Article pubs.acs.org/crystal
Pressure Effects on the Size of Type‑I and Type-II Si-Clathrates Synthesized by Spark Plasma Sintering S. Stefanoski, M. C. Blosser, and G. S. Nolas* Department of Physics, University of South Florida, Tampa, Florida 33620, United States S Supporting Information *
ABSTRACT: The crystallinity of clathrate-I Na8Si46 and clathrate-II Na24Si136 was investigated as a function of applied pressure and temperature employing spark plasma sintering. Na8Si46 and Na24Si136 with full Na and Si occupancies were obtained at 450 and 600 °C, respectively. Microcrystalline powders were obtained at 60 MPa, and single crystals were obtained at 100 MPa. The size of the crystals increased with pressure for the clathrate-II Na24Si136, while a threshold value in pressure separated microcrystalline and single-crystal clathrate-I Na8Si46.
1. INTRODUCTION In spark plasma sintering (SPS), a pulsed dc current is sourced through a specimen enclosed in a punch and die assembly placed between two electrodes. The two electrodes, which source current through the specimen and die assembly, also act for application of uniaxial pressure. The application of electrical current in SPS influences the generation and mobility of point defects.1 The fast heating rates and more uniform heating, as compared with other consolidation techniques, generally result in faster densification, and have typically been employed for the densification of refractory materials.2−4 These characteristics make SPS a state-of-the-art consolidation technique.2−4 In addition to consolidation, SPS was employed for the synthesis of various materials ranging from ceramics to metals, polymers, and semiconductors.5−7 The SPS approach allows for mass transport of ions to take place within a precursor while under pressure. It can, therefore, be used for clathrate formation and crystal growth. Recently, SPS was used for the synthesis of single-crystal clathrate-II Na24Si136.8 Intermetallic clathrates represent open-framework materials in which guest atomic species are enclosed within the host framework.9,10 The interest in group 14 clathrates stems from the unique properties that have been reported in these materials,9,11−19 making them promising for a range of applications, including thermoelectric,9,14,15 magnetocaloric,16,17,20 optoelectronic,12,21 and superconductivity.19 Single crystals of clathrate-II Na24Si136 were synthesized via SPS at 600 °C under uniaxial pressure of 100 MPa for 3 h.8 At 550 °C, approximately 10% of clathrate-I Na8Si46 was observed as an impurity, whereas reactions performed at 700 °C yielded a similar fraction of α-Si.8 The synthesis procedure reported here allows for the selective synthesis of single-crystal Na8Si46 and Na24Si136 using SPS. Size and crystallinity of the clathrate © 2012 American Chemical Society
products were investigated as a function of the applied pressure and temperature.
2. EXPERIMENTAL SECTION The Na4Si4 precursor was synthesized by direct reaction of elemental Na (Alfa Aesar, 99.98%) and Si powder (Alfa Aesar, 99%). A Na/Si atomic ratio of 1.1:1 was placed inside a tungsten crucible that was inside a sealed stainless steel canister and reacted at 650 °C for 36 h. The resulting product was a dark gray crystalline material and was used as the precursor in the SPS process. The SPS investigations were carried out using the Thermal Technology, Inc. 10-3 system. Na4Si4 precursor was loaded in a graphite die with uniaxial pressures of 60, 80, and 100 MPa applied using graphite punches. The maximum pressure applied with graphite die and punches was 100 MPa. Tantalum foil was used to prevent direct reaction between the precursor and the graphite die and punches, as well as to provide a tight fit of the die assembly. Pulsed dc current, with a pulse-on time of 36 ms and a pulse-off time of 2 ms, was sourced through the precursor and die from the bottom electrode (anode) to the top electrode (cathode) while under uniaxial pressure in a vacuum of 10−3 Torr for 3 h after flushing the chamber three times with high-purity N2. The product of the reaction was separated from any unreacted Na4Si4 by washing with ethanol and distilled water. Single-crystal XRD data of Na8Si46 and Na24Si136 crystals were obtained at 293 K on a STOE diffractometer using a graphite monochromator and a Mo Kα fine-focus sealed tube with a wavelength of 0.71073 Å. Structural refinements were performed using SHELXL-97 (Tables S1−S5 in the Supporting Information). Powder X-ray diffraction (XRD) patterns were collected with a Bruker D8 Focus diffractometer in Bragg−Brentano geometry using Cu Kα,β radiation and a graphite monochromator. NIST Si 640c internal standard was used for determination of lattice parameters. Received: September 14, 2012 Revised: November 16, 2012 Published: November 20, 2012 195
dx.doi.org/10.1021/cg3013443 | Cryst. Growth Des. 2013, 13, 195−197
Crystal Growth & Design
Article
other.22−27 Also, impurities of α-Si have been typically observed in NaxSi136 using this technique.22−27 It is also known that an insulating oxide layer can readily form on the grains of microcrystalline specimens28 and presents difficulties in densification and interpretation of the measured data.29,30 Less than 5 wt % α-Si and no clathrate impurities were observed in the clathrate-II Na24Si136 microcrystalline specimen using this SPS approach (Figure S1, Supporting Information). Only single-crystal Na24Si136 are free of α-Si and other impurities and allow for intrinsic physical properties measurements.31 It is, therefore, of interest to investigate the size and crystallinity of single-crystal clathrates employing SPS. Details of the SPS synthesis parameters, including temperature, T, pressure, P, and average size, L, of the resulting products, are given in Table 1. The grain size of the powders was calculated using the Debye−Scherrer equation.32,33 The average size of the single crystals was obtained from SEM analyses.
Scanning electron micrographs (SEM) were collected using a JEOL JSM-6390LV, and energy-dispersive X-ray spectroscopic (EDS) data were collected using an Oxford INCA X-Sight 7582M.
3. RESULTS AND DISCUSSION Specimens with the clathrate-I and clathrate-II crystal structures were synthesized by SPS processing of Na4Si4 at the respective temperatures of 450 and 600 °C, and uniaxial pressures of 60, 80, and 100 MPa for 3 h. As described in ref 8, Na transport occurs during SPS, thereby resulting in a redox reaction. The temperature ramp rates were 100 °C/min for the clathrate-I phase, and 25 °C/min up to 450 °C and then 10 °C/min up to 600 °C for the clathrate-II phase. Selective synthesis was achieved by choosing the appropriate temperature, 450 °C for the clathrate-I phase and 600 °C for the clathrate-II phase (Figure 1; Figure S1 in the Supporting Information). No
Table 1. Processing Conditions for SPS Synthesis from the Precursor Na4Si4 T (°C)
P (MPa)
phase
crystallinity
L (μm)
450
60 80 100 60 80
clathrate-I clathrate-I clathrate-I clathrate-I + II clathrate-I + II
10 10 150 10 10/150
100
clathrate-I + II
60 80 100 100
clathrate-II clathrate-II clathrate-II silicon
microcrystalline microcrystalline single crystal microcrystalline microcrystalline + single crystal microcrystalline + single crystal microcrystalline single crystal single crystal nanocrystalline
550
600
Figure 1. X-ray diffraction patterns of powdered Na8Si46 and Na24Si136, collected from crushed single crystals, Na4Si4, and Si demonstrating the selective synthesis of single-crystal clathrates at 100 MPa.
650
change in the precursor was observed at temperatures below 400 °C. At 550 °C, a mixture of both phases was observed. SPS processing at 650 °C yields the α-Si phase. The highest yield of single crystals was achieved at 450 and 600 °C for the clathrateI and clathrate-II phases, respectively, at 100 MPa. This approach allowed for the selective synthesis of single-crystal (Figure 1) and microcrystalline (Figure S1, Supporting Information) Na8Si46 and Na24Si136 as a function of the applied pressure. At 60 MPa, high-purity microcrystalline products were observed. When the pressure was increased from 60 to 80 MPa, clathrate-II single crystals started to form but clathrate-I single crystals formed at 100 MPa. Compositions with stoichiometries of Na8Si46 and Na24Si136 were obtained from refinement of the single-crystal XRD data (Tables S1−S5, Supporting Information). Compositions with stoichiometries of Na8Si46 and Na24Si136 were obtained from the refinement of powder XRD data collected on microcrystalline clathrate-I and clathrate-II specimens, respectively (Tables S6−S8, Supporting Information). EDS analyses of these single crystal stoichiometries, Na7(1)Si46 and Na23(1)Si136, corroborate the single-crystal XRD results. The crystal structure for the microcrystalline specimens of both phases was obtained from Rietveld refinement (Figures S2 and S3, Supporting Information). Microcrystalline Na8Si46 and Na24−xSi136 (0 < x < 24) clathrates have traditionally been synthesized by thermal decomposition of the Na4Si4 precursor under dynamic vacuum with one clathrate phase present as an impurity in the
10/150 15 50 250 0.065
The average size of the clathrate-I phase does not change as the pressure is increased from 60 to 80 MPa (Table 1), in contrast to the clathrate-II phase. The average crystal size for the clathrate-II phase at 80 MPa is smaller than that at 100 MPa, suggesting that the crystal size increases with increasing pressure. In the case of the clathrate-I phase, 100 MPa seems to be the optimum pressure for graphite tooling at which singlecrystal growth takes place. SPS processing at 60 and 80 MPa resulted in only microcrystalline products, whereas at 100 MPa, only single crystals were observed at the appropriate temperatures. The average size of the single-crystal products, 150 and 250 μm for the clathrate-I and clathrate-II phases, respectively, (Table 1) is comparable to that obtained from other singlecrystal synthesis methods.34 At 550 °C and 100 MPa, a mixture of powders and single crystals of both clathrate phases was observed (Table 1). The relative clathrate fraction in the synthesis product as a function of reaction time was also investigated. When the precursor was reacted at 600 °C for 30 min, the clathrate-I phase was estimated to be approximately 60%, whereas the clathrate-II phase was approximately 40%, from powder XRD analyses (Figure S4, Supporting Information). As the reaction time increased at the same temperature, the intensity of the clathrate-I reflections in the XRD spectra decreased. For a reaction time of 3 h, only the clathrate-II phase was observed. The average size of Si obtained by SPS processing of the Na4Si4 precursor at 650 °C is 65 nm (Table 1). Si nanoparticles 196
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Article
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of 3 nm can been synthesized by a microwave assisted technique35 and are of interest for the green chemistry as they are considered to be nontoxic, biocompatible, and biodegradable.35,36 The high heating rates and localized heating in the SPS, considered two of the most important factors for the synthesis of Si nanoparticles,35 may reveal SPS to be a promising method for the synthesis of Si nanoparticles. Further investigations are necessary in order to determine the full potential of SPS in this direction.
4. CONCLUSION Selective synthesis of single-crystal and microcrystalline clathrate-I Na8Si46 and clathrate-II Na24Si136 was achieved via SPS. The size of the product increased with pressure in the case of clathrate-II Na24Si136, in contrast with the clathrate-I Na8Si46. Microcrystalline specimens were obtained at 60 MPa and single-crystal specimens at 100 MPa, at temperatures of 450 and 600 °C for the clathrate-I and clathrate-II phases, respectively. This approach may also have promise for the preparation of nanocrystalline Si, as Si nanoparticles with an average size of 65 nm were synthesized by SPS.
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ASSOCIATED CONTENT
S Supporting Information *
Figures showing XRD patterns and Reitveld refinement data, tables containing XRD and Rietveld refinement data, and crystallographic data in CIF format.This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge support by the U.S. Department of Energy, Basic Energy Sciences, Division of Materials Science and Engineering, under Award No. DE-FG02-04ER46145 for single-crystal synthesis, structural analyses, and measured and simulated pXRD, SEM, and EDS measurements and analyses. We also acknowledge C. D. Malliakas and M. G. Kanatzidis for collecting the single-crystal XRD data.
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REFERENCES
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