spherical silica powder is a highly crystalline, hard and chemically inert material that has many applications in the microelectronics industry. Its spherical shape makes it suitable for use as a filler in semiconductor encapsulants, where it provides good arc insulation and UV radiation resistance and enhances the packaging process.
Methods of producing spherical fused silica powder are known in the art, wherein starting silica powder is fed to a burner flame formed by combustion of oxygen with a flammable gas such as propane. The heat of combustion imparted by the flame is used to melt the powder, thereby spheroidizing it. The flame must have sufficient thermal power to raise the temperature of the silica powder to a melting point or higher and also must have a long enough flame length to uniformly melt silica powder having a broad particle size distribution.
In order to achieve this, an auxiliary flame containing a large heat of combustion is used to increase the flame temperature and lengthen the flame. The auxiliary flame may be fed to the burner as a liquid or as a superheated vapor, depending on the desired production method.
The auxiliary flame may be fed to the combustion burner from a same burner as that used to feed the starting silica powder or from a different burner. The auxiliary flame is fed at a rate of 4.4 Nm 3/h for example to achieve the same heat of combustion as the propane gas and oxygen gas fed to the burner in Example 1.
A further advantage of using a large heat of combustion in a composite flame of the fuel gas combustion flame and the siloxane or alkoxysilane auxiliary flame is that the fine particles of starting silica powder that form as a result of oxidative combustion coalesce with the molten silica powder and are ultimately recovered as part of the final powder product. This increases the speed at which the spherical silica powder is produced and reduces the amount of flammable gas required for the production.