Resistive paste for the formation of electrically...

Compositions – Electrically conductive or emissive compositions – Free metal containing

Reexamination Certificate

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C252S520300

Reexamination Certificate

active

06406646

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a resistive paste composition for the formation of an electrically heat-generating thick film, particularly to a paste composition which can be calcined at about 600° C. or lower to provide a film having good stability, uniformity and heat-up characteristics.
BACKGROUND OF THE INVENTION
A resistive paste composition is used for forming a thick film resistive element on an electrically insulating substrate by a screen printing or dipping method, and it generally comprises conductive materials and inorganic and organic binder materials. The process for forming such a film involves a high temperature calcination step wherein the organic component is removed and the inorganic component is molten down to increase the integrity of the conductive components to the substrate, and hitherto, various resistive pastes have been developed.
For example, Japanese Unexamined Patent Publication No. sho 53-100496 discloses a paste for a resistor element, which comprises ruthenium oxide particles and glass frit powders dispersed in an organic medium composed of an organic solvent and a resin; and U.S. Pat. No. 5,510,823 assigned to Fuji Xerox Co., Ltd. describes a resistive element film-forming paste comprising an organic metal compound and at least one organic binder compound and an asphalt solution to improve the dispersion of the compounds. Further, Korean Patent No. 130831 issued to DuPont teaches a thick film resistor composition comprising 5 to 30 wt % of ruthenium pyrochlore oxide (PbRuO
3
)and 10 to 90 wt % of a glass binder.
However, the process of preparing resistor elements using the paste compositions disclosed in the above patents requires a high calcination temperature ranging from about 600 to 1,000° C. Accordingly, their application is limited to substrates which can stand such a high temperature.
Recently, a method of preparing a film-type heat-generating element using a self-heating resistive paste composition has attracted attention since it requires no separate heating source and can reduce the weight of a device.
U.S. Pat. No. 5,900,295 assigned to Fuji Electric Co., Ltd. relates to a fixing roller used for fixing a toner on a paper sheet and installed on an electro-photographic devices such as a copying machine, a printer or a facsimile machine, which comprises a cylinder-shaped metallic base body, an insulation layer and a resistance layer. In this patent, the resistance layer is formed from a resin composition comprising a resin selected from polyphenylene sulfide, polyphthalamide and a liquid crystal polymer, and a mixture of carbon black and carbon fiber. However, the resistance layer must be formed by a cumbersome injection molding process due to the characteristics of the resin component used in this patent, and thus, it is difficult to provide a film having a uniform thickness and a good heat-generating property.
SUMMARY OF THE INVENTION
Accordingly, it is a primary object of the invention to provide an improved resistive paste composition which can be applied on a substrate by a printing method and can be calcined at a low temperature to form on the substrate a heat-generating thick film having good stability, uniformity and heat-up characteristics.
In accordance with an aspect of the present invention, there is provided a resistive paste composition for the formation of an electrically heat-generating layer, which comprises
(a) 5 to 75% by weight of particles of ruthenium(Ru) metal or an oxide thereof having a specific surface area of 5 to 30 m
2
/g,
(b) 5 to 75% by weight of particles of silver(Ag) metal or a compound thereof having an average particle size of 0. 1 to 3 &mgr;m, the maximum particle size not exceeding 8 &mgr;m,
(c) 5 to 40% by weight of a glass frit powder having a softening point of 400 to 550° C. and
(d) 5 to 45% by weight of an organic binder.
DETAILED DESCRIPTION OF THE INVENTION
In the inventive composition, the conductive Ru and Ag components are employed to control the electrical and mechanical properties of the paste composition. Further, the glass frit is employed to increase the adhesive strength of the resulting thick film to the substrate, and the organic binder functions to enhance the dispersion of the conductive materials and the inorganic binder in the composition.
(a) Ru component
The Ru component employed in the present invention may be ruthenium metal or an oxide thereof, e.g., RuO
2
, GdBiRu
2
O
6-7
, Pb
2
Ru
2
O
6-7
, Co
2
Ru
2
O
6-7
, PbBiRu
2
O
6-7
, Cu
x
Bi
2−x
Ru
2
O
6-7
(0<x<1) and Bi
2
Ru
2
O
6-7
.
The Ru component in the form of particles has a specific surface area of 5 to 30 m
2
/g, preferably 10 to 25 m
2
/g, and an average particle diameter of 0.01 to 0.1 &mgr;m, preferably 0.02 to 0.08 &mgr;m. When the specific surface area is less than 5 m
2
/g or the average particle diameter is greater than 0.1 &mgr;m, the particle size is too great to provide a uniform film having smooth surface. Further, when the specific surface area is greater than 30 m
2
/g or the average diameter is less than 0.01 &mgr;m, the resulting paste composition has poor printing and calcining properties, leading to a film having a low density.
In the present invention, the particles of the Ru component are employed in an amount ranging from 5 to 75% by weight, preferably from 5 to 20% by weight of the composition. If the amount is greater than 75%, the surface smoothness of the film deteriorates, and if the amount is less than 5%, the film has an excessively high resistance.
(b) Ag component
The Ag component employed in the present invention may be metallic silver, or an oxide thereof (e.g., Ag
2
O) or an alloy thereof (e.g., AgPd, Ag
0.1
Pd
0.9
RhO
2
). In order to obtain a paste which can be calcined at a low temperature, it is particularly preferred to employ the Ag component in the form of plate-shaped particles.
The Ag component in the form of particles has preferably a specific surface area of 0.5 to 3.5 m
2
/g and an average particle diameter of 0.1 to 3 &mgr;m and a maximum particle size of 8 &mgr;m or less. When the specific surface area is less than 0.5 m
2
/g or the average particle diameter is greater than 3 &mgr;m, the resulting film has a rough surface and the resolution of the printed film pattern becones poor. Further, when the specific surface area is greater than 3.5 m
2
/g or the average particle diameter is less than 0.1 &mgr;m, the paste tends to agglomerate easily, has poor printability, and undergoes an excessive shrinkage during the calcination process, which leads to the formation of cracks on the film surface.
The Ag component preferably has a density of 2.5 to 6 g/cm
3
. If the density is not within the specified range, the printability of the paste may become poor.
In the present invention, the particles of the Ag component are employed in an amount ranging from 5 to 75% by weight, preferably from 20 to 40% by weight of the composition. If the amount does not fall within the specified range, the film does not have a proper resistance.
(c) Glass frit
The glass frit employed in the present invention functions as an inorganic binder for bringing to the particles of the Ru component into contact with each other. Also, the glass frit powder which agglomerates during the calcination process enhances the adhesive strength of the resulting film to a substrate.
The glass frit used in the present invention has a softening point ranging from 400 to 550° C., preferably from 420 to 500° C. When the softening point is lower than 400° C., the resulting film tends to have blisters on the surface thereof due to inclusion of organic components. When the softening point is higher than 550° C., the adhesive strength of the resulting film to the substrate becomes poor.
In the present invention, the glass frit is employed in an amount ranging from 5 to 40% by weight, preferably from 10 to 40% by weight of the composition. If the amount is less than 5%, the adhesive strength of the calcined film to the substrate is poor, whereas if the amount is greater than 40%, the film&ap

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