Formation of different Si3N4 nanostructures by salt-assisted nitridation

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Functional Nanostructured Materials (including low-D carbon)

Formation of different Si3N4 nanostructures by salt-assisted nitridation Xiongzhang Liu, Ran Guo, Senjing Zhang, Qingda Li, Genki Saito, Xuemei Yi, and Takahiro Nomura ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16952 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 14, 2018

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ACS Applied Materials & Interfaces

Formation of different Si3N4 nanostructures by saltassisted nitridation Xiongzhang Liu1, Ran Guo1, Sengjing Zhang1, Qingda Li1, Genki Saito2, Xuemei Yi 1*, Takahiro Nomura3 1

College of Mechanical and Electronic Engineering, Northwest A&F University, XinongRoad 22, Yangling, Shaanxi 712100, China

2

Faculty of Engineering, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo 060-8628, Japan

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Center for Advanced Research of Energy and Materials, Hokkaido University, Kita 13 Nishi 8, Kitaku, Sapporo 060-8628, Japan

ACS Paragon Plus Environment

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ABSTRACT Silicon nitride (Si3N4) products with different nanostructure morphologies and different phases for Si3N4 ceramic with high thermal conductivity were synthesized by a direct nitriding method. NaCl and NH4Cl were added to raw Si powders, and the reaction was carried out under a nitrogen gas flow of 100 mL/min. The phase composition and morphologies of the products were systemically characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HR-TEM). At 1450°C, the NaCl content was 30 wt%, the NH4Cl content was 3 wt%, and the maximum αSi3N4 content was 96 wt%. The process of Si nitridation can be divided into three stages by analyzing the reaction schemes: in the first stage (25–900 °C), NH4Cl decomposition and the generation of stacked amorphous Si3N4 occurs; in the second stage (900–1450 °C), NaCl melts and Si3N4 generates; and in the third stage (>1450 °C), α-Si3N4 →β-Si3N4 phase change and evaporation NaCl occurs. The products are made of two layers: a thin upper layer of nanowires containing different nanostructures and a lower layer mainly comprising fluffy, blocky, and short needle-like products. The introduction of NaCl and NH4Cl facilitated the evaporation of Si powders and the decomposition of Al2O3 from porcelain boat and furnace tube, which resulted in the mixing of N2, O2, Al2O, and Si vapors and generated AlxSiyOz nanowires with rough surfaces, and lead to thin Si3N4 nanowires, nanobranches by the VS (vapor–solid), VLS (vapor– liquid–solid) and the double-stage VLS-base and VS-tip growth mechanisms. KEYWORDS: Silicon nitride; Si nitridation; thermal conductivity; nanostructure; sodium chloride; ammonium chloride; growth mechanism


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INTRODUCTION Because electrical energy is efficient, clean, and environment-friendly, it is being increasingly used in many fields such as transportation, manufacturing industry, space industry, agriculture, and electronics industry. Therefore, development of electronic equipment that can efficiently use electric energy is very important. In order to adapt to complex and volatile service environment and to meet the increasing stiff service demand, power device technology is advancing toward higher voltage, larger current, and greater power density. However, this trend is leading to serious difficulties in heat dissipation in electronic equipment. New types of electronic equipment are being developed that are low cost, and have high electrical insulation, high stability, high thermal conductivity and coefficient of thermal expansion (CTE) to match the chip, smoothness, and high strength demands for radiation materials1. In order to meet these requirements, researchers have investigated Al2O3, AlN, BeO, SiC, BN, and Si as heat dissipation substrate materials for electronic equipment. However, these materials suffer from certain limitations: Al2O3 has low thermal conductivity, and the sintering of high-purity Al2O3 is expensive2. Moreover, AlN has poor mechanical properties, high sintering temperature, and easily form meta-aluminic acid in water. The linear expansion coefficient of BeO is different from that of Si and its thermal conductivity falls sharply at high temperatures3. Also, as a substrate material, BN is too expensive. The dielectric constant of SiC4 is large. In addition, the processing of Si is difficult. Hence, the above materials cannot meet the strict requirements of next-generation high-power radiation substrate materials. The electronics industry is desperately looking for new heat-dissipation substrate materials with excellent mechanical properties and high thermal conductivity. Thus, silicon nitride ceramics are attracting increasing attention5. An initial study has shown that silicon


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nitride ceramic has excellent hardness, thermal shock resistance, high corrosion resistance6, and good mechanical reliability, but low thermal conductivity; therefore, it is regarded as a hightemperature structural material. In 1995, Haggerty7 predicted that the thermal conductivity of βSi3N4 ceramic can reach to 200–320 W/(m • K). In 1999, Watar8 prepared a silicon nitride ceramic with a thermal conductivity of 155 W/(m • K) by using the hot isostatic pressing method at 2773 K and 200 MPa nitrogen pressure, proving, for the first time, that silicon nitride ceramics have high thermal conductivities. Because of its excellent mechanical performance and high thermal conductivity, silicon nitride is considered a potential heat dissipation substrate material for high power electronic devices5, 9–11 and solar industries12. To synthesize silicon nitride materials, imide thermal decomposition method, reduction nitriding method, direct nitriding method, sol-gel method, and vapor-phase processes are well known13. However, the sol-gel method and gas-phase methods are complex, expensive, and environmentally unfriendly, while the reduction nitriding method tends to introduce impurities into the final product. In contrast, the direct nitridation method for the preparation of silicon nitride through the reaction 3Si+2N2→Si3N4 provides efficiency and convenience; thus, it has been widely used in the industry. Si3N4 has two kinds of crystal structures: α and β. α-Si3N4 is stable at low temperatures and can transform to β-Si3N4 at elevated temperatures14. It is well known that sintered Si3N4 has excellent toughness and thermal conductivity, which is attributed to the α→β phase transformation that accelerates the grain growth and densification at high temperatures. Therefore, the preparation of Si3N4 powder with a high content of α-Si3N4 is the key to prepare Si3N4 ceramics with high thermal conductivity15. On the one hand, adding NH4Cl into the reactants is an effective way of decreasing the temperature at which Si powder is completely


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nitrided and preventing self-sintering of the Si powder10. NH4Cl can increase the content of αSi3N4 in products16. On the other hand, a previous work reported that NaCl determines how much α-Si3N4 occurs in products. Adding NaCl and β-Si3N4 in raw materials can generate different Si3N4 microstructures, but the growth mechanism of the Si3N4 microstructure remains to be studied17. NaCl also limited the growth of crystals18; molten NaCl can facilitate the migration and complete nitridation of materials19, 20. The evaporation of liquid NaCl reduces the heat of the reaction environment, which reduces the extent to which α-Si3N4 changes to βSi3N421. Hence, to prepare Si3N4 powders with high contents of α-Si3N4 and study the morphology of Si3N4 products, we investigated the effect of temperature and the contents of NaCl and NH4Cl on the α-Si3N4 content in Si3N4 products, and studied the influence mechanism of NaCl and NH4Cl on the morphology of the product synthesized by the direct nitriding method. EXPERIMENTAL PROCEDURE Si powders (99.99 wt% purity, 300 mesh, Adamas Reagent Co. Ltd., Shanghai) were used as the starting raw materials, and high-purity N2 (purity > 99.999%) was used as the nitrogen source. Analytical-grade NaCl (≥ 99.5 wt% purity, Guangzhou Jinhuada Chemical Reagent Co. Ltd.) and NH4Cl (≥99.5 wt% purity, Tianjing Beilianjingxi Chemical Reagent Co. Ltd.) were used as salts and added in concentrations of 0, 10, 20, 30, and 40 wt% and 0, 1, 3, 4, and 5 wt%, respectively. The chemical compositions of NaCl and NH4Cl are listed in Table 1 and Table 2, respectively. The starting powders comprising the components (weight ratios) are shown in Table 3. A zirconium oxide ball mill was used to dry-mix the starting powders for 10 min at 1200 rpm, setting the weight ratio of the ball to the raw materials as 3:1. A mixed powder sample of 2 g was


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loaded into an alumina crucible and placed at the center of a long alumina tube inserted into an electric furnace. The air was evacuated from the alumina tube by using a vacuum pump. The furnace was heated from room temperature at 5 °C min-1 to 1300, 1350, 1400, 1450, and 1500°C (the melting temperature of Si powder is 1410 °C), and Si nitridation was carried out for 4 h in a continuous flow of N2 at 0.1 L/min under atmospheric pressure. The fired samples were then allowed to cool to room temperature in the furnace. The samples were repeatedly washed with distilled water using ultrasonic cleaning equipment and filtered several times to remove the residual salt. The phase composition of the powders resulting from nitridation was characterized by powder X-ray diffraction (XRD, X’Pert Pro MPD) with Cu Kα radiation, and the microstructures were observed by field emission scanning electron microscopy (FESEM, Nova 450 Nano). The sample powder was dispersed in ethanol and then deposited on a Cu micro grid for analysis by transmission electron microscopy (TEM, JEOL, JEM-2010F). RESULTS Different Si3N4 powders were prepared through Si nitridation by varying the temperature and NaCl and NH4Cl contents. The effect of temperature on the synthesis of Si3N4 is shown in Fig. 1. As can be seen, α-Si3N4 and β-Si3N4 peaks begin to appear at 1300°C, and continue to increase until the temperature increases to 1400°C. The α-Si3N4 peaks increased more rapidly than the βSi3N4 peaks did. Thus, at temperatures between 1300 and 1350°C, the powder produced is essentially composed of α-Si3N4, β-Si3N4, and Si. However, the Si peaks continued to decrease and even disappeared when the temperature was above 1400°C, and the content of α-Si3N4 was the highest at 1450°C. Subsequently, the content of α-Si3N4 decreased when the temperature was


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increased to 1500°C. Therefore, it can be said that the amount of α-Si3N4 first increases and then decreases with temperature during heat treatment. Figure 2 shows the XRD patterns of Si3N4 containing 0–40 wt% NaCl prepared for 4 h at 1450°C. As can be seen, α-Si3N4 and β-Si3N4 are present in the reference sample without impurities or unreacted silicon. Under this condition, silicon powder was completely nitrided, which was independent of the content of NaCl. It can be seen that an increase in α-Si3N4 and decrease in β-Si3N4 were accompanied by an increase in the NaCl content. Moreover, the content of α-Si3N4 was the highest when the NaCl content was 30 wt%. However, increasing the amount of NaCl to 40 wt% decreased the α-Si3N4 content and increased the β-Si3N4 content. This implies that the increased amount of NaCl significantly affected the contents of α- and β-Si3N4 in the products when the NaCl content was increased from 0 wt% to 40 wt%. This means the amount of α-Si3N4 first increased and then decreased with the amount of NaCl. The XRD patterns in Fig. 3 were obtained from the products prepared for 4 h at 1450 °C with 0–5 wt% NH4Cl. As can be seen, α-Si3N4 and β-Si3N4 are present in the reference sample without any impurities or unreacted silicon. Under this condition, Si powder was completely nitrided, which was independent of the content of NH4Cl. Similar to the case of NaCl, an increase in the α-Si3N4 content and a decrease in the β-Si3N4 content were accompanied by an increase in the NH4Cl content; the α-Si3N4 content was the highest when the amount of NH4Cl was 4 wt%. However, increasing the amount of NH4Cl to 5 wt% sequentially decreased the α-Si3N4 content and increased the β-Si3N4 content. This implies that similar to NaCl, the increased amount of NH4Cl significantly affected the contents of α- and β-Si3N4 in the products when its amount was increased from 0 wt% to 5 wt%. This means that the α-Si3N4 content first increased and then decreased with the amount of NH4Cl.


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The microstructures of the products were observed with a scanning electron microscope (SEM). It was found that the products were composed of two layers: a thin upper layer of nanowires and a lower layer of fluffy, blocky products. The effect of NaCl and NH4Cl contents on the microstructures of the products are as follows. SEM images in Fig. 4 show the microstructures of the Si3N4 products synthesized from raw materials of A0H3, A20H3, and A40H3 at 1450 °C for 4 h under N2 atmosphere. The SEM images in Fig. 4a, b, and c indicate that the upper layer comprised thin nanowires with smooth surfaces, thick nanowires with rough surfaces, nanobranches and some clastic floaters (shown in Fig. 4 A, B, C, D, E). The length of the thin nanowires was in the range of ∼50–200 µm with an average diameter of around 40 nm. The length of the thick nanowires was in the range of ∼20– 50 µm with an average diameter of around 2 µm. No traces of liquid droplets were found on the tops of the thin nanowires. Therefore, the thin nanowires reported here may grow via a vapor– solid (VS) process22–25. At the same time, Fig. 4a shows that the Si3N4 nanowires were straight and had smooth surfaces, without any particles on the surfaces of the nanowires. Fig. 4b shows that the Si3N4 nanowires, containing a few thick nanowires, demonstrate bifurcate and disorder phenomena. Fig. 4c shows more thick nanowires with bifurcate and disorder phenomena, compared to that observed in Fig. 4b. The results show that NaCl can affect the morphologies of the products in the upper layer. The SEM images in Fig. 4d, e, and f indicate that the microstructures of the products in the lower layer were blocky and short needle-like. At high NaCl concentrations, the content of short needle-like products increased. Thus, it is obvious that the generation of short needle-like product was related to the NaCl content, which is discussed in detail later.


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The structure of the nanowires was further investigated, as shown in Fig. 5. The TEM image (Fig. 5a) of the sample synthesized from raw materials of A20H3 shows a thin nanowire without any droplet on the top of it. It can be seen that the nanowire, with a diameter of around 40 nm, is straight and has a smooth surface. The high-resolution TEM (HRTEM) image (Fig. 5b) and the corresponding fast Fourier transform (FFT) pattern (Fig. 5c) show more clearly the singlecrystalline nature of the nanowire. At the same time, the lattice-fringe spacings of 0.557 and 0.683 nm conformed well to the (001) and (100) planes of α-Si3N4, respectively. Therefore, the thin nanowires in the upper layer were α-Si3N4. The TEM image in Fig. 5d shows a thick nanowire with rough surface of the sample synthesized from raw materials of A20H3. It can be seen that there are no droplet on the top of it. Energy dispersive spectroscopic (EDS) analysis (Fig. 5e) revealed that the nanowire contained Al and O in the ratio of 2.5, and a small amount of Si, which does not conform to the stoichiometric ratio of 2/3 in Al2O3. According to Table 1 and 2, as there is no Al in the raw materials, it may have come from the porcelain boat and furnace tube. Thus, the thick nanowires with rough surface are the mixture of Al, O, and Si, whose formation mechanism is discussed in detail later. Figure 6 presents the TEM and HRTEM images, and the FFT and EEL spectra of the branched nanostructure. A typical Y-shaped (Fig. 6a) nanobranch can be seen in the TEM image. EEL spectra were collected for three different areas of the nanobranch, as shown in Fig. 6a. Figs. 6a 1, b show the HR-TEM image and FFT pattern, respectively, recorded for a branch of the nanobranch. The branch grew along (100) and (001), with lattice-fringe spacings of 0.619 and 0.543 nm, respectively, which conformed well to the α-Si3N4. Figs. 6a 3, c show the HR-TEM image and FFT pattern, respectively, recorded for a stem of the nanobranch. The stem grew -

along (111) and (101), with lattice-fringe spacings of 0.433 and 0.317 nm, respectively, which


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conformed well to the α-Si3N4. Analysis of the EEL spectra (Fig. 6d) reveals that all three areas primarily contained Si and N, with little Al and O; moreover, a comparison of their elemental content revealed nearly identical content for the three areas. Figure 7 shows the TEM image and EEL spectra of the broad nanobranch. A typical broad nanobranch (Fig. 7a) is shown in the TEM image. EEL spectra were recorded for three different areas of the broad nanobranch. Analysis of the EEL spectra (Fig. 7b) reveals that all three areas contained Al, Si, O and N; the content of each element in all three areas was nearly identical, respectively. Figure 8 shows the TEM, HRTEM, FFT, and EDS results of the nanowire with the clastic floater. A typical nanowire with a clastic floater (Fig. 8a) can be seen in the TEM image. EDS spectra were recorded for two different areas of the nanowire with the clastic floater, as shown in Fig. 8a. Figs. 8b and c show the HR-TEM image and FFT pattern, respectively, recorded at -

-

locations 1 and 2 of the nanowire. The growth directions of the nanowire were (100) and (131), for which the lattice-fringe spacings were 0.619 and 0.231 nm, respectively, which conformed well to α-Si3N4. Hence, the nanowire was identified as an α-Si3N4 nanowire. EDS analysis (Fig. 8d) reveals that both areas contained a small amount of Al and O. The content of each corresponding element was also nearly identical for both areas, indicating that the surface of the nanowire with clastic floater has little mixture containing Al, O, and Si. The morphology of the products synthesized from the raw materials A20H0 and A20H4 was further investigated by SEM. Figs. 9a and b show the upper layers of the products; further, it can be seen that the morphologies of the upper layers are nearly identical for both raw materials. The SEM images in Figs. 9c and d show the morphologies of the lower Si3N4 layers; they also reveal


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that NH4Cl plays an important role in the morphology of the lower layers. On addition of 0 wt% NH4Cl, the microstructure of the lower layer consisted primarily of short rod-like and small blocky structures. On adding 4 wt% NH4Cl, the microstructure of the lower layer consisted of short needle-like and stacked blocky structures. According to the XRD patterns presented in Fig. 3, the short rod-like and small blocky structures in Fig. 9c correspond to β-Si3N4 and α-Si3N4, respectively, whereas the small stacked blocky structure in Fig. 9d was α-Si3N4. The highmagnification SEM image shown in Fig. 9e indicates that some β-Si3N4 grew together, which may be attributed to the nucleation of Si3N4 in the initial stage. The high-magnification SEM image of α-Si3N4 shown in Fig. 9f indicates that α-Si3N4 deposited gradually, which probably is related to the addition of NH4Cl. DISCUSSION Reaction schemes As is known from previous studies on the direct nitridation of Si powders, the conversion rate and content of α-Si3N4 in the product is low20, and excess heat is generated, because of the selfsintering of the Si powder27. The results of this study indicated that the addition of NaCl and NH4Cl into the reactants is an effective way of decreasing the temperature at which Si powder is completely nitrided, thereby increasing the content of α-Si3N4 in the product10. As shown in Figs. 1, 2 and 3, the increase in the temperature and the content of NaCl and NH4Cl in the raw materials caused an initial increase followed by a dramatic decrease in the formation of α-Si3N4. It is obvious that NH4Cl and NaCl played an important role in dissipating the heat of the Si nitridation system. In other words, the addition of the inexpensive additives NH4Cl and NaCl to


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the Si powder reactants has the same effect as the Si3N4 diluent, in controlling the heat and increasing the content of α-Si3N4 in the product11-13. The total reaction schemes are as follows28-31: NH4Cl(s) → NH3(g) + HCl(g); △H = 177.24 kJ/mol; >350 °C

(1)

Si + 4HCl → SiCl4 + 2H2; △H = −265.23 kJ/mol; >400°C

(2)

3SiCl4(g) + 4NH3(g) → Si3N4(s) + 12HCl(g); >400 °C

(3)

SiCl4(g) + 2NH3(g) → Si(NH)2 + 4HCl(g); △H = 12.57 kJ/mol; >400 °C

(4)

nxSi(s) +

ny ny N2(g) + NH3(g) →[Si x (NH) y ] n (s); >500 °C 3 3

(5)

3Si(NH)2 → Si3N4 (amorph) + 2NH3; △H = 198.61 kJ/mol; >800 °C

(6)

[Si x (NH) y ] n (s) → Si3N4 (amorph) + NH3(g); >800 °C

(7)

[Si x (NH) y ] n (s) → Si3N4(s) + NH3 (g); >800 °C

(8)

Si3N4 (amorph) → α-Si3N4; >1200 °C

(9)

α-Si3N4 → β-Si3N4; >1200 °C

(10)

Si + N2 → α-Si3N4

(11)

Si + N2 → β-Si3N4

(12)

It is considered that the reaction of Si powders containing NaCl and NH4Cl with N2 can be divided into three stages by analyzing the reaction schemes.


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The first stage In the first regime (25–900 °C), when the temperature is ≥350 °C, NH4Cl decomposes to NH3 and HCl and consumes some thermal energy from the reaction environment14 (reaction scheme 1), which decreases the concentration of N2 inside the reacting mixture. A small amount of NH3 and HCl reacts with SiO2 on the surface of the Si powder, which exposes the Si powder to a gaseous mixture of HCl, NH3, and N2. Some part of the Si powders also reacts with N2, HCl, and NH3 to generate Si3N4(s), Si(NH)2, and [Six(NH)y]n, according to reaction schemes (2)–(5). Reactions (2)–(5) occurred during Si nitridation at a temperature ≥ 400–500 °C. Among them, in reactions (2)–(4), Si participated in reaction (2) to generate gaseous SiCl4, following which the gaseous SiCl4 reacted with NH3 to generate Si3N4(s) and Si(NH)2 (reactions (3) and (4)). With time, Si3N4(s) and Si(NH)2 stacked onto the surface of the Si3N4(s) and Si(NH)2 generated previously. Therefore, stacked structures of Si3N4(s) and Si(NH)2 were generated. The other part of the Si powder reacted with N2 and NH3 to generate [Six(NH)y]n, according to reaction (5). When the temperature reached 800 °C or higher, Si(NH)2 and [Six(NH)y]n decomposed to stacked Si3N4(s) and stacked amorphous Si3N4, according to reactions (6)–(8)26. In the initial stage, the reaction consumed vast amounts of HCl and NH3, but also finally generated nearly the same amount of HCl and NH3, implying that HCl and NH3 played the role of a catalyst in the first regime. The second stage During the second regime (900–1450 °C), NaCl melted into a liquid and was adsorbed on the surface of the raw materials. NH3, HCl, and N2 continued to react with the rest of the Si powder to generate amorphous Si3N4. With increasing temperature, stacked amorphous Si3N4 formed


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stacked α-Si3N4 through reaction (9), which was the growth mechanism of the stacked α-Si3N4 in the lower layer20. At the same time, liquid NaCl increased Si powders’ activity on the surface, with increasing temperature, due to the high temperature stimulus and liquid NaCl, some β-Si3N4 was also generated16. Hence, in the second regime, both α-Si3N4 and β-Si3N4 were formed. The third stage In the third regime (>1450 °C), NH3 and HCl played the role of catalysts in the reaction26. However, most of the NH3 and HCl were taken away by flowing N2, leaving only a small amount of NH3 and HCl, which causing their concentration in the mixed gases was extremely low. Due to the high temperature and the presence of liquid NaCl, some α-Si3N4 changes into βSi3N432 (reaction 10); the residual Si powders reacted with N2 to generate a little α-Si3N4 and βSi3N4 (reactions 11 and 12). Hence, in the third stage, there was more β-Si3N4 in the product. Simultaneously, increasing temperature caused the evaporation of liquid NaCl, which reduced the heat of the reaction environment. Based on the results and discussions above, the effect of temperature and the content of NaCl and NH4Cl on the α-Si3N4 content and the growth mechanism of the products, can be simply illustrated. Effect of temperature Increasing the heating temperature has an influence on the second and third regime, even for Si nitridation. Therefore, in the second regime, due to the catalytic action of NH3 and HCl, a greater amount of α-Si3N4 is obtained from Si nitridation. At the same time, When NaCl melts into liquid, Si powders react with N2 to generate β-Si3N4, due to the combined effect of the liquid


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NaCl and high temperature. Hence, increasing the temperature results in an increase in the formation of both α-Si3N4 and β-Si3N4; however, the generation velocity of α-Si3N4 is more than β-Si3N4. In the third stage, on the one hand, the evaporation of liquid NaCl reduces the heat of the reaction environment; hence, the amounts of liquid NaCl as well as the heat both decrease in reaction. Due to lack of the high temperature and liquid NaCl, less α-Si3N4 changes into β-Si3N4, which led to less β-Si3N4 formed. On the other hand, the increasing temperature also promotes the evaporation of Si powders, which increase the content of α-Si3N4 nanowires. Hence, at some specific high temperature, the content of α-Si3N4 will reach a maximum. However, when the temperature is too high, the heat of reaction environment increases, which can also increase the content of β-Si3N4 in the products. Thus, the amount of α-Si3N4 continues to increase and then decline with temperature during heat treatment. Effect of NaCl content As stated above, NaCl mainly is melted into liquid in the second regime and evaporated to gas in the third regime, which absorb the heat of Si nitridation. Therefore, NaCl plays an important role of adjusting the heat in the reaction environment. In the second stage, increasing the content of NaCl results in greater consumption of heat of the reaction environment when NaCl melts into liquid, which inhibits the endothermic reactions (6) and (7), and reduces the generation of amorphous Si3N4; this also prevents the transformation of amorphous Si3N4 to α-Si3N4 and then β-Si3N4 under the combined effects of high temperature and liquid NaCl. In the third stage, the evaporation of liquid NaCl reduces the heat of the reaction and the amount of liquid NaCl in the reaction environment, which also led to less α-Si3N4 changes to β-Si3N421,33. Therefore, with the increase in the NaCl content, that of α-Si3N4 also increases. However, when NaCl is in excess, the residual liquid NaCl causes α-Si3N4 to transform into β-Si3N4 by the solution-reprecipitation


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mechanism, and the residual Si powders easily react with N2 to generate β-Si3N4. Thus, the amount of α-Si3N4 continues to increase and then decline with the content of NaCl. Effect of NH4Cl content Increasing the content of NH4Cl increase the NH3 and HCl generated by the chemical decomposition of NH4Cl, which increases the amorphous Si3N4 generated in the first regime. Hence, more α-Si3N4 is generated in the second and third regime. However, when NH4Cl is in excess, NH3 and HCl are also generated in excess by the chemical decomposition of NH4Cl, which promotes the exothermic reactions (2) and (3). Due to the combined effect of the heat from the exothermic reactions (2) and (3) and the liquid NaCl, β-Si3N4 can be easily obtained. Thus, the amount of α-Si3N4 continues to increase and then decline with the content of NH4Cl. Further investigations are required to confirm the effective mechanism of the content of NH4Cl on the α-Si3N4 content in product. Growth mechanism As stated in Figs. 4, 5, 6, 7, and 8, some Si3N4 nanowires with different nanostructure morphologies were synthesized, the growth mechanism of them were discussed as follows. Fig. 10a shows the growth mechanism of thick nanowire with rough surface. In initial stage, the liquid NaCl adsorbed on the surface of some Si powders, absorbing a little SiO from reaction (13) and Al2O from the porcelain boat and furnace tube decomposition in reaction (14)34 to form a small amount of Si and Al2O3 seed35 (the reaction 15). When SiO2 around the Si powders was consumed in the end, the liquid NaCl droplet on the Al2O3 seed absorbed the Al2O and O2 from N2 and reacted with Si to generate Al2O3, AlxSiyOz (reactions 16 and 17). After the initial


16

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formation of the Al2O3 seed, Al2O3 and AlxSiyOz from reaction (16) and (17) continued to stack on the Al2O3 seed. Finally, the liquid NaCl located on the top of nanowires evaporated, thus forming thick nanowires with a rough surface morphology, as shown in Fig. 5d. It can be concluded that increasing the content of NaCl could have increased the number of NaCl droplets; hence, more Al2O, Si vapor, and O2 could be absorbed, resulting in the more generation of thick nanowires with a rough surface containing Al2O3 and AlxSiyOz. This result is consistent with the observations in Fig. 4a, b, c. SiO2 + Si → 2SiO

(13)

Al2O3 → Al2O + O2

(14)

Al2O + 2SiO → Al2O3 + Si

(15)

Al2O + O2 → Al2O3

(16)

Al2O +

O2 + Si →AlxSiyOz (17)

For other Si powders located on the upper layer of the raw materials, due to the effect of weight of liquid NaCl, no NaCl droplets adsorbed on most of them (Fig. 10b). With increasing temperature, N2 adsorbed on the Si powder surface and generated the Si3N4 nucleus36, following which Si vapor and N2 continued to stack on the Si3N4 nucleus, thus sustaining the growth of the Si3N4 nanowires. In this manner, Si3N4 nanowires were formed on the upper layer of the product37, as shown in Fig. 5a. Based on the results shown in Figs. 6 and 7, and their relevant discussions above, all three areas on the nanobranch primarily contained Si and N, with a little Al and O; moreover, their


17


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elemental contents in all three areas were nearly identical. It is generally recognized that the growth mechanism of the nanobranch is the double-stage VLS-base (vapor–liquid–solid mechanism) and VS-tip (vapor–solid mechanism)23, 25, 38-39, which is established by considering that the catalytic Al element content in the root of the branch is higher than in other areas. However, as seen in Figs. 6 and 7, the contents of the Al elements all three areas were nearly identical, which may not conform to the double-stage VLS-base and VS-tip growth mechanism. Hence, there may be two growth mechanisms: First growth mechanism: As shown in Fig. 10c I, in the initial stage, liquid NaCl deposited on the surface of some Si3N4 nanowires, resulting in the absorption of Si vapor and N2. When these NaCl droplets were oversaturated with Si and N, Si3N4 may have started to nucleate and then grow from the droplets. The melting of NaCl and decomposition of NH4Cl facilitated the migration and evaporation of Si, respectively; thus, the liquid NaCl located on the Si3N4 nucleus could absorb Si vapor and N2 at a relatively low temperature (1250 °C) (even though this temperature was lower than the melting point of Si at 1414 °C), to generate Si3N4. At elevated temperatures, the liquid NaCl located on the top of the Si3N4 branches evaporated, finally leading the formation of Si3N4 nanobranches. It is evident that when a small amount of liquid NaCl deposited on the surface of some Si3N4 nanowires, a Si3N4 nanobranch (Fig. 10c I) was generated, whereas when a large amount of liquid NaCl deposited on the surface of some Si3N4 nanowires, a Si3N4 broad nanobranch (Fig. 10c I) was generated. Second growth mechanism: First, the first-generation α-Si3N4 nanowires (stems) were formed via the VS growth mechanism, as already stated above (Fig. 10b). Second, catalytic Al was randomly deposited onto the surfaces of the stems (Figs. 6, 7, and Fig. 10cⅡ). There are two


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conditions in the process. When the catalytic Al particles were deposited to a lesser extent onto the surfaces of the stems, second-generation nanobranches would nucleate and grow via the aggregation of Si and N vapors into the catalyst, followed by VLS-base growth (Fig. 6a, Fig. 10c Ⅱ ). Then, those nanobranches continually grew along specific directions to form Y-type branched nanostructures via the VS-tip growth mechanism (Fig. 6a). With the growth of the Ytype branched nanostructures, the Al at the root of the branch diffused, which caused the Al content in all three areas was nearly identical. When the catalytic Al particles were deposited to a greater extent onto the surfaces of the stems, second-generation broad nanobranches would nucleate and grow via the aggregation of Si and N vapors into the catalyst, followed by VLSbase growth (Fig. 7a, Fig. 10cⅡ). Then, those broad nanobranches continually grew along specific directions to form short but broad branched nanostructures via the VS-tip growth mechanism (Fig. 7a). Finally, with the growth of the broad nanobranches, the Al located at the root of the broad branch diffused, which caused the Al content in all three areas was nearly identical. Comparison revealed that these growth mechanisms were different (Fig. 10b and c). A possible reason for this is that as seen in Fig. 10b, N2 absorbed on the Si powders’ surface and generated the Si3N4 nucleus, finally forming Si3N4 nanowires. As can be seen in Fig. 10c I, N2 adsorbed on the surface of the Si3N4 nanowires (stem) and could not generate the Si3N4 nucleus. Hence, when Al2O adsorbed on the surface of the Si3N4 nanowires (stem) and generated Al, the Si3N4 nanobranches would be generated by the VLS or VLS-base and VS-tip growth mechanisms. At the same time, as seen in Fig. 8, there was also a small amount of Al2O from the porcelain boat and furnace tube decomposition reaction (14) that had not been absorbed by NaCl droplets, which reacted with O2 and Si vapor to generate AlxSiyOz, adsorbing on the surface of


19


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nanowires, nanobranches or floating on the nanowires. Hence, Al, Si, and O were detected on the surface of nanowires, nanobranches and the floater on the nanowires. SEM images in Fig. 4d, e, and f show the microstructures of the lower Si3N4 nanowires that were synthesized from the raw materials A0H3, A20H3, and A40H3 at 1450 °C for 4 h. For Si powders located on the lower layer of the raw materials, due to the effect of weight of liquid NaCl, most of NaCl droplets adsorbed on them (Fig. 10d). For one, NaCl droplets increased Si powders’ activity on the surface, causing Si powders on the surface react with N2 adsorbed by the NaCl droplets to generate Si3N4; For another, Liquid NaCl can absorb Si vapor and N2, causing the formation of Si3N4. With increasing content of NaCl, the amount of the lower layer short needle-like Si3N4 increased, whose growth mechanism is similar with the first growth mechanism of the nanobranches (Fig. 10c I), which can be explained from Fig. 10d. CONCLUSIONS Silicon nitride (Si3N4) nanowires with different nanostructure morphologies and different phases were synthesized through a direct nitriding method by adding NaCl and NH4Cl in raw Si powders under nitrogen atmosphere. The conclusions of this study can be summarized as follows: (1) When the temperature was 1450°C, the NaCl content was 30 wt%, the NH4Cl content was 3 wt%, the maximum of α-Si3N4 content reached 96 wt%. The process of Si nitridation was divided into three stages by analyzing the reaction schemes: The first stage (25–900 °C): NH4Cl decomposition and the stacked amorphous Si3N4 generation; the second stage (900–1450 °C): NaCl melting and Si3N4 generation; the third stage (>1450 °C): α-Si3N4→β-Si3N4 phase change and NaCl evaporation.


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(2) The products showed two layers: the upper layer consisted of a thin layer of nanowires, in which there were thin nanowires with a smooth surface, thick nanowires with a rough surface, nanobranches; the lower layer was mainly a fluffy, blocky, and short needle-like product. (3) The introduction of NaCl and NH4Cl facilitated the evaporation of Si (vapor) and the decomposition of the Al2O3 porcelain boat and furnace tube; this resulting in mixing of N2, O2, Al2O, and Si vapor, resulting in the formation of different products with different morphologies, such as AlxSiyOz nanowires with a rough surface and Si3N4 nanowires. This was followed by growth of the nanowires, according to the VS, VLS and the double-stage VLS-base and VS-tip growth mechanisms. Silicon nitride (Si3N4) containing high α-Si3N4 with various single crystal nanostructure morphologies can be obtained by the direct nitriding method. However, the products contain a little amount of Al and O, which could affect the thermal conductivity of Si3N4 ceramic. Therefore, removing Al and O in Si3N4 products is an issue that needs to be solved urgently in future. In conclusion, it can be envisioned that the simple preparation method and Si3N4 products can be in high power electronic devices and solar industries.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *Xuemei Yi, Email: [email protected] (X. Yi). Tel.: +862987092391. Funding Sources


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The Fundamental Research Funds for the Central Universities (NO. Z109021534) The Seed Fund for International Science and Technology Cooperation Project of Northwest A&F University (NO. A213021607). ACKNOWLEDGMENT This work was supported by the Fundamental Research Funds for the Central Universities (NO. Z109021534) and the Seed Fund for International Science and Technology Cooperation Project o f Northwest A&F University (NO. A213021607). We gratefully acknowledge the Institute of Water-saving Agriculture in Arid Areas of China (IWSA), Northwest A&F University, for XRD analyses. REFERENCES (1) Li, H.; Chai, G.; Peng, W.; Niu, H. Package and MCPCB for High-power LEDs. Semiconductor Optoelectronics 2007, 28, 47-50. (2) Chen, Q.; Tan, D.; Yu, F.; Chen, F. New Development of Research on Heat-release Substrates of High-power LEDs. Mater. Rev. 2009, 23, 61-62. (3) Zhao, Z.; Tang, Z.; Can, X.; Li, Q.; Zhang, X. Comparing some packaging Board Materials for Large power LED. Equipment Manufacturing Technology 2006, 26, 81-84. (4) Shi, G.; Wang, J.; Ding, P. Present Research Situation of Ceramic Substrate material. J Funct Mater 1993, 24, 176-180. (5) Hirao, K.; Watari, K. High Thermal Conductivity Silicon Nitride Ceramic. Journal of the Korean Ceramic Society 2001, 49, 451-455.


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Page 23 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6) Ke, Z.; Miao, D.; Lei, H.; Meng, J.; Wang, J.; Jing, M.; Liu, X. Highly Corrosion Resistant and Sandwich-Like Si3N4/Cr-CrNx/Si3N4 Coatings used for Solar Selective Absorbing Applications. Acs Appl Mater Inter 2016, 8, 34008-34012. (7) Haggerty, J. S.; Lightfoot, A. Opportunities for Enhancing the Thermal Conductivities of SiC and Si3N4 Ceramics through Improved Processing. John Wiley & Sons 2008, 16, 475-487. (8) Watari, K.; Hirao, K.; Brito, M. E. Hot Isostatic Pressing to Increase Thermal Conductivity of Si3N4 Ceramics. J. Mater. Res. 1999, 14, 1538-1541. (9) Kasori, M.; Hhiorigu, A.; Sumino, H.; Silicon nitride ceramic circuit substrates and the use of the ceramic substrate of semiconductor devices [P]. CN.1149666 C. 1996. (10) Wasanapiarnpong, T.; Wada, S.; Imai, M.; Yano, T. Effect of post-sintering heattreatment on thermal and mechanical properties of Si3N4ceramics sintered with different additives. J Eur Ceram Soc 2006, 26, 3467-3475. (11) Weidlich, H.; Pettenpaul, E.; Petz, F. A. Gallium arsenide integrated circuits for professional broadband applications up to 3 GHz approximately. Acs Appl Mater Inter 2012, 4, 5643-5649. (12) Delgadosanchez, J. M.; Guilera, N.; Francesch, L.; Alba, M. D.; Lopez, L.; Sanchez, E. Ceramic barrier layers for flexible thin film solar cells on metallic substrates: a laboratory scale study for process optimization and barrier layer properties. Acs Appl Mater Inter 2014, 6, 18543. (13) Jiang, J. A New Synthesis Method of α-Silicon Nitride Powder-Reductive Combustion Synthesis from Silicon and Silicon Dioxide. J Am Ceram Soc 2009, 92, 3095-3097.


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(14) Zhang, Y. The Study on High Performance Nitride Ceramic Powders. Zhejiang University 2015. (15) Chung, Y. K.; Koo, J. H.; Kim, S. A.; Chi, E. O.; Hahn, J. H.; Park, C. Optimization of reaction parameters for synthesis of amorphous silicon nitride powder by vapor phase reaction. Ceram Int 2014, 40, 14563-14568. (16) Chen, Y. X.; Li, J. T.; Lin, Z. M.; Liu, G. H.; Yang, S. L.; Du, J. S. Combustion Synthesis of Si3N4 from Si/NH4Cl at Low Nitrogen Pressure. Key Engineering Materials 2008, 368-372, 1767-1770. (17) Liu, X.; Yi, X.; Guo, R.; Li, Q.; Nomura, T. Formation mechanisms of Si3N4 microstructures during silicon powder nitridation. Ceram Int 2017, 43, 16773-16779. (18) Niu, J.; Yi, X.; Nakatsugawa, I.; Akiyama, T. Salt-assisted combustion synthesis of βSiAlON fine powders. Intermetallics 2013, 35, 53-59. (19) Ding, J.; Zhu, H.; Li, G.; Deng, C.; Chai, Z. Catalyst-assisted synthesis of α-Si3N4 in molten salt. Ceram Int 2016, 42, 2892-2898. (20) Niu, J.; Suzuki, S.; Yi, X.; Akiyama, T. Fabrication of AlN particles and whiskers via saltassisted combustion synthesis. Ceram Int 2015, 41, 4438-4443. (21) Yi, X.; Suzuki, S.; Liu, X.; Guo, R.; Akiyama, T. Combustion Synthesis of β-SiAlON Using 3D Ball Milling. Materials Science Forum 2017, 898, 1717-1723. (22) Gao, F.; Yang, W.; Fan, Y.; An, L. Aligned ultra-long single-crystalline α-Si3N4 nanowires. Nanotechnology 2008, 19, 105602.


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(23) Huang, J.; Huang, Z.; Yi, S.; Liu, Y.; Fang, M.; Zhang, S. Fe-catalyzed growth of onedimensional α-Si3N4 nanostructures and their cathodoluminescence properties. Sci Rep-Uk 2013, 3, 3504-3513. (24) Du, H.; Zhang, W.; Li, Y. Effects of growth parameters on the yield and morphology of Si3N4 microcoils prepared by chemical vapor deposition. Mater Res Bull 2014, 50, 57-62. (25) Hu, P.; Dong, S.; Fang, C.; Cheng, Y.; Sun, B.; Chen, G. Ultra-long Si3N4 nanobelts prepared by nanosilicon, nanosilica and graphite. Mat Sci Semicon Proc 2016, 56, 189-195. (26) Yi, S.; Wang, L.; Liu, C.; Tong, L.; Sun, S. The study on the preparation of silicon nitride powders by direct nitridation at normal pressure. Bull Chin Ceramic Soc 2008, 27, 230-235. (27) Liu, Y. D.; Kimura, S. Fluidized-bed nitridation of fine silicon powder. Powder Technol 1999, 106, 160-167. (28) Li, J.; Li, K.; Li, G.; Yan, D. Preparation of Si3N4 powder by combustion synthesis. J Ceram Process Res 2009, 10, 296-300. (29) Yue, H.; Tian, G.; Li, S. Effects of additives on the Content of α-Phase in Si3N4 Synthesized by Direct Nitridation of Si Powder. Bull Chin Ceramic Soc 2008, 27, 370-375. (30) Moulson, A. Review Reaction-bonded silicon nitride: its formation and properties. J. Mater. Sci. 1979, 14, 1017-1051. (31) Wang, Y.; Cheng, L.; Guan, J.; Zhang, L. Effect of dilution and additive on direct nitridation of ferrosilicon. J Eur Ceram Soc 2014, 34, 1115-1122.


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(32) Li, J.; Li, K.; Li, G.; Yan, D. Preparation of Si3N4 powder by combustion synthesis. J Ceram Process Res 2009, 10, 296-300. (33) Suzuki, S.; Yi, X.; Niu, J.; Akiyama, T. Combustion Synthesis of High Purity α-Si3N4 by Premixing of Raw Materials. Journal of the Society of Powder Technology Japan 2016, 53, 301305. (34) Guthrie, R.; Riley, F. Effect of oxide impurities on the nitridation of high purity silicon. J. Mater. Sci. 1974, 9, 1363-1365. (35) Tang, C. In situ catalytic growth of Al2O3 and Si nanowires. J Cryst Growth 2001, 224, 117-121. (36) Pigeon, R.G.; Varma, A. Quantitative kinetic analysis of silicon nitridation. J. Mater. Sci. 1993, 28, 2999-3013. (37) Huang, J.; Zhang, S.; Huang, Z.; Fang, M.; Liu, Y. G.; Chen, K. Co-catalyzed nitridation of silicon and in-situ growth of α-Si3N4 nanorods, Ceram Int 2014, 40, 11063-11070. (38) Huang, J.; Zhang, S.; Huang, Z.; Liu, Y.; Fang, M. Growth of α-Si3N4 nanobelts via Nicatalyzed thermal chemical vapour deposition and their violet-blue luminescent properties. Crystengcomm 2012, 15, 785-790. (39) Yang, W.; Xie, Z.; Li, J.; Miao, H.; Zhang, L.; An, L. Growth of plate like and branched single-crystalline Si3N4 whiskers. Solid State Commun 2004, 132, 263-268.


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Table and Figure captions

Table. 1 Chemical compositions of NaCl. Table. 2 Chemical compositions of NH4Cl. Table. 3 Compositions of the initial reactants. Fig. 1 XRD patterns of the product synthesized from raw materials of A30H3 after heating for 4 h at different temperatures. Fig. 2 XRD patterns of the product synthesized from different raw materials after heating for 4 h at 1450 ℃. Fig. 3 XRD patterns of the product synthesized from different raw materials after heating for 4 h at 1450 ℃. Fig. 4 SEM images of upper (U) and lower (L) layers in the products synthesized from raw materials of A0H3, A20H3 and A40H3 heated for 4 h at 1450 ºC, in which A, E are thin nanowires with smooth surfaces, B is some clastic floaters, C is the nanobranch, D is the thick nanowires with rough surfaces. Fig. 5 TEM images of upper nanowires in the products synthesized from raw materials of A20H3 heated for 4 h. (a) A typical TEM image of a thin nanowire with smooth surface without droplet. (b) HR-TEM image and (c) corresponding FFT pattern of (a). (d) A typical TEM image of a thick nanowire with rough surface. (e) EDS spectra of (d).


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Fig. 6 TEM images of the Si3N4 nanobranch synthesized from raw materials of A20H3. (a) A typical nanobranch and HR-TEM images of location 1, 3. (b) and (c) FFT patterns of location 1, 3. (d) EEL spectra of location 1, 2 and 3. Fig. 7 TEM images of the Si3N4 broad nanobranch synthesized from raw materials of A20H3. (a) A typical broad nanobranch TEM image. (b) EEL spectra of location 1, 2 and 3. Fig. 8 TEM images of the Si3N4 nanowire with clastic floater synthesized from raw materials of A20H3. (a) and (b) A typical nanowire and HR-TEM images of the nanowire. (c) FFT patterns of the nanowire. (d) EDS of location 1, 2. Fig. 9 SEM images of the products synthesized from raw materials of A20H0 and A20H4 at 1450 ℃ for 4 h. (a) The upper product of A20H0. (b) The upper product of A20H4 (c) The lower product of A20H0. (d) The lower product of A20H4. (e) The magnification of (c). (f) The magnification of (d). Fig. 10 Simplified growth models of the different nanostructures. (a) The thick nanowire with rough surface in the upper products. (b) The thin nanowire with smooth surface in the upper products. (c) The nanobranch in the upper products. (d) The short needle-like nanowire in the lower products.


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29

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 30 of 41

Table 1 Chemical compositions of NaCl Major impurity elements (wt.%) NaCl K 99.5 0.02

Br

Ca

Mg

I

S

Ba

N

Fe

Loss on drying

0.0002

0.4558

P

0.01 0.005 0.002 0.002 0.002 0.001 0.001 0.001

Table 2 Chemical compositions of NH4Cl Major impurity elements (wt.%) NH4Cl

99.5

Na

K

S

Mg

Ca

Fe

P

Loss on drying

0.005

0.005

0.005

0.001

0.001

0.0005

0.0005

0.482


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Table 3 Compositions of the initial reactants

Samples (A:NaCl; H:NH4Cl) A0H3 A10H3 A20H3 A30H3 A40H3 A20H0 A20H2 A20H3 A20H4 A20H5

Si (wt. %)

NaCl (wt. %) NH4Cl (wt. %)

97 87 77 67 57 80 79 77 76 75 ACS Paragon Plus Environment

0 10 20 30 40 20 20 20 20 20

3 3 3 3 3 0 1 3 4 5


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Fig. 1 XRD patterns of the product synthesized from raw materials of A30H3 after heating for 4 h at different temperatures. ACS Paragon Plus Environment

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Fig. 2 XRD patterns of the product synthesized from different raw materials after heating for 4 h at 1450 ℃. ACS Paragon Plus Environment


Page 34 of 41

Fig. 3 XRD patterns of the product synthesized from different raw materials after ACS Paragon Plus Environment heating for 4 h at 1450 ℃.

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A40H3-L

(f)

5um

Fig. 4 SEM images of upper (U) and lower (L) layers in the products synthesized from raw materials of A0H3, A20H3 and A40H3 heated for 4 h at 1450 ºC. ACS Paragon Plus Environment

(a) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


Page 36 of 41

(b)

FFT

(c)

0.557nm

0.683nm

(d)

(e) EDS

Fig. 5 TEM images of upper nanowires in the products synthesized from raw materials of A20H3 heated for 4 h. (a) A typical TEM image of a thin nanowire with smooth surface without droplet. (b) HR-TEM image and (c) corresponding FFT pattern of (a). (d) A typical TEM image of a thick nanowire with rough surface. (e) EDS spectra of (d). ACS Paragon Plus Environment

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(d)

Materials & Interfaces 0.619nm 1ACS Applied

(a)

1 0.543nm

3

3

1

2

FFT of 1

0.433nm

0.317nm

(b) FFT of 3 α (𝟏𝟎𝟎)

α (𝟎𝟎𝟏)

2

(c) α (1𝟏𝟏)

3

α (𝟏𝟎𝟏)

Fig. 6 TEM images of the Si3N4 nanobranch synthesized from raw materials of A20H3. (a) A typical nanobranch and HR-TEM images of location 1, 3. (b) and (c) FFT patterns of location 1, 3. (d) EEL spectra of location 1, 2 and 3. ACS Paragon Plus Environment


(a)

1

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(b)

2 2 3

1 3

Fig. 7 TEM images of the Si3N4 broad nanobranch synthesized from raw materials of A20H3. (a) A typical broad nanobranch TEM image. (b) EEL spectra of location 1, 2 and 3. ACS Paragon Plus Environment

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α-Si3N4

1

(b) 0.653nm

(a)

2

EDS

1

FFT

(c) α (1𝟎𝟎) α (𝟏𝟑𝟏)

2

0.231nm

Fig. 8 TEM images of the Si3N4 nanowire with clastic floater synthesized from raw materials of A20H3. (a) and (b) A typical nanowire and HR-TEM images of the nanowire. (c) FFT patterns of the nanowire. (d) EDS of location 1, 2.


(d)


Page 40 of 41

Fig. 9 SEM images of the products synthesized from raw materials of A20H0 and A20H4 at 1450 ℃ for 4 h. (a) The upper product of A20H0. (b) The upper product of A20H4 (c) The lower product of A20H0. (d) The lower product of A20H4. (e) The magnification of (c). (f) The magnification of (d). ACS Paragon Plus Environment

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A. The upper layer in products

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27B. The lower layer in products 28 29 30 31 32 33 34 35 36 37 38 39 40 41


(a)

(b)

(c)

(d)

Fig. 10 Simplified growth models of the different nanostructures. (a) The thick nanowire with rough surface in the upper products. (b) The thin nanowire with smooth surface in the upper products. (c) The in the upper products. (d) The short ACSnanobranch Paragon Plus Environment needle-like nanowire in the lower products.

Formation of different Si3N4 nanostructures by salt-assisted nitridation

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