Characterization of crystalline SiCN formed during the nitridation of silicon and cornstarch powder compacts

Journal of Alloys and Compounds 725 (2017) 326e333

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Characterization of crystalline SiCN formed during the nitridation of silicon and cornstarch powder compacts Wenjie Yuan*, Ling Qu, Jun Li, Chengji Deng, Hongxi Zhu The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, 430081, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 April 2017 Received in revised form 6 July 2017 Accepted 17 July 2017 Available online 20 July 2017

This work reported that crystalline SiCN formed during the nitridation of silicon and cornstarch powder compacts when the temperature exceeded 1350
C. The resultant samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric (TG) and transmission electron microscopy (TEM). By comprehensive analysis, crystalline SiCN possessed cubic and hexagonal structure derived from SiC and Si3N4. SiCN crystals had a rod like shape, and its length could reach several mm. The element molar ratios and the crystal structure of SiCN depended on the soaking temperature as well as the formation mechanism. The nitridation of the compacted pellet composed of silicon and cornstarch presented a facile route to synthesize crystalline SiCN. © 2017 Elsevier B.V. All rights reserved.

Keywords: SiCN Nitridation Characterization

1. Introduction The ternary Si-C-N ceramics possessing oxidation and creep resistance as well as novel optical, electrochemical and electromagnetic properties [1e5] caught more and more attention of the researchers since nonoxide ceramics were made from molecular precursors in 1960s [6]. Si-C-N system ceramics and their composites have great potential applications in the harsh environment or at high temperatures [7,8]. Si-C-N ceramics in the form of bulks, fibers, coatings and thin films were prepared by polymer derived ceramics (PDCs) route, ion implantation, physical vapor deposition (PVD) and chemical vapor deposition (CVD), respectively [9e13]. Moreover, most of Si-C-N ceramics fabricated by PDCs were amorphous, which had higher glass transformation point [14]. Though the crystallization behaviors of amorphous Si-C-N ceramics in the range from 1100 to 1600
C depended on their composition, the type of polymeric precursors and the atmosphere [15,16], the crystallization products were thermodynamically stable phases Si3N4 and SiC. In other words, crystalline SiCN could not be obtained by the crystallization process. The further disintegration of Si-C-N ceramics was as the consequence of the reaction between Si3N4 and carbon to form SiC with releasing of N2 above 1484
C [17], which prevented their applications at higher temperatures. Nevertheless, crystalline SiCN with high hardness and a wide

* Corresponding author. E-mail address: [email protected] (W. Yuan). http://dx.doi.org/10.1016/j.jallcom.2017.07.170 0925-8388/© 2017 Elsevier B.V. All rights reserved.

band gap attracted more interests [18,19]. Despite Riedel discovered two new phases (SiC2N4 and Si2CN4) by pyrolysis of polysilylcarbodiimide in 1997 [20], crystalline SiCN was mainly synthesized by microwave plasma CVD and magnetron sputtering PVD [21]. The drawbacks of the deposition route included the limited size of the sample, the low yield, the use of highly expensive equipment and raw materials [22]. In our previous work, SiC, silicon and cornstarch powders were used as the starting materials. Porous SiC/SiCN composite ceramics with rod-like crystalline SiCN grains were fabricated by foaming and reaction sintering at 1650
C [23]. Normally, Si3N4/SiC composites were obtained from the reaction of silicon and carbon in nitrogen [24]. This naturally raised the question of why crystalline SiCN formed during the nitridation processing. Above-mentioned case could be simplified to the incorporated reactions of nitridation and carbonization. The purpose of this work is to interpret the mechanism by characterizing crystalline SiCN formed during the nitridation of silicon and cornstarch powder compacts via different methods. In addition, this paper also presented a facile route to synthesize crystalline SiCN. 2. Experimental procedure Silicon powder (d < 74 mm, 99.99% purity) and cornstarch powder (d50~9.4 mm) were selected as raw materials. After mixing, the mixture of Si and cornstarch powders with the weight ratio of 1.037:1 (approximately molar ratio of Si:C ¼ 2.22:1) was uniaxially pressed at 250 MPa into compacts with 20 mm diameter. The nitridation process was carried out in nitrogen (99.999% purity) as

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follows: 1) heat up to 1350
C at a rate of 3
C/min followed by soaking for 1 h; 2) sequentially heat up to 1450e1650
C at a rate of 5
C/min and soaked for 3 h. All samples were milled into fine powders for the characterization. The phase composition of the as-prepared sample was determined by X-ray powder diffractometer (XRD, X'pert Pro MPD, Philips, Netherlands). Spectra in the range of 10
and 90
(2q) were recorded at 40 kV and 40 mA using Cu Ka radiation (l ¼ 0.15406 nm) radiation with a step size of 0.033
and counting time of 12.06 s/step. X-ray photoelectron spectra of samples were collected by X-ray photoelectron spectroscopy (XPS, PHI Quantera II, ULVCA-PHI, Japan) with X-ray lamp (Al Ka 1486.6 eV). Energy step size of 0.1 eV and spot size of 200 mm were used. Thermogravimetric analysis (TG) was performed using a simultaneous thermal analyzer (STA 449F3, NETZSCH, Germany) up to 1500
C at a heating rate of 10
C under air with a flow rate of 50 ml/min. Meanwhile TG curves of chemical reagents including SiC, a-Si3N4 and b-Si3N4 as the references were measured for the comparison. The microstructure of the sample was observed by high-resolution transmission electron microscope (TEM, JEM-2100 UHR, JEOL, Japan) equipped with energy dispersive X-ray spectroscopy (EDS, IET 200, OXFORD, UK). 3. Results and discussion 3.1. Characterization of silicon and cornstarch powder compacts after the nitridation 3.1.1. X-ray diffraction (XRD) XRD patterns of the samples synthesized at different temperatures are shown in Fig. 1(a). There were unreacted silicon and a small amount of SiC in the sample after maintained at 1350
C for 1 h. The carbonization of silicon was dominant in this stage. Amorphous carbon (a-C) formed as the product from the pyrolysis carbonization of cornstarch at high temperatures [25], so none of diffraction peaks corresponded to pyrolytic carbon. With the increasing of the firing temperature, a and b-Si3N4 gradually formed. The diffraction peaks of silicon disappeared when the firing temperature went up to 1650
C, which indicated silicon was completely consumed. Furthermore, the intensity of b-Si3N4 peaks obviously rose at 1650
C. Despite the diffraction peaks of SiCN and SiC were very similar, SiCN phase was still identified according to the coincidence degree of the strongest peak for SiCN and the more deviation of SiC peaks at higher degree by carefully comparing with the powder diffraction file (PDF) in the inorganic crystal structure database. The detailed analysis of this case was as follows: Silicon (111) was chosen as the reference, which was also prevalent in XRD quantitative analysis. The relative positions of (220) and (111) peaks of SiC (PDF 73-1708) or SiCN (PDF 74-2308) comparing with (111) peak of silicon (PDF 77-2108) were calculated and denoted with broken lines in Fig. 1(b). According to the difference of peak's position between A or B and silicon (111), the peaks (A and B) shifted to lower angle with the temperature, which indicated the larger lattice parameters. That is to say, the position of selected peaks was moved from near SiC side to SiCN side. Moreover, the difference of silicon (111) and selected peak B at higher position was more prominent. Consequently, SiCN was identified in samples calcined above 1350
C from XRD patterns. More evidence will be discussed in the following section. 3.1.2. X-ray photoelectron spectroscopy (XPS) Chemical bonding information of samples synthesized at different temperatures was obtained from high resolution XPS spectra as shown in Figs. 2e4. Three peaks (100.5, 101.6 and 102.6 eV) in the Si 2p spectra of the sample calcined at 1350
C

Fig. 1. XRD patterns (a) and peaks' shift (b) of the samples synthesized at different temperatures.

corresponded to Si-Si, Si-C and Si-N bonds respectively (Fig. 2(a)) while only Si-C and Si-N bonds presented in samples heated up to 1450e1650
C (Fig. 2(b)-(d)). The Si-Si bond was not detected because of the low content and uneven distribution of unreacted silicon for 1450e1550
C and the complete consumption of silicon for 1650
C. From the fitted photoelectron peaks of N 1s (Fig. 3), it can be seen that the peaks centered at 398 eV was attributed to N-Si bond, and N-C (sp3) as well as N¼C (sp2) bond [26] formed in sequence with the temperature rising. As shown in Fig. 4, the XPS C 1s spectra can be fitted into peaks with the binding energies at around 283.3, 284.6 and 285.4 eV, corresponding to C-Si, C-C and CN, respectively [18]. The crucial difference between this work and other Si-C-N ceramics made by PDCs, CVD and DC sputtering was that C-N bonds formed in samples calcined above 1450
C [6,9,27]. C atoms in polymer-derived SiCN were not bonded to N atoms, which was determined by nuclear magnetic resonance [28]. Because the contribution of shorter C-N bonds to the hardness was more than Si-C and Si-N bonds [19], it can be deduced that crystalline SiCN possessed ultrahigh hardness [29]. Regarding C-N bonds as the indication of SiCN phase, the proportion of C-N bonds represented the amount of SiCN. The percentage of peak's area for C-N bonds increased with the increasing temperature as shown in Fig. 5, which implied that the yield of SiCN in the reaction products gradually rose. Above all, the formation of SiCN phase was

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Fig. 2. XPS spectra of Si2p for samples synthesized at different temperatures.

effectively validated by XPS spectra.

3.1.3. Thermogravimetric analysis (TG) The thermogravimetric analysis was carried out in order to provide the evidence of the phase composition in samples. TG curves of samples synthesized at different temperatures and the references (SiC, a-Si3N4 and b-Si3N4) under air are shown in Fig. 6. The results indicated that a continuous weight gain for all specimens except the sample fired at 1350
C above a certain temperature in the heating procedure. The weight gains of SiC and a-Si3N4 were larger than b-Si3N4 and other samples when the testing temperature was raised to 1500
C. The oxidation of SiC and Si3N4 was controlled by the reaction with oxygen at the initial stage in non-isothermal testing [30,31]. The mass loss for the sample synthesized at 1350
C in the temperature range of between 550 and 850
C was related to the oxidation of unreacted pyrolytic carbon derived from cornstarch. Otherwise, little or no free carbon was left in samples synthesized at higher temperatures. On the basis of the carbon content of cornstarch, it could be estimated that a small amount of pyrolytic carbon attended the reaction when the sample was soaked at 1350
C. The weight of this sample increased up to 1500
C, and the weight gain reached 5% compared with the minimum weight associating with the oxidation of silicon and SiC in air. However, the weight gain of samples after nitriding at 1450e1650
C was equivalent or slightly below b-Si3N4, which verified the existence of SiCN. According to XRD results, either the complete consumption of silicon or less formation of a-Si3N4 also accounted for the lowest weight gain for the sample calcined at

1650
C. If some amount of SiC formed rather than SiCN, more weight gain could be expected. Therefore, the oxidation resistance of SiCN may be better than SiC and Si3N4. 3.1.4. Transmission electron microscopy (TEM) The microstructure of received samples was further characterized by transmission electron microscopy. TEM image, EDS spectra and selected area electron diffraction (SAED) pattern of samples synthesized at different temperatures are shown in Fig. 7. The chemical compositions of micro-areas determined by EDS are also listed in Table 1. From Fig. 7(a), there were different regions corresponding to SiC, SiCN and Si3N4 in one particle of the sample calcined at 1450
C, which was detected by EDS. It can be seen that SAED patterns toward (100), (101) and (001) of crystalline SiCN and a-Si3N4 were very similar and distinct from b-SiC with the cubic structure, which was consistent with polycrystalline films deposited by electron cyclotron resonance plasma chemical vapor deposition [32]. At the initial stage, hexagonal SiCN phase with less carbon and nitrogen had the same orientation in the diffraction position with adjacent a-Si3N4. Consequently, this kind of SiCN phase was interpreted as corresponding to a pseudo a-Si3N4 [33]. When the nitridation temperature was raised to 1550
C, there were two types of crystalline SiCN with a rod like shape and the length of several mm as shown in Fig. 7(b) and (c). Although the difference of chemical composition between micro-area 2 and 3 was not significant, their crystal structure of cubic and hexagonal respectively was determined by analyzing the SAED patterns. Cubic SiCN crystal formed at 1550
C was isomorphic to b-SiC (Fig. 7(b)),

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Fig. 3. XPS spectra of N1s for samples synthesized at different temperatures.

which was unlike that at 1450
C. The epitaxial growth of SiCN on the surface of b-SiC probably should be feasible due to the same crystal system [34]. Hexagonal SiCN in the sample calcined at 1650
C was identified as same as that in SiC/SiCN composites [23]. SAED pattern of SiCN generated at 1650
C was not analogous to aSi3N4 but a-SiC according to the different axial ratio of c/a for the hexagonal lattice parameter. The presence of a-SiC was detected in polysilazanes-derived Si-C-N ceramics annealed above 1500
C in nitrogen [35]. So it could be assumed that cubic SiCN would transform into hexagonal structure accompanying the variation of element contents at higher temperatures. Besides, the molar percentage of C þ N in SiCN phase became greater as the temperature increased (Table 1), which demonstrated that the reactions involved carbon and nitrogen proceeded gradually. Another noticeable feature was that the composition of area 2 and 4 located closed to the tie line of SiC and Si3N4 with various C and N contents. The Si-N-C phase containing 3-6 at% C with higher hardness than Si3N4 and SiC also corresponded to the same line in the phase diagram [36]. It implied that as-received SiCN might be much harder. 3.2. Mechanism of phase evolution Taking into account the crystal structure, composition and morphology, the possible reaction path of SiCN formation was as following considerations. First (1350
C), a series of reactions happened in the thermal degradation of cornstarch resulted in the formation of small molecular species such as hydrocarbon, aldehyde, H2 and CO with increasing temperature [37]. Amorphous

carbon generated by the carbonized reaction above 600
C [38]. As the temperature went up to 1350
C, b-SiC formed from the reactions of a-C and either silicon or the thin silica layer on the surface of silicon particles [39]. But the direct contact between a-C and silicon was limited because a large amount of gases were released from the compacts. So the yield of b-SiC was not much. Despite the initial nitridation of silicon was so quick, a trace of Si3N4 was generated at this stage due to the kinetic factor [40]. Second (1450
C), the loose a-C could be infiltrated with the silicon melt due to the capillary effect. The b-SiC layer formed at the interface of silicon melt and carbon in a narrow reaction zone [41]. The growth of a continuous b-SiC layer followed a fourth-power rate law [42]. The contact angle was reduced further to 10
for Si/ SiC/C system [43], which promoted the spreading of the silicon melt. In contrast, the contact angle between of molten silicon and Si3N4 was about 90
[44]. In the light of the heterogeneous structure of SiC/SiCN/Si3N4 in the sample calcined at 1450
C as shown in Fig. 7(a), the reaction process could be deduced. Assuming the original particle at a lower temperature was a joint of SiC and Si as well as a little unreacted carbon, silicon melt and nitrogen reacted to generate Si3N4 at 1450
C. Normally, a- and b-Si3N4 both formed at the temperature ranging from 1400 to 1700
C [45]. But the fine SiC particle could inhibit the a-b phase transformation of Si3N4 [46]. On the other hand, the competition of the formation of SiC and Si3N4 led to the conversion of a part of silicon to Si3N4 [47]. Though the individual solubility of N and C in silicon melt was very limited, the interactions of carbon and nitrogen as well as the higher temperature in a local region as the consequence of exothermic

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Fig. 4. XPS spectra of C1s for samples synthesized at different temperatures.

Fig. 6. TG curves for SiC, a-Si3N4, b-Si3N4 and samples synthesized at different temperatures under air. Fig. 5. The percentage of C-N bond calculated by peak fitting result of Fig. 4.

reactions could enlarge their solubility [48]. The mass transfer of carbon within the silicon was extremely rapid [49]. Above factors were presumably benefit to the nucleation of SiCN cluster in silicon melt. Additionally, it was confirmed that the nitrogen concentration in amorphous carbon films could reach up to 30 at% at the extreme condition [50]. Thus, SiCN with a pseudo a-Si3N4 structure

formed on the interface, and the possible reactions to form SiCN were given by the following formulas: 2Siþ2C þ N2 ¼ 2SiCN

(1)

SiC þ Si þ C þ N2 ¼ 2SiCN

(2)

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Fig. 7. TEM image, EDS spectra and electron diffraction pattern of SiCN in samples synthesized at 1450
C (a), 1550
C (b) (c) and 1650
C (d).

2SiC þ N2 ¼ 2SiCN(cubic)

(4)

2Si3N4þ6C ¼ 6SiCN(hexagonal)þN2

(5)

nanowires with Fe nanoparticles [54], no droplet at the tip of rodlike crystal was observed in the present case. Rod-like b-SiC with the size of less than 2 nm was generated through the reaction of amorphous carbon and Si3N4 at 1500
C in PDCs [55]. The growth of rod-like SiCN with cubic structure was probably governed by the liquid phase epitaxial under nitrogen on the above nano b-SiC as the template, which was similar with the process for SiC [56]. Regarding hexagonal SiCN rods, the gaseous reaction of Si vapor and nitrogen at a suitable temperature region played a role in the formation process, which was dominated by a vapor-solid mechanism [57]. Finally, the phase transformation of SiCN occurred similar to b-a for SiC when the temperature reached 1650
C, which means SiCN(cubic) / SiCN(hexagonal). The formation and growth mechanisms of crystalline SiCN required more systematic investigations. In summary, the variation in the element content and the crystal structure of crystalline SiCN indicated that its formation was more complex than previously considered. The phase evolution for the silicon and cornstarch powder compacts with the calcining temperatures was followed by SiC (1350
C) / SiC þ a-Si3N4 þ bSi3N4 þ SiCN (hexagonal) (1450
C) / SiCN (cubic and hexagonal) þ a-Si3N4 þ b-Si3N4 (1550
C) / SiCN (hexagonal) þ aSi3N4 þ b-Si3N4 (1650
C). The preparation cost of SiCN could be remarkably reduced by using the cheap raw materials such cornstarch and a simple process.

Si3N4þSiCþ3C ¼ 4SiCN(hexagonal,

(6)

4. Conclusions

Table 1 EDS analysis results of areas in TEM images for the samples. Area No.

1 2 3 4

Atomic% Si

C

N

76.3 54.6 61.4 48.8

7.7 12.1 9.0 31.6

16.0 33.3 29.6 19.6

Si3N4þSiþ4C ¼ 4SiCN

(3)

Third (1550
C), the reactions would be more severe with the increasing of the temperature. The crystal structure of SiCN depended on the dominant reaction of SiC and Si3N4 formations with the incorporation of nitrogen and carbon respectively during the nucleation stage. An alternative mechanism was that nitrogen atom substituted carbon in SiC [51]. Combined with the preceding analysis, possible paths of phase evolution for SiCN with cubic and hexagonal structure at 1550
C were proposed by reactions as below:

cubic)

The volume expansion on conversion of silicon to Si3N4 and the gas release in former stage provided the more space for the growth of crystals. The morphology of the product was very strongly controlled by the parameters of the nitridation process [52]. The growth of the rod-like crystals was controlled by vapor-solid (VS) and vapor-liquid-solid (VLS) mechanisms [53]. Unlike SiCN

A facile route to synthesize crystalline SiCN with a rod like shape and the length of several mm through the nitridation of silicon and cornstarch powder compacts was presented. Combining XRD patterns and XPS spectra with the EDS and SAED results, crystalline SiCN with cubic and hexagonal structure derived from SiC and Si3N4 was identified with different element contents. The

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incorporation of silicon, nitrogen and amorphous carbon from the cornstarch played a decisive role on the growth of crystalline SiCN. Acknowledgement This research was financially supported by the National Nature Science Foundation of China (No. 51502214).

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