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KIIYETICS OF REACTION OF ELEMESTAL FLCORINE
Kinetics of Reaction of Elemental Fluorine.
11.
The Fluorination
of Hafnium Carbide and H[afnium Boride
by A. K. Kuriakose and J. L. Margrave Department of Chemistry, Rice Uniuersity, Houston, Texas
77001
(Received April 9 , 1964)
The corrosion kinetics of high purity hafnium carbide and hafnium diboride in elemental fluorine under low fluorine partial pressures (2.8-52.5 torr) in helium have been investigated up to !390" for HfC and 1020" for HfB2. Although HfC produces solid hafnium tetrafluoride from 300 to 500", the tetrafluoride does not adhere to the HfC. HfB,, on the other hand, breaks down in the same temperature range, apparently because of reaction a t pre-existing cracks when the fluorine partial pressure is 13 torr. Due to the volatility of the fluorides around 600", both HfC and HfBa begin l o lose weight in fluorine with linear reaction rates and the samples stay unbroken even to the highest temperatures examined in this investigation. The reactions are highly exothermic and the surface temperatures of the samples increase rapidly, almost proportionally to the fluorine partial pressure in the system, as fluorine is admitted. For HfC the reaction rate varies directly as the fluorine partial pressure while for HfBz it varies directly as the square root of fluorine pressure. A comparison of the behavior of the carbides and diborides of zirconium and hafnium toward fluorine is given.
I n the first paper of' this series,I measurements of the fluorination rates of ZrC and ZrBz were reported. Because of the well-known close resemblances between zirconium and hafnium compounds, similar behaviors of HfC aiid HfBz with fluorine are to be expected. However, the slightly higher volatility of HfF4 than ZrF4 should make the Hf compounds slightly more reactive a t lower temperatures.
Experimental The apparatus and experimental procedure were essentially the same as for the fluorination of ZrC and ZrBz. The HfC sain,ples were prepared from an arcmelted piece of composition HfC,. 952 with the following impurities (yo):Zr, 0.035; N, 0.031; Fe, 0,005; 0, 0.003; Ti and Si, 0.002 each; H, Cu, M n ,and N g , 0.001 each. The HfB, specimens were cut from zoiie-refined cylindrical bars which had a corriposition HfB,.,3, with impurities: Zr, 0 Ol-O.lyG;C, 0. 16y0;Cr, Cu, and M g , 0.001% each; S.42 p.p.in.; Ti and Fe, 30 p.p.ni. each; 0, 26 p.p.m.; and Si, 10 p.p.in. The surfaces were polished by inechaiiical means and their areas were calculated from geornetrical dimensions.
Results Both HfC and HfB, were found to withstand fluorine at room temperature to a pressure of 1 atm. The exposures a t higher temperatures mere done only a t low fluorine partial pressures. Fluorznation of HfC. Experiments with HfC exposed to fluorine a t 2.8 torr partial pressure indicated that a detectable reaction started a t about 300". There was formation of solid HfF4, but the reaction rate could not be measured since the product was not adherent to the HfC surface up to 400". The HfF4 was collected a t the bottom of the furnace aiid the HfC surfaces were practically clean. At 500°, however, the product was more adhesive ; but still no reproducible rate data could be obtained because of the abrupt cruinbliiig of the product layer. From a fern initial readings at this temperature, the reaction rate appeared t o be linear. At 335" the HfC began to lose weight, and the reaction became smoother. The rate data froin 565 to 890" are given in Table I. The reaction rate was almost inde(1) A. K. Kuriakose and (1964).
J. L. Margrave, J . Phys. Chem., 6 8 ,
290
Volume 68, S u m b e r 8 August, 1984
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A. K. KURIAKOSE AND J. L. MARGRAVE
pendent of temperature above 650" and the calculated activation energy practically zero. At 565" the rate was considerably lower, probably due to the incomplete volatilizatioii of the HfF4. The reaction rate was directly proportional to the fluorine partial pressure up to 52.5 torr above 600" (Fig. 1).
the reaction, but the fluorine partial pressure had a marked influence on the reaction.
Table 11: Rate Data for the HfBt-F2 Reaction Temp., OC.
Table I : Rate Data for the HfC-F2 Reaction kl, Temp,, OC.
565" 650" 720 765 800 a
PF~, torr
2.8 2.8 2.8 2.8 6.7
kl,
mg./om.2/ min.
Temp., OC.
PFZ, torr
mg./om.2/ min.
2.06 5.36 6.14 5.18 8.26
800 820 825 890
13.2 52.5 2.8 2.8
11.71 49.80 6.32 6.08
765 830 900 975 810 840 940 1020
PF~, torr
2.8 2.8 2.8 2.8 6.7 6.7 6.7 6.7
hi, mg./cm.Z/ min.
Temp., *C.
torr
5.31 5.24 4.86 5.16 6.92 6.15 9.37 8.82
610" 835 880 985 610a 890 950 610a
13.2 13.2 13.2 13.2 30.6 30.6 30.6 52.5
PFZ,
kl, mg./om.a/ min.
9.82 7.94 9.62 11.87 13.56 12.95 15.77 19.44
Furnace temperature.
Furnare temperature.
Fluorination of HfB,.Although no measurable reaction occurred between HfBs and fluorine a t low fluorine pressures up to ZOO", the samples broke down presumably a t pre-existing cracks even a t about 350" a t 13.2 torr fluorine pressure. Sniall quantities of white HfF, were found wit,hin the cracks, but not on the outer surface. HfBS began to lose weight a t about 550' but it stayed together even up to 1020". The
10
20
30 40 P n , torr.
50
As the fluorine pressure was increased, there was a proportional rise in the sample surface temperature and above 1000" sparks began to appear as the specimen was observed through the optical pyrometer. Shortly after the appearance of the sparks, the nickel wire burned up and the sample fell down. Although the sample surface temperatures rose above lOOO", the actual furnace temperature was never raised above about 900". I n Table I11 are shown the actual surface temperatures of a sample of weight about 0.7 g. and surface area about 1.3 cm.2under various fluorine partial pressures a t a furnace temperature of 740". Figure 2 is a plot of the rate constants against the square root
BO
Figure 1. Effect of fluorine partial pressure on the HfC-Fz reaction above 600'. 2
reaction was linear as expected and the rate constants under various conditions are recorded in Table 11. Here again, temperature had practically no effect on The Journal o j Physical Chemistry
4 PFil'a,
6 torr.
Figure 2. Effect of fluorine partial pressure on the HfBZ-F2 reaction above 600".
8
10
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KINETICSOF REACTION OF ELEMENTAL FLUORINE
of fluorine partial pressure. The linearity of the plot indicates that the rate of fluorination of HfB, varies directly as the square root of the fluorine partial pres-. sure. ~
Table I11 : Surface Temperatures of HfBz Samples with Various Fluorine Partial Pressures a t a Furnace Temperature of 740"
Pel, torr
0 2.8 6.7 13.2 30.6
Surface temperature, OC.
740 765 810 835 890
Discussion As expected, the behaviors of both Zr and Hf carbides and borides toward fluorine are similar. The borides of both meta,ls are more resistant to fluorine than the carbides. Whereas the carbides begin to react a t around 300" under low fluorine partial pressure conditions forming solid product,s, the borides are attacked only above the volatilization points of the
metallic fluorides. Thus, a coating of the fluoride is never obtained on the ZrBz and HfB2. At temperatures above 600" all the reactions are gas diffusion controlled which is characterized by the insensitiveness of the rates to surface temperature changes. ZrC differs from WfC in that while the former forms thick scales of ZrFl below 500°, the latter does not grow any scales on it. HfBz crumbles in fluorine even a t low fluorine partial pressure a t 350-550", but the ZrBz stays undamaged up to 500" under the same conditions. The fluorination of HfC and HfB2 is much faster than that of ZrC and ZrB2. Even after converting the rate constants to moles, the HfC and HfBz are found to react with fluorine about four to five times faster than ZrC and ZrBz. The dependence of the reaction rates on the fluorine partial pressure is also similar for both Zr and Hf carbides and borides, except that ZrC shows a 1.5 order with respect to fluorine while the HfC shows first order.
Acknowledgments. This research was supported, in part, by the United States Air Force under a subcontract with A. D. Little, Inc., and adniiiiistered by Dr. Leslie A. McClaine. Samples of HfC and HfB, were furnished through the courtesy of Dr. George Feick.
Volume 68, Number 8
August, 1064
