材料表征教學(xué)資料 interaction

1、1interaction of radiations with mattersthe probe to microstructures2why ?nthe 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 to

2、 perturb the material by interacting in some way with it.3examplesnone sees a matter only because the matter reflects visible light.n x-ray and high energy electron beam could “see” crystal lattices.4the dilemmaninteraction of radiation with materials under probe may cause damage to or change the mi

3、crostructure of them.nhigher resolution requires radiation with smaller wavelength, or higher energy. thus, the probability of radiation damage increases.5the criterionnevery characterization should acquire the most information on the expense of the least damage.ntherefore, knowledge on interaction

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

5、er mean free distance determines the analysis volume.7penetration depth: photonsnp h o t o n s a r e d i s c r e t e q u a n t a o f 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

6、 and c the velocity of light./hche8penetration depth: photonsnelectromagnetic spectrum spans a vast range with wavelength varying from 106 m to 10-14 m. nif we want to make use of electromagnetic radiation to characterize the microstructure of materials, the wavelength of photons must be in comparis

7、on with the features we want to observe.ntherefore, we do not need photons with wavelength larger than 10-4 m or smaller than 10-10 m.9penetration depth: photonsnthe penetration depth of photons depends sensitively on materials and photon energies. it is not possible or instructive to give a detaile

8、d relation over the whole spectrum.nhowever, it is possible to give a rough description over some specific and important wavelength. 10penetration depth: photonsnvisible light: 500 nm, penetration depth 50 - 300 nm information from visible light are averaged over a few hundreds of atomic layers.nult

9、raviolet light: severe absorption, only suitable to surface analysisnx-ray: generally a few micrometers, dependent of absorption coefficient increases with increasing atomic number.)exp(0tii11penetration depth: electronsnthe penetration depth of electrons varies dramatically with electron energy and

10、 atomic number of materials.nthe higher the energy, the deeper the penetration.10 kev 0.? m30 kev 2 m12penetration depth: electronsnthe smaller the atomic number, the deeper the penetration.nthis affects the characterization methods using electron beams as probes because common materials are compose

11、d of elements with different atomic numbers.atomic numberpenetration depth 10 m 40 2 m13penetration depth: electronsnthe scattering of electrons in the materials limits the space resolution.14interaction of electrons with matterssecondary electronsbackscattered electronsauger electronsx-raysdiffract

12、ed electrons15secondary electronsnsecondary electrons are used to give scanning electron microscope (sem) images.nsecondary 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.ntherefore, the resol

13、ution of a sem is basically determined by the incident beam area.16auger electronsnt h e e n e r g y o f a u g e r 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.nlight atoms or l

14、ow energy bonds are prone to generate auger electrons.vacuumecefvalence bandl3l2l1k17characteristic x-raysnx-ray photons have strong ability to penetrate through the sample. therefore, x-r a y s g e n e r a t e d i n a relatively large volume can be detected.nheavy atoms have large cross-sections to

15、 generate characteristic x-rays.ncharacteristic x-rays are suitable to detect heavy atoms.vacuumecefvalence bandl3l2l1kcharacteristic x-rays18radiation damage: photonsnphotons have the least damage to materials. ngenerally, the damage from photons comes from heating effect. the amplitude of the dama

16、ge depends on penetration depth, photon energy and current density.nexample: strong laser can cause melt and evaporation in a short time, e.g. the interaction of laser with target in pulse laser deposition.19radiation damage: electronsnradiolysis: inelastic scattering (mainly ionization) breaks chem

17、ical bonds in materials, especially polymers and alkali-halogen chemicals.nknock-on damage: direct knock out the atoms from lattice sites to form point defects, effective to all kinds of materials.nheating 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.20radiation damage: electronsnradiolysis is due to