Microstructures alumina-based ceramics

Padture NP (1991) Crack resistance and strength properties of some alumina-based ceramics with tailored microstructures. Ph.D. Thesis, Lehigh University... 

Considerable development has occurred on sintered ceramics as bone substitutes. Sintered ceramics, such as alumina-based ones, are uru eactive materials as compared to CBPCs. CBPCs, because they are chemically synthesized, should perform much better as biomaterials. Sintered ceramics are fabricated by heat treatment, which makes it difficult to manipulate their microstructure, size, and shape as compared to CBPCs. Sintered ceramics may be implanted in place but cannot be used as an adhesive that will set in situ and form a joint, or as a material to fill cavities of complicated shapes. CBPCs, on the other hand, are formed out of a paste by chemical reaction and thus have distinct advantages, such as easy delivery of the CBPC paste that fills cavities. Because CBPCs expand during hardening, albeit slightly, they take the shape of those cavities. Furthermore, some CBPCs may be resorbed by the body, due to their high solubility in the biological environment, which can be useful in some applications. CBPCs are more easily manufactured and have a relatively low cost compared to sintered ceramics such as alumina and zirconia. Of the dental cements reviewed in Chapter 2 and Ref. [1], plaster of paris and zinc phosphate... 

From the point of view of the volume of production, polycrystalline alumina is the material most frequently used as ceramics for structural applications. However, in comparison with for example, silicon nitride, where the influence of various additives on microstructure and properties has been well characterized and understood, and despite several decades of lasting research effort, alumina remains a material with many unknown factors yet to be revealed. Alumina-based materials can be divided roughly into three groups ... 

Alumina based fibers are subject to creep, even at low temperatures, This phenomenon is exacerbated by the fine grain size of the fibers [20] [54] [67] [70] [73] [79-80]. At 1200°C and with an applied stress of 70 MPa, the strain rate for the fine-grained Nextel 610 a-alumina fiber is higher than that of Fiber FP with a coarser microstructure. Both strain rates are about one order of magnitude higher than that for a bulk alumina ceramic with a grain size of 1.2 jm. The aeep of Nextel 610 is already measurable at a temperature as low as 900 C under an applied stress of 200-500 MPa [70]. 

Wilson, D. M., Lueneburg, D. C., and Leider, S. L. (1993). High temperature properties of Nextel 610 and alumina-based nanocomposite fibers. Ceram Eng Sci. Proc 14 609-621. Wilson, D. M., Lueneburg, D. C., and Leider, S. L. (1995). Microstructure and high temperature properties of Nextel 720 fibers. Ceram Eng Sci. Proc 16 1005-1014. 

The possibility to obtain a uniformly dispersed composite powder was shown for the a-Fe-Al203 system where metal particles with an average size of 55 nm were formed in an amorphous/nano alumina matrix.18 Other studies attempting to obtain dense bulk composites based on the sol-gel route using conventional pressure-assisted sintering ( 1400°C and an applied force of 10 MPa) resulted in a coarse microstructure.16 However, if reaching theoretical density is not a necessary requirement, a porous ceramic microstructure containing nanometer-sized metal particles can be used as a catalytic material.19 Certain combinations of composite materials demand... 

The present study aims at investigating the Reaction Bonded Silicon Carbide (RBSC) process to produce porous mullite-bonded SiC ceramics. Wu and Claussen (1991) reported a technique to produce mullite ceramics starting from Al, SiC and AI2O3 powder mixtures. However for the purpose of this study it was decided to use only SiC and Al 03 as the precursor powders with SiC as the major component so that after completion of the reaction the microstructure would be SiC bonded with mullite phase, with no residual alumina. This material was then tested for its mechanical properties like Young s modulus. Modulus of Rupture. Properties of Silicate-based SiC refractories have been reported to a certain extent by Reddy and others. Its potential use as a refractory material has been evaluated by measuring its thermal shock resistance. A sample refractory that has been designed in the... 

This latest trend in load-bearing materials for arthroplastic applications involves the development of highly fracture-resistant alumina/zirconia composites, as an alternative choice to alumina and zirconia monolithic ceramics. Composite materials are designed from both chemical and microstructural viewpoints in order to prevent environmental degradation and fracture events in vivo. Based on the experimental determination of an activation energy value for an environmentally driven tetragonal to monoclinic transformation, the long-term in vivo environmental resistance of prostheses made from these composite materials can be predicted (Chevalier et al., 2009). 

One of the routes to dense, fine-microstructured, polycrystaUine abrasive minerals considered by 3M at that time (-1974) was sol-gel technology. Prior to work on abrasives, 3M s Dr. Harold Sowman had been successful utilizing sol-gel approaches for the synthesis of nuclear fuel pellets and production of ceramic fibers. He, along with Dr. Melvin Leitheiser, initiated an effort to develop new aluminous abrasive minerals based on sol-gel technology. They focused their effort on the conversion of colloidal boehmite, y-AlOOH, into dense, polycrystalline alumina. Since that time, boehmite has become, not only the precursor of choice for sol-gel abrasive grains, but the precursor of choice for virtually every sol-gel alumina process. 

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