Apparent auxetic to non-auxetic crossover driven by Co2+ redistribution in CoFe2O4 thin films

Oxide spinels of general formula AB2O4 (A = Mg2+, Fe2+; B = Al3+, Cr3+, etc.) constitute one of the most abundant crystalline structures in mineralogy. In this structure, cations distribute among octahedral and tetrahedral sites, according to their size and the crystal-field stabilization energy. The cationic arrangement determines the mechanical, magnetic, and transport properties of the spinel and can be influenced by external parameters like temperature, pressure, or epitaxial stress in the case of thin films. Here, we report a progressive change in the sign of the Poisson ratio, ν, in thin films of CoFe2O4, defining a smooth crossover from auxetic (ν 0) behavior in response to epitaxial stress and temperature. Microstructural and magnetization studies, as well as ab initio calculations, demonstrate that such unusual elastic response is actually due to a progressive redistribution of Co2+ among the octahedral and tetrahedral sites of the spinel structure. The results presented in this work clarify a long standing controversy about the magnetic and elastic properties of Co-ferrites and are of general applicability for understanding the stress-relaxation mechanism in complex crystalline structures.This work has received financial support from Ministerio de Economía y Competitividad (Spain) under Project No. MAT2016-80762-R and MAT2017-82970-C2-R, Xunta de Galicia (Centro singular de investigación de Galicia accreditation 2016-2019, No. ED431G/09), the European Union (European Regional Development Fund-ERDF), and the European Commission through the Horizon H2020 funding by H2020-MSCA-RISE-2016-Project No. 734187–SPICOLOST. I.L.d.P. and B.R.-M. thank the funding under the ESTEEM2 project and the researchers L.A. Rodríguez and E. Snoeck for preliminary Lorentz Microscopy (L.M.) and electron holography (EH) studies in CoFe2O4 samples synthesized by PAD method performed at CEMES (Toulouse)S

Composite phenolic resin-based carbon molecular sieve membranes for gas separation

Composite carbon molecular sieve membranes (c-CMSM) were prepared from phenolicresin loaded with boehmite by a single dippingdryingpyrolysis step. The composite membranewas analyzed by scanning electron microscopy, high resolution transmission electronmicroscopy, X-ray diffraction, thermogravimetric analysis, mercury porosimetry, CO2adsorption and permeation experiments. It was produced a 2 lm thick composite uniformlayer on top of a a-Al2O3 support. The composite top layer exhibited nanowires of Al2O3 12 nm thick and 1030 nm long well dispersed in a microporous carbon matrix. The microporesnetwork accounted for 63% of the total pore volume (DR isotherm). The c-CMSM exhibitedideal O2/N2 and C3H6/C3H8 permselectivities of 5 and 15, respectively. The performanceof the c-CMSM for pair C3H6/C3H8 was above the upper bound curve for polymeric membranes,making it a promising vehicle for olefin purification