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At least 1,531 records · Page 85
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

The enhancing performance of (Ba{sub 0.85}Ca{sub 0.15}Ti{sub 0.90}Zr{sub 0.10})O{sub 3} ceramics by tuning anatase–rutile phase structure

Graphical abstract: Titanium dioxide (TiO{sub 2}) with different phase structure had interesting influence on the crystal structure, microstructure, the sintering temperature and electrical properties. - Highlights: • BCZT ceramics were prepared using either anatase or rutile structures as Ti source. • Orthorhombic and tetragonal mixture structure was exhibited by adjusting Ti source. • The optimal properties were observed in BCZT ceramics with rutile titanium dioxide. - Abstract: To research effect of raw materials TiO{sub 2} with the phase structures on the crystal structure, microstructure and electrical properties of lead-free (Ba{sub 0.85}Ca{sub 0.15})(Ti{sub 0.90}Zr{sub 0.10})O{sub 3} (BCZT) ceramics, BCZT ceramics using either anatase or rutile as Ti source were synthesized by solid-state reaction. Titanium dioxide (TiO{sub 2}) with anatase/rutile phase structures had interesting influence on the crystal structure, microstructure and the sintering temperature by the X-ray diffraction and SEM, which also played an important role in improved electrical properties. The BCZT ceramics with rutile titanium dioxide demonstrated optimal piezoelectric and dielectric properties: d{sub 33} = 590 pC/N, k{sub p} = 0.46, ε{sub r} = 2810, tanδ = 0.014 and T{sub c} = 91 °C, which was obviously superior to BCZT ceramics with anatase titanium dioxide.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Microstructures and martensitic transformation behavior of superelastic Ti-Ni-Ag scaffolds

Highlights: • The B2-R-B19′ transformation occurred in 49Ti-50.3Ni-0.7Ag alloy fibers. • Annealing treated alloy fibers showed superelastic recovery ratio of 93%. • Ageing treated scaffold had an elastic modulus of 0.67 GPa. • Ageing treated scaffold exhibited good superelasticity at human body temperature. - Abstract: Ti-Ni-Ag scaffolds were prepared by sintering rapidly solidified alloy fibers. Microstructures and transformation behaviors of alloy fibers and scaffolds were investigated by means of electron probe micro-analyzer (EPMA), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The B2-R-B19′ transformation occurs in alloy fibers. The alloy fibers have good superelasticity with superelastic recovery ratio of 93% after annealing heat treatment. The as-sintered Ti-Ni-Ag scaffolds possess three-dimensional and interconnected pores and have the porosity level of 80%. The heat treated Ti-Ni-Ag scaffolds not only have an elastic modulus of 0.67 GPa, which match well with that of cancellous bone, but also show excellent superelasticity at human body temperature. In terms of the mechanical properties, the Ti-Ni-Ag scaffolds in this study can meet the main requirements of bone scaffold for the purpose of bone replacement applications.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Crystal structure and dielectric properties of a new Na2O-Nd2O3-CeO2 ceramic system at microwave frequencies

Highlights: • A large amount of Na evaporation can make τ{sub f} more negative in present ceramics. • Na-excess evaporation results in the formation of Ce{sub 1-x}Nd{sub x}O{sub 2-x/2} solid solution. • Ce{sub 1-x}Nd{sub x}O{sub 2-x/2} (x = 0.25 and 0.5) solid solutions belong to the same fluorite-type. - Abstract: Na{sub 2}O-Nd{sub 2}O{sub 3}-CeO{sub 2} ceramic systems of the form Na{sub 3x−1}Nd{sub 1−x}CeO{sub 3} (x = 0.4, 0.5, and 0.6) were prepared via a solid-state reaction method. X-ray diffraction analyses indicated that the samples with x = 0.4, 0.6 and those with x = 0.5 formed Nd{sub 0.25}Ce{sub 0.75}O{sub 1.875} and Nd{sub 0.5}Ce{sub 0.5}O{sub 1.75} solid solutions, respectively, which resulted from a large degree of Na{sup +} ion evaporation. Both the Nd{sub 0.25}Ce{sub 0.75}O{sub 1.875} and Nd{sub 0.5}Ce{sub 0.5}O{sub 1.75} phases belonged to the same cubic structure with an Fm-3m (225) space group. Additionally, the microstructures, sintering behaviors, and microwave dielectric properties were considered to be strongly related to the Na content in the Na{sub 3x−1}Nd{sub 1−x}CeO{sub 3} ceramics. A relatively large degree of Na{sup +} ion evaporation could increase the negative temperature coefficient of the resonant frequency (τ{sub f}) for these ceramic systems. Moreover, after sintering at 1380 °C for 2 h, the x = 0.6 specimen possessed a low permittivity (ε{sub r}) of approximately 9.13, a moderate quality factor (Q × f) of approximately 23,800 GHz (at 9.14 GHz) and a negative τ{sub f} value of approximately −61.7 ppm/°C. As a result, it could be a promising microwave dielectric material for achieving a near-zero τ{sub f} value by solidifying one or more microwave dielectric ceramics with a high positive τ{sub f} value.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Nucleation and growth behaviors of hydriding and dehydriding reactions of Mg2Ni

Highlights: • Preparation of Mg{sub 2}Ni by sintering under Ar in a stainless steel crucible at 823 K. • Application of Johnson-Mehl equation for the nucleation and growth mechanism. • Reverse dependences on temperature of initial hydriding rate and quantity of H absorbed for 60 min. • Hydriding and dehydriding reactions progressed by a nucleation and growth mechanism. - Abstract: Mg{sub 2}Ni has drawn attention due to its high reaction rates, good reversibility, and possibility to be applied as a negative electrode in the Ni-MH (metal hydride) batteries. Mg{sub 2}Ni samples were prepared by sintering under Ar in a stainless-steel crucible at 823 K. Johnson-Mehl equation for nucleation and growth mechanism was applied to the behaviors of hydriding and dehydriding reactions of Mg{sub 2}Ni. At 571 K under 30 bar H{sub 2}, the activation of the Mg{sub 2}Ni sample was completed at the number of cycles of three (n = 3). At n = 3, hydrided fractions of the sample were 0.51 (1.92 wt% H) at 0.50 min and 0.90 (3.38 wt% H) at 2.00 min. Reverse temperature dependences of the initial hydriding rate and the quantity of hydrogen absorbed for 60 min appeared from 571 K to 620 K. Hydriding and dehydriding reactions progressed by the nucleation and growth mechanism and could be expressed by the Johnson-Mehl equation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Strong photoluminescence emission at low dopant amount in YAG:Ce and YAG:Eu phosphors

Highlights: • YAG:Ce and YAG:Eu materials were prepared by solid state reaction. • Pure YAG phase can be obtained at 1300 °C. • High density materials were synthesized at 1600 °C for 10 h. • The highest PL emission was observed for the YAG:Ce{sub 0.01} and YAG:Eu{sub 0.1} samples. - Abstract: YAG:Ce and YAG:Eu phosphors have been prepared by solid state reaction. The use of submicrometric powders allows preparing pellets with high density, enhancing the photoluminescence (PL) properties. Pellets were prepared by uniaxial pressing and the influence of the thermal treatment on the microstructural and optical properties were also studied. Pure YAG phase was obtained at 1400 °C during 1 h and high density pellets (97%) after sintering at 1600 °C for 10 h. The highest PL emission was achieved for the sintered YAG:Ce and YAG:Eu pellets doped with low dopant concentrations (0.01 mol% for Ce and 0.1 mol% for Eu). Finally, increasing the Ce and Eu concentration the PL intensity decreases and the dopants occupy different positions than the Y one in the YAG lattice.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Novel preparation of an ancient ceramic pigment BaCuSi4O10 and its performance investigation

Highlights: • The BaCuSi{sub 4}O{sub 10} (Chinese blue) was prepared firstly by coprecipitation and hydrothermal process at 180 °C for 20 h, and then sintered at 800 °C, resulting in pigment with superior performance. • The as-prepared pigment possesses bright blue coloration, preferable flower-like mesoporous microspheres morphology and qualified purity. • This novel process could fabricate the BaCuSi{sub 4}O{sub 10} pigment with broadband optical absorption and better emission in the near-infrared region. • This novel process decreases the calcination temperature compared with the traditional solid state method and also lowers the hydrothermal temperature and time compared with only a sole hydrothermal process. - Abstract: The BaCuSi{sub 4}O{sub 10} (Chinese blue) was prepared by coprecipitation and hydrothermal process at 180 °C for 20 h prior to sintering at 800 °C with a final pigment of superior performance. The crystalline configuration and microstructure of the product were characterized by XRD, FTIR, SEM and TEM. The electronic structure and the valence situation were determined by XPS. The coloration was studied with colorimeter. The absorption and the luminescence properties were examined with an UV−VIS-NIR spectrometer and a spectrofluorimeter, respectively. The results show that the BaCuSi{sub 4}O{sub 10} pigment prepared by this combined fabrication process possesses flower-like mesoporous microspheres morphology, better crystallization, clear coloration and rather qualified purity. The as-prepared BaCuSi{sub 4}O{sub 10} pigment also shows well optical property with broadband absorption and better emission in the 800–1200 nm range. Compared with traditional solid state fabrication or only sole hydrothermal process, this novel process can also reduce hydrothermal temperature and time, and decrease calcination temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Generation of electric-field stabilized zirconium monoxide secondary phase within cubic zirconia

Flash sintering of cubic yttria-stabilized zirconia has been studied via in situ energy dispersive diffraction. A secondary rocksalt-phase zirconium monoxide was observed emerging from the primary fluorite-phase zirconium dioxide, previously only observed as a thin interlayer at metal-oxide interfaces, upon application of a direct current electric field. The average secondary phase intensity rose over extended flow of direct current through the ceramic, faded upon removal of the field and disappeared entirely upon application of an alternating current electric field. The results indicate that the sustained application of a direct current electric field can result in significant electrochemical reduction of zirconia under high current density conditions. Poor sinterability and cracking under direct current is apparent and potential mechanisms discussed therein.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Advantages of MgAlOx over gamma-Al2O3 as a support material for potassium-based high temperature lean NOx traps

MgAlOx mixed oxides were employed as supports for potassium-based lean NOx traps (LNTs) targeted for high temperature applications. Effects of support compositions, K/Pt loadings, thermal aging and catalyst regeneration on NOx storage capacity were systematically investigated. The catalysts were characterized by XRD, NOx-TPD, TEM, STEM-HAADF and in-situ XAFS. The results indicate that MgAlOx mixed oxides have significant advantages over conventional gamma-Al2O3-supports for LNT catalysts, in terms of high temperature NOx trapping capacity and thermal stability. First, as a basic support, MgAlOx stabilizes stored nitrates (in the form of KNO3) to much higher temperatures than mildly acidic gamma-Al2O3. Second, MgAlOx minimizes Pt sintering during thermal aging, which is not possible for gamma-Al2O3 supports. Notably, combined XRD, in-situ XAFS and STEM-HAADF results indicate that Pt species in the thermally aged Pt/MgAlOx samples are finely dispersed in the oxide matrix as isolated atoms. This strong metal-support interaction stabilizes Pt and minimizes the extent of sintering. However, such strong interactions result in Pt oxidation via coordination with the support so that NO oxidation activity can be adversely affected after aging which, in turn, decreases NOx trapping ability for these catalysts. Interestingly, a high-temperature reduction treatment regenerates essentially full NOx trapping performance. In fact, regenerated Pt/K/MgAlOx catalyst exhibits much better NOx trapping performance than fresh Pt/K/Al2O3 LNTs over the entire temperature range investigated here. In addition to thermal aging, Pt/K loading effects were systemically studied over the fresh samples. The results indicate that NOx trapping is kinetically limited at low temperatures, while thermodynamically limited at high temperatures. A simple conceptual model was developed to explain the Pt and K loading effects on NOx storage. An optimized K loading, which allows balancing between the stability of nitrates and exposed Pt surface, gives the best NOx trapping capability.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2024

Dynamic Copper Site Redispersion through Atom Trapping in Zeolite Defects

Single-site copper-based catalysts have shown remarkable activity and selectivity for a variety of reactions. However, deactivation by sintering in high-temperature reducing environments remains a challenge and often limits their use due to irreversible structural changes to the catalyst. Here, we report zeolite-based copper catalysts in which copper oxide agglomerates formed after reaction can be repeatedly redispersed back to single sites using an oxidative treatment in air at 550 °C. Under different environments, single-site copper in Cu–Zn–Y/deAlBeta undergoes dynamic changes in structure and oxidation state that can be tuned to promote the formation of key active sites while minimizing deactivation through Cu sintering. For example, single-site Cu 2+ reduces to Cu 1+ after catalyst pretreatment (270 °C, 101 kPa H 2 ) and further to Cu0 nanoparticles under reaction conditions (270–350 °C, 7 kPa EtOH, 94 kPa H 2 ) or accelerated aging (400–450 °C, 101 kPa H 2 ). After regeneration at 550 °C in air, agglomerated CuO was dispersed back to single sites in the presence and absence of Zn and Y, which was verified by imaging, in situ spectroscopy, and catalytic rate measurements. Ab initio molecular dynamics simulations show that solvation of CuO monomers by water facilitates their transport through the zeolite pore, and condensation of the CuO monomer with a fully protonated silanol nest entraps copper and reforms the single-site structure. Importantly, the capability of silanol nests to trap and stabilize copper single sites under oxidizing conditions could extend the use of single-site copper catalysts to a wider variety of reactions and allows for a simple regeneration strategy for copper single-site catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Stable platinum nanoparticles on specific MgAl 2 O 4 spinel facets at high temperatures in oxidizing atmospheres

The development of thermally stable, nanometer-sized precious metal-based catalysts remains a daunting challenge. Such materials, especially those based on the use of costly platinum metal, are essential and, to date, non-replaceable for a large number of industrially important catalytic processes. Here we report a well-defined cuboctahedral MgAl 2 O 4 spinel support material that is capable of stabilizing platinum particles in the range of 1–3 nm on its relatively abundant {111} facets during extremely severe aging at 800 °C in air for 1 week. The aged catalysts retain platinum dispersions of 15.9% with catalytic activities for methanol oxidation being ~80% of that of fresh ones, whereas a conventional Pt/γ-Al 2 O 3 catalyst is severely sintered and nearly inactive. Finally, we reveal the origin of the markedly superior ability of spinel {111} facets, resulting from strong interactions between spinel surface oxygens and epitaxial platinum {111} facets, inspiring the rational design of anti-sintering supported platinum group catalysts.

anti-sintering↗
DOE OSTI2013

Effect of composition on electrical properties of lead-free Bi{sub 0.5}(Na{sub 0.80}K{sub 0.20}){sub 0.5}TiO{sub 3}-(Ba{sub 0.98}Nd{sub 0.02})TiO{sub 3} piezoelectric ceramics

Lead-free piezoelectric ceramics with the composition of (1-x)Bi{sub 0.5}(Na{sub 0.80}K{sub 0.20}){sub 0.5}TiO{sub 3}-x(Ba{sub 0.98}Nd{sub 0.02})TiO{sub 3} or (1-x) BNKT-xBNdT (with x = 0-0.20 mol fraction) have been synthesized by a conventional mixed-oxide method. The compositional dependence of phase structure and electrical properties of the ceramics were systemically studied. The optimum sintering temperature of all BNKT-BNdT ceramics was found to be 1125 Degree-Sign C. X-ray diffraction pattern suggested that BNdT effectively diffused into BNKT lattice during sintering to form a solid solution with a perovskite structure. Scanning electron micrographs showed a slight reduction of grain size when BNdT was added. It was found that BNKT-0.10BNdT ceramic exhibited optimum electrical properties ({epsilon}{sub r} = 1716, tan{delta} = 0.0701, T{sub c} = 327 Degree-Sign C, and d{sub 33} = 211 pC/N), suggesting that this composition has a potential to be one of a promising lead-free piezoelectric candidate for dielectric and piezoelectric applications.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Development of radon sources with a high stability and a wide range

A solid {sup 222}Rn (radon) source using a fibrous and porous SiC ceramic disk was developed. The emission rate of radon emanated from the disk depended on the content of {sup 226}Ra and the sintering temperature. A {sup 226}Ra sulfate ({sup 226}RaSO{sub 4}) solution was dropped on a fibrous SiC ceramic disk (33 mmφ) of 1 mm in thickness, and sintered at 400 °C. The radon concentration from a disk containing {sup 226}Ra of 1.85 MBq was measured to be 38 kBq m{sup −3} at a carrier airflow rate of 0.5 L min{sup −1}. By adjusting the {sup 226}Ra content or the sweep airflow rate, the radon concentrations were easily controlled over a wide range of over three orders of magnitude. The concentration was very stable for a long term. The compactness of the source disk made is easy for handling the source container and the shielding of gamma radiation from {sup 226}Ra and its decay products. Such advantages in a radon generation system are desirable for experiments of high-level, large-scale radon exposure.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2014

Dielectric strength of voidless BaTiO{sub 3} films with nano-scale grains fabricated by aerosol deposition

In order to investigate the dielectric strength properties of the BaTiO{sub 3} films with nano-scale grains with uniform grain size and no voids, BaTiO{sub 3} films were fabricated with a thickness of 1 μm by an AD process, and the fabricated films were sintered at 800, 900, and 1000 °C in air and reducing atmosphere. The films have superior dielectric strength properties due to their uniform grain size and high density without any voids. In addition, based on investigation of the leakage current (intrinsic) properties, it was confirmed that the sintering conditions of the reducing atmosphere largely increase leakage currents due to generated electrons and doubly ionized oxygen vacancies following the Poole-Frenkel emission mechanism, and increased leakage currents flow at grain boundary regions. Therefore, we conclude that the extrinsic breakdown factors should be eliminated for superior dielectric strength properties, and it is important to enhance grain boundaries by doping acceptors and rare-earth elements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Room temperature strain rate sensitivity in precursor derived HfO{sub 2}/Si-C-N(O) ceramic nanocomposites

Investigation on the room temperature strain rate sensitivity using depth sensing nanoindentation is carried out on precursor derived HfO{sub 2}/Si-C-N(O) ceramic nanocomposite sintered using pulsed electric current sintering. Using constant load method the strain rate sensitivity values are estimated. Lower strain rate sensitivity of ∼ 3.7 × 10{sup −3} is observed and the limited strain rate sensitivity of these ceramic nanocomposites is explained in terms of cluster model. It is concluded that presence of amorphous Si-C-N(O) clusters are responsible for the limited flowability in these ceramics.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Low temperature diffusion process using rare earth-Cu eutectic alloys for hot-deformed Nd-Fe-B bulk magnets

The low temperature grain boundary diffusion process using RE{sub 70}Cu{sub 30} (RE = Pr, Nd) eutectic alloy powders was applied to sintered and hot-deformed Nd-Fe-B bulk magnets. Although only marginal coercivity increase was observed in sintered magnets, a substantial enhancement in coercivity was observed when the process was applied to hot-deformed anisotropic bulk magnets. Using Pr{sub 70}Cu{sub 30} eutectic alloy as a diffusion source, the coercivity was enhanced from 1.65 T to 2.56 T. The hot-deformed sample expanded along c-axis direction only after the diffusion process as RE rich intergranular layers parallel to the broad surface of the Nd{sub 2}Fe{sub 14}B are thickened in the c-axis direction.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Al insertion and additive effects on the thermoelectric properties of yttrium boride

The aluminoboride Y{sub x}Al{sub y}B{sub 14} (x ∼ 0.57, 0.41 ≤ y ≤ 0.63) has been found to show striking p-n control of the thermoelectric properties through variations of the y occupancy of the Al site. The effect of Al was investigated in further extremes. Polycrystalline samples of Al-free Y{sub x}B{sub 14}(x ∼ 0.55; “YB{sub 25}”) were successfully synthesized in sufficient amounts for bulk spark plasma sintering (SPS) samples and their thermoelectric properties were investigated. Y{sub 0.56}Al{sub 0.57}B{sub 14} was also prepared in comparison, and further Al was added to the samples through SPS treatment. We observed that Y{sub 0.55}B{sub 14} exhibits large positive Seebeck coefficients, ∼1000 μV K{sup −1}, around room temperature and the absolute value of the Seebeck coefficient largely decreases with increase of temperature while that of Y{sub 0.56}Al{sub 0.57}B{sub 14} is proportional to T{sup −1/2}, indicating a strong effect of Al on the electronic structure around the Fermi level. Y{sub 0.55}B{sub 14} was found to be strongly disordered with a relatively low thermal conductivity and short localization length of 0.65 Å which is close to that previously determined for the disordered and thermally glass-like compound YB{sub 66}. Occupancy of Al could not be increased further for the Al-rich sample, although Al was discovered to act as a sintering aid to enhance density and ZT could be significantly improved by 50%.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Extrinsic mechanism for giant dielectric response in Ba{sub 0.70}Sr{sub 0.30}(Fe{sub 0.5}Nb{sub 0.5})O{sub 3} ceramic

To obtain the high dielectric constant, the effect of sintering process on the electrical properties of Ba{sub 0.70}Sr{sub 0.30}(Fe{sub 0.5}Nb{sub 0.5})O{sub 3} ceramics were investigated. X-ray diffraction pattern of the samples at room temperature shows a monoclinic structure. Microstructure analysis shows well-grown and dense microstructure in all the samples. We found giant dielectric constant (∼3.59 × 10{sup 5}) with low dielectric loss (∼0.49) at room temperature for 2 hr sintered sample at 1250 °C. The extrinsic phenomena like interfacial polarization due to space charge accumulation at grain boundaries are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Synthesis and characterization of black, red and yellow nanoparticles pigments from the iron sand

The aim of this research is to synthesize nanoparticles of black pigment of Magnetite (Fe{sub 3}O{sub 4}), red pigment of hematite (α-Fe{sub 2}O{sub 3}), and yellow pigment of ghoetite (α-FeOOH) from the iron sand. The black pigment of Fe{sub 3}O{sub 4} and the yellow pigment α-FeOOH nanoparticles were synthesized by coprecipitation method with variation of pH. Whereas, the red pigment Fe{sub 2}O{sub 3} was synthesized by sintering Fe{sub 3}O{sub 4} nanoparticles at temperature between 400 °C and 700 7°C for 1 hour. All the pigments has been characterized using X-ray diffraction and SEM. The XRD results shown that the particle size of the black pigmen Fe{sub 3}O{sub 4}, red pigment Fe{sub 3}O{sub 4} and yellow pigment α-FeOOH are around 12, 32, and 30 nm respectively. The particle size of Fe{sub 2}O{sub 3} nanoparticles increase by increasing sintering temperature from 32 nm at 400 °C to 39 nm at 700 °C. For yellow pigment of α-FeOOH, the particle size increase by increasing pH from 30,54 nm at pH 4 to 48,60 nm at pH 7. The SEM results shown that the morphologies of black, yellow and red pigments are aglomarated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗