Prime pagine RA2010FUS:Copia di Layout 1 - ENEA - Fusione

094
progress report
2010
Intensity (Arb. units)
1200
800
400
As grown
Annealed
42 43
(200) MgO
0
35 45 55
2θ (deg)
Figure 4.8 – X–ray diffraction θ–2θ scan of MgO
film grown on Pd–buffered Ni–Cu–W. The
spectra refer to the as–grown sample (full
dots) and after annealing at 800 °C in vacuum
(empty dots). In the inset, a detail of (200)MgO
peaks. The thickness of MgO, Pd and Ni–Cu–W
is 120 nm, 10 nm and 50 mm, respectively
Realtive–peak–to–peak
intensity (Arb. units)
2×10 3
a)
(200) Pd
b)
(200) Ni–Cu–W
Zr
Y
O
Ce
Ni
Pd
0 400 800
oriented (fig. 4.8). The MgO layer is adequate for the
deposition of a cap layer before YBCO deposition as tested by
the heat treatment at 800 °C in vacuum.
Oxidation behaviour of the Ni–W and CeO 2
interface: role of
Pd inter–layer
Considering that oxidation at the substrate interface could
influence the epitaxial growth and the mechanical stability of
the whole coating architecture, the role of the Pd interlayer at
the interface between NiW and CeO 2
/YSZ/CeO 2
buffer layer
structure has been studied by x–ray techniques and electron
Auger spectroscopy. Extended x–ray absorption fine structure
(EXAFS) analyses reveal that the inter–diffusion process
between the Pd layer and the substrate modifies the substrate
interface composition due to the formation of an ordered
Ni–Pd alloy even at temperatures as low as 600°C. At high
temperatures, the oxidation mechanism is dependent on the Pd
layer thickness, and competition between the NiO and the
NiWO 4
formation is observed. Oxidation also affects the CeO 2
interface with the substrate. Auger electron spectroscopic (AES)
analyses, reported in figure 4.9, reveal that the interface region
is more extended than that of samples in vacuum annealed,
and that outward migration of Ni and Pd in the CeO 2
layer
occurs. The CeO 2
layer contamination results to be reduced as
the Pd layer increases. The lower CeO 2
contamination and the
lower NiO formation could be associated to the good adhesion
obtained in coated conductor samples with a Pd interlayer.
4×10 3 400 800 1200 1600
Realtive–peak–to–peak
intensity (Arb. units)
0
0
8×10 4
4×10 4
Sputtering time (s)
c)
Zr
Y
O
Ce
Ni
Pd
0
0 400 800 1200 1600
Sputtering time (s)
Figure 4.9 – AES depth profiles for CeO 2 /YSZ
samples deposited on Ni–W substrate buffered
with a 50 nm Pd over layer annealed at 800 °C
a) in vacuum and b) in 10 mTorr of oxygen back
ground pressure. c) AES depth profiles for
CeO 2 /YSZ samples deposited on Ni–W
substrate buffered with a 200 nm Pd over layer
annealed at 800 °C in 10 mTorr of oxygen back
ground pressure. W signal is multiplied for 50
Low fluorine YBCO MOD
In this low fluorine method, only the Ba precursor is introduced
in the coating solution as a trifluoroacetate, while other
precursors are (Cu and Y)–acetate treated with an excess of
propionic acid. The reaction path for YBCO formation was
investigated by x–ray photoelectron spectroscopy (XPS) and
diffraction (XRD). From these analyses it resulted that the
YBCO formation occurs through a rather complex mechanism
involving hydrolysis of both Y and Ba fluorides and the
reaction with CuO. However, this path is probably hindered by
a competing reaction taking place at the same temperature
range as YBCO formation (around 700–800°C). In figure 4.10,
the evolution of the reaction path can be derived through
x–ray diffraction θ–2θ patterns. Within the 700–795°C
temperature range the YBCO phase coexists with both
Y 2
Cu 2
O and BaF 2
phases. At 795°C, the XRD spectrum
shows the presence of sharp and intense (00l) reflection of
YBCO together with other distinct peaks ascribable to the
presence of residual Y 2
Cu 2
O 5
phase, while BaF 2
phase
disappears.
Transport properties improvement in low fluorine YBCO MOD with artificial pinning sites
The possibility of enhancing the pinning efficiency by means of artificial pinning sites created by addition of
BaZrO 3
(BZO) has been investigated in YBa 2
Cu 3
O 7–x
(YBCO) films grown by metallorganic decomposition
(MOD) methods. YBCO and BZO coating solutions were prepared and subsequently mixed in molar ratios
corresponding to 5, 7.5, 10, 15 mol.% BZO. A marked increase in the J c
(0) value has been measured in MOD