Search NASA⌕ Search

SEARCH · Search NASA

Results for “CHROMITES”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Enhancing the stability of copper chromite catalysts for the selective hydrogenation of furfural with ALD overcoating (II) – Comparison between TiO2 and Al2O3 overcoatings

TiO2 atomic layer deposition (ALD) overcoatings were applied to copper chromite catalysts to increase the stability for 2-furfuraldehyde (“furfural”) hydrogenation. After overcoating, about 75% activity was preserved compared to neat copper chromite: much higher activity than an alumina ALD overcoated catalyst with a similar number of ALD cycles. The effects of ALD TiO2 on the active Cu nanoparticles were studied extensively using both in-situ TPR/isothermal-oxidation and in-situ furfural hydrogenation via Cu XAFS. The redox properties of Cu were modified only slightly by the TiO2 ALD overcoat. However, a subtle electronic interaction was observed between the TiO2 ALD layers and the Cu nanoparticles. With calcination at 500 °C the interaction between the TiO2 overcoat and the underlying catalyst is strong enough to inhibit migration and site blocking by chromite, but is sufficiently weaker than the interaction between the Al2O3 overcoat and copper chromite that it does not strongly inhibit the catalytic activity of the copper nanoparticles.

Zhang, Hongbo↗
DOE OSTI2016

Magnetic and magnetocaloric properties of iron subs tituted holmium chromite and dysprosium chromite

In this work, structural, magnetic, and magnetocaloric properties of HoCrO 3 and Fe substituted HoCrO 3 and DyCrO 3 (i.e. HoCr 0.7 Fe 0.3 O 3 and DyCr 0.7 Fe 0.3 O 3 ) powder samples were synthesized via a solution route. The structural properties of the samples were examined by Raman spectroscopy and x-ray diffraction techniques, which were further confirmed using first-principle calculations. The dc magnetic measurements indicate that the Cr 3+ ordering temperatures for the HoCrO 3 , HoCr 0.7 Fe 0.3 O 3 , and DyCr 0.7 Fe 0.3 O 3 samples are 140 K, 174 K, and 160 K, respectively. The ac magnetic measurements not only confirmed the Cr 3+ ordering transitions in these samples (obtained using dc magnetic measurements), but also clearly showed the Ho 3+ ordering at ~10 K in the present HoCrO 3 and HoCr 0.7 Fe 0.3 O 3 samples, which to our knowledge, is the first ac magnetic evidence of Ho 3+ ordering in this system. The effective magnetic moments were determined to be 11.67μB, 11.30μB, and 11.27μB for the HoCrO 3 , HoCr 0.7 Fe 0.3 O 3 , and DyCr 0.7 Fe 0.3 O 3 samples, respectively. For the first time, the magnetocaloric properties of HoCrO 3 and HoCr 0.7 Fe 0.3 O 3 were studied here, showing their potential for applications in magnetic refrigeration. In an applied dc magnetic field of 7 T, the maximum magnetocaloric value were determined to be 7.2 (at 20 K), 6.83 (at 20 K), 13.08 J/kg K (at 5 K) and the relative cooling power were 408, 387, and 500 J/kg for the HoCrO 3 , HoCr 0.7 Fe 0.3 O 3 , and DyCr 0.7 Fe 0.3 O 3 samples, respectively.

36 MATERIALS SCIENCE↗
NASA NTRS2006

Chromite-Plagioclase Assemblages as a New Shock Indicator; Implications for the Shock and Thermal Histories of Ordinary Chondrites

Chromite in ordinary chondrites (OC) can be used as a shock indicator. A survey of 76 equilibrated H, L and LL chondrites shows that unshocked chromite grains occur in equant, subhedral and rounded morphologies surrounded by silicate or intergrown with metallic Fe-Ni and/or troilite. Some unmelted chromite grains are fractured or crushed during whole-rock brecciation. Others are transected by opaque veins; the veins form when impacts cause localized heating of metal-troilite intergrowths above the Fe-FeS eutectic (988 C), mobilization of metal-troilite melts, and penetration of the melt into fractures in chromite grains. Chromite-plagioclase assemblages occur in nearly every shock-stage S3-S6 OC; the assemblages range in size from 20-300 microns and consist of 0.2-20-micron-size euhedral, subhedral, anhedral and rounded chromite grains surrounded by plagioclase or glass of plagioclase composition. Plagioclase has a low impedance to shock compression. Heat from shock-melted plagioclase caused adjacent chromite grains to melt; chromite grains crystallized from this melt. Those chromite grains in the assemblages that are completely surrounded by plagioclase are generally richer in Al2O3, than unmelted, matrix chromite grains in the same meteorite. Chromite veinlets (typically 0.5-2 microns thick and 10-300 microns long) occur typically in the vicinity of chromite-plagioclase assemblages. The veinlets formed from chromite-plagioclase melts that were injected into fractures in neighboring silicate grains; chromite crystallized in the fractures and the residual plagioclase-rich melt continued to flow, eventually pooling to form plagioclase-rich melt pockets. Chromite-rich chondrules (consisting mainly of olivine, plagioclase-normative mesostasis, and 5-15 vol.% chromite) occur in many shocked OC and OC regolith breccias but they are absent from primitive type-3 OC. They may have formed by impact melting chromite, plagioclase and adjacent mafic silicates during higher-energy shock events. The melt was jetted from the impact site and formed droplets due to surface tension. Crystallization of these droplets may have commenced in flight, prior to landing on the parent-body surface. Chromite-plagioclase assemblages and chromite veinlets occur in 25 out of 25 shock-stage S1 OC of petrologic type 5 and 6 that I examined. Although these rocks contain unstrained olivine with sharp optical extinction, most possess other shock indicators such as extensive silicate darkening, numerous occurrences of metallic Cu, polycrystalline troilite, and opaque veins. It seems likely that these rocks were shocked to levels at least as high as shock-stage S3 and then annealed by heat generated during the shock event. During annealing, the olivine crystal lattices healed but other shock indicators survived. Published Ar-Ar age data for some SI OC indicate that many shock and annealing events occurred very early in the history of the parent asteroids. The common occurrence of shocked and annealed OC is consistent with collisions being a major mechanism responsible for metamorphosing OC.

Rubin, Alan E.↗
NASA NTRS1999

Massive Chromite in the Brenham Pallasite and the Fractionation of Cr During the Crystallization of Asteroidal Cores

Large (greater than or equal to 2 mm) chromite grains are present in IIIAB iron meteorites and in the main-group a pallasites (PMG), closely related to high-Au IIIAB irons, Pallasites seem to have formed by the intrusion of a highly evolved metallic magma from a IIIAB-like core into fragmented olivine of the overlying dunite mantle. High Cr contents are commonly encountered during the analyses of metallic samples of high-Au IIIAB irons and main-group pallasites, an indication that Cr contents were high in the intruding liquid and that Cr behaved as an incompatible element during the crystallization of the IIIAB magma, contrary to expectations based on the negative IIIAB Cr-Ni and Cr-Au trends among low-Au IIIAB irons. In a region about 10 cm across in the Brenham main-group pallasite massive chromite fills the interstices between olivine grains, the site normally occupied by metal in Brenham and other pallasites. The massive chromite may have formed as a late cumulus phase; because Fe-Ni was also crystallizing, its absence in the chromite-rich region suggests a separation associated with differences in liquid buoyancy. The coexisting chromite and olivine are zoned; in the olivine FeO is highest in pallasitic (olivine-metal) regions, lowest in rims adjacent to chromite, and intermediate in the cores of these olivines. Chromite shows the opposite zoning, with the highest FeO contents at grain edges adjacent to olivine. The observed gradients are those expected to form by Fe-Mg exchange between olivine and chromite during slow cooling at subsolidus temperatures. Compared to normal Brenham, contents of phosphoran olivine and phosphates are higher in the chromitic pallasitic region. We also report data for large-to-massive chromites present in PMG Molong and in high-Au IIIAB Bear Creek that, like Brenham, formed from a highly evolved magma. The Bear Creek chromite has a much lower Mg content than that in the pallasites, implying that, in the PMG, the Mg was extracted from the olivine during high-temperature reaction with the precipitating chromite. There are other circumstantial arguments indicating that Cr was incompatible in the metal during the crystallization of the IIIAB magma, with the concentration in die residual magma rising from an initial value of about 300 micrograms/g to a value around 700 micrograms/g when Bear Creek and Brenham were formed. We consider possible explanations for these negative Cr-Au and Cr-Ni trends and find the most probable one to be that they reflect sampling artefacts resulting from analysts avoiding visible chromite (and the commonly associated phase FeS) when choosing metal samples.

Wasson, John T.↗
DOE OSTI2017

Shocked chromites in fossil L chondrites: A Raman spectroscopy and transmission electron microscopy study

Abstract Chromites from Middle Ordovician fossil L chondrites and from matrix and shock‐melt veins in Catherwood, Tenham, and Coorara L chondrites were studied using Raman spectroscopy and TEM . Raman spectra of chromites from fossil L chondrites showed similarities with chromites from matrix and shock‐melt veins in the studied L chondrite falls and finds. Chromites from shock‐melt veins of L chondrites show polycrystallinity, while the chromite grains in fossil L chondrites are single crystals. In addition, chromites from shock‐melt veins in the studied L chondrites have high densities of planar fractures within the subgrains and many subgrains show intergrowths of chromite and xieite. Matrix chromite of Tenham has similar dislocation densities and planar fractures as a chromite from the fossil meteorite Golvsten 001 and higher dislocation densities than in chromite from the fossil meteorite Sextummen 003. Using this observation and knowing that the matrix of Tenham experienced 20–22 GP a and <1000° C, an upper limit for the P,T conditions of chromite from Golvsten 001 and Sextummen 003 can be estimated to be 20–22 GP a and 1000° C (shock stage S3–S6) and 20 GP a and 1000° C (S3–S5), respectively, and we conclude that the studied fossil meteorite chromites are from matrix.

Rout, Surya S.↗
DOE OSTI2018

Calcium Chloride-Assisted Segregation Reduction of Chromite: Influence of Reductant Type and the Mechanism

This paper describes an innovative process for the production of ferrochromium alloy via segregation reduction of chromite at 1300 °C in the presence of calcium chloride. Both charcoal and petroleum coke were used as the reductant. Individual polycrystalline ferrochrome carbide particles were produced, with their particle size and shape resembling that of the starting carbon particles. Interactions among calcium chloride, clinochlore, and chromite resulted in the formation of Ca-bearing chromite, wadalite, and gaseous chlorides. Monocrystalline ferrochrome carbide whiskers were formed only when charcoal in the presence of calcium chloride was used to reduce the chromite fraction containing large amounts of siliceous gangue. Incorporating thermodynamic evaluations, a possible segregation reduction mechanism is suggested based on the characterization of the products using scanning electron microscopy, energy dispersive spectroscopy, X-ray powder diffraction, and electron probe microanalysis, combined with thermogravimetric analysis. Metallization within chromite particles was minimized, which is suggested to be due to the ubiquitous presence of molten and gaseous calcium chloride between the chromite and carbon particles, and especially in the porous rim of the chromite particles during reduction, resulting in restricted metallization only on the carbon particles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1993

The origin of chromitic chondrules and the volatility of Cr under a range of nebular conditions

We characterize ten chromatic chondrules, two spinelian chondrules andd one spinel-bearing chondrule and summarize data for 120 chromitic inclusions discovered in an extensive survey of ordinary chondrites. Compositional and petrographic evidence suggests that chromitic chondrules and inclusions are closely related. The Cr/(Cr + Al) ratios in the spinal of these objects range from 0.5 to 0.9 and bulk Al2O3 contents are uniformly high (greater than 10 wt%, except for one with 8 wt%). No other elements having comparable solar abundances are so stongly enriched, and alkali feldspar and merrillite are more common than in normal chondrules. The Cr/Mg ratios in chromitic chondrules are 180-750 times the ratios in the bulk chondrite. With the possible exception of magnetic clumping of chromite in the presolar cloud, mechanical processes cannot account for this enrichment. Examination of nebular equilibrium processes shows that 50%-condensation temperatures of Cr at pH2/pH2O of 1500 are several tens of degrees below those of Mg as Mg2SiO4; the condensation of Cr is primarily as MgCr2O4 dissolved in MgAl2O4 at nebular pressures of 10(exp -4) atm or below. At pH2 = 10(exp -3) atm condesation as Cr in Fe-Ni is favored. Making the nebula much more oxidizing reduces the difference in condensation temperatures but Mg remains more refractory. We conclude that nebular equilibrium processes are not responsible for the enhanced Cr/Mg ratios. We propose that both Cr and Al became enriched in residues formed by incomplete evaporation of presolar lumps. We suggest that spinals remained as solid phases when the bulk of the silicates were incorporated into the evaporating melt; vaporization of Al and Cr were inhibited by the slow kinetics of diffusion. Subsequent melting and crystallization of these residues fractionated Cr from Al. The resulting materials constituted major components in the precursors of chromitic chondrules. Our model implies that chromitic chondrules and inclusions preserve the Cr isotopic record of presolar sources.

Krot, Alexander↗
DOE OSTI2011

High Performance Ceramic Interconnect Material for Solid Oxide Fuel Cells (SOFCs): Ca- and Transition Metal-doped Yttrium Chromite

The effect of transition metal substitution on thermal and electrical properties of Ca-doped yttrium chromite was investigated in relation to use as a ceramic interconnect in high temperature solid oxide fuel cells (SOFCs). 10 at% Co, 4 at% Ni, and 1 at% Cu substitution on B-site of 20 at% Ca-doped yttrium chromite led to a close match of thermal expansion coefficient (TEC) with that of 8 mol% yttria-stabilized zirconia (YSZ), and a single phase Y0.8Ca0.2Cr0.85Co0.1Ni0.04Cu0.01O3 remained stable between 25 and 1100 degree C over a wide oxygen partial pressure range. Doping with Cu significantly facilitated densification of yttrium chromite. Ni dopant improved both electrical conductivity and dimensional stability in reducing environments, likely through diminishing the oxygen vacancy formation. Substitution with Co substantially enhanced electrical conductivity in oxidizing atmosphere, which was attributed to an increase in charge carrier density and hopping mobility. Electrical conductivity of Y0.8Ca0.2Cr0.85Co0.1Ni0.04Cu0.01O3 at 900 degree C is 57 S/cm in air and 11 S/cm in fuel (pO2=5×10^-17 atm) environments. Chemical compatibility of doped yttrium chromite with other cell components was verified at the processing temperatures. Based on the chemical and dimensional stability, sinterability, and thermal and electrical properties, Y0.8Ca0.2Cr0.85Co0.1Ni0.04Cu0.01O3 is suggested as a promising SOFC ceramic interconnect to potentially overcome technical limitations of conventional acceptor-doped lanthanum chromites.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2018

Dissolution Behavior of Mg from Magnesia-Chromite Refractory into Al-killed Molten Steel

Magnesia-chromite refractory materials are widely employed in steel production, and are considered a potential MgO source for the generation of MgO·Al{sub 2}O{sub 3} spinel inclusions in steel melts. In this study, a square magnesia-chromite refractory rod was immersed into molten steel of various compositions held in an Al{sub 2}O{sub 3} crucibles. As the immersion time was extended, Mg and Cr gradually dissolved from the magnesia-chromite refractory, and the Mg and Cr contents of the steel melts increased. However, it was found that the inclusions in the steel melts remained as almost pure Al{sub 2}O{sub 3} because the Mg content of the steel melts was low, approximately 1 ppm. On the surface of the magnesia-chromite refractory, an MgO·Al{sub 2}O{sub 3} spinel layer with a variable composition was formed, and the thickness of the MgO·Al{sub 2}O{sub 3} spinel layer increased with the immersion time and the Al content of the steel melts. At the rod interface, the formed layer consisted of MgO-saturated MgO·Al{sub 2}O{sub 3} spinel. The MgO content decreased along the thickness direction of the layer, and at the steel melts interface, the formed layer consisted of Al{sub 2}O{sub 3}-saturated MgO·Al{sub 2}O{sub 3} spinel. Therefore, the low content of Mg in steel melts and the unchanged inclusions were because of the equilibrium between Al{sub 2}O{sub 3}-saturated MgO·Al{sub 2}O{sub 3} layer and Al{sub 2}O{sub 3}. In addition, the effects of the Al and Cr contents of the steel melts on the dissolution of Mg from the magnesia-chromite refractory are insignificant.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Damring Formation During Rotary Kiln Chromite Pre-reduction: Effects of Pulverized Carbonaceous Fuel Selection and Partial Pellet Melting

Electricity consumption is the largest cost contributing factor in the production of ferrochrome. Currently the pelletized chromite pre-reduction process (solid-state reduction of chromite) is the process option with the lowest specific electricity consumption (MWh/ton). In this process, pelletized chromite is fed into a rotary kiln at 1573 K (1300 °C), where partial pre-reduction takes place. Damring formation (material build-up) in the rotary kiln causes routine shutdowns, resulting in loss of revenue. The damring formation is possibly caused by melting of the ash of the pulverized coal used to fire the kiln and/or the partial melting of the chromite pellets. Ash fusion temperatures of twenty different samples were evaluated to assess the temperature at which the pulverized coal ash will start to contribute to damring formation. Sessile drop tests were used to assess the softening behavior of different ore types (e.g., UG2, MG, and LG metgrade), as well as softening of composite chromite pellets made from these ores. Actual damrings were also analyzed using scanning electron microscopy with energy dispersive X-ray spectroscopy. Results indicate that it is mainly the pulverized coal ash that will contribute to damring formation, and not ore or pellet softening. Multiple-linear regression was used to derive equations to predict the ash fusion temperatures of the pulverized coal ash, which can be used by ferrochrome producers to optimize pulverized coal selection.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Direct Production of Ferrochrome by Segregation Reduction of Chromite in the Presence of Calcium Chloride

A solid reduction process is described whereby chromite is reduced with the help of calcium chloride to produce ferrochrome alloy powders with high metal recovery. The process involves segregation reduction of chromite using graphite as the reductant and calcium chloride as the segregation catalyst. Experiments were performed in the temperature range of 1200–1400 °C to evaluate the influences of various design parameters using both a thermogravimetric analyzer and an electric tube furnace with continuous off-gas analysis. The reduced products were characterized by scanning electron microscopy, X-ray powder diffraction, synchrotron X-ray absorption spectroscopy, and were subjected to wet chemical analysis. It was concluded that the addition of calcium chloride not only accelerated the carbothermic reduction of chromite but also promoted the formation and growth of individual ferrochrome alloy particles. The alloy formation within chromite particles was minimized, enabling the effective separation of ferrochrome alloy particles from the unwanted gangue without the need for fine grinding. Majority of the calcium chloride remained in a recoverable form, with a small percentage (<10 wt %) consumed by reacting with the siliceous gangue forming wadalite. Pure ferrochrome alloy powders were successfully produced with high metal recovery using elutriating separation.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Investigation of nanocrystalline zinc chromite obtained by two soft chemical routes

Graphical abstract: - Highlights: • Two soft chemical routes to synthesize zinc chromites are described. • Glycine is used as chelating agent (precursor method) and fuel (solution combustion method). • The synthesized chromites have crystallite size in the range of 18–27 nm. • An antiferromagnetic (AFM) transition is observed at about T{sub N} ∼ 18 K. - Abstract: Zinc chromite (ZnCr{sub 2}O{sub 4}) nanocrystalline powders were obtained by two different chemical routes: the precursor method and the solution combustion method involving glycine-nitrates. The complex compound precursors, [ZnCr{sub 2}(NH{sub 2}CH{sub 2}COO){sub 8}]·9H{sub 2}O and [ZnCr{sub 2}(NH{sub 2}CH{sub 2}COOH){sub 4.5}]·(NO{sub 3}){sub 8}·6H{sub 2}O, were characterized by chemical analysis, infrared spectroscopy (IR), ultraviolet–visible spectroscopy (UV–vis) and thermal analysis. The structure, morphology, surface chemistry and magnetic properties of ZnCr{sub 2}O{sub 4} powders were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), infrared and Raman spectroscopy (RS), ultraviolet–visible spectroscopy (UV–vis) and magnetic measurements. X-ray diffraction patterns indicated the chromite spinel phase with good crystallinity and an average crystallite size of approximately 18–27 nm. The band gap values ranged between 3.31 and 3.33 eV. The magnetic measurements indicated an antiferromagnetic transition at T{sub N} ∼ 17.5/18 K.

36 MATERIALS SCIENCE↗
DOE OSTI2012

From spin induced ferroelectricity to dipolar glasses: Spinel chromites and mixed delafossites

Magnetoelectric multiferroics showing coupling between polarization and magnetic order are attracting much attention. For instance, they could be used in memory devices. Metal-transition oxides are provided several examples of inorganic magnetoelectric multiferroics. In the present short review, spinel and delafossite chromites are described. For the former, an electric polarization is evidenced in the ferrimagnetic state for ACr{sub 2}O{sub 4} polycrystalline samples (A=Ni, Fe, Co). The presence of a Jahn-Teller cation such as Ni{sup 2+} at the A site is shown to yield larger polarization values. In the delafossites, substitution by V{sup 3+} at the Cr or Fe site in CuCrO{sub 2} (CuFeO{sub 2}) suppresses the complex antiferromagnetic structure at the benefit of a spin glass state. The presence of cation disorder, probed by transmission electron microscopy, favors relaxor-like ferroelectricity. The results on the ferroelectricity of ferrimagnets and insulating spin glasses demonstrate that, in this research field, transition-metal oxides are worth to be studied. - Graphical abstract: Electric polarization as a function of temperature is measured up to T{sub C} in three chromite ferrimagnetic spinels. Largest values are reached for spinels with Jahn-Teller cations at the A site (Ni or Fe). Highlights: Black-Right-Pointing-Pointer Electric polarization is evidenced in the ferrimagnetic state of the chromite spinels. Black-Right-Pointing-Pointer Jahn-Teller cations at the A site of these spinels lead to larger polarization values. Black-Right-Pointing-Pointer Vanadium substituted at the Cr (or Fe) site of delafossites changes the antiferromagnetic state to spin glass. Black-Right-Pointing-Pointer Electric polarization is not the result of magnetic ordering but magnetic disordering in Cr or Fe delafossites. Black-Right-Pointing-Pointer Relaxor-type ferroelectricity or spin induced ferroelectricity can be observed in the delafossites.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗