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The preparation of uranium layers by high voltage electrophoresis
NUCLEAR
INSTRUMENTS
AND METHODS
60 (I968) I 2 5 - I 2 8 ;
© NORTH-HOLLAND
PUBLISHING
CO.
THE PREPARATION OF U R A N I U M LAYERS BY HIGH VOLTAGE E L E C T R O P H O R E S I S V. V E R D I N G H and K. F. L A U E R
Central Bureau for Nuclear Measurements, Euratom, Geel. Belgium Received 8 December 1967 A method is described that allows the preparation of thick and homogeneous uranium oxide layers on metallic backings. Thelayers were used in neutron physics experiments.
1. Introduction
Electrophoresis, known already for many years, was used for the preparation of relatively thick uranium oxide layers of various shapes and dimensions. The method was studied and adapted for the fabrication of layers above the mg/cm 2 range, where electrospraying 5) or electrolysis proved to be time consuming or impossible. Whilst settling techniques give not to well adhering and homogeneous samples. Samples were prepared on different backings (e.g. aluminium, platinum, stainless steel and copper). Homogeneous samples with good adherence to the backing can be prepared in a minimum of time. 2. Principle of the method
Lyophobic colloids always migrate under the action of an electric field, the particles often give up their surface energy and pass to a more stable form. Under certain experimental conditions the particles precipitate
on the electrode, thus forming an adherent film. In the case of suspended neutral particles, an ionogenic part or complex can be added to the neutral particle, giving it a charge and allowing it to behave similar to a colloid. In view of the high electrical charge deposited on the particle, the neutral attraction of the counter-ions its surroundings is very strong so that a considerable electrical field is necessary to separate them and to permit a migration with a certain velocity. 3. Experimental
3.1. EQUIPMENT - Pleuger (Belgium) High Voltage power supply with up to 5000 V a continuously variable voltage and capable of delivering 500 mA. - Ultra sonic vibration unit. - Cells: different types of cells were used. The cells resemble strongly the ones used usually for electrolysis. The support that has to be covered is always suspended from the top of the cell. In this way all coarser particles will settle out and only the fine particles will migrate upwards to the electrode (fig. 1). Figs. 2 and 3 show sample holders as used for the preparation of uranium layers. The main parts of these holders are made in PVC. A teflon gasket is pressed against the backing by the diaphragm. 3.2. REAGENTS Isopropylic alcohol, benzoic acid, nitric acid, ammonium hydroxide. All reagents were p.a. grade (Merck). 3.3. THE PREPARATION OF FINE URANIUMOXIDE POWDERS ( U 3 0 8 )
Fig. 1. Support to be covered, suspending from top of cell.
MARCh 1968
The ideal particle size for electrophoresis according to literature is 6/~ml). Such powders were mostly prepared by milling, a method that is liable to introduce undesirable chemical impurities. A fairly simple method is the quantitative precipitation and calcination of the substances. Uranylnitrate solution is precipitated under vigo125
126
V. V E R D I N G H AND K. F. L A U E R SAMPLE HOLDER
Figs. 2, 3. Sample holders as used for the preparation of uranium layers.
Electrical
contc
Backing - Teflon gasket and diafragm
7O
2 (Fig. 2.)
rous stirring (ultra sonic vibration), with concentrated ammonia. A quasi colloidal precipitate of ammoniumdiuranate is obtained which is centrifuged and to avoid recristallization, quickly washed with water and absolute alcohol. The precipitate is dried with ether. To obtain the stoechiometric UaOs, the ammoniumdiuranate is calcined. The calcination is made under temperature control, by increasing the temperature stepwise by 50 ° C each half an hour, until a temperature of 900 ° C is reached. Measurements of different oxide powders by immersion microscopy showed an overall particle size of about 1 pm and less. The microscopic measurements were confirmed by Debye-Scherrer diagrams indicating an overall grain size smallei than 1 l~m.
4. Determination of the experimental parameters 4.1. COMPOSITION OF T H E SUSPENSION Although in preliminary experiments ammoniumdiuranate was suspended and precipitated, the advantage of using a compound with a higher U-content directed us to use U 3 0 s as suspended matter. The U 3 0 s powders were suspended in dry iso-
0
0
\N
eo
y
'XN\\ \
o
~,
\
50
/ 40 Total quantity of suspended matter
3O
0 0
50 rng U3 08 7Oral
•
140 m g
•
280rag
t rain 0
2
3
4
5
D-
6
Fig. 4. Influence of electrophoresis time in the yield of the deposition for different total quantities of UzO8 in the cell.
THE PREPARATION
OF U R A N I U M
LAYERS
127
propylic alcohol. Two drops of diluted nitric acid and about 0.05 mg benzoic acid were added. The nitric acid was added to obtain a slight electrical conductance of the solution and the benzoic acid served as the ionogenic pa~t of the neutral particles. Experiments using ammonia as additive gave unsatisfactory, non adherent deposits. 4.2. ELECTRODEVOLTAGEAND CURRENT DENSITY Experiments at voltages between 400 and 900 V did not lead to good results. The current flowing through the solution was small and of the order of a few mA. It was observed that an increase of the electrode voltage produced a current of 20 mA, but still no improvement of the quality of the deposits was observed. By augmenting the acidity with H N O 3 and hence the conductivity of the solutions, the current flowing through the cell was raised to a value of 100 to 150 mA. Now the yield and the quality of the deposits were good.
Fig. 6. U-samples on the inner side of an aluminium hemisphere. It seems that under these conditions the maximum yield obtainable is about 60-65%. This can be explained by the fact that the remaining material consists of agglomerated oxide particles which drop to the bottom of the cell. In later experiments the yield was improved by homogenizing and suspending the oxide by means of ultra-sonic vibration. The yield could be raised to 85-95%. The homogeneity of the prepared samples of 5 mg cm 2 thickness was of the order of 5%. The stability and adherence of the prepared layers is remarkable. An enriched uranium sample of 5 mg U/cm 2. deposited on both sides of a platinum backing
Fig. 5. U-samples on thin aluminium backings. Backing: aluminium 8 #m thick, 22x 22 ram; layer size: circular 20 mm dia.; thickness of layers: 12 mg U/cmL The sample holder used for the preparation of the3e targets is shown in fig. 2. 4.3. RESULTS Fig. 4 shows the influence of the electrophoresis time on the yield of the deposition for different total quantities of U308 in the cell. These experiments were made with aluminium cathodes of 20 x 20 mm, the distance of the anode to the cathode was 4 cm. Suspended matter: various quantities of U308 in about 200 ml isopropylic alcohol. Additives: 2 drops HNO3(conc ) and 0.05 mg benzoic acid. In these experiments the maximum deposition yield was of the order of 60%. Even if the experiment was continued no further deposition occurred but the layer started to detach itself from the electrode.
Fig. 7. U-samples on both sides of Pt backings. Sample size: circular 52 mm on both sides; Mass of the samples: 6 mg U/cm2 on each side.
128
V. V E R D I N G H
remained intact for more than two years of experimenting with it. To improve the homogeneity of the sample, the sample holder rotates during the electrophoresis at a speed of 20-100 rpm. As the temperature of the solvent rises above the boiling point after a few minutes of working, the evolution of gas sometimes causes troubles when bubbles form under the electrode. By positioning the cell and electrodes obliquely these gasses can freely escape. Figs. 5-7 show some types of samples that have been prepared by electrophoresis. To obtain a homogeneous deposit on the inner surface of a hemisphere a special cell had to be prepared. Experiments using the normal cell with a flat anode did not allow a complete deposition, because of the asymmetric distribution of the electrical field. A metal rod
A N D K. F. L A U E R
that could be positioned in equidistance from the cathode was then used as anode. A typical example of such a sample is shown in fig. 6. The inside of the hemisphere was covered with a layer of 10 mg U/cm 2. The authors wish to thank H. Mast for his technical assistance. The grain size measurements were kindly done by Mr. F. Verheyen. References 1) G. P. Gutierrez et al., J. Electrochem. Soc. 109 (1962). 2) F. M. Lang et al., Rapport CEA 545 (1956). 8) C. G. Bazell and R. V. Davis, AEEW-R391 (1964). 4) A. Cranston and B. Mc. Creary, Rev. Sci. Instr. 108 (1956) 973. 5) K. F. Lauer and V. Verdingh, Nucl. Instr. and Meth. 2I (1963) 161; 31 (1964) 355; 49 (1967) 179.
INSTRUMENTS
AND METHODS
60 (I968) I 2 5 - I 2 8 ;
© NORTH-HOLLAND
PUBLISHING
CO.
THE PREPARATION OF U R A N I U M LAYERS BY HIGH VOLTAGE E L E C T R O P H O R E S I S V. V E R D I N G H and K. F. L A U E R
Central Bureau for Nuclear Measurements, Euratom, Geel. Belgium Received 8 December 1967 A method is described that allows the preparation of thick and homogeneous uranium oxide layers on metallic backings. Thelayers were used in neutron physics experiments.
1. Introduction
Electrophoresis, known already for many years, was used for the preparation of relatively thick uranium oxide layers of various shapes and dimensions. The method was studied and adapted for the fabrication of layers above the mg/cm 2 range, where electrospraying 5) or electrolysis proved to be time consuming or impossible. Whilst settling techniques give not to well adhering and homogeneous samples. Samples were prepared on different backings (e.g. aluminium, platinum, stainless steel and copper). Homogeneous samples with good adherence to the backing can be prepared in a minimum of time. 2. Principle of the method
Lyophobic colloids always migrate under the action of an electric field, the particles often give up their surface energy and pass to a more stable form. Under certain experimental conditions the particles precipitate
on the electrode, thus forming an adherent film. In the case of suspended neutral particles, an ionogenic part or complex can be added to the neutral particle, giving it a charge and allowing it to behave similar to a colloid. In view of the high electrical charge deposited on the particle, the neutral attraction of the counter-ions its surroundings is very strong so that a considerable electrical field is necessary to separate them and to permit a migration with a certain velocity. 3. Experimental
3.1. EQUIPMENT - Pleuger (Belgium) High Voltage power supply with up to 5000 V a continuously variable voltage and capable of delivering 500 mA. - Ultra sonic vibration unit. - Cells: different types of cells were used. The cells resemble strongly the ones used usually for electrolysis. The support that has to be covered is always suspended from the top of the cell. In this way all coarser particles will settle out and only the fine particles will migrate upwards to the electrode (fig. 1). Figs. 2 and 3 show sample holders as used for the preparation of uranium layers. The main parts of these holders are made in PVC. A teflon gasket is pressed against the backing by the diaphragm. 3.2. REAGENTS Isopropylic alcohol, benzoic acid, nitric acid, ammonium hydroxide. All reagents were p.a. grade (Merck). 3.3. THE PREPARATION OF FINE URANIUMOXIDE POWDERS ( U 3 0 8 )
Fig. 1. Support to be covered, suspending from top of cell.
MARCh 1968
The ideal particle size for electrophoresis according to literature is 6/~ml). Such powders were mostly prepared by milling, a method that is liable to introduce undesirable chemical impurities. A fairly simple method is the quantitative precipitation and calcination of the substances. Uranylnitrate solution is precipitated under vigo125
126
V. V E R D I N G H AND K. F. L A U E R SAMPLE HOLDER
Figs. 2, 3. Sample holders as used for the preparation of uranium layers.
Electrical
contc
Backing - Teflon gasket and diafragm
7O
2 (Fig. 2.)
rous stirring (ultra sonic vibration), with concentrated ammonia. A quasi colloidal precipitate of ammoniumdiuranate is obtained which is centrifuged and to avoid recristallization, quickly washed with water and absolute alcohol. The precipitate is dried with ether. To obtain the stoechiometric UaOs, the ammoniumdiuranate is calcined. The calcination is made under temperature control, by increasing the temperature stepwise by 50 ° C each half an hour, until a temperature of 900 ° C is reached. Measurements of different oxide powders by immersion microscopy showed an overall particle size of about 1 pm and less. The microscopic measurements were confirmed by Debye-Scherrer diagrams indicating an overall grain size smallei than 1 l~m.
4. Determination of the experimental parameters 4.1. COMPOSITION OF T H E SUSPENSION Although in preliminary experiments ammoniumdiuranate was suspended and precipitated, the advantage of using a compound with a higher U-content directed us to use U 3 0 s as suspended matter. The U 3 0 s powders were suspended in dry iso-
0
0
\N
eo
y
'XN\\ \
o
~,
\
50
/ 40 Total quantity of suspended matter
3O
0 0
50 rng U3 08 7Oral
•
140 m g
•
280rag
t rain 0
2
3
4
5
D-
6
Fig. 4. Influence of electrophoresis time in the yield of the deposition for different total quantities of UzO8 in the cell.
THE PREPARATION
OF U R A N I U M
LAYERS
127
propylic alcohol. Two drops of diluted nitric acid and about 0.05 mg benzoic acid were added. The nitric acid was added to obtain a slight electrical conductance of the solution and the benzoic acid served as the ionogenic pa~t of the neutral particles. Experiments using ammonia as additive gave unsatisfactory, non adherent deposits. 4.2. ELECTRODEVOLTAGEAND CURRENT DENSITY Experiments at voltages between 400 and 900 V did not lead to good results. The current flowing through the solution was small and of the order of a few mA. It was observed that an increase of the electrode voltage produced a current of 20 mA, but still no improvement of the quality of the deposits was observed. By augmenting the acidity with H N O 3 and hence the conductivity of the solutions, the current flowing through the cell was raised to a value of 100 to 150 mA. Now the yield and the quality of the deposits were good.
Fig. 6. U-samples on the inner side of an aluminium hemisphere. It seems that under these conditions the maximum yield obtainable is about 60-65%. This can be explained by the fact that the remaining material consists of agglomerated oxide particles which drop to the bottom of the cell. In later experiments the yield was improved by homogenizing and suspending the oxide by means of ultra-sonic vibration. The yield could be raised to 85-95%. The homogeneity of the prepared samples of 5 mg cm 2 thickness was of the order of 5%. The stability and adherence of the prepared layers is remarkable. An enriched uranium sample of 5 mg U/cm 2. deposited on both sides of a platinum backing
Fig. 5. U-samples on thin aluminium backings. Backing: aluminium 8 #m thick, 22x 22 ram; layer size: circular 20 mm dia.; thickness of layers: 12 mg U/cmL The sample holder used for the preparation of the3e targets is shown in fig. 2. 4.3. RESULTS Fig. 4 shows the influence of the electrophoresis time on the yield of the deposition for different total quantities of U308 in the cell. These experiments were made with aluminium cathodes of 20 x 20 mm, the distance of the anode to the cathode was 4 cm. Suspended matter: various quantities of U308 in about 200 ml isopropylic alcohol. Additives: 2 drops HNO3(conc ) and 0.05 mg benzoic acid. In these experiments the maximum deposition yield was of the order of 60%. Even if the experiment was continued no further deposition occurred but the layer started to detach itself from the electrode.
Fig. 7. U-samples on both sides of Pt backings. Sample size: circular 52 mm on both sides; Mass of the samples: 6 mg U/cm2 on each side.
128
V. V E R D I N G H
remained intact for more than two years of experimenting with it. To improve the homogeneity of the sample, the sample holder rotates during the electrophoresis at a speed of 20-100 rpm. As the temperature of the solvent rises above the boiling point after a few minutes of working, the evolution of gas sometimes causes troubles when bubbles form under the electrode. By positioning the cell and electrodes obliquely these gasses can freely escape. Figs. 5-7 show some types of samples that have been prepared by electrophoresis. To obtain a homogeneous deposit on the inner surface of a hemisphere a special cell had to be prepared. Experiments using the normal cell with a flat anode did not allow a complete deposition, because of the asymmetric distribution of the electrical field. A metal rod
A N D K. F. L A U E R
that could be positioned in equidistance from the cathode was then used as anode. A typical example of such a sample is shown in fig. 6. The inside of the hemisphere was covered with a layer of 10 mg U/cm 2. The authors wish to thank H. Mast for his technical assistance. The grain size measurements were kindly done by Mr. F. Verheyen. References 1) G. P. Gutierrez et al., J. Electrochem. Soc. 109 (1962). 2) F. M. Lang et al., Rapport CEA 545 (1956). 8) C. G. Bazell and R. V. Davis, AEEW-R391 (1964). 4) A. Cranston and B. Mc. Creary, Rev. Sci. Instr. 108 (1956) 973. 5) K. F. Lauer and V. Verdingh, Nucl. Instr. and Meth. 2I (1963) 161; 31 (1964) 355; 49 (1967) 179.