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Crystallization of Amorphous Precursors in the Calcia–Alumina System: A Differential Scanning Calorimetry Study
Author(s) -
Douy André,
Gervais Monique
Publication year - 2000
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.2000.tb01150.x
Subject(s) - differential scanning calorimetry , crystallization , amorphous solid , orthorhombic crystal system , calcination , materials science , calorimetry , monoclinic crystal system , phase diagram , solid solution , crystallography , phase (matter) , metastability , analytical chemistry (journal) , mineralogy , chemical engineering , chemistry , thermodynamics , crystal structure , metallurgy , organic chemistry , physics , engineering , catalysis
Amorphous powders were synthesized by spray‐drying aqueous solutions of calcium and aluminum nitrates, followed by calcination in order to completely decompose the salts. The formation of pure calcium aluminates—C 3 A, C 5 A 3 , C 12 A 7 , CA, CA 2 , and CA 6 (C = CaO and A = Al 2 O 3 )—has been studied by DSC and XRD. A number of compositions first crystallized to metastable structures without partitioning, i.e., without phase separation. This is the case for orthorhombic C 5 A 3 , orthorhombic CA, and γ‐Al 2 O 3 solid solution which may incorporate up to about 25 mol% CaO. o ‐C 5 A 3 ultimately partitioned into a mixture of C 12 A 7 and CA according to the equilibrium diagram. o ‐CA transformed partitionless into the stable monoclinic m ‐CA, and the stable CA 6 compound was obtained pure at low temperature from γ‐Al 2 O 3 solid solution through a second exotherm. Among all the calcium aluminates studied, C 3 A was the only one for which an intermediary amorphous precursor could not be obtained, CaO crystallizing as the nitrates were decomposed. Phase transitions involving metastable materials are often fast and release heat over a narrow temperature range leading to steep exothermal DSC signals even at a relatively low heating rate (5°C/min). The present results are consistent with current understanding of partitionless crystallization. They clearly show the ability of the synthesis used to achieve homogeneous mixing at the molecular level. Amorphous precursors formed in a large composition range may be advantageously used as glass models.