Ivan Davoli

The ReflEXAFS station at the GILDA beamline (BM08) of ESRF

The experimental station for measuring X-ray absorption spectra in total reflection geometry operative at the GILDA CRG beamline of ESRF is described. The main features of the station are shown, namely: the possibility of detecting very small signals from thin (a few ML) samples, of depositing thin films under controlled conditions and thermal treating the samples in order to study dynamical processes. Case studies are reported in order to show the performances of the apparatus.

Indentation modulus and hardness of viscoelastic thin films by atomic force microscopy: A case study

We propose a nanoindentation technique based on atomic force microscopy (AFM) that allows one to deduce both indentation modulus and hardness of viscoelastic materials from the force versus penetration depth dependence, obtained by recording the AFM cantilever deflection as a function of the sample vertical displacement when the tip is pressed against (loading phase) and then removed from (unloading phase) the surface of the sample. Reliable quantitative measurements of both indentation modulus and hardness of the investigated sample are obtained by calibrating the technique through a set of different polymeric samples, used as reference materials, whose mechanical properties have been previously determined by standard indentation tests. By analyzing the dependence of the cantilever deflection versus time, the proposed technique allows one to evaluate and correct the effect of viscoelastic properties of the investigated materials, by adapting a post-experiment data processing procedure well-established for standard depth sensing indentation tests. The technique is described in the case of the measurement of indentation modulus and hardness of a thin film of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate), deposited by chronoamperometry on an indium tin oxide (ITO) substrate.

Quantitative measurement of indentation hardness and modulus of compliant materials by atomic force microscopy

An atomic force microscopy (AFM) based technique is proposed for the characterization of both indentation modulus and hardness of compliant materials. A standard AFM tip is used as an indenter to record force versus indentation curves analogous to those obtained in standard indentation tests. In order to overcome the lack of information about the apex geometry, the proposed technique requires calibration using a set of reference samples whose mechanical properties have been previously characterized by means of an independent technique, such as standard indentation. Due to the selected reference samples, the technique has been demonstrated to allow reliable measurements of indentation modulus and hardness in the range of 0.3-4.0 GPa and 15-250 MPa, respectively.

Electrical and Mechanical Characterization of Coated Conductors Lap Joints

Electrical and mechanical characterizations of YBCO coated conductors diffusion joints obtained in a 2 mm wide, Cu stabilizer free commercial tape provided by Superpower Inc. were reported. Two conductor pieces were overlapped for a length of 20 mm and kept in contact under a pressure as high as 80 MPa. A face-to-face approach was used, with or without an additional Ag tape insert. The joint was realized by Ag diffusion at a temperature of 200 and 400°C in N2 and O2 atmosphere, respectively. The resistivity of the joint was measured at 77 K in N2 bath. The mechanical properties of the joint were characterized by means of single-lap tensile test operated at room temperature at a rate of 0.5 mm/min. The best results were obtained by high temperature joining in O2 atmosphere. The resistivity of the joints was as low as 3.1 × 10-12 ¿ m2. This value is almost halved if an Ag insert is used. The mechanical properties of the joints are strongly dependent on the use of an Ag insert, since maximum load as high as 60 N can be applied if no additional insert is used. This value enhances to about 100 N when an Ag insert is employed. However, the joint obtained at low temperature in N2 atmosphere is also interesting since the joint resistivity was slightly increased to 4.7 × 10-12 ¿ m2 and a maximum load as high as 50 N was obtained indicating that diffusion joints can be safely handled. These results are promising in perspective of oxygen free diffusion joints in Cu stabilized coated conductors.

Structural and chemical evolution of propionate based metal–organic precursors for superconducting YBa2Cu3O7 − δ epitaxial film growth

The structural and chemical evolution of propionate based low fluorine YBa2Cu3O7 − δ (YBCO) precursor during the conversion thermal treatment to obtain superconducting film has been investigated by both x-ray photoelectron and diffraction techniques in a set of partially converted films on SrTiO3 single crystals. The pyrolysis temperature within the range 400–480 °C mainly affects the copper valence state with an increase of the Cu2 + fraction with temperature with respect to the Cu1 + oxidation state. During the subsequent thermal treatment up to 700 °C, the reduction of fluorine content is mainly ascribed to the hydrolysis of YF3. At higher temperatures, Ba hydrolysis, Y2Cu2O5 and YBCO phase formation (nucleation at 700 °C and 725 °C, respectively) have been observed. The temperature dependences of the formation and decomposition of YBCO, Y2Cu2O5 and Ba-oxyfluoride were evaluated by x-ray diffraction measurements. The reaction path emerging from these analyses agrees with the one observed for YBCO films obtained with the standard MOD method based on metal tri-fluoroacetate precursors.

Discoloration of the smalt pigment: experimental studies and ab initio calculations

Smalt is a blue pigment used by many European artists in mural and easel paintings, mainly in the period from the XV to XVIII century. It is a potassium glass where cobalt is added to the glassy matrix to get the blue hue. The pigment deteriorates with age, changing its colour from an intense blue to a grey-yellowish hue, causing severe problems in the conservation of the paintings. In this study a set of specimens of smalt dispersed in linseed oil was prepared and artificially aged to simulate the progressive deterioration of the pigment in a painting on canvas. The artificially aged smalt specimens were compared with some samples of naturally aged smalt taken from a banner painted at the end of XV century by Luca Signorelli, the “Baptism of Jesus”. A multi-technique approach, including SEM-EDX, spectro-colorimetry, X-ray absorption spectroscopy and ab initio calculations, was used to understand the progressive discoloration and to reveal its correlation with changes occurring in the pigment structure.

MOD Derived Pyrochlore Films as Buffer Layer for All-Chemical YBCO Coated Conductors

We report a detailed study performed on La2Zr2O7 pyrochlore material grown by the metal-organic decomposition method as buffer layers for YBa2Cu3O7-x (YBCO) coated conductors. High-quality epitaxial LZO thin films have been obtained on single crystal (SC) and Ni-5 %at.W substrates. In order to evaluate structural and morphological properties, films have been characterized by means of X-ray diffraction analyses, atomic force microscopy, and scanning electron microscopy. Precursor solutions and heat treatments have been studied by thermogravimetric analyses and infrared spectra with the aim of optimizing the annealing process. Thin films of YBCO have been deposited by pulsed laser ablation on this buffer layer. The best results obtained on SC showed YBCO films with critical temperature values above 90 K, high self-field critical current density values (Jc >; 1 MA/cm2) and high irreversibility field values (8.3 T) at 77 K together with a rather high depinning frequency νp (0.5 T, 77 K) >; 44 GHz as determined at microwaves. The best results on Ni-5% at.W has been obtained introducing in the heat treatment a pyrolysis process at low temperature in air in order to remove the residual organic part of the precursor solution.

Deposition and Characterization of Metal Propionate Derived Epitaxial

A new modified TFA-MOD method for the YBa2Cu3O7-x (YBCO) deposition, using only barium trifluoroacetate, is presented. The yttrium and copper trifluoroacetates were replaced by the alcoholic solutions of Cu and Y acetates dispersed in propionic acid to prepare a stable coating solution. Due to the smaller amount of evolved hydrofluoric acid, this method permits the shortening of the pyrolysis time by a factor 4, with respect to conventional trifluoroacetate metal organic deposition (TFA-MOD) method. The X-ray photoelectron spectroscopy (XPS) analyses, performed on a pyrolyzed sample, showed the complete decomposition of precursors. The as obtained films exhibit good morphological, structural and superconducting properties with T c(R = 0) greater than 90 K. Transport measurements at different temperatures and applied magnetic fields have revealed promising superconducting properties of the critical current density (J c) vs. the applied magnetic fields and the value of J c at 77 K and zero magnetic field greater than 3 MA/cm2.

{\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7-{\rm x}}

We studied the growth and oxidation of niobium nitride (NbN) films that we used to fabricate superconductive tunnel junctions. The thin films were deposited by dc reactive magnetron sputtering using a mixture of argon and nitrogen. The process parameters were optimized by monitoring the plasma with an optical spectroscopy technique. This technique allowed us to obtain NbN as well as good quality AlN films and both were used to obtain NbN/AlN/NbN trilayers. Lift-off lithography and selective anodization of the NbN films were used, respectively, to define the main trilayer geometry and/or to separate electrically, different areas of the trilayers. The anodized films were characterized by using Auger spectroscopy to analyze compounds formed on the surface and by means of a nano-indenter in order to investigate its mechanical and adhesion properties. The transport properties of NbN/AlN/NbN Josephson junctions obtained as a result of the above described fabrication process were measured in liquid helium at 4.2 K.

Films for Coated Conductor Fabrication

The interface between the cube textured Ni-W substrate, both bare and Pd-buffered, and the CeO2/YSZ buffer layer structure-required to growth YBCO coated conductors-samples have been subjected to different oxidizing conditions and have been investigated with different techniques: X-ray diffraction (XRD), X-ray absorption and Auger electron spectroscopy. Due to the Ni-Pd interdiffusion, the presence of a Pd over layer significantly modifies the substrate surface composition and the substrate oxidation mechanism. Depending on the Pd layer thickness, the formation of NiO can be completely prevented while the formation of ternary oxides and of W oxide is favored. Moreover, the Pd layer affects the extension of interface between the substrate and the CeO2 layer and the portion of uncontaminated CeO2.