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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Fast-airflow tumble clothes dryer with small thermoelectric heat pump: Experimental evaluation

Residential clothes drying accounts for about 5% of the total residential-sector energy consumption in the United States. Most dryers use electric resistance heaters to dry clothes and have low efficiencies. Higher-efficiency dryers that use vapor compression heat pumps are expensive and complex and have not gained a large market share in the United States. A novel tumble clothes dryer using a small thermoelectric heat pump with faster airflow than typical dryers is presented in this work. The benchtop performance of the thermoelectric heat pump and high-speed blower are presented, and the development of the prototype dryer is described. The dryer was tested for efficiency and dry time for a range of airflow rates and applied currents to the thermoelectric heat pump. The combined efficiency factor was 5.09–6.29 lb BDW /kWh (specific moisture extraction rate of 1.23–1.53 kg w /kWh) with 100–138 min dry times for these tests. The measured efficiency was 36 %–68 % greater than the minimum efficiency standard in the United States, and compared with vapor compression heat pump–based clothes dryers, the prototype dryer had less expensive, less complex components and did not use refrigerants. Finally, the performance of this small thermoelectric heat pump clothes dryer is also compared with previous iterations of the thermoelectric tumble clothes dryer described in the literature.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2022

Grafted polythiophene pendent polymer brushes and their electronanopatterning

Here electrochemical nanolithography using grafted polymer brushes with terthiophene (3T) pendent was investigated. The film was fabricated by surface electropolymerization of a reversible addition-fragmentation chain-transfer agent macroinitiator containing a 3T pendant molecule and the subsequent polymer brush growth from a 3T functionalized methacrylate monomer (3T-Methacrylate). Electrochemical nanolithography and patterning were done using conducting atomic force microscopy (C-AFM) enabling precise nanopattern fabrication. In contrast to direct nanowriting, dip pen nanolithography, and nanolithographic additive manufacturing, the electrochemical nanolithography can be readily facilitated by applying a bias voltage between a conductive AFM tip and the grafted polymer brush film at ambient conditions and without ink transfer. The height and electrical resistance of the nanopatterns were dependent on the writing parameters (i.e. applied bias voltage and scan rate).

36 MATERIALS SCIENCE↗
DOE OSTI2025

Sputter deposition of ultrathick Bi coatings onto rotating substrates for inertial confinement fusion

Sputter-deposited Bi is a potential candidate material for the production of inertial confinement fusion (ICF) hohlraums, which are sphero-cylindrical canisters that serve as the housing for hydrogen-fuel-filled capsules in ICF implosions. The fabrication of hohlraums involves deposition onto rotating sphero-cylindrical substrates and, hence, requires an understanding of oblique angle deposition phenomena. Here, we systematically study the effect of dynamic substrate tilt on properties of Bi films deposited onto rotating planar substrates. Results show that film morphology, porosity, and electrical resistivity depend on substrate tilt. In comparison with films deposited onto substrates adhered to a stationary sample holder, films deposited on rotating substrates have more uniform microstructure with fewer void defects. Based on Monte Carlo simulations, these changes in film morphology and microstructure are attributed to averaging and geometric effects related to sample position and rotation. In conclusion, the fabrication of an ∼ 50-µm-thick Bi film on a rotating sphero-cylindrical mandrel demonstrates a significant step toward the production of Bi hohlraums.

Bismuth↗
DOE OSTI2021

Unexpected Hydride: Ce 4 B 2 C 2 H 2.42 , a Stuffed Variant of the Nd 2 BC Structure Type

Ce 4 B 2 C 2 H 2.42 was grown as large crystals from a cerium/copper eutectic flux. The structure was characterized by single-crystal X-ray and neutron diffraction and was found to be a stuffed variant of Nd 2 BC with the addition of two interstitial hydrogen positions. The tetrahedral hydrogen position is fully occupied, while the octahedral position has an occupancy of 42(3)%. Initial synthesis was due to hydrogen contamination of the cerium metal but has been successfully repeated using anthracene as a carbon and hydrogen source. Density of states calculations suggest that the incorporation of hydrogen stabilizes the compound with respect to the nonhydrided model. Magnetic susceptibility data show a complex magnetic ordering at 7.7 K that originates from the localized electron on the Ce 3+ in the structure. The trivalent state is also supported by X-ray photoelectron spectroscopy measurements. Heat capacity and electrical resistivity data show that the phase transition is broad in temperature, which may be due to structural disorder. Furthermore, the large low temperature value of C/T also indicates possible heavy fermion behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Is a Homologous Series Member Equal to or Greater than the Sum of its Subunits? A Focus on a Few Fe-based BaNiSn3 subunits: LnFeGe3 (Ln = La, Pr)

The homologous series Lnn+1MnGe3n+1 (Ln = Ce, Pr, M = Fe, Co) has proven to be a fruitful platform for tuning physical properties as a function of subunit stacking. In this study, we investigate the crystal growth, structure, and physical properties of PrFeGe3, aiming to deconstruct Pr4Fe3Ge10 (n = 3) into its subunits, including the BaNiSn3 and CeNiSi2 structure types. The electrical resistivity and magnetic susceptibility of PrFeGe3 were measured. The magnetic properties reveal PrFeGe3 to be antiferromagnetic (5 K) with an effective moment of eff = 4.38 B/F.U., larger than the spin-only moment from Pr3+ (3.58 B). Due to the large magnetic moment observed, the oxidation state of the elements was determined using spectroscopic methods including X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. The neutral oxidation state of iron determined from X-ray photoelectron spectroscopy supports itinerant magnetic behavior of iron. Annealing, in tandem with differential scanning calorimetry, revealed PrFeGe3 to be a precursor for the formation of new homologous series members.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Cooling-Rate-Driven Polymorphism and Vacancy Ordering in Ce 2 MnGe 6 Intergrowth Phases

Intergrowth phases in intermetallic systems provide a compelling framework for investigating structure–property relationships as a function of crystallographic subunit stacking. In this study, we examine the influence of Mn vacancy ordering on subunit stacking and reassign the structure of Ce 2 MnGe 6 to a monoclinic C2/m space group (a = 8.3486(17) Å, b = 8.6181(18) Å, c = 10.778(2) Å, β = 101.17(2)°). Additionally, we report a monoclinic polymorph of the Ln 2 MGe 6 family characterized as a tripled c-axis supercell derivative of monoclinic Ce 2 MnGe 6 , 3×c-Ce 2 MnGe 6 (a = 8.3482(16) Å, b = 8.6179(18) Å, c = 31.813(6) Å, β = 93.763(7)°). This polymorph emerges under rapid cooling conditions during the synthesis and offers insight into the structural relationship between the orthorhombic and monoclinic variants of the Ln 2 MGe 6 phases. Notably, the supercell form of Ce 2 MnGe 6 (3×c-Ce 2 MnGe 6 ) exhibits increased electrical resistivity and suggests enhanced Zintl-like behavior, potentially indicating greater thermodynamic stability relative to the parent monoclinic phase.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Lithium thiostannate spinels: air-stable cubic semiconductors.

Lithium chalcogenide materials have been studied less than other alkali metal analogs and are of interest as ion conductors and semiconductors capable of thermal neutron detection. Herein, we describe the structural, thermal, magnetic, electronic, and ionic properties of a new family of cubic lithium thiospinels Li2MSn3S8 (M = Mg, Fe, Mn, Ni, Co) and Li1.66CuSn3.33S8. All compounds crystallize in the Fd (3) over barm space group and exhibit site occupancy disorder with Sn and M on the octahedral site for M = Mg, Fe, Mn, Ni, Co. The Li1.66CuSn3.33S8 compound also exhibits occupancy disorder on the tetrahedral sites between Li and Cu, as well as on the octahedral sites between Li and Sn. We demonstrate the ability to tune the physical properties of the compounds by changing their composition. By tuning the identity of the transition metal in the structure, the optical band gaps can vary between 0.91(2) and 2.19(2) eV, and electrical resistivities between 2.8 x 10(3) Omega-cm and 2.2 x 10(7)Omega-cm can be achieved. Density functional theory calculations for Li2MgSn3S8 and Li1.66CuSn3.33S8 suggest that both materials have indirect band gaps and significantly dispersive conduction bands. A bandwidth dispersion of 0.75 and 0.70 eV along the Gamma -> M direction in the conduction band are predicted for Li2MgSn3S8 and Li1.66CuSn3.33S8, respectively, with an electron effective mass of 0.49 and 0.55 m(e) at the Gamma point. The hole effective mass in the valence band is calculated to be 1.25 me at the M point and 0.75 me at the L point for Li2MgSn3S8 and Li1.66CuSn3.33S8, respectively. The ionic/electronic conductivity of each compound was determined using AC electrochemical impedance spectroscopy.

Quintero, Michael A.↗
DOE OSTI2025

Growth and Characterization of Epitaxial FeWO 4 Thin Films with Controlled Oxygen Stoichiometry

Here, we report the growth of single-phase epitaxial FeWO 4 thin films, using plasma-assisted molecular beam epitaxy, and investigate structural, optical, and electronic properties. The FeWO 4 films grow in (100) orientation on c-plane sapphire (0001) substrates and exhibit 3 rotational twin variants where FeWO 4 [001] is aligned to sapphire [100] equivalent in-plane directions. X-ray diffraction measurements indicate that the epitaxial FeWO 4 (100) structure is optimized when 80-100 W of rf power is applied to an atomic oxygen source during growth, yielding films with minimal strain and impurity phases or other orientations. In films grown with 120 W of rf power, FeWO 4 crystallites develop inhomogeneous and homogeneous strains and are potentially contaminated with Fe 3+ oxide phase impurities. In films grown with 60 W of rf power, FeWO 4 crystallites do not form fully epitaxial layers. X-ray photoelectron spectroscopy indicates that the structural changes are correlated with the Fe 3+ /Fe 2+ oxidation state ratio increasing from 0.6-1.4 with rf power from 60-120 W. X-ray fluorescence spectroscopy indicates that the Fe/W composition ratio is also increasing from 1.1-1.8 with rf power from 60-120 W. Ultraviolet and visible optical absorption spectra indicate a 1.8 ± 0.1 eV band gap with an additional interband absorption feature at 3.1 ± 0.1 eV in the 80-100 W films, with similar onsets observed in the 60 W films. In the 120 W films, the higher lying transition is shifted to 2.7 ± 0.1 eV due to the Fe 3+ enrichment. Electrical resistivity decreases over 2 orders of magnitude with oxidation from 10 4 -10 5 Ω cm in 60 W films to 120 ± 10 Ω cm in 120 W films. Thermopower measurements show p-type to n-type conductivity conversion when oxidation states shift from Fe 2+ majority in the 100 W films to Fe 3+ majority in the 120 W films. We conclude that electron polaron hopping driven by Fe 3+ is a dominant transport mechanism and a source of n-type conductivity in overoxidized FeWO 4 films.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Ambient and High Pressure CuNiSb 2 : Metal-Ordered and Metal-Disordered NiAs-Type Derivative Pnictides

The mineral Zlatogorite, CuNiSb 2 , was synthesized in the laboratory for the first time by annealing elements at ambient pressure (CuNiSb 2 -AP). Rietveld refinement of synchrotron powder X-ray diffraction data indicates that CuNiSb 2 -AP crystallizes in the NiAs-derived structure ( P 3 m 1, #164) with Cu and Ni ordering. The structure consists of alternate NiSb 6 and CuSb 6 octahedral layers via face-sharing. The formation of such structure instead of metal disordered NiAs-type structure ( P 6 3 / mm c, #194) is validated by the lower energy of the ordered phase by first-principle calculations. Interatomic crystal orbital Hamilton population, electron localization function, and charge density analysis reveal strong Ni-Sb, Cu-Sb, and Cu-Ni bonding and long weak Sb-Sb interactions in CuNiSb 2 -AP. The magnetic measurement indicates that CuNiSb 2 -AP is Pauli paramagnetic. First-principle calculations and experimental electrical resistivity measurements reveal that CuNiSb 2 -AP is a metal. The low Seebeck coefficient and large thermal conductivity suggest that CuNiSb 2 is not a potential thermoelectric material. Single crystals were grown by chemical vapor transport. The high pressure sample (CuNiSb 2 -8 GPa) was prepared by pressing CuNiSb 2 -AP at 700 °C and 8 GPa. However, the structures of single crystal and CuNiSb 2 -8 GPa are best fit with a disordered metal structure in the P 3 m 1 space group, corroborated by transmission electron microscopy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Two Polymorphs of BaZn 2 P 2 : Crystal Structures, Phase Transition, and Transport Properties

Here, the novel α-BaZn 2 P 2 structural polymorph has been synthesized and structurally characterized for the first time. Its structure, elu-cidated from single crystal X-ray diffraction, indicates that the compound crystallizes in the orthorhombic α-BaCu 2 S 2 structure type, with unit cell parameters a = 9.7567(14) Å, b = 4.1266(6) Å, and c = 10.6000(15) Å. With β-BaZn 2 P 2 being previously iden-tified as belonging to the ThCr 2 Si 2 family, and with the precedent of structural phase transitions between the α-BaCu 2 S 2 type and the ThCr 2 Si 2 type, the potential for the pattern to be extended to the two different structural forms of BaZn 2 P 2 was explored. Thermal analysis suggest that a first order phase transition occurs at ~1123 K, whereby the low-temperature orthorhombic α-phase transforms to a high-temperature tetragonal β-BaZn 2 P 2 ; the structure of which was also studied and confirmed by single-crystal X-ray diffraction. Preliminary transport properties and band structure calculations indicate that α-BaZn 2 P 2 is a p-type, narrow-gap semiconductor with a direct bandgap (Eg) of 0.5 eV. The Seebeck coefficient, S(T), for the material increases steadily from the room temperature value of 119 µV/K to 184 µV/K at 600 K. The electrical resistivity of α-BaZn 2 P 2 is relatively high, on the order of 40 mΩ·cm, and the ρ(T) dependence shows gradual decrease upon heating. Such behavior is comparable to those of the typical semimetals or degenerate semiconductors. Although the observed values of the as-synthesized samples are not optimal, the carrier concentration can apparently be tuned to fall between the values of metals and semiconductors, thus providing an open window for optimizing this phase towards achieving an enhanced thermoelectric figure of merit zT. The calculated indirect band gap for the β-BaZn 2 P 2 phase is Eg = 0.03 eV which is about an order of magnitude lower than that of α-BaZn 2 P 2 . It is expected that the cage-like structural motif and layered structure possessed by α-BaZn 2 P 2 and β-BaZn 2 P 2 phases, respectively, would promote the realiza-tion of a low thermal conductivity in both compounds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Structural Complexity and Tuned Thermoelectric Properties of a Polymorph of the Zintl Phase Ca 2 CdSb 2 with a Non-centrosymmetric Monoclinic Structure

The Zintl phase Ca 2 CdSb 2 was found to be dimorphic. Besides the orthorhombic Ca 2 CdSb 2 (-o), here we report on the synthesis, the structural characterization, and the thermoelectric transport properties of its monoclinic form, Ca 2 CdSb 2 (-m), and its Lu-doped variant Ca 2–x Lu x CdSb 2 (x ≈ 0.02). The monoclinic structure exhibits complex structural characteristics and constitutes a new structure type with the non-centrosymmetric space group Cm (Z = 30). The electrical resistivity ρ(T) measured on single crystals of both phases portrays a transition from a semiconductor to a degenerate p-type semiconductor upon doping with Lu and with an attendant change in the Hall carrier concentration nH from 7.15 × 10 18 to 2.30 × 10 19 cm –3 at 300 K. The Seebeck coefficient S(T) of both phases are comparable and indicate a hole-dominated carrier transport mechanism with magnitudes of 133 and 116 μV/K at 600 K for Ca 2 CdSb 2 (-m) and Ca 2–x Lu x CdSb 2 , respectively. The convoluted atomic bonding with an attendant large unit cell volume of ~4365 Å 3 drives a putative low thermal conductivity in these materials resulting in a power factor PF of 1.63 μW/cm K 2 and an estimated thermoelectric figure of merit zT of ~0.5 for Ca 2–x Lu x CdSb 2 at 600 K. Furthermore, differential scanning calorimetry results reveal the stability of these phases up to about 960 K, making them candidates for moderate temperature thermoelectric materials.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Rb 4 CuSb 2 Cl 11 and Rb 2 In 0.91(0.2) Sb 0.09 Cl 5 ·H 2 O: Wide Band Gap 0D Metal Halide Semiconductors

Herein, we report the discovery, structural and photophysical characterization of a new zero-dimensional (0D) lead-free all-inorganic halide, Rb 4 CuSb 2 Cl 11 , which adopts a new structure type. Single-crystal X-ray diffraction (SCXRD) shows that the structure consists of isolated, distorted seesaw [SbCl 4 ] − and trigonal planar [CuCl 3 ] 2− units, separated by Rb + cations that provide charge balance. Optoelectronic measurements and density functional theory (DFT) calculations indicate an indirect band gap of 2.89 eV, making it a candidate for wide-bandgap optoelectronic applications. Electrical resistivity was measured at 1.29 × 10 10 Ω·cm, and the trap-state density (n trap ) was found to be 7.44 × 10 10 cm −3 . Attempts to synthesize substitution analogs of Rb 4 CuSb 2 Cl 11 led to the synthesis of Rb 2 In 0.91(0.2) Sb 0.09 Cl 5 ·H 2 O, which was erroneously reported as Rb 2 SbCl 5 O in a previous study. Rb 2 In 0.91(0.2) Sb 0.09 Cl 5 ·H 2 O adopts a vacancyordered perovskite structure and exhibits broad-band yellow emission under UV excitation. The measured photoluminescence quantum yield (PLQY) for Rb 2 In 0.91 (0.2)Sb 0.09 Cl 5 ·H 2 O is 18.2%. These findings add to the growing class of quaternary metal halides with multiple cation and anion compositions, expanding the chemical phase space for the discovery of new materials with functional properties.

Crystal structure↗
DOE OSTI2021

Electronic Conductivity of Nanoporous Indium Oxide Derived from Sequential Infiltration Synthesis

Sequential infiltration synthesis (SIS) is a vapor phase synthetic method that enables the selective nucleation and growth of metal oxides within polymer volumes. The expanding palette of SIS materials and process designs enables tunability of the resulting material properties. We derive porous indium oxide thin films from the SIS of indium oxide using trimethyl indium and H 2 O 2 in polymethyl methacrylate films and observe the strong effect of SIS and post-deposition processing, which affords 7 orders of magnitude of tunability in electrical resistivity. While as-deposited hybrid nanocomposites show no measurable conductivity, high-temperature treatment in O 2 removes the polymer matrix and creates a porous nanocrystalline In 2 O 3 film with resistivities ranging from 10 3 to 10 5 Ω*cm, with lower resistivity correlated to larger grain sizes. Subsequent annealing in H 2 decreases the resistivity of films to less than 10 –2 Ω*cm. A clear correlation between increasing In 2 O 3 volume fraction, grain size, and carrier mobility is observed, which arises from increased percolation pathways, path length, and contact area in nanocrystalline In 2 O 3 films. Furthermore, this wide tunability demonstrates the importance of understanding growth mechanisms and processing conditions for functional materials development.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Synthesis and Physical Properties of Manganese Chromium Nitride Thin Films Grown via Molecular Beam Epitaxy

Here, we demonstrate the ability to grow epitaxial thin films of manganese–chromium nitride (Mn x Cr 1–x N) across the entire composition range on magnesium oxide (MgO) (1 0 0) surfaces by molecular beam epitaxy. By independently controlling the ratio of the atomic fluxes of manganese and chromium, the compositional dependence of structural and electrical transport properties is determined. Soft X-ray absorption shows that the valence states of Cr( 3+ ) and Mn( 3+ ) remain identical among these MnCrN epitaxial films. The metallic transport behavior of binary manganese nitride (MnN) and chromium nitride (CrN) is suppressed in the ternary alloys. Changes in room temperature electrical resistivity of up to 100 times and 5 orders of magnitude at cryogenic temperatures are observed. This work expands the variety of nitride compounds available for epitaxial integration and could provide additional flexibility for the development of metal–semiconductor–insulator junctions for spintronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Emerging Magnetic Interactions in van der Waals Heterostructures

Vertical van der Waals (vdWs) heterostructures based on layered materials are attracting interest as a new class of quantum materials, where interfacial charge-transfer coupling can give rise to fascinating strongly correlated phenomena. Transition metal chalcogenides are a particularly exciting material family, including ferromagnetic semiconductors, multiferroics, and superconductors. Here, we report the growth of an organic–inorganic heterostructure by intercalating molecular electron donating bis(ethylenedithio)tetrathiafulvalene into (Li,Fe)OHFeSe, a layered material in which the superconducting ground state results from the intercalation of hydroxide layer. Molecular intercalation in this heterostructure induces a transformation from a paramagnetic to spin-glass-like state that is sensitive to the stoichiometry of molecular donor and an applied magnetic field. Besides, electron-donating molecules reduce the electrical resistivity in the heterostructure and modify its response to laser illumination. In conclusion, this hybrid heterostructure provides a promising platform to study emerging magnetic and electronic behaviors in strongly correlated layered materials.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Ultrathin Ruthenium Films on Graphene Buffered SiO 2 via Quasi Van der Waals Epitaxy

In this study, we demonstrate a quasi van der Waals epitaxy approach to prepare single crystalline Ru ultrathin film on large scale, monolayer graphene. Physical and epitaxial properties of bulk, near surface and surface of ultrathin Ru films were comprehensively studied using various structural, morphological, compositional, and electrical characterization techniques. Here, we confirm that Ru can epitaxially grow on single, monolayer graphene using magnetron sputtering at elevated temperature of 600 °C. The epitaxial Ru films with film thickness ranging from 94.2 nm down to 3.9 nm show the (0001) out-of-plane orientation. The epitaxial relationships between Ru and graphene are out-of-plane Ru(0001) || graphene(0001) and in-plane Ru[$11\bar20$] || graphene[$11\bar20$]. All the Ru films show smooth surfaces with root-mean-square roughness less than 0.8 nm and have negligible oxide layer on the surfaces. The Ru films on graphene demonstrate significantly reduced electrical resistivity comparing to their counterpart grown on bare SiO 2 , which show polycrystalline nature. For 7.1 to 3.9 nm film thicknesses, the resistivity of Ru on graphene shows 38 to 45% resistivity decrease from that of Ru film on bare SiO 2 without graphene. Our observations suggest the existence of the above-classical van der Waals interaction between Ru and graphene. On the other hand, graphene is capable of effectively blocking the inter-diffusion/interaction between Ru and SiO 2 during a 1000 °C annealing process.

36 MATERIALS SCIENCE↗
DOE OSTI2023

High Thermoelectric Performance in 2D Sb 2 Te 3 and Bi 2 Te 3 Nanoplate Composites Enabled by Energy Carrier Filtering and Low Thermal Conductivity

Thermoelectrics are an important class of materials with great potential in alternative energy applications. In this study, two-dimensional (2D) nanoplates of the layered chalcogenides, Sb 2 Te 3 and Bi 2 Te 3 , are synthesized and composites of the two are investigated for their thermoelectric properties. The two materials, Sb 2 Te 3 and Bi 2 Te 3 , were synthesized as hexagonal, 2D nanoplates via a colloidal polyol route. The as-synthesized Sb 2 Te 3 and Bi 2 Te 3 vary drastically from one another in their lateral and vertical dimensions as revealed by scanning electron microscopy and atomic force microscopy. The single crystalline nanoplate nature is deduced by high-resolution transmission electron microscopy and selected area electron diffraction. Nanoplates have well-defined hexagonal facets as seen in the scanning and transmission electron microscopy images. The nanoplates were consolidated as an anisotropic nanostructured pellet via spark plasma sintering. Preferred orientation observed in the powder X-ray diffraction pattern and scanning electron microscopy images of the fractured pellets confirm the anisotropic structure of the nanoplates. Thermoelectric properties in the parallel and perpendicular directions were measured, revealing strong anisotropy with a significant reduction to thermal conductivity in the perpendicular direction due to increased phonon scattering at nanoplate interfaces. All compositions, except that of the 25% Bi 2 Te 3 nanoplate composite, behave as degenerate semiconductors with increasing electrical resistivity as the temperature increases. The Seebeck coefficient is also increased dramatically in the nanocomposites, the highest reaching 210 μV/K for 15% Bi 2 Te 3 . The increase in Seebeck is attributed to energy carrier filtering at the nanoplate interfaces. Overall, these enhanced thermoelectric properties lead to a drastic increase in the thermoelectric performance in the perpendicular direction, with zT ~ 1.26, for the 15% Bi 2 Te 3 nanoplate composite at 450 K.

36 MATERIALS SCIENCE↗