Synthesis and characterization of Sr2+ and Mg2+ doped LaGaO3 by co-precipitation method followed by hydrothermal treatment for solid oxide fuel cell applications

In the present study, LSGM (La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8) powder has been synthesized using precipitation route followed by hydrothermal treatment. Quantitative phase analyses of different powders, have been done by Rietveld analyses of the XRD data and they reveal formation of single phase orthorhombic LSGM at 1400 °C, 8 h. Morphology of the calcined powder and microstructure of the sintered pellets are observed by transmission electron microscope (TEM) and scanning electron microscope (SEM), respectively. Thermal analysis has been carried out to find out the thermal expansion co-efficient. Successive electrical characterization of the 99% dense sintered pellet has been done by impedance spectroscopic analysis. The diffused semicircles observed in the Nyquist plots have been simulated as (RQ)(RQ) circuit and the total ionic conductivity obtained is found to be the highest for LSGM synthesized by similar routes.     

Effect of Sr/Ca substitution on phase structure and photoluminescence properties of micro-SrxCa1−xAlSiN3: Eu2+ phosphor for high CRI white LEDs

Sr_xCa_1−xAlSiN₃: Eu²⁺ phosphors were prepared by using the high temperature solid state reaction in a 1.1 Mpa N₂ atmosphere. The phase structures, photoluminescence (PL) properties, and chromaticity properties of the phosphors affected by Sr/Ca Substitution have been investigated in detail. With increasing Sr content (x value), the crystal grain became bigger and the average grain size increased from 5 µm to 10 µm. PL emission bands of Sr_xCa_1−xAlSiN₃: Eu²⁺ showed a blue-shift from 660 (x=0) to 617 nm (x=0.8), the shoulder of the excitation spectra around 550 nm showed a slightly blue-shift and decay lifetime shortened from 776.96 (x=0.2) to 642.35 ns (x=0.8). Both the emission and excitation intensity of peak position increased with Sr content increased. The ideal white light with high CRI (Ra>88) can be obtained by mixing the Sr_xCa_1−xAlSiN₃: Eu²⁺ phosphors and commercial green phosphors with appropriate proportion of the components.     

Synthesis and characterization of Ba1−xSrxTiO3 nanopowders by citric acid gel method

Stoichiometric and monophasic Ba_1−xSr_xTiO₃ (x = 0.3) nanopowders were successfully prepared by the citric acid gel method using barium nitrate, strontium nitrate and tetra-n-butyl titanate as Ba, Sr, Ti sources and citric acid as complexing reagent. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), infrared (IR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the thermal decomposition behavior, the crystallization process and the particle size and morphology of the calcined powders. The results indicated that single-phase and well-crystallized Ba_1−xSr_xTiO₃ (x = 0.3) nanopowders with particle size around 80 nm could be obtained after calcining the dried gel at 950 °C for 2 h.     

Continuous hydrothermal synthesis of Ca1−xSrxTiO3 solid-solution nanoparticles using a T-type micromixer

In this work, a continuous hydrothermal synthesis method in supercritical water was applied to environmentally benign production of Ca_1−xSr_xTiO₃ (x = 0.0–1.0) solid-solution nanoparticles as key materials in conducting, electric, magnetic, and photocatalytic fields. A T-type micromixer (330 μm id) was introduced for rapid heating of stating solutions of Ca(NO₃)₂, Sr(NO₃)₂, and TiO₂ sol using turbulent flow mixing with preheated NaOH aqueous solution and also for exact control of reaction temperature. At 673 K and 30 MPa for 5.0 s mean residence time, Ca_1−xSr_xTiO₃ solid-solution nanoparticles having crystallite diameters of around 20 nm with monomodal diameter distributions were obtained without byproducts and production of CaTiO₃ and SrTiO₃ separately over the whole composition range. Effects of NaOH molality, Ca and Sr compositions in the starting solutions, and mean residence time on the reaction were examined. The results showed that TiO₂ sol dissolution and Ca_1−xSr_xTiO₃ precipitation were almost finished within 0.25 s mean residence time, and after that Ostwald ripening proceeded.     

Sol–gel synthesis of La0.6Sr0.4CoO3−x and Sm0.5Sr0.5CoO3−x cathode nanopowders for solid oxide fuel cells

Nano-powders of La_0.6Sr_0.4CoO_3?x (LSC) and Sm_0.5Sr_0.5CoO_3?x (SSC) compositions, which are being investigated as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) with La(Sr)Ga(Mg)O_3?x (LSGM) as the electrolyte, were synthesized by low-temperature sol–gel method using metal nitrates and citric acid. Thermal decomposition of the citrate gels was followed by simultaneous DSC/TGA methods. Development of phases in the gels, on heat treatments at various temperatures, was monitored by X-ray diffraction. Sol–gel powders calcined at 550–1000 °C consisted of a number of phases. Single perovskite phase La_0.6Sr_0.4CoO_3?x or Sm_0.5Sr_0.5CoO_3?x powders were obtained at 1200 °C and 1300 °C, respectively. Morphological analysis of the powders calcined at various temperatures was done by scanning electron microscopy. The average crystallite size of the powders was ～15 nm after 700 °C calcinations and slowly increased to 70–100 nm after heat treatments at 1300–1400 °C.     

Phase and microstructure evolution during hydrothermal solidification of clay–quartz mixture with marble dust source of reactive lime

Marble dust (200 μm size), air and water polluting waste, generated by the marble cutting industries has been used as cheap source of lime for hydrothermal solidification of clay–quartz mixtures at different saturated steam pressure (0.525–1.225 MPa). Marble dust was calcined at two different temperatures (900 and 1000 °C) and then added to the clay–quartz mixture, with clay/(clay + quartz) ratio 0.9, at two different amounts. The hydrothermally solidified samples were characterized by bulk density, apparent porosity, flexural strength, porosimetric study, phase and microstructural analysis and the results were compared against similarly treated chemical grade CaCO₃ containing compositions. Tobermorite [Ca₅(Si₆O₁₈H₂)·4H₂O] and hydrogarnet [Ca₃Al₂(SiO₄)(OH)₈] were the major phases formed in the treated samples, which are responsible for the strength development. Microstructural study supports the generation of these hydrated phases. Porosimetric study reveals that higher exposed area of the chemical grade CaCO₃ containing composition results higher extent of hydrothermal reaction and higher strength. Though marble dust containing compositions showed relatively lower strength values, the combined properties of the hydrated products are suitable as new building material where such mesopore containing products with inherent micron range pore size distribution are important parameters.     

Sono-synthesis of nanohydroxyapatite: Effects of process parameters

Hydroxyapatite (HA) [Ca₁₀(PO₄)₆(OH)₂] is a bioactive ceramic with excellent osteoconductive properties. This characteristic helps HA to be integrated into the bone without provoking an immune reaction, thus making it a useful biocompatible material for load bearing bone implant. In this study, nanohydroxyapatite (NHA) was synthesised using a precipitation method assisted with ultrasonication. The process parameters such as ultrasonic time (t) (10–30 min), ultrasonic amplitude (A) (50–70%), solution temperature (T) (50–90 °C), and solution pH (7–9) were varied on the basis of single factor and their effects on NHA synthesis was investigated. Besides that, the effect of calcination on the NHA powder morphology was also studied by varying the calcination time (2, 4 and 6 h) and temperature (400, 800 and 1200 °C). The characterisations of the synthesised NHA powder were conducted using thermogravimetric analysis (TGA), field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), transmission electron microscope (TEM), zeta-sizer and Fourier transform infrared spectroscopy (FTIR). It was found that nano-sized HA particles can be produced at optimum set of process parameters of t=25 min, T=90 °C, A=65%, and pH=8. Results revealed that the thermal stability, morphology and crystallinity of the NHA powder was further improved by calcinating the powder at optimum temperature and time of 800 °C and 2 h, respectively.     

Sr-doped forsterite nanopowder: Synthesis and biological properties

Nanoscale forsterite (Mg₂SiO₄) has recently been proposed for bone tissue engineering application. Due to the special role of strontium (Sr) in bone remodeling, the stimulation of bone formation and reduction in bone resorption, the modification of forsterite by doping with Sr is expected to increase bioactivity and biocompatibility. The aim of this study was to incorporate Sr (0, 0.05, 0.1, 0.2 and 0.4 at%) into forsterite using sol-gel method and to investigate the effect of Sr content on the phase composition, in vitro apatite-formation ability as well as osteoblast-like MG63 viability. Results demonstrated that while forsterite was the main phase of all Sr-doped forsterite nanopowders, Sr₂MgSi₂O₇, MgO and MgSiO₃ were present as the minor phases depending on the Sr content. Moreover, the presence of Sr atom influenced the crystallite and particle size as well as lattice parameters of the forsterite powder, while did not significantly change the morphology of particles. Noticeably, the incorporation of Sr up to 0.2 at% enhanced the average crystallite size (from 25.3 nm to 45.9 nm) and particle size (31.0 ± 3.9 nm to 62.9 ± 11.8 nm) of pure forsterite powder. Additionally, according to the Rietveld refinement, the incorporation of Sr up to 0.2 at% increased the lattice parameters of forsterite more than 0.1%, depending on the Sr content. In vitro bioactivity assessment in simulated body fluid (SBF) revealed while all Sr-forsterite samples possessed greater bioactivity than pure forsterite nanopowder, the incorporation of 0.1 at% Sr revealed improved bioactivity compared to other Sr-forsterite samples. However, according to MTT assay, while all forsterite-based ceramics significantly improved the cell proliferation compared to tissue culture plate (TCP) and forsterite nanopowder, Sr-forsterite nanopowders consisting of 0.05–0.1 at% Sr revealed a considerably promoted cell proliferation. In conclusion, Sr-forsterite nanopowder could be a promising candidate for bone tissue engineering and reconstruction of bone defects such as osteoporosis.     

Mechanical properties of nano-grain SiO2 glass prepared by spark plasma sintering

Silicon carbide (SiC) layers were deposited on silica (SiO₂) glass powder by rotary chemical vapor deposition (RCVD) to form SiO₂ glass (core)/SiC (shell) powder; this powder was consolidated by spark plasma sintering (SPS). SiO₂ glass powder with a particle size of 250 nm was coated with 5–10-nm-thick SiC layers. The resultant SiO₂ glass (core)/SiC (shell) powder was consolidated to form a nano-grain SiO₂ glass composite at a relative density above 90% by SPS in the sintering temperature range of 1573–1823 K. The Vickers hardness and fracture toughness of the SiO₂ glass composite at 1723 K were found to be 14.2 GPa and 5.4 MPa m^1/2, respectively.     

Wet chemistry approach to the preparation of tantalum-doped hydroxyapatite: Dopant content effects

Tantalum-doped hydroxyapatite (Ta-doped HA) nanopowders with different Ta contents were synthesized by a wet-chemical precipitation route. The structure modification and charge compensation mechanism were investigated by various characterization techniques. Due to the smaller size of tantalum ions compared to the Ca²⁺ size, it was assumed that the tantalum ions occupy either the Ca²⁺ and/or the interstitial positions in the HA lattice, where the charge imbalance from to this substitution was compensated by the Ca²⁺ vacancies. From the XRD patterns, the as-synthesized nanopowders were poorly crystalline apatite in the absence and presence of different dopant contents. The hexagonal HA and tricalcium phosphate (β-TCP) phases as biphasic calcium phosphate mixtures were formed after heating at 900 °C. In addition to the β-TCP phase, minor extra phases such as calcium oxide (CaO) and calcium pyrophosphate (Ca₂P₂O₇) were identified from the HA decomposition. The FTIR results indicated that the decrease of structural hydroxyl groups depended on both tantalum oxyanions and carbonate contents. In the XPS profile, the Ta 4 f peak of the doped sample could be decomposed into four main components, which showed different oxidation states for tantalum (TaO₂ oxide). According to the TEM observations, the doped calcined powder at 900 °C was composed of uniform nanoneedles with an average length and width of 120 ± 50 and 10 ± 5 nm, respectively.     

Synthesis and photocatalytic performance of hollow sphere particles of SiO2-TiO2 composite of mesocellular foam walls

Hollow Microspheres of SiO₂-TiO₂ photocatalysts whose walls are made up of mesoporous cellular foams were synthesized with the aid of hexane as a swelling agent and P123 as a pore template by an emulsion templating method. Pore structure of materials and crystal phase of titanium oxide was tailored by hydrothermal and calcination temperature during synthesis of samples. The samples were characterized with field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), N₂ adsorption–desorption experiments, X-ray photoelectron spectroscopy (XPS) and X ray diffraction (XRD) techniques. The effect of pore structure and titania phase on photoactivity were evaluated by methylene blue (MB) degradation test under UV light as well. Results showed that hydrothermal temperature during synthesis process has a significant effect on pore and window sizes of mesostructured cellular foam. Interestingly, for the sample hydrothermally treated at higher temperature (130 °C), anatase to rutile transformation was avoided after calcination treatment as high as 800 °C. The highest photocatalytic activity was detected from the sample hydrothermally treated at 130 °C and calcined at 800 °C for which the highest degree of crystallinity and anatase phase as well as enhanced pore connectivity was obtained.     

Low-temperature synthesis of CdWO4 nanorods via a hydrothermal method

Cadmium tungstate (CdWO₄) nanorods were successfully synthesized via a hydrothermal process and characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and photoluminescent spectra techniques (PL). A pure monoclinic phase of well-crystallized CdWO₄ nanorods, with lengths of 250–400 nm and widths of 30–60 nm, could be readily synthesized at as low temperature as 70 °C.The CdWO₄ nanorods showed a PL emissions peak at 435 nm.     

Densification of SiO2–cBN composites by using Ni nanoparticle and SiO2 nanolayer coated cBN powder

Cubic boron nitride (cBN) powder was coated with Ni nanoparticle and SiO₂ nanolayer (abbreviated as cBN/Ni and cBN/SiO₂, respectively) by rotary chemical vapor deposition (RCVD), and compacted with SiO₂ powder by spark plasma sintering at 1473–1973 K for 0.6 ks. The effects of Ni and SiO₂ coatings on the densification, phase transformation of cBN and hardness of SiO₂–cBN composites were compared. The phase transformation of cBN to hBN was identified at 1973 K in SiO₂–cBN/SiO₂ composites, 300 K higher than that in SiO₂–cBN/Ni composites, indicating that SiO₂ retarded the transformation of cBN. The relative density of SiO₂–cBN/SiO₂ with 50 vol% cBN sintered at 1873 K was 99% with a hardness of 14.5 GPa.     

Preferential occupancy of metal ions in the hydroxyapatite solid solutions synthesized by hydrothermal method

The solid solutions in the systems of Ca-Cd HAp [Ca_10−xCd_xHAp (x = 0–10)], Ca-Sr HAp [Ca_10−xSr_xHAp (x = 0–10)] and Ca-Pb HAp [Ca_10−xPb_xHAp (x = 0–10)], were successfully synthesized at 200 °C for 12 h under hydrothermal conditions. The site of the metal ions in the solid solutions was analyzed by the Rietveld method. The results of the Rietveld analysis indicated that the metal ions of Pb²⁺, Sr²⁺, and Cd²⁺ all preferentially occupied M (2) sites in the apatite structure. The preferential occupancy of the metal ions in M (2) sites were explained mainly by their ionic radius and electronegativity.     

Reactivity and phase composition of Ca2SiO4 binders made by annealing of alpha-dicalcium silicate hydrate

This study investigates the production of highly reactive dicalcium silicate Ca₂SiO₄ (C₂S).To that end, binders were synthesised by annealing of alpha-dicalcium silicate hydrate (α-C₂SH) between 400 and 800 °C. Two different heating sequences were tested. The phase compositions were determined by means of XRD. Depending on the annealing temperature and the heating conditions the cementitious materials consist of an X-ray-amorphous content as well as x-Ca₂SiO₄ (x-C₂S) and γ-Ca₂SiO₄ (γ-C₂S). The hydration kinetics of some selected binders were investigated by means of isothermal calorimetry. The specific reactivity of the phases produced by means of annealing was determined during the first 40 h of hydration by use of XRD and TGA.The resulting binders show the highest reactivity when low annealing temperatures (< 500 °C) were used. After 72 h, degrees of hydration of about 89% are achieved. The most reactive component is the X-ray-amorphous content, followed by x-C₂S.     

Structural characterization of plasma sprayed basalt–SiC glass–ceramic coatings

In the present study, the effect of SiC addition on properties of basalt base glass–ceramic coating was investigated. SiC reinforced glass–ceramic coating was realized by atmospheric air plasma spray coating technique on AISI 1040 steel pre-coated with Ni + 5 wt.%Al bond coat. Composite powder mixture consisted of 10%, 20% and 30% SiC by weight were used for coating treatment. Controlled heat treatment for crystallization was realized on pre-coated samples in argon atmosphere at 800 °C, 900 °C and 1000 °C which determined by differential thermal analysis for 1–4 h in order to obtain to the glass–ceramic structure. Microstructural examination showed that the coating performed by plasma spray coating treatment and crystallized was crack free, homogeneous in macro-scale and good bonded. The hardness of the coated samples changed between 666 ± 27 and 873 ± 32 HV_0.01 depending on SiC addition and crystallization temperature. The more the SiC addition and the higher the treatment temperature, the harder the basalt base SiC reinforced glass–ceramic coating became. X-ray diffraction analysis showed that the coatings include augeite [(CaFeMg)–SiO₃], diopside [Ca(Mg_0.15Fe_0.85)(SiO₃)₂], albite [(Na,Ca)Al(Si,Al)₃O₈], andesine [Na_0.499Ca_0.492(Al_1.488Si_2.506O₈] and moissanite (SiC) phases. EDX analyses support the X-ray diffraction analysis.     

Synthesis of La0.9Sr0.1Ga0.8Mg0.2O2.85 powder by gel combustion route with two-step doping strategy

A two-step doping strategy was applied to the synthesis of La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 (LSGM1020) powder by a gel combustion method. The Mg-doped LaGaO₃ powder was prepared in the first step, and Sr incorporation in the Mg-doped LaGaO₃ powder was done in the second step to obtain the final LSGM1020 powder. The two-step procedure is effective in preparing higher purity powders than the traditional one-step procedure. Rietveld refinement of X-ray powder diffraction (XRD) patterns shows that incorporation of Mg in LaGaO₃ in the first step enlarges the LaGaO₃ lattice: this facilitates the incorporation of Sr in the second doping step and thus high purity powder is obtained. Relatively phase pure LSGM1020 powder with only 3.1% of LaSrGaO₄ was obtained after calcination at 1300 °C for 5 h. Therefore, the two-step doping strategy is an effective procedure for the preparation of LSGM powders with high Sr- and Mg-doping levels.     

Synthesis and characterization of nanometric gadolinia powders by room temperature solid-state displacement reaction and low temperature calcination

Nanometric-sized gadolinia (Gd₂O₃) powders were obtained by applying solid-state displacement reaction at room temperature and low temperature calcination. The XRD analysis revealed that the room temperature product was gadolinium hydroxide, Gd(OH)₃. In order to induce crystallization of Gd₂O₃, the subsequent calcination at 600 ∼ 1200 °C of the room temperature reaction products was studied. Calculation of average crystallite size (D) as well as separation of the effect of crystallite size and strain of nanocrystals was performed on the basic of Williamson-Hall plots. The morphologies of powders calcined at different temperatures were followed by scanning electron microscopy. The pure cubic Gd₂O₃ phase was made at 600 °C which converted to monoclinic Gd₂O₃ phase between 1400° and 1600 °C. High-density (96% of theoretical density) ceramic pellet free of any additives was obtained after pressureless sintering at 1600 °C for 4 h in air, using calcined powder at 600 °C.     

Color tuning in (Ca1−xSrx)8MgLu(PO4)7:Eu2+,Mn2+ phosphor via host composition design and energy transfer

In this work, a series of Eu²⁺-doped (Ca_1−xSr_x)₈MgLu(PO₄)₇ and Eu²⁺/Mn²⁺-codoped Ca_6.5Sr_1.5MgLu(PO₄)₇ phosphors were prepared via the combustion-assisted solid-state reaction process. XRD patterns and Rietveld refinements were used to verify the incorporations of Sr into Ca₈MgLu(PO₄)₇:Eu²⁺. Upon the same excitation wavelength of 380 nm, the emission peaks of Eu²⁺-doped (Ca_1−xSr_x)₈MgLu(PO₄)₇ (0≤x≤1) phosphors red-shifted from 453 to 519 nm with increasing Sr/Ca ratio. The red-shift of the Eu²⁺ emission with increasing Sr/Ca ratio was ascribed to the change of Eu²⁺ emission at different lattice sites. With variation of the Mn²⁺ content, the emission color of Eu²⁺/Mn²⁺ codoped Ca_6.5Sr_1.5MgLu(PO₄)₇ phosphors exhibited the luminescence tunable from greenish blue to white and eventually to red. The energy transfer from Eu²⁺ to Mn²⁺ in Ca_6.5Sr_1.5MgLu(PO₄)₇ host matrix was demonstrated to be of a resonant type via a dipole- dipole mechanism with the critical distance of ∼16.7 Å. By the Sr substitution for Ca and properly tuning by the relative composition change of Eu²⁺/Mn²⁺, chromaticity coordinates of (0.329, 0.326) can be reached at near UV light excitation. The combination of host composition design and energy transfer may provide a novel strategy to obtain white light and tunable luminescence.     

Processing and mechanical properties of hydroxyapatite pieces obtained by the gelcasting method

The aim of this work was to prepare hydroxyapatite (HA) pieces, by the gelcasting method, with appropriate mechanical properties for using as bone implants. The mechanical properties of the obtained pieces were evaluated. The influence of the solid content on the rheological behaviour of the slurries and, in turn, on the mechanical properties, was investigated. Pieces were prepared by using slurries containing 55 and 60 vol.% of an HA calcined at 1200 °C and 47 vol.% of HA calcined at 1100 °C. HA powders calcined at lower temperature than 1100 °C were not suitable for producing concentrated slurries. The raw and calcined HA were characterised by XRD, FTIR, X-ray fluorescence and elemental analysis. The pieces sintered at different temperatures and times were characterised by XRD, and the sintering treatment applied to the specimens for mechanical testing (1300 °C for 24 h) was selected taking into account the phases present after sintering.The mechanical properties of the green bodies were higher than obtained by other methods and the ones of the final pieces were comparable to those obtained by other techniques. The results indicated that slurries with a solid content higher than a limit value (for which limited contraction occurs) must be used and that the rheological properties of the slurries play an important role in the mechanical properties of the resulting pieces.