METHOD AND APPARATUS FOR PRODUCING BORON CARBIDE CRYSTALS

Abstract: The invention is a method and apparatus for producing boron carbide crystals in which a major portion of the crystals are of sub-micrometer size, and the crystals have a low combined nitrogen content. A nitrogen-free, particulate mixture of a boric oxide compound and a carbon compound (35) are dropped into the hot zone of a high temperature furnace (17), through a vertical liquid-cooled feed tube (26). The feed tube is inserted into the furnace roof (11). The particles fall from the feed tube into boat members (40) that move out of the hot zone to a collection point, along the furnace floor (14). As the mixture falls through the hot zone, it is rapidly heated above the initiation reaction temperature of boron carbide by heaters (15). The result is a product in which most of the boron carbide crystals are less than one (1) micrometer in size.

Description: METHOD AND APPARATUS FOR PRODUCING BORON CARBIDE CRYSTALS This invention relates to a method and apparatus for producing boron carbide crystals. More specifically, the invention is directed to the production of boron carbide crystals of a submicron size. Boron carbide (B^C) is a ceramic material having a high degree of hardness, good structural integrity at high temperatures, and chemical inertness. These properties make boron carbide a useful material for fabricating devices such as armor plating, sand blasting nozzles, bearings, dies, control rods for nuclear reactors, and refractory liners. In many of these applications it is desirable to use a high purity, monodispersed, boron carbide powder in which the crystals are less than one (1) micrometer in size. The narrow particle size distribution gives the product certain advantages. One advantage is optimum reactivity. Another is that the material can be , hot-pressed to yield a uniform, fine-grained material that is free of pores, excess carbon, and low melting metallic carbide impurities. The usual method for producing boron carbide crystals is to place a particulate mixture of a boric oxide compound and a carbon compound in a crucible and pass the crucible through the hot zone of a high temperature furnace. A major problem with this method is that the mixture is heated to its reaction temperature at a rate which produces only a very broad range of crystals in the micron size range, and essentially no crystals that are smaller than one (1) micrometer in size. The invention is directed to an apparatus and method for producing a quantity of boron carbide crystals in which a major portion of the crystals are of sub-micrometer size. The apparatus used in the practice of this invention is a modified version of a graphite resistance push-type furnace that operates at extremely high temperatures. The furnace unit includes a floor member and a roof member, with the space between these two members defining a hot zone. Heat is delivered to the hot zone by heater means located Inside the furnace unit adjacent to the roof member. Another component of the furnace unit is a vertical feed tube, which Is positioned above the furnace hot zone. The feed tube is designed for feeding a nitrogen-free particulate mixture of a boric oxide compound and a carbon compound into the hot zone. A cooling fluid is circulated around the feed tube, which cools the tube enough to maintain the boric oxide feed compound below its melting point. The furnace unit also includes a group of boat members designed to move along the floor of the furnace hot zone in a path that passes directly below the feed tube. In a typical operation of the furnace unit, as the particulate mixture falls from the feed tube through the furnace hot zone, the temperature of the hot zone is maintained above 1570°C. At this temperature the boric oxide compound will react with the carbon compound to form boron carbide crystals. As each boat member moves underneath the the feed tube, the boat is filled with a load of the boron carbide crystals, which are carried in the boat to a collection point outside of the furnace hot zone. Figure 1 is a front elevation view, mostly in schematic, of a high temperature furnace used in making boron carbide according to this invention. Figure 2 is a detail view, partly in schematic, of components of the furnace " shown in Fig. 1 which are used to feed a starting material for boron carbide into the furnace. In the drawing, referring particularly to Figure 1, the high temperature furnace of this invention is generally designated by the letter F. The outside of the furnace is defined by a metal shell 10. The inside part of the furnace is defined generally by a roof section and a floor member. The roof section consists of three decks, namely, an upper deck 11, intermediate deck 12, and lower deck 13» with the floor member . H being located below deck 13. The roof section decks and the floor member are constructed of graphite. The space between the shell and the roof section, and the shell and the floor member provides an insulation section 15. In the furnace illustrated herein the insula¬ tion section 15 is filled with lampblack 16. The space between the deck 12 and floor member 14 defines the hot zone 17 of the furnace. Heater boards 18, which are fabricated of graphite, are positioned directly above the hot zone 17 in a space 19 between the upper deck 11 and the intermediate deck 12. A DC current is passed through each board as the heating medium. Above the hot zone 17 of the furnace is a 10 vertical chute structure that consists of three pieces. The lower part of the chute is defined by a sleeve 20, that fastens into the deck 12 and extends up through deck 11. The upper part of the chute is defined by a -,- sleeve 21, of smaller diameter than sleeve 20. The lower end of sleeve 21 is coupled to the upper end of sleeve 20 by a transition piece 22. The top end of sleeve 21 is fitted with a packing gland 23 and gland nut 23a, which provide a seal assembly. 0 As shown particularly in Fig. 2, a cooling jacket is mounted lengthwise inside the sleeve 21 of the chute. The jacket is defined by an inside tube 24, which is enclosed within an outside tube 25. A vertical 5 feed tube 26 is mounted lengthwise inside tube 24, and an annulus between these tubes defines a passage 27 for circulating a cooling fluid along the outside of the feed tube. The cooling fluid enters passage 27 through an inlet fitting 28. Another annulus between the out¬ 30 side of tube 24 and the inside of tube 25 defines a passage 29 for the cooling fluid to leave the jacket through an outlet fitting 30. In the practice of this invention, the starting material for producing boron carbide is a particulate mixture of a boric oxide compound and a carbon compound. A device for feeding the particulate mixture into the feed tube 26 is located above the upper end of the feed tube. As shown in Figure 2, one type of feeder device that may be used is a screw feeder 31. An outlet spout 31a on the screw feeder is connected to the top end of a glass sight tube 32 with a flexible coupling 33- At its bottom end the sight tube is connected to the top end of feed tube 26 by another flexible coupling 34. Operation A typical example will now be given to illustrate the production of boron carbide crystals according to the practice of this invention. The particulate mixture consisted of a physical blend of technical grade boric acid (U.S. Borax), size -200 mesh (less than 74 micrometers) and 50 percent compressed acetylene carbon black (Gulf Oil Co.). The mixture was blended for 30 minutes in a modified mortar mixer coated with an epoxy film. The mixture was prepared to give an excess of boron over carbon of approximately 20 percent, based on the reaction stoichiometry: 4 mols B: 7 mols C = 100% 4 x 1.2 mols B: 7 mols C = 20 excess B The mixture was heated in titanium pans for 3.5 hours at 350°C to dehydrate it to B 2 0 3 + C. The pans measured 2 in. high x 24 in. wide x 72 in. (5 cm x 61 cm x 183 cm) long and they were closed with titanium covers having 1/2 in. (1.3 cm) dia. holes therein to allow water vapor to escape from the mixture during heating. After cooling the dried mixture is loosely agglomerated, but it can be easily broken up into <10 mesh (1.68 mm) aggregates. At this point in the preparation, the mixture has a bulk density of about 15 lbs/cu. ft. (240.3 kg/m-3), since it contains a substantial amount of entrapped air. The entrapped air includes about 80 percent N 2 , which is undesire-able because it can react to form boron nitride. To correct the problem, a vacuum is pulled on the mixture to deaerate it, and argon is used to fill the evacuated feed mixture. The deaerating step thus reduces the N 2 concentration in the boron carbide product to about 0.3 to 0.4 percent. In situations where the feed mixture is not deaerated, the nitrogen content of the submicrometer boron carbide crystals can be as high as 1 to 3 percent. Following the deaerating step, the particulate mixture 35 is loaded into a hopper 36, which is mounted on the screw conveyor 31 at the end opposite from the outlet spout 31a. During the production operation, argon, as a purge gas, is directed into the hopper through a purge line 37, and into the screw conveyor through a purge line 38. The screw shaft 31b in the conveyor is driven by a motor 39, which has a variable speed drive. The argon purge gas provides an inert environment for the reaction of the boric oxide and carbon compounds in the mixture 35. As the mixture 35 moves from the conveyor 31 into the feed tube 26, it passes through the glass sight tube 32. The sight tube thus provides a "window" for the furnace operator to periodically view the flow of the reactive mixture into the furnace, and take corrective action if it becomes necessary. From the screw conveyor, the mixture is delivered into the feed tube at a rate of not more than 0.3 lbs/min. (0.7 kg/min), and preferably 0.1 to 0.2 lbs/min. (0.2-0.4 kg/min). As the mixture falls through the feed tube, water (or some other suitable cooling fluid) is continuously circulated through the passages 27 and 29 of the cooling jacket. Cooling the feed tube as described herein keeps the temperature inside the tube below 300°C, which is the softening point of boric anhydride. If the particulate mixture is not cooled as it moves through the feed tube, it will rapidly convert to a semi-liquid phase and plug off the tube. From the feed tube 26, the particulate mixture 35 falls downwardly through the hot zone 17 of furnace F and into a product boat 40, which is being pushed along the floor 14 of the furnace. As shown in Figure 1, there is a continuous string of the product boats, and they are moved by a suitable conveyor system (not shown). The boats are pushed through the hot zone 17 at a rate of about 2 in. to 3 in. per minute (5 to 18 cm/min.). As the par-ticles 35 enter the hot zone 17 at the discharge end 26a of the feed tube 26, the temperature is about 1500°C. The temperature increases to about 2000°C at the point where the particles fall into the product boats. When each product boat 40 passes directly below the discharge end of the feed tube, it is filled with a load of boron carbide crystals, and the product is carried to a collection point (not shown) outside of the hot zone. During the production operation, the hot zone 17 is filled with carbon monoxide and argon gas, to provide a desireable inert environment for the reaction of the starting material to boron carbide. As shown in Figure 1, the argon is introduced into the hot zone at two points; one point is through the feed inlet pipe 26, and the other point is at the left end of the zone, as indicated by arrow 42. The argon stream entering the hot zone from. the left end also serves another purpose. This stream moves in a direction countercurrent to the path followed by the product boats 40 as they leave the hot zone. The gas stream thus acts as a barrier to prevent the particles in the mixture 35 from being swept out of the hot zone before the boron carbide reaction is completed. A key to obtaining boron carbide crystals in a sub-micrometer size is to be able to heat the starting material above its initiation reaction temperature very rapidly. In the practice of this invention, therefore, it is critical that the temperature of the hot zone be maintained above 1570°C. Another critical factor is the time it takes to heat the particles above the reaction temperature. In the operation described herein the temperature of the hot zone will be 1600°C to 2100°C and the rate at which the particles are heated as they move through the hot zone is at least 200°C per second. The amount of boron carbide crystals produced was about one (1) pound per hour (0.46 hg/hr), of which about 90 percent by weight of the crystals were of sub-micrometer size. The actual size was 0.1 to 0.3 micrometers.

Claims: 1. A method for producing boron carbide crystals in which a major portion of the crystals are less than one micrometer in size and have a low combined nitrogen content, comprising: mixing a boric oxide compound and a carbon compound to form a particulate feed mixture, said mixture containing a substantial amount of nitrogen; deaerating the particulate feed mixture to remove the nitrogen contained therein such that said nitrogen is removed and replaced with an inert gas; dropping the nitrogen-free particulate feed mixture through a vertical feed tube having a discharge end that extends into a hot zone of a high temperature furnace; and allowing the nitrogen-free particulate feed mixture to fall from the discharge end of the vertical tube through the furnace hot zone, while maintaining the temperature of the hot zone above 1570°C such that the boric oxide compound reacts with the carbon compound to form boron carbide crystals as the nitrogen-free particulate feed mixture falls through the hot zone of the furnace. 2. The method of Claim 1, further comprising: moving a group of boat members through the furnace hot zone along a path that passes directly below the discharge end of the vertical feed tube; filling each boat member with a load of boron carbide crystals as the boat passes under the discharge end of the feed tube; and carrying the load of boron carbide crystals in each boat member to a collection point outside of the furnace hot zone. 3- The method of Claim 1 in which the furnace hot zone is maintained at a temperature of 1600°C to 2100°C. 4. The method of Claim 1 in which the nitrogen-free particulate feed mixture is dropped into the vertical feed tube at a rate of 0.1 to 0.3 lbs. per minute (0.05-0.13 kg/min). 5. The method of Claim 1 in which the boat members move through the hot zone of the furnace at a rate of 1 to 3 inches per minute (2.5-7.6 cm/min). 6. The method of Claim 1 in which the boric oxide compound is boric anhydride and the carbon compound is acetylene carbon black. 7. The method of Claim 1 in which at least 50 percent by weight of the boron carbide crystals are of less than one micrometer in size. 8. The method of Claim 7 in which the boron carbide crystals contain not more than 0.5 percent by weight nitrogen. 9. The method of Claim 1 in which the size of the boron carbide crystals is 0.1 micrometers to 0.3 micrometers. 10. A high temperature furnace unit for producing boron carbide crystals, in which a major portion tof the crystals are less than one micrometer in size and have a low combined nitrogen content, the furnace unit comprising: a floor member (14) and a roof member (11) spaced from the floor member, the space between the floor and roof members defining a hot zone (17); heaters (18) for delivering heat to the furnace hot zone (17), the heaters being located inside the furnace adjacent to the roof member (11); a vertical feed tube (26) positioned above the furnace hot zone (17), the feed tube being designed for feeding a nitrogen-free particulate mixture (35) of a boric oxide compound and a carbon compound into said hot zone, and the feed tube being cooled by a cooling fluid to maintain the boric oxide feed compound below its melting point; the furnace hot zone (17) being at a temperature which causes the particulate mixture to form boron carbide crystals as said mixture moves through said hot zone; and a group of boats (40), each boat being adapted to move along the floor (14) of the furnace hot zone (17) in a path that passes directly below the vertical feed tube (26), and each boat (40) being adapted to carry a load of the boron carbide crystals to a collection point outside of said hot zone.

Inventors: MOORE WILLIAM G, MOORE, WILLIAM, G.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1989
Main classification: C01B3
IPC codes: C01B31