Reactivity of Titanium Hydride d
d
with Air KERMIT ANDERSON AND W. S. FLESHMAN Fairchild Engine and Airplane Corporation, Oak Ridge, Tenn.
A
preliminary investigation of the rate of oxidation of the hydrides of tantalum, zirconium, columbium, and titanium in static laboratory air showed that titanium hydride was most resistant to oxidation. A further investigation of the reactivity of titanium hydride, using a dynamic method, indicated that with dried air, reaction rate decreased with increasing density and increased with increasing teniperature and air flow rate. The rate of reaction seemed to be strongly influenced by diffusional processes. Diffusion of decomposing hydrogen from within the pelletized sample and of atmospheric components across the gaseous boundary lager and into the sample were apparently important rate determining processes.
T
HE hydrides of tantalum, zirconium, columbium, and titanium, stable in normal atmospheric conditions, will begin t o decompose when heated above 100" C. Although numerous vapor pressure studies have been made with these hydrides (1-4) little was known of their reactivity on contact with air at elevated temperatures. This subject was recently investigated on the NEPA project (Nuclear Energy for Propulsion of Aircraft) and this paper gives a summary of the information obtained. MATERIALS, APPARATUS, AND METHODS
Samples of each of the hydrides, titanium, zirconium, columbian, and tantalum were obtained from stock supplies of Metal Hydrides, Incorporated, of Beverly, Mass. Single samples of TaH, ZrHl.,e and CbH were used throughout this work, but two separate samples of TiH1.92 were required to complete the investigation. All of these samples were specified by the manufacturer to pass 99% through 325-mesh screen. A small open-type furnace, equipped with thermocouple connections, was used in static air tests. A 1.25-inch inside diameter combustion furnace, equipped with a heat-resistant glass tube and thermocouple connections, was employed in the dynamic laboratory air tests. A conventional type combustion train was utilized for dynamic dry-air tests. Static-air tests were conducted by heating a hydride sample until ignition took place. The reaction during ignition was observed and the effect of ignition on the weight of the sample wm determined. Dynamic-air tests were carried out in a stream of normal laboratory air passing through a glass tube holding the hydride sample. The same observations and determinations were made to find the effect of ignition on the hydride material. The dynamic dry-air tests were all made in the combustion train t o find the amount of hydrogen relBased from the hydride material by the action of heat. STATIC AND DYNAMIC TESTS WITH LABORATORY AIR
Some preliminary experiments were first made with all of these hydrides, in pelletized form, in both static and dynamic laboratory air. The pellets were pressed in a cylindrical die, 8.4 mm. inside diameter, and were approximately 10 mm. in length. The purpose was to investigate such factors as the relative stability of each in air at elevated temperatures; the approximate tempera-
ture at which each ignited; the reaction during ignition; and the effect of ignition on the pelletized form and the weight of the sample. The results indicated definite trends. The order of increasing stability, based on ignition temperatures, was TaH, ZrHl.ra, CbH, and TiHl.ez. Although ignition temperatures varied with changes in heating conditions, that for TaH was close to 300" C. and increased t o approximately 612' C. for TiHI.92. No violent reaction took place during ignition. Flaming action occurred during ignition in static-air tests, but only a glow accompanied ignition in dynamic tests. The hydrogen evolved in static tests tended t o remain near the pellets t o account for the flame during ignition. However, the hydrogen liberated in dynamic tests w f ~ constantly s swept away to account for only a glow during ignition. The pellets of titanium hydride never lost their shape under any conditions of these tests, but pellets of all the other hydrides disintegrated to powder form during ignition. All pellets gained weight during heating with the greatest gain taking place with ignition. Weight gain was essentially due t o oxidation. DYNAMIC TESTS WITH DRY AIR
Since the preliminary experiments showed that titanium hydride was most resistant t o air a t elevated temperatures, the remainder of this investigation was confined t o the use of pellets of this hydride. This work wm divided into three groups of tests t o study the following factors:
1. The effect oE apparent density a t constant temperature and 'air flow rate 2. The effect of temperatures at constant air flow rate and apparent density 3. The effect of air flow rates a t constant temperature and apparent density Since dry air was used, reactivity wm measured primarily by the rate of loss of hydrogen. Compressed air was dried in eonventional manner and forced through a preheater in the forward furnace. After p w i n g over the hydride sample and picking u p hydrogen, the dry air was forced through a column of copper oxide in the second furnace t o effect complete conversion of t h e hydrogen t o water vapor. This water vapor was absorbed in drying tubes, weighed periodically, and calculated as hydrogen. I n all tests, observations were made of any activity taking place during heating, and samples were weighed before and after heating to determine percentage gain in weight. Effect of Density on Reactivity. It was assumed that the outward diffusion of hydrogen from within a heated hydride pellet would decrease as the hydride particles became more densely packed. Accordingly, three pellets of titani'um hydride were pressed t o different apparent densities and tested. I n each run, the temperature of the furnace was first raised t o 500" C. and wm then maintained a t this temperature. The air flow was regulated for a constant rate of 250 ml. of dry air per minute; the sample was quickly inserted into the tube and the system closed; heating was continued for 4 hours under these constant conditions. Hydrogen was determined a t intervals as water vapor absorbed in the drying tubes. The data are given in Table I and results presented graphically in Figure 1. 1381
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 42, No. 7
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'TIME- M I NUTES 16
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2 TI ME - M INU TES
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I
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64
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256
144
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60 120 180 240 T I M E MI NUTES
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Figure 1. Effect of DensitJ on Reactivity of Titanium Hydride with Dynamic Dr:, Air
Figure 2. Effect of Tetnperature on Reactivity of Dry Air with Titanium Hydride
Flow rate wnstant at 259 ml./mm. and temperature constant a t 5ooo C.
Air flowrate constant a t 250 m l . / m i n . and d m q i t y r o n s t a n t a t 2.6 g. ml.
7 / ~M!E - M I N. Figure 3. Effect of Air Flow Rate on Reactivity of Titanium Hydride with Dynamic Dry Air Temperature constant at 500' d e n s i t y a t 2.6 g./ml.
C . and
ary film a t the surface of the hydride pellet, some dependence on TABLE I. COMPOSITE DATAFOR TITAXILXI HYDRIDE REACTIVITY air flow rate would be expected. Increasing the air flow rate will WITH DYNAMIC DRYAIR decrease the boundary film thickness, and there should result an (Effect of density a t constant te, m e m a t u r e and air flow rate) increase in hydride reactivity with air. Pellets of titanium hy-Pellet density. g./ml. 2.37 2.77 3.15 dride of comparable densities were used in these tests. The 4 4 4 Heating time, hours Gain i n weight, % B 43 4.57 3.93 temperature of the pellet was maintained a t 500" C. in each run HQLost, % while constant air flow rates of 50, 450, and 1000 ml. of dry air E n d of 30 min. 28.6 19.5 16.7 E n d of 6 0 min. 20.4 32.8 23.9 per minute were used in the separate tests. The previously E n d of 120 min. 38.8 31.5 25.0 29.4 42.3 35.5 E n d of 180 min. described equipment and techniques were used. The dat'a are E n d of 240 min. 32.6 44.7 38.3 given in Table 111, the results presented graphically in Figure 3. Figure 3 has been plotted as percentage of hydrogen lost as it function of the square root of time. The linear relat,ionship displayed emphasizes the extent t o which diffusional processes conThe results show that the lowest rate of hydrogen loss \$as trol the rat,e of reaction. found in the pellet with the highest density. This was probablj The results substantiate the assumption that hydride reardue to the greater compactness of the hydride particles arting as t,ivity, as measured by the rate of loss of hydrogen, would be increased interference to the outward diffusion of hydrogen. I t accelerated by increased air flow rates when temperat'ure and was also found that the percentage gain in weight decreased with increasing density. This was probably due to the increased resistance t o the inward diffusion of oxygen caused by greater compactness of the hydrides a t higher densities. The small percentTABLE 11. COMPOSITE DATA FOR TITANIUM HYDRIDE age gain in weight was characteristic of all these hydrides when REACTIVITY WITH DYXAMIC DRYAIR heated below their ignition temperature. (Effect of temperature a t constant air flow r a t e a n d density) Effect of Temperature a t Constant Air Flow Rate and Density. 500 Temperature, C. 400 600 Under constant conditions of air flow rate and density, increasing 2 77 2.81 Pellet density, g./ml. 2.85 do Heating time, hours 4 4 the temperature of a hydride body should increase the outward Gain i n weight, % 2.37 4.57 1i.40 HQLost, diffusion rate of hydrogen and the inward diffusion rate of oxygen. 17.4 99.7 E n d of 15 min. Accordingly, three pellets of titanium hydride of comparable 19.6 E n d of 30 min. 7.0 23.9 E n d of 60 min. densities were pressed and tested in a constant air flow rate of 250 E n d of 120 min. 8.5 31.5 10.1 35.5 E n d of 180 min. ml. of dry air per minute to study the effect of temperature on E n d of 240 min. 10.8 38.3 hydride reactivity. The tests mere run a t 400" C., 500" C., and a Test a t 600' C. was continued for same length of time although ignition 600" C. for 4 hours. The same equipment and techniques pietook place during t h e first 15-minute i n t e r i a l . viously described were used. The data are given in Table IJ and presented graphically in Figure 2. TABLE 111. COMPOSITE DATAFOR TITAXIUM HYDRIDEO The results show that rate of hydrogen loss (hydride decomREACTIVITY WITH DYNAMICDRYAIR position) W B S accqlerated by increased temperatures, probably by (Effect of temperatiire a t constant air flow r a t e a n d density) some such function as might be predicted by theoretical kinetics. 1000 Air flow r a t e ml./rnin. 50 450 The ignition of the sample tested a t 600" C. with the subsequent 2.78 2.79 2.78 Pellet densit; g./ml. loss of practically all its hydrogen was expected, since this tem4 4 4 Heating time,'hours 6.70 6.69 6.36 Gain i n weight, Yo perature is very close t o the critical ignition temperature of this Hz lost, % 13.3 5.6 10.2 E n d of 30 min. hydride. The results also indicate that the rate of oxidation of 15.6 20.4 9 7 E n d of 60 min. the metal increases with rising temperature, when other factors 30.6 E n d of 120 nlin. 17.3 22.7 37.4 22.5 E n d of 180 min. 27.6 are constant. 43.4 E n d of 240 min. 26.7 32.3 Effect of Air Flow Rates at Constant Temperature and DenD a t a of Table I11 were obtained from tests with t h e second sample of sity. Since hydride reactivity with air must depend on the rate TiH1.m used in this investigation. of outward diffusion of gaseous reaction products across a boundO
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INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
density were constant. The percentage gain in weight was found to be almost independent of air flow rates when samples were heated below ignition temperature. SUMMARY
The results of this investigation indicate t h a t the hydrides of tantalum, zirconium, columbium, and titanium will ignite in air and lose their hydrogen when heated sufficiently. This activity is not explosive when small quantities are tested. Each hydride has a characteristic ignition temperature which varies slightly as heating conditions vary. Tantalum hydride has the lowest and titanium hydride the highest ignition temperature. This ignition temperature is affected by the physical form of the hydride and by whether it is heated in static or dynamic air. These hydrides gain weight during heating essentially because of oxidation although it is possible some gain is due t o nitride formation. The rate of gain in weight is dependent on the rate of diffusion of oxygen into the pellet, a t least up to the ignition point. When other factors are constant, t,he percentage gain in weight
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decreases with increase in density and increases with increase in temperature. The reactivity of titanium hydride with dry dynamic air is reduced as the density increases, when the factors of temperature and air flow rate are constant. Temperature increase, when density and air flow rate are constant, results in increased reactivity. When the temperature and density are constant, reactivity will increase when the air flow rate is accelerated. LITERATURE CITED
(1) Gibb, T. R. P., Jr., Electrocham. Soc., 93,199 (1948). (2) Gibb, T. R. P., Jr., "Preparation and Properties of Hydrides,"
Metal Hydrides Incorporated, Beverly, Mass.
(3) Huttig, G. F., Z.angew Chem., 39,67 (1926). (4) Kirschfeld, L., and Sieverts, A , , Ber., 59, 2891 (1926). RECPIVEDOctober 3, 1949. This document is based on work performed under oontract for t h e United States Air Force by the N E P A Division, Fairahild Engine a n d Airplane Corporation at Oak Ridge, Tenn. Presented before t h e Division of Industrial a n d Engineering Chemistry at the 116th CHEMICAL SOCIETY,Atlantic City, N. J. Meeting of the AMERICAN
Iodine Heptafluoride Preparation and Some Properties WALTER C. SCHUMB AND MAURICE A. LYNCH, JR.' Massachusetts Institute of Technology, Cambridge, Mass. Iodine pentafluoride, as one of the interhalogen compounds, has chemical interest both of a theoretical and practical nature. It is unique in being the only simple binary compound in which sevenfold co-ordination is exhibited, and in common with other halogen fluorides it is a possible substitute for fluorine in certain fluorination processes. A detailed account of the preparation and purification of iodine heptafluoride is presented. No stable addition compounds of the type M(IF8) were obtained from the interaction of iodine heptafluoride and several alkali halides. The reactions between iodine heptafluoride and several chlorofluorocarbons are presented along with a brief synopsis of previous work.
M
OISSAN ( 4 ) observed that when iodine pentafluoride, prepared by the combustion of iodine in fluorine, was heated to 500" C . iodine w m liberated, and he concluded that another higher fluoride of iodine must simultaneously be formed. This observation led Ruff and Keim (6) to the discovery in 1930 of iodine heptafluoride. At ordinary temperatures, fluorine and iodine pentafluoride fail to react, but a t elevated temperatures the pentafluoride vapors react in the presence of excess fluorine to produce the heptafluoride, a white solid, which sublimes a t 4 " C. forming a colorless gas with a distinctive odor resembling neither fluorine nor iodine. Under pressures slightly above atmospheric the solid melts a t 5 ' to 6 O C. to a colorless, mobile liquid ( 2 ) . The compound has particular interest not only because it represents a unique example of a neutral binary compound with 7-co-ordination of the central atom, the molecular structure of which has recently been shown (3)to be pentagonal bipyramidal, likewise unique, but also because it may conceivably serve as a substitute for elementary fluorine in fluorination processes. Certain of the other halogen fluorides, such as chlorine trifluoride and bromine pentafluoride, have become commercially available for this purpose. 1
Present address, Linde Air Products Company, Tonawanda, li.Y.
PREPARATION
The iodine heptafluoride needed both for the determination of the infrared and Raman spectra of this compound and for the study of its fluorinating action was prepared in quantity and in a condition of high purity by the following modification of the method employed by Ruff and Keim (6): The reactor em lo ed, shown in Figure 1, was constructed of copper, nickel, and'donel, as indicated. Quarter-pound samples of reagent grade iodine were introduced by removing the soft copper gasket, 4, a t the top of the column, or else samples of iodine pentafluoride, previously prepared, were introduced through the fluorine inlet valve, 3. (The preparation of the pentafluoride was carried out in a similar apparatus, except that the condenser assembly was omitted and a nickel stirrer, motor driven, passed centrally down through the column, I , to the bottom of the Monel sphere, 2.) In the former case, but not the latter, the Monel sphere, 2, was chilled in an ice bath. Fluorine was admitted at a rate of approximately 0.4to 0.5 mo!e per hour, a t which rate absorption was essentially complete. The fluorine was supplied from a commercial 70-ampere 100" C. cylindricaltype generator manufactured and donated for the authors' use by the Hanhaw Chemical Company of Cleveland, Ohio. The electrolyte was of composition approximating KF-1.9 HF, containing a small proportion of lithium fluoride which reduces the incidence of anodic polarization (7). Hydrogen fluoride was removed from the gas by passage through a copper trap cooled to -78' C. and through a copper tube containing sodium fluoride a t 100" C. Waste fluorine was absorbed by passage through a 6-foot length of 2-inch iron pipe packed with roll sulfur t o convert the gas t o a mixture of sulfur fluorides. After passage of the effluent gas through a 1-inch Monel tube heated t o about 400" C. in order t o yrolyze any disulfur decafluoride, the effluent gas was allowe~ftoescape into the air through a copper tube leading directly t o the roof. When the absorption of fluorine had appreciably decreased, the temperature of the column was raised t o 280' t o 290 O C. and the Monel flask was brought t o 70" t o 80" C. by means of a water bath, thereby increasing the concentration of iodine pentafluoride vapor passing up the column. The rate of flow of fluorine was reduced at the same time to 0.2 to 0.3 mole per hour. The iodine heptafluoride leaving the column was condensed out of the stream of excess fluorine in Monel traps cooled to -78' C. by a mixture of solid carbon dioxide and trichloroethylene. Based on the uantities of iodine used, estimates of the amount of heptafluori3e obtained indicated nearly quantitative yields of the product.
