76
G. G. LIBOWITZ, H. F. HAYESAND T.R. P. GIBB,JR.
Vol. 62
THE SYSTEM ZIRCONIUM-NICKEL AND HYDROGEN‘ BY GEORGEG. LIBOWITZ,~ HERBERT F. HAYESAND THOMAS R. P. GIBB,JR. Contribution NO.246 from Department of Chemistrg, Tufts Universill;, Illedford, Mass. Received July 18, 1967
The interaction of the intermetallic compound ZrNi with hydrogen was studied, and pressurecomposition isotherms were obtained a t 100,200 and 250’. There was no resemblance to the zirconium-hydrogen system. The alloy formed a definite hydride a t a limiting composition ZrNiHa, and possibly a second hydride a t ZrNiH. X-Ray and density meaaurements on both the alloy and the hydrides indicated a distorted cubic structure for all phases. Hysteresis was observed on the isotherms and a possible explanation for this phenomenon is given on the assumption of lattice defects.
Introduction Zirconium-nickel alloy is an example of a binary alloy consisting of one metal, which reacts exothermically with large amounts of hydrogen (Zr), and another which occludes only traces of hydrogen endothermically (Ni). As has been shown by many in~estigators,~--6 zirconium reacts with hydrogen to form at least two hydride phases and possibly one or more minor transition phases. The hydride phase having the maximum hydrogen composition is ZrHz. The compound is normally non-stoichiometric, the H to Zr ratio usually being somewhat less than two. I n contrast to zirconium, the maximum hydrogen to nickel ratio is only 3 x at 820” and one atmosphere of hydrogen.’ This solubility deereases with decreasing temperature. No hydride of nickel ever has been established definitely, although the adsorption of hydrogen by finely divided nickel is quite large. The only reliable work on the zirconium-nickel alloy system has been done by Hayes, Roberson and Paasche.6 These investigators studied the zirconium-rich portion of the system and found two intermetallic compounds present at ZrNi and ZrzNi. Although some X-ray work was done, the structures of these compounds were not determined. The compound ZrNi was chosen for the present study. Equilibrium pressure measurements, Xray studies and density measurements were made on the alloy and its hydrides. Experimental Apparatus.-The ap aratus used for dissociation pressure measurements of the Kydrides consisted of a quartz combustion tube containing the sample, a resistance furnace with a temperature control system, pressure measuring gages and a gas handling system. The tubular furnace was constructed from 30 ft. of I6 gage Kanthal wire wound around a 12 in. core, 1.3 in. in diameter. Alumina and fire brick were used for thermal insulation. The density of resistance wire windings along the core was such that the length covered by the sample was at a constant temperature to within 2.5” at 400’. The furnace was mounted horizontally on a movable transite (1) This research was supported by the Atomio Energy Commission. (2) Materials Researoh Group, Atomics International, Canoga Park, California. (3) M. N. A. Hall, 8. L. H. Martin and A. L. G. Rees, Trans. Faraday Soc., 50, 376 (1954). (4) E. A. Gulbransen and K. F. Andrews, J . Eleelrochem. Soc., 101, 474 (1954); J . Metale, 7, 136 (1955). ( 5 ) P. K. Edwards, P. Levesque and D. Cubicciotti, J . Am. Chen. Soc., 77, 1307 (1955). (6) G. Hagg, 2. physik. Chem., 11, 439 (1930). (7) D. P. Smith, “Hydrogen in Metals,” Univ. of Chicago Press, Chicago, Ill,, 1948,p. 62. (8) E. T.Hayes, A. H. Roberson and 0. a. Pawche, TTans. Am. SOC. kfCtal8, 46, 893 (1953).
platform, with a track long enough to permit variable positioning of the furnace over a twenty inch length. The quartz combustion tube was connected to the vacuum system through a breakseal. The thermocouple placed alongside the combustion tube in the furnace was made from 20-gage chrome1 and alumel wires and calibrated against a standard thermocouple. Corrections were made for the difference between the thermocouple reading and the temperature inside the tube. A photocell-reflecting gdvanometer control circuit was used to control the temperature. The arrangement was such that upon activation of the photocell, the circuit was broken and the heating current shunted through a 3 ohm resistance. When not activated, a relay closed, and the furnace was heated directly. A Bristol Indicating Pyrometer Controller was put into the circuit to prevent overheating if the light beam overshot the photocell. After reaching equilibrium, the temperature fluctuation of the sample was less than rtO.3’. Pressures from 1 to 950 mm. were measured with an open end mercury manometer. Lower ressures were measured with two McLeod gages connecteJ to the system through cold traps. These gages had ranges of 1 to 10-4 mm. and 5 X 10-2 to 1 x 10-6 mm. A Pirani gage was attached to the manifold of the vacuum line for indicating the location of leaks. The gas handling system consisted of B 2.2 liter calibrated flask and a 5.4 liter calibrated flask with adjacent manometer for storing and measuring hydrogen. A three-way stopcock between diffusion and mechanical pumps permitted pumping of hydrogen from the sample into the storage flasks with the mercury diffusion pump. The volumes of the various sections of the system were calculated by expanding a gas from a known volume into the one to be determined. Boyle’s law, with suitable corrections for volume changes due to mercury movement in the manometer, was used. I n order to accurately calculate the composition of the hydride, i t was necessary t o know what portion of the sample tube was a t the temperature of the furnace, and also what percentage of the volume of the system was at Dry Ice-acetone temperatures. These “hot” and “cold” volumes were determined a t various temperatures and pressures using argon and hydrogen gas. Materials.-The hydrogen gas used for hydriding the sample was purified by passing over Drierite and hot uranium metal powder. The zirconium-nickel alloy used in these studies was prepared from hafnium-free crystal bar zirconium and electrolytic nickel by Nuclear Metals, Inc., in an arc meltingQunit under argon atmosphere. Analysis of the sample gave a zirconium to nickel ratio of 1.02 to 1, and the over-all assay of zirconium plus nickel was 99.1%. Of the 0.9%impurity, 0.4% was found to be copper, probably introduced by the water-cooled copper electrode used in arc melting. The sampling was prepared for hydriding by buffing on a wire wheel to a high metallic luster. It was then taken into an argon dry box, polished with emery paper, weighed and transferred to the combustion tube under argon. Experimental Procedure.-After eyacuating and outgassing the system, hydrogen was admltted into the storage system where i t was measured. The stopcocks to the sample tube were then opened and reaction took place a t whatever temperature was selected for hydriding. Pressure-composition isotherms were obtained by both dehydriding and hydriding the sample. Before a dehydrid(9) J. F. Kuchta and 8.Isserow, A.E.C. Report NMI-1142,Feb. 13* 1956.
Jan., 1958
THESYSTEM ZIRCONIUM-NICKEL AND HYDROGEN
ing run, residual gas was evacuated in all parts of the system except the sample tube containing fully hydrided material. The sample tube was then opened to the evacuated manometer and manifold region, and the temperature raised to the desired value. When the system reached equilibrium, the dissociation pressure was measured and recorded. The sample tube was then closed, and the hydrogen remaining in the manifold and manometer region removed. Maintaining the sample at constant temperature, the reaction tube was re-opened to the manifold and manometer region, and an equilibrium pressure again recorded after dissociation. Pressures were considered at equilibrium only when no change occurred over a 20 minute or longer interval. However, in the range from 1 to 10 mm., changes in pressure due to lack of equilibrium were often less than those due to atmospheric pressure changes. It was necessary, when in this range, to extend the time in which no discernible changes in pressure occurred to 3 hours or longer. The process of dissociation, measurement and removal of hydrogen was repeated, opening new portions of evacuated space as the pressure decreased. At lower pressures, the mercury diffusion pump was used to pull hydrogen from the sample and push it into the storage bulbs where i t was measured, and subsequently removed. At still lower pressures, removals were made by heating the sample to drive off the hydrogen, and cooling to the desired temperature before measuring the equilibrium pressure. These last measurements were always in the single phase metal region. Previous to hydriding runs, the sample was completely dehydrided under vacuum at SOO", and then brought to the desired temperature. A measured amount of hydrogen was transferred from the storage system to the manifold, manometer, and McLeod gage line. The stopcock to the sample was opened, and the sample permitted to react with the hydrogen. The hydrogen pressure over the sample was always initially higher than the dissociation pressure of the hydride at a corresponding composition. When equilibrium was reached, the pressure was recorded, and more hydrogen added. This process was repeated until the dissociation pressure reached one atmosphere. Since the time to reach equilibrium for hydriding at 100' was extremely long, the method of taking equilibrium pressure measurements was varied somewhat in the plateau region. After the addition of hydrogen and the subsequent recording of what seemed to be an equilibrium pressure, the fraction of a cc. of hydrogen in the manometer, manifold and McLeod gage line was removed. The sample was then opened to the evacuated space and dissociation took place with a corresponding drop in pressure. On repeating this several times, however, the pressure remained reasonably constant. Therefore, this pressure, on reaching equilibrium, was considered characteristic of a hydriding isotherm. X-Ray Studies.-For crystal structure studies, a 2.6-g. slice was cut from the ZrNi ingot, and prepared for reaction with hydrogen in exactly the same manner as the sample used in dissociation pressure studies. After hydriding, it was ground to a fine powder, passed through a 200 mesh sieve, and packed into glass capillaries all under argon atmosphere. X-Ray powder patterns were obtained with a G.E. XRD-4 unit using copper radiation. To obtain X-ray patterns of the zirconium-nickel alloy, the remaining hydride was resealed in a combustion tube, and completely dehydrided by heating to over 500' under vacuum. This sample, and the ZrNiH sample, obtained by discontinuing the 200"run at the appropriate hydrogen composition, were handled in air. Density Measurements.-The density of the Z r N i alloy was determined on a 20 g. ingot by suspending it under water on a Westphal Specific Gravity balance. Before determining the density, the ingot was cleaned with 10:9:1 water-HNOa-HF solution and rinsed with distilled water, alcohol and ether. For density determinations of the hydride, a 7-g. sample of ZrNi alloy was cleaned as above and hydrided to ZrNiHz.08. The hydride was then transferred anaerobically to a helium densitometer,lO and the density measured by helium displacement. Two separate samples were measured in this way.
Results and Discussion Zirconium-nickel alloy reacted readily with hy(10) W. C. Schumb and E. S. Rittner, J . Am. Chem. Soc., 66, lG92 (1943).
77
100:
3
-
0
DEHYDRlDlNG HYDRlDlNG
OJ
015
'
i.0
i5
210
n/zr.
i.5
30
Fig. 1.-Pressurecomposition isotherms for the ZrNi hydrogen system.
-
drogen to form a brittle powder with metallic appearance. The maximum amount of hydrogen absorbed by the two samples studied was 2.93 and 2.94 atoms of hydrogen per atom of zirconium. These values were a t room temperature and one atmosphere pressure. The pressure-composition isotherms obtained are shown in Fig. 1. There was a pronounced hysteresis effect between hydriding and dehydriding isotherms similar to that observed on other metal-hydrogen The most interesting result of this investigation is the unique behavior of the zirconium-nickel alloy with hydrogen. Although nickel is relatively inert toward hydrogen, its effect on zirconium is such that there is no similarity between this system and the zirconium-hydrogen system as evidenced by the following observations. (1) The limiting hydrogen content in the alloy-hydrogen system is close to three atoms of hydrogen per atom of zirconium, as compared to a rat,io of two in the zirconium-hydrogen system. (2) Plateau pressures are much higher than those at corresponding temperatures in the zirconium-hydrogen system, e.g., the plateau pressure in the zirconium-hydrogen system at 250" would be about 10-8 mm. (3) The phase boundaries of the two-phase region are a t (11) Ref. 7, p. Si'-100. (12) F. H. Spedding, A. S. Newton, .I. C. Warf, 0. Johnson, R. W. Nottorf, I. B. Johns and A. H. Uaane, Nucleonics. 4, 4 (1949). (13) T. R. P. Gibb, Jr., and H. W. Kruschwits, Jr., J . Am. Chpm. SOC., 7 8 , 5305 (1950). (14) R. N. R. Mulford and G. E. Sturdy, ibld., 78, 3897 (1956).
78
G. G. LIBOWITZ, H. F. HAYESAND T. R. P. GIBB,JR.
completely different hydrogen compositions from those of the Zr-H system. (4) X-Ray studies of the alloy hydride show no evidence of the presence of a zirconium hydride phase. From Fig. 1, it can be seen that there is a definite hydride phase with a limiting composition approaching ZrNiHt. In addition, the almost vertical inclination of the 100" isotherm at ZrNiH, plus the fact that the phase boundary occurs at this composition for each temperature probably denotes that a hydride phase of composition ZrNiH also exists. Additional evidence for a phase at ZrNiH is the slight horizontal deviation and hysteresis occurring on the 250" isotherms at low hydrogen compositions. This seems to indicate another two-phase region at a temperature somewhat below 250". Since pressure measurements under one micron were unreliable, and equilibrium at low temperatures could not be attained easily, this probable plateau could not be investigated. A n interpretation of the isotherms can be given as follows: as hydrogen is added to the ZrNi alloy, it dissolves forming a one phase solid solution with corresponding increase in equilibrium hydrogen pressure. When the solubility limit is reached, a new phase, ZrNiH, is formed giving rise to a constant pressure region characteristic of the coexistence of two solid phases. When all the alloy has been converted to ZrNiH, the pressure again rises, and the addition of more hydrogen causes the formation of the second hydride phase, non-stoichiometric ZrNiH3. Another pressure plateau is observed indicative of the coexistence of the two phases ZrNiH and ZrNiH3. With further addition of hydrogen, the ZrNiH is completely converted to non-stoichiometric ZrNiHt (actually ZrNiHz,6) which then approaches stoichiometry with a cossesponding rise in pressure. The reaction occurring in the plateau region can be written ZrNiH
+ ?+)
Hz
ZrNiH3-,
(1)
or, neglecting the non-stoichiometry of the hydride ZrNiH
+ Hz JT ZrNiHs
(2)
The heat of reaction can then be calculated from the van't Hoff equation d In K ,
-=
dT
log P H z =
d In PIC,-'= dT R TP 303RT AH constant
+ ( 7 )
Vol. 62
Hysteresis.-A possible explanation for the hysteresis in this system as well as other hydride systems can be given on the basis of a defect theory of metallic hydridesl7 assuming that the nonstoichiometry of the hydride is due to hydrogen vacancies in the lattice. As hydrogen is withdrawn from the stoichiometric ZrNiHa, hydrogen vacancies are formed, and the hydride becomes nonstoichiometric. At the composition where the lattice becomes saturated with vacancies, further removal of hydrogen causes the lattice to break down thus forming a two-phase system ZrNiHt-. and ZrNiH. Therefore, the "plateau" pressure is actually the dissociation pressure of non-stoichiometric hydride ZrNiHt-,. On hydriding, it is possible, because of the longer time to reach equilibrium, that a rather "stable" metastable hydride is formed having fewer vacancies (and, therefore, higher hydrogen composition) than the stable hydride. This metastable hydride, therefore, has a correspondingly higher dissociation or "plateau" pressure. Because the hydriding plateau represents a hydride of fewer vacancies, it should extend further to the right.(higher hydrogen content) than the dehydriding plateau, since the boundary of the single phase hydride region is at the over-all hydrogen content corresponding to the composition of the hydride. Such is actually the case, as both the 100 and 250" isotherms clearly show. On the 100" isotherms, the dehydriding plateau ends at a hydrogen content of 2.4 H/Zr whereas the hydriding plateau ends at 2.55 H/Zr. Therefore, the higher hydrides present in the two-phase region are ZrNiH2.4 and ZrNiH2.66,respectively. TABLE I d-SP..icmm FOR ZrNi ALLOY Distorted cubic: a. = 6.98 A. Obsd.
Index
Calcd.
3.12 2,465 2.305 2.225 2.115 2.04 2.01 1.621
210 220 300,221 3 10 311
3.121 2.467 2.326 2.207 2.104
222
2.015
411
1.645
420
1.561
42 1 422 510,431
1.523 1.425 1.369
520
1.296
1
1,561 1.528. 1.444 1.358 1.292 1.272 1.247 1.19 1.176
1.274 where K , is the equilibrium constant for the reac52 1 1.234 440 tion, P His~ the plateau pressure, and AH is the heat 1.197 530 of reaction. A plot of log P H US. ~ ' / T using the 1.180 three dehydriding plateau pressures shown in Fig. 1 531 yielded a linear relationship and a value of - 18.4 f X-Ray and Density Results.-As mentioned 0.2 kcal./mole hydride for the reaction monohydride to trihydride. The apparent linearity of the above the X-ray powder patterns of the alloy plot indicated that AH is reasonably constant with hydride showed no lines characteristic of zirconium temperature and variation in stoichiometry as has hydride, thus denoting a novel phase. The strucbeen found in other metal-hydrogen system~.~J68'6 tures of the ZrNi alloy, ZrNiH and ZrNiH3 were found to be identical except for a slight expansion (15) R. N. R. Mulford and C. E. Holley, Jr., THE JOURNAL, 69, of the ZrNi lattice in the case of ZrNiH and a larger 1222 (1955). (16) G. G. Libowitm and T. R. P. Gibb, Jr., {bid., 61, 793 (1957).
(17) G . G. Libowits, J . Chem. P h w . , 27, 514 (1957).
DECOMPOSITION OF MALONIC ACIDIN NON-AQUEOUS SOLVENTS
Jan., 1958
expansion for ZrNiHa. An attempt t o index the d-spacings of the ZrNi alloy yielded what seemed to be a primary cubic structure with a lattice parameter of 6.98 A. as shown in Table I. The lattice para%eters of ZrNiH and ZrNiHa were 7.045 and 7.40 A., respectively. The splitting of the ( 2 2 2 ) , (420) and (520) lines, however, indicated that the structure is actually one which deviates slightly from cubic. Additional confirmation for this deviation is obtained from the density measurements. The density of the alloy was found t o be 7.46 g./ cm.3. A comparison of the density value with the X-ray data yields a value of 10.18 ZrNi “mole-
79
cules” per unit cell. Similarly, the density of the ZrNiH,, 6.38 g./cm.3, gives a value of 10.17 ZrNiHa “nzolecules” per unit cell, The fact that these are not integers is further indication that the lattice is not quite cubic. Unfortunately, powder patterns do not give enough data t o deduce definitely the structure of this alloy and its hydrides; however, these compounds seem to have a complex (because of the large number of missing reflections) slightly distorted primary cubic structure (possibly rhombohedral) with 10 “molecules” per unit cell. Xray patterns of single crystals would be necessary for an unequivocal identification of the structure.
A SYSTEMATIC STUDY OF THE KINETICS OF THE DECOMPOSITION OF MALONIC ACID I N NON-AQUEOUS SOLVENTS BY LOUISWATTSCLARK Contribution from the Department of Chemistry, Saint Joseph College, Emmilsburg, Maryland Received July 18, 1967
Revised data are reported on the decarboxylation of malonic acid in five solvent,s previously studied and new data on eight solvents not previously studied. The data for these and eleven other solvents previously studied are summarized. The data reveal that an increase in the effective negative charge on the nucleophilic atom of the solvent lowers the enthnlpyof activation of the reaction according to predictions which would be made on the basis of the electron theory of organic chemistry.
It was postulated by Fraenkel and co-workers that, in the decomposition of malonic acid in quinoline, a transition complex is formed between the carboxyl carbon atom of the acid and the unshared pair of electrons on the nitrogen atom of the amine.’ This view has been substantiated by studies on the decomDosition of malonic acid in other non-aqueous, ba& type solvents.2 A review of the kinetic data available for the decomposition of malonic acid in the various solvents studied2 revealed that the energy of activation for the reaction appeared to decrease as the availability of the unshared pair of electrons on the nucIeophilic atom increased. This observation is in harmony with the well known principle that an increase in the attraction between two reagents tends to lower the activation energy.3 I n the light of this principle the data for the reaction in several of the solvents studied failed to show the anticipated trend and appeared to be in need of further study. These were : aniline, o-toluidine, m-toluidine, p toluidine, and 4-picoline. Experiments were repeated in each of these solvents, using every possible precaution to ensure the purity of reagents, the accuracy of temperature measurements, and adherence to the proper technique. The reaction also was studied in eight additional solvents, namely, N-eth ylaniline, N-n-propylaniline N-n- butylaniline, o-anisidine, o-phenetidine, ochloroaniline, m-chloroaniline and o-bromoaniline. I
(1) G. Fraenkel, R. L. Belford and P. E. Yankwich, J. Am. Chem. Soc., 76, 16 (1954). (2) (a) L. W. Clark, THIS JOURNAL, 60, 825 (19%); (b) 60, 1150 (1956); (c) 60, 1340 (1956); (d) 60, 1583 (1956); (e) 61, I009 (1957); (f) 61, 1975 (1957).
(3) K. J. Laidler, “Chemical Kinetics,” McGraw-Hill Book Co., Inc., 1950,New York, N. Y., p. 138.
This brings the number of solvents in which this reaction has been studied to date in this Laboratory to a total of 24. The results of this investigation are reported herein, Included also in this report is a summary of the data for the reaction in all the 24 solvents studied. Experimental Reagents.-( 1) The malonic acid used was analytical reagent grade, 100.0% assay. To ensure perfect dryness I t was stored in a desiccator containing sulfuric acid. (2) . Solvents. The solvents used in this investigation were either analytical reagent grade or highest purity. Each sample of each liquid was distilled a t atmospheric pressure into the reaction flask immediately before the beginning of the decarboxylation experiment. This was a precaution which had not been taken in the case of the five solvents previously studied, data for which were out of line with the rest. Apparatus and Technique.-The details of the apparatus and technique have been described In these experiments a sample of malonic acid weighing 0.1857 g. (corresponding to 40.0 ml. of COz at STP) was weighed into a fragile glass capsule weighing approximately 0.1 g. and blown from 7 mm. soft glass tubing. Fifty ml. of solvent (saturated with dry COZ gas) was placed in the 100-ml. 3neck, standard taper reaction flask immersed in the 011bath. The temperature of the thermostat controlled oilbath was determined using a thermometer graduated in tenths of a degree and calibrated by the U. S. Bureau of Standards to which stem corrections and other necessary corrections were applied.
Results and Discussion Table I lists the apparent first-order rate constants for the decomposition of malonic acid in the five solvents previously studied and the eight solvents not previously studied. These values were obtained in the usual manner from the slopes of the logarithmic plots. The data for aniline obtained in this Laboratory
