Field Effects On Reactivity And Field Assisted Reaction Sintering Of Ceramics

Author : Jorgen Fredrick Rufner
Publisher :
ISBN 13 : 9781321609813
Total Pages : 0 pages
Book Rating : 4.6/5 (98 download)

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Download or read book Synthesis, Microstructure, and Electric Field Assisted Sintering of Nanocrystalline Magnesium Aluminate written by Jorgen Fredrick Rufner and published by . This book was released on 2014 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: In the powder processing community there has been a large drive to develop techniques for generating bulk nanostructured polycrystalline ceramic materials. To that end, various types of nanoparticle and interface engineering have been performed through the modification or exploitation of surface and grain boundary energies. Not only have interface energies been explored, processing techniques have also developed to aid in the consolidation of nanopowders through the application of an electric field or current during sintering. These processes go by many names but EFAS techniques has been shown to provide improvements during sintering such as enhancing rates of densification, decreasing sintering temperatures, reducing required applied pressures, and grain growth suppression across a wide range of materials, however the underlying mechanisms responsible are still unclear and widely debated. This work had 2 primary goals: 1) create and characterize nanoparticles of such a small size that their large excess energy generates a driving force for densification that allows for the reduction of traditional sintering time and temperature toward achieving dense nanocrystalline magnesium aluminate; 2) deconvolute the complex system that is EFAS and determine the influence of electrostatic fields (non-contact electric field) on the sintering and grain growth behavior of electrically non-conductive dielectric ceramic materials. The motivation for this lies in delineating the role of each individual variable during EFAS so the process can be tuned with the a-priori knowledge of what the resultant microstructure will be. Through tuning solution synthesis methods, magnesium aluminate nanoparticles were successfully synthesized via a co-precipitation method with an average size 5-6 nm. When traditionally sintered in a tube furnace at 1300°C in air, densification rates were high enough that only after 10 minutes sample relative density reached above 90% with an ultimate final density of 96% after only 20 minutes. This was a noticeable improvement from the typical conditions required for isothermal sintering of magnesium aluminate which are at temperatures above 1600°C for hours rather than minutes. In addition, a second synthesis method, the polymer precursor or Pechini method, was investigated for its well-known ability to create multi-cation oxide ceramics but gave very different results. For the case of MgAl2O4 it resulted in porous, tightly bound agglomerates made up of approximately 20nm nanocrystals and a volume fraction porosity of [phi][subscript vol] = 21.6 ± 3.7%. Scanning and transmission electron microscopy (TEM) was performed on individual agglomerates showcasing their bimodal pore structure of large sub-micron to micron sized pores alongside a dominant network of smaller nanosized pores. The pore structure was found to be very stable and significantly inhibit powder densification during sintering to only 55% relative density, regardless of sintering temperature or time. The mechanical strength of the agglomerates was quantitatively measured through TEM in-situ nanoindentation and was found to be the chief inhibiting factor during sintering. Deformation behavior of the agglomerates under compression followed three distinct failure mechanisms, unique to honeycomb/nanofoam structures, with complete fracture occurring at a stress of 2.75 ± 0.35GPa. Details as to how and why the synthesis method generated the porous structure and not small individual nanoparticles, as with the co-precipitated powder, are explained. Since surface and interface energetics play a critical role for densification during sintering, it was critical that the surface reactivity of such small nanoparticles be studied. This was performed by surface water adsorption where small controlled doses of water vapor were adsorbed onto anhydrous particle surfaces and the associated enthalpy of adsorption measured. It was apparent from the heat of adsorption data that more than just surface adsorption was occurring when water interacted with the nanoparticles. Upon further investigation, water was found to react with the particle surface creating magnesium hydroxide as well as atomic disordering of the surface. This endothermic surface reaction continues with the addition of more water such that when powder was completely submerged, large 2-dimensional amorphous hydroxide structures were seen cutting through the particle agglomerates. The adsorption of even small amounts of water by metal oxide nanoparticles has implications in that environmental humidity during processing can alter the way in which these nanomaterials sinter and densify. Finally, the synthesized nanopowders were used to study the effect of a 1000V/cm DC electric field on the densification and grain growth behavior during sintering. No appreciable change was observed in final density or densification rate with the application of the electric field, however, the average grain size of the field sintered sample was larger than that sintered with no field. To further clarify the experimental system, samples were annealed from full density to isolate grain growth mechanisms from densification mechanisms. The enhanced grain boundary mobility (grain growth) was again observed to be higher with the applied field. Nuclear resonance spectroscopy (NMR) and various TEM techniques including orientation image mapping (OIM), electron energy loss spectroscopy (EELS), and high resolution microscopy (HRTEM) were all used to determine local chemistry, structure, and bonding environments to elucidate how electric fields affect grain boundary mobility in electrically non-conductive ceramics.