CONTENTS 3.2. Procedure. 5 4. Result and

CONTENTS
 
1.    Aim.. 2
2.    Theory. 2
2.1.     Priniple of SEM… 2
2.2.     Interaction of electron beam and sample. 3
2.3.     CONTRAST. 4
3.    Experiment 5
3.1.     Device and Samples. 5
3.2.     Procedure. 5
4.    Result and Discussion. 6
4.1.     NiTi Sample. 6
4.2.     Ceramic Sample. 8
4.3.     ZnO Tertrapodal Structure. 11
5.    Conclusion. 14
I Bibliography. 15
II List of Figures. 16
 

 

 

 

 

 

 

 

 

1.    Aim

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To gain knowledge on the basics and working principle of a scanning electron microscope. Fracture Study and analysis of the given NiTi sample, microscopic analysis and fracture behaviour comparison of ceramic and ZnO tetra pods are done.

 

2.    Theory

 

2.1.         Priniple of SEM

Scanning electron microscopy can be explained by the construction which starts with the emission of high energy beam of electrons from the cathode which passes through the Wehnelt cylinder. This cylinder which is at slightly higher negative potential accelerates the electrons and its made to pass through the anode. The beam passing through the anode travels towards condenser lens. The condenser lens further reduces the beam diameter. They focus the beam to the deflection coil which deflects the beam on to the sample. To vary the magnification, the current in the deflection coil is either increased or decreased. The imaging process happens through raster type scanning.

The Signal Processing System consists of various detectors like Everhart Thornley Detector and in-lens detector. A potential of approximately 300 KV to 400 KV is applied between the specimen and the collector. A high voltage of approximately KV is applied to the collector and the scintillator. This further accelerates the secondary electrons towards the scintillator. The photons thus produced passes through the optical fibre and hit the photo multiplier which results in the production of electrons.

 

 

2.2.         Interaction of electron beam and sample

The scanning electron microscope uses a beam of high energy electrons to produce different kinds of signals. The electron beam, accelerated through high voltages interacts with the surface layers of the specimen which results in the release of Secondary electrons (SE), Back scattered electrons (BSE), X-rays, Cathodoluminescence (CL) and Auger electrons (AE). This zone of interaction is called the “Interaction volume” and its size depends on the energy of the electron. Figure 1 shows us the interaction between the primary electron beam and the various types of electrons produced thereafter.

Secondary electrons (SE) are produced from the in-elastic scattering between the electron beam and the sample. The secondary electrons (SE) gives information on the topography of the sample. The peaks and edges appear brighter in the image. The escape level of SE usually varies for metals and insulators. For metals it is 5 nm, while for insulators it is 50 nm. (1)

The electrons which are produced at the point of impact of the primary electron beam are secondary electrons (SE1) which are approximately more than half of all electrons produced. The secondary electrons which are produced approximately 0.1 microns to some microns form the point of impact are SE2.

The electrons which have high energy undergoes elastic interaction in the electron diffusion cloud are the back scattered electrons (BSE). When the BSE hits the wall of the specimen chamber, the third kind of secondary electrons(SE3) is produced.

The secondary electrons are classified by the escape energy which is less than or equal to 50 eV while the escape energy of back scattered electron is greater than or equal to 50 eV.

When an electron jumps from a higher energy state to the lower most shell (K- Shell) of an ionized atom, it relases energy which is further absorbed by an electron in the outer most shell which leaves the atom as an auger electron. This is the principle behind the production of Auger electrons.

 

Figure 1: Interaction between the primary electron beam and the specimen (2)

 

2.3.         CONTRAST

There are different types of contrast that occur and can be seen in the SEM imaging. The contrast which occurs when the beam of electrons is incident on the sample at an angle less than 90°, it is called as an inclination contrast. But when the beam of electrons penetrates the surface structures such as tips, edges at the sample, it is called as edge contrast. This is more clearly visible as a bright part in SEM images. These bright and dark regions in the SEM image are due to differene in the number of electrons coming to the detectors. When more number of the electrons is detected, it appears as a bright region while if less number of the electrons is detected it appears as a dark region.

 

 

3.    Experiment

 

3.1.         Device and Samples

 

·         SEM: Ultra Plus, ZEISS

·         Sample 1: Nickel Titanium (NiTi)

·         Sample 2: Ceramic

·         Sample 3: ZnO

 

3.2.         Procedure

In the first step, samples were placed on the sample holder and vacuum inside the chambers was noted. In the next step, the specimen exchange chamber (S.E.C.) was vented and the samples were placed inside the S.E.C. Now the S.E.C. door was closed, vacuumed and the air tight door to the specimen chamber (S.C.) was opened. The samples were then placed inside S.C. from S.E.C. through the sample holder. The air tight door of S.C. was closed afterward. A high accelerating voltage was applied and the display power was turned on. The samples were then focused from lower to higher magnification and adjusted with different brightness and contrast to capture the picture. The images were taken by varying different parameters such as magnification, accelerating voltage and working distance. To remove the samples again, the air tight door to S.C. was opened. The samples were transferred to the S.E.C. Then the air tight door to S.C. was closed. The S.E.C was vented.

 

 

 

 

 

4.    Result and Discussion

The following images were obtained at different conditions – different magnification, sites and different intensities.

4.1.         NiTi Sample

Figure 2: SEM-Image of NiTi-sample made under a  magnification of 1500 X

 

The given NiTi sample is fractured due to tensile testing which is very evident from the necking of the sample.  Figure 2 shows an overall SEM image of the sample. Using the ImageJ software the image was post-processed to analyse its dimensions. It was found that the edge of the NiTi sample has an average width of 22.725 µm. Figure 3 shows an SEM image under a magnification of 10000 X. Here we can observe two features – small voids in between cavities (white arrow) and edge contrast (black arrow). Presence of edge constrast is very evident from the fact that the surface from the edge of the sample appears bright when compared to the remaining regions of the image. These voids are very possibly due to the manufacturing process, because when melting the elemental nickel and titanium ingots, the speed of diffusion of nickel atoms into the titanium is different from that of titanium atoms inside the nickel.

This accounts for the presence of voids known as Kirkendall porosities (or Kirkendall effect). (3)

Figure 3: SEM-Image of NiTi-sample made under a magnification of 10 KX

 

Figure 4 shows a high magnification image of the dimpled surface. The surface also shows other defects like cavities and longitudinal peaks. These defects made the material more vulnerable to fracture, serving as notches that would act as a stress concentrated area. NiTi is a shape memory material which undergoes a stress-induced phase transition from austenite to martensite (4), along the edge shows it is tending to smudge towards the right.

   Figure 4: SEM-Image of NiTi-sample made under a magnification of 15 KX

 

4.2.         Ceramic Sample

Figure 5: SEM-Image of ceramic sample made under a magnification of 1500 X

.

Figure 6: SEM-Image of ceramic sample made under a magnification of 1000 X showing the image as perceived from the SE2 detector

 

Figure 7: SEM-Image of ceramic sample made under a magnification of 1000 X showing the image as perceived from the in-lens detector

Figure 8: SEM-Image of ceramic sample made under a magnification of 15 KX

 

Figure 5 shows an overview of the orientation of the grains in the ceramic sample. There are voids and cracks that are homogenously spread all over the sample surface. Figure 6 shows one of the cracks as perceived through the SE2 detector. Also, we can see that network of small cracks emerges from the big crack. Figure 7 shows the same crack as perceived through the in-lens detector. The advantage of using an in-lens detector is that we can observe and analyse to a greater depth of the defects. While the in-lens detector is not good enough for topographical analysis. Figure 8 shows a high magnified image of the ceramic sample. We observe some striations like structures (as shown by the black arrow) and some crater with concentric circles (as shown by the white arrow). 

The ceramic sample was produced by sintering at 1500°C. During this process, the grains recovers the stress induced, which results in recrystallization and grain growth. During the sintering process, the free volume of the sample decreases, leading to the formation of twinning. As we cool down the sample, it undergoes a phase transition. Twinning occurs in order to retain the shape of the structure. The crater like defects could also have arisen during this cooling process. The curves on these craters are created or are formed at an earlier stage than the twinning. Because these occur during grain growth which happens during the diffusion of atoms and it is in the most energy favourable state.

 

4.3.         ZnO Tertrapodal Structure

 

The ZnO sample is analysed for its tetrapodal growth and analysed. The ZnO sample usually exhibits a typical hexagonal-wurtzite crystal structure. This offers very easy growth of different one-dimensional (1D) nano- and microstructures. (5) These crystals consist of a ZnO core in the zinc blende structure. From the core of this zinc blende structure four ZnO arms in the wurtzite structure radiate outwards. The arms are cylindrical in shape with the hexagonal cross section and each arm are usually of equal length and diameter (6).

Figure 9:  SEM-Image the ZnO-sample under a magnification of 500 X

Figure 10: A high magnification SEM image of one of the tetrapod tip

Figure 11: SEM image of ZnO sample for the measurement of the arm length

Figure 12: SEM image of ZnO sample showing a multipod growth (Defect)

 

The size and morphology of the tetrapod nanocrystals can be varied by growth parameters such as the O2 content.  (5)

One can have control over the shape and size of tetrapods by simply varying the growth times, reaction temperature and partial pressure of oxygen. (7)

Figure 9 shows us an overview of the ZnO tetrapods when taken from a magnification of 500X. It shows inhomogenously grown tetrapods with varied shapes and sizes all over the surface. Figure 10 shows the high magnification image of the tip of a single tetrapod structure. Using the ImageJ software the dimensions of the hexagonal cross-section as well as the length of the arms of the tetrapods were found out. From the dimensions we obtained, it is quite evident that the dimensions of the hexagonal structure are unhomogenous. Figure 11 shows a magnified image of tetrapods where one particular tetrapod(marked with white lines) was considered for the measurement of its dimensions. In this case also, we see that all the arm lengths are not equal This could either be due to defects or also because we are measuring all the dimensions at a particular direction, which may result in perceiving the distance as a projection of the actual length. Figure 12 shows the result of a probable defect which has made the formation of a multipod structure.

 

5.    Conclusion

In this laboratory, we got hands on experience in the working of an SEM. Furthermore, we also learned to how SEM is used to characterize the fracture behaviour of a metallic and a ceramic sample. Learning the importance of proper selection of parameters was achieved.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I Bibliography
1. Institute of Materials science, CAU Kiel. Scanning electron microscopy (SEM). Kiel: s.n., 2017. M109.
2. Electron microscope. Wikipedia. Online Cited: 25 Jan 2018. https://en.wikipedia.org/wiki/Electron_microscope.
3. Quantitative and qualitative elemental analysis of different nickel-titanium rotary instruments by using Scanning electron microscopy and energy dispersive spectroscopy. Ounsi HF, Al-Shalan T, Salameh Z, Grandini S, Ferrari M. 53, s.l. : J Endod, 2008, Vol. 34. 5.
4. Direct observation of NiTi martensitic phase transformation in nanoscale volumes. Jia Ye, Raj K. Mishra, Alan R. Pelton, Andrew M. Minor. 2, s.l. : Elsevier, 2010, Vol. 58, pp. 490-498.
5. ZnO: Material, Physics and Applications. Klingshirn, Prof. Dr. C. 6, s.l. : Wiley, 2007, Vol. 8.
6. ZnO tetrapod nanocrystal. Marcus C. Newton, Paul A. Warburton. 5, s.l. : materials today, 2007, Vol. 10, pp. 50 – 54.
7. Morphogenesis of One-Dimensional ZnO Nano- and Microcrystals. H. Yan, R. He, J. Pham, P.Yang. 5, s.l. : Advanced Materials, 2003, Vol. 15, pp. 402 – 405.
 

 

 

 

 

 

 

II List of Figures

Figure 1: Interaction between the primary electron beam and the specimen (2) 3

Figure 2: SEM-Image of NiTi-sample made under a  magnification of 1500 X.. 5

Figure 3: SEM-Image of NiTi-sample made under a magnification of 10 KX.. 6

Figure 4: SEM-Image of NiTi-sample made under a magnification of 15 KX.. 7

Figure 5: SEM-Image of ceramic sample made under a magnification of 1500 X.. 7

Figure 6: SEM-Image of ceramic sample made under a magnification of 1000 X showing the image as perceived from the SE2 detector 8

Figure 7: SEM-Image of ceramic sample made under a magnification of 1000 X showing the image as perceived from the in-lens detector 8

Figure 8: SEM-Image of ceramic sample made under a magnification of 15 KX.. 9

Figure 9:  SEM-Image the ZnO-sample under a magnification of 500 X.. 10

Figure 10: A high magnification SEM image of one of the tetrapod tip. 11

Figure 11: SEM image of ZnO sample for the measurement of the arm length. 11

Figure 12: SEM image of ZnO sample showing a multipod growth (Defect) 12