Thermogravimetric evaluation of the suitability
of precursors for MOCVD
G V Kunte, S A Shivashankar and A M Umarji,
Meas. Sci. Technol, 2008
Abstract:
A method based on the Langmuir equation for the estimation
of vapour pressure and enthalpy of sublimation of subliming
compounds is described. The variable temperature thermogravimetric/differential
thermogravimetric (TG/DTG) curve of benzoic acid is used
to arrive at the instrument parameters. Employing these
parameters, the vapour pressure-temperature curves are derived
for salicylic acid and camphor from their TG/DTG curves.
The values match well with vapour pressure data in the literature,
obtained by effusion methods. By employing the Clausius-Clapeyron
equation, the enthalpy of sublimation could be calculated.
Extending the method further, two precursors for metal-organic
chemical vapour deposition (MOCVD) of titanium oxide bis-isopropyl
bis tert-butyl 2-oxobutanoato titanium, Ti(OiPr)2(tbob)2,
and bis-oxo-bis-tertbutyl 2-oxobutanoato titanium, [TiO(tbob)2]2,
have been evaluated. The complex Ti(OiPr)2(tbob)2 is found
to be a more suitable precursor. This approach can be helpful
in quickly screening for the suitability of a compound as
a CVD precursor.
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Mechanical properties of tricalcium phosphate single
crystals grown by molten salt synthesis
Viswanath, B., Raghavan, R., Gurao, N.P., Ramamurty, U.,
Ravishankar, N.
Accepted in Acta Biomaterialia (In Press).
Abstract:
Mechanical properties of flux-grown tricalcium phosphate
(TCP) single crystals ranging in size from 50 to 75 ?m have
been characterized by performing micro- and nanoindentation
on their facets. Notwithstanding the inherent brittleness
and anisotropy, these single crystals exhibit nanoscale
plasticity in the form of pile-up around the edges of indents.
A similar plastic response was observed in hydroxyapatite
(HA) single crystals during nanoindentation in an earlier
study. The hardness and elastic modulus obtained during
nanoindentation are discussed in comparison with the polycrystalline
forms of both TCP and HA found in the literature. The indentation
fracture toughness values of TCP single crystals were found
to be higher than those of HA single crystals. The higher
values are attributed not only to the difference in crystal
structure and corresponding differences in surface energy,
but also to extensive crack bridging by ligament formation
across crack faces during crack propagation.
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Predicting the Growth of Two-Dimensional Nanostructures
Viswanath, B., Paromita Kundu, Mukherjee, B., Ravishankar,
N.
Nanotechnology 2008
Abstract:
The ability to predict the morphology of crystals formed
by chemical reactions is of fundamental importance for the
shape-controlled synthesis of nanostructures. Based on the
atomistic mechanism for crystal growth under different driving
forces, we have developed morphology diagrams to predict
regimes for the growth of two-dimensional crystals. By using
controlled reactions for crystal growth in the absence of
surfactants/capping agents, we demonstrate the validity
of this approach for the formation of 2D structures of Au,
Ag, Pt and Pd.
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A one-step technique to prepare aligned arrays
of carbon nanotubes
Pitamber Mahanandia and Karuna Kar Nanda
Nanotechnology 2008
Abstract: A simple effective pyrolysis
technique has been developed to synthesize aligned arrays
of multi-walled carbon nanotubes (MWCNTs) without using
any carrier gas in a single-stage furnace at 700 °C.
This technique eliminates nearly the entire complex and
expensive machinery associated with other extensively used
methods for preparation of CNTs such as chemical vapour
deposition (CVD) and pyrolysis. Carbon source materials
such as xylene, cyclohexane, camphor, hexane, toluene, pyridine
and benzene have been pyrolyzed separately with the catalyst
source material ferrocene to obtain aligned arrays of MWCNTs.
The synthesized CNTs have been characterized by scanning
electron microscopy (SEM), x-ray diffraction (XRD), transmission
electron microscopy (TEM), thermogravimetric analysis (TGA)
and Raman spectroscopy. In this technique, the need for
the tedious and time-consuming preparation of metal catalysts
and continuously fed carbon source material containing carrier
gas can be avoided. This method is a single-step process
where not many parameters are required to be monitored in
order to prepare aligned MWCNTs. For the production of CNTs,
the technique has great advantages such as low cost and
easy operation
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