Suchergebnisse

Despite the large photovoltaic performance recently achieved, many aspects of the working principles of hybrid organic–inorganic perovskite solar cells remain to be unveiled. We analyze the experimental features observed in the decay of photovoltage and provide an interpretation of the different depolarization regimes at distinct time scales. We introduce an instantaneous relaxation time that shows the type of relaxation for each separate mechanism. The decay of photovoltage is characterized by electronic events at the ms time scale followed by a power law relaxation in the 10–100 s time window. The latter process is associated with the slow dielectric relaxation of CH3NH3PbI3 perovskite and it points to cooperative kinetics of polarization and depolarization of ferroelectric domains. These findings provide an important tool for interpretation of kinetic features in the perovskite ferroic solar cells. ; The research leading to these results has received funding from the European Union Seventh Framework Program [FP7/2007 – 2013] under grant agreement 316494, MINECO of Spain under project MAT2013-47192-C3-1-R, Generalitat Valenciana (project ISIC/2012/008) and the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea under contract no. NRF- 2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy System). HW Han and LF Liu acknowl- edge  nancial support from the National Natural Science Foundation of China (91433203, 61474049) and the Ministry of Science and Technology of China (863, SS2013AA50303).

In a repository for radioactive waste hosted in a clay formation, hydrogen and other gases may be generated due to the corrosion of metallic materials under anoxic conditions, the radioactive decay of waste and the radiolysis of water. If the gas production rate exceeds the gas diffusion rate within the pores of the clay, a discrete gas phase will form and accumulate until its pressure becomes large enough to exceed the entry pressure of the surrounding material, at which point dilatant, advective flow of gas is expected to occur. The purpose of Task An under DECOVALEX-2019 is to better represent the processes governing the advective movement of gas in both low-permeability argillaceous repository host rocks and clay-based engineered barriers within numerical codes. In this paper special attention is given to the mechanisms controlling gas entry, flow and pathway sealing and their impact on the performance of the engineered clay barrier. Previous work suggests gas flow is accompanied by the creation of dilatant pathways whose properties change temporally and spatially within the medium. Thus, four new types of approaches have been developed: (i) standard two-phase flow models (continuous techniques) incorporating a range of different mechanical deformation behaviours, (ii) enhanced two-phase flow models in which fractures are embedded within a plastic material (continuous techniques) or incorporated into the model using a rigid-body-spring network (discrete approaches), (iii) a singlephase model incorporating a creep damage function in which only gas flow is considered, and (iv) a conceptual approach used to examine the chaotic nature of gas flow. The outputs from these different approaches are compared. This is an essential step as the choice of modelling approach strongly impacts the representation and prediction of gas flow in a future repository. In addition, experience gained through this task is of direct relevance to other clay-based engineering issues where immiscible gas flow is a consideration including hydrocarbon migration, carbon capture and storage, shale gas and landfill design. This paper summarises the outcomes of work in Task A conducted between May 2016 and May 2019 and provides a brief overview of the experimental data and a synthesis of the work of the participating modelling teams. ; DECOVALEX is an international research project comprising participants from industry, government and academia, focusing on development of understanding, models and codes in complex coupled problems in sub-surface geological and engineering applications; DECOVALEX-2019 is the current phase of the project. The authors appreciate and thank the DECOVALEX-2019 funding organisations Andra, BGR/UFZ, CNSC, US DOE, ENSI, JAEA, IRSN, KAERI, NWMO, RWM, SÚRAO, SSM and Taipower for their financial and technical support of the work described in this paper. The statements made in the paper are, however, solely those of the authors and do not necessarily reflect those of the funding organisations. ; Peer Reviewed ; Postprint (published version)