The development of ceramic materials resistance in various aggressive media combined with required mechanical properties is of considerable importance for enabling the wider application of ceramics. The corrosion resistance of ceramic materials depends on their purity and microstructure, the kind of aggressive media used, and the ambient temperature. Therefore, the corrosion resistance of alumina ceramics in aqueous HNO3 solutions of concentrations of 0.50 mol dm−3, 1.25 mol dm−3, and 2.00 mol dm−3 and different exposure times—up to 10 days—have been studied. The influence of temperature (25, 40, and 55 °C) was also monitored. The evaluation of Al2O3 ceramics corrosion resistance was based on the concentration measurements of eluted Al3+, Ca2+, Fe3+, Mg2+, Na+, and Si4+ ions obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES), as well as density measurements of the investigated alumina ceramics. The response surface methodology (RSM) was used for the optimization of parameters within the experimental “sample-corrosive media” area. The exposure of alumina ceramics to aqueous HNO3 solutions was conducted according to the Box–Behnken design. After the regression functions were defined, conditions to achieve the maximum corrosion resistance of the sintered ceramics were determined by optimization within the experimental area.
The present study examines the potential of microwave heating as an emerging and innovative energy-efficient alternative to conventional heating techniques used for different materials, with a focus on the processing of ceramic materials. Modern ceramics are studied extensively, and their use and different applications are wide due to many advantages of these materials. The most important factor in microwave sintering which differentiates it from conventional heating techniques is a unique heat transfer mechanism. Microwave energy is absorbed by the material, hence the transfer of energy takes place at the molecular level. This way, the heat is generated throughout the material, i.e. on the inside as well on the outside. This allows a very low temperature gradient throughout the material cross section. When conventional sintering is used, typically at high heating rates, high temperature gradients pose a problem. The accelerated microwave heating occurs through the whole volume, so the heating is uniform, which limits the grain growth and coarsening, and leads to a uniform and fine microstructure. The densification is accelerated as well during the unique heat transfer mechanism of microwave sintering, which enhances the mechanical properties of the sintered materials.This paper discusses the use of microwave sintering in the manufacturing of different modern technical materials, namely ceramics, composites, metals and alloys, and glasses. The improvement of different properties is described using the available literature.
Noise is an all-present environment pollutant, considered to be one of the greatest contemporary pollutants. World-wide, co-ordinated actions are conducted in order to develop systems which minimise the noise influence onto society.In this article we argue that novel approach to suppression of influence of noise is useful. Furthermore, we argue that the efficient approach is formulation of the efficient, broadly applicable, ubiquituous, adaptive noise-protection system. The approach combines the natural noise-protection form based on plants with the artificially formed coatings.Elements of the system are discussed, its formation and maintenance analysed and perspectives conjectured.
The goal of this study is to compare the properties of cold isostatically pressed (CIP) alumina (A2O3) samples sintered by conventional (electrical) and nonconventional (hybrid microwave) techniques. X-ray diffraction was used to determine phase composition of A2O3 samples (raw powder and granules). Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were used to investigate the thermal behaviour of the Al2O3 powder and granules during the thermal treatment. Compaction of spray dried A2O3 granules into green compact bodies was performed by CIP, followed by sintering of green bodies at 1600 °C in an electrical and hybrid microwave kiln, respectively. Scanning electron microscopy (SEM) was used to analyse morphology of the Al2O3 granules and fracture surface of Al2O3 compacts derived by both sintering techniques. Higher linear shrinkage and densification were obtained for alumina samples sintered in electrical kiln (conventional method), while sintering by faster and more energy efficient hybrid microwave kiln (non-conventional sintering method) yielded alumina samples with finer grain size. Alumina samples sintered by electrical kiln displayed higher relative densities and lower porosities.
The goal of this research is the statistical optimisation of the chemical stability of hybrid microwave-sintered alumina ceramics in nitric acid. The chemical stability of ceramic materials in corrosive media depends on many parameters, such as the chemical and phase composition of the ceramics, the properties of the aggressive medium (concentration, temperature, and pressure), and the exposure time. Therefore, the chemical stability of alumina ceramics in different aqueous nitric acid solution concentrations (0.50 mol dm−3, 1.25 mol dm−3, and 2.00 mol dm−3), different exposure times (up to 10 days), as well as different temperatures (25, 40, and 55 °C), was investigated, modelled, and optimised. The chemical stability of high purity alumina ceramics (99.8345 wt.% of Al2O3) was determined by measuring the amount of eluted ions (Al3+, Ca2+, Fe3+, Mg2+, Na+, and Si4+) obtained by inductively coupled plasma atomic emission spectrometry. The changes in the density of alumina ceramics during the chemical stability monitoring were also determined. The Box–Behnken approach was employed to reach the optimum conditions for obtaining the highest possible chemical stability of alumina at a given temperature range, exposure time, and molar concentration of nitric acid. It was found that an increase in exposure time, temperature, and nitric acid concentration led to an increase in the elution of ions from hybrid microwave-sintered alumina. Higher amounts of eluted ions, Al3+ (14.805 µg cm−2), Ca2+ (7.079 µg cm−2), Fe3+ (0.361 µg cm−2), Mg2+ (3.654 µg cm−2), and Na+ ions (13.261 µg cm−2), were obtained at 55 °C in the 2 mol dm− 3 nitric acid. The amount of eluted Si4+ ions is below the detection limit of inductively coupled plasma atomic emission spectrometry. The change in the alumina ceramic density during the corrosion test was negligible.
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