High-frequency EPR of Tb<Superscript>3+</Superscript>-doped KPb<Subscript>2</Subscript>Cl<Subscript>5</Subscript> crystal

High-frequency electron paramagnetic resonance (EPR) spectra of the KPb₂Cl₅:Tb³⁺ crystal have been investigated. Three types of spectra were observed in the frequency range of 74–200 GHz. The most intensive spectrum with the resolved hyperfine structure corresponded to transitions between sublevels of the¹⁵⁹Tb³⁺ ground quasi-doublet with the zero-field splitting (ZFS) close to 48 GHz. Experimental results were analyzed by the exchange charge model of the crystal field affecting terbium ions in low-symmetry Pb²⁺ positions with the chlorine sevenfold coordination and the charge compensating vacancy in the nearest potassium site. The calculated values ofg-factors and ZFS were in agreement with the experimental data. The nature of a broad EPR line with ZFS of about 180 GHz and of additional weak EPR lines observed as satellites of the main Tb³⁺ lines was discussed.     

Application of large amplitude oscillatory shear for the analysis of polymer material properties in the nonlinear mechanical behavior

The results of studying the nonlinear viscoelastic properties obtained through the generation of large strain amplitudes are interpreted via plotting of Lissajous-Bowditch figures in two different systems of coordinates: (i) stress versus strain and (ii) the derivative of stress with respect to the phase angle versus strain. The former system yields the integral characteristic of dissipative loss in the deformation cycle, and the latter yields a measure of elasticity of a material. The generality of such approach to analyze large deformations, even in the case of an extremely complex shape of the nonlinear response, is due to the fact that it is not related to the a priori choice of any rheological constitutive equation. The developed method was tested on supramolecular structures and polymer solutions and melts. Novel results allow estimation of the character of nonlinearity development, i.e., the dependences of pseudoplasticity, dilatancy, and stiffening or softening of the medium on the shear. The comparison between the proposed measures of nonlinearity and large strain nonlinearity characteristics described in the literature shows that the integral characteristics are in good qualitative agreement with other measures of nonlinearity. However, in some cases, the proposed approach gives a more objective and consistent estimation than other measures of nonlinearity give.     

Frequency stabilization in free-electron masers with 2D and 1D distributed feedback

We analyze the dynamics of free-electron masers (FEMs) with two-mirror hybrid Bragg resonators using the coupling between running and quasi-critical modes in the input mirror. Such a mirror may have the form of a 2D Bragg structure with coaxial geometry or a segment of a cylindrical waveguide with axisymmetric corrugation having a period close to the wavelength. The output mirror has the traditional Bragg structure coupling two counterpropagating running waves (the corrugation period is close to half the wavelength). It is shown that a stable unimodal lasing with a radiation frequency close to the cutoff frequency of the quasicritical mode excited in the input mirror can be attained using this scheme under optimal conditions. Such a regime is insensitive to variations of the electron beam parameters. Simulation of experimentally implemented FEMs and those being developed is carried out.     

Rheokinetics of urea-formaldehyde resins gelation

Rheokinetics of gelation of aqueous suspensions of urea-formaldehyde oligomers was studied. Experiments were carried out with a rotational viscometry method using two regimes of deformation: constant stress or constant shear rate. It was shown that a maximum on a viscosity vs. time curve is observed and it reflects sedimentation of rather large gelled particles. Then, gelation of a whole system (determined as the point of the unlimited growth of viscosity) takes place. Experiments carried out at different temperatures gave us an apparent activation energy which depends on the oligomer content in a composition. Characteristic points of a rheokinetic curve do not depend on the concentration of a curing agent. The problem of the temperature increase due to heat dissipation is also discussed. It was demonstrated that shear stress is a kinetic factor influencing the rate of a reaction. A master curve for a rheokinetic curve was constructed, time scale being reduced by the gel time.     

Experimental verification of the theory of thixotropy of viscoelastic polymer media

Previous studies [1, 2] were devoted to the formulation of a phenomenological theory of the reversible changes (thixotropy) in the physicomechanical properties of viscoelastic media, in particular, polymer systems. The basis of this theory is the idea that the reversible changes in the structure and properties of viscoelastic thixotropic media under the influence of mechanical action are due to a change in their relaxation spectrum. In this case the behavior of the mechanical properties is entirely and uniquely determined by two material functions which completely characterize the given material: the relaxation time (frequency) distribution function, which describes the behavior of the material in the linear region of deformation, and the thixotropy function, which determines the nature of the changes compression) of the relaxation spectrum on transition to the nonlinear region. The object of this approach is to reflect the effect of the change in supramolecular and molecular structures associated with the flow of polymer system, or in the bonds and orientation of the particles in disperse systems, on the viscoelastic and viscosity properties of the systems concerned. The essential validity of this approach was demonstrated in [3], where it was shown that at sufficiently high strain rates changes occur in the structure of the polymer leading to a change in mechanical properties. In [4] the basic structural functions were found. These, in conjunction with the proposed theory of thixotropy, characterize the behavior of a broad group of polymer systems in both the linear and the nonlinear regions of deformation. In this connection it should be noted that whereas for the linear region the accuracy of the laws obtained lies within the limits ± 100%, on transition to the nonlinear region the error may increase, Therefore, for the purposes of a rigorous quantitative verification of the theory we shall use not the universal functions obtained in [4], but the more precise characteristics of the specific material on which the experiments are performed, since those effects with respect to which it is desired to test the theory usually lie within the limits of 30% of the measured quantities.The authors thank G. V. Vinogradov for organizing and discussing their work.     

Illumination of the effect of the overlap of lone-pairs on indirect nuclear spin-spin coupling constants

The effect of electron lone-pairs on the Fermi-contact (FC) contribution to indirect nuclear spin-spin coupling constants is analyzed using new tools for their interpretation. In particular, visualization of spin-spin coupling pathways using the coupling deformation density (CDD) has been employed. Furthermore, the recently developed perturbation-stable localization procedure has been applied for decomposition of CDD and the calculated value of couplings into contributions from localized molecular orbitals (LMOs). Correlation between the overlap of densities of LMOs representing lone-pairs and the Fermi-contact contribution to spin-spin coupling constants has been demonstrated. A new way for analyzing spin-spin couplings using the expansion of CDD as a linear combination of the products of molecular orbitals has been suggested. The considered examples include two- and three-bond phosphor-phosphor couplings. Significance of the obtained insight is not restricted to spin-spin couplings of nuclei possessing lone-pairs, as demonstrated in the example of vicinal hydrogen-hydrogen coupling in ethane.