Metallizing Compositions

This invention relates to metallizing compositions containing an electrically conductive metal and a crystallizable frit, to a method for applying these compositions to heat resistant substrates and particularly glass and other ceramic substrates, and to articles of heat resistant substrates having an electrically conductive metal film fused to at least a portion of their surfaces.

Metallizing compositions and particularly paste-like or liquid metallizing compositions containing finely-divided silver particles and inorganic binder dispersed in a liquid vehicle are known to the art. In such compositions the inorganic binder component serves to bond the conductive silver firmly to the substrate to which the composition is applied and the liquid vehicle is present to form a metallizing ink, paint or paste for ease of application to the substrate in a desired pattern.

The conventional inorganic binders for metallizing compositions which provide fired-on conductive metal coatings on ceramic substrates are frits of soft glasses such as, for example, the metal borate glasses, the borosilicate glasses containing at least as much boron as silica, the borosilicate high density glasses containing high amounts of bismuth and/or lead oxides, the high barium and/or strontium oxide-containing glasses, and the like. Frits of the above types in which silica is the minor glass-forming ingredient give metallizing compositions of good adhesion and solder acceptability. However, because the metallizing compositions have high coefficients of thermal expansion ranging usually from about +155.degree. to +180.degree. .times. 10.sup.-7 /.degree. C., they are not suitable for use with many substrates and particularly with the low expansion glass-ceramics or thin flint glass substrate due to their weakening effect on the substrate. The designing of satisfactory metallizing coating compositions which will provide good substrate wetting as well as coefficients of expansion equal to or below that of the substrate so that weakening does not occur has not been achieved. Thus, the search continues for metallizing compositions having a broader range of utility and better characteristics than those presently known.

Now, in accordance with the present invention, it has been found that metallizing compositions formulated with certain alkali metal, crystallizable high silica frits not only produce on firing a conductive metal coating having good adherence to a wide variety of substrates but that the resulting composite structure has a greater stength value than that of the non-coated substrate to which the coating is applied. The finding that any improvement of strength can be realized with the metallizing compositions of this invention is indeed surprising because it has generally been recognized in the ceramic art that weakening, not strengthening, results when a substrate is fusion coated with a material having a significantly higher coefficient of expansion.

Accordingly, the present invention relates to a conductive metal composition adapted to be applied to and fired on a heat resistant substrate to form thereon an electrically conductive, adherent film which does not adversely affect the overall strength of the substrate, said composition comprising, by weight, (a) from 50 to 88% of at least one finely-divided, electrically conductive metal; (b) from 2 to 40% of a crystallizable glass frit binder comprising by weight 40 to 70% of SiO.sub.2, 10 to 31% of Al.sub.2 O.sub.3, 3 to 20% of Li.sub.2 O, 2 to 15% of B.sub.2 O.sub.3, 0 to 4% of As.sub.2 O.sub.3, 0 to 5% of Na.sub.2 O, 0 to 5% of K.sub.2 O, 0 to 6% of Bi.sub.2 O.sub.3, and at least one oxide component selected from the group consisting of 4 to 19% of ZrO.sub.2 and 1 to 10% to TiO.sub.2 ; (c) from 5 to 48% of an inert liquid vehicle; and (d) from 0 to 20% of an inert filler or pigment. The invention also relates to a method for forming an electrically conductive film on a heat resistant substrate using the composition of this invention and to the articles so formed.

The crystallizable glass frits which serve as the binder component in the conductive metal compositions of this invention are, as indicated, high silica glasses comprising as base constituents on the oxide basis

at least one of ZrO.sub.2 or TiO.sub.2 being present in the above amount as a nucleating agent. The frits will usually also contain at least one of As.sub.2 O.sub.3, Na.sub.2 O, K.sub.2 O or Bi.sub.2 O.sub.3 as refining agents or modifiers, in which case the total amount of such agents or modifiers will preferably range from about 1 to about 10%.

The following frit compositions are given as examples of preferred compositions which will crystallize .beta.-spodumene or .beta.-eucryptite when fired under appropriate conditions. All thermal expansion values reported in the Table and the examples which follow were measured according to ASTM E-228-71.

Typically, the crystallizable frits are made by mixing the batch components together, melting or smelting the mixture, pouring the metal into water to produce a frit and then grinding the resulting frit, usually with a ball mill to provide a frit composition which will pass a 325-mesh screen.

The frit may include minor amounts of impurities or optional constituents other than those noted above, provided, of course, that the inclusion of such constituents or the amount utilized does not detract from the advantages of the invention.

The composition of the invention, as indicated, contains a major amount which is from 50 to 88% by weight of the composition of at least one electrically conductive metal such as the noble metals, gold, silver, platinum, palladium, rhodium and iridium in particulate form. Silver is the preferred noble metal and most preferably will be present in an amount from 60 to 86% by weight of the composition. The particulate metal is generally present in its elemental form and the particles thereof are usually in powder or flake form. Preferably, the metal is a powder which will pass through a 200 mesh or smaller screen. Particulate compounds of the metal can also be used provided the compound can be reduced to the elemental metal during processing such as by carrying out the firing in the presence of a reducing atmosphere.

The liquid vehicle portion of the composition functions to hold the conductive metal particles and the frit binder together and to permit the facile application of the composition to the substrate. The liquid should be inert to the other components of the composition and to the substrate to which it is applied and should not interfere with the formation of a metallic film during firing. Many liquids satisfy these general requirements and are known to perform well in metallizing compositions. Particularly suitable are pine oil, turpentine, mineral oils, glycols, clean burning heavy bodied oils and like. The method of application and the thickness of the desired coating will, of course, influence the proportion and type of liquid vehicle in the composition.

The metallizing composition can also contain, if desired, a small amount and preferably up to 20% by weight of the composition of inert fillers and/or pigments. The inclusion of fillers such as silica or alumina usually results in films having improved abrasion durability. Although the inclusion of pigments is generallly for the purpose of enhancing the coloration of the substrate, certain pigments can also function as fillers and in this dual capacity can be used to impart color to the composite structure and to modify the properties of the coating in a desired manner.

The compositions of the invention can be applied to any substrate which retains its integrity during the firing operation such as ferrous and non-ferrous metal substrates and ceramic substrates but is particularly suited to the ceramic substrates such as glass, china, porcelain and glass-ceramics. The method of application of the composition to the substrate is conventional and typically includes such well known techniques as spraying, brushing, dipping, banding or screen or stencil printing.

Following application of the composition to the substrate in the desired pattern or design, the coating is preferably, but not necessarily, dried prior to firing. Air drying will usually suffice when the vehicle is volatile at room temperature. Under other circumstances, dry air currents or mild baking at elevated temperature may be required to remove the diluent. The temperature of actual firing will vary, depending upon the maturation temperature of the frit but should be below the point at which the substrate will physically deform or change chemically. Usually, firing at a temperature of at least about 590.degree. C. and which is within the tempering cycle of the substrate will fuse the frit and effect a firm bond between the composition and the substrate. Preferably, firing is carried out for 2 to 15 minutes at from about 600.degree. to about 900.degree. C.

The compositions of the invention are particularly useful in the production of electrical components since the fired on metallic films, in an electrical pattern or design on ceramic objects, are easily connected in electrical circuits. Among the many uses are conductors, resistors and other components in printed circuits and other electronic applications, capacitors, glass electrodes, electroconducive heating elements on aircraft, architectural and automobile windows to remove moisture or ice deposits, ornamental metallic coatings or designs, and the like.

The invention is further described by reference to the following examples wherein all parts and percentages are by weight unless otherwise indicated.

Various silver metallizing pastes were prepared by blending 236 parts of particulate silver as powder or flake, from 23.6 to 28 parts of a crystallizable frit or frit mixture, and 41 parts of a pine oil vehicle and subjecting the blends to the shearing action of a three-roll mill. The resulting silver pastes were screen printed in an electrical grid pattern on 1/4" float glass plates and in an overall wraparound design on 1/4" flint glass rods, and the printed plates and rods were dried at 425.degree. C. for 10 minutes and then fired at 621.degree. C. for 7 minutes. A control paste was also prepared in the manner of Example 2 except that the 22.4 parts of Frit F were replaced with 22.4 parts of the control frit and 44.7 parts of the vehicle were used. Details of the paste compositions of these examples and the control and the properties of the printed glass articles are given below in Table II.

The procedure of Example 5 was repeated except that in these examples an equal amount of frit A (Example 6), frit B (Example 7), frit C (Example 8) and frit D (Example 9) was substituted for frit E. The modulus of ruptures values for the fired flint glass rods printed with the pastes of these examples ranged from 17,000 to 20,000 p.s.i. and the electrical resistance values ranged from 0.26 to 0.36 .OMEGA./inch.

If desired, the strength of ceramic substrates and particularly glass metallized with the compositions of this invention can be further increased by staining that area of the substrate to which the conductive metal composition is to be applied. Stains are well known and commercially available and typically comprise a silver compound such as silver chloride, a modifier such as one or more compounds of iron or copper, inert filler ingredients, and an inert liquid vehicle. The stain is applied to the substrate conventionally as by screen printing and then the stained substrate, with or without drying, is fired to the maturation temperature which usually ranges from about 510.degree. to about 540.degree. C., depending upon the particular stain employed. After removal of any residue from the firing step, the stained substrate is next overprinted in the area of stain with the conductive paste and then fired as above. This results in a metallized substrate having outstanding strength properties. It is also possible to incorporate the stain ingredients directly with the paste constituents and arrive at metallized substrates of improved strength. The use of a stain in combination with the paste of Example 5 is demonstrated below.

Flint glass rods having a diameter of 0.25 inch and a length of 3 inches were printed with a stain composed of, by weight, 9.25 of silver chloride, 15.2% of ferrous sulfide, 15.2% of cuprous sulfide, 38.4% of silica and 22% of pine oil vehicle by screen printing and the printed rods were dried at 425.degree. C. for 10 minutes and then fired at 510.degree. C. for 10 minutes. The resulting stained rods were next overprinted in the area of the stain with the paste of Example 5 and the overprinted rods were dried at 425.degree. C. for 10 minutes and then fired at 620.degree. C. for 7 minutes. For the sake of comparison, a flint glass rod was also treated in the manner of these examples except that no stain was used and the paste was printed directly onto the glass rod after firing the rod at 510.degree. C. for 10 minutes. The strength values for the metallized rods and a glass rod control fired at the same temperature-time cycles as above except no stain or paste was used are reported below.