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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Sintering behavior of Lanthana-bearing nanostructured ferritic steel consolidated via spark plasma sintering

Elemental powder mixture of Fe–14Cr–1Ti–0.3Mo–0.5La 2 O 3 (wt%) composition is mechanically alloyed for different milling durations (5, 10 and 20 h) and subsequently consolidated via spark plasma sintering under vacuum at 950 °C for 7 min. The effects of milling time on the densification behavior and density/microhardness are studied. The sintering activation energy is found to be close to that of grain boundary diffusion. The bimodal grain structure created in the milled and sintered material is found to be a result of milling and not of sintering alone. The oxide particle diameter varies between 2 and 70 nm. As a result, faceted precipitates smaller than 10 nm in diameter are found to be mostly La–Ti–Cr-enriched complex oxides that restrict further recrystallization and related phenomena.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2018

Effect of Sintering Temperature on Density and Mechanical Properties of Solid-State Sintered Silicon Carbide Ceramics and Evaluation of Failure Origin

Densified silicon carbide (SiC) is prepared through dry pressing of SiC powder followed by pressureless sintering in the presence of boron carbide and carbon as the additives. Sintering of SiC parts shows the increase in density with the increase in temperature (1950-2180 °C) by resulting in higher than 98% relative density (RD) at 2150 °C and above for 1 h. Sintered specimens are evaluated concerning the phase, microstructure and mechanical properties including hardness and flexural strength. The machined surface of SiC parts with about 98% RD is studied for the origin of failure. The estimated size of critical flaws (32-110 µm) based on flexural strength and fracture toughness indicates that machining defects are one of the primary reasons for failure in SiC ceramics. The order of magnitude of defects on machined surfaces of sintered SiC by SEM studies is found to be comparable with the estimated size of critical flaws. The failure behavior of SiC is discussed with the help of Weibull statistics with respect to the variation of four-point flexural strength.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of heating rate on properties of transparent aluminum oxynitride sintered by spark plasma sintering

High heating rates ranging from 50 to 250°C/min are selected to rapidly sinter transparent aluminum oxynitride (AlON) ceramics by spark plasma sintering (SPS) at 1600°C under 60 MPa using a bimodal AlON powder synthesized by the carbothermal reduction and nitridation method. With 1 minute holding time before cooling, all the specimens show high density and high transparence. The maximum transmittance is up to 74.5%-80.6%, where the maximum transmittance is positively correlated with the heating rate. Further analysis reveals that faster heating rates enable the decrease in the amount of the AlON phase decomposed into the α-Al 2 O 3 and AlN phases during heating. These α-Al 2 O 3 and AlN phases have to be converted back to AlON at the final stage of sintering, which indicates that a decrease in the amount of the α-Al 2 O 3 and AlN phases via the boosted heating leads to the higher transmittance of the AlON ceramics. The high heating rates and short holding duration of the SPS utilized in this study result in the fine grain size of the obtained ceramics (1-6 μm) compared to that of the AlON ceramics fabricated by the conventional sintering method. Finally, this effect of high heating rates is confirmed by the coupled densification-grain growth modeling. In turn, the obtained AlON specimens exhibit a Vickers hardness of 15.87-16.62 GPa.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Selective laser sintering and spark plasma sintering of (Zr,Nb,Ta,Ti,W)C compositionally complex carbide ceramics

Abstract Two advanced manufacturing processes, spark plasma sintering (SPS) and selective laser sintering (SLS), have been developed for synthesis of (Zr,Nb,Ta,Ti,W)C compositionally complex carbide (CCC) via reactive sintering of a powder mixture of constitute monocarbides. X‐ray diffraction analysis confirmed that the single‐phase CCC can be formed by both SPS and SLS. While a homogenous microstructure with uniform metal element distributions was developed during SPS, three‐layer microstructures with a thin TiC‐rich layer and two TaC‐rich layers along with a TiO 2 ‐rich surface layer containing W nanoparticles were formed during SLS. In addition, cellular structures with W, Zr, and Ti element segregation and dislocations on cell boundaries were observed in the SLS‐CCC sample, indicating the effect of nonequilibrium conditions on microstructure formation during laser melting followed by rapid cooling and solidification process. Compared to the SPS‐CCC sample, the SLS‐CCC showed enhanced hardness and reduced thermal conductivity, which may be related to their unique cellular structures.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Fabrication and thermophysical properties of UO 2 -UB 2 and UO 2 -UB 4 composites sintered via spark plasma sintering

Uranium dioxide (UO 2 ) composites with uranium diboride (UB 2 ) and uranium tetraboride (UB 4 ) have been proposed as advanced fuel candidates due to their high thermal conductivity, high melting point, high fissile density and their ability to incorporate a built-in burnable poison by tailoring the targeted 10 B/ 11 B ratio. As such, it is important to assess the fabrication, and thermal and micromechanical properties of such composites. In this work, UO 2 -UB 2 and UO 2 -UB 4 samples with boride phase fractions of 5, 15 and 30 wt% were fabricated to high densities (above 95 % theoretical density) via spark plasma sintering (SPS). This enabled sintering at relatively low temperatures and short timescales. SPS also aided in maintaining the target phase fractions of the samples as reactions between the constituent phases were suppressed due to the short timescales and reducing environment during sintering. Here, thermal diffusivity measurements from 299 to 1273 K were conducted through laser flash analysis (LFA). The diffusivity increased as a function of boride weight fraction, and UB 2 additions increased the thermal diffusivity of the composites more than UB 4 additions. Assessment of the LFA results indicated in-situ reactions between the UO 2 and boride phases that suppress the thermal diffusivity occur above 800 K for all samples. Oxidation of the boride phase was proposed as the underlying reaction. This was supported by thermodynamic assessments from the literature, as well as microstructural, crystallographic, and nanoindentation characterization performed on these samples.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2012

Effect of sintering in ball-milled K{sub 2}Bi{sub 8}Se{sub 13} thermoelectric nano-composites

K{sub 2}Bi{sub 8}Se{sub 13} has many attractive features for thermoelectric applications. Recently, K{sub 2}Bi{sub 8}Se{sub 13}-based nanocomposite materials, consisting of nano-crystalline, micro-crystalline and amorphous phases, have been fabricated based on powder technology techniques. The Seebeck coefficient has been enhanced while the thermal conductivity has been decreased presenting, thus, interesting behavior. The behavior of the materials under heat treatment conditions is now of interest, as the application of sintering process is necessary for the development of thermoelectric modules. In this work, the crystallization of the K{sub 2}Bi{sub 8}Se{sub 13}-based nano-composites is studied using Differential Scanning Calorimetry. The results show that crystallization follows a multiple-step process with different activation energies. The thermoelectric properties are also discussed in the range that crystallization occurs. - Graphical Abstract: {beta}-K{sub 2}Bi{sub 8}Se{sub 13}-based nanocomposites follow a multiple-step crystallization process. Highlights: Black-Right-Pointing-Pointer K{sub 2}Bi{sub 8}Se{sub 13}-based composites consisting of nanocrystalline and amorphous phases. Black-Right-Pointing-Pointer Sintering results multiple-step crystallization with variable activation energies. Black-Right-Pointing-Pointer Thermoelectric properties follow a step-like behavior during sintering. Black-Right-Pointing-Pointer Properties are attributed to the strain relaxation, nucleation and grain growth.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

3D Conformal Printing and Photonic Sintering of High‐Performance Flexible Thermoelectric Films Using 2D Nanoplates

Abstract Flexible thermoelectric (TE) devices hold great promise for energy harvesting and cooling applications, with increasing significance to serve as perpetual power sources for flexible electronics and wearable devices. Despite unique and superior TE properties widely reported in nanocrystals, transforming these nanocrystals into flexible and functional forms remains a major challenge. Herein, demonstrated is a transformative 3D conformal aerosol jet printing and rapid photonic sintering process to print and sinter solution‐processed Bi 2 Te 2.7 Se 0.3 nanoplate inks onto virtually any flexible substrates. Within seconds of photonic sintering, the electrical conductivity of the printed film is dramatically improved from nonconductive to 2.7 × 10 4 S m −1 . The films demonstrate a room temperature power factor of 730 µW m −1 K −2 , which is among the highest values reported in flexible TE films. Additionally, the film shows negligible performance changes after 500 bending cycles. The highly scalable and low‐cost fabrication process paves the way for large‐scale manufacturing of flexible devices using a variety of high‐performing nanoparticle inks.

Saeidi‐Javash, Mortaza↗
DOE OSTI2024

Effects of processing conditions on flash sintering of commercial ZrN

Flash sintering (FS) is a potentially rapid and low-cost manufacturing technique for advanced ceramics. There are still many unknowns about ceramic FS, especially for highly conductive high-temperature ceramics (HTC), which often display decreased bulk conductivity with increasing temperature. This study qualitatively characterizes the flash behavior of highly conductive HTC materials using zirconium nitride (ZrN) as an example. The effects of processing parameters (e.g., DC voltage/electrical field strength, voltage ramp rate, post-flash holding time, mechanical pressure, sample milling, and choice of electrode materials) on ZrN flash behavior are also qualitatively studied and linked to samples microstructure and hardness. It is observed that best densification was achieved using 5 min Spex-milled ZrN powder under 25 MPa of applied pressure and constant 8 V DC. The potential mechanism for ZrN FS and the similarity and difference from FS of conventional oxides like YSZ have been proposed based on experimental observations, and the directions for future research are also pointed out.

Flash Sintering↗
DOE OSTI2019

Thermophysical and mechanical property assessment of UB 2 and UB 4 sintered via spark plasma sintering

Uranium diboride (UB 2 ) and uranium tetraboride (UB 4 ) are candidate constituents for multi-phase accident tolerant fuel due to their anticipated high thermal conductivity. These fuels have high uranium density that contributes to fission, and by tailoring the ratio of 10 B/ 11 B, can also act as an integrated burnable poison. Understanding the thermophysical and mechanical properties of uranium borides, for which only limited data are available in the literature, is of importance to determine their accident tolerance. In this work UB 2 and UB 4 have been synthesized via arc melting and sintered to high densities via spark plasma sintering (SPS). High density samples, >90% theoretical density, were used to measure the thermal diffusivity and thermal expansion of UB 2 and UB 4 and, in conjunction with specific heat literature data, their thermal conductivities were calculated from 298 to 1773 K. Additionally, resonance ultrasound spectroscopy (RUS) and nanoindentation were performed to investigate the mechanical properties of the uranium borides. Our results are discussed in the context of available literature. Both UB 2 and UB 4 exhibit thermal conductivities higher than that of UO 2 , with UB 2 having the highest. The thermal conductivity of UB 2 increases with temperature above 874 K, while for UB 4 there is a linear increase over the entire measured range. X-ray diffraction (XRD) results indicate that impurity phases were present in the fabricated materials, which could explain why literature density functional theory (DFT) results predict higher values. As a result, this suggests that if impurity phases or any microstructural defects can be eliminated then the thermal conductivity can be further increased.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Microstructure and mechanical properties of joints in sintered SiC fiber-bonded ceramics brazed with Ag Cu Ti alloy

Active metal brazing of a new high thermal conductivity sintered SiC-polycrystalline fiber-bonded ceramic (SA-Tyrannohexs) has been carried out using a Ti-containing Ag Cu active braze alloy (Cusil-ABAs). The brazed composite joints were characterized using scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM EDS). The results show that this material can be successfully joined using judiciously selected off-the shelf active braze alloys to yield metallurgically sound joints possessing high integrity. Uniform and continuous joints were obtained irrespective of differences in the fiber orientation in the substrate material. Detailed interfacial microanalysis showed that the titanium reacts with C and Si to form TiC layer and a Ti Si compound, respectively. Furthermore, the evaluation of shear strength of the joints was also conducted at ambient and elevated temperatures in air using the single-lap offset (SLO) shear test. The perpendicular-type SA-Tyrannohex joints exhibited apparent shear strengths of about 42 MPa and 25 MPa at 650 1C and 750 1C, respectively. The fracture at the higher temperature occurred at the interface between the reactionformed TiC layer and braze. This might be caused by generation of stress intensity when a shear stress was applied, according to m-FEA simulation results.

Singh, Mrityunjay↗