Juras Banys

Study of the tetragonal-to-cubic phase transition in PbTiO3 nanopowders

The CPP (combined polymerization and pyrolysis) preparation route, in its enhanced liquid-precursor-based version, was combined with consecutive soft milling. For studies of temperature- and size-dependent structural changes occurring in ferroelectric lead titanate, this combined route yields a nanopowder series covering the relevant particle-size region at target quality. This material basis enables consistent SEM, TEM, XRD, Raman, EPR and dielectric measurements, which furnish a comprehensive picture of the cooperation between temperature rise and size reduction to eliminate tetragonality and concomitant ferroelectricity. Our previous original EPR studies on nanosized barium titanate are now extended to the lead titanate case. Furthermore, as compared to the pertinent literature standard, the materials basis is extended to powder samples of smaller mean particle sizes, comprising the critical size at which a PbTiO3 particle undergoes a transition into cubic paraelectric phase. Thus, the size-driven phase transition can be observed in a direct way (at 7 nm, which compares to 40 nm for BaTiO3), and the EPR data suggest a much less spacious gradient shell at the particle surfaces (thickness ≈ 2 nm) than in previous analogous investigations on BaTiO3 (15 nm).

Electromagnetic shielding efficiency in Ka-band: carbon foam versus epoxy/carbon nanotube composites

Abstract. The wide application of microwaves stimulates searching for new materials with high electrical conductivity and electromagnetic (EM) interference shielding effectiveness (SE). We conducted a comparative study of EM SE in Ka-band demonstrated by ultra-light micro-structural porous carbon solids (carbon foams) of different bulk densities, 0.042 to 0.150  g/cm3, and conventional flexible epoxy resin filled with carbon nanotubes (CNTs) in small concentrations, 1.5 wt.%. Microwave probing of carbon foams showed that the transmission through a 2 mm-thick layer strongly decreases with decreasing the pore size up to the level of 0.6%, due to a rise of reflectance ability. At the same time, 1 mm thick epoxy/CNT composites showed EM attenuation on the level of only 66% to 37%. Calculating the high-frequency axial CNTs’ polarizability on the basis of the idea of using CNT as transmission lines, we devised a strategy to improve the EM SE of CNT-based composites: because of the high EM screening of inner shells of multi-walled CNTs in the GHz range, it is effective to use either single-walled CNT or multi-walled CNTs with a relatively small number of walls (up to 15, i.e., those taking part in the EM interaction, if the CNT length is 20 μm).

Broadband dielectric spectroscopy of water confined in MCM-41 molecular sieve materials : low-temperature freezing phenomena

Dielectric properties of water adsorbed in pure siliceous and aluminium containing mesoporous MCM-41 materials have been investigated in the frequency range from 20 Hz to 1 MHz. The dielectric spectra revealed three dispersion regions, liquid-like free water in the centre of the mesopores, an intermediate water layer with reduced mobility, and an interfacial water layer at the inner surface of the mesopores. The analysis of the relaxation time distribution by means of a double-well potential indicates a strong dependence of the barrier height of reorienting water molecules dipoles in the interfacial layer on the Si/Al ratio in the framework.

Dielectric Relaxation in Ba2NaNb5(1-x)Ta5xO15 Single Crystrals.

This paper presents the results of dielectric dispersion in Ba 2 NaNb 5(1- x ) Ta 5 x O 15 (BNNT) single crystals over the frequency range 1 kHz to 4 GHz. The fundamental dielectric dispersion occurs above 10 7 Hz and is caused by a soft relaxational mode. This mode gives the main contribution to the high static permittivity which fits the Curie-Weiss law. The results indicate the anharmonic motion of the Nb(Ta) atoms and the order-disorder nature of the proper ferroelectric phase transition in BNNT.

Broadband dielectric/electric properties of epoxy thin films filled with multiwalled carbon nanotubes

Abstract. Many attempts have been made to fully explore flexibility, resistance to corrosion, and processing advantage of epoxy resin filled with carbon nanotubes (CNTs) as conductive filler, although sometimes with a certain degradation of polymers’ intrinsic properties. It is important to move the percolation threshold into the region of smaller CNTs’ concentration. The results of a broadband dielectric investigation of multiwalled CNT (MWCNT)/epoxy resin composites in wide temperature range from room temperature to 450 K were analyzed for percolation. Far below the percolation threshold (0.25 wt. % MWCNT) the dielectric properties of the composite are mostly determined by alpha relaxation in pure polymer matrix and the freezing temperature decreases due to the extra free volume at the polymer–filler interface. Close to the percolation threshold, the composite shows the negative temperature coefficient effect in the temperature region, where the pure polymer matrix becomes conductive. The activation energy of DC conductivity increases with the MWCNT concentration far below the percolation threshold and decreases close to it (1.5 wt. % MWCNT). The dielectric analysis of the MWCNT/epoxy resin reveals a significant influence of the polymer matrix on the temperature dependence of composite dielectric properties.

Electronic Structure and Phase Transition in Ferroelectic Sn2P2S6 Crystal

An analysis of the P2S6 cluster electronic structure and its comparison with the crystal valence band in the paraelectric and ferroelectric phases has been done by first-principles calculations for Sn2P2S6 ferroelectrics. The origin of ferroelectricity has been outlined. It was established that the spontaneous polarization follows from the stereochemical activity of the electron lone pair of tin cations, which is determined by hybridization with P2S6 molecular orbitals. The chemical bonds covalence increase and rearrangement are related to the valence band changes at transition from the paraelectric phase to the ferroelectric phase.

Dielectric Properties of Sodium Nitrite Confined in Porous Glass

In this paper the results of dielectric investigation of nanoconfined NaNO2 is presented. The phase transition is shifted to lower temperatures. Only one phase transition can be seen in the dielectric spectra and the intermediate incommensurate phase is suppressed. The increase in the frequency dependent dielectric permitivity on heating is mainly the result of a conductivity effects. The relaxational soft mode contribution is as well present, but is small as compared to the conductivity contribution. Such huge conductivity is caused by premelting phase, which is visible even at temperatures below the phase transition

The Characterization of Two Dimensional Electrostrictive CuInP2S6 Materials for Transducers

The electromechanical properties of layered, two-dimensional materials of CuInP2S6 family have been investigated. It was shown that, at room temperature, which is above phase transition and under DC bias electric field, these materials behave as a piezoelectric because of electrostriction. In this case, the piezoelectric and electromechanical coupling coefficients are odd functions of the bias field and have a linear dependence on the bias field. The relative changes of ultrasonic velocity are found to have a quadratic dependence on the bias DC field. In bias fields of about 20 kV/m, the values of square of electromechanical coupling coefficient could be high enough (>20%) for longitudinal vibrations in thin plates of investigated CuInP2(S,Se)6 materials in the paraelectric phase. In the ferroelectric phase, the external DC electric field acts as polarizing field and electromechanical coupling coefficients sufficiently increase. At the transitions, the piezoelectric anomalies have been observed.

Peculiar Bi-ion dynamics in Na1/2Bi1/2TiO3from terahertz and microwave dielectric spectroscopy

Dynamics of the main dielectric anomaly in Na1/2Bi1/2TiO3 (NBT) were studied by time-domain THz and microwave spectroscopy, using also previously published data and their new overall fits. Above the dielectric maximum temperature Tm ≈ 600 K, the response consists of coupled sub-THz oscillator and a relaxation mode, assigned to strongly anharmonic Bi-ion vibrations and hopping, whose slowing down explains the paraelectric-like permittivity increase to Tm. Below Tm, the main relaxation continues slowing down and additional relaxation, assigned to quasi-Debye losses, appears in the 1011 Hz range. The oscillator hardens on cooling and takes over the whole oscillator strength. The permittivity decrease below Tm is caused by the reduced strength of the relaxations due to dominance of the rhombohedral phase within the coexistence region with the tetragonal phase. The anharmonic dynamics of Bi are supported by previous structural studies. NBT represents a hybrid between standard and relaxor ferroelectric behaviour.

Dielectric investigations of polycrystalline samarium bismuth ferrite ceramic

Results of broadband dielectric investigations of samarium doped bismuth ferrite ceramics are presented in wide temperature range (20–800 K). At temperatures higher than 400 K, the dielectric properties of samarium bismuth ferrite ceramics are governed by Maxwell-Wagner relaxation and electrical conductivity. The DC conductivity increases and activation energy decreases with samarium concentration. In samarium doped bismuth ferrite, the ferroelectric phase transition temperature decreases with samarium concentration and finally no ferroelectric order is observed at x = 0.2. At lower temperatures, the dielectric properties of ferroelectric samarium doped bismuth ferrite are governed by ferroelectric domains dynamics. Ceramics with x = 0.2 exhibit the relaxor-like behaviour.