Tez özetleri Astronomi ve Uzay Bilimleri Anabilim Dalı 2

Preparatıon, Characterızatıon Of Aerogel Doped Polymer Composıtes And Investıgatıon Of Dıelectrıc Propertıes
In this study, the first surface modification at ambient pressure drying process performed with a hydrophobic silica-based aerogels were synthesized. In different percentages of aerogel were homogeneously doped onto the polar thermoplastic polyurethane and aerogels doped with different percentages polyurethane composite films were prepared. Structural, thermal, surface and morphological feature of prepared in pure and aerogel doped thermoplastic polyurethane (TPU) composite films were examined by FTIR analysis, TGA-DSC system, SEM-EDS and the surface contact angle measurements, respectively. Using Alpha-A High Resolution Dielectric, Conductivity and Impedance Analyzer, dielectric properties and conductivity of samples were examined in detail that determining their capacities, energy loss factors and their behaviors depent to frequency and temperature of the applied alternating electric field. In addition, all this analyzes were performed for factory-made thermoset glass fiber containing polyester and aerogel doped containing glass fiber polyester composites which is used as electrical insulating materials.

In FTIR graph of aerogel was seen vibration peaks of hydrophobic methyl (CH₃) which attaching on silika backone, succesfully. On the other hand, in FTIR graphs of doped polyurethane composites films were observed all the peaks expected for polyurethane structure and new peaks in the 1000-1100 cm^-1 range of siloxane groups in the silica-based aerogel structure was also take place in the FTIR curve of composite film. Thermogravimetric analysis was carried out from 30^oC to 1100^oC for synthesized aerogel. From results of TGA, the weight loss to 200^oC was attributed to exist of water or ethanol. Also TG curves showed that thermal resistance of aerogel doped polyurethane composite films and glass fiber containing polyester products are up to 300-320^oC.

SEM images was indicated that the aerogel powder with porosity could be successfully synthesized by surface modification. Also SEM images of aerogel doped polyurethane films were confirmed that aerogel was dispersed homogeneously in the TPU. Contact angle of silica-based aerogel prepared with TMCS co-precursor was determined 106^o which was its indicated hydrophobic feature. Pure polyurethane has hydrophilic nature but 7% aerogel doped polyurethane composite film showed the highest hydrophobic behavior. So hydrophobicity of aerogel doped PU composite films was developed with increasing amount of aerogel doped onto polyurethane film.

When the dielectric measurements were analyzed; capacity and dielectric constant aerogel doped polyurethane composite films increased with increasing temperature at low frequencies, the dielectric constant decreases at high frequencies. It can be concluded from the dielectric measurement data that capacity and dielectric constant aerogel doped polyurethane composite films increased with increasing temperature at low frequencies, the dielectric constant decreases at high frequencies. Also, silica aerogel doped composite films have lower dielectric constant than pure PU film because hydrophobicity and voids in the composite structure increased. From results, whereas dielectric constant of polyurethane was 5,1, this value was reduced to 3,3 for 2% doped silica aerogel composite film at 1kHz.

Dopping aerogel effects on electrical properties of glass fiber containing polyester bulk products were also investigated to develop the end product which has lower dielectric constant for electrical insulating systems. In this context, the dielectric constant of the %3 aerogel doped polyester was compared with neat polyester at room temperature and found that dielectric constant was decreased from 5,5 to 5 by dopping aerogel.

Also, it was observed that the relation between AC conductivity and applied frequency obeys equation for all samples. It was determined that s cofficient values are between 0,1-1 and these values are depend on temperature. It was found that polarization mechanism in structure depend on temperature and AC conductivity mechanism is Correlated Barrier Hopping.