Patent Number: 048266302
Section: description

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 shows a neutron-absorber rod 21 including inner hollow cylinder 23 and outer hollow cylinder 25. These cylinders are sealed at their ends and define an annular chamber 27 in which pellets 29 made in accordance with this invention are stacked coaxially with the cylinders. Orr application shows the rod 21 in more detail. FIG. 1 shows typical dimensions of the cylinders 23 and 25 and the pellet 29. It is emphasized that the diameters of the cylinders must be maintained to within plus or minus 0.0015" and the diameter of the pellet to within plus or minus 0.002". This invention concerns itself with the producing of these pellets to size with a minimum of grinding or other machining processes. The pellets 29 are ceramics produced by following the steps shown in the flow chart of FIG. 2. In the first step 31 the Al.sub.2 O.sub.3 and B.sub.4 C powders are mixed. The starting mean size of the B.sub.4 C powder is about 1 to 30 microns; 5 to 15 microns mean size is preferred. The starting mean size of the Al.sub.2 O.sub.3 is 1 to 20 microns. To homogenize the powders and eliminate coarse agglomerates several hundred microns in size, the powders are milled and ground in a ball mill in the second step 33. This process permits intimate mixing of the constituents Al.sub.2 O.sub.3 and B.sub.4 C. To aid in the comminution and homogenation, the powders are mixed in a liquid, typically deionized water. Small but effective quantities of a wetting agent, surfactant and deflocculant are added to the liquid. A small but effective quantity of a thixotropic agent may be added. The surfactant aids in imparting wetting qualities to the liquid. The deflocculant suppresses the formation of agglomerates. The thixotropic agent imparts a fluid property to the powder when it is agitated preventing the larger particles from settling out. The powders are milled for about 1 to 2 hours producing a slurry including about 40% by weight of the powder. Organic binders and plasticizers are added to the slurry in the third step 35 and the milling is continued for 1/2 hour to 1 hour. The binders and plasticizers may also be added earlier in the processing, in the first or second step 31 or 33. The slurry is then spray dried in the next step 37 and screened in step 39. The screening eliminates large agglomerates from the powder. The result of the drying and screening is to produce free-flowing spheres of 30 to 50 microns mean diameter. The spheres are predominantly Al.sub.2 O.sub.3 with B.sub.4 C particles embedded therein. The spheres may be smaller than 30 microns or larger than 50 microns, depending on the spray-drying equipment or its operation. Typically the slurry is spray dried in a centrifugal-separator apparatus 41 as shown in FIG. 3. Such apparatus may be procurred from Niro Atomizer, Inc., Columbia, Md. This apparatus 41 includes a chamber 43 mounted on a movable support 45 formed of metal tubes. Under the top 47 of the chamber a rotatable centrifugal atomizer 49 is mounted. A feed device 51 for the slurry, which may be a hopper or the like, is mounted above the top 47 and is connected to the atomizer 49 through conductor 53. Heated air is supplied to dry the slurry emitted by the atomizer 49. The air is heated by a gas heater 55 and an electric heater 57 and flows to the region around the atomizer 49 through a conductor 59. Arrows 61 show the path of the heated air. The resulting mixture of gas and particulate flows through the chamber 43 and through conductor 63 as shown by the arrows 65 to a cyclone 67. The powder is separated from the gas in the cyclone and is deposited in container 69. An exhaust fan 71 controlled by a damper 73 is provided for exhausting the air as represented by the arrows 75. The air heated by the heaters 55 and 57 enters the chamber 43 at a temperature of about 300.degree. C. and is at a temperature of between 100.degree. C. and 125.degree. C. in the region of the atomizer 49. In the next step (77, FIG. 2) the dried powder is poured into a mold 79. The mold 79 (FIGS. 4-9) is of the multiple type. It includes a body 81 in which there are a plurality of cavities 83 (7 in the mold shown in FIG. 4). The mold 79 is formed of a material such as polyurethane which is capable of transmitting pressure. The mold with its cavities may itself be formed by molding in a die. Each cavity 83 is cylindrical terminating at the top in an expanded volume 85 (FIG. 5) of circular cross section capable of accommodating a funnel 87 (FIGS. 6,7,8) for depositing the powder. The diameters of the cavity 83 are precisely dimensioned. Typically the core which forms the lower portion of the cavity has a diameter which is maintained to plus or minus 0.001 inch. The diameter of this core in the lower region is typically about 0.430 inch. A rod or mandrel 89 is precisely centered in each cavity 83. Each rod is composed of tool steel and is precisely dimensioned. The length of the rod 89 which is typically about 8 inches, for a cavity of about 7 inches in length, is maintained within plus or minus 0.001 inch; its diameter is maintained within plus or minus 0.001 inch. Typical diameters of the rod 89 are 0.2870 inch and 0.2830 inch. The funnel 87 (FIGS. 6, 7) includes an outer shell 91 and an inner cylinder 93. The shell 91 and cylinder 93 are connected by radial plates 95. The inner diameter of the cylinder 93 typically is a slip fit on the rod 89 which extends into it. Typically the outer diameter of the cylinder 93 is 0.400 inch and is maintained to within plus 0.020 inch and minus 0.002 inch. The rod 89 is aligned in each cavity by the cylinder 93 and a precisely dimensioned groove 94 (FIG. 5) in the base of each cavity 83. The projection from the core which forms the groove 94 has a diameter which is maintained to within plus or minus 0.0005 and a height which is maintained to within plus or minus 0.005. Typically this diameter is 0.3100.+-.0.0005 inch and this height is 0.125.+-.0.005. The outer shell 91 includes a tapered section 95 interposed between a cylindrical section 97 above and a thickened short cylindrical section 99, from which a cylindrical lip 101 extends, below. The lip 101 is a slip fit in the upper rim of the wall 103 of the expanded volume 85 of the cavity (FIG. 8). When the powder is to be deposited in the cavities 83, the mold is placed in a cylinder 105 (FIG. 8). The cylinder is a slip fit on the mold 81 and serves to maintain the mold rigid and to prevent the walls of its cavities 83 from becoming deformed. The mold 81 and cylinder 105 are vibrated on a vibrating table or the like while the powder 107 (FIG. 5) is being deposited in the cavities 83 through the funnel 87. The vibration distributes the powder 107 so that it is deposited uniformly in each cavity in the annular space between the rod 89 and the cylindrical wall of the cavity. The powder is deposited along this annular space up to the junction 109 of the cylindrical portion of the cavity and the start of the taper 111 to the space of larger volume 83. The funnel 87 is removed after the powder is deposited. The rod 89 is then maintained aligned by the powder 107 and the groove 94. After the funnel is removed a plug 113 (FIG. 9) of polyurethane or the like is inserted in the opening 85. The plug is a slip fit in the wall 103 of this opening. The plug 113 has a central wall 115 onto which the top of rod 89 is a slip fit. The next step 121 (FIG. 2) is to compress the powder in the cavities 83 to form green bodies of tubular configurations. The green bodies are porous and the percent of theoretical density is evaluated. The higher the percent of theoretical density the less shrinkage of the tube during sintering. To produce the green bodies, cavities 83 are closed by plugs 113 and the mold 81 is placed in an isostatic press. Such a press can be procured from Autoclave Engineers, Inc. of Erie, Pa. The pressure may vary from 5,000 to 60,000 pounds per square inch. A pressure of 30,000 pounds per square inch is convenient. This pressure is the limit of most typical isometric presses. It has been found that the increase in percent of theoretical density of the green body with increasing pressure is not significant. This is demonstrated by the graph shown in FIG. 10. This graph was produced by compressing masses of Al.sub.2 O.sub.3 into green bodies by applying different isostatic pressures and measuring the percent of theoretical density for each pressure. Percent theoretical density is plotted vertically and pressure horizontally. The percent theoretical density of the body compressed at about 10,000 psi was 50 as compared to 60 at about 80,000 psi. A change of 1,000 psi produces only 0.14 change in the percent of theoretical density. The data on the Al.sub.2 O.sub.3 applies to mixtures of Al.sub.2 O.sub.3 and B.sub.4 C. In the next step 123 (FIG. 2) the green body is presintered at a temperature sufficient to remove the organic binder and plasticizer. This step is optional. The presintering step is followed by a sintering step 125. The sintering is at a temperature between 1400.degree. C. and 1800.degree. C.. During sintering the atoms of the Al.sub.3 O.sub.3 matrix diffuse and the mass shrinks. The B.sub.4 C particles remain essentially unchanged. The sintering should be carried out in such manner that the resulting body is sintered to size requiring only a minimum of external grinding. It is necessary to form the pellets so as to prevent excessive swelling and possible destruction of the pellets by reason of neutron bombardment and release of helium gas during reactor operation. The sintering should be carried out so that the pellets are porous. Typically the porosity should be such that the density of the pellets is equal to or less than 70% of theoretical density. The porosity should be open so as to permit evolution of the helium. Since the density is substantially less than theoretical density, it is necessary to control the percent theoretical density accurately for all boron loadings to a predetermined magnitude. It is necessary to ensure that the green bodies sinter identically, or at least predicatably, from batch to batch and lot-to-lot so that the shrinkage during sintering can be controlled to obtain sintered tubes of requisite dimensions. This object is achieved by maintaining the same powder compositions and green body density during pressing and to use the same sintering schedule including temperature, environment and time of sintering. The powders which are used must be of consistent quality. It has been found that to achieve the desired relatively low percent of theoretical density (.ltoreq.70%), the sintering should take place in an inert gas such as argon at about atmospheric pressure. By sintering in argon the percent of theoretical density can be controlled over a wide range of B.sub.4 C content. There is no appreciable vaporization. Other gases present problems. N.sub.2 can only be used at relatively low temperatures and for short intervals, typically 1400.degree. C., and 3 hours. At higher temperatures or for longer times boron nitride is formed. Sintering in carbon dioxide results in oxidation of B.sub.4 C to B.sub.2 O.sub.3. Sintering in hydrogen is on the whole satisfactory but it results in lower densities of the sintered ceramic than sintering in argon. Also density of the ceramic progressively decreases with increase in temperature. Vacuum sintering can be used at lower temperatures, typically 1600.degree. C. or lower. At higher temperatures, B.sub.4 C and Al.sub.2 O.sub.3 are lost by vaporization in the vacuum. Also, control of present or theoretical density of the vacuum-sintered ceramic is not effective. FIG. 11 shows the relationship between the percent theoretical density of green bodies of Al.sub.2 O.sub.3 and B.sub.4 C and the content of B.sub.4 C in the green bodies. The Al.sub.2 O.sub.3 was powder sold under the designated A-16 by Alcoa. The sintering was carried out in argon at 1400.degree. C. and at 1500.degree. C. for 3 hours. Percent theoretical density is plotted vertically and weight percent of B.sub.4 C in the green body horizontally. The percent theoretical density rises sharply for B.sub.4 C content less than 2.5 percent but for higher B.sub.4 C content, the change is relatively small. Between 2.5 and 25%, the percent decrease is from about 70 to about 65 at 1400.degree. C. and from about 71 or 72 to about 64 or 63 at 1500.degree. C.. Neither the sintering temperature nor the content in the green body of above 2.5% of B.sub.4 C have a marked effect on the percent theoretical density. Sintering time is an important parameter where the time is substantially higher than about 3 hours. In FIG. 12 percent theoretical density is plotted vertically as a function of sintering temperature plotted horizontally. A family of curves for three times, 1 hour, 3 hours and 8 hours are presented. The curves were plotted for ceramics composed of 80% by weight of Alcoa-A16 Al.sub.2 O.sub.3 and 20% B.sub.4 C sintered in argon. The percent theoretical density is substantially the same for sintering for 1 hour and 3 hours and for sintering for 8 hours at temperatures below 1500.degree. C. But above 1500.degree. C. for 8 hours the percent theoretical density decreases sharply with increase in temperature; i.e., the porosity rises sharply. This increase in porosity results from the reaction of B.sub.4 C with the residual oxygen in the argon-forming gaseous species at the higher temperatures during the extended heating interval. It is desirable that the argon gas be as pure as practicable for higher-temperature or extended-time sintering. That the percent theoretical density may be set by appropriate selection of the Al.sub.2 O.sub.3 powder is shown in FIG. 13. In this graph percent theoretical density is plotted vertically and temperature horizontally. A family of curves is presented. Each curve is plotted for the sintering in argon of a green body composed of 80% by weight of a selected Al.sub.2 O.sub.3 and 20% B.sub.4 C. The green body with Reynolds 172-DBM manifests the lowest percent theoretical density; i.e., the highest porosity; Linde-A manifests the highest percent theoretical density. Alcoa-A16 and Reynolds HP-DBM did not differ significantly. The differences are governed by the sinterability of the powder; i.e., by the extent of the diffusion of the molecules and atoms of the powder during the sintering operation. Changes in percent theoretical density can also be effected by appropriate selection of the particle size of the B.sub.4 C. Higher percent theoretical densities of the ceramic are obtained with fine powders, typically less than 400 mesh, than with coarse powder, typically greater than 200 mesh. In the final steps, 127 and 129 the outer surface of the tube can be ground and the pellet lengths are accurately finished to length. The pellet lengths may also be cut from the green tube or from the presintered tube. If desired the green or presintered pellets can be machined to size prior to sintering. Tables I and II below show typical constituents, based on 100 grams of Al.sub.2 O.sub.3 and B.sub.4 C powder, of feed material for isostatic pressing in the practice of this invention: A slurry of the constituents in Tables I and II was formed with about 150 grams of water. Lomar is procured from Process Chemical Division, Norristown, N.J. Carbowax 200 and Ucon 2000 are procured from Union Carbide, New York, N.Y. Triton, Tamol, and Rhoplex are procured from Rohn & Hass, Pelham Manor, N.J. Santicizer is procured from Monsanto Chemical Company While preferred embodiments of this invention have been disclosed herein, many modifications thereof are feasible. This invention is not to be restricted except insofar as is necessitated by the spirit of the prior art.