Preparation and Characterization Techniques Used for Bulk and Thin Films of MgTiO 3
2.2 Characterization techniques used for the bulk MTO ceramics
2.1.9 Liquid phase sintering
When a wetting liquid is present, bulk viscous flow can cause volume shrinkage [11].
When liquid coats at each grain, the material can often be sintered to a higher density at lower temperatures with less of a tendency for exaggerated grain growth. Less than 1 vol%
liquid phase is sufficient to coat the grains, if the liquid is distributed uniformly and grain size is about 1 µm. The wetting liquid concentrates at the particle contacts and forms a meniscus, which exerts an effective compressive pressure on the compact. There is a rapid rearrangement of particles into higher density configuration. After the initial rearrangements, further densification takes place as particle contacts flatten under the compressive stress applied to the point contacts by capillary pressure. The contact flattening occurs through dissolution at the particle contacts and transport of the materials towards stress free interfaces. This leads to appreciable grain growth compared to solid state sintering.
Figure 2.2: Schematic block diagram of X-ray diffractometer
In the present work, two types of X - ray diffractometeres were used to characterize the samples. One is Seifert X - ray diffractometer (XRD) with CuKα (λ =1.5406 Å) radiation operating at 40 kV and 30 mA (1.2 kW). The other one is Rigaku (TTRAX-III, 18 kW) X-ray diffractometer with CuKα (λ = 1.5406 Å) radiation. Calibration using a Si standard was done to account for the instrumental line broadening and the value was approximately 0.15o.
The XRD patterns were analyzed with the help of Reitveld refinement method using Fullprof program [13]. The background was refined using a polynomial function.
2.2.2 Surface area analyzer
The surface area of the prepared nanopowders was measured using the Brunauer, Emmett and Teller (BET) theory by gas adsorption [14]. The schematic block diagram of a surface area analyzer is shown in Figure 2.3. The specific surface area of a powder is determined by physical adsorption of a gas on the surface of the solid. The amount of adsorbed gas was calculated corresponding to a monomolecular layer on the surface.
Physical adsorption results from relatively weak forces (Vander waals forces) between the adsorbed gas molecules and the adsorbent surface area of the test powder. The determination is usually carried out at the temperature of liquid nitrogen (77 K) so - called adsorption isotherms (BET isotherm). The amount of gas adsorbed can be measured by a volumetric or continuous flow procedure. Before determining the specific surface area of the sample, it is
necessary to remove gases and vapors that may have become physically adsorbed onto the surface after manufacture and during treatment, handling and storage are called out gassing.
In the present work, surface area of the samples prepared in semi alkoxide precursor method were obtained using surface area analyzer Autosorb iQ automated gas sorption analyzer (Quantachrome instruments).
Figure 2.3: A schematic block diagram of surface area analyzer.
2.2.3 Particle size analyzer
The particle size distribution of calcined powders was measured using electrophoretic light scattering method [15]. It is the method used to determine the velocity of the particles suspended in a liquid medium under an applied electric field. In order to determine the speed of the particles movement, the particles are irradiated with a laser light, and the scattered light emitted from the particles is detected. Since the frequency of the scattered light is shifted from the incident light in proportion to the speed of the particles movement, the electrophoretic mobility of the particles could be measured from the frequency shift of the scattered light. In the present work, Delsa Nano C (Beckman coulter, Model no: A53878) was used to measure the particle size of the calcined powders using semi alkoxide precursor method.
2.2.4 Density Measurement
The densities of the sintered samples were measured by Archimedes's setup along with weighing machine. According to Archimedes's principle, when a body is immersed partially or fully in a fluid, it experiences an upward force (buoyant force) that is equal to the weight of the liquid displace by it. The apparent densities of the sintered pellets calculated using the following expression
3 3
2
1 w gm/cm
a w w
w ρ
ρ ×
= − (2.5) where w1 is the weight of the sintered pellet in air,
w2 is the weight of the sintered pellet immersed in liquid medium (distilled water)
w3 is the weight of the sintered pellet after removed from liquid medium (distilled water) and ρw density of liquid medium (water =1 gm/cm3). The relative densities of sintered pellets were calculated using the formula
density Relativedensity Theoriticaρal
= (2.6) 2.2.5 Scanning electron microscopy
The schematic block diagram of Scanning Electron Microscope (SEM) is shown in Figure 2.4. In SEM, electrons are thermionically emitted from a tungsten cathode and are accelerated towards an anode. Alternatively, electrons can be emitted via field emission (FE).
The electron beam, which typically has an energy ranging from a few hundred eV to 50 keV, is focused by one or two condenser lenses into a beam with a fine focal spot size of 1 nm to 5 nm. The beam passes through pairs of scanning coils in the objective lens, which deflect the beam in a raster fashion over a rectangular area of the sample surface. Through these scattering events, the primary electron beam effectively spreads and fills a teardrop - shaped volume, known as the interaction volume, extending from less than 100 nm to around 5 µm into the surface. Interactions in this region lead to the subsequent emission of electrons which are then detected to produce an image. X - rays, which are also produced by the interaction of electrons with the sample, may as well be detected in an SEM equipped for energy-dispersive
X - ray spectroscopy or wavelength dispersive X - ray spectroscopy. The most common imaging mode monitors low energy (< 50 eV) secondary electrons. Due to their low energy, these electrons originate within a few nanometers from the surface. The electrons are detected by a scintillator -photomultiplier device, and the resulting signal is rendered into a two-dimensional intensity distribution that can be viewed and saved as a digital image. This process relies on a raster - scanned primary beam.
Figure 2.4: Schematic diagram of SEM.
The brightness of the signal depends on the number of secondary electrons reaching the detector. If the beam enters the sample perpendicular to the surface, then the activated region is uniform about the axis of the beam and a certain number of electrons "escape" from within the sample. As the angle of incidence increases, the "escape" distance of one side of the beam will decrease, and more secondary electrons will be emitted. Thus steep surfaces and edges tend to be brighter than flat surfaces, which results in images with a well - defined, three - dimensional appearance. Using this technique, resolutions less than 1 nm is possible.
In addition to the secondary electrons, backscattered electrons can also be detected.
Backscattered electrons may be used to detect contrast between areas with different chemical compositions.