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Detection of nitrogen impurities in aluminium by positron annihilation
Volume 71A, number 2,3
PHYSICS LETTERS
30 April 1979
DETECTION OF NITROGEN IMPURITIES IN ALUMINIUM BY POSITRON ANNIHILATION B.G. HOGG’ and R. PAULIN Institut National des Sciences et TechniquesNucléaires Saclay, 91190 Gifsur Yvette, France
and T.D. TROEV Institute of Nuclear Research and Nuclear Energy, Sofia 13, Bulgaria Received 5 December 1978 Revised manuscript received 28 February 1979 An increase both in the peak (4%) and in the tails (20% for 0 hilation 7-rays when Al + 0.05 at% N is compared with pure Al.
Aluminium (99.9999% pure) was alloyed from the melt with nitrogen gas at 50 atm. Angular correlation experiments were performed [1] at room temperature on pure Al annealed at 500°C for two hours and on the unrefined Al + 0.0005 N. Runs were also made on Al + 0.0005 N samples which had been annealed at 240°C and 550°C. All curves, normalized to equal numbers of counts, are compared in fig. 1. The sharp increase in peak height for the Al ÷ 0.0005 N unrefined sample curve is clearly resolved. The intermediate curve for the separate Al + 0.0005 N samples annealed at 240°C and 550°C, respectively, show that the effect is still resolved after appreciable loss of nitrogen from the sample. Even after the 550°C treatment, enough nitrogen remains to identify its presence or to confirm that it was formerly present. The effect in the tails of the curves is difficult to see at the scale in fig. 1 and therefore a difference curve has been plotted in fig. 2. The logarithmic ordinate emphasizes the situation at high momenta while retaining all the data over the total momentum range for 0 > 8 mrad. Triftshaüser [2] has shown that with single crystals of pure aluminium, “the difference curve obtained between the angular correlation curve measured at 563°C (where almost all the positrons
On leave from the University of Winnipeg, Canada.
240
>
8 mrad) is observed in the angular distribution of anni-
are trapped at vacancies) and at 25°C(where practically no vacancies are present)” becomes negative at 8 mrad and remains negative. Shulman and Berko [3] reported the same results and found the effect independent of crystal orientation and hence applicable to polycrystalline aluminium. In contrast, our difference curve, obtained with polycrystalline samples, is positive over the region 0 > 8 mrad, but displays the same features at low momentum as determined by the previous workers [2,3] and attributed to vacancy trapping. In our sample, the presence of a vacancy bound to a nitrogen impurity is very improbable because of the repulsive interaction between an aluminium vacancy and nitrogen predicted from theory [4]. Therefore, the positron trapping in a vacancy cannot be retained even to explain the enhancement at the peak. It appears reasonable to interpret the enhancement at the tails as resulting from the nitrogen’s presence since an indication of such enhancement has been observed in the alloying of nitrogen with zinc [5]. So we conclude that these data reveal some positron trapping by nitrogen impurities. In these localized states, the overlap of the positron wavefunction with core electrons is larger than for unbound positrons annihilating freely in the pure metal. To confirm this hypothesis, it is necessary to do experiments at various nitrogen
60K.
B
A
C
D
~ 50K0 U U C
40K
C., C 0
C.)
30K
201c
10K-
0
ii
—19
—16
II
—14
—12
111111
—10
—8
I
—6
—4
—2
II
0
III
2
6
liii
6
8
10
I
12
14
I
16
r
19
8 (mrd). Angle between annihiLation photons Fig. 1. All the curves were obtained at room temperature. A is Al + 0.0005 N as produced from the melt. B is Al + 0.0005 N after annealing at 240°C.C is Al + 0.0005 N after annealing at 550°C.D is pure aluminium. All samples were gently polished to remove oxide film. Experimental points are only shown in the difference curve for the sake of clarity.
~
z In 0 C C
P:~
L2~1O-8~6~hF0
8 (mrd). AngLe between annihilation photons
The difference in the number of counts for Al + 0.0005 N as produced from the melt and pure Al. Beyond 8 mrad, the difference remains positive. 241 Fig. 2.
Volume 71A, number 2,3
PHYSICS LETTERS
concentrations * ~. To estimate the mean electronic density experienced by the positron localized near the nitrogen, we measured the positron lifetime spectra in pure Al and in Al + 0.0005 N at room temperature. The single lifetime, 25% longer, found in the sample containing nitrogen reveals a reduction of electronic density similar to that experienced by a positron in a dislocation. Material science has a paucity of nondestructive techniques for ~ T.D. Troev, samples with various nitrogen concentrations are under preparation at the Bulgarian Academy of Sciences where Doppler broadening experiments are in progress.
242
30 April 1979
the study of gases in metals. Positron annihilation provides a new method for in situ study of gases in metals. References [1] G. Coussot and R. Paulin, J. Appl. Phys. 43 (1972) 1325.
[2] W. Triftshaüser, Phys. Rev. B12 (1975) 4634. [31 M.A. Shulman and S. Berko, in: Proc. 4th Intern. Conf.
on Positron annihilation (Helsmgor, Denmark, 1976) ~. E31. [4] A. Blandin, J.L. Deplanté and J. Friedel, J. Phys. Soc. Japan 18 Suppl. 11(1963) 85. [5] T.D. Troey, in: Proc. 4th Intern. Conf. on Positron annihilation (Helsingor, Denmark, 1976) p. D10.
PHYSICS LETTERS
30 April 1979
DETECTION OF NITROGEN IMPURITIES IN ALUMINIUM BY POSITRON ANNIHILATION B.G. HOGG’ and R. PAULIN Institut National des Sciences et TechniquesNucléaires Saclay, 91190 Gifsur Yvette, France
and T.D. TROEV Institute of Nuclear Research and Nuclear Energy, Sofia 13, Bulgaria Received 5 December 1978 Revised manuscript received 28 February 1979 An increase both in the peak (4%) and in the tails (20% for 0 hilation 7-rays when Al + 0.05 at% N is compared with pure Al.
Aluminium (99.9999% pure) was alloyed from the melt with nitrogen gas at 50 atm. Angular correlation experiments were performed [1] at room temperature on pure Al annealed at 500°C for two hours and on the unrefined Al + 0.0005 N. Runs were also made on Al + 0.0005 N samples which had been annealed at 240°C and 550°C. All curves, normalized to equal numbers of counts, are compared in fig. 1. The sharp increase in peak height for the Al ÷ 0.0005 N unrefined sample curve is clearly resolved. The intermediate curve for the separate Al + 0.0005 N samples annealed at 240°C and 550°C, respectively, show that the effect is still resolved after appreciable loss of nitrogen from the sample. Even after the 550°C treatment, enough nitrogen remains to identify its presence or to confirm that it was formerly present. The effect in the tails of the curves is difficult to see at the scale in fig. 1 and therefore a difference curve has been plotted in fig. 2. The logarithmic ordinate emphasizes the situation at high momenta while retaining all the data over the total momentum range for 0 > 8 mrad. Triftshaüser [2] has shown that with single crystals of pure aluminium, “the difference curve obtained between the angular correlation curve measured at 563°C (where almost all the positrons
On leave from the University of Winnipeg, Canada.
240
>
8 mrad) is observed in the angular distribution of anni-
are trapped at vacancies) and at 25°C(where practically no vacancies are present)” becomes negative at 8 mrad and remains negative. Shulman and Berko [3] reported the same results and found the effect independent of crystal orientation and hence applicable to polycrystalline aluminium. In contrast, our difference curve, obtained with polycrystalline samples, is positive over the region 0 > 8 mrad, but displays the same features at low momentum as determined by the previous workers [2,3] and attributed to vacancy trapping. In our sample, the presence of a vacancy bound to a nitrogen impurity is very improbable because of the repulsive interaction between an aluminium vacancy and nitrogen predicted from theory [4]. Therefore, the positron trapping in a vacancy cannot be retained even to explain the enhancement at the peak. It appears reasonable to interpret the enhancement at the tails as resulting from the nitrogen’s presence since an indication of such enhancement has been observed in the alloying of nitrogen with zinc [5]. So we conclude that these data reveal some positron trapping by nitrogen impurities. In these localized states, the overlap of the positron wavefunction with core electrons is larger than for unbound positrons annihilating freely in the pure metal. To confirm this hypothesis, it is necessary to do experiments at various nitrogen
60K.
B
A
C
D
~ 50K0 U U C
40K
C., C 0
C.)
30K
201c
10K-
0
ii
—19
—16
II
—14
—12
111111
—10
—8
I
—6
—4
—2
II
0
III
2
6
liii
6
8
10
I
12
14
I
16
r
19
8 (mrd). Angle between annihiLation photons Fig. 1. All the curves were obtained at room temperature. A is Al + 0.0005 N as produced from the melt. B is Al + 0.0005 N after annealing at 240°C.C is Al + 0.0005 N after annealing at 550°C.D is pure aluminium. All samples were gently polished to remove oxide film. Experimental points are only shown in the difference curve for the sake of clarity.
~
z In 0 C C
P:~
L2~1O-8~6~hF0
8 (mrd). AngLe between annihilation photons
The difference in the number of counts for Al + 0.0005 N as produced from the melt and pure Al. Beyond 8 mrad, the difference remains positive. 241 Fig. 2.
Volume 71A, number 2,3
PHYSICS LETTERS
concentrations * ~. To estimate the mean electronic density experienced by the positron localized near the nitrogen, we measured the positron lifetime spectra in pure Al and in Al + 0.0005 N at room temperature. The single lifetime, 25% longer, found in the sample containing nitrogen reveals a reduction of electronic density similar to that experienced by a positron in a dislocation. Material science has a paucity of nondestructive techniques for ~ T.D. Troev, samples with various nitrogen concentrations are under preparation at the Bulgarian Academy of Sciences where Doppler broadening experiments are in progress.
242
30 April 1979
the study of gases in metals. Positron annihilation provides a new method for in situ study of gases in metals. References [1] G. Coussot and R. Paulin, J. Appl. Phys. 43 (1972) 1325.
[2] W. Triftshaüser, Phys. Rev. B12 (1975) 4634. [31 M.A. Shulman and S. Berko, in: Proc. 4th Intern. Conf.
on Positron annihilation (Helsmgor, Denmark, 1976) ~. E31. [4] A. Blandin, J.L. Deplanté and J. Friedel, J. Phys. Soc. Japan 18 Suppl. 11(1963) 85. [5] T.D. Troey, in: Proc. 4th Intern. Conf. on Positron annihilation (Helsingor, Denmark, 1976) p. D10.