Oct., 1954
ELECTRON DIFFRACTION STUDIES IN
THE
SYSTEM BeO-InzOa
817
ELECTRON DIFFRACTION STUDIES I N THE SYSTEM Be0-In2O3I BY C. R. ADAMS~ AND W. 0. MILLIGAN The Rice Institute, Houston, Texas Receiued March 6 ,1064
Electron diffraction atterns have been obtained for a series of eleven dual oxide, heat-treated (2 hr., 500') gels in the system BeO-Inz03. T i e gels were the identical ones previously studied by X-ray diffraction methods, gas adsorption and electron microscope techniques, and which exhibited two composition zones of (a) mutual protection against crystallization, (b) enhanced adsorptive capacities, (e) enhanced water surface areas, and (d) enhanced differential and integral heats of adsorption. The electron diffraction patterns exhibit enhanced line broadening in the same two composition zones. The positions of the diffraction lines are identical with those from well-crystallized In203, and the observed broadening is interpreted as indicating the presence of extremely small crystals within the dual zones of mutual protection. The secondary particles (aggregates), as viewed in the electron microscope, consist of several crystallites. The number of crystallites per aggregate increased rapidy in the two composition zones. The enhanced surface properties, occurring in the zones of mutual protection, are attributed primarily to the smaller crystal size. From a comparison of the water surface area with the geometrical area of the primary crystallite, it is deduced that more than half of the crystallite surface is accessible to water vapor. The electron diffraction pattern of BeO, as a separate phase, is detectable a t a concentration as low as 60 mole % Be0 in gels heat-treated at 600'.
Introduction In previous reports from this Laboratory systematic X-ray diff raction3 and sorption-desorption4 studies of the dual hydrous oxide system BeOInz03 demonstrated the existence of two composition zones of mutual protection6 against crystallization. These composition zones showed enhanced adsorptive capacities, water surface areas, and differential and integral heats of adsorption. I n a more recent report16a detailed high magnification electron microscope study of the secondary particle (aggregate) size in this system showed that the zones of mutual protection do not result from variation in secondary particle size. I n an effort to elucidate the structure of these secondary particles, a systematic electron diffraction study was carried out on this system. It is the purpose of this paper to report the results obtained.
Experimental The electron diffraction patterns were taken in a Philips electron microscope using 100-kv. electrons. The photographic densities were obtained from the plates by the use of a Moll microphotometer and a "Photopen" recorder. The photographic densities were converted to relative electron intensities and transferred to large graph paper. The incoherent scattering was obtained by drawing m a smooth curve and the structure-sensitive diffraction peaks obtained by subtracting. In Fig. 1 are plotted, on a relative intensity basis, the coherent scattering patterns for the system heat-treated for 2 hours a t temDerature levels of 500 and 600'. It will be ndted in Fig. 1 that enhanced line broadening occurs a t compositions corresponding to the two previously observed zones of mutual protection. The increased line broadening may result from smaller crystal size, or from strains and distortions. The positions of the diffraction lines are identical, within the limits of observation, with those obtained from large, well-formed crystals of In& In the following interpretations of the observed line broadening, it is assumed that broadening from strain or distortion may be neglected, or is independent of the temperature (1) Presented before the twenty-eighth National Colloid Symposium which was held under the auspices of the Division of Colloid Chemistry of the American Clieniical Society in Troy, New York, June 24-26, 1954. (2) Ethyl Coiporation Fellow in Chemistry a t The Rice Institute, 1953-1954. (3) L. M. Watt and W. 0. Milligan, THIBJOURNAL, 67, 883 (1953). (4) W.0.Milligan and C. R. Adams, W d . , 57,885 (1953). (5) W. 0 . Milligan, ibid., 55, 497 (1951). (6) C. R.Adanis and W. 0. Milligan, ibid., 58,219 (1954)
of heat-treatment. The assumption accounting the line broadening to crystal size effects leads to results agreeing closely with water surface areas measured previously by gas adsorption techniques.4 It appears reasonable that the 1i.ne:broadening may be interpreted on the basis of crystal size. To obtain a measure of the crystallite size from line broadening one must know the instrumental broadening due to the particular experimental apparatus. The instrumental broadening was obtained in this case for each particular line by heating samples of pure InzOaand Be0 to a sufficiently high temperature to obtain single crystals large enough to see and measure in the electron microscope. In the samples employed for standardization the single crystal cubes of InSOa were about 250 A. on an edge. The single crystals of Be0 tended to be plate-like and were about 250 A. in length. In these samples the broadening due to crystal size amounted to only a few per cent. of the total broadening. The electron diffraction rings were slightly spotty in appearance.
Results and Conclusions In Fig. 1 are shown the coherent scattering patterns for the system heat-treated a t 500 and 600". From a consideration of the patterns for the system heat-treated a t 500" it is apparent that the same zones of composition which had previously shown enhanced adsorptive capacity, surface areas and differential and integral heats of adsorption, correspond to a more poorly crystalline condition. Visual inspection of the patterns suggests that the crystal size is much smaller in these regions. In a previous study with X-rays3 no pattern for Be0 as a separate phase was detected in the dual gels a t any heat-treatment. From an examination of the 90 mole % ' Be0 sample heated a t 500°, it is apparent that the stronger diffracting power of electrons has detected the presence of a crystalline phase of BeO. However, the crystal size is very small for the samples heated a t 500" and, therefore, a complete electron diffraction study was carried out on samples heat-treated for two hours a t 600". From these latter patterns it is apparent that the Be0 exists as a separate crystalline phase to as low a concentration as 60 mole % BeO, although X-ray diffraction patterns of these samples fail to show the presence of a separate crystalline phase of Be0 for any samples containing InzOa. The apparent contradiction of these experiments is attributed to the much greater diffracting power of Be0 for electrons relative to X-rays. Figure 2 shows the size of the crystallites as measured by line-broadening
818
C. R. ADAMSAND W. 0. MILLIGAN
Vol. 58 Mole S Be0 In203
0
100
30
70
n
7
0
5
10
e
15
20 0
x 103 500*,
Fig. 1.-Relative
5
10
15
40
60
50
50
60
40
70
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80
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e . 103 ~ 6000,
coherent electron diffraction intensities for the system BeO-IngOl, heat-treated for 2 hr. a t 500 and 600".
for the system heated at 500" as compared to the secondary particle size. The open points give the volume average of the secondary particle diameter as measured in the electron microscope.6 The closed points give the edge of a c,rystalline cube of In2O3 as deduced from line broadening. The haif-open circles give the diameter of a sphere of BeO. It is noted that the secondary particle size is a linear function of composition, thus indicating that the interaction has no effect on the secondary particle size. However, the crystallite size shows a decided decrease in the regions of the zones of mutual protection. It follows that the secondary particles are composed of several smaller crystallites, the number rising very rapidly in the zones of mutual protection, w shown in the top part of Fig. 2.
In Fig. 3 is shown a plot of the fraction of crystallite surface accessible to mater vapor as measured by a calculation of the water surface area.4 It is noted that more than half of the crystallite surface is accessible to water vapor. In view of the uncertainty of the crystallite size measurements (15-20%) and a lack of a detailed knowledge of the exact shape of the crystallites, one can only conclude that most of the crystallite surface is accessible to water vapor. However, since all of the crystallite surface is not accessible to water vapor, a consideration of the packing of the crystallites in the secondary particle must account for this area which is not accessible to the water vapor. Since the water vapor does have access to a much larger area than the external area of the secondary particles,6 it fol-
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Oct., 1954
819
ELECTRON DIFFRACTION STUDIESIN THE SYSTEMBe0-lii.103 100
o S e c o n d a r y Particle S i z e 0 ln203 C r y s t a l S i z e o Be0 Crystal S i z e 50
n
0
200 0 n
0
0
0
0
1.0
0.0
-
1
I
I
I
L
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C. R. ADAMSAND W. 0. MILLIGAN
820
water molecule for close-packed spheres of diameter equal to the crystallite size. The fair agreement indicates that the crystallites are fairly tightly packeg in the secondary particles, but it would be difficult to distinguish between similar idealized models.
DISCUSSION F. M. FowKEs.-The calc,ulation of surface areas from weight average diameters of particles is a rather inaccurate process. It seems that more conclusions are drawn from these calculated areas (as in Fig. 3) than are justified.
C. R. ADAMS(comrnunicaled).-It is noted in the last paragraph of the papcr that the values of areas obtained from crystal sizes are uncertain and can only be considered
VQl. 58
approximate. For this remon the only conclusions emphasized by the authors are ( a ) some area inside the aggregate is accessible to water, (b) not all of crystal area is accessible, and ( c ) these effects are due to packing. The roughness factor values from electron microscopic data would have to be in error by several hundred per cent to invalidate the first conclusion, whereas errors of the order nf one hundred per cent wo:ild be necessary in Fig. 3 to invalidat,e the second conclusion. The third conclusion i R a natural conse uence of the first two conclusions. It is obvious that %e true area of the crystals would be slightly higher than areas calculated from weight average diameters since the latter are always slightly lzrger than areas from average diameters. This would be reflected in slightly lowcr values in Fig. 3, which certainly would not tend to invalidate any conclusion stated above. The idealized model of close packed spheres is not intended as a true representation but merely as an illustration of the magnitude of the effect of packing on available surface.
