Synthesis of SiC matrix in ceramic matrix composite from Gaseous SiO using Ni

Silicon carbide (SiC) can be synthesized from gaseous silicon monoxide (SiO) and toluene vapor in the presence of a nickel (Ni) component, in the form of Ni or Ni oxide form at a temperature of 1400 °C with very high catalytic activation. With the aid of the catalytic e ﬀ ect, SiC can be formed from continuously supplied gaseous SiO and toluene vapor such that: ﬁ brous SiC is formed in a free space containing a Ni component having a particle size of several micrometers; a dense SiC body is formed from a SiC powder compact carried with a Ni component, and particularly, SiC matrix is formed between SiC ﬁ laments of SiC fabric when a SiC powder carried with a Ni component is placed within the SiC fabric. The latter two cases are believed due to SiC formation to embed the small spaces between SiC particles or ﬁ laments from gaseous SiO and toluene vapor at the positions of Ni component spread substantially over the entire surface of SiC particles, in which the outer surface of the SiC powder compact or SiC fabric is far from blocking since the Ni component is not placed at the outer surface. Accordingly, this study proposes a novel chemical vapor in ﬁ ltration process for creating the SiC matrix in a ceramic matrix composite.


Introduction
Silicon carbide (SiC) has exceptional properties such as high temperature strength, oxidation resistance, and abrasion resistance. Thus, it is widely preferred in the ceramic matrix composite (CMC). 1), 2) Researchers have previously reported that the combination of gaseous silicon monoxide (SiO) and carbon compounds can be beneficial for the formation of SiC in the presence of a particular catalyst at relatively lower temperatures. For example, it has been determined that the presence of Fe supports the formation of the SiC coating. The formation of the SiC layer through the combination of gaseous SiO and carbon compounds 3) can be promoted on the surface of the Si metal. 4) Further, SiC synthesis with Ni catalyst using polycarbosilane or Si tetrachloride is described in the literatures. 5), 6) This study investigated the effect of Ni as a catalyst to synthesize SiC through the combination of gaseous SiO and toluene vapor. The results showed the possibility of realizing a new chemical vapor infiltration (CVI) process that could assist in SiC formation. The researchers acknowledge that one of the key points in current CVI processes is to keep the temperature below 1400°C, thereby preventing the degradation of fibers in the CMC. 7) 2. Experimental procedures

SiC formation in free space
A horizontal tube furnace with an inner diameter of 42 mm, an outer diameter of 50 mm, and a length of 200 mm was designed in the middle of a core tube with an inner diameter of 52 mm, an outer diameter of 60 mm, and a length of 1000 mm of a horizontal tube furnace. A support plate, sample, and SiO were placed inside the inner tube, as shown in Fig. 1.
The commercially available Ni powder (with an average particle diameter of 500 nm) was mixed with the same weight of isopropyl alcohol. The prepared paste was applied and dried on an alumina plate with 10 © 10 © 0.5mm thickness, hence 3.0-mg Ni powder was deposited in  Granular SiO of 500 mg was placed on both sides of the sample, as shown in Fig. 1. Then, 1 g of an inorganic fiber felt was soaked in 5 g of toluene, and the felt with the toluene was placed at the outer end of the inner tube (right side of Fig. 1); 2 g of another inorganic fiber felt was soaked in 10 g of toluene, and the felt with the toluene was deposited at the inner end of the core tube (right side of Fig. 1); 1000 mg of SiO was placed inside the inner tube and 15 g of toluene was placed inside the core tube; then, both ends of the core tube were closed with Si rubber stoppers.
The Si rubber stopper (left side of Fig. 1) was pierced by a stainless tube with an outer diameter of 8 mm to communicate between the inside and outside of the core tube.
Under the abovementioned conditions, calcination was carried out at an elevated temperature of the middle outer surface (core tube) and held at 1400°C for 10 h. This process was repeated 10 times by giving 15 g of toluene and 1000 mg of SiO at ³20°C before each calcination process. It was observed that after each calcination cycle, ³90 wt % toluene escaped from inorganic fiber felts and a small amount of gas always flowed out of the stainless tube during the 10 h heating period; the total SiO weight of 1000 mg had reduced to half of the initially deposited positions. Thus, the sample was largely exposed to gaseous SiO and toluene vapor during all calcination periods for 10 h at 1400°C. Researchers observed that toluene was the most effective in the abovementioned experiment among various C-containing compounds, including ethanol, butanol, acetone, xylene, ethylene glycol, benzene, and naphthalene.
The prepared product was characterized using a scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD).

SiC formation in green body
SiC powder (alpha-type, produced via the silica reduction process, BET: 15 m 2 /g) was processed to support Ni(III) oxide in an amount of 0.5 part by weight of Ni per 100 parts by weight of SiC powder by mixing an aqueous solution of 5 wt % Ni(II) nitrate hexahydrate with the SiC powder, followed by drying at 100°C and heating the mixture in an air atmosphere at 700°C for 1 h to support the Ni oxide on the SiC powder.
The SiC powder was uniaxially pressed in a mold to form a cylindrical tablet with a diameter of 10 mm and a height of 2 mm. The resulting tablet was calcinated while being exposed to gaseous SiO and toluene vapor at 1400°C for 10 h under the placement shown in Fig. 1. The calcination was repeated 10 times under the same placement, in which 1000 mg of SiO and 15 g of toluene in total were deposited, as described above before each of the calcination processes.
The prepared product was evaluated using SEM and Vickers hardness tester.

SiC formation in woven fabric
The SiC woven fabric (Hi-Nicalon Type S, 8 satin weaves, filament diameter: 13¯m) was commercially obtained from NGS Advanced Fibers Co., Ltd. The SiC powder, supporting Ni oxide as 0.5 part by weight of Ni per 100 parts by weight of SiC powder by weight (prepared in the same manner as described above), was mixed and made into a slurry using a ball mill. The SiC woven fabric was infiltrated with the slurry under a vacuum and gradually dried to allow the Ni oxide-supporting SiC powder supporting the Ni oxide into the SiC woven fabric; then the SiC woven fabric was cut with scissors to take a sample.
The sample was 30 © 20 mm in size with a thickness of 0.5 mm and the rate of SiC to woven fabric was 39:61 wt %.
The sample was calcined and exposed to gaseous SiO and toluene vapor at 1400°C for 10 h. The calcination was repeated 10 times using the placement shown in Fig. 1 similar to the aforementioned process.

SiC formation in free space
The Ni powder on the alumina plate before calcination is shown in Fig. 2(a)  after 10 calcination cycles at 1400°C for 10 h. After calcination, fibrous material was generated instead of the Ni powder, and the volume of the fibrous material increased along with the calcination cycle. From XRD analysis, the fibrous material generated via calcination was determined to be a highly crystalline beta-type SiC [ Fig. 3(a)].
Further, the catalytic effect of the Ni to produce SiC was high and the weight of the fibrous material became 345 mg after 10 calcination cycles; this weight is ³100© more than the initial weight of Ni powder (3.0 mg). Figure 4(a) reveals that the material has a fibrous mass containing multiple filaments. The EDX result of SiC revealed that Ni appeared at each end of the filament [ Fig. 4(b)]. Figure 5(a) presents the elemental composition of SiC, and it was seen that a ball-like substance contains Ni, Si, and C; Fig. 5(b) shows a rod-like substance containing Si, C, and traces of Ni. Based on these results, the ball-like material was considered to be the part where SiC was synthesized from a source of gaseous SiO and a C, extending the length of the rod-like part.

SiC formation in green body
During the calcination (10 times) of the cylindrical tablet composed of SiC powder, the tablet gradually increased its weight by 18 % without changing its size. In the end, a hard tablet was obtained.
The tablet after the calcination cycles is shown in Fig. 6(a) and SiC single-crystal wafer is shown in Fig. 6(b). The indentation length of the tablet was 1.16 times that of SiC single-crystal wafer by measuring and averaging 5 indentations of them. This demonstrates that the tablet starting from a powder compact, having almost no hardness, could take a Vickers hardness of as high as      Figure 7 shows the cross-section of the prepared cylindrical tablet, and it indicates that the condensation is progressing in the tablet at a vertical gradient from the surface. Conversely, Ni-rich sites, such as those in Fig. 5(a), were analyzed via SEM and EDX. However, these sites could not be found, and the elemental analysis indicated that Ni existed over the entire tablet at low concentrations, which were barely detectable via EDX analysis.

SiC formation in woven fabric
During the 10 calcination cycles to form the SiC woven fabric and the SiC powder supporting Ni oxide as 0.5 part Ni by weight per 100 parts SiC powder by weight, the weight gradually increased by 21 % to obtain a CMC material without changing its size. Figure 8 shows the CMC material comprising SiC filaments of SiC woven  fabric as a reinforcing material as well as SiC matrix comprised of the original SiC powder and the SiC synthesized from gaseous SiO and toluene vapor around the original SiC powder. Figure 9 shows a cross-section SEM image of the CMC material containing the SiC matrix and SiC woven fabric; Fig. 9(a) shows cross-sections of laterally/vertically extending SiC filaments and SiC matrix, whereas Fig. 9(b) shows cross-sections of vertically extending SiC filaments and SiC matrix. Moreover, the EDX results of the crosssection images shown in Fig. 9 were conducted. In this result, Si and C elements with percentages of 45 % to 55 atm % were obtained, respectively. Ni was detected over the entire tablet at very low concentrations.
In addition, the density of the CMC matrix, comprising original SiC powder and synthesized SiC, was calculated as 86 % at the SiC density of 3.21 g/cm 3 . Figure 3(b) shows an XRD pattern of the SiC woven fabric; it indicates that the material is a beta-type SiC whose crystallinity is lower than the SiC formed in free space [Figs. 3(a) and 3(b)]. Figure 3(c) shows an XRD pattern of the CMC material containing the SiC matrix and SiC woven fabric. This figure demonstrates that the peaks include beta-type SiC and alpha-type 6H SiC. The beta-type SiC was considered to have been derived from the SiC woven fabric, as well as the SiC synthesized from the gaseous SiO and toluene vapor, whereas the alpha-type SiC had been derived from the alpha-type SiC powder that was placed within the SiC woven fabric before the calcination.

Consideration
For the SiC formation in free space, highly crystalline beta-type SiC can be synthesized from gaseous SiO and toluene vapor in the presence of Ni at 1400°C, which was considerably lower than the conventional temperatures of 1600°C or higher for producing highly crystalline betatype SiC materials. 8)10) It is believed that the low temperature of 1400°C and the resulting large weight ratio of SiCNi are due to the high Ni catalytic effect.
Gaseous SiO is reduced to Si by Ni and thereby preventing migration, similar to the catalytic effect of Fe. 3) The reaction to form the SiC is as follows: The resulting Si will be active in this stage and can thus react with C in the toluene vapor at a lower temperature of 1400°C.

Si þ C ! SiC
Furthermore, NiO is reduced to Ni by C, H, and/or other reducible components in the pyrolyzed toluene vapor; this reaction cycle occurs repeatedly to produce SiC, as long as gaseous SiO and toluene vapor are supplied. Hence, the oxygen in SiO will turn to CO, H 2 O, etc.
The reaction mechanism is reflected in Figs. 4 and 5, defined as the so-called vapor, liquid, and solid mechanism, i.e., it is believed that the spherical tips in Fig. 5 were liquid droplets at 1400°C and SiC was synthesized therein while absorbing gaseous SiO and toluene vapor.
Conversely, in the case of the Ni-carrying SiC powder in the tablet or in the SiC woven fabric described in Sections 3.2 and 3.3, the SiC powder was gradually densified, and Ni was detected at very low concentrations throughout the tablet and CMC. These phenomena were considered to occur because the catalytic effect of Ni was maintained (even at low concentrations) and because Ni changed its shape and/or position within the SiC powder spaces while acting to produce SiC from gaseous SiO and toluene vapor, as long as fresh gaseous SiO and toluene vapor were supplied.
In this way, gaseous SiO and toluene vapor can easily flow into the small spaces between SiC particles or filaments due to its gaseous condition without closing up the inlets of the SiC powder compact or woven fabric since the Ni component exists within the compact or woven fabric; consequently, the spaces are embedded by the synthesized SiC and densification of SiC powder or formation of SiC matrix can be attained.

Conclusion
In this study, SiC can be synthesized from gaseous SiO and toluene vapor in the presence of Ni catalyst. Various SiC configurations, such as fibrous SiC and densified SiC powder, can be produced; particularly, SiC matrix can be synthesized within the SiC woven fabric to prepare a CMC via a process that uses raw materials in the form of continuously supplied gaseous SiO and toluene with catalytic aid of the Ni. These materials represent steam or gas at a temperature of 1400°C for the synthesis of SiC; this process is defined as a CVI process. During the CVI process, raw materials of SiO and toluene in gaseous condition and the catalyst disposed within the SiC woven fabric may lead to dense SiC matrix in the CMC.
This CVI process may also present advantages such that the temperature of the SiC fiber may not necessarily be above 1400°C for this process, the step requiring the scrubbing of halogen-containing waste gas may not be necessary, 11)15) and the resulting SiC matrix is highly crystalline SiC matrix.