Measurement of mechanical properties of alkaline-earth metal hexaboride one-dimensional nanostructures by nanoindentatio
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The divalent alkaline-earth metal hexaboride MB6 (M 5 Ca, Sr, Ba) one-dimensional (1D) nanostructures are promising n-type thermoelectric materials for high temperature power generation. Understanding fundamental physical and mechanical properties of these new nanostructures is critical for their future applications. Current work focuses on reliable study of mechanical properties of MB6 1D nanostructures by nanoindentation. Factors affecting the measured nanostructure-onsubstrate system modulus, such as the stiffness of a supporting substrate, the width and cross section of a nanostructure, the interaction between a nanostructure and a substrate, were systematically studied by both experimental investigation and numerical simulation. The intrinsic modulus of a nanostructure, extracted from the measured system modulus, was determined between two bounds set by the receding contact and the perfect bond interaction between a nanostructure and a substrate, respectively. The extracted modulus increases as the width-to-thickness ratio of a nanostructure increases from 1 to 2.
I. INTRODUCTION
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.82
Nanoindentation is capable of giving insights to both Young’s modulus and hardness of bulk elastic–plastic materials based on the Oliver–Pharr method.14 The assumptions behind this method are that the material being tested is a homogeneous half-space. Cares must be taken to extract properties of tested materials when those assumptions are broken down, such as in cases when an indentation is close to a free edge and/or a tested system is inhomogeneous.15–20 For example, Jakes et al.15 demonstrated that artifacts, due to an indentation near a free edge, could obscure the real trend of orientation dependent materials properties. A SYS (Stone, Yoder, Sproul) correlation capable of removing the artifacts was proposed. Chen et al. used a finite element method to study the effect of substrate on measurements of mechanical properties of thin films. A substrate effect factor was defined. In addition, hardness and stiffness maps were constructed, serving as a guideline in the interpretation of indentation results for thin films with extremely thin thickness (e.g., under 50 nm).20 Recently nanoindentation has been extended to study mechanical properties of 1D nanostructures.21–24 The Young’s moduli of as-tested nanostructures were usually obtained by the Oliver–Pharr method. However, during nanoindentation, individual nanostructures were laid on a substrate such as silicon. This experimental configuration obviously does not satisfy the aforementioned assumptions, indicating the Oliver–Pharr method cannot be directly used to extract the intrinsic properties of nanostructures. Since a nanoindenter senses the response of both the nanostructure and the substrate as a system, the measured modulus given by the software associated with the nanoindenter (which is
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Ó Materials Research Society 2012
The divalent alkaline-earth metal hexaborides MB6 (M 5 Ca, Sr,
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