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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

The Change of X‐ray Diffraction Peak Width During in situ Conventional Sintering of Nanoscale Powders

Diffraction peaks of nanoscale particles of 3 mol% yttria‐stabilized zirconia become sharper as the powder sinters. The reduction in the peak width is correlated with the increase in density. The sharpening of the peak agrees reasonably well with the remaining free surface area as the sample sinters. Therefore, high curvature of the free surface of the pores is assumed to lead to peak broadening (the grain boundaries that grow at the expense of the free surfaces of the pores do not have this curvature). The change in the grain size during sintering does not make a significant contribution to peak width.

Lebrun, Jean‐Marie↗
DOE OSTI2016

Thermal Runaway in Mold‐Assisted Flash Sintering

Analyzing the temperature evolution in pressureless mold‐assisted flash sintering, we found the same onset condition as in standard flash sintering: When sample's DC or AC Joule heating replaces environment's radiation heating as the dominant power input term, thermal runaway ensues. Various serial and parallel components connected to the sample, including the mold, insulation, and punches, can affect Joule heating and conduction heat loss, thus play an important role in successful mold‐assisted flash sintering.

Dong, Yanhao↗
DOE OSTI2018

Microwave sintering of complex shapes: From multiphysics simulation to improvements of process scalability

The microwave sintering homogeneity of large and complex shape specimens is analyzed. A new approach enabling the fabrication of complex shapes ceramics via 3D printing and microwave sintering is presented. The use of a dental microwave cavity is shown to enable a substantial level of densification of complex shape components while restricting the grain growth. The homogeneity of the processed samples during microwave sintering is studied by an electromagnetic-thermal-mechanical simulation. The realistic densification behavior, that phenomenologically takes into account the microwave effect, is included in the modeling framework. We observe that the simulation indicates the sharp correlation between the microwave field distribution in the cavity, the temperature profile, and the specimen's shape distortion.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Reactive flash sintering of powders of four constituents into a single phase of a complex oxide in a few seconds below 700°C

Abstract Recent work on reactive flash sintering of powders of two oxides, bismuth and of iron oxide, into pure single phase bismuth ferrite, which was accomplished in a few seconds at low furnace temperatures, is expanded to four constituents, alumina, lithia, zirconia, and lanthana, to produce reasonably dense polycrystals of a predominantly single phase, cubic LLZO(Al). Transformation and sintering occur concurrently at a furnace temperature near 700°C, in ambient atmosphere, in just a few seconds. The process may simplify the preparation of complex ceramics with new chemistries and dopants, which are predicted from ab intio calculations to have special attributes, not only because the powders sinter quickly at low temperatures, but also because the need for stoichiometric powders as starting materials is obviated.

Avila, Viviana↗
DOE OSTI2021

The Impact of Sintering Atmosphere and Temperature on the Phase Evolution of High Surface Area LSCF Prepared by In Situ Carbon Templating

The thermochemical stability of lanthanum strontium cobalt ferrite (LSCF) processed between 1000 °C–1200 °C via the in situ carbon templating method was studied. This method generates high surface area ceramics at traditional solid oxide fuel cell (SOFC) sintering temperatures by generating a carbon template in situ and subsequently removing the template by oxidation at 700 °C. Argon processed samples produced an amorphous carbon template, whereas nitrogen tended to form graphitic carbon. Prior to the oxidation step, nitrogen samples comprised larger La 2 O 3 crystallites (22–40 nm) compared to argon (9–17 nm). Upon oxidation, argon samples resulted in a pure LSCF phase with surface areas in the 21–29 m 2 ·g −1 range, whereas nitrogen samples contained significant impurities. This demonstrates that the size of La 2 O 3 crystallites formed during inert processing limited the ability to produce a pure LSCF phase. Symmetrical cells comprising nano-LSCF electrodes generated by the templating method were compared to cells sintered directly in air. Impedance results suggest that nano-LSCF cells and cells processed in air were dominated by interfacial charge transfer resistance and gas diffusion, respectively. The results map out conditions for preparing and integrating high surface area, nanostructured LSCF into SOFC electrodes at traditional sintering temperatures. Strategies for improving the interfacial resistance of nano-LSCF electrodes are discussed.

Muhoza, Sixbert P. (ORCID:0000000299864241)↗
DOE OSTI2019

Modeling Sintering Processes of Nanoparticle Inks

The In-Pile Instrumentation program conducts research on advanced manufacturing of sensors for use within nuclear reactor experiments. The process for making these sensors involves the sintering of nanoparticles within solvent. The nanoparticles must be sintered in a way that minimizes pores and maximizes continuity. It has also been observed that the nanoparticle films may fall off the substrate, so maximizing adhesion is also important. To model the sintering process, the phase field method was employed for one and two nanoparticle - substrate systems and the contact angle was measured and compared to existing experimental and theoretical studies.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Embedding of Optical Fibers with Electric Field Assisted Sintering

The Electric Field Assisted Sintering (EFAS) technique was used for embedding the fibers, involving rapid heating via an electric current and pressure, reducing fiber exposure to high temperatures. Stainless steel guide tubes were inserted at the fiber-matrix junctions to prevent fiber breakage during the sintering process. The study varied key EFAS parameters—temperature (800°C to 980°C), pressure (40-50 MPa), and hold time (5-10 minutes)—to assess how they influenced fiber embedding. After embedding, the fibers were inspected using optical frequency domain reflectometry (OFDR) to evaluate optical losses and strain along the fiber length. The OFDR scans confirmed that the fibers remained intact, and subsequent transmission tests demonstrated no macroscopic fractures. The embedded samples were also analyzed using scanning electron microscopy (SEM) and X-ray computed tomography (CT) scans. These tests confirmed good bonding between the fiber and matrix with no cracking, though some porosity was present in samples fabricated at lower temperatures. At higher temperatures (980°C), the fiber-matrix bonding was continuous and defect-free, with metal coatings aiding in the bonding process. Elemental analysis showed interdiffusion between the fiber coatings and matrix materials, particularly between the gold coating and stainless steel. Helium leak tests revealed that lower sintering temperatures resulted in matrix porosity, causing leaks, but samples fabricated at 980°C were leak-tight.

46 - INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AN↗
DOE OSTI2018

Laser sintering of Al nanoparticles for Al-air batteries

With the growing price of lithium material and associated safety concerns, there is an urgent need for developing new efficient, cost-effective battery systems as a successor for lithium ion batteries. Metal-air batteries have drawn huge attention and interest as energy storage devices due to their high theoretical energy density when Al, Zn, Ni, and Fe are used. In particular, Al-air batteries have the potential to become a promising primary battery system with their relatively low prices and significantly high energy density. In this paper, the authors report a printed Al anode for Al-air battery combined with a laser sintering method. The anode was fabricated by Al nanoparticle ink using squeezing printing. Furthermore, a laser sintering method was applied to remove the organic solvent in the slurry and increase the conductivity of the printed anode. By application of infrared laser sintering, the authors significantly improved electrical conductivity of Al nanoparticles and electrochemical performance of Al-air cells. A Pt/C coated hydrophobicity carbon paper was used for the air cathode to provide a good electrical conductivity, an oxygen flow ability, and the water vapor sealing. Gel-style KOH was introduced as the electrolyte, and waterways were also utilized in the chemical reaction. The authors show a full battery cell with a printed Al anode on flexible substrates. This microbattery can provide a 227 mWh/g energy density and a 239 mAh/g capacity at an operation voltage of 0.95 V.

25 ENERGY STORAGE↗
NASA NTRS1972

Activated sintering of ThO2 and ThO2-Y2O3 with NiO.

Scanning electron microscopy and isothermal shrinkage measurements were used to study the effects of additives on the sintering of Thoria-Yttria compacts and loose powders. Small amounts of the oxides of Ni, Zn, Co, and Cu were found to reduce the sintering temperature (which is normally above 2000 C). In particular, NiO, at a concentration of 0.8 wt %, was found to yield high-density bodies at temperatures below 1500 C. Densification occurs very rapidly, and is followed by a much slower sintering process typical of volume diffusion.

Halbfinger, G. P.↗
NASA NTRS1973

Sintering and hot pressing of Fra Mauro composition glass and the lithification of lunar breccias.

It is proposed that Apollo 14 type breccias lithify by sintering of particles of matrix glass under stress-free conditions. Meteorite impacts generate the heat necessary for sintering. Compacted angular particles of glass with the composition of an Apollo 14 rock were sintered experimentally. Loose clods of sub 37 micron grains form in several days at 700 C. Synthetic rocks, which texturally resemble the breccias, were produced at 795 C and above in 7.5 hours. Glass devitrified with increasing temperature. At 850 C, devitrification prevents much further densification of the synthetic breccias after 2 hours. Experiments compressing glass at 500 bars at 600 C produced dense vitreous masses which did not resemble lunar breccias. Confining pressure does not play a major role in lithifying the breccias. It is concluded that Apollo 14 type lunar breccias form at shallow depths at 800 C+ in a period of minutes to days.

Simonds, C. H.↗
NASA NTRS1973

Dislocation generation during early stage sintering.

Discussion of the effects of capillarity-induced stresses on dislocations during early stage sintering. A special version of Hirth's (1963) theoretical calculation procedures modified to describe dislocation nucleation on planes meeting the sintering body's neck surface obliquely is shown to predict plastic flow at stress levels know to exist between micron size metal particles in the early stages of sintering.

Sheehan, J. E.↗
NASA NTRS1974

Liquid phase sintered compacts in space

A model that will explain the effect of gravity on liquid phase sintering was developed. Wetting characteristics and density segregation which are the two important phenomena in liquid phase sintering are considered in the model development. Experiments were conducted on some selected material combinations to study the gravity effects on liquid phase sintering, and to verify the validity of the model. It is concluded that: (1) The surface tension forces acting on solid particles in a one-g environment are not appreciably different from those anticipated in a 0.00001g/g sub 0 (or lower) environment. (2) The capillary forces are dependent on the contact angle, the quantity of the liquid phase, and the distance between solid particles. (3) The pores (i.e., bubbles) do not appear to be driven to the surface by gravity-produced buoyancy forces. (4) The length of time to produce the same degree of settling in a low-gravity environment will be increased significantly. (5) A low gravity environment would appear to offer a unique means of satisfactorily infiltrating a larger and/or complex shaped compact.

Mookherji, T. K.↗
NASA NTRS1975

Silicon nitride-aluminum oxide solid solution (SiAION) formation and densification by pressure sintering

Stirred-ball-mill-blended Si3N4 and Al2O3 powders were pressure sintered in order to investigate the mechanism of solid solution formation and densification in the Si3N4-Al2O3 system. Powder blends with Si3N4:Al2O3 mole ratios of 4:1, 3:2, and 2:3 were pressure sintered at 27.6-MN/sq m pressure at temperatures to 17000 C (3090 F). The compaction behavior of the powder blends during pressure sintering was determined by observing the density of the powder compact as a function of temperature and time starting from room temperature. This information, combined with the results of X-ray diffraction and metallographic analyses regarding solutioning and phase transformation phenomena in the Si3N4-Al2O3 system, was used to describe the densification behavior.

Yeh, H. C.↗
NASA NTRS1978

Rub tolerance evaluation of two sintered NiCrAl gas path seal materials

Two strength level variations of sintered NiCrAl (about 40 percent dense), candidate high pressure turbine seal materials, were subject to rub tolerance testing against AM 355 steel blade tips. The high strength material (17 N/sq mm tensile strength) showed frictional and radial loads that were 20 to 50 percent higher than those measured for the low strength material (15.5 N/ sq mm tensile strength). Measured wear to the AM 355 blade tips was not significantly different for the two sintered NiCrAl seal materials. Wear of the sintered NiCrAl was characterized by material removal to a depth greater than the depth to which blade tips were driven into the seal, indicating self-erosion effects.

Bill, R. C.↗
NASA NTRS1980

Effect of milling and leaching on the structure of sintered silicon

Sintering was performed in He for 16 hours at 1200, 1250, and 1300 C. Compacts of as-received Si did not densify during sintering. Milling reduced the average particle size to below 0.5 micrometer and enhanced densification (1.75 g/cc). Leaching milled Si further enhanced densification (1.90 g/cc max.) and decreased structural coarsening. After sintering, the structure of the milled and leached powder compacts appears favorable for the production of reaction bonded silicon nitride.

Yeh, H. C.↗
NASA NTRS1982

Sintered plaque characteristics

The structural transformations occurring during sintering, the fabrication of a slurry produced sintered plaque, are detailed. Degradation of the positive electrode in performance in cycling in a nickel hydrogen battery were traced to the quality of the sintered plaque. Electrode degradation was found to be a limiting factor in the battery cycle life. Details of microstructural characterization and distribution of pores, examination of plastic flow during shrinkage, and observations of the rounding of nickel powder particles during the slurry process are presented.

Vaidyanathan, H.↗
NASA NTRS1982

Effects of various additives on sintering of aluminum nitride

Effects of thirty additives on sintering A/N were investigated. The addition of alkali earth oxides and rare earth oxides gave fully densified aluminum nitride. This is due to the formation of nitrogen-containing aluminate liquid in the system aluminum nitride-alkali earth oxides or rare earth oxides. Microstructural studies of the sintered specimens with the above two types of additives suggested that the densification was due to the liquid phase sintering. Additions of silicon compounds resulted in poor densification by the formation of highly refractory compounds such as A/N polytypes.

Komeya, K.↗