Heat-resistant unfired phosphate ceramics with carbon nanotubes for electromagnetic application

Composite materials, containing low concentrations of carbon nanotubes (CNTs) of three different diameters and heat-resistant phosphate ceramic as a matrix were prepared by cold-pressing method. Their dielectric properties were studied at room temperature in a wide frequency range (20 Hz–1 MHz). It was found experimentally and proved theoretically via modeling of the composites as a random capacitor–resistor–diode network that electrical percolation concentration depends significantly on the diameter of the nanoinclusions. The main conclusion is that the best candidate providing both the lower percolation threshold and high absolute values of ac conductivity is thinner carbon nanotubes (in …

Dielectric properties of graphite-based epoxy composites

International audience; Composite materials based on epoxy resin filled with various kinds of graphite particles: exfoliated graphite, natural graphite, and coarse, medium and fine artificial graphites have been prepared. Results of broadband dielectric investigations of such materials in wide temperature (25-450 K) and frequency (20 Hz-3 THz) ranges are presented. The dielectric permittivity strongly increases with graphite particle size. The graphite particle size and shape also have a strong impact on freezing temperature, conductivity activation energy and composite electromagnetic absorption properties at room temperature. The lowest percolation threshold is observed for exfoliated gra…

Broadband Dielectric Investigation of Sodium Potassium Niobate Ceramic Doped 8% of Antimony

In this work we present dielectric permittivity measurement results of KNN ceramic doped antimony (K0.5Na0.5)(Nb0.92Sb0.08)O3 with sintering aid MnO2 (0.5 mol% MnO2) (KNNSbx + 0.5Mn). Its dielectric properties were investigated in broad frequency (from 20 Hz to 32 GHz) and temperature (from 26 K to 780K) ranges. Above room temperature obtained results show two diffused phase transitions. Below room temperature investigation revealed dielectric permittivity dispersion similar to the relaxor one. The obtained mean relaxation times fulfills Arrhenius relationship with dipole activation energy Ea/kB = 2282 K.

Electrical transport in carbon black-epoxy resin composites at different temperatures

Citation: J. Appl. Phys. 114, 033707 (2013); doi: 10.1063/1.4815870 (Received 3 May 2013; accepted 27 June 2013; published online 17 July 2013) Results of broadband electric/dielectric properties of different surface area—carbon black/epoxy resin composites above the percolation threshold are reported in a wide temperature range (25–500 K). At higher temperatures (above 400 K), the electrical conductivity of composites is governed by electrical transport in polymer matrix and current carriers tunneling from carbon black clusters to polymer matrix. The activation energy of such processes decreases when the carrier concentration increases, i.e., with the increase of carbon black concentration…

DIELECTRIC PROPERTIES OF EPOXY RESIN COMPOSITES FILLED WITH NANOCARBON INCLUSIONS

The epoxy resin composites with various carbon additives were investigated in the frequency range of 20 Hz - 3 GHz at temperatures from room to 500 K. The dielectric properties were found to be strongly impacted by percolation threshold. The lowest percolation threshold (< 0.25 wt.%), was observed in composites with single-walled carbon nanotubes.

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

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 K a -band demonstrated by ultra-light micro-structural porous carbon solids (carbon foams) of different bulk densities, 0.042 to 0.150  g/cm 3 , 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…

Epoxy composites filled with high surface area-carbon fillers

Citation: J. Appl. Phys. 114, 164304 (2013); doi: 10.1063/1.4826529 (Received 24 July 2013; accepted 6 October 2013; published online 22 October 2013) 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, go…

Epoxy Resin/Carbon Black Composites Below the Percolation Threshold

International audience; A set of epoxy resin composites filled with 0.25-2.0 wt.% of commercially available ENSACO carbon black (CB) of high and low surface area (CBH and CBL respectively) has been produced. The results of broadband dielectric spectroscopy of manufactured CB/epoxy below the percolation threshold in broad temperature (200 K to 450 K) and frequency (20 Hz to 1 MHz) ranges are reported. The dielectric properties of composites below the percolation threshold are mostly determined by alpha relaxation in pure polymer matrix. The glass transition temperature for CB/epoxy decreases in comparison with neat epoxy resin due to the extra free volume at the polymer-filler interface. At …

Broad Distribution of Relaxation Times in 0.6PMN-0.4PZN Relaxor Ceramics

In this paper dielectric spectra of 0.6PMN-0.4PZN ceramic obtained from broadband dielectric spectroscopy is presented. Measurements revealed an unusual relaxor like nature. Two different processes in the dielectric spectra have been observed. High frequency, or short relaxation times, behaviour is typical for ferroelectric crystals. Longest relaxation times do not follow Vogel—Fulcher dependence, which is unusual for relaxors.

Dipolar Glass-Like Perovskite Sr0.8Bi0.2TiO3Ceramic

In this work lead-free Sr0.8Bi0.2TiO3 ceramic have been investigated in the frequency range from 20 Hz to 36 GHz. The obtained results show pronounced dispersion below room temperature. It looks more similar to the one observed in dipolar glasses as in relaxors. Calculated relaxation time distribution function broadens towards longest relaxation times with decreasing temperature. The obtained most probable relaxation time fulfills Vogel-Fulcher relationship with the following parameters: dipole activation energy EA/kB = 1718 K, freezing temperature T0 = 65 K, attempt relaxation time, referring to dipole relaxation at very high temperatures, τ0 = 5.5·10−14 s.

Isostructural phase transition, quasielastic neutron scattering and magnetic resonance studies of a bistable dielectric ion-pair crystal [(CH3)2NH2]2KCr(CN)6

We have synthesised and characterised a novel organic–inorganic hybrid crystal, [(CH3)2NH2]2KCr(CN)6. The thermal DSC, TMA, DTG and DTA analyses indicate two solid-to-solid structural phase transitions (PTs). According to the X-ray diffraction experiments, the first PT at 220 K is isostructural, since it does not involve a change of the space group. This transition occurs between the states, where the (CH3)2NH2+ cations are orientationally disordered and ordered (frozen). The other reversible PT at 481 K leads to a melt-like phase similar to the one observed in plastic crystals or polar liquids. Dielectric spectroscopy has been used to characterise the switching properties of the dipole mom…

Anomalous Broad Distribution of Relaxation Times in Mixed PMN-PSN Ceramics

Dielectric permittivity of 0.8PMN-0.2PSN ceramic was measured in the frequency range from 20 Hz to 3 GHz. The typical ferroelectrics relaxor dielectric dispersion was observed. From the frequency dependence of complex dielectric permittivity, the distribution of relaxation times was calculated. The low frequency limit of the distribution of relaxation times diverges according to the Vogel–Fulcher law with the freezing temperature T f = 226 K which is lower than in PMN ceramics, obtained previously. The comparison of distributions of relaxation times between 0.8PMN-0.2PSN and pure PMN tends to the conclusion about smaller polar nano regions in 0.8PMN-0.2PSN ceramic.

Broadband Dielectric Spectroscopy of PSN-Rich PMN-PSN Ceramics

The broadband dielectric permittivity of ordered 0.1PMN-0.9PSN and disordered 0.2PMN-0.8PSN ceramics was investigated. The anomaly of dielectric permittivity is clearly seen in the region of 370 K for 0.1 PMN-0.9 PSN at all frequencies. 0.2 PMN-0.8 PSN shows relaxor behaviour with huge frequency dispersion below 350 K. Calculated distribution of relaxation times shows noticeable influence of polar nano regions to the total dielectric spectrum below 320 K with the longest relaxation times edge diverging according to the Vogel-Fulcher law. In 0.1 PMN-0.9 PSN two a little overlapped peaks present in the distribution of relaxation times, influenced by displacive lead motions at high frequencies…

Anomalous Broad Dielectric Dispersion of 0.4PZN-0.3PSN-0.3PZN Relaxor Ceramics at Lower Temperatures

Dielectric properties of 0.4PbZn1/3Nb2/3O3-0.3PbSc1/2Nb1/2O3-0.3PbMg1/3Nb2/3O3- (0.4PZN- 0.3PSN-0.3PMN) ceramics are presented for 200 &lt; T &lt; 500 K and 20 Hz &lt; ν &lt; 1 MHz. Dielectric constant is very high (more 14000) in the vicinity of the peak. Anomalous broad dielectric relaxation have been observed near the temperature of the maximum permittivity, Tm (at 1 kHz). External bias field considerably lowers the value of dielectric losses at low frequencies due to decrease of polar nano regions contribution to the dielectric permittivity.

Dielectric properties of 0.4Na0.5Bi0.5TiO3–(0.6-x)SrTiO3–xPbTiO3 solid solutions

Abstract In this paper we present our measurements of the linear and nonlinear dielectric permittivity of 0.4 Na 0.5 Bi 0.5 TiO 3 – ( 0.6 - x ) SrTiO 3 – xPbTiO 3 solid solutions (x = 0, 0.05, 0.1, 0.15). The dielectric anomaly increases in the system with respect to the concentration of lead, showing that interactions between dipolar entities are modified. The system exhibits dipolar-glass-like behaviour at low values of x ( 0 ⩽ x 0.1 ). Relaxor behaviour emerges in the sample where x = 0.1 . Furthermore, a spontaneous first-order phase transition from relaxor to normal ferroelectric is observed at x ⩾ 0.15 . A few peculiar dispersion regions are observed in the ferroelectric phase, which …

Dielectric investigation of lead‐free perovskite strontium titanate with 25% bismuth ceramics

In this work lead-free 0.75SrTiO3-0.25BiTiO3 ceramics have been investigated in broad frequency range. Obtained results showed pronounced dispersion below room temperature typical for the dipolar glasses. The calculated relaxation time distribution function broadens toward longest relaxation times part on cooling. The obtained most probable relaxation time diverges according to the Vogel-Fulcher relationship with the following parameters: dipole activation energy EA=2123 K, glassy state temperature T0=52.7 K, shortest relaxation time, referring to dipole relaxations at very high temperatures, τ0=2.1·10–14 s. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dielectric investigation of sodium potassium niobate ceramic doped 7% of antimony

Abstract Electric properties of sodium potassium niobate ceramic doped antimony (Na0.5K0.5)(Nb0.93Sb0.07)O3 with sintering aid MnO2 (0.5 mol% MnO2) (KNNSbx + 0.5Mn) were investigated in a broad frequency (from 1 Hz to 54 GHz) and temperature (from 110 K to 800 K) ranges. The mean relaxation times calculated from the dielectric permittivity measurements fulfill Arrhenius relationship with dipole activation energy EA = 3409 K (EA/kB = 0.29 eV). The DC conductivity has been extracted from experimental results and the activation energy was calculated EA = 11733.78 K (EA/kB = 1.011 eV).

High Temperature Dielectric Properties of PMN‐PSN‐PZN Relaxors

Dynamics of Polar Clusters in PMN Ceramics: Comparison with PMN Single Crystal

The dielectric permittivity of PMN ceramics was measured in the frequency range from 20 Hz to 37 GHz. From the frequency dependence of the real and imaginary parts of dielectric permittivity, the distribution of relaxation times f(τ) was calculated. The f(τ) function broadens significantly, and the second maximum appears below the 270 K temperature. The dielectric response and also relaxation time distribution of PMN ceramics was compared with the previous results of PMN single crystal. The results showed that dynamics of polar nano regions in PMN ceramics is suppressed due to the small size of grains in the ceramics.