Hermetic sealing composition

A hermetic sealing composition consisting essentially of from 60 to 99 wt % of a bismuth type low melting glass powder and from 1 to 40 wt % of a low expansion ceramic filler powder, wherein the low melting glass has a composition consisting essentially of: ______________________________________ Bi.sub.2 O.sub.3 77 to 95 wt %, MgO + ZnO 1 to 20 wt %, B.sub.2 O.sub.3 2 to 10 wt %, SiO.sub.2 0 to 1 wt % and CeO.sub.2 0 to 10 wt %. ______________________________________

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates to a hermetic sealing composition which can 
be sealed by heat treatment at a low temperature and which is useful for 
hermetically sealing a panel and a funnel of a cathode ray tube (CRT) and 
a hermetic sealing composition which is useful for hermetically sealing a 
plasma display panel (PDP) or a fluorescent character display tube (VFD). 
Discussion of Background 
Heretofore, for hermetically sealing a panel and a funnel of a color CRT, 
it has been common to employ a PbO-B.sub.2 O.sub.3 -ZnO-SiO.sub.2 
crystalline low melting glass of the type as disclosed in JP-B-36-17821 
and to carry out hermetic sealing by maintaining it at a temperature of 
from 440.degree. to 450.degree. C. for from 30 to 40 minutes. The panel 
and funnel assembly thus sealed, will then be evacuated while being heated 
at a temperature of from 300.degree. to 380.degree. C., to have the 
interior highly vacuumed to a level of 10.sup.-6 Torr or less. 
On the other hand, for hermetically sealing glass substrates for PDP or 
VFD, it has heretofore been common to employ a low melting glass and to 
carry out hermetic sealing by maintaining it at a temperature of from 
440.degree. to 500.degree. C. In the case of PDP, the panel thus sealed is 
evacuated while being heated at a temperature of from 250.degree. to 
380.degree. C., and a gas for electric such as Ne or He-Xe will be sealed 
in to a level of from 100 to 500 Torr. In the case of VFD, the sealed 
panel will be evacuated while being heated at a temperature of from 
250.degree. to 380.degree. C., to attain vacuum and then hermetically 
sealed. 
In conventional hermetic sealing glass powders, a glass containing a lead 
component has been used. Recently, however, it has been desired to use a 
glass containing no lead component, which is advantageous from the view 
point of e.g. recycling. 
Conventional lead-free hermetic sealing glass powders have had drawbacks 
such that due to mismatching of their thermal expansion coefficients with 
glasses to be sealed, panels are likely to break, and the solder is likely 
to flow on glass substrates or form bubbles, or the sealed portions tend 
to crack by heating during the evacuation. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a glass ceramic 
composition suitable for hermetically sealing CRT, PDP and VFD by using a 
glass powder containing no lead component. 
The present invention provides a hermetic sealing composition consisting 
essentially of from 60 to 99 wt % of a bismuth type low melting glass 
powder and from 1 to 40 wt % of a low expansion ceramic filler powder, 
wherein the low melting glass has a composition consisting essentially of: 
______________________________________ 
Bi.sub.2 O.sub.3 77 to 95 wt %, 
MgO + ZnO 1 to 20 wt %, 
B.sub.2 O.sub.3 2 to 10 wt %, 
SiO.sub.2 0 to 1 wt % and 
CeO.sub.2 0 to 10 wt %. 
______________________________________ 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The compositional ranges of the bismuth type low melting glass in the 
present invention will be described below. 
In the present invention, a low melting glass having a softening point of 
at most 500.degree. C. is employed so that it is capable of sufficiently 
flow at a relatively low temperature of from 400.degree. C. to 550.degree. 
C. and in a short period of time (from 6 minutes to 1 hour) for hermetic 
sealing. The low melting glass of the present invention is preferably 
crystalline in a case where hermetic sealing is required to be completed 
in a short time as in the case of hermetic sealing of CRT. On the other 
hand, in the case of an application where hermetic sealing is carried out 
via a plurality of heating steps, the low melting glass may sometimes be 
preferably non-crystalline. 
Here, crystalline glass is the one which shows an exothermic peak in a 
differential thermal analysis (DTA) when heated at a rate of 10.degree. 
C./min and maintained at the hermetic sealing temperature (from 
400.degree. to 500.degree. C.) for two hours. 
The low melting glass has the following compositional ranges: 
______________________________________ 
Bi.sub.2 O.sub.3 77 to 95 wt %, 
MgO + ZnO 1 to 20 wt %, 
B.sub.2 O.sub.3 2 to 10 wt %, 
SiO.sub.2 0 to 1 wt % and 
CeO.sub.2 0 to 10 wt %. 
______________________________________ 
If the content of Bi.sub.2 O.sub.3 is less than 77 wt %, the softening 
point tends to be too high, the fluidity tends to be poor, and the 
strength and air tightness of the sealed portion are likely to be 
impaired, whereby hermetic sealing may not be carried out at a temperature 
of from 400.degree. to 550.degree. C. On the other hand, if the content 
exceeds 95 wt %, vitrification tends to be difficult. The content is 
preferably at least 79% and at most 93%. 
With respect to ZnO and MgO, at least one of them may be incorporated. If 
their total content is less than 1 wt %, crystallization tends to proceed 
too much during heating of the low melting glass, whereby the fluidity 
tends to be poor. On the other hand, if the total content exceeds 20 wt %, 
the softening point tends to be too high, and the fluidity tends to be 
poor. In either case, the strength and air tightness of the sealed portion 
are likely to be impaired, whereby hermetic sealing may not be carried out 
at a temperature of from 400.degree. to 550.degree. C. Their total content 
is preferably at least 2 wt % and at most 15 wt %. 
Specifically, MgO is preferably from 0 to 8 wt %. If it exceeds 8 wt %, 
vitrification tends to be difficult, whereby a stabilized glass may not be 
obtained. ZnO is preferably from 0 to 20 wt %. If it exceeds 20 wt. %, the 
softening point tends to be high. 
Further, the total content of ZnO and Bi.sub.2 O.sub.3 is preferably from 
85 to 98 wt %. If the total content is less than 85 wt %, the hermetic 
sealing temperature tends to be too high. Particularly preferably, the 
total content is at least 87 wt %. 
If the content of B.sub.2 O.sub.3 is less than 2 wt %, the fluidity of the 
glass tends to be poor, and the strength and air tightness of the sealed 
portion are likely to be impaired. If the content exceeds 10 wt %, the 
softening point of the glass tends to be high, whereby hermetic sealing 
may not be carried out at a temperature of from 400.degree. to 550.degree. 
C. The content is preferably at least 3 wt % and at most 8 wt %. 
SiO.sub.2 is not an essential component. However, by incorporating it, it 
is possible to suppress crystallization of the low melting glass and to 
increase the fluidity. However, if its content exceeds 1 wt %, the 
softening point tends to be too high. When it is required to carry out 
hermetic sealing at a low temperature, it is preferred not to incorporate 
it. CeO.sub.2 is also not an essential component. However, it is capable 
of suppressing precipitation of Bi.sub.2 O.sub.3 in the glass composition 
in the form of metallic bismuth in the molten glass and suppressing a 
decrease of the electrical insulation property of the hermetic sealing 
composition. However, if the content exceeds 10 wt %, the softening point 
tends to be too high, whereby hermetic sealing may not be carried out at a 
temperature of from 400.degree. to 550.degree. C. The content is 
preferably at least 0.01 wt % and at most 5 wt %. 
Further, CaO, SrO and BaO may be incorporated within a range not to 
substantially increase the thermal expansion coefficient of the hermetic 
sealing composition. Furthermore, In.sub.2 O.sub.3, TiO.sub.2, SnO.sub.2 
and ZrO.sub.2 may be incorporated within a range not to substantially 
increase the softening point, and Li.sub.2 O, Na.sub.2 O, K.sub.2 O, Cl 
and F may be incorporated within a range not to adversely affect the 
properties of CRT, PDP or VFD. If incorporated, such components should 
preferably be at most 10 wt %, particularly preferably at most 5 wt. %, in 
their total amount. CuO may deteriorate a phosphor in an electronic 
application. In such a case, it is advisable not to incorporate CuO. 
The glass transition temperature of the glass obtained is preferably within 
a range of from 280.degree. to 360.degree. C. to obtain a proper hermetic 
sealing temperature. Especially for a hermetic sealing composition to be 
used in the field where a low hermetic sealing temperature is required, 
such as CRT, the glass transition temperature is preferably from 
280.degree. to 350.degree. C., more preferably from 280.degree. to 
340.degree. C. 
In the present invention, the low expansion ceramic filler is a ceramic 
filler having a thermal expansion coefficient of at most 
70.times.10.sup.-7 /.degree.C. within a range of from room temperature to 
300.degree. C. Such a low expansion ceramic filler is preferably at least 
one member selected from the group consisting of zircon, cordierite, 
aluminum titanate, alumina, mullite, silica, .beta.-eucryptite, 
.beta.-spodumene and .beta.-quartz solid solution. Particularly preferred 
are cordierite and zircon, since they are excellent in the sealing 
strength. 
In the present invention, the content of the low melting glass powder is 
within a range of from 60 to 99 wt %, based on the total amount of the low 
melting glass powder and the low expansion ceramic filler powder, and the 
content of the low expansion ceramic filler powder is within a range of 
from 1 to 40 wt %, based on the total amount of the low melting glass 
powder and the low expansion ceramic filler. 
If the low melting glass powder exceeds 99 wt %, the amount of the low 
expansion ceramic filler powder tends to be so small that the thermal 
expansion coefficient after firing of the hermetic sealing composition, 
tends to be too large, whereby the thermal expansion coefficient tends to 
mismatch with the glass to be sealed, and cracking is likely to result. If 
its content is less than 60 wt %, the glass content tends to be small, 
whereby the fluidity tends to be poor, and air tightness of the sealed 
portion tends to be impaired. 
The average thermal expansion coefficient after firing of the hermetic 
sealing composition thus obtained, is preferably from 65.times.10.sup.-7 
to 100.times.10.sup.-7/ .degree.C. within a range of from room temperature 
to 250.degree. C. If the average thermal expansion coefficient departs 
from this range, matching of the thermal expansion coefficient with the 
glass to be sealed, tends to be difficult. 
When the hermetic sealing composition of the present invention is used for 
hermetically sealing a panel and a funnel of CRT, it is preferred that the 
composition consists essentially of from 70 to 99 wt % of the bismuth type 
low melting glass powder and from 1 to 30 wt % of the low expansion 
ceramic filler powder, and the composition has an average thermal 
expansion coefficient after firing of from 80.times.10.sup.-7 to 
100.times.10.sup.-7/ .degree.C. within a range of from room temperature to 
300.degree. C. 
Such a hermetic sealing composition is capable of hermetically sealing a 
panel and a funnel of CRT by maintaining it at a temperature of from 
400.degree. to 550.degree. C. for from 5 minutes to 1 hour, and it is free 
from flowing, bubbling or deterioration of the mechanical strength, by 
heating during evacuation at a temperature of from 300.degree. to 
380.degree. C. after hermetic sealing. 
In the case where the hermetic sealing composition of the present invention 
is used for hermetically sealing a panel and a funnel of CRT, if the 
content of the low melting glass powder exceeds 99 wt %, the amount of the 
low expansion ceramic filler powder tends to be small, whereby the thermal 
expansion coefficient tends to be too large and mismatches with the panel 
and the funnel, whereby breakage is likely to result. If the content is 
less than 70 wt %, the glass content tends to be small, whereby the 
fluidity tends to be poor, and no adequate vacuum for CRT tends to be 
obtained. 
Further, if the average thermal expansion coefficient of the hermetic 
sealing composition after firing is outside the range of from 
80.times.10.sup.-7 to 100.times.10.sup.-7/ .degree.C. within a range of 
from room temperature to 300.degree. C., a tensile stress will be exerted 
to the panel glass or the funnel glass after hermetic sealing or to the 
sealed portion, whereby the strength of the bulb against pressure tends to 
be low. 
In a case where the hermetic sealing composition of the present invention 
is used for sealing PDP or VFD, it is preferred that the composition 
consists essentially of from 60 to 98 wt % of the bismuth type low melting 
glass powder and from 2 to 40 wt % of the low expansion ceramic filler 
powder, and the composition has an average thermal expansion coefficient 
after firing of from 65.times.10.sup.-7 to 90.times.10.sup.-7/ .degree. C. 
within a range of from room temperature to 250.degree. C. 
Such a hermetic sealing composition is capable of hermetically sealing PDP 
or VFD by maintaining it at a temperature of from 400.degree. to 
500.degree. C. for from 5 minutes to 1 hour, and it is free from flowing, 
bubbling or deterioration of the mechanical strength by heating at a 
temperature of from 280.degree. to 380.degree. C. during evacuation after 
the sealing. 
In the case where the hermetic sealing composition of the present invention 
is used for sealing PDP or VFD, if the content of the low melting glass 
powder exceeds 98 wt %, the amount of the low expansion ceramic filler 
powder tends to be small, whereby the thermal expansion coefficient after 
firing tends to be too large and tends to mismatch with the substrate 
glass, a tensile stress tends to remain at the seal frit portion after 
hermetic sealing, and thus such a portion is likely to break. If the 
content is less than 60 wt %, the glass content tends to be small, whereby 
it tends to be difficult to attain air tightness of the sealed portion 
adequate for PDP or VFD. 
Further, if the average thermal expansion coefficient of the hermetic 
sealing composition after firing is outside the range of from 
65.times.10.sup.-7 to 90.times.10.sup.-7/ .degree.C. within a range of 
from room temperature to 250.degree. C., a tensile stress will be exerted 
to the substrate glass or to the sealed portion after hermetic sealing, 
and an strength against pressure tends to be low. 
To this composition, a coloring agent such as a pigment may be incorporated 
for coloring. 
Now, the present invention will be described in further detail with 
reference to Examples. However, it should be understood that the present 
invention is by no means restricted to such specific Examples.