GB2150129A

GB2150129A - Transparent aluminum oxynitride and method of manufacture

Info

Publication number: GB2150129A
Application number: GB08429792A
Authority: GB
Inventors: Thomas M Hartnett; Richard L Gentilman; Edward A Mcguire; Leonard E Dolhert
Original assignee: Raytheon Co
Current assignee: Raytheon Co
Priority date: 1981-08-31
Filing date: 1984-11-26
Publication date: 1985-06-26
Also published as: GB2150129B; GB2126568B; GB8429792D0; GB2126568A

Abstract

A method of preparing the optically transparent aluminum oxynitride is also provided comprising the steps of forming a green body of substantially homogeneous aluminum oxynitride powder and pressureless sintering said green body in a nitrogen atmosphere and in the presence of predetermined additives which enhance the sintering process. Preferred additives are boron and yttrium in elemental or compound form.

Description

1 GB2150129A 1

SPECIFICATION

Transparent alurninum oxynitride and method of manufacture This invention relates to durable transparent ceramic compounds. There is a need for these compounds in applications requiring substantial transmission and imaging capabilities in the visible range and the infrared range. These requirements can be found in both military and commercial applications. For example, infrared transparent domes are needed for missiles and transparent envelopes are needed in different types of vapor lamps. Many transparent materials are not adequately durable for these applications, thus, the search has been directed towards developing transparent ceramics. Although many ceramic compounds satisfy the durability requirement, they are not transparent to a sufficient degree for these applications. For instance, alumina is a very hard material but the main problem is that it is not sufficiently transparent and scat- ters light to an excessive degree. An additional consideration for a candidate material is the cost of manufacturing, thus, methods that require individual processing of these windows are bound to remain an unfeasible alternative from a cost point of view. From this perspective, forging and hotpressing methods are not desirable. This leaves batch processing methods as a desirable feasible alternative and sintering lends itself to the manufacture of a plurality of units in a single run. However, the sintering of transparent ceramics is not widely known or practiced.

Aluminum oxynitride is a promising candidate for applications requiring multi-spectral transmission capabilities. The only known prior attempt at producing a sintered aluminum oxynitride body is found in U.S. Patent No. 4,241,000, wherein precursor powders are mixed and the sintering step is used to both react and sinter the precursor powders to 110 produce an aluminum oxynitride body. The problem is that the resulting material is not sufficiently transparent for the applications mentioned hereinabove.

These and other problems are solved by the present invention which provides for the sin tering of cubic aluminum oxynitride to pro duce a transparent ceramic window. It has been found that starting with a homogenous powder of cubic aluminum oxynitride and using doping additives leads to an adequately transparent window in both the visible and infrared range. It has further been found that the sintering parameters of the present inven tion are such that a liquid phase at grain boundaries occurs during the early stages of sintering.

This invention provides a sintered body of polycrystalline cubic aluminum oxynitride ma terial having a density of at least 98% prefera-130 bly at least 99% of theoretical density, a sample of said material having an in-line transmission of at least 20% in the wavelength range of 0.3 to 5 microns, said sample having a thickness of 1.90 millimeters and preferably having a resolving angle of better than 3 mrad.

The invention also provides a sintered body of doped poly-crystal line aluminum oxynitride material having a density greater than 98% of theoretical density, a sample of said material having an in-line transmissin of at lest 20% in the wavelength range of 0.3 to 5 microns, said sample having a thickness of 1.90 milli- meters and preferably having a resolving angle of better than 3 mrad.

This invention further provides a method of manufacturing a substantially transparent aluminum oxynitride body comprising the steps of forming a homogeneous single phase aluminum oxynitride powder, pressing said powder into a green body of predetermined shape, and sintering in a nitrogen atmosphere said green body in the presence of dopants.

Preferably, said aluminum oxynitride powder has an average particle size less than 1.0 micrometers, said dopants preferably comprise boron and/or a lanthanide, such as yttrium or lanthanum, or compounds thereof, and said sintering step is carried out at a temperature higher than 1 90WC, but lower than the solidus temperatures of aluminum oxynitride and for a period of at least 20 hours.

The method of the present invention uses a substantially homogenous aluminum oxynitride powder to produce a transparent sintered aluminum oxynitride body. The homogenous cubic aluminum oxynitride powder may be prepared, for example, according to copend- ing U.S. patent application No. 297897 or by the following method. A mixture is prepared comprising of 30-37 mole % akuminum nitride and 70-63 mole % aluminum oxide (alumina). The aluminum oxide normally consists of the alpha type. Both components are preferably fine powders having a particle size not greater than 74 micrometers. The aluminum nitride has a purity of 97-98% while the aluminum oxide primarily is 99.9% or greater. The mixture is preferably ball-milled in alumina mills with alumina grinding media and with methanol as the milling medium. The preferred duration of milling is 16 hours. The mixture is then dried and placed in alu- mina crucibles for calcination. Calcining is preferably carried out at a temperature range of 1600-1 75WC for four hours in a stagnant atmosphere of nitrogen at a pressure range of 0-5 psig (0-34.5 KPa). During the calcination, alumina reacts with the aluminum nitride to form a cubic aluminum oxynitride compound. The calcined aluminum oxynitride powder is preferably also ball-milled in alumina mills with alumina grinding media and methanol as the milling medium. The pre- 2 GB2150129A 2 ferred duration of the milling is 72 hours. This results in a single phase aluminum oxynitride powder having an average particle size smaller than 1.0 micrometers. The slurry is then thor- oughly dried. A small amount of doping additive, as discussed herein below, may be added at this point to the aluminum oxynitride or prior to the second milling step. Alternatively, the doping additive may be added later. The only requirement is that the total amount of the additive present in a body during the sintering step does not exceed approximately 0.5% by weight of the green body. The aluminum oxynitride powder is preferably placed in rubber molds having predetermined shapes and is isostatically pressed at pressures greater than 15,000 psi(l 03420 KPa) to produce green bodies to be used in sintering. The fabricated green bodies are then set in containers in the sintering chamber. The containers may be composed either entirely of boron nitride or partly of boron nitride and partly of molybdenum metal. The small quantity of the doping additive may alternatively be added now in the containers with the fabricated green bodies. The additive may be in the form of a mixture of aluminum oxide, aluminum nitrode and the additive, with the additive comprising up to 100% by weight of the mixture. The preferred additive is yttrium oxide, but elemental yttrium or other compound forms may also be used. Sintering is then performed in a stagnant atmosphere of nitrogen at 0-5 psig. Sintering temperatures used were higher than 1 90WC, but lower than the solidus temperature of aluminum oxynitride which is approximately 2140'C. Sintering may be performed for a minimum of 20 hours up to 100 hours. The resulting polycrystalline sintered body preferably has an average grain size of 200 microns.

The unexpected transparency obtained by the method of the present invention was discovered after the sintering in a boron nitride container of a first undoped oxynitride green body together with a second oxynitride green body containing 5 weight percent yttrium oxide. Spectrographic analysis revealed that the first sample contained trace amounts of both boron and yttrium. For a sintering temperature of 1 925C and a duration of 24 hours, the transparent oxynitride body has an additive doping of 100 parts per million (ppm) boron and 600 plam yttrium. If yttrium oxide were to be used in the mixture as the source of yttrium, then the weight percent Of Y203 needed would be 0.075% of the green body. The density of this aluminum oxynitride sin tered sample is greater than 99% of theoretical density, the in-line transmission, for a sample thickness of 1.78mm, at 4 microns is 43% and the resolving angle is 0.5 mrad.

The in-Une transmission is at least 40% in the range of 0.3 to 5 microns.

The density was measured by the Archi- 130 medes method, in-line transmission was measured with a 457 Perkin-Elmer Grating Infrared S pectro photometer, and the resolving angle was measured by using the Standard USAF 1951 Resolution Test Pattern.

A similar sample was sintered at a temperture of 1 94WC keeping the other process parameters the same. The boron doping remained the same at 100 ppm while the yttrium doping increased slightly to 800 ppm resulting in a Y,O, requirement of 0. 10 weight percent for an equivalent doping. The density remained the same at over 99% of theoretical density, the in-line transmission, for a thickness of 1. 78nim, was 41 % at 4 microns, and the resolution remained at 0.5 mrad.

Another sample was sintered at 1 94WC for 20 hours and resulted in a sintered body having a boron doping of 100 ppm, and a yttrium doping of 1500 ppm, corresponding to an equivalent Y,O, weight percent of 0. 19, and having a density of over 98% of theoretical density, an in-line transmission, for a sam- ple thickness of 1.90 mm, of 21 % at 4 microns and a resolution of 3 mrad. Trace amounts up to an equivalent 0.5 weight percent of yttrium oxide doping r-nay be used for producing a transparent aluminum oxynitride window having an in-line transmission of at least 20% across the 0.3 to 5 micron range.

A yttrium-doped sample with a yttrium doping equivalent to a Y20, amount of 0.0 13 weight percent was sintered at 1 925C for 24 hours. it resulted in an opaque sample, thus pointing to the need of a minimum amount of yttrium doping to achieve the improved optical qualities. By extrapolating the values of the samples available, the minimum amount of yttrium doping is believed to be equivalent to 0.02 weight percent of Y203.

It should be understood that the yttrium additive itself need not be present in a nearby sample nor in a vapor form. For convenience, the additive may be mixed in with the aluminum oxynitride powder prior to sintering, but otherwise it need not be placed in direct contact with the green body. Again, it is sufficient that the selected additive be available within the sintering chamber for vapor doping the aluminum oxynitride. Similarly, it should be understood that the boron is also an additive, even though it is not mixed in with the other compounds. Its presence in the sintering chamber, as part of the compounds forming the container, is sufficient to provide boron vapor doping of aluminum oxynitride. Thus, this invention is considered to encompass other methods of introducing the addi- tives in the sintering chamber to produce in situ vapor doping of the aluminum oxynitride compact.

The sintering is found to be enhanced by the presence of specific additives, and more specifically, by the yttrium used and the boron 3 GB2150129A 3 found in the container. It is believed that the mechanism is as follows. At sintering temperatures, the mixture of aluminum oxynitride and the excess aluminum that might be pre5 sent in the sample has a significantly high vapor pressure of AixCi, gas species. The Al,,0, gas reacts with nearby boron nitride present in the container to produce B203 gas and/or A1B02 gas plus AIN solid. The 1320, and/or Ail302 vapors transport to and react with alu- 75 minum oxynitride to produce a liquid phase at grain boundaries which enhances the early stages of sintering. The 1320, also interacts with the source of yttrium doping such as the yttrium oxide vapor from an adjacent source 80 or from the Y203 added to the sample to produce Y1302 gas. The Y1302 vapor transports to the aluminum oxynitride and dopes it with the boron and yttrium. It is believed that this additive doping aids the final stages of sinter- 85 ing by causing either solute drag or second phase precipitates to pin grain boundaries and thus preventing excessive grain growth which might otherwise trap pores within the grains.

Claims

1. A method of manufacturing a dense aluminium oxynitride body comprising the steps of forming a single phase aluminium oxynitride powder; pressing said powder into a green body of predetermined shape; and sintering said green aluminium oxynitride body in a nitrogen atmosphere.

2. A method according to Claim 1, wherein said sintering step takes place in the 100 presence of at least one dopant.

3. A method according to Claim 1 or Claim 2, wherein said sintering step is carried out at a temperature and for a length of time sufficient to eliminate porosity by diffusion of 105 said solid single phase aluminium oxynitride while preventing excessive grain growth.

4. A method according to any one of Claims 1 to 3, wherein said single phase aluminium oxynitride powder has an average particle size smaller than 1.0 microns.

5. A method according to any one of Claims 1 to 4, wherein said aluminium oxyni tride forming step comprises the steps of preparing an homogeneous mixture of alumi nium nitride and aluminium oxide, said alumi nium oxide forming 63 to 70 mole percent of said mixture; calcining said mixture until sub stantially changed to single phase aluminium oxynitride powder; and reducing said alumi nium oxynitride powder to an average particle size less than 1.0 microns.

6. A method according to Claim 5, wherein said homogeneous mixture is pre pared by mixing aluminium nitride and alumi nium oxide powders having a purity of at least 97% and 99%, respectively, and a particle size smaller than 74 microns, and ball milling the mixture until fully mixed with an average particle size reduced to approximately 1.0 microns.

7. A method according to any one of Claims 1 to 5, wherein said sintering is carried out at a temperature higher than 1 90WC but lower than the solidus temperature of aluminium oxynitride and for a period of time of at least 20 hours.

8. A method according to any one of Claims 2 to 7, wherein the dopant is a lanthanide and/or boron.

9. A method according to Claim 7, wherein said dopant comprises yttrium and/or lanthanum and/or boron and/or a compound thereof.

10. A method according to any one of Claims 2 to 9, wherein said dopants are introduced in trace amounts.

11. A method according to any one of Claims 2 to 10, wherein said dopants are in a vapor phase during a portion of said sintering step.

12. A method according to Claim 11, wherein in said sintering step, the dopant vapor transports to and diffuses throughout said body.

13. A method according to any one of Claims 10 to 12, wherein said sintered body comprises not more than 0.5 weight percent of said dopants.

14. A method according to any one of Claims 2 to 12, wherein said dopants produce a liquid phase at grain boundaries during said sintering step.

15. A method according to any one of Claims 1 to 14, further comprising the step of adding yttrium oxide to the mixture of alumi nium nitride and aluminium oxide in amounts up to 0.5 weight percent.

16. A method according to any one of Claims 1 to 15 conducted substantially as described herein.

17. An aluminium oxynitride body when ever produced by a method according to any one of Claims 1 to 16.

Printed in the United Kingdom for Her Majesty's Stationery Office. Dd 8818935. 1985, 4235Published at The Patent Office. 25 Southampton Buildings. London, WC2A l AY, from which copies may be obtained-