Solid-State Phase Transformation as a Route for the Simultaneous

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Solid-State Phase Transformation as a Route for the Simultaneous Synthesis and Welding of Single-Crystalline Mg2Si Nanowires Yongmin Kang† and Sreeram Vaddiraju*,†,‡ †

Department of Materials Science & Engineering, Texas A&M University, College Station, Texas 77843, United States Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States

‡

S Supporting Information *

ABSTRACT: A simple, but elegant, strategy for the simultaneous synthesis and welding of single-crystalline Mg2Si nanowires is presented. For the synthesis of Mg2Si nanowires, the solid-state phase transformation of presynthesized silicon nanowires was employed. For assembling the Mg2Si nanowires via the formation of Mg2Si bridges, the phase transformation of silica nanoparticle-decorated silicon nanowires was employed. To circumvent the formation of multiple Mg2Si nuclei and hence the phase transformation of single-crystalline Mg2Si nanowires into polycrystalline Mg2Si nanowires, solid-state reaction of silicon nanowire tips with magnesium foils at elevated temperatures of 350−400 °C was employed. In this procedure, the supersaturation of the sharp tips of the silicon nanowires with magnesium led to the formation of only one Mg2Si nucleus per nanowire. Growth of these lone nuclei led to the formation of single-crystalline Mg2Si nanowires. Extension of this procedure for the phase transformation of silica nanoparticle-coated silicon nanowires led to the formation of Mg2Si nanowires with Mg2Si bridges between them. The formation of Mg2Si bridges was confirmed by high-resolution transmission electron microscopy analysis and further verified by electrical conductivity measurements. Such simultaneous synthesis and assembly of nanowires will be highly useful in the fabrication of thermoelectric modules from not only Mg2Si but also other metal silicide nanowires.

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INTRODUCTION The large-scale deployment of solid-state thermoelectrics offers two exciting possibilities: increasing the efficiencies of existing processes and systems (e.g., automobiles)1,2 and the generation of renewable energy (e.g., from solar thermal energy).3−5 These devices that convert waste heat into electricity have no moving parts, require minimal maintenance, and are reliable.6 These characteristic traits make them an attractive option for either waste heat scavenging or renewable energy generation. The reliability of solid-state thermoelectrics in electricity generation can be clearly inferred from their use in satellites and space probes.7,8 The first aspect that needs to be addressed for realizing the large-scale deployment of thermoelectrics in the energy generation and usage chain consists of pathways for enhancing their efficiencies, beyond that obtained by the state of the art. The performance of thermoelectrics is dependent upon the figure of merit (zT) of thermoelectric materials used in their fabrication. The zT of materials, in turn, is dependent upon the magnitudes of electrical and thermal transport through them, in accordance with the relationship zT = (S2σT)/(κe + κl), where zT is the figure of merit of the thermoelectric material, S is the Seebeck coefficient, σ is the electrical conductivity, κe is the electronic contribution to the thermal conductivity, and κl is the lattice contribution to the thermal conductivity. Therefore, enhancing the performance of thermoelectric materials requires reducing the extent of thermal transport through them, without © 2014 American Chemical Society

simultaneously reducing the extent of electrical transport through them. A review of the literature indicates that one way to accomplish this, while also successfully circumventing the Wiedemann−Franz law constraint, is through a selective reduction of the lattice thermal conductivities of materials (κl).6,9,10 Synthesizing materials in nanowire form is a pathway for selectively reducing the κl of materials. Recent theoretical and experimental studies have clearly proven this possibility.11−13 A second aspect that needs to be addressed for the large-scale deployment of thermoelectrics in the energy generation and usage chain consists of pathways for lowering their cost. This can be accomplished by designing thermoelectric materials from earth-abundant elements. Furthermore, the nontoxic nature of these elements will be all the more beneficial. A material system that perfectly satisfies these criteria is magnesium silicide (Mg2Si). Mg2Si is a low-bandgap semiconductor and has a bandgap of 0.78 eV.14 In the bulk, Mg2Si is known to be very brittle.15 The zT values of bulk Mg2Si alloyed with Sn14 and Bi15,16 have been reported to be 0.717 and in the range of 0.5−1.2,14,18 respectively. It was also predicted through theoretical modeling by Satyala and Vashaee that reduction of grain sizes led to an increase in the zT values of Mg2Si.19 Received: January 15, 2014 Revised: April 12, 2014 Published: April 14, 2014 2814

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Therefore, it is possible to achieve zT values of ∼1.2 by making Mg2Si in nanowire form with diameters on the order of 5−20 nm.19 A primary requirement for accomplishing this task is the mass production and assembly of single-crystalline Mg2Si nanowires into bulk thermoelectric devices. The singlecrystalline nanowire form of Mg2Si is ideal for this purpose because it offers the possibility of independently tuning electrical and thermal transport. While the diameter of the nanowires can be used as a lever to tune lattice thermal conductivity, the single-crystalline nature of the nanowires offers the possibility of realizing enhanced electrical conductivity.20 Another requirement for realizing enhanced zT values in bulk assemblies of nanowires comes during their assembly. Any assembly strategy developed has to ensure that the interfaces between the nanowires are oxide-free. In other words, there should be a path for electrical conduction between the nanowires in the assembly. No reports of the synthesis of single-crystalline Mg2Si nanowires exist in the literature. Similarly, reports that discuss tailoring the interfacial chemistries of Mg2Si nanowire assemblies to ensure that the interfaces between the nanowires are oxide-free also do not exist in the literature. However, synthesis of other metal silicide nanowires has been accomplished in the past and reported. These reports include the phase transformation of silicon nanowires into MnSi1.75,21−23 CoSi,24,25 GdSi1.75,26 NiSi,27 NiSi2,28 and PtSi29 nanowires. In addition to the synthesis of PtSi nanowires, PtSi− Si heterojunctions have also been synthesized and reported.29 Most of these reports relied on the supply of either metal or metal halides through the vapor phase onto presynthesized silicon nanowires for their conversion into metal silicide nanowires. A few others relied on the solid-state diffusion of the metal into silicon nanowires for the synthesis of metal silicide nanowires.27,29 Previously, we have also demonstrated that solid-state phase transformation of presynthesized silicon nanowires can be employed for the formation of Mg2Si nanowires.30 This procedure allowed for circumventing the problems typically encountered during the direct reaction of silicon and magnesium, namely, the strong propensity of magnesium to be oxidized and the large vapor pressure difference between silicon and magnesium.30 However, the phase transformation of single-crystalline silicon nanowires by reacting them with magnesium supplied via the vapor phase led to the formation of polycrystalline Mg2Si nanowires in that case.30 Additionally, the presence of an MgO sheath was observed on top of nanowires. The formation of MgO is believed to be the result of the reaction of Mg with the native SiO2 layer present on top of the silicon nanowires (4Mg + SiO2 → 2MgO + Mg2Si). The assembly of these nanowires using hot isostatic pressing or spark plasma sintering will result in the presence of electrically insulating MgO at the interfaces between the nanowires. This lowers the overall electrical conductivity of the Mg2Si nanowire assemblies. In this context, the aim of this paper is twofold: (i) to tune the nucleation and growth steps involved in the solid-state phase transformation of silicon nanowires and realize singlecrystalline Mg2Si nanowires and (ii) to extend the solid-state phase transformation strategy for the simultaneous synthesis and welding of Mg2Si nanowires presented in this work. Herein, we will demonstrate that solid-state phase transformation (i.e., reaction of magnesium with silicon or SiO2) can be employed for the simultaneous formation and welding of single-crystalline

Mg2Si nanowires. More specifically, the welding of Mg2Si nanowires through the formation of Mg2Si bridges between them will be demonstrated. Finally, the effect of this welding process on the electrical properties of Mg2Si nanowire assemblies will be discussed.

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EXPERIMENTAL SECTION

Silicon nanowires necessary for the synthesis and welding of Mg2Si nanowires were obtained using electroless etching (Figure 1a). This procedure was described in detail previously.30−32 Boron-doped ⟨100⟩oriented silicon wafers (obtained from University Wafer) were employed as the raw materials for the synthesis of silicon nanowires. Following electroless etching, the obtained silicon nanowires were additionally etched using a 3 wt % KOH aqueous solution for 2 min to ensure that they had sharp tips.33 The diameters of the nanowires obtained using electroless etching typically ranged from 50 to 100 nm, while the lengths ranged from 4.9 to 5.3 μm. All the phase transformation experiments were performed using a solid-state reaction. Typically, these experiments involved bringing the asobtained silicon nanowire arrays or silica nanoparticle-decorated silicon nanowire arrays in contact with a polished magnesium foil and heating them to 350−400 °C in a vacuum chamber (Figure S1 of the Supporting Information). Mild manual pressure was employed to ensure a good contact between the nanowires and the foil before the start of the phase transformation experiments. The flexible nature of the polished magnesium foil allowed for the formation of a good contact. A boron nitride ceramic plate weighing 45 g was placed on top of the silicon nanowires, and the magnesium foil experimental setup aided in ensuring that this contact remained in place throughout the phase transformation process (Figure S1 of the Supporting Information). These experiments were performed in the presence of hydrogen and at a pressure of 100 mTorr. The typical duration of these experiments was 20−60 min. The lower reaction temperature ensured that the supply of magnesium into silicon nanowires for the formation of Mg2Si occurred only through solid-state diffusion. No appreciable evaporation of magnesium is expected to occur at these temperatures. Therefore, the supply of magnesium via the vapor phase onto silicon nanowires resulting in Mg2Si nanowire formation is not expected to occur at these low reaction temperatures. For the synthesis of Mg2Si nanowires, as-obtained silicon nanowires were phase transformed using this solid-state reaction. For welding the obtained nanowires and realizing Mg2Si nanowires welded together with Mg2Si bridges between them, phase transformation of silica nanoparticle-decorated silicon nanowires was employed. Silica nanoparticle-decorated silicon nanowires necessary for this purpose were obtained using the following procedure. Silicon nanowires were first exposed to oxygen plasma for 5 min for the formation of -OH groups on their surfaces. The silicon nanowire arrays were then dipped in a dilute solution of a (3-aminopropyl)trimethoxysilane-functionalized silica nanoparticle dispersion in water for 10 min. Electrostatic attraction between the NH3+ end groups on top of the silica nanoparticles and the OH− groups on top of the silicon nanowires leads to the formation of silica nanoparticledecorated silicon nanowires.34 The nanowires were then cleaned with excess deionized (DI) water to remove excess silica nanoparticles. The silica nanoparticles used in these experiments had an average diameter of 200 nm, sufficient to bridge the gap (or pitch) between two adjacent silicon nanowires in the array. These silica nanoparticle-coated silicon nanowires were also phase transformed using the same procedure described above. The obtained Mg2Si nanowires were characterized using an array of techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffractometry (XRD). For the measurement of the electrical conductivities of the nanowires, they were scraped off the wafers onto pyrolytic BN substrates in the form of mats. Silver paste was employed to make four contacts with each nanowire mat. The electrical conductivities of these mats were then measured using the four-point probe method. These measurements were performed in vacuum at temperatures in the range 2815

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of 325−625 K. The thicknesses of the nanowire mats necessary for the determination of the electrical conductivities were measured using profilometry and confirmed using electron microscopy measurements.

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RESULTS AND DISCUSSION A scanning electron micrograph of an array of as-obtained silicon nanowires is depicted in Figure 1a. These silicon

Figure 2. (a) XRD pattern of welded Mg2Si nanowires obtained by solid-state phase transformation of silica nanoparticle-coated silicon nanowires. The complete transformation of the silica nanoparticlecoated silicon nanowires into Mg2Si was observed. (b) XRD pattern of Mg2Si nanowires obtained by solid-state phase transformation of silicon nanowires. Similar to the case of welded Mg2Si nanowires, the complete transformation of the silicon nanowires into Mg2Si nanowires was observed in this case.

transformation of the as-obtained silicon nanowires resulted in the formation of single-crystalline Mg2Si nanowires (Figure 3a). The analysis indicated the presence of an MgO sheath around the Mg2Si nanowires. Analysis of the diffraction pattern from the Mg2Si nanowire shown in Figure 3a indicated that their growth direction was [202] (inset of Figure 3a). TEM analysis of welded nanowires indicated that after phase transformation they are composed of Mg2Si nanowires welded together via Mg2Si bridges (Figure 3b−d). The phase transformation led to the formation of a single-crystalline Mg2Si bridge between adjacent single-crystalline Mg2Si nanowires, and the bridge is devoid of the presence of any electrically insulating MgO. This result (Figure 3b−d), in conjunction with the fact that the phase transformation of only silicon nanowires led to the formation of Mg2Si nanowires (Figure 3a), clearly indicated that the presence of silica nanoparticles between the silicon nanowires is essential for the formation of Mg2Si bridges between the nanowires. Both the formation of single-crystalline Mg2Si nanowires and the oxide-free welding of Mg2Si nanowires using phase transformation can be explained using the following mechanisms. Typically, the supply of magnesium through the vapor phase onto silicon nanowires and the subsequent supersaturation of the nanowires with magnesium lead to the formation of multiple Mg2Si nuclei inside each nanowire. Further reaction of silicon with additional magnesium diffusing into the nanowires leads to the growth of these nuclei and the formation of polycrystalline Mg2Si nanowires.30 However, tuning the experimental conditions to allow for the formation of only one Mg2Si nucleus per nanowire should lead to the growth of this nucleus into a single-crystalline Mg2Si nanowire. The experimental procedure employed in this study allows for this possibility as illustrated in Figure 4a. Heating the silicon nanowire arrays with sharp tips placed in contact with a magnesium foil leads to the formation of a single nucleus at the tip of each nanowire. The formation of a single Mg2Si nucleus inside each nanowire can be confirmed by the fact that at substrate temperatures in the range of 350−900 °C, Mg2Si nuclei 3−4 nm in size will be formed. The size of the nuclei was estimated using nucleation theory as explained below. The critical size of the nuclei of the second phase (R*) formed inside a parent phase during phase transformation is determined by the equation R* = (2γ)/(ΔGv + ΔGs),36−38 where ΔGv and ΔGs are the total volume free energy and the

Figure 1. Scanning electron micrographs of (a) as-obtained silicon nanowires and (b) silicon nanowires after solid-state phase transformation into Mg2Si nanowires. (c) Micrograph of silica nanoparticlecoated silicon nanowires. (d) Micrograph of Mg2Si nanowires welded via the formation of Mg2Si bridges between them. These welded nanowires were obtained by phase transforming silica-coated silicon nanowires depicted in panel c. The phase transformation was performed by bringing the nanowires into contact with a magnesium foil and heating the resulting setup to elevated temperatures.

nanowires had diameters in the range of 50−100 nm (Figure 1a). As observed in the figure, the nanowires also had sharp tips. A micrograph of these nanowires after solid-state phase transformation into Mg2Si nanowires is presented in Figure 1b. As is clearly evident in the figure, the phase transformation allowed for the retention of the nanowire morphology. A micrograph of silica nanoparticle-decorated silicon nanowires is presented in Figure 1c. A micrograph of these nanowires after phase transformation is presented in Figure 1d. Similar to the case of silicon nanowires, the nanowire morphology was still retained after phase transformation (Figure 1d). The spherical bridges between the nanowires are also clearly observed after phase transformation (Figure 1d). The XRD pattern of the silica nanoparticle-decorated silicon nanowires after phase transformation is presented in Figure 2a. Additionally, the XRD pattern of bare silicon nanowires after phase transformation is presented in Figure 2b. It is clearly evident from the data that the nanowires are composed of only Mg2Si after phase transformation (cubic crystal structure with a lattice parameter of 0.639 nm).35 This result clearly indicates the complete transformation of both silicon nanowires and silica nanoparticle-decorated silicon nanowires into Mg2Si nanowires. TEM analysis of the phase-transformed nanowires was performed to determine not only whether the Mg2Si nanowires formed by the phase transformation process are singlecrystalline or polycrystalline but also whether the Mg2Si bridges formed between the nanowires ensure the formation of an oxide-free path (i.e., devoid of MgO) for electrical conduction between them. The results indicated that the phase 2816

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Figure 3. HRTEM image and the corresponding SAED pattern of single-crystalline Mg2Si nanowires obtained by solid-state phase transformation of silicon nanowires with sharp tips. (b−d) Representative TEM micrographs of welded Mg2Si nanowires obtained by solid-state phase transformation of silica nanoparticle-coated silicon nanowires. As seen in the images, this procedure led to the seamless welding of Mg2Si nanowires. The formation of Mg2Si bridges between the nanowires after welding was clearly observed in the high-resolution TEM image provided in panel d. No MgO phase was observed at the interface between the welded nanowires.

Figure 4. (a) Schematic representing the steps involved in the solid-state phase transformation of single-crystalline silicon nanowires into singlecrystalline Mg2Si nanowires. The sharp tips of the silicon nanowires allow for the formation of a single nucleus within each nanowire when they are brought into contact with a magnesium foil and heated. The growth of this lone nucleus within each nanowire leads to the formation of singlecrystalline Mg2Si nanowires. (b) Schematic representation of the steps involved in the solid-state phase transformation process for obtaining Mg2Si nanowires welded via the formation of Mg2Si bridges. The path for the diffusion of Mg is through the Mg2Si first and then through the silica nanoparticles bridging them. As magnesium diffuses through the silica nanoparticle, it reacts with it and forms an Mg2Si bridge between two adjacent Mg2Si nanowires.

free energy change resulting from strain, respectively, and γ is the surface energy.39 The free energy change resulting from strain (ΔGs) can be expressed as ΔGs = [2Y(1 + ν)ε2]/(1 − ν),36−38 where Y is the Young’s modulus of the second phase, ν is the Poisson ratio of the second phase, and ε is the lattice mismatch between the two phases.36 Estimation of the variation in the size of the Mg2Si nuclei formed inside Si nanowires with phase transformation temperature indicated that the size of the nuclei decreases with an increase in the phase transformation temperature (see Figure S2 of the Supporting Information). The following data aided in this estimation. The crystal structures of both Mg2Si and Si are cubic (lattice parameters a = 0.639 nm35 and a = 0.543 nm);35 Y(Mg2Si) = 7.6 × 1010 Pa,40 and ν(Mg2Si) = 0.161.41 Therefore, at substrate temperatures in the range of 350−900 °C, Mg2Si nuclei 3−4 nm in size will be formed (Figure S2 of the Supporting Information). As the nanowire diameters are in the range of 50−100 nm, the possibility of forming multiple Mg2Si nuclei inside each

nanowire exists in silicon nanowires with a uniform diameter all along their lengths. However, in tapered nanowires, the size of the nanowires at the tip is reduced to sizes on the order of the size of the Mg2Si nuclei (see Figure S3 of the Supporting Information for micrographs of the tips of silicon nanowires before and after KOH etching). This essentially leads to the formation of only one Mg2Si nucleus per nanowire. The growth of this nucleus into a single-crystalline Mg2Si nanowire proceeds via the reaction of silicon of the nanowire with additional magnesium diffusing through it. The step involved in the growth of Mg2Si nuclei into single-crystalline Mg2Si nanowires also explains the phenomenon of nanowire welding. As depicted in Figure 3, upon addition of silica nanoparticles to the silicon nanowires, phase transformation leads to the formation of Mg2Si nanowires with Mg2Si bridges between them. The mechanism underlying this phenomenon is pictorially depicted in Figure 4b. It is well-known that reaction of Mg with silica leads to the formation of Mg2Si and MgO, 2817

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according to the reaction 4Mg + SiO2 → Mg2Si + 2MgO.42−44 Although previous studies have indicated that at the microscale this reaction leads to the formation of alternating layers of Mg2Si and MgO,42,43 TEM analyses of the structures reveal a random distribution of MgO and Mg2Si within a region at nanoscale.44 The diffusion of magnesium and its reaction with SiO2 are responsible for the formation of Mg2Si and MgO. Previous studies have indicated that the diffusion of magnesium occurs preferentially though the Mg2Si phase, and not the MgO phase.43 Therefore, if silicon nanowires bridged together with silica nanoparticles are brought into contact with a magnesium foil and heated (Figure 4b), the formation of a lone Mg2Si nucleus inside and its growth lead to the formation of Mg2Si nanowires (Figure 4a). Any additional magnesium diffusing through the Mg2Si nanowires diffuses through the silica nanoparticles bridging them and leads to the formation of an Mg2Si bridge between the nanowires. Had the reaction of magnesium with silica nanoparticles led to the formation of a Mg2Si/MgO core/shell nanoparticle, no further diffusion of magnesium would have occurred and the reaction would not have reached completion. Further confirmation of the formation of Mg2Si nanowires bridged together with Mg2Si bridges comes from electrical conductivity measurements. The electrical conductivities of both mats of Mg2Si nanowires and mats of Mg2Si nanowires welded together via Mg2Si bridges were measured and are presented in Figure 5. As expected, both the Mg2Si nanowire

believed to be the result of the more porous nature of the welded nanowire mats employed in this study. It is well-known that the porosity of nanomaterial assemblies impacts their electrical conductivities.47 An examination of the electrical conductivities of welded Mg2Si nanowire assemblies as a function of their densities is currently ongoing and will be reported at a later date.

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CONCLUSIONS In summary, a simple solid-state phase transformation strategy for the synthesis and assembly via welding of Mg2Si nanowires is presented. In this strategy, presynthesized silicon nanowires, obtained by electroless etching, were phase transformed into single-crystalline Mg2Si nanowires. To circumvent the formation of multiple Mg2Si nuclei within each nanowire and the formation of polycrystalline Mg2Si nanowires, solid-state reaction of sharp silicon nanowires with magnesium foils was employed. The supersaturation of the sharp tips of the silicon nanowires with the diffusing magnesium led to the formation of a single nucleus within each nanowire. The growth of this single nucleus within each nanowire led to the formation of singlecrystalline Mg2Si nanowires. The phase transformation strategy was also extended to the phase transformation of silica nanoparticle-bridged silicon nanowires into Mg2Si nanowires welded together with Mg2Si bridges. It is believed that the magnesium diffusing through the silicon nanowires first and then the silica nanoparticle bridging the nanowires react with them and leave an Mg2Si path between the nanowires, thereby welding them together. In the absence of silica nanoparticles, no welding of the nanowires was observed. The formation of Mg2Si bridges between Mg2Si nanowires was further confirmed by the electrical conductivity measurements. Welded nanowire assemblies exhibited conductivities 2 orders of magnitude higher than those exhibited by nonwelded nanowires. This strategy is simple and can be extended to obtain welded nanowire assemblies of many other metal silicides, in addition to Mg2Si.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 5. Plot comparing the variation of the electrical conductivities with temperature of both nonwelded Mg2Si nanowires and welded Mg2Si nanowires. The process of welding enhanced the electrical conductivity of Mg2Si nanowires by approximately 2 orders of magnitude.

A pictorial representation of the experimental setup, a plot indicating an estimate of the variation of the size of Mg2Si nuclei formed inside silicon nanowires at various temperatures, and high-resolution micrographs of the silicon nanowire tips before and after KOH etching. This material is available free of charge via the Internet at http://pubs.acs.org.

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mats and welded Mg2Si nanowire mats exhibited semiconducting behavior. Their conductivities increased exponentially with temperature. However, the electrical conductivities of the welded Mg2Si nanowire mats were observed to be 2 orders of magnitude higher than those of the bare Mg2Si nanowire mats that are not welded. The enhanced electrical conductivity in the welded nanowire mats cannot be attributed to density changes caused by to the addition of silica nanoparticles. This is because only 1 wt % silica nanoparticles were added to the silicon nanowires to weld them using phase transformation. The higher conductivities of the welded Mg2Si nanowire mat, compared to those of the bare Mg2Si nanowire mats, are therefore believed to be the result of the absence of insulating MgO layers at the interfaces between welded Mg2Si nanowires. Still, the electrical conductivities of welded Mg2Si nanowire mats (25 S/m at 625 K) were observed to be lower than those reported for bulk Mg2Si (850 S/m at 623 K).45,46 This is

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the NSF/DOE thermoelectrics partnership (CBET 1048702) program. Access to the materials characterization housed inside the Conn Center for Renewable Energy at the University of Louisville is gratefully acknowledged. The aid of Dr. Jasek Jasinski of the Center for the TEM characterization of the nanowires is also acknowledged. 2818

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dx.doi.org/10.1021/cm500153h | Chem. Mater. 2014, 26, 2814−2819