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Formation mechanisms of Si3N4 microstructures during silicon powder nitridation
Author’s Accepted Manuscript Formation mechanisms of Si3N4 microstructures during silicon powder nitridation Xiongzhang Liu, Xuemei Yi, Ran Guo, Qingda Li, Takahiro Nomura www.elsevier.com/locate/ceri
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S0272-8842(17)32001-1 http://dx.doi.org/10.1016/j.ceramint.2017.09.072 CERI16243
To appear in: Ceramics International Received date: 23 August 2017 Revised date: 7 September 2017 Accepted date: 10 September 2017 Cite this article as: Xiongzhang Liu, Xuemei Yi, Ran Guo, Qingda Li and Takahiro Nomura, Formation mechanisms of Si3N4 microstructures during silicon powder nitridation, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.09.072 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Formation mechanisms of Si3N4 microstructures during silicon powder nitridation
Xiongzhang Liu a, Xuemei Yi a *, Ran Guo a, Qingda Li a, Takahiro Nomura b a
College of Mechanical and Electronic Engineering, Northwest A&F University, Xinong Road 22, Yangling, Shaanxi 712100, China b
Center for Advanced Research of Energy and Materials, Hokkaido University, Kita 13 Nishi 8, Kitaku, Sapporo 060-8628, Japan
*Corresponding author. Tel.:+86 2987092391 E-mail address: [email protected]
(X. Yi)
Abstract Silicon nitride (Si3N4) was synthesized under a nitrogen gas flow (100 mL/min) using a molten salt nitriding method to investigate the effects of the temperature and NaCl content on the α-Si3N4 content in products and their micro-morphologies. Adding NaCl and β-Si3N4 in silicon powders resulted in Si nitridation products divided into two layers. Analysis of the lower product using X-ray diffraction revealed a change in the α-Si3N4 content with changes in the temperature and NaCl content. Analysis of the lower and upper layers using scanning electron microscopy revealed that the upper layer contained Si3N4 nanowires, Si3N4 nanobelts, and clastic oxide impurities; the lower one contained short needle-like and blocky Si3N4. From the microstructures of the products, the product morphology related to that the dry mixing procedure did not correspond to homogenization of the starting Si-Si3N4-NaCl mixtures and the different concentrations of raw materials resulted in different morphologies.
Keywords: Silicon nitride; Molten salt; Microstructure; Si3N4 nanobelts.
1. Introduction Silicon nitride (Si3N4) is a promising engineering material as it exhibits excellent mechanical properties such as good refractoriness, thermal shock resistance, chemical stability, creep resistance, high thermal conductivity, and wear resistance even at high temperatures [1-4]. Hence, it is used extensively. Because of its low thermal expansion coefficient, silicon nitride is not easily deformed or cracked when the environmental temperature changes suddenly. Therefore, it is a good refractory material. In the field of metal processing, silicon nitride ceramic is an important material in metal cutting [5]. Due to its excellent mechanical performance and high thermal conductivity, silicon nitride is considered a potential heat dissipation substrate material in high-power electronic devices [6,7]. To synthesize silicon nitride materials, the imide thermal decomposition method, reduction nitriding method, direct nitriding method, and vapor-phase process are common [2,8]. However, the sol-gel method and gas phase method are complex, expensive, and environmentally unfriendly, while the reduction nitriding method tends to introduce impurities in the final product [9]. Direct nitridation method to prepare silicon nitride using 3Si + 2N2
Si3N4 provides
efficiency and convenience, but also needs high heating temperature. Therefore, there is a need to develop more convenient, economical, and effective methods for producing Si3N4 powder. Molten salt nitriding method is a method that adds molten salt or other material in raw material. It can be used for modifying ZrC by flake graphite [10]; It can also be used for the preparation of porous materials [11], titanium nitride [12] and SiAlON
[13], especially in preparing silicon nitride at low temperature [9]. As such, it has been used to produce silicon nitride with whisker, nanowire, or hollow sphere morphologies [14-18]. Ding and Zhu [9] added NaCl, NaF, and Co into raw Si powders to study the effect of nitriding temperature and Co content on nitriding product and microstructure. The study found that Co can promote the nitriding process and the production of silicon nitride nanorods and nanoparticles was governed by the vapor-liquid-solid and vapor-solid mechanisms. Gu et al. [19] investigated the effect and influencing mechanism of Co nanoparticles (NPs) on the nitridation of silicon. The study found that Co significantly reduces the nitriding temperature and forms silicon nitride whiskers among the products. To the author's knowledge, most researches are focused on the effects of nitridation conditions on the microstructures of products; and the related microstructures are mostly nanowires as well as nanoparticles. However, few research is concentrated on the other types of microstructure and formation mechanisms. Hence, the product's microstructures, the formation mechanisms and the influence of nitridation conditions on the microstructures need to be further studied. This article mainly studies the addition of β-Si3N4 and molten salt NaCl in raw Si powders, the influence of nitriding temperature and NaCl content on the nitriding process, and the formation mechanisms of different product's microstructures.
2. Materials and methods Si powder (99.99 wt% purity, 300 mesh, Adamas Reagent Co. Ltd.) was used as the starting raw material, β-Si3N4 powder (99.9 wt% purity, 20 nm, Shanghai
Titanchem Co. Ltd.) was used as the diluting agent, and high-purity N2 (purity > 99.999%) was used as the nitrogen source. Analytical-grade (≥ 99.5 wt% purity, Guangzhou Jinhuada Chemical Reagent Co. Ltd.) NaCl was used for the molten salt, added at levels of 0, 10, 20, 30, and 40 wt%. The starting powders comprising the components (weight ratios) are shown in Table 1. A zirconium oxide ball mill was used to dry mix the mixed starting powders for 10 min at 1200 r/min and the weight ratio of ball to starting powder was 3:1. The mixed powder sample (1.2 g) was loaded in an alumina crucible and placed at the center of a long alumina tube inserted into an electric furnace. The air in the alumina tube was dried using a vacuum pump. The furnace was heated from room temperature at 5 °C/min to 1250 °C, 1300 °C, 1350 °C, 1400 °C, and 1450 °C (the melting temperature of Si powder is 1410 °C). The nitridation of silicon powder was performed for 4 h in a 0.1 L/min continuous flow of N2 at atmospheric pressure. The heated samples were allowed to cool to room temperature in the furnace. The samples were repeatedly washed with distilled water by using ultrasonic cleaning equipment and filtered to remove residual salt. The phases of the powders resulting from nitridation were characterized by powder X-ray diffraction (XRD, X’Pert Pro MPD) with Cu-Ka radiation, and the microstructures were observed by scanning electron microscopy (SEM, Nova 450 Nano).
3. Results and discussion Si nitridation experiments concerning the change of temperature and the variation
of NaCl content were conducted. As shown in Fig. 1, the products were divided into two layers: the upper was a thin layer of fibrous product, with more at the porcelain boat wall and less in the middle of the porcelain boat. It could be determined that the fibrous product first grew along the wall of the porcelain boat and then expanded to the middle of porcelain boat. The lower was a fluffy blocky product. It was found that when the temperature and NaCl content increased, the upper fibrous products gradually increased and the lower fluffy blocky products reduced. To study the upper fibrous product and the lower blocky product, the products synthesized from raw material set D (40 wt% NaCl + 30 wt% Si3N4 + 30 wt% Si) after heating for 4 h at 1450 °C were analyzed by XRD. As shown in Fig. 2, the lower blocky products were Si3N4 without impurities or unreacted silicon and the content of α-Si3N4 was ~30 wt%. Due to the low quantity of the upper fibrous product, the sample containing 2/3 lower blocky product and 1/3 upper fibrous product was analyzed by XRD. It can be seen that the phase compositions of hybrid materials—except for a small amount of mullite and sialon—were identical to the phase composition of the lower blocky products. Therefore, the upper fibrous product contained O and Al impurities. The effect of temperature on the synthesis of Si3N4 is presented in Fig. 3; the α-Si3N4 peaks appeared at 1250 °C and then continued to increase in height with temperature. Thus, at temperatures between 1250 °C and 1350 °C, the powder produced was composed of α-Si3N4, β-Si3N4, and Si. The Si peaks continued to decrease and disappeared at 1400 °C. And more α-Si3N4 phase was obtained when the
temperature increased to 1450 °C. Therefore, the amount of α-Si3N4 increased with the temperature during heat treatment. The XRD patterns in Fig. 4 were obtained from products containing 0–40 wt% NaCl for 4 h at 1450 °C. As the figure shows, α-Si3N4 and β-Si3N4 were present in the reference sample. Under this condition, silicon powder was completely nitrided, which was independent of the content of NaCl. Comparing the NaCl contents of 0 wt% and 10 wt%, it was found that adding NaCl reduced the intensity of the α-Si3N4 peak and increased the intensity of the β-Si3N4 peak. The intensity of the β-Si3N4 peak increased and the intensity of the α-Si3N4 peak decreased when the NaCl content increased further to 40 wt%, but the intensities of the α-Si3N4 and β-Si3N4 peaks changed slowly. This means that the increased amount of NaCl had little impact on the content of α-Si3N4 and β-Si3N4 when the NaCl content increased from 10 wt% to 40 wt%. SEM images in Fig. 5a and d show microstructures of the lower products synthesized from raw materials of set C (30 wt% Si3N4 + 20 wt% NaCl + 50 wt% Si) at different temperatures for 4 h. Short needle-like and fluffy blocky Si3N4 were obtained in the samples prepared at 1400 °C and 1450 °C, as shown in Fig. 5a and d. The SEM images in Fig. 5b show the microstructure of the upper fibrous product at 1400 °C. Cluster distribution can be found in the upper fibrous product (Fig. 5d, position A), that is to say the upper fibrous product may grow around from one point. The SEM images in Fig. 5c show partial microstructures of the upper fibrous product at 1400 °C. The microtopography of the upper fibrous product revealed a lot of Si3N4
nanowires and few Si3N4 nanobelts. There may be metal droplets on the heads of the Si3N4 nanobelts, as shown in Fig. 5c, position B. The SEM image in Fig. 5e shows the microstructure of the upper fibrous product produced at 1450 °C as a scattered distribution. Fig. 5c and f show that Si3N4 nanowires decreased and Si3N4 nanobelts increased in size and quantity with increasing reaction temperature. For example, the fibrous products in Fig. 5c mostly consisted of Si3N4 nanowires and a few nanobelts with typical widths in the range 1–3 µm at large scale. The fibrous products in Fig. 5f almost entirely consisted of Si3N4 nanobelts with typical widths in the range 3–6 µm. SEM images in Fig. 6 show microstructures of the products synthesized from raw material sets C-E (C: 20 wt% NaCl + 30 wt% Si3N4 + 50 wt% Si; D: 30 wt% NaCl + 30 wt% Si3N4 + 40 wt% Si; E: 40 wt% NaCl + 30 wt% Si3N4 + 30 wt% Si)
at
1450 °C for 4 h. Short needle-like and fluffy blocky Si3N4 were obtained in the lower products with 20 wt%–40 wt% NaCl, as shown in Fig. 6a, d, and g. Comparing the microstructures in Fig. 6a, d, and g, short needle-like Si3N4 gradually decreased. Si3N4 nanobelts and clastic oxide impurities were obtained in the upper products prepared with 20–40 wt% NaCl, as shown in Fig. 6 b, e, and h. Comparing the microstructures in Fig. 6b, e, and h, Si3N4 nanobelts and clastic oxide impurities increased gradually. Fig. 6c, f, and i are typical SEM images of the upper fibrous Si3N4, which indicates that fibrous Si3N4 mainly consisted of nanobelts with typical widths in the range 1–5 µm. Si3N4 nanobelts increased in size and quantity with increasing content of NaCl. More careful observation reveals that the widths of nanobelts were uniform along their entire lengths. There were smooth surfaces and jagged surfaces on the nanobelts
(shown in Fig. 6f, positions A and B). There were more nanobelts with smooth surface than those with jagged surface (shown in Fig. 6f). No metal droplets were observed on the heads of Si3N4 nanobelts, as shown in Fig. 6f, position C. The microstructures of experimental products were slender nanowires, fluffy blocky, nanobelts, short needle-like, and clastic. To study the formation mechanisms of the product microstructures based on the results of previous studies, the relevant references were obtained. Based on the phenomena described, the mechanism by which Si3N4 can form with distinct morphologies is schematically illustrated in Fig. 7a. The dry mixing procedure did not ensure homogenization of starting Si-Si3N4-NaCl mixtures, which caused agglomeration in the raw materials. Fig. 7a shows four agglomeration models in the raw materials. In model A, Si gathers with Si3N4 (shown in Fig. 7a, A), while in model B, Si aggregates alone in raw materials (shown in Fig. 7a, B). The formation mechanism of model A is similar to that of model B. When the agglomeration locates at the lower of the raw mixtures, due to the catalytic and seed action of Si3N4 from raw mixtures or generated by Si nitridation in axial, as well as the hindrance of the materials around the agglomeration, short needle-like and fluffy blocky Si3N4 can be obtained (shown in Fig. 6a, d, and g). When the agglomeration locates at the upper, no hindrance of the raw materials around the agglomeration, only Si3N4 nanowires (shown in Fig. 6i, position D) can be generated through vapor–vapor–solid (VVS) mechanism [20]. In model C, Si gathers with NaCl in raw materials (shown in Fig. 7a, C), high temperature causes part of NaCl, Si, and metal impurities to evaporate during the
process of nitridation. On the one hand, the airflow takes some Si to the upper of the raw mixtures. These Si, together with the agglomeration of Si and NaCl locating at the upper, generate Si3N4 nanowires. The formation mechanism is similar to model B. At the same time, the Si3N4 generated by Si nitridation and NaCl in the upper region common catalyzes the reaction that Si vapor with N2 generates a few Si3N4 nanobelts; Si3N4 plays the role of seed, so the growth mechanism of the upper silicon nitride nanobelts is VVS [20]. Some Si powders not carried to the upper region by airflow and remaining in the lower form short needle-like and blocky Si3N4, as in the case of raw materials where Si aggregates. On the other hand, Si vapor reacts with N2, O2, and metal impurities to generate clastic oxide impurities floating among the Si3N4 nanobelts (shown in Fig. 6b, e, h). In model D, Si gathers with NaCl and Si3N4 in raw materials (shown in Fig. 7a, D). With increasing temperature, NaCl molten salt in raw materials melts, which makes Si dissolve in liquid NaCl to generate Si-NaCl droplets. Part of the Si-NaCl droplets adheres to Si3N4 particles, another part of the Si-NaCl droplets existes alone. Si nitridation is exothermic and the reaction temperature is high, which cause NaCl, Si, and some metal impurities to form vapors during reaction. On the one hand, NaCl airflow takes some Si and Si3N4 from raw mixtures or generated by Si nitridation to the upper, with the agglomeration locating at the upper, generate a lot of Si3N4 nanobelts and few Si3N4 nanowires. This is similar to model C. Other Si powders and Si3N4 locating at the lower region not carried to the upper region by airflow forms short needle-like and blocky Si3N4, as in the case where Si gathers with Si3N4.
Therefore, increasing the content of NaCl in Si powders and temperature causes more NaCl airflow, which bring more Si3N4 particles and silicon powders of the lower raw materials, and decreasing the Si3N4 particles and Si powders of the lower raw materials. Hence, the airflow reduces the lower Si3N4 diluent and Si powders, causing the lower Si3N4 product more compact and less than the reaction without NaCl airflow. On the other hand, Si vapor reacts with N2, O2, and metal impurities from NaCl to generate clastic oxide impurities floating among the silicon nitride nanobelts (shown in Fig. 6b, e, h). Increasing the content of NaCl increases the metal impurities from NaCl, increasing the upper clastic oxide impurities, which conforms to the rule described in Fig. 6b, e, h. At the same time, raw Si powders and NaCl contain small amounts of metal impurities, and the heads of a small amount of Si3N4 nanobelts may contain metal droplets (as shown in Fig. 5c, position B). Hence, a small amount of Si3N4 nanobelts may grow by VLS (vapor–liquid–solid mechanism) [20]. Thus, the experimental product's micro-morphologies contain slender nanowires, fluffy blocky, nanobelts, short needle-like, and clastic (shown in Fig. 7b). To verify the above hypothesis, related experiments were conducted. The microstructures of products synthesized from raw materials of A (70 wt% Si + 30 wt% Si3N4) for 4 h at 1450 °C mainly were the lower short needle-like, fluffy blocky, and upper nanowires (shown in Fig. 8a, e), which conforms to the phenomenon of Si gathered with Si3N4 in raw materials. The microstructure of products synthesized from raw materials of F (100 wt% Si) for 4 h at 1450 °C mainly were the upper slender nanowires and the lower short needle-like and fluffy blocky (shown in Fig.
8b,f), which conforms to the phenomenon of the Si aggregated alone in raw materials. The microstructure of products synthesized from raw materials of G (60 wt% Si + 40 wt% NaCl) for 4 h at 1450 °C mainly were the upper slender nanowires and few nanobelts and the lower blocky clusters (shown in Fig. 8c, g), which conforms to the phenomenon of Si gathered with NaCl in raw materials. The microstructure of products synthesized from raw materials of E (30 wt% Si + 30 wt% Si3N4 + 40 wt% NaCl) for 4 h at 1450 °C mainly were the lower short needle-like and fluffy blocky, and upper nanowires and nanobelts (shown in Fig. 8d,h), which conforms to the phenomenon of Si gathered with Si3N4 and NaCl in
raw materials. The Si3N4
nanowires in figure. 8f are longer and straighter without bifurcation of Si3N4 nanowires, but the Si3N4 nanowires in Fig. 8g are shorter, more curved, and appear bifurcate phenomenon, which may due to the influence of NaCl airflow. That is to say the NaCl airflow has a significant impact on the morphology of Si3N4. Therefore, the experimental phenomenon confirms the hypothesis. It was found that Si3N4 microstructure have nanowires, blocks and slender nanobelts. Several scholars have performed research on the formation mechanism of Si3N4 morphology. It is widely believed that the nanowire growth mechanism is VLS or VVS [21]. Huang et al. [20] studied the influence of adding Co on the growth mechanism of Si3N4 nanorods and found that there were metal particles on top of some of the nanorods but another not; they believed that the Si3N4 nanowires growth mechanism was VLS and VVS. Chai et al. [22] studied the microstructure of the hexagonal cylindrical blocky α-Si3N4 and found that the generation of hexagonal
plate-like a-Si3N4 phase was related to the formation of Ni-Si in the eutectic salt at 1050–1350 °C and the content of Ni had no effect on the morphology of α-Si3N4. The formation mechanism of nanobelts is generally believed to be VLS, VS (vapor–solid mechanism), and VVS [23-26]. According to Rodriguez and Makhonin [27], the Si3N4 nanobelts obtained in experiment are single crystal β-Si3N4. The growth of nanobelts contains radial growth and axial growth. The effect of metal droplet catalysis [28] or Si3N4 seeding accelerated the axial growth of Si3N4, which led to axial growth speed exceeding the radial growth speed. During radial growth, on the one hand, the anisotropic crystal led to different radial growth speeds, causing inhomogeneity in radial growth, finally forming nanobelts [29]; on the other hand, because the particular regions of Si3N4 particles are at their higher energy states, they can absorb the molecular or atomic vapor species and as a result, the growth rate as well as the dimensions of the nanoforms increase. Therefore, in order to minimize the surface energy, the lower energy facets appears, resulting into the formation of nanobelts [30]. Some curved nanobelts may be caused by the internal stress in nanobelts [24] or the airflow of the NaCl and N2 during the growth process [31]. It was also observed that the surfaces of some nanobelts were zigzag shape (shown in Fig. 5c, position B), which is assumed to be caused by the different growth rates on various crystals [32], this remains to be studied.
4. Conclusions The impact of the temperature and NaCl content on the Si3N4 phase composition
and morphology of Si3N4 prepared by the molten salt nitriding method were studied. This study found that when β-Si3N4 and NaCl were added to the raw material, the nitride products were divided into two layers. The upper product was fibrous Si3N4 and contained small amounts of oxide impurities; the lower product appeared blocky and short needle-like. When the temperature was 1400 °C, the silicon powders were completely nitrided. With an increase in the temperature, the Si3N4 nanobelts content increased but the Si3N4 nanowires content reduced among the upper products. The amount of α-Si3N4 gradually increased in the lower products. With increasing of NaCl content, the amount of Si3N4 nanobelts and clastic oxide impurities in the upper products increased gradually and the amount of α-Si3N4 among the lower products reduced gradually. The reason of different morphologies of products was different concentrations of the raw materials. Finally, the formation mechanism of Si3N4 nanobelts had been discussed.
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Table and Figure captions
Table 1 Compositions of the initial reactant mixtures Fig. 1. Image of the Si nitridation product. The products are divided into two layers: the upper is a thin layer of fibrous product and the lower is a fluffy blocky product. Fig. 2. XRD patterns of the lower blocky product and the lower blocky product + the upper fibrous product synthesized from raw materials of D after heating for 4 h at 1450 °C. Fig. 3. XRD patterns of the lower blocky product synthesized from raw materials of C after heating for 4 h at different temperatures. Fig. 4. XRD patterns of the lower blocky product synthesized from raw materials of A-E after heating for 4 h at 1450 °C. Fig. 5. SEM images of products synthesized from raw materials of C heated for 4 h at different temperatures. (a) The lower blocky, short needle-like product at 1400 °C. (b) and (c) The upper fibrous products at 1400 °C. (d) The lower blocky, short needle-like product at 1450 °C. (e) and (f) The upper fibrous products at 1450 °C. Fig. 6. SEM images of products synthesized from raw materials of C-E at 1450 °C for 4 h. (a), (d), and (g) The lower blocky, short needle-like products synthesized from raw materials of C-E, respectively. (b), (e), and (h) The upper fibrous products synthesized from raw materials of C-E, respectively. (c), (f), and (i) The high-magnification SEM images of the upper fibrous products synthesized from raw materials of C-E, respectively.
Fig. 7. (a) The phenomenon of agglomeration in raw materials: A is a mixture of Si and Si3N4, B is mixture of Si, C is a mixture of Si and NaCl, D is a mixture of Si, NaCl, and Si3N4. (b) Images of Si3N4 products, the upper products are fibrous Si3N4, the lower products are short needle-like and fluffy blocky Si3N4. Fig. 8. SEM images of products synthesized from different raw materials of A, F, G, D at 1450 °C for 4 h. (a)-(d) The lower products synthesized from different raw materials of A, F, G, D, respectively. (e)-(h) The upper products synthesized from different raw materials of A, F, G, D, respectively.
Figure
60 30 10
70
Si(wt%)
Si3N4(wt%) 30
NaCl(wt%) 0
B
A
Samples
20
30
50
C
30
30
40
D
40
30
30
E
0
0
100
F
Table 1 Compositions of the initial reactant mixtures
40
0
60
G
Fig. 1 Image of the Si nitridation product. The products are divided into two layers: the upper is a thin layer of fibrous product and the lower is a fluffy blocky product.
Fig. 2 XRD patterns of the lower blocky product and the lower blocky product + the upper fibrous product synthesized from raw materials of D after heating for 4 h at 1450℃.
Fig. 3 XRD patterns of the lower blocky product synthesized from raw materials of C after heating for 4 h at different temperatures.
Fig. 4 XRD patterns of the lower blocky product synthesized from raw materials of A-E after heating for 4 h at 1450 ℃.
Fig. 5 SEM images of products synthesized from raw materials of C heated for 4 h at different temperatures. (a) The lower blocky, short needle-like product at 1400 ℃.(b) and (c) The upper fibrous products at 1400 ℃. (d) The lower blocky, short needle-like product at 1450 ℃. (e) and (f) The upper fibrous products at 1450 ℃.
Fig. 6 SEM images of products synthesized from raw materials of C-E at 1450 ℃ for 4 h. (a), (d), and (g) The lower blocky, short needle-like products synthesized from raw materials of C-E, respectively. (b), (e), and (h) The upper fibrous products synthesized from raw materials of C-E, respectively. (c), (f), and (i) The high-magnification SEM images of the upper fibrous products synthesized from raw materials of C-E, respectively.
Fig. 7 (a) The phenomenon of agglomeration in raw materials: A is a mixture of Si and Si3N4, B is mixture of Si, C is a mixture of Si and NaCl, D is a mixture of Si, NaCl, and Si3N4. (b) Images of Si3N4 products, the upper products are fibrous Si3N4, the lower products are short needle-like and fluffy blocky Si3N4.
Fig. 8 SEM images of products synthesized from different raw materials of A, F, G, D at 1450 ℃ for 4 h. (a)-(d) The lower products synthesized from different raw materials of A, F, G, D, respectively. (e)-(h) The upper products synthesized from different raw materials of A, F, G, D, respectively.
PII: DOI: Reference:
S0272-8842(17)32001-1 http://dx.doi.org/10.1016/j.ceramint.2017.09.072 CERI16243
To appear in: Ceramics International Received date: 23 August 2017 Revised date: 7 September 2017 Accepted date: 10 September 2017 Cite this article as: Xiongzhang Liu, Xuemei Yi, Ran Guo, Qingda Li and Takahiro Nomura, Formation mechanisms of Si3N4 microstructures during silicon powder nitridation, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.09.072 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Formation mechanisms of Si3N4 microstructures during silicon powder nitridation
Xiongzhang Liu a, Xuemei Yi a *, Ran Guo a, Qingda Li a, Takahiro Nomura b a
College of Mechanical and Electronic Engineering, Northwest A&F University, Xinong Road 22, Yangling, Shaanxi 712100, China b
Center for Advanced Research of Energy and Materials, Hokkaido University, Kita 13 Nishi 8, Kitaku, Sapporo 060-8628, Japan
*Corresponding author. Tel.:+86 2987092391 E-mail address: [email protected]
(X. Yi)
Abstract Silicon nitride (Si3N4) was synthesized under a nitrogen gas flow (100 mL/min) using a molten salt nitriding method to investigate the effects of the temperature and NaCl content on the α-Si3N4 content in products and their micro-morphologies. Adding NaCl and β-Si3N4 in silicon powders resulted in Si nitridation products divided into two layers. Analysis of the lower product using X-ray diffraction revealed a change in the α-Si3N4 content with changes in the temperature and NaCl content. Analysis of the lower and upper layers using scanning electron microscopy revealed that the upper layer contained Si3N4 nanowires, Si3N4 nanobelts, and clastic oxide impurities; the lower one contained short needle-like and blocky Si3N4. From the microstructures of the products, the product morphology related to that the dry mixing procedure did not correspond to homogenization of the starting Si-Si3N4-NaCl mixtures and the different concentrations of raw materials resulted in different morphologies.
Keywords: Silicon nitride; Molten salt; Microstructure; Si3N4 nanobelts.
1. Introduction Silicon nitride (Si3N4) is a promising engineering material as it exhibits excellent mechanical properties such as good refractoriness, thermal shock resistance, chemical stability, creep resistance, high thermal conductivity, and wear resistance even at high temperatures [1-4]. Hence, it is used extensively. Because of its low thermal expansion coefficient, silicon nitride is not easily deformed or cracked when the environmental temperature changes suddenly. Therefore, it is a good refractory material. In the field of metal processing, silicon nitride ceramic is an important material in metal cutting [5]. Due to its excellent mechanical performance and high thermal conductivity, silicon nitride is considered a potential heat dissipation substrate material in high-power electronic devices [6,7]. To synthesize silicon nitride materials, the imide thermal decomposition method, reduction nitriding method, direct nitriding method, and vapor-phase process are common [2,8]. However, the sol-gel method and gas phase method are complex, expensive, and environmentally unfriendly, while the reduction nitriding method tends to introduce impurities in the final product [9]. Direct nitridation method to prepare silicon nitride using 3Si + 2N2
Si3N4 provides
efficiency and convenience, but also needs high heating temperature. Therefore, there is a need to develop more convenient, economical, and effective methods for producing Si3N4 powder. Molten salt nitriding method is a method that adds molten salt or other material in raw material. It can be used for modifying ZrC by flake graphite [10]; It can also be used for the preparation of porous materials [11], titanium nitride [12] and SiAlON
[13], especially in preparing silicon nitride at low temperature [9]. As such, it has been used to produce silicon nitride with whisker, nanowire, or hollow sphere morphologies [14-18]. Ding and Zhu [9] added NaCl, NaF, and Co into raw Si powders to study the effect of nitriding temperature and Co content on nitriding product and microstructure. The study found that Co can promote the nitriding process and the production of silicon nitride nanorods and nanoparticles was governed by the vapor-liquid-solid and vapor-solid mechanisms. Gu et al. [19] investigated the effect and influencing mechanism of Co nanoparticles (NPs) on the nitridation of silicon. The study found that Co significantly reduces the nitriding temperature and forms silicon nitride whiskers among the products. To the author's knowledge, most researches are focused on the effects of nitridation conditions on the microstructures of products; and the related microstructures are mostly nanowires as well as nanoparticles. However, few research is concentrated on the other types of microstructure and formation mechanisms. Hence, the product's microstructures, the formation mechanisms and the influence of nitridation conditions on the microstructures need to be further studied. This article mainly studies the addition of β-Si3N4 and molten salt NaCl in raw Si powders, the influence of nitriding temperature and NaCl content on the nitriding process, and the formation mechanisms of different product's microstructures.
2. Materials and methods Si powder (99.99 wt% purity, 300 mesh, Adamas Reagent Co. Ltd.) was used as the starting raw material, β-Si3N4 powder (99.9 wt% purity, 20 nm, Shanghai
Titanchem Co. Ltd.) was used as the diluting agent, and high-purity N2 (purity > 99.999%) was used as the nitrogen source. Analytical-grade (≥ 99.5 wt% purity, Guangzhou Jinhuada Chemical Reagent Co. Ltd.) NaCl was used for the molten salt, added at levels of 0, 10, 20, 30, and 40 wt%. The starting powders comprising the components (weight ratios) are shown in Table 1. A zirconium oxide ball mill was used to dry mix the mixed starting powders for 10 min at 1200 r/min and the weight ratio of ball to starting powder was 3:1. The mixed powder sample (1.2 g) was loaded in an alumina crucible and placed at the center of a long alumina tube inserted into an electric furnace. The air in the alumina tube was dried using a vacuum pump. The furnace was heated from room temperature at 5 °C/min to 1250 °C, 1300 °C, 1350 °C, 1400 °C, and 1450 °C (the melting temperature of Si powder is 1410 °C). The nitridation of silicon powder was performed for 4 h in a 0.1 L/min continuous flow of N2 at atmospheric pressure. The heated samples were allowed to cool to room temperature in the furnace. The samples were repeatedly washed with distilled water by using ultrasonic cleaning equipment and filtered to remove residual salt. The phases of the powders resulting from nitridation were characterized by powder X-ray diffraction (XRD, X’Pert Pro MPD) with Cu-Ka radiation, and the microstructures were observed by scanning electron microscopy (SEM, Nova 450 Nano).
3. Results and discussion Si nitridation experiments concerning the change of temperature and the variation
of NaCl content were conducted. As shown in Fig. 1, the products were divided into two layers: the upper was a thin layer of fibrous product, with more at the porcelain boat wall and less in the middle of the porcelain boat. It could be determined that the fibrous product first grew along the wall of the porcelain boat and then expanded to the middle of porcelain boat. The lower was a fluffy blocky product. It was found that when the temperature and NaCl content increased, the upper fibrous products gradually increased and the lower fluffy blocky products reduced. To study the upper fibrous product and the lower blocky product, the products synthesized from raw material set D (40 wt% NaCl + 30 wt% Si3N4 + 30 wt% Si) after heating for 4 h at 1450 °C were analyzed by XRD. As shown in Fig. 2, the lower blocky products were Si3N4 without impurities or unreacted silicon and the content of α-Si3N4 was ~30 wt%. Due to the low quantity of the upper fibrous product, the sample containing 2/3 lower blocky product and 1/3 upper fibrous product was analyzed by XRD. It can be seen that the phase compositions of hybrid materials—except for a small amount of mullite and sialon—were identical to the phase composition of the lower blocky products. Therefore, the upper fibrous product contained O and Al impurities. The effect of temperature on the synthesis of Si3N4 is presented in Fig. 3; the α-Si3N4 peaks appeared at 1250 °C and then continued to increase in height with temperature. Thus, at temperatures between 1250 °C and 1350 °C, the powder produced was composed of α-Si3N4, β-Si3N4, and Si. The Si peaks continued to decrease and disappeared at 1400 °C. And more α-Si3N4 phase was obtained when the
temperature increased to 1450 °C. Therefore, the amount of α-Si3N4 increased with the temperature during heat treatment. The XRD patterns in Fig. 4 were obtained from products containing 0–40 wt% NaCl for 4 h at 1450 °C. As the figure shows, α-Si3N4 and β-Si3N4 were present in the reference sample. Under this condition, silicon powder was completely nitrided, which was independent of the content of NaCl. Comparing the NaCl contents of 0 wt% and 10 wt%, it was found that adding NaCl reduced the intensity of the α-Si3N4 peak and increased the intensity of the β-Si3N4 peak. The intensity of the β-Si3N4 peak increased and the intensity of the α-Si3N4 peak decreased when the NaCl content increased further to 40 wt%, but the intensities of the α-Si3N4 and β-Si3N4 peaks changed slowly. This means that the increased amount of NaCl had little impact on the content of α-Si3N4 and β-Si3N4 when the NaCl content increased from 10 wt% to 40 wt%. SEM images in Fig. 5a and d show microstructures of the lower products synthesized from raw materials of set C (30 wt% Si3N4 + 20 wt% NaCl + 50 wt% Si) at different temperatures for 4 h. Short needle-like and fluffy blocky Si3N4 were obtained in the samples prepared at 1400 °C and 1450 °C, as shown in Fig. 5a and d. The SEM images in Fig. 5b show the microstructure of the upper fibrous product at 1400 °C. Cluster distribution can be found in the upper fibrous product (Fig. 5d, position A), that is to say the upper fibrous product may grow around from one point. The SEM images in Fig. 5c show partial microstructures of the upper fibrous product at 1400 °C. The microtopography of the upper fibrous product revealed a lot of Si3N4
nanowires and few Si3N4 nanobelts. There may be metal droplets on the heads of the Si3N4 nanobelts, as shown in Fig. 5c, position B. The SEM image in Fig. 5e shows the microstructure of the upper fibrous product produced at 1450 °C as a scattered distribution. Fig. 5c and f show that Si3N4 nanowires decreased and Si3N4 nanobelts increased in size and quantity with increasing reaction temperature. For example, the fibrous products in Fig. 5c mostly consisted of Si3N4 nanowires and a few nanobelts with typical widths in the range 1–3 µm at large scale. The fibrous products in Fig. 5f almost entirely consisted of Si3N4 nanobelts with typical widths in the range 3–6 µm. SEM images in Fig. 6 show microstructures of the products synthesized from raw material sets C-E (C: 20 wt% NaCl + 30 wt% Si3N4 + 50 wt% Si; D: 30 wt% NaCl + 30 wt% Si3N4 + 40 wt% Si; E: 40 wt% NaCl + 30 wt% Si3N4 + 30 wt% Si)
at
1450 °C for 4 h. Short needle-like and fluffy blocky Si3N4 were obtained in the lower products with 20 wt%–40 wt% NaCl, as shown in Fig. 6a, d, and g. Comparing the microstructures in Fig. 6a, d, and g, short needle-like Si3N4 gradually decreased. Si3N4 nanobelts and clastic oxide impurities were obtained in the upper products prepared with 20–40 wt% NaCl, as shown in Fig. 6 b, e, and h. Comparing the microstructures in Fig. 6b, e, and h, Si3N4 nanobelts and clastic oxide impurities increased gradually. Fig. 6c, f, and i are typical SEM images of the upper fibrous Si3N4, which indicates that fibrous Si3N4 mainly consisted of nanobelts with typical widths in the range 1–5 µm. Si3N4 nanobelts increased in size and quantity with increasing content of NaCl. More careful observation reveals that the widths of nanobelts were uniform along their entire lengths. There were smooth surfaces and jagged surfaces on the nanobelts
(shown in Fig. 6f, positions A and B). There were more nanobelts with smooth surface than those with jagged surface (shown in Fig. 6f). No metal droplets were observed on the heads of Si3N4 nanobelts, as shown in Fig. 6f, position C. The microstructures of experimental products were slender nanowires, fluffy blocky, nanobelts, short needle-like, and clastic. To study the formation mechanisms of the product microstructures based on the results of previous studies, the relevant references were obtained. Based on the phenomena described, the mechanism by which Si3N4 can form with distinct morphologies is schematically illustrated in Fig. 7a. The dry mixing procedure did not ensure homogenization of starting Si-Si3N4-NaCl mixtures, which caused agglomeration in the raw materials. Fig. 7a shows four agglomeration models in the raw materials. In model A, Si gathers with Si3N4 (shown in Fig. 7a, A), while in model B, Si aggregates alone in raw materials (shown in Fig. 7a, B). The formation mechanism of model A is similar to that of model B. When the agglomeration locates at the lower of the raw mixtures, due to the catalytic and seed action of Si3N4 from raw mixtures or generated by Si nitridation in axial, as well as the hindrance of the materials around the agglomeration, short needle-like and fluffy blocky Si3N4 can be obtained (shown in Fig. 6a, d, and g). When the agglomeration locates at the upper, no hindrance of the raw materials around the agglomeration, only Si3N4 nanowires (shown in Fig. 6i, position D) can be generated through vapor–vapor–solid (VVS) mechanism [20]. In model C, Si gathers with NaCl in raw materials (shown in Fig. 7a, C), high temperature causes part of NaCl, Si, and metal impurities to evaporate during the
process of nitridation. On the one hand, the airflow takes some Si to the upper of the raw mixtures. These Si, together with the agglomeration of Si and NaCl locating at the upper, generate Si3N4 nanowires. The formation mechanism is similar to model B. At the same time, the Si3N4 generated by Si nitridation and NaCl in the upper region common catalyzes the reaction that Si vapor with N2 generates a few Si3N4 nanobelts; Si3N4 plays the role of seed, so the growth mechanism of the upper silicon nitride nanobelts is VVS [20]. Some Si powders not carried to the upper region by airflow and remaining in the lower form short needle-like and blocky Si3N4, as in the case of raw materials where Si aggregates. On the other hand, Si vapor reacts with N2, O2, and metal impurities to generate clastic oxide impurities floating among the Si3N4 nanobelts (shown in Fig. 6b, e, h). In model D, Si gathers with NaCl and Si3N4 in raw materials (shown in Fig. 7a, D). With increasing temperature, NaCl molten salt in raw materials melts, which makes Si dissolve in liquid NaCl to generate Si-NaCl droplets. Part of the Si-NaCl droplets adheres to Si3N4 particles, another part of the Si-NaCl droplets existes alone. Si nitridation is exothermic and the reaction temperature is high, which cause NaCl, Si, and some metal impurities to form vapors during reaction. On the one hand, NaCl airflow takes some Si and Si3N4 from raw mixtures or generated by Si nitridation to the upper, with the agglomeration locating at the upper, generate a lot of Si3N4 nanobelts and few Si3N4 nanowires. This is similar to model C. Other Si powders and Si3N4 locating at the lower region not carried to the upper region by airflow forms short needle-like and blocky Si3N4, as in the case where Si gathers with Si3N4.
Therefore, increasing the content of NaCl in Si powders and temperature causes more NaCl airflow, which bring more Si3N4 particles and silicon powders of the lower raw materials, and decreasing the Si3N4 particles and Si powders of the lower raw materials. Hence, the airflow reduces the lower Si3N4 diluent and Si powders, causing the lower Si3N4 product more compact and less than the reaction without NaCl airflow. On the other hand, Si vapor reacts with N2, O2, and metal impurities from NaCl to generate clastic oxide impurities floating among the silicon nitride nanobelts (shown in Fig. 6b, e, h). Increasing the content of NaCl increases the metal impurities from NaCl, increasing the upper clastic oxide impurities, which conforms to the rule described in Fig. 6b, e, h. At the same time, raw Si powders and NaCl contain small amounts of metal impurities, and the heads of a small amount of Si3N4 nanobelts may contain metal droplets (as shown in Fig. 5c, position B). Hence, a small amount of Si3N4 nanobelts may grow by VLS (vapor–liquid–solid mechanism) [20]. Thus, the experimental product's micro-morphologies contain slender nanowires, fluffy blocky, nanobelts, short needle-like, and clastic (shown in Fig. 7b). To verify the above hypothesis, related experiments were conducted. The microstructures of products synthesized from raw materials of A (70 wt% Si + 30 wt% Si3N4) for 4 h at 1450 °C mainly were the lower short needle-like, fluffy blocky, and upper nanowires (shown in Fig. 8a, e), which conforms to the phenomenon of Si gathered with Si3N4 in raw materials. The microstructure of products synthesized from raw materials of F (100 wt% Si) for 4 h at 1450 °C mainly were the upper slender nanowires and the lower short needle-like and fluffy blocky (shown in Fig.
8b,f), which conforms to the phenomenon of the Si aggregated alone in raw materials. The microstructure of products synthesized from raw materials of G (60 wt% Si + 40 wt% NaCl) for 4 h at 1450 °C mainly were the upper slender nanowires and few nanobelts and the lower blocky clusters (shown in Fig. 8c, g), which conforms to the phenomenon of Si gathered with NaCl in raw materials. The microstructure of products synthesized from raw materials of E (30 wt% Si + 30 wt% Si3N4 + 40 wt% NaCl) for 4 h at 1450 °C mainly were the lower short needle-like and fluffy blocky, and upper nanowires and nanobelts (shown in Fig. 8d,h), which conforms to the phenomenon of Si gathered with Si3N4 and NaCl in
raw materials. The Si3N4
nanowires in figure. 8f are longer and straighter without bifurcation of Si3N4 nanowires, but the Si3N4 nanowires in Fig. 8g are shorter, more curved, and appear bifurcate phenomenon, which may due to the influence of NaCl airflow. That is to say the NaCl airflow has a significant impact on the morphology of Si3N4. Therefore, the experimental phenomenon confirms the hypothesis. It was found that Si3N4 microstructure have nanowires, blocks and slender nanobelts. Several scholars have performed research on the formation mechanism of Si3N4 morphology. It is widely believed that the nanowire growth mechanism is VLS or VVS [21]. Huang et al. [20] studied the influence of adding Co on the growth mechanism of Si3N4 nanorods and found that there were metal particles on top of some of the nanorods but another not; they believed that the Si3N4 nanowires growth mechanism was VLS and VVS. Chai et al. [22] studied the microstructure of the hexagonal cylindrical blocky α-Si3N4 and found that the generation of hexagonal
plate-like a-Si3N4 phase was related to the formation of Ni-Si in the eutectic salt at 1050–1350 °C and the content of Ni had no effect on the morphology of α-Si3N4. The formation mechanism of nanobelts is generally believed to be VLS, VS (vapor–solid mechanism), and VVS [23-26]. According to Rodriguez and Makhonin [27], the Si3N4 nanobelts obtained in experiment are single crystal β-Si3N4. The growth of nanobelts contains radial growth and axial growth. The effect of metal droplet catalysis [28] or Si3N4 seeding accelerated the axial growth of Si3N4, which led to axial growth speed exceeding the radial growth speed. During radial growth, on the one hand, the anisotropic crystal led to different radial growth speeds, causing inhomogeneity in radial growth, finally forming nanobelts [29]; on the other hand, because the particular regions of Si3N4 particles are at their higher energy states, they can absorb the molecular or atomic vapor species and as a result, the growth rate as well as the dimensions of the nanoforms increase. Therefore, in order to minimize the surface energy, the lower energy facets appears, resulting into the formation of nanobelts [30]. Some curved nanobelts may be caused by the internal stress in nanobelts [24] or the airflow of the NaCl and N2 during the growth process [31]. It was also observed that the surfaces of some nanobelts were zigzag shape (shown in Fig. 5c, position B), which is assumed to be caused by the different growth rates on various crystals [32], this remains to be studied.
4. Conclusions The impact of the temperature and NaCl content on the Si3N4 phase composition
and morphology of Si3N4 prepared by the molten salt nitriding method were studied. This study found that when β-Si3N4 and NaCl were added to the raw material, the nitride products were divided into two layers. The upper product was fibrous Si3N4 and contained small amounts of oxide impurities; the lower product appeared blocky and short needle-like. When the temperature was 1400 °C, the silicon powders were completely nitrided. With an increase in the temperature, the Si3N4 nanobelts content increased but the Si3N4 nanowires content reduced among the upper products. The amount of α-Si3N4 gradually increased in the lower products. With increasing of NaCl content, the amount of Si3N4 nanobelts and clastic oxide impurities in the upper products increased gradually and the amount of α-Si3N4 among the lower products reduced gradually. The reason of different morphologies of products was different concentrations of the raw materials. Finally, the formation mechanism of Si3N4 nanobelts had been discussed.
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Table and Figure captions
Table 1 Compositions of the initial reactant mixtures Fig. 1. Image of the Si nitridation product. The products are divided into two layers: the upper is a thin layer of fibrous product and the lower is a fluffy blocky product. Fig. 2. XRD patterns of the lower blocky product and the lower blocky product + the upper fibrous product synthesized from raw materials of D after heating for 4 h at 1450 °C. Fig. 3. XRD patterns of the lower blocky product synthesized from raw materials of C after heating for 4 h at different temperatures. Fig. 4. XRD patterns of the lower blocky product synthesized from raw materials of A-E after heating for 4 h at 1450 °C. Fig. 5. SEM images of products synthesized from raw materials of C heated for 4 h at different temperatures. (a) The lower blocky, short needle-like product at 1400 °C. (b) and (c) The upper fibrous products at 1400 °C. (d) The lower blocky, short needle-like product at 1450 °C. (e) and (f) The upper fibrous products at 1450 °C. Fig. 6. SEM images of products synthesized from raw materials of C-E at 1450 °C for 4 h. (a), (d), and (g) The lower blocky, short needle-like products synthesized from raw materials of C-E, respectively. (b), (e), and (h) The upper fibrous products synthesized from raw materials of C-E, respectively. (c), (f), and (i) The high-magnification SEM images of the upper fibrous products synthesized from raw materials of C-E, respectively.
Fig. 7. (a) The phenomenon of agglomeration in raw materials: A is a mixture of Si and Si3N4, B is mixture of Si, C is a mixture of Si and NaCl, D is a mixture of Si, NaCl, and Si3N4. (b) Images of Si3N4 products, the upper products are fibrous Si3N4, the lower products are short needle-like and fluffy blocky Si3N4. Fig. 8. SEM images of products synthesized from different raw materials of A, F, G, D at 1450 °C for 4 h. (a)-(d) The lower products synthesized from different raw materials of A, F, G, D, respectively. (e)-(h) The upper products synthesized from different raw materials of A, F, G, D, respectively.
Figure
60 30 10
70
Si(wt%)
Si3N4(wt%) 30
NaCl(wt%) 0
B
A
Samples
20
30
50
C
30
30
40
D
40
30
30
E
0
0
100
F
Table 1 Compositions of the initial reactant mixtures
40
0
60
G
Fig. 1 Image of the Si nitridation product. The products are divided into two layers: the upper is a thin layer of fibrous product and the lower is a fluffy blocky product.
Fig. 2 XRD patterns of the lower blocky product and the lower blocky product + the upper fibrous product synthesized from raw materials of D after heating for 4 h at 1450℃.
Fig. 3 XRD patterns of the lower blocky product synthesized from raw materials of C after heating for 4 h at different temperatures.
Fig. 4 XRD patterns of the lower blocky product synthesized from raw materials of A-E after heating for 4 h at 1450 ℃.
Fig. 5 SEM images of products synthesized from raw materials of C heated for 4 h at different temperatures. (a) The lower blocky, short needle-like product at 1400 ℃.(b) and (c) The upper fibrous products at 1400 ℃. (d) The lower blocky, short needle-like product at 1450 ℃. (e) and (f) The upper fibrous products at 1450 ℃.
Fig. 6 SEM images of products synthesized from raw materials of C-E at 1450 ℃ for 4 h. (a), (d), and (g) The lower blocky, short needle-like products synthesized from raw materials of C-E, respectively. (b), (e), and (h) The upper fibrous products synthesized from raw materials of C-E, respectively. (c), (f), and (i) The high-magnification SEM images of the upper fibrous products synthesized from raw materials of C-E, respectively.
Fig. 7 (a) The phenomenon of agglomeration in raw materials: A is a mixture of Si and Si3N4, B is mixture of Si, C is a mixture of Si and NaCl, D is a mixture of Si, NaCl, and Si3N4. (b) Images of Si3N4 products, the upper products are fibrous Si3N4, the lower products are short needle-like and fluffy blocky Si3N4.
Fig. 8 SEM images of products synthesized from different raw materials of A, F, G, D at 1450 ℃ for 4 h. (a)-(d) The lower products synthesized from different raw materials of A, F, G, D, respectively. (e)-(h) The upper products synthesized from different raw materials of A, F, G, D, respectively.