الفهرس 
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Abstract
Ultrasonic waves are mechanical waves with a frequency above 20 kHz. Their attenuation coefficient and velocity are dependent on the wave type and the medium in which it is traveling. This makes ultrasonic technique a very convenient sensitive tool to characterize the behaviour of materials and to investigate their physical properties. It is of technological importance to be able to control systematically the crystallization in glass systems. The ability to control crystallization allows one to utilize glasses for high technology applications such as advanced ceramics for high temperature aerospace and automotive uses. Generally, investigation of amorphous-to-crystalline transformation and estimation of degree of crystallization are of vital importance from the scientific and technical point of view~ There are many well established methods and techniques for structure study, such as X-ray and neutron diffraction which are the Widely used methods for investigating the crystallization process.
This work aims to investigate the applicability of ultrasonic measurements to study crystallization in some borate glasses. XRD is utilized to justify the structural variation during annealing of such glasses with different Li- and AI-contents. The relation between crystallization temperature and ultrasonic parameters has to be proved that it is linear in order to use such relation for quantitative analysis.Study of crystallization ultrasonic parameters relationship is also investigated to enable using such technique for quantitative analysis.
A series of lithium borate glasses were prepared by the melt quenching method at temperature of 1000 DC. The glass melts were casted into a rectangular stainless steel mold and was transferred to a
muffle furnace which slowly cooled to room temperature. DTA measurements were performed to the prepared glasses with heating of rate 5 °C/min in order to determine the crystallization temperature. Accordingly, the annealing temperatures were selected and the glass samples were heated for 5 hr at each temperature. Density was measured by Archmides method in pure toluene. XRD patterns were recorded in 28 range of 2-70° using Ni-filtered Cu-Ka radiation.
The used technique in ultrasonic parameters determination was the pulse echo technique, in which the longitudinal velocity and, hence, the longitudinal elastic moduli and the acoustic impedance were determined. from the curves of the attenuation coefficient versus temperature, the activation energy of the relaxation process could be calculated.
According to DTA results, as Lj... and AI-contents increased, the temperature of the exothermic peak ascended indicating less tendency for crystallization during preparation. This is in contrast to what is expected, where addition of Li improve the crystallization process. Therefore, this reality seems to be due to the anomaly of lithium borate glasses.
The density was found to increase systematically with the Li- and AIIcontents. Also, the density increased as either the time or temperature of crystallization ascended.
X-ray diffractograms show that the least Li-content sample of crystallize during processing while other samples show complete amorphous structure for the as-received glasses. As the annealing temperature ascends the crystallization improved but the degree of crystallization depends on the temperature. As the Li-content increases for the same heat treatment conditions the degree of crystallization diminish. Thus, the XRD results
verify the above conclusion about the effect of lithium and aluminum on the crystallization process.
At the same treatment conditions, it was found that the ultrasonic parameters (longitudinal velocity, longitudinal elastic moduli and acoustic impedance) increase linearly with increasing Li-content. Either elevating annealing temperature or time causes an increase in the crystallization process showing a linear relationship with ultrasonic parameters. Also, the calculated activation energy was found to vary (decrease) linearly with annealing temperatures. Thus, a good correlation between the density and XRD analysis, on one hand, and the ultrasonic results, on other hand, was obtained. This enables the use of ultrasonic technique as useful tool for quantitative determination of degree of crystallinity.