Nuclear Graphite

U.S. Pat. No. 3,393,085 which is incorporated herein by reference for various process details therein describes thermally stable carbon substrates comprising fine spherical graphite particles having a mesh size of 10 to 150 (Tyler Mesh) with fillers including graphitized gilsonite flour shaped by hot pressing in an electrically and thermally insulate mold and baked. The resulting product is not a fine grained article because of the relative coarseness of the graphite particles.

The main object of the invention is to provide a high strength, high coefficient of thermal expansion (CTE), fine-grain isotropic graphite suitable as a nuclear graphite in high performance applications.

Another object is to provide a fine-grain and more isotropic material than previously available, whose advantages include higher strength and better isotropy than any presently available nuclear graphite.

These objects are attained by the invention in its product aspect in an article containing from 30 to 70 weight percent, preferably 50-65 weight percent, of attritor milled gilsonite coke or other high CTE carbon fillers having a particle size up to 150 microns and preferably up to 75 microns and which is formed by warm isostatic molding.

In its process aspect, the invention comprises the steps of attritor milling high CTE isotropic fillers, preferably gilsonite, and pitch cokes, blending the milled material, mixing with a carbonaceous binder, heating the mixture, cooling the mix, milling the mix, molding the milled mix in an isostatic mold, baking the shape, densifying the baked shape and graphitizing the baked shape in any conventionally known manner.

The invention employs a filler material that is noted for producing isotropic properties, "Gilso coke" or gilsonite coke, which had been used for producing what was in the 1960's considered to be an excellent grade of graphite for nuclear applications, specifically where silicon carbide coating were to be applied. Gilsonite coke is an exceptionally hard carbonaceous material with spherically-shaped particles, making it difficult to mill by conventional means without suffering contamination from the milling operation, such as wear particles from the mill itself. The advent of a device called the attritor mill brought to this technology a new milling method that is fast, clean, and efficient with a minimum of contamination of the product. Attritor milling of gilsonite coke produces a fine-grained filler necessary for this process. However, other high CTE isotropic carbonaceous fillers can also be used in this process.

The isostatic forming method used in this process is unique in itself. This forming method increases the density and stength of the product. For comparison purposes, the usual compressive strength of the product of this process is approximately 16,000 psi, as compared with 6,000 to 7,000 psi for standard grade of "nuclear graphite" (AGOT), and 13,000 psi for coarser-grained products made using hot pressing methods at 320.degree. C. and 1,000 psi as is taught in U.S. Pat. No. 3,393,085.

Fine-grain graphites were prepared using Gilso coke and pitch coke fillers. The CTE's were 5.3 and 3.5.times.10.sup.-6 /.degree. C., respectively, at 100.degree. C. and their anistropy ratios were under 1.07 for all properties. The flexural and compressive strengths were 8024 and 16,772 psi for the Gilso coke filled graphite and 6746 and 12,746 psi for the pitch coke variation.

Other properties are set forth in Table I.

The process steps are detailed below:

I. Filler Preparation--High CTE Isotropic Fillers Such as Gilso and Pitch Coke.

(a) Attritor mill coke to fine flour

(b) Screen out particles greater than 75 microns (200 mesh)*

(c) Remill oversize

(d) Remove magnetic impurities

(e) Blend coke

II. Mix Binder and Filler

(a) Determine binder level

(b) Heat and mix correct proportions of binder and filler

(c) Cooling the resulting mix

III. Mill Cooled Mix

(a) Mill mix

(b) Screen through -100 mesh

(c) Remill oversize particles

(d) Blend

(e) Remove magnetic impurities

IV. Form

(a) Load isostatic mold

(b) Preheat

(c) Mold

V. Bake

VI. Densify

(a) Pitch impregnation

(b) Bake

(c) Pitch Impregnation

VII. Graphitization

(a) Carried out in conventional manner

Additional details of the process steps are set forth below:

Milling problems which may be encountered on producing an 85 to 95* flour from the extremely hard filler cokes (Gilso and pitch coke) employed in the invention are solved by using an attritor mill, which achieves dry grinding in a stationary tank by means of a rotating shaft with arms that agitate the grinding media into a random state of motion of internal porosity forcing the grinding media to impinge and reduce particle size of the material. Milling to a 90 flour was accomplished on a Union Process, Inc. 30S (52 gallon) dry batch attritor. The attritor consisted of a stationary cylindrical vessel in which the produce and 3/8-inch diameter steel balls were agitated by means of a rotating center shaft with arms. A 135-lb batch of crushed pitch coke was milled for 20 minutes, while Gilso coke required 60 minutes of milling time. After running 25 batches, the mill balls remained within the same tolerances as new balls.

Eleven milling lots of Gilso coke and 14 of pitch coke resulted in 89 and 91 flours (percent through 200 mesh), respectively. The individual batch analyses are shown in Table I-A.

The attritor milled cokes were screened to -200 mesh and blended to yield uniform lots of Gilso and pitch coke 100 flours. The average particle size distribution of the screened and blended flours are shown in Table II.

The 100 flour cokes were mixed with milled binder pitch (67 flour) in an oil heated Sigma blade mixer. The mixes each contained 200 lbs of filler flour and a sufficient quantity of binder. Each mix was heated slowly, over a period of about two hours, to a temperature of 160.degree. C., and then discharged and cooled. The mix compositions are shown in Table III. Binder levels of 52.5, 55, and 57.5 pph filler were used for the pitch coke and 60, 62.5, an 65 pph for the Gilso coke. The cooled mixes were crushed, milled, and screened to yield molding particles 100% through 100 mesh. The particle size distribution of the six mixes are shown in Table IV.

Seventy pounds of molding particles were loaded into a 14-inch diameter.times.16-inch long flexible mold which was then evacuated and sealed. Three such assemblies were placed in a 24-inch diameter.times.84 inch deep water-filled autoclave, and held at 62.degree. C. for 16 hours. The system was then pressured to 1000 psi over a period of approximately 15 minutes and held at pressure for five minutes. The molds were then removed from the autoclave, the flexible tooling stripped from the cylinders, and the cylinders were cooled to room temperature under ambient conditions. After compression, the billets were approximately 11.5 inch diameter.times.12.5 inch long.

The pitch coke primary particles were formed to a green density averaging 1.58 g/cc at all three binder levels. The Gilso coke base billets were formed to a density of 1.45 g/cc at the lower binder level.

The billets were sagger baked in coke and sand pack using the schedule shown in Table V. The load was topped with 30 inches of coke and sand and two-inch layers of charcoal, green mix, and sand. Three layers of seven billets each were held in a 48-inch diameter sagger. All of the billets shrunk in bake. Typically, the greatest diameter changes were in the top layer and the greatest length changes were in the bottom layer of the sagger.

The low binder level (52.5 pph) pitch coke billets and the 62.5 pph binder level Gilso coke billets survived the baking process.

The changes in the volume and weight in going from the green to the first bake condition of the successfully processed billets are shown in Table VI. The data are based on measurements of the as-formed surfaces which are less accurate than subsequent measurement on machine surfaces. The weight and volume changes during the baking of the pitch coke base billets were similar, therefore, their average density, 1.55 g/cc, was unchanged in going from the green to the baked state. Greater shrinkage in the baking of the Gilso coke base billets led to an increase of about two points in density to an average of about 1.48 g/cc.

The bake billets were machined to uniform diameter and length and impregnated using a coal tar pitch in a conventional process. The billets were placed in an autoclave under molten pitch at a pressure of 100 psi and a temperature of 150.degree. C. for 20 hours to effect impregnation. The pitch pickup averaged 16.6 w/o for the pitch coke base product and 18.5 w/o for the less dense Gilso coke base material. Subsequent to a sagger rebake to 900.degree. C. (using the conditions shown in Table V), the products averaged 1.68 and 1.61 g/cc in density, an increase of about 9% as compared to the baked density shown in Table VI.

The second impregnation was run in a manner identical to the first. The pickups averaged 11.1 and 12.6 w/o for the pitch and Gilso coke base cylinders, respectively.

The second rebake and graphitization schedules are also included in Table V. The cylinders were induction graphitized, two in a furnace, to a final temperature of 2800.degree. C. Significant shrinkage, 7 v/o for the pitch coke and 12 v/o for the Gilso coke base billets resulted in average product densities of 1.85 g/cc, respectively.

The graphitized products, cylinders P1-2, P1-3, made using pitch coke, and G1-1, G1-2, and G1-3 made using Gilso coke, were examined using X-ray radiography and sonic techniques. X-ray radiographs were evidence of structural uniformity. The ratios of the sonic velocities in the axial and radial directions were 1.023 and 1.010 for the pitch coke and Gilso coke base cylinders, respectively. The sonic transit time data, shown in Table VII, are clear evidence of the isotropy and the homogeneous nawture of both types of graphite.

A three-inch thick cylinder of material was removed from billets numbered P1-2 and G1-2. Twenty tensile strength and strain samples, ten flexural strength, five compressive strength, and samples for thermal conductivity and thermal expansion were taken from both the axial and the perpendicular to the axial directions of each material. Permeability was run on compressive samples and Rockwell hardness on the flexural samples. In addition, ash and mercury porosimetry data were obtained. A summary of the data is shown in Table I.

The properties shown are the high average regardless of sample orientation and the percent difference between the high and the low average. In general, the density and strength properties of both ptich and Gilso coke base graphite far exceeded the goals established at the beginning of the program which lead to the subject invention. For example, the density, tensile, flexural, and compressive strength goals were 1.81 g/cc, 1600, 600, and 13,000 psi, respectively. The corresponding averages for the Gilso carbon base graphite were 1.86 g/cc, 6695, 8024, and 16,772 psi. These data emphasize the superior properties of the product of this invention.