A2−αAα′BO4-type oxides as cathode materials for IT-SOFCs (A = Pr,Sm; A′ = Sr; B = Fe,Co)

Preparation, thermal expansion, electrical conductivity and polarization of A_2−αA _α′BO₄-type oxides (A = Pr, Sm, A′ = Sr, B = Fe, Co) were investigated systematically to evaluate their potential as cathode materials for IT-SOFCs. Within 0.8 ≤ α ≤ 1.5, A_2−αSr_αBO_4−δ (A = Pr, Sm, B = Fe, Co) could be obtained as a single K₂NiF₄-structural phase. Thermal expansion coefficients (TECs) of the specimens increase with increasing Sr²⁺ content, TECs of cobaltites are much higher than that of ferrites. The electrical conductivity of cobaltites is in the order of 10² S cm^− 1 near 800 °C, which is acceptable for the cathode of IT-SOFC. Polarization measurements showed that Sm_0.5Sr_1.5CoO_4−δ exhibited the lowest cathodic overpotential at 700–900 °C (72 mV at 500 mA/cm² at 800 °C), being a high potential candidate of cathode material for IT-SOFCs.     

On the synthesis and dielectric studies of (1 − x)Bi(Mg1 / 2Zr1 / 2)O3 − xPbTiO3 piezoelectric ceramic system

Scandium free piezoelectric ceramics of the composition (1 − x)Bi(Mg_1 / 2Zr_1 / 2)O₃ − xPbTiO₃ (BMZ − xPT) were fabricated by the solid state reaction method. Dielectric and structural properties were measured and phase diagram was constructed from the temperature dependent dielectric and impedance data. The morphotropic phase boundary (MPB) was found to be located in the range 0.55 < x < 0.60 with paraelectric–ferroelectric phase transition temperature, T_C (∼ 280 °C). The ceramics near the MPB showed high room temperature dielectric constant (∼ 1387). The room temperature values of the remanent polarization (P_r) and coercive filed (E_C), were ∼ 29 μC/cm² and ∼ 23 kV/cm, respectively.     

Microwave dielectric ceramics in (Ca1− xBax)(Zn1 / 3Nb2 / 3)O3 system

(Ca_1−xBa_x)(Zn_1 / 3Nb_2 / 3)O₃ (x = 0–1.0) microwave dielectric ceramics were prepared and investigated. The Ba(Zn_1 / 3Nb_2 / 3)O₃-based solid solution was observed for x = 0.9, and the compositions with x = 0.1–0.7 resulted in the mixture of two phases. Dielectric constant ε_r and temperature coefficient of resonant frequency τ_f of the present ceramics varied anomalously and reached their maximum at x = 0.7–0.9, and these phenomena were originated from the partial substitution of small Ca²⁺ ions for larger Ba²⁺ at A-site. On the other hand, a good combination of microwave dielectric properties (ε_r = 36, Qf = 16,170 GHz, τ_f = − 12 ppm/°C) were obtained at x = 0.1, while the decreased Qf value was observed in other compositions.     

Ultraviolet–green photoluminescence of β-Ga2O3 films deposited on MgAl6O10 (1 0 0) substrate

β-Ga₂O₃ films were grown on double-side polished MgAl₆O₁₀ (1 0 0) substrate by metal organic chemical vapor deposition (MOCVD) at 600, 650 and 700 °C. The refractive index dispersive behaviors of Ga₂O₃ films have the typical shape of the normal dispersion curve. Photoluminescence (PL) spectra measured at room temperature revealed that all the films exhibited intense ultraviolet (UV)–green emission from 300 to 650 nm. A minor deep UV emission around 275 nm (∼4.51 eV) was observed for the sample prepared at 700 °C. The intensity of the emission increased markedly when measured at low temperature. The corresponding PL mechanisms were discussed in detail and a schematic diagram was proposed.     

Impedance spectroscopy analysis of TinO2n − 1 Magnéli phases

This letter presents a comprehensive impedance spectroscopy characterisation of Magnéli phases (Ti_nO_2n ? 1) over a range of temperatures, which are of interest in electrochemistry and sensing applications, with the aim to enhance the understanding of their electrical properties and influence their microstructure. The impedance of the Ti_nO_2n ? 1 can be resolved into two different contributions, namely the grain bulk (R_B) and grain boundaries (R_GB). The ac conductivity increases with frequency and temperature, following a universal power law. The high relative permittivity (10⁵–10⁶), which is relatively frequency independent from 0.1 Hz to 100 kHz, is attributed to the presence of insulating grain boundaries (R_GB >> R_B) creating an Internal Barrier Layer Capacitor (IBLC) effect. Above 100 kHz, the grain boundaries begin to contribute to the ac conductivity and the permittivity drops sharply.     

The rock salt oxide Li2MgTiO4: Type I dielectric and ionic conductor

The exploration of the Li–Ti–Mg–O system, using both sol–gel technique and solid state reaction method, allowed a new phase, Li₂MgTiO₄, with disordered rock salt structure (a = 4.159 Å) to be synthesized. The latter is shown to be a good type I dielectric material, with a relative constant of 15 at high frequency and low dielectric loss (tanδ < 10⁻³) over the temperature range −60 to 160 °C. It is also observed that the sintering temperature of this phase is strongly lowered by adopting the sol–gel technique compared to solid state reaction (1150 °C instead of 1300 °C). Finally we show that this phase exhibits cationic conductivity above 400 °C (σ_600 °C = 9 × 10⁻⁵ S cm⁻¹).     

Characterization of M(H2PO4)2·2H2O (M = Mn,Co, Ni) and their in situ thermal decomposition by magnetic measurements

It has been established that M(H₂PO₄)₂·2H₂O (M = Mn, Co, Ni) are paramagnetics between 173 and 353 K with weak antiferromagnetic exchange interaction between the metal ions. In situ magnetic measurements during the thermal decomposition of the salts show that the oxidation state and the octahedral coordination of M²⁺ are preserved. From the data obtained it could be supposed that in M(H₂PO₄)₂·2H₂O (M = Co, Ni) this process is topotactic with no long-range diffusion transport. In Mn(H₂PO₄)₂·2H₂O, the formation of the large variety of intermediate products probably requires more drastic rearrangement and diffusion of the manganese ions during the complex transformations, which reflect on both the value and the sign of the θ constants. M₂P₄O₁₂ (M = Mn, Co, Ni), which are the final decomposition products of the corresponding dihydrogen phosphates are paramagnetics in the temperature range of 295–573 K with antiferomagnetic interactions between the metal ions. The lattice parameters of Ni(H₂PO₄)₂·2H₂O have been calculated. It crystallizes in the monoclinic system with a = 7.228(1) Å; b = 9.778(1) Å; c = 5.306(1) Å; β = 94.50(1)°, SG P2₁/n with Z = 2.     

Low temperature synthesis,structural characterization,and zero-field resistivity of nanocrystalline La1−xSrxMnO3+δ (0.0 ≤ x ≤ 0.3) manganites

This paper reports the synthesis of nanocrystalline strontium doped manganites by a simple and instantaneous solution combustion method, which is a low temperature initiated route to prepare fine-grained oxide powders with good surface area. Rietveld refinement of X-ray data indicates that the as-formed manganites exhibit a cubic phase with space group Pm3m. However calcined (900 °C, 6 h) samples transform into rhombohedral phase with space group R-3c. The scanning electron micrographs of the powders reveal that the sizes of very fine primary particles which are in an agglomerated form, decrease as strontium content increases. The infrared spectra of lanthanum manganites show two active modes at 400 and 600 cm⁻¹. From zero-field resistivity measurements, it is found that all the samples exhibit metal–insulator transition (T_M–I) in the range of 219–252 K, which is low as compared to the samples prepared by ceramic route. The lower (T_M–I) values might be due to smaller particle sizes in the range of 25–45 nm.     

Synthesis and characteristics of nature-superlattice-structured Bi3TiNbO9–Bi4Ti3O12 ferroelectric thin films by MOD

Natural-superlattice-structured ferroelectric thin films, Bi₃TiNbO₉–Bi₄Ti₃O₁₂ (BTN–BIT), have been synthesized on Pt/Ti/SiO₂/Si by metal organic decomposition (MOD) using BTN–BIT (1 mol:1 mol) solution. BTN–BIT films show natural-superlattice peaks below 2θ = 20° in X-ray diffraction patterns, which indicate that the BTN–BIT films annealed at 700–800 °C in O₂ ambient are consisted of iteration of two unit cells of Bi₃TiNbO₉ and one unit cell of Bi₄Ti₃O₁₂. As the annealing temperature increases from 600 to 750 °C, uniform and crack-free films, better crystallinity and ferroelectric properties can be obtained, but the pyrochlore phase in BTN–BIT films annealed over 800 °C would impair the ferroelectric properties. With the increase of O₂ flow rate from 0.5 to 1.5 L/min, both remanent polarization P_r and coercive electric field E_C increase, which are mainly attributed to reduction of the vacanvies of Bi and oxide ions in the films. Natural-superlattice-structured BTN–BIT thin films having 2–1 superlattice annealed at 750 °C in O₂ ambient with a flow rate of 1.5 L/min exhibit superior ferroelectric properties of P_r = 23.5 μC/cm² and E_C = 135 kV/cm.     

Synthesis of RuSr2GdCu2O8 − δ superconductors by the sol–gel route

The RuSr₂GdCu₂O_8 − δ (Ru-1212) superconductors have been prepared through the sol–gel route. We found that 1030 °C is the optimum annealing temperature for the formation of the Ru-1212 superconductors synthesized by the sol–gel route. X-ray powder diffraction analysis indicates that nearly all the peaks from the samples can be indexed to a single phase of Ru-1212, tetragonal symmetry with lattice parameters a = b = 3.382 Å, c = 11.478 Å and space group I4/mmm. The RuSr₂GdCu₂O_8 − δ superconductors prepared by the sol–gel method exhibit onset transition temperature T_c-onset near 55 K and zero resistance temperature T_c-zero at 45 K.     

Strain profiles and defect structure in 6H–SiC crystals implanted with 2 MeV As+ ions

Highly perfect (00.1) oriented 6H–SiC wafers were implanted with 2 MeV As⁺ ions to a number of fluencies in the range from 5 × 10¹² cm^?2 to 1 × 10¹⁴ cm^?2 and examined with synchrotron X-ray diffraction methods and RBS/channeling method. The X-ray methods included the investigation of rocking curves recorded with a small 50 × 50 μm² probe beam and white beam Bragg-Case section and projection topography.The implanted layers provided distinct interference maxima in the rocking curves and interference fringes in Bragg-Case section topographies (strain modulation fringes). A good visibility of interference maxima enabled effective evaluation of the strain profile by fitting the theoretical rocking curves to the experimental ones. The evaluated strain profiles approximated by browsed Gaussian curves were similar to the distribution of point defects calculated with SRIM 2008 code. The profiles were similar to the defect distribution determined from the channeling measurements.     

Investigation of La3 + Doped Yb2Sn2O7 as new thermal barrier materials

Low thermal conductivity is one of the key requirements for thermal barrier coating materials. From the consideration of crystal structure and ion radius, La^3 + Doped Yb₂Sn₂O₇ ceramics with pyrochlore crystal structures were synthesized by sol–gel method as candidates of thermal barrier materials in aero-engines. As La^3 + and Yb^3 + ions have the largest radius difference in lanthanoid group, La^3 + ions were expected to produce significant disorders by replacing Yb^3 + ions in cation layers of Yb₂Sn₂O₇. Both experimental and computational phase analyses were carried out, and good agreement had been obtained. The lattice constants of solid solution (La_xYb_1 − x)₂Sn₂O₇ (x = 0.3, 0.5, 0.7) increased linearly when the content of La^3 + was increased. The thermal properties (thermal conductivity and coefficients of thermal expansion) of the synthesized materials had been compared with traditional 8 wt.% yttria stabilized zirconia (8YSZ) and La₂Zr₂O₇ (LZ). It was found that La^3 + Doped Yb₂Sn₂O₇ exhibited lower thermal conductivities than un-doped stannates. Amongst all compositions studied, (La_0.5Yb_0.5)₂Sn₂O₇ exhibited the lowest thermal conductivity (0.851 W·m^− 1·K^− 1 at room temperature), which was much lower than that of 8YSZ (1.353 W·m^− 1·K^− 1), and possessed a high coefficient of thermal expansion (CTE), 13.530 × 10^− 6 K^− 1 at 950 °C.     

Microwave dielectric properties of xNd(Zn1/2Ti1/2)O3–(1 − x)CaTiO3 ceramics

The microwave dielectric properties of xNd(Zn_1/2Ti_1/2)O₃–(1 − x)CaTiO₃ have been investigated. The system has been prepared by a conventional solid state ceramic route. Nd(Zn_1/2Ti_1/2)O₃ (NZT) possesses a dielectric constant (ε_r) of 32, a high quality factor (Q × f) of 170,000 GHz and a temperature coefficient of resonant frequency (τ_f) of − 42 ppm/°C. In order to produce a temperature-stable material, the addition of CaTiO₃ leads to a near-zero temperature variation of resonant frequency. In general, the microwave quality factor (Q × f) decreased as x increased and the temperature coefficient of resonant frequency (τ_f) was approximately linearly proportional to permittivity. The dielectric constant decreases from 77 to 32 as x varies from 0.2 to 1.0. The dielectric constant (ε_r) of 45, Q × f value of 56,000 (at 6 GHz) and temperature coefficient of resonant frequency (τ_f) of 0 ppm/°C were obtained for 0.5Nd(Zn_1/2Ti_1/2)O₃–0.5CaTiO₃ ceramics sintered at 1300 °C for 4 h. As the content of x increases, the highest Q × f value of 136,200 GHz for x = 0.8 is achieved at the sintering temperature 1300 °C.     

Chemical effects in Zr- and Co-implanted sapphire

Sapphire samples were implanted with Zr and Co ions to a fluence of 5 × 10¹⁷ ions/cm² in MEVVA implanter operated at 65 kV. XPS and RBS spectra were studied in the course of annealing within the temperature range of 200–1400 °C. It is concluded that Zr diffuses to the surface and forms ZrO₂ precipitates, whereas Co migrates to the bulk and forms CoAl₂O₄ spinel which gives the crystal a bluish color. These changes are consistent with thermodynamical properties of the implanted ions.     

Photoluminescence properties of Y0.95 − xMxBO3:Eu3+ (M = Ca,Sr, Ba,Zn, Al, 0 ≤ x ≤ 0.1) in 100–450 nm regions

The single phases of Y_0.95 − xM_xBO₃:5%Eu³⁺ (M = Ca, Sr, Ba, Zn, Al, 0 ≤ x ≤ 0.1) were synthesized successfully by solid-state reaction. Their luminescent properties were studied under UV and VUV excitation. The results indicated that with the incorporation of Ca²⁺, Sr²⁺, Ba²⁺, Zn²⁺ or Al³⁺ into the host lattice of YBO₃:Eu³⁺, the high symmetry around Eu³⁺ was destroyed and the ratio of red emission(⁵D₀–⁷F₂) to orange one (⁵D₀–⁷F₁) increased, leading to a better chromaticity. Furthermore, the co-doping ions such as Ca²⁺, Zn²⁺ and Al³⁺ were beneficial to enhance the luminescent intensity of Eu³⁺. These phenomena were evaluated, and possible explanations were proposed.     

Thermal stability and photoluminescence of Zr1−xCexO2 (0 ≤ x ≤ 1) nanoparticles synthesized in a non-aqueous process

Complete range Zr_1−xCe_xO₂ (0 ≤ x ≤ 1) nanoparticles are synthesized by the thermal decomposition of metal-organic precursors (zirconium acetylacetonate and cerium acetylacetonate) in oleylamine. XRD and HRTEM indicate that all of the as-prepared nanoparticles are single-crystal, and the crystallinity becomes better with increasing Ce content. The Zr_1−xCe_xO₂ nanoparticles with Ce content larger than 1/8 crystallize in cubic fluorite phase. XRD measurements on the as-prepared and calcinated ZrO₂ samples reveal that the tetragonal ZrO₂ nanoparticles are stable below 600 °C, and the lattice parameters of ZrO₂ nanoparticles decrease with decreasing particle size. The thermal stability of the cubic phase increases with increasing Ce content. The UV–vis absorption spectra reveal that the band gap energy increases with increasing cerium content. Room-temperature photoluminescence (PL) spectra of pure ZrO₂ nanoparticles show strong emission peaks centered at about 441 nm at room temperature, which is attributed to the ionized oxygen vacancies in the nanoparticles. On the other hand, room-temperature PL spectra of the as-prepared CeO₂ nanoparticles shows two peaks at 417 and 436 nm, which might arise from the transition from the cerium 4f and to the oxygen 2p band (valence band) in CeO₂ and the presence of oxygen vacancies, respectively.     

New dielectric materials of xSrTiO3–(1 − x)Ca(Mg1/3Nb2/3)O3 ceramic system at microwave frequency

The crystal structures and the microwave dielectric properties of the xSrTiO₃–(1 − x)Ca(Mg_1/3Nb_2/3)O₃ perovskite ceramic system have been investigated. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. SrTiO₃ has dielectric properties of dielectric constant ε_r ∼ 205, Q × f value ∼ 4200 GHz and a large positive τ_f value ∼ 1700 ppm/°C. Ca(Mg_1/3Nb_2/3)O₃ possesses high dielectric constant (ε_r ∼ 28), high quality factor (Q × f value ∼ 58,000 at 7 GHz) and negative τ_f value (− 48 ppm/°C). As the x value varies from 0.2 to 0.8, the xSrTiO₃–(1 − x)Ca(Mg_1/3Nb_2/3)O₃ system has the dielectric properties as follows: 40 < ε_r < 123, 4600 < Q × f < 33,400 and − 23 < τ_f < 600. A new microwave dielectric material, 0.3SrTiO₃–0.7Ca(Mg_1/3Nb_2/3)O₃, applicable in microwave devices is suggested and possesses the dielectric properties of a dielectric constant ε_r ∼ 46, a Q × f value ∼ 29,300 GHz (at 6.8 GHz) and a τ_f value ∼− 2 ppm/°C. A near-zero τ_f value can be achieved by adjusting the x value of xSrTiO₃–(1 − x)Ca(Mg_1/3Nb_2/3)O₃ ceramics.     

First observation of the reversible O3 ↔ P2 phase transition: Crystal structure of the quenched high-temperature phase Na0.74Ni0.58Sb0.42O2

According to thermal expansion data, O3-type phase Na_xNi_(1+x)/3Sb_(2−x)/3O₂ (x ≈ 0.8) undergoes at ca. 1270 K a reversible transition to a less dense form. The high-temperature phase quenched to liquid nitrogen belongs to P2 type, space group P6₃/mmc (no. 194), a = 3.0123 Å(2), c = 11.2264 Å(7) for x ≈ 0.74 at 298 K. The stabilisation of P2 versus O3-type structure at high temperatures seems to be due to alkali distribution over greater number of sites thus increasing entropy and decreasing Na⁺–Na⁺ repulsion.     

(C2N2H10)[FexV1−x(HPO3)F3] (x = 0.44, 0.72): Two new organically templated phosphites: Solvothermal synthesis and structural,thermal, spectroscopic and magnetic studies

(C₂N₂H₁₀)[Fe_xV_1−x(HPO₃)F₃] (x = 0.44, 0.72) have been synthesized using mild solvothermal conditions under autogenous pressure and the ethylenediamine molecule as templating agent. The crystal structures have been determined from X-ray single-crystal diffraction data. The compounds crystallize in the orthorhombic P2₁2₁2₁ space group with Z = 4 and unit-cell parameters a = 12.8494(9), b = 9.5430(6), c = 6.4372(5) Å, and a = 12.8578(1), b = 9.5342(1), c = 6.4370(7) Å for (C₂N₂H₁₀)[Fe_0.44V_0.56(HPO₃)F₃] and (C₂N₂H₁₀)[Fe_0.72V_0.28(HPO₃)F₃], (1) and (2), respectively. These isostructural compounds exhibit a monodimensional crystal structure formed by pillared double anionic chains with the formula [M(HPO₃)F₃]²⁻, extended along the [0 0 1] direction. These doubled ionic chains are the result of the linking of two simple chains in which there are alternating octahedral [MO₃F₃] and tetrahedral groups [HPO₃]. The ethylendiammonium cations are placed in the space delimited by three different chains. The metallic ions are interconnected by the pseudo-pyramidal (HPO₃)²⁻ phosphite oxoanions, adopting a slightly distorted octahedral geometry. The IR spectra show bands corresponding to the phosphite oxoanion and the ethylendiamonium cation at 2400 and 1600 cm⁻¹, respectively. The thermogravimetric analyses show that these phases are stable up to ca. 280 °C, at higher temperatures, the decomposition of the crystal structure begins by calcination of the organic cation and the elimination of the fluoride anions. The diffuse reflectance spectra show bands of the V³⁺ ion (d²) in octahedral symmetry. The values of the Dq (1540, 1540 cm⁻¹), and Racah parameters, B (560, 535 cm⁻¹) and C (3055, 3140 cm⁻¹) for (1) and (2), respectively, correspond with those usually found for octahedrically coordinated V(III) compounds. Magnetic measurements, performed on a powered sample from 5.0 to 300 K at 1000 G, in the ZFC and FC modes, indicate the existence of antiferromagnetic interactions.     

Solvothermal synthesis of graphene sheets at 300 °C

Graphene nanosheets (GS) had been solvothermally synthesized through reducing hexachloro-1,3-butadiene (C₄Cl₆) by metallic sodium (Na) in polyethylene glycol-600 (PEG-600) at 300 °C. Atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM) investigations indicated that 1–3 graphite layers could be observed. The Raman spectrum showed that the peak of 2D band at 2693 cm^? 1 of GS had a smaller wave number and stronger intensity compared to the 2717 cm^? 1 of commercial graphitic flakes. Meanwhile, the I_D/I_G value of GS was 0.40 indicating a lower density of defects of GS. The possible reaction process was that C₄Cl₆ was dechlorinated by Na in the presence of PEG-600 to produce carbon framework, then these newly produced carbon framework would connect to each other to form the hexagonal network of graphene.