Formation mechanisms of Si3N4 and Si2N2O in silicon powder nitridation

Solid State Sciences 66 (2017) 50e56

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Formation mechanisms of Si3N4 and Si2N2O in silicon powder nitridation Guisheng Yao a, Yong Li a, Peng Jiang a, *, Xiuming Jin a, Menglong Long a, Haixia Qin a, R. Vasant Kumar b a b

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB23QZ, United Kingdom

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 February 2017 Received in revised form 3 March 2017 Accepted 4 March 2017 Available online 6 March 2017

Commercial silicon powders are nitrided at constant temperatures (1453 K; 1513 K; 1633 K; 1693 K). The X-ray diffraction results show that small amounts of Si3N4 and Si2N2O are formed as the nitridation products in the samples. Fibroid and short columnar Si3N4 are detected in the samples. The formation mechanisms of Si3N4 and Si2N2O are analyzed. During the initial stage of silicon powder nitridation, Si on the outside of sample captures slight amount of O2 in N2 atmosphere, forming a thin film of SiO2 on the surface which seals the residual silicon inside. And the oxygen partial pressure between the SiO2 film and free silicon is decreasing gradually, so passive oxidation transforms to active oxidation and metastable SiO(g) is produced. When the SiO(g) partial pressure is high enough, the SiO2 film will crack, and N2 is infiltrated into the central section of the sample through cracks, generating Si2N2O and short columnar Si3N4 in situ. At the same time, metastable SiO(g) reacts with N2 and form fibroid Si3N4. In the regions where the oxygen partial pressure is high, Si3N4 is oxidized into Si2N2O. © 2017 Elsevier Masson SAS. All rights reserved.

Keywords: Nitridation mechanism Silicon monoxide Silicon nitride Silicon oxynitride Thermodynamics Commercial silicon powder

1. Introduction Si3N4 ceramics have been widely used as high-performance structural materials in high-temperature industries of engineering, metallurgy, aerospace and atomic energy due to their superior strength, great electric insulation, low density and coefficient of thermal expansion, excellent resistance of abrasion and corrosion, outstanding thermal shock resistance and creep resistance at high temperatures [1,2]. As reaction-bonded Si3N4 ceramics are demonstrating excellent properties, the nitridation mechanism of silicon has attracted a lot of research focuses in order to improve their physical properties [3]. The prevailing theory of the formation mechanism of Si3N4 is direct nitridation of silicon powder and several explanations were raised [4e10]. Moulson [11] reported that the initial stage of high-purity silicon nitridation at constant temperature involves the formation of Si3N4 nuclei on the Si surface. Followed by their growth, the reaction is completed between chemisorbed nitrogen and silicon, where silicon arrives at the reaction site by a combination of surface

* Corresponding author. E-mail address: [email protected] (P. Jiang). http://dx.doi.org/10.1016/j.solidstatesciences.2017.03.002 1293-2558/© 2017 Elsevier Masson SAS. All rights reserved.

diffusion and an evaporation/condensation process. As the nitrided layer extends over the surface, the supply of Si to reaction sites decreases. Finally, as the film effectively separates the reactants, the reaction rate falls to nearly zero. And the thermodynamics calculation from Moulson's work proves that direct nitridation of silicon powder requires p(O2)<10
21 MPa at 1623 K. Jennings et al. [12,13] pointed out that the formation mechanisms of a- and b-Si3N4 exists in different forms: a-Si3N4 involves the reaction between molecular nitrogen and gaseous silicon, while b-Si3N4 is generated by the reaction between atomic nitrogen and liquid or solid silicon. Ziegler et al. [14] postulated that fibroid a-Si3N4 is mainly formed by a gasphase reaction between gaseous silicon and nitrogen. At the same time, Ziegler stated that the reaction between SiO(g) and nitrogen may be important during the silicon nitridation. Though theories stated above were raised to analyze the nitridation mechanism of Si, it's difficult to meet the requirement of low oxygen pressure either in industry or in laboratory. Besides, when preparing bulk samples with larger size, e.g. refractory bricks, the penetration depth of the reaction layer is limited, so the existing direct nitridation theories are hardly applied to this cases directly. Therefore, research and analysis on the nitridation of Si in bulk samples are necessary for complete understanding of Si powder nitridation. Our previous work [15,16] has revealed the formation

G. Yao et al. / Solid State Sciences 66 (2017) 50e56 Table 1 Chemical compositions of commercial silicon powder (Wt%). Si

97.49

Major impurity elements Fe

Al

Ca

Ti

O

0.90

0.20

0.30

0.10

1.00

Wt% ¼ mass fraction.

mechanism of Si3N4 in SiC-Si3N4 composites. And in this work, the nitridation of pure silicon powder is completed at various temperatures. The research [17] showed that an initial mass gain of about 12% at 1523e1573 K corresponds to the formation of a product layer of about 0.2 mm thickness (assuming spherical particles), and Si is rapidly converted to a mixture of a- and b-Si3N4 above 1573 K, and the amount of b-Si3N4 is predominant with temperature above 1673 K. During the process of producing Si3N4 related refractories composites, the nitriding rate of Si would increase rapidly from the temperature of 1423 K. Therefore, 1453 K, 1513 K, 1633 K, 1693 K were selected as the nitridation temperatures of commercial silicon powder in this work. The assynthesized samples are analyzed through X-ray diffraction and scanning electronic microscope. The thermodynamic assessment and nirtriding mechanism of silicon powders are also provided. 2. Experiment 2.1. Samples preparation

51

product of the reaction between residua C in dextrin and Si. And aSi3N4, b-Si3N4 and Si2N2O are detected as the nitridation products in the proposed system. The amounts of each phase in the samples are analyzed by HighScore software which compares the areas of main peak (semi-quantitative method) and the results are summarized in Table 3. With the rising of nitridation temperatures, the content of residue Si decreases while the content of Si2N2O increases, which means a greater nitriding rate. Small amounts of a-Si3N4 were obtained in the samples sintered at 1453 K and 1513 K, and slight amounts of b-Si3N4 were detected in the samples sintered at 1633 K and 1693 K. It is worth noticing that the samples at all four temperatures didn't achieve complete nitridation, which is consistent with the experimental results of Moulson. Because the nitriding rate of the sample sintered at 1453 K is very low, the sample is too loose to be polished flatly for complete observation in SEM. The microstructure of the samples sintered at 1513 K, 1633 K and 1693 K are observed and shown in Fig. 2. Two different morphologies of Si3N4 which are fibroid and short columnar are shown in the nitrided products of silicon powder. The Si3N4 fibers are mainly dispersed in the pores of samples. With less quantities observed, the short columnar Si3N4 are appeared in dense state and bonding with each other. So it can be deduced that there are two kinds of formation mechanisms of Si3N4 in the process of silicon powder nitridation.

3.2. Thermodynamic assessment

Commercial silicon powder (grain diameter F ¼ 75 mm) was used as starting materials and 3.5% dextrin was used as a binding agent. The chemical compositions of the silicon powder is listed in Table 1. The commercial silicon powder and dextrin were mixed intensively and pressed into green compacts in cylindrical shape (diameter d ¼ 50 mm; height h ¼ 80 mm). The samples were prepared by sintering the green compacts at constant temperatures for 8 h in a furnace with constant nitrogen flow. The parameters of sample preparation are presented in Table 2. 2.2. Test methods

The purity of N2 used in the silicon powder nitridation is 99.999%, so the partial pressure of oxygen (p(O2)) could reach to 1.0  10
6 MPa, which does not meet the p(O2) requirement of direct reaction between Si and N2. Therefore, in the proposed synthesis method, the reactivity between Si(s) and N2(g) in N2 atmosphere with slight amounts of O2 was analyzed. In Si-N-O system, Si, Si3N4, Si2N2O and SiO2 can exist in stable condensed phases. In the system includes Si, N2 and O2, the following reactions [18] could occur:

SiðsÞ þ O2 ðgÞ ¼ SiO2 ðsÞ; DGq

The samples were crushed to a fine powder (45 mm) for phase composition analysis by using X-ray diffraction (XRD, Ultima IV of Rigaku); The microstructure is observed by environmental scanning electron microscope (ESEM, FEI Quanta 250); The element analysis in the micro area is detected using energy disperse spectroscopy (EDS, Ametek Apollo-X).

¼
904760 þ 173:38T$J$mol
1

(1a) Sið1Þ þ O2 ðgÞ ¼ SiO2 ðsÞ; DGq ¼
946350 þ 197:64T$J$mol
1

3. Results and discussion 3.1. Analysis of phase composition and microstructure

ð298  1685KÞ

ð1685  1973KÞ

(1b)

3SiðsÞ þ 2N2 ðgÞ ¼ Si3 N4 ðsÞ; DGq ¼
722836

The XRD patterns of the samples after nitridation at different temperatures are shown in Fig. 1. The major phase shown at various sintering temperatures is Si which has not been nitrided. Small amounts of SiO2 and SiC are observed in the samples as well. SiC is a

þ 315:01T$J$mol
1

ð298  1685KÞ

(2a)

Table 2 Parameters of sample preparation. Samples ID

Grain diameter/mm

Size of compacts/mm

Temperature/K

Atmosphere

Time/h

A1 A2 A3 A4

F ¼ 75

d ¼ 50 h ¼ 80

1453 1513 1633 1693

N2 Flowing Purity: 99.999% Ordinary pressure

8

52

G. Yao et al. / Solid State Sciences 66 (2017) 50e56

2Sið1Þ þ N2 ðgÞ þ 1=2O2 ðgÞ ¼ Si2 N2 OðsÞ; DGq ¼
951651 þ 290:57T$J$mol
1 ð1685  1973KÞ (3b) 2Si3 N4 ðsÞ þ 3=2O2 ðgÞ ¼ 3Si2 N2 OðsÞ þ N2 ðgÞ; DGq ¼
1106047 þ 61:69T$J$mol
1

(4)

2=3Si2 N2 OðsÞ þ O2 ðgÞ ¼ 4=3SiO2 ðsÞ þ 2=3N2 ðgÞ; DGq ¼
627233 þ 69:81T$J$mol
1

Fig. 1. XRD patterns of the samples after nitridation at different temperatures.

Table 3 Composition content of the samples after nitridation at different temperatures (Wt %). Samples ID

Si

a-Si3N4

b-Si3N4

Si2N2O

SiC

SiO2

A1 A2 A3 A4

94 92 76 48

/ 1 7 14

/ / 2 3

3 4 11 28

1 1 2 4

2 2 2 3

(5)

Based on the above thermodynamic data, the stable regions of Si, Si3N4, Si2N2O and SiO2 as a function of p(O2) and p(N2) when temperature is 1453 K, 1513 K, 1633 K or 1693 K are depicted in Fig. 3. And the p(O2) ranges are calculated respectively when SiO2, Si2N2O and Si3N4 are stable. In the N2 atmosphere with high-purity, Si2N2O can maintain stable at a higher p(O2) and this region of p(O2) is enlarged at a higher temperature according to Table 4, that's the reason why the amount of Si2N2O phase is increased with the rising of nitridation temperatures. SiO2 is an unstable phase in the condition of high temperature and low p(O2), so there is a mutual transformation of SiO2(s) and SiO(g) [19e21].

SiOðgÞ þ 1=2O2 ðgÞ ¼ SiO2 ðsÞ; DGq ¼
800070 þ 250:66T$J$mol
1

3Sið1Þ þ 2N2 ðgÞ ¼ Si3 N4 ðsÞ; DGq

ð298  1685KÞ (6a)

¼
874456 þ 405:01T$J$mol
1

ð1685  1973KÞ

SiOðgÞ þ 1=2O2 ðgÞ ¼ SiO2 ðsÞ; DGq (2b)

¼
791120 þ 244:92T$J$mol
1

ð1685  1973KÞ (6b)

2SiðsÞ þ N2ðgÞ þ 1=2O2 ðgÞ ¼ Si2 N2 OðsÞ; DGq ¼
850571 þ 230:57T$J$mol
1 ð298  1685KÞ (3a)

DG ¼ DGq þ ln 10,RT,lg 

1   1=2 pO pSiO , pq2 pq

(7)

When the reaction reaches equilibrium which means DG ¼ 0,

Fig. 2. SEM photographs of the samples after nitridation at different temperatures. (a) A2: 1513 K; (b) A3: 1633 K; (c) A4: 1693 K.

G. Yao et al. / Solid State Sciences 66 (2017) 50e56

53

Fig. 3. Phase stability diagram in Si-N-O system at different temperatures. (a) 1453 K; (b) 1513 K; (c) 1633 K; (d) 1693 K.

Table 4 The ranges of p(O2) when SiO2, Si2N2O and Si3N4 are stable at different temperatures. Temperature/K

The ranges of p(O2)/MPa

1453 1513 1633 1693

1.26 1.00 3.89 2.00

Si2N2O

SiO2    

10
20 10
19 10
18 10
17

1.26 1.00 3.89 2.00

   

Si3N4 10
20e4.37 10
19e4.90 10
18e3.72 10
17e2.51

   

10
26 10
25 10
23 10
22

4.37 4.90 3.72 2.51

   

10
26 10
25 10
23 10
22

the relationship of p(SiO)/pq and p(O2)/pq can be deduced from equation (7) at 1453 K, 1513 K, 1633 K and 1693 K, as shown in Fig. 4. Fig. 4 shows that at a certain temperature, the product is metastable SiO(s) when the reaction is in the region below the diagonal line, while the product is solid SiO2 when the reaction is in the region above the diagonal line. With different p(O2) values, the corresponding p(SiO) when Si3N4, Si2N2O and SiO2 are stable at different temperatures are calculated, as shown in Table 5. For

example, when p(O2) decreases to lg[p(O2)/pq] ¼ -21.43 at 1633 K, p(SiO) equals to 1.64  10
3 MPa and the production of Si3N4 could occur through direct nitridation between Si and N2 as shown in equation (1) (see Table 6). 3.3. Analysis of nitridation mechanism During the initial stage of silicon powder nitridation, Si on the outside of green compacts captures the slight amounts of O2 in N2 atmosphere, forming a thin film of SiO2 on the surface of compacts which prevents the residual silicon inside from further reaction. The p(O2) between the SiO2 film and free silicon is decreasing gradually, then passive oxidation transforms to active oxidation and metastable SiO(g) is generated. The SiO2 film becomes thinner due to its own decomposition, and the metastable SiO(g) which was derived from reactive oxidation forms a SiO gas film between the SiO2 film and free Si. The SiO2 film will crack when p(SiO) is increased to a critical value. Then SiO(g) will escapes to atmosphere

54

G. Yao et al. / Solid State Sciences 66 (2017) 50e56 Table 6 The atom ratios of selected areas in Fig. 5 analyzed by EDS.

Fig. 4. The relationship of p(SiO)/pq and p(O2)/pq in Si-O system at different temperatures.

through the cracks, while N2 will infiltrate into the central section of compacts and react with non-bonded free Si, forming short columnar Si3N4 in situ, as shown in Fig. 5a. In the partial regions where the p(O2) is high, equation (3) occurs and Si2N2O is appeared as the incomplete nitridation product on the surface of the silicon particles, as shown in Fig. 5c. According to Table 5, it can be derived that the higher nitridation temperature is corresponding to the greater equilibrium vapor pressure of SiO. For example, while the temperature is increased from 1453 K to 1693 K, p(SiO) is increased by 100 times

Table 5 The p(SiO) corresponding to certain p(O2) at different temperatures. Temperature/K 1453

1513

1633

1693

lg[p(O2)/pq]
5.00
18.90
24.36
5.00
18.00
23.31
5.00
16.41
21.43
5.00
15.70
20.60

lg[p(SiO)/pq]
13.17
6.22
3.49
12.03
5.53
2.88
10.00
4.30
1.79
9.12
3.77
1.32

p(SiO)/pq 6.76 6.03 3.24 9.33 2.95 1.33 1.00 5.07 1.64 7.59 1.70 4.79

           

p(SiO)/MPa
14

10 10
7 10
4 10
13 10
6 10
3 10
10 10
5 10
2 10
10 10
4 10
2

6.76 6.03 3.24 9.33 2.95 1.33 1.00 5.07 1.64 7.59 1.70 4.79

           

10
15 10
8 10
5 10
14 10
7 10
4 10
11 10
6 10
3 10
11 10
5 10
3

Area

Si

N

O

C

A B C

53.16 41.23 47.90

40.95 51.47 28.45

2.10 5.47 15.94

3.79 1.83 7.71

approximately. High p(SiO) is conducive to destroy the SiO2 film, providing a better kinetics condition for the direct reaction between N2 and Si. Therefore, the nitriding rate of samples increases with the rising of nitridation temperatures, similar to the results in Table 3. When the nitridation temperature is low, p(SiO) is too low to destroy the SiO2 film effectively. There is no diffusion path for N2 to infiltrate into the central section of compacts and react with Si, so that there is the residual Si in the central part of the samples could not complete nitridation. The metastable SiO(g) in the system reacts with N2 as in equation (8), forming fibroid Si3N4 shown in Fig. 5b. Then O2 which is derived from equation (8) will reacts with Si, generating SiO(g) again. With this process occurring repeatedly, large amounts of fibroid Si3N4 is formed in the pores of the samples, So Si3N4 in the nitriding products are generated by direct nitridation of Si(s) and indirect nitridation of SiO(g).

3SiOðgÞ þ 2N2 ðgÞ ¼ Si3 N4 ðsÞ þ 3=2O2 ðgÞ; DGq ¼
445090 þ 767:60T$J$mol
1

(8)

The short columnar Si3N4 formed by direct nitridation of Si(s) continuously fills the pores between the silicon particles, contributing to the densification of the samples. The O2 which is derived from indirect nitridation of SiO(g) is blocking the pores and the p(O2) in the microcell increases. So part of Si3N4 is further oxidized into Si2N2O which is isolated in the pores in the samples. A schematic of silicon powder nitridation model is shown in Fig. 6. 4. Conclusions The samples were fabricated by nitriding commercial silicon powder at constant temperatures. Thermodynamics and the process of silicon nitridation has been analyzed. 1) In the products of silicon powder compacts nitridation, Si3N4 exists in two different morphologies: fibroid and short

Fig. 5. SEM photographs of the products of silicon powder nitridation.

G. Yao et al. / Solid State Sciences 66 (2017) 50e56

55

Fig. 6. The schematic of silicon powder nitridation model. (a) Green compacts of commercial silicon powder; (b) Si on the outside of compacts captures slight amounts of O2 in N2 atmosphere firstly, forming a thin film of SiO2 on the outside of compacts: Si(s)þO2(g) ¼ SiO2(s); (c) Passive oxidation is transformed to active oxidation and metastable SiO(g) is generated: Si(s)þSiO2(s) ¼ 2SiO(g), 2Si(s)þO2(g) ¼ 2SiO(g); (d) The SiO2 film cracks when p(SiO) is increased to a critical value; (e) The direct nitridation between Si(s) and N2(g) generates fibroid Si3N4 and the indirect nitridation between SiO(g) and N2(g) generates short columnar Si3N4 in situ; (f) Part of Si3N4 is oxidized: 2Si3N4(s)þ3/2O2(g) ¼ 3Si2N2O(s)þ N2(g). (1) Here dO2 is the thickness of the O2(g) boundary layer and dSiO is the thickness of the SiO(g) boundary layer; (2) The close region of silicon particles; (3) The outside of Si3N4 was oxidized and its surface becomes smooth.

columnar. This indicates that there are two kinds of formation mechanisms of Si3N4 in the process of Si nitridation which are direct nitridation of Si(s) and indirect nitridation of SiO(g). 2) The indirect nitridation is from the reaction between metastable SiO(g) derived from active oxidation and N2(g). The process belongs to the gas-gas reaction, so the reaction rate is high and the Si3N4 formed from the gas phase was fibroid. 3) As the indirect nitridation consumes O2 which reduces p(O2) of the system, thermodynamic conditions for direct nitridation is therefore created. When p(O2) is low enough, Si(s) directly reacts with N2(g), forming short columnar Si3N4 in situ. 4) At a higher temperature, Si2N2O can maintain stable at a higher p(O2) and this region of p(O2) is enlarged, that's the reason why the amount of Si2N2O phase is increased with the rising of nitridation temperatures.

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