Title of Invention	AN IMPROVED PROCESS FOR THE PREPARATION OF ZIRCONIA-MULLITE COMPOSITE
Abstract	An improved process for the preparation of zireonia - mullite composite which comprises intimately mixing 30 to 50% wt alumina and 50 to 70% wt zircon in ethanol medium, drying the above said mixture by conventional methods followed by melting at temperature 1900°C- 3000°C for a period of 5 to 10 minutes followed by cooling to room temperature to obtain zircon-mullite composite.

Full Text

This invention relates to an improved process for the preparation of zirconia mullite composite. More particularly this invention relates to preparation of zirconia mullite composite which finds application in fabrication of high temperature structural components, as refractories in metallurgical and glass industries. Hitherto known process (Reference : Mechanical properties of sintered in-situ reacted mullite zirconia composites; N. Claussen and J. Jahn, J. Am. Ceram. Soc. 63 [3-4], 228-229 (1980)]) the preparation of zirconia toughened mullite composite follows the reaction sintering method wherein a mixture of zircon and alumina of predetermined weight and particle size is attrition milled in water/alcohol medium for 6-7 hours, spray dried and isostatically pressed to form the green body which is then sintered in two steps. The first step involves sintering at lower temperature (1250-1300°C) for densification of zircon and alumina while second step involves sintering at higher temperature (1500-1600°C) for reaction to occur between zircon and alumina to form zirconia toughened mullite composite. The hitherto known process has following drawbacks : 1) In reaction-sintering process, the reaction occurs in solid-state, (Indian patent No. 178844). Hence the kinetics of the process is slow. Also to get a completely reacted mullite composite, it is required to sinter for longer period. 2) The process also requires milling of raw materials for a longer period of time for size reduction as well as for the intimate contact between the reactants to effect reaction sintering as well as densification of mullite composite. 3) The process also involves two step sintering which makes it cumbersome. The main object of the present invention is to develop a process for preparation of zirconia toughened mullite composite from a mixture of zircon and alumina with improved reaction kinetics. It is also the object of the invention to eliminate the extensive milling time and make the process simpler and faster. Yet another object of the invention is to reduce the energy consumption of the said process. The present invention relates to the preparation of zirconia mullite composite having 25 to 50 wt% zirconia in mullite matrix wherein the presence of zirconia particles toughen the matrix by dispersion-strengthening as well as by transformation-toughening. The new principle relates to the preparation of zirconia toughed mullite composite by reacting zircon and alumina in liquid state f ollowed by cooling, wherein the microstructure of the resultant composite is controlled by the rate of cooling and the initial composition of the melt. According to another new principle the mixture of zircon and alumina is melted by thermal plasma where the high temperature and high heat content of latter allow the melting to occur within a very short duration. Since the reaction occurs in liquid state, kinetics improve considerably. Further, milling is eliminated for increasing the surface area of the reactants since the reaction occurs in liquid state. The new result is the preparation of zirconia toughened mullite composite by reacting zircon and alumina in liquid state followed by the cooling of the melt, thereby, controlling the microstructure of the composite by rate of cooling and by the initial composition of the melt. Thermal plasma is used to melt the mixture of zircon and alumina and, hence, reducing the reaction time required between zircon and alumina to form zirconia toughened mulliteComposite. The invented process also has improved reaction kinetics since reaction is occurring in liquid - state. It is faster than the hitherto known process and also simpler since it involves two steps, namely mixing of raw material and melting in a thermal plasma reactor followed by cooling. Accordingly the present invention provides an improved process for the preparation of zirconia - mullite composite which comprises intimately mixing 30 to 50 % wt alumina and 50 to 70 % wt zircon in ethanol medium, drying the above said mixture by conventional methods followed by melting at temperature above 1900 °C/for a period of 5 to 10 minutes followed by cooling to room temperature to obtain zircon-mullite composite. In an embodiment of the invention the percentage composition of alumina is 30 to 50 wt % while rest is zircon. In another embodiment of the invention melting of mixture is done in thermal plasma reactor. In yet another embodiment of the invention melting of mixture is done at temperature above 1600°C. In still yet another embodiment of the present invention zircon in the mixture can be replaced by plasma dissocaiated zircon, which is a physical mixture of zirconia and silica and is produced following a patented process (pending Indian patent : 189/del/91 dated 11.3.91) The invention is now described in detail. Zircon above 1500°C react with alumina to form zircoma toughened mullite composite according to the following equation. (Equation Removed) The reaction occurs in solid state, however, kinetics improves considerably in liquid state. The examples given below are to illustrate how the invention is carried out in actual practice and should not be construed to limit the scope of the invention. Example 1: A typical experiment is carried out in 200 gm batch scale. Zircon of MK grade and alumina of laboratory grade of particle size -150 µm + 75 µm in ratios according to equation (1) arc mixed in a ball mill in ethanol medium for 15 min. The mixture is dried at 100 °C and then melted in a 50 kW dc transferred arc thermal plasma reactor for 5 minutes. Product is then furnace cooled. Product is finally characterized by XRD which shows presence of monoclinic zirconia and mullite phase. The process yields fully reacted and dense zirconia toughened mullite composite. Example 2: Still yet another example, experiment is carried out 250 gm scale. Stoichiometric requirements of zirconia and silica are calculated from the equation (1). In this experiment 10 % excess of Stoichiometric requirement of alumina is mixed with ground zircon ( particle size: -150 + 75 (µm) in ethanol medium for 15 min. The mixture is then dried at 110° C. The mixture thus formed is melted under plasma for 6 minutes. The product is furnace cooled and characterized by XRD. This confirms formation of zirconia mullite composites. The main advantages of the invention are : 1) The invented process has improved reaction kinetics, since the reaction occurs between zircon and alumina in liquid state to form zirconia toughened mullite composite. 2) The process does not require milling of the reactants mixture. 3) The said process is faster than the hitherto known process since thermal plasma is used to melt the mixture and very high temperature (~ 10,000°C) and high heat content of plasma allow the mixture to melt within a short duration. 4) The process is also simpler simce it nivolves selting the mixture in a thermal. plasma reactor followed by cooling to yield fully reacted and dense zirconia toughened mullite composite. 5) The invented process also reduces energy consumption compared to hither to known process by i) eliminating attrition-milling of reactants ii) reducing reaction time considerably between the reactants by carrying out the reaction in liquid state. We Claim: 1. An improved process for the preparation of zireonia - mullite composite which comprises intimately mixing 30 to 50% wt alumina and 50 to 70% wt zircon in ethanol medium, drying the above said mixture by conventional methods followed by melting at temperature 1900°C- 3000°C for a period of 5 to 10 minutes followed by cooling to room temperature to obtain zircon-mullite composite. 2. An improved process as claimed in claim 1 wherein the weight % composition of alumina is 45. 3. An improved process as claimed in claims 1 & 2 wherein the weight % composition of zircon is 55. 4. An improved process as claimed in claims 1 to 3 wherein the melting of the mixture is done at temperature 1900°C. 5. An improved process for the preparation of zireonia mullite composite as described herein with reference to examples.

Documents:

623-del-2001-abstract.pdf

623-del-2001-claims.pdf

623-del-2001-correspondence-others.pdf

623-del-2001-correspondence-po.pdf

623-del-2001-description (complete).pdf

623-del-2001-form-1.pdf

623-del-2001-form-18.pdf

623-del-2001-form-2.pdf

623-del-2001-form-3.pdf