Amorphous Nanowires and Crystalline Thin Films of SiO 2 -Li 2 O Compounds obtained by Combustion Chemical Vapor Depositi
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Amorphous Nanowires and Crystalline Thin Films of SiO2-Li2O Compounds obtained by Combustion Chemical Vapor Deposition M. D. Lima, S.S. Stein, M.J. de Andrade, C.P. Bergmann. Federal University of Rio Grande do Sul, DEMAT, Av. Osvaldo aranha, 99/705, Bairro Centro, Porto Alegre 900135-190, Brazil
ABSTRACT Amorphous silica films deposited by Combustion Chemical Vapor Deposition (CCVD) were modified by lithium addition in the precursor solution. The modified films were characterized by X-ray diffraction and scanning and transmission electron microscopy. The addition of lithium promoted the crystallization of Li2O-SiO2 compounds, mainly crystalline phases like Li2SiO3, Li2Si2O5, quartz and cristobalite. Besides that, the morphology of the film was modified, leading to the formation of acicular structures and nanowires. The acicular structures were identified through TEM associated with SAED as crystalline phases, mainly constituted by Li2SiO3 and Li2SiO5. TEM and SEM analysis indicated that the nanowire diameter is between 20 and 80nm. In addition to this, SAED and microprobe EDS analysis indicated that these nanowires are constituted by amorphous silica. The probable growth mechanism of these nanowires is the vapor-liquid-solid (VLS) catalyzed by a liquid particulate composed by Li2O-SiO2. INTRODUCTION The CCVD technique is a relatively recent technique [1] and has advantages over the traditional methods of CVD due to its low cost of equipment and precursors employed. The energy needed to activate the nucleation and diffusion processes during the film formation is provided by a flame containing hydrogen, methane, acetylene or other combustible that does not contaminate the film. The deposition process generally occurs at open atmosphere. The precursors are dissolved in a combustible solvent, which is atomized and burned. The flame produced by the solvent could be the main heat source but, in this case, there is a second flame that acts as a pilot. The deposition of amorphous silica films by the CCVD technique was reported by Hampikian [2]. In his work, tetraethylorthosilicate (TEOS) diluted in alcohol in a concentration of 0.0025M was used as a precursor, and the films were deposited onto a Ni-20Cr substrate. Many other oxides, amorphous or crystalline, have been deposited by this technique [3]. Several investigations are concerned with the crystallization phenomena of the SiO2-Li2O system, especially in the glass-ceramics field [4,5]. The compounds formed from these two oxides have a great range of coefficient of thermal expansion (CTE). According to Strnad [6], this property could be adjusted through the control of the nature and quantity of the crystallized lithium silicates. It is then possible to create not only materials with a high thermal shock resistance, but also glass-ceramic films with good adherence and compatibility with metallic or ceramic substrates. Amorphous silica nanowires have potential applications as optical nanometric devices for light conduction and focalization. Furthermore, such s
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