Synthesis and electrical properties of nanostructured Ba2SnYO5.5 proton conductor

Ba₂SnYO_5.5 nanopowders were synthesized by a gel polymerization method. In this process, a gel containing Ba, Sn and Y cations has been obtained by the polymerization of acrylic acid using N,N′-methylene bis-acrylamide as a cross-linking reagent and hydrogen peroxide as an igniting reagent. The gel was calcined at 1200 °C, giving rise to the Ba₂SnYO_5.5 single-phase nanopowders with the grain size ranging from 50 nm to 70 nm. Nanopowders were sintered at 1150 °C by spark plasma sintering (SPS) to obtain dense nanostructure materials (> 95%) containing grains whose size ranges between 70 nm and 100 nm. Nanostructured Ba₂SnYO_5.5 shows a good chemical stability in wet atmosphere. However, its protonic conductivity decreases compared with that of microcrystallin Ba₂SnYO_5.5 ceramics.     

Effect of Ti5Si3 on wear properties of Ti3Si(Al)C2

Near-fully dense Ti₃Si(Al)C₂/Ti₅Si₃ composites were synthesized by in situ hot pressing/solid–liquid reaction process under a pressure of 30 MPa in a flowing Ar atmosphere at 1580 °C for 60 min. Compared to monolithic Ti₃Si(Al)C₂, Ti₃Si(Al)C₂/Ti₅Si₃ composites exhibit higher hardness and improved wear resistance, but a slight loss in flexural strength (about 26% lower than Ti₃Si(Al)C₂ matrix). In addition, Ti₃Si(Al)C₂/Ti₅Si₃ composites maintain a high fracture toughness (K_IC = 5.69–6.79 MPa m^1/2). The Ti₃Si(Al)C₂/30 vol.%Ti₅Si₃ composite shows the highest Vickers hardness (68% higher than that of Ti₃Si(Al)C₂) and best wear resistance (the wear resistance increases by 2 orders of magnitude). The improved properties are mainly ascribed to the contribution of hard Ti₅Si₃ particles, and the strength degradation is mainly due to the lower Young's modulus and strength of Ti₅Si₃.     

Effects of grinding and firing conditions on CaAl2Si2O8 phase formation by solid-state reaction of kaolinite with CaCO3

The effects of grinding and firing conditions on CaAl₂Si₂O₈ phase formation by solid-state reaction of kaolinite with CaCO₃ were investigated by differential thermal analysis (DTA)–thermogravimetry (TG), X-ray powder diffraction (XRD) and ²⁹Si and ²⁷Al MAS NMR. Unground and ground samples showed similar crystallization behavior at about 850 °C, and the crystallizing temperature was relatively unaffected by grinding. On the other hand, the crystalline products were strongly influenced by the grinding. Gehlenite (Ca₂Al₂SiO₇) was the dominant phase in the unground samples but layer-structured CaAl₂Si₂O₈ was dominant in the ground samples, together with a small amount of anorthite, which is the stable phase. The amount of anorthite gradually increased with higher firing temperature, the sample fired at 1000 °C being almost completely anorthite. Grinding treatment before firing was effective in accelerating the decomposition of CaCO₃ and extending the temperature range for the formation of CaAl₂Si₂O₈, a phase with local structure similar to that of layered CaAl₂Si₂O₈.     

TiO2 photocatalyst loaded on hydrophobic Si3N4 support for efficient degradation of organics diluted in water

TiO₂ photocatalyst loaded on Si₃N₄ (TiO₂/Si₃N₄) was prepared by a conventional impregnation method and its photocatalytic performance for the degradation of organics (2-propanol) diluted in water was compared with that of TiO₂ photocatalysts (TiO₂/SiO₂, TiO₂/Al₂O₃, and TiO₂/SiC) loaded on various types of supports (SiO₂, Al₂O₃, and SiC). The formation of the well-crystallized anatase phase of TiO₂ was observed on the calcined TiO₂/Si₃N₄ photocatalyst, while a small anatase phase of TiO₂ was observed on the TiO₂/SiC photocatalyst and amorphous TiO₂ species was the main component on the TiO₂/SiO₂ and TiO₂/Al₂O₃ photocatalysts. The measurements of the water adsorption ability of photocatalysts indicated that the TiO₂/Si₃N₄ photocatalyst exhibited more hydrophobic surface properties in comparison to other support photocatalysts. Under UV-light irradiation, the TiO₂/Si₃N₄ photocatalyst decomposed 2-propanol diluted in water into acetone, CO₂, and H₂O, and finally, acetone was also decomposed into CO₂ and H₂O. The TiO₂/Si₃N₄ photocatalyst showed higher photocatalytic activity than TiO₂ photocatalyst loaded on other supports. The well-crystallized TiO₂ phase deposited on Si₃N₄ and the hydrophobic surface of Si₃N₄ support are important factors for the enhancement of photocatalytic activity for the degradation of organic compounds in liquid-phase reactions.     

Thermal shock resistance of in situ formed Si2N2O–Si3N4 composites by gelcasting

Thermal shock resistance of Si₂N₂O–Si₃N₄ composites was evaluated by water quenching and subsequent three-point bending tests of strength diminution. Si₂N₂O–Si₃N₄ composites which was prepared with in situ liquid pressureless sintering process using Yb₂O₃ and Al₂O₃ powders as sintering additives by gelcasting showed no macroscopic cracks and the critical temperature difference (ΔT_c) could be up to 1400 °C. A mass of pores existed in the sintered body and the irregular shaped fibers extended from the pores increased the thermal shock property.     

The preparation of Mg3Si2O5(OH)4 nanotubes under solvothermal conditions

With magnesium carbonate hydroxide and nanoporous silica as the starting materials, chrysotile (Mg₃Si₂O₅(OH)₄) nanotubes were prepared by using a solvothermal method at 400^∘C within four hours. This new method needs no strong alkali medium and the reaction time is very short. EDX analysis showed a molar ratio of 3Mg:2Si:9O of the product. Selected Area Electron Diffraction (SAED) pattern indicated that the tube axis is along [100] direction. HRTEM image showed the nanotubes were multi-walled and the distance between the two close layers was about 0.75 nm, which is very near to the distance of {001} planes. Thus, combining the results of SAED and HRTEM, we can conclude that the {001} planes of serpentine roll up along the [100] direction to form the tubular structure. The effects of various reaction conditions and the formation mechanism were also discussed.     

Fluorescence improvement of Ba1.3Ca0.7SiO4:Eu,Mn phosphors via Dy addition and their color-tunable properties

A series of novel single-phase white phosphors Ba_1.3Ca_0.69−x−ySiO₄:0.01Eu²⁺,xMn²⁺, yDy³⁺ were synthesized by the solid-state method. The excitation spectra of these phosphors exhibit a broad band in the range of 260–410 nm, which can meet the application requirements for near-UV LED chips (excited at 350–410 nm). The emission spectra consist of two broad bands positioned around 455 nm and 596 nm, which are assigned to 5d→4f transition of Eu²⁺, and ⁴T₁→⁶A₁ transition of Mn²⁺, respectively. The luminescence intensity of phosphors enhances obviously by doping Dy³⁺ ions, and the intensity of two bands reaches an optimum when Dy³⁺ amounts to 2 mol%. In addition, thermoluminescence investigation of phosphor was conducted, getting two shallow trap defects with activation energy of 0.43 eV and 0.45 eV, which demonstrates the energy transfer mechanism of Dy–Eu through the process of hole and electron traps. By precisely tuning the Mn²⁺ content, an optimized white light with color rendering index (CRI) of R_a=84.3%, correlated color temperature (CCT) of T_c=8416 K and CIE chromaticity coordinates of (0.2941, 0.2937) is generated. The phosphor could be a potential white phosphors for near-UV light emitting diodes.     

Dielectric and mechanical properties of porous Si3N4 ceramics prepared via low temperature sintering

Borophosphosilicate bonded porous silicon nitride (Si₃N₄) ceramics were fabricated in air using a conventional ceramic process. The porous Si₃N₄ ceramics sintered at 1000–1200 °C shows a relatively high flexural strength and good dielectric properties. The influence of the sintering temperature and contents of additives on the flexural strength and dielectric properties of porous Si₃N₄ ceramics were investigated. Porous Si₃N₄ ceramics with a porosity of 30–55%, flexural strength of 40–130 MPa, as well as low dielectric constant of 3.5–4.6 were obtained.     

Improvement of the high temperature properties of the LiYO2–Si3N4 system by removing residual Li

Effects of the vaporization of residual Li on the microstructure, oxidation behavior and high temperature properties of a low-temperature pressureless sintered Si₃N₄ using LiYO₂ additive were investigated. The oxidation and creep resistance of the Si₃N₄ was improved after an annealing at 1650 °C because residual Li, which deteriorated the high temperature properties of the Si₃N₄, could be mostly removed. The high temperature deformation of the Si₃N₄ was strongly suppressed after the annealing treatment. The annealed specimens retained 64% of the room temperature strength at 1300 °C in air. The present investigation reports a method to improve the high temperature properties of Si₃N₄.     

Microwave-assisted solvothermal preparation and photoluminescence properties of Y2O3:Eu phosphors

Europium doped yttrium oxide phosphors were synthesized by a rapid microwave-assisted solvothermal method. The microwave processing time for synthesizing the precursors of Y₂O₃:Eu³⁺ powders was as short as 5 min. After calcination at 600 °C, a well-crystallized pure phase of Y₂O₃:Eu³⁺ was obtained. The morphology of the precipitated powders was spherical and composed of nano-sized grains. As the microwave irradiation time was increased, the average particle size of the spherical powders increased, and the crystallinity of heat-treated powders was also enhanced. The synthesized powders retained the spherical morphology after heating treatments. An intense red emission at 611 nm was assigned to the ⁵D₀-⁷F₂ transition of Eu³⁺.     

Luminescent properties of a novel afterglow phosphor Sr3Al2O5Cl2:Eu,Ce

A series of Eu²⁺ and Ce³⁺ doped/co-doped Sr₃Al₂O₅Cl₂ afterglow phosphors that presented various bright colors were successfully synthesized via high temperature solid state reaction. The structure and luminescence properties of the obtained samples were characterized by X-ray powder diffraction (XRD), photoluminescence (PL) spectra and decay curves as well as the thermoluminescence (TL) glow curves. The XRD results showed that all the phase could be indexed to the orthorhombic structure with the space group P2₁2₁2₁. After being exposed to a 254 nm or 365 nm mercury lamp, blue/yellow-orange afterglow emissions with broad bands peaking around 620 nm/435 nm, which were ascribed to the characteristic 4f⁶5d–4f⁷/5d¹–4f¹ transitions of Eu²⁺/Ce³⁺, could be observed in phosphors of Sr₃Al₂O₅Cl₂:Eu²⁺/Sr₃Al₂O₅Cl₂:Ce³⁺, respectively. Because of the overlap spectral range between the Sr₃Al₂O₅Cl₂:Eu²⁺ and Sr₃Al₂O₅Cl₂:Ce³⁺ phosphors, the energy transfer (ET) from Ce³⁺ to Eu²⁺ occurred. The related ET process was discussed in detail. Moreover, the incorporation of Ce³⁺ could significantly prolong the afterglow duration of Sr₃Al₂O₅Cl₂:Eu²⁺ phosphor, which was due to the increase of trap concentration. Consequently, 6 h of the afterglow duration could be observed in Sr₃Al₂O₅Cl₂:1.0%Eu²⁺, 0.5%Ce³⁺ sample, exhibiting much longer than that of Sr₃Al₂O₅Cl₂: 1.0%Eu²⁺ (3 h). From the afterglow decay curves and the fitting results, the optimal concentration of Ce³⁺ for the enhanced afterglow property was experimentally determined to be 0.5%.     

Synthesis and optical characterization of novel Sr3Ga2O6:Eu phosphor

Alkaline earth metal gallets have been identified as an important ceramic material. The crystal chemistry of many of these gallets is well explored; however, very rare studies regarding optical properties of rare earth (RE) ions doped in such gallets, particularly in Sr₃Ga₂O₆ host, have been carried out. The present study reports on synthesis and characterization of novel Sr₃Ga₂O₆:Eu³⁺ phosphors. The phosphors have been synthesized using a conventional solid state reaction method. Crystal structure, morphology and luminescence properties (excitation, emission and CIE coordinate) of these phosphors have been studied as a function of sintering temperature and Eu³⁺ concentration. X-ray diffraction study reveals that the phosphor sintered at low temperature (900 °C) contains an impurity phase which is removed at higher sintering temperatures and results into cubic crystalline phase of Sr₃Ga₂O₆. Particle size of the phosphor increases with an increase in sintering temperature which results to a red shift in the peak position of excitation band lying in a broad range from 250 to 370 nm. Optimum emission intensity is attained for 0.12 mol% concentration of Eu³⁺ ions; above this concentration, a quenching in emission intensity is observed.     

XANES study of polyaniline-V2O5 and sulfonated polyaniline-V2O5 nanocomposites

In this work, two materials for secondary lithium battery cathodes formed by polyaniline-V₂O₅ and sulfonated polyaniline-V₂O₅, which have a higher charge capacity than the V₂O₅ xerogel, were synthesized. X-ray absorption and Fourier transform infrared spectroscopies were employed to analyze the short-range interactions in these materials. Based on these experiments, it was possible to observe significant differences in the symmetry of the VO₅ units, and this was attributed to the intimate contact between V₂O₅ and the polymers, and to some flexibility of the VO₅ square pyramids due to the low range order of the nanocomposites.     

Fabrication of SiCN-Sc2Si2O7 coatings on C/SiC composites at low temperatures

SiCN-Sc₂Si₂O₇ environmental barrier coatings were fabricated on the surface of C/SiC composites at low temperatures by adding Li₂CO₃ as sintering aids. With this addition, the fabrication temperature could be lowered about 100-200 °C. The shrinkage of the polysilazane-Sc₂Si₂O₇ bars with and without Li₂CO₃ was tested by dilatometer. The results indicate that the shrinkage speed of the polysilazane-Sc₂Si₂O₇ bar with Li₂CO₃ is faster than the one without Li₂CO₃, indicating that the Li₂CO₃ greatly promotes the sintering of polysilazane-Sc₂Si₂O₇. Water-vapor corrosion behavior of the SiCN-Sc₂Si₂O₇ coated C/SiC composites was carried out at 1250 °C. The results reveal that the SiCN-Sc₂Si₂O₇ coatings can effectively protect the C/SiC composites. The corrosion resistance of SiCN-Sc₂Si₂O₇ coatings is not degraded by adding Li₂CO₃.     

Si3N4 with comparable permeability to SiC

Porous sintered reaction-bonded silicon nitrides (SRBSNs) with comparable permeability to SiC were fabricated using presintered Si-additive mixture granules. By increasing the granule strength through the adjustment of the presintering conditions, the strengthened sturdiness of the granules led to an increase in the pore channel size. Porous SRBSNs with ≥60% porosity were achieved without employing a pore former due to the formation of intragranular narrow pore channels as well as intergranular wide pore channels. As the specific pore area of the developed SRBSN is nearly 26 times that of SiC with similar permeability, superior filtering efficiency for nano-sized particulate matter is expected.     

The influence of mechanochemical activation on combustion synthesis of Si3N4

This study addresses itself in the performance of Si₃N₄ combustion synthesis, occurred in the presence of Si₃N₄ and NH₄Cl powders in N₂ atmosphere of 6 MPa. Mechanochemical activation of Si powder, achieved via high-energy attrition milling up to 24 h, increases the intensity and the efficiency of the reactions between Si and N₂ as well as combustion temperature. Benign processing conditions, anticipated with lower mechanochemical activation of Si powder, low N₂ pressures, and low combustion temperatures, favor formation of α-Si₃N₄.     

Mixed oxides Ca2Fe2O5 and Ca2Co2O5 as anode materials for Li-ion batteries

The electrochemical performance of mixed oxides, Ca₂Fe₂O₅ and Ca₂Co₂O₅ for use in Li-ion batteries was studied with Li as the counter electrode. The compounds were prepared and characterized by X-ray diffraction and SEM. Ca₂Fe₂O₅ showed a reversible capacity of 226 mAh/g at the 14th cycle and retained 183 mAh/g at the end of 50 cycles at 60 mA/g in the voltage window 0.005-2.5 V. A reversible capacity in the range, 365-380 mAh/g, which is stable up to 50 charge-discharge cycles is exhibited by Ca₂Co₂O₅ in the voltage window, 0.005-3.0 V and at 60 mA/g. This corresponds to recycleable moles of Li of 3.9±0.1 (theoretical: 4.0). Significant improvement in the cycling performance and attainable reversible capacity were noted for Ca₂Co₂O₅ on cycling to an upper cut-off voltage of 3.0 V as compared to 2.5 V. Coulombic efficiency for both compounds is >98%. Electrochemical impedance spectroscopy (EIS) data clearly indicate the reversible formation/decomposition of polymeric surface film on the electrode surface of Ca₂Co₂O₅ in the voltage window, 0.005-3.0 V. Cyclic voltammetry results compliment the galvanostatic cycling data.     

Dielectric and ferroelectric properties of Ba0.8Sr0.2Ti1−5x/4NbxO3 ceramics

Ba_0.8Sr_0.2Ti_1−5x/4Nb_xO₃ ceramics, x = 0, 0.01, 0.05, 0.10, were fabricated by conventional solid-state reaction. With increasing niobium content the ferroelectric phase transition temperature decreases linearly, and the dispersivity of the transition increases. Niobium B-site decreases transition temperature more pronounced than Sr²⁺ at A-site. The heterovalent substitution of Nb⁵⁺ in low content causes local defect dipole, while more substitutions introduce disorder to disturb the long-range dipole correlation. Ba_0.8Sr_0.2Ti_1−0.5/4Nb_0.1O₃ ceramic shows weak ferroelectric loop at room temperature far from its transition temperature, 153 K.     

Effect of carboxymethyl cellulose addition on the properties of Si3N4 ceramic foams

The effect of carboxymethyl cellulose (CMC) addition on the preparation of Si₃N₄ ceramic foam by the direct foaming method was investigated. The addition of CMC in the foam slurry can reduce the surface tension, increase the viscoelasticity of foams, and improve their stability and fluidity. The foam ceramics show low shrinkage during drying owing to the CMC and the gelation of acrylamide monomers. The surface structure of dried foam is uniform, and there are no macropores and cracks on the surface. The sintered Si₃N₄ foam ceramics have very uniform pore distribution with average pore size of about 16 μm; the flexure strength is as high as 3.8–77.2 MPa, and the porosity is about 60.6–82.1%.     

Microstructure and mechanical properties of Si3N4/42CrMo joints brazed with TiNp modified active filler

Si₃N₄ ceramic/42CrMo steel joints were obtained by employing TiNp modified Ag–Cu–Ti active filler and subsequently the effect of TiNp content on the microstructure and mechanical properties of the joints was investigated. Microstructural examination revealed that TiN+Ti₅Si₃ reaction layer was adjacent to the Si₃N₄ ceramic while a TiC reaction layer was close to the steel substrate. With the increase of TiNp content, more fine grains and less Ag–Cu eutectic appeared in the joint and the reaction layers near the two base materials became thinner. The flexural strength of the joint obtained by four-point bending test climbed about 100% with the optimum TiNp content of 5 vol%, comparing with the case without TiNp. Thermal stress distributions in the joint were analyzed using finite element modeling computations, which accorded well with the bending test results.