RESUMO
CaMnO3-δ-based perovskites find application in a variety of thermochemical cycles, e.g. oxygen partial pressure adjustment, chemical looping processes, and thermochemical energy storage. The applicability of these materials is governed by their thermodynamic and kinetic properties. Therefore, tunability of these properties is desirable to adapt the material to the required conditions. In this study, the effect of Sr content in Ca1-xSrxMnO3-δ on thermodynamics and kinetics is investigated by thermogravimetric analysis. The thermodynamics are measured in the temperature range of 873 K to 1473 K with oxygen partial pressures of 1 × 10-4 bar to 0.8 bar. The oxidation kinetics were characterized in the temperature range from 473 K to 673 K with oxygen partial pressures of 0.01 bar to 1 bar. The reduction kinetics were very rapid in the temperature range of 873 K to 1023 K, with the measured rates limited by the constraints of the measurement device. The results show that with increasing Sr content the structural changes of the material decrease the reduction enthalpy and the oxidation activation energy. This not only leads to a tunability of material properties, but can also be used to predict changes of these properties when only the structural changes are known.
RESUMO
Perovskite oxides of the general formula ABO3-δ, with A and B being metal cations, present themselves in various crystal structures that originate from a distorted ideal cubic perovskite. Understanding how composition, temperature, atmosphere and reduction extent of these non-stoichiometric redox materials induce structural changes on an atomic, as well as macroscopic, level is crucial to transfer newly developed materials to industrial scale applications in the redox-based energy conversion sector. Herein, Ca1-xSrxMnO3-δ (x ∈ [0,0.2]) and its micro- and macroscopic structural changes at elevated temperatures and varying oxygen partial pressure are analyzed by means of in situ high temperature XRD, DSC and dilatometry. Results are expanded by room temperature XRD of compositions with higher Sr-content up to x = 0.4. By adjusting the Sr-content, the formed crystal structure can be governed and thermal expansion can be impacted beneficially in the context of future applications utilizing monolithic structures. Phase transitions from orthorhombic to cubic were found to shift from 900 °C to 830 °C under air and to even lower temperatures under 1% O2 atmosphere. Small amounts of Sr-content (5-10%) stabilize the macroscopic structural integrity by improving the reversibility of the cyclic thermal expansion and contraction in a 1% O2 atmosphere. However, at Sr-contents of 20% an increased irreversible residual expansion within each thermal cycle becomes apparent and shows that such improvements do not follow a linear dependency with Sr-content, but most benefits in this context can be found at Sr-contents below 20%. The results demonstrate the sensitivity of such materials micro- and macroscopic characteristics to composition. In the context of utilization of monolithic structures, fabricated entirely from Ca1-xSrxMnO3-δ, in thermochemical or thermoelectric applications, the results have considerable significance as minor A-site Sr-substitution can substantially improve macroscopic stability of monolithic structures over multiple thermal cycles. Besides the often solely regarded thermodynamic characteristic, this work demonstrates the importance to consider the impact of composition on structural behavior in materials design processes including perovskites for thermochemical applications.
RESUMO
The Martian atmosphere contains 0.16% oxygen, which is an example of an in-situ resource that can be used as precursor or oxidant for propellants, for life support systems and potentially for scientific experiments. Thus, the present work is related to the invention of a process to concentrate oxygen in the oxygen-deficient extraterrestrial atmosphere by means of a thermochemical process and the determination of a suitable best-case apparatus design to carry out the process. The perovskite oxygen pumping (POP) system uses the underlying chemical process, which is based on the temperature-dependent chemical potential of oxygen on multivalent metal oxide, to release and absorb oxygen in response to temperature swings. The primary goal of this work is therefore to identify suitable materials for the oxygen pumping system and to optimize the oxidation-reduction temperature and time, required to operate the system, to produce 2.25 kg of oxygen per hour under the Martian most-extreme environmental conditions and based on the thermochemical process concept. Radioactive materials such as 244Cm, 238Pu and 90Sr are analyzed as a heating source for the operation of the POP system, and critical aspects of the technology as well as weaknesses and uncertainties related to the operational concept are identified.
RESUMO
The current COVID-19 pandemic has aggravated pre-existing oxygen supply gaps all over the world. In fact, oxygen shortages occurred in affluent areas with highly developed healthcare systems. The state-of-affairs created much suffering and resulted in potentially preventable deaths. Meanwhile, several international activities have been initiated to improve oxygen availability in the long-term by creating new networks of oxygen plants and supply channels. However, disasters such as the current pandemic may require rapid, autarkic oxygen production. Therefore, we determined whether oxygen resilience could conceivably be improved through self-made oxygen generators using material that is easily available even in remote areas. The team comprised engineers and physicians with hands-on experience in low- and middle-income countries. We constructed and tested self-made setups for water hydrolysis and membrane-based oxygen purification. We must conclude, however, that the massive amounts of oxygen patients with COVID-19 require cannot be reasonably met with such simple measures, which would require high efforts and hold potential risks.