• PDF / 914,027 Bytes
• 4 Pages / 612 x 792 pts (letter) Page_size
• 70 Downloads / 274 Views

DOWNLOAD

REPORT

---

apid Communications

In situ thermomechanical testing for micro/nanomaterials Wonmo Kang and M. Taher A. Saif, Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206 West Green Street, Urbana, Illinois 61801 Address all correspondence to M. Taher A. Saif at [email protected] (Received 14 June 2011; accepted 27 July 2011)

Abstract A novel method for the in situ thermomechanical test of micro/nanoscale samples at high temperature is presented. During the in situ test, the stage is resistively heated while the temperature is measured by a cofabricated temperature sensor. For experimental demonstration of the thermomechanical testing method, we fabricate the Micro-Electro-Mechanical Systems (MEMS) stage using silicon carbide (SiC) and carry out in situ uniaxial tests for single-crystal silicon (SCS) microsamples at temperatures from room temperature to 400 °C. We recover the known elastic modulus of SCS within 1% error in this temperature range.

Micro/nanomaterials exhibit significantly different mechanical behaviors from their bulk counterparts due to sizedependent material properties.[1–3] Although size-dependent material properties at room temperature have been extensively studied by various in situ methods[4–9] in the literature, the thermomechanical behavior of micro/nanomaterials is not well understood. For example, there are increasing numbers of experimental data for single-crystal silicon,[10–12] MgAl2O4,[13] and silicon carbide (SiC),[14] which indicate a significant reduction in brittle-to-ductile transition (BDT) temperature with a decrease in sample size. However, sizedependent BDT temperature is not conclusive as Zhu et al.[15] tested single-crystal silicon (SCS) nanowires where no ductility was observed even for nanowires with 16 nm diameter. The reason for the limited understanding and available data as well as the controversy is the lack of a robust and comprehensive in situ mechanical testing method at high temperatures that allows direct observation of the underlying mechanism for the size-dependent and temperature-sensitive material properties. In situ thermomechanical characterization of micro/nanoscale samples involves several challenges, including (i) the fabrication, handling, and gripping of small samples, (ii) high resolution in strain/stress measurements, (iii) limited available space in analytical chambers, and (iv) in situ heating of a sample with quantitative temperature measurement. For in situ thermomechanical tests, force measurement at high temperature becomes more difficult since traditional force measurement methods (load cell or strain gauge) and a MEMS-based method (microfabricated silicon force sensing beams) are often not applicable due to strong coupling between temperature and fundamental mechanisms of force measurement. Also, a large variation of temperature imposes unavoidable

challenges in the loading condition. For example, a stage and sample can have a different coefficient of thermal expansion, which may cause undesired loading/unl