Superplastically foaming method for inclusion of closed pores in fully densified ceramics

Abstract

Porous ceramics incorporate pores to improve material properties, including thermal insulation, while maintaining inherent ceramic properties such as corrosion resistance and high mechanical strength. Conventional porous ceramics are usually fabricated by insufficient sintering that excludes pores; thus, it must be terminated in an early stage to maintain high porosity. The premature termination leads to degraded strength and durability. We developed a superplastic-foaming method to fabricate ceramic foams in the solid state. In this method, the inserted foam agent evaporates after full densification of the matrix at the sintering temperature. Closed pores expand by superplastic deformation driven by the gas pressure. The pores are introduced after sintering the solid polycrystal. Then, only closed pores are introduced, improving the insulation of gas, sound and heat. The pore walls are fully densified for high mechanical strength. Compared to the melt-foaming method, this technique is practical because the fabrication temperature is far below the material's melting point, and it does not require moulds. In addition, the size and location of the pores can be controlled by the amount and position of the foam agent.

Closed porosity ceramics and glasses

“…In this simple method, few percent (generally below 10 wt%) of foaming agent was coated or mixed with the target composition, pressed and sintered for extended periods (eg, 8 hours) at elevated temperatures. While the process does not yield with highly porous samples (ϕ T was generally below 40%), yttria‐stabilized zirconia (3YSZ and 8YSZ), alumina (with superplasticity‐facilitating dispersoids; 3YSZ, 0YZ or spinel) and titania‐based closed porosity ceramics were produced . Similarly, recycled materials (eg, granite scrap and fired clay tailings) were used with SiC to cause gas generation and form closed porosity ceramic foams .…”

“…While the process does not yield with highly porous samples (ϕ T was generally below 40%), yttria-stabilized zirconia (3YSZ and 8YSZ), alumina (with superplasticity-facilitating dispersoids; 3YSZ, 0YZ or spinel) and titania-based closed porosity ceramics were produced. [72][73][74][75][76][77][78][79][80] Similarly, recycled materials (eg, granite scrap and fired clay tailings) were used with SiC to cause gas generation and form closed porosity ceramic foams. 81 In a different technique, closed porosity aluminum oxynitride (AlON) (ϕ C = 80%) foams were made by combustion synthesis in between 2165°C and 2650°C for a very short time (10 seconds).…”

Optical interferometry - the sharpest tool in the box

“… Size–luminosity relationship for the continuum emission from AGN torii (interferometric measurements are from Kishimoto et al 2011). The K‐band sizes scale with L ½ .…”

Effect of K2SO4 additions on properties of porous fibrous alumina ceramics prepared by DCC and lost‐mold method

“…Alumina fiber, as a new type of inorganic material and a significant candidate for porous ceramics, has attracted increasing interest due to their excellent mechanical behavior under high temperature, good resistance to chemical attack, low thermal conductivity, etc . There are many traditional preparation methods applied to form pores in ceramic components, such as freeze drying, sol‐gel method, and foaming method . However, the development of these methods is greatly obstructed in modern industry for the shape limitation, low performance, pollution issues, etc.…”

“…1,2 There are many traditional preparation methods applied to form pores in ceramic components, such as freeze drying, 3,4 sol-gel method, 5,6 and foaming method. 7,8 However, the development of these methods is greatly obstructed in modern industry for the shape limitation, low performance, pollution issues, etc. Gel-casting is a well-established colloidal processing technology which has been used to prepare complex-shaped ceramics with high performance and proven to be a promising method to fabricate porous ceramics.…”