Showing papers by "Xingcheng Xiao published in 2005"

Thermionic field emission from nanocrystalline diamond-coated silicon tip arrays

Abstract: Thermionic field emission properties of nitrogen doped ultrananocrystalline diamond (UNCD) coated silicon tip arrays are examined using thermionic field emission electron microscopy (TFEEM). Nitrogen doping has been shown to enhance the emission properties of diamond by the introduction of a donor level $1.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the conduction band minimum. The field enhancing geometry of the films initiates accelerated electron emission at the tipped structures which may be beneficial to thermionic energy converter design where space charge effects can significantly limit attainable current densities. Two temperature regimes of electron emission are observed; $600--800\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, where the emission is enabled because of the H passivation and $900--1100\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, where the emission is attributed to tunneling from nitrogen related states through the barrier of a clean diamond surface.

Cover Picture: Synthesis of a Self‐Assembled Hybrid of Ultrananocrystalline Diamond and Carbon Nanotubes (Adv. Mater. 12/2005)

Abstract: The cover shows self-assembled hybrids of ultrananocrystalline diamond (UNCD) and carbon nanotubes (CNTs). These hybrids were successfully prepared by their simultaneous growth within an argon-rich Ar/CH4 plasma, in work reported on p. 1496 by Carlisle and co-workers. Various methods demonstrated the coexistence of UNCD and CNTs, and the capability of controlling the relative fraction and configuration of UNCD and CNTs in the hybrid material. This new synthesis pathway enables the development of new nanocarbons with unique mechanical, tribological, and electrochemical properties.

A comparison of mechanical properties of three MEMS materials - Silicon carbide, ultrananocrystalline diamond, and hydrogen-free tetrahedral amorphous carbon (Ta-C)

Abstract: Many MEMS devices are based on polysilicon because of the current availability of surface micromachining technology. However, polysilicon is not the best choice for devices where extensive sliding and/or thermal fields are applied due to its chemical, mechanical and tribological properties. In this work, we investigated the mechanical properties of three new materials for MEMS/NEMS devices: silicon carbide (SiC) from Case Western Reserve University (CWRU), ultrananocrystalline diamond (UNCD) from Argonne National Laboratory (ANL), and hydrogen-free tetrahedral amorphous carbon (ta-C) from Sandia National Laboratories (SNL). Young's modulus, characteristic strength, fracture toughness, and theoretical strength were measured for these three materials using only one testing methodology - the Membrane Deflection Experiment (MDE) developed at Northwestern University. The measured values of Young's modulus were 430GPa, 960GPa, and 800GPa for SiC, UNCD, and ta-C, repectively. Fracture toughness measurments resulted in values of 3.2, 4.5, and 6.2 MPa x m{sup 1/2}, respectively. The strengths were found to follow a Weibull distribution but their scaling was found to be controlled by different specimen size parameters. Therefore, a cross comparison of the strengths is not fully meaningful. We instead propose to compare their theoretical strengths as determined by employing Novozhilov fracture criterion. The estimated theoretical strength formore » SiC is 10.6GPa at a characteristic length of 58nm, for UNCD is 18.6GPa at a characteristic length of 37nm, and for ta-C is 25.4GPa at a characteristic length of 38nm. The techniques used to obtained these results as well as microscopic fractographic analyses are summarized in the article. We also highlight the importance of characterizing mechanical properties of MEMS materials by means of only one simple and accurate experimental technique.« less