NOTES
2462
Wavelength (microns) a
s
IO
12
I, the observed value is 1.404. Unfortunately, the spectral data do not permit an unambiguous distiriction between v4 and v 5 in HBBrz and it is possible that these two assignments in Table I could be reversed.
IS
Graphitic Acid of Pyrolytic Carbon and Its Heat Treatment by Eitaro llatuyama Faculty of Engineering, Yamaguchi University, Ube, J a p a n (Received December 1.4, 1964) IO00 800 Frequency (cm-1)
600
Figure 1. Portion of the infrared spectra of HBBrl (upper curve) and DBI\r2 (lower ciirve). Bands at 820 and 856 cm.? are diie to BBr3: slit width -2 cm.-'.
attributed to B-H and B-D stretching vibrations, respectively. In the low-frequency range (Figure l ) , two strong bands of about equal intensities are observed for HBBrz and one strong band and one weak band are observed for DBBr2. The band a t 872 cm.-' in DBBr2 lies close to the B-Br asymmetric stretching vibration in BBra9and is assigned to v4. The assignment of v4 in HBBrz was complicated since the choice of either the 1040- or 773-cm1.-' band would necessitate either an unusually large deuterium isotope shift (168 cm.-') or an isotope shift in the direction opposite to that normally expected. h plausible explanation is that the two strong bands in HBBrz are v 4 and ~5 (the in-plane B-H bend) but are split by strong Fermi resonance mhile the two bands in DBBr2 do not coincide favorably for such an interaction. This explanation is also consistent with the observed variation in relative band intensities for the two molecules. For the B, symnietry type, the product rulelo gives ~(vAG)H
( V 4 V B ID
-
):({
(.W,BBr2) -lfDBBr2
(I~I)\"' IZ(D)
where mH arid mD are the masses of H and D atoms, is the molecular weight of HBBrz or DBBr2, and IZ is the monwnt of inertia about an axis perpendicular to the plane of the molecule. From the moments of inertia for €IBBr2 and DBBr2 reported by Lynds and Bass," we calculate for the I'B molecule a value of 1.399 for the product of frequency ratios. From Table (9) L. P. Lindeinan and 11,K . Wilson, J . Chem. Phys., 2 4 , 2 4 2 (1956). (IO) G . Herzberg. "Infrared and Raman Spectra of Polyatomic Molecules," D. Van Nostrand Co., Inc., New York, N. Y . , 1945, p. 231. (11) L. Lynds tind C . D . B a ~ s J, . Chem. Phys.. 41, 3165 (1964).
The Journal of Physical Chemistry
We have previously reported' the crystallographic change of several lamellar compounds of natural graphite by heat treatment. In these compounds, the original graphite structure was restored as soon as most of the foreign atoms were driven off from the layers. The graphitic acid (graphite oxide) was an exception; it required 1800" to be completely restored. I n recent years, pyrolytic carbon (pyrographite)2 has attracted considerable interest as a material for high temperature applications or as a material having unusually high anisotropic physical properties. Using this material we have prepared graphitic arid and have performed X-ray studies as a supplement to the previous work. This modification of carbon is usually formed by the thermal decomposition of a hydrocarbon vapor a t 1600-2500". The basal layer planes are highly oriented parallel to the substrate, but the successive hexagonal layers are randomly displaced laterally to each other, ie., turbostratic. In the as-deposited material, the crystallite dimension i s of the order of 100 A, As the grains grow when heat-treated a t higher temperatures, the two-dimensional ordering gradually transforms to the three-dimensional one. In the present experiment, the carbon was prepared as follows. Benzene vapor diluted with an inert carrier gas was pyrolyzed at 1600" on a carbon rod 8 m m . in diameter, and the deposited carbon shell was heated at 2000" under vacuum for 30 min. Figure 1 shows the logarithmic plot of intensity of the diffracted X-rays as a function of the Bragg angle for Co K a radiation. The lowest curve designated "original" is for the pyrolytic carbon obtained in this way. The (0002) and (0004) lines having a syni(1) E. Matuyarna, J . Phys. Chem., 58, 215 (1954). (2) C . A . Klein, J . A p p l . Phys., 33, 3338 (1962).
NOTES
2463
decreased with temperature. The first stage was due to the loss of water and continued until 100". The second state starting at 150" was an exothernic decomposition which accelerated the decrease and finally resulted in an explosion at about 200" if the heating rate was not carefully controlled. In this temperature range most of the inclusions were driven off, hence, the diffraction pattern showed a rapid and coniplicated change. The observed splitting of the (0002) peak presumably originated from temporal overlapping of the interniediate states. The observed distortion was slowly removed when the sample was heated to higher temperatures, and a t 1800" the original state was almost restored. The diffraction patterns of the successive stages are shown in the figure in which the temperatures of the heat treating are designated on the respective curves. At each temperature, the sample was kept for 30 min. in a stream of nitrogen gas and then evacuated a t I I I I I I I I temperatures above 300". At 300°, the (0002) peak I 0 IO 20 30 50 e DEG 40 is already in the correct position, but the line width is Figure 1. Successive change of the X-ray diffractions. The remarkably wide. This would seem that the hexagonal intensity as B function of the Bragg angle for layers are wavy or indicate a combination of lattices Co Ka radiation. having different interlayer spacings. The breadth decreased slowly with the temperature. At 300", the (lOi0) and (1150) lines are also broader than a t metrical shape are the three-dimensional reflections from SO", and, in addition, they are shifted to the highthe layer planes. The (1070) and (1130) lines angle side which corresponds to a contraction of the which are distinctly asymmetrical are the reflections layer planes by O.55Oj0. A slightly larger value of from the planes perpendicular to the- layers, often 0.75% was obtained in the previous experiment. The indexed (10) and ( l l ) . 3 This curve is similar to the diffractonieter curve published recently by G ~ e n t e r t . ~ discrepancy may be partially attributed to errors due to the large line width of the pyrolytic carbon. As can He used a pyrolysis temperature of 2100". This be seen in the figure, the line shift decreases slowly with temperature is higher than that of our experiment; the temperature. We have assumed buckled layers hence, his lines are slightly narrower than the present to account for these shifts. A sniall line shift on the curve. Graphitic acid was prepared by treating the opposite side is shown on the "80°C" curve. This pyrolytic carbon with Staudenmaier's reagent5Jj which indicates expansion of the layer planes. Such trends is a mixture of nitric acid, sulfuric acid, and potassium can be understood if we assume the effect of layer perchlorate. The treatment was carried out as depuckering has been overcome by increased interatomic scribed in the previous report. The chemical comdistance caused by the bonding of foreign atoms position of the resulting sample depended on many between the layers. experimental cbonditions, especially on the amount of I n pyrolytic carbon as well as the natural graphite, water introduced. However, the X-ray patterns were complete curing of the buckled layers occurs a t a much similar except for a difference in the diffraction angle lower temperature than that required for the conof the (0002) line. The curve of the sample dried a t struction of the layer lattice as in the graphitizing 80" is shown in the figure. On the curve designated process. The former is effected by heat treating at "80°C" only three peaks appear corresponding, respectively, to the (0002j, ( l O i O ) , and (1190) lines as in the sample prepared from the natural graphite; however, their widths are too large by nearly a factor (3) B. E. Warren, Phys. Reu., 59, 693 (1941). (4) 0. J. Guentert, J. Chem. P h y s . , 37, 884 (1962). of 3. The shift of the (0002) line indicates an increase ( 5 ) U. Hofmann and E. Konig, 2. anorg. allgem. Chem., 234, 311 of the interlayer spacing by 2.5 times. The swelling (1937). is nearly equal to that of the sample prepared from (6) A. R. Ubbelohde and F. A. Lewis, "Graphite and its Crystal natural graphite. The spacing as well as the mass Compounds," Clarendon Press, Oxford, 1960, p. 151. 8
I
Volume 69, Sumber 7
July 1966
NOTES
2464
1800"; the latter requires a temperature between 2000 and 3000". This situation seems to be similar in some respects to graphite damaged by neutron irradiation in which complete recovery occurs when the annealing temperature approaches 2000°.7-9 ~~
~
~
(7) M .Burton and T. J. Neubert, J . A p p l . P h y s . , 27, 557 (1956). (8) A. Herpin, J . phys. radium, 24, 499 (1963). (9) E. A . Kellott and H. P. Rooksby. G.E C. Journal, 31, 28 (1964).
The Heat of Formation of Lanthanum Oxide' by George C. Fitzgibbon, Charles E. Holley, Jr., Uniaersity of California, Los Alamos Scientific Laboratory, Los Alamos, S r w Mexico
and Ingeniar Wadso Thermochemistry Laboratory, University of L u n d , L u n d , Sweden (Receired December 83, 2964)
Values of the heats of solution of lanthanum metal and lanthanum oxide are presented as determined in calorinieters previously d e s ~ r i b e d and, , ~ ~ ~in conjunction with the heat of formation of liquid water from its e l e ~ n e n t sare , ~ used to determine the heat of formation of Laz03.
Experimental The calorimeter used in Los Alamos is an isothermal solution calorimeter whose environmental temperature niay be kept constant a t any setting between 23 and 33" to within 0.001" in an 800-1. thermostatically controlled bath. The vacuum-j acketed, silver-bodied, platinum-lined calorimeter reaction vessel has a volume of -450 cc., a thermal leak modulus of O.O05/min., and a heat capacity of 420 cal. Within the reaction chaniber are a heater, a thermistor, a Pyrex rod to which is attached a platinum stirrer, and a glass sample bulb. The heater consists of a lrj.23-cni. length of 0.64-cni. 0.d. platinum tubing, the lower end gold soldered, the upper end sealed to glass tubing which extends through the calorimeter lid and carries the heater leads. The platinum tubing contains 23 o h m of bifilarly wound, helirally coiled, sill,-covered iiiangaiiin wire with leads to measure the voltage drop, located at the solution level. A Fenwall 2300-ohm tht~rniistorIS used as the sensing element to iiieasure temperature differences up to 1.6" to within 0.0001". h Brown recorder was modified to an automatic changing iiiultiscale self-balancing WheatThe Journal of Physical Chemistry
stone bridge, whose arm position is an indication of the resistance of the thermistor. The energy equivalent is determined by passing a current from a precision voltage-regulated supply through the calorimeter heater and a 0.1-ohm standard resistor in series and measuring the voltage drops using a Rubicon Type B potentiometer and a Rubicon reflecting galvanometer. The input time is read directly from an electronic decade counter whose time base is derived from a 100-kc. crystal-controlled oscillator, accurate and stable to O . O l ~ o . The lanthanum metal was obtained from the Anies Laboratory, hmes, Iowa, in the form of a small ingot, through the courtesy of Prof. F. H. Spedding. I t was prepared before each run by filing the surfaces to a shiny luster before cutting off several small pieces. The sample size was chosen to give a temperature rise The composition of the saiiiple, calculated of -1". from an analysis, is: La, 98.810; JIg, 0.003; Ca, 0.010; Fe, 0.050; L a x , 0.164; Laz03, 0.146; Lac2, 0.047; LaH2,0.768. The hydrated sesquioxide mas obtained from the Michigan Chemical Co. I t was treated by heating in an induction furnace, under vacuum, to 1200" in an alunduni crucible for about 1 hr. It was then transferred to a drybox for weighing and sealing in a sample bulb. The oxide sample used in Lund was further treated before use by heating in air at 1030" for about 30 min. in a platinum crucible. It was then stored under dry S p for about 1 hr. before being put into a calorimetric ampoule. The coniposition of the oxide sample, calculated from an analysis, is : La203, 99.882; SiOz, 0.002; BaO, 0.006; CaO, 0.040; Fe203, 0.070. The heats of solution of this lanthanum metal and oxide were determined in 1.00 HC1 saturated with hydrogen. The average duration of a run was 10 min. from the breaking of the sample bulb until equilibrium was reached in the after period. Weights are given in vacuo. The defined calorie (4.1840 absolute joules) is used in expressing the results. The atomic weight used for lanthanuni is 138.91. The sample sizes of metal and oxide were adjusted so ihat the final solution in both cases had approximately the same composition, i.e., ca. 0.0065 -11 in LaC13 and with enough excess of HC1 that its concentration changed only (1) Work done in part under the auspices of the U. S.Atomic Energy Commission. (2) G . C . Fitzgibbon, D. Pavone, E. J. Huber, Jr., and C . E. Holley, Jr.. Los Alamos Scientific Laboratory Report. LA-3031 (1964). (3) S.Sunner and I. Wadso, Acta Chem. Scand.. 13, 97 (1959). (4) "Selected Values of Chemical Thermodynamic Properties," National Bureau of Standards Circular 500, U. S. Government Printing Office, Washington, D. C . . 1952.
