ACKNOWLEDGMENT
The authors are grateful to D. R. Petersen for programming some of the more extcnsive calculation.. LITERATURE CITED
( 1 ) Barrdt, E. P., Joyner, L. G., Halenda, P. P., J . Am. Chem. Soc. 73,373 ( I 951 ). (21 Bell, J. R., Grosberg, P., Eature 189, 980 (1961). (3) de Boer, J. H., “Structure and Prop-
erties of Porous Materials,” D. H. Everett and F. S. Stone, eds., pp. 68-94, Academic Press, New York, 1958. (4) Drake, I,. C., Ritter, H. L., IND. ENG.CHERI.,4 x 4 ~ ED. . 17, 787 (1945). ( 5 ) Emmett, P. H., Advan. Catalysis 1, 86 11948). (6) Everett, D. H., “Structure and Properties of Porous Materials,” D. H.
Everett and F. S. Stone, eds., p. 95, Academic Press, New York, 1958. (7) Graton, L. C., Fraser, H. J., J. Geol. 43, 785 (1935). (8) Joyner, L. G., Barrett, E. P., Skold, R., J . Am. Chem. SOC.73,3155 (1951). (9) Katzoff, S., Ott, E., Z. Krist. (A) 86, 311 (1933). (10)Kiselev, A. V., “Structure and Prop-
erties of Porous Materials,” D. H. Everett and F. S. Stone, eds., p. 128, Academic Press, h’ew York, 1958. (11) Kreyszig, E., “Differential Geometry,” p. 244, Univ. of Toronto Press, Toronto, 1959. (12) Kruyer, S., “Structure and Properties of Porous Materials,” D. H. Everett and F. S. Stone, eds., p. 121, Academic Press, New York, 195s. 113) Lorenz. P. B.. h‘atim 189. KO. 4762. SOC.44, 513 (196
(15) Meyer, H. I., J . Appl. Phys. 24, 510 (1953). (16) Ritter, H. L., Drake, L. C., IND. ENG.CHEM.,ANAL.ED. 17, 782 (1945). (17) Swanson, H. E., Cook, H. I., Evans,
E. H., de Groot, J. H., Natl. Bur. Stand., Circ. 539, 10, 50 (1960). (18) Swanson, H. E., Fuyat, R. K., Ugrinic, G. AI., Zhid., 539, 3, 24 (1954). (19) Villet, R. H., Wilhelm, R. H., Znd. Enq. Chem. 53, 837 (1961). (20) Washburn, E. W., Phys. Rea. 17, 273 (1921).
)2) Washburn, E. IT., BI J . A m . Ceram. SOC.4,983 (192 ( 2 3 ) Ibzd., 5, 48 (1922). (24) Winslow, N. M., Shapiro, J. J., A S T J f Bull., No. 236, 39 (1959).
RECEIVED for review December 12, 1962. Accepted May 21, 1963.
Determination of Oxygen in Pyrolytic GraphiteCoated Uranium Carbide by Use of a Current-Concentrator Furnace M. E. SMITH, J. M. HANSEL, R. B. JOHNSON, and G. R. W A T E R B U R Y The los Alamos Scientific Laborafory, University of California, Los Alamos, N. M.
b An inert carrier-gas
method was modified for the determination of low concentrations of oxygen in highly refractory materials b y incorporating the use of a current-concentrator furace to obtain higher temperatures. Operating temperatures ranging between 2400’ and 2800’ C. were readily attainable, and relatively low blanks, between 10 and 30 pg. of oxygen, were obtained with the modified apparatus. This method was applied to the determination of oxygen in uranium carbide particles coated with pyrolytic graphite. A temperature of 2500’ C. is sufficient to rupture the shells of these particles and release the oxygen, which is converted to carbon dioxide and measured manometrically. The relative standard deviations of this method for coated particles are & l o % at the 1000p.p.m. oxygen concentration level, based upon 79 determinations, and f12% at the 75-p.p.m. level, based upon 15 determinations.
T
determination of low concentrations of oxygen in some refractory materials requires higher temperatures than are used in most existing methods for determining oxygen. Difficulties encountered in attaining the required temperatures while keeping the blank a t a satisfactory level indicated that a furnace of different design was necessary. An induction furnace with a HE
1502 *
ANALYTICAL CHEMISTRY
current concentrator proved very satisfactory for this purpose. Furnaces of this type have been used for various heating operations (3-6), but no application to the determination of oxygen in refractory materials is known. The current concentrator furnaces, as the name implies, concentrate the generator output current in the vicinity of the crucible. This technique differs in principle from that commonly used to heat crucibles in many vacuum fusion and inert-gas fusion furnaces in which high temperatures are attained by surrounding the crucible with carbon insulation contained in a quartz thimble. As the furnaces with concentrators do not contain hot carbon in contact with quartz, high blanks due to the reduction of the silica at high temperature (7) are not encountered. The inert carrier-gas method adapts readily for use with the current concentrator furnaces. This method, as described by Smiley (8), consists of reaction of the oxygen in the sample with carbon to form carbon monoxide, which is oxidized by iodine pentoxide and measured manometrically as carbon dioxide in a capillary trap manometer. The incorporation of a furnace with a current concentrator necessitated increasing the carrier gas flow rate and the length of the capillary trap to ensure quantitative collection of the carbon dioxide. The modified method using the hightemperature furnace was applied first to
the determination of oxygen in uranium carbide particles coated with pyrolytic graphite. Uncoated uranium carbide is very reactive, and its oxygen content can be determined without difficulty by the usual inert-gas fusion and vacuum fusion methods a t temperatures of 2000’ to 2100’ C. However, if the uranium carbide particles are coated with pyrolytic graphite (2, 6), the oxygen cannot be determined at these temperatures because the coatings are not ruptured sufficiently to permit complete release of osygen. Crushing of the coated particles, preferably in a diamond mortar, destroys the integrity of the coatings but introduces the difficult problem of preventing hydrolysis or other oxygen pickup during the crushing and transferring of the sample to the furnace. At 2500’ C., the pyrolytic graphite coatings are effectively ruptured, probably because of the formation of a uranium carbide-carbon eutectic mixture, and oxygen is released without use of a metal flux or any special pretreatment of the sample. Application of this high temperature inert carrier-gas method to other refractory materials is under investigation a t this time. EXPERIMENTAL
Apparatus. The current concentrators (Figure 1) were made of highpurity, vacuum-cast copper following an established design ( I ) . Details of their construction are pending publica-
tion. Three concentrators '/a inch inch in diameter X 3 4 inch high, in diameter X 1 inch high, or 1 X 1 inch were tried. Temperatures greater than 3000" C. were reached with each; however, the largest was the most practical from the standpoint of crucible capacity. The concentrator is inserted in an envelope consisting of a borosilicate glass tube 4 inches in diameter and 15 inches long with ends ground flat to seal against silicone rubber gaskets in the base and top plates (Figure 2). The induction coil consists of 20 turns of slightly flattened, 'l4-inch copper tubing wrapped around the glass envelope. Purified argon enters the furnace through '/4-inch copper tubing soldered to the base plate. The sightglass cap located a t the top of the furnace can he unscrewed so that samples may be poured through a funnel into the crucible. A graphite crucible (AUC grade), 1 inch long with 3/&-inch o.d. and a '/kincb wall thickness. is sunnorted in the center of the
I LTt
manamas) using a wine mngszenfilament strip lamp. Corrections were made for the errors introduced by the quart5 sight glass and mirror used in the optical path. The temperatures measured are estimated to be within &Zoo C. of the true temperature.
as a single unit facilitates replacement and aids in alignment. Power was supplied through several feet of coaxial leads either by a 2 0 - k ~ . Lepel Model T20-3 or a Thermonic Model 1070 induction. heater. Generators having a lower output would suffice if shorter leads are convenient. Temperature measurements were made with a Pyro Micro optical pyrometer manufactured by the Pyrometer Instrument Co.. Inc. This instrument
Figure 2. Diagram of high temperature furnace with current concentrator and graphite crucible
Figure 1. Induction current concentrator Oblique view from below
systems (IFig&e 3) are similar to those described by Sniiley (S), except that the manostat bottles were replaced with a pressure regulator, and a longer capillary trap of spiral design (Figure 4) was used. The trap and manometer were ma.de removable by sealing the semiball joints to modified bellows valves (shown in Figure 5) with Apieaon W wax. The capillary trap and manometer were calibrated in the manner described by Smiley (8). Procedure. To degas the apparatas, the argon supply regulator was adjusted to about 1 p s i . and valve VI (Figure 3) opened. The screw cap, C,' was removed, a flowmeter was fitted in the opening by means of a rubber stopper, and the pressure regulator was readjusted to provide a flow between 4 and 5 liters per minute through the furnace. This high flow rate is necessary to prevent air from entering the furnace when the cap is removed. When the furnace is closed, a flow rate of 1. liter per
1
.~,I.~IIyU..Y "..~
11'
a1 vacuum pump started. Leaving valves V,, V4, and V , closed, valves Ti6 and V S were opened in sequence and valve V , was adjusted until the manometer read 300 mm., indicating a Aow rate of about 1 liter per minute. The cooling water was turned on in the concentrator and base plate and the induction heater started. New crucibles were heated to about 1000" C. for 10 minutes and then increased by 250" C. intervals to 2550" C., allowing 5 minutes between temperature changes. This program of gradual heating was used as a precaution against rapid localized degassing, which tends to produce arcing between the crucible and the concentrator core. After the operating temperature was attained, valve Vs was opened, valve V z closed, and the flow of gas through the reagent regulated with valve V4 to re-establish the 1 liter per minute flow rate. Heating was continued for another 20 minutes before determining a blank. Blanks were determined following a simulated sample addition in which valve V6 w a closed and furnace cap C removed for 1 minute. The furnace cap was replaced, valve V s opened, and the system flushed for about 1 minute. The spiral trap was immersed in liquid nitrogen contained in a Dewar flask, and the crucible was heated a t 2550" C. for 5 minutes. On completion of the heating, the poiver was shut off, valve tlUu
Ll.
Figure 4. Capillary trap and manometer Dimensions in mm.
VOL. 35, NO, IO, SEPTEMBER 1963
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Figure 5. Modified bellows valve
V , was closed, and a few seconds were allowed for the pump to remove excess argon before closing valve Ve. The liquid nitrogen was removed and the trap was warmed to room temperature with warm water. The manometer was dried, tapped gently, and read to the nearest 0.5 mm. Blank determinations were repeated until they agreed to within 1 to 2 mm. Blanks were then determined before and following each sample analysis.
Table 1.
Weight of Thoz, mg. 1.62 i.04 1.38 1.37 1.48 1.18 1.23 1.38 n.74
Recovery of Oxygen from Thorium Dioxide
Flow
Recovery, %
rate, m!./ min.
Short tended capillary capillary
EX-
120 120 580 580 con
98 108 74 78
...
...
...
vrl
JOU
1'5
...
91 91 500 500
Table
...
... , . .
...
II.
93 101 99 102
Samples varying in size between 0.1 and 1 gram were placed in small vials and weighed accurately. With the induction heater power off, valve V S was closed and C replaced with a 1-hole Teflon stopper. The end of the longstemmed funnel used for adding the samples was inserted into the hole in the stopper, aiid air was flushed from the funnel by the argon issuing from the furnace. JT7hile the funnel was held in the stream of argon, the Teflon stopper was removed from the furnace and the funnel lowered into the furnace until the end of the funnel was inserted into the hole in the crucible cover. The sample was poured into the crucible, the funnel was removed, and the furnace was cap replaced. The empty vial was weighed to determine the weight of the added sample, and the oxygen was determined by the procedure described for measuring the blank. The manometer readings were converted t o micrograms of oxygen using previously obtained calibration data, and this quantity was corrected for the apparatus blank by subtracting the average value for blanks determined immediately before and following the annly4.i of the sample. RESULTS A N D DISCUSSION
Flow Rate of Carrier Gas. An argon flow rate of 800 to 1000 ml. per minute was necessary to obtain complete and reproducible recoveries of oxygen. Slower flow rates proved inadequate because of the large volume of the current concentrator furnaces. To test the efficiency of collection of carbon dioxide a t this high flow rate, samples of high purity thorium oxide were analyzed using an inert-gas fusion apparatus (8) first with the regular trap and second with the extended-length spiral trap described in this paper. The samples were wrapped in tared platinum foils and weighed accurately, using a Cahn electrobalance, Model R. M. Osyqen recoveries were significantly low (Table I) when the regular (.;hart) trap was used a t a flow rate of only 580 ml. per minute. The spiral trap effectively collected the
Effect of Temperature on Oxygen Recovery
Oxygen found, yo 2230' C. 0.08 0.07 0.04 0.06 0.04 0.04 0.04 0.04 0.07 0.Oi 0.10 0.18 0.05
1504
2330" C. 0.10 0.07 0.07 0.07 0.08 0.11 0.06 0.09 0.10 0.12 0.14 0.11 0.12 0.la
2420" C. 0.12 0.12 0.12 0.12 0.11 0.12 0.11 0.10 0.14
ANALYTICAL CHEMISTRY
2500' C. 0.12 0.12 0.12 0.12 0.11 0.13 0.12 0.13 0.14 0.13 0.13 0.14 0.14 0.13
330' C .
0.11 0.11 0.10 0.12 0.12
2660' C . 0.13 0.15 0.12 0.12 0.10 0.16 0.13 0.12 0.13 0.12 0.12 0.14 0.10 0.13
carbon dioxide a t a flow rate of 1 liter per minute. Operating Temperature. The effect of temperature on the recovery of oxygen from a single sample of uranium carbide coated with pyrolytic graphite is shown in Table 11. The range between 2420' and 2660' C., where melting of the uranium carbide-carbon mixture occurs, gave reproducible results, and 2550" C. was selected t o enwre t h a t any instability of the induction heating equipment would not cause a decrease below the minimum of 2420' C. Provided proper degassing and crucible alignment procedures are observed, the graphite crucibles generally withstand 2550' C. during successive determinations until the crucible is filled with samples. Operation a t 2700' C. frequently results in excessive vaporization of carbon and arcing between the crucible and the concentrator core. This produces rapid deterioration of the crucible by cracking and channeling, which immediately decreases the temperature because of the loss of effective induction coupling. In spite of these difficulties, two determinations of oxygen in the coated uranium carbide sample were made a t 2800' C. and the results were in agreement with those obtained a t 2550' C. Time of Heating. A heating period of 5 minutes was selected for the pyrolytic graphite-coated samples. Longer periods did not increase the recovery of oxygen, and blanks determined after the 5-minute heating periods were not appreciably larger than initial blanks, showing t h a t oxygen was effectively removed from the samples in 5 minutes. Blanks. Blank determinations vary from 10 to 30 pg. of oxygen, depending on the history of the furnace. Variations between initial and final blanks were usually 2 to 3 pg. No correlation was observed between the size of the blank and the size of the concentrator core. Purification of the copper used in the construction of the concentrators by vacuum casting did not reduce the blank. Placing a desiccant (Linde Molecular Sieve 5A) in the furnace overnight reduced the time of degassing, indicating that water adsorbed on the surfaces of the concentrator and glass envelope may be the major contributor t o the blank. Precision and Accuracy. The relative standard deviation was calculated to be =t3Oj,, based on 42 determinations of oxygen (Table 11) in a single ,ample of coated uranium carbide made a t operating temperatures between 2420" and 2660' C. 4 n additional 37 determinations were made on the same sample a t the selected operating temperature of 2550' C. The relative standard deviation c:tlcu-
lated for the total 70 determinations is =!= 10%. The dai,a used in calculating this prrcision wc'w obtained diiring a time period of one year by three different operators v sing three different apparatus. The relative standard deviation calculated for 15 determinations of oxygen in a coatrd uranium carbide sample containing only 75 p.p.m. of oxygen is =t12yo. Because the degree of homogeneity of the samples is unknown, the measure of precision includes the variance due t o heterogeneity and experimental errors in the method. At present, over 4,50 saniples having various oxygen contents ha1 e been analyzed in duplicate. and the two results obtained for each sample have not differed by more than 10% in the majority of the determinations. KOstandards are rivailable for oxygen in uranium carbide samples coated with pyrolytic graphite, so that a good estimate of the accu-acy is not possible. However, an attempt was made to determine the recov:rp of oxygen from a sample of pure uranium dioxide. Enclosing the oxide sample in a graphite pellet or capsule W M unsuccessful because of the tendencv of the graphite to be blown out of the crucible and cause arcing between the c-ucible and the concentrator core. A series of determinations was made by removing the crucible
from the furnace and adding a weighed quantity of oxide. Twelve samples weighing from 0.4 to 0.7 mg. gave an average oxygen recovery of 96%, with a relative standard deviation of &IO%. The errors involved in the change of procedure plus the weighing errors are included in the results. The slightly low recovery may be due to the tendency of these samples of high oxygen content to be blown from the crucible before intimate contact with the graphite is achieved. A sample of uranium carbide particles coated with pyrolytic graphite was analyzed for oxygen by this method, using the unground material, and also by an outside laboratory using a vacuum fusion method subsequent to grinding the coated particles in an inert atmosphere. The results obtained by the two methods did not differ by more than 10%. Agreement also was found with values obtained here with the usual inert-gas fusion method when the sample was ground in an inert atmosphere prior to analysis. It should be possible to apply this method to the determination of oxygen in other pyrolytic graphite-coated materials such as uranium-thorium carbide particles or other coated carbides, providing their oxides are reduced and a metal carbide-carbon eutectic mixture is formed below 2550' C. Under
optimum conditions, 10 analyses in duplicate can be made in one day. ACKNOWLEDGMENT
The authors thank C. F. Metz, under whose supervision this work was performed, for his valuable advice and assistance, and D. E. Hull and D. E. Carlson for advice in the installation of the induction furnaces with current concentrators. LITERATURE CITED
(1) Hull, D. E., Los Alamos Scientific Laboratory, Los Alamos, N. h'l., private communication, 1962. (2) Kempter, C. P., J. Less-Common Metals 4, 419 (1962); U. 6. At. Energy Comm. Rept. TID-15185(1962). (3) Krupka, 1LI. C., Los Alamos Scientific Laboratory Rept. LA2611 (1962). (4)Leitnaker, J. M., Ibid., LA-2402 (1960). (5) Leitnaker, J. M., Bowman, M. G., Gilles, P. W., J. Chem. Phys. 36, 350 (1962). (6) Litz, L. M., Ph.D. dissertation, Ohio
State University, Columbus, Ohio,
1948. (7) Potter, J. L., Murphy, J. E., Heady, H. H., ANAL.CHEM.34, 1635 (1962). (8) Smiley, W. G., Zbid., 27, 1098 (1955).
RECEIVEDfor review April 26, 1963.
Accepted July 1, 1963. Loa Alamos Scientific Laboratory is operated under the auspices of the Atomic Energy Comrmssion.
Carbon Determination in Elemental Boron CHEN-WIN KUO, GARY T. BENDER, and JOE M. WALKER Department of Chemistry, Kansas State College of Pittsburg, Pittsburg, Kan. b A convenient, rapid, and highly precise method for the determination of the carbon content of elemental boron has been developed. Under controlled conditions, powdered boron, mixed with tin accelerator in a prefired steel crucible, is cornbusted in a highfrequency induction furnace. The carbon content is converted into carbon dioxide and detected electroconductometrically. National Bureau of Standards steel samples were used for standardization purposes. The method shows a standard deviation of fl.7 to f21 using standard steel Samples in which the carbon content ranged from 20 to 400 p.p.m. The results of boron analysis gave a standard deviation of f5.8 to =t25 in the range of 350 to 1000 p.p.rn. carbon. The prefiring of the crucibles was necessary when working with lowcarbon materials. The critical role of boron particle size regarding combustion was demonstrated, as well as errors introduced by mixing boron with steel. The time required for prefiring
the crucible, loading and combusting the sample, and data recording of a single run was approximately 25 minutes.
T
HERMODYNAMICALLY, boron is an excellent solid fuel for rockets because of its large heat of combustion which releases considerable energy per unit volume of fuel. Unfortunately, elemental boron does not oxidize easily. Confronted with the rapid growth of rocket propulsion, the combustion of boron is now being extensively investigated. Boron can also be used in many technical devices such as windows, filters, thermistors, and resistors, because of its electrical and optical properties. However, a t the Boron Conference (1) held in Asbury Park, N. J., in 1959 (the specific objective being the bringing together of scientists for the discussion and exchange of information related to solid state science), S. Benedict Levin pointed out that further progress in the use of boron has been
frustrated by the lack of very pure boron in any form adequate for research. Both governmental and industrial organizations have tried very hard to isolate high-purity boron in their laboratories and to develop an analytical method for the determination of the carbon content. Carbon represents the largest impurity in the high-purity boron thus far produced. Not a single article related to the determination of carbon in boron has appeared in the literature. This subject involves essentially two major problems. The first is the conversion of the carbon to carbon dioxide, and the second is that of the quantitative measurement of the evolved carbon dioxide. A method for oxidizing the boron sample and conversion of the carbon content into carbon dioxide was developed in this laboratory. The procedure consisted of firing a powdered boron sample in an oxygen atmosphere with tin accelerator under carefully controlled conditions in a high-frequency induction field. The electroconductoVOL 35, NO. 10, SEPTEMBER 1963
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