Desarrollo de sensores ópticos de presión y temperatura basados en la luminiscencia de iones de tierras raras para experimentos bajo condiciones extremas

Resumen

This thesis is focused on the search for new optical pressure and temperature sensors to be used in experiments under extreme conditions. The research carried out and shown in this memory may be summarized as: 1. Rate equations for excited ions relative populations were raised for a system in contact only with a heat source and for another in contact with a heat source and laser excitation. The solution of these equations shows that the ratio of populations thermalized levels, whose emission is used as a temperature probe, remains in stationary equilibrium, almost thermal equilibrium, always that the laser power is low. 2. The influence of the ligand type and the symmetry of the local environment on its optical properties are factors that have to be considered in the development of optical temperature sensors. This influence can be characterized using the prediction capability of the Judd¿Ofelt Theory. For Er3+ ions, host matrices with highly distorted local symmetry environments, with highly covalent Er3+¿ligand bonds and strong crystal-field strengths, show large fluorescence intensity ratios and thermal sensitivities, requirements easily met by oxide glasses. In order to show that Judd-Ofelt Theory is an important tool to predict the sensor sensitivity two different matrices, with different ligands and local symmetry: an oxyfluoride glass and a fluoride-type nanocrystalline glass-ceramic were studied. 3. The concentration of the optically active ion is another relevant factor that must be taken into account when developing an optical temperature sensor. A high concentration produces a reduction in the ratio of the emissions of the thermalized levels and, therefore, in the sensitivity of the sensor due to radiative transfer processes between the optically active ion as a consequence of the large overlapping between the emission and the absorption. The re-absorption mechanism explains the changes in the profiles of the samples due to re-emitted photons in different solid angles to that of the originally excited ions. This transfer processes are especially pronounced in the emission with high oscillator strength absorption band as the 2H11¿4I15/2 emission for the Er3+ ion. This problem can only be eliminated if the sensor matrix is doped at a very low concentration. 4. Taking into account the prediction capability of the Judd¿Ofelt Theory and the effect of the optically active concentration ion on the sensitivity of a temperature optical sensor, it has synthesized a sensor matrix doped with Er3+ ions with one of the highest sensitivities found. Furthermore, this glass was used to study the errors in the value of the temperature due to laser heating. The complete theoretical investigation on this glass has allowed to propose a prototype temperature sensor. 5. On the other hand, pressure optical sensors are based on a garnet crystal doped with Nd3+ ions. This sensor uses the behaviour of the transition 4F3/2¿4I9/2 as probe. In particular, two Stark peaks of this emission, R1,2¿Z5 present a high sensitivity the pressure. Their displacement in different garnet crystals (GSGG, GGG, CGGG, YAG) have been calibrated. Also, some nanocrystalline garnet doped Nd3+ was studied. 6. When the displacement of these two peaks is compared in different garnets, it is found that a high initial separation between them guarantees a wider working range for the sensor. Initial separation of these bands R1,2¿Z5 is the result of splitting of the level 4F3/2 by the crystal field. For this reason, it was studied the relationship between the crystal field parameters and their influence in the initial splitting level 4F3/2. In this analysis, the local environment D2 (orthorhombic) that Nd3+ ions occupy within the garnet structure is approached a cubic environment distorted by orthorhombic terms not included in the cubic symmetry. The simulations indicate that garnet crystals where the ionic radii of the cations (Ga3+, Gd3+, Ca2+, ...) are the most diverse possible have higher orthorhombic distortion. This allows to understand why CGGG crystal has a higher initial separation of the R1 and R2 peaks and working range is the largest of the garnets studied. 7. GSGG doped Cr3+-Nd3+ permits to determinate simultaneously the pressure (up to 12 GPa) and temperature (10-300 K) from the luminescence spectra of the 4F3/2¿4I9/2. Two intense and isolated lines corresponding to R1¿Z5 and R2¿Z5 centred in the range from 10600 to 10800 cm¿1,respectively, are the most sensitive to pressure with coefficients of -11.3 cm¿1/GPa and -8.8 cm¿1/GPa, respectively, while the relative intensities of the R1,2¿Z1 transitions show a large dependence with temperature.