This study is led by Dr. Yang Shen (State Key Lab of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University). The team report a highly stretchable/compressible piezoelectric composite composed of ferroelectric ceramic skeleton, elastomer matrix and relaxor ferroelectric-based hybrid at the ceramic/matrix interface as dielectric transition layers, exhibiting a giant piezoelectric coefficient of 250 picometers per volt,high electromechanical coupling factor keff of 65% and highcyclic stability under 50% compression strain. The superior flexibility and piezoelectric properties are attributed to the electric polarization and mechanical load transfer paths formed by the ceramic skeleton, and the dielectric mismatch mitigation between ceramic fillers and elastomer matrix by the dielectric transition layer. The synergistic fusion of ultrahigh piezoelectric properties and superior flexibility in these polymer composites is expected to drive emerging applications in flexible smart electronics.
Piezoelectric composites have been pioneered by Newnham et.al. in the early 1970s, where piezoelectric ceramic fillers have been introduced in the form of spheres or fibers into the polymer matrix to fabricate '0-3' composite ('0'means the ceramic filler is not connected in a continuous path in any dimension throughout the polymer matrix, while '3' means the polymer matrix is connected in all the 3 dimensions. The large difference in dielectric constant between ferroelectric ceramic and polymer leads to a discontinuity in electric polarization at the filler/matrix interfaces and forms an electric double-layer that shields the dielectric response of ceramic fillers. The dielectric difference also leads to severe concentration of the local electric field in polymer matrix hence substantially reduces the local electric field in the ferroelectric ceramic filler, thereby compromising the overall piezoelectric response of the composites. Besides, the large difference in elastic modulus between ceramic and polymer also results in the concentration of local stress field in the polymer matrix. Therefore, the mechanical load may not be efficiently transferred to ceramic filler to induce high piezoelectric response.
The team from Tsinghua university proposed a combination approach to simultaneously address both dielectric and mechanical mismatch issues and achieve stretchable polymer composites with giant piezoelectric properties. Researchers first assemble PZT particles into an interconnected skeleton inside the polydimethylsiloxane (PDMS) polymer matrix to form '3-3' type composites, in which both PZT and PDMS form global networks. We then used a thin relaxor ferroelectric polymer layer, polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene (P(VDF-TrFE-CFE), terpolymer) and carbon nanotubes (CNT), to modulate the local electric field distribution in the PZT skeletons (denoted as '3-3-3' and CNT@'3-3-3' composites). Researchers demonstrate that this combination approach gives rise to a record high piezoelectric property in stretchable polymer composites. In addition, the composites exhibit a very low acoustic impedance of ~3 MRayl, one order of magnitude lower than that of PZT ceramic, being beneficial to the design of ultrasonic transducer considering the complex acoustic impedance matching layers in ceramic-based transducers.
