Grain size effect and microstructure influence on the energy storage properties of fine‐grained BaTiO3‐based ceramics

The dependence of energy storage properties on grain size was investigated in BaTiO₃‐based ferroelectric ceramics. Modified BaTiO₃ ceramics with different grain size were fabricated by two‐step sintering method from BaTiO₃ powders doped with Al₂O₃ and SiO₂ by aqueous chemical coating. For samples doped with ZnO sintering aid in addition to Al₂O₃‐SiO₂, the density and breakdown strength increased significantly. In general, samples with smaller grains have lower polarization but higher energy storage efficiency. Al₂O₃‐SiO₂‐ZnO‐doped samples with average grain size of 118±2 nm have an energy density of 0.83±0.04 J/cm³. Obvious segregation of doping elements in second phase and grain boundary was observed by TEM‐EDS. Impedance spectroscopy further explains the relationship between microstructure and properties. Compared to common energy storage ceramics, the grain size of this low‐cost ceramics sintered at relatively low temperature is small, and the pilot scale production has been well completed. All these features make the utilization in multilayer devices and industrial mass production possible. In addition, the obtained rules are helpful in further development of energy storage ceramics.     

0.73ZrTi2O6–0.27MgNb2O6 microwave dielectric ceramics modified by Al2O3 addition

0.73ZrTi₂O₆–0.27MgNb₂O₆ ceramics with various Al₂O₃ contents (0‐2.0 wt%) were prepared by conventional ceramic route. The effects of Al₂O₃ on the phase composition, microstructure, conductivity, and microwave dielectric properties were systematically investigated. The coexistence of a disordered α–PbO₂‐type phase and a rutile second phase was found in all compact ceramics with low Al₂O₃ contents (x = 0, 0.5, and 1.0 wt%), while a corundum phase was detected when Al₂O₃ additive increased to 1.5 and 2.0 wt% based on X‐ray diffraction results. With the addition of Al₂O₃, the decreased grain size of the matrix phase was observed using field‐emission scanning electron microscope, accompanied with increased resistivity and band‐gap energy. Additionally, Al₂O₃ additives efficiently improved the quality factor of the ceramics. After sintering at 1360°C for 3 hours, the ceramic with 1.0 wt% Al₂O₃ exhibited excellent microwave dielectric properties: a dielectric constant of 43.8, a quality factor of 33 900 GHz (at 6.6 GHz), and a near‐zero temperature coefficient of resonant frequency (3.1 ppm/^°C).     

Selective laser sintering of porous Al2O3-based ceramics using both Al2O3 and SiO2 poly-hollow microspheres as raw materials

Porous Al₂O₃-based ceramics with improved mechanical strength and different pore size were fabricated using Al₂O₃ and SiO₂ poly-hollow microspheres (PHMs) as raw materials by selective laser sintering (SLS). The effects of different contents of SiO₂ PHMs on phase compositions, microstructures, mechanical properties and pore size distribution of the prepared ceramics were investigated. It is found that moderate content of SiO₂ PHMs (≤30 wt%) could work as a sintering additive, which could enhance the bonding necks between Al₂O₃ PHMs. When the content of SiO₂ PHMs increased from 0 wt% to 30 wt%, the compressive strength of Al₂O₃-based ceramics increased from 0.3 MPa to 4.0 MPa, and the porosity decreased from 77.0% to 65.0% with open pore size decreased from 52.0 μm to 38.3 μm. However, SiO₂ PHMs could provide pores by keeping its integrity when the content of SiO₂ PHMs increased to 40 wt%, which could result in the porosity increasing to 66.8% and pore size decreasing to 30.1 μm. Selective laser sintering of different kinds of ceramic PHMs is a feasible method to fabricate porous ceramics with complex shape, controllable pore size and improved properties.     

The properties of Al2O3 coated fine‐grain temperature stable BaTiO3‐based ceramics sintered in reducing atmosphere

Chemical coating, an effective doping modification method, was employed to fabricate fine‐grain BaTiO₃‐based ceramics. Based on the consideration of subsequently using base metal as inner electrodes in multilayer ceramic devices, green bodies are generally sintered in reducing atmosphere, which generates more charged point defects and thus affects the electric properties. According to the elements distribution analysis, Al element is greatly enriched in the grain boundary and shell region. Coating Al₂O₃ achieves not only a smaller grain size and narrower distribution but also a higher breakdown strength, discharge energy density and energy efficiency at ambient temperature. In addition, temperature dependences of dielectric and energy storage properties under a same field were also investigated. Over the whole measuring temperature range, the sample with Al₂O₃ remains higher discharge energy density and energy efficiency.     

Enhanced microwave absorption and electromagnetic shielding property of (1-x)K0.5Na0.5NbO3 ~ xAl2O3 nano-ceramics

(1-x) K_0.5Na_0.5NbO₃ ~ xAl₂O₃ (x = 0, 0.2, 0.4, 0.6) ceramics were prepared via a traditional solid-state reaction method. The phase structure, micro-morphology, dielectric properties and electromagnetic properties of ceramic samples were studied and analyzed. Results indicate that all the samples are similar to K_0.5Na_0.5NbO₃ (KNN) in perovskite structure. With the increase of Al₂O₃ content, the X-ray diffraction peaks move to a large angle region, suggesting the substitution of niobium ions by aluminium ions and the distortion of the KNN lattice with a new phase arising. With the increase of Al₂O₃ content the grain size reduces and the dielectric constant decrease, yielding to the decrease of the electromagnetic shielding performance of ceramic. When the x is 0.4, the minimum value of reflectivity of sample is −28 dB at the frequency of 11.6 GHz. It can be concluded that both the grain size and Al₂O₃ content can obviously affect the electromagnetic properties of ceramics, which can be easily turned through a multi-layer SiO₂ heterojunction structure.     

The formation of Y5V-type fine-grained ceramics based on spherical submicron BaZr0.1Ti0.9O3@Al2O3 particles

To improve multilayer ceramic capacitors (MLCCs), thinner dielectric layers are necessary. To achieve this goal, both grain size and uniformity of the MLCC particles must be controlled effectively. In this study, the core-shell structure of submicron-sized multi-function ceramic capacitors powder was synthesized using a novel precipitation route, which controls both dispersion and particle size of BaZr_0.1Ti_0.9O₃ and BaZr_0.1Ti_0.9O₃@Al₂O₃ particles. In this paper, we investigate the effect of Al₂O₃ coating on the microstructure and the dielectric properties of BaZr_0.1Ti_0.9O₃. We found that both average grain size and maximum dielectric constant (ε_max) of the ceramics decrease with increasing concentration of Al₂O₃. Our results demonstrate that fine-grained ceramic materials can meet the specifications of the Electronic Industries Alliance Y5V with a concentration of Al₂O₃-coated of 0.25 mol percent, a permittivity of 3393 at room temperature, and an average particle size of about 400 nm.     

Significant enhancement in breakdown strength and energy density of the BaTiO3/BaTiO3@SiO2 layered ceramics with strong interface blocking effect

The BaTiO₃/BaTiO₃@SiO₂ (BT/BTS) ceramics with layered structure, where grain size was about 1–2 μm in the BT layer while it was about 300–400 nm in the BTS layer, were fabricated by the tape-casting and lamination method. With the increasing of SiO₂ content in the BTS layer, the dielectric constant decreased gradually, and the breakdown strength was remarkably improved. Compared to the SiO₂-added BaTiO₃ bulk ceramics, the layered ceramics displayed significant enhancements in dielectric properties, breakdown properties and energy storage properties. The enhancement in dielectric properties was mostly attributed to the diluting effects created by this structure to SiO₂. Based on the finite element analysis with the dielectric breakdown mode, it was regarded that the electric field redistribution and the interface blocking effect led to the enhancement of breakdown strength. Finally, the maximum energy density of 1.8 J/cm³ was obtained at a breakdown strength of 301.4 kV/cm for the BT/BTS3 ceramic.     

Enhanced dielectric characteristics of Ba0.94Bi0.04(Fe0.5Nb0.5)O3 coated with SiO2 and SiO2/Al2O3 over a broad frequency and temperature range

Pure phase of Ba_0.94Bi_0.04(Fe_0.5Nb_0.5)O₃ (BBFN) nano-particles were obtained by chemical co-precipitation method. The core-shell structure of BBFN@SiO₂ and BBFN@SiO₂/Al₂O₃ particles and the target ceramics were successfully prepared by aqueous chemical coating approach. The microstructures and dielectric properties of BBFN@SiO₂ and BBFN@SiO₂/Al₂O₃ were studied. Both the BBFN@SiO₂ and BBFN@SiO₂/Al₂O₃ samples show significantly decreased dielectric loss and good frequency and temperature stability on relative permittivity. Compared to the rapid decline of relative permittivity of BBFN@SiO₂, the synergistic effect of SiO₂ and Al₂O₃ in BBFN@SiO₂/Al₂O₃ ceramics made the relative permittivity of which remains a relatively high level with very low dielectric loss, making it more suitable in colossal permittivity applications. Based on the impedance analysis, the grain boundary effect and IBLC models play the important role for the improvement of dielectric properties of BBFN@SiO₂/Al₂O₃ samples.     

Role of Nano‐ and Micron‐Sized Particles of TiO2 Additive on Microwave Dielectric Properties of Li2ZnTi3O8 – 4 wt% TiO2 Ceramics

Microwave dielectric ceramics with the composition of Li₂ZnTi₃O₈ – 4 wt% TiO₂ were synthesized by the conventional solid‐state reaction. 4 wt% TiO₂ powders with different particles size were added to the Li₂ZnTi₃O₈ ceramic. Then the ceramic samples were sintered at temperatures 1075°C, 1050°C, 1000°C, and 950°C for 4 h. The effect of the particles size of TiO₂ additive on the microwave dielectric properties of the ceramics has been investigated. In the study two categories of particles size of TiO₂ additive have been used; (i) Nanoparticle (50 nm), (ii) Micron sized (40, 5, 1 μm) powder. X‐ray showed that the TiO₂ additive has not solved in the LZT structure and has not almost undergone chemical reaction with the LZT ceramic. The results showed that the addition of TiO₂ nanoparticles to the LZT ceramics significantly improved the density and a dense and uniform microstructure and also abnormal grain growth were observed by SEM. The use of TiO₂ nanoparticle reduces porosity and leads to an increase in green density. The maximum density was found to be 98.5% of the theoretical density and the best relative permittivity of 28, quality factor of 68000 GHz and τ_f value of ?2 ppm/°C were obtained for the samples added with 4 wt% of the TiO₂ nanoparticles, sintered at 1050°C for 4 h.     

Microwave dielectric properties of Al2O3 ceramics sintered at low temperature

The sintering behavior, microstructure and microwave dielectric properties of Al₂O₃ ceramics co-doped with 3000ppmCuO₂+6000ppmTiO₂+500ppmMgO (Cu/Ti/Mg) have been investigated. The results show that 1 wt% Cu/Ti/Mg can reduce the sintering temperature of Al₂O₃ ceramics effectively. Samples with relative densities of ≥97% and uniform microstructure can be obtained when sintered at 1150 °C. Higher temperature can further increase the density of the sample, but it inevitably leads to abnormal grain growth. Meanwhile, the investigation results show that the low-firing Al₂O₃ ceramics have good microwave dielectric properties especially high Q × f value. A high Q × f value of 109616 GHz is able to be obtained for the 1150 °C sintered sample. The reason for the low temperature densification, abnormal grain growth behavior and the changing trend of the microwave dielectric properties are discussed in the paper.     

The effect of BaTiO3 addition on the dielectric constant of Si3N4 ceramics

Si₃N₄ ceramics with different BaTiO₃ contents have been fabricated by pressureless sintering in a N₂ atmosphere at 1680°C for 2 h. Al₂O₃ and Nd₂O₃ were used as sintering additives to promote the densification of Si₃N₄ ceramics. The effect of BaTiO₃ addition on the densification, mechanical properties, phase compositions, microstructure, and dielectric properties of Si₃N₄ ceramics was investigated. The relative density and flexural strength of Si₃N₄ ceramics increased with the addition of BaTiO₃ up to 15 wt% and then decreased, while the dielectric constant increased continuously as the BaTiO₃ contents increased. The dielectric constant of Si₃N₄ ceramics can be tailored in the range from 8.42 to 12.96 by the addition of 5 wt%‐20 wt% BaTiO₃. Meanwhile, these Si₃N₄ ceramics all had flexural strength higher than 500 MPa.     

Fabrication of Highly Textured Fine‐Grained α‐Alumina by Templated Grain Growth of Nanoscale Precursors

[0001] textured alumina ceramics with a fine grain size were fabricated between 1400°C and 1600°C via templated grain growth (TGG) using fine alumina platelets (~0.6 and ~3 μm diameter) aligned by tape casting in either a 50 nm α‐Al₂O₃ matrix powder, or in a seeded boehmite sol. The 3 μm templates could be readily aligned by tape casting in both matrices (orientation parameters r = 0.27 and 0.18, respectively), whereas 0.6 μm diameter templates were well aligned in the seeded boehmite sol only (r = 0.29). Improved alignment in boehmite sols is attributed to inorganic gelation, resulting in a strongly pseudo‐plastic rheology that preserves template alignment against the influence of Brownian motion. The in situ formation of fine α‐Al₂O₃ matrix after transformation in the seeded boehmite system results in a higher driving force for TGG and improves texture development. The combination of 3 μm templates with a seeded boehmite matrix results in extremely high texture qualities (texture fraction f = 0.97–0.99, r = 0.17) while maintaining a relatively fine grain size (5–10 μm in diameter and 1.5–3 μm in thickness). Although undoped samples can be fully textured at 1600°C, adding as little as ~0.25 wt% CaO/SiO₂ dopant improves TGG kinetics and yields full texture at 1400°C.     

Fabrication of Al2O3/AlN composite ceramics with enhanced performance via a heterogeneous precipitation coating process

To improve the thermal and mechanical properties of Al₂O₃/AlN composite ceramics, a novel heterogeneous precipitation coating (HPC) approach was introduced into the fabrication of Al₂O₃/AlN ceramics. For this approach, Al₂O₃ and AlN powders were coated with a layer of amorphous Y₂O₃, with the coated Al₂O₃ and AlN powders found to favor the formation of an interconnected YAG second phase along the grain boundaries. The interconnected YAG phase was designed to act as a diffusion barrier layer to minimize the detrimental interdiffusion between Al₂O₃ and AlN particles. Compared with samples prepared by a conventional ball-milling method, the HPC Al₂O₃/AlN composites exhibited less AlON formation, a higher relative density, a smaller grain size and a more homogeneous microstructure. The thermal conductivity, bending strength, fracture toughness and Weibull modulus of the HPC Al₂O₃/AlN composite ceramics were found to reach 34.21 ± 0.34 W m⁻¹ K⁻¹, 475.61 ± 21.56 MPa, 5.53 ± 0.29 MPa m^1/2 and 25.61, respectively, which are much higher than those for the Al₂O₃ and Al₂O₃/AlN samples prepared by the conventional ball-milling method. These results suggest that HPC is a more effective technique for preparing Al₂O₃/AlN composites with enhanced thermal and mechanical properties, and is probably applicable to other composite material systems as well.     

Thermal Reaction of Cristobalite in Nano‐SiO2/α‐Al2O3 Powder Systems for Mullite Synthesis

Nanoscaled cristobalite and α‐Al₂O₃ powders were used as the starting materials for synthesizing mullite by solid‐state reaction. The thermal reaction of the cristobalite with α‐Al₂O₃ during the thermal treatment was examined. Cristobalite powder with a D₅₀ value of 430 nm was adopted to mix with α‐Al₂O₃ powders with a D₅₀ values of 230, 310, and 400 nm in a stoichiometric composition of 3Al₂O₃?2SiO₂ (71.8 wt% α‐Al₂O₃ and 28.2 wt% SiO₂). Samples for thermal reaction were prepared using uniaxial pressed from the three mixtures that showed various particle number ratios of SiO₂/Al₂O₃ due to the different particle sizes of α‐Al₂O_3. Examinations were performed by differential thermal analysis, X‐ray diffraction, scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, and transmission electron microscopy techniques. The results showed that cristobalite particles amorphized during the thermal treatment, and then reacted with the α‐Al₂O₃ particle to form mullite via nucleation and growth. The amorphization temperature can be reduced by using finer‐sized α‐Al₂O₃ powders, thus leading to a lower temperature for mullite formation. Mullite crystals with a multidomain structure were observed in the α‐Al₂O₃ particle matrixes. The crystal orientation of the mullite was controlled by the α‐Al₂O₃ matrix, that is, [001] α‐Al₂O₃ → [001] mullite. These results indicate that the amorphization of cristobalite may trigger the reaction of SiO₂ with α‐Al₂O₃, initiating the nucleation of mullite. The α‐Al₂O₃ particles act as the hosts for mullite formation and determine the size of the mullite particles.     

Grain size effect on electrical and reliability characteristics of modified fine-grained BaTiO3 ceramics for MLCCs

Fine-grained BaTiO₃-based ceramics of different grain sizes (118–462 nm) with core–shell structures were prepared by a chemical coating method, having good dielectric properties and gentle temperature stability. The grain size effect on the dielectric properties and insulation resistivity of modified fine-grained BaTiO₃ ceramics under high temperatures and electric fields were investigated. The DC bias shows a strong effect on the dielectric properties with decreasing grain size. In the finest ceramics, the absolute value of the capacitance stability factor was the smallest, indicating that the modified-BaTiO₃ ceramic capacitor with smaller grains had higher reliability under the DC bias voltage. The highly accelerated lifetime test results showed that with decreasing the grain size, samples exhibited higher insulation resistance under elevated temperatures and high voltages. Impedance analysis proved that the finer-grained ceramic with core–shell structure had higher activation energy for both grain and grain boundary, whereas the proportion of ionic conductivity was lower.     

Uniform Coating of BaTiO3–Dy2O3–SiO2 Compound Nano Layer on Ni Particles for MLCC Electrode

Uniform coating of nanometer‐scale BaTiO₃–Dy₂O₃–SiO₂ layers on spherical Ni particles are achieved by controlled hydrolysis of tetrabutyl titanate (TBT), hydrothermal reaction with Ba(OH)₂, and co‐precipitation of tetraethylorthosilicate (TEOS) and Dy(NO₃)₃. The composition of the coating layer is similar to rare earth oxide‐silica–doped BaTiO₃, which is the main component of dielectric layer for base metal electrode (BME) multilayer ceramic capacitors (MLCCs). After coating, the shrinkage onset temperature of Ni particles is significantly increased. After sintered to pellets, the electrode has good electrical conductivity. This electrode material has good compatibility with rare earth oxide and silica‐doped BaTiO₃ dielectric materials, and could serve as promising candidate for application in the next generation BME‐MLCCs.     

Microstructural evolution and dielectric properties of SiO2-doped CaCu3Ti4O12 ceramics

The abnormal grain growth (AGG) behavior of undoped and SiO₂-doped CaCu₃Ti₄O₁₂ (CCTO) ceramics were investigated. With the addition of 2 wt.% SiO₂, the AGG-triggering temperature decreased from 1100 to 1060 °C, and the temperature for obtaining a uniform and coarse microstructure decreased from 1140 to 1100 °C. The lowering of the AGG temperature by SiO₂ addition was attributed to the formation of a CuO-SiO₂-rich intergranular phase at lower temperature. The apparent dielectric permittivity of coarse SiO₂-doped CCTO ceramics was ∼10 times higher than that of fine SiO₂-doped CCTO ceramics at the frequency of 10³–10⁵ Hz. The doping of SiO₂ to CCTO ceramics provides an efficient route of improving the dielectric properties via grain coarsening. The correlation between the microstructure and apparent permittivity suggests the presence of a barrier layer near the grain boundary.     

A lightweight CNWs-SiO2/3Al2O3·2SiO2 porous ceramic with excellent microwave absorption and thermal insulation properties

To obtain excellent microwave absorption and thermal insulation properties, carbon nanowires reinforced SiO₂/3Al₂O₃·2SiO₂ composite ceramics (CNWs-SiO₂/3Al₂O₃·2SiO₂) were fabricated by catalytic chemical vapor deposition (CCVD) using C₂H₄ as the carbon source. The content of CNWs in SiO₂/3Al₂O₃·2SiO₂ porous ceramics can be adjusted by controlling the concentration of the catalyst precursor and the CCVD time. A higher concentration of catalyst precursor and longer CCVD time are beneficial for the growth of CNWs and for improving the electromagnetic wave (EMW) absorption properties of CNWs-SiO₂/3Al₂O₃·2SiO₂. However, CNWs are harmful to impendence matching due to the strong reflection and weak absorption when the content exceeds the threshold (30 wt%) in SiO₂/3Al₂O₃·2SiO₂ porous ceramics. CNWs are also harmful to the thermal insulation properties due to their high thermal conductivity. The results show that CNWs-SiO₂/3Al₂O₃·2SiO₂ can attain good EMW absorption and thermal insulation properties if the content of CNWs is 30 wt% when the concentration of the catalyst precursor is 3 wt% and the CCVD time is 15 min. The effective absorption bandwidth (EAB) can cover from 8.2 to 12.4 GHz (the whole X-band), and the minimum reflection coefficient (RC_min) is -31 dB at 9.1 GHz. The temperature gradient is 218 °C, which can satisfy the design requirement. Thus, the dielectric and thermal insulation properties are designable for CNWs reinforced SiO₂/3Al₂O₃·2SiO₂ porous ceramics to obtain excellent EMW absorption and thermal insulation properties.     

Novel Al2Mo3O12-PTFE composites for microwave dielectric substrates

Nowadays, microwave dielectric substrate materials have been extensively investigated to meet the requirements of rapid development in modern communications. Among them, the composites of ceramic powder filled polytetrafluoroethylene (PTFE) have been a hot topic. However, the compatibility and connectivity between the surface of ceramics and PTFE molecular chains in the samples are usually low. Herein novel PTFE based composites with different contents of Al₂Mo₃O₁₂ (20–60 wt%) modified by C₁₄H₁₉F₁₃O₃Si (F8261) coupling agent were designed and prepared. The coupling agent F8261 has been successfully grafted to the surface of Al₂Mo₃O₁₂ powders, effectively promoting the densification and dielectric properties of the composites. As the content of the modified Al₂Mo₃O₁₂ powders increases from 20 to 60 wt%, the ε_r value increases from 3.4 to 4.2, and tanδ almost remains constant at the beginning and increases with much more Al₂Mo₃O₁₂ added. The Al₂Mo₃O₁₂-PTFE composites filled with 30 wt% Al₂Mo₃O₁₂ present the optimal dielectric properties of ε_r = 3.6 and tanδ = 0.0018 with a high density of 95.6%. In addition, the electromagnetic and multiphysic simulation of a 24 GHz substrate integrated waveguide filter on the basis of the 30 wt% Al₂Mo₃O₁₂ - 70 wt% PTFE composite was carried out. It was revealed that the filter presented high stability on the electrical parameters caused by self-heating and dimension deformation due to the good microwave dielectric, thermal and mechanical properties of the substrate. These results indicate that the as-prepared 30 wt% Al₂Mo₃O₁₂ - 70 wt% PTFE composite would be a promising candidate for high-performance microwave dielectric substrates.     

Liquid phase sintering of Al2O3/SiC nanocomposites

Al₂O₃/SiC nanocomposites are usually prepared by hot pressing or using high sintering temperatures, viz. 1700°C. This is due to the strong inhibiting effect of the nano-sized SiC particles on the densification of the material. Liquid phase sintering (LPS) can be used to improve densification. This work explored two eutectic additive systems, namely MnO₂.SiO₂ (MS) and CaO.ZnO.SiO₂ (CZS). The additive content in Al₂O₃/5 wt% SiC nanocomposite material varied from 2 to 10 wt%. Densities of up to 99% of the theoretical value were achieved at temperatures as low as 1300°C. Characterisation of the materials by XRD, indicated the formation of secondary crystalline phases in addition to Al₂O₃ and SiC. SEM and TEM analysis showed the presence of a residual glassy phase in the grain boundaries, and an increase in the average grain size when compared to nanocomposites processed without LPS additives.