Compositions: ceramic – Ceramic compositions – Aluminum compound containing
Reexamination Certificate
2000-05-19
2002-07-09
Koslow, C. Melissa (Department: 1755)
Compositions: ceramic
Ceramic compositions
Aluminum compound containing
C501S127000
Reexamination Certificate
active
06417127
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a translucent polycrystalline ceramic and a method of making the same and in particular relates to a translucent polycrystalline Al
2
O
3
ceramic and a method of making the same.
There are many applications of the translucent polycrystalline ceramic. For instance, an outer tube for a metal halide or sodium lamp, a light emitting tube and a window plate for high-temperature furnace are made of translucent alumina ceramic, wherein not only a electrical resistance and/or chemical corrosion resistance, but also a high strength and hardness from low temperature (ambient) up to as high as 1000-1200° C. are often required. This is partly because the translucent alumina ceramic may be thinned to gain more light transmittance therethrough. In addition to a high straight-line light transmission or rather called as liner-light transmittance (corresponding to transparency), some applications need a fine texture and high abrasion resistance (namely ceramic particles does not fall off from its surface maintaining a smooth and less roughness surface). Such applications includes optical connectors, optical filters, medical articular heads in medical use, cutting tools, bearings, high-Q dielectrics for some electronic components.
2. Related Arts
It is known that some alumina ceramics (polycrystalline) can be made translucent or partially transparent, in other words, light-transmittable therethrough.
However, the translucent alumina ceramic that satisfies both transparency and high mechanical strength has not been reported. For example, in Japanese Patent Laid-Open No. H03-285865/1991, a translucent alumina ceramic made by using alumina grains of high-purity (99.99% purity) and a small amount of MgO is disclosed, however, its bending strength is insufficiently low as about 500 MPa. On the other hand, in other Japanese Patent Nos. 2729204 and 2663191, the translucent alumina ceramic having high strength and hardness made by controlling a mean particle size under HIP is disclosed, however, a straight-line transmission of light propagating through a thickness of 1 mm thereof is insufficiently lower than 50%.
In addition, it has been conventionally considered that large particle sizes of alumina crystal grains constituting the alumina ceramic contributes to gaining a high translucency or transparency of the alumina ceramic because chances of light-scattering(:reflection and/or refraction occurring at grain boundaries by the light) are reasoned to be lesser than the alumina having much smaller grains inside.
In the conventional translucent alumina ceramic, alumina particles constituting the polycrystalline alumina ceramic are liable to fall off from its surface. In other words, it has been difficult to attain a precisely or rather extremely mirror-polished surface of the translucent alumina ceramic. When the conventional translucent alumina ceramic is subjected under a large contact-stress as in use for bearings or cutting tools or even in a surface polishing process thereof, such a stress-concentrated site thereof tends to break, probably due to large size crystals are formed with magnesia (MgO) binding the crystals inside the conventional translucent alumina ceramic.
SUMMARY OF THE INVENTION
An object of the invention is therefore to provide a translucent polycrystalline ceramic having a good strength and hardness, capable of transmitting a light through the ceramic.
Another object of the invention is to provide a translucent polycrystalline alumina ceramic having excellent strength and hardness or abrasion resistance, capable of light transmittance and/or straight-line light transmission therethrough and withstanding a high temperature.
Still another object of the invention is provide a method of making a translucent polycrystalline alumina ceramic. This method enables manufacture of an excellent translucent alumina ceramic having high temperature bending strength and hardness and/or abrasion resistance, wherein particles/grains constituting the ceramic are hard to fall off from its surface and the surface can be ground and polished into a smooth surface with less surface roughness and be mechanically less injured for instance under a high contact stress applied to the ceramic in use maintaining its translucency.
In a first embodiment of the invention, there is provided a translucent polycrystalline ceramic capable of transmitting a light that enters the ceramic, comprising crystal particles, wherein a mean facet length of the crystal particles is not longer than a maximum wave length of the light that transmits through the polycrystalline ceramic product; the mean facet length being defined as an average of lengths of sides forming polygons that appear in cross sections of the crystal particles constituting the ceramic.
A characteristic feature of the translucent polycrystalline ceramic according to the invention is that the mean facet length as defined above is shorter than a maximum wave length of the light that transmits through the translucent polycrystalline ceramic.
In an aspect, in the case that a visible light that has a wave length of about 380-780 nm, if all of the facet lengths (meaning facet side lengths of the alumina crystal particles) are less than 380 nm, highest translucency of the polycrystalline ceramic transmitting most of the visible light is attained with the translucent polycrystalline ceramic. Even when the mean facet length(meaning an average of the facet side lengths of the crystal particles) is less than 700 nm, the translucent polycrystalline ceramic with thickness of 1 mm can transmit more than 50% of the visible light therethrough, as will be later described in detail.
When the facet length is shorter, the better translucency and transparency is attained, and this is in contrast to the conventional technology that requires larger particles or grains (resulting in longer facet lengths) for attaining a better translucency through the ceramic.
In a preferred embodiment of the invention, the best translucency is attained when all the facet lengths are shorter than all wave lengths of the lights that transmit through the ceramic. The better light translucency as high as 70% is attained with the mean facet length of less than 500 nm and the best one (more than 75%) is attained with that of less than 400 nm.
In another preferred embodiment according to the invention is that the polycrystalline ceramic should is substantially poreless. In other words, a relative density of the fired ceramic should be at least 99.8% or substantially 100% with a minimum binder connecting the transparent crystal particles (or grains) formed inside the translucent ceramic. This is because the pores decrease translucency and/or transparency of the polycrystalline ceramic, and in addition decreases strength and hardness thereof. In the case of a translucency polycrystalline alumina ceramic, the density thereof should be at least 3.98 g/cm
3
(substantially 100% in relative density).
A material candidate for the crystal particles is Al
2
O
3
, AlN, ZrO
2
, spinel and so on, so long as large optical anisotropy or crystal anisotropy is not formed with the crystal particles. In other words, if a mean aspect ratio of the crystal particles is 1-1.5 (preferably 1-1.3) and a mean particle size of the crystal particles formed inside the ceramic is not larger than about 1 &mgr;m, the mean facet length becomes less than the maximum wave length or most of wave lengths of the visible light. Among them, Al
2
O
3
(alumina or sappier) is best selected for the crystal particles. Because a crystal structure of the alumina belongs to a hexagonal system, difference of refractive index for the light between its crystal facets formed along a-axis and c-axis in crystallography is theoretically only about 0.008, which renders the reason why majority of the visible light can transmit through the translucent polycrystalline ceramic comprising crystal particles having the mean aspect ratio of 1-1.5 and the mean crystal parti
Iio Satoshi
Mitsuoka Takashi
Yamamoto Hiroshi
Koslow C. Melissa
NGK Spark Plug Co. Ltd.
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