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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 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Preparation of Nb{sub 3}Al Superconducting Tapes by a Powder-in-Tube Method Combined with Hot-Pressed Sintering

The precursors of Nb-Al tape were fabricated by a powder-in-tube (PIT) technique. Supersaturated solid solutions of Nb-Al powder were obtained by high-energy ball milling, followed by a powder-in-tube process to prepare series of Nb{sub 3}Al precursor tapes. Compared with sintering under normal pressure, the hot-pressing sintering greatly increased the critical current density of the tapes. The results showed that the Nb{sub 3}Al tapes prepared via the powder-in-tube method and hot-pressed sintering could significantly improve the J{sub c} performance, and the value of J{sub c} (8 K, 0 T) was higher than 6 × 10{sup 5} A/cm{sup 2}. The Nb{sub 3}Al tapes after pressing under the pressure of 20 MPa and sintering at 950 {sup ∘}C for 3 h had the best magnetic flux pinning performance, and the sample reached the maximum flux pinning force (F{sub p,max}) of 3.28109 N/m{sup 3} in the magnetic field of 2.2 T.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Central Band Structures: New Insights into the Coupling Effects Between the Pores and Minerals of Sinter

It is important to elucidate the coupling effects that exist between the pores and minerals of sinter to establish the relationships that exist among the technological parameters, microstructures, and performance of the sinter. Based on the formation mechanism of the pores within sinter, four types of central band structures (CBSs) were defined to represent the microstructures of the sinter. During reduction, the cracking behavior of each CBS was studied through a series of experiments conducted at a temperature of 873 K.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Neutron Diffraction and Neutron Radiography Investigation into Powder Sintering of Ti/Al and TiH{sub 2}/Al Compacts

Sintering behaviors of two types of powder compact, i.e., Ti/Al and TiH{sub 2}/Al, under vacuum were studied using in situ neutron diffraction, in situ thermal dilatometric measurement, and ex situ neutron radiography. Their densification, microstructure, hydrogen concentration, and phase transformation features were compared. The results indicate that the predominant phase identified in the samples sintered from both compacts is γ-TiAl phase with minor Ti{sub 3}Al phase. Dehydrogenation of TiH{sub 2} plays a decisive role in the sintering behavior and the resultant microstructure. In comparison with the Ti/Al compact, dehydrogenation occurring in the TiH{sub 2}/Al compact leads to a higher level of densification and faster sintering kinetics.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of SiO{sub 2} on the Formation of Acicular Calcium Ferrite in Sinter

SiO{sub 2} greatly influences the formation of acicular calcium ferrite (ACF), which is the main binder phase in sinter, but the mechanism of its influence is not yet clear. Experiments were carried out under air at 1200 °C with different amounts of SiO{sub 2} mixed with Fe{sub 2}O{sub 3} and Ca(OH){sub 2}. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and three-dimensional (3D) reconstruction were used to characterize the phase and morphological changes of the sintered samples. The results show that SiO{sub 2} can form silicon-ferrite of calcium (SFC), which is beneficial to the formation of ACF in sinter. The SFC content first increased and then decreased with the increasing SiO{sub 2} content. The sintered sample contained the highest contents of SFC and ACF when w(SiO{sub 2}) was 3 to 4 pct. The morphological changes in calcium ferrite observed with the increasing SiO{sub 2} content followed a sequence of massive → platy → acicular; both platy calcium ferrite and ACF were the longest in one direction in their 3D morphology. The inclusion of Si{sup 4+} in calcium ferrite crystals changes the crystal growth habit from having no obvious orientation to having a preferential orientation along the (040) or (320) surfaces, as determined by the first-principles calculations and XRD analysis of block samples. Viscosity was the main environmental factor influencing the size of ACF in samples with sufficient liquid phase.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Influence of Particle Size on Apparent Diffusivity During Spark Plasma Sintering of Crystalline Powders

Abstract:: A theoretical framework is presented to determine an apparent diffusivity D during powder sintering and further elucidate the underlying interrelation between D and shrinkage behaviors of powders. Furthermore, to eliminate the influence of crystalline defects that are typically present in raw powders, complete crystallization of metallic glass powders is performed to allow the investigation of the influence of particle size solely on powder densification. Furthermore, to validate the framework developed, Ti{sub 40.6}Zr{sub 9.4}Cu{sub 37.5}Ni{sub 9.4}Sn{sub 3.1} crystalline alloy powders constituting particles with different sizes are examined to establish a correlation between D and the sintering behaviors of the powders. The findings show that the value of the apparent diffusivity D increases with the increasing particle size, which accelerates powder densification during spark plasma sintering. Furthermore, the results quantitatively support the argument that particle size can affect atomic diffusion during powder sintering.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of Gold on the Microstructural Evolution and Integrity of a Sintered Silver Joint

There is a need for next-generation, high-performance power electronic packages and systems employing wide band gap devices to operate at high temperatures in automotive and electric grid applications. Sintered silver joints are currently being evaluated as an alternative to Pb-free solder joints. Of particular interest is the development of joints based on silver paste consisting of nanoscale or micron scale particles that can be processed without the application of an external pressure. Microstructural evolution at the interface of a pressureless sintered silver joint formed between a SiC die with a Ti/Ni/Au metallization and an Active Metal Brazed substrate with Ag metallization at 250 °C was evaluated using Scanning Electron Microscopy, X-ray microanalysis, and X-ray Photo Electron Spectroscopy. Results from Focused Ion Beam cross-sections show that during sintering, the pores in the sintered region close to the Au layer tend to be smaller and are oriented predominantly with their longer dimension oriented parallel to the interface. With further densification, this results in the alignment of small pores parallel to the interface, creating a path for easy crack propagation. Lastly, X-ray microchemical analyses results confirm interdiffusion between Au and Ag and that a region with poor mechanical strength is formed at the edge of this region of interdiffusion.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effects of Sintering Temperature on the Porosity and Mechanical Behavior of Porous Titanium Scaffolds Prepared by Freeze-Casting

The purpose of this study was to investigate the effects of sintering temperature on the microstructure, and associated mechanical properties of porous titanium scaffolds prepared by freeze-casting. Sintering temperature significantly influenced the porosity and the mechanical properties of the titanium scaffolds. The porosity decreased from 60 to 20% as the sintering temperatures increased from 800 to 1100 °C. The scaffolds were of an open pore structure with pore sizes ranging from 2 to 20 μm. The average wall thickness of the porous titanium was measured, using a micro-computed tomography, to be in the range from 9.2 to 26.2 μm. Elastic modulus was between 2 to 7 GPa, depending on vacuum annealing conditions. The compression strength of the scaffolds may exceed 1000 MPa as the sintering temperature higher than 1000 °C. The increase in titanium strength and stuffiness could be mainly attributed to the decrease in porosity as well as the increase in wall thickness.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Functionally Graded W-Cu Materials Prepared from Cu-Coated W Powders by Microwave Sintering

Three-layered W-Cu (W-10Cu/W-20Cu/W-30Cu, weight percentage) functionally graded materials were fabricated by laminate pressing of Cu-coated W composite powders with different Cu contents (10-30 wt.% Cu) and W particle sizes in the powders and microwave sintering of the powder compacts. The microstructure and the physical and mechanical properties of the graded materials were investigated. The experimental results show that the relative density of the graded W-Cu samples sintered at 1250 °C for 20 min is above 97%. Cu has a gradient layer distribution in the thickness direction, and there is good combination between the layers in the graded samples. Furthermore, the graded W-Cu materials exhibit good physical and mechanical properties. The sintered samples have thermal conductivities above 210 W m K{sup −1}, and the maximum bending strength and hardness reach 1004 MPa and 346 HV, respectively. The present results show that functionally graded W-Cu materials with high density, gradient Cu and W distribution, and good properties can be successfully prepared by laminate pressing and microwave sintering.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Spark plasma sintering (SPS) densified U 3 Si 2 pellets: Microstructure control and enhanced mechanical and oxidation properties

Dense U 3 Si 2 pellets with controlled grain structure and enhanced thermal-mechanical and oxidation properties are synthesized with spark plasma sintering (SPS). Microstructure and phase composition of the SPS densified pellets are characterized systematically using SEM, EDS, and XRD. Thermal-mechanical properties and oxidation behavior of the sintered silicide fuel pellets are analyzed by laser flash, indentation, and dynamic thermogravimetric analysis. Dense U 3 Si 2 pellets are consolidated by combining high energy ball milling and rapid sintering by SPS, and the microstructure structures are controlled from micron-sized (~5.7 μm grain size) for conventional silicide to a nanocrystalline matrix with an average grain size of ~280 nm. A dominant phase of distorted U 3 Si 2 was identified with lattice expansion due to residual thermal stress upon SPS consolidation and rapid cooling processes. Both micron-sized and nano-sized pellets show exceptional thermal transport properties, consistent with monolithic silicides reported in literature. The SPS-densified pellets possess simultaneously high hardness and fracture toughness. The SPS-densified silicide pellets also demonstrate exceptional oxidation performance with extended onset oxidation temperature above 500 °C and reduced oxidation kinetics, particularly for nano-sized pellets. A strong strain effect was proposed in which compressive stress in nano-sized pellets enhances the oxidation resistance of silicide fuels, as evidenced by the degradation of oxidation performance upon strain relaxation by isothermal annealing. The correlation among the sintering process – microstructure control – physical properties and fuel behavior is established. A new concept of strain engineering is proposed further properties optimization, enabling the development of potential oxidation and corrosion-resistant silicides with extended performance, the key technological challenge of U 3 Si 2 as the leading concept of accident tolerant fuels.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

Magnetic and mechanical properties of grain-refined Dy-free Nd-Fe-B sintered magnets

In this work, the effects of grain size on magnetic and mechanical properties of Dy-free Nd-Fe-B sintered magnets were studied. The grain size of the sintered magnets was varied by using different sized Dy-free Nd-Fe-B alloy feedstock powders which were ball milled to different extents. Increasing the ball milling time from 7 to 11 h caused the average particle size of the powders to decrease from 3.6 to 2.1 µm, while the average grain size of the corresponding sintered magnets decreased from 5.0 to 4.0 µm. The intrinsic coercivity Hcj and maximum energy product (BH) max of these magnets increased from 11.6 to 13.7 kOe and 41.6 to 43.6 MGOe, respectively. The finer grain magnets exhibited better thermal stability, as indicated by a smaller temperature coefficient of H cj (-0.71%/°C). With extending ball milling time to 15 h, the magnetic properties of the magnets deteriorate, while the flexural strengths first decrease and then increase. The flexural strength improvement is attributed to the increased Oxygen content in the magnets due to finer grains. The coercivity enhancement by reduction of grain size is a promising approach to develop Dy-free Nd-Fe-B sintered magnets with better thermal stability. It is important to control the Oxygen contents in the magnets to compromise both magnetic and mechanical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Thermoelectric transport properties of polycrystalline titanium diselenide co-intercalated with nickel and titanium using spark plasma sintering

Polycrystalline samples of nickel intercalated (0-5%) TiSe{sub 2} were attempted via solid-state reaction in evacuated quartz tubes followed by densification using a spark plasma sintering process. X-ray diffraction data indicated that mixed NiSe{sub 2} and TiSe{sub 2} phases were present after initial synthesis by solid-state reaction, but a pure TiSe{sub 2} phase was present after the spark plasma sintering. While EPMA data reveals the stoichiometry to be near 1:1.8 (Ti:Se) for all samples, comparisons of the measured bulk densities to the theoretical densities suggest that the off stoichiometry is a result of the co-intercalation of both Ni and Ti rather than Se vacancies. Due to the presence of excess Ti (0.085-0.130 per formula) in the van der Waals gap of all the samples, the sensitive electron-hole balance is offset by the additional Ti-3d electrons, leading to an increase in the thermopower (n-type) over pristine, stoichiometric TiSe{sub 2}. The effects of the co-intercalation of both Ni and Ti in TiSe{sub 2} on the structural, thermal, and electrical properties are discussed herein. - Graphical abstract: Co-intercalation of nickel and excess titanium into the van der Waals gap of TiSe{sub 2} via solid state synthesis followed by spark plasma sintering results in a systematic shift in the ratio of hole and electron carrier concentration, which is close to unity for pristine TiSe{sub 2}. This directly affects the electrical transport properties, and as the structural disorder induced by intercalation suppresses the lattice thermal conductivity, co-intercalation is an effective route to enhance the thermoelectric properties of transition metal diselenides. Highlights: Black-Right-Pointing-Pointer Single phase bulk Ni and Ti co-intercalated TiSe{sub 2} samples prepared by spark plasma sintering. Black-Right-Pointing-Pointer Density and X-ray diffraction suggest that the Ni and excess Ti are ordered in the Van der Waals gap. Black-Right-Pointing-Pointer Co-intercalation of Ni and Ti can be used to control electron-hole ratio and structural disorder.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Effects of sintering temperature and duration on the structural and electrical properties of CuBiS{sub 2} bulks

CuBiS{sub 2} bulks were prepared by reactive sintering the mixture of Cu{sub 2}S and Bi{sub 2}S{sub 3} at 300, 350, 400, and 450 °C for 2 h and at the sintering temperature of 400 °C for 1, 1.5, 2, 2.5, and 3 h under a compensation disc of CuS for atmospheric control. Composition, structure, morphology, and electrical properties of the sintered bulks were analyzed. The compositions of Cu, Bi, and S did not change until the temperature reached at 450 °C.The highest electrical conductivity of 4.3 S cm{sup −1} and the highest Hall mobility of 11.1 cm{sup 2} V{sup −1} s{sup −1} were obtained for CuBiS{sub 2} sintered at 400 °C for 2 h. The deviation in the S/(Cu+Bi) ration caused the degradation of electrical properties, though the CuBiS{sub 2} remained as a single phase. Therefore, CuBiS{sub 2}, a less studied ternary copper based sulfide, is the p-type semiconductor for potential energy-related application and needs to have a good control in composition. - Graphical abstract: CuBiS{sub 2}, a less studied ternary copper based sulfide, is the p-type semiconductor and needs to have a good control in composition to achieve the optimal electrical properties. - Highlights: • CuBiS{sub 2}, a less studied copper-based sulfide, is a p-type semiconductor. • Electrical properties of CuBiS{sub 2} are important for its photovoltaic applications. • Deviation in composition leads a change in the electrical properties of CuBiS{sub 2}. • n of 2.4×10{sup 18} cm{sup −3}, μ of 11.1 cm{sup 2} V{sup −1} s{sup −1}, and σ of 4.3 S cm{sup −1} were obtained. • The process control in CuSbS{sub 2} is critical to achieve consistent performance.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Microwave sintering of MoSi{sub 2}-Mo{sub 5}Si{sub 3} to promote a final nanometer-scale microstructure and suppressing of pesting phenomenon

This work shows an innovative sintering process for molybdenum disilicide (MoSi{sub 2}) and molybdenum silicide (Mo{sub 5}Si{sub 3}) in order to have a final nanometer-scale microstructure. Sintered MoSi{sub 2}-Mo{sub 5}Si{sub 3} was fabricated by microwave sintering. The final material showed a significant reduction of pesting phenomena for MoSi{sub 2}. The final microstructure is a mix of micrometer-scale MoSi{sub 2} and nanometer-scale Mo{sub 5}Si{sub 3} immersed in an amorphous SiO{sub 2} phase. This material was characterized by X-Ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Thermal Gravimetric Analysis (TGA). - Highlights: Black-Right-Pointing-Pointer We sintered the MoSi{sub 2}-Mo{sub 5}Si{sub 3} by microwave and obtained high final density material. Black-Right-Pointing-Pointer We got a final nanostructured material. Black-Right-Pointing-Pointer We inhibited pesting phenomenon for MoSi{sub 2}.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Enhancement of thermal shock resistance of reaction sintered mullite–zirconia composites in the presence of lanthanum oxide

Mullite–zirconia composites containing 20 wt.% zirconia were prepared by reaction sintering of zircon flour, sillimanite beach sand and calcined alumina. 0 to 8 mol% of La{sub 2}O{sub 3} with respect to zirconia was used as sintering aid. The effect of additive on the various physical, microstructures, mechanical and thermo-mechanical properties was studied. Quantitative phase analysis shows the change in tetragonal zirconia content with incorporation of lanthanum oxide. La{sub 2}O{sub 3} addition has significantly improved the thermal shock resistance of the samples. Samples without additive retained only 20% of initial flexural strength after 5 cycles, whereas samples containing 5 mol% La{sub 2}O{sub 3} retained almost 78% of its initial flexural strength even after 15 thermal shock cycles. - Highlights: • Mullite–zirconia composites were prepared by reaction sintering route utilizing zircon and sillimanite beach sand. • Lanthanum oxide was used as sintering aid. • The presence of lanthanum oxide decreased the densification temperature. • Lanthanum oxide significantly improved the thermal shock resistance of the composites.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Densification, microstructure and properties of TiB2 ceramics fabricated by spark plasma sintering

Highlights: • Nearly full densification of in-house processed TiB{sub 2} ceramic powders at 1500 °C. • Microstructures revealing the size and shape of TiB{sub 2} grains after SPS. • The effect of sub-micron particles on the densification during SPS. • The effect of sintering pressure on the microstructure. • Microhardness and nano-indentation measurements along with oxidation studies. - Abstract: This study reports the fabrication of high-density TiB{sub 2} ceramics at reduced temperatures (1500 °C) by using field assisted sintering technology/spark plasma sintering (FAST/SPS) technique. In-house processed TiB{sub 2} powders with high purity were employed as starting materials. The effects of SPS conditions on the sintering behavior and microstructure of TiB{sub 2} were investigated. Sub-micron character of TiB{sub 2} particles has a strong influence on the densification during SPS. Microstructural analyses revealed that the high pressure affected both the grain size and the shape of TiB{sub 2} crystallites. Elongated grains with average size of 6 μm were perceived in TiB{sub 2} sample fabricated by SPS at 1500 °C under 60 MPa pressure with a relative density of 96.7%. The microhardness and elastic modulus were measured as 27.4 GPa and 685.2 GPa, respectively. This sample revealed an improved oxidation resistance due to the formation of a thin oxidation layer on the surface when heated up to 1200 °C.

36 MATERIALS SCIENCE↗
DOE OSTI2011

One component metal sintering additive for {beta}-SiC based on thermodynamic calculation and experimental observations

Graphical abstract: . Standard Gibbs formation free energies vs. temperature for various metal carbides. The heavy line represents the standard Gibbs free energy for {beta}-SiC. The hatched area denotes the typical liquid phase hot pressing temperature of {beta}-SiC (1973-2123 K). Highlights: {yields} Various metal elements were examined as a sintering additive for {beta}-SiC. {yields} Al and Mg enhanced the density significantly without decomposing {beta}-SiC, as predicted by thermodynamic simulation. {yields} Cr, Fe, Ta, Ti, V and W additives formed metal carbide and/or silicide compounds by decomposing {beta}-SiC. {yields} This approach would be useful for selecting effective sintering additive for high temperature ceramics. -- Abstract: Various types of metals were examined as sintering additives for {beta}-SiC by considering the standard Gibbs formation free energy and vapor pressure under hot pressing conditions (1973-2123 K), particularly for applications in nuclear reactors. Metallic elements having the low long-term activation under neutron irradiation condition, such as Cr, Fe, Ta, Ti, V and W, as well as widely used elements, Al, Mg and B, were considered. The conclusions drawn from thermodynamic considerations were compared with the experimental observations. Al and Mg were found to be effective sintering additives, whereas the others were not due to the formation of metal carbides or silicides from the decomposition of SiC under hot pressing conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Influence of sintering conditions on the microstructure and optical properties of Eu: CaF{sub 2} transparent ceramic

Highlights: • Eu: CaF2 ceramics sintering at different conditions were fabricated by HP method. • The results show that the temperatures have great effect on the microstructure. • The ceramic sintered at 600 °C/30 MPa has high transmittance and pore-free structure. • The diffusion and densification process mainly occurs between 350 °C and 500 °C. • The optical losses in ceramic were caused by structure defects and polishing damage. - Abstract: Nanopowders of the 5 at.% Eu doped CaF{sub 2} have been synthesized by co-precipitated method. X-ray diffraction (XRD) indicated that the samples corresponds to the cubic CaF{sub 2} phase and scanning electron microscopy (SEM) showed that the morphology of nanopowders were approximately spherical. The Eu: CaF{sub 2} transparent ceramics were fabricated by hot-pressing (HP) method at different P-T conditions in a vacuum environment. The fracture surface, the relative density, average grain size and shrinkage displacements were also studied to analysis the influence of sintering conditions on Eu: CaF{sub 2} ceramic. In comparison with other samples, the proper sintering temperature and pressure was 600 °C/30 MPa. The transmittance of ceramic was up to 79.7% and 88.85% at the wavelength of 760 nm and 2400 nm. Furthermore, the emission spectra of Eu: CaF{sub 2} transparent ceramics under 393.5 nm excitation was also discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Chemically Inert Hydrocarbon-Based Slurries for Rapid Laser Sintering of Thin Proton-Conducting Ceramics

The process of rapid laser sintering of thin BaZrO 3 –BaCeO 3 -based proton-conducting electrolytes is being developed for easy fabrication of ceramic fuel cells and electrolyzers. However, cracks on the electrolytes caused by volume change due to chemical reactions between the basic ceramic constituents and the polar solvents during wet processing has been problematic. In order to address this issue, the use of chemically inert saturated-hydrocarbon-based slurries comprised of hexadecane, polybutene, and a long-chain saturated fatty acid were investigated in this work. By optimizing slurry composition and laser sintering conditions, a 20 mm long, 4 mm wide, 13.5-μm-thick and 97%-dense BaCe 0.7 Zr 0.1 Y 0.07 Sm 0.13 O 3-d membrane showing proton conductivity on the order of 10 –4 S•cm –1 at 600 °C was successfully prepared in just three seconds by laser sintering. As a result, the use of saturated-hydrocarbon-based slurries will facilitate wet processing and rapid laser sintering of proton-conducting ceramic electrolytes.

36 MATERIALS SCIENCE↗