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At least 1,639 records · Page 91
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Green micromachining of ceramics using tungsten carbide micro-endmills

This paper presents an experimental analysis on micromachining of green-state silicon carbide (SiC) and aluminum nitride (AlN) using uncoated tungsten carbide (WC) micro-endmills. Although the unique properties of ceramics make them ideal materials for many applications involving micro-scale features, the industrial adaptation of ceramics at the micro-scale has been hindered due to the lack of viable ceramic micro-manufacturing techniques. Green micromachining (GMM), where micro-scale features are created by micromachining green-state ceramics that contain ceramic particles and a polymer binder, offers a potential solution for this problem by affording tremendous improvements in micromachinability with respect to sintered ceramics. After the completion of GMM, the green parts are debound and sintered to obtain the final component with the desired microscale features. In this work, we evaluated GMM characteristics of powder-injection molded green-state SiC and AlN to correlate process conditions and green-material compositions with the micromachining forces, quality, and tool wear. An experimental study that involves full immersion micromilling tests with 254 μm diameter micro-endmills at different cutting speeds and feed rates is conducted. For each of the two materials, two binder states (with and without wax) and two powder states (with micro particles only, or with both micro- and nanoparticles) are investigated. Green micromachining forces, specific energies and the resulting surface roughness are analyzed quantitatively. A qualitative analysis of burr formation and channel quality, as well as a preliminary study on micro-tool wear are also performed. Both the machining conditions and the material compositions are seen to have a strong effect on micro-machinability of green-state ceramics. Overall, we conclude that GMM can be a viable ceramic micro-manufacturing strategy if favorable (or optimal) micromachining and material parameters are identified through the presented (or a similar) micromachinability study.

36 MATERIALS SCIENCE↗
DOE OSTI2025

High temperature tensile properties of H13 tool steel fabricated via binder jet additive manufacturing

H13 is one of the most used steels for hot work tooling applications like die casting. Binder jet additive manufacturing (BJAM) offers a cost-effective solution to fabricate tools with performance enhancing features like internal cooling channels. While full densification of BJAM H13 has been demonstrated in the literature, the room and high temperature tensile properties of the material, and the effect of post-process treatments like hot isostatic pressing (HIP), and heat treatment (HT) are unknown. Here we use HIP+HT to obtain a combination of ultimate tensile strength (UTS) and elongation to failure that is superior to H13 fabricated via laser powder bed fusion and wire arc additive manufacturing. HIP+HT BJAM H13 has a superior UTS (1836 MPa) compared with conventionally processed (1671 MPa) H13 but has lower plastic elongation. We report that as-sintered samples have an extensive network of continuous grain boundary (GB) carbides that contribute to brittle fracture. HIP+HT results in partial dissolution of GB carbides and formation of a fine martensitic microstructure resulting in superior tensile strength and elongation compared with as-sintered H13. We use thermo-kinetic calculations to rationalize the effects of HIP+HT on microstructure and properties.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Short communication on " In-situ TEM ion irradiation investigations on U 3 Si 2 at LWR temperatures"

Here, the radiation-induced amorphization of U 3 Si 2 was investigated by in-situ transmission electron microscopy using 1 MeV Kr ion irradiation. Both arc-melted and sintered U3Si2 specimens were irradiated at room temperature to confirm the similarity in their responses to radiation. The sintered specimens were then irradiated at 350 °C and 550 °C up to 7.2 × 10 15 ions/cm 2 to examine their amorphization behavior under light water reactor (LWR) conditions. U 3 Si 2 remains crystalline under irradiation at LWR temperatures. Oxidation of the material was observed at high irradiation doses.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2017

Radiation damage of hollandite in multiphase ceramic waste forms

Radiation damage was simulated in multiphase titanate-based ceramic waste forms using an ion accelerator to generate high energy alpha particles (He + ) and an ion implanter to generate 7 MeV gold (Au 3+ ) particles. X-ray diffraction and transmission electron microscopy were used to characterize the damaged surfaces and nearby regions. Simulated multiphase ceramic waste forms were prepared using two processing methods: spark plasma sintering and melt-processing. Both processing methods produced ceramics with similar phase assemblages consisting of hollandite-, zirconolite/pyrochlore-, and perovskite- type phases. The measured heavy ion (Au 3+ ) penetration depth was less in spark plasma sintered samples than in melt-processed samples. Furthermore, structural breakdown of the hollandite phase occurred under He + irradiation indicated by the presence of x-ray diffraction peaks belonging to TiO 2 , BaTiO 5 , and other hollandite related phases (Ba 2 Ti 9 O 20 ). The composition of the constituent hollandite phase affected the extent of damage induced by Au 3+ ions.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The defect chemistry of U O 2 ± x from atomistic simulations

Control of the defect chemistry in UO 2±x is important for manipulating nuclear fuel properties and fuel performance. For example, the uranium vacancy concentration is critical for fission gas release and sintering, while all oxygen and uranium defects are known to strongly influence thermal conductivity. Here the point defect concentrations in thermal equilibrium are predicted using defect energies from density functional theory (DFT) and vibrational entropies calculated using empirical potentials. Electrons and holes have been treated in a similar fashion to other charged defects allowing for structural relaxation around the localized electronic defects. Predictions are made for the defect concentrations and non-stoichiometry of UO 2±x as a function of oxygen partial pressure and temperature. If vibrational entropy is omitted, oxygen interstitials are predicted to be the dominant mechanism of excess oxygen accommodation over only a small temperature range (1265 K to 1350 K), in contrast to experimental observation. Conversely, if vibrational entropy is included oxygen interstitials dominate from 1165 K to 1680 K (Busker potential) or from 1275 K to 1630 K (CRG potential). Below these temperature ranges excess oxygen is predicted to be accommodated by uranium vacancies, while above them the system is hypo-stoichiometric with oxygen deficiency accommodated by oxygen vacancies. Our results are discussed in the context of oxygen clustering, formation of U 4 O 9 , and issues for fuel behavior. In particular, the variation of the uranium vacancy concentrations as a function of temperature and oxygen partial pressure will underpin future studies into fission gas diffusivity and broaden the understanding of UO 2±x sintering.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2018

Glass-bonded iodosodalite waste form for immobilization of 129I

Immobilization of radioiodine (e.g., 129I, 131I) is an important need for current and future nuclear fuel cycles. For the current work, iodosodalite [Na8(AlSiO4)6I2] was synthesized hydrothermally from metakaolin, NaI, and NaOH. Following hydrothermal treatment, dried unwashed powders were used to make glass-bonded iodosodalite waste forms by heating pressed pellets at 650, 750, or 850 °C with two different types of sodium borosilicate glass binders, i.e., NBS-4 and SA-800. These heat-treated specimens were characterized with X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, thermal analysis, porosity and density measurements, neutron activation analysis, and inductively-coupled plasma mass spectrometry. The pellets mixed with 10 mass% of NBS-4 or SA-800 and heat-treated at 750 °C contained relatively high percentage iodine retention (~44-47 % of the maximum iodine loading) with relatively low porosities, while other pellets with higher percentages iodine retention either contained higher porosity or were not completely sintered. ASTM C1308 chemical durability tests of monolithic specimens showed a large initial release of Na, Al, Si, and I on the first day, possibly from water-soluble salt crystals or non-durable amorphous phases. Release rates of Na and Si were higher than for Al and I, probably due to a poorly durable Na-Si-O phase from the glass bonding matrix. The cumulative normalized release of iodine was 12.5 g m-2 for the first 10 1-d exchanges, suggestive of coherent dissolution. The average release rate from 10-24 days during the 7-d exchange intervals was 0.2336 g m-2 d-1.

sodalite, iodosodalite, waste forms, iodine↗
DOE OSTI2020

Additive manufacturing of silicon carbide for nuclear applications

Additive manufacturing (AM) is a rapidly evolving technology being considered for nuclear applications. A special focus on AM to fabricate nuclear-grade silicon carbide (SiC) is explored in this paper. First, we present currently available AM processing options for SiC. AM methods commonly used for other ceramics, in which the feedstocks are forms of polymers, powders, and/or reactive chemical vapors, are also applicable to SiC. SiC phases are formed by pyrolysis of pre-ceramic polymer, direct reaction of powder precursors, sintering of SiC powders, or chemical vapor deposition/infiltration. Second, we discuss how the different microstructures of SiC materials fabricated by various processing methods affect their behavior in nuclear environments. Third, we discuss state-of-the-art AM technologies for the fabrication of relatively pure SiC, which show great potential to retain its strength under neutron irradiation: (1) binder jet printing followed by chemical vapor infiltration, (2) laser chemical vapor deposition, and (3) selective laser sintering of SiC powders.

36 MATERIALS SCIENCE↗
DOE OSTI2025

UO2 microstructural evolutions induced by Ni, Mo, and W dopants for intentional forensics

The concept of tagging nuclear fuel with a chemical barcode to enable forensics analysis across the nuclear fuel cycle is an area of active investigation, particularly to ensure fabrication viability without disrupting current fuel performance. This study explored the feasibility of using Ni, Mo, and W isotopic double-spikes as dopants in UO2 fuel from the perspective of fuel fabrication. Doped UO2 pellets were produced using conventional fuel fabrication processes, including powder mixing, sieving, pressing, and sintering in a reductive atmosphere. Two composition levels, 100 and 1000 ppm, were evaluated for each dopant element with isotopic double-spike configurations. For the Ni system, additional dopant concentrations of 250 and 500 ppm were produced with nonperturbed isotopic ratios. The results demonstrated that successful incorporation of Ni, Mo, and W double-spikes into UO2 pellets occurred with minimal shift in final density or dopant loss during pellet fabrication. Isotopic analysis confirmed the presence of the double-spike signature even when diluted with natural isotopic material in ratio of 1:5 in the fabrication process. Microstructural examinations revealed different impacts on grain size compared with undoped UO2. This study showed that Ni incorporation up to ∼500 ppm promoted moderate grain growth, whereas the Mo and W systems caused grain size reduction at all concentrations. Changes in the UO2 lattice parameter as a function of composition were detected exclusively for Ni up to 500 ppm, indicating that the Ni solid solution was the main factor for the observed grain growth. Insoluble (Mo and W) or supersaturated (Ni > 500 ppm) conditions produced grain size reduction. The Ni-doped pellets in the solution range resulted in a final microstructure within fuel specifications, demonstrating its potential benefits of employing complex dopant systems for potential nuclear forensic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2026

The influence of iron incorporation on the microstructure and crystal chemistry of UO 2 fuel

The incorporation of dopants into UO 2 is of interest for both doped advanced fuel and nuclear forensics research. Suitable forensics dopants, or taggants, for nuclear fuels are ones that do not adversely affect the performance of the parent material while still being traceable during key stages of the fuel’s life cycle. In this paper, we present our efforts to understand the effects of doping UO 2 with Fe 2 O 3 under multiple parameters, such as Fe concentration (0–3000 wppm), sintering temperature (1673 K, 1773 K, and 1973 K), sintering time (2–12 h), and atmosphere (reducing and inert). Here, we investigated densification behavior, microstructure evolution, and lattice defects based on the geometric densities, scanning electron microscopy (SEM), X-ray diffractograms, and Raman spectra. The effect of Fe on the microstructure and crystal chemistry of UO 2 is very complex, especially at the intermediate temperature (1773 K), where we found that densification kinetics are enhanced and that lattice contraction occurs for doping concentrations greater than 1000 wppm Fe.

Crystallography↗
DOE OSTI2019

A high-performance reversible protonic ceramic electrochemical cell based on a novel Sm-doped BaCe 0.7 Zr 0.1 Y 0.2 O 3-δ electrolyte

The present work describes a doping strategy to improve the electrochemical performance of BaCe(Zr)O 3 -based reversible protonic ceramic electrochemical cells (RePCECs). BaCe 0.7 Zr 0.1 Y 0.2-x Sm x O 3-δ (0 ≤ x 0 ≤.2) synthesized by a solid-state reactive sintering method. Enhanced sintering with Sm-doping is observed with a significant increase of grain size as compared to un-doped samples. The improvement of the proton conductivity by proper Sm-doping is also achieved. This work fabricates and tests an electrochemical cell with a 25 μm thick electrolyte in both protonic ceramic fuel cell (PCFC) and protonic ceramic electrolysis cell (PCEC) operation modes. At the intermediate temperature of 600°C, a maximum power density of 410 mWcm -2 is observed in PCFC mode under Air/H2 gradient, while a current density of 370 mAcm -2 is obtained with an applied voltage of 1.3 V in PCEC mode using 12 vol% H 2 O humidified air as feed gas and 5% H 2 as sweep gas

08 HYDROGEN↗
DOE OSTI2025

Etching-assisted upcycling of Ni-lean to Ni-rich cathode materials

Upcycling is recognized as a sustainable recycling approach for spent lithium-ion batteries. However, existing upcycling methods typically involve intricate pretreatment or post-treatment steps, complicating their practical application. Here, in this study, we propose a straightforward, etching-assisted upcycling method that effectively transforms polycrystalline Ni-lean cathodes into high-performance single crystal Ni-rich cathodes. During the etching step, nickel acetate was dissolved into acetic acid and then polycrystalline NMC111 are etched in the solution. Finally, polycrystalline NMC111 are converted into single crystal particles coated with amorphous nickel acetate. This significantly enhances elemental diffusion during subsequent sintering by minimizing both particle size and the contact distance between NMC111 and nickel acetate. As elemental diffusion is improved and acetate ions decompose completely during sintering, the process requires neither additional pretreatment nor post-treatment. The resulting cathode materials (Etched-UP622) exhibit superior structural and electrochemical properties compared to the Control622, achieving an energy density of 719.7 Wh/kg, approximately 56.7 mAh/g higher than Control622 and 125.5 Wh/kg higher than NMC111. Etched-UP622 also delivers higher discharge capacity, improved rate performance and cycling stability, surpassing Control622 and NMC111. Meanwhile, NMC811 also can be synthesized by the proposed strategy, and the discharge capacity can reach 166.9 mAh/g at 1C, similar to 14 mAh/g higher than Control811. Overall, this etching-assisted strategy simplifies the upcycling process and offers a scalable, sustainable route for producing high-quality cathode materials.

Acid etching↗
DOE OSTI2026

The structure, composition, and performance impact of a YSZ-GDC interdiffusion layer in solid oxide electrolysis cells

This study provides a combined experimental and computational investigation into the structure and impact of the cation interdiffusion layer that appears at the gadolinium doped ceria (GDC)/yttria stabilized zirconia (YSZ) interface in solid oxide electrolysis cells (SOECs). Scanning transmission electron microscopy (STEM) illustrates that a ∼0.4 μm interdiffusion layer (IDL) with an intermixed cation distribution and fine grain size forms upon sintering. STEM identifies that the interdiffusion layer exists in the cubic fluorite structure despite changes in cation composition. The interdiffusion layer microstructure formed during sintering does not change during SOEC testing at either 1.3V or heightened voltage pulse testing. Modeling predicts that ionic conductivity may decrease in the interdiffusion layer due to Coulombic trapping between mobile oxygen vacancies and excess Gd 3+ acceptor dopants. Yet, the density and continuous nature of the layer should benefit cell stability by substantially reducing the formation of SrZrO 3 , which is corroborated by STEM and Synchrotron X-ray diffraction (XRD). We conclude that the interdiffusion layer acts as a beneficial barrier to Sr diffusion, when operating in a regime where electrolyte void formation is not observed.

organic↗
DOE OSTI2013

Structural studies of TiC{sub 1−x}O{sub x} solid solution by Rietveld refinement and first-principles calculations

The lattice parameters, structural stability and electronic structure of titanium oxycarbides (TiC{sub 1−x}O{sub x}, 0≤x≤1) solid solution were investigated by Rietveld refinement and first-principles calculations. Series of TiC{sub 1−x}O{sub x} were precisely synthesized by sintering process under the vacuum. Rietveld refinement results of XRD patterns show the properties of continuous solid solution in TiC{sub 1−x}O{sub x} over the whole composition range. The lattice parameters vary from 0.4324 nm to 0.4194 nm decreasing with increasing oxygen concentration. Results of first-principles calculations reveal that the disorder C/O structure is stable than the order C/O structure. Further investigations of the vacancy in Ti{sub 1−Va}(C{sub 1−x}O{sub x}){sub 1−Va} solid solution present that the structure of vacancy segregated in TiO-part is more stable than the disorder C/O structure, which can be ascribed to the Ti–Ti bond across O-vacancy and the charge redistributed around Ti-vacancy via the analysis of the electron density difference plots and PDOS. - Graphical abstract: XRD of series of titanium oxycarbides (TiC{sub 1−x}O{sub x}, 0≤x≤1) solid solution prepared by adjusting the proportion of TiO in the starting material. Highlights: • Titanium oxycarbides were obtained by sintering TiO and TiC under carefully controlled conditions. • Rietveld refinement results show continuous solid solution with FCC structure in TiC{sub 1−x}O{sub x}. • The disorder C/O structure is stable than the order C/O structure. • Introduction of vacancy segregated in TiO-part is more stable than disorder C/O structure. • Ti–Ti bond across O-vacancy and the charge redistributed around Ti-vacancy enhance structural stability.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Mg dopant in Cu{sub 2}ZnSnSe{sub 4}: An n-type former and a promoter of electrical mobility up to 120 cm{sup 2} V{sup −1} s{sup −1}

Mg-doped Cu{sub 2}ZnSnSe{sub 4} (CZTSe) bulk materials with the (Cu{sub 2−x}Mg{sub x})ZnSnSe{sub 4} formula at x=0, 0.1, 0.2, 0.3, and 0.4 were prepared at 600 °C for 2 h with soluble sintering aids of Sb{sub 2}S{sub 3} and Te. Defect chemistry was studied by measuring structural and electrical properties of Mg-doped CZTSe as a function of dopant concentration. Except at x=0, all Mg-doped CZTSe pellets showed an n-type behavior. The Mg-doped CZTSe pellets showed an n-type behavior. n-Type Mg-CZTSe pellets at x=0.1 showed the highest electrical conductivity of 24.6 S cm{sup −1} and the net hole mobility of 120 cm{sup 2} V{sup −1} s{sup −1}, while they were 11.8 S cm{sup −1} and 36.5 cm{sup 2} V{sup −1} s{sup −1} for the undoped p-type CZTSe. Mg dopant is a strong promoter of electrical mobility. Mg dopant behaves as a donor defect in CZTSe at a 5% doping content, but is also used as an acceptor at a high content above 5%. Mg doping has further developed CZTSe into a promising semiconductor. - Graphical abstract: The effects of extrinsic doping of Mg{sup 2+} on the electrical properties of Cu{sub 2}ZnSnSe{sub 4} bulks. - Highlights: • (Cu{sub 2−x}Mg{sub x})ZnSnSe{sub 4} bulks were fabricated by liquid-phase sintering at 600 °C. • All Mg-x-CZTSe pellets except at x=0 exhibited n-type conductivity. • Electrical properties of CZTSe pellets changed with the Cu and Mg ratios. • Mg{sup 2+} goes to the Cu{sup 1+} site to form the Mg{sub Cu}{sup 1+} donor defect for the n-type CZTSe. • n-Type Mg-0.1-CZTSe bulk with 5% Mg showed the highest mobility of 120 cm{sup 2} V{sup −1} s{sup −1}.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Material properties of perovskites in the quasi-ternary system LaFeO{sub 3}–LaCoO{sub 3}–LaNiO{sub 3}

An overview is presented on the variation of electrical conductivity, oxygen permeation, and thermal expansion coefficient as a function of the composition of perovskites in the quasi-ternary system LaFeO{sub 3}–LaCoO{sub 3}–LaNiO{sub 3}. Powders of thirteen nominal perovskite compositions were synthesized under identical conditions by the Pechini method. The powder X-ray diffraction data of two series, namely La(Ni{sub 0.5}Fe{sub 0.5}){sub 1−x}Co{sub x}O{sub 3} and LaNi{sub 0.5-x}Fe{sub x}Co{sub 0.5}O{sub 3}, are presented after the powders had been sintered at 1100 °C for 6 h in air. The measurements revealed a rhombohedral structure for all compositions except LaNi{sub 0.5}Fe{sub 0.5}O{sub 3} for which 60% rhombohedral and 40% orthorhombic phase was found. The maximum DC electrical conductivity value of the perovskites at 800 °C was 1229 S cm{sup −1} for the composition LaCoO{sub 3} and the minimum was 91 S cm{sup −1} for the composition LaCo{sub 0.5}Fe{sub 0.5}O{sub 3}. The oxygen permeation of samples with promising conductivities at 800 °C was one order of magnitude lower than that of La{sub 0.6}Sr{sub 0.4}Co{sub 0.8}Fe{sub 0.2}O{sub 3} (LSCF). The highest value of 0.017 ml cm{sup −2} min{sup −1} at 950 °C was obtained with LaNi{sub 0.5}Co{sub 0.5}O{sub 3}. The coefficients of thermal expansion varied in the range of 13.2×10{sup −6} K{sup −1} and 21.9×10{sup −6} K{sup −1} for LaNi{sub 0.5}Fe{sub 0.5}O{sub 3} and LaCoO{sub 3}, respectively. {sup 57}Fe Mössbauer spectroscopy was used as probe for the oxidation states, local environment and magnetic properties of iron ions as a function of chemical composition. The substitution had a great influence on the chemical properties of the materials. - Graphical abstract: Compositional dependence of isothermal electrical conductivity at 800 °C and thermal expansion coefficient in the series La(Fe{sub 0.5}Ni{sub 0.5}){sub (1−x)}Co{sub x}O{sub 3} and LaNi{sub 0.5−x}Fe{sub x}Co{sub 0.5}O{sub 3}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗