Field-assisted selective-melt sintering: a novel approach to high-density ceramics
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esearch Letters
Field-assisted selective-melt sintering: a novel approach to high-density ceramics J. Narayan, Department of Materials Science and Engineering, North Carolina State University, Centennial Campus, EB-1, Raleigh, NC 27695-7907, USA Address all correspondence to J. Narayan at [email protected] (Received 3 April 2013; accepted 22 July 2013)
Abstract Electrical fields can be used to heat selectively dislocations and grain boundaries to a much higher temperature compared with the bulk. This selective joule heating, if uncontrolled by limiting the current flow, can lead to melting of grain boundaries and sintering of poly- and nanocrystalline materials close to the theoretical density in a much shorter time due to fast diffusivities of the order of 10−4 to 10−5 cm2/s in the liquid. I refer to this sintering mode as selective-melt sintering, which can occur at lower overall temperatures with much lower energy consumption compared with conventional sintering involving solid-state diffusion.
Introduction It is well established that electrical fields (DC and AC) affect ionic conduction and diffusion transport in ceramics in a significant way.[1–4] Narayan and co-workers (ORNL researchers) in the late 70s and early 80s[1–4] studied extensively the characteristics of electrical conductivity in pure (ORNL) and impurity doped (Norton) MgO single crystals as a function of temperature and applied field, and analyzed the role of defects and impurity clusters in enhancing electrical conductivity and mass transport which led to an avalanche and thermal electric breakdown. These studies established the presence of high concentrations of intrinsic (anion and cation vacancies) defects and impurity precipitates in highly conductive regions between the electrodes after cooling. The enhancement of conductivity at high temperatures exhibited an avalanche effect, which, if uncontrolled by limiting the current flow, led to melting and eventually evaporation of materials between the electrodes. These results suggested a predominant role of vacancies and impurities in electronic conduction and diffusion transport. Although ORNL work was conducted at higher fields (>1000 V/cm) and higher temperatures (>1000 °C), other researchers[5–15] have obtained interesting results on mechanical properties and sintering at lower fields, which can be correlated with earlier results at higher fields. At lower fields Conrad and co-workers have found interesting effects on polycrystalline NaCl, MgO, and yttria stabilized zirconia materials related to grain growth retardation and reduction in flow stress and enhancement in ductility.[5–8] Applied electric (AC and DC) fields have been shown to produce a host of interesting phenomena related to sintering of solid-state materials.[9–15] More recently Raj and coworkers[13–16] have shown that the application of DC electrical fields in the intermediate range
of about 30–100 V/cm produced sintering of polycrystalline yttria-stabilized zirconia in mere seconds at furnace temperatures of 850 °C, well below the normal
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