Small-angle X-ray Scattering Tomography And Its Applications

The development of modern biomedical and material science immensely depends on the understanding of nanostructure.Small-angle X-ray scattering is a common characterization method to detect the nano-structural properties inside the sample.However,traditional SAXS measurements can only be used to study the homogenous sample.For the heterogenous systems,traditional SAXS measurements usually need destructive sample slicing.X-ray micro-CT is a non-destructive three-dimensional imaging method but limited by the imaging resolution and scale range.Other characterization methods like electron and optical microscopy can only obtain the local information about the sample.Consequently,new methods must be developed for three-dimensional,non-destructive,nano-structural characterization of bulky and heterogenous samples.To the limits of the existing nano-structural characterization methods,a highefficiency small-angle x-ray scattering tomography(SAXS-CT)is established at Shanghai Synchrotron Radiation Facility(SSRF).The method can non-destructive three-dimensional imaging for bulky and heterogenous sample and can obtain each local scattering information in reciprocal space and analyze the distribution of nanostructural information in heterogenous systems.The main contributions are summarized as follows:(1)The SAXS-CT system using micro-focusing KB mirrors was successfully developed at Shanghai Synchrotron Radiation Facility.System optimization design,X-ray micro-focusing,fast and effective data acquisition and high-accuracy image reconstruction were resolved in the process of system research.The samples of bamboo,polyethylene and injection-molded polylactide were selected asexperimental verification for SAXS-CT system.For the bamboo sample,compared with the phase-contrast CT result,the distribution of vascular bundle and parenchyma cell have been accurately obtained by the developed SAXS-CT method.Moreover,the scattering difference and orientation characteristics of nanofibers have also been obtained.For the polyethylene sample,the difference of scattering characteristics and crystal structure inside the sample were detected by SAXS-CT while the traditional phase-contrast CT is invalid.For the injectionmolded polylactide sample,the results show that lamella structure inside the sample was layered and the distribution of lamella structure and its long spacing has also been acquired.The experimental results have confirmed the reliability and practicality of the SAXS-CT system.(2)A high-efficiency SAXS-CT method based on OSEM algorithm was developed.For the traditional SAXS-CT mehotd,the problem of inefficient,vast amounts of data,long experimental time and high radiation dose limit its development.The OSEM algorithm is introduced to improve the efficiency of SAXS-CT.For the simulation results,compared with the traditional FBP algorithm,the OSEM algorithm can effectively eliminate streaking artefacts in limited projection angles and the projection data can be reduced to at least 1/3 on the premise of maintaining the image quality.In addition,through the research about the effect of subsets and iterations,the optimal reconstruction parameters were selected and applied in the reconstruction of real data.The experimental results of bamboo and polyethylene samples show that OSEM reconstructions can still maintain the high-contrast image quality under few projection angles while the FBP reconstructions have hardly distinguish the interior structure about the sample.(3)The evolution of three-dimensional crystalline structure under shear flow wasquantitatively studied by the established SAXS-CT method.Oscillation shear is the key process of injection molding technology for polylactide(PLA)parts.This technology usually makes PLA parts form a hierarchical crystalline structure with several layers due to the existence of shear and temperature field gradients inside the mold.Technology optimization directly affects material performance and accurate characterization of internal structure becomes the key to contact technology and performance.The existing characterization methods can only obtain local information of sample and often require destructive sample slicing.The SAXS-CT method is employed to quantitatively reveal the three-dimensional structure of PLA parts.The three-dimensional distribution of the complete crystalline structure and a series of morphological parameters have been quantitatively extracted.The results show that shish-kebab structure mainly exists in shear layer while skin layer and core layer show a spherulitic structure,which are in line with the electron microscopy results.The area of shear layer significantly enlarges,and core layer quickly shrinks with the increase of shear duration.Moreover,the area of core layer shrinks much more rapidly in TD than in ND.A multi-layered shish-kebab morphology is formed within the shear layer.The crystalline structure along MD direction shows no obvious change.The results provide reliable theory basis for the establishment of the relationship between the internal structure and performance of injection-molded PLA and the link about “technology-structure-performance”.