Exuded Transition Metal Spinel Films on Glass-Ceramic Articles

Transition-metal-containing crystalline compounds of spinel structure are important constituents of inorganic coatings employed for an extremely wide variety of applications. For example, iron-containing spinels such as magnetite (Fe.sub.3 O.sub.4), and also other ferrites such as manganese, cobalt and nickel ferrites, have been applied to a variety of organic and inorganic substrates in order to impart desirable electromagnetic properties thereto. Processes for applying such coatings and controlling the properties thereof in the course of deposition comprise a large body of technology and include vapor deposition, precipitation, evaporation and thermal decomposition techniques.

Similarly, a large number of transition metal spinels are included within the classes of catalytically-active compounds, and have been applied in the form of coatings to a variety of metal, glass and ceramic supports for use in catalytic processes. Among the transition metal spinels useful in catalysis are Mn.sub.3 O.sub.4, MnAl.sub.2 O.sub.4, MnCr.sub.2 O.sub.4, FeAl.sub.2 O.sub.4, CoAl.sub.2 O.sub.4, NiAl.sub.2 O.sub.4, CuAl.sub.2 O.sub.4 and CuCr.sub.2 O.sub.4.

In virtually all applications wherein transition metal spinel coatings are employed in combination with inorganic substrates, coating adherence and durability are extremely important properties. In many applications, binders are required to enable these coatings to demonstrate the toughness needed to withstand the shocks and abrasion of use. In the case of inorganic substrates exhibiting low surface porosity, bonding agents or high sintering temperatures are required to obtain coatings demonstrating a useful degree of adherence to the support. In the case of magnetic recording media such as magnetic memory discs, extreme surface flatness is required, compounding the difficulty of providing a durable, adherent coating.

In catalytic applications, particularly where high temperatures are involved, interactions between active spinel coatings and incompatible binders and/or supporting materials can cause reductions in activity due to the growth of inactive reaction phases or sintering. Yet the attainment of good adherence to commercial supports often requires the use of binders and/or high-temperature sintering treatments.

What is therefore desired is a means for providing transition-metal-containing spinel surface layers on compatible inorganic supports wherein bonding agents, binders, and high sintering temperatures are not required to obtain adherence, toughness and stability in the spinel film.

I have now devised a means of providing transition metal spinel surface films on glass-ceramic articles which overcomes the various problems of adherence, flatness, durability and thermal stability encountered in the prior art. Glass-ceramic articles are a relatively recent development in ceramic technology, the first commercially practicable articles being described by Stookey in U.S. Pat. No. 2,920,971. Briefly, the manufacture of glass-ceramics conventionally involves first compounding and melting a batch for a glass to which an amount of a crystallization-promoting or nucleating agent has been added, thereafter forming a glass article from the melt and cooling the article at least below the transformation range of the glass, and finally reheating the glass article according to a defined time-temperature schedule to produce internal nucleation and crystallization in situ thereof. The resulting article is uniformly crystallized, non-porous, free of voids, and retains the shape of the parent glass article. For a more complete description of glassceramic manufacturing theory and practice, reference may be made to the aforementioned Stookey patent and numerous subsequently published patents, articles and texts.

The present invention is based on the discovery that certain base glass-ceramic compositions, when modified by the addition of specified quantities of transition metal oxide modifiers, can be crystallized and thereafter heat-treated in a manner which will promote the formation and growth of transition metal spinel surface films thereon. In this way, glassceramic articles having integral surface films demonstrating desirable electrical, magnetic, lightabsorptive and/or catalytic properties may be produced. The integral nature of these so-called exuded transition metal spinel films inherently provides the desired properties of adherence, durability and substrate compatibility.

Transition metals suitable for inclusion in glass-ceramic base compositions according to the invention include manganese, iron, cobalt, copper, chromium, nickel and vanadium. Among the crystalline phases which may be present in exuded transition metal spinel films produced according to the invention are Mn.sub.3 O.sub.4, Co.sub.3 O.sub.4, Fe.sub.3 O.sub.4, NiFe.sub.2 O.sub.4, MnFe.sub.2 O.sub.4, CoFe.sub.2 O.sub.4, CuCr.sub.2 O.sub.4, FeCr.sub.2 O.sub.4, MnCr.sub.2 O.sub.4, CoMn.sub.2 O.sub.4, CoTiO.sub.3, Co.sub.2 TiO.sub.4, MnTiO.sub.4, FeTiO.sub.3, VAlO.sub.4, CuAl.sub.2 O.sub.4, CrAl.sub.2 O.sub.4, NiAl.sub.2 O.sub.4, FeAl.sub.2 O.sub.4, CoAl.sub.2 O.sub.4, MnAl.sub.2 O.sub.4, Mn.sub.2 Al.sub.2 (SiO.sub.4).sub.2, MnNb.sub.2 O.sub.6, NiNb.sub.2 O.sub.6 and Ti.sub.2 Nb.sub.10 O.sub.29.

In many cases, exuded transition-metal-containing spinel films produced according to the invention also demonstrate useful light reflecting and transmitting properties. Decorative films over a wide range of colors may be produced, or transparent or translucent bodies exhibiting sharp electromagnetic cutoff values may result.

The end properties of the particular spinel film produced depends on the composition of the base glass-ceramic composition selected as a support and upon the transition metal oxide additives selected for incorporation into the base composition.

Whereas certain of the base glass-ceramic compositions amenable to treatment according to the present invention are generally known, the capability of producing useful exuded transition-metal-containing spinel films on glass-ceramics has been found to depend not only upon the composition of the glass-ceramic material selected as a base but also upon the nature of the subsequent treatment of the material to cause the formation and growth of useful exuded spinel films thereon. In general, the formation and growth of transition-metal-containing spinel films is promoted by heat-treating transition metal-doped glass-ceramic articles under reducing conditions at elevated temperatures for a period of time sufficient to attain the desired degree of film growth.

There is some evidence that the initiation of film formation occurs in some systems in the course of crystallization in situ of the glass precursor body. While such films are not typically sufficiently developed to be useful, the existence of the phenomenon indicates that crystallization need not be entirely complete prior to the commencement of the reduction heat treatment which is employed to achieve the growth of the exuded spinel film. However, completion of the bulk crystallization process prior to or at least concurrently with the formation of the exuded transition-metal-containing spinel film is normally required.

For the purposes of the present description, exuded transition-metal-containing spinel films include not only films composed essentially entirely of transition metal spinels such as MnAl.sub.2 O.sub.4, Fe.sub.3 O.sub.4, and the like, but also films wherein such spinels are in solid solution or combination with other crystalline species such as transition-metal-free aluminates, silicates, titanates, or similar compounds which may or may not be of spinel structure. Glassy matrix phases commonly found in semi-crystalline glass-ceramic materials produced from the crystallization in situ of glasses may also be present in these exuded films. In the preferred embodiments, however, transition metal spinels normally comprise a predominate proportion (at least about 50% by volume) of the exuded film.

A particularly useful class of base glass-ceramic compositions suitable for treatment according to the invention are the aluminosilicate and lithium aluminosilicate compositions characterized by the presence of beta-spodumene and/or beta quartz solid solutions as principal crystal phases. This base glass composition area includes compositions consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 55-80% SiO.sub.2, 14-35% Al.sub.2 O.sub.3, 0-5% Li.sub.2 O, 0-7% TiO.sub.2, 0-10% ZrO.sub.2, 3-13% total of TiO.sub.2 + ZrO.sub.2, and 0-3% F, to which may be added a total of 0.1-10% of transition metal oxide additives. Suitable transition metal oxide additives include one or more oxides selected in the indicated proportions from the group consisting of 0-5% MnO.sub.2, 0-5% Fe.sub.2 O.sub.3, 0-3% CoO, 0-2% CuO, 0-2% Cr.sub.2 O.sub.3, 0-3% V.sub.2 O.sub.5, and 0-10% NiO. Minor amounts of other compounds may, of course, be included within these compositions as aids in melting, to modify properties, or for other known purposes. Examples of additives which have been employed are La.sub.2 O.sub.3, Nb.sub.2 O.sub.5, BaO, B.sub.2 O.sub.3, P.sub.2 O.sub.5, MgO, CaO, ZnO, Na.sub.2 O, K.sub.2 O, Ta.sub.2 O.sub.5, Cl, Br, As.sub.2 O.sub.3, and Sb.sub.2 O.sub.3. The total amount of these additives, however, is generally held to not more than about 10% by weight, so that the basic constituents SiO.sub.2, Al.sub.2 O.sub.3, Li.sub.2 O, TiO.sub.2, ZrO.sub.2, F, and transition metal oxides will comprise at least about 90% by weight of the glass-ceramic composition.

Glass-ceramic compositions within the above described composition range may be compounded and melted in accordance with conventional glass-making practice, and thereafter formed into glass articles by conventional means such as pressing, rolling, casting, spinning or the like. The batch materials may consist of oxides or may comprise any other compounds which will decompose at melting temperatures to yield molten batches having calculated oxide compositions within the aforementioned range. For these compositions, melting typically requires temperatures in the range of about 1600.degree.-1650.degree.C. for times in the range of about 6-16 hours.

Glass articles formed from the above compositions may be converted by crystallization in situ into semicrystalline glass-ceramic articles according to processes conventional for beta-spodumene and beta-quartzcontaining glass-ceramics. Preferred crystallization treatments comprise exposure of the articles to temperatures in the nucleation range of about 700.degree.-800.degree.C. for times in the range of about 1-4 hours, followed by exposure to temperatures in the crystallization range of about 800.degree.-1200.degree.C. for times in the range of about 1-8 hours.

Following crystallization, the semicrystalline glass-ceramic articles are subjected to further heat treatment under reducing conditions to promote the development of an exuded surface phase comprising transition metal compounds of spinel structure. Exuded spinel films formed in the described composition system include spinels such as Mn.sub.3 O.sub.4, Fe.sub.3 O.sub.4, NiAl.sub.2 O.sub.4, CoAl.sub.2 O.sub.4, CuCr.sub.2 O.sub.4, MnCr.sub.2 O.sub.4, CrAl.sub.2 O.sub.4, Co.sub.3 O.sub.4, FeCr.sub.2 O.sub.4, CoFe.sub.2 O.sub.4, MnFe.sub.2 O.sub.4 and CoMn.sub.2 O.sub.4. These may be found alone, in combination with each other, or in solid solution or combination with MgAl.sub.2 O.sub.4, VAlO.sub.4, CuAl.sub.2 O.sub.4, MnAl.sub.2 O.sub.4, FeAl.sub.2 O.sub.4, CoTiO.sub.3 or Co.sub.2 TiO.sub.4.

The film-producing heat treatments suitably comprise heating at temperatures in the range of about 500.degree.-1000.degree.C. in a reducing atmosphere. Preferred atmospheres include hydrogen and hydrogen-containing atmospheres such as forming gas (H.sub.2, N.sub.2). These atmospheres may contain additional constituents such as water vapor, CO, CO.sub.2, Cl.sub.2 or sulfur. Of course, other conventional reducing atmospheres such as hexane, methane, ammonia or the like may also be employed if desired. Typical treatment times range from at least about 1/2 hour up to about 10 hours or more. Longer treatment times may be employed, if desired, but long treatments are of no practical benefit and are commercially undesirable.

After sufficient growth of the transition metal spinel film has been attained in accordance with the above-described treatment, it may be desirable to further treat the article to modify the properties of the surface film for certain applications. Leaching is sometimes useful to remove residual glassy phases and/or to modify the porosity of the film. Supplemental oxidizing and/or reducing treatments may also be employed to modify the oxidation states of certain of the film constituents. The precise nature of the supplemental treatment employed, if any, will depend on the properties desired in the film and the nature of the use for which the article is intended.

Examples of thermally-crystallizable glass compositions suitable for forming beta-spodumene and beta-quartz glass-ceramics having exuded surface films containing transition metal spinel compounds according to the invention are set forth in Table I below. Compositions are given in parts by weight on the oxide basis as calculated from the batch. These compositions were batch melted in platinum crucibles at 1625.degree.C. for 16 hours, and then poured into steel molds to form 4 .times. 4 .times. 1/2 inch slabs and annealed at 650.degree.C. Most of the compositions shown also include minor amounts of As.sub.2 O.sub.5 as a fining agent; however, the amount remaining in the glass after melting is negligible and is therefore not reported.

The thermally-crystallizable glass articles in Table I, produced as above described, are thereafter treated as set forth below in Table II in order to produce glass-ceramic articles having beta-spodumene and/or beta-quartz solid solutions as principal crystalline phases and exuded surface films containing transition metal spinels. Table II reports the crystallizing heat treatments employed to convert each thermally-crystallizable glass article to the semicrystalline state, the principal crystal phase present in the articles after ceramming, the reducing heat treatments employed to promote the growth of transition metal spinel compounds present in the exuded surface films, the appearance of the articles after the growth treatments, and the dominant properties of the exuded films. The principal crystalline phases listed are generally solid solutions rather than specific compounds. In instances where forming gas is used as the reducing atmosphere, a gas consisting of 8% H.sub.2 and 92% N.sub.2 by volume was employed. Typcial film thicknesses over the range of growth treatments employed range about 0.1-2 microns.

From the foregoing examples it is readily apparent that a broad range of aluminosilicate and lithium aluminosilicate glass-ceramic compositions containing transition metal additives may be treated according to the invention to provide exuded transition metal spinel films thereon having a variety of uses.

Compositions which are particularly preferred for producing transition metal spinel films having desirable magnetic and electrical properties are titania-nucleated lithium aluminosilicate compositions consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 58-64% SiO.sub.2, 19-21% Al.sub.2 O.sub.3, 2-5% Li.sub.2 O, a total of 3-7% of nucleating agents selected in the indicated proportion from the group consisting of 2-7% TiO.sub.2 and 0-1% ZrO.sub.2, 0-1% F, and 1-6% total of transition metal additives, essentially including iron, selected in the indicated proportion from the group consisting of 1-5% Fe.sub.2 O.sub.3, 0-5% MnO.sub.2, 0-5% CoO and 0-3% NiO. Exuded transition metal films produced from articles of these compositions typically include Co.sub.3 O.sub.4, Fe.sub.3 O.sub.4, Mn.sub.3 O.sub.4, MnAl.sub.2 O.sub.4, FeAl.sub.2 O.sub.4, CoAl.sub.2 O.sub.4, MnFe.sub.2 O.sub.4, NiAl.sub.2 O.sub.4, NiFe.sub.2 O.sub.4 and CoFe.sub.2 O.sub.4. Example I of Table I represents the presently preferred composition for producing a spinel film having particularly desirable magnetic properties according to the invention.

Compositions which are preferred for producing glass-ceramic articles having exuded transition metal spinel films demonstrating useful catalytic activity consist essentially, in weight percent on the oxide basis as calculated from the batch of about 68-74% SiO.sub.2, 14-19% Al.sub.2 O.sub.3, 0-5% Li.sub.2 O, 0-6% TiO.sub.2, 0-3% ZrO.sub.2, 5-9% total TiO.sub.2 + ZrO.sub.2, 0-5% B.sub.2 O.sub.3, 0-3% F, and 1-10% total of transition metal additives selected in the indicated proportion from the group consisting of 0-5% Fe.sub.2 O.sub.3, 0-5% CoO, 0-5% MnO.sub.2, 0-3% CuO, 0-2% Cr.sub.2 O.sub.3, and 0-3% NiO. Exuded transition metal spinel films from articles of these compositions typically contain CoTiO.sub.3, Co.sub.2 TiO.sub.4, CoMn.sub.2 O.sub.4, Co.sub.3 O.sub.4, Mn.sub.3 O.sub.4, Fe.sub.3 O.sub.4, CoFe.sub.2 O.sub.4, CoAl.sub.2 O.sub.4, MnTiO.sub.3 and FeTiO.sub.3. Example 8 of Table I represents the presently preferred composition for producing a catalytically-active spinel film in this system.

Types of glass-ceramic articles other than aluminosilicate and lithium aluminosilicate beta-quartz and beta-spodumene articles are also useful in providing exuded transition-metal-containing spinel films according to the invention. Another preferred composition area is found to include somewhat diverse silicate, aluminosilicate, and boroaluminate base compositions wherein manganese is a major constituent, comprising at least about 10% by weight of the compositions. The generally preferred composition area includes compositions consisting essentially, in weight percent on the oxide basis, as calculated from the batch, of about 10-60% MnO.sub.2, at least one oxide selected in the indicated proportion from the group consisting of 10-70% SiO.sub.2, 13-43% Al.sub.2 O.sub.3, and 0-35% B.sub.2 O.sub.3, essentially including at least about 5% B.sub.2 O.sub.3 and 20% Al.sub.2 O.sub.3, when SiO.sub.2 is absent, not exceeding about 5% B.sub.2 O.sub.3 when Al.sub.2 O.sub.3 is absent, and not exceeding about 10% B.sub.2 O.sub.3 when both SiO.sub.2 and Al.sub.2 O.sub.3 are present, the sum total of MnO.sub.2 + SiO.sub.2 + Al.sub.2 O.sub.3 + B.sub.2 O.sub.3 comprising at least about 60% by weight of the composition. Optional useful additional constituents of these compositions include 0-10% ZrO.sub.2, 0-20% TiO.sub.2, 0-30% Nb.sub.2 O.sub.5, 0-35% La.sub.2 O.sub.3, 0-10% Ta.sub.2 O.sub.5, 0-15% BaO, 0-10% SnO.sub.2, 0-4% ZnO, 0-10% K.sub.2 O, 0-5% Fe.sub.2 O.sub.3, 0-10% NiO, 0-3% Cr.sub.2 O.sub.3, and 0-3% CoO.

Minor amounts of other compounds may, of course, be included within these compositions as aids in melting, to modify properties and so forth, including, for example, Li.sub.2 O, Na.sub.2 O, WO.sub.3, P.sub.2 O.sub.5, MgO, Cl, F, MnO.sub.3, Cu.sub.2 O, V.sub.2 O.sub.5, As.sub.2 O.sub.3, and Sb.sub.2 O.sub.3.

Glass-ceramic compositions within the aforementioned composition range may be melted according to conventional practice, typically at temperatures in the range of about 1500.degree.-1600.degree.C. for times on the order of about 6-16 hours. The molten glasses may be formed into glass articles by conventional means such as pressing, rolling, casting, drawing or the like. Batch materials for these glasses may comprise oxides or other compounds which will decompose at melting temperatures to yield molten batches having oxide compositions within the aforementioned range.

Glass articles formed from the above compositions may be converted by crystallization in situ into glassceramic articles by heat treatment at temperatures in the range of about 600.degree.-1200.degree.C. for times in the range of about 4-24 hours. Preferred crystallization treatments comprise a nucleation step wherein the article is heated at temperatures in the range of about 600.degree.-800.degree.C. for times on the order of 1-4 hours, and a crystallization step wherein the article is heated at temperatures in the range of about 800.degree.-1200.degree.C. for times on order of 1-4 hours. Principal crystal phases in these composition systems include MnAl.sub.2 O.sub.4, Mn.sub.3 O.sub.4, Mn.sub.2 Al.sub.2 (SiO.sub.4).sub.2 and MnSiO.sub.3 depending somewhat on the composition of the MnO.sub.2 -(B.sub.2 O.sub.3, Al.sub.2 O.sub.3, SiO.sub.2) base glass.

Following crystallization, the growth of transition-metal-containing spinel films on the surface of these glass-ceramic articles is promoted using reduction heat treatments substantially the same as those above described for beta-spodumene and beta-quartz-containing articles. Such treatments typically comprise heating to temperatures in the range of about 500.degree.-1000.degree.C. in a reducing atmosphere, preferably an atmosphere comprising hydrogen or nitrogen-hydrogen forming gas, for treatment times in the range from about 1/2 hour up to about 10 hours, or more. Again, longer treatments may be employed if desired, but these are not deemed of practical benefit.

Transition-metal-containing exuded spinel films which may form in this composition system include Mn.sub.3 O.sub.4, Fe.sub.3 O.sub.4, MnAl.sub.2 O.sub.4, NiAl.sub.2 O.sub.4, NiFe.sub.2 O.sub.4, MnFe.sub.2 O.sub.4, MnCr.sub.2 O.sub.4, MnNb.sub.2 O.sub.6, NiNb.sub.2 O.sub.6 and Ti.sub.2 Nb.sub.10 O.sub.29. The spinels in this system may be found either alone or in solid solution or combination with other crystalline species such as MnSiO.sub.3, ZrO.sub.2, and Mn.sub.2 Al.sub.2 (SiO.sub.4).sub.2. Residual glassy phases may also be present. Whereas the transition metal spinel films produced in these systems typically differ in composition from the interior of the article, it is possible that in certain cases the predominant surface spinel is also one which predominates in the article as a whole. Nevertheless treatment according to the invention is effective to increase spinel formation in the surface layers of the article such that improved surface properties are obtained.

Examples of thermally-crystallizable glass compositions suitable for forming silicate, aluminosilicate, and boroaluminate glass-ceramics having exuded surface films containing transition metal spinel compounds according to the invention are set forth in Table III below. Compositions are given in parts by weight on the oxide basis as calculated from the batch. The compositions were batch melted in platinum crucibles at 1600.degree.C. for about 6 hours, and then poured into steel molds to form 3/8 .times. 5 .times. 5 inch slabs and annealed at about 600.degree.C. A few of the compositions additionally contained minor amounts of As.sub.2 O.sub.5 as a fining agent, but the amount remaining in the glass after melting is small and is therefore not reported.

The thermally-crystallizable glass articles of Table III, produced as above described, are thereafter treated as set forth below in Table IV in order to produce glass-ceramic articles having exuded surface films containing transition metal spinels. Table IV reports the crystallization heat treatments employed to obtain bulk crystallization in situ of the articles, the principal crystalline phases present in the articles after ceramming, the reducing heat treatments employed to promote the growth of transition metal spinel films on the articles, the transition metal spinels present in the exuded surface films, the appearance of the articles after growth treatment, and the dominant properties of the exuded films. Film dielectric constant (K') and loss tangent (tan .delta.) are reported where determined on individual samples. In instances where forming gas is reported as present in the reducing atmosphere, a gas consisting of 8% H.sub.2 and 92% N.sub.2 by volume was employed. Typical film thicknesses for these exuded films over the range of growth treatments employed range about 0.1-4 microns.

While the foregoing examples indicate that a broad range of manganese-containing compositions may be treated according to the invention to provide spinel films thereon, the best film properties are produced over a somewhat narrower range of composition. Among the aluminosilicate glass-ceramic compositions amenable to treatment according to the invention, those consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 19-40% SiO.sub.2, 13-43% Al.sub.2 O.sub.3, and 15-50% MnO.sub.2, optionally including 0-4% ZnO, 0-10% TiO.sub.2, 0-10% ZrO.sub.2, 0-10% SnO.sub.2, 0-10% NiO, 0-5% Fe.sub.2 O.sub.3, and 0-30% Nb.sub.2 O.sub.5, are preferred. These compositions may provide spinel films containing Mn.sub.3 O.sub.4, Fe.sub.3 O.sub.4, MnAl.sub.2 O.sub.4, NiAl.sub.2 O.sub.4, MnFe.sub.2 O.sub.4, NiFe.sub.2 O.sub.4, MnNb.sub.2 O.sub.6, NiNb.sub.2 O.sub.6, and Ti.sub.2 Nb.sub.10 O.sub.29, many of which provide desirable electrical, magnetic and/or catalytic properties.

Preferred boroaluminate glass-ceramic compositions according to the invention are those consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 20-35% Al.sub.2 O.sub.3, 5-35% B.sub.2 O.sub.3, and 28-60% MnO.sub.2, optionally including 0-15% BaO and 0-35% La.sub.2 O.sub.3. These compositions provide Mn.sub.3 O.sub.4 and/or MnAl.sub.2 O.sub.4 -containing films of good quality having desirable electrical properties.

From the foregoing description it is apparent that a large number of exuded transition metal spinel films may be provided on glass-ceramic base articles according to the invention to impart a variety of useful properties thereto. Thus articles having configurations suitable for use as magnetic memories, such as discs, may be conventionally formed, crystallized and provided with flat surfaces, and thereafter heat treated to exude magnetic transition metal spinel films thereon. Similarly glass-ceramic tubes, honeycombs, or the like may be formed, heat treated to exude catalytically-active films thereon, and incorporated into catalytic reactors to provide stable, durable active elements. Of course, these examples are merely illustrative of the numerous applications for glass-ceramic articles having integral exuded spinel films which may be practiced within the scope of the present invention as defined by the appended claims.