FR3026737A1 - REFRACTORY MATERIAL AND FUSED ALUMINUM GRAIN - Google Patents

Abstract

Ceramic material, comprising: - a main phase comprising more than 90% by weight of alumina in corundum form, - a secondary phase having the following composition, in weight percentages on the basis of the oxides: ○ from 40 to 90% of ZrO 2, 2 to 35% of TiO 2, 2 to 20% of Y 2 O 3, less than 5% of Al 2 O 3 less than 2% of impurities, the alumina phase representing more than 70% by weight of said material and said phases main and secondary together represent more than 95% of the weight of said material. Melted grains of the same composition for obtaining said material

Description

FIELD OF THE INVENTION The invention relates to refractory materials for ceramics applications consisting mainly of aluminum oxide. The invention also relates to a process for producing such materials, as well as to ceramic grains, in particular melted, for the manufacture thereof, used in particular, but not only, in the field of metallurgy. . In the remainder of the description, for convenience and in accordance with the habits in the field of ceramics, said oxides comprising elements Al, Ti, Zr and Y (or others) will be described with reference to the corresponding simple oxides, that is to say A1203, h02, ZrO2, Y2O3. Thus, in the description which follows, unless otherwise stated, the proportions of the various elements in the grains according to the invention are given by reference to the weight of the corresponding simple oxides, reported as a percentage by weight relative to all the oxides present detectable in said grains, even if they are not necessarily present in this simple form in said grains. By contrast, in the present description it is meant a composition actually present with an oxide in said grains by "phase" or "oxide phase". In the remainder of the description, the application of the materials or grains according to the invention and their advantages in the specific field of refractory products used in metallurgy for casting molten metals is described more particularly. However, it is understood that such materials / grains, by the advantages they provide, are likely to be used advantageously in many other applications in the field of ceramics, especially in any field for which a high resistance to thermal shocks to high temperatures, especially greater than 1200 ° C, is sought, that is to say in any field requiring high stability of the initial mechanical properties after said thermal shocks. In particular, many areas of furnaces manufacturing or metal processing require the use of refractory products resistant to high temperatures but also to large temperature variations over very short periods. For example, the slide plates, or "slide gates" according to the English term, are parts used during a continuous casting of the steel to open or close distributors or outlets of ladles in communication with fluid, via a sliding nozzle ("sliding nozzle" in English), with ingot molds. All parts of this device and particularly the drawer plates must thus have optimum resistance to thermal shock to prevent breakage. Thus, US Pat. No. 5,363,995 describes a slide plate made from a refractory material obtained from a composition comprising from 70 to 95% by weight of alumina, from 1 to 10% of reactive alumina, from 1 to 5% silica and 3 to 15% silicon carbide. US Pat. No. 5,954,989 alternatively describes as a possible material a refractory comprising up to 95% alumina, up to 15% carbon and up to 10% silica, in the presence of less than 3% of an antioxidant. . US Pat. No. 4,646,950 describes a refractory material for a sliding nozzle, obtained from a composition incorporating from 2 to 50% by weight of particles comprising from 42 to 79.3% by weight of alumina, from 20 to 40% by weight of zirconia. and from 0.5 to 16% by weight of titanium oxide and less than 2% of impurities.

There is still at present a need for refractory materials and raw materials (in the form of grains) for the manufacture of such refractory materials, particularly in the field of metallurgy, having improved resistance to thermal shocks.

The object of the invention is to satisfy such a need. The object of the present invention therefore relates firstly to a ceramic material, essentially comprising: a main phase comprising more than 90% by weight of alumina in corundum form, a secondary phase having the following composition, in weight percentages on the oxides: 40 to 90% ZrO 2, 2 to 35% TiO 2, 2 to 20% Y 2 O 3, less than 5% Al 2 O 3, less than 2% impurities. According to the invention, the alumina phase represents more than 70% by weight of said material. According to the invention, the main phase of alumina and the secondary phase together represent more than 95% of the weight of said material. According to preferred embodiments of the material according to the present invention, which can of course be combined with one another as appropriate: The composition of the secondary phase comprises more than 45% by weight of ZrO 2, more preferably more than 50% by weight ZrO2 or even more than 55% by weight of ZrO2. - The composition of the secondary phase comprises less than 30% by weight of h02, more preferably less than 25% by weight of h02, or even less than 20% by weight of h02. - The main phase comprises more than 95% by weight of alumina. More preferably, the main phase comprises more than 99% by weight of alumina. The main alumina phase and the secondary phase together (ie in total) represent more than 98% of the weight of said material. The alumina phase represents more than 80% of the weight of said material, or even more than 85% of the weight of said material or even more than 90% of the weight of said material. The secondary phase represents more than 1% of the weight of said material, more preferably more than 2% of the weight of said material. - The secondary phase is less than 25% of the weight of said material, preferably less than 15% of the weight of said grains, more preferably less than 10% of the weight of said material. - The ceramic material according to the invention has the following overall composition, in weight percentages based on simple oxides: - more than 70.0% and less than 97.0% of A1203, - more than 0.3% and less than 6.0% of h02, - more than 0.5% and less than 20.0% of Zr02, - more than 0.3% and less than 4.0% of Y203, - less than 2.0% in total, other oxides. In the above overall composition, the level of Al 2 O 2 is preferably greater than 75% by weight, preferably greater than 80% by weight or even greater than 85% by weight. In the above overall composition, the TiO 2 content is preferably less than 4% by weight. In the above overall composition, the level of ZrO 2 is preferably less than 8% by weight, or even less than 6% by weight. According to a particular advantageous embodiment, the ceramic material according to the invention has the following overall composition, based on simple oxides: - more than 85.0% and less than 97.0% of A1203, - more than 0 , 3% and less than 4.0% of h02, - more than 0.5% and less than 8.0% of Zr02, - more than 0.3% and less than 2.0% of Y203, - less than 2.0%, in total, other oxides.

The present invention also relates to a method of manufacturing a piece of ceramic material as described above, comprising the following steps: a) the mixture of raw materials consisting of aluminum oxide, titanium oxide, oxide of zirconium, yttrium oxide or precursors of said oxides, in the following respective proportions, on the basis of the oxides: - more than 70,0% and less than 97,0% of A1203, - more than 0, 3% and less than 6.0% of h02, - more than 0.5% and less than 20.0% of Zr02, - more than 0.3% and less than 4.0% of Y203, - less than 2% 0%, in total, other oxides. b) complete melting of said mixture, c) obtaining from said molten mixture a powder of melted grains by steps comprising: - cooling the molten mixture to room temperature, - dividing the molten mixture into individualized grains, d) a step of shaping said workpiece from the powder of melted grains, optionally mixed with carbonaceous or organic materials such as graphite or phenolic resins, e) a temperature treatment step at a temperature between 500 ° C and 1500 ° C to obtain said refractory ceramic piece.

According to the invention, the two cooling and dividing steps previously described in c) are not necessarily performed in this order. The order of said two steps depends in particular on the technique used to obtain the melted grains. For example, first the cooling step and then the dividing step in the case where the division of the melt consists of grinding. Alternatively, the dividing step and then the cooling step will first be carried out in the case where the division of the melt consists of a molding such as for example described in application WO2012 / 045302. According to the invention, the raw materials are obviously chosen in step a) in proportions such that the grains finally obtained are in accordance with the invention. Any conventional process for producing molten grains may be carried out, provided that the composition of the feedstock makes it possible to obtain grains having a composition in accordance with that of the grains according to the invention. In step a), the titanium may be introduced in any form, in particular in oxide form, or in metallic form or in the form of a mixed zirconia-titanium oxide compound. Similarly, yttrium can be introduced in the form of a zirconia-yttrium oxide compound. In step b), an electric arc furnace is preferably used, but all known furnaces are conceivable, such as an induction furnace or a plasma furnace or torch, provided that they allow the charge to be completely melted. departure. The firing is preferably carried out under neutral conditions, for example under argon, or oxidising, preferably at atmospheric pressure. In step c), the solid mass is divided according to conventional techniques until a granulometry suitable for its subsequent use is obtained. Said feedstock may comprise unavoidable impurities in the limited amounts specified above. By "impurities" is meant inevitable constituents necessarily introduced with the raw materials or resulting from reactions with these constituents. The impurities can in particular be introduced during the preliminary step of manufacturing the melted grains. Impurities are not necessary constituents, but only tolerated. It is considered that a total content of impurities of less than 2%, preferably less than 1%, does not substantially modify the results obtained. The invention also relates, in another aspect, to melted grains for synthesizing the material according to the invention by the implementation of the method 30 described above, especially in the form of powder. Said melted grains according to the invention essentially comprise: a main oxide phase comprising more than 90% by weight of alumina in the corundum form, a secondary oxide phase having the following composition, in weight percentages on the basis of the oxides: 40 to 90% of ZrO 2, o 2 to 35% of H 2 O, 2 to 20% of Y 2 O 3, less than 5% of Al 2 O 3, less than 2% of impurities, said phases of alumina and secondary oxide together representing more than 95% of the weight of the grains and said alumina phase representing more than 70% of the weight of said grains.

According to preferred embodiments of the melted grains according to the present invention, which can of course be combined with one another if appropriate: the secondary phase is essentially present in said grains at the grain boundaries of the crystallites of the main phase . - The composition of the melted grains corresponds to the following general formula: t 0.4 x (y + z) in which: - t is the weight percentage of h02, - y is the weight percentage of Y203, - z is the weight percentage of Zr02. - The composition of the melted grains preferably corresponds to the general formulation: t 0.3 x (y + z), more preferably to the general formulation: t 0.2 x (y + z), and very preferably to the general formulation: t 0.1 x (y + z). Melted grains according to the invention also advantageously have the following characteristics: The composition of the secondary phase comprises more than 45% by weight of ZrO 2, more preferably more than 50% by weight of ZrO 2, or even more than 55% by weight of ZrO 2. - The composition of the secondary phase comprises less than 30% by weight of h02. - The main phase comprises more than 95% by weight of alumina. More preferably, the main phase comprises more than 99% by weight of alumina. The main alumina phase and the secondary phase together (ie in total) represent more than 98% of the weight of said material. The alumina phase represents more than 80% of the weight of said material, or even more than 85% of the weight of said material or even more than 90% of the weight of said material. - The secondary phase represents more than 1% of the weight of said material, or even more than 2% of the weight of said material. - The secondary phase is less than 25% of the weight of said material, preferably less than 15% of the weight of said grains, more preferably less than 10% of the weight of said material. - The melted grains according to the invention have the following overall composition, in weight percentages based on simple oxides: - more than 70.0% and less than 97.0% of A1203, - more than 0.3% and less than 6.0% of h02, - more than 0.5% and less than 20.0% of Zr02, - more than 0.3% and less than 4.0% of Y203, - less than 2.0% in total, other oxides. In the above overall composition, the Al 2 O 3 content is preferably greater than 75% by weight, preferably greater than 80% by weight or even greater than 85% by weight. In the above overall composition, the TiO 2 content is preferably less than 4% by weight. In the above overall composition, the level of ZrO 2 is preferably less than 8% by weight, or even less than 6% by weight. According to a particular advantageous embodiment, the melted grains according to the invention have the following overall composition, on the basis of simple oxides: more than 85.0% and less than 97.0% of Al 2 O 3, more than 0.3% and less than 4.0% of h02, - more than 0.5% and less than 8.0% of Zr02, - more than 0.3% and less than 2.0% of Y203, - less a total of 2.0% of other oxides. Preferably, the grains according to the invention comprise less than 2% of silica 5iO 2, even less than 1%, or even less than 0.5%, or even less than 0.4%. Thus, according to one of its essential aspects, the invention relates to a ceramic material that can be obtained by sintering the melted grains as previously described (or a powder of said grains) at a temperature preferably between 1100.degree. ° C and 1500 ° C, optionally mixed with grains of alumina. The invention thus relates to refractory products, especially for use in the field of metallurgy, obtainable by sintering raw materials comprising or preferably constituted by the melted grains based on alumina according to the invention, said grains being sintered together to form said refractory materials.

According to another aspect, the invention relates to a ceramic material that can be obtained by consolidating the melted grains as described above (or a powder of said grains), at a temperature preferably between 500 ° C. and 1100 ° C. C, optionally mixed with grains of alumina. Without departing from the scope of the present invention, an alternative method of manufacturing a piece of ceramic material as previously described comprises the following steps: a) the mixture of raw materials, consisting mainly of an alumina powder and a powder of grains having the following composition, in weight percentages on the basis of the oxides: from 40 to 90% of ZrO 2, from 2 to 35% of H 2, from 2 to 20% of Y 2 O 3, less than 5% by weight, A1203, less than 2% impurities, said alumina powder and said grain powder together accounting for more than 95% by weight of said raw materials and the alumina powder representing more than 70% by weight of said mixture, b) a step of shaping said workpiece from the mixture of raw materials, optionally mixed with carbonaceous or organic materials such as graphite or phenolic resins, c) a heat treatment step at a temperature between e 500 ° C and 1500 ° C to obtain said ceramic piece. The following definitions are given: A "grain" is a grain obtained by a manufacturing process comprising at least one melting step, a solidification step and a dividing step, in particular by grinding, molding or any other equivalent known means. . A powder according to the invention is a set of grains according to the invention, the granulometry of which is adapted to a particular use. By "grain" is meant an individualized particle 10 especially within a powder. By "grain boundary" is meant the interface between two crystals of the polycrystalline structure of said grain. The term "fusion" of a mixture of precursors or oxides means a heat treatment at a temperature sufficiently high that all the constituents of the mixture are in the molten (liquid) state. Conventionally in the field of ceramics, the term "sintering" of a set of grains, a heat treatment allowing the junction and the development of their contact interfaces by movement of the atoms inside and between the grains, meaning given in ISO 836: 2001 (point 120). According to the invention, the sintering temperature of the melted grains is normally between 1100 ° C. and 1500 ° C., in particular between 1300 ° and 1500 ° C. Alternatively, consolidation means a thermal treatment of the grains at a more moderate temperature specific to the simple shaping of a ceramic part, without however strong bonds between the grain interfaces, as opposed to the sintering process previously described, the bond that can be provided by a binder, for example a phenolic resin.

According to the invention, the consolidation temperature of the melted grains is normally between 500 ° C. and 1100 ° C., in particular between 500 ° C. and 1100 ° C. The "median size" or median diameter of a set of particles, denoted D50, is defined as the size dividing the particles of this set into a first and a second population equal in mass, these first and second populations comprising only particles having a larger or smaller size, respectively, than said median size. The size of the grains is measured according to the well-known techniques of laser granulometry up to 20 micrometers and then by conventional sieving techniques beyond 20 micrometers. The invention and its advantages will be better understood on reading the following nonlimiting examples. In the examples, all percentages are given by weight. Examples: Comparative example 1 is a melted grain (that is to say obtained by electrofusion then grinding) constituted for more than 99% of its weight of alumina and median diameter D50 of the order of 5 microns . The example according to the invention is prepared from the necessary proportions of the following raw materials: Alumina AR75 comprising more than 98% of Al 2 O 3, marketed by Alcan, H 2 O, in rutile form marketed by Altichem comprising more than 98% of H02, - Zirconia with a purity level greater than 98.5% marketed under the reference CC10 by the company Saint-Gobain ZirPro, Yttrium oxide Y203 marketed by the company Altichem with a degree of purity greater than 99%. The mixture of the initial reactants thus obtained is electro-fired in an electric arc furnace, under air. The molten mixture is cast in ingot. The cooled ingot obtained is crushed and sieved to obtain a powder of melted grains whose average diameter D50 is equal to 4.8 micrometers as measured on a LA950V2 laser granulometer of Horiba® brand.

Structures: The samples according to Examples 1 and 2 are then analyzed. The overall chemical composition of the grains, indicated in weight percent based on the oxides, was determined by X-ray fluorescence. The results of the analyzes performed on each of the samples of Examples 1 and 2 are summarized in Table 1 which follows. A1203 ZrO 2 Y 2 O 3 TiO 2 SiO 2 Na 2 OO MgO Example 1 complement 1.02 <0.01 <0.01 <0.05 0.40 0.03 0.04 (comparative) Example 2 complement 6.39 0.70 2.80 0.08 0.09 0.06 0.11 (invention) Table 1 The qualitative analysis of the phases present in the melted grains according to Examples 1 and 2 is then carried out by conventional techniques using the microprobe of Castaing (in English electron microanalyser probe or EPMA).

More precisely, the grains are polished and analyzed by scanning electron microscope. The single figure attached shows the electronic plate corresponding to a melted grain according to Example 2 according to the invention.

The molten grain according to the invention consists of two oxide phases, main 1 and secondary 2 as indicated in the figure, the secondary phase being essentially present at the grain boundaries of the crystals of the corundum main phase. Analysis by Castaing microprobe of the different zones (phases) distinct from the grain according to the invention (Example 2) makes it possible to determine the respective compositions thereof, as shown in the figure. The results obtained, in weight percentages on the basis of simple oxides, are reported in Table 2 below. As also reported in Table 2, the weight percent of each phase in the material is determined by calculation, based on the overall chemical composition of the material as determined by X-ray fluorescence (see Table 1) and the composition of the main and secondary phases, as determined by microprobe analysis, the results of which are also reported in table 2 below.

Example 2 Main Phase Secondary (1) A1203 99.4 3.5-4.2 TiO2 0.5 26.8-30.3 Zr02 53-55.8 Y203 11.8-12.2% Weight in 92.5 7.5 the grain (calculated) Table 2 (1) Range of values obtained by the measurement at different points of the secondary phase, by microprobe analysis.

Properties: The properties of the materials obtained from grains according to the invention and compared are measured as follows: First, cylinders with a diameter of about 3 centimeters and a length of about 20 centimeters are synthesized. isostatic pressing and sintering of a powder of said grains by a heat treatment at 1650 ° C or 1470 ° C respectively for Example 1 and Example 2 for 4 hours. These different sintering temperatures make it possible to obtain sintered pieces having a comparable porosity of approximately 16% for the two examples. These sintered pieces are then machined with a series of bars of dimensions 45 mm x 4 mm x 3 mm for each of the two examples. To compare the properties of the materials constituting the bars thus obtained, the evolution of the modulus of rupture in flexion of said bars is compared with a thermal shock. More specifically, the thermal shock resistance of the materials is evaluated by placing said bars for 15 minutes in an oven preheated to 1250 ° C. and then removing these bars and immersing them immediately in water at ambient temperature (25 ° C.). The rupture modules (MOR) of the bars obtained from the melted grains according to Examples 1 and 2 are determined at room temperature in 3-point bending, according to standard NF EN 993-6. More precisely, for each example, the initial MOR without heat shock is measured from the first 5 bars and the residual MOR on a series of other bars having this time undergone thermal shock. The results reported in Table 2 below correspond to the average value found for each series of 5 bars. In Table 3 below, the value of residual flexural modulus of rupture after a thermal shock test is denoted "MOR res" and the variation of MOR measured at ambient temperature ("MOR res" - the initial MOR) is noted. "AMOR" in Table 3. A substantially identical or slightly positive value of the AMOR indicates that the mechanical strength of the material remained substantially unchanged due to thermal shock, a strongly negative value indicates that the mechanical strength of the material has greatly decreased following the Thermal shock. Example (inv.) 2 Example 1 (comp.) Initial MOR 17 149 MOR res. (MPa) 27 AMOR (MPa) +10 -129 Table 3 The results reported in Table 3 show that the material according to the invention (Example 2), obtained from molten grains according to the invention, exhibits a stability AMOR greatly improved compared to fused alumina corundum grains. The comparative sample is characterized by a very sharp decrease in its mechanical performance after thermal shock, unlike the sample according to the invention (Example 2). Finally, the sample according to the invention has a small variation in its mechanical strength after the thermal shock. In addition, its residual resistance 30 MOR is greater than that of the comparative sample.

Claims (16)

REVENDICATIONS1. Ceramic material, comprising: a main phase comprising more than 90% by weight of alumina in corundum form, a secondary phase having the following composition, in weight percent based on the oxides: o from 40 to 90% of ZrO 2, o 2 to 35% of hydrogen, o of 2 to 20% of Y 2 O 3, less than 5% of Al 2 O 3, less than 2% of impurities, the alumina phase representing more than 70% by weight of said material and said main phases and secondary representing together more than 95% of the weight of said material. 2. ceramic material according to the preceding claim, wherein the main phase comprises more than 95% by weight of alumina and preferably wherein the main phase comprises more than 99% by weight of alumina. 3. Ceramic material according to one of the preceding claims, wherein said alumina and secondary oxide phases together representing more than 98% of the weight of said material. 4. Ceramic material according to one of the preceding claims, wherein the alumina phase represents more than 80% of the weight of said material, or even more than 85% of the weight of said material. 5. Ceramic material according to one of the preceding claims, wherein the secondary phase represents more than 1% of the weight of said material. 6. ceramic material according to one of the preceding claims, wherein the secondary phase is less than 25% of the weight of said material. Ceramic material according to one of the preceding claims, having the following overall composition, based on single oxides: more than 70.0% and less than 97.0% Al 2 O 3, more than 0.3% and less than 6.0% of h02, - more than 0.5% and less than 20.0% of Zr02, - more than 0.3% and less than 4.0% of Y203, - less than 2.0 %, in total, other oxides. Ceramic material according to one of the preceding claims, having the following overall composition, based on simple oxides: - more than 85.0% and less than 97.0% Al 2 O 3, - more than 0.3% and less than 4.0% of h02, - more than 0.5% and less than 8.0% of Zr02, - more than 0.3% and less than 2.0% of Y203, - less than 2.0 %, in total, other oxides. 9. A method of manufacturing a piece of ceramic material according to one of the preceding claims, comprising: - the mixture of raw materials consisting of aluminum oxide, titanium oxide, zirconium oxide, yttrium oxide or precursors of said oxides, in the following respective proportions, on the basis of the oxides: - more than 70.0% and less than 97.0% of A1203, - more than 0.3% and less than 6.0% of h02, - more than 0.5% and less than 20.0% of Zr02, - more than 0.3% and less than 4.0% of Y203, - less than 2.0%, total, other oxides. the complete melting of said mixture, obtaining from said melted mixture of a powder of melted grains by steps comprising: cooling the molten mixture to room temperature, dividing the melted mixture into individualized grains a step of shaping said workpiece from the powder of melted grains, optionally mixed with carbonaceous or organic materials such as graphite or phenolic resins, a heat treatment step at a temperature of between 500 ° C. C and 1500 ° C to obtain said refractory ceramic piece. 10. A method of manufacturing a piece of ceramic material according to one of claims 1 to 8, comprising: - the mixture of raw materials, consisting mainly of an alumina powder and a grain powder having the following composition, in weight percentages on the basis of the oxides: - from 40 to 90% of ZrO 2, - from 2 to 35% of h 2, - from 2 to 20% of Y 2 O 2, - less than 5% of Al 2 O 2, - less than 2% impurities, said alumina powder and said powder of grains together representing more than 95% by weight of said raw materials and the alumina powder representing more than 701 wt. of said mixture, - a step of shaping said piece from the mixture of raw materials, optionally mixed with carbonaceous or organic materials such as graphite or phenolic resins, a heat treatment step at a temperature between 500 ° C and 1500 ° C to obtain said ceramic part. 10 11. Melted grains, especially in the form of a powder, comprising essentially: a main oxide phase comprising more than 90% by weight of alumina in corundum form, a secondary oxide phase having the following composition, in weight percentages on the basis of oxides: from 40 to 90% of ZrO 2, from 2 to 35% of TiO 2, from 20 to 20% of Y 2 O 3, less than 5% of Al 2 O 3, less than 2% of impurities, alumina and secondary oxide together represent more than 95% of the weight of the grains and said alumina phase representing more than 70% of the weight of said grains. 12. Melted grains according to the preceding claim, wherein the secondary phase is essentially present in said grains at the grain boundaries of the crystallites of the main phase. 13. Melted grains according to one of claims 11 or 12, wherein said composition further meets the following general formula: wherein Y 22 0.4 x + z) is the weight percentage is the weight percentage z is the percentage weight of TiO 2, Y 2 O 3, ZrO 2. 14. Melted grains according to the preceding claim, wherein said composition further meets the following general formula: 10 t 0.1 x (y + z) 15. Melted grains, according to one of claims 11 to 14, having the following overall chemical composition, in weight percentages on the basis of the oxides: greater than 70.0% and less than 97.0% of Al 2 O 3 , - more than 0.3% and less than 6.0% of T102, - more than 0.5% and less than 20.0% of Zr02, - more than 0.3% and less than 4.0% of Y203, 20 - less than 2.0%, in total, other oxides. 16. Melted grains according to one of claims 11 to 15, having the following, in oxides: more than - more than - more than - of the overall chemical composition weight percentages on the basis of 85.0% and less of 97.0% Al 2 O 3, 0.3% and less than 4.0% TiO 2, 0.5% and less than 8.0% ZrO 2, 0.3% and less than 2.0% Y 2 O 3 - less than 2.0%, in total, of other oxides,