JOURNAL OF RESEARCH of the National Bureau of Standards -A. Physics and Chemistry Vol. 73A, No.3, May- June 1969 The Heat of Combustion of Beryllium in Fluorine* K. L. Churney and G. T. Armstrong Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234 (February 11, 1969) An expe rimental dete rmination of the e ne rgies of combustion in Auorine of polyte traAuoroethylene film and Q.o wder and of mixtures of beryllium with polytetraAuoroethyle ne gi ves for reacti on ( 1)f).H ~.or= - 1022.22 kJ 111 0 1- 1 (- 244.32 kcal mol - I) wit h a n ove ra ll precision of 0.96 kJ 111 0 1- 1 (0. 23 kcal 111 0 1- 1 ) at the 95 pe rce nt confid ence limit s. The tota l un cert a int y is estimated not to exceed ±3.2 kJ mol- I (±0.8 kcal mol - I). The measureme nts on polytetraflu oroeth yle ne giv e for reaction (2a) and reacti on (2 b) f).H ~. o c =- 10 369. 7 and - 10392.4 Jg- I, respective ly. Overall precisions e xpressed at the 95 pe rcent confide nce Ijmits are 3.3 and 6.0 Jg- I, respective ly. Be(c)+ F,(g) = BeF2(a morphous) (1) C,F.(polym e r powd er) + 2F2(g) = 2CF.(g) (2a) C2F.(polyme r film ) + 2F2 (g) = 2CF.(g) (2b) Be2C and Be metal were observed in a small carbonaceous residue from the co mbustion of the beryll iul11 -polytetraAuoroethylene mixtures. Methods of analys is for these s ubstances we re de veloped. Cases res ul t ing from the so lution of the so lid residues in aqueous KOH were analyzed for I-I, and CH. by differential absorption in molecular sieves at low temperatures. Key wo rds : Analysis of meth ane- hydrogen mixtures; beryllium flu oride; beryllium metal; com· bustion calorimetry; flu orine; heat of formation; mol ecular-sieve gas analys is; poly· tet raA uoroet hyl ene. i 1. Introduction I tion of BeF2(s) that is inde pende nt of the heats of formation of BeO(s) and HF(aq). Better than 99 percent No direct determination of the heat of formation of conversion of beryllium to beryllium fluoride was crystalline beryllium fluoride by combustion of obtained by the de vi ce of burning a mixture of beryl­ beryllium in fluorine has been published, probably lium and polytetraAuoroethyle ne (PTFE) in fluorine . because of the difficulty of obtaining reasonably complete combustion and the diffic ulty of obtaining X-ray analysis of the solid combustion products a crystalline beryllium fluoride product. showed that amorphous rather than crystalline BeF2 All the published determinations are indirect had been formed. The products also contained un­ and involve the heats of formation of BeO(s) and burned beryllium, appreciable quantities of Be2 C, and a residue containing carbon and fluorine. The HF(aq). A heat of formation of BeF 2(s) near - 242 magnitudes and the uncertainties of the energy cor· kcal mol- t (see [1]1) is obtained jf one assumes !J.Hr [B eO(s)] is -I43.I kcal mol- I based on Cosgrove rections for the various constitue nts of the combus­ and Snyder's [2] study of the direct oxidation reaction. tion products are relatively larger fraction s of the An examination by Parker r3] of other values [4, 5, energy liberated by formation of BeF 2 than would 6, 7] for the heat of formation of BeO(s) suggests the be indicated by the degree of comple teness of co m­ above value may be too positive by approximately one bustion since 65 to 70 percent of the total energy or more kcal mol- I. The uncertainty in the heat of was due to the combustion ofPTFE. formation of HF(aq), a few tenths of a kilocalorie Failure to correct for the formation of Be2 C and the per mole, introduces a smaller though still signifi cant carbonaceous residue would have led to errors of effect. the order of 0.1 percent and 1 percent, respectively, -Direct combination of beryllium with fluorine was in the heat of formation of BeFz. Consequently, an undertaken to obtain a value for the heat of forma- accurate method for determining the amounts of Be2 C and the residue as well as unburned beryllium metal *This work was supported by the Ai ," Forc e Office of Scientifi c Research under Orde r No. OAB ISSA 65- 8 and the Advan ced Besea rch Projects Age ncy unde r Order No. 20- 60. was required, and its development constituted an I Figures in brac kets indicate ihe lil erature references at the end of this paper. important part of the experimental work. 281 2. Preliminary Combustion Experiments of the uncertaintles in the suppliers' analyses. An effort to obtain a check on the assay of metal from Attempts by other workers to burn beryllium foil measurements of the amount of hydrogen evolved on [8], powder [9], or rod [9] to completion in fluorine dissolving the powders in aqueous HF, done in con­ have been notably unsuccessful (in general, less than juction with other work performed in this laboratory 50% of the sample has burned). Similar difficulties [11], was only suggestive of the general accuracy of encountered with aluminum were overcome in this the impurity analysis in table 1, because the quantita­ laboratory by burning a pelleted mixture of aluminum tive technique for collection of hydrogen had not and polytetrafluoroethylene powders in 20 atm pres­ been fully worked out. sure of fluorine [10]. The applicability of this method PolytetraAuoroethylene (PTFE). The powder to combustion of beryllium was tested using metal was obtained as a commercial preparation desig­ powders with two sizes of particles. The first was a nated as TFE Fluorocarbon Resin, "Teflon 7". It was powder of 12 /-tm or finer particle size (passing 200 composed of irregularly shaped _particles with an mesh) and 98.9 percent purity, designated as type average size of 35 /-tm. Polytetrafluoroethylene film, BB; and the second was a powder of 25 /-tm or finer designated as FEP Fluorocarbon Resin film, of particle size (passing 100 mesh) and 99.7 percent 0.0025 cm thickness, was used to enclose the beryl­ purity, designated as type NM. lium-PTFE mixtures. Neither the film nor the powder Type NM metal powder in mixtures with ratios of was modified or specially treated before use. the weight of polytetrafluroethylene to weight of Fluorine. Two commercial preparations of fluorine beryllium ranging from 2 to 20 burned with- approxi­ were used, one of which was purified specially for mately the same percentage combustion, 75 to 90 the bomb calorimetry work. Samples were periodi­ percent. Spattering of beryllium metal onto the com­ cally analyzed by absorbing the fluorine in mercury bustion-bomb walls, formation of glassy (lump) and observing the residual pressure of the unreacted gases [16]. The residual gas was analyzed in a mass BeF2 , and corrosion of the holder were minimal when 0.1 g of metal was mixed with 1. 7 g of polymer. spectrometer. Typical analyses appear in table 2. Combustions of type BB beryllium powder in pellets The estimated uncertainties in the amounts of indi­ of the last mentioned composition were 99 to 100 vidual impurities include the uncertainty in our percent complete. measurement of the total mole percent as well as the uncertainties in the relative amounts of impuri­ Of several sample supports tested only nickel re­ ties that were estimated by the mass spectrometrist. sisted corrosion. Stainless steel and Monel reacted In the case of the purified fluorine a variation was ob­ with the beryllium in the pellet to varying degrees. served in oxygen content, which gave rise to differ­ Calcium fluoride disks tended to crack or melt. ing total impurities_ Data obtained during composi­ Both type BB and NM beryllium powders were tion analyzis definitely suggests that the major por­ burned in the preliminary combustion experiments and tion of SiF4 and its variability are due to the reac­ some calorimetric measurements were made with each. tion of fluorine with the glass walls of the gas analysis Only measurements made with the finer powder, bulbs. It may be that the oxygen arose in this or a type BB, were used in the final experiments because related process. the disadvantage of a lower sample purity in the case of the finer powder was largely offset by its higher completeness of conversion to BeF2 • On the basis 4. Calorimetric Apparatus of experience obtained in the preliminary combus­ tions, some changes in procedure and apparatus were Heat measurements were made with an isothermal­ made and are mentioned in the following sections. jacket, stirred-water calorimeter of the Dickinson A method of determining unburned beryllium ac­ design [17] as modified by Prosen and co-workers [18]. curate to 30 /-tg or better was required. An analysis One small change was made. The jacket and calorim­ based upon the reaction of beryllium metal with eter vessel stirrers were coupled by rubber O-rings concentrated KOH to give hydrogen fulfilled this and pulleys to individual motors mounted on an insu­ requirement and is summarized in section 9. lated bracket on the jacket wall to minimize heat transfer between the motors and calorimeter. The jacket-water temperature was held constant to 3. Materials ± 0.002 °C near a value of 31°C. The connections for the electrical leads between the calorimeter vessel Beryllium. The amounts and assumed states of and jacket were designed to insure good thermal the impurities of the two beryllium samples are given contact with the calorimeter jacket.