Concentration Of Paramagnetic Centers Research Articles

Comparative e.p.r. analysis of interactions between macerals and atmospheric oxygen

Interactions of paramagnetic centres in exinite, vitrinite and inertinite from durain, vitrain and clarain of Polish medium-rank coal (85.6 wt% C) with atmospheric oxygen were studied by electron paramagnetic resonance in the X-band (9.3 GHz). The concentrations of paramagnetic centres, the number of component lines in the e.p.r. spectra and their linewidths, g-values and proportions in the total spectrum were determined. Total concentration of paramagnetic centres increases from exinite to inertinite. It decreases after oxygen evacuation from the exinite and vitrinite environment, whereas it increases for inertinite. Four groups of paramagnetic centres exist in these macerals, with e.p.r. lines of different widths: 0.78–0.85 mT (Gauss), 0.48–0.60 mT (Lorentz), 0.34–0.36 mT (Lorentz) and 0.14–0.20 mT (Lorentz). The linewidths of the broad Gauss line increases after oxygen evacuation. Narrowing of the Lorentz lines after evacuation was observed, g-Values were the same in air and in vacuum. Both decrease and increase in concentration of paramagnetic centres of the individual groups after oxygen evacuation were detected. The most active interactions with atmospheric oxygen were observed for paramagnetic centres with the narrowest Lorentz lines existing in all the types of macerals and for paramagnetic centres in exinite and vitrinite with the broadest Lorentz lines.

Paramagnetism and Electrical Conductivity Centre Relation in Semiconductor Oligoresorcinol

The electrical properties of oligoresorcinols with various concentrations of paramagnetic centres (N S ) in constant and variable electric fields have been investigated. It has been established that the electrical conductivity and the tangent of the dielectric loss angle are increased with the growth of N S in the composition of the oligoresorcinol. With increase of frequency the electrical conductivity of oligoresorcinol increases according to the usual law. It has been shown that the electrical conductivity and concentration of paramagnetic centres of oligoresorcinol are interrelated and charge transfer in a variable field predominates at the expense of jumps of charge bearers between localized states with energy near the Fermi level. The densities of the states near the Fermi level for resorcinol examples have been calculated. The results show that, in this case, paramagnetic centres of the aroxyl type determine the density of the localized states near the Fermi level.

E.p.r. studies of interactions between paramagnetic centres of exinite, vitrinite and inertinite during thermal decomposition

Electron paramagnetic resonance in the X-band (9.3 GHz) was used for investigations of petrographically complex coal samples (densities 1.24–1.28 and 1.30–1.36 g cm −3) from Polish medium-rank coal (85.6 wt% C) heated in an inert atmosphere at 300–650°C. A strong increase in total concentration of paramagnetic centres in the sample of lower density at 300–350 and 450–550°C was measured. Relatively smaller changes in the total amount of paramagnetic centres in the heavier sample at 300–650°C were observed. There are three different types of paramagnetic centres in both samples, and their e.p.r. spectra are a superposition of one broad Gauss and two Lorentz (broad and narrow) lines. The most active group in the thermal decomposition of the two coal samples is formed by paramagnetic centres responsible for the broad Lorentz lines. Mainly dipolar interactions of unpaired electrons influence the linewidths of the e.p.r. signals. The microwave saturation power of all the e.p.r. lines detected increases with temperature. According to the e.p.r. results for pure macerals and the complex coal samples, the hypothesis of independent progress of reactions in individual macerals during the thermal decomposition of coal can be rejected.

Effects of vacuum annealing on the optical properties of porous silicon

The effects of vacuum annealing on the optical absorption spectra in the visible and infrared ranges, photoluminescence intensity, and concentration of paramagnetic centers in free-standing porous silicon films were investigated in a temperature range of 100–600 °C. It was found that heat-induced hydrogen desorption decreased the porous silicon band gap, which suggests that band-gap energy depends on hydrogen coverage of nanoparticles. The annealing also leads to increasing concentration of defects that were identified as silicon dangling bonds. The energy distribution of the dangling-bond states was estimated from the absorption spectrum.

Dependence of the low-temperature specific heat of YBa2Cu3O7?? on ? and magnetic field: The H1/2T term for H?0; absence of evidence for a T2 term for H=0

Specific-heat measurements with improved precision on two new samples of YBa2Cu3O7−δ with relatively low concentrations of paramagnetic centers are reported. For one sample the measurements were made for different values of δ. The data are analyzed, and the data reported earlier for another sample reanalyzed, with an improved fitting expression. In each case, the data give a well defined H1/2T term in the mixed state. No persuasive evidence for a T2 term in the superconducting state is found.

Dependence of the low-temperature specific heat of YBa2Cu3O7?? on ? and magnetic field: Concentrations of spin-2 and spin-1/2 paramagnetic centers

Specific heat data on two samples of YBa2Cu3O7−δ with relatively low concentrations of paramagnetic centers show the presence of both spin-2 and spin-1/2 moments. Measurements on one sample following incremental increases in δ show that for δ≤0.2 approximately one spin-1/2 moment is produced for each 100 O vacancies, with no substantial increase in the linear term of the specific heat. For several reasons it is concluded that these O-vacancy related moments are different from those that have figured in previously reported correlations with parameters characteristic of the superconductivity.

Nuclear-spin-lattice relaxation in natural clays via paramagnetic centers.

Nuclear-spin-lattice relaxation time measurements were made on $^{29}\mathrm{Si}$ nuclei in a series of 2:1 natural clay minerals. The magnetization recovery was found to be nonexponential. The nuclear relaxation arises from coupling to fixed paramagnetic impurities in the absence of spin diffusion. A theory is developed in which the nuclear spin relaxation behavior is expressed as an error function. These clays have very similar structures which allow the relative concentration of paramagnetic centers to be obtained from the fit of the magnetization recovery curves. \textcopyright{} 1996 The American Physical Society.

E.p.r. studies of thermal decomposition of exinite and inertinite

Exinite and inertinite from Polish medium-rank coal (85.6 wt%C) heated in inert atmosphere at 300–650°C were investigated by electron paramagnetic resonance in the X-band (9.3 GHz). Numerical analysis of the line-shape of the e.p.r. spectra was performed. The g-factor, the linewidth of the components and their fraction in the total spectrum were determined. Three groups of paramagnetic centres with one Gauss and two Lorentz lines were detected for exinite. The e.p.r. spectra of inertinite were two superimposed Lorentz lines. During thermal decomposition the paramagnetic centres of exinite with broad Gauss and broad Lorentz lines and all the paramagnetic centres of inertinite were very active. The total concentration of paramagnetic centres in exinite considerably increased from 550 to 600°C and decreased at 650°C. A visible decrease in total concentration of paramagnetic centres in inertinite began at 500°C. The microwave saturation of all the measured e.p.r. lines depended on the heat-treatment temperature.

Concentration of paramagnetic centers in boron doped polycrystalline diamond films

Though no boron signal is identified, with respect to their boron content the narrow and broad components of the electron spin resonance signal in boron doped polycrystalline film decrease from 1018 to 1016 cm−3. This is ascribed to two different structural defects with concentration decreasing with the boron content, in agreement with theoretical predictions. Finally, we suggest a spatial location for these defects.

E.p.r. studies of thermal decomposition of vitrinite

Vitrinite from Polish medium-rank coal (85.6 wt% C) heated in an inert atmosphere at 300–650°C was investigated by electron paramagnetic resonance in the X-band (9.3 GHz). The e.p.r. spectra of the vitrinite, recorded using different microwave powers, were approximated by superposition of a Gauss line and two Lorentz lines. The g factor, linewidth of the components and their fraction in the total spectrum were determined. A sharp increase in total concentration of paramagnetic centres after heating above 550°C was observed. Strong quenching and production of paramagnetic centres with broad Gauss and broad Lorentz lines above 550°C were detected. The heating of the vitrinite only slightly influenced the amount of paramagnetic centres with narrow Lorentz lines. The saturation behaviour of the all measured e.p.r. lines depended on heat-treatment temperature.

Latent tracks generated in micro-crystalline α-L-alanine and standard bone powder by 59Co ion beams as investigated by EPR-method

Latent tracks created by 59Co ions in l-α-alanine and standard bone powder were examinated by EPR-method. For 3.4 MeV/amu cobalt ions the mean radius of tracks (considered as the distance from the ion path on which a detectable concentration of paramagnetic centers exists) was estimated as (190±30) Å for l-α-alanine and (220±30) Å for bone powder.

Photochemical transformations of sulfophthaleine dyes in thin film state

Photochemical transformations of phenol red, thymol blue, bromthymol blue, bromcresol purple and bromphenol blue were studied in a thin film state by spectroscopy techniques. Sulfophthaleines in thin film state have been shown to be a mixture of three associated tautomeric forms, lactone, quinone and zwitterion, which differ in their stability in vacuum and in air, the ratio of concentrations of the tautomers being dependent on the chemical structure of the dyes. Reactions in the dark in sulfophthaleine films led to new equilibrium tautomer concentrations, the stable forms being predominant; the dark reactions are accelerated with an increase in temperature. Simultaneous photochromic transformations (equilibrium first-order reaction) and photobleaching and photoetching (non-equilibrium zero-order reactions) occur during photoirradiation of sulfophthaleine films. The concentration of paramagnetic centers (PMCs) in unexposed film is practically equal to that in powders and substantially higher than that in UV-exposed films. A synchronous change in concentration of the zwitterion form, in the quantity of oxygen consumed, and in the concentration of PMCs is observed in photoirradiated sulfophthaleine layers.

EPR Studies of E-Beam and Gamma Irradiated ZnGeP2: A Nonlinear Optical Material for the Infrared

AbstractZinc germanium phosphide is a nonlinear optical material for efficient frequency conversion in the mid-IR spectral region. One challenge in the development of ZnGeP2 is to reduce the near band edge absorption in the 0.7 to 2.5 micron region. Several methods have been used to ascertain the origin of this absorbance. One method, electron paramagnetic resonance (EPR), has been used to characterize the paramagnetic native acceptor in ZnGeP2 by studying as-grown, thermally annealed, electron-beam irradiated, and gamma irradiated single crystals. Each of the these processing routines improves the optical transparency with a concurrent decrease in the concentration of paramagnetic centers and defect site symmetry as seen in the EPR spectra. High energy e-beam and gamma irradiation may cause compensation, movement, or creation of new defects. In addition, nuclear magnetic resonance (NMR) has been used to further characterize several samples of as-grown, annealed and polycrystalline ZnGeP2. Preliminary calculations of the defect energetics have been conducted using atomistic simulation techniques employing the shell model to describe the lattice. Interionic potentials between the constituent ions were obtained by performing quantum cluster calculations on ZnGeP2.

Influence of feedstock oxidation temperature on stability of paving asphalts

When the oxidation temperature in the production of asphalts is increased, breakdown processes are accelerated, lowering the yield and polarity of the asphalt [1, 2]. Also, because of the high yields of gaseous products and black solar oil, environmental pollution becomes a problem. At the 4th World Petroleum Congress (1955), it was indicated that a temperature of 250~ is optimal for the oxidation of feedstocks to produce asphalt. Since that time, new technologies have been developed for rapid oxidation, yielding highly polar and more chemically inert binders at lower process temperatures. Outstanding properties were demonstrated in [2] for a plastic bitumen obtained by oxidation at a temperature far below 250~ The influence of the oxidation temperature on the thermal and thermooxidative stability of paving asphalt has received very little attention. When asphalt is held in tanks for the preparation of asphaltic concrete mixes for road construction, and during the course of the mixing process itself, the asphalt is exposed to temperatures of 140-160~ During these stages, the asphalt properties undergo their most severe changes. This period is termed the first stage of aging; it can be simulated quite accurately under laboratory conditions. The second stage of aging begins when the asphaltic concrete pavement is laid down, and it continues until the pavement fails (up to 20 years). During this time, the softening point of the asphalt changes (approximately) from 45 ~ to 70~ As a result, the road coating becomes brittle during the winter; it will not sustain dynamic stresses generated by moving vehicles, and is subject to cracking and breakdown. In studying the change of properties of paving asphalt under service conditions, extended observations under natural conditions are necessary [3]. However, hardly any such observations have been made. In the work reported here, aging tests were performed on asphalts obtained by the oxidation of one particular charge stock, from West Siberian crude, in a column-type unit; all oxidations were performed with the same air input rate, but at different temperatures (220-280~ The first stage of aging was evaluated on the basis of property changes due to thermal and thermooxidative action at 160~ The thermal action (this term is somewhat arbitrary) on the asphalt during storage in the hot state in the refinery, in the course of transportation, and upon heating in tanks at the asphaltic concrete plant was simulated by thermostating the test sample at 160_+2~ in a steel cup used for penetration tests (the specific surface area of contact between the test sample and air was 2 cm2/g). Thermooxidative action on the asphalt during the preparation of the asphaltic concrete mix was simulated in tests on a 10-/xm layer of asphalt applied to steel plates (5 x 5 cm), with a 1000 cm2/g specific surface area of contact with air. The penetration and softening point were determined at intervals on samples that had been held in penetration cups in the oven at 160~ the shearing stress at 25~ was determined on plates that were cemented together by a layer of the asphalt (tested in a cohesiometer designed by SoyuzDorNII [State All-Union Scientific-Research Street and Highway Institute]), The charge stock used in preparing the oxidized asphalts had the following physicochemical properties: nominal [Engler] viscosity 26 sec at 80~ chemical group composition (wt. %) paraffins + naphthenes 15; monocyclic, bicyclic, and polycyclic aromatics 10, 29, and 8, respectively; resins 25; asphaltenes 13. With increasing oxidation temperature in the production of the asphalts, the softening point increased at a faster rate (Fig. 1). The results of tests on samples that were practically identical in depth of oxidation at 220 ~ and 280~ (Table 1) confirmed the conclusion stated in [2] that the standard asphalt property indexes (penetration at 25~ softening point, and ductility at 25~ are independent of the oxidation temperature. The low-temperature properties (penetration at 0~ breaking point, and ductility at 0~ are appreciably better for the asphalt obtained at 220~ than for the asphalt obtained at 280~ These two asphalts differ the most significantly in their concentrations of paramagnetic centers.

Light induced charge and absorption changes in BaTiO3as studied by simultaneous ESR and optical extinction measurements

Abstract By investigating the concentrations of paramagnetic centers in two nominally undoped BaTiO3 single crystals at low temperatures as depending on illumination with light of varying wavelengths the resulting charge transfer processes could partly be identified. The corresponding photochromic behavior of the crystals was studied by measuring optical absorption simultaneously with ESR. Essentially two trap levels are involved: a deep one (Fe4+/3+) and shallow ones, which have been characterized by joint ESR and optical absorption measurements.

Effects of oxidation conditions on the formation of paramagnetic centers at and near the Si/SiO2 interface

To discern the most important parameter determining the concentration of paramagnetic centers, the effects of chemical treatments before and/or after oxidation, the type of furnaces, the cooling rate after oxidation, and oxidation temperature were examined. The effects of chemical treatments for cleaning on Pb and E′ centers before and/or after oxidation were negligible. Two types of furnaces were prepared: One where contamination by the outer air could be prevented and one where it could not be prevented, called an air-removed furnace and a conventional furnace, respectively. The concentration of Pb and E′ centers at the Si/SiO2 interface fabricated in a conventional furnace was, however, drastically smaller than that fabricated in an air-removed furnace. The cooling rate after oxidation did not determine the concentration of E′ centers but affected Pb centers. The oxidation temperature also determined the concentration of both Pb and E′ centers. It was found that the parameter most effective in determining the concentration of paramagnetic centers was the type of furnace, whether there was contamination from air or not. The paramagnetic centers (such as Pb and E′ centers) react with some element(s) in the air. It is expected that such elements are not N2 gas but hydrogen-related impurities in the air, perhaps H2O.

An EPR study of the relaxation parameters of paramagnetic centres in polyacetylene

Transmission effects in the 2-mm EPR range and the induced spin-echo in the 3-cm range have been used to investigate the relaxation parameters of neutral and of modified (by irradiation or doping) polyacetylene. The spin-spin and spin-lattice relaxation times of cis- and trans-polyacetylene have been determined between 90 and 320 K. These parameters have been shown to depend on the specimen thickness, degree of π-conjugation and the concentration of paramagnetic centres. The fact that the rate of spin-lattice relaxation depends on the square of the temperature is evidence that Raman relaxation processes predominate in the polymer. Irradiation leads to stabilization of the parameters of polyacetylene.

Electronic paramagnetic resonance (EPR) study of the structure of ZnO varistors prepared by various chemical methods

The structure of ZnO varistors prepared by different chemical methods was studied by the electronic paramagnetic resonance (EPR) method. The presence of Mn2+ ions in both ZnO lattice and electroconductive phase was used as a sensitive “probe” for the analyses of structural changes which occur during sintering of ZnO varistors. Potential mechanisms which can contribute to the formation of resonant lines were considered. The concentration of paramagnetic centres was quantitatively analysed. The variation of EPR signals of Mn2+ ions in ZnO phase was registered as a function of the chemical methods used for the preparation of powders only when samples were sintered at lower temperatures and non-linear characteristics of varistor ceramics had not yet been reached. At higher sintering temperatures EPR signals of Mn2+ ions in electroconductive phase differed only in the case of powders obtained by NaOH coprecipitation.

Ge xS 1− x glasses. Defects, density and structure

The density, X-ray scattering and ESR ofGe x S 1− x glasses within0.10 ⩽ x ⩽ 0.44 have been measured. The region of compositions in question is subdivided into three intervals: 0.10 ⩽ x ⩽ 0.325, 0.325 ⩽ x ⩽ 0.30and0.30 ⩽ 0.44 according to the types of paramagnetic centers (PC) and the specific features of the concentration dependences of the experimental quantities under study. It is found that the density, the PC concentration and the features of the radial distribution functions are correlated in each interval. The concentration dependence of the relative amount of two types of PC is the first interval is obtained. The influence of γ-irradiation at 300 K upon the structure and concentration of PC is studied. It is found that under γ-irradiation the amount of PC either increases (first and second interval) or decreases (third interval) and the unpaired spin density is redistributed among different types of structural defects. New types of PC do not arise under γ-irradiation. In the framework of the structural chemical analysis the models of structural units and their transformation with glass composition are suggested. Based on the proposed model for the glass network, the experimental data are analyzed.

ChemInform Abstract: Ammoxidation of Toluene on Coke‐Covered Alumina.

Abstract The catalytic activity of coke formed during the ammoxidation of toluene was studied as a function of time-on-stream. The changes in catalytic activity could be explained in terms of competing rates of formation of active centres (coke) and of their encapsulation within the growing coke mass. The acid strength of coke-forming centres on alumina, the concentration of paramagnetic centres in the coke and the elemental composition of the coke were examined and related to the nature of the ammoxidation process. Some of the chemical character of the coke was evaluated using IR spectrophotometry.