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44245-G10
Novel Boron-based Nanomaterials for Thermoelectric Energy Conversion
During the period from September 1, 2006 to August 31, 2007, the following tasks have been completed.
Experimental set-up:
Chemical vapor deposition using B2H6 as the boron source is the primary technique used to produce proposed boron-based one-dimensional (1D) nanostructures.
Two low pressure chemical vapor deposition LPCVD systems, designed and assembled for synthesis of boron-based one-dimensional (1D) nanostructures have been set up in the PI's lab. The reaction chamber of one system is a quartz tube located between two semi-cylindrical ceramic fiber heaters. The reaction chamber of the other system is a quartz tube between two infrared heaters. All systems can be operated in the temperature range of 20°C to 1100°C, and in the pressure range from a few mTorr to several atm. However, the systems with infrared heaters can achieve quick (< 10 minutes from room temperature to 1100 °C) and localized (<1 inch2 area) heating, in favor of synthesizing uniform 1D nanostructures. A special nozzle system designed to facilitate the doping of boron-based 1D nanostructures has also been implemented.
Synthesis and characterization of alkaline-earth metal hexaboride (MB6, M=Sr, Ba) 1D nanostructures:
The MB6 1D nanostructures are appealing candidates for high temperature thermoelectric energy conversion. Catalyst-assisted growth of MB6 1D nanostructures was achieved by pyrolysis of B2H6 over alkaline-earth metal oxide (MO) or alkaline-earth metal carbonate (MCO3) powders at elevated temperature (~890-960 ºC) and low pressure (~165 mTorr). Nickel, gold and palladium are effective catalytic materials. The as-synthesized MB6 1D nanostructures were characterized by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. Results (Fig. 1) show that the MB6 nanostructures are single crystalline with preferred growth direction along [001]. The MB6 nanostructures are several tens of nanometer in diameter and up to ten micrometer in length. The growth of these nanostructures involved both vapor-liquid-solid and vapor-solid growth mechanisms. A manuscript based on the finding from this project is currently under preparation, and will be submitted this October.
Figure 1. Preliminary results of as-synthesized MB6 (M=Sr, Ba) 1D nanostructures. (a) An SEM image of as-synthesized BaB6 1D nanostructures. (b) An HRTEM image shows one BaB6 1D nanostructures having a crystalline core and amorphous surface sheath. (c) EDX spectra show the compositional information from both core and sheath. A new seed project: A seed project titled “Design and Instrumentation of a Micro-Compression Apparatus in a Scanning Electron Microscope” has started using the funding from this ACS-PRF award. The motivation of the project is to study the mechanical properties of boron-based 1D nanostructures. Good thermoelectric materials not only need to have remarkable thermoelectric figure of merit, but also need to be robust. Therefore, it is important to study their mechanical properties. The seed project focuses on design and implementation of an ultra high precision compression apparatus for probing micro/nano scale mechanical properties of materials. To visualize the deformation processes in real time, this device is designed to operate in a scanning electron microscope. The project is a joint project with two other colleagues at UNC Charlotte. Training of a SUMR scholar: The SUMR scholar: Ms. Nise Kaneza worked on preparation of large area (~4 cm2) of catalytic particles arrays by Nanosphere Lithography. The catalytic particles arrays will be used to facilitate controlled-growth of boron-based 1D nanostructures. During the summer, Nise not only established her research skills, but also got chances to improve her presentation skills by attending the final oral presentation in a departmental-level Summer Research Experience for Undergraduates (SREU) program which the PI serves as a committee member to design and coordinate the program. 
