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Degassing unit is a crucial step after mixing (sometimes degassing is also required after casting) to eliminate residual pores in the slurry. These pores can be introduced during either mixing or the chemical reaction, or they can form as a result of entrapped air during casting. Another detrimental effect of entrapped air on gelcasting is its inhibition of the free radical polymerization reaction. Figure 5 shows the significance of degassing in reducing the flaw size of gelcast ceramics. The degassing time needs to be precise; it cannot be either too short or too long. If the degassing time is too short, it will not be adequate to remove the pores, whereas too long a degassing time may cause unwanted evaporation of liquid and a change in slurry rheology.
Ultrasonic degassing has been tried in many fields of application with demonstrated advantages, although it is not currently the mainstream degassing technology in any of them. Ultrasonic degassing of optical glass was one of the first successful applications. To overcome the problem of sonotrode material selection, Eckardt and Eden (1952) used a high-frequency induction furnace with permanent magnets to induce contactless vibration in the melt. Degassing with the aid of ultrasound has been reported for extracting gases from hydrocarbons, oils, and resins, in which case ultrasound application accelerates the process of degassing (Kapustina, 1970). The food industry is one potential niche for ultrasonic degassing; successful trials have been performed for degassing juices, beer, chocolate, and vegetable oils. Removal of oxygen improves the quality and preservation of foods (Kapustina, 1970), whereas removal of carbon dioxide accelerates fermentation (Patist and Bates, 2008). Purification of water from heavy metals (mercury ions) assisted by ultrasonic degassing was confirmed in laboratory experiments (Gill et al., 2008). Acoustic degassing can be applied in materials production. For example, the quality of electrochemical plating can be substantially improved by removing gas products of interfacial reactions with acoustic streaming, accelerating the deposition (Kapustina, 1970). Liquid scintillators widely used in radiochemistry can benefit from hydrogen and oxygen removal without the loss of radioactive components (Chleck and Ziegler, 1957; Danielsson and Greber, 1993). There are also some possible applications in medicine, for example, ultrasonic degassing of dialysis solution may open the way to miniaturization of delivery systems (Yang et al., 2002).
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