132
J. R. SOULEN, P. STHAPITANONDA AND J. L. MARGRAVE
Vol. 59
VAPORIZATION OF INORGANIC SUBSTANCES : B203,TeOz AND Mg3N21 BY JOHN R. SOULEN, PRASOM STHAPITANONDA AND JOHN L. MARGRAVE Department of Chemistry, University of Wisconsin, Madison, Wisconsin Received October 12, 1964
The necessity for auxiliary experiments in the study of vaporization of substances at high tem eratures is discussed. Earlier work pertaining to the vaporization of B2Oa and TeOz is reviewed and new vapor pressure &terminations are presented which extend the temperature ranges of the measurements. In the temperature ranges studied BzOa and TeOz are found to vaporize as BzO,(g) and TeOz(g) molecules, and vapor pressure equations are given. A H : of sublimation has been calculated from vapor pressure data for BzOs with the aid of free energy functions, and the heats of sublimation, vaporization and fusion are given for Te02. Vapor pressure measurements have been supplemented by spectroscopic observations and bands attributable to the Bz03(g)molecule have been found. The vaporization of MgsN2 has also been studied and found to be more complex. It is found that the Mgz(g) molecule is involved in the process of vaporization of MgaN2. From the Mg2(g) band spectrum the dissociation energy of the molecule has been estimated. Final products of MgaN2 vaporization are Mg(g) and N2(g).
important in the vapor, or if a reactive atmosphere is used, vaporization may be greater than calculated. (7) Comparison of flow, effusion, static and other vapor pressure measurements should be made to see if they all agree, since molecular species are often incorrectly assumed in effusion and flow calculations and erroneous pressures calculated. One must know what atoms or molecules are coming out of an effusion cell before the equilibrium pressure inside the cell can be calculated from weight loss or gain alone. I n this work we have used flow and effusion vapor pressure measurements, and spectroscopic observations in the visible and ultraviolet regions t o elucidate the vaporization processes for three inorganic compounds: Bz03,TeOz and Mg3Nz. Vapor Pressure of BzOs.-Liquid boric oxide begins to vaporize appreciably a t temperatures above 1000". The logical possibilities for the vaporization reaction are
I n most cases where vaporization or evaporation processes a t high temperatures have been studied carefully by modern techniques, the reactions have been found to be more complex than formerly believed. The idea that stable species a t high temperatures are necessarily simple is found not to be true, and many indications of large and/or new molecules are found through careful interpretation of vapor pressure, spectroscopic and mass spectrographic data. Because of this possibility of a complex vaporization process, it is usually necessary to perform several different experiments to establish without doubt the vaporization equilibrium species and equilibrium constant, and even more data may be necessary to establish the mechanism of a vaporization reaction. A systematic vapor pressure study of any material may well be accompanied by the following auxiliary studies besides the standard flow, effusion, or other type of vapor pressure measuremen t . (1) Identification of all solid phases present a t all temperatures of interest. This can best be done with the aid of low and high temperature X-ray diffraction studies. (2) Analysis of the compound before and after several vaporization runs to determine whether or not one constituent is being lost preferentially. Use of the same sample for several consecutive runs might also show this effect through changing analyses. (3) A search by spectrographic means for emission or absorption spectra of gaseous molecules and atoms being formed. This method is very sensitive, and may detect partial pressures down to about ntm. (4) A search with a mass spectrograph to identify important vapor species, ( 5 ) A determination of the vapor density to confirm vapor composition and indicate whether dimers, trimers or other complex species are important in vaporization. ( G ) Calculation of the equilibrium pressure of the elements over a compound assuming decomposition only t80 the elements. This will give the lowest possible pressure over a compound a t equilibrium in a vacuum. If there are any other species
Purified boric oxide from Baker and Adamson was prepared for vapor pressure runs by slow heating of samples in a platinum boat up to 1000° and maintaining this temperature for an hour or more to remove all traces of water. Two series of flow vapor pressure studies were carried out, both with dry, oxygen-free nitrogen as the carrier gas. In one series of runs a platinum boat containing Bz03 was placed in the flow stream and in the hot zone of a McDanel high temperature porcelain tube heated in a S i c hollow tube furnace. In the other series of runs the porcelain tube was lined with platinum foil, and the vaporization carried out essentially in a platinum system. The runs made in the unprotected tube gave weight losses per mole of flow gas similar to those predicted from the early work of Cole and TaylorZ and much higher than expected on the basis of the effusion vapor prcssure studies of Speiser, Naiditch and Johnston.3 Runs in the platinum foil protected tube agreed closely with those of Speiser, Naiditch and Johnston in regions where experimental temperatures allowed direct comparison.
(1) Presented at the Symposium on High Temperature Chemical Reactions a t the 12Gtli Meeting of the American Chemical Society, New York, N. Y . , Septeinber 15, 1954.
(2) S. S. Cole and N. W. Taylor, J . Am. Cer. Soc., 18, 82 (1035). (3) R. Speiaer, S. Nniditch and H. L Johnston, J . Am. Chcm. S o c . , 71, 2678 (1950).
BzOa(1) BzOa(g) zBz03(1) (B@a)z(g) '/zOz(g) Bz03(1) = 2BO(g)
+
.
VAPORIZATION OF INORGANIC SUBSTANCES :
Feb., 19.55
Treatment of the weight loss type of vapor pressure data obtained in these measurements requires knowledge of the gaseous species present, and Bradt4has recently shown in a mass spectrographic study of the vapor effusing from a Ihudsen cell containing Bz03(1) a t about 1300” that monomeric B203(g) is the only important vapor species under these conditions. In part (a) of Table I are listed the pressures of B203(g)from Speiser, Naiditch and .Johnston calculated with the assumption of monomeric Bz03(g),the change of free energy function for vaporization, A F / T , A H : / T and AH:, the heat of sublimation of B203 a t 0°K. Free energy functions for gaseous and liquid Bz03were computed from the tables of Huff, Gordon and MorrelL6 Values of A H : are seen to be essentially constant, and the average is 89.4 kcal./mole. An uncertainty of a t least 0.5 kcal./mole should be allowed because of the doubt about the true structure of Bdh(g). If one corrects this calculated heat up t o 15OO0K., the middle of the experimental range studied by Speiser, Naiditch and Johnston, by means of the heat content functions of Huff, Gordon and Morrell, one obtains A H M N = 77.6 kcal./mole, which is exactly the slope of the log p vs. 1/T plot of their data. TABLE I
&Os, T E O AND ~ MG~N~
133
- 2.0
- 3.0
\
\
\ 1s I
E
24.0
a c3
0 -I
VAPORIZATION cw B2O3 F - H
T.
PB208,
atm.
OK.
e.u.
T e.u.
T
ex.
AH;, kcal./ mole
(a) Effusion vapor pressure data (from ref. 3) 1331 1350 1369 1380 1390 1449 1476 1490 1497 1510 1540 1550 1569 1631 1642
7.74 1.88 2.46 5.02 4.00 9.90 1.59 2.25 2.88 4.25 6.53 7.36 7.21 2.31 2.87
1567 1579 1772 1806 1808
3.6 3.1 4.1 3.5 2.6
X IO-’ X 10-6 X 10-6
X X X X X X X X X X X X
10-6 10-6
10-6 10-5 10-5 10-6
10-8 10-5
10-6 10-6
10-4 10-4
39.80 39.68 39.58 39.52 39.47 39.14 39.00 38.92 38.89 38.80 38.65 38.61 38.51 38.21 38.15
27.96 26.20 25.67 24.25 24.70 22.90 21.96 21.27 20.78 20.00 19.15 18.91 18.95 16.64 16.20
67.76 65.88 65.25 63.77 64.17 62.04 60.96 60.19 59.67 58.80 57.80 57.52 57.46 54.85 54.35
(b) Flow vapor pressure data X 10-4 X 10-4 X 10-8
X 10-1 X 10-8
38.51 38.47 37.72 37.44 37.44
15.74 16.08 10.90 11.22 11.82
54.25 54.55 48.62 48.66 49.26
90.2 88.9 89.3 88.0 89.2 89.2 90.0 89.7 89.3 88.8 89.0 89,2 90.1
89.5 89.2 85.0 86.2 86.3 88.0 89.0
Part (b) of Table I includes calculations similar to those above but based on our flow vapor pressure measurements in platinum, and again assumes the vapor to consist of monomeric BzOa(g). AH: values calculated here agree fairly well with the results of the effusion measurements. Figure 1 shows that, over the range 1331 to 1808”K.,one can satisfactorily describe the vaporization of Bz03by assuming monomeric B203(g)as (4) P. Bradt. NBS Report 3016, January, 1954. ( 5 ) V. N . Huff, 9. Gordon and V. E. Morrell, National Advisory Committee for Aeronautics Report 1037 (1951). These authors assume BiOa(g) to be a symmetrical molecule in which the oxygen atoms form an equilateral triangle, and the boron atoms lie on each side of the plane of this triangle and are both singly bonded to eaah of the three oxygen atoms. This structure was suggested by P. F. Wacker, H. W. Wooley and M. F. Fair in a Teohnical Report to the Bureau of Aeronautics, Navy Department, January 25, 1945.
- 5.0
- 6.0 6
4
Fig. 1.-Log p vs. 1/T plot for BzOa vaporization: A, Speiser, Naiditch and Johnstona; e, Cole and Taylors; 0, this work.
the main vapor species. The older data of Cole and Taylor2 are also shown. The vapor pressure is given by the equation log P.tm
E:
6.742
-16960 T
(1331 < T
< 1808’K.)
The molecule B203(g)is not well known. Various spectra from flames and electrical discharges into which boric acid has been introduced show BO(g) bands as well as the so-called “boric acid fluctuation bands.”6 By heating BZO3in porcelain tubes to 1400-1700” we have obtained emission and absorption spectra of the gas over B203(1)with a small Hilger quartz spectrograph. These are shown in Fig. 2. The bands are the same as the “fluctuation bands,” but do not have the BO background spec(6) R. W. B. Pearse and A. G. Gaydon, “The Identification of Molecular Spectra,” John Wiley and Sons, Inc., New York, N. Y., 1950, p. 60.
134
J. R. SOULEN, P. STHAPITANONDA AND J. L. MARGRAVE
trum. From the analysis of the vapor pressure data, they must be associated with an electronic transition for the B20a(g) molecule. We are now re-photographing these spectra with a high resolution grating, and hope to obtain more information about the structure of the molecule.
The flow vapor pressure measurements were carried out with five flow rates of either nitrogen or oxygen: 559, 340, 155, 80.2 and 34.4 m l . / d n . Graphical extrapolation to zero flow rate gave the extrapolated vapor pressures at the various experimental temperatures as shown in part (a) of Table 11. Effusion vapor pressure data are given jn part
B203
-
VOl. 59
TABLE I1
-____
VAPORIZATION OF TeO2 Flow vapor pressure data (both Nz and On as carrier gases) -log p , a t m . (extrapd. t o zero T, OK. flow rate)
(a)
h
3.30 2.93 2.38 1.90
1064 1100 1151 1211
1965OK.
'E
(b) Effusion vapor pressure d a t a Totel Area of -+&loss, effusion hole, Time, mg. cm.2 min.
T, OK.
846 888
0.2 1.0
8.58 8.58
x x
10-3
35 60
10-3
-alog P, tm. 6.07 5.60
(b) of Table 11. A graph showing the log of the vapor pressure in atm. vs. 1/T is given in Fig. 3.
-4
1805°K.
I
I
V A P O R PRESSURE
of TeO, -3 0 A Uano's
,E
A
0 Thls work
-
p3.0 (3
s 1670 K .
E
-
-LO
-6.0
I
I
8
9
I
lo
Fig. 2.-Absorption (A) and emission (E) spectra of gas over Bz03(1) a t various temperatures. Exposure times for the absorption photographs were 0.1 seo. (upper) and 1 sec. (lower). For emission, exposure times of 1 sec. (upper) and 10 sec. (lower) were used.
Vapor Pressure of TeOz.-The vapor pressure of solid and liquid TeOz has been measured over the range 846 to 1211°K. by effusion and flow vapor pressure studies. The main species in equilibrium with both solid and liquid TeOz in this range is TeOz(g) as established by spectroscopic observations.' TeO(g) is observed a t still higher temperatures. Spectroscopically pure TeOz was used. It was checked by X-ray diffraction, and showed the desired powder pattern with no impurity lines. Samples were vaporized from porcelain, quartz and MgO containers and no signs of attack were observed for any of these materials. The melting point of TeOz was determined in a quartz capillary tube, and found to be 733 l o ,in good agreement with previous work.8
Fig. 3.-Log
( 8 ) A. Simek and
I I2
p us. 1/T plot for TeOl vaporization.
Also shown here are the effusion data of Ueno9 for solid TeOz. From the difference in the slopes of the vapor pressure curves for liquid and solid, one evaluates the heat of fusion of TeOz as 3.2 0.5 kcal./mole. The vapor pressure equations are
*
TeOz(s) = TeOz(g); AH,,b = 54.9 kcal./mole log p r t m = 8.067
-7 12000
(846 < T
< 1006OI
