Neutron Absorbing Composite Material and Method of Manufacture

Abstract

A method of producing a neutron absorbing plate constructed of a boron carbide aluminum matrix composite material is disclosed. The method includes mixing a 30-50 micron average particle size B4C powder with an aqueous organic binder component to form a slurry; then drying the slurry at a temperature from about 20 to about 90 degrees Celsius until a dried cake comprising 1-20 percent organic binder of the total weight of said dry cake is formed; then granulating said dried cake to yield a granule size from about 0.5 mm to about 3 mm; then compressing said granules under pressure to create a particulate preform having an interior open porosity; and finally infiltrating the preform under pressure with a liquid metal, to form a metal matrix composite with uniform B4C particle loading.

Claims

1. A method of producing a neutron absorbing Metal Matrix Composite, comprising the steps of: mixing a 30-50 micron average particle size B4C powder with an aqueous organic binder component to form a slurry; drying said slurry at a temperature from about 20 to about 90 degrees Celsius until a dried cake comprising 1-20 percent organic binder of the total weight of said dry cake is formed; granulating said dried cake to yield a granule size from about 0.5 mm to about 3 mm; compressing said granules under pressure to create a particulate preform having an interior open porosity; infiltrating said preform under pressure with a liquid metal, said metal infiltrating said interior open porosity of said preform to form a metal matrix composite, said metal matrix composite having uniform B4C particle loading.

2. A method of producing a neutron absorbing Metal Matrix Composite as in claim 1, wherein the step of compressing said granules further includes the steps of placing said granules in a mold cavity; then applying low pressure from about 10 to about 15 PSI to allow said resultant preform to conform to the dimensions of said mold cavity.

3. A method of producing a neutron absorbing Metal Matrix Composite as in claim 1, wherein said preform compresses from about 20 to about 50 percent of its original volume subsequent to said compression step.

4. A method of producing a neutron absorbing Metal Matrix Composite as in claim 1, wherein said mixing step further includes the addition of a 1-5 micron average particle size B4C powder mixed with said 30-50 micron average particle size B4C powder to form a bi-modal distribution of B4C powder.

5. A method of producing a neutron absorbing Metal Matrix Composite as in claim 1, wherein said mixing step continues up to a point where said binder and said B4C form a low viscosity slurry with a solids content between from about 30% to about 50%.

6. A method of producing a neutron absorbing Metal Matrix Composite as in claim 1, wherein said preform has an interior open porosity between about 30% and about 75%, and has a percentage of B4C between about 70% and about 25%.

7. A method of producing a neutron absorbing Metal Matrix Composite as in claim 1, wherein said metal matrix composite has a density from about 2.6 to about 3.0 grams/cubic centimeter.

8. A method of producing a neutron absorbing Metal Matrix Composite as in claim 2, wherein said step of applying low pressure is accomplished with a lid exerting force downward against said preform.

9. A method of producing a neutron absorbing Metal Matrix Composite as in claim 2, further including the step of stacking a plurality of preforms prior to said infiltration step.

10. A method of producing a neutron absorbing Metal Matrix Composite as in claim 2, further including the step of: placing a layer of fiber paper on the top and bottom of said resultant preform, said fiber paper having an interior porosity of about 95%.

11. A method of producing a neutron absorbing Metal Matrix Composite as in claim 9, wherein said stacking step further includes mixing a plurality of ceramic fibers on top and around said plurality of said preform to increase creep and heat resistance.

12. A method of producing a neutron absorbing metal matrix composite, comprising the steps of: mixing a 30-50 micron size B4C powder with an aqueous organic binder component to form a slurry; drying said slurry at a temperature from about 20 to about 90 degrees Celsius until a dried cake comprising 1-20 percent organic binder of the total weight of said dry cake is formed; granulating said dried cake to yield a granule size from about 0.5 mm to about 3 mm; compressing said granules under pressure to create a particulate preform having an interior open porosity.

13. A method of producing a neutron absorbing metal matrix composite as in claim 12, further including the step of: infiltrating said preform under pressure with a liquid metal, said metal infiltrating said interior open porosity of said preform to form a metal matrix composite, said metal matrix composite having uniform B4C particle loading.

14. A neutron absorbing metal matrix composite, comprising: at least one stacked preform having an interior porosity between about 30% to about 75%, said preform further comprising between about 25% to about 70% of B4C; said at least one stacked preform positioned between a top and bottom layer of fiber paper, said fiber paper having an interior porosity of about 95%; said at least one stacked preform and said top and bottom layer further comprising a metal, said metal infiltrated within said stacked preform interior open porosity and said top and bottom fiber paper layers interior open porosity, said metal infiltration forming a neutron absorbing metal matrix composite; wherein said metal matrix composite comprises about 5% fiber loading and a ductility greater than 1%, and wherein said B4C is distributed uniformly throughout the entire volume of said metal matrix composite.

15. A neutron absorbing metal matrix composite as in claim 14, wherein said preform is between about 0.020 to about 0.2 inches in thickness and said top and bottom fiber paper layers are each about 0.020-0.040 inches in thickness.

16. A neutron absorbing metal matrix composite as in claim 14, wherein said neutron absorbing metal matrix composite has a density of 2.6 to about 3.0 grams/cubic centimeter.

17. (canceled)

18. A method of producing a neutron absorbing Metal Matrix Composite as in claim 2, wherein said slurry is placed into said mold cavity prior to said drying step.

19. A method of producing a neutron absorbing Metal Matrix Composite as in claim 1, wherein said mixing step further includes the steps of: Mixing ceramic powders with said 30-50 micron size B4C powder up to a point where particulate loading is about 50 percent, said powders selected from the group consisting of alumina, SiC, oxide, nitride, and carbide.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0014] A method of producing a neutron absorbing plate to be utilized standalone or in a pre-fabricated assembly is described herein. It is understood that the inventive neutron absorbing plate can be used in any environment (and in conjunction with any other equipment) where neutron absorption is desirable and compatible with aluminum metal matrix composites.

[0015] As space concerns within the fuel pond increase, it has become desirable that the neutron absorbing plate take up as little room as possible in the cell of the fuel rack. Thus, the Plate is preferably constructed of an aluminum boron carbide metal matrix composite material having a percentage of boron carbide between 25% and 70%. The method of the present invention, as described below, has mad it possible to fabricate sheets of boron carbide aluminum matrix composite material to a variety of Net-Shapes and thicknesses to meet end user requirements.

[0016] The method of the present invention begins with the production of a Metal Matrix Composite (MMC) of B4C and aluminum. The method of producing such a composite involves creating a preform suitable for molten metal infiltration, the preform including a B4C powder, or mixture of B4C with other powders, that is mixed with a binder component. In one embodiment of the present method, an average particle size of between 30-50 microns B4C powder can be utilized or alternatively a bi-modal distribution including both 30-50 micron average particle sizes and an average 1-5 micron particle size B4C powder. A bi-modal distribution will help to control powder packing and the ultimate powder fraction present in the final composite. Other powders may be mixed with the B4C powder to further control total B4C content. Such ceramic powders include but are not limited to alumina, SiC, and a variety of other oxide, nitride, and carbide ceramic powders. Metal powders, such as stainless steel, tungsten, may also be utilized in the B4C powder mixture .

[0017] A typical ceramic processing aqueous binder component to be added to the B4C powder is next prepared, and comprises at a minimum both a binder and a surfactant. The binder is present to provide adhesive bond between the B4C particles, providing green body structure and strength to the particulate body. The dispersant is present to help uniformly distribute the powder into individual particles that remain separate and suspended in the aqueous media during drying.

[0018] The binder and B4C powder are mixed to produce a low viscosity slurry with a solids content from about 30 to about 50%. The slurry is then ambient air or hot oven dried at a temperature of about 20 to about 80 degrees Celsius for several hours until a dried cake is created, with softness and flexibility imparted by the organic binder constituents.

Drying times vary depending on the volume of slurry mixture to be dried. In the preferred embodiment, and after drying the binder component is 1-20% the total weight of the resultant preform with the B4C being 80-95 percent by weight.

[0019] After drying, the resultant cake is granulated and passed through metal sieves to yield a granule size of about 0.5 mm to 3 mm. Alternative methods of granulating the slurry include spray drying the slurry directly to form granules or mixing the slurry to create a dry mix prior to milling through the metal sieves. Granule size of about 0.5 mm-3 mm allow leveling in a mold cavity and compression under relatively low pressure of between 10-50 psi to form a particulate preform directly within the casting mold. The compression may be accomplished by utilizing the lid of the mold cavity or any external workpiece for exerting force and compressing the preform. The granules compress in the resultant preform from about 20 to about 50 percent of the original volume of the granulated cake, and are compressed within the mold cavity to conform to the dimensions of the mold. Alternatively, a particulate preform may be formed outside of the mold cavity then placed within the mold.

[0020] In an alternative embodiment, the resultant slurry can be poured into a flat plate mold comprised of an aluminum ring frame placed atop an aluminum plate or other suitable substrate. The mold is vibrated or tapped to completely fill the frame with the slurry. The frame/slurry combination is allowed to dry in ambient air, for several hours. After drying the resultant particulate panels (aka preform layers) can be further hardened by heating in air to about 80C-100C.

[0021] In yet another alternative embodiment, the resultant slurry is added to a pressurized spray gun, and sprayed direct onto either an Al sheet substrate or fiber paper substrate. Both the Al sheet or fiber paper are placed on a hot plate set for 195F. The slurry is sprayed under pressure until the desired dry powder thickness is achieved.

[0022] At this point multiple preform layers may be stacked within the mold if desired to impart structural rigidity to the final plate structure. Each preform layer has a typical thickness of about 0.020 inches to about 0.200, inches however, a wide range of thickness can be achieved. In the example described below the particulate preform has a thickness of 0.085 inches. The presence of the binder helps to keep the particulate preform structure intact during subsequent casting steps without gross particle rearrangement. The resultant B4C preform has an interior open porosity between about 30% and about 75% prior to metal infiltration and has a predetermined fraction of void volume or open structure throughout the material structure. Following infiltration casting the B4C preform becomes metal rich throughout its open porosity. The resultant MMC has a density from about 2.6 to about 3 grams/cubic centimeter.

[0023] If combined with fiber reinforcement, then prior to placing the preforms in the mold cavity, a fiber paper sheet of either discontinuous alumina sheets or quartz veil sheets may be placed on the bottom of the mold cavity. The fiber paper may have a nominal thickness of about 0.020 inches. The B4C containing particulate preform is then placed atop the fiber paper. Next, another matching fiber paper sheet having a nominal thickness of about 0.020 inches is placed on top of the preform and the mold is closed. Ceramic fibers may be added between and around the preforms to increase the overall creep and heat resistance and ductility of the resultant MMC plate structure. Examples of such fibers include but are not limited to Saffil fiber paper, nominally about 5% fiber volume of short, discontinuous alumina fiber, or fabrics woven from continuous ceramic fiber, such as 3M Nextel, achieving about 30% fiber loading by volume. Quartz, and glass fiber use can also be anticipated for this application, whether as continuous or discontinuous fiber structures.

[0024] The mold is next infiltrated with aluminum. The aluminum infiltration process causes aluminum to penetrate throughout the overall structure and solidifies within the open porosity of the material layers. In cases where multiple layers are present, the liquid metal extends from one layer to the next, binding the layers together and integrating the structure. While molten aluminum is the embodiment illustrated other suitable metals include but are not limited to aluminum alloys, copper, titanium and magnesium and other metal alloys cast from the molten liquid phase. The liquid metal infiltration process is described in U.S. Pat. No. 3,547,180 and incorporated herein by reference for all that it discloses. Subsequent to the liquid metal infiltration step, the metal matrix composite is next demolded or removed from the closed mold.

[0025] In this embodiment, Aluminum infiltration permeates throughout the fiber reinforced surfaces and the B4C particulate core to create a three layer MMC sandwich comprised of about 5% fiber loading MMC skin cladding at 0.020 thickness with about 50 vol% B4C particulate filled aluminum metal core at 0.085 or a total thickness of 0.125. This structure provides sufficient B4C content at this thickness and volume fraction for most neutron absorber applications, and the 95% aluminum reinforced with 5% ceramic fiber skins provide overall ductility to the structure, nominally greater than 1% elongation of the sandwich body and imparts greater high temperature creep resistance.

[0026] Alternatively, the fiber paper sheets positioned both on the top and bottom of the preform can be replaced with Al foil sheets at 0.020 thickness. This structure is then placed in a closed mold and aluminum infiltrated to permeate the preform with aluminum while bonding the aluminum foil sheets to the top and bottom sides of the preform.