Low thermal expansion and high rigidity ceramic sintered body

Details Low thermal expansion and high rigidity ceramic sintered body Low thermal expansion and high rigidity ceramic sintered body

2000-12-05

2002-04-16

Brunsman, David (Department: 1755)

Compositions: ceramic

Ceramic compositions

Refractory

C501S112000, C501S120000

Reexamination Certificate

active

06372677

ABSTRACT:

TECHNICAL FIELD
This invention relates to a low thermal expansion and high rigidity ceramic sintered body which is excellent in thermal stability and specific rigidity and usable in members of high precision control device, members of optical devices, and members demanding high thermal shock resistance which invariably abhor changes in size and changes in shape due to a thermal expansion or contraction induced by changes in temperature.
BACKGROUND ART
As materials which have been heretofore used under conditions demanding thermal stability, low thermal expansion metallic materials such as an Invar alloy (Fe-Ni type) and a super Invar alloy (Fe-Ni-Co type), low thermal expansion glasses such as quartz glass (SiO
2
) and quartz glass containing titanium oxide (SiO
2
-TiO
2
), and low thermal expansion ceramics such as aluminum titanate (TiO
2
.Al
2
O
3
), eucryptite (Li
2
O.Al
2
O
3
.2SiO
2
), &bgr;-spundumene (Li
2
O.Al
2
O
3
.4SiO
2
), petalite (Li
2
O.Al
2
O
3
.8SiO
2
), and cordierite (2MgO.2Al
2
O
3
. 5SiO
2
) have been known. These materials are excellent in thermal stability because they have such a small thermal expansion coefficient as of not more than 1.2×10
−6
/°C. in the neighborhood of a room temperature. Nevertheless they usually have a specific rigidity expressed by a ratio of a Young's modulus to a specific gravity such values lower than 45 GPa/g/cm
3
and, when used in members demanding dimensional stability and thermal shock resistance, therefore, are at a disadvantage in being readily deformed by an external force or under its own weight, offering only a low resonance frequency to the vibration of the relevant member, and generating a large amplitude.
The Invar alloy, for example, has a relatively small thermal expansion coefficient of about 1.2×10
−6
/°C. at near a room temperature, a Young's modulus of 144 GPa, a value rating high among low thermal expansion materials, a large specific gravity, and a small specific rigidity of 18 GPa/g/cm
3
. The super Invar alloy, though enjoying a small thermal expansion coefficient of 0.13×10
−6
/°C., is deficient in mechanical stability because of a small specific rigidity of 17 GPa/g/cm
3
.
The quartz glass has a small thermal expansion coefficient of 0.48×10
−6
/°C. and such an insufficient specific rigidity as of 33 GPa/g/cm
3
. The quartz glass containing titanium oxide has an extremely small thermal expansion coefficient of about 0.05×10
−6
/°C. and is deficient in mechanical stability because of an insufficiently high specific rigidity of 33 Gpa/g/cm
3
.
Further, the aluminum titanate manifests negative expansion as evinced by a thermal expansion coefficient of −0.8×10
6
/°C. and has an extremely small specific rigidity of about 2 GPa/g/cm
3
. The lithium aluminosilicate type low thermal expansion ceramics such as eucriptite, &bgr;-spodumene, and petalite have a small thermal expansion coefficient in the range of −5 to 1×10
−6
/°C., a not very high specific rigidity of about 35 GPa/g/cm
3
, and are deficient in mechanical stability. The compact sintered body of cordierite, though excelling various low thermal expansion materials mentioned above by exhibiting a specific rigidity of about 50 GPa/g/cm
3
, has a thermal expansion coefficient of 0.5×10
6
/°C., which is not deserving to be called sufficiently low.
The invention described in JP-A-61-72,679, with a view to reducing a thermal expansion coefficient of cordierite capable of producing a relatively high specific rigidity, discloses a method which consists in attaining coexistence of a cordierite phase with a &bgr;-spodumene phase as crystal phases and an auxiliary crystal phase as of spinel. This method has been reported to be capable of lowering a thermal expansion as compared with a simple phase of cordierete. With the same view as above, the invention described in JP-A-10-53,460 discloses a compact ceramics allowing the coexistence of a petalite phase, a spodumene phase, and a cordierite phase in a crystal phase. This compact ceramics has been demonstrated to excel in thermal shock resistance. Further, the invention described in JP-A-58-125,662 discloses a method for producing a ceramics allowing the coexistence of zircon in cordierite by adding a zirconium compound and a phosphorus compound to the cordierite. The sintered body obtained by this method has been reported to excel in thermal shock resistance. These materials, however, do not deserve to be rated as having a sufficiently low thermal expansion coefficient. As structural parts to be used for members in precision control devices, members in optical devices, and members demanding high thermal shock resistance, they cannot be said as having satisfactory thermal mechanical stability. Such has been the true state of the materials of interest.
Since conventional low thermal expansion ceramic materials are such that those having a small thermal expansion coefficient show a low specific rigidity and those having a high specific rigidity show no sufficiently low thermal expansion coefficient as described above, no low thermal expansion ceramic materials developed today have secured thermal stability such that the absolute value of thermal expansion coefficient does not exceed 0.1×10
−6
/°C. while the specific rigidity is retained at a high level of not less than 45 Gpa/g/cm
3
, for example. Thus, conventional low thermal expansion ceramic materials have been at a disadvantage in being deficient in thermal reliability for members in precision structures.
It is, therefore, an object of this invention to provide a low thermal expansion ceramic sintered body which excels in thermal and mechanical stability and manifests both high specific rigidity and low thermal expansion coefficient.
DISCLOSURE OF THE INVENTION
The low thermal expansion and high rigidity ceramic sintered body of this invention is characterized by assuming as a crystal structure a hexagonal close-packed structure and substantially comprising solid solution crystal grains represented by the formula: Mg
a
Li
b
Fe
c
Al
d
Si
e
O
f
(wherein a is in the range of 1.8 to 1.9, b is in the range of 0.1 to 0.3, c is in the range of 0 to 0.2, d is in the range of 3.9 to 4.1, e is in the range of 6.0 to 7.0, and f is in the range of 19 to 23).
In the ceramic sintered body mentioned above, the solid solution crystal grains may preferably have lattice constants in such ranges, i.e., a
0
in the range of 9.774 to 9.804 Å and c
0
in the range of 9.286 to 9.330 Å. The ceramic sintered body mentioned above may more preferably have a relative density of not less than 98%.
BEST MODES OF CARRYING OUT THE INVENTION
The present inventors, as a result of various studies, have found that in a sintered body formed solely of a single solid solution phase having as a crystal structure a hexagonal close-packed structure substantially represented by the formula: Mg
a
Li
b
Fe
c
Al
d
Si
e
O
f
excepting inevitable impurities, an absolute value of a thermal expansion coefficient can be controlled to not more than 0.1×10
−6
/°C. and a specific rigidity to not less than 45 GPa/g/cm
3
by controlling the ratios of each the component element within respectively prescribed range. When a second phase such as an amorphous phase having a large thermal expansion coefficient or a spinel phase assuming a cubic crystal structure is present in addition to the solid solution phase represented by the formula: Mg
a
Li
b
Fe
c
Al
d
Si
e
O
f
, the sintered body can not attain a sufficiently low thermal expansion coefficient. In order to obtain a sintered body having a high specific rigidity, it is desirable to incorporate a second phase such as an amorphous phase having a small specific rigidity, a &bgr;-spodumene phase having a cubic crystal structure, or a &bgr;-quartz solid solution phase in addition to the solid solution phase represented by the formula: Mg
a
Li
b
Fe
c
Al
d
Si
e
O
f
.
The solid solution crystal grains according to this invention may

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