Effect of cooling rate on the microstructure and porosity of alumina produced by freeze casting

Abstract

Freeze casting is a well-known shaping technique to produce materials with directional porosity. One of the major problems is the difficulty to control the cooling rate thus leading to gradients in pore size and homogeneity. This work deals with the manufacture of alumina ceramics with directional porosity by freeze casting of aqueous suspensions. An experimental set-up was prepared in order to apply different cooling rates. Freeze casting tests were done with an aqueous alumina suspension after optimization of its rheological behavior. The porosity and microstructural features of sintered bodies produced under different experimental conditions were studied and analyzed. It is concluded that the cooling rate influences the microstructure while final temperature has a much lower influence. Also, the microstructural analysis shows that there is a gradient in the directionality of pores, being lower at the bottom and the top and higher in the central region of the specimens

A homemade freezing unit for ice-templating

“…The final structure depends on the nature of the solvent, on the particle distribution in the slurry and on the liquid-solid interaction. This means that the structure is strongly influenced by the freezing conditions, highlighting the importance of controlling the cooling rate, time and temperature Rodríguez-Parra et al, 2012;Waschkies et al, 2011). Some authors describe the process they used to produce porous materials and provide simple schemes of the freezing system as well as the chosen parameters (Araki & Halloran, 2005c;Deville et al, 2007;Du et al, 2018;Hong et al, 2011;Huang et al, 2017;Waschkies et al, 2011;Wegst et al, 2010;Xie et al, 2013).…”

“…An important component of the freezing unit is the temperature sensor, although some authors do not report its use (Araki & Halloran, 2005c;Choi et al, 2012;Dong et al, 2016;Du et al, 2018;Fukasawa et al, 2001). It can be a thermocouple, a thermostat or a resistance temperature detector (RTD) (Deville et al, 2007;Huang et al, 2017;Rodríguez-Parra et al, 2012;Waschkies et al, 2011;Wegst et al, 2010;Xie et al, 2013). An RTD was used in this project because, although it is more expensive than the other options, because it is more accurate and works over a wider temperature range.…”

“…Although some authors report manual control of the heating element (Rodríguez-Parra et al, 2012;Xie et al, 2013), it would be hard to operate the system described in this work without an automatic controller to monitor and adjust freezing parameters such as cooling rate, time and temperature (Deville et al, 2007;Waschkies et al, 2011;Wegst et al, 2010). The controller used in this project was the COEL® KM5P (Coelamtic 2016).…”

Influence of the processing variables on the microstructure and properties of gelatin‐based scaffolds by freeze‐drying

“…Regarding the modification of the processing variables, the different parameters to be considered in this process are: the container used during the freeze‐drying process, the freezing temperature, the pH value of the solution, and the solvent used. Among these, freezing temperature is the most studied variable, obtaining better properties for those scaffolds fabricated at a lower temperature . With respect to the mold where the solution is added to remove the solvent, the use of a hydrophobic container has been reported to favor the elimination of aqueous solvents, improving the process…”

Carbon Nanotube Modified Sepiolite Porous Ceramics for High-efficient Oil/Water Separation

“…Key words: sepiolite porous ceramics; freeze drying; carbon nanotubes; superhydrophobic/superlipophilic; oil/water separation 近年来, 频发的石油泄漏以及工业和生活含油 废水的随意排放对人类生态系统和健康产生巨大威 胁。对油污及有机污染物的收集和处理引起了研究 人员的广泛关注 [1] 。 基于仿生学原理, 具有荷叶结构 的自清洁表面、蚊子眼睛结构的防雾涂层以及甲虫 翅膀结构的超疏水表面等被成功制备, 并在油水分 离领域显示出较大的应用潜力 [2] 。 其中, 三维多孔材 料由于其高孔隙率及大比表面积, 具有远高于一维 与二维材料的水油分离效率。 但它们通常是由海绵 [3] 、 石墨烯 [4][5] 和碳纳米管 [6][7] 等组装而成, 存在机械强 度低及耐酸碱腐蚀性差的问题, 在一定程度上限制 了其在极端环境下的应用。因此, 开发和制备具有 高油水分离效率、较高的力学强度及良好化学稳定 性的三维多孔材料具有重要意义。 多孔陶瓷具有显气孔隙率高、比表面积大、耐 磨性高及化学稳定性好等特点 [8][9] , 是理想的过滤 吸附材料 [10] 。其制备方法主要有: 浸渍法 [11] 、添加 造孔剂法 [12] 、发泡法 [13] 及冷冻干燥法 [14] 等。其中冷 冻干燥法具有操作简单、适用范围广、环保可靠等 优点, 并且能够制备具有特殊定向孔道结构的材料, 已被广泛用于制备氧化铝 [15] 、硼化锆-碳化硅 [16] 、 氮化硅 [17] 及氧化钛 [18] 等多孔陶瓷材料。但传统的多 孔陶瓷往往既亲水又亲油, 因此, 需对其进行表面 改性, 使其疏水亲油、具有良好的油水分离性能。 本课题组前期的研究结果表明 [19] , 经石墨烯/ 碳纳米带改性后, 亲油疏水的硅藻土多孔陶瓷具有 良好的的油水分离能力和较高的力学强度。但是该 工艺需要先采用发泡法制备硅藻土多孔陶瓷, 再用 石墨烯进行表面改性, 最后用催化裂解法在多孔陶 瓷表面原位化学沉积碳纳米带, 制备工艺复杂; 同 时, 所制备样品的油水分离能力还有待进一步优化。 纤维状海泡石具有成本低、来源广、储量大、 安全环保、比表面积大及吸附性能良好等特点, 在 吸附和净化领域应用广泛 [20][21][22][23] 。海泡石优良的吸附 性能取决于其独特的结构: 1) 硅氧四面体和镁氧八 面体形成的层链状结构使其具有了巨大的理论比表 面积; 2) 纤维状海泡石内部分布着分子尺寸的微孔 通道, 这些贯通的通道赋予其较大的理论孔容积。 海泡石的这些特性是其在物理吸附方面得以广泛应 用的关键 [24][25][26] [27][28] , 最终制得 CNTs 改性海泡石多孔陶瓷。 Modified silica powders Motor oil 1, 7-octadiene 0.330 [30] Diatomite/silicalite-Ⅰcomposite powders Benzene Silicalite-Ⅰ 0.095-0.246 [31] Modified diatomite porous ceramics Toluene Graphene/carbon nanobelts 1.090 [19] Modified diatomite porous ceramics Vacuum pump oil Graphene/carbon nanobelts 1.025 [19] Boron nitride aerogel Salad oil -5 [32] Modified alumina porous ceramics Hexane Polydimethylsiloxane 2 [33] Modified rock wool Diesel oil Polydim...…”