PRISMAP Deliverable D10.2 - Report on target material characterization

The target material characteristics play a crucial role in the radionuclide production, especially for ISOL targets. On one hand, the microstructure deeply affects the target release capabilities, on the other hand the thermal and structural properties are strongly related to the target integrity and stability during operation. This report summarizes the activity related to the production and characterization of two materials relevant as ISOL targets, in particular titanium carbide and uranium carbide.
Titanium carbide is regarded as a possible target material for the production of the Sc ion beams [1] and was studied at INFN using two different production techniques: a powder-based approach and the Additive Manufacturing Direct Ink Writing (DIW) process.
According to the first approach, titanium carbide was produced following the classical carbothermal reduction from titanium dioxide (TiO2) as a precursor and graphite powder as a carbon source, with an over-stoichiometric carbon content, achieving thus a titanium carbide/carbon composite. Disk shaped samples were produced and employed for the characterization activities. Microstructural observations confirmed the presence of a disperse graphite phase and a total porosity above 50% was achieved. The thermal characterization was performed with a custom set-up available at INFN-LNL and highlighted a thermal conductivity in the 8-10 W/mK range. The mechanical characterization foresaw the induction of thermal stresses with the samples and failures occurred above 1200°C. To analyse the data collected from the mechanical tests the Weibull statistical approach was adopted, providing information on the survival probability of a target as a function of the applied thermal stress.
For the DIW process, a custom ink based on a commercial TiC powder was developed and characterized, and solid disk-shaped samples with a log-pile structure were successfully produced. Also in this case, a total porosity above 50% was achieved, most of which proved to be open porosity. Mechanical tests were performed on the produced disk at room temperature according to the ball on three balls approach. The collected data were again analysed with the Weibull statistical approach providing the correlation between the material survival probability and the applied load. The high temperature thermal characterization provided a thermal conductivity divided by a factor of 2 respect to the TiC/C composite.
Uranium carbide is another fundamental ISOL target material and bimetallic uranium carbides (U1-xMxCz) with Hf, Nb, Ta, Zr or Ti as second metal were produced at IST. Samples were produced with the arc melting technique and a custom routine was identified. XRD (X-Ray Diffraction) analyses on the produced specimens highlighted the actual formation of the U1-xMxCz carbide and SEM-EDS (Scanning Electron Microscope - Energy Dispersive X-ray Spectroscopy) observations allowed for the qualitative identification of the achieved phases. Image elaboration of the collected SEM pictures allowed for the quantification of the ratio of the different phases. LHM (Laser Heating Method) was employed to study the thermal behaviour at high temperatures of U1-xMxC (0.05 ≤ x ≤ 0.2; M = Hf, Nb, Ta) samples, providing the confirmation that the addition of small amounts of refractory metal carbides had a beneficial effect by significantly increasing the compound melting temperature. However, for the systems containing Nb and Ta the solubility in UC2 seems to be smaller than for Hf and Zr, not following the linear behaviour for all the studied composition range observed in the other systems.