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At least 1,315 records · Page 73
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Effect of Bi doping on morphotropic phase boundary and dielectric properties of PZT

In our present attempt, Pb{sub (1-x)}Bi{sub x}Zr{sub 0.52}Ti{sub 0.48}O{sub 3} [PBZT] {where x = 0, 0.05, 0.1} is synthesized by sol-gel route. Effect of Bi addition on structure, sinterability and dielectric properties are observed. The presence of morphotropic phase boundary (coexistence of tetragonal and rhombohedral symmetry) is confirmed by X-ray diffraction. Enhancement of sinterability after Bi doping is observed through a systematic sintering program. Frequency and temperature dependent dielectric constant are studied. Bi doping in PZT is found to enhance room temperature dielectric constant. However, at high temperature the dielectric constant of pure PZT is more than that of doped PZT.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Aerosol jet printed 3 omega sensors for thermal conductivity measurement

The 3 omega (3ω) method is a trusted technique for measuring thermal conductivity—a fundamental material property of critical importance in a broad range of applications. However, traditional 3ω sensor processing requires some form of physical vapor deposition, such as metal evaporation or sputtering. These 3ω sensor deposition techniques limit the materials and sample sizes applicable to the 3ω method. This work demonstrates an aerosol jet printing method to directly print silver 3ω sensors that yield accurate temperature-dependent measurement up to 300 °C on materials with thermal conductivity ranging from 1 to 150 W/m K. The interrelationship between printed sensor geometry, sensor thermal stability, and applicability to the 3ω method is examined. Thermal conductivity measurement with 3ω sensors conventionally sintered at 300 °C agrees to independent laser flash measurement within 4% from room temperature to 150 °C. An unconventional rapid high-temperature sintering method is shown to produce sensors that agree within 3% of the laser flash measurements from room temperature to 300 °C. The rapid sintering profiles also reduced the sensor–substrate thermal boundary resistance of the printed sensors by as much as 88%. The direct printing of 3ω sensors creates opportunities for measurement of thermal transport properties in applications previously inapplicable to the 3ω method.

47 OTHER INSTRUMENTATION↗
DOE OSTI2020

Fabrication of UN-Mo CERMET Nuclear Fuel Using Advanced Manufacturing Techniques

Ceramic-metallic nuclear fuels are a candidate fuel for nuclear thermal propulsion systems due to their high heat transport properties, which are necessary in very high-temperature environments. The conventional fabrication of uranium nitride–molybdenum fuel has been thoroughly studied in the past, but modern manufacturing techniques have presented a unique opportunity for further development within this field. This work demonstrates the use of advanced manufacturing techniques to produce nuclear fuel pellets composed of uranium nitride microspheres encased in a molybdenum matrix. Binder jetting is used to print molybdenum disks that are filled with uranium nitride microspheres and afterward sintered using spark plasma sintering. Two fuel pellets were fabricated to demonstrate the methodology and to provide a baseline analysis of the effects of temperature and pressure processing conditions. Characterization of the sintered fuel pellets includes detailed microstructural analysis and thermal conductivity measurements.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

Ni-base superalloy powder-processed porous layer for gas cooling in extreme environments

Extreme high temperature conditions demand novel solutions for hot gas filters and coolant access architectures, i.e., porous layers on exposed components. These high temperatures, for example in current turbine engines, are at or exceeding current material limits for high temperature oxidation/corrosion, creep resistance, and, even, melting temperature. Thus novel blade designs allowing greater heat removal are required to maintain airfoil temperatures below melting and/ or rapid creep deformation limits. Gas atomized Ni-base superalloy powders were partially sintered into porous layers to allow full-surface, transpirational cooling of the surface of airfoils. Furthermore, these powder-processed porous layers were fully characterized for surface, morphology, cross-sectional microstructure, and mechanical strength characteristics. A sintering model based on pure Ni surface diffusion correlated well with the experimental results and allowed reasonable control over the partial sintering process to obtain a specified level of porosity within the porous layer.

36 MATERIALS SCIENCE↗
DOE OSTI2016

The modification of BaCe{sub 0.5}Zr{sub 0.3}Y{sub 0.2}O{sub 3–δ} with copper oxide: Effect on the structural and transport properties

The effect of the content of CuO additive on the sinterability, phase composition, microstructure, and electrical properties of BaCe{sub 0.5}Zr{sub 0.3}Y{sub 0.2}O{sub 3–δ} proton-conducting material is studied. Ceramic samples were produced by the citrate–nitrate synthesis method with the addition of 0, 0.25, 0.5, and 1% CuO. It is shown that the relative density of the samples containing 0.5 and 1% CuO is higher than 94% at a sintering temperature of 1450°C, whereas the relative density of the material is no higher than 85% at a lower content of the sintering additive. From the data of X-ray diffraction analysis and scanning electron microscopy, it is established that the introduction of a small CuO content (0.25%) is inadequate for single-phase and high-dense ceramics to be formed. The conductivity and scanning electron microscopy data show that the sample with BaCe{sub 0.5}Zr{sub 0.3}Y{sub 0.2}O{sub 3–δ} + 0.5% CuO composition possesses high total and ionic conductivities as well as a high degree of microstructural stability after hydrogen reduction of the ceramics. The citrate–nitrate method modified by the introduction of a small CuO content can be recommended for the production of single-phase, gas-tight, and high-conductivity electrolytes based on both BaCeO{sub 3} and BaZrO{sub 3}.

36 MATERIALS SCIENCE↗
DOE OSTI2016

M3FT-16OR020202112 - Report on viability of hydrothermal corrosion resistant SiC/SiC Joint development

Hydrothermal corrosion of four types of the silicon carbide (SiC) to SiC plate joints were investigated under PWR and BWR relevant chemical conditions without irradiation. The joints were formed by metal diffusion bonding using molybdenum or titanium interlayer, reaction sintering using Ti-Si-C system, and SiC nanopowder sintering. Most of the formed joints withstood the corrosion tests for five weeks. The recession of the SiC substrates was limited. Based on the recession rate of the bonding layers, it was concluded that all the joints except for the molybdenum diffusion bond are promising under the reducing activity environments. The SiC nanopowder sintered joint was the most corrosion tolerant under the oxidizing activity environment among the four joints.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2020

Coupling between interfacial charge and mechanical deformation at high temperatures in ceramics. Final report

Basic research, under this project, has led to new fundamental discoveries in the effect of electric field (and temperature) on diffusion kinetics, electroluminescence and electronic conductivity in ceramic materials. For example, yttria stabilized zirconia sinters in just seconds well below 1000°C, while, at the same time, exhibiting a highly non-linear rise in conductivity, and intense electroluminescence. The phenomenon has been shown to occur in nearly all ceramics whether they are ionic or electronic conductors, or insulators. The rise in conductivity leads to Joule heating but in-situ measurements with a platinum standard at APS and Brookhaven (NSLS-II) have shown that the temperatures are much below what would have been needed to sinter in a few seconds. We have explored the hypothesis that huge concentrations of Frenkel pairs of vacancies and interstitials are produced. While these point defects accelerate sintering (by the interstitials being transported to the pores and the vacancies to the grain boundaries), they may also produce electron-hole pairs which can impart high electrical conductivity or recombine to emit photons. Many measurements, including in-situ measurements of lattice expansion are consistent with this hypothesis. However, the energy barrier for generating Frenkels in zirconia for example is much larger than can be expected at temperature and electrical fields in these experiments. Most recent work suggests that short wavelength phonons play a critical role in the genesis of the “flash” phenomenon. For example, the Debye temperature has been shown to be the lower bound for initiating flash. Molecular dynamics simulations have shown that large concentrations of Frenkel pairs can form under the proliferation of “optical” phonons at the edge of the Brillouin zone, that is with a wavelength at or below the lattice parameter. This project has led to the development of new experimental techniques as well as a software hardware interface for process and control and data analysis in real time, with a time scale of less than one second. This approach spells a new paradigm in materials science research. It potential impact on high rate manufacturing is self evident.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Influence of Carbides and Alloying Elements on Intercrystallite Corrosion in Cutting-Ceramic Inserts

It is established that intercrystallite corrosion of cutting-ceramic inserts arises when the chromium content in the peripheral zone of the grains is depleted by the deposition of chromium carbides at the grain boundaries, in the presence of stress due to the fast cooling of the inserts from high temperatures. This is especially apparent when 12Kh18N10Т, 40Kh13, 14Kh17N2, and other steels that are hard to machine are worked by a tool equipped with inserts of VOK-60M modified cutting ceramic. Research shows that the intercrystallite corrosion of cutting-ceramic inserts may be attributed to the stress arising in the surface layers of the grains as a result of carbide or nitride deposition. Heating cutting-ceramic inserts at 650–800°С lowers this stress and restores their resistance to intercrystallite corrosion. Research also shows that chromomanganese cutting-ceramic inserts are less susceptible to intercrystallite corrosion than are chromium–manganese–nickel inserts, while chromium–manganese–nickel cutting-ceramic inserts are less susceptible than are chromonickel inserts. The benefits of chromium–manganese–nickel cutting-ceramic inserts are especially pronounced with relatively brief machining of 12Kh18N10Т, 40Kh13, 14Kh17N2, and other such steels, in rigorous conditions, with a temperature of 600–650°С in the workpiece–tool contact zone. A new theory of the intercrystallite corrosion of VOK-60M modified cutting ceramic is based on the role of the kinetic factor at the relatively low tempering temperatures associated with the deposition of metastable carbide, although this does not lead to maximum change in free energy of the system but only to a relative change. In the cutting ceramic, this carbide may be chromium carbide (Cr, Тi){sub 23}C{sub 6}, which is greatly enriched with titanium. With increase in tempering temperature, the deposited carbide tends toward the stable composition Cr{sub 23}С{sub 6}. Prolonged holding at the tempering temperature also tends to favor thermodynamic equilibrium. In other word, the chromium carbide is gradually enriched to reach the stable composition Cr{sub 23}С{sub 6}. This process is more rapid at higher temperatures. Obviously, the variation in composition of the carbide deposited at the grain boundaries in VOK-60M cutting ceramic with change in temperature and holding time will affect the kinetics of electrochemical solution of the system consisting of the carbide in contact with a solid solution. Research shows that vacuum sintering and annealing markedly improve the thermal stability. For the cutting ceramic, this is explained not only by the removal of fusible impurities but also by the larger quantity of aluminum and other alloying elements that remain in the strengthening phase, rather than being bound in oxides. Sintering in vacuum ensures greater plasticity of cutting ceramic based on nickel and cobalt in cutting 12Kh18N10Т and 14Kh17N2 steel at high speeds, with high temperatures in the machining zone. In addition, on sintering in vacuum furnaces, the content of small nonmetallic inclusions in the basic structure is 5–7 times less than in regular cutting ceramic. That significantly improves the deformability of the ceramic inserts.

36 MATERIALS SCIENCE↗
DOE OSTI2023

A Study of a Surrogate Nuclear Thermal Propulsion Fuel Element

We report a surrogate fuel, Zirconium Carbide/Titanium Nitride has been prepared using a simple method to obtain a more uniform distribution of titanium nitride in the zirconium carbide matrix. Powders were sintered using Spark Plasma Sintering at 1750oC and 1850oC using a modified sintering profile. This resulted in a fairly uniform distribution of titanium nitride in the zirconium carbide matrix with a density of 95% of theoretical at 1850oC. Scanning electron microscopy, indentation hardness and x-ray diffraction results are given.

36 MATERIALS SCIENCE↗
NASA NTRS2023

Homogeneity Study of ZrC, NbC, and TaC Binary Carbide Fuels for the Application of Nuclear Thermal Propulsion

In alignment with NASA and DARPA goals, efforts towards Nuclear Thermal Propulsion (NTP) have increased in recent years. As progress is made, a main challenge still exists; the fuel in an NTP system must survive the extreme environment experienced during operating conditions. Three fuel forms have been studied including a ceramic-metallic matrix, a ceramic-ceramic matrix, and solid solution carbides. Solid solution carbides were studied previously as a fuel for NTP and was shown to be a promising technology; however, the programs were canceled before demonstration in an NTP engine test. Of the refractory metal carbides of interest, zirconium carbide (ZrC), niobium carbide (NbC) and tantalum carbide (TaC) were studied. ZrC, NbC, and TaC powders were consolidated in monocarbide (ZrC, NbC, TaC) and bi-carbide (ZrC-NbC, ZrC-TaC, NbC-TaC) forms using spark plasma sintering (SPS), with 50-50 mol% compositions of the bi-carbides of interest. The time and temperature conditions to achieve a completely homogeneous solid solution for the mixed carbides is of interest for NTP fuel applications, since this homogeneity is considered to be essential for this fuel form. Homogeneity is typically not achieved during sintering, as surface diffusion (predominant for sintering to near ideal density) tends to occur at lower temperatures than bulk volumetric diffusion (required for solute interdiffusion to achieve a homogeneous solid solution). High temperature isochronal annealing was performed to identify the conditions to achieve a chemically homogeneous sample. The solute interpenetration vs annealing temperature was quantified using energy dispersion spectroscopy (EDS) in a scanning electron microscope (SEM). X-ray diffraction (XRD) was used to accurately measure the phase stability and lattice parameter as a function of annealing conditions. The diffusion data from each sample was then used to determine self-diffusion coefficients for interdiffusion of the Nb, Zr, and Ta solutes and compared to existing literature values. The minimum annealing conditions to achieve complete chemical homogeneity for each carbide was determined.

nuclear thermal propulsion↗
DOE OSTI2017

Performance and Reliability of Bonded Interfaces for High-Temperature Packaging

Sintered silver has proven to be a promising candidate for use as a die-attach and substrate-attach material in automotive power electronics components. It holds promise of greater reliability than lead-based and lead-free solders, especially at higher temperatures (>200 degrees C). Accurate predictive lifetime models of sintered silver need to be developed and its failure mechanisms thoroughly characterized before it can be deployed as a die-attach or substrate-attach material in wide-bandgap device-based packages. Mechanical characterization tests that result in stress-strain curves and accelerated tests that produce cycles-to-failure result will be conducted. Also, we present a finite element method (FEM) modeling methodology that can offer greater accuracy in predicting the failure of sintered silver under accelerated thermal cycling. A fracture mechanics-based approach is adopted in the FEM model, and J-integral/thermal cycle values are computed.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2014

Synthesis and Characterization of Supported Cobalt-Manganese Nanoparticles as Model Catalysts for Fischer-Tropsch Synthesis

Supported Co is an effective catalyst for the Fischer-Tropsch synthesis of various hydrocarbon products that can be converted to diesel. Recent studies have shown that the formation of methane can be suppressed and the formation of C 5+ products enhanced by promoting Co with Mn. Because the activity and product selectivity of Co-based catalysts are dependent on the size of Co nanoparticles and the extent of Co promotion by Mn, it is desirable to understand these effects by investigating the performance of Co nanoparticles with well-defined size and elemental composition. The present study was undertaken with the aim of producing well-defined nanoparticles of Co and Co-Mn and then supporting them on silica. Co and Co-Mn particles were synthesized through the polyol reduction of Co and Mn acetylacetonates. By controlling synthesis conditions, Co particles with diameters of 7-10 nm and similarly sized Co-Mn (Mn/Co=0.1) particles were prepared. XRD and elemental mapping with scanning TEM-energy-dispersive X-ray spectroscopy and scanning TEM-electron energy loss spectroscopy studies suggested that most of the Mn species was associated with the Co particles. Ex situ prepared Co and Co-Mn nanoparticles were first supported on silica and then investigated for the catalytic activity for the Fischer-Tropsch synthesis. The turnover frequencies and product distributions obtained with silica-supported Co and Co-Mn nanoparticles were similar to those obtained with catalysts prepared by using the conventional incipient wetness impregnation method. However, the rate of CO consumption per mass of Co was much lower for the catalysts produced by supporting ex situ prepared nanoparticles. This effect was attributed to the sintering of the nanoparticles during their calcination and reduction. Magnetic interactions among nanoparticles during their immobilization and thermal pretreatment were identified as the primary cause of sintering. Balancing yin and yang energy: The immobilization of well-defined cobalt-manganese nanoparticles on porous supports leads to the formation of Fischer-Tropsch catalysts with turnover frequencies and product selectivities comparable to those of conventionally prepared catalysts. The catalytic activity per cobalt mass is found to be significantly lower owing to sintering. Finally, magnetic interactions among nanoparticles during immobilization and thermal pretreatment are identified as the primary cause of this phenomenon.

cobalt↗
DOE OSTI2020

Understanding the Deactivation of Ag–ZrO 2 /SiO 2 Catalysts for the Single-step Conversion of Ethanol to Butenes

Ag–ZrO 2 /SBA-16 has recently been found to be efficient for catalyzing the single-step conversion of ethanol to butene (1- and 2-butene mixtures) in the presence of H 2 . The reaction proceeds via a cascading sequence of reactions over mixed metal and Lewis sites, with the catalyst composition tuned to selectively favor butene formation. However, the catalyst slowly deactivates when evaluated over long reaction times. Here, we evaluated the lifetime of the Ag–ZrO 2 /SBA-16 catalyst system for ethanol-to-butene conversion at 325 °C for up to 800 hours on stream. Several characterization techniques were used to elucidate the mechanism(s) by which catalyst deactivation occurs. Coke deposition, Ag particle sintering, and Ag 0 -to-Ag + oxidation state change were identified to be the major causes of catalyst deactivation. Coke deposits cover primarily Lewis acid sites which are responsible for aldol condensation, Meerwein-Ponndorf-Verley (MPV) reduction, and dehydration reactions. Ag particle sintering and Ag oxidation state change leads to a reduction in the number of metallic Ag sites responsible for the dehydrogenation/hydrogenation steps. The fresh catalyst likely experiences hydrothermal sintering in the early stage of reaction and permanently loses some active Lewis acid sites before reaching a new structural steady state. The deactivation of Lewis acid sites leads to a decrease in overall ethanol conversion, whereas the deactivation of the metallic Ag sites decreases the butene selectivity. For catalyst regeneration, oxidative calcination (at 500 °C) followed by reduction (at 325 °C) successfully removes all the coke species on the catalyst surface and restores the metallic Ag particles of the 4Ag–4ZrO 2 /SBA-16 catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Investigation of the Surface Chemistry of Magnesium and Magnesium Alloy Powders

Magnesium (Mg) and Mg alloy powders are generally difficult to sinter at atmospheric pressure in inert atmospheres purportedly due to the presence of powder particle surface oxide layers. To better understand sintering of Mg and Mg alloy powders, x-ray photoelectron spectroscopy (XPS) was performed to characterize the surface chemistry of gas atomized pure Mg and Mg alloys AZ91D and WE43 powders. These results are used to quantitatively determine the surface chemistry of these powders. Additionally, equilibrium thermodynamic modeling of these Mg/Mg alloys materials exposed to oxygen down to part per billion concentrations are presented for relative comparison to experimental XPS results. Analysis of these results are intended to improve sintering of Mg and Mg alloy powder materials through more quantitative knowledge of powder surface chemistry.

25 ENERGY STORAGE↗
DOE OSTI2025

Design, fabrication, simulation, and testing of additively manufactured lattice-based copper heat sinks

Here, this study investigates the potential advantages of lattice structure-based bound metal material extrusion (MEX) 3D printing for fabricating high-performance copper heat sinks. Copper powder-filled polymer filaments, with a copper content of > 90 wt.%, were developed specifically for the bound metal MEX 3D printing process. Three types of structures—planar, strut, and surface lattices—were 3D printed to facilitate efficient heat transfer pathways within the heat sinks. Subsequent post-processing steps, including polymer removal and sintering, were performed to achieve dense copper parts. Hot isostatic pressing was further employed to enhance the sintered density from 93 to 98%. Finite element analysis (FEA) simulations were conducted to assess the heat transfer efficiency of the designs, and heat transfer experiments were performed using a custom setup to validate the simulation results. Additionally, this research explores the use of extended hold times during pre-sintering and a reduced atmosphere to enhance the %IACS values (electrical conductivity) and thermal performance of the bound metal MEX 3D printed copper heat sinks. The investigation combines experimental analysis, including simulations and heat transfer experiments, to gain insights into the structure-material property relationships and optimize the thermal performance of the printed heat sinks.

Ajjarapu, Kameswara Pavan Kumar [Oak Ridge Nationa↗
DOE OSTI2016

Processing of crack-free high density polycrystalline LiTaO3 ceramics

Our work achieved high density (99.9%) polycrystalline LiTaO 3 . The keys to the high density without cracking were the use of LiF-assisted densification to maintain fine grain size as well as the presence of secondary lithium aluminate phases as grain growth inhibitors. The average grain size of the hot pressed polycrystalline LiTaO 3 is less than 5 μm, limiting residual stresses caused by the anisotropic thermal expansion. Dilatometry results clearly indicate liquid phase sintering via the added LiF sintering aid. Efficient liquid phase sintering allows densification during low temperature hot pressing. Electron microscopy confirmed the high-density microstructure. Furthermore, Rietveld analysis of neutron diffraction data revealed the presence of LiAlO 2 and LiAl 5 O 8 minority phases and negligible substitutional defect incorporation in LiTaO 3 .

36 MATERIALS SCIENCE↗
DOE OSTI2018

Structural and Thermal Validations of Y{sub 3}Ba{sub 5}Cu{sub 8}O{sub 18} Composites Synthesized via Citrate Sol-Gel Spontaneous Combustion Method

The fabrication of single-phase superconductors enabled promising advancements in a wide range of technological applications. Numerous parameters including processing temperature, thermal cycle, chemical composition, doping, and atmosphere will be carefully addressed and optimized. This paper investigates phase stability and compound formation of Y{sub 3}Ba{sub 5}Cu{sub 8}O{sub y} (Y-358) preform powders which were sintered at different temperatures. The precursor powder synthesized via chemical route and employing spontaneous combustion technique, respectively. The presence of Y-358 and YBa{sub 2}Cu{sub 3}O{sub 7-δ} (Y-123) phases in composites sintered at different temperatures with varied phase fractions were further confirmed through Rietveld refinement. The formation of Y-123 phase is dominant in all samples and composite was sintered at 900 °C, which was exhibited higher as compared to Y-358 phase content. The decomposition of different phases present in the composites and reaction temperatures were investigated with simultaneous differential thermal and thermogravimetry analysis.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗