Ultra-thin alumina films with self-ordered cylindrical vertical pores were fabricated
on a p-type silicon substrate by anodization of Al films with thickness in the
range of 30–500 nm in sulfuric or oxalic acid aqueous solutions. In both cases
the pores were arranged in hexagonal cells in a close-packed structure and their
diameter and density depended on the electrochemical solution used. In the case of
sulfuric acid both 30 and 500 nm Al films resulted in a similar uniform porous
structure using exactly the same anodization conditions for both thicknesses, the
pore diameter being in the range of 10–30 nm and their density of the order of
6–8 × 1010 pores cm−2. In the case of oxalic acid the 500 nm thick films resulted in a uniform porous structure
with larger pores than in sulfuric acid, of diameter in the range of 20–40 nm and a density
of the order of . On the other hand, with oxalic acid it was impossible to form a uniform porous structure
from the 30 nm thick Al film at the same conditions as used for the 500 nm thick
film. Plan-view and cross-sectional transmission electron microscopy was used to
investigate systematically the structure and morphology of the alumina films.
Cross-sectional TEM images showed that the alumina/Si interface was sharp,
but a void was observed beneath each pore, separated from the pore by a thin
alumina layer. The same structure was obtained with both electrolytes. The effect of
pre-annealing of the Al films on the anodic alumina layers was also investigated in
detail.
The observation of rapid reactions in nanoscale multilayers present challenges that require sophisticated analysis methods. We present high-resolution in situ x-ray diffraction analysis of reactions in nanoscale foils of Ni 0.9 V 0.1-Al using the Mythen II solid-state microstrip detector system at the Material Science beamline of the Swiss Light Source Synchrotron at Paul Scherrer Institute in Villigen, Switzerland. The results reveal the temperature evolution corresponding to the rapid formation of NiAl intermetallic phase, vanadium segregation and formation of stresses during cooling, determined at high temporal ͑0.125 ms͒ and angular ͑0.004°͒ resolution over a full angular range of 120°.
In this study, we performed simulations of self-propagating reactions of nanoscale nickel-aluminum multilayers using numerical methods. The model employs two-dimensional heat transfer equations coupled with heat generation terms from, (1) 1D parabolic growth of intermetallic phases Ni2Al3 and NiAl in the thickness direction and (2) phase transformations such as melting and peritectic reactions. The model uses temperature dependent physical and chemical data, such as interdiffusion coefficients, specific heat capacities, and enthalpy of reactions obtained from previous independent work. The equations are discretized using a lagged Crank–Nicolson method. The results show that initially, the reaction front velocity is determined by the rapid growth of Ni2Al3 and the front temperature is limited by the peritectic reaction at ∼1406 K. After the front completely traverses the foil and the temperature reaches the peritectic point, the reaction slows down and the temperature rises by the growth of NiAl which is the only stable phase at these temperatures. The reaction is completed when the initial constituents are consumed and the temperature reaches the melting point of NiAl. Subsequently, the foil cools and solidifies to the final phase dictated by the overall composition. The computational results show excellent fit to experimental velocity and temperature measurements.
In this work, we present the fabrication and full characterization of stoichiometric
SiO2
nanoisland arrays (dots) on silicon, grown through an anodic porous alumina
template. Atomic force and transmission electron microscopy (AFM, TEM) were
used to characterize the morphology, size, size distribution and density of the
dots as a function of the anodization time used. It was found that dot density is
lower for very short anodization times, and it stabilizes after a certain time. The
dot height increases rapidly after nucleation, reaching values of 8–10 nm. With
prolonged oxidation the dots continue to nucleate to fill the available area on the
silicon surface underneath the porous alumina, while the well developed dots grow
in height and width, reaching saturation values at 14 and 55 nm respectively.
X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy
(EELS) were used to investigate the stoichiometry and surface coverage of the dots.
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