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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 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Self-assembly synthesis of BiVO4/Polydopamine/g-C3N4 with enhanced visible light photocatalytic performance

Highlights: • BiVO{sub 4}/PDA/g-C{sub 3}N{sub 4} was synthesized via simple self-assembly at the room temperature. • PDA has a strong binding affinity to BiVO{sub 4} and g-C{sub 3}N{sub 4} to form BiVO{sub 4}/PDA/g-C{sub 3}N{sub 4}. • BiVO{sub 4}/PDA/g-C{sub 3}N{sub 4} exhibits good stability and reusability in the photocatalytic reaction of glyphosate. • The enhanced activity of BiVO{sub 4}/PDA/g-C{sub 3}N{sub 4} is mainly attributed to the increased charge separation. - Abstract: BiVO{sub 4}/PDA(Polydopamine)/g-C{sub 3}N{sub 4} was synthesized by facile ultrasonic dispersion and self-assembly at the room temperature, and monoclinic BiVO{sub 4} was embedded on PDA/g-C{sub 3}N{sub 4} sheet, which PDA was first formed in situ on the surface of g-C{sub 3}N{sub 4} via self-polymerization of dopamine. The dependence of photocatalytic activity on the structural, electronic and optical properties of the BiVO{sub 4}/PDA/g-C{sub 3}N{sub 4} nanostructure has been explored using XRD, XPS, TEM, SEM, DRS, PL, BET and photocurrent. The BiVO{sub 4}/PDA/g-C{sub 3}N{sub 4} photocatalysts show higher activity of glyphosate degradation than BiVO{sub 4}, g-C{sub 3}N{sub 4} and BiVO{sub 4}/g-C{sub 3}N{sub 4} under visible light irradiation. The enhanced activity of BiVO{sub 4}/PDA/g-C{sub 3}N{sub 4} can be significantly attributed to the improved visible light absorption, enhanced specific surface area and increased charge separation. PDA has a strong and irreversible binding ability onto BiVO{sub 4} and g-C{sub 3}N{sub 4} to form BiVO{sub 4}/PDA/g-C{sub 3}N{sub 4}. This simple method at the room temperature is promising for the synthesis of high performance composite photocatalysts for different applications, such as pollutant degradation, photocatalytic splitting water, etc.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Nitrogen-doped nanostructured porous carbon derived from monosodium glutamate for high-performance lithium sulfur battery

Highlights: N-doped porous carbon nanosheets were prepared by a low-cost carbonization approach. The product possesses a special micro/mesoporous structure with high surface area. The S/NPCN-800 exhibits excellent electrochemical cycling and rate performance. The enhancement is attributed to synergistic effect of hierarchical porous structure and N-doping. - Abstract: Carbon host materials play an important role in improving the electrochemical performances of lithium-sulfur batteries. In this work, we have successfully prepared nitrogen-doped porous carbon nanosheets (NPCN) by using low-cost raw materials (monosodium glutamate) via a facile carbonization approach without any activation process. The obtained NPCN-800 possesses a special micro/mesoporous structure, with a relatively high BET surface area and appropriate nitrogen doping. By employing the NPCN-800 as a reservoir to impregnate sulfur as the cathode material of lithium-sulfur batteries, the resulting sulfur/carbon composite with 65.5 wt% sulfur exhibits an initial discharge capacity of 1070 mAh g{sup −1} at 0.2 C and enhanced capacity retention of 680 mAh g{sup −1} after 150 cycles. Moreover, a capacity of 452 mAh g{sup −1} at 0.5 C is achieved over 200 cycles with 98.6% Coulombic efficiency. The superior electrochemical performance is attributed to the synergistic effect of the hierarchical porous structure (physisorption) and N-doping (chemisorption).

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Dual oxidation state induced oxygen vacancies in Pr substituted BiFeO3 compounds: An effective material activation strategy to enhance the magnetic and visible light-driven photocatalytic properties

Highlights: • BiFeO{sub 3} with tunable oxygen vacancy is achieved through Pr substitution. • Dual oxidation of Pr ions is found to be the origin of oxygen vacancies in BiFeO{sub 3}. • Mechanism of the observed photocatalytic activity could be due to the oxygen vacancy. - Abstract: Bi{sub 1-x}P{sub x}FeO{sub 3} (BP{sub x}FO) nanoparticulates with tunable oxygen vacancy have been synthesized via sol-gel process. Substitution induced oxygen vacancy defects in BP{sub x}FO compositions were evidenced from their X-ray photoelectron spectra, where the mechanism behind the formation of oxygen vacancy was essentially found to be the dual oxidation states of Pr (+3/+4) ions. The origin of the observed enhanced magnetic properties in BP{sub x}FO nanoparticulates is attributed to be the oxygen vacancy mediated alterations in FeOFe bond angle and the alignment of Pr-3d magnetic moments with Fe-3d magnetic moments, which is resulted the ferromagnetic exchange interactions in the BP{sub x}FO system. The observed enhanced photocatalytic activity of BP{sub x}FO compositions is attributed to the formation of defective states in the valence band due to the oxygen vacancies that favored the dispersion of band edge position with required photocatalytic redox potential as well as the acceleration of the charge separation process in these defective BP{sub x}FO systems.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Metal ion-mediated structure and properties of α-Fe2O3 nanoparticles

Highlights: • Cu- and Zn-doped Fe{sub 2}O{sub 3} nanoparticles were successfully obtained by a simple solvothermal method. • The sizes and morphologies of the doped Fe{sub 2}O{sub 3} nanoparticles could be regulated by doping with divalent cations. • The doped Fe{sub 2}O{sub 3} nanoparticles showed enhanced remanent magnetization and coercivity. • The doped Fe{sub 2}O{sub 3} nanoparticles showed excellent catalytic activity toward thermal decomposition of ammonium perchlorate. • The enhanced catalytic activity could be attributed to the defect structure induced by doping with divalent cations. - Abstract: To assess the relationship between metal ion-mediated microstructures and the macro-performance, Cu- and Zn-doped α-Fe{sub 2}O{sub 3} nanoparticles were prepared via a solvothermal method. The morphologies of the as-synthesized nanoparticles were investigated by scanning electron microscopy and transmission electron microscopy. The results revealed that the sizes and morphologies could be regulated by doping with divalent cations. The structures of the as-synthesized nanoparticles were characterized by X-ray diffraction and Raman spectroscopy, indicating that Cu{sup 2+} and Zn{sup 2+} ions had diffused into the lattice of α-Fe{sub 2}O{sub 3} matrix. The magnetization behaviors of these nanoparticles were measured to analyze the effect of doping on α-Fe{sub 2}O{sub 3}. Furthermore, the catalytic activities of Cu- and Zn-doped α-Fe{sub 2}O{sub 3} nanoparticles demonstrated that the high-temperature decomposition temperature of ammonium perchlorate could be lowered by 115 °C and 107 °C, respectively, compared to that of ammonium perchlorate without catalyst. The enhanced catalytic activity could be attributed to the defect structure induced by doping with divalent cations.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Ultraviolet light induced degradation of luminescence in CsPbBr3 perovskite nanocrystals

Highlights: • The degradation of PL in CsPbBr{sub 3} nanocrystals under UV illumination was studied. • The degradation was attributed to the photo-oxidation of lead atoms on NC surface. • The PbCO{sub 3} and PbO phases were observed after long illumination time. • The change in PL intensity, peak energy, and linewidth as well as lifetimes under illumination was discussed. - Abstract: Perovskite nanocrystals (NCs) have been applied in light emitting diodes, but their stability is always the decisive factor to limit the practical application of devices. In this work, the influence of ultraviolet light illumination on the structural and luminescent properties of CsPbBr{sub 3} NC films with emissions of 495 and 520 nm was studied. It was demonstrated that the CsPbBr{sub 3} NC films exhibited significant degradation of photoluminescence (PL) under the illumination in air. The PbCO{sub 3} and PbO phases in CsPbBr{sub 3} NC films were observed after long illumination time with X-ray photoemission spectroscopy. The temperature-dependent PL spectra revealed that large amount of nonradiative traps were formed with increasing illumination time. The PL intensities, band energies, and bandwidths as well as PL lifetimes of the illuminated CsPbBr{sub 3} NC films were analyzed. The experimental results indicated that the PL degradation was attributed to the photo-oxidation of NCs under the illumination.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Anomalous Raman and photoluminescence blue shift in mono- and a few layered pulsed laser deposited MoS2 thin films

Highlights: • Mono- and a few layered MoS{sub 2} films deposited using pulsed laser deposition technique by varying the deposition time. • The films experienced an epitaxial compression due to its adhesion with the substrate. • Lattice strain of the films led to a significant Raman frequency shift. • An anomalous PL peak was observed which is attributed to quantum confinement. - Abstract: We report on the Raman blue shift of layered MoS{sub 2} films as well as an anomalous blue shift of the photoluminescence spectra of nanocrystallite MoS{sub 2} films grown by pulsed laser deposition. Monolayer, bilayer and trilayer MoS{sub 2} films were fabricated by ablating MoS{sub 2} target for a deposition time of around 14, 24 and 30 s at a laser fluence of 2.2 Jcm{sup −2}. The two Raman active modes showed lattice strain-induced blue shift of 1–3 cm{sup −1}. The layered films showed an out-of-plane compressive strain as well as an in-plane tensile strain confirmed by Riet-Veld refinement of the X-ray diffraction pattern. TEM image of the MoS{sub 2} film for 14 s demonstrated the presence of nanocrystallites of an average size of 13 nm along with the layered film of MoS{sub 2}. An anomalous photoluminescence peak was observed corresponding to the MoS{sub 2} nanocrystallites at 2.18 eV, attributed to quantum confinement effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Observation of oxo-bridged yttrium in TiO2 nanostructures and their enhanced photocatalytic hydrogen generation under UV/Visible light irradiations

Highlights: • Synthesis of yttrium doped TiO{sub 2} nanoparticles (NPs) and nanorods (NRs) through a facile wet chemical method. • The doped Y ions are found to be oxo-bridged with the host TiO{sub 2} through Y-O-Ti like formation. • The NPs of Y:TiO{sub 2} showed increased photocatalytic hydrogen production efficiency. • The observed efficiency of NPs is attributed to their three-dimensional confinement. - Abstract: Yttrium doped TiO{sub 2} (Y:TiO{sub 2}) nanoparticles (NPs) and nanorods (NRs) are synthesized using facile wet chemical methods. Interestingly, the doped Y ions are found to be oxo-bridged with the host TiO{sub 2} through Y-O-Ti like formation as confirmed from their XRD and XPS studies. The photocatalytic hydrogen production efficiency of the synthesized nanostructures was studied under UV and visible light irradiations. The NPs of Y:TiO{sub 2} showed increased activity as compared to NRs. The observed efficiency of NPs is attributed to their three-dimensional confinement, where it facilitated a stronger oxo-bridging formation that improved the metal-to-metal charge transfer process, reducing the band gap and surface energies as compared to NRs. As investigated in this study, the oxo-bridging formation in the doped metal-oxide systems is relatively promising as this configuration largely favors the photocatalytic process by re-designing the charge separation and transfer processes as compare to the other conventional systems.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Highly stable and efficient hybrid perovskite solar cells improved with conductive polyanilines

Highlights: • MAPbI{sub 3} perovskite film has been fabricated and modified with polyanilines for highly stable and efficient solar cells. • For the C-PNAI modified perovskite solar cells, a PCE enhanced from 7.91% to 9.43% as the fill factor improved. • The C-PNAI surfaces of perovskite film assists in the hole transport to the conductive carbon in the device. • The improved stability is attributed to the –NH{sub 2}{sup +}– of C-PNAI cations on the surface of the perovskites. - Abstract: Methyl ammonium lead iodide (MAPbI{sub 3}) perovskite film has been fabricated and modified with conductive polyanilines (C- PANI) and demonstrated for highly stable and efficient solar cells. For the C-PANI modified perovskite solar cells (PSCs), the average short circuit current density increased by 13.8% compared with pristine perovskite film (i.e., from 15.9 to 18.1 mA/cm{sup 2}) and the fill factor also improved, eventually leading to a power conversion efficiency (PCE) enhancement from 7.91% to 9.43% with high reproducibility. The C-PANI acts as a scaffold for crystallization of the perovskite layer, and improves the perovskite photovoltaic performance and stability, attributed to the favorable interfacial interaction between –NH{sub 2}{sup +}– groups of C-PANI and I{sup −} of MAPbI{sub 3} perovskite. The C-PANI-rich surfaces of MAPbI{sub 3}/PNAI perovskite film may also assists in the hole transport to the conductive carbon in the device. This work demonstrates that C-PANI can enhance the PCE and stability of PSCs.

14 SOLAR ENERGY↗
DOE OSTI2018

In situ construction of conjugated polymer P3HT coupled hierarchical ZnO composite with Z-scheme enhanced visible-light photocatalytic activity

Highlights: • P3HT@H-ZnO photocatalysts were fabricated via a two-step solution method. • The P3HT@H-ZnO composite. exhibited high photodegredation of RhB under visible-light. • Enhanced photocatalytic activity attributed to the closely contacted interface between P3HT and H-ZnO and the matched band gap structure. • The fabricated P3HT@H-ZnO composite follow a Z-scheme mechanism rather than type II heterojunction. - Abstract: Conjugated polymer P3HT coupled with hierarchical ZnO (H-ZnO) photocatalysts were fabricated by a two-step solution method with assembling P3HT nanoparticles on the surface of H-ZnO micro-flowers. Optical properties revealed that the fabricated H-ZnO micro-flowers were rich in defects. The as-prepared P3HT@H-ZnO composite exhibited higher photocatalytic activity than H-ZnO for photocatalytic degradation of Rhodamine B (RhB) under visible light irradiation. The enhanced photocatalytic activity was attributed to the strengthened visible light absorption and the closely contacted interface between P3HT and H-ZnO. Based on the reactive species trapping experiments and band gap structure analysis, it can be identified that the effective separation and transfer of the photo-induced electron-hole pairs within the P3HT@H-ZnO composite follow a Z-scheme mechanism rather than type II heterojunction. The present work will provide a new insight to design Z-scheme photocatalysts with enhanced photocatalytic performance via metal-free polymers modification.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Intrinsic and extrinsic effects on the fracture toughness of ductile metallic glasses

The potential application of metallic glasses (MGs) as structural materials demands accurate measurements of their toughness and the understanding of the underlying factors affecting it. Currently, it is challenging to precisely measure the toughness of MGs, especially ductile MGs. Measured toughness values are widely scattered, even for MGs with identical compositions. That is attributed to the combined effect of intrinsic and extrinsic factors including processing, sample geometries, and loading conditions. A fundamental understanding of the influences of these elements is thus of great significance. In the present study, molecular dynamics simulations are performed to investigate the influence of intrinsic and extrinsic effects on the fracture toughness of CuZr MGs. In particular, focused is placed on the effects of cooling rate, notch shape and size, strain rate, and temperature. The results indicate that the fracture toughness of a MG scales with the cooling rate used to prepare it. This is attributed to increased free volume content generated at high cooling rates, which enhances plastic deformation and amplifies the associated energy dissipation during plastic deformation events. Fracture toughness also increases with the strain rate, arguably due to strain rate-induced crack extension delay. Overall, the results demonstrate that the largest fracture toughness are achieved when MG samples are fabricated at high cooling rates and subjected to high strain rate deformation. In addition, results suggest that the fracture toughness decreases with increasing temperature, due to the significant decrease in strength. Here, the correlations revealed between these crucial intrinsic and extrinsic parameters and the calculated MG fracture toughness support the development of a framework to understand the root of the discrepancies in the measurement of the toughness of MGs and provide insights into the design of tough MGs for structural applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Cryogenic mechanical behaviors of CrMnFeCoNi high-entropy alloy

The CrMnFeCoNi high-entropy alloy (HEA) exhibits higher yield strength, ultimate strength and ductility at lower temperature. To further clarify the effect of the testing temperature on microstructure evolution, in this study in-situ synchrotron-based high-energy X-ray diffraction tensile tests were carried out from 298 K down to 123 K. The enhanced yield strength of the alloy at cryogenic temperatures can be attributed to the greater lattice distortion prior to plastic deformation. Higher strain hardening rate leads to the simultaneously enhanced strength and ductility of the studied HEA below room temperature. Both dynamic Hall-Petch hardening (twinning) and dislocation hardening provide high work hardening capacity for this alloy during the plastic deformation at cryogenic temperatures. The increased dislocation density and nano-twins at cryogenic temperatures can be attributed to the decrease in the stacking fault energy as the deformation temperature decreases. These studies could provide an in-depth understanding for the strengthening mechanisms of the HEA in different temperature conditions and guide the exploration of HEAs with superb mechanical properties at cryogenic environments.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Boron Nitride-Driven Strengthening of Aluminum Composites via Friction Stir Processing

Friction stir welding and processing (FSW/P) has emerged as an effective solid-state joining technique for fabricating metal matrix composites (MMCs), offering improved mechanical properties through refined microstructural evolution. In this study, an aluminum-boron nitride nanoparticle (Al-BNNP) composite was synthesized via FSW, and its indentation-based mechanical properties were systematically evaluated. Microhardness mapping across the weld cross-section revealed a progressive increase in hardness toward the stir zone (SZ), attributed to severe plastic deformation, dynamic recrystallization (DRX), and the reinforcing effect of BNNPs. Profilometry-based indentation plastometry (PIP) inferred yield strength (YS) demonstrates a 47.8% increase compared to the base metal (BM) and a 75% improvement compared to FSP pure aluminum reported in literature. This enhancement is attributed to strengthening mechanisms, including grain boundary pinning, load transfer, and increased dislocation density. The strain rate sensitivity (SRS) measurements at the nanoscale demonstrated a substantial decrease in the SZ, correlated with ultrafine grain structures and strong BNNP-matrix interactions. Activation volume analysis revealed a significant reduction in the SZ, suggesting that dislocation motion is increasingly restricted by dislocation-dislocation and dislocation-particle interactions. These findings suggest that incorporating BNNPs in FSW/P enables tailoring the microstructure without thermal degradation of the secondary particles, thereby significantly enhancing the mechanical performance of aluminum composites, particularly for structural applications in aerospace and automotive industries.

Aluminum↗
DOE OSTI2026

Influence of the as-built microstructure on the recrystallization of an additively manufactured Inconel939 Ni-based superalloy

This study investigates the influence of the as-built microstructure on the recrystallization (RX) behavior and mechanical properties of the Ni-based superalloy Inconel 939 produced by laser powder bed fusion (PBF-LB/M). Two distinct as-built microstructures were obtained by varying the hatch distance (h d ): a columnar, strongly textured condition (h d =50, termed h d 50) and an equiaxed, weakly textured condition (h d =70, termed h d 70)). Both were subjected to nine solution treatments combining three temperatures (1100, 1150, and 1200 °C) and three holding times (1, 4, and 8 h). Comprehensive microstructural characterization was conducted to assess grain morphology, texture, grain boundary character, dislocation density, and precipitate distribution. Recrystallization was found to be significantly slower than in cast counterparts, requiring higher temperatures and longer times for completion. The initial microstructure plays a decisive role: full RX was achieved only in hd70 specimens after treatment at 1200 °C for 8 h, whereas hd50 samples exhibited delayed and incomplete RX under identical conditions. This behavior is attributed to the finer grain size and higher fraction of high-angle grain boundaries in hd70, which promote recrystallization. Mechanical testing revealed that hd70 samples subjected to a 1200 °C/8 h treatment followed by standard double ageing show higher yield and tensile strengths across the investigated temperature range than both printed and cast Inconel939 processed under conventional conditions, albeit with slightly reduced ductility. The enhanced mechanical performance is attributed to the larger grain size, which limits grain boundary sliding. These results demonstrate the critical importance of controlling the as-built microstructure and tailoring post-processing strategies to optimize high-temperature performance of PBF-LB/M Inconel939.

Inconel939↗
DOE OSTI2022

Multicycle flash sintering of cubic Y 2 O 3 -stabilized ZrO 2 : An in situ energy dispersive synchrotron x-ray diffraction study with high temporal resolution

The current induced unit cell volume changes, (111) Bragg peak full width at half maximum (FWHM) and its integrated intensity in 8 % Y 2 O 3 stabilized ZrO 2 (8 %YSZ) solid state electrolyte was monitored during a triple-flash sintering experiment by in situ energy dispersive x-ray diffraction using a polychromatic synchrotron probe (max, photon energy 200 keV) with 2 second temporal resolution. The first spontaneous singularity in the unit cell volume (+0.54 %) was observed at 899 °C under 15 V/mm applied field intensity, which was associated with 13 mA/mm 2 current draw and an increase in density to 97 %. Following anelastic relaxation of the unit cell volume under open circuit conditions, the same applied field was applied twice in a row which resulted in additional induced singularities at 925 °C (+0.48 %) and 944 °C (+0.42 %). A floating baseline, which was above the thermal expansion baseline, was observed from 833 to 969 °C and was attributed to Joule heating. In conclusion, the singularity at 899 °C is associated with a sharp change in (111) FWHM and a 34 % decrease in integrated peak area that was attributed to changes in the distribution of oxygen vacancies and the changes in their concentration as induced by the applied field in the spontaneous transient stage of flash sintering.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Atomic Diffusion, Segregation, and Grain Boundary Migration in Nickel-Based Alloys from Molecular Dynamics Simulations

Grain boundary diffusion and metal mobility in alloys control material performance in many applications and yet remain poorly understood at a mechanistic level. With advances in accessible time and length scales for computational molecular simulations, and recent force field developments, we now possess tools to help unravel those mechanisms. Using large-scale molecular dynamics simulations, here we examined vacancy-mediated diffusion processes in Ni-5Cr alloy with low and high-energy grain boundaries. We show that atomic diffusion inside the grain boundary plane is about four times higher than bulk diffusion, at any temperature, and exhibits a typical Arrhenius behavior with a very small energy barrier (0~.8 eV for Cr and 0.7 eV for Ni within 1300-1600 K). Additionally, the fastest diffusing species inverts; Cr diffusion was faster than Ni in the bulk but slower in the grain boundaries. This is attributed to the creation of high cohesive energy clusters of Cr at the grain boundary. Grain boundary migration was also observed to be temperature dependent and appears to be two times higher in the 5% Cr alloy than in pure Ni, highlighting the important role of the alloying element on grain boundary motion.

Simonnin, Pauline GN↗
DOE OSTI2022

Process and feedstock driven microstructure for laser powder bed fusion of 316L stainless steel

Here in the pursuit of improving additively manufactured (AM) component quality and reliability, fine-tuning critical process parameters such as laser power and scan speed is a great first step toward limiting defect formation and optimizing the microstructure. However, the synergistic effects between these process parameters, layer thickness, and feedstock attributes (e.g. powder size distribution) on part characteristics such as microstructure, density, hardness, and surface roughness are not as well-studied. In this work, we investigate 316L stainless steel density cubes built via laser powder bed fusion (L-PBF), emphasizing the significant microstructural changes that occur due to altering the volumetric energy density (VED) via laser power, scan speed, and layer thickness changes, coupled with different starting powder size distributions. This study demonstrates that there is not one ideal process set and powder size distribution for each machine. Instead, there are several combinations or feedstock/process parameter ‘recipes’ to achieve similar goals. This study also establishes that for equivalent VEDs, changing powder size can significantly alter part density, GND density, and hardness. Through proper parameter and feedstock control, part attributes such as density, grain size, texture, dislocation density, hardness, and surface roughness can be customized, thereby creating multiple high-performance regions in the AM process space.

316L stainless steel↗
DOE OSTI2018

Heat capacity of Mg 3 Sb 2 , Mg 3 Bi 2 , and their alloys at high temperature

The thermoelectric figure of merit reported for n-type Mg 3 (Sb,Bi) 2 compounds has made these materials of great engineering significance, increasing the need for accurate evaluations of their thermal conductivity. Thermal conductivity is typically derived from measurements of thermal diffusivity and determination of the specific heat capacity. The uncertainty in this method (often 10% or more) is frequently attributed to measurement of heat capacity such that estimated values are often more accurate. Inconsistencies between reported thermal conductivity of Mg 3 (Sb,Bi) 2 compounds may be attributed to the different values of heat capacity measured or used to calculate thermal conductivity. The high anharmonicity of these materials can lead to significant deviations at high temperatures from the Dulong-Petit heat capacity, which is often a reasonable substitute for measurements at high temperatures. Herein, a physics-based model is used to assess the magnitude of the heat capacity over the entire temperature range up to 800 K. The model agrees in magnitude with experimental low-temperature values and reproduces the linear slope observed in high-temperature data. Owing to the large scatter in experimental values of high-temperature heat capacity, the model is likely more accurate (within ±3%) than a measurement of a new sample even for doped or alloyed materials. It is found that heat capacity for the solid solution series can be simply described (for temperatures: 200K≤T≤800K ) by the polynomial equation: c p [Jg –1 K –1 ]=3NR/M W (1 + 1.3 × 10 –4 T – 4 × 10 3 T –2 ), where 3 NR = 124.71 J mol –1 K –1 , M W is the molecular weight [gmol –1 ] of the formula unit being considered, and T is temperature in K . This heat capacity is recommended to be a standard value for reporting and comparing the thermal conductivity of Mg 3 (Sb,Bi) 2 including doped or alloyed derivatives. Furthermore, a general form of the equation is given which can be used for other material systems

36 MATERIALS SCIENCE↗
DOE OSTI2020

In-situ measurement of irradiation behavior in LiNbO 3

In-situ measurement of LiNbO 3 based surface acoustic wave (SAW) crystal resonator device under irradiation was demonstrated and used to characterize the impact of radiation on physical properties of this material. The resonant frequency of the SAW device was monitored as the output power of the reactor was varied. Upon step increase of the reactor power, a gradual shift in the device’s resonant frequency was observed. This frequency shift initially exhibits a linear growth and eventually reaches an equilibrium value proportional to the reactor power. The observed behavior can be attributed to two competing processes: increase of temperature due to gamma heating or accumulation of irradiation induced defects. In both cases, the response is attributed to changes in the physical properties of LiNbO 3 , particularly the elastic constants. This demonstrated ability to measure materials properties under irradiation is attractive for development of sensors and performing materials science under irradiation.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗