Giant pyroelectric effect for energy conversion via tuning lattice compatibility

Searching for performant pyroelectric materials is gathering research interest for its potential in clean electricity generation and waste heat harvesting. Here we report achieving the giant pyroelectric effect in materials undergoing a first-order phase transformation with high reversibility by tuning the crystallographic compatibility. The single crystalline Ba0.95Ca0. 05Ti0.99Zr0. 005Ce0.005O3 shows a pyroelectric coefficient of 1.93 μC/(cm2K), substantially higher than that of most of the state-ofthe-art pyroelectric generators by orders of magnitude. We demonstrate that it lights a LED directly without attaching an external power source and consistently generates about 6μA/cm2 current density by small thermal fluctuations near 100◦C over 540 complete phase transformation cycles with no degradation. This promising material candidate brings pyroelectric energy conversion closer to a practical realization. Electrically polarized crystals naturally exhibit the pyroelectric effect: a change in temperature causes a change in its spontaneous electrical polarization. The earliest documented observation of pyroelectric effect in stones such as tourmaline can date back to ancient Greece. [1] Since the 1960s, pyroelectric materials have attracted profound attention due to their suitability for thermal sensing and thermal imaging [2, 3]. Later, some theoretical models were proposed by Olsen, Evans and Drummond to explore the heat-to-electricity conversion as utilizing these materials as the dielectric layer in capacitors, called dielectric power converter (DPC) [4–6]. Their work underlies the thermodynamics of energy conversion by pyroelectric effect. This idea was further optimized and demonstrated in relaxor ferroelectrics. [7, 8] As the nano-fabrication techniques flourished, pyroelectric energy conversion designs based on Olsen cycles have been revitalized in thin film devices. [9–12] One of the obstacles preventing pyroelectric energy conversion from broad commercialization is that the pyroelectric effect is weak in common relaxor thin 1 ar X iv :2 10 5. 07 20 1v 1 [ co nd -m at . m tr lsc i] 1 5 M ay 2 02 1 films, e.g. 0.055 μC/(cm K) in PMN–0.32PT at room temperature [12]. Several attempts, such as actively sweeping the bias electric field between two values far apart [6, 12], have been made to circumvent this weakness. Without connection to external bias field, the pyroelectric current generation becomes rather marginal, only at several nano ampere scale. [10–14] Recent advances in innovative thermal management [12, 15] and exquisite nano-devices [10, 13, 14] have pushed the device performance towards its current limit. This field urgently calls for new materials to intrinsically improve the multiferroic-thermal response. Core material development strategies Pyroelectric effect is characterized by the pyroelectric coefficient defined as dP/dT , where P is the polarization and T is the temperature. The foremost task of material development for pyroelectric energy conversion is to search for a material having a large pyroelectric coefficient. In the temperature vicinity of a phase transformation, dP/dT could potentially reach its maximum value. For example, the pyroelectric coefficient of single crystal BaTiO3 during its first-order phase transformation is ∆Pr ∆T ≈ 1μC/cm K, as calculated from experimental measurements [16]. That is about twice of the highest dP/dT reported in lead-based relaxor PMN-0.32PT single crystal along [111] polar axis at 180 ◦C [17], and 20 times larger than relaxor ferroelectric thin films[12]. Utilizing this fact, people has indeed demonstrated promising energy conversion performance using single crystal BaTiO3 by a small temperature fluctuation near the first-order phase transformation. [18] Reserved energies stored as small thermal fluctuations are abundant and rapidly growing in nature and from human activities. These rich thermal energies are mainly around 100◦C. Hence, finding the ferroelectric material undergoing first-order phase transformation at a proper temperature with a giant pyroelectric coefficient is a core strategy of material development. From energetic analysis, the cost of pyroelectric energy conversion is the heat in the heating half cycle or the work required to cool the active material in the cooling half cycle, while the return is the change of polarization stored as the electrostatic energy. The ratio (1/c)(dP/dT ) where c is the heat capacity at a fixed bias field, is a natural choice of figure of merit, which is in fact equivalent to one of the proposed figures of merit in this research field. [19, 20] Across a phase transformation, the effective heat capacity integrated over temperature is the latent heat `, while the integrated dP/dT is the jump of polarization [[P ]]. Based on thermodynamic analysis of a new energy conversion design without application of a bias field [21], a new figure of merit was proposed to consider the conversion in a temperature range covering a complete loop of phase transformation. That is, FOM = κ[[P ]] ` , (1)