This project will be conducted in connection with the Nation Nuclear Laboratory to determine the envelop density of the flakes of debris that come off the side of a pipe that caries radioactive material. It is important to measure the density due to the radioactivity of a certain density will determine how it is disposed safely. Since the fragments are radioactive and fragile, a simulant low density sample like cornflake is required for experiment. Scanning electron microscopy is currently used to estimate the volume of fragments which are low atomic number material, since the density can be determined once the volume and mass are know. Alternative methods as confocal microscopy, pycnometry and X-ray tomography will also be investigated in this project.
These methods all contain limitations. The sample should be transparent of light in order to use the confocal microscopy. The pycnometry cannot measure the envelop density since the gas will fulfill the cavities inside the sample. If the sample is twisted more than 90 degrees, the image output from the X-ray tomography cannot be reconstructed due to the twisted part would not be defined.
New method called Wax-Cutting is proposed in the recommendation. Without any limitation of the sample, it becomes the most efficient and accurate method to determined the envelop density of any sample.
Contents
Figures ………………………………….……..……..………………..……................
Tables ……………………….…………..………………….……...............................
Notations …………………………………….………..………………..……............
Introduction ………………………………….…..………………..…….........
Background of Scanning electron microscope……………..
Background of Confocal Microscopy…………………………..
Background of Pycnometry………………………………………
Background of Tomography ………………………………….
Projection-based X-ray microscope…………………….
Lens-Based X-ray Microscope……………………………
Scanning X-ray Microscope……………………………
Theory……………………………………………………………………………
Scanning electron microscope (SEM)…………………………..
Wavelength of electrons………………………………
Focus method……………………………………………..
Confocal Microscopy (specific LSCM)………………………
Pycnometry ……………………………………………………………
X-Ray Tomography…………………………………………………..
Experimental Arrangements…………………………………………..
Results …………………………………….....….……………..…….........…
Thickness of the sample…………………………………………..
Calculation for Sample A…………………………………………..
Find shadow area of the sample A…………………………..
Find actual area…………………………………………………….
Calculation for Sample B…………………………………………………..
Find shadow area of sample B……………………………………
Find actual area…………………………………………………….
Discussions ………………………………………………………..…….........
Scanning electron microscope…………………………………….
Laser Scanning Confocal Microscopy……………………………….
Pycnometry……………………………………………………………
X-ray tomography……………………………………………………..
Project management……………………………………………………………
Conclusions …………………………………………...…………..…………..
Recommendations………………………………………………………….
New suggestion method……………………………………………………
Disadvantages of using cornflake as simulant sample………………
Suggestion material…………………………………………………………………
Further Recommendation experiment …………………………………….
Acknowledgements……………………………………………………………………….
References ………………………………………………………..……...……
Appendices ………...………………………………………………………………
Section numbering stops when you get to these final sections:
Tables  Each individual table or figure must be numbered with full title.
Figures 
Appendices - identified in letter series; include full titles.
Notation.
A list of symbols used, on a separate page, with definition in words and symbols. Arrange symbols alphabetically, English capitals, then lower case, and then Greek. SI units must be included where appropriate.
1. Introduction
This project is to measure the density of the fragile samples. Since the actual material is radioactive, it has been replaced by a simulant low-density sample that is cornflake. Similarly, it is fragile and does not have a well-defined geometry. Currently, the scanning electron microscopy is used to measure the 3-D surface of the sample in order to calculate the volume. However the company NNL is interested in some other way of measurement such as confocal microscopy, pycnometry and X-ray tomography.
1.1 Background of Scanning electron microscope
Scanning electron microscope has much higher resolution compare to the optical microscopy. It uses electrons instead of light to form the image. While the incident beam hits the sample, the detectors collect these X-rays, backscattered electrons and secondary electrons and convert them into a sign that is sent to the screen. Before the measurement, all water must be removed on both equipment and sample due to the vaporized. In addition, generally, all samples should be solid and conductive. In this project the cornflake needs to be made conductive by coving the sample with a thin layer of conductive material, normally gold powder. However, in a certain case, the sample does not need to be coated. When the numbers of high-speed electrons is equal to the low speed electron diffused out of the sample, it is on the electric balance. Hence, the coating process can be dispensed with.
Due the radiation come from the backscattered as well as the X-ray, the SEMs should be extremely well shielded and the exposure rate lower than 0.5mrem/hr.
Apart from SEM, confocal microscope is also a specific instrument to measure the volume of the sample. Typically, there are three varieties of c available confocal microscopes are commercially: confocal laser scanning microscopes (CLSM or LSCM), spinning-disk (Nipkow disk) confocal microscopes and programmable Array Microscopes (PAM).
1.2 Background of Confocal Microscopy
Generally, confocal microscopy allows better observation of fine details that is achieved by excluding most of the light from the specimen that is not from the microscope's focal plane. Apart from this, it is possible to build three-dimensional reconstructions of a volume of the sample by re-assembled series of images of thin slices taken along the vertical axis. (Denis, Eric R 2005)
During the measurement, only the in-focus light is detected, while the out-of-focus light is blocked by a pinhole. A pinhole is located at the image plane and front an electronic light detector. Due to this method, it only allows one point in the specimen to be focused upon at a time. The image of the specimen can be constructed by having a laser beam scan over the entire focal plane and can mechanically move the specimen to change the depth of the optical plane through the specimen.
1.3 Background of Pycnometry
Pycnometry is another method should be considered. It mainly comprises a core cell called pycnometer. Pycnometer is a container with a determined volume that used to determine the density of liquids and their dispersions. Density and dispersions can be measured by weighing the defined volumes of powders and granules. Pycnometer can also be used to determine the density of the solid phase on porous solids. Doing this, a sample must first be crushed, ground, or powdered to the point that all pores are opened. Whereas, sometimes, the sample should keep its original appearance, powder method apparent cannot achieve the target. To prevent damage the sample, the new generation pycnometer has been invented. It works by measuring the amount of displaced gas. The pressures observed upon filling the sample chamber allow computation of the sample solid phase volume. There are varieties volumes of the chamber which can be selected to provide the best with the certain samples. However, corresponding to the aim of this project, the objective is to measure the envelop density of a sample that means. It seems pycnometry cannot achieve the target.
1.4 Background of Tomography
Tomography is imaging by sections or sectioning, through the use of any kind of penetrating wave. A tomography is a device used in tomography while the image produced is a tomogram. Similar with confocal microscopy, it is based on the mathematical procedure to foam the 3-D reconstruction image. Due to different kinds of source, there are several varieties of tomogram:
Physical Phenomenon
Type of tomogram
X-Ray
CT
Gamma ray
SPECT
Radio-frequency waves
MRI
Electron-positron annihilation
PET
Electrons
Electron tomography or 3D TEM
ions
Atom probe
1.4.1Projection-based X-ray microscope
In this project, X-Ray is the penetrating wave that needed. There are three main different kinds of X-Ray Microscopy: Projection X-ray Microscope, Lens-Based X-ray Microscope and Scanning X-ray Microscope.
In a projection-based X-ray microscope, magnification is achieved by positioning the sample close to a point source of X-rays. A magnified projection image of the object is formed on the detector, while the magnification is equal to the ratio of the source - detector and source - object distance.
By rotating the sample or the point - source, a series of projection images at different angles is acquired from which the internal 3 - D structure can be formed based on tomographic reconstruction algorithms.
1.4.2 Lens-Based X-ray Microscope
Lens-Based X-ray Microscope has a similar structure to the standard light microscope; thereby it can attain much higher resolution. The X-rays emitted by the source are concentrated using a condenser lens on the sample to be imaged. After through the sample, the transmitted X-rays are imaged by an objective lens onto an area detector.
1.4.3 Scanning X-ray Microscope
In scanning X-ray Microscope; a zone plate lens is used to focus the X-ray beam to a probe, through which the specimen is raster-scanned. At each scan position, transmitted, fluorescent or diffracted X-rays can be detector, mapping the chemical, elemental or crystallographic phase properties of the sample. However, due to the requirement for higher intensity X-ray beams, the use of scanning X-ray microscopes is limited to synchrotron radiation laboratories.
2. Theory
2.1 Scanning electron microscope (SEM)
Electron microscopes were developed due to the limitation of light microscopes which are limited by the physics of light that is the wave length of the light which is much longer than the electron.
Resolution∝ =
d is the distance between two pixel
λ is the wave length
θ is the half angle of the cone of light either acceptable by the objective lens or emerging from the condenser lens and the refractive indexes of the imbibing medium between specimen and lens.
2.1.1 Wavelength of electrons
The wavelength of an electron is given by the de Broglie equation
Here h is Planck's constant and p the relativistic momentum of the electron. λ is called the de Broglie wavelength. The electrons are accelerated in an electric potential U to the desired velocity:
m0 is the mass of the electron, and e is the elementary charge. The electron wavelength is then given by:
However, in an electron microscope, the accelerating potential is usually several thousand volts causing the electron to travel at an appreciable fraction of the speed of light. An SEM may typically operate at an accelerating potential of 10,000 volts (10 kV) giving an electron velocity approximately 20% of the speed of light. It can be shown to take relativistic effects into account that the electron wavelength is then modified according to:
c is the speed of light. The first term in this final expression is regarded as the non-relativistic expression derived above, while the last term is a relativistic correction factor. The wavelength of the electrons in a 10 kV SEM is then 12.3 x 10−12 m (12.3 pm). In comparison the wavelength of X-rays usually used in X-ray diffraction is in the order of 100 pm (Cu kα: λ=154 pm).
While visible light has a wavelength in a range from about 380 or 400 nanometres to about 760 or 780 nm which is approximately 40000 times longer than the electron.
Based on the much less wave length, the scanning electron microscope has many advantages over traditional microscopes. The SEM has a large depth of field, which allows more of a specimen to be in focus at one time. The SEM also has much higher resolution, so closely spaced specimens can be magnified at much higher levels. All of these advantages, as well as the actual strikingly clear images, make the scanning electron microscope one of the most useful instruments in research today.
2.1.2 Focus method
Unlike the optical microscopy, the Scanning Electron Microscopy use electron beam instead of the normal light. Since the electron would not change the direction through the optical lens, there is a specified section called electromagnetic lenses do the same job that is to focus the beam. Based on the Lorentz force equation: , the lenses will force the electron change its direction.
Tow component to the B field.
Nonaxial electrons will experience a force both down the axis and one radial to it. Thereby, only downwards electrons can balance the radial force from all sides of the lens. The unequal force felt by off-axis electron cause spiraling about the optic axis.
2.2 Confocal Microscopy (specific laser scanning confocal microscopy)
When a light is incident on a molecule, it may absorb the light and then emit light of a different color, a process known as fluorescence. Fluorescence is a common fluorophore, emitting green light when stimulated with blue excitation light. The wavelengths of the excitation light and the color of the emitted light are material dependent. When molecules absorbed energy (like a photon of light), the electron would jump to a discrete singlet excited state. However, due to collision, the molecules drop to a lower energy level. If surround molecules not able to accept the larger energy difference needed to further lower the molecule to its ground state, it may undergo spontaneous emission, thereby, emitting light of a longer wavelength. Microscopy in the fluorescence mode has several advantages over the reflected or transmitted modes. It can be more sensitive. Consequently, it is possible to attach florescent molecules to specific parts of the specimen, making them the only visible ones in the microscope and it is also possible to use more than one type of fluorophore. Thus, by switching the excitation light different parts of the specimen can be distinguished.
A confocal microscope creates sharp images of a specimen that would otherwise appear blurred when viewed with a conventional microscope. This is achieved by excluding most of the light from the specimen that is not from the microscope's focal plane. The image has less haze and better contrast than that of a conventional microscope and represents a thin cross-section of the specimen. Thus, apart from allowing better observation of fine details it is possible to build three-dimensional (3D) reconstructions of a volume of the specimen by assembling a series of thin slices taken along the vertical axis.
2.3 Pycnometry
Initially a Pycnometer works by measuring the amount of displaced gas. The pressures observed upon filling the sample chamber gas and then discharging it into a second empty chamber allow computation of the sample solid phase volume. Gas molecules rapidly fill the tiniest pores of the sample so that the truly solid phase of the sample displaces the gas. The simplest type of gas pycnometer consists of two chambers, one (with a removable gas-tight lid) to hold the sample and a second chamber of fixed, know internal volume-referred to as the reference volume or added volume.
The device additionally comprises a valve to admit a gas under pressure to one of the chambers, a pressure measuring device usually a transducer - connected to the first chamber, a valve pathway connecting the two chambers, and a valve vent from the second of the chamber. Hence the volume of the sample can be calculated:
vs : sample volume
vc : volume of the empty sample chamber
vr : reference volume
p1 : the first pressure (in the sample chamber only)
p2 : the second (lower) pressure after expansion of the gas into the combined
volumes of sample chamber and reference chamber
2.4 X-Ray Tomography
A zone plate is a device used to focus light in X-Ray Tomography. Unlike lenses however, zone plates use diffraction instead of refraction. The zone plate's focusing ability is an extension of the Arago spot phenomenon caused by diffraction from an opaque disc. A zone plate consists of a set of radially symmetric rings, known as Fresnel zones, which alternate between opaque and transparent. Light hitting the zone plate will diffract around the opaque zones. The zones can be spaced so that the diffracted light constructively interferes at the desired focus, creating an image there. Zone plates produce equivalent diffraction patterns no matter whether the central disk is opaque or transparent (due to Babinet's principle), as long as the zones alternate in opacity.
Fresnel Zone Plate with absorbing and transmitting zones
Phase Zone Plate
Phase Fresnel Lens
Further knowledge will show up at the appendix end of the report.
3. Experimental Arrangements
Since the lack of equipments, only SEM was used in this project. Once got the sample, cornflakes, they needed to be covered with a thin layer of conductive material to make the sample to be conductive. This device is called sputter coater. The sample is placed in a small vacuum chamber that contains argon gas. When the chamber affected by the electric field, the argon ions become attracted to a negatively changed gold foil. The argon ions knock gold atoms from the surface of the gold foil. These gold atoms fall and settle onto the surface of the sample producing a thin gold coating.
While the SEM not in use, the chamber was vacuumed to protect the hinge of the door. Since the SEM which used in this experiment need a vacuum condition, any damage to the hinge may cause the SEM out of work. By holding the button VENT three seconds, the chamber would be pumped in air. When the pressure remained same between inside the chamber and outside, the button VENT would stop lighting.
In order to protect the hinge, one hand must hold the door while opening. Loading the specimen carefully and do not ever touch the lens. Enter the button EVAC to vacuum the chamber after sealing the door. Meanwhile, since the vacuum process cost about 10 minutes, several program settings can be done. Select the shape of the holder at first to ensure the sample can be detected easily by clicking the location on the screen. If the sample's height is more than approximately 2 mm, the initial height of the sample should be set to impede the crash between lens and sample. Consequential, the initial distance between sample and lens was set to 20 mm while keep other data zero. Since the vacuum process accomplished, click the button HT to activate the lens. By changing the x-y coordinate or sample dragging the image, the sample could be found on the screen. As the sample cornflake was relative huge, the lowest magnification that was 8 times need to be used in first stage.
Shoot a picture after adjusting the focus, contrast and brightness. There were four kinds of scan modules: 1.scan the central part; 2.normal scan; 3.slow scan; 4. picture. In this experiment, only 2 and 4 were needed since the project does not contain the analysis of the microstructure or the component of the sample. Since got a picture of the sample, a stereo pair can be obtained by changing the angle of the sample normally ranges from 0 to 9 degrees. Afterwards, increase the magnification to 9 times and repeat the process again. Noticed, if the sample was not totally in the image, the magnification process would be neglected. When finishing the experiment, the height of the lens should be set to 80 mm and then chamber needs to be vacuumed when the sample was moved out of the chamber. After acquired the proper images from SEM, the area of the sample can be calculated through mathematical simulation.
The next stage is to define the thickness of the sample. Base on its own geometry property, the average thickness can be obtained by measuring the overall thickness. Travelling microscopy is the instrument that was used in the experiment. Similar with the screw micrometer, the accuracy of travelling microscopy is 10 micrometer. Since it can be rotated in both x and y direction, the thickness of the sample would be defined well.
The final stage was to define the mass of the sample which is the easiest part in the experiment. Micro Balance is the equipment should be used. With its 10 microgram accuracy, the mass of the sample can be acquired successfully.
4. Results
4.1 Thickness of the sample
thickness (m)
sample 1
sample 2
1
690
524
2
1107
801
3
630
815
4
499
568
5
839
539
6
755
290
7
801
676
8
608
648
9
624
567
10
659
408
11
681
789
12
850
543
13
465
615
14
764
676
15
648
644
16
771
668
17
1004
397
18
508
702
19
565
660
20
532
544
21
363
658
22
712
804
23
393
939
24
619
634
25
437
675
26
751
675
27
460
903
28
581
386
29
404
402
30
480
451
average
640
620
4.2 Calculation for Sample A
4.2.1 Find shadow area of the sample
A`= 64.8767 cm2 (Direct drag the image into program)
A``=36.8823 cm2 (1:1 ratio from the image of SEM)
AB = 8.506 mm
CD = 7.150 mm
AB` = 6.6834 cm
CD `= 5.5287 cm
AB``= 8.7525 cm
CD``= 7.2929 cm
Ratio` = 7.7938
Ratio``= 10.2413
Area` = 0.60714 cm2
Area``= 0.61858 cm2
Area = 0.61286 cm2
4.2.2 Find actual area
Actual area = 0.62428 cm2
4.3 Calculation for Sample B
4.3.1 Find shadow area of sample B
A`= 54.0339 cm2 (Direct drag the image into program)
A``=37.2682 cm2 (1:1 ratio from the image of SEM)
4.3.2 Find actual area
AB = 6.815 mm
AC = 6.522 mm
AB` = 4.441 cm
AC `= 4.274 cm
AB``= 3.741 cm
AC``= 3.560 cm
Ratio` = 7.7938
Ratio``= 10.2413
Area` = 1.2653 cm2
Area``= 1.24378 cm2
Area = 1.25454 cm2
5. Discussion
As the project is to determine the envelop density of the sample, there is no absolutely accuracy value since the relative volume can be defined in different ways. Based on its variety, each method has its own advantages on certain samples. Furthermore, the accuracy of the relative volume is not proportional to the accuracy of the equipment. Sometimes, the relationship is even inversely. In this condition, some microscope with relative low accuracy can obtain the best value.
For the cornflakes, how to define the big open air bubbles are the core issues. As its particular shape, all the microscopes will neglect the area since there is no surface lay on the bubbles. However, due to the definition of envelop density, these kinds of areas should be included since they are mostly surrounded.
5.1 Scanning Electron Microscopy
SEM can easily handle the sample properly with large depth of field and high resolution if 2-D image is required in the experiment. However, the requirement in this project is to measure the volume, the image got from SEM can only help us to simulate the outside boundary more precisely. As the SEM cannot directly reconstruct the 3-D image from 2-D images, the error magnified due to the process of manual simulation. In the experiment, the shadow area of the sample has the accuracy as high as 1 y that is precisely enough for measuring the area. Similarly, the accuracy of travelling microscopy is 10 micrometer that is far more what the experiment need. As the property of cornflakes, the surface are fulfill with the random size pores and cavities. Due to that, the only way to minimize the error is to take as much data as possible. In the experiment, the average thickness for both samples is generated by 30 series of thickness around the sample. Furthermore, simulation is the core cell of the whole process. In this project, the sample cornflakes are treated as a piece with certain thickness so that the volume can only be devised to At where A stands for area and t stands for average thickness. Since the cavities and air bubbles are every in the sample, it is assumed that the embossment and recess take same contribution. Under this circumstance, the volume of the cornflake can be measured.
Since the advantages of SEM are due to the property of electron, the defect is the sample need to be a conductor. However, it is possible that if the sample is not a conductor theoretically.
Schematic illustration of the total electron coefficient as a function of beam energy (after Goldstein et. al.).
At the two cross point, there is no charge on the sample which means the sample do need to be conductive if the election energy satisfied the specific requirement. However, since the variety of the electron beam energy is fixed in the SEM that has been used in the experiment, it is impossible to find the electron balance point. In this case, the coating process should be included during the measurement. As the gold do not oxidation, the sample only need a layer as thin as one gold atom thickness. In which case, the volume of the sample remains constant.
As mentioned before, the lowest magnification of the SEM used in this experiment is 8 times. Even under this relatively low magnification circumstance, the sample cannot be shoot into the image completely. Furthermore, since the phenomenon of the aberration, the out circle image is not as sharp as inside which may cause problems while define the edge of the sample later. In this project, as the initial sample is too big for SEM, it is easy to get another small cornflake. However, if the sample is unique, the SEM would not help due to its lowest magnification was not low enough.
As the stereo pair image can be obtained from the SEM, it is possible to reassemble the three dimensional image of the sample theoretically. Since the two were shoot as the very same situation apart from the shooting angle, the surface height can be simulated by defined a reference point.
MeX is a strand software that turns any SEM with digital imaging into a true a surface metrology. Using stereoscopic mages the software automatically retrieves 3D information and presents a high accurate, robust and dense 3D dataset which is then used to perform traceable metrology examination. It is conceivable that if the software is used in the project, the volume of the sample can be easily obtained.
5.2 Laser Scanning Confocal Microscopy
The accuracy of LSCM relate to the specific sample. During the measurement, one of the factors of the accuracy is depended on the number of photons that pass through the sample. Meanwhile, more photons passing through, more accuracy will acquire. In the project, the cornflake can be transparent to light initially. However, since the cornflakes were coated with a gold layer, it became opaque. While measuring, the laser will hit the gold coating and simply be reflected. Under this circumstance, the confocal cannot make optical sections of the interior.
Additional limitations are imposed on the rate at which these measurements can be made by the effects of photo damage to specimen, finite source brightness, and fluorescence saturation.
Finally, limitations are imposed by the fact that continuous specimen must be measured in terms of discrete volume elements called Voxels.
However, the kind of the laser souse will also affect the accuracy. As the blue laser contains highest energy among the visible light, it is the best source of LSCM. Since it is much harder to generate a blue laser beam than other colors, the price of a LSCM using blue laser source is incredible expensive.
5.3 Pycnometry
Since theory of pycnometry is measure the gas displacement due to the change of internal volume, the cornflake sample will fulfilled with gas. As the sample contains lots pores and bubbles, the density which measured form pycnometry is essential far away from the aim of the project. Whereas, if the sample is covered with a certain layer to prevent the gas goes inside, the density would be accepted.
Consequently, the sample needs to be placed into a plastic bag which should be vacuumed before measurement. As the atmospheric pressure force the plastic bag fitting tightly, the envelop density can be calculated. Since the surface tension vary from materials, the plastic bags with different materials can be selected based on the sample's condition. If the sample has large cavities on the surface which should be contained into the relative volume, a certain plastic bag with high surface tension should be used so as to prevent the bag suck into the cavity.
5.4 X-ray tomography
Using both projection X-ray microscope and Lens-Based X-ray Microscope can achieve the aim of the project by reconstructed the image from all the angles. The resolution of projection-based X-ray microscopy is limited by the size of the point X-ray source and the resolution of the X-ray detector. Generally, the resolution of projection X-ray microscope is approximately 1 micrometer.
From figure , the system resolution vary from 0.9 micrometer to 150 micrometer due to the increasing of the point X-ray source. Furthermore, the resolutions of the detector also affect the system resolution. In line 1 and line 2, since the magnification grandly increase, the system resolution raises synchronous. This is due to the low resolution of the detector works better with larger image. However, when the resolution of the detector is high enough, the system resolution only depends on the size of the X-ray spot. When the system is on high magnification, smaller size the X-ray spot has, better resolution will get since the larger image close the gap between the differences of the detectors' resolution.
However, the X-ray tomography may not be chosen in some specific condition. If the sample is twisted more than 90 degrees, the image got from detector cannot be defined that which part is twisted. Furthermore, since the sample is fulfilled with cavities so that even the image of a plate might contain varieties grayscales, it cannot be distinguished from the level of grayscales.
6. Project management
Initially, this project was separated to two parts that were theoretical research and experiment. The theoretical research was prepared to be investigated in semester one and experiment would be left until semester two.
Generally, the investigation in semester one was fully according to the Gantt chart. Each method was evaluated successfully in three weeks. However, when the project steps into the second part, it was quite different as planed before.
Since the lack of equipments, only SEM was used during the whole project. Therefore there is no result for confocal microscopy, pycnometry and X-ray tomography. During the experiment with SEM, the initial samples were too large. Furthermore, those cornflakes were attached onto a base, so that the side view cannot be obtained. As a result, new samples were selected with small scale and fully flexibility.
Consequently, as the limitations were found during investigating the theoretical background of each method, the final results would not be acquired even some of the equipments were available to use. Whereas, if all the equipments can be used in the project, the final result would be more convinced that compared with purely theoretical analysis.
Overall, the project management can be improved by rearranging the experiment time. The available time of using those equipments should be confirmed at beginning of the semester instead of semester two so that the experiment time can be set with much more flexibility.
7. Conclusions
During the session, it is successful acquired the aim the object of the target. Each method has limitation about the sample. The specimen of the scanning electron microscopy should be conductor. Otherwise, it needs to be coated. The laser scanning confocal microscopy is only available when the sample can be transparent of light, meanwhile, it should be thick enough. Additionally, both pycnometry and X-ray tomography have no strict about the samples' physical properties. However, some samples with specific structures and geometry would not be measured by these methods. If the specimen fulfills with cavities, the pycnometry would not make sense. Whereas, the X-ray tomography cannot be used to measure those sample with more than 90 degrees twisted geometry.
Furthermore, scanning electron microscopy is one of the options if any surface of the sample is less than 1 cm2. With relative big sample, other three methods should be selected.
Generally, X-ray microscopes are particularly powerful in providing tomographic information on the 3D structure of samples too large for electron microscopy, where the resolution required is better than what visible light microscopes can deliver, or where the sample is opaque to visible light.
However, since the actual sample in the project which is a conductor, opaque to visible light and does not complex twisted geometry, both scanning electron microscopy and X-ray tomography should be chosen to measure the envelop density.
8. Recommendations
8.1 New suggestion method
Wax - Cutting, a method with simple experiment steps, easy operation, high accuracy and the most important is that this method can be regarded as a flow process.
Apparatus needed: optical microscopy with digital image output and linear cutting machine (laser cutting machine)
The process of this method is constant no matter the difference of the material or shape of the samples. Firstly, the sample should be placed in a cylinder which fulfill with the liquefied wax. The cylinder should have different scales to fit the varieties of the samples. Before injection the wax, the inside should be coated with little oil to make sure the wax can be easily taken out when solidified.
Then, the columniform wax which contains sample should be cut in to slices with constant thickness. The piece of slices variety of the shape of the sample and the accuracy acquired. More slices provide more accuracy volume meanwhile cost more time and money. After that, all the slices which contain sample should be pictured by the optical microscopy. Due to define the edges of each picture, the total amount of area can be obtained. Thus, multiplied by the thickness of the slice, the final volume can be acquired.
This method does not have any problems with the shape of the samples which may not be measured by confocal microscopy and X-ray tomography. Furthermore, there is not any limitation of the property of the sample. However, since the sample needs to be cut into slices, it cannot keep its original form. If there is some certain requirement to keep the sample remains itself, this method should not be selected.
8.2 Disadvantages of using cornflake as simulant sample
The cornflakes are twisted far more than the actual sample, sometimes even more than 90 degree that may cause problems while using the X-ray tomography.
The cornflake is not a conductor so that it needs to be coated. However, it became opaque to light so that the confocal microscopy cannot be used in the experiment.
8.3 Suggestion material
Foam Material Rohacell-51 WF
Since the foam material can be easily formed another shape, it can make a better simulation than cornflake.
The cavities and pores can be easily modified to fit the different condition.
More relatively information will showed in appendix at the end of the report.
8.4 Further Recommendation experiment
A new experiment associated to Pycnometry that is to find the relationship between the scale of the cavity and the surface tension should be lunched in order to define which material should be used.
Acknowledgements
References:
Encyclopedia.Com
Iowa State SEM Homepage
http://www.purdue.edu/rem/rs/sem.htm
SE Yield
Y Lin, D Joy and H Meyer (2005). Absolute Calibration of Auger Spectrometer for Measuring SE and BSE Yield. Microscopy and Microanalysis, 11(Suppl. 02), pp 768-769 doi:10.1017/S1431927605500886
