Effect of B-site doping on Sm0.5Sr0.5MxCo1−xO3−δ properties for IT-SOFC cathode material (M = Fe, Mn)

The crystal structure, thermal expansion rate, electrical conductivity and electrochemical performance of Sm_0.5Sr_0.5M_xCo_1−xO_3−δ (M = Fe, Mn) have been investigated. Two crystal structures have been observed in the specimens of Sm_0.5Sr_0.5Fe_xCo_1−xO_3−δ (SSFC) at room temperature, the perovskite structure of SSFC has an orthorhombic symmetry for 0 ≤ x ≤ 0.4 and a cubic symmetry for 0.5 ≤ x ≤ 0.9. The specimens of Sm_0.5Sr_0.5Mn_xCo_1−xO_3−δ (SSMC) crystallize in an orthorhombic structure. The adjustment of thermal expansion rate to electrolyte, which is one of the main problems of SSC, can be achieved to lower TEC values with more Fe and Mn substitution. Especially, Sm_0.5Sr_0.5Mn_0.8Co_0.2O_3−δ exhibits good thermal compatibility with La_0.8Sr_0.2Ga_0.8Mg_0.2O₃. High electrical conductivities are obtained for all the specimens and they demonstrate above 100 S/cm at 800 °C in SSFC system. The polarization resistance increases with increasing Mn content, Nevertheless, the polarization resistance of SSFC increases with increasing Fe content, but when the amount of Fe reaches to 0.4, the maximum is obtained while the resistance will decrease when the amount of Fe reaches above 0.4. Sm_0.5Sr_0.5Fe_0.8Co_0.2O_3−δ electrode exhibits high catalytic activity for oxygen reduction operating at temperature from 700 to 800 °C.     

Enhancement microwave dielectric properties of La(Mg0.5Sn0.5)O3 ceramics by Substituting Mg for Co

The microwave dielectric properties of La(Mg_0.5−xCo_xSn_0.5)O₃ ceramics were examined with a view to exploiting them for mobile communication. The La(Mg_0.5−xCo_xSn_0.5)O₃ ceramics were prepared using the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the La(Mg_0.4Co_0.1Sn_0.5)O₃ ceramics revealed that La(Mg_0.4Co_0.1Sn_0.5)O₃ is the main crystalline phase, which is accompanied by small extent of La₂Sn₂O₇ as the second phase. Formation of this Sn-rich second phase was attributed to the loss of MgO upon ignition. Increasing the sintering temperatures seemed to promote the formation of La₂Sn₂O₇. An apparent density of 6.67 g cm⁻³, a dielectric constant (?_r) of 20.3, a quality factor (Q.F.) of 70,500 GHz, and a temperature coefficient of resonant frequency (τ_f) of −77 ppm °C⁻¹ were obtained for La(Mg_0.4Co_0.1Sn_0.5)O₃ ceramics that were sintered at 1550 °C for 4 h.     

Microwave dielectric properties of La1−xBix(Mg0.5Sn0.5)O3 ceramics

The La_1−xBi_x(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the La_0.97Bi_0.03(Mg_0.5Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. An apparent density of 6.50 g cm⁻³, a dielectric constant (?_r) of 20.2, a quality factor (Q × f) of 58,100 GHz and a temperature coefficient of resonant frequency (τ_f) of −84.2 ppm °C⁻¹ were obtained for La_0.97Bi_0.03(Mg_0.5Sn_0.5)O₃ ceramics that were sintered at 1550 °C for 4 h.     

Enhanced magnetoelectric coupling in La-modified Bi<Subscript>5</Subscript>Co<Subscript>0.5</Subscript>Fe<Subscript>0.5</Subscript>Ti<Subscript>3</Subscript>O<Subscript>15</Subscript> multiferroic ceramics

Multiferroic properties of La-modified four-layered perovskite Bi_5?x La _x Fe_0.5Co_0.5Ti₃O₁₅ (0 ≤ x ≤ 1) ceramics were investigated, by analyzing the magnetodielectric effect, magneto-polarization response and magnetoelectric conversion. X-ray diffraction indicated the formation of pure Aurivillius ceramics, and Raman spectroscopy revealed the Bi ions displacement and the crystal structure variation. The enhancement of ferromagnetic and ferroelectric properties was observed in Bi_5?x La _x Fe_0.5Co_0.5Ti₃O₁₅ after La modification. The evidence for enhanced ME coupling was determined by magnetic field-induced marked variations in the dielectric constant and polarization. A maximum ME coefficient of 1.15 mV/cm·Oe was achieved in Bi_4.25La_0.75Fe_0.5Co_0.5Ti₃O₁₅ ceramic, which provides the possible promise for novel magnetoelectric device application.     

Ferroelectric and pyroelectric properties of (Na0.5Bi0.5)TiO3-BaTiO3 based trilayered thin films

Trilayered Bi_3.25La_0.75Ti₃O₁₂ (25 nm)/(Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃ (300 nm)/Bi_3.25La_0.75Ti₃O₁₂ (25 nm) and Pb(Zr_0.4Ti_0.6)O₃ (25 nm)/(Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃ (300 nm)/Pb(Zr_0.4Ti_0.6)O₃ (25 nm) thin films without undesirable phases have been deposited on Pt/Ti/SiO₂/Si substrates. It was found that the Bi_3.25La_0.75Ti₃O₁₂ and Pb(Zr_0.4Ti_0.6)O₃ layers are very effective to inhibit the charge transport in the trilayered films. Much better insulating properties than those of (Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃ films have been achieved in the trilayered films. The trilayered films show good dielectric, ferroelectric and pyroelectric properties. Remnant polarizations 2P_r of 16 µC/cm² and 34 µC/cm², pyroelectric coefficients of 4.8 × 10^− 4 C m^− 2 K^− 1 and 7.0 × 10^− 4 C m^− 2 K^− 1 have been obtained for the Bi_3.25La_0.75Ti₃O₁₂/(Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃/Bi_3.25La_0.75Ti₃O₁₂ and Pb(Zr_0.4Ti_0.6)O₃/(Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃/Pb(Zr_0.4Ti_0.6)O₃ thin films, respectively. The trilayered films are promising candidates for sensor and actuator applications.     

Microwave dielectric properties of Ba2 − xSrxLa3Ti3NbO15 ceramics

High dielectric constant and low loss ceramics in the system Ba_2 − xSr_xLa₃Ti₃NbO₁₅ (x = 0-1) have been prepared by conventional solid-state ceramic route. Ba_2 − xSr_xLa₃Ti₃NbO₁₅ solid solutions adopted A₅B₄O₁₅ cation-deficient hexagonal perovskite structure for all compositions. The materials were characterized at microwave frequencies. They show a linear variation of dielectric properties with the value of x. Their dielectric constant varies from 48.34 to 43.03, quality factor Q_u × f from 20,291 to 39,088 GHz and temperature variation of resonant frequency from 8 to 1.39 ppm/°C as the value of x increases. These low loss ceramics might be used for dielectric resonator (DR) applications.     

New substitutions and novel derivatives of the Aurivillius phases, Bi5TiNbWO15 and Bi4Ti3O12

We investigated isomorphous substitution of several metal atoms in the Aurivillius structures, Bi₅TiNbWO₁₅ and Bi₄Ti₃O₁₂, in an effort to understand structure-property correlations. Our investigations have led to the synthesis of new derivatives, Bi₄LnTiMWO₁₅ (Ln = La, Pr; M = Nb, Ta), as well as Bi₄PbNb₂WO₁₅ and Bi₃LaPbNb₂WO₁₅, that largely retain the Aurivillius (n = 1) + (n = 2) intergrowth structure of the parent oxide Bi₅TiNbWO₁₅, but characteristically tend toward a centrosymmetric/tetragonal structure for the Ln-substituted derivatives. On the other hand, coupled substitution, 2Ti^IV → M^V + Fe^III in Bi₄Ti₃O₁₂, yields new Aurivillius phases, Bi₄Ti_3−2xNb_xFe_xO₁₂ (x = 0.25, 0.50) and Bi₄Ti_3−2xTa_xFe_xO₁₂ (x = 0.25) that retain the orthorhombic noncentrosymmetric structure of the parent Bi₄Ti₃O₁₂. Two new members of this family, Bi₂Sr₂Nb₂RuO₁₂ and Bi₂SrNaNb₂RuO₁₂ that are analogous to Bi₂Sr₂Nb₂TiO₁₂, possessing tetragonal (I4/mmm) Aurivillius structure have also been synthesized.     

Low loss Sr1−xCaxLa4Ti5O17 microwave dielectric ceramics

Microwave dielectric ceramics in the Sr_1−xCa_xLa₄Ti₅O₁₇ (0 ≤ x ≤ 1) composition series were prepared through a solid state mixed oxide route. All the compositions formed single phase ceramics within the detection limit of in-house X-ray diffraction when sintered in the temperature range 1450-1580 °C. Theoretical density and molar volume decreased due to the substitution of Ca²⁺ for Sr²⁺ which was associated with a decrease in the dielectric constant (?_r) and temperature coefficient of resonant frequency (τ_f) but an increase in quality factor, Qf_o. Optimum properties were achieved for Sr_0.4Ca_0.6La₄Ti₅O₁₇ which exhibited, ?_r ∼ 53.7, Qf_o ∼ 11,532 GHz and τ_f ∼ −1.4 ppm/°C.     

V-doping effects on ferroelectric properties of K0.5Bi4.5Ti4O15 thin films

V-doped K_0.5Bi_4.5Ti₄O₁₅ (K_0.5Bi_{4.5 − x/3}Ti_4 − xV_xO₁₅, KBTiV-x, x = 0.00, 0.01, 0.03, and 0.05) thin films were prepared on a Pt(111)/Ti/SiO₂/Si(100) substrate by a chemical solution deposition method. The thin films were annealed by using a rapid thermal annealing process at 750 °C for 3 min in an oxygen atmosphere. Among them, KBTiV-0.03 thin film exhibited the most outstanding electrical properties. The value of remnant polarization (2P_r) was 75 μC/cm² at an applied electric field of 366 kV/cm. The leakage current density of the thin film capacitor was 5.01 × 10⁻⁸ at 100 kV/cm, which is approximately one order of magnitude lower than that of pure K_0.5Bi_4.5Ti₄O₁₅ thin film capacitor. We found that V doping is an effective method for improving the ferroelectric properties of K_0.5Bi_4.5Ti₄O₁₅ thin film.     

Synthesis and electromagnetic properties of microwave absorbing material: Cox(Cu0.5Zn0.5)1−xFe2O4 (0 < x < 1) nanoparticles

Spinel ferrite Co_x(Cu_0.5Zn_0.5)_1−xFe₂O₄ over a compositional range 0 < x < 1 was prepared using a simple hydrothermal method. Particle sizes could be varied from 14 to 25 nm by changing the x value. X-ray diffraction results confirmed that all the as-prepared nanoparticles revealed typical spinel structure and transmission electron microscopy images showed that the particle size of the samples increased with increasing x value. The magnetic properties of the as-prepared Co_x(Cu_0.5Zn_0.5)_1−xFe₂O₄ nanoparticles have been systematically examined. The maximum saturation magnetization existed at the highest Co content (x = 1). The electromagnetic properties of all the samples have been measured by an Agilent network analyzer and the results showed that Co_0.1(Cu_0.5Zn_0.5)_0.9Fe₂O₄ possessed the best microwave absorbing properties.     

Composition dependent phase connectivity, dielectric and magnetoelectric properties of magnetoelectric composites with Pb(Mg1/3Nb2/3)0.67Ti0.33O3 as piezoelectric phase

Ferrite (Ni_0.6Co_0.4Fe₂O₄) phase, ferroelectric (Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃) phase and magnetoelectric composites of (x)Ni_0.6Co_0.4Fe₂O₄ + (1 − x)Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃ with x = 0.15, 0.30 and 0.45 were prepared using solid-state reaction technique. Presence of Ni_0.6Co_0.4Fe₂O₄ and Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃ was confirmed using X-ray diffraction technique. The scanning electron microscopic images were used to study the microstructure of the composites. Connectivity scheme present in the magnetoelectric (ME) composites are discussed from the microscopic images. Variation of dielectric constant and dielectric loss with temperature for all the composites was studied. Here we report the effect of Ni_0.6Co_0.4Fe₂O₄ mole fraction on connectivity schemes between Ni_0.6Co_0.4Fe₂O₄ and Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃ composite. The variation of magnetoelectric voltage coefficient with dc magnetic field shows peak behaviour. The maximum value of magnetoelectric voltage coefficient of 9.47 mV/cm Oe was obtained for 0.15Ni_0.6Co_0.4Fe₂O₄ + 0.85Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃ composites. Finally we have co-related the effect of Ni_0.6Co_0.4Fe₂O₄ content and dielectric properties on magnetoelectric voltage coefficient.     

Dielectric properties of Ba(Ti1 − xZrx)O3 solid solutions

This paper reports the structural and dielectric properties of Ba(Ti_1 − xZr_x)O₃ (x = 0-0.3) ceramics. Single-phase solid solutions of the samples were determined by X-ray diffraction. Microscopic observation by scanning electron microscope revealed dense, single-phase microstructure with large grains (20-60 μm). The evolution of dielectric behavior from a sharp ferroelectric peak (for x ≤ 0.08) to a round dielectric peak (for 0.15 ≤ x ≤ 0.25) with pinched phase transitions and successively to a ferroelectric relaxor (for x = 0.3) was observed with increasing Zr concentration. Compared with pure BaTiO₃, broaden dielectric peaks with high dielectric constant of 25,000-40,000 and reasonably low loss (tanδ: 0.01-0.06) in the Ba(Ti_1 − xZr_x)O₃ ceramics have been observed, indicating great application potential as a dielectric material.     

Properties of La0.75Sr0.25MnO3 films grown on Si substrate with Si1−xGex and Si1−yCy buffer layers

The structural and electrical properties of La_0.75Sr_0.25MnO₃ (LSMO) film on Bi₄Ti₃O₁₂ (BTO)/CeO₂/YSZ buffered Si_1−xGe_x/Si (0.05 ≤ x ≤ 0.2 for compressive strain), blank Si, and Si_1−yC_y/Si (y = 0.01 for tensile) were studied. X-ray high resolution reciprocal lattice mapping (HRRLM) and atomic force microscopy (AFM) show that structural properties of LSMO and buffer oxide layers are strongly related to the strain induced by amount of Ge and C contents. The RMS roughness of LSMO on Si_1−xGe_x/Si has a tendency to increase with increasing of Ge content. Electrical properties of LSMO film with Ge content up to 10% are slightly improved compared to blank Si whereas higher resistivity values were obtained for the samples with higher Ge content.     

Structural, magnetic and dielectric properties of Bi1−ySryFe(1−y)(1−x)Sc(1−y)xTiyO3 (x = 0-0.2, y = 0.1-0.3) ceramics

In this study, bulk ceramics with general formula Bi_1−ySr_yFe_{(1−y)(1−x)}Sc_(1−y)xTi_yO₃ (x = 0-0.2, y = 0.1-0.3 mol%) were prepared by traditional solid-state reaction method. As a comparison, bulk BiFeO₃ (BF) was also sintered by rapid sintering method. Their structural, magnetic, dielectric properties were investigated. X-ray diffraction analysis indicated that apart from a small amount of secondary phase detected in BF, all other samples crystallized in pure perovskite structure and maintained original R3c space group. The room temperature M-H curves were obtained. While BF had a coercive magnetic field (H_c) of 150 Oe, Bi_1−ySr_yFe_1−yTi_yO₃ solid solutions had a much larger value (for y = 0.1, 0.2, 0.3, H_c were 4537, 5230 and 3578 Oe, respectively). Sc³⁺ substitution decreased the H_c values of these solid solutions remarkably, and resulted in soft magnetic properties, as well as a decrease of the dielectric loss. At 1 MHz, the tan δ of Bi_0.7Sr_0.3Fe_0.7(1−x)Sc_0.7xTi_0.3O₃ with x = 0.05, 0.1, 0.15, 0.2 were 0.1545, 0.1078, 0.1046 and 0.1701, respectively.     

Properties of La-substituted Na0.5Bi4.5Ti4O15 ferroelectric thin films

Ferroelectric Na_0.5La_0.5Bi₄Ti₄O₁₅ (NaLaBTi) thin films were prepared by a chemical solution deposition method. The NaLaBTi thin films annealed at 750 °C under oxygen atmosphere were randomly oriented polycrystalline. Electrical properties of the NaLaBTi thin films were compared to Na_0.5Bi_4.5Ti₄O₁₅ thin films and better properties were observed in the NaLaBTi thin films. Remnant polarization (2P_r) and coercive electric field (2E_c) were 43 µC/cm² and 204 kV/cm at an applied electric field of 478 kV/cm, respectively. Leakage current density was 1.95 × 10^− 6 A/cm² at 100 kV/cm. Dielectric constant and dielectric loss were 805 and 0.05 at 1 kHz, respectively. Switchable polarization was suppressed by 15% after 1.44 × 10¹⁰ switching cycles.     

Structural,transport and spectroscopic properties of Ti substituted magnetite: Fe3−xTixO4

The effect of Ti⁴⁺ ion on the formation of magnetite, which were prepared by solid-state route reaction method, were studied by resistivity, Raman and ⁵⁷Fe Mössbauer spectrometry. Resistivity measured in the range of 10 < T < 300 K for Ti⁴⁺ magnetite Fe_3−xTi_xO₄ exhibit first order phase transformations at the Verwey transition T_v for Fe₃O₄, Fe_2.98Ti_0.02O₄ and Fe_2.97Ti_0.03O₄ at 123 K, 121 K and 118 K, respectively. No first order phase transition was observed for Fe_2.9Ti_0.1O₄ and small polaron model retraces the semiconducting resistivity behavior with activation energy of about 72 meV. The changes in Raman spectra as a function of doping show that the changes are gradual for samples with higher Ti doping. The Raman active mode for Fe_2.9Ti_0.1O₄ at ≅634.4 cm⁻¹ is shifted as compared to parent Fe₃O₄ at ≅670 cm⁻¹, inferring that Mn²⁺ ions are located mostly on the octahedral sites. ⁵⁷Fe Mössbauer spectroscopy probes the site preference of the substitutions and their effect on the hyperfine magnetic fields confirms that Ti⁴⁺ ions are located mostly on the octahedral sites of the Fe_3−xTi_xO₄ spinel structure.     

Structure, dielectric and ferroelectric anisotropy of Sr2−xCaxBi4Ti5O18 ceramics

Sr_2−xCa_xBi₄Ti₅O₁₈(x = 0, 0.05) powders synthesized by solid state route were uniaxially pressed and sintered at 1225 °C for 2 h. The obtained dense ceramics exhibited crystallographic anisotropy with a dominant c axis parallel to the uniaxial pressing direction which was quantified in terms of the Lotgering factor. Microstructure anisotropy of both compositions (x = 0, 0.05) consisted of plate like grains exhibiting their larger surfaces mostly perpendicular to the uniaxial pressing direction. Dielectric and ferroelectric properties of Sr_2−xCa_xBi₄Ti₅O₁₈ ceramics were measured under an electric field (E) applied parallel and perpendicularly to uniaxial pressing direction. The obtained dielectric ?_R(T) and polarization (P-E) curves depended strongly on E direction thus denoting a significant effect from microstructure and crystallographic texture. Sr_2−xCa_xBi₄Ti₅O₁₈ properties were also significantly affected by Ca content (x): Curie temperature increased from 280 °C (x = 0) to 310 °C (x = 0.05) while ?_R and remnant polarization decreased for x = 0.05. The present results are discussed within the framework of the processing and crystal structure-properties relationships of Aurivillius oxides ceramics.     

Structural and transport properties of stoichiometric Mn-doped magnetite: Fe3−xMnxO4

The polycrystalline samples of Fe_3−xMn_xO₄ (0.10 ≤ x ≤ 0.50) were prepared by a solid-state route reaction method. X-ray diffraction pattern shows that Mn²⁺ doped magnetites are in single phase and possess cubic inverse spinel structure. The resistivity measurements (10 < T < 300 K) for x = 0.0 and 0.01 confirms the first order phase transition at the Verwey transition T_V = 123 K and 117 K, respectively. No first order phase transition was evidenced for Fe_3−xMn_xO₄ (0.10 ≤ x ≤ 0.50). Small polaron model has been used to fit the semiconducting resistivity behavior and the activation energy ?_a, for samples x = 0.10 and 0.50 is about 72.41 meV and 77.39 meV, respectively. The Raman spectra of Fe_3−xMn_xO₄ at room temperature reveal five phonons modes for Fe_3−xMn_xO₄ (0.01 ≤ x ≤ 0.50) as expected for the magnetite (Fe₃O₄). Increased Mn²⁺ doping at Fe site leads to a gradual changes in phonon modes. The Raman active mode for Fe_3−xMn_xO₄ (x = 0.50) at ≅641.5 cm⁻¹ is shifted as compared to parent Fe₃O₄ at ≅669.7 cm⁻¹, inferring that Mn⁺² ions are located mostly on the octahedral sites. The laser power is fixed to 5 mW causes the bands to broaden and to undergo a small shift to lower wave numbers as well as increase in the full width half maxima for A_1g phonon mode with the enhancement of Mn²⁺ doping. Mössbauer spectroscopy probes the site preference of the substitutions and their effect on the hyperfine magnetic fields confirms that Mn⁺² ions are located mostly on the octahedral sites of the Fe_3−xMn_xO₄ spinel structure.     

Polymorphism of Bi1−xLnxO1.5 phases (0<x<0.40): Characterization of a new compound Bi4Ln2O9 (x=0.33; Ln=La, Pr, Nd)

The Bi_1−xLn_xO_1.5 solid solutions (Ln=La, Pr, Nd), of the β₂/β₁/ε (Bi-Sr-O) structural type, have been investigated in their Ln-rich domains. For Ln=La, Pr, and Nd, the upper limits are 0.35, 0.35 and 0.33, respectively. The Bi₄Ln₂O₉ ε phase (x=0.33) appears to be the single definite compound. For Bi₄La₂O₉, Bi₄Pr₂O₉ and Bi₄Nd₂O₉, the ε-type cells are respectively: a=9.484(4) Å, b=3.982(2) Å, c=7.030(3) Å, β=104.75(3)°; a=9.470(5) Å, b=3.945(2) Å, c=6.968(4) Å, β=104.73(3)° and a=9.439(3) Å, b=3.944(2) Å, c=6.923(2) Å, β=105.03(3)°. Upon heating, each monoclinic (ε) compound transforms successively into rhombohedral phases (β₂/β₁) and finally into a cubic fluorite-type phase. For La- and Pr-based compounds, all transitions are reversible; for Nd, depending on the thermal treatment, the reversibility of ε→β₂ can be incomplete. These transformations are characterized using X-ray thermodiffractometry, differential thermal analysis, dilatometry and impedance spectroscopy versus temperature. Examination of Bi₄(Ln, Ln′)₂O₉ samples allows to correlate the evolution of the thermal behavior and of the unit cell parameters, to the lanthanide size. A partial plot of the (Bi₂O₃)_1−x-(La₂O₃)_x phase diagram (0≤x≤0.40) is proposed.