In situ transmission electron microscopy of solid–liquid phase transition of silica encapsulated bismuth nanoparticles Jianjun Hu, * ac Yan Hong, b Chris Muratore, c Ming Su b and Andrey A. Voevodin c Received 18th April 2011, Accepted 5th June 2011 DOI: 10.1039/c1nr10394f The solid–liquid phase transition of silica encapsulated bismuth nanoparticles was studied by in situ transmission electron microscopy (TEM). The nanoparticles were prepared by a two-step chemical synthesis process involving thermal decomposition of organometallic precursors for nucleating bismuth and a sol–gel process for growing silica. The microstructural and chemical analyses of the nanoparticles were performed using high-resolution TEM, Z-contrast imaging, focused ion beam milling, and X-ray energy dispersive spectroscopy. Solid–liquid–solid phase transitions of the nanoparticles were directly recorded by electron diffractions and TEM images. The silica encapsulation of the nanoparticles prevented agglomeration and allowed particles to preserve their original volume upon melting, which is desirable for applications of phase change nanoparticles with consistently repeatable thermal properties. Introduction Phase change material (PCM) can absorb and release a signifi- cant amount of heat during solid–liquid–solid phase transitions. 1 Dissipating thermal energy into PCMs results in an isothermal process for transferring heat. Various PCMs have been studied for thermal energy storage and heat transfer applications. 2–5 The size effects at the nanoscale on melting point depression and isothermal heat absorption are well-known, 6 however the effects of nucleation at the nanoscale are not as widely reported, but are significant for the use of nanoparticle PCMs for thermal management, as investigated here. In addition, encapsulation of PCMs had been proposed to prevent their agglomeration and leakage upon melting that could result in variable thermal properties in the following melting–solidifying cycles. 2 Dielectric encapsulation of PCM nanoparticles in heat transfer fluids was required for cooling high-power-density-electronic circuits with novel microchannel heat sinks. 5 Although metallic nanoparticles are often produced by the colloidal method 7–9 and encapsulated by the sol–gel process, 10–12 there were limited reports on encap- sulation and studies of PCM nanoparticles of low-melting-point metals. Recently, we proposed the application of encapsulated PCM nanoparticles for increasing the heat capacity of a colloidal suspension at a specific temperature to transfer heat, 13 and for nano-heat-sinks in the thermal management of heterogeneous chemical reactions. 14 However, there is still interest in investi- gating the microstructural changes of PCM nanoparticles at melting temperatures. Transmission electron microscopy (TEM) is a common tech- nique to study the microstructure of static materials with high- spatial resolution. Information on the real-time response of microstructures to changes in temperature is useful for the investigation of PCM nanoparticles because their properties strongly depend on their temperature-dependent microstruc- tures. In situ TEM techniques have proved to be useful methods for direct measurement of thermodynamically driven changes in microstructure with increasing temperatures. 15,16 In this paper, we focus on phenomena observed during annealing of bare and silica-encapsulated bismuth nanoparticles in a TEM. Bismuth has a melting temperature of 271.5  C, which is higher than indium (156.60  C) but lower than lead (327.46  C). Bismuth is known for its highly anisotropic Fermi surface, long Fermi wavelength and mean-free path of carriers. A strong quantum finite-size effect has been examined in nanoscale bismuth parti- cles. 17 Silica encapsulation was applied to contain the metallic core upon melting and was chosen because of its excellent stability at high temperatures. 18 The results show the benefits of silica-encapsulation of bismuth nanoparticles for prevention of agglomeration and leakage upon melting. Such experiments have not been reported in detail or employed in PCM nanoparticle investigations before. Experimental details Bismuth acetate, tetraethoxysilane (TEOS), poly-vinyl- pyrrolidone (PVP), ammonium hydroxide (NH 4 OH) and a University of Dayton Research Institute, Dayton, Ohio, 45469, USA. E-mail: Jianjun.Hu@WPAFB.AF.MIL; Fax: +1 937 2552176; Tel: +1 937 2559073 b NanoScience Technology Center, University of Central Florida, Orlando, Florida, 32826, USA c Materials and Manufacturing Directorate, Air Force Research Laboratory (AFRL/RXBT), Wright-Patterson Air Force Base, Bldg. 654, 2941 Hobson Way, Room 10, Dayton, Ohio, 45433-7750, USA 3700 | Nanoscale, 2011, 3, 3700–3704 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < Nanoscale Cite this: Nanoscale, 2011, 3, 3700 www.rsc.org/nanoscale PAPER Published on 28 July 2011. Downloaded by Northeastern University on 03/02/2015 20:13:15. View Article Online / Journal Homepage / Table of Contents for this issue