PHASE TRANSFORMATIONS OF SUPERSATURED SPUTTERED Nb Co FILMS

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PHASE TRANSFORMATIONS OF SUPERSATURED SPUTTERED Nb Co FILMS

W. Biegel, W. Schaper, H.-U. Krebs, J. Hoffmann, H. Freyhardt, R. Busch, R.

Bormann

To cite this version:

W. Biegel, W. Schaper, H.-U. Krebs, J. Hoffmann, H. Freyhardt, et al.. PHASE TRANSFORMA-

TIONS OF SUPERSATURED SPUTTERED Nb Co FILMS. Journal de Physique Colloques, 1990,

51 (C4), pp.C4-189-C4-195. �10.1051/jphyscol:1990423�. �jpa-00230783�

PHASE TRANSFORMATIONS OF SUPERSATURED SPUTTERED Nb CO FILMS

W. BIEGEL, W. SCHAPER, H.-U. KREBS, J. HOFFMANN, H.C. FREYHARDT, R. BUSCH and R. BORMANN*

Institut fiir Metallphysik, Universitdt Gbttingen, Hospitalstrasse 3 / 5 , p-3400 Gbttingen, F.R.G.

Forschungszentrum Geesthacht GmbH (GKSS), Max-Planck-Strasse, 0-2054 Geesthacht, F.R.G.

Abstract

-

Supersaturated NbCo thin films were prepared in the concentration range 4 -15 at.% CO by triode co-sputtering. Based on thermodynamic calculations (CALPHAD) for the Nb-CO system, we expect during heating a transformation of the supersaturated bcc NbCo phase into a metastable two-phase system of bcc NbCo and an amorphous phase, and for temperatures above 750°C, the crystallization into the equilibrium phases. Structure and structural transformation of the annealed supersaturated NbCo phase were studied by means of X-ray diffraction and transmission electron microscopy. The kinetics of the phase formation could be investigated by in-situ X-ray diffraction experiments at constant temperature of 600 "C. The shift of the NbCo-(110) peak position obeys a s e law, indicating a linear decrease of the CO concentration in the NbCo phase with

fi.

The super- conducting transition temperature and the critical current of the films were used as a probe t o detect precipitations and t o determine the concentration of the NbCo phase.

1 - INTRODUCTION

In recent years the amorphization of binary alloys by solid state reaction was successfully carried out by several methods: reaction of multilayers of the two components, mechanical alloying from crystalline elemental powders or, in some cases, by milling of an appropriate intermetallic compound.

Another way to produce an amorphous phase is via the transformation of a metastable, super- saturated homogenius AB crystal into a metastable equilibrium of a crystalline A phase and an amorphous AB phase. In this case, but also for the processes mentioned above, it is necessary that the formation of the crystalline equilibrium phases of the AB system are kinetically supressed /l/.

In this contribution we describe the transformation of co-sputtered NbCo films and discuss the results t o the thermodynamics of this system calculated by the CALPHAD method.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990423

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2

-

EXPERIMENTAL METHODS

Nbloo-, CO, thin films ( 4 L X L 15 ) with a thickness of about 1 ym were deposited on A1203 or KBr single crystals by triode sputtering in a vacuum system with a base pressure of about 10-a mbar. The concentration of the films was established by EDX analysis. After dissolving the KBr substrates, some films were heated at 600 OC for 30

k

and investigated by transmission electron microscopy (TEM). For some samples, in-situ X-ray experiments with Cu-Ka radiation were per- formed on a hot stage of a diffractometer under a vacuum of better than 10-' mbar using a constant heating rate of 1.5 K/min. X-ray scans were recorded every 6 minutes. In addition, iso- thermal X-ray measurements at a temperature of 600 OC were used t o study the kinetics of the transformation. Measurements of the superconducting transition temperature T, and the critical current I, of the Nb bcc phase sewed as an additional method t o characterize the films before and after heat treatments. For the NbCo system, the Gibbs free energies of the metastable and stable phases and their respective thermodynamic equilibria were determined by the CALPHAD method (calculation of phase diagrams) /2/. Thereby the thermodynamic functions of the meta- stable and stable phases are calculated by using the experimental thermodynamic data of the alloy system. The amorphous phase is treated as an undercooled liquid at the glass transition. The superconducting transition temperatures, T,, were determined resistively and the critical current, I,, was measured as a function of the magnetic field by a standard four probe method using a 1 y~cm-' voltage criterion.

3

-

RESULTS AND DISCUSSION

All of the as deposited NbCo films are bcc. CO is found t o dissolve in the bcc phase up t o maximum content of 15 at.%, whereas the equilibrium solubility of CO in bcc Nb amounts t o about 2 at.% at temperatures of about 800 OC /3/. The films are homogeneous but show some texturing, which is also known for sputtered NbFe films /4/. Thermodynamic calculations, i. e. CALPHAD calculations, based on experimental data, are used t o discuss phase relations and transformations.

As an example, Fig.1 shows the Gibbs free energies for various phases in the NbCo system at 580 OC.

On the Nb-rich side of the system the following three phases are important: the bcc Nb phase, the intermetallic NbCo compound and the amorphous phase. The arrow in the diagram at 14 at.% CO indicates the particular composition for which details of the transformation sequence will be reported. This supersaturated bcc phase can undergo a transformation during heating into a meta- stable equilibrium of a bcc phase with 4 at.% CO and an amorphous phase, provided that the formation of the intermetallic NbCo compound can be avoided.

The lattice constant of the supersaturated bcc Nb phase was found t o decrease linear with increasing CO content, according t o /S/. Therefore we suppose that the CO atoms (rGoldschmidt=l.2S

H)

^sub-

stitute the Nb atoms (rG=1.43

H)

in this structure. To characterize the transformation the strong (110) peak of the bcc Nb phase was observed by a high resolution X-ray diffraction technique. Fig.2 shows the shift of the (110) peak position with temperature for a Nb film with 13 at.% Co. A heating rate of 1.5 K/min was chosen For temperatures up t o 550 OC, the linear peak shift is ascribed t o the thermal expansion of the bcc lattice. The reason for the deviation from this linear behavior at about 350 OC is not clear up t o now. However, it should not be an experimental artefact. Upon increasing temperature from SS0 up t o 680 OC the peak position significantly decreases. This is the result of an increase of the lattice constant, which could only be caused by a decrease of the CO content in the Nb phase. Therefore, this peak shift directly indicates the formation of a CO-rich phase. Obviously at a temperature of 600 OC a sufficiently high mobility of the CO atoms (which are known t o be a fast diffusor in NbCo /6/) allows the formation of a new phase.

Fig.1 : The free energy of t h e metastable and equilibrium phases a t S80 OC. The free energies of bcc Nb and fcc CO are chosen as energy reference points. The transformation of supersaturated NbCo into the metastable equilibrium between NbCo and the amorphous phase is marked.

381) ^{I I} I

100 2M3 300 LOO 500 600 700 800 900

T/ 'C

Fig.2 : Change of t h e Nb (110) peak position with temperature during annealing of a Nbs7CoI3 film a t a heating rate of 1.5 K/rnin.

The X-ray characterizations of t w o films with similar compositions after different heat treatments (600 OC for 30 h and 850 OC) reveal different s t a t e s of t h e films (Fig.3). The film treated a t 850 OC (Fig.3b) shows a strong Nb (110) peak with its KP satellite (due t o the filtered Cu-Kcc radiation we used) and t h e (110) peak of t h e intermetallic NbCo compound a t 2 0 = 36.20. On t h e other hand t h e film which was heated for 30 h a t 600 OC (Fig.3a) does not exhibit any other crystalline peaks except Nb (110) nd its KB satellite. Both films show a significant shift of the Nb (110) peak, indicating the formation of CO-rich phases. Obviously. during the 600 OC heat treatment an amorphous phase is formed rather than the NbCo intermetallic compound, but its volume fraction is t o o small t o be detected by X-ray diffraction.

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l . . . . . . . . - l

2 8 38 4 8

(b) 2 81 degree

Fig.3 : X-ray diffraction patterns (Cu Ka radiaion) of a Nb86C~14 film after a heat treatment a t 600 O C f o r 30 h (a) and a NbS7CoI3 film after heating a t 850 O C (b).

0 2 L 6 8 10 12 1L 16

at.% CO

Fig.4 : The superconducting transition temperature of NbCo films with different compositions. The transition width AT was typical 0.3 K. The T, of t h e NbS6CoI4 film after a heat treatment a t 600 OC for 30 h is marked.

The superconducting transition temperature, T,, is a very sensitive probe f o r the CO content in the bcc Nb phase. This can be seen from Fig.4, where t h e T, of some a s prepared films is plotted versus t h e CO content.

T,

decreases linearly with increasing CO content. An extrapolation t o X = 0 at.% CO yields t h e bulk value of 9.2 K of pure Nb. For t h e film heated a t 600 OC a T, of 6.1 K was obtained, which corresponds t o a CO concentration of about S at.% in t h e bcc phase. This is close to t h e calculated content of t h e bcc phase in a metastable equilibrium with the amorphous phase (Fig.1).

In order t o get some information about the kinetics of this transformation isothermal X-ray investigations a t 600 OC were performed. The Nb (110) peak position of a Nb86C~14 film is plotted versus

G

in Fig.5. The peak position exhibits a good linear square-root-time behavior over a time scale of 45 h, indicating a decrease of the CO content in the NbCo grains during the formation of the amorphous phase with fi. From this i t can be deduced t h a t the volume fraction of the amorphous phase grows with

6 .

In t h e light of the described results the question about the microstructure of these films arose.

The superconducting properties of t h e bcc Nb phase were used t o get some insight. Flux can interact with microstructural features when the latter induce variations in t h e superconducting properties relative t o those of t h e bulk. The strength and nature of t h e interaction are dependent upon the way in which t h e microstructural features differ from t h e matrix, a s well a s upon their size, shape and distribution. A decrease in normal electron mean free path could lead t o a local increase in the Ginzburg-Landau-Parameter, X, and this change will result in a pinning interaction /7/. On t h e basis of the change in free energy of t h e flux-line lattice as X changes t o x + A x within a sharp boundary, the expression for the pinning force per unit volume is given by

where Bc2 is t h e upper critical field and S, t h e pinning area of t h e boundary per unit volume.

Nb,, C06

as deposited 1. ^{heatedat 600eC}

Fig.6 : Volume pinning force FP and overall critical current density J, with t h e magnetic field a t 1.7 K for an as deposited Nbg4C06 film (a) and t h e Nba6Co14 film after a heat treatment a t 600 OC for 30 h (b).

C4-194 COLLOQUE DE PHYSIQUE

Fig.6a shows t h e volume pinning force. FP, and t h e overall critical current density, J,, of a n a s prepared film with 6.4 at.% CO depending o n t h e applied magnetic field B. A comparison with t h e heated and already decomposed film with amorphous precipitates dispersed in a bcc phase with about 5 at.% CO (Fig.6b) yields similar shapes and values f o r Fp(B). The small differences in the Fp(max) and BC2 can be due t o t h e different CO contents of t h e bcc phases. From this observation it can be concluded t h a t t h e amorphous phase of t h e heat-treated film does not cause pinning. If t h e pinning sites are grain boundaries than S, * D - ~ . where D is the grain diameter. A pinning function o f t h e form B(Bc2-B1 is often found f o r bcc Nb alloys. For reasonable values of %*0.016 and x%30, we estimated an average grain diameter of 1000

W.

The TEM investigations of t h e films we carried o u t t o check this statement: The bright field micro- graph of t h e Nbs6Co14 film heated for 30 h a t 600 OC (Fig.7) shows t h e newly formed amorphous precipitates. The selected area diffraction pattern does not exhibit any other crystalline peaks except those of t h e bcc Nb phase. The distribution of t h e amorphous phase is roughly consistent with a distance of 1000

8.

In a high resolution TEM picture of this film (Fig.8). t h e visible lattice fringes of one Nb grain end in t h e grain boundary t o the neighboring grain. In this grain boundary an amorphous precipitation can be seen. It is known f o r other systems, e.g. ZrNi /8/ t h a t grain boundaries in t h e crystal of the immobile atom species are necessary f o r t h e amorphous phase formation a s nucleation sites. We found t h e same behavior f o r the NbCo system: t h e precipitates were formed preferentially in grain boundaries and d o therefore not influence t h e pinning.

Fig.7 : Transmission electron micrograph of t h e Nbe6CoI4 film after a heat treat- ment a t 600 OC f o r 30 h.

4 - CONCLUSIONS

In t h e NbCo system, t h e amorphous phase formation from supersaturated bcc Nb, predicted by the

CALPHAD calculations, was monitored and characterized by means of X-ray diffraction, supercon- ducting properties and TEM. We found a transformation of a Nb 14 at.% CO film during a heat treatment a t 600 OC f o r 30 h into a bcc Nb phase with S at.% CO and an amorphous phase. This phase formation obeys a K e law and occurs preferentially a t grain boundaries.

ACKNOWLEDGEMENT

This work was supported by t h e Sonderforschungsbereich 345 Gottingen and t h e Deutsche Forschungs- gemeinschaft. We especially would like t o thank Christine Borchers very much f o r her help during t h e HREM investigations.

REFERENCES:

/l/ W.L. Johnson, Progr. Mat. Science 30 (19861, 81

/2/ R. Bormann and R. Busch, Proc. 7th Int. Conf. on Liquid and Amorphous Metals, Kyoto, Japan, J. Non-Cryst. Solids (1990)

/3/ J.K. Pargeter and W. Hume-Rothery, J. Less-Common Met. 12 (19671, 366

/4/ H.-U. Krebs, W. Biegel, R. Bormann and R. Busch, Proc. 7th Int. Conf. on Liquid and Amorphous Metals, Kyoto, Japan, J. Non-Cryst. Solids (1990)

/S/ H. Schliiter, H.C. Freyhardt, H.-U. Krebs and R. Bormann, Zt. Phys. Chemie 157 (19881, 407 /6/ F. Wenwer, N.A. Stolwijk and H. Mehrer, Z. Metallkde. 8 0 (19891, 204

/7/ D. Dew-Hughes, Phil. Mag. B55 (19861, 459

/8/ W.J. Meng, C.W. Nieh, E. Ma, B. Fultz and W.L. Johnson, Mat. Sc. Eng. 97 (19881, 87