Near‐Net‐Shaped Porous Ceramics for Potential Sound Absorption Applications at High Temperatures

Abstract

We present an interesting processing route for obtaining alumina/mullite‐based ceramics with controlled porosity and airflow resistance leading to promising microstructures for application as sound absorbers. The use of ceramic materials aims for potential applications where high temperatures or corrosive atmospheres are predominant, e.g., in combustion chambers of gas turbines. For the production of the porous ceramics we combined freeze gelation and sacrificial templating processes to produce near‐net‐shaped parts with low shrinkage (<3%) based on environmental‐friendly and low cost conditions. The obtained microstructure presents a bimodal pore size distribution, with small pores derived from the freeze gelation process (~30 μm) connecting large pores (2–5 mm diameter) originated from the expanded polystyrene template particles. These connections, called “windows” in this study, show a significant impact on the sound absorption properties, allowing the pressure diffusion effect to take place, resulting in a significant improvement of the sound absorption coefficient. By varying the template particle content and the slurry solid content, it is possible to control the sound absorption behavior at different frequencies of the open‐celled ceramics. These ceramics feature a high open porosity, from 77% to 82%, combined with sufficient compressive strength ranging from 0.27 to 0.68 MPa and sound absorption coefficients of 0.30–0.99, representing a highly promising combination of properties for noise control and reduction at corrosive environments and high temperatures.

Open‐cell porous sound‐absorbing ceramics prepared by a new procedure that combines foamed and templated methods

“…Previous studies found that sound-absorption performance was determined by the apparent porosity. 8,31,32 An open cellular structure in porous medium caused irregular transmission routes for sound wave. 32 The diffuse reflection or refraction produced by the acoustic wave entering materials increased with increased apparent porosity, impelling the increment of the vibration of the hole wall and consuming sound energy.…”

“…From Figures 5 and 6, small difference of pore-size distribution between FAC was 0 and 1 wt%, but the latter one has high pore connectivity and better acoustic properties. 31,32 The increased numbers of various sizes and morphology open cells with the increment of FAC were beneficial to the absorption of sound waves under different frequencies, resulting in better sound-absorption performance and wider sound-absorption frequency band. The samples with FAC of 4 wt% had a smaller pore-size distribution compared with FAC of 5 wt% ( Figure 6).…”

“…This transformation decreased the ability of the samples to consume sound energy. 31,33 Although larger cavities also appeared at 4 wt% TAC, the effect of co-making open cell showed the best effort. As shown in Figures 8 and 9, there were many small size of open cells in larger cavities, preventing the reduction of the apparent porosity.…”

Thermal conductivity of ceramic/metal composites from preforms produced by freeze casting

“…This is the last calculation step where Eq. (6). is used again with the thermal conductivity obtained by second step calculation as input.…”

“…As examples, porous ceramic structures are used as filters for molten metal [1], water [2] or gas [3] and microfiltration by membranes, but also to process fuel cell electrodes [4], for thermal energy storage devices [5], and for sound [6] or thermal insulation [7]. All these applications are typically developed with isotropic porous materials.…”