HK1159595B - Method for producing high-purity silicon nitride - Google Patents

Description

The present invention relates to a two-stage process for the production of high purity silicon nitride by nitrating metallic silicon powder.

Silicon nitride powder is a technically important starting material for the manufacture of ceramic materials with high temperature and corrosion resistance and strength. This material is used in particular to manufacture parts that are subjected to high temperature loads.

The properties of articles formed from silicon nitride are largely determined by its purity.

In accordance with the state of the art, a large number of processes have already been developed for the production of high purity silicon nitride, which can be divided into four process variants.

In particular, many processes involve the thermal decomposition of silicon tetrachloride or chlorosilanes in the presence of ammonia or hydrogen/nitrogen mixtures.

For example, according to EP 365 295 A1, crystalline silicon nitride powder is produced by a gas-phase reaction of ammonia with silane at temperatures between 900 and 1,450 °C.

US Patent No 4.122.155 describes a process for the production of amorphous silicon nitride powder by which silane and ammonia are converted to a temperature range of 600 to 1,000 °C and the amorphous reaction product is then calcined at a temperature of at least 1,100 °C to produce ultrafine silicon nitride powder of high purity.

According to Japanese disclosure JP 06-345412, a high-purity silicon nitride/silicon carbide complex is produced by the thermal decomposition of organo-silicon compounds such as hexamethyldisilazan, and the corresponding reaction product is used to produce superconductors.

US patents 4,952,715 and 5,008,422 describe the thermal decomposition of polysilazanes to form mixtures of silicon nitride, silicon carbide and silicon oxynitride, if any.

The disadvantage of all these variants is that they require ammonia, which is technically extremely difficult and requires special safety measures. e other alternative, namely the production of silicon nitride from organo-silicon compounds such as silazans, must be considered to be technically very expensive and largely uneconomic for large-scale production of silicon nitride.

The second process variant, which is the precipitation and thermal decomposition of silicon dimide, is similar.

For example, European patent No 479 050 describes a silicon dimide with a carbon content of not more than 0.5% by weight and a chlorine content of not more than 20 ppm which is converted into silicon nitride in a nitrogen atmosphere.

The fact that silicon dimid has to be produced by reaction of silicon tetrachloride or silicon sulphide with liquid or gaseous ammonia, which is technically very expensive, is considered to be a particular disadvantage.

The same disadvantages are also present in the process described in US Patent 4,405,589, which reveals a process for the production of silicon nitride powder by first converting a chloralkylsilane with ammonia in the liquid or gas phase and then calcining the resulting silicon dimide in an inert gas atmosphere at a temperature of between 1,200 and 1,700 °C.

The third process variant is the carbothermic reduction of silicon dioxide-containing materials in the presence of nitrogen.

For example, Japanese disclosure JP 60-122706 discloses the production of silicon nitride powder by calcining a mixture of silicon dioxide with carbon and silicon nitride in an inert gas atmosphere consisting of nitrogen.

US Patent 5,378,666 is known to produce whisker-free silicon nitride particles obtained by reaction of SiO2 and carbon in a porous carbon-containing matrix.

In addition, US Patent No. 5.662.875 describes a continuous process for the production of fine silicon nitride by reacting silicon dioxide and carbon in a nitrogen atmosphere in the presence of vaccine crystals.

The main disadvantages of this process are that it requires relatively pure starting materials and that the reaction conditions must be carefully controlled to avoid the formation of the unwanted by-product silicon carbide.

The fourth process variant for the production of silicon nitride is the nitratation of silicon powder, which is particularly suitable for the large-scale production of silicon nitride powder.

According to the Japanese patent application JP 06-219715 the production of high purity silicon nitride powder with a high α-phase content is described by converting metallic silicon powder with nitrogen in the presence of calcium oxide.

The direct production of silicon nitride by reaction of silicon and nitrogen without further additives is known, for example, from EP 377 132 B1. In this reaction, silicon powder is converted with nitrogen at 1,000 to 1,800 °C in the first reaction step up to a nitrogen content of 5 to 25% by weight and the partially azotized product is finished azotized in a mixture of nitrogen and inert gas in a second reaction step at 1,100 to 1,600 °C. This process only achieves purity levels of up to 99,8%.

The present invention was therefore intended to develop a process for the production of high-purity silicon nitride which does not present the abovementioned disadvantages in accordance with the state of the art but which makes it possible to produce high-purity silicon nitride in a technically simple and cost-effective manner.

This problem was solved by using the invention (a) High-purity silicon with nitrogen in a rotary kiln with an initial temperature range of 1.150 to 1.250 °C and at least a further temperature range of 1.250 to 1.350 °C in the presence of a gas mixture consisting of argon and hydrogen, up to a nitrogen content of 10 to 30% by weight; and (b) the partially aerosol product of stage a) in a chamber or set oven in a bed of rest at 1.100 to 1.450 °C with a mixture of nitrogen and, where appropriate, argon and/or, where appropriate, hydrogen, preferably with a mixture of nitrogen and argon and, where appropriate, hydrogen, until nitrogen uptake is terminated.

Surprisingly, it has been shown that the method of the invention can be used to produce high purity silicon nitride with a purity > 99,9% in a technically simple manner.

The method according to the present invention is to azotate silicon in two steps: in the first reaction step (a) high-purity silicon is converted with nitrogen in a rotary kiln to a nitrogen content of 10 to 30%, preferably 15 to 20%.

The purity of high purity silicon should be preferably > 99,9%, in particular > 99,99%, and the metallic impurities in silicon should be preferably < 100 ppm and preferably < 50 ppm.

The particle size of the silicon powder used can vary widely, but it has proved particularly advantageous to use silicon powder with a grain size < 100 μm, preferably 1 μm to 50 μm and in particular < 20 μm.

It is essential to the invention that the partial acidification at step (a) be carried out in a rotary kiln having a first temperature zone of 1.150 to 1.250 °C and at least one further temperature zone of 1.250 to 1.350 °C, preferably with temperatures higher than the previous temperature zone in each of the other temperature zones. Preferably, each subsequent further temperature zone has a temperature zone at least 5 °C higher, in particular a temperature at least 10 °C higher than the previous temperature zone. According to a preferred embodiment, the rotary kiln has a first temperature zone of 1.150 to 1.250 °C and two further temperature zones of 1.250 to 1.350 °C.

This method ensures that the reaction heat from the exothermic reaction is immediately released into the lathe and thus largely avoids overheating of the reaction product.

The partial acidification in step (a) is carried out in the presence of a gas mixture consisting of argon and hydrogen, with argon content, in particular, between 5 and 30% by volume and, preferably, between 10 and 20% by volume, in relation to nitrogen content.

The hydrogen content is set at a) preferably at 1 to 10% vol. and preferably at 3 to 7% vol. in relation to the sum of nitrogen and argon.

In one embodiment, in addition to the starting materials silicon and nitrogen and the gas mixture of argon and hydrogen, no other substances are present in the conversion in step (a).

The reaction conditions with respect to pressure in reaction step (a) are relatively uncritical, but it has proved particularly advantageous to perform partial asotherisation in the rotary drill in the pressure range of 1.01 to 1.8 bar, particularly 1.1 to 1.7 bar.

The residence time of the silicon powder in reaction stage a) can vary greatly depending on the size of the spindle and the flow rate of the silicon.

In the second reaction step (b), the partially azotated product from step (a) is converted in a chamber or set-top oven in a bed rest at 1,100 to 1,450 °C in the presence of a gas mixture consisting of nitrogen, argon and, where appropriate, hydrogen.

In a preferred embodiment, no other substances are present in addition to the starting materials, the partially azotized product of step (a) and nitrogen and the gas mixture of nitrogen, argon and/or hydrogen, if any, in step (b).

In this case, the nitration is continued until the nitrogen consumption per unit time has been reduced to virtually zero, so that the product is fully reacted and has reached a nitrogen content of up to 39,5%.

Preferably, a mixture of nitrogen, argon and, if necessary, hydrogen is used first and then replaced during the reaction with up to 100% nitrogen by volume.

The hydrogen content in step (b) may preferably be up to 10% vol, in particular between 1% vol and 8% vol, based on the sum of nitrogen and argon.

The residence time of the partially acidified silicon nitride in the chamber or set oven again depends essentially on the size of the oven and the flow rate of the silicon nitride and is on average 1 to 14 days.

The procedure is preferably to start the conversion in step (b) at a temperature in the range of 1 100 °C to 1 250 °C. The temperature is then increased preferably during the conversion in step (b), in particular to 1 300 °C to 1 450 °C. The temperature increase is preferably gradual.

The silicon nitride obtained in step (b) is usually a porous, essentially spherical granule with a particle size of 0.1 to 30 mm, preferably 0.5 to 25 mm. If desired, the spherical granule can be ground into fine silicon nitride powder depending on the further processing.

The silicon nitride produced by the method of the invention preferably has an α-phase > 60% by weight.

The method of the invention thus allows the manufacture of high purity silicon nitride with a purity of > 99,9%, in particular > 99,99%, in a technically simple manner, without any further purification steps such as leaching with inorganic acids.

The following examples are intended to explain the invention in more detail.

Examples Example 1

Silicon powder of a grain size < 20 μm and a purity > 99.99% Si, with a total metallic impurity of < 40 ppm, was continuously poured into a rotary tube filled with nitrogen (99.99%), argon (> 99.99%) and hydrogen (> 99.99%) to set the hydrogen content of the gas mixture to 3.5 vol. % relative to the sum of nitrogen and argon. The argon content relative to nitrogen was 12 vol. %. The heating zones of the rotary tube were heated to 1.250, 1.330 and 1.350 °C. The rotation rate of the tube was 1.2 m, resulting in an exothermic reaction at an upper gas pressure of 1.01 x 105 °C.The porous, essentially spherical granulate, which was deposited in the 1 to 25 mm grain size range, had an average nitrogen content of 19.4 g. After transferring this granulate to a gastight chamber furnace with a nitrogen/argon/hydrogen atmosphere ratio of 34:60:6 vol., with the nitrogen/argon/hydrogen ratio controlled according to the progressive reaction course (with a rapid reaction course, increased argon in the gas mixture as compared to a slow reaction course), the product was initially heated at 1.150 °C by a corresponding increase in nitrogen to 1.410 °C, with the nitrogen completely heated at a temperature of 10 per cent in the reaction gas phase, up to a 100 per cent increase in the nitrogen concentration.The nitrogen content of the silicon nitride obtained was 39,2% and the product was obtained as a loose porous granulate corresponding to the unchanged grain size range of the partially acidified granulate used. The phase analysis of the resulting product showed an α content of 85% in silicon nitride, the remainder being present in the β modification.

Example 2

Silicon powder of a grain size < 20 μm and a purity > 99.99% Si, with a total metallic impurity of < 40 ppm, was continuously added to a rotary tube filled with nitrogen (99.99%), argon (> 99.99%) and hydrogen (> 99.99%) and a hydrogen content of 3.5% vol. in the gas mixture was set to the sum of nitrogen and argon. The argon content of nitrogen was 19 vol. The heating zones were heated to 1.150, 1.250 and 1.260 °C. The pipe rotation rate was 1.1 m, the gas upper pressurization was 1.01 x 105 bar. The duration of the powder was 180 minutes (1.01 bar).A porous spherical granule with a diameter of 0.5 to 15 mm and an average nitrogen content of 17.8% by weight was produced. The resulting partially acidified granulate was further acidified in a loose pouring process in a gas-tight chamber oven heated to 1200 °C with nitrogen/argon/hydrogen atmosphere, with the nitrogen/argon/hydrogen atmosphere set to 54:40:6 vol. %. The oven temperature was gradually increased to 1,405 °C in 7 days until nitrogen uptake was complete. The nitrogen to argon ratio was increased to 100% nitrogen as the reaction progressed. The oven was equipped with silicon with an average of 38.8% less porosity than the porous granules, according to the unchanged granular size range of the corticosteroid granules added.The silicon nitride granulate was then ground on a beam mill < 10 μm and the phase analysis showed an α content of 65%.

The average analysis of the ground products showed the following impurities. Other

Element	Konzentration
	ppm
Al	2
B	<2
Ca	2
Co	<1
Cr	<2
Cu	<1
Fe	6
K	<1
Li	<1
Mg	<1
Mn	<2
Mo	<1
Na	<2
Ni	<2
Ti	<3
V	<1
W	<1

Claims (15)

1. Method for producing high purity silicon nitride in two steps, characterised in that:

   a) high purity silicon is reacted with nitrogen in a rotary kiln having a first temperature zone of from 1150 to 1250 °C and at least one further temperature zone of 1250 to 1350 °C in the presence of a gas mixture consisting of argon and hydrogen, up to a nitrogen content of 10 to 30 wt. %; and,

   b) the partially azotised product from step a) is reacted in a chamber or batch kiln in a stationary bed at 1100 to 1450 °C with a mixture of nitrogen and optionally argon and/or optionally hydrogen until nitrogen absorption ceases.
2. Method according to claim 1, characterised in that the high purity silicon powder has a grain size of < 100 µm, more particularly a grain size of < 20 µm.
3. Method according to either claim 1 or claim 2, characterised in that the high purity silicon has a purity of > 99.9 %, more particularly > 99.99%.
4. Method according to any of claims 1 to 3, characterised in that the metallic impurities in the silicon amount to < 100 ppm, preferably < 50 ppm.
5. Method according to any of claims 1 to 4, characterised in that the rotary kiln has a first temperature zone of from 1150 to 1250 °C and two additional temperature zones of from 1250 to 1350 °C.
6. Method according to any of claims 1 to 5, characterised in that the argon content in step a) is 5 to 30 vol. %, preferably 10 to 20 vol. %, relative to the nitrogen content.
7. Method according to any of claims 1 to 6, characterised in that hydrogen is used in step a) in a quantity of from 1 to 10 vol. %, preferably 3 to 7 vol. %, relative to the sum of nitrogen and argon.
8. Method according to any of claims 1 to 7, characterised in that the reaction step a) is carried out in the rotary kiln in the pressure range of from 1.01 to 1.8 bar.
9. Method according to any of claims 1 to 8, characterised in that the retention time of the silicon powder in reaction step a) is 60 to 180 minutes.
10. Method according to any of claims 1 to 9, characterised in that the partial azotisation according to step a) is carried out up to a nitrogen content of 15 to 20 wt. %.
11. Method according to any of claims 1 to 10, characterised in that in step b), the proportion of nitrogen in the reaction gas is set to 20 to 80 vol. % and is increased up to 100 vol. % as the reaction progresses.
12. Method according to any of claims 1 to 11, characterised in that in step b, the proportion of hydrogen amounts to 0 to 10 vol. %, relative to the sum of nitrogen and argon.
13. Method according to any of claims 1 to 12, characterised in that the retention time of the partially azotised silicon nitride in step b) is 1 to 14 days.
14. Method according to any of claims 1 to 13, characterised in that the silicon nitride produced in step b) has an α-phase of > 60 wt. %.
15. Method according to any of claims 1 to 14, characterised in that the granular silicon nitride produced in step b) has a particle size of 0.1 to 30 mm, preferably 0.5 to 25 mm.