SEARCH · Engineering Papers
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.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
CERN-LHC. To assess the properties of the quark–gluon plasma formed in ultrarelativistic ion collisions, the ATLAS experiment at the LHC measures a correlation between the mean transverse momentum and the flow harmonics.
Hybrid organic-inorganic perovskites such as methylammonium lead iodide have emerged as promising semiconductors for energy-relevant applications. The interactions between charge carriers and lattice vibrations, giving rise to polarons, have been invoked to explain some of their extraordinary optoelectronic properties. Here, time-resolved optical spectroscopy is performed, with off-resonant pumping and electronic probing, to examine several representative lead iodide perovskites. The temporal oscillations of electronic bandgaps induced by coherent lattice vibrations are reported, which is attributed to antiphase octahedral rotations that dominate in the examined 3D and 2D hybrid perovskites. The off-resonant pumping scheme permits a simplified observation of changes in the bandgap owing to the A(g) vibrational mode, which is qualitatively different from vibrational modes of other symmetries and without increased complexity of photogenerated electronic charges. The work demonstrates a strong correlation between the lead-iodide octahedral framework and electronic transitions, and provides further insights into the manipulation of coherent optical phonons and related properties in hybrid perovskites on ultrafast timescales.
Methylammonium lead iodide (MAPbIx) perovskites are organic-inorganic semiconductors that serve as the light-harvesting component in photovoltaic devices. Their long carrier diffusion lengths yield power conversion efficiencies in excess of 22%. Conventional growth techniques apply spin-coated precursors on an oxide or polymer substrate followed by annealing; however, use of toxic solvents and high temperature hinder device stability and performance. Through a nucleophilic substitutional growth of MAPbIx in polar protic alcohols, we show evidence for their substrate- and annealing- free production. These variable-sized crystals (~100 nm-10 µm) are found to be tetragonally single-phased in alcohols, and precipitated as powders that are metallic-lead-free with a negligibly small amount of unreacted PbI2 (<10%). On the basis of this chemical approach, we obtain chemically stable MAPbIx in air up to ~1.5 months with thermal stability at ≤300°C. This method is amendable for large-area deposition, large-scale manufacturing, and ultimately leads to energy-efficient, low-cost, and stable optoelectronics.
Not Available
Residents in older homes may experience increased lead (Pb) exposures due to release of lead from interior paints manufactured in past decades, especially pre-1960s. The objective of the study was to determine the speciation of Pb in settled dust from an urban home built during WWII. X-ray absorption near-edge structure (XANES) and micro-X-ray diffraction (XRD) analyses were performed on samples of paint (380-2,920 mg Pb kg{sup -1}) and dust (200-1,000 mg Pb kg{sup -1}) collected prior to renovation. All dust samples exhibited a Pb XANES signature similar to that of Pb found in paint. Bulk XANES and micro-XRD identified Pb species commonly found as white paint pigments (Pb oxide, Pb sulfate, and Pb carbonate) as well as rutile, a titanium-based pigment, in the <150 {micro}m house dust samples. In the dust fraction <36 {micro}m, half of the Pb was associated with the Fe-oxyhydroxides, suggesting additional contribution of outdoor sources to Pb in the finer dust. These results confirm that old paints still contribute to Pb in the settled dust for this 65-year-old home. The Pb speciation also provided a clearer understanding of the Pb bioaccessibility: Pb carbonate > Pb oxide > Pb sulfate. This study underscores the importance of taking precautions to minimize exposures to Pb in house dust, especially in homes where old paint is exposed due to renovations or deterioration of painted surfaces.
Not provided.
Assay methods for measuring 238U, 232Th, and 210Pb concentrations in refined lead are presented. The 238U and 232Th concentrations are assayed via inductively coupled plasma mass spectrometry (ICP-MS) after anion exchange column separation on dissolved lead samples. The 210Pb concentration is inferred through α-spectroscopy of a daughter isotope, 210Po, after chemical precipitation separation on dissolved lead samples. Subsequent to the 210Po α-spectroscopy assay, a method for evaluating 210Pb concentrations in solid lead samples was developed via measurement of bremsstrahlung radiation from β-decay of a daughter isotope, 210Bi, by employing a 14-crystal array of high purity germanium (HPGe) detectors. Ten sources of refined lead were assayed. The 238U concentrations were <34 microBq/kg and the 232Th concentrations ranged <0.6 – 15 microBq/kg, as determined by the ICP-MS assay method. The 210Pb concentrations ranged from ~0.1 – 75 Bq/kg, as inferred by the 210Po α-spectroscopy assay method.
Impurities in semiconductors, for example, lead-based hybrid perovskites, have a major influence on their performance as photovoltaic (PV) light absorbers. While impurities could create harmful trap states that lead to nonradiative recombination of charge carriers and adversely affect PV efficiency, they could also potentially increase absorption via midgap energy levels that act as stepping stones for subgap photons or introduce charge carriers via doping. To unearth trends in impurity energy states, we use first principles density functional theory calculations to extensively study partial substitution of Pb in methylammonium lead bromide (MAPbBr 3 ), a representative lead-halide perovskite. Investigation of the density of states and energy levels related to the transition of the substitutional defect from one charge state to another reveals that several elements create midgap energy states in MAPbBr 3 . We machine learned trends and design rules from the computational data and discovered that a few easily computed properties of the bromide compounds of any element can be used to predict the energetics and energy levels of the substitutional defect related to that element. The calculated Fermi level-dependent defect formation energies lead to the observation that substitution by transition metals, Zr, Hf, Nb, and Sc, and group V element Sb can shift the equilibrium Fermi level and change the perovskite conductivity, as determined by the dominant intrinsic point defects. Finally, metal-substituted MAPbBr 3 compounds of Bi, Sc, Ni, and Zr were experimentally investigated, and while there was an improvement in the thin-film morphology and an enhancement in charge carrier lifetimes, no clear evidence of subgap absorption features owing to the substituent being incorporated in the MAPbBr 3 lattice could be seen.
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.
We report the results of a computation of the full next-to-leading order QCD corrections to the production of two bb pairs at the LHC. This calculation at the parton level provides predictions for well separated b jets. The results show that the next-to-leading order corrections lead to an enhancement of the cross section for the central scale choice by roughly 50% with respect to the leading order result. The theoretical uncertainty estimated by variation of the renormalization and factorization scales is strongly reduced by the inclusion of next-to-leading order corrections.
Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low-frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in the cubic phase of both hybrid (CH 3 NH 3 PbBr 3 ) and all-inorganic (CsPbBr 3 ) lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. Furthermore, MD simulations indicate that head-to-head Cs motion coupled to Br face expansion, occurring on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr 3 .
Lead-acid batteries (LABs) play a pivotal role in the energy storage sector with applications spanning from starting-lighting-ignition batteries to grid energy storage. Passivation of lead negative electrodes by PbSO 4 particles is a fundamental mechanism limiting the performance of LAB. In this regard, an electrochemical model is developed that simulates the nucleation and growth (N&G) dynamics of PbSO 4 particles on a flat lead electrode, responsible for its passivation. The model considers the electrochemical reactions between lead electrode and sulfuric acid, N&G of PbSO 4 particles, passivation of the lead surface, and the ternary transport of PbSO 4 (aq), bisulfate, and protons in H 2 SO 4 electrolyte. The model is validated with a dataset of cyclic voltammetry (CV) responses collected at several scan rates and H 2 SO 4 concentrations. The model shows remarkable qualitative and quantitative agreement with the experimental data including CV peak features, discharge capacity, and particle size. The model was employed to explore key N&G quantities, such as supersaturation, nucleation rate, particle count, growth rate, particle size, and surface coverage, and to examine how their interactions influence electrode utilization. Parametric studies were also conducted to evaluate how scan rates and acid concentrations influence the previously mentioned N&G quantities and, subsequently, the utilization of the electrode.
Lead exposures from legacy sources threaten children’s health. Soil in Omaha, Nebraska, was contaminated by emissions from a lead smelter and refinery. The U.S. Environmental Protection Agency excavated and replaced contaminated soil at the Omaha Lead Superfund Site between 1999 and 2016. The goal of this study was to assess the association of soil lead level (SLL) and soil remediation status with blood lead levels (BLLs) in children living near or on the site. We linked information on SLL at residential properties with children’s BLLs and assigned remediation status to children’s BLL measurements based on whether their measurements occurred during residence at remediated or unremediated properties. We examined the association of SLL and remediation status with elevated BLL (EBLL). We distinguished the roles of temporal trend and the intervention with time-by-intervention-status interaction contrasts. All analyses estimated odds ratios (ORs) with a generalized estimating equations approach to ensure robustness under the complex correlations among BLL measurements. All analyses controlled for relevant covariates including children’s characteristics. EBLL (> 5 μg/dL ) was associated with both residential SLL [e.g., OR=2.00; 95% confidence interval (CI): 1.83, 2.19; > 400–800 vs. ≤ 200 ppm] and neighborhood SLL [e.g., OR=1.85 (95% CI: 1.62, 2.11; > 400–800 vs. ≤ 200 ppm)] before remediation but only with neighborhood SLL after remediation. The odds of EBLL were higher before remediation [OR 1.52 (95% CI: 1.34, 1.72)]. Similarly, EBLL was positively associated with preremediation status in our interaction analysis [interaction OR=1.18 (95%CI: 1.02, 1.37)]. Residential and neighborhood SLLs were important predictors of EBLLs in children residing near or on this Superfund site. Neighborhood SLL remained a strong predictor following remediation. Our data analyses showed the benefit of soil remediation. Results from the interaction analyses should be interpreted cautiously due to imperfect correspondence of remediation times between remediation and comparison groups. https://doi.org/10.1289/EHP8657
Many technologically critical materials are metastable under ambient conditions, yet the understanding of how to rationally design and guide the synthesis of these materials is limited. This paper presents an integrated approach that targets a metastable lead-free piezoelectric polymorph of SrHfO 3 . First-principles calculations predict that the previous experimentally unrealized, metastable P4mm phase of SrHfO 3 should exhibit a direct piezoelectric response (d 33 ) of 36.9 pC N -1 (compared to d 33 = 0 for the ground state). Combining computationally optimized substrate selection and synthesis conditions lead to the epitaxial stabilization of the polar P4mm phase of SrHfO 3 on SrTiO 3 . The films are structurally consistent with the theory predictions. A ferroelectric-induced large signal effective converse piezoelectric response of 5.2 pm V -1 for a 35 nm film is observed, indicating the ability to predict and target multifunctionality. Finally, this illustrates a coupled theory-experimental approach to the discovery and realization of new multifunctional polymorphs.
Heterogeneous nanocomposites of p-type bismuth antimony telluride (Bi{sub 2-x}Sb{sub x}Te{sub 3}) with lead telluride (PbTe) nanoinclusions have been prepared by an incipient wetness impregnation approach. The Seebeck coefficient, electrical resistivity, thermal conductivity and Hall coefficient were measured from 80 to 380 K in order to investigate the influence of PbTe nanoparticles on the thermoelectric performance of nanocomposites. The Seebeck coefficients and electrical resistivities of nanocomposites decrease with increasing PbTe nanoparticle concentration due to an increased hole concentration. The lattice thermal conductivity decreases with the addition of PbTe nanoparticles but the total thermal conductivity increases due to the increased electronic thermal conductivity. We conclude that the presence of nanosized PbTe in the bulk Bi{sub 2-x}Sb{sub x}Te{sub 3} matrix results in a collateral doping effect, which dominates transport properties. This study underscores the need for immiscible systems to achieve the decreased thermal transport properties possible from nanostructuring without compromising the electronic properties. - Graphical abstract: PbTe nanoparticles introduced into p-type Bi{sub 2}Te{sub 3} by incipient wetness results in decreased lattice thermal conductivity, but also acts as an electronic dopant, resulting in an overall decrease in thermoelectric performance. Highlights: Black-Right-Pointing-Pointer Composites of PbTe nanoparticles in Bi{sub 2-x}Sb{sub x}Te{sub 3} were formed by incipient wetness. Black-Right-Pointing-Pointer PbTe nanoparticles leads to decreased {kappa}{sub l}, consistent with phonon scattering. Black-Right-Pointing-Pointer PbTe nanoparticles lead to decreased S and {rho}, due to increased carriers. Black-Right-Pointing-Pointer Collateral doping from PbTe leads to decreased ZT with increasing concentration. Black-Right-Pointing-Pointer Immiscible systems are preferred for improved ZT.
Lead oxide (PbO) nanoparticles were chemically synthesized using Lead (II) acetate as precursor. The effects of organic capping agents such as Oleic acid, Ethylene Diamine Tetra Acetic acid (EDTA) and Cetryl Tri Methyl Butoxide (CTAB) on the size and morphology of the nanoparticles were studied. Characterization techniques such as X-ray diffraction (XRD), Fourier Transform-Infrared spectroscopy (FT-IR), Photoluminescence (PL) Field Emission Scanning Electron Microscopy (FE-SEM), Energy Dispersive Spectroscopy (EDS) and Transmission Electron Microscopy (TEM) were used to analyse the prepared nanoparticles for their physical, structural and optical properties. The characterization studies reveal that the synthesized PbO nanoparticles had well defined crystalline structure and sizes in the range of 25 nm to 36 nm for capping agents used and 40 nm for pure PbO nanoparticles.
Two new hybrid lead halides (H{sub 2}BDA)[PbI{sub 4}] (1) (H{sub 2}BDA=1,4-butanediammonium dication) and (HNPEIM)[PbI{sub 3}] (2) (HNPEIM=N-phenyl-ethanimidamidine cation) have been synthesized and structurally characterized. X-ray diffraction analyses reveal that compound 1 features a two-dimensional corner-sharing perovskite layer whereas compound 2 contains one-dimensional edge-sharing double chains. The N-phenyl-ethanimidamidine cation within compound 2 was generated in-situ under solvothermal conditions. The optical absorption spectra collected at room temperature suggest that both compounds are semiconductors having direct band gaps, with estimated values of 2.64 and 2.73 eV for 1 and 2, respectively. Results from the density functional theory (DFT) calculations are consistent with the experimental data. Density of states (DOS) analysis reveals that in both compounds 1 and 2, the energy states in the valence band maximum region are iodine 5p atomic orbitals with a small contribution from lead 6s, while in the region of conduction band minimum, the major contributions are from the inorganic (Pb 6p atomic orbitals) and organic components (C and N 2p atomic orbitals) in compound 1 and 2, respectively. - Graphical abstract: Two new hybrid lead halides built on one-dimensional edge-sharing double chains and two-dimensional corner-sharing perovskite layers are synthesized and their structural and electronic properties are analyzed. - Highlights: • Two new hybrid lead iodides are designed, synthesized, and characterized. • They are closely related to, but different from, perovskite structures. • The electronic properties of both compounds are analyzed by DFT calculations.
In this work, we have used the thermal lens technique combined with conventional spectroscopy to characterize the thermo-optical properties of Er{sup 3+}-doped lead pyrophosphate-based glasses. More precisely, we have investigated and quantified experimentally the fluorescence quantum efficiencies of the Er{sup 3+} levels, and we describe the role of concentration quenching effects. The fluorescence quantum efficiency of the {sup 4}I{sub 13/2} level is very high when compared to other phosphate glasses, while that of the green-coupled levels is very small. Other important photonic materials parameters, such as the thermal diffusivity and temperature coefficient of the optical path length change, were obtained and compared with those of other glass systems. The cumulative results obtained here for the Er-doped lead pyrophosphate glass show that this material is a good candidate for photonic applications with a characteristic Er{sup 3+} infrared emission around 1550 nm.