BiFeO3 Coupled Polysulfide Trapping in C/S Composite Cathode Material for Li-S Batteries as Large Efficiency and High Rate Performance

We demonstrated the efficient coupling of BiFeO3 (BFO) ferroelectric material within the carbon–sulfur (C-S) composite cathode, where polysulfides are trapped in BFO mesh, reducing the polysulfide shuttle impact, and thus resulting in an improved cyclic performance and an increase in capacity in Li-S batteries. Here, the built-in internal field due to BFO enhances polysulfide trapping. The observation of a difference in the diffusion behavior of polysulfides in BFO-coupled composites suggests more efficient trapping in BFO-modified C-S electrodes compared to pristine C-S composite cathodes. The X-ray diffraction results of BFO–C-S composite cathodes show an orthorhombic structure, while Raman spectra substantiate efficient coupling of BFO in C-S composites, in agreement with SEM images, showing the interconnected network of submicron-size sulfur composites. Two plateaus were observed at 1.75 V and 2.1 V in the charge/discharge characteristics of BFO–C-S composite cathodes. The observed capacity of ~1600 mAh g−1 in a 1.5–2.5 V operating window for BFO30-C10-S60 composite cathodes, and the high cyclic stability substantiate the superior performance of the designed cathode materials due to the efficient reduction in the polysulfide shuttle effect in these composite cathodes.

Селективный магнитный контроль толщины и степени упрочнения поверхностных слоев на стальных объектах

The dependences of measured locally magnetic characteristics of surface-hardened steel objects on the thickness and physical properties of their surface layers are studied. It is shown theoretically and experimentally that a change in the thickness of the hardened layer on the surface of steel objects affects on the magnitude of the tangential field component on the surface of the object in the interpolar space significantly , as well as the change in the strength properties of the layer affects the magnitude of the magnetic flux in the "transducer-object" circuit. It is proposed to use this difference in magnetic parameters for selective testing of the surface hardening quality. It is shown that the coercive force measured locally by the internal field and the maximum value of the magnetic flux, which can be measured using a single transducer in single measuring cycle, can be used as diagnostic parameters.

Theoretical explanation of rotational flow in the liquid-film motor

A liquid film that is under the action of two electric forces, an external electric field parallel to the film and a lateral voltage difference applied to both edges of the film, exhibits a universal rotational flow. In this article, we revisit this phenomena by considering an idealized so-called liquid-film motor and provide a theoretical description of the underlying physical mechanism that is responsible for the rotation. Based on this theory, the external electric field induces a non-uniform distribution of freely moving charges on the film. Then the internal field that is mainly resulted from the lateral voltage difference, will exert forces on induced charges and subsequently will result the rotational flow. We show, how the fields contribute in developing a universal flow pattern.

Multi-Dimensional Nuclear Magnetic Resonance Methods in the Inhomogeneous Magnetic Field

<p>This thesis introduces new NMR techniques which use the inhomogeneous internal magnetic fields present in the pore space of a porous medium exposed to an external magnetic field to obtain information about the pore size and heterogeneities of the the sample. Typically internal field inhomogeneities are regarded as unwanted due to their effect on various material properties such as relaxation and diffusion. However, in the experiments presented here, we choose samples specifically for their inhomogeneous internal fields and use multi-dimensional NMR methods and simulations to obtain our pore space and heterogeneity information. We first describe software developed to specifically simulate the internal magnetic field and diffusion through the pore space of a simple sphere pack system. This software generates a sphere pack and calculates the internal magnetic field generated by z-aligned magnetic dipoles placed at the center of each sphere. The internal magnetic field gradient is also calculated in the pore space. From there, a random walk method is developed and a realistic reflection off a sphere is introduced. We work through the development of this software and the mathematics behind the algorithms used. This simulation is used in all subsequent experimental chapters. We then use a two-dimensional exchange experiment to separate the susceptibility induced line broadening with the broadening caused by diffusion through the inhomogeneous field. We observe off-diagonal line broadening as the mixing time increases. We attempt to quantify this off-diagonal growth by selecting points on either side of the off-diagonal maximum and plotting their average as a function of mixing time. A biexponential fit to the average intensities with respect to mixing time results in a characteristic time and from that a characteristic length as a fraction of bead diameter. This experiment is simulated and a biexponential growth is also observed in the simulated off-diagonal with characteristic lengths comparable to experiment. To obtain a correlation length directly from experiment and not deduce one from a characteristic time, we add a spatial dimension to our exchange experiment in the form of a propagator dimension. This dimension allows us to select 2D spectra based on their Z-displacement. We observe off-diagonal growth due to both an increase in Z-displacement and an increase in mixing time. We move away from the biexponential fit and move to a relationship based on mixing time, effective diffusion, and Z-displacement to directly calculate a characteristic length. We see these same traits in the simulated data which agrees well with experiment. Lastly, we move away from exchange experiments and move to correlating the transverse relaxation time with the internal field offset. We find that there is correlation at large magnetic field offsets and small T2 times which appear to be indicative of sample heterogeneities. To confirm this we use a highly heterogeneous rock core sample which increases the correlations seen at the previous offsets and times. This experiment is more qualitative than the previous two as we do not have a concrete value for the heterogeneity of our samples. The simulation used throughout the thesis, while showing a definite correlation between field offset and T2 relaxation, is unable to accurately simulate the experiment and requires more development.</p>

Multi-Dimensional Nuclear Magnetic Resonance Methods in the Inhomogeneous Magnetic Field

<p>This thesis introduces new NMR techniques which use the inhomogeneous internal magnetic fields present in the pore space of a porous medium exposed to an external magnetic field to obtain information about the pore size and heterogeneities of the the sample. Typically internal field inhomogeneities are regarded as unwanted due to their effect on various material properties such as relaxation and diffusion. However, in the experiments presented here, we choose samples specifically for their inhomogeneous internal fields and use multi-dimensional NMR methods and simulations to obtain our pore space and heterogeneity information. We first describe software developed to specifically simulate the internal magnetic field and diffusion through the pore space of a simple sphere pack system. This software generates a sphere pack and calculates the internal magnetic field generated by z-aligned magnetic dipoles placed at the center of each sphere. The internal magnetic field gradient is also calculated in the pore space. From there, a random walk method is developed and a realistic reflection off a sphere is introduced. We work through the development of this software and the mathematics behind the algorithms used. This simulation is used in all subsequent experimental chapters. We then use a two-dimensional exchange experiment to separate the susceptibility induced line broadening with the broadening caused by diffusion through the inhomogeneous field. We observe off-diagonal line broadening as the mixing time increases. We attempt to quantify this off-diagonal growth by selecting points on either side of the off-diagonal maximum and plotting their average as a function of mixing time. A biexponential fit to the average intensities with respect to mixing time results in a characteristic time and from that a characteristic length as a fraction of bead diameter. This experiment is simulated and a biexponential growth is also observed in the simulated off-diagonal with characteristic lengths comparable to experiment. To obtain a correlation length directly from experiment and not deduce one from a characteristic time, we add a spatial dimension to our exchange experiment in the form of a propagator dimension. This dimension allows us to select 2D spectra based on their Z-displacement. We observe off-diagonal growth due to both an increase in Z-displacement and an increase in mixing time. We move away from the biexponential fit and move to a relationship based on mixing time, effective diffusion, and Z-displacement to directly calculate a characteristic length. We see these same traits in the simulated data which agrees well with experiment. Lastly, we move away from exchange experiments and move to correlating the transverse relaxation time with the internal field offset. We find that there is correlation at large magnetic field offsets and small T2 times which appear to be indicative of sample heterogeneities. To confirm this we use a highly heterogeneous rock core sample which increases the correlations seen at the previous offsets and times. This experiment is more qualitative than the previous two as we do not have a concrete value for the heterogeneity of our samples. The simulation used throughout the thesis, while showing a definite correlation between field offset and T2 relaxation, is unable to accurately simulate the experiment and requires more development.</p>

Symmetry-protected solitons and bulk-boundary correspondence in generalized Jackiw–Rebbi models

We investigate the roles of symmetry and bulk-boundary correspondence in characterizing topological edge states in generalized Jackiw–Rebbi (JR) models. We show that time-reversal (T), charge-conjugation (C), parity (P), and discrete internal field rotation ($$Z_n$$ Z n ) symmetries protect and characterize the various types of edge states such as chiral and nonchiral solitons via bulk-boundary correspondence in the presence of the multiple vacua. As two representative models, we consider the JR model composed of a single fermion field having a complex mass and the generalized JR model with two massless but interacting fermion fields. The JR model shows nonchiral solitons with the $$Z_2$$ Z 2 rotation symmetry, whereas it shows chiral solitons with the broken $$Z_2$$ Z 2 rotation symmetry. In the generalized JR model, only nonchiral solitons can emerge with only $$Z_2$$ Z 2 rotation symmetry, whereas both chiral and nonchiral solitons can exist with enhanced $$Z_4$$ Z 4 rotation symmetry. Moreover, we find that the nonchiral solitons have C, P symmetries while the chiral solitons do not, which can be explained by the symmetry-invariant lines connecting degenerate vacua. Finally, we find the symmetry correspondence between multiply-degenerate global vacua and solitons such that T, C, P symmetries of a soliton inherit from global minima that are connected by the soliton, which provides a novel tool for the characterization of topological solitons.

Liquid-film Motor: Physical Mechanism

A liquid film that is under the action of two electric forces, an external electric field parallel to the film and a lateral voltage difference applied to both edges of the film, exhibits a universal rotational flow. In this article, we revisit this phenomena by considering an idealized so-called liquid-film motor and provide a theoretical description of the underlying physical mechanism that is responsible for the rotation. In this theory, the external electric filed induces a non-uniform distribution of free charges on the film then the internal field, resulted mainly from the voltage difference, will exert forces on these charges and subsequently induce a rotational flow in the ambient fluid. We show, how the fields contribute in developing a universal flow pattern.

Electronic performance of GaN nanowires cathode under electric field

The built-in electric field of exponential doping can promote the concentration of the photogenerated carrier center to the top surface of the nanowire. The external electric field also can bend the motion trajectory of the emitted electrons toward the collecting side. These field-assisted methods promote the quantum efficiency. In this paper, the emission theory of a single GaN nanowire photocathode is studied for the first time. The effects of height and width of the nanowire, wavelength, intensity of electric field on quantum efficiency of uniformly doped or exponentially doped GaN nanowire photocathodes were explored. It shows that the top of the exponentially doped cathode has a higher quantum efficiency than uniformly doped cathode. With the absence of the field, quantum efficiency of a uniformly doped cathode reaches a maximum value of 55.29% when the width is 150 nm and the wavelength is 220 nm. The form of exponentially doped cathode can generate an internal field. With the internal field, a maximum value rises 56.73% when the height is 900 nm and the wavelength is 230 nm. The theoretical results can direct the preparation of this kind of photocathode.