NANOFABRICATION
Electronic device miniaturization has led to the emergence of nanotechnology research. The ability to obtain variations in device properties due to the nanodimensional spatial restrictions has stimulated the interest of those preoccupied with new applications. Although more traditional fabrication methods such a electron beam lithography and scanning probe microscopy were undisputedly favored at first, alternate nanofabrication methods emerged slowly. The top down approach made space for more inexpensive bottom up means of obtaining nanostructures.
different types of ... NANOSTRUCTURES
Nanodots
Metal or semiconductor nanodots are increasingly being associated with the emerging new generation of magnetic storage, optoelectronic or piezoelectric devices. Novel properties of devices comprising nanodots have been demonstrated for example, in photovoltaic solar cells, light-emitting diodes, varistors, and ion-insertion batteries. However, nanodots have also shifted their applications from materials science to medicine and biology, by showing far-reaching possibilities for high resolution in-vivo imaging of live cells, diagnostics or even tumor targeting.
The process of obtaining nanodots starts with identifying the particular application for which they intend to be used, and then creating the optimum nanofabrication conditions. Fluorescence microscopy, a commonly employed technique for labeling of proteins in live cells uses colloidal nanodots synthesized in organic solvents. Conversely, nanoelectromechanical systems require a solid substrate to host the nanodots. Furthermore, a regular array of dots can be beneficial for applications such as quantum computation where neighbors need to be alike and equally spaced apart.
A rather new technique for obtaining nanodots is through electron beam evaporation of a metal or semiconductor through a mask. The latter can be an anodized membrane or some other self-assembled nanopatterned template.
AFM image of alumina mask displaying a
hexagonal array of nanopores.
ZnO nanodots obtained by e-beam evaporation.
Nanowires
Uniquely structured two-dimensional networks of nanowires are fabricated in alumina templates by electrodeposition. By changing the material that is being deposited in the nanopores, several layers are obtained on top of each other. These newly engineered nanowires exhibit properties that are characteristic of the new structure, and not of the individual materials that they contain. It is expected that such arrays of nanowires can lead to a new generation of computers where storage of information is based on electron spin rather than electron charge.
Nondestructive Testing
Some of the previous nondestructive testing work comprised a
comprehensive study of elastic and plastic deformation influences on surface magnetic Barkhausen noise (MBN) which is a phenomenon that
ultimately depends upon nanoscale events in the material (e.g. steel). In
summary, MBN signal behavior displayed in the elastic range was compared to the
one observed beyond the macroscopic yield point. A mild steel specimen was deformed
plastically by successively loading it in tension up to 13.71 % longitudinal
strain. While still in the elastic range of deformation, various strain levels
displayed an abrupt increase in a parameter termed 'MBNenergy'.
Overall, the MBNenergy increased significantly in the elastic, yet
only slightly in the plastic range of deformation. Variations in the angular
dependence of the MBNenergy with strain indicated changes in
magnetic anisotropy. In this investigated case, an easy axis
of magnetization was produced parallel to the applied elastic tensile
stress, and became more pronounced with plastic stress.
In another study, several mild steel specimens were stress cycled and
analyzed after unloading to separate elastic from plastic
deformation effects. Plastic strains obtained ranged from 0.02 % to 39.69
%. The magnetic easy axis revealed during loading became less pronounced with
unloading due to the presence of axial compressive residual stresses. Build-up
of tensile residual stresses in the transverse direction increased MBNenergy
values. The increased number of large voltage pulses seen by the MBN detection
system indicated slip along planes of maximum shearing stress.
Effects of bending a mild steel specimen to 0.21 % longitudinal surface
strain were investigated magnetically, and results confirmed MBN behavior
observed during uniaxial loading. The tensile stress on the outer surface of
the specimen produced abrupt changes in MBNenergy before
the macroscopic yield point. An overall marked increase in MBNenergy
values, and the development of an easy axis of magnetization were notable.
In a more advanced study, cold rolling of nuclear reactor pressure vessel
steel specimens to reduction ratios between 0 % and 60 % caused changes in
magnetic anisotropy. Redistribution of internal stresses through reorientation
of crystallographic planes destroyed the magnetic easy axis
present in the undeformed state. The number of large voltage pulses detected
along the rolling direction increased initially. Residual stress configurations
present at higher reduction ratios restored the easy axis, and increased the
number of voltage pulses.
Results of these studies have implications for other nondestructive evaluation (NDE) techniques. The magnetic flux leakage (MFL) technique is the main beneficiary of this work's experimental findings. The volume surrounding a defect is either elastically or plastically deformed, therefore a detailed understanding of the effects of these stress states on magnetic behavior is necessary. After all, it is the strains around the defect that cause the failure of the engineering component.