摘要
在合成材料中引入梯度孔结构，可以制备出许多新用途的功能材料。由于孔梯度泡沫陶瓷的气孔率和孔尺寸连续梯度变化，与一般泡沫陶瓷材料相比其过滤效率较高，并且在骨组织材料，热障材料及医学移植等方面都有着潜在的应用价值。然而目前制备完全连通的孔梯度泡沫陶瓷鲜见报道，制备具有可控的连续变化通孔梯度结构仍然是一个巨大挑战。
本文通过研究多孔陶瓷和泡沫陶瓷的制备工艺及其特点，提出了一种新的孔梯度成型方法——凝胶注模工艺和聚合物球结合起来，制备可控孔梯度的泡沫陶瓷。本工艺基于有机单体原位聚合，制备的坯体结构均一、净尺寸成型，可以满足机加工要求，而且还适用于多种不同的材料组分和体系。实验采用Al2O3材质，通过合理选择聚合物球尺寸和数量，可以进行梯度多孔陶瓷的孔连通性和梯度层高度的设计。主要的研究结果如下：
(1)剪切速率、分散剂、固含量、球磨时间及pH值对浆料粘度的影响研究表明，剪切速率较低时，不同固含量浆料的粘度都比较大。随着剪切速率增大到60s-1后，粘度显著降低且趋于平缓。随着浆料固含量的增加，粘度逐渐增加，其中60Vol.%的浆料粘度增加最快。实验综合得出球磨8h、pH为9、分散剂为0.5～0.7%及固含量为55Vol.%的浆料具有较好分散效果，粘度低且流动性好，成型的坯体宏观结构和微观性能都表现良好。
(2)采用了不同方法对EPS表面进行改性，结果表明三乙醇胺对聚合物球的改性效果较差，氧等离子体和UV的改性效果较好。注浆成型时，浆料对未改性的球体表面润湿性较差，成型坯体中存在微孔等缺陷。而改性后球与浆料的相容性较好，没有微孔等缺陷。从球在模板中的受力变形模拟图及实验得知，要提高孔梯度陶瓷的气孔率，既需要对模板施加外力来减小模板间的孔隙，又要防止模板过分变形造成的泥浆渗透性降低及成型后模板的弹性后效造成坯体变形开裂。
(3)研究分析了模板热熔法和溶出法对坯体和烧结体的影响。试验表明，直接烧结去除模板时，由于加热时球之间相互粘结收缩对骨架产生了拉力，造成骨架中细丝的断裂和裂纹的产生。而在二氯甲烷溶剂中预先移排模板，烧结后孔结构和形貌较完整。
(4)对于烧结温度相同不同梯度层结构来说，随着梯度层数的增加，梯度层间结构的交错较多，抗压强度增加。当梯度层数相同时，随着球直径增大，骨架的厚度增粗，坯体的抗压强度也略有增加。用球径为2.1mm,2.6mm和3.2mm制备出了具有分级的孔结构的泡沫陶瓷在1500℃烧结时，孔隙率和抗压强度分别是70.5%和3.16Mpa。
The incorporation of gradient porous structure in synthetic materials may lead to thecreation of new functional materials for many innovative applications. Due to thecontinuous change of gradient in porosity and pore size, filter efficiency and highselectivity of pore-gradient ceramic foams (PGCF) are higher than that of traditionalceramic foams. PGCF offer an interesting potential for being used as bone tissuematerials, thermal barrier materials and medical implants. However, few techniqueshave been reported on the fabrication of PGCF with fully interconnected open cell.Creation of a continuously varying and spatially controlled gradient in porosity withinterconnected open pores is still a challenge for these processing steps.
The forming process and their characteristic of porous ceramics and pore-gradientceramics are generalized in the paper. A novel approach was introduced to preparepore-gradient Al2O3ceramic foams with association of gelcasting process andpolymeric sphere template. Gelcasting process, based on the in situ polymerization oforganic monomers, shows the ability to fabricate homogeneous, near-net-shape andcomplex-shaped bodies with high strength required for machining. The approach can beadapted for many different compositions and systems, and allow the design of poreconnectivity and gradient height of ceramic foams by the appropriate selection of spheresizes and numbers. The limitation of ceramic foams fabricated by polymeric spongeprocess in sponge carrier can be resolved by this approach. The main results of studiesare as follows:
(1) Influence of shear rate, dispersant, solid content, ball milling time and pH valueon the viscosity of Al2O3slurry was studied. Results show that viscosity of different solid content slurries is comparatively high at low shear rates. The viscosity decreasesdramatically and hardly changes after60s-1of shear rate. The viscosity decreases withthe increase of dispersant and slightly enhances with excessive dispersant.0.5-0.7%dispersant can effectively decreases viscosity of slurries. The viscosity increases withthe increase of solid content and enhances quickly at60Vol.%solid content. Slurryparticles can disperse well and accordingly viscosity is very low when ball milling timeis8h and pH value is9. Samples with55Vol.%solid content slurry maintaining goodfluidity perform well in both macro-structure and micro-performance.
(2) Studies of different modification methods on EPS surfaces show thatmodification by triethylolamine is ineffective and modification by oxygen plasma andVacuum Ultraviolet is effective. Compared with treated sphere surface, wettability ofslurry for untreated surface is not good and micro-pores were found in gelcasting bodies.From the stress-strain simulation pictures of spheres in the template and experimentalresults we know, in order to enhance the porosity of PGCF, it is needed not only toenhance the applied force on template for reducing pores in the template, but to avoidthe reducing of slurry permeability due to excessive deformation of template, and thecrack of bodies linked to flexibility of formed template.
(3) The influence of pre-removal of EPS template on the integrity of networks wasinvestigated. Some defects such as edge-delamination and micropores disappeared bysurface modification. The dendritic solidification structure, flawless filamentary of thenetwork and spherical-shaped walls perfectly maintained through template pre-removedwith organic solvent. This method was better than direct pre-removal of template byheating, because closely ordered spheres bonded together by adhesive force, which willcause strain to struts of sample and result in rupture of solid struts and cracks ofreticulation points.
(4) As for the same sintering temperature and different gradient layers, compressivestrength enhanced along with the increase of numbers of gradient layers and intersectionof gradient structure. When gradient layer is same and diameter of spheres increases,compressive strength enhanced slightly due to the increase of strut thickness. The hierarchical pore structure was fabricated with EPS spheres of2.1mm,2.6mm and3.2mm diameters. Porosity and compressive strength of the pore-gradient Al2O3ceramicfoam were68.5%and3.06MPa at1500℃, respectively.
引文
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