Optimal design on the mechanical and thermal properties of porous alumina ceramics based on fractal dimension analysis

In order to investigate the relationship between pore structure and thermal conductivity as well as mechanical strength, porous alumina ceramics (PAC) with various pore structures were fabricated, using starch as the pore‐forming agent. Fractal theory was employed to characterize the pore size distribution more accurately than ever used parameters. The results show that the increase in starch content in PAC leads to an enhanced porosity, a higher mean pore size, and reduced fracture dimension, thermal conductivity and strength. The fractal analysis indicated that the fractal dimension decreases gradually and reaches its minimum value with increasing the starch content up to 25 wt%, but the further incorporation results in an opposite trend. It is suggested from micro‐pore fractographic analysis that the optimization of both thermal insulation performance and mechanical strength are positively correlated with the increase in the mean pore size and proportion of 2‐14 μm pores but negatively corrected with the porosity. These results provide a new perspective and a deeper understanding for fabrication of PAC with both excellent thermal insulation and mechanical performance.     

Preparation and characterization of porous alumina ceramics through starch consolidation casting technique

This work aims at studying the influence of thermal treatment on the microstructure, resistivity and technological properties of porous alumina ceramics prepared via starch consolidation casting (SCC) technique. Colloidal suspensions were prepared with three different contents of alumina solid loading (55, 60 and 65 mass%) and corn starch (3, 8 and 13 mass%). The sintered samples at 1400, 1500, 1600 and 1700 °C, show open porosity between 46 and 64%, depending on the starch content in the precursor suspensions and sintering temperature. The pore structures were analyzed by SEM. The effect of corn starch content on the apparent porosity, pore size distribution, linear shrinkage and electrical resistivity as well as cold crushing strength of the sintered porous alumina ceramics was also measured. These porous alumina ceramics are promising porous ceramic materials for using in a wide range of thermal, electrical and bioceramics applications as well as filters/membranes and gas burners, due to their excellent combination properties.     

Effect of starch on pore structure and thermal conductivity of diatomite-based porous ceramics

Considering the low-cost and environmental protection, the porous ceramics with high porosity using natural diatomite powder were successfully prepared by utilizing hot injection moulding and sacrificial fugitives. The impacts of different content of starch as a pore-forming agent on the phase composition, mechanical properties, thermal conductivity, and micro-structure of porous ceramics were investigated. The results demonstrate that starch content can significantly affect the mechanical properties and thermal conductivity of diatomite-based porous ceramics. When the starch content increased from 0 wt % to 50 wt %, the porosity increased from 61.2% to 80%, while the thermal conductivity decreases from 0.239 W/(m K) to 0.098 W/(m K). The low thermal conductivity of porous ceramics may be related to the macroporous–mesoporous composite structure. With the starch content increased, a greater chance of starch granule contact, higher internal pore sizes and a wider pore size distribution in the prepared samples, which resulting in lower mechanical strength, such as the three-point bending strength from 2.83 MPa to 0.46 MPa.     

Porous cordierite-based ceramics processed by starch consolidation casting – Microstructure and high-temperature mechanical behavior

Porous cordierite-based ceramics with different microstructural features and mechanical behavior were formed by starch consolidation casting (SCC) using native potato and corn starches and sintered at 1275, 1300 and 1330 °C. The composition and microstructure of the ceramic materials were investigated via quantitative phase analysis using X-ray diffraction (with Rietveld refinement), the Archimedes method, mercury porosimetry, scanning electron microscopy and optical microscopy with stereology-based image analysis. The mechanical behavior of samples was evaluated by diametral compression tests at room temperature, 1000 and 1100 °C. The type of starch used and the sintering temperatures were the main factors determining the characteristics of the developed porous microstructures. Materials prepared with corn starch achieved the lowest porosity and the lowest values of mean chord length, mean pore distance and pore throat size. Because of these features, these materials thus presented, in general, higher values of apparent Young's modulus, elastic limit and mechanical strength than those prepared with potato starch. Despite the presence of a silicate glassy phase, both porous materials, mainly those prepared with corn starch, still enhanced the basic mechanical properties at high temperature, in particular, the mechanical strength and the apparent Young's modulus due to the special combination of the porous microstructure features.     

Porosity measurements in suspension plasma sprayed YSZ coatings using NMR cryoporometry and X-ray microscopy

A large variety of coatings are used to protect structural engineering materials from corrosion, wear, and erosion, and to provide thermal insulation. In this work, yttria-stabilized zirconia coatings produced by suspension plasma spraying were investigated with respect to their microstructure and especially their porosity, as the porosity affects the thermal insulation of the underlying component. To determine porosity, pore size distribution, and pore shape, the coatings were investigated using novel advanced characterization techniques like NMR cryoporometry and X-ray microscopy. In general, the porosity is inhomogeneously distributed and the coatings showed a large variety of pore sizes ranging from a few nanometers to micrometers.     

Influence of pore distribution on the equivalent thermal conductivity of low porosity ceramic closed-cell foams

The microstructures of porous alumina materials with different porosities were established by introducing the departure factor of pore position and acentric factor of pore diameter to describe the distribution of pores in space and in size, respectively. The contribution of radiation and influence of pore distribution on the equivalent thermal conductivity were discussed based on numerical simulations by the finite volume method (FVM) considering both thermal conduction and radiation. When the pore diameter was less than 10?µm, the radiation component was less than 2%, and radiation could be neglected. Radiative heat transfer played a dominant role for materials with high porosity and large pore size at high temperatures. For micro pore materials (<?100?µm), broad pore size and non-uniform pore space distribution decreased the thermal conductivity across the entire temperature range. For materials with macro pores (>1?mm), broad pore distribution decreased the thermal conductivity at low temperatures and increased it at high temperatures. The basic prediction model of effective thermal conductivity for a two-component material, the Maxwell–Eucken model (ME1) and its modified model were corrected by introducing the pore structure factor. The results from experiments prove that the numerical values were satisfactory.     

Correlation of structure and thermal conductivity of highly disperse porous-fiber materials under variations of temperature and moisture

The effect of the structure (pore type, pore size, fiber diameter) of highly disperse proous materials on their thermal conductivity is considered.     

淀粉基多孔碳材料的制备及其吸附CO2/CH4性能

以淀粉为原料,使用水热法将其碳化后用活化剂KOH对其活化,制备了淀粉基多孔碳材料,并对其进行结构表征和CO₂/CH₄的吸附性能测试,计算吸附热以及材料对CO₂/CH₄的吸附选择性,讨论了碳材料结构对其吸附性能的影响。结果表明：在制备过程中,随着活化剂KOH用量比例的增大,所制得的材料其比表面积和孔容增大,其孔径分布也就越宽。所制得的碳材料其比表面积可达2972 m²·g^-1。这些淀粉基多孔碳材料对水蒸气的吸附等温线呈现出Ⅳ类等温线。所制备材料对CO₂吸附容量主要取决于其孔径小于0.8 nm的累积孔容（Vd < 0.8 nm）。材料的超微孔的孔容越大,其对CO₂吸附容量也越大。所制备的C-KOH-1材料在101325 Pa和298 K条件下,对CO₂的吸附量达到4.2 mmol·g^-1,其对CO₂的吸附热明显高于其对CH₄吸附热,其对CO₂/CH₄吸附选择性为3.7～4.26,同时本文通过对材料的水蒸气吸附等温线进行测试,结果表明所得材料主要表现为中等憎水性,这对材料在实际工况的应用奠定了基础。     

Effect of various pore formers on the microstructural development of tape-cast porous ceramics

Various types of pore formers have been used for the fabrication of ceramics with controlled porosity. This study addresses a detailed and systematic comparison of different pore formers (e.g. graphite, polymethyl methacrylate, sucrose and polystyrene) with distinct features such as size, distribution and morphology of particles and decomposition/oxidation behavior. Investigations also involve their effect on the rheological properties of the slurries and the microstructural development of laminated porous ceramic tapes.Morphological features of the pore former particles were characterized using laser diffraction, B.E.T. surface area measurement and scanning electron microscopy (SEM) techniques as their thermal decomposition/oxidation behavior were determined by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) methods. Tape compositions were developed and optimized in order to incorporate identical volumetric loadings of the materials in the tape formulations with different pore formers for a reliable comparison of their pore forming characteristics. Porous yttria stabilized zirconia (YSZ) ceramics were fabricated without macroscopic defects (e.g. cracks, warpage and delamination) by developing heating profiles based on the identified thermal properties of the pore formers. Characterization of the sintered porous ceramics by SEM and mercury intrusion porosimetry techniques revealed novel relationships between the physical properties of the utilized pore formers, processing parameters and final pore structures.     

降低蜂窝状堇青石陶瓷载体热膨胀的途径

详细叙述了降低蜂窝状堇青石陶瓷载体热膨胀的各种途径。主要包括设计适宜的化学组成 ,控制各种原料的种类、杂质含量、形貌、粒度以及对其进行必要的预处理 ,选用合适的添加剂 ,选择最佳烧成工艺 ,调节气孔率、孔径分布和烧成后陶瓷体的酸处理。     

Porosity and pore size distribution influence on thermal conductivity of yttria-stabilized zirconia: Experimental findings and model predictions

Porous yittria-stabilized zirconia is an important advanced ceramic material for technological applications. One of the most important characteristics of this material is low thermal conductivity, which is greatly influenced by the presence of pores into the microstructure. In fact, air trapped in the pores represents a better thermal insulator. The role of the pore volume fraction on porous material characteristics has been extensively studied. On the other hand, the influence of the structure disorder, the pore size range and pore size distribution have been studied much less. In this study, an intermingled fractal model capable of relating thermal properties of ceramic materials and their pore microstructure has been proposed. Model predictions are found confirming the experimental data fairly well, even better than the others models available in the literature.     

Microwave processing of starch‐based porous structures for tissue engineering scaffolds

A novel microwave (MW) processing technique was used to produce biodegradable scaffolds for tissue engineering from different types of starch‐based polymers. Potato, sweet potato, corn starch, and nonisolated amaranth and quinoa starch were used to produce porous structures. Water and glycerol were used as plasticizers for the different types of starch. Characterization of the pore morphology of the scaffolds was carried out with scanning electron microscopy. Three‐dimensional structures with variable porosity and pore size distribution were obtained with the MW foaming technique. The amount of remaining water in the scaffolds and their corresponding densities showed important variations among the different types of starch. Compressive mechanical properties were assessed by indentation tests, and a strong dependence of the indentation stress on the average pore size was found. Studies in simulated body fluid were used to assess the in vitro bioactivity, degradability, and surface topology evolution in the scaffolds. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1332–1339, 2007     

Thermal decomposition behavior of 3D printing filaments made of wood-filled polylactic acid/starch blend

Dynamic thermogravimetric analysis under nitrogen environment was used to understand the thermal decomposition process of 3D printing filaments made of wood-filled polylactic acid (PLA)/starch blend. The characteristic temperatures and apparent activation energy (AAE) of the filaments with various starch contents were calculated with well-known kinetic models by Friedman, Flynn–Wall–Ozawa, Coats–Redfern, and Kissinger. With the increased starch content in the filament, the onset thermal decomposition temperatures of the filaments decreased gradually from 272.4 to 155.1°C. The thermal degradation degree became smaller, and the transitional temperature interval became larger with increased starch proportion. The AAE values of the three types of filaments with different starch ratios varied between 97 and 114 kJ/mol, depending on material composition and method of calculation. The four kinetics methods provide complementary techniques for analyzing thermal stability behavior of composite materials. The improved understanding of thermal decomposition behavior of PLA-starch-wood composites can help develop PLA/starch-based filaments for 3D printing.     

Enhancing thermal insulation and mechanical strength of porous ceramic through size-graded MA Hollow Spheres

In this study, a series of porous ceramics were prepared using different ratios of small and large size MA hollow ceramic spheres as pore-forming agents, and their thermal insulation properties were investigated. The results showed that increasing the proportion of small size hollow ceramic spheres could effectively decrease the thermal conductivity and improve the compressive strength of the porous ceramics. The optimal porous ceramic was prepared with a ratio of 10∼50 of small and large size hollow ceramic spheres, which had a thermal conductivity of 0.368 W/(m·K) at 800 °C and a compressive strength of 22.43 MPa. Microscopic analysis indicated that the enhanced thermal insulation and mechanical properties were due to the improved pore structure and the enhanced bonding strength between the ceramic spheres and the matrix. The findings provide valuable insights for the development of high-performance thermal insulation materials.     

水醇淀粉凝胶的形成及多孔淀粉的制备

以木薯淀粉为原料,水和乙醇为混合介质,利用溶胶-凝胶法及超临界CO₂干燥制备多孔淀粉,分别考察了水醇比、糊化时间、固含量、冷藏时间、乙醇置换用量、置换时间等对凝胶体积收缩率和质量损失率的影响,通过扫描电子显微镜、比表面积及孔径分布测试仪、X射线衍射仪表征了多孔淀粉形貌及孔结构,并考察了多孔淀粉的吸油性能。结果发现,水醇比为19∶1,固含量为13%,糊化时间为30min,冷藏时间为5天,醇置换量为5mL/g,置换时间为30min,水醇凝胶的体积收缩程度和质量损失相对较小,多孔淀粉的比表面积为122m²/g,平均孔径为25.6nm,大豆油的吸附率可达457%（质量比）。研究结果表明,适当调整固含量,能改善凝胶的三维网孔密度,增大多孔淀粉的比表面积及孔容;水醇淀粉凝胶中的淀粉分子能形成一定结晶结构的网络骨架,利于维持水醇凝胶的骨架稳定性。     

Thermally bonded nonwoven filters composed of bicomponent polypropylene/polyester fiber. I. Statistical approach for minimizing the pore size

A statistical approach involving the uniform design of experiments and regression analysis is used to investigate the effects of thermal bonding process parameters, dwell time, thermal bonding temperature, and hot air velocity, on the pore size of three‐dimensional (3D) nonwoven filters. These filters are produced from polypropylene (PP)/polyester (PET) (sheath/core) bicomponent staple fibers. The pore structures of the filter samples were examined using the capillary flow porometer. Results reveal that the statistical approach is effective in identifying the effects of the investigated process parameters on both the bubble point pore diameter and the mean flow pore diameter for the thermally bonded nonwoven filter samples. Under the optimized processing parameters for achieving the minimized pore size, the predicted minimum bubble point pore diameter is 111.12 μm and the predicted minimum mean flow pore diameter is 63.4 μm for the filter sample with the area density of 60 g/m². They are in good agreement with the experimental values of 111.71 and 60.91 μm, respectively. Microstructure features observed using scanning electron microscope indicate that the effects of the investigated process parameters on the pore size are closely related to the thermal energy delivered to the fibers and the pressure drop acting on the fabrics. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2689–2699, 2006     

Preparation of porous Al2O3 ceramics by starch consolidation casting method

Porous alumina ceramics were fabricated by starch consolidation casting using corn starch as a curing agent while their microstructure, mechanical properties, pore size distribution, and corrosion resistance were examined. Results showed that the porous alumina ceramics with the flexural strength of about 44.31MPa, apparent porosity of about 47.67% and pore size distribution in the range of 1‐4 μm could be obtained with 3wt% SiO₂ and 3wt% MgO additives. Corrosion resistance results showed mass losses: hot H₂SO₄ solution and NaOH solution for 10 hours were 0.77% and 2.19%, which showed that these porous alumina ceramics may offer better corrosion resistance in acidic conditions.     

Fabrication and characterisation of cellular alumina articles produced via radiation curable dispersions

A general and versatile method for the production of cellular materials from radiation curable solvent-free colloidal ceramic dispersions containing pore formers has been developed. By this technique cellular ceramic articles with a precisely controlled porosity, cell size and shape are obtained for compositions containing solid pore formers. Monolithic bulk samples are obtained by thermal curing, whereas thin films and multi-layered articles are advantageously produced by UV curing. In this work the influence of three different spherical pore former types, PE, PS and PMMA, on the processing and final properties of the porous materials using alumina as model material is studied. The effect of pore former type and concentration on rheology, curing behaviour, debinding and sintering steps as well as thermal conductivity and mechanical strength of the sintered cellular materials is presented. It is also shown that the choice of pore former type modifies the sintering behaviour and resulting properties.     

Heat transfer in high porous alumina: Experimental data interpretation by different modelling approaches

Advanced porous ceramics are a remarkable class of materials with important applications in engineering fields. Porosity features have received wide attention for their capability to influence all properties. In this paper, the correlation between pore structure and heat transfer has been studied. Different analytical procedures found in literature as well as an Intermingled Fractal Units’ model are proposed. Models predictions are compared with experimental data. It has been observed that IFU is particularly suitable to predict thermal conductivity values very close to experimental ones. This fact is related to its capability to replicate porous microstructures in terms of pore volume fraction, pore size range and pore size distribution.     

Porous anorthite ceramics with ultra-low thermal conductivity

Porous anorthite ceramics with an ultra-low thermal conductivity of 0.018 W/m K have been fabricated by hydrous foam-gelcasting process and pressureless sintering method using γ-alumina, calcium carbonate and silica powders as raw materials. Microstructure and phase composition were analyzed by SEM and XRD respectively. Properties such as porosity, pore size distribution and thermal conductivity were measured. High porosity (69–91%) and low thermal conductivity (0.018–0.13 W/m K) were obtained after sintering samples with different catalyst additions at 1300–1450 °C. Porosity, pore size, pore structure and grain size had obvious effect on heat conduction, resulting in the low thermal conductivity. The experimental thermal conductivity data of porous anorthite ceramics were found to be fit well with the computed values derived from a universal model.