Experiment On X Ray Diffraction Biology Essay

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XRD is a useful method for measuring the inter-planar spacing of crystal lattices; these measurements can then be used to find the unit cell of different materials. This method relies on the regular rows of atoms so that the monochromatic beam of x-rays being used can then be diffracted by the spacing in-between atoms in the sample. This of course makes this experimental method only useful on crystalline regions of a sample.

X-rays are produced by heating a filament of tungsten at one end of a tube, which in turn fires electrons towards a copper sample. Once these electrons hit, in this analysis from the machine at around 40keV, they cause the displacement of electrons and as other electrons from higher energy shells drop down to replace these electrons they enter a lower energy state and thus must lose energy. This energy is then released in the form of x-rays.

When this happens a single source of x-rays is not formed as any combination of x-rays can form from electrons dropping from one shell to a different one. The x-rays produced are defined by their energy state which will always be the same quantised energy depending on which shell they end up in and how many energy states they dropped down to get there. In the case of CuKα this is a copper sample and the x-ray is produced by an electron dropping α (one shell) into the K (first) shell. A graphical representation of this can be seen below:

Nucleus

M

L

K

Figure x, Showing the different x-ray emission definitions from dropping energy states

Once these x-rays are emitted filters are used so that only the CuKα remains and this beam is then fired at the sample.

X-rays are difficult to manipulate but their ability to pass through most samples without losing intensity and their wavelength make them useful. This is because the wavelength (λ) is similar in size to atomic spacing and thus allows for large diffraction which is required to get good readings.

dsinθ

Figure x, showing the 2dsinθ path difference between the diffracted x-rays.x

By using Bragg's equation the size of the interplanar spacing can then be found:

[x]

Where n=the number of wavelengths in the path difference, d=interplanar spacing, θ=angle of diffraction and λ=wavelength of incident ray.

For this case study the Bragg equation (equation x) is used as all results for this experiment are done using the high angle diffraction method. Bragg's equation works by finding the placement of the constructive and destructive interference of the diffracted radiation to find d. It can be found that the intensity of the diffracted x-ray beams will only find a maximum when the path difference of two waves (2dsinθ) is equal to an integer, n, of the wavelength of the x-rays, showing constructive interference. This is why when the results are analysed there is peaks of intensity in relationship to the amount of lattice spacings in that orientation within the sample.

It should be noted all attained values will be given as double the angle which needs to be applied to Bragg's law. Therefore when the sample is put in the XRD machine it is known that the incident beam when moved by 1 degree, the detector will move by 2 degrees to keep the results equal. This is usually done by a gear shaft connecting the two within the machine.

Samples are mounted onto a zero-background substrate, which is a single crystal of silicon so it has no diffraction peak. Once mounted depending on the sample either the sample or detector and emitter may be rotated as the sample is analysed. This is so that an integration of the orientation of grains can be calculated so it is more likely to gain results. This is due to the random orientation of grains within samples so all equal lattice spacings do not face in the same axis. For this analysis a range of around 10o to 100o is used for the diffraction beam hitting the sample. This material analysis is an example of a non-destructive test and so is usually done first as it does not damage the sample.

When the results are plotted the height of the peak shows intensity is greatest when the diffraction angle is hitting the d-spacing at the exact diffraction angle. When looking at XRD graphical results it should also be noted that the sharpness of the peak is related to how crystalline the sample is. The sharper the peak is the more crystalline the material is as a perfectly crystalline material would only diffract at the diffraction angle showing a straight line within no readings either side. Broader peaks show a more amorphous sample as there will be a wide range of correct diffraction angles making a broader diffraction peak as there will be many different orientations present.

After a diffraction pattern is recorded two pieces of data are required to identify the material; the angle for d-spacing of each peak and the relative intensity. This data is then passed through the ICDD (international centre for diffraction data), to find the compound of the material. Unlike other analysis techniques this will tell you the compound of the material as each material has a specific fingerprint of d-spacings. It is found that when a composite material is put into the XRD machine that many peaks will be seen.

There is however a few errors which can occur with the use of XRD. When composite materials are analysed there can be a matching of peaks which are close to each other leading to there sometimes being more than one compound which is very similar to the fingerprint of that being analysed so it is hard to know which it is. Also when using XRD as is explained in figure x there is the emission of more than just the CuKα1 being measured. As the energy levels of CuKα2 are so close to those of CuKα1 it is impossible to remove these x-rays by filters. This means that some peaks can be broadened by a second peak of these other emissions. When the angle of diffraction is much higher these peaks can separate and be seen much more easily.

When passing data through the ICCD it is also possible for there to be a peaks missing from the data. This is due to the fact that some crystalline peaks in a composite are so low in intensity that when the data filter cuts off the amorphous halo of a sample that some peaks are lost or cannot be seen at all to begin with. Peaks can also be lost from the data if they are not within the angle of diffraction range measured by the XRD. If for example there are some critical peaks to be seen above 110o or if the XRD only goes up to 80o for a particular sample then peaks may be missed changing the fingerprint.

Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Microscopy (EDX)

SEM is a type of electron microscopy that fires a focused beam of electrons at a target sample surface. This can allow you to get a highly detailed look at the topography of a sample as well as tell you about the composition, such as in EDS. An SEM can gather many different types of data from a sample by the detection of different signals. As the focused electron beam fires at the surface of the sample you can detect secondary electrons as well as back-scattered electrons (BSE). Also importantly to do with EDX it is possible to see characteristic x-rays being produced from the sample.

Samples being prepared for SEM and EDX should not in general be larger than 2 by 2cm in area and 0.5cm in height. If the sample is not metal it most likely be needed to be coated in either a gold or chromium plasma coating. This is a very thin coating that will ensure the material is electrically conductive so that electrical charge does not build up within the samples and cause errors in results. It should also be ensured that the samples are grounded for the same reason. Even if the material being analysed is electrically conductive it may be worth coating so as to increase the surface resolution of the sample as both secondary and back-scattering electron emission will be increased.

Figure x showing the general setup within an SEM and EDHX analysis machine

The beam of electrons is produced by the heating of a tungsten filament, which acts as the electron gun in the SEM and is positioned at the top of the chamber as is shown by figure x. The tungsten filament is shaped with a twisted loop at the bottom of the element, which is positively charged. By passing a current through the filament the electrons (charge carriers) are attracted to the lower curve, which then acts as the illumination source. The electrons are thermionically emitted from the filament as it is heated. The increase in energy causes the electrons to be pushed into an energy state greater than their fermii level thus causing them to be emitted. A tungsten filament is favourably used due to its high melting temperature and it's relatively low cost.

'The electron beam has an energy ranging from about 0.5-40KeV and is focused to a width of about 0.4-5nm'x.

To aim the electron beam the electrons are controlled by two pairs of scan coils. One set of these controls the x-axis whilst the other controls the y-axis. These act by a combination of attraction and repulsion by the oppositely charge coils. The scan coils are controlled by an electrical circuit found on the console, which allow you to focus on any particular part of the sample. Before it is possible to aim the beam it is necessary to condense the beam by use of condenser lens as can be seen in figure x. These lenses cannot be conventional like those used in opto-electrics and a series of electro-magnetic lens is used to focus the electron beam.

The SEM produces very highly detailed topographies of the sample and allows you to get a good picture of the morphology of the top of the sample. A good SEM allows you to see objects of approximately the size of 1-3nm in size. 'SEM can magnify about 250 times greater than the best light microscopes and can reach magnifications as high as 500,000 times'x.

Due to back scattering of electrons however it is possible to penetrate far into the sample and so a 3D image of the top of samples can be seen. The electrons can penetrate as far as 6000nm into a sample; this is known as the interaction volume. The level of penetration of a sample is dependent on the density and atomic weight of the elements present. As the atomic weight increases the electrons cannot penetrate as far into the sample. It can be seen that the brighter the material is in the SEM image the greater the atomic weight of the elements present are as the electron scatterings produce a stronger intensity at the surface of the sample.

It is found that one problem with the SEM is the level of resolution. The resolution is how close two points can be seen before they cannot be distinguished. It is a problem in SEM that the resolution cannot be refined further and this is due in part to the inefficiency of electro-magnetic lens. If it was possible to focus a beam leaving the electron gun there would be no need for the condenser lens. In an optical microscope the red/blue light is changed to change wavelength and the extent to which and image can be seen. In SEM the wavelength (λ) is around 0.012nm.

The resolution can be improved by reducing the energy (eV) of the electrons hitting the sample. This will allow the there to be a much greater detection of the scattering of secondary electrons as they would not penetrate far into the sample and not have the required energy to escape the surface and reach the detector, as such the detected electrons only originate within a few nm of the surface. These are detected by a scintillator system, which 'attracts secondary electrons towards an electrical grid by maintaining a +400V charge'. This cannot be higher as it would then affect the original focused beam. These beams are then charge towards the scintillator by a +2000V charge and can be detected and converted via a photomultiplier outside the SEM to show digital images. This type of detection can greatly increase both resolution and can allow images smaller than 5nm to be seen. Back-scattered electrons can be detected in much the same way and are usually a donut-shaped detector positioned in a 3600 range around the sample on the platform.

EDX is a specific analytical technique to do with SEM that gives an elemental overview of the elements present in the sample in a target area and can help to find the overall composition of the sample by weight percentage. EDX is measured by firing the concentrated electron beam in a specified area, which then measures the x-ray emissions detected in spacial dependent timed signals. The reason that this analysis works is that each element has its own atomic structure, thus allowing the machine to distinguish between them.

The x-rays produced are like that in the XRD experimental for the emission of CuKα. As electrons bombard the surface of the sample they can displace electrons from the inner shells within the atoms. This causes an electron hole to form, which another electron will then fill from a higher energy state. Depending upon which shell this electron has come from a specific amount of energy will be lost in the form of an x-ray emission. This specific x-ray can then be quantified by an energy dispersive spectrometer. Depending upon the number of x-rays of a certain type detected this gives a weight percentage of the amount of this element and thus a elemental composition can be found. Graphically this data can be plotted as the energy of the x-rays versus the density of those x-rays.

There is however error present in using EDX as it cannot measure the x-rays emitted by elements with an atomic number less that 5. This is mainly due to the fact that they may not make it to the detector or they will dissipate when they hit the glass present in front of detectors. Also if x-rays are emitted from deeper penetrating electrons in the sample they may not make it to the surface and so inaccuracies in the analysis may be present. The analysis can also be inaccurate due to the similar peaks produced by some emissions. There can be inaccuracies due to the preparation of the sample also as if it is uneven then surfaces closer to the beam may receive unequal focus from the electron beam and become more excited thus increasing intensity of element peaks in that area of the sample. It could also be due to the energy of the incoming electron beam, if a higher energy beam comes in it will be more preferential to high energy peaks and in turn reduce the size of small peaks as the high energy favours the more common larger energy state changes at the right end of the spectrum produced.

Motherboard Section

Introduction

This section concentrates on the material analysis of the motherboard of the mobile phone. The motherboard is the central printed circuit board (PCB) of the mobile phone and is the back board by which all other electrical components are connected. The PCB acts as both an electrical and mechanical support for the mobile phone. PCBs in general are a series of conductive pathways and/or wires, which are then coated in a non-conductive substrate.

The PCB was first invented and used in as part of a radio in 1936. The PCB was invented by an Austrian engineer, Paul Eisler (1907-1995) whilst working in England at the time. By the end of 1943, during world war two PCBs began to be used in large scale production to make a handheld radio for use in the military.

Before and a while after the invention of the PCB another system was used to electronically connect all the components of machines. This was known as point to point construction where the components were screwed onto a non-conductive board such as wood and then wires connected the components and were soldered into place. At this time the only other alternative in the early 1930s to late 1940s was a bakelite board which was then coated in a metallic spray and holes for the components were drilled into the board.

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