ELECTRON MICROSCOPY

2.6. ELECTRON MICROSCOPY

2.6.1. Scanning Electron Microscope

In ‘Scanning Electron Microscopy’ (SEM), a film beam of electron (10-40KeV) is caused to scan the sample in a series of parallel tracks. These electrons interact with the samples producing sec- ondary emission of electron (SEE), the back scattered electrons (BSE), the light of cathode lumines- cence and the X-ray. Each of these signals can be detected and deployed on the screen of a cathode ray tube like a television picture. The examinations are generally made on the photographic records of the screen.

The SEM is considerably faster and gives more of three dimensional details than TEM. The samples as large as 25 mm × 25 mm can be accommodated and parts viewed at magnifications varying from 20 to 100,000 at a resolution of 15 nm - 20 nm as compared to 0.3 nm - 0.5 nm for transmission electron microscopy. Due to its great depth of focus of a particle and its surface morphology, its depth of focus is nearly 300 times that of an optical microscope. In both, the scanning electron microscope and back scattered electron modes, the particles appear to be viewed from the above.

In SEM mode, where the particles appear as diffusely illuminated, the particle size can be meas- ured and the aggregation behaviour can be studied, but there is little indication of the height. The BSE mode in which the particles appear to be illuminated from the point source gives a good impression of the height due to the shadows. Several of the current methods of particle size analysis can be adopted for the quantitative measurement of images in SEM photographic records.

2.6.2. Sample Preparation for Microstructural Study

The micro-structural evolution for sintered α-silicon carbide, for various sintering atmospheres, was studied by Scanning Electron Microscope (SEM). The ‘small chips’ of the samples are mounted in resin for grinding and polishing. The mounted samples are initially ground by alpha silicon carbide of grit size of – 200 and – 400. These are then polished with a diamond paste upto 1 micron. They are finally polished by 0.5 micron alumina powder in a vibratory polisher. The optically smooth polished surfaces are etched to reveal the full details of the microstructure according to the requirement.

Alpha Etch : sintered α-silicon carbide samples are boiled in 100 ml of water containing Murakami reagent [10 gm of Sodium Hydroxide (NaOH) and 10 gm of Potasium Ferro Cyanide (K 3 Fe(CN) 6 ] for 30 mins. This etching attacks α-phase preferentially. The microscopy of the samples was studied by scanning electron microscope (Model - JSM 5200, JEOL, Japan).

A fused salt mixture of Potassium Hydroxide and Potassium Nitride at 480°C for 5 minutes is used. This mixture preferentially attacks β- Silicon carbide revealing α/β interfaces, and also etches the grain boundaries between the β-grains. The microscopy of the samples was also studied by scanning electron microscopy (Model - JSM 5200, JEOL, Japan).

Beta Etch :

2.6.3. Transmission Electron Microscope

In ‘Transmission Electron Microscope’ (TEM), a thin specimen is irradiated with an electron beam of uniform current density : the electron energy is in the range of 60 -150 KeV (usually, 100 keV), or 200 KeV-1 MeV in case of the ‘high voltage electron microscope’ (HVEM) or ‘high resolution transmission electron microscope’ (HRTEM).

SILICON CARBIDE

93 The electrons are emitted in the electron gun by the 'thermionic emission' from tungsten cathodes

or LaB 6 rods or by the field emission from the pointed tungsten filaments. The latter are used when high gun brightness is needed. A two-stage condenser-lens system permits the variation of the illumi- nated aperture, and the area of the specimen is imaged with a three- or four-stage lens system onto a fluorescent screen. The image can be recorded in emulsion inside the vacuum.

The lens aberrations of the objective lens are so great that it is necessary to work with very small objective apertures, of the order of 10-25 mrad, to achieve a resolution of the order of 0.2 nm -0.5 nm. The bright-field contrast is produced either by the adsorption of the electrons scattered through the angles, which are larger than the objective aperture (i.e. scattering contrast), or by the interference between the scattered wave and the incident wave at the image point (i.e. phase contrast). The phase of the electron waves behind the specimen is modified by the wave aberration of the objective lens. This aberration, and the energy spread of the electron gun, which is of the order of 1-2 eV, limits the contrast transfer (i.e. Fourier transform) of high spatial frequencies.

The electrons interact strongly with the atoms by elastic and inelastic scattering. The specimen must therefore be very thin, typically of the order of 5 nm - 0.5 µµµµµm for 100 KeV electrons, depending on the density and the elemental composition of the object, and the resolution desired. The special preparation techniques are needed for this purpose.

The TEM can provide high resolution, because the elastic scattering is an interaction process that is highly localized to the region occupied by the screened Coulomb potential of an atomic nucleus, whereas the inelastic scattering is more diffuse. It spreads out over about a nanometer.

A further capability of the modern TEM is the formation of very small electron probes, 2 nm -

5 nm in diameter, by means of a three-stage ‘condenser-lens’ system, the last lens field of which is the objective pre-field in front of the specimen. This enables the instrument to operate in a scanning trans- mission mode with a resolution determined by the electron probe-diameter. This has the advantage for imaging thick or crystalline specimens, and for recording secondary electrons and back-scattered elec- trons, cathode-luminescence and electron-beam-induced currents.

The main advantage of equipping a TEM with a STEM attachment is the formation of a very small electron probe, with which the elemental analysis and micro-diffraction can be performed on extremely small areas. The X-ray production in thin foils is confined to small volumes excited by the electron probe, which is only slightly broadened by the multiple scattering. Therefore, a better ‘spatial resolution’ is obtainable for the ‘segregation effects’ at crystal interfaces or precipitates, for example, than in an X-ray micro-analyser with the bulk specimens, where the spatial resolution is limited to 0.1-

1 mm by the diameter of the electron-diffusion cloud.

2.6.4. Sample Preparation for TEM Study

For TEM study, the cylindrical specimen of 3 mm diameter and 1 mm high, which is a suitable size for the fabrication of TEM specimens, is cut directly from the bulk sintered pellets of alpha silicon carbide. The specimens are prepared from these samples by mechanical thinning to ≅ 75 µm, which is followed by dimpling and subsequent low-energy (5 to 6 kV) and low angle (15°) Ar + ion beam milling. The films are then examined in a transmission electron microscope (Model - TEM-400CX, JEOL, Japan), which was operated at an accelerating voltage of 100 KeV.

NANO MATERIALS