Phase Transformation of Andalusite-Mullite and Its Roles in the Microstructure and Sinterability of Refractory Ceramic
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Phase Transformation of Andalusite-Mullite and Its Roles in the Microstructure and Sinterability of Refractory Ceramic BOWEN LI, MENGSHENG HE, and HUAGUANG WANG Andalusite has been realized as a special mineral for the production of refractory ceramics due to its unique property to automatically decompose into mullite and silica during heating at high temperature. The phase transformation from andalusite to mullite plays a critical role for the effective applications of andalusite. This study investigated the microstructural characteristics and sinterability of andalusite powder during high-temperature decomposition. The andalusite powder was bonded with kaolin and prepared as a cylinder green body at 20 MPa; it was then fired at 1423 K to 1723 K (1150 °C to 1450 °C). The microstructures and mechanical strengths of the sintered ceramics were studied by the compressive test, X-ray diffraction, and scanning electron microscopy. The results showed that newly born mullite appeared as rodlike microcrystals and dispersed around the initial andalusite. At 1423 K (1150 °C), the mullitization of andalusite was started, but the complete mullitization was not found until firing at 1723 K (1450 °C). The compressive strength of the ceramics increased from 93.7 to 294.6 MPa while increasing the fire temperature from 1423 K to 1723 K (1150 °C to 1450 °C). Meanwhile, the bulk density of the ceramics was only slightly changed from 2.15 to 2.19 g/cm3. DOI: 10.1007/s11661-017-4092-z Ó The Minerals, Metals & Materials Society and ASM International 2017
I.
INTRODUTION
AS a refractory mineral, mullite (3Al2O3Æ2SiO2) has attracted broad attention with excellent properties such as high refractoriness, low thermal conductivity and expansion, excellent chemical stability, dielectric strength, and mechanical properties at high temperatures.[1–3] It has been extensively used for producing various mullite-based ceramic products or for improving strength and toughness of other ceramic products.[4–6] In general, mullite ceramics were synthesized using kaolin, bauxite, and other minerals containing high alumina.[7–12] Andalusite (Al2O3ÆSiO2) is a natural mineral that consists of alumina and silica. It has been realized as a special mineral for the production of refractory ceramics due to its unique property to automatically decompose into mullite and silica during heating at high temperature.[13,14] The main reaction is
BOWEN LI is with the Department of Materials Science and Engineering, Michigan Technological University, 1400 Townsend Dr., Houghton, MI, 49931, and also with the Technology Center of Wuhan Iron & Steel (Group) Corp., Wuhan, 430080, China. Contact e-mail: [email protected] MENGSHENG HE is with the Technology Center of Wuhan Iron & Steel (Group) Corp. HUAGUANG WANG is with the Department of Materials Science and Engineering, Michigan Technological University. Manuscript submitted January 14, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS A
3ðAl2 O3 SiO2 Þ ! 3 Al2 O3 2SiO2 þ SiO2
½1
This phase transformation from andalusite to mul
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