Thermoelectric coupling effect in BNT-based pyroelectric ceramics for low-grade temperature driven energy harvesting

doi:10.21203/rs.3.rs-2756826/v1

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Thermoelectric coupling effect in BNT-based pyroelectric ceramics for low-grade temperature driven energy harvesting

https://doi.org/10.21203/rs.3.rs-2756826/v1

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published 30 Nov, 2023

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Pyroelectric energy harvesting has received increasing attention in recent years due to its ability to convert low-grade waste heat into electricity for sustainable development. However, the low output energy density driven by low-grade temperature limits its practical applications. Here, we report a high-performance hybrid BNT-BZT-xGaN thermal energy harvesting system with environmentally friendly lead-free BNT-BZT pyroelectric matrix and high thermal conductivity GaN as dopant. The theoretical analysis of BNT-BZT and BNT-BZT-xGaN with x = 0.1 wt% suggests that the introduction of GaN facilitates the resonance vibration between Ga and Ti, O atoms, which not only contributes to the enhancement of the lattice heat conduction, but also improves the vibration of TiO₆ octahedra, resulting in simultaneous improvement of thermal conductivity and pyroelectric coefficient. Therefore, a thermoelectric coupling enhanced energy harvesting density of 80 µJ cm⁻³ has been achieved in BNT-BZT-xGaN ceramics with x = 0.1 wt% driven by a temperature gradient of 2 ^oC, at the optical load resistance of 600 MΩ. This theoretical understanding provides a guidance to establish new principles for designing pyroelectric materials with further improved thermal energy harvesting performance.

Physical sciences/Materials science/Materials for energy and catalysis/Thermoelectrics

Earth and environmental sciences/Environmental sciences/Environmental chemistry/Environmental monitoring

Implementation of self-powered and battery-free sensors and other devices made from green materials composition is demanded for realizing energy and environmental sustainability [1–2]. Among them, the system capable of harvesting thermal waste is attracting special attention, because a large amount of energy produced by non-regeneration resources, such as coal, gas and oil, is wasted as heat [3–4]. Eco-friendly methods to convert the waste heat into electricity can not only achieve reduction in carbon emissions, but also furnish renewable and clear energy for self-powered sensors. Thermoelectric energy harvesting based on semiconductor materials has been widely explored due to their ability to turn spatial temperature gradients into electricity [5–6]. As a supplement of thermoelectrics, thermal energy harvesters based on pyroelectric effect in a ferroelectric material can convert waste heat of low temperature grade into electricity and work autonomously almost without requiring any external maintenance. Such pyroelectric energy harvesters are suitable for providing green power for passive sensor network, medical health monitoring, and intelligent residential system [7–9].

The current of pyroelectric energy collector can be expressed as:

$${I}_{p}=p\times A\times \frac{dT}{dt}$$

where p, A, and dT/dt are the pyroelectric coefficient (p), the effective electrode area, and the rate of temperature change (dT/dt), respectively [8]. It is well known from the equation that the performance of pyroelectric energy harvester is proportional to pyroelectric coefficient (p) of the material and the rate of temperature change (dT/dt). There are different methods proposed for improving the pyroelectric energy harvesting performance, and almost all of them are achieved by increasing the p or/and dT/dt [7, 10–12]. For example, extensive attempts have been made to enhance p by constructing morphotropic phase boundary (MPB) or polar nanoscale regions (PNRs) to improve the performance of pyroelectric energy harvesting [13–14]. Many efforts have also been made to increase the dT/dt by building meshed electrodes or porous structures [15–16]. Simultaneous increase in p and dT/dt has been reported in PZT-based pyroelectric materials by introducing AlN or BN high thermal conductivity network to enlarge their thermal energy harvesting performance [17–18]. However, why the high thermal conductivity filler simultaneously leads to enhanced p and dT/dt in pyroelectric ceramics still remain an open theoretical question. This limits the further improvement in output energy density of the pyroelectric energy harvester for practical applications.

To reveal the mechanism of the high thermal conductivity of fillers for enhancing thermal energy harvesting density and obtain environmentally friendly thermal energy harvester, we designed a hybrid system comprising lead-free BNT-BZT pyroelectric matrix and high thermal conductivity GaN as dopant. The hybrid ceramic BNT-BZT-xGaN was fabricated by conventional solid-state reaction method. When x = 0.1 wt%, the peak pyroelectric coefficient reaches a large value of 850×10^− 4 C m^− 2 K^− 1, which generates a short-circuit peak current of 0.12 µA and an open-circuit peak voltage of 58 V driven by a low-grade temperature difference of 2 ^oC. Our systematic study on the performance evolution with the GaN doping content, material microstructure, lattice vibrations and phonon structures, reveals the mechanism underlying the superior energy harvesting performance. The introduction of GaN facilitates the resonance vibration between Ga and Ti, O atoms, leading to the enhancement of the lattice vibration of TiO₆ octahedra. This not only contributes to the enhancement of the lattice heat conduction, but also improves the spontaneous polarization, resulting in the simultaneous improvement of dT/dt and p as shown in Fig. 1. Therefore, a high thermoelectrical energy harvesting density of 80 µJ cm^− 3 at the optimal load resistance of 600 MΩ has been obtained in BNT-BZT-xGaN ceramic with x = 0.1 wt%. The understanding from this work provides a valuable guidance for designing pyroelectric materials with further improved thermal energy harvesting performance.

2.1 Sample preparation

The solid-state reaction method was used to prepare 0.94Na_0.5Bi_0.5TiO₃-0.06BaTi_0.75Zr_0.25O₃ ceramic samples. According to the chemical formula, reagent grade metal oxide Bi₂O₃ (99%, Sinopharm), Na₂CO₃ (99.8%, Sinopharm), BaCO₃ (99%, Sinopharm), TiO₂ (98%, Sinopharm), and ZrO₂ (99%, Sinopharm) were ball milled in ethanol with zirconia for 10 ~ 18 h. After drying and calcination at 800 ~ 900 ºC for 2 h, the mixtures were ball milled for 10 ~ 18 h again, then dried and mixed with commercial GaN nanoparticle according to the chemical formula (1-x) 0.94Na_0.5Bi_0.5TiO₃-0.06BaTi_0.75Zr_0.25O₃-xGaN (x = 0, 0.05, 0.1, 0.15, 0.2 wt%) (BNT-BZT-xGaN). After granulated with 5% polyvinyl alcohol, the mixed powders were pressed into a disk in a prototype abrasive tool at 5 ~ 10 MPa. Finally, sintering was carried out in aluminium oxide crucibles at 1130 ~ 1160 ^oC for 2 h with decreasing temperature rate of 3 ^oC min^− 1.

2.2 Compositional heterogeneity analysis and microstructure characterization

Field emission scanning electron microscopy (FE-SEM) (Zeiss Geminisem 300) and energy-dispersive X-ray spectroscope (EDS) (Oxford instruments X-MaxN SN 78861) were employed to analyze the compositional heterogeneity and microstructures of the samples. The X-ray diffraction (XRD) (D8 Advance, Bruker, Germany) was utilized to characterize crystal structure. The refined data were obtained through Rietveld method on the basis of the slow-scan XRD data by using the GSAS-EXPGUI software and the Na_0.5Bi_0.5TiO₃ (space group R3c) CIF was chosen as the initial model [19–20].

2.3 Calculation of phonon dispersion, DOS and phonon group velocity

The Raman scattering spectra of the samples were measured by laser Raman spectrometer (LabRAM HR Evolution, Horiba, Japan). To analyze the phonon spectra, phonon density of state (PDOS), and phonon group velocity, first-principles calculations were performed within the Cambridge Serial Total Energy Package (CASTEP). The PDOS is calculated by the equation: ${g}^{j}\left({\omega },\widehat{n}\right)=\frac{1}{N}{\sum }_{{\lambda }}{\delta }\left({\omega }-{{\omega }}_{{\lambda }}\right){\left|\widehat{n}\bullet {e}_{\lambda }^{j}\right|}^{2}$, where N is the number of unit cells, j is the atom indices, δ is the Sommerfeld constant, ω is specified arbitrary using DOS-RANGE, ${\omega }_{\lambda }$ is phonon frequency, $\widehat{n}$ is the unit projection direction vector, and e is the polarization vector [19, 21]. The phonon group velocity is calculated by the formula:${v}_{g}\left(q{\nu }\right)=\frac{1}{2{\omega }\left(q{\nu }\right)}⟨e\left(q\nu \right)\frac{\partial D\left(q\right)}{\partial q}e\left(q\nu \right)⟩$, where q is the wave vector, $\nu$ is the index of phonon mode, ω is the phonon frequency, and D(q) is dynamical matrix. The optimized lattice parameters and IFCs are obtained from the total energy calculations by using plane augmented wave method based on density functional theory with Vienna ab initio simulation package (VASP) [22]. The electronic wave functions are expanded in a plane wave basis set with an energy cutoff of 680 eV. Such a high cutoff was found necessary to converge to the phonon dispersion curves. The Brillouin zone was sampled with 6 × 6 × 6 k-points Monkhorst-Pack meshes for primitive cells of R3c phases of BNT-BZT-xGaN [23–24].

2.4 Measurement of thermal parameters

The thermal conductivity (k) was calculated according to the equation $k=\alpha \rho {C}_{p}$, where α is the thermal diffusivity and measured by a LFA427 Microflash (NETZSCH-Gerätebau GmbH, Germany), C_p is the specific heat capacity and measured by Differential Scanning Calorimeter (DSC-200F3, NETZSCH-Gerätebau GmbH, Germany), and ρ is the density of the samples and characterized by the Archimedes method [17].

2.5 Electrical and pyroelectric properties characterization

To study the electrical properties, the samples were grinded into 0.3 mm and coated with silver paste on both sides with the electrode diameter of 6 mm and then sintered at 600 ^oC for 15 min. After that, all the samples were poled in silicone oil using a high-voltage power supply (ET2673D-10kV) with applied field of 5 kV mm^− 1 for 20 min at room temperature. The relative permittivity and dielectric loss were measured by the Dielectric Properties Testing System (DPTS-RT-1000, Wuhan Yanhe Technology Co.,Ltd., China). The Polarization-electric field (P-E) hysteresis loops were measured with the Ferroelectric Analyzer System (TF2000, aixACCT, Germany). The pyroelectric properties were measured using the Pyroelectric Test System (PCTS-3000, Wuhan Yanhe Technology Co.,Ltd., China) for poled samples at a heating rate of 2°C min^− 1.

2.6 Measurement of energy harvesting

The Thermal Energy Harvesting Testing System comprises a signal generator, power amplifier, Peltier cell, an electrometer, computer and display screen as shown in Fig. S1. A signal generator and power amplifier are employed to actuate the Peltier cell to produce a temperature gradient of 2 ^oC as shown in Fig. 8(a). A Keithley 6517B electrometer is utilized to record the output current and voltage with various electrical loads, and computer and screen display and store the corresponding data.

3.1 Compositional heterogeneity and microstructure

To analyze the lattice structures of BNT-BZT-xGaN samples, the XRD patterns and XRD Rietveld refinement are presented in Fig. 3(a)-(d). As shown in Fig. 3(a), the pure perovskite structures can be observed for all BNT-BZT-xGaN samples, suggesting that GaN has diffused into BNT-BZT lattice (as shown in Fig. 2(d)), being consistent with the analysis of EDS. The perovskite phase structure of BNT-BZT-xGaN ceramics can be confirmed by assessing the splitting peaks at around 40.0^o and 46.5^o as magnified in Fig. 3(b). For the rhombohedral phase, only one peak can be observed at 46.5^o, while the (200) peak turns into two peaks of tetragonal phase [25–26]. As shown in Fig. 3(b), both the (100) peak at 40.0^o and the (200) peak divide into two peaks, implying the coexistence of rhombohedral and tetragonal phase in BNT-BZT-xGaN system [27]. The rhombohedral phase dominates in the BNT-BZT-xGaN ceramic matrix because of the agglomeration of BaTiO₃ as shown in Fig. 2. The main crystal phase with R3c space group (as shown in Fig. 3(d)) can be identified by the XRD Rietveld refinement as shown in Fig. S5(a)-(e). The reliability factors of weighted patterns (R_wp), patterns (R_p) and the goodness-of-fit indexes (χ²) are under 7.5%, 5.5% and 5.5, respectively, indicating that the crystal structure mode is valid and the refinement result is reliable. The lattice parameters (a = b = c, α = β = γ and volume) and the distance of Ti-O are obtained as shown in Fig. 3(c). The angles (α = β = γ) tend to decrease, while both the lattice parameters (a, b, c) and volumes first increase and then decrease with addition of GaN. In addition, the similar tendency is also observed in the Ti-O bond distances of BNT-BZT-xGaN samples. As the GaN content increases to 0.1 wt%, the Ti-O (1) and Ti-O (2) bond lengths reach the maximum value of 1.894 Å and 2.044 Å.

3.2 Lattice vibrations and phonon structure

In BNT-based dielectric solids, lattice vibrations dominate the heat transport and a phonon refers to the quantum of atomic vibrational energy. The thermal transport in solids is regarded as the diffusion of phonons actuated by a heat gradient [28]. To analyze the relation between lattice vibration and phonon structure of BNT-BZT-xGaN samples, the calculated phonon-dispersion curves along the high-symmetry lines of Brillouin zone of BNT-BZT and BNT-BZT-xGaN with x = 0.1 wt% are presented in Fig. S6. There are three low-frequency acoustic modes observed near the Г-point and a lack of imaginary line in phonon band structures proves the dynamical stability for both BNT-BZT and BNT-BZT-xGaN with x = 0.1 wt% samples [29–30]. Meanwhile, the dispersion curves show some optical modes above 80 cm^− 1 and a band gap between 600 ~ 680 cm^− 1. The optical modes below 800 cm^− 1 and the acoustic modes determine the lattice thermal conductivity, playing an important role in heat transport. To find out intrinsic nature of lattice thermal conduction, the phonon density of state (PDOS) of BNT-BZT and BNT-BZT-xGaN with x = 0.1 wt% is presented in Fig. 4(a)-(b). As shown in Fig. 4(a)-(b), there are different shadowed areas representing Bi, Na, Ti, O, Ga and N atom phonon DOS, respectively. The atoms of BNT-based matrix mainly possess phonon distribution within the low-frequency part blew 800 cm^− 1. This means that atoms for BNT-based matrix vibrate at the phonon modes below 800 cm^− 1 and conduct heat, corresponding to the phonon band dispersion as shown in Fig. S6(b). In addition, the phonon distribution of Ga atom is located between 10 and 375 cm^− 1, which has overlapped regions with the atom phonon DOS of Na, Ti, and O. These overlapped phonon distributions stand for the resonance vibration between Ga and Na, Ti, O atoms, which contributes to the enhancement of the lattice vibration of BNT-BZT. Thus, the enhancement of the lattice vibration of BNT-BZT not only gives rise to the enhancement of the lattice heat conduction, but also improves the spontaneous polarization of the samples. Because the spontaneous polarization of BNT-based ceramics is prevailingly derived from the movement of Ti and O atoms, the resonance vibration among Ga and Ti, O atoms (as shown in Fig. 4(b)) improve the vibration amplitude of Ti-O (as shown in Fig. 4(f) and supplementary Video V1), which can be confirmed by the group velocity of BNT-BZT and BNT-BZT-xGaN with x = 0.1 wt% as shown in Fig. 4(c)-(d). The phonon group velocity of BNT-BZT-xGaN with x = 0.1% at about 300 cm^− 1, roughly corresponding to the Ti-O vibration (shown in Fig. 4(e) and Fig. S7), is 684.1 m s^− 1. This value is higher than 651.0 m s^− 1 of pristine BNT-BZT sample as shown in Fig. 4(c). Meanwhile, as shown in Fig. 4(e) and Fig. S7, the intensity of Raman spectrum between 10 and 375 cm^− 1 for BNT-BZT-xGaN with x = 0.1% is significantly stronger than that of pure BNT-BZT sample, further confirming the resonance vibration among Ga and Ti, O atoms. These results show that the introduction of GaN achieves the simultaneous improvement of thermal conductivity and spontaneous polarization, which can be verified by the measurement of heat conductivity and electrical properties as shown in Fig. 5 and Fig. 6, respectively.

3.3 Thermal conductivity and heat transport

Based on the kinetic theory of gases and Debye’s specific heat theory, the lattice thermal conductivity of a crystalline solid can be expressed as: $\kappa =\frac{1}{3}{C}_{v}{v}_{g}^{2}\tau$, where ${C}_{v}$ is the specific heat per unit volume, ${v}_{g}$ is the phonon group velocity, $\tau$is the relaxation time [31]. According to the equation, the thermal conductivity is proportional to the square of the phonon group velocity. As discussed in Fig. 4(c)-(d), with the modification of GaN, the phonon group velocity of BNT-BZT-xGaN with x = 0.1 wt% is higher than that of pristine BNT-BZT sample. Theoretically, the thermal conductivity of BNT-BZT-xGaN with x = 0.1 wt% is correspondingly higher than that of pristine BNT-BZT sample. To evaluate the actual heat conductivity, Fig. 5(a) and Fig. S8 give the temperature-dependent thermal parameters of BNT-BZT-xGaN ceramics with different amount of GaN. As shown in Fig. 5(a), while the thermal conductivity dose not vary much with an increase in temperature, the introduction of GaN contributes to an increase in the thermal conductivity from 1.48 W m^− 1 K^− 1 to 1.61 W m^− 1 K^− 1 when the GaN content increases from 0 to 0.1 wt% at room temperature. This may be contributed to the resonance vibration and enhancement in the phonon group velocity resulted from introduction of GaN. In addition, the temperature distributions of the pristine and BNT-BZT-xGaN sample with x = 0.1 wt% calculated by COMSOL Multiphysics software at the same time duration are presented in Fig. 5(b). It can be observed that BNT-BZT-xGaN sample with x = 0.1 wt% is able to conduct heat faster for the same thermal input in comparison with the pure ceramic. Namely, the construction of BNT-BZT-xGaN hybrid ceramic can be an effective method to improve the heat transfer performance compared to the pure BNT-BZT ceramic samples.

3.4 Dielectric and ferroelectric properties

Figure 6(a) and Fig. S9 present the dielectric constant (ε_r) and loss (tanδ) with temperature range from 25 to 400 ^oC for polarized BNT-BZT-xGaN samples. As shown in Fig. 6(a) and Fig.S9, three dielectric peaks can be observed in dielectric constant curves for all samples, which correspond to ferroelectric to paraelectric phase transition (T_m) at around 300 ^oC, ergodic relaxor to ferroelectric state (T_s) at about 150 ^oC, and non-ergodic to ergodic relaxor state (T_FR) near ambient temperature, respectively [32]. The T_m decreases slightly while the T_s and T_FR increase appropriately with the increase in GaN content as presented in Fig. 5(b). In particular, the temperature of non-ergodic to ergodic relaxor state (T_FR) for BNT-BZT-xGaN samples shifts from 40 to 65 ^oC with increasing GaN, which is corresponding to the peak pyroelectric temperature as shown in Fig. 7(a). In addition, the dielectric constant (ε_r) and loss (tanδ) at corresponding ambient temperature range decrease with the increase in GaN content, especially in room temperature from ~ 1750 to ~ 1400 and from ~ 0.074 to ~ 0.05, respectively. Because the pyroelectric energy harvesting figure of merit is inversely proportional to the dielectric constant, the decrease in dielectric constant (ε_r) facilitates the improvement of pyroelectric energy harvesting performance. The polarization-electric field (P-E) hysteresis loops of BNT-BZT-xGaN samples with various content of GaN at room temperature are presented in Fig. 6(c). All the samples exhibit typical ferroelectric hysteresis loops. Both the saturation polarization (P_s) and remnant polarization (P_r) first increase and then decrease with increasing GaN content. As the GaN content increases to 0.1 wt%, both the P_s and P_r gradually increase to the maximum value of 40 µC cm^− 2 and 23 µC cm^− 2, respectively, which is attributed to the enhancement of Ti-O vibration as discussed in Fig. 4.

3.5 Pyroelectric effect and mechanism of pyroelectric energy harvesting

Figure 7(a) presents the temperature dependent pyroelectric coefficient of BNT-BZT-xGaN ceramics with different content of GaN in the temperature range of 20 ~ 90 ^oC. It can be observed from Fig. 7(a) that the peak pyroelectric coefficient of BNT-BZT-xGaN samples shows a similar trend as that of saturation polarization and remnant polarization. The peak pyroelectric coefficient firstly increases and then decreases with the addition of GaN, and reaches the maximum value of 850×10^− 4 C m^− 2 K^− 1 when the content of GaN increases to 0.1 wt%. Meanwhile, the peak position gradually shifts to higher temperature with the increase in GaN, in accordance with the temperature of non-ergodic to ergodic relaxor state as shown in Fig. 7(b). The mechanisms of pyroelectric energy harvesting for BNT-BZT and BNT-BZT-xGaN with x = 0.1 wt% are illustrated in Fig. 7(c) and Fig. 7(d), respectively. At a steady state (dT/dt = 0), for BNT-BZT samples, the polarized dipoles reorient along the applied electric field and oscillate randomly to reach an equilibrium condition, concomitantly obtaining the attracted charges on both surfaces of samples as shown in Fig. 7(c). For BNT-BZT-xGaN with x = 0.1 wt%, the poled dipoles are easier to reorient and absorb a greater quantity of opposite electrical charge on both sides of samples (as presented in Fig. 7(d)) due to the enhancement of Ti-O vibration, which provides the potential for giant pyroelectric effect. When the sample is heated, the dipoles in BNT-BZT ceramics oscillate within a large degree of alignment, leading to the decrease in spontaneous polarization and absorbed electrical charges, and thus the reduced currents flow in the external circuit. With the introduction of GaN, heat transfer is faster in BNT-BZT-xGaN with x = 0.1 wt% owing to the improved thermal conductivity, which contributes to dipoles’ oscillation in even larger degree and larger currents in the circuits as illustrated in Fig. 7(d).

3.6 Pyroelectric energy harvesting

Figure 8 presents the energy harvesting performance of BNT-BZT-xGaN samples in response to temperature change between 25 to 50 ^oC. The corresponding time dependent temperature variation curves are exhibited in Fig. 8(a). It can be seen that the thermal distribution is a combination of continuous rising trend with periodic temperature fluctuation of about 2 ^oC every 10 s. The short-circuit current and open-circuit voltage of BNT-BZT-xGaN ceramic induced by the low temperature gradient of 2 ^oC are shown in Fig. 8(b)-(c). As shown in Fig. 8(b)-(c), both short-circuit current and open-circuit voltage increase first and then decrease with the increase in GaN content. As the content of GaN reaches 0.1 wt%, the BNT-BZT-xGaN sample with x = 0.1 wt% can generate a short-circuit peak current of 0.12 µA, or an open-circuit peak voltage of 58 V. The improvements in pyroelectric short-circuit current and open-circuit voltage are mainly owing to the thermoelectrical coupling effect in BNT-BZT-xGaN sample with x = 0.1 wt% modulated by GaN. Meanwhile, the output energy density of BNT-BZT-xGaN ceramics with x = 0.1 wt% has been investigated by measuring the pyroelectric current and voltage with a range of load resistance and calculation based on the integral formula $\text{w}={\int }_{t=0}^{t=T}\frac{I\times U}{S\times d}dt$, where I, U, T, S and d are pyroelectric current, voltage, cycle time, sample area and thickness, respectively. The pyroelectric current and voltage obtained at different load resistance are presented in Fig. 7(d)-(e). Figure 7(f) shows the load resistance dependent energy density curves of BNT-BZT-xGaN ceramics with x=0.1 wt% induced by the low temperature gradient of 2 ^oC. As shown in Fig. 7(f), the output energy density firstly increases and then decreases with the increase in load resistance and reaches the maximum value of 80 µJ cm⁻³ at the load resistance of 600 MΩ. In addition, Fig. 7(g) presents the electrical circuit diagram of the pyroelectric energy harvesting device where a 4.8 µF load capacitor and BNT-BZT-0.1 wt%GaN sample are connected by a phase-controlled rectifier. The pyroelectric samples generate alternating current driven by a low thermal gradient, which is converted by the phase-controlled rectifier to direct current and stores the electricity in the capacitor. When the voltage of the capacitor reaches 2.2 V, the capacitor is triggered to discharge and light up the LED bulb. The charging-discharging voltage variation curves of the capacitor are presented in Fig. S10. Also, the practical applicability of the thermal energy harvesting device induced by low temperature gradient is illustrated in supplementary Video V2. The BNT-BZT-xGaN ceramics with x = 0.1 wt% exhibit superior pyroelectric energy density driven by unit temperature gradient in comparison with state-of-the-art pyroelectric energy harvesting systems (as shown in Table 1), which have great potential application in low temperature driven thermal energy harvesting devices [16, 33–45].

Table 1

Comparison of pyroelectric energy harvesting performance of BNT-BZT-xGaN ceramic with that of other state-of-the-art pyroelectric energy harvesting systems driven by unit temperature gradient.
Materials	ΔT (K)	p (10^− 4 C m^− 2 K^− 1)	U_o (V K^− 1)	I_o (µA K^− 1)	Energy density for volume (µJ cm^− 3 K^− 1)	Power Density for area (µw cm^− 2 K^− 1)	Ref.
ITO/BNT-BZT/Ag	30.5	5.0	1.67	0.003	~	~	[33]
BZT/BCT/STO	11.8	34.3	~	0.11	~	~	[34]
I⁻/I³⁻/MC/KCl (TGC)	15.0	~	0.01	~	5.34	~	[35]
PPGO	110.0	~	0.02	0.10	~	0.17	[36]
PMN-PMS-PZT: xCNT	20.0	43.3	0.67	~	~	~	[16]
HfO₂/Hf_0.5Zr_0.5O₂:La	160.0	0.7	~	~	~	~	[37]
PyEHs	4.0	~	15	~	~	~	[38]
PEG	70.0	~	0.43	0.27	~	0.09	[39]
SnS:Na	3.9	~	0.001	24.51	~	~	[40]
PEDOT: Tos	13.8	~	5.41	0.001	~	0.03	[41]
WKF-based PTM	80.0	~	~	~	~	0.002	[42]
S-PENG	29.0	~	4.14	0.072	~	0.009	[43]
Al/PVDF/Al	5.0	17.9	9.60	0.005	0.23	~	[44]
PPNG	38.0	~	0.001	~	~	0.003	[45]
BNT-BZT-GaN	2.0	850.0	29.0	0.06	40	~	This work

BNT-BZT pyroelectric ceramics with embedded high thermal conductive GaN are designed and fabricated by conventional solid-state reaction method to realize high pyroelectric energy harvesting performance. The hybrid BNT-BZT-xGaN ceramic exhibited a peak pyroelectric coefficient of 850×10^− 4 C m^− 2 K^− 1, when x = 0.1 wt%, generating a short-circuit peak current of 0.12 µA or an open-circuit peak voltage of 58 V driven by a low temperature change of 2 ^oC, which exhibits superior pyroelectric energy density driven by unit temperature gradient in comparison with state-of-the-art pyroelectric energy harvesting systems. With a systematic study on the thermoelectrical output, microstructure, lattice vibrations and phonon characteristics of the materials with the varied GaN doping content, the mechanism underlying the enhancement in the energy harvesting performance has been revealed. The introduction of GaN facilitates the resonance vibration with interactions among Ga and Ti, O atoms, which contributes to the enhancement of not only the lattice heat conduction, but also the vibration of Ti-O. This results in the simultaneous improvement of thermal conductivity and spontaneous polarization, and hence the significant improvement in the thermoelectric coupling effect. A high energy harvesting density of 80 µJ cm^− 3 at the optimal load resistance of 600 MΩ has been achieved in the BNT-BZT-x wt% GaN ceramic with x = 0.1. The theoretical understanding provides a guidance to establish new principles for designing pyroelectric materials with further improved thermal energy harvesting performance.

Acknowledgement:

This work is supported by the National Natural Science Foundation of China (Grant No. 52102128, 52272107), the Natural Science Foundation of Hubei Province (Grant No. 2021CFB117), and the Open Project Fund of Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices (Grant No. K202010). We knowledge the Innovation Program of Wuhan-Shuguang (Grant No. 2022010801020329), and we also gratefully acknowledge the support by A*STAR, Singapore, RIE2020 Advanced Manufacturing and Engineering (AME) Programmatic Fund (Grant No. A20G9b0135).

Author contributions:

M. S., S. J., and Y. C. conceived the idea of this work. K. L. fabricated the BNT-based pyroelectric ceramics. M. S. and K. L. carried out the measurements of microstructure, electrical and pyroelectric properties. G-H. Z. performed the calculation of the phonon dispersion, DOS and phonon group velocity. Q. L. performed the measurement of thermal parameters. Q. Z., and G. Z. carried out the measurement of energy harvesting. M. S. H. Z., and S. J. analyzed all the data and prepared the manuscript. G. Z. and K. Y. revised the manuscript. All authors discussed the results and commented on the paper. S. M., Y. C. and K. Y. supervised the research.

Competing interests:

The authors declare no competing interests.

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V1FrequencyofTiO6Vibration.mp4
frequency of Ti-O vibration
V2EnergyHarvesting.mp4
the practical applicability of the thermal energy harvesting device
SupportingInformation.docx
supporting information

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published 30 Nov, 2023

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Thermoelectric coupling effect in BNT-based pyroelectric ceramics for low-grade temperature driven energy harvesting

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Abstract

Figures

1. Introduction

2. Experimental Procedure

2.1 Sample preparation

2.2 Compositional heterogeneity analysis and microstructure characterization

2.3 Calculation of phonon dispersion, DOS and phonon group velocity

2.4 Measurement of thermal parameters

2.5 Electrical and pyroelectric properties characterization

2.6 Measurement of energy harvesting

3. Results And Discussion

3.1 Compositional heterogeneity and microstructure

3.2 Lattice vibrations and phonon structure

3.3 Thermal conductivity and heat transport

3.4 Dielectric and ferroelectric properties

3.5 Pyroelectric effect and mechanism of pyroelectric energy harvesting

3.6 Pyroelectric energy harvesting

4. Conclusion

Declarations

References

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