Bi2O3 Addition Effects on the Sintering Mechanism,Magnetic Properties,and DC Superposition Behavior of NiCuZn Ferrites

The Bi₂O₃ addition effects on the densification mechanism, magnetic properties, and DC superposition behavior of NiCuZn ferrites are investigated in this study. Bi₂O₃ addition can effectively promote NiCuZn ferrite densification. The densification controlling mechanism is dominated by the liquid formation resulting from the Bi₂O₃ and Bi₂CuO₄ eutectic reaction as Bi₂O₃ addition is increased above 2wt%. A suitable compromise between the initial permeability and DC‐bias superposition characteristic can be obtained by adding a proper amount of Bi₂O₃ to adjust the nonmagnetic copper and bismuth‐rich precipitate thickness at the grain boundaries.     

Sintering and cooling atmosphere effects on the microstructure,magnetic properties and DC superposition behavior of NiCuZn ferrites

This study investigated the sintering and cooling atmosphere effects on the microstructure and magnetic properties of NiCuZn ferrites. The copper-rich phase segregated during sintering near the grain boundaries. The sample sintered in N₂ and cooled in air (N₂-Air) exhibited the highest amount of segregated copper-rich second phase, followed by the sample sintered and cooled both in air (Air-Air). However, no precipitate was observed for the sample sintered and cooled both in N₂ (N₂-N₂). This can be explained by the fact that Cu²⁺ was reduced into Cu⁺, promoting Cu₂O phase precipitation and then reacting with CuO to form a liquid phase. The Air-Air sample exhibited the highest initial permeability (μ_i ≈ 250) and saturation magnetization (B_s ≈ 88 emu/g), followed by N₂-N₂ (μ_i ≈ 180, B_s ≈ 82) and N₂-Air (μ_i ≈ 150, B_s ≈ 80). The highest DC superposition behavior was obtained for N₂-Air sample due to the highest nonmagnetic copper-rich precipitate thickness at the grain boundaries.     

Magnetic Properties of a Novel Ceramic Ferroelectric–Ferromagnetic Composite

A series of [0.8Pb(Ni_1/3Nb_2/3)O₃–0.2PbTiO₃]/[(Ni_0.2Cu_0.2 Zn_0.6)Fe₂O₄] (PNNT/NiCuZn) ferroelectric–ferromagnetic composites were prepared by the conventional solid-state route. The composites show good co-firing behaviors. X-ray diffraction and scanning electron microscope studies confirmed the coexistence of ferroelectric PNNT phase and ferromagnetic NiCuZn phase in the composites. No significant chemical interaction has occurred between PNNT phase and NiCuZn phase. All sintered samples exhibit typical magnetic hysteresis loops at room temperature. The saturation magnetization of composites rises linearly with the increase in NiCuZn content. Frequency dependence of initial permeability was also measured. With an increase in NiCuZn, the initial permeability increases and the cutoff frequency tends to decrease. The Maxwell–Garnett (MG) effective medium theory was used to model the magnetic properties of composites. Although the MG equation cannot give an accurate prediction for the initial permeability of composites because of the oversimplified assumption, it gives upper and lower limits for the initial permeability of compositions.     

以C为自蔓延高温合成燃料制备高磁导率NiCuZn铁氧体

以不同质量的C粉替代Fe粉作为合成燃料,通过自蔓延高温合成（SHS）反应制备NiCuZn铁氧体粉体,并将合成粉体压成环进行烧结。采用X射线衍射、扫描电子显微镜、振动探针式磁强计和阻抗分析仪研究了C粉含量对NiCuZn铁氧体烧结样品的物相、形貌、密度及磁性能的影响。结果表明：相对于Fe粉,以C粉为SHS燃料合成NiCuZn铁氧体不会引入杂质;在C含量为5％时,获得NiCuZn铁氧体固相反应充分,具有单一的尖晶石相;其比饱和磁化强度为45．5A·m^2／kg,比剩余磁化强度为12．1Am^2/kg,矫顽力为13．1A／m。975℃烧结2h的样品在1MHz频率下的磁导率高达5936。     

Sintering Behavior of 0.8 mol%-CuO-Doped 3Y-TZP Ceramics

In recent years, 3 mol% yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) doped with copper oxide has obtained increasing interest due to its enhanced superplastisity and good potential in tribological applications. In this work, the effect of addition of small amounts (0.8 mol%) of copper oxide on the sintering behavior of 3Y-TZP was studied using a dilatometer and high-temperature X-ray diffraction (XRD). A qualitative sintering model was established based on several reactions during sintering as indicated by thermal analysis and XRD. Some of these reactions remarkably retard densification and consequently result in low final density (86%) of the sample sintered at 1400°C in air. The reaction between molten Cu₂O and yttria as segregated to the Y-TZP grain boundaries at around 1180°C leads to the depletion of yttria from Y-TZP grains, which results in the formation of monoclinic phase during cooling. A relatively higher oxygen partial pressure can inhibit the dissociation of CuO to Cu₂O. This inhibition in dissociation is one of the reasons why a dense (>96%) 0.8 mol% CuO-doped 3Y-TZP ceramic can be obtained after sintering at 1400°C in flowing oxygen.     

Electrically Active and Inactive Single Grain Boundaries in Semiconducting BaTiO3

Bi‐crystal specimens were prepared from Nb and Mn‐doped BaTiO₃ poly‐crystals with giant grains of millimeter order in size, and the resistance (R) versus temperature (T) characteristics of these individual grain boundaries was investigated. The electrically active grain boundaries that show normal positive temperature coefficient resistor (PTCR) behavior had no second phases or they were partially distributed along boundaries. On the other hand, electrically inactive grain boundaries that show flat R‐T characteristics were also observed, where continuous Ti‐rich second phases of Ba₄Ti₁₂O₂₇ could be detected. Different interface state density and its resultant R–T characteristics were observed for each individual active boundary, which indicates the degree of oxidation and the formation of potential barrier can be different depending on the character of the grain‐boundary plane. The resistance of inactive boundaries was determined by that of insulating second phase showing negative temperature coefficient resistor (NTCR) behavior. These results demonstrate that continuous second phase surrounding a grain deactivates the electrical properties of grain boundary, and thus should be distinguished from insulating depletion layer near grain boundary.     

Inner Stress Induced by Cu Metal Precipitation at Grain Boundaries in Low-Temperature-Fired Ni-Zn-Cu Ferrite

Multilayer chip inductors were characterized using analytical electron microscopy. The chip inductors consist of silver internal electrode sandwiched between low-temperature-fired, Ni-Zn-Cu ferrite layers. It was found that the cooling rate affected the inductance of the chip inductor. Elastic strain fields were observed at the grain boundaries in the rapidly cooled samples, causing degradation of the inductance. The strain field causing a decrease in the effective permeability of the ferrite was found to be due to interstitial-type Cu precipitates at the grain boundaries. The occurrence of Cu metal precipitates was dependent on the starting chemical composition, oxygen partial pressure, and thermal equilibrium during cooling after the sintering process.     

Role of Liquid Phase in PTCR Characteristics of (Ba0.7Sr0.3)TiO3 Ceramics

The role of liquid phase in the enhancement of the PTCR (positive temperature coefficient of resistance) effect in (Ba_0.7Sr_0.3)TiO₃ (BST) with the addition of AST (4Al₂O₃· 9SiO₂· 3TiO₂) is investigated in this paper. The AST–BST samples were characterized with optical microscopy, transmission electron microscopy, energy-dispersive spectroscopy, and impedance spectroscopy. Microscopic observations showed that slower cooling might facilitate the precipitation of the (Ba,Sr)TiO₃ phase from the liquid phase on matrix grains since the amount of liquid phase was reduced with a decreasing cooling rate. Impedance spectroscopy indicated that this variation accompanied the change in the intrinsic properties of grain boundaries, which could not be explained by well-known oxidation effects. With the aid of a brick-layer model and high-resolution transmission electron microscopy (HRTEM), it appeared that the change in electrical characteristics of grain boundaries with decreasing cooling rate originated from the precipitation of (Ba,Sr)TiO₃. Finally, the effect of precipitated (Ba,Sr)TiO₃ on the PTCR characteristics is discussed in terms of the acceptor-state density and the polarization state at grain boundaries.     

Toward Knowledge‐Based Grain‐Boundary Engineering of Transparent Alumina Combining Advanced TEM and Atomistic Modeling

Transition element dopants (e.g., Y, La) are commonly used as sintering aids in polycrystalline alumina ceramics, which segregate to the grain boundaries and control the grain‐boundary mobility. However, due to the extremely thin (<2 nm) layer of segregated dopants, the experimental characterization of the segregated alumina grain boundaries is a complex task. Computational studies have focused only on tilt grain boundaries, which are only a small fraction in a sintered alumina sample. In this study, a quantitative characterization of the segregation of Y and La at general high angle grain boundaries in transparent alumina is carried out using a unique combination of advanced TEM and near coincidence grain‐boundary atomistic simulations. The result show that high angle grain boundaries may lead to enhanced grain growth in comparison to symmetric tilt twin grain boundaries due to the reduced configuration entropy for dopant segregation and higher order grain‐boundary complexions. On the other hand, multidoping with different dopants was shown to be more beneficial than single doping due to its contribution in increasing the configurational entropy for segregation. The advanced TEM analysis showed Y and La distributions and concentrations on a series of general grain boundaries in very good agreement with the atomistic simulations. This validation of atomistic modeling technique used in this study means, as it a generic method, can be used as a predictive tool to design ceramic microstructure and properties.     

Effects of solid-state reaction temperature on the activation energy and DC-bias superposition of NiCuZn ferrites based on master sintering curve

This work reports our recent studies on the microstructure and properties of NiCuZn ferrites, with particular interests in effects of solid-state reaction temperature on the sintering activation energy and DC-bias superposition characteristic. The theory of master sintering curve was used to investigate the densification evolutions of NiCuZn ferrites during sintering process. Experimental results demonstrated that materials with solid-state reaction temperature of 850°C exhibited the lowest value of activation energy, 577.6?kJ?mol^?1. When the solid-state reaction temperature was lower than 800°C, reaction could not be perfectly completed, which would hinder the consolidation process during heating up. In addition, increased solid-state reaction temperature could facilitate the homogeneous grain growth, reduce the final grain size and improve DC-bias superposition characteristic. Possible mechanisms behind are discussed.     

High-resolution FIB-TEM-STEM structural characterization of grain boundaries in the high dielectric constant perovskite CaCu3Ti4O12

In this work, the grain boundaries composition of the polycrystalline CaCu₃Ti₄O₁₂ (CCTO) was investigated. A Focused Ion Beam (FIB)/lift-out technique was used to prepare site-specific thin samples of the grain boundaries interface of CCTO ceramics. Scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectrometry (EDXS) and Electron Energy Loss Spectroscopy (EELS) systems were used to characterize the composition and nanostructure of the grain and grain boundaries region. It is known that during conventional sintering, discontinuous grain growth occurs and a Cu-rich phase appears at grain boundaries. This Cu-rich phase may affect the final dielectric properties of CCTO but its structure and chemical composition remained unknown. For the first time, this high-resolution FIB-TEM-STEM study of CCTO interfacial region highlights the composition of the phases segregated at grain boundaries namely CuO, Cu₂O and the metastable phase Cu₃TiO_4.     

Modification of Sintering Kinetics by Solute Segregation in Al2O3

The segregation of MgO solute at grain boundaries in Al₂O₃ controls the grain-growth kinetics during sintering. In the initial stages of sintering where grain growth can be neglected, the MgO solute causes a decreased sintering rate by reducing the surface energy and/or the diffusion coefficient of the rate-determining species. During the later stages of sintering, the segregated solute decreases the grain-growth rate which dominates the sintering kinetics.     

Thermal and morphological evaluation of very low density polyethylene/high density polyethylene blends

Blends of very low density polyethylene (VLDPE) and high density polyethylene (HDPE) were prepared by melt extrusion. These blends exhibit a tendency to phase segregate when they are slow cooled from the melt. If they are cooled at increasingly faster rates, a finite population of co‐crystals can be isolated from the rest of the phase segregated material, indicating that this system is probably miscible in the melt but phase separates during cooling. Transmission electron microscopy observations are consistent with the blend melt miscibility since inter‐lamellar mixing was clearly appreciated in the samples examined. Other effects arising from interactions between the polymers were the nucleation of VLDPE rich phase by HDPE rich phase, and a melting point depression of HDPE rich phase caused by a dilution effect exerted by molten VLDPE rich phase. After a successive self‐nucleation and annealing thermal fractionation procedure is applied to the blends, phase separation dominates the behavior, although some small fraction of co‐crystals was still present.     

Transmission Electron Microscopy at Grain Boundaries of PTC-Type BaTiO3 Ceramics

Transmission electron microscopy of the grain boundaries of Ti-rich semiconducting PTC-type BaTiO₃ ceramics is described. At a width of 2 to 10 nm, there were no indications of intergranular second-phase layers covering the grains. Second phase was segregated only at the contacts of three or more grains; electron diffraction confirmed the amorphous structure of this phase. Observed ferroelectric domains at the grain boundaries do not indicate a PTC-specific orientation of ferroelectric domains due to the negative grain surface charge.     

Cubic-Formation and Grain-Growth Mechanisms in Tetragonal Zirconia Polycrystal

The microstructure in Y₂O₃-stabilized tetragonal zirconia polycrystal (Y-TZP) sintered at 1300°–1500°C was examined to clarify the role of Y³⁺ ions on grain growth and the formation of cubic phase. The grain size and the fraction of the cubic phase in Y-TZP increased as the sintering temperature increased. Both the fraction of the tetragonal phase and the Y₂O₃ concentration within the tetragonal phase decreased with increasing fraction of the cubic phase. Scanning transmission electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) measurements revealed that cubic phase regions in grain interiors in Y-TZP generated as the sintering temperature increased. High-resolution electron microscopy and nanoprobe EDS measurements revealed that no amorphous layer or second phase existed along the grain-boundary faces in Y-TZP and Y³⁺ ions segregated at their grain boundaries over a width of ∼10 nm. Taking into account these results, it was clarified that cubic phase regions in grain interiors started to form from grain boundaries and the triple junctions in which Y³⁺ ions segregated. The cubic-formation and grain-growth mechanisms in Y-TZP can be explained using the grain boundary segregation-induced phase transformation model and the solute drag effect of Y³⁺ ions segregating along the grain boundary, respectively.     

TEM Observations on 0.65Pb(Zr0.42Ti0.58)O3‐0.35Pb(Ni0.33Nb0.67)O3 Ceramics with CuO Additive

The microstructural properties and the chemical compositions of a 0.65Pb(Zr_0.42Ti_0.58)O₃‐0.35Pb(Ni_1/3Nb_2/3)O₃ (0.65PZT58‐0.35PNN) ceramic system sintered at 900°C for 4 h with 10 mol% CuO additives were studied using transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS). CuO pockets were found as new microstructural constituents. The liquid phase has formed by the partial melting of CuO additives at the sintering temperature by reacting with Pb element in the matrix. The reaction started at the interfaces and then proceeded into the pocket through the diffusion of the Pb element. The presence of the Pb‐rich precipitates during cooling was confirmed by EDS analyses. Cu‐rich crystals in the pocket were observed near the boundaries between the matrix grain and the pocket. Rather smaller Pb‐Cu‐O‐contained particle segments were detected around the center of the pocket, demonstrating that the reaction of melting of CuO has occurred with the Pb element which was diffused from the matrix. Due to the existence of the liquid phase, a dense microstructure was achieved during the sintering process and abnormal grain growth occurred in the process.     

Microstructural Development in Partially Stabilized ZrO2 in the System CaO-ZrO2

The microstructures of 3 zirconias partially stabilized with CaO were investigated using scanning electron microscopy and qualitative and quantitative X-ray analysis. The structure was closely related to the heat treatments involved in fabrication. A bimodal structure with small grains of pure ZrO₂ dispersed along the grain boundaries of larger cubic solid-solution grains developed during slow cooling from 1850° to 1300°C. The presence of a liquid phase greatly enhances the growth of the pure ZrO₂ phase. An anneal at 1300°C induces precipitation of fine ZrO₂ particles within the solid-solution grains. The relative mechanical strengths of the materials are explained in terms of the weakening of the grain boundaries associated with the transformation of the grain-boundary phase on cooling.     

Consolidation of undoped,monoclinic zirconia polycrystals by flash sintering

Consolidated, monoclinic ZrO₂ polycrystal was produced from undoped ZrO₂ powders in air by flash sintering at the sintering temperature of 1350°C for 5 minutes or 3 hours under an applied DC electric field of 175 V/cm. When the ZrO₂ was heated under the applied DC field, the electric current of the specimen steeply increased at the furnace temperature of 1335°C below the sintering temperature of 1350°C. When the furnace temperature was decreased from the sintering temperature of 1350°C to room temperature, volumetric expansion associated with tetragonal‐to‐monoclinic phase transformation gradually took place at the furnace temperature from 1000°C to 750°C, and monoclinic ZrO₂ body was remained consolidated even at room temperature in both specimens. In contrast, conventionally sintered ZrO₂ without applying DC field exhibited the abrupt volumetric expansion at about 1000°C, and shattered. SEM observation revealed the presence of grain‐boundary second phase in the flash‐sintered specimen for 3 hours, which is a possible origin of keeping a bulk body at room temperature. The thinner second phase is considered to be formed also in the flash‐sintered specimen for 5 minutes, although the formation of the phase could not be observed clearly by SEM observation. On the other hand, XRD measurements showed that <001> directions of the monoclinic ZrO₂ grains were oriented along the applied DC field after the isothermal flash sintering for 3 hours while the grain alignment could not be observed in flash‐sintered specimen for 5 minutes. The alignment of ZrO₂ grains observed in the isothermal flash sintering is considered to be closely related to the preferential direction of oxygen ionic conduction and the second phase formed along grain boundaries.     

Effects of Yttrium Doping α-Alumina: I, Microstructure and Microchemistry

The influence of yttrium doping on the microstructure and microchemistry of hot-pressed α-alumina was investigated using a combination of electron microscopy techniques. The implications of microstructure and microchemistry on the improved creep behavior of the doped material are discussed. The samples doped only with yttrium had a bimodal grain-size distribution that was strongly correlated to the frequency and distribution of Y₃Al₅O₁₂ (YAG) precipitates in the microstructure. Yttrium segregated to most of the grain boundaries, with a normal excess concentration of Gamma= 3.3 ± 0.9 atoms/nm² at random boundaries. Two types of twin boundaries (Sigma3 and Sigma7) accommodated no yttrium. None of the boundaries or triple-point junctions contained a glassy grain-boundary phase. Strong interaction of the grain boundaries and dislocations with YAG precipitates indicated a pinning mechanism by the precipitates. Yttrium doping did not appear to favor formation of special boundaries in α-alumina.     

Microdomain Formation,Oxidation, and Cation Ordering in LaCa2Fe3O8+y

The compound LaCa₂Fe₃O_8+y, also known as the Grenier phase, is known to undergo an order–disorder transformation (ODT) at high temperatures and oxidation has been observed when the compound is cooled in air after the ODT. In this study, we have synthesized the Grenier compound in air using traditional solid‐state reactions and investigated the structure and composition before and after the ODT. Thermal analysis showed that the material undergoes an ODT in both oxygen and argon atmospheres with dynamic, temperature dependent, oxidation upon cooling. Results from scanning transmission electron microscopy (STEM) suggest that the Grenier phase has preferential segregation of Ca and La on the two crystallographic A sites before the ODT, but a random distribution above the ODT temperature. Furthermore, STEM images suggest the possibility that oxygen excess may exist in La‐rich regions within microdomains rather than at microdomain boundaries.