Pressure-Induced Dislocations and Their Influence on Ionic Transport in Li+-Conducting Argyrodites.

The influence of the microstructure on the ionic conductivity and cell performance is a topic of broad scientific interest in solid-state batteries. The current understanding is that interfacial decomposition reactions during cycling induce local strain at the interfaces between solid electrolytes and the anode/cathode, as well as within the electrode composites. Characterizing the effects of internal strain on ion transport is particularly important, given the significant local chemomechanical effects caused by volumetric changes of the active materials during cycling. Here, we show the effects of internal strain on the bulk ionic transport of the argyrodite Li6PS5Br. Internal strain is reproducibly induced by applying pressures with values up to 10 GPa. An internal permanent strain is observed in the material, indicating long-range strain fields typical for dislocations. With increasing dislocation densities, an increase in the lithium ionic conductivity can be observed that extends into improved ionic transport in solid-state battery electrode composites. This work shows the potential of strain engineering as an additional approach for tuning ion conductors without changing the composition of the material itself.

On the Discrepancy between Local and Average Structure in the Fast Na+ Ionic Conductor Na2.9Sb0.9W0.1S4.

Aliovalent substitution is a common strategy to improve the ionic conductivity of solid electrolytes for solid-state batteries. The substitution of SbS43- by WS42- in Na2.9Sb0.9W0.1S4 leads to a very high ionic conductivity of 41 mS cm-1 at room temperature. While pristine Na3SbS4 crystallizes in a tetragonal structure, the substituted Na2.9Sb0.9W0.1S4 crystallizes in a cubic phase at room temperature based on its X-ray diffractogram. Here, we show by performing pair distribution function analyses and static single-pulse 121Sb NMR experiments that the short-range order of Na2.9Sb0.9W0.1S4 remains tetragonal despite the change in the Bragg diffraction pattern. Temperature-dependent Raman spectroscopy revealed that changed lattice dynamics due to the increased disorder in the Na+ substructure leads to dynamic sampling causing the discrepancy in local and average structure. While showing no differences in the local structure, compared to pristine Na3SbS4, quasi-elastic neutron scattering and solid-state 23Na nuclear magnetic resonance measurements revealed drastically improved Na+ diffusivity and decreased activation energies for Na2.9Sb0.9W0.1S4. The obtained diffusion coefficients are in very good agreement with theoretical values and long-range transport measured by impedance spectroscopy. This work demonstrates the importance of studying the local structure of ionic conductors to fully understand their transport mechanisms, a prerequisite for the development of faster ionic conductors.

Correction: The importance of phase equilibrium for doping efficiency: iodine doped PbTe.

Correction for 'The importance of phase equilibrium for doping efficiency: iodine doped PbTe' by James Male et al., Mater. Horiz., 2019, 6, 1444-1453, DOI: 10.1039/C9MH00294D.

Estimating the lower-limit of fracture toughness from ideal-strength calculations.

Fracture mechanics is a fundamental topic to materials science. Fracture toughness, in particular, is a material property of great technological importance for device design. The relatively low fracture toughness of many semiconductor materials, including electronic and energy materials, handicaps their use in applications involving large external stresses. Here, it is shown that quantum-mechanical density functional theory calculations of ideal strength, in conjunction with an integral stress-displacement method, can be used to estimate the fracture energy needed to calculate fracture toughness. Using the fracture energy associated with the weakest crystallographic direction provides an estimation for the lower-limit of the fracture toughness of a material. The lower-limit values are in good agreement with experimental single crystal measurements across several orders-of-magnitude of fracture toughness. Furthermore, the proposed methodology is useful for benchmarking experimental measurements of fracture toughness in polycrystalline materials and can serve as a starting point for the construction of more detailed fracture models and the computational design of new materials and devices.

Inherent Anharmonicity of Harmonic Solids.

Atomic vibrations, in the form of phonons, are foundational in describing the thermal behavior of materials. The possible frequencies of phonons in materials are governed by the complex bonding between atoms, which is physically represented by a spring-mass model that can account for interactions (spring forces) between the atoms (masses). The lowest-order, harmonic, approximation only considers linear forces between atoms and is thought incapable of explaining phenomena like thermal expansion and thermal conductivity, which are attributed to nonlinear, anharmonic, interactions. Here, we show that the kinetic energy of atoms in a solid produces a pressure much like the kinetic energy of atoms in a gas does. This vibrational or phonon pressure naturally increases with temperature, as it does in a gas and therefore results in a thermal expansion. Because thermal expansion thermodynamically defines a Grüneisen parameter γ, which is a typical metric of anharmonicity, we show that even a harmonic solid will necessarily have some anharmonicity. A consequence of this phonon pressure model is a harmonic estimation of the Grüneisen parameter as γ≈3/23-4x2/1+2x2, where x=vt/vl is the ratio of the transverse and longitudinal speeds of sound. We demonstrate the immediate utility of this model by developing a high-throughput harmonic estimate of lattice thermal conductivity that is comparable to other state-of-the-art estimations. By linking harmonic and anharmonic properties explicitly, this study provokes new ideas about the fundamental nature of anharmonicity, while also providing a basis for new material engineering design metrics.

Phase Transformation Contributions to Heat Capacity and Impact on Thermal Diffusivity, Thermal Conductivity, and Thermoelectric Performance.

The accurate characterization of thermal conductivity κ, particularly at high temperature, is of paramount importance to many materials, thermoelectrics in particular. The ease and access of thermal diffusivity D measurements allows for the calculation of κ when the volumetric heat capacity, ρcp , of the material is known. However, in the relation κ = ρcp D, there is some confusion as to what value of cp should be used in materials undergoing phase transformations. Herein, it is demonstrated that the Dulong-Petit estimate of cp at high temperature is not appropriate for materials having phase transformations with kinetic timescales relevant to thermal transport. In these materials, there is an additional capacity to store heat in the material through the enthalpy of transformation ΔH. This can be described using a generalized model for the total heat capacity for a material [Formula: see text] where φ is an order parameter that describes how much latent heat responds "instantly" to temperature changes. Here, Cpφ is the intrinsic heat capacity (e.g., approximately the Dulong-Petit heat capacity at high temperature). It is shown experimentally in Zn4 Sb3 that the decrease in D through the phase transition at 250 K is fully accounted for by the increase in cp , while κ changes smoothly through the phase transition. Consequently, reports of κ dropping near phase transitions in widely studied materials such as PbTe and SnSe have likely overlooked the effects of excess heat capacity and overestimated the thermoelectric efficiency, zT.

Lattice Softening Significantly Reduces Thermal Conductivity and Leads to High Thermoelectric Efficiency.

The influence of micro/nanostructure on thermal conductivity is a topic of great scientific interest, particularly to thermoelectrics. The current understanding is that structural defects decrease thermal conductivity through phonon scattering where the phonon dispersion and speed of sound are assumed to remain constant. Experimental work on a PbTe model system is presented, which shows that the speed of sound linearly decreases with increased internal strain. This softening of the materials lattice completely accounts for the reduction in lattice thermal conductivity, without the introduction of additional phonon scattering mechanisms. Additionally, it is shown that a major contribution to the improvement in the thermoelectric figure of merit (zT > 2) of high-efficiency Na-doped PbTe can be attributed to lattice softening. While inhomogeneous internal strain fields are known to introduce phonon scattering centers, this study demonstrates that internal strain can modify phonon propagation speed as well. This presents new avenues to control lattice thermal conductivity, beyond phonon scattering. In practice, many engineering materials will exhibit both softening and scattering effects, as is shown in silicon. This work shines new light on studies of thermal conductivity in fields of energy materials, microelectronics, and nanoscale heat transfer.

The importance of phase equilibrium for doping efficiency: iodine doped PbTe.

Semiconductor engineering relies heavily on doping efficiency and dopability. Low doping efficiency may cause low mobility and failure to reach target carrier concentrations or even the desired carrier type. Semiconducting thermoelectric materials perform best with degenerate carrier concentrations, meaning high performance in new materials might not be realized experimentally without a route to optimal doping. Doping in the classic PbTe thermoelectric system has been largely successful but reported doping efficiencies can vary, raising concerns about reproducibility. Here, we stress the importance of phase equilibria considerations during synthesis to avoid undesired intrinsic defects leading to sub-optimal doping. By saturation annealing at 973 K, we decidedly fix the composition in single crystal iodine-doped PbTe samples to be Pb-rich or Te-rich without introducing impurity phases. We show that, regardless of iodine concentration, degenerate n-type carrier concentrations with ideal doping efficiency require Pb-rich compositions. Electrons in Te-rich samples are heavily compensated by charged intrinsic Pb vacancy defects. From Hall effect measurements and a simple defect model supported by modern defect calculations, we map out the 973 K ternary Pb-Te-I phase diagram to explicitly link carrier concentration and composition. Furthermore, we discuss unintentional composition changes due to loss of volatile Te during synthesis and measurements. The methods and concepts applied here may guide doping studies on other lead chalcogenide systems as well as any doped, complex semiconductor.

Compliant and stretchable thermoelectric coils for energy harvesting in miniature flexible devices.

With accelerating trends in miniaturization of semiconductor devices, techniques for energy harvesting become increasingly important, especially in wearable technologies and sensors for the internet of things. Although thermoelectric systems have many attractive attributes in this context, maintaining large temperature differences across the device terminals and achieving low-thermal impedance interfaces to the surrounding environment become increasingly difficult to achieve as the characteristic dimensions decrease. Here, we propose and demonstrate an architectural solution to this problem, where thin-film active materials integrate into compliant, open three-dimensional (3D) forms. This approach not only enables efficient thermal impedance matching but also multiplies the heat flow through the harvester, thereby increasing the efficiencies for power conversion. Interconnected arrays of 3D thermoelectric coils built using microscale ribbons of monocrystalline silicon as the active material demonstrate these concepts. Quantitative measurements and simulations establish the basic operating principles and the key design features. The results suggest a scalable strategy for deploying hard thermoelectric thin-film materials in harvesters that can integrate effectively with soft materials systems, including those of the human body.

Suppression of atom motion and metal deposition in mixed ionic electronic conductors.

Many superionic mixed ionic-electronic conductors with a liquid-like sublattice have been identified as high efficiency thermoelectric materials, but their applications are limited due to the possibility of decomposition when subjected to high electronic currents and large temperature gradients. Here, through systematically investigating electromigration in copper sulfide/selenide thermoelectric materials, we reveal the mechanism for atom migration and deposition based on a critical chemical potential difference. Then, a strategy for stable use is proposed: constructing a series of electronically conducting, but ion-blocking barriers to reset the chemical potential of such conductors to keep it below the threshold for decomposition, even if it is used with high electric currents and/or large temperature differences. This strategy not only opens the possibility of using such conductors in thermoelectric applications, but may also provide approaches to engineer perovskite photovoltaic materials and the experimental methods may be applicable to understanding dendrite growth in lithium ion batteries.

Manipulating Band Structure through Reconstruction of Binary Metal Sulfide for High-Performance Thermoelectrics in Solution-Synthesized Nanostructured Bi13 S18 I2.

Reconstructing canonical binary compounds by inserting a third agent can significantly modify their electronic and phonon structures. Therefore, it has inspired the semiconductor communities in various fields. Introducing this paradigm will potentially revolutionize thermoelectrics as well. Using a solution synthesis, Bi2 S3 was rebuilt by adding disordered Bi and weakly bonded I. These new structural motifs and the altered crystal symmetry induce prominent changes in electrical and thermal transport, resulting in a great enhancement of the figure of merit. The as-obtained nanostructured Bi13 S18 I2 is the first non-toxic, cost-efficient, and solution-processable n-type material with z T=1.0.

Micro- and Macromechanical Properties of Thermoelectric Lead Chalcogenides.

Both n- and p-type lead telluride (PbTe)-based thermoelectric (TE) materials display high TE efficiency, but the low fracture strength may limit their commercial applications. To find ways to improve these macroscopic mechanical properties, we report here the ideal strength and deformation mechanism of PbTe using density functional theory calculations. This provides structure-property relationships at the atomic scale that can be applied to estimate macroscopic mechanical properties such as fracture toughness. Among all the shear and tensile paths that are examined here, we find that the lowest ideal strength of PbTe is 3.46 GPa along the (001)/⟨100⟩ slip system. This leads to an estimated fracture toughness of 0.28 MPa m1/2 based on its ideal stress-strain relation, which is in good agreement with our experimental measurement of 0.59 MPa m1/2. We find that softening and breaking of the ionic Pb-Te bond leads to the structural collapse. To improve the mechanical strength of PbTe, we suggest strengthening the structural stiffness of the ionic Pb-Te framework through an alloying strategy, such as alloying PbTe with isotypic PbSe or PbS. This point defect strategy has a great potential to develop high-performance PbTe-based materials with robust mechanical properties, which may also be applied to other materials and applications.

Thin-film metallic glass: an effective diffusion barrier for Se-doped AgSbTe2 thermoelectric modules.

The thermal stability of joints in thermoelectric (TE) modules, which are degraded during interdiffusion between the TE material and the contacting metal, needs to be addressed in order to utilize TE technology for competitive, sustainable energy applications. Herein, we deposit a 200 nm-thick Zr-based thin-film metallic glass (TFMG), which acts as an effective diffusion barrier layer with low electrical contact resistivity, on a high-zT Se-doped AgSbTe2 substrate. The reaction couples structured with TFMG/TE are annealed at 673 K for 8-360 hours and analyzed by electron microscopy. No observable IMCs (intermetallic compounds) are formed at the TFMG/TE interface, suggesting the effective inhibition of atomic diffusion that may be attributed to the grain-boundary-free structure of TFMG. The minor amount of Se acts as a tracer species, and a homogeneous Se-rich region is found nearing the TFMG/TE interface, which guarantees satisfactory bonding at the joint. The diffusion of Se, which has the smallest atomic volume of all the elements from the TE substrate, is found to follow Fick's second law. The calculated diffusivity (D) of Se in TFMG falls in the range of D~10-20-10-23(m2/s), which is 106~107 and 1012~1013 times smaller than those of Ni [10-14-10-17(m2/s)] and Cu [10-8-10-11(m2/s)] in Bi2Te3, respectively.

Nanocomposites from Solution-Synthesized PbTe-BiSbTe Nanoheterostructure with Unity Figure of Merit at Low-Medium Temperatures (500-600 K).

A scalable, low-temperature solution process is used to synthesize precursor material for Pb-doped Bi0.7 Sb1.3 Te3 thermoelectric nanocomposites. The controllable Pb-doping leads to the increase in the optical bandgap, thus delaying the onset of bipolar conduction. Furthermore, the solution synthesis enables nanostructuring, which greatly reduces thermal conductivity. As a result, this material exhibits a zT = 1 over the 513-613 K range.

Highly Porous Thermoelectric Nanocomposites with Low Thermal Conductivity and High Figure of Merit from Large-Scale Solution-Synthesized Bi2 Te2.5 Se0.5 Hollow Nanostructures.

To enhance the performance of thermoelectric materials and enable access to their widespread applications, it is beneficial yet challenging to synthesize hollow nanostructures in large quantities, with high porosity, low thermal conductivity (κ) and excellent figure of merit (z T). Herein we report a scalable (ca. 11.0 g per batch) and low-temperature colloidal processing route for Bi2 Te2.5 Se0.5 hollow nanostructures. They are sintered into porous, bulk nanocomposites (phi 10 mm×h 10 mm) with low κ (0.48 W m-1 K-1 ) and the highest z T (1.18) among state-of-the-art Bi2 Te3-x Sex materilas. Additional benefits of the unprecedented low relative density (68-77 %) are the large demand reduction of raw materials and the improved portability. This method can be adopted to fabricate other porous phase-transition and thermoelectric chalcogenide materials and will pave the way for the implementation of hollow nanostructures in other fields.

Is there material loss at the backside taper in modular CoCr acetabular liners?

BACKGROUND: Metal wear and corrosion products generated by hip replacements have been linked to adverse local tissue reactions. Recent investigations of the stem/head taper junction have identified this modular interface as another possible source of metal debris; however, little is known regarding other modular metallic interfaces, their ability to produce metal debris, and possibly to provide insight in the mechanisms that produce metal debris. QUESTIONS/PURPOSES: We asked three questions: (1) can we develop a reliable method to estimate volumetric material loss from the backside taper of modular metal-on-metal liners, (2) do backside tapers of modular metal-on-metal liners show a quantifiable volumetric material loss, and, if so, (3) how do regions of quantitatively identified material loss correspond to visual and microscopic investigations of surface damage? METHODS: Twenty-one cobalt-chromium (CoCr) liners of one design and manufacturer were collected through an institutional review board-approved retrieval program. All liners were collected during revision surgeries, where the primary revision reason was loosening (n=11). A roundness machine measured 144 axial profiles equally spaced about the circumference of the taper region near the rim to estimate volume and depth of material loss. Sensitivity and repeatability analyses were performed. Additionally, visual and scanning electron microscopy investigations were done for three liners. RESULTS: Our measurement method was found to be reproducible. The sensitivity (how dependent measurement results are on experimental parameters) and repeatability (how consistent results are between measurements) analyses confirmed that component alignment had no apparent effect (weak correlation, R2=0.04) on estimated volumetric material loss calculations. Liners were shown to have a quantifiable material loss (maximum=1.7 mm3). Visual investigations of the liner surface could identify pristine surfaces as as-manufactured regions, but could misidentify discoloration as a possible region of material loss. Scanning electron microscopy more accurately distinguished between as-manufactured and damaged regions of the taper. CONCLUSIONS: The roundness machine has been used to develop a repeatable method for characterizing material loss; future work comparing a gravimetric standard with estimations of material loss determined from the roundness machine may show the accuracy and effectiveness of this method. Liners show rates of material loss that compare with those reported for other taper junctions. Visual inspection alone may misidentify as-manufactured regions as regions of material loss. CLINICAL RELEVANCE: This study identifies the acetabular liner/shell interface in modular metal-on-metal devices as a potential source of metal wear or corrosion products. The relation between metal debris and clinical performance, regardless of the type of bearing couple, is a concern for clinicians. Therefore, it is important to characterize every type of modular junction to understand the quantity, location, and mechanism(s) of material loss.