Structural and spectroscopic properties of Nd:Y2Si2O7 phosphors

Phosphors of α-Y₂Si₂O₇ doped with Nd³⁺ ions were prepared using the sol–gel technique. Nano-sized crystalline phosphor powders were obtained by annealing the dried gels at 960 °C. The crystallization properties of the phosphor powders were determined from their XRD patterns. The α-Y₂Si₂O₇ phase was the only phase observed in all compositions. As the amount of amorphous SiO₂ in the composition was increased, the crystalline sizes and the widths of the size distribution curves were found to decrease from 17.8 nm to 10.6 nm and from 15.6 nm to 12.2 nm, respectively. The spectroscopic properties of the powders were studied by measuring the luminescence and the decay patterns of the ⁴F_3/2→ ⁴I_9/2 and ⁴F_3/2→ ⁴I_11/2 transitions between 50 K and 310 K. No appreciable effect of the crystallite sizes on the average lifetime of the ⁴F_3/2 level was observed at temperatures below 100 K. The effect of temperature, however, becomes relevant above 100 K as the size of α-Y₂Si₂O₇ nano-crystal becomes smaller.     

Synthesis of LiMn2O4 from a gelled ovalbumin matrix

A simple and inexpensive route for the preparation of LiMn₂O₄ using ovalbumin (egg white) as a gelating agent is described. Gelation of freshly extracted ovalbumin was effected by changes in ionic strength brought about by the addition of nitrate precursors to an aqueous solution of ovalbumin and subsequent warming. This resulted in tiny pockets of precursor materials getting trapped in the matrix of the gelled ovalbumin. Heat treatment of the gelled mass yielded submicron-sized LiMn₂O₄ crystals at temperatures as low as 400 °C.     

Bioactive HA/TiO2 coating on magnesium alloy for biomedical applications

Magnesium alloys are new class of biodegradable alloys having many favourable properties to overcome the limitations of currently used biomedical alloys. Recently, several coatings have been developed to overcome their higher degradation rate. In this regard, a new attempt has been made to develop Hydroxyapatite and Hydroxyapatite/TiO₂ coatings on magnesium alloys to increase the biocompatibility and reduce the corrosion rate. The coated surfaces were characterized by Fourier-Transform Infrared Spectroscopy (FT-IR), X-ray Diffraction Analysis (XRD), Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray (EDX) Spectroscopy, Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). Contact angle measurements proved higher hydrophilic nature of HA/TiO₂ coating compared to HA coating. In-vitro studies showed that HA–TiO₂ coated alloy exhibited higher osteoinduction compared to HA coated alloy. Hydrogen evolution studies and corrosion studies confirmed greater reduction in degradation rate of HA/TiO₂ coated alloy. Vickers microhardness test also showed enhancement in mechanical strength of the composite coated alloy compared to HA coated alloy. Three point bend test depicted better adherence of the HA/TiO₂ coating compared to HA coating on the substrate. Cell culture studies proved higher cell attachment and proliferation on composite coated alloy by controlling the release of magnesium ions into the surrounding body tissue.     

Template assisted fabrication of TiO2 and WO3 nanotubes

Anodic aluminum oxide (AAO) templates with diameters of 200–500 nm were generated by anodizing a commercial aluminum (Al) substrate (99.7%) in 1 vol% phosphoric acid (H₃PO₄), with an applied voltage of 195 V. Titania and tungsten oxide nanotubes (NTs) were successfully grown on AAO template by the sol–gel process. Thermal gravimetric analyzer (TGA) curves showed that gel can be transfered to nanocrystalline particles after 19% weight loss of water molecule by evaporation. The results showed that the nanocrystalline TiO₂ NTs presented at 200 °C, and grains grew as temperature increased. At a temperature of 550 °C, the (101), (103), (004), (112), (200), (105), and (211) planes of anatase TiO₂ were detected clearly, whereas tungsten oxide NTs are amorphous after heat treatment at 200 °C or 300 °C. But the (110), (111), (002), (022), (222), and (004) planes of γ-WO₃ phase can be observed obviously after the heat treatment at 400 °C.     

Enhanced mechanical properties of 3 Y2O3 stabilized tetragonal ZrO2 incorporating tourmaline particles

The current study reports on the improvement of mechanical properties of 3?mol% Y₂O₃ stabilized tetragonal ZrO₂ (3Y-TZP) by introduction of tourmaline through ball milling and subsequent densification by pressureless sintering at 800, 1200, 1300, 1400?°C. Findings demonstrate that no matter which sintering temperature the 3Y-TZP ceramic containing 2?wt% tourmaline reach a maximum value in flexural strength and fracture toughness as compared to other composite ceramics. As the tourmaline content is 2?wt% and the sintering temperature is 1300?°C, the flexural strength and fracture toughness of the composite ceramics are the highest, increases of 36.2% and 36.6% over plain 3Y-TZP ceramic respectively. The unique microstructure was systematically investigated through X-ray diffraction, scanning electron microscopy, energy dispersive spectrum, and flourier transform-infrared. The strengthening and toughening mechanism of tourmaline in 3Y-TZP ceramic were also discussed.     

Preparation of Y2O3:Eu nanopowders via polymer complex solution method and luminescence properties of the sintered ceramics

Herein we presented polymer complex solution method for production of well crystalline europium doped Y₂O₃ nanopowders. Polyethylene glycol (PEG) of five different molecular weights is used both as a fuel and as a nucleation agent for the crystallization. Powders were cold-pressed and sintered to obtain ceramics. SEM images taken from ceramic pellets indicate formation of a dense structure, with a pronounced grain growth and low pore concentration. Luminescence emission spectra of powders and ceramics are similar, and in good agreement with theoretical data. Lifetimes of Eu³⁺⁵D₀ level in nanocrystalline powders are higher compared to one observed in bulk, confirming in this case theory of lifetime lengthening in nanophosphors due to the change of effective refraction index. As expected, lifetime values in ceramic samples decrease toward the value in bulk Y₂O₃. The optical filling factor is calculated from observed decay times, providing a measure of discrepancy between powder and bulk state regarding their luminescent properties.     

Glass transition and crystallization of ZnO-B2O3-SiO2 glass doped with Y2O3

The effect of Y₂O₃ on the glass transition kinetics, crystallization kinetics, phase separation and crystallization behavior of 60ZnO–30B₂O₃–10SiO₂ glass has been investigated by non-isothermal differential thermal analysis, scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). The glass transition activation energies E_g calculated by using both Kissinger and Moynihan model decrease from 668?kJ/mol to 573?kJ/mol for Kissinger model, and 682?kJ/mol to 587?kJ/mol for Moynihan model with the increase of yttrium oxide doping content from 0 to 6?mol%. And the glass crystallization kinetics parameters, crystallization activation energy E_c and Avrami exponent n stands for crystal growth, are also obtained on the basis of several well developed equations. Increase of about 58?kJ/mol in Ec values obtained by different theoretical equations is caused by addition of 6?mol% yttrium oxide into 60ZnO–30B₂O₃–10SiO₂ glass, and the Avrami exponent (n close to 2) suggests that crystal growth in 60ZnO–30B₂O₃–10SiO₂ glass doped with or without yttrium is mainly one-dimensional growth of crystals. The results on the phase separation and crystallization behavior occurred at 893?K and 993?K respectively for base and doped glass, are well consistent with the glass transition and crystallization kinetics results. Hence, addition of yttrium oxide into 60ZnO–30B₂O₃–10SiO₂ glass decrease the glass transition activation energy while increase the crystallization activation energy of glass, thereby the stability of glass structure is improved. Phase separation phenomenon and crystallization behavior occurred at glass surface provide some useful information for preparing glass ceramics with micro- or nano-crystals in surface.     

Synthesis,structural and luminescence properties of near white light emitting Dy-doped Y2CaZnO5 nanophosphor for solid state lighting

Novel near white light emitting Y₂CaZnO₅ (YCZ) nanocrystalline powders doped with Dy³⁺ ions were synthesized via the citrate gel combustion method. The structure of the compound is found to be triclinic with a particle size in the range of 20–30 nm. Luminescence properties have been characterized using photoluminescence (PL), excitation spectra and decay time measurements. The PL spectra have shown a broad blue band due to ⁴F_9/2→⁶H_15/2 transition and sharp yellow band corresponding to ⁴F_9/2→⁶H_13/2 transition of Dy³⁺ ions. From the concentration dependent PL studies, the optimum concentration of Dy³⁺ ions in YCZ is found to be 1.0 mol%, where intense near white light emission was observed. The Dy³⁺:YCZ nanophosphor has shown relatively better white color properties than the reported Dy³⁺:Y₂O₃ nanophosphor. The yellow to blue intensity ratios, CIE chromaticity coordinates and correlated color temperature studies have shown the possibility of using this compound for white light emission.     

Carbonate-precipitation synthesis of Yb:Y2O3 nanopowders and its characteristics

Ytterbium-doped yttria (Yb³⁺:Y₂O₃) nanopowders for transparent ceramics were synthesized by using a carbonate-precipitation method. The characteristics of precursor and powders calcined at different temperatures were investigated. The pure yttria phase can form through calcining at 700 °C. The Yb³⁺:Y₂O₃ nanopowders calcined at 1100 °C were well dispersed with a spherical morphology, and had a narrow particle size distribution with a mean particle size of about 70 nm. By using 1100 °C-calcined powders, nearly full dense Yb³⁺:Y₂O₃ ceramics were fabricated at 1750 °C for 8 h without any additives under vacuum conditions. The fluorescence spectrum of the sintered ceramics illustrates that there are two emission peaks locating at 1028 and 1071 nm respectively, all corresponding to the ²F_5/2 → ²F_7/2 transitions of Yb³⁺ ion. Homogeneous Yb³⁺:Y₂O₃ nanopowders synthesized by carbonate-precipitation method are suitable for the fabrication of IR-transparent ceramics.     

Correlation of compaction pressure, green density, pore size distribution and sintering temperature of a nano-crystalline 2Y-TZP-Al2O3 composite

Highly sinter-active nano crystalline composite powder of 2 mol% yttria doped tetragonal zirconia polycrystal (2Y-TZP) with 2 wt% alumina was synthesized by co-precipitation method. Crystallization temperature of the amorphous precursor powder, measured from simultaneous thermogravimetric (TG) and differential thermal analysis (DTA) techniques was found to be ∼470 °C. The powder was calcined at different temperatures in the range of 700-1000 °C. XRD patterns of the calcined powders revealed the presence of a single tetragonal phase. Particle size of the calcined powder measured by different techniques (X-ray line broadening, BET surface area and laser scattering technique) indicated an increase in the average particle size with calcination temperature. The study of compaction behaviour revealed the presence of soft agglomerates in the calcined powder. Pore size distribution of the green compacts obtained from a mercury porosimeter was found to be monomodal above a critical pressure. The onset temperature of sintering was found to increase with calcination temperature. Powders calcined at 800 °C and 900 °C had shown better sinterability at 1200 °C owing to the presence of finer pores with a narrow size distribution in the green compacts. Sintering behaviour of the powder calcined at 700 °C was found to be marginally poorer in comparison to the other samples, whereas the powder calcined at 800 °C had demonstrated best densification behaviour, especially when compacted at 300 MPa.     

Low-temperature synthesis of ZnNb2O6 powders via hydrothermal method

ZnNb₂O₆ powder was successfully synthesized via hydrothermal method with Nb₂O₅ and Zn(NO₃)₂·6H₂O as raw materials and cyclohexane as solvent. Phase composition, morphology, and chemical composition were determined via a combination of XRD, SEM, TEM and EDS techniques. The effects of synthesis temperature and reaction time on phase composition and particle morphology were investigated in this paper. The results showed that fine ZnNb₂O₆ powders could be obtained at a hydrothermal temperature of 190 °C or above under different reaction time.     

A kinetic study of lithium transport in the sol–gel Cr0.11V2O5.16 mixed oxide

The kinetics of the electrochemical lithium insertion reaction in the sol–gel chromium–vanadium mixed oxide Cr_0.11V₂O_5.16 has been investigated using ac impedance spectroscopy. The chemical lithium diffusion coefficient is found to be in the range 10⁻⁸/10⁻¹² cm²/s depending the Li content over the wide Li composition range 0 < x < 2 in Li_xCr_0.11V₂O_5.16. The evolution of the cathode impedance is investigated as a function of the lithium content and cycles. The results are discussed in relation with the cycling properties of the electrode material and the unusual structural response of the sol–gel mixed oxide which consists of a single phase behaviour with a continuous cell volume expansion of 6–7% in the 0 < x < 2 range for Li_xCr_0.11V₂O_5.16. For x < 1, a comparison with available kinetic data for the parent oxide indicates a very close behaviour. Cr_0.11V₂O_5.16 is shown to be the best V₂O₅-based cathode material with an initial specific capacity of 280 mAh/g at C/10 rate and still 240 mAh/g after 50 cycles in the 3.8–2 V potential range. The present kinetic data seem to indicate its better cycling behaviour mainly originates from its specific structural response rather than from kinetic reasons.     

Low-temperature reaction-sintering of mullite ceramics with an Y2O3 addition

Mullite ceramics were fabricated at relatively low temperatures from powder mixtures of -Al₂O₃ and quartz, with an Y₂O₃ addition. The mullitization process was analyzed by X-ray diffraction. The densification behavior was investigated as a function of the Y₂O₃ content, sintering temperature and holding time as well as mullite seeds. It has been shown that mullitization occurs via a nucleation and growth mechanism within an yttrious aluminosilicate glass, but lattice and grain-boundary diffusion becomes important during the densification process. Moreover, the incorporation of mullite seeds was observed to enhance both mullitization and densification. At 1400°C for 5 h or 1450°C for 2 h, 15 mol% Y₂O₃-doped and 5 mol% mullite-seeded specimens can be sintered to almost full density.     

Sintering of Al2O3-SiC composite from sol-gel method with MgO, TiO2 and Y2O3 addition

Al₂O₃-SiC composite ceramics were prepared by pressureless sintering with and without the addition of MgO, TiO₂ and Y₂O₃ as sintering aids. The effects of these compositional variables on final density and hardness were investigated. In the present article at first α-Al₂O₃ and β-SiC nano powders have been synthesized by sol-gel method separately by using AlCl₃, TEOS and saccharose as precursors. Pressureless sintering was carried out in nitrogen atmosphere at 1600 °C and 1630 °C. The addition of 5 vol.% SiC to Al₂O₃ hindered densification. In contrast, the addition of nano MgO and nano TiO₂ to Al₂O₃-5 vol.% SiC composites improved densification but Y₂O₃ did not have positive effect on sintering. Maximum density (97%) was achieved at 1630 °C. Vickers hardness was 17.7 GPa after sintering at 1630 °C. SEM revealed that the SiC particles were well distributed throughout the composite microstructures. The precursors and the resultant powders were characterized by XRD, STA and SEM.     

Synthesis and sintering of Y2O3-doped ZrO2 powders using two Pechini-type gel routes

Yttria-tetragonal zirconia polycrystal (ZrO₂ + 4.5 mol% Y₂O₃) nanocrystalline powder was synthesized by two Pechini-type gel routes, the in situ polymerized complex (IPC) method and the PEG/AF method. FTIR spectra confirmed coordination of metal ions with the polymer by different routes, depending on the method used. The crystallite size of the powder increased from 5 nm to 8 nm when the temperature was increased from 450 °C to 600 °C and calcination times increased from 2 h to 24 h. The morphology of the powders differed only when the organic impurities were not completely eliminated. After calcination, the morphology of the powders produced by the two methods showed porous agglomerates composed of smaller particles. All the resulting microstructures were very similar, regardless of the method employed to obtain the powder or the calcination times and temperatures.     

Microstructure and optical limiting properties of multicomponent inorganic gel-glasses: A focus on SiO2, TiO2 and PbO gel-glasses

Two kinds of multicomponent inorganic gel-glasses, namely, SiO₂–TiO₂ binary and SiO₂–TiO₂–PbO ternary gel-glasses, were obtained via the hydrolysis and co-condensation of tetraethyloxysilane (Si(OC₂H₅)₄, TEOS), titanium tetraisopropoxide (Ti(C₄H₉O)₄, TTIP), and lead (II) acetate trihydrate (Pb(Ac)₂). X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and pore structure measurements were performed to investigate the morphology, structure, and texture properties of the gel-glasses. The samples were mainly composed of amorphous phase without any detected phase separation. The introduced Ti⁴⁺ and Pb²⁺ were partially involved in the hydrolysis and condensation processes. Furthermore, the composition had a significant influence on the pore structure of silica-based gel-glasses, and the introduction of TiO₂ in the gel-glasses caused the formation of bimodal-distributed pore shapes and dimensions in multicomponent inorganic gel-glasses. The optical limiting (OL) properties were measured at 532 nm by using the open aperture Z-scan technique. The SiO₂–TiO₂ binary and SiO₂–TiO₂–PbO ternary gel-glasses exhibited greatly enhanced OL behaviors than the SiO₂ unitary gel-glass. The observed OL phenomenon in the SiO₂–TiO₂ binary gel-glass originated from two-photon absorption (TPA), whereas TPA followed by nonlinear scattering (NLS) contributed to the increased OL performance of the SiO₂–TiO₂–PbO ternary gel-glass. SiO₂–TiO₂ binary and SiO₂–TiO₂–PbO ternary gel-glasses are potential materials for practical applications in the fields of optics, all-optical switching, and related optical devices.     

Influence of calcium concentration on formation of tetravalent chromium doped Y3Al5O12 ceramics

Yttrium Aluminium Garnet (YAG) ceramics doped with chromium were prepared by solid-state reactive sintering in a vacuum. The influence of the charge compensator Ca²⁺ concentration on microstructure, optical properties and efficacy of Cr³⁺ oxidation to Cr⁴⁺ under air annealing was investigated. A non-monotonic dependence of these features on the amount of CaO as an additive was found. The changes in ceramic transparency and microstructure were explained considering the interaction between CaO and Cr₂O₃ at the ceramic grain boundaries, which leads to a different pore evolution in distinct samples during sintering. The efficacy of the oxidation of Cr³⁺ to Cr⁴⁺ strongly depends on the concentration of Ca dissolved in the YAG. The calcium solubility decreases due to the higher oxygen partial pressure of the extra phases on the grain boundaries that decreases the amount of generated Cr⁴⁺ ions. Such phenomenon explains the lower concentration of Cr⁴⁺ ions in the sample with 0.8% of Ca against the one with 0.5%. The experiment shows that the ceramic with 0.5% of Ca has a better in-line transmission and a higher concentration of Cr⁴⁺ ions in comparison with samples with a different Ca concentration.     

Microstructure of LaNiO3 thin films obtained by the spin-on technique from citrate precursors

The morphology of perovskite LaNiO₃ (LNO) thin films is a very important feature considering the fact that they could be used as an electrode material in ferroelectric devices. In this work, LNO films were prepared from citrate precursors. The films were deposited on Si (100) and Pt covered Si substrates [Pt (111)/Ti/SiO₂/Si] using the spin-on technique. The phase composition of the films was confirmed by X-ray powder diffraction analysis, while the microstructures were investigated by atomic force microscopy. The influence of the metal ion:citric acid:ethylene glycol ratio on the morphology of the films was investigated. After optimization of these parameters, very homogenous, uniform and crack-free LNO films were obtained.     

Investigation of optical,mechanical, and thermal properties of ZrO2-doped Y2O3 transparent ceramics fabricated by HIP

Highly transparent ZrO₂-doped Y₂O₃ ceramics were successfully synthesized using the hot isostatic pressing (HIP) process. The effects of the ZrO₂ content on the sintering behaviors, optical transmission spectra, Vickers hardness, grain size, size distribution, and Raman spectra were determined. The results indicated that decreased ZrO₂ content could promote increased transmittance, red-shifted infrared cutoff wavelength, increased thermal conductivity, decreased Vickers hardness, and increased lattice ordering. According to the optical transmission spectra, the optimized ZrO₂ content was 0.50 at%, at which point the ceramic exhibited a larger pre-sintering temperature range of 1650–1750 °C and the average grain size of 3.35 µm at 1750 °C. The grain size was significantly decreased at lower pre-sintering temperatures. Furthermore, a moderate Vickers hardness of 8.42 GPa and high thermal conductivity of 10.85 W/m K at room temperature were obtained for the optimized ceramic.     

Sodium impurities in ZnO-Bi2O3-Sb2O3 based varistors

Sodium impurities are frequently present in the raw oxides that constitute the complex formulation of ZnO-Bi₂O₃-Sb₂O₃ based varistors. But actually little is known about their effect on the microstructure and the electrical response of these materials. This is the main goal of the present contribution and according to the obtained results an excessive presence of this alkaline impurity can lead to a compositional change in the Bi-rich skeleton of the varistor microstructure. As a consequence the grain growth kinetics, the densification rate and the characteristic non-linear I-V response of these electroceramics are seriously affected.