116 research outputs found
Structural Disorder, Octahedral Coordination, and 2-Dimensional Ferromagnetism in Anhydrous Alums
The crystal structures of the triangular lattice, layered anhydrous alums
KCr(SO4)2, RbCr(SO4)2 and KAl(SO4)2 are characterized by X-ray and neutron
powder diffraction at temperatures between 1.4 and 773 K. The compounds all
crystallize in the space group P-3, with octahedral coordination of the
trivalent cations. In all cases, small amounts of disorder in the stacking of
the triangular layers of corner sharing MO6 octahedra and SO4 tetrahedra is
seen, with the MO6-SO4 network rotated in opposite directions between layers.
The electron diffraction study of KCr(SO4)2 supports this model, which on
average can be taken to imply trigonal prismatic coordination for the M3+ ions;
as was previously reported for the prototype anhydrous alum KAl(SO4)2. The
temperature dependent magnetic susceptibilities for ACr(SO4)2 (A = K,Rb,Cs)
indicate the presence of predominantly ferromagnetic interactions. Low
temperature powder neutron diffraction reveals that the magnetic ordering is
ferromagnetic in-plane, with antiferromagnetic ordering between planes below 3
K.Comment: Accepted to the Journal of Solid State Chemistr
K0.8Ag0.2Nb4O9AsO4
The title compound, potassium silver tetraÂniobium nonaÂoxide arsenate, K0.8Ag0.2Nb4O9AsO4, was prepared by a solid-state reaction at 1183â
K. The structure consists of infinite (Nb2AsO14)n chains parallel to the b axis and cross-linked by corner sharing via pairs of edge-sharing octaÂhedra. Each pair links together four infinite chains to form a three-dimensional framework. The K+ and Ag+ ions partially occupy several independent close positions in the interÂconnected cavities delimited by the framework. K0.8Ag0.2Nb4O9AsO4 is likely to exhibit fast alkali-ion mobility and ion-exchange properties. The Wyckoff symbols of special positions are as follows: one Nb 8e, one Nb 8g, As 4c, two K 8f, one Ag 8f, one Ag 4c, one O 8g, one O 4c
Vanadium(V) oxide arsenate(V), VOAsO4
The vanadyl arsenate, VOAsO4, has been isolated by a solid-state reaction. The structure consists of distorted VO6 octaÂhedra and AsO4 tetraÂhedra sharing corners to build up VAsO7 layers parallel to ac linked by edge-sharing of VO6 octaÂhedra, forming a three-dimensional framework
AgNa2Mo3O9AsO4
The title compound, silver disodium trimolybdenum(VI) nonaoxide arsenate, AgNa2Mo3O9AsO4, was prepared by a solid-state reaction at 808â
K. The structure consists of an infinite (Mo3AsO13)n ribbon, parallel to the c axis, composed of AsO4 tetraÂhedra and MoO6 octaÂhedra sharing edges and corners. The Na and Ag ions partially occupy several independent close positions, with various occupancies, in the inter-ribbon space delimited by the one-dimensional framework. The composition was refined to Ag1.06(1)Na1.94(1)Mo3O9AsO4
La variĂ©tĂ© ÎČ-NaMoO2(AsO4)
The title compound, sodium dioxidomolybdenum(VI) arsenate(V), ÎČ-NaMoO2AsO4, was prepared by solid-state reaction at 953â
K. In the crystal structure, the AsO4 tetraÂhedra and MoO6 octaÂhedra (both with m symmetry) share corner atoms to form a three-dimensional framework that delimits cavities parallel to [010] where disordered six-coordinated sodium cations (half-occupation) are located. Structural relationships between the different orthoarsenates of the AMoO2AsO4 series (A = Ag, Li, Na, K and Rb) are discussed
Conception dâobservateurs pour la commande dâun systĂšme pile Ă combustible embarquĂ© en vue dâoptimiser performances et durabilitĂ©
Fuel cells are considered as a promising source of energy for the future, thanks to their non-polluting aspect. However, the deployment of these solutions on a large scale is still conditioned by the improvement of their performance and especially of their durability in order to guarantee a low cost industrialization. The transport application also imposes a variable power demand, which complicates the improvement of performance and durability. The approach adopted for this work consists of the design of a system management law that generates the optimal operating conditions to be applied to the stack (pressures, temperature, current, stoichiometries) as a function of the power demand, the state of health (active surface loss) and current humidity. Optimality is understood in the sense of increasing system efficiency and decreasing the degradation of the membrane and the platinum dissolution. This law is based on degradation and performance models of a fuel cell system. This management law requires in real time the data of the state of health of the fuel cell and the humidity rate. The assessment of the state of health is already the subject of many diagnostic work. On the other hand, the humidity rate must be estimated by a state observer because the humidity sensors are not reliable for a transport application. Therefore, a state observer was developed to estimate the relative humidities in the stack channels and also the membrane water content, the hydrogen at the anode as well as the nitrogen saturation at the anode. This last data makes it possible to propose a purge strategy for a dead-end architecture, based on nitrogen saturation, which limits the losses in hydrogen and reduces the damage associated with this architecture.Les piles Ă combustibles sont considĂ©rĂ©es comme une Ă©nergie dâavenir, notamment grĂące Ă leur caractĂšre non polluant Ă lâusage. Cependant, le dĂ©ploiement de ces solutions Ă grande Ă©chelle est encore conditionnĂ© par lâamĂ©lioration de leurs performances et surtout de leur durabilitĂ© afin de garantir une industrialisation Ă faible coĂ»t. Lâapplication de la pile Ă combustible au domaine des transports impose en plus un fonctionnement Ă puissance variable, ce qui complique lâamĂ©lioration des performances et de la durabilitĂ©. Lâapproche retenue pour ces travaux consiste en la conception dâune loi de gestion du systĂšme qui gĂ©nĂšre les conditions opĂ©ratoires optimales Ă appliquer au stack (pressions, tempĂ©rature, courant, stoechiomĂ©tries) en fonction de la demande en puissance, de lâĂ©tat de santĂ© de la pile (perte de surface active) et du taux dâhumiditĂ© actuel. LâoptimalitĂ© est entendue au sens de lâaugmentation du rendement systĂšme et de la diminution des dĂ©gradations du platine et de la membrane. Cette loi se base sur des modĂšles de dĂ©gradations et de performances dâun systĂšme pile Ă combustible. Cette loi de gestion requiert pour fonctionner les donnĂ©es de lâĂ©tat de santĂ© de la pile et du taux dâhumiditĂ©. LâĂ©valuation de lâĂ©tat de santĂ© de la pile fait dĂ©jĂ lâobjet de nombreux travaux de diagnostic. En revanche, le taux dâhumiditĂ© doit ĂȘtre estimĂ© par un observateur dâĂ©tat car les capteurs dâhumiditĂ© ne sont pas fiables pour une application transport. Pour cela, un observateur dâĂ©tat a Ă©tĂ© dĂ©veloppĂ© pour estimer les humiditĂ©s relatives dans les canaux du stack et aussi le chargement en eau de la membrane, la quantitĂ© dâhydrogĂšne Ă lâanode ainsi que la saturation dâazote Ă lâanode. Cette derniĂšre donnĂ©e permet de proposer une stratĂ©gie de purge pour une architecture dead-end basĂ©e sur la saturation dâazote, qui limite les pertes en hydrogĂšne et rĂ©duit les dĂ©gradations liĂ©es Ă cette architecture
- âŠ