Preparation and properties of porous Al ₅ BO ₉ for high‐temperature wave‐transparent and thermal insulating applications

Abstract Porous Al ₅ BO ₉ is a promising high‐temperature wave‐transparent material. However, method for the preparation of this material is not readily available. Herein, porous Al ₅ BO ₉ ceramics with controlled porosity and small volume shrinkage are successfully prepared by using Al ₂ O ₃ and B ₂ O ₃ as starting materials without pore formers. The SEM and pore size distribution studies show that the as‐prepared porous Al ₅ BO ₉ ceramics exhibit a uniform pore structure and a narrow pore size distribution. Intriguingly, simply adjusting the densities of the green bodies, the density and porosity of the porous Al ₅ BO ₉ ceramics can be controlled. The pore‐forming mechanism is presumed to be a combination of boron oxide volatilization during the high‐temperature synthesis and lap of elongated grains. Porous Al ₅ BO ₉ ceramics have good high‐temperature stability, which can maintain dimensional and composition stability up to 1673 K. The compressive strength can reach 211 MPa at 32.4% porosity and the dielectric constant can be as low as 3.02 at 43.2% porosity. In addition, the dielectric constant and loss tangent keep almost unchanged with temperature.