3,529,046 United States Patent Office Patented Sept. 15, 1970 2 of die materials utilized in the hot-pressing operation is 3,529,046 UTILIZENG LITHIUM oxide AND PRECURSORS somewhat limited because of the high temperature and ASSINTERING AID FOR HOT PRESSING BERYL pressures necessitated for the fabrication. LUMOXDE It is the aim of the present invention to obviate or sub Rudolph Hendricks, Jr., Oak Ridge, Tenn., assignor to 5 stantially minimize the above and other shortcomings or the United States of America as represented by the drawbacks suffered by practicing previously known tech United States Atomic Energy Commission niques for fabricating BeO structures by providing an im No Drawing. Filed June 20, 1968, Ser. No. 738,418 proved hot-pressing process wherein significant reductions Int, C. CO4b 35/08 in the cost of preparing theoretically dense or near theo U.S. C. 264-66 4. Claims 10 retically dense, high-purity BeO structures are realized. In accordance with the method of the present invention, lithium oxide (Li2O) is employed as a fugitive-type sinter ABSTRACT OF THE DISCLOSURE ing aid with BeO powders of either the so-called "pressa High-purity, theoretically dense beryllium oxide prod ble” or "non-pressable' variety so as to promote rapid ucts are produced at reduced temperatures and pressures 5 densification of the BeO at temperatures substantially low by utilizing a fugitive-type sintering aid and beryllium er than previously useable and to facilitate the fabrication oxide powder. A mixture of beryllium oxide powder and of BeO structures by utilizing relatively inexpensive hot about 0.5 weight percent lithium oxide, added either as pressing equipment such as a single-acting die. lithium oxide or a precursor of lithium oxide is hot-pressed An object of the present invention is to provide a new at a temperature of about 950° C. and a pressure of 1500 20 and improved method of fabricating structures of beryl 2000 p.s.i. to form theoretically dense products. The prod lium oxide in a substantially less expensive and more uct may be subsequently heat-treated at about 1000 readily reproducible manner than heretofore available. 1400° C. for removing virtually all the lithium oxide re Another object of the present invention is to provide maining in the product to provide a product of high purity. for the fabrication of beryllium oxide structures near or 25 at theoretical density at significantly lower temperatures and pressures than previously useable while maintaining desired product purity. The present invention relates generally to hot-pressing A still further object of the present invention is to beryllium oxide products or structures of theoretical fabricate beryllium oxide structures by hot-pressing a density or densities near theoretical, and more particular 30 mixture of beryllium oxide powder and a fluxing agent ly to a method of producing such structures at lower consisting of lithium oxide or a lithium compound which pressures and temperatures than previously employed. will decompose to lithium oxide when heated to a tempera This invention was made in the course of, or under, a ture less than about 950 C. contract with the U.S. Atomic Energy Commission. Other and further objects of the invention will be ob Beryllium oxide (beryllia or BeO) enjoys nuclear and 35 vious upon an understanding of the illustrative process physical properties which are highly advantageous in about to be described, or will be indicated in the appended nuclear power and space applications. However, because claims, and various advantages not referred to herein will of the difficulty of fabricating complicated structural com occur to one skilled in the art upon employment of the ponents from beryllium oxide with desirable density, puri invention in practice. ty, and strength characteristics, the employment of this 40 The mixture of beryllium oxide powder and lithium material in such applications has heretofore been some oxide found suitable for providing structures of theo what limited. retical density or, if desired, near theoretical density, e.g., Previous efforts utilized for the fabrication of beryllium 95 percent or greater, consists of 99.5 weight percent beryl oxide structures of desired densities and close dimensional lium oxide powder and 0.5 weight percent lithium oxide. tolerances for minimizing or obviating the high cost of 45 This mixture is hot-pressed at a temperature less than machining the structures with diamond tooling include 1000 C., preferably about 950 C., and a pressure less hot-pressing techniques as well as cold-pressing techniques than about 2000 p.s. i., preferably about 1500 p.s. i. for a followed by a sintering operation. These techniques result duration ranging from about 15 minutes for smaller struc in undesirable density gradients unless structures of near tures, e.g., wafers of about 2 inches in diameter by 1 inch theoretical density are obtained. It is also known that 50 thick, to about 30 minutes for larger structures. In other beryllium oxide structures produced by hot-pressing tech Words, the duration of the hot-pressing operation is de niques are somewhat more desirable and superior in many pendent upon the mass of the structure. respects to BeO structures made by practicing cold-press The high degree of densification of the BeO structure ing and sintering techniques. In some applications, the obtained by hot-pressing at these relatively low tempera only means of obtaining densities near or at theoretical 55 tures and pressures is believed to be due to the formation density, i.e., 3.0 grams per cubic centimeter, as well as of a Small amount of liquid phase resulting from the inter precise dimensions, is by employing a relatively expensive action of LiO and BeO to form a compound believed to hot-isostatic-pressing operation. be LibeC2 (an empirical formula for a compound not To fabricate beryllium oxide structures of near theo listed in the ASTM X-ray diffraction index and conse retical density by hot-pressing, the use of sintering aids, 60 quently identified as an unknown compound). The high high pressures, and temperatures in excess of 1500° C. degree of mobility promoted by this small amount of has heretofore been required. For example, the fabrica liquid phase causes the novel rapid densification of the tion of beryllium oxide components having a density of BeO. Also, since the thermal stability of LiBeO is ap 95 percent theoretical or greater required temperatures of parently very poor above its melting point of less than 1500 C. at 1500 p.s. i. while utilizing known sintering aids, 65 950 C., this compound acts as a fugitive-type flux or e.g., MgO. Without the use of sintering aids, an even high sintering aid for effecting and facilitating liquid phase er temperature of 1800° C. and a higher pressure of 4000 sintering. Approximately 50 percent of the LiO is lost p.s. i. are required for producing a product of about 95 during the hot-pressing operation and virtually all, if not percent thoretical density, with the temperature and pres all, of the remaining LiO may be removed by sublimation Sure requirements further increasing for fabricating struc 70 with a subsequent, or prolonged, heating of the product tures of theoretical density. In addition to the difficulties at a temperature of about 1000-1500 C, without de of producing these high-temperature products, the choice creasing the density of the beryllium oxide product. 3,529,046 3 4 While a specific quantity of LiO in the BeO-TiO mix the graphite flakes to the wetted surface. The hot-pressing ture does not appear to be critical for the formation of operation can be readily accomplished in a vacuum, inert the theoretically dense BeO products, it has been found atmosphere, or oxidizing atmosphere, depending on the that about 0.5 weight percent lithium oxide is satisfac particular die materials. tory and that a greater quantity of lithium oxide adds very In order to provide a more facile understanding of the little, if anything, to the effectiveness of the process. With present invention, an example of a typical hot-pressing op less than about 0.3 weight percent lithium oxide, the quan eration utilizing the novel lithium oxide sintering aid is tity of liquid phase present in the mixture may be insuf set forth below. This example is merely illustrative and ficient to provide a product of theoretical density. The is not intended to limit the scope of the present invention, lithium oxide employe as the fugitive-type sintering aid which is limited only by the scope of the appended claims. may be provided by lithium oxide or by any other lithium O compound capable of decomposing to lithium oxide when EXAMPLE heated to a temperature less than 950 C. For example, A beryllium oxide product of 3.00 gms./cc. was pre lithium hydroxide (LiOH) or lithium carbonate (Li2CO3) pared by admixing 1000 grams of a commercially avail have provided satisfactory results when mixed with the 5 able and “pressable' beryllium oxide powder of the de beryllium oxide in an amount or quantity equivalent to sired purity with 8.13 grams lithium hydroxide in a twin about 0.5 weight percent lithium oxide upon decomposi shell-type blender; loading the beryllium oxide-lithium hy tion of the lithium compound. When using lithium hy droxide mixture in a graphite die assembly which had been droxide as a sintering aid, it may be desirable to employ coated with approximately 0.005 mill of natural-flake a water vehicle for facilitating the admixture of the lithi graphite bonded with a suitable plastic binder; placing the um hydroxide with the beryllium oxide. A quantity of loaded graphite die assembly in an induction furnace; water corresponding to 6-8 weight percent of the reaction pressing the assembly at 100 p.s.i. to lock the die parts in mixture has been found to be satisfactory. This water is place; purging the furnace with an inert gas to prevent readily driven from the mixture during the hot-pressing oxidation of the graphite assembly; heating the loaded as operation. 25 sembly to 950 C. at a rate of 700° C. per hour to pro Five different grades of commercially available beryl duce Li2BeO2; pressing the mixture contained in the graph lium oxide powders having average particle sizes ranging ite die assembly to 1500 p.s.i.; maintaining a temperature from 2 to 10 microns have been evaluated to determine of 950° C. and a pressure of 1500 p.s. i. for about 30 min the effects of LiO as a liquid phase sintering aid for beryl utes; pushing the core assembly from the die after the lium oxide powders considered to be hot-pressable as well 30 pressure had been removed; cooling the assembly core and as those considered to be non-hot-pressable, or at least removing it from the furnace; and disassembling the die highly difficult to hot-press. Commercial sources from core and removing the beryllium oxide structure. which the pressable beryllium oxide were obtained recom It will be seen that the present invention provides a new mended that the BeO powder be hot-pressed at a tempera and highly advantageous approach to the fabrication of ture in a range of 1470 to 1600° C. in order to obtain 95 35 beryllium oxide components due to the significant reduc percent of theoretical density. In addition to recommended tions in the temperatures and pressures utilized in the hot temperatures, various sintering aids such as Al2O3, SiO2, pressing operation. The rapid rate of densification and MgO, or a combination thereof were also suggested for plastic creep rate after densification also make the use of achieving these high densities. To evaluate the advantage lithium oxide as a sintering aid extremely advantageous of LiO as a liquid phase sintering aid, mixtures of each 40 for fabricating complicated shapes to desired density, in grade of BeO powder were prepared with LiOH and cluding theoretical density, with much simpler die con Li2CO3 in amounts equivalent to 0.5 weight percent Li2O Struction and process equipment than previously useable. and hot-pressed in single- and double-acting graphite dies Further, the method of the present invention permits the at a temperature of 950 C. and at a pressure of 1500 to use of relatively large grain size powder since no precom 2000 p.s. i. in an argon atmosphere. The densities attained 45 paction of the powders is necessary for increasing the bulk for these experiments ranged from 2.92 grams per cubic density of the mixture prior to the hot-pressing operation. centimeter (97 percent of theoretical density) to 3.00 Additionally, the savings from the reduced cost of high grams per cubic centimeter (100 percent of theoretical temperature equipment and electricity, the improved nu density). Three of the five powders used were hot-pressed clear properties, and the use of so-called “non-pressable' to densities of 99.5 to 100 percent of theoretical. The 50 powders, which result in a material cost saving of ap properties and characteristics of the beryllium oxide struc proximately 40 percent, are highly desirable features in ture produced by the process of the present invention are the production of beryllium oxide structures. illustrated in the following table. AS Various changes may be made in the types of lithi TABLE um compounds used for providing the LiO sintering 55 aid and in the arrangement of the method steps herein Density (gms./cc.)-2.98 to 3.00 without departing from the spirit and scope of the inven Chemical assay-99.50 a BeO,.25 LiO, 99.90 b BeO, .0005 tion and without sacrificing any of its advantages, it is LiO to be understood that all matter herein is to be inter Hardness-61.5 Rockwell 'A' scale preted as illustrative and not in a limiting sense. Flexure strength-33,100 p.s.i. 60 What is claimed is: Flexure modulus of elasticity-4.9X106 1. A method preparing a beryllium oxide product, Tensile strength-10,000 p.s.i. comprising the steps of forming a mixture of beryllium Tensile modulus of elasticity-52.4x106 Oxide powder having an average particle size ranging a Hot-pressed at 950-1000° C. and 2000 p.s.i. from 2 to 10 microns and a sintering aid selected from gressed at 950-1000 C. and 2000 p.s. i., post-heated 65 the group consisting of lithium oxide and a lithium com at: C. pound decomposable to lithium oxide when heated to a As briefly mentioned above, the hot-pressing of the temperature less than about 950° C., said sintering aid BeO-LiO mixture may be readily accomplished in a sim being present in an amount sufficient to provide a con ple graphite die assembly of the common single- or dou centration of at least 0.3% lithium oxide in said mix ble-action type. The die is preferably provided with a O ture, confining the mixture, heating the confined mixture natural-flake graphite core or layer for preventing or mini to a temperature in the range of about 950° C. to about mizing a reaction between the graphite die and the lithium 1000 C. to cause a reaction between the beryllium oxide oxide. The natural-flake graphite core may be held in and the lithium oxide for forming a liquid phase com place in any suitable manner Such as by wetting the graph pound of beryllium and lithium, stressing the heated mix ite die with a clear lacquer and then applying a layer of ture with a force corresponding to a pressure in the range 3,529,046 5 6 of about 1500 to about 2000 p.s. i., and maintaining the lithium hydroxide with the beryllium oxide, and wherein mixture under said stress at a temperature of approxi the water vehicle is of a quantity corresponding to 6-8 mately the reaction temperature for a duration sufficient Weight percent of the mixture. to affect the formation of a beryllium oxide product characterized by a density in a range between about 95 References Cited to 100 percent of theoretical density and a sintering aid UNITED STATES PATENTS content less than that in said mixture prior to the heat ing and stressing thereof. 3,067,048 12/1962 Gion et al. ------106-55 2. The method of preparing a beryllium oxide prod 3,141,782 7/1964 Livey et al. ------264-125 uct as claimed in claim 1, wherein each of the sintering 3,341,425 9/1967 Chu ------106-55 aids as provided by the lithium oxide and the lithium O 3,226,456 12/1965 Ryshkewitch et al. --- 264-56 compound when decomposed to lithium oxide is of a 3,346,681 10/1967 White et al. ------264-332 quantity corresponding to about 0.5 weight percent of OTHER REFERENCES the mixture. 3. The method of preparing a beryllium oxide prod J. E. Burke: "Progress in Ceramic Science,” vol. 4, uct as claimed in claim 1, including the additional step 15 1966, Pergamon Press, New York, pp. 100-101, 103, of heating the product subsequent to said formation there and 119. of at a temperature of at least about 950 C. for a period of time sufficient to sublime virtually the entire quantity JULIUS FROME, Primary Examiner of the sintering aid from said product. J. H. MILLER, Assistant Examiner 4. The method of preparing a beryllium oxide prod 20 uct as claimed in claim 1, wherein the lithium compound U.S. C. X.R. decomposable to lithium oxide is lithium hydroxide con 106-55; 264-125, 332 tained in a water vehicle to facilitate the mixing of the