Kinetics of Hygrothermal Treatment of Capillary Porous Materials

The kinetics of moisture and heat exchange between a capillary porous body and the surrounding single-phase gas–vapor medium is described. The physical aspects of various modes of the process are considered. The importance of taking into account the internal filtration of moisture under the action of the external field of body forces is demonstrated.     

Sintering Kinetics of Porous Ceramics from Natural Diatomite

The sintering kinetics of the porous ceramics from natural diatomite is investigated by means of stepwise isothermal dilatometry (SID) accompanied with XRD, scanning electron microscope (SEM), and porosity measurement. We find that the isothermal shrinkage data from SID could be well analyzed to get kinetic parameters according to the empirical rate equation developed by Makipirtti–Meng , where Y is the fractional volume shrinkage during sintering process and n a dimensionless exponent. The apparent activation energy Δ E values obtained for 850°–1000°C and 1050°–1150°C are 911.8 and 610.5 kJ/mol, respectively. Correspondingly, the exponent n values for the two temperature ranges are also much different and can be served to reveal the morphology changing during the sintering process, which is well consistent with the SEM observation.     

Porous Filtering Ceramics with Engobed Pores

The authors described the results of modernizing ceramic filters made of chamotte-bentonite ceramics through decreasing the pore size of the main carrier by means of introducing engobe into pores in the form of a multiphase solid solution of anorthite and leucite, which after secondary firing constitutes molecular sieves with pores 1.5 – 2.0 m in diameter and ensures effective filtration of suspensions.     

Kinetics of Moisture and Heat Transfer between a Capillary Porous Body and an Aerosol

The kinetics of moisture and heat transfer between a capillary porous body and a gas–vapor–liquid medium with a dispersed liquid phase (aerosol) is described theoretically.     

Precursor-Derived Si-B-C-N Ceramics: Oxidation Kinetics

The oxidation behavior of three precursor-derived ceramics—Si_4.46BC_7.32N_4.40 (AMF2p), Si_2.72BC_4.51N_2.69 (AMF3p), and Si_3.08BC_4.39N_2.28 (T2/1p)—was investigated at 1300° and 1500°C. Scale growth at 1500°C in air can be approximated by a parabolic rate law with rate constants of 0.0599 and 0.0593 μm²/h for AMF3p and T2/1p, respectively. The third material does not oxidize according to a parabolic rate law, but has a similar scale thickness after 100 h. The results show that at least within the experimental times these ceramics develop extremely thin scales, thinner than pure SiC or Si₃N₄.     

Reticulated Porous Foam Ceramics with Different Surface Chemistries

The aim of this study was to discuss the influence of different filter surface chemistries on the properties of foam filters. For reliable results, it is essential to ensure comparable structural properties (cell size and strut thickness) for all different surface chemistries (Al₂O₃, MgAl₂O₄, 3Al₂O₃·2SiO₂, SiO₂, and TiO₂) possess the same structural properties (cell size and strut thickness). Filters made of 100% of the investigated materials and alumina skeletons coated with the investigated materials were prepared. The coated alumina samples were sintered in one and two steps. The processing route with two sintering steps resulted in improved mechanical properties and comparable shrinkage and strut thickness. The 100% bulk foams possessed different pore sizes due to the differences of the material shrinkage. In this study, a comparison of the experimental investigated properties of the ceramic foam filters and the theoretically calculated values for foam materials derived from the bulk material properties is established.     

Hertzian Contact Damage in Porous Alumina Ceramics

A study has been made of Hertzian contact damage in porous and dense liquid-phase-sintered aluminas. Indentation stressstrain curves show increasing nonlinearity as the materials become more porous, illustrating an increasing component of "quasi-plasticity" in the contact damage. Observations of the surface and subsurface damage patterns using a bonded-interface sectioning technique reveal a transition in the Hertzian damage process, from classical tension-driven cone cracks in the high-density material, to distributed shear- and compression-driven subsurface damage and deformation in the porous materials. Strength tests on specimens subjected to cyclic indentations reveal a substantially higher susceptibility to fatigue in the most porous structure.     

多孔低介电氧化硅陶瓷材料的制备

采用氧化硅为原料,木屑作为造孔剂制备了多孔的氧化硅陶瓷材料。借助于气孔率测试、抗弯强度测试、介电性能测试和SEM测试手段分析了造孔剂和烧结助剂的添加量对材料性能的影响。结果表明:加入BN作为添加剂烧成的氧化硅抗弯强度最大可达到14.80MPa。加入木屑作为造孔剂制备的陶瓷可以形成明显的气孔,气孔率最高可达到48.40%,介电常数最低可以达到3.0。     

氮化硅基多孔陶瓷的制备技术、孔隙结构及其相关性能

氮化硅多孔陶瓷是近年来得到广泛关注的一类新型的结构?功能一体化陶瓷材料,在航空航天、机械、化工、海洋工程等重要领域有着广阔的应用前景。本文介绍了氮化硅基多孔陶瓷的主要制备技术,回顾了氮化硅基多孔陶瓷力学性能和介电性能的研究进展。考虑到高孔隙率氮化硅基多孔陶瓷力学性能难以提高,磷酸盐结合氮化硅基多孔陶瓷已经逐渐成为新的研究热点,因此,本文进一步对磷酸盐结合氮化硅基多孔陶瓷的制备技术、力学性能、介电性能、热学性能进行了综合评述,并对氮化硅基多孔陶瓷的应用前景进行了展望。     

Fabrication of Porous Ceramics with Unidirectionally Aligned Continuous Pores

Porous alumina ceramics with unidirectionally aligned continuous pores were fabricated via the slurry coating of fugitive fiber. Cotton thread was coated with ceramic slurry by pulling it through the slurry, and specimens were produced by spooling the coated thread. The obtained porous alumina ceramics had an average pore diameter of 165 μm, 35% open porosity, and a bending strength of 160 MPa. It was suggested that the pore size and the porosity could be adjusted using the diameter of the cotton thread and the solids concentration of the slurry, respectively.     

Modeling of Matter Extraction from a Capillary Porous Particle with a Bidisperse Capillary Structure

A model is suggested for a particle with a bidisperse capillary structure. A numerical analysis of the model demonstrates that, for the liquid extraction of a desired substance from the particle, there exists an optimum range of oscillation frequencies of the liquid in large pores.     

Highly Porous SiOC Bulk Ceramics with Water Vapor Assisted Pyrolysis

Micro/mesoporous SiOC bulk ceramics with the highest surface area and the narrowest pore size distribution were prepared by water‐assisted pyrolysis of polysiloxane in argon atmosphere at controlled temperatures (1100°C–1400°C) followed by etching in hydrofluoric acid (HF) solution. Their pyrolysis behaviors, phase compositions, and microstructures were investigated by DSC, FTIR, XRD, and BET. The Si–O–Si bonds, SiO₂‐rich clusters, and SiO₂ nanocrystals in the pyrolyzed products act as pore‐forming species and could be etched away by HF. Water injection time and pyrolysis temperature have important effects on phase compositions and microstructures of the porous SiOC bulk ceramics, which have a maximum‐specific surface area of 2391.60 m²/g and an average pore size of 2.87 nm. The porous SiOC ceramics consist of free carbon phase, silicon carbide, and silicon oxycarbide.     

Mathematical Model of Removal of a Polymeric Component from a Porous Composite by Capillary Gel Flow

A mathematical model of removal of low-density polyethylene formed in processing synthetic leather base with alkylbenzene vapors and condensate was obtained. The model allows calculating the number of base compression cycles required for exhaustive removal of LDPE as a function of the initial concentration of the polymer in the material and the relative deformation of the material.     

Characterization of Porosity in Porcelain-Bonded Porous Alumina Ceramics

A water removal method was used to determine pore sizes in highly porous large-pore-size specimens. The maximum, minimum, and equivalent pore sizes, as well as complete size distributions (number of pores vs pore sizes), were obtained. The results agree with those obtained using a mercury porosimeter. Characterizing parameters such as permeable porosity, labyrinth factor, and structure factor were estimated from the water removal porosimeter data. Electrolytic resistance was measured to determine the tortuosity and inert porosity. A physical meaning of Archie's law which correlates the formation resistivity factor with porosity is also given. Sonic Young's and shear moduli depended on both the porosity and particle sizes. The Young's modulus was linearly related to the modulus of rupture.     

Capillary Infiltration Rates into Porous Media with Applications to Silcomp Processing

The rate of capillary rise of a liquid into a porous medium made up of consolidated particulates is analyzed. The infiltration distance is parabolic in time and can be modeled using the Washburn analysis. The effective pore radius is measured to be one to two orders of magnitude smaller than the particle size, particle spacing, and the median pore size as measured by mercury porosimetry. This result is interpreted using a modification of the Washburn model which models the porous medium as a single pore with varying diameter. Using a two-sized single pore model, the predicted infiltration rate is consistent with the measured values. In applying the two-sized single pore model to the reactive capillary infiltration of silicon into a carbonaceous preform in the Silcomp process, the effect of pore closure by the conversion of carbon to SiC is predicted. Using pore closure dimensions measured in a partial infiltration experiment, a decrease in the infiltration rate constant is predicted and is consistent with the measured infiltration rate.     

Lumped- Differential Formulations for Drying in Capillary Porous Media

ABSTRACT

Simultaneous heat and mass transfer, during drying of moist capillary porous media, is modeled through different lumped-differential formulations, obtained from spatial integration of the original set of Luikov's equations for temperature and moisture potential. Besides the classical lumped system analysis, improved formulations are proposed, of same mathematicai simplicity, based on approximate evaluations of both temperature and moisture gradients within the medium. The relative merits of such improved zero-dimensional formulations are established, through critical comparisons against analytical solutions for the full partial diflerential system, over a wide range of the governing parameters.     

Lumped- Differential Formulations for Drying in Capillary Porous Media

Simultaneous heat and mass transfer, during drying of moist capillary porous media, is modeled through different lumped-differential formulations, obtained from spatial integration of the original set of Luikov's equations for temperature and moisture potential. Besides the classical lumped system analysis, improved formulations are proposed, of same mathematicai simplicity, based on approximate evaluations of both temperature and moisture gradients within the medium. The relative merits of such improved zero-dimensional formulations are established, through critical comparisons against analytical solutions for the full partial diflerential system, over a wide range of the governing parameters.     

Porous PZT Ceramics with Aligned Pore Channels for Energy Harvesting Applications

In this study, aligned porous lead zirconate titanate (PZT) ceramics with high pyroelectric figures‐of‐merit were successfully manufactured by freeze casting using water‐based suspensions. The introduction of aligned pores was demonstrated to have a strong influence on the resultant porous ceramics, in terms of mechanical, dielectric, and pyroelectric properties. As the level of porosity was increased, the relative permittivity decreased, whereas the Curie temperature and dielectric loss increased. The aligned porous structure exhibited improvement in the compressive strength ranging from 19 to 35 MPa, leading to easier handling, better processability and wider applications for such type of porous material. Both types of pyroelectric harvesting figures‐of‐merit (F_E and F′_E) of the PZT ceramics with a porosity level of 25–45 vol% increased in all porous ceramics, for example, from 11.41 to 12.43 pJ/m³/K² and 1.94 to 6.57 pm³/J, respectively, at 25°C, which were shown to be higher than the dense PZT counterpart.     

Synthesis of Porous Ceramics with Complex Pore Structure by Freeze-Dry Processing

Porous ceramics with complex pore structure were synthesized by a freeze-dry process. Freezing-in of a water-based ceramics slurry was done while controlling the growth direction of the ice. Sublimation marks of the ice were generated by drying under reduced pressure. Porous ceramics having a complex pore structure were obtained by sintering the green body: aligned macroscopic open pores contained micropores in their internal walls. The pore structure was substantially affected by the starting slurry concentration and sintering temperature. The pore formation mechanism is discussed in relation to these effects.     

多孔陶瓷的制备和应用研究进展

由于多孔陶瓷材料在化学工业、信息通讯，生物技术、环境能源等领域的广泛应用，近年来倍受关注。本文综述丁多孔陶瓷的制备方法和应用的最新研究进展。