Improved Thermoelectric Performance of p-Type PbTe by Entropy Engineering and Temperature-Dependent Precipitates

Entropy engineering is aneffective scheme to reduce the thermal conductivity of thermoelectric materials, but it inevitably deteriorates the carrier mobility. Here, we report the optimization of thermoelectric performance of PbTe by combining entropy engineering and nanoprecipitates. In the continuously tuned compounds of Pb0.98Na0.02Te(1-2x)SxSex, we show that the x = 0.05 sample exhibits an exceptionally low thermal conductivity relative to its configuration entropy. By introducing Mn doping, the produced temperature-dependent nanoprecipitates of MnSe cause the high-temperature thermal conductivity to be further reduced. A very low lattice thermal conductivity of 0.38 W m-1 K-1 is achieved at 825 K. Meanwhile, the carrier mobility of the samples is only slightly influenced, owing to the well-controlled configuration entropy and the size of nanoprecipitates. Finally, a high peak zT of ∼2.1 at 825 K is obtained in the Pb0.9Na0.04Mn0.06Te0.9S0.05Se0.05 alloy.

Thermal Stability of Cu-Al-Ni Shape Memory Alloy Thin Films Obtained by Nanometer Multilayer Deposition

Cu-Al-Ni is a high-temperature shape memory alloy (HTSMA) with exceptional thermomechanical properties, making it an ideal active material for engineering new technologies able to operate at temperatures up to 200 °C. Recent studies revealed that these alloys exhibit a robust superelastic behavior at the nanometer scale, making them excellent candidates for developing a new generation of micro-/nano-electromechanical systems (MEMS/NEMS). The very large-scale integration (VLSI) technologies used in microelectronics are based on thin films. In the present work, 1 μm thickness thin films of 84.1Cu-12.4 Al-3.5Ni (wt.%) were obtained by solid-state diffusion from a multilayer system deposited on SiNx (200 nm)/Si substrates by e-beam evaporation. With the aim of evaluating the thermal stability of such HTSMA thin films, heating experiments were performed in situ inside the transmission electron microscope to identify the temperature at which the material was decomposed by precipitation. Their microstructure, compositional analysis, and phase identification were characterized by scanning and transmission electron microscopy equipped with energy dispersive X-ray spectrometers. The nucleation and growth of two stable phases, Cu-Al-rich alpha phase and Ni-Al-rich intermetallic, were identified during in situ heating TEM experiments between 280 and 450 °C. These findings show that the used production method produces an HTSMA with high thermal stability and paves the road for developing high-temperature MEMS/NEMS using shape memory and superelastic technologies.

Nanoprecipitates to Enhance Radiation Tolerance in High-Entropy Alloys

The growth of advanced energy technologies for power generation is enabled by the design, development, and integration of structural materials that can withstand extreme environments, such as high temperatures, radiation damage, and corrosion. High-entropy alloys (HEAs) are a class of structural materials in which suitable chemical elements in four or more numbers are mixed to typically produce single-phase concentrated solid solution alloys (CSAs). Many of these alloys exhibit good radiation tolerance like limited void swelling and hardening up to relatively medium radiation doses (tens of displacements per atom (dpa)); however, at higher radiation damage levels (>50 dpa), some HEAs suffer from considerable void swelling limiting their near-term acceptance for advanced nuclear reactor concepts. In this study, we developed a HEA containing a high density of Cu-rich nanoprecipitates distributed in the HEA matrix. The Cu-added HEA, NiCoFeCrCu_0.12, shows excellent void swelling resistance and negligible radiation-induced hardening upon irradiation up to high radiation doses (i.e., higher than 100 dpa). The void swelling resistance of the alloy is measured to be significantly better than NiCoFeCr CSA and austenitic stainless steels. Density functional theory simulations predict lower vacancy and interstitial formation energies at the coherent interfaces between Cu-rich nanoprecipitates and the HEA matrix. The alloy maintained a high sink strength achieved via nanoprecipitates and the coherent interface with the matrix at a high radiation dose (∼50 dpa). From our experiments and simulations, the effective recombination of radiation-produced vacancies and interstitials at the coherent interfaces of the nanoprecipitates is suggested to be the critical mechanism responsible for the radiation tolerance of the alloy. The materials design strategy based on incorporating a high density of interfaces can be applied to high-entropy alloy systems to improve their radiation tolerance.

High-Mg calcite nanoparticles within a low-Mg calcite matrix: A widespread phenomenon in biomineralization

Biominerals are extraordinarily intricate and possess superior mechanical properties compared with their synthetic counterparts. In this study, we show that the presence of high-Mg calcite nanoparticles within a low-Mg calcite matrix is a widespread phenomenon among marine organisms whose skeletons are composed of high-Mg calcite. It seems most likely that formation of such a complex structure is possible because of the phase separation that occurs as a result of spinodal decomposition of an amorphous Mg–calcium carbonate precursor and is followed by crystallization. We demonstrate that the basis of such phase separation stems from chemical composition rather than from biological similarities. The presence of high-Mg calcite nanoparticles increases the skeletons’ toughness and hardness.

During the process of biomineralization, organisms utilize various biostrategies to enhance the mechanical durability of their skeletons. In this work, we establish that the presence of high-Mg nanoparticles embedded within lower-Mg calcite matrices is a widespread strategy utilized by various organisms from different kingdoms and phyla to improve the mechanical properties of their high-Mg calcite skeletons. We show that such phase separation and the formation of high-Mg nanoparticles are most probably achieved through spinodal decomposition of an amorphous Mg-calcite precursor. Such decomposition is independent of the biological characteristics of the studied organisms belonging to different phyla and even kingdoms but rather, originates from their similar chemical composition and a specific Mg content within their skeletons, which generally ranges from 14 to 48 mol % of Mg. We show evidence of high-Mg calcite nanoparticles in the cases of six biologically different organisms all demonstrating more than 14 mol % Mg-calcite and consider it likely that this phenomenon is immeasurably more prevalent in nature. We also establish the absence of these high-Mg nanoparticles in organisms whose Mg content is lower than 14 mol %, providing further evidence that whether or not spinodal decomposition of an amorphous Mg-calcite precursor takes place is determined by the amount of Mg it contains. The valuable knowledge gained from this biostrategy significantly impacts the understanding of how biominerals, although composed of intrinsically brittle materials, can effectively resist fracture. Moreover, our theoretical calculations clearly suggest that formation of Mg-rich nanoprecipitates greatly enhances the hardness of the biomineralized tissue as well.

Nanoprecipitate-Strengthened High-Entropy Alloys

Multicomponent high-entropy alloys (HEAs) can be tuned to a simple phase with some unique alloy characteristics. HEAs with body-centered-cubic (BCC) or hexagonal-close-packed (HCP) structures are proven to possess high strength and hardness but low ductility. The faced-centered-cubic (FCC) HEAs present considerable ductility, excellent corrosion and radiation resistance. However, their strengths are relatively low. Therefore, the strategy of strengthening the ductile FCC matrix phase is usually adopted to design HEAs with excellent performance. Among various strengthening methods, precipitation strengthening plays a dazzling role since the characteristics of multiple principal elements and slow diffusion effect of elements in HEAs provide a chance to form fine and stable nanoscale precipitates, pushing the strengths of the alloys to new high levels. This paper summarizes and review the recent progress in nanoprecipitate-strengthened HEAs and their strengthening mechanisms. The alloy-design strategies and control of the nanoscale precipitates in HEAs are highlighted. The future works on the related aspects are outlined.

Enhanced Thermoelectric and Mechanical Performance in n-Type Yb-Filled Skutterudites through Aluminum Alloying

Herein, we demonstrate a synergistic combination of novel mechanisms in aluminum (Al)-alloyed Yb_0.3Co₄Sb₁₂-based thermoelectric materials to address both reduction in thermal conductivity and concomitant enhancement in power factor (PF). Upon Al alloying, CoAl nanoprecipitates are embedded in the matrix, leading to (1) significant local strain and thus intensified phonon scattering and (2) carrier injection because of interphase electron transfer. Moreover, by decreasing the Yb filling fraction, not only is the electronic thermal conductivity significantly suppressed but also the carrier concentration is modulated to the optimum range, thus resulting in the dramatically boosted PF, especially below 773 K. As a result, a peak ZT value of 1.36 at 873 K and ZT_ave of 0.96 from 300 to 873 K were obtained in Yb_0.21Co₄Sb₁₂/0.32CoAl. Last but not the least, the mechanical properties of the Al-alloyed samples were considerably improved through CoAl precipitate hardening, offering great potential for commercial applications.

Effect of Zr Addition on the Microstructure and Mechanical Properties of CoCrFeNiMn High-Entropy Alloy Synthesized by Spark Plasma Sintering

As a classic high-entropy alloy system, CoCrFeNiMn is widely investigated. In the present work, we used ZrH₂ powders and atomized CoCrFeNiMn powders as raw materials to prepare CoCrFeNiMnZr_x (x = 0, 0.2, 0.5, 0.8, 1.0) alloys by mechanical alloying (MA), followed by spark plasma sintering (SPS). During the MA process, a small amount of Zr (x ≤ 0.5) can be completely dissolved into CoCrFeNiMn matrix, when the Zr content is above 0.5, the ZrH₂ is excessive. After SPS, CoCrFeNiMn alloy is still as single face-centered cubic (FCC) solid solution, and CoCrFeNiMnZr_x (x ≥ 0.2) alloys have two distinct microstructural domains, one is a single FCC phase without Zr, the other is a Zr-rich microstructure composed of FCC phase, B2 phase, Zr₂Ni₇, and σ phase. The multi-phase microstructures can be attributed to the large lattice strain and negative enthalpy of mixing, caused by the addition of Zr. It is worth noting that two types of nanoprecipitates (body-centered cubic (BCC) phase and Zr₂Ni₇) are precipitated in the Zr-rich region. These can significantly increase the yield strength of the alloys.

Enhancing Thermoelectric Performances of Bismuth Antimony Telluride via Synergistic Combination of Multiscale Structuring and Band Alignment by FeTe2 Incorporation

It has been a difficulty to form well-distributed nano- and mesosized inclusions in a Bi₂Te₃-based matrix and thereby realizing no degradation of carrier mobility at interfaces between matrix and inclusions for high thermoelectric performances. Herein, we successfully synthesize multistructured thermoelectric Bi_0.4Sb_1.6Te₃ materials with Fe-rich nanoprecipitates and sub-micron FeTe₂ inclusions by a conventional solid-state reaction followed by melt-spinning and spark plasma sintering that could be a facile preparation method for scale-up production. This study presents a bismuth antimony telluride based thermoelectric material with a multiscale structure whose lattice thermal conductivity is drastically reduced with minimal degradation on its carrier mobility. This is possible because a carefully chosen FeTe₂ incorporated in the matrix allows its interfacial valence band with the matrix to be aligned, leading to a significantly improved p-type thermoelectric power factor. Consequently, an impressively high thermoelectric figure of merit ZT of 1.52 is achieved at 396 K for p-type Bi_0.4Sb_1.6Te₃-8 mol % FeTe₂, which is a 43% enhancement in ZT compared to the pristine Bi_0.4Sb_1.6Te₃. This work demonstrates not only the effectiveness of multiscale structuring for lowering lattice thermal conductivities, but also the importance of interfacial band alignment between matrix and inclusions for maintaining high carrier mobilities when designing high-performance thermoelectric materials.

Contrasting the Role of Mg and Ba Doping on the Microstructure and Thermoelectric Properties of p-Type AgSbSe2

Microstructure has a critical influence on the mechanical and functional properties. For thermoelectric materials, deep understanding of the relationship of microstructure and thermoelectric properties will enable the rational optimization of the ZT value and efficiency. Herein, taking AgSbSe2 as an example, we first report a different role of alkaline-earth metal ions (Mg(2+) and Ba(2+)) doping in the microstructure and thermoelectric properties of p-type AgSbSe2. For Mg doping, it monotonously increases the carrier concentration and then reduces the electrical resistivity, leading to a substantially enhanced power factor in comparison to those of other dopant elements (Bi(3+), Pb(2+), Zn(2+), Na(+), and Cd(2+)) in the AgSbSe2 system. Meanwhile, the lattice thermal conductivity is gradually suppressed by point defects scattering. In contrast, the electrical resistivity first decreases and then slightly rises with the increased Ba-doping concentrations due to the presence of BaSe3 nanoprecipitates, exhibiting a different variation tendency compared with the corresponding Mg-doped samples. More significantly, the total thermal conductivity is obviously reduced with the increased Ba-doping concentrations partially because of the strong scattering of medium and long wavelength phonons via the nanoprecipitates, consistent with the theoretical calculation and analysis. Collectively, ZT value ∼1 at 673 K and calculated leg efficiency ∼8.5% with Tc = 300 K and Th = 673 K are obtained for both AgSb0.98Mg0.02Se2 and AgSb0.98Ba0.02Se2 samples.

Thermoelectric Properties of Ga/Ag Codoped Type-III Ba₂₄Ge₁₀₀ Clathrates with in Situ Nanostructures

Because of the low thermal conductivity and high electrical conductivity, type-III Ba24Ge100 clathrates are potentially of interest as power generation thermoelectric materials for midto-high temperature operations. Unfortunately, their too high intrinsic carrier concentration results in a quite low Seebeck coefficient. To reduce the carrier concentration, we prepared a series of Ga/Ag codoped type-III Ba24Ge100 clathrate specimens by vacuum melting and subsequently compacted by spark plasma sintering (SPS). Doping Ga-Ag on the sites of Ge reduces the concentration of electrons and, at higher concentrations, also leads to the in situ formation of BaGe2 nanoprecipitates detected by the microstructural analysis. As a result of doping, the Seebeck coefficient increases, the thermal conductivity decreases, and the dimensionless figure of merit ZT reaches a value of 0.34 at 873 K, more than three times the value obtained with undoped Ba24Ge100.