Droplet Micro‐Reactor for Internal Gelation to Fabricate ZrO2 Ceramic Microspheres

A droplet micro‐reactor was developed to realize the internal sol–gel reaction of ZrO₂ inside the droplet. The reactants are only allowed to combine when the droplet forms, meanwhile, they are able to mix well and react to form gel particles with a proper designed structure of the micro‐reactor. In this condition, the mixing efficiency was determined by the structure of the micro‐reactor and the flow rate ratio of the two components. After sintering, ZrO₂ ceramic microspheres are acquired with high sphericity and narrow size distribution. This droplet micro‐reactor overcomes the gelation of the premixed sol and reactant in the channel, and it has been demonstrated to be more stable with a longer running time.     

An Improved Internal Gelation Process for Preparing ZrO2 Ceramic Microspheres Without Cooling the Precursor Solution

ZrO₂ microspheres were prepared with an improved internal gelation process without cooling the precursor solution. The stability of the broth for internal gelation process has been systematically investigated, and the results show that the preparation and storage temperature, the concentration of NO₃^? and the urea in the broth have important effects on the stability of the broth. Through optimizing the broth formulation the broth prepared can be stable for 14 h at 25°C. The prepared ZrO₂ ceramic microspheres have uniform size and good sphericity, with a density of 5.87 g/cm³.     

Synthesis of ZrO2 -SiC Composite by Carbothermal Reduction of Zircon

Zircon （mesh size ≤ 44μm ） and carbon black （mesh size ≤30μm ） were used as the starting materials, weighed with re（zircon） ： re（carbon black） of 100 ： 20 and mixed fully. The specimens with the diameter of 20ram and length of 5ram were prepared by pressing at 100 MPa, then dried at 120℃ for 12h, put into a furnace with 1. 5L ·min^-1 argon gas and fired at 1450℃, 1500℃, 1550℃, 1600℃ and 1650℃ for 4h, respectively. The chemical composition, phase composition and microstructure of the specimens were studied by chemical analysis, X-ray diffraction and scanning electronic microscope, and the carbothermal reduction reaction process was discussed by thermodynamic analysis. The results showed that the ZrO2-SiC composite could be synthesized by carbothermal reduction reaction using zircon and carbon black as the starting materials in argon atmosphere. The composite with different composition was obtained by controlling the firing temperature and partial pressure of CO gas. The proper temperature to synthesize ZrO2-SiC composite was 1600℃ in this experiment.     

Solidification of welded SiC–ZrB2–ZrC ceramics

A silicon carbide‐based ceramic, containing 50 vol% SiC, 35 vol% ZrB₂, and 15 vol% ZrC was plasma arc welded to produce continuous fusion joints with varying penetration depth. The parent material was preheated to 1450°C and arc welding was successfully implemented for joining of the parent material. A current of 138 A, plasma flow rate of ~1 L/min or ~0.5 L/min, and welding speed of ~8 cm/min were utilized for repeated joining, with full penetration fusion zones along the entire length of the joints. Solidification was determined to occur through the crystallization of β‐SiC (3C), then the simultaneous solidification of SiC and ZrB₂, and lastly through the simultaneous solidification of SiC, ZrB₂, and ZrC through a ternary eutectic reaction. The ternary eutectic composition was determined to be 35.3 ± 2.2 vol% SiC, 39.3 ± 3.8 vol% ZrB₂, and 25.4 ± 3.0 vol% ZrC. A dual fusion zone microstructure was always observed due to convective melt pool mixing. The SiC content at the edge of the fusion zone was 57 vol%, while SiC content at the center of the fusion zone was 42 vol% although the overall SiC content was still nominally 50 vol% throughout the entire fusion zone.     

Residual Stress and Biaxial Strength in Sc2O3–CeO2–ZrO2/Y2O3–ZrO2 Layered Electrolytes

Multi‐layered (Y₂O₃)_0.08(ZrO₂)_0.92/(Sc₂O₃)_0.1(CeO₂)_0.01‐(ZrO₂)_0.89(YSZ/SCSZ) electrolytes have been designed, so that the inner SCSZ layers provided superior ionic conductivity and the outer YSZ skin layers maintained good chemical and phase stability. Due to the mismatch of coefficients of thermal expansion between layers of different compositions, the thermal residual stresses were generated. The theoretical residual stress and strain were calculated for different thickness ratios of the electrolytes. In order to study the residual stress effect on the mechanical properties, the biaxial flexure tests of electrolytes with various layered designs were performed via a ring‐on‐ring method at room temperature and 800 °C. The maximum principal stress at the fracture indicated improved flexure strength in the electrolytes with layered designs at both temperatures. It is believed to be the result of the residual compressive stress in the outer YSZ layer. In addition, the Weibull statistics of the stress at the fracture at room temperature was studied, and the values of residual stress presented at the outer layer were well verified.     

Fabrication of Open-Cell, Microcellular Silicon Carbide Ceramics by Carbothermal Reduction

A novel processing route for developing open-cell, microcellular SiC ceramics has been developed. The strategy adopted for making microcellular SiC ceramics involved the following: (i) fabricating a formed body from a mixture of polysiloxane, phenol resin (used as a carbon source), polymer microbeads (used as sacrificial templates), and Al₂O₃–Y₂O₃ (an optional sintering additive); (ii) cross-linking the polysiloxane in the formed body; (iii) transforming the polysiloxane and phenol resin by pyrolysis into silicon oxycarbide and C, respectively; and (iv) synthesizing SiC by carbothermal reduction. By controlling the microbead and additive contents, it was possible to adjust the porosity so that it ranged from 60% to 95%.     

Reactive Hot Pressing and Properties of Zr1−xTixB2–ZrC Composites

Reactive hot pressing was used to prepare Zr_1?xTi_xB₂–ZrC composites with advantageous microstructure and mechanical properties from ZrB₂–TiC powders. The reaction mechanisms and the effects of different levels of TiC on the physical and mechanical properties of the resulting composite were explored in detail and compared to conventionally hot‐pressed ZrB₂ and ZrB₂–ZrC. Incorporation of 10 to 30 vol% TiC enabled full densification and restrained grain growth, reducing the final average grain size from 5.6 μm in pure ZrB₂ to a minimum of 1.4 μm in samples with 30 vol% TiC. The flexural strengths and hardnesses of the composites sintered with TiC were consequently greater than the conventionally processed ZrB₂–ZrC materials, increasing from 440 MPa and 17.4 GPa to a maximum of 670 MPa and 24.2 GPa at 10 vol% TiC. However, despite a decrease in the total average grain size, the flexural strength at higher TiC levels was limited by an increase in ZrC grain growth, which was observed to determine the flexural strength of the reaction sintered composites similar to the case of ZrB₂–SiC.     

Synthesis of Al_2OC Compound by Carbothermal Reduction Process with Boron Oxide Additive

The synthesis method of Al2OC by adding B2O3 was studied to modulate the traditional synthesis process.The mixtures of active carbon,alumina and boron oxide with different carbon contents were heated at 1 700 ℃ for 2 h in flowing argon atmosphere to get the Al2OC product.The results indicate that the addition of B203 promotes the formation of Al2OC,which is dependent on the addition of B2 O3,and the Al2 OC content in the products increases with the increase of carbon.By systematically exploring the ratio of active carbon,alumina and boron oxide,the best formulation and the corresponding reaction mechanism were determined.     

Phase Equilibria in the ZrO2–MgO–MnOx System

Phase equilibria were experimentally investigated in the MgO–MnO_x and the ZrO₂–MgO–MnO_x systems for different oxygen partial pressures by powder X‐ray diffractometry, scanning electron microscopy, and differential thermal analysis. The formation of two compositionally and structurally different β‐spinel solid solutions was observed in the MgO–MnO_x system in air in the temperature interval 1473–1713 K. Isothermal sections of the ZrO₂–MgO–MnO_x phase diagram were constructed for air conditions ( = 0.21 bar) at 1913, 1813, 1713, 1613, and 1523 K. In addition, isothermal sections at 1913 and 1523 K were constructed for = 10^?4 bar. The β‐spinel and halite phases of the MgO–MnO_x system were found to dissolve up to 2 and 5 mol% ZrO₂. A continuous c‐ZrO₂ solid solution forms between the boundary ZrO₂–MnO_x and ZrO₂–MgO systems. It stabilizes in the ZrO₂–MgO–MnO_x system down to at least 1613 K in air and down to 1506 K at = 10^?4 bar.     

ZrO2粉体及陶瓷的制备

叙述了ZrO2的基本性能和应用,介绍了ZrO2微粉和超细粉的制备方法及目前ZrO2结构陶瓷生产常用的成型方法,比较了几种成型方法的优缺点,指出了成型和烧成是较关键的两道工序。同时详细分析了干压成型中分层和烧成过程中开裂出现的原因,并提出了解决的办法。     

碳热还原法合成氮化铝陶瓷粉末的研究

本文以Al2O3和活性碳为原料，采用碳热还原法制备了氮化铝陶瓷粉末。通过对原料的DSC－TG及制备物XRD、SEM、EDAX的分析，表明该方法在较低的温度下即能制备出晶形发育完善、粒度均匀、纯度高的氮化铝陶瓷粉末，并对其反应机理进行了初步探讨。     

Phase Evolution of SiO2–Al2O3–ZnO–CaO–ZrO2–TiO2‐Based Glass with Added Y‐PSZ Particles

Yttria partially stabilized zirconia Y‐PSZ/glass‐ceramic composites were prepared by reaction sintering using powder mixtures of a SiO₂–Al₂O₃–ZnO–CaO–ZrO₂–TiO₂‐based glass and yttria partially stabilized zirconia (Y‐PSZ). The glass crystallized during sintering at temperatures of 1173, 1273, and 1373 K to give a glass‐ceramic matrix for high‐temperature protecting coatings. With the increasing firing time, the added zirconia reacted with the base glass and a glass‐ceramic material with dispersed zircon particles was prepared in situ. Furthermore, the added zirconia changed the crystallization behavior of the base glass, affecting the shape, amount, and distribution of zircon in the microstructure. The bipyramid‐like zircon grains with imbedded residual zirconia particles turned out to have two growth mechanisms: the inward growth and the outward growth, and its rapid growth was mainly dominated by the later one. For comparison, the referenced glass‐ceramic was prepared by sintering using exclusive glass granules and its crystallization behavior at 1173–1373 K was examined as well. Scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDS), transmission electron microscopy (TEM), and X‐ray diffraction (XRD) were used to characterize the crystallization behavior of the base glass and the phase evolution of the Y‐PSZ/glass‐ceramic composites.     

Methane combustion on sol–gel Rh/ZrO2–SiO2 catalysts

Rh/ZrO₂–SiO₂ catalysts prepared by impregnation of mixed ZrO₂–SiO₂ oxides obtained by using the sol–gel method were characterised and used in the combustion of CH₄. It was found that the specific area and porosity depend strongly on the pH of gelation. Two different Zr precursors were used: Zr(acac)₄ and Zr alkoxide. When the former precursor is used, no significant changes in the surface area are detected, however, important changes in pore size distribution are observed. In those samples prepared from Zr alkoxide, both the surface area and the porosity change significantly. No important changes in rhodium dispersion were observed by TEM, in agreement with XPS results. On the contrary hydrogen chemisorption results indicate that the hydrogen uptake of the sulfated samples drops drastically compared with those in which sulfate ions are absent. This behaviour has been attributed to a poisoning of rhodium particles by sulfide species produced by reduction of sulfate ions, as was revealed by TPR. The catalytic activity in the methane combustion is affected by the presence of chloride ions, which induce an inhibitory effect on the combustion. Additionally, the presence of micropores also decreases the activity due to diffusional limitations. © 1999 Society of Chemical Industry     

Fabrication of Transparent Spinel Honeycomb Structures by Methyl Cellulose–Based Thermal Gelation Processing

Transparent MgAl₂O₄ spinel of flat as well as honeycomb structure was fabricated by employing thermally induced gel casting of the slurry with 56 wt% solid loading containing 0.2 wt% of methylcellulose. The green specimens were pressureless sintered to 98%–99% of theoretical density with no open porosity at an optimum temperature of 1700°C. Final densification by hot isostatic pressing of both the specimens at the optimum temperature of 1800°C and 195 MPa pressure enabled further elimination of residual porosities and full densification resulting in theoretical density of 3.58 g/cc. The design of the honeycomb was such that it exhibited a surface area to volume ratio of 0.65 cm²/cm³ and a relative density of 0.69. The hardness of the honeycomb specimens has been found to be 13 GPa, which is at par with the solid specimens processed under identical conditions. Solid specimens of around 4 mm thickness exhibited a transmission of >80% in the visible (0.4–0.8 μm) region. Specimens were also tested according to ASTM procedures and have shown a flexural strength (σ_f) of 195 MPa and plane‐strain fracture toughness (K_Ic) of 1.87 MPa·m^1/2 as reported in this study.     

Preparation of Y2Si2O7/ZrO2 Composites and Their Composition – Mechanical Properties – Tribology Relationships

In this work, novel Y₂Si₂O₇/ZrO₂ composites were developed for structural and coating applications by taking advantage of their unique properties, such as good damage tolerance, tunable mechanical properties, and superior wear resistance. The γ‐Y₂Si₂O₇/ZrO₂ composites showed improved mechanical properties compared to the γ‐Y₂Si₂O₇ matrix material, that is, the Young's modulus was enhanced from 155 to 188 GPa (121%) and the flexural strength from 135 to 254 MPa (181%); when the amount of ZrO₂ was increased from 0 to 50 vol%, the γ‐Y₂Si₂O₇/ZrO₂ composites also presented relatively high facture toughness (>1.7 MPa·m^1/2), but this exhibited an inverse relationship with the ZrO₂ content. The composition–mechanical property–tribology relationships of the Y₂Si₂O₇/ZrO₂ composites were elucidated. The wear resistance of the composites is not only influenced by the applied load, hardness, strength, toughness, and rigidity but also effectively depends on micromechanical stability properties of the microstructures. The easy growth of subcritical microcracks in Y₂Si₂O₇ grains and at grain boundaries significantly contributes to the macroscopic fracture toughness, but promotes the pull‐out of individual grains, thus resulting in a lack of correlation between the wear rate and the macroscopic fracture toughness of the composites.     

The Influence of ZrO2 Additions on Al2O3 Evaporation and AlN Crystal Growth by Thermal Nitridation of Al2O3–ZrO2 Pellets

The effects of ZrO₂ additions to Al₂O₃ were investigated to improve the evaporation rate of Al₂O₃ for bulk AlN crystal growth. The evaporation rate of Al₂O₃ increased concomitantly with increasing ZrO₂ concentration under a nitrogen gas stream at 2223 K. The ZrO₂ was predominantly nitrided. The nitridation of ZrO₂ kept the local oxygen partial pressure high at the pellet surface, which suppressed the nitridation of Al₂O₃. The nitridation of ZrO₂ caused the outward diffusion of ZrO₂ (Zr⁴⁺ and O^2?) in the pellet, which was accelerated further by the presence of Al₂O₃–ZrO₂ liquid phase in grain boundaries, leading to the prompt formation of ZrN porous layer on the pellet surface. The suppressed nitridation of Al₂O₃ and the formation of porous ZrN layer were the reasons for the enhanced evaporation of Al₂O₃, leading to enhanced bulk AlN growth.     

干凝胶法制备空心玻璃微球炉内成球过程的数值模拟

基于对干凝胶粒子在干凝胶炉内吸热、封装、发泡、精炼和冷却过程的传热、传质和运动分析,建立了空心玻璃微球炉内成球过程的数学模型,并模拟了干凝胶粒子在炉内自由下落过程中温度、速度和直径的变化、粒子/微球与载气之间的温差及热传递系数在成球过程中的变化趋势.结果表明,在吸热封装阶段,干凝胶粒子表面升温速率非常高(1000～2000℃/s),能在很短时间(1s)和距离(4.5cm)内完成封装过程.提高载气的传热性能并不能显著增大液态玻璃微球的降温速率,提高制备炉冷却区的轴向温度梯度是增加其降温速率以提高空心玻璃微球几何对称性和表面粗糙度的关键.实验与模拟结果基本一致.     

Corrosion behavior and microstructural evolution of BN–ZrO2–SiC composites in molten steel

The influence of SiC content on corrosion resistance and microstructural evolution of BN–ZrO₂–SiC composites after corrosion in molten steel at 1600°C has been investigated. The corrosion resistance of BN–ZrO₂–SiC composite decreases with increasing the SiC content. When the SiC content is more than 15 vol%, the corrosion rate begins to accelerate sharply. A resintered ZrO₂ bilayer structure with a distinctly reduced porosity in the corrosion layer contributes significantly to the enhanced corrosion resistance. Thus, optimizing the SiC content and tailoring the microstructural feature of residual corrosion layer are effective ways to improve the corrosion resistance of BN–ZrO₂–SiC composite.     

Characterization of ZrN,ZrO2 and β′–Zr7O11N2 nanoparticles synthesized by pulsed wire discharge

Nanoparticles of ZrN, ZrO₂ and β′–Zr₇O₁₁N₂ were synthesized by pulsed wire discharge using Zr wire in various O₂ and N₂ gas mixtures with different oxygen partial pressures of up to 40 kPa and a total pressure of 100 kPa. The syntheses were carried out at a relative energy ratio (K) of 6.4 which was defined by a charged energy in a capacitor of the synthesis apparatus divided by the evaporation energy of the Zr wire. Morphology and phase analyses were carried out on these nanoparticles by X‐ray diffraction and field‐emission transmission electron microscopy. ZrN and Zr₂N were observed in a sample synthesized in 100% N₂ gas while with increasing the O₂ from 1% of total pressure, formation of β′–Zr₇O₁₁N₂ and ZrO₂ was seen. By bright field image observation, electron energy loss spectroscopy (EELS) and selected area electron‐diffraction analyses, nanoparticles of ZrN, β′–Zr₇O₁₁N₂ and ZrO₂ were separately characterized. In these syntheses, nanoparticles of β′–Zr₇O₁₁N₂ existed in much smaller size and different shape than the ordinary spherical nanoparticles of ZrN and ZrO₂. In gas mixtures where O₂ contents were larger than 22% (dry air composition), ZrN was not detected or detected with just a fractional amount compared to the two major phases of β′‐Zr₇O₁₁N₂ and ZrO₂. EELS data for ZrN and β′–Zr₇O₁₁N₂ were obtained and compared by separated analyses of nanoparticles of these phases. From these data, it was concluded that, in nuclear accidents, small amount of particles can indicate the accident atmosphere around the Zr alloys fuel cladding.     

Precisely Controlling Preparation of Ceria‐Stabilized Zirconia Microspheres of ~100 μm by External Gelation

Ceria‐stabilized zirconia (CSZ) microspheres of ~100 μm were prepared by external gelation process. The inner diameter of nozzle was 0.2 mm. The key process parameters including broth pH value, Zr concentration, flow rate, and vibration frequency were optimized for precise control of the diameter. The optimum parameters were found to be flow rate of 11.08 mL/min, Zr concentration of 0.72 mol/L, and vibration frequency of 2650 Hz. CSZ microspheres with an average diameter of 103.1 μm and standard deviation of 4.4 were prepared under these conditions. CSZ microspheres showed good monodispersity, good sphericity, and high density.