Transformation Potential of KALEMADEN CLAY–220 to Technological Ceramics

1. Table 1. Chemical and mineral composition of clay (Designation Code: Kil-220, from Kalemaden A. Ş. Çan-Çanakkale / Turkey) used as a raw material for the SiAlON formation. Fig. 1.XRD patterns of the powders synthesized at different temperatures for 4 hour in N2flow (1 lt/min). (β: β-SiAlON; M: Mullite; s: β-SiC; *: SiO2; α: Si3N4) Fig. 3.Holding time effect seen on the XRD pattern of the samples tested at constant temperature (1350oC) and N2 flow (1 lt/min). (β: β-Si3Al3O3N5; M: Al6Si2O13, s: β-SiC; *: SiO2; α: Si3N4) Elem Wt % At % ----------------------------- N K 26.84 40.59 O K 6.85 9.07 Al K 10.88 8.54 Si K 55.43 41.81 Total 100.00 100.00 Fig. 2.a) General view, and b) focusing the needle-like morphologyin figure (a) of -SiAlON powders synthesised at 1475oC for 4h in 1 lt/min N2 flow. c) EDS pattern of the powders in “a”. Transformation Potential of KALEMADEN CLAY–220 to Technological Ceramics Ali Osman KURT 1 andRamazanYILMAZ 2 1 Department of Metallurgy and Materials Engineering, Sakarya University, 54187 Sakarya - TURKIYE 2 Technical Education Faculty, Sakarya University, 54187Sakarya - TURKIYE 1 aokurt@sakarya.edu.tr, 2 ryilmaz@sakarya.edu.tr 1. OBJECTIVES 2. METHODS This is an initial part and first outcomes of an ongoing series of research works concentrating mainly on the synthesis of advanced ceramic materials from natural earth minerals used as starting raw materials. SiAlONs (solid solutions of Si3N4 and Al2O3) are promising advanced ceramic materials for use in environments where high temperature and high stresses with abrasive conditions are existing. These ceramics exhibit high temperature strength, excellent thermal shock resistance, better wear and corrosion resistance and high reliability with low thermal expansion coefficients as compared to other ceramics. Those are the reason why they are being in use for the engineering applications notably in making cutting tools and structural components. SiAlONs are prepared using Si3N4 and Al2O3 along with other minor additives used in powder form. In this work, the Clay – 220 mineral of Turkish origin from Kalemaden A.Ş., having a composition of the quarts, kaolinite, feldspar and the others with the weight percent of 36.20, 12.66, 48.03 and 3.11, respectively, was used as a raw material to investigate its transferability potential to SiAlON ceramic. The mineral was in the form of conglomerate rocks, which were prepared in the consecutive mineral processing procedures before taken it to the transformation stage where carbothermal reduction - nitridation (CRN) was performed in an atmosphere controlled environment. Carbon black was the main reducing agent used in the CRN process. The influence of reaction temperature, holding time and N2-flow rate on the formation of SiAlON powder has been investigated. (Results for the effect of N2-flow are not shown here.) 3. RESULTS The effect of temperature on the formation of the product powders during CRN was tested using various temperatures from 1300 to 1475 oC for 4 hours with N2 flow rate of 1 lt/min (Fig. 1). The results of XRD analysis show that mullite and silica phases are dominant in the powders produced at low reaction temperatures (below 1350 oC), while β-SiAlON exists as a major phase in the powders prepared at higher synthesising temperatures in the range of 1350 to 1450 oC. SiO2 composition in the product gradually disappears as the reaction temperature increases. Using a very high temperature, i.e., 1475oC, the resultant powder synthesised for 4 hours included mainly of β-SiAlON phase among with a significant amount of β-SiC and minor amount of Si3N4 phase. During the production of the SiAlON powder by CRN process, a little amount of N2 involves the reaction, most of N2 is used as a carriage gas for drive away of CO(g), which may otherwise cause the formation of SiC when remained in the system. XRD patterns of the powder produced by CRN method at 1350 oC as a function of holding time are given in Figure 3. As clearly seen from the figure that the holding time has a significant influence on the powder composition. Powders produced at short holding time exhibited of mullite and silica phases. Whereas, an increase in holding time resulted in a reduction of the phases formed early on to the formation of new phases, mainly β-SiAlON and SiC. A 6-hour holding time obviously fulfilled the CRN reaction; therefore, the final powder Main and intermediate reaction steps during the formation of β-SiAlON from clay 1… 3[Al2O3.2SiO2.2H2O] + 15C+ 5 N2(g) 2Si3Al3O3N5(s) + 15CO(g) + 6 H2O(g) or with the reaction below from the calcined mineral; 2… [3Al2O3 · 6SiO2] + 15C + 5 N2(g) 2Si3Al3O3N5(s) + 15CO(g) the decomposition of kaolinite to mullite appears at low temperatures 3… 3Si2Al2O7  Al6Si2O13 + 4SiO2 Mullite is formed by the reaction of carbon with kaolinite following the reaction, 4… 3[SiO2.Al2O3] + C  3Al2O3.2SiO2+ SiO(g) + CO(g) Silicon carbide may form after following reactions, 5… SiO2 + 3C  SiC + 2CO(g) 6… SiO(g) + 2C  SiC + CO(g) or SiO(g) may result in the formation of α-Si3N4 7… 3SiO(g) + 3C + 2N2  Si3N4 + 3CO (g) 8… Al6Si2O13 + 4SiC + 3C + 5 N2(g)  2Si3Al3O3N5 + 7CO (g) may also express as following equation: 9… 3Al2O3.2SiO2 + 2Si2Al3O7N + 3Si2N2O + 6N2 + 18C  Si3Al3O3N5 + 18CO (g) produced was mainly of β-SiAlON. Once optimising the temperature where reaction proceeds the holding time may be set to adjust the required product phase and its quantity. 4. CONCLUSIONS After series of experimental works, the Clay – 220 mineral of Turkish origin from KALEMADEN A.Ş. exhibited a high transformation potential to SiAlON type ceramics in powdered form. Using different sets of the process parameters, fine powders formed at temperature of 1350 oC, which is the lowest value reported among the current literature, contain mainly of SiAlON of β-phase. The conditions for optimising the processes are as follows. • The optimum condition to have high amount of SiAlON was found as: at temperature 1350 oC, holding time of 4 hours with N2-flow rate of 1 lt/min. Although, there was a small amount of other phases (i.e., mullite and SiC) present they were small in quantities. • After 6 hours reaction at 1350oC the mullite phase was completely removed and the final product was fully transformed to a β-SiAlON phase plus a very small amount of retained SiC. • Increase in temperature or N2 flow rate, both of which had a marked effect on the β-SiAlON phase content, should be ajust in such a way to have efficiently transform the constituents to high yield of β-SiAlON ceramic. The microstructural photographs of powder synthesised at 1475 oC for 4 hours are shown in Figure 2. Two main morphologies are existing; one is the needle-like grain morphology with sub-micrometer sized cross-section (actually less than 500 nm) while others are equiax-sized small particles agglomerated in a few micrometer size. Acknowledgments This work was carried out as apart of the project (Grant No. DPT-2003K120970) supported by the DPT (State Planning Organisation of Turkey). Kalemaden A. Ş. generously provided us with raw materials. We acknowledge those people who involved in this study with their valuable helping in experimental works and discussions bits. Tel. +90 264 295 5778 Fax. +90 264 295 5608 aokurt@sakarya.edu.tr www.sakarya.edu.tr/~aokurt