Dipolar Processes Research Articles

Influence of relative humidity on CO2 interaction mechanism for Gd-doped SnO2 with respect to pure SnO2 and Gd2O3

The paper aims to identify the CO2 interaction mechanism for chemical sensors based on Gd-doped SnO2, SnO2 and Gd2O3 powders deposited as thick sensitive layers. The low reactivity of CO2 conferred by the thermodynamic stability and chemical inertia can be offset by the presence of relative humidity. The sensitive powders were prepared by wet chemical co-precipitation method. The Gd concentration was varied from 1% to 20 at% in order to determine the limit for Gd integration as a doping ion prior to chemical segregation as a secondary phase. Analytical transmission electron microscopy points to a homogeneous Gd doping of the nanostructured SnO2 powders for low doping concentrations and the formation of a nanocomposite based on SnO2 as main phase and cubic Gd2O3 as secondary phase for the highly doped samples. The electrical resistance is either influenced by the density of oxygen vacancies, or is the result of compensation for two opposite behaviours into the SnO2-Gd2O3 nanocomposite structures. The CO2 exposure to humid atmosphere determines distinct behaviours corresponding to SnO2 and Gd2O3 as constitutive elements. The associated CO2 interaction mechanism is based on simultaneous DC electrical resistance and Contact Potential Difference measurements, which allow decoupling the ionosorption from the dipolar processes, thus highlighting specific chemical interactions on the SnO2 and Gd2O3 surfaces.

Biomass-Derived Activated Carbon/Epoxy Composite as Microwave Absorbing Material

An activated carbon/epoxy composite was synthesized in which waste mango leaves biomass was used as the carbon source. To create a sufficient amount of porosity in the biomass, a simple process of carbonization followed by its activation was used. The morphology and porosity of the activated carbon material were studied by SEM, TEM, and BET analysis, which confirmed the presence of macro- and mesopores in the material. The activated carbon/epoxy composite with 3 mm thickness gave the highest reflection loss (RL) of − 39.57 dB at 6.48 GHz. At a thickness of just 2 mm the maximum effective absorption bandwidth (RL < − 10 dB) of 2.14 GHz was achieved. This excellent microwave absorption performance was attributed to the porous and bowl-type structure, which resulted in large surface area, high impedance matching, and subsequent attenuation of microwaves by conduction losses, as well as interfacial and dipolar polarization processes. This study provides a porous activated carbon/epoxy composite that is lightweight and economical, and has a high structural stability for microwave absorption applications.Graphical Abstract

Low-temperature dipolar polarization mechanisms in Ba0·9La0·1Fe12O19 ceramic hexaferrite

The complex eletrical permittivity and the thermally stimulated depolarization current responses of the M-type Ba0·9La0·1Fe12O19 ceramic hexaferrite was investigated in the 10–300 K temperature range. The sample was obtained by using the conventional solid-state reaction method. From X-ray diffraction patterns we have found, by using the Rietveld refinement method, that most of the indexed reflections belong to BaM hexaferrite with spatial group P63mc (186), and less than 5% of α-Fe2O3 secondary phase. It was found from magnetization measurements at room temperature values of saturation magnetization, remnant magnetization, and coercivity of 61.69 emu/g, 27.13 emu/g, and 1.71 kOe, respectively. Measurements of the real, ε', and imaginary, ε'', parts of the complex electric permittivity, revealed two different polarization processes: a dipolar polarization process, characterized by a relaxation peaks of ε'' dependence, in the temperature range 50–130 K and the interfacial polarization in the temperature interval 150–300 K. The analysis of all experimental data suggested that dipolar polarization at low-temperature can be related to the off-equatorial displacement of the Fe3+ at 2b and 6c sites, respectively. In association with the dipolar polarization process, a ferroelectric behavior between 10 and 80 K, was detected by using measurements of depolarization current as a function of temperature and at zero applied magnetic field.

Dielectric Spectroscopy of Water Dynamics in Functionalized UiO-66 Metal-Organic Frameworks.

We present a dielectric spectroscopy study of dipolar dynamics in the hydrated UiO-66(Zr) type metal-organic frameworks (MOFs) functionalized with −NH2 and −F groups. Experiments are performed in a broad temperature and frequency ranges allowing us to probe several dipolar relaxations. For both samples at temperature below 220 K, we observe confined supercooled water dynamics, which can be described by the Arrhenius law. At slightly higher temperature, a second less pronounced dipolar relaxation is identified, and its origin is discussed. At even higher temperature, the dielectric permittivity exhibits anomalous increase with increasing temperature due to the proton conductivity. Upon further heating, the permittivity shows a sudden decrease indicating a reversible removal of water molecules. Measurements of the dehydrated samples reveal absence of all three dipolar processes.

Photoelectrochemical Study of the Delafossite AgNiO2 Nanostructure: Application to Hydrogen Production

Abstract AgNiO2 is a semiconductor crystallizing in the delafossite structure; it is prepared by the hydrothermal route, and the photoelectrochemical properties are studied for the first time. The TG/DSC analyses show a low stability not exceeding 290 °C before its reduction into Ag and NiO. The direct bandgap energy of the bulk material is 0.87 eV, due to the d–d transition of Ag+ linearly coordinated. AgNiO2 is chemically stable in the pH region (4–14); a flat band potential of −0.022 VRHE with p-type behavior, inferred to oxygen insertion is reported in KOH solution (10−2 M). The holes density (1.92 × 1022 cm−3) agrees with a semi-metallic behavior. Positive potentials give rise to surface oxidation of AgNiO2 in the diffusion plateau before oxygen evolution. The electrochemical oxygen insertion, investigated by chrono-amperometry, is found to be slow with a diffusion coefficient of ∼8 × 10−16 cm2 s−1. The Nyquist plot exhibits a semicircle centered below the abscissa axis, whose diameter 4200 Ω cm2 decreases down to 760 Ω cm2 under visible illumination. Such results indicate dipolar and multi-relaxation processes and confirm the existence of the optical gap. The conduction band (−0.88 VRHE) derived from Ag+: 4d orbital is more cathodic than the potential of H2O/H2 (∼−0.64 VRHE) level and hydrogen is evolved under visible irradiation. An evolution rate of 1.43 mL g−1 min−1 at pH ∼ 12.8 is obtained with a light-to-chemical energy efficiency of 2.40%.

Optimal control of the self-bound dipolar droplet formation process

Dipolar Bose–Einstein condensates have recently attracted much attention in the world of quantum many body experiments. While the theoretical principles behind these experiments are typically supported by numerical simulations, the application of optimal control algorithms could potentially open up entirely new possibilities. As a proof of concept, we demonstrate that the formation process of a single dipolar droplet state could be dramatically accelerated using advanced concepts of optimal control. More specifically, our optimization is based on a multilevel B-spline method reducing the number of required cost function evaluations and hence significantly reducing the numerical effort. Moreover, our strategy allows to consider box constraints on the control inputs in a concise and efficient way. To further improve the overall efficiency, we show how to evaluate the dipolar interaction potential in the generalized Gross–Pitaevskii equation without sacrificing the spectral convergence rate of the underlying time-splitting spectral method.

Solvent-Modulated Charge-Transfer Resonance Enhancement in the Excimer State of a Bay-Substituted Perylene Bisimide Cyclophane.

Excimer, a configurational mixing between Frenkel exciton and charge-transfer resonance states, is typically regarded as a trap state that hinders desired energy or charge-transfer processes in artificial molecular assemblies. However, in recent days, the excimer has received much attention as a functional intermediate in the excited-state dynamics such as singlet fission or charge-separation processes. In this work, we show that the relative contribution to charge-transfer resonance of the excimer state in a bay-substituted perylene bisimide dimer cyclophane can be modulated by dielectric properties of the solvents employed. Solvent-dependent time-resolved fluorescence and absorption measurements reveal that an enhancement of charge-transfer resonance in the excimer state is reflected by incomplete symmetry-breaking charge-separation processes from the structurally relaxed excimer state by means of dipolar solvation processes in the high dielectric environment.

On the protons involvement in the electrical conductivity of the urea-based linear supramolecular polymers

The correlations between the dc conductivity or the charge relaxation time and the dipolar relaxation time have been found in such range of N,N′-diethylurea concentrations in nonpolar solvent, (mol. fr. >0.04), where the dipolar reorientation process concerns already not the whole supramolecular polymers chains, but their fragments resulting from the thermal breaking of the hydrogen bonds. That experimental fact seems to be an important evidence for the participation of the protons in the electrical conductivity of the supramolecular system. The most probable, in the conductivity process are involved the parts of the polymers chains formed by cracking of the hydrogen bonds COH+⋯N− with the transferred protons in their double potential well. The cascading rotation of the protonated fragments spreading along the chains can be an efficient way of proton transport in the linear supramolecular polymers. It is just the case, where one can expect the compatibility of the time constants in the dipolar and the electric charge dynamics, what was found in our experiment.

Synthesis of Benzodihydrofurans by Asymmetric C-H Insertion Reactions of Donor/Donor Rhodium Carbenes.

Metal carbenes appended with two electron-donating groups, known as "donor/donor" carbenes, undergo diastereo- and enantioselective rhodium-catalyzed C-H insertion reactions with ether substrates to form benzodihydrofurans. Unlike the reactions of metal carbenes with electron-withdrawing groups attached, the attenuated electrophilicity enables these reactions to be conducted in Lewis basic solvents (e.g., acetonitrile) and in the presence of water. The diazo precursors for these species are prepared in situ from hydrazone using a mild and chemoselective oxidant (MnO2 ). Although this sequence often can be performed in one-pot, control experiments have elucidated why a "two-pot" process is often more efficient. A thorough screening of achiral catalysts demonstrated that sterically encumbered catalysts are optimal for diastereoselective reactions. Although efficient insertion into allylic and propargylic C-H bonds is observed, competing dipolar cycloaddition processes are noted for some substrates. The full substrate scope of this useful method of benzodihydrofuran synthesis, mechanisms of side reactions, and computational support for the origins of stereoselectivity are described.

1H-31P CPVC NMR method under Very Fast Magic Angle Spinning for analysis of dipolar interactions and dynamics processes in the crystalline phosphonium tetrafluoroborate salts

We present an NMR methodology which can be used to study the dynamical processes occurring in organophosphorus compounds that belong to the group of the organic ionic plastic crystals (OIPCs). As model samples we employed two phosphonium tetrafluoroborate salts; (t-Bu)3PH+BF4− (1) and (Me)3PH+BF4− (2). Both samples possess in their structures direct H-P bonds, and both undergo complex thermal processes in the solid state, forming below the melting point three or four phases, respectively. 1H-31P CPVC (Cross-Polarization Variable Contact) measurements were performed under Very Fast Magic Angle Spinning with speed equal to 50 or 60 kHz, in order (i) to establish the hydrogen-phosphorus dipolar couplings, and (ii) to correlate the dipolar splitting values with molecular motions of the cation.Our project is divided into three sections. In the first part we present DSC studies of (1) and (2), to verify whether these samples fulfill the requirements that define them as OIPC. The second part is dedicated to a discussion of the theoretical aspects of 1H-31P CPVC and especially an analysis of the influence of different parameters, e.g. CSA31P, H-H mismatch, rf-inhomogeneity, dipolar truncation, and the type of dynamics through the motionally averaged <ηD> asymmetry value on the NMR response. The third part shows experimental 1H-31P CPVC data and applicability of these measurements to study H-P distances and dynamics. The complex molecular motion for sample (2), including rotation and diffusion, versus temperature is then postulated on the bases of the changes of H-P dipolar splitting.

Handling Hazards Using Continuous Flow Chemistry: Synthesis of N1-Aryl-[1,2,3]-triazoles from Anilines via Telescoped Three-Step Diazotization, Azidodediazotization, and [3 + 2] Dipolar Cycloaddition Processes

The conversion of commercially available anilines into triazole products was realized using a telescoped three-reactor flow diazotization, azidodediazotization, and [3 + 2] dipolar cycloaddition process. The diazotization–azidodediazotization sequence was accelerated by means of an ultrasonic bath resulting in a degassed, segmented effluent. An automated continuous flow unit controlled by custom software created in-house was used to collect the aryl azide stream and restore it to a continuous column of the reagent. When combined with a variety of dipolarophiles, 1-aryl-[1,2,3]-triazoles were thus assembled by either copper catalyzed alkyne–azide cycloaddition (CuAAC) or Huisgen cycloaddition reactions.

Monitoring molecular dynamics of bacterial cellulose composites reinforced with graphene oxide by carboxymethyl cellulose addition

Broadband Dielectric Relaxation Spectroscopy was performed to study the molecular dynamics of dried Bacterial Cellulose/Carboxymethyl Cellulose-Graphene Oxide (BC/CMC-GO) composites as a function of the concentration of CMC in the culture media. At low temperature the dielectric spectra are dominated by a dipolar process labelled as a β-relaxation, whereas electrode polarization and the contribution of dc-conductivity dominate the spectra at high temperatures and low frequency. The CMC concentration affects the morphological structure of cellulose and subsequently alters its physical properties. X-ray diffractometry measurements show that increasing the concentration of CMC promotes a decrease of the Iα/Iβ ratio. This structural change in BC, that involves a variation in inter- and intramolecular interactions (hydrogen-bonding interactions), affects steeply their molecular dynamics. So, an increase of CMC concentration produces a significantly decrease of the β-relaxation strength and an increase of the dc-conductivity.

Implementation of polarization processes in a charge transport model applied on poly(ethylene naphthalate) films

One of the difficulties in unravelling transport processes in electrically insulating materials is the fact that the response, notably charging current transients, can have mixed contributions from orientation polarization and from space charge processes. This work aims at identifying and characterizing the polarization processes in a polar polymer in the time and frequency-domains and to implement the contribution of the polarization into a charge transport model. To do so, Alternate Polarization Current (APC) and Dielectric Spectroscopy measurements have been performed on poly(ethylene naphthalene 2,6-dicarboxylate) (PEN), an aromatic polar polymer, providing information on polarization mechanisms in the time- and frequency-domain, respectively. In the frequency-domain, PEN exhibits 3 relaxation processes termed β, β* (sub-glass transitions), and α relaxations (glass transition) in increasing order of temperature. Conduction was also detected at high temperatures. Dielectric responses were treated using a simplified version of the Havriliak-Negami model (Cole-Cole (CC) model), using 3 parameters per relaxation process, these parameters being temperature dependent. The time dependent polarization obtained from the CC model is then added to a charge transport model. Simulated currents issued from the transport model implemented with the polarization are compared with the measured APCs, showing a good consistency between experiments and simulations in a situation where the response comes essentially from dipolar processes.

Structural, electrical and dielectric characterization of TeO2–WO3–Y2O3–Er2O3–Yb2O3 glasses

TeO2 based glasses with molar composition (70-x-y)TeO2–20WO3–10Y2O3–xEr2O3–yYb2O3, where x=0 and 0.5mol% and y=0, 0.5, 1, 2 and 4mol%, were prepared by the melt quenching technique. At room temperature all the samples are transparent, mechanical and thermally stable. The XRD results showed the amorphous nature of all the samples and the Raman spectra showed a band distribution related with TeO and WO bond vibrations. The introduction of WO3 results in a bandwidth substantially larger when compared to SiO2 based glasses and even other TeO2 based glasses, making this vitreous system promising for Raman optical amplification.In this work, the measurement and analysis of the electrical properties of this glass system was made. It was found that the hopping of small radius polarons, for small concentrations of networks modifiers, is the dominant process in the charge transport mechanism for the dc conductivity. For higher concentrations of ions structurally inserted in the glass network, the ionic conductivity starts to have an important role in the transport mechanism. The ac conduction mechanism was attributed to a dipolar process between the interstitial modifier cations and the nonbridging oxygen ions. For all samples, the dielectric constant is practically independent of the temperature and the frequency. This quantity tends to decrease with the increase of the concentration of Yb3+ cations. No dielectric relaxation phenomenon was observed in the temperature and frequency measurement range.

Dissociating space charge processes from orientation polarization in poly(ethylene naphthalate) films

Thermo-stimulated depolarization current (TSDC) measurements and space charge measurements were performed on poly(ethylene naphthalene 2,6-dicarboxylate) (PEN), an aromatic and polar polyester. The aim is to develop an understanding of the dipolar and conduction processes at play in this material and in particular to understand the effects of temperature. For the TSDC measurements, when polarizing at 130 and 170 °C, the sub-glass transition and the glass transition relaxations are observed. However, in the case of a polarization temperature of 170 °C, one more current peak, labelled ρ peak, is observed at temperatures above the glass transition. This peak is not only of dipolar origin and could be associated with charge detrapping in the material. To unravel the mechanisms behind this process, a TSDC was combined with space charge measurements using the pulsed electroacoustic method (PEA) and the partial heating method was used. It is shown that the ρ peak is predominantly associated with the release of the negative charge build-up in the material.

Dipolar atomic spin ensembles in a double-well potential

We experimentally study the spin dynamics of mesoscopic ensembles of ultracold magnetic spin-3 atoms located in two separated wells of an optical dipole trap. We use a radio-frequency sweep to selectively flip the spin of the atoms in one of the wells, which produces two separated spin domains of opposite polarization. We observe that these engineered spin domains are metastable with respect to the long-range magnetic dipolar interactions between the two ensembles. The absence of inter-cloud dipolar spin-exchange processes reveals a classical behavior, in contrast to previous results with atoms loaded in an optical lattice. When we merge the two subsystems, we observe spin-exchange dynamics due to contact interactions which enable the first determination of the s-wave scattering length of 52Cr atoms in the S=0 molecular channel a_0=13.5^{+11}_{-10.5}a_B (where a_B is the Bohr radius).

Infrared-dressed entanglement of cold open-shell polar molecules for universal matchgate quantum computing

Implementing a scalable quantum information processor using polar molecules in optical lattices requires precise control over the long-range dipole–dipole interaction between molecules in selected lattice sites. We present here a scheme using trapped open-shell polar molecules that allows dipolar exchange processes between nearest and next-nearest neighbors to be controlled in order to construct a generalized transverse Ising spin Hamiltonian with tunable XX, YY and XY couplings in the rotating frame of the driving lasers. The scheme requires a moderately strong bias magnetic field together with near-infrared light to provide local tuning of the qubit energy gap, and mid-infrared pulses to perform rotational state transfer via stimulated Raman adiabatic passage. No interaction between qubits occurs in the absence of the infrared driving. We analyze the fidelity of the resulting two-qubit matchgate, and demonstrate its robustness as a function of the driving parameters. We discuss a realistic application of the system for universal matchgate quantum computing in optical lattices.

XPS for probing the dynamics of surface voltage and photovoltage in GaN

We describe application of two different data gathering techniques of XPS for probing the dynamics of surface voltage and surface photovoltage (SPV) developed in microseconds to seconds time-domain, in addition to the conventional steady-state measurements. For the longer (seconds to milliseconds) regime, capturing the data in the snapshot fashion is used, but for the faster one (down to microseconds), square wave (SQW) electrical pulses at different frequencies are utilized to induce and probe the dynamics of various processes causing the surface voltage, including the SPV, via the changes in the peak positions. The frequency range covers anywhere from 10−3 to 105Hz for probing changes due to charging (slow), dipolar (intermediate), and electronic (fast) processes associated with the external stresses imposed. We demonstrate its power by application to n- and p-GaN, and discuss the chemical/physical information derived thereof. In addition, the method allows us to decompose and identify the peaks with respect to their charging nature for a composite sample containing both n- and p-GaN moieties.

Mechanism of dilute-spin-exchange in solid-state NMR

In the stationary, aligned samples used in oriented sample (OS) solid-state NMR, (1)H-(1)H homonuclear dipolar couplings are not attenuated as they are in magic angle spinning solid-state NMR; consequently, they are available for participation in dipolar coupling-based spin-exchange processes. Here we describe analytically the pathways of (15)N-(15)N spin-exchange mediated by (1)H-(1)H homonuclear dipolar couplings. The mixed-order proton-relay mechanism can be differentiated from the third spin assisted recoupling mechanism by setting the (1)H to an off-resonance frequency so that it is at the "magic angle" during the spin-exchange interval in the experiment, since the "magic angle" irradiation nearly quenches the former but only slightly attenuates the latter. Experimental spectra from a single crystal of N-acetyl leucine confirm that this proton-relay mechanism plays the dominant role in (15)N-(15)N dilute-spin-exchange in OS solid-state NMR in crystalline samples. Remarkably, the "forbidden" spin-exchange condition under "magic angle" irradiation results in (15)N-(15)N cross-peaks intensities that are comparable to those observed with on-resonance irradiation in applications to proteins. The mechanism of the proton relay in dilute-spin-exchange is crucial for the design of polarization transfer experiments.

Electric modulus and polarization studies on piezoelectric polyimides

The piezoelectric polymeric sensors and actuators are widely valued because of their low density, availability to obtain complex shapes, and easy processing among other characteristics. However, there are not many useful piezoelectric polymers. Aromatic polyimides are high-performance polymeric materials characterized by thermal stability, chemical resistance, and excellent mechanical properties. Besides, some of these polymers have been reported as candidates to present piezoelectric properties. Our research focuses in the electric characterization of three piezoelectric amorphous aromatic polyimides, containing CN groups in different number and positions. The piezoelectricity in amorphous polymers is mainly due to the orientation polarization of the molecular dipoles, which is induced by the application of an external electrical field to a temperature over their glass transition temperature (Tg). Polyimides are measured by thermally stimulated depolarization currents and dielectric spectroscopy and the analysis of the dielectric data has been performed using the electric modulus formalism to separate conductive and dipolar processes. These measurements have evidenced that the frozen-in polarization is mainly due to the dipole orientation of the dipolar CN groups, allowing us to understand in depth the mechanism of polarization that contributes to the piezoelectric properties. This will facilitate the obtention of materials with the best possible piezoelectric properties, comparable to those currently available but with higher mechanical and thermal performance. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012