A new method of fabricating internally sol–gel coated capillary tubes

A new method for fabricating internally sol–gel coated long capillary tubes was developed to solve the problems of the capillary vibration and the difficulty in gelation reaction inner the capillary during sol–gel processing. Firstly, a sol was coated on the inner wall of a standing long silica glass capillary filled with the sol solution by lowering the sol. Then the air was very slowly pumped into the capillary to create an air condition similar to the outside space. By this way the gelation reaction went as well as it usually goes in the dip-coating process. The multilayer or thick coating could easily be fabricated by repeating the sol up-and-down and air flowing process. Subsequently, the capillary was subjected to heat treatment in a special stove while oxygen was kept flowing through the capillary. In this way the organic groups in the gel burned as completely as possible, and the burned gases could also be easily removed out of the capillary. The equipment specially designed for the fabrication of this coating and such processing parameters as the sol-dropping speed, the fluxes of the air and oxygen and temperature programs are also discussed in the paper. The results show that this method is not only an effective method for fabricating inner sol–gel coating in a capillary but also a good supplement for the dip-coating technique under some circumstances where a dip-coating method is limited to be used.