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All sourcesNASA NTRSDOE OSTI
NASA NTRS2016

Evaluation of Observed and Modelled Aerosol Lifetimes Using Radioactive Tracers of Opportunity and an Ensemble of 19 Global Models

Aerosols have important impacts on air quality and climate, but the processes affecting their removal from the atmosphere are not fully understood and are poorly constrained by observations. This makes modelled aerosol lifetimes uncertain. In this study, we make use of an observational constraint on aerosol lifetimes provided by radionuclide measurements and investigate the causes of differences within a set of global models. During the Fukushima Dai-Ichi nuclear power plant accident of March 2011, the radioactive isotopes cesium-137 (Cs-137) and xenon-133 (Xe-133) were released in large quantities. Cesium attached to particles in the ambient air, approximately according to their available aerosol surface area. Cs-137 size distribution measurements taken close to the power plant suggested that accumulation mode (AM) sulfate aerosols were the main carriers of cesium. Hence, Cs-137 can be used as a proxy tracer for the AM sulfate aerosol's fate in the atmosphere. In contrast, the noble gas Xe-133 behaves almost like a passive transport tracer. Global surface measurements of the two radioactive isotopes taken over several months after the release allow the derivation of a lifetime of the carrier aerosol. We compare this to the lifetimes simulated by 19 different atmospheric transport models initialized with identical emissions of Cs-137that were assigned to an aerosol tracer with each model's default properties of AM sulfate, and Xe-133 emissions that were assigned to a passive tracer. We investigate to what extent the modelled sulfate tracer can reproduce the measurements, especially with respect to the observed loss of aerosol mass with time. Modelled Cs-137and Xe-133 concentrations sampled at the same location and times as station measurements allow a direct comparison between measured and modelled aerosol lifetime. The e-folding lifetime e, calculated from station measurement data taken between 2 and 9 weeks after the start of the emissions, is 14.3 days (95% confidence interval 13.1-15.7 days). The equivalent modelled e lifetimes have a large spread, varying between 4.8 and 26.7 days with a model median of 9.42.3 days, indicating too fast a removal in most models. Because sufficient measurement data were only available from about 2 weeks after the release, the estimated lifetimes apply to aerosols that have undergone long-range transport, i.e. not for freshly emitted aerosol. However, modelled instantaneous lifetimes show that the initial removal in the first 2 weeks was quicker (lifetimes between 1 and 5 days) due to the emissions occurring at low altitudes and co-location of the fresh plume with strong precipitation. Deviations between measured and modelled aerosol lifetimes are largest for the northernmost stations and at later time periods, suggesting that models do not transport enough of the aerosol towards the Arctic. The models underestimate passive tracer (Xe-133) concentrations in the Arctic as well but to a smaller extent than for the aerosol (Cs-137) tracer. This indicates that in addition to too fast an aerosol removal in the models, errors in simulated atmospheric transport towards the Arctic in most models also contribute to the underestimation of the Arctic aerosol concentrations.

Radioactive isotopes↗
DOE OSTI2020

CsI-Antisolvent Adduct Formation in All-Inorganic Metal Halide Perovskites

The excellent optoelectronic properties shown by hybrid organic/inorganic metal halide perovskites are all predicated on precisely controlling the exact nucleation and crystallization dynamics that occur during film formation. In general, high-performance thin films are obtained by a method commonly called solvent engineering (or antisolvent quench) processing. The solvent engineering method removes excess solvent, but importantly leaves behind solvent that forms chemical adducts with the lead-halide precursor salts. These adduct-based precursor phases control nucleation and the growth of the polycrystalline domains. There has not yet been a comprehensive study comparing the various antisolvents used in different perovskite compositions containing cesium. In addition, there have been no reports of solvent engineering for high efficiency in all-inorganic perovskites such as CsPbI 3 . In this work, inorganic perovskite composition CsPbI 3 is specifically targeted and unique adducts formed between CsI and precursor solvents and antisolvents are found that have not been observed for other A-site cation salts. These CsI adducts control nucleation more so than the PbI 2 -dimethyl sulfoxide (DMSO) adduct and demonstrate how the A-site plays a significant role in crystallization. The use of methyl acetate (MeOAc) in this solvent engineering approach dictates crystallization through the formation of a CsI-MeOAc adduct and results in solar cells with a power conversion efficiency of 14.4%.

14 SOLAR ENERGY↗
DOE OSTI2024

The Surface Chemistry of Methanol on Pd(111) and H–Pd(111) Surfaces: C–O Bond Cleavage and the Effects of Metal Hydride Formation

Palladium catalysts are frequently employed in processes where methanol is an energy vector or carrier, being useful for the synthesis of methanol from mixtures of carbon dioxide and hydrogen (CO 2 /H 2 ) or its steam reforming on demand. Results of synchrotron-based ambient pressure X-ray photoelectron spectroscopy for the adsorption of methanol on a Pd(111) model catalyst show a rich surface chemistry and complex phenomena that strongly depend on pressure and temperature. At low pressures (< 10 -6 Torr) and temperatures (< 300 K), CO is the dominant decomposition product. Further, as the pressure increases, cleavage of C-H, O-H and C-O bonds is observed, and at elevated temperatures (400-600 K) the formation of CO and CH x /C fragments compete on the surface. Thus, existing reaction networks for methanol decomposition must be modified. Furthermore, surface and subsurface hydrogen (coming from PdH x ) play a significant role in the stability and removal of CH x and C species.

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

Alternative Treatment of Defense Waste Processing Facility Recycle via Reuse of Existing Liquid Waste Facilities - 20097

The Defense Waste Processing Facility (DWPF) at Savannah River Site (SRS) has been immobilizing high level waste since 1996. The chemical process within DWPF generates a large volume of condensate, which is recycled to the SRS H-Area Tank Farm. The recycle stream includes a small quantity of sludge solids, as well as soluble cesium that is volatilized during melter operation. The recycle waste is currently received into a large, underground waste tank that separates insoluble solids via decanting. The supernate is treated by an evaporator with a concentrate stream that is stored for future processing. The evaporator overheads are collected and sent to the Effluent Treatment Project (ETP) for final polishing and testing prior to discharge to local surface water. Recycle storage and treatment as described above complicates the overall mission within the tank farms, which are primarily engaged in waste retrieval and preparation activities that support sludge and salt disposition, as well as tank characterization and closure. The need to devote a portion of available storage space to recycle treatment limits operational flexibility, and ultimately the DWPF recycle stream must be diverted to fully close all the SRS waste tanks. An alternative treatment process is being explored to decouple the recycle stream from the tank farm. The proposed treatment process will accomplish solids separation via crossflow filtration and will utilize a wiped film evaporator to volume-reduce the filtrate stream. Evaporator overheads will continue to be further processed in ETP while the solids stream and evaporator concentrate stream will be returned for reprocessing with the DWPF and Salt Waste Processing Facility (SWPF). This process will utilize existing facilities within DWPF and the tank farm that were previously dedicated to Interim Salt Disposition (ISD), but no longer have an identified mission with the startup of SWPF. The reuse of these facilities will remove several constraints from the current Liquid Waste (LW) System Plan without expanding the current footprint of Department of Energy Environmental Management (EM) infrastructure within LW. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2020

Wash Penalty Factors: Insight into the Effects of Water Wash Down of Building Materials for Radionuclide/Nuclear (Rn) Decontamination - 20153

The release of radiological material to the urban environment can have a devastating socio-economic impact. In an urban release scenario, areas around the dispersal epicenter and any areas contaminated via atmospheric deposition may require extensive decontamination. Most recently, our group reported the decontamination efficacy of pressurized water washing of common porous building materials, and compared the penetration depth of the contaminations before and after exposure to pressurized tap water. Here, we continue evaluation of the decontamination efficacy of wash solutions based on potassium (K{sup +}) salt for cesium (Cs), strontium (Sr), and europium (Eu) contaminations on common building surfaces. The surfaces included: the cement used in concrete, the aggregate used in concrete, intact concrete, roadway asphalt, asphalt shingles, and latex paint. We deposited solutions of {sup 137}Cs, {sup 85}Sr, and {sup 152}Eu onto numerous common building materials and exposed these materials to a static bath or low-pressure flow of tap water, 0.1 M potassium chloride (KCl), and 0.5 M KCl. The decontamination efficacy and the depth profile for residual contamination were measured to determine the conditions under which applying a wash solution has benefit compared to physically removing the surface material. To aid in these analyses, we introduced a 'wash penalty factor' to quantify the degree to which contamination migrated into the bulk material by the action of the wash method. On asphalt, 70-80% of the radionuclides were found to be within 0.02 mm of the surface. Cs{sup +} bonded to negative charge sites on the surface of the mineral aggregate (used in concrete) so decontamination was improved by ion exchange with K{sup +}. Sr{sup 2+} is soluble and removed by water, while not bonding to aggregate or demonstrating solubility in the liquid asphalt. Eu{sup 3+} is precipitated and not removed in gentle wash but displays mobility within the pore space that suggests its presence as a colloid. Concrete is more porous than asphalt, and 80% of the radionuclides were within 0.2 mm of the surface for {sup 137}Cs and {sup 152}Eu and 50- 80% for {sup 85}Sr. On concrete, Cs{sup +} bonds similarly to asphalt. Sr{sup 2+} bonds to the cement or precipitates from the alkaline pore water. Eu{sup 3+} sorbs to the cement and then precipitates. In brick, the results for Cs{sup +} and Sr{sup 2+} suggested that diffusion of ions from deeper in the coupon was important. Eu{sup 3+} is removed poorly due to precipitation. We will explain and report example wash penalty factors that vary from 0 to > 1 for the various methods and the implications for deciding when to implement wash down methods. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2020

Repurposing Hanford Waste Tank AP-106 for Interim Storage of Treated Low-Activity Waste - 20044

Hanford Double-Shell Tank AP-106 is being repurposed to serve as the lag storage tank holding feed for the Direct Feed Low-Activity Waste (DFLAW) process at the Waste Treatment and Immobilization Plant (WTP). The repurposing process is the sequence of tank additions and transfers that will remove the Cs 137 inventory from AP-106. The repurposing process steps are designed to remove Cs-137 more effectively and efficiently as compared to regular supernatant transfers. The process steps are estimated to take 2-months to complete. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2021

Cesium Ion Exchange Testing Using Crystalline Silicotitanate with Hanford Tank Waste 241-AP-107

At the time of this testing, the Low-Activity Waste Pretreatment System (LAWPS) was to provide for the initial production of immobilized low-activity waste by feeding Hanford tank supernate from tank farms to the Hanford Tank Waste Treatment and Immobilization Plant (WTP) Low-Activity Waste (LAW) Facility for immobilization. Washington River Protection Solutions requested that Hanford tank waste collected from tank 241-AP-107 (hereafter called AP-107) be processed using conceived pretreatment steps (suspended solids removal by filtration, Cs removal by ion exchange) then vitrified. A small-scale test platform to demonstrate the solids filtration, Cs removal, and LAW vitrification was constructed and installed at Pacific Northwest National Laboratory. Bench-scale ion exchange testing with approximately 9 L of AP-107 supernate was conducted using crystalline silicotitanate (CST) ion exchange media. The IONSIV R9140-B CST was provided by Honeywell UOP, LLC in 2018 (Batch 2081000057). The ion exchange media was first tested with simulant and was previously described. This report describes the Cs ion exchange batch contact and column test results with the AP-107 tank waste. Batch contact testing helps to evaluate CST performance on tank waste supernate prior to processing it in the ion exchange columns. Batch contacts were performed with the waste at four Cs concentrations at a phase ratio of 200 (liquid volume to exchanger mass) with AP-107. The distribution coefficient (K d ) at the equilibrium condition of 8.57 µg Cs/mL (AP-107 feed condition) was determined to be 669 mL AP-107/g CST. With a CST bed density of 1.00 g/mL, this K d corresponded to a predicted 50% Cs breakthrough of 669 bed volumes (BVs). The Cs load capacity at the equilibrium feed condition was determined to be 7.5 mg Cs/g dry CST. The column testing was prototypic to the intended LAWPS operations in a lead-lag column format, although on a small-scale basis with 10-mL CST beds. The feed was processed downflow through the lead column and then through the lag column at ~2.2 BV/h. Loading continued until the lag column reached the WTP waste acceptance criteria (WAC) for receiving supernatant waste for vitrification (a function of the Na and 137 Cs concentrations). For AP-107, the WAC is 0.114% of the influent 137 Cs concentration; this required a Cs decontamination factor of 876. The Cs effluent from the lag column reached the WAC after processing ~410 BVs. To keep the subsequent product effluent below the WAC, a replacement lag column was prepared, the lead column was removed from service (after processing a total of 471 BVs), the lag column was put into the lead column position, and the replacement lag column was installed. Feed processing continued and after another ~290 BVs the Cs effluent from the lag column again exceeded the WAC. In both cases, the lead columns only reached 25% Cs breakthrough before removal. Although 50% Cs breakthrough was not reached, this value was estimated and averaged based on extrapolation of the loading curves (640 BVs) and agreed within 4% of the predicted 50% Cs breakthrough from batch contact test results (669 BVs). Table ES.1 summarizes the observed column performance and relevant Cs loading characteristics.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2020

Life cycle Management of 324 Project High-Activity, Mixed/Low-Level Radioactive Waste - 20506

U.S. DOE contractor CH2M Hill Plateau Remediation Company (CHPRC) manages the 324 Building and is preparing to remotely excavate and disposition the radioactive soil beneath the building to allow further deactivation of the building. The 324 Building is a non-reactor Category 2 Nuclear Facility located in the 300 Area of the Hanford Site. Records indicate that in October 1986, approximately 516 liters of a concentrated liquid waste stream containing cesium-137 ({sup 137}Cs) and strontium-90 ({sup 90}Sr) spilled onto the floor of a hot cell, B-Cell, in the 324 Building. The spill contained an estimated 1.3 million curies (Ci) of radioactivity. A breach in the sump of B-Cell was discovered in November 2009 during characterization of the soil under the building. Exposure rates in excess of 10,000 R/hr were detected through a system of access pipes installed under the hot cell, and an estimated 1.557 E+05 Ci of {sup 137}Cs and 6.842 E+04 Ci of {sup 90}Sr are in the soil beneath B-Cell. The magnitude of the soil contamination below B-Cell represents one of the most challenging remediation activities in the DOE complex. The objective of CHPRC's 324 Project is to remotely excavate and disposition the highly contaminated soil under the hot cell so building deactivation and stabilization activities can resume. Depending on the volume and radioactivity of contaminated soil under B-Cell, the material will be dispositioned either at the Hanford Site's Environmental Restoration Disposal Facility (ERDF) or in grout monoliths that will be created in the hot cells (A, C and D) next to B-Cell during future building demolition activities. The current project scope includes installing structural supports under the walls of B-Cell and using remotely operated equipment to remove debris from the hot cell, remove debris mixed with grout on the floor, saw through and remove the cell floor and liner, and excavate contaminated soil under B-Cell to a depth of up to 3.66 meters (12 feet). Addition or removal of debris in A-Cell may be required to support B-Cell cleanout and excavation activities. The base approach is to disposition debris and soil bins in the Radiochemical Engineering Cell (REC) monoliths, and subsequently disposition the monoliths during building demolition. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2021

Tank 48H Tetraphenylborate Mitigation: Simulant Studies using Sodium Permanganate

Tank 48H currently holds legacy material containing organic tetraphenylborate (TPB) compounds from the operation of the In-Tank Precipitation (ITP) process. TPB was added during the ITP process to precipitate the otherwise soluble cesium as insoluble cesium TPB (CsTPB), but excessive benzene generation from TPB degradation curtailed this treatment method. The contents of Tank 48H, which include approximately 26.000 kg of potassium TPB (KTPB) and trace CsTPB, are not compatible with the waste treatment facilities at the Savannah River Site (SRS) since the organic content and the associated flammability issues pose a challenge to the salt processing and sludge processing facilities within the liquid waste system. An in-tank process to remove (or decompose) TPB safely would be of great value. Previous testing at Savannah River National Laboratory (SRNL) demonstrated the destruction of glycolate via chemical oxidation using sodium permanganate with simulated and radioactive waste. Scoping tests were performed to study the destruction of TPB to determine if the contents of Tank 48H would be amenable to the same type of destruction. Partial destruction of TPB was observed in Tank 48H simulants under mild conditions (e g., pH 11, room temperature) with no definitive indication of benzene generation. To build upon the success of the scoping tests, an additional study was requested to provide a better understanding of the underlying chemistry for Tank 48H content destruction using sodium permanganate. Three experiments were performed with Tank 48H simulants at 40 °C to determine the efficacy of using sodium permanganate for TPB destruction. Three starting pH values were selected: 1) pH 11 for comparison with the previous work at room temperature, 2) pH 10 as the minimum pH recommended by the Corrosion Control Program (CCP) for in-tank processing, and 3) pH 8 to determine the effectiveness of TPB destruction at near neutral pH. While below the allowable pH for the CCP, the experiment at pH 8 was performed to study the TPB-Permanganate reaction under more extreme conditions and further verify the potential for out-of-tank processing.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2022

Synthesis and structural properties of a 2D Zn( II ) dodecahydroxy- closo -dodecaborate coordination polymer

Here, in this work, we discuss the synthesis and characterization of a 2D coordination polymer composed of a dianionic perhydroxylated boron cluster, [B 12 (OH) 12 2– ], coordinated to Zn( II )—the first example of a transition metal-coordinated [B 12 (OH) 12 ] 2– compound. This material was synthesized via cation exchange from the starting cesium salt and then subjected to rigorous characterization prior to and after thermal activation. Numerous techniques, including XRD, FTIR, SEM, TGA, and solid-state NMR revealed a 2D coordination polymer composed of sheets of Zn( II ) ions intercalated between planes of boron clusters. The as-synthesized material was then evacuated of solvent via thermal treatment, and atomic-level changes from this transformation were elucidated through a combination of 1D and 2D solid-state NMR analyses of 11 B and 1 H nuclei, suggesting the full removal of coordinated solvent molecules. Evidence also suggested that [B 12 (OH) 12 2– ] can adjust its coordination to Zn( II ) in the solid-state through hemilability of its numerous –OH ligands.

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

Colorimetric Detection and Surface Decontamination of Metal Ions by Using Strippable Polymers - 20024

Strippable coating is a low cost and efficient decontamination method far more compatible for the cleaning of surfaces. This method allows contaminant entrapment and removal from different types of surfaces and also characterized by low-secondary waste generation after decontamination. The present work deals with the elaboration and testing of a new composition of Polyvinyl alcohol (PVA) aqueous solutions utilize the TTA (Thenoyl trifluoroacetone) ligand, employed as a strippable coating for the removal of metal ions e.g., UO{sub 2}{sup +2}, Ni{sup +2}, Cu{sup +2}, Fe{sup +3}, V{sup +5}, Eu{sup +3} and Cs{sup +} from 'contaminated' surfaces. The study aims at developing contaminant-sensing and decontaminating coatings for the detection and removal of metal ions from non porous surfaces. This 'smart' coating is shown to exhibit a clear contaminant dependent response in the form of color variation, and a detection sensitivity less than 0.5 μg per cm{sup 2}. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2025

Efficient Mn 2+ Doping in Non-Stoichiometric Cesium Lead Bromide Perovskite Quantum Dots

Doping magnetic transition metal ions (e.g., Mn 2+ ) into colloidal quantum dots endows novel optical and magnetic properties to the host materials. CsPbBr 3 quantum dots (QDs) are emerging light-emitting materials with high structural and chemical flexibility in the visible spectral regime. However, efficiently doping Mn 2+ ions in CsPbBr 3 QDs remains challenging, especially when size confinement and ensemble uniformity are needed for understanding the underexplored exciton-dopant exchange interaction. Here, we introduce a doping mechanism based on electrostatic surface Mn 2+ adsorption that enables efficient Mn 2+ incorporation in strongly confined CsPbBr 3 QDs. The resultant QDs are found to have a Cs-deficient stoichiometry compared to their undoped counterparts. A redox reaction-based purification method was developed to remove Mn 2+ cations that are tightly adsorbed on the surface to determine the concentration of lattice-incorporated Mn 2+ . Our synthesis enables a Mn 2+ doping/alloying concentration of up to ∼44% with a Mn 2+ photoluminescence efficiency exceeding 90%. This allows for the determination of the intrinsic exciton-to-dopant energy transfer rate.

Hidayatova, Lamia [Univ. of Oklahoma, Norman, OK (↗
DOE OSTI2022

Room temperature doping of Ln 3+ in perovskite nanoparticles: a halide exchange mediated cation exchange approach

This study presents a halide exchange mediated cation exchange strategy for a room temperature doping of trivalent lanthanide cations (Ln 3+ ) in cesium lead halide (CsPbX 3 ) nanoparticles (NPs). Post-synthetic addition of LnCl 3 [Ln = Nd, Sm, Eu, Tb, Dy, and Yb] to a solution of CsPbBr 3 NPs generates the corresponding lanthanide doped NPs which display host sensitized Ln 3+ emission. Structural and spectroscopic characterizations indicate a successful halide exchange and substitutional displacement of Pb 2+ by Ln 3+ . The effect of halide identity in controlling the Ln 3+ sensitization was also evaluated. Here, a photophysical framework is presented that can be used to predict the Ln 3+ sensitization in perovskite NPs semiempirically, thereby removing the constraints of trial and error in designing a perovskite NP-Ln 3+ host–guest combination.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS1976

A target for production of radioxenons

A liquid cesium target has been developed which permits the production and identification of the neutron deficient isotopes of xenon. The thick target yields of Xe-127 and Xe-129 m were measured for proton energies between 28 and 40 MeV. Production of Xe-129 m, as indicated by the presence of a 196 keV line in the gamma ray spectra, occurred at all proton energies; Xe-127 was produced at proton energies above 33 MeV. Both accumulated on the target with the same time constant, and the cross sections were comparable in magnitude. The radioxenons were removed from the target by a stream of carrier gas so that chemical separation was unnecessary.

Blue, J. W.↗
DOE OSTI2021

Influence of Cs Loading on Pt/m-ZrO 2 Water–Gas Shift Catalysts

Certain alkali metals (Na, K) at targeted loadings have been shown in recent decades to significantly promote the LT-WGS reaction. This occurs at alkali doping levels where a redshift in the C-H band of formate occurs, indicating electronic weakening of the bond. The C-H bond breaking of formate is the proposed rate-limiting step of the formate associative mechanism, lending support to the occurrence of this mechanism in H2-rich environments of the LT-WGS stage of fuel processors. Continuing in this vein of research, 2%Pt/m-ZrO 2 was promoted with various levels of Cs in order to explore its influence on the rate of formate intermediate decomposition, as well as that of LT-WGS in a fixed bed reactor. In situ DRIFTS experiments revealed that Cs promoter loadings of 3.87% to 7.22% resulted in significant acceleration of the forward formate decomposition in steam at 130 °C. Of all of the alkali metals tested to date, the redshift in the formate ν(CH) band with the incorporation of Cs was the greatest. XANES difference experiments at the Pt L 2 and L 3 edges indicated that the electronic effect was not likely due to an enrichment of electronic density on Pt. CO 2 TPD experiments revealed that, unlike Na and K promoters, Cs behaves more like Rb in that the decomposition of the second intermediate in LT-WGS, carbonate species, is hindered due to (1) increased basicity of Cs, (2) the tendency of Cs to cover Pt sites that facilitate CO 2 decomposition, and (3) the tendency of Cs to increase Pt particle size as shown by EXAFS results, resulting in fewer Pt sites that facilitate CO2 decomposition. As such, the LT-WGS rate was hindered overall and the rate-limiting step shifted to carbonate decomposition (CO 2 removal). Like its Rb counterpart, low levels of added Cs (e.g., 0.72%Cs) were found to improve the stability of the catalyst relative to the unpromoted catalyst; the stability comparison was made at similar CO conversion level as well as similar space velocity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2008

NaK Variable Conductance Heat Pipe for Radioisotope Stirling Systems

In a Stirling radioisotope power system, heat must continually be removed from the General Purpose Heat Source (GPHS) modules to maintain the modules and surrounding insulation at acceptable temperatures. The Stirling convertor normally provides most of this cooling. If the Stirling convertor stops in the current system, the insulation is designed to spoil, preventing damage to the GPHS, but also ending use of that convertor for the mission. An alkali-metal Variable Conductance Heat Pipe (VCHP) was designed to allow multiple stops and restarts of the Stirling convertor. In the design of the VCHP for the Advanced Stirling Radioisotope Generator, the VCHP reservoir temperature can vary between 40 and 120 C. While sodium, potassium, or cesium could be used as the working fluid, their melting temperatures are above the minimum reservoir temperature, allowing working fluid to freeze in the reservoir. In contrast, the melting point of NaK is -12 C, so NaK can't freeze in the reservoir. One potential problem with NaK as a working fluid is that previous tests with NaK heat pipes have shown that NaK heat pipes can develop temperature non-uniformities in the evaporator due to NaK's binary composition. A NaK heat pipe was fabricated to measure the temperature non-uniformities in a scale model of the VCHP for the Stirling Radioisotope system. The temperature profiles in the evaporator and condenser were measured as a function of operating temperature and power. The largest delta T across the condenser was 2S C. However, the condenser delta T decreased to 16 C for the 775 C vapor temperature at the highest heat flux applied, 7.21 W/ square cm. This decrease with increasing heat flux was caused by the increased mixing of the sodium and potassium in the vapor. This temperature differential is similar to the temperature variation in this ASRG heat transfer interface without a heat pipe, so NaK can be used as the VCHP working fluid.

Tarau, Calin↗
DOE OSTI2023

Tailoring Crystallization Dynamics of CsPbI 3 for Scalable Production of Efficient Inorganic Perovskite Solar Cells

All-inorganic perovskite cesium lead triiodide (CsPbI 3 ) with inorganic nature, low-temperature synthesis, and a suitable bandgap is desirable for high-performance photovoltaics. However, the scalable production of CsPbI 3 photovoltaics is still challenging due to a large nucleation energy barrier and slow phase transition during unassisted natural crystallization. Here, the crystallization dynamics of CsPbI 3 thin films is tailored via lead acetate (PbAc 2 ) substitution in the perovskite precursor ink, allowing the scalable fabrication of efficient all-inorganic perovskite solar cells and minimodules. Introducing PbAc 2 enlarges CsPbI 3 colloid size in the precursor and reduces the nucleation energy barrier. Additionally, reactions between acetate and dimethylammonium in the wet film accelerate the removal of dimethylammonium additives and generate solvent vapors for self-regulate internal solvent annealing, resulting in densely packed, uniform, and pinhole-free CsPbI 3 perovskite films over large areas. This strategy demonstrates inverted CsPbI 3 solar cells with 20.17% efficiency and good operational stability (retaining 95.5% of initial efficiency after continuous operation for 1800 h) and 15.1%-efficient CsPbI 3 minimodules with an active area of 26.8 cm 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2022

Cesium intercalation of graphene: A 2D protective layer on alkali antimonide photocathode

Alkali antimonide photocathodes have wide applications in free-electron lasers and electron cooling. The short lifetime of alkali antimonide photocathodes necessitates frequent replacement of the photocathodes during a beam operation. Furthermore, exposure to mediocre vacuum causes loss of photocathode quantum efficiency due to the chemical reaction with residual gas molecules. Theoretical analyses have shown that covering an alkali antimonide photocathode with a monolayer graphene or hexagonal boron nitride protects it in a coarse vacuum environment due to the inhibition of chemical reactions with residual gas molecules. Alkali antimonide photocathodes require an ultra-high vacuum environment, and depositing a monolayer 2D material on it poses a serious challenge. In the present work, we have incorporated a novel method known as intercalation, in which alkali atoms pass through the defects of a graphene thin film to create a photocathode material underneath. Initially, Sb was deposited on a Si substrate, and a monolayer graphene was transferred on top of the Sb film. Heat cleaning around 550–600 °C effectively removed the Sb oxides, leaving metallic Sb underneath the graphene layer. Depositing Cs on top of a monolayer graphene enabled the intercalation process. Atomic force microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, low energy electron microscopy, and x-ray diffraction measurements were performed to evaluate photocathode formation underneath the monolayer graphene. Our analysis shows that Cs penetrated the graphene and reacted with Sb and formed Cs 3 Sb.

2D materials↗