Ontwikkeling van carbide materialen voor de productie van radioactieve ionenbundels in hoog vermogen ISOL-installaties

The isotope separation on-line (ISOL) technique consists of on-line evaporation, ionization, acceleration, and mass purification of radioisotopes produced in a target material irradiated by an accelerated particle beam. At SCK CEN, the ISOL@MYRRHA project envisages the construction of a high power ISOL facility that will utilize a 600 MeV proton beam of 200 µA of intensity extracted from the beam to the ADS (Accelerator Driven System) MYRRHA. Aside from exotic nuclei used in fundamental physics research, ISOL@MYRRHA will produce radioactive isotope beams (RIBs) for applications in nuclear medicine. In this framework, this PhD project aims to develop novel target materials based on tantalum carbide for the production of radiolanthanides, such as 149,152,155Tb, an isotope proposed for cancer radio-theragnostics. In order to increase the release and yield of isotopes, the target is usually operated at 2200°C to boost the diffusion of the isotope reaction products, for operation times up to 4 weeks. In these harsh environments, the target materials must possess several characteristics such as refractoriness and stable open porosity, so as to enable the fast evaporation of short-lived reaction products. A new innovative and promising processing route was developed to produce microporous TaCx by exploiting the thermal decomposition of MAX phase ternary carbides, such as Ta4AlC3. The Mn+1AXn phases are nanolaminated ternary ceramics composed of a transition metal (M), a metalloid (A) and carbon or nitrogen (X). Typical A elements such as aluminum have a relatively high vapor pressure compared to the other components. Therefore, these ternary ceramics are unstable at high temperatures in vacuum, and tend to decompose into porous binary Mn+1Xn phases and A-element vapors. A processing route for the production of pure bulk Ta4AlC3 was initially developed and several heat treatments in vacuum (10-5 mbar) were conducted at 1200°C and 1250°C with the aim to evaporate the aluminum and degrade this MAX phase into a binary TaCx. On fully dense bulk material, the decomposition takes place across a layer of which the depth depends on the temperature and treatment time. Due to the complex nature of the Ta-C phase diagram, a biphasic TaCx + Ta2C material was obtained. It was concluded that it is not beneficial to produce target materials by degradation of bulk Ta4AlC3, because the resulting pores are not homogenously distributed and interconnected. Therefore, the Ta4AlC3 was milled and used as precursor in powder form in a ceramic - wax feedstock, which was mold-casted and pyrolyzed in order to remove the wax. This route was chosen with the aim to maximize the surface to volume ratio and allow for efficient decomposition and retention of a porous structure. This technique, inspired by the injection molding technique, was combined with the addition of sacrificial polyamide pore formers (sacrificial templating), which introduced a bimodal pore size distribution in the final product. Commercial TaC was also used in parallel with the same process as a reference to the Ta4AlC3 MAX phase-based route. A campaign of heat treatments in a specifically designed thermal test bench was conducted at SCK CEN in order to prove the stability of the microstructure at an operating temperature of 2200°C for up to 12 h. The obtained materials had an interconnected porous structure with a percentage of the theoretical density in the 35-50% range. The specific surface areas were measured between 0.2 and 0.8 m2/g. The stability under operating conditions was remarkably high, without loss of open porous structure, and only compositional changes in the carbon stoichiometry and phase content. MAX phase precursors have an advantage compared to commercial TaC in terms of machinability allowing the processing of tailored microstructures, and their lower carbon stoichiometry might result in more efficient isotope release. Moreover, MAX phases have a potential for alloying, and creating target materials with complex compositions. The formation of new MAX phases in the Ta-Hf-Al- C and Ta-Nb-Al-C systems was therefore investigated. MAX phases with Hf, Nb solid solutions on the M sites were discovered with the following formulas: (TaxHf1-x)4AlC3, (TaxHf1-x)2AlC, (TaxNb1-x)4AlC3 and (TaxNb1-x)2AlC, with x = 0.9, 0.85, 0.8, 0.75; (Ta0.75,Nb0.25)4(Al0.5,Sn0.5)C3 and (Ta0.75,Nb0.25)2(Al0.5,Sn0.5)C. The materials were characterized in terms of microstructure/porosity by the Archimedes method combined with solvent intrusion, and mercury intrusion porosimetry. Crystal structures were characterized by X-ray diffraction (XRD) and selected area electron diffraction (SAED). Imaging of the microstructure and elemental analysis were conducted by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersion spectroscopy (EDS/EDX), wavelength dispersion spectroscopy (WDS) and electron back-scatter diffraction (EBSD). Ultimately, an off-line release experiment was proposed and designed with the aim to be conducted at the ISAC-TRIUMF facility in Canada. Such a test will allow to assess the performance of porous TaCx targets in terms of the release of lanthanide isotopes.

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