ZnO and CuO crystal precipitation in sintering Cu-doped Ni-Zn ferrites. II. Influence of sintering temperature and sintering time

This paper presents a study of the precipitation of ZnO and CuO crystals during the sintering of Cu-doped Ni-Zn ferrites. The nature of the resulting crystal precipitates were analysed using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), confirming the findings of a previous paper (Part I). This study examines the influence of sintering temperature and sintering time of the thermal cycle on their formation, and on their microstructure and electromagnetic properties. The same two consecutive chemical reactions proposed in Part I can be used to explain the bulk precipitation and subsequent re-dissolution of the zinc and copper oxides observed during sintering. The effect of these crystal precipitates on the final properties of this type of soft ferrite was also analysed, revealing a deterioration in its electromagnetic performance.     

Coherent Precipitation of Copper Metal in Low-Temperature-Fired Ni-Zn-Cu Ferrite

Coherent copper-metal precipitates at grains adjacent to the Σ= 5 coincident-site lattice (CSL) boundary in a low-temperature-fired Ni-Zn-Cu ferrite have been observed and analyzed using analytical electron microscopy. This precipitation behavior is related to the copper solubility limits in the Ni-Zn-Cu ferrite during cooling after sintering, and preferential solute segregation may be dominant at stacking faults rather than at the Sigma = 5 CSL boundary. Copper-metal precipitates have a 〈100〉 matrix ∥〈100〉 Cu coherent relationship to the ferrite matrix.     

Precipitation During Controlled Cooling of Magnesia-Partially-Stabilized Zirconia

A study has been made of the precipitation and growth processes which occur during cooling from solution treatment and under isothermal hold conditions in a magnesia-partially-stabilized zirconia alloy. Three types of precipitate have been identified which develop during cooling or during isothermal hold treatments just above and below the eutectoid temperature. These precipitate forms are termed (i) primary, (ii) large random, and (iii) secondary. Further precipitation, slow growth of existing precipitates, and subeutectoid decomposition result when an additional 1100°C aging treatment is given to the previously cooled material. It is shown that type (iii) secondary precipitates form rapidly within the temperature range of 1300° to 1375°C. The secondary precipitates are largely responsible for the improved room-temperature strength properties of the heat-treated samples. The formation and effects on mechanical properties of each type of precipitate are discussed.     

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.     

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.     

Phase Identification and Electrical Properties in ZnO–Glass Varistors

Ceramic varistors based on ZnO with lead zinc borosilicate glass instead of Bi₂O₃ were prepared. The effect of sintering conditions on the electrical properties was studied by sintering samples at various temperatures and cooling them at different rates. The sample sintered at 1250°C for 1 h, then furnace cooled, possessed the best electrical properties, as characterized by the highest nonlinear coefficient, lowest leakage current, and lowest degradation. The microstructure and crystal structure of the glass phase of ZnO–glass varistors were examined by means of scanning electron microscopy, transmission electron microscopy, and powder X-ray diffractometry. The glass phase was originally amorphous, but crystallized as an intergranular layer in the sintered and furnace-cooled samples. This crystallized phase was a zinc borate phase (5ZnO·2B₂O₃), which was identified by X-ray diffractometry, transmission electron microscopy, and Auger electron spectroscopy. The zinc borate phase at the grain boundary of ZnO–glass samples enhanced the nonohmic characteristics of the ceramic varistors.     

Preparation and Sintering of Homogeneous Silicon Nitride Green Compacts

Interactions between silicon nitride particles and hydroxide precipitates were investigated using electrophoresis measurements. Conditions under which stable suspensions of silicon nitride particles and flocculation and heteroflocculation of silicon nitride/hydroxide mixtures occur were Identified. On the basis of the observations, a method for producing uniform mixtures of silicon nitride powders and additive precipitates was formulated and used to produce green compacts of improved compositional homogeneity. The effect of the mixing process on the sintering of green silicon nitride compacts was investigated and compared to the sintering behavior of conventionally prepared green compacts. The results show that the improved homogeneity obtained using the precipitation mixing process leads to enhanced sintering of the green compacts.     

An investigation of the precipitation process during ageing of a sintered low-carbon copper steel

The precipitation process during ageing (at 500° C) of a low-carbon copper steel containing 2% Cu and 0.03% C produced by sintering was studied by means of transmission electron microscopy. The precipitates, consisting of a solid solution of iron in copper, appear in two orientations relative to the iron matrix, and their growth is controlled by the diffusion of copper atoms within the matrix.     

Microstructure and Physicochemical Studies of Pure and Zinc-Substituted Lithium Ferrites Sintered above 1000°C

The effects of sintering temperature above 1000°C and of cooling rate on the microstructual development of pure and Zn-substituted spinel lithium ferrites are discussed. The strong dependence of microstructural features on cooling rate can be explained on the basis of the precipitation of α-Fe₂O₃ or the formation of a solid solution of ferrite phase with Fe₃O₄ occurring during sintering above 1000°C. These two phenomena are studied by detailed characterization analyses: SEM, XRD, magnetization, and ac electrical resistivity measurements.     

Current–Voltage Characteristics of Cobalt-Doped Inversion Boundaries in Zinc Oxide Bicrystals

Undoped and cobalt-doped basal inversion boundaries were fabricated in ZnO bicrystals to investigate their current–voltage characteristics. High-resolution transmission electron microscopy observations and energy-dispersive X-ray spectroscopy analyses for a cobalt-doped bicrystal revealed that the boundary was highly coherent and free from intergranular phases and precipitates, but a certain amount of cobalt was present near the boundary. The cobalt-doped bicrystals exhibited nonlinear characteristics that depended on cooling rates from annealing temperature, in contrast to linear characteristics of the undoped bicrystals. It is suggested that the presence of cobalt impurities enhances the formation of acceptor-like native defects near the boundaries to generate electrostatic potential barriers.     

The Effect of Monoglyceride Polymorphism on Cold-Flow Properties of Biodiesel Model Fuel

The cloud point (CP) of biodiesel refers to the temperature at which crystallization begins on cooling. However, solid precipitates are often formed at a temperature higher than the CP during storage. Such precipitates are known to consist largely of monoglycerides (MGs) as intermediate compounds. MGs have high melting points, which are detrimental to the cold‐flow properties of biodiesel. MGs have several polymorphic forms, including α, β′ and β, with different melting points (α < β′ < β), and this fact makes the behavior of biodiesel at low temperature complicated. In this study, the precipitation behavior of MGs in biodiesel is discussed focusing on polymorphism using 1‐monopalmitin and methyl oleate mixtures as a model biodiesel fuel. The CPs measured were close to the calculated solid‐liquid equilibrium curve for α‐type 1‐monopalmitin. However, precipitates formed at temperatures higher than the CP when the mixtures were held at temperatures lower than the equilibrium curve of the β′ form. This indicates that the β′ form causes a risk of precipitation at temperatures above the CP and that the CP is not a suitable indicator of the cold‐flow properties of biodiesel.     

纳米氧化钇的制备与表征

以Y(NO3)3为原料,聚乙烯醇为分散剂,采用碳酸氢铵沉淀工艺制备了氧化钇前驱体。用FTIR、TG-DTA分析了沉淀前驱体的成分及其热分解过程。结果表明,前驱体为NH4Y(OH)2CO3。对形成前驱体的机理进行了初步探讨。用XRD、UV-Vis和TEM分析了不同温度下煅烧所得粉体的结构和形貌,结果表明,前驱体在750℃保温2h,得到分散性良好、纯度高、立方体型的氧化钇,平均颗粒尺寸20nm左右。实验证明,适量的分散剂和(NH4)2SO4的添加对氧化钇颗粒形貌和粒径有显著影响。     

Effects of irradiation energy and nanoparticle concentrations on the structure and morphology of laser sintered magnesia with alumina and iron oxide nanoparticles

The laser sintering mechanism of composites based on magnesia and oxide nanoparticles was studied in terms of nanoparticle concentration and laser energy fluence. Iron oxide and aluminum oxide nanoparticles were mechanically mixed with magnesia (MgO) powder (5, 7 and 10 wt%) and the compacted pellets were irradiated with the fundamental output (1064 nm) of a pulsed Nd:YAG laser at 2.5 and 3.0 J/cm². Crystal structure, elemental composition and morphology were characterized by X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. X-ray diffraction results confirmed the crystalline phases and spinel formation by addition of oxide nanoparticles and laser sintering. X-ray photoelectron spectroscopy analysis confirmed their surface composition and chemical states of the corresponding elements. Morphological changes were observed due to the laser fluence and the oxide nanoparticle concentrations. Results show that a coarsening mechanism was predominant with a high energy fluence and concentration of oxide nanoparticles.     

Effect of sintering temperature on the morphology,crystallinity and mechanical properties of carbonated hydroxyapatite (CHA)

Effect of sintering temperature on the physical and mechanical properties of synthesized B-type carbonated hydroxyapatite (CHA) over a range of temperature in CO₂ atmosphere has been investigated. The B-type CHA in nano size was synthesized at room temperature by using a direct pouring wet chemical precipitation method. The synthesized CHA powders were subsequently consolidated by sintering treatment from 800 to 1100 °C. The sintered CHA samples were evaluated using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrometry, X-ray fluorescence (XRF), carbon-hydrogen-nitrogen-sulfur-oxygen (CHNS/O) elemental analyzer, Field emission scanning electron microscopy (FESEM), and Vicker's indentation technique. The results obtained from XRD and FESEM indicated that the synthesized B-type CHA powders were nanometer in size. The crystallinity and crystallite size of the sintered CHA samples were increased due to increasing sintering temperature. The heat treatment between 800 °C and 1000 °C has resulted in coarsening and increased hardness of the sintered CHA samples. However, these properties began to deteriorate when sintering beyond 1100 °C due the formation of calcium oxide.     

Development of matrix microstructure in polycrystalline cubic boron nitride ceramics

The development of matrix microstructure in polycrystalline cubic boron nitride cutting tool materials has been investigated by X-ray diffraction and scanning and transmission electron microscopy combined with energy dispersive X-ray spectrometry. The materials had a matrix based on Ti(C,O,N) and Al and were fabricated from powder mixtures milled with either WC-Co cemented carbide or (Ti,W)(C,N)-Co cermet milling media. The introduction of WC-Co debris resulted in the formation of an Al- and W-rich liquid phase during high pressure high temperature sintering. Crystalline intergranular phases rich in W partitioned from this liquid. Debris from the cermet milling media resulted in the formation of intergranular Co₂B; the W was retained in the (Ti,W)(C,N) structure during sintering. Al-rich reaction zones on cBN grain surfaces developed locally during high pressure high temperature sintering.     

Calcium Phosphate Materials Prepared from Precipitates with Various Calcium:Phosphorus Molar Ratios

Depending on the calcium:phosphorus molar ratio of the initial precipitates determined by precipitation conditions (calcium:phosphorus molar ratio of reactants and pH of reaction environment), after sintering at 1250°C, monophase, biphase, or triphase ceramics consisting of hydroxyapatite, β-tricalcium phosphate, α-tricalcium phosphate, and calcium oxide were obtained. The phase composition and properties—i.e., density, shrinkage, hardness, bending strength, and roughness—of the fractured surfaces of the isostatically re-pressed sinters were determined.     

Aggregation behaviour of stereoregular poly(methyl methacrylates) in dilute solution: light scattering and proton n.m.r. study

The aggregation behaviour of syndiotactic and isotactic poly(methyl methacrylate) in methyl ethyl ketone, n-butyl acetate, and 2-ethoxyethanol was investigated by light scattering and ¹H n.m.r. spectroscopy. Syndiotactic poly(methyl methacrylate) remains in its molecular form in solution at temperatures above 60°–70°C; on cooling it undergoes aggregation followed by macroscopic separation (precipitation) of the polymer from solution. The rate of these processes depends on temperature, concentration of the polymer, and solvent. Molecular solutions of isotactic poly(methyl methacrylate) can be prepared only by long-term heating at temperatures above 100°–130°C. During cooling, isotactic macromolecules prior to separation form stable associates in the region limited by discrete temperatures, and below this region polymer precipitates. The individual stereo forms of poly(methyl methacrylate) are separated from solution at different temperatures which are above the θ-temperatures of the atactic polymer.     

Batch crystallization and solid-liquid separation of potassium sulphate

The physical form of potassium sulphate crystals produced in laboratory scale batch crystallizers by both cooling and by precipitation is studied in terms of its subsequent effect on solid-liquid separation characteristics. Control of operating conditions improves the crystal size distribution in both cases and considerably increases the permeability of the precipitates.     

2223 Phase Formation in Bi(Pb)─Sr─Ca─Cu─O: III, The Role of Atmosphere

The formation of second phases during the preparation of the 2223 phase and their stability in the Bi system under various annealing temperatures and atmospheres have been studied. The 2201 precipitates developed at ∼830°C, and their conversion to the 2223 phase can be completed in the temperature range 810°C ≤ T < 830°C. A Ca₂PbO₄-like phase can precipitate from the liquid phase at ∼830°C during cooling. A (Sr,Ca)₁₄Cu₂₄O₄₁ phase is usually found accompanying the synthesis of the 2223 phase. This secondary phase is stable in an oxidizing atmosphere but can be eliminated by annealing under a low oxygen atmosphere or by choosing a suitable starting composition and set of sintering conditions. The precipitation of Ca₂PbO₄-like phase can be avoided by using a relatively fast cooling rate. Unlike the YBa₂Cu₃O _x superconductor, the 2223 phase can be stable under a wide range of atmospheres, such as argon, air, and oxygen.     

Kinetics of NiFe2O4 Precipitation in NiO

The kinetics of the precipitation of NiFe₂O₄ from an Fedoped NiO matrix has been studied. Polycrystalline samples were cooled at a constant rate through the solvus temperature, and the various stages of precipitation were monitored by periodically interrupting the cooling to observe the evolving size, distribution, and morphology of the precipitates. The precipitation occurred primarily by homogeneous nucleation and growth. The coherent precipitates have a dendritic morphology with a large interparticle spacing characteristic of fast precipitate growth. Reasonable rates for the nucleation of NiFe₂O₄ were observed to occur between 850° and 800°C, resulting in a 350° to 400°C undercooling. Preferential nucleation occurred at grain boundaries, but these particles were only twice as long as those nucleated homogeneously. The size and shape of the precipitates are independent of the distance from the surface, indicating that precipitation occurs by a local arrangement of the Ni and Fe cations and the preexisting cation vacancies.