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The effect of annealing temperature on flexural strength, dielectric loss and thermal conductivity of Si3N4 ceramics
Journal of Alloys and Compounds 813 (2020) 152203
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The effect of annealing temperature on flexural strength, dielectric loss and thermal conductivity of Si3N4 ceramics Huanbei Chen a, 1, Weide Wang b, c, 1, Xing Yu d, Kai-hui Zuo b, Yongfeng Xia b, Jinwei Yin b, Hanqin Liang b, Dongxu Yao b, *, Yu-Ping Zeng b a
Nanjing Electronic Devices Institute, Nanjing, 210016, China State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China c University of Chinese Academy of Sciences, Beijing, 100049, China d Shanghai Radio Equipment Research Institute, Shanghai, 200090, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2019 Received in revised form 15 August 2019 Accepted 7 September 2019 Available online 8 September 2019
Effect of annealing temperatures on flexural strength, dielectric and thermal properties of Si3N4 ceramics was investigated with different amounts of Yb2O3 (3.5 mol% and 7 mol%) as sintering additives. High density of >99% were achieved after hot pressing. The post annealing heat treatment promoted the crystallization of the grain boundary glass phase. Low dielectric loss of <4 104 were obtained for all samples, a low dielectric loss of 1.8 104 was achieved by annealing at 1500 C for 24 h with 7 mol% Yb2O3 as sintering additives. A mild anisotropic microstructure was obtained due to merit of hot pressing method. High thermal conductivity of >75 W/(m$K) and >90 W/(m$K) were obtained in the direction parallel and perpendicular to the hot-pressing direction in all samples, respectively. The low flexural strength of ~600 MPa may attribute to the large grain in the matrix and lack of elongated abnormal grains. © 2019 Elsevier B.V. All rights reserved.
Keywords: Si3N4 ceramics Annealing Dielectric Thermal conductivity
1. Introduction Silicon nitride (Si3N4) ceramics with high strength, low dielectric loss and high thermal conductivity is a promising candidate for usage in structural components in the microwave devices, such as radio frequency windows for gyrotron and klystron applications [1e3]. Low dielectric constant (ε) and low dielectric loss (tan d) can reduce the dielectric absorption of microwave power, high thermal conductivity can accelerate heat transfer, both can contributed to avoiding an inhomogeneous temperature profiles and thermal stress of the products, thereby enhancing the stability of the device. High flexural strength can also guarantee a high thermal shock resistance according to the equation [4]: 0
R ¼
sf ð1 mÞ $f Ea
(1)
where sf is flexural strength, m is Poisson's ratio, E is elastic
* Corresponding author. Shanghai Institute of Ceramics, Chinese Academy of Sciences, Heshuo Road 585, Shanghai, 201899, PR China. E-mail address: [email protected] (D. Yao). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jallcom.2019.152203 0925-8388/© 2019 Elsevier B.V. All rights reserved.
modulus, a coefficient of thermal expansion, f is thermal diffusivity. High thermal conductivity Si3N4 ceramics has been widely researched and high thermal conductivity up to 177 W/m K has been achieved by sintered reaction bonded Si3N4 method with holding time of 60 h at 1900 C followed by annealing at a very slow rate (0.2 C/min), which might help reduce structural imperfections in b-Si3N4 grains and enhance devitrification of amorphous grain boundary phases [5]. In the case of low tan d of Si3N4 ceramics, it has not been fully investigated. H. Miyazaki and et al. [6,7] have carried out some researches on Yb2O3 based sintering additive followed by annealing, Si3N4 ceramics with high thermal conductivity of ~100 W/m$K and low tan d of 1.4 104 was achieved. This result of tan d is much lower than the conventional gas pressing sintered or reaction sintered Si3N4 (2e11 103), even lower than that of the CVD Si3N4 (3 104) [8], and equivalent to a commercial product Kyocera Si3N4 (SN287, 1.0 104@30e40 GHz). In their experiments, the heat treatments were carried out at 1300 C for 24 h due to the report that the devitrification of intergranular glass phase took place between 1250 C and 1450 C for 12e24 h [9]. While in the papers of Guo [10] and Liu [11], they have shown that different grain boundary phases, such as Yb4Si2O7N2, Yb2SiO5, and Yb2Si2O7, were detected under varied annealing temperature
2
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ranging from 1350 C to 1700 C with different content of Yb2O3. Since the grain boundary phase plays an important role in decrease the tan d of Si3N4 ceramics. It is worthwhile investigating the content of sintering additive and annealing temperatures on phase formation and the consequent variation of the comprehensive strength-thermal-dielectric properties. In this paper, two different amounts of Yb2O3 (3.5 mol% and 7 mol%) were used as sintering additives. The Si3N4 ceramics were prepared by hot pressing method, further annealing is performed at 1300e1700 C for about 24 h. The phase transformation of the grain boundary phases, flexural strength, dielectric properties and thermal conductivity were fully investigated.
2. Experimental procedure The starting powders used in this study were Si3N4 powder (1.2 mm, 4 N, a phase ~50%, Hebei Corefra Silicon Nitride Material Co., Ltd, China) and Yb2O3 (5.0 mm, purity99.99 wt%, Yuelong Company, China). Two types of composition, one is with 3.5 mol% of Yb2O3 (hereafter 3.5 Yb), and the other with 7 mol% of Yb2O3 (hereafter 7 Yb) were fabricated by the following procedure. The addition of 7 mol% Yb2O3 was proved to achieve low dielectric loss [6]. In those experiments, the starting powder used was a-Si3N4 powder (SN-E10, Ube Industries, Ltd., Japan), the oxygen content was 1.3e1.6 wt%, in this experiment, the oxygen content of the Si3N4 powder was 0.7e0.8 wt%, which is only half of the SN-E10 powder. So a similar Yb2O3/SiO2 in molar ratio as in Miyazaki’ research [6] were also prepared with Yb2O3 content of 3.5 mol%. The starting compositions were point in Fig. 1. The starting powders were mixed by planetary milling in ethanol for 3 h. The slurry was dried, and then passed through 100 mesh sieve. The mixed powders were hot-pressed in a graphite die with inner diameter of 70 mm at 1800 C under pressure of 30 MPa in a steady-flow N2 atmosphere for 2 h, the sample is labeled as HP. Post annealing of the as-sintered bodies were performed subsequently at 1300 C, 1500 C and 1700 C for 24 h using the same equipment, the sample are labeled as HP-A-1, HP-A-2, HP-A-3. One batch as-hot pressed samples were post heated at 1900 C for 4 h with gas pressure of 1 MPa and further annealed at 1700 C for 24 h, labeled as HP-A-4. X-ray diffraction (XRD, D8 discover, Bruker, Germany) was used to characterize the phase composition of the as-prepared and asannealed Si3N4 ceramics at the surfaces perpendicular to the hotpressing direction. The Lotgering orientation factor, f, is used to evaluate the degree of texture in the sintered samples. It can be expressed as follow [12]:
Fig. 1. Composition of Si3N4eYb2O3eSiO2 [14].
this
work's
specimens
in
the
ternary
system
of
f¼(PeP0)/(1-P0)
(2)
where P and P0 ¼ SI(hk0)/SI(hkl). SI(hk0) is the sum of peak intensities of the (hk0) and SI(hkl) is the sum of peak intensities of all the (hkl) on the surface perpendicular to the direction of hot pressing direction. The values of P are obtained from the sample and the values of P0 are obtained from the standard PDF card of b-Si3N4 (33e1160). If f ¼ 0, grains randomly align; if f ¼ 1, grain orientation is formed completely. The bending strength of the samples was measured by the three-point bending test with a span length of 30 mm and across head speed of 0.5 mm/min. The reported value was the average of five samples. Disk samples (4 60 2 mm) were machined for dielectric constant and dielectric loss test at 5 GHz by the perturbation method using a cavity resonator and a vector network analyzer (Keysight E5071C). Archimedes method was used to calculate the density of samples. Laser flash thermal analyzer (LFA467, Netzsch, Germany) was used to test the thermal diffusivity of samples. The thermal diffusivity parallel (PA) and perpendicular (PE) to the hot-pressing direction were used with samples of 4 12.6 2 mm and 4 25.4 0.5 mm, respectively. The thermal conductivity (k) was calculated according to the equation [13]:
k ¼ rCpa
(3)
where r, Cp and a are the bulk density, specific heat and thermal diffusivity, respectively. A constant value of specific heat, 0.68 J g1 K1, was used in this work to calculate the thermal conductivity at room temperature [13]. 3. Results and discussion 3.1. Phase formation of the hot pressed and annealed samples Fig. 2 shows the XRD patterns of the samples from TS direction. It can be seen that the major phase is b-Si3N4 and the minor phase is Yb based silicates. The b-Si3N4 peak intensities of (200) and (210) compared to (101) and (002) are very high. These results show that b-Si3N4 grains can be oriented under the hot pressing process. However, the calculated Lotgering orientation factor are in the range of 0.3e0.4 as listed in Table 1. This is relatively lower than the common hot pressed result (~0.7) with submicron Si3N4 powder as raw material [15]. In this experiment, the coarse Si3N4 powder may form larger b-Si3N4 grains which are much harder to orient under pressure than fine b-Si3N4 grains. The orientation will lead to a difference of thermal conductivity in different direction that will be discussed in the following section. A detailed XRD patterns of grain boundary phase at 27e33 are shown in Fig. 2 (c) and (d). For the 3.5 Yb samples, the main grain boundary phase was Yb2SiO5 for samples of HP, HP-A-1 and HP-A-2, and Yb4Si2O7N2 for samples of HP-A-3 and HP-A-4. In the hot pressing process, Yb2O3 formed Yb-Si-O-N glass phase with Si3N4 and its native SiO2 surface, densification of Si3N4 ceramics was promoted with the existence of Yb-Si-O-N glass phase and the applied pressure. Yb2SiO5 was precipitated from the Yb-Si-O-N glass phase during cooling. The formation of Yb4Si2O7N2 phase was may through the reaction of Yb2SiO5 with Si3N4 and the precipitation of the residue Yb-Si-O-N glass phase during annealing. Some researchers have shown that a designed Yb2O3/SiO2 in molar ratio may not necessarily lead to a desired precipitated phase assumed from the phase diagram as shown in Fig. 1 [16]. In this experiment, the Yb2O3/SiO2 in molar ratio was 1.18:1 and 2.45:1 for 3.5 Yb and 7 Yb samples, respectively. These composition were located in the range of Yb2SiO5 (1:1) and Yb4Si2O7N2 (4:1). The grain boundary phases were strongly influenced by the cooling as
H. Chen et al. / Journal of Alloys and Compounds 813 (2020) 152203
3
Fig. 2. XRD patterns of hot pressed and annealed samples from top surface with (a) 3.5 Yb, (b) 7 Yb and detailed grain boundary phase with (c) 3.5 Yb, (d) 7 Yb.
well as annealing program. For the 7 Yb samples, the main grain boundary phase was Yb4Si2O7N2 for all annealed samples. The strong peak of Yb4Si2O7N2 of sample HP-A-2 implies that the annealing can effectively promote the crystallization of the amorphous phase. The peaks are weakened for HP-A-3 and HP-A-4 samples compared with HP-A-2, this may indicate a decreasing in the content of grain boundary phase. The evidently decrease in density for HP-A-3 (3.51 g/cm3) and HP-A-4 (3.42 g/cm3) may also prove this conjecture, as listed in Table 1. 3.2. Microstructure of the hot pressed and annealed samples Based on the calculation by Archimedes method, the 3.5 Yb and
7 Yb samples reached densities of 3.37 g/cm3 and 3.56 g/cm3, and relative densities of 99.2% and 99.1%, respectively. The result indicated that Yb2O3 alone can effective achieving high densification of Si3N4 ceramics by hot pressing sintering, the SEM pictures of the Si3N4 ceramics also verify it, as shown in Fig. 3 (a)and Fig. 4 (a) The grain size is quite large, while the aspect ratio of the grains is low. The large grain size may attribute to the relatively larger size of the raw Si3N4 powder (1.2 mm) compared with common used UBE-10 powder (0.5 mm). The low aspect ratio may due to the low a content of the raw Si3N4 powder. As the sintering temperature is 1800 C for 2 h, the temperature is not high enough to promote Oswald ripening [17], so the phase transformation is governed by dissolution-precipitation process. a-Si3N4 particles would
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Table 1 Dielectric and thermal properties of the samples after hot pressing and annealing. Samples
Heat treatment
Lotgering orientation
Density (g/cm3)
Dielectric constant
Dielectric loss (104)
Heat capacity (mm2/s) PA/PE
Thermal conductivity (W/(m$K)) PA/PE
3.5 Yb
HP HP-A-1 HP-A-2 HP-A-3 HP-A-4 HP HP-A-1 HP-A-2 HP-A-3 HP-A-4
0.38 0.32 0.36 0.36 0.34 0.33 0.34 0.38 0.31 0.34
3.37 3.37 3.37 3.36 3.35 3.56 3.56 3.55 3.51 3.42
7.8 7.8 7.8 7.8 7.8 8.0 8.0 8.0 7.9 7.9
3.2 2.4 2.1 2.3 2.6 2.8 2.0 1.8 2.4 2.5
33.94/42.43 33.34/42.05 33.03/41.32 35.68/45.24 35.15/44.19 32.09/40.08 31.68/39.69 32.84/41.02 34.06/42.34 35.31/43.96
77.8/97.2 76.4/96.4 75.7/94.7 81.5/103.4 80.1/100.7 77.7/97 76.7/96.1 79.3/99.02 81.3/101.1 82.1/102.2
7 Yb
Fig. 3. SEM micrographs of polished surface perpendicular to the hot-pressing direction in (a)HP, (b) HP-A-1, (c) HP-A-2, (d) HP-A-3, (e) HP-A-4 of 3.5 Yb samples.
Fig. 4. SEM micrographs of polished surface perpendicular to the hot-pressing direction in (a)HP, (b) HP-A-1, (c) HP-A-2, (d) HP-A-3, (e) HP-A-4 of 7 Yb samples.
H. Chen et al. / Journal of Alloys and Compounds 813 (2020) 152203
dissolution in the Yb-Si-O-N glass phase, after reaching the saturation point, it would precipitation onto the b-Si3N4 nuclei and promote anisotropic grain growth. So in ideal condition, high aspect ratio b-Si3N4 grains can be obtained with high a-Si3N4 raw materials combined with free space for grain growth, such as porous Si3N4 ceramics prepared by freeze drying method [18]. But in this experiment, low a-Si3N4 raw materials and coarse raw Si3N4 materials limited the anisotropic grain growth. Beside the Si3N4 grains, the grain boundary phase (white) can clearly observe in the SEM pohtos, as show in Figs. 3 and 4 that indicated the grain boundary phase can definitely not ignorable on the high temperature strength [19e21], as well as dielectric and thermal properties. For 3.5 Yb samples, the grains show no much difference after annealing treatment. For 7 Yb samples, the grains are relatively fine and separated by the grain boundaries in HP sample, while some larger grains emerged in HP-A-2e4 samples. In considering the density have decreased to 3.51 g/cm3 and 3.42 g/cm3 for samples HP-A-3 and HP-A-4, respectively, and the weakening peak in XRD results. This can be explained as follow: In the case of high amount of sintering additive, such as 7 mol% Yb2O3, the migration of grain boundary and the Oswald ripening of the grains may “purge” the grain boundary phase to the surface during annealing at temperature >1700 C, that lead to a decrease in grain boundary phase as well as the density. 3.3. Dielectric properties of the hot pressed and annealed specimens Table 1 listed the dielectric constant and dielectric loss of the samples. A dielectric constant of 7.8e8 were obtained. The dielectric constant is not influenced by the annealing condition. Only in 7 Yb samples, the HP-A-3 and HP-A-4 samples show a slight decrease which may also connected to the decreased amount of grain boundary phase. The dielectric loss is strongly influenced by the annealing conditions in this experiment. Actually, the dielectric loss is primarily controlled by the sintering additives and the crystalline phase of the grain boundary. Miyazaki and et al. [7,9] have investigated different Yb2O3eSiO2 ratio on the dielectric loss of Si3N4 ceramics and have found out that the glassy phase have a great influence on the dielectric loss, a low dielectric of 1.4 104 can be achieved even without annealing treatment in the sample with 7Yb5Si and 7 Yb. In this experiment, dielectric loss <4 104 were obtained for all samples, a low dielectric loss of 1.8 104 in 7 Yb sample by annealing at 1500 C for 24 h. A Y2SiO5 single crystal was tested, the dielectric constant is 10.6, the dielectric loss is 8.5 105,which is much lower than Si3N4. It is reasonable to infer that Yb2SiO5 may also possess low dielectric loss due to the substation of Yb for Y, and that Yb4Si2O7N2 may also possess low dielectric loss since no much difference in dielectric loss with these two phases in different annealed samples. So the Si3N4 ceramics with these crystalline phases can achieve lower dielectric than the CVD Si3N4 (3 104).
were obtained in the hot pressed 3.5 Yb and 7 Yb samples, however, the post annealing only have slight improvement on thermal conductivity, even in the annealing condition of 1900 C-4h & 1700 C24 h. Many researchers have investigated a hot-pressing combined post sintering method to prepare high thermal conductivity Si3N4 ceramics [15,25], normally the thermal conductivity were increased by ~30% after post-sintering heat treatments. The main reason is related with strong grain growth after post-sintering. In this experiment, the grain size is not remarkable increased, so the increment of thermal conductivity is only 5% after post-sintering at 1900 C-4h and annealing at 1700 C-24 h. 3.5. Flexural strength of the hot pressed and annealed samples The flexural strength of hot pressed and annealed samples are presented in Fig. 5. Many researches have indicated that a bimodal grain microstructure can achieve high strength in Si3N4 ceramics [26,27]. Lee and et al. [28] have investigated the effect of starting Si3N4 powders with different a-to-b ratios on the microstructure evolution and the consequent influence on the flexural strength, they have found out that a distinctive bimodal microstructure occurred as a result of the abnormal grain growth with 50% a-Si3N4 as raw powder (specimen B), a flexural strength >900 MPa and thermal conductivity of 86W/m$K were obtained. While a broader range of grains with only monomial distribution was obtained with only 2~4 wt% a-Si3N4 as raw powder. In the meantime, the size of the large grains was smaller, whereas the size of the matrix grains was larger than that of specimen B, that resulting in lower flexural strength (<700 MPa). In this experiment, a ~50% a-Si3N4 content was used, but the microstructure is not distinctly bimodal, both large grain in the matrix and lack of elongated abnormal grains were resulting in lower flexural strength of ~600 MPa. It is quite rare to see a increasing in flexural strength with increased annealing temperature, the reason may related to the emerged elongated grains, as shown in Fig. 4 (d) and (e). 4. Conclusions The effect of annealing temperatures on flexural strength, dielectric and thermal properties of Si3N4 ceramics was investigated with different amounts of Yb2O3 (3.5 mol% and 7 mol%) as sintering additives. The XRD results indicated a mild anisotropic thermal conductivity was obtained due to merit of hot pressing method, the calculated Lotgering orientation factor are in the range of 0.3e0.4. Moreover, the post annealing heat treatment promoted the crystallization of the grain boundary glass phase. Low dielectric loss of <4 104 were obtained for all samples, a low dielectric loss of 1.8 104 by annealing at 1500 C for 24 h with 7 mol% Yb2O3 as
3.4. Thermal conductivities of the hot pressed and annealed samples Table 1 listed the heat capacity and thermal conductivity of the samples. Due to slight orientation of the samples, anisotropic thermal behavior was obtained. High thermal conductivity in Si3N4 ceramics with high anisotropic microstructure have been achieved by tape casting [22], extrusion [23], and magnetic field treatment [24] with b-Si3N4 seed,the thermal conductivity along the c-axis direction can be remarkably improved. The thermal conductivity is higher in the direction perpendicular to the hot-pressing direction (~100 W/(m$K)) than in the direction parallel to the hot-pressing direction (~80 W/(m$K)). Thermal conductivity of ~77 W/(m$K)
5
Fig. 5. Flexural strength of the hot pressed and annealed samples.
6
H. Chen et al. / Journal of Alloys and Compounds 813 (2020) 152203
sintering additives. High thermal conductivity >75 W/(m$K) and >90 W/(m$K) were obtained in the direction parallel and perpendicular to the hot-pressing direction in all samples, respectively, the annealing and post heat treatment at 1900 C did not lead to a tremendous increasing in thermal conductivity. The low flexural strength of ~600 MPa may attribute to the large grain in the matrix and lack of elongated abnormal grains that closely related to the coarse raw Si3N4 powder and low a-Si3N4 content.
[11]
[12] [13]
[14]
Acknowledgement [15]
This work was supported by National Key R&D Program of China (2017YFB0406200), the Youth Innovation Promotion Association CAS (No. 2019254), Science Foundation for Youth Scholar of State Key Laboratory of High Performance Ceramics and Superfine Microstructures (SKL201701), State Key Laboratory of New Ceramic and Fine Processing Tsinghua University (KF201806).
[16]
[17]
[18]
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The effect of annealing temperature on flexural strength, dielectric loss and thermal conductivity of Si3N4 ceramics Huanbei Chen a, 1, Weide Wang b, c, 1, Xing Yu d, Kai-hui Zuo b, Yongfeng Xia b, Jinwei Yin b, Hanqin Liang b, Dongxu Yao b, *, Yu-Ping Zeng b a
Nanjing Electronic Devices Institute, Nanjing, 210016, China State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China c University of Chinese Academy of Sciences, Beijing, 100049, China d Shanghai Radio Equipment Research Institute, Shanghai, 200090, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2019 Received in revised form 15 August 2019 Accepted 7 September 2019 Available online 8 September 2019
Effect of annealing temperatures on flexural strength, dielectric and thermal properties of Si3N4 ceramics was investigated with different amounts of Yb2O3 (3.5 mol% and 7 mol%) as sintering additives. High density of >99% were achieved after hot pressing. The post annealing heat treatment promoted the crystallization of the grain boundary glass phase. Low dielectric loss of <4 104 were obtained for all samples, a low dielectric loss of 1.8 104 was achieved by annealing at 1500 C for 24 h with 7 mol% Yb2O3 as sintering additives. A mild anisotropic microstructure was obtained due to merit of hot pressing method. High thermal conductivity of >75 W/(m$K) and >90 W/(m$K) were obtained in the direction parallel and perpendicular to the hot-pressing direction in all samples, respectively. The low flexural strength of ~600 MPa may attribute to the large grain in the matrix and lack of elongated abnormal grains. © 2019 Elsevier B.V. All rights reserved.
Keywords: Si3N4 ceramics Annealing Dielectric Thermal conductivity
1. Introduction Silicon nitride (Si3N4) ceramics with high strength, low dielectric loss and high thermal conductivity is a promising candidate for usage in structural components in the microwave devices, such as radio frequency windows for gyrotron and klystron applications [1e3]. Low dielectric constant (ε) and low dielectric loss (tan d) can reduce the dielectric absorption of microwave power, high thermal conductivity can accelerate heat transfer, both can contributed to avoiding an inhomogeneous temperature profiles and thermal stress of the products, thereby enhancing the stability of the device. High flexural strength can also guarantee a high thermal shock resistance according to the equation [4]: 0
R ¼
sf ð1 mÞ $f Ea
(1)
where sf is flexural strength, m is Poisson's ratio, E is elastic
* Corresponding author. Shanghai Institute of Ceramics, Chinese Academy of Sciences, Heshuo Road 585, Shanghai, 201899, PR China. E-mail address: [email protected] (D. Yao). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jallcom.2019.152203 0925-8388/© 2019 Elsevier B.V. All rights reserved.
modulus, a coefficient of thermal expansion, f is thermal diffusivity. High thermal conductivity Si3N4 ceramics has been widely researched and high thermal conductivity up to 177 W/m K has been achieved by sintered reaction bonded Si3N4 method with holding time of 60 h at 1900 C followed by annealing at a very slow rate (0.2 C/min), which might help reduce structural imperfections in b-Si3N4 grains and enhance devitrification of amorphous grain boundary phases [5]. In the case of low tan d of Si3N4 ceramics, it has not been fully investigated. H. Miyazaki and et al. [6,7] have carried out some researches on Yb2O3 based sintering additive followed by annealing, Si3N4 ceramics with high thermal conductivity of ~100 W/m$K and low tan d of 1.4 104 was achieved. This result of tan d is much lower than the conventional gas pressing sintered or reaction sintered Si3N4 (2e11 103), even lower than that of the CVD Si3N4 (3 104) [8], and equivalent to a commercial product Kyocera Si3N4 (SN287, 1.0 104@30e40 GHz). In their experiments, the heat treatments were carried out at 1300 C for 24 h due to the report that the devitrification of intergranular glass phase took place between 1250 C and 1450 C for 12e24 h [9]. While in the papers of Guo [10] and Liu [11], they have shown that different grain boundary phases, such as Yb4Si2O7N2, Yb2SiO5, and Yb2Si2O7, were detected under varied annealing temperature
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ranging from 1350 C to 1700 C with different content of Yb2O3. Since the grain boundary phase plays an important role in decrease the tan d of Si3N4 ceramics. It is worthwhile investigating the content of sintering additive and annealing temperatures on phase formation and the consequent variation of the comprehensive strength-thermal-dielectric properties. In this paper, two different amounts of Yb2O3 (3.5 mol% and 7 mol%) were used as sintering additives. The Si3N4 ceramics were prepared by hot pressing method, further annealing is performed at 1300e1700 C for about 24 h. The phase transformation of the grain boundary phases, flexural strength, dielectric properties and thermal conductivity were fully investigated.
2. Experimental procedure The starting powders used in this study were Si3N4 powder (1.2 mm, 4 N, a phase ~50%, Hebei Corefra Silicon Nitride Material Co., Ltd, China) and Yb2O3 (5.0 mm, purity99.99 wt%, Yuelong Company, China). Two types of composition, one is with 3.5 mol% of Yb2O3 (hereafter 3.5 Yb), and the other with 7 mol% of Yb2O3 (hereafter 7 Yb) were fabricated by the following procedure. The addition of 7 mol% Yb2O3 was proved to achieve low dielectric loss [6]. In those experiments, the starting powder used was a-Si3N4 powder (SN-E10, Ube Industries, Ltd., Japan), the oxygen content was 1.3e1.6 wt%, in this experiment, the oxygen content of the Si3N4 powder was 0.7e0.8 wt%, which is only half of the SN-E10 powder. So a similar Yb2O3/SiO2 in molar ratio as in Miyazaki’ research [6] were also prepared with Yb2O3 content of 3.5 mol%. The starting compositions were point in Fig. 1. The starting powders were mixed by planetary milling in ethanol for 3 h. The slurry was dried, and then passed through 100 mesh sieve. The mixed powders were hot-pressed in a graphite die with inner diameter of 70 mm at 1800 C under pressure of 30 MPa in a steady-flow N2 atmosphere for 2 h, the sample is labeled as HP. Post annealing of the as-sintered bodies were performed subsequently at 1300 C, 1500 C and 1700 C for 24 h using the same equipment, the sample are labeled as HP-A-1, HP-A-2, HP-A-3. One batch as-hot pressed samples were post heated at 1900 C for 4 h with gas pressure of 1 MPa and further annealed at 1700 C for 24 h, labeled as HP-A-4. X-ray diffraction (XRD, D8 discover, Bruker, Germany) was used to characterize the phase composition of the as-prepared and asannealed Si3N4 ceramics at the surfaces perpendicular to the hotpressing direction. The Lotgering orientation factor, f, is used to evaluate the degree of texture in the sintered samples. It can be expressed as follow [12]:
Fig. 1. Composition of Si3N4eYb2O3eSiO2 [14].
this
work's
specimens
in
the
ternary
system
of
f¼(PeP0)/(1-P0)
(2)
where P and P0 ¼ SI(hk0)/SI(hkl). SI(hk0) is the sum of peak intensities of the (hk0) and SI(hkl) is the sum of peak intensities of all the (hkl) on the surface perpendicular to the direction of hot pressing direction. The values of P are obtained from the sample and the values of P0 are obtained from the standard PDF card of b-Si3N4 (33e1160). If f ¼ 0, grains randomly align; if f ¼ 1, grain orientation is formed completely. The bending strength of the samples was measured by the three-point bending test with a span length of 30 mm and across head speed of 0.5 mm/min. The reported value was the average of five samples. Disk samples (4 60 2 mm) were machined for dielectric constant and dielectric loss test at 5 GHz by the perturbation method using a cavity resonator and a vector network analyzer (Keysight E5071C). Archimedes method was used to calculate the density of samples. Laser flash thermal analyzer (LFA467, Netzsch, Germany) was used to test the thermal diffusivity of samples. The thermal diffusivity parallel (PA) and perpendicular (PE) to the hot-pressing direction were used with samples of 4 12.6 2 mm and 4 25.4 0.5 mm, respectively. The thermal conductivity (k) was calculated according to the equation [13]:
k ¼ rCpa
(3)
where r, Cp and a are the bulk density, specific heat and thermal diffusivity, respectively. A constant value of specific heat, 0.68 J g1 K1, was used in this work to calculate the thermal conductivity at room temperature [13]. 3. Results and discussion 3.1. Phase formation of the hot pressed and annealed samples Fig. 2 shows the XRD patterns of the samples from TS direction. It can be seen that the major phase is b-Si3N4 and the minor phase is Yb based silicates. The b-Si3N4 peak intensities of (200) and (210) compared to (101) and (002) are very high. These results show that b-Si3N4 grains can be oriented under the hot pressing process. However, the calculated Lotgering orientation factor are in the range of 0.3e0.4 as listed in Table 1. This is relatively lower than the common hot pressed result (~0.7) with submicron Si3N4 powder as raw material [15]. In this experiment, the coarse Si3N4 powder may form larger b-Si3N4 grains which are much harder to orient under pressure than fine b-Si3N4 grains. The orientation will lead to a difference of thermal conductivity in different direction that will be discussed in the following section. A detailed XRD patterns of grain boundary phase at 27e33 are shown in Fig. 2 (c) and (d). For the 3.5 Yb samples, the main grain boundary phase was Yb2SiO5 for samples of HP, HP-A-1 and HP-A-2, and Yb4Si2O7N2 for samples of HP-A-3 and HP-A-4. In the hot pressing process, Yb2O3 formed Yb-Si-O-N glass phase with Si3N4 and its native SiO2 surface, densification of Si3N4 ceramics was promoted with the existence of Yb-Si-O-N glass phase and the applied pressure. Yb2SiO5 was precipitated from the Yb-Si-O-N glass phase during cooling. The formation of Yb4Si2O7N2 phase was may through the reaction of Yb2SiO5 with Si3N4 and the precipitation of the residue Yb-Si-O-N glass phase during annealing. Some researchers have shown that a designed Yb2O3/SiO2 in molar ratio may not necessarily lead to a desired precipitated phase assumed from the phase diagram as shown in Fig. 1 [16]. In this experiment, the Yb2O3/SiO2 in molar ratio was 1.18:1 and 2.45:1 for 3.5 Yb and 7 Yb samples, respectively. These composition were located in the range of Yb2SiO5 (1:1) and Yb4Si2O7N2 (4:1). The grain boundary phases were strongly influenced by the cooling as
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3
Fig. 2. XRD patterns of hot pressed and annealed samples from top surface with (a) 3.5 Yb, (b) 7 Yb and detailed grain boundary phase with (c) 3.5 Yb, (d) 7 Yb.
well as annealing program. For the 7 Yb samples, the main grain boundary phase was Yb4Si2O7N2 for all annealed samples. The strong peak of Yb4Si2O7N2 of sample HP-A-2 implies that the annealing can effectively promote the crystallization of the amorphous phase. The peaks are weakened for HP-A-3 and HP-A-4 samples compared with HP-A-2, this may indicate a decreasing in the content of grain boundary phase. The evidently decrease in density for HP-A-3 (3.51 g/cm3) and HP-A-4 (3.42 g/cm3) may also prove this conjecture, as listed in Table 1. 3.2. Microstructure of the hot pressed and annealed samples Based on the calculation by Archimedes method, the 3.5 Yb and
7 Yb samples reached densities of 3.37 g/cm3 and 3.56 g/cm3, and relative densities of 99.2% and 99.1%, respectively. The result indicated that Yb2O3 alone can effective achieving high densification of Si3N4 ceramics by hot pressing sintering, the SEM pictures of the Si3N4 ceramics also verify it, as shown in Fig. 3 (a)and Fig. 4 (a) The grain size is quite large, while the aspect ratio of the grains is low. The large grain size may attribute to the relatively larger size of the raw Si3N4 powder (1.2 mm) compared with common used UBE-10 powder (0.5 mm). The low aspect ratio may due to the low a content of the raw Si3N4 powder. As the sintering temperature is 1800 C for 2 h, the temperature is not high enough to promote Oswald ripening [17], so the phase transformation is governed by dissolution-precipitation process. a-Si3N4 particles would
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Table 1 Dielectric and thermal properties of the samples after hot pressing and annealing. Samples
Heat treatment
Lotgering orientation
Density (g/cm3)
Dielectric constant
Dielectric loss (104)
Heat capacity (mm2/s) PA/PE
Thermal conductivity (W/(m$K)) PA/PE
3.5 Yb
HP HP-A-1 HP-A-2 HP-A-3 HP-A-4 HP HP-A-1 HP-A-2 HP-A-3 HP-A-4
0.38 0.32 0.36 0.36 0.34 0.33 0.34 0.38 0.31 0.34
3.37 3.37 3.37 3.36 3.35 3.56 3.56 3.55 3.51 3.42
7.8 7.8 7.8 7.8 7.8 8.0 8.0 8.0 7.9 7.9
3.2 2.4 2.1 2.3 2.6 2.8 2.0 1.8 2.4 2.5
33.94/42.43 33.34/42.05 33.03/41.32 35.68/45.24 35.15/44.19 32.09/40.08 31.68/39.69 32.84/41.02 34.06/42.34 35.31/43.96
77.8/97.2 76.4/96.4 75.7/94.7 81.5/103.4 80.1/100.7 77.7/97 76.7/96.1 79.3/99.02 81.3/101.1 82.1/102.2
7 Yb
Fig. 3. SEM micrographs of polished surface perpendicular to the hot-pressing direction in (a)HP, (b) HP-A-1, (c) HP-A-2, (d) HP-A-3, (e) HP-A-4 of 3.5 Yb samples.
Fig. 4. SEM micrographs of polished surface perpendicular to the hot-pressing direction in (a)HP, (b) HP-A-1, (c) HP-A-2, (d) HP-A-3, (e) HP-A-4 of 7 Yb samples.
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dissolution in the Yb-Si-O-N glass phase, after reaching the saturation point, it would precipitation onto the b-Si3N4 nuclei and promote anisotropic grain growth. So in ideal condition, high aspect ratio b-Si3N4 grains can be obtained with high a-Si3N4 raw materials combined with free space for grain growth, such as porous Si3N4 ceramics prepared by freeze drying method [18]. But in this experiment, low a-Si3N4 raw materials and coarse raw Si3N4 materials limited the anisotropic grain growth. Beside the Si3N4 grains, the grain boundary phase (white) can clearly observe in the SEM pohtos, as show in Figs. 3 and 4 that indicated the grain boundary phase can definitely not ignorable on the high temperature strength [19e21], as well as dielectric and thermal properties. For 3.5 Yb samples, the grains show no much difference after annealing treatment. For 7 Yb samples, the grains are relatively fine and separated by the grain boundaries in HP sample, while some larger grains emerged in HP-A-2e4 samples. In considering the density have decreased to 3.51 g/cm3 and 3.42 g/cm3 for samples HP-A-3 and HP-A-4, respectively, and the weakening peak in XRD results. This can be explained as follow: In the case of high amount of sintering additive, such as 7 mol% Yb2O3, the migration of grain boundary and the Oswald ripening of the grains may “purge” the grain boundary phase to the surface during annealing at temperature >1700 C, that lead to a decrease in grain boundary phase as well as the density. 3.3. Dielectric properties of the hot pressed and annealed specimens Table 1 listed the dielectric constant and dielectric loss of the samples. A dielectric constant of 7.8e8 were obtained. The dielectric constant is not influenced by the annealing condition. Only in 7 Yb samples, the HP-A-3 and HP-A-4 samples show a slight decrease which may also connected to the decreased amount of grain boundary phase. The dielectric loss is strongly influenced by the annealing conditions in this experiment. Actually, the dielectric loss is primarily controlled by the sintering additives and the crystalline phase of the grain boundary. Miyazaki and et al. [7,9] have investigated different Yb2O3eSiO2 ratio on the dielectric loss of Si3N4 ceramics and have found out that the glassy phase have a great influence on the dielectric loss, a low dielectric of 1.4 104 can be achieved even without annealing treatment in the sample with 7Yb5Si and 7 Yb. In this experiment, dielectric loss <4 104 were obtained for all samples, a low dielectric loss of 1.8 104 in 7 Yb sample by annealing at 1500 C for 24 h. A Y2SiO5 single crystal was tested, the dielectric constant is 10.6, the dielectric loss is 8.5 105,which is much lower than Si3N4. It is reasonable to infer that Yb2SiO5 may also possess low dielectric loss due to the substation of Yb for Y, and that Yb4Si2O7N2 may also possess low dielectric loss since no much difference in dielectric loss with these two phases in different annealed samples. So the Si3N4 ceramics with these crystalline phases can achieve lower dielectric than the CVD Si3N4 (3 104).
were obtained in the hot pressed 3.5 Yb and 7 Yb samples, however, the post annealing only have slight improvement on thermal conductivity, even in the annealing condition of 1900 C-4h & 1700 C24 h. Many researchers have investigated a hot-pressing combined post sintering method to prepare high thermal conductivity Si3N4 ceramics [15,25], normally the thermal conductivity were increased by ~30% after post-sintering heat treatments. The main reason is related with strong grain growth after post-sintering. In this experiment, the grain size is not remarkable increased, so the increment of thermal conductivity is only 5% after post-sintering at 1900 C-4h and annealing at 1700 C-24 h. 3.5. Flexural strength of the hot pressed and annealed samples The flexural strength of hot pressed and annealed samples are presented in Fig. 5. Many researches have indicated that a bimodal grain microstructure can achieve high strength in Si3N4 ceramics [26,27]. Lee and et al. [28] have investigated the effect of starting Si3N4 powders with different a-to-b ratios on the microstructure evolution and the consequent influence on the flexural strength, they have found out that a distinctive bimodal microstructure occurred as a result of the abnormal grain growth with 50% a-Si3N4 as raw powder (specimen B), a flexural strength >900 MPa and thermal conductivity of 86W/m$K were obtained. While a broader range of grains with only monomial distribution was obtained with only 2~4 wt% a-Si3N4 as raw powder. In the meantime, the size of the large grains was smaller, whereas the size of the matrix grains was larger than that of specimen B, that resulting in lower flexural strength (<700 MPa). In this experiment, a ~50% a-Si3N4 content was used, but the microstructure is not distinctly bimodal, both large grain in the matrix and lack of elongated abnormal grains were resulting in lower flexural strength of ~600 MPa. It is quite rare to see a increasing in flexural strength with increased annealing temperature, the reason may related to the emerged elongated grains, as shown in Fig. 4 (d) and (e). 4. Conclusions The effect of annealing temperatures on flexural strength, dielectric and thermal properties of Si3N4 ceramics was investigated with different amounts of Yb2O3 (3.5 mol% and 7 mol%) as sintering additives. The XRD results indicated a mild anisotropic thermal conductivity was obtained due to merit of hot pressing method, the calculated Lotgering orientation factor are in the range of 0.3e0.4. Moreover, the post annealing heat treatment promoted the crystallization of the grain boundary glass phase. Low dielectric loss of <4 104 were obtained for all samples, a low dielectric loss of 1.8 104 by annealing at 1500 C for 24 h with 7 mol% Yb2O3 as
3.4. Thermal conductivities of the hot pressed and annealed samples Table 1 listed the heat capacity and thermal conductivity of the samples. Due to slight orientation of the samples, anisotropic thermal behavior was obtained. High thermal conductivity in Si3N4 ceramics with high anisotropic microstructure have been achieved by tape casting [22], extrusion [23], and magnetic field treatment [24] with b-Si3N4 seed,the thermal conductivity along the c-axis direction can be remarkably improved. The thermal conductivity is higher in the direction perpendicular to the hot-pressing direction (~100 W/(m$K)) than in the direction parallel to the hot-pressing direction (~80 W/(m$K)). Thermal conductivity of ~77 W/(m$K)
5
Fig. 5. Flexural strength of the hot pressed and annealed samples.
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sintering additives. High thermal conductivity >75 W/(m$K) and >90 W/(m$K) were obtained in the direction parallel and perpendicular to the hot-pressing direction in all samples, respectively, the annealing and post heat treatment at 1900 C did not lead to a tremendous increasing in thermal conductivity. The low flexural strength of ~600 MPa may attribute to the large grain in the matrix and lack of elongated abnormal grains that closely related to the coarse raw Si3N4 powder and low a-Si3N4 content.
[11]
[12] [13]
[14]
Acknowledgement [15]
This work was supported by National Key R&D Program of China (2017YFB0406200), the Youth Innovation Promotion Association CAS (No. 2019254), Science Foundation for Youth Scholar of State Key Laboratory of High Performance Ceramics and Superfine Microstructures (SKL201701), State Key Laboratory of New Ceramic and Fine Processing Tsinghua University (KF201806).
[16]
[17]
[18]
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