Microstructure of TiO2 porous ceramics by freeze casting of nanoparticle suspensions

During freeze casting of TiO₂ porous ceramics, the porous architecture is strongly influenced by TiO₂ particle size, solids loading, and cooling temperature. This work investigates the influences of particle size, freezing substrate, and cooling temperature on the TiO₂ green bodies prepared by freeze casting. The results show that the lamellar channel width with 100 nm particles is larger than that of 25 nm particles, yet the ceramic wall thickness is noticeably decreased. The lamellar structure is more ordered when using a copper sheet than glass as its freezing substrate. A finer microstructure results when frozen at − 50 ℃ than − 30 ℃. Such porous materials have application potentials in a wide range of areas such as photocatalysis, solar cells, and pollutant removal and should be further studied.     

Preparation and properties of high-porosity ZrB2-SiC ceramics by water-based freeze casting

The development of novel cermet composites based on porous ceramics with high porosity, interconnected pore structure and good mechanical property has attracted considerable attention in engineering application. In this work, water-based freeze casting process was employed to fabricate ZrB₂-SiC porous ceramic with aligned lamellar-channels structure using PAA-NH₄ as the dispersant. The results revealed that the well-dispersed suspension with best rheological behavior was obtained using 1.0 wt% PAA-NH₄ at pH 9. The crack-free porous ceramic exhibited small volume shrinkage ranging from 2.59 % to 1.87 %. By varying the solid loading, the fabricated samples displayed a tailored porosity ranging from 76.12% to 59.37% and an excellent compressive strength of 7 MPa to 78 MPa. After oxidation, the samples displayed a decreased porosity and an increased compressive strength. The ZrB₂­SiC porous ceramic fabricated in this work will be a promising candidate for the framework of cermet composite.     

Theoretical analysis of colloidal interaction energy in nanoparticle suspensions

This study is focused on understanding interaction energies for Al₂O₃ nanoparticle suspensions at high solids loadings. Among the four interaction energies: van der Waals interaction energy, electrostatic interaction energy, steric interaction energy, and depletion interaction energy, the study found that the van der Waals attraction is the dominant mechanism in destabilizing the dispersion system; the steric repulsion is a more effective stabilization mechanism than the electrostatic repulsion. The fundamental cause for an unstable suspension due to the particle–particle close contact and thus attraction at high solids loading can be overcome by increasing the particle–particle repulsion. When a dispersant is used in a suspension, the polymer chain length must be carefully selected so that the dispersant provides enough stabilization but does not compromise the maximum achievable solids loading. If an appropriate dispersant is chosen, up to 45 vol% solids loading suspension can be obtained for the Al₂O₃ nanoparticles.     

Pore structure and thermal oxidation curing behavior of porous polymer derived ceramics with superhigh porosity fabricated by freeze casting

Porous ceramics with porosity up to 92.5 % have been successfully fabricated by freeze casting of polycarbosilane (PCS) solution. The effect of PCS concentration and thermal oxidation curing on the pore structure and compressive properties was investigated. Curing mechanism and thermodynamics were illuminated through analyzing the molecular structure, curing activation energy, and curing degree. Porous ceramics, mainly composed of SiC and a small amount of SiO₂, have dendritic pore structure which well replicates the solidification morphology of camphene solvent. Results of FT-IR and Gaussian computation of PCS electron density show that Si–H and Si–CH₃ bonds play a dominant role in thermal oxidation curing reaction. Both curing degree and ceramic yield increase with the increase in curing temperature and time. The curing degree of Si–H bond is close to 52 % and the corresponding ceramic yield is about 83 % when the porous PCS was cured at 200 °C for 90 min. Both polymer concentration and curing time have influences on the compressive strength of porous ceramics.     

Effects of SiC,SiO2 and CNTs nanoadditives on the properties of porous alumina-zirconia ceramics produced by a hybrid freeze casting-space holder method

Highly porous alumina-zirconia ceramics were produced by adding space-holder materials during freeze casting. To increase the strength of porous ceramics, different amounts of nanoadditives (silicon carbide-SiC, silica-SiO₂, and multi-wall carbon nanotubes-CNTs) were added. Space-holder materials were removed by preheating, and solid samples were produced by sintering. Up to 68% porosity was achieved when 40% space-holder was added to the solid load of slurry. Wall thicknesses between pores were more uniform and thinner when nanoadditives were added. Compressive tests revealed that SiC nanoparticles increased the strength more than other nanoadditives, and this was attributed to formation of an alumina-SiC phase and a uniform distribution of SiC nanoparticles. Results indicated that by including 20% space-holder materials and 15% SiC nanoparticles, the density decreases by 33.8% while maintaining a compressive strength of 132.5 MPa and porosity of 43.4%. Relatively low thermal conductivities, less than 3.5 W/K-m, were measured for samples with SiC nanoparticles.     

Rheology and gelation behavior of gel-cast cordierite-based glass suspensions

A cordierite-based glass suspension was shaped by gel-casting method. Effects of various parameters like pH, volume fraction of glass particles, gelling agents and dispersant on the rheology of the prepared slurries were investigated. The results demonstrated that using sodium tripolyphosphate (STPP) dispersant brings about a suspension of minimum viscosity. The bending strength of the dried gels was increased with the gelling agents. In addition, the utilization of cross linkers at a fixed concentration of monomer led to the gradual enhancement of the bending strength of the dried bodies. A bending strength of ∼11 MPa was obtained for the most promising dried gel cast bodies.     

Multiphase imaging of freezing particle suspensions by confocal microscopy

Ice-templating is a well-established processing route for porous ceramics. Because of the structure/properties relationships, it is essential to better understand and control the solidification microstructures. Ice-templating is based on the segregation and concentration of particles by growing ice crystals. What we understand so far of the process is based on either observations by optical or X-ray imaging techniques, or on the characterization of ice-templated materials. However, in situ observations at particle-scale are still missing. Here we show that confocal microscopy can provide multiphase imaging of ice growth and the segregation and organization of particles. We illustrate the benefits of our approach with the observation of particles and pore ice in the frozen structure, the dynamic evolution of the freeze front morphology, and the impact of PVA addition on the solidification microstructures. These results prove in particular the importance of controlling both the temperature gradient and the growth rate during ice-templating.     

Microstructure and permeability of porous YSZ ceramics fabricated by freeze casting of oil-in-water suspension

Porous yttria-stabilized zirconia (YSZ) ceramics are fabricated through freeze casting of oil-in-water suspension followed by sintering at 1250−1550 °C. The pore structure, compressive strength and permeability of porous YSZ ceramics are tailored via altering the emulsion content and sintering temperature. The samples obtained using higher emulsion content or at lower sintering temperature show larger Darcian and non-Darcian constants due to their higher open porosity and larger pore size. Furthermore, the investigation on individual contributions of viscous and inertial resistances on the total pressure drop during permeation process indicates that the viscous resistance increases but the inertial resistance decreases with increasing the emulsion content or decreasing the sintering temperature for samples. Porous YSZ ceramics obtained in this work with a k₁ range of 3.14 × 10⁻¹³–1.12 × 10⁻¹² m² are appropriate for applications in filters and membrane supports.     

Investigation of photocatalytic properties of TiO2 nanoparticle coating on fly ash and red mud based porous ceramic substrate

Titanium dioxide is one of the best semiconducting photocatalysts available for photocatalytic cleaning applications. Especially nano-sized TiO₂ particles deposited on porous substrates can be utilized as a filter for solid and liquid media. On the other hand, red mud and thermal plant fly ash are hazardous wastes that are produced in large quantities. Recycling/reuse of these waste material in a porous ceramic production would be beneficial both for environmental and economical issues. In the present study, a porous substrate was produced from red mud and fly ash with varying ratios and additives of H₃BO₃, CaCO₃, and MgCO₃ for lowering the melting temperature and porosity formation. Sintered ceramics were then coated with nano-sized TiO₂ particles by the sol-gel method. Ultrasonic dispersion of nano-sized TiO₂ nanoparticles was also utilized as an alternative method for impregnation of nanoparticles into the porous structure of the ceramic substrate. Finally, photocatalytic activities and degradation of methylene blue (MB) under UV radiation of substrates were investigated. According to the SEM investigations, the sol-gel method was observed to be a better way of nanoparticle deposition because deposited particles are homogenous throughout the ceramic body. Also, this method provides lower particle sizes than the ones that were deposited by the ultrasonic dispersion method. This results in higher surface area and better photocatalytic activities.     

Novel freeze-casting fabrication of aligned lamellar porous alumina with a centrosymmetric structure

A novel freeze-casting method is used to fabricate aligned lamellar porous alumina with a centrosymmetric structure from aqueous alumina slurries. Two cold fronts oriented perpendicularly to each other, originating from the bottom and side of the cylindrical copper mold, induce the growth of ice crystals in specific directions along the radius of the cylindrical mold. Lamellar channels of porous alumina are arranged centrosymmetrically along the radial axis. The pore distribution of the currently prepared porous ceramics is more regular when compared with that of porous ceramics prepared by conventional freeze casting. This affords porous ceramics with improved mechanical properties and stability. The current method addresses the issue of partial failure as induced by the randomly distributed channels in lamellar porous ceramics.     

Fabrication of homogenous pellets by freeze granulation of optimized TiO2-Y2O3 suspensions

MOX fuels are used in nuclear reactors. To ensure the manufacture of MOX, wet processes are investigated. Freeze granulation of water-based powder suspension seems a promising way to yield homogenous and easy-to-press UO₂-PuO₂ granules. These granules are expected to form dense and homogenous pellets by uniaxial pressing. Granules and pellet properties are affected by suspension formulation and atomization conditions. Therefore, these conditions must be studied and optimized to produce granules with good processability and thus MOX pellets with compliant density, homogeneity and absence of defects. In this scope, key properties of water-based suspensions of UO₂ and PuO₂ surrogate powders, TiO₂ and Y₂O₃ respectively, were firstly studied to assess their suitability for the freeze granulation process. These properties were compared to those of UO₂ and PuO₂ suspensions to verify and confirm the correctness in the choice of surrogate powders. Then, the freeze granulation process itself was investigated focusing on TiO₂-Y₂O₃ suspensions.     

Porous SiC using polycarbosilane/camphene solutions: Roles of freeze casting parameters

In order to control the pore characteristics and macroscopical performance of porous ceramics, roles of the freeze casting parameters are the key points. Herein, aligned dendritic porous SiC was fabricated by freeze casting of PCS-camphene solutions with different solid loading, freeze front velocity, temperature gradient, and freezing temperature. Influence of these parameters on the microstructure and compressive strength of porous SiC was investigated. With increasing the PCS content, freeze temperature, freeze front velocity or temperature gradient, degree of undercooling of the camphene was increased, resulting in the formation of smaller pore size, decreased porosity and increased compressive strength. Compared to variables of freeze temperature and temperature gradient, increased freeze front velocity was more efficiency in improving the compressive strength of porous SiC, owing to the formation of smaller pore size and longer secondary dendritic crystals. Promising micron-sized porous SiC with high porosity (79.93 vol%) and satisfactory strength (15.84 MPa) was achieved for 10% PCS-camphene solution under optimized freezing conditions.     

Freeze-cast alumina pore networks: Effects of processing parameters in steady-state solidification regimes of aqueous slurries

Aqueous alumina slurries with varying solids loading and particle size were freeze cast under seven freezing conditions to investigate the influence of these on pore network characteristics including pore size and geometric specific surface area. Slurry temperatures were recorded in situ to determine freezing front position and velocity during solidification, which were then analyzed via regression and modeled using solidification theory. Classic mathematical models for the time dependence of freezing front position and velocity were found to hold for freeze-cast slurries. Building on these, a one-phase Stefan problem was used to describe freezing kinetics. Models for freezing front velocity were combined with solidification theory to obtain predictions of microstructural feature size from freezing kinetics. Results showed that, while there may be a dependence on solids loading, the combination of mathematical modeling of solidification and classic solidification theory is applicable to freeze-cast ceramics and accurately describes pore network characteristics from processing parameters.     

Fabrication of highly porous mullite microspheres via oil-drop molding accompanied by freeze casting

Porous mullite microspheres with a highly open porosity and average diameter of more than 800 µm were fabricated via an oil-drop molding method accompanied by a freeze casting process. After sintering, a highly porous structure was formed due to interlocking whisker-shaped mullite grains and formation of interconnected skeletons during the freeze-casting process. Additionally, it was found that a high porosity and large pore size in the microspheres green bodies are favorable for the synthesis of mullite whiskers with high aspect ratio.     

Controlling micro-porous size in TiO2 pellets processed by sol-gel and rapid liquid phase sintering

A fast and lower electric energy consumption process to synthesize TiO₂ pellets with interconnected micropores, is proposed. Pellets were prepared by rapid liquid-phase sintering (RLPS) at different temperatures (900, 1000 and 1100 °C) and times (2, 5, 7 and 10 min). The density of these samples increases when temperature rises and decreases for longer sintering times; the highest density, of 2.78 g/cm³ was obtained when sintering at 1100 °C/2min. The addition of PEG and the annealing at 450 °C/2 min produced pores of 38.51 ± 27.51 μm and 48.98 ± 32.34 μm when PEG3350 and PEG8000 respectively, were used. An additional RLPS at 1100 °C/2 min gives rise to TiO₂ pellets in a rutile phase, with pores of 76.82 ± 34.23 μm and 173.04 ± 68.03 μm for PEG3350 and PEG8000, respectively. Interconnectivity of pores is obtained in all samples. The elastic module of these pellets was 39.22 ± 0.16 GPa, for the sample prepared with PEG3350; and 121.30 ± 0.04 GPa for the one made with PEG8000. The achieved pore size and interconnectivity at 1100 °C/2 min are a result of the optimized sintering conditions and the better control of PEG vapor pressure released when the intermediate annealing at 450 °C/2 min is introduced.     

Preparation of porous TiO2 by a novel freeze casting

Highly porous and open interconnected pore structural TiO₂ were prepared by a novel freeze casting method. In the experiment, the well-dispersed aqueous slurries were first frozen, and then dried at a reduced vacuum. Since the sublimation of ice crystals developed in the freezing process, the green bodies with highly porous were obtained. The phase composition and the microstructure of the sintered samples were characterized by XRD, SEM, porosity and the pore size distribution was measured by mercury porosimetry. The results demonstrated that the PVA concentration in the slurries remarkably affect the microstructure of TiO₂ ceramics. The pore morphology of TiO₂ ceramics with 3 wt.% polyvinyl alcohol (PVA) addition was dendritic, and however, the pore morphology of TiO₂ ceramics with 6 wt.% PVA addition changed into columnar. The reason for the variation of the pore morphology was ascribed to the effect of the PVA gelation on the growth behavior of the ice crystals.     

多孔Al2O3-ZrO2陶瓷的微观结构

以Al₂O₃、ZrO₂陶瓷粉体为溶质,以莰烯为溶剂,以Texaphor963作为添加剂,制备出低粘度高稳定性的陶瓷浆料,采用冷冻注模工艺制备出具有较高强度的陶瓷坯体,采用无压烧结工艺,得到了多孔Al₂O₃-ZrO₂陶瓷制品,并对其微观结构进行了研究。     

纳米TiO_2的制备及其光催化性能

以钛酸四丁酯为原料,采用溶胶 凝胶法制备了纳米级TiO2颗粒,通过甲苯在样品上的光催化氧化过程评价了样品的光催化活性。考察了制备过程中的焙烧温度对样品颗粒的晶型、粒径和光催化性能的影响。X射线衍射结果表明,焙烧温度低于500℃时得到的样品都是锐钛矿型TiO2,700℃下得到的已基本是金红石型;随着焙烧温度的升高样品的粒径增大,光催化活性下降;甲苯在TiO2上的气相光催化氧化符合一级反应规律。     

Al2O3多孔陶瓷的制备和性能研究

采用乙基纤维素为成孔剂制备了Al2O3多孔陶瓷,探讨了工艺参数对其性能的影响。研究结果表明,造孔剂含量、球磨时间及烧结温度均对多孔陶瓷的气孔率和抗折强度有影响。烧结温度的升高使得气孔率降低,但变化不明显,抗折强度明显提高。随造孔剂含量的升高,使得气孔率逐渐上升,超过20％后变化趋于平稳。随着球磨时间的增加,试样呈现气孔率下降和抗折强度升高的趋势。以烧结温度为1580℃,造孔剂含量20％;球磨时间为2．5h的条件下,可获得高显气孔率、抗折强度较高的舢203多孔陶瓷。     

Ag/TiO2纳米催化剂的制备及性能

采用溶胶-凝胶法制备了Ag/TiO₂光催化剂。通过X射线衍射(XRD)、场发射扫描电子显微镜(FE-SEM)、N₂吸附-脱附(BET)、透射电子显微镜(TEM)对产物进行了表征。以亚甲基蓝(MB)为降解物,考察了不同Ag含量和不同煅烧温度对样品的光催化性能影响。结果表明,掺杂Ag后,增大了样品的比表面积,800℃时,Ag的引入抑制了TiO₂锐钛矿向金红石相的转变,掺杂后TiO₂的光催化活性大大提高,在500℃煅烧温度下,当Ag的摩尔分数为1%时,在紫外光照射下,经过180min光催化实验,对MB的降解率达到90%.