Papers by Lorenzo Malavasi
Solid oxide fuel cell (SOFC) technology offers substantial advantages for clean and efficient pow... more Solid oxide fuel cell (SOFC) technology offers substantial advantages for clean and efficient power generation. However, the high working temperatures impose severe restrictions on the materials selection. The key to improve the stack lifetime, widen the selection of materials and reduce the costs lies in the lowering of working temperatures to 600÷800°C. Nevertheless, the decreasing of operating temperatures causes the reduction of electrolyte ionic conductivity, resulting in a rapid decay of cells performance. To overcome this problem, two main approaches have been applied so far: i) looking for materials of higher ionic conductivity at lower temperatures or ii) decreasing the electrolyte thickness. Doped-ceria materials, such as Gd doped CeO 2 (GDC), have been considered to be promising candidates as electrolyte operating in the 500÷700°C range. Considerable efforts have been recently directed toward intermediate temperatures SOFCs (IT-SOFCs) based on thin-film electrolytes of doped-ceria, in order to minimize the internal ohmic loss and possibly reduce the time and energy for start-up. At present, different methods have been studied for the deposition of fully dense thin coatings. Among them, physical vapor deposition methods resulted to be suitable to deposit high quality, ultra-clean thin films for a wide variety of applications. In particular, the radio frequency (RF) magnetron sputtering technique is a powerful and versatile tool to deposit, on various substrates, metals as well as mixed oxide thin films in a wide thickness range (100 nm ÷ 10 µm). The sputtering method has been widely used in the SOFCs technology for YSZ or GDC thin film depositions. In this report, the results about the deposition by RF magnetron sputtering of 1÷3 µm GDC films on the anode substrates will be presented. The GDC targets were fabricated using commercial or in-house prepared powders. The investigation has been focused on the study of the correlation between the process parameters (deposition rate, target÷substrate distance, sputtering atmosphere composition, etc.) and the transport properties of the Ce 0.8 Gd 0.2 O 2-y solid solutions. The differences between conductivity characteristics of sintered bulk materials and films prepared by means of RF-magnetron sputtering were carefully analyzed. Dense and smooth GDC coatings were initially deposited on quartz substrates. The as-prepared samples were partially crystalline with cubic fluorite structure. However, a prolonged heating of coatings in O 2 atmosphere at 650÷850°C considerably increased the crystallinity of the films. The as-prepared films exhibited a noteworthy conductivity at low temperatures, higher than the one of annealed coatings and sintered bulk samples. Nevertheless, poor stability at temperatures higher than 400°C was observed, probably related to a progressive crystallization process. Higher stability was achieved thermally treating the coatings, despite a slight loss in conductivity. Some tests conducted in reactive O 2 atmosphere (P O 2 ≈5x10 -3 mbar) showed, despite the decrease of sputtering rate, a remarkable increase on the crystallization rate of the film; in addition, more homogeneous morphologies and stable conductivities have been obtained compared to the ones of sintered bulk samples. After optimization of the process on quartz substrates, the depositions were made on 5x5 cm tape-casted NiO-YSZ substrates. In this case, nanostructured, smooth and dense, around 3 µm thick, GDC coatings were obtained in Ar plasma with no need of thermal treatments. Electrical characterizations of the coatings showed conductivities comparable to the ones of annealed GDC films deposited on quartz. Preliminary tests performed on NiO-GDC and reduced Ni-YSZ anodes confirmed the possibility to obtain dense and crystalline films by RF sputtering. The electrochemical properties of these coatings are under investigation.
Journal of Physics: Condensed Matter, 2012
Recently, Mitsuhashi et al observed superconductivity with a transition temperature up to 18 K in... more Recently, Mitsuhashi et al observed superconductivity with a transition temperature up to 18 K in potassium doped picene (C 22 H 14 ), a polycyclic aromatic hydrocarbon compound (Mitsuhashi et al 2010 Nature 464 76). Theoretical analysis indicates the importance of electron-phonon coupling in the superconducting mechanisms of these systems, with different emphasis on inter-and intra-molecular vibrations, depending on the approximations used.
Chemistry of Materials, 2006
J. Mater. Chem. A, 2014
Lithium metal orthosilicates with general formula Li 2 MSiO 4 (M ¼ Mn, Fe and Co) are among the m... more Lithium metal orthosilicates with general formula Li 2 MSiO 4 (M ¼ Mn, Fe and Co) are among the most attractive new materials as potential high-specific-energy cathodes for lithium batteries. All the members of this family present a rich polymorphism with at least three clearly identified crystal structures of each Li 2 MSiO 4 compound. Several theoretical investigations have highlighted that the energy stability of the different polymorphs is very close to each other irrespective of their average crystal structures. At the same time, the calculated and experimental electrochemical performances are again very similar among different polymorphs. By means of neutron total scattering investigation of different polymorphs (monoclinic and orthorhombic) of Li 2 FeSiO 4 and Li 2 MnSiO 4 orthosilicates coupled to Pair Distribution Function (PDF) analysis we showed that, at the local scale, all the polymorphs have the same structure (in particular the structure of the monoclinic polymorph) irrespective of the average structure they possess. This experimental evidence of a strong similarity at the local scale can be correlated with the observed electrochemical similarity (such as the lithium extraction voltages) among the different orthosilicate polymorphs, thus providing an approach to elucidate the relevance of local versus longrange structure.
The Journal of Physical Chemistry C, 2013
Picene is a polycyclic aromatic hydrocarbon belonging to the class of phenacenes which have been ... more Picene is a polycyclic aromatic hydrocarbon belonging to the class of phenacenes which have been recently found to behave as hightemperature superconductors upon alkali metal doping. The electronic properties of organic crystals can be finely and largely modified by the density changes obtained by the application of an external pressure. In this work, the role of pressure in tuning the optical properties of crystalline picene has been investigated from room conditions up to 15 GPa through the measurement of UV−visible absorption spectra, two-photon excitation profiles, and one-and two-photon excited fluorescence spectra in a diamond anvil cell. The pressure dependence of the optical band gap was determined, and the frequencies of several vibronic bands belonging to electronic transitions from the ground state (S 0 ) to the four lowest-energy excited singlet states (S 1 to S 4 ) were determined as a function of pressure. We evidence a very different density dependence of the transition energy of S 0 → S 1 , which undergoes a remarkable red shift of ∼400 cm −1 /GPa, and of the transitions from S 0 to the higher excited states, which remain constant in the whole investigated range. This is consistent with a S 1 state of 1 L a character in solid picene. The high-pressure chemical stability of solid picene was investigated through visible absorption and Fourier transform infrared spectroscopy (FTIR). A chemical transformation involving the bulk picene crystal occurs above ∼23 GPa, giving rise to a disordered material similar to the amorphous hydrogenated carbon obtained in the pressure-induced reactivity of benzene. The combination of electronic and vibrational data allows us to identify the presence of reaction intermediates at ∼10 GPa, preferentially forming at crystal defects.
Physical Review B, Jul 31, 2006
The results of the synthesis and characterization of the optimally doped (La) 1.4 (Sr 1-y Ca y ) ... more The results of the synthesis and characterization of the optimally doped (La) 1.4 (Sr 1-y Ca y ) 1.6 Mn 2 O 7±δ solid solution with y=0, 0.25 and 0.5 are reported. By progressively replacing the Sr with the smaller Ca, while keeping fixed the hole-concentration due to the divalent dopant, the "size effect" of the cation itself on the structural, transport and magnetic properties of the bilayered manganite has been analysed. Two different annealing treatments of the solid solution, in pure oxygen and in pure argon, allowed also to study the effect of the oxygen content variation. Structure and electronic properties of the samples have been investigated by means of X-ray powder diffraction and X-ray absorption spectroscopy measurements. Magnetoresistivity and static magnetization measurements have been carried out to complete the samples characterization. Oxygen annealing of the solid solution, that showed a limit for y~0.5, induces an increase of the Mn average valence state and a transition of the crystal structure from tetragonal to orthorhombic while the argon annealing induces an oxygen under-stoichiometry and, in turn, a reduction of the Mn average valence state. Along with the Ca substitution, the Jahn-Teller distortion of the MnO 6 octahedra is reduced. This has been directly connected to a general enhancement of the transport properties induced by the Ca-doping.
In this work we present the result of the first experimental investigation of the role of oxygen ... more In this work we present the result of the first experimental investigation of the role of oxygen content on the properties of the HoBaCo 2 O 5+δ layered cobaltite. We have measured the variation of oxygen content as a function of temperature and oxygen partial pressure by means of thermogravimetry coupled to chemical titration analysis. On selected samples of accurately known oxygen content we have undertaken a systematic investigation of their structural, thermal and magnetic properties by means of x-ray diffraction, differential scanning calorimetry and magnetometry. The overall results gained by this study confirm the central role of oxygen content on the properties of these materials suggesting that, for the Ho composition, even very slight variation of the order of δ=0.01 has a dramatic influence on the magnetic and transport properties of the samples. In addition, we have presented results showing the strategy to check the quality of samples prepared at selected oxygen contents by annealing procedures.
RSC Adv., 2015
ABSTRACT Picenes 2a-c were readily obtained via photocyclization of dinaphthylethenes 1a-c by a s... more ABSTRACT Picenes 2a-c were readily obtained via photocyclization of dinaphthylethenes 1a-c by a straightforward solar light-induced Mallory reaction. The product was easily recovered by simple filtration after exposure of the reaction mixture to sunlight. Similar results have been The polyene obtained was then characterized by means of Raman spectroscopy and X-ray diffraction.
We present here non-stoichiometry and electrical conductivity measurements in the low-T and high-... more We present here non-stoichiometry and electrical conductivity measurements in the low-T and high-T range for the Eu1+xBa2-xCu3Oy system with x=0, 0.2 and 0.4. We found an anomalous trend of the Tc with y for EuBa2Cu3Oy (Eu-123) relative to other HTSC of REBa2Cu3Oy family (RE=Y, Nd, Sm). On the contrary high-T data for the whole system show a behavior similar to
Nanoscale, 2015
In this paper we report on the deposition of La0.99Ca0.01NbO4 thin films with scheelite-type crys... more In this paper we report on the deposition of La0.99Ca0.01NbO4 thin films with scheelite-type crystal structure. Thanks to the film's nanostructure, we were able to stabilize the tetragonal scheelite-type structure phase at room temperature, which involves a full removal of the fergusonite-scheelite phase transition.
The Journal of Physical Chemistry B, 2003
In this paper we report the determination of oxygen diffusion coefficient for optimally doped man... more In this paper we report the determination of oxygen diffusion coefficient for optimally doped manganite thin films of La 0.85 Na 0.15 MnO 3 (LNMO). The study was performed by means of isothermal conductivity measurements on two different epitaxially oriented thin films grown on SrTiO 3 (100) and SrTiO 3 (110). The values found for the oxygen diffusion coefficients are around 10 -14 cm 2 s -1 . The diffusion seems to be faster for the c-oriented film with respect to the other one (growth on SrTiO 3 (110)). The activation energy for the diffusion process is 0.35 eV for the LNMO film on SrTiO 3 (100) and 0.66 for the LNMO film on SrTiO 3 (110). A possible reason to account for the different behaviour between the two films is proposed.
The Journal of Physical Chemistry B, 2005
The local structure and electronic properties of Mn in Na doped lanthanum manganites have been in... more The local structure and electronic properties of Mn in Na doped lanthanum manganites have been investigated by means of Mn-K edge XAFS, at various temperatures ranging from RT to 30 K. The hole content depends on the amount of doping, both with Na which injects two holes per doping atom and with oxygen nonstoichiometry. The holes are annihilated by oxygen under-stoichiometry via the formation of oxygen vacancies; the presence of a greater static disorder in the orthorhombic manganites is evidenced by both EXAFS and XANES analysis. In the rombohedral samples, a single Mn-O distance is found. In addition, the trend of the EXAFS Debye-Waller factors for the Mn-O distance can be fitted with a simple Einstein model, with no static disorder at low temperature. This evidence suggests small disorder of polaronic and of Jahn-Teller origin in the rombohedral samples at low temperatures, which is in contrast with what was found for the orthorhombic samples below T(c).
Solid State Communications, 2001
X-ray powder diffraction, electron paramagnetic resonance and static magnetisation measurements o... more X-ray powder diffraction, electron paramagnetic resonance and static magnetisation measurements of slowly cooled Mg 12x Mn 21x O 4 solid solutions for 0 # x # 1 have been performed in order to investigate the effects of the Mg/Mn ratio on the spinel inversion. Our results prove that a small inversion degree exists at room temperature for the samples with x , 0:5 and suggest that the inversion decreases with increasing x, that is by reducing the Mg 21 content. The multi-technique approach for the de®nition of the correct synthetic and cooling procedures enables the formation of single-phase high purity materials. q
Solid State Communications, 2001
Electron paramagnetic resonance (EPR) and static magnetic susceptibility measurements on (Cd 1Ϫx ... more Electron paramagnetic resonance (EPR) and static magnetic susceptibility measurements on (Cd 1Ϫx Mn x )Mn 2 O 4 samples with 0:03 Ͻ x Ͻ 0:5; characterized by X-ray powder diffraction (XRPD) and with defined oxygen content, are reported to clarify the nature and amount of the Cd/Mn(II) cation substitution. The EPR results are in agreement with the existence of Cd 1Ϫx t Mn(II) x t -Mn(III) 2 o O 4 solid solution, where o and t refer to the octahedral and tetrahedral sites of the spinel structure. The EPR and magnetization techniques also allowed detecting the presence of spurious phases, like Cd 2 Mn 3 O 8 and relating them to the sample preparation. ᭧
Physical Review B, 2006
The results of the synthesis and characterization of the optimally doped (La) 1.4 (Sr 1-y Ca y ) ... more The results of the synthesis and characterization of the optimally doped (La) 1.4 (Sr 1-y Ca y ) 1.6 Mn 2 O 7±δ solid solution with y=0, 0.25 and 0.5 are reported. By progressively replacing the Sr with the smaller Ca, while keeping fixed the hole-concentration due to the divalent dopant, the "size effect" of the cation itself on the structural, transport and magnetic properties of the bilayered manganite has been analysed. Two different annealing treatments of the solid solution, in pure oxygen and in pure argon, allowed also to study the effect of the oxygen content variation. Structure and electronic properties of the samples have been investigated by means of X-ray powder diffraction and X-ray absorption spectroscopy measurements. Magnetoresistivity and static magnetization measurements have been carried out to complete the samples characterization. Oxygen annealing of the solid solution, that showed a limit for y~0.5, induces an increase of the Mn average valence state and a transition of the crystal structure from tetragonal to orthorhombic while the argon annealing induces an oxygen under-stoichiometry and, in turn, a reduction of the Mn average valence state. Along with the Ca substitution, the Jahn-Teller distortion of the MnO 6 octahedra is reduced. This has been directly connected to a general enhancement of the transport properties induced by the Ca-doping.
Physica B: Condensed Matter, 2003
ABSTRACT
Journal of the American Chemical Society, 2007
La 2 Mo 2 O 9 (LAMOX) is a fast oxygen ion conductor which shows high oxygen ion conductivities c... more La 2 Mo 2 O 9 (LAMOX) is a fast oxygen ion conductor which shows high oxygen ion conductivities comparable to those of yttria-sabilized zirconia (YSZ) . LAMOX is subject to a structural phase transition from the non-conductive monoclinic form to the highly-conductive cubic form at about 580°C. The origin of the conductivity in cubic LAMOX has been suggested to be due to a "disorder" in the O sub-lattice without any insight into the real distribution of the oxygen ion. In this paper, thanks to the application of the neutron atomic pair distribution function (PDF) analysis, we provided evidences that the local structure of the cubic polymorph of LAMOX is exactly the same of that of the monoclinic phase thus indicating that the structural phase transition is actually a transition from a static to a dynamic distribution of the oxygen defects. This work represent the first application of the atomic-pair distribution function analysis to the study of an oxygen fast-oxide ion conductor and clearly indicates that a more reliable and detailed description of their local structure, particularly in the highly conductive phases, and can lead to a better comprehension of the structure-property correlation, which is the starting point for the design of new and optimized functional materials.
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Papers by Lorenzo Malavasi