X-RAY AND NEUTRON DIFFRACTION ON LASER HEATED LEVITATED SAMPLES: SUCCESSES AND CHALLENGES

Development of new nuclear reactors, fuels and actinide transmutation matrices requires high temperature data. We demonstrated that X-ray and neutron diffraction on solid laser heated levitated samples can be used on many refractory oxide compounds to obtain thermal expansion, refine high temperature crystal structure and for in situ study of phase transitions and melting in variable atmosphere.

Aerodynamic levitators with laser heating are accessible as sample environment for X-ray diffraction experiments at beamline 6-ID-D at Advanced Photon Source at Argonne National Laboratory and for neutron diffraction experiments on NOMAD BL1B instrument at Spallation Neutron Source at Oak Ridge National Laboratory. In these instruments, solid spherical sample is suspended and rotated by a gas stream through a conical nozzle, heated by 400 W CO₂ laser and temperature is monitored by pyrometer. X-ray diffraction pattern suitable for Rietveld refinement can be collected in less than 10 seconds and neutron diffraction pattern in ~10 minutes. The maximum temperature is inherently limited only by the sample and its amenability to laser heating.

The method was successfully used to collect diffraction data up to the melting temperature on Hf, Zr, La, Nd, Sm, Eu, Gd, Ho, Er, Yb and Lu oxides and on La, Sm and Nd zirconates and hafnates. Due to fast equilibration at high temperature, in-situ determination of phase diagrams is possible. The challenges lie in improvement of accuracy of temperature measurement, thermal gradient reduction and reproducible sample rotation to assure random orientation.

This methodology is promising for high temperature materials relevant to nuclear reactors, including new nuclear fuels, lanthanide contaminated UO₂, and other applications.