The Neutron Diffraction Study Of The Phase Transition Of Metal Cerium At Low Temperature And High Pressure Respectively

Ce is a typical felectron transition metal element.Under the influence of f electron,the phase structure of Ce and actinides has many similarities.Without radioactivity or poisonousness,cerium is regarded as an ideal simulation material for studying the properties of actinides by researchers.Therefore,the study of cerium phase transition can help people to establish models to study actinides.Although many years of researches have been done,there are still a lot of disputes about the phase boundary of the pressure-temperature phase diagram of cerium.In this paper,the phase transition characteristics of cerium at low temperature and at high pressure are studied by using neutron diffraction technique and a 30 tons press,respectively.The 30-tons press can not reach a higher pressure,in order to the follow-up study of higher pressure,the Paris-Edinburgh press is used to carry out the pressure calibration experiment.At the same time,the heating assemblies are designed and tested.The main conclusions are as follows:(1)The low-temperature neutron diffraction experiments on Ce samples show that there is no B phase in the whole heating and cooling process.The phase transition of ?-Ce??-Ce occurs between 100 K and 150 K,accompanied by volume collapse of 16.8%.When the temperature nses,it is found that the transition temperature of the phase transition from a-Ce to y-Ce is between 180 K and 250 K.The ?-Ce?a-Ce phase transformation has a large hysteresis and latent heat.According to Scheller formula,the increase of half-width of the diffraction peak of y-Ce during cooling is caused by the formation of a phase.It is inferred that the interface of the two phases is semi-coherent with dislocations,based on the variation of the half-width of the diffraction peak of ?-Ce and the mismatch of the two phases.So,the phase transformation of y-Ce??-Ce belongs to displacement phase transformation,not martensitic phase transformation.(2)High-pressure neutron diffraction experiments of Ce metal at room temperature are carried out on a 30-ton press.The neutron diffraction spectra at atmospheric pressure,0.7 GPa,1.3 GPa and 2.2 GPa are successfully obtained by using iron as a pressure calibration material.The results show that the y phase still does not appear at 0.7 GPa.When pressure reaches 1.3 GPa,the y phase disappears and the a phase peaks appear,and the two phases do not coexist.This indicates that the phase transition between y phase and a phase has ended at 1.3 GPa.(3)High-pressure neutron diffraction experiments are earried out on a Paris-Edinburgh cell.The neutron diffraction spectra of iron samples under different pressures are successfully obtained.The stability of single-toroidal and double-toroidal assemblies is analyzed.The results show that the double-toroidal WC anvil has higher pressure efficiency and better stability.At the same time,the double-toroidal assembly can withstand 100 MPa load pressure stably,while the single-toroidal assembly starts to be unstable under 80 MPa load pressure and blow out.By comparing the thickness of the gasket before and after pressurization,it is found that the thickness of the edge of the gasket during pressurization is very important to the stability of the assembly.The groove enhances the lateral support ability of the gasket and keeps the thickness of the edge of the gasket,which is the main reason for ensuring the better stability of the double-toroidal assembly.