NASICON materials - a long neglected class of solid electrolytes

The so-called NASICON materials AT2P3O12 (A = alkaline metal, T = tetravalent transition metal) are known since the 1970s [1] and are derived from the first “Na+ super-ionic conductor”, Na3Zr2Si2PO12, of this group of materials. The aims of current investigations are on the one hand the better understanding of the ionic conduction and on the other hand the search for new materials with very high ionic conductivity. For this purpose, new and simple synthesis methods have been developed, which deliver very homogeneous powders with reduced temperatures for the preparation of ceramics. In this way a lithium ion conductor with the composition Li1.5Al0.5Ti1.5P3O12 was manufactured. After sintering to highly dense ceramics a total conductivity of 0.7 mS/cm was achieved at room temperature [2] and therefore this material belongs to the best known solid oxidic Li+ ion conductors. NMR and impedance spectroscopy investigations [3-5] have shown that the bulk conductivity amounts to 3-5 mS/cm and that the grain boundaries determine the quality of the material. In the case of Na+ ion conductors, the prototype Na3Zr2Si2PO12 was newly synthesized and gave a previously not achieved conductivity of 1 mS/cm [6]. The modification of the composition by substitution with scandium delivered conductivities of 0.8 mS/cm (Na3.4Sc2Si0.4P2.6O12) [7] and 4 mS/cm (Na3.4Zr1.6Sc0.4Si2PO12) [8]. The latter composition possesses one of the highest known Na+ ion conductivities of oxide ceramics and reaches the conductivity of liquid electrolytes. The mentioned compositions confirm the empirical criteria which are necessary for achieving high ionic conductivities in NASICON materials [9]. References: [1] H. Y. P. Hong, Mater. Res. Bull. 11 (1976) 173-182; H. Y. P. Hong, J. B. Goodenough, J. A. Kafalas, Mater. Res. Bull. 11 (1976) 203-220 [2] Q. Ma, Q. Xu, C.-L. Tsai, F. Tietz, O. Guillon, J. Am. Ceram. Soc., (2016), in press [3] V. Epp, Q. Ma, F. Tietz, M. Wilkening, Phys. Chem. Chem. Phys., 17 (2015) 32115-32121 [4] S. Breuer, D. Prutsch, V. Epp, Q. Ma, F. Preishuber-Pflügl, F. Tietz, M. Wilkening, J. Mater. Chem. A, 3 (2015) 21343-21350 [5] D. Rettenwander, A. Welzl, S. Pristat, F. Tietz, S. Taibl, G. J. Redhammer, J. Fleig, J. Chem. Mater. A, 4 (2016) 1506-1513 [6] S. Naqash, Q. Ma, Tietz, O. Guillon, in preparation [7] M. Guin, F. Tietz, O. Guillon, in preparation [8] Q. Ma, M. Guin, S. Naqash, C.-L. Tsai, F. Tietz, O. Guillon, in preparation [9] M. Guin, F. Tietz, J. Power Sources, 273 (2015) 1056-106

Lattice instabilities of cubic NiTi from first principles

The phonon dispersion relation of NiTi in the simple cubic B2 structure is computed using first-principles density-functional perturbation theory with pseudopotentials and a plane-wave basis set. Lattice instabilities are observed to occur across nearly the entire Brillouin zone, excluding three interpenetrating tubes of stability along the (001) directions and small spheres of stability centered at R. The strongest instability is that of the doubly degenerate M5' mode. The atomic displacements of one of the eigenvectors of this mode generate a good approximation to the observed B19' ground-state structure.Comment: 11 pages, 3 figure

Quasi-normal frequencies: Key analytic results

The study of exact quasi-normal modes [QNMs], and their associated quasi-normal frequencies [QNFs], has had a long and convoluted history - replete with many rediscoveries of previously known results. In this article we shall collect and survey a number of known analytic results, and develop several new analytic results - specifically we shall provide several new QNF results and estimates, in a form amenable for comparison with the extant literature. Apart from their intrinsic interest, these exact and approximate results serve as a backdrop and a consistency check on ongoing efforts to find general model-independent estimates for QNFs, and general model-independent bounds on transmission probabilities. Our calculations also provide yet another physics application of the Lambert W function. These ideas have relevance to fields as diverse as black hole physics, (where they are related to the damped oscillations of astrophysical black holes, to greybody factors for the Hawking radiation, and to more speculative state-counting models for the Bekenstein entropy), to quantum field theory (where they are related to Casimir energies in unbounded systems), through to condensed matter physics, (where one may literally be interested in an electron tunelling through a physical barrier).Comment: V1: 29 pages; V2: Reformatted, 31 pages. Title changed to reflect major additions and revisions. Now describes exact QNFs for the double-delta potential in terms of the Lambert W function. V3: Minor edits for clarity. Four references added. No physics changes. Still 31 page

Solid oxide fuel cell reactor analysis and optimisation through a novel multi-scale modelling strategy

The simulation of a solid oxide fuel cell (SOFC) that incorporates a detailed user-developed model was performed within the commercial flowsheet simulator Aspen Plus. It allows modification of the SOFC's governing equations, as well as the configuration of the cell's fuel-air flow pattern at the flowsheet level. Initially, the dynamic behaviour of single compartment of a cell was examined with a 0D model, which became the building block for more complex SOFC configurations. Secondly, a sensitivity analysis was performed at the channel (1D) scale for different flow patterns. Thirdly, the effect of fuel and air flow rates on the predominant distributed variables of a cell was tested on a 2D assembly. Finally, an optimisation study was carried out on the 2D cell, leading to a robust, optimal air distribution profile that minimises the internal temperature gradient. This work forms the foundation of future stack and system scale studies

Multilocus microsatellite analysis of European and African Candida glabrata isolates

This study aimed to elucidate the genetic relatedness and epidemiology of 127 clinical and environmental Candida glabrata isolates from Europe and Africa using multilocus microsatellite analysis. Each isolate was first identified using phenotypic and molecular methods and subsequently, six unlinked microsatellite loci were analyzed using automated fluorescent genotyping. Genetic relationships were estimated using the minimum-spanning tree (MStree) method. Microsatellite analyses revealed the existence of 47 different genotypes. The fungal population showed an irregular distribution owing to the over-representation of genetically different infectious haplotypes. The most common genotype was MG-9, which was frequently found in both European and African isolates. In conclusion, the data reported here emphasize the role of specific C. glabrata genotypes in human infections for at least some decades and highlight the widespread distribution of some isolates, which seem to be more able to cause disease than others.This research was supported in part by the EU Mare Nostrum (EUMN-III Call) program of the European Union, grant agreement number 2011-4050/001-EMA2. Dr Sanae Rharmitt was the recipient of a scholarship (10 months) signed within the EUMN program for PhD students (F.S. 1.04.11.01 UORI) under the supervision of Prof Orazio Romeo.info:eu-repo/semantics/publishedVersio

Development of lanthanum strontium cobalt ferrite composite cathodes for intermediate- to low-temperature solid oxide fuel cells

Solid oxide fuel cells (SOFCs) offer high energy conversion, low noise, low pollutant emission, and low processing cost. Despite many advantages, SOFCs face a major challenge in competing with other types of fuel cells because of their high operating temperature. The necessity to reduce the operational temperature of SOFCs has led to the development of research into the materials and fabrication technology of fuel cells. The use of composite cathodes significantly reduces the cathode polarization resistance and expands the triple phase boundary area available for oxygen reduction. Powder preparation and composite cathode fabrication also affect the overall performance of composite cathodes and fuel cells. Among many types of cathode materials, lanthanum-based materials such as lanthanum strontium cobalt ferrite (La1-xSrxCo1-yFeyO3-δ) have recently been discovered to offer great compatibility with ceria-based electrolytes in performing as composite cathode materials for intermediate- to low-temperature SOFCs (IT-LTSOFCs). This paper reviews various ceria-based composite cathodes for IT-LTSOFCs and focuses on the aspects of progress and challenges in materials technology