ALUMINUM-RESISTANT TITANIUM DIBORIDE ARTICLES AND METHOD OF MAKING SAME

Abstract: This invention deals with aluminum-resistant titanium diboride and method of making same. High strength titanium diboride (TiB₂) articles which are obtained by sintering articles preformed from TiB₂ powder mixed with epoxy resin by means of isostatic molding using sintering temperatures found to be a critical variable for achieving maximum strength. Resistance to aluminum attack is achieved by removing contaminating oxides, nitrides and carbides of titanium by selecting either annealing the sintered article in a boron nitride (BN) pack or by including a small amount of BN or elemental boron in the original TiB₂ powder, such that the finished article is substantially free of such con­ taminants.

Description: ALUMINUM-RESISTANT TITANIUM DIORIDE ARTICLES AND METHOD OF MAKING SAME FIELD OF THE INVENTION This invention relates generally to shaped titanium diboride (TiB2) articles and more particularly to shapes suitable for use in Hall cells due to their improved resistance to attack by aluminum. Aluminum metal is commonly obtained from cryolite or bauxite by their electrolysis in a Hall cell. The aluminum industry has been developing improvements in the Hall cell over the years. Baffles and other devices are used in the pool of molten aluminum employed in Hall cells to reduce or eliminate waves or other disruptions in the surface. These disruptions are caused by eddy currents and other effects, both physical and electromagnetic, in the bath. The smoother interface thus produced allows for a reduced gap between the anode and the cathode, resulting in lower power costs. the material of construction to be used in the aluminumwetted portions of the cell must be electrically conductive, °°hanically strong, thermally and chemically stable in the Hall cell environment, and resistant to thermal shock. Theoretically, fiB2 is a suitable material, but tests have shown that the material -ssMerçialJy mrnufactzured lacks stability in the Hall cell eDvironment'. The.aluminum-wetted portions of the cell are the bottom and walls of the cell and any protrusions or inlays in or On. te cel bottom, which is the cathodic contact to the molten aluminum. STATEMENT OF THE PRIOR ART The use of TiB2 tiles in aluminum reduction cells is discussed in U.S. Patents Nos. 4,231,853 and 4,341,611; in both of These patents, the process for making the tiles is described as a hot-pression operation and no mention is made of the damaging cubic phase contaminants addressed by the present invention. Purification of TiB2 powder by mixing the powder with a boron-supplying compound, such as BN, and heating the mixture in a crucible to remove the cubic phase composed of borides, oxides, nitrides, and carbides by reaction with boron is taught in U.S. Patent No. 3,316,062, but the patent is directed to powders and does not teach the purification of formed bodies. Undesirable contamination may be introduced during the forming and sintering operations, even if powders purified by following this prior art are used. OBJECTS AND SUMMARY OF THE INVENTION The principal object of this invention is to provide an article of the disclo;se & type which combines simplicity, strength and durability in a fi gh degree together with inexpensiveness of construction and a method of making the same. Other objects of the invention will in part be obvious and in part hereinafter pointed out. In summary, this invention resides in high strength titanium diboride (TiB2) articles which are obtained by sintering articles preformed from TiB2 powder mixed with epoxy resin by means of isostatic molding using sintering temperatures found to be a critical variable for achieving maximum strength and wherein resistance to aluminum attack is achieved by removing contaminating oxides, nitrides, and carbides of titanium by selecting either annealing the sintered article in a boron nitride (BN) pack or by including a small amount of BN or elemental boron in the original TiB2 powder, such that the finished article is substantially free of such contaminants. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow diagram illustrating the method of the invention; FIG. 2 is a flow diagram illustrating a modified version of the invention; FIGS. 3a, 3b, 3c, and 3d are a group of scanning electron microscope (SEM) photos enlarged 5,000 and 20,000 times showing the microstructures of sintered Tii shapes not following this invention after 28 and 72 hours in contact with molten aluminum. FIGS. 4a and 4b are a pair of photomicrographs enlarged 2,000 and 5,000 times and showing the microstructures of TiW articles sintered and annealed according to this invention after 100 hours in contact with molten aluminum. FIG. 5 is a graph, the curve of which shows the flexural strength of TiB2 at 250C versus the sinter temperature that demonstrates the criticality of sinter temperature whether this invention is used or not. The data shown were obtained while not using any form of the present invention. FIG. 6 is a graph, the curve of which shows the electrical resistivity of TiB2 at 250C versus sinter temperature. The data shown were obtained while not using any form of the present invention. DETAILED DESCRIPTION OF THE INVENTION The present invention is based on the unexpected and unobvious discoveries that: (i) failure in the use of TiB2 structures in the Hall cells was due to the presence therein of separate phase consisting of cubic crystals of titanium carbide (TiC), titanium nitride (TiN), titanium oxide (TiO), or mixtures thereof. (ii) there is a best sintering temperature; (iii) a formed and sintered TiB2 article can be purified by a thermal treatment in a BN pack; and (iv) the discovery that a purified article can also be made by including BN in the TiB2 blend prior to forming and intering. The articles of the invention can be made by annealing at high temperature in a boron nitride (BN) pack a sintered article containing the harmful impurities. The impurities are converted to TiB2 in situ, but the annealing operation causes an unavoidable loss in strength. It has been discovered that the initial sintering temperature of the TiB2 article should be 2000"C+ 250C for maximum strength of the finished product. In practice, however, a product can be made at initial sintering temperatures ranging from about 1600"C to about 21000C if strength and conductivity can be sacrificed. The TiB2 articles of the invention can also be obtained by combining a small amount of BN in the powdered TiB2 blend prior to the forming and sintering of the article. The amount of BN used is calculated from the amount of impurity in the powder, so that little or no residual BN is present is the finished article after sintering. This method does not require any additional annealing after sintering, Referring to the flow diagrams of FIGS. 1 and 2, the method of the invention comprises mixing from about 9O.5 to about 97.5 weight percent of TiB2 powder, preferably having a particle size ranging from about 2.5 to about 3.0 microns with from about 2.0 to about 3.5 weight percent of a sintering aid such as chromium diboride; adding to the resulting blend an amount of from about 2.9 to about 3.0 of an epoxy binder and a catalyst such as triethanolamine, isostatically molding the mixture, followed by sintering at 2000 C+ 25"C in a particle pack comprising sintered crushed TiB2 under an inert atmosphere, suitably argon, or high vacuum (pressures below about 5 x 10 3 Torr) with argon under high vacuum preferred. In the embodiment of FIG. 1, the shape is annealed at about 18000C while in that of FIG. 2 from about 0.3 to about 1.2 weight percent of BN powder is added to the starting blend to remove the impurities. Thus it will be seen that the sintered articles of the invention can be annealed at 18000C in a BN pack in an argon atmosphere, to convert any oxide, nitride, and carbide of titanium to the diboride (TiB2). In practice, one may use annealing temperatures in the range pf about 1700 to 19000C with 1800 C being the optimum. The loss of strength associated with this annealing process amplifies the need for maximizing the strength during sintering. Alternatively, a small amount of BN or elemental boron can be blended with the TiB2 powder before the article is formed by isostatic pressing. The BN or boron then reacts with the impurities during the sintering operation. This is to be contrasted with TiBz-BN composites wherein large amounts of BN are admixed with TiB2. In the present case when BN is employed it is present only as a minor component that is essentially completely consumed by reaction with the impurities during sintering. This fact is important to the production of a TiB2 product having low resistivity as the presence of residual BN increases the resistivity of the product. In accordance with the foregoing, a TiB2 shape was developed by changing the sintering temperature to improve the structure. The physical properties are shown in Table I, where a material made by using the reduced sintering temperature of this invention is compared with a prior art material that is the same except for sintering temperature. TABLE 1 Comparative Values of Physical Properties Made By Using The Sintering Temperature Physical Made By of this Properties Prior Art Invention Change 1. Flexural Strength a) 25"C 192 MPa 413 MPa 115% higher b) 1000"C 103 MPa 148 MPa 44% higher 2. Sonic Modulus (GPa), 250C 528 700 32% higher 3. Electrical flesistivity (r,cn), =50C 0.60 0.18 70% less 4. Density, % of Theoretical 94.9 94.6 5. Coefficient of Thermal Expansion (oC)-1 x 10-6 8.2 7.8 5% lower 6. Micro Cracks Some None Improved From Table I, it can be seen that strength is improved dramatically and microcracks are eliminated. The improved strength results in greater resistance to thermal shock, a property that is essential in the Hall cell application. Thermal shock tests to 675"C at 1500/minute and to 8100C at 210 C/minute showed no cracking due to thermal shock. An added advantage can be gained by removing impurities from the TiB2. The common impurities are TiO, TiN, and TiC. When the TiB2 with these impurities is exposed to molten aluminum, aluminum oxide, aluminum nitride, and aluminum carbide are formed. The crystal size for these reaction products is larger than the titanium compounds so the structure of the TiB2 body is disrupted and microcracks form. This is believed to be the principal mechanism of aluminum penetration and attack. The subject invention is further illustrated in a nonlimiting fashion by the following examples. EXAMPLE I Purification by Annealing Ninety-seven grams of TiB2 powder (98.4% pure) was ballmilled with 3 g CrB2 powder (99.5% pure) for 96 hours or until the particle size of the combination was 2.5 sum or finer, as measured on a Fisher Sub-Sieve Sizer. The ball-milled powder was mixed in a Hobart mixer with a solution of 33 cm3 acetone, 3 g Epoxy ERL 3794 (an epoxy resin product of Union Carbide Corporation) and 0.3 g triethanolamine until all the acetone evaporated. The mix was dried in an oven at 65.60C overnight and then micro-milled (i.e., milled using a swing-hammer mill) to break agglomerates. The micro-milled powder was then isostatically molded into testing shapes, cured in sand to 185"C in 24 hours in a conventional oven and then sintered in an induction vacuum furnace at 2000"C in 2 hours under an argon flow rate of 170 dm3/h (6 SCFH). The shapes were sintered in sintered crushed TiB2 pack to support the shapes to keep them from warping and to provide a uniform atmosphere. The sintered shapes had densities equivalent to 98% of the theoretical value. Finally, the testing shapes were annealed in a resistance heated tube furnace at 1800"C in 2 hours under an argon flow rate of 142 dm3/h (5 scFs). The shapes were packed entirely in BN powder pack in a BN tube for che annealing process. Ten- TiB2 samples (25.4 mm x 6.35 mm x 6.35 mm) sintered to 20000C in two hours and ten samples (25.4 mm x 6.35 mm x 6.35 mm) sintered to 20000C in two hours followed by annealing to 18000C for two hours were evaluated in molten aluminum under the electrolytic action of a Hall cell. The samples were immersed in molten aluminum as described in Table II: TABLE II Hers. in No. of Samples Process Molten Aluminum 4 Sintered 75* 6 Sintered 28 8 Sintered & 100 Annealed * These four samples were immersed in molten aluminum for 28 hours at first; they werè removsed*hd exposed to molten'aluminum again for another 47 hours. The samples that were exposed to aluminum were each soaked in 100 cm3 of a solution made up of 50 vol. % concentrated hydrochloric acid and 50 vol. % deionized water for 24 hours to dissolve aluminum from the surface that might affect the results of the measurement of physical properties of the samples. The physical properties of the tested samples are listed in Table III: TABLE III Density Flexural Strength(MPa) Sonic Percent of Room Resist. Modulus Samples Theoretical Temp. 1000"C (Sin) (GPa) Sintered 94.7 322 133 0.25 533 Sintered 93.3 284 173 0.25 518 and Annealed in BN Pack As can be seen from Table III, the annealing process reduces flexural strength somewhat. As this appears to be unavoidable, it is all the more important that the optimum sintering temperature have been used in making the sample to be annealed in SN pack for purification. Electron microscopy was used on representative samples of the materials in Table III to try to detect aluminum. The photographs in FIGS. 3a, 3b, 3c, and 3d show that aluminum concentrates at the grain boundaries between the TiB2 particles, showing up as fillets between grains after 28 and 75 hours exposure. This aluminum remains although the sample was soaked in the hydrochloric acid solution, which removed only surface aluminum. The samples shown in FIG. 3 were not purified according to this invention. In contrast to FIGS. 3a, 3b, 3c, and 3d, FIGS. 4a, and 4b show no evidence of aluminum at the grain boundaries or alsewhere in the samples that were annealed in the BN pack after sintering. The aluminum has not penetrated from the molten bath in 100 hours of exposure. EXAMPLE 2 Purification by Using BN Powder Ninety-seven grams of TiB2 powder (98.7% pure) was ball-milled with 3 g CrB2 powder (99.5% pure) and 0.5 g BN powder (oxygen-free powder) for 96 hours or until the particle size of the combination was 2.5 um or finer as measured on a Fisher Sub-Sieve Sizer. The ball-milled powder was mixed in a Hobart mixer with a solution of 33 cm3 acetone, 3 g Epoxy ERL 3794 and 0.3 g triethanolamine until all the acetone evaporated. The mix was dried in an oven afterward at 65.6"C overnight and then micro-milled to break up agglomerates. The micro-milled powder was then isostatically molded into testing shapes, cured in sand to 185"C in 24 hours in a conventional oven and then sintered in an induction furnace at 2000"C in two hours under an argon flow rate of 113 dm3/h (4SCFH). The shapes were sintered in sintered crushed TiB2 pack to provide uniform atmosphere and to support the shapes to prevent warping. The sintered shapes had densities equivalent to 98 pf tje theoretical value and were free of cubic phases. Fifty plates measuring nominally 127 mm x 127 mm x 9.5 mm were manufactured as above. The density of test samples cut from these plates were all in the range of 97% to 98% of the theoretical value. Representative samples of these plates were analyzed for nitrogen, carbon and oxygen with the results shown in Table IV: TABLE IV CHEMICAL ANALYSIS N2 (Wt. %) C (wot. %) 2 (Wt. %) Sample A 0.000 0.130 0.820 Sample B 0.000 0.360 0.600 Sample C 0.000 0.290 0.610 Average 0.000 0.260 0.677 Analysis of representative samples by X-ray diffraction techniques detected no measurable cubic phase (TiO, TiN, TiB, or TiC) in the material produced. Analysis of the TiB2 powder from which these plates were made showed the following contaminants (before addition of the BN powder): 0.4 weight % nitrogen; 0.5 weight % carbon; and 1.3 weight % oxygen. At these concentrations, the contaminants would generate sufficient cubic phase to be readily detected by using X-ray diffraction techniques. The graph of FIG. 5 shows that a sintering temperature of 2000"C+ 25"C is optimum. Higher temperatures cause drastic loss of strength and lower temperatures do not develop sufficient strength. Electrical resistivity is another important variable in TiB2 articles. FIG. 6 shows that the resistivity is minimum at the preferred sintering temperature of 2000"C. The use of higher temperatures is detrimental and results in drastically increased resistivity. The article of the invention has been thoroughly tested under actual use conditions and has been found to be completely successful for the accomplishment of the above stated objects of the invention. The practice of the invention hereinabove described will be evident to those skilled in the art to which it relates from a consideration of the foregoing. It will thus be seen that there is provided an article in which the several objects of this invention are achieved, and which is well adapted to meet the conditions of practical use.

Claims: CLAIMS 1. A method for making shaped TiB2 articles comprising: preforming said articles from finely divided TiB2 and a sintering aid; sintering said articles at a temperature of between about 16000C and about 2100"C; and removing from said articles substantially all of the TiO, TiN and TiC impurities present therein. 2. The method of claim 1 wherein said sintering temperature is between about 1975"C and about 2025"C. 3. The method of claim 1 or 2 wherein said removing is effectuated by annealing the sintered shape under an inert atmosphere at a temperature of from about 1700 to about 1900"C. 4. The method of claim 1 or 2 wherein said removing is effectuated by incorporating with said TiB2 and said sintering aid from about 0.3 to about 1.2 weight percent of finely divided powdered material selected from the group consisting of BN and elemental boron. 5. The method of claim 3 wherein said annealing is effectuated with said sintered shape in a BN pack. 6. The method of claim 1, 2, 3, 4, or 5 wherein said preforming is effectuated by isostatic molding. 7. An aluminum-resistant shaped TiB2 article suitable for use in a Hall cell made by the method of claim 1, 2, 3, or 4. 8. In a Hall cell of conventional design, the improvement consisting in the use therein of the shaped TiB2 article of claim 7.

Inventors: PARRISH, LIEN NGUYEN
Applicants: UNION CARBIDE CORPORATION
Application year: 1984
Main classification: C04B35
IPC codes: C25C3, C04B35