Energy Storage and Conversion investigations in ferroelectric / antiferroelectric materials

Abstract

With the ever-increasing global demand for new energy and modern technology, the investigations in advanced ferroelectrics (FE) and antiferroelectric (AFE) ceramics is getting more and more urgent. The phase structure and phase transition behaviors of FE and AFE materials can be influenced by various external stimuli, such as temperature and pressure, and thus offer great possibilities for different applications. With this bearing in mind, several energy related effects in FE/AFE materials are studied in this thesis, including the pyroelectric effect, the explosive energy conversion effect, and the electrocaloric effect. The studies will offer more understanding about FE/AFE materials and pave the way for speeding up the development of FE/AFE materials for real energy harvesting applications. Firstly, the pyroelectric effect of 0.97(0.99Bi0.5Na0.5TiO3-0.01BiAlO3)-0.03K0.5Na0.5NbO3 (BNT-BA-KNN) is investigated. The depolarization temperature of the BNT-BA-KNN ceramics is 118 oC. At room temperature, a large pyroelectric coefficient (p ~3.7*10-8Ccm-2K-1) is achieved. Moreover, the Figure of merits Fi, Fv, and Fd are determined as high as 1.32*10-10m/V, 2.89*10-2m2/C, 1.15*10-5Pa-1/2 at 1 kHz. The temperature dependent study suggests an excellent temperature stability of p and figures of merits during RT~85oC. The high pyroelectric properties and excellent thermal stability of BNT-BA-KNN reveal its promising potential as a lead-free candidate for infrared temperature detector materials. Secondly, the explosive energy conversion effect is studied in the (Ag1-xKx)NbO3 FE materials. Optimized (Ag0.935K0.065)NbO3 (AKN) is demonstrated to show the best explosive energy conversion performance, being able to display a FE-AFE phase transition under a low hydrostatic pressure of 350 MPa. The AKN ceramics also possess a stable high polarization from room temperature to 150 oC. A device has been designed and fabricated and the shock wave experiment shows that the AKN possesses a record-high energy storage density of 5.401 J/g, and enable a pulse current of 22 A within 1.8 microseconds, which is better than that of the commercially employed Pb(Zr,Ti)O3. A detailed study on the structural transition has been conducted using the TEM and in-situ neutron diffraction technique. With increasing hydrostatic pressure, the disappearance of 1/2(301)p, 1/2(321)p and 1/2(341)p reflections and appearance of 1/4(443)p and 1/4(229)p further confirms the pressure driven FE-AFE phase transition. A phenomenological theory is also employed to rationalize the pressure driven FE-AFE phase transition. The explosive energy conversion effect has also been demonstrated in NaNbO3 (NN) ceramics. With increasing pressure to 450 MPa, a sharp FE-AFE transition and depolarization behavior can be observed for NN ceramics. The FE-AFE phase transition was further supported structurally via in-situ neutron diffraction measurement. The polarization of NN can keep stable (~30 uC/cm2) over a wide temperature range of 20-180oC, which adds to its suitability for explosive energy conversion application. The investigation results of AKN and NN under pressure will serve as a guidance for further development of new FE materials and devices for energy conversion application. Finally, the electrocaloric effect of a compositionally modulated (Pb0.97-xLa0.02Bax)(Zr0.58Sn0.29Ti0.13)O3 (PLZST) (x=0, 0.08, 0.09, 0.11) ceramics is investigated. With increasing Ba doping level, the sharp FE-AFE-paraelectric (PE) phase transition sequence with increasing temperature is gradually immerged into a diffused FE-PE phase transition. Moreover, the Curie temperature decreases down to around room temperature. As a result, a large electrocaloric effect (1.2 K) and wide electrocaloric temperature span (112 K), are simultaneously achieved in PLZST with x=0.11. This work provides a novel method to simultaneously realize high electrocaloric effect and large temperature span.