- Email: [email protected]
Spark plasma sintering of fine Mg2Si particles
Powder Technology 228 (2012) 295–300
Contents lists available at SciVerse ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Spark plasma sintering of fine Mg2Si particles Etienne Savary ⁎, Franck Gascoin, Sylvain Marinel, Romain Heuguet Laboratoire CRISMAT UMR CNRS 6508, 6 Bd Maréchal Juin, 14050 CAEN Cedex 4, France
a r t i c l e
i n f o
Article history: Received 11 January 2012 Received in revised form 4 April 2012 Accepted 12 May 2012 Available online 19 May 2012 Keywords: Microwave synthesis Mg2Si Thermoelectricity Spark Plasma Sintering Nanoparticles Ball milling
a b s t r a c t Nanostructuration is an efficient way to improve thermoelectric properties. In this paper, the densification of micro and nanocrystalline Mg2Si is studied using Spark Plasma Sintering process. It seems very problematic to successfully sinter Mg2Si nanoparticles because the nano-size promotes oxidation at grain boundaries and prevent obtaining high green densities of the compacts. The in situ synthesis of Mg2Si under reductive atmosphere, during SPS process, has not led to dense samples, demonstrating that the grain size is the key factor in densifying magnesium silicide powders. Moreover, the sintering of micronic Mg2Si grains has been successfully carried out confirming this assumption. Our results show that mixing powders made of small and large particles give access to 95% dense composite materials with a large amount of fine grains homogeneously distributed in the large grain matrix. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectricity is a very promising way to reduce fuel consumption and CO2 emissions through the conversion of waste heat into electric energy. However, the high cost of the materials and the relatively low efficiency of the thermoelectric generators limit their large scale development. The performance of a thermoelectric material is quantified by its dimensionless figure of merit ZT which is equal to α 2T/ρκ, where α represents the Seebeck coefficient, T is the absolute temperature, ρ is the electrical resistivity and κ is the thermal conductivity [1]. Furthermore, κ is the sum of two contributions: one is due to the phonons, labeled κlat and one is due to the charge carrier, labeled κel. It has been often reported that κlat can be significantly decreased by the multiplication of grain boundaries, enhancing the phonon scattering at the interfaces and consequently increasing ZT [2]. Thus, many efforts have been devoted in past years to the elaboration of nanostructured bulk materials such as Bi2Te3 [3–5], PbTe [4,6] or SiGe [7,8]. Magnesium silicide is now considered as a very good candidate for thermoelectric applications. Indeed, ZT values higher than unity have already been reported for n-type Mg2Si [9–15]. Moreover, this material is made of widely abundant, cheap, light, and non-toxic elements; all criteria that fit with large scale production requirements. Different techniques are used to synthesize Mg2Si phase such as mechanical alloying [16–18], conventional high temperature reaction [19–22], Spark Plasma Sintering (SPS) [22,23] or microwave irradiation [24] of the metallic precursors.
⁎ Corresponding author. Tel.: + 33 2 31 45 13 69; fax: + 33 2 31 45 13 09. E-mail address: [email protected] (E. Savary). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.05.036
Unfortunately, independent of the method used, it remains very difficult to produce nanosized powders. In a recent paper, we reported the possibility to synthesize nanocrystalline Mg2Si by a very fast microwave irradiation of the ground metallic precursors [25]. The next step is now to elaborate dense bulk materials with fine grains to improve the thermoelectric properties of the Mg2Si compounds. For the sake of retaining the “nano” size of the powders, the SPS densification, due to the short processing time necessary, appears to be very well suited for such a task. In this contribution we report on the SPS processing of powders with different grain sizes to clarify the influence of the granulometry on the densification of Mg2Si compounds. 2. Experimental part 2.1. Powders processing Silicon lumps (Alfa Aesar 99.9999%) and magnesium turnings (Alfa Aesar Puratronic 99.98%) were ground under nitrogen in a high energy ball mill (Fritsch Pulverisette 7 Premium Line). The elemental precursors were placed in a 20 ml air-proof tungsten carbide bowl containing six 10 mm diameter tungsten carbide balls. All the manipulations were performed in a glove box to prevent any reaction with air and moisture. The grinding cycles were composed of 2 periods of 2, 10 or 30 min at a speed of 450 rounds per minute with a pause of 1 min in between. After completion of the milling program, the resulting powder was cold pressed under 3 tons in glove box to obtain 500 mg pucks. Each puck was subsequently inserted in a 12 mm outside diameter fused silica tube equipped with a valve so that the microwave heating of the puck could be performed under nitrogen.
296
E. Savary et al. / Powder Technology 228 (2012) 295–300
Fig. 1. SEM micrograph of Mg2Si particles synthesized by microwave.
Fig. 3. SEM micrographs of Si particles after 2 h of ball milling.
to think that the size of the Mg2Si grains after synthesis is mainly directed by the size of the Si particles.
2.2. Microwave synthesis of the Mg2Si particles The microwave furnace consists of a microwave generator (2.45 GHz Sairem GMP20KSM) delivering a variable power of up to 2000 W. The microwave radiation passes through a rectangular waveguide (WR340) ended by a TE10p cavity. A coupling iris and a short circuit piston allow tuning the cavity length to excite two different modes of resonance called TE102 and TE103. When the cavity is excited within the TE102 mode the sample is located in a maximum of magnetic field whereas in the TE103 mode it is located in a maximum of electric field (for more details see [25]). 2.3. Granulometry of the powders A laser granulometer (Malvern Mastersizer 2000 — Dispersion unit: Malvern Hydro 2000 S) was used to estimate the particle size distribution. This method is based on the measurement of the light diffusion by particles dispersed in a liquid medium. Concerning the powders obtained after the milling process, the sharp difference in mechanical properties between the hard and easy to crush silicon and the soft magnesium leads to a mixture composed of Si particles embedded in a magnesium “mush” preventing granulometric measurements. However, the magnesium can be dissolved in diluted nitric acid solution allowing the measure of the granulometry of the Si particles. A pH = 9 aqueous solution has been found to be an appropriate liquid medium for dispersing silicon particles. Considering the Mg2Si powders after microwave irradiation, the high sensitivity of Mg2Si powders towards water prevents measurements in aqueous solution. Moreover, the dispersion of these particles in any liquid medium remains problematic. Nevertheless considering the respective mechanical properties of Si and Mg, it sounds reasonable
2.4. SPS processing The sintering processes were carried out under vacuum in a Spark Plasma Sintering apparatus (FCT HP D 25/1). For each sample between 2 g and 3 g of powder were poured in 15 mm or 20 mm diameter graphite dies. The temperature and the pressure are raised simultaneously to their maximum values. The maximum temperature used was 775 °C since at higher temperature the graphite foil insulating the pistons from the silicide starts to react with the latter. The maximum pressure was 50 MPa, limit that can sustain the graphite dies utilized. In some cases, a plateau at 450 °C was also necessary in order to prevent the evaporation of elemental magnesium. 2.5. Characterization The sizes of the particles before and after sintering were determined with a scanning electron microscopy (Zeiss Supra 55 operated at 5 kV) using the Mendelson's method [26]. The chemical composition of the different phases were analysed by energy dispersion spectroscopy (EDAX-EDS). 3. Results and discussion 3.1. SPS of powders synthesized by microwave As described elsewhere [25], microwave process allows producing in a very short time (4 periods of 30 min of grinding and merely 2 min of microwave irradiation) cauliflower-like aggregates of Mg2Si nanoparticles (Fig. 1). The size of these particles is smaller than 100 nm
Fig. 2. SEM micrographs of Mg2Si pellets after SPS treatment.
E. Savary et al. / Powder Technology 228 (2012) 295–300
297
Fig. 6. Granulometry of silicon particles for different grinding times.
Fig. 4. Pressure evolution and piston displacement profiles during the SPS process of MgH2 + Si mixture.
and the grain size distribution is very homogeneous. The crucial point is now to elaborate dense Mg2Si bulk while limiting the grain growth. In this aim, the SPS method seems to be interesting because of the high heating rates and the short processing times usually associated with this process. Different SPS programs have been tested but whatever the dwell temperature and the pressure applied (though limited by the graphite dies), it has not been possible to get a dense and cohesive pellet. In all cases, SEM analysis revealed the presence of MgO phase at the grain boundaries (Fig. 2) which was confirmed by X-ray diffraction analysis. At this stage, two assumptions can be advanced to explain the difficulty to densify these powders. First, the MgO phase at the grain boundary could limit the solid state diffusion at the interfaces which is the main driving force for the sintering. Secondly, the nanosize of the Mg2Si particles and the narrow granulometric distribution could reduce the defect gradients which promote this solid state diffusion. A better understanding of the respective contributions of these assumptions could be achieved by avoiding the oxidation of the magnesium. This oxidation can be due to either the presence of MgO in the commercial Mg turnings used or to an accidental oxidation during the different steps of the elaboration process. To partially clarify this incertitude, a reactive SPS heating of a mixture of MgH2 and Si based on a Schmidt et al. patent [23] has been performed to avoid the use of possibly oxidized Mg turnings and to strengthen the reductive conditions of SPS process. 3.2. Reactive SPS process from magnesium hydride The size of the silicon particles obtained after the 2 × 30 min ball milling process is between 50 and 400 nm (Fig. 3) which is comparable with the granulometry of the magnesium silicide after microwave
synthesis. In the SPS set up, the thermal decomposition of MgH2 occurs at about 410 °C, dihydrogen gas is thus released and elemental magnesium “produced”. Thus, Mg and Si precursors can react to form in situ the Mg2Si phase. One can see at 410 °C an important increase of the pressure inside the reactor which corresponds to the H2 emanation (Fig. 4). At the same time, a huge displacement of the piston is also evidenced (Fig. 4). The H2 production increases the reducing character of the atmosphere during the SPS process. Together with the fact that the magnesium produced should be oxide-free, this reduces drastically the amount of MgO phase as shown on the SEM micrograph (Fig. 5). Moreover, it can be seen that the grain size is between 1 and 4 μm which is similar to the values obtained after the SPS process of the powders synthesized by microwave. However, although the diminution of the MgO phase has been achieved, it has not been possible to successfully produce dense and cohesive pellets. Considering these results, it can be assumed that the difficulty to sinter the Mg2Si is strongly correlated to the small size of the silicon particles. A granulometric study is required for a better understanding of the silicon particle size influence on the densification process. 3.3. Granulometry of the powders a) Silicon and magnesium mixture As explained before, it is not possible to realize any granulometric measurement on Mg2Si powder after the grinding process. Thus the silicon particle size is measured as a function of the grinding time. The granulometric curves are represented by Fig. 6 and two SEM micrographs are shown Fig. 7. First of all, as expected, the longer the grinding time the smaller the particles. The mode of the equivalent spherical diameter (ESD) varies from 3 μm for a grinding time of 2 × 2 min to about 1 μm for a grinding time of 2 × 30 min. Moreover, it can be noticed that the particle size reaches a limit after 1 h of grinding. Thus, the mixture processed
Fig. 5. SEM micrographs of Mg2Si processed by SPS from MgH2 and Si mixture.
298
E. Savary et al. / Powder Technology 228 (2012) 295–300
Fig. 7. SEM micrographs of Si particles after (a) 2 × 10 min; (b) 2 × 30 min of grinding.
Fig. 8. SEM micrographs of (a) Si particles before the microwave irradiation; (b) Mg2Si particles after the microwave irradiation for a grinding time of 2 × 2 min.
during 2 h shows the same evolution than that milled during 1 h. This can be explained by the physical limits of the equipment used such as the size of the balls or the rotation speed and the detection limit of the granulometer. The formation of agglomerates has also to be considered for the longest grinding times leading to the smallest particles. b) Mg2Si after microwave processing The powders previously ground are heated by microwave to synthesize the Mg2Si phase. The granulometry of Mg2Si particles after microwave irradiation follows the same trend than that observed on the Si grains. So the grinding time and mainly the Si particle
Fig. 9. Temperature and pressure profiles during the SPS treatment.
size can be used afterwards as key parameters for the granulometry of the Mg2Si synthesized powder. However, a small increase of the particle size can be observed after the microwave process as for each grinding time as shown on the SEM micrographs (Fig. 8) for the powders ground 2 × 2 min. 3.4. Densification of large grains Mg2Si powders Different SPS experiments have been conducted on large Mg2Si particles to confirm the previously assumed influence of the granulometry on the densification process. For the following of this paper a unique SPS program whose temperature and pressure cycles are plotted in Fig. 9 has been used. First of all, it can be noticed that the pellets have a good cohesion after the thermal treatment as shown
Fig. 10. Photograph of a pellet of large grains Mg2Si powders after SPS treatment.
E. Savary et al. / Powder Technology 228 (2012) 295–300
299
Fig. 11. SEM micrographs of SPS sintered samples after a grinding time of (a) 2 × 2 min; (b) 2 × 10 min.
on the photograph (Fig. 10). Two short grinding times 2 × 2 min and 2 × 10 min have been used leading to samples whose relative densities are 95% and 85% respectively. Again, these experiments demonstrate that the difficulties encountered previously to densify the fine Mg2Si powders are strongly correlated to the grain size. The final grain size of the sintered pellets is about 8 μm and 4.5 μm for the powder grinded 2 × 2 min and 2 × 10 min respectively which is not relevant to our goal of elaborating nanostructured Mg2Si bulks. Besides, the SEM micrographs (Fig. 11) clearly show that the oxidation of the magnesium is drastically reduced which demonstrates that this oxidation is not due to the beforehand presence of MgO within the commercial magnesium turnings. It can be postulated that the oxidation occurs during the microwave irradiation because of the proximity between our powder and the fused silica tube walls. 3.5. Towards fine microstructures As discussed before, the sintering of nanosized Mg2Si samples, required for impacting the thermal transport, seems to be very problematic. Regarding the previous results a reasonable way to elaborate dense materials while reducing the thermal conductivity of Mg2Si bulks should be to mix powders with different grain sizes. Powders with two different granulometries were chosen to realise these mixtures: one grinded 2 × 2 min (with large particles, called LP) and another one ground 2 × 30 min (with small particles, called SP). A first mixture composed of 50% of LP powder and 50% of SP powder was first realised. The sintered pellet relative density is about 75% and the particle size distribution is very large with grain sizes between about 1 and 7 μm (Fig. 12a). A second mixture composed of 67% of
LP and 33% of SP was then realized. The increase of LP proportion allows achieving a relative density of 95%. Moreover, according to the SEM observations, it can be noticed that Mg2Si dense composite bulks have been elaborated with a large amount of small particles (≈1 μm diameter) homogeneously distributed in a Mg2Si matrix composed of larger grains (≈8 μm diameter) as shown on Fig. 12b. Forthcoming work will focus on the thermal properties of such composites on optimized composition samples in order to assess the interest of such mixtures on the transport properties. 4. Conclusion In this paper, we studied the densification of fine Mg2Si powders by SPS process. It sounds clear that it is very problematic to sinter samples composed of Mg2Si nanoparticles. Indeed the small size of the grains favors the magnesium oxidation at grain boundaries and prevents high green densities which lead to non cohesive pellet after SPS treatment. The problem of the oxidation has been solved by means of an in-situ synthesis of the Mg2Si phase under reductive conditions during SPS process. Despite this, it has not been possible to elaborate dense and cohesive pellets showing that the powder granulometry is the key parameter to be controlled. This assumption has been confirmed by the elaboration of 95% dense sintered samples when starting from large grain powders. A new approach consisting in adding Mg2Si nanoparticles in micronic powder, has been carried out. The resulting dense composites show original microstructures with small grains homogeneously distributed in a large grain matrix opening new ways for improving the thermoelectric properties of Mg2Si-based materials.
Fig. 12. SEM micrographs of SPS sintered samples made from mixtures of (a) 50% LP and 50% SP; (b) 67% LP and 33% SP.
300
E. Savary et al. / Powder Technology 228 (2012) 295–300
Acknowledgments The authors wish to acknowledge financial support from the Thermomag Project, which is co-funded by the European Commission in the 7th Framework Programme (contract NMP4-SL-2011-263207), by the European Space Agency and by the individual organisations. The authors would like to thank Ronan Macanjo for the technical support. Etienne Savary thanks the French Ministry of Research for the financial support. The ANR SONDE (grant no. ANR-06-BLAN-0331) is cordially acknowledged for supporting in part the cost of the ball milling equipment. References [1] H.J. Goldsmid, Thermoelectric Refrigeration, Plenum Press, New-York, 1964. [2] M.S. Dresselhaus, G. Chen, M.Y. Tang, R.G. Yang, H. Lee, D.Z. Wang, et al., New directions for low-dimensional thermoelectric materials, Advanced Materials 19 (2007) 1043–1053. [3] S. Li, M.S. Toprak, H.M.A. Soliman, J. Zhou, M. Muhammed, D. Platzek, et al., Fabrication of nanostructured thermoelectric bismuth telluride thick films by electrochemical deposition, Chemistry of Materials 18 (2006) 3627–3633. [4] X. Ji, B. Zhang, T.M. Tritt, J.W. Kolis, A. Kumbhar, Solution-chemical syntheses of nano-structured Bi2Te3 and PbTe thermoelectric materials, Journal of Electronic Materials 36 (2007) 721–726. [5] H. Ni, T. Zhu, X. Zhao, Thermoelectric properties of hydrothermally synthesized and hot pressed n-type Bi2Te3 alloys with different contents of Te, Materials Science and Engineering B 117 (2005) 119–122. [6] C.H. Kuo, H.S. Chien, C.S. Hwang, Y.W. Chou, M.S. Jeng, M. Yoshimura, Thermoelectric properties of fine-grained PbTe Bulk materials fabricated by cryomilling and spark plasma sintering, Materials Transactions 52 (2011) 795–801. [7] X.W. Wang, H. Lee, Y.C. Lan, G.H. Zhu, G. Joshi, D.Z. Wang, et al., Enhanced thermoelectric figure of merit in nanostructured n-type silicon germanium bulk alloy, Applied Physics Letters 93 (2008) 193121. [8] G. Joshi, H. Lee, Y. Lan, X. Wang, G. Zhu, D. Wang, et al., Enhanced thermoelectric figure-of-merit in nanostructured p-type silicon germanium bulk alloys, Nano Letters 8 (2008) 4670–4674. [9] J. Tani, H. Kido, Thermoelectric properties of Bi-doped MgSi semiconductors, Physica B: Condensed Matter 364 (2005) 218–224. [10] M.I. Fedorov, V.K. Zaĭtsev, I.S. Eremin, E.A. Gurieva, A.T. Burkov, P.P. Konstantinov, Transport properties of Mg2X0.4Sn0.6 solid solutions (X = Si, Ge) with p-type conductivity, Physics of the Solid State 48 (2006) 1486–1490.
[11] J.I. Tani, H. Kido, Thermoelectric properties of Sb-doped Mg2Si semiconductors, Intermetallics 15 (2007) 1202–1207. [12] G. Nolas, D. Wang, M. Beekman, Transport properties of polycrystalline Mg2Si1−y Sby (0≤yb0.4), Physical Review B 76 (2007). [13] M. Akasaka, T. Iida, A. Matsumoto, K. Yamanaka, Y. Takanashi, T. Imai, et al., The thermoelectric properties of bulk crystalline n- and p-type Mg2Si prepared by the vertical Bridgman method, Journal of Applied Physics 104 (2008) 013703. [14] Q. Zhang, H. Yin, X.B. Zhao, J. He, X.H. Ji, T.J. Zhu, Thermoelectric properties of n-type Mg2Si0.6-ySbySn0.4 compounds, Physica Status Solidi (a) 205 (2008) 1657–1661. [15] K. Mars, H. Ihou-Mouko, G. Pont, J. Tobola, H. Scherrer, Thermoelectric properties and electronic structure of Bi- and Ag-doped Mg2Si1−xGex compounds, Journal of Electronic Materials 38 (2009) 1360–1364. [16] J. Schilz, Synthesis of thermoelectric materials by mechanical alloying in planetary ball mills, Powder Technology 105 (1999) 149–154. [17] T. Aizawa, R. Song, Mechanically induced reaction for solid-state synthesis of Mg2Si and Mg2Sn, Intermetallics 14 (2006) 382–391. [18] R. Song, T. Aizawa, J. Sun, Synthesis of Mg2Si1−xSnx solid solutions as thermoelectric materials by bulk mechanical alloying and hot pressing, Materials Science and Engineering: B 136 (2007) 111–117. [19] K. Kondoh, H. Oginuma, E. Yuasa, T. Aizawa, Solid-state synthesis of Mg2Si from Mg–Si mixture powder, Materials Transactions 42 (2001) 1293–1300. [20] L. Zhang, Y. Leng, H. Jiang, L. Chen, T. Hirai, Synthesis of Mg2Si1−xGex thermoelectric compound by solid phase reaction, Materials Science and Engineering B 86 (2001) 195–199. [21] R. Song, Y. Liu, T. Aizawa, Solid state synthesis and thermoelectric properties of Mg–Si–Ge system, Journal of Materials Science and Technology 21 (2005) 619. [22] W. Luo, M. Yang, F. Chen, Q. Shen, H. Jiang, L. Zhang, Fabrication and thermoelectric properties of Mg2Si1−xSnx (0≤x≤1.0) solid solutions by solid state reaction and spark plasma sintering, Materials Science and Engineering: B 157 (2009) 96–100. [23] M. Schmidt, J. Schmidt, Y. Grin, A. Böhm, B. Kieback, R. Scholl, et al., Production of Mg2Si and ternary compounds Mg2(Si, E; E = Ge, Sn, Pb and Transition Metals; b10wt. %) Made of MgH2 and Silicon and Production of Magnesium Silicide Moulded Bodies by Pulse-Plasma Synthesis, U.S. Patent PCT/EP02/03953, 2002. [24] S.C. Zhou, C.G. Bai, Microwave direct synthesis and thermoelectric properties of Mg2Si by solid-state reaction, Transcations of Nonferrous Metals Society of China 21 (2011) 1785–1789. [25] E. Savary, F. Gascoin, S. Marinel, Fast synthesis of nanocrystalline Mg2Si by microwave heating: a new route to nano-structured thermoelectric materials, Dalton Transactions 39 (2010) 11074. [26] M.I. Mendelson, Average grain size in polycrystalline ceramics, Journal of the American Ceramic Society 52 (1969) 443–446.
Contents lists available at SciVerse ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Spark plasma sintering of fine Mg2Si particles Etienne Savary ⁎, Franck Gascoin, Sylvain Marinel, Romain Heuguet Laboratoire CRISMAT UMR CNRS 6508, 6 Bd Maréchal Juin, 14050 CAEN Cedex 4, France
a r t i c l e
i n f o
Article history: Received 11 January 2012 Received in revised form 4 April 2012 Accepted 12 May 2012 Available online 19 May 2012 Keywords: Microwave synthesis Mg2Si Thermoelectricity Spark Plasma Sintering Nanoparticles Ball milling
a b s t r a c t Nanostructuration is an efficient way to improve thermoelectric properties. In this paper, the densification of micro and nanocrystalline Mg2Si is studied using Spark Plasma Sintering process. It seems very problematic to successfully sinter Mg2Si nanoparticles because the nano-size promotes oxidation at grain boundaries and prevent obtaining high green densities of the compacts. The in situ synthesis of Mg2Si under reductive atmosphere, during SPS process, has not led to dense samples, demonstrating that the grain size is the key factor in densifying magnesium silicide powders. Moreover, the sintering of micronic Mg2Si grains has been successfully carried out confirming this assumption. Our results show that mixing powders made of small and large particles give access to 95% dense composite materials with a large amount of fine grains homogeneously distributed in the large grain matrix. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectricity is a very promising way to reduce fuel consumption and CO2 emissions through the conversion of waste heat into electric energy. However, the high cost of the materials and the relatively low efficiency of the thermoelectric generators limit their large scale development. The performance of a thermoelectric material is quantified by its dimensionless figure of merit ZT which is equal to α 2T/ρκ, where α represents the Seebeck coefficient, T is the absolute temperature, ρ is the electrical resistivity and κ is the thermal conductivity [1]. Furthermore, κ is the sum of two contributions: one is due to the phonons, labeled κlat and one is due to the charge carrier, labeled κel. It has been often reported that κlat can be significantly decreased by the multiplication of grain boundaries, enhancing the phonon scattering at the interfaces and consequently increasing ZT [2]. Thus, many efforts have been devoted in past years to the elaboration of nanostructured bulk materials such as Bi2Te3 [3–5], PbTe [4,6] or SiGe [7,8]. Magnesium silicide is now considered as a very good candidate for thermoelectric applications. Indeed, ZT values higher than unity have already been reported for n-type Mg2Si [9–15]. Moreover, this material is made of widely abundant, cheap, light, and non-toxic elements; all criteria that fit with large scale production requirements. Different techniques are used to synthesize Mg2Si phase such as mechanical alloying [16–18], conventional high temperature reaction [19–22], Spark Plasma Sintering (SPS) [22,23] or microwave irradiation [24] of the metallic precursors.
⁎ Corresponding author. Tel.: + 33 2 31 45 13 69; fax: + 33 2 31 45 13 09. E-mail address: [email protected] (E. Savary). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.05.036
Unfortunately, independent of the method used, it remains very difficult to produce nanosized powders. In a recent paper, we reported the possibility to synthesize nanocrystalline Mg2Si by a very fast microwave irradiation of the ground metallic precursors [25]. The next step is now to elaborate dense bulk materials with fine grains to improve the thermoelectric properties of the Mg2Si compounds. For the sake of retaining the “nano” size of the powders, the SPS densification, due to the short processing time necessary, appears to be very well suited for such a task. In this contribution we report on the SPS processing of powders with different grain sizes to clarify the influence of the granulometry on the densification of Mg2Si compounds. 2. Experimental part 2.1. Powders processing Silicon lumps (Alfa Aesar 99.9999%) and magnesium turnings (Alfa Aesar Puratronic 99.98%) were ground under nitrogen in a high energy ball mill (Fritsch Pulverisette 7 Premium Line). The elemental precursors were placed in a 20 ml air-proof tungsten carbide bowl containing six 10 mm diameter tungsten carbide balls. All the manipulations were performed in a glove box to prevent any reaction with air and moisture. The grinding cycles were composed of 2 periods of 2, 10 or 30 min at a speed of 450 rounds per minute with a pause of 1 min in between. After completion of the milling program, the resulting powder was cold pressed under 3 tons in glove box to obtain 500 mg pucks. Each puck was subsequently inserted in a 12 mm outside diameter fused silica tube equipped with a valve so that the microwave heating of the puck could be performed under nitrogen.
296
E. Savary et al. / Powder Technology 228 (2012) 295–300
Fig. 1. SEM micrograph of Mg2Si particles synthesized by microwave.
Fig. 3. SEM micrographs of Si particles after 2 h of ball milling.
to think that the size of the Mg2Si grains after synthesis is mainly directed by the size of the Si particles.
2.2. Microwave synthesis of the Mg2Si particles The microwave furnace consists of a microwave generator (2.45 GHz Sairem GMP20KSM) delivering a variable power of up to 2000 W. The microwave radiation passes through a rectangular waveguide (WR340) ended by a TE10p cavity. A coupling iris and a short circuit piston allow tuning the cavity length to excite two different modes of resonance called TE102 and TE103. When the cavity is excited within the TE102 mode the sample is located in a maximum of magnetic field whereas in the TE103 mode it is located in a maximum of electric field (for more details see [25]). 2.3. Granulometry of the powders A laser granulometer (Malvern Mastersizer 2000 — Dispersion unit: Malvern Hydro 2000 S) was used to estimate the particle size distribution. This method is based on the measurement of the light diffusion by particles dispersed in a liquid medium. Concerning the powders obtained after the milling process, the sharp difference in mechanical properties between the hard and easy to crush silicon and the soft magnesium leads to a mixture composed of Si particles embedded in a magnesium “mush” preventing granulometric measurements. However, the magnesium can be dissolved in diluted nitric acid solution allowing the measure of the granulometry of the Si particles. A pH = 9 aqueous solution has been found to be an appropriate liquid medium for dispersing silicon particles. Considering the Mg2Si powders after microwave irradiation, the high sensitivity of Mg2Si powders towards water prevents measurements in aqueous solution. Moreover, the dispersion of these particles in any liquid medium remains problematic. Nevertheless considering the respective mechanical properties of Si and Mg, it sounds reasonable
2.4. SPS processing The sintering processes were carried out under vacuum in a Spark Plasma Sintering apparatus (FCT HP D 25/1). For each sample between 2 g and 3 g of powder were poured in 15 mm or 20 mm diameter graphite dies. The temperature and the pressure are raised simultaneously to their maximum values. The maximum temperature used was 775 °C since at higher temperature the graphite foil insulating the pistons from the silicide starts to react with the latter. The maximum pressure was 50 MPa, limit that can sustain the graphite dies utilized. In some cases, a plateau at 450 °C was also necessary in order to prevent the evaporation of elemental magnesium. 2.5. Characterization The sizes of the particles before and after sintering were determined with a scanning electron microscopy (Zeiss Supra 55 operated at 5 kV) using the Mendelson's method [26]. The chemical composition of the different phases were analysed by energy dispersion spectroscopy (EDAX-EDS). 3. Results and discussion 3.1. SPS of powders synthesized by microwave As described elsewhere [25], microwave process allows producing in a very short time (4 periods of 30 min of grinding and merely 2 min of microwave irradiation) cauliflower-like aggregates of Mg2Si nanoparticles (Fig. 1). The size of these particles is smaller than 100 nm
Fig. 2. SEM micrographs of Mg2Si pellets after SPS treatment.
E. Savary et al. / Powder Technology 228 (2012) 295–300
297
Fig. 6. Granulometry of silicon particles for different grinding times.
Fig. 4. Pressure evolution and piston displacement profiles during the SPS process of MgH2 + Si mixture.
and the grain size distribution is very homogeneous. The crucial point is now to elaborate dense Mg2Si bulk while limiting the grain growth. In this aim, the SPS method seems to be interesting because of the high heating rates and the short processing times usually associated with this process. Different SPS programs have been tested but whatever the dwell temperature and the pressure applied (though limited by the graphite dies), it has not been possible to get a dense and cohesive pellet. In all cases, SEM analysis revealed the presence of MgO phase at the grain boundaries (Fig. 2) which was confirmed by X-ray diffraction analysis. At this stage, two assumptions can be advanced to explain the difficulty to densify these powders. First, the MgO phase at the grain boundary could limit the solid state diffusion at the interfaces which is the main driving force for the sintering. Secondly, the nanosize of the Mg2Si particles and the narrow granulometric distribution could reduce the defect gradients which promote this solid state diffusion. A better understanding of the respective contributions of these assumptions could be achieved by avoiding the oxidation of the magnesium. This oxidation can be due to either the presence of MgO in the commercial Mg turnings used or to an accidental oxidation during the different steps of the elaboration process. To partially clarify this incertitude, a reactive SPS heating of a mixture of MgH2 and Si based on a Schmidt et al. patent [23] has been performed to avoid the use of possibly oxidized Mg turnings and to strengthen the reductive conditions of SPS process. 3.2. Reactive SPS process from magnesium hydride The size of the silicon particles obtained after the 2 × 30 min ball milling process is between 50 and 400 nm (Fig. 3) which is comparable with the granulometry of the magnesium silicide after microwave
synthesis. In the SPS set up, the thermal decomposition of MgH2 occurs at about 410 °C, dihydrogen gas is thus released and elemental magnesium “produced”. Thus, Mg and Si precursors can react to form in situ the Mg2Si phase. One can see at 410 °C an important increase of the pressure inside the reactor which corresponds to the H2 emanation (Fig. 4). At the same time, a huge displacement of the piston is also evidenced (Fig. 4). The H2 production increases the reducing character of the atmosphere during the SPS process. Together with the fact that the magnesium produced should be oxide-free, this reduces drastically the amount of MgO phase as shown on the SEM micrograph (Fig. 5). Moreover, it can be seen that the grain size is between 1 and 4 μm which is similar to the values obtained after the SPS process of the powders synthesized by microwave. However, although the diminution of the MgO phase has been achieved, it has not been possible to successfully produce dense and cohesive pellets. Considering these results, it can be assumed that the difficulty to sinter the Mg2Si is strongly correlated to the small size of the silicon particles. A granulometric study is required for a better understanding of the silicon particle size influence on the densification process. 3.3. Granulometry of the powders a) Silicon and magnesium mixture As explained before, it is not possible to realize any granulometric measurement on Mg2Si powder after the grinding process. Thus the silicon particle size is measured as a function of the grinding time. The granulometric curves are represented by Fig. 6 and two SEM micrographs are shown Fig. 7. First of all, as expected, the longer the grinding time the smaller the particles. The mode of the equivalent spherical diameter (ESD) varies from 3 μm for a grinding time of 2 × 2 min to about 1 μm for a grinding time of 2 × 30 min. Moreover, it can be noticed that the particle size reaches a limit after 1 h of grinding. Thus, the mixture processed
Fig. 5. SEM micrographs of Mg2Si processed by SPS from MgH2 and Si mixture.
298
E. Savary et al. / Powder Technology 228 (2012) 295–300
Fig. 7. SEM micrographs of Si particles after (a) 2 × 10 min; (b) 2 × 30 min of grinding.
Fig. 8. SEM micrographs of (a) Si particles before the microwave irradiation; (b) Mg2Si particles after the microwave irradiation for a grinding time of 2 × 2 min.
during 2 h shows the same evolution than that milled during 1 h. This can be explained by the physical limits of the equipment used such as the size of the balls or the rotation speed and the detection limit of the granulometer. The formation of agglomerates has also to be considered for the longest grinding times leading to the smallest particles. b) Mg2Si after microwave processing The powders previously ground are heated by microwave to synthesize the Mg2Si phase. The granulometry of Mg2Si particles after microwave irradiation follows the same trend than that observed on the Si grains. So the grinding time and mainly the Si particle
Fig. 9. Temperature and pressure profiles during the SPS treatment.
size can be used afterwards as key parameters for the granulometry of the Mg2Si synthesized powder. However, a small increase of the particle size can be observed after the microwave process as for each grinding time as shown on the SEM micrographs (Fig. 8) for the powders ground 2 × 2 min. 3.4. Densification of large grains Mg2Si powders Different SPS experiments have been conducted on large Mg2Si particles to confirm the previously assumed influence of the granulometry on the densification process. For the following of this paper a unique SPS program whose temperature and pressure cycles are plotted in Fig. 9 has been used. First of all, it can be noticed that the pellets have a good cohesion after the thermal treatment as shown
Fig. 10. Photograph of a pellet of large grains Mg2Si powders after SPS treatment.
E. Savary et al. / Powder Technology 228 (2012) 295–300
299
Fig. 11. SEM micrographs of SPS sintered samples after a grinding time of (a) 2 × 2 min; (b) 2 × 10 min.
on the photograph (Fig. 10). Two short grinding times 2 × 2 min and 2 × 10 min have been used leading to samples whose relative densities are 95% and 85% respectively. Again, these experiments demonstrate that the difficulties encountered previously to densify the fine Mg2Si powders are strongly correlated to the grain size. The final grain size of the sintered pellets is about 8 μm and 4.5 μm for the powder grinded 2 × 2 min and 2 × 10 min respectively which is not relevant to our goal of elaborating nanostructured Mg2Si bulks. Besides, the SEM micrographs (Fig. 11) clearly show that the oxidation of the magnesium is drastically reduced which demonstrates that this oxidation is not due to the beforehand presence of MgO within the commercial magnesium turnings. It can be postulated that the oxidation occurs during the microwave irradiation because of the proximity between our powder and the fused silica tube walls. 3.5. Towards fine microstructures As discussed before, the sintering of nanosized Mg2Si samples, required for impacting the thermal transport, seems to be very problematic. Regarding the previous results a reasonable way to elaborate dense materials while reducing the thermal conductivity of Mg2Si bulks should be to mix powders with different grain sizes. Powders with two different granulometries were chosen to realise these mixtures: one grinded 2 × 2 min (with large particles, called LP) and another one ground 2 × 30 min (with small particles, called SP). A first mixture composed of 50% of LP powder and 50% of SP powder was first realised. The sintered pellet relative density is about 75% and the particle size distribution is very large with grain sizes between about 1 and 7 μm (Fig. 12a). A second mixture composed of 67% of
LP and 33% of SP was then realized. The increase of LP proportion allows achieving a relative density of 95%. Moreover, according to the SEM observations, it can be noticed that Mg2Si dense composite bulks have been elaborated with a large amount of small particles (≈1 μm diameter) homogeneously distributed in a Mg2Si matrix composed of larger grains (≈8 μm diameter) as shown on Fig. 12b. Forthcoming work will focus on the thermal properties of such composites on optimized composition samples in order to assess the interest of such mixtures on the transport properties. 4. Conclusion In this paper, we studied the densification of fine Mg2Si powders by SPS process. It sounds clear that it is very problematic to sinter samples composed of Mg2Si nanoparticles. Indeed the small size of the grains favors the magnesium oxidation at grain boundaries and prevents high green densities which lead to non cohesive pellet after SPS treatment. The problem of the oxidation has been solved by means of an in-situ synthesis of the Mg2Si phase under reductive conditions during SPS process. Despite this, it has not been possible to elaborate dense and cohesive pellets showing that the powder granulometry is the key parameter to be controlled. This assumption has been confirmed by the elaboration of 95% dense sintered samples when starting from large grain powders. A new approach consisting in adding Mg2Si nanoparticles in micronic powder, has been carried out. The resulting dense composites show original microstructures with small grains homogeneously distributed in a large grain matrix opening new ways for improving the thermoelectric properties of Mg2Si-based materials.
Fig. 12. SEM micrographs of SPS sintered samples made from mixtures of (a) 50% LP and 50% SP; (b) 67% LP and 33% SP.
300
E. Savary et al. / Powder Technology 228 (2012) 295–300
Acknowledgments The authors wish to acknowledge financial support from the Thermomag Project, which is co-funded by the European Commission in the 7th Framework Programme (contract NMP4-SL-2011-263207), by the European Space Agency and by the individual organisations. The authors would like to thank Ronan Macanjo for the technical support. Etienne Savary thanks the French Ministry of Research for the financial support. The ANR SONDE (grant no. ANR-06-BLAN-0331) is cordially acknowledged for supporting in part the cost of the ball milling equipment. References [1] H.J. Goldsmid, Thermoelectric Refrigeration, Plenum Press, New-York, 1964. [2] M.S. Dresselhaus, G. Chen, M.Y. Tang, R.G. Yang, H. Lee, D.Z. Wang, et al., New directions for low-dimensional thermoelectric materials, Advanced Materials 19 (2007) 1043–1053. [3] S. Li, M.S. Toprak, H.M.A. Soliman, J. Zhou, M. Muhammed, D. Platzek, et al., Fabrication of nanostructured thermoelectric bismuth telluride thick films by electrochemical deposition, Chemistry of Materials 18 (2006) 3627–3633. [4] X. Ji, B. Zhang, T.M. Tritt, J.W. Kolis, A. Kumbhar, Solution-chemical syntheses of nano-structured Bi2Te3 and PbTe thermoelectric materials, Journal of Electronic Materials 36 (2007) 721–726. [5] H. Ni, T. Zhu, X. Zhao, Thermoelectric properties of hydrothermally synthesized and hot pressed n-type Bi2Te3 alloys with different contents of Te, Materials Science and Engineering B 117 (2005) 119–122. [6] C.H. Kuo, H.S. Chien, C.S. Hwang, Y.W. Chou, M.S. Jeng, M. Yoshimura, Thermoelectric properties of fine-grained PbTe Bulk materials fabricated by cryomilling and spark plasma sintering, Materials Transactions 52 (2011) 795–801. [7] X.W. Wang, H. Lee, Y.C. Lan, G.H. Zhu, G. Joshi, D.Z. Wang, et al., Enhanced thermoelectric figure of merit in nanostructured n-type silicon germanium bulk alloy, Applied Physics Letters 93 (2008) 193121. [8] G. Joshi, H. Lee, Y. Lan, X. Wang, G. Zhu, D. Wang, et al., Enhanced thermoelectric figure-of-merit in nanostructured p-type silicon germanium bulk alloys, Nano Letters 8 (2008) 4670–4674. [9] J. Tani, H. Kido, Thermoelectric properties of Bi-doped MgSi semiconductors, Physica B: Condensed Matter 364 (2005) 218–224. [10] M.I. Fedorov, V.K. Zaĭtsev, I.S. Eremin, E.A. Gurieva, A.T. Burkov, P.P. Konstantinov, Transport properties of Mg2X0.4Sn0.6 solid solutions (X = Si, Ge) with p-type conductivity, Physics of the Solid State 48 (2006) 1486–1490.
[11] J.I. Tani, H. Kido, Thermoelectric properties of Sb-doped Mg2Si semiconductors, Intermetallics 15 (2007) 1202–1207. [12] G. Nolas, D. Wang, M. Beekman, Transport properties of polycrystalline Mg2Si1−y Sby (0≤yb0.4), Physical Review B 76 (2007). [13] M. Akasaka, T. Iida, A. Matsumoto, K. Yamanaka, Y. Takanashi, T. Imai, et al., The thermoelectric properties of bulk crystalline n- and p-type Mg2Si prepared by the vertical Bridgman method, Journal of Applied Physics 104 (2008) 013703. [14] Q. Zhang, H. Yin, X.B. Zhao, J. He, X.H. Ji, T.J. Zhu, Thermoelectric properties of n-type Mg2Si0.6-ySbySn0.4 compounds, Physica Status Solidi (a) 205 (2008) 1657–1661. [15] K. Mars, H. Ihou-Mouko, G. Pont, J. Tobola, H. Scherrer, Thermoelectric properties and electronic structure of Bi- and Ag-doped Mg2Si1−xGex compounds, Journal of Electronic Materials 38 (2009) 1360–1364. [16] J. Schilz, Synthesis of thermoelectric materials by mechanical alloying in planetary ball mills, Powder Technology 105 (1999) 149–154. [17] T. Aizawa, R. Song, Mechanically induced reaction for solid-state synthesis of Mg2Si and Mg2Sn, Intermetallics 14 (2006) 382–391. [18] R. Song, T. Aizawa, J. Sun, Synthesis of Mg2Si1−xSnx solid solutions as thermoelectric materials by bulk mechanical alloying and hot pressing, Materials Science and Engineering: B 136 (2007) 111–117. [19] K. Kondoh, H. Oginuma, E. Yuasa, T. Aizawa, Solid-state synthesis of Mg2Si from Mg–Si mixture powder, Materials Transactions 42 (2001) 1293–1300. [20] L. Zhang, Y. Leng, H. Jiang, L. Chen, T. Hirai, Synthesis of Mg2Si1−xGex thermoelectric compound by solid phase reaction, Materials Science and Engineering B 86 (2001) 195–199. [21] R. Song, Y. Liu, T. Aizawa, Solid state synthesis and thermoelectric properties of Mg–Si–Ge system, Journal of Materials Science and Technology 21 (2005) 619. [22] W. Luo, M. Yang, F. Chen, Q. Shen, H. Jiang, L. Zhang, Fabrication and thermoelectric properties of Mg2Si1−xSnx (0≤x≤1.0) solid solutions by solid state reaction and spark plasma sintering, Materials Science and Engineering: B 157 (2009) 96–100. [23] M. Schmidt, J. Schmidt, Y. Grin, A. Böhm, B. Kieback, R. Scholl, et al., Production of Mg2Si and ternary compounds Mg2(Si, E; E = Ge, Sn, Pb and Transition Metals; b10wt. %) Made of MgH2 and Silicon and Production of Magnesium Silicide Moulded Bodies by Pulse-Plasma Synthesis, U.S. Patent PCT/EP02/03953, 2002. [24] S.C. Zhou, C.G. Bai, Microwave direct synthesis and thermoelectric properties of Mg2Si by solid-state reaction, Transcations of Nonferrous Metals Society of China 21 (2011) 1785–1789. [25] E. Savary, F. Gascoin, S. Marinel, Fast synthesis of nanocrystalline Mg2Si by microwave heating: a new route to nano-structured thermoelectric materials, Dalton Transactions 39 (2010) 11074. [26] M.I. Mendelson, Average grain size in polycrystalline ceramics, Journal of the American Ceramic Society 52 (1969) 443–446.