New Analysis Methods,Pulsed Beam Technique and Applications in Positron Annihilation Lifetime

Author:Gu Bing Chuan

Supervisor:ye bang jiao


Degree Year:2019





Research in the field of modern materials science often revolves around three important issues:the physical and chemical properties of materials,the microstructure of materials,and the preparation and synthesis of materials.The microstructure of materials determines its macro properties.Over the years,many efforts have been made to explore and understand the relationship between material structure and its properties,and various tools and methods have been developed to detect the microstructure of materials.Defects have always played a very important role in numerous types of microstructures of materials.For example,in materials such as semiconductors,even a small amount of atomic defects may have a significant impact on the electrical and optical properties of the samples.As a result,it is particularly important to characterize the internal defects of materials.Positrons,as the anti-particles of electrons,are one of the sensitive probes to detect defects in materials,because they can be trapped by defects and annihilate with surrounding electrons,and the annihilated photons will carry information of the electrons such as the energy,momentum,position and defect status.From the earliest measurement of the positron annihilation spectra in the 1950s to the present,positron annihilation spectroscopy has developed various means and analytical methods for measuring and characterizing material microstructure,which has become one of the most powerful tools to characterize atomic-level defects in materials.However,with the deepening of people’s exploration in the field of material science and the shrinking of the micro-scale of research,the requirements for characterization methods are also increasingly high.It is hoped that defects as small as atomic scales can be characterized more precisely,and for thin film samples,it is hoped that more quantitative description parameters,such as the annihilation lifetime of positrons,can be obtained.It is still a challenge for positron annihilation spectroscopy,which has one-decade history of development.Lifetime Spectrum is the most commonly used technology in positron annihilation spectroscopy.This paper will be based on three directions,the analysis of lifetime spectrum,technology,and application,trying to solve some difficulties and bottlenecks during the application of lifetime spectrum nowadays,aims to make the analysis of positron annihilation technology more accurate,the scope of application is more extensive,the applications in the study of material science more extensive.In this paper,we study the development of new positron annihilation lifetime analysis method,the establishment of pulsed slow positron beam and the application of positron to thermoelectric materials.The main contents of the study are as follows:On the lifetime spectrum analysis,we have developed a CosmoMC software package based on the MCMCI-BI method,which is used to analyze the positron annihilation lifetime spectrum.The program can be directly applied to the analysis of the lifetime spectrum and gives a more reliable and robust global optimal solution with the result almost independent of the initial parameter guesses.We tested the reliability of the program with simulated theoretical life spectra and experimental standard samples,and the accuracy of the results reached to the same level with the conventional spectral software LTv9.On the other hand,this method gives more abundant information such as parameter posterior probability distribution and confidence interval.etc.and avoids the loss of the global optimal solution.We also carefully analyzed detailed results such as correlations between the various parameters.Compared with the PAScual program which uses the similar algorithm,this method is more accurate,and the calculation of the posterior probability distribution of the parameters is more reliable.In the aspect of slow positron technology development,we design and construct the pulsed slow positron beam device by ourselves.These include the DC slow positron generation part the RGM and the time modulation part for the pulsing beam.With the simulation of G4beamine software,the working conditions of the two kinds of choppers and the two bunchers under different parameters,as well as the parameters such as voltage and phase relationship required by the two kinds of choppers working together are obtained.In the actual measurement,we find that the pass rate of the deflection chopper is too low,so our final selection was the reflective chopper.The beam pulsing system and time measuring system including chopper pre-buncher and main buncher were built and adjusted,and the pulsed beam with a beam spot diameter of about 10 mm was obtained,with a time distribution of 650 ps and a peak to background ratio of about 400:1.In the application of positrons,we applied the lifetime spectrum to the characterization of thermoelectric materials,and the Cu1-xAg1-ySe(x=0,0.2,0.4;y=0,0.2)is synthesized by the solid phase reaction method.XRD,SEM photographs and DSC curves show that the CuAgSe sample with ideal proportion is in pure phase,Ag-deficient sample(CuAg0.9sSe)is close to pure phase,and other Cu-deficient samples have obvious impurity phases.The positron annihilation lifetime spectrum proves that there are not only Ag vacancies in the Ag-deficient sample,but also Ag vacancies in the Cu-deficient samples due to the formation of Ag2Se,leading to the increase of the second lifetime intensity.At room temperature,the thermoelectric properties of elements-deficient materials(Cu0.98AgSe,CuAg0.98Se,Cu0.98Ag0.98Se and Cu0.96AgSe)are similar to that of CuAgSe in ideal proportions.When the temperature increases,Cu vacancies and Ag vacancies provide more holes for the samples.It leads to the dominant of p-type carriers,the n-p transition of the material,and rapid decreases of the absolute values of S.The changes of S dominate the changes of the ZT values of the samples,and the ZT of the element-deficient samples decrease rapidly with the increase of temperature until it is reduced to 0 at around 400 K.However,the ideal proportion of CuAgSe samples does not have significant fluctuations in S.There is a small raise from 0.4 to 0.5 of ZT from room temperature to 450 K.Our experiment confirmed that the existence of Ag vacancies and Cu vacancies in CuAgSe-based compounds dominate the n-p transition of the material before the β-α phase transition temperature,and clarified the intrinsic relationship between the defects and thermoelectric properties,which directly proved that the defects in CuAgSe-based compounds are the most important that affecting their ZT values.