Elena Kashchieva

SILICA GELS AND GEL GLASSES CONTAINING SILVER AND PLATINUM METAL PARTICLES

Silica gels containing between 0.05 and 20.0 mol% Ag2O and between 0.1 and 1.0 mol% PtO2 have been synthesized using TEOS, C2H5OH, H2O, HNO3, HCl, AgNO3 and PtCl4 as precursors. The gels obtained with Ag2O are colorless, while the gels containing PtO2 are yellow. The gels have been heat treated in the 25–1000°C temperature range. The processes of structural evolution of gels have been studied by means of IR-spectroscopy, X-ray diffraction analysis, transmission electron microscopy (TEM) and transmission electron microdiffraction (TEMD). It has been established that the temperature of gel to gel-glass transition decreases when the Ag2O content of the gels increases. The same tendency was established for the gel to gel-glass transition in the SiO2-PtO2 system.Special attention was paid to the formation of silver and platinum metallic particles in amorphous materials. The microstructure of the gels has been observed and the sizes of the metal particles were determined to be from 3 to 25 nm.The crystallization processes in the gels heat treated at 1000–1200°C have been examined and besides the silver and platinum particles α-cristobalite was formed.It has been shown that nanocomposite materials containing ultrafine silver and platinum particles in SiO2 amorphous or polycrystalline matrixes can be obtained.

Electron diffraction study of the short-range order in glasses of the system TeO2B2O3

Abstract The short-range order in vitreous TeO 2 and B 2 O 3 and in TeO 2 B 2 O 3 glasses has been studied using electron diffraction. The amorphous samples investigated were prepared by two methods. Powdered samples were produced either by rapid quenching using the roller technique (TeO 2 glass) or by slow cooling (B 2 O 3 and TeO 2 B 2 O 3 glasses) and amorphous TeO 2 and TeO 2 B 2 O 3 thin films were deposited by vacuum evaporation with resistive heating. According to the electron diffraction data, the TeO 4 polyhedron is shown to be the main structural unit in the TeO 2 glass. The boron atoms in the B 2 O 3 glass are found to be threefold-coordinated with respect to the oxygen and the presence of B 3 O 6 groups is indicated. In the binary glasses mainly TeO or BO distances are resolved, depending on their composition. The results for the powdered and thin film samples are compared and a change in the TeO distances for the films is established.

Microstructure of gel-glasses in the SiO2−Nd2O3 system

The microstructure of thermally treated neodymium containing silica gels is investigated by transmission electron microscopy and electron diffraction. Microheterogeneities with amorphous nature are found. Information for the short range order is obtained by the Radial Distribution Functions: SiO4 tetrahedra and NdO6 groups are formed and transition from four-fold to six-fold ring structure is observed with increasing of the heat treatment temperature.

Microstructure and chemical composition of Thracian medallions with enamel in millefiori technique found in Bulgaria

Abstract The objects of investigation are three medallions. Both of them are made with copper alloy plating from Thracian chariot found in the Plovdiv region, Bulgaria. The dating of medallions is made in accordance with the construction of the chariot: 1st–4th centuries AD. The medallions are richly decorated with enamel in millefiori technique showing a variety of colored glasses. The third medallion is from Alfatar, Bulgaria. Adornments of this type are unique for present day Bulgaria. The aim of this work is to begin a detailed study of these rare archaeological objects to determine their technological features and to compare them with some similar artefacts from other lands. A combination of relatively non-destructive, structurally sensitive, methods as: transmission electron microscopy (TEM), selected area electron diffraction SAED and electron probe microanalysis (EPMA) is applied on small samples of the medallions. From obtained data we conclude that the composition corresponds to the composition of similar enamels dating from the same period. The observed microstructure of some glasses in the decoration of the medallions is due to the weathering processes and the technological history of the medallions.

Microaggregation processes in B2O3-TeO2 glasses

The purpose of the study is to trace the appearance and development of heterogeneities in the structure of slowly cooled compositions in the B 2 O 3 -rich part of the B 2 O 3 -TeO 2 system. The main methods applied were transmission electron microscopy (TEM) and computer simulation

Structure of gel materials in the system Ce2O3Al2O3P2O5

By using the sol-gel method, amorphous materials have been obtained in the Ce 2 O 3 -Al 2 O 3 -P 2 O 5 system. Their phase formation and microstructural changes have been investigated as a function of thermal treatment in the temperature range of 300-1400°C by X-ray diffraction, infrared spectroscopy and transmission electron microscopy. It is established that gels containing Ce 3+ possess high thermal stability. According to the X-ray data, they are amorphous up to 500°C. After crystallization, phases characteristic of the Ce 3+ ions are present. The network of the gel materials is built up from PO 4 phosphate complexes, by contrast with the gel products containing Ce 4+ .

Glass-crystalline materials in the system P2O5–CaO–ZnO–Ti ☆

Abstract Calcium-phosphate ceramic and glasses in combination with titanium and titanium alloys could be successfully used in clinical practice. In the present work the interaction between titanium and phosphate glasses from the system P2O5–CaO–ZnO is investigated. The precursors used are NH4H2PO4, CaCO3, ZnO and Ti. The initial glasses in the system P2O5–CaO–ZnO are synthesized by melting in corundum crucibles. The thermal treatment of samples is performed in the temperature range from 800 to 1250°C for 20 min prepared by several variants of contact between initial phases: (i) homogenized mixture of glassy powder and titanium powder; (ii) a monolithic glass in contact with titanium plate; (iii) packets from bulk titanium substrate and powdered phosphate glass mixed with Ti or Ca3(PO4)2. For the characterization of the specimens the following methods are used: X-ray diffraction (XRD), infrared (IR) spectroscopy and transmission electron microscopy (TEM). It is established that the obtained final products present glass-crystalline polyphase materials containing different crystal phosphates, titanium phosphide, TiO2, TiO and titanium. As a result of the solid state reaction between the metal and phosphate glass a transition region is found. The applied technique of preparation is an initial step towards the development of gradient materials with controlled properties.

Glass formation and immiscibility in the TeO2-B2O3-Fe2O3-MnO system

The glass formation in the quaternary TeO2-B2O3-MnO-Fe2O3 system and in its ternary systems was investigated. A range of liquid immiscible phases, located near to the binary TeO2-B2O3 and B2O3-MnO systems was established. Using transmission electron microscopy, a trend to metastable liquid-phase separation in the single-phase glasses, located near to the boundary of immiscibility was observed. With an increase in the Fe2O3 and MnO content still in the process of cooling of the melts, it was possible for a fine glassy crystalline structure to be formed in them. It was shown that by changing the upper limit of the melting temperature and the cooling rate, the glassy crystalline structure and the Fe3O4 content could be modified.

Electrical Characteristics of Bismuth Titanate Ceramics and Glass-Ceramics, Containing SiO2 and Nd2O3 as Additives

Bismuth-titanate ceramics containing SiO2 and Nd2O3 as additives are synthesized by melt quenching method in the system Bi2O3-TiO2-Nd2O3-SiO2 at temperature 1450 oC. By X-ray diffraction analysis are determined the crystal phases participating in the materials: Bi4Ti3O12, Bi12TiO20 and δ-Bi2O3 . The addition of SiO2 allows to control the glass formation ability of the supercooled melts. The addition of Nd2O3 stimulates the thermal stability of the materials due to formation of solid solutions Bi4-xNdxTi3O12. Measurements of the electrical conductivity, capacitance and dielectric losses of selected samples are performed by DC resistible bridge and digital capacity meter (with frequency of 1 kHz) using two-terminal method and a suitable sample holder with graphite electrodes. It is established that for the investigated samples the activation energy of conductivity is in the range 1 – 1,3 eV, dielectric permittivity from 1000 to 3000 at (at 400 oC) and dielectric losses tgδ are between 0,0002 and 0,1.

Thermal Stability and Microstructure of Glasses and Glass-Ceramics in the System La2O3-PbO-MnO-B2O3

Glasses have been synthesized in the system La2O3-PbO-MnO-B2O3 varying the MnOcontent in large compositional range between 10 and 70 mol%. Glass-ceramics containing La0,6Pb0,4MnO3 phase have been obtained directly by melt quenching method. Transition and crystallization temperatures have been determined by DTA. The increasing of the B2O3-content improves the thermal stability of the glasses. By TEM analysis, different type microheterogeneous structures have been observed in compositions rich in B2O3.