Olena Okhay

Enhancement of tetragonality and role of strontium vacancies in heterovalent doped SrTiO3

The effect of Sr vacancies on the behavior of strontium titanate with trivalent dopants (La3+, Gd3+, and Y3+) substituting Sr2+ ions is reported. A remarkable shift of the antiferrodistortive transition temperature Ta is revealed by Raman spectroscopy for just a small content of dopant. It is shown that a unique linear dependence of Ta versus tolerance factor is obtained when Sr-vacancies are taken into account. A vacancy size value of ∼1.54 A is estimated, which is ∼7% larger than Sr2+ radius. This size difference enables explaining the unexpected increase of lattice parameter with increasing Bi3+ content in Sr1−1.5xBixTiO3.

High dielectric constant and tunability of strontium titanate ceramics modified by chromium doping

The incorporation of chromium on the strontium site of the perovskite lattice of SrTiO3 ceramics and its influence on the structural, microstructural and low-frequency dielectric properties is studied in this work. Dense Sr1−1.5xCrxTiO3 (x = 0, 0.0005, 0.0010, 0.0015) ceramics are prepared from powders obtained by a sol–gel method. The lattice parameter ~3.906 A and average grain size ~2.5 µm are almost identical for all the ceramic samples under study. On the other hand, the dielectric constant increases from approximately 7000 for x = 0 to >16 000 for x = 0.0005. Detailed investigations of the low-temperature dielectric properties show a remarkable increase of the tunability and nonlinearity of the dielectric constant as a function of dc bias field in strontium titanate ceramics modified by chromium doping but no ferroelectric or relaxor-like behaviour. These results imply the possible incorporation of small Cr amounts on the Sr site of strontium titanate ceramics and expand the tunable applications of SrTiO3-based materials.

Hydrogen Storage for Energy Application

The rising population and increasing demand for energy supply urged us to explore more sustainable energy resources. The reduction of fossil fuel dependency in vehicles is key to reducing greenhouse emissions [1-2]. Hydrogen is expected to play an important role in a future energy economy based on environmentally clean sources and carriers. As a fuel of choice it is light weight, contains high energy density and its combustion emits no harmful chemical by-products. Moreover, hydrogen is considered as a green energy, because it can be generated from renewable sources and is non-polluting [3-5].

Conductivity enhancement and resistance changes in polymer films filled with reduced graphene oxide

The electrical properties of polymer composites based on polycarbonate (PC) and panipol CXM (CX), filled with reduced graphene oxide (rGO), were investigated. The composite preparation conditions allowed good dispersion of rGO in the polymer matrix. We show here that when used as a nanofiller in polymers, rGO offers an appreciable improvement of the electrical current in 3 orders of magnitude (from 10−10 A to 10−7 A at 10 V), as observed in current-voltage (I-V) data for both PC and CX polymers with rGO. The suggested mechanism for the observed switching effects is the migration of oxygen groups aided by both the electrical field and Joule heating. Moreover, some reset- and set- like changes similar to resistive switching were observed in the I-V data of PC and CX-based films upon the addition of rGO. Clockwise (resembling a memristive system type II) and counter-clockwise (resembling a memristive system type I) directions were detected in the I-V data of the analyzed films. According to the obtained resu...

Dielectric relaxation of Sr1–1.5xBixTiO3 sol-gel thin films

The dielectric response of Sr1–1.5xBixTiO3 films (0.002 ≤ x ≤ 0.167), prepared by sol-gel and deposited on Si/SiO2/TiO2/Pt substrates, is analyzed as a function of frequency and temperature. The hysteretic behavior of the polarization versus the electric field is studied as well. Between 100 Hz and 1 MHz, the real part of the dielectric permittivity ɛ′ exhibit a relaxation between ∼60 and 260 K, shifting to high temperatures with increasing the Bi content. In the imaginary part of the dielectric permittivity ɛ′′ of these films two relaxations are induced by Bi doping below the temperature of the ɛ′ relaxation. The first relaxation observed in films with 0.002 ≤ x ≤ 0.10 follows the Arrhenius law with an activation energy of U = 64–80 meV and a preexponential term τ0 = (0.3–10.8) × 10−14 s almost independent on the Bi content and is ascribed to the individual hopping of dipoles created by the off-center Bi ions. The second relaxation observed in the films with 0.04 ≤ x ≤ 0.167 is described by the Vogel–Ful...

Thin film versus paper-like reduced graphene oxide: Comparative study of structural, electrical, and thermoelectrical properties

We report fabrication of reduced graphene oxide (rGO) films using chemical reduction by hydrazine hydrate and rGO paper-like samples using low temperature treatment reduction. Structural analysis confirms the formation of the rGO structure for both samples. Current-voltage (I–V) measurements of the rGO film reveal semiconductor behavior with the maximum current value of ∼3 × 10−4A. The current for the rGO paper sample is found to be, at least, one order of magnitude higher. Moreover, bipolar resistance switching, corresponding to memristive behavior of type II, is observed in the I–V data of the rGO paper. Although precise values of the rGO film conductivity and the Seebeck coefficient could not be measured, rGO paper shows an electrical conductivity of 6.7 × 102 S/m and Seebeck coefficient of −6 μV/ °C. Thus, we demonstrate a simplified way for the fabrication of rGO paper that possesses better and easier measurable macroscopic electrical properties than that of rGO thin film.

Manipulation of dielectric permittivity of sol-gel SrTiO 3 films by deposition conditions

Structure, microstructure and low-temperature dielectric properties of undoped SrTiO3 (STO) thin films prepared by sol‐gel and deposited on Si/SiO2/TiO2/Pt substrates are studied. The effect of annealing temperature and of buffer layers on properties of STO films is analysed. Dielectric permittivity e � , relative tunability nr and polarization P are lowest for STO films prepared without buffer layers and annealed at 750 ◦ C and are highest for films prepared with buffer layers and annealed at 900 ◦ C. The increase of c/a ratio for films with buffer layer and of the grain size for films annealed at higher temperature is used to explain the improved dielectric response. The dielectric permittivity of STO films prepared by low-cost sol‐gel technique with optimized deposition conditions is found to be comparable to that of STO films fabricated by more sophisticated and expensive methods such as pulsed laser deposition. (Some figures may appear in colour only in the online journal)

Low temperature dielectric characterization of Mg-doped SrTiO3 thin films prepared by sol-gel

Voltage dependence of dielectric constant e′ of ferroelectrics and low dielectric loss tan δ of incipient ferroelectrics make them attractive for tuning elements in microwave circuits. In this letter, field dependence of the low-temperature dielectric permittivity and polarization of Mg-doped SrTiO3 films is studied. Incorporation of Mg on both Sr and Ti sites decreases the e′ and relative tunability nr of sol-gel derived SrTiO3 films, whereas polarization is reduced by Ti site substitution only. tan δ of the studied films is ≤0.012, decreasing at low temperatures down to 0.001 when Ti is substituted by 5% of Mg.

Dielectric response of polycrystalline Sr1−1.5xBixTiO3 thin films under direct current bias

The effect of dc electric field Edc on the dielectric behavior of sol-gel derived Sr1−1.5xBixTiO3 films on Si/SiO2/TiO2/Pt substrates is systematically studied between 10 and 300 K. The rounded peak in the temperature dependence of the dielectric constant e′(T) of all Sr1−1.5xBixTiO3 films is strongly suppressed under dc bias. The observed dielectric behavior is attributed to the formation of polar dipoles by the introduction of Bi into Sr site. A high tunability (∼40%) and communication quality factor (∼10 000) under 100 kV/cm make moderately Bi-doped SrTiO3 films a promising material for tunable devices.

Magnetic Anomaly and Dielectric Tunability of (Sr,Mn)TiO3 Thin Films

Polycrystalline Sr0.98Mn0.02TiO3 thin films are prepared by sol-gel spin-coating method on Si/SiO2/TiO2/Pt substrates. Their dielectric permittivity, polarization, and magnetization are investigated as a function of temperature (from 10 to 300 K), electric and magnetic fields. Temperature dependences of the magnetization M ZFC-FH(T), real part of the dielectric permittivity ϵ′(T), and dissipation factor tanδ(T) of Sr0.98Mn0.02TiO3 films show anomalies around 41–45 K, implying an interrelation between polar and magnetic order. “Butterfly”-like dc bias dependences of the real part of the dielectric permittivity ϵ′(E dc), as well as slim polarization and magnetization hysteresis loops are observed, suggesting that Sr0.98Mn0.02TiO3 thin films belong to “multiglass” systems.