Advanced Materials Research Vols. 39-40

Abstract: The characterization of the early reaction steps of the corrosion process in liquid media represents a tremendous challenge within the field of chemical analysis. In response to this, some known instrumental methods like AFM, SIMS and SNMS have proven themselves useful. Within the present work on the subject, we will introduce an optimized direct chemical etching technique which allows the quantification of the primary removed glass surface layers. It is based on the established etching procedure using hydrofluoric acid in combination with other mineral acids. The above mentioned method is modified and appropriately extended by enhancing the precision of the selected exposure time with the help of an automatic dipping device. Other improvements include the minimization of the dissolution rate by lowering the test temperature to – 10°C and also the decrease of the aqueous activity by adding alcohol instead of water into the acid solutions. Furthermore, by applying sequential etching with low concetrations of HF and HNO3, very small removal depths within the range below 10 nm per step are obtained. The equivalent components from the dissolved surface layers of the examined float glass are determined by subsequent chemical analysis of the etchants via AAS and ICP-OES. Their dissolution character can be well distinguished through a comparison between the gravimetric mass loss qgravim and the so-called normalized mass release per surface area qsol, calculated from the chemical composition of the corrosive solutions.

Abstract: Flow – through leaching experiments have been conducted on E – glass fibres in distilled water and buffered potassium borate solution at temperatures 50, 70 and 90°C and at flow rates 8, 16 and 32 ml/hour. Time dependencies of normalized amount of leached elements in corrosive solutions have been determined. Values of stationary leaching rates were calculated and values of activation energies were found.

Abstract: One of the general symptoms of ageing is a volume change due to the moisture exposure, known as an irreversible moisture expansion (IME). It is assumed IME of fired clay-based materials is governed by the presence of inner-surface and interlamellar OH-groups, present in clay minerals and assumed to be also abundant in amorphous intergranular phases. IME was measured on a series of clay-based ceramic bodies with a variable content of clay raw minerals under two different hydrothermal conditions. IR spectroscopy was used to correlate IME with the increasing amount of inner and interlamellar hydroxyl groups.

Abstract: Glass fiber is commonly used as thermal insulation for the equipment in nuclear power plants. In case of LOCA (Loss Of Coolant Accident), the insulation can be damaged by the jet impact and carried to the screens of the emergency systems needed to cool the reactor. In this case, the fibers can be strongly corroded by particular cooler solutions and their behavior under this environment can modify the head loss of the screens. The pH value of coolant solution can be stabilized respectively by borate or phosphate buffering systems depending of the plants. A new type of small scale laboratory equipment allowing simultaneous study of the chemical durability of glass fibers and hydrodynamic conditions on the glass fiber bed placed on the screen was developed and tested. The results obtained for leaching of glass fibers in sodium borate and sodium phosphate solutions show the significant difference between the saturation levels of Ca, Al, and Si in tested leaching solutions. The observed difference in leaching chemistry caused significant differences in the head loss value and its time development. This preliminary conclusion opens a new research field to improve the NPPs but it has to be substantiated by additional records and sensitivity studies including uncertainties analysis.

Abstract: One of the major attributes of glass-ceramics is an ability to tailor their thermal expansion characteristics and this makes them ideal candidates for sealing to a wide variety of metals and alloys; however, during the sealing process, reaction of diffusing metal species with glass constituents may occur, and this can lead to the formation of undesirable phases within the interfacial region. In addition, diffusion of metal species into the bulk glass away from the interface may affect the overall crystallization kinetics and can result in the formation of unwanted crystalline phases which may be detrimental to the lifetime behaviour of a seal component. This contribution outlines and discusses the factors affecting the crystallization behaviour of glasses employed in seal manufacture and describe methods by which undesirable reactions can be alleviated or minimized through effective control of the process parameters and starting glass composition.

Abstract: The sintering of Low Temperature Co-fired Ceramics prepared from alumoborosilicate glass- and Al2O3 powders of similar small particle size was studied by dilatometry, heating microscopy, microstructure analysis, glass- and effective viscosity measurements. The steric effect of Al3O3 inclusions was studied using a “non-reactive” model composite. With increasing Al3O3 volume fraction ( Φ ≤ 0.45), sintering decelerates and its final stage shifts to higher temperature. The attainable shrinkage is reduced as Al2O3 particle clusters bearing residual pores become more frequent. The kinetics of sintering could be described formally superposing the weighed contributions of differentially sized and randomly composed glass-crystal particle clusters and assuming a sintering rate controlled by the effective matrix viscosity, which increases with Φ and with progressive wetting of Al2O3 particles during densification. The “reactive” model composite shows significant dissolution of Al2O3 into the glass, which has two opposed effects on sintering: reducing Φ and increasing glass viscosity. For the present case ( Φ = 0.25), the latter effect dominates and sintering is retarded by Al2O3 dissolution. Crystallization of wollastonite starts after full densification. Dissolution of Al2O3 was found to promote the subsequent growth of anorthite.

Abstract: To survey new method for controlling thermal expansion of glass-ceramic material, we studied the coefficient of thermal expansion (CTE) and the microstructure of β-spodumene glass-ceramics. We found that the CTE of the β-spodumene glass-ceramics is remarkably reduced by introducing interstices at the boundary between the crystalline and glass phases. Despite its relatively low crystallinity (45vol%), a newly developed glass-ceramic showed an anomalous low thermal expansion of 7x10-7 /oC, which compares to that of silica glass. The mechanical strength of this glass-ceramic was at the same level as that of natural stones: marble and granite.

Abstract: The electrochemically induced nucleation and crystallization in a glass with the basic composition (mol%) 52.6 SiO2 · 18.7 Al2O3 · 14.3 MgO · 7.8 TiO2 · 4.7 B2O3 · 2.0 CaO leads to the formation of oriented mullite crystals. Electron microscopy techniques (energydispersive X-ray microanalysis and electron backscatter diffraction) were used to analyze the composition and the orientation of the crystals in the glassy matrix. The mean composition of the mullite crystals matches the formula Al4,65Ti0,11Si1,24O9,675. The electron backscatter diffraction pattern is consistent with data from the literature that needle-like mullite crystals are fast-growing parallel to the crystallographic c-axis [001]. The composition of the melt grown mullite crystals does not depend on the melting temperature in the range between 1230 °C and 1350 °C.

Abstract: α-Bi2Mo3O12 and β-Bi2Mo2O9 crystal phases have been synthesized by crystallization of a glass for a short time. Amorphous samples with the compositions corresponding to the alpha and beta bismuth molybdates have been obtained at high cooling rates (104-105 K/s). XRD, DTA and SEM studies were performed to examine the phase formation, thermal behavior of the glasses and the microstructure of obtained crystalline products. By scanning electron microscopy it was shown that the obtained bismuth molybdates consist of dense agglomerates, containing uniform distributed crystals with submicron size.

Abstract: Photo-thermo-refractive (PTR) glass belongs to a class of photosensitive multicomponent silicate glasses which demonstrate photo-thermo-induced precipitation of a crystalline phase. Photo-induced crystallization of sodium fluoride results in refractive index variations in exposed areas of PTR glass which was successfully applied for phase hologram recording. However, even if this glass is widely used for advanced optical applications such as the stabilization of laser diodes or spectral beam combining, the detailed mechanism of its photosensitivity is not well known. We present in this paper an analysis of the evolution of the absorption spectra of UVexposed PTR glass at the early stages of the thermal treatment (nucleation). Absorption spectra were measured on samples nucleated at constant temperature for different periods. Analysis of the structure of these spectra shows that the optimum of the nucleation process is associated with the appearance of a new absorption band in visible range that disappears when the crystal growth process starts. Analogous study was performed during the nucleation process. High temperature spectroscopic measurements were performed during the thermal treatment. It was found that absorption spectra evolve with a change of temperature, i.e. the band structure of these spectra is different whether or not the temperature is decreased below a temperature of ~400°C. We associate this process with the change of phase of the silver bromide nuclei (liquid/solid) that appears during the cooling or heating processes.