Layered Mn Oxides Research Articles

Improvement of a Single‐Chamber Solid‐Oxide Fuel Cell and Evaluation of New Cell Designs

The performance of a single‐chamber solid‐oxide fuel cell (SOFC) made from an yttria‐stabilized zirconia solid electrolyte with a (GDC)‐containing Ni anode and a cathode was found to be significantly enhanced by the deposition of Mn, Ga, Cr, Ce, and Lu oxide layers on the YSZ surface. In particular, the deposition of the Mn oxide layer increased the maximum power density from 161 to in a mixture of methane and air having a volume ratio of methane to oxygen of 1/1 at a flow rate of (methane , oxygen , nitrogen ), and at an operating temperature of 950°C. This effect was the result of the promoted anodic and cathodic reactions. Two types of cell designs were examined for the single‐chamber SOFC; the two electrodes were deposited on opposite surfaces (A‐type cell) and on the same face (B‐type cell) of the solid electrolyte. The A‐type cell showed an increasing power density with decreasing thickness of the solid electrolyte. The maximum power density was at a solid electrolyte thickness of 0.3 mm. The B‐type cell showed an increased power density for a decreased gap between the two electrodes. The maximum power density was for a gap of 0.5 mm between the two electrodes. In addition, the long‐term stability of the single‐chamber SOFC was also studied and found to have a direct relationship with the carbon deposition on the GDC‐containing Ni electrode. © 2000 The Electrochemical Society. All rights reserved.

Hydrothermal manganese crusts from Enarete and Palinuro seamounts in the Tyrrhenian Sea

Manganese crusts were recovered from Enarete and Palinuro Seamounts in the Tyrrhenian Sea. The crusts consist of porous, black, layered Mn oxides up to 45 mm thick. In some cases, the surface has a black metallic sheen similar to that observed in crusts from the Tonga‐Kermadec Arc. The crusts overlie substrates such as calcareous sediment, siltstone, and oyster shells. They consist dominantly of 10‐Å manganate and 7‐Å manganate, with minor quartz, calcite, aragonite, illite, mont‐morillonite, plagioclase, and goethite. The sample having the highest Mn content contained 52.8% Mn, 0.15% Fe, 46 ppm Ni, 84 ppm Cu, 26 ppm Zn, 6 ppm Pb, and 2,130 ppm Ba, with a Mn/Fe ratio of 347. It also had a low rare‐earth element (REE) abundance (La 2.7 ppm) and a negative Ce anomaly (Ce/La ratio 0.5). A TV profile on Palinuro showed the occurrence of yellow (Fe oxyhydroxide or nontronite), black (Mn oxide), and white (sulfate) halos in the sediment and opaqueness in the overlying water. Nontronite and massive sulfides were also recovered at some locations on Palinuro. The morphology, mineralogy, and composition of these crusts as well as ancillary geological evidence confirm that the crusts are of hydrothermal origin. Similar crusts occur in a number of island‐arc environments, particularly in the Pacific, and are known to be formed by low‐temperature hydrothermal venting. The origin of the Tyrrhenian Sea is complex but it is believed to be a back‐arc basin formed above a NW‐dipping subduction zone. The Aeolian islands and seamounts consist of 16 volcanic edifices which form an anticlockwise arc around the Marsili abyssal plain. Palinuro is the youngest of these. Our data support the idea that the Mn deposits formed as a result of submarine hydrothermal activity associated with subduction‐related processes.

Giant magnetoresistance in layered Mn oxides

The magnetic phase diagram and giant magnetoresistance in perovskite (LaX)MnO 3 with XBa, Ca or Sr are studied by using a model where t 2g and e g electrons of Mn ions are treated as localized spins and strongly correlated itinerant electrons, respectively, and a Hund coupling exists between them. It is shown in a mean field approximation that the antiferromagnetism of LaMnO 3 changes to a spiral state, not the canted state proposed before, by doping of mobile carriers. Giant magnetoresistance is studied in a strong coupling limit of the model. The strong Hund coupling is shown to account for a strong dependence of the resistivity on the magnetization induced by an external magnetic field in agreement with experimental results.

Rare earth element evidence of origin and depositional environment of Late Proterozoic ironstone beds and manganese-oxide deposits, SW Brazil and SE Bolivia

Ironstone beds and high grade Mn-oxide deposits (layers or concretions) of Late Precambrian age occur in SW Brazil and SE Bolivia. These deposits, referred to here as Jacadigo-Boqui Fe-Mn deposits after the stratigraphic units hosting most of them, share many characteristics with other siliceous, Precambrian iron formations. Rare earth element (REE) distributions in the Jacadigo-Boqui Fe-Mn deposits were determined by instrumental neutron activation analysis (INAA) and compared with published data for other iron formations and for various water types. REE patterns of both ironstone beds and Mn-oxide deposits are light REE (LREE) enriched on chondrite-normalized plots. Average REE ratios of all samples are: La/Sm = 5.66, La/Ce = 0.63, Eu/Sm = 0.19, Sm/Lu = 8.09. Sm/Lu values for all samples range from 2 to 16, and Sm/Lu values also vary among samples of ironstone beds and Mn-oxide deposits. REE ratios are generally not related to other chemical parameters. Comparison to REE distributions in river, estuarine, and oceanic waters and to particulate matter forming in the oceans and anoxic basins suggests that the Jacadigo-Boqui Fe-Mn deposits formed in waters most similar to those of estuaries or coastal oceans. Formation in river or lake water appears unlikely. The lower Sm/Lu values of oxide layers suggest that these layers formed in a more marine environment than the jasper layers. The balance of continental and marine inputs may have been changed by glacial cycles or sea level changes, either eustatic or tidal. REE patterns of the Jacadigo-Boqui Fe-Mn deposits are roughly similar to those for most other iron formations. However, La/Ce and Eu/Sm values are much lower. A glaciogenic model suggests that the ironstone beds formed in a basin under an ice shelf as a result of freezing. Nodular, detritus-rich Mn-oxide deposits probably formed off the edge of the ice shelf. Layered Mn-oxide deposits most probably formed rapidly during glacial melting and retreat of the shelf as a result of quantitative oxidation of basin water. Glacial melting or lower sea level should result in higher Sm/Lu and lower La/Ce and La/Sm values of chemical sediments (i.e., values more like those of rivers). However, the similarity of REE ratios of layered Mn-oxides and ironstone beds suggest that overturn and oxidation dominated over mixing of marine and river waters during deposition of layered Mn-oxides. The negative Eu anomalies of the Jacadigo-Boqui Fe-Mn deposits distinguish them from most natural waters and other iron formations and are surprising in light of the hypothesis that banded iron formations derive their Fe and Mn from sea-floor, hydrothermal vent waters. It is unlikely that these anomalies were produced during precipitation of Fe- and Mn-oxides from waters with oceanic or mid-ocean and ridge hydrothermal REE patterns. Mixing of REE from a continental water with a negative Eu anomaly and REE from oceanic and ridge hydrothermal waters is believed unlikely because it can not explain all the REE ratios and presents problems regarding the source of Fe and Mn. Alternatives that must be evaluated are derivation of substantial Fe, Mn, and REE from either a continental source or a hydrothermal fluid that had a REE pattern different from that of mid-ocean ridge hydrothermal fluids.

Ferromanganese deposits from the Gulf of Alaska Seamount Province: mineralogy, chemistry, and origin

Ferromanganese-oxide deposits dredged from four seamounts (Welker, Miller, Murray, and Patton) in the Gulf of Alaska Seamount Province include poorly crystallized microlaminated crusts on basalt substrate, well-crystallized Mn-oxide veins in epiclastic sedimentary rocks, and crystalline Mn-oxide layers and micronodules in phosphorite. The principal rock types dredged are alkali-basalt pillow fragments and tuffaceous conglomerate and sandstone. The glassy rims of pillow fragments, the glassy goundmass of large volcanic clasts, and the tuffaceous component of the sediment are altered to palagonite. Other low-temperature alteration products include phillipsite, smectite, and carbonate-apatite.Thick (10–50 mm) Fe–Mn crusts consist mainly of δ-MnO2; straight and cuspate growth laminae indicate variable growth rates and periods of nondeposition. A larger number of detrital particles toward the top of thick crusts record the increasing influence of active volcanoes of the Aleutian arc during northwestward movement of the Pacific plate. Thick crusts on basalt substrate have higher Mn/Fe ratios and lower Co content than Fe–Mn crusts from low-latitude seamounts of the central Pacific region. Thin (< 10 mm) crusts on volcaniclastic substrate contain todorokite and birnessite and have higher Mn/Fe ratios, Ni, and Cu and lower Fe and Co than thick Gulf of Alaska crusts.Veins of todorokite and cryptomelane with complex internal structure occur in altered tuffaceous sandstone and conglomerate from Miller Seamount. Fibrous todorokite has a composition similar to those of other marine examples but may contain up to 7% Mn2+ in M2 sites. Microprobe analysis of the marine cryptomelane indicates a composition that is approximately (K,Ba)1–2(Mn4+,Co)7–8O16∙x(H2O).A third type of Fe–Mn deposit in phosphorite is an intimate mixture of todorokite (and minor δ-MnO2)-bearing layers and micronodules, carbonate-apatite, and phillipsite that encloses grains of altered volcanic glass and lithic fragments.The microlaminated structure, mineralogy (predominantly δ-MnO2), and composition (Mn/Fe ratio and transition metal, rare earth element, U, and Th contents) of the thick crusts are characteristic of hydrogenetic Fe–Mn crusts elsewhere in the Pacific. Conversely, the crystallinity and chemical composition of the Mn-oxide veins and thin crusts indicate formation during diagenesis of the volcanogenic sediment substrate. Mn and other transition metals are mobilized during low-temperature oxidative alteration (palagonitization) of basaltic volcanic glass; the oxidation of Fe2+ to Fe3+ during palagonitization and the dissolution of the dilute biogenic fraction of the sediment combine to lower the Eh of ambient pore fluid and enhance the mobility of Mn2+. Diagenesis in the phosphatic sandstone from Patton Seamount involves organic-rich sediment and pore waters elevated in phosphorus owing to upwelling above a large volcanic edifice.