J. Banys

Dielectric dispersion of the relaxor PLZT ceramics in the frequency range 20 Hz-100 THz

The dielectric dispersion of the transparent relaxor ferroelectric ceramics PLZT 8/65/35 and 9.5/65/35 was determined in a wide frequency range including the microwave and infrared range. The number of observed polar phonons in infrared spectra gives evidence about the locally broken cubic symmetry and the presence of polar nanoclusters in the whole investigated temperature range up to 530 K. A single broad and symmetric dispersion that occurs below the polar phonon frequencies was fitted with the Cole-Cole formula and a uniform distribution of Debye relaxations. On decreasing temperature, the distribution of relaxation times becomes extremely broad which indicates increasing correlation among the clusters. The mean relaxation time diverges according to the Vogel-Fulcher law with the same freezing temperature 230±5 K for both ceramics, but different activation energies 1370 K and 1040 K for the 8/65/35 and 9.5/65/35 sample, respectively. The shortest relaxation time is about 10-12 s and remains almost temperature independent. Below room temperature, the loss spectra become essentially frequency independent and the permittivity increases linearly with decreasing logarithm of frequency. The slope of this dependence is proportional to T 4 in the investigated temperature range (above 210 K) which indicates appreciable anharmonicity of the potential for polarization fluctuations.

Crossover from ferroelectric to relaxor behavior in BaTi1− x Sn x O3 solid solutions

Dielectric relaxation of BaTi1− x Sn x O3 ceramics is investigated by means of dielectric spectroscopy. The gradual crossover from ferroelectric to relaxor behavior is characterized by vanishing of the contribution due to domain walls and appearance of relaxation related to reorientation of polar nanosized regions. Typical behavior of relaxors is observed only in ceramics with x = 0.30, while the compositions with 0.175 ≤ x ≤ 0.25 show coexistence of both ferroelectric and relaxor features. The relaxor properties are supposed to be due to both weak random fields and disorder inherent in pure BaTiO3.

Terahertz Emission from Tubular Pb(Zr,Ti)O3 Nanostructures

We report intense terahertz emission from lead zirconate titanate (PZT) tubular nanostructures, which have a wall thickness around 40 nm and protrude on n-type Si substrates. Such emission is totally absent in flat PZT films or bulk; hence the effect is attributed to the nanoscale geometry of the tubes. The terahertz radiation is emitted within 0.2 ps, and the spectrum exhibits a broad peak from 2 to 8 THz. This is a gap in the frequency spectrum of conventional semiconductor terahertz devices, such as ZnTe, and an order of magnitude higher frequency peak than that in the well-studied p-InAs, due to the abnormally large carrier concentration gradient in the nanostructured PZT. The inferred mechanism is optical rectification within a surface accumulation layer, rather than the Dember effect. The terahertz emission is optically pumped, but since the tubes exhibit ferroelectric switching, electrically driven emission may also be possible. EPR reveals 02 molecules adsorbed onto the nanotubes, which may play some role in the emission.

Epoxy composites filled with high surface area-carbon fillers: Optimization of electromagnetic shielding, electrical, mechanical, and thermal properties

A comprehensive analysis of electrical, electromagnetic (EM), mechanical, and thermal properties of epoxy resin composites filled with 0.25–2.0 wt. % of carbon additives characterized by high surface area, both nano-sized, like carbon nanotubes (CNTs) and carbon black (CBH), and micro-sized exfoliated graphite (EG), was performed. We found that the physical properties of both CNTs- and CBH-based epoxy resin composites increased all together with filler content and even more clearly for CBH than for CNTs. In the case of EG-based composites, good correlation between properties and filler amount was observed for concentrations below 1.5 wt. %. We conclude that CBH and, to a lower extent, EG could replace expensive CNTs for producing effective EM materials in microwave and low-frequency ranges, which are, in addition, mechanically and thermally stable.

High dielectric permittivity of percolative composites based on onion-like carbon

The broadband dielectric properties of the onion-like carbon (OLC) and polyurethane based composites of different compositions are presented for the frequency range of 20 Hz–3 GHz and 250–450 K temperatures. It is shown that an abrupt increase in the complex dielectric permittivity at the volume fraction of OLC 10% is associated with the onset of percolation. The revealed critical exponent and percolation point reasonably agree with theoretically predicted values. Composites with 10% of OLC exhibit dielectric permittivity as high as 2000 and conductivity as high as 0.01 S/m at 100 Hz.

Dielectric and ultrasonic investigation of phase transition in cuinp2s6 crystals

Investigation results of dielectric and ultrasonic properties of layered CuInP2S6 crystals are presented. At low frequencies, dielectric spectra are highly influenced by the high ionic conductivity with the activation energy of 7357.4 K (0.635 eV). The high-frequency part of the spectra is determined by relaxational soft mode. The critical slowing down and Debye-type dispersion show the order–disorder type of the phase transition. The temperature dependence of the relaxational soft mode and dielectric contribution show a quasi-one-dimensional behaviour. Ultrasonic velocity exhibits critical slowing down which is accompanied by attenuation peaks in the phase transition region. Layered CuInP2S6 crystals have extremely large elastic nonlinearity in the direction perpendicular to layers. The nonlinear elastic parameters substantially increases at the PT temperature.

High-temperature properties of lithium tetraborate Li2B4O7

High-temperature coherent neutron powder diffraction experiments were carried out on Li2B4O7 with boron isotope ratio 11B : 10B as high as 99.6 : 0.4%. Neither traces of phase transformations nor discontinuous changes of physical properties were observed. Anomalies in the thermal expansion of lithium tetraborate were considered in terms of first-order Gruneisen approximation. Extended bond length analysis revealed significant modifications of the boron–oxygen framework which is supplemented by dynamic lithium disorder. Impedance spectroscopy studies revealed a complicated conduction mechanism in single crystalline lithium tetraborate. The lithium diffusion pathway for bulk conductivity along the polar axis was established using both maximum entropy and anharmonic refinement techniques.

Ultrasonic dispersion in the phase transition region of ferroelectric materials

Abstract Measurements of longitudinal ultrasonic velocity and attenuation have been performed in several ferroelectric materials. Interesting behaviour of the ultrasonic velocity was observed in Ca 2 Sr(C 2 D 5 CO 2 ) 6 (DDSP) single crystals. The temperature dependence of τ in the DDSP crystal was estimated to be 3.5 × 10 −9 /( T − T c ) s.

Asymmetric phase diagram of mixed CuInP2(SxSe1 x)6 crystals

In this article mixed CuInP2(SxSe1−x)6 crystals were investigated by broadband dielectric spectroscopy (20 Hz 3 GHz). The complete phase diagram has been obtained from these results. The phase diagram of investigated crystals is strongly asymmetric the decreasing of ferroelectric phase transition temperatures in CuInP2(SxSe1−x)6 is much more flat with small admixture of sulphur then with small admixture of selenium. In the middle part of the phase diagram (x=0.4-0.9) the dipolar glass phase has been observed. In boundary region between ferroelectric order and dipolar glass disorder with small amount of sulphur (x=0.2-0.25) at low temperatures the nonergodic relaxor phase appears. The phase diagram was discussed in terms of random bonds and random fields.

Investigation of Ultrasonic and Acoustoelectric Properties of Ferroelectric-Semiconductor Crystals

The ultrasonic velocity and attenuation anomalies were observed near phase transitions (PT) in CuInP2(S,Se)6 semiconductor crystals, where S was partially substituted by Se. It was shown that, crystals became piezoelectric not only in low temperatures below PT in ferroelectric phase but also at room temperature, which is far above the phase transition and under DC bias electric field, the electrostriction induced piezoelectricity appears. The anomalies of ultrasonic velocity, attenuation, second harmonic, and acoustoelectric voltage were also observed near the phase transitions in Cu 6 PS 5 I single crystals. These crystals belong to the polar symmetry group 3m at room temperature and exhibit a ferroelastic phase transition at 270 K.