Method of Making Aluminum-Resistant Titanium Diboride Articles

This invention relates generally to shaped titanium diboride (TiB.sub.2) 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 aluminum-wetted portions of the cell must be electrically conductive, mechanically strong, thermally and chemically stable in the Hall cell environment, and resistant to thermal shock. Theoretically, TiB.sub.2 is a suitable material, but tests have shown that the material as commercially manufactured lacks stability in the Hall cell environment. The aluminum-wetted portions of the cell are the bottom and walls of the cell and any protrusions or inlays in or on the cell bottom, which is the cathodic contact to the molten aluminum.

The use of TiB.sub.2 tiles in aluminum reduction cells is discussed in U.S. Pat. Nos. 4,231,853 and 4,341,611; in both of these patents, the process for making the tiles is described as a hot-pressing operation and no mention is made of the damaging cubic phase contaminants addressed by the present invention.

Purification of TiB.sub.2 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. Pat. 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.

The principal object of this invention is to provide an article of the disclosed type which combines simplicity, strength and durability in a high 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 (TiB.sub.2) articles which are obtained by sintering articles preformed from TiB.sub.2 powder mixed with epoxy resin by means of isostatic molding using sintering temperatures found to be a critical variable for achieving maximum strength and wherin 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 TiB.sub.2 powder, such that the finished article is substantially free of such contaminants.

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 TiB.sub.2 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 TiB.sub.2 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 TiB.sub.2 at 25.degree. C. 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 TiB.sub.2 at 25.degree. C. versus sinter temperature. The data shown were obtained while not using any form of the present invention.

The present invention is based on the unexpected and unobvious discoveries that:

(i) failure in the use of TiB.sub.2 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 TiB.sub.2 article can be purified by a thermal treatment in a BN pack; and

(iv) the discover that a purified article can also be made by including BN in the TiB.sub.2 blend prior to forming and sintering.

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 TiB.sub.2 in situ, but the annealing operation causes an unavoidable loss in strength. It has been discovered that the initial sintering temperature of the TiB.sub.2 article should be 2000.degree. C..+-.25.degree. C. for maximum strength of the finished product. In practice, however, a product can be made at initial sintering temperatures ranging from about 1600.degree. C. to about 2100.degree. C. if strength and conductivity can be sacrificed. The TiB.sub.2 articles of the invention can also be obtained by combining a small amount of BN in the powdered TiB.sub.2 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 in 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 96.5 to about 97.5 weight percent of TiB.sub.2 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.degree. C..+-.25.degree. C. in a particle pack comprising sintered crushed TiB.sub.2 under an inert atmosphere, suitably argon, or high vacuum (pressures below about 5.times.10.sup.-3 Torr) with argon under high vacuum preferred.

In the embodiment of FIG. 1, the shape is annealed at about 1800.degree. C. 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 1800.degree. C. in a BN pack in an argon atmosphere, to convert any oxide, nitride, and carbide of titanium to the diboride (TiB.sub.2). In practice, one may use annealing temperatures in the range of about 1700.degree. to 1900.degree. C., with 1800.degree. 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 TiB.sub.2 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 TiB.sub.2 --BN composites wherein large amounts of BN are admixed with TiB.sub.2. 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 TiB.sub.2 product having low resistivity as the presence of residual BN increases the resistivity of the product.

In accordance with the foregoing, a TiB.sub.2 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.

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.degree. C. at 150.degree./minute and to 810.degree. C. at 210.degree. C./minute showed no cracking due to thermal shock.

An added advantage can be gained by removing impurities from the TiB.sub.2. The common impurities are TiO, TiN, and TiC. When the TiB.sub.2 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 TiB.sub.2 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.

Ninety-seven grams of TiB.sub.2 powder (98.4% pure) was ball-milled with 3 g CrB.sub.2 powder (99.5% pure) for 96 hours or until the particle size of the combination was 2.5 .mu.m 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 cc 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 150.degree. F. overnight and then micro-milled (i.e., milled using a swing-hammer mill) to break up agglomerates. The micro-milled powder was then isostatically molded into testing shapes, cured in sand to 365.degree. F. in 24 hours in a conventional oven and then sintered in an induction vacuum furnace at 2000.degree. C. in 2 hours under an argon flow rate of 6 SCFH. The shapes were sintered in sintered crushed TiB.sub.2 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.degree. C. in 2 hours under an argon flow rate of 5 SCFH. The shapes were packed entirely in BN powder pack in a BN tube for the annealing process.

Ten TiB.sub.2 samples (2".times.1/4".times.1/4") sintered to 2000.degree. C. in two hours and ten samples (2".times.1/4".times.1/4") sintered to 2000.degree. C. in two hours followed by annealing to 1800.degree. C. 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:

The samples that were exposed to aluminum were each soaked in 100 ml 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.

As can be seen from Table III, the annealing process reduces room temperature 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 BN 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 TiB.sub.2 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 elsewhere 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.

Ninety-seven grams of TiB.sub.2 powder (98.7% pure) was ball-milled with 3 g CrB.sub.2 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 .mu.m 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 cc 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 150.degree. F. overnight and then micro-milled to break up agglomerates. The micro-milled powder was then isostatically molded into testing shapes, cured in sand to 365.degree. F. in 24 hours in a conventional oven and then sintered in an induction furnace at 2000.degree. C. in two hours under an argon flow rate of 4 SCFH. The shapes were sintered in sintered crushed TiB.sub.2 pack to provide uniform atmosphere and to support the shapes to prevent warping. The sintered shapes had densities equivalent to 98% of the theoretical value and were free of cubic phases.

Fifty plates measuring nominally 5".times.5".times.3/8" 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.

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 TiB.sub.2 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.degree. C..+-.25.degree. 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 TiB.sub.2 articles. FIG. 6 shows that the resistivity is minimum at the preferred sintering temperature of 2000.degree. 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.