Two-dimensional image characterization of powder mixing and its effects on the solid-state reactions

In this paper, the fundamental problem of powder mixing and its influences to the subsequent solid-state reactions were studied. A two-dimensional chessboard-like mixing efficiency evaluation was used to determine the mixing efficiency of powders. Mixings of two powder systems (Ti/SiO2 and C/SiO2) were studied using dry mixing (hand shaking or mechanical vibration) as well as wet mixing (with different dispersion media and stirring speed, with or without an additional ultrasonic treatment) methods. An optical microscope was used to obtain the images of mixed powders, and the powder images were analyzed by an image processing system to determine the mixing efficiency. From the experimental results, the dry mixing of C/SiO2 can reach a 0.82 mixing efficiency (the perfect powder mixture has a 1.0 mixing efficiency), while the Ti/SiO2 only had a value of 0.62. In wet mixing, the mixing efficiency of Ti/SiO2 was not affected by the dispersion medium. But, for C/SiO2 system the mixing efficiency could differ by a value of 0.26 when different dispersion medium was used. The increase of stirring speed can slightly raise the mixing efficiency. Additional ultrasonic treatment on powder suspension could either increase or decrease the mixing efficiency, depending on the powder system. Two important factors were found to be important in wet mixing of powders: density difference of powders, as well as the suspensibility of the powders in the dispersion medium. The reaction yield of the solid-state reaction increased with the mixing time of the reactant powders, and the reaction yield was decreased as the amount of reactant powders increased. Moreover, the reaction yield was roughly proportional to the mixing efficiency.