Maksim Ivanov

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).

Tannin-Based Carbon Foams for Electromagnetic Applications

Broadband dielectric analysis of tannin-based carbon foams produced at nine different densities, ranging from 0.036 to 0.114 g/cm3, was carried out in wide frequency (20 Hz-35 GHz) and temperature (25-500 K) ranges. Both dielectric constant and electrical conductivity of carbon foams in quasi-static regime, at 129 Hz, were high (more than 106 and close to 70 S/m, respectively), and increased with carbon foams density according to power laws. Significant electromagnetic interference shielding efficiency (EMI SE) was found at microwave (MW) frequencies for all produced samples. The highest value of electromagnetic attenuation at the level of 20 dB for 2-mm thick sample was observed for the foam having the highest density and the lowest mean cell size. For thicker samples (4 mm and 1 cm), EMI SE reached 39 and 73 dB, respectively. On cooling, dc conductivity decreased with temperature according to Motts law but foams still remained opaque to the MW radiation. MW performances of carbon foams reported here, along with other valuable properties such as “green” origin, low price, lightness, chemical inertness, thermal stability and high conductivity, open new routes for producing effective EMI shields from tannin-based carbonaceous porous structures.

Polarization reversal in organic-inorganic ferroelectric composites: Modeling and experiment

Tailoring of ferroelectric properties of copolymer P(VDF-TrFE) by incorporation of ceramic inclusions in the polymer matrix is promising for advanced applications in sensorics. We have observed experimentally that in composites of P(VDF-TrFE) with barium-doped lead zirconate titanate (BPZT), the remanent polarization increases, while the coercive field substantially decreases in comparison with the pure polymer samples. Results of simulation in framework of the modified Weiss model have shown that the changes of the hysteresis loops characteristics are due to increase of the dielectric susceptibility of the composite as compared to pure PVDF-TrFE. This originates from the strong dispersion of the mean field constant α, which describes the feedback of the polarization on the electric field at the location of the dipoles and is closely related with the local increase of composite susceptibility in the vicinity the BPZT inclusions. This phenomenon effectively becomes macro-scale due to the long-range nature of the inhomogeneous elastic and electric fields occurring at the interfaces between the matrix and inclusions.

Dielectric and Conductive Properties of Hydrotalcite

Hydrotalcite-type materials are made of positively charged two-dimensional sheets of mixed hydroxides with water and exchangeable charge-compensating anions. The crystal structure of hydrotalcite consists of octahedral brucite-like layers. The interlayer space between each brucite sheets is filled with water and anions that compensate the positively charged layers. This region is highly disordered and the amount of water, which influences heavily physical properties of the material, is a function of temperature, of water vapour pressure and of nature of the anions present. We present an initial study of dielectric and conductive properties of synthetic hydrotalcite in this contribution.

Dynamics of Phase Transition in 0.4NBT-0.4ST-0.2PT Solid Solution

In this paper we present dielectric spectroscopy results of NBT-(0.6-x)ST-xPT with x = 0.2 solid solution. Dielectric investigations clearly showed a relaxor—normal ferroelectric phase transition at TPT = 419 K and low temperature and low frequency dispersion similar to coexistence of dipolar glass and ferroelectric phase. The mean relaxation time above the phase transition follows Vogel—Fulcher law with following parameters: E A = 0.179 eV, τ 0 = 3.39·10−14 s, T VF = 223 K.

Dielectric and Pyroelectric Properties of PMN-29PT Single Crystals near MPB

This paper presents a systematic investigation of the dielectric and pyroelectric properties of PMN-29PT single crystals, oriented and poled along three different crystallographic directions, <001>, <011> and <111>as a function of temperature and frequency. Dielectric measurements were performed over a temperature range of 300 K–500 K and at frequencies of 20 Hz–1 MHz. During the heating cycle, two phase transitions are detected: one at ~380 K from rhombohedral to tetragonal phase, another at ~410 K from tetragonal to cubic phase. However, during the cooling cycle only one smeared phase transition is observed, i.e. phase transition from cubic to rhombohedral phase. Pyroelectric measurements were performed from 220 K to 420 K during the heating cycle. The variations of pyroelectric coefficients with temperature shows two maxima, one at 370 K and another at 400 K for the <111>- and <011>-oriented crystals, respectively, while only one maximum is observed at 360 K for the crystal oriented along the <001> direction.

Size effects in a relaxor: further insights into PMN

Dielectric measurements of PbMg1/3Nb2/3O3 (PMN) powder and dense ceramics with grain sizes between 15 nm and two microns were carried out in a broad frequency range (20 Hz-1 GHz). Clear grain size dependence of relaxor behavior was evidenced. A progressive transformation from Vogel-Fulcher behavior towards the Arrhenius process in the PMN with reduction of grain size in both ceramics and powder was observed. In the case of ceramics we were able to extract deeper information from the distributions of relaxation times and an analysis using the Vogel-Fulcher law, revealing two main contributions: a fast part of distribution of relaxation times with a maximum close to 10(-11) s, which is almost grain-size independent and has a non-polar origin; whereas, a process with long relaxation times (in the time range of 10(-8) to 10(-5) s) is associated with the dynamics of the polar nanoregions and is strongly suppressed with reduction of grain size. The results of dielectric investigations are confirmed by Nuclear Magnetic Resonance experiments.

Dielectric and Impedance Spectroscopy of BaSnO3 and Ba2SnO4

Dielectric and impedance spectroscopy studies of barium stannate (BaSnO3) and barium orthostannate (Ba2SnO4) nano-grained ceramics were performed in 300–1000 K temperature and 20 Hz–1 MHz frequency ranges. No dielectric anomalies were observed. Both materials are sensitive to humidity of air. Conductivity is dominated by electronic conductivity at low temperatures, but above 700 K it could be either intrinsic electronic conductivity or ionic one.

THz Emission from PZT Nanotubes

We report THz emission from lead zirconate-titanate (PZT) nanotube arrays, which is totally absent in flat films or bulk PZT; hence the effect is due to the nano-scale geometry of the tubes, which are 1 μ m in total diameter, 40 nm in wall thickness and standing normal to the Si substrates. The THz radiation is emitted within 0.2 ps and the spectrum exhibits a broad peak from 2 to 8 THz due to the abnormally large carrier concentration gradient and large piezoelectric coefficient in PZT; the inferred mechanism is optical rectification within a surface accumulation layer, rather than the Dember effect.

DIELECTRIC BEHAVIOUR OF A NANOGRAIN PMN POWDERS

ABSTRACT Dielectric permittivity of 150 nm mean grain size Pb(Mn1/3Nb2/3)O3 (PMN) nanopowder has been investigated by dielectric spectroscopy in the 20 Hz–1 MHz frequency range and 100 K–400 K temperature range. The broad and diffused dielectric dispersion has been observed, but the dispersion region and the maximum of the real part of dielectric permittivity is shifted to lower temperatures compare to PMN single crystal and ceramics. The mean relaxation time, obtained from fits of the frequency dependences of the dielectric permittivity with Cole-Cole formula, changes according to the Vogel-Fulcher law with the freezing temperature T 0 = 88 K which is much lower than in bulk PMN materials.