Papers by Keyword: Devitrification

Abstract: Dental prostheses made of ceramic composites infiltrated with glasses have been used due to their biocompatibility and possibility to mimic the natural teeth. In this study, the devitrification behavior of 20SiO₂-25B₂O₃-25Al₂O₃-15La₂O₃-15TiO₂ glass during the infiltration process in a porous alumina preform was investigated. Glass frits were prepared by melting the raw materials at 1500 °C for 60 min. The glass was infiltrated into the alumina preform at 1,150 or 1,200 °C for 60 min. The specimens were characterized by X-ray diffraction analysis and scanning electron microscopy. After the infiltration, it was possible to note that the devitrification process occurred in the remaining glass (excess glass that did not infiltrate in the preform), forming mostly aluminum borate and mullite crystalline phases. However, within the infiltrated composite no devitrification was noticed in the infiltrated glass. Possible explanations for this behavior are discussed.

Abstract: Nanoparticles in solution offer unique electrical, mechanical and thermal properties due to their physical presence and interaction with the state of dispersion. This work aimed to study the effects of hydroxyapatite (HA) nanoparticles on the behavior of devitrification and recrystallization of glycerol (60% w/w) and PEG-600 (50% w/w) solutions during warming. HA nanoparticles of different sizes (20nm40nm60nm) and contents (0.1%0.5%,w/w) were incorporated into solutions, and were observed under a cryomicroscope. Experimental results showed that the morphologies of glycerol and PEG-600 solutions are dendritic and spheric respectively. The ice fraction of glycerol solution containing 0.1% HA with the size of 60nm decreased to 2/5 of that of solution without nanoparticles. The ice fractions of PEG-600 solutions with and without nanoparticles increased significantly between-64 °C and-54 °C, and the ice fraction of PEG-600 solution without nanoparticles increased by 92% within the temperature range. The findings have significant implications for biomaterial cryopreservation, food manufacturing, and cryosurgery.

Abstract: By copper mold casting, a bulk Fe₆₃Co₇Nb₄Zr₆B₁₉Si₁ metallic glass was synthesized in the diameter of 2mm. The Fe-based amorphous alloy exhibits soft ferromagnetic characteristics with a saturation magnetization of 77Am²/kg and a saturated magnetostriction of 25.4×10^-6. The saturation magnetization is reduced while the coercive force is slightly lowered by annealing treatment below the crystallization temperature. The devitrification gives rise to a remarkable enhancement both in saturation magnetization and coercive force.

Abstract: The application of moderate cooling rates to metal alloys of certain composition can generate metals that exhibit an amorphous microstructure on a bulk scale. This phenomenon is related to the avoidance of the nucleation of the competing crystalline phases associated with the alloy during solidification. This work describes the devitrification behaviour of the bulk glass forming Mg60Cu29Gd11 system through the use of a number of analytical techniques including DSC, laser confocal microscopy, SEM and XRD. Attention is drawn to the correlation between the more common analytical techniques and the observation of phase transformations on the surface of the metal, evident using a laser scanning confocal microscope fitted with a heating stage.

Abstract: This paper provides an overview of the crystallisation of an oxynitride glass likely to remain in a silicon nitride ceramic following firing. The crystallisation process was studied using both differential thermal analysis (DTA) and separate isothermal heat treatments in a tube furnace under nitrogen. The activation energy for the crystallisation process was determined by DTA. The nucleation temperature, Tg + 40°C, which corresponded to the maximum volume fraction of crystalline phases, agreed closely with the optimum nucleation temperature of Tg + 35°C, found from DTA. The optimum crystal growth temperature was observed to be 1210°C and yielded the - and -polymorphs of yttrium disilicate. Heat treatments at other temperatures indicated the development of phase assemblages which contained different polymorphs of yttrium disilicate as well as silicon oxynitride. Not all of the polymorphic transformations of yttrium disilicate were observed by DTA unless some crystallisation exotherms were deconvoluted, indicating that DTA analysis of the crystallisation of complex systems requires careful interpretation. It is, however, possible to simplify the system by substituting some yttrium by lanthanum. This stabilises the -polymorph of yttrium disilicate. The activation energy for crystallisation was observed to be similar to that for viscous flow of Y-Si-Al-O-N glasses.

Abstract: Selected oxyfluoroborate glasses have been investigated before and after heat treatment. Transparent glass-ceramics (TGC) were obtained during controlled crystallization (devitrification). X-ray diffraction studies confirmed that material was partially crystallized. Diffraction lines due to orthorhombic PbF2 phase were identified for heat-treated samples at various temperatures and times. Results were compared to that ones obtained for as-melted glass.

Abstract: Mineral wool products can be used for thermal and acoustic insulation as well as for fire protection. The high temperature properties and the crystallization behaviour (devitrification) of the amorphous fibres during heating have been examined. Commercial stone wool and commercial hybrid wool (stone wool produced by a glass wool process) have been compared, as well as specially produced stone wool fibres. The fibres differed in chemical compositions and degree of oxidation given by Fe3+/Fetotal ratios. The materials were studied by thermal stability tests, X-ray diffraction, Mössbauer spectroscopy, secondary neutral mass spectroscopy, differential scanning calorimetry and thermal gravimetric analysis. When stone wool fibres were heated at 800 °C in air, oxidation of Fe2+ to Fe3+ occurred simultaneously with migration of divalent cations (especially Mg2+) to the surface. Decreasing Fe3+/Fetotal ratios resulted in increasing migration and improved thermal stability. The cations formed a surface layer mainly consisting of MgO. When heated to above 800 °C, bulk crystallization of the fibres took place with diopside and nepheline as the main crystalline phases. Commercial stone wool and the specially made fibres were considerably more temperature stable than the commercial hybrid wool. Commercial hybrid wool has a high Fe3+/Fetotal ratio of 65% resulting in less migration of cations during heat treatment.