THERMODYNAMICS OF VAPORIZATION IN THE A120a&03 SYSTEM
2469
Thermodynamics of Vaporization in the Aluminum Oxide-Boron Oxide Systemla
by Paul E. Blackburn,lbAlfred Biichler, and James L. Stauffer Arthur D . Little, Inc., Cambridge, Massachusetts O3l4O
(Received August 2 , 1566)
The vaporization in the aluminum-boron-oxygen system has been studied by means of Knudsen effusion weight loss and mass spectrometry. Over the binary A1203-B203 system, B203was the only vapor species found. Heats and entropies of formation of the mixed oxides AI4B20~(c)and Al18B403a(c)were determined. Under reducing conditions, in a system containing excess aluminum, gaseous aluminum monometaborate AlB02(g) ( A H t l s ~ o = - 136 f 3 kcal), aluminum boron suboxide AlBO(g) (AHr 1600 = -28 3 kcal), and boron monoxide BO(g) (AHt 298 = 3 3 kcal) were identified.
*
Introduction In earlier papers,2 we have been concerned with gaseous lithium, sodium, and beryllium metaborates. In each case a similarity was found with the corresponding metal halides. In this paper we discuss gaseous aluminum borate, boron monoxide, and the thermodynamics of the condensed phases in the aluminumboron-oxygen system. The condensed alumina-boric oxide system has been examined by MallardJ3ScholzeJ4 and Baumann and MooreJ6all of whom obtained optical constants, X-ray parameters, and composition of the mixed oxide. Mallard, the first investigator, reported the constitution of the mixed oxide to be A1,B2012 (3A1203. B203). Baumann and Moore, and later Scholze, found the compound to have the composition Al18B4033 (9A120a*4B203).Scholze detected, in addition, the presence of a second compound, A14B209 (2A1203.B202). He found that both these compounds are rhombic and melt incongruently, A118B4033 at 1440' and A14Bz09 at 1050'. While the two condensed mixed oxides have been characterized in X-ray studies, there are no data at all relating to gaseous species or to the thermodynamics of the condensed or vapor phases. We have studied this system by measuring vapor pressures by Knudsen effusion using a vacuum balance and a mass spectrometer. The results of this research and the derived thermal properties are reported here. Experimental Section Experiments were made by (1) vacuum microbalance measurements to determine phase boundaries and heats
*
of vaporization of B204g) and (2) mass spectrometric measurements to identify vapor species and determine their heats of vaporization from the mixed system. Microbalance Measurements. The vacuum microbalance system has been described previously.2b It consists of a quartz beam null balance sealed inside a high-vacuum stainless apparatus. The weight change of a 0.63-cm diameter X 0.95-cm high Knudsen cell was measured. The platinum cells were fabricated by welding the lid onto the crucible. Two furnace systems were used. The first of these2b contained a platinum-40% rhodium resistance furnace. The temperature of this furnace is measured with a thermocouple and is continuously controlled and recorded. The second system consists of a stainless steel induction furnace with the work coil placed inside the vacuum system. The induction furnace was controlled by monitoring the high-frequency output. This output is held constant with a feedback unit acting through a Leeds and Northrup C.A.T. controller, magnetic amplifier, and saturable reactor. Temperatures are measured by sighting an optical pyrometer through a (1) (a) This work was supported by the U. S. Air Force O5ce of Scientific Research, Contract No. AF 49(638)-1171, ARPA Order No. 31562, and by the U. S. Army Research O5ce (Durham), Contract No. DA-31-124ARO-D-315; (b) Argonne National Laboratory, Argonne, Ill. (2) (a) A. Btichler and J. B. Berkowit%-Mattuck,J . Chem. Phys., 39,286 (1963); (b) P. E. Blackburn and A. Btichler, J . Phys. Chem., 69,4250 (1965). (3) E. Mallard, Compt. Rend., 105, 1260 (1887). (4) H. Scholze, 2. Anorg. Allgem. Chem., 284, 272 (1956). (5) H. N. Baumann, Jr., and C. H. Moore, Jr., J. A m . Ceram. Soc., 2 5 , 391 (1942).
Volume 70,Number 8 August 1966
2470
side window and between the work coil turns. Adsorption by the window is corrected with a standard lamp calibration. High-Temperature Mass Spectrometry. A Nuclide Corp. 30-cm radius, 60" sector magnetic deflection mass spectrometer was used in these experiments. A platinum-lined two-piece crucible has been described earlier.2b Cells were constructed of platinum and alumina. The small size of these cells (0.63 cm in diameter and 0.95 cm high, or 1.2 cc) reduced thermal gradients. Materials. Crystalline B203 furnished by the U. S. Borax and Chemical Corp. was found to contain 4.5% water. This was removed by heating under vacuum. The aluminum oxide, 99.9% pure, was obtained from A. D. MacKay, Inc. Aluminum powder was filed from a single crystal prepared in our laboratory. We are grateful to Dr. D. L. Hildenbrand for sending us A14B209 and A118B4033 samples prepared in his laboratory. Both samples contained 2 to 5y0 excess alumina.
Results and Discussion 1 . The A120&403 Binary System: B203 Activity and Condensed Phase Thermodynamics. (a) Pressure us. Composition. The B203 pressure (as shown mass spectrometrically, BzOa constituted over 99% of the vapor) was measured as a function of composition a t constant temperature over A120rB203. This was achieved by measuring the weight change with time of a Knudsen cell containing B203-rich samples. As the B203 was lost, the sample became richer in A1203. Constant pressures should indicate two-phase regions, while a decrease in pressure should indicate the phase boundary. A number of preliminary experiments were made in which the loss of B203 from AlzOrBzOa mixtures was measured in an attempt to check phase boundaries and determine activities over the twophase regions. In these experiments, measurements were made at temperatures from 990 to 1285" using orifices from to 5 X cm2. With the temperature at 1054" and above, no change in B203 activity was observed up to 75% A l 2 0 3 . From this composition to 82% A1203 the pressure of B203 dropped. Beyond this point, no further weight loss could be measured. Subsequent measurements described below indicate that the activity should have dropped to about loP2 and the rate of weight loss for the largest orifices (because of a low evaporation coefficient) should have The decreased to about times that over BzOs. ~. rate Of effusion at these lower pressures (lo-* to lo-' atm) was too low to detect under our experimental conditions. The Journal of Phyakal Chemistry
P. E. BLACKBURN, A. BUCHLER,AND J. L. STAUFFER
I n a measurement at 990", a decrease in activity was observed at 67% AlaOa. The activity continued to fall with increasing A I 2 0 3 concentration. At 70% AI203 the B203 activity was about 0.4, but no plateau was seen in the curve. The temperature was increased to 1057" where the activity rose to unity and then decreased above 77% Al203, reaching 0.012 at 87% AlzOa. These experiments confinned the existence of the two mixed oxides found previously, &B20s (67% A120a) and AllJ34Oaa (82% A120~),but indicated that it was necessary, because of large changes in activity, a low evaporation coefficient, and slow equilibration in the condensed phases, to carry the experiments to higher temperatures and use smaller orifices. (b) Pressure over AlleB40arAZz03. Pressures over the pseudo-binary system AllsB4033-AI2O3were measured using the induction furnace. The platinum cell was suspended directly in the work coil. There was an apparent weight change as power to the work coil increased or decreased, due to the interaction of the cell and the magnetic field. However, this did not affect our isothermal measurement since the power was automatically fixed. The constancy of the power was confirmed by temperature measurements, which were stable within the limits of the Leeds and Northrup optical pyrometer (*2"). Two sets of measurements were made. First, a charge of 69 mg of 9A1203.2B203 and 18 mg of A1203 was added to a 0.63-cm diameter platinum cell with an orifice of 2.4 X cm2, Temperatures were measured on the platinum surface and were later corrected for the measured emissivity of the cell ( e 0.31). A number of isothermal measurements were made. These are given in Table I and plotted in Figure 1. In the second set of measurements, a hole was drilled in the side of the cell without altering the remaining sample. This hole was designed to measure emissivity and to determine the effects of the orifice area. The combined orifice areas, corrected for the Clausing factor,6 were 7.8 X lom3cm2, or 32 times larger than the original top orifice. These results are also given in Table I and plotted in Figure 1. In treating the data for orifice effects, use is made of the simplified Motzfeldt' equation to establish the magnitude of the evaporation coefficient. In this equation
( 8 ) s, Dushman, "Vscuum Technique," John \IViley and Sons, Inc,, New york, N. y.,1949, pp 98.99. (7) K. Motzfeldt, J . Phye. Chem., 59, 139 (1955).
THERMODYNAMICS OF VAPORIZATION IN THE A1209-BzOs SYSTEM
Table I: B20a Pressure over 9A1208.2BzOa
T,"K
SBC
1638 1638 1690 1663 1603 1570 1540 1632 1728b 175gb
2 ,400 2,700 975 1 ,650 6 ,600 8,400 10,800 3,600 1,500 2,250
1538 1603 1568 1636 1661
15,000 1,650 3 ,450 7,800 24,600
Orifice aa 50 55 60 60 65 48 25 64 200 510
2B2Os(l)
+ &Oa
Weight Effuaion rate, loss, g om -1 aec -1 la x 106
Time,
87.5 85.5 258 158 41.3 24.0 9.72 74.7 560 952
Orifice be 106 0.912 83 6.49 161 6.02 1260 20.8 3710 19.5
Pressure, atm X 108
9.56 9.35 28.6 17.4 4.47 2.57 1.03 8.15 62.9 108.0 0.0966 0.702 0.644 2.28 2.16
P, and P,, are the measured and equilibrium pressures, a is the orifice area, W. its Clausing factor, B is the sample area, and cye is the evaporation coefficient. The orifice area effects are quite large, indicating a very small evaporation coefficient for this system. The value for 1/Bcy, in eq 1 is 800. Inasmuch as the effective area B of the powdered sample cannot be established, it is only possible to state that cu, is less than 4 X This is the upper limit for (Ye calculated for a solid, nonporous, plane sample. Even with an orifice area as small as 2.4 X om2, the measured pressures must be increased 20%. I n earlier measurements on this system, in which heating was continued until no further weight was lost, the weight of the residue was found to be that of the A1203 initially present. Mass spectrometric examination showed that only BzO3 is present in the vapor. The data in Table I were calculated on this basis. The heat of reaction calculated by least squares for
-
+
Q/2A1208(c) B20a(g)
+ gA120dc)
+AGsB40~(c)
(3) the thermal values AHlsao = -42 f 5 kcal/mole of 3 eu/mole of Al18B4Os3. &B& and A81600 = -14 These data are summarized in Table I1 with those for A14B20eand the vapor species.
*
Table I1 : Thermodynamic Values for the A1-B-0 System
Orifice area corrected for Clausing factor = 2.38 X 10-4 Orifice area cm2. 'Above the melting point of All&Osa. corrected for Clausing factor = 7.77 x 10-8 cm2.
1/2A48B4033(c)
247 1
This same system was studied with a mass spectrometer to establish the vapor composition. At temperatures up to 1740°K only B203+ and its fragments were found. The second-law heat for reaction 2 was AH = 112 1 kcal/mole of B203, in excellent agreement with the gravimetric measurements. No aluminum-containing species were detected. Estimates of A H r = -500 kcal and S1600= 160 eu for Al(B02)3(g), based on comparison with A1C13, BeC12, and Be(B02)2,2bindicated that this species, if present, would have at 1740°K a pressure below our sensitivity , (c) Pressure over A I & O Q - A ~ ~ ~ B ~Since O ~ ~A14B209 . decomposes above its melting point of 1050 f 20", it waa necessary to measure pressures below this
*
€ \
ORIFICE AREAs2.4 X l 8 C d
(2)
is 113 i 2 kcal/mole of B203, and the entropy change for this reaction is 47 1 eu/mole of B203 where a correction has been made for the orifice effect. Taking = 92 3 for the vaporization of Bz03the values AHI~OO kcal/mole and ASla~a = 40 1 eu/mole, we found for the reaction
*
*
*
+
1. B208 pressure over AllsB40ss effect of orifice size.
Volume 70,Number 8 August 1966
P. E. BLACKBURN, A. B ~ C H L EAND R , J. L. STAUFFER
2472
combined with the heat and entropy of formation of A118B4033 shown above, give for
+
AFTER MELTlNO AH 92 t 2 kOo(
mi
*
I+T
THROUOH MELTING POINT A H * S O t 3
Id
0 DATA BEFORE MELTING
.(IO
.ea0
.?PO
.TOO
.eo0
.am
IO~IT,G~
Figure 2. B20a pressure over A ~ ~ B ~ O B - A I ~ ~ B ~ O ~ ~ .
temperature. The low evaporation coefficient (
