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Microwave sintering and kinetic analysis of Y2O3–MgO composites
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 10211–10215 www.elsevier.com/locate/ceramint
Microwave sintering and kinetic analysis of Y2O3–MgO composites Haibin Suna,b, Yujun Zhanga,b,n, Hongyu Gonga,b, Teng Lia,b, Qisong Lia,b a
Key Laboratory for Liquid–Solid Structural Evolution & Processing of Materials of Ministry of Education, Shandong University, Jinan 250061, PR China b Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, Shandong University, Jinan 250061, PR China Received 21 January 2014; received in revised form 25 February 2014; accepted 25 February 2014 Available online 7 March 2014
Abstract To obtain fine grained Y2O3–MgO composites for infrared application, the kinetics of Y2O3–MgO nanopowders ( 50 nm) during microwave sintering process was analyzed to track details of densification evolution. Finer structure and higher density were exhibited during microwave sintering process compared with conventional sintering process. The values of grain growth exponent n indicate that grain boundary diffusion is the main migration mechanism for microwave sintering when sintering temperature is below 1300 1C, while volume diffusion and grain boundary diffusion coexist at higher temperature (1400 1C). The calculated grain growth activation energy of microwave sintered samples (108.22 kJ/mol) is much lower than that of conventional sintered ones (160.42 kJ/mol), indicating that microwave sintering process can effectively promote the densification. Based on the kinetic analysis data, microwave sintering parameters were optimized, and Y2O3–MgO composites with an average grain size of 300 nm and Vickers hardness of 11.27 0.3 GPa were obtained. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Y2O3–MgO composites; Microwave sintering; Kinetic analysis; Densification behavior
1. Introduction Y2O3–MgO composites have recently acquired a high degree of research interest for infrared window materials due to the advantages of low cost and increased damage threshold compared with single-crystal infrared transparent materials. To obtain high infrared transmittance, a requirement for low optical scatter in a composite material is that grain size must be substantially smaller than the wavelength of light to reduce optical scatter to tolerable levels [1]. For instance, the Y2O3– MgO composites with grain size of 310 nm can exhibit a high infrared transmittance of 80%, which is very close to the theoretical value [2]. However, the sintering of nanopowders to full or nearly full density without appreciable grain growth continues to present a significant practical challenge [3]. In view of this, a sintering process which can effectively densify materials without inducing the grain growth is thus n
Corresponding author at: Key Laboratory for Liquid–Solid Structural Evolution & Processing of Materials of Ministry of Education, Shandong University, Jinan 250061, PR China. Tel./fax: þ 86 531 88399760. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.ceramint.2014.02.106 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
demanded [4]. Previously, many materials have been successfully sintered, and fine grain size, uniform microstructure and significant energy saving were achieved by microwave sintering [5–8]. As reported [9], the advantages of microwave sintering were found to include not only higher energy efficiency, higher post sintering density and lower sintering temperature, but also reduced activation energy compared with those of conventional sintering. No report on microwave sintering of Y2O3–MgO composites has been undertaken to this date. In this study, the kinetics of the microwave sintering process of Y2O3–MgO composites was analyzed to track the details of densification evolution. Conventional sintering process was also analyzed as a comparison. Furthermore, microwave sintering parameters were optimized, and submicron-grained Y2O3–MgO composites were achieved. 2. Experimental procedures The as-received raw material used in this study was 50:50 vol% Y2O3–MgO nanopowders, synthesized previously by sol–gel combustion technique using yttrium acetate and magnesium nitrate [10]. To improve the phase homogeneity, a
H. Sun et al. / Ceramics International 40 (2014) 10211–10215
ball-milling process for 12 h with ZrO2 ball media was applied to the fluffy powder. The milled powders were pressed into green compacts of 30 mm diameter by uniaxial pressing at 30 MPa, followed by isostatic cold pressing at 200 MPa. At this stage, samples had a green density of around 38% of the theoretical density. Y2O3–MgO composites were sintered by microwave sintering and conventional sintering process. Microwave sintering was realized in an automated microwave laboratory furnace (SLWSQF14X10-1800B). During Microwave sintering, two magnetrons were used with the operating frequency of 2.45 GHz750 MHz. In all cases, the samples were placed at the same position in the microwave furnace in order to avoid the influence of geometric factor. The sintering temperatures of samples were monitored with an thermocouple at the back of the furnace. For nonisothermal experiments, the compacted samples were heated to 800 1C, 900 1C, 1000 1C, 1100 1C, 1150 1C, 1200 1C, 1300 1C, 1350 1C, 1400 1C and 1500 1C, with a constant heating rate 10 1C/min, and the furnace was shut down without any holding when each temperature was reached. For isothermal experiments, the samples were held at 1100 1C, 1200 1C, 1300 1C and 1400 1C for 15 min, 30 min, 45 min, and 60 min, respectively. The densities of the sintered samples were determined based on Archimedes' principle and calculated using the theoretical Y2O3–MgO (50:50 vol%) density. Phase analysis was conducted in an X-ray diffraction technique (D/MAX-γA). Microstructural observation was carried out using a field emission high-resolution scanning electron microscope (JSM6380LA). The average particle size and grain size were measured from SEM graphs using the line-intercept method, and taking into account at least 100 grains. The Vickers hardness measurement was performed using a hardness tester (LECO DM-400FT) with 300 g load and a dwell time of 15 s. The hardness value is the average of 10 measurements.
3.1. Characterization of Y2O3–MgO nanopowder The results from X-ray diffraction (Fig. 1) indicated that the Y2O3–MgO nanopowder was a two phase material consisting
Relative Intensity
Cubic Y2O3 Cubic MgO
20
40
60
80
dm ~50 nm
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
10
20
30
40
50
60
70
80
90 100
Grain size (nm)
Fig. 2. (a) Low magnification SEM micrographs of Y2O3–MgO nanopowders, (b) high magnification SEM micrographs of Y2O3–MgO nanopowders and (c) particle size distributions of Y2O3–MgO nanopowders.
of cubic Y2O3 and cubic MgO phase. Most powders were agglomerated to 1.5 μm size (Fig. 2(a)) and the particle size was normally below 70 nm, with the average value being 50 nm (Fig. 2(b), (c)).
3. Results and discussion
0
0.40
Frenquency
10212
100
2 Theta (degree) Fig. 1. XRD pattern of Y2O3–MgO nanopowders.
3.2. Densification evolution Fig. 3 shows the change in the relative density of Y2O3– MgO composites upon heating from 800 1C to 1500 1C. Based on the typical “S” shape curve, the evolution of density during both microwave and conventional sintering can be both viewed as consisting of three stages: the initial, intermediate, and final stages. For microwave sintering, the initial stage of sintering refers to the slow densification at temperatures below 1150 1C, and corresponding density increases from 38% (green density) to 50%. The intermediate stage refers to the rapid densification between 1150 1C and 1400 1C, during which the majority of densification is completed and the density increased from 50% to 95%. The final stage of sintering involves the continued densification, with full or nearly full densification achieved during this stage. As a comparison, for a given temperature,
H. Sun et al. / Ceramics International 40 (2014) 10211–10215
Relative density (%)
100
10213
Microwave sintering Conventional sintering
90 80 70 60 50 40
800
1000
1200
1400
1600
Temperature (r&) Fig. 3. Densification behavior curves on microwave and conventional sintering.
0.0030
Convenrional sintering Microwave sintering
dρ / dT
0.0025
Tmax
0.0020 0.0015 0.0010
Tsep
0.0005
Fig. 5. (a) SEM micrographs of Y2O3–MgO composites sintered by microwave sintering and (b) SEM micrographs of Y2O3–MgO composites sintered by conventional sintering.
0.0000
800
900
1000 1100 1200 1300 1400 1500
Temperature (°C) Fig. 4. Densification rate on microwave and conventional sintering of Y2O3– MgO composites.
the densities of all the samples under conventional sintering are lower than those under microwave sintering. Fig. 4 shows the densification rate curves of Y2O3–MgO composites during the microwave and conventional sintering process. The densification rates between the conventional and the microwave sintering curves start to shift at 900 1C, and a higher densification rate is illustrated on microwave sintering process. The temperature corresponding to the maximum densification rate is labeled as Tmax, which is about 1275 1C for microwave sintering, much lower than that of conventional sintering, and the gap between them ΔTmax is about 80 1C. It can be concluded that, microwave sintering may promote the densification of Y2O3–MgO composites at a lower temperature compared with conventional sintering process. When samples are sintered at 1350 1C for 30 min, microwave sintered samples exhibit a finer microstructure with an average grain size of 200 nm (Fig. 5(a)), while markedly larger grain size and more pores are observed on conventional sintered ones (Fig. 5(b)). According to the above experimental results, microwave sintering process is observed to densify the Y2O3–MgO composites without inducing appreciable grain growth. 3.3. Kinetic analysis To understand the large difference between microwave and conventional sintering, sintering kinetics of Y2O3–MgO
composites is analyzed by isothermal experiments. The kinetics can be analyzed using Eq. (1), which is derived from linear shrinkage kinetics of two equal-size-sphere model [11] by translating linear shrinkage into density change, assuming the densification process is isotropic. ΔL 1 ð1Þ ¼ In AðTÞ þ In t In L0 n where A(T) is a constant related to sintering temperature, n is grain growth exponent. At each temperature T, the shrinkage rate (ΔL/L) and dwell time t are derived from experimental results, and exponent n could be determined by plotting Eq. (1). Based on conventional sintering models, the value of exponent n is 2.5 for volume diffusion and 3 for grain boundary diffusion [12], both being used for the following kinetic analysis. Fig. 6 plots ln(ΔL/L) versus ln(t) according to Eq. (1). Exponent n can be obtained by the slope of the sintering shrinkage dynamic line, and the calculated values under different sintering conditions are listed in Table 1. It is clear that, for microwave sintering, grain boundary diffusion is the main migration mechanism when sintering temperature is below 1300 1C because n4 3, while volume diffusion and grain boundary diffusion coexist at higher temperature (1400 1C) because 2.5 o no 3. However, on conventional sintering process, volume diffusion starts at a relatively lower temperature (1300 1C) because n is close to 2.5. That is to say, grain boundary diffusion is the main diffusion mechanism during microwave sintering densification process, while volume diffusion plays a main role in the conventional sintering process, which can be used to explain the reason
10214
H. Sun et al. / Ceramics International 40 (2014) 10211–10215 3.0
ln [ (LL) %]
2.6
ln(t / min)
1400 r& 1300 r& 1200 r& 1100 r&
2.8
2.4 2.2 2.0 1.8 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2
6.4
6.6
6.8
7.0
7.2
7.4
7.0
7.2
7.4
5.5
2.0 1.8 1.6
5.0
Q=160.42KJ/mol
4.5 4.0 3.5
1.4 1.2
6.2
6.0
ln(t / min)
ln [ (LL) %]
2.2
6.0
1/T (10-4 K-1)
1400 r& 1300 r& 1200 r& 1100 r&
2.4
Q=108.22 KJ/mol
5.8
ln (t / min) 2.6
3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6
3.0 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2
5.8
ln (t / min) Fig. 6. (a) Sintering kinetic lines of Y2O3–MgO composites during microwave sintering and (b) sintering kinetic lines of Y2O3–MgO composites during conventional sintering.
Table 1 Values of exponent n of Y2O3–MgO composites sintered under different sintering conditions.
Microwave sintering Conventional sintering
1100 1C
1200 1C
1300 1C
1400 1C
3.63 3.83
3.28 2.94
3.14 2.67
2.76 2.48
6.0
6.2
6.4
6.6
6.8
1/T
(10-4
K-1)
Fig. 7. (a) Relationship between ln t and (1/T) of Y2O3–MgO composites sintered by microwave sintering and (b) relationship between ln t and (1/T) of Y2O3–MgO composites sintered by conventional sintering.
explain the reason why microwave sintering can achieve full dense samples at a lower temperature compared with conventional sintering. Moreover, relatively low value of activation energy for microwave sintering further infers that grain boundary diffusion is the main diffusion mechanism of the densification process. 3.4. Preparation of submicron Y2O3–MgO composites
why microwave sintering is conducive to achieving a finer structure. The relationship between sintering temperature T and reaction rate coefficient K can be described by Eq. (2) [13,14], where Q is grain growth activation energy, R is molar gas constant with a value of 8.314 J/(mol K), B is a constant. K¼
1 ¼ B exp Q=ðRTÞ t
ð2Þ
Eq. (2) can also be described as follows: In t ¼
Q þ In B0 RT
ð3Þ
The activation energy Q can be determined by plotting ln(t) versus (1/T) from data of Fig. 6 while determinate ln(ΔL/L) as a fixed value, the results are shown in Fig. 7. The calculated activation energy for grain growth is 108.22 kJ/mol for the microwave sintered samples, which is much lower than that for conventional sintering (160.42 kJ/mol). It can be used to
To obtain fully dense samples without appreciable grain growth, microwave sintering process with two-step mentioned in Ref. [8] emerged as a useful technology. The key of this technology is to choose appropriate T1 (the temperature of first step) and T2 (the temperature of second step). T1 must be sufficient to achieve the critical density (at which the pores become thermo-dynamically unstable against shrinkage), and T2 must be lower but sufficient to yield high density without grain growth (at which the grain boundary diffusion can continue but volume diffusion should be inhibited) [15]. Based on the kinetic analysis data mentioned above, microwave sintering parameters are optimized. When samples are heated up to T1 (1380 1C) with a heating rate of 10 1C/min and retained for 2 min at the first step, and then cooled to T2 (1285 1C) and held for 55 min at the second step, near fully dense (99.4 T.D.%) Y2O3–MgO composites with an average grain size of 300 nm are achieved (Fig. 8). The grain size is much smaller than 0.5–1 μm of Y2O3–MgO composites
H. Sun et al. / Ceramics International 40 (2014) 10211–10215
10215
cooled to 1285 1C with a dwell time of 55 min at the second step. References
Fig. 8. SEM micrograph of Y2O3–MgO submicron composites sintered by microwave sintering process.
sintered by conventional sintering process mentioned in Ref. [10] due to the different diffusion mechanisms between microwave and conventional sintering process, and similar to 310 nm of Y2O3–MgO composites sintered by hot isostatic pressing process which is another useful technology to prepare fine grained composites mentioned in Ref. [2]. The Vickers hardness of the Y2O3–MgO composites is 11.2 70.3 GPa, which is significantly higher than 6.8 GPa of coarse grained single-phase MgO and 7–9 GPa of coarse grained single-phase Y2O3 [16,17].
4. Conclusions The microwave sintering and kinetic analysis of densification behavior of Y2O3–MgO composites were investigated. Compared with conventional sintering, microwave sintering process can effectively promote the densification behavior without inducing grain growth at a lower temperature. The values of grain growth exponent n for microwave sintering indicate that grain boundary diffusion is the main diffusion mechanism during microwave sintering densification process, while volume diffusion plays a main role in the conventional sintering process. We also found that the calculated grain growth activation energy for microwave sintering (108.22 kJ/mol) is much lower than that for conventional sintering (160.42 kJ/mol). Based on the kinetic analysis data, submicron-grained ( 300 nm) Y2O3–MgO composites with Vickers hardness of 11.2 7 0.3 GPa were achieved by microwave sintering process, when samples are heated up to 1380 1C with a dwell time of 2 min at the first step, and then
[1] D.C. Harris, L.R. Cambrea, L.F. Johnson, R.T. Seaver, M. Baronowski, R. Gentilman, et al., Properties of an infrared-transparent MgO:Y2O3 nanocomposite, J. Am. Ceram. Soc. (2013) 1–8. [2] J.W. Wang, D.Y. Chen, E.H. Jordan, M. Gell, Infrared-transparent Y2O3– MgO nanocomposites using sol–gel combustion synthesized powder, J. Am. Ceram. Soc. 93 (11) (2010) 3535–3538. [3] U.A. Tamburini, J.E. Garay, Z.A. Munir, Fast low-temperature consolidation of bulk nanometric ceramic materials, Scr. Mater. 54 (2006) 823–828. [4] C.S. Chen, P.Y. Chen, C.C. Chou, C.S. Chen, Microwave sintering and grain growth behavior of nano-grained BaTiO3 materials, Ceram. Int. S38 (2012) S117–S120. [5] C.S. Chen, C.C. Chou, W.C. Yang, I.N. Lin, Microwave sintering of basemetal-electrode BaTiO3 capacitor materials co-doped with MgO/ Y2O3 additives, J. Electroceram. 13 (2004) 573–577. [6] D.L. Johnson, Microwave heating of grain boundaries in ceramics, J. Am. Ceram. Soc. 74 (1991) 849–850. [7] B. Meng, J. Booske, R. Cooper, S. Freeman, Microwave absorption in NaCl crystals with various controlled defect conditions, Mater. Res. Soc. Symp. Proc. 347 (1994) 467–472. [8] D.J. Veljovic, E. Palcevskis, I. Zalite, R. Petrovic, D.J. Janackovic, Twostep microwave sintering – a promising technique for the processing of nanostructured bioceramics, Mater. Lett. 93 (2013) 251–253. [9] J.D. Katz, Microwave sintering of ceramics, Annu. Rev. Mater. Sci. 22 (1992) 153–170. [10] C.K. Muoto, E.H. Jordan, M. Gell, M. Aindow, Effects of precursor chemistry on the structural characteristics of Y2O3–MgO nanocomposites synthesized by a combined sol–gel/thermal decomposition route, J. Am. Ceram. Soc. 94 (2) (2011) 372–381. [11] W.D. Kingery, M. Berg, Study of the initial stages of sintering solids by viscous flow, evaporation–condensation, and self-diffusion, J. Appl. Phys. 26 (1955) 1205–1212. [12] R.G. Lange, Z.A. Munir, Sintering kinetics of pure and doped boron carbide, Mater. Sci. Res. Int. 13 (1989) 311–321. [13] K. Matsuiw, A. Matsumoto, M. Uehara, N. Enomoto, J. Hojo, Sintering kinetics at isothermal shrinkage: effect of specific surface area on the initial sintering stage of fine zirconia powder, J. Am. Ceram. Soc. 90 (2007) 44–49. [14] L.P. Maqueda, J.M. Criado, C. Real, Kinetics of the initial stage of sintering from shrinkage data: simultaneous determination of activation energy and kinetic model from a single nonisothermal experiment, J. Am. Ceram. Soc. 85 (2002) 763–768. [15] I.W. Chen, X.H. Wang, Sintering dense nanocrystalline ceramics without final-stage grain growth, Nature 404 (2000) 168–171. [16] D.Y. Chen, E.H. Jordan, M. Gell, Pressureless sintering of translucent MgO ceramics, Scr. Mater. 59 (7) (2008) 757–759. [17] R.W. Rice, C.C. Wu, F. Boichelt, Hardness–grain-size relations in ceramics, J. Am. Ceram. Soc. 77 (10) (1994) 2539–2553.
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 10211–10215 www.elsevier.com/locate/ceramint
Microwave sintering and kinetic analysis of Y2O3–MgO composites Haibin Suna,b, Yujun Zhanga,b,n, Hongyu Gonga,b, Teng Lia,b, Qisong Lia,b a
Key Laboratory for Liquid–Solid Structural Evolution & Processing of Materials of Ministry of Education, Shandong University, Jinan 250061, PR China b Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, Shandong University, Jinan 250061, PR China Received 21 January 2014; received in revised form 25 February 2014; accepted 25 February 2014 Available online 7 March 2014
Abstract To obtain fine grained Y2O3–MgO composites for infrared application, the kinetics of Y2O3–MgO nanopowders ( 50 nm) during microwave sintering process was analyzed to track details of densification evolution. Finer structure and higher density were exhibited during microwave sintering process compared with conventional sintering process. The values of grain growth exponent n indicate that grain boundary diffusion is the main migration mechanism for microwave sintering when sintering temperature is below 1300 1C, while volume diffusion and grain boundary diffusion coexist at higher temperature (1400 1C). The calculated grain growth activation energy of microwave sintered samples (108.22 kJ/mol) is much lower than that of conventional sintered ones (160.42 kJ/mol), indicating that microwave sintering process can effectively promote the densification. Based on the kinetic analysis data, microwave sintering parameters were optimized, and Y2O3–MgO composites with an average grain size of 300 nm and Vickers hardness of 11.27 0.3 GPa were obtained. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Y2O3–MgO composites; Microwave sintering; Kinetic analysis; Densification behavior
1. Introduction Y2O3–MgO composites have recently acquired a high degree of research interest for infrared window materials due to the advantages of low cost and increased damage threshold compared with single-crystal infrared transparent materials. To obtain high infrared transmittance, a requirement for low optical scatter in a composite material is that grain size must be substantially smaller than the wavelength of light to reduce optical scatter to tolerable levels [1]. For instance, the Y2O3– MgO composites with grain size of 310 nm can exhibit a high infrared transmittance of 80%, which is very close to the theoretical value [2]. However, the sintering of nanopowders to full or nearly full density without appreciable grain growth continues to present a significant practical challenge [3]. In view of this, a sintering process which can effectively densify materials without inducing the grain growth is thus n
Corresponding author at: Key Laboratory for Liquid–Solid Structural Evolution & Processing of Materials of Ministry of Education, Shandong University, Jinan 250061, PR China. Tel./fax: þ 86 531 88399760. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.ceramint.2014.02.106 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
demanded [4]. Previously, many materials have been successfully sintered, and fine grain size, uniform microstructure and significant energy saving were achieved by microwave sintering [5–8]. As reported [9], the advantages of microwave sintering were found to include not only higher energy efficiency, higher post sintering density and lower sintering temperature, but also reduced activation energy compared with those of conventional sintering. No report on microwave sintering of Y2O3–MgO composites has been undertaken to this date. In this study, the kinetics of the microwave sintering process of Y2O3–MgO composites was analyzed to track the details of densification evolution. Conventional sintering process was also analyzed as a comparison. Furthermore, microwave sintering parameters were optimized, and submicron-grained Y2O3–MgO composites were achieved. 2. Experimental procedures The as-received raw material used in this study was 50:50 vol% Y2O3–MgO nanopowders, synthesized previously by sol–gel combustion technique using yttrium acetate and magnesium nitrate [10]. To improve the phase homogeneity, a
H. Sun et al. / Ceramics International 40 (2014) 10211–10215
ball-milling process for 12 h with ZrO2 ball media was applied to the fluffy powder. The milled powders were pressed into green compacts of 30 mm diameter by uniaxial pressing at 30 MPa, followed by isostatic cold pressing at 200 MPa. At this stage, samples had a green density of around 38% of the theoretical density. Y2O3–MgO composites were sintered by microwave sintering and conventional sintering process. Microwave sintering was realized in an automated microwave laboratory furnace (SLWSQF14X10-1800B). During Microwave sintering, two magnetrons were used with the operating frequency of 2.45 GHz750 MHz. In all cases, the samples were placed at the same position in the microwave furnace in order to avoid the influence of geometric factor. The sintering temperatures of samples were monitored with an thermocouple at the back of the furnace. For nonisothermal experiments, the compacted samples were heated to 800 1C, 900 1C, 1000 1C, 1100 1C, 1150 1C, 1200 1C, 1300 1C, 1350 1C, 1400 1C and 1500 1C, with a constant heating rate 10 1C/min, and the furnace was shut down without any holding when each temperature was reached. For isothermal experiments, the samples were held at 1100 1C, 1200 1C, 1300 1C and 1400 1C for 15 min, 30 min, 45 min, and 60 min, respectively. The densities of the sintered samples were determined based on Archimedes' principle and calculated using the theoretical Y2O3–MgO (50:50 vol%) density. Phase analysis was conducted in an X-ray diffraction technique (D/MAX-γA). Microstructural observation was carried out using a field emission high-resolution scanning electron microscope (JSM6380LA). The average particle size and grain size were measured from SEM graphs using the line-intercept method, and taking into account at least 100 grains. The Vickers hardness measurement was performed using a hardness tester (LECO DM-400FT) with 300 g load and a dwell time of 15 s. The hardness value is the average of 10 measurements.
3.1. Characterization of Y2O3–MgO nanopowder The results from X-ray diffraction (Fig. 1) indicated that the Y2O3–MgO nanopowder was a two phase material consisting
Relative Intensity
Cubic Y2O3 Cubic MgO
20
40
60
80
dm ~50 nm
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
10
20
30
40
50
60
70
80
90 100
Grain size (nm)
Fig. 2. (a) Low magnification SEM micrographs of Y2O3–MgO nanopowders, (b) high magnification SEM micrographs of Y2O3–MgO nanopowders and (c) particle size distributions of Y2O3–MgO nanopowders.
of cubic Y2O3 and cubic MgO phase. Most powders were agglomerated to 1.5 μm size (Fig. 2(a)) and the particle size was normally below 70 nm, with the average value being 50 nm (Fig. 2(b), (c)).
3. Results and discussion
0
0.40
Frenquency
10212
100
2 Theta (degree) Fig. 1. XRD pattern of Y2O3–MgO nanopowders.
3.2. Densification evolution Fig. 3 shows the change in the relative density of Y2O3– MgO composites upon heating from 800 1C to 1500 1C. Based on the typical “S” shape curve, the evolution of density during both microwave and conventional sintering can be both viewed as consisting of three stages: the initial, intermediate, and final stages. For microwave sintering, the initial stage of sintering refers to the slow densification at temperatures below 1150 1C, and corresponding density increases from 38% (green density) to 50%. The intermediate stage refers to the rapid densification between 1150 1C and 1400 1C, during which the majority of densification is completed and the density increased from 50% to 95%. The final stage of sintering involves the continued densification, with full or nearly full densification achieved during this stage. As a comparison, for a given temperature,
H. Sun et al. / Ceramics International 40 (2014) 10211–10215
Relative density (%)
100
10213
Microwave sintering Conventional sintering
90 80 70 60 50 40
800
1000
1200
1400
1600
Temperature (r&) Fig. 3. Densification behavior curves on microwave and conventional sintering.
0.0030
Convenrional sintering Microwave sintering
dρ / dT
0.0025
Tmax
0.0020 0.0015 0.0010
Tsep
0.0005
Fig. 5. (a) SEM micrographs of Y2O3–MgO composites sintered by microwave sintering and (b) SEM micrographs of Y2O3–MgO composites sintered by conventional sintering.
0.0000
800
900
1000 1100 1200 1300 1400 1500
Temperature (°C) Fig. 4. Densification rate on microwave and conventional sintering of Y2O3– MgO composites.
the densities of all the samples under conventional sintering are lower than those under microwave sintering. Fig. 4 shows the densification rate curves of Y2O3–MgO composites during the microwave and conventional sintering process. The densification rates between the conventional and the microwave sintering curves start to shift at 900 1C, and a higher densification rate is illustrated on microwave sintering process. The temperature corresponding to the maximum densification rate is labeled as Tmax, which is about 1275 1C for microwave sintering, much lower than that of conventional sintering, and the gap between them ΔTmax is about 80 1C. It can be concluded that, microwave sintering may promote the densification of Y2O3–MgO composites at a lower temperature compared with conventional sintering process. When samples are sintered at 1350 1C for 30 min, microwave sintered samples exhibit a finer microstructure with an average grain size of 200 nm (Fig. 5(a)), while markedly larger grain size and more pores are observed on conventional sintered ones (Fig. 5(b)). According to the above experimental results, microwave sintering process is observed to densify the Y2O3–MgO composites without inducing appreciable grain growth. 3.3. Kinetic analysis To understand the large difference between microwave and conventional sintering, sintering kinetics of Y2O3–MgO
composites is analyzed by isothermal experiments. The kinetics can be analyzed using Eq. (1), which is derived from linear shrinkage kinetics of two equal-size-sphere model [11] by translating linear shrinkage into density change, assuming the densification process is isotropic. ΔL 1 ð1Þ ¼ In AðTÞ þ In t In L0 n where A(T) is a constant related to sintering temperature, n is grain growth exponent. At each temperature T, the shrinkage rate (ΔL/L) and dwell time t are derived from experimental results, and exponent n could be determined by plotting Eq. (1). Based on conventional sintering models, the value of exponent n is 2.5 for volume diffusion and 3 for grain boundary diffusion [12], both being used for the following kinetic analysis. Fig. 6 plots ln(ΔL/L) versus ln(t) according to Eq. (1). Exponent n can be obtained by the slope of the sintering shrinkage dynamic line, and the calculated values under different sintering conditions are listed in Table 1. It is clear that, for microwave sintering, grain boundary diffusion is the main migration mechanism when sintering temperature is below 1300 1C because n4 3, while volume diffusion and grain boundary diffusion coexist at higher temperature (1400 1C) because 2.5 o no 3. However, on conventional sintering process, volume diffusion starts at a relatively lower temperature (1300 1C) because n is close to 2.5. That is to say, grain boundary diffusion is the main diffusion mechanism during microwave sintering densification process, while volume diffusion plays a main role in the conventional sintering process, which can be used to explain the reason
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H. Sun et al. / Ceramics International 40 (2014) 10211–10215 3.0
ln [ (LL) %]
2.6
ln(t / min)
1400 r& 1300 r& 1200 r& 1100 r&
2.8
2.4 2.2 2.0 1.8 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2
6.4
6.6
6.8
7.0
7.2
7.4
7.0
7.2
7.4
5.5
2.0 1.8 1.6
5.0
Q=160.42KJ/mol
4.5 4.0 3.5
1.4 1.2
6.2
6.0
ln(t / min)
ln [ (LL) %]
2.2
6.0
1/T (10-4 K-1)
1400 r& 1300 r& 1200 r& 1100 r&
2.4
Q=108.22 KJ/mol
5.8
ln (t / min) 2.6
3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6
3.0 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2
5.8
ln (t / min) Fig. 6. (a) Sintering kinetic lines of Y2O3–MgO composites during microwave sintering and (b) sintering kinetic lines of Y2O3–MgO composites during conventional sintering.
Table 1 Values of exponent n of Y2O3–MgO composites sintered under different sintering conditions.
Microwave sintering Conventional sintering
1100 1C
1200 1C
1300 1C
1400 1C
3.63 3.83
3.28 2.94
3.14 2.67
2.76 2.48
6.0
6.2
6.4
6.6
6.8
1/T
(10-4
K-1)
Fig. 7. (a) Relationship between ln t and (1/T) of Y2O3–MgO composites sintered by microwave sintering and (b) relationship between ln t and (1/T) of Y2O3–MgO composites sintered by conventional sintering.
explain the reason why microwave sintering can achieve full dense samples at a lower temperature compared with conventional sintering. Moreover, relatively low value of activation energy for microwave sintering further infers that grain boundary diffusion is the main diffusion mechanism of the densification process. 3.4. Preparation of submicron Y2O3–MgO composites
why microwave sintering is conducive to achieving a finer structure. The relationship between sintering temperature T and reaction rate coefficient K can be described by Eq. (2) [13,14], where Q is grain growth activation energy, R is molar gas constant with a value of 8.314 J/(mol K), B is a constant. K¼
1 ¼ B exp Q=ðRTÞ t
ð2Þ
Eq. (2) can also be described as follows: In t ¼
Q þ In B0 RT
ð3Þ
The activation energy Q can be determined by plotting ln(t) versus (1/T) from data of Fig. 6 while determinate ln(ΔL/L) as a fixed value, the results are shown in Fig. 7. The calculated activation energy for grain growth is 108.22 kJ/mol for the microwave sintered samples, which is much lower than that for conventional sintering (160.42 kJ/mol). It can be used to
To obtain fully dense samples without appreciable grain growth, microwave sintering process with two-step mentioned in Ref. [8] emerged as a useful technology. The key of this technology is to choose appropriate T1 (the temperature of first step) and T2 (the temperature of second step). T1 must be sufficient to achieve the critical density (at which the pores become thermo-dynamically unstable against shrinkage), and T2 must be lower but sufficient to yield high density without grain growth (at which the grain boundary diffusion can continue but volume diffusion should be inhibited) [15]. Based on the kinetic analysis data mentioned above, microwave sintering parameters are optimized. When samples are heated up to T1 (1380 1C) with a heating rate of 10 1C/min and retained for 2 min at the first step, and then cooled to T2 (1285 1C) and held for 55 min at the second step, near fully dense (99.4 T.D.%) Y2O3–MgO composites with an average grain size of 300 nm are achieved (Fig. 8). The grain size is much smaller than 0.5–1 μm of Y2O3–MgO composites
H. Sun et al. / Ceramics International 40 (2014) 10211–10215
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cooled to 1285 1C with a dwell time of 55 min at the second step. References
Fig. 8. SEM micrograph of Y2O3–MgO submicron composites sintered by microwave sintering process.
sintered by conventional sintering process mentioned in Ref. [10] due to the different diffusion mechanisms between microwave and conventional sintering process, and similar to 310 nm of Y2O3–MgO composites sintered by hot isostatic pressing process which is another useful technology to prepare fine grained composites mentioned in Ref. [2]. The Vickers hardness of the Y2O3–MgO composites is 11.2 70.3 GPa, which is significantly higher than 6.8 GPa of coarse grained single-phase MgO and 7–9 GPa of coarse grained single-phase Y2O3 [16,17].
4. Conclusions The microwave sintering and kinetic analysis of densification behavior of Y2O3–MgO composites were investigated. Compared with conventional sintering, microwave sintering process can effectively promote the densification behavior without inducing grain growth at a lower temperature. The values of grain growth exponent n for microwave sintering indicate that grain boundary diffusion is the main diffusion mechanism during microwave sintering densification process, while volume diffusion plays a main role in the conventional sintering process. We also found that the calculated grain growth activation energy for microwave sintering (108.22 kJ/mol) is much lower than that for conventional sintering (160.42 kJ/mol). Based on the kinetic analysis data, submicron-grained ( 300 nm) Y2O3–MgO composites with Vickers hardness of 11.2 7 0.3 GPa were achieved by microwave sintering process, when samples are heated up to 1380 1C with a dwell time of 2 min at the first step, and then
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