Electromagnetic characteristics of biosilica from rice husk

Abstract

Rice husk, being a widely available natural plant renewable agricultural resource, can be transformed into effective reinforcing fillers of special concrete and gypsum building materials. The samples of silica from rice husks were synthesized by thermal oxidative pyrolysis and their electromagnetic and microstructural characteristics were investigated. It was found that the rice husk itself is practically EM-wave transparent material in the frequency range of 0.1-7 GHz, while the products of its thermal oxidative pyrolysis have different microwave absorbing properties, depending on the amount of oxidizing agent used. The X-ray powder diffraction data showed the predominant presence of amorphous silica in the samples of rice husk ash with a small amount of α-quartz, α-cristobalite and α-tridymite. At a pyrolysis reaction temperature of rice husk of about 560 ± 20°C, the resulting product, in addition to amorphous silica and crystalline phases of silicon dioxide, contains traces of graphite particles, which leads to a sharp increase in dielectric characteristics and effective microwave absorption. When the temperature of the pyrolysis reaction of rice husk rises above 700°C the EM-wave absorption of such materials decreases. Thus, on the basis of the experiments carried out, the optimal ratios of rice husk and the used oxidizer of ammonium nitrate were revealed to obtain environmentally friendly ecological low-cost powder nanostructured biosilica additives for concrete and gypsum building compositions with increased effective radio absorption in the frequency range of the electromagnetic field above 1 GHz.

Study of Electromagnetic Shielding Properties of Composites Based on Glass Fiber Metallized with Metal Films

“…According to the data obtained in the course of the performed investigation, glass fibers metallized with submicron films of brass, neusilber, and stainless steel are a good dielectric filler for polymer composite and building materials. At the same time, the studied dielectric micriwave-absorbing metallized glass fiber fillers are more effective at EMI shielding when compared to the previously studied dielectric materials such as rice husk ash [15] or brass micropowder [17]. It is possible to recommend the addition of glass fibers metallized with submicron films of brass, neusilber and stainless steel in the form of a microwave-absorbing and EMI-shielding reinforcing filler to poured concrete.…”

“…Previously, it was shown that from an economic and environmental point of view, it is promising to use the products resulting from processing various industrial wastes as raw materials for low-cost microwave-absorbing/EMI-shielding fillers for concrete building and structural materials [10][11][12][13][14][15][16][17]. However, as the results of studies [10][11][12][13][14][15][16][17] show, the use of a large mass fraction of such fillers (from 15 to 50%) is required to achieve effective electromagnetic shielding in the microwave range with electromagnetic radiation frequencies above 3 GHz. Additionally, it is necessary to take into account the practical fact that microsized powder radio-absorbing/radio-shielding fillers mostly lead to a significant decrease in the strength of concrete that is based on them.…”