Attività di ricerca della sezione di
1) Anelastic and dielectric spectroscopies of complex materials
- Francesco Cordero (I Ricercatore)
- Floriana Craciun (Ricercatore)
- Fabrizio Corvasce, (CTER 4°)
- Paolo Massimiliano Latino (CTER 7°)
- Setups for anelastic spectroscopy measurements: complex elastic modulus of resonating bar shaped samples (20-50 mm long) electrostatically excited on their flexural modes (1.1-300 K and 70-1150 K in vacuum; 0.1-100 kHz)
- Resistivity measurements in cryocooler (12-300 K)
- Setup for dielectric spectroscopy measurements on bulk samples and films: impedance bridge HP 4194 A with Delta Design 9023 climatic chamber (100-600 K; 100Hz-40MHz
- High sensitivity interferometric probe for detecting mechanical vibrations
- Setup for measuring the ferroelectric hysteresis
- Piezoelectric characterisation of thin films
- UHV systems for annealing, outgassing and doping samples in vacuum or controlled static atmosphere up to 1100 oC
- Setup for reducing/oxidising samples in flux of H2, CO, O2 with induction heating
The laboratories include the facilities for measuring the anelastic and dielectric properties of solids, for characterising ferroelectric materials and for annealing, outgassing, reducing and oxidising samples. The anelastic (0.1-100 kHz) and dielectric (0.1 kHz-40 MHz) responses provide information on the dynamics of structural and ferroelectric domains, defects and excitations, including ionic mobility. Also the glassy and non-equilibrium dynamics (ageing, memory) can be studied of materials in which intrinsic inhomogeneities of structural, electric or magnetic type on nanoscopic scale make the physical properties interesting for technological applications.
The materials that are studied are mainly ferroelectric, ion conducting, superconducting and magnetic perovskites, and metal-H systems.
Ferroelectrics and relaxors
Relaxors are materials in which the chemical or structural disorder inhibits the long range ferroelectric order giving rise to fluctuating polar domains on nanoscale. These materials may have extremely high values of the dielectric, piezoelectric, electrostrictive and electro-optic constants over wide temperature ranges (applications in sensors, actuators, permanent memories, electro-optic devices). These materials may also be deposited and characterised as thin films. The main results regard the observation of memory also above the polar freezing temperature, and separation of polar and non-polar fluctuations.
Also ferroelectrics are studied, particularly regarding the polar and elastic response in regions of the phase diagram where coexistence of domains with different structures or having an intermediate structure allow for a high electromechanical response. (Project of CNR RSTL “Materiali ferroelettrici e rilassori”)
Anelastic and dielectric studies are being conducted on doped cerates (Ba(Ce/Y)O3-x) used as solid electrolytes, in order to probe the dynamics of O vacancies and protons, and separate the contributions from diffusion, reorientation around dopants, ordering, tunnelling in the case of H/D. In parallel, simpler systems like SrTiO3 are studied (Project of MIUR FIRB “Fuel cells with polymeric and ceramic electrolytes”)
Giant magnetoresistance manganites
In these materials there is formation of fluctuating charge and magnetic inhomogeneities on nanoscopic scale, which cause a strong dependence of resistivity on the magnetic field (applications as sensors, heads for hard-disks). The anelastic spectroscopy allows one to study the strong coupling of charge and spin with the lattice and to probe the glassy dynamics of the charge-ordered versus ferromagnetic inhomogeneities.
High Tc superconductors
In the superconducting cuprates (YBCO and LSCO families) and ruthenocuprates (Ru-1212) the main results regard: mobility and ordering of the non-stoichiometric O, which determines the superconducting properties; spin and charge inhomogeneities that seem to characterise the HTS, measuring the transversal dynamics of the charge stripes.
The main results have been obtained on reorientational and tunnelling dynamics of H/D trapped by substitutional or interstitial impurities in bcc transition metals and alloys (e.g. Nb, ZrV2) and rare earths (Y).