Dielectric properties and conductivity of zinc ferrite and zinc ferrite doped with yttrium

The electrical properties of spinel ferrites are determined by the different factors, of which synthesis method, chemical composition and grain size have the most important role. In this paper we investigate dielectric behavior and conductivity of the nanosized ZnFe₂O₄ and Y_0.15Zn_0.85Fe₂O₄ powders obtained by the coprecipitation method. The frequency dependence of the dielectric permittivity, the loss factor, and the conductivity of the samples are determined in the frequency range from 1 Hz to 100 kHz, at temperatures from 300 to 350 K. The dielectric behavior of ferrites is explained by the interface polarization, arising from the heterogeneous nature of its structure. Increasing trend of electrical conductivity (σ) and decreasing trend of the dielectric permittivity () with increasing frequency are explained by the Koops model. The maximum values of tan δ are noticed in the frequency range from 1 to 10 kHz.     

Dielectric properties and current–voltage nonlinear behavior of Ca1−xSrxCu3Ti4O12 ceramics

The effects of Sr doping on the dielectric properties and current–voltage nonlinear behavior of CCTO were investigated. By combining the observations of dielectric properties, current–voltage nonlinearity and impedance spectroscopy, we have found that Sr doping has influenced the electrical properties by adjusting the impedance characteristics of the grain and grain boundary. Among the Sr-CCTO specimens in this work, as Sr doping concentration is 10%, the specimen (Sr-CCTO-2) has the highest permittivity and lowest nonlinear coefficient.     

Low-temperature thermoelectric properties of α- and β-Zn4Sb3 bulk crystals prepared by a gradient freeze method and a spark plasma sintering method

The low temperature thermoelectric properties of Zn₄Sb₃ samples prepared by the gradient freeze (GF) method and sintering have been characterized. With decreasing temperature a dramatic rise in the thermal expansion is observed associated with the structural transition from β- to α-phase; Δl/l=2.8×10⁻⁴ at T_s^GF=257.4 K for GF and Δl/l=1.6×10⁻⁴ at T_s^S=236.5 K for sintered samples. Enhancement is observed in electrical conductivity and p-type thermopower at T_s^GF and T_s^S, while a reduction is observed in the magnetic susceptibility. The GF sample exhibits higher thermoelectric performance than the sintered sample. The power factor of the α-phase in the GF sample is twice as large as that of the β-phase; it exceeds 20 μW/cm·K² between 120 and 240 K, indicating that the α-phase Zn₄Sb₃ is one of the prime candidates for thermoelectric materials for cryogenic use.     

Growth and dielectric properties of Ca3NbGa3Si2O14 crystals

A new langasite type single crystal Ca₃NbGa₃Si₂O₁₄ (CNGS) was grown by Czochralski (CZ) method. The structure of CNGS crystal was determined by X-ray powder diffraction, the lattice parameters were a=0.8087 ± 0.0001 nm, c=0.4974 ± 0.0002 nm, V=0.2817 ± 0.0002 nm³; The congruency of CNGS was examined by measuring the chemical composition of the grown crystal by quantitative X-ray fluorescent (XRF) analysis. The melting point of CNGS crystal was measured by using the differential scanning calorimetry (DSC). Dielectric properties of (1 1 0) wafer plate were studied in the temperature range from 298.15 to 873.15 K; the frequency dependence of dielectric loss in the frequency range 10 Hz–13 MHz was measured.     

Magnetic and electrical transport properties of La0.5A0.5CoO3 nanoparticles (A = Sr, Ca, and Ba)

We have investigated the effect of particle size on the electrical transport and magnetic properties of La_0.5A_0.5CoO₃ (A = Sr, Ca, and Ba) nanoparticles synthesized by sol-gel technique. A size-induced metal to insulator transition is observed in the resistivity behaviour of Sr- and Ba-doped samples as the dimension changes from higher to lower ones. The magnetoresistance exhibits almost linear behaviour throughout the studied field range. The zero-field-cooled (ZFC) and field-cooled (FC) magnetizations display a broad paramagnetic to ferromagnetic transition at T_C with a large magnetic irreversibility. The magnetization results indicate that Co³⁺ ions are in the intermediate spin state but Co⁴⁺ ions stay in a mixture of intermediate and high spin states. The observed frequency dependent shoulder in the in-phase and the peak in the out of phase component of the ac susceptibility indicate the glassy nature of the samples. The analysis of the ac magnetization results suggests that the magnetic behaviour is consistent with the cluster glass model.     

Characterization of hydrothermally prepared BaTi1−xZrxO3

BaTiO₃ (BTO) and BaTi_0.8Zr_0.2O₃ (BZT) powders were prepared using the hydrothermal method, starting from BaO, TiO₂ and Zr(NO₃)₂, 7H₂O. X-ray diffraction analysis showed that the cubic phase is stable at room-temperature and the pure perovskite phase is obtained after heating the powders for 2 h at 1280 °C. The temperature dependence of the dielectric constant points to ferroelectric behavior. This ferroelectric behavior can likely be due to the presence of a possible quadraticity gradient in the grains since the cubic phase may not be ferroelectric. The diffuse character of the transition is attributed to this quadraticity gradient, to grain size distribution and (for BZT) to spatial fluctuations in the concentrations of the substituted ion (Zr) leading to the coexistence of regions of different Curie temperatures.     

Microwave irradiation effects on reversible hydrogen desorption in sodium aluminum hydrides (NaAlH4)

The effect of microwave irradiation on the reversible desorption reaction in sodium aluminum hydride (NaAlH₄) is explored. NaAlH₄ is doped with 2 mol% TiCl₂ and pre-activated by high energy ball milling and aging to show the presence of metallic aluminum phase. As a catalyst, Ti²⁺ has been used to improve desorption kinetics in sodium alanate. X-ray diffraction was performed on the samples exposed to microwave irradiation for 10, 20, 30, 40 and 50 min. Results show that when the powders show the presence of aluminum, a steady increase in the formation of the hexahydride (Na₃AlH₆) phase and Al occurs during microwave irradiation; and is accompanied by a steady reduction in the NaAlH₄ phase XRD peak (h k l) intensities. This data suggests that microwave irradiation drives the reversible H₂ desorption reaction in NaAlH₄. NaAlH₄ doped with 2 mol% TiCl₂ which does not show the presence of Al phase, undergoes a reduction in NaAlH₄ peak intensities with increasing microwave exposure (and no reversible product phases are detected in this case). Dielectric studies on NaAlH₄ indicate that microwave penetration is low. Therefore, it is proposed that microwave irradiation heating of the Al particulate phase is responsible for the hydrogen desorption reaction pathway which is similar to that of conventional heating.     

Crystal structure of the Ag2SiS3 compound

The crystal structure of the ternary compound Ag₂SiS₃ was determined on the basis of X-ray powder diffraction. The compound belongs to a new structure type, space group P2₁/c, a = 0.66709(1), b = 0.66567(2), c = 1.31748(3) nm, and β = 118.658(1)°. Ag₂SiS₃ contains isolated [Si₂S₆] anionic units consisting of pairs of edge-shared tetrahedra. The Ag atoms are situated in the interstices formed by these fragments.     

Relaxation mechanisms and microstructure in glass and glass-ceramics

Two LAS (Li₂O–Al₂O₃–SiO₂)-type glass-ceramics and their parent glass have been studied by isothermal mechanical spectroscopy. These materials have the same chemical composition but the two glass-ceramics differ in microstructure: one is a ‘β-quartz’ glass-ceramic whereas the other one is of ‘β-spodumene’ type. The isothermal internal friction measurements performed in a frequency sweep [10⁻⁴–31.6 Hz] with an inverted torsion pendulum, submitted to subresonant forced oscillations, at temperatures between 93 and 820 K, have revealed several mechanical relaxation peaks. A single internal friction peak is observed in the glass sample whereas two peaks occur in the ‘β-quartz’ and ‘β-spodumene’ glass-ceramics. A detailed microstructure analysis (XRD, IRTF, SEM, TEM and DTA) and dielectric loss measurements have allowed to interpret these relaxation phenomena. The mechanical relaxation peak observed in the glass (290 K for 1 Hz) is assigned to the stress-induced movement of lithium ions. In each glass-ceramic the ‘low-temperature’ peak (340 K for 1 Hz) is linked with the ion mobility in the respective main crystalline phase. As for the ‘high-temperature’ peak, its origin is totally different for the two glass-ceramics; in the ‘β-quartz’ glass-ceramic it is due to the Mg²⁺ and Zn²⁺ ion relaxation in the crystalline phase, whereas in the ‘β-spodumene’ glass-ceramic it is linked with a complex entity within the residual vitreous phase.     

Structural and piezoelectric properties of LiNbO3-modified BiScO3–PbTiO3 ceramics

Piezoelectric perovskite materials based on the solid solution (1 − x)BiScO₃–xPbTiO₃ (BSPT) have been attracting attention for their high Curie temperature (T_c = 450 °C) and excellent piezoelectric properties. The LiNbO₃ (LN), which has a T_c as high as 1150 °C, has been recently reported forming a phase pure perovskite solid solution with some perovskite structure compounds. In the current work, the effects of LN substitution on the structural and electrical properties of BSPT ceramics were investigated in the 0.36BiScO₃–0.64{(1 − x)PbTiO₃–xLiNbO₃} (BSPTLNx) system. The results of LN addition in the BSPT ceramics show significant enhancement of the piezoelectric properties. The piezoelectric constant d₃₃, planar electromechanical coupling coefficient and remnant polarization P_r values reached 465 pC/N, 0.57 and 48 μC/cm², respectively, for x = 0.04. The T_c gradually decreases with increasing LN content in the BSPTLNx system, due to the structure transform from the tetragonal to the rhombohedral. A typical relaxor behavior is also produced with the LN substitution in the BSPTLNx system.     

Fabrication and electrical and thermal properties of Ti2InC, Hf2InC and (Ti,Hf)2InC

In this paper we report on the characterization of predominantly single phase, fully dense Ti₂InC (Ti_1.96InC_1.15), Hf₂InC (Hf_1.94InC_1.26) and (Ti,Hf)₂InC ((Ti_0.47,Hf_0.56)₂InC_1.26) samples produced by reactive hot isostatic pressing of the elemental powders. The a and c lattice parameters in nm, were, respectively: 0.3134; 1.4077 for Ti₂InC; 0.322, 1.443 for (Ti,Hf)₂InC; and 0.331 and 1.472 for Hf₂InC. The heat capacities, thermal expansion coefficients, thermal and electrical conductivities were measured as a function of temperature. These ternaries are good electrical conductors with a resistivity that increases linearly with increasing temperatures. At 0.28 μΩ m, the room temperature resistivity of (Ti,Hf)₂InC is higher than the end members (0.2 μΩ m), indicating a solid solution scattering effect. In the 300 to 1273 K temperature range the thermal expansion coefficients are: 7.6×10⁻⁶ K⁻¹ for Hf₂InC, 9.5×10⁻⁶ K⁻¹ for Ti₂InC, and 8.6×10⁻⁶ K⁻¹ for (Ti,Hf)₂InC. They are all good conductors of heat (20 to 26 W/m K) with the electronic component of conductivity dominating at all temperatures. Extended exposure of Ti₂InC to vacuum (10⁻⁴ atm) at 800 °C, results in the selective sublimation of In, and the conversion of Ti₂InC to TiC_x.     

Comparative studies on structural and electrical properties of lead titanate synthesized by ceramic and co-precipitation method

Lead titanate, PbTiO₃ (PT), ceramic material is a useful piezoelectric material for high-temperature and high-frequency applications such as non-volatile memories, infrared sensors and capacitors. However, it is very difficult to obtain a pure-phase and dense PT as a result of a maximum c/a ratio. In this paper, we have reported a conformable PT ceramics with standard double sintering as well as chemical co-precipitation method were successfully synthesized by means of carefully controlled processing parameters, viz., pH value, sintering temperature, soaking time and heating/cooling rates. This novel method was developed to synthesize PbTiO₃ sample so as to gain better control on chemical homogeneity and stoichiometry. Moreover, relatively inexpensive laboratory grade (LR) lead nitrate [Pb(NO₃)₂] and titanium tetra-isopropoxide [Ti{OCH(CH₃)₂}₄] are used as starting materials. Single phase formation was confirmed through X-ray diffraction (XRD) analysis. The room temperature conductivity study was done by measuring AC conductivity in the frequency range from 100 Hz to 1 MHz and DC resistivity in the temperature range from room temperature to 800 K. Dielectric measurements were carried out at selected frequencies from room temperature to well above transition temperature (T_c). Discussion on comparison between both preparation methods was made on the basis of observed results from electrical properties.     

Lattice parameter values and phase transitions for the Cu2Cd1−zMnzSnSe4 and Cu2Cd1−zFezSnSe4 alloys

X-ray powder diffraction measurements and differential thermal analysis (DTA) were made on polycrystalline samples of the Cu₂Cd_1−zMn_zSnSe₄ and Cu₂Cd_1−zFe_zSnSe₄ alloy systems. The diffraction patterns were used to show the equilibrium conditions and to derive lattice parameter values. For Cu₂Cd_0.8Fe_0.2SnSe₄ as well as for Cu₂Cd_0.2Fe_0.8SnSe₄ the crystal structures were refined using the Rietveld method. It was found that the internal distortion parameter σ decreases as Cd is replaced by either Mn and/or Fe. For the Cu₂Cd_1−zMn_zSnSe₄ and Cu₂Cd_1−zFe_zSnSe₄ alloy systems, only two single solid phase fields, the tetragonal stannite α and the wurtz–stannite δ (Pmn2₁) structures were found to occur in the diagram. In addition to the tetragonal stannite α phase extra X-ray diffraction lines due to MnSe and/or FeSe₂ were observed for as grown samples in the range 0.7 < z < 1.0. However, it was found that the amount of the extra phase decreased for the compressed samples.