Pranati Sahoo

Partial indium solubility induces chemical stability and colossal thermoelectric figure of merit in Cu2Se

High thermoelectric figure of merit, ZT ∼ 2.1 at 1000 K, have been reported in Cu2−xSe-based materials. However, their deployments in operational devices have been hampered by chemical instability and low average ZT (ZTave) values. Here, we demonstrate improved chemical stability and a record high ZTave ∼ 1.5 over a broad temperature range (T ≤ 850 K) in Cu2Se/CuInSe2 nanocomposites, with ZT values ranging from 0.6 at 450 K to an unprecedentedly large value of 2.6 at 850 K for the sample with 1 mol% In. This remarkable performance is attributed to the localization of Cu+ ions induced by the incorporation of In into the Cu2Se lattice, which simultaneously boost the electrical conductivity and reduce the thermal conductivity of the nanocomposites. These findings pave the way for large-scale utilization of Cu2Se-based materials in thermoelectric generators.

Coexistence of high-Tc ferromagnetism and n-type electrical conductivity in FeBi2Se4

The discovery of n-type ferromagnetic semiconductors (n-FMSs) exhibiting high electrical conductivity and Curie temperature (Tc) above 300 K would dramatically improve semiconductor spintronics and pave the way for the fabrication of spin-based semiconducting devices. However, the realization of high-Tc n-FMSs and p-FMSs in conventional high-symmetry semiconductors has proven extremely difficult due to the strongly coupled and interacting magnetic and semiconducting sublattices. Here we show that decoupling the two functional sublattices in the low-symmetry semiconductor FeBi2Se4 enables unprecedented coexistence of high n-type electrical conduction and ferromagnetism with Tc ≈ 450 K. The structure of FeBi2Se4 consists of well-ordered magnetic sublattices built of [FenSe4n+2]∞ single-chain edge-sharing octahedra, coherently embedded within the three-dimensional Bi-rich semiconducting framework. Magnetotransport data reveal a negative magnetoresistance, indicating spin-polarization of itinerant conducting electrons. These findings demonstrate that decoupling magnetic and semiconducting sublattices allows access to high-Tc n- and p-FMSs as well as helps unveil the mechanism of carrier-mediated ferromagnetism in spintronic materials.

Electronic and phonon transport in Sb-doped Ti0.1Zr0.9Ni1+xSn0.975Sb0.025 nanocomposites

The thermoelectric behavior of n-type Sb-doped half-Heusler (HH)-full-Heusler (FH) nanocomposites with general composition Ti(0.1)Zr(0.9)Ni(1+x)Sn(0.975)Sb(0.025) (x = 0, 0.02, 0.04, 0.1) was investigated in the temperature range from 300 to 775 K. Samples used for structural characterization and transport measurements were obtained through the solid-state reaction of high purity elements at 950 °C and densification of the resulting polycrystalline powders using a uniaxial hot press. X-ray diffraction study of the powder samples suggested the formation of single-phase HH alloys regardless of the Ni concentration (x value). However, high resolution transmission electron microscopy investigation revealed the presence of spherical nanoprecipitates with a broad size distribution coherently embedded in the HH matrix. The size range and dispersion of the precipitates depend on the concentration of Ni in the starting mixture. Well dispersed nanoprecipitates with size ranging from 5 nm to 50 nm are observed in the nanocomposite with x = 0.04, while severe agglomeration of large precipitates (>50 nm) is observed in samples with x = 0.1. Hall effect measurements of various samples indicate that the carrier concentration within the Sb-doped HH matrix remains nearly constant (~7 × 10(20) cm(-3)) for samples with x = 0.02 and x = 0.04, whereas a significant increase of the carrier concentration to ~9 × 10(20) cm(-3) is observed for the sample with x = 0.1. Interestingly, only a marginal change in thermopower value is observed for various samples despite the large difference in the carrier density. In addition, the carrier mobility remains constant up to x = 0.04 suggesting that the small nanoprecipitates in these samples do not disrupt electronic transport within the matrix. Remarkably, a large reduction in the total thermal conductivity is observed for all nanocomposites, indicating the effectiveness of the embedded nanoprecipitates in scattering phonons while enabling efficient electron transfer across the matrix/inclusion interfaces.

Microstructural and thermal investigations of HfO2 nanoparticles

Monodispersed HfO2 nanoparticles can be readily prepared at room temperature by the ammonia catalyzed hydrolysis and condensation of hafnium(IV) tert-butoxide in the presence of a surfactant. The nanoparticles are faceted with an average diameter of about 4 nm. The as-synthesized amorphous nanoparticles crystallize upon post-synthesis heat treatment. The crystallization temperature of the nanoparticles can be controlled by adjusting the annealing atmosphere. The HfO2 nanoparticles have a narrow size distribution, large specific surface area and the thermal conductivity of pressed pellets is drastically reduced compared to the bulk counterpart. The specific surface area was about 239 m2 g−1 on as-prepared samples while those annealed at 500 °C have a surface area of 221 m2 g−1 showing that the heat treatment produced no significant increase in particle size. Transmission electron microscopy (TEM) further confirmed that the nanoparticles annealed at different temperatures while X-ray diffraction studies of the crystallized nanoparticles revealed that HfO2 nanoparticles were monoclinic in structure. High density pellets of the as-synthesized HfO2 nanoparticles were obtained, using both spark plasma sintering and uniaxial hot pressing, and their thermal conductivity was measured in the temperature range from 300 to 775 K. A large reduction of the thermal conductivity was observed for HfO2 nanoparticles as compared to that of bulk HfO2. The decrease in thermal conductivity is discussed in terms of the microstructure of the compacted samples. The synthetic procedure used in this work can be readily modified for large scale production of monodispersed HfO2 nanoparticles.

Nanometer-scale interface engineering boosts the thermoelectric performance of n-type Ti0.4Hf0.6Ni1+zSn0.975Sb0.025 alloys

Engineering the internal structure of bulk semiconductors through the introduction of nanometer-scale interfaces affords opportunities for simultaneous optimization of electronic and thermal transport properties, enabling the fabrication of nanocomposite materials with superior thermoelectric performance. Several compositions of n-type Ti0.4Hf0.6Ni1+zSb0.975Sn0.025 half-Heusler (HH)/full-Heusler (FH) composites were synthesized by a high temperature solid state reaction of the elements. Polycrystalline powders of the resulting HH(1 − z)/FH(z) composites were further processed by mechanical alloying using high-energy ball milling. Transmission electron microscopy (TEM) studies revealed the presence of small (∼20 nm) FH precipitates coherently embedded in micron-scale grains of HH/FH composites obtained after the solid state reaction, whereas nanometer-scale grains (∼20 nm) of the HH/FH composites containing very small (<10 nm) FH precipitates were observed in mechanically alloyed samples (SS-MA). The as-synthesized HH(1 − z)/FH (z) samples (SS) showed similar electrical conductivity, thermopower and thermal conductivity at all temperatures regardless of the fractions of FH inclusions. This suggests that the FH inclusions are electronically inert with regard to the matrix. Interestingly, drastic alterations of all three properties were observed on mechanically alloyed samples and the magnitude of change in various properties increases with the mole fractions of FH inclusions. For example, large reductions in the thermal conductivity (8.5 W K−1 m−1 to 4.0 W K−1 m−1 at 300 K) as well as electrical conductivity (4750 S cm−1 to 1750 S cm−1 at 300 K) were observed in all SS-MA samples compared to SS samples with similar compositions. This behavior is attributed to enhanced scattering of phonons and electrons at multiple grain boundaries generated by the mechanical alloying process. Remarkably, the magnitude of the reduction in the electrical and thermal conductivities increases with the mole fraction of FH inclusions in the samples suggesting (1) reduction in the carrier density due to trapping of low-energy carriers and (2) additional phonon scattering at nanometer-scale HH/FH interfaces. This results in large increases in the thermopower (−60 μV K−1 to −92 μV K−1 at 300 K) of SS-MA samples, which mitigates the drop in the electrical conductivity leading to large improvement of the figures of merit of mechanically alloyed Ti0.4Hf0.6Ni1+zSb0.975Sn0.025 samples.

Synthesis and thermal stability of HfO2 nanoparticles

A simple method is reported for the synthesis of monodispersed HfO 2 nanoparticles by the ammonia catalyzed hydrolysis and condensation of hafnium (IV) tert-butoxide in the presence of surfactants at room temperature. Transmission electron microscopy shows faceted nanoparticles with an average diameter of 3-4 nm. As-synthesized nanoparticles are amorphous in nature and crystallize upon moderate heat treatment. The HfO 2 nanoparticles have a narrow size distribution, large specific surface area and good thermal stability. Specific surface area was about 239 m 2 /g on as-prepared nanoparticle samples while those annealed at 500 °C have specific surface area of 221 m 2 /g indicating that there was no significant increase in particle size. This result was further confirmed by TEM images of nanoparticles annealed at 300 °C and 500 °C. X-ray diffraction studies of the crystallized nanoparticles revealed that HfO 2 nanoparticles were monoclinic in structure. The synthetic procedure used in this work can be readily modified for large scale production of monodispersed HfO 2 nanoparticles.

Distribution of impurity states and charge transport in Zr{sub 0.25}Hf{sub 0.75}Ni{sub 1+x}Sn{sub 1−y}Sb{sub y} nanocomposites

Abstract Energy filtering of charge carriers in a semiconducting matrix using atomically coherent nanostructures can lead to a significant improvement of the thermoelectric figure of merit of the resulting composite. In this work, several half-Heusler/full-Heusler (HH/FH) nanocomposites with general compositions Zr 0.25 Hf 0.75 Ni 1+ x Sn 1− y Sb y (0≤ x ≤0.15 and y =0.005, 0.01 and 0.025) were synthesized in order to investigate the behavior of extrinsic carriers at the HH/FH interfaces. Electronic transport data showed that energy filtering of carriers at the HH/FH interfaces in Zr 0.25 Hf 0.75 Ni 1+ x Sn 1− y Sb y samples strongly depends on the doping level ( y value) as well as the energy levels occupied by impurity states in the samples. For example, it was found that carrier filtering at HH/FH interfaces is negligible in Zr 0.25 Hf 0.75 Ni 1+ x Sn 1− y Sb y ( y =0.01 and 0.025) composites where donor states originating from Sb dopant dominate electronic conduction. However, we observed a drastic decrease in the effective carrier density upon introduction of HH/FH interfaces for the mechanically alloyed Zr 0.25 Hf 0.75 Ni 1+ x Sn 0.995 Sb 0.005 samples where donor states from unintentional Fe impurities contribute the largest fraction of conduction electrons. This work demonstrates the ability to synergistically integrate the concepts of doping and energy filtering through nanostructuring for the optimization of electronic transport in semiconductors.

Correction to "Coexistence of high-Tc ferromagnetism and n-type electrical conductivity in FeBi2Se4".

■ ACKNOWLEDGMENTS The synthesis and structural characterization portion of this work was supported by the National Science Foundation (NSF) Career Award DMR-1237550. C.U. and P.F.P.P. gratefully acknowledge financial support for electrical conductivity, Hall effect, and magnetic measurements from the Department of Energy, Office of Basic Energy Sciences, under Award No. DE-SC-0008574. D.P.Y. acknowledges support for heat capacity and magnetoresistance measurements from the NSF under grant no. DMR-1306392. Magnetic data were recorded on a SQUID magnetometer at the University of Michigan purchased using an MRI grant from the NSF (CHE104008).