Dendritic Deposition of NH₄CI Crystals with Free Surface as Simulations of Surface Solidification of Materials

Junming LIU Zhiguo LIU Zhuangchun WU National Lab.of Solid State Microstructures,Nanjing University,Nanjing 210008,China

摘 要：<正> As a simulation of surface solidification of materials,quasi-two dimensional dendritic deposition ofammonium chloride（NH₄Cl）crystals from an aqueous solution film with free surface byvapourization was investigated by the in- situ observation technique.The depositing morphologiesof NH₄Cl crystals are faceted and of growth laws differing from the three-dimensional case.Underlow supersaturation,faceted dendrites formed,whereas the anisotropy in growth kinetics decreaseswith the increase of supersaturation.When vapourizing rate（supersaturation）is not very low,oscillations of growth rate.tip-radius and second arm spacing of the dendrites were observed.It issuggested that this dynamic phenomenon is attributed to the correlation between theconvection-transfer effect induced by the surface tension gradient of solution film.and the drivingforce for deposition.

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