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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 3609−3615
Ag-Segregation to Dislocations in PbTe-Based Thermoelectric Materials Yuan Yu,†,‡ Siyuan Zhang,§ Antonio Massimiliano Mio,† Baptiste Gault,§ Ariel Sheskin,∥ Christina Scheu,§ Dierk Raabe,§ Fangqiu Zu,*,‡ Matthias Wuttig,†,⊥ Yaron Amouyal,*,∥ and Oana Cojocaru-Mirédin*,†,§ †
I. Physikalisches Institut (IA), RWTH Aachen, 52074, Aachen, Germany Liquid/Solid Metal Processing Institute, School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China § Max-Planck Institut für Eisenforschung GmbH (MPIE), 40237, Düsseldorf, Germany ∥ Department of Materials Science and Engineering, Technion-Israel Institute of Technology, Technion City, 32000 Haifa, Israel ⊥ JARA-Institut Green IT, JARA-FIT, Forschungszentrum Jülich GmbH and RWTH Aachen University, 52056 Aachen, Germany
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‡
S Supporting Information *
ABSTRACT: Dislocations have been considered to be an efficient source for scattering midfrequency phonons, contributing to the enhancement of thermoelectric performance. The structure of dislocations can be resolved by electron microscopy whereas their chemical composition and decoration state are scarcely known. Here, we correlate transmission Kikuchi diffraction and (scanning) transmission electron microscopy in conjunction with atom probe tomography to investigate the local structure and chemical composition of dislocations in a thermoelectric Ag-doped PbTe compound. Our investigations indicate that Ag atoms segregate to dislocations with a 10-fold excess of Ag compared with its average concentration in the matrix. Yet the Ag concentration along the dislocation line is not constant but fluctuates from ∼0.8 to ∼10 atom % with a period of about 5 nm. Thermal conductivity is evaluated applying laser flash analysis, and is correlated with theoretical calculations based on the Debye−Callaway model, demonstrating that these Ag-decorated dislocations yield stronger phonon scatterings. These findings reduce the knowledge gap regarding the composition of dislocations needed for theoretical calculations of phonon scattering and pave the way for extending the concept of defect engineering to thermoelectric materials. KEYWORDS: thermoelectric material, dislocation, correlative microscopy, atom probe tomography, Ag segregation, lead-telluride compound
1. INTRODUCTION
It has been shown that low-frequency phonons are most efficiently scattered by precipitates and interfaces,9,10 whereas high-frequency phonons are mainly scattered by point defects,11 and the midfrequency phonons are effectively scattered by dislocation cores and strain fields.12−14 Such frequency-dependent scattering has recently been modeled.15 Kim et al.16 used a liquid-phase compaction method to induce dense dislocation arrays embedded in grain boundaries, leading to a substantial scattering of midfrequency phonons and hence low lattice thermal conductivity. Similarly, the presence of numerous dislocations at low-angle grain boundaries in n-type skutterudites has also been proven to enhance TE perform-
Thermoelectric (TE) materials, which can directly convert thermal energy into electricity and vice versa, have received wide attention in recent decades driven by the demand for sustainable energy consumption and carbon-free emission.1,2 A good TE material should have large Seebeck coefficient and electrical conductivity to enable high power output, while exhibiting low thermal conductivity to maintain a large temperature difference. Approaches such as band engineering,3 resonance doping,4 and modulation doping5 have been successfully adopted to enhance the electrical properties of TE materials. In addition, by utilizing alloys with intrinsically low thermal conductivity6 or introducing hierarchical defects to enhance multiscale phonon scattering,7,8 the thermal conductivity can be strongly reduced. © 2018 American Chemical Society
Received: November 10, 2017 Accepted: January 8, 2018 Published: January 8, 2018 3609
DOI: 10.1021/acsami.7b17142 ACS Appl. Mater. Interfaces 2018, 10, 3609−3615
Research Article
ACS Applied Materials & Interfaces
disks were finally aged at 380 °C for 48 h in sealed quartz ampules under a 120 Torr Ar−7% H2 atmosphere followed by ice−water quenching. The thermal conductivity was measured based on direct measurements of temperature-dependent thermal diffusivity, heat capacity, and density. The thermal diffusivity and heat capacity were simultaneously determined by LFA-457 MicroFlash laser flash analysis (LFA) (Netzsch GmbH, Selb, Germany) using a pure Al2O3-reference sample with similar geometry of the sample. The electrical conductivity of the pellet was determined by the SBA-458 Nemesis apparatus (Netzsch GmbH, Selb, Germany) applying the four-point probe technique.36,37 The electronic thermal conductivity was calculated based on the Wiedemann−Franz law with the Lorenz number of 2.4 × 10−8 W Ω K−2, which was subtracted from the measured thermal conductivity, resulting in the lattice component of thermal conductivity. TEM lamellae and APT needle-shaped specimens were prepared using a dual-beam scanning electron microscope/focused ion beam (Helios NanoLab 650, FEI) following the site-specific “lift-out” method described elsewhere.38 We performed correlative TKD− STEM to first localize the position of low-angle grain boundaries which contain a high density of dislocations, and second to investigate the structure of these dislocations. STEM investigations were carried out using an aberration-corrected FEI Titan Themis microscope operated at 300 kV. The convergence angle of the probe was 24 mrad, and the collection angles of the annular bright field (ABF) and highangle annular dark field (HAADF) detectors were 8−16 and 73−352 mrad, respectively. TEM investigations were performed using a FEI Tecnai F20 microscope operated at 200 kV. TKD was performed for the TEM lamella at an acceleration voltage of 20 kV and a beam current of 1.6 nA using the Helios NanoLab 650 equipped with a Hikari high-speed camera (EDAX/TSL). Electron backscatter diffraction (EBSD) was performed on the same equipment with the same parameters but a different inclined angle for the sample. The TKD and EBSD data were processed by the TSL OIM Analysis 7.0 software. Finally, the composition of dislocations was determined by APT. APT measurements were conducted on a local electrode atom probe (LEAP 4000X Si, Cameca) by applying 10-ps, 30-pJ ultraviolet (wavelength = 355 nm) laser pulses with a detection rate of 1 ion per 100 pulses on average, a pulse repetition rate of 200 kHz, a base temperature of the specimen of 40 K, and an ion flight path of 90 mm. The detection efficiency of this microscope is limited to 50% owing to the open area left between the microchannels on the detector plates. The APT data were processed using the commercial software package IVAS 3.6.12.
ance.17 Recently, Pei and co-workers12,14 demonstrated that dislocations inside the grains are also beneficial for lowering the lattice thermal conductivity and enhancing TE properties. It is plausible that not only the structure but also the composition of matrix and defects play a significant role in tuning their phonon scattering efficiency and thus their TE performance.18 The structural configuration of dislocations can be determined by high-resolution transmission electron microscopy (TEM),12,14,16,17 but the local chemical composition of dislocations remains for most systems practically inaccessible by energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS), due to projection effects and limited spatial and/or chemical resolution. Only a few studies report on the chemical composition of dislocations using TEMbased methods.19,20 Hitherto, the local chemical composition of specific defects within subnanometers resolution is still one of the missing pieces in the puzzle of TE materials design. To investigate the chemical decoration state of dislocations, we use atom probe tomography (APT) which has a unique ability to achieve three-dimensional, near-atomic-scale analytical imaging with a sensitivity in the ppm range.21,22 APT has been successfully applied to investigate the composition of nanoprecipitates,7,23,24 and elemental segregation to grain boundaries7,25−27 and to dislocations.28,29 Considering the outstanding TE performance of PbTe-based alloys and the high diffusion coefficient and low solubility of Ag in PbTe,30−34 in this work, a (PbTe)0.97(Ag2Te)0.03 alloy has been investigated to determine the local structure and composition of dislocations within grains and at low-angle grain boundaries. The misorientation values, geometry, strain field, and Burgers vector of the dislocations were resolved by correlating high-resolution transmission Kikuchi diffraction (TKD)35 and aberration-corrected scanning transmission electron microscopy (STEM). The chemical composition of dislocations within grains and at low-angle grain boundaries was determined by APT. We find that Ag atoms strongly segregate to dislocations and exhibit compositional fluctuations along the dislocation line. The highest Ag concentration measured at the dislocation core is 1 order of magnitude higher than that in the matrix. Moreover, the localized composition fluctuation might perturb the charge balance, the electrical and strain fields, and thus, the TE properties. Our results provide the missing information for analyzing the influence of dislocations on TE performance and may help to guide the design of TE materials.
3. RESULTS AND DISCUSSION Figure 1 shows the correlative TKD−TEM measurement performed for a lamella. Two grain boundaries are observed from the bright-field (BF) and dark-field (DF) micrographs as shown in Figures 1a and 1b, respectively. Some dislocations can also be observed in the left corner highlighted by yellow arrows. To determine the nature of the grain boundaries, we performed TKD analysis directly on the TEM lamella. TKD is based on forward-scattering phenomenon and thus can reach a higher spatial resolution of 2 in p-Type AgSbTe2−xSex Alloys via Exploring the Extra Light Valence Band and Introducing Dense Stacking Faults. Adv. Energy Mater. 2017, 1702333.
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DOI: 10.1021/acsami.7b17142 ACS Appl. Mater. Interfaces 2018, 10, 3609−3615
