Freeze cast porous barium titanate for enhanced piezoelectric energy harvesting

Abstract

Energy harvesting is an important developing technology for a new generation of self-powered sensor networks. This paper demonstrates the significant improvement in the piezoelectric energy harvesting performance of barium titanate by forming highly aligned porosity using freeze casting. Firstly, a finite element model demonstrating the effect of pore morphology and angle with respect to poling field on the poling behaviour of porous ferroelectrics was developed. A second model was then developed to understand the influence of microstructure-property relationships on the poling behaviour of porous freeze cast ferroelectric materials and their resultant piezoelectric and energy harvesting properties. To compare with model predictions, porous barium titanate was fabricated using freeze casting to form highly aligned microstructures with excellent longitudinal piezoelectric strain coefficients, d33. The freeze cast barium titanate with 45 vol.% porosity had a d33 = 134.5 pC/N, which compared favourably to d33= 144.5 pC/N for dense barium titanate. The d33 coefficients of the freeze cast materials were also higher than materials with uniformly distributed spherical porosity due to improved poling of the aligned microstructures, as predicted by the models. Both model and experimental data indicated that introducing porosity provides a large reduction in the permittivity (𝜀 "" # ) of barium titanate, which leads to a substantial increase in energy harvesting figure of merit, 𝑑 "" % /𝜀 "" # , with a maximum of 3.79 pm 2 /N for barium titanate with 45 vol.% porosity, compared to only 1.40 pm 2 /N for dense barium titanate. Dense and porous barium titanate materials were then used to harvest energy from a mechanical excitation by rectification and storage of the piezoelectric charge on a capacitor. The porous barium titanate charged the capacitor to a voltage of 234 mV compared to 96 mV for the dense material, indicating a 2.4-fold increase that was similar to that predicted by the energy harvesting figures of merit.

Modified energy harvesting figures of merit for stress- and strain-driven piezoelectric systems

“…The effect of porosity on the effective piezoelectric strain coefficients, d 33 and d 31 , is shown in Figure 3a, and the effect of porosity on the compliance is shown in Figure 3b. Introducing porosity into a ferroelectric ceramic leads to inhomogeneous poling field distributions that result in ferroelectric regions in the vicinity of pores remaining unpoled [6,13,14], i.e. the net spontaneous polarisation after poling remains zero.…”

“…the output electrical power. As discussed in Section 1, porosity has been shown previously to be a useful tool for tuning the properties of piezoelectric ceramics to enhance the energy harvesting capabilities [5,6]. The following section draws on the analysis to this point in the context of porous piezoelectrics for energy harvesting to understand how the relevant properties (piezoelectric coefficient, permittivity and compliance) vary due to the introduction of porosity, and depending on the mode of harvesting, i.e.…”

“…Finite element modelling has been used to provide a dataset to demonstrate the theory discussed in Section 2. The model presented is a porous poling model, used previously to explain the behaviour of porous lead zirconate titanate (PZT) materials with equiaxed pores [13], porous sandwich layer barium titanate [14] and porous barium titanate with aligned, anisotropic pores formed via the freeze casting method [6]. This model takes into account the complex electric field distribution during poling, which is the process required to align ferroelectric domains to yield a spontaneous polarisation.…”

“…(19)). The stored mechanical and output electrical energy have been calculated using equations (5) and (6), and equation (19), respectively. Porous barium titanate with equiaxed, randomly distributed porosity will be the focus of this study, but the effect of porosity on both barium titanate and other ferroelectric ceramics, such as PZT, is comparable.…”

“…This concept is also relevant to the use of porosity to enhance the performance of piezoelectric materials, whereby pores are intentionally introduced into a piezoceramic in order to reduce the effective permittivity, whilst maintaining relatively high piezoelectric strain coefficients. This leads to increased longitudinal harvesting figures of merit, FoM X 33 , with increasing porosity [5,6]. The elastic compliance, which is used to determine k 2 , is difficult to measure for porous piezoceramics as quality factors (Q m ) decrease compared to dense piezoceramics [7].…”