Advanced Materials Research Vols. 39-40

Abstract: Homogeneous yellowish and well transparent glasses (Li2O)x(TiO2)x(TeO2)1-2x and (BaO)x(TiO2)x(TeO2)1-2x , (x = 0.075, 0.1, 0.125) were prepared from corresponding oxides (purity 99+%). For prepared glasses the optical gap values were found in the region 3.2 eV – 3.4 eV and the temperature (T) coefficient of the optical gap (γ) varies in the narrow region 5.3×10-4 eV/K to 5.9×10-4 eV/K for 300 K < T < 580 K. Estimated values of non-linear refractive index (n2) were found in the region n2 [m2W-1] = 2.7×10-18 – 3.5×10-18 close to n2 values for similar glasses. Raman spectra measured indicate that connectivity of (BaO)x(TiO2)x(TeO2)1-2x network is more evolved than that one for (Li2O)x(TiO2)x(TeO2)1-2x glasses. TiO2 most probably assists to continuous network formation due to appearance of Te-O-Ti bridges.

Abstract: The work reports on the preparation and properties of binary alumina-rare-earth oxide glass microspheres with high contents of aluminium oxide. Because Al2O3 is not a typical glass former, preparation of these glasses in bulk is difficult due to high melting temperatures and high tendency to crystallisation, which requires high cooling rates. One of the possibilities is preparation of glass microspheres by flame synthesis and rapid quenching of microspheres by spraying them with water. Microspheres with dimensions ~10 µm and with various compositions from the systems RE2O3-Al2O3 (RE = Y, Yb, La) have been prepared with the use of an in-house built equipment. The prepared microspheres have been characterised by SEM-EDX, IR, XRD and DTA.

Abstract: The new generation of high silica materials with high thermal resistance was created by leaching of chopped glass fibre. These materials with low thermal conductivity are inert to the majority of chemical reagents, resistant to organic and mineral acids, weak alkali, water and highpressure steam. High silica chopped strand mats are non-woven fabrics designed for using in a wide range of insulation and protection applications at temperature till 11000C. The technology and quality of leaching process of initial Si-Al-Na glass widely depends on quality of fibre surface characteristics, i.e., roughness of surface of glass filaments. The surface roughness of the fibre before leaching is a function of chemical durability, therefore it depend on content of Al2O3. The thermal conductivity (within 20…10000C) of chopped strand mats directly depends on the surface roughness. The morphology and compositional profiles of surface of glass fibre before and after leaching were investigated using AFM, SEM, X-ray microanalysis and X-ray powder diffractometer. The different defects for fibre with content of Al2O3 <2.5% and high roughness namely cracking and crystalline deposits of Na2SO4 on top and into pores of fiber after leaching have been identified. The presence of sodium ions on surface of fibre decreases the heat insulation properties of mats. The structure of glass filaments surface was investigated in order to clarify the influence of surface characteristics on thermal conductivity of high silica glass fibre non-woven fabrics.

Abstract: Spectral behaviour of the real and imaginary parts of the optical constants of glass has been widely investigated to obtain band structural parameters of optically induced transitions. Curve fitting procedures for obtaining a classical oscillator fit to reflectance and/or transmittance data play an important role in this application. The present form of the dispersion model that is widely used to describe disordered media like glass uses a convolution of the Lorentzian oscillator function and the Gaussian Probability Density Function. This model is capable of describing symmetrical band shapes only. In this paper a convolution model is proposed based on using the Beta function as PDF. The maximum of the PDF, which corresponds to the frequency with the highest probability, corresponds to the central frequency of the particular oscillator species in the convolution model. For a certain value of the parameter that describes the degree of asymmetry, the new model is equivalent with the present convolution model and the remaining parameters have an identical meaning yielding practically identical values. The new model also solves a problem related to the infinite integration intervals of the Gaussian PDF based convolution model.

Abstract: Amorphous oxide thin films of Fe2O3-R2O3 (R = La, Gd and Tb) systems have been deposited on silica glass substrates by using a radio frequency sputtering method, and magnetic properties of the thin films have been examined. The Fe2O3-La2O3 thin films exhibit cusp-like maxima of dc magnetic susceptibility in their temperature dependences as well as magnetic aging and memory effects characteristic of typical spin glasses. For Fe2O3-Gd2O3 and Fe2O3-Tb2O3 systems, magnetic moments of iron ions take part in formation of a spin glass state, as indicated by the magnetic aging effects, while those of rare-earth ions remain to be in a paramagnetic state even at very low temperatures.

Abstract: Spectral, radiation optical, and radiation shielding properties of new phosphate glasses, TPF, with high lead content (up to 40 mol. % PbO) were studied. They are characterized by the heightened radiation optical resistance at exposure doses up to 5*107 R, and by the improved radiation shielding properties as compared with similar data of silicate glasses with the same lead content. A TPF boundary of semi-transmission for a 1 cm thick glass layer lies approximately at 355 nm, its absorption coefficient, µ, for γ-quanta of 137Cs (0.663 MeV) and 60Co (1.25 MeV) reach 0.373 and 0.240 cm-1 whereas µ’s of the Russian radiation resistant strong flint TF110 (40 mol. % PbO) are equal to 0.355 and 0.214 cm-1. These properties allowed to use TPF instead of strong silicate flints as material for observation radiation schielding windows of facilities of special usage.

Abstract: Compositions and heat treatment conditions were determined at which absorption and luminescence properties of aluminosilicate transparent glass-ceramics are defined mainly by tetrahedrally coordinated Co2+ ions located in spinel nanocrystals. Optical properties of cobaltdoped aluminosilicate transparent glass-ceramics have been studied in visible – near infrared (IR) spectral range. It was demonstrated that the concentration of tetrahedrally coordinated Co2+ ions in the spinel nanocrystals is determined by the CoO content in the initial glass and is independent of the heat-treatment schedule. Absorption saturation and bleaching relaxation under excitation of the 4A2→4T1(4F) transition of tetrahedrally coordinated Co2+ ions were studied. The value of groundstate and excited state absorption cross-sections of tetrahedrally coordinated Co2+ ions was estimated. Comparative study of output pulse parameters of Q-switched Er:glass laser using cobaltdoped glass-ceramics and Co2+:MgAl2O4 single crystal is presented.

Abstract: High-power optical multimode fibers are essential components for materials processing and surgery and can limit the reliability of expensive systems due to breakdown at the end faces. The breakdown threshold of fibers is determined by intrinsic materials properties and parameters of the technology applied. The aim of this paper is the identification of technological parameters that are crucial for the fiber quality. Fibers were drawn from preforms of Heraeus SWU with core material F300 and a low amount of OH-. Both, the cladding (fluorine doped SiO2) to core diameter ratio (CCDR) and the drawing speed were varied. CCDR values between 1.05 and 1.4 were used. Afterwards, the laser-induced damage thresholds (LIDT) of the fibers were determined. For comparison, also samples from preforms, which underwent different thermal treatments above the transition temperature, were tested with respect to their damage resistivity. Single and multi pulse LIDT measurements were done in accordance with the relevant ISO standards. Nd:YAG laser pulses with durations of 15 ns (1064 nm wavelength) and 8.5 ns (532 nm) at a repetition rate of 10 Hz were utilized. For the fibers, LIDT values (1-on-1, 1064 nm and 532 nm) increased with growing CCDR and with decreasing drawing velocities.

Abstract: Glasses in the composition range in mol% 35-50 ZnO – 10-15 Al2O3 – 40-55 SiO2 were prepared, undoped and doped with 1 x 1019 and 1 x 1020 Sm3+ per cm3 glass as luminescent species for the visible region, especially for blue and red emission. Phase separation occurs in glass samples with high SiO2 content. SiO2-rich droplets in a Zn2+- Al3+- enriched matrix were formed. Sm3+ ions prefer the Zn2+-Al3+-rich glass matrix. By thermal treatment glass samples were transformed into glass ceramics with the main crystal phases: Zn2SiO4 (willemite), ZnAl2O4 (gahnite) and SiO2-mixed crystals. XRD, SEM and TEM measurements were carried out. The luminescent Sm3+ ions are concentrated in the glass matrix. Photoluminescence excitation and emission spectra were recorded in the UV-VIS-region and the time resolved emission behavior was measured by excitation with a N2-laser at 337 nm. The glasses and glass ceramics have interesting chemical and physical properties, high Tg around 700°C, low coefficient of thermal expansion ~ 4 ppm/K, and high chemical stability.

Abstract: Monovalent silver ions have been introduced in a sodium niobium borophosphate glass by poling the glass under heating and high voltage. A silver thin film electrode was previously deposited on the glass surface and put in contact with the anode. Second harmonic generation (SHG) has been obtained after the thermal poling treatment. The SHG signal has been studied through an analysis of transmitted polarized Maker-fringe patterns. The transmitted current curves during poling are registered and compared to the curves obtained from poling of the natural glass by the use of ndoped silicon anode. It indicates a complex space-charge-migration process during the poling treatment resulting in modification of the nonlinear SHG glasses signal profile.