Electrocaloric effect based on the depolarization transition in (1−x)Bi0.5Na0.5TiO3–xKNbO3 lead-free ceramics

Lead-free ferroelectric ceramics (1−x)Bi_0.5Na_0.5TiO₃−xKNbO₃ (BNT–xKN) with x=0.00, 0.04, 0.06 and 0.08 were synthesized by the conventional solid state reaction method. The effects of the KNbO₃ addition on the dielectric behavior, ferroelectric properties, as well as electrocaloric effect of the ferroelectric ceramic BNT–xKN were investigated. The results show that the depolarization temperature decreases with the increment of KN content. A high ECE of 1.73 °C is achieved at 76 °C in BNT–0.06KN. The relation between electrocaloric effect and depolarization transition was discussed. This investigation indicates that the depolarization transition below Curie transition in BNT-based ceramics is a promising approach in ECE technique.     

Synthesis and dielectric properties of polyaniline/titanium dioxide nanocomposites

Two series of polyaniline–TiO₂ nanocomposite materials were prepared in base form by in situ polymerization of aniline with inorganic fillers using TiO₂ nanoparticles (P25) and TiO₂ colloids (Hombikat), respectively. The effect of particle sizes and contents of TiO₂ materials on their dielectric properties was evaluated. The as-synthesized polyaniline–TiO₂ nanocomposite materials were characterized by transmission electron microscopy (TEM), Fourier transform infrared (FTIR), thermal analysis (DTA/TGA), and X-ray diffraction (XRD). Dielectric properties of polyaniline–TiO₂ nanocomposites in the form of films were measured at 1 KHz–1 MHz and a temperature range of 35–150 °C. Higher dielectric constants and dielectric losses of polyaniline–TiO₂ nanocomposites than those of neat PANI were found. PANI–TiO₂ nanocomposites derived from P25 exhibited higher dielectric constants and losses than those from Hombikat TiO₂ colloids. Electrical conductivity measurements indicate that the conductivity of nanocomposites is increased with TiO₂ content. The dielectric properties and conductivities are considered to be enhanced due to the addition of TiO₂, which might induce the formation of a more efficient network for charge transport in the base polyaniline matrix.     

Tuning the electrocaloric effect by tailoring phase fraction in BaTiO3-based ferroelectrics

The giant electrocaloric effect (ECE) in ferroelectrics arouses enthusiasm on the electrocaloric refrigeration for its zero global-warming-potential. Previous studies have observed that large ECE response occurs around ferroelectric-paraelectric phase transition in BaTiO₃-based ferroelectrics. However, few attention is paid to the effect of phase fraction on ECE performance. In this work, Ba_1-xSr_xTi_0.95Sn_0.05O₃ ceramics which display varied phase fraction of orthorhombic and tetragonal (O and T) phase at room temperature, are developed to reveal the influence of phase fraction on the electric properties, especially ECE performance. The electrocaloric temperature change (ΔT) is enhanced in composition with equal phase fraction of O and T, which also shows excellent piezoelectric property. Intrinsically, a number of nanodomains are formed due to the incorporation of Sr²⁺, greatly benefiting the invertibility of domains under the electric field. It is believed that this study opens a new way for designing and understanding of ECE in BT-based ceramics.     

Direct measurement and dynamic mechanical analysis on the coexistence of positive and negative electrocaloric effects in Bi0.5Na0.5TiO3-xBaTiO3 solid solutions

Abnormal electrocaloric effect (ECE) is investigated in (1-x)Bi_0.5Na_0.5TiO₃-xBaTiO₃ solid solutions by calorimetry measurements, manifesting itself by the differences in the heats released and absorbed under the application and withdrawal of electric fields, respectively. The abnormal ECE is explained by the coexistence of positive and negative ECEs close to the ferroelectric-to-antiferroelectric transition temperature, which is demonstrated by the structural and ferroelectric properties determined from dynamic mechanical analyses and thermodynamics calculations on the solid solutions under an applied electric field, respectively. A threshold electric field below which the negative ECE occurs is obtained, in comparisons with that predicted by the thermodynamics Maxwell's relations. The abnormal ECE obtained from direct measurement is compared with that calculated by the indirect measurement, and the discrepancy is discussed.     

High electrocaloric effect in hot‐pressed Pb0.85La0.1(Zr0.65Ti0.35)O3 ceramics with a wide operating temperature range

The temperature stability of the electrocaloric effect (ECE) in relaxor ferroelectric Pb_0.85La_0.1(Zr_0.65Ti_0.35)O₃ (PLZT) prepared by the hot‐press sintering method has been investigated. Compared to the PLZTs prepared via the conventional sintering process, the hot‐pressed PLZTs exhibit larger ECE and superior temperature stability. The hot‐pressed sample with an appropriate content of excess PbO presents a high ΔT of 2.4°C and ΔS of 2.3 J kg^?1·K^?1, both of which are 30% greater than those of the conventionally sintered samples measured at 100 kV·cm^?1. More importantly, the hot‐pressed specimens display great stable electrical properties, including the dielectric breakdown strength and electrical resistivity in the temperature range from 0°C to 100°C, whose ECE instability, especially, is only one‐half that of the samples prepared by the conventional solid‐state method. In addition, the ECE and its stability of the hot‐pressed sample can be further enhanced by increasing the operating electric field to a relatively high level of 200 kV·cm^?1. This work demonstrates hot‐press sintering is an effective method to fabricate ferroelectric ceramics with high ECE as well as desirable temperature stability.     

Non-ergodic – ergodic transition and corresponding electrocaloric effect in lead-free bismuth sodium titanate-based relaxor ferroelectrics

The ferroelectric refrigeration technology based on electrocaloric effect (ECE) is a promising environmental way to replace the gas compression refrigeration. Lead-free bismuth sodium titanate (NBT)-based relaxor ferroelectric ceramics have advantages of a large ECE within a wide temperature range around the depolarization temperature (T_d), but the T_d of pure NBT is too high to meet the application requirements. Here, we systematically studied the electrocaloric effect in (0.95-x)(Na_0.5Bi_0.5)TiO₃-0.05SrTiO₃-x(K_0.5Na_0.5)NbO₃ ((0.95-x)NBT-0.05ST-xKNN) ceramics and its relation with the relaxor behavior. The addition of KNN enhances the relaxor character, and shifts the T_d from 120 °C to a low temperature even below 0 °C. The directly measured ECE shows a peak of high ?T_max = 0.88 K (@ 60 °C) for x = 0.05 under an electric field of 40 kV/cm and a wide temperature span of 79 °C (≥ 80% of ?T_max). The x = 0.07 sample exhibits the optimal room-temperature ECE performance with a high ?T_30 °C = 0.71 K because the non-ergodic – ergodic phase transition occurs near room temperature. The electric field–temperature phase diagram reveals the origin of the enhanced ECE as the electric-field-induced transition between polar nanoregions (PNRs) and long-range ferroelectric domain.     

Preparation and properties of single-walled carbon nanotubes/poly(butylene terephthalate) nanocomposites

A neat poly(butylene terephthalate) (PBT) polymer and functionalized single-walled carbon nanotubes (F-SWNTs)/PBT nanocomposite films were prepared by solution casting technique. The SWNTs were functionalized by acid treatment, which introduced carboxylic groups onto the SWNTs. The morphological studies showed that the F-SWNTs were embedded and dispersed well within the PBT polymer matrix. The POM study illustrated that a neat PBT showed Maltese-type spherulites. It was also observed that the size of neat PBT spherulites was larger than F-SWNTs/PBT nanocomposite spherulites, which might be due to the nucleation effect of F-SWNTs in the case of nanocomposites. The thermal stabilities and mechanical properties such as stress yield and moduli of F-SWNTs/PBT nanocomposites were enhanced as compared to neat PBT. The DSC study showed that the melting temperature (T _m) of PBT was slightly increased by addition of F-SWNTs. This increase in T _m might be due to the formation of compact structure, which was formed through different types of molecular interactions with addition of F-SWNTs. It was also found that initially the solvent (distilled water, kerosene, 2 M HNO₃ solution) uptake by neat PBT polymer and its nanocomposites increased gradually, which became steady after specific intervals for each sample. The results also exhibited that the solvent uptake of F-SWNTs/PBT nanocomposites was less than neat PBT.     

Electrocaloric effect and pyroelectric energy harvesting of (0.94 − x)Na0.5Bi0.5TiO3-0.06BaTiO3-xSrTiO3 ceramics

Ceramics with the composition (0.94 − x)Na_0.5Bi_0.5TiO₃–0.06BaTiO₃–xSrTiO₃ (NBBSTx) where x = 0.10, 0.15, 0.20, and 0.25 were synthesized by a conventional solid-state reaction method to investigate their electrocaloric effect (ECE) and pyroelectric energy harvesting (PEH) properties. The ferroelectric, dielectric, and pyroelectric properties of the prepared ceramics were measured and discussed. It is found that the strontium titanate (ST) content and bias field greatly affect the ferroelectric–relaxor transition. Increasing ST content lowers the depolarization temperature of the ceramics, and both the ECE and PEH behavior of the ceramics strongly depend on their ST content because of the composition-induced decrease of the ferroelectric–relaxor transition temperature. The present investigation demonstrates that the ECE and PEH properties of NBBSTx ceramics can be tuned by introducing ST. Furthermore, a high PEH density of 425 kJ/m³ is obtained for NBBST0.20, which is much higher than those of conventional Pb-based ferroelectrics.     

Effect of sintering temperature on the ferroelectric property and electrocaloric effect of Pb0.8Ba0.2ZrO3 ceramics

This work presents the effects of sintering temperature ranging from 1200 °C to 1300 °C at intervals of 20 °C on the crystal structure, ferroelectric properties, and electrocaloric effect (ECE) of Pb_0.8Ba_0.2ZrO₃. Samples sintered at 1240 °C, 1260 °C, and 1280 °C have large remanent polarization and small coercive field. Meanwhile, samples sintered at 1260 °C, 1280 °C, and 1300 °C possess large breakdown field strength. Samples sintered at 1260 °C for 4 h exhibit the optimal ferroelectric properties. Antiferroelectricity-ferroelectricity (AFE-FE) phase transition occurs at room temperature T₁ (279 K). Directly examining ECE at this temperature is meaningful, and the temperature change is 0.068 K at approximately 60 °C and 30 kV/cm. Results laid the foundation for studying the performance of ferroelectric and ECE within this phase-transition temperature range and provide a reference for new solid-state refrigeration technology.     

Ethylene–propylene‐diene terpolymer/halloysite nanocomposites: Thermal,mechanical properties,and foam processing

Ethylene–propylene‐diene terpolymer (EPDM)/halloysite nanotube (HNT) nanocomposites were prepared by melt mixing in an internal mixer using a commercially available maleated semicrystalline EPDM and HNT. Transmission electron microscopy analysis of the EPDM/HNT composites revealed that the HNTs are uniformly dispersed at a nanometer scale in the matrix. Differential scanning calorimeter studies indicated that the HNT caused an increase in the nonisothermal crystallization temperature of the EPDM. Tensile and dynamic mechanical analysis exhibited that a small amount of the HNTs effectively enhanced the stiffness of the EPDM without adversely affecting its elongation‐at‐break. The EPDM/HNT nanocomposites were used to produce foams by using a batch process in an autoclave, with supercritical carbon dioxide as a foaming agent. The nanocomposite foams showed a smaller cell size and higher cell density as compared to the neat EPDM foam, and the nanocomposite with 10 phr HNT produced a microcellular foam with average cell size as small as 7.8 μm and cell density as high as 1.5 × 10¹⁰ cell/cm³. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40307.     

Effect of layered silicate on EPDM/EVA blend nanocomposite: Dynamic mechanical,thermal, and swelling properties

Polymer blend nanocomposites have been developed by solution method using ethylene propylene diene terpolymer (EPDM), ethylene vinyl acetate (EVA‐45) copolymer, and organically modified layered silicate. Morphological investigation made by wide‐angle X‐ray diffraction and transmission electron microscopic analysis indicates intercalated structure of EPDM/EVA nanocomposites with partial disorder. Scanning electron microscopic studies exhibit the phase behavior of EPDM/EVA blend nanocomposites. Dynamic mechanical thermal analysis shows a significant increase in storage modulus in the rubbery plateau. The decrease in damping (tan δ) value and enhanced glass‐transition temperature (T_g) demonstrate the reinforcing effect of layered silicate in the EPDM/EVA blend matrix. The tensile modulus of these nanocomposites also showed a significant improvement with the filler content. The main chain scission of EPDM/EVA blend nanocomposites compared with the neat EPDM/EVA blend showed substantial improvement in thermal stability in nitrogen, whereas a sizeable increase is observed in air. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Giant electrocaloric effect of free-standing Pb0.85La0.1(Zr0.65Ti0.35)O3 thick films fabricated by the self-lift-off screen printing method

Free-standing Pb_0.85La_0.1(Zr_0.65Ti_0.35)O₃ (PLZT) ceramic thick films have been prepared via a facile and low-cost self-separating screen printing method for electrocaloric cooling, and the relation among the fabrication processes, phase composition, microstructure, dielectric characteristics, ferroelectric properties and electrocaloric effect (ECE) has been systematically investigated. Compared to the conventional ceramic thick films supported by substrates, the free-standing feature enables the EC cooling of the free-standing PLZT thick films to be fully used for cooling down different thermal loads rather than be futilely absorbed by the substrates. Furthermore, without the mechanical restriction of the substrates, the free-standing PLZT thick films can freely shrink during the high-temperature densification process, leading to their high density and favorable microstructures. Additionally, by introducing an adequate amount of excess PbO, the pyrochlore phase can be removed from the samples to yield high-purity perovskite PLZTs. With the comprehensive improvement in phase composition, microstructure and the elimination of mechanical strain between the active materials and substrates, the free-standing PLZT thick films exhibited an optimized ECE including changes of temperature and entropy of 1.95 °C and 2.09 J kg^?1 K^?1, which are almost 3 times that of the samples deposited on the Al₂O₃ substrates without excess PbO. This work would contribute to the development of ferroelectric ceramics, especially thick films, for practical EC cooling.     

Phase–composition and temperature dependence of electrocaloric effect in lead–free Bi0.5Na0.5TiO3–BaTiO3–(Sr0.7Bi0.2□0.1)TiO3 ceramics

The (0.94–x)Bi_0.5Na_0.5TiO₃–0.06BaTiO₃–x(Sr_0.7Bi_0.2□_0.1)TiO₃ (BNT–BT–xSBT, 0 ≤ x ≤ 0.24) solid solution ceramics were synthesized via a conventional solid–state reaction method and the correlation of phase structure, piezoelectric, ferroelectric properties and electrocaloric effect (ECE) was investigated in detail. The ECE in lead–free BNT–BT–xSBT ceramics was measured directly using a home–made adiabatic calorimeter with maximum adiabatic temperature change ΔT = 0.4 K with x = 0.08 under the electric field E = 6 kV/mm at room temperature. The position of maximum ECE was found in the vicinity of nonergodic and ergodic phase boundary, where the maximum change in entropy occurs as a result of the field–induced phase transformation between the ergodic and long–range ferroelectric phase. Besides, the mechanism for the shift of ECE peak is discussed in detail. Finally, the temperature dependence of ECE for BNT–BT–xSBT (x = 0, 0.04 and 0.08) was also investigated. This work may present a guideline for designing BNT–based ferroelectric relaxor ceramics for EC cooling technologies.     

Smart,lightweight, flexible NiO/poly(vinylidene flouride) nanocomposites film with significantly enhanced dielectric,piezoelectric and EMI shielding properties

We report a facile technique to fabricate flexible and self-standing NiO/PVDF nanocomposite films. Detail structural and thermal characterizations of nanocomposite films show the gradual increase of electroactive β-phase of PVDF with increasing NiO nanoparticles content. The enhancement of β phase in the NiO/PVDF nanocomposites has been explained from physicochemical point of view. Electrical properties of the nanocomposites indicate fair improvement in dielectric properties at low filler loading with less dielectric loss. The value of dielectric constant of 0.75 wt% NiO/PVDF films at 100 Hz is five times higher than that of neat PVDF. Series-parallel model was used to describe the filler concentration dependence of the dielectric constant of the nanocomposites. These nanocomposites also exhibit excellent ferroelectric properties. Nanocomposite films having thickness 300 μm were also successfully employed for microwave shielding application. This work suggests that these films would be very useful for thinner, lighter energy harvesting storage and EMI shielding applications.     

Surface modification of BaTiO3 inclusions in polydicyclopentadiene nanocomposites for energy storage

A new nanocomposite system displaying high breakdown strength, improved permittivity, low dielectric loss, and high thermal stability is presented. Free‐standing nanocomposite films were prepared via a solvent‐free in‐situ polymerization technique whereby 5 vol % BaTiO₃ (BT) nanocrystals with tailored surface chemistry were dispersed in dicyclopentadiene (DCPD) prior to initiation of ring opening metathesis polymerization by a second generation Grubbs catalyst. The relative permittivity was enhanced from 1.7 in the neat poly(DCPD) film to a maximum of 2.4 in the composite, while the dielectric loss tangent was minimized below 0.7%. Surface modification of the BT nanocrystals mitigated reduction in breakdown strength of the resulting nanocomposites such that only a 13% reduction in breakdown strength was observed relative to the neat polymer films. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40290.     

The electrocaloric effect of Ba1-xLaxTi0.9Sn0.1O3 ceramics with excellent temperature stability near room temperature

The Ba_1-xLa_xTi_0.9Sn_0.1O₃ ceramics (x = 0, 0.006, 0.007, 0.008) were prepared by the traditional solid-state reaction method. The influence of La³⁺ on the phase, dielectric properties, ferroelectric properties, and electrocaloric effect (ECE) was analyzed in detail. The results of refinement show that all ceramics are multiphase coexistence at room temperature, including the cubic phase, the tetragonal phase, and the orthogonal phase. With the increase of La³⁺, the polar phases decrease but the non-polar phase increases, which is the main reason for the decline in adiabatic temperature change (ΔT). The analysis of dielectric properties and ferroelectric properties demonstrate that the diffuse phase transition is strengthened by introducing La³⁺. It also means that polar nanoregions (PNRs) might be formed. Therefore, the temperature stability of the Ba_1-xLa_xTi_0.9Sn_0.1O₃ ceramics in a wide temperature range near room temperature is improved. Simultaneously, the PNRs provide additional entropy to improve ECE. A higher ΔT = 0.88 K is obtained under 60 kV/cm for x = 0.007, which also possesses excellent temperature stability in the temperature range of 298 K–378 K. The doping of La³⁺ also improves the electric field threshold of the electrocaloric strength (ΔT_max/ΔE) and stabilizes the ΔT_max/ΔE under a higher electric field, which is conducive to improving ECE under a higher electric field and providing another possible solution for promoting the practical application of ECE.     

Friction spot welding of PMMA with PMMA/silica and PMMA/silica-g-PMMA nanocomposites functionalized via ATRP

In the present study, the feasibility of Friction Spot Welding (FSpW) of a commercial-grade poly(methyl methacrylate) (PMMA) (PMMA GS) and PMMA 6N/functionalized silica (SiO₂) nanocomposites was investigated. The silica nanoparticles were functionalized via atom transfer radical polymerization (ATRP) with PMMA chains to achieve a uniform dispersion in the polymer matrix. The successful functionalization of silica nanoparticles with PMMA chains via ATRP was evaluated by ATR-FT-IR and TGA measurements. Rheological investigations of the silica nanocomposites showed a plateau of the storage modulus G′ at low frequencies (0.01–0.03 rad/s) as a result of elastic particle–particle interactions. Overlap friction spot welds consisting of PMMA GS and a 2 wt% SiO₂-g-PMMA nanocomposite were successfully prepared and compared to spot joints of PMMA GS welded with PMMA 6N and PMMA 6N/silica nanocomposite with 2 wt% unfunctionalized silica nanoparticles. Raman mappings of selected areas of cross-sectional plastographic specimens revealed an increased mixing behavior between the two polymer plates in the case of PMMA GS/2 wt% SiO₂-g-PMMA joints. Although the joints welded with PMMA 6N/silica nanocomposites showed a reduction of 22% in lap shear strength and 21% displacement at peak load compared with the neat PMMA spot welds, they can compete with other state-of-the-art PMMA welding techniques such as thermal bonding and ultrasonic welding, which indicates the potential of friction spot welding as an alternative fabrication technology for joining future nanocomposite engineering parts.     

The influence of phase transition on electrocaloric effect in lead‐free (Ba0.9Ca0.1)(Ti1−xZrx)O3 ceramics

In this paper, the influence of phase evolution on polarization change and electrocaloric response in lead‐free (Ba_0.9Ca_0.1)(Ti_1?xZr_x)O₃ ceramics (BCTZ) was systematically investigated. With increasing Zr/Ti ratio, the phase structure and phase transition behavior were greatly changed, resulting in various temperature and electric field dependence of electrocaloric responses. For x=0.05, a peak electrocaloric temperature change 1.64 K (at 130°C) and corresponding entropy change 1.78 J·kg^?1·K^?1 were obtained for 0‐7 kV·mm^?1 electric field. Negative electrocaloric temperature change in ?0.1 K was obtained below Curie temperature (T_c), which may be induced by the orthorhombic‐tetragonal ferroelectric phase transition. With the increase in x, the peak value of the electrocaloric response decreased but much better temperature stability was observed. Simultaneously the negative electrocaloric response gradually disappeared with the disappearance of the low temperature ferroelectric‐ferroelectric phase transition. For x=0.2, electrocaloric response showed good temperature stability ranging from room temperature to 130°C, attributing to the relaxor ferroelectric feature.     

Large electrocaloric effect with ultrawide temperature span in Na1/2Bi1/2TiO3-based lead-free ceramics

Innovative cooling technologies are recognized by many industries as a crucial part of their system design. A large electrocaloric effect (ECE) and extended working temperature are the key issues hindering the realization of electrocaloric refrigeration technology. In this work, large ECE (ΔT = 0.8–0.9°C @ 4 kV/mm) with an ultrawide temperature span from 30 to 120°C is noted for lead-free (Na_1/2Bi_1/2)_0.80Sr_0.20(Zn_1/3Nb_2/3)_xTi_1-xO₃ ceramics. The excellent ECE performance can be ascribed to the evolution of polar nanoregions. Our work suggests that this material is promising for applications in solid-state refrigeration systems with a broad range of operating temperatures.     

Direct measurement of electrocaloric effect in lead-free (Na0.5Bi0.5)TiO3-based multilayer ceramic capacitors

Multilayer ceramic capacitors (MLCCs) of lead-free NBT-based ceramics are produced and their electrocaloric effect (ECE) is characterised for the first time. Dense MLCCs with 97μm-thick active layers are successfully produced by tape casting. Dielectric permittivity measurements reveal the MLCCs to have properties similar with that reported for the corresponding bulk ceramics, including T_d∼50°C and T_S∼100°C. Direct ECE measurements also reveal agreement and confirm the previously reported tendency of the high-field ECE peak to shift towards T_S. The highest value of ECE, ΔT_max∼1.7K is measured at 90°C under 90kV/cm. A low breakdown strength of 93kV/cm needs to be solved to realise stronger electric fields and achieve commercially viable ECE values.