Entropy Engineering Research Articles

A novel (Sm0.2Eu0.2Gd0.2Ho0.2Yb0.2)CrO3 high-entropy ceramic nanofiber as a negative temperature coefficient thermistor

The electrical properties of high-entropy ceramics (HECs) have been extensively studied in recent years due to their unique structural characteristics and fascinating functional properties induced by entropy engineering. Novel high-entropy (Sm0.2Eu0.2Gd0.2Ho0.2Yb0.2)CrO3 (HE-RECrO3) nanofibers were prepared by electrospinning. This work demonstrates that HE-RECrO3 nanofibers were successfully synthesized at a low temperature (800 °C), which is approximately 400 °C lower than the temperatures at which chromate ceramics were synthesized via the sol–gel method and the solid-state reaction method. The resistivity of HE-RECrO3 nanofibers decreases exponentially with increasing temperature from 25 to 600 °C. The logarithm of the resistivity is linearly related to the inverse of the temperature, confirming the negative temperature coefficient property of HE-RECrO3 nanofibers. The B25/50 value of the HE-RECrO3 nanofibers reaches 4072 K. In conclusion, HE-RECrO3 nanofibers are expected to be potential candidates for negative-temperature-coefficient (NTC) thermistors.

Cubic phase stabilization and thermoelectric performance optimization in AgBiSe2–SnTe system

AgBiSe2 is a lead-free thermoelectric (TE) material that demonstrates excellent TE performance in its high-temperature cubic phase. However, the known order-disorder transition at around 580 K induces internal stress caused by the sudden change in thermal expansion coefficient, negatively impacting TE device operation. In this study, we heavily alloyed AgBiSe2 with SnTe to increase configurational entropy (ΔS) and maintain disordered cubic phase throughout the entire temperature range. This structure variation from hexagonal to cubic increased the density-of-state effective mass as demonstrated by experimental results and first-principles calculations. Additionally, the lattice thermal conductivity was reduced by multiple point defects. Overall, the average ZT value from 300 K to 773 K increased to 0.3 after SnTe alloying. Our study provides an additional perspective of entropy engineering on thermoelectric properties beyond the conventional phonon scattering, which helps to tailor properties in other TE materials with phase transition.

Towards a new chalcopyrite high-performance thermoelectric semiconductor Cu3InSnSe5 by entropy engineering

A new semiconductor Cu3InSnSe5 is introduced by entropy driven alloying equimolar ratio CuInSe2 and Cu2SnSe3. Cu3InSnSe5 is a p-type semiconductor (Eg ∼0.41 eV) and crystallizes in the chalcopyrite structure with I4_2 m space group. Our density functional theory calculations show that Cu3InSnSe5 exhibits an electronic band structure with multiple valance band peaks, rendering a large density of state effective mass m* of 1.8 me. Therefore, Cu3InSnSe5 has a high power factor of 7.59 μW cm−1 K−2 at 773 K. In addition, Cu3InSnSe5 also has a low lattice thermal conductivity (0.34 W m−1 K−1 at 773 K) owing to its severe lattice distortion arose by entropy enhancement. As a result, the figure of merit reaches as high as 1.08 at 773 K for Cu3InSnSe5. Furthermore, electronic band engineering is adopted to enhance the carrier concentration and m* of Cu3InSnSe5 via the substitution of In for Sn, resulting in ∼28% enhancement in power factor, ∼21% increment in ZT (1.31 at 773 K), and ∼10% enhancement in average ZT (∼ 0.67 at 473 – 773 K) for Cu3In1.01Sn0.99Se5 compared with Cu3InSnSe5 itself. Our work shows Cu3InSnSe5 should be a promising candidate for thermoelectric power generation at medium temperature.

Phonon entropy engineering for caloric cooling

Electrocaloric cooling, with the advantages of zero global warming potential, high efficiency, smart size, etc., is regarded as a promising next-generation technology for green refrigeration. The exotic negative electrocaloric effect (ECE) in antiferroelectric materials forms the basis to improve the caloric cooling power density, but the underlying mechanism remains elusive. By using a fully first-principles method, we successfully simulate the electric field-triggered structural phase transition from antiferroelectric to ferroelectric in a prototypical antiferroelectric material PbZrO3 (PZO). Through tracking the phonon entropy evolution and measuring the temperature-dependent polarization along the transition path, we disclose that the negative ECE in PZO originates from the latent heat associated with phonon entropy rather than the previously recognized dipolar entropy. Accordingly, a new concept of phonon entropy engineering is proposed that engineering the density of states especially for low-frequency phonons can modulate the phonon entropy, which provides an effective route to enhance the cooling power density.

A novel valence-balanced double half-Heusler Ti2Zr2Hf2NbVFe5Ni3Sb8 alloy by high entropy engineering

The lattice thermal conductivity of stable 18-electron Ti2FeNiSb2 compound is remarkably lower than that of the traditional ternary half-Heusler alloy, and it is considered a promising thermoelectric material. Owing to the distinctive performance of high entropy alloys, we designed a novel valence-balanced double half-Heusler Ti2Zr2Hf2NbVFe5Ni3Sb8 alloy with high entropy sublattice. The experimental results showed that enhanced electrical transport properties and reduced thermal conductivity of the alloy were achieved. The thermal conductivity at room temperature was two fifths of that of reported Ti2FeNiSb2 and the maximum power factor improved three times. The ultralow theoretical minimum lattice thermal conductivity was estimated to be less than 0.2 W m−1K−1, showing the possibility for further reduction. Phonon spectrum calculation showed that the high entropy strategy was valid for scattering phonons and limiting lattice thermal conductivity. A peak ZT ∼0.027 at 823 K was achieved, which is almost five times that of the Ti2FeNiSb2. This work demonstrates the effectiveness of the high entropy strategy in double half-Heusler compounds and the potential of the new concept of HE-DHH compounds as high-performance thermoelectric materials.

Lead-free medium-entropy (Na0.47(1-x)Bi0.47(1-x)Ba0.06(1-x)Sr0.7xNd0.2x)TiO3 relaxor ceramics with robust energy-storage performance

High-entropy engineering is becoming a new strategy in dielectric energy storage capacitors. However, high-entropy strategy alone is insufficient to produce good energy storage performance, as the polarization intensity is greatly reduced by the highly disordered state. We, herein, propose an “entropy engineering + polyphase” strategy and highlight that materials with medium entropy values, no necessity of high entropy values, can yield good energy storage performance under condition that polyphases are introduced in the materials. To illustrate the effectiveness of this strategy, the medium-entropy ceramics (Na0.47(1-x)Bi0.47(1-x)Ba0.06(1-x)Sr0.7xNd0.2x)TiO3 with x = 0.1, 0.2, 0.3, 0.4, and 0.5 were designed by doping ions with different ionic radii and valence states at the A-site. Investigations on the energy storage performance reveal that the sample of x = 0.4, with the maximum entropy value of 1.39R, has the largest recyclable energy storage density Wrec = 8.71 J/cm3 with a high efficiency η = 88.5%. The sample also shows excellent thermal stability of dielectric constant (Δε′/ε25oC′≤ ±15%) over a temperature range of −61–144 °C. The excellent performance is due to the introduction of multiphase in the medium entropy ceramics, which effectively refines the grain and domain sizes, leading to a high polarization intensity (44.23 μC/cm2) and excellent energy storage performance. This work shows that “entropy engineering + polyphase” is an effective strategy for excellent energy storage performance and paves a way of developing new energy storage materials by entropy engineering.

Development and Applications of Thermoelectric Oxide Ceramics and Devices

Thermoelectric materials have gained wide attention to realize multilevel efficient energy management to alleviate the increasingly severe energy crisis. Oxide ceramics were well-explored as potential thermoelectric candidates because of their outstanding merits, including abundance, eco-friendliness, high-temperature stability, and chemical stability. In this work, we aim to provide a comprehensive summary of the diversified state-of-the-art oxide ceramics and establish the links between composition designing, preparation process, structural characteristics, and properties to summarize the underlying chemistry and physics mechanism of band engineering, doping, composited with the second phase, defects engineering, and entropy engineering. Furthermore, advanced device design and applications such as thermoelectric modules, miniature generators, sensors, and coolers were reviewed. Ultimately, the challenges and future perspective of oxides ceramics for the device design and thermoelectric applications in the development of energy harvesting technology have been prospected.

Entropy regulation in LaNbO4-based fergusonite to implement high-temperature phase transition and promising dielectric properties

High-entropy effect is a novel design strategy to optimize properties and explore novel materials. In this work, (La_1/5Nd_1/5Sm_1/5Ho_1/5Y_1/5)NbO₄ (5RNO) high-entropy microwave dielectric ceramics were successfully prepared in the sintering temperature (S.T.) range of 1210–1290 ℃ via a solid-phase reaction route, and medium-entropy (La_1/3Nd_1/3Sm_1/3)NbO₄ and (La_1/4Nd_1/4Sm_1/4Ho_1/4)NbO₄ (3RNO and 4RNO) ceramics were compared. The effects of the entropy (<i>S</i>) on crystal structure, phase transition, and dielectric performance were evaluated. The entropy increase yields a significant increase in a phase transition temperature (from monoclinic fergusonite to tetragonal scheelite structure). Optimal microwave dielectric properties were achieved in the high-entropy ceramics (5RNO) at the sintering temperature of 1270 ℃ for 4 h with a relative density of 98.2% and microwave dielectric properties of dielectric permittirity (<i>ε</i>_r) = 19.48, quality factor (<i>Q</i>×<i>f</i>) = 47,770 GHz, and resonant frequency temperature coefficient (<i>τ</i>_f) = –13.50 ppm/℃. This work opens an avenue for the exploration of novel microwave dielectric material and property optimization via entropy engineering.

Enhanced Energy Storage Properties in Paraelectrics via Entropy Engineering

AbstractElectrostatic energy storage capacitors based on dielectrics have attracted much attention due to their wide applications in advanced electrical technology and electronic devices. Generally, high energy density is achieved at a high electric field, while conduction loss becomes nonnegligible, which harms practical applications. Here distinctly suppressed leakage current in BaZr0.5Ti0.5O3‐based films by entropy engineering is realized. With increased entropy, the leakage current density decreases by two orders of magnitude at the electric field of 3 MV cm−1, leading to a markedly improved energy efficiency of 87% at an ultrahigh breakdown strength of 8 MV cm−1 in high‐entropy films. Thereby, a high energy density of 51.9 J cm−3 is achieved. This work demonstrates the effectiveness of entropy engineering in improving the breakdown strength of dielectric films and shows great potential in enhancing the energy storage performance of capacitors.

Unraveling the Role of Entropy in Thermoelectrics: Entropy-Stabilized Quintuple Rock Salt PbGeSnCdxTe3+x.

Entropy-engineered materials are garnering considerable attention owing to their excellent mechanical and transport properties, such as their high thermoelectric performance. However, understanding the effect of entropy on thermoelectrics remains a challenge. In this study, we used the PbGeSnCdxTe3+x family as a model system to systematically investigate the impact of entropy engineering on its crystal structure, microstructure evolution, and transport behavior. We observed that PbGeSnTe3 crystallizes in a rhombohedral structure at room temperature with complex domain structures and transforms into a high-temperature cubic structure at ∼373 K. By alloying CdTe with PbGeSnTe3, the increased configurational entropy lowers the phase-transition temperature and stabilizes PbGeSnCdxTe3+x in the cubic structure at room temperature, and the domain structures vanish accordingly. The high-entropy effect results in increased atomic disorder and consequently a low lattice thermal conductivity of 0.76 W m-1 K-1 in the material owing to enhanced phonon scattering. Notably, the increased crystal symmetry is conducive to band convergence, which results in a high-power factor of 22.4 μW cm-1 K-1. As a collective consequence of these factors, a maximum ZT of 1.63 at 875 K and an average ZT of 1.02 in the temperature range of 300-875 K were obtained for PbGeSnCd0.08Te3.08. This study highlights that the high-entropy effect can induce a complex microstructure and band structure evolution in materials, which offers a new route for the search for high-performance thermoelectrics in entropy-engineered materials.

High symmetry structure and large strain field fluctuation lead enhancement of thermoelectric performance of quaternary alloys by tuning configurational entropy

Quaternary Cu2MnSnSe4 alloys are promising thermoelectric materials due to their earth-abundant nature and environmentally friendly components. Recently, effectiveness of entropy engineering in phonon transport regulation has been demonstrated, while maintaining well electrical properties remains challenging. Here, first principles calculation predicts Fe Co and Zn have weak effect on electronic band edges of Cu2.1Mn0.9SnSe4, so compositions of Cu2.1Mn0.8Fe0.1SnSe4, Cu2.1Mn0.7Fe0.1Co0.1SnSe4, Cu2.1Mn0.6Fe0.1Co0.1Zn0.1SnSe4 are designed with tetragonal structure. Their thermodynamic stabilities are confirmed by analysis of entropy, enthalpy and Gibbs free energy. The increased configurational entropy leads to a well-maintained tetragonal structure with high symmetry, thus obtaining high Seebeck coefficients, which compensate for reduction in electrical conductivity. A highest power factor of 732.1 μW m−1 K−2 at 673 K is obtained for Cu2.1Mn0.9SnSe4 sample. Meanwhile, increased configurational entropy also results in large strain field fluctuation and strong anharmonicity (γ = 1.59–1.80) in crystal lattice, which suppress lattice thermal conductivity (κL) together. Consequently, the Cu2.1Mn0.7Fe0.1Co0.1SnSe4 sample obtains the lowest κL of 0.80 W m−1 K−1 at 673 K. Finally, a high Vickers Hardness of 206 Hv and zTmax of 0.40 are achieved in Cu2.1Mn0.8Fe0.1SnSe4. Entropy engineering opens up a channel to optimize the electrical-thermal properties of complex compounds simultaneously.

A reversible perovskite air electrode for active and durable oxygen reduction and evolution reactions via the A-site entropy engineering

The performance of reversible solid oxide electrochemical cells (R-SOECs) is largely hindered by the insufficient electroactivity and poor durability of the bifunctional air electrodes, where the oxygen reduction and evolution reactions (ORR and OER) occur. Here, we report our findings in boosting the electrochemical activity and durability of an air electrode with Pr0.2Ba0.2Sr0.2La0.2Ca0.2CoO3-δ (PBSLCC) via an A-site entropy engineering. The PBSLCC electrode shows enhanced oxygen reaction activity and excellent durability compared to binary and ternary double perovskites (PrBaCo2O5+δ and Pr0.8Ba0.8Ca0.4Co2O5+δ, respectively). A low and nearly unchanged area-specific resistance of 0.042 Ω cm2 is achieved at 750 °C during the 225-h stability test. La0.8Sr0.2Ga0.8Mg0.2O3-δ electrolyte-supported cells with the PBSLCC air electrode show remarkable performance at 800 °C, demonstrating a peak power density of 1.2 W cm−2 in the fuel cell mode, and a current density of −1.1 A cm−2 at 1.3 V in the electrolysis mode while maintaining the excellent cycling durability of 228 h at ±0.5 A cm−2 under humidified H2 (10% H2O). A bulk oxygen p-band center model is applied to verify that tailoring of the A-site entropy strongly influences the surface exchange coefficients (k*chem), leading to higher oxygen reaction activity of PBSLCC than the binary and ternary perovskites. This study opens a new class of high-entropy perovskites for the rational design of air electrodes for R-SOECs with high activity and durability.

High-Entropy Enhanced Microwave Attenuation in Titanate Perovskites.

High-entropy oxides (HEOs), which incorporate multiple-principal cations into single-phase crystals and interact with diverse metal ions, extend the border for available compositions and unprecedented properties. Herein, a high-entropy-stabilized (Ca0.2 Sr0.2 Ba0.2 La0.2 Pb0.2 )TiO3 perovskite is reported, and the effective absorption bandwidth (90% absorption) improves almost two times than that of BaTiO3 . The results demonstrate that the regulation of entropy configuration can yield significant grain boundaries, oxygen defects, and an ultradense distorted lattice. These characteristics give rise to strong interfacial and defect-induced polarizations, thus synergistically contributing to the dielectric attenuation performance. Moreover, the large strains derived from the strong lattice distortions in the high-entropy perovskite offer varied transport for electron carriers. The high-entropy-enhanced positive/negative charges accumulation around grain boundaries and strain-concentrated location, quantitatively validated by electron holography, results in unusual dielectric polarization loss. This study opens up an effective avenue for designing strong microwave absorption materials to satisfy the increasingly demanding requirements of advanced and integrated electronics. This work also offers a paradigm for improving other interesting properties for HEOs through entropy engineering.

Enhanced thermoelectric properties and electrical stability for Cu1.8S-based alloys: Entropy engineering and Cu vacancy engineering

Cu1.8S-based thermoelectric (TE) materials have garnered considerable interest due to their pollution-free, low-cost, and superior performance characteristics. However, high Cu vacancy and Cu migration inhibit their performance and electrical stability improvement. Through mechanical alloying and spark plasma sintering, a series of Cu1.8S and MnxCu1.8-S0.5Se0.5 (0.01 ≤ x ≤ 0.06) bulk samples were prepared in this study. With Se alloying and Mn doping, the configuration entropy of MnxCu1.8S0.5Se0.5 increases from low-entropy 0.4R* for pristine Cu1.8S to medium-entropy 1.2R* for MnxCu1.8S0.5-Se0.5. MnxCu1.8S0.5Se0.5 subsequently crystallized in a cubic phase with enhanced symmetry and Mn solid solubility. High solubility enables the filling of excessive Cu vacancies, the reduction of carrier concentration, the adjustment of band structure, the enhancement of the Cu migration energy barrier, and the inhibition of Cu migration. Even at current densities exceeding 25 A cm−2 at 750 K, the resistance of Mn0.03Cu1.8S0.5Se0.5 remained hardly changed, indicating a vastly improved electrical stability. In addition, the ultralow thermal conductivity of the lattice is achieved by decreasing the sound velocity and softening the lattice. At 773 K, the bulk ZT of Mn0.06Cu1.8S0.5Se0.5 reaches a maximum of 0.79, which is twice that of pure Cu1.8S. The results indicate that combining entropy engineering and Cu vacancy engineering is an effective strategy for developing high-performance Cu1.8S TE materials.

Ultralow lattice thermal conductivity and improved thermoelectric performance in a Hf-free half-Heusler compound modulated by entropy engineering

A great improvement in the thermoelectric and mechanical properties of a TiNiSn-based compound has been achieved by entropy engineering.

Designing rare earth-free high entropy oxides with a tungsten bronze structure for thermoelectric applications.

In recent years, forming high entropy oxides has emerged as one of the promising approaches to designing oxide thermoelectrics. Entropy engineering is an excellent strategy to improve thermoelectric performance by minimizing the thermal conductivity arising from enhanced multi-phonon scattering. In the present work, we have successfully synthesized a rare-earth-free single phase solid solution of novel high entropy niobate (Sr0.2Ba0.2Li0.2K0.2Na0.2)Nb2O6, with a tungsten bronze structure. This is the first report on the thermoelectric properties of high entropy tungsten bronze-type structures. We have obtained a maximum Seebeck coefficient of -370 μV K-1 at 1150 K, which is the highest among tungsten bronze-type oxide thermoelectrics. The minimum thermal conductivity of 0.8 W m-1 K-1 is obtained at 330 K, which is so far the lowest reported value among rare-earth-free high entropy oxide thermoelectrics. This synergistic combination of large Seebeck and record low thermal conductivity gives rise to a maximum ZT of 0.23 which is so far the highest among rare-earth free high entropy oxide-based thermoelectrics.

Enhancing the solubility of Mn in AgSbSe2 for high thermoelectric performance through entropy engineering

Point defects play an important role in regulating the electrical and thermal transport properties of thermoelectric materials, but they are always restricted by the solubility of dopants in the matrix.

Entropy engineering of La-based perovskite for simultaneous photocatalytic CO2 reduction and biomass oxidation.

Herein, the high-entropy perovskite, i.e. La(FeCoNiCrMn)O3, was prepared for simultaneous CO2 reduction and biomass upgrading. Based on the synergistic effect between the elements in the high-entropy material, an excellent CO evolution rate of 131.8 μmol g-1 h-1 and a xylonic acid yield of 63.9% were gained.

Entropy Engineering in Tellurium-Free Thermoelectric Cu8GeSe6 with a Stable Cubic Structure

Argyrodite-type semiconductors, as prototypes of the “phonon-liquid electron crystal” concept, have attracted intensive attention due to their intrinsically low thermal conductivity. Like other members of this family, Cu8GeSe6 undergoes structural phase transition at 330 K from a low-temperature hexagonal phase to a high-temperature hexagonal phase. Here, by employing the entropy engineering strategy through S alloying on the Se site, the phase transition temperature is lowered to below room temperature, so that the stable cubic phase is maintained in the whole operating temperature region. Cubic Cu8GeSe6 exhibits a favorable electronic structure, which leads to improved carrier mobility and electrical performance. In addition, benefiting from the weak chemical bonding between Cu atoms and the [GeSe6] sublattice and the disordered cation occupancy, cubic Cu8GeSe6 possesses an ultralow thermal conductivity, and the sample with a nominal composition of Cu8GeSe4.2S1.8 reaches a zT value of 0.5 at 627 K. These results provide insights for suppressing phase transition in argyrodite-type thermoelectrics.

Enhancing thermoelectric properties of MCoSb-based alloys by entropy-driven energy-filtering effects and band engineering

MCoSb-based medium-entropy half-Heusler (HH) alloy with low lattice thermal conductivity (κL) is a promising medium- and high-temperature thermoelectric material. However, the unexpected band structure of the alloy leads to a sudden decrease in the Seebeck coefficient (S), which severely restricts the optimization of the figure-of-merit (ZT). In this study, we demonstrate a new concept for decoupling S and electrical conductivity (σ) based on entropy engineering. This method involves band engineering and the energy-filtering effect, where the entropy-driven quantum confinement effect is manipulated to filter low-energy electrons, and the increasing entropy promotes a rapidly changing density of state near Fermi level. Consequently, an excellent S of ∼ −211.6 μV K−1 at 923 K was achieved for the M0.82Nb0.18CoSb HH alloy, which is 62.3% higher than that of the pristine M0.85N0.15CoSb (M = Ti, Zr, Hf; N= V, Ta; equimolar) HH alloy (∼−130.4 μV K−1). Moreover, benefiting from the optimization of S and κL, a peak ZT was enhanced from ∼0.17 for the pristine M0.85N0.15CoSb medium-entropy HH alloy to 0.58 for the M0.85Nb0.15CoSb high-entropy HH alloy. These results broaden the applications of entropy-engineering to decrease κL and decoupling between S and σ.