Adjustable giant negative electrocaloric effect in Pb 1+x ZrO 3 thin films

Electrocaloric effect (ECE) driven by electric field is suitable for implementation of built-in cooling in electronic devices. However, most of the known electrocaloric materials show low adiabatic temperature change ( ∆T ) near room temperature and usually require high electric field. Here, the investigation of ECE in Pb 1+x ZrO 3 (x=0, 0.1, 0.15) thin films, which were prepared on Pt/Ti/SiO 2 /Si substrates by sol-gel method, reveals that both the magnitude and the present temperature range of ∆T can be controlled by Pb concentration. Through increasing the dosage of PbO, decreased lead vacancies and enhanced interface layer are induced, which postpone the transition from antiferroelectrics to ferroelectrics of PbZrO 3 films under a given electric field ( E ), which thus controls the appearance temperature range of negative ECE. As a result, large isothermal entropy change ( ∆S) and 𝛥T are observed in the temperature range from 260 K to 494 K, depending on the applied electric field and Pb concentration. Giant ECE ( ∆T~ − 24.9 K , ∆𝑇 ∆𝐸 ~0.054) at room temperature (303 K) is obtained in Pb 1.1 ZrO 3 films under 460 kV/cm. This result provides a convenient method for modulating ECE of PbZrO 3 -based materials and will benefit its applications in cooling devices.


Introduction
The electrocaloric effect (ECE) refers to the reversible thermal change of a polar material under applying/removing an external electric field, where the change of polarization by electric field causes adiabatic temperature change (∆T) and isothermal entropy change (∆S) [1,2]. The ECE refrigeration has been considered as a promising alternative in modern refrigeration industry. Compared to the traditional vapor compression cooling technology, electrocaloric solid-state cooling has drawn much attention because of its high energy conversion efficiency, environmental-friendly process, low-cost, and small volume [3][4][5]. The ECE is characterized by Δ under external electric fields, where the positive ΔT is classified as the positive ECE [1,6] and the reverse belongs to the negative ECE [7,8]. The coexistence of positive and negative ECEs in one cooling cycle can effectively improve the coefficient of performance and benefit the application in ECE devices.
PbZrO3 (PZO) is a typical antiferroelectric (AFE) material, which possesses positive ECE around the phase transition temperature (Tc) from AFE to paraelectric (PE) phase and negative ECE at the transition from AFE to ferroelectric (FE) under the applied electric field [9,10]. The large positive ΔT can be achieved around Tc and usually needs high measuring electric field, while the negative ECE, which is considered to originate from the noncollinearity between the applied electric field and ferroelectric dipoles, can be obtained at various temperatures under a moderate field [11,12]. Therefore, the negative ECE of PZO not only relates to the measured temperature and electric field, but also can be controlled by its structure, chemical constituent, interface, defects, etc. Li [16]. Large ΔT of about -10 K was obtained in 1.0 mol% Yb doped and Nb and Sn co-doped PZO thin films [17,18]. In addition, cation nonstoichiometry shows a significant effect on the evolution of material properties, especially for the perovskite systems where the polarization and lattice modes are strongly coupled [19,20]. For PZO, the excess or deficiency of Pb not only affects the defect types but also the amount of antisite defects, where lead ions enter into the site of zirconium ions [21,22]. As a result, the antiferroelectrics of PZO can be obviously influenced by Pb concentration, which thus controls the temperature-dependent ECE behavior. However, in spite of intense studies about PZO, few works have focused on the impact of Pb nonstoichiometry on ECE. In this paper, Pb1+xZrO3 (x=0, 0.1, 0.15) thin films were fabricated on Pt/Ti/SiO2/Si substrates by sol-gel method. It is found that the temperature ranges of ECE of PZO films can be tuned effectively by Pb concentration. As x varies from 0 to 0.15, the peak temperature of ΔT changes from 260 K to 309 K under an applied electric field of 460 kV/cm. For Pb1.1ZrO3 thin films, large ΔT of about -24.9 K is obtained under 460 kV/cm at room temperature (303 K), favoring the application of PZO as a room temperature ECE materials.
Pb(OAc)2· 3H2O and C12H28O4Zr were used as the precursors of Pb and Zr, which were dissolved in 2-methoxyethanol, respectively. After adding acetylacetone, the two solutions were mixed at 40 o C for 2 hours. The concentration of final solution was adjusted to 0.2 mol/L by 2-methoxyethanol. After aging for 24 h, the precursor solution was deposited on Pt/Ti/SiO2/Si substrates by spin-coating. Then, the films were dried at 200 o C for 5 min in air on a heating stage and annealed at 700 o C for 8 min in O2 by rapid thermal annealing. The above processes were repeated several times to get the desired film thickness. Finally, the coated films were crystallized at 700 °C for 15 min in O2. To measure the electrical properties of the films, Pt dot electrodes were deposited by sputtering through a shadow mask. X-ray diffraction (XRD, Bruker D8) with Cu Kα radiation was used for the phase analysis. Surface morphology and thickness were studied by Atomic Force Microscope (AFM, Icon) and scanning electron microscopy (SEM, Nova NanoSEM230). The antiferroelectric and dielectric properties were measured by TF2000 standard ferroelectric test unit and HP4194A impedance analyzer coupled with a heating/cooling stage (TMS94). The combination states of Pb 4f and O 1s electrons were examined by X-ray photoelectron spectroscopy (XPS, PHI5000 VersaProbe).  In addition, lead-antisite defects, which have smaller formation energy than that of zirconium vacancies under oxygen-rich growth conditions, can be formed even there is only a small amount of lead excess [19]. In order to explore the evolution of antisite defects with the variation of x, Pb1+xZrO3 films were investigated by XPS technique.  around Ef, the polarization increases quickly with the increase of electric field, contributing to negative ECE. Therefore, the variation of Ef can directly affects the appearing temperature range of ECE. As aforementioned, the adding of excess PbO can compensate for the lead loss during the high-temperature annealing process and decrease the concentration of VPb. Such a decrease can postpone the transition from antiferroelectric to ferroelectric and contribute to the increase of Ef since VPb benefits the movement of domain walls and thus promotes the polarization reorientation [26].

Results and discussion
Moreover, compared with PZO, the interface layer between films and Pt bottom electrodes is more obvious in Pb1.10ZrO3 and Pb1.15ZrO3 films (Fig. 2). When an electric field is applied, it firstly concentrates on such interface layer due to its lower dielectric constant and then switches the Pb1+xZrO3 films to ferroelectrics, also contributing to the enhanced Ef with increasing x. In addition, it is reported that the antisite defects in PZO can increase its saturation polarization (Ps) while the residual of PbO shows the opposite effect [19,21]. The dielectric measuring results can also confirm the variation of defects. Figure   5(a)-(c) shows the temperature dependent dielectric constant at various frequencies.
Compared with pure PZO, the Tc of Pb1.10ZrO3 and Pb1.15ZrO3 films slightly increases probably due to the improvement of crystalline perfection by compensating for the lead loss. Meanwhile, a small dielectric anomaly at around 380 K is observed in Pb1.10ZrO3 and becomes more predominant in Pb1.15ZrO3, which possibly stems from the antisite defects. Though more antisite defects are possibly involved in Pb1.15ZrO3 films, the excessive residual of PbO decreases its dielectric constant and Ps (Fig. 4(c)).
On the basis of the above results, fascinating ECE is expected in Pb1+xZrO3 thin films. In order to evaluate the ECE, P-E loops of the Pb1+xZrO3 thin films were measured under different electric fields and temperatures. As shown in Fig. 5(d)-(f), at a given electric field, the Pb1+xZrO3 films gradually translate from AFE to FE with the increase of temperature, resulting in a rapid increase of polarization. Figure 6  (1) Where is the density, C p is the heat capacity, P is the polarization, and E is the electric field (here, 1 = 0 is chosen as the initial electric field and 2 is the applied electric field. = 8.22 g/ 3 and C p = 330 /( * ) are used in the calculation [27]. The values of ( ) are obtained by fitting the P-T curves using a ninth-order polynomial. It should be noted that a constant value of C p is used for simplicity though it usually changes with the variation of temperature [28]. As is shown in Fig. 6 .054) is obtained at room temperature (303 K) in Pb1.1ZrO3 films under 460 kV/cm. The present work provides a feasible alternative method for modulating ECE of PZO-based materials and will favor its applications in solid-state refrigeration.