Title of Invention

AN IMPROVED PROCESS FOR THE PREPARATION OF ß-SIALON POWDER

Abstract

The present invention reports an improved process for the preparation of P-sialon powder which comprises reacting aluminosilicate powder of particle size of 0.5 to 10 urn being placed in a graphite boat and ammonia gas at a flow rate of about 1000 ml /minute at a temperature in the range of 1200-1450°C for a period ranging 1-7 hours to obtain P-sialon powder. Sialons find application in cutting tools and structural parts of combustion engine such as valves , pistons , turbocharger , turbinre rotors and high temperature bearings.

Full Text

This invention relates to an improved process for the preparation of (3-sialon powder. This invention particularly relates to an improved process for the preparation of P-sialon from naturally occurring aluminosilicates. Sialon is the acronym given to the phases formed in the Si-Al-O-N system. These are built up of (Si,Al)(0,N)4 tetrahedra in the same way as the mineral silicates are built up of SiO4 tetrahedra. The mutual replacement of silicon by aluminium and of nitrogen by oxygen in P-S13N4 gives P-sialons. They can be represented by the formula Si6-zAlzOzNg.z. The best properties are exhibited by a sialon in which Si/Al ratio is unity (z=3). In general, sialons have the hardness and thermal shock resistance of silicon nitride and offer better chemical stability at high temperatures. Therefore, sialons are technologically important materials and they find applications in cutting tools and structural parts of combustion engines such as valves, pistons, turbocharger, turbine rotors and high temperature bearings. Sialons are commercially produced by reacting together silicon nitride, silica, alumina and aluminium nitride. The presently known process is covered by large number of publications and patents. However, the cost of the sialons produced by the known process is very high because of the high temperatures involved. So, there have been many attempts all over the world to prepare sialons by carbothermal reduction and nitridation of naturally occurring aluminosilicates, or synthetically prepared aluminosilicate gels. Kwong Keyi Sing prepared a variety of sialons (Patent Appl. No.US 341,227 dt 1.8.95) by intercalating Kaolin, halloysite and montmorillonite with organic compounds and 2 firing at high temperature in nitrogen atmosphere. Nishi Yoshiji, Shiogai Tatsuya, Suzuki Kazunari and Yamagishi Senjo prepared fine P-sialon powder (Patent No.JP 04,16564 dt 21.1.92) by heating mullite powder with carbon in nitrogen atmosphere. A. Seron, F. Beguin and J. Thebault (J.Mater.Res. 9, 2079,1994) prepared almost pure P-sialon by reduction and nitridation of films of kaolinite of thickness 50-150 um under a flow of a mixture of nitrogen and hydrogen in different ratios in the temperature range of 1100-1450°C. There are very few reports on the use of ammonia for reducing and nitriding aluminosilicates. In our pending patent application No.322/DEL/94 dt 23.3.94, we have described a process for the synthesis of OC-S13N4 powder and whiskers from silica using ammonia. We have described another process in our pending patent application No.252/DEL/97 dt 31.1.97 to prepare P-S13N4 powder from silica and an alkaline earth oxide using ammonia. In these processes, silica or silica with an alkaline earth oxide is heated for a period of 0.5 to 20 hours in flowing ammonia in the temperature range of 1150-1400°C to produce α-Si3N4 or P-S13N4. Due to the technological importance of P-sialon, we carried out R&D work to develop an improved process for the preparation of p-sialon powder. The main objective of the present invention is to provide an improved process for the preparation of P-sialon powder. 3 Another objective of this invention is to provide an improved process for the preparation of -sialon starting from naturally occurring aluminosilicates which are cheap and abundantly available. Yet another objective of this invention is to use ammonia to reduce and nitride the aluminosilicates. Still another objective of this invention is to prepare (i-sialon in which Si/Al ratio is unity (with z=3), which is very important for technological applications. Accordingly, the present invention provides an improved process for the preparation of P-sialon powder which comprises reacting aluminosilicate powder of particle size of 0.5 to 10 urn being placed in a graphite boat and ammonia gas at a flow rate of about 100-2000 ml /minute at a temperature in the range of 1200-14500c for a period ranging 1-7 hours to obtain P-sialon powder. In an embodiment of the present invention the aluminosilicate powder used may be of particle size less than 10 uxn. In another embodiment of the present invention the ammonia gas used may be pure and dry. In yet another embodiment of the present invention the ammonia gas may be allowed to flow at the rate of 100 to 2000 ml per minute. 4 In still another embodiment of the present invention the reaction may be effected by placing aluminosilicate powder in a graphite boat. The scientific basis for this invention is that ammonia can reduce and nitride various oxides at high temperatures. However, this reaction is very slow in the case of very stable oxides such as alumina. Therefore, it may be expected that aluminosilicates can be partly reduced and nitrided into silicon aluminium oxynitrides (sialons) by this reaction. We have studied the reaction of chemically processed aluminosilicates with ammonia in the temperature range of 1200-1450°C by varying the experimental conditions such as the duration of the reaction, container material and the rate of flow of ammonia. We have observed that mullite is the major product in the initial stages of the reaction and prolonged reaction produces a mixture of p-sialon, aluminium nitride and other phases. We have observed that pure p-sialon can be prepared by optimising the conditions of the process of the present invention. The novelty of this invention is that pure P-sialon has been prepared starting from aluminosilicates by the reaction of ammonia. In the improved process of the present invention, pure p-sialon with z=3 is obtained by reacting Kaolinite with ammonia at a temperature range of 1350-1450°C for a duration of 2-6 hours in a suitable container preferably graphite. 5 The steps involved in the process of the present invention are (1) selecting the aluminosilicate powder with small particle size preferably less than 10 µm, (2) selecting proper container for heating the powder, (3) heating the powder taken in the container in a furnace at a temperature in the range of 1200-1450°C preferably at 1350-1400°C, (4) passing pure and dry ammonia gas over the sample at a required rate of 100-2000 ml per minute during the heating and cooling and (5) continuing the reaction for a period of time ranging from 1-8 hours. The invention is described in detail in the examples given below which are provided by way of illustration only and therefore should not be construed to limit the scope of the invention. Example 1 Chemically processed kaolinite with the analysis data shown in the table was used as the starting material. Composition of Kaolinite used in weight % A1203 39.00 CaO 0.06 S1O2 46.00 MgO 0.06 Ti02 0.50 K20 0.03 Fe203 0.52 Na20 0.08 Loss on ignition 14.3 6 The particle size of this powder ranged between 0.5 - 10 µm with an average of 2 µm. 5 grams of this powder was taken in a graphite boat of length 20 cm and width 7 cm and placed in the uniform hot zone of a tubular mullite furnace of 7.5 cms diameter and 100 cms length. The mullite tube used was impervious to gases and it was provided with end fittings to pass dry ammonia gas over the sample. Heating was done using silicon carbide elements and the temperature of the furnace was controlled to better than 5°C. The air present in the furnace tube was removed by flushing with dry ammonia gas for half an hour. Heating was then started and the temperature was raised to 1000°C in 2 hours and to 1400°C in another 2 hours. The rate of flow of ammonia was maintained at 1 litre/minute. The reaction was done for 1 hour at this temperature. The furnace was then cooled gradually to room temperature. The product was taken out after the flow of ammonia was switched off. It was a grey coloured powder. X-ray diffraction analysis of this powder showed that it was a mixture of mullite, -sialon and x-phase sialon with mullite as the major phase. Example 2 5 grams of the chemically processed kaolinite was taken in a graphite boat to give a powder bed of thickness of about 10 mm and it was kept inside the mullite tubular furnace. Ammonia gas was passed over the sample at a rate of 1 litre/min. The temperature of the furnace was raised to 1400°C in a period of 3 hours and maintained for 3 hours. The furnace was allowed to cool. The flow of ammonia was then stopped and the product was taken out. It was a grey powder and x-ray diffraction analysis of this powder 7 showed that it contained only P-sialon with no other phases present. The lattice parameter measurements clearly suggested that the sialon obtained had a z value of 3. Electron micrographs of the powder showed that the size of the particles was in the range of 0.5 to 2.0 µm. Example 3 Chemically processed kaolinite, used in the examples 1 and 2, was used in this experiment also. 5 grams of this powder was taken in a graphite boat and placed in the uniform hot zone of the mullite tubular furnace. The furnace tube was flushed with ammonia gas and the temperature was raised to 1400°C in 3 hours. The furnace was maintained at this temperature for a period of 6 hours and then cooled to room temperature. Flow of ammonia gas was maintained at a rate of 1 litre/minute throughout the heating and cooling. The product from this experiment also was grey in colour. X-ray diffraction analysis of this powder showed it to be a mixture of P-sialon, aluminium nitride and a-silicon nitride with p-sialon as the major phase. Example 4 Chemically processed pyrophillite with the composition of SiO2:64 wt% and Al2O3:28 wt% and with average particle size of 2 µm was used as the starting material in this experiment. 5 grams of this material was taken in a graphite boat similar to that used in example 1 and placed in the mullite tubular furnace. The furnace tube was flushed with 8 ammonia gas and the temperature of the furnace was raised to 1350°C in 3 hours and it was maintained at this temperature for 2 hours while passing ammonia gas at a rate of 1 litre/minute. The furnace was then cooled and the flow of ammonia was stopped. The product was grey in colour and x-ray diffraction analysis showed that it contained mullite, silica and P-sialon with mullite as the major phase. Example 5 5 grams of chemically processed kaolinite was taken in an alumina boat of 7 cms width and 20 cms length. It was kept inside the mullite tubular furnace and dry ammonia gas was passed over the sample at a rate of 1 litre/minute. The temperature of the furnace was raised to 1400°C in 3 hours and was maintained for 5 hours. The furnace was then cooled to room temperature. The flow of ammonia was maintained throughout the heating and cooling. The product was characterised by x-ray diffraction and it was found to be a mixture of p-sialon and mullite. These examples clearly show that single phasic P-sialon can be obtained by the reaction of ammonia with aluminosilicates for an optimum time in the temperature range of 1200-1450°C when a suitable container is used, preferably graphite. It may be noted that by this process, p-sialon powder with particle size in the range of 0.5 - 2.0µm can be prepared. When kaolinite is used as the starting material, this process gives a sialon with z=3. The main advantages of this process are : 1. The raw materials namely the aluminosilicates are naturally occurring clay minerals which are abundantly available and therefore cheap. 2. Other methods such as carbothermal reduction and nitridation of aluminosilicates yield only mixtures of sialons with other phases whereas the present process produces single phasic sialons. 3. This process is applicable to bulk samples because of the more reactive nature of the nitrogen and hydrogen produced by the in-situ decomposition of ammonia whereas the process in which mixtures of nitrogen and hydrogen are used, is applicable to only films. 4. Unlike in other processes, there is no loss of silicon during this process and therefore by starting with an aluminosilicate of known ratio of silicon to aluminium, one can produce a sialon with the same ratio of silicon to aluminium. We Claim: 1. An improved process for the preparation of ß-sialon powder which comprises reacting aluminosilicate powder of particle size of 0.5 to 10 urn being placed in a graphite boat and ammonia gas at a flow rate of about 100-2000 ml /minute at a temperature in the range of 1200-1450°C for a period ranging 1-7 hours to obtain P-sialon powder. 2. An improved process as claimed in claims 1, wherein the ammonia gas used is pure and dry. 3. An improved process as claimed in claims 1-2, wherein the reaction is effected by placing the aluminosilicate powder in a graphite boat and the said boat placed in the uniform hot zone of a tubular mullite furnace which helps in keeping oxygen partial pressure low important for nitridation reaction. 4. An improved process for the preparation of p-sialon substantially as herein described with reference to the examples accompanying the specification.

Documents:

1997-del-1998-abstract.pdf

1997-del-1998-claims.pdf

1997-del-1998-complete specification (granded).pdf

1997-del-1998-correspondence-others.pdf

1997-del-1998-correspondence-po.pdf

1997-del-1998-description (complete).pdf

1997-del-1998-form-1.pdf

1997-del-1998-form-19.pdf

1997-del-1998-form-2.pdf