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The preparation of thin foils from sintered uranium carbide for examination by transmission electron microscopy
JOURNAL
OF NUCLEAR
THE
MATERIALS
PREPARATION FOR
34 (1070) 202-205.
OF THIN
EXAMINATION
BY
FOILS
0
FROM
TRANSMISSION
J. E. BAI~RIDGE
Metauurgy
Division,
UKAEA
ELECTRON
URANIUM
31 July
mioroscopy. Previous transmission microscopy of UC 2* 3, has been confined to lightly irradiated arc-cast
Microstructure
of the. sintered
CO., AMSTERDAM
CARBIDE
MICROSCOPY
and L. THORPE
In a recent paper Harrison 1) desoribed a model of the radiation-induced swelling of uranium carbide (UC) which predicted volume increases in close agreement with experimental results up to 1300 “C. He found also that below 600-700 “C, fission gas bubbles larger than about 20 A diameter are unlikely to occur in this fuel until high burn-up has been achieved. A useful experimental test of this model would therefore be to search for evidence of fission gas formation in the material by thin film electron
I.
SINTERED
PUBLISHING
Research Uroup, Atomic Energy BeipecarchEstablishment, Received
Fig.
NORTH-HOLLAND
Hartudi,
Berka.,
UK
1969
material, whereas practical fuel elements almost certainly will contain porous sintered UC [or (U, Pu)C], not only because such material is cheaper to fabricate, but also because its internal voidage may act as a sink for fission product swelling. The present paper describes a double jet thinning technique which has been used to prepare the foils from unirradiated sintered UC pellets, as Al prelude to examining the irradiated fuel. The same technique has been applied already with some success to irradiated UOs 4), and no practical difficulties are envisaged in examining high burn-up carbide when such material becomes available.
uranium
202
c&bide
used in this work.
x
800
THE
PREPARATION
Experimental techniques: The starting materi-
OF
TBIK
203
FOILS
The thinning
apparatus
(fig. 3) is shown as
al in this experiment was standard sintered uranium carbide of 94% theoretical density. The typical microstructure of this material (fig. 1) shows the UC2 platelets in the slightly hyperstoichiometric carbide. The specimen to be thinned is taken from the bulk sample as follows: a 0.3 mm thick slice is cut from the pellet using a 92 mm thick
installed in a small lead cell developed
Carborundum wheel, and ground down between two revolving 600 grit discs to 0.2 mm. This grinding operation gives two flat parallel faces, and from this slice 3.05 mm-diameter discs are hollow trepanned using a diamond-coated drill 4). The combined slicing and trepanning unit is shown in fig. 2.
The 3.05 mm-diameter specimen is inserted into a Tufnol holder which protects the periphery and exposes the central area of > 1 mm diameter on each face. Several sealing compounds have been tried to prevent the electrolyte leaking round the edge of the specimen but excellent results, including ease of specimen
Fig.
2.
by the
authors to have high accessibility for the easy manipulation of relatively small quantities of irradiated material (N 10 mg). These 10 cmwall cells (since called a mini-cell), about 75 cm x 75 cm x 75 cm have proved very convenient for the various stages of preparation of these irradiated 4) thin films.
Slioing and trepannhg
apparatus.
204
J.
E.
BAINBRIDGE
Fig. 3.
AND
t.
THORNE
‘LIn-cell” thinning unit.
removal after thinning, are obtained by placing a small natural rubber ‘0’ ring over the disc before screwing the two halves together. The essential feature of our method is that electrical contact is made through the jets of electrolyte pumped to both sides of the specimen from stainless steel nozzles which are also electrodes of reversible polarity. No mechanical contact with the specimen is required and damage to the specimen in its final stage due to such contact is elimina~d. The holder is mounted between the nozzles in such a way that the specimen forms the only electrical return path between the jets, and the electrolyte, 90-95 o/0 orthophosphoric acid + 5--10% methanol, is pumped round the closed circuit by means of two flow-inducers (fig. 3). The DC supply to the electrodes is set at 2%30 V (open circuit) which, with the pumps on, gives a current flow through the specimen of 140-160 mA (4 A/cm-z), while to give a uniform rate of attack the polarity is reversed at one minute intervals. Although there is no obvious indication when perforation has taken
place, specimens of equal thickness take the same time to reach this perforation point (i.e. to within 5%). A pumping speed of 300 ccs/min is used. The major problem encountered so far has been to get the specimens clean after thinning. Several solvents have been tried with little success, the best results being obtained after prolonged washing in acetone or in three individual baths of acetone. It has however been difficult to obtain large clean areas. The foil quality shown in figs. 4-6 has been obtained by modifying the apparatus slightly to include a probe to the specimen through the top of the holder. This is the positive connection and both jets are run at negative potential. Very clean specimens are obtained, the washing problem is eliminated and better thin foils are produced. The results from both methods have been examined and found to be identical except for the degree of cleanliness. Conclusions : The techniques developed for the preparation of thin foils for electron transmission work from irradiated UOs have been
THE
PREPARATION Fig.
OF
THIN
205
FOILS
4.
Fig.
6.
Diffraction
pattern
taken
from fig. 4 from
region of UC2 platelets.
Fig.
5.
successfully used to prepare foils of UC. As with any thinning process, the aim has been to produce a final foil which contains large areas of material less than 1000 A thick and sufficiently strong and coherent to withstand both handling and thermal shock from the beam. Large thin areas are required to allow a fully representative examination to be made of the specimen and to give an overall appreciation of what effects have taken place. The essential features, i.e. ease of operation, reduced complexity and numbers of handling operations, and the ability to keep intact the fragile foils, have been maintained. The most significant development is that sintered uranium carbide can successfully be thinned with the bipolar double electrolyte method of electropolishing. The authors wish to thank Mr. J. W. Harrison and Mr. J. D. B. Lambert for their support and helpful discussions. References 1) J. W. Harrison, J. Nucl. Mat. 30 (1969) 319
Figs.
4 and 5.
Thinned
uranium
UC2 pBtelets.
carbide
x 4000
showing
2)
B. L. Eyre and M. J. Sole, J. Nucl. Mat. 18 (1966) 314
3)
J. L. Whitton,
4)
J.
E.
AERE-R
T.R.G.
Bainbridge, 5677
Report
UKAEA
(1968)
612(D) (Harwell)
(1963) Report
OF NUCLEAR
THE
MATERIALS
PREPARATION FOR
34 (1070) 202-205.
OF THIN
EXAMINATION
BY
FOILS
0
FROM
TRANSMISSION
J. E. BAI~RIDGE
Metauurgy
Division,
UKAEA
ELECTRON
URANIUM
31 July
mioroscopy. Previous transmission microscopy of UC 2* 3, has been confined to lightly irradiated arc-cast
Microstructure
of the. sintered
CO., AMSTERDAM
CARBIDE
MICROSCOPY
and L. THORPE
In a recent paper Harrison 1) desoribed a model of the radiation-induced swelling of uranium carbide (UC) which predicted volume increases in close agreement with experimental results up to 1300 “C. He found also that below 600-700 “C, fission gas bubbles larger than about 20 A diameter are unlikely to occur in this fuel until high burn-up has been achieved. A useful experimental test of this model would therefore be to search for evidence of fission gas formation in the material by thin film electron
I.
SINTERED
PUBLISHING
Research Uroup, Atomic Energy BeipecarchEstablishment, Received
Fig.
NORTH-HOLLAND
Hartudi,
Berka.,
UK
1969
material, whereas practical fuel elements almost certainly will contain porous sintered UC [or (U, Pu)C], not only because such material is cheaper to fabricate, but also because its internal voidage may act as a sink for fission product swelling. The present paper describes a double jet thinning technique which has been used to prepare the foils from unirradiated sintered UC pellets, as Al prelude to examining the irradiated fuel. The same technique has been applied already with some success to irradiated UOs 4), and no practical difficulties are envisaged in examining high burn-up carbide when such material becomes available.
uranium
202
c&bide
used in this work.
x
800
THE
PREPARATION
Experimental techniques: The starting materi-
OF
TBIK
203
FOILS
The thinning
apparatus
(fig. 3) is shown as
al in this experiment was standard sintered uranium carbide of 94% theoretical density. The typical microstructure of this material (fig. 1) shows the UC2 platelets in the slightly hyperstoichiometric carbide. The specimen to be thinned is taken from the bulk sample as follows: a 0.3 mm thick slice is cut from the pellet using a 92 mm thick
installed in a small lead cell developed
Carborundum wheel, and ground down between two revolving 600 grit discs to 0.2 mm. This grinding operation gives two flat parallel faces, and from this slice 3.05 mm-diameter discs are hollow trepanned using a diamond-coated drill 4). The combined slicing and trepanning unit is shown in fig. 2.
The 3.05 mm-diameter specimen is inserted into a Tufnol holder which protects the periphery and exposes the central area of > 1 mm diameter on each face. Several sealing compounds have been tried to prevent the electrolyte leaking round the edge of the specimen but excellent results, including ease of specimen
Fig.
2.
by the
authors to have high accessibility for the easy manipulation of relatively small quantities of irradiated material (N 10 mg). These 10 cmwall cells (since called a mini-cell), about 75 cm x 75 cm x 75 cm have proved very convenient for the various stages of preparation of these irradiated 4) thin films.
Slioing and trepannhg
apparatus.
204
J.
E.
BAINBRIDGE
Fig. 3.
AND
t.
THORNE
‘LIn-cell” thinning unit.
removal after thinning, are obtained by placing a small natural rubber ‘0’ ring over the disc before screwing the two halves together. The essential feature of our method is that electrical contact is made through the jets of electrolyte pumped to both sides of the specimen from stainless steel nozzles which are also electrodes of reversible polarity. No mechanical contact with the specimen is required and damage to the specimen in its final stage due to such contact is elimina~d. The holder is mounted between the nozzles in such a way that the specimen forms the only electrical return path between the jets, and the electrolyte, 90-95 o/0 orthophosphoric acid + 5--10% methanol, is pumped round the closed circuit by means of two flow-inducers (fig. 3). The DC supply to the electrodes is set at 2%30 V (open circuit) which, with the pumps on, gives a current flow through the specimen of 140-160 mA (4 A/cm-z), while to give a uniform rate of attack the polarity is reversed at one minute intervals. Although there is no obvious indication when perforation has taken
place, specimens of equal thickness take the same time to reach this perforation point (i.e. to within 5%). A pumping speed of 300 ccs/min is used. The major problem encountered so far has been to get the specimens clean after thinning. Several solvents have been tried with little success, the best results being obtained after prolonged washing in acetone or in three individual baths of acetone. It has however been difficult to obtain large clean areas. The foil quality shown in figs. 4-6 has been obtained by modifying the apparatus slightly to include a probe to the specimen through the top of the holder. This is the positive connection and both jets are run at negative potential. Very clean specimens are obtained, the washing problem is eliminated and better thin foils are produced. The results from both methods have been examined and found to be identical except for the degree of cleanliness. Conclusions : The techniques developed for the preparation of thin foils for electron transmission work from irradiated UOs have been
THE
PREPARATION Fig.
OF
THIN
205
FOILS
4.
Fig.
6.
Diffraction
pattern
taken
from fig. 4 from
region of UC2 platelets.
Fig.
5.
successfully used to prepare foils of UC. As with any thinning process, the aim has been to produce a final foil which contains large areas of material less than 1000 A thick and sufficiently strong and coherent to withstand both handling and thermal shock from the beam. Large thin areas are required to allow a fully representative examination to be made of the specimen and to give an overall appreciation of what effects have taken place. The essential features, i.e. ease of operation, reduced complexity and numbers of handling operations, and the ability to keep intact the fragile foils, have been maintained. The most significant development is that sintered uranium carbide can successfully be thinned with the bipolar double electrolyte method of electropolishing. The authors wish to thank Mr. J. W. Harrison and Mr. J. D. B. Lambert for their support and helpful discussions. References 1) J. W. Harrison, J. Nucl. Mat. 30 (1969) 319
Figs.
4 and 5.
Thinned
uranium
UC2 pBtelets.
carbide
x 4000
showing
2)
B. L. Eyre and M. J. Sole, J. Nucl. Mat. 18 (1966) 314
3)
J. L. Whitton,
4)
J.
E.
AERE-R
T.R.G.
Bainbridge, 5677
Report
UKAEA
(1968)
612(D) (Harwell)
(1963) Report