Conductivity and dielectric properties of SrLaxBixYxFe12−3xO19 (0.0≤x≤0.33) hexaferrites

Both Y-La-Bi substituted and non-substituted Sr-hexaferrites (SrM) were synthesized by sol-gel auto combustion technique and the influences of Y, La and Bi ions on ac/dc conductivities, dielectric constant/loss properties as well as complex dielectric modulus were investigated extensively for a variety of substitutions. It was observed that ac conductivity of SrM increases slightly with increasing Y³⁺ La³, and Bi³⁺ substitutions at first, and then decreases with its further increments. Furthermore, ac conductivity increased with increasing frequency which could be regarded as an origin indication of both electronic and polaron hopping mechanism. It is also worthy to note that the activation energy, which is an indication of both mechanisms depicted in the Arrhenius plot, is increased as the enhanced contributions of substitutions in SrM with the illustration of a better stability and tunability of electrical bonds among substituted ions and ferrous and ferric ions. The dielectric constant, dielectric loss and dielectric modulus represent a very interesting tunability and optimised consequences regarding to the variation of frequencies, temperatures and substitutions of SrM.     

Crystal structure,magnetic and electric properties of highly coercive ferrites Sr1 − x Sm x Fe12 − x Zn x O19 (0 ≤ x ≤ 0.4)

The samples of highly coercive ferrites Sr_{1 ? x} Sm _x Fe_{12 ? x} Zn _x O₁₉ (0 ≤ x ≤ 0.4) with the structure of magnetoplumbite have been obtained by the solid-phase method. It has been determined that the samples Sr_{1 ? x} Sm _x Fe_{12 ? x} Zn _x O₁₉ are single-phase at x ≤ 0.2 and contain the impurity phases of α-Fe₂O₃, SmFeO₃, and ZnFe₂O₄ at x ≥ 0.3. In the magnetic fields up to 14 T, the specific magnetization has been measured and the values of coercive force (_σ H _c ) have been determined at 5 and 300 K. It has been shown that semiconductor electroconductivity of single-phase samples Sr_{1 ? x} Sm _x Fe_{12 ? x} Zn _x O₁₉ gradually decreases with the increase in the degree of substitution of x from 0 to 0.2.     

Preparation and dielectric properties of (Sr1−xCax)Fe0.5Nb0.5O3; (x=0.0, 0.1, 0.2) ceramics

In the present work, strontium calcium iron niobate ((Sr_1?xCa_x)Fe_0.5Nb_0.5O₃; SCFN) (x=0, 0.1, and 0.2) powders were synthesized for the first time using a molten salt technique. The pure phase perovskite obtained at a relative low calcination temperature of 800 °C was characterized using the X-ray diffraction technique (XRD). SCFN ceramics were fabricated and their properties were investigated. The XRD data of the SCFN ceramics was consistent with an orthorhombic symmetry. However, the solubility of Ca in the SCFN ceramics had an upper limit at x=0.1. All ceramics showed a large dielectric constants. The Ca doping inhibited grain growth, but produced an improvement in dielectric–temperature stability. Furthermore, the doping reduced loss tangent, especially for the x=0.1 sample. These results suggest that the SCFN ceramics prepared from molten salt synthesis exhibit a good dielectric performances, compared to many high dielectric materials that have been prepared using the conventional method.     

Enhanced magnetic,ferroelectric, and photocatalytic properties of (1-x)BiFeO3-xBaTiO3 (0.0 ≤ x ≤ 0.4) powders

The magnetic, ferroelectric, and photocatalytic properties of (1-x)BiFeO₃-xBaTiO₃ (0.0 ≤ x ≤ 0.4) powders synthesized by sol-gel method have been investigated. X-ray diffractometry confirms that the phase of the samples changed from rhombohedral to cubic with the increase in BaTiO₃ content. The grain size decreases and the particle shape becomes homogeneous with the introduction of BaTiO₃. BaTiO₃ substitution enhances the multiferroic properties of the ceramics and the maximum remnant magnetization (0.261 emu/g) and remnant polarization (20 μC/cm²) have acquired in 0.8BiFeO₃-0.2BaTiO₃ and 0.7BiFeO₃-0.3BaTiO₃, respectively. The absorbance in ultraviolet and visible light regions is improved obviously for powder with x = 0.3. The energy band gap of the samples decreases from 2.06 eV to 1.57 eV with the introduction of BaTiO₃, indicating that the excitation rate of photogenerated electron-hole pairs is improved. The highest methylene blue degradation efficiency of ~62% within 3 h under the visible light is achieved in the 0.7BiFeO₃-0.3BaTiO₃ which can be attributed to its suitable energy band gap and large remnant polarization.     

Structural,magnetic and magnetocaloric properties of CoFe2−xMoxO4 (0.0≤x≤0.3) ferrites

We have investigated structural, magnetic and magnetocaloric properties of CoFe_2-xMo_xO₄ (0.0≤x≤0.3) ferrites. Polycrystalline samples were prepared by the sol gel method and characterized by the powder X-ray diffraction and scanning electron microscopy. X-ray diffraction patterns show that all samples have a cubic spinel structure and the lattice parameter, a, decreases monotonically with increase in Mo concentration. Scanning electron micrographs indicate that most of the particles are in the range of 400–850 nm size. Magnetic measurements, performed by using a cryogen free vibrating sample magnetometer, show that these samples are soft ferromagnets in the measured temperature range. The saturation magnetization, M_s, values of Mo-doped samples are larger than the parent compound with a maximum value of ~106 emu/g for x=0.2 sample. The magnetic entropy change (−ΔS) increases with increase in applied magnetic field and shows a peak in the vicinity of blocking temperature. A maximum value of 0.56 J kg⁻¹ K⁻¹ at 5 T field has been observed for x=0.2 sample.     

Enhanced piezoelectric properties of Sr(1.7)(Na0.5Bi0.5)0.3Bi4Ti5O18 ceramics in the system Sr(2−x)(Na0.5Bi0.5)xBi4Ti5O18 (where 0 ≤ x ≤ 0.5)

Fine powders synthesized via sol-gel route were employed to fabricate Sr_(2−x)(Na_0.5Bi_0.5)_xBi₄Ti₅O₁₈ (SNBT, where x = 0, 0.1, 0.25, 0.3, 0.4, and 0.5) ceramics. The composition (x)-dependent structural changes associated with SNBT ceramics were analyzed using X-ray powder diffraction, transmission electron microscopy, and Raman spectroscopic techniques. Average grain size analyses carried out on the SNBT ceramics by scanning electron microscopy revealed an important role played by the dopants in inhibiting the grain growth. Dielectric constants and the Curie temperature of the ceramics were found to decrease and increase, respectively, with increase in x. The increase in Curie temperature with increase in x was attributed to the decrease in the tolerance factor. The specific composition (x = 0.3) of the SNBT ceramics exhibited improved piezo- and ferroelectric properties associated with a higher Curie temperature (569 K). The piezoelectric coefficient (d₃₃) and the planar electromechanical coupling coefficient (k_p) of SNBT(x = 0.3) were enhanced by 25% and 42%, respectively, as compared to the undoped ceramics.     

Synthesis,sintering, and thermoelectric properties of Co1-xMxO (M = Na, 0 ≤ x ≤ 0.07; M = Ag, 0 ≤ x ≤ 0.05)

The structural and thermoelectric properties of Na- and Ag-substituted CoO dense ceramics have been investigated. X-ray diffraction shows that pure phase and Ag/CoO composites have been obtained for Na-doped and Ag-doped CoO, respectively. Raman spectroscopy shows an effect of Na dopants on the lattice disorder of CoO. The chemical composition, element distribution, and valence states of the samples have been characterized by Auger electron microscopy and X-ray photoelectron spectroscopy. Substitution of Co by 5 at. % Na enhances the power factor to 250 μW m⁻¹ K^-2 at 1000 K, similar to that of Ca₃Co₄O₉. The corresponding thermal conductivity is also reduced to 3.55 W.m⁻¹ K⁻¹ at 1000 K. Consequently, Co_0.95Na_0.05O exhibits the best thermoelectric figure of merit (ZT), which is 0.07 at 1000 K. On the other hand, the substitution of Ag into CoO leads to the formation of CoO/Ag composites and deteriorates ZT values.     

Synthesis,structure, optical and magnetic properties of Nd1−xAxMn0.5Co0.5O3−δ (A = Ba,Sr and Ca; x = 0 and 0.25)

The structural, optical, and magnetic properties of polycrystalline Nd_1-xA_xMn_0.5Co_0.5O_3−δ (A = Ba, Sr and Ca; x = 0 and 0.25) perovskite oxides were investigated. The powder XRD pattern demonstrates that the unit cell volume decreases with the changing A-site dopant type. The estimated bandgap energy (Eg) from UV–vis spectroscopic for NdMn_0.5Co_0.5O_3−δ, Nd_0.75Ba_0.25Mn_0.5Co_0.5O_3−δ, Nd_0.75Sr_0.25Mn_0.5Co_0.5O_3−δ and Nd_0.75Ca_0.25Mn_0.5Co_0.5O_3−δ are 3.27, 3.82, 3.79 and 3.53 eV respectively. The substitution of divalent element alters the absorption spectrum, while the redshift optical transition was observed with an increasing ionic radius of dopant. Temperature-dependent magnetization exposes that the Curie temperature (TC) gradually decreases with the decreasing size of alkaline earth metals, and glassy nature was observed at a lower applied magnetic field. The observation of TC can be well explained by the considering of the cationic size disorder parameter in A-site than the random distribution of B-site ions.     

Structure,dielectric, and ferroelectric properties of (1-x)Bi0.5Na0.5TiO3 – X K0.5Na0.5NbO3 (0 ≤ x ≤ 0.1) solid solution

The correlation of the phase structure, dielectric, and ferroelectric properties of lead-free (1-x)(Na_0.5Bi_0.5)TiO₃–xK_0.5Na_0.5NbO₃ (NBTKNx) (0 = x ≤ 0.1) polycrystalline ceramics, fabricated via a solid state reaction technique, were investigated. The Rietveld refinement allowed identifying the crystallographic transformation from a rhombohedral to a coexisting rhombohedral-tetragonal or tetragonal long range-ordered ferroelectric (FE) phase. The dielectric investigations showed an increase of the dielectric diffuseness (1.53 = γ ≤ 1.73) and a clear shift of the depolarization temperature (T_d) to a lower temperature while increasing substitution. More importantly, the lattice disorder also generated a plateau-like dielectric anomaly, leading to a thermally stable ϵ_r ∼2859 ± 20% (120–500 °C) and ∼3112 ± 10% (120–420 °C) for x = 0.075 and 0.1 samples, respectively. At room temperature (RT), Raman spectroscopy investigations revealed a downshift of the frequencies as a function of the composition with an inhomogeneous broadening of the Raman lines. On heating, Raman spectra showed changes in the region where the dielectric transitions are observed. Moreover, the composition dependence of the current peaks in the I-E loops confirmed the occurrence of a phase transition from a non-ergodic polar phase to an ergodic weakly polar after the applying of an electric field of 60 kV/cm⁻¹.     

Synthesis,structural evolution,and dielectric properties of a new perovskite solid solution (Pb0.5Sr0.5)(Zr0.5Ti0.5)O3–PbTiO3

To explore lead-reduced dielectric materials in the SrTiO₃–PbTiO₃–PbZrO₃ ternary system, a novel solid solution between relaxor ferroelectric (Pb_0.5Sr_0.5)(Zr_0.5Ti_0.5)O₃ and ferroelectric PbTiO₃, namely (1 − x)(Pb_0.5Sr_0.5) (Zr_0.5Ti_0.5)O₃–xPbTiO₃ (lead–strontium–zirconate–titanate [PSZT]–PT), has been synthesized in the perovskite structure by high-temperature solid-state reaction method in the form of ceramics. The crystal structure and phase symmetry of the materials synthesized were analyzed and resolved based on X-ray powder diffraction (XRD) data through both the Pawley and Rietveld refinements. The results of the structural refinements indicate that at low PT-concentration end of the solid solution system, for example, x = 0.05, the PSZT–PT solid solution exhibits a cubic structural symmetry (with the space group Pm-3m). As the PT concentration (x) increases, the structure of (1 − x)PSZT–xPT gradually transforms from the cubic to a tetragonal (P4mm) phase. In the composition range of x = 0.10–0.25, a mixture of the cubic and tetragonal phases was identified. As the concentration of PT increases, the proportion of the tetragonal phase increases at the expense of the cubic phase. For a composition of x > 0.25, a pure tetragonal phase is observed. The dielectric properties of the materials were studied by measuring the permittivity as a function of temperature at various frequencies. For the composition of x = 0.05, the temperature dependence of dielectric constant shows typical relaxor behavior. For x = 0.35, the dielectric peaks indicate a normal ferroelectric phase transition. Overall, a structural transformation from a central-symmetric, nonpolar cubic phase to a non-centrosymmetric, polar tetragonal phase is induced by the substitution of PT for PSZT in the pseudo-binary solid solution of (1 − x)PSZT–xPT, which also reveals an interesting relaxor to ferroelectric crossover phenomenon.     

The PZT system (PbZr1-xTixO3, 0.0≤x≤1.0): Specific features of recrystallization sintering and microstructures of solid solutions (Part 1)

This work is concerned with the specific features of sintering of solid solutions of the PZT system (PbZr_1?xTi_xO₃, 0.0≤x≤1.0) depending on composition. The microstructure of the solid-solution ceramics has been found to be a fairly homogeneous, mosaic, sufficiently close packing of isometric crystallites with a range of section from 3 to 11 μm, but there exists a number of special features related to the component and phase composition of the objects. It has been found that the changes in multifractal parameters of grain structure of these ceramics reflect with high sensitivity the processes of phase transformations in the solid solutions. The obtained results are useful in developing piezoelectric materials based on the PZT system.     

Polyaniline (PANI)–Co0.5Mn0.5Fe2O4 nanocomposite: Synthesis,characterization and magnetic properties evaluation

Polyaniline (PANI)/Cobalt-manganese ferrite, (PANI)/Co_0.5Mn_0.5Fe₂O₄, nanocomposite was synthesized by oxidative chemical polymerization of aniline in the presence of ammonium peroxydisulfate (APS). Microwave assisted synthesis method was used for the fabrication of core CoFe₂O₄ nanoparticles. The structural, morphological, thermal and magnetic properties of the nanocomposite were investigated in detail by X-ray diffraction (XRD), fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). The average crystallite size of (PANI)/Co_0.5Mn_0.5Fe₂O₄ nanocomposite by the line profile method was 20±9 nm. The magnetization measurements revealed that (PANI)/Co_0.5Mn_0.5Fe₂O₄ nanocomposite has superparamagnetic behavior with blocking temperature higher than 300 K. The saturation magnetization of the composite is considerably low compared to that of CoFe₂O₄ nanoparticles due to the partial replacement of Co²⁺ ions and surface spin disorder. As temperature decreases, both coercivity and strength of antiferromagnetic interactions increase which results in unsaturated magnetization of the nanocomposite.     

Microwave dielectric properties of SrLa[Ga1−x(Mg0.5Ti0.5)x]O4 and SrLa[Ga1−x(Zn0.5Ti0.5)x]O4 (x = 0.2-0.8) ceramics

SrLa[Ga_1−x(R_0.5Ti_0.5)_x]O₄ (R = Mg, Zn) ceramics were prepared by a standard solid state sintering method. The single-phase ceramics with K₂NiF₄-type layered perovskite structure and I4/mmm space group were obtained, indicating that SrLa(R_0.5Ti_0.5) and SrLaGaO₄ can form the unlimited solid solutions. With increasing x for R = Mg and Zn, ε_r increases monotonously, the Qf value first increases and then decreases, while τ_f increases from a negative to a positive value. The optimized microwave dielectric properties were obtained as following: ε_r = 23.3, Qf = 89 400 GHz, τ_f = −0.8 ppm/°C for SrLa[Ga_0.6(Mg_0.5Ti_0.5)_0.4]O₄ and ε_r = 23.3, Qf = 76 200 GHz, τ_f = 0.2 ppm/°C for SrLa[Ga_0.7(Zn_0.5Ti_0.5)_0.3]O₄, indicating that the present solid solution ceramics are the promising candidates as microwave resonator materials for the telecommunication applications.     

Investigation on phase structure,spectral characteristics,microstructure and microwave dielectric properties of Li2Zn[Ti1-x(Co1/3Nb2/3)x]3O8 (0.0 ≤ x ≤ 0.4) ceramics

In this paper, a series of Li₂Zn[Ti_1-x(Co_1/3Nb_2/3)_x]₃O₈ (0.0 ≤ x ≤ 0.4) ceramics were prepared via the conventional solid-state method. The influences of (Co_1/3Nb_2/3)⁴⁺ complex ions on the phase composition, spectral characteristics, microstructure, and microwave dielectric properties of Li₂Zn[Ti_1-x(Co_1/3Nb_2/3)_x]₃O₈ ceramics were studied systematically. XRD analysis accompanied with Rietveld refinements showed that pure Li₂ZnTi₃O₈ solid solution ceramics with the cubic spinel structure were obtained at x = 0.2–0.4. New Raman-active mode of about 858 cm⁻¹ should be attributed to the vibrations of NbO₆ due to the high bond energy of Nb–O bonds, exerting a certain impact on the structure and performance of Li₂Zn[Ti_1-x(Co_1/3Nb_2/3)_x]₃O₈ ceramics. XPS results indicated that Nb⁵⁺ ion donor suppressed the deoxidation process and therefore resulted in the disappearance of Ti³⁺ ion and oxygen vacancy. The downward trend variation in the ε_r value with the increase of (Co_1/3Nb_2/3)⁴⁺ content could be explained by the presence of “compressed” cations and “rattling” cations effect. In addition, the Q × f of the current ceramics was closely dependent on relative density, grain size, FWHM, and oxygen vacancy. Good combined microwave dielectric properties of ε_r = 24.5, Q × f = 91,250 GHz, and τ_f = −16.8 ppm/°C were achieved for the Li₂Zn[Ti_0.8(Co_1/3Nb_2/3)_0.2]₃O₈ ceramic sintered at 1120 °C. High quality factor gives evidence that the Li₂Zn[Ti_0.8(Co_1/3Nb_2/3)_0.2]₃O₈ ceramic is an appealing candidate for highly selective microwave devices.     

Synthesis and thermoelectric properties of (NaxCa1−x)3Co4O9 ceramics

(Na_xCa_1−x)₃Co₄O₉ (x=0.05−0.2) ceramics with a layered crystal structure were prepared by a sol–gel method followed by a low-temperature sintering procedure. The electrical conductivity and Seebeck coefficient of the complex oxide ceramics were measured from 400 to 900 °C. Their electrical conductivity and power factor increase with increasing temperature, while the thermal conductivity is very weakly dependant on the temperature. Na dopant amount has a remarkable effect on electrical and thermal transport properties. The figure of merit in the ceramic samples is smaller than that of traditional thermoelectric alloys.     

Annealing temperature-dependent structural and electrical properties of (Ta2O5)1-x - (TiO2)x thin films,x ≤ 0.11

(Ta₂O₅)_1-x- (TiO₂)_x (TTO_x) thin films, with x = 0, 0.03, 0.06, 0.08, and 0.11, were deposited using magnetron direct current (DC) sputtering method onto the P/boron-silicon (1 0 0) substrates by varying areas of Tantalum and Titanium metallic targets, in oxygen environment at ambient temperature. The as-deposited thin films were annealed at temperatures ranging from 500 to 800 °C. Generally, the formation of the Ta₂O₅ structure was observed from the X-ray diffraction measurements of the annealed films. The capacitance of prepared metal– oxide– semiconductor (MOS) structures of Ag/TTO_x/p-Si was measured at 1 MHz. The dielectric constant of the deposited films was observed altering with varying composition and annealing temperature, showing the highest value 71, at 1 MHz, for the TTO_x films, x = 0.06, annealed at 700 °C. With increasing annealing temperature, from 700 to 800 °C, the leakage current density was observed, generally decreasing, from 10^?5 to 10^?8 A cm^?2, for the prepared compositions. Among the prepared compositions, films with x = 0.06, annealed at 800 °C, having the observed value of dielectric constant 48, at 1 MHz; and the leakage current density 2.7 × 10^?8 A cm^?2, at the electric field of 3.5 × 10⁵ V cm^?1, show preferred potential as a dielectric for high-density silicon memory devices.     

Crystal phases and electric properties of (Na0.5K0.5)1−xNb1+x/5O3:yCuO,zLiSbO3 piezoceramics

In this study the electric property and the formation of crystal phases are characterized along with the increase of the A-site alkali deficiency(x) in the non-stoichiometric (Na_0.5K_0.5)_1?xNb_1+x/5O₃:yCuO + zLiSbO₃ (x = ?0.01 to 0.1; y = 0, 0.01; z = 0, 0.05) ceramics. Quantitative crystal phase analysis has been carried out using Rietveld method. The crystal structure of tetragonal tungstenbronze phase is discussed in relation with the P–E hysteresis and dielectric properties. The stoichiometric and the slightly alkali deficient samples show very leaky P–E loop. With increasing the alkali deficiency the electrical leakage decreases and the P–E loop shows the saturation. CuO and LiSbO₃ doping in the alkali deficient sample (x = 0.05, y = 0.01, z = 0) leads to the slim and pinched P–E loop shape. By CuO doping the P_r and P_s decreases to 13.9 and 20.87 μC/cm² from 25.6 and 27.2 μC/cm², respectively.     

Crystal structure and luminescence properties of green-emitting Sr1−xAl12O19:xEu2+ phosphors

In this paper, a series of novel luminescent Sr_1−xAl₁₂O₁₉:xEu²⁺ phosphors were synthesized by a high temperature solid-state reaction. The phase structure, photoluminescence (PL) properties, as well as the decay curves were investigated. The quenching concentration of Eu²⁺ in SrAl₁₂O₁₉ was about 0.15 (mol). Upon excitation at 378 nm, the composition-optimized Sr_0.85Al₁₂O₁₉:0.15Eu²⁺ exhibited strong broad-band green emission at 530 nm with the CIE chromaticity (0.2917, 0.5736). The results indicate that Sr_1−xAl₁₂O₁₉:xEu²⁺ phosphors have potential applications as green-emitting phosphors for UV-pumped white-light LEDs.     

Improved magnetic and electromagnetic absorption properties of xSrFe12O19/(1−x)NiFe2O4 composites

xSrFe₁₂O₁₉/(1−x)NiFe₂O₄ composites (0 ≤ x ≤ 1.0) were synthesized by using a conventional solid-state synthetic route. The results show that magnetic hysteresis loops of the xSrFe₁₂O₁₉/(1−x)NiFe₂O₄ composites are similar to those of individual component ferrites, except for the 0.1SrFe₁₂O₁₉/0.9NiFe₂O₄ and 0.3SrFe₁₂O₁₉/0.7NiFe₂O₄, suggesting that the hard/soft magnetic phases are well exchange-coupled. The saturation magnetization, coercivity, and remanent magnetization of the xSrFe₁₂O₁₉/(1−x)NiFe₂O₄ composites are increased with increasing content of SrFe₁₂O₁₉, with maximal values of 42.1 Am² kg⁻¹, 78.7 kA m⁻¹, 17.2 Am² kg⁻¹, respectively, as the content x is about 0.5. They are higher than those of the individual components, implying that interface coupling is present in the magnetic composites. The coercivity and remanent magnetization of the composites are increased initially with increasing sintering temperature and then show a downward tendency. For the component SrFe₁₂O₁₉ and NiFe₂O₄, the minimum reflection losses are −12.5 dB and −18.3 dB at match thicknesses of 2.5 mm and 2 mm, respectively. Compared with those of the component SrFe₁₂O₁₉ and NiFe₂O₄, the microwave absorption performances of the xSrFe₁₂O₁₉/(1−x)NiFe₂O₄ composites are improved remarkably, especially for the samples of x = 0.3 and x = 0.9. The minimum reflection losses values of the 0.3SrFe₁₂O₁₉/0.7NiFe₂O₄ composite are −31.6 dB (12.7 GHz) and −20.2 dB (13 GHz), while those of the 0.9SrFe₁₂O₁₉/0.1NiFe₂O₄ composites are −23.7 dB (16.3 GHz) and −33.5 dB (15.8 GHz), as the matching thicknesses are 2.5 mm and 2 mm, respectively. Therefore, the xSrFe₁₂O₁₉/(1−x)NiFe₂O₄ composites could be used as potential microwave absorption materials.