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- Esmaeilzadeh, Saeid, et al.
(author)
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Glass forming region in Ca-Si-O-N system using CaH2 as Ca source
- 2008
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In: Journal of the European Ceramic Society. - : Elsevier BV. - 0955-2219 .- 1873-619X. ; 28:14, s. 2659-2664
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Journal article (peer-reviewed)abstract
- The present work explores the glass-forming region in the Ca–Si–O–N system, particularly in the N-rich part of the system. Bulk oxynitride glasses in the Ca–Si–O–N system were prepared by melting mixtures of CaH2, SiO2 and Si3N4 powders in a nitrogen atmosphere at 1500–1650 °C. The glasses were characterized by X-ray powder diffraction, differential thermal analysis, scanning electron microscopy and transmission electron microscopy. Glass compositions were calculated from analyses obtained by energy dispersive X-ray (EDX) spectroscopy and combustion analysis, for cation and anion compositions, respectively. The glasses were found to be X-ray amorphous and gray to black in color. The glasses retain up to 58 e/o of nitrogen and 42 e/o of calcium. The glass formation depends on reaction kinetics and the precursor used. A strong exothermic reaction is observed at 850–1000 °C, leading to formation of amorphous and crystalline oxynitride phases that melt at high temperatures upon further heating. Glass transition temperatures (Tg) were observed to vary between 798 °C and 1050 °C, and crystallization occurs typically 130 °C above the glass transition temperature. The glass densities vary between 2.79 g/cm3 and 3.25 g/cm3.
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- Iftekhar, Shahriar, et al.
(author)
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Phase formation of CaAl2O4 from CaCO3-Al2O3 powder mixtures
- 2008
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In: Journal of the European Ceramic Society. - : Elsevier BV. - 0955-2219 .- 1873-619X. ; 28:4, s. 747-756
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Journal article (peer-reviewed)abstract
- Calcium aluminate is the main constituent in calcium aluminate cements, used in a wide range of applications in construction and mining industries and recently also as biomedical implant. In applications that demand very precise reaction features, such as the biomedical ones, the phase purity is of very high importance. In this paper the formation of CaAl2O4 from CaCO3–Al2O3 powder mixtures has been studied, varying holding times between 1 and 40 h and temperatures between 1300 and 1500 °C. Phase formation was studied in samples both quenched from the holding temperatures and in samples slowly cooled. Samples were characterized by X-ray powder diffraction (XRPD), using Guinier-Hägg film data and the Rietveld method, and scanning (SEM) and transmission (TEM) electron microscopy. Samples for TEM with very high site accuracy were produced using focused ion beam microscopy. In addition to CA (CaAl2O4) the samples contained major amounts of CA2 (CaAl4O7), C12A7 (Ca12Al14O33) and minor amounts of un-reacted A (Al2O3). Trace amounts of C3A (Ca3Al2O6) were observed only for samples heated to 1500 °C. The amount of the Ca-rich phase C12A7 was found to decrease with time as it reacts with A and, to a less degree, CA2 to form CA. In agreement with previous studies the amount of CA2 formed decreases comparatively slowly with time. Its un-reactivity is due to that it is concentrated in isolated porous regions of sizes up to 100 μm. The formation of the Ca aluminates is found to be in response to local equilibriums within small inhomogeneous regions, with no specific phase acting as an intermediate phase. Samples quenched from 1500 °C were found to contain smaller amounts of poorly crystallized phases. A reaction between C and A takes place already at 900 °C, forming a meta-stable orthorhombic modification of CA. The orthorhombic unit cell with a = 8.732(2) Å, b = 8.078(2) Å, c = √3·a = 15.124(4) Å was verified by electron diffraction, revealing frequent twinning and disorder of the crystallites.
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