In this paper, co_2 laser pulse of 0.1μs was sued to heat instantly the specimens of Al, constantant, Ag, amorphous alloy, BN thin pieces those are thickness of 20-100μm, and a system of the slit diffraction by He-Ne laser has been designed. Then, by an installation of photoelectric converting, the changes among the length of the specimen, light strength, voltage has been realized, we used this method to observe and measure the changing process of the thermal expansion amount of the specimen with time when the specimen was heated instantly. For the first time, the dynamic-process of thermal expansion has been observed directly, and the photograph of the relaxation curve of thermal expansion has also been taken. On the basis of the experiment above, taking the specimen Al for example, we examed the relation between the relaxation time and the length of specimen. At last, the mechanism of the dynamic-process of thermal expansion has been discussed.
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