Title of Invention	MOULD PARTS OF SILICON NITRIDE AND METHOD FOR PRODUCING SUCH MOULD PARTS
Abstract	The present invention relates to silicon nitride mould parts, particularly crucibles for use in connection with directional solidification and pulling of silicon single crystals. The mould parts consist of Si3N4 having a total open porosity between 40 and 60% by volume and where more than 50% of the pores in the surface of the mould parts have a size which is larger than the means size of the Si3N4 particles. The invention further relates to a method for producing the silicon nitride mould parts.

Title of Invention

MOULD PARTS OF SILICON NITRIDE AND METHOD FOR PRODUCING SUCH MOULD PARTS

Abstract

The present invention relates to silicon nitride mould parts, particularly crucibles for use in connection with directional solidification and pulling of silicon single crystals. The mould parts consist of Si3N4 having a total open porosity between 40 and 60% by volume and where more than 50% of the pores in the surface of the mould parts have a size which is larger than the means size of the Si3N4 particles. The invention further relates to a method for producing the silicon nitride mould parts.

Full Text	Title of invention Mould parts of silicon nitride and method for producing such mould parts. Field of invention The present invention relates to mould parts of silicon nitride for use in connection with molten metal, particularly molten silicon, and to a method for producing such mould parts. Background art It is known to use quartz cmcibles during crystallising of pure molten silicon by directional solidification and by crystal pulling from pure molten silicon, Quartz crucibles have, however, the drawback that quartz is wet by molten silicon, and solidified silicon will thereby affix to the walls in the quartz cnjcibles. Further, quartz and silicon have different coefficients of thermal expansion, resulting in that when molten silicon solidifies in a quartz crucible, thermal stress will be introduced in the quartz cmcible which will destroy the crucible. Quartz cmcibles can therefore only be used once. From JP-59-62199 it is known a method for the production of silicon nitride cmcibles for use in pulling silicon crystals. The cmcible according to JP-59-62199 can be produced by fomning silicon powder by cold pressing, whereafter the cmcible is heated in an Inert atmosphere in a first step and thereafter nitridation is earned out at a higher temperature in a second step. According to JP-59-62199 the produced crucibles have a density of 85% of the theoretical density for silicon nitride. The crucibles according to JP-59-62199 have a good strength, but are wet by molten silicon to such an extent that the solidified silicon ingot sticks to the walls in the crucible. The ingot can thus not be removed from the crucible without destroying the cmcible. The crucibles according to JP-59-62199 can thus only be used once when they are used for directional solidification of silicon. The same is tme also when the crucibles according to JP-59-62199 are used for crystal pulling of silicon single crystals. Description of Invention The object of the present invention is to provide silicon nitride mould parts, such as crucibles, which are not wet by molten silicon thus avoiding that molten silicon which is solidified in the crucibles sticks to the wail of the crucible. The present invention thus relates to silicon nitride mould parts, particularly crucibles for use in connections with directional solidification and pulling of silicon single crystals, which mould parts consist of Si3N4 having a total open porosity between 40 and 60% by volume and where more than 50% of the pores in the surface of the mould parts have a size which is larger than the mean size of the Si3N4 particles. / ■ According to a preferred embodiment the mould parts are coated with silicon nitride particles having an average particle size of less than SOprn. It has surprisingly by been found that S\z^4 mould parts having such an open porosity are not wet by molten silicon and have a strength making it possible to reuse the crucibles a number of times for directional solidification of molten silicon. According to another aspect, the present invention relates to a method for the production of silicon nitride mould parts, particularly crucibles for use in connection with directional solidification of silicon, where particulate silicon having a particulate size of less then 100pm is fornied to a mould part and subjected to nitridation for conversion of the silicon particles to Si3N4, which ^ method is characterized in that the forming is carried out under such a pressure and with such a particle size distribution of the silicon particles that the finished silicon nitride mould part has an open porosity between 40 and 60% by volume. Further, more than 50% of the surface of the finished mould parts that consist of pores that breaks through the surface, have pores which are greater than the mean size of the Si3N4 particles. The shaping of mould parts from the silicon particles is preferably carried out at a pressure of below 200 MPa, and It is particularly preferred to carry out the shaping of the mould parts using vibration. Tests with the use of Si3N4 crucibles according to the present invention for directional solidification of molten silicon have shown that the solidified ingot did not, or to a very limited extent, stick to the walls of the crucibles. It was very surprising that crucibles having a high open porosity of between 40 and. 60% by volume showed this property. By providing the crucibles with a layer of silicon nitride powder with an average particle size of maximum 50pm, any attachment of solidified silicon to the walls of the cmcibles is avoided. Short description of the figures Figure 1 shows the shape and dimensions of the crucible produced according to example 1. Detailed description of the Invention Example 1 A Si3N4 crucible was produced according to the method of the present invention. Silicon powder having a particle size below 75pm sold by Elkem ASA under the trademark SILGRAIN was filled into a mould having the shape and dimensions as shown in Figure 1. The silicon powder was compacted by vibration whereafter the crucible was nitrided at a temperature between 1105 and 1380°C in a vertical tube furnace until a conversion of silicon to Si3N4 of 97% of theoretic conversion was achieved. The produced crucible had an open porosity of 41.25% by volume and a density of 1.85 g/cm^. The crucible according to the invention was used for directional solidification of silicon. The directional solidification was done by filling particulate silicon in the crucible. The crucible was then placed in a vertical tube furnace and argon was supplied to the furnace in order to prevent oxidation of the crucible and of the molten silicon. The silicon in the crucible was melted at a temperature of 1500°C. The crucible was thereafter slowly lowered down through the furnace until the bottom of the crucible was positioned outside the hot zone. In this position the temperature was lowered 60°C pr. hour until the temperature reached 1375°C. The crucible was then cooled to room temperature. Upon examination it was found that the solidified silicon ingot was only affixed to the crucible on a few spots at the walls of the crucible where the open porosity was below 40% by volume and where the size of the pores was smaller than the Si3N4 particles. Example 2 A crucible produced as described in example 1 was coated on its inside walls with Si3N4 powder. The crucible was used for directional solidification of silicon according to the procedure described in example 1. Examination showed that the solidified silicon ingot was not affixed to the crucible. WE CLAIM : 1. Mould parts for use in connection with directional solidification and drawing of silicon single crystals, in which the mould parts consist of Si3N4 particles, the mould parts have a total open porosity between 40 and 60% by volume, and in which more than 50% of the pores in the surface of the mould parts have a size which is greater than the mean size of the Si3N4 particles. 2. Mould parts according to claim 1, wherein the mould parts are coated with silicon nitride particles having an average particle size of less than 50μm. 3. A method for the production of silicon nitride mould parts for use in connection with directional solidification of silicon, comprising the steps of forming a mould part from particulate silicon having a particle size of less than 1 OOμm and subjecting the mould parts to nitridation for conversion of the silicon particles to Si3N4; in which the forming step is carried out under such a pressure and with such a particle size distribution of the silicon particles that the finished silicon nitride mould part has an open porosity between 40 and 60% by volume and where more than 50% of the pores in the surface of the finished mould parts are greater than the mean size of the Si3N4 particles. 4. Method according to claim 3, wherein the shaping of the mould parts from the silicon particles is carried out at a pressure of below 200 Mpa. 5. Method according to claim 3, wherein the shaping of the mould parts are carried out using vibration.

Full Text

Title of invention
Mould parts of silicon nitride and method for producing such mould parts.
Field of invention
The present invention relates to mould parts of silicon nitride for use in connection with molten metal, particularly molten silicon, and to a method for producing such mould parts.
Background art
It is known to use quartz cmcibles during crystallising of pure molten silicon by directional solidification and by crystal pulling from pure molten silicon, Quartz crucibles have, however, the drawback that quartz is wet by molten silicon, and solidified silicon will thereby affix to the walls in the quartz cnjcibles. Further, quartz and silicon have different coefficients of thermal expansion, resulting in that when molten silicon solidifies in a quartz crucible, thermal stress will be introduced in the quartz cmcible which will destroy the crucible. Quartz cmcibles can therefore only be used once.
From JP-59-62199 it is known a method for the production of silicon nitride cmcibles for use in pulling silicon crystals. The cmcible according to JP-59-62199 can be produced by fomning silicon powder by cold pressing, whereafter the cmcible is heated in an Inert atmosphere in a first step and thereafter nitridation is earned out at a higher temperature in a second step. According to JP-59-62199 the produced crucibles have a density of 85% of the theoretical density for silicon nitride.
The crucibles according to JP-59-62199 have a good strength, but are wet by molten silicon to such an extent that the solidified silicon ingot sticks to the walls in the crucible. The ingot can thus not be removed from the crucible without destroying the cmcible. The crucibles according to JP-59-62199 can thus only be used once when they are used for directional solidification of silicon. The same is tme also when the crucibles according to JP-59-62199 are used for crystal pulling of silicon single crystals.

Description of Invention
The object of the present invention is to provide silicon nitride mould parts, such as crucibles, which are not wet by molten silicon thus avoiding that molten silicon which is solidified in the crucibles sticks to the wail of the crucible.
The present invention thus relates to silicon nitride mould parts, particularly
crucibles for use in connections with directional solidification and pulling of
silicon single crystals, which mould parts consist of Si3N4 having a total open porosity between 40 and 60% by volume and where more than 50% of the pores in the surface of the mould parts have a size which is larger than the
mean size of the Si3N4 particles.
/ ■
According to a preferred embodiment the mould parts are coated with silicon nitride particles having an average particle size of less than SOprn.
It has surprisingly by been found that S\z^4 mould parts having such an open porosity are not wet by molten silicon and have a strength making it possible to reuse the crucibles a number of times for directional solidification of molten silicon.
According to another aspect, the present invention relates to a method for the production of silicon nitride mould parts, particularly crucibles for use in connection with directional solidification of silicon, where particulate silicon having a particulate size of less then 100pm is fornied to a mould part and subjected to nitridation for conversion of the silicon particles to Si3N4, which ^ method is characterized in that the forming is carried out under such a pressure and with such a particle size distribution of the silicon particles that the finished silicon nitride mould part has an open porosity between 40 and 60% by volume. Further, more than 50% of the surface of the finished mould parts that consist of pores that breaks through the surface, have pores which are greater than the mean size of the Si3N4 particles.

The shaping of mould parts from the silicon particles is preferably carried out
at a pressure of below 200 MPa, and It is particularly preferred to carry out the
shaping of the mould parts using vibration.
Tests with the use of Si3N4 crucibles according to the present invention for directional solidification of molten silicon have shown that the solidified ingot did not, or to a very limited extent, stick to the walls of the crucibles. It was very surprising that crucibles having a high open porosity of between 40 and. 60% by volume showed this property.
By providing the crucibles with a layer of silicon nitride powder with an average particle size of maximum 50pm, any attachment of solidified silicon to the walls of the cmcibles is avoided.
Short description of the figures
Figure 1 shows the shape and dimensions of the crucible produced according to example 1.
Detailed description of the Invention
Example 1
A Si3N4 crucible was produced according to the method of the present invention. Silicon powder having a particle size below 75pm sold by Elkem ASA under the trademark SILGRAIN was filled into a mould having the shape and dimensions as shown in Figure 1. The silicon powder was compacted by vibration whereafter the crucible was nitrided at a temperature between 1105 and 1380°C in a vertical tube furnace until a conversion of silicon to Si3N4 of 97% of theoretic conversion was achieved.
The produced crucible had an open porosity of 41.25% by volume and a density of 1.85 g/cm^.
The crucible according to the invention was used for directional solidification of silicon. The directional solidification was done by filling particulate silicon in the crucible. The crucible was then placed in a vertical tube furnace and argon

was supplied to the furnace in order to prevent oxidation of the crucible and of the molten silicon. The silicon in the crucible was melted at a temperature of 1500°C. The crucible was thereafter slowly lowered down through the furnace until the bottom of the crucible was positioned outside the hot zone. In this position the temperature was lowered 60°C pr. hour until the temperature reached 1375°C. The crucible was then cooled to room temperature. Upon examination it was found that the solidified silicon ingot was only affixed to the crucible on a few spots at the walls of the crucible where the open porosity was below 40% by volume and where the size of the pores was smaller than the Si3N4 particles.
Example 2
A crucible produced as described in example 1 was coated on its inside walls with Si3N4 powder. The crucible was used for directional solidification of silicon according to the procedure described in example 1. Examination showed that the solidified silicon ingot was not affixed to the crucible.

WE CLAIM :
1. Mould parts for use in connection with directional solidification and drawing of
silicon single crystals, in which the mould parts consist of Si3N4 particles, the
mould parts have a total open porosity between 40 and 60% by volume, and in
which more than 50% of the pores in the surface of the mould parts have a size
which is greater than the mean size of the Si3N4 particles.
2. Mould parts according to claim 1, wherein the mould parts are coated with
silicon nitride particles having an average particle size of less than 50μm.
3. A method for the production of silicon nitride mould parts for use in
connection with directional solidification of silicon, comprising the steps of
forming a mould part from particulate silicon having a particle size of less than
1 OOμm and subjecting the mould parts to nitridation for conversion of the
silicon particles to Si3N4; in which the forming step is carried out under such a
pressure and with such a particle size distribution of the silicon particles that
the finished silicon nitride mould part has an open porosity between 40 and
60% by volume and where more than 50% of the pores in the surface of the
finished mould parts are greater than the mean size of the Si3N4 particles.
4. Method according to claim 3, wherein the shaping of the mould parts from the
silicon particles is carried out at a pressure of below 200 Mpa.
5. Method according to claim 3, wherein the shaping of the mould parts are
carried out using vibration.

Documents:

0177-chenp-2005 abstract-duplicate.pdf

0177-chenp-2005 abstract.pdf

0177-chenp-2005 claims-duplicate.pdf

0177-chenp-2005 claims.pdf

0177-chenp-2005 correspondence-others.pdf

0177-chenp-2005 correspondence-po.pdf

0177-chenp-2005 description(complete)-duplicate.pdf

0177-chenp-2005 description(complete).pdf

0177-chenp-2005 drawings.pdf

0177-chenp-2005 form-1.pdf

0177-chenp-2005 form-18.pdf

0177-chenp-2005 form-26.pdf

0177-chenp-2005 form-3.pdf

0177-chenp-2005 form-5.pdf

0177-chenp-2005 pct search report.pdf

0177-chenp-2005 pct.pdf

0177-chenp-2005 petition.pdf

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Patent Number

218770

Indian Patent Application Number

177/CHENP/2005

PG Journal Number

21/2008

Publication Date

23-May-2008

Grant Date

16-Apr-2008

Date of Filing

14-Feb-2005

Name of Patentee

CRUSIN AS

Applicant Address

Sem Saelandsvei 12, N-7465 Trondheim,

Inventors:

#	Inventor's Name	Inventor's Address
1	OLSEN, Espen	Eidsvollsgt. 14, N-7030 Trondheim,
2	SOLHEIM, Arve	Martin Kregnesv. 31, N-7091 Tiller,
3	SORHEIM, Havard	Hoiliveien 12, N-7052 Trondheim,

PCT International Classification Number

C30B 15/10

PCT International Application Number

PCT/NO03/00274

PCT International Filing date

2003-08-13

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	20023865	2002-08-15	Norway