NULL

NULL

NULL

Silica aerogel is a lightweight and low-permittivity dielectric material that possesses attractive features for use as an antenna substrate. In this paper, we characterize the radio frequency and microwave dielectric permittivity properties of substrates composed of silica aerogel encapsulated in polymer aerogel in the frequency range from 10 MHz to 8.5 GHz. Characterized silica-based aerogel substrates show relative permittivity values varying between 1.055 and 1.25 and loss tangent values ranging from 5.08 × 10−4 to 0.0206. Silica-based aerogel substrates thus have the potential of use in designing antennas with high gain and large bandwidth. Validation is presented by characterizing the performance of a manufactured C-band circular patch antenna on silica-based aerogel substrate. The performance is also compared to a design that uses Rogers Duroid RT5880 substrate. The results reveal that the silica aerogel substrate antenna at 7.2 GHz provides 1.5 dB increase in gain, 88% enhancement in bandwidth and 68.5% reduction in mass, in comparison with the antenna on RT5880 substrate.

NULL

NULL

NULL

NULL

The present work investigates the influence of magnetic field on the friction coefficient displayed by sliding of steel pin on aluminium, steel and polyamide discs lubricated by paraffin oil and dispersed by different lubricant additives such as zinc dialkyldithiophosphates (ZDDP), molybdenum disulphide (MoS2), heteropolar organic based additive (CMOC), graphite (C), detergent additive (calcium sulphonate) (DA), polytetrafluroethylene (PTFE) and polymethyl methacrylate (PMMA). The experiments showed that, aluminium as a paramagnetic material recorded higher values of friction coefficient than steel and polyamide. That observation confirms that in spite of the increase of the attractive force generated from the magnetic field for steel/steel and the electrostatic charge for polyamide/steel, oil molecules were more adhered into the surfaces of steel and polyamide than aluminium and consequently friction coefficient significantly decreased. When the oil was dispersed by additives, it was found that, friction coefficient slightly increased with increasing magnetic field for oil dispersed by ZDDP additive. For oil dispersed by MoS2 friction coefficient displayed by aluminium disc showed relatively lower values in the presence of the magnetic field than that displayed by polyamide and steel discs. Magnetic field drastically decreased friction coefficient displayed by aluminium and steel disc. As for polyamide disc friction coefficient slightly increased with increasing the magnetic field. Aluminium disc displayed the lowest friction coefficient in the presence of C, which decreased with increasing magnetic field. For steel disc friction coefficient displayed the highest values. Polyamide disc showed no change in friction values as the intensity of the magnetic field increased. For oil dispersed by DA, steel disc showed significant friction decrease. Aluminium disc showed slight friction increase with increasing magnetic field, while polyamide disc showed slight friction decrease. PTFE particles dispersed in the oil were much influenced by the magnetic field, where the friction coefficient displayed by steel drastically decreased with increasing magnetic field. Aluminium and polyamide discs showed an increasing trend as the magnetic field increased. As for PMMA particles dispersed in oil aluminium disc showed slight friction increase, while steel and polyamide discs gave decreasing trend of friction as the magnetic field increased.

NULL