Qazi, Muhammad Pervez Iqbal (1999) STUDY OF CRYSTAL GROWTH AND CHARACTERIZATION OF Li2O-SiO2-B2O3 BASED GLASS CERAMICS. PhD thesis, University of the Punjab, Lahore.
Ternary Li2O-SiO2-B2O3system has been investigated with regard to its glass forming ability and crystallisation behaviour. The compositions studies were divided into two groups, (i) glasses containing 46, mol % silica and (ii) glasses containing 50 mol % silica and above. In each group various compositions were made by varying the B2O3 and Li2O content. It was concluded that SiO2 /Li2O ratio plays an important role in predicting the glass forming ability and thermal stability. It was found that most of clear glasses were formed where Li2O-SiO2 ratio lies between 1 and 2. Thermal stability criteria have also indicated a maximum working temperature gap of 175ÃÂÃÂÃÂÃÂ°C for the glasses with SiO2 /Li2O ratio around 1.5. Thermal expansion coefficient of these glasses was also determined. At a heating rate of 10ÃÂÃÂÃÂÃÂ°C/min its value lies between 65 x 10-7 / 0C to 98 x 10-7 / 0C in the measuring range of 30-300ÃÂÃÂÃÂÃÂ°C. Isothermal heat treatments were carried out at various temperatures for different time intervals to monitor the crystallization route of these glasses. Various phases formed as a result of heat treatments were identified by X-ray diffraction technique. It was established that when SiO2 /Li2O ratio is above 1.5 lithium disilicate was the major crystalline phase. Below this limit lithium meta-silicate appeared as the major phase, however, in certain cases lithium disilicate was also accompanied in significant amount. Scanning Electron Microscopy (SEM) of the heat treated samples has confirmed the occurrence of phase separation in these systems which seemed to be the precursor for crystallisation. An energy barrier (activation energy) is always required to be surmounted for crystallisation process to proceed. Activation energy for crystallisation calculated from the data obtained by carrying out non-isothermal heat treatment comes out to be 280 kJ/mol. This is quite comparable with the binary Li2O-SiO2 glasses. Thus indicating that B2O3 content has not strengthened the energy barrier. The electrical properties of glasses were restricted to the measurement of dielectric constant only. Its relationship with temperature, frequency and composition has been established. Its value in the present study has been found to lie between 1.114 to 21.97 at these measuring frequencies.
|Item Type:||Thesis (PhD)|
|Uncontrolled Keywords:||Crystal Growth, Li2O-SiO2-B2O3, Glass Ceramics, Silica, Thermal Stability, Crystallisation, Quartz, Glass Formation|
|Subjects:||Biological & Medical Sciences (c) > Biological Sciences(c1) > Biology (c13)|
|Deposited By:||Mr Ghulam Murtaza|
|Deposited On:||28 Jun 2006|
|Last Modified:||04 Oct 2007 21:00|
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