Ag-Segregation to Dislocations in PbTe-Based Thermoelectric Materials

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Ag-segregation to dislocations in PbTe-based thermoelectric materials Yuan Yu, Siyuan Zhang, Antonio Massimiliano Mio, Baptiste Gault, Ariel Sheskin, Christina Scheu, Dierk Raabe, Fang-Qiu Zu, Matthias Wuttig, Yaron Amouyal, and Oana Cojocaru-Miredin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17142 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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ACS Applied Materials & Interfaces

Ag-segregation to dislocations in PbTe-based thermoelectric materials Yuan Yu,a,b Siyuan Zhang,c Antonio Massimiliano Mio,a Baptiste Gault,c Ariel Sheskin,d Christina Scheu,c Dierk Raabe,c Fangqiu Zu,b,* Matthias Wuttig,a,e Yaron Amouyal,d,* and Oana Cojocaru-Mirédina,c,* a

I. Physikalisches Institut (IA), RWTH Aachen, 52074, Aachen, Germany

b

Liquid/solid Metal processing Institute, School of Materials Science and Engineering, Hefei

University of Technology, Hefei 230009, China c

Max-Planck Institut für Eisenforschung GmbH (MPIE), 40237, Düsseldorf, Germany

d

Department of Materials Science and Engineering, Technion-Israel Institute of Technology,

Technion City, 32000 Haifa, Israel e

JARA-Institut Green IT, JARA-FIT, Forschungszentrum Jülich GmbH and RWTH Aachen

University, 52056 Aachen, Germany

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 1 ACS Paragon Plus Environment

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dislocations with a tenfold 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 at.% to ~10 at.% 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 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. 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 mid-frequency phonons are effectively scattered by dislocation cores and strain fields.12-14 Such 2 ACS Paragon Plus Environment

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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 mid-frequency 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 performance.17 Recently, Pei and coworkers12, 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 plays 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 TEM based methods.19-20 Hitherto, the local chemical composition of specific defects within sub-nanometers resolution is still one of the missing pieces in the puzzle of TE materials design. In order 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 3 ACS Paragon Plus Environment


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 one 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. 2. Experimental section Ag-alloyed PbTe specimens were synthesized from pure elemental Pb powder (99.96%, Riedelde Haën®), Te ingots (99.99%, STREM CHEMICALS®), and Ag ingots (99.999%, Alfa Aesar®) by mixing in appropriate molar ratios to obtain the average composition of (PbTe)0.97(Ag2Te)0.03. We chose this composition since it is within the single-phase regime at temperatures above 600 °C, and is expected to decompose into a two-phase mixture, namely Ag2Te + PbTe, via solid-state precipitation at lower temperatures. The pure Pb, Te, and Ag raw materials were poured into a quartz ampoule, which was evacuated and refilled with a 120 torr Ar-7 % H2 gas mixture to avoid oxidation. The sealed ampoule was subsequently heated to 1000 °C for 6 h in a vertical programmable tube furnace for melting, followed by moderate cooling down to 700 °C and dwelling for 48 h to enable homogenization. Termination of the latter step was performed by quenching in an ice-water bath. The ingot, made of a super-saturated PbTe-based solid solution, was then grinded and hot-pressed at 650 °C (within the single-phase region) in a 12.5 mm diameter die under a pressure of 45 MPa for 30 minutes and flowing Ar-7 % H2 gas mixture to 4 ACS Paragon Plus Environment

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prevent oxidation, followed by ice-water quenching. The hot-pressed disks were finally aged at 380 °C for 48 h in sealed quartz ampoules 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 and 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 Ω , 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 high angle annular dark field (HAADF) detectors were 8~16 mrad 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 Hikari high speed camera (EDAX/TSL). 5 ACS Paragon Plus Environment


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 picosecond 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. 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. In order 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 I> (K) (" O ()B QM

(3)

where ;R is the Boltzmann constant, S is the average sound velocity, ℏ is the reduced Planck constant, is the Debye temperature, T is the frequency of phonons, and K = ℏT/;R U. 5>I> is the total relaxation time, and is calculated according to the Matthiessen’s rule:14 ( ( ( 5>I> = 56( $ 57( $ 58( $ 59 $ 5:

(4)

The associated equations and parameters for each phonon scattering process are provided in the supporting information and Table S1. Klemens has pointed out that for an alloy comprising dislocations, the concentration of solute atoms could be modulated by the strain fields around the dislocations. This can reinforce or oppose the scattering of dislocation strain fields depending on atomic mass and volume differences.48 The modified scattering due to dislocation strains can be calculated by changing the Grüneisen parameter,16, 48 which is equivalent to changing the prefactor C in Equation (S8). Since the integral of the spectral lattice thermal conductivity (Ks) with respect to the phonon frequency equals to the lattice thermal conductivity (Kl), we can clarify which sort of phonon scatterings contributes to Kl by evaluating Ks.16, 49 Thus, we calculated the Ks using two different pre-factors for dislocation strain scattering. The calculated Ks at room temperature and temperature dependent Kl are shown in Figure 4a and 4b, respectively. It is indicated that the model using a pre-factor of 0.96,14 disregarding the variation of solute composition at dislocations, contributes to less phonon scattering and exhibits larger values of calculated lattice thermal conductivity compared with the experimental data. However, considering the dislocation composition using a modified pre-factor of 5, the calculated data are well-fitted to the experimental one, which highlights the influence of dislocation composition on 14 ACS Paragon Plus Environment

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phonon scattering. To the best of our knowledge, only the density of dislocations has been modified up to now for tuning TE transport coefficients, i.e. the option of controlling their compositions has been overlooked so far.12-14, 16 Here, we find that dislocations can be decorated by solute atoms, whose local distribution and their effects on stoichiometry can be detected by APT. These decorated dislocations provide an alternative and feasible approach to modulate the structure and chemical composition of dislocations to stabilize them and render them more efficient phonon scattering sites. Both factors can act as potent parameters for optimizing TE alloys.

Figure 4. (a) Calculated spectral lattice thermal conductivity at room temperature using DebyeCallaway’s model; (b) Calculated lattice thermal conductivity compared with the experimental data, which was obtained by subtracting the electronic thermal conductivity from the measured total thermal conductivity. 4. Conclusions and outlook The structural and chemical features of dislocations in a (PbTe)0.97(Ag2Te)0.03 TE alloy were analyzed using state-of-the-art correlative TKD and aberration-corrected STEM in conjunction with APT. We found numerous Ag-decorated dislocations inside the grain and inside a low-angle grain boundary. The highest concentration of Ag within the dislocation line is ~10 at.%, which is 15 ACS Paragon Plus Environment


nearly one order of magnitude higher than the Ag content in the matrix. Such Ag-decorated dislocations further lower the lattice thermal conductivity owing to extra dislocation strain field phonon scattering. Our results may pave the way to tailor the structure of TE materials further and provide missing information regarding the dislocation composition required for theoretical calculations of the contribution of dislocations to phonon scattering. Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Poles depicted by the iso-density map and scheme of the inverse point-projection model for APT data reconstruction (Figure S1); Gibbsian interfacial excess of Ag at the dislocations (Figure S2); Multiscale structures of the analyzed sample including grain boundaries, Ag2Te precipitates, dislocations, and nanoscale defects (Figure S3); Calculation of the lattice thermal conductivity according to the Debye-Callaway model and the associated parameters (Table S1). Corresponding Author *E-mail: [email protected] (F.Q.Z.); [email protected] (Y.A.); [email protected] (O. C.-M.) Notes The authors declare no competing financial interest.

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Acknowledgments This work was funded by the Excellence Initiative of the German Federal and State Governments, by the international Max Planck Research School for Interface Controlled Materials for Energy Conversion, and by RWTH Aachen University through SEED FUNDS 2016 program. Y.A. acknowledges generous support from the Israel Science Foundation (ISF), grant no. 698/13, and from the German-Israeli Foundation for Research and Development (GIF), grant no. I-23331150.10/2012. Y.Y. acknowledges the support from the National Natural Science Foundation of China (grant No. 51371073) and the Chinese Scholarship Council for providing his visiting scholar stipend. References 1.

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Ag-Segregation to Dislocations in PbTe-Based Thermoelectric Materials

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