Compared with dense ceramics, porous ceramics exhibit lower density and acoustic impedance, as well as excellent thermal resistance, enabling their broader application in hydrophones, high-temperature ultrasonic transducer backing layers, biopiezoelectric devices, and related fields. However, conventional fabrication methods for porous ceramics, such as freeze-casting and gelcasting, are often associated with complex processing procedures. The digital light processing (DLP) additive manufacturing technique offers several advantages, i.e., high resolution, rapid curing, uniform surface exposure, and low production cost, making it a promising alternative for fabricating complex porous ceramic structures. Lead-free barium titanate piezoelectric ceramics have attracted much attention due to their favorable piezoelectric performance. In this study, porous barium titanate ceramics with varying porosity levels were fabricated via DLP 3D printing by adjusting the content of polymethyl methacrylate (PMMA) from 0 to 20%. The influence of porosity on the phase structure, microstructure, and electrical properties of the ceramics was systematically investigated.

A ceramic slurry containing 82% barium titanate (BTO), composed of BTO powder (500 nm), photosensitive resin, and dispersant, was mixed with PMMA microspheres (15 μm) at different mass fractions of 5%, 10%, 15% and 20%, respectively. The mixture was homogenized at 2000 r/min by a homogenizer (ZYE, ZYMC-180V) for 2 min. Subsequently, the designed 3D model in STL format was imported into the control computer for DLP printing. The printed samples had dimensions of Ф12mm×2 mm, with a layer thickness of 50 μm. After printing, the green bodies were subjected to a two-step debinding process (i.e., firstly under vacuum and then in an air atmosphere) and then sintered. The sintered samples were ground, polished, and coated with gold electrodes. The poling was performed in an electric field of 3 kV/mm in silicone oil at 60 ℃ for 15 min. The microstructure of the samples was determined by field emission scanning electron microscopy (LYRA 3 XMU, Tescan Co., CZ). The phase composition of the BTO ceramics was determined by X-ray diffraction (X'Pert PRO, PANalytical B.V., the Netherlands). The sample density was determined based on the Archimedes principle. The dielectric properties were measured by precision impedance analysis (6500B, Wayne Kerr Co., UK), while the piezoelectric coefficient (d₃₃) was evaluated by a d₃₃ meter (ZJ-3A, Institute of Acoustics, Chinese Academy of Sciences, China).

The results show that the porosity of barium titanate ceramics gradually increases as the PMMA content increases from 0 to 20%. The porous ceramics exhibit closed, spherical pores, with pore sizes closely matching the particle size of the employed PMMA (approximately 15 μm). The axial shrinkage ratio of the ceramics increases from 11% to 29%, and the radial shrinkage increases from 14% to 26%. This anisotropic shrinkage behavior is attributed to the layer-by-layer printing process, resulting in a greater axial shrinkage relative to the radial direction. Besides, after the introduction of the air phase, the bulk density decreases from 5.7 g/cm³ to 4.5 g/cm³, the porosity increases from 6.0% to 25.7%, and the acoustic impedance reduces from 23.12 MRayls to 15.25 MRayls. Regarding the electrical properties, the piezoelectric coefficient d₃₃ gradually decreases from 223 pC/N to 135 pC/N, and the dielectric constant ε_r declines from 1255 to 407. This is primarily attributed to the reduction in the effective volume fraction of the piezoelectric phase caused via increasing porosity, as well as the non-uniform electric field distribution within the material, which impairs a polarization efficiency. Meanwhile, the dielectric loss tanδ remains in the range of 0.02-0.04, exhibiting minor fluctuations with an increasing porosity and maintaining a low value.

The effect of PMMA content on the microstructure and electrical properties of BTO ceramics was investigated. The piezoelectric constant and dielectric constant of porous barium titanate ceramics both decreased, while the dielectric loss remained a low range as the PMMA content increased, indicating a good stability. An increase in porosity reduced the bulk density of the BTO ceramics, thereby lowering the acoustic impedance and enhancing acoustic impedance matching. DLP-based 3D printing, by combining high precision with rapid prototyping capability, could provide an efficient and cost-effective route for the fabrication of porous BTO ceramics in acoustic device applications.