Scanning Electron Microscopy (SEM) Study

Scanning electron microscopy (SEM) technique is on the mainstay among the vital techniques used to reveal the information about the topography, morphology, compositions and crystallographic information of solid samples. The scanning electron microscope uses a focused beam of high energy electrons to produce the images of the sample surface by scanning in raster scan pattern with a spatial resolution greater than 1 nm. The data are collected over a selected area of the sample surface and a 2-dimensional image of the sample is generated. SEM can also be performed on the selected point locations on the sample.

The accelerated electrons in SEM carry significant amount of kinetic energy, which is dissipated to produce variety of signals due to the interaction of electrons with the atoms in the sample. The signals include secondary electrons (SE), back scattered electrons (BSE), characteristic X-rays, visible light and heat as shown in Fig. 2.3. The SEs are produced during ionization of atoms in the material due to incident electrons. BSEs are the electrons scattered either elastically or inelastically from the sample surface. The signals detected from secondary and backscattered electrons are commonly used for imaging samples. SEs are valuable for showing morphology and topography on samples and BSEs are most valuable for illustrating contrasts in composition in multiphase samples.

Another important point in SEM study is the sample preparation. The samples must be electrically conductive because the non-conductive samples get charged when scanned by the electron beam and detect faults in the secondary electron imaging mode. So the non-conducting samples are coated with a thin layer of electrically conducting material deposited on the sample

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surface either by low vacuum sputtering or by high vacuum evaporation. The conducting material used for specimen coating include gold/ palladium alloy, graphite, platinum, tungsten, osmium, iridium, chromium, etc. Alternatively, an electrically insulating sample can be examined without a conductive coating in an instrument capable of "low vacuum" operation. SEM analysis is considered as a "non-destructive" technique as the x-rays generated by electron interactions do not lead to volume loss of the sample. So the same material can be used to analyze repeatedly [22,23].

Fig. 2.3 Interaction of the electron beam with a sample [22].

2.6.1Energy Dispersive X-ray Analysis (EDX)

EDX analysis technique is a commonly used technique in conjunction with SEM instrument to determine the chemical composition of the material. EDX analysis is based on the principle that each element has a unique atomic structure allowing unique set of peaks on its X-ray spectrum [24]. When an electron beam with energy of ~10-20 keV strikes the material, X-rays are emitted along with other electromagnetic radiations. These X-rays are generated from the atoms present in a region of material surface to a depth of 2 microns. These X-rays are detected by suitable detectors and the signals are analyzed to know the chemical composition. The limitation of EDX analysis is that elements with low atomic number (<11) cannot be detected.

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2.7 Density & Porosity Measurements

For measuring the experimental bulk density of the ceramics/composite samples Archimedes principle was used.

The formulas employed for the calculation of the density & apparent porosity of the ceramic samples are as follows:

(2.3)

(2.4)

To measure dry weight, the sintered samples were first heated to 100oC to avoid the moisture content in the samples. Then the weights of the samples were measured by a digital electronic balance. For measuring the soaked weight, the samples were put in a beaker with kerosene oil. Kerosene oil is used as it has better wetting capacity compared to water, easier for drying and suitable for hygroscopic materials. The beaker containing the samples dipped in kerosene oil was kept in a desiccator attached with a suction pump. As the air is sucked out by the pump, vacuum is created and air trapped pores of the samples are replaced by kerosene showing bubbling. When there were no more bubbles coming out of the sample, the suction pump was stopped and the vacuum was slowly released. Then the beaker was taken out of the desiccator. The soaked weight of the samples were measured by the digital electronic balance. The suspended weight of the samples were measured by suspending the samples in kerosene oil with the help of a specially designed light weight hanging pan for holding the samples inside the kerosene oil. The experimental density and porosity of the ceramic samples was calculated by using the above formulas. The given Archimedes relation cannot account the porosity of ceramic-polymer composites due to closed pores. The porosity due to the closed pores of ceramic-polymer composites can be accounted by non-destructive techniques (NDTs).

We can also use the following formulae for calculating the density, porosity and relative density of the composite samples.

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(2.6)

(2.7) Where, ρf is the density of the filler (ceramic), ρm is the density of the matrix (polymer), V is the volume fraction of the filler, P is the porosity of the composites and ρrel is the relative density of the composite samples.

The experimental density can be compared with the theoretical density for better application point of view. The theoretical density can be determined by using the lattice parameters from XRD. The formula for calculating the theoretical density is as follows,