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Journal of Sol-Gel Science and Technology

, Volume 89, Issue 1, pp 295–302 | Cite as

From sol–gel prepared porous silica to monolithic porous Mg2Si/MgO composite materials

  • N. Hayati-Roodbari
  • R. J. F. Berger
  • J. Bernardi
  • S. Kinge
  • N. Hüsing
  • M. S. ElsaesserEmail author
Open Access
Original Paper: Sol-gel and hybrid materials for energy, environment and building applications

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

  • We present a new and versatile method to prepare macroporous Mg2Si/MgO composites.

  • Both components, Mg2Si and MgO, are homogeneously distributed in the final composite.

  • Our approach can easily be extended to other highly porous silica templates.

  • The Mg2Si/MgO network comprises nanosized MgO particles in a 3D interconnected Mg2Si network.

Keywords

Hierarchically structured silica Nanocomposite material Magnesiothermic reduction Magnesium silicide 

1 Introduction

Mg2Si is the only intermetallic alloy formed in the Mg-Si system (Eq. 1) [1]. In recent years, it has attracted an increasing amount of interest for a wide range of applications in various fields, such as thermoelectrics and optics and in lithium-ion batteries, largely due to its low toxicity, unique optical and electrical properties, low costs of production, and environmental compatibility [2, 3, 4, 5, 6, 7, 8].
$$2{\mathrm{Mg + Si}} \to {\mathrm{Mg}}_{\mathrm{2}}{\mathrm{Si}}$$
(1)

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].

Adding a second phase, such as a metal oxide, into the system also reduces the lattice thermal conductivity by enabling an additional scattering mechanism and thus, improving thermoelectric performance [15, 16]. Cederkrantz et al. [17] investigated the influence of the addition of TiO2 nanoparticles to Mg2Si grains of > 100 nm on the thermoelectric properties of the latter. The addition of only 1 vol% TiO2 nanoparticles to Mg2Si resulted in a significant improvement of ZT at 300 °C, reaching ZT values of 0.042. This is a factor 2.75 higher than for pure Mg2Si at the same temperature. Similar effects can be expected from using MgO instead of TiO2 as a dielectric component in Mg2Si, which seems to be easily achievable through magnesiothermic reduction of silica with an excess of magnesium (see equations 2 and 3). This has been shown by Szczech et al. [18] who converted diatomaceous earth particles to Mg2Si/MgO composite powders via a gas-solid displacement reaction with magnesium vapor. Not only the micrometer sized morphology of the diatoms was preserved, but both components were also homogeneously mixed with MgO crystallites within a nanometer size range (~30 nm). This reaction represents a possible method for the production of large quantities of low-cost nanoscale thermoelectric materials with enhanced thermoelectric performance. Furthermore, for optimization of the thermoelectric performance an adjustment of the ratio of thermoelectric to dielectric phase is highly desirable [17].
$${\mathrm{SiO}}_{\mathrm{2}}{\mathrm{ + 2Mg}} \to {\mathrm{Si + 2MgO}}$$
(2)
$${\mathrm{SiO}}_{\mathrm{2}}{\mathrm{ + 4Mg}} \to {\mathrm{Mg}}_{\mathrm{2}}{\mathrm{Si + 2MgO}}$$
(3)

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].

In the present study, we have developed a two stage synthesis strategy to prepare monolithic Mg2Si/MgO composite materials comprising a macroporous network and an adjustable ratio of both components. This approach opens a simple route towards a variety of Mg2Si/MgO materials with respect to chemical composition and morphology compared to previously described processes [19]. By magnesiothermically reacting hierarchically structured macro/mesoporous silica with a low amount of Mg vapor, a mixture of Si and remaining silica is obtained. If this material is reacted with Mg vapor again a complete conversion to Mg2Si and MgO is obtained according to the parallel reactions given in Eqs. 1 and 3. The overall process has the following advantages:
  • Easily adjustable ratio of Mg2Si and MgO

  • Nanosized MgO particles homogeneously embedded in a macroporous Mg2Si network

  • Low equipment cost

  • 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

Magnesiothermic reduction [1, 23, 24] was performed using a “mesh-boat” set-up to ensure spatial separation of silica and Mg (see Fig. 2). The silica slices were placed on a stainless steel mesh with the magnesium powder beneath in a small boat, and together these were inserted into a stainless steel tube filled with an argon atmosphere. This was then placed in a tube furnace under a slight argon stream and heated to 650 °C with a heating rate of 1 K min−1 and 2 h holding time. In order to obtain Mg2Si/MgO composites in a direct one-step reaction, silica and Mg were used in a molar ratio of 1:4. In this case no further purification treatment was performed. To prepare three different Si/SiO2 composite materials for further treatment as described in “Synthesis of Mg2Si/MgO composites” section, the amount of Mg was kept below 2 moles (0.50–1.61) in relation to SiO2 (see detailed calculated values in Table 1 for samples A1, B1, and C1). Removal of the by-product (MgO) was conducted by immersing the reaction product in 1 mL of degassed water, followed by addition of 20 mL of 2 M hydrochloric acid and 10 mL of diluted acetic acid for 3 h at 40 °C. Afterwards, washing with degassed water was repeated until a neutral pH-value was achieved. The resulting brown monoliths were dried in vacuum.
Table 1

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

To prepare three different Mg2Si/MgO composite samples (A2, B2, and C2), the same set-up as described before was used. The silica/silicon monoliths (A1, B1, or C1) were placed on a mesh and separated from Mg powder in the boat beneath (see detailed amounts in Table 2). Afterwards, the set-up was placed in a stainless steel tube, which was heated in a tube furnace under argon atmosphere to 650 °C with 1 K min-1 heating rate and 2 h holding time.
Table 2

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

Based on the experiments by Szczech et al. [18] with diatomaceous earth SiO2, and also on our own previous experiments [20], we reacted hierarchically structured silica, consisting of a cellular network comprising struts with hexagonally ordered mesopores with a diameter of 8 nm and a pore wall thickness of 3–5 nm (see Supplementary Fig. S1 ESI), directly with four moles of Mg vapor at a temperature of 650 °C for 2 h following Eq. 3. The PXRD pattern in Fig. 1b confirms the complete conversion to a Mg2Si/MgO composite with a ratio of 44:56 by weight (molar ratio Mg2Si/MgO = 0.41). Starting with amorphous silica, indicated by a very broad peak in the PXRD diffraction pattern at 22° 2 Theta (Fig. 1a), a product with crystallite sizes of 73 ± 5 nm (Mg2Si) and 21 ± 5 nm (MgO) was obtained according to the Scherrer equation. Mg vapor can easily diffuse into and through the macroporous network, followed by condensation and reaction with the SiO2 on the struts (see Supplementary Fig. S1 ESI), which are typically of about 1 µm in length and 100 nm in diameter. This build-up of a pure crystalline material upon the magnesiothermic conversion is accompanied by the destruction of pores in the mesoscopic range. However, a perfect preservation of the macroporous network of Mg2Si/MgO is observed (see Fig. 1b). Similar results were obtained recently for the reaction of Si with Mg to yield pure Mg2Si [20].
Fig. 1

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.

In order to gain better control over the molar ratio between Mg2Si and MgO in the final composite material, we developed a route based on two successive magnesiothermic reductions making use of the two parallel reactions as given in Eqs. 1 and 3. The aim is to first convert silica with a very low amount of Mg to a silica/silicon composite material, which is reacted in a second step with Mg to give a Mg2Si/MgO composite (see Fig. 2).
Fig. 2

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)

In principle, the reaction of silica and Mg is performed as described before with the exception that the amount of Mg is now adjusted to values lower than four moles (thus sub-stoichiometric with respect to Equation 2).
$${\mathrm{SiO}}_{\mathrm{2}}{\mathrm{ + }}\left( {{\mathrm{2 - 2}}}x \right){\mathrm{Mg}} \to \left( {{\mathrm{1 - }}}x \right){\mathrm{Si + }x}\,{\mathrm{SiO}}_{\mathrm{2}}{\mathrm{ + }}\left( {{\mathrm{2 - 2}}}x \right){\mathrm{MgO}}$$
(4)
\(x_{\mathrm{A}}={{0.75}}; x_{\mathrm{B}}={{0. 44}};x_{\mathrm{C}}={{0.20}}\)
$$\left( {{\mathrm{1 -}}}x \right){\mathrm{Si}} + \left( {{\mathrm{2 - 2}}}x \right){\mathrm{Mg}} \to \left( {{\mathrm{1 -}}}x \right){\mathrm{Mg}}_{\mathrm{2}}{\mathrm{Si}}$$
(5a)
$$x\,{\mathrm{SiO}}_{\mathrm{2}}{\mathrm{ + }}\left( {{\mathrm{2}}}x \right){\mathrm{Mg}} \to \left( {{\mathrm{2}}}x \right){\mathrm{MgO}} + x{\mathrm{Mg}}_{\mathrm{2}}{\mathrm{Si}}$$
(5b)

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.

To prove our concept we prepared three different samples with theoretical Mg2Si/MgO ratios of 0.66, 1.14, and 2.50 indicated with A2, B2, and C2 (Table 2). For the first reaction, we reacted (2 – 2x) mole of Mg and 1 mole of SiO2, with x = 0.20, 0.44, and 0.75 being the amount of remaining silica after the first step. A detailed overview of the calculated reaction compositions is given in Table 2. Since Mg powder contains traces of MgO impurities we take advantage of the boat-mesh set-up, which ensures spatial separation from the reactants, since MgO always remains beneath the mesh. In addition, the chosen Ar atmosphere avoids formation of MgO traces due to Mg oxidation. We investigated the intermediate SiO2/Si composites by PXRD diffraction as shown in Fig. 3a after extraction of MgO. All three samples (A1, B1, and C1) show an amorphous signal with a broad reflection at ~22° 2 Theta indicating remaining silica accompanied by Bragg reflexes for crystalline silicon. Unfortunately, the amorphous character of silica made detailed calculations of the composition by Rietveld refinement impossible.
Fig. 3

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

The three Mg2Si/MgO composite samples were characterized by PXRD diffraction (see Fig. 3b). For all three samples only Mg2Si and MgO are present (reference pdf files: Mg2Si-PDF-87-0651 and MgO-PDF-35-0773). For comparison, we normalized the XRD patterns and calculated the ratios of Mg2Si and MgO by Rietveld refinement. As shown in Table 2, the obtained values for the molar ratio between Mg2Si and MgO match very well with the calculated contents of silica and Mg and show the great versatility and reliability of the method. Expectedly, the absolute experimental amounts obtained in the final product are lower than the theoretically calculated ones (c.f. Tables 2 and 3) due to material loss during washing, processing by use of the mesh or even handling. The crystallite sizes calculated by the Scherrer equation are in the range 193 to 269 nm (for Mg2Si) and ~30 nm (for MgO) (see Table 2). The morphology of the obtained composite samples was investigated by SEM and TEM. Figure 4 shows the microstructure indicating in all cases a preservation of the macroporous cellular network. The TEM images in Fig. 4 reveal the fine dispersion of Mg2Si and MgO crystallites in each strut with the absence of pores in the mesoscopic range, which proves the contention that Si and SiO2 are also homogeneously distributed in the intermediate product. We examined the distribution of Mg2Si and MgO also by EDX and obtained a homogeneous distribution in an area of several square µm of the sample (see Fig S3 ESI). The loss of mesopores during processing is also supported by nitrogen sorption analysis and very low SSAs ( < 50 m2 g-1) in comparison to the highly porous silica precursor (700 m2 g−1).
Table 3

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

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

Crystallite sizes were calculated by use of the Scherrer equation.

Fig. 4

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

  1. 1.

    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

10971_2018_4778_MOESM1_ESM.docx (2 mb)
Supplementary information

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Authors and Affiliations

  1. 1.Materials ChemistryParis Lodron University SalzburgSalzburgAustria
  2. 2.USTEM, Technische Universität WienViennaAustria
  3. 3.Toyota Motors Company EuropeAntwerpBelgium

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