Low-temperature Dispersion Research Articles

Ordered mesoporous CrCeOx catalyst prepared by hard template for toluene and formaldehyde combustion

The meso-Cr-Ce catalysts with different proportions of chromium and cerium were prepared using three-dimensional ordered mesoporous silica (KIT-6) as hard template by vacuum assisted impregnation method. The physicochemical properties of the catalysts were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), nitrogen adsorption-desorption (BET), transmission electron microscopy (TEM), Scanning electron microscopy (SEM) and temperature programmed reduction (TPR) techniques. The oxidation removal performance of toluene and formaldehyde was evaluated. The results show that all catalysts have better catalytic oxidation activity than the bulk counterpart. The meso-Cr-Ce-1 catalyst of same Cr/Ce molar ratio was larger in surface area (up to 186 m2/g) and could be reduced at lower temperatures, and exhibited the highest catalytic oxidation performance. At 30,000 h−1 of space velocity and 1000 ppm of VOC, toluene and formaldehyde conversions reached 90 % at 240 and 160 °C, respectively, and the apparent activation energies were 64.1 and 37.2 kJ/mol. The excellent performance of the catalyst is associated with its well-developed mesoporous structure, high specific surface area, good low temperature reducibility and dispersion and synergistic effect of active components.

Intrinsic and extrinsic dielectric response of Sr0.95Nd0.05Fe12O19 nanoceramics doped with Sc3+ and Al3+ ions

ABSTRACT The dielectric response of M-hexaferrite nanoceramics is complex since it is determined by crystal structure and microstructure. We studied the dielectric behavior of Nd3+ modified Sr2 + Fe12O19 nanoceramics with ferric ions, responsible for magnetic properties, substituted by Sc3+ and Al3+. To compare the effect of different ionic radii we discussed the dielectric response of Sr0.95Nd0.05Fe12-xScxO19 and Sr0.95Nd0.05Fe12-xAlxO19 at the doping level of x = 1.08. The dielectric response was found to consist of four contributions: (i) Low-temperature dispersion of dielectric polarization created by doping-induced spin-canting. The polarization, described by the inverse Dzyaloshinskii-Moriya model, is in nanoceramics limited by the shape and size of the crystallites. (ii) Changes in relaxation modes of polar vacancies due to interaction with electric dipole moments modified by deformation of oxygen octahedra. (iii) Dielectric dispersion in room temperature range related to space charge relaxation in grain boundaries. (iv) High-temperature electric conductivity of the grain interiors.

Investigation and Modeling of Multifrequency CV characteristics for 10-nm Bulk FinFETs at Cryogenic Temperatures

In this work, multifrequency capacitance–voltage characteristics have been investigated from 300K to 10K for high-k HfO2-based 10-nm bulk n-channel FinFETs. The dispersion observed in the accumulation region for the multifrequency capacitance characteristics with respect to temperature is explored taking into consideration the temperature dependence of border traps, series resistance, tunneling current, and the substrate time constant. It was found that the temperature-dependent dispersion arises due to the increase in the substrate time constant with temperature reduction. Multifrequency conductance measurements were performed from 300K to 10K to experimentally investigate the variation of substrate time constant with temperature. Furthermore, the temperature dependence of the accumulation region surface potential quotient is also explored in order to examine the direct tunneling current. Finally, modifications to existing accumulation region BSIM-CMG 110.0.0 compact model equations are proposed to include the low-temperature dispersion effects.

Dynamics of magnetic collective modes in square- and triangular-lattice Mott insulators at finite temperature

We study the equilibrium dynamics of magnetic moments in the Mott insulating phase of the Hubbard model on the square and triangular lattice. We rewrite the Hubbard interaction in terms of an auxiliary vector field and use a recently developed Langevin scheme to study its dynamics. A thermal noise, derivable approximately from the Keldysh formalism, allows us to study the effect of finite temperature. At strong coupling, $U\ensuremath{\gg}t$, where $U$ is the local repulsion and $t$ the nearest-neighbor hopping, our results reproduce the well known dynamics of the nearest-neighbor Heisenberg model with exchange $J\ensuremath{\sim}\mathcal{O}({t}^{2}/U)$. These include crossover from weakly damped dispersive modes at temperature $T\ensuremath{\ll}J$ to strong damping at $T\ensuremath{\sim}\mathcal{O}(J)$, and diffusive dynamics at $T\ensuremath{\gg}J$. The crossover temperatures are naturally proportional to $J$. To highlight the progressive deviation from Heisenberg physics as $U/t$ reduces we compute an effective exchange scale ${J}_{\mathrm{eff}}(U)$ from the low-temperature spin-wave velocity. We discover two features in the dynamical behavior with decreasing $U/t$: (i) the low-temperature dispersion deviates from the Heisenberg result, as expected, due to longer range and multispin interactions, and (ii) the crossovers between weak damping, strong damping, and diffusion take place at noticeably lower values of $T/{J}_{\mathrm{eff}}$. We relate this to enhanced mode coupling, in particular to thermal amplitude fluctuations, at weaker $U/t$. A comparison of the square and triangular lattice reveals the additional effect of geometric frustration on damping.

ANISOTROPY OF THE BACKGROUND MICROWAVE RADIATION OF THE UNIVERSE AND MASSIVE PHOTON PAIRS

The paper considers the issue of the anomalously low temperature dispersion of the cosmic microwave background (CMB) of the Universe, which was discovered by researchers R. Penrose and V. Gurzadyan according to the data of the WMAP and Planck missions. A new explanation of this phenomenon is given as evidence of residual traces in the CMB after the occurrence of multiple instabilities in the vacuum phase of matter in the first moments of the development of our Universe. It is these instabilities that have become the centers of the emergence of the fundamental mass of matter – massive photon pairs (ultralight scalar bosons). Theoretical and experimental evidence of the connection of CMB with massive photon pairs is presented using data from the Interball-Tail Probe, RHESSI and XMM-Newton missions.

Сomparative study of the hydrocarbon resins production of by the С9 fraction emulsion and suspension oligomerization

The production of a hydrocarbon resins (co-oligomers) by low-temperature dispersion co-oligomerization of C9 fraction of diesel fuel pyrolysis liquid by-products was researched. This method allows to reduce the temperature of the process. The synthesis was carried out by emulsion and suspension co-oligomerization. The main regularities of the suspension and emulsion

Hydrothermal Synthesis of 6mol% Yttria Stabilized Cubic ZrO2 Nano Powders

YSZ (Yttria-stabilized zirconia) is a ceramic material that is used for electronic and structural materials due to its excellent mechanical properties and specific electrical characteristics according to the Yttrium addition. Hydrothermal synthesis has several advantages such as fine particle size, uniform crystalline phase, fast reaction time, low process temperature and good dispersion condition. In order to synthesize YSZ nanoparticles with high crystallinity, hydrothermal synthesis was performed at various concentrations of NaOH. The hydrothermal process was held at a low temperature (100 °C), with a short process time (2,4,8 hours); the acidity or alkalinity of solution was controlled in a range of pH 2~12 by addition of NaOH. The optimum condition was found to be pH 12, at which high solubility levels of Y(OH) and Zr(OH) were reported. The synthesized nano powder showed high crystallinity and homogenous composition, and uniform particle size of about 10 nm.

Electric field-induced strains, conductivities and energy-storage properties in Na1/2Bi1/2TiO3–Ba(Mg1/3Nb2/3)O3 ceramics

In this work, an electric field-induced giant strain response and electric displacement, as well as temperature-dependence dielectric, impedance and energy storage densities were studied in lead-free BNT-based ferroelectric system. The XRD analysis reveals that all the investigated ceramics have a pure perovskite structure. A maximum energy storage density up to 0.907 J/cm3 at 80 kV/cm is exhibited for (1−x) Na1/2Bi1/2TiO3–xBa(Mg1/3Nb2/3)O3 ceramics at an x of 0.07. Follow-up experiments confirm that an electric-field-induced phase transition, which results in giant strain up to 0.12% at 10 Hz, is also the origin of initially weak piezoelectricity. After doping a certain amount of Ba(Mg1/3Nb2/3)O3, it is obvious that the dielectric peaks shift to low temperature and present dispersion characteristic. The increase in composition (x) can suppress grain conductivity heavily in high temperatures, thus gives a way how to adjust the nominal BNT composition for the reduction of leakage conductivity.

Structural, dielectric and electrical analysis of Ba2−xLa4 + 2x/3Ti8O24ceramics system with frequency and temperature

In the present investigation, the study of structural, dielectric, and electrical properties of lanthanum doped barium titanate ceramics with structural formula Ba2−xLa4 + 2x/3Ti8O24 (x = 0.0, 0.2, 0.4, 0.6) was reported. The target samples were obtained by mixed oxide route technique. Preliminary X-ray diffraction studies revealed the single phase formation in the orthorhombic crystal structure. The surface morphology of the samples showed that there were well-defined grains of different sizes and shapes that were homogenously distributed on the surface of samples. The approximate grain size was also calculated. The dielectric properties like dielectric constant (er), loss tangent (tan δ), and AC conductivity (σac) and temperature coefficient of resonant frequency were studied in the temperature range of 298—623 K and frequency range of 10 KHz–1 MHz. Dielectric response showed low temperature dispersion in dielectric properties at low frequency. This showed the space charge phenomenon arising due to defects. DC resistivity measured at room temperature for different compositions decreased with increase in substitution. All four samples gave very small value of temperature coefficient of resonant frequency. Sample with x = 0.6 showed minimum value of τf. Thus, the prepared samples could be the dielectric materials for the practical applications like resonators, filters, antennas, etc. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1679–1686, 2016

Dissipation in Non-Kramers Doublet of PrMg3

The low-temperature ultrasonic dispersion in the transverse ( C 11 - C 12 )/2 mode of PrMg 3 with the non-Kramers-doublet ground state has been investigated in temperatures down to 20 mK. The dispersion is caused by a thermally activated motions characterized by an Arrhenius-type relaxation time of τ=τ 0 exp ( E / k B T ) with low activation energy, E = 500 mK, and slow attempt time, τ 0 =8.5×10 -10 s. The coupling of the electric quadrupoles of the non-Kramers-doublet ground state to transverse lattice vibrations leads to a vibronic ground state with dissipation. The vibronic state in PrMg 3 releases the entropy of k B ln 2 with lowering temperature across the activation energy. A Kondo-like singlet state due to the binding of the non-Kramers doublet to the lattice vibrations appears at low temperatures far below the activation energy.

Dynamics of Phase Transition in 0.4NBT-0.4ST-0.2PT Solid Solution

In this paper we present dielectric spectroscopy results of NBT-(0.6-x)ST-xPT with x = 0.2 solid solution. Dielectric investigations clearly showed a relaxor—normal ferroelectric phase transition at TPT = 419 K and low temperature and low frequency dispersion similar to coexistence of dipolar glass and ferroelectric phase. The mean relaxation time above the phase transition follows Vogel—Fulcher law with following parameters: E A = 0.179 eV, τ 0 = 3.39·10−14 s, T VF = 223 K.

Chemical Vapor Deposition of Nanocarbon-Supported Platinum and Palladium Catalysts for Oxygen Reduction

A direct scheme for the synthesis of carbon nanotube (CNT) supported Pt and Pd catalysts is reported. Pt and Pd nanocatalysts deposited on nitrogen doped CNTs (NCNTs) were prepared via metal organic chemical vapor deposition (MOCVD) using thermally labile metal organic precursors. These materials were characterized by a host of techniques including TEM, XPS, TGA and voltammetry. The MOCVD based synthetic route offers several advantages including the circumvention of a majority of processing steps that are normally required to disperse and activate such catalysts on carbon supports. The low temperature dispersion and loading of catalysts also allows for study of the synergistic role of the carbon support on the oxygen reduction reaction (ORR). The activity of NCNT supported Pt catalysts for ORR was comparable to Pt/ Vulcan catalysts. Results from preliminary experiments and prospects for extending the MOCVD synthesis route for preparing viable carbon supported catalysts for ORR are discussed.

The temperature-dependent spectral properties of filter substrate materials in the far-infrared (6–40 μm)

This paper presents the experimental results on the low temperature absorption and dispersion properties for a variety of frequently used infrared filter substrate materials. Index of refraction ( n) and transmission spectra are presented for a range of temperatures 300–50 K for the Group IV materials silicon (Si) and germanium (Ge), and Group II–VI materials zinc selenide (ZnSe), zinc sulphide (ZnS) and cadmium telluride (CdTe).

Dielectric relaxation and porosity determination of porous silicon

Results of the dielectric spectroscopy study of porous silicon (PS) samples in the frequency range, 20 Hz–1 MHz, and in the temperature range, 173–493 K, are presented. Three relaxation processes that dominate at low, moderate and high temperatures respectively were observed. The low temperature dispersion follows the Cole–Cole type with two activation energies of 0.28–0.4 and 0.2–0.3 eV for samples with thickness 20 and 30 μm, respectively. At moderate temperatures the time domain dielectric response function demonstrates stretch exponential behaviour associated with the percolation of charge carries near threshold. At temperatures above 400 K, we found a strong contribution of dc-conductivity, with approximately the same activation energy of 0.47±0.01 eV for both samples. An additional Havriliak–Negami relaxation process with typical relaxation times of the order of 10 −3 s is observed in this temperature interval. The dielectric response is found to be very sensitive to the geometrical micro- and meso-structural features of the PS. The dynamical aspects of the processes are discussed.

Thermally stimulated depolarization and infrared studies in single-crystal dolomite (CaMg(CO3)2)

The thermally stimulated depolarization current technique is employed so as to study the dipole relaxation in dolomite. We traced two dominant low-temperature dispersions; one located at 140K and another at 188K. The first is characterized by the distribution in the relaxation time and the latter one has single-valued relaxation parameters. Standard characterization techniques and annealing treatments on dolomite samples with different micro-structural features showed that the dipoles contributing to the 140K peak are matrix defects, rather than hydrophilic sites in the pore network. The infrared spectroscopy established the absence of water molecules and the presence of hydroxyls, which probably constitute one type of defect dipoles.

Low energy phonon spectrum and its parameterization in pure KTaO3 below 80 K

High resolution data on low energy phonon branches (acoustic and soft optic) along the three principal symmetry axes in pure KTaO3 were obtained by cold neutron inelastic scattering between 10 and 80 K. Additional off-principal axis measurements were performed to characterize the dispersion anisotropy (away from the <100> and <110> axes). The parameters of the phenomenological model proposed by Vaks are refined in order to successfully describe the experimental low temperature (10 < T < 100 K) dispersion curves, over an appreciable reciprocal space volume around the zone center (|q| < 0.25 rlu). The refined model, which involves only 4 temperature-independent adjustable parameters, is intended to serve as a basis for quantitative computations of multiphonon processes.

Low energy phonon spectrum and its parameterization in pure KTaO

High resolution data on low energy phonon branches (acoustic and soft optic) along the three principal symmetry axes in pure KTaO3 were obtained by cold neutron inelastic scattering between 10 and 80 K. Additional off-principal axis measurements were performed to characterize the dispersion anisotropy (away from the and axes). The parameters of the phenomenological model proposed by Vaks (28) are refined in order to successfully describe the experimental low temperature (10 < T < 100 K) dispersion curves, over an appreciable reciprocal space volume around the zone center (|q| < 0.25 rlu). The refined model, which involves only 4 temperature-independent adjustable parameters, is intended to serve as a basis for quantitative computations of multiphonon processes.

Dielectric studies of protein hydration

This work deals with dielectric studies of the binding modes of water in hydrated powders of the proteins casein and lysozyme by means of the thermally stimulated depolarization currents (TSDC) method in the temperature range 77–300 K. The water content was varied between 0.01 and 0.61 w/w. The measurements revealed the existence of two complex dispersions, a low-temperature dispersion with maxima in the temperature region 115–165 K and a high-temperature dispersion with maxima in the temperature region 155–280 K. Both dispersions were studied in detail by using several experimental techniques offered by the TSDC method. The low-temperature dispersion is due to the dielectric relaxation of loosely bound and free water molecules. The fractions of tightly bound, loosely bound and free water molecules are obtained from the dependence of the characteristics of this dispersion on water content. Apart from dc conductivity, two main relaxation mechanisms contribute to the high-temperature dispersion : the relaxation of polar side chains and the displacement of protons on the surface of the protein molecules, a mechanism first proposed by Careri and coworkers. Both mechanisms are strongly affected by the presence of water. They are characterized by distributions of relaxation times.

Structural change of polyorganosilsesquioxanes during mechanical dispersion

Low-temperature adsorption of argon, volumometry, ESR and mass-spectroscopy were used to study processes taking place during mechanical treatment of polyorganosilsesquioxanes in vibratory mills at 80-to-300°K. After mechanical treatment the specific surface of polymers reaches 300-to-400 m 2/g and is stabilized by crosslinks formed during mechanical breakdown. It was found that crosslinking and an increase in the specific surface of polyphenylsilsesquioxane only occur during low-temperature (80-to-240°K) dispersion; a system was proposed for free-radical reactions resulting in crosslinking of polymer chains at low temperature.

Vogel-Fulcher Scaling of the Susceptibility in a Mixed-Crystal Proton Glass

Low-temperature dispersion of the electric susceptibilities in a mixed crystal of ${\mathrm{Rb}}_{1\ensuremath{-}x}{(\mathrm{N}{\mathrm{H}}_{4})}_{x}{\mathrm{H}}_{2}\mathrm{P}{\mathrm{O}}_{4}$ ($x=0.35$) is reported. The frustrated proton ordering of this system relates it to spin-glasses. The measurements reveal a broad distribution of relaxation times. The data scale using a temperature-independent distribution of activation energies linked to the relaxation times by a Vogel-Fulcher law with a freezing temperature of 10 K.