Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1997-05-14
2001-05-01
Morris, Terrell (Department: 1771)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C252S508000, C252S518100, C252S519140, C252S520300, C252S514000, C252S519330, C106S001140, C501S018000, C501S021000
Reexamination Certificate
active
06225392
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a conductive paste containing silver as the main component. Particularly, it relates to a conductive paste which is printed on a substrate and fired to form an electric conductor.
2. Discussion of Background
As a method for preventing fogging of a rear window glass of an automobile, a method has been employed wherein a conductive paste is printed on the glass surface to form a plurality of conductive strips for heating and bus bars connected to both ends of the strips and then fired to form an electric conductor, and metallic terminals for lead wires are attached to the bus bars by soldering, and a voltage is applied across the metallic terminals, so that the surface temperature of the glass is maintained at a level higher than the dew point by heat generation of the conductor.
In this case, a constant voltage power source is used, and the heating value is accordingly governed by the resistivity of the conductor. Accordingly, in order to control the amount of heat exerted to a unit area of glass to be constant irrespective of the shape of the glass, electric conductors are required which have various resistivities and to which metallic terminals can be soldered.
Heretofore, as a method for forming such conductors, a method has been known as disclosed in JP-B-57-41763 in which silver powder, low-melting glass frit and an organic varnish are mixed to obtain a paste, which is then fired to a glass plate, and then copper is electroplated to adjust the resistivity.
However, the electroplating treatment has had problems with respect to the operation efficiency and costs, since the number of process steps thereby increases, although desired levels of the resistivity and bond strength with the metallic terminals can be obtained.
Further, it has been proposed to incorporate a metal oxide or a metal having high resistivity such as Ni, Al, Sn, Pb, Pt or Pb as a resistivity-controlling agent to the above mentioned paste, without carrying out such electroplating.
However, when such a resistivity-controlling agent is used, there has been a drawback that the bond strength to the metallic terminals can not be adequately increased. The reason is considered to be such that when the above mentioned metal or its oxide is added as a resistivity-adjusting agent, sintering of silver particles will be hindered, and the sintered structure of silver will be coarse, and accordingly, when metal terminals are soldered thereto, the solder tends to erode the grain boundaries of silver, and silver tends to melt in the solder, whereby the silver structure will be broken, thus leading to so-called solder-erosion defects, whereby the bond strength is believed to deteriorate. Even if the solder-erosion phenomenon does not occur, the silver structure breakage is still likely to occur, since the sintered structure of silver is coarse, whereby the bond strength tends to be poor.
It has been proposed to incorporate an organic rhodium compound in order to overcome such a drawback (JP-B-57-41763). However, such a proposal has a drawback from the viewpoint of costs, since the organic rhodium compound is expensive.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a conductive paste anew, whereby the resistivity can easily be adjusted, and high bond strength is obtainable between the conductor and the metallic terminals for lead wires.
The present invention provides a conductive paste comprising conductive powder and low-melting glass frit, wherein the low-melting glass frit constituting the conductive paste crystallizes crystals during firing to increase the resistivity of the conductive paste.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Now, the present invention will be described in detail.
The low-melting glass frit constituting the conductive paste of the present invention is a constituent which bonds the conductive powder governing electrical conductivity to the substrate glass or to a ceramic color layer formed on the substrate glass, and as such, it is essential to the present invention.
In the present invention, the low-melting glass frit crystallize at least one type of crystals. The crystals to be crystallized are preferably crystals of at least one member selected from lead titanate, lead zirconate, lead borate and lead stannate. These crystal components will be crystallized in the form of crystals in a temperature range of from 570 to 700° C. as the temperature for bending a glass sheet for an automobile. It is advantageous that the conductive paste can be fired at the same time as the glass bending operation.
Such crystals crystallized from low-melting glass frit, will be present in grain boundaries of silver to hinder the electrical conductivity of the silver grain boundaries, thus providing an effect for increasing the resistivity. Namely, it is possible to adjust the resistivity to a desired level by controlling the amount of crystals crystallized from the low-melting glass frit.
In general, the resistivity can be increased by increasing the amount of titanium, zirconium, tin or boron as a constituting element of crystals and the amount of lead as the other constituting component. Further, such crystals will be crystallized by the reaction with lead. Accordingly, it is possible to increase the amount of crystallized crystals and increase the resistivity also by lowering the glass transition temperature at which the reaction starts.
In the present invention, two or more different types of the above mentioned crystals may be crystallized, and crystals other than those described above may also be crystallized. The effect of increasing the resistivity can be obtained, if crystallization takes place to such an extent that crystallization of crystals can be ascertained by a mineral X-ray analyzer.
With a view to facilitating crystallization of crystals, the low-melting glass frit of the present invention preferably has substantially the following composition:
PbO
65 to 85 wt %,
B
2
O
3
5 to 13 wt %,
SiO
2
2 to 7 wt %,
TiO
2
0 to 20 wt %,
ZrO
2
0 to 15 wt %,
SnO
2
0 to 15 wt %,
TiO
2
+ ZrO
2
+ SnO
2
3 to 25 wt %,
Al
2
O
3 + Ce
2
O
3
0.1 to 2 wt %,
Li
2
O
3
+ Na
2
O + K
2
O
0 to 1 wt %,
MgO + CaO + SrO + BaO
0 to 5 wt %, and
P
2
O
5
0 to 3 wt %.
PbO is essential in this preferred composition as a flux component and a component for crystallization. If it is less than 65 wt %, the softening temperature of glass tends to be high, and if it exceeds 85 wt %, the chemical durability tends to deteriorate. It is preferably from 68 to 83 wt %, more preferably from 70 to 80 wt %.
B
2
O
3
is essential in this preferred composition as a flux component and a component for crystallization. If it is less than 5 wt %, the softening temperature of glass tends to be high, and if it exceeds 13 wt %, the chemical durability tends to deteriorate. It is preferably from 8 to 12 wt %, more preferably from 10 to 12 wt %.
SiO
2
is a network former of glass and an essential component in this preferred composition for controlling the chemical, thermal and mechanical properties. If it is less than 2 wt %, the chemical durability tends to be poor, and if it exceeds 7 wt %, the glass softening point tends to be high, and the effect for bonding silver to the primer ceramics color layer tends to be low. It is preferably from 3 to 6 wt %.
At least one member selected from TiO
2
, ZrO
2
and SnO
2
is essential in this preferred composition to improve the chemical durability and crystallization of crystals during baking. If their total amount is less than 3 wt %, the chemical durability tends to be poor, and crystallization tends to be difficult during the firing. If their total amount exceeds 25 wt %, devitrification tends to result during melting of glass. Preferably, their total amount is from 4 to 15 wt %.
Among them, if TiO
2
exceeds 20 wt %, th
Asahi Glass Company Ltd.
Guarriello John J.
Morris Terrell
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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