M. Lutfy Öveçoğlu

Crystallization Behaviour of some TeO2-ZnO Glasses

Zinc-tellurite glasses, comprising 80 mol% TeO2 20 mol% ZnO (hereafter referred to as 0.8TeO2 – 0.2ZnO) and 60 mol% TeO2 40 mol% ZnO (hereafter referred to as 0.6TeO2 – 0.4ZnO), were crystallized at different temperatures on the basis of DTA analyses. Using XRD, SEM and EDS analyses, the crystallizing phases, with their shapes and sizes, were determined in these glass compositions. Thus, different effects of ZnO content on crystallization of binary zinc-tellurite glasses were studied. It was found that crystallizing phases were different for two different samples. In the 80TeO2 20ZnO glasses, α-TeO2 (paratellurite) crystallizes first at around 428 o C, and Zn2Te3O8 phase crystallizes at around 458 o C. On the other hand, in the 60TeO2 40ZnO glass, only the Zn2Te3O8 phase crystallizes. Since this crystallization takes place at around 432 o C, it can be concluded that increasing the ZnO content in zinc-tellurite glasses slightly increases the crystallization temperature.

Sintering Effect on Mechanical Properties of Composites Made of Bovine Hydroxyapatite (BHA) and Commercial Inert Glass (CIG)

Composite biomaterials of calcinated bovine bone derived hydroxyapatite (HA) doped with 5 and 10 wt% commercial inert glass (CIG) were prepared by sintering. The production of HA from natural sources satisfies economic and time-saving aims, while the use of CIGs is directed by economic and ecological aspects. The produced composites were subjected to scanning electron microscopy (SEM) and X-ray diffraction analysis. Measurements of compression strength, microhardness, and density were also carried out. The experimental results and their discussion showed that the type and the composition of incorporated glass are of crucial importance. Consequently, provided that an optimum amount of glass is incorporated, the resultant materials can exhibit good values of compression strength and microhardness and hence they can be suggested for potential use in load-bearing biomedical applications.

Development of Si-O-C Based Ceramic Matrix Composites Produced via Pyrolysis of a Polysiloxane

Proceedings of the 8th Conference and Exhibition of the European Ceramic Society; Istanbul; Turkey; 29 June 2003 through 3 July 2003

Crystallization Behavior of some TeO2-GeO2-ZnO Glasses

Abstract. Crystallization behaviors of 80 mol% TeO2-10 mol% GeO2-10 mol% ZnO, 85 mol% TeO2-5 mol% GeO2-10 mol% ZnO and 90 mol% TeO2-5 mol% GeO2-5 mol% ZnO glasses were investigated by using differential thermal analysis (DTA), scanning electron microscopy (SEM) and X-ray diffraction (XRD). DTA curves in the temperature range from 25 °C to 750 °C with the heating rate of 10 °C/min are used to determine the thermal properties such as glass transition temperature Tg, and crystallization peak temperatures Tp. Glass samples were crystallized in the vicinity of these crystallization peak temperatures. Phase analyses were carried out by XRD and SEM techniques.

Chemical Degradation of Restorative Materials

The chemical environment is an important aspect of the oral environment that could have an appreciable influence on restorative materials. The effects of chemical media on surface roughness, Vickers hardness and weight loss of a ceramic material, a hybrid ceramic resin, an ormocer and a posterior composite were investigated. The specimens were made for each material and were stored in artificial saliva at 37oC for 24 h. The weight of the specimens were recorded and then randomly divided into two groups and stored in the following chemicals for 2 weeks at 37oC: 0.02 N citric acid and 0.02 N lactic acid. After conditioning, the specimens were weighed again and subjected to Vickers hardness and surface roughness tests. The specimens were also examined in the scanning electron microscopy (SEM). All results were analyzed statistically. After a 2 week storage period, there was a significant weight loss in P60 TM stored in citric acid and Admira TM stored in lactic acid. Vickers hardness values of all materials except Finesse were increased significantly (p<0.05).

Mechanochemical Synthesis of WC Powders by Mechanical Alloying

A WC powder alloy was synthesized by mechanical alloying of elemental tungsten and graphite powders. Mechanical alloying runs were carried out in a heavy-duty attritor and subsequently in a Spex Mixer-Mill at room temperature. X-ray investigations conducted on the attritor-milled powders revealed only the presence of the W phase and the indication of carbon amorphization with milling time. When the attritor-milled powders were annealed at 1100 °C, metastable and stable WCx phases emerged in the microstructure. Progressive Spex milling of the powders attritor-milled for 30 h was carried out to follow the transformation of into the stable WC phase which was achieved between 20 and 30 h during Spex milling. Analytical transmission electron microscopy experiments on the Spex milled powders verified the presence of the WC powder particles.

Characterization of Fly Ash-Based Glass-Ceramic Samples Quenched in Air Followed by Crystallization Stage

The effect of quenching in air on the microstructure and the mechanical properties of the fly ash-based glass-ceramic samples were investigated. SEM observations clearly revealed that the presence of homogeneously dispersed tiny crystallites and crystalline grain size which increased with holding time at the nucleation stage. Furthermore, extensive cracks on the surface of the glass-ceramics were present due to quenching in air. On the other hand, XRD analyses clearly showed that the only crystalline phase that observed was diopside (Ca(Mg, Al)(Si, Al) 2O6). The result of the Vickers microhardness tests indicated that the hardness values of the fly ash-based glass-ceramics decreased with increasing holding time at the nucleation temperature. Resistance to wear decreased with the increase in crystalline grain size, similar to hardness values.

Optical Properties and Crystallization Kinetics of TeO2-GeO2-ZnO Glasses

Ultraviolet visible near-infrared (UV/VIS/NIR) and differential thermal analysis (DTA) measurements were carried out in order to investigate the optical and thermal properties of TeO 2- GeO 2-ZnO glasses. The samples were prepared by fusing the mixture of their respective reagent grade powders in a platinum crucible at 850 °C for 1h. According to the DTA scan curves of 0.85TeO 2-0.05GeO 2-0.10ZnO and 0.90TeO 2-0.05GeO 2-0.05ZnO glasses taken at different heating rates, activation energy values and the Avrami constants for the crystallization peaks were determined. The absorption spectra measured between 300 - 800 nm wavelength region were used to determine the optical band-gap and Urbach energies of the glasses.

Production of Fly Ash Based Glass-Ceramics by Sintering

The properties of glass-ceramics produced from Çayırhan Thermal Power Plant fly ash by sintering was investigated by means of Differential Thermal Analysis(DTA), X-ray Diffraction(XRD) and SEM techniques. The mechanical, physical and chemical properties of the samples were also determined. The crystallites occurred in the microstructure of the samples were observed from the SEM micrographs of the samples. XRD patterns of the samples clearly showed that the main crystalline phase was diopside[Ca(Mg,Al)(Si,Al)2O6] . It was also found that the mechanical and physical properties of the sintered glass-ceramic samples depended on the amount of the crystalline phase.

Effect of Modifier Content on the Crystallization and the Optical Gaps of a TeO2-ZnO Glass

On the basis of DTA analyses, crystallization mechanism and crystallization activation energy of 0.6TeO 2-0.4ZnO glass were determined. Also, the effect of ZnO content on the optical band-gap and Urbach energies of (1-x) TeO 2 - (x)ZnO (where x=0.2, 0.3 and 0.4 in molar ratio) glasses have been examined by ultraviolet- visible- near infrared (UV / VIS / NIR) absorption measurements between 300 - 800 nm wavelength region. It is found that they both are functions of glass composition.