Temperature-Induced Structural Transformations in Undoped and Eu3+-Doped Ruddlesden–Popper Phases Sr2SnO4 and Sr3Sn2O7: Relation to the Impedance and Luminescence Behaviors

We report that luminescence of Eu3+ ion incorporated into Ruddlesden–Popper phases allows monitoring phase transition in powders (instead of single crystals), in a time-efficient manner (compared to neutron diffraction), and importantly, with greater sensitivity than previous methods. Crystal structure and dielectric response of undoped and 0.5%Eu3+-doped Sr3Sn2O7 ceramics were studied as a function of temperature over the temperature range of 300–800 K. The luminescence studies of 0.5%Eu3+-doped Sr2SnO4 and Sr3Sn2O7 samples were performed in the temperature range of 80–500 K. These results were compared with the respective dependences for the undoped compounds. The structural transformations in 0.5%Eu3+-doped Sr3Sn2O7 were found at 390 and 740 K. The former is associated with the isostructural atomic rearrangement that resulted in a negative thermal expansion along two of three orthorhombic crystallographic axes, while the latter corresponds to the structural transition from the orthorhombic Amam phase to the tetragonal I4/mmm one. A similar temperature behavior with the structural transformations in the same temperature ranges was observed in undoped Sr3Sn2O7, although the values of lattice parameters of the Eu3+-doped and undoped compounds were found to be slightly different indicating an incorporation of europium in the crystal lattice. A dielectric anomaly associated with a structural phase transition was observed in Sr3Sn2O7 at 390 K. Optical measurements performed over a wide temperature range demonstrated a clear correlation between structural transformations in Eu3+-doped Sr2SnO4 and Sr3Sn2O7 and the temperature anomalies of their luminescence spectra, suggesting the efficacy of this method for the determination of subtle phase transformations

Estimation of parameters of diffusions having a stationary distribution

We consider the estimation of unknown parameters in the drift and diffusion coefficients of a one-dimension diffusion X when the observation is a discrete sample. For the estimation we use stationary distribution function. Using numerical methods we approximate SDE and realize the algorithm with computer

Remarks on the SLLN for linear random fields

We consider random linear fields on generated by ergodic or mixing (in particular case, independent identically distributed (i.i.d.)) random variables. Our main results generalize the classical Strong Law of Large Numbers (SLLN) for multi-indexed sums of i.i.d. random variables. These results are easily obtained using ergodic theory. Also we compare the results for SLLN obtained using ergodic theory and with the help of the Beveridge-Nelson decomposition.

Fibers of Thermoplastic Copolyamides with Carbon Nanotubes for Electromagnetic Shielding Applications

Polymer composites containing carbon nanofillers are extensively developed for electromagnetic shielding applications, where lightweight and flexible materials are required. One example of the microwave absorbers can be thermoplastic fibers fabricated from copolyamide hot melt adhesives and 7 wt% of multi-walled carbon nanotubes, as presented in this paper. A broadband dielectric spectroscopy confirmed that the addition of carbon nanotubes significantly increased microwave electrical properties of the thin (diameter about 100 μm) thermoplastic fibers. Moreover, the dielectric properties are improved for the thicker fibers, and they are almost stable at the frequency range 26–40 GHz and not dependent on the temperature. The variances in the dielectric properties of the fibers are associated with the degree of orientation of carbon nanotubes and the presence of bundles, which were examined using a high-resolution scanning microscope. Analyzing the mechanical properties of the nanocomposite fibers, as an effect of the carbon nanotubes addition, an improvement in the stiffness of the fibers was observed, together with a decrease in the fibers’ elongation and tensile strength

Dielectric properties of epoxy-matrix composites with tungsten disulfide nanotubes

Addition of conductive nanotubes to an insulating polymer matrix has been proven as an efficient strategy that can improve the electromagnetic shielding performance, due to the high aspect ratio of nanotubes. Herein, a set of epoxy-matrix composites filled with 0.15-1.6 vol% of tungsten disulfide (WS2) nanotubes being of 30-120 nm in diameter and 5-20 μm in length has been produced. Electromagnetic properties of the prepared composites were probed in the frequency range from 20 Hz to 1 MHz in a temperature range from 250 K to 500 K. Broadband properties of these materials are controlled by the dynamics of epoxy resin molecules, and no electrical percolation was observed up to the highest concentration (1.6 vol%) of WS2 nanotubes. The value of dielectric permittivity for all composites is not bigger than 6 at room temperature and 1 kHz frequency, and the electrical conductivity of composites is about 10-6 S/m at 500 K, which demonstrate that the composites are suitable for antistatic applications at higher temperatures. The relaxation time follows the Vogel-Fulcher law, and the Vogel temperature has the minimum for the WS2 nanotube concentration 0.15 vol%. Above 410 K, the electrical conductivity determines the properties of the investigated composites due to nonzero electrical conductivity of epoxy resin. The value of DC electrical conductivity for pure epoxy at T = 450 K is 0.3 μS/m, while the DC conductivity of the composites slightly increases with the WS2 concentration. Therefore, the electrical contacts between WS2 nanotubes and polymer matrix are rather ohmic. Additionally, the activation energy is almost independent on the concentration of WS2. However, it is higher in composites than in pure epoxy resin

Fine tuning of electrical transport and dielectric properties of epoxy/carbon nanotubes composites via magnesium oxide additives

The dielectric properties of epoxy/MWCNT (multi-walled carbon nanotubes)/MgO hybrid composites with a fixed MWCNT amount of 0.12 vol.% (0.2 wt.%) and varying MgO concentrations up to 3 vol.% were investigated in broad frequency (20–40 GHz) and temperature (20–500 K) ranges. The composites with up to 2 vol.% MgO nanoparticles concentration showed a significant increase of DC conductivity in relation to their non MgO-containing counterparts. The optimal content of MgO was found, i.e., 0.46 vol.%, which gave up to 2.5 orders of magnitude larger DC conductivity than those of the samples prepared without MgO additives. Using various amounts of MgO, it is possible to predictably vary the broadband electromagnetic properties of the composites, even entirely eliminating the electrical percolation. Electrical transport at different temperatures can be substantially controlled by the addition of given amounts of MgO. The broadband properties are discussed in terms of the distribution of relaxation times, which are proven to be an effective, noninvasive, and simple tool for checking composite fabrication issues, such as the distribution of MWCNT aggregates within the epoxy matrix

Multilayered Composites with Carbon Nanotubes for Electromagnetic Shielding Application

Bulk polylactic acid (PLA)/multiwall carbon nanotube (MWCNT) composites were prepared and investigated in wide frequency ranges (20 Hz–1 MHz and 24–40 GHz). It was determined that the percolation threshold in bulk PLA/MWCNT composites is close to 0.2 vol.% MWCNT. However, the best microwave dielectric properties and absorption were observed in composites with 3.0–5.0 vol.% MWCNT. Therefore, for future investigations, we selected layered (laminate) polymeric structures with gradual changes in MWCNT concentration from 0.2 to 8.0 vol.% MWCNT. Two approaches to laminate structure designs were examined and compared: a five-layer composite and a nine-layer composite that included four pure PLA middle layers. The addition of MWCNT enhanced the elastic modulus by up to 1.4-fold and tensile strength by up to 1.2-fold, with the best performance achieved at 5.0 vol.% loading. High microwave shielding was observed for these layered PLA/MWCNT structures with a gradient change in MWCNT concentration (up to 26 dB in both transmission and absorption coefficients) in the broad frequency range (from 24 to 40 GHz). Obtained structures are highly anisotropic, and the absorption coefficient is 2–5 dB higher in the direction of MWCNT concentration increase; however, the transmission coefficient is the same in both directions. The properties of microwave absorption are mainly unaffected by the additional polymeric layers. The absorption of the layered structure is greater than the absorption of single-layer composites with an optimal MWCNT concentration of the same thickness. The proposed laminate structure design is promising in the field of efficient electromagnetic shielding