Structural properties

83 stopping). Effectively, these particles have measurable less energy than a particle which backscatters from the same element on the sample surface. The amount of energy a projectile loses through the sample depends on the projectile type, its velocity, the chemical composition and the density of the sample material. Dedicated application program have been studied to calculate and simulate different stopping power in order to interpret the very intricate data collected from multi-layer systems.

The orientation degree of the (200) direction relative to the others eventually present is calculated by means the ratio between the (200) intensity line and the sum of the latter and the other peaks taken into consideration. Every peak-line is normalized to a reference XRD powder spectrum of the compound analyzed. In the table above it is clear that the doping permits degree of orientation along (200) direction as high as in pure-Ceria compound.

The values of critical thickness showed in the last column might be underestimated since they are not theoretical calculation but values measured by alpha-step profilometer in our laboratories evidencing no cracks formation for the reported layer thickness. They highlight that critical thickness enhancement has been reached by CeO₂ doping.

As well-know in many research works presented in paragraph 4.5, also for our deposition tests, the reasons for the critical thickness behaviour as a function of type and amount of doping have been verified. Structural improvement plays a key role in these compounds, but mechanical properties are not less important. Indeed, using Yb³⁺ as dopant, the mismatch with the substrate decreases from 8,4% to 7,5% varying the doping proportion.

The same effect has been found in the compound doped with Zr⁴⁺. Although in this case it is limited to only one tenth of a percentage point and cannot justify the great increase in tc.

Surprisingly, in the case of Sm doping the opposite phenomenon occurs: the mismatch with Ni-W is slightly higher moving up to 8,6% but the highest critical thickness (more than 260 nm) is reached. We can attribute this effect to the increased fracture toughness which is therefore related to an increase in mechanical properties of the material, probably generated by the crystal lattice of defects provided by oxygen vacancies. These defects allow the growing layer to accumulate greater amounts of elastic energy, thanks to a higher

“flexible ability” given by a different distribution of bonding energies.

Returning to, It is important to emphasize that the change of the lattice parameter is consistent with the bibliographic data presented for all the three dopant. The experimental measures were made on the basis of the diffraction pattern obtained by X-ray Bragg-Brentano geometry, taking as reference peak (200) of ceria, as shown in fig. 4.13. The shift of this peak toward higher angles corresponds to a crystalline cell contraction and then to a smaller mismatch with the substrate for Yb³⁺ and Zr⁴⁺, while Sm³⁺ doping involves a reduction of the diffraction angle corresponding to an enlargement of the elementary cell

85 The calculation of the (200) peak shifts of doped-CeO2 relative to pure CeO2 has been measured fixing the (200) Ni-W peak as reference and normalizing the other peaks.

0 2000 4000 6000 8000 10000 12000 14000 16000

32.9 33.1 33.3 33.5 33.7 33.9 34.1 34.3 34.5

2θθθθ [deg]

Intensity [arb.un.]

6% Sm-doped ceria (180nm) Undoped ceria (90 nm) 15% Zr-doped ceria (200 nm) 45% Yb-doped ceria (210nm)

Fig. 4.13 – Peak shifts taken from θ/2θ XRD pattern of doped CeO2 relative to pure CeO2 compound

The most interesting results were obtained for 6at.%Sm³⁺, 45at.%Yb³⁺ and 15at.%Zr⁴⁺

doping which permit to grow samples characterized by very good values of bi-axial texturing and high thickness, maintaining a compact crack-free structure.

The spectra of X-ray diffraction in θ/2θ geometry of these tapes, shown in fig. 4.14, 4.15 and 4.16, confirmed that (h00) plans of the buffer layer are correctly parallel-aligned to the surface of the tape.

The analysis of these data indicates also that in most cases the addition of trivalent and tetravalent cations to CeO2 compound allows to keep the fluorite-type crystalline structure of Ceria and does not lead to the formation of unwanted secondary phases, which would prevent optimal epitaxial growth of YBCO superconducting layer. In only one case a secondary phase of ZrO2 has been noticed after the growth at low temperature but thanks

to a brief annealing treatment at 600°C in oxygen atmosphere it disappeared. This might be due to the solubility limit of the Zr⁴⁺ in CeO2 system and will be deepen in next studies.

Ce_0.94Sm_0.06O₂/Ni-W

0 2000 4000 6000 8000 10000 12000 14000

20 25 30 35 40 45 50 55 60 65 70

2θθθθ [deg]

Intensity [arb.un.]

(111) Ceria

(200) Ceria

(200) Ni-W

(200) Ni-W kβ (200)

Ceria kβ

(400) Ceria

Fig. 4.14 – θ/2θ XRD pattern of Sm-doped CeO2 (Ce0,94Sm0,06O2)

Ce_0.55Yb_0.45O₂/Ni-W

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000

20 25 30 35 40 45 50 55 60 65 70

2θθθ [deg]θ

Intensity [arb.un.]

(111) Ceria

(200) Ceria

(200) Ni-W

(200) Ni-W kβ (200)

Ceria kβ

(400) Ceria

Fig. 4.15 – θ/2θ XRD pattern of Yb-doped CeO2 (Ce0,55Yb0,45O2)

87 Ce_0.85Zr_0.15O₂/Ni-W

0 3000 6000 9000 12000 15000

20 25 30 35 40 45 50 55 60 65 70

2θθθθ [deg]

Intensity [arb.un.]

(200) Ceria

(200) Ni-W

(200) Ni-W kβ (200)

Ceria kβ

(400) Ceria

Fig. 4.16 – θ/2θ XRD pattern of Zr-doped CeO2 (Ce0,85Zr0,15O2)

The values on the orientation in-plane and out-of-plane have been derived from rocking curves and pole figures (fig. 4.17, 4.18, 4.19) that show a high degree of bi-axial texturing, expressed by the ∆ω and ∆φ average below 5° and 6,5°, with a minimum at 2,75° and 4°

respectively for Yb³⁺ and Zr⁴⁺ doped samples. These features are particularly significant in order to guarantee the optimal texturing properties that the buffer layer have to transmit to the next layer of YBCO. Indeed, only in this case superconducting layer can reach the ideal shape to achieve high critical current values.

0 2000 4000 6000 8000 10000 12000 14000 16000

0 10 20 30

ωωω ω [deg]

Intensity [arb.un.]

Fig. 4.17 – Rocking curve (on the left) with a ∆ω = 6° and pole figure (on the right) with ∆ϕ = 6° of a Ce0,94Sm0,06O2 grown on Ni-W RABiTS^TM tape

∆ω

∆ω ∆ω

∆ω ^{= 6°} ∆ϕ∆ϕ∆ϕ∆ϕ _{= 6°}

0 5000 10000 15000 20000 25000 30000

0 10 20 30

ω ωω ω [deg]

Intensity [arb.un.]

Fig. 4.18 – Rocking curve (on the left) with a Dw = 2,75° and pole figure (on the right) with Dj = 5° of a Ce0,55Yb0,45O2 grown on Ni-W RABiTS^TM tape

0 2000 4000 6000 8000 10000 12000 14000 16000

0 10 20 30

ω ωω ω [deg]

Intensity [arb.un.]

Fig. 4.17 – Rocking curve (on the left) with a ∆ω = 4° and pole figure (on the right) with ∆ϕ = 8° of a Ce0,85Zr0,15O2 grown on Ni-W RABiTS^TM tape