2019材料表征教学资料ppt课件

1、1,Interaction of Radiations with Matters,the probe to microstructures,2,Why ,The interaction of electromagnetic radiation with crystalline solids is now understood in considerable detail so that it can be exploited to provide the necessary information,To characterize a microstructure it is necessary

2、 to perturb the material by interacting in some way with it,3,Examples,One sees a matter only because the matter reflects visible light. X-ray and high energy electron beam could “see” crystal lattices,4,The dilemma,Interaction of radiation with materials under probe may cause damage to or change th

3、e microstructure of them. Higher resolution requires radiation with smaller wavelength, or higher energy. Thus, the probability of radiation damage increases,5,The criterion,Every characterization should acquire the most information on the expense of the least damage. Therefore, knowledge on interac

4、tion of radiation with matters is indispensable,6,Penetration depth,Penetration depth, or mean free distance, of the incident determines the depth and volume in the sample that can be analyzed. In most cases, the type of radiation generated in the sample is different from the incident radiation. The

5、 smaller mean free distance determines the analysis volume,7,Penetration depth: Photons,Photons are discrete quanta of electromagnetic radiation. The photon is identified by the wavelength, , energy, E, and frequency, , all of which are related by the equation where h is Plank constant and c the vel

6、ocity of light,8,Penetration depth: Photons,Electromagnetic spectrum spans a vast range with wavelength varying from 106 m to 10-14 m. If we want to make use of electromagnetic radiation to characterize the microstructure of materials, the wavelength of photons must be in comparison with the feature

7、s we want to observe. Therefore, we do not need photons with wavelength larger than 10-4 m or smaller than 10-10 m,9,Penetration depth: Photons,The penetration depth of photons depends sensitively on materials and photon energies. It is not possible or instructive to give a detailed relation over th

8、e whole spectrum. However, it is possible to give a rough description over some specific and important wavelength,10,Penetration depth: Photons,Visible light: 500 nm, penetration depth 50 - 300 nm Information from visible light are averaged over a few hundreds of atomic layers. Ultraviolet light: se

9、vere absorption, only suitable to surface analysis X-ray: generally a few micrometers, dependent of absorption coefficient increases with increasing atomic number,11,Penetration depth: Electrons,The penetration depth of electrons varies dramatically with electron energy and atomic number of material

10、s. The higher the energy, the deeper the penetration,12,Penetration depth: Electrons,The smaller the atomic number, the deeper the penetration. This affects the characterization methods using electron beams as probes because common materials are composed of elements with different atomic numbers,13,

11、Penetration depth: Electrons,The scattering of electrons in the materials limits the space resolution,14,Interaction of electrons with matters,Secondary electrons Backscattered electrons Auger electrons X-rays Diffracted electrons,15,Secondary electrons,Secondary electrons are used to give scanning

12、electron microscope (SEM) images. Secondary electrons are low energy electrons. Thus, only those generated beneath the incident beam or in the vicinity of incident beam can escape out of the surface. Therefore, the resolution of a SEM is basically determined by the incident beam area,16,Auger electr

13、ons,The energy of Auger electrons is about 0 2 keV, absorbed easily by the materials. Only Auger electrons generated within a few atomic layers beneath surface could escape out of the surface. Light atoms or low energy bonds are prone to generate Auger electrons,17,Characteristic X-rays,X-ray photon

14、s have strong ability to penetrate through the sample. Therefore, X-rays generated in a relatively large volume can be detected. Heavy atoms have large cross-sections to generate characteristic X-rays. Characteristic X-rays are suitable to detect heavy atoms,18,Radiation damage: Photons,Photons have

15、 the least damage to materials. Generally, the damage from photons comes from heating effect. The amplitude of the damage depends on penetration depth, photon energy and current density. Example: Strong laser can cause melt and evaporation in a short time, e.g. the interaction of laser with target i

16、n pulse laser deposition,19,Radiation damage: Electrons,Radiolysis: inelastic scattering (mainly ionization) breaks chemical bonds in materials, especially polymers and alkali-halogen chemicals. Knock-on damage: direct knock out the atoms from lattice sites to form point defects, effective to all kinds of materials. Heating effect: negligible to metal or other good heat conductors. It can be severe problems for insulators. Ceramic micropowders can be heated to 1700 0C in transmission electron microscope,20,Radiation damage: Electrons,Radiolysis is due to electron-e