Synthesis of glass ceramics containing finely dispersed particles of aluminum-doped M-type strontium hexaferrite

Glasses with nominal compositions SrFe₁₀Al₂O₁₉+4(SrB₂O₄+Sr₂B₂O₅) (1) and SrFe₉Al₃O₁₉+4(SrB₂O₄+Sr₂B₂O₅) (2) were prepared by rapid quenching of melts. Thermal treatment of glass samples at 600–900 °C resulted in crystallization of the magnetic phase SrFe_12−x Al_xO₁₉ (x = 1.1±0.1) and strontium borates. Platelet hexaferrite particles with average sizes from (250×60) nm² to (450×140) nm² were prepared. The coercive force of glass ceramics is 580 and 475 kA m⁻¹ for glasses 1 and 2, respectively. The coercive force of 580 kA m⁻¹ is the highest known value compared to hexaferrite particles prepared earlier by glass crystallization.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 74–77, January, 2005.     

Tunable red-green upconversion luminescence in novel transparent glass ceramics containing Er: NaYF4 nanocrystals

To develop NaYF(4) as bulk luminescence material, transparent glass ceramics containing Er(3+): NaYF(4) nanocrystals were fabricated for the first time, and the influences of heat-treatment temperature and Er(3+) doping level on their upconversion luminescence were investigated. With increasing heating temperature, the upconversion intensity enhanced accordingly, attributing to the incorporation of more Er(3+) into the grown NaYF(4). Notably, when the heating temperature reached 650 degrees C, the upconversion intensity augmented drastically due to the occurrence of phase transition from the cubic NaYF(4) to the hexagonal one. Interestingly, for the samples heat-treated at 620 degrees C, when the Er(3+) doping level was increased from 0.05 to 2.0 mol %, the upconversion emission was whole-range tunable from monochromatic green to approximately monochromatic red, which could be mainly attributed to the cross-relaxation between Er(3+) ions. The excellent optical properties and its convenient, low-cost synthesis of the present glass ceramic imply that it is an excellent substitution material for the unobtainable bulk NaYF(4) crystal, potentially applicable in many fields.     

Rare-earth-doped transparent glass ceramics

Glass ceramics are a known class of polycrystalline ceramic materials, where, depending on the glass matrix and the particular crystalline phases, one can obtain materials with improved mechanical, thermal, electrical or optical properties. The characteristics and applications of optical glass ceramics are reviewed, with particular emphasis on rare-earth-doped transparent glass ceramics for photonics, including the search for new transparent glass ceramic compositions and the development of suitable methods to process such materials into functional devices.     

Behaviour of glass and silver/silver chloride electrodes in some non-aqueous media

The stability of glass electrodes and silver/silver chloride electrodes in isopropanol, methyl ethyl ketone and a mixture of equal volumes of these has been measured, and found to be of the order of 1 mV. Changes in the electrode potential due to addition of water and to addition of supporting electrolyte have been investigated. The glass electrode responded reversibly to hydrogen ion activity changes in buffers of picric acid-tetraethylammonium picrate, and perchloric acid-di-isopropylamine. The autoprotolysis constants at 25 degrees were calculated to be 2 x 10(-19) in isopropanol, 2 x 10(-26) in the ketone and 8 x 10(-19) in the mixture.     

Structural study of ternary iron-lead-germanate glass ceramics

Glass ceramics with the composition xFe(2)O(3)·(100-x)[7GeO(2)·3PbO(2)] where 0≤x≤60 mol% were obtained and studied using XRD, FTIR and UV-vis spectroscopy investigations. Heat treatment of glass samples at 400°C for 8 h led to the formation of α, γ-PbGe(4)O(9), Pb(3)Fe(2)Ge(4)O(14) and PbO(1.44) crystalline phases. The content of these crystalline phases depends of Fe(2)O(3) concentration. FTIR spectroscopy data suggest that the lead ions have a pronounced affinity towards [GeO(5)] structural units containing non-bridging oxygens and [FeO(4)] anions producing formation of the Pb(3)Fe(2)Ge(4)O(14) crystalline phase. The introduction of low concentrations of Fe(2)O(3) into the host matrix results in the formation of new absorption UV bands between 320 and 450 nm. These bands arise from to the d-d transitions of the Fe(+3) ions. The light absorption in the range from 250 to 600 nm increases with increasing iron oxide content in matrix network, accompanied with the changes on color from white to brown yellow and darker brown.     

Preparation process and upconversion luminescence of Er3+-Doped glass ceramics containing Ba2LaF7 nanocrystals

The preparation process and upconversion luminescence of the Er(3+)-doped glass ceramics containing Ba(2)LaF(7) nanocrystals were investigated. The formation of Ba(2)LaF(7) nanocrystals in the glass ceramics was confirmed by X-ray diffraction. Er(3+)-doped glass ceramics containing Ba(2)LaF(7) nanocrystals exhibited highly efficient upconversion luminescence in comparison with glasses. With the increase of heat treatment temperature the upconversion luminescence intensity increased gradually. The composition of glasses was also found to have significant influence on the crystallization process of glass ceramics. The mixture of Ba(2)LaF(7) and La(2)O(3) nanocrystals and the mixture of La(2)F(3) and La(2)O(3) nanocrystals in the glass ceramics could be obtained by controlling different compositions of glasses. The upconversion luminescence intensity also varied significantly with different nanocrystals in the glass ceramics.     

Electrolytic nucleation and growth of silver crystals in silver nitrate melts containing oxidants

Silver crystal nucleation is studied in AgNO₃ melts containing Pb(NO₃)₂ and HNO₃ at 250°C by the galvanostatic method. The maximum nucleation overpotential is higher as compared with pure AgNO₃ and the exchange current of silver deposition-dissolution is lower. It is found that the added substances and the products of their reactions with the melt are strong oxidants, take part in the electrochemical processes, and induce passivation of the cathodic substrate and the surface of growing silver crystals. Substantial changes in the morphology of the cathodic deposit and certain changes in the silver lattice parameter were observed depending on the oxidant content.     

Kinetics of silver dissolution in sulfide containing thiocarbamide electrolytes

Effective values of reaction order with respect to ligand P, transfer coefficient α, and exchange current i ₀ at constant silver surface coverages θ by sulfide ions are measured. The employed solutions contained from 0.4 to 0.05 M thiocarbamide, 0.5 M HClO₄, 10^?4 M AgNO₃, and from 10^?5 to 10^?4 M Na₂S. It is shown that the exchange current grows approximately linearly from 10^?5 to 1.5 × 10^?4 A/cm² at θ increase in the range from zero to 0.8, while α and P values grow negligibly in the ranges of 0.4–0.45 and 0.9–1.1, accordingly. The obtained results are compared with the data of similar studies of the gold behavior in acidic thiocarbamide solutions. The possible reasons for the different effects of sulfide ion chemisorption on the anodic dissolution of gold and silver in the studied solutions are discussed.     

Fluorescence of pyrene in inhomogeneous media containing silver nanoparticles

Pyrene fluorescence in inhomogeneous media based on ionic detergents containing silver nanoparticles with different morphologies is investigated. An increase in pyrene monomer emissions in the spectral range of 400–500 nm is observed, due to the resonance between electronic transitions in pyrene molecules in that region and the plasmonic oscillations of silver nanoparticles.     

The low temperature heat capacity of some glass ceramics

The heat capacities between 1.5 and 25 K have been measured for glassy and devitrified samples of two substances: one is a commercial material (“Cer-Vit”) and the other is diopside (CaMgSi₂O₆). All samples, whether glassy or crystalline, have an “excess” heat capacity at the lowest temperatures roughly proportional to T and of similar magnitude to that found in a wide variety of inorganic glasses.     

The tetragonal structure of nanocrystals in rare-earth doped oxyfluoride glass ceramics

Rare-earth doped oxyfluoride glasses and nanocrystalline glass ceramics have been prepared and studied by energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) aiming at investigating the structure and the symmetry of the nanocrystal as well as the site of the rare-earth ion. To solve the problem encountered by previous researchers due to glass host interference, we etched off glass matrix and released the fluoride nanocrystal, which is more convenient for EDS measurement. A tetragonal phase model with the chemical formula as PbREF(5) proved by quantitative EDS and XRD analyses has been proposed in this paper for the first time. Two specific crystalline phases with the same space group have been observed at 460 °C-500 °C and 520 °C-560 °C, respectively. Moreover, a super "pseudo-cubic" cell based on our tetragonal model may give a good explanation to the probable previous cubic-symmetry misunderstanding by researchers. Additionally, the thermodynamic mechanism of phase transition and the thermal stability related to the structure of nanocrystals in glass ceramics have been studied and supported by ab initio calculations and experimental methods. The structure and thermal stability of the nanocrystal and clear environment of the rare-earth ion reported here have far-reaching significance with respect to the optical investigations and further applications of rare-earth doped oxyfluoride glass ceramics.     

Deposition of silver coatings onto sodium borosilicate glass microspheres

A technique for preparation of silver coatings on the surface of sodium borosilicate microspheres was examined. The strength and stability of the coatings were studied as influenced by pre-irradiated titanium(IV) tetrabutoxide employed as surface modifier.     

Structural properties of silver nanoclusters–phosphate glass composites

The structure of the xAg₂O(100 − x)[50P₂O₅·30CaO·20Na₂O] glasses, with x = 0, 3 and 5 mol%, was investigated by means of Raman and infrared spectroscopy. The structural changes induced by the Ag₂O presence into the soda-calcium–phosphate matrix were evidenced and discussed in terms of the network depolymerization process and distortion of the PO₄ tetrahedra. The existence of silver nanoclusters inside the glass matrix, considered to be mainly responsible for the found structural behavior, was supposed by results obtained from the analysis of the UV–vis absorption spectra and further proved by transmission electron microscopy images.     

Characterisation of the surface structure and bioactivity of glass and glass ceramics using surface topography

The present paper reports the results of the relationship between the surface topography, microstructure and the in vitro bioactivity of samples with and without fluorapatite content in simulated body fluid. Glasses and glass ceramics belonging to the Li₂O-SiO₂-CaO-P₂O₅-CaF₂ system were prepared by using conventional melting technique following by heat treatment to obtain glass ceramics. This current study demonstrates the benefits of combining two microscopic methods for better investigation of the surface structure. The formation of apatite layer on the surface and the increase in surface roughness proved that the glasses and glass ceramics with bioactive fluorapatite content could satisfy to the requirements for biomaterial applications. The results also showed that the roughness of apatite layer formed after immersion in body fluid on the surface of glasses with fluorapatite was more pronounced than that of equivalent glass ceramic samples cured under the same conditions.      

Electric characteristics of cellular structures containing colloidal silver

Electrophysical characteristics of yeast cells in which precipitates are formed by the chemical-microbiological method from colloidal solutions with different concentrations are studied. The cell ζ-potential and dc surface conductivity at frequencies of 100–600 kHz are calculated using the data from measuring the electrophoretic mobility and dispersion of cell conductivity. In our calculations, we used known electric cell model (isolating cytoplasmic membrane that separates conducting cytoplasm and external solution) supplemented by an external ion-exchange layer that models the cell wall. A comparison of changes in the dc and ac surface conductivities of cell testifies to the formation of clusters of metal silver nanoparticles in the cell wall.     

Preparation of antibacterial polyimide films containing silver nanoparticles

Polyimide/silver composite films were successfully prepared by in situ polymerization. A precursor, AgNO₃ was used as the source of the silver nanoparticles. The structure and morphology of resulting films were characterized by FTIR spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Consequently, the silver nanoparticles were well dispersed in polyimide matrix. Meanwhile, thermal properties from thermal gravimetric analyses (TGA) and mechanical properties from tensile test which confirmed composites were kept good performance as compared to pure polyimide. In addition, the antimicrobial activity of polyimide/silver composite films against three different bacteria, B. subtilis, S. aureus, and E. coil, illustrated excellent activity. This composite is potential useful as antimicrobial material with good thermal performance in a wide variety of biomedical and general use applications.     

Silicone composites containing stabilized silver clusters or nanoparticles

Silver nanoparticles are of high importance due to their electrical, magnetic, and optical properties, as well as catalytic and biocidal activity that are superior to the bulk silver and other metals. To prepare certain devices, generally, silver is incorporated into a matrix either as preformed or in situ‐generated particles. Silver nanoparticles were generated in situ into a silicone matrix formed by cohydrolysis of the mixture of silanes, each of them having a certain role: dimethyldiethoxysilane (DMDES) as a precursor for highly flexible polydimethylsiloxane, methyltriethoxysilane (MTES) as a cross‐linker highly compatible with polydimethylsiloxane, and 3‐aminopropyltriethoxysilane as a stabilizer, since it can readily complex to silver atoms through its amine functionality. Dimethylformamide (DMF) was used as a solvent for the silver nitrate and reducing agent. The samples were investigated both in sol state and as aged coating films deposited on glass substrate. The complexation of the silver and the matrix formation were emphasized by FTIR. The size of the formed silicone particles encapsulating silver was estimated by dynamic light scattering (DLS) (about 100 nm) in sol and by AFM in film (about 90 nm). The formation of the clusters or nanoparticles depending on the ratio between the reducing and complexing agents was evidenced by UV–Vis absorption spectra. Thus, it would create conditions to stop and isolate clusters at the desired size by precise control of the experimental conditions. The composites could be used alone as antibacterial‐coating materials but also, porous silica having incorporated silver clusters with potential applicability in catalysis may result after their calcination. Copyright © 2010 John Wiley & Sons, Ltd.