Nucleation and Growth of the First Phase in Sputter-Deposited Nb/Al Multilayer Thin Films
- PDF / 2,013,063 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 29 Downloads / 201 Views
Mat. Res. Soc. Symp. Proc. Vol. 398 ©19 9 6 Materials Research Society
experiments in a Siemens D5000 x-ray diffractometer. Films on sapphire, that were annealed into the first stage of NbAl 3 formation in the heat-flux DSC furnace, were made into electron transparent cross-sections and the microstructure of the reactant and product phases was studied in a Philips EM400T transmission electron microscope (TEM). The phases in these samples were identified by electron diffraction. RESULTS Figure 1 shows the enthalpy release rate versus temperature for experiments carried out at a constant heating rate of 20 K/min in the heat-flux DSC. Three peaks are present in the traces, marked as A, B and C. X-ray and electron diffraction confirm that both peaks A and B are associated with the formation of the same product phase, NbAl3. Peak C arises from the formation of the A15 Nb 3+xA1 and the sigma Nb 2,YA] phases [1]. Power-compensated DSC plots are presented in Fig. 2 for comparison and also clearly show two exothermic peaks for the formation of NbA13. Peak C is not accessible in these traces, because the maximum operating temperature of this instrument is limited to 1000 K. The height ratio of peak A to peak B shows a systematic increase with decreasing periodicity until at a periodicity of 10 nm only peak A is observed. Using the Kissinger [3] (or modified Kissinger [4]) method, from the shift of the peak temperatures with heating rate, the activation energies for peaks A and B have been determined to be 1.7±0.2 eV and 2.5±0.6 eV, respectively. These values are in good agreement with the values of 1.7±0.3 eV and 2.5±0.1 eV obtained from the power-compensated DSC scans. The kinetics of transformation associated with peak A was studied isothermally for a film with A = 23 nm. Figure 3 shows the exothermic calorimetry trace for this film at 590 K. The trace is "bell shaped" and, therefore characteristic of a nucleation and growth process. Following the Johnson-Mehl-Avrami (JMA) theory for transformations occurring by nucleation and growth [5], the isothermal trace of Fig. 3 was fitted to a weighted sum of functions of the form rate of heat flow (t) = AH. n- k. t'-. exp(-kt')
(1)
where AH is the total enthalpy change for the transformation, k = k(T) is the rate constant, and n is the JMA exponent. The best fit to the data of Fig. 3 was found for a weighted sum of two equations of the form of eq. (1), with exponents 2.1 and 1.2 and two different values of k. w ,
'I
'
I
'
1
,
I'
'
I7 '
-r' -
I
I
B C
AA 143-69 nm
sw
143-45 nm 143-34 nm 72-17 nm
200
400
600
800
1000
1200
1400
Temperature (K) Fig. 1 - Heat-flux differential scanning calorimetry traces for sputter-deposited Nb/Al multilayers annealed at 20 K/min. The numbers, in nanometers, associated with each curve denote the periodicity of the film and the thickness of the Al layer, respectively.
258
A = 143 nm
72 nm
.
54.5 nm
0
23 nm
10 nm
300
500
700
900
Temperature (K) Fig. 2 - Power-compensated differential scanning calorimetry traces fo
Data Loading...
