A flexible metal–organic framework (MOF) of Zn3(btca)2(OH)2·(guest) n (H2btca = 1,2,3-benzotriazole-5-carboxylic acid) that exhibits guest molecule-controlled dynamic gas adsorption is reported in ...which carbon dioxide molecules rather than N2, He, and Ar induce a structural transition with a corresponding appearance of additional steps in the isotherms. Physical insights into the dynamic adsorption behaviors of flexible compound 1 were detected by gas adsorption at different temperatures and different pressures and confirmed by Fourier transform infrared spectroscopy and molecular simulations. Interestingly, by taking advantage of the flexible nature inherent to the framework, this MOF material enables highly selective adsorption of CO2/N2, CO2/Ar, and CO2/He of 36.3, 32.6, and 35.9, respectively, at 298 K. This class of flexible MOFs has potential applications for controlled release, molecular sensing, noble gas separation, smart membranes, and nanotechnological devices.
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Computer modelling techniques have been employed to qualitatively and quantitatively investigate the dehydration of hydroxyapatite to oxyapatite and the defect chemistry of calcium-deficient ...hydroxyapatite, where a number of vacancy formation reactions are considered. The dehydration of hydroxyapatite into oxyhydroxyapatite is calculated to be endothermic by E = +83.2 kJ mol(-1) in agreement with experiment, where thermal treatment is necessary to drive this process. Calcium vacancies are preferentially charge-compensated by carbonate ions substituting for phosphate groups (E = -5.3 kJ mol(-1)), whereas charge-compensating reactions involving PO4 vacancies are highly endothermic (E > or =652 kJ mol(-1)). The exothermicity of the charge compensation of a Ca vacancy accompanied by a PO4/CO3 substitution agrees with their co-occurrence in natural bone tissue and tooth enamel. Our calculations of a range of defect structures predict (i) that calcium vacancies as well as substitutional sodium and potassium ions would occur together with carbonate impurities at phosphate sites, but that other charge compensations by replacement of the phosphate groups are unfavourable, and (ii) that the hydroxy ions in the channel are easily replaced by carbonate groups, but that the formation of water or oxygen defects in the channels is thermodynamically unfavourable. Calculated elastic constants are reported for the defect structures.
The reaction between Zn and a pyrene-based ligand decorated with benzoate fragments (H4TBAPy) yields a 2D layered porous network with the metal coordination based on a paddlewheel motif. Upon ...desolvation, the structure undergoes a significant and reversible structural adjustment with a corresponding reduction in crystallinity. The combination of computationally assisted structure determination and experimental data analysis of the desolvated phase revealed a structural change in the metal coordination geometry from square-pyramidal to tetrahedral. Simulations of desolvation showed that the local distortion of the ligand geometry followed by the rotation and displacement of the pyrene core permits the breakup of the metal-paddlewheel motifs and the formation of 1D Zn–O chains that cross-link adjacent layers, resulting in a dimensionality change from the 2D layered structure to a 3D structure. Constrained Rietveld refinement of the powder X-ray diffraction pattern of the desolvated phase and the use of other analytical techniques such as porosity measurements, 13C CP MAS NMR spectroscopy, and fluorescence spectroscopy strongly supported the observed structural transformation. The 3D network is stable up to 425 °C and is permanently porous to CO2 with an apparent BET surface area of 523(8) m2/g (p/p° = 0.02–0.22). Because of the hydrophobic nature, size, and shape of the pores of the 3D framework, the adsorption behavior of the structure toward p-xylene and m-xylene was studied, and the results indicated that the shape of the isotherm and the kinetics of the adsorption process are determined mainly by the shape of the xylene isomers, with each xylene isomer interacting with the host framework in a different manner.
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•An experiment to study nitrogen/steam flowing in concrete cracks in is presented.•Mass flow regulators and sensors accurately monitor fluctuations in the mixture.•Embedded thermocouples register ...fluctuations and phase changes in the crack.
An experimental setup to study nitrogen/steam mixtures flowing through cracks in concrete is presented. Mass flow regulators placed upstream of an evaporator allow the flow rate to be controlled precisely while thermocouples and pressure sensors placed near the entry and exit of the cracks accurately monitor the pressure and temperature fluctuations as the mixture flows. Thermocouples embedded just below the surfaces of the crack provide details of the temperature fluctuations within the crack, which indicate the formation of condensation and transfer of heat to the concrete.
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•Linking mass flow rates to pressure differences for steam in concrete cracks.•Condensation forming within the crack changes the flow in a complex manner.•The pressure difference for a mass flow rate ...is reduced depending on steam content.
Experiments in which heated nitrogen/steam mixtures are passed through model cracks in concrete while monitoring the temperature and the pressure at the entry and exit of the crack provide links between the mass flow rate and the pressure difference across the crack. Cooling of the nitrogen/steam mixture by the concrete, and particularly the formation of condensation, alter the relationship between the pressure difference and the flow rate. At low flow rates, the nitrogen/steam mixture is cooled to the temperature of the concrete and the flow is practically the same as for the cold nitrogen portion. At higher flow rates, the pressure differences approach and can rise above the values they would have if the concrete were heated to the same temperature as the nitrogen/steam mixture.
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Experiments in which heated nitrogen/steam mixtures are passed through model cracks in concrete with thermocouples embedded along the inner surfaces of the cracks provide insights about the ...thermodynamic processes occurring within them. The thermal behaviour that is observed results from several interlinked processes that influence one another and the resulting transfer of nitrogen, steam, water and heat. By correlating the temperatures measured at different times and positions within cracks with different separations, one can observe how the flow is modified by the formation of condensation. When pure nitrogen is passed through the cracks, only a small amount of water evaporates from the concrete and partial obstruction of the cracks by condensed water only occurs in the cracks with small separations (∼30 µm). As the proportion of steam in the mixture is increased, the thermal fluctuations caused by the formation of condensation become more important and evolve over time as the concrete is heated.
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Slices through the valence electron density calculated using DFT+U for low energy terminations of, from left to right, (110), (100) and (111) slabs of uranium dioxide. Display omitted
► DFT+U ...calculations are used to model the (100), (110) and (111) surfaces of UO2. ► A novel method to bypass metastable states in the calculations is presented. ► The calculations agree with experimentally observed surface terminations.
We present a study of the bulk and the (100), (110) and (111) surfaces of uranium dioxide (UO2) using DFT+U in combination with a mixed Gaussian and plane waves basis set method, as implemented in the CP2K program package. The compromise of accuracy against computational efficiency by which this method enables calculations of relatively large UO2 systems is found to be acceptable in light of the almost 50-fold improvement in calculation times. A novel scheme is presented which reliably allows the system to escape the lower-lying metastable states that arise from the improved treatment of the strongly correlated 5f electrons of uranium. Based on the electronic configurations obtained by an f-occupation smearing combined with U ramping, this method relaxes the local energy minima by averaging the f-occupation matrices used to calculate the corrections in DFT+U. Various surface terminations are investigated and their calculated formation energies are found to be consistent with the experimentally observed morphologies. The direction of the antiferromagmentic ordering in relation to the surfaces exerts little influence on the results of the calculations while surface reconstructions can induce significant structural changes extending well into the bulk.
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8.
Steam flows in concrete cracks: Thermal behaviour Rabone, Jeremy; Carre, Hélène; La Borderie, Christian
Nuclear engineering and design,
November 2023, 2023-11-00, 2023-11, Volume:
413
Journal Article
Peer reviewed
•Temperature profiles of steam passed through model cracks in concrete give insights into the processes affecting the flow.•Condensing steam produces localised heat spikes that are detected by the ...embedded thermocouples.•The correlations of the heat spikes extracted from temperature profiles are compared between cracks with different openings.
Experiments in which heated nitrogen/steam mixtures are passed through model cracks in concrete with thermocouples embedded along the inner surfaces of the cracks provide insights about the thermodynamic processes occurring within them. The thermal behaviour that is observed results from several interlinked processes that influence one another and the resulting transfer of nitrogen, steam, water and heat. By correlating the temperatures measured at different times and positions within cracks with different separations, one can observe how the flow is modified by the formation of condensation. When pure nitrogen is passed through the cracks, only a small amount of water evaporates from the concrete and partial obstruction of the cracks by condensed water only occurs in the cracks with small separations (∼30 µm). As the proportion of steam in the mixture is increased, the thermal fluctuations caused by the formation of condensation become more important and evolve over time as the concrete is heated.
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The diffusion rate of hydrogen in zirconium oxides comes into play in both the steam oxidation and the hydriding of zirconium alloys. In view of the low measured uptake and diffusion of neutral ...hydrogen species in zirconium oxides, it has been suggested that the measured rates of hydrogen uptake in zircaloys exposed to high-temperature steam can be explained by the diffusion of protons through the surface oxide layers. This paper investigates the diffusion of protons in tetragonal zirconia at 1500 K using density functional theory-based molecular dynamics and Metadynamics simulations. An average value of 4 x 10 super(-9)/m super(2) s super(-1) is calculated for the diffusion rate, which, considering the contrived basis of the simulations, compares qualitatively with the value of 3.2 x 10 super(-10)/m super(2) s super(-1) obtained by fit to experimentally-determined diffusivities of hydrogen in yttrium-stabilized zirconia. The simulations described show that the "proton" is present as the hydrogen atom in a hydroxide ion, and the analysis of the electronic structure reveals that the diffusion of the proton is mediated by two-electron-three-centre bonds that form between hydroxide and adjacent oxide ions.
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The diffusion rate of hydrogen in zirconium oxides comes into play in both the steam oxidation and the hydriding of zirconium alloys. In view of the low measured uptake and diffusion of neutral ...hydrogen species in zirconium oxides, it has been suggested that the measured rates of hydrogen uptake in zircaloys exposed to high-temperature steam can be explained by the diffusion of protons through the surface oxide layers. This paper investigates the diffusion of protons in tetragonal zirconia at 1500K using density functional theory-based molecular dynamics and Metadynamics simulations. An average value of 4×10−9/m2s−1 is calculated for the diffusion rate, which, considering the contrived basis of the simulations, compares qualitatively with the value of 3.2×10−10/m2s−1 obtained by fit to experimentally-determined diffusivities of hydrogen in yttrium-stabilized zirconia. The simulations described show that the “proton” is present as the hydrogen atom in a hydroxide ion, and the analysis of the electronic structure reveals that the diffusion of the proton is mediated by two-electron-three-centre bonds that form between hydroxide and adjacent oxide ions.
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