P. Akhter

High spatial resolution surface potential measurements using secondary electrons

Abstract High spatial resolution surface potential and work function measurements have been made by observing the onset energy of secondary electron emission using a band-pass analyser (CMA) in an ultra high vacuum scanning electron microscope (UHV-SEM). Multiple secondary electron spectra can be displayed for closely spaced positions on the sample. Alternatively a feedback loop is used to maintain a constant surface potential as the SEM beam is scanned across the sample. Both methods have been shown to yield linear measures of surface potential, and of work function differences, with a sensitivity of

UHV-SEM study of the nucleation and growth of Ag/W(110)

The nucleation and growth of Ag on W(110) single crystals has been studied by ultra-high vacuum SEM, AES and electron backscattering techniques. The system is confirmed to grow in the Stranski-Krastanov growth mode with 2 stable monolayers (ML) in the intermediate layer. Island density measurements are reported in the temperature range 473 < T < 823 K at a deposition rate R = 0.3 ML min−1, and at T = 573 and 773 K for 0.03 < R < 0.8 ML min−1. These results are interpreted in terms of nucleation theory. They are consistent with critical nucleus sizes in the range 7–10 atoms, and binding energies of small, two-dimensional Ag clusters which are less than expected on the simplest pair-bonding arguments. More qualitative observations on defect-induced nucleation and the shape of the growing islands are also reported.

A novel approach to estimate the clear day global radiation

A model based on the modified version of Gaussian distribution function has been proposed to estimate the clear day global radiation. The model fits very well on the recorded data which is further exploited to develop a relationship between the ratio of hourly to daily global radiation and day length (time between sunrise to sunset). This has helped us to calculate the distribution of the broad-band global radiation on any clear day of the year.

Accurate microcrystallography at high spatial resolution using electron back-scattering patterns in a field emission gun scanning electron microscope

Electron back-scattering patterns (EBSPS) have been obtained in a field emission gun scanning electron microscope, and the crystallographic information they contain has been shown to come from a small volume <or approximately=20*80*10 nm. A small angle back-scattered electron detector has been used to take pictures with enhanced crystallographic contrast, and is capable of clearly delineating sub-grain boundaries in a Ni (100) crystal with misorientations of order 0.5 degrees . The EBSP technique has been applied to determine epitaxial orientation relationships between sub- mu m crystals grown in situ on a single crystal substrate (Ag/W (110)) with an accuracy of +or-0.3 degrees .

Surface studies in a UHV field emission gun scanning electron microscope

Recent instrumental advances have enabled scanning electron microscopy using a field emission gun (FEG) to be combined with an ultra high vacuum specimen chamber. The resulting instrument, the UHV‐FEG‐SEM, allows high spatial resolution microstructural and micro‐analytic information to be obtained reliably from clean surfaces for the first time.

SEM observations of caesium monolayers on polycrystalline tungsten

Abstract The large work function changes, and associated secondary electron yield changes, has made the visualization of adsorbed sub-monolayers of Cs on polycrystalline W possible by SEM and related work-function techniques. But the Cs rapidly reacts with CO in a 5 × 10 −10 Torr vacuum to produce contrast reversals, which can be changed due to redistribution of molecular positions on gentle heating. Diffusion of first-layer Cs atoms have been studied during heating up to 200°C, and the diffusion energy, E d1 , estimated as 0.50 ± 0.1 eV . A sharp monolayer boundary is formed on evaporation past a shutter, due to second-layer diffusion. This effect is observed dynamically, and interpreted to give a second-layer Cs diffusion energy, E d2 , of 0.17 ± 0.03 eV . The importance of vacuum better than 2 × 10 −10 Torr and fast experimental procedures for quantitative experiments on reactive monolayers is emphasised.

EFFECT OF COATING THICKNESS ON THE PROPERTIES OF TiN COATINGS DEPOSITED ON TOOL STEELS USING CATHODIC ARC PVD TECHNIQUE

Titanium nitride (TiN) widely used as hard coating material, was coated on tool steels, namely on high-speed steel (HSS) and D2 tool steel by physical vapor deposition method. The study concentrated on cathodic arc physical vapor deposition (CAPVD), a technique used for the deposition of hard coatings for tooling applications, and which has many advantages. The main drawback of this technique, however, is the formation of macrodroplets (MDs) during deposition, resulting in films with rougher morphology. Various standard characterization techniques and equipment, such as electron microscopy, atomic force microscopy, hardness testing machine, scratch tester, and pin-on-disc machine, were used to analyze and quantify the following properties and parameters: surface morphology, thickness, hardness, adhesion, and coefficient of friction (COF) of the deposited coatings. Surface morphology revealed that the MDs produced during the etching stage, protruded through the TiN film, resulting in film with deteriorated surface features. Both coating thickness and indentation loads influenced the hardness of the deposited coatings. The coatings deposited on HSS exhibit better adhesion compared to those on D2 tool steel. Standard deviation indicates that the coating deposited with thickness around 6.7 μm showed the most stable trend of COF versus sliding distance.

INFLUENCE OF NITROGEN FLOW RATE ON FRICTION COEFFICIENT AND SURFACE ROUGHNESS OF TiN COATINGS DEPOSITED ON TOOL STEEL USING ARC METHOD

In the present study, the effect of various N2 gas flow rates on friction coefficient and surface roughness of TiN-coated D2 tool steel was examined by a commercially available cathodic arc physical vapor deposition (CAPVD) technique. A Pin-on-Disc test was carried out to study the Coefficient of friction (COF) versus sliding distance. A surface roughness tester measured the surface roughness parameters. The minimum values for the COF and surface roughness were recorded at a N2 gas flow rate of 200 sccm. The increase in the COF and surface roughness at a N2 gas flow rate of 100 sccm was mainly attributed to an increase in both size and number of titanium particles, whereas the increase at 300 sccm was attributed to a larger number of growth defects generated during the coating process. These ideas make it possible to optimize the coating properties as a function of N2 gas flow rate for specific applications, e.g. cutting tools for automobiles, aircraft, and various mechanical parts.

On the measurement of clear day solar irradiance components at Islamabad

An experiment has been designed where only one instrument (pyranometer) has been used to record direct, total and global irradiances on an half hourly basis on every 15th or nearest clear day of each month and on four special days when the sun was passing through an equinox or solstice. The results have been exploited to calculate the diffuse components of total, diffuse and direct components of global radiation, their percentages and sun hour durations, etc. The correlations between the length of the day, sun hours and noontime intensities of different components have been discussed. Also a simple approach has been developed to observe the changes in the clear day atmospheric turbidity over the day and also over the year.

Comments on Stranski-Krastanov Growth of Bi and Ag on Si(100)

T. Yamada and H. Abe (Jpn. J. Appl. Phys. 29 (1990) L1192) have interpreted their results of adsorption of Bi and Ag on Si(100) as Stranski-Krastanov growth. We have analysed their data and have shown that both the metals grow in island mode instead of Stranski-Krastanov.