先进陶瓷胶态成型新工艺的研究进展

总结了先进陶瓷胶态成型新工艺在清华大学的研究进展.简要介绍了几种高固相含量的浓悬浮体和高质量坯体的制备新技术,提出胶态成型新工艺固化机制的新见解.采用新工艺去除坯体中缺陷,实现高可靠性、复杂形状高性能陶瓷部件低成本的自动化、批量化制备,为先进陶瓷产业化铺平了道路.     

陶瓷成型新方法及其应用的研究

介绍和讨论了作为一种借助酶催化化学反应实现原位凝固的崭新近净尺寸陶瓷成型概念的直接凝固注浆成型方法与技术，以及通过与陶瓷粉料混合形成浓悬浮胶体的有机单体在加人偶联剂、催化剂和引发剂后的聚合反应促成原位聚合凝固的注疑成型方法与技术。利用这两种成型技术可以获得均匀、无密度梯度的近净尺寸坯体和致密陶瓷制品。这里也简单介绍和讨论了喷墨打印成型技术。它是一种利用计算机控制实现多层打印、逐层叠加制出三维陶瓷坯体的计算机辅助制造(CAM)陶瓷的成型新技术。     

直接凝固注模成型Si_3N_4及SiC陶瓷──基本原理及工艺过程

直接凝固注模成型（ｄｉｒｅｃｔｃｏａｇｕｌａｔｉｏｎｃａｓｔｉｎｇ，ＤＣＣ）是一种崭新的（准）净尺寸陶瓷成型方法。本文报道了采用此法成型Ｓｉ_３Ｎ_４及ＳｉＣ陶瓷的基本原理和工艺过程。ＤＣＣ成型工艺过程为把高固相含量低粘度的陶瓷浆料浇注到无孔模具中，事先加入到浆料中的生物酶及化学物质通过改变浆料的ｐＨ或电解质浓度来改变浆料的胶体化学行为，从而使浆料原位凝固，得到有足够脱模强度的陶瓷坯体。ＤＣＣ成型的特点为坯体密度高（理论密度的５５％～７０％），坯体均匀，不用或只需少量的有机添加剂（少于１％），可成型大尺寸、复杂形状、高可靠性的陶瓷部件。     

Freeform Fabrication of Ceramics via Stereolithography

Ceramic green bodies can be created using stereolithography methods where a ceramic suspension consisting of 0.40–0.55 volume fraction ceramic powder is dispersed within an ultraviolet-curable solution. Three ceramic materials were investigated: silica for investment casting purposes, and alumina and silicon nitride for structural parts. After mixing the powders in the curable solution, the ceramic suspension is photocured, layer by layer, fabricating a three-dimensional ceramic green body. Subsequent binder removal results in a sintered ceramic part. Three-dimensional objects have been fabricated from a 0.50 volume fraction silica suspension.     

Recent developments in gelcasting of ceramics

Gelcasting is a well-established colloidal processing method with a short forming time, high yields, high green capacity and low-cost machining, and has been used to prepare high-quality and complex-shaped dense/porous ceramic parts. In this article, we reviewed recent developments in gelcasting technology for ceramic preparation. For environmental pollution reduction during ceramic preparation by gelcasting, the development of low-toxic and nontoxic gelcasting systems is discussed. The occurrence and control of inner stress and surface-exfoliation in ceramic green bodies prepared by gelcasting are analyzed, and then some methods to control and eliminate the inner stress and surface-exfoliation in ceramic gelcast green bodies, especially for colloidal injection molding of ceramics (CIMC) are proposed. Finally, the applications of gelcasting for the fabrication of porous ceramics and complex-shaped ceramics (e.g., microbeads, rutile capacitor, thin-wall rutile tube, refractory nozzle) are summarized.     

Influence of Green Formulation and Pyrolyzable Particulates on the Porous Microstructure and Sintering Characteristics of Tape Cast Ceramics

Porous morphology and total porosity produced in sintered ceramic tapes was controlled by the amount and distribution of pyrolyzable graphite particles added to a colloidal suspension during a tape casting operation. A conceptual model of the green tape microstructure was used to explain the influence of graphite and tape formulation on sintering characteristics. The creation of a connected, open porous network in the sintered body was the result of graphite particle percolation within the green body. Additional voidage introduced by particle bridging was the source of excess porosity and also resulted in a bimodal pore size distribution. Sintering shrinkage was determined by the ceramic packing density, which was primarily determined by the tape formulation.     

关于陶瓷成型工艺的讨论

陶瓷制造经历数千年历史,直到20世纪中叶因为烧结理论的创立获得了飞速发展,上世纪七八十年代关于超细粉体制备和表征的发展,促使陶瓷工艺第二次大发展,当前阻碍陶瓷材料进一步发展的关键之一是成型工艺技术没有突破,压力成型不能满足形状复杂性和密度均匀性的要求,传统的湿法成型工艺可以获得复杂形状,但不能保证坯体的均匀性,90年代以来发展起来的多种胶体原位成型工艺和气相成型工艺有望促使成型工艺获得突破。     

Freeze Casting as a Nanoparticle Material-Forming Method

Nanoparticle material forming is challenging because of loose packing and agglomeration issues intrinsic to nanoparticles. Liquid processing shows great potential to overcome such hurdles. This study is focused on nanoparticle colloidal processing and freeze-casting forming. Al₂O₃ nanoparticle suspensions are examined, and microstructure evolution of Al₂O₃ nanoparticle suspension during freeze casting is discussed. The "Fines" effect influences nanoparticle packing on freeze-cast sample surfaces. Trapped air bubbles in the suspension lead to a porous bulk microstructure. Prerest is necessary for dense and homogeneous green microstructure formation. The green strength, fracture mode, and ability to form fine features by freeze casting are also evaluated.     

α-Al2O3悬浮体的流变性及凝胶注模成型工艺的研究

陶瓷凝胶注模型工艺是一种新颖的成型工艺,具有很好的应用前景。它将高分子化学单体聚合的思路引入到陶瓷的成型工艺中,可制备高强度、高均匀性的陶瓷坯体。     

Freeze Casting of Aqueous Alumina Slurries with Glycerol

Freeze-drying concepts were utilized in the shape forming of alumina parts by pressureless slip molding using aqueous slurries. A water solidification modifier, glycerol, was utilized to eliminate the defects associated with the expansion and ceramic particle rejection of water during freezing. Castable alumina slurries with solids loading up to 60 vol% were prepared and characterized using viscosity and zeta-potential measurements with and without glycerol additions. Frozen parts were dried under vacuum by sublimation of ice to obtain net-shape green bodies. The combined effects of high-solids-loading slurries, >57.5 vol%, and glycerol additions were essential for freeze casting to achieve highly dense alumina bodies with a uniform microstructure.     

精密陶瓷原位凝固制备技术的研究

七十年代末，世界范围内伴随着陶瓷热机部件的热潮，精密陶瓷受到了各国政府、研究部门及产业界的充分重视。进入九十年代后，精密陶瓷的发展遇到了许多问题，其中形状复杂结构部件的成型工艺是制造高性能陶瓷材料最为关键的环节之一。本文综合分析了近年来精密陶瓷成型工艺的研究进展，着重强调指出砂位凝固成型技术是保证坯体均匀性和解决高性能陶瓷可靠性的重要环节，同时，指出成型工艺今后需要加强和解决的几个关键技术。     

Self-supported ceramic substrates with directional porosity by mold freeze casting

Manufacture of thin-film ceramic substrates with high permeability and robustness is of high technological interest. In this work thin (green state thickness ∼500 μm) porous yttria-stabilized zirconia self-supported substrates were fabricated by pouring stable colloidal aqueous suspensions in a mold and applying directional freeze casting. Use of optimized suspension, cryoprotector additive and mold proved to deliver defect free ceramic films with high dimensional control. Microstructure analysis demonstrated the formation of desirable aligned porosity at macro-structural scale and resulted to be highly dependent on colloidal behaviour and freeze casting conditions. Manufactured green films were joined by lamination at room temperature and sintered to obtain symmetrical cells consisting of two porous self-supported substrate electrodes (∼420 μm) and dense yttria stabilized zirconia electrolyte (∼10 μm).     

Hydrolysis-induced simultaneous foaming and coagulation casting of secondary aluminum dross aqueous suspension

The reactive AlN, heavy metal ions, and salts in secondary aluminum dross make them harmful to the environment and difficult to handle, and therefore categorized as hazardous solid waste. A novel, simple, and versatile technique has been proposed here to prepare hierarchically porous ceramics using secondary aluminum dross as the sole raw material, namely, the hydrolysis-induced simultaneous foaming and coagulation casting method. As the main component, Al and AlN could be hydrolyzed to generate aluminum hydroxide sol, which can form three-dimensional network and lead to the outstanding compressive strength of green body (7.03 MPa). Gas release during hydrolysis, high-temperature oxidation of AlN, and metallic Al based on Kirkendall effect could account for the final multilevel pores. The utilization of harmful components in secondary aluminum dross, supplys an alternative colloidal forming way for porous green body with complex shape when combined with other ceramic powders or inorganic solid waste. Low sintering shrinkage of 1.38%–3.60% for hierarchically porous ceramics offers this highly effective and low-cost method to be easily promoted for industrial production.     

Gelcasting of a rutile mixture applied to extrusion forming

To improve sintering and forming behaviors, many additives are normally included in the rutile powder during extrusion forming. Hence, this complex rutile mixture is difficult to be dispersed into an aqueous solution of acrylamide monomers for gelcasting. In this work, with the aid of a dispersant, the mixture is easy to be dispersed by calcining at high temperature and then ball-milling, and a concentrated rutile suspension with low viscosity was successfully obtained. The suspension can be used to form in-situ a ceramic green body. After sintering the green body, the microstructure and performance of the product are both improved. For example, breakdown strength improved from 12.6 kV/mm to more than 23.6 kV/mm.     

The role of plasticizer in optimizing the rheological behavior of ceramic pastes intended for stereolithography-based additive manufacturing

Adding plasticizer is an efficient way to regulate the rheological behavior of ceramic paste and quality of green body in stereolithography-based additive manufacturing. The type and content of plasticizers (polyethylene glycol (PEG) and dibutyl phthalate (DBP)) had substantial effects on the rheological behavior and solid loading of ceramic paste, leading to varied macro / micro structure and strength of the green and sintered parts. DBP significantly reduced the viscosity and increased solid loading, and could adjust the flexibility of the green body by reducing the crosslinking density of UV curing system. PEG could inhibit crack initiation to some extent, but it was less effective for preventing cracking than DBP on ceramic parts with large-sized cross sections. It was concluded that DBP was more suitable as a plasticizer in alumina paste for SL additive manufacturing to form dense parts without defects.     

新型高性能陶瓷成型法的研究

探讨了一种新的非化学反应直接凝聚固化成型方法。利用高分子电解质分散剂可以制备固体含量高、流动性良好的陶瓷泥浆。这种泥浆注入非多孔性模具后 ,无需生物酶催化反应 ,只需将温度升至70～80℃ ,泥浆便由分散状态转为凝聚状态 ,泥浆即达到固化成型。这种方法具有工艺简单、容易控制的特点 ,并且可以获得高致密、高均匀性的陶瓷成型体。通过对氧化铝 -高分子电解质 -水系统的吸附及流动特性进行的系统的研究 ,提出了覆盖率理论 ,解释了用经典的DLVO理论难以解释的胶体稳定性与温度敏感特性之间的关系     

琼脂糖凝胶大分子在陶瓷原位凝固成型中的应用

研究了一种新的陶瓷原位凝固成型方法,其原理是依据琼脂糖凝胶大分子在水溶液中加热时溶解,冷却时凝固这一物理变化。对于固相体积分数大于50％陶瓷悬浮体中引入约1％（质量分数）琼脂糖大分子,即可凝胶注模进行各种形状复杂的陶瓷部件成型,获得表面光洁、内部孔隙尺寸和密度分布均匀的陶瓷坯体,并烧结出均匀致密内部无明显缺陷的涡轮转子等异型部件。     

Colloidal processing: enabling complex shaped ceramics with unique multiscale structures

Colloidal processing of fine ceramic powders enables the production of complex shaped ceramics with unique micro and macro structures which are not possible to produce via conventional dry processing routes. Because of this enhanced structural control and shaping capabilities, colloidal processing has been exploited to produce ceramic components with ever increasing complexity and functionalities. In this review, we revisit some of the research efforts on this topic to highlight its relevance and growing importance for the advanced manufacturing of functional ceramics. Selected examples of colloidal systems with increasing level of complexity are discussed to showcase the wide range of structures that can be generated through wet processing approaches. The historical development and background knowledge pertaining to colloids and surface interactions is first briefly reviewed. The major colloidal shape forming and additive manufacturing processes that utilize colloidal pastes and inks are then reviewed, highlighting the control of suspension rheology needed in these techniques. Next, methodologies that combine suspended particles with a pore‐forming phase are discussed as a means to produce porous ceramic materials. Further control over the interactions between anisotropic particles and their alignment in suspensions can be gained via externally applied fields (such as magnetic) to produce texturally aligned green bodies. This leads to bioinspired ceramics that can programmably morph into complex shaped objects upon sintering. Hierarchical porous structures with high mechanical efficiency are also shown as an example of the multiscale designs that can be generated through advanced colloidal processing. As drying of ceramic bodies is an inevitable consequence of wet colloidal processing, the current understanding of this critical processing step is reviewed. Finally, the gaps in knowledge in these fields are discussed to provide our perspective on where the field may support advances in ceramics in the future.     

A novel casting forming for ceramics by gelatine and enzyme catalysis

A new kind of gelation process for forming a ceramic green body is discussed. The forming method is based on a process in which gelatine, enzyme (urease) and urea are used. Urea is employed to prevent attraction between gelatine molecules when the dissolved hot gelatine solution cools down for preparing a well-dispersed suspension by ball milling and degassing at ambient conditions. After urease is added into the suspension, gelatine molecules attract each other and form three-dimensional network through hydrogen bonding due to urea decomposition by urease. Slurry containing alumina and gelatin could be consolidated in situ by the above gelation process at room temperature. A green body with homogeneous microstructure and smooth surface was obtained.     

Aqueous tape casting of super-hard B4C laminates with rGO-enriched reinforcing interlayers

Superhard B₄C parts with microarchitectures constituted by ceramic layers and evenly-spaced rGO-enriched reinforcing interlayers were fabricated for the first time. To this end, a concentrated slurry of B₄C with its Ti-Al sintering additive was first prepared by aqueous colloidal processing, optimizing its total solids loading and content of both binder and plasticizer to obtain, by tape casting, handleable and flexible green tapes. A semi-dilute aqueous suspension of B₄C with Ti-Al and abundant GO was also prepared to dip-coat those green tapes with a GO-enriched layer, optimizing the withdrawal rate and the dipping time. The bare and GO-coated B₄C+Ti-Al tapes were then sequentially laminated, thus yielding green multilayered laminates that finally were appropriately debinded and densified by spark plasma sintering. Vickers indentation tests demonstrated that these multilayered laminates are superhard (～31 GPa), and that their rGO-enriched reinforcing interlayers are effective in arresting crack propagation.