Energetics of an adiabatically-loaded excited anharmonic oscillator

The dependence of the energy characteristics on an adiabatically slowly increasing external force is determined analytically for an anharmonic oscillator. The analytical results are confirmed by a numerical calculation. The nature of the force dependences of the energy characteristics are determined and discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 153–157 (January 1997)     

Characteristics of elementary acts in the kinetics of metal fracture

Experimental studies of the fracture kinetics of polycrystalline metals have led to the problem of the barrier and activation volume of elementary fracture acts. A model is proposed where the field binding one atom to its environment in a metal is represented by equivalent bonds directed along three orthogonal axes. These bonds are described using the Morse potential, whose parameters are found from the values of the Young’s modulus and the linear thermal expansion coefficient for metals. The validity of the model is checked by comparing the results obtained with metal sublimation data. The values of the barrier and activation volume of elementary fracture acts are determined for 15 polycrystalline metals. The levels of local overstresses are estimated. The theoretical breaking strengths of the metals are calculated.     

A mechanism for generation of strong energy fluctuations in one-dimensional systems

A mechanism for the generation of strong energy fluctuations in a chain of harmonically and anharmonically coupled atoms in a state of thermodynamic equilibrium has been investigated by a molecular dynamics method. Energy fluctuations multiply exceeding the mean value were localized in a group of three to five atoms with lifetime on the order of an atomic vibrational period. They arise as a consequence of collisions of density waves propagating along the chain. A collision of waves of the same density sign leads to a fluctuation of the potential energy, while for differing signs a fluctuation in the kinetic energy occurs. In particular, collision of dilitatin waves leads to thermal breaking of the chain. The energy concentration at the moment of collision is 30% greater in a harmonic chain than in an anharmonic one.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 47–52, May, 1991.     

Investigation of the thermophysical properties of polymers by the temperature wave technique

The technique of temperature waves to measure the complex thermophysical characteristics of materials has been proposed. The relations for the moduli and arguments of these characteristics for the case of plane temperature waves have been calculated. The temperature dependences of the moduli and arguments of complex thermophysical characteristics of polyvinylacetate (PVAc) at different frequencies and also the temperature dependences of its vibrational and configurational heat capacities have been determined. It has been shown that the vibrational heat capacity of PVAc in the region of softening grows sharply and its configurational heat capacity goes through a peak. The segmental mobility has been found to affect the heat transfer in the polymer in an ambivalent way causing both a decrease and increase of thermal conductivity.

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Zusammenfassung Zur Bestimmung der komplexen thermophysischen Eigenschaften von Stoffen wurde eine Temperaturwellentechnik vorgeschlagen. Für den Fall zweidimensionaler Temperatur-wellen wurden die Verhältnisse von Moduli und Argumenten dieser Eigenschaften berechnet. Die Temperaturabhängigkeit von Moduli und Argumenten der komplexen thermophysischen Eigen-schaften von Polyvinylacetat (PVAc) bei verschiedenen Frequenzen und auch die Temperatur-abhängigkeit seiner Vibrations- und Konfigurations-Wärmekapazitäten wurden bestimmt. Es wurde gezeigt, daß die Vibrations-Wärmekapazität von PVAc im Erweichungsbereich scharf zunimmt und die Konfigurations-Wärmekapazität durch einen Peak geht. Man fand, daß die Segment-Mobilität den Wärmetransport im Polymer auf eine ambivalente Weise beeinflußt, in dem die Wärmeleit-fähigkeit entweder gesenkt oder angehoben wird.

     

On the determination of the energy of activation of relaxation transitions in polymers by differential scanning calorimetry

The determination of the energy of activation (barrier height) of elementary events involved in relaxation (fluctuation) transitions in polymers from the temperature dependence of the specific heat is discussed. The dependence is derived by the method of differential scanning calorimetry. It is emphasized that the correct determination of the energy of activation must include the temperature variation of the barrier. The deviation of the preexponential in the Arrhenius temperature dependence from the value predicted theoretically demonstrates that the barrier does depend on temperature. Experimental data from which the realistic energy of activation of the α relaxation in polymers can be found are given.     

Effect of hydrostatic pressure on submicrocrack formation in oriented polymers

Small-angle x-ray scattering has been used to obtain a quantitative estimate of the concentration and size of the submicrocracks in Kapron and polypropylene equally extended at 1 and 1500 atm. Direct evidence of the retarding effect of pressure on fracture development has been obtained.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga: Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 5, pp. 917–921, September–October, 1971.     

Deformations and stresses around submicroscopic cracks in loaded polymers

Structure deformations upon loading of oriented films of polycaproamide (Kapron) or polypropylene when submicroscopic cracks (dimensions of hundreds of Ångstroms) have appeared in the samples in high concentration (up to 2 · 10¹⁵ cm^–3) have been studied by x-ray diffraction methods at low and large angles. It has been established that the appearance of submicro-cracks causes relieving of the regions adjacent to them along the loading axis (extent about 1000 Å) and an increase in stress in the lateral zone relative to the crack, which is manifested in an intensification of the stretching of these zones.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 1, pp. 51–58, January–February, 1972.     

Supramolecular structure and incipient cracks in three-dimensional stitched amorphous polymers

The electron-microscopic method was used to study the structure of unsaturated polyester films. The initial submicrocracks developing in the loaded films were studies by the low-angle x-ray diffraction method. It is shown that for these polymers a typical feature is the globular structure with a globule diameter of 10³–3·10³ Å, and the transverse dimension of the submicrocracks developing under the action of a load practically coincides with the globule diameter.     

Total absorption of an electromagnetic wave by an overdense plasma

We show both theoretically and experimentally that an electromagnetic wave can be totally absorbed by an overdense plasma when a subwavelength diffraction grating is placed in front of the plasma surface. The absorption is due to dissipation of surface plasma waves (plasmons polaritons) that have been resonantly excited by the evanescent component of the diffracted electromagnetic wave. The developed theoretical model allows one to determine the conditions for the total absorption.     

Dependence of the magnetic properties of an amorphous metal alloy on its nanoporosity

Magnetic and structural properties of an amorphous alloy based on iron are investigated. The properties and state of the alloy are changed under hydrostatic pressure of up to 1.3 GPa. An increase in the pressure leads to a linear increase in the maximum magnetic induction and a linear decrease in the remanence. Two nanopore fractions with average sizes of ～20 and 150 nm are revealed in the alloy with the use of small-angle x-ray diffraction. The application of the pressure decreases the size of nanopores and increases the average distance between their boundaries, i.e., increases the average sizes of continuity regions in the alloy. It is established that the relative change in the magnetic characteristics is linearly related to the change in the size of the continuity regions in the alloy. Upon extrapolation to zero nanoporosity, the remanence reduces to zero. The inference is made that the nanopores play a dominant role (as compared to other structural defects) in the magnetic properties of the alloy.