- Email: [email protected]
Preparation of pure nano-grained Si2N2O ceramic
Int. Journal of Refractory Metals and Hard Materials 36 (2013) 97–100
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Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Preparation of pure nano-grained Si2N2O ceramic Shoujun Wu ⁎, Xiangming Li College of Water Resources and Architectural Engineering, Northwest A&F University, Yangling Shaanxi, 712100, People's Republic of China
a r t i c l e
i n f o
Article history: Received 30 March 2012 Accepted 21 July 2012 Keywords: Si2N2O Nano‐particles Thermodynamics calculation Sintering Formation mechanism
a b s t r a c t Pure Si2N2O ceramic is fabricated by nitridizing a powder mixture of Si and SiO2. Results show that the prepared Si2N2O is composed of nano-particles with a size of about 50 nm. Analysis and discussion are focused on the formation mechanism of the nano-grained Si2N2O ceramic based on thermodynamics calculation, micro-morphologies and phase composition analysis. Preferential formation of Si3N4 on the surface of nano-SiO2 particles at below melting point of Si and then successive in-situ transformation of SiO2 and Si3N4 into Si2N2O at above melting point of Si are considered as the key reasons to form the pure nano-grained Si2N2O ceramic. Crown Copyright © 2012 Published by Elsevier Ltd. All rights reserved.
1. Introduction Silicon oxynitride (Si2N2O) is the only compound in the SiO2–Si3N4 system [1]. It has many good properties such as good thermal shock resistance, high thermodynamic stability temperature (about 1800 °C) and excellent oxidation resistance in air up to 1600 °C and high flexural strength up to 1400 °C without degradation [2–7]. In addition, Si2N2O possesses low dielectric constant than that of Si3N4 [8]. Therefore, Si2N2O is considered as a new great potential structural/functional material in place of Si3N4 in some fields, especially as a new high-temperature functional ceramic material with a variety of potential applications such as high-temperature electric insulator, nuclear-reactor moderator or reflector and solid electrolyte [9]. Sintering of Si2N2O is similar to that of Si3N4, as both materials have strongly covalent bonds and low diffusion coefficient, requiring high sintering temperatures. When Si2N2O is synthesized by reactive sintering of equimolar mixtures of Si3N4 and SiO2, oxide additives are usually used to form a liquid phase with a eutectic melting point low enough to permit sintering without excessive dissociation [2,5,6]. Si2N2O is in-situ fabricated by nitridizing a powder mixture of Si and SiO2, usually much of Si3N4 is produced during the fabrication process, as well as residual Si and SiO2 due to incomplete reaction with each other. As properties of Si2N2O will be badly influenced by minor amounts of other phases, such as Si3N4, Si, SiO2 and oxide additives, it is necessary to fabricate pure Si2N2O ceramic material in the absence of other phases. Bulk nano-ceramics with a grain size in the nanometer range are expected to exhibit attractive properties such as high strength [10], and dielectric properties [11], but achieving nanocrystalline grain
⁎ Corresponding author. Tel.: +86 29 87082902; fax: +86 29 87082901. E-mail address: [email protected] (S. Wu).
sizes requires careful attention to processing. Therefore, it is supposed that preparation of pure nano-grained Si2N2O ceramic is quiet important for applications of this material. In the present work, a nano-grained Si2N2O ceramic without second phases is fabricated by in-situ reactive method by nitridizing a powder mixture of Si and SiO2. Much of the analysis and discussion are focused on the mechanism of formation of pure nano-grained Si2N2O.
2. Experimental procedures 2.1. Fabrication of Si2N2O Si and SiO2 powders are used as initial powders. Si powder with a particle size of ~45 μm is used in the present experiment. The Si powder is ball-milled with agate balls in an agate barrel for 6 h to a final particle size of ~1–10 μm which is randomly checked under SEM. Aerosil 200, an amorphous high-purity SiO2 powder with very high specific area of 200 ± 25 m 2/g and low mean particle size of 50 nm, is used as another initial powder. Powder mixture of the SiO2 and the milled Si with a molar ratio of Si to SiO2 of 3:1 is firstly wet-milled in ethanol for 24 h. The slurry is dried by freeze-drying method and sieved through a 50 mesh screen. Then the powder mixture is uniaxially pressed under 100 MPa to form green bodies of rectangular bars with a dimension of 5 × 10× 70 mm. The green bodies are placed in an alumina crucible and sintered in fluid nitrogen atmosphere with a gas pressure of 0.1 MPa by a multi-step sintering approach: firstly, temperature is increased to 1200 °C with a heating rate of 5 °C/min, then increased to 1400 °C with heating rate of 0.2 °C/min. Secondly, the temperature is further increased to 1700 °C with a heating rate of 5 °C/min and held for 2 h. As a comparison, samples treated with the same first sintering while in the secondary sintering process with a final sintering temperature of
0263-4368/$ – see front matter. Crown Copyright © 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2012.07.007
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S. Wu, X. Li / Int. Journal of Refractory Metals and Hard Materials 36 (2013) 97–100
1400 °C and 1450 °C with a heating rate of 5 °C/min and held for 2 h are prepared, respectively. 2.2. Characterization Open porosity and relative bulk density of the samples are measured by Archimedes method using distilled water firstly, and then the total porosity is calculated according to the relative density and theoretical density of the Si2N2O (2.81 g/cm 3) [12]. Phase composition of the sintered samples is identified by X-ray diffraction (XRD, X'Pert Pro, Philips, Netherlands) analysis at 40 kV and 35 mA using Cu Kα radiation with a step width of 0.02°. The XRD measurement is performed on the surface of the samples, except for those of the secondary sintering temperature is 1700 °C, of which center of the samples are also checked. Microstructure and morphology are characterized by Scanning Electron Microscopy (SEM, JEOL JSM-6360, Japan). Five samples with a dimension of 3 (height)×4 (width)×36 (length) mm are tested to obtain the average flexural strength by the three-point bending tests (SANS CMT 4304, Sans Materials Testing Co., Shenzhen, China) with a span of 30 mm and a loading speed of 0.5 mm/min. Fracture toughness is measured using Single Edge Notched Beam (SENB) method. Five samples with a dimension of 2.5 (width)×5 (height)×30 (length) mm are tested to obtain the average toughness. The edge notch of about 2.5 mm is cut by a diamond cutting wheel and then finished with a razor blade and diamond paste. Span of 20 mm and loading speed of 0.5 mm/min were used. Fracture toughness is calculated from the following equation: pc s f ðc=hÞ b h3=2 f ðc=hÞ ¼ 2:9ðc=hÞ1=2 −4:6ðc=hÞ3=2 þ 21:8ðc=hÞ5=2 −37:6ðc=hÞ7=2
K IC ¼
9=2
þ38:7ðc=hÞ
where c is notch length, b is width of rectangular bar samples, h is height of rectangular bar samples, s is support span, pc is the maximum load. Vickers hardness (Hv) of the samples is measured using a Digital Hardness Tester (HBV-30A, Huayin Experimental Apparatus Co, Shandong, China) with a pyramidal Vickers indenter. The tests are performed under loads of 5 kg for 15 s and an average of eight indents is analyzed. 3. Results and discussion Fig. 1 shows XRD patterns of the prepared samples. It can be seen, XRD patterns of the surface and center of samples of which the secondary sintering temperature is 1700 °C, only sharp peaks of Si2N2O can be detected, no peaks of Si, SiO2 or Si3N4 can be detected, suggesting the produced ceramic is Si2N2O scarcely with a second crystalline phase. While surface XRD pattern of samples of which
the secondary sintering temperature is 1400 °C showed the obtained ceramic is mainly consisted of Si3N4 and minor amounts of Si2N2O. And a certain amounts of residual unreacted Si and SiO2 could also be detected. Moreover, due to the constraining effect of nitrogen [13] and the produced Si3N4 [14], there is little cristobalite produced, though almost all of the residual SiO2 still remained amorphous phase. Those of the secondary sintering temperature is 1450 °C, the primary phase in the obtained ceramic was Si2N2O with a little secondary phase of Si3N4. During sintering of the mixture of Si and SiO2 in nitrogen, mainly possible reactions are as follows: 3=2Si þ 1=2SiO2 þ N2 ¼ Si2 N2 O
ð1Þ
3Si þ 2N2 ¼ Si3 N4
ð2Þ
3=4Si þ 1=2SiO2 þ 1=4Si3 N4 þ 1=2N2 ¼ Si2 N2 O
ð3Þ
According to thermochemical data of the related materials [15] involved in the above reactions, variation of Gibbs free energy values (ΔG) of the above reactions with temperatures are shown in Fig. 2. When the sample is sintered at below the melting point of Si, ΔG2 of reaction (2) is lower than that of ΔG1 of reaction (1), indicating Si is prone to be nitrided into Si3N4 instead of Si2N2O. Moreover, Si and SiO2 both are existed as solid form, reaction (1) could only take place at the contact interface between Si and SiO2 particles depending on lattice diffusion, while reaction (2) can rapidly process to produce masses of Si3N4 on the surface of Si particles due to Si particles are well contacted with nitrogen. Above 1470 °C, ΔG2 becomes higher than that of ΔG1 suggesting Si together with SiO2 trends to be nitrided into Si2N2O. Furthermore, the SiO2 particles are well surrounded by liquid Si, which made the reaction (1) takes place more easily. Above 1600 °C, ΔG1 and ΔG3 are lower than ΔG2, indicating Si3N4 is difficult to form instead of the Si3N4 pre-formed at lower temperatures which reacts with SiO2 to produce Si2N2O theoretically when liquid Si and gas N2 joined according to reaction (3). As the final sintering temperature is above 1600 °C and molar ratio of Si to SiO2 is 3:1, there is no redundant Si or SiO2 according to reaction (1) or (3) and the formed Si3N4 will be completely consumed. As a result, only Si2N2O can be formed and no second phase can be found in the samples. Therefore, it is supposed that formation of pure Si2N2O ceramic is directly related to the two-step sintering process. As pure Si2N2O ceramic only can be obtained with a secondary sintering temperature of 1700 °C, the following results and discussion are only referred to samples with this sintering approach.
-30
Si 2N2O
surface
(c)
Center
ΔG (kJ/mol)
Intensity
(d)
(b)
-80
Si2 N2 O
Si3 N4
3/4Si+1/2SiO2 +1/4Si3 N4 +1/2N 2
Si2 N2 O
-130 -180 -230
Si
(a)
3/2Si +1/2SiO2 + N 2 3Si +2N2
Si 3N 4
SiO2
(1470°C)
-280 15
20
25
30
35
40
45
2θ (º) Fig. 1. XRD patterns of the prepared Si2N2O samples final sintered at: (a) 1400 °C (surface); (b) 1450 °C (surface); (c) & (d)1700 °C.
1200
1300
1400
1500
1600
1700
1800
Temperature (°C) Fig. 2. Variation of Gibbs free energy of the related reactions in Si–SiO2–N2 system with temperatures.
S. Wu, X. Li / Int. Journal of Refractory Metals and Hard Materials 36 (2013) 97–100
Fig. 3 shows morphologies and EDS of the fracture surface of samples with final sintering at 1700 °C. It can be seen that there are big pores of about micrometers as shown in Fig. 3(a) and masses of small pores of tens of nanometers in the produced ceramic. Moreover, as shown in Fig. 3(b) it seems that the produced ceramic is composed of nano-particles with a size of ~ 50 nm, which is nearly equal to the size of SiO2 powder used in the present work. EDS result of the fracture surface of the samples shows peaks of Si, N and O can be detected, suggesting these particles are Si2N2O, which consists of XRD pattern at the center of the sample. As the sample body is porous, nitrogen can diffuse inward of the samples. When the sample is finally sintered above melting point of Si (1412 °C), liquid Si together with nitrogen reacted with SiO2 particles quickly to produce Si2N2O on the surface of SiO2 particles. The early produced Si2N2O on the surface of SiO2 particles connected to each other to stop the movement of SiO2 meanwhile the liquid Si and gas nitrogen could only infiltrate into SiO2 particles for reaction thereafter. In addition, as discussed above, below melting point of Si, due to solid-phase sintering and constrain from Si3N4 formation, densification and grain growth are difficult. During final sintering at higher temperature, though liquid-phase sintering made the densification become easier, constrain of grain growth from Si3N4 still works until it is completely consumed. Therefore, the produced ceramic which showed fine grains with size are nearly equal to that of initial SiO2 powder. According to the above discussion, combined with the XRD results of samples with different sintering procedures, it also can be concluded that formation of nano-grained Si2N2O ceramic body without other crystalline phase is also directly related to the applied sintering approach. Preferential formation of Si3N4 on nano-SiO2 particles at below melting point of Si and then successive in-situ transformation of SiO2 and Si3N4 to Si2N2O at 1700 °C are the key reasons to form pure nano-grained Si2N2O ceramic. Volatilization of Si becomes faster at 1700 °C [16] and results in more pores formed. However, easier densification of liquid sintering makes growth of pores from volatilization of Si became difficult. Therefore, nano-pores exist in the produced ceramics. On the other hand, constrain from formation of Si3N4 and the successive shrinkage induced by the in-situ phase transformation result in some big pores formed. Table 1 shows open porosity, relative bulk density, flexural strength, fracture toughness and Vickers hardness of the produced Si2N2O ceramic final sintered at 1700 °C. Though the produced Si2N2O is composed of nano-grains, there are lots of pores, which make relative density, flexural strength and fracture toughness of the samples lower than properties of denser ceramics reported by others works [6,8,12]. For the same reason, the samples showed a low Vickers hardness, besides the load is large. Though the prepared material is porous and has lower strength as shown in Table 1, it can be candidate materials as environmental filters for the purification systems of polluted air and water or high performance radomes [17,18] if the related properties improved. And it is also conceived that, decreasing porosity, especially for big pores is favorable
(a)
99
Table 1 Properties of produced Si2N2O ceramic. Relative density (g/cm3)
Porosity (%)
Flexural strength (MPa)
Fracture toughness (MPa·m1/2)
Vickers hardness (GPa)
2.0
29
180 ± 14
1.9 ± 0.3
4.53 ± 0.45
to improve properties of the produced Si2N2O ceramics, with which hot pressure sintering in nitrogen is considered more favorable. 4. Conclusions Nano-grained porous Si2N2O ceramic without second phases can be fabricated by in-situ reactive method by nitridizing a powder mixture of Si and SiO2 with a molar ratio of Si to SiO2 of 3:1 based on a multi-step sintering composed of pre-sintering at below melting point of Si and successive sintering at 1700 °C. Preferential formation of Si3N4 on the nano-SiO2 particles at below melting point of Si and then successive in-situ transformation of SiO2 and Si3N4 into Si2N2O at 1700 °C are the key reasons to form pure Si2N2O and obtain nano-grained ceramic body. Acknowledgement The authors gratefully acknowledge the financial support from the Human Resources Foundation of Northwest A&F University (No. Z111021101). References [1] Hillert M, Jonsson S, Sundman B. Thermodynamic calculation of the Si–N–O system. Z Metallkd 1992;83:648-54. [2] Rocabois P, Chatillon C, Bernard C. Thermodynamics of the Si–O–N system: I, high-temperaure study of the vaporization behavior of silicon nitride by mass spectrometry. J Am Ceram Soc 1996;79:1351-60. [3] Bressiani JC, Izhevskyi V, Bressiani Ana HA. Development of the microstructure of the silicon nitride based ceramics. Mater Res 1999;2:165-72. [4] Lee BT, Kim HD. Effect of sintering additives on the nitridation behavior of reaction-bonded silicon nitride. Mater Sci Eng, A 2004;364:126-31. [5] Ohashi M, Kanzaki S, Tabata H. High-temperature flexural strength of hot-pressed silicon oxynitride ceramics. J Mater Sci Lett 1988;7:339-40. [6] Ohashi M, Kanzaki S, Tabata H. Processing, mechanical properties, and oxidation behavior of silicon oxynitride ceramics. J Am Ceram Soc 1991;74:109-14. [7] Huang ZK, Greil P, Petzow G. Formation of silicon oxinitride from Si3N4 and SiO2 in the presence of Al2O3. Ceram Int 1984;10:14-7. [8] Tong QF, Wang JY, Li ZP, Zhou YC. Low-temperature synthesis/densification and properties of Si2N2O with Li2O additive. J Eur Ceram Soc 2007;27:4767-72. [9] Marchand R, Laurent Y, Guyader J, L'Haridon P, Verdier P. Nitrides and oxynitrides: preparation, crystal chemistry and properties. J Eur Ceram Soc 1991;8:197-213. [10] Chokshi AH, Rosen A, Karch J, Gleiter H. On the validity of the Hall–Petch relationship in nanocrystalline materials. Scr Metall 1989;23:1679-83. [11] Park MB, Kim CD, Lee SK, Cho NH. Phase transition and dielectric characteristics of nano-grained BaTiO3 ceramics synthesized from surface-coated nano-powders. Appl Surf Sci 2002;190:416-21.
(b)
(c)
Si
Nano pores O
Big pores
N
0.00 1.00 2.00 3.00 4.00 5.00
Energy (KeV)
Fig. 3. (a) Morphologies of the fracture surface of prepared Si2N2O sample final sintered at 1700 °C; (b) magnified view of (a); (c) corresponding area EDS result of (b).
100
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[12] Larker R. Reaction sintering and properties of silicon oxynitride densified by hot isostatic pressing. J Am Ceram Soc 1992;75:62-6. [13] Li XM, Yin XW, Zhang LT, Cheng LF, Qi YC. Mechanical and dielectric properties of porous Si3N4–SiO2 composite ceramics. Mater Sci Eng, A 2009;500:63-7. [14] Xu CM, Wang SW, Huang XX, Guo JK, Zhou GH. Crystallization and amorphization of cristobalite. J Inorg Mater 2007;22:577-82. [15] Knacke O, Kubaschewski O, Hesselmann K, editors. Thermochemical properties of inorganic substance. second edition. Springer-Verlag; 1991.
[16] Ashkin D. Nitridation behaviour of silicon with clay and oxide additions: rate and phase development. J Eur Ceram Soc 1997;17:1613-24. [17] Lee BT, Paul RK, Lee CW, Kim HD. Fabrication and microstructure characterization of continuously porous Si2N2O–Si3N4 ceramics. Mater Lett 2007;61:2182-6. [18] Barta J, Manela M, Fischer R. Si3N4 and Si2N2O for high performance radomes. Mater Sci Eng 1985;71:265-72.
Contents lists available at SciVerse ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Preparation of pure nano-grained Si2N2O ceramic Shoujun Wu ⁎, Xiangming Li College of Water Resources and Architectural Engineering, Northwest A&F University, Yangling Shaanxi, 712100, People's Republic of China
a r t i c l e
i n f o
Article history: Received 30 March 2012 Accepted 21 July 2012 Keywords: Si2N2O Nano‐particles Thermodynamics calculation Sintering Formation mechanism
a b s t r a c t Pure Si2N2O ceramic is fabricated by nitridizing a powder mixture of Si and SiO2. Results show that the prepared Si2N2O is composed of nano-particles with a size of about 50 nm. Analysis and discussion are focused on the formation mechanism of the nano-grained Si2N2O ceramic based on thermodynamics calculation, micro-morphologies and phase composition analysis. Preferential formation of Si3N4 on the surface of nano-SiO2 particles at below melting point of Si and then successive in-situ transformation of SiO2 and Si3N4 into Si2N2O at above melting point of Si are considered as the key reasons to form the pure nano-grained Si2N2O ceramic. Crown Copyright © 2012 Published by Elsevier Ltd. All rights reserved.
1. Introduction Silicon oxynitride (Si2N2O) is the only compound in the SiO2–Si3N4 system [1]. It has many good properties such as good thermal shock resistance, high thermodynamic stability temperature (about 1800 °C) and excellent oxidation resistance in air up to 1600 °C and high flexural strength up to 1400 °C without degradation [2–7]. In addition, Si2N2O possesses low dielectric constant than that of Si3N4 [8]. Therefore, Si2N2O is considered as a new great potential structural/functional material in place of Si3N4 in some fields, especially as a new high-temperature functional ceramic material with a variety of potential applications such as high-temperature electric insulator, nuclear-reactor moderator or reflector and solid electrolyte [9]. Sintering of Si2N2O is similar to that of Si3N4, as both materials have strongly covalent bonds and low diffusion coefficient, requiring high sintering temperatures. When Si2N2O is synthesized by reactive sintering of equimolar mixtures of Si3N4 and SiO2, oxide additives are usually used to form a liquid phase with a eutectic melting point low enough to permit sintering without excessive dissociation [2,5,6]. Si2N2O is in-situ fabricated by nitridizing a powder mixture of Si and SiO2, usually much of Si3N4 is produced during the fabrication process, as well as residual Si and SiO2 due to incomplete reaction with each other. As properties of Si2N2O will be badly influenced by minor amounts of other phases, such as Si3N4, Si, SiO2 and oxide additives, it is necessary to fabricate pure Si2N2O ceramic material in the absence of other phases. Bulk nano-ceramics with a grain size in the nanometer range are expected to exhibit attractive properties such as high strength [10], and dielectric properties [11], but achieving nanocrystalline grain
⁎ Corresponding author. Tel.: +86 29 87082902; fax: +86 29 87082901. E-mail address: [email protected] (S. Wu).
sizes requires careful attention to processing. Therefore, it is supposed that preparation of pure nano-grained Si2N2O ceramic is quiet important for applications of this material. In the present work, a nano-grained Si2N2O ceramic without second phases is fabricated by in-situ reactive method by nitridizing a powder mixture of Si and SiO2. Much of the analysis and discussion are focused on the mechanism of formation of pure nano-grained Si2N2O.
2. Experimental procedures 2.1. Fabrication of Si2N2O Si and SiO2 powders are used as initial powders. Si powder with a particle size of ~45 μm is used in the present experiment. The Si powder is ball-milled with agate balls in an agate barrel for 6 h to a final particle size of ~1–10 μm which is randomly checked under SEM. Aerosil 200, an amorphous high-purity SiO2 powder with very high specific area of 200 ± 25 m 2/g and low mean particle size of 50 nm, is used as another initial powder. Powder mixture of the SiO2 and the milled Si with a molar ratio of Si to SiO2 of 3:1 is firstly wet-milled in ethanol for 24 h. The slurry is dried by freeze-drying method and sieved through a 50 mesh screen. Then the powder mixture is uniaxially pressed under 100 MPa to form green bodies of rectangular bars with a dimension of 5 × 10× 70 mm. The green bodies are placed in an alumina crucible and sintered in fluid nitrogen atmosphere with a gas pressure of 0.1 MPa by a multi-step sintering approach: firstly, temperature is increased to 1200 °C with a heating rate of 5 °C/min, then increased to 1400 °C with heating rate of 0.2 °C/min. Secondly, the temperature is further increased to 1700 °C with a heating rate of 5 °C/min and held for 2 h. As a comparison, samples treated with the same first sintering while in the secondary sintering process with a final sintering temperature of
0263-4368/$ – see front matter. Crown Copyright © 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2012.07.007
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S. Wu, X. Li / Int. Journal of Refractory Metals and Hard Materials 36 (2013) 97–100
1400 °C and 1450 °C with a heating rate of 5 °C/min and held for 2 h are prepared, respectively. 2.2. Characterization Open porosity and relative bulk density of the samples are measured by Archimedes method using distilled water firstly, and then the total porosity is calculated according to the relative density and theoretical density of the Si2N2O (2.81 g/cm 3) [12]. Phase composition of the sintered samples is identified by X-ray diffraction (XRD, X'Pert Pro, Philips, Netherlands) analysis at 40 kV and 35 mA using Cu Kα radiation with a step width of 0.02°. The XRD measurement is performed on the surface of the samples, except for those of the secondary sintering temperature is 1700 °C, of which center of the samples are also checked. Microstructure and morphology are characterized by Scanning Electron Microscopy (SEM, JEOL JSM-6360, Japan). Five samples with a dimension of 3 (height)×4 (width)×36 (length) mm are tested to obtain the average flexural strength by the three-point bending tests (SANS CMT 4304, Sans Materials Testing Co., Shenzhen, China) with a span of 30 mm and a loading speed of 0.5 mm/min. Fracture toughness is measured using Single Edge Notched Beam (SENB) method. Five samples with a dimension of 2.5 (width)×5 (height)×30 (length) mm are tested to obtain the average toughness. The edge notch of about 2.5 mm is cut by a diamond cutting wheel and then finished with a razor blade and diamond paste. Span of 20 mm and loading speed of 0.5 mm/min were used. Fracture toughness is calculated from the following equation: pc s f ðc=hÞ b h3=2 f ðc=hÞ ¼ 2:9ðc=hÞ1=2 −4:6ðc=hÞ3=2 þ 21:8ðc=hÞ5=2 −37:6ðc=hÞ7=2
K IC ¼
9=2
þ38:7ðc=hÞ
where c is notch length, b is width of rectangular bar samples, h is height of rectangular bar samples, s is support span, pc is the maximum load. Vickers hardness (Hv) of the samples is measured using a Digital Hardness Tester (HBV-30A, Huayin Experimental Apparatus Co, Shandong, China) with a pyramidal Vickers indenter. The tests are performed under loads of 5 kg for 15 s and an average of eight indents is analyzed. 3. Results and discussion Fig. 1 shows XRD patterns of the prepared samples. It can be seen, XRD patterns of the surface and center of samples of which the secondary sintering temperature is 1700 °C, only sharp peaks of Si2N2O can be detected, no peaks of Si, SiO2 or Si3N4 can be detected, suggesting the produced ceramic is Si2N2O scarcely with a second crystalline phase. While surface XRD pattern of samples of which
the secondary sintering temperature is 1400 °C showed the obtained ceramic is mainly consisted of Si3N4 and minor amounts of Si2N2O. And a certain amounts of residual unreacted Si and SiO2 could also be detected. Moreover, due to the constraining effect of nitrogen [13] and the produced Si3N4 [14], there is little cristobalite produced, though almost all of the residual SiO2 still remained amorphous phase. Those of the secondary sintering temperature is 1450 °C, the primary phase in the obtained ceramic was Si2N2O with a little secondary phase of Si3N4. During sintering of the mixture of Si and SiO2 in nitrogen, mainly possible reactions are as follows: 3=2Si þ 1=2SiO2 þ N2 ¼ Si2 N2 O
ð1Þ
3Si þ 2N2 ¼ Si3 N4
ð2Þ
3=4Si þ 1=2SiO2 þ 1=4Si3 N4 þ 1=2N2 ¼ Si2 N2 O
ð3Þ
According to thermochemical data of the related materials [15] involved in the above reactions, variation of Gibbs free energy values (ΔG) of the above reactions with temperatures are shown in Fig. 2. When the sample is sintered at below the melting point of Si, ΔG2 of reaction (2) is lower than that of ΔG1 of reaction (1), indicating Si is prone to be nitrided into Si3N4 instead of Si2N2O. Moreover, Si and SiO2 both are existed as solid form, reaction (1) could only take place at the contact interface between Si and SiO2 particles depending on lattice diffusion, while reaction (2) can rapidly process to produce masses of Si3N4 on the surface of Si particles due to Si particles are well contacted with nitrogen. Above 1470 °C, ΔG2 becomes higher than that of ΔG1 suggesting Si together with SiO2 trends to be nitrided into Si2N2O. Furthermore, the SiO2 particles are well surrounded by liquid Si, which made the reaction (1) takes place more easily. Above 1600 °C, ΔG1 and ΔG3 are lower than ΔG2, indicating Si3N4 is difficult to form instead of the Si3N4 pre-formed at lower temperatures which reacts with SiO2 to produce Si2N2O theoretically when liquid Si and gas N2 joined according to reaction (3). As the final sintering temperature is above 1600 °C and molar ratio of Si to SiO2 is 3:1, there is no redundant Si or SiO2 according to reaction (1) or (3) and the formed Si3N4 will be completely consumed. As a result, only Si2N2O can be formed and no second phase can be found in the samples. Therefore, it is supposed that formation of pure Si2N2O ceramic is directly related to the two-step sintering process. As pure Si2N2O ceramic only can be obtained with a secondary sintering temperature of 1700 °C, the following results and discussion are only referred to samples with this sintering approach.
-30
Si 2N2O
surface
(c)
Center
ΔG (kJ/mol)
Intensity
(d)
(b)
-80
Si2 N2 O
Si3 N4
3/4Si+1/2SiO2 +1/4Si3 N4 +1/2N 2
Si2 N2 O
-130 -180 -230
Si
(a)
3/2Si +1/2SiO2 + N 2 3Si +2N2
Si 3N 4
SiO2
(1470°C)
-280 15
20
25
30
35
40
45
2θ (º) Fig. 1. XRD patterns of the prepared Si2N2O samples final sintered at: (a) 1400 °C (surface); (b) 1450 °C (surface); (c) & (d)1700 °C.
1200
1300
1400
1500
1600
1700
1800
Temperature (°C) Fig. 2. Variation of Gibbs free energy of the related reactions in Si–SiO2–N2 system with temperatures.
S. Wu, X. Li / Int. Journal of Refractory Metals and Hard Materials 36 (2013) 97–100
Fig. 3 shows morphologies and EDS of the fracture surface of samples with final sintering at 1700 °C. It can be seen that there are big pores of about micrometers as shown in Fig. 3(a) and masses of small pores of tens of nanometers in the produced ceramic. Moreover, as shown in Fig. 3(b) it seems that the produced ceramic is composed of nano-particles with a size of ~ 50 nm, which is nearly equal to the size of SiO2 powder used in the present work. EDS result of the fracture surface of the samples shows peaks of Si, N and O can be detected, suggesting these particles are Si2N2O, which consists of XRD pattern at the center of the sample. As the sample body is porous, nitrogen can diffuse inward of the samples. When the sample is finally sintered above melting point of Si (1412 °C), liquid Si together with nitrogen reacted with SiO2 particles quickly to produce Si2N2O on the surface of SiO2 particles. The early produced Si2N2O on the surface of SiO2 particles connected to each other to stop the movement of SiO2 meanwhile the liquid Si and gas nitrogen could only infiltrate into SiO2 particles for reaction thereafter. In addition, as discussed above, below melting point of Si, due to solid-phase sintering and constrain from Si3N4 formation, densification and grain growth are difficult. During final sintering at higher temperature, though liquid-phase sintering made the densification become easier, constrain of grain growth from Si3N4 still works until it is completely consumed. Therefore, the produced ceramic which showed fine grains with size are nearly equal to that of initial SiO2 powder. According to the above discussion, combined with the XRD results of samples with different sintering procedures, it also can be concluded that formation of nano-grained Si2N2O ceramic body without other crystalline phase is also directly related to the applied sintering approach. Preferential formation of Si3N4 on nano-SiO2 particles at below melting point of Si and then successive in-situ transformation of SiO2 and Si3N4 to Si2N2O at 1700 °C are the key reasons to form pure nano-grained Si2N2O ceramic. Volatilization of Si becomes faster at 1700 °C [16] and results in more pores formed. However, easier densification of liquid sintering makes growth of pores from volatilization of Si became difficult. Therefore, nano-pores exist in the produced ceramics. On the other hand, constrain from formation of Si3N4 and the successive shrinkage induced by the in-situ phase transformation result in some big pores formed. Table 1 shows open porosity, relative bulk density, flexural strength, fracture toughness and Vickers hardness of the produced Si2N2O ceramic final sintered at 1700 °C. Though the produced Si2N2O is composed of nano-grains, there are lots of pores, which make relative density, flexural strength and fracture toughness of the samples lower than properties of denser ceramics reported by others works [6,8,12]. For the same reason, the samples showed a low Vickers hardness, besides the load is large. Though the prepared material is porous and has lower strength as shown in Table 1, it can be candidate materials as environmental filters for the purification systems of polluted air and water or high performance radomes [17,18] if the related properties improved. And it is also conceived that, decreasing porosity, especially for big pores is favorable
(a)
99
Table 1 Properties of produced Si2N2O ceramic. Relative density (g/cm3)
Porosity (%)
Flexural strength (MPa)
Fracture toughness (MPa·m1/2)
Vickers hardness (GPa)
2.0
29
180 ± 14
1.9 ± 0.3
4.53 ± 0.45
to improve properties of the produced Si2N2O ceramics, with which hot pressure sintering in nitrogen is considered more favorable. 4. Conclusions Nano-grained porous Si2N2O ceramic without second phases can be fabricated by in-situ reactive method by nitridizing a powder mixture of Si and SiO2 with a molar ratio of Si to SiO2 of 3:1 based on a multi-step sintering composed of pre-sintering at below melting point of Si and successive sintering at 1700 °C. Preferential formation of Si3N4 on the nano-SiO2 particles at below melting point of Si and then successive in-situ transformation of SiO2 and Si3N4 into Si2N2O at 1700 °C are the key reasons to form pure Si2N2O and obtain nano-grained ceramic body. Acknowledgement The authors gratefully acknowledge the financial support from the Human Resources Foundation of Northwest A&F University (No. Z111021101). References [1] Hillert M, Jonsson S, Sundman B. Thermodynamic calculation of the Si–N–O system. Z Metallkd 1992;83:648-54. [2] Rocabois P, Chatillon C, Bernard C. Thermodynamics of the Si–O–N system: I, high-temperaure study of the vaporization behavior of silicon nitride by mass spectrometry. J Am Ceram Soc 1996;79:1351-60. [3] Bressiani JC, Izhevskyi V, Bressiani Ana HA. Development of the microstructure of the silicon nitride based ceramics. Mater Res 1999;2:165-72. [4] Lee BT, Kim HD. Effect of sintering additives on the nitridation behavior of reaction-bonded silicon nitride. Mater Sci Eng, A 2004;364:126-31. [5] Ohashi M, Kanzaki S, Tabata H. High-temperature flexural strength of hot-pressed silicon oxynitride ceramics. J Mater Sci Lett 1988;7:339-40. [6] Ohashi M, Kanzaki S, Tabata H. Processing, mechanical properties, and oxidation behavior of silicon oxynitride ceramics. J Am Ceram Soc 1991;74:109-14. [7] Huang ZK, Greil P, Petzow G. Formation of silicon oxinitride from Si3N4 and SiO2 in the presence of Al2O3. Ceram Int 1984;10:14-7. [8] Tong QF, Wang JY, Li ZP, Zhou YC. Low-temperature synthesis/densification and properties of Si2N2O with Li2O additive. J Eur Ceram Soc 2007;27:4767-72. [9] Marchand R, Laurent Y, Guyader J, L'Haridon P, Verdier P. Nitrides and oxynitrides: preparation, crystal chemistry and properties. J Eur Ceram Soc 1991;8:197-213. [10] Chokshi AH, Rosen A, Karch J, Gleiter H. On the validity of the Hall–Petch relationship in nanocrystalline materials. Scr Metall 1989;23:1679-83. [11] Park MB, Kim CD, Lee SK, Cho NH. Phase transition and dielectric characteristics of nano-grained BaTiO3 ceramics synthesized from surface-coated nano-powders. Appl Surf Sci 2002;190:416-21.
(b)
(c)
Si
Nano pores O
Big pores
N
0.00 1.00 2.00 3.00 4.00 5.00
Energy (KeV)
Fig. 3. (a) Morphologies of the fracture surface of prepared Si2N2O sample final sintered at 1700 °C; (b) magnified view of (a); (c) corresponding area EDS result of (b).
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