1-3 type piezoelectric composites are widely used to fabricate high-sensitivity and broadband ultrasonic transducers due to the advantages like a low dielectric constant, a low acoustic impedance, and a high electromechanical coupling coefficient. However, some conventional manufacturing methods such as the cut-and-fill method, the arrangement-casting process, and the injection molding method have challenges to emanate from geometric complexity and structural integrity. Stereolithography technology based on the mask projection approach is an alternative method for manufacturing piezoelectric ceramics with a rapid prototyping and a high precision. Lead-free barium titanate BaTiO₃ (BTO) ceramics are widely investigated for piezoelectric sensors and dielectric capacitors applications because of their high dielectric constant, good piezoelectric coefficient, and favorable electromechanical coupling coefficient. In this work, digital light processing (DLP) technology was used to manufacture barium titanate based 1-3 type piezoelectric composites. The influence of piezoelectric phase volume fraction (i.e., 20%-45%) on the micrograph, acoustic impedance and electrical properties of BTO 1-3 composites was investigated.

The printed ceramic slurries with 84% (in mass fraction) BTO content, including BTO powders (d₅₀=500 nm), dispersant, and liquid photopolymers, were mixed by a model ZYMC-180V homogenizer (ZYE) at 2 000 r/min for 90 s. The printed models were built by a software named Computer-Aided Design (CAD) and sliced to a thickness of 7 μm. The piezoelectric pillars (150 μm) with a support were printed layer by layer to form the physical models (i.e., Soon solid, TC-Ⅱ). The printed samples were washed in alcohol by aultrasonic cleaner to remove the residual slurry. Afterwards, the green bodies were treated by a two-step heating treatment, i.e., a debonding process and a sintering process, to obtain the piezoelectric phase structures. Two-time vacuum treatments below -100 kPa were introduced to remove air bubbles in epoxy resin (EPO-TEK301) that filled in the kerfs between piezoelectric pillars (width of 120 μm, height of 330 μm). The samples filled with epoxy resin were heat treated at 60 ℃ for 12 h and then coated with gold electrodes. The BTO 1-3 composites were polarized in an electric field of 4 kV/mm in silicone oil. The microstructures of samples were characterized by a model TM4 000 Plus scanning electron microscope (Hitachi Co., Lted., Japan). The dielectric constant ε_r, dielectric loss δ, and frequency impedance spectrum were measured by a model 6500B precision impedance analyzer (Wayne Kerr Co., Ltd., UK). Piezoelectric constant d₃₃ was determined by a model ZJ-3A piezo-d₃₃ meter (Institute of Acoustics, Chinese Academy of Sciences, China). The ferroelectric hysteresis loop was measured by a model 2000E ferroelectric analyzer (aixACCT Co., Germany).

The uniform piezoelectric pillars with a width of (150±10) μm are printed through optimizing the slurry formula, and the minimum distance between pillars is approximately 100 μm. The bending deformation of BTO pillars is negligible, and the lateral shrinkage rate is 20%. The forming mechanism of DLP technology results in a clear lamination of the piezoelectric pillar along the printing direction. The shrinkage of ceramics and the growth of grains fill the gap between interlayers, thus reducing the lamination after sintering. There are no residual bubbles in the epoxy resin after vacuum treatment. The density and acoustic impedance of the piezoelectric composites increase with the growth of the piezoelectric phase volume proportion. Compared with the acoustic impedance of BTO ceramics (i.e., Z=31.64 MRayl, 1Rayl=1 Pa·s/m=10^-6 MRayl), the acoustic impedance of composites (45%, in volume fraction) is decreased by 67%.

The piezoelectric constant d₃₃ and electromechanical coupling coefficient k_t enhance with the piezoelectric phase volume proportion of the composites, meeting their maximum values of 74 pC/N and 0.48, respectively. Compared with the BTO ceramics (k_t=0.37), the electromechanical coupling coefficient is improved by 30%. According to the frequency impedance spectra of piezoelectric composites, 1-3 type composites have a pure thickness vibration mode. The dielectric constant ε_r increases linearly with increasing the piezoelectric phase volume fraction, reaching its maximum of 1 167. The dielectric loss tanδ is lower than 3%, which can enhance the receiving sensitivity of ultrasound transducers. The residual polarization intensity increases with the increase of volume fraction of the piezoelectric phase. The residual polarization intensity of the piezoelectric composite with a volume fraction of 45% reaches a maximum of 1.77 μC/cm² at 4 kV/mm.

1-3 type barium titanate/epoxy resin piezoelectric composites were fabricated by the DLP technology. The effect of volume fraction of piezoelectric phases on the electrical performances of composites was investigated. The electrical properties such as piezoelectric constant d₃₃, electromechanical coupling coefficient k_t, and dielectric constant ε_r were optimized as the volume fraction of the piezoelectric phase (≤45%) was increased. The polymer phase could effectively reduced the acoustic impedance of piezoelectric materials, thus obtaining an acoustic matching between the piezoelectric materials and tissues or water. Compared to pure barium titanate ceramics, 1-3 type composites as a material of ultrasound transducers could effectively suppress the lateral vibration of the material and enhance the thickness vibration mode, which could improve the pulse echo response. The DLP technology with a high resolution provides an effective and low-cost method for fabricating the piezoelectric composites.