第五章 TEM显微图象解释-1精要.ppt

* 第五章透射电镜显微图像解释 * TEM image 衬度来源 质厚衬度:样品中不同部位由于原子序数不同或者密度不同、样品厚度不同，入射电子被散射后能通过物镜光阑参与成像的电子数量不同，从而在图像上体现出的强度的差别； 衍射衬度:由于晶体试样的不同部位满足布拉格衍射条件的程度不同而引起衍射强度的差异而导致的衬度； 相位衬度:样品内不同部位对入射电子作用不同引起相位变化,相位差转化为强度差。 * Cr-Mo钢中一些小析出物的萃取复型样品的质厚衬度像 钢的衍射衬度像 高分辨像 * 第一节 质厚衬度 1. 成象原理 入射电子与试样中原子作用产生的电子散射,散射本领越大，电子束发散的程度也越大，穿过光阑孔参与成像的电子数目越少，相应的像就暗些。而散射本领小的区域，发散角小的电子数目多，穿过光阑孔参与成像的电子数目多，荧光屏就亮些。由于试样各部分对电子束散射本领的差异，经光阑作用后，在荧光屏上造成强度不一，便构成了通常所说的像。 * Mass-thickness formula The scattering cross section of a single atom of specimen is ?0: where Z is atomic number, V is the applied voltage, ? is the scattering angle, e is the charge of electron. If the interaction between the atoms is ignored, the total scattering cross section of material with N atoms per cubic centimeter is Q=N ?0= ?0 N0? /A (5-1-2) where N0 is Avogadro’s number, ? is density of the specimen, A is atomic weight of the specimen. (5-1-1) 2. 质厚衬度公式 Consider a volume of specimen with cross section of 1cm2 and thickness of dt. Its scattering cross section is This is the probability of an electron be scattered outside the objective aperture after passing through the specimen with thickness of dt. The larger the scattering cross section, the better the chances of scattering. Assume number of electrons arrived in the specimen with cross section of 1cm2 is n. After passing through the specimen with thickness of dt, dn electrons are scattered outside the objective aperture. The rate of electron reduction is (5-1-3) Z=t Z=0 n0-n n n0 dt t Z As TEM specimen is very thin, we can ignore the electron absorption. Assume the number of the incident electrons is n0 at Z=0. After scattering of specimen with thickness of t, only n electrons pass through the objective aperture to form image. Integrating of (5-1-3), we obtain Solve it to obtain The intensity of electron beam is (5-1-4) (5-1-5) Z=t Z=0 n0-n n n0 dt t Z where I0=n0e . Critical thickness When Qt=