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Uranium dioxide crystals grown by a solar furnace
Journal of Crystal Growth 2 (1968) 326—327 ~ North- tb/land Publishing Co., Amsterdam
URANIUM DIOXIDE CRYSTALS CROWN BY A SOLAR FURNACE
TAKEMARO SAKURAI, OSAMU KAMADA and MAREO ISHIGAME The Research Institute for Scientific Measurements, TJhok a University, Sendai, Japan
Received 4 April 1968
3 in dimension was irraU0 diated by a large solar 2 powder pressed intofurnace. a cube By 1 cmirradiation, minute single crystals grew which had a pyramidal form with optically flat
crystal surfaces. Some considerations on the crystal growth proccss are also presented.
U0 2 is usually available in black crystalline powder. In order to make minute crystals grow larger, the powder was irradiated by a large solar furnace in our laboratory’). For a black target, the furnace can attain a temperature as high as 3500 ~C. To prevent oxidation, the specimen was kept in helium atmosphere. When helium is at an atmospheric pressure, the heated specimen hardly shows fusion except slight sublimation. It has been found, however, that numerous single crystals grow in the irradiated surface. The experimental arrangement, the feature of single crystals and some considerations on the crystal growth will be described briefly. U02 powder was prepared by deoxidizing U305. Chemical analysis showed that the value of x in UO~ is 2.000. The observed lattice constant is 5.468 A. To make a specimen, the powder pressed into a 3 in dimension. Fig. I has is a been schematic represencube 1 cm tation of the mounting of the U0 2 specimen. The pressed powder S is mounted on a T-shaped tungsten plate W supported by a graphite rod G, and is placed at the focus of a paraboloidal concentrator. The specimen is covered by a fused silica bulb B, 25 cm in diameter, into which helium is introduced during irradia~ tion. Figs. 2a and 2b are the micrographs of the irradiated surface at different points, in which the grown single crystals can be seen clearly. These crystals represent triangular crystal surfaces which are optically flat enough to give regular reflection. In the microscopic observation of the surface, an unidirectional illumina326
tion is not adequate because it gives the image of only a few crystal surfaces which happen to be oriented to reflect the light in the direction of the optical axis.
S
w
He Fig. 1.
Schematic representation of the mounting of the UO~ specimen.
Therefore, the micrographs in fig. 2 have been taken by using nearly hemispherical illumination to give the image of surfaces as much as possible. A large surface at the top of fig. 2a is about 0.15 mm in size. It is oriented parallel to the optical axis, hence it gives dark image in spite of the hemispherical illumination. This figure indicates that the crystal surface makes an equilateral triangle exactly through two apexes which are missing in this case. As is seen in figs. 2a and 2b, the crystals grown are the whole or the part of a pyramid
URANIUM DIOXIDE CRYSTALS GROWN BY A SOLAR FURNACE
327
~1
k: _
(al
Fig. 2.
Micrographs of irradiated surface at different points (a) and (b).
obtained by halving an octahedron. It is quite reasonable since uranium dioxide has the same crystal structure as fluorite which is common to grow in an octahedral morphology. Triangular crystal surfaces which grow easily are (Ill) planes. By the use of a stereomicroscope, it has been found that the grown crystals are separated by a deep valley. Some of the valleys show a bottom with a vitreous surface and the others are covered by minute crystals. The detail of the bottorn cannot be seen in fig. 2 since the focus is adjusted to the elevated crystal surfaces, The stoichiometry of grown crystals has been examined by various methods. Chemical analysis shows the value of x in UO~to be practically 2.000. A lattice constant observed by X-ray analysis is 5.470 A. By the relation between the lattice constant and 2’3),the thisoxygen value content which has hitherto been reported of lattice constant leads to the conclusion that the value of x does not exceed 2.000. Metallographic technique developed by Schaner4) has also been used to see the etching figures on the optically flat crystal surfaces. The etching by a solution of H202 and H2 SO4 gives neither pits nor boundaries, which shows that the value of x is not less than 2.000. From these results, the crystals may be considered to have excellent stoichiometry. U02 powder, dark in color, is a good absorber of solar radiation and reaches extremely high temperature by irradiation. On the irradiated surface, the sintering of the powder may occur, but it seems that the evaporation and the deposition play an important role in the
(b)
growth of single crystals. U02 vapor evaporates from some points of the surface and leaves small cavities behind. It deposits then on microcrystals of powder and makes them grow. En the solar furnace, a cavity is a very effective target in absorbing solar energy. Moreover, the inner surface of the cavity will still be datk in color. Further evaporation takes place by which the cavity grows larger and larger. On the other hand, the growing microcrystal becomes reflective for solar radiation and its temperature decreases by which the deposition is promoted and the crystal grows larger. Thus, the crystals grow to make plateaus and the cavities make deep valleys. When the vapor density in the valley becomes too high, however, the crystallization takes place giving rise to the growth of minute crystals there. growth of this is an isunique limitedCrystal in the case where the kind specimen heatedprocess by an imaging furnace. The authors are indebted to Prof. S. Yajima and Dr. H. Furuya for the preparation of U0 2 powder and the chemical analysis of grown crystals. References .
I) T. Sakurai 0. Kamada, K. Shishido and K. Inagaki, Solar Energy 8 (1964) 117. 2) K. Sudo and A. Kogoshi, Sci. Rept. Res. Inst. TOhoku Univ. 13A (1961) 31. 3) and J. S. M. Anderson J. 0.Nature Sawyer,185H.(1960) W. Worner, G. M. Willis J. Bannister, 915. 4) B. E. Schaner, J. NucI. Mater. 2 (1960) 110.
URANIUM DIOXIDE CRYSTALS CROWN BY A SOLAR FURNACE
TAKEMARO SAKURAI, OSAMU KAMADA and MAREO ISHIGAME The Research Institute for Scientific Measurements, TJhok a University, Sendai, Japan
Received 4 April 1968
3 in dimension was irraU0 diated by a large solar 2 powder pressed intofurnace. a cube By 1 cmirradiation, minute single crystals grew which had a pyramidal form with optically flat
crystal surfaces. Some considerations on the crystal growth proccss are also presented.
U0 2 is usually available in black crystalline powder. In order to make minute crystals grow larger, the powder was irradiated by a large solar furnace in our laboratory’). For a black target, the furnace can attain a temperature as high as 3500 ~C. To prevent oxidation, the specimen was kept in helium atmosphere. When helium is at an atmospheric pressure, the heated specimen hardly shows fusion except slight sublimation. It has been found, however, that numerous single crystals grow in the irradiated surface. The experimental arrangement, the feature of single crystals and some considerations on the crystal growth will be described briefly. U02 powder was prepared by deoxidizing U305. Chemical analysis showed that the value of x in UO~ is 2.000. The observed lattice constant is 5.468 A. To make a specimen, the powder pressed into a 3 in dimension. Fig. I has is a been schematic represencube 1 cm tation of the mounting of the U0 2 specimen. The pressed powder S is mounted on a T-shaped tungsten plate W supported by a graphite rod G, and is placed at the focus of a paraboloidal concentrator. The specimen is covered by a fused silica bulb B, 25 cm in diameter, into which helium is introduced during irradia~ tion. Figs. 2a and 2b are the micrographs of the irradiated surface at different points, in which the grown single crystals can be seen clearly. These crystals represent triangular crystal surfaces which are optically flat enough to give regular reflection. In the microscopic observation of the surface, an unidirectional illumina326
tion is not adequate because it gives the image of only a few crystal surfaces which happen to be oriented to reflect the light in the direction of the optical axis.
S
w
He Fig. 1.
Schematic representation of the mounting of the UO~ specimen.
Therefore, the micrographs in fig. 2 have been taken by using nearly hemispherical illumination to give the image of surfaces as much as possible. A large surface at the top of fig. 2a is about 0.15 mm in size. It is oriented parallel to the optical axis, hence it gives dark image in spite of the hemispherical illumination. This figure indicates that the crystal surface makes an equilateral triangle exactly through two apexes which are missing in this case. As is seen in figs. 2a and 2b, the crystals grown are the whole or the part of a pyramid
URANIUM DIOXIDE CRYSTALS GROWN BY A SOLAR FURNACE
327
~1
k: _
(al
Fig. 2.
Micrographs of irradiated surface at different points (a) and (b).
obtained by halving an octahedron. It is quite reasonable since uranium dioxide has the same crystal structure as fluorite which is common to grow in an octahedral morphology. Triangular crystal surfaces which grow easily are (Ill) planes. By the use of a stereomicroscope, it has been found that the grown crystals are separated by a deep valley. Some of the valleys show a bottom with a vitreous surface and the others are covered by minute crystals. The detail of the bottorn cannot be seen in fig. 2 since the focus is adjusted to the elevated crystal surfaces, The stoichiometry of grown crystals has been examined by various methods. Chemical analysis shows the value of x in UO~to be practically 2.000. A lattice constant observed by X-ray analysis is 5.470 A. By the relation between the lattice constant and 2’3),the thisoxygen value content which has hitherto been reported of lattice constant leads to the conclusion that the value of x does not exceed 2.000. Metallographic technique developed by Schaner4) has also been used to see the etching figures on the optically flat crystal surfaces. The etching by a solution of H202 and H2 SO4 gives neither pits nor boundaries, which shows that the value of x is not less than 2.000. From these results, the crystals may be considered to have excellent stoichiometry. U02 powder, dark in color, is a good absorber of solar radiation and reaches extremely high temperature by irradiation. On the irradiated surface, the sintering of the powder may occur, but it seems that the evaporation and the deposition play an important role in the
(b)
growth of single crystals. U02 vapor evaporates from some points of the surface and leaves small cavities behind. It deposits then on microcrystals of powder and makes them grow. En the solar furnace, a cavity is a very effective target in absorbing solar energy. Moreover, the inner surface of the cavity will still be datk in color. Further evaporation takes place by which the cavity grows larger and larger. On the other hand, the growing microcrystal becomes reflective for solar radiation and its temperature decreases by which the deposition is promoted and the crystal grows larger. Thus, the crystals grow to make plateaus and the cavities make deep valleys. When the vapor density in the valley becomes too high, however, the crystallization takes place giving rise to the growth of minute crystals there. growth of this is an isunique limitedCrystal in the case where the kind specimen heatedprocess by an imaging furnace. The authors are indebted to Prof. S. Yajima and Dr. H. Furuya for the preparation of U0 2 powder and the chemical analysis of grown crystals. References .
I) T. Sakurai 0. Kamada, K. Shishido and K. Inagaki, Solar Energy 8 (1964) 117. 2) K. Sudo and A. Kogoshi, Sci. Rept. Res. Inst. TOhoku Univ. 13A (1961) 31. 3) and J. S. M. Anderson J. 0.Nature Sawyer,185H.(1960) W. Worner, G. M. Willis J. Bannister, 915. 4) B. E. Schaner, J. NucI. Mater. 2 (1960) 110.