1,721,088 research outputs found
Local and average structure of Yb-doped ceria through synchrotron and neutron pair distribution function
No full textAs transport properties of doped ceria electrolytes depend significantly on the nature of the dopant and the defectivity, the design of new materials and devices requires proper understanding of the defect structure. Among lanthanide dopants, Yb shows some peculiar characteristics that call for a possible different defect structure compared to Gd and Sm conventional dopants, which could be linked to its poorer performance. For this purpose, we combine synchrotron and neutron powder diffraction exploiting the Rietveld and Pair distribution Function. By increasing its concentration, Yb produces qualitatively the same structural distortions as other dopants, leading to a domain structure involving the progressive nucleation and growth of nanodomains with a Yb2O3-like (C-type) structure hosted in a fluorite CeO2 matrix. However, when it comes to growing the C-type nanodomains into a long-range phase, the transformation is less pronounced. At the same time, a stronger structural distortion occurs at the local scale, which is consistent with the segregation of a large amount of oxygen vacancies. The strong trapping of VOs by Yb3+ explains the poor performance of Yb-doped ceria with respect to conventional Sm-, Gd-, and Y-doped samples at equal temperature and dopant amount
Rare earth doped ceria: The complex connection between structure and properties
No full textThe need for high efficiency energy production, conversion, storage and transport is serving as a robust guide for the development of new materials. Materials with physical-chemical properties matching specific functions in devices are produced by suitably tuning the crystallographic- defect- and micro-structure of the involved phases. In this review, we discuss the case of Rare Earth doped Ceria. Due to their high oxygen diffusion coefficient at temperatures higher than ~500°C, they are very promising materials for several applications such as electrolytes for Solid Oxide Fuel and Electrolytic Cells (SOFC and SOEC, respectively). Defects are integral part of the conduction process, hence of the final application. As the fluorite structure of ceria is capable of accommodating a high concentration of lattice defects, the characterization and comprehension of such complex and highly defective materials involve expertise spanning from computational chemistry, physical chemistry, catalysis, electrochemistry, microscopy, spectroscopy, and crystallography. Results coming from different experimental and computational techniques will be reviewed, showing that structure determination (at different scale length) plays a pivotal role bridging theoretical calculation and physical properties of these complex materials
Probing complex disorder in Ce 1-xGd xO 2-x/2 using the Pair Distribution Function analysis
No full textIn this work the first Pair Distribution Function (PDF) study on Ce 1-xGd xO 2-x/2 (CGO) electrolytes for solid oxide fuel cells is presented, aiming to unveil the complex positional disorder induced by gadolinium doping and oxygen vacancies formation in these materials. The whole range of Gd concentration x Gd (0 ≤ x Gd ≤ 1) of the CGO solid solutions was investigated through high resolution synchrotron radiation powder diffraction. The reciprocal space Rietveld analysis revealed in all the solid solutions the presence of positional disorder, which has been explicitly mapped into the real space. The average structural models, as obtained by the Rietveld method, fit well the experimental PDF data only for a spatial range r > ∼10 Å. The same models applied at lower r values fails to reproduce the experimental curves. A clear improvement of the fit quality in the 1.5 < r < ∼6 Å range was obtained for all the CGO samples applying a biphasic model encompassing both a fluorite CeO 2-like and a C-type Gd 2O 3-like phases. This provides evidence that extended defects at local scale exist in the CGO system. Gdrich and Ce-rich droplets coexist in the subnanometric range. © 2012 American Chemical Society
Successful Treatment of Dercum's Disease by Transcutaneous Electrical Stimulation: A Case Report
No full textDercum's disease is a rare condition of painful subcutaneous growth of adipose tissue. Etiology is unknown and pain is difficult to control. We report the case of a 57-year-old man with generalized diffuse Dercum's disease, who improved after the treatment with transcutaneous frequency rhythmic electrical modulation system (FREMS). Treatment consisted in 4 cycles of 30 minutes FREMS sessions over a 6-month period. Measures of efficacy included pain assessment (visual analogue scale, VAS), adipose tissue thickness by magnetic resonance imaging, total body composition and regional fat mass by dual-energy X-ray absorptiometry, physical disability (Barthel index), and health status (Short Form-36 questionnaire). After FREMS treatment the patient's clinical conditions significantly improved, with reduction of pain on the VAS scale from 64 to 17 points, improvement of daily life abilities (the Barthel index increased from 12 to 18) and amelioration of health status (higher scores than baseline in all Short Form-36 domains). Furthermore, we documented a 12mm reduction in subcutaneous adipose tissue thickness at the abdominal wall and a 7040 g decrease in total body fat mass. FREMS therapy proved to be effective and safe in the treatment of this rare and disabling condition
Defect structure of Y-doped ceria on different length scales
No full textAn exhaustive structural investigation of a Y-doped ceria (Ce 1-xYxO2-x/2) system over different length scales was performed by combining Rietveld and Pair Distribution Function analyses of X-ray and neutron powder diffraction data. For low doping amounts, which are the most interesting for application, the local structure of Y-doped ceria can be envisaged as a set of distorted CeO2- and Y 2O3-like droplets. By considering interatomic distances on a larger scale, the above droplets average out into domains resembling the crystallographic structure of Y2O3. The increasing spread and amount of the domains with doping forces them to interact with each other, leading to the formation of antiphase boundaries. Single phase systems are observed at the average ensemble level. © 2013 American Chemical Society
Growth and structural characterization of needlelike metastable crystals in the Nd-Ba-Cu-(Al)-O system
No full textAn unconventional technique was used to grow crystals in the Nd-Ba-Cu-O system. A Nd4Ba2-Cu2O10 (Nd-422) bar was supported on two NdBa2Cu3Ox (Nd-123) pellets in an alumina crucible and was heated in a horizontal furnace at a higher temperature than the peritectic decomposition point of Nd-422. Brown Al-doped Nd-Ba-Cu-O needlelike crystals belonging to a new phase were grown directly on the crucible. The structure of the new phase, whose composition is Nd6Ba9.78Sr0.22Cu2Al6O30, has been determined by single-crystal X-ray diffraction. The space group is P63mc; the unit cell parameters are a ) 11.526(2) Å, c ) 6.963(1) Å, and Z ) 1. A full matrix least-squares refinement yielded R(F) ) 0.0405 for 3131 independent reflections. This new phase is a metastable polymorphic modification of Nd-422 solid solution. The preferential directional growth of the needles lies along the c-axis. Many needles are affected by twinning by merohedry, with the twinning plane parallel to the (001) crystallographic plane
Local disorder in yttrium doped ceria (Ce1-xYxO 2-x/2) probed by joint X-ray and neutron powder diffraction
No full textYttrium doped ceria materials (Ce1-xYxO 2-x/2) are widely studied for their application in Solid Oxide Fuel Cells devices. An anomalous decrease in the isothermal ionic conductivity at increasing Y3+ concentration above a critical value has been observed and attributed to the formation of defect clusters / domains at the nanometric scale, the crystallographic structure of which is still under debate. In this context we present a combined Synchrotron Radiation and Neutron Powder Diffraction study. In particular, neutrons allow to determine accurately oxygen related parameters, the contribution of which in terms of X-ray scattering power is almost negligible when compared to that of cations. The effect of doping on the average structure is investigated using conventional Rietveld analysis, while the Pair Distribution Function (PDF) technique is used to explore structural distortions and the spatial extent of disorder as well. The local structure observed in the real space is not consistent with the mean crystallographic one and is better modeled considering a biphasic model
- …
