Optimisation of SrBi2(Nb,Ta)2O9 Aurivillius phase for lead-free electrocaloric cooling

The influence of different substitutional mechanisms on the electrocaloric effect of a lead-free SrBi₂(Nb_0.2Ta_0.8)₂O₉ Aurivillius phase was studied for application in electrocaloric cooling systems. The A-site substitution with barium efficiently reduced the temperature of maximum permittivity from about 300?°C to 100?°C. The A-site substitution induced phenomena that are typical of strong relaxor ferroelectrics such as significant broadening of the permittivity peak and an increase in its frequency dispersion and with a depolarization temperature below room temperature. These features directly influenced the electrocaloric effect. A direct measurement system, based on a modified-differential scanning calorimeter, was used to analyze the EC effect of the dense (Sr_0.5Ba_0.5)Bi₂(Nb_0.2Ta_0.8)₂O₉ ceramics. In accordance with the relaxor characteristics, the EC effect was found to increase continuously over a broad temperature range above the room temperature. This was attributed to the alignment of field induced polar nanodomains. Directions for optimization towards a high-performing EC ceramic were identified.     

Controlled synthesis of Bi2O3–YSZ composite powders and their sintering behavior for high-performance electrolytes

Bismuth oxide (Bi₂O₃) is a promising additive to decrease the sintering temperature of yttria-stabilized zirconia (YSZ)-based electrolyte for solid oxide fuel cell application. However, Bi₂O₃ tends to grow into large column bars (>50 µm) in a chemical coprecipitation method, which dramatically limits the mixing uniformity of Bi₂O₃ and YSZ, even much worse than that of mechanical mixing. In this study, the reaction temperature was increased from room temperature to 90°C to increase the number of nucleation during the violate reaction between Bi³⁺ solution and YSZ suspension in NaOH. On this basis, the violence of the reaction was further moderated by adding half of NaOH first, then YSZ powders and the other half of an NaOH solution. The size of Bi₂O₃ was further decreased to sub-micrometer and Bi₂O₃ was homogeneously mixed with YSZ particles, even when its addition amount was as large as 20 mol%. These composite powders effectively promoted the sintering behavior of YSZ. The sintering temperature of YSZ was decreased to 900 and 1000°C with 10 and 5 mol% Bi₂O₃ doping, respectively. Increasing the doping ratio induced severe volatilization of Bi₂O₃ and pore formation. Raising the sintering temperature (no more than 1200°C) enhanced the doping effect of Bi₂O₃ into the YSZ lattice but induced instability in the YSZ crystal structure.     

Dielectric,ferroelectric and induced strain behavior of PLZT 9/65/35 ceramics modified by Bi2O3 and CuO co-doping

PLZT 9/65/35 (Pb_0.91La_0.09(Zr_0.65Ti_0.35)_0.9775O₃) ceramics with addition of 0.25, 0.5 and 1.0 wt% of Bi₂O₃/CuO (where the ratio of Bi₂O₃:CuO=9:1 by mole) were prepared by sintering at the temperatures between 1000 and 1200 °C. It was found that Bi₂O₃/CuO could bring the sintering temperature down ~50 °C to obtain PLZT with no second phase. Dielectric and ferroelectric properties were investigated. Bi₂O₃/CuO decreased both coercive field and remnant polarization, which was caused by an increase of the degree of diffuseness in relaxor ferroelectric materials. Electric field induced strain behavior was also investigated and it was found that the addition of Bi₂O₃/CuO increased the maximum induced strain and maximized electrostrictive effect. Therefore, Bi₂O₃/CuO was useful as a sintering aid, which improved the dielectric and the relaxor ferroelectric properties as well as the electric field induced strain of PLZT ceramics.     

Influence of the composition-induced structure evolution on the electrocaloric effect in Bi0.5Na0.5TiO3-based solid solution

The present work investigated the influence of the composition induced structure evolution on the electrocaloric effect in lead-free (0.935−x)Bi_0.5Na_0.5TiO₃–0.065BaTiO₃–xSrTiO₃ (BNBST, BNBSTx) ceramics. It was found that broad ∆T peak could be observed for all compositions and the electrocaloric strength α (α=ΔT_max/ΔE) in BNBST0.02 could reach as high as 0.27 K mm/kV. The increase of the SrTiO₃ concentration led to a shift of ∆T_max to a lower temperature, resulting in a large near room-temperature electrocaloric strength α of 0.17 K mm/kV in BNBST0.22.     

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.     

Highly-efficient electromagnetic interference shielding and microwave dielectric behavior of a (Bi2O3 + B2O3)-doped MWCNT/BaTiO3 ceramic nanocomposite

Microwave dielectric properties along with electromagnetic interference shielding effectiveness (EMI SE) of a multi-walled carbon nanotube (MWCNT)/barium titanate (BaTiO₃) nanocomposite are investigated in this paper. Appropriate amount of sintering additive (Bi₂O₃ +?B₂O₃) was doped into some nanocomposites to reduce the sintering temperatures. The dielectric properties of the nanocomposites with various MWCNT and sintering additive contents were evaluated at different microwave frequency ranges. It was found that the incorporation of optimized amount of (Bi₂O₃ +?B₂O₃) can give rise to significantly good dielectric properties. Results also indicated that incorporation of 6?wt% (Bi₂O₃ +?B₂O₃) into 1.5?mm-thick nanocomposite containing 8?wt% MWCNT led to an EMI SE greater than 28?dB, suggesting this novel nanocomposite as a promising candidate for microwave absorption and electromagnetic interference applications.     

The near room temperature electrocaloric cycling refrigeration in Bi5Ti3FeO15/BiFeO3 mesoscopic composites: Experiment and simulation

Recently, the high efficient and environment–friendly electrocaloric cooling technology has become a hot topic. Unfortunately, the single process and extreme conditions have greatly limited its application. Due to the similar perovskite structure, Bi₅Ti₃FeO₁₅ and BiFeO₃ were selected to fabricate mesoscopic composites in this work to realize the double consecutive cooling effect. We propose that the non–collinear interfacial polarization is the origin of near room temperature electrocaloric cycling refrigeration. The electrons are trapped at the Bi₅Ti₃FeO₁₅/BiFeO₃ interfacial potential well, where the electrons possess discontinuous energy and jump by thermal activity. The structure, dielectric capacitance and ferroelectric polarization of the composite films were further analyzed. Finally, the performance of the mesoscale cooling device is simulated, which shows a promising avenue to high efficient double cycling refrigeration. More importantly, the results are helpful to understand the cycling principle of electrocaloric cooling.     

Enhanced piezoelectric properties and electrocaloric effect in novel lead‐free (Bi0.5K0.5)TiO3‐La(Mg0.5Ti0.5)O3 ceramics

The crystal structure, electromechanical properties, and electrocaloric effect (ECE) in novel lead‐free (Bi_0.5K_0.5)TiO₃‐La(Mg_0.5Ti_0.5)O₃ ceramics were investigated. A morphotropic phase boundary (MPB) between the tetragonal and pseudocubic phase was found at x = 0.01‐0.02. In addition, the relaxor properties were enhanced with increasing the La(Mg_0.5Ti_0.5)O₃ content. In situ high‐temperature X‐ray diffraction patterns and Raman spectra were characterized to elucidate the phase transition behavior. The enhanced ECE (ΔT = 1.19 K) and piezoelectric coefficient (d₃₃ = 103 pC/N) were obtained for x = 0.01 at room temperature. Meanwhile, the temperature stability of the ECE was considered to be related to the high depolarization temperature and relaxor characteristics of the Bi_0.5K_0.5TiO₃‐based ceramics. The above results suggest that the piezoelectric and ECE properties can be simultaneously enhanced by establishing an MPB. These results also demonstrate the great potential of the studied systems for solid‐state cooling applications and piezoelectric‐based devices.     

Effects of CoO and Bi2O3 single/dual sintering aids doping on structure and properties of Ce0.8Nd0.2O1.9

Nd_0.2Ce_0.8O_3-δ (NDC) is one of the most common solid electrolyte materials used in solid oxide fuel cells (SOFCs). However, the densification temperature of NDC electrolyte is above 1400 °C. In this work, Bi₂O₃ and CoO sintering aids were individually or synergistically added to Nd_0.2Ce_0.8O_3-δ (NDC) electrolytes through the sol-gel method to lower its sintering temperature. Effects of Bi₂O₃-CoO dual sintering aid on the sintering behavior, phase composition, microstructure, and electrochemical properties of NDC electrolyte were all investigated. The data revealed that Bi₂O₃-CoO dual-sintering aid doped-NDC (labeled as NDC-CB) possessed high density and superior conductivity at low temperatures, better than that of Bi₂O₃ or CoO single sintering aid. NDC electrolyte doped with Bi₂O₃-CoO dual-sintering aid achieved highest relative density of 95.3% at 1100 °C and total conductivity of 5.765 × 10^-2 S cm^-1 at 800 °C. Furthermore, NDC-CB displayed excellent physical and chemical compatibility with La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (LSCF) cathode and NiO-NDC anode. Oxygen reduction reaction at LSCF/NDC-CB interface was improved by about 40% when compared to NDC. In sum, Bi₂O₃-CoO looks promising as dual-sintering additive for lowing sintering temperature and increasing electrical conductivity of NDC. Therefore, NDC-CB might be potential electrolyte for future intermediate-temperature solid oxide fuel cells (IT-SOFCs).     

Effect of Li2CO3–Bi2O3 doping on the low-temperature sintering,structure, and dielectric and mechanical properties of MgO–TiO2(w) ceramics

Dense MgO–12% TiO₂(w) ceramics containing 12 wt% TiO₂, which were doped with Li₂CO₃–Bi₂O₃ composite sintering aids, were prepared at a low sintering temperature of 950 °C in this study. The effects of sintering additives on the sintering characteristics, phase composition, microstructure, and dielectric and mechanical properties of the ceramic samples were systematically investigated, and the influences of their phase composition and microstructure on the dielectric and mechanical properties were examined. The introduction of sintering aids produced a new Bi₄Ti₃O₁₂ phase in the sample structure, while the residual Bi₂O₃ mixed with the newly formed Mg₂TiO₄ and Bi₄Ti₃O₁₂ phases distributed at MgO grain boundaries formed a structure surrounding MgO grains. This structure filled the pores in the ceramic sample, which increased its density and enhanced the mechanical properties. At a Li₂CO₃–Bi₂O₃ content of 15 wt%, the density, flexural strength, and Vickers hardness of the ceramic samples reached their maximum values of 3.4 g/cm³, 218.9 MPa, and 778.7 HV, respectively. However, the further increase in the Li₂CO₃–Bi₂O₃ content deteriorated their dielectric properties although the dielectric constant and dielectric loss remained below 13.4 and 2.1 × 10⁻³, respectively. The findings of this work indicate that Li₂CO₃–Bi₂O₃ sintering aids can significantly lower the sintering temperature of MgO–12% TiO₂(w) ceramics and control their dielectric and mechanical properties through microstructural changes.     

Dielectric and magnetic studies of 0.5Bi2/3Cu3Ti4O12 - 0.5Bi3LaTi3O12 nano-composite ceramic synthesized by semi-wet route

A nano-composite electro ceramic with the chemical composition of 0.5Bi_2/3Cu₃Ti₄O₁₂ - 0.5Bi₃LaTi₃O₁₂ was synthesized by a semi-wet route using high purity metal nitrate and solid TiO₂ in a stoichiometric ratio. X-ray diffraction (XRD) analysis showed the presence of Bi₃LaTi₃O₁₂ (BLTO) and Bi_2/3Cu₃Ti₄O₁₂ (BCTO) phases in the composites sintered at 900 °C for 8 h. Transmission electron microscope (TEM) analysis of the composite shows the presence of nanoparticles in the range of 55±3 nm. Atomic force microscopy (AFM) study also substantiates the presence of nanoparticles in the composite. Scanning electron microscope (SEM) images show that the surface morphology consists of plates like and spherical grains. The study of PE hysteresis loop revealed no saturation polarization which suggested lossy capacitor behavior of the composite. Magnetic behavior of the composite shows the weak ferromagnetic nature in M-T and M-H curve. The High observed value of dielectric constant (ε’=13.94×10³) of the composite may be due to the presence of space charge polarization.     

Improving the multicaloric properties of Pb(Fe0.5Nb0.5)O3 by controlling the sintering conditions and doping with manganese

Designing multicaloric single-phase materials with combined electro- and magnetocaloric effects is still at its initial stage and presents a number of challenges. One of the main challenges encountered so far is to reduce the excessive electrical conductivity, which leads to the appearance of Joule heating that might completely degrade the electrocaloric response. In this work, multicaloric Pb(Fe_0.5Nb_0.5)O₃ material was successfully prepared exhibiting pronounced electrocaloric effect above room temperature and maximum magnetocaloric effect at cryogenic temperature. The conductivity was suppressed by controlling the sintering temperature. The ceramic sintered at 1000 °C exhibits maximum electrocaloric effective cooling of 0.88 °C at 28 °C and maximum magnetocaloric effect of 0.14 °C at ?271 °C. The caloric properties can be further improved by doping Pb(Fe_0.5Nb_0.5)O₃ with manganese. In comparison to the undoped sample, Pb(Fe_0.5Nb_0.5)O₃ doped with 0.5 mol% of manganese exhibits three times higher maxima of electrocaloric effective cooling (2.47 °C at 80 °C) and magnetocaloric temperature change (0.44 °C at ?271 °C).     

Sintering behavior and characteristics study of BaTiO3 with 50 wt% of B2O3-Bi2O3-SiO2-ZnO glass

The thermal analysis of B₂O₃-Bi₂O₃-SiO₂-ZnO (BBSZ) glass with different particle sizes and LiF addition was researched to study its temperature behavior. Next the composites with 50 wt% BaTiO₃–50 wt% BBSZ glass were prepared for shrinkage, microstructures and dielectric properties investigations. The differently treated BBSZ glass showed that the smaller glass particles clearly decreased its softening and crystallization temperatures. LiF addition had the same but much weaker effect.The composites showed two-stage shrinkage related to the softening of the glass and new phase generation of Bi₂₄Si₂O₄₀ at 385–450 °C, and Bi₄BaTi₄O₁₅ over 680 °C. The microstructures of the composites sintered at 720 °C showed Bi₄BaTi₄O₁₅, BaTiO₃ and Bi₂₄Si₂O₄₀ with residual ZnO phase. LiF addition increased the amount of Bi₄BaTi₄O₁₅, thus increasing the loss value. However the particle size of the glass did not effect to the dielectric properties of the composites showing permittivity of 248–256 and loss of 0.013 at 100 kHz.     

Effects of Sb2O3 on the microstructure and electrical properties of ZnO–Bi2O3-based varistor ceramics fabricated by two-step solid-state reaction route

In this work, the ZnO–Bi₂O₃–Cr₂O₃–Co₂O₃–MnO₂ varistors doped with different content of Sb₂O₃ were prepared by two-step solid-state reaction route, including a pre-calcining of the mixtures of nanosized ZnO and the other additives at an optimized temperature, followed by a consequent sintering process at different temperatures. Meanwhile, the effects of Sb₂O₃ on the sintering temperature, microstructure and electrical properties of the objective varistors were investigated. It was found the densification temperature went up in a proper range and the content of pyrochlore phase, spinel phase and β-Bi₂O₃ phase increased with the increasing content of Sb₂O₃, while the grain size of ZnO–Bi₂O₃-based varistor reduced. The results demonstrated that at the same sintering temperature, the second particles increased with the increasing amount of Sb₂O₃, which was helpful to control the grain growth, leading to a higher breakdown voltage. However, the decrease of α-Bi₂O₃ phase (melting point of α-Bi₂O₃ phase is 825 °C), which is the main component of the liquid Bi₂O₃ phase in the sample during sintering process, leads to the increase of the sintering temperature of the green pallet. As a result, the ZnO varistor doped with 3.0 mol% Sb₂O₃ sintered at 1000 °C exhibited the highest breakdown voltage of 1863.3 V/mm. By contrast, the ZnO varistor without Sb₂O₃ doping sintered at 900 °C had the optimum nonlinear coefficient of 59.8.     

Development of a lead‐free copper co‐fired BNT‐based piezoceramic sintered at low temperature

Compatibility of Bi‐based piezoelectric ceramic and copper electrodes is demonstrated by co‐firing 0.88Bi_1/2Na_1/2TiO₃–0.08Bi_1/2K_1/2TiO₃–0.04BaTiO₃ (BNKBT88) with copper. A combination of Bi₂O₃, CuO, ZnO, Li₂CO₃, and B₂O₃ are used as additives to reduce firing temperature to 900°C with minimal effect on the electromechanical properties compared to sintering at 1150°C without additives. Co‐firing with copper electrodes requires controlled oxygen sintering at low temperature. The atmosphere is controlled using carbon dioxide and hydrogen gas to maintain an oxygen partial pressure of 6.1 × 10^?8 atm, which is necessary for the coexistence of Cu metal and Bi₂O₃. The thermodynamic activity of bismuth oxide in BNKBT88 is calculated to be 0.38. BNKBT88 ceramics were successfully co‐fired with internal as well as surface Cu metal electrodes. The copper co‐fired ceramics were successfully polarized and the dielectric and piezoelectric properties are evaluated.     

Effect of Bi2O3 content on sintering and crystallization behavior of low-temperature firing Bi2O3–B2O3–SiO2 glasses

The sintering behavior of a Pb-free Bi₂O₃–B₂O₃–SiO₂ glass system was examined as a function of Bi₂O₃ content. The glass transition temperature and the crystallization temperature of the glasses decreased with different decreasing gradients as the Bi₂O₃ content increased. The change in temperature affected the sintering behaviors of the glasses. In the case of the 40 mol% Bi₂O₃ addition, large pore accompanied over-firing phenomenon was observed when the sample was sintered over the optimum sintering temperature. However, over-firing was not observed in the sample with 45 mol% of Bi₂O₃ because of the crystallized phases during sintering. When the Bi₂O₃ content was 50–55 mol%, the crystallization temperature became lower than the glass transition temperature, which resulted in the crystallization of glass and it hindered densification.     

Investigation of grain boundary diffusion and grain growth of lithium zinc ferrites with low activation energy

Activation energy and diffusion kinetics are important factors for grain growth and densification. Here, Bi₂O₃ was introduced into Li_0.43Zn_0.27Ti_0.13Fe_2.17O₄ ferrite ceramics via a presintered process to lower the reaction activation energy and to achieve low temperature sintering. Interestingly, Bi³⁺ ions entered the lattice and substituted for Fe³⁺ in the B‐site (i.e., a pure LiZn spinel ferrite). Also, SEM image results show that Bi₂O₃‐substituted LiZn ferrite ceramics have low critical temperature for grain growth (920°C), which is very advantageous for LTCC technology. This indicates that Bi₂O₃ is an excellent dopant for ceramics. Furthermore, to promote normal grain growth of the ceramics at low temperatures, different volumes of V₂O₅ additive were added at the final sintering stage. Results indicate that an optimal volume of V₂O₅ additive promotes grain growth (with no abnormal grains) and enhances magnetic performances of the ceramics at low sintering temperature. Finally, adding the optimal volume of V₂O₅ additive resulted in a homogeneous and compact LiZnTiBi ferrite ceramic with larger grains (average size of ~8 μm), high 4πM_s (~4100 gauss), and low ΔH (~190 Oe) obtained (at 900°C). Moreover, the doping method reported in this study also provides a reference for other low temperature sintered ceramics.     

Low-temperature constrained sintering of YSZ electrolyte with Bi2O3 sintering sacrificial layer for anode-supported solid oxide fuel cells

Solid oxide fuel cells (SOFCs) have strong potential for next-generation energy conversion systems. However, their high processing temperature due to multi-layer ceramic components has been a major challenge for commercialization. In particular, the constrained sintering effect due to the rigid substrate in the fabrication process is a main reason to increase the sintering temperature of ceramic electrolyte. Herein, we develop a bi-layer sintering method composed of a Bi₂O₃ sintering sacrificial layer and YSZ main electrolyte layer to effectively lower the sintering temperature of the YSZ electrolyte even under the constrained sintering conditions. The Bi₂O₃ sintering functional layer applied on the YSZ electrolyte is designed to facilitate the densification of YSZ electrolyte at the significantly lowered sintering temperature and is removed after the sintering process to prevent the detrimental effects of residual sintering aids. Subsequent sublimation of Bi₂O₃ was confirmed after the sintering process and a dense YSZ monolayer was formed as a result of the sintering functional layer-assisted sintering process. The sintering behavior of the Bi₂O₃/YSZ bi-layer system was systematically analyzed, and material properties including the microstructure, crystallinity, and ionic conductivity were analyzed. The developed bi-layer sintered YSZ electrolyte was employed to fabricate anode-supported SOFCs, and a cell performance comparable to a conventional high temperature sintered (1400 °C) YSZ electrolyte was successfully demonstrated with significantly reduced sintering temperature (<1200 °C).     

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.     

Preparation and properties of novel,flexible, lead‐free X‐ray‐shielding materials containing tungsten and bismuth(III) oxide

Novel, flexible, lead‐free X‐ray‐shielding composites were prepared with a high‐functional methyl vinyl silicone rubber (VMQ) matrix with W and Bi₂O₃ as filler materials. To verify the advanced properties of the lead‐free material, composites with the same mass fraction of PbO were compared. With the X‐ray energy ranging from 48 to 185 keV, the W/Bi₂O₃/VMQ composites exhibited higher X‐ray‐shielding properties. As the filler volume fraction decreased, the tensile strength, elongation, tear strength, and flexibility of the W/Bi₂O₃/VMQ composites increased. The Shore hardness of the W/Bi₂O₃/VMQ composites had a maximum value of 46.6 HA and was still very flexible. With decreasing filler volume fraction, the water‐vapor transmission performances of the W/Bi₂O₃/VMQ composites increased, and the W/Bi₂O₃/VMQ composites also showed better water‐vapor permeability. The heat‐transfer properties of the W/Bi₂O₃/VMQ composites increased with increasing W content, and when the W content exceeded 70 wt %, the thermal conductivity of the W/Bi₂O₃/VMQ material was about 70.45% higher than that of the PbO/VMQ composite. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43012.