Porous ceramics have been used widely in many areas of industry involving catalyst carriers, membrane supports, filters, and biomaterials.^1-3) For these applications, many excellent properties including high porosity and good mechanical strength are typically required.³⁾ Generally, higher porosity tends to result in a certain decrease in mechanical strength.⁴⁾ Thus, it is important to improve the mechanical strength of porous ceramics with higher porosity, and many studies have been carried out to investigate the relationship between porosity and mechanical strength of porous ceramics.^5-9) For example, Chang⁷⁾ reported that fine Al₂O₃ grains with a lower degree of agglomeration could increase the bending strength of porous Al₂O₃ ceramics with a slight increase in porosity. Liu⁸⁾ found that pores smaller than 2 μm were helpful for improving strength whereas pores larger than 14 μm had the opposite effect. Tang⁹⁾ developed a routine including directional freeze casting and annealing in a tertiary butyl alcohol (TBA)-water-sucrose system that could significantly improve the compressive strength of porous alumina ceramics. However, until now, the effects of Al(H₂PO₄)₃ addition on the porosity and flexural strength of porous Al₂O₃ ceramics had not been investigated in detail. In this work, Al(H₂PO₄)₃ was used as an additive to fabricate porous Al₂O₃ ceramics, and the effects of Al(H₂PO₄)₃ addition on the porosity and flexural strength of the ceramics were studied extensively.

Al₂O₃ powders with purity > 99.0 wt% were used in this work. The average particle size of these powders was about 0.9 μm. The Al₂O₃ powders were mixed with 0.5 wt% polyvinyl alcohol (PVA) solution (concentration of 5 wt%) and chemically pure Al(H₂PO₄)₃ with additions of 0, 5, 10, 15, and 20 wt%. Then, green rectangular bodies of 5.0 mm × 10.0 mm × 50.0 mm were bidirectionally pressed under the pressure of 35 MPa. After drying at 50°C for 10 h, the green bodies were heated to1300°C, 1350°C, and 1400°C at a heating rate of 5°C/min and then soaked for 2 h in air.

The water absorption and open porosity of the as-prepared Al₂O₃ ceramics were measured using Archimedes principle. Ignition loss (IL) was calculated by measuring the weight of the bodies before and after sintering. Flexural strength was tested via a three-point bending test (ATOGRAPH AG-I, Shimadzu) with a support distance of 30.0 mm and cross-head speed of 0.50 mm/min. The crystal phase compositions of the sintered Al₂O₃ ceramics were determined with an X-ray diffractometer (XRD; D8 Advance, Brucker, Germany) using a Ni-filtered Cu Kα radiation source (λ = 0.154178 nm). The microstructures of fracture surfaces of the sintered specimens were characterized with a field emission scanning electron microscope (FESEM; Sirion 2000, FEI, Netherlands). Pore size distribution was determined by mercury porosimetry (AutoPore IV 9510, Micromeritics, USA).

Porous Al₂O₃ ceramics were successfully fabricated with 0, 5, 10, and 20 wt% Al(H₂PO₄)₃ additions. The open porosity and average pore size of the as-sintered porous ceramics increased remarkably with an increase in Al(H₂PO₄)₃ addition. The flexural strength of the porous ceramics first increased to a peak value when 5 wt% Al(H₂PO₄)₃ was added and then decreased as additional Al(H₂PO₄)₃ was added. The average Al₂O₃ grain size of the porous ceramics changed in an opposite way as the flexural strength. It should be noted that the ceramics with 10 wt% Al(H₂PO₄)₃ addition exhibited flexural strength comparable to that of the ceramics without Al(H₂PO₄)₃ addition, although the latter had much higher porosity.