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Thermal conductivity of β-SiAlONs prepared by a combination of combustion synthesis and spark plasma sintering
Thermochimica Acta 576 (2014) 56–59
Contents lists available at ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Thermal conductivity of -SiAlONs prepared by a combination of combustion synthesis and spark plasma sintering Xuemei Yi a,∗ , Weiguo Zhang a , Tomohiro Akiyama b a b
College of Mechanical and Electronic Engineering, Northwest A&F University, Xinong Road 22, Yangling, Shaanxi 712100, China Center for Advanced Research of Energy and Materials, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo 060-8628, Japan
a r t i c l e
i n f o
Article history: Received 21 October 2013 Received in revised form 20 November 2013 Accepted 2 December 2013 Available online 7 December 2013 Keywords: Combustion synthesis Sintering Defects Thermal conductivity SiAlON
a b s t r a c t -Si6−z Alz Oz N8−z s (z = 1–3) have been synthesized by combustion synthesis (CS) and the as-received powders were densified by spark plasma sintering (SPS). The thermal properties of the dense -SiAlONs have been investigated by the laser-flash method from room temperature to a high temperature of 800 ◦ C. The highest thermal conductivity of 9.45 W m−1 K−1 was obtained when z = 1 at room temperature. Both thermal diffusivity and thermal conductivity decreased with an increase in the z value. In the -Si6−z Alz Oz N8−z , with z value increasing, more Si4+ and N3− ions get replaced by Al3+ and O2− ions, thereby more lattice defects are formed. These defects cause phonon scattering, accordingly reduce the thermal conductivity.
1. Introduction -SiAlON, a solid solution of -Si3 N4 in which part of Si–N has been replaced by Al–O, is most commonly described by the general formula -Si6−z Alz Oz N8−z (z = 0–4.2) [1,2]. Ever since it was discovered in the early 1970s [3], -SiAlON materials have been attracting considerable attention on account of their being suitable for hightemperature applications owing to their excellent mechanical and thermal properties. In our previous researches, we have synthesized -SiAlONs (z = 1–3) by a very simple method of combustion synthesis and have obtained dense products by spark plasma sintering method [4], and we have also measured their mechanical properties and corrosion behavior in different conditions [5–7]. Lots of reports concern the thermal properties for they are the very important property for many kinds of materials [8,9], in which many studies have been focused on the thermal conductivity of Si3 N4 [10–17]. Watari et al. reported that the thermal conductivity at room temperatures of sintered -Si3 N4 was 80 W m−1 K−1 in the direction parallel to the hot pressing direction, where the materials were hot-pressed at 1800 ◦ C and further hot-isostatic pressing to 2400 ◦ C [11]. It has also been reported that when alumina was added to silicon nitride, it resulted in reduction of thermal
© 2013 Elsevier B.V. All rights reserved.
conductivity to only 30 W m−1 K−1 [10]. It represents that the substitution of aluminum and oxygen into silicon and nitrogen sites in the -Si3 N4 structure increase the crystal defects, thereby decrease the thermal conductivity. In addition, the small grain size and intergranular phases also cause loss in thermal conductivity in SiAlON ceramics. Thermal conductivity of -SiAlON at room temperature has been reported to be 12.44 W m−1 K−1 by Liu et al. [18]. The maximum thermal conductivity was reported around 17 W m−1 K−1 for -SiAlON at ambient temperature by Joshi et al., which was prepared by hot pressing under 1850 ◦ C for 1 h with 2 wt.% Y2 O3 addition [19]. In our previous study, thermal conductivities of spark plasma sintered -SiAlONs (Si3 Al3 O3 N5 ) procured from combustion synthesis (CS) with no sintering additive were measured by the laser flash method at room temperature [20]. The results showed that thermal conductivity values increased with sintering temperature and attained a maximum of 5.49 W m−1 K−1 for fully densified -SiAlONs sintered at 1700 ◦ C for 10 min. However, no measurement has been performed on the thermal conductivity of CS-SPSed -SiAlONs (z = 1–3) as a function of temperature. Therefore, the purpose of this study was to obtain the thermal conductivity of CS-SPSed -SiAlONs (z = 1–3) as a function of temperature. 2. Experiments 2.1. Sample preparation
∗ Corresponding author. Tel.: +86 2987092391. E-mail addresses: xuemei [email protected], [email protected] (X. Yi). 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.12.002
The synthesis method has been described in detail elsewhere [21]. Here we only repeated the preparation method briefly.
X. Yi et al. / Thermochimica Acta 576 (2014) 56–59
57
Table 1 Characteristics of the CS-SPSed -SiAlONs used in this study. Samples
Phase compositions from XRD
Bulk density (g/cm3 )
Theoretical densitya (g/cm3 )
Relative density (%)
Z1 (z = 1) Z2 (z = 2) Z3 (z = 3)
-Si5 AlON7 -Si4 Al2 O2 N6 -Si3 Al3 O3 N5
3.13 3.12 3.073
3.168 3.122 3.082
98.8 99.9 99.7
where z takes values of 1–3. The mass percent of the diluent was determined according to our preliminary experiment. The CSed powder was first subjected to planetary ball milling for 60 min, then was compacted into a carbon die of 10 mm in inner diameter and sintered using a SPS system (Sumitomo Coal Mining Co. Ltd., Tokyo, Japan) under vacuum of lower than 4 Pa at a compressive stress of 50 MPa. The resulting compacts were heated from room temperature to 600 ◦ C in 5 min, and then were heated to 1600 ◦ C at a rate of 30 ◦ C/min. The compacts were maintained at this temperature for 10 min before the power was turned off. The phases of the CS-SPSed products were analyzed using an X-ray diffraction (XRD) (Mini Flex, Rigaku Corporation, Tokyo, Japan). The morphology was examined by scanning electron microscopy (SEM) (FE-SEM JSM-7400F, JEOL, Tokyo, Japan). 2.2. Thermal properties measurements Specimens with a dimension of 10 mm in diameter and 2–3 mm in thickness were cut from the spark-plasma-sintered discs, and were polished using emery paper until to No. 1200. Prior to measurement, a thick layer of colloidal graphite was sputter-coated to the surface of the specimen to enhance absorption of the flash energy. The thermal diffusivity and specific heat capacity were measured by the laser-flash method (TC-7000, ULVAC Sinku Riko Co., Yokohama, Japan) from room temperature to 800 ◦ C. The thermal diffusivity was analyzed with the t1/2 method. The bulk density was measured according to the Archimedean principle, using distilled water as the medium. All the experiments were carried out under a flowing argon gas atmosphere. The thermal conductivity (K) of -SiAlONs was determined by following equation: K = Cp ˛
(2)
where represents the bulk density, Cp is the specific heat capacity and ˛ is the thermal diffusivity. 3. Results 3.1. Microstructure of the CS-SPSed samples Table 1 shows the characteristics of the CS-SPSed -SiAlONs (z = 1, z = 2, and z = 3). The bulk density decreases from 3.13 to 3.073 g/cm3 with an increase in z value, and all of the relative densities of the as-received samples are higher than 98.8% theoretical density. Fig. 1 gives the XRD patterns of -SiAlONs (z = 1, z = 2, and z = 3) before and after SPS. The XRD patterns of CSed products before SPS show a little un-reacted Si except -SiAlON peaks; after SPS, only peaks due to -SiAlONs are observed. Fig. 2 shows the SEM images of the as-received -SiAlONs (z = 1, z = 2, and z = 3) after combustion synthesis and also the images
(220) (310) (301)
(201)
: Si
(311)
(221) (311)
(210) (111)
: β-SiAlON
(111)
(100)
Z3-C Intensity (a. u.)
(6−1.5z)Si+zAl+0.5zSiO2 +(4 − 0.5z)N2 → ˇ − Si6−z Alz Oz N8−z (1)
Z3-S
(200)
Commercially available powders of Si (98% purity, 1–2 m in size), Al (99.9% purity, 3 m in size), and SiO2 (99.9% purity, 0.8 m in size) were used as starting materials. -SiAlON powders (CSed product, unknown purity, 0.5 m in size) were added as the diluent. The chemical reaction for the synthesis of -SiAlON from the abovementioned starting materials can be shown as follows:
(101)
See Ref. [22].
(110)
a
Z2-S Z2-C Z1-S Z1-C 10
20
30
40
50
60
Diffraction angle (2θ) Fig. 1. XRD patterns of -SiAlONs (Z1, Z2, and Z3). “C” means after combustion synthesis (CS) and before spark plasma sintering (SPS); “S” means after SPS.
after spark plasma sintering. For the CSed powders, the primary microstructure is rod-like crystal with an average diameter of ∼500 nm and a length of one to several micrometers. Some particles and whiskers can also be found in the microstructure. After SPS process, the product looks dense solid, and the grain size increases clearly with an increase in the z value. 3.2. Thermal properties Table 2 gives the thermal properties of -SiAlONs and -Si3 N4 at room temperature. -Si3 N4 sintered at 1900 ◦ C for 36 h and subsequent annealed at 1700 ◦ C for 100 h shows very high thermal diffusivity and thermal conductivity [14]. This was attributed to the reduction of internal defects of the -Si3 N4 grains with sintering and annealing time as the grains grew. In contrary, the thermal conductivity of -SiAlONs is much less by nearly two orders of magnitude when compared to that of -Si3 N4 . The -SiAlON shows a little higher thermal conductivity of 12.44 W m−1 K−1 than those of our products [18]. This could also be attributed to the difference of the sintering method. In addition, we could not know the exact z value of this sample. For our products, it shows that both thermal diffusivity and thermal conductivity decrease with an increase in the z value. The highest thermal conductivity of 9.45 W m−1 K−1 was obtained when z = 1 at room temperature. Fig. 3 represents the temperature dependence of thermal diffusivity of the -SiAlONs from room temperature to 800 ◦ C comparing with sintered -SiAlON. The thermal diffusivity decreases with increasing temperature for all of the samples. And with z value increasing, the thermal diffusivity shows decreasing. The sintered -SiAlON showed a little higher than our data, but we could not know the exact z value of their sample. Fig. 4 shows the temperature dependence of heat capacity of the -SiAlONs from room temperature to 800 ◦ C comparing with sintered -SiAlON. Our data shows higher than that of sintered -SiAlON. For the CSed beta-SiAlON, Z2 shows the highest heat capacity, and Z3 shows the lowest data.
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X. Yi et al. / Thermochimica Acta 576 (2014) 56–59
0.08
0.06 0.05 0.04 0.03 0.02 0.01 200
1.6
-1
0.07
-1
1.8 beta-sialon (Ref.[18]) Z1 (this study) Z2 (this study) Z3 (this study)
Heat capacity Cp / J g K
Thermal Diffusivity α / 10-4 m2 s-1
Fig. 2. SEM images of -SiAlONs (Z1, Z2, and Z3). “C” means after combustion synthesis (CS) after spark plasma sintering (SPS); “S” means after SPS.
Z1 (this study) Z2 (this study) Z3 (this study) beta-sialon (Ref.[18])
1.4 1.2 1.0 0.8 0.6
400
600
800
1000
1200
200
Measurement Temperature T / K
400
600
800
1000
1200
Measurement Temperature T / K Fig. 3. Temperature dependence of thermal diffusivity of -SiAlONs. Fig. 4. Temperature dependence of the heat capacity of -SiAlONs.
The calculated thermal conductivity of -SiAlONs as a function of temperature is plotted in Fig. 5. The thermal conductivity of SiAlONs gradually decreases with the increase of temperature up to 1000 K, which is attributed to the lattice thermal conduction. However, the thermal conductivity of Z3 increases slightly above 1000 K, which may be attributed to the increased radiation, also known as photon thermal conductivity, with the increase of temperature [23]. From Fig. 5, the thermal conductivity of -SiAlONs gradually decreases with the increase of the z value under identical temperature conditions. The thermal conductivity of sintered SiAlON shows higher than our data when the temperature is lower than 473 K, contrarily it shows lower than that of our data of Z1 when the temperature is higher than 473 K.
4. Discussion From the literature, it is known that crystallinity, structural discontinuities such as pores, micro-cracking and glassy phases; micro-structural features, such as grain size, grain orientation and atomic mass differences of phases can affect the thermal conductivity of materials [24]. In our study of -SiAlONs, the solid solution of -Si3 N4 , where Si4+ and N3− ions get replaced by Al3+ and O2− ions turn out to be substitutional impurities in the crystal. From the SEM images of these samples, the grain size increases with the increasing in z value, but the true density decreases with the z value increasing, although the SPS conditions are the same. It
Table 2 Thermal properties of the -SiAlONs and -Si3 N4 at room temperature. Samples
Relative density (%)
Specific heat (103 J kg−1 K−1 )
Thermal diffusivity (10−4 m2 s−1 )
Thermal conductivity (W m−1 K−1 ) 28 ◦ C
Z1 (present work) Z2 (present work) Z3 (present work) -SiAlONa -Si3 N4 b
98.8 99.9 99.7 >98.5 Near full density
0.728 0.760 0.722 0.561 0.67
0.0415 0.0354 0.0264 0.0700 0.6723
9.45 8.40 5.86 12.44 150
a b
Sintered pressurelessly at 1800 ◦ C for 2 h from Si3 N4 , AlN, and Al2 O3 [18]. Sintered from -Si3 N4 , Yb2 O3 , and ZrO2 at 1900 ◦ C for 36 h and subsequent annealed at 1700 ◦ C for 100 h [14].
X. Yi et al. / Thermochimica Acta 576 (2014) 56–59
59
51302221); the Fundamental Research Funds for the Central Universities (Z109021204), and by the Scientific Research Staring Foundation for the Returned Overseas Chinese Scholars from the Northwest A&F University (Z111021201). References
Fig. 5. Temperature dependence of the thermal conductivity of -SiAlONs.
has been demonstrated that the thermal conductivity of -Si3 N4 is basically governed by the dissolved oxygen in the lattice of Si3 N4 , which causes phonon-defect scattering, thereby lowering thermal conductivity. The vacancies formed in the lattices and the mass difference due to substitution act as the phonon scattering sites, thereby reducing the thermal conductivities [14,15]. Their results revealed that the thermal conductivity at room temperature is independent of grain size, but the internal defect in -Si3 N4 grains such as point defects and dislocations are the significant factors that affect the conductivity values. Therefore, in the -Si6−z Alz Oz N8−z system, with z value increasing, more Si4+ and N3− ions get replaced by Al3+ and O2− ions, thereby more lattice defects are formed. These defects will cause phonon scattering, accordingly reduce the thermal conductivity. An increased thermal conductivity of Liu et al. [18] than our value might be mainly attributed to the Y2 O3 addition as a sintering aid and long sintering time (2 h) at high temperatures of 1800 ◦ C that produces large grains, in turn possess less crystal defects and decrease phonon scatterings. 5. Conclusions The thermal conductivity of the CS-SPSed -Si6−z Alz Oz N8−z s (z = 1–3) has been investigated by the laser-flash method from room temperature to a high temperature of 800 ◦ C. (1) The highest thermal conductivity of 9.45 W m−1 K−1 was obtained when z = 1 at room temperature. (2) Both thermal diffusivity and thermal conductivity decreased with an increase in the z value. (3) In the -Si6−z Alz Oz N8−z system, with z value increasing, more Si4+ and N3− ions get replaced by Al3+ and O2− ions, thereby more lattice defects are formed. These defects cause phonon scattering, accordingly reduce the thermal conductivity. Acknowledgements This research was supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant No.
[1] K.H. Jack, Review SiAlONs and related nitrogen ceramics, Journal of Materials Science 11 (1976) 1135–1158. [2] T. Ekström, P.O. Käll, M. Nygren, P.O. Olssen, Dense single-phase -SiAlON ceramics by glass-encapsulated hot isostatic pressing, Journal of Materials Science 24 (1989) 1853–1861. [3] Y. Oyama, O. Kamigaito, Solid solubility of some oxides in Si3 N4 , Japanese Journal of Applied Physics 10 (1971) 1673. [4] X. Yi, K. Watanabe, T. Akiyama, Fabrication of dense -SiAlON by a combination of combustion synthesis (CS) and spark plasma sintering (SPS), Intermetallics 18 (2010) 536–541. [5] X. Yi, K. Watanabe, T. Akiyama, Vickers hardness of -SiAlON prepared by a combination of combustion synthesis and spark plasma sinterng, Journal of the Ceramic Society of Japan 118 (2010) 250–252. [6] X. Yi, A. Yamauchi, K. Kurokawa, T. Akiyama, Oxidation of -SiAlONs prepared by a combination of combustion synthesis and spark plasma sintering, Corrosion Science 52 (2010) 1738–1745. [7] X. Yi, A. Yamauchi, K. Kurokawa, T. Akiyama, Corrosion of combustionsynthesized -SiAlONs in supercritical water, Corrosion Science 56 (2012) 153–157. [8] T. Yiamsawasd, A.S. Dalkilic, S. Wongwises, Measurement of the thermal conductivity of titania and alumina nanofluids, Thermochimica Acta 545 (2012) 48–56. [9] S. Aksöz, N. Maras¸lı, K. Kes¸lio˘glu, F. Yıldız, Variations of thermal conductivity with temperature and composition of Zn in the Bi–[x] at.% Zn–2 at.% Al alloys, Thermochimica Acta 547 (2012) 1–5. [10] K. Watari, Y. Seki, K. Ishizaki, Thermal properties of HIP sintered silicon nitride, Journal of the Ceramic Society of Japan 97 (1989) 56–92. [11] K. Watari, K. Hirao, M. Toriyama, K. Ishizaki, Effect of grain size on the thermal conductivity of Si3 N4 , Journal of the American Ceramic Society 82 (1999) 777–779. [12] M. Kitayama, K. Hirao, A. Tsuge, K. Watari, M. Toriyama, S. Kanzaki, Thermal conductivity of -Si3 N4 : II. Effect of lattice oxygen, Journal of the American Ceramic Society 83 (2000) 1985–1992. [13] H. Hayashi, K. Hirao, M. Toriyama, S. Kanzaki, K. Itatani, MgSiN2 addition as a means of increasing the thermal conductivity of -silicon nitride, Journal of the American Ceramic Society 84 (2001) 3060–3062. [14] H. Yokota, H. Abe, M. Ibukiyama, Effect of lattice defects on the thermal conductivity of -Si3 N4 , Journal of the European Ceramic Society 23 (2003) 1751–1759. [15] H. Yokota, M. Ibukiyama, Effect of lattice impurities on the thermal conductivity of -Si3 N4 , Journal of the European Ceramic Society 23 (2003) 55–60. [16] X. Zhu, Y. Zhou, K. Hirao, Effects of processing method and additive composition on microstructure and thermal conductivity of Si3 N4 ceramics, Journal of the European Ceramic Society 26 (2006) 711–718. [17] M. Ma, Z. Liu, Y. Li, Y. Zeng, D. Yao, Thermal conductivity of low-temperature sintered calcium aluminosilicate glass–silicon nitride whisker composites, Ceramics International 39 (2013) 4683–4687. [18] D.-M. Liu, C.-J. Chen, R.R.-R. Lee, Thermal diffusivity/conductivity in SiAlON ceramics, Journal of Applied Physics 77 (1995) 494–496. [19] B. Joshi, G. Gyawali, H. Wang, T. Sekino, S.W. Lee, Thermal and mechanical properties of hot pressed translucent Y2 O3 doped Mg-␣/-SiAlON ceramics, Journal of Alloys and Compounds 557 (2013) 112–119. [20] R. Sivakumar, K. Aoyagi, T. Akiyama, Thermal conductivity of combustion synthesized -SiAlONs, Ceramics International 35 (2009) 1391–1395. [21] X. Yi, K. Watanabe, T. Akiyama, Preparation of -Si6−z Alz Oz N8−z (z = 1–3) by combustion synthesis, IOP Conference Series: Materials Science and Engineering 18 (2011). [22] M. Haviar, A. Johannsen, Unit-cell dimention of -SiAlON, Advanced Ceramics Materials 3 (1988) 405–407. [23] Z.-G. Liu, J.-H. Ouyang, Y. Zhou, X.-L. Xia, Structure and thermal conductivity of Gd2 (Tix Zr1−x )2 O7 ceramics, Materials Letters 62 (2008) 4455–4457. [24] G.B. Raju, B. Basu, A.K. Suri, Thermal and electrical properties of TiB2 –MoSi2 , International Journal of Refractory Metals and Hard Materials 28 (2010) 174–179.
Contents lists available at ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Thermal conductivity of -SiAlONs prepared by a combination of combustion synthesis and spark plasma sintering Xuemei Yi a,∗ , Weiguo Zhang a , Tomohiro Akiyama b a b
College of Mechanical and Electronic Engineering, Northwest A&F University, Xinong Road 22, Yangling, Shaanxi 712100, China Center for Advanced Research of Energy and Materials, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo 060-8628, Japan
a r t i c l e
i n f o
Article history: Received 21 October 2013 Received in revised form 20 November 2013 Accepted 2 December 2013 Available online 7 December 2013 Keywords: Combustion synthesis Sintering Defects Thermal conductivity SiAlON
a b s t r a c t -Si6−z Alz Oz N8−z s (z = 1–3) have been synthesized by combustion synthesis (CS) and the as-received powders were densified by spark plasma sintering (SPS). The thermal properties of the dense -SiAlONs have been investigated by the laser-flash method from room temperature to a high temperature of 800 ◦ C. The highest thermal conductivity of 9.45 W m−1 K−1 was obtained when z = 1 at room temperature. Both thermal diffusivity and thermal conductivity decreased with an increase in the z value. In the -Si6−z Alz Oz N8−z , with z value increasing, more Si4+ and N3− ions get replaced by Al3+ and O2− ions, thereby more lattice defects are formed. These defects cause phonon scattering, accordingly reduce the thermal conductivity.
1. Introduction -SiAlON, a solid solution of -Si3 N4 in which part of Si–N has been replaced by Al–O, is most commonly described by the general formula -Si6−z Alz Oz N8−z (z = 0–4.2) [1,2]. Ever since it was discovered in the early 1970s [3], -SiAlON materials have been attracting considerable attention on account of their being suitable for hightemperature applications owing to their excellent mechanical and thermal properties. In our previous researches, we have synthesized -SiAlONs (z = 1–3) by a very simple method of combustion synthesis and have obtained dense products by spark plasma sintering method [4], and we have also measured their mechanical properties and corrosion behavior in different conditions [5–7]. Lots of reports concern the thermal properties for they are the very important property for many kinds of materials [8,9], in which many studies have been focused on the thermal conductivity of Si3 N4 [10–17]. Watari et al. reported that the thermal conductivity at room temperatures of sintered -Si3 N4 was 80 W m−1 K−1 in the direction parallel to the hot pressing direction, where the materials were hot-pressed at 1800 ◦ C and further hot-isostatic pressing to 2400 ◦ C [11]. It has also been reported that when alumina was added to silicon nitride, it resulted in reduction of thermal
© 2013 Elsevier B.V. All rights reserved.
conductivity to only 30 W m−1 K−1 [10]. It represents that the substitution of aluminum and oxygen into silicon and nitrogen sites in the -Si3 N4 structure increase the crystal defects, thereby decrease the thermal conductivity. In addition, the small grain size and intergranular phases also cause loss in thermal conductivity in SiAlON ceramics. Thermal conductivity of -SiAlON at room temperature has been reported to be 12.44 W m−1 K−1 by Liu et al. [18]. The maximum thermal conductivity was reported around 17 W m−1 K−1 for -SiAlON at ambient temperature by Joshi et al., which was prepared by hot pressing under 1850 ◦ C for 1 h with 2 wt.% Y2 O3 addition [19]. In our previous study, thermal conductivities of spark plasma sintered -SiAlONs (Si3 Al3 O3 N5 ) procured from combustion synthesis (CS) with no sintering additive were measured by the laser flash method at room temperature [20]. The results showed that thermal conductivity values increased with sintering temperature and attained a maximum of 5.49 W m−1 K−1 for fully densified -SiAlONs sintered at 1700 ◦ C for 10 min. However, no measurement has been performed on the thermal conductivity of CS-SPSed -SiAlONs (z = 1–3) as a function of temperature. Therefore, the purpose of this study was to obtain the thermal conductivity of CS-SPSed -SiAlONs (z = 1–3) as a function of temperature. 2. Experiments 2.1. Sample preparation
∗ Corresponding author. Tel.: +86 2987092391. E-mail addresses: xuemei [email protected], [email protected] (X. Yi). 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.12.002
The synthesis method has been described in detail elsewhere [21]. Here we only repeated the preparation method briefly.
X. Yi et al. / Thermochimica Acta 576 (2014) 56–59
57
Table 1 Characteristics of the CS-SPSed -SiAlONs used in this study. Samples
Phase compositions from XRD
Bulk density (g/cm3 )
Theoretical densitya (g/cm3 )
Relative density (%)
Z1 (z = 1) Z2 (z = 2) Z3 (z = 3)
-Si5 AlON7 -Si4 Al2 O2 N6 -Si3 Al3 O3 N5
3.13 3.12 3.073
3.168 3.122 3.082
98.8 99.9 99.7
where z takes values of 1–3. The mass percent of the diluent was determined according to our preliminary experiment. The CSed powder was first subjected to planetary ball milling for 60 min, then was compacted into a carbon die of 10 mm in inner diameter and sintered using a SPS system (Sumitomo Coal Mining Co. Ltd., Tokyo, Japan) under vacuum of lower than 4 Pa at a compressive stress of 50 MPa. The resulting compacts were heated from room temperature to 600 ◦ C in 5 min, and then were heated to 1600 ◦ C at a rate of 30 ◦ C/min. The compacts were maintained at this temperature for 10 min before the power was turned off. The phases of the CS-SPSed products were analyzed using an X-ray diffraction (XRD) (Mini Flex, Rigaku Corporation, Tokyo, Japan). The morphology was examined by scanning electron microscopy (SEM) (FE-SEM JSM-7400F, JEOL, Tokyo, Japan). 2.2. Thermal properties measurements Specimens with a dimension of 10 mm in diameter and 2–3 mm in thickness were cut from the spark-plasma-sintered discs, and were polished using emery paper until to No. 1200. Prior to measurement, a thick layer of colloidal graphite was sputter-coated to the surface of the specimen to enhance absorption of the flash energy. The thermal diffusivity and specific heat capacity were measured by the laser-flash method (TC-7000, ULVAC Sinku Riko Co., Yokohama, Japan) from room temperature to 800 ◦ C. The thermal diffusivity was analyzed with the t1/2 method. The bulk density was measured according to the Archimedean principle, using distilled water as the medium. All the experiments were carried out under a flowing argon gas atmosphere. The thermal conductivity (K) of -SiAlONs was determined by following equation: K = Cp ˛
(2)
where represents the bulk density, Cp is the specific heat capacity and ˛ is the thermal diffusivity. 3. Results 3.1. Microstructure of the CS-SPSed samples Table 1 shows the characteristics of the CS-SPSed -SiAlONs (z = 1, z = 2, and z = 3). The bulk density decreases from 3.13 to 3.073 g/cm3 with an increase in z value, and all of the relative densities of the as-received samples are higher than 98.8% theoretical density. Fig. 1 gives the XRD patterns of -SiAlONs (z = 1, z = 2, and z = 3) before and after SPS. The XRD patterns of CSed products before SPS show a little un-reacted Si except -SiAlON peaks; after SPS, only peaks due to -SiAlONs are observed. Fig. 2 shows the SEM images of the as-received -SiAlONs (z = 1, z = 2, and z = 3) after combustion synthesis and also the images
(220) (310) (301)
(201)
: Si
(311)
(221) (311)
(210) (111)
: β-SiAlON
(111)
(100)
Z3-C Intensity (a. u.)
(6−1.5z)Si+zAl+0.5zSiO2 +(4 − 0.5z)N2 → ˇ − Si6−z Alz Oz N8−z (1)
Z3-S
(200)
Commercially available powders of Si (98% purity, 1–2 m in size), Al (99.9% purity, 3 m in size), and SiO2 (99.9% purity, 0.8 m in size) were used as starting materials. -SiAlON powders (CSed product, unknown purity, 0.5 m in size) were added as the diluent. The chemical reaction for the synthesis of -SiAlON from the abovementioned starting materials can be shown as follows:
(101)
See Ref. [22].
(110)
a
Z2-S Z2-C Z1-S Z1-C 10
20
30
40
50
60
Diffraction angle (2θ) Fig. 1. XRD patterns of -SiAlONs (Z1, Z2, and Z3). “C” means after combustion synthesis (CS) and before spark plasma sintering (SPS); “S” means after SPS.
after spark plasma sintering. For the CSed powders, the primary microstructure is rod-like crystal with an average diameter of ∼500 nm and a length of one to several micrometers. Some particles and whiskers can also be found in the microstructure. After SPS process, the product looks dense solid, and the grain size increases clearly with an increase in the z value. 3.2. Thermal properties Table 2 gives the thermal properties of -SiAlONs and -Si3 N4 at room temperature. -Si3 N4 sintered at 1900 ◦ C for 36 h and subsequent annealed at 1700 ◦ C for 100 h shows very high thermal diffusivity and thermal conductivity [14]. This was attributed to the reduction of internal defects of the -Si3 N4 grains with sintering and annealing time as the grains grew. In contrary, the thermal conductivity of -SiAlONs is much less by nearly two orders of magnitude when compared to that of -Si3 N4 . The -SiAlON shows a little higher thermal conductivity of 12.44 W m−1 K−1 than those of our products [18]. This could also be attributed to the difference of the sintering method. In addition, we could not know the exact z value of this sample. For our products, it shows that both thermal diffusivity and thermal conductivity decrease with an increase in the z value. The highest thermal conductivity of 9.45 W m−1 K−1 was obtained when z = 1 at room temperature. Fig. 3 represents the temperature dependence of thermal diffusivity of the -SiAlONs from room temperature to 800 ◦ C comparing with sintered -SiAlON. The thermal diffusivity decreases with increasing temperature for all of the samples. And with z value increasing, the thermal diffusivity shows decreasing. The sintered -SiAlON showed a little higher than our data, but we could not know the exact z value of their sample. Fig. 4 shows the temperature dependence of heat capacity of the -SiAlONs from room temperature to 800 ◦ C comparing with sintered -SiAlON. Our data shows higher than that of sintered -SiAlON. For the CSed beta-SiAlON, Z2 shows the highest heat capacity, and Z3 shows the lowest data.
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0.08
0.06 0.05 0.04 0.03 0.02 0.01 200
1.6
-1
0.07
-1
1.8 beta-sialon (Ref.[18]) Z1 (this study) Z2 (this study) Z3 (this study)
Heat capacity Cp / J g K
Thermal Diffusivity α / 10-4 m2 s-1
Fig. 2. SEM images of -SiAlONs (Z1, Z2, and Z3). “C” means after combustion synthesis (CS) after spark plasma sintering (SPS); “S” means after SPS.
Z1 (this study) Z2 (this study) Z3 (this study) beta-sialon (Ref.[18])
1.4 1.2 1.0 0.8 0.6
400
600
800
1000
1200
200
Measurement Temperature T / K
400
600
800
1000
1200
Measurement Temperature T / K Fig. 3. Temperature dependence of thermal diffusivity of -SiAlONs. Fig. 4. Temperature dependence of the heat capacity of -SiAlONs.
The calculated thermal conductivity of -SiAlONs as a function of temperature is plotted in Fig. 5. The thermal conductivity of SiAlONs gradually decreases with the increase of temperature up to 1000 K, which is attributed to the lattice thermal conduction. However, the thermal conductivity of Z3 increases slightly above 1000 K, which may be attributed to the increased radiation, also known as photon thermal conductivity, with the increase of temperature [23]. From Fig. 5, the thermal conductivity of -SiAlONs gradually decreases with the increase of the z value under identical temperature conditions. The thermal conductivity of sintered SiAlON shows higher than our data when the temperature is lower than 473 K, contrarily it shows lower than that of our data of Z1 when the temperature is higher than 473 K.
4. Discussion From the literature, it is known that crystallinity, structural discontinuities such as pores, micro-cracking and glassy phases; micro-structural features, such as grain size, grain orientation and atomic mass differences of phases can affect the thermal conductivity of materials [24]. In our study of -SiAlONs, the solid solution of -Si3 N4 , where Si4+ and N3− ions get replaced by Al3+ and O2− ions turn out to be substitutional impurities in the crystal. From the SEM images of these samples, the grain size increases with the increasing in z value, but the true density decreases with the z value increasing, although the SPS conditions are the same. It
Table 2 Thermal properties of the -SiAlONs and -Si3 N4 at room temperature. Samples
Relative density (%)
Specific heat (103 J kg−1 K−1 )
Thermal diffusivity (10−4 m2 s−1 )
Thermal conductivity (W m−1 K−1 ) 28 ◦ C
Z1 (present work) Z2 (present work) Z3 (present work) -SiAlONa -Si3 N4 b
98.8 99.9 99.7 >98.5 Near full density
0.728 0.760 0.722 0.561 0.67
0.0415 0.0354 0.0264 0.0700 0.6723
9.45 8.40 5.86 12.44 150
a b
Sintered pressurelessly at 1800 ◦ C for 2 h from Si3 N4 , AlN, and Al2 O3 [18]. Sintered from -Si3 N4 , Yb2 O3 , and ZrO2 at 1900 ◦ C for 36 h and subsequent annealed at 1700 ◦ C for 100 h [14].
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51302221); the Fundamental Research Funds for the Central Universities (Z109021204), and by the Scientific Research Staring Foundation for the Returned Overseas Chinese Scholars from the Northwest A&F University (Z111021201). References
Fig. 5. Temperature dependence of the thermal conductivity of -SiAlONs.
has been demonstrated that the thermal conductivity of -Si3 N4 is basically governed by the dissolved oxygen in the lattice of Si3 N4 , which causes phonon-defect scattering, thereby lowering thermal conductivity. The vacancies formed in the lattices and the mass difference due to substitution act as the phonon scattering sites, thereby reducing the thermal conductivities [14,15]. Their results revealed that the thermal conductivity at room temperature is independent of grain size, but the internal defect in -Si3 N4 grains such as point defects and dislocations are the significant factors that affect the conductivity values. Therefore, in the -Si6−z Alz Oz N8−z system, with z value increasing, more Si4+ and N3− ions get replaced by Al3+ and O2− ions, thereby more lattice defects are formed. These defects will cause phonon scattering, accordingly reduce the thermal conductivity. An increased thermal conductivity of Liu et al. [18] than our value might be mainly attributed to the Y2 O3 addition as a sintering aid and long sintering time (2 h) at high temperatures of 1800 ◦ C that produces large grains, in turn possess less crystal defects and decrease phonon scatterings. 5. Conclusions The thermal conductivity of the CS-SPSed -Si6−z Alz Oz N8−z s (z = 1–3) has been investigated by the laser-flash method from room temperature to a high temperature of 800 ◦ C. (1) The highest thermal conductivity of 9.45 W m−1 K−1 was obtained when z = 1 at room temperature. (2) Both thermal diffusivity and thermal conductivity decreased with an increase in the z value. (3) In the -Si6−z Alz Oz N8−z system, with z value increasing, more Si4+ and N3− ions get replaced by Al3+ and O2− ions, thereby more lattice defects are formed. These defects cause phonon scattering, accordingly reduce the thermal conductivity. Acknowledgements This research was supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant No.
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