Novel Sintering Processes and News in Traditional Sintering and Grain Growth: Field Assisted and NanoSintering I
Sponsored by: MS&T Organization
Program Organizers: Ricardo H. R. Castro, University of California at Davis; Douglas Gouvêa, Universidade de São Paulo

Tuesday 8:00 AM
October 18, 2011
Room: C222
Location: Greater Columbus Convention Center

8:00 AM
Thermal Stability of Cryomilled Al Alloy during Spark Plasma Sintering: Yuhong Xiong¹; Dongming Liu¹; Ying Li¹; Chris Haines²; Joseph Paras²; Darold Martin²; Deepak Kapoor²; Enrique Lavernia¹; Julie Schoenung¹; ¹University of California; ²US Army
    Spark plasma sintering (SPS) technology has emerged as a viable approach to sinter materials due to its rapid heating and high pressure application features. Al alloys have been widely used because of their combination of light weight and high strength. In this study, SPS was chosen to produce dense ultrafine-grained bulk samples using cryomilled nanostructured Al alloy powder. Thermal stability of the nanostructured powder was investigated through TEM at various sintering temperatures and holding times. A bimodal microstructure and a banded structure were observed in the consolidated bulk samples. The generation and evolution of such microstructures can be attributed to the starting powder and the process conditions, which are associated with the thermal, electrical and pressure fields present during SPS.

8:20 AM
Synthesis of Nanostructured (Bi,Sb)₂Te₃ Alloys by Spark Plasma Sintering: Zhihui Zhang¹; Yizhang Zhou¹; Enriqure Lavernia¹; Nancy Yang²; Ryan Nishimoto²; Peter Sharma²; ¹UC Davis; ²Sandia National Laboratories
    Nanostructured (Bi, Sb)₂Te₃ alloys have attracted increasing attention as a result of enhanced thermoelectric and mechanical behavior. In this work, nanostructured p-type (Bi_0.2Sb_0.8)₂Te₃ alloy was produced by high energy ball milling mixtures of Bi₂Te₃ and Sb₂Te₃ compounds followed by spark plasma sintering (SPS). X-ray diffraction Rietveld analysis suggested a complete solid solution had formed with a grain size of 15 nm following ball milling. In subsequent SPS, it was found that nano-pores of approximately 20-30 nm in size formed mostly along grain boundaries. The effects of SPS sintering temperature and pressure on the grain size and nano-pore evoluton was studied in the range of 300-450 °C and 50~500 MPa. The thermal conductivity, electrical resistivity, Seebeck coefficient, and Hall mobility were measured along and perpendicular to the loading directions. The resultant thermoelectric and mechanical properties were discussed in terms of the effects of grain size refinement and presence of nanoscale defects.

8:40 AM
Thermal Stability of Al 5356 Processed by Cryomilling and Spark Plasma Sintering: Bamidele Akinrinlola¹; Mathieu Brochu¹; Raynald Gauvin¹; ¹McGill University
    Nanostructured materials possess improved properties when compared to their conventional counterparts. This quality is especially of interest for high strength-to-weight ratio metals like Aluminum alloys. While nanostructured powders can be produced by various methods, maintaining the nanocrystalline structure during consolidation is a challenge. In this study the thermal stability of cryomilled Al-5356 powder is investigated during annealing treatments and consolidation via Spark Plasma Sintering. The powder is subjected to thermal treatments at temperatures ranging from 350 to 500 °C, and durations of 5 to 60 minutes; powders are then sintered at similar parameters. Grain growth and relaxation of micro strain during annealing and sintering are determined from XRD peak broadening. The microstructure in the powder resulting from the milling process and during consolidation is observed with the Electron Channeling Contrast Imaging (ECCI) technique. Post-consolidation testing includes micro-hardness and three-point bending tests.

9:00 AM  Invited
Field Assisted Sintering of Oxide Ceramics: New Mechanisms: Sebastian Schwarz¹; Raschid Baraki¹; Andrew Thorn²; Klaus van Benthem²; Olivier Guillon¹; ¹Technische Universität Darmstadt; ²UC Davis
     After having shown that the densification mechanism for insulating to semi-conducting oxides is identical in FAST or SPS and hot pressing, we concentrate on two interesting effects: high heating rates and high electrical loading. It appears that an increase of the heating rate significantly enhances the sinterability of nanocrystalline zinc oxide. This enables to get almost dense specimens at a temperature as low as 400°C. To explain the positive effect of high heating rates, microstructure, inter-particle necks and grain size are examined using TEM/SEM. For nanocrystalline cubic zirconia, the effect of AC field/current is investigated by means of a unique sinter-forging apparatus. By decoupling the effect of field and current during the sintering cycle, it is possible to rationalize the observed improvement in densification kinetics. Emphasize is also put on the differences between these conditions and those met in standard FAST experiments.

9:40 AM Break

10:00 AM  Invited
New Insights on SPS Sintering Mechanisms: Vincent Garnier¹; Yann Aman²; Elisabeth Djurado³; ¹Université de Lyon - Insa de Lyon; ²Insa de Lyon; ³INP Grenoble
    At the origin of the SPS process, it was supposed that the application of pulsed D.C. currents could generate a plasma discharge between the particles of powdered material, thus enhancing the creation of necks at low temperatures. The microstructure of necks of SPS sintering of alumina was characterized in comparison with an electrical conductive material (copper). It reveals non-conventional particular morphologies. A model is described to account for the effect of the pulsed currents on the mechanism of creation of these necks. Physico-chemical mechanisms that govern the densification and grain growth kinetics of alumina were also studied. Microstructural analyzes reveal that the sintering paths are influenced by the heating rates, and that densification at low temperature is governed by fast mechanisms of diffusion such as grain boundaries diffusion. While at high temperature, the densification seems to be controlled by slow diffusion mechanisms such as volume diffusion and grain boundary sliding.

10:40 AM  Invited
Densification and Preservation of the Ceramic Nanocrystalline Character by Spark Plasma Sintering: Rachman Chaim¹; Rachel Marder¹; Claude Estournes²; Zhijian Shen³; ¹Technion - Israel Institute of Technology; ²CNRS, Institut Carnot Cirimat; ³Stockholm University
    Spark plasma sintering (SPS) is a hot-pressing technique where rapid heating by dc electric pulses and applied pressure were used for rapid densification of ceramic nano-powders. The main densification of the green compact into the intermediate stage sintering occurs during the heating by particle sliding and rotation. Further densification to the final stage sintering may take place by plastic yield of the nanoparticles. Full densification at the final stage sintering is associated with diffusional processes. Nanoparticle sliding and rotation during the heating may lead to grain growth with faster kinetics than normal grain growth at high temperatures. Therefore, densification of the ceramic nanoparticles to fully dense nanocrystalline compact necessitates optimization of the SPS temperature – pressure – time. Here we systematically consider the different aspects of the processes and the material properties, during the SPS of the nanoparticles, and define their respective effects and control through accepted materials science theories.

11:20 AM
On the Effects of Local Joule Heating during the Electric Field Assisted Sintering of Ionic Ceramics: Troy Holland¹; Umberto Anselmi-Tamburini²; Dat Quach¹; Tien Tran¹; Joanna Groza¹; Amiya Mukherjee¹; ¹University of California, Davis; ²University of Pavia
    Recent investigations regarding the role of applied fields on the grain growth and/or densification behavior of ionic ceramics are providing strong insights into explanations of the efficacy of Electric Field Assisted Sintering (EFAS). The explanations based on grain growth suppression, or locally increased kinetics, argue that local joule heating provides an energetic balance for reducing grain boundary mobility. Three aspects of the applicability of this mechanism to EFAS experiments need addressing: the thermodynamics and kinetics of the mechanism, grain boundary versus grain core conductivities in the EFAS environment, and most importantly the time dependent nature of the temperature difference between the interface regions and the grain cores. In this paper we analyze the thermal, energetic, and practical details of this process in the context of the commonly accepted stages of sintering. This investigation was supported by the Office of Naval Research. (ONR Grant #N00014-10-1-0632 Program Manager: Dr. Lawrence Kabacoff)

11:40 AM
On the Local Field Strengths during Early Stage Electric Field Assisted Sintering of Ionic Ceramics: Troy Holland¹; Umberto Anselmi-Tamburini²; Dat Quach¹; Tien Tran¹; Joanna Groza¹; Amiya Mukherjee¹; ¹University of California, Davis; ²University of Pavia
    Determination of the effects of an applied field during electric field assisted sintering of ionic ceramics must reflect the effects on the requisite stages of sintering. Here we describe the contributions of a dielectric material's polarization to the local field strengths extant during the initial stage of sintering with an applied field. Using numerical models it is shown that significant increases in the local field strengths can be expected during the initial neck formation and continue, with decreasing enhancement, through approximately half of the first stage of sintering. The field strengths achievable in the early stage sintering in Electric Field Assisted Sintering (EFAS) processing are found to be consistent with strengths at which electric fields have been shown to have contributory effects relevant to the densification behavior of ionic ceramics.