Abstract:
The synthesis of actinide tetraborides including uranium tetraboride (UB 4 ), plutonium tetraboride (PuB 4 ) and thorium tetraboride (ThB 4 ) by a solid-state metathesis reaction are demonstrated. The present method significantly lowers the temperature required to ≦850° C. As an example, when UCl 4  is reacted with an excess of MgB 2 , at 850° C., crystalline UB 4  is formed. Powder X-ray diffraction and ICP-AES data support the reduction of UCl 3  as the initial step in the reaction. The UB 4  product is purified by washing water and drying.

Description:
This invention was made with government support under Contract No. W7405-ENG-36 awarded by the U.S. Department of Energy to The Regents of The University of California. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the preparation of actinide boride materials suitable for intermediate storage of actinide elements and, more particularly, to the use of low-temperature, solid-state metathesis reactions to prepare these materials. 
     BACKGROUND OF THE INVENTION 
     Department of Energy strategy for plutonium has shifted in focus within the past decade from production and recycling to stabilization and disposal. This change results from the reduction in the nuclear stockpile and the accompanying need for plutonium disposition. Today&#39;s strategy uses plutonium oxide as the optimum intermediate (i.e. ≦50-year) storage form, even though John M. Haschke et al. reported in “Reaction of Plutonium Dioxide with Water: Formation and Properties of PuO 2+X ”, Science 287, 285 (2000), that PuO 2  slowly reacts with moisture to form hydrogen, causing numerous safety and storage concerns. 
     The area of actinide borides is underdeveloped, in part due to the high temperatures required to produce these materials (See, e.g., H. A. Eick and R. N. R. Mulford, J. Inorg. Nucl. Chem. 31, 371 (1969). The list of known binary thorium- and uranium-boride phases includes only ThB 4 , ThB 6 , ThB 66 , UB 2 , UB 4 , UB 12 , with little information reported on their chemical properties (See, e.g., J. J. Katz et al.  The Chemistry of Actinide Elements , Chapman and Hall; New York, N.Y. (1986), pages 56, 280 and 317 for Th, U and Pu, respectively). In contrast, plutonium borides have been synthesized at lower temperatures but require the use of molten plutonium (800° C.), which is extremely corrosive (See, e.g., H. A. Eick, Inorg. Chem. 4, 1237 (1965)), or the use of PuH 3  (900° C.) (See, e.g., R. E. Skavdahl et al.,  Trans. Am Nucl. Soc.  7, 403 (1964)). Plutonium borides are known to be refractory, but other properties such as chemical behavior and stability have not been evaluated. In contrast, many transition metal and lanthanide borides, such as ZrB 2  (See, e.g., K. Su and L. G. Sneddon, Chem. Mater. 5, 1659 (1993) and L. Rao et al., J. Mater. Res. 10, 353 (1995)), and LaB 6  (See, e.g., S. S. Kher and J. T. Spencer, J. Phys. Chem. Solids 59, 1343 (1998)), have been extensively studied and have been used as refractory materials and corrosion-resistant coatings. It is therefore expected that some actinide-boride phases will also be corrosion resistant. 
     During the past two decades significant advances have been made in the low-temperature synthesis of highly refractory materials (See, e.g., K. H. Wynne and R. W. Rice, Ann. Rev. Mater. Sci. 14, 297 (1984) and R. W. Rice, Am. Ceram. Soc. Bull. 62, 889 (1983)). New methods, such as molecular precursors, pre-ceramic polymers, chemical vapor deposition, sol-gel and hydrothermal syntheses, low-temperature molten salts, self-propagating high-temperature synthesis (SHS), and solid-state metathesis reactions (SSM), virtually eliminate the problems associated with slow solid-state diffusion by mixing the constituents of the ceramic at a molecular level. SHS and SSM methods have been used successfully to synthesize transition metal borides, nitrides, and oxides and actinide oxides and nitrides at low- to moderate-temperatures (See, e.g., E. G. Gillan and R. B. Kaner, Chem. Mater. 8, 333 (1996), I. P. Parkin and J. C. Fitzmaurice, J. Mat. Sci. Lett. 13, 1185 (1994), and I. P. Parkin and J. C. Fitzmaurice, New J. Chem. 18, 825 (1994)). The key to low-temperature synthesis is identification of suitable precursors that lead to ceramic materials having the desired physical characteristics described above. However, no mention is made of generating actinide borides using low-temperature SSM. 
     Accordingly, it is an object of the present invention to provide a low-temperature method for preparing stable actinide boride ceramic compositions from commonly available or readily prepared actinide compounds. 
     Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the Invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     SUMMARY OF THE INVENTION 
     To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the method for preparing an actinide boride of an actinide element hereof includes the step of heating a mixture of a halide of the actinide element with a chosen amount of magnesium diboride such that a metathesis reaction occurs. 
     In another aspect of the present invention, in accordance with its objects and purposes, the method for preparing an actinide boride of an actinide element hereof includes the step of heating a mixture of an oxide of the actinide element with a chosen amount of magnesium diboride such that a metathesis reaction occurs. 
     In still another aspect of the present invention, in accordance with its objects and purposes, the method for preparing an actinide boride of an actinide element hereof includes the step of heating a mixture of an oxyhalide of the actinide element with a chosen amount of magnesium diboride such that a metathesis reaction occurs. 
     Benefits and advantages of the present invention include the conversion of already existing or readily generated actinide compounds into less-reactive, safer storage forms utilizing metathesis reactions which take place at readily attainable temperatures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate an embodiment of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
     FIG. 1 shows the powder X-ray diffraction pattern for the water-washed solid product from the reaction of UCl 4 +2 MgB 2  for 1 d at 850° C. (top), and a powder X-ray diffraction pattern of UB 4  calculated from single-crystal data (bottom). 
     FIG. 2 shows X-ray powder diffraction patterns of the crude products of the reaction of UCl 4  with: (a) 0.5 MgB 2 ; (b) 1 MgB 2 ; (c) 2 MgB 2 ; and (d) 2.5 MgB 2 , at 850° C. for 1 d. The powder patterns-are normalized to a constant MgCl 2  peak height. The x-axis corresponds to pattern (a), while patterns b-d are offset by +0.5 degrees in 2 Θfrom pattern (a) for clarity. All powder patterns were taken with the sample under a dry, inert atmosphere. 
    
    
     DETAILED DESCRIPTION 
     Briefly, the present invention includes a method for the preparation of actinide borides by low-temperature, solid-state metathesis reactions as an alternative intermediate storage form for actinide elements. These materials are highly-refractory boride phases of plutonium and other actinides that are stable to moisture, are highly compact, and have many of the desired nonproliferation physical characteristics such as chemical inertness and stability to radiolytic decay. The basic reaction involves a source of boron, such as magnesium diboride with a halide, oxide or oxyhalide of the actinide. 
     Chlorides, bromides and iodides are all suitable halides since the respective heats of reaction readily drive the reactions to completions. Actinide halides include trichlorides and tetrachlorides, oxides include dioxides and oxyhalides include oxychlorides. Although the method of the present invention has been demonstrated for chlorides of several actinide elements and oxides thereof, it is anticipated that the above-described metathesis reactions will proceed well with bromides and iodides and actinide oxyhalides having chlorine, bromine, and iodine; as examples, UOCl 2  and ThOCl 2  and PuOCl can be used. 
     For actinide halides and oxyhalides, a fluxing agent such as potassium chloride, lithium chloride or calcium chloride, or eutectic mixtures of these salts is optional and assists in the intimate mixing of the reacting species at low temperatures by dissolving the actinide compound without taking part in the reaction. This results in shorter reaction times. As an example, a eutectic mixture of potassium and lithium chlorides melts at 352° C., and allows the metathesis reactions to take place at this low temperature. Sodium and cesium chlorides could also be used; the choice of fluxing agent relates to its low melting point and lack of reactivity with the species of interest. As an example, since UCl 4  has a low melting temperature, a fluxing agent would not be required. For the actinide oxides, a fluxing agent is mandatory since these materials have high melting temperatures and slow reaction times. Upper limits for the reaction temperature are limited by materials compatibility, expense and convenience; 850° C. has been found to be a preferred upper temperature limit, although higher reaction temperatures can be utilized. 
     Quartz and titanium reaction vessels have been successfully used. It is anticipated that tungsten would also be suitable. 
     When mixtures of borides are formed in the metathesis reaction, as in the case when uranium and plutonium trichlorides are converted, the addition of elemental boron permits the reaction to proceed to essential completion with the generation of substantially only the actinide tetraboride. 
     In accordance with the teachings of the present invention, suitable actinides include plutonium, uranium, thorium, americium, and neptunium. 
     Having generally described the invention, the following EXAMPLES provide additional details. 
     EXAMPLE 1 
     Preparation of UB 4  from UCl 4  and MgB 2 : 
     Typically, 50-250 mg of UCl 4  and 2 equivalents of MgB 2  were reacted in a vacuum-sealed 1.5 mL quartz tube by heating the mixture to 850° C. for 1 d (Although depleted uranium ( 238 U) was used, care must be taken in the handling and disposal of the starting materials and products since they are alpha particle emitters.). Sealed ampoules were run in furnaces in a fume hood to prevent contamination in the case of a tube rupture: 
       UCl   4 +2MgB 2  →UB 4 +2MgCl 2 (ΔH rxn =−58 kcal/mole) 
     After the reaction, the tubes were opened and the contents washed with water to remove soluble salts. The sample was then rinsed with ethanol and dried. The bromide of uranium may also be employed in place of the chloride. 
     Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are shown in the accompanying drawings. Turning now to FIG. 1, a powder X-ray diffraction (pXRD) pattern of the washed product is compared with a UB 4  pattern calculated from single-crystal data. From the similarity of the experimental pattern and the calculated pattern it is clear that UB 4  is the only crystalline phase readily identified in the remaining solid. 
     The reaction of UCl 4  with substoichiometric amounts of MgB 2  produces a mixture of crystalline phases composed of UB 4 , UCl 3 , and MgCl 2 . X-ray powder diffraction patterns of the reactions of UCl 4 +xMgB 2 , where x=0.5, 1, 2, and 2.5 for reactions conducted at 850° C. for 1 d are shown in FIG. 2 hereof. As the amount of MgB 2  in the reaction is increased, the amount of UCl 3  remaining in the reaction decreases while the amount of UB 4  produced increases. The presence of UCl 3  in the reaction mixture was also confirmed by single-crystal X-ray diffraction. This evidence suggests that MgB 2  acts as a reducing agent as well as a source of boron: 
       UCl   4 +0.5 MgB 2  →UCl 3 +0.5 MgCl 2 +B. 
     To support this hypothetical reduction of UCl 4  to UCl 3  and in order to better understand the nature of the reaction products, two sets of samples were taken for ICP-AES analysis. First, the reaction products were washed with 40 mL of distilled water and the filtrate collected. Separately, the water-insoluble, solid products were dissolved in 20 mL of 3.2 M HNO 3 . Any unreacted UCl 4 , as well as UCl 3  and MgCl 2  produced, will dissolve in the water; unreacted MgB 2  and UB 4  produced are insoluble in water and will remain in the solid. The resulting samples were analyzed for uranium, magnesium and boron content and compared with the initial amounts present in the reaction. At 0.5 equivalents (eq) of MgB 2 , almost all of the uranium (96%) and magnesium (83%) are present in the filtrate, which is consistent with the production of soluble UCl 3  and MgCl 2  as proposed above. This is also consistent with the lack of UB 4  and MgB 2  in the pXRD pattern of this sample (See, FIG. 2 a ). The lack of significant amounts of magnesium (1%) in the x=0.5 solid is consistent with formation of elemental boron as the only boron-containing production in the reduction reaction proposed above (i.e., no MgB 2  remains and no UB 4  has yet been formed). 
     With increasing amounts of MgB 2  (i.e., x≧0.5), the amount of uranium in the filtrate is expected to decrease and the amount in the solid increase due to the conversion of soluble UCl 3  into insoluble UB 4 . At x=2.5, 90% of the uranium was found to be in the solid product. If all of the uranium in the solid samples is in the form of UB 4 , the elemental boron in x=0.5 must be converted to UB 4  as x approaches 2.5 (See FIG. 2 d ). For all values of x, less than 4% of the magnesium is in the solid product; thus, all of the magnesium is dissolved in the filtrate (presumably as MgCl 2 ). This is consistent with the observation that no magnesium uranium boride ternary phases are present in pXRD patterns. 
     Experiments were also conducted to determine the effect of time (1-5 d) and temperature (600-850° C.) on the reaction of UCl 4  with 2 eq of MgB 2 , UCl 3 , and MgCl 2 . The major difference in these reactions is the crystallinity of the UB 4 ; as might be expected, longer reaction times and higher temperatures yield a more crystalline product. 
     Thus UCl 4  has been demonstrated to be converted in high yield (90+% by ICP-AES based on total uranium) to the refractory boride UB 4  by solid-state metathesis methods. No other uranium boride compounds were observed in these studies. These methods allow for the formation of crystalline boride materials at low temperatures (i.e., between 600 and 850° C.) using quartz reaction vessels. 
     EXAMPLE 2 
     Preparation of ThB 4  from ThCl 4 : 
     To 0.200 g of ThCl 4  (0.535 mmol), 0.0123-0.1474 g of MgB 2  (0.268-3.210 mmol; 0.5-6 equivalents (eq)) were added, and the mixture placed In a tantalum reaction vessel that was sealed under 1 atm of He. The reaction was heated to 850° C. over and held at that temperature for 5 d. After cooling, the reaction tubes were opened in an inert-atmosphere, dry glovebox. Powder X-ray diffraction samples were prepared, and the samples analyzed. It was found that both ThB 4  and ThB 6  form in the reactions. 
     EXAMPLE 3 
     Preparation of UB 4  from UCl 3 : 
     0.200 g of UCl 3  (0.581 mmol) were combined with 0.040 g of MgB 2  (0.871 mmol; 1.5 eq) or with 0.040 g of MgB 2  (0.871 mmol; 1.5 eq) and 0.0753 g of B (6.969 mmol; 12 eq) in a tantalum reaction vessel that was sealed under 1 atm of He. The reaction was heated to 850° C. over and held at that temperature for between 1 h and 30 d. After cooling, the reaction tubes were opened in an inert-atmosphere, dry glovebox. Powder X-ray diffraction samples were prepared, and the samples analyzed. It was found that both a 50% mixture of UB 2  and UB 4  formed in the first case and only UB 4  when boron was added to the reactions. Thus, the addition of elemental boron permits the reaction to proceed to completion. 
     EXAMPLE 4 
     Preparation of UB 4  from UO 2 : 
     0.200 g of UO 2  (0.741 mmol), 0.0680 g of MgB 2  (1.481 mmol; 2 eq) and 0.55 g of KCl (0.74 mmol; 10 eq) as a flux material were placed into a tantalum reaction vessel that was sealed under 1 atm of He. The reaction was heated to 850° C. and held at that temperature for 5 d. After cooling, the reaction tube was opened in an inert-atmosphere, dry glovebox. A powder X-ray diffraction sample was prepared and the sample analyzed. It was found that UB 4  was the only uranium boride present in the reaction products. The use of LiCl and CaCl 2 , NaCl and CsCl, and eutectic mixtures thereof, as flux materials are expected to yield similar results. The use of low-melting fluxing agents should permit the metathesis reactions to occur at as low as about 350° C.; for example, a 44% eutectic mixture of LiCl in KCl melts at 352° C. 
     EXAMPLE 5 
     Preparation of PuB 4  from PuCl 3 : 
     5.0 g of PuCl 3  (14.5 mmol) are combined with up to 3.0 g of MgB 2  (65.2 mmol; 4.5 eq) in an open tantalum cylinder. The reaction was heated to 850° C. under a dry Ar atmosphere and held at that temperature for 1 d. After cooling, a powder X-ray diffraction sample was prepared and the sample analyzed. Powder X-ray diffraction revealed that PuB 2  and PuB 4  were present in the reaction products. 
     The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. 
     The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.