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About this book

This book discusses several new, near-net-shape techniques for fabricating highly reliable, high-performance, complex ceramic parts. In the context of materials design, the creation of high-performance ceramic products of desired shapes has led to the need for new ceramic forming processes. The near-net-shape techniques combine both injection-molding and colloidal-forming processes. Reviewing and summarizing the research and latest advances, the book is divided into 6 parts: (1) the basic theory, development, and application of the colloidal injection molding of ceramics; (2) the tape casting technology; (3) the reliability of the product; (4) the colloidal injection molding of Si3N4 and SiC; (5) low-toxicity systems; and (6) the novel in-situ coagulation casting of ceramic suspensions via controlled release of high-valence counter ions and dispersant removal. It is intended for researchers and graduates in materials science and engineering.

Table of Contents

Frontmatter

Chapter 1. Aqueous Colloidal Injection Molding of Ceramics (CIMC) Based on Gelation

Abstract
In this chapter, aqueous colloidal injection molding (CIMC), which is based on gelation of monomer polymerization, like a reactive injection molding of double slurry of polyester, is discussed systematically. The solidification mechanism is based on gelation of monomer polymerization. It was found that pressure can induce the polymerization of the monomer. The gelation time can be effectively controlled by both temperature and pressure. The testing equipment of gel point was developed and used to test the gel reaction of the monomer in the suspension under different pressures. A machine of for ceramic colloidal injection molding was developed. It was observed that copper accelerated the polymerization of the monomer. Therefore, copper parts are forbidden for use in a CIMC machine.
Jinlong Yang, Yong Huang

Chapter 2. Gel Tape Casting of Ceramic Substrates

Abstract
In this chapter, aqueous gel tape casting (in short form, novel gel-tape-casting) processing is introduced systematically. The solidification mechanism and factors influencing the gel-tape-casting are analyzed. Alumina green tapes have been prepared through gel-tape-casting successfully. Experimental results show that the efficiency of styrene-acrylic binder is obvious for water-based tape casting. Sodium alginate has been used successfully to consolidate alumina tapes from aqueous suspensions. Based on gel-tape-casting, another novel process, spray inducing instant solidification of gel tape casting, was developed to overcome the limitation of operating time. This process is ideal for forming ceramic sheets with thickness less than 0.3 mm. Research studies have demonstrated that the ceramic sheet products fabricated by the new gel-tape-casting process have good quality, reduce pollution, lower costs, and yield significant social and economic benefits.
Jinlong Yang, Yong Huang

Chapter 3. Gelation Forming Process for Toxicity-Free or Low-Toxicity System

Abstract
In this chapter, the forming process of ceramics by gelling of agarose, gelatin, and sodium algaecide is presented, and another gelcasting system with low toxicity by using HEMA is also described. This study shows that gelled green bodies by the addition of agarose of 0.7–1.05 wt% and a solid loading of 53–55 vol% had a homogeneous density distribution and a green strength >3 MPa. Some simple and complex parts, such as a turbine rotor, were formed by this method. Gelatin can be dissolved in water above 40 °C, and the solutions gel at 15–20 °C. Wet green compacts can be formed from the alumina slurry containing 4–5 wt% gelatin (based on water) by gelation. A novel casting forming method by gelatin and enzyme catalysis is also described. As urease is added into the suspension, the gelatin molecules attract each other and change to a three-dimensional (3D) network structure through hydrogen bonding due to urea decomposition by urease. Then, the slurry containing ceramic powder and gelatin will be consolidated in situ by the gelation under ambient conditions. For another gel-casting process by alginate, in which solidified agent and chelator are added into sodium alginate solution simultaneously; thus, the gelation between calcium ions and sodium alginate is avoided in this stage before casting. Free calcium ions are released when hexanedioic acid is added into the suspension. They react with sodium alginate and form a 3D network. Therefore, the ceramic particles are fixed in this 3D network. The slurry is consolidated and near-net-shape green parts are formed. In addition, a new innoxious gelcasting system has been successfully developed by using HEMA or HEMA-PVP, by which dense and homogeneous complex-shaped alumina and silicon carbide parts have been successfully fabricated.
Jinlong Yang, Yong Huang

Chapter 4. Generation, Development, Inheritance, and Control of the Defects in the Transformation from Suspension to Solid

Abstract
This chapter focuses on the generation, development, inheritance, and control of the defects in colloidal forming process, by which the complex near-net shape ceramic products with a homogeneous microstructure and high reliability were consequentially achieved. The rheological performance of the ceramics suspension is very important to the colloidal forming processes. In our studies, the rheological behaviors of aqueous alumina suspensions were investigated, and the various ways of modifying the rheological behaviors of the suspensions were discussed. A critical solid volume fraction between the shear-thinning and shear-thickening behavior was found in an aqueous alumina suspension. Moreover, the gelation of the suspension with high solid volume fraction of alumina (SVF) got delayed and the strength of the green body decreased, which implies that the fast polymerization of monomers in high SVF alumina suspension was inhibited, and the flexibility of the gelcasting was improved. We found that there was an optimal SVF for the ZTA suspension to obtain the highest bending strength and the highest Weibull modulus for the sintering bodies. Micro mechanisms of agglomerations’ generation in ceramic suspensions used in colloidal forming processing were investigated. On the basis of the potential energy between particles in liquid, a microscopic model of agglomerations in ceramic suspensions was presented. Two types of agglomerations, tight and loose, were observed by using an environmental scanning microscope as a quasi-direct method. It was observed that their generation is caused by the deviation of the solid-phase content in the slurry from the stable solid loading. The influences of the idle time on the microstructures and the mechanical properties of the green bodies are also described in this chapter. Vertically laminated cracks were observed in the coagulated green bodies after casting. The cracks were attributed to the shrinkage of the particle network (syneresis) during coagulation. This process in combination with sedimentation during the reaction affected the vertical cracks, which was not observed when a certain idle time before casting was introduced. The idle time diminished the final shrinkage still having a castable slurry of reasonably low viscosity. The study indicated that the solids volume loading in the suspension depended on high valence counterions and ion conductivity constants. Further research revealed that the inner stress in the ceramic green bodies originates from the nonuniform shrinkage during the solidification of the precursor suspension and the drying of the green bodies. The gradients of temperature, the initiator concentration, and the moisture are important original factors causing the inner stress. It is found that a proper amount of a water-soluble polymer, such as hydroxyethyl acrylate (HEA), polyethylene glycol (PEG), and polyvinylpyrrolidone (PVP) added into the concentrated suspension can adjust the polymer network structure and thus reduce the inner stress and cracking in the ceramic green body. The debindering time of large ceramic parts can be significantly shortened by reducing the harmful inner stresses in the green body.
Jinlong Yang, Yong Huang

Chapter 5. Gelcasting of Non-oxide Ceramics

Abstract
In this chapter, the non-oxide powder surface characteristics before and after treatment, and the effect of surface modification on the solids loading of its aqueous suspension were studied. For Si3N4 powders, a universal method to prepare highly concentrated suspensions has been established. The results show that Si3N4 with high reliability was obtained via processing strategy, for example, using a combination of coating, oxidizing, gelcasting, cold isostatic pressing, and gas-pressure sintering (GPS).
Jinlong Yang, Yong Huang

Chapter 6. Applications of New Colloidal-Forming Processes

Abstract
The colloidal-forming process based on gelcasting was applied for preparing microbeads of ceramics, improving the breakdown strength of the rutile capacitor, developing the thin-wall rutile tube for ozone generator, and producing the refractory nozzle of zirconia (ZrO2) and lead zirconate titanate (PZT). The microbeads of ceramics were used as the grinding media, gel-pen ball, and healthcare product with far-infrared function for the human body. Owing to the low-cost associated with the production of the gel beads as well as because of its superior properties, they have been widely used and marketed. When compared with the extrusion formation, the breakdown strength of rutile capacitor prepared by gelcasting was significantly improved. Rutile ceramics with high dielectric constant and high breakdown strength have been used in the thin-wall tube of the ozone generator. The ZrO2 refractory nozzle prepared by gelcasting has a high melting point above 2680 ℃ and a good thermal shock resistance to withstand the temperature rising from the room temperature to 2200 ℃ in several seconds. Hence, it is considered as a suitable material for making nozzles for precision casting of Cu–Cr molten alloy. When compared with die pressing, gelcasting can provide a more homogenous microstructure as well as more homogenous piezoelectric property of the PZT samples. By using the same sintering procedure, gelcast samples were observed to exhibit slightly stronger piezoelectric effects than the die-pressed ones, and this can be attributed to both density difference and pore size.
Jinlong Yang, Yong Huang

Chapter 7. New Methods and Techniques Based on Gelation

Abstract
The reliability and cost of the products, which are closely related to the preparation process, are the two major concerns in the industrial production of high-performance ceramics. The preparing, forming, and sintering processes of the ceramic powders are thus becoming increasing important in the recent decades in the ceramic research field. As a precondition of the materials design and realization of the high-performance ceramic products of the desired shapes, the ceramic-forming process usually plays a crucial role in determining the reliability and cost of the products. The traditional methods of producing ceramic products have been widely used in the field of fine ceramics, but they do not meet the requirements of high-performance ceramics. Therefore, numerous research efforts have been directed to devise new methods for the ceramic-forming process. In this chapter, some new methods and techniques based on gelation are introduced, such as solid freeform fabrication (SFF) of alumina ceramic parts; freeform fabrication of aqueous alumina–acrylamide gelcasting suspensions; ceramics with special porous structures fabricated by freeze-gelcasting; solidifying concentrated Si3N4 suspensions for gelcasting by ultrasonic effects; laser machining technique of alumina green ceramic; etc. Furthermore, the forming mechanism is analyzed and the characteristics of the green and sintered bodies of the ceramic parts using the new forming methods or new techniques are introduced.
Jinlong Yang, Yong Huang

Chapter 8. Novel In-situ Coagulation Casting of Ceramic Suspensions

Abstract
In this chapter, direct coagulation casting of ceramics via controlled release of high valence counter-ions and novel in-situ coagulation casting of ceramic suspensions via dispersion removal are discussed systematically. Direct coagulation casting of ceramics via high valance counter-ions (DCC–HVCI), as a new direct coagulation casting method, is proposed based on the strong coagulation ability of high valance counter-ions (HVCI) on ceramic suspension. The influence of types and addition content of dispersant, pH value, and solid loading on zeta potential and viscosity of ceramic suspensions has been systematically investigated. Several new methods for controlled release of HVCI have been proposed, such as using temperature as the leading controlling factor, chemical reaction and pH value as assisting controlling means, the increase of solubility of calcium iodate, calcium phosphate reacting with hydrochloric acid and decomposition of citric acid salt, and so on. It has been proved from both experimental and theoretical calculation results that the suspension of DCC–HVCI is coagulated at the primary minimum. This avoids the problems such as cracks formation and inner stress resulted from monovalence ions coagulation method which has been proved to coagulate at the second minimum. The results of this chapter are the basis for the deep research and applications of DCC. Based on the stabilization mechanism of ceramic suspension, we systemically investigate another new ceramic colloidal forming method, namely, dispersion removal coagulation casting (DRCC). Dispersant reaction, hydrolysis, crosslink, and separation have been used to control the dispersion removed processing. In the second part, the coagulation mechanisms of different ceramic suspension systems with different stabilization mechanisms are systematically investigated and the idea of the dispersion removal has been proposed. Different kinds of oxide and non-oxide ceramics and green bodies with excellent performance have been successfully prepared by dispersion removal method. The results are the basis for the further research and applications of ceramic colloidal forming. It provides a new idea and new method for the further study of the colloidal processing of advanced ceramics and also provides a new way for the industrialization of advanced ceramics with complex shapes.
Jinlong Yang, Yong Huang

Backmatter

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