Coating processes – Optical element produced – Polarizer – windshield – optical fiber – projection screen – or...
Reexamination Certificate
2000-09-22
2003-03-11
Barr, Michael (Department: 1762)
Coating processes
Optical element produced
Polarizer, windshield, optical fiber, projection screen, or...
C427S126200, C427S165000, C427S376200, C427S376300
Reexamination Certificate
active
06531181
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for producing conductive coatings, in particular strip conductors, and substrates selected from glass and enamelled steel, coated by this process. A screen-printable aluminum paste based on aluminum powder, at least one glass frit and an organic medium is used. The invention also relates to coated substrates obtainable by this process.
BACKGROUND INFORMATION
Sheets of glass and ceramic substrates for electronic uses are for various purposes often provided partially or over the entire surface with a conductive layer, the coating containing a metal as the component imparting conductivity and a glass composition as binder. Such coatings are often based on precious metals such as, for example, silver in the case of strip conductors on glass or gold and/or platinum metals in the case of strip conductors or electrodes on ceramic substrates, such as semiconductor ceramics.
The production of these strip conductors comprises the application of a metal powder and a liquid or paste-like composition containing glass frit and a subsequent firing at a firing temperature appropriately selected for the substrate.
Whereas the precious metals can be replaced partially by aluminum in the production of conductive coatings on ceramic substrates, it has not hitherto been possible to produce high-quality aluminum-based strip conductors on sheets of glass. Known compositions containing solely aluminum powder as conductive component require a heating temperature which is too high for glass and/or result in a coating having inadequate adhesion and/or inadequate conductivity.
U.S. Pat. No. 3,833,348 describes a process for bonding two parts made of silicon nitride. A composition to be used for this is based on a glass frit and aluminum. The preferred glass frits based on SiO
2
, Al
2
O
3
and MnO have a melting point which is too high for application to glass.
U.S. Pat. No. 5,562,972 discloses a conductive paste for the production of moisture-resistant electrodes having a favorable ohmic contact between the electrode and a semiconductor element. The paste contains aluminum powder and a glass frit based on barium borosilicate or/and calcium borosilicate in a quantity of 5 to 40 wt. %, based on aluminum. Again, these pastes cannot be applied to glass, owing to their high melting point.
The composition containing aluminum powder and glass frit according to U.S. Pat. No. 5,856,015 does in fact contain a glass frit having a dilatometric softening point below 600° C., but it is a glaze for a ceramic substrate. The aluminum content of this glaze—30 wt. % at most—is too low for use as conductive paste if an adequate conductivity is to be ensured.
According to U.S. Pat. No. 4,039,721, thick-layer strip conductors on a ceramic substrate are obtained by applying a paste consisting of 3 to 28 wt. % silver powder, 30 to 61 wt. % aluminum powder, 24 to 44 wt. % borosilicate glass frit to the substrate by means of screen printing and heating for a period of 0.2 to 1 hour at 850 to 105° C. Although a “glassy substrate” is referred to in the claims, only ceramic substrates are used in the Examples. The firing temperature does not favor the use of the paste on sheets of glass. It is necessary to use silver in addition to aluminum in order to achieve an adequate conductivity.
U.S. Pat. No. 5,358,666 discloses an ohmic electrode material consisting of 10 to 50 wt. % glass frit selected from zinc borosilicate and lead-zinc borosilicate and of a conductive component containing 48 to 96 wt. % aluminum and 4 to 52 wt. % silicon. These components, in the form of a paste in an organic medium, are applied to a ceramic substrate and heated at 600 to 800° C. In order to ensure that the specific resistance is sufficiently stable even after storage in a moist atmosphere, it is imperative that the paste contain silicon. This document is not concerned with strip conductors on glass.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a process whereby, with the use of a paste containing glass frit and metal powder, conductive coatings can be produced on substrates such as glass and enamelled steel, for example, strip conductors on glass for automobiles and the like. The paste to be used should preferably be silver-free and, after application to glass, capable of being heated under conventional firing conditions, in particular in the rapid firing conventional for sheets of glass for automobiles, and give rise to firmly adhering conductive layers.
A further object is the provision of a glass substrate, in particular a sheet of glass, supplied with a conductive coating and obtainable according to the invention, the coating being characterized by a specific resistance of less than 100 &mgr;ohm·cm and preferably less than 50 &mgr;ohm·cm, at the same time with good adhesion.
DETAILED DESCRIPTION
The present invention provides a process for producing a conductive coating, in particular strip conductors, on a substrate selected from glass and enamelled steel, comprising applying to the substrate a layer comprising a metal powder, at least one glass frit and a paste containing a liquid or thermoplastic medium and heating of the coating at a temperature in the range of 500 to 750° C., which is characterized in that a screen-printable aluminum paste is used which contains (i) aluminum powder having a d
50
value in the range of 1 to 10 &mgr;m in a quantity of 40 to 80 wt. %, (ii) one or more glass frits having an initial softening temperature in the range of 400 to 700° C. and a d
50
value in the range of 1 to 10 &mgr;m in a quantity of 5 to 40 wt. %, (iii) a liquid or thermoplastic medium consisting of one or more polymers and/or solvents in a total quantity of 10 to 35 wt. %, (iv) sintering aids in pulverulent form in a quantity of 0 to 10 wt. % and (v) 0 to 40 wt. % silver powder.
The aluminum paste can be applied in the form of thin layers to the substrate by means of screen printing or other method known in the practice of decorating. The term “screen-printable” includes direct printing onto the substrate and the transfer technique.
Surprisingly, the aluminum pastes to be used according to the invention can be heated onto glass as substrate without difficulty in an atmosphere of air, even in a rapid firing, with firmly adhering, conductive layers based on aluminum, having a specific resistance of less than 100 &mgr;ohm·cm, being obtained. Depending on the glass frit used in the paste, layers having a specific resistance of less than 70 &mgr;ohm·cm are also obtainable, and particularly preferred pastes result in a specific resistance in the range of 20 to 50 &mgr;ohm·cm. Surprisingly, the heated conductive layers are also distinguished by having a high adhesive strength—the scratch resistance is above 20 N, as determined by means of an Erichsen pin, which is drawn across the strip conductor produced according to the invention on glass. This scratch resistance is substantially preserved even after storage for 5 days at 70° C. and 90% relative humidity. These positive properties can be achieved in the aluminum paste not only with lead-containing glass frits, but also with lead-free glass frits, with even better values surprisingly being obtainable by using zinc-containing glass frits.
Neither was it foreseeable—and this is a particular advantage—that, with the use of preferred aluminum pastes containing a zinc borosilicate glass frit, the specific resistance remains substantially constant within a range of about ±20° C. below or above the conventional firing temperature.
The aluminum pastes contain preferably at least 50 wt. %, in particular 50 to 75 wt. % Al powder. The Al powder used is preferably granular and in particular spherical. Commercially available Al powder in the form of flakes is less suitable owing to the large diameter of the flakes (d
90
equal to/greater than 30 &mgr;m). The d
50
value is suitably in the range of 1 to 10 &mgr;m and in particular from 1 to 8 &mgr;m. The d
10
value is to be greater than 0.2 &mgr;m, in particular greater th
Niemann Axel
Roche Guy
Barr Michael
Cillo Daniel P.
DMC
Eckert Seamans Cherin & Mellott , LLC
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