From sol–gel prepared porous silica to monolithic porous Mg2Si/MgO composite materials
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Abstract
Mg2Si is apart from its conductivity properties expected to be a promising candidate for thermoelectric applications due to its low toxicity, low costs, and the high abundance of its precursor chemicals. Through the addition of a homogeneous distribution of nanoparticles (e.g. MgO) and by reducing the size of Mg2Si to the nanometer regime, it is possible to decrease the thermal conductivity by increasing phonon-interface scattering and, as a result, improve the thermoelectric properties. However, classical approaches do not allow for the synthesis of nanocomposites from Mg2Si and MgO. In this work, a straightforward route is presented towards homogeneously mixed Mg2Si/MgO via a two-step magnesiothermic reduction process starting from sol–gel derived hierarchically organized porous silica. Monolithic materials composed of Mg2Si and MgO in variable molar ratios are built up from a macroporous network of Mg2Si with homogeneously distributed MgO particles exhibiting a crystallite size in the range of 24–37 nm.
Highlights
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We present a new and versatile method to prepare macroporous Mg2Si/MgO composites.
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Both components, Mg2Si and MgO, are homogeneously distributed in the final composite.
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Our approach can easily be extended to other highly porous silica templates.
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The Mg2Si/MgO network comprises nanosized MgO particles in a 3D interconnected Mg2Si network.
Keywords
Hierarchically structured silica Nanocomposite material Magnesiothermic reduction Magnesium silicide1 Introduction
It has been proven that nanostructuring of materials for thermoelectric applications, e.g., as nanofibres, results in improved performances compared to their bulk counterparts [9, 10]. Nanostructured thermoelectric materials are designed to introduce nanometer-sized polycrystallites and interfaces into bulk materials, which can improve the figure of merit ZT1 for thermoelectric materials [11, 12, 13, 14].
Nanostructured composites are typically fabricated by hot pressing or spark plasma sintering of fine powders prepared by grinding, milling, or wet chemical processing [11]. Such an approach creates a large number of interfaces between neighboring nanoparticles, thereby improving the thermoelectric performance [19]. However, typical drawbacks of these methods are that (1) a deliberate tailoring of the nanostructure is difficult to achieve, (2) the reaction is often incomplete, and (3) the final product is often contaminated with side-products [20]. In a previous study, we showed that magnesiothermic reduction of porous, hierarchically organized silica produced similarly structured meso/macroporous silicon, which could be converted to monolithic porous magnesium silicide via a specifically designed set-up for the gas-solid displacement reaction [20].
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Easily adjustable ratio of Mg2Si and MgO
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Nanosized MgO particles homogeneously embedded in a macroporous Mg2Si network
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Low equipment cost
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No further purification steps are required
2 Experimental procedure
2.1 Materials
Ethylene glycol (Sigma-Aldrich) was purified by drying over Na2SO4 (Prolabo) and filtering. Tetraethoxysilane (Merck), trimethylchlorosilane (TMCS, Sigma-Aldrich), Pluronic P123 (EO20PO70EO20, BASF), petroleum ether (40–60 °C, Prolabo), magnesium powder (for synthesis, Merck), hydrochloric acid (37%, Merck), and acetic acid (glacial, Merck) were used without further purification.
2.2 Preparation of hierarchically organized silica
Hierarchically organized meso/macroporous silica (SiO2) was prepared according to Brandhuber et al. by sol–gel processing of tetrakis(2-hydroxyethyl)orthosilicate (EGMS) combined with a templating approach in an aqueous medium containing Pluronic P123™ and 1 M hydrochloric acid (HCl), with a composition by weight of Si/P123/0.1 M HCl = 8.4/30/70 [21, 22]. The surface hydrophobization concomitant with surfactant extraction was performed by immersing the gel bodies in TMCS (10% in petroleum ether (PE)). After washing with PE three times, the wet silica gels were cut into small, monolithic slices (with 2–3 mm in height and 3–4 mm in diameter (c.f. Fig. 2)). After drying at 80 °C the slices were calcined at 550 °C for 4 h.
2.3 Reaction of SiO2 with Mg with various molar ratios to obtain Mg2Si/MgO or Si/SiO2 composites
Calculated amounts of reactants and content after magnesiothermic reduction of Si/SiO2 composites
Sample | SiO2 [mol] | Mg [mol] | Product SiO2 [mol] | Product Si [mol] |
---|---|---|---|---|
Mg2Si: MgO A1 | 1.00 | 0.50 | 0.75 | 0.25 |
Mg2Si: MgO B1 | 1.00 | 1.12 | 0.44 | 0.56 |
Mg2Si: MgO C1 | 1.00 | 1.61 | 0.20 | 0.80 |
2.4 Synthesis of Mg2Si/MgO composites
Calculated amounts of reactants and content after reaction of the Mg2Si/MgO composites
Sample | SiO2 [mol] | Si [mol] | Mg [mol] | Product Mg2Si [mol] | Product MgO [mol] | Mg2Si/MgO molar ratio |
---|---|---|---|---|---|---|
Mg2Si: MgO A2 | 0.75 | 0.25 | 3.50 | 1.00 | 1.50 | 0.66 |
Mg2Si: MgO B2 | 0.44 | 0.56 | 2.88 | 1.00 | 0.88 | 1.14 |
Mg2Si: MgO C2 | 0.20 | 0.80 | 2.39 | 1.00 | 0.40 | 2.50 |
3 Methods
Powder X-ray diffractograms (PXRD) were recorded using a Bruker D8 diffractometer with a DaVinci Design and CuKα radiation. Evaluation of crystallite sizes was done according to the Scherrer equation and Rietveld refinement was performed using TOPAS V4-2 software (Bruker).
Sample morphology was examined using a Zeiss Ultra Plus scanning electron microscope (SEM) operated at an accelerating voltage of 2 kV with an in-lens detector. Silica samples were sputtered with gold to provide an electrically conductive surface. An Oxford Instruments X-Max energy dispersive X-ray (EDX) detector was used for elemental analysis.
The microstructure of the samples was studied with transmission electron microscopy (TEM) using a TECNAI F20 field emission microscope operated at an accelerating voltage of 200 kV. Images were recorded with a Gatan Orius SC 600 charge-coupled device (CCD) camera.
Nitrogen sorption isotherms were recorded at 77 K using a sorption porosimeter (Micromeritics, ASAP 2420). Prior to the measurement, samples were degassed for 3 h at 100 °C in vacuum. The Brunauer-Emmett-Teller (BET) surface area was evaluated using adsorption data in a relative pressure range p/p0 0.05–0.25. The mesopore size distribution was calculated on the basis of the desorption branch using the Barrett–Joyner–Halenda (BJH) model.
4 Results and discussions
PXRD pattern of a (1) hierarchically structured, amorphous silica and (2) Mg2Si/MgO composite material after reaction at 650 °C for 2 h (molar ratio SiO2/Mg 1:4); b SEM image of the Mg2Si/MgO composite material of (2)
It is very likely that the presence of MgO mitigates the locally released heat from the exothermic reaction helping to preserve the macro-morphology [23]. Elemental analysis by EDX confirmed a homogeneous distribution of Mg2Si and MgO in the macroporous network (see Supplementary Information, Fig. S2) again supporting the contention of the benefit of the better diffusion in a highly porous precursor material. The loss of mesopores is indicated by a decrease of the specific surface area (SSA) from 700 m2 g−1 in silica to less than 50 m2 g−1 in the composite sample. This data proves that we can successfully prepare a monolithic, but nanosized, Mg2Si/MgO composite material via a direct one-step magnesiothermic reaction. However, any adjustment of the ratio of the components to each other does not seem to be possible. Variations in the amounts of Mg for the magnesiothermic reaction would result in either silica and/or silicon containing samples or residual unreacted Mg.
Schematic presentation of the two-step process to yield a composite material with adjustable ratio of Mg2Si/MgO (upper part); photographs of the precursor silica gel (bottom left), the Si/SiO2 monolith (bottom middle) and the (Mg2Si/MgO) monolith (bottom right)
This leads to an incomplete reaction resulting in a monolithic material consisting of silicon, MgO and remaining silica. The given amount of Mg, indicated by “x” in Eq. 4, is always adjusted to < 2. In this series, the values have been set to (A, B, and C) xA = 0.75; xB = 0.44, xC = 0.20, and thus the amount of Mg determines the content of residual silica. MgO is removed by HCl etching leaving a macroporous silicon/silica network behind. Although the distribution of Si and SiO2 in this network is unknown, we conclude that it is homogeneous since it is homogeneously distributed in the final product (see Supplementary Information; EDX data in Fig. S3). MgO removal is necessary to be able to adjust the desired ratios in the final product. The dried Si/SiO2 composite monolith is then subjected to a second reaction with Mg. The amount of Mg is calculated on the basis of the calculated ratios of Si/SiO2 (see Table 1) and the stoichiometry given in Eqs. 5a and b. While silica is now converted to magnesium silicide and MgO (Eq. 5b), silicon directly gives magnesium silicide according to Eq. 5a. In this step, Si and SiO2 form the final macroporous Mg2Si network with homogeneously distributed MgO crystallites. Depending on the amount of silica in this step, a number of different compositions (A2, B2, and C2) are accessible without further purification (see also photographs in Fig. 2).
The monolithic shape of the starting silica gel is preserved upon all processing steps to the final composite material without significant shrinkage. As expected, the color change from white to slight brownish and furthermore to bluish demonstrates the transformation from silica to silicon/silica and Mg2Si/MgO. Along with the transformation from silica to the Mg2Si/MgO composite materials, we obtain a slight decrease in density overall: 0.35 g cm−3 for silica and 0.10–0.20 g cm−3 for Mg2Si/MgO (A2 0.10; B2 0.13; C2 0.20). Interestingly, the densities of the intermediate stage of Si and silica (after removal of MgO) are the lowest, with measured values of 0.067 g cm−3 for A1, 0.085 for B1, and 0.095 for C1. Considering the low densities of amorphous silica (~2.20 g cm−3), silicon (2.32 g cm−3) and Mg2Si (1.99 g cm−3) compared to MgO (3.58 g cm−3), this is not unexpected.
a PXRD patterns of the intermediate silica/silicon product A1, B1, and C1; b PXRD patterns of the final Mg2Si/MgO composite materials A2, B2, and C2
Product compositions of Mg2Si/MgO composites
Samples | Product Mg2Si [mol] | Product MgO [mol] | Mg2Si crystallite size [nm] | MgO crystallite size [nm] | Mg2Si/MgO molar ratio |
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Mg2Si/MgO A2 | 0.76 | 1.17 | 193 | 37 | 0.65 |
Mg2Si/MgO B2 | 0.78 | 0.69 | 241 | 29 | 1.13 |
Mg2Si/MgO C2 | 0.85 | 0.35 | 269 | 24 | 2.45 |
SEM images of a Mg2Si/MgO composite A2, b Mg2Si/MgO composite B2, c Mg2Si/MgO composite C2, and TEM images of d Mg2Si/MgO composite A2, e Mg2Si/MgO composite B2, f Mg2Si/MgO composite C2
5 Conclusion
In summary, we developed a facile and versatile method to prepare macroporous Mg2Si and MgO composite materials with adjustable molar ratios of both components. Magnesiothermic reduction of hierarchically structured amorphous silica with an amount of Mg powder that results in an incomplete reaction produced silicon/silica composites. The latter could be reacted in a second treatment with Mg to give Mg2Si/MgO composites. The ratio between magnesium silicide and magnesium oxide is easily controlled via the amount of Mg used in both reactions.
Both components are homogeneously distributed in the final product, implying good diffusion of Mg into the porous scaffolds of SiO2 and Si/SiO2. From a structural point of view, the monolithic shape as well as the macroporous network are preserved during the reactions with magnesium. Smaller mesopores, however, are lost in the reaction process. Both components Mg2Si and MgO are obtained as crystalline materials with crystallite sizes in the upper nanometer regime ( > 150 nm) and about 30 nm, respectively. Both reaction steps can be conducted in the same set-up.
As a broad variety of porous silica structures are availiable, this approach can easily extended to many other composite morphologies of Mg2Si and MgO in different molar ratios. To overcome a low electrical conductivity of the porous composite material in thermoelectric applications, typically a sintering process (plasma sintering) is followed up to yield a bulk form with homogeneous distribution of MgO nano crystals.
Footnotes
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ZT defined as dimensionless scalar: ZT = S2ρ-1κ-1T with S (Seebeck coefficient), ρ (electrical resistivity), κ (thermal conductivity), and T (absolute temperature)
Notes
Acknowledgements
Open access funding provided by Paris Lodron University of Salzburg. The help of G. Tippelt for the X-ray diffraction measurements and M. Suljic for the N2 sorption measurements is acknowledged. Financial support of Toyota Motors Company Europe is kindly appreciated.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
Supplementary material
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