Classifications

• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
  • C04B35/58—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides
  • C04B35/5805—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on borides
  • C04B35/58064—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on borides based on refractory borides
  • C04B35/58071—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on borides based on refractory borides based on titanium borides
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
  • C04B35/52—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbon, e.g. graphite
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
  • C04B35/52—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbon, e.g. graphite
  • C04B35/528—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbon, e.g. graphite obtained from carbonaceous particles with or without other non-organic components
  • C04B35/532—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbon, e.g. graphite obtained from carbonaceous particles with or without other non-organic components containing a carbonisable binder
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/622—Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/64—Burning or sintering processes
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/622—Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/64—Burning or sintering processes
  • C04B35/645—Pressure sintering
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes

Definitions

• Refractory hard metals as a class are hard, dense materials with high melting points, and are generally of low solubility and resistant to corrosive attack by most acids and alkalis.
• RHMs have high electrical conductivity due to their metallic structure; consequently, this combination of properties has made them candidates for use as electrodes in molten salt electrolysis processes where their corrosion resistance and conductivity are vital properties needed for economical performance.
• the RHMs have other properties which have limited their usage up to the present time. Tney are usually brittle, haye little resistance to thermal shock, and are quite expensive to produce and fabricate into useful articles.
• RHM articles have been produced by a number of processes including hot pressing of the granular or powdered materials, chemical vapor deposition, and in situ reduction of metals by carbon or other reducing agents.
• Hot pressing is the most commonly used process for production of shapes.
• a die and cavity mold set is filled with powder, heated to about 300 ⁇ -800 ⁇ C and placed under pressure of about 2 x 10 8 Pa, then removed from the mold and heated at about 1500 ⁇ -2000 ⁇ C, or higher, or sintered in the mold.
• Hot pressing has the limitations of applicability to simple shapes only, erosion of the mold, and slow production.
• the pieces produced by hot pressing are subject to a high percentage of breakage in handling, making this process expensive in terms of yield of useful products.
• the RHMs of most interest include the carbides, borides, and nitrides of the metals of Iy ⁇ , IVB, VB, and VIB of the periodic table, particularly Ti, V, Si, and W.
• the borides are of most interest as electrodes in high temperature electrolysis applications due to their electrical conductivity, and of the borides, TiB 2 has been extensively investigated for use as a cathode or cathodic element in the Hall-Heroult cell.
• the Hall cell is a shallow vessel, with the floor forming the cathode, the side walls a rammed coke-pitch mixture, and the anode a block suspended in the molten cryolite bath at an anodecathode separation of a few centimeters.
• the anode is typically formed from a pitch-calcined petroleum coke blend, prebaked to form a monolithic block of amorphous carbon.
• the cathode is typically formed from a pre-baked pitch-calcined anthracite or coke blend, with cast-in-place iron over steel bar electrical conductors in grooves in the bottom side of the cathode.
• the anode-cathode spacing is usually about 4-5 cm., and attempts to lower this distance result in an electrical discharge from the cathode to the anode through aluminum droplets.
• the molten aluminum is present as a pad in the. cell, but is not a quiescent pool due to the factors of preferential wetting of the carbon cathode surface by the cryolite melt in relation to the molten aluminum, causing the aluminum to form droplets, and the erratic moyements of the molten aluminum from the strong electromagnetic forces generated by the high current density.
• the wetting of a solid surface in contact with two immisci ble liquids is a function of the surface free energy of the three surfaces, in which the carbon cathode is a low energy surface and consequently is not readily wet by the liquid aluminum.
• the angle of a droplet of aluminum at the cryolite-aluminum-carbon junction is governed by the relationship where ⁇ 12 , ⁇ 13 , and ⁇ 23 are the surface free energies at the aluminum carbon, cryolite-carbon, and cryolite-aluminum boundaries, respectively. If the cathode were a high energy surface, such as would occur if it were a ceramic instead of carbon, it would have a higher contact angle and better wettability with the liquid aluminum.
• amorphous carbon is a low energy surface, but also is quite durable, lasting for several years duration as a cathode, and relatively inexpensive.
• a cathode or a cathode component such as a TiB 2 stud which has better wettability and would permit closer anode-cathode spacing could improve the thermodynamic efficiency and be very cost-effective.
• Titanium Diboride, TiB 2 has been proposed for use as a cathodic element in Hall cells, giving an improved performance oyer the amorphous carbon and semi-graphite cathodes presently used.
• Titanium Diboride (TiB 2 ) was useful as a cathode component in the electrolytic production of aluminum, when retrofitted in the Hall cell as a replacement for the carbon or semi-graphite form.
• the electrical efficiency of the cell was improved due to better conductivity, due mainly to a closer anodecathode spacing; and the corrosion resistance was improved, probably due to increased hardness, and lowex solubility and chemical inertness as compared to the carbon and graphite, forms. If the anode-cathode (A-C) distance could be lowered, the % savings in electricity would be as follows:
• TiB 2 as a Hall cell cathode
• the principal deterrent to the use of TiB 2 as a Hall cell cathode has been the sensitivity to thermal shock and the great cost, approximately $25/lb. as compared to the traditional carbonaceous compositions, which cost about $0.60/lb.
• the method is markedly more economical, and also produces an unexpectedly improved cathode when its performance is compared to the traditional carbonaceous material.
• Our method of producing TiB 2 articles involves a sintering process in which powdered TiB 2 , other RHMs or mixture of a KBM and carbon powder are simply poured or packed vertically into a mold with slightly larger dimensions than the desired article, and then fired in a con trolled atmosphere to the sintering temperature for the particular
• Our process has the advantages of savings of time, capital investment, and operating costs due to the fewer operations, improved yield, less wear on the equipment by abrasive RHMs, lower density and ability to use low cost fillers, adaptability to automated production, and less critical control needed for heating and cooling rates.
• the articles produced by our process have improved thermal and mechanical shock resistance and a more active surface area, when compared to conventionally pressed and sintered pieces.
• a core with lower strength and more elasticity than the sintered TiB 2 may be used to form a composite mechanically bonded article.
• the article formed may be further treated by impregnation with a carbonizable. binder, baked, and graphitized to form an impervious carbon-TiB 2 structure.
• the carbon particulate matter found most useful includes fine particle size graphite, calcined petroleum coke, metallurgical coke, and wood charcoal.
• Impregnating carbonizable binders that are useful include petroleum and coal tar pitches, phenolic type condensation resins, vegetable pitches, and lignosulfonates from wood.
• a TiB 2 shape after impregnation with a carbonizable impregnant we find that we obtain a higher coke yield (75-80%) from the impregnant than when baking a similar shape of baked carbon particu lates after impregnation with the same impregnant (70-75%), apparently due to a catalytic effect by the TiB 2 during the coking reaction.
• a mixture of dif ferent sized particles will normally sinter to a higher density piece than one of uniformly sized particles.
• the strength, density, electrical conductivity, chemical resistance, and other parameters can be controlled by varying the particle sizes and mixtures, heat treating temperature, impregnation, and graphitization processes.
• Figure 1 is a yertical view of a cylinder of 100% TiB 2 processed in argon for 2 hours at 2615 + 15°C with a maximum temperature of 2630°C.
• the TiB has partially melted and reacted with the mold as shown in Figure 2 in a cutaway view.
• Figure 3 is a cylinder of 100% TiB 2 processed for 2 hours @ 2490° + 40°C with a maximum temperature of 2530°C in argon, showing fusion of the granules.
• Figure 4 is a sintered molding of 100% TiB 2 processed @ 2450°C + 50°C in argon for 1.5 hours
• Figure 5 is a section of the same piece in the mold. There is some slight eyidence of attack on the graphite mold, leading to the conclusion that the ⁇ upper limit of temperature for this particular raw material and process is slightly less than 2450° or probably about 2400°C.
• Figure 6 is a piece of 100% TiB 2 sintered in argon for 2 hours @ 2260°C + 50°C,and Figure 7 shows the article taken out of the mold.
• Figure 8 is a piece of 100% TiB 2 processed for 3 hours @ 2200°
• Figure 9 is the article out of the mold.
• the filled cavity was 2.86 cm in diameter and the finished piece was 2.41 cm in diameter, for a shrinkage of approximately 15%, although shrinkage
• the method is generally adaptable to any of the various processes for the production of RHM containing articles at temperatures over approximately 2000°C.
• These processes include sintering of either loose filled or pressed articles; the heat treatment of mixtures of a RHM and carbonaceous materials, e.g., TiB 2 plasticized with coal tar pitch; heat treatment of reactants to form the RHM in situ, e.g., B 4 C + C + TiO 2 ; RHM deposited by chemical vapor deposition, e.g., TiCl 4 + 2 BCl 3 + 5 H 2 TiB 2 + 10 HCl.
• Powdered TiB 2 is poured into a mold and heated to a sintering temperature of 1700°-2400°C in an inert atmosphere, preferably argon, for about 1 to 4 hours.
• a graphite mold may be used and is the preferred material of construction.
• TiB 2 of the proximate analysis below is useful: Ti - 69.06% B - 31.24% C - 2270 P.P.M. (parts per millon) O 2 - 3490 P.P.M. N 2 - 150 P.P.M.
• the particle size mean is approximately 7 ⁇ , with 90% falling in the 5-15 ⁇ range and none over 44 ⁇ .
• the articles produced by this method have unexpectedly good resistance to thermal and mechanical shocks, probably due to the point contact bonding of the particles and the porosity.
• This porosity may also be filled by impregnants such as pitch or phenolic resins if special enhanced properties of strength, thermal shock resistance or non-porosity are required.
• a petroleum pitch having a softening point from 110°-120°C is the preferred impregnant, applied under alternate cycles of vacuum and pressure at 175°-250° C and 2-15 x 10 5 Pa, baked on a cycle rising to 700°-1100° C over a period of 1 to 10 days, then further heated to about 2000°-2400°C to graphitize the carbonized residue.
• the process in general comprises gravity filling a mold with the RHM powder, with a small amount of vibration sometimes needed to eliminate voids and air pockets.
• the mold is then heated to the sintering temperature in a controlled atmosphere and held there for a period of about one to four hours.
• the atmosphere is generally an inert gas, and preferably a noble gas such as argon. If the RHM being formed is a nitride, nitrogen may be used, but may react undesirably with borides and carbides to form the nitrides.
• a piece (Figure 19) was made from a mixture of 15% graphite flour, having a particle size similar to the TiB 2 powder, and 85% TiB by wt., sintered in a graphite mold in argon at 1900°C. The piece was unloaded from the furnace after 16 hours cooling time, and was strong and homogenous in appearance.
• a mold was filled with a mixture of the graphite flour described in Example 1 and TiB 2 at a 50/50 wt. ratio. It did not sinter at 1900°C in argon. About 20-30% graphite by wt. appears to be a maximum concentration for this system.
• a sample of TiB 2 powder was sintered to form a strong article when processed in argon @ 1700°-2400°C.
• a series of cylinders treated at temperatures in this range had a positive correlation of apparent density (A.D.) with sintering temperature:
• the pieces were unloaded from the furnace after about 40 hours cooling time.
• EXAMPLE 5 A TiB 2 piece with a graphite core was made by placing a preformed core insert of Great Lakes Carbon H-303 graphite with the following characteristics into a graphite mold:
• Powdered TiB 2 was poured into the mold, the mold placed in an induction furnace, and heat treated at 1900°C in argon.
• the sample piece was sintered successfully ( Figures 12, 13), with the following parameters:
• Example 5 The same type of core used in Example 5 was used, with the mixture of the powdered graphite (15% by wt.) and TiB 2 (85% by wt.) used in Example 1 poured into the mold surrounding the core, and processed in the same manner at 1900°C in argon.
• a TiB 2 pipe was made by filling a cylindrical mold having a centered wooden dowel rod with the above powdered TiB 2 and sintering as in Example 1. The dowel rod burned out leaving a TiB 2 cylinder with uniform walls ( Figures 23, 24, 25).
• a sintered 100% TiB 2 piece was impregnated by heat ing it to 240°C, placing it in an autoclaves, and drawing a vacuum. After 1 hour, the piece was impregnated with molten pitch at 240°C under 7 x 10 3 Pa pressure (100 PSI).
• the A. D. of the piece before impregnation was 2.9, giving a 36% pore volume. Pitch pickup on impregnation was 16%. by wt.
• the piece was baked for six days on a cycle rising to 720°.
• the article of sintered TiB 2 iaay be re-impregnated and rebaked as above, to fully impregnate the available porosity. It may further be heated to a graphitizing temperature of 1800 °C to 2400°C.
• Pressures used for impregnation may vary widely from about 2 to 15 x 10 5 Pa, with the fluidity and the temperature of the impregnant used. Temperatures used will usually be in the range of 175° to 250°C.
• the impregnant preferred is a petroleum pitch having a softening point of 110°-120°C.
• the baking cycle is frcm 1 to 10 days, typically 6 days, with the tem ⁇ erature in the range of 700°-1100°C.
• a graphite mold was filled with TiB 2 powder having a maximum particle size of 44 ⁇ , and a mean particle size of 7 ⁇ . It was heated to 2615 + 15°Cin an argon atmosphere and held there for two hours. The TiB was partially melted, and had attacked the wall of the graphite mold.
• Example 2 The same TiB 2 powder used in Example 1 was dispersed in molten coal tar pitch at about 175°C, using 85% TiB 2 - 15% pitch by wt. The plastic mixture was molded into a cylinder, baked on a six day cycle rising to
• Example 4 The same materials and procedures were used as in Example 4 above, except that the atmosphere in the furnace was argon and the final temperature was 2400°C. The piece produced had good performance when tested in the Hall cell.
• a piece was produced from the following raw material composition: TiB 2 - 72.0 % by wt.
• Example 13 The mixture was heated to 175°C and the solids dispersed in the molten pitch. The mixture was cooled, then molded to a cylindrical shape. It was baked on a six day cycle rising to 720°C, then cooled, placed in a furnace with a nitrogen atmosphere, and heated tc 2100°C. The piece produced showed good durability with little corrosion and no cracking when tested as a cathodic element in a Hall aluminum cell.
• a mixture of the following composition was used to produce a cathodic element for a Hall cell.
• Example 10 The same raw materials and procedures used in Example 10 are used to make a cathodic element, with a nitrogen atmosphere used up to
• the article is then further heated to 2400°C in an argon atmo sphere by purging the nitrogen with argon and maintaining the argon atmosphere up to the final temperature and during coolinc of the element .
• Example 12 The same materials and procedures used in Example 12 are used to make a cathodic element for a Hall cell.
• the atmosphere is purged with argon and further heated to 2400°C with argon.
• the power is cut off and the furnace allowed to cool.
• the temperature has cooled to about 2000°C, the small argon flow required to maintain the atmosphere is replaced by nitrogen and the nitrogen atmosphere maintained to ambient or slightly above.
• a cathodic element for a Hall cell was produced from a mixture of 50% TiB 2 (85% assay), 27% prilled pitch (coal tar pitch, 110° softening point), and 23% calcined sponge coke (particle size 3 mm mean diem.). The mixture was heated and the particulate matter dispersed in a sigma mixer at 170°C, then molded at 1.4 x 10 7 Pa (2000 PSI). The element was baked to about 720° over a six day period to carbonize the pitch, impregnated with petroleum pitch, re-baxed, and beared in argcn to 2400°C to graphitize the carbon. The element formed had an A.D. (Apparent Density) of 2.26. After a test run in a Hall cell, the element was fully wetted by the aluminum and edges were sharp, indicating good resistance of the element to corrosion by the electrolyte.
• the puddles found in the above samples were analyzed by x-ray diffraction and found to contain TiB 2 , TiO, BN , and C.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Ceramic Engineering (AREA)
• Manufacturing & Machinery (AREA)
• Materials Engineering (AREA)
• Organic Chemistry (AREA)
• Structural Engineering (AREA)
• Inorganic Chemistry (AREA)
• Ceramic Products (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Metallurgy (AREA)

Applications Claiming Priority (4)

Publications (2)

Family

ID=26964272

Family Applications (1)

Country Status (4)

Families Citing this family (9)

Citations (6)

Family Cites Families (6)

• 1982
  • 1982-07-22 JP JP50260582A patent/JPS58501172A/ja active Pending
  • 1982-07-22 BR BR8207805A patent/BR8207805A/pt unknown
  • 1982-07-22 EP EP19820902610 patent/EP0085093A4/de not_active Withdrawn
  • 1982-07-22 WO PCT/US1982/001004 patent/WO1983000325A1/en not_active Application Discontinuation

Patent Citations (6)

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events