Elastic moduli of grain boundaries in nanocrystalline MgO ceramics
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Rachman Chaima) Department of Materials Engineering, Technion–Israel Institute of Technology, Haifa 32000, Israel
Zhijian Shen and Mats Nygren Department of Inorganic Chemistry, BRIIE Center for Inorganic Interfacial Engineering, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden (Received 5 September 2004; accepted 13 December 2004)
Dense MgO ceramics with nanometer to submicrometer grain size were fabricated by high-temperature hot-isostatic pressing, low-temperature hot-pressing, and spark plasma sintering. The elastic properties were determined by sound wave velocity measurements. Young’s and shear moduli of nanocrystalline MgO were lower by 13% than those with submicrometer grain size. Softening of the elastic properties was analyzed and related to the lower density and lower elastic moduli of the grain boundaries compared to the crystal interior. Young’s and shear moduli of the grain boundaries were evaluated as 90 and 34 GPa, respectively. This leads to a more than 3-fold decrease in the effective elastic moduli with the decrease of grain size into the nanometer range.
I. INTRODUCTION
Elastic properties are important for understanding the physical and mechanical behavior of materials. Since these properties are microstructure dependent, their values as well as their grain size dependence become of a paramount importance for material design and application, especially in the nanometer-sized devices. Several investigations were carried out to reveal the grain size effect on the elastic properties of nanocrystalline (nc) metals1–5 and ceramics;6,7 however, only a few used fully dense materials.8,9 Young’s modulus in nanocrystalline cubic metals such as Ag, Cu, and Fe, formed by different techniques was lower by 10–20% than that of their counterpart materials with micrometer-sized grains. However, no data were reported on similar effects in fully dense nanocrystalline ceramics. Different types of atomistic simulations support the effects of the nanometer grain size on elastic properties.10–12 Calculations by the embedded-atom model have shown up to 50% decrease in the elastic moduli of the ⌺5 twist grain boundaries in copper and gold.10 Molecular dynamic simulations were used to show a 15–28% decrease in the
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0094 J. Mater. Res., Vol. 20, No. 3, Mar 2005
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Young’s modulus of nanocrystalline aggregates with high-angle grain boundaries.11,12 Fundamental understanding of such effects in ceramic systems necessitates examination of fully dense ceramics within a wide grain-size range. However, no experimental data are available on this grain size effect in fully dense nanocrystalline ceramics. This is mainly due to the difficulty of fabricating fully dense nanocrystalline ceramics at low temperatures at which nanometer grain size may be preserved. Nevertheless, recently, fully dense nanocrystalline MgO (nc-MgO) was fabricated using controlled ho
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