Exploring Ortho–Para Hydrogen Conversion CatalystsBased on Surface Electric Field Gradient

Flexoelectric effect is a novel electro-mechanical coupling effect that has drawn a lot of attention in the past few decades. Compared with piezoelectric materials, flexoelectric materials have the inherent characteristics of gradient coupling effect between electric field and elastic field. Since no complex pre-polarization process is required, the flexoelectric materials do not have depolarization or aging problems during the trial period. Similar to piezoelectricity, flexoelectricity exhibits two different effects: direct and converse flexoelectric effect. The direct flexoelectric effect indicates that the strain gradient produces an electrical response, and can be used in sensor design and energy harvesting. The converse flexoelectric effect represents mechanical stress or strain caused by inhomogeneous electric field or polarization gradient, and then as a response, the induced membrane force and the corresponding control moment will occur in the structure. This study will take the mechanic electric coupling characteristics of flexoelectric effect as the core, establish the dynamic equation, deduce the modal response of plate structure under the effect of flexoelectric and obtain the relationship between modal response and electric field gradient. To validate the theoretical method, a finite element model of flexoelectric actuated plate will be established in the commercial software COMSOL Multiphysics. With no existing flexoelectric module, the finite element model of flexoelectric plate will be built based on the original dynamic model. A non-uniform electric field is firstly constructed in COMSOL with the electrostatic module. The influence of different structural parameters on the vibration control effect caused by flexoelectric excitation will be analyzed and optimized, which will provide a theoretical basis for the application of flexoelectric materials in the vibration control of novel smart structures.

We discuss the temperature dependence of the nuclear quadrupolar spin relaxation rate in metals and alloys. Four different relaxation processes must be distinguished: Hard collisions, diffusion, decay of associates and rotational diffusion. The latter two processes are important in alloys of strongly interacting alloying partners, the former in pure metals (collision) and ideal or regular mixtures. Diffusion is important if the alloying partners produce different electrical field gradients.

The effect of the spatially inhomogeneous electric field on the quantum thermodynamic property of the particle confined in a quantum well has been investigated theoretically. The Schrodinger equation was solved for the particle subjected to a spatial confinement by an inhomogeneous electric field and energy eigen-values were obtained. Using the energy eigen-values, we have calculated the average energy, free energy, entropy, and heat capacity of this system. As an important parameter in the inhomogeneous electric field, the inhomogeneity gradient of the field plays an important role. Compared with the spatially homogenous electric field, some novel quantum effect appears. The energy eigen-value for the particle corresponding to each bound state increases as the inhomogeneity gradient increases, resulting in the increase of the average energy, free energy, and heat capacity; however, the entropy, which measures the disorder of the particle confined in the inhomogeneous field, gradually decreases with the increase of the inhomogeneity gradient. In addition, the confinement effect of the quantum well on the thermodynamic properties of this system has also investigated in great detail. The results show that the larger size of the quantum well is, the stronger the influence of inhomogeneous field becomes, which makes the thermodynamic properties of the particle in the spatially inhomogeneous field quite different from those in the homogeneous field. Our work may provide some references for the future research on the thermodynamic property of the confined atom and molecule in the inhomogeneous field and has some practical applications in statistical physics, chemical physics, and condensed physics, etc.

A comparison of the data on nuclear spin relaxation and ultrasonic relaxation is made. It is suggested the correlation time, τc, used in explaining nuclear spin effects is related to the shear and volume relaxational processes occurring in a liquid. The results of comparing the nuclear and ultrasonic processes indicate the following. (1) The average shear relaxation time found for glycerol is close in value to τc (2) The value for τc for water is shown to be very close to the volume relaxation time calculated by Hall. (3) The variation in the nuclear relaxation times in going from water to glycerol are closely related to the variation in the volume relaxation times. (4) Setting τc, proportional to the average shear relaxation time offers an explanation as to why the pressure dependence of the nuclear relaxation time is not a simple function of the viscosity as predicted by present nuclear relaxation theory.

We present a detailed analytical and numerical analysis of the nuclear spin dynamics in parabolic quantum wells. The shallow potential of parabolic quantum wells permits substantial modification of the electronic wave function in small electric fields. The nuclear spin relaxation via the hyperfine interaction depends on the electronic local density of states, therefore the local nuclear relaxation time depends sensitively on the electric field. For an inhomogeneous nuclear magnetization, such as generated by dynamic nuclear polarization, the total nuclear magnetization dynamics can similarly be altered. We examine this effect quantitatively and the effect of temperature, field, well thickness, and nuclear spin diffusion.

Additional comments on “A proposed method for measuring the electric dipole moment of the neutron using acceleration in an electric field gradient and ultracold neutron interferometry”

The nuclear quadrupole interactions at $^{111}\mathrm{Cd}$ probe nuclei in Re and Zr hosts are measured in the temperature range 4-900 K employing time differential perturbed ($\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$) angular correlation of the 173-247-keV cascade. Using the known quadrupole moment of the 247-keV state of $^{111}\mathrm{Cd}$, the electric field gradient (EFG) at 293 K for $^{111}\mathrm{Cd}Re=1.60\ifmmode\times\else\texttimes\fi{}{10}^{17}$ V/${\mathrm{cm}}^{2}$ and for $^{111}\mathrm{Cd}Zr=0.79\ifmmode\times\else\texttimes\fi{}{10}^{17}$ V/${\mathrm{cm}}^{2}$. Both metallurgically prepared and implanted sources were used. The temperature variation of the EFG for $^{111}\mathrm{Cd}\mathrm{Re}$ is quite small and is about 12% over the temperature range 4-860 K. Since the sign of the EFG is known from ($\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$) angular correlation measurements reported earlier, we are able to obtain the electronic part precisely. The electronic part is about 2.1 times the lattice part and is essentially constant over the temperature range studied showing that the entire temperature variation comes from the lattice part. For $^{111}\mathrm{Cd}\mathrm{Zr}$ the electronic part is about 1.9 times the lattice part if we assume that the sign of the electronic contribution is opposite to the sign of the lattice part. The EFG is highly temperature dependent and changes by about 57% over 4-890 K. For this system also, the temperature dependence is neither given by the linear $T$ dependence nor by the ${T}^{\frac{3}{2}}$ law. The results are discussed in the light of the systematic trends and the currently available semiempirical and theoretical models.

Quantum mechanical equations of motion are strictly linear in density operators, but equations describing chemical kinetics and hydrodynamics may be nonlinear in concentrations. This incompatibility is fundamental, but special cases can be handled—for example, in magnetic resonance where nuclear spin interactions may be too weak influence concentration dynamics. For isolated spins and first-order reactions, this is a well-researched topic, but time evolution of complex nuclear spin systems in the presence of second-order kinetics, diffusion, and flow has so far remained intractable. In this communication, we report a numerically stable formalism for time-domain description of nuclear spin dynamics and relaxation in the simultaneous presence of diffusion, flow, and second-order chemical reactions. As an illustration, we use Diels-Alder cycloaddition of acrylonitrile to cyclopentadiene in the presence of diffusion and flow in a microfluidic NMR probe (a finite element model with thousands of Voronoi cells) with a spatially localized stripline radio frequency coil.

Theory required for estimating the electric field gradients (EFGs) in ionic solids incorporating the effects of the ligand distortions due to the internal crystalline electric fields has been described. The EFGs have been derived from the distorted electronic charge distributions surrounding the nucleus under consideration in addition to the appropriate contributions from the multipole distributions in the lattice. Application of the theory has been made to study extensively the nuclear quadrupole interactions of the excited states of $^{111}\mathrm{Cd}$ and $^{117}\mathrm{In}$ in ${\mathrm{CdCl}}_{2}$. The results have been compared with those obtained under the cases: (i) "without ligand distortion"---which neglects the distortions of the ligands but considers the electronic as well as the "monopole and dipole" distributions in the crystal in a manner described previously, (ii) "monopoles and dipoles only" on the entire lattice, and (iii) "monopoles only" on the entire lattice. The ligand-distortion effects have been found to be very significant and the net EFGs change sign because of the ligand distortions. The need for obtaining the experimental sign of the quadrupole interaction frequencies in ${\mathrm{CdCl}}_{2}$ has been emphasized to understand the ligand-distortion effects on the EFG. Analysis of the EFGs from the cases considered reveals that the electronic contributions are dominant. This requires reanalysis of the experimental quadrupole interaction frequencies observed by the time-differential perturbed-angular-correlation technique in some Cd salts. The reanalysis yields the quadrupole moment $|Q(^{111}\mathrm{Cd})|$ very close to that obtained by Behrend and Budnick from the study of the circular polarization of the 247-keV $\ensuremath{\gamma}$ ray of $^{111}\mathrm{Cd}$.

The ${}^{27}\mathrm{Al}\mathrm{}\mathrm{NMR}$ spectrum and the spin-lattice relaxation rate of an icosahedral ${\mathrm{Al}}_{72.4}{\mathrm{Pd}}_{20.5}{\mathrm{Mn}}_{7.1}$ quasicrystal were studied in the temperature range from 300 to 4 K at two magnetic fields. The results are compatible with the hypothesis that a screening electron cloud is formed around the Mn ions in an aluminum-rich environment, which together with the core electrons produces a large electric-field gradient (EFG) at the sites of the Al nuclei. The EFG is temperature dependent due to the polarization of the screening cloud and the core electrons by the magnetic moments of the Mn unpaired d electrons. The shift of the ${}^{27}\mathrm{Al}$ central line shows an inverse magnetic-field dependence and appears to be of an electric quadrupolar origin. The ${}^{27}\mathrm{Al}$ spin-lattice relaxation rate shows an ${aT+bT}^{3}$ temperature dependence that can be explained by approximating the quasicrystalline electronic structure by a two-band model, consisting of a broad s and a narrow d conduction bands. The ${T}^{3}$ term originates from the narrowness of the d band. The ${}^{27}\mathrm{Al}$ spin-lattice relaxation in ${\mathrm{Al}}_{72.4}{\mathrm{Pd}}_{20.5}{\mathrm{Mn}}_{7.1}$ is governed by two relaxation mechanisms of similar strength, the electric quadrupole and the magnetic hyperfine. The quadrupole mechanism is stronger at high temperatures such as room temperature whereas the magnetic relaxation dominates at low temperatures.

The membrane-bound heterotrimeric nitrate reductase A (NarGHI) catalyzes the oxidation of quinols in the cytoplasmic membrane of Escherichia coli and reduces nitrate to nitrite in the cytoplasm. The enzyme strongly stabilizes a menasemiquinone intermediate at a quinol oxidation site (Q(D)) located in the vicinity of the distal heme b(D). Here molecular details of the interaction between the semiquinone radical and the protein environment have been provided using advanced multifrequency pulsed EPR methods. (14)N and (15)N ESEEM and HYSCORE measurements carried out at X-band ( approximately 9.7 GHz) on the wild-type enzyme or the enzyme uniformly labeled with (15)N nuclei reveal an interaction between the semiquinone and a single nitrogen nucleus. The isotropic hyperfine coupling constant A(iso)((14)N) approximately 0.8 MHz shows that it occurs via an H-bond to one of the quinone carbonyl group. Using (14)N ESEEM and HYSCORE spectroscopies at a lower frequency (S-band, approximately 3.4 GHz), the (14)N nuclear quadrupolar parameters of the interacting nitrogen nucleus (kappa = 0.49, eta = 0.50) were determined and correspond to those of a histidine N(delta), assigned to the heme b(D) ligand His-66 residue. Moreover S-band (15)N ESEEM spectra enabled us to directly measure the anisotropic part of the nitrogen hyperfine interaction (T((15)N) = 0.16 MHz). A distance of approximately 2.2 Abetween the carbonyl oxygen and the nitrogen could then be calculated. Mechanistic implications of these results are discussed in the context of the peculiar properties of the menasemiquinone intermediate stabilized at the Q(D) site of NarGHI.

A number of isotopes have been implanted into thin foils of highly oriented pyrolytic graphite in order to investigate the possibility of using this material as a catcher for on-line LT-NO experiments to measure the quadrupole interaction of short-lived nuclei. Pure nuclear electric quadrupole interaction of nuclei of188,189Ir,203Hg,69mZn,182m,183Re and111In has been observed. Values for the electric field gradient along theC-axis are: +10.0(3) +8.2(9), +5.4(2.6) and +1.5(1)·1022 V/m2, respectively, for Ir, Hg, Re and In in graphite. A value ofQ=+0.79(6) b is deduced for the quadrupole moment of189Ir. Two theoretical models provide a better understanding of the origin of the electric field gradient in graphite. The first is based on induced electric polarization of graphite atoms, while in the second one hybridization of impurity and graphite electronic wave functions is calculated.

Liquids entrapped in cavities and containing quadrupole nuclei are considered. The interaction of the quadrupole moment of a nucleus with the electric field gradient is studied. In such a system, molecules are in both rotational and translational Brownian motions which are described by the diffusion equation. Solving this equation, we show that the intra- and intermolecular nuclear quadrupole interactions are averaged to zero in cavities with the size larger than several angstroms.

The results are presented of the experimental detection and investigation of the phenomenon of the drift of excitons in an inhomogeneous electric field. The investigations were performed on silicon structures with Shottky barriers at 4.2 K. A study was made of the influence of the photoexcitation intensity and electric field strength on the breakdown dynamics of the structure. The experimental results were used to show that breakdown of a reverse-biased Shottky barrier involves a narrowing of the width of the space-charge region due to a drift of excitons in the inhomogeneous electric field and to recombination of electrons on ionized impurity centers of the barrier.

A general linear theory of the E×B instability is developed which considers an ambient electric field that is at an arbitrary angle to the density gradient and allows the electric field component parallel to the density gradient to be inhomogeneous. A differential equation is derived which describes the mode structure of the unstable waves in the direction of the inhomogeneities. The theory (1) includes ion inertia effects, (2) allows for arbitrary density and electric field profiles, and (3) is valid in the long‐wavelength regime, i.e., kyL < 1, where L is the width of the boundary layer. The main results of the analysis are as follows. First, the inhomogeneous velocity flow caused by the inhomogeneous electric field can stabilize the instability. Second, short‐wavelength modes are preferentially stabilized over longer‐wavelength modes. Third, the stabilization mechanism is associated with the velocity shear due to an x‐dependent resonance [ω − kyVy(x)]−1, where Vy(x) = −cEx(x)/B, and not velocity shear terms explicitly proportional to ∂Vy/∂x or ∂²Vy/∂x². Fourth, the marginal stability criterion is weakly dependent on the magnitude of νin/ω. Applications of these results to ionospheric phenomena are discussed, viz., barium cloud striations and high‐latitude F region irregularities.