Title of Invention

"A PROCESS FOR THE PREPARATION OF SINTERED CUBIC γ-ALUMINIUM OXYNITRIDE"

Abstract

This invention relates to a process for the preparation of sintered gamma-alumnium ox nitride (y-AlON). The material is suitable for manufacturing components to be used as window material. Process steps comprises: preparing a homogeneous mixture by known methods of 59-65 mole% A^Os and 41-35 mole% AIN, passing the powder through 100 mesh, pressing the powder by known methods to form green compacts, sintering the green compacts at a temperature in the range of 1775°C to 1950°C in nitrogen atmosphere.

Full Text

This invention relates to a process for the preparation of sintered gamma-aliiminiiim ownitride fv-AlONl aluminium oxynitride (y-AlON) y-AlON is one of the compounds formed in the binary system containing A1N and A12O3. Although the chemical composition of this compound is close to the formula Al^C^Ns, this is not a strictly stoichiometric compound and is rather a solid solution containing a range of the starting materials. This is a promising material for window applications because it exhibits the necessary combination of good strength along with an in-line transmission capability of a wide range of electromagnetic radiation in the n^arjUtraviplet, visible and infrared to approximately 5 microns wavelength. The IR detectors are commonly protected by a magnesium fluoride (MgF2) dome, y-A1ON is an attractive alternative material to replace MgF2 because of its higher strength and thermal shock resistance. Hence, this is a more suitable protector material. In general, the material is suitable for manufacturing components, to be used as window material, for transmission of wavelengths from 0.2 to 5 (j. and which operate under high stress. Although the detailed data involving the densification of A1ON is lacking, three earlier work have reported the formation of dense and transparent cubic y-AlON as follows: 1) T.M. Harriett, R.L. Gentilman, E.A. Me Guire, L.E. Dolhert, "Transparent aluminium oxyuitridc and method of manuiaclmc", UK Patent Application No. 8224836, dt. 31.8.82 l- .u 2) R.L. Gentilman, E.A. Me Guire, L.E. Dolhert, "Transparent aluminium oxynitride and method of manufacture", US Patent No. 4,520,116 dt. 28.5.85 3) R.L. Gentilman, E.A. Me Guire, L.E. Dolhert, "Transparent aluminium oxynitride and method of manufacture", US Patent No. 4,720,362 dt. 19.1.88 4) All the three works considered compositions on the ACV rich side (65 mole%) of the compositional zone for the formation of cubic y-AJON in the A12O3-A1N binary phase system. Additionally, a third material Boron Nitride (BN) and/or Yttrium Oxide (Y2O3) was doped as a sintering additive. The presence of the additive materials might cause either solute drag or secondary phase precipitation to pinning grain boundaries thereby preventing excessive grain growth which otherwise traps pores within the grains. In another work (J.P. Mathers, R.J. Frey, "Transparent aluminium oxynitride- based ceramic articles", US Patent No. 4241000 dt. 27.01.93), Magnesium oxide (MgO) was used as an additive. The main difficulty encountered for such compositions was that the desired product development depended on the control of the additives used and a very long soaking period of at least 20 h to as long as 100 h at sintering temperature was required to be employed. The main object of the present invention, is to provide a process for the preparation of sintered cubic y-AION. Another object of the present invention is to provide a process wherein the A1N rich side of the compositional zone is used to obtain sintered cubic y-A1ON in the system Al-O-N, using A12O3 and A1N as starting materials. Yet another object of the present invention is to provide a process wherein no sintering additive is used. Still another object is to provide a process wherein a lower sintering time is i required thus making the process economic. In the process of the present invention, compositions in the A1N rich region of the compositional plane A12O3-A1N in the system Al-O-N were taken which displays an easier sintering behaviour in comparison to the other compositions taken for the additive-Al-O-N systems where the main difficulty encountered was the requirement of precise control over the additives used in association with very long sintering time. The main finding of the present invention is that the product obtained using the A1N rich compositional zone exhibits y-AION as single crystalline phase with excellent sintcrability and possess an in-line transmission capability nol less than 10% of the electromagnetic radiation in the wavelength range of 0.3 to 5 u,. The products are suitable for the uses mentioned above. Accordingly, the present invention provides a process for the preparation of sintered cubic y-aluminium oxy nitride which comprises; a) Characterised in that preparing a homogenous mixture of 59-65 mole% A12O3 and 35-41 mole % Aluminium nitride by milling in a liquid medium of pure acetone with water content b) prefiring the said homogenous mixture at a temperature in the range to 1500 to 1770°C for a period of 2 to 4 hrs to obtain preformed cubic y -A1ON, c) milling the preformed y -A1ON by known methods, passing the powder through 100 mesh, d) pressing the powder by known methods to form green compacts, e) sintering the green compacts at a temperature in the range of 1775 to 1950°C in nitrogen atmosphere, to obtain sintered cubic y-aluminium oxy nitride, pgeeeciaEactorigod in that In UGing-AM-rieh 3ide-gfcjtomtRJsili01ial /one" Ab63-AlN 111 Ihe In an embodiment of the present invention, AbOs may be purity 90%. In another embodiment of the present invention, AIN used may have an oxygen content around 1 to 1.5%. In yet another embodiment of the present invention, the homogeneous mixture prior to pressing may be prefired at a temperature in the range of 1500°C to 1770°C for at least 5 minutes to obtain preformed cubic y-AlON milling the preformed y-AlON so obtained by known methods and passing the powder through 100 mesh. In still another embodiment of the present invention, the sintering time is at least 8 hours in the case of direct sintering and is at least 1 hour in the case of preformed y-AlON. The process of the present invention for preparing sintered y-AlON is described below in detail. Pure and fine grade of a-AJ2O3 and A1N were taken as starting materials. Accurately weighed appropriate proportions of starting materials were taken in A12O3 pot in an attritor mill along with A12O3 balls (size around 2 to 3 mm). The ball:powder ratio were kept in-between 6:1 and 9:1. The milling was done in a liquid medium of pure acetone with water content The process of the present invention wherein the prefiring is done is described below in detail as follows: In another batch, accurately weighed appropriate proportions of starting materials A12O3 and A1N were taken in A12O3 pot in an attritor mill along with A12O3 balls (size around 2 to 3 mm). The ball:powder ratio were kept in-between 6:1 and 9:1. The milling was done in a liquid medium of pure acetone with water content 0.5%. The milling time was ranged between 2 to 8 hours. After milling, the powder was taken in a boron nitride crucible and was fired in a graphite resistance heating furnace under nitrogen atmosphere within a temperature range of 1550°C to 1750°C for a holding period of 2 to 4 h. After firing, the lump was initially broken in an agate mortar for 5 to 15 min. and then was taken with the AOj balls in the attritor mill. The similar procedure of attrition milling of the powder was undertaken, as mentioned above. The milling time was varied from 5 to 15 h. After this second milling, the powder was dried, sieved and green pressed as narrated above. The pressed green billets were taken in a graphite resistance heating furnace and were fired in a nitrogen atmosphere at a temperature in the range of 1800°C to 1950°C. The soaking time was varied in between 1 and 9 h. The sintering characteristics of the fired samples like firing shrinkage, weight loss, density, etc., were evaluated following conventional procedures. To prepare the samples for transmission measurements, small discs of thickness up to 2 mm were cut from sintered billets and both surfaces of the discs were ground and polished up to 1 fj,m. The novelty of the present invention lies in obtaining lower firing and/or sintering times and also avoiding^ the use of a sintering additive. The inventive stepJs-using the A1N jrich side, of the compositional zone_ MjOr A1N in the system Al-O-N. The sintering is found to be enhanced in the A1N richer compositional zone of the y-AlON solid solution. It is believed that the mechanism is as follows: y-AlON is found to be stable in the y-Al2O3 structure with five nitrogen anions replacing oxygen which corresponds to about 35.7 mole% A1N representing the stoichiometric formula ACNs. In the higher A12O3 region than stoichiometric composition, the cation vacancy increases. At the sintering temperature, this larger vacancy is supposed to favour taster movements of the cations. However, to avoid polarity and to maintain electrical neutrality, the corresponding movements of the anions arc required, due to ionic size of which are not as diffusive. Therefore a mismatch in the defects may be created which in turn, may not enhance densification. On the other hand, at lower concentration of cation vacancies, the thermally activated "matching-defects" may occur in the compositions containing larger amount of A1N than stoichiometry, which in turn, is believed to produce better densification. Another sintering mechanism in the region of around 40 mole% A1N may be as follows: As the represented boundary lines of the A1ON solid solution in the A12O3-A1N binary phase diagram were drawn by basing only on few experimental points, it may believed that the A1N richer compositions of around 40 mole% AJN may contain minor amounts of secondary phases or liquid. These phases crystallise in the grain boundary thereby pinning the boundary of the AJON grains to prevent the exaggerated grain growth. The exaggerated grain growth otherwise may trap pores within grains hindering densification. The liquid which may appear in the product during firing may also enhance densification. The following examples are given by way of illustration of the present invention and should not be construed to limit the scope of the invention : Example 1 One composition containing A12O3- 61 mole% and A1N- 39 mole% was attrition milled for 3 h, dried, cold pressed at a pressure of 65 MPa and was fired at 1850°C for 2 h under 10 psi nitrogen gas pressure. The firing shrinkage perpendicular and parallel to the pressing direction were 16.02% and 14.65%, respectively and the fired density achieved was 98% of the theoretical value. Example 2 One composition containing A12O3- 61 mole% and A1N- 39 mole% was attrition milled for 3 h, dried, cold pressed at a pressure of 65 MPa and was fired at 1930°C for 4 h under 10 psi nitrogen gas pressure. The firing shrinkage perpendicular and parallel to the pressing direction were 17.51% and 17.16%, respectively and the fired density achieved was 98.1% of the theoretical value. Example 3 One composition containing A12O3- 64.3 mole% and AIM- 35.7 mole% was attrition milled for 3 h, dried, cold pressed at a pressure of 65 MPa and was fired at 1930°C for 4 h under 10 psi nitrogen gas pressure. The firing shrinkage perpendicular and parallel to the pressing direction were 15.29% and 15.12%, respectively and the fired density achieved was 98.5% of the theoretical value. Example 4 One composition containing A12O3- 64.3 mole% and A1N- 35.7 mole% was attrition milled for 3 h, dried and sieved. The powder was fired at 1750°C for 2.5 h under 5 psi nitrogen gas pressure. The product was attrition milled for 10 h, dried and sieved. The powder median diameter was 0.98 fim whereas the highest population data were from 1.5 uin to 1.0 jim to be 28%. The range of 2.0 j4.m to 0.6 ju.ni was 56% whereas the submicron sized particles were around 50%. The powder was cold pressed at a pressure of 65 MPa and was fired at 1820°C for 4 h under 10 psi nitrogen gas pressure. The firing shrinkage perpendicular to the pressing direction were 18.89% and the fired density achieved was 98.1% of the theoretical value. Example 5 One composition containing A12O3- 64.3 mole% and AIN- 35.7 mole% was attrition milled for 3 h, dried and sieved. The powder was fired at 1750°C for 2.5 h under 5 psi nitrogen gas pressure. The product was attrition milled for 10 h, dried and sieved. The powder median diameter was 0.98 jim whereas the highest population data were from 1.5 jim to 1.0 jim to be 28%. The range of 2.0 jj.m to 0.6 u.m was 56% whereas the submicron sized particles were around 50%. The powder was cold pressed at a pressure of 65 MPa and was fired at 1855°C for 4 h under 10 psi nitrogen gas pressure. The firing shrinkage perpendicular to the pressing direction were 18.77% and the fired density achieved was 99.5% of the theoretical value. The transmission capability of the polished specimens at both ends of thickness 0.7 mm were around 12% in the visible spectral range. Example 6 One composition containing A12O3- 64.3 mole% and AIN- 35.7 mole% was attrition milled for 3 h, dried and sieved. The powder was fired at 1750°C for 2.5 h under 5 psi nitrogen gas pressure. The product was attrition milled for 10 h, dried and sieved. The powder median diameter was 0.98 ja.ni whereas the highest population data were from 1.5 jj.ni to 1.0 jj.m to be 28%. The range of 2.0 jim to 0.6 jam was 56% whereas the submicron sized particles were around 50%. The powder was cold pressed at a pressure of 65 MPa and was fired at 1945°C for 4 h under 10 psi nitrogen gas pressure. The firing shrinkage perpendicular to the pressing direction were 18.9% and the fired density achieved was 99.55% of the theoretical value. The transmission capability of the polished specimens at both ends of thickness 1.0 mm were around 26% in the visible spectral range. We find that in case of direct sintering the sintering time required is at least 8 hours and the fired density obtained is of the order of 98%. When the optional step of prefiring is followed the sintering time reduces to 1 hour and the fired density obtained is in the order of 99%. The main advantages are : 1) Near full density materials based on y-AlON ceramic can be prepared by using the compositions (in the Al-O-N system) of the present invention. 2) The compositions display excellent sinterability under normal sintering conditions at ambient nitrogen gas pressure and at temperature at around il800°-1960°C. 3) Using the compositions of the present invention, the dense sintered material shows the presence of cubic y-AJON as only crystalline phase. 4) The density achievable using the compositions is above 98% of the theoretical value. 5) The sintered materials using the compositions display transparency not less than 10% in the wavelength region of 0.3 jam to 5 jim. We Claim: 1. A process for the preparation of sintered cubic y-aluminium oxy nitride which comprises; a) Characterised in that preparing a homogenous mixture of 59-65 mole% AlaOs and 35-41 mole % Aluminium nitride by milling in a liquid medium of pure acetone with water content b) Pre-firing the said homogenous mixture at a temperature in the range to 1500 to 1770°C for a period of 2 to 4 hrs, to obtain preformed cubic y -A1ON, c) milling the preformed y -A1ON by known methods, passing the powder through 100 mesh, d) pressing the powder by known methods to form green compacts, e) sintering the green compacts at a temperature in the range of 1775 to 1950°C in nitrogen atmosphere, to obtain sintered cubic y-aluminium oxy nitride. 2. A process as claimed in claim 1 wherein the AbOs is of purity >99%. 3. A process as claimed in claims 1-2, wherein the sintering time is at least 8 hours in the case of direct sintering and is at least 1 hour in the case of preformed y- A1ON. 4. A process for the preparation of sintered cubic y-Aluminium oxynitride substantially as herein described with reference to the examples.

Documents:

151-del-2000-abstract.pdf

151-del-2000-claims.pdf

151-del-2000-complete specification (granted).pdf

151-del-2000-correspondence-others.pdf

151-del-2000-correspondence-po.pdf

151-del-2000-description (complete).pdf

151-del-2000-form-1.pdf

151-del-2000-form-19.pdf

151-del-2000-form-2.pdf

151-del-2000-form-3.pdf