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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Dynamically Tunable Terahertz Emission Enabled by Anomalous Optical Phonon Responses in Lead Telluride

Lead telluride (PbTe), a narrow bandgap semiconductor commonly used in infrared detectors, exhibits anomalous vibrational and structural properties, making it appealing for thermoelectrics. Despite significant fundamental interest in the microscopic origins of its unusual vibrational properties, the optical functionalities stemming from phonons and electron–phonon coupling in PbTe have not been closely investigated. This work reports measurements of terahertz (THz) radiation from a PbTe single crystal following ultrafast optical excitation and investigates IR-active phonon responses as a function of excitation fluence and temperature. We uncover a spectrally tunable THz emission peak enabled by an epsilon-near-zero response of the coupled plasmon–longitudinal optical phonon mode that can be dynamically shifted via tuning photocarrier density. Spectral tunability (Δω/ω = 25%) is significant and beyond what has been achieved by any other THz emitter. In addition, the emitted THz fields reveal signatures of a zone center transverse optical phonon anomaly and unveil a new mode at 0.3 THz that diminishes in amplitude under increasing photocarrier density. Temperature-dependent measurements suggest that the transverse-like modes at 1 and 1.5 THz are possibly of different origins. These results indicate that the unusual phononic properties in PbTe are tunable via photoexcitation and enable new optical functionalities in THz applications, such as spectrally tunable emitters and all-optical modulators.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Thermal conductivity of bulk nanostructured lead telluride

Thermal conductivity of lead telluride with embedded nanoinclusions was examined using Monte Carlo simulations with intrinsic phonon transport properties obtained from first-principles-based lattice dynamics. The nanoinclusion/matrix interfaces were set to completely reflect phonons to model the maximum interface-phonon-scattering scenario. The simulations with the geometrical cross section and volume fraction of the nanoinclusions matched to those of the experiment show that the experiment has already reached the theoretical limit of thermal conductivity. The frequency-dependent analysis additionally identifies that the thermal conductivity reduction is dominantly attributed to scattering of low frequency phonons and demonstrates mutual adaptability of nanostructuring and local disordering.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Design of Lead Telluride Based Thermoelectric Materials through Incorporation of Lead Sulfide Inclusions or Ligand Stripping of Nanosized Building Blocks

Design of thermoelectric materials focuses on the optimization of several unfavorably coupled factors: electrical conductivity, Seebeck coefficient, and thermal conductivity. Recent work in thermoelectrics has focused on decreasing lattice thermal conductivity by nanostructuring thermoelectric materials, while recent work in photovoltaics has demonstrated ligand stripping as a means to increased electron mobility in thin films of nanoparticles. In the present work, these two features are combined. A multigram scale synthesis of dispersible, lead telluride nanocrystals (25–50 nm) is developed using hot-injection methods in common organic solvents. These nanocrystals (NCs) are ligand stripped with sulfide (PbTe-S) or iodide (PbTe-I) sources to result in p-type or n-type materials with large Seebeck coefficients at room temperature of 520 or –540 μV·K –1 , respectively. Sequential stripping with sulfide and then iodide (PbTe-SI) resulted in a small Seebeck due to counter doping. PbTe-S and PbTe-SI are found to generate nanostructured composites by growth of lead sulfide nanocrystals (~50–60 nm) in situ upon annealing. However, the electrical conductivities are low (<1 S·cm –1 ) due to excess doping during the ligand stripping. Intentional formation of a nanocomposite (PbTe–PbS) is achieved by combining PbTe NCs with 4–6 nm diameter lead sulfide particles via mixing by incipient wetness with a target of 8 mol % lead sulfide. The resulting nanocomposite is n-type with a Seebeck coefficient of –160 μV·K –1 and an electrical conductivity of 42 S·cm –1 at room temperature. The lattice thermal conductivities of all materials at room temperature are substantially lower than those of bulk lead telluride (2.0 W m –1 ·K –1 ). However, thermoelectric figure of merit (ZT) values are low for all samples (maximum ZT = 0.03 for PbTe–PbS), attributed primarily to the low electrical conductivities. As a result, this work underscores the importance of developing new methods for augmenting electrical conductivity if nanoparticle assemblies are to be practically employed in thermoelectrics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Preparation and Characterization of Niobium Doped Lead-Telluride Glass Ceramics

Niobium-lead-telluride glass ceramics of composition xNb{sub 2}O{sub 5}-(20-x) pbO-80TeO{sub 2}(where x = 0.1 mol% to 0.5 mol%) were prepared by using conventional melt quenching method. The prepared glass samples were initially amorphous in nature after annealed at 400 deg. c all samples were crystallized. This was confined by X-ray diffraction and scanning electron microscopy. The particle size of these glass ceramics have been calculated by using Debye-Scherer formula and the particle size is in the order of 15 nm to 60 nm. The scanning electron microscopy (SEM) photograph shows the presence of needle-like crystals in these samples.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Synthesis and structure of undoped and indium-doped thermoelectric lead telluride nanoparticles

Undoped and indium (In)-doped lead telluride (PbTe) nanostructures were synthesized via solvothermal/hydrothermal route. The crystalline structure of the as-prepared undoped and In-doped PbTe samples was examined by X-ray diffraction (XRD) which indicated the formation of face-centered single-phase cubic crystal. A first principle calculation on indium doping shows that the indium atoms are more likely to replace lead (Pb) rather than to take the interstitial sites. Laser-induced breakdown spectroscopy (LIBS) analysis confirms that indium is incorporated into the PbTe matrix of the indium-doped PbTe samples. The effects of surfactant and synthesis temperature on the structure and morphology of the undoped PbTe were also investigated; it was found that PbTe nanostructures synthesized with the addition of surfactants exhibited uniform shapes and their size increased with the synthesis temperature.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Microscopic mechanism of low thermal conductivity in lead telluride

Themicroscopic physics behind low-lattice thermal conductivity of single-crystal rock salt lead telluride (PbTe) is investigated. Mode-dependent phonon (normal and umklapp) scattering rates and their impact on thermal conductivity were quantified by first-principles-based anharmonic lattice dynamics calculations that accurately reproduce thermal conductivity in a wide temperature range. The low thermal conductivity of PbTe is attributed to the scattering of longitudinal acoustic phonons by transverse optical phonons with large anharmonicity and small group velocity of the soft transverse acoustic phonons. This results in enhancing the relative contribution of optical phonons, which are usually minor heat carriers in bulk materials.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Temperature effects on the energy bandgap and conductivity effective masses of charge carriers in lead telluride from first-principles calculations

We determined the temperature effects on the electronic properties of lead telluride (PbTe) such as the energy bandgap and the effective masses of charge carriers by incorporating the structural changes of the material with temperature using ab-initio density functional theory (DFT) calculations. Though the first-principles DFT calculations are done at absolute zero temperatures, by incorporating the lattice thermal expansion and the distortion of Pb{sup 2+} ions from the equilibrium positions, we could determine the stable structural configuration of the PbTe system at different temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Microscopic mechanism of low thermal conductivity in lead telluride

The microscopic physics behind low-lattice thermal conductivity of single-crystal rock salt lead telluride (PbTe) is investigated here. Mode-dependent phonon (normal and umklapp) scattering rates and their impact on thermal conductivity were quantified by first-principles-based anharmonic lattice dynamics calculations that accurately reproduce thermal conductivity in a wide temperature range. The low thermal conductivity of PbTe is attributed to the scattering of longitudinal acoustic phonons by transverse optical phonons with large anharmonicity and small group velocity of the soft transverse acoustic phonons. This results in enhancing the relative contribution of optical phonons, which are usually minor heat carriers in bulk materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗