Phonon Glass Electron Crystal Research Articles

Influence of electronic transport mechanism optimization on the thermoelectric properties of ZnO based functional ceramics

ZnO is one of the most promising mid-high temperature thermoelectric materials because of low cost, easy preparation and favorable chemical stability. Herein, the influence mechanism of Alkaline earth metal Ba doping on the thermoelectric properties of ZnO was studied. It is found that Ba doping can effectively improve the carrier mobility and electrical conductivity for that Ba doping can shorten the Zn–O bond and enhance the overlap and hybridization of electron orbitals. The electronic density of states of Zn and O is significantly enhanced. The highest power factor of ∼7.95 μWcm−1K−2 was obtained at 873 K, which is ∼1.4 times higher than that of the undoped sample. Due to the decrease of Young’s modulus caused by Ba doping and the enhancement of phonon scattering of heavy element, the thermal conductivity decreases obviously in the whole temperature range. Ba doping enhanced the effect of phonon glass-electron crystal, and the ZT value of ∼0.345 was obtained at 873 K, which is ∼3.52 times as that of the pristine ZnO. The results indicated that alkaline earth metal doping is an effective way to improve the thermoelectric properties of ZnO functional materials.

Lithium‐Filled Skutterudites by Intercalation at Ambient‐Temperature

AbstractThe incorporation of lithium as a filler species in Co1‐2xFexNixSb3 skutterudites was accomplished by intercalation at 60 °C, using n‐BuLi as a reducing agent. Solid state 7Li NMR and ICP‐MS analysis confirm the presence of lithium in the product phases and provide an estimate of the lithium content. The maximum uptake of lithium increases as cobalt is progressively substituted by an equimolar mixture of iron and nickel. Difference Fourier maps, calculated during Rietveld structure refinement using powder neutron diffraction data, locate the lithium cations at the 2a (0,0,0) sites within the cavities of the skutterudite framework. The intercalation of lithium results in reductions in thermal conductivity of up to 47 %, indicative of phonon glass electron crystal (PGEC) type behaviour. Charge transfer from lithium to the framework that accompanies intercalation results in a substantial decrease in electrical resistivity in lithiated phases and a more metal‐like temperature dependence. The increased carrier concentration also decreases the Seebeck coefficient, with the consequence that modest increases in the figure of merit occur, despite the reduced thermal conductivity.

Effect of Cu distribution on thermoelectric properties of evaporated Cu2−xSe films with various Ag contents

Cu2Se is one kind of popular thermoelectric materials due to its performance of the phonon glass-electron crystal. However, the poor thermal stability limits its application. Regulating the distribution of the Cu+ by elemental dopant is a general method to solve the stability and to improve the thermoelectric performance. In this study, the Ag-doped β-Cu2−xSe films were prepared by vacuum thermal evaporation method. The Cu+ distribution on the octahedral 32(f) sites in the Cu2Se crystal is tuned by the Ag content. It found that when the Ag content is 1.0%, the Cu+ and Ag+ contents in the 32(f) sites reach the maximum. In this case, the power factor of the film is 1750 μW·m−1·K−2 at 530 K. As the Ag content rises, the second phase of CuAgSe appears. This results in the decrease of the power factor. It shows that controlling the distribution of the Cu+ by the Ag dopant is conducive to improve the thermoelectric properties of the β-Cu2−xSe films.

Crystalline-liquid duality of specific heat in halide perovskite semiconductor

Crystalline-liquid duality in specific heat is observed in lead-free bismuth halide perovskite A3Bi2×9 [A= FA, MA, Cs, and X = I, Br, Cl] semiconductor. All crystals exhibit unusual liquid-like behavior and show substantial deviation from the classical Dulong-Petit law at room temperature. This unusual liquid-like behavior of specific heat is attributed to the dynamic disorder and strong anharmonicity in the soft crystal lattice. The low-temperature specific heat was modeled using the combined Debye-Einstein model and obtained Einstein modes demonstrate strong anharmonic coupling of the low-energy optical modes with acoustic modes resulting in ultrasoft frequency dampening of acoustic phonons. The combination of strong acoustic−optical phonon coupling, soft elastic layered structure, and abundance of low-energy optical phonons results in anomalous thermodynamic specific heat duality in these bismuth-based phonon glass electron crystals. Our study is of fundamental interest in thermodynamics and will help to design novel thermoelectric materials.

Ultralow thermal conductivity and anharmonic rattling in two-dimensional WB4 monolayer

Two-dimensional (2D) WB4 monolayer is a typical graphene analog with high electrical conductivity and structural stability. Yet, its thermal transport properties are not available. By using first-principles calculations and iteratively solving the linearized Boltzmann transport equation, we predict an ultralow in-plane lattice thermal conductivity (κlat) of 0.28 W/m K at T = 300 K. Such an ultralow κlat is attributed to WB4 monolayer's predominantly large phonon scattering rates and flat acoustic phonon dispersion caused by strong anharmonicity. By analyzing the vibrational patterns and bonding environment, we confirm the origin of the strong anharmonicity to be tungsten atom rattling inside the framework of two boron sheets. Such a mechanism fulfills the concept of phonon glass-electron crystal, making the WB4 monolayer an outstanding 2D thermoelectric material. The rich formation mechanism, including multiple interactions in the WB4 monolayer, provides us inspiration for searching for materials with ultralow κlat.

Dwelling deep into the anisotropic transport of Zintl Ca3Sb2

Phonon-glass-electron-crystal (PGEC) is a class of materials that exert electron crystal like pronounced electronic conduction and glass like supressed thermal conduction leading to astonishing thermoelectric (TE) performance. The density functional theory based investigation of Zintl Ca3Sb2 is presented. We find that a narrow band gap of 0.35 eV and the spatially dispersed phonon branches of Ca3Sb2 result in improved power factor and ultra-low thermal conductivity. Comparing with conventional TE materials PbTe, Bi2Te3 and Mg3Sb2, the Ca3Sb2 possesses significantly low average thermal conductivity (κL) of 0.26 Wm-1K−1 at 300 K. The root-cause of such a feeble κL is reciprocated in terms of the acoustic modes with supressed vibrational frequency that when coupled with lower lying optic phonon modes, results in significantly high phonon scattering rates and phonon anharmonicity. Interestingly, the thermal conduction along the cross-plane direction is found to be much more supressed than the in-plane direction suggesting appreciable anisotropy. The TE efficiency of Ca3Sb2 is envisaged from the temperature dependent profile of figure of merit ZT which is found to attain magnitude of ∼ 0.72 (in z-direction) within temperature range 300 K-600 K for both charge carriers. Besides providing in-depth insight to the thermal conduction, the study provides motivation to investigate this yet-to-be-synthesized potential Zintl compound.

Phase Transitions, Structure Evolution, and Thermoelectric Properties Based on A2MnSb2 (A = Ca, Yb)

Layered structures can facilitate “phonon-glass, electron-crystal (PGEC)” properties, which are crucial for the design of the state-of-the-art thermoelectric materials. In this report, two Mn-containing metastable Zintl phases with layered structures, Ca2MnSb2 and Yb2MnSb2, are discovered for the first time. Their structures are isotypic to that of LiZnSb with transition-metal sites partially occupied. Interestingly, these compounds feature very broad homogeneity ranges, which enable flexible tuning on the Mn sites. In addition, close relationships to previously reported Yb9Mn4+xSb9 series have been well established by phase exploration, and structure transition from Yb2MnSb2 to Yb9Mn4+xSb9 was experimentally observed after annealing. The thermal stability of A2MnSb2 (A = Ca, Yb) can be significantly improved through Ag doping on the Mn sites, based on which, systematic investigations on the crystal structures and thermoelectric properties were carried out. Benefited from the complex anionic structures, the thermal conductivity of these phases is extremely low, that is, the Yb-containing material Yb2(Mn0.9Ag0.2)Sb2 features glasslike thermal conductivity with a minimum value of 0.42 W·m–1·K–1 at 673 K, and meanwhile, a high Seebeck coefficient of 208 μV·K–1 was obtained at 673 K. The discovery of such A2MnSb2 (A = Ca, Yb) phases completed the critical missing members of the A–Mn–Sb ternary family, which is crucial for understanding the structure and properties related to these novel phases, and with the progress of such studies, the design of the state-of-the-art thermoelectric materials based on such Zintl phases is very promising.

N-type amorphous silicon-germanium thin films with embedded nanocrystals as a novel thermoelectric material of elevated ZT

The state-of-art thermoelectric (TE) materials with high efficiency (i.e., ZT ~ 1) at room temperature require meeting essential features, such as being environmentally friendly (i.e., avoiding the use of rare and toxic elements), cheap to produce, and high ZT. However, the enhancement of ZT has always been a challenge because the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature are interdependent. In this work, we report the fabrication of amorphous silicon-germanium (a-SiGe) thin films deposited by low-frequency plasma-enhanced chemical vapor deposition (LF-PECVD) with embedded nanocrystals that have a ZT of 2.61 at room temperature. The above is achieved by enhancing the thermoelectric power factor and, simultaneously, keeping the lowest thermal conductivity; both are in concordance with the phonon-glass electron-crystal (PGEC) approach. The power factor enhancement is accomplished by increasing the electrical conductivity through N-type doping and an annealing process at 500 °C to which thin films were subjected to improve their transport properties while avoiding crystallization. Therefore, the room-temperature electrical conductivity increased two orders of magnitude from 1.11E-01 up to 42.79 S/cm. In addition, the growth of nanocrystals (5–15 nm) embedded into the amorphous matrix contribute to the transport of charge carriers. The measured thickness and Seebeck coefficient of thin films were 200 nm and −1.038 mV/K, respectively. On the other side, the lowest thermal conductivity is reached because the material’s amorphous phase is kept despite the applied post-deposition thermal annealing. The experimental value for thermal conductivity was 0.53 W/m·K, almost half of the minimum thermal conductivity proposed by Slack. In summary, the structural characterization (developed by X-ray diffraction, Fourier transform infrared spectroscopy, and Raman spectroscopy) shows that a-SiGe material not only has an amorphous phase despite the applied thermal annealing, but it also possesses nanocrystals, which is demonstrated by transmission electron microscopy in high-resolution mode. Therefore, the a-SiGe material obtained in this research is proposed as a novel and potential thermoelectric material because it has optimized TE properties and meets desirable features, such as being inexpensive, non-hazardous, and environmentally friendly for energy harvesting applications.

Point defect approach to enhance the thermoelectric performance of Zintl-phase BaAgSb

Zintl-phase compounds have great potential in thermoelectric applications owing to their “phonon glass-electron crystal” (PGEC) structures. In this paper, a new Zintl-phase thermoelectric material BaAgSb is reported. Ba deficiency increased the carrier concentration, and then suppressed the intrinsic excitation. The peak ZT value of Ba0.98AgSb reached ∼0.56 at 773 K. Moreover, Eu alloying at Ba site not only lowered the lattice thermal conductivity by inducing point-defect scattering, but also improved the electrical properties by increasing the carrier mobility. Finally, a peak ZT of ∼0.73 was achieved in Ba0.78Eu0.2AgSb.

Thermoelectric materials with crystal-amorphicity duality induced by large atomic size mismatch

Summary Discovering novel materials and attaining higher performance are the eternal pursuit of thermoelectric materials research. Here, we report a material series, (Cu1−xAgx)2(Te1−ySy) (0.16 ≤ x ≤ 0.24, 0.16 ≤ y ≤ 0.24), which adopts a complex orthorhombic structure differing from any known crystal structure of (Cu/Ag)2(S/Te). This material series is featured by the crystal-amorphicity duality induced by the large anionic size mismatch: a crystalline sublattice of highly size-mismatched anions Te/S coexists with an amorphous-like sublattice of cations Cu/Ag. In the context of structure-property correlation, the crystal-amorphicity duality gave rise to not only interesting electrical properties but also exceptionally low lattice thermal conductivities from 300 to 1,000 K. A state-of-the-art figure of merit zT of 2.0 is obtained in the x = y = 0.22 sample at 1,000 K. These results give insights into crystal-amorphicity duality as a paradigm-shifting materials design approach to develop high-performance thermoelectric materials.

Ultralow Lattice Thermal Conductivity at Room Temperature in Cu4TiSe4

AbstractUltralow thermal conductivity draws great attention in a variety of fields of applications such as thermoelectrics and thermal barrier coatings. Herein, the crystal structure and transport properties of Cu4TiSe4 are reported. Cu4TiSe4 is a unique example of a non‐toxic and low‐cost material that exhibits a lattice ultra‐low thermal conductivity of 0.19 Wm−1 K−1 at room temperature. The main contribution to the unusually low thermal conductivity is connected with the atomic lattice and its dynamics. This ultralow value of lattice thermal conductivity (kL) can be attributed to the presence of the localized modes of Cu, which partially hybridize with the Se atoms, which in turn leads to avoidance of crossing of acoustic phonon modes that reach the zone boundary with a reduced frequency. Like a phonon glass electron crystal, Cu4TiSe4 could also open a route to efficient thermoelectric materials, even, with chalcogenides of relatively high electrical resistivity and a large band gap, provided that their structures offer a sublattice with lightly bound cations.

Ultralow Lattice Thermal Conductivity at Room Temperature in Cu4 TiSe4.

Ultralow thermal conductivity draws great attention in a variety of fields of applications such as thermoelectrics and thermal barrier coatings. Herein, the crystal structure and transport properties of Cu4 TiSe4 are reported. Cu4 TiSe4 is a unique example of a non-toxic and low-cost material that exhibits a lattice ultra-low thermal conductivity of 0.19 Wm-1 K-1 at room temperature. The main contribution to the unusually low thermal conductivity is connected with the atomic lattice and its dynamics. This ultralow value of lattice thermal conductivity (kL ) can be attributed to the presence of the localized modes of Cu, which partially hybridize with the Se atoms, which in turn leads to avoidance of crossing of acoustic phonon modes that reach the zone boundary with a reduced frequency. Like a phonon glass electron crystal, Cu4 TiSe4 could also open a route to efficient thermoelectric materials, even, with chalcogenides of relatively high electrical resistivity and a large band gap, provided that their structures offer a sublattice with lightly bound cations.

Synergistic effect of In doping on electrical and thermal properties of Cu2SnSe3 thermoelectric system

Cu2SnSe3 has been considered as a potential thermoelectric material owing to its tunable transport properties and its phonon-glass-electron-crystal (PGEC) characteristics. Here, p-type pure and In-doped Cu2SnSe3 samples are synthesized by the solid-state sintering technique. Cubic structure with Foverline{4}3m space group is maintained for all the samples, and a linear increase in lattice parameter with increasing In concentration has been observed. The nature of electrical resistivity changes from semiconducting to metallic behavior for samples with x > 0.10. The decrease in both electrical resistivity and Seebeck coefficient with an increase in x is attributed to the increased hole concentration. Such a scenario is confirmed from the room-temperature Hall effect measurements. Indium doping also reduces the thermal conductivity of the Cu2SnSe3 system as a result of increased phonon scattering due to the mass fluctuation. Concurrently, enhancement of thermoelectric power factor (PF) and figure of merit (ZT) is achieved with In doping at Sn site of Cu2SnSe3. The maximum ZT of 0.04 has been exhibited by the sample with x = 0.25 at 400 K, which is six times higher than that of the undoped Cu2SnSe3.

Strain-Induced Ultrahigh Electron Mobility and Thermoelectric Figure of Merit in Monolayer α-Te.

In line with the classic phonon-glass electron-crystal (PGEC) paradigm, semiconducting and semimetallic multinary compounds remain the cornerstone of the state-of-the-art thermoelectric materials. By contrast, elemental PGEC is very rare. In this work, we report a thermoelectric study of monolayer α-Te by first-principles calculations and solving the parameter-free Boltzmann transport equation. It is found that monolayer α-Te possesses high electron mobility (about 2500 cm2 V-1 s-1) at room temperature due to small effective mass, low phonon frequencies, and thus a restricted phase space for electron-phonon scattering. In monolayer α-Te, the electrons near the conduction band edge are mainly scattered by the heavily populated quadratically dispersing out-of-plane acoustic (ZA) phonon modes. The thermoelectric figure of merit (ZT) for n-type monolayer α-Te is 0.55 at 300 K and 1.46 at 700 K. Notably, tensile strain stiffens the ZA modes, yielding a linear energy-momentum dispersion relation and the removal of the diverging thermal population of ZA phonons. Consequently, the electron mobility is enhanced. At a 4% tensile strain, the electron mobility can reach up to 8000 cm2 V-1 s-1 at room temperature while the thermal conductivity is almost unaffected, yielding a state-of-the-art ZT value of 0.94 and 2.03 in n-type monolayer α-Te at 300 and 700 K, respectively. For completeness, the thermoelectric study of p-type monolayer α-Te is also conducted. These results beckon further experiments toward high-performance α-Te-based thermoelectric materials via doping, alloying, and compositing.

Characterization of rattling in relation to thermal conductivity: Ordered half-Heusler semiconductors

The factors that affect the thermal conductivity of semiconductors is a topic of great scientific interest, especially in relation to thermoelectrics. Key developments have been the concept of the phonon-glass-electron-crystal (PGEC) and the related idea of rattling to achieve this. We use first principles phonon and thermal conductivity calculations in order to explore the concept of rattling for stoichiometric ordered half-Heusler compounds. These compounds can be regarded as filled zinc blende materials, and the filling atom could be viewed as a rattler if it is weakly bound. We use two simple metrics, one related to the frequency and the other to bond frustration and anharmonicity. We find that both measures correlate with thermal conductivity. This suggests that both may be useful in screening materials for low thermal conductivity.

Secondary phase induced electrical conductivity and improvement in thermoelectric power factor of zinc antimonide films

The urgent global need to develop sustainable energy conversion methods and to minimize green-house gas emissions has intensified interest in finding more efficient and non-polluting means for power generation. Thermoelectric materials directly convert heat into electricity which can be used for power generation and waste heat recovery. The waste heat is mostly in the intermediate temperature range and current materials are economically unviable for the recovery of heat loss. Zinc antimonide, β-Zn4Sb3, has one of the highest figures of merit (ZT) owing to its phonon-glass-electron-crystal (PGEC) structure with potential application in the intermediate temperatures (500 K–900 K). It shows poor thermal stability and often includes secondary phases zinc antimony (ZnSb) and zinc (Zn) which deteriorate its thermoelectric properties. Zn4Sb3 films were prepared by ion beam sputtering and their electrical and thermoelectric properties were studied following annealing at temperatures up to 400 °C. Rutherford backscattering spectroscopy (RBS) and electron diffraction spectroscopy results showed formation of Zn-rich Zn4Sb3 films. X-ray diffraction (XRD) and Raman results corroborated the formation of the β-Zn4Sb3 phase along with the presence of a ZnSb secondary phase. Annealing the zinc antimonide films increased electrical conductivity (σ) by three orders of magnitude with σ ∼ 3.7 × 104 Sm−1 for films annealed at 400 °C. Increased conductivity is associated with increased charge carrier density and mobility due to decomposition of the ZnSb secondary phase and improved crystalline quality of the films. The highest Seebeck coefficient of α ∼171 μVK−1 was found for films annealed at 100 °C. The power factor (α2σ) increased with annealing temperature, and the maximum value of α2σ ∼7.5 μWm−1K−2 was obtained for films annealed at 400 °C due to the correspondingly high electrical conductivity. Our findings indicate that the thermoelectric properties of Zn4Sb3 films can be tuned via annealing-induced suppression of the high-resistance ZnSb secondary phase, leading to multifold changes in the electrical conductivity and Seebeck coefficient.

Intrinsic anharmonic localization in thermoelectric PbSe

Lead chalcogenides have exceptional thermoelectric properties and intriguing anharmonic lattice dynamics underlying their low thermal conductivities. An ideal material for thermoelectric efficiency is the phonon glass–electron crystal, which drives research on strategies to scatter or localize phonons while minimally disrupting electronic-transport. Anharmonicity can potentially do both, even in perfect crystals, and simulations suggest that PbSe is anharmonic enough to support intrinsic localized modes that halt transport. Here, we experimentally observe high-temperature localization in PbSe using neutron scattering but find that localization is not limited to isolated modes – zero group velocity develops for a significant section of the transverse optic phonon on heating above a transition in the anharmonic dynamics. Arrest of the optic phonon propagation coincides with unusual sharpening of the longitudinal acoustic mode due to a loss of phase space for scattering. Our study shows how nonlinear physics beyond conventional anharmonic perturbations can fundamentally alter vibrational transport properties.

Oxygen vacancy revived phonon-glass electron-crystal in SrTiO3

Controlling oxygen vacancy in undoped SrTiO3 was used to realize the phonon-glass electron-crystal (PGEC) concept. The undoped SrTiO3 samples were synthesized through solid-state-reaction method in air and then reduced through annealing under low oxygen partial pressure conditions. We show that decreasing the oxygen partial pressure (Po2) results in increase of the electrical conductivity, and the oxygen vacancies act as phonon scattering centers, which lead to decrease in the lattice thermal conductivity. Both the enhanced electrical conductivity (σ) and reduced thermal conductivity (κ) were achieved simultaneously through oxygen vacancy in undoped SrTiO3. This significantly increases the ratio of σ/κ that results in an enormously enhanced ZT value of about 13 times larger as compared with the samples reduced under relatively high Po2. Density-functional theory calculations also reveal that oxygen vacancies in SrTiO3 reduce the band-gap, and consequently increase the electrical conductivity.

Thermoelectric Properties of p-type Polycrystalline Si Prepared by Melt-Spinning Process

Thermoelectric energy conversion has attracted much attention as an efficient and environment-friendly energy technology. Thermoelectric energy conversion performance is directly influenced by the figure of merit (ZT) of the thermoelectric material, however, most known thermoelectric materials with high ZT contain toxic and/or scarce elements. Herein, we report the thermoelectric properties of a polycrystalline p-type Si composite. Si is not only one of the most abundant elements in the earth's crust and but also non-toxic. The composite was prepared by consolidating melt-spun B-doped Si ribbons using spark plasma sintering to achieve a nanocrystalline composite. However, unexpectedly, the resulting material was composed of micron-sized grains with submicron-sized pores. This microstructural character provided unexpected benefits from a thermoelectric point of view. First, the mobility of the composite was quite compatible with that of single crystalline B-doped Si wafer, as elucidated by our calculations based on Masseti's formula. Furthermore, the hole concentration was increased in the compound compared to the B-doped Si wafer, due to unintentional Cu-doping during the melt-spinning process, which resulted in enhanced electrical conductivity. Notably, the lattice thermal conductivity was significantly reduced by the existence of pores, while the electrical conductivity was enhanced. This phonon-glass electron-crystal (PGEC), realized in the polycrystalline p-type Si composite, could lead to an increase in ZT. Key words: thermoelectric, Si, melt-spinning, spark plasma sintering

Thermoelectric phonon-glass electron-crystal via ion beam patterning of silicon

Ion beam irradiation has recently emerged as a versatile approach to functional materials design. We show in this work that patterned defective regions generated by ion beam irradiation of silicon can create a phonon glass electron crystal (PGEC), a longstanding goal of thermoelectrics. By controlling the effective diameter of and spacing between the defective regions, molecular dynamics simulations suggest a reduction of the thermal conductivity by a factor of $\approx$20 is achievable. Boltzmann theory shows that the thermoelectric power factor remains largely intact in the damaged material. To facilitate the Boltzmann theory, we derive an analytical model for electron scattering with cylindrical defective regions based on partial wave analysis. Together we predict a figure of merit of ZT$\approx$0.5 or more at room temperature for optimally patterned geometries of these silicon metamaterials. These findings indicate that nanostructuring of patterned defective regions in crystalline materials is a viable approach to realize a PGEC, and ion beam irradiation could be a promising fabrication strategy.