HEATOF FORMATIOX OF TITANIUM DIBORIDE
Dec., 1961
meaningless, but the oxime pK does appear to go through a maximum very near Oo.
it may be pointed Out that the values are characteristic. Ionization of the pyridinium group presumably follows the equation
+ HzO
RNH+
2213 RNH
+ HaO+
whose symmetry suggests that the entropy change should be small; but ionization of the oxime group is of the same kind as that of carboxylic acids, which decrease of usually is accompanied by a significant entropy.
THE HEAT OF FORiIlATION OF TITAN1U.M DIBORIDE : EXPERIMENTAL AND ANALYTICAL RESOLUTION OF LITERATURE CONFLICT BY &‘ENDELL
s. WILLIAMS
Research Luboratory of National Carbon Company, Division of Union Carbide Corporation, Parma SO, Ohio Received Ju2u 10, 1961
-
Although the literature contains three independent values for the heat of formation of TiBz of -70 kcal./mole, the present work shows that this agreement is fortuitous and that each value is in error for a different reason. Brewer and Haraldsen’s value, -71.4 kcal., obtained from the reaction TiN 2BN = TiBz 3/2 NP,is questionable because of experimental difficulties and a mistake in the tabular data employed. Samsonov’s experimental value, -70.04 kcal., obtained BIC = 2TiBz COZ, is incorrect because of the use of unreliable thermodynamic data. Samfrom the reaction 2Ti0 sonov’s calculated value, -73 kcal., obtained from an empirical relation between heat of formation and volume change, could not be duplicated. Another value for the heat, -32 kcal., obtained by Schissel and Williams with a mass spectrometer and Knudsen cell, is shown to be low by a stability comparison with T i c and B&. When corrected, within appropriate limits of error, the three experiments yield results in agreement with the recent calorimeter value of Lowell and Williams, -50 f 5 kcal./mole.
+
+
+
+
I. Introduction While investigating the vaporization of the refractory hard metal TiBz with a mass spectrometer, Schissel and the writer’ obtained and reported a value of -32 kcal./mole for the heat of formation. This value differs substantially from the four other literature values, all of which a,re -70 kcal./mole. By a study of the reaction of titanium and boron in a nitrogen atmosphere, Brewer and Haraldsen2 obtained -71.4 kcal. ; by a study of the reduction of Ti02 by carbon and boron carbide, SamsonovS obtained -70.04 kcal. ; by use of an empirical formula of Kubaschewski,4 Samsonov6 calculated -73 kcal.; and by analysis of the literature data Krestovnikov and Vendribs selected the value -70.00 kcal. In an attempt to reconcile the conflict, the writer performed several additional experiments and analyzed the papers mentioned. The additional experiments were of three types: (1) stability comparisons in vhich bounds were placed on the unknown heat of formation by comparison with other compounds; (2) direct reaction of the elements in a high temperature calorimeter, with ow ell,^ and (3) a refinement of the Brewer and Haraldsen experiment.2 A discussion of each of th.ese experiments follows as Section 11. In Section 111, an analysis of each of the earlier determinat,ions of AHf(TiB2)is presented.
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(1) P. 0. Schissel and W. S. Williams, Bull. Am. Phus. Soc. Ser. II, 4,No. 3 (1959). ( 2 ) L. Brewer and H. Haraldsen, J . Electrockem. Soc., 102, 399 (1955). (3) G.V. Samsonov, Zhur. PrikZad. Khim., 28,1018 (1955). (4) 0. Kubaschewski and E. L. Evans, “Metallurgical Thermochemistry,” Third Ed., Pergamon Press, 1958. ( 5 ) G. V. Samsonov, Zhur. Fiz. Khdm., 30,2057 (1956). (6) A. N. Krestovnikov and M. S. Vendrikh, Izvest. Vyssikh Ucheb. Zavednil Tsoet. Metall., No. 2, 54 (1959). (7) C. E. Lowell and W. S. Williams, Rev. Sei. Instr., in press.
11. Description of Present Work Stability Comparisons.-The results of the (1) first stability comparison, presented in Table I, establish the coexistence of TiBz and C at temperatures up to 2250’, in agreement with the ternary diagram presented by Brewer and Haraldsen. Thus A F and AH > 0 for the reaction TiBz 3/2 C = TiC 1/2 B4C. From the heats of formation of B4C and AHr(TiB2) < -51 f 5 kcal./mole. This result shows that the -32 value for AHt(TiB2) must be in error. Other comparisons were made against various titanium and boron compounds, but because of a deficiency of thermodynamic data for these materials the results are principally of qualitative interest (Table 11).
+
+
TABLE I RESULTSOF INVESTIGATION OF THE REACTION TiBz 3/2C = TiC 1/2 B4C
+
Reactants
+C +C + 2B Tic + 2B 2TiC + B& Ti& TiBt TiC
+
Products
+ + + +
TiBz C TiB2 C TiBz C TiBz C 2 T i B ~ f 3C
Temp. Crucible
(“(2.)
Time (hr.)
TiB2 carbon carbon carbon carbon
1900 2250 2050 2100 2000
1.5 8
4 3 1
(2) Direct Reaction Calorimeter.-In the calorimeter used by Lowell and Williams,? titanium and boron powders were mixed in the correct ratio tJoyield TiB2, packed in a thermally isolated graphite capsule and heated in vucuo. At a temperature of 1500’ an exothermic reaction occurred, raising the temperature of the capsule above that of the heater by 1000° in 0.2 second. By X-ray diffraction analysis of a pulverized sample the product was shown to be all TiB2. The temperature rise of the capsule was followed with a calibrated photo-
WENDELLS. WILLIAMS
2214
Vol, 65
TABLE I1 ADDITIONAL STABILITY COMPARISONS Reactants
Products
TiBz
+ hfo
TaBz TiBz
+ Ta
TiB TiB
TiBz
+W
FVzB
+ Ti
Type of analysis
p(Mo-Ti)B
+ TaB + TaB
X-Ray diff. and emission spectroscopy X-Ray diff. X-Ray diff. X-Ray diff.
tube feeding a recording galvanometer. From the corrected AT, the number of moles of C, Ti and B determined by weighing, and heat capacities for the elements given by Stull and Sinkel8 the heat of formation of Ti& was found to be AH* (TiB2) = -50 5 kcal./mole. (3) Nitride Transition.-Brewer and Haraldsen studied the reaction TiN 2BN = TiBz 3/2N2 and reported a temperature a t which A F reaction S 0, hereafter called the transition temperature Tt. Above this temperature AFreaction < 0, and below, AFreaction > 0. Once this temperature was known, they used extrapolated literature values of AFf for TiN and BN to compute AHf (TiB2). It was decided to repeat their experiment, avoiding the use of a molybdenum crucible, which reacts with TiB2 as shown in Table 11, and using more than the two reaction temperatures employed by these workers. Two different experimental geometries were used, both leading to a value of Tt = 2150 f 25OK. (Brewer and Haraldsen’s value was 1820OK.) In the first method, BN capsules filled with BX-TiN mix were heated in a miniature graphite tube furnace. Five runs were made a t different temperatures in 1/2 atmosphere of K2. Pyrometer readings were made on a small hole in the side of the graphite tube and corrected for losses through the chamber wall. The contents of the BN capsule were analyzed by X-ray diffraction after each run. The results are summarized in Table 111. Visually, the three phases in runs 3 and 5 were randomly dispersed, indicating temperature uniformity. The transition temperature indicated by this experiment is 214OOK.
*
+
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TABLE 111 RESULTSOF SEARCHFOR TRANSITION TEMPERATURE IN REACTION 2BN TiN = TiBz 3/2 N2.
+
T ,OK.
(Control sample) 2300 2033 2163 2128 2148 1870
+
Time (hr.)
Products
BN, TiN 0.5 1.25 1.5 1
3.5 30
TiB2 BN, TiN BN, TiN, TiB2 BN, TiN BY, TIN TiB,( weak) BN, TIN
The second geometry employed induction heating. The powders were contained in a BN crucible in a graphite susceptor. Temperature readings were made via a prism and Pyrex flat in a hole in the top of the crucible, The transmission coefficient for the optical system was determined using a flat-filament lamp, and checked against the (5) D. R. Stull and C.C . Sinke, “Thermodynamic Properties of the Elements ” Adv. in Chem. No. 18.Am. Chem. SOC., 1956.
Container
TiB2 crucible T a tube TiBz crucible
Mode of heating Temp. (OC.)Time (hr.)
Induction
1900
Resistance 2250 Electron 2ooo\ bombardment 2200 W strip (free surface) Resistance 2300
1.5
8 5 1/4 1/2
melting point of gold. X-Ray analysis was used after three runs a t different but constant tempers ture in Nz and placed bounds on the location of Tt. In several later runs a manometer was connected to the system and the Kz pressure monitored as a function of temperature. A rise in pressure was taken as an indication of the conversion of the two solid nitrides to the boride, releasing nitrogen to the gas phase. The value of Tt found in this way (2159OK.) lay between the limits set by the X-ray results on the second set of constant-temperature runs. The value adopted on comparing the two sets of measurements was Tt = 2150 f 25OK. The above result can be criticized on the grounds that equilibrium has not been demonstrated: the transition from the nitrides to the boride might be rate-limited by diffusion or some other process and hence the true value of T , should be lower than observed. In an attempt to test this hypothesis, the transition was sought from the high-temperature side. KOreversal in the direction of the reaction was detected on lowering the temperature. Then TiBz powder was heated in a graphite tube in successive atmospheres of N2 and ammonia a t temperatures below even Brewer and Haraldsen’s reported transition. In each case the TiBz showed no evidence of conversion to the nitrides of boron and titanium. Because of this experimentally observed lack of reversibility of the reaction, due possibly to surface contamination of the TiB2 with oxides, the value of Tt obtained in these experiments represents an upper bound. The agreement between runs and under slightly varying experimental conditions-e.g., amount of previous outgassing before introduction of N2 atmosphere, extent of compaction of powders-suggests, homever, that 215OOK. might indeed be a good value. Additional evidence that Tt is higher than that reported by Brewer and Haraldsen comes from the writer’s observation that mixed T i S and BN in Nz do not convert to TiB2 a t 187OOIi. after holding at temperature for 30 hours. This result is significant because Brewer and Haraldsen found TiB2 after reacting Ti, B, and Nz at a temperature 50’ lower. Using the above value of T k for the purposes of calculation, the heat of formation of TiBz is obtained as follows: entropy, enthalpy and heat capacity data for Kz and B are from Stull and Sinke; the values for BN are from the Kutional Bureau of Standards tabulation,$ Tbhich is more reliable for this compound than The U. S. Bureau of Nines Bulletins’O KO.426 or 584. A discussion of this (9) National Bureau of Standards, “Selected Val>ies of Chemical Thermodynamic Properties,” Series 111. (IO) K. K. Kelley, “Contributions to the Data on Theoretical Metallurgy,” U. S. Bur. Mines Bull. 476 and 584.
HEATOF FORMATION OF TITANIUM DIBORIDE
Dec., 1961
point is given in Section I11 (1). The Dworkin, Sasmor and Van Artsdalen" value of AHr(BN) = -60.7 kcal./mole a t 298'K. is adopted. The highest temperature given for the thermodynamic quantities in the WBS table is 1200'K., leading directly to AHr(BN) = -59.8 at 1200'K. Adjusting to the observed transition temperature gives aHf(Bh') = --59.0 at 2150'K. Using A S appropriate to 2150'K. gives AFf(BN) = -15.9 kcal./mole at 2150'K. The other quantity needed is AFr(TiN) at 2150'K. The KBS table gives values up to 1500'K. Using a linear extrapolation, we obtain -34.6 kcal./mole for the required quantity. Then AFf(TiB2) = AFf(TiN) 2AF(BN) = -66 kcal./mole at 2150'K. Except for a slight correction for the A F of fusion of Ti, this value represents the LWf(TiBz) a t 298'K. generated by this experiment and the supplementary data available. A substantial difference exists between the calorimeter value of Lowell and Williams, -50 kcal./mole, and the nitride transition value described above, -66 kcal./mole. This difference cannot be attributed to lack of equilibrium in the present nitride transition experiment since if the true transition temperature were lower than that observed, the computed heat of formation would be even more negative than -66 kcal./ mole. Cumulative uncertainty in the thermodynamic data employed and the high temperature extrapolations of these quantities probably are responsible. Ten per cent. uncertainty in each value, which is reasonable, makes them overlap. 111. Analysis of Earlier Experiments (1) Brewer and Haraldsen.*-In the nitride transition experiment discussed in Section I1 (3) and originally performed by Brewer and Haraldsen, two general sources of error can exist. One is the experimental determination of the transition temperature, which established A F = 0 in the reaction TiN 2BN = TiBz 3/2 Nz, and the other is the thermodynamic values from the literature for AF,(TiN) and dbFf(BN). The present work indicates that both types of error were included. In computing AF!(Br\i) a t their transition temperature, Brewer and Haraldsen evidently used heat content and heat capacity data from U. S. Bureau of &lines Bulletin 476, tabulated by K. K. Kelley. These values, in turn, were taken from the experimentd work of Magnus and Danz,I2 but their data referred to one-half mole. Thus, Bulletin 476 and its successor, Bulletin 584, contain values for BN heat content and heat capacity that are too low by a factor of two since they are intended t o refer to one mole. This point has been confirmed by Kelley.la With the use of the corrected heat capacity equation, the Brewer and Haraldsen experiments yield AHr(TiBz) = -S4 kcal./mole . The above remarks have dealt only with the second type of error. As mentioned in Section I1 (3), the value reported here for the transition
+
+
+
(11) A. S. Dworkin, D. J. Sasinor and E. R. Van Artsdalen, J . Chem. Phus., 22, 837 (1954).
(12) A. Magnus and H. Dam, Ann. Phyuik, 81, [41,407 (1826). (13) K. K. Kelley. private communication.
2215
temperature Tt does not agree with that found by Brewer and Haraldsen. In their original experiment, two runs were made in the attempt to fmd Tt: at 2270'K., TiB2 formed in abundance from a mixture of Ti, B and BN heated in Nz; at 1820°K., only the elemental powders of Ti and B were used as solid reactants, and a small amount of TiBz was formed along with a large amount of TiN and possibly a trace of BN. This temperature, 1820°K., was taken by Brewer and Haraldsen as the desired Tt for computing aHf(TiB2). The absence of BN X-ray diffraction peaks comparable in intensity with those found for TiN requires explanation. A possibility is that the amorphous boron did not react with Nz at 1820'K. in the period of time employed (50 minutes), but instead reacted with the Ti to form TiB2. To investigate this point, the writer heated amorphous boron in one atmosphere of N2 to a temperature 50' higher than the Brewer and Haraldsen value of Tt. At the end of one hour no X-ray diffraction peaks for BN were found. Presumably the lack of reaction is due to surface contamination of the boron particles, although in the writer's experiment the boron powder was outgassed in vacuo a t 1500'K. for an hour before the NZ was admitted to the system. (At 227OoK., the same experiment yielded about 50% conversion of amorphous boron to BN.) In the calorimeter experiment of Lowell and Williams it was shown that Ti and amorphous B react to form TiBz at a temperature below 1820°K., and other work with the same apparatus demonstrated that TiN is formed at 13OOOK. Thus a mixture of elemental Ti and B heated in an Nz atmosphere even below Tt would be expected to yield only TiBz. In the Brewer and Haraldsen experiment, much of the boron reacted with the molybdenum crucible, leaving an excess of titanium over that required to combine with the remaining boron as TiB2. The excess titanium formed TiN. As discussed in Section I1 (3), the writer has found that TiBz does not react in a period of several hours with Nz at any temperature, due again to some surface contamination, so the TiBLformed would be stable during a 50-minute experiment. A false conclusion about Tt is the result. Thus even if corrections are made in the thermodynamic data employed by Brewer and Haraldsen, the resulting value of AHf(TiB,) is too negative because of experimental problems in determining Tt. (2) Samsonov.9--SamsoiiovS studied the reduction of Ti02 with carbon and boron carbide. The analysis given by him is appropriate to the reaction 2Ti0 B,C C = 2TiBz CO. Since the value of AHt(TiB2) obtained from his data and this reaction does not agree with his value, it is likely that the reaction investigated was really that given by Iirestovnikov and Vendrikhs in discussing SamB4C = 2TiBz COZ. sonov's work: 2Ti0 Lack of identification of solid or gaseous products makes evaluation difficult. If the latter reaction is correct, the value of AHf(TiB2) calculated by Samsonov is still unreliable because of the choice of some questionable thermodynamic quantities from the Russian literature. In particular, the
+
+
+
+
+
R. A. KRAUSE,N. B. COLTHUP AND D. H. BUSCH
2216
heat of formation of boron carbide used differs from the value of Smith, Dworkin and Van Artsdalen14 by 52 kcal./mole. If the latter value is substituted, the value of AHf(TiB2) generated by Samsonov’s experiment is -44 i 6 kcal./ mole-a value consistent with the calorimeter value of Lowell and Williams. (3) Samsono~.~-In a separate paper Samsonov6 gives the value AHr(TiB2) =+ -73 kcal./ mole which he obtained from an empirical relation by Kubaschewsky and Evans4 between the heat of formation of a compound and the change in molar volume on forming the compound from the elements. The writer has not been able to duplicate this calculation using the Kubaschewsky and Evans formula and both Russian and American values of atomic volumes of Ti and B. (4) Krestovnikov and Vendrikh.6-An abstract15 of a paper by Krestovnikov and Vendrikh6 implied that an experimental study of the stability of TiBz has been made, but the paper itself contains only review and discussion of (l), (2) and (3) above. mass spec( 5 ) Schissel and Williams.l-The trometer experiment performed by Schissel and Williams’ involved the measurement of the equilibrium titanium partial pressure over TiB2 mixed with excess boron in a TiBz Knudsen cell contained in a tungsten jacket. To bring their value, - 32 kcal./mole, into agreement with the calorimeter value of -50, the temperature determination for the former would have to have been in error (too high) by several hundred degrees or the Ti vapor pressure measurement would have to have been in error (too high) by two orders of magnitude. Neither of these errors is likely. The geometry of the Knudsen cell radiation sighting hole was favorable for blackbody conditions (depth = 10 X radius, diameter = (14) D. Smith, A. S. Dworkin and E. R. Van Artsdalen, J . Am. Chem. SOC., 7 7 , 2854 (1955).
(15) A. N. Krestovnikov and M. S. Vendrikh, Chem. Abstr., 62. 18613g (1959).
Vol. 65
l/*”), and the window was calibrated with a flatfilament lamp and protected with an internal shield when readings were not being taken. A 50’ uncertainty was allowed by the authors. The Ti pressure was measured using both an absolute calibration against a weighed amount of silver and a relative calibration against the known silver vapor pressure; the Ti pressures so determined agreed within a factor of two. A more plausible explanation is that the Ti pressure was in fact too high because of the vaporization of Ti from some other source. Analysis of material chipped from around the orifice of the tungsten outer jacket of the Knudsen cell showed W2B by X-ray diffraction and only W and B by spectrographic analysis. Thus the titanium that must have interacted with the tungsten cell via the vapor phase evidently re-evaporated. (This conclusion is in agreement with the results of the experiment listed in Table 11: TiBz powder heated on a tungsten strip in vacuo formed WZBand lost Ti by vaporization.) The collimation of the molecular beam effusing from the Knudsen cell was such as to give line geometry, while the orifice of the cell was circular. Thus Ti atoms vaporizing from the tungsten lid on either side of the orifice could enter the beam, giving rise to a spurious Ti flux. Recently the mass spectrometer experiment has been repeated by Schissel and Trulson,I6using TiBz powder in a graphite crucible. The resulting value for AHt(TiBZ), -52 6 kcal./mole, is in agreement with the calorimeter result of Lowell and Williams. Acknowledgments.-The writer is grateful to Alan W. Searcy for his continued interest in this problem and for many helpful comments on the manuscript. P. 0. Schissel and 0. C. Trulson have generously allowed mention of their results before full publication. Appreciat,ion also is due t o C. E. Lowell , J. Weigel and R. D. Schaal for assistance during the work. (16) P.
0. Schissel and C. Trulson, private communication.
ISFRARED SPECTRA OF COMPLEXES OF 2-PYRIDINALDOXIME BY RONALD A. KRAUSE,NORMAN B. COLTHUP AND DARYLE H. BUSCH A Contribution from the McPherson Chemical Laboratmy of The Ohio State University, Columbus, Ohio, and the Central Research Division, Chemical Research Department and the Research Service Department, American Cyanamid Company Received J u l y 1 1 , 1961
Infrared spectra of nickel( 11),palladium( 11),palladium( IV), platinum(11) and platinum( IV) complexes of 2-pyridinaldoxime are reported. Assignments have been made for four pyridine ring bands, the acyclic C=N vibration and the N-0 stretching mode. The pyridine ring bands appear in the range expected for 2-substituted pyridines; the frequency of the C==N stretching mode, however, is very dependent on the complex type. As oxime protons are removed from the complex, the C=N vibration shifts from the normal range (1654-1614 cm.-’) to the range 1519-1505 cm.-l for the uncharged complexes. The N-0 vibration shifts toward higher frequencies as oxime protons are removed.
Introduction During the course of a study of the nickel(II), palladium(II), palladium(IV), platinum(I1) and platinum(IV) complexes of 2-pyridinaldoxime, 1
the infrared spectra of these compounds were recorded. In the Present Paper assignments for the C=N, N-0 and four pyridine ring bands have been made for these compounds. Perhaps the most
(1) R. A . Krause and D. H. Busch, J . Am. Chem. Soc., 82, 4830 ladium(1V) and dibromobis-(2-pyridinaldoximo)-platinurn(IV) has been described by R. A . Krause, Ph.D. dissertation, The Ohio State (1960); R. A. Krauae, D. C. Jicha and D. H. Busch, ibid., 88, 528 (1961). The preparation of dibromobis-(2-pyridinaldoximo)-pal- University. 1959.
