Structure and properties of SiC nanowires and SiC-CNT-diamond composites obtained at high pressure and high temperature

1. Structure and properties of SiC nanowires and SiC-CNT-diamond composites obtained at high pressure and high temperature T. Waldek Zerda, Yuejian Wang, and Bogdan PaloszDepartment of Physics, TCU, Fort Worth, TX 76129 . Introduction Diamond-SiC composites are extremely hard materials but their applications are limited by relatively low fracture toughness. Diamond composites are produced from micron size diamond and silicon at high pressure and high temperature. SiC is obtained due to a chemical reaction between diamond and silicon. Experiments showed that composite fracture toughness increases with decreasing size of SiC crystallites. The infiltration technique produces 50 to 1 mm crystallites, high impact ball milling may lead to 50 nm crystallites, and to seek further reduction in SiC sizes we plan to include carbon nanotubes in the production process. During this study we realized that silicon reacts readily with carbon nanotubes to produce SiC nanowires. Therefore, we produced SiC nanowires and studied their physical and mechanical properties. SiC nanowires • When Si was in the liquid phase we produced silicon carbide coated carbon nanotubes. • The core composed of carbon nanotubes survived heating in oxygen at 1000 C. • When Si was in the solid phase we produced pure SiC nanowires of diameter about 20 nm. SiC nanowires were obtained at normal pressure by sintering CNT and Si at high temperatures TEM Carbon nanotubes SiC 800 C Raman spectra of specimens produced at high (A) and (low (B) temperatures 1200 C Materials and sintering technique Silicon nanopowder of particle size 30-50 nm and purity better than 98% and carbon multiwall nanotubes of outside diameters between 60 nm and 100 nm and length up to several microns were mixed together by high energy sonication in ethanol. Below is a TEM image of the mixture. Si particles and CNT are well mixed. After drying, the mixtures were a) sintered directly in a toroid high pressure cell at 2 and 8 GPa and temperatures between 1500 K and 1970 K. b) micron size diamond crystals were added to the Si-carbon nanotubes mixture and the new mixture was sintered at 2, 5.5 and 8 GPa and T=1970 K. X-Ray diffraction patterns of : a: initial mixture b: sintered at 1473 K. c: sintered sample after oxidization d: after all treatments (sintering, oxidization, chemical washing) toroids High pressure press

2. Publications Raman spectra of silicon carbide small particles and nanowires, M. Wieligor, Y. Wang, T. Zerda J. Phys. Condens. Matter 17, 2387 (2005) Solid state reation between carbon nanotubes and nanocrystalline silicon under high pressure and high temperature, Y. Wang, T. W. Zerda J.Phys. Condens. Matter, 18, 2995 (2006) SiC-CNT nanocomposites, high pressure reaction synthesis and characterization, Y. Wang, G. Voronin, A. Winiarski, T. W. Zerda J. Phys. Condens. Matter, 18, 275 (2006) Kinetics of SiC nanowire formation Plot of degree of SiC yield vs. time. Solid, dash, and dot lines represent the best fit of the Avrami-Erofeev equation to experimental data for the (111) reflections at 1270 K, 1320 K, and 1370 K, respectively. In each case m=0.4. X-Ray diffraction pattern of sample obtained at 2 GPa, 1370 K and sintering for 60s. The insert represents measured (open circles) and fitted (solid line) diffraction profiles of SiC (220) using Voigt function. From peak intensities we calculated concentration of SiC. Activation energy is about half the value of activation energy for SiC formation from diamond and silicon (260 kJ/mol) or graphite and silicon (230 kJ/mol). • Avrami-Erofeev model • a(t)=1-exp[-kt)m] • is the degree of the reaction a=mSireacted/mSiinitial • k is the reaction rate • When m=0.5 it indicates a 1-dimensional growth • Conclusions • Different SiC nanowires can be obtained by heating mixtures of Si and CNTs. CNT coated with SiC crystallites were obtained at 1723 K. At lower temperatures pure SiC nanowires were obtained. • 2) The specific microstructure of CNT (surface defects - 5 and 7 member rings) leads to a lower activation energy of the reaction with Si than with other forms of carbon (diamond and graphite). • CNT: 97 kJ/mol • Graphite: 230 kJ/mol • Diamond: 260 kJ/mol • 3) SiC/CNT composites were produced for the first time and showed many promising properties, such as high fracture toughness and low density. • 4) Diamond/SiC/CNT composites were obtained and characterized. CNT is a promising candidate to enhance fracture toughness of ceramic. Further reduction of SiC crystalline sizes is needed. SiC – CNT composites Diamond–SiC-CNT composites Microstrain was evaluated from x-ray patterns 8 GPa 1970 K 2 GPa 1770 K Initial mixture 2 GPa SiC CNT Mechanical properties of diamond-SiC-CNT composites ♦ CNT ■ Si, ▼SiC 8 GPa X-ray patterns of composites Raman spectrum of composite Mechanical properties of diamond-SiC-CNT composites The domain sizes of diamond decrease slightly with increasing temperature. However, for the SiC phase, the crystallites grow rapidly with temperature. For example, at the final temperature, the diamond domain size decrease by less than 20% compared to the starting materials. At the same time, the SiC domain size increases by more than 90%. Both hardness and fracture toughness increase with increasing temperature. Composites manufactured at 8 GPa have large density, and their high fracture toughness is related to reinforcing properties of carbon nanotubes. Acknowledgments This work was supported by NSF DMR-0502136 grant.