490
W. 0 . MILLIQAN AND HARRY E. W W E R
mum chloride at room temperature, and (3) the large decreese in electrid
conductivities as the temperature was dropped to -73°C. Our findings indicate the following solvent-electrolyte systems as the more promising to be used at -73OC.: ( 1 ) eutectic hydrochloric acid, (8) 43 per cent fluoboric acid, (3) methylamine hydrochloride in methanol, (4) eutectic lithium chloride and eutectic lithium bromide in water, and ( 6 ) methylamine hydrochloride and lithium chloride in water. REFERENCES PARDO, B. G.: Unpublished data in National Bureau of Standards report entitled “Densities of Perchloric Acid Solutions from +50° t o -SO’C.,” January, 1944. BRICKWEDDE,L. H., A N D PARDO, B. G.: Unpublished data in National Bureau of Standards report entitled “Viscosity of Perchloric Acid Solutions from +Meto -boDC.,” August, 1944. FAUNS, K., AND HASSEL,0.: 2. Elektrochem. Is, 495 (1923). GARRETT,A. B.,AND COOPER, R.: J. Phys. & Colloid Chem. 64,4374 (1960). GABBETT,A. B., A N D HEIKS,J.: To be submitted for publication. GAB~ETT, A. B., AND WELSH,J.: To be submitted for publication. G,AB~ETT, A. B., A N D WELSH,J.: To be submitted for publication. GABBETT,A. B., WELSH,J., WOODRUFF, 8.A., COOPER,R., AND HEIKS, J.:J. Phys. & Colloid Chem. 69, 505 (1949). Intenational Critical Tables, Vol. 111, pp. 60,64, 77. McGraw-Hill Book Company, Inc., New York (1928).
(1) BRICKWEDDE, L. H., (2)
(3) (4)
(5) (6)
(7) (8) (9)
AND
ORIENTATION EFFECTS IN TRANSPARENT ALUMINA FILMS* W. 0. MILLIGAN AND HARRY B. WEISER Deparfment of Chemistry, The Rice Institute, Houston, Tezas Received January IS, 1060
The hydrous particles in alumina sols prepared by the peptization of precipitated alumina consist of minute crystals of -/-AlnOs.H20or y-Al00H (for a survey of the literature we reference 14). X-ray Mraction examination of (a) airdried precipitated gels, (b) moist gels, (c) the alumina particles in the sol d t e , and electron diflraction examination of thin films from evaporated sols show that the dispersed phase has a crystal structure identical withthat of relatively large crystals of y A l 0 0 H (boehmite). The structure of y A l 0 0 H haa not been determined by single-crystal methods, but de Lapparent (2) obmrved that the crystals are orthorhombic. Comthe x-ray powder photographs of r-Al00H and the corresponding r-FeOOH, Hocard and de Lappsrent (7) and Goldstaub (6)concluded, from the close similarity of the powder 1 P m n t e d before the Division of Colloid Chemistry at the 104th Meeting of the Ameriian Chemioal &&ty, which WIW held in Bu5810, New York, September 7-11, 1942.
ORIENTATION EFFECTS IN ALUMINA FILMS
491
x-radiograms, that the structures are analogous. More recently Reichertz and Yost (10) deduced from powder photographs that the space group is D:;, with a0 = 2.850, bo = 12.24, and co = 3.691 A. These investigators reported a very small hydrogen-bond distance of 2.47 i= 0.07 A. Assuming a close relationship between the structures of y-A100H and yFeOOH (3), it may be concluded that the yA100H crystals are tablet-shaped and hence the crystals under proper conditions may form sheets of oriented lamellar layers. In 1939 it was reported (9) that electron diffraction patterns of thin alumina films, formed by evaporation of a sol, exhibited definite indications of orientation. Germer and Storks (5) likewise have shown conclusively from electron diffraction studies that T-AlOOH films may exhibit orientation. The existence of orientation in extremely thin films such as are used for electron diffraction studies suggested that the x-ray diffraction technique may detect orientation in relatively thick layers of alumina which are transparent to light. EXPERIMENTAL
Preparation of transparent alumina films A highly thixotropic yA100H sol was prepared by peptization with hydrochloric acid of alumina gel obtained from the action of water on amalgamated aluminum. The details of the procedure have been described previously (8, 12). The colloidal solution employed in this present investigation was the identical sol used for the earlier x-ray diffraction examination of moist alumina gels (12) and for the direct x-ray examination of alumina particles in the sol state (8). The alumina content was determined (8) by evaporating a definite volume of the sol to dryness at room temperature in a porcelain crucible, followed by ignition at 800°C. I t was observed that the sample dried at room temperature to a clear transparent continuous film about 1 mm. in thickness. Ignition at 800°C. did not destroy the continuous character of the film. It was concluded, therefore, that this type of sol would be especially suitable for preparing transparent alumina films for x-ray examination for possible orientation effects. Vollrath (11) has prepared translucent films of alumina by the electrolytic oxidation of aluminum foil in oxalic acid solution. Electron diffraction examination The sol described above was diluted to a concentration of about 1 mg. of alumina per milliliter, and two drops were allowed to evaporate on a thin collodion film on a 300-mesh stainless-steel gauze (13). Electron diffraction patterns were photographed with the sample ( a ) perpendicular to the electron beam, and ( b ) rotated to an angle of about 30" from the perpendicular. The resulting patterns are shown in figure 1. The difference in intensity of some of the lines is attributed to a partial orientation of the tablet-shaped or lamellar crystals of y-AlOOH. This orientation effect is demonstrated more clearly by the numerous and more accurate observations of Germer and Storks (5).
HCATCD 600- C
493
ORIBNTATION EPFECTB IN ALUMINA FILMS
. .. 1.'
B
4
FIG.3. X-ray diffraction pattern; incidence angle 90' Fro. 4. X-ray diffraction pattern; inoidenco angle 0'
TRANSPARENT F I L M , ANGLE SOo
iz-i ,A..,AF
.n.*
,2 Pd'
TRANSPARENT F I L M , ANGLE 60-
&.--f TRANSPARENT F I L M , ANGLE 4 5 O
Fic. 5. Miomphotometer tmcinga of x-ray diffraction patterns. Incidence angles 90' to 45'.
494
W. 0. MILLIGAN AND HARRY B. WEZSER
The clear transparent character of the resulting films is illustrated in figure 2. Film ahout 0.3mm. in thicknea were mounted in an x-ray diffraction camera, using a flat photographic plate, and diffractionpatterns were obtained at various angles of incidence from 9 0 O to O* of the x-ray heam. Cu K. x-radiation filtered
1
I / u PIG. 6 . Electron photomiemgraph of thin film of .i-AIOOH
TRANSPARENT FILM, ANGLE
SO0
Fie. 7. Miorophotometcr tracinp of x-ray diffraction pntterne or 7-AI2O0,.Incidence angles 90' and Bo'.
through nickel foil waa employed, the expasure time hexring ahout 1-2 hr. For incidence angles of 90' and ,'O x-ray diffraction patterns were also obtained, wing Cu K. x-radiation from a quarts crystal monochromator. The resulting monochromatic x-ray diffraction patterns are given in figures 3 and 4. X-ray diffraction photographs were a160 obtained with the slumina. films at
ORIEIUTATION EFFECTS IN .4LUMINA FILMS
495
various angles of incidence in a cylindrical camera. Microphotometer tracings are shown in figure 5. An electron microscopic photograph of R sample of yAlOOH taken by Mr. W. H. Gitzen is reproduced in figure 6 through the courtesy of Dr. J. W. Kewsome and Mr. A. H. Riesmeyer of the Aluminum Ore Company, East St. Louis, Illinois. This photograph demonstrates clearly the lamellar form of the crystals of y-Al00H in this particular sample. The results indicate that, like the relatively thin films used in electron diffraction studies, the thick films exhibit a very pronounced preferred orientation. A similar preferred orientation in transparent films of silver chloride was recently described by Fugassi and hlcKinney (4).Additional quantitative interpretations of the orientation effect must await a more complete knowledge of the exact lattice structure of y-Al00H crystals. The transparent y-Al00H films described above often develop cracks at the boundaries. I n an attempt to eliminate the cracks, some films were formed on glass plates covered with a thin coating of vaseline deposited by evaporating an ether solution. Since this procedure proved unsatisfactory, films were made by evaporating drops of sol on a mercury surface. These films did not crack, but they had a tendency to curl or pucker a t the edges, suggesting that strains are set up in the film when it dries. I t was observed that ( a ) cracks develop if the film adheres to the surface, and ( b ) the film bends or curls so as to minimize the strain if the film does not adhere to the surface. A close connection may exist between orientation and these strains in the dry film.
X-ray diffraction examination of transparent y-A1203jilms Transparent yAl00H films prepared as described above were heated to 600°C. for 2 hr. in an electric furnace. The resulting Y-A1203 films were slightly less transparent than the original y-Al00H films (figure 2) and maintained their continuous character. X-ray diffraction patterns were taken in a cylindrical camera at various inclinations of the sample. The corresponding microphotometer tracings are given in figure 7. The results show that y-A1203crystals produced by heating underwent orientation. This surprising result suggests a close relationship between the structures of y-Al00H and y-A-1208, such that the removal of water (or the elements of water) from y-Al00H causes the lattice to rearrange only slightly, and thus permits the y-A.1203 crystals in the continuous heated film to remain in the same oriented position as the y-Al00H crystals in the parent film. In this connection Bussem and Koberich (1) observed thLt brucite [Mg(OH)2] crystals decompose upon heating to form periclase [MgO], with the trigonal axes of the original and final crystals in parallel orientation. It does not seem likely that decomposition of oriented yA100H crystals produces y-AbO3 crystals in random orientation which subsequently rearrange into oriented positions within the continuous film. SUMMARY
The following is a brief summary of the results of this investigation: 1. Transparent films of alumina (y-A1203”20 or y-Al00H) have been made
496
W. 0. MILLIGAN AND HARRY B. WEBER
by evaporating a highly thixotropic alumina sol to dryness at room temperature. 2. Transparent Y-AlOOH films remain transparent after being transformed into -y-Al& by heating to 600-800"C. 3. Electron diffraction examination of certain thin yA100H films shows that the y A l 0 0 H crystals are oriented. 4. X-ray diffraction examination of certain relatively thick films of yA100H and yA1~03shows that the crystals of both phases are oriented in their respective films. 5. Transparent alumina films formed on clean or vaseline-coated glass plates tend to develop cracks at the edges, whereas films formed on a mercury surface tend to curl or pucker at the edges. This behavior suggests that the orientation process is closely connected with the strain which results when the gel is dried. REFERENCES (1) BUSSEMAND KBBERICH: 2. physik. Chem. 17, 310 (1932). (2) DE LAPPARENT: Bull. soc. franc. mineral. 68,255 (1930). (3) EWING: J . Chem. Phys. 3, 420 (1935). (4) FUGASSI AND MCKINNEY: Rev. Sci. Instruments 13, 335 (1942). (5) GERMER AND STORKS: Ind. Eng. Chem., Anal. Ed. 11,583 (1939). (6) GOLDSZTAUB: Bull. soc. franc. mineral 69, 348 (1935). (7) HOCART AND DE LAPPARENT: Compt. rend. 189, 995 (1929). (8) MILLIGAN ASD WEISER:J . Phys. Chem. 40, 1095 (1936). (9) MILLIGAX AND WEISER:Preliminary results presented before the Division of Colloid Chemistry at the 98th meeting of the American Chemical Society, which P 6s held in Boston, Massachusetts, September 11-15, 1939. (10) REICAERTZ AND YOST:J. Chem. Phys. 14,495 (1946). (11) VOLLRATH: J. Phys. Chem. 44, 401 (1940). (12) WEISERAND MILLIGAN: J. Phys. Chem. 40, 1 (1936). (13) WEISERAND MILLIGAN: J. Phys. Chem. 44, 1081 (1940). (14) WEISERAND MILLIGAN:Advances in Colloid Science, 1'01. 1, p. 227. Interscience Publishers, Inc., New York City (1942).
