Acceptor doping of Ln2Ti2O7 (Ln = Dy, Ho, Yb) pyrochlores with divalent cations (Mg, Ca, Sr, Zn)

New LANTIOX high-temperature conductors with the pyrochlore structure, (Ln_1−xA_x)₂Ti₂O_7−δ (Ln = Dy, Ho, Yb; A = Ca, Mg, Zn; x = 0, 0.01, 0.02, 0.04, 0.07, 0.1), have been prepared at 1400-1600 °C using mechanical activation, co-precipitation and solid-state reactions. Acceptor doping in the lanthanide sublattice of Ln₂Ti₂O₇ (Ln = Dy, Ho, Yb) with Ca²⁺, Mg²⁺ and Zn²⁺ increases the conductivity of the titanates except in the (Ho_1−xCa_x)₂Ti₂O_7−δ system, where the conductivity decreases slightly at low doping levels, x = 0.01-0.02. The highest conductivity in the (Ln_1−xA_x)₂Ti₂O_7−δ (Ln = Dy, Ho, Yb; A = Ca, Mg, Zn) systems is offered by the (Ln_0.9A_0.1)₂Ti₂O_7−δ and attains maximum value for (Yb_0.9Ca_0.1)₂Ti₂O_6.9 and (Yb_0.9Mg_0.1)₂Ti₂O_6.9 solid solutions:∼2 × 10⁻² and 9 × 10⁻³ S cm⁻¹ at 750 °C, respectively. Ca and Mg are best dopants for Ln₂Ti₂O₇ (Ln = Dy, Ho, Yb) pyrochlores. Using impedance spectroscopy data, we have determined the activation energies for bulk and grain-boundary conduction in most of the (Ln_1−xA_x)₂Ti₂O_7−δ (Ln = Dy, Ho; A = Ca, Mg, Zn) materials. The values obtained, 0.7-1.05 and 1-1.4 eV, respectively, are typical of oxygen ion conductors. We have also evaluated defect formation energies in the systems studied.     

Structure and thermal conductivity of Gd2(TixZr1 − x)2O7 ceramics

The Gd₂(Ti_xZr_1 − x)₂O₇ (x = 0, 0.25, 0.50, 0.75, 1.00) ceramics were synthesized by solid state reaction at 1650 °C for 10 h in air. The relative density and structure of Gd₂(Ti_xZr_1 − x)₂O₇ were analyzed by the Archimedes method and X-ray diffraction. The thermal diffusivity of Gd₂(Ti_xZr_1 − x)₂O₇ from room temperature to 1400 °C was measured by a laser-flash method. The Gd₂Zr₂O₇ has a defect fluorite-type structure; however, Gd₂(Ti_xZr_1 − x)₂O₇ (0.25 ≤ x ≤ 1.00) compositions exhibit an ordered pyrochlore-type structure. Gd₂Zr₂O₇ and Gd₂Ti₂O₇ are infinitely soluable. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ increases with increasing Ti content under identical temperature conditions. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ first decreases gradually with the increase of temperature below 1000 °C and then increases slightly above 1000 °C. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ is within the range of 1.33 to 2.86 W m^− 1 K^− 1 from room temperature to 1400 °C.     

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.     

Synthesis and characterizations of A-site deficient perovskite Sr0.9Ti0.8−xGaxNb0.2O3

The effects of reduction and Ga-doping on the physicochemical properties of A-site deficient perovskites Sr_0.9Ti_0.8−xGa_xNb_0.2O₃ (x = 0, 0.05, 0.1, 0.15 and 0.2) are reported. With 10% Ga doping, the sample sintered in air and treated at 1400 °C in H₂ atmosphere exhibits the highest electrical conductivity. It is found that the Ga-doping lowers the sinterability but promotes the reduction of Sr_0.9Ti_0.8−xGa_xNb_0.2O₃. The XRD analysis on the reduced samples suggests that some cations are reduced during the treatment. However, without high temperature pre-reduction, the improvement of Ga-doping is limited and the overall cell performance using Sr_0.9Ti_0.8−xGa_xNb_0.2O₃ as an anode without catalysts is still relatively low.     

A yellow-emitting Ca3Si2O7: Eu phosphor for white LEDs

A series of yellow-emitting phosphors based on a silicate host matrix, Ca_3 − xSi₂O₇: xEu²⁺, was prepared by solid-state reaction method. The structure and photoluminescent properties of the phosphors were investigated. The XRD results show that the Eu²⁺ substitution of Ca²⁺ does not change the structure of Ca₃Si₂O₇ host and there is no impurity phase for x < 0.12. The SEM images display that phosphors aggregate obviously and the shape of the phosphor particle is irregular. The EDX results reveal that the phosphors consist of Ca, Si, O, Eu and the concentration of these elements is close to the stoichiometric composition. The Ca_3 − xSi₂O₇: xEu²⁺ phosphors can be excited at a wavelength of 300-490 nm, which is suitable for the emission band of near ultraviolet or blue light-emitting-diode (LED) chips. The phosphors exhibit a broad emission region from 520 to 650 nm and the emission peak centered at 568 nm. In addition, the shape and the position of the emission peak are not influenced by the Eu²⁺ concentration and excitation wavelength. The phosphor for x = 0.045 has the strongest excitation and emission intensity, and the Ca_3 − xSi₂O₇: xEu²⁺ phosphors can be used as candidates for the white LEDs.     

Novel transparent conducting oxide: Anatase Ti1−xNbxO2

Single-crystalline Ti_1−xNb_xO₂ (x = 0.2) films of 40 nm thickness were deposited on SrTiO₃ (100) substrates by the pulsed laser deposition (PLD) technique. X-ray diffraction measurement confirmed epitaxial growth of anatase (001) film. The resistivity of Ti_1−xNb_xO₂ films with x ≥ 0.03 is 2-3 × 10^− 4 Ω cm at room temperature. The carrier density of Ti_1−xNb_xO₂, which is almost proportional to the Nb concentration, can be controlled in a range of 1 × 10¹⁹ to 2 × 10²¹ cm^− 3. Optical measurements revealed that internal transmittance in the visible and near-infrared region for films with x = 0.03 was more than 97%. These results demonstrate that the presently developed anatase Ti_1−xNb_xO₂ is one of the promising candidates for the practical TCOs.     

Effect of photocatalytic activities of nano-sized copper molybdate (CuMoO4)-doped bismuth titanate (Bi2Ti4O11) (CMBT) alloy

Different compositions of [CuMoO₄]_x-doped Bi₂Ti₄O₁₁ nanophotocatalyst (x = 0.05, 0.1, 0.5) have been prepared by chemical precursor decomposition (CPD) method using triethanolamine (TEA) and HNO₃. Cu(II) is one of reactive species on the catalyst surface and Mo(VI) ion helps to generate charge compensation of lattice having poor catalytic properties. The photocatalytic properties based on the prepared samples for photodecolorization of thymol blue (TB) solutions are examined by Hg-lamp. The crystal structures of the prepared nano-powders are characterized by XRD, EDAX, UV-vis spectra, specific surface area (BET), and HRTEM analyses. The average particle size of copper molybdate-doped bismuth titanate ranges 32 ± 5 nm measured from TEM. Results show doping of copper molybdate of 5 mol% with bismuth titanate can significantly increase the photoactivity of bismuth titanate compared all the compositions studied except degussa P25 titania. The observed increased photocatalytic activity of copper molybdate-doped bismuth titanate ((CuMoO4)_x(Bi₂Ti₄O₁₁)_1−x; CM_xBT_1−x) is attributed to the strong absorption of OH groups at the surface of the catalyst.     

Order-disorder phase transitions and high-temperature oxide ion conductivity of Er2+xTi2−xO7−δ (x = 0, 0.096)

The Er_2+xTi_2−xO_7−δ (x = 0.096; 35.5 mol% Er₂O₃) solid solution and the stoichiometric pyrochlore-structured compound Er₂Ti₂O₇ (x = 0; 33.3 mol% Er₂O₃) are characterized by X-ray diffraction (phase analysis and Rietveld method), thermal analysis and optical spectroscopy. Both oxides were synthesized by thermal sintering of co-precipitated powders. The synthesis study was performed in the temperature range 650-1690 °C. The amorphous phase exists below 700 °C. The crystallization of the ordered pyrochlore phase (P) in the range 800-1000 °C is accompanied by oxygen release. The ordered pyrochlore phase (P) exists in the range 1000−1200 °C. Heat-treatment at T ≥ 1600 °C leads to the formation of an oxide ion-conducting phase with a distorted pyrochlore structure (P2) and an ionic conductivity of about 10⁻³ S/cm at 740 °C. Complex impedance spectra are used to separately assess the bulk and grain-boundary conductivity of the samples. At 700 °C and oxygen pressures above 10⁻¹⁰ Pa, the Er_2+xTi_2−xO_7−δ (x = 0, 0.096) samples are purely ionic conductors.     

Study on properties of BaxSmyTi7O20 ceramics with CaO-SiO2-B2O3 glass

The effect of CaO-SiO₂-B₂O₃ (CSB) glass addition on the sintering temperature and dielectric properties of Ba_xSm_yTi₇O₂₀ ceramics has been investigated using X-ray diffraction, scanning electron microscopy and differential thermal analysis. The CSB glass starts to melt at about 970 °C, and a small amount of CSB glass addition to Ba_xSm_yTi₇O₂₀ ceramics can greatly decrease the sintering temperature from about 1350 to about 1260 °C, which is attributed to the formation of liquid phase. It is found that the dielectric properties of Ba_xSm_yTi₇O₂₀ ceramics are dependent on the amount of CSB glass and the microstructures of sintered samples. The product with 5 wt% CSB glass sintered at 1260 °C is optimal in these samples based on the microstructure and the properties of sintering product, when the major phases of this material are BaSm₂Ti₄O₁₂ and BaTi₄O₉. The material possesses excellent dielectric properties: ?_r = 61, tan δ = 1.5 × 10⁻⁴ at 10 GHz, temperature coefficient of dielectric constant is −75 × 10⁻⁶ °C⁻¹.     

Effect of Gd substitution on ferroelectric and magnetic properties of Bi4Ti3O12

Polycrystalline samples Bi_4 − xGd_xTi₃O₁₂ (x = 1, 1.5, 2) were investigated by X-ray diffraction, piezoresponse force microscopy and SQUID-magnetometry techniques. Increasing the gadolinium content was shown to suppress the spontaneous polarization in Bi_4 − xGd_xTi₃O₁₂, resulting in a polar-to-nonpolar phase transition near x = 1.5. In contrast to previous expectations, all these samples were found to be paramagnets. It was thus proven that introducing magnetically-active Gd ions into the lattice of the ferroelectric Aurivillius-type compound should not be considered as an effective way to achieve multiferroic behavior.     

The effect of Ca-rich on the electric properties of Ca1+xCu3−xTi4O12 polycrystalline system

Ca_1+xCu_3−xTi₄O₁₂ (x = 0, 0.25, 0.5, 1) ceramics were prepared using the conventional solid-state reaction method. The XRD patterns show that Ca_1+xCu_3−xTi₄O₁₂ compounds are comprised of CaCu₃Ti₄O₁₂ (CCTO) and CaTiO₃ (CTO) phases compared with the traditional CCTO and the content of CTO phase increases with the increase of x. The micrographs demonstrate that the sample for x = 0 has larger grain size 8-10 μm. However, for the samples (x = 0.25, 0.5, 1), the mean grain size decreases markedly with the increase of CTO phase. The measurement for electric properties indicates that the permittivity values decrease with the increase of Ca atoms, but the breakdown electric field E_b and the nonlinear coefficient α values have a behavior reverse to it. The nonlinear coefficient α reaches 28 for x = 1, yet it is only 11.4 for x = 0 in the current range of 1-10 mA. This can be ascribed to the reduction of grain size and the changes in the electric conductivity of main grains and grain boundary with increasing Ca/Cu ratio. Imbalances between Ca and Cu atoms with Ca in excess can favor the non-ohmic properties in detriment to the dielectric property and a suitable Ca/Cu ratio can be selected to adjust the permittivity and I-V nonlinearity, according to different desired device applications.     

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.     

Structural and optical properties of Er-doped Y2Ti2O7 thin films by sol-gel method

Er³⁺-doped Y₂Ti₂O₇ and Er₂Ti₂O₇ thin films were fabricated by sol-gel spin-coating method. A well-defined pyrochlore phase Er_xY_2-xTi₂O₇ was observed while the annealing temperature exceeded 800 °C. The average transmittance of the Er_xY_2-xTi₂O₇ thin films annealed at 400 to 900 °C reduces from ∼ 87 to ∼ 77%. The refractive indices and optical band gaps of Er_xY_2-xTi₂O₇ (x = 0-2) annealed at 800 °C/1 h vary from 2.20 to 2.09 and 4.11 to 4.07 eV, respectively. The ∼ 1.53 μm photoluminescence spectrum of Er³⁺ (5 mol%)-doped Y₂Ti₂O₇ thin films annealed at 700 °C/1 h exhibits the maximum intensity and full-width at half maximum (∼ 60 nm).     

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.     

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.     

Microfabrication of dilute magnetic semiconducting Ti1−xCoxO2 films using photosensitive sol-gel method

Photosensitive Ti_1−xCo_xO₂ gel films are prepared by chemical modification with benzoylacetone, and then their Fourier transform infrared spectra and ultraviolet visible spectra are measured. Results show that the chelate rings of benzoylacetone with Ti ions are formed in the Ti_1−xCo_xO₂ gel films. With irradiation of UV light, the chelate rings are photolyzed, which leads to a change of the solubility of the Ti_1−xCo_xO₂ gel films in methanol. Based on this, the micro-patterns of Ti_1−xCo_xO₂ gel films are obtained. After the patterned gel films are dried at 200 °C and are fired at 550 °C for 1 h, negative patterned Ti_1−xCo_xO₂ inorganic films are obtained. The Ti_1−xCo_xO₂ films have an anantase structure and exhibit a room temperature ferromagnetic property.     

Morphotropic phase boundary in high temperature ferroelectric xBi(Zn1/2Ti1/2)O3 − yPbZrO3 − zPbTiO3 perovskite ternary solid solution

A bismuth and lead oxide based perovskite ternary solid solution xBi(Zn_1/2Ti_1/2)O₃ − yPbZrO₃ − zPbTiO₃ (xBZT − yPZ − zPT) was investigated as an attempt to develop a high T_C ferroelectric material for piezoelectric sensor and actuator applications. A morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases was determined through an XRD study on a pseudobinary line 0.1BZT − 0.9[xPT − (1 − x)PZ] for composition 0.1Bi(Zn_1/2Ti_1/2)O₃ − 0.5PbZrO₃ − 0.4PbTiO₃. Enhanced piezoelectric and ferroelectric activities were observed for MPB composition with dielectric constant ε_r′ ~ 23,000 at Curie temperature (T_C) ≈ 320 °C, remanent polarization (P_r) = 35 μC/cm², piezoelectric coefficient (d₃₃) = 300 pC/N, unipolar strain = 0.15%, and electromechanical coupling coefficient (k_P) = 0.45.     

Electronic structure and photoluminescence properties of Eu-activated Ca4Si2O7F2

The blue-emitting phosphors Ca_(4−x)Eu_xSi₂O₇F₂ (0 < x ? 0.05) have been prepared by solid-state reaction and the photoluminescence properties have been studied systematically. The electronic structure of calcium fluoride silicate Ca₄Si₂O₇F₂ was calculated using the CASTEP code. The calculation results of electronic structure show that Ca₄Si₂O₇F₂ has an indirect band gap with 5 eV. The top of the valence band is dominated by O 2p and Si 3p states, while the bottom of the conduction band is mainly composed of Ca 3d states. Under the 350 nm excitation, the obtained sample shows a broad emission band in the wavelength range of 400-500 nm with peaks of 413 nm and 460 nm from two different luminescence centers, respectively. The relative intensity of the two peaks changes with the alteration of the Eu²⁺ concentration. The strong excitation bands of the powder in the wavelength range of 200-420 nm are favorable properties for the application as lighting-emitting-diode conversion phosphor.     

Structural, magnetic and dielectric properties of Bi5−xLaxTi3Co0.5Fe0.5O15 ceramics

The Bi_5−xLa_xTi₃Co_0.5Fe_0.5O₁₅ (0 ≤ x ≤ 0.4) ceramics were successfully synthesized by a modified Pechini process. The samples were characterized by X-ray diffraction and no impurity phase has been detected. The cell volume of the composites increases monotonously with the increase of La content, which indicates that La ions have been incorporated into the lattice of Bi₅Ti₃Co_0.5Fe_0.5O₁₅. The magnetic measurements show that La doping on Bi sites has enhanced the magnetization of Bi_5−xLa_xTi₃Co_0.5Fe_0.5O₁₅ (0 ≤ x ≤ 0.4). Both the dielectric constants and loss tangent of all the samples decrease on increasing frequency and then become almost constant at room temperature. The La doped Bi₅Ti₃Co_0.5Fe_0.5O₁₅ samples exhibit improved dielectric and ferroelectric properties, with higher dielectric constant enhanced remnant polarization and lower losses at room temperature.     

Studying trivalent/bivalent metal ion doped TiO2 as p-TiO2 in bipolar heterojunction devices

Trivalent/bivalent metal ions doped TiO₂ thin films (M_xTi_1−xO₂, M = Cr³⁺, Fe³⁺, Ni²⁺, Co²⁺, Mn²⁺ and x = 0.01, 0.05, 0.1, 0.15, 0.2) were deposited on Indium–tin oxide (ITO) coated glass substrates by spin coating technique. X-ray photoelectron spectroscopy (XPS) showed Ti⁴⁺ oxidation state of the Ti2p band in the doped p-TiO₂. The homogenous M_xTi_1−xO₂ was used to support n-ZnO thin films with thickness ∼40–80 nm and vertically aligned n-ZnO nanorods (NR) with length ∼300 nm and 1.5 μm. Current (I)–voltage (V) characteristics for the Ag/n-ZnO/M_xTi_1−xO₂/ITO/glass assembly showed rectifying behavior with small turn-on voltages (V₀) < 1 V. The ideality factor (η) and the resistances in both forward and reverse bias were calculated. The temperature dependence performance of these bipolar devices was performed and variation of the parameters with temperature was studied.