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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Thermionic converter performance with oxide collectors

Thermionic converters using a variety of metal oxide collector surfaces have been fabricated and tested. Both work function and power output data are presented and evaluated. Oxides of barium, strontium, zinc, tungsten and titanium have been incorporated into a variable spacing converter. Tungsten oxide was found to give the highest converter performance and to furnish oxygen for the emitter at the same time. Oxygenated emitters operate at reduced cesium pressure with an increase in electrode spacing. Electron spectroscopy for chemical analysis (ESCA) performed on several tungsten oxide collectors showed cesium penetration of the oxide layer, possibly forming a cesium tungstate bronze. Titanium oxide showed high performance but did not furnish oxygen for the emitter; strontium oxide, in the form of a sprayed layer, appeared to dissociate in the presence of cesium. Sprayed coatings of barium and zinc oxides produced collector work functions of about 1.3 eV, but had excessive series resistance. Lanthanum hexaboride, in combination with oxygen introduced through a silver tube, and cesium produced a low work function collector and better than average performance.

Lieb, D.↗
NASA NTRS2003

Development of Oxidation Protection Coatings for Gamma Titanium Aluminide Alloys

Metallic material systems play a key role in meeting the stringent weight and durability requirements for reusable launch vehicle (RLV) airframe hot structures. Gamma titanium aluminides (gamma-TiAl) have been identified as high-payoff materials for high-temperature applications. The low density and good elevated temperature mechanical properties of gamma-TiAl alloys make them attractive candidates for durable lightweight hot structure and thermal protection systems at temperatures as high as 871 C. However, oxidation significantly degrades gamma-TiAl alloys under the high-temperature service conditions associated with the RLV operating environment. This paper discusses ongoing efforts at NASA Langley Research Center to develop durable ultrathin coatings for protecting gamma-TiAl alloys from high-temperature oxidation environments. In addition to offering oxidation protection, these multifunctional coatings are being engineered to provide thermal control features to help minimize heat input into the hot structures. This paper describes the coating development effort and discusses the effects of long-term high-temperature exposures on the microstructure of coated and uncoated gamma-TiAl alloys. The alloy of primary consideration was the Plansee alloy gamma-Met, but limited studies of the newer alloy gamma-Met-PX were also included. The oxidation behavior of the uncoated materials was evaluated over the temperature range of 704 C to 871 C. Sol-gel-based coatings were applied to the gamma-TiAl samples by dipping and spraying, and the performance evaluated at 871 C. Results showed that the coatings improve the oxidation resistance, but that further development is necessary.

Wallace, T. A.↗
NASA NTRS1976

Selected fretting-wear-resistant coatings for titanium - 6-percent-aluminum - 4-percent-vanadium alloy

A titanium - 6-percent-aluminum - 4-percent-vanadium alloy (Ti-6Al-4V) was subjected to fretting-wear exposures against uncoated Ti-6Al-4V as a baseline and against various coatings and surface treatments applied to Ti-6Al-4V. The coatings evaluated included plasma-sprayed tungsten carbide with 12 percent cobalt, aluminum oxide with 13 percent titanium oxide, chromium oxide, and aluminum bronze with 10 percent aromatic polyester; polymer-bonded polyimide, polyimide with graphite fluoride, polyimide with molybdenum disulfide (MoS2), and methyl phenyl silicone bonded MoS2, preoxidation surface treatment, a nitride surface treatment, and a sputtered MoS2 coating. Results of wear measurements on both the coated and uncoated surfaces after 300,000 fretting cycles indicated that the polyimide coating was the most wear resistant and caused the least wear to the uncoated mating surface.

Bill, R. C.↗
NASA NTRS1975

Investigation to develop a method to apply diffusion barrier to high strength fibers

A radio frequency powered ion plating process was used to apply the diffusion barriers of aluminum oxide, yttrium oxide, hafnium oxide and titanium carbide to a substrate tungsten fiber. Each of the coatings was examined as to its effect on both room temperature strength and tensile strength of the base tungsten fiber. The coated fibers were then overcoated with a nickel alloy to become single cell diffusion couples. These diffusion couples were exposed to 1093 C for 24 hours, cycled between room temperature and 1093 C, and given a thermal anneal for 100 hours at 1200 C. Tensile testing and metallographic examinations determined that the hafnium oxide coating produced the best high temperature diffusion barrier for tungsten of the four coatings.

Veltri, R. D.↗
DOE OSTI2022

Controlled site coverage of strong metal–support interaction (SMSI) on Pd NP catalysts

Here, strong metal–support interaction catalysts have been shown to improve desired product selectivity at the cost of fractional rates due to active site coverage. The goal of this study was to determine if the active site coverage of metallic nanoparticles could be controlled to lower levels than have been previously reported in SMSI catalysts with the aim of improving the rate while maintaining high selectivity. 2Pd–XTi/SiO 2 (2 wt% Pd, X wt% Ti) strong metal–support interaction (SMSI) catalysts with Ti loadings between 0–1.0 wt% were synthesized to control Pd nanoparticle coverage. Calcination at 450 °C and reduction at 550 °C were sufficient for forming ~2 nm sized Pd particles in all catalysts. Increasing the Ti loading from 0.1 to 1.0 wt% increased the surface coverage from 40 to 85% at a fixed reduction temperature of 550 °C. The IR spectra of the SMSI catalysts were similar with a high fraction of linear bonded CO which was much higher than that of Pd nanoparticles of similar size. The SMSI overlayer could be removed by oxidation at 350 °C and re-reduction at 200 °C. EXAFS of the oxidized catalysts indicates that nearly full oxidation of the metallic nanoparticle was required to remove the SMSI overlayer. Oxidation temperatures from 30 to 300 °C partially oxidized the Pd nanoparticles and subsequent re-reduction at 200 °C partially decreases the SMSI coverage. The fractional surface coverage was determined by measuring the rate of propylene hydrogenation with and without the SMSI overlayer. Increasing the reduction temperature from 200 to 550 °C increased the SMSI coverage from 0 to 85% depending on the Ti loading and temperature. In conclusion, after reduction at 550 °C and oxidation at 350 °C, the range of coverages varied between ~10% with 0.1 wt% Ti after re-reduction at 300 °C and ~85% with 1 wt% Ti after reduction at 550 °C.

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

A Low-Voltage Layered Na 2 TiGeO 5 Anode for Lithium-Ion Battery

Titanium-based anode materials have achieved much progress with the wide studies in lithium-ion batteries. However, these known materials usually possess high discharge voltage platforms and limited energy densities. In this work, a titanium-based oxide of Na 2 TiGeO 5 with layered structure, two-dimensional lamellar frame and exposed highly active (001) facet, exhibiting good electrochemical performance in terms of high capacity (410 mAh g –1 with a current density of 50 mA g –1 ), excellent rate capability and cycling stability with no obvious capacity attenuation after 4000 cycles, is reported. The appropriate discharge voltage plateau at around 0.2 V endows the Na 2 TiGeO 5 anode material high security compared with graphite and high energy density compared with spinel Li 4 Ti 5 O 12 . Combining the electrochemical tests and the density functional theory calculations, the Li + storage mechanism of Na 2 TiGeO 5 is elucidated and the conversion reaction process is revealed. More importantly, this study provides a way to develop low-voltage and high-capacity titanium-based anode materials for efficient energy storage.

25 ENERGY STORAGE↗
DOE OSTI2024

Light-Induced Transformation of Virus-Like Particles on TiO 2

Titanium dioxide (TiO 2 ) shows significant potential as a self-cleaning material to inactivate severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and prevent virus transmission. This study provides insights into the impact of UV-A light on the photocatalytic inactivation of adsorbed SARS-CoV-2 virus-like particles (VLPs) on a TiO 2 surface at the molecular and atomic levels. X-ray photoelectron spectroscopy, combined with density functional theory calculations, reveals that spike proteins can adsorb on TiO 2 predominantly via their amine and amide functional groups in their amino acids blocks. We employ atomic force microscopy and grazing-incidence small-angle X-ray scattering (GISAXS) to investigate the molecular-scale morphological changes during the inactivation of VLPs on TiO 2 under light irradiation. Notably, in situ measurements reveal photoinduced morphological changes of VLPs, resulting in increased particle diameters. These results suggest that the denaturation of structural proteins induced by UV irradiation and oxidation of the virus structure through photocatalytic reactions can take place on the TiO 2 surface. The in situ GISAXS measurements under an N 2 atmosphere reveal that the virus morphology remains intact under UV light. This provides evidence that the presence of both oxygen and UV light is necessary to initiate photocatalytic reactions on the surface and subsequently inactivate the adsorbed viruses. The chemical insights into the virus inactivation process obtained in this study contribute significantly to the development of solid materials for the inactivation of enveloped viruses.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Synergy of Ion Doping and Spiral Array Architecture on Ti 2 Nb 10 O 29 : A New Way to Achieve High-Power Electrodes

Ameliorating electronic/ionic transport and structural stability of electrode materials is important to the development of power-intensive lithium ion batteries. Despite its great potential as a high-power anode, titanium niobium oxide (Ti 2 Nb 10 O 29 , TNO) still underperforms due to its unsatisfactory electronic/ionic conductivity. In this work, a powerful synergistic strategy by combining ion doping and spiral array architecture to boost high-rate performance of TNO is reported. Cr 3+ doped TNO nanoparticles (Cr-TNO) of 5-10 nm intimately grow on a conductive vertical graphene@TiC-C (VGTC) skeleton, forming novel Cr-TNO@VGTC spiral arrays. The unique spiral growth of TNO is achieved due to the confinement effect of VGTC skeleton. Meanwhile, a more open TNO crystal structure with faster ion transfer paths and enhanced structural stability is realized by Cr 3+ doping, demonstrated via density functional theory calculation and in situ synchrotron X-ray diffraction technique. Benefiting from the superior conductive network, enhanced intrinsic electronic/ionic conductivity of Cr-TNO and reinforced structural stability, the Cr-TNO@VTC arrays show prominent high-power performance with a large capacity of 220 mAh g -1 at 40 C (power density of ≈11 kW kg -1 ) and superior durability (91% retention after 500 cycles). This work provides a new path for the construction of widespread high-power electrodes for fast energy storage.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Solution-Based, Anion-Doping of Li 4 Ti 5 O 12 Nanoflowers for Lithium-Ion Battery Applications

Solution–based, anionic doping represents a convenient strategy with which to improve upon the conductivity of candidate anode materials such as Li 4 Ti 5 O 12 (LTO). As such, novel synthetic hydrothermally–inspired protocols have primarily been devised herein, aimed at the large–scale production of unique halogen–doped, micron–scale, three–dimensional, hierarchical LTO flower–like motifs. Although fluorine (F) doping has been explored, the use of chlorine (Cl) dopants is the primary focus here. Several experimental variables, such as dopant amount, lithium hydroxide concentration, and titanium butoxide purity, were probed and perfected. Furthermore, the Cl doping process did not damage the intrinsic LTO morphology. The analysis, based on interpreting a compilation of SEM, XRD, XPS, and TEM–EDS results, was used to determine an optimized dopant concentration of Cl. Electrochemical tests demonstrated an increased capacity via cycling of 12 % for a Cl–doped sample as compared with pristine LTO. Moreover, the Cl–doped LTO sample described in this study exhibited the highest discharge capacity yet reported at an observed rate of 2C for this material at 143mAh g –1 . Overall, these data suggest that the Cl dopant likely enhances not only the ion transport capabilities, but also the overall electrical conductivity of our as–prepared structures. Furthermore, to help explain these favorable findings, theoretical DFT calculations were used to postulate that the electronic conductivity and Li diffusion were likely improved by the presence of increased Ti 3+ ion concentration coupled with widening of the Li migration channel.

25 ENERGY STORAGE↗
DOE OSTI2020

Ionic Liquid–Directed Nanoporous TiNb 2 O 7 Anodes with Superior Performance for Fast–Rechargeable Lithium–Ion Batteries

Nanoporous TiNb 2 O 7 (NPTNO) material is synthesized by a sol–gel method with an ionic liquid (IL) as the nanoporous structure directing template. NPTNO exhibits a high reversible capacity of 210 mAh g –1 even at the charging rate of 50 C and an excellent cyclability of half-cell capacity retention of 74% for 1000 cycles at 5 C and LiNi 0.5 Mn 1.5 O 4 -coupled full-cell capacity retentions of 81% and 87% for 1000 cycles at 1 C and 2 C, respectively. The studies of the 1000 cycled NPTNO electrode illustrate that the IL-directed mesoporous structure can enhance the cyclability of NPTNO cells due to the alleviation of repetitive mechanical stress and volume fluctuation induced by the repetitive Li + insertion-extraction processes. The measured Li + diffusion coefficients from the galvanostatic intermittent titration technique suggest that the IL-templating strategy indeed ensures the fast rechargeability of NPTNO cells based on the fast Li + diffusion kinetics. Benefitting from the nanoporous structure, NPTNO with unhindered Li + diffusion pathways achieves a superior rate capability in the titanium-based oxide materials and the best full-cell cyclability in the TNO materials. Furthermore, the templating potential of IL is demonstrated, and the superb electrochemical performance establishes the IL-directed NPTNO as a promising anode candidate for fast-rechargeable LIBs.

25 ENERGY STORAGE↗
DOE OSTI2021

Colloidal immobilized protein based on stable colloid of TiO 2 nanoparticles at neutral pH for protein microarray

A stable colloid of titanium dioxide nanoparticles was specially prepared at neutral pH so that horse radish peroxidase as a model protein can interact and be immobilized with colloidal titanium dioxide nanoparticles, while usually titanium dioxide colloid is only stable at acidic condition. Colloidal immobilized peroxidase based on titanium dioxide nanoparticles at neutral pH were highly active and stably dispersible in aqueous solution. The resulting colloidal immobilized peroxidase can be readily printed using a standard microarray printer for facile fabrication of protein microarrays. After printing, the colloidal immobilized protein formed uniform membrane of proteinated nanoparticles with 3-dimensional nanoconfiguration. Finally, the protein microarray sensitivity based on peroxidase-titanium dioxide nanoparticles was nearly 20 times as high as that using conventional technology.

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

Vacancy Control in TiNb 2 O 7 : Implications for Energy Applications

Rapid global electrification, including for transportation, has dramatically increased demand for long-lasting and faster-charging batteries. Titanium niobium oxide (TiNb 2 O 7 ) is one of the most promising anode materials for high-power lithium-ion batteries (LIBs). However, the intrinsic low electronic conductivity of TiNb 2 O 7 is a significant drawback. Herein, an almost 10 orders of magnitude increase in conductivity is achieved via reduction of TiNb 2 O 7 in H 2 at 900 °C. The observed dramatic increase in electron conductivity upon reduction is unprecedented and opens new possibilities to produce niobium-based conductive materials for next-generation energy storage. Upon extended reduction, TiNb 2 O 7 converts into a distorted rutile TiNb 2 O 6 structure, which can be reoxidized back into the crystallographic shear phase. In addition, TiNb 2 O 7 can be thermally reduced in an inert atmosphere and reoxidized by CO 2 with excellent oxygen exchange capacity. Thus, the TiNb 2 O 7 Wadsley–Roth phase demonstrates outstanding potential for solar-driven thermochemical CO 2 splitting at 1400 °C. In conclusion, these findings manifest that controlling defect chemistry paves the way for developing advanced materials for LIBs and solar-driven thermochemical fuel production.

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

Silver(I) Supported Liquid Membranes for Selective Ethylene Recovery from Mixed-Gas Streams of Tandem CO 2 Electrolysis

Large-scale olefin separations from unreacted paraffins and other byproduct gases are primarily done by energy-intensive cryogenic distillation processes at refineries. Silver(I) supported liquid membranes (Ag SLMs) can be implemented at smaller production scales of ethylene (C 2 H 4 ), a critical industrial chemical, such as its electrocatalytic (EC) production from CO 2 . Challenges of EC C 2 H 4 production mainly stem from reducing gases like hydrogen (H 2 ), where the redox reactions pertinent to Ag(I) facilitators diminish olefin transport. Herein we report that aqueous Ag(I) solution in a composite Ag SLM can operate in mixed-gas conditions containing H 2 gas utilizing reduced titania compounds, such as titanium(III) oxide (Ti 2 O 3 ). Embedding Ti 2 O 3 in the polydimethylsiloxane layer of Ag SLM assisted in selectively separating C 2 H 4 from mixed-gas feed streams related to CO 2 electrolysis containing H 2 . The direct exposure of a mixed-gas stream containing C 2 H 4 , CO 2 , CO, N 2 , and CH 4 with as high as 50 vol % H 2 maintained excellent C 2 H 4 separations for 7 days of continuous operation. The Ag SLM provided effective separations of C 2 H 4 from CO (at detection limits), CH 4 (selectivity ratio (α) = 20–30), and H 2 (α = ∼20), but C 2 H 4 from CO 2 (α = 2–4) revealed a slightly lower separation. These data show that aqueous Ag(I) solution(s) used in the SLMs can separate C 2 H 4 for extended periods, even under highly reducing gas conditions. Also, we report C 2 H 4 recovery from the gas mixture produced in the EC CO 2 reduction process. In conclusion, the Ag SLM gave C 2 H 4 selective separation from a five-component complex mixed-gas stream, relevant to tandem CO 2 electrolysis.

37 - INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL C↗
DOE OSTI2025

Cobalt promotion of the InO x –TiO 2 heterojunction for dual photothermal reduction of CO 2

A series of cobalt-promoted indium–titanium composite oxides was synthesized using a microemulsion method. Their functional properties were investigated for the photothermal reduction of carbon dioxide. In this series, the indium content varied between 2.5 and 20%, while the cobalt percentage was kept constant at 4% throughout the series. In all cases, carbon monoxide formation occurred selectively through the reverse water gas shift reaction. The sample with 2.5% In maximized the synergistic use of the two energy sources, while the sample with 10% In showed the highest catalytic activity under both thermal and dual photo-thermo conditions. A physicochemical characterization was performed for all samples. The use of microscopy and X-ray absorption spectroscopy demonstrated that sub-nanometric indium entities, in combination with atomically dispersed cobalt oxide entities, achieved a balance between high thermal activity and significant synergy between light and heat in a catalytic process. The system with 10% indium is thus able to improve the thermal catalytic process through the use of light, providing an intensification procedure for the classic process.

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

Synthesis of TiO{sub 2} nanoparticles decorated with gold nanoclusters using pulsed laser ablation in liquid

Amorphous spherical titanium dioxide (TiO{sub 2}) nanoparticles, decorated gold nanoparticles, are shown to be formed under irradiation of commercial TiO{sub 2} nanopowders dispersed in aqueous solution of hydrogen tetrachloroaurate (HAuCl{sub 4}) by second-harmonic nanosecond pulses of an Nd : YAG laser. It is found that these hybrid nanostructures are formed during laser-induced remelting of the initial nanoparticles, stimulated by gold nanoclusters (providing more efficient absorption of visible laser radiation by titanium dioxide) recovered on their surface. The morphology and chemical composition of the newly formed hybrid nanomaterials are investigated in detail by electron microscopy, Raman spectroscopy, and energy-dispersive analysis. It is shown that the average size and number of gold nanoparticles recovered on the TiO{sub 2} surface can be controlled by varying the concentration ratio of the initial nanomaterial and HAuCl{sub 4}. A spectroscopic analysis of light scattering by single hybrid nanoparticles and the results of numerical calculation of the structure of electromagnetic fields near their surface indicate good prospects of these hybrid nanomaterials for various applications in modern optics, optoelectronics, and nanophotonics; for example, for designing chemo- and biosensor platforms and new-generation solar cells. (nanoparticles)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Atmospheric Pressure Plasma Deposition of TiO2: A Review

Atmospheric pressure plasma (APP) deposition techniques are useful today because of their simplicity and their time and cost savings, particularly for growth of oxide films. Among the oxide materials, titanium dioxide (TiO2) has a wide range of applications in electronics, solar cells, and photocatalysis, which has made it an extremely popular research topic for decades. Here, we provide an overview of non-thermal APP deposition techniques for TiO2 thin film, some historical background, and some very recent findings and developments. First, we define non-thermal plasma, and then we describe the advantages of APP deposition. In addition, we explain the importance of TiO2 and then describe briefly the three deposition techniques used to date. We also compare the structural, electronic, and optical properties of TiO2 films deposited by different APP methods. Lastly, we examine the status of current research related to the effects of such deposition parameters as plasma power, feed gas, bias voltage, gas flow rate, and substrate temperature on the deposition rate, crystal phase, and other film properties. The examples given cover the most common APP deposition techniques for TiO2 growth to understand their advantages for specific applications. In addition, we discuss the important challenges that APP deposition is facing in this rapidly growing field.

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

Spectroscopic Orbits of 11 Nearby, Mid-to-late M-dwarf Binaries

We present the spectroscopic orbits of 11 nearby, mid-to-late M dwarf binary systems in a variety of configurations: 2 single-lined binaries (SB1s), 7 double-lined binaries (SB2s), 1 double-lined triple (ST2), and 1 triple-lined triple (ST3). Eight of these orbits are the first published for these systems, while five are newly identified multiples. We obtained multi-epoch, high-resolution spectra with the TRES instrument on the 1.5 m Tillinghast Reflector at the Fred Lawrence Whipple Observatory located on Mt. Hopkins in AZ. Using the TiO molecular bands at 7065–7165 Å, we calculated radial velocities for these systems, from which we derived their orbits. We find LHS 1817 to have in a 7 hr period a companion that is likely a white dwarf, due to the ellipsoidal modulation we see in our MEarth-North light-curve data. We find G 123-45 and LTT 11586 to host companions with minimum masses of 41 M {sub Jup} and 44 M {sub Jup} with orbital periods of 35 and 15 days, respectively. We find 2MA 0930+0227 to have a rapidly rotating stellar companion in a 917 day orbital period. GJ 268, GJ 1029, LP 734-34, GJ 1182, G 258-17, and LTT 7077are SB2s with stellar companions with orbital periods of 10, 96, 34, 154, 5, and 84 days; LP 655-43 is an ST3 with one companion in an 18 day orbital period and an outer component in a longer undetermined period. In addition, we present radial velocities for both components of L 870-44AB and for the outer components of LTT 11586 and LP 655-43.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Search for TiO and Optical Nightside Emission from the Exoplanet WASP-33b

With a temperature akin to an M dwarf, WASP-33b is among the hottest Jupiters known, making it an ideal target for high-resolution optical spectroscopy. By analyzing both transmission and emission spectra, we aim to substantiate previous reports of atmospheric TiO and a thermal inversion within the planet’s atmosphere. We observed two transits and six arcs of the phase curve with the Echelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) on the Canada–France–Hawaii Telescope and High Resolution Echelle Spectrometer (HIRES) on the Keck telescope, which provide high spectral resolution and ample wavelength coverage. We employ the Doppler cross-correlation technique to search for the molecular signatures of TiO and H{sub 2}O in these spectra, using models based on the TiO line list of Plez. Though we cannot exclude line-list-dependent effects, our data do not corroborate previous indications of a thermal inversion. Instead we place a 3σ upper limit of 10{sup −9} on the volume mixing ratio of TiO for the T–P profile we consider. While we are unable to constrain the volume mixing ratio of water, our strongest constraint on TiO comes from dayside emission spectra. This apparent absence of a stratosphere sits in stark contrast to previous observations of WASP-33b as well as theoretical predictions for the atmospheres of highly irradiated planets. The discrepancy could be due to variances between line lists, and we stress that detection limits are only as good as the line list employed, and are only valid for the specific T–P profile considered due to the strong degeneracy between lapse rate (dT/dlogP) and molecular abundance.

79 ASTRONOMY AND ASTROPHYSICS↗