes because of its high viscosity and poor fluidity. The B-Si-doped carbon materials obtained by carbonization treatment at 1,200 °C contain the small grain sizes of SiC and B₂O₃. The materials had a relatively dense structure and oxygen diffused relatively slowly into the internal. In the process of oxidation, the SiO₂ formed by the oxidation of SiC might be soluble in liquid B₂O₃ and would form liquid borosilicate, which existed in the form of B₂O₃-SiO₂ eutectic solutions [14]. The protective glassy film had a self-healing capability. The glassy film had much lower volatility and oxygen permeability so that it could inhibit volatilization of B₂O₃ and oxidation of carbon substrate. The B₂O₃-SiO₂ eutectic solutions were synergistic oxidation resistance effects, which could been better oxidation resistance of the B-Si-doped carbon materials obtained by carbonization treatment at 1,200 °C.