Temperature Dependence Of Relaxation Time Research Articles

Synergistic phases in isopropanol electrospun for Fe2O3/Fe3O4@C@MoS2 nanofibers for enhanced electromagnetic interference shielding characteristics and impedance analysis

Co-existence of phases in Fe2O3/Fe3O4@C@MoS2 nanofibers composite is synthesized via electrospinning and hydrothermal processes, where iso-propanol is utilized as a solvent. An extensive analysis is performed to evaluate the different properties, such as dielectric, morphological, structural, impedance, and electromagnetic shielding efficiency. A crystallite size of 24 nm is calculated using the Debye-Scherer formula. By using impedance spectroscopy, the electrical behavior of Fe2O3/Fe3O4@C@MoS2 nanofibers is described. Variations in frequency and temperature-dependent relaxation times are compared in an impedance plane plot. Changes in the conduction mechanism at different temperatures are observed. In Fe2O3/Fe3O4@C@MoS2 nanofibers composites, a number of distinctive characteristics in this work are explained, including stable dielectric constant, low tangent loss small polaronic hopping (SPH), and variable range hoping (VRH) model. The localization length of Fe2O3/Fe3O4@C@MoS2 nanofibers is calculated, 1/α = 1.2 Å. The typical EMI shielding efficiency of about 23 dB is demonstrated by the Fe2O3/Fe3O4@C@MoS2 nanofibers composite in the X-band frequency range. This extraordinary shielding capacity is observed in 3 mm thick nanofibers.

Tuning the Physical State of Aripiprazole by Mesoporous Silica.

The main purpose of our studies is to demonstrate that commercially available mesoporous silica (MS) can be used to control the physical state of aripiprazole (ARP). The investigations performed utilizing differential scanning calorimetry and broadband dielectric spectroscopy reveal that silica can play different roles depending on its concentration in the system with amorphous ARP. At low MS content, it activates recrystallization of the active pharmaceutical ingredient and supports forming the III polymorphic form of ARP. At intermediate MS content (between ca. 27 and 65 wt %), MS works as a recrystallization inhibitor of ARP. At these concentrations, the formation of III polymorphic form is no longer favorable; therefore, it is possible to use this additive to obtain ARP in either IV or X polymorphic form. At the same time, employing MS in concentrations >65 wt % amorphous form of ARP with high physical stability can be obtained. Finally, regardless of the polymorphic form it crystallizes into, each composite is characterized by the same temperature dependence of relaxation times in the supercooled and glassy states.

The Universal Scaling of Dielectric Response as a Tool in the Description of a Complex Dynamic of 4′-Butyl-4-(2-methylbutoxy)azoxybenzene (4ABO5*)

The results of dielectric relaxation spectroscopy of the chiral liquid crystal 4′-butyl-4-(2-methylbutoxy)azoxybenzene in the crystal phase are presented. The scaling procedure showed complex molecular dynamics and allows one to decompose the observed relaxation process into two closely located relaxation processes around the short molecular axis. Temperature dependences of relaxation times characterizing flip-flop motions (rotation around the short molecular axis) and rotation around the long molecular axis are of the Arrhenius type.

Probing the heterogeneity of molecular level organization of ionic liquids: a comparative study using neutral Nile red and cationic Nile blue sulfate as fluorescent probes for butyrolactam-based protic ionic liquids.

Ionic liquids (ILs) are liquid salts composed of cations and anions, known for their significant local heterogeneity at the molecular level. To understand the microheterogeneity with regard to their local polarity and local viscosity, we have used two structurally similar but chemically distinguishable fluorescent probes: Nile red (NR), a neutral molecule, and Nile blue sulfate (NBS), a charged molecule. A comparative study of the response of the two probes to the molecular level heterogeneity of ILs is expected to provide a better clarity of understanding regarding the charged polar domain and the uncharged hydrophobic domain of ILs. Towards this, we synthesized two butyrolactam-based protic ionic liquids (PILs), i.e., BTF and BTD, with the same ionic headgroup ([BT]+) and different alkyl tails ([RCOO]-), where {R = H, C11H23}. BTF has no significant hydrophobic domain, whereas BTD has a larger hydrophobic domain. Temperature-dependent fluorescence parameters such as fluorescence intensity, lifetime, and anisotropy were measured for both NR and NBS molecules. The use of a pair of structurally similar but ionically different probes enables differential estimation of parameters like the microviscosity of a domain using the fluorescence anisotropy parameter (r). The absorption and emission spectra of both probe molecules are observed to be blue shifted upon going from BTF to BTD. NR showed a significant blue shift in absorption and emission band maxima. Conversely, NBS exhibited a small wavelength shift, possibly influenced by the preferred location of their charged head group domain. Temperature-dependent rotational relaxation time (θ) of NR in BTD is smaller than that of NBS by 60-70%, indicating that stronger charge-charge interactions exist between the polar domain of BTD and NBS. Moreover, it is observed that the local viscosity of the BTF IL around both probes is similar, whereas there is a considerable difference for the BTD IL. These results are an indication that NBS being charged prefers to locate itself in the charged head group region of the IL, whereas NR being neutral tends to reside both in the hydrophobic domain and in the head group but is predominantly located in the hydrophobic domain.

Structural, dielectric and electrical transport properties of thermal stable (1-x)K0.5Bi0.5TiO3-xBaTiO3 ceramics

Polycrystalline samples of (1-x)K0.5Bi0.5TiO3 - xBaTiO3 [(1-x)KBT - xBT] were synthesized using a solid-state reaction pathway. X-ray diffraction analysis confirmed the presence of a single-phase perovskite structure. To explore relaxation and conduction mechanisms, complex impedance spectroscopy was employed. Reduction in Curie temperature was observed as BT content increased. The grain and grain boundary contributions to relaxation was examined through complex impedance and modulus plots and quantified via fitting experimental data to an equivalent circuit model. Examination of the Complex impedance spectra uncovered the existence of dielectric relaxation characteristic of a non-Debye nature. Furthermore, the activation energy determined suggests commonalities in governing the conduction and relaxation processes in relation to temperature-dependent conductivity and relaxation time. A negative temperature coefficient of resistance behaviour was obtained from the temperature-dependent conductive study. Dielectric spectra revealed that these ceramic capacitors are useful in reduction of signal interference, signal attenuation and impedance matching circuits applications.

Slow magnetic relaxation behaviors and diamagnetic dilution studies of a carboxyl bridged dinuclear dysprosium(III) complex

By using a rigid bulky carboxylic ligand α-cyanocinnamic acid (CCA), a dinuclear dysprosium (III) complex [Dy2(CCA)6 (MeOH)4] (1) was synthesized. Single crystal X-ray crystallography reveals that the two eight-coordinate dysprosium ions are bridged by four deprotonated carboxyl groups, forming a centrosymmetric paddle-wheel-like structure. Dynamic magnetic property measurements indicate that comple×1 displays field-induced slow magnetic relaxation. The temperature-dependent relaxation times can be fitted using Orbach and Raman processes with parameters of n = 2.8 (2), C = 27 (8) s−1/Kn, τ0 = 5 (2) × 10−10 s and Ueff = 40 (3) cm−1. Magnetic studies on the diamagnetic YIII-diluted analogue [Dy0.206Y1.794(CCA)6 (MeOH)4] (2) reveal its slow magnetic relaxation behavior without external dc field and the antiferromagnetic coupling between the DyIII ions in 1. Fits on the obtained relaxation times of 2 lead to the parameters of n = 4.5 (3), C = 0.7 (2) s−1/Kn, τ0 = 2.8 (2) × 10−9 s and Ueff = 38 (2) cm−1. The results suggest that slow magnetic relaxation originates from the single-ion relaxation of DyIII ions. Moreover, the diamagnetic dilution can suppress other fast relaxation pathways at low temperature, on account of the elimination of magnetic coupling and dipolar interaction. Ab initio calculations were then performed on the single DyIII ion species {YDy} and indicate that the first excited Kramers doublets (KDs) lie at ca. 76 cm−1, which is slightly higher than the experimental Ueff value. The intramolecular magnetic interactions were also calculated and indicate a weak ferromagnetic dipole-diploe magnetic interaction and an antiferromagnetic exchange coupling.

Optoelectronic and thermoelectric properties of LiCuM (M=S, Se and Te) half-Heuslers: insights first principle calculations

The work is performed to study the structural stability and optoelectronic properties as well as thermoelectric properties of LiCuM (M=S, Se and Te) half-Heusler semiconductors using density functional theory (DFT) and semi-classical Boltzmann transport. The ground state results show that the compounds exhibit semiconducting behavior with a direct band-gap. The elastic parameters indicate that the present compounds are mechanically, dynamically stable and brittle. The calculated optical properties in GGA and GGA+U approaches show that the dominant response in the low ultraviolet and visible energy regions. The thermoelectric properties are evaluated using the Slack model and temperature dependent relaxation time in the temperature range of 100 K to 1000 K. The response of thermoelectric properties to temperature is evaluated and discussed in detail. The figure of merit with relaxation time is found to increase with temperature and reaches the optimal values in GGA and GGA+U at 1000 K are 0.69(0.01), 0.66(0.665) and 0.67(0.778) for LiCuS, LiCuSe and LiCuTe, respectively. The lattice thermal conductivity decreases with increasing temperature. These properties make these compounds promising candidates for optoelectronic and thermoelectric devices.

Low-frequency electrical response related to oxygen vacancies in Al3+ substituted M-type hexaferrite

The presence of oxygen vacancies is unavoidable in many complex metal oxide materials such as hexaferrites. This article investigates the role of crystal defects, such as oxygen vacancies, and different oxidation states of Fe ions, on the electrical conduction behaviors of Al3+ substituted Ba0.4La0.1Sr0.5Al x Fe12−x O19 hexaferrites by analyzing the temperature dependent AC impedance and AC conductivity. The optimal substitution of La3+ ions and high-temperature sintering is responsible for forming oxygen vacancies while reducing Fe from Fe3+ to Fe2+ cations, as observed in structural and XPS analysis, which affects electrical conduction behaviors. The departure from ideal Debye-type relaxation behavior is also one of the effects of crystal defects. Temperature variation of relaxation time shows a sharp change at around 403 K. This may be due to different electrical entities influencing the mode of electron hopping with temperature. The activation energy calculated from the temperature dependent relaxation time confirmed the presence of electron hopping through the bridging effects of the first ionization of oxygen vacancies Fe2+––Fe3+ below a temperature of 403 K. Above 403 K, electron hopping through different oxidation states of Fe3+ + e− → Fe2+ and migration of oxygen vacancies toward the bulk surface are responsible for the conduction mechanism. The DC resistivity and AC conductivity show that the bridging effects of oxygen defects can also induce Coulomb interaction between the defect sites resulting in Efros–Shklovskii variable range hopping and correlation barrier hopping below 403 K. Above 403 K, polaron-assisted electron hopping is more desirable, as confirmed by the non-overlapping small polaron tunneling model.

Chemically ordered structure and dynamics in Al80Ti20 liquids

We conducted ab initio molecular dynamics simulations to study the structural and dynamical properties in Al80Ti20 liquids. The prepeak that represents some chemically ordered structure, emerges in the partial static structure factors at low temperatures. By calculating coordination numbers, we found a preferred connection for Al–Ti pairs, which induces an excess Ti/Al concentration around the central Al/Ti atoms. An excess Ti concentration is even found in the second neighbour shells around Ti atoms, rationalizing the prepeak as the result of chemical concentration fluctuation in these neighbour shells. Concomitantly, slow structural relaxation occurs below 1400K, exhibited as the second step relaxation in self-intermediate scattering function. The slow dynamics induces a non-Arrhenius transition for the temperature-dependent diffusivity and structural relaxation time, as well as the breakdown of the Stokes–Einstein relation. These results unveil the detailed ordering structure for the chemical interaction and the associated dynamical behaviour in Al80Ti20 liquids.

Study of dielectric, impedance and transport properties of co-doped Ba0.5Sr0.5TiO3 ceramics

Dielectric properties and Electrical response of BST ceramics having composition Ba0.5Sr0.5Ti0.98Sn0.01Zr0.01O3 synthesized by solid state reaction route are studied in this piece of work. The structural part analysis is done using XRD technique and scanning electron microscopy and is confirmed to be of cubic structure. Electrical characterization through impedance and modulus spectroscopy is made over a frequency region of 1 kHz to 1 MHz from ambient temperature to 400 °C. Near constant value of Ɛr and significantly low and flat loss tangent at room temperature are achieved in the material which suggest possible application in electronic circuits. Temperature dependent broadened loss peaks observed beyond 250 °C indicated thermally activated conductivity/ relaxation process in the material. Impedance spectroscopy results were examined in alternate formalism for the role of grain boundary and grain in various processes. The computed activation energy for charge carriers provides information of the transport phenomena. The estimated values suggest that oxygen vacancies play important role in conduction process. The activation energy is estimated by fitting temperature dependent relaxation time with Arrhenius equation and the type of charge carriers involved in the relaxation process are suggested. Frequency dependent ac conductivity plots at various temperatures obey Johnscher’s law. Variation of exponent in Johnsher’s power law with temperature well agrees with correlated barrier hopping (CBH) model for ac conduction mechanism.

Temperature-dependent thermal phonon relaxation time and its influence on ballistic-diffusive heat transport in SOI transistor

A simple model of the average thermal phonon relaxation time including its temperature dependency for silicon (Si) is proposed and incorporated with the gray version of the phonon Boltzmann transport equation to investigate its effect on the temperature distribution of the Si-layer in a silicon-on-insulator (SOI) transistor with heat generation (qv). Our analysis showed that the peak temperatures both in the nonlocal heating region and at the Si/SiO2 interface increase quadratically with increasing values of qv, while in the case of the temperature-independent, however, they show a linear increase with increasing qv. The results from this work will help in understanding the phonon transport at the device-level based on Si and will provide a way how to do an efficient thermal analysis of the device-level including all the frequency and temperature dependencies of the phonon transport properties.

Janus 2H-MXTe (M = Zr, Hf; X = S, Se) monolayers with outstanding thermoelectric properties and low lattice thermal conductivities.

Two-dimensional (2D) materials have been one of the most popular objects in the research field of thermoelectric (TE) materials and have attracted substantial attention in recent years. Inspired by the synthesized 2H-MoSSe and numerous theoretical studies, we systematically investigated the electronic, thermal, and TE properties of Janus 2H-MXTe (M = Zr and Hf; X = S and Se) monolayers by using first-principles calculations. The phonon dispersion curves and AIMD simulations confirm the thermodynamic stabilities. Moreover, Janus 2H-MXTe were evaluated as indirect band-gap semiconductors with band gaps ranging from 0.56 to 0.90 eV using the HSE06 + SOC method. To evaluate the TE performance, firstly, we calculated the temperature-dependent carrier relaxation time with acoustic phonon scattering τac, impurity scattering τimp, and polarized scattering τpol. Secondly, the calculation of lattice thermal conductivity (κl) shows that these monolayers possess relatively poor κl with values of 3.4-5.4 W mK-1 at 300 K, which is caused by the low phonon lifetime and group velocity. After computing the electronic transport properties, we found that the n-type doped Janus 2H-MXTe monolayers exhibit a high Seebeck coefficient exceeding 200 μV K-1 at 300 K, resulting in a high TE power factor. Eventually, combining the electrical and thermal conductivities, the optimal dimensionless figure of merit (zT) at 300 K (900 K) can be obtained, which is 0.94 (3.63), 0.51 (2.57), 0.64 (2.72), and 0.50 (1.98) for n-type doping of ZrSeTe, HfSeTe, ZeSTe, and HfSTe monolayers. Particularly, the ZrSeTe monolayer shows the best TE performance with the maximal zT value. These results indicate the excellent application potential of Janus 2H-MXTe (M = Zr and Hf; X = S and Se) monolayers in TE materials.

Second order hydrodynamics based on effective kinetic theory and electromagnetic signals from QGP

We study the thermal dilepton and photon production from relativistic heavy ion collisions in presence of viscosities by employing the recently developed second order dissipative hydrodynamic formulation estimated within a quasiparticle description of thermal quantum chromo-dynamics (QCD) medium. The sensitivity of shear and bulk viscous pressures to the temperature dependence of relaxation time is analyzed within one- dimensional boost invariant expansion of quark gluon plasma (QGP). The dissipative corrections to the phase-space distribution functions up to first order in gradients are obtained from the Chapman–Enskog like iterative solution of effective Boltzmann equation in the relaxation time approximation. Thermal dilepton and photon production rates for QGP are calculated by employing this viscous modified distribution function. Yields of these particles are quantified for the longitudinal expansion of QGP with different temperature dependent relaxation times. Our analysis employing this second order hydrodynamic model indicates that the spectra of dileptons and photons get enhanced by both bulk and shear viscosities and are well behaved. Also, these particle yields are found to be sensitive to relaxation time. Further, we do a comparison of these particle spectra with a standard hydrodynamic formulation.

Far-from-equilibrium attractors for massive kinetic theory in the relaxation time approximation

We investigate whether early and late time attractors for non-conformal kinetic theories exist by computing the time-evolution of a large set of moments of the one-particle distribution function. For this purpose we make use of a previously obtained exact solution of the 0+1D boost-invariant massive Boltzmann equation in relaxation time approximation. We extend prior attractor studies of non-conformal systems by using a realistic mass- and temperature-dependent relaxation time and explicitly computing the effect of varying both the initial momentum-space anisotropy and initialization time on the time evolution of a large set of integral moments. Our findings are consistent with prior studies, which found that there is an attractor for the scaled longitudinal pressure, but not for the shear and bulk viscous corrections separately. We further present evidence that both late- and early-time attractors exist for all moments of the one-particle distribution function that contain greater than one power of the longitudinal momentum squared.

Augmented thermoelectric performance of LiCaX (X = As, Sb) Half Heusler compounds via carrier concentration optimization

The present study is focussed on the detailed physical insight into the structural, thermal, and dynamical properties of 8-valence electron Half Heusler (HH) compounds LiCaX (X = As, Sb) using Density functional theory. The thermal and dynamic stabilities of the compounds are assessed via ab-initio molecular dynamic simulations and phonon dispersion calculations, respectively. The Tran-Blaha modified Becke Johnson potential is used to accurately predict the band gap of investigated compounds. It is found that they are indirect band gap semiconductors with band gaps of 2.52 eV (LiCaAs) and 2.09 eV (LiCaSb). The transport parameters are obtained for p-type and n-type doping at temperatures ranging from 300 K to 800 K by solving the Boltzmann Transport equation. The deformation potential theory is employed to calculate the temperature dependent relaxation time for both compounds. The results of the various thermoelectric parameters obtained using actual values of time-dependent relaxation time are compared with that calculated under constant relaxation time approximation. The maximum power factor is 10.95 × 1011 (4.99 × 1011) Wm−1K−2s−1 for p-type (n-type) LiCaAs and 12.53 × 1011 (5.30 × 1011) Wm−1K−2s−1 for p-type (n-type) LiCaSb at optimized carrier concentration. The obtained low lattice thermal conductivities for LiCaSb (0.66 Wm−1K−1) and LiCaAs (0.88 Wm−1K−1) are explicated it in terms of different phonon modes. The Figure of Merit at 800 K for p-type (n-type) LiCaAs is as high as 0.90 (0.73) and 0.93 (0.84) for LiCaSb at optimum carrier concentration ∼1020 cm−3 (∼1019 cm−3), which has been experimentally realized in other 8-valence electron Li-based HH compounds. The good thermoelectric performance of p-type LiCaX in comparison to n-type suggests that p-type LiCaX alloys are viable candidates for high temperature energy harvesting applications.

Temperature Dependent Dielectric Relaxation Studies of 2-Nitrotoluene-DMSO mixture using Time Domain Relectometry from 10MHz to 50GHz

The dielectric relaxation properties of 2-Nitrotoluene (2-NT) with DMSO for 11 different concentrations has been studied in the frequency range from 10 MHz to 50 GHz using time domain reflectometry (TDR) method. In this frequency range Dielectric relaxation spectra of 2-NT-DMSO mixtures shows Debye type behavior. The temperature dependent static dielectric constant and relaxation time have been determined from 20oC to 5oC. Effective Kirkwood correlation factor is calculated to determine the effect of strong directional forces such as microwaves on heterogeneous molecules under different temperatures. The excess permittivity and excess inverse excess relaxation time is found to be negative for 2-NT-DMSO system with weak molecular interactions. The activation enthalpy have been calculated to find the energy required to rotate the molecules

Mononuclear Heptacoordinated 3d-Metal Helicates as a New Family of Single Ion Magnets

The series of Co(II), Fe(II), and Ni(II) mononuclear coordination compounds of [CoL(NCS)2]·3DMSO (1), [CoL(H2O)2](ClO4)2·DMSO (2), [CoL(H2O)(EtOH)][CoCl4]·2H2O (2a), [FeL(NCS)2]·DMSO (3), and [NiL(NCS)2]·CH3CN (4) composition (where L is 2,6-bis(1-(2-(4,6-dimethylpyrimidin-2-yl)hydrazineylidene)ethyl)pyridine), with an [MLA2] coordination unit (where A is a pair of apical monodentate ligands), was synthesized. In compounds 1, 2, 2a, and 3, the ligand L is pentadentate, and cobalt and iron ions are placed in a heavily distorted pentagonal pyramidal coordination environment, while in 4 the Ni(II) ion is hexacoordinated. Easy plane-type magnetic anisotropy (D = 13.69, 11.46, 19.5, and 6.2 cm−1 for 1, 2, 2a, and 4, respectively) was established for cobalt and nickel compounds, while easy axis-type magnetic anisotropy (D = −14.5 cm−1) was established for iron compound 3. The cobalt coordination compounds 1 and 2 show SIM behavior under a 1500 Oe external magnetic field, with effective magnetization reversal barriers of 65(1) and 60(1) K for 1 and 2, respectively. The combination of Orbach and Raman relaxation mechanisms was shown to adequately describe the temperature dependence of relaxation times for 1 and 2. CASSCF/NEVPT2 calculations were performed to model the parameters of the effective spin Hamiltonian for the compounds under study.

SPH simulations of transient non-isothermal viscoelastic flows with free surfaces

• Transient non-isothermal viscoelastic free surface flows are simulated by SPH. • SPH discretization of the temperature equation is derived. • The impacting droplet and the filling process of circular disc with core are studied. • Both Oldroyd-B and UCM fluid models are considered. • The work provides references to handle non-isothermal viscoelastic flows in an entirely meshfree framework. This study presents a smoothed particle hydrodynamics (SPH) method for simulating transient non-isothermal viscoelastic flows with free surfaces. The basic equations governing the non-isothermal free surface flow of an Oldroyd-B fluid are considered and approximated by SPH. To model non-isothermal fluid flows, a working SPH discretization of the temperature equation is derived, and the temperature-dependent viscosity and relaxation time are represented by the time–temperature superposition principle. The proposed SPH method is first validated by solving the viscoelastic Poiseuille flow problem under isothermal and non-isothermal conditions and comparing the SPH solutions with the analytical solutions or those obtained by other methods. Then, the SPH method is extended to deal with the impact of a non-isothermal viscoelastic droplet with a cold wall and the non-isothermal filling process of a circular disc with a core. The convergence of the method is verified by four particle spacings of different levels of refinement. The effects of some important parameters, including the temperature dependency coefficient, reference temperature of the fluid, and Péclet number, on the flow behavior are analyzed in detail. All the results demonstrate that the proposed SPH method is capable of accurately and stably simulating transient non-isothermal viscoelastic free surface flows in an entirely meshfree framework.

Semiconductorlike photocarrier dynamics in Dirac-semimetal Cd3As2 films probed with transient terahertz spectroscopy

The topological three-dimensional Dirac semimetal ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ has drawn great attention for the novel physics and promising applications in optoelectronic devices operating in the infrared and terahertz (THz) regimes. Among the extensive studies in the past decades, one intriguing debate is the underlined mechanism governing the nonequilibrium carrier dynamics following photoexcitation. In this study, the temperature-dependent photocarrier dynamics in ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ film has been investigated with time-resolved terahertz spectroscopy. The experimental results demonstrate that photoexcitation results in abrupt increase in THz photoconductivity, and the subsequent relaxation shows a single exponential relaxation for various temperatures and pump fluences. The relaxation time increases from 4.7 ps at 5 K to 7.5 ps at 220 K, while the lifetime remains almost constant at $\ensuremath{\sim}7.5$ ps with temperature above 220 K. A Rothwarf-Taylor model was employed to fit the temperature-dependent relaxation time, and a narrow energy gap of $\ensuremath{\sim}35\ifmmode\pm\else\textpm\fi{}6$ meV is obtained, which occurs around the Dirac node. Our THz spectroscopy results demonstrate that the photocarrier relaxation in ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ shows a semiconductorlike behavior, rather than hot-carrier scatterings in graphene and most of metals.

(Invited) Unravelling Water-Ion Dynamics in Reverse Osmosis Membranes with Nuclear Magnetic Resonance Spectroscopy

Fundamental understanding of the charge (ions and water) transport in polymeric membranes used at the forefront of the water-energy nexus challenge is yet uncertain and depends on the complex interplay between polymer structure and dynamics that facilitate transport. Herein, we investigated the dynamics-transport of proton and ion transport in a rather neutrally charged organic membrane (i.e., polyamide) using solid-state NMR spectroscopy. Specifically, the role of competing ions' effect in complex ionic mixtures at the membrane interface has been explored on a molecular level. Proton and sodium ion diffusion measurements are made using pulsed-field gradient NMR diffusometry. The energy barriers for proton and ion transport within the active layer of the reverse osmosis (RO) membrane are assessed using magic angle spinning (MAS) and static NMR spectral line shape analysis. We have investigated the dynamics-transport of water and sodium ions in complex ionic mixtures of polyamide polymeric media using solid-state NMR spectroscopy. Through monitoring 13C, 23Na, and 1H NMR nuclei, quantitative insights into the diffusion of water (1H), sodium mobile ion (Na+), and polymer membrane dynamics (13C) are extrapolated. Furthermore, the spin-lattice (T1) relaxation time of sodium and water ions is investigated to obtain quantitative insight into how sodium and water ions’ rotational movements occur in the absence and presence of other competing seawater cations such as potassium. Room temperature spectra and spin-lattice relaxation times indicated the presence of both mobile and rigidly held ionic species in the membrane at different levels of relative humidity. The concentration and temperature dependence of relaxation times and chemical shifts have been investigated. NMR relaxation measurements detect how the water rotation is modified by sodium and potassium cations in the membrane. Results show that at low relative humidity and in the presence of competing ions, there are sodium ion resonances for crystalline sodium ions and rigidly held sodium ions on the surface. With increasing relative humidity and decreasing competing ions, the resonance suggests solution-like dynamic rotational movements of sodium ions in polyamide. The results confirm the change in activation energy barrier for sodium and hydrogen ion permeation in polyamide upon the presence of potassium ions.