High-Tc superconductor/silver composites A new direct preparation process

PHYSICA ELSEVIER

Physica C 262 (1996)111-119

High- Tc superconductor / silver composites A new direct preparation process B. b/it Salah a, M. Mansori b, M.A. Fremy c, M.H. Pischedda c, M. Roubin b, A. Benlhachemi c, H. Benyaich a, J.R. Gavarri c,. a Lab. Chimie des Solides, Facuh~ des Sciences, Universit~ Ibnou Zohr. Agadir. Maroc b Lab. MFS, Universit~ de Toulon et du Var, BP 132, 83 957 La Garde, France c Lab. MMI. Universit~ de Toulon et du Var, BP 132.83957 La Garde, France

Received 12 December 1995; revised manuscript received 10 February 1996

Abstract

A new wet process has been used to directly prepare high-To superconductor/silver composites from mixed precursors of the whole constituents including silver. In an aqueous solution of metal nitrates, either oxalic acid or potassium carbonate solutions have been introduced in order to coprecipitate oxalates or carbonates, respectively. The pyrolysis of the dried powder finally led to the desired final composite. Two series of high-Tc superconducting oxide (HTS) have been studied; bismuth cuprates, Bi-2212 (Tc = 80-85 K) and YBa2Cu30 7 (YBCO-123). The volume fractions ( ~ ) of silver (expected in a final metallic form) ranged between 0 and 50%. The different phases produced during the synthesis were then characterized by coupled DTA-TGA analyses (differential thermal analysis and thermogravimetric analysis), X-ray diffraction, and Fourier transform infrared spectroscopy (FTIR). Measurements of levitation forces, magnetic susceptibility and electrical resistance at low temperature were carried out to control the final superconducting properties. In the case of YBa2Cu307 some changes in structural parameters and physical properties are observed ( ~ > 0.20).

I. Introduction

The general aim of this study was to prepare various two-phase composite materials based on a high-Tc superconducting (HTS) phase and a ductile metallic phase acting as an elastic shock-absorber. To simultaneously improve the mechanical properties and hold the superconducting properties in a granular material sintering processes are generally needed: weak links and interfaces can be formed,

* Corresponding author. Fax: + 33 94 14 23 11.

thus involving both elastic, magnetic and electrical modifications. Such m e t a l / H T S composites prepared with either ductile [1-7] or stiff metallic additions have already been studied [8,9] using powders that were compressed and then sintered. It was previously shown that Ag additions [1-6], Sn additions [7] or even organic polymeric matrices [10,11] could bring about interesting mechanical properties in the system. However, in the case of organic matrices, the percolation of superconducting currents cannot be obtained. In the case of tin composites [7], it was shown that after a thermal treatment under air the percolation could be obtained even for significant proportions of Sn (25 vol.%) but,

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A. A'it Salah et al./Physica C 262 (1996) 111-119

metallic tin was changed into tin oxide, thus limiting the desired mechanical improvement. To date, silver is considered as an interesting candidate to obtain a mechanical improvement in composites, given its ductility and its stability at high temperature. To manufacture ceramics/metal composites, the classical preparation route generally involves a cold-pressing treatment followed by a high-temperature sintering process. In such a preparation process, the homogeneity of the initial and final samples is rarely achieved. So, to improve the homogeneity of samples, another route is presently proposed. It consists of mixing all the constituents in an aqueous solution, with the metal atoms being present as cations diluted in the solution [12,13]. This solution is then evaporated and the precipitated powders heated. In the case of silver, the oxide is not stable at high temperature: thus after heating under air environment, the initial Ag ÷ cations are expected to be finally found under the form of metal precipitates, with small Ag particles distributed through the superconducting matrix. This is only possible if no Ag atom substitutes for an atom of the superconducting lattice. In this work, the direct preparation of Ag based superconducting composites is shown to be an interesting alternative route leading to homogeneous sampies. The superconducting phases are the following: (1) YBa2Cu30 7 (denoted YBCO) ; (2) * Bil.6Pb0.4Sr2Calfu20(8+~s ) (denoted Bi-2212). Some of the characterizations of YBCO and Bi2212 composites are presented next.

2. Sample preparation The preparation process [13] is firstly described for each type of composites; then an analysis of the chemical steps is carried out using FTIR spectroscopy and thermal analyses. 2.1. The Ag / Bi cuprate composites

The composites are prepared from organometallic complexes using oxalic acid as complexing agent [12]. The various nitrates (Bi 3÷, Pb 2+, Sr 2÷, Ca 2+, Cu 2÷, Ag +) in aqueous solutions react with an ox-

Table 1 The direct synthesis of Ag/Bi 2212 composites 1.6[Bi(NO3)3•5H20] + 0.4 Pb(NO3)2 + 2 Sr(NO3) 2 + Ca(NO3)2- 4H sO + 2(Cu(NO3)2. 2.5 H 20) + AgNO3 + + H2C204 + OHCH2CH~OH+ (C 2Hs)3N Oxalate precipitation = (thermal treatment at 550°C under air) ~ oxi-carbonates oxi-carbonates (750°C) ~ homogeneousoxide mixture = (847°C) ~ Bi 1.6Pb0.4Sr2CaiCu20~s+,~ + Ag.

alic acid solution and glycol ethylen. The full precipitation of the complexes occurs at pH = 11. The pH is increased by adding triethylamine, a base without any complexing action on the metallic elements previously cited. The precursors, first dried, are then subjected to a pyrolysis which involves a decomposition in several steps and leads to a polyphasic material composed of metallic silver and oxides containing one or more cations. This is in agreement with the thermodynamical prediction of the dissociation of A g 2 0 into Ag and 0 2. The different chemical steps leading to A g / B i - 2 2 1 2 composites (Tc about 85 K) can be represented as in Table 1. 2.2. The Ag / YBCO composites

The composite materials A g / Y B a 2 C u 3 0 7 are prepared in a different way, given the presence of yttrium which does not allow the formation of complex of metallic elements to occur. As previously described in the case of cuprate-based composites, those elements are obtained from nitrates in aqueous solution: then they precipitate giving the corresponding carbonates. The CO32- species are brought about by the addition of K2CO 3 in aqueous solution. As previously indicated, the pyrolysis of such precursors finally leads to the formation of metallic silver in presence of superconducting oxide YBCO. The various steps are described in Table 2. Table 2 The direct synthesis of Ag/YBCO composites Y(NO3)3" 5H20 + 2" Ba(NO3)2 + 3(Cu(NO3)2. 2.5 H 20) + K 2CO3 + KOH(aq.)+ AgNO3 = (precipitation of carbonates)~ Hydrated Carbonates (Y,Ba,Cu,Ag) = (350°C) Anhydrous Carbonates = (800°C) Homogeneousmixture of oxides = (920°C, air) YBa2Cu3Ou- 8) + Ag

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2.3. The chemical steps

(a)

To limit the crystal growth and the chemical diffusions in the final composite pellets it is necessary to know each chemical step of the preparation process: to control each chemical step, a Fourier transform infrared spectroscopy analysis (FTIR-RS Nucleus from Unicam/Matson) has been used. Several steps have been observed. In the case of Y B C O / A g precursors, the FTIR spectra reported in Fig. l(a) are characteristic of the decomposition steps in which water and carbonate species are involved. After heating at 60°C during 5 h, the absorption bands of lattice water and carbonate ions (CO 2- ) are observed with some metal-oxygen bands (500-600 cm-l).

After heating at 300°C during 5 h, the absorption bands present the characteristics of the sole CO32species with some modification in the M - O bands (one M O - C O 3 band appears). After heating at 800°C during 12 h, all the CO 2bands disappear. Only M - O vibrational bands (500600 cm -1 ) are observed; however, they are different from the initial ones. In addition the evolution of the continuous transmittance range 4000-500 c m - i is characteristic of some increase in grain sizes. In the case of Bi2212/Ag samples, the FTIR spectra reported in Fig. lb present the characteristic of the decomposition steps of oxalate precursors. The 60°C spectra show the presence of water and H 2 C 2 0 4 species ( C = O and C - C bands), with the existence of M - O bands. The 550°C spectra show the presence of carbonates (CO 2- specific bands) resulting from the oxalate modification. The 750°C spectra present the characteristic of carbonate decomposition and oxide formation ( M - O bands). The thermal decomposition of precursors has been studied using differential thermal analyses (DTA) coupled with thermogravimetric analyses (TGA) from room temperature to 1000°C, under air. The heating rate was A T / A t = 10°C/min. Each precursor was initially dried at 60°C under air. Fig. 2 describes the disappearance of chemical species in the case of YBCO and Bi-2212/Ag composites. In the case of oxi-carbonate precursors, the disap-

40o0

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2boo

lobo (b)[

C-C

/

c-o c- M

c=o

SO'C

4000

3~oo

2doo

ldoo

W~,emmb~s

Fig. 1. Thermal decomposition of precursors: FTIR vibrational spectra of the successive phases involved in each chemical step. (a) (CO~-) species observed from YBCO precursor; (b) C2042species and carbonate ions observed from Bi-2212 precursor.

A. Ait Salah et al./ Physica C 262 (1996) 111-119

114

pearance of HzO and CO 2 molecules is first observed. In Fig. 2(a), a DTA peak is observed at about 800°C: it can be connected with the endothermic decomposition of the barium carbonate and the appearance of the specific MO infrared band of the YBCO phase (Fig. l(a)). Then, above 900°C, the endothermic formation of the superconducting phase is observed with characteristic DTA signals. In the case of oxalate precursors, three steps in weight loss are observed on the T G / D T A curves: they correspond to H20, CO 2 then CO gas disappearing. These steps are due to the decomposition: H2C204 =*' H 2 0 + CO 2 + CO. The last step (exothermic DTA signal oberved at 425°C) corresponds to the oxidation of CO into CO 2. Finally the expected oxides are formed.

2.4. The final composites The final YBCO samples are obtained as follows: (1) compacted pellets are firstly formed by subjecting the precipitated powders to a pressure of 5 kbar; (2) the composites are then heated firstly at 920°C during 2 h then at 400°C during 10 h under air; (3) the samples are finally cooled down to 200°C then quenched at room temperature. The Bi composites are obtained in a similar way but with an unique sintering temperature of 847°C for a limited duration of 12 h. These conditions have been chosen to limit the crystal growth during the thermal treatment. The Ag volume fractions (qb) range between 0 to 0.20 for Bi samples and 0 to 0.50 for YBCO samples.

3. Characterizations of the composites Ca) TG

HEATFLOW(mto'oV) DTA% s

A%

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.-10

-20 Temperature 25

425

225

625

(*C)

825

(b) TGA

96

HEAr~LOW( ~ o V) DTA

•

•-10

CO2

20

Temperature (*C) 25

2~;5

425

6"Z5

825

Fig. 2. TG-DTA analyses of the thermal decomposition in the case of (a) Y B C O / A g precursor and (b) B i - 2 2 1 2 / A g precursor. (a) H 2 0 , CO 2 disappearing; (b) H 2 0 , C O 2 then CO disappearing.

3.1. Effective volume fractions and Ag distribution The final Ag concentrations and distributions have been studied using scanning electron microscopy (SEM). In Fig. 3(a) and (b) the Ag distribution in Y B C O / A g composites is observed: (a) in a backscattered electron micrograph (X2000, q)= 0.10) some bright Ag particles can be observed; (b) in a secondary electron image ( x 7 9 2 4 , q~= 0.10) silver is observed at the grain boundaries and on the grain surfaces: the X-ray emission analyses show that Ag is distributed as small particles or threads (cotton aspect). At very low concentrations ( < 0.10), silver is regularly distributed along the grain boundaries and cavities. At higher concentrations, silver is agglomerated at the junctions and on the surfaces, forming threads with a general cotton aspect. The crystal growth of the Bi-2212 phase is anisotropic which is not the case for the YBCO phase. As shown in Figs. 3(c) and (d), the cuprate crystals form a complex combination of plates: some of the apparent needles observed in the micrographs are in fact the sections of Bi-2212 plates oriented because of compression effects induced by the manufacturing process (micrograph of a section of a composite pellet having 13 mm of diameter, 3 mm of thickness). For the highest silver contents, some

A. A'it Salah et al. / Physica C 262 (1996) 111-119

small Ag particles are observed either at some grain boundaries or on the surfaces of the plate crystals. They frequently appear as bright spheroidal particles in the micrographs (Fig. 3(d)).

3.2. X-ray diffraction Using X-ray diffraction the Bi-2212 and YBCO phases have been identified, using the JCPDS standards 41-0317 (Bi compound) and 39-0486 (YBCO compound), respectively. The Bragg peaks of the silver lattice (JCPDS standard 4-0783) are observable and roughly proportional to the initial volume fractions. No other crystalline phase is detected in such powder diffraction analyses.

115

For high Ag fractions ( > 20 vol.%), some slight modifications in the orthorhombic lattice of YBCO are observed. This corresponds to the formation of an intermediate chemical state in relation with the decrease of the oxygen content (8 in YBa2Cu 307_~5 increases). This variation involves the evolution of the orthorhombic YBCO phase to the YBa2Cu306.56 phase (Tc = 60 K)having the orthorhombic parameters a = 3 . 8 3 3 6 A, b = 3 . 8 8 1 0 A, c = 1 1 . 7 3 5 5 ~, (see JCPDS). Another well known YBaECU306 phase presents the tetragonal symmetry with a = 3.857 A, c = 11.839 A. In Table 3 the refined cell parameters are reported with the orthorhombic distortion 8 = b a as a characteristic of the influence of Ag during the composite preparation process. o

-

Fig. 3. Scanning electron micrograph. (a) Backscattered electron (BSE) imaging of YBCO/Ag ( × 2000) with qb = 0.10. (b) Secondary electron imaging of YBCO/Ag (X 7924) with • = 0.10; the X-ray emission analyses show that Ag is distributed as small particles or threads (cotton aspect) on the grain surfaces. (c) BSE imaging of pure Bi-2212 sample ( x 908); anisotropic growth with oriented plates. (d) BSE imaging of Bi-2212/Ag (X 1931) with ~ = 0.10; modification of lamellar aspect, presence of bright particles of Ag.

116

A. A~t Salah et al./Physica C 262 (1996) 111-119

3.3. Magnetic properties

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3.3.1• Meissner effect: Levitation Using a home-made device allowing one first to control the existence of the expected superconducting phase, then to correlate the measured levitation forces with the Ag compositions [10], the YBCO based composites have been tested. The relative density of each pellet (ratio of the observed density over the calculated density) has been determined: in this composition range, it is roughly constant, about 80%. The levitation force F ( ~ , h) is measured as a function of the Ag composition (~), and of the distance h between the opposite surfaces of the sample and of the magnet• As expected, the levitation force is a decreasing function of the parameters h and ~. In Fig. 4 the levitation measurements are reported. However, the observed linear variation of F(q0, h) strongly depends on the force of the magnetic field, i.e. of the h distance. For low h values, F is roughly proportional to the Ag fraction. This is not the case for high values of h. These two behaviors could be explained using the following description that we previously proposed (see Ref. [14]): (1) for small h values (high magnetic field), a type-II superconducting behavior should occur in the samples with some penetration of the magnetic flux

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60

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Fig. 4. Levitation forces in YBCO/Ag composites [10,11]: F / M (in N kg- ' ) is a function of the distance h and Ag vol. fraction. The distance h is taken between the opposite faces of a cylindrical magnet put on a balance and of the composite pellets. The variation of F / M strongly depends on the distance h.

0,1 0,0 0

10

20

30

A g volume fraction (%) Fig. 5. (a) Magnetic susceptibility measurements in YBCO samples. (b) Evolution of Tc and (c) amplitude of signals X(~, T = l0 K) extrapolated at l0 K vs. volume fraction.

A. A'it Salah et al./Physica C 262 (1996) 111-119

inside the ceramics pellet; (2) for large h values (low magnetic field), a total exclusion of the magnetic field from the pellets should probably occur. As a consequence, the levitation force is expected to behave quite differently in these two h ranges: for the largest h values (the magnetic flux being fully expelled in pure ceramics), the presence of Ag particles allows some penetration of this magnetic flux, thus inducing a strong decrease in the levitation force. In the case of the smallest h values, a magnetic flux penetration already exists in pure samples: thus, the presence of Ag particles does not modify the levitation law but only the relative content of the total vortex volume. This fact could be at the origin of the linear variation of the levitation force in this range. In the case of the Bi-2212 composites, the levitation forces cannot be observed because the T~° is too low ( < 80 K) in comparison with the value obtained in liquid-nitrogen temperature.

3.3.2. Susceptibility measurements In the case of YBCO samples the susceptibility measurements were carried out in the range 10 K 300 K using a home-made device. The AC magnetic susceptibility was measured using two secondary coils coaxially placed in the primary coil. The frequency of the alternative magnetic field is 80 Hz. In Fig. 5(a) some of the data are reported. For a volume fraction of 0.20 a significant decrease in T~ is observed. This evolution is well connected with the structural modification of the YBCO composite. This might argue in favor of a loss in oxygen stoichiometry due to the limited sintering time.

117

In Figs. 5(b) and 5(c) the curves Tc(~) and X(@, T = 10 K) are reported. The extrapolated zero value of X(q~, T = 10 K) is reached for • = 0.28 _ 0.05: in the case of a two-phase material (pure YBCO-123 + metal" Ag), the extrapolated • value should have been @ = 1. In the Ag volume fraction range 0 to 0.10, the T~ seems to be unchanged. However, at • = 0.20 a strong decrease in Tc is observed: this can be directly connected with the small structural modification of YBCO. Both results (Tc and X0 variations) are in full agreement with the fact that there is a structural modification in the YBCO superconducting phase. In the case of Bi-2212/Ag composites, the superconducting properties were only observed for low values ( < 0.20). The X(qb, T) values are not reported here because they behave in the standard way ( L = 80-85 K).

3.4. Resistances R(T) To characterize the superconducting currents, resistance measurements have been carried out at low temperature (10 K - 3 0 0 K) and using the four-probe method. The results are reported in Figs. 6(a-c) in the case of Y B C O / A g (0, 15, 20 vol.% Ag) and in Fig. 6(d) in the case of the Bi-2212/Ag composite (10 vol.% Ag). In the pure YBCO sample the zeroresistance value is observed at T° = 89 + 1 K with Tc onset = 91 K. In the Y B C O / A g samples the zero value of the resistance is not reached because the sintering process has been deliberately limited in time: as a consequence, the Josephson junctions are not completely formed through the sample and in addition, some oxygen deficiency may occur. In Bi

Table 3 Evolution of the YBCO orthorhombic cell in the presence of Ag a 1004

0

5

20

30

40

50

a (~,)

3.823(1)

3.825(1)

3,830(1)

3.835(2)

3.840(4)

b (A)

3.891(1)

3.889(1)

3.880(1)

3.878(2)

3.870(4)

3.870(4)

c (,~)

11.703(4)

11.705(5)

11.720(4)

11.725(5)

11.730(8)

11.730(8)

e = b - a (A)

0.068

0.064

0.050

0.043

0.030

a Between the parentheses 0 the errors are reported as 2o" values (o" being the classical mean square amplitude). Sixteen dthkt ) data were used in a classical refinement calculation.

3.840(4)

0.030

A. A'it Salah et al./Physica C 262 (1996) 111-119

118

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Temperature (K) 50 45

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250

300

..... 0

50

100

150

Temperature

200

250

300

(K)

Fig. 6. Resistances R(T) and s u p e r c o n d u c t i n g transitions in A g / Y B C O composites. The v o l u m e fractions are (a) 0, (b) 0.15, (c) 0.20. Tc o ~ s e t = 91 K and ATc = 5 K with a residual R o value due to limited sintering time. (d) Resistance R(T) and superconducting transition in " a s - p r e p a r e d " 10% A g / B i - 2 2 1 2 composites: Tc onset : 85 K and A Tc = 25 K with a residual resistance value. The R(T) curve obtained from the same sample after sintering during 3 d a y s under air is given a n d referred to as " s i n t e r e d " .

2212/Ag samples (O < 0.20), the T~'s are 85 K with a AT~ of about 25 K. In both types of composites, the superconducting phase transitions have been obtained even without optimized sintering conditions but with a residual R 0 value (no Tc°). In previous works [6], it was shown that, in Ag composites, a significant increase of the sintering time could improve the quality of superconducting properties. However, it is necessary to recall that the present study is not devoted to the optimization of superconductivity in such composites, but mainly to the direct preparation of composites having homogeneous distributions of Ag particles, small grain sizes and obtained without any mechanical intermediate process (i.e. grinding of individual phases). Some sam-

pies were re-heated for 3 days: as expected, an improvement of the superconductivity characteristics has been observed but with a correlated crystal growth of Ag particles. In Fig. 6(d) the effect of the sintering on a Bi-2212/10% Ag composite is shown: the "as-prepared" sample presents a residual resistance below T~ o,set while the re-heated sample (3 days at 847°C) is characterizedby a typical zero resistance below about 45°C.

4. Conclusion

The main interest of the present approach was to

directly obtain solid two-phase composites using

A. A'it Salah et al. / Physica C 262 (1996) 111-119

precursors obtained in aqueous solutions. From the compacted precursor powders, it has been possible to directly synthesize Ag based superconductor composites without any intermediate mechanical treatment. The final product contains metallic Ag particles or Ag threads (mean linear dimensions of 0.1 to 5 p,m) located in cavities, grain boundaries and probably on the grain surfaces of the superconducting phases. The homogeneity of such composites is improved in comparison with classical composites obtained after mixing, grinding, pressing then sintering the powder constituents. This chemical approach can be applied to other superconducting systems: the Bi(Pb)-2223/Ag composites have been synthesized using a similar process. The results will be published later. In the case of high Ag fractions ( > 20 Ag vol.%) in YBCO composites, some modifications in the superconducting state have been observed because of the deliberately limited annealing duration. In this case, the homogeneously distributed Ag particles might play the role of barriers for oxygen diffusion, thus involving an oxygen deficiency in the YBCO lattice. Some controlled post-annealing could now be optimized, firstly to recover the standard superconducting properties, and secondly to hold the ductility brought about by the regular Ag distribution. One interesting purpose of this new approach was to obtain composites based on a superconducting phase and a soft metal, and having improved bulk mechanical properties. The first mechanical observations we obtained from recent Young modulus measurements, seem to confirm that the macroscopic ductility of the ceramics pellets should be improved in such samples. Mechanical measurements are now planned to systematically control the elastic behaviors of such "as-prepared" materials.

119

Acknowledgements We gratefully acknowledge Jannie Marfaing and C. Alfred-Duplan for the low-temperature resistance measurements in MATOP Laboratory (CNRS, University Aix-Marseille III-St-Jdrfme).

References [1] W. Gao and J.B. Vander Sande, Mater. Sci. Eng. B 10 (1990) 247. [2] A. Matsumuro, K. Kasumi and U. Mizutani, J. Mater. Sci. 26 (1991) 737. [3] Y. Ishida, J. Matsuzaki, T. Kizuka and H. Ichinose, Physica C 190 (1991) 67. [4] S.M. Cassidy, L.F. Cohen, M.N. Cuthbert, S.X. Dou and A.D. Caplin, Cryogenics 32 (1992) 11. [5] K. Tachikawa, T. Inoue, K. Zama and Y. Hikichi, Supercond. Sci. Technol. 5 (1992) 386. [6] K.H. Song, S.X. Dou and C.C. Sorrell, Physica C 185-189 (1991) 2387. [7] A. Ouammou, O. Pena, A. Benlhachemi, J.R. Gavarri and C. Carel, Ann. Chim. Fr. 19 (1994) 493. [8] A. Siwek, 1. Suliga, M.H. Pischedda, J.R. Gavarri and St. Jasienska, Solid State Ion. 80 (1995) 45. [9] C. Alfred-Duplan, J. Marfaing, G. Vacquier, A. Benlhachemi, J. Musso and J.R. Gavarri, Mat. Res. Ing. (1994), to be published. [10] A. Benlhachemi, S. Golec and J.R. Gavarri, Physica C 209 (1993) 353. [11] A. Benlhachemi, J. Musso, J.R. Gavarri. C. Alfred-Duplan and J. Marfaing, Physica C 230 (1994) 246. [12] X.D. Chen, S.Y. Lee, J.P. Golben, S.I. Lee, R.D. McMichael, Y.S. Tae, W. Noh and J.R. Gaines, Rev. Sci. Instr. 58 (1987) 1565. [13] M. Mansori, P. Satre, C. Breandon, M. Roubin and A. Sebaoun, Ann. Chim. Fr. 18 (1993) 537. [14] A. Benlhachemi, H. Tatarenko and J.R. Gavarri, Physica C 230 (1994) 246.