【作者】 苏鹏；

【导师】 郭学益；

【作者基本信息】 中南大学 ， 材料冶金， 2009， 博士

【摘要】 泡沫陶瓷特有的三维开放孔隙结构,与陶瓷自身的优良属性相结合,使其在熔融金属过滤方面表现出特殊的优越性。采用有机泡沫浸渍法生产铝熔体过滤用泡沫陶瓷过滤器(Ceramic Foam Filter,以下简称CFF),具有工艺简单、操作方便、成本低的优势,发展前景广阔。铝用CFF目前存在的主要问题是制品的热强度及抗热冲击性能差,使用寿命短。这些均制约着铝合金产品性能的提升,同时也是CFF研究的热点。本文从干压实体陶瓷成瓷机理研究入手,通过优化烧结制度、调整粘结剂比例,以及添加混合稀土、滑石、蓝晶石、工业氧化铝、TiO2等手段,大幅提高烧结体强度。在优化挂浆性能的基础上,采用有机泡沫浸渍法进行泡沫陶瓷的实验室制备,并对其力学性能、抗热震稳定性、孔径分布、比表面积等进行表征。在此基础上,结合现场生产条件,制备出新型铝熔体过滤用CFF,并通过A356铝合金现场熔铸过滤试验,从合金成分、金相组织、抗拉强度、延伸率等方面对新型CFF进行了效能评价。对以刚玉、硅微粉、钾长石、高岭土等为原料的原始配方干压实体陶瓷的力学性能及成瓷机理的研究表明：基体内缺少活性氧化铝、α-方石英生成过多、二次针状莫来石生成量少等,是烧结体强度偏低的主要原因。研究了磷酸添加量对烧结体力学性能的影响,确定了最佳的磷酸加入量。当磷酸与去离子水的质量比达到20／65时,烧结体的抗压与抗弯强度比原始配方分别提高50.5%与55.4%,这是因为合适的磷酸加入,使基体内活性Al2O3含量相对增加,促进了莫来石转化,并使基体内形成牢固的聚合磷酸铝陶瓷网络。优化的烧结制度对实体陶瓷力学性能有重要影响。其中1350℃优化条件下的烧结体抗压及抗弯强度,比原始烧结体分别提高6.8%及13.1%。而1400℃优化条件下的烧结体抗压及抗弯强度,比原始烧结体分别提高44.3%及52.8%。系统研究了多种添加剂对实体陶瓷力学性能的比较,在磷酸强化配方(A)的基础上,确定了三种添加剂优化配方,即6%生滑石+4%蓝晶石配方(B)、3%混合稀土配方(C)及TiO2+工业Al2O3高能球磨配方(D)。·添加6%生滑石的实体陶瓷抗压及抗弯强度比磷酸强化配方分别提高108.9%和33%；在此基础上添加4wt%的蓝晶石球磨粉,烧结体抗压及抗弯强度进一步提高20.1%及29.1%。滑石的强化效应主要源于其酸溶反应及相变产生的大量非晶态物质成为促进石英及莫来石转化的活化中心,以及铝镁尖晶石的细晶强化作用。而蓝晶石的强化与补缩作用主要源于其高温下的不可逆膨胀及莫来石转化。·添加3%的混合稀土可使实体陶瓷的抗弯及抗压强度比磷酸强化配方分别提高23.3%和25.4%；其强化机理在于：混合稀土中的CeO2高温变价产生的晶格缺陷促进了液相的生成、聚集,及莫来石的转化,并产生细晶强化作用。·加入TiO2+工业Al2O3高能球磨粉的烧结体,其抗压及抗弯强度分别比磷酸强化配方提高80.2%及85.3%。其强化作用源于球磨产生的高活性Al2O3、TiO2及其介稳相对合成莫来石-钛酸铝固溶体的促进作用、以及莫来石、Fe2O3等对钛酸铝高温分解的抑制作用。浆料吸光度与沉降度研究表明：在相同固液比下,上述四种优选配方分散性能排序为：D-A-C-B。生滑石因其固有的疏水作用能及表面气膜,使浆料分散性变差；蓝晶石表面铝离子的优先酸解,可在一定程度上减弱滑石的不利影响；CeO2表面离子极化弱,并造成Al2O3双电层压缩,使浆料分散性变差；而TiO2+工业Al2O3高能球磨增加了浆料固-液界面的相容性,有利于浆料分散。浆料的流变曲线分析表明：当固液比为1：0.4时,四种浆料流变曲线均表现为一定的切稀及应力过冲行为,其触变性的改善程度排序为：B-C-A-D,这与不同添加剂对浆料内“卡片结构”的影响有关。通过水解和表面改性,确定了聚氨酯泡沫最佳挂浆条件,将聚氨酯泡沫浸泡于15%NaOH溶液中,在60℃条件下水解40min,可有效去除其孔筋间膜,增加孔筋表面粗糙度。采用羧甲基纤维素钠(CMC)、十二烷基硫酸钠(SDS)、硅溶胶、自制铝溶胶等改善聚氨酯泡沫挂浆量,效果最好的是自制铝溶胶。通过调整对辊间距,确定当挤压比为5：1时,浆料涂覆效果最优。在上述研究的基础上,采用有机泡沫浸渍法进行了上述四种配方泡沫陶瓷的实验室制备及性能表征。结果表明：B、C、D配方烧结体的常温抗压强度比A配方分别提高64.4%、28.8%、48.1%；抗弯强度分别提高24.9%、15%、72.6%。抗热震破坏循环次数最多的是配方D,其次为C、B、A。而经一次热震循环后抗弯强度的下降幅度最小的是配方B,其次为配方C、D、A。采用压汞法测定泡沫陶瓷表面孔径分布和比表面积,发现在低压区,配方A烧结体的表面孔径分布最广,函数峰值最高、其次为配方C、B、D,这主要与基体内液相量及颗粒重排有关。而高压区内纳米孔径的存在,则与烧结过程中气体排出、相变收缩、微裂纹、莫来石生成等有关。而经冷冻干燥的烧结体的孔径分布范围、函数峰值及比表面积比均大幅增加。采用实验室配方A、B、C进行了新型铝用CFF的中试制备,结果表明：新型CFF通孔率高、外观整洁、无掉渣、裂纹等缺陷。A、B、C三种过滤器的常温抗压强度分别比原始配方提高17.6%、104%、64.9%；抗弯强度分别提高20.8%、163.6%、74%。采用新型CFF进行了A356铝合金熔体现场过滤试验。结果表明：新型CFF高温性能稳定,对铝合金熔体中小于10μm细微夹杂的去除效率达到50%以上。同时对熔体中的氢具有一定的去除作用。A、B、C新型CFF,过滤后合金试样的抗拉强度比过滤前分别提高18.9%、13.1%、12.9%,延伸率分别比过滤前提高4%、11.4%、8.3%。更多还原

【Abstract】 Foam ceramics, with particular structure of 3D open porosity and excellent texture properties, have showed great superiority in the field of molten metal filtration. Organic sponge impregnation technology, due to its simple process and low cost, is an economical and practical method for aluminum ceramic foam filters(CFF). Presently, the main technical problems for aluminum CFF lie in its low thermal strength, poor thermal shock resistance and long-term stability. All these restrain the elevation for the aluminium alloy properties, and therefore having become the hotspots of CFF research.Laboratory studies of this paper started from the porcelain-forming mechanism investigation for the dry-pressed concrete ceramic. By means of binder ratio optimizing, sintering enhancement, and the addition of mixed rare earth, talc, kyanite, industrial alumina and TiO2, ceramic matrixs with high-strength have been obtained. Based on slurry coating optimization, foam ceramics were prepared in lab using organic foam impregnation technique, which is followed by characterization of mechanical properties, thermal shock stabilities, pore size distributions, specific surface areas, etc. Then, according to the optimized formulas and slurry coating techniques, pilot studies were carried out firstly to prepare new types of aluminium-used CFF, then the new filters were applied to on-site A356 aluminum alloy filtration tests, and comprehensive comparison and evaluation were done on the aspects of specimen mechanical properties, alloy composition, etc.Form the mechanical property and porcelain-forming mechanism studies of the original formula dry-pressed concrete ceramics, which induded coruadum, silica, K-feldspar and kaolinite, as raw materials, it showed that:low sintering density and matrix intensity can be mainly attributed to the lack of active Al2O3, the excessiveα-cristobalite and low secondary acerate mullite generated in the ceramics.By studing the effect of the additive concentration, on the mechanical preoperties of the matrix, it is found that the optimal phosphoric acid addition amount was 85% phosphoric acid and deionized with the ratio of 20/65 in matrix. In this case, the compressive and flexural strength at room temperature of the dry-pressed ceramics were 50.5% and 55.4% respectively, which was more than the original formula. This is mainly due to the low stoichiometric phosphoric acid which supplies more activiteγ-Al2O3. And the activeγ-Al2O3 is much propitious to the formation of secondary mullite and firm aluminum phosphate ceramic net.Sintering system has great impact on the mechanical properties of the sintered body. Two optimized sintering curves with their highest temperatures being 1350℃and 1400℃were determined. The compressive and flexural strength of ceramic matrix were increased by 6.8% and 13.1% respectively for 1350℃sinters, and 44.3% and 52.8% respectively for 1400℃sinters.Effects of multiform additives on the mechanical properties of the dry-pressed ceramics entities were systematically studied. Three kinds of optimized formulas were determined based on the phosphoric acid strengthended formula(A), which were the 6% talc+4% kyanite modified formula(B); the 3% mixed rare earth modified formula(C); and the TiO2+industrial Al2O3 co-milled formula(D). The main results and achievementsa are as following:·Adding 6wt% raw talc, the compressive and flexural strength of the dry-pressed sintered bodies can be increased by 108.9% and 33% respectively compared to that of A. Then further adding 4% ball milled kyanite on this basis, the elevation of the compressive and flexural strength further reached 20.1% and 29.1% respectively. The strengthening mechanism of talc addition mainly lies in the mullitization promoting effect by the generation of large number of glass phase, as well as the fine-grain strengthening effect of the generated aluminum-magnesium spinel solid solution. The role of kyanite in strengthening is mainly due to its high temperature inreversible mullitization, and its volume expansion effects fetching up the matrix shrinkage.·Adding 3% CeO2-riched mixed rare earth(formula C), the compressive and flexural strength of the dry-pressed ceramics entities can be improved by 25.4% and 23.3% respectively compared to that of formula A. The strengthening effects mainly lie in the CeO2 lattice defects generated at high-temperature which acts as activation centers for mullite crystallization, and promotes liquid-phase formation and aggregation, and the fine-grain strengthening function.·Adding TiO2+industrial Al2O3 co-milled powder(formula D), the compressive and flexural strength of the dry-pressed ceramic sintered body can be increased by 80.2% and 85.3% respectively compared to that of formula A. The strengthening mechanism of mainly lies in the promoting effect of the highly activated Al2O3 and TiO2, together with the metastabilized phase generated during the ball-milling process, and the stabilizing effect of mullite, silica and Fe2O3 on the aluminium titanate crystal.Dispersity and thixotropy of the above mentioned four optimized formula slurries were studied. formula D showed best dispersity followed by formula A、C、and B. The bad dispersity for formula B is due to the inherent hydrophobic interaction and surface air film on the talc particles; and for formula C, weaker surface ionic polarization for CeO2 particles resulted in compressure to the Al2O3 electric double layer. Well for formula D, high energetic ball milling improved the solid-liquid interface compatibility, and thus improving the slurry dispersity. Thixotropy studies showed that:when the solid-liquid ratio exceeded 1:4, the four kinds of slurries all put up the feature of pseudo-plastic fluid to some extent with shear thinning behavior and different stress overshoots. Formula B showed best thixotropy properties followed by formulaC、A and D. These are mainly associated with the different effects of the additives on the kaolinite "card structure" in the slurries.The polyurethane foam interosseous membrane can be removed effectively when hydrolyzed in 15% NaOH solution at 60℃for 40min, with its surface roughness increasing and no significant decline in elasticity. Idea slurry coating was derived when adjusting the compression ratio to 5:1. Soaked in certain solutions containing sodium carboxymethyl cellulose (CMC), sodium dodecyl sulfate (SDS), silica sol, and a home-made aluminum sol respectively, the slurry coating performances for the polyurethane foam were improved to different degrees, among which the home-made aluminum sol showed best coating effect. Laboratory preparation and property characterizations of foam ceramics based on above studies have been carried out with formula A as comparison. Mechanical properties studies showed that:the compressive strength for formula B、C and D were increased by 64.4%,28.8% and 48.1% respectively compared to that of formula A; as for the bending strength, the elevations were 24.9%,15%,72.6% respectively. Formula D showed highest number of thermal shock cycles, followed by formula C、B and A, this showed the effects of aluminum titanate stability on the improvement of matrix thermal shock resistance. As the bending strength variation after one thermal shock cycle, formula B showed smallest decline, followed by formula C、D and A.Mercury injection method was used to test the surface pore size distribution and the specific surface area of the sinters. At low-pressure, pore size distribution curve for formula A showed highest functional peak and widest pore size distribution, followed by formula C、B and D. This may be attributed to liquid aggregation and particle rearrangement in the matrix. While the presence of nano-sized pores in the matrix may be associated with gas emission, phase change shrinkage, micro-cracks and the mullitation in the sintering bodies. It was ulteriorly found that: inflicting the adobe of formula with vacuum freeze drying, the pore size distribution of the sinter was much wider than the unfrozen sinter. This is due to the formation of amouts of large pores with 10:1 aspect ratio on the cross-bars between the struts during freeze-drying process.Formula A、B and C were adopted in the pilot tests to prepare new types of aluminum CFF. The new products showed the advantages of high porosity, clean appearance, with no dregs, cracks and other defects. The compressive strength for the specimen of formula A、B and C were increased by 17.6%、104%、64.9% respectively, while the flexural strength,20.8%、163.6%、74% respectively, both of which are more than that of original formula.The on-site A356 aluminum alloy filtration tests showed that:the new filters met the actual production needs completely with high temperature stability, and good removing effects on fine inclusions. Hydrogen content testing to the various casting stages indicated that: filtration processes had certain removing effects on the hydrogen. Tensile strength and elongation tests for the alloy samples before and after filtration showed that:the tensile strength elevation rates for filters of the fomula A、B、C were 18.9%、13.1%、12.9%、respectively; while for the elongation, elevation rate for fomula A、B、C were 4%、11.4% and 8.3% respectively.更多还原