GB2105316A - Porous silicon nitride - Google Patents

Abstract

The method comprises forming a preform of silicon powder, a liquid binder and a filler composed of material adapted for burn-out or vaporization at temperatures at which silicon powder reaction in air is negligible. The filler is displaced at 750 DEG C, and the resulting porous silicon body is nitrided by heating in a nitrogen atmosphere with stepwise temperature increases. The filler may be a mixture of coarse and fine particles, such as 1.2mm diameter hollow polystyrene spheres and 0.086mm diameter powdered polymethylmethacrylate. The method is particularly suitable for producing silicon nitride catalyst support stones having a high degree of open porosity.

Description

SPECIFICATION Production of porous silicon nitride This invention relates to the production of porous silicon nitride, and more particularly although not exclusively to its production in a form suitable inter alia for catalyst support applications.

The production of silicon nitride by reaction bonding or hot-pressing techniques is well-known in the prior art. The objective has generally been to achieve highly dense material with few voids, approaching theoretical density, in order to achieve maximum strength. However, for some applications of ceramics, catalyst supports in particular, low void content or porosity is a disadvantage because high specific surface area is required. Moreover, reaction bonded silicon nitride has comparatively low porosity even at densities well below theoretical.

It is an object of the present invention to provide highly porous silicon nitride.

The present invention provides a method of making silicon nitride including steps of: (1) forming a preform of silicon powder, a liquid and a filler material composed of divided material adapted to vaporize or burn at temperatures at which reaction of silicon powder in air is negligible, (2) heating the preform sufficiently to displace the material and produce a porous silicon body.

(3) nitriding the silicon body.

In a preferred embodiment of the invention, the method comprises mixing silicon powder with a small quantity of water and water-soluble binder and a filler comprising ccarse and fine particles of plastics material. From this an approximately spherical preform is moulded. The preform is heated at substantially 750"C in air to burn out the plastics material, which may preferably be a combination of hollow polystyrene spheres and finely powdered polymethylmethacrylate. Nitriding may conveniently be performed by heating in a nitrogen atmosphere at stepwise temperature increases in the range 1100-1425"C. Silicon nitride produced in this way has been found to have in the order of 90% porosity with good interconnection of pores.

Despite its porosity, the material surprisingly retains a high degree of strength as compared to other porous ceramics, which tend to be undesirably friable, and is therefore well suited to use in catalyst support applications in which catalyst agitation occurs.

In an alternative embodiment, the method comprises forming a suspension of silicon powder in a mixture of water and liquid dispersing agent, and impregnating a porous body of plastics material with the liquid suspension. The plastics material is burnt out at 750"C to form a porous silicon body which is then nitrided. The porous plastics material may conveniently be a polyurethane foam capable of being shaped as required.

The invention will now be described with reference to the examples.

Table 1 shows examples 1 to 1 3 of preform constituents for use in the method of the invention. In each example, a preform was made from a dough of wet cake consistency comprising a mixture of silicon powder, a divided and combustible filler and an aqueous binder solution of polyvinyl alcohol or methylcellulose. The filler was variously hollow polystyrene spheres (HPS), linear shreds of ashless filter paper, short lengths (2-3mm) of polyethylene rope fibres, and polymethymethacrylate spherical powder. The polystyrene spheres were heated at 130"C before use to inflate to about 1.2mm particle size, and the polymethylmethacrylate powder had a fine particle size averaging 0.086mm, or approximately 1mm and 0.1 mum respectively.Table 1 gives details of the various combinations of these filler and binder solutions used to form dough as previously mentioned.

Preforms were made from doughs by pressing in stainless steel dies. This produced approximately spherical preforms, or spheroids. The filler material was in all cases burnt out by heating in air at 750"C. This produced combustion of the filler to leave a porous silicon powder structure. Subsequent thermogravimetric analysis showed that oxidation of the silicon structure at 750"C was negligible. The structures were nitrided by a multistage process of heating in a nitrogen atmosphere with stepwise temperature increases, the process comprising heating at 1100, 1150, 1200, 1250, 1300, 1 350 and 1425"C for respective periods of 5, 10, 5, 5, 10, 60 and 10 hours. This procedure produced porous silicon nitride stones with void contents in the range 65 to 95% as listed in Table 1.The void content or porosity of the stones was measured by boiling in distilled water for two hours, cooling, removal from the water followed by rapid weighing after displacing excess surface water. The weight of the stone and its internally absorbed water was then compared with its dry weight to yield the internal volume of free space. This gave a reasonable measure of porosity, since there was substantially no closed porosity in the stones.

Examples 1 to 5 had fillers of linear morphology, paper shreds and polyethylene fibres.

Example 6 to 1 2 fairly large (1.2mm mean size) hollow polystyrene spheres (HPS) and fine acrylic powder (0.086mm mean size), ie a coarse and fine powder mixture of approximately 1 mm to 0.1 mm size ratio. It was found that the Example 1 3 filler had high porosity by virtue of the HPS, and that by inspection the connectivity of the porosity was improved as compared to Examples 1 to 1 2 by the addition of the fine acrylic powder, although the degree of porosity was not substantially affected. Reaction bonded silicon nitride Is not usually a very porous material, and the silicon powder preform after fillber burn-out tended in Examples 1 to 1 2 to have a hollow sphere morphology with a comparatively impervious outer shell.The addition in Example 1 3 of fine acrylic powder resulted in the appearance of holes in the outer shell. This greatly increased the connectivity of the porosity without substantially affecting the degree of porosity itself. The Example 1 3 silicon powder preform preserved its highly connected porosity through the nitridation stage.

For the purposes of producing porous ceramic stone for catalyst support application, liquid access to the central region of the stone should be facilitated. It was found surprisingly ihat there was no substantial benefit in porosity connectivity gained by using fillers of linear morphology as in Examples 1 to 5. Moreover, the use of hollow polystyrene spheres as a filler in Examples 5 to 1 3 was found to be a simple and highly effective way of increasing porosity, particularly when combined with fine acrylic powder as in Example 1 3. The use of a filler comprising a mixture of fine and coarse spherical particles is therefore preferred. The fine particles were solid and the coarse particles hollow in Example 1 3.

The catalyst support properties of the Example 1 2 and 1 3 silicon nitride stones were assessed by treating them with catalyst material and then using them to decompose an 85% solution of H202. Catalyst treating was carried out by boiling firstly in sodium permanganate solution and secondly in a mixed solution of sodium permanganate and potassium chromate. The stones were subsequently baked at 140"C. The dydrogen peroxide decomposition test consisted of immersing each stone in successive 50cm3 aliquots of H202. As catalytic activity falls with length of use, the time taken to decompose successive aliquots lengthened.The test was terminated when the time taken to decompose an aliquot exceeded ten minutes, at which point the total amount of H202 decomposed was noted, being the sum of previously decomposed aliquots. The results are given in Table 2, with the performance of similarly catalyst-treated conventional stones being given for comparison.

As shown in Table 2, silicon nitride catalyst support stones produced in accordance Examples 1 2 and 1 3 of the invention decomposed from 22 to over 32 times as much H202 as conventional foamed silicon nitride or porous alumina, the same catalyst treatment being used throughout. It is therefore believed that silicon nitride made in accordance with the invention is particularly suitable for use as a catalyst support.

Despite their very high void content of around 90%, or 10% solid content, silicon nitride stones of the invention have been found to retain a surprising degree of strength. Stones in accordance with the examples of Table 1 survived repeated dropping from a height of 5 metres onto hard surface with only minor surface cracking or crushing at the point of impact. Such robustness is of importance in the catalysis of violent reactions, or in applications where the catalyst is stirred or otherwise mechanically agitated, in order that the catalyst support might survive for retreatment and reuse. Conventional porous ceramics such as pottery or alumina are by comparison less strong and may be undesirably friable.

In a further embodiment of the invention, silicon nitride stones were produced from preforms made by impregnating a polyurethane foam with silicon powder in aqueous suspension with a dispersing agent. 25 gm of silicon powder was mixed with 30 cm3 of a 3% aqueous solution of methylcellulose. The suspension was impregnated into cubes of polyurethane foam by squeezing the cubes in the suspension. After drying in air, the cubes were heated at 750"C in air to burn away the polyurethane leaving a silicon powder skeleton, The powder skeleton was nitrided by the staged process previously described. Porosities of 74.22 + 2.14 and 73.57 + 2.36 were obtained for silicon nitride produced in this way. Such porosities are lower than those obtained for the Examples of Table 1, but the skeietal morphology (hollow struts of reaction bonded silicon nitride) of the samples gave a highly accessible internal structure. The impregnated polyurethane foam or preform dictated the shape of the final silicon nitride structure, and it may easily be preshaped to any form. Accordingly, this approach avoids any requirement to mould a silicon/filler/liquid dough to a final shape.

TABLE 1 FORMULATION USED TO MAKE CATALYST SPHERES, AND POROSI TI ES OBTAINED AFTER NITRIDATION Mean % Void Content of spheres + standard EXAMPLE Composition deviation (ASTM Method) 20 g silicon powder (diameter about 25mm) 68.17 + 2.05 1.75 9 polythene rope, cut into 2-3 mm 68.33 i 4.70 lenghs 65.47 i 1.13 1.75 9 shredded fiber paper 67.65 + 2.04 0.87 9 of Shell 491/79 hollow 67.27 + 1.52 polystyrene spheres (HPS) Made into a dough with 10% polyvinyl alcohol solution, and coated with Shell R751 HPS 40 9 silicon powder 73.32 + 6.50 2 9 polythene rope, as Example 1 2 3.5 9 shredded filter paper 5 9 R 751 HPS Mixed with 10% pva soln 120 9 silicon powder 74.12 + 1.53 6 g polythene rope as Example 1 69.33 + 3.62 3 10.5 9 shredded filter paper 72.35 + 6.06 16 9 R 751 HPS 74.85 + 0.73 Mixed with 10% pva soln 73.07 + 5.66 76.47 + 4.05 120 9 silicon powder 83.64 + 0.65 6 g polythene rope, as Example 1 83.15 + 1.75 4 10.5 9 shredded filter paper 18 9 R 751 HPS Mixed with 10% pva soln 120 9 silicon powder 75.73 + 2.53 6 9 polythene rope, as Example 1 77.38 + 4.40 10.5 9 shredded filter paper 75.89 + 3.15 5 15 9 larger HPS (R 751) 79.21 j0.82 3 9 smaller HPS (491/79) 78.10 + 1.66 Mixed with 10% pva soln. Rolled spheres coated with R 751 HPS 10 9 silicon powder 94.98* 6 6 9 491/78 HPS Mixed with 10% pva soln *Single Value 10 9 silicon powder 89.50* 7 3 9 491/78 HPS Mixed with 10% pva soln 15 9 silicon powder 90.45* 8 6 9 491/78 HPS Mixed with 10% pva soln 10 9 silicon powder 95.09* 9 6 9 R 751 HPS Mixed with 10% pva soln TABLE 1 continued 10 g silicon powder 91.45* Mean % Void Content of spheres i standard EXAMPLE Composition deviation (ASTM Method) 10 3 g R 751 HPS Mixed -with 10% pva soln 15 g silicon powder 90.76* 11 6 g R 751 HPS Mixed with 10% pva soln 75 g silicon powder 86.85 + 0.83 12 25 g R 751 HPS Mixed with 2% methylcellulose soln 60 g silicon powder 89.18 + 1.40 25 g R 751 HPS 1 3 25 g polymethylmethacrylate spherical powder (0.086mm mean particle size) Mixed with 2% methylcellulose Single Value TABLE 2 Catalytic Total H202 (85% Solution) Stone Decomposed Example 1 2 stone 3500 cm3 Example 1 3 (stone 1 4550 cm3 (stone 2 6750 cm3 Porous Alumina Stone 2000 cm3 (Damp Control Insert Type) Conventional Foamed Silicon Nitride Stone ) 2000 cm3

Claims (11)

1. A method of making porous silicon nitride including the steps of: (1) forming a preform of silicon powder, a liquid and a filler material adapted to vapourize or burn at temperatures at which reaction of silicon powder in air is negligible, (2) heating the preform sufficiently to displace the filler and produce a porous silicon body, and (3) nitriding the silicon body.

2. A method according to Claim 1 wherein the liquid is a mixture of water and water-soluble binder.

3. A method according to Claim 2 wherein the filler is a mixture of coarse and fine particulate material.

4. A method according to Claim 3 wherein the filler is a mixture of hollow polystyrene spheres and finely powdered polymethylmethacrylate.

5. A method according to Claim 4 wherein the polystyrene spheres and the powdered polymethylmethacrylate have diameters in the order of 1 mum and 0.1 mm respectively.

6. A method according to Claim 5 wherein the diameters average substantially 1.2mm and 0.086mm respectively.

7. A method according to Claim 1 wherein the filler is displaced at substantially 750"C.

8. A method according to Claim 7 wherein the porous silicon body is nitrided by heating in a nitrogen atmosphere with stepwise temperature increases.

9. A method according to Claim 8 wherein the porous silicon nitride body is heated in steps at 1100 C, 1150"C, 1200"C, 1250"C, 1300"C, 1350"C and 1425"C for respective periods of 5, 10, 5, 5, 10, 60 and

10 hours.

1 0. A method according to Claim 1 wherein the preform is produced by impregnating the powder, liquid and filler into a polyurethane foam, the liquid being a mixture of water and and a dispersing agent and the silicon powder and filler being in suspension in the liquid.

11. Porous silicon nitride made in accordance with the method of any preceding claims.

1 2. A method of making porous silicon nitride in accordance with any on of examples 1 to 13.