Showing papers on "Calcium aluminates published in 2006"

Effects of CaCO3 content on the densification of aluminum nitride

Abstract: The effect of adding up to 134 wt% CaCO 3 on the densification behavior of aluminium nitride (AlN) was investigated during pressureless sintering between 1100 and 2000 °C The presence of second-phases, weight losses, Ca contents, and microstructures of sintered samples were correlated with the densification curves Two microstructural aspects determined the densification of aluminum nitride with CaCO 3 : second-phase evolution path and formation of large pores Additions of small amounts of CaCO 3 caused the formation of higher melting point calcium aluminates (mainly CA 2 ) that increased the temperature at which liquid-phase sintering process started, but once activated rapid densification was observed For larger CaCO 3 amounts, liquid-phase started to form at lower temperature, but the initial densification was slow, diminishing the advantage of lower C 12 A 7 related eutectic temperature Irrespective of the initial CaCO 3 content, all second-phase evolution paths converged to CA phase above 1600 °C, suggesting that during sintering of AlN with CaO at high temperatures, a liquid phase with composition of CA phase is more stable than others compositions The effect of this composition changing on densification is discussed Large pores were formed in the sites originally occupied by large particles of CaCO 3 and retarded the bulk densification in samples with high additive contents

The effect and its mechanism of calcium carbonate on the cement based materials

Abstract: The effect and its mechanism of calcium carbonate on the cement based materials were studied in this paperThe relationship between fineness of limestone and early stress,long-term stress of the mortar were investigated respectivelyResult show that the hydration of cement is accelerated by limestone flour at early agesWhile the later stress is enhanced chiefly by activity of limestone as calcium aluminates monocarbonate are formedSo the stress and durability of the cement based materials are increased

Synthesis of calcium aluminates through the polymeric precursor route

Abstract: The Pechini method is a polymeric precursor route for the synthesis of polycrystalline ceramics of high purity. The conventional solid state synthesis of calcium aluminate compounds requires long-term lasting sintering at high temperatures. In this study, the Pechini method was applied for the preparation of pure CaO.Al2O3 and 3CaO.Al2O3. Solid state synthesis was also used in order to compare the final products obtained by the two methods. The intermediate and final products obtained were characterized by XRD and FTIR spectroscopy. It was concluded that the Pechini method can be successfully applied for the preparation of high purity CaO.Al2O3 and 3CaO.Al2O3. The formation of 3CaO.Al2O3 and CaO.Al2O3 requires three-hour sintering at 1000°C and 900°C, respectively. The main intermediate product was 5CaO.3Al2O3 in the case of 3CaO.Al2O3, while in the case of CaO.Al2O3 there were no other crystalline intermediate products. The samples prepared through solid state synthesis, contain a considerable amount of secondary phases, even after sintering at 1350-1400°C for more than 15 hours.

Energetics of Formation and Oxidation of Microporous Calcium Aluminates: A New Class of Electrides and Ionic Conductors.

Abstract: Microporous calcium aluminates with a common formula [Ca12Al14O33-δ]2δ+(2δe-), (0 < δ ≤ 1), represent a new family of electrides with conducting extraframework electrons encaged within the framework. Their enthalpies of oxidation to eliminate these electrons and obtain a stoichiometric Ca12Al14O33 compound, and formation enthalpies relative to the most stable annealed calcium aluminate Ca12Al14O33.46 (which contains excess oxygen probably as O- and O2- species), are studied by transposed temperature drop calorimetry and drop solution calorimetry in molten lead borate at 1080 K. The standard enthalpy of oxidation per O2 consumed is −579.4 ± 9.4 kJ/mol (or −144.8 ± 9.4 kJ per encaged electron). The enthalpies obtained for samples with variable amounts of electrons (and less than 33 oxygens per formula unit) are compared to that for oxidation of a stoichiometric Ca12Al14O33 sample to an oxygen-rich composition. The study supports the idea that the oxidation of the stoichiometric Ca12Al14O33 sample results in...

Characterization of W-Ir Mixed Metal Matrix Scandate Cathode

Abstract: In this paper development and characterization of W-Ir mixed metal matrix scandate cathodes prepared by impregnating W-Ir (80% - 20% by weight) mixed metal matrices with 3% scandium oxide doped 5:3:2 barium calcium aluminates are discussed Emission characterization, Auger electron spectroscopy and life test results are discussed

Hydrothermal reaction of fly ash/hydrated lime: characterization of the reaction products

Abstract: Class F coal fly ash was slurried with hydrated lime at 90°C in 1/3, 5/3, 9/3, and 15/3 weight ratios and for 3, 5, 7, and 9 hours of hydration, in a process to prepare sorbents for SO2 removal. The amounts of aluminum, silicon, and calcium in the product of the pozzolanic reaction were determined in order to study the evolution of product composition with the initial raw materials ratio and hydration time and to relate this composition to the desulfurization capability of the material. Al, Si, and Ca were present in the solid product for any raw materials ratio and hydration time, showing that calcium silicates, calcium aluminates, and/or calcium aluminum silicates were obtained simultaneously. The products formed show a nearly constant molar ratio of Al2O3/SiO2 independent of the experimental conditions tested and similar to the Al2O3/SiO2 ratio in the fly ash. The SiO2/CaO molar ratio in the products decreased as the initial fly ash/Ca(OH)2 ratio decreased, being approximately constant for each ratio w...