Theoretical description of electron transport in homo-and molecularly doped polymers

The general aspects of electron transport in disordered organic media (polymers, organic glasses, molecularly doped polymers) are considered. A critical analysis of experimental results and their interpretation in terms of basic theoretical models and experimental methods used in research in this area is presented. Particular attention is given to the comparative analysis of the data obtained by the time-of-flight technique and in studying the radiation conductivity of polymers, in particular, to the possibility of describing the conductivity data in terms of the Gaussian disorder model.     

Thermodynamic analysis and purifying an amorphous phase of frozen crystallization centers

The possibility of dissolving frozen crystallization centers in amorphous alloys of the Fe–B system is considered by means of thermodynamic calculations. This can in turn improve the thermal stability of an amorphous alloy. The effect isothermal annealing has on the thermal stability of multicomponent amorphous alloys based on iron is investigated via the highly sensitive dilatometric technique, measurements of microsolidity, and electron microscopic investigations. The annealing temperature is determined empirically on the basis of the theses of the thermodynamic theory of the high temperature stability of multicomponent amorphous alloys, according to which there exists a range of temperatures that is characterized by a negative difference between the chemical potentials of phases in a heterogeneous amorphous matrix–frozen crystallization centers system. The thermodynamic condition of the possible dissolution of frozen crystallization centers is thus met. It is shown that introducing regimes of thermal processing allows us to expand the ranges of the thermal stability of iron-based amorphous alloys by 20–40 K through purifying an amorphous matrix of frozen crystallization centers. This conclusion is proved via electron microscopic investigations.     

Formation of a plateau on time-of-flight curves of molecularly doped polymers

A program for numerical computation of the curves of the transient current in samples of molecularly doped polymers has been developed with allowance for the presence of a defective surface layer in them. Time-of-flight curves are calculated with the multiple-trapping model with a Gaussian energy distribution of traps. The model parameters are determined from the results of independent measurements. Numerical computations are in qualitative agreement with experimental data for a typical molecularly doped polymer. The features of the formation of a flat plateau in samples of different thickness are discussed in terms of the proposed model.     

A theoretical description of geminal recombination and radiation-pulse-induced electric conduction in polymers in the approximation of probability of survival of ionic pairs

An approximate analytical theory of geminal recombination and geminal radiation-induced electric conduction of polymers is developed. The theory is based on an ion-pair mechanism of radiolysis and the concept of dispersion of electron transport and is valid at fairly strong fields and not overly low temperatures. The proposed approach was used to perform a preliminary analysis of the published data on pulsed-radiation-induced electroconductivity of low-density polyethylene.     

Scandium-containing layered hydroxides

A scandium analogue of hydtotalcite and a mixed sample containing both scandium and aluminum have been synthesized and characterized. It is known that both samples have a layered structure; the unit cell parameters of the scandium-containing samples are slightly higher than the respective values of an ordinary aluminum-containing sample. The mixed-metal scandium-aluminum sample easily regains its layered structure after a dehydration/rehydration cycle, while the scandium analogue after this treatment is restored only partially.     

On the theory of classic mesodiffusion

A simple model of the classical random walk of particles with a constant speed and anisotropic angular distribution is used to study the characteristic features of mesodiffusion, that is, of an intermediate stage between the ballistic regime (short times) and ordinary diffusion (long times). In the extreme case of anisotropy, namely, walking along a straight line, the process can be described by the telegraph equation, whose solution contains δ-functions accounting for the ballistic component. As the anisotropy becomes less pronounced, the δ-singularity transforms into a frontal burst (the quasi-ballistic component), beyond which the distribution can be satisfactorily described by the telegraph approximation. In the other extreme case of isotropic walking, the frontal burst disappears and the telegraph approximation, contrary to general belief, proves to be cruder than the diffusion approximation.     

Hole transport in polystyrene and polycarbonate doped with polar dopants

Hole transport in PS and PC doped with low-molecular-mass dopants with dipole moments larger than 3D is investigated. Both near-surface and bulk charge-carrier generation techniques are used. There is good agreement between the experimentally measured values of hole mobility and the published data. It is shown that, for the investigated systems, the hole transport is nonequilibrium and is well described by the multiple-trapping model with the Gaussian trap distribution over energy. The presence of a flat plateau on the time-of-flight curves does not necessarily mean establishment of the quasi-equilibrium hole transport in the system.     

The nature of a defective layer in samples of molecularly doped polymers

The shape of time-of-flight curves in the mode of subsurface irradiation of samples with lowenergy electrons is analyzed for free-standing films of a typical molecularly doped polymer of different thicknesses (11?C45 ??m). Special attention is paid to comparison of curves registered for both sides of the samples. The data confirm the hypothesis that the defective layer is formed owing to sublimation of dopant molecules during sample preparation and qualitatively agree with predictions of the two-layer multiple-trapping model.     

Spectroscopic Observations in the 3-mm Wavelength Range at CrAO RT-22 Radiotelescope Using an Acousto-Optical Spectrum Analyzer

Within the framework of a multi-year program of dense interstellar gas studies, we performed a new series of observations at the CrAO RT-22 radiotelescope in July 1998 using an improved 3-mm wavelength receiving facility. An acousto-optical spectrum analyzer (AOS) was used for the first time for the spectral analysis of incoming radiation. The AOS has larger bandwidth compared to the filter spectrum analyzer used previously and simplifies tuning SIS-mixer circuits and frequency converters. Using this facility, we observed a number of star-forming regions including clouds associated with bright IR sources and ultracompact HII regions as well as clouds with high-velocity outflows in the HCO⁺, ¹³CO, HCN, and CS molecule lines.     

The effect of charged centers on the transport of charge carriers in molecularly doped polymers

The effect of charged centers on the mobility of charge carriers in molecularly doped polycarbonate is experimentally studied by the method of accelerated electrons for the generation of charge carriers in the surface layer and in the polymer bulk. The hypersensitivity of mobility toward the presence of charged sites that is predicted by the theory of correlated disorder (dipolar glass) is not found. With accumulation of charged centers, transformation of the time-of-flight curves with a well-pronounced plateau region, which is indicative of quasiequilibrium transport according to theory, proceeds in full agreement with the classical concepts on the role of a space charge or bimolecular recombination during high-signal irradiation. Transport of charge carriers in the molecularly doped polymer is not quasiequilibrium but dispersive.