Factors governing the particle size of silicon carbide precursor gel in the preparation of monosized spherical precursor gel powders through the sol-gel method and the thermal behavior of these powders were described.
In the preparation of gel powders, the size and morphology were controlled by hydrolysis and condensation of phenyltrimethoxysilane (PTMS) or a mixture of PTMS and tetramethylorthosilicate(TMOS) in $NH_4OH$ solution.
The experimental procedures were conducted through two routes; Base-Catalyzed and Acid-Base Catalyzed Route. The Base-Catalyzed Route consisted of 1 step process in which hydrolysis, condensation, and gellation reactions were proceeded consecutively in a $NH_4OH$ solution. Acid-Base Catalyzed Route consisted of 2 step process in which prehydrolysis was first conducted in acidic solution and then condensation and gellation were occurred in $NH_4OH$ solution. In this work, two kinds of $NH_4OH$ solutions were used; one was ${H_2O}-{NH_4OH}$ solution with varing $NH_4OH$ content to define the effect of catalyst and the other was ${H_2O}-{MeOH}-{NH_4OH}$ solution with varing volume ratio of MeOH to $H_2O$ to define the effect of dielectric constant.
The factors effecting on the particle size and morphology prepared from this work were summarized as follows;
the effect of starting material
the volume ratio of MeOH to $H_2O$(=the effect of dielectric constant)
the effect of prehydrolysis
the effect of $N_4OH$ content
and the roles of these factors were described below.
The growth behavior of powders coincided with that of nucleation and agglomeration theory which was suggested by Bogushand Zukoski and the phenomena occurred in this work could be explained by DLVO theory which defines the relation between the electrostatic repulsion force and van der Waals attraction force.
The roles of starting material and the volume ratio of MeOH to $H_2O$ could be explained by energy barrier which is a function of particle size, dielectric constant, and surface potential.
The particles derived from PTMS were easily grown to $\mu$m size but those derived from PTMS-TMOS were 0.1 $\mu$m. This phenomenon was attributed to the difference of surface potential between the particles of PTMS and PTMS-TMOS, in which the number of surface phenyl group were affected by existence of TMOS. As the volume ratio of MeOH to $H_2O$ increased, the particle size increased because of dielectric constant decrease.
The particles produced without prehydrolysis showed monosized spherical shape. But by treating prehydrolysis the particle size distribution went broad. This was due to the rapid agglomeration and short period of nuclei supply.
As the content of $NH_4OH$ increased, the particle size decreased in the Base-Catalyzed Route because of increase of number of nuclei produced and particle size and distribution decreased in Acid-Base Catalyzed Route because of enhanced reverse reaction of alcoholysis which producing unreacted monomer.
The powders heat-treated at 1500℃ for 2h. exhibited well crystallized β-SiC phase, but those at 1400℃ were still amorphous phase. When heat-treated, the powders derived from PTMS underwent redistribution and the size and shape differences which were occurred during gel preparation were disappeared. But the powders derived from PTMS-TMOS sustained their size and shape. These phenomena were considered as soft agglomeration between the nucleis in PTMS derived powders by insufficiency of hydroxyl group.
