JPH0420850B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to durable transparent ceramic compounds. These compounds are required for applications requiring substantial transparency and imaging capabilities in the visible and infrared ranges. These requirements can be found in military and commercial applications.
For example, infrared transparent domes are required for missiles, and transparent envelopes for various types of steam lamps. Many transparent materials are not adequately durable for these applications, and thus research has been directed toward the development of transparent ceramics. Although many ceramic compounds meet the durability requirements, they are not sufficiently transparent for these applications. For example, alumina is a sufficiently hard material, but the main problem is that it is not transparent enough;
And it scatters light too much. An additional consideration for candidate materials is the cost of manufacturing, and thus methods that require individual processing of these windows remain an unfeasible alternative from a cost standpoint. From this point of view, the step forming method and the hot pressing method are not desirable. This leaves batch processing as a desirable viable alternative, and sintering is suitable for manufacturing multiple units in a single experiment. However, sintering of transparent ceramics is not widely known or practiced. Aluminum oxynitride is a promising candidate material for applications requiring multispectral transmission capabilities. The only known prior art attempting to produce sintered aluminum oxynitride objects is U.S. Pat.
No. 4,241,000, in which precursor powders are mixed and a sintering process is used to react and sinter the precursor powders to produce aluminum oxynitride objects. The problem is that the resulting material is not transparent enough for the aforementioned uses. The inventors have discovered substantially homogeneous cubic aluminum oxynitride, particularly useful for sintering to produce durable transparent ceramic windows, produced from aluminum oxide and carbon black. It has been discovered that when the powder is sintered with special additives, a reasonably transparent window in the visible and infrared range can be obtained. Such homogeneous aluminum oxynitride is produced by placing aluminum oxide powder and carbon black in a reaction chamber, supplying nitrogen to the chamber, and heating the chamber to cause the powder and gas to react. It can be manufactured by producing a reacted powder that becomes a target. The reacted powder may also contain up to 15% by weight of aluminum oxide and aluminum nitride, such that the ratio of aluminum oxide to aluminum nitride is within the composition of cubic aluminum oxynitride. According to the invention, a mixture of aluminum oxide and carbon black is prepared, the mixture is reacted in the presence of nitrogen at a temperature in the range of 1550-1850°C, and the mixture is pre-prepared from said mixture. Shaping a pressed green body of a determined shape, placing said green body in a sintering chamber and supplying said chamber with a doping additive, said additive being one selected from the group of yttrium and lanthanum or Consisting of more than one element or a compound thereof,
There is then provided a method for producing a transparent sintered aluminum oxynitride object, comprising the step of sintering said green object at a temperature above 1900° C. but below the solidus temperature of the aluminum oxynitride object. Preferably, the doping agent is in the vapor phase during a portion of the sintering process and the vapor migrates to and diffuses throughout the body. Doping additives 0.5% by weight of raw matter
Configure the following. Preferred starting mixtures range from 5.4 to
It has a carbon content in the range of 7.1% by weight. Preferably, the reacted mixture is crushed into particles in the size range of 0.5 to 5 microns and the reacted mixture is heated in air or oxygen to remove any organic contaminants that may be present. Furthermore, according to the present invention, at least the theoretical density
A sample with a density of 99% and a thickness of 1.45mm is 0.3~
A polycrystalline doped cubic aluminum oxynitride object is provided that is characterized by having an in-line transmission of at least 50% in the 5 micron wavelength range. First, the production of aluminum oxynitride powder will be explained. By reacting gamma aluminum oxide and carbon in a nitrogen atmosphere, substantially homogeneous cubic aluminum oxynitride powder can be produced. More specifically, aluminum oxide (alumina) and carbon black are dry mixed, for example, in a Patterson-Kelly twin-shell blender for up to two hours. Preferably the aluminum oxide is at least 99.98
% purity and an average particle size of 0.06 microns, and carbon black has a purity of over 97.6% and a volatile content of 2.4% and an average particle size of 0.027 microns. The carbon content of this mixture is 5.4
It can range from ~7.1% by weight. A preferred mixture is 5.6% carbon black and
Consisting of 94.4% by weight aluminum oxide. The aluminum oxide/carbon mixture is placed in an alumina crucible and reacted at a maximum temperature of up to 2 hours at a temperature of 1550 DEG C. to 1850 DEG C. in an atmosphere of flowing nitrogen. A preferred heat treatment is a two-step process. In the first step, a temperature of approximately 1550°C was used for approximately 1 hour, thereby partially converting the temperature-labile gamma-aluminum oxide with carbon and nitrogen in order to obtain a suitable alumina to carbon ratio. only reacts to produce both alpha aluminum oxide and aluminum nitride. Soaking for 1 hour at 1550° C. is sufficient to convert the appropriate amount of Al ₂ O ₃ to AIN. In the second step, the temperature is 1750℃ or the solidus temperature of aluminum oxynitride (2140℃).
C.) for approximately 40 minutes, thereby combining the alpha-aluminum oxide and aluminum nitride to form cubic aluminum oxynitride. The reacted material obtained from this heat treatment is composed primarily of cubic aluminum oxynitride, but in amounts up to 15% by weight such that the ratio of aluminum oxide to aluminum nitride is within the composition range of cubic aluminum oxynitride. and contains alumina and/or aluminum nitride. The amount of alumina and aluminum nitride can be controlled by the heat treatment and the amount of aluminum nitride formed in the first heating step, the amount of aluminum nitride depending on the amount of carbon in the starting mixture. Except for sample 5, which was treated at 1620°C, for the first step with a preferred 1 hour soak at 1550°C, the table lists various amounts of carbon in the starting mixture and during the second step of heat treatment. The effect of using temperature is shown.

TABLE The preferred heat treatment produces a composition consisting essentially of 100% aluminum oxynitride, which corresponds to sample 5. Another preferred resulting composition is that of Sample 1. The resulting aluminum oxynitride powder consists of agglomerated particles, which are easily broken down during ball milling into particles with a size of 0.5 to 5 microns. The reacted material is ball milled in a polyurethane or rubber lined mill using methanol as the milling fluid and high alumina grinding balls. Milling time is 16 hours. Pass the milled powder through a 400 mesh sieve and
Dry at °C for up to 24 hours. After drying, the powder is heated to 600° C. for 2 hours in air to remove organic contaminants. Sintering aids are now added in the form of small amounts of up to 0.5% by weight of the aluminum oxynitride powder of the preselected doping additive. In addition, the additive is an element selected from the group of yttrium and lanthanum,
or a compound thereof. It is believed that other elements of the lanthanide series can be used as well. Preferably, oxides of the selected elements are used. Combinations of doping additives may also be used as long as the total amount of additives does not exceed 0.50% by weight. A preferred combination consists of 0.08% by weight yttrium oxide (Y ₂ O ₃ ) and 0.02% by weight lanthanum oxide (La ₂ O ₃ ). Alternatively, doping agents can be added during ball milling of aluminum oxynitride powder. The additive-containing aluminum oxynitride powder is placed into a rubber mold of predetermined shape and isostatically pressed at a pressure greater than 15000 psi to produce a green body for use in sintering. The produced green object is solidified in a container within a sintering chamber. The vessel may be made entirely of boron nitride or may be constructed of part boron nitride and part molybdenum metal.
Sintering is then carried out in a nitrogen free atmosphere at 0-5 psig. To obtain a substantially transparent material, the sintering temperature is above 1900°C, but below the solidus temperature of aluminum oxynitride, which is approximately 2140°C. Sintering is carried out for a minimum of 24 to 48 hours.

TABLE The table shows to some extent the effect of additives, time and temperature on the resulting clarity of aluminum oxynitride. Density was measured by the Archimedes method, in-line transmittance was measured by a Perkin-Elmer 457 grating infrared spectrophotometer, and the angle of resolution was standard
Measured by using the USAF1951 resolution angle test pattern. The temperature is precisely within 10°C. 1900
A temperature of 0.degree. C. is the minimum temperature found to consistently produce a transparent material given the appropriate amount of _{Y.sub.2 O.sub.3} and/or _{La.sub.2 O.sub.3} . The optimum amount of additive is the minimum amount necessary to create a liquid phase at the grain boundaries that is not initially present but is present as a second phase after sintering. 0.1% by weight gives the best results, but it is expected that trace amounts of about 0.05% by weight will be adequate. That is, a liquid phase forms around 1900° C. which promotes rapid densification and pore removal. This liquid phase disappears as Y and La become a solid solution with aluminum oxynitride. This liquid phase sintering process is believed to exist at the sintering temperature early in the sintering process. After this, solid state diffusion is the means by which residual porosity is eliminated and substantial transparency is achieved. Porosity elimination by solid state diffusion is a very slow means of pore elimination requiring longer times, with 24 hours being the minimum preferred period. This is samples 2 and 6
, where even with the use of appropriate amounts of additives, the samples remain translucent because the sintering period was limited to 1 h and 8 h, respectively. It should be understood that the additives discussed above do not need to be mixed with the pre-sintered oxynitride powder or placed in direct contact with the green body. Again, for the doping of the aluminum oxynitride, it is sufficient for the selected additives to be present in the sintering chamber. In fact, a green body consisting strictly of aluminum oxynitride powder, together with an adjacent green body containing yttrium oxide, on a platform of boron nitride,
It has been discovered that after sintering, the transparency of sintered aluminum oxynitride unexpectedly improves. Thus, the present invention provides another method for introducing an additive into the sintering chamber to produce in-situ vapor doping of a compacted body of aluminum oxynitride.
include. The explanation for the doping of the in-situ steam by the presence of other additives to enhance sintering is believed to be as follows. At the sintering temperature, the mixture of aluminum oxynitrides has a significantly higher vapor pressure of the Al _x O _y gas species. This Al _x O _y gas reacts with nearby boron nitride present in the container,
Generate B ₂ O ₃ gas and/or AlBO ₂ gas + AlN solid. The B ₂ O ₃ and/or AlBO ₂ vapor goes to the aluminum oxynitride and reacts with it, creating a liquid phase at the grain boundaries, which enhances the initial state of sintering. When using yttrium oxide as an additive, _{B2O3 also} reacts with yttrium-doped _aluminum oxynitride or pure _Y2O3 to produce _YBO2 gas. YBO ₂ vapor transfers to aluminum oxynitride and dopes it with boron and yttrium. When using other elements as additives, B ₂ O ₃ reacts similarly to provide the corresponding vapor doping of aluminum oxynitride. This additive doping is achieved by either damping the solute or penetrating the grain boundaries with a second phase precipitate, thus preventing excessive grain growth that might otherwise trap intragrain pores. , is believed to facilitate the final stage of sintering. According to another explanation, yttrium, or its components, forms a liquid phase. This liquid phase promotes rapid densification and significant pore removal in the early stages of sintering, so that in the final stages of sintering, fewer pores have to be eliminated and high density and transparency are achieved. . In this mechanism, boron is only needed to transfer yttrium to aluminum oxynitride. The method of the present invention overcomes many of the problems normally encountered when producing aluminum oxynitride by mixing aluminum oxide and aluminum nitride and reacting them, such as purity variations, commercially available Inorganic impurities in aluminum oxynitride result from the large particle size and wide size distribution of aluminum nitride, the long time required to form aluminum oxynitride, and the long milling time required to reduce the particle size. Increase in content, eliminate. Furthermore, the method of the present invention avoids the use of more expensive aluminum nitride as a starting component, requires less time to form aluminum oxynitride, and
and reduces manufacturing costs by requiring less milling time to obtain a homogeneous sinterable powder of suitable particle size. The aluminum oxynitride powder produced by the method described above also improves the reproducibility of the sintering process and improves the transparency of the sintered product.

Claims (1)

[Claims] 1. Having a density of at least 99% of the theoretical density,
Polycrystalline, at least one member selected from the group consisting of yttrium and lanthanum, characterized in that a 1.45 mm thick sample has an in-line transmission of at least 50% in the wavelength range of 0.3 to 5 microns. Cubic aluminum oxynitride object doped with elements. 2. The aluminum oxynitride object is formed from substantially single-phase aluminum oxynitride and is doped with boron and at least one element selected from the group consisting of yttrium and lanthanum. The object described in item 1. 3. The object of claim 1, wherein the sample of the object has a resolution angle of less than 1 mrad. 4. has a density of at least 99% of the theoretical density,
Polycrystalline, yttrium and A cubic aluminum oxynitride object doped with at least one element selected from the group consisting of lanthanum. 5. The object of claim 4, wherein the object is doped with boron, yttrium and lanthanum. 6. The object of claim 5, wherein the aluminum oxynitride object is formed from substantially single phase aluminum oxynitride. 7. The object of claim 5, wherein the sample of the object has a resolution angle of less than 1 mrad. 8. Prepare a mixture of aluminum oxide powder and carbon black, the content of carbon black in the mixture is in the range of 5.4 to 7.1% by weight, and the mixture is heated in the presence of nitrogen at a temperature in the range of 1550 to 1850 °C. reacting to form a pressed green body of a predetermined shape from said mixture; said green body being placed in a container consisting entirely of boron nitride or partially consisting of boron nitride and partially molybdenum metal; placing the untreated object in a sintering chamber, supplying at least one doping agent into the chamber, the doping agent being selected from the group consisting of yttrium, lanthanum or compounds thereof; A method for producing a transparent sintered aluminum oxynitride object comprising sintering in a nitrogen atmosphere at a temperature above 1900°C but below the solidus temperature of the aluminum oxynitride. 9. The reaction step includes reacting the mixture for a first predetermined period of time to convert the mixture into a second mixture containing aluminum oxide and aluminum nitride in an alpha phase;
9. The method of claim 8, comprising reacting at a temperature in the range of 1750 to 2140<0>C to convert said second mixture to substantially single phase aluminum oxynitride. 10. The method of claim 8, wherein the doping agent is in the vapor phase during a portion of the sintering process. 11. The method of claim 8, wherein in the sintering step, the doping agent migrates into the object and diffuses throughout it. 12. The doping agent is used during the sintering process.
12. The method according to claim 11, wherein a liquid phase is generated at grain boundaries. 13. The method of claim 9, wherein the doping agent is mixed with the mixture. 14. The method of claim 13, wherein the doping agent is less than 0.5% by weight of the mixture. 15. The method of claim 8, further comprising the steps of: comminuting the reacted mixture to a particle size in the range of 0.5 to 5 microns; and heating the reacted mixture to remove organic contaminants.