Effects of Bi2O3-Nb2O5 additives on microstructure and magnetic properties of low-temperature-fired NiCuZn ferrite ceramics

NiCuZn ferrite with superior magnetic performance is vital ceramic material in multilayer chip inductors (MLCI) applications. In this study, low-temperature-sintered Ni_0.22Cu_0.2Zn_0.58Fe₂O₄ ferrite ceramic doped with 1.0?wt% Bi₂O₃-x?wt% Nb₂O₅ (where x?=?0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) was synthesized via solid-state reaction method. Effects of Bi₂O₃-Nb₂O₅ additives on microstructures and magnetic properties of NiCuZn ferrite ceramics sintered at 900?°C were systematically investigated. Results indicate that an appropriate amount of Bi₂O₃-Nb₂O₅ composite additives can significantly promote grain growth and densification of NiCuZn ferrite ceramics when sintered at low temperatures. Specifically, samples doped with 1.0?wt% Bi₂O₃ and 0.4?wt% Nb₂O₅ additives exhibited excellent initial permeability (~ 410 @ 1?MHz), high cutoff frequency (~ 10?MHz), high saturation magnetization (~ 54.92?emu/g), and low coercive force (~ 20.32?Oe). These observations indicate that NiCuZn ferrite ceramics doped with appropriate amounts of Bi₂O₃-Nb₂O₅ additives are great candidate materials for MLCI applications.     

Enhanced electromagnetic properties of low-temperature sintered NiCuZn ferrites by doping with Bi2O3

NiCuZn ferrite is a material suitable for low-temperature co-fired ceramic (LTCC) technology due to its high permeability and relatively low sintering temperature. The main research questions regarding NiCuZn ferrites are focused on reducing the sintering temperature of the NiCuZn ferrites to achieve compatibility with the Ag electrodes and improve their electromagnetic properties. In this study, the electromagnetic properties of NiCuZn (Ni_0.29Cu_0.14Zn_0.60Fe_1.94O_3.94) ferrites were enhanced by doping with Bi₂O₃, resulting in a reduction of the sintering temperature to 925 °C. The findings show that a suitable concentration of Bi₂O₃ doping could promote the growth of grains and result in NiCuZn ferrites with denser microstructures sintered at a low temperature. Furthermore, adding 0.30 wt% Bi₂O₃ to NiCuZn ferrite enhances its electromagnetic properties, such as a high real part of permeability (～937.6 @ 1 MHz), high saturation magnetization (～60.353 emu/g), low coercivity (～0.265 kA/m), and excellent dielectric constant (～14.71 @ 1 MHz). In addition, the chemically compatible Ag electrodes suggest that the NiCuZn +0.30 wt% Bi₂O₃ ceramics may be acceptable for LTCC technology.     

Enhanced gyromagnetic properties of NiCuZn ferrite ceramics for LTCC applications by adjusting MnO2-Bi2O3 substitution

The demand for high performance microwave devices is increasingly promoting the development of miniaturization, integration and multifunctionalization. Here, a uniform and dense NiCuZn ferrite ceramic with high saturation magnetization and low ferromagnetic resonance linewidth was obtained at 950?°C by adjusting the MnO₂-Bi₂O₃ composite additives. The MnO₂-Bi₂O₃ composite additives were composed of 0.5?wt% MnO₂ and x wt% Bi₂O₃ (x?=?0.0, 0.5, 1.0, 1.5, 2.0, and 3.0). The phase structure, microstructures and magnetic properties were systematically studied by means of modern measurement techniques. SEM images reveal that appropriate MnO₂-Bi₂O₃ additions can promote grain growth and reduce sintering temperatures, which is very advantageous for LTCC technology. In addition, the content of MnO₂-Bi₂O₃ additives can significantly reduce ferromagnetic resonance linewidth (FMR) by promoting grain growth and densification at low temperatures. Finally, a uniform and compact NiCuZn ferrite ceramic with an improved 4πM_s (~?3812.5 Gauss), a narrow ΔH (~?144.6?Oe), and a reduced H_c (~?85.2?A/m) were obtained (at 950?°C) by adding the optimal volume of Bi₂O₃ additive. It is expected that the improved gyromagnetic performances will allow the NiCuZn ferrite ceramics to be promising candidates for X-band microwave devices.     

Low temperature firing of Co2Y–NiCuZn ferrite composites

Hexagonal structure magnetoplumbite ferrites have revealed a higher dispersion frequency than that of nickel ferrites because of the magnetoplumbite's magnetic anisotropy. The magnetoplumbite ferrite densification temperature always exceeds 1000 °C and the initial low temperature firing permeability of magnetoplumbite ferrites with added glass is too low (μ_i = 2–4). Therefore, it is desirable to develop a material that has a higher permeability at above 300 MHz and can be densified at temperatures below 900 °C. The Bi₂O₃–B₂O₃–ZnO–SiO₂ (BBSZ) glass addition effects on the densification and magnetic properties of Co₂Y–NiCuZn ferrite composites with various Co₂Y/NiCuZn ferrite ratios were investigated. The densification of Co₂Y–NiCuZn ferrite composites was enhanced by the addition of glass at low sintering temperatures (<900 °C) due to the liquid phase sintering. Co₂Y–NiCuZn ferrite composites with 4 wt% BBSZ glass sintered at 900 °C show a relative density above 90%, a high-initial-permeability of 5–6, a quality factor of above 30 in the 200–300 MHz frequency and a resonance frequency above 1 GHz, which can be used in high frequency multilayer chip inductors.     

Effect of ZnBi2O4 and Bi2O3 addition on the densification,microstructure, and varistor properties of ZnO varistors

In this study, 1 wt% Bi₂O₃ (1B), 1 wt% ZnBi₂O₄ (1BZ), and a composite (a mixture of 1 wt% Bi₂O₃ and various amounts (1-4 wt%) of ZnBi₂O₄ ,1B1BZ-1B4BZ) were added to ZnO varistors to investigate the effects of additives on the densification, microstructure, and varistor performance. The results showed that the addition of ZnBi₂O₄ can lower the densification temperature to about 850^oC. When the additive was changed from 1 wt% Bi₂O₃ to 1 wt% ZnBi₂O₄, the α value increased from 42 to 54, the breakdown voltage increased from 775 V/mm to 1011 V/mm, and the leakage current decreased to 0.11 μA. Additions of ZnBi₂O₄ and transition metal cations as donor dopants for the ZnO varistors promote oxygen chemisorption at grain boundaries, resulting in greater α value and lower leakage currents. This suggests the addition of ZnBi₂O₄ can effectively promote densification and improve the varistor properties of ZnO varistors.     

Electrical properties of low-temperature-fired ferrite–dielectric composites

The effects of the BaO·(Nd_0.8Bi_0.2)₂O₃·4TiO₂ (BNBT) to NiCuZn ferrite ratio and addition of Bi₂O₃–B₂O₃–SiO₂–ZnO (BBSZ) glass on the sintering behavior, microstructure evolution, dielectric and magnetic properties of BNBT–NiCuZn ferrite composites were investigated in developing low-temperature-fired composites for high frequency electromagnetic interference (EMI) devices. The results indicate that these composites can be densified at 900 °C and exhibit superior dielectric and magnetic properties with the addition of BBSZ glass. The dielectric system used in the ferrite–dielectric composites reported in the previous studies mostly belong to the ferroelectricity group, which are not suitable for use in the high frequency range (>800 MHz) due to the selfresonance frequency limit. In this study, the dielectric constant remains nearly a constant over a wide range of frequencies (100 MHz to 1 GHz) and the magnetic resonance frequencies are larger than 100 MHz for the BNBT + BBSZ glass–NiCuZn ferrite composites. Therefore, the BNBT + BBSZ glass–NiCuZn ferrite composites can be a good candidate material for high frequency EMI device applications.     

Constrained Sintering of a Low‐Fire,Polycrystalline Bi2(Zn1/3Nb2/3)2O7 Dielectric

The densification behavior of a low‐fire, polycrystalline Bi₂(Zn_1/3Nb_2/3)₂O₇ (BZN) dielectric under constrained sintering at 800°C–900°C is investigated. Although the constrained densification is retarded in relative to free sintering, a high sintered density of >95% is obtained at 900°C. No significant anisotropy with similar grain sizes is developed under free and constrained sintering. The densification behavior and stress development during constrained sintering of BZN is thus analyzed by using the well‐known isotropic constitutive laws.     

Microstructure,magnetic, and power loss characteristics of low-sintered NiCuZn ferrites with La2O3-Bi2O3 additives

Low-temperature-sintered Ni_0.5Cu_0.125Zn_0.375Fe_1.98O₄ ferrites co-added with x wt% (x = 0.00-0.25 wt%) La₂O₃ and 0.25 wt% Bi₂O₃ were successfully prepared via conventional solid-phase reaction method. The phase composition, microstructure, magnetic properties, and especially power loss variation of the samples were systematically studied. The results showed that all samples possessed a single spinel phase structure at a sintering temperature of 900°C, exhibiting high degree of densification and uniform grains. The appropriate amount of La₂O₃ additive improved the saturation flux density and permeability of NiCuZn ferrites, simultaneously reducing the coercivity and power loss. The maximum permeability and the lowest power loss were achieved at x = 0.15 wt%. The corresponding sample had the homogeneous microstructure and excellent magnetic properties, being a promising low-temperature co-fired ferrite candidate for magnetic power components.     

Synthesis,structural characterization,and photocatalytic activity of Bi‐based nanoparticles

Bi‐based nanoparticles were successfully synthesized (500°C/2 h) following two intermediate‐temperature routes, Pechini and a combined method of mechanical milling/molten salts. Herein, α‐Bi₂O₃/Bi₂O₂CO₃ heterostructures were identified. These types of heterostructures have shown advantages to improve the visible light responsive activity of Bi‐based nanoparticles to facilitate the separation of photogenerated electron‐hole pairs. By fixing the experimental parameters, the synthesis route and Ag contents showed to play a very important role in the phase composition of the Ag‐containing samples; the formation of Bi/Ag‐Bi₂O₃/Bi₂O₂CO₃ and Ag/Ag‐Bi₂O₃/Bi₂O₂CO₃ complex hybrid structures were revealed. The α‐Bi₂O₃/Bi₂O₂CO₃ heterostructure prepared by combined method exhibited higher photocatalytic efficiency (66.84%) for degrading methylene blue (MB) depending on Bi₂O₂CO₃ component proportion. For the Ag‐containing hybrid structures, the performance of their catalytic activity did not show an increase with respect to α‐Bi₂O₃/Bi₂O₂CO₃. However, in these samples, the degradation efficiency was slightly enhanced with the efficient doping of Ag into the α‐Bi₂O₃ structure and the existence of Bi when the Pechini method was used. Also, a possible photocatalytic process for degrading MB is presented.     

Preparation and sintering behaviour of nanostructured SrBi2Ta2O9 ceramics

Abstract

Nanostructured SrBi₂Ta₂O₉ powders have been prepared by sol-gel processing and the resulting precursors characterised after calcination by DTA-TG and XRD. The results show that pure powders can be obtained above 600°C with a particle size of 60 nm, as determined by TEM . Pressed pellets sintered in air increased in density with increasing sintering temperature, reaching a maximum of 8·42 g cm ^{- 3} after 2 h at 1250°C. In order to improve densification, Bi₂O₃ contents between 3 and 10 wt-% were added to pure SrBi₂Ta₂O₉. The results show that these additions tend to increase density, but that decomposition by volatilisation of Bi₂O₃ at high temperature is retarded to a degree depending on the Bi₂O₃ content. This behaviour is attributed to the increased density resulting from Bi₂O₃ addition, which reduces volatilisation, and hence decomposition, at higher temperatures.     

Kinetic field approach to study liquid phase sintering of ZnO based ceramics

Liquid phase sintering kinetics in the system ZnO–Bi₂O₃–Sb₂O₃ was studied using closed crucibles and an optical dilatometer. A modified kinetic field technique was applied for the first time to investigate the densification rates. The values obtained were assessed with existing liquid phase sintering models. Grain growth data were derived from the kinetic field diagram and compared to those obtained from microstructure analysis of quenched samples. Good agreement was obtained between both techniques. Values for both the activation energies (activation energies for grain growth and densification) were also reported for the ZnO–Bi₂O₃–Sb₂O₃ system for the first time. In the initial sintering stage mechanisms were identified which retard densification and are essentially unaffected by temperature. It was shown how the position and slope of the iso-strain lines in the modified kinetic field diagram can be used for a qualitative understanding of the interaction of coarsening, liquid redistribution and densification during sintering.     

Glass Additive Influence on the Sintering Behaviors,Magnetic and Electric Properties of Bi–Zn Co‐Doped Co2Y Ferrites

The Bi₂O₃–B₂O₃–ZnO–SiO₂ (BB35SZ) glass effects on the sintering behavior and magnetic properties of Bi–Zn co‐doped Co₂Y ferrites were investigated in developing low‐temperature‐fired ferrites. The results indicate that BB35SZ glass can be used as a sintering aid to reduce the densification temperature of Co₂Y ferrites from 1300°C to 900°C. The 2(Ba_0.9Bi_0.1O)·2(Zn_0.4Co_0.4Cu_0.2O)·6(Fe_1.97Zn_0.03O₃) ferrite with 4 wt% BB35SZ glass can be densified below 900°C, exhibiting an initial permeability of 3.4 and quality factor of 55. This process provides a promising candidate for multilayer chip magnetic devices for microwave applications.     

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.     

Cooling Rate Effects on the Microstructure,Magnetic Properties,and DC Superposition Behavior of NiCuZn Ferrites

This study investigated the cooling rate effects on the magnetic properties of NiCuZn ferrites. A copper‐rich phase segregated near the grain boundaries during sintering was observed. The amount of copper‐rich precipitates depends strongly on the cooling rate and decreases with decreasing cooling rate. The quenched sample exhibited superior initial permeability and DC‐bias‐superposition characteristics due to the highest saturation magnetization and a thick nonmagnetic second phase segregated at the grain boundaries. A NiCuZn ferrite with superior initial permeability and DC superposition characteristics can be obtained by controlling the cooling rate to adjust the copper‐rich precipitate thickness at the grain boundaries.     

Effects of trace of Bi2O3 addition on the morphology of strontium ferrite particles

A strontium ferrite powder added with trace of Bi₂O₃ was prepared by the conventional high-temperature solid phase reaction. The effects of Bi₂O₃ addition on the morphology of Sr-ferrite particles fired at different temperatures and times were investigated. The results show that a small quantity of Bi₂O₃ addition accelerates the reaction of SrO and Fe₂O₃ to form SrM ferrite and obviously improves the morphology and size of the strontium ferrite particles. A possible mechanism was proposed to interpret the influence of trace of Bi₂O₃ addition on the morphology of strontium ferrite particles.     

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.     

Particle atomic layer deposition of alumina for sintering yttria-stabilized cubic zirconia

The addition of aluminum oxide (Al₂O₃) as a sintering aid to yttria-stabilized zirconia (YSZ) reduces the required densification temperature. Sintering aids are incorporated using a number of processes which can lead to ambiguity when determining the effect of the sintering aid on the densification mechanism. In this study, a novel method for sintering aid addition, Particle Atomic Layer Deposition (ALD), was used to deposit an amorphous Al₂O₃ thin film on YSZ particles. Transmission electron microscopy confirmed the deposition of conformal Al₂O₃ thin films on the surface of the YSZ particles. The addition of Al₂O₃ to YSZ reduced the temperature at which densification began by ~75°C, and 2.2 wt% Al₂O₃ addition resulted in a minimum activation energy for the intermediate stage of densification. This concentration is well in excess of the solubility limit of Al₂O₃ in YSZ, showing that Al₂O₃ does not enhance the densification of YSZ solely by dissolving into the YSZ lattice and activating volume diffusion. The addition of 0.7 wt% Al₂O₃ with one Particle ALD cycle enhanced the ionic conductivity of YSZ by 23% after sintering at 1350°C for 2 hours, demonstrating that dense parts with high oxygen ion conductivities can be produced after sintering at reduced temperatures. One Particle ALD cycle is a fast, easily scaled-up process that eliminates the use of solvents and has substantial cost/performance advantages over conventional processing.     

Synthesis,Crystal Structure,and Enhanced Luminescence of Garnet‐Type Ca3Ga2Ge3O12:Cr3+ by Codoping Bi3+

Garnet‐type compound Ca₃Ga₂Ge₃O₁₂ and Cr³⁺‐doped or Cr³⁺/Bi³⁺ codped Ca₃Ga₂Ge₃O₁₂ phosphors were prepared by a solid‐state reaction. The crystal structure of Ca₃Ga₂Ge₃O₁₂ host was studied by X‐ray diffraction (XRD) analysis and further determined by the Rietveld refinement. Near‐infrared (NIR) photoluminescence (PL) and long‐lasting phosphorescence (LLP) emission can be observed from the Cr³⁺‐doped Ca₃Ga₂Ge₃O₁₂ sample, and the enhanced NIR PL emission intensity and LLP decay time can be realized in Cr³⁺/Bi³⁺ codped samples. The optimum concentration of Cr³⁺ in Ca₃Ga₂Ge₃O₁₂ phosphor was about 6 mol%, and optimum Bi³⁺ concentration induced the energy‐transfer (ET) process between Bi³⁺ and Cr³⁺ ions was about 30 mol%. Under different excitation wavelength from 280 to 453 nm, all the samples exhibit a broadband emission peaking at 739 nm and the intensity of NIR emission increases owing to the ET behavior from Bi³⁺ to Cr³⁺ ions. The critical ET distance has been calculated by the concentration‐quenching method. The thermally stable luminescence properties were also studied and the introduction of Bi³⁺ can also improve the thermal stability of the NIR emission.     

Microstructural and electromagnetic study of low temperature fired nano crystalline MgCuZn ferrite with Bi2O3 addition

The present study investigates electromagnetic properties, as well as microstructural and thermal stability, of low temperature fired MgCuZn ferrite with the addition of various amounts of Bi₂O₃. To achieve better performance at low sintering temperature, ceramic specimens were fabricated using nano-sized precursor powders through the nitrate-citrate auto-combustion method. X-ray diffraction study shows the formation of single phase spinel structure without any impurity phases. Compared with the additive-free sample, the addition of Bi₂O₃ increases the density of all specimens. Scanning electron microscopy micrographs of samples indicate that Bi₂O₃ content significantly affects densification through grain growth promotion. Excessive amounts of Bi₂O₃, however, lead to abnormal grain growth. Moreover, the sample with 0.25 wt% Bi₂O₃ shows the highest density and grain shape uniformity as well as the lowest porosity. Dynamic magnetic properties were studied in a frequency range of 1–90 MHz, using an impedance analyzer. Results reveal that the sample with a low amount of additive (0.25 wt%) has the highest saturation magnetization, initial permeability (at 1 MHz), and quality factor, whereas it's Curie temperature slightly decreases. With the further increase of Bi₂O₃ content, however, the initial permeability and saturation magnetization of samples deteriorate gradually.     

Effect of dual liquid phase sintering aids on the densification and microstructure of low temperature sintered alumina ceramics

Alumina ceramics was prepared by pressureless sintering technology in which a CuO–TiO₂–Bi₂O₃ mixture (0–4.0 wt% Bi₂O₃ and 4.0 wt% CuO and TiO₂) was added as dual liquid phase sintering aids. The phase compositions, microstructural feature, and sintering behaviour of the alumina ceramics were analyzed. The results showed that adding 2.5 wt% Bi₂O₃ to alumina ceramics can increase the contribution rate of initial stage of sintering to the sintering process. The relative density of the sample reached 97.63% after sintering at 1200 °C for 90 min. Measurements from differential scanning calorimetry, with the addition of CuO–TiO₂–Bi₂O_3, demonstrated the formation of two liquid phase points, 827.4 and 936.8 °C. Notably, the solid solution temperature of TiO₂ and Al₂O₃ ceramics diminished thanks to the dual liquid phase sintering aids, and at the same time the activation energy required also dropped from 368.96 to 137.31 kJ/mol. Research indicates that the combined action of dual liquid phase sintering and solid-state reaction sintering has promoted the densification of alumina ceramics during the sintering process while at the same time inhibiting the growth of abnormal grains so that a homogeneous microstructure can be formed.