Influence of composition and defects on the properties of transparent ceramics and crystals for laser and scintillator applications.

Project leaders

Guido Toci, Martin Nikl

Agreement

REPUBBLICA CECA - CAS (ex AVCR) - Czech Academy of Sciences

Call

CNR/AVCR 2013-2015

Department

Materials and Devices

Thematic area

Physical sciences and technologies of matter

Status of the project

New

Research proposal

 
The research program is based on the experience gained during the long term cooperation between Institute of Applied Physics "Nello Carrara" (IFAC-CNR, Sesto F.no, Italy) and Institute of Physics (IP-AVCR, Prague, Czech Republic), on the characterization of crystalline materials for scintillators and laser applications (see e.g. P. Fabeni et al., J. Lumin. 129, 767, 2009; M. Nikl, et. al., J. Lumines. 129, 1564; 2009; M. Nikl et al. , J. Physics: Conference Series 249 012018, 2010; Pirri A. et al. Opt. Express, 17 18312, 2009).
Recently, Transparent Optical Ceramics (TOC) with different compositions have reached a good level of development, often reaching an adequate quality for the realization of laser devices and scintillators. The influence of composition and defects for the two kinds of applications is rather different. In laser materials, the most important characteristics are the low scattering losses due to the residual porosity and interfacial secondary phases, the absence of quenching mechanisms for the excited states of the lasing ion, as well as macroscopic thermomechanical properties (e.g. thermal conductivity, fracture toughness). For scintillator applications other aspects are important such as the absence of trap states or decay processes with long life time. The optimization of the composition and preparation methods for the two kinds of applications can then be different. The general aim of this project is to study the laser characteristics and the scintillation and luminescence properties on TOC and precursor powder samples with different compositions, in order to get useful information regarding stoichiometry, doping levels and preparation methods optimized for the two applications, also by comparison with the characteristics of the related single crystal samples.
Regarding the scintillating materials, a new efficient phosphor SrHfO3:Ce was discovered (S. L. Dole, et al. 1992), with emission at 400-410 nm due to the 5d-4f transition of Ce3+, further studied by Y. M. Ji et al. (2005) including. Li+ codoping to stabilize 3+ charge state of Cerium ion. Because of its high melting point (>2400 oC) it is not available as a single crystal but it can be fabricated as a transparent ceramics. Influence of Sr/Hf ratio and annealing treatment on structural and scintillating properties of sol-gel Ce3+ -doped SrHfO3 powders are studied by M. Villanueva-Ibanez et al. (2005).
Scintillation properties of Ce-doped SrHfO3 and BaHfO3 ceramics were studied by E. V. Van Loef (2007). They both have nearly isotropic optical properties allowing them to be fabricated in the form of fully transparent optical ceramics. Powders of the materials were prepared by solid state synthesis and TOCs were fabricated by vacuum hot press or hot isostatic press at temperatures between 1440 and 1700 oC and pressures up to 30 kpsi. Comparing to BGO the integrated RL spectra of both TOC are substantially higher. Both materials may become of interest for medical imaging applications, such as PET and CT.
Recently, undoped non-stoichiometric SrHfO3 and SrZrO3 have been revealed (M. Nikl et al, 2011; P. Bohacek et al. 2012). Also the heavily Ce-doped (up to 15at%) SrHfO3 was reported and the dependence of scintillation efficiency on its stoichiometry was noticed.
During the project, powders of mixed compositions (Sr-Ba)MeO3, (Me=Zr, Hf), with varying stoichiometry, undoped and Ce-doped, will be prepared by the solid state reaction in IP-AVCR to study the dependence of luminescence and scintillation properties on the composition. Their structure and morphology will be checked as well. Time-resolved spectroscopy methods will be applied both at IFAC-CNR and IP-AVCR laboratories. While in IP-AVCR the classical (flashlamp excited) methods will be used, in IFAC-CNR the pulsed UV laser excitation will be used to achieve enhanced dynamical resolution, essential to study the ionization and afterglow processes in such powders.
As for the laser materials, the research will mostly focus on materials doped with rare earths i.e. Ytterbium and Erbium.
During the project, investigations will be carried out on TOCs activated with Yb, Er and Yb-Er, in particular Lu2O3, Sc2O3, YAG and its isomorph LuAG, focusing in particular on the effects of the increasing doping levels. IFAC will, in particular, perform characterizations on the laser properties (e.g. threshold, slope efficiency, tuning range) under semiconductor laser pumping, and characterization of the transition lifetimes. IP-AVCR will carry out absorption spectroscopy, photoluminescence (PL) spectroscopy under X-ray excitation, as well as PL and photoexcitation spectroscopy under UV excitation, for the characterization of the host band structure, of the color centers and of the dopant-host charge transfer transitions. Measurements will be also performed on the related crystalline samples for a clear comparison.
 

Research goals

Scintillators: the aim is to prepare and study solid solutions with minimum Ba content (with cubic phase at RT) of mixed compositions (Sr-Ba)MeO3 (Me=Zr, Hf), with varying stoichiometry, undoped and Ce-doped. In Ce-doped samples the maximum efficiency of Ce emission, stability against excited state ionization and minimal afterglow will be searched depending on material non-stoichiometry and Ce content. Powder phosphors will be prepared by solid state reaction, by repeatedly calcining and homogenizing the starting powders in an agate mortar. Energy transfer from the host to emission centers will be studied with time-resolved emission spectroscopies, evaluating the results with our phenomenological models developed for ns ion and exciton centers in a wide range of inorganic hosts.
Lasers: spectroscopic and laser properties of Yb and Er doped TOCs (in particular Sc2O3, Lu2O3, LuAG, YAG) will be studied. The influence of Yb content on population quenching in Yb doped hosts and on the energy transfer process in Er-Yb doped hosts will be
studied, also by comparison with the related crystals. We will study the laser properties under CW and quasi-CW pumping, and the laser transitions lifetimes, looking for effects of radiation self-trapping and concentration quenching; Yb-Er excitation transfer will be studied with time resolved spectroscopies. The host band structure, color centers and dopant-host charge transfer transitions will be studied mainly with absorption, PL and PE spectroscopy.

Last update: 25/04/2024