Note: Descriptions are shown in the official language in which they were submitted.
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En 25 CA
Process for Joining Alumina Ceramic Bodies
FIELD of the INVENTION
The present invention relates to a process for forming a
long-time vacuum-tight, corrosion-resistant, and high-
strength joint between a first body and a second body each
made of sintered alumina ceramic or sapphire.
BACKGROUND of the INVENTION
Alumina ceramic used in technical processes common contains
more than approximately 92 wt.o alumina and less than ap-
proximately 8 wt.% of an auxiliary sintering agent, such as
silicon dioxide, titanium dioxide, magnesium oxide, or cal-
cium oxide. In most cases a 96 wt.% alumina ceramic is
used, which thus contains 4 wt.% auxiliary sintering agent
and whose sintering temperature is approximately 1,500 °C.
In special cases, the alumina ceramic may also be particu
larly pure (99.5 wt.o alumina), i.e., the ceramic contains
only an extremely small percentage of an auxiliary sinter
ing agent; the sintering temperature is then approximately
1,600 °C.
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A special application of such alumina ceramic is
ceramic pressure sensors. Such sensors, as is well known,
comprise a ceramic substrate and a ceramic diaphragm which is
jointed to the substrate near its edge to form an internal
chamber.
As a material for joining the sintered alumina
ceramic bodies, U.S. Patent 4,177,496 describes a glass frit,
and U.S. Patent 5,005,421 or 5,334,344 an active brazing
solder.
Because of the more or less differing thermal
expansion coefficients of glass frit or active brazing solder
and alumina ceramic, both materials pose problems, particularly
if an optimum match is required over a wide temperature range.
Although active brazing alloys can be found where this match
appears satisfactory, cf. Laid-open Canadian Patent Application
No. 2,217,471, the need for optimally matched thermal expansion
coefficients of alumina ceramic and joining material still
exists.
SUMMARY OF THE INVENTION
Therefore, the inventors have based the solution of
this problem on the idea not to use a glass frit or an active
brazing solder for the joining material, but to try to use
alumina itself as the joining material, because this results in
the thermal expansion coefficients of alumina ceramic and
joining material being identified.
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While unsintered alumina bodies, i.e., so-called green com-
pacts, readily unite during the sintering process without a
joining material, the realization of a joint between alrea-
dy sintered alumina bodies by means of high-purity alumina
presents considerable practical difficulties and meets with
theoretical doubts, if not prejudices. "High purity" as
used herein means a purity of at least 99.9 wt.%.
These doubts are based on the mere fact, for example, that
an already sintered high-purity alumina ceramic body and a
high-purity alumina green compact cannot be sintered to-
gether, since the green compact shrinks isotropically by
about 50%.
The inventors nevertheless have looked for ways to join
sintered alumina ceramic by means of alumina at tempera-
tures far below the sintering temperature. The joint,
particularly if it is to be part of a ceramic pressure
sensor, must be vacuum-tight over a long period of time,
exhibit high strength, and be resistant to corrosion.
To solve these problems, the invention provides a process
for forming a long-time-vacuum-tight, high-strength, and
corrosion-resistant joint, by means of a joining material,
between a first body and a second body each made of sinter-
ed, polycrystalline alumina ceramic with a purity greater
than 92 wt.% or of sapphire, the process comprising steps
of
- interposing the joining material in the form of a paste,
a foil, or a slip between the first body and the second
body,
- the joining material having been made from
-- an agglomerate-free, highly disperse, high-purity
a-alumina powder with particles of as high a degree of
calcination as possible and of a size not exceeding
100 nm (= 10-~ m),
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-- an inorganic oxidic powder of an auxiliary sintering
agent with particles of approximately the same size as
the particle size of the a-alumina powder, and an or-
ganic vehicle which is dissolved or suspended in an
organic or aqueous solvant and in which the particles
of the respective powder are distributed as evenly as
possible,
- or with the joining material having been made from
-- an agglomerate-free, highly disperse, high-purity
a-alumina powder with particles of as high a degree of
calcination as possible and of a size not exceeding
100 nm (= 10-7 m),
-- an inorganic oxidic auxiliary sintering agent produced
chemically as envelopes around or attachment to the
particles of the a-alumina powder, and
-- an organic vehicle dissolved or suspended in an organic
or aqueous solvant,
--- with the total a-alumina and auxiliary-sintering-agent
content of the paste, the foil, or the slip ranging
between 50% and 70%; and
- heating the bodies to a temperature of not more than
1,300 °C and then allowing them to cool down,
-- with the alumina content after cooling being at least
95 wt.% and the auxiliary-sintering-agent content after
cooling being at most 5 wt.%.
In a preferred embodiment of the invention, the inorganic
content of the paste, the foil, or the slip is at least
99.9 wt.% a-alumina.
According to a development of the invention or of this pre-
ferred embodiment of the invention, in addition to the
temperature, a pressure acts on the bodies.
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A preferred use of the process of the invention serves to
produce a ceramic, particularly capacitive, absolute-pres-
sure or differential-pressure sensor in which the first
body is a substrate and the second body a diaphragm which
5 is connected with the substrate near its edge so as to form
a long-time-vacuum-tight, high-strength, corrosion-resis-
tant joint and an internal chamber.
The invention finally consists in the provision of a capa-
1o citive absolute-pressure sensor or a capacitive differen-
tial-pressure sensor having a substrate of alumina ceramic
and a diaphragm of alumina ceramic which is mechanically
connected with the substrate near its edge by means of a
joining layer of alumina ceramic so as to form a long-time-
vacuum-tight, high-strength, corrosion-resistant joint and
an internal chamber, the opposite surfaces of the substrate
and the diaphragm being each provided with at least one
electrode.
A principal advantage of the invention is that, as the ex-
amination of micrographs of the joint shows, one body is
connected with the other without any phase boundary, which,
of course, is present in the case of glass frit or active
brazing solder. The bodies are monolithically connected
with one another interface-free, and the joint, like the
bodies, is thus made of alumina ceramic.
Thus, the joint has the same properties as the bodies,
i.e., it is just as vacuum tight over a long period of
time, mechanically strong, and resistant to corrosion as
the alumina ceramic itself. A joint formed from glass frit
can bear much less stress in comparison. Moreover, the tem-
perature to which the bodies must be heated is far below
the above-mentioned sintering temperature of green com-
pacts.
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Further advantages of the invention will become apparent
from the following detailed description of the invention,
for which no drawing is necessary.
DETAILED DESCPRIPTION of the INVENTION
As the parent substance of a material for joining a first
body and a second body of sintered, polycrystalline alumina
ceramic with the above-mentioned purity, a-alumina is used.
Pure a-alumina has a melting point of approximately
2,100 °C. It is produced by heating aluminum hydroxide
(A1-O-OH, bauxit) to a temperature of approximately 1100 °C
(this is the so-called "calcination"), and remains stable
after cooling down to the ambient temperature, so that it
can be processed into powders, e.g., by grinding.
The sintering of 96 wt.% alumina green compacts into alu-
mina ceramic bodies commonly takes place at approximately
1,500 °C, and that of 99.5 wt.~ alumina green compacts at
approximately 1,600 °C. Thus, the alumina contained in the
sintered ceramic is virtually completely a-alumina.
The a-alumina powder with the aforementioned properties is
then processed into a joining material in the form of a
paste, a foil, or a slip. A slip contains, inter alia,
water and is pourable into a mold, e.g., a plaster mold,
which isotropically removes the water from the slip to form
a dimensionally stable body of joining material. By con-
trast, a paste is thixotropic, so that dimensionally stable
bodies of joining material formed therefrom can be applied
directly to at least one of the bodies to be joined, using a
silk-screening technique, for example.
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To produce the joining material, two variants are possible.
According to the first variant, the joining material is
formed from the above-mentioned a-alumina powder, an inor-
ganic oxidic powder of an auxiliary sintering agent, such
as silicon dioxide, titanium dioxide, magnesium oxide, or
calcium oxide, and an organic vehicle in the form of a
paste, a foil, or a slip. The size of the particles of the
auxiliary sintering agent is approximately the same as the
particle size of the a-alumina powder.
l0
The vehicle - this term is commonly used in connection with
the production of thick films and solder pastes, cf., for
example, EP-A 619 161 - contains, for example, a solvant, a
dispersant, such as fish coil, a bonding agent, such as
polyvinyl butyral, a plastifier, such as polyethylene gly-
col, and, if necessary, suitable further substances. The
vehicle is dissolved or suspended in an organic or aqueous
solvant, such as an alcohol, in which the particles of the
respective powder are distributed as evenly as possible.
According to the second variant, the joining material is
formed from the above-mentioned a-alumina powder, the in-
organic oxidic auxiliary sintering agent, which was pro-
duced chemically, e.g., by the so-called sol-gel process,
as envelopes around or attachments to the particles of the
a-alumina powder, and the same organic vehicle as in the
first variant. In both variants, the total a-alumina and
auxiliary-sintering-agent content of the paste, the foil,
or the slip ranges between 50% and 70%.
Next, a required and sufficient amount of joining material
is interposed between the two bodies. The two bodies are
then heated to a temperature of not more than 1,300 °C and
subsequently allowed to cool down.
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After the cooling, the alumina content is at least 95 wt.%
and the auxiliary-sintering-agent content is not more than
wt.%. This is due to the fact that all constituents of
the vehicle burn into carbon dioxide or vaporize and escape
5 from the joint.
In the first variant, the inorganic content of the joining
material can be readily increased to more than 99.9 wt.%
a-alumina, so that practically no auxiliary sintering agent
is used. Since the auxiliary-sintering-agent content of the
joining material reduces the temperature required to join
the two bodies as compared with the case mentioned in the
preceding paragraph, a joining material with, e.g., 95 wt.%
a-alumina and 5 wt.% auxiliary sintering agent only re-
quires a temperature of about 1,100 °C to 1,200 °C.
In view of the total a-alumina and auxiliary-sintering-
agent content of the joining material ranging between 50%
and 70% it is to be expected that nonpermissible shrinkage
of the joining material occurs during heating and cooling,
all the more so since only microfilters, i.e., porous bo-
dies, and thus no vacuum-tight high-strength bodies, have
so far been producible with the composition provided by the
invention for the joining material.
It has turned out, however, that no isotropic shrinkage
occurs, but only as anisotropic, namely uniaxial, shrinkage
in the direction of the perpendicular to the two surfaces
of the bodies to be joined, but no undesirable lateral
shrinkage parallel to these surfaces. Obviously a re-
arrangement of particles occurs during the heating process.
To avoid this lateral shrinkage, it may also be advanta-
geous to apply to the two bodies during the heating a
pressure of up to l0 MPa (= 10o bars). The application of
such a pressure also makes it possible to increase the
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tightness of the joint, to lower the heating temperature,
and to shorten the production time.
In the case of high-purity a-alumina powder with particles
of a size less than 100 nm (= 10-~ m) an unexpected viscous
phase occurs in the joint at the heating temperature, so
that any cracks occurring or existing in the bodies and
lying in the submicrometer ormicrometer range will be
closed.
It was thus quite surprising for the inventors that the
object of the invention is attainable with the above-de-
scribed joining materials in a highly satisfactory manner,
which also overcomes a prejudice held by the experts.
_
The process of the invention is especially suitable for use
in the fabrication of a ceramic, particularly capacitive,
absolute-pressure sensor or differential-pressure sensor.
In that case, the first body is the substrate of the pres-
sure sensor and the second body the diaphragm, which is
connected with the substrate near its edge to form a long-
time-vacuum-tight, high-stength, and corrosion-resistant
joint and an internal chamber.
It is thus possible to produce a capacitive absolute-pres-
sure sensor or a capacitive differential-pressure sensor
with a substrate of alumina ceramic and a diaphragm of
alumina ceramic. The diaphragm is mechanically connected
with the substrate near its edge by means of a joining
layer which is also made exclusively of alumina ceramic.
The opposite surfaces of the substrate and the diaphragm.
are each provided with at least one electrode, so that a
pressure acting on and deflecting the diaphragm will result
in a change in the capacitance existing between the elec-
trodes. This change in capacitance can be measured in the
usual manner.
