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Journal ArticleDOI

Studies of electrical transport properties of Sr2Fe(Mo, V)O6 compound

10 Mar 2004-Journal of Alloys and Compounds (Elsevier)-Vol. 366, Iss: 1, pp 28-33
TL;DR: In this article, an analysis of X-ray, electrical resistivity and thermoelectric power on polycrystalline Sr 2 FeMoO 6 (x = 0) samples has been performed over a wide temperature range (20-900 K).
Abstract: Systematic study of X-ray, electrical resistivity ( ρ ) and thermoelectric power (TEP, S ) on polycrystalline Sr 2 FeMo 1− x V x O 6 ( x =0–0.5) samples have been performed over a wide temperature range (20–900 K). From the analysis of the resistivity data we find that the temperature dependence of ρ of the undoped Sr 2 FeMoO 6 ( x =0) sample is similar to that of metallic sample. For the doped samples ( x ≠0) ρ scales well with T 2 nature only in the high temperature region and the analysis suggests that the electron–electron scattering plays a major role governing the conduction mechanism. At the low temperature semiconducting region ρ ( T ) data starts deviation from the T 2 variation and can be explained considering three-dimensional variable range hopping (VRH) processes implying weak localization of the charge carriers. For all the samples with x >0 thermoelectric power ( S ) measurement reveals almost linear temperature dependencies similar to metallic samples. For x =0 sample the linear variation of S ( T ) is observed only in the temperature interval ( 170 K K ) and below 170 K S ( T ) shows a small curvature down to 77 K. At T S from negative to positive values has been observed for samples with x =0.1–0.3. The estimated Fermi energy ( E F ) calculated from the slope of S ( T ) curves lies in the range 1.12–0.61 eV for x =0.0–0.5 samples, respectively.
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01 May 2010
TL;DR: In this paper, the performance and durability of a solid oxide cell applied for co-electrolysis of CO2 and H2O to produce syngas (CO/H2 mixture) is investigated.
Abstract: Recycling CO2 into Sustainable Hydrocarbon Fuels: Electrolysis of CO2 and H2O Christopher Ronald Graves Great quantities of hydrocarbon fuels will be needed for the foreseeable future, even if electricity based energy carriers begin to partially replace liquid hydrocarbons in the transportation sector. Fossil fuels and biomass are the most common feedstocks for production of hydrocarbon fuels. However, using renewable or nuclear energy, carbon dioxide and water can be recycled into sustainable hydrocarbon fuels in non-biological processes which remove oxygen from CO2 and H2O (the reverse of fuel combustion). Capture of CO2 from the atmosphere would enable a closed-loop carbon-neutral fuel cycle. The purpose of this work was to develop critical components of a system that recycles CO2 into liquid hydrocarbon fuels. The concept is examined at several scales, beginning with a broad scope analysis of large-scale sustainable energy systems and ultimately studying electrolysis of CO2 and H2O in high temperature solid oxide cells as the heart of the energy conversion, in the form of three experimental studies. The contributions of these studies include discoveries about electrochemistry and materials that could significantly improve the overall energy use and economics of the CO2-to-fuels system. The broad scale study begins by assessing the sustainability and practicality of the various energy carriers that could replace petroleum-derived hydrocarbon fuels, including other hydrocarbons, hydrogen, and storage of electricity on-board vehicles in batteries, ultracapacitors, and flywheels. Any energy carrier can store the energy of any energy source. This sets the context for CO2 recycling – sustainable energy sources like solar and wind power can be used to provide the most energy-dense, convenient fuels which can be readily used in the existing infrastructure. The many ways to recycle CO2 into hydrocarbons, based on thermolysis, thermochemical loops, electrolysis, and photoelectrolysis of CO2 and/or H2O, are critically reviewed. A process based on high temperature co-electrolysis of CO2 and H2O to produce syngas (CO/H2 mixture) is identified as a promising method. High temperature electrolysis makes very efficient use of electricity and heat (near-100% electricity-to-syngas efficiency), provides high reaction rates, and the syngas produced can be catalytically converted to hydrocarbons in well-known fuel synthesis reactors (e.g. Fischer-Tropsch). The experimental studies of high temperature electrolysis are made at different scales – at the cell level, electrode level, and in materials and microstructure development. The results include cell performance and durability, insight into electrode reaction mechanisms, and new high-performance electrode materials. The experimental studies make extensive use of electrochemical impedance spectroscopy and systematic variation of test conditions to examine the electrochemical phenomena. Variation of the material composition itself within families of related materials was an additional parameter used in the electrode level and materials studies that revealed more information than studying a single material would have. Using full cells, the performance and durability of a solid oxide cell applied for coelectrolysis of CO2 and H2O was investigated. High initial performance was observed but the long-term durability needs to be improved. Based on these results, an analysis of the energy balance and economics of an electrolysis-based synthetic fuel production process, including CO2 air capture and Fischer-Tropsch fuel synthesis, determined that the system can feasibly operate at 70% electricity-to-liquid fuel efficiency (higher heating value basis) and that the price of electricity needed to produce competitive synthetic gasoline (at USD$2/gal, or $0.53/L, wholesale) is 2-3 U.S. cents per kWh. For $3/gal ($0.78/L) gasoline, 4-5 cents per kWh is needed. Fuel production may already be economical in some regions that have inexpensive renewable electricity, such as Iceland. The dominant costs of the process are the electricity cost and the capital cost of the electrolyzer, and this capital cost is significantly increased when operating intermittently (on renewable power sources such as solar and wind). Low cell internal resistance, low degradation, and low manufacturing cost each contribute to a low electrolyzer capital cost, and can be traded off. One straightforward path to affordability is by improving the durability of the high current density cell operation (≥1 A/cm) that is already possible with these cells. The negative-electrode, a composite of nickel and yttria-stabilized zirconia (YSZ), is often the major site of cell degradation, including in the co-electrolysis results presented here. To better understand the reaction mechanisms at the negative-electrode that limit performance and durability, different metal electrodes including nickel were studied using a simplified point-contact electrode geometry with a well-defined three-phase boundary (TPB; the electrode/electrolyte/gas interface where the electrochemical reactions take place). The simple geometry is useful for isolating the electrochemical properties without the effects of the complex microstructure of technological porous electrodes. Widely different impedance responses of the different metals to the same changes in test conditions (gas composition, temperature, and polarization) were observed, indicating that the same reaction mechanisms are not shared by the different metals, contrary to some recent studies. Evidence was also found that supports the explanation that impurities segregated to the TPB play a major role and are largely responsible for inconsistencies in the electrode kinetics literature. The significance of microstructure at the TPB was also revealed – the electrode polarization resistance was reduced by an order of magnitude when subjected to extreme conditions of oxidation-reduction and strong cathodic polarization, which induced the formation of a micro/nanostructured TPB. Possible reaction mechanisms for H2O/CO2 reduction and H2/CO oxidation are discussed. Novel ceramic materials based on molybdates with varying Mo valence were synthesized as possible alternative negative-electrode materials. The phase, stability, microstructure and electrical conductivity were characterized. The electrochemical activity for H2O/CO2 reduction and H2/CO oxidation was studied using simplified geometry electrodes, similar to the metals study. Unique phenomena were observed for some of the molybdate materials – they decomposed into multiple phases and formed a nanostructured surface upon exposure to operating conditions (in certain reducing atmospheres). The new phases and surface features enhanced the electronic conductivity and electrocatalytic activity. Preparing an electrode by performing controlled decomposition to form multiple desirable phases and a desirable microstructure (which can take place in situ) using these materials is a novel way to produce potentially high-performance electrodes for solid oxide cells. By modifying the composition, it was possible to prevent decomposition. Other members of the molybdate family exhibited similarly high electronic conductivity and electrocatalytic activity but did not decompose. The high activity was the result of a different mechanism, probably related to the defect chemistry of the material. The polarization resistances of the best molybdate materials were two orders of magnitude lower than that of donor-doped strontium titanates. Many of the molybdate materials were significantly activated by cathodic polarization, and they exhibited higher performance for cathodic (electrolysis) polarization than anodic (fuel cell) polarization, which makes them especially interesting for use in electrolysis electrodes. Whereas nearly all of the molybdates showed higher performance for H2O electrolysis than CO2 electrolysis, one with vanadium showed nearly equal performance, and a non-molybdate which exhibits some complementary properties to the best molybdates, Gd-doped ceria in nanoparticle form, was found to be an excellent electrocatalyst for CO2 electrolysis and CO oxidation (moreso than for H2O/H2 for which it is known to be good).

51 citations

Journal ArticleDOI
TL;DR: In this paper, the influence of successive sintering treatments on the structural, magnetic and electric properties of high ordered Sr2FeMoO6 (SFMO) samples belonging to the same starting batch was investigated.
Abstract: The influence of successive sintering treatments on the structural, magnetic and electric properties of high ordered Sr2FeMoO6 (SFMO) samples belonging to the same starting batch was investigated The solid-state synthesis conditions influence the microstructure of the samples and the Fe/Mo order Successive sintering treatments with intermediate grindings improve the saturation magnetization (38μB) which became close to the ideal value (4μB) and the low field magnetoresistance (LFMR) as a consequence of microstructural changes Above the Curie temperature the evolution of effective magnetic moment values depicts the presence of mixed valency states: FeII/MoVI and FeIII/MoV where the contribution of FeII/MoVI configuration is favoured by the enhancement of Fe/Mo order during successive sintering treatments

26 citations

Journal ArticleDOI
TL;DR: Sr2MgMo0.95V0.05O6−−d synthesized in 5%H2/Ar showed a conductivity higher than the samples with x < 0.05, but close to the sample without V. This sample is promising for the anode of SOFCs using biogas fuel.
Abstract: Sr2MgMo1 − xVxO6 − d (x = 0–0.2) materials with double perovskite structure were synthesized by sol–gel method, and studied for the possibility of being the anode of solid oxide fuel cells (SOFCs) with biogas as the direct fuel. The sample of Sr2MgMo0.95V0.05O6 − d synthesized in 5%H2/Ar showed a conductivity higher than the samples with x > 0.05, but close to the sample without V. Besides, it showed better catalytic activity, stability, and H2S tolerance (up to 1% of H2S in the feed gas) for biogas combustion than the sample without V. This sample is promising for the anode of SOFCs using biogas fuel.

13 citations

Journal ArticleDOI
TL;DR: In this paper, it was shown that the a-b plane is the active facet for the propylene oxide reaction and the acidity of Mo-V-O catalysts is associated with the active crystal facet.
Abstract: Crystalline Mo–V–O oxides have been used as a catalyst for the hydrolysis and alcoholysis of propylene oxide to diols and ethers, respectively. Relationships between the active crystal facet, the acidity of Mo–V–O catalysts and the activity have been established. Our results indicate that the a–b plane is the active facet for the hydrolysis reaction.

12 citations

Journal ArticleDOI
TL;DR: Lopez, Carlos Alberto, et al. this paper, this paper presented a paper entitled "Consejo Nacional de Investigaciones Cientificas y Tecnicas.
Abstract: Fil: Lopez, Carlos Alberto. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico San Luis. Instituto de Investigaciones En Tecnologia Quimica; Argentina

6 citations

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Journal ArticleDOI
TL;DR: In this article, the theory of double exchange was applied to perovskite-type manganites and detailed qualitative predictions about the magnetic lattice, the crystallographic lattice and the electrical resistivity were made.
Abstract: The theory of semicovalent exchange is reviewed and applied to the perovskite-type manganites $[\mathrm{La}, M(\mathrm{II})]\mathrm{Mn}{\mathrm{O}}_{3}$. With the hypothesis of covalent and semicovalent bonding between the oxygen and manganese ions plus the mechanism of double exchange, detailed qualitative predictions are made about the magnetic lattice, the crystallographic lattice, the electrical resistivity, and the Curie temperature as functions of the fraction of ${\mathrm{Mn}}^{4+}$ present. These predictions are found to be in accord with recent findings from neutron-diffraction and x-ray data as well as with the earlier experiments on this system by Jonker and van Santen.

3,148 citations

Journal ArticleDOI
15 Oct 1998-Nature
TL;DR: In this paper, an ordered double perovskite (Sr2FeMoO) was shown to exhibit intrinsic tunnelling-type magnetoresistance at room temperature.
Abstract: Colossal magnetoresistance—a huge decrease in resistance in response to a magnetic field—has recently been observed in manganese oxides with perovskite structure. This effect is attracting considerable interest from both fundamental and practical points of view1. In the context of using this effect in practical devices, a noteworthy feature of these materials is the high degree of spin polarization of the charge carriers, caused by the half-metallic nature of these materials20,21; this in principle allows spin-dependent carrier scattering processes, and hence the resistance, to be strongly influenced by low magnetic fields. This type of field control has been demonstrated for charge-carrier scattering at tunnelling junctions2,3 and at crystal-twin or ceramic grain boundaries4,5, although the operating temperature of such structures is still too low (⩽150 K) for most applications. Here we report a material—Sr2FeMoO6, an ordered double perovskite6—exhibiting intrinsic tunnelling-type magnetoresistance at room temperature. We explain the origin of this behaviour with electronic-structure calculations that indicate the material to be half-metallic. Our results show promise for the development of ordered perovskite magnetoresistive devices that are operable at room temperature.

2,065 citations

Journal ArticleDOI
TL;DR: The results show that the notion of ``double exchange'' must be generalized to include changes in the Mn-Mn electronic hopping parameter as a result of changes inThe Mn-O-Mm bond angle.
Abstract: A detailed study of doped LaMn${\mathrm{O}}_{3}$ with fixed carrier concentration reveals a direct relationship between the Curie temperature ${T}_{c}$ and the average ionic radius of the La site $〈{r}_{A}〉$, which is varied by substituting different rare earth ions for La. With decreasing $〈{r}_{A}〉$, magnetic order and significant magnetoresistance occur at lower temperatures with increasing thermal hysteresis, and the magnitude of the magnetoresistance increases dramatically. These results show that the notion of ``double exchange'' must be generalized to include changes in the Mn-Mn electronic hopping parameter as a result of changes in the Mn-O-Mn bond angle.

1,654 citations

Journal ArticleDOI
24 Sep 1998-Nature
TL;DR: In this article, it was shown that the atmosphere of solid planets is capable of exerting dynamic pressure on their surfaces, thereby exciting free oscillations with amplitudes large enough to be detected by modern broad-band seismographs.
Abstract: Seismology provides a powerful tool for probing planetary interiors1,2, but it has been considered inapplicable to tectonically inactive planets where earthquakes are absent. Here, however, we show that the atmospheres of solid planets are capable of exerting dynamic pressure on their surfaces, thereby exciting free oscillations with amplitudes large enough to be detected by modern broad-band seismographs. Order-of-magnitude estimates of these forces give similar amplitudes of a few nanogals for the Earth, Venus and Mars despite widely varying atmospheric and ambient conditions. The amplitudes are also predicted to have a weak frequency dependence. Our analysis of seismograms, recorded continuously from 1992 to 1993 at 13 globally distributed stations, shows strong evidence for continuously excited fundamental-mode free oscillations on the Earth. This result, together with other recent studies3,4,5, is consistent with our estimate of atmospheric forcing and we therefore propose that it may be possible to detect atmospheric excitation of free oscillations on Venus and Mars as well.

1,048 citations