Rice hull-derived silica: applications in Portland cement and mullite whiskers

Silica was extracted from rice hulls by three processing routes, starting with acid treatment followed by burning and milling. The amorphous white silica powder showed a surface area of 260 to 480 m²/g, purity above 99% and average particle size of 2.0 to 0.6 mm. This silica is suitable for the preparation of mullite whiskers employing the rare earth aluminosilicate glass technique. Due to its highly pozzolanic reaction, this prepared silica is used as an additive in high performance concrete

Nucleation and growth of mullite whiskers from lanthanum-doped aluminosilicate melts

Nucleation and growth of mullite whiskers in the La2O3-Al2O3-SiO2 system were investigated in the 1500degrees-1700degreesC temperature range. A differential thermal analysis (DTA) showed that the mullitization temperature decreases from 1350degreesC to 1240degreesC as a result of lanthania doping. In the temperature range of 1250-1500degreesC, most of the mullite grains have an Al2O3/SiO2 = 1.5 composition throughout the ceramic body; however, from 1400degreesC upward, the number of anisotropic grains with the Al2O3/SiO2 = 1.3 composition begins to increase. The concentration of alumina in the composition of the grain-boundary phase decreases as firing temperatures increase. At temperatures > 1500degreesC, alumina grains and whiskers grow on the internal and external surfaces of the ceramic body with the characteristic Al2O3/SiO2 = 1.3 composition. Removal of the mullite whisker layer by acid attack revealed an alumina-rich, rosace-like patterned microstructure correlated with the process of whisker nucleation and growth. In the early stages, whisker growth rates were found to be near 60 mum/h. Experimental evidence pointed to nucleation inside the thin glass layer on the external surface

Mullite whiskers grown from erbia-doped aluminum hydroxide-silica gel

Mullite whiskers and anisotropic grains that were derived from erbia-doped aluminum hydroxide-silica gel were studied. Firing 3.0-mol%-erbia-doped isostatically pressed pellets at 1600 degrees C for 1.0-8.0 h resulted in a high surface concentration of mullite whiskers. Their c-axes were aligned preferentially along the pellet surface; the maximum length was 50 mu m, and the maximum aspect ratio was 23. The pellet surface was fully covered by mullite whiskers, and small anisotropic grains with a low aspect ratio were observed in the bulk. The voids that were observed in the fracture surfaces were covered fully by mullite whiskers. The large number of voids resulted in an apparent density of 1.60 g/cm(3) in the sintered pellets. The molar ratio of alumina to silica in the whiskers was in the range of 1.30-1.45 tan average value of 1.31), regardless of whether the alumina/silica powder compositions were mixed in a 3:2 or 2:1 ratio

Anisotropic grain growth in mullite powders as a result of high-energy ball milling

10.1111/j.1551-2916.2007.02063.xJournal of the American Ceramic Society90124055-405

Effectiveness of the 40-Minute Handmade Manikin Program to Teach Hands-on Cardiopulmonary Resuscitation at School Communities

Bystander training in cardiopulmonary resuscitation (CPR) is crucial to improve the victims' survival and quality of life after sudden cardiac arrest. This observational study aimed to determine the success rate of 2 different programs of CPR training for children, adolescents, and adults in school communities. We assessed the development and acquisition of the following CPR skills checking local safety, assessing victim's responsiveness, calling for help, assessing victim's breathing, and performing chest compression (hands and straight arms placement on the chest, compression velocity, depth, and chest release) using a 40-minute program with handmade manikins or the 120-minute program using intermediate-fidelity manikins. There were 1,630 learners (mean age 16 years, 38% male) in the 40-minute program, and 347 learners (mean age 27 years, 32% male) in the 120-minute program. The lowest successful pass rate of learners that developed CPR skills was 89.4% in the 40-minute program and 84.5% in the 120-minute program. The chances of success increased with age in the same program (compression rate and depth). The success rate also increased with the more extended and intermediate-cost program at the same age (assessing victim's responsiveness, calling for help, and assessing the victim's respiration). In conclusion, a 40-minute and cheaper (low-cost handmade manikin) CPR program was adequate to develop and acquire the overall CPR skills for >= 89% at school communities, independently of gender. However, some individual CPR skills can be further improved with increasing age and using the longer and intermediate-cost program. (C) 2020 Elsevier Inc. All rights reserved

Influência do Y2O3 na sinterização rápida de mulita comercial Influence of Y2O3 on rapid sintering of commercial mullite

A mulita tem se tornado um material forte candidato para uso como cerâmica avançada e funcional. Nesse sentido, muito interesse tem sido devotado no estudo de aditivos de sinterização. Assim, este trabalho tem por objetivo analisar a influência do Y2O3 na queima rápida de mulita comercial. Pó de mulita comercial foi dopado com 0,5, 1,0 e 2,0% de Y2O3 prensado isostaticamente e submetida a ciclos de sinterização rápida a 1600 ºC. Foi determinada a densidade dos corpos sinterizados e executada sua análise por difração de raios X e microscopia eletrônica. A sinterização rápida da mulita gera corpos com baixa densidade, enquanto a adição de Y2O3 possibilita atingir elevadas densificações nos corpos mulíticos. O aumento do teor de Y2O3 não provoca alterações significativas na distribuição de tamanho de grão da mulita quando de sua sinterização rápida. Elevadas taxas de aquecimento permitem a obtenção de microestruturas com distribuições de tamanho de grão mais estreitas.<br>Mullite has become a strong candidate material for advanced structural and functional ceramics. Much interest has recently focused on sintering aids for mullite. The aim of this work was to evaluate Y2O3 influence on rapid sintering of commercial mullite. Mullite powder was doped with Y2O3, pressed under a cold isostatic pressure and fast sintered at 1600 ºC. Sintered mullite was characterized by density determination, X ray diffraction and electronic microscopy. Rapid sintering produces low density mullite bodies, however, Y2O3 addition significantly increase the densification of fast fired doped bodies. Rise of Y2O3 content do not alter the grain size distribution of rapid sintered mullite. High heating rates produce microstructures with narrower grain size distributions