Influence of Processing and Microstructure on Internal Friction of Ceramics (Invloed van de verwerking en de microstructuur op de inwendige wrijving in keramische materialen)

Abstract

Internal friction or damping is a phenomenon of converting an applied mechanical energy of vibrations by a solid body into internal energy, which dissipates over a period of time. Knowledge of the resonance frequencies and the internal friction is extremely useful in characterising fundamental properties such as elastic modulus, defects in materials, glass transitions, etc. even in materials with complex microstructures. An established characterisation technique called Impulse Excitation Technique (IET) is extensively used in the present study to analyse the internal friction properties of two material systems: crystalline zinc oxides (ZnO) and amorphous ionomer glasses (4.5SiO2-3Al2O3-1.5P2O5-3MO-2MF2, where M is alkaline earth metal). The interpretation of results is further corroborated by carrying out phase and micro structural analysis using other characterization techniques such as high temperature X-ray diffraction (HT-XRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC).ZnO varistors are used for the protection of electronic circuits against excessive transient voltages. Doping is a crucial step to achieve better electrical and thermal properties of a ZnO varistor because it results in a homogenous and dense microstructure. In the first part, the temperature dependent elastic modulus and damping properties of ZnO varistors doped with bismuth and antimony oxides are investigated. The damping peaks are associated with point defects relaxation and phase transformation mechanisms. Ionomer glasses are widely used in dentistry as a component in dental glass ionomer cements (GIC). The internal friction of ionomer glasses containing alkaline earth ions in the structure are studied in the second part of the thesis. Substitution of calcium by barium or strontium in the glass composition results in a disrupted glass network, whereas magnesium leads to a more packed network. The glass containing magnesium results in high Young s modulus due to its small ionic radius and high field strength. A theoretical model proposed by Makishima and Mackenzie is applied for estimation of Young s modulus at room temperature. By incorporating the structural information in the classical Makishima and Mackenzie model a more accurate estimate can be obtained as compared to the measured Young s modulus. An original approach is presented to describe the temperature derivative of the Young s modulus using an empirical relation based on the Makishima and Mackenzie model. The ionomer glasses showed anelastic relaxation arising from oxygen diffusion and mixed alkaline effect or both, depending on the number of alkaline earth cations (single or double) in the network. The glass transition and crystallization behaviour are interpreted using XRD, DSC and internal friction results. The crystallization studies on ionomer glasses showed apatite phase formation, which has similar structure as bone and can be used for bio-medical applications. The internal friction of calcium- fluoro-aluminosilicate glass annealed at different temperatures exhibits glass transition and crystallization peaks. The impulse excitation technique is used as a characterisation tool for a qualitative analysis of residual glass remains after crystallization.nrpages: 253status: publishe