Oxygen transport in La0.5Sr0.5Fe1−yTiyO3− δ ( y =0.0, 0.2) membranes

The influence of partial substitution of Fe with Ti on the oxygen transport properties of La1−x Sr x FeO3 membranes was investigated in view of their application for oxygen separation. Samples of composition $$La_{{0.5}} Sr_{{0.5}} {\text{Fe}}_{{1 - y}} {\text{Ti}}_{y} {\text{O}}_{{3 - \delta }}$$ (y=0, 0.2) were prepared and their oxygen transport properties characterised by potential step relaxation and by oxygen permeation measurement in an air/argon gradient. With the first technique, chemical diffusion $${( {\widetilde{D}} )}$$ and surface exchange (k S) coefficients were obtained by fitting of the current relaxation data to a single expression valid over the complete time range. The Ti-substituted composition gave slightly larger values of $${\widetilde{D}}$$ and k S. The trend was opposite for the measured oxygen permeation flux. In the latter experience, ordering of oxygen vacancies was observed at lower temperature, reducing significantly the performance of the materia

Properties of B-site substituted La0.5Sr0.5FeO3−δ{\mathbf{La}}_{{0.5}} {\mathbf{Sr}}_{{0.5}} {\mathbf{FeO}}_{{3 - {\mathbf{\delta }}}} perovskites for application in oxygen separation membranes

Mixed ionic-electronic conducting $$La_{0.5} Sr_{0.5} Fe_{1 - x} B_x O_{3 - \delta }$$ (B: Al, Cr, Zr, Ga, Ti, Sn, Ta, V, Mg, and In with x = 0, 0.1, 0.2) perovskite materials were produced via solid-state synthesis. In order to study the effect of B-site substitution on the expansion behavior of these materials, their thermal expansion in air up to 900°C and isothermal expansion at the same temperature from air to Ar were measured by dilatometry. Ti and Ta were found to be the most effective substitutions in suppressing the isothermal expansion. The isothermal expansion at 900°C from air to Ar was reduced by 50% by substitution of 20% Ti or 10% Ta. Therefore, these compositions were further characterized by 4-probe total DC conductivity and permeation measurements under air/Ar gradient. The total conductivity of $$La_{0.5} Sr_{0.5} FeO_{3 - \delta }$$ was decreased by more than one order of magnitude at low temperatures and from 430S/cm, which is the maximum, to around 100S/cm at 500°C with the addition of Ti and Ta. The normalized oxygen permeation of LSF at 900°C decreased from 0.18 to 0.05μmol/cm2s and 0.07μmol/cm2s with the substitution of 20% Ti and 10% Ta, respectivel

One-year Body Mass Index Change in Adult Renal Transplant Recipients and Its Relationship with Glomerular Filtration Rate and Creatinine Level: A Retrospective Study

Objective: Renal transplantation is a challenging process for the recipients. One of the important problems in this process is unwanted weight gain. This study aimed to determine the change in body mass index (BMI) and to evaluate the effect of recipient characteristics on BMI during one-year period after renal transplantation. Methods: The article was conducted in a retrospective design. In the study, files of 170 patients who underwent renal transplantation between 2015 and 2016 were reviewed retrospectively. T-test, ANOVA, and correlation analysis were used in the analysis of data. Results: It was determined that the patients had a tendency to have increased BMI after transplantation, with a higher rate in the first three months. The increase in BMI was higher in singles than in married participants (p=0.01 and p0.05). Conclusion: In the study, it was determined that the increase in BMI was higher especially in the first three months after renal transplantation. For this reason, it is an important requirement to address the counseling and support to patients and their relatives regarding weight management in the early period

La0.5Sr0.5Fe1-yMyO3-[delta] (M = Ti, Ta) perovskite oxides for oxygen separation membranes

Mixed ionic electronic conducting perovskite materials have been receiving considerable attention for the application of oxygen separation membranes. These membranes have potential to be integrated in industrial processes that require pure oxygen and to provide advantages considering economical and environmental aspects. The used perovskite materials can accommodate a high concentration of disordered oxygen vacancies and provide high oxygen permeation flux in the presence of an oxygen partial pressure gradient. However, the materials with high oxygen flux are observed to have low chemical and mechanical stability, originating from oxygen vacancy formation and reduction of the B-site transition metal ion. Based on the fact that the stability of the materials at reducing pO2 is dependent on the redox stability of the B-site cation, a partial substitution of the B-site transition metal with a more stable cation is considered in this thesis to improve the stability of the base perovskite oxide. The approach used for the identification of the suitable material based on the host composition La0.5Sr0.5FeO3-δ (LSF) was to first explore the B-site substituting elements in terms of their effect on the thermal expansion and the isothermal expansion measured during the atmosphere change from air to argon. The elements Al, Cr, Zr, Ga, Ti, Sn, Ta, In, V, and Mg were used for 10 or 20% substitution of Fe. The isothermal expansion of the host material was decreased by substitution, which was attributed to decreased amount of oxygen vacancy formation as it was evidenced later by TGA of chosen compositions. As a result, the compositions substituted with higher valence ions (particularly, 20% Ti4+, LSFTi2, and 10% Ta5+, LSFTa1) were identified as the least expanding materials in isothermal conditions. The effect of substitution was further characterized extensively including structural, mechanical, and oxygen transport properties. The oxygen vacancy (VO••) concentration changes induced by substitution of Fe were reflected in unit cell sizes of heat-treated samples under argon atmosphere. The increase in the unit cell size was attributed to the isothermal expansion due to B-site reduction and lowered to half in case of substituted LSFTi2 and LSFTa1 compared to the host material LSF. Similar trends were observed for interrelated properties such as the coefficient of thermal expansion, isothermal expansion, and the actual amount of oxygen vacancies formed under argon at 900°C, all of which were higher for LSF. The mechanical properties of LSF, LSFTi2, and LSFTa1 were characterized at room temperature. The Young's modulus and the bending strength of the materials were in the same range (141-147 GPa and ∼120 MPa, respectively) while the fracture toughness of the LSF sample was improved by Ti and especially Ta substitution. The fractography of the samples provided evidence that the several LSF samples showed extended cracks possibly related to residual stresses forming during cooling, due to non-equilibrated oxygen stoichiometry across the sample. The potential step measurements showed that the chemical diffusion and the surface exchange coefficients measured during oxidation of the samples were higher than the ones measured during reduction of the samples. The tendency was reversed at lowered pO2 along with the decrease of both coefficients. The influence of B-site substitution on both coefficients was not substantial. The oxygen permeation flux of LSF, LSFTi2, and LSFTa1 was measured under air/argon gradient on planar membranes. The permeation rates of the membranes with thicknesses close to 1 mm were considered to be limited by surface exchange kinetics at temperatures above 875°C. The permeation rate of LSF was decreased (from 0.2 µmolcm-2s-1) to its third by Ti and Ta additions. The permeation rate of Ti and Ta-substituted samples showed a drop around 875°C, keeping a similar activation energy at lower and higher temperature regions. Oxygen vacancy ordering in the perovskite structure was given as a possible explanation, which provided explanation for the slow equilibrium in the lower temperature region. Tubular membranes of LSF and LSFTi2 (0.47 mm and 0.36 mm thick, respectively) and a planar LSFTa1 membrane were characterized using an air/(Ar-CH4) gradient. The measurements conducted using pure Ar on the lean-side provided the possibility to compare the thickness dependence of the permeation. In agreement with the previous observations, the permeation of the materials at and above 900°C was independent of the thickness, therefore, surface exchange limited. Stability under reducing atmospheres was increased by substitution. The LSF membrane failed shortly after the introduction of CH4 to the system, while LSFTi2 survived 5% CH4 and its oxygen permeation was improved substantially by a factor of 14. LSFTa1 membrane was measured with pure CH4 and the permeation was increased by a factor of 9, reaching 0.7 µmolcm-2s-1 at 900°C. The membrane operated stably over 2000h with pure CH4, 1000h. A slow degradation of 3.7% per 1000h was observed at 1000°C, most probably due to cation migration. The compositions identified as a result of B-site substitution screening were effective in improving the stability of the materials without extensive loss of the oxygen permeation rate. Ta-substituted material was shown to be promising with its long-term stability as a partial oxidation membrane

İsmet İnönü’nün foto muhabiri : Hüseyin Ezer

Ankara : İhsan Doğramacı Bilkent Üniversitesi İktisadi, İdari ve Sosyal Bilimler Fakültesi, Tarih Bölümü, 2017.This work is a student project of the The Department of History, Faculty of Economics, Administrative and Social Sciences, İhsan Doğramacı Bilkent University.by Özer, Abdürrahim

Optimization of mixed conducting perovskites for the application in dense ceramic oxygen separation membranes, used in partial oxidation of natural gas to synthesis gas

The aim in this project was the optimization of LaFeO3-based perovskite materials for oxygen separation membranes (OSM). The work was carried out by the Laboratory for High Performance Ceramics, Empa in collaboration with the Laboratory for Industrial Energy Systems, LENI, EPFL and the Ceramics Laboratory, LC, EPFL. The ceramic material of interest is a mixed conductor for oxygen ions and electrons at high temperature (800°-900°C) and therefore can separate oxygen from nitrogen with 100% selectivity when exposed to an oxygen partial pressure gradient (e.g. air vs. natural gas). In this capacity, such materials are attractive for the application of partial oxidation (POX) of natural gas (NG) to synthesis gas in an important and growing business. The crucial point in this developing technology is the chemical and mechanical stability of the membrane under the operating conditions (high temperature, PO2 gradient). Because of thermal and chemical expansion, large stress can be induced in the material, which can lead to fracturing. Additional destabilizing processes such as kinetic demixing (elemental segregation under influence of the oxygen gradient) or slow phase transformation can lead to decreased performance with time. The overall goal in this Ph.D. thesis was to address these stability problems from a combined experimental and theoretical point of view in order to develop a more detailed understanding and strategy for improving the materials properties. Experimentally, the ceramic membrane materials were synthesized with different B-site substitutions with the aim of decreasing chemical expansion. The focus lied on LaFeO3 materials substituted with Sr on A-site. Detailed characterizations of the most promising compositions were done including structural, transport, and mechanical properties

Oxygen transport in La0.5Sr0.5Fe1-yTi yO3-δ (y=0.0,0.2) membranes

The influence of partial substitution of Fe with Ti on the oxygen transport properties of La1- xSrxFeO3 membranes was investigated in view of their application for oxygen separation. Samples of composition La0.5Sr0.5 Fe1-yTiyO3-δ (y = 0, 0.2) were prepared and their oxygen transport properties characterised by potential-step relaxation and by oxygen permeation measurement in an air/argon gradient. With the first technique, chemical diffusion (D) and surface exchange (kS) coefficients were obtained by fitting of the current relaxation data to a single expression valid over the complete time range. The Ti- substituted composition gave slightly larger values of D and kS. The trend was opposite for the measured oxygen permeation flux. In the latter experience, ordering of oxygen vacancies was observed at lower temperature, reducing significantly the performance of the material

Properties of B-site substituted La0:5Sr0:5FeO3 δ perovskites for application in oxygen separation membranes

Mixed ionic–electronic conducting La0:5Sr0:5 Fe1 xBxO3 δ (B: Al, Cr, Zr, Ga, Ti, Sn, Ta, V, Mg, and In with x=0, 0.1, 0.2) perovskite materials were produced via solid-state synthesis. In order to study the effect of Bsite substitution on the expansion behavior of these materials, their thermal expansion in air up to 900°C and isothermal expansion at the same temperature from air to Ar were measured by dilatometry. Ti and Ta were found to be the most effective substitutions in suppressing the isothermal expansion. The isothermal expansion at 900°C from air to Ar was reduced by 50% by substitution of 20% Ti or 10% Ta. Therefore, these compositions were further characterized by 4-probe total DC conductivity and permeation measurements under air/Ar gradient. The total conductivity of La0:5Sr0:5FeO3 δ was decreased by more than one order of magnitude at low temperatures and from 430 S/cm, which is the maximum, to around 100 S/cm at 500°C with the addition of Ti and Ta. The normalized oxygen permeation of LSF at 900°C decreased from 0.18 to 0.05 μmol/cm2s and 0.07 μmol/cm2s with the substitution of 20% Ti and 10% Ta, respectively

Improved stability of La0.5Sr0.5FeO3 by Ta-doping for oxygen separation membrane application

(La,Sr)FeO3 mixed conducting perovskites are considered as interesting candidates for oxygen separation membranes but they suffer from limited structural stability in a large oxygen partial pressure (pO2) gradient, because of their propensity for chemical expansion. Partial substitution of Fe with more stable elements tends to improve the stability while penalizing the electronic and ionic conductivities. In this study, we investigate the effect of 10% Ta substitution on the oxygen transport properties and stability of La0.5Sr0.5FeO3. For this purpose, the material was evaluated as a membrane in a CPOX reactor. The oxygen permeation through a ~ 3 cm2 pellet sample was first measured under air/Ar gradient in the temperature range of 800 to 1000 °C. The measured flux was 0.1 µmol cm− 2 s− 1 at 900 °C, which was a factor of 2 lower than for the Ta-free material. Methane was then introduced into the system and reacted in a catalytic bed with oxygen that has permeated through the membrane to form syngas (H2, CO). As a result, the oxygen flux increased by a factor of 9, reaching 0.9 µmol cm− 2 s− 1 at 900 °C. The reactor was operated at 1000 °C for another 1000 h. During this time, the oxygen permeation flux decayed by ca. 4%/1000 h. The test was stopped after more than 2000 h of operation and the membrane analyzed by electron microscopy