Naslov
Chelation Aided Sol-Gel Synthesis of Calcium Aluminate: Gel Structure and Thermal Evolution

Autori
Kurajica ; Stanislav

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
POLYCHAR19 Book of abstracts / Adhikari ; Rameshwar - Kathmandu : Nepal Polymer Institute, 2011

Skup
POLYCHAR19 – World Forum on Advanced Materials

Mjesto i datum
Kathmandu, Nepal, 20.03.2011. - 24.03.2011

Vrsta sudjelovanja
Pozvano predavanje

Vrsta recenzije
Nije recenziran

Ključne riječi
Calcium Aluminate; Sol-Gel; Chelation

Sažetak
Calcium aluminate (CaAl2O4, CA) is the main crystalline phase of aluminate cement and an important refractory material. The traditional technique of preparing pure calcium aluminates is by solid state reactions of calcium and aluminium oxides. This process takes a lot of time and energy and does not allow the fine control of the phase composition and the microstructure due to limitations of physical mixing to the micrometer scale, yielding multiple, unwanted phases. Recently, new applications for calcium aluminate have emerged in optical and structural ceramics. The advanced CA applications require sophisticated and more efficient preparation process such as sol-gel process. The advantages of sol-gel method are generally related to good mixing of starting reagents yielding more homogeneous products with greater reactivity. Consequently, the crystallization is achieved by subsequent heating of amorphous gel and lower processing temperature. The common aluminium sol-gel precursors are aluminium alkoxides, known for their high affinity for water. Uncontrolled hydrolysis of those precursors results in an aluminium hydroxide precipitate. In order to achieve control over sol to gel transformation the alkoxide hydrolysis rate has to be controlled. This is best achieved by using chelating agents such as -diketones. Chemical modification of metal alkoxide with chelating agents like -diketones enables reduction of the hydrolytic activity, i.e. the rate of hydrolysis and therefore control of condensation process of reactive metal alkoxides. The structure of modified alkoxide affects the structure of gel formed by its hydrolysis and physical properties of the final material1. Therefore, CA powder was prepared by sol–gel technique using aluminium-sec-butoxide (Al(OsBu)3, Asb) and calcium nitrate tetrahydrate (Ca(NO3)24H2O) as starting materials and ethyl acetoacetate (C6H10O3, Eaa) as a chelating agent in order to control the rate of hydrolysis of Asb. Chemical modification of the Asb precursor with Eaa has been carried out in ratio Eaa/Asb = 3/2. The dried gel and thermally treated samples were characterized by means of Fourier Transform Infrared spectroscopy (FTIR), 1H, 13C Nuclear Magnetic Resonance (NMR) spectroscopy, solid-state 27Al Magic Angle Spinning (MAS) NMR, 3Q MAS NMR spectroscopy, simultaneous Differential Thermal and Thermo-Gravimetric Analysis (DTA/TGA) and X-Ray Diffraction (XRD). From the results obtained, the effect of modification of the starting Asb on the hydrolysis process, hydrolyzed gel structure, thermal evolution and crystallization behavior is discussed. It has been established that Eaa reacts completely with Asb forming chelate. Spontaneous gellation has been observed in the sols slowly hydrolyzed by exposing to air moisture and a transparent gel was obtained. The reactivity towards hydrolysis of chelated alkoxide depends on the number of chelating ligands bonded to aluminium. Sec-butoxy groups were primarily hydrolyzed ; ethyl acetoacetate groups in less chelated units are much less susceptible to hydrolysis, while trichelated units were not hydrolyzed. Thus, in hydrolyzed gel a partially chelated oligomers and trichelated Al(Eaa)3 units exist. Crystal phase, not described previously, related to Al(Eaa)3 chelate crystallizes in the gels with higher Eaa/Asb ratio. Hydrolysis leads to formation of three kinds of Al coordination sites: six coordinated Al(Eaa)3, and five and six coordinated Al atoms in oligomers2. In the course of thermal treatment the decomposition of chelate and the evaporation of Eaa, followed by an auto-combustion process, occur in lower temperature range. The crystallization process starts at temperatures higher than 900°C. Calcium aluminate, CaAl2O4, appears as the major crystalline phase, and calcium dialuminate, CaAl4O7, and dodecacalcium heptaaluminate, Ca12Al14O33, as minor phases. The crystallization processes proceed simultaneously and independently. Thermal treatment at higher temperature increases the amount of calcium aluminate and decrease the amount of minor components3. References: 1. Kurajica, S ; Tkalcec, E. ; Sipusic, J ; Matijasic, G. ; Brnardic, I ; Simcic, I. J. Sol-Gel Sci. Tech. 2008, 46, 152. 2. Kurajica, S. ; Mali, G. ; Gazivoda, T. ; Sipusic J. ; Mandic, V. J. Sol-Gel Sci. Tech. 2009, 50, 58. 3. Kurajica, S. ; Mandic, V. ; Sipusic, J accepted for publication in J. Ceram. Sci. Tech.

Izvorni jezik
Engleski

Znanstvena područja
Kemijsko inženjerstvo

POVEZANOST RADA