N. D. Gavrilova

Synthesis and ionic conductivity of polymer ionic liquids

Four vinyl monomers containing a covalently bonded cation ethylimidazolium and various anions—Br−, (CF3SO2)2N−, (CN)2N−, and CF3SO 3 − —have been synthesized. High-molecular-mass polymers (M w up to 1.84 × 106) having the structure of ionic liquids have been prepared via the free-radical polymerization of 1-vinyl-3-ethylimidazolium in bulk and molecular and ionic solvents. The thermal stability and heat resistance of the resulting polymer salts have been estimated. It has been demonstrated that the thermal characteristics of these salts significantly depend on the nature of anions. The glass-transition temperatures of the polymers range from 19 to 235°C. The ionic conductivity of the polymer salts and their compositions with individual ionic liquids has been studied in the frequency range 50–106 Hz. The highest conductivity (1.5 × 10−5 S/cm) is exhibited by the polymer containing the (CN)2N− anion.

Relationship between the pyroelectric coefficient and the lattice dynamics of the pyroelectrics

Abstract The experimental results for a temperature dependence of the pyroelectric coefficient are represented for twenty different crystals in a wide temperature range. Current theories are used for an analytical fitting of this result. It is shown that in the ferroelectrics the pyroelectric coefficient γσ is defined mainly by optical phonon modes in comparison with the linear pyroelectrics in which γσ follows the T3 law due to acoustic phonon. The possibility of an appearance of the linear temperature dependence of γσ ≈ T suggested by Born is discussed briefly. Our experimental results show that such behavior indeed exists but only in very imperfect crystals and at very low temperature.

The low-temperature behaviour of the pyroelectric coefficient

Abstract The results of both theoretical and experimental investigations of the temperature dependence of the pyroelectric coefficient γδ (T) are given. It is shown that in ideal crystals the γδ (T) may be represented by the sum of Debyes and Einsteins functions, which describe the contributions of acoustic and optical modes, respectively. Experimentally obtained γδ (T) dependences for different kinds of linear pyroelectrics and ferroelectrics are listed. The experimental data for linear pyroelectrics (except the region of the lowest temperatures) are described well by the mentioned sum of Debyes and Einsteins functions. In ferroelectrics the acoustic modes contribution to the γδ (T) is not found out in the frames of the accuracy of measurements. The simple estimations of the coefficient of anharmonicity and of the nonlinear dipole moment show that in ferroelectrics the relative contribution of the optical modes is greater than of the acoustic ones to the extent of the ratio (ω D /ω oj )2, where ω D i...

Ferroelectric properties of colemanite

The electrical conductivity, dielectric permittivity, and loss tangent of colemanite single crystals are measured as functions of temperature, and the fractal scaling law exponent is evaluated. The origin of the colossal dielectric response of hydrogen-bonded crystals is discussed.

The conductivity, dielectric permittivity, and electric module of erbium formate crystallohydrate at frequencies of 0.07 Hz-1 MHz

The influence of heating after sharp cooling to −60°C on the low-frequency dielectric response of erbium formate crystallohydrate is investigated. Such research has been performed previously for objects of different chemical natures and structural organizations (in colemanite, diglycinenitrate, triglycinetellurate crystals, and several polymers with water). There is one structural fragment in these substances, viz., a weak hydrogen bond O...H-O that is 0.26–0.29 nm in length. A common feature of these measurements is a sharp increase of dielectric permittivity of the orders at frequencies of 0.1–4 Hz close to 0°C, upon heating after sharp cooling, and decay in the region of 40°C. To explain similar phenomena one can use the deformation model of the double minimum potential in the field of the thermoelastic stresses and other molecular fields.

On the role of a weak hydrogen bond OH…O in the formation of the anomalous dielectric response of crystals and polymers near 40°C

In some crystals, polymers, and gels that contain hydrogen bonds OH…O, NH…O of length 2.8–3 Å or water molecules, gigantic anomaly of dielectric permittivity (ɛ ∼ 103–106) is observed in certain circumstances at frequencies of 1–106 Hz, which is accompanied by peculiarities in conductivity σ and dielectric losses tanδ. In crystals this effect appears after a sudden cooling to −50°C and it is observed at slow heating in the range of 20–40°C. At the return temperature course from 40°C dependences ɛ(T), σ(T), and tanδ(T) have their usual values. Anomalies in objects that differ by their compositions are unified by their temperatures, which are all close to 40°C. Authors have made an attempt to explain the similarity of these phenomena by the features of hydrogen bonds that are present in the objects.

Ionic transport and microwave spectra of TGS crystals

Complex investigations have been made in order to show the correlations between the microwave dielectric response dispersion and both thermodepolasation currents and conductivity. It was also shown that activation energy in TGS crystals in the vicinity of phase transition takes on the same value as in superionic conductors in superionic phase. Apparent absence of high conductivity at low frequencies can be explained by the fact of channel conductivity.

Point defects and singularities of the low-temperature (T < 15 K) behavior of the pyroelectric coefficient and the spontaneous polarization of TGS, LiTaO3 and LiNbO3

Abstract The common phenomenon of the formation of the additional component Psi of the spontaneous polarization resulting from the ordered localization of impurity ions (charged point defects) in the field of the host polar lattice for polar dielectrics (linear pyroelectrics and ferroelectrics) is established. The additional component is not, in the general case, collinear to the matrix lattice, the spontaneous polarization Psm, and correlates with it as |Psi| |Psm|−1 < 10−4. In spite of the small value of |Psi|, its temperature dependence at T ⩽ 7 K is the main contribution to the value of the pyroelectric coefficient. Based on the results of investigations of single crystals TGS, LiTaO3 and LiNbO3 with different contents of point defects, the following mechanisms are responsible for changes of the spontaneous polarization, as the temperature decreases from the paraelectric phase: for pure crystals, the mechanism of phase transition (order-disorder or displacive type) → some optical modes anharmonism; fo...

Dielectric loss as an indication of the kinetics of the ferroelectric phase transition

The dependences tanδ(f, T) for triglycine sulfate and diglycine nitrate single crystals are investigated in the frequency range f = 1−2 × 107 Hz at temperatures from −150 to 50°C for triglycine sulfate and from −150 to −60°C for diglycine nitrate. The results are discussed in the framework of the model according to which two or more types of media with different dielectric properties are formed in the ferroelectric phase. The polar volumes of domains themselves and the volumes of domain walls with different symmetries are treated as individual media. The dependences tanδ(f, T) make it possible to separate the contribution of a particular medium and to trace its evolution with temperature. The results of the analysis of the relaxation properties confirm that the phase transitions lead to the formation of two media (the polar medium and domain walls) in triglycine sulfate and three media (the polar medium and two types of domain walls) in diglycine nitrate. It is demonstrated that the relaxation properties are not steady-state characteristics of the samples but depend on their prehistory.

Low-Temperature Pyroelectricity

The current state of the set of problems associated with the study and application of the pyroelectric effect at low (T<30 K) temperatures is outlined. The reasons for the qualitatively different temperature dependences of the total pyroelectric coefficient γσ(T) in linear pyroelectrics and ferroelectrics are discussed. An analysis is given of the reasons why the theoretical temperature dependences obtained for the primary pyroelectric coefficient γε(T) adequately describe the experimental γσ(T) dependences for all materials studied. In this connection, the correctness of determining the secondary pyroelectric coefficient γS(T) from the macroscopically measured coefficients of thermal expansion is considered. The potential of thermodynamically nonequilibrium polar media and low-temperature pyroelectric materials is substantiated. The review consists of the following sections: (1) an introduction; (2) the theory of low-temperature pyroelectricity; (3) experimental procedures; (4) discussion of the experimental data: (A) perfect single crystals, (B) the problem of the secondary pyroelectric coefficient, (C) imperfect single crystals; (5) pyroelectric materials for low-temperature applications; and (6) a conclusion.