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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Sintering Mechanism of Pt/Al 2 O 3 in Complex Emission Gases Elucidated via In Situ Environmental STEM

Emission control catalysts are crucial for protecting human health by preventing the release of harmful gases and unburnt fuel into the atmosphere. These catalysts often face deactivation through sintering processes in high-temperature, chemically reactive environments containing multiple gas species. Here, we use in situ environmental scanning transmission electron microscopy to monitor the sintering behavior and transient morphologies of Pt/Al 2 O 3 in various relevant gas environments through controlled experiments. Our results reveal the particle migration and atomic ripening behavior of Pt/Al 2 O 3 at the atomic scale in the presence of water vapor and oxygen, which differs from behaviors observed in single gas environments. We identify an atomic ripening mechanism involving the dissociation and migration of Pt adatom chains from Pt nanoparticles, observed only in combinational gases. Finally, these findings provide valuable insights into catalyst degradation behavior in complex gas environments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Revealing Sintering Kinetics of MoS 2 -Supported Metal Nanocatalysts in Atmospheric Gas Environments via Operando Transmission Electron Microscopy

The decoration of two-dimensional (2D) substrates with nanoparticles (NPs) serve as heterostructures for various catalysis applications. Deep understanding of catalyst degradation mechanisms during service conditions is crucial to improve the catalyst durability. In this work, we studied the sintering behavior of Pt and bimetallic Au-core Pt-shell (Au@Pt core–shell) NPs on MoS 2 supports at high temperatures under vacuum, nitrogen (N 2 ), hydrogen (H 2 ), and air environments by in situ gas-cell transmission electron microscopy (TEM). The key observations are summarized as effect of environment: while particle migration and coalescence (PMC) was the main mechanism that led to Pt and Au@Pt NPs degradation under vacuum, N 2 , and H 2 environments, the degradation of MoS 2 substrate was prominent under exposure to air at high temperatures. Pt NPs were less stable in H 2 environment when compared with the Pt NPs under vacuum or N 2 , due to Pt–H interactions that weakened the adhesion of Pt on MoS 2 . Effect of NP composition: under H 2 , the stability of Au@Pt NPs was higher in comparison to Pt NPs. This is because H 2 promotes the alloying of Pt–Au, thus reducing the number of Pt at the surface (reducing H 2 interactions) and increasing Pt atoms in contact with MoS 2 . Effect of NP size: The alloying effect promoted by H 2 was more pronounced in small size Au@Pt NPs resulting in their higher sintering resistance in comparison to large size Au@Pt NPs and similar size Pt NPs. The present work provides key insights into the parameters affecting the catalyst degradation mechanisms on 2D supports.

2D materials↗
DOE OSTI2011

End Uses Mechanical Properties Settled By The Modified Sintering Conditions Of The Metal Injection Molding Process

Most common mechanical applications require parts with specific properties as hard faced features. It is well known that treating parts under suitable atmospheres may improve hardness and strength yield of steels. Heat treatment process and more particularly thermo-chemical diffusion processes (such as carburizing or its variation: carbonitriding) can be performed to reach the industrial hardness profile requirements. In this work, a low-alloyed steel feedstock based on water soluble binder system is submitted to the MIM process steps (including injection molding, debinding and sintering). As-sintered parts are then treated under a low pressure carbonitriding treatment. This contribution focuses on preliminary results such as microstructural analyses and mechanical properties which are established at each stage of the process to determine and monitor changes.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Electric Discharge Sintering and Joining of Tungsten Carbide--Cobalt Composite with High-Speed Steel Substrate

Simultaneous electro discharge sintering of high strength structure of tungsten carbide-cobalt composite and connection it with high-speed steel substrate is investigated and suitable operating parameters are defined. Tungsten carbide-cobalt and high-speed steel joining was produced by the method of high voltage electrical discharge together with application of mechanical pressure to powder compact. It was found that the density and hardness of composite material reach its maximum values at certain magnitudes of applied pressure and high voltage electrical discharge parameters. We show that there is an upper level for the discharge voltage beyond which the powder of composite material disintegrates like an exploding wire. Due to our results it is possible to determine optimal parameters for simultaneous electro discharge sintering of WC-Co and bonding it with high-speed steel substrate.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Microstructure and magnetic properties of sintered Nd-Fe-B magnets with high hydrogen content

The effects of hydrogen contents after the process of hydrogen decrepitation on the magnetic properties and corrosion resistance of sintered Nd-Fe-B permanent magnets have been studied. The intrinsic coercivity of the magnets increases and the remanence decreases with increasing hydrogen content. High residue hydrogen content in the starting ingot leads to the reaction of hydrogenation, disproportionation, desorption and recombination in the sinter process resulting in magnets with low remanence. The results of highly accelerated temperature/humidity stress test indicate that excessive hydrogen contents are harmful to the corrosion resistance of the magnets. The influences of residue hydrogen contents on the corrosion resistance of magnets dominate only when the residue hydrogen content exceeds 1300 parts per million.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Phase, Dielectric and Ferroelectric Properties of Microwave Sintered La and Ca Modified BaTiO{sub 3} Ceramics

Lanthanum and calcium modified barium titanate, Ba{sub (1-x)}La{sub x}Ti{sub (1-x/4)}O{sub 3}/BLT and Ba({sub 1-x})Ca{sub x}TiO{sub 3}/BCT, where x = 0.02, ferroelectric ceramic samples were synthesized in single perovskite phase by microwave processing technique. Sintering temperature was optimized at 1100 deg. C for 1h. Presence of pore free micron size uniform grains suggested the advantage of using microwave sintering process. Transition temperature (T{sub c}) decreases with the substitution of La content in BT system. Temperature coefficient of capacitance is negligible from RT temperature to 75 deg. C for BLT ceramic samples. Polarization vs. electric field (P-E) hysteresis loop study confirms the ferroelectric nature of the modified BT ceramic samples.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Zn migration during spark plasma sintering of thermoelectric Zn{sub 4}Sb{sub 3}

The phase homogeneity of spark plasma sintered thermoelectric Zn{sub 4}Sb{sub 3} pellets along the pressing direction has been studied by potential Seebeck microprobe scanning and spatially resolved x-ray diffraction. Significant variations in the Seebeck coefficient reflect presence of different crystalline phases. The emergence of the ZnSb phase at the bottom of the pellet and metallic Zn impurity at the top explains the variation in the Seebeck coefficients. Quantitative phase distributions along the pressing axis were determined from the Rietveld refinements of spatially resolved x-ray diffraction patterns. These reveal a migration of highly mobilized Zn atoms under the direct current applied during spark plasma sintering.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Spark-plasma-sintering magnetic field assisted compaction of Co{sub 80}Ni{sub 20} nanowires for anisotropic ferromagnetic bulk materials

We developed a two-step process showing the way for sintering anisotropic nanostructured bulk ferromagnetic materials. A new reactor has been optimized allowing the synthesis of several grams per batch of nanopowders via a polyol soft chemistry route. The feasibility of the scale-up has been successfully demonstrated for Co{sub 80}Ni{sub 20} nanowires and a massic yield of ∼97% was obtained. The thus obtained nanowires show an average diameter of ∼6 nm and a length of ∼270 nm. A new bottom-up strategy allowed us to compact the powder into a bulk nanostructured system. We used a spark-plasma-sintering technique under uniaxial compression and low temperature assisted by a permanent magnetic field of 1 T. A macroscopic pellet of partially aligned nanowire arrays has been easily obtained. This showed optimized coercive properties along the direction of the magnetic field applied during compaction (i.e., the nanowires' direction)

36 MATERIALS SCIENCE↗
DOE OSTI2014

Grain size effect on the giant dielectric constant of CaCu{sub 3}Ti{sub 4}O{sub 12} nanoceramics prepared by mechanosynthesis and spark plasma sintering

In the present work, CaCu{sub 3}Ti{sub 4}O{sub 12} (CCTO) nanoceramics with different grain sizes were prepared by spark plasma sintering (SPS) at different temperatures (SPS-800, SPS-900, SPS-975, and SPS-1050) of the mechanosynthesized nano-powder. Structural and microstructural properties were studied by XRD and field-emission scanning electron microscope measurements. The grain size of CCTO nanoceramics increases from 80 nm to ∼200 nm for the ceramics sintered at 800 °C and 975 °C, respectively. Further increase of SPS temperature to 1050 °C leads to micro-sized ceramics of 2–3 μm. The electrical and dielectric properties of the investigated ceramics were studied by impedance spectroscopy. Giant dielectric constant was observed in CCTO nanoceramics. The dielectric constant increases with increasing the grain size of the nanoceramics with values of 8.3 × 10{sup 3}, 2.4 × 10{sup 4}, and 3.2 × 10{sup 4} for SPS-800, SPS-900, and SPS-975, respectively. For the micro-sized SPS-1050 ceramics, the dielectric constant dropped to 2.14 × 10{sup 4}. The dielectric behavior is interpreted within the internal barrier layer capacitance picture due to the electrical inhomogeneity of the ceramics. Besides the resistive grain boundaries that are usually observed in CCTO ceramics, domain boundaries appear as a second source of internal layers in the current nanoceramics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effect of sintering on structural and dielectric properties of PLZT ferroelectrics

Here we report synthesis of (Pb{sub 0.93}La{sub 0.07}Zr{sub 0.6}Ti{sub 0.4}O{sub 3}) PLZT by chemical co-precipitation route. The precipitate obtained was subjected to different processing methods namely furnace sintering and hot press sintering. The product obtained was characterized using X-ray diffraction and dielectric measurements techniques. The dielectric properties obtained are correlated with the structural properties of PLZT.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Dynamic compressive and tensile strengths of spark plasma sintered alumina

Fully dense submicron grain size alumina samples were manufactured from alumina nano-powder using Spark Plasma Sintering and tested in two kinds of VISAR-instrumented planar impact tests. In the first kind, samples were loaded by 1-mm tungsten impactors, accelerated to a velocity of about 1 km/s. These tests were aimed at studying the Hugoniot elastic limit (HEL) of Spark Plasma Sintering (SPS)-processed alumina and the decay, with propagation distance, of the elastic precursor wave. In the tests of the second kind, alumina samples of 3-mm thickness were loaded by 1-mm copper impactors accelerated to 100–1000 m/s. These tests were aimed at studying the dynamic tensile (spall) strength of the alumina specimens. The tensile fracture of the un-alloyed alumina shows a monotonic decline of the spall strength with the amplitude of the loading stress pulse. Analysis of the decay of the elastic precursor wave allowed determining the rate of the irreversible (inelastic) strains in the SPS-processed alumina at the initial stages of the shock-induced inelastic deformation and to clarify the mechanisms responsible for the deformation. The 1-% addition of Cr{sub 2}O{sub 3} decreases the HEL of the SPS-processed alumina by 5-% and its spall strength by 50% but barely affects its static properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Self-biased large magnetoelectric coupling in co-sintered Bi{sub 0.5}Na{sub 0.5}TiO{sub 3} based piezoelectric and CoFe{sub 2}O{sub 4} based magnetostrictive bilayered composite

In this work, magnetoelectric properties of a co-sintered bilayered composite of non-lead based piezoelectric 0.97(Bi{sub 0.5}Na{sub 0.5}TiO{sub 3})–0.03(K{sub 0.47}Na{sub 0.47}Li{sub 0.06}Nb{sub 0.74}Sb{sub 0.06}Ta{sub 0.2}O{sub 3}) and magnetostrictive Co{sub 0.6}Zn{sub 0.4}Fe{sub 1.7}Mn{sub 0.3}O{sub 4} are presented. Similar optimal sintering conditions of the individual components lead to a very clean interface as evidenced in the scanning electron microscopy, angle dispersive X-ray diffraction, and energy-dispersive X-ray (EDX) results. Clean interface results in strong intimate mechanical coupling between both components and causes a maximum transfer of induced strain, leading to a large magnetoelectric coupling ∼142 mV/cm·Oe measured in longitudinally magnetized-transversely polarized configuration (L-T mode). Remnant polarization ∼32 μC/cm{sup 2}, remnant magnetization ∼0.50 emu/g, and sufficiently high self biased magnetoelectricity ∼135 mV/cm Oe (L-T mode) were observed for this composite.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Sintered tantalum carbide coatings on graphite substrates: Highly reliable protective coatings for bulk and epitaxial growth

Highly reliable low-cost protective coatings have been sought after for use in crucibles and susceptors for bulk and epitaxial film growth processes involving wide bandgap materials. Here, we propose a production technique for ultra-thick (50–200 μmt) tantalum carbide (TaC) protective coatings on graphite substrates, which consists of TaC slurry application and subsequent sintering processes, i.e., a wet ceramic process. Structural analysis of the sintered TaC layers indicated that they have a dense granular structure containing coarse grain with sizes of 10–50 μm. Furthermore, no cracks or pinholes penetrated through the layers, i.e., the TaC layers are highly reliable protective coatings. The analysis also indicated that no plastic deformation occurred during the production process, and the non-textured crystalline orientation of the TaC layers is the origin of their high reliability and durability. The TaC-coated graphite crucibles were tested in an aluminum nitride (AlN) sublimation growth process, which involves extremely corrosive conditions, and demonstrated their practical reliability and durability in the AlN growth process as a TaC-coated graphite. The application of the TaC-coated graphite materials to crucibles and susceptors for use in bulk AlN single crystal growth, bulk silicon carbide (SiC) single crystal growth, chemical vapor deposition of epitaxial SiC films, and metal-organic vapor phase epitaxy of group-III nitrides will lead to further improvements in crystal quality and reduced processing costs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

High magnetic-refrigeration performance of plate-shaped La{sub 0.5}Pr{sub 0.5}Fe{sub 11.4}Si{sub 1.6} hydrides sintered in high-pressure H{sub 2} atmosphere

La(Fe, Si){sub 13} hydride is regarded as one of the most promising room-temperature refrigerants. However, to use the alloys in an active magnetic regenerator machine, it is vital to prepare thin refrigerants. In this work, a high H{sub 2} gas pressure of 50 MPa was employed to suppress the desorption of hydrogen atoms during the sintering process of plate-shaped La{sub 0.5}Pr{sub 0.5}Fe{sub 11.4}Si{sub 1.6} hydrides. At 330 K, a high-density sintered thin plate shows a large magnetic-entropy change ΔS{sub m} of 15.5 J/kg K (106 mJ/cm{sup 3 }K) for a field change of 2 T. The volumetric ΔS{sub m} is almost twice as large as that of bonded La(Fe,Si){sub 13} hydrides. Favorably, hysteresis is almost absent due to the existence of micropores with a porosity of 0.69% which has been analyzed with high-resolution X-ray microtomography.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Phase stability, porosity distribution and microstructural evolution of amorphous Al{sub 50}Ti{sub 50} powders consolidated by electrical resistance sintering

The effect of intensity and duration of the electrical resistance sintering process on the phase stability, porosity distribution and microstructural evolution of Al{sub 50}Ti{sub 50} amorphous powders is studied. The phase transformations during the consolidation process were determined by X-ray diffraction. The porosity distribution was observed by optical and scanning electron microscopy. The amorphous phase is partially transformed to the crystalline phase during the sintering process, and formation of AlTi and AlTi{sub 3} intermetallic compounds occurs for temperatures higher than 300 °C. Finally, it is observed that the compacts core have lower porosity and a higher tendency to the amorphous-crystalline phase transformation than the periphery.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Welding of titanium and nickel alloy by combination of explosive welding and spark plasma sintering technologies

A possibility of titanium and nickel-based alloys composite materials formation using combination of explosive welding and spark plasma sintering technologies was demonstrated in the current research. An employment of interlayer consisting of copper and tantalum thin plates makes possible to eliminate a contact between metallurgical incompatible titanium and nickel that are susceptible to intermetallic compounds formation during their interaction. By the following spark plasma sintering process the bonding has been received between titanium and titanium alloy VT20 through the thin powder layer of pure titanium that is distinguished by low defectiveness and fine dispersive structure.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Effect of sintering temperature on the structural and electrical properties of Zn{sub 0.95}Cr{sub 0.05}O

In this communication, we report the results of the studies on the sintering temperature dependent structural properties and dielectric behavior of 5% Cr doped Zn{sub 0.95}Cr{sub 0.05}O samples synthesized by cost effective sol-gel technique. Crystallographic orientations, crystallinity and crystallite size (CS) have been understood by performing X-ray diffraction (XRD) measurement at room temperature. Values of CS have been further verified by carrying out Williamson – Hall (W-H) plot analysis and the difference between the CS from XRD and W-H analyses has been discussed. Dielectric behavior has been studied and its dependence on the sintering temperature has been understood in the light of crystallite size, crystal boundary density and oxygen vacancies induced possible conductivity in the samples.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Sintering temperature effect on electric transport properties and magnetoresistance of La{sub 0.67}Ca{sub 0.33}MnO{sub 3}

A compound of La{sub 0.67}Ca{sub 0.33}MnO{sub 3} (LCMO) is known as a famous colossal magnetoresistance material. Many studies have been carried in order to get more information about the electric and magnetic characteristic of the sample. The effect of two different sintering temperatures on manganite material La{sub 0.67}Ca{sub 0.33}MnO{sub 3} (LCMO) which prepared by solid state reaction, has been investigated in order to find the relations between the sintering temperature and the manganite’s electric transport properties. Characterization using X-Ray Diffractometer shows that the two samples have a single phase pattern. However, from the resistivity measurement both samples show a different value, but similar behavior. The resistivity reach maximum value in specific temperature T{sub p}, before it decreases once again to its lowest value in low temperature region.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗