Figure 2 shows the XRD patterns obtained from the F4N3 sample after the synthesis of Si3N4 at different temperatures. α-Si3N4 and β-Si3N4 began to be produced at 1250 °C, and by the end the α-Si3N4 content was 39.2 wt%, but the product still contained residual 48.2 wt% Si powder. The α-Si3N4 content increased from 75.2 wt% to 88.5 wt% when the temperature increased from 1350 °C to 1450 °C, and the Si powder was fully reacted. Moreover, further increasing the temperature to 1550 °C reduced the content of α-Si3N4 to 54.9 wt%. The α-phase content was estimated from the intensity of the XRD peak. Therefore, as the temperature increased, the content of Si3N4 first increased and then decreased. Significantly, the content of α-Si3N4 increased rapidly at 1450 °C.
To analyze the effect of temperature on the synthesis of Si3N4 content, the calcination process of the product at different temperatures is shown in Fig. 1. The calcination profile of the product at 1250 °C revealed that the appearance structure of the product can be divided into the three layers. As we all know, Si and N2 were reacted directly to obtain Si3N4 at 1300–1400 °C. However, according to reaction schemes (1)-(7), shown in Table 2, adding NH4Cl to the raw materials effectively reduce the nitridation temperature of Si powder and produced α- and β-Si3N4 at 1250 °C. Further, when the temperature was above 1350 °C, the product images showed two layers. Thus, it is obvious that the appearance structure of product was related to the temperature, which is discussed in detail later.
Table 2
Seven potential reaction schemes present in this study
Chemical reaction schemes at different temperatures |
NH4Cl (s)→NH3 (g) + HCl (g) | T > 368 °C (1) |
Si (s) + 4HCl (g)→SiCl4 (g) + 2H2 (g) | T > 400 °C (2) |
3SiCl4 (g) + 4NH3 (g)→Si3N4 (s) + 12HCl (g) | T > 400 °C (3) |
SiCl4 (g) + 2NH3 (g)→Si(NH)2+4HCl (g) | T > 400 °C (4) |
3Si(NH)2→Si3N4 (amorph) + 2NH3 (g) | T > 800 °C (5) |
Si3N4 (amorph)→α-Si3N4 | T > 1200 °C (6) |
α-Si3N4→β-Si3N4 | T > 1200 °C (7) |
Fe (s) + 2NH4Cl (s)→2NH3 (g) + FeCl2 (s) + H2 (g) | T > 204 °C (8) |
Fe (s) + HCl (g)→FeCl2 (s) + H2 (g) | T > 946 °C (9) |
FeCl2 (s)→FeCl2 (g) | T > 1023 °C (10) |
Figure 3 shows the XRD patterns of Si3N4 obtained from the F2N3, F3N3, F4N3, F5N3 and F6N3 samples prepared for 4 h at 1450 °C. The XRD results indicate that the main substance in the products were α- and β-Si3N4 without any impurities. It can be seen that the α-Si3N4 content increased from 78.3 wt% to 94.8 wt%, meanwhile the β-Si3N4 content gradually decreased when the amount of Fe powder content increasing from 2 to 5 wt%. However, when the Fe powder content increased to 6 wt%, the α-Si3N4 content decreased and the β-Si3N4 content rapidly increased. This implies that the Fe powder content has an important effect on both the α- and β-Si3N4 content in the products.
According to analysis of reaction schemes (8)-(9) in Table 2, it is obvious that Fe powder plays an important role in the synthesis of Si3N4. When the temperature was above 204 °C, a small amount of Fe powder reacted with NH4Cl to generate solid FeCl2, gas NH3, and H2, which promoted reactions (3)-(5), and increased the generation of amorphous Si3N4. The amorphous Si3N4 then converted to α-Si3N4 and β-Si3N4 (reactions (6) and (7)). Further, increasing Fe powder content, some parts of the Fe powder reacted with HCl to generate solid FeCl2 at temperatures above 946 °C. When the temperature continued to increase above 1023 °C, solid FeCl2 powder began to sublimate, usually FeCl2 melts at 946 °C and evaporates at 1023 °C. The phase change of FeCl2 decreased the temperature of the reaction system, caused the formation of Si3N4 fibers at certain temperatures, meanwhile inhibited the α to β phase transition, which led to less β phase being formed [22]. Nevertheless, after increasing the Fe powder to 6 wt %, the excess Fe powder and Si formed a liquid Si-Fe alloy at high temperatures. The evaporation of liquid Si-Fe alloy produced Si vapor, which reacted with N2 to generate α-Si3N4 on the surface of the products, but the solid-liquid transition led to a large number of gaps and defects on the surface of the products, which caused N2 to easily diffuse to the lower layer of the product, and promoted the formation of β-Si3N4 at high temperatures [23]. Therefore, the content of α-Si3N4 first increased and then decreased with an increasing amount of Fe powder.
Figure 4 shows the XRD patterns of products obtained from the F4N1, F4N2, F4N3, F4N4, and F4N5 samples prepared at 1450 °C for 4 h. Similar to the case of Fe powder, the α-Si3N4 content increased from 80.6 to 89.2 wt%, and the β-Si3N4 content gradually decreased with an increase in the NH4Cl content from 1 to 4 wt%. However, when the NH4Cl content was increased to 5 wt%, the content of α-Si3N4 decreased. This implied that the NH4Cl content significantly affected the α-Si3N4 content.
According to the above analyses, Fe powder played an important role when adding a small amount of NH4Cl, which was able to produce a large amount of β-Si3N4. However, as the NH4Cl content increases, the heat of the reaction is absorbed by the NH4Cl, and it decompose to NH3 and HCl (reaction (1)), which further facilitates the generation of amorphous Si3N4, then α-Si3N4 (reactions (5) and (6)). Meanwhile, the latent heat of transition state FeCl2 absorbs heat generated during nitridation, reducing the reaction system, and inhibiting the formation of β-Si3N4. Nevertheless, a large amount of gas generated by excess NH4Cl can produce a fluffiness on the surface of the product, which destroys its surface structure, and increases the β-Si3N4 content. Hence, the content of α-Si3N4 first increases and then decreases with increasing NH4Cl.
Figure 5 shows the SEM images of the Si3N4 products synthesized from F2N3 and F5N3 samples at 1450 °C for 4 h (U and L represent the upper and lower layers of product, respectively). Figure 5 (a) and (b) show that the upper layer morphologies of product are comprised of thin nanowires with rough surfaces, thick nanowires with smooth surfaces, and floating particles (circle 2 in Fig. 5 (b)). The average diameter of the thin nanowires was about 1 µm, and the average diameter of the thick nanowires was about 3 µm. The SEM images in Fig. 5 (a) indicate that the Si3N4 nanowires surface was rough. Figure 5 (b) shows that Si3N4 nanowires were straight with smooth surfaces, containing some floating particles. Figures 5 (c) and (d) show the lower layer morphologies of the products were loose and short rod-like nanowires, long needle-like nanowires, and blocky particles.
Figure 6 shows the SEM images of the Si3N4 products synthesized from the F4N1 and F4N5 samples at 1450 °C for 4 h (U and L represent the upper and lower layers of the product, respectively). Figures 6 (a) and (b) show that the upper layer morphologies are comprised of thin nanowires with smooth surfaces and some floating particles (circle 2 in Fig. 6 (a)). The average diameter of nanowires was about 1 µm, which was more uniform, compared to those in Figs. 5 (a) and (b). However, some of the floating particles were adsorbed on the surface of the nanowires. Figures 6 (c) and (d) show that the lower layer product morphologies contained short rod-like nanowires, long needle-like nanowires, and dense and blocky Si3N4.
Currently, it is generally accepted that the growth mechanism of nanowires includes the double-stage vapor-liquid-solid (VLS) and vapor-solid (VS) mechanisms at the tip. [24–26]. Figure 5 (b) 1 and Fig. 6 (b) 1 show that there were no traces of liquid generation at the nanowire tips, Hence, the nanowires may grow via a vapor-solid (VS) process. However, according to reactions 6, 9, and 10 and the melting and boiling points of the Si-Fe alloy and FeCl2, nanowires should theoretically grow via the VLS mechanisms. Therefore, the formation mechanism of Si3N4 fiber may have two growth mechanisms.
The simplified growth models of the fibers are shown in Fig. 7. The fiber growth of the upper layer mainly depends on temperature. On the one hand, when the temperature is above 946 °C, a large amount of amorphous Si3N4 is produced (reaction (5)), meanwhile the solid FeCl2 is transformed into a liquid, which is adhered onto the surface of the amorphous Si3N4. When the temperature increases above 1023 °C, amorphous Si3N4 gradually generates fibrous Si3N4 form liquid FeCl2, and the FeCl2 droplets gradually evaporate and disappear with temperature increases. On the other hand, when the temperature is above 1300 °C, the Si-Fe alloy produces Si vapor, which reacts with N2 to nucleate and then grow into Si3N4, then the Fe droplets gradually evaporate and disappear at high temperatures. Moreover, at high temperatures, a small amount of Si vapor reacts directly with N2 to generate Si3N4, which was then absorbed onto the fiber surface (circle 2 in Figs. 5 (b) and 6 (a)). It is clear that when the ratio of Fe powder and NH4Cl in the raw material is low, the product mainly produces Si3N4 fibers with a rough surface and a small diameter. However, as the Fe powder content increases, the surface of the Si3N4 fiber becomes smoother, and the average diameter of the Si3N4 fiber gradually increases, and the amount and density of the floating particles gradually increases.
In addition, the product growth formed on the lower layer product includes fibers and stacked blocks, increasing the ratio of Fe powder to NH4Cl, the number of long needle-like fibers, and the density of blocky structures, while decreasing the number of short rod-like fibers. There could be two reasons for this growth analysis of products in the lower layer. On the one hand, the fiber growth mechanism in the lower layer could be similar to the fiber growth mechanism in the upper layer, but because of the effect of the weight of FeCl2 and Si-Fe droplets, most of the FeCl2 and Si-Fe droplets are adsorbed on the surface of the Si powders, causing the Si on the surface to react with the N2 adsorbed by the droplets to generate fibers, but the Fe powder and NH4Cl produced gas to destroy the formation of nanowires during nitridation, thus forming short rod-like and long needle-like fibers. On the other hand, a small amount of Si powder particles in the lower layer could gradually react with N2 and continuously stack to form a blocky structure.