Effect of yttrium (Y) substitution on the structure and dielectric properties of BaTiO3

As the rate of application of multilayer ceramic capacitors (MLCCs) in small electronic devices increases, the use of the raw material barium titanate (BaTiO₃) with a small particle size and excellent dielectric properties becomes needed. Due to the size effect, small-sized BaTiO₃ generally has a cubic phase structure with a low dielectric constant, which limits its use in MLCCs. We report the preparation of small cubic phase Y-doped BaTiO₃ (BYT) nanoparticles by a hydrothermal method and the preparation of highly dielectric tetragonal phase BYT ceramics based on this method. XRD and Raman analysis showed that the BYT nanoparticles are in substable cubic phases. The particle size of the BYT nanoparticles, measured by TEM, XRD, and BET, was approximately 35 nm. The dielectric properties of the BYT ceramics were tested by an impedance analyzer, and the dielectric constant of the BYT ceramics was 7547 when the Y³⁺ doping amount was 0.5 mol%. In addition, the substitution mechanism of Y³⁺ doping in BaTiO₃ crystals was proposed from XPS and EPR analysis. The results demonstrate for the first time that the 50 nm cubic phase BaTiO₃ powder can meet the needs of next-generation high-capacity MLCCs. This work provides a reference for small cubic phase BaTiO₃ as a dielectric material for high-capacity MLCCs.     

Grain size engineered high-performance nanograined BaTiO3-based ceramics: Experimental and numerical prediction

The ultra-thin multilayer ceramic capacitors (MLCCs) with layer thickness less than 1 μm or even 0.5 μm are in urgent demand due to the rapid development of modern electronic industries. Notably, the dielectric and ferroelectric properties of nanograined BaTiO₃-based ceramics, which are widely used as dielectric materials in MLCCs, are highly related to grain size. In this work, nanograined BaTiO₃-based ceramics with various grain sizes (50-100 nm) were prepared via the chemical coating method. The grain size effect on the dielectric and energy storage properties were systematically investigated. TEM and EDS images demonstrate that the typical core-shell structure is obtained inside ceramic grains even if the grain size is reduced to 50 nm. The fine-grain ceramic displays a lower maximal polarization but a higher breakdown strength, which ascribes to its weaker ferroelectric contribution and higher grain boundary ratio, respectively. As a result, it is confirmed that there exists an optimal grain size around 70 nm where maximum discharge energy density is achieved under the synergy effect of breakdown strength and polarization, which is also verified by a finite element analysis based on a modified hyperbolic tangent model. All these features provide important guidance towards the design of ultra-thin layer MLCCs by optimizing the dielectric properties and energy storage performance while pursuing miniaturization.     

Importance of uniformity of grain size to reduce dc degradation and improve reliability of ultra-thin BaTiO3-based MLCCs

Excellent direct current (dc)-bias and reliability have become increasingly important for ultra-thin BaTiO₃-based multilayer ceramic capacitors (MLCCs). Herein, X5R-MLCCs with a thickness of ～1 μm are fabricated using BaTiO₃ with varied grain size. It is shown that the uniformity of the grain size plays an important impact on the direct current (dc)-bias and the reliability of ultra-thin MLCCs. Uniform grain size, which is indicative of good distribution of doping element contributes to improved temperature stability. By contrast, abnormal large grains induce reinforced space charge polarization. Chips with non-uniform grains exhibit dramatic dielectric constant change under dc-bias due to the high tetragonality and irreversible domain-wall motion. Weibull distribution and highly accelerated life test (HALT) reveal that non-uniform grains contain more oxygen vacancies supported by impedance spectra analysis at 300–450 °C. This study provides a feasible strategy to improve the dc-bias and the reliability of the ultra-thin MLCCs.     

Synthesis and performance of tetragonal BaTiO3 fine powders via a facile tartaric acid assisted method

A facile tartaric acid assisted method using metatitanic acid, barium hydroxide and tartaric acid as starting materials is proposed to prepare tetragonal BaTiO₃ fine powders. Owing to the ultrafine character of the as-formed BaCO₃ with high chemical activity, and its arrangement coating the surface of intermediate TiO₂ nanoscale needles, pure cubic phase BaTiO₃ homogeneous powders with size of about 50 nm was obtained via calcining treatment at 650 ℃, which subsequently transformed as tetragonal BaTiO₃ uniform powders with size of about 240 nm and high tetragonality (c/a=1.0095) after calcining treatment at 1050 ℃. The BaTiO₃ ceramic prepared from the BaTiO₃ uniform powders displayed much improvement performances with high permittivity of about 5980 and low dielectric loss of about 0.014 at room temperature in comparison to the BaTiO₃ ceramic prepared respectively from the BaTiO₃ synthesized via a traditional solid-state method, and commercial BaTiO₃, suggesting it’s compatible in application of MLCCs with high performance.     

Simultaneous enhancement of dielectric properties and temperature stability in BaTiO3-based ceramics enabling X8R multilayer ceramic capacitor

Modified BaTiO₃ ceramics that possess high dielectric permittivity and acceptable temperature stability have been widely utilized as multilayer ceramic capacitors (MLCCs) for high-frequency bypass and power filtering in automotive applications. However, since the increasing demand for high-capacity and small-size, high-permittivity materials that can serve as dielectric layers in MLCCs are urgently required. In this work, we design and fabricate a special BaTiO₃-0.03Mg-0.02Y-0.02CaZrO₃ ceramic with a high dielectric permittivity of 3000 and the dielectric variation below ±13% in the temperature range of -55–150°C, fulfilling the requirements of X8R capacitors. To achieve these results, we employed grain size engineering and cation doping, using BaTiO₃ precursors with a particle size of 240 nm to prepare the BaTiO₃-based ceramics with fine grains, while Mg and Y co-doping was used for improving the temperature stability due to dielectric dispersion. Utilizing these high-permittivity BaTiO₃-based materials, we fabricated MLCCs that satisfy the X8R criterion, possessing a high dielectric constant of 2950 and a high breakdown field (410 kV/cm).     

Shrinkage behavior and interfacial diffusion in Ni-based internal electrodes with BaTiO3 additive

The effects of particle size of starting materials and amount of a BaTiO₃ additive on the shrinkage behavior and elemental diffusion in Ni-based internal electrodes have been investigated in order to control the shrinkage of the internal electrode in multilayer ceramic capacitors (MLCCs). Two kinds of Ni and BaTiO₃ powders were used with different particle sizes. Volume shrinkage over the range of 700–1300 °C at 150 °C intervals and linear shrinkage during sintering were measured for starting materials and composites in a reducing atmosphere. The interfaces of Ni/BaTiO₃ composites with 90:10 and 70:30 volume ratios, respectively, were investigated using TEM. Composites with bimodal Ni powder show less shrinkage than those with monomodal Ni powder, showing less shrinkage in monolith Ni of bimodal particle size. The shrinkage behavior is changed during sintering with increasing amounts of BaTiO₃ additives in both Ni-based composites. The particle size of the BaTiO₃ additive affects the shrinkage behavior of composites, without the additional amount affecting the final shrinkage. A reaction layer of about 300 nm wide is observed at the interface between the Ni and BaTiO₃ powders in composites, in which elemental Ni diffuses into the BaTiO₃ without counterdiffusion.     

Synthesis and Characterization of Bowl-Like Single-Crystalline BaTiO3 Nanoparticles

Novel bowl-like single-crystalline BaTiO₃ nanoparticles were synthesized by a simple hydrothermal method using Ba(OH)₂·8H₂O and TiO₂ as precursors. The as-prepared products were characterized by XRD, Raman spectroscopy, SEM and TEM. The results show that the bowl-like BaTiO₃ nanoparticles are single-crystalline and have a size about 100–200 nm in diameter. Local piezoresponse force measurements indicate that the BaTiO₃ nanoparticles have switchable polarization at room temperature. The local effective piezoelectric coefficient d₃₃ ^* d_{33}^{ * } is approximately 28 pm/V.     

Investigation of thin film end-termination on multilayer ceramic capacitors with base-metal-electrode

The thin film of copper, chromium and titanium as end-termination studies were performed on multilayer ceramic capacitors (MLCCs) based on BaTiO₃ ceramic with nickel internal electrodes. A green sheet was prepared by tape casting using the X7R/BME powders. Nickel paste was attached to the green sheet as an internal electrode. After lamination, the green chips were sintered at 1300 °C for 2 h, then the external electrodes were sputtered as thin films for end-termination. There is no extra curing process, so that thermal shock of the MLCCs is reduced. To improve the adhesion between thin film end-termination and dielectric body, chromium and titanium were applied as media in this study. The mechanical and electrical properties of the MLCCs were investigated subsequently. The results showed that end-termination with chromium/copper has good performances on electrical and mechanical properties of MLCC, compared to conventional end-termination.     

Structure and dielectric properties of barium titanate thin films for capacitor applications

BaTiO₃ is a typical ferroelectric material with high relative permittivity and has been used for various applications, such as multilayer ceramic capacitors (MLCCs). With the tendency of miniaturization of MLCCs, the thin films of BaTiO₃ have been required. In this work, BaTiO₃ thin films have been deposited on Pt-coated Si substrates by RF magnetron sputtering under different deposition conditions. The films deposited at the substrate temperature from 550 °C–750 °C show a pure tetragonal perovskite structure. The films deposited at 550 °C–625  °C exhibit (111) preferential orientation, and change to (110) preferential orientation when deposited above 650 °C. The film morphologies vary with working pressure and substrate temperature. The film deposited at 625 °C and 4.5 Pa has the relative permittivity of 630 and the loss tangent of 2% at 10 kHz.     

Synthesis and surface modification of submicron BaTiO3 powders via a facile surfactant-assisted method

Due to their unique properties, well-dispersed barium titanate (BaTiO₃) ultrafine powders can be used in wide-ranging fields. In the present work, by using barium hydroxide octahydrate (Ba(OH)₂·8H₂O) and α titanic acid (H₄TiO₄) as raw materials, uniform submicron BaTiO₃ powders with tetragonal structure and high degree of crystallinity were prepared via a solid-state reaction method at relatively low temperatures. Moreover, by simply using the stearic acid (St) as the modifier to modify the surface of the aggregated BaTiO₃ powders, well-dispersed BaTiO₃ particles could be obtained, which were then examined by complementary characterizations such as XRD, TEM, HRTEM, SEM, Raman, FT-IR, XPS and EDS. The results indicated that the tetragonal BaTiO₃ particles with submicron-size, good uniformity, and high crystallinity could be prepared at 800 °C for 1 h. Moreover, the addition of St for surface modification proved to be an effective way to avoid the agglomeration of the BaTiO₃ particles to get well-dispersed products, where 1 wt % of St was found to be the optimum concentration. The demonstrated surfactant-assisted surface modification method is expected to be applicable for other ultrafine powders to get well-dispersed particles.     

Percolative BaTiO3–Ni composite nanopowders from alkoxide-mediated synthesis

BaTiO₃–Ni nanopowders have been synthesized via an alkoxide-mediated synthesis route through the hydrolysis and condensation of barium hydroxide octahydrate and titanium (IV) isopropoxide in the presence of submicron sized, spherical Ni particles originating from a commercial Ni paste, that was introduced during the preparation procedure. X-ray diffraction (XRD) patterns indicate that nanocomposite powders of the phases BaTiO₃ and Ni could be successfully prepared and tailor-made composition control was confirmed. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that the synthesized BaTiO₃ nanoparticles were aggregates of nanosized primary particles as small as 40 nm in diameter. The average Ni particle size was estimated to be about 200 nm. Dilatometric measurements on green compacts of these powders revealed that the shrinkage of BaTiO₃–Ni composites is retarded compared to both, pure BaTiO₃ and Ni. Thermogravimetric analysis (TGA) shows weight losses due to the decomposition of organic binder from Ni paste, the release of water from the surface and of hydroxyl ions from inside the lattice of the BaTiO₃ nanoparticles. With the addition of nickel, the dielectric constant increased slightly due to the percolation effect.     

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.     

Direct Observation of Thickness Dependence of Ferroelectricity in Freestanding BaTiO3 Thin Film

In this work, we focus on the thickness dependence of the electric polarization in a freestanding BaTiO₃ thin film. The locations of titanium and oxygen ions as well as their relative displacements were directly observed and measured by using aberration‐corrected transmission electron microscopy (AC‐TEM). It is found that the freestanding BaTiO₃ thin film keeps the tetragonal distortion when its thickness is thicker than 4.8 nm, while completely loses its ferroelectricity when thinner than 4.4 nm. The method of combining high‐resolution AC‐TEM imaging with the multiparameter simulation unit cell by unit cell was developed to accurately determine each local thickness of the sample. This method is powerful for quantitative analysis in AC‐TEM images.     

Dy-modified barium calcium titanate sintered in a reducing atmosphere: Crystal structure,microstructure, and electrical characteristics

The microstructure, crystal structure, and electrical properties of calcium-doped BaTiO₃ (BCT) sintered at 1180 °C for 1 h in a reducing atmosphere were investigated in this study. A detailed mechanism for improving the reliability of Dy-modified BCT sintered in a reducing atmosphere has been proposed. Ca in BCT mostly substituted for Ba and occupied A-site, according to the Rietveld analysis. The Dy³⁺-added sample revealed a core-shell structure with lamellar domain walls in the core region, as seen in TEM images. Curie temperature and dielectric constant increased as calcium concentration increased. Dy³⁺ doped samples had a higher Curie temperature and dielectric constant than non-doped samples. The Dy-doping and substitution of Ca²⁺ for Ba²⁺ can significantly increase the grain conductivity activation energy for Dy-doped BCT sintered under reduction atmosphere. It suggests that Dy³⁺ and Ca²⁺ doping can effectively suppress the formation of oxygen vacancies and inhibit their movement, potentially improving the HALT of BME-MLCC.     

Exceptional reliability of MLCCs enabled by defect-engineered BaTiO3

It has generally been believed that the reliability of BaTiO₃-based multilayered ceramic capacitors (MLCCs) is mainly contributed by hydroxyl (OH⁻), and the contribution of CO₃²⁻ can be neglected. However, in this work, we demonstrated that the contributions of Ba/Ti ratio and CO₃²⁻ play important roles in the delivering high reliability for BaTiO₃-based MLCCs. The structure and performance of MLCC devices and ceramic chips based on BaTiO₃ powders prepared by different approaches were studied. It is found that the intracrystalline pores in ceramics or MLCCs are mainly derived from the decomposition of BaCO₃ during sintering, which has been demonstrated by ceramic derived from hydrothermal method powder and its modified powders. The point defects of Ba and Ti vacancies mainly originating from nonstoichiometric Ba/Ti rather than thermally stimulated have substantial influence on the migration of grain boundary that determines the grain size and whether the pores can be annihilated from the bulk material. Particularly, the Ti vacancies have a strong pinning effect and inhibit the migration of grain boundary effectively, due to their shorter migration distance comparing to Ba vacancies. Therefore, the synergetic effect of the second phase BaCO₃ and point defects leads to the differences in the structure and performance.     

Low temperature synthesis of single-phase α-Fe2O3 nano-powders by using simple but novel chemical methods

Two simple chemical methodologies have been described for the preparation of single-phase α-Fe₂O₃ nano powder with particle size ∼ 20-30 nm. Precursor powders were synthesized by reacting aqueous solutions of ferric nitrate and PVA and sucrose (method 1) and EDTA (method 2) and then evaporating the resulting solutions to dryness. The precursors were subsequently calcined in air for 2.30 hr at different temperatures ranging from 250 to 450 °C. The synthesized powders were characterised using TG-DSC analysis, X-Ray Diffraction, BET surface area measurement, SEM and TEM. DC electrical resistivity of the synthesized materials was measured by the two-probe method. The synthetic routes described here provide simple but cost-effective methods to produce single phase, α-Fe₂O₃ nanopowder at a comparatively low temperature.     

Defect control of Yb-doped dielectric ceramics on improving the reliability for MLCC application

A fine-grained Yb-doped (Ba_1-xCa_x)_mTiO₃ (BCT)-based ceramic, possessing high dielectric properties and outstanding reliability under a reducing atmosphere sintering, was synthesized by the chemical coating method. The mean grain size of the ceramics was 170 nm. A distinct core-shell structure of ceramics was observed by the TEM-EDS measurement. With increasing content of Yb, the curie temperature increased and the DC-bias capacitance change rate decreased under a DC field at 40 kV/cm. The optimal component showed a dielectric constant of 2200 and met the requirement of the X8R MLCCs application. Moreover, the outstanding highly accelerated life-time and higher grain boundary activation energy substantiated that Yb dopant prominently improved the reliability of dielectric materials of MLCCs. All these studies provide meaningful recommendations for the research on dielectric materials of high-performance, high-reliability base-metal MLCCs.     

Preparation and characterization of polyimide/silica‐barium titanate nanocomposites

The silica‐barium titanate (SiO₂‐BaTiO₃) nanocomposites coated with polyimide had been synthesized successfully by a dispersion polymerization method. The conformation, structure, and size of SiO₂‐BaTiO₃ nanocomposites coated with polyimide were investigated by using FT‐IR, EDAX, XRD,TEM, SEM, and TGA. The results indicate that there is a thin layer polymer of SiO₂‐BaTiO₃ nanocomposites surface, in which the polymer thickness is about 10 nm and the size of them are about 50–60 nm, and the particles are well‐dispersed with even particle size. In addition, the crystal structure of BaTiO₃ is stable in preparing composite process and the chemical bond is formed between the inorganic phase and the polymer matrix. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Low temperature synthesis of tetragonal BaTiO3 by a novel composite-hydroxide-mediated approach and its dielectric properties

High purity tetragonal BaTiO₃ powders were synthesized by a composite-hydroxide-mediated approach at low temperature using a novel hydrothermal reaction apparatus with a rolling system. The optimum synthesis conditions were explored, and the obtained samples were characterized by their XRD, TEM, TG-DTA and SEM. The powders with an average size of 150 nm in diameter were sintered to almost full theoretical density (ca. 99%) at 1200 °C for 5 h and the obtained ceramics presented a high dielectric constant (9500 at the Curie temperature).     

Tailoring and improving the strong-electric-field electrical properties of the BNT-BT ferroelectric ceramics by a functional-group-doping

Lead-free ferroelectric ceramics with high tailored strong-electric-field electrical properties (energy storage, electrocaloric cooling, and energy harvesting, etc.) are attractive to many fields, such as modern electronics, medical and military, etc. We demonstrated that the strong-electric-field electrical properties of the 0.89(Bi_0.5Na_0.5TiO₃)-0.11(BaTiO₃) (BNT-BT) relaxor ceramics not only could be tailored easily after doping the (BaTiO₃)_0.5-(BiMg_0.5Ti_0.5O₃)_0.5 (BT-BMT, the functional-group), but also could be improved largely. As a result, an optimized electrostrain (S ~ 0.4%) was acquired at x = 0.12, and an optimized energy storage density (W ~ 0.775 J/cm³) with a high efficiency (η ~ 50%) was achieved at x = 0.24, as well as a phase-induced negative electrocaloric (EC) effect (ΔT ~ 3.72 K) was harvested at x = 0.04. It is concluded that the functional-group-doping can be regarded as a new strategy to tailor and improve the strong-electric-field electrical properties of ferroelectric materials.