The preparation of ZrO2–ZrC ceramic microspheres by internal gelation process with sucrose as a carbon source

ZrO₂–ZrC (ZrCO) ceramic microspheres were fabricated by internal gelation and carbothermal reduction. The effect of various carbon sources on the stability of the broth was investigated, the carbon sources which have little effect on the stability of the broth were chosen as the carbon sources for carbothermal reduction, and the component distribution and microstructure of the sintered ZrCO microspheres were analyzed. The results indicate that the broth was stable at room temperature when sucrose was used as the carbon source, and crack-free ZrC–ZrO₂ ceramic microspheres with good sphericity and uniform distribution of ZrC and ZrO₂ could be successfully prepared. Moreover, the ZrC–ZrO₂ ceramic microspheres fabricated have no obvious pores and free carbon.     

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.     

The Fabrication of ZrO2–ZrC Microspheres by Internal Gelation and Carbothermal Reduction Process

ZrO₂–ZrC (ZrCO) ceramic microspheres were fabricated by combination of internal gelation and carbothermal reduction process. The internal gelation broth containing carbon powders was prepared and stored at room temperature of 25°C to form a precursor of microspheres. By dropping droplets of the broth into hot silicone, the gel microspheres would solidify within a few seconds by decomposition of hexamethylenetetramine (HMTA). The dispersability of the carbon powders in the broth is the key factor to producing ZrCO microspheres with uniform density distribution, as zirconium carbides are generated by the direct reduction of ZrO₂ with carbon. The ZrCO ceramic microspheres fabricated with various mass ratio of ZrO₂ and ZrC have good sphericity and no cracks through optimizing washing and heat treatment process.     

An improvement of microfluidic-assisted internal gelation in the preparation of millimeter-sized ceramic microspheres

The microspheres prepared by the microfluidic-assisted internal gelation process are still challenging to reach the millimeter size due to the large gravity. The gravity would affect the sphericity and size uniformity of microspheres. The improved microfluidic-assisted internal gelation process is designed and optimized to produce large-sized monodisperse ceramic microspheres with good sphericity. The movement mechanism of millimeter-sized sol droplets and gel microspheres in the microchannel is summarized and the state of the dispersed phase entering the measuring cylinder is controlled. The friction between the gel microspheres and the tubing wall is reduced as much as possible, and ZrO₂ ceramic microspheres of 500 ± 5 μm with good sphericity are prepared with the broth at room temperature. The mechanism of sol droplets entering the oil surface is explored to demonstrate the superiority of the improved microfluidic-assisted internal gelation process. The improved microfluidic-assisted internal gelation process could be directly extended to preparing other millimeter-sized monodisperse ceramic microspheres.     

Preparation of monodisperse ZrO2 ceramic microspheres (>200 μm) by co-axial capillary microfluidic device assisted internal gelation process

The internal gelation process was combined with a co-axial capillary microfluidic device to prepare monodisperse ZrO₂ ceramic microspheres. Large sized gel microspheres in the range of hundreds of micrometers could be produced with the capillary microfluidic device with co-axial flow structure and the large sized outlet tubing. After drying and sintering, large sized ZrO₂ ceramic microspheres (>200 μm) which had good sphericity and narrow size distribution were prepared.     

Influence of compatibility between sol and intermediate layer on the performance of yttria-stabilized zirconia nanofiltration membrane

This paper reports the synergistic effect of the sol and intermediate layer on the performance of yttria-stabilized zirconia (YSZ) nanofiltration (NF) membranes. We have focused on the characterization of the microstructure, pure water permeance, and molecular weight cut-off (MWCO) of the NF membranes derived from zirconia sols of different precursor concentrations on two types of supported ZrO₂ ultrafiltration (UF) membranes. We found that the performance of YSZ membranes strongly depends on the sol concentration and the pore size of the intermediate layer. In addition, YSZ gel membrane formation was found to follow the filtration process. Therefore, it is essential to maintain the compatibility between the sol and intermediate layer to fabricate high-performance NF membranes. A crack-free thin YSZ layer with an MWCO of 816 Da (pore size: 1.4 nm) and a water permeance of 25 L m^-2 h^-1 bar^-1 was fabricated using a precursor concentration of 0.03 mol/L, on ZrO₂ UF membrane with a pore size of 5.5 nm. The YSZ NF membrane exhibited a relatively high retention rate towards MgCl₂ (71%), whereas a lower retention rate was observed for NaCl (35%).     

Electro‐Sintering of Yttria‐Stabilized Cubic Zirconia

Electro‐sintering, i.e., electrically enhanced densification without the assistance of Joule heating, has been observed in 70% dense 8 mol% Y₂O₃‐stabilized ZrO₂ ceramics at temperatures well below those for conventional sintering. Remarkably, full density can be obtained without grain growth under a wide range of conditions, including those standard for solid oxide fuel cell (SOFS) and solid oxide electrolysis cell (SOEC), such as 840°C with 0.15 A/cm². Microstructure evidence and scaling analysis suggest that electro‐sintering is aided by electro‐migration of pores, made possible by surface flow of cations across the pore meeting lattice/grain‐boundary counter flow of O^2?. This allows pore removal from the anode/air interface and densification at unprecedentedly low temperatures. Shrinkage cracking caused by electro‐sintering of residual pores is envisioned as a potential damage mechanism in SOFC/SOEC 8YSZ membranes.     

微通道内双重孔结构SiO2微球的制备

利用微流控技术制备双重孔结构SiO₂微球具有微观结构和宏观形貌可控的优点。在同轴环管微通道中,通过pH和温度变化引发快速凝胶过程制备得到了具有双重孔结构的SiO₂微球,考察了有机相溶剂性质、有机相流速以及凝胶温度等因素对微球宏观形貌以及微观结构的影响规律。实验结果表明,制备得到的SiO₂微球粒径在300～600 μm可调,比表面积可以达到1000 m²·g^-1,介孔孔径在4～10 nm之间,大孔孔径在400～1500 nm之间。实验发现有机相流速的增大会导致微球粒径的减小,提高三辛胺对盐酸的萃取速率,加快二氧化硅溶胶粒子的凝胶过程,更易生成松散的网状大孔结构。较高的凝胶温度会增大SiO₂微球介孔的孔容和孔径。     

Microfluidic fabrication of ZrO2 microspheres using improved external gelation aided by polyacrylamide networks

Microfluidic generation of emulsion microdroplets, combined with external gelation, is a promising method for the continuous production of high-quality ceramic microspheres. However, the external gelation mode is sensitive to the environmental concentration of sol-gel triggering agent, resulting in gel deformation due to uneven infiltration and reaction within the sol microdroplets. In this work, a polyacrylamide (PAM) gel network was used to prevent the microdroplets from undergoing apparent deformation throughout the gelation process, and dense ZrO₂ microspheres with high monodispersity and sphericity were fabricated successfully. Importantly, owing to the assistance of PAM network, the external gelation process becomes less sensitive and can be treated with a substantially higher triggering agent concentration, thus allowing a faster and more efficient sol-gel production of ZrO₂ ceramic microspheres.     

Effect and mechanism of ZrO2 doping on the cracking behavior of melt-grown Al2O3 ceramics prepared by directed laser deposition

Directed laser deposition (DLD) is a new method for rapidly preparing melt-grown ceramics, but cracking problem greatly limited its application. In this study, cracking behavior of Al₂O₃ ceramics was suppressed by doping ZrO₂. Crack suppression mechanism of ZrO₂ doping in melt-grown ceramics was also analyzed. Process parameters which are prone to generating cracks were adopted in the experiments, and they contribute to showing the crack clearly. Results show that ZrO₂ doping has remarkable crack suppression effects. It is most obvious when ZrO₂ content is 37 mol%. Compared with those of pure Al₂O₃ ceramics, crack density reduces by 43.2%, and the number of longitudinal main cracks reduces by 63.2%. Doping of ZrO₂ forms dense composite microstructure with primary α-Al₂O₃ grains discretely distributing in eutectic continuous matrix. Therefore, initial crack sources are effectively reduced. Morphology of primary Al₂O₃ grains transforms from cellular to dendritic, which changes crack propagation mode from inter-granular to trans-granular. Mismatch of thermo-physical properties of different phase promotes the arrest, deflection, and bridging phenomena in crack propagation, contributing to crack suppression. On the basis of ZrO₂ doping, we have realized the preparation of crack-free eutectic ceramic (37 mol%ZrO₂) samples through further process optimization. The maximum size of the sample reaches 230 mm.     

Synthesis of the ZrO2–SiO2 microspheres as a mesoporous candidate material

A series of mesoporous ZrO₂?CSiO₂ microspheres with different amounts of silica were synthesized by a polymerization-induced colloid aggregation process, using zirconyl chloride and commercial SiO₂ colloids as the raw materials. The microspheres were characterized by scanning electron microscopy (SEM), N₂ adsorption?Cdesorption isotherms, X-ray powder diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). According to the SEM results, the ZrO₂ and ZrO₂?CSiO₂ microspheres had spherical morphologies and the sizes of the ZrO₂?CSiO₂ microspheres increased with increasing SiO₂/ZrO₂ weight ratios. The XRD spectrum of the pure ZrO₂ microspheres contained characteristic peaks for a monoclinic crystalline zirconia structure. The XRD spectrum of the ZrO₂?CSiO₂ microspheres showed a tetragonal crystalline structure. The specific surface areas of the ZrO₂?CSiO₂ microspheres increased with increasing SiO₂/ZrO₂ weight ratios, and the pore volumes also increased. The average pore size of the ZrO₂?CSiO₂ microspheres was 3?C5?nm. The FT-IR spectrum of the ZrO₂?CSiO₂ microspheres confirmed the formation of Zr-O-Si bonds. The Zr-O-Si bonds make the metastable tetragonal zirconia stable at room temperature.     

Fabrication and characterization of porous ZrO2 with a high volume fraction of fine closed pores

A novel technique for the fabrication of porous ZrO₂ with a high volume fraction of fine closed pores was investigated. A partially stabilized ZrO₂ (3 mol% Y₂O₃; Y-PSZ) body, with a 97–99% relative density and containing a small amount of impurities, exhibited a large volume expansion related to the formation of closed pores after heating at 1700 °C for 10 min in N₂. These closed pores seemed to mainly form due to the vaporization of hydroxyl apatite: Ca₁₀(OH)₂(PO₄)₆ as an impurity and superplasticity of the ZrO₂ during heating. Porous ZrO₂ with approximately 24.6% closed pores (total porosity: 26.7%) was successfully fabricated by the addition of 1 mass% SiO₂, 1 mass% TiO₂, and 1 mass% hydroxyl apatite. The closed pore size and morphology of the resultant porous ZrO₂ bodies were investigated, and the formation mechanism of the closed pores was examined on the basis of chemical thermodynamics.     

Synthesis,surface characteristics,and electrochemical capacitance of Cu-doped carbon xerogel microspheres

Small-amplitude oscillatory shear tests were used to determine the rheological properties of a copper acetate-doped resorcinol–formaldehyde mixture at between 30 and 40 °C. The apparent activation energy of the sol–gel transition was 76.6 ± 0.6 kJ/mol. Organic gel microspheres were only obtained when the sol was emulsified immediately before the gelation point and not at the gelation point itself, due to the fast gelation kinetics of the copper acetate-doped resorcinol–formaldehyde mixture. The microspherical shape was preserved after carbonization. Cu-doped carbon xerogel microspheres were steam-activated at 840 °C. All samples comprised isolated well-formed microspheres, whose size increased with higher degree of activation. The surface area and porosity varied with the activation degree. Copper was detected as CuO, which acted as gasification catalyst during activation, and its size increased with higher activation degree. Electrochemical measurements were conducted with a three-electrode cell in 1 M H₂SO₄. A very large volumetric capacitance, 146 F/cm³, was found for the 30%-activated Cu-doped activated carbon xerogel, attributable to the high particle density resulting from the very compact packing of the microspheres. This sample also showed the lowest equivalent series resistance, due to its pore texture and high surface Cu content.     

Effect of high alumina cement on permeability and structure properties of ZrO2 composites

Porous ZrO₂ based ceramics are widely used for filtration/separation processes due to the good chemical and thermal stability. For these applications it is desirable that the material have a controlled porous structure in order to obtain good permeability. In this study Ca stabilized ZrO₂ composites were developed from a starting mixture of pure ZrO₂ containing different mole proportions of calcium aluminate cement. Ceramics disks were uniaxially pressed and subsequently sintered at 1300–1450 °C. The influence of process parameters such as chemical compositions and sintering temperature on textural characteristics (volume fraction of pores, pore size distribution) and permeability was followed by apparent density measurements, Hg porosimetry and N₂ permeation, respectively. Sintered microstructure was examined by scanning electron microscopy SEM. The XRD analysis showed that m-ZrO₂ transformed to tetragonal and/or cubic ZrO₂, these phases probably coexisted at relatively low CaO addition. For 30 mol% addition, amount of the cubic Ca_0.15Zr_0.85O_1.85 phase appreciably increased. At 50 mol% CaO, CA₂ was the major phase of the composite with minor CaZrO₃ formation whereas relative content c-ZrO₂ is slightly reduced.The composites had 30–40 vol% porosity with typical pore radius of 1–1.3 μm and the corresponding Darcian permeability k₁ values varied between 2 and 4 × 10^?14 m², such structure parameters slightly increased for high cement addition. The k₁ of ceramics produced from 50 mol% CaO composition remained nearly constant up to 1450 °C due to similar densification degree. The experimental permeability dependence on pore structure parameters as well as the comparison with the value estimated by Ergun's equation are showed.     

Processing of microcellular silicon carbide ceramics with a duplex pore structure

A novel processing route for producing microcellular SiC ceramics with a duplex pore structure has been developed using a polysiloxane, carbon black, SiC, Al₂O₃, Y₂O₃, and two kinds of pore former (expandable microspheres and PMMA spheres). The duplex pore structure consists of large pores derived from the expandable microspheres and small windows in the strut area that were replicated from the PMMA spheres. The presence of these small windows in the strut area improved the permeability of the porous ceramics. The gas permeability coefficients of porous SiC ceramics were 0.13 × 10¹² m² for the porous SiC without PMMA spheres, 0.47 × 10¹² m² for the porous SiC with 10 wt% PMMA spheres, and 0.82 × 10¹² m² for the porous SiC with 20 wt% PMMA.     

Desert Rose‐Shaped Zircon Synthesized by Low‐Temperature Hydrothermolysis

This study has presented synthesis, characterization, and formation mechanism of a kind of novel porous zircon with desert rose‐shaped morphology, which was hydrothermally fabricated by a two‐step quasi‐in situ reaction in a system of silica hydrogel and well‐dispersed ZrO₂ precursor microspheres. The micro–mesoporous zircon product possesses a sole crystalline phase of hydroxyl‐fluorinated ZrSiO₄ without unreacted ZrO₂ or SiO₂ in the final resultant. It has a specific surface area exceeding 120 m²/g and can maintain more than 50% micro‐mesoporic pores after calcination at 800°C due to its magnificent thermal stability. A four‐stage formation mechanism has been proposed to elucidate the process of quasi‐in situ crystallization and growth for the rose‐shaped zircon.     

Novel micro-spherical Si3N4 nanowire sponges from carbon-doped silica sol foams via reverse templating method

A novel kind of nanowire sponges, namely Si₃N₄ nanowire-weaving microspheres, synthesized from a simple, convenient, high-efficient approach are proposed here. As the reverse template, three-dimensional foam skeleton structure with uniform pores and ultrathin pore walls is constructed via the effective particle-stabilized foam method, where the silica sol and carbon black are chosen as the raw materials, providing the sufficient space for the growth of nanowires during the carbothermal reduction reaction process. The formation mechanism of this novel sponge is studied via multiple characterization methods. Si₃N₄ nanowires formed microspheres possess uniform and curving morphology due to the stable environment for growing via vapor–solid mechanism, leading to the relatively high specific surface area of 86.77 m²/g. Owing to in-situ oxidation process, micro-spherical SiO₂ nanowire sponges with similar morphology are synthesized, which present diameter in range of 20-40 nm and specific surface area of 50.47 m²/g. This work provides insights for the design of high-performance nanowire sponges with promising applications in the filtration, thermal insulators, and catalyst supports fields.     

Synthesis and characterization of mesoporous materials: Silica–zirconia and silica–titania

An experimental strategy was developed to obtain mesoporous SiO₂–ZrO₂ and SiO₂–TiO₂ mixed oxides by a sol–gel method, treating the gels hydrothermally. The solids were characterized by nitrogen physisorption, pyridine thermodesorption, ²⁹Si nuclear magnetic resonance, SEM and X-ray diffraction. The effects of ZrO₂ content, the generated pressure in the synthesis vessel and further modification of this type of procedure on the solids properties were studied. It was found that SiO₂–ZrO₂ and SiO₂–TiO₂ mixed oxides dried at atmospheric pressure developed type I isotherms. On the other hand, for the SiO₂–ZrO₂ and SiO₂–TiO₂ mixed oxides that were treated under pressure in the autoclave (at high SiO₂ content) the porosity was improved and mesoporous materials exhibiting type IV adsorption isotherms. Specific surface area and pore size distribution were a function of ZrO₂ and TiO₂ content. The materials exhibited narrow pore size distributions with pore diameters in the region of mesopores at about 4 nm and high surface areas, the highest being 481 m²/g for the 10 wt% ZrO₂ Si–Zr material. Differences in acidity as determined by pyridine thermodesorption were observed to depend on the synthesis parameters and ZrO₂ and TiO₂ concentration.     

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.     

Microfluidic fabrication of ceramic microspheres with controlled morphologies

In this study, the diffusion‐induced external gelation is combined with a microfluidic technique to prepare monodisperse ZrO₂ ceramic microspheres. The gelation of sol droplets is traced by fluorescence visualization of the local pH, and it illustrates the effect of the external concentration of triggering agent (tetramethylethylenediamine, TMEDA) on the formation of the gel network, which results in 3 kinds of deformation of the gel particles. The deformation mechanism mainly lies in the imbalanced Laplace pressure exerted on the gel network during the competition between the gelation and the drying processes. By regulating the concentration of TMEDA, the monodisperse ZrO₂ ceramic microspheres with high sphericity can be readily fabricated.