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Results for “Thermogravimetric analysis”

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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Physicochemical evolution of uranium nitride kernel microstructure with varying carbon distribution for advanced TRISO fuel forms

Uranium nitride (UN) has emerged as a fuel candidate for advanced nuclear reactor concepts due to its superior uranium density, thermal conductivity, and high melting temperature. However, the fabrication route for converting UO 2 to UN is complex and difficult to standardize. Although the chemistry of this conversion process is well-studied, more insight into the physicochemical dynamics of this conversion using advanced characterization techniques can help further our understanding of this material system. This work leveraged thermogravimetric analysis (TGA), X-ray diffraction (XRD), and nondestructive 3D X-ray computed tomography (XCT) to characterize dynamic microstructural changes in the UO 2 → UCO → UN fabrication pathway for two kernels with a varying carbon distribution in the starting composition. TGA and XRD were used to quantify changes in the mass, density, and chemical composition of the two kernels, while three-dimensional image processing and segmentation of XCT data were used to quantify the volume, surface area, and spatial distribution of features within each kernel for multiple steps along the fabrication pathway. The analysis indicates distinct differences between the two kernels that are correlated to downstream conversion efficiency. In conclusion, this work is among the first to perform 3D quantification of physicochemical evolution during UN conversion, providing quantitative correlation between processing, properties, and expected fuel performance.

Nuclear fuel↗
DOE OSTI2024

Pyrolysis of high-density polyethylene: Degradation behaviors, kinetics, and product characteristics

Pyrolysis is a promising technology for converting plastic waste into valuable raw materials while offering a potential solution to the global plastic pollution crisis. In this study, the thermal pyrolysis of high-density polyethylene (HDPE) is investigated in a drop tube reactor under nearly isothermal conditions. The impact of reaction temperature and gas/volatile residence time on carbon conversion and product distribution is examined across a range of 500–900°C and 3.6–32.2s, respectively. Non-condensable gas products detected by online mass spectrometry are H 2 , CH 4 , C 2 H 4 , C 2 H 6 , C 3 H 6 , and C 3 H 8 . At elevated temperatures and prolonged residence time, H 2 yield reaches as high as 8.6 wt% of the initial HDPE mass due to intensified cracking reactions of C 2 –C 3 hydrocarbons and long-chain aliphatic compounds. Consequently, pyrolysis tars consist mainly of polycyclic aromatic hydrocarbons (PAHs) with 5–7 rings, accompanied by visible coke deposition within the reactor. HDPE decomposition to volatiles is an endothermic process and it is complete at a temperature between 492°C and 525°C, depending on the heating rate employed, from non-isothermal thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) measurements. The thermal degradation of HDPE pellets follows the two-dimensional nucleation growth model for conversion levels up to 0.8 with an apparent activation energy of 259–270 kJ/mol and a pre-exponential factor of 4.83 × 10 17 –1.37 × 10 19 min -1 , determined from various isoconversional methods such as Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS), and Starink, along with Criado's master plots. Further, these findings provide valuable insights into optimizing process parameters and refining reactor design for pyrolysis, which can be integrated with gasification and reforming processes to enhance hydrogen production on a larger scale.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Thermodynamic investigation of the NaCl-KCl salt system from 25 to 950 °C

NaCl-KCl molten salt system has been proposed as a primary component of several promising heat transfer eutectics and fuel host for Generation IV molten salt reactors (MSR). In this work, several key thermodynamic parameters, including high temperature enthalpies, isobaric heat capacities (C p ) and molar enthalpies of mixing (ΔH mix ), of the NaCl -KCl system (NaCl, 75mol% NaCl – 25mol% KCl, 51mol% NaCl – 49mol% KCl, 25mol% NaCl – 75mol% KCl, and KCl) were measured by high temperature drop calorimetry (HTDC) utilizing laser sealed nickel and aluminum crucibles. Salts were checked for phase purity by X-ray diffraction (XRD), and water content by Karl Fischer coulometric titrimetry and thermogravimetric analysis (TGA). Thermo-mechanical analysis (TMA) utilizing custom-built boron nitride (BN) crucibles was employed to determine temperatures of phase transitions (solid $\rightarrow$ liquid) for the salts and plotted against the pseudobinary phase diagram for the NaCl-KCl system. Small deviations (3-10 %) were found among C p values obtained from HTDC, differential scanning calorimetry (DSC), and NIST, comparable to the average uncertainty based on two standard deviations of measured data that are ~6% for DSC and ~3% for HTDC. Additionally, the measured enthalpies and C p values of mixed NaCl-KCl salts (e.g., 51mol% NaCl – 49mol% KCl eutectic) were shown to behave as statistical mixtures of its pure endmembers, consistent with ΔH mix = ~ 0 kJ/mol at 800 °C, and a regular interaction parameter of Ω = 1.20 ± 0.76 kJ/mol. In conclusion, this study represents a step towards improved accuracy and precision in determinations of thermodynamic parameters of molten salts by using DSC, TMA, and HTDC, in completion of the Molten Salt Thermal Properties Database (MSTDB).

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

Highly thermally stable copolymers of epoxy and trifunctional polybenzoxazine

Here, in this work, we report the copolymerization of a triamine-based benzoxazine and a commercial difunctional epoxy resin, and investigate their synergistic effect on improved thermal properties using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Results showed that a glass transition temperature increase of up to 268 °C was obtained for the copolymer with as low as 25 wt% epoxy. A higher thermal performance was also achieved with an onset degradation of nearly 400 °C and char yield of 39 wt% at 800 °C for the copolymer with 52 wt% epoxy. The behavior was compared to previously reported benzoxazine/epoxy combinations and was found to be advantageous. The curing behavior of different copolymer compositions was also investigated via DSC and a methodology for matching each peak with its corresponding reaction was developed. Results showed that the alternating favorability of the reactions was dependent on the benzoxazine/epoxy ratio. Overall, the prepared PBZ/epoxy copolymers displayed higher thermal stability than their individual components, indicating suitability for a number of potential extended uses including utility in higher temperature applications.

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

Hexagonal boron nitride (h-BN) irradiated with 140 MeV protons

Here, hexagonal boron nitride was irradiated with 140 MeV protons to a fluence of ~6 10 20 p/cm 2 at T irr ~200 °C. Isotropic graphite was also irradiated alongside h-BN under similar conditions and fluence to enable direct comparison of the two similar structures. The effects of proton irradiation on dimensional stability and microstructure were studied using precision dilatometry and energy dispersive Xray diffraction techniques revealing by direct comparison to graphite that h-BN can better resist radiation damage from bombardment when irradiated with protons impinging normal to the crystallographic planes or along the crystallographic c-axis. X-ray diffraction experiments also revealed a preferred orientation of the crystallites in bulk samples near the sample surface, an orientation influenced by irradiation. Thermal studies using differential scanning calorimetry and thermogravimetric analysis to 740 °C, augmented by precision dilatometry, provided evidence of subtle phase transitions attributed to residual w-BN in the matrix. Irradiation appears to induce shifting of such transitions.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Exploring competitive metal binding and crystallization of UO 2 2+ and Cu 2+ tetrahydrofuran-2,3,4,5-tetracarboxylic acid complexes

Solvent extractions are used to separate actinide elements from fission products in nuclear waste streams and the successful isolation of specific species utilize subtle differences in metal ligand binding. Metal ligand binding is also important in crystallization of metal organic materials and herein we explore the importance of competitive binding in the tetrahydrofuran-2,3,4,5-tetracarboxylic acid (THFTCA) system. This ligand has been previously evaluated for the selective extraction of uranium from other lanthanides and actinides and in the current research, we evaluate the crystallization of uranyl-THFTCA complexes in the presence of Cu 2+ , Sr 2+ , Th 4+ , and Ce 3+ . Three major phases were formed in the crystallization experiments and characterized with single crystal X-ray diffraction, powder X-ray diffraction, thermogravimetric analysis, and Raman spectroscopy. Two of the resulting phases were novel (UTHF1 ((C 4 H 10 N 2 )[UO 2 (C 8 H 8 O 9 ) 2 ]∙2H 2 O) and UTHF2 (Na[(UO 2 )(C 8 H 5 O 9 )(H 2 O)]∙ 3.5 H 2 O)), whereas the third (CuTHF1) was previously reported in the literature. Raman spectroscopy was utilized to evaluate spectral changes in the mother liquor of UTHF1, UTHF2, and CuTHF1 over time to assess the crystallization process. Further, isothermal titration calorimetry was used to determine the binding constants for UO 2 2+ and Cu 2+ to the THFTCA ligand in solution and evaluate the role of competitive metal binding in this system. The thermodynamic parameters and the crystallographic data were used to justify the formation of a weaker UO 2 2+ -THFTCA complex. Formation of a weaker UO 2 2+ -THFTCA complex supports our findings that UTHF1 only forms in homomeric systems, but suggests that the crystallization of UTHF2 and CuTHF1 is reliant on the presence of additional counter ions and ligands to change the amount of available THFTCA ligand.

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

Local conformations and heterogeneities in structures and dynamics of isotactic polypropylene adsorbed onto carbon fiber

Carbon fiber (CF) reinforced polymers (CFRPs) have experienced widespread use in various industries. One of the most important parameters that controls the macroscopic property of CFRPs is the interface between a polymer matrix and CF. There is growing evidence to suggest the formation of a bound polymer layer (BPL), i.e., polymer chains that physically adsorb on a filler surface. However, this interface is always in contact with the thicker part of a polymer matrix, rendering its understanding a difficult task. Therein, we use CF-reinforced isotactic polypropylene (iPP) as a rational CFRP. To characterize the BPL on the CF surface, we extracted it from the CFRP using solvent-rinsing with p-xylene. The physical and thermal properties of the BPL were characterized by differential scanning calorimetry and thermogravimetric analysis, while its microscopic structures and dynamics were probed by small-angle neutron scattering and quasi-elastic neutron scattering (QENS) techniques. Subsequently, we employed atomistic molecular dynamics (MD) simulations to complement the QENS results above the bulk melting temperature and reveal details that were experimentally inaccessible. In this work, we observed that the degree of crystallinity of the BPL was quite lower than the bulk, while the melting temperature of the BPL remained the same as the bulks. Within the given length and time scales probed by QENS and MD, we also observed that most of the bound chains were mobile, with the formation of a high-density region (less than 1 nm in thickness) near the CF surface. The segmental dynamics of the bound chains probed by both QENS and MD were also much faster than those of the free chains, possibly due to the presence of a free surface region at the topmost surface of the BPL. Furthermore, the MD results demonstrated that the backbone chains and side groups lie nearly flat on the CF surface, which is the driving force for the flattening process of the iPP BPL to overcome the conformational entropy loss in the total free energy.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Comparison of volatiles evolving from selected highland and mare lunar regolith simulants during vacuum sintering

Volatiles evolving from JSC-1A, NU-LHT-4M and CSM-LHT-1G lunar regolith simulants during in vacuo thermal processing were analyzed using mass spectrometry as a function of temperature. Two high-fidelity simulants, JSC-1A (mare) and NU-LHT-4M (highland), were compared to a newly developed CSM-LHT-1G highland simulant, modified to closely match lunar geochemistry. Large autogenous gas loads were observed for all investigated materials. Mineralogical knowledge was used to identify and attribute individual volatile species to reacting, transforming, or decomposing constituents (hydrates, carbonates, sulfates, sulfides, clays, etc.) of the respective regolith simulant in the self-generated gas environment. Cumulative mass losses for individual simulant components as a function of temperature were quantified using mass spectrometry in conjunction with thermogravimetric analysis. Investigation of the four components of CSM-LHT-1G – anorthosite, basalt, augite, and glass – aided the attribution of volatile species to specific compounds and their respective sources. The results showed significant decomposition of non-lunar phases present in the man-made regolith simulants below the typical glass crystallization temperatures, which paves the way to devising methods for enhancing the fidelity of the simulants. Finally, high gas loads and corrosive gases (HF and HCl) were recognized as potential hazards, pertaining to the development of large testbed facilities.

42 ENGINEERING↗
DOE OSTI2023

Determination of the regeneration energy of direct air capture solvents/sorbents using calorimetric methods

Regeneration energy and recyclability are important parameters that dictate the viability of a CO 2 direct air capture (DAC) process. In this study, differential scanning calorimetry (DSC) is used in conjunction with thermogravimetric analysis (TGA) and Fourier Transform Infrared Spectroscopy (FTIR) to evaluate the regeneration energy required for CO 2 -loaded 3 M aqueous potassium sarcosinate (K-SAR) solvent and crystalline methyl-glyoxal-bisiminoguanidine (MGBIG) sorbent. Based on calorimetric measurements, for aqueous K-SAR, the sensible heat amounts to 1.5 GJ/tCO 2 and the enthalpy of desorption is estimated at 3.68 GJ/tCO 2 . The total energy of regeneration will also include the enthalpy of solvent vaporization, which will depend on the thermodynamic conditions and the level of regeneration. For MGBIG, the total regeneration energy required was measured at approximately 7.0 GJ/tCO 2 . Further, FTIR measurements were used to observe CO 2 and H 2 O release during the thermal regeneration process. For K-SAR, CO 2 release is seen to take place at temperatures greater than 80 °C and, for MGBIG, the CO 2 release takes place at temperatures greater than 100 °C. The moderate temperatures required for K-SAR and MGBIG regenerations compared to other DAC solvents/sorbents suggest that low-cost sources of heat, such as geothermal heat, could potentially be used for regeneration to drive down the cost of direct air capture.

42 ENGINEERING↗
DOE OSTI2021

Cohesive and adhesive degradation in PET-based photovoltaic backsheets subjected to ultraviolet accelerated weathering

Delamination between layers in photovoltaic (PV) backsheets is often reported in the literature, causing voids that can collect moisture, diminish module backside heat dissipation, and reduce the backsheet’s effectiveness as a physical barrier. While backsheets with weathering-resistant fluoropolymer outer layers have traditionally been used in modules, more recent backsheets using non-fluoropolymer outer layers, such as polyethylene terephthalate (PET), have been developed. These backsheets have shown signs of premature degradation, and their adhesion degradation, in particular, has not been widely studied. In the present work, the single cantilever beam (SCB) adhesion test was used to quantify the adhesion energy in two commercially available PET-based backsheets. To study the effect of minor changes in formulation, the backsheets were obtained from same manufacturer and product line but during different years. To study the effect of environmental variables on adhesion degradation, the backsheets were subjected to artificial weathering at controlled temperature, humidity, and ultraviolet (UV) radiation in an indoor weathering chamber, and the adhesion energy was quantified at several intervals of exposure time. Layering structure, composition, and adhesion failure mode were compared between the backsheets, using Raman and infrared spectroscopy and thermogravimetric analysis. The results show a large difference in initial (unexposed) adhesion energy between the backsheets, despite very similar structures and compositions. Following exposure, adhesion energy dropped significantly, primarily due to thermo-hydrolytic degradation of the polyurethane (PU)-based adhesive layers. Significant UV-induced adhesion degradation of the PET outer layer surface was also observed. The study represents an important step in understanding adhesion degradation in PET-based backsheets, suggesting ways in which adhesion integrity – and, correspondingly, module service life – can be improved.

14 SOLAR ENERGY↗
DOE OSTI2020

Ambient pressure synthesis and characterization of layered honeycomb Li 2 PdO 3

Single-phase polycrystalline Li 2 PdO 3 has been synthesized at 640°C in oxygen for the first time under ambient pressure. X-ray and neutron diffraction analyses show that the sample possesses a monoclinic layered structure belonging to the C2/m space group. Rietveld refinements of neutron powder diffraction data indicate ~10% Li–Pd site exchange and DIFFaX modelling manifest ~2% stacking faults present within LiPd 2 layers. A band gap of ~2.23 eV was calculated for the golden Li 2 PdO 3 using absorbance measurements. Thermogravimetric analysis of the sample shows that Li 2 PdO 3 is stable up to 730°C under oxygen. Here a Curie tail is observed at low temperature magnetic measurements (T < 50K), yielding an effective moment of 0.038 μ B , possibly due to spin ½ impurities.

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

Thermodynamics, local structure, and transport of protons in triple-conducing oxide, BaCo 0.4 Fe 0.4 Zr 0.1 Y 0.1 O 3-δ (BCFZY4411)

Triple-conducting oxides (TCOs) are an emerging class of mixed ionic and electronically conducting materials that show great promise for oxygen reduction/oxygen evolution (ORR/OER) electrocatalysis—primarily in high-temperature ceramic electrochemical cells— but also in aqueous alkaline environments. Their high activity is attributed, at least in part, to their ability to incorporate and transport three mobile charge carriers: protons, oxygen vacancies, and electron-holes Despite their promise, fundamental studies of TCOs are challenging, as transport dynamics from three charge carriers cannot be fully disentangled via traditional electrical measurement techniques. Characterizing proton dynamics in TCOs is particularly difficult as protons are generally the minority carrier, and their conduction response is typically obscured by the oxygen vacancies and electron holes. Here, we demonstrate successful isolation of the proton behavior in an archetypal TCO, BaCo 0.4 Fe 0.4 Zr 0.1 Y 0.1 O 3-δ (BCFZY4411), using a combination of non-electrical techniques. We determine proton uptake and oxygen non-stoichiometry (δ) using thermogravimetric analysis (TGA). X-ray absorption near edge structure (XANES) and neutron diffraction (ND) are used to validate the oxidation state of Co and the δ values obtained through TGA. We apply 1H solid-state magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) to provide insights into local structure, dynamics, and proton kinetics. Finally, the proton transport properties are further quantified using tracer isotope exchange with time-of-flight secondary ion mass spectrometry (ToF-SIMS). Despite the very low proton concentrations in BCFZY4411 (<0.2% under most conditions), our analysis suggests that the oxygen Manuscript File Click here to view linked References 2 reduction and evolution reactions are nevertheless limited by the oxygen ion kinetics (e.g., oxygen surface exchange) rather than the proton kinetics at the reduced operating temperatures (<500 °C) that are targeted for electrochemical cell applications. These findings provide a comprehensive understanding of proton behavior in BCFZY4411 and pave the way for advancing the fundamental study of TCOs.

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

Thermodynamics and dehydration/hydration behaviors of Na-sulfate, -borate, and -bicarbonate minerals

Here, detailed thermal experiments using relative humidity- (RH) and temperature-controlled X-ray diffraction (XRD) and H 2 O adsorption/desorption measurements were applied to understand the dehydration/hydration behaviors of thenardite/mirabilite (Na 2 SO 4 /Na 2 SO 4 ·10H 2 O), tincalconite/borax (Na 2 B 4 O 7 ·5H 2 O/Na 2 B 4 O 7 ·10H 2 O), nahcolite (NaHCO 3 ), and qilianshanite (NaH 4 (BO 3 ) (CO 3 )·2H 2 O or NaHCO 3 ·H 3 BO 3 ·2H 2 O). The thermal behaviors of thenardite/mirabilite and tincalconite/borax were evaluated at room temperature as a function of RH. Hydration of thenardite to mirabilite is very sluggish and does not begin until at least 85 % RH, whereas dehydration of mirabilite to thenardite occurs at ~76 % RH. The reactions are accessible in the solid state and were not completely reversible on the time scale of these measurements. Qilianshanite appears stable under room conditions, as long as the RH is >20 %, supported by 2.5 months of observation during exposure to room air of known RH. Qilianshanite persists metastably for at least two days when exposed to both wet and 0% RH atmospheres at room temperature, observed during environment-controlled XRD. Below 20 % RH, qilianshanite sluggishly reacts to form tincalconite and nahcolite. When heated, qilianshanite broke down at 70 °C based on XRD results, and thermogravimetric analysis (TGA) confirmed a dehydration temperature of ~75 °C. Tincalconite hydrated to borax at 80 % RH, and dehydration from borax to tincalconite occurred at 40–50 % RH, illustrating the effective reversibility of the reaction under room conditions. The minor phase nahcolite originally in qilianshanite remained in the residue of decomposed qilianshanite after heating to 125 °C, demonstrated by continuous XRD measurements, whereas nahcolite was largely dehydrated when heated to ~100 °C during TGA. The difference can be attributed to the slower reaction kinetics in the environmental XRD. These results shed considerable light on the behavior of these minerals in the environment, particularly in evaporite deposits in Antarctica, where all of these minerals are reacting with the atmosphere on a diurnal-to-seasonal basis.

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

Effect of natural carbon filler on thermo-oxidative degradation of thermoplastic-based composites

Here thermo-oxidative degradation of thermoplastic-based composites filled with two types of carbon filler including bituminous (Pittsburgh No.8, P8) and sub-bituminous (Powder River Basin, PRB) coals were investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis. Oxidation induction time (OIT) and activation energy (AE) for coal plastic composites (CPCs), HDPE, and commercially available wood plastic composites (WPCs) were determined using isothermal and isoconversional kinetic methods. OIT values for CPC increased with coal content, indicating higher thermo-oxidative stability. AE values obtained using isothermal method were 52–116 kJ/mol for all CPC formulations, 32 kJ/mol for HDPE, and 33–114 kJ/mol for WPCs. AEs for CPCs obtained using isoconversional methods were not constant but rather dependent on the degree of degradation with CPC/ P8 coal possessing higher overall AE. Incorporating coal into HDPE increased thermo-oxidative stability of the resulting composite, indicating coal potentially acts as a natural primary and secondary antioxidant for polymer materials.

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

Enhanced Guest@MOF Interaction via Stepwise Thermal Annealing: TCNQ@Cu 3 (BTC) 2

Confinement of guest molecules in porous materials such as metal organic frameworks (MOFs) promises to deliver emergent properties separate from those of the individual components. Understanding the confinement mechanism is therefore important for the development of new synthesis routes that adjust MOF properties for specific applications. In this work, we developed a new synthetic method to confine guest molecules into MOF pores through a stepwise thermal annealing process, wherein the confinement of 7,7,8,8-tetracyanoquinodimethane (TCNQ) guest molecules into Cu 3 (BTC) 2 (BTC = benzene-1,3,5-tricarboxylic acid) MOF host is used as an example of how novel materials can be created with new physical properties. The stepwise thermal annealing process includes 1) an activation process of pristine Cu 3 (BTC) 2 MOF to maximizes the TCNQ guest loading in the MOF host by effectively removing the residual solvents and 2) post-annealing of the TCNQ infiltrated MOF to enhances the interaction of the confined guest molecules with the MOF host. Obtained experimental results based on thermogravimetric analysis, N 2 gas adsorption, electron microscopy, X-ray diffraction and infrared absorption, combined with density functional theory calculations provide evidence that the use of a stepwise thermal annealing process yields enhancements in the guest loading, packing and interaction between the TCNQ guest and the MOF host. The new hybrid TCNQ@Cu 3 (BTC) 2 system is stable and shows no significant signs of structural degradation even after submersion in water. This is due to the presence of significantly stronger interactions of TCNQ with the frame-work metal ions compared to those of the water molecules competing for the same framework binding sites. It was also found that TCNQ@Cu 3 (BTC) 2 system maintains a significant CO 2 and CH 4 adsorption potential compared to the pristine MOF. The synthetic route developed in this work yields novel guest@MOF hybrid systems that will be useful for many MOF-based applications such as gas separations and chemical sensors performed under humid conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Crystal Growth of Quaternary RE 2 EuSi 2 S 8 ( RE = Ce–Nd, Sm, Gd, Tb) Using Flux-Assisted Boron Chalcogen Mixture (BCM) Method: Investigation of Magnetic and Luminescence Properties

A series of quaternary rare-earth containing thiosilicates with the general formula RE 2 EuSi 2 S 8 (RE = Ce–Nd, Sm, Gd, Tb) has been synthesized via the flux-assisted boron chalcogen mixture (BCM) crystal growth method. High-quality single crystals were obtained, and their crystal structures were determined by single-crystal X-ray diffraction. The RE 2 EuSi 2 S 8 series crystallizes in the trigonal system, adopting the space group R-3c. Polycrystalline samples were employed for physical property measurements, including magnetic susceptibility measurements, UV–visible diffuse reflectance, and photoluminescent response. Magnetic data of RE 2 EuSi 2 S 8 ( RE = Ce, Nd, and Gd) were collected over the 2–300 K temperature range. The samples were paramagnetic behavior with negative Weiss constants (θ W = −11.05, −10.55, and −1.35 K respectively). Their thermal stability was investigated using thermogravimetric analysis (TGA). Optical band gaps, estimated from diffuse reflectance spectra, were determined to be 2.2(1) eV for Ce 2 EuSi 2 S 8 , 1.8(1) eV for Nd 2 EuSi 2 S 8 , and 1.7(1) eV for Gd 2 EuSi 2 S 8 respectively. Finally, photoluminescence measurements were collected on Ce 2 EuSi 2 S 8 and Tb 2 EuSi 2 S 8 single crystals.

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

Favorable Redox Thermodynamics of SrTi 0.5 Mn 0.5 O 3–δ in Solar Thermochemical Water Splitting

Two-step, solar thermochemical splitting of water using nonstoichiometric redox-active metal oxides has emerged as an intriguing approach for large-scale hydrogen production. Perovskites have been proposed as alternatives to state-of-the-art fluorite CeO 2–δ because of their potential for lowering reduction temperature while maintaining high fuel productivity. Guided by computational insights, we explore the thermodynamic properties and water splitting efficacy of the cubic perovskite SrTi 0.5 Mn 0.5 O 3–δ (STM55). Thermogravimetric analysis is performed under controlled oxygen partial pressures (pO 2 ) and temperatures up to 1500 °C, from which both the enthalpy and entropy of reduction as a function of oxygen nonstoichiometry are determined. STM55 provides an attractive combination of moderate enthalpy, 200–250 kJ (mol-O) –1 , and high entropy, with unusual δ dependence. Using a water splitting cycle in which the material is thermally reduced at 1350 °C (pO 2 , ~10 –5 atm) and subsequently exposed to steam at 1100 °C (steam partial pressure of pH 2 O = 0.4 atm), we demonstrate a hydrogen yield of 7.4 mL g –1 . Through both half-cycles, the material remains largely in quasi-equilibrium with the gas phase, as reflected in the agreement of the measured data with predicted profiles based on the thermodynamic data. This behavior indicates rapid surface and bulk diffusion kinetics. Cyclic operation showed the material to be free of degradation and always resulted in a 2:1 yield of H 2 /O 2 . Altogether, STM55 provides outstanding performance characteristics for thermochemical hydrogen production.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗