X-ray tubes are devices that produce x-ray radiation, which is useful in various applications. For example in medicine this high energy electromagnetic radiation is used for imaging your body. In airports x-rays are used to scan your luggage for prohibited items. X-rays can also be used for materials characterization in techniques such as x-ray fluorescence spectroscopy or photoelectron spectroscopy.
An x-ray tube consists of an anode and a cathode within a casing that can hold vacuum. The cathode is heated to high temperatures, where it starts emitting electrons – this process is known as thermionic emission. A high voltage applied between the cathode and the anode accelerates the emitted electrons towards the anode. When these high energy electrons interact with the anode some of the energy is converted into x-ray radiation and some into heat. Thats why water cooling is needed to prevent the overheating of the anode.The emitted x-ray radiation consists of two components – bremsstrahlung and characteristic x-rays. In the case of bremsstrahlung the electromagnetic radiation is emitted from the negative electron when its trajectory is changed by a positively charged atoms nucleus. This radiation has a very broad energy range. Its energy and intensity depends on the voltage between the anode and the cathode, on the cathode filaments heating current and on the atomic number of the anode material. Characteristic x-rays however have a very specific energy, which strongly depends on the anode material. This radiation is generated when the accelerated electrons excite the anode atoms by kicking out inner shell electrons. In the relaxation process a higher shell electron moves to the vacant spot and the excess energy is emitted in the form of x-rays. The energy of these characteristic x-rays depend on the binding energy of the electron that was kicked out and the binding energy of the electron that occupied the vacant spot. The generated x-rays leave the tube through a beryllium window. Beryllium is used as a window material because it doesnt absorb much of the x-rays as it has a low atomic number. Be sure to follow us in youtube for more awesome videos in the future!
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Atomic Layer Deposition – A Method for Making Ultra-Thin Invisible Corrosion Resistant Coatings
Ever thought how nice it would be if the replicator (synthesizer) from Star Trek actually existed? Assembling different materials in an atomic scale however, has been possible already for decades! This exciting technique is called „Atomic Layer Deposition“ (ALD). The deposition process is carried out in a specially designed ALD reactor, where different chemicals enter the reaction chamber one at a time and react with the substrates surface in a self limiting manner. With each deposition cycle a thin layer is deposited and by repeating the cycle thicker material layers can be obtained. An easy example would be the deposition of titanium dioxide by using titanium(IV) chloride and water as reacting chemicals (precursors) and nitrogen as carrier gas.
Although this method is not suitable for creating macroscopic objects, it can be used to significantly enhance their properties such as corrosion resistance, wetting (self cleaning surfaces) or even biocompatibility (brain chips). This method is also used in the production of some solar cells, microelectronic devices and nanostructures. The huge benefit of this method is the possibility to apply films with well defined thickness and composition even on sophisticated three-dimensional objects. This makes ALDep perfect for applying ultra thin (nanometric) corrosion resistant coatings on many small devices (including jewelery), where thick coatings cannot be used.
In order to get a better understanding of this method, watch the video above.
Sony Vegas Pro 13 Suite was used for making this video – check out their website below:
Sony Creative Software Inc.
Vacuum Systems and Technologies
Vacuum can be understood as space from where matter (for example air) has been removed. It naturally exists in outer space but for certain applications, like materials characterization techniques, it needs to be achieved artificially. The desired level of vacuum is obtained with the help of a suitable vacuum pump. For example low vacuum (low quality vacuum with higher pressure) can be generated with a diffusion pump, scroll compressor pump, rotary vane pump, diaphragm pump or a sorption pump. High vacuum (high quality vacuum with very low pressures) however, can be obtained with high vacuum pumps such as the turbomolecular pump, ion pump, titanium sublimation pump and cryopump. The level of vacuum is measured with devices called vacuum gauges (vacuum meters) like the thermocouple gauge, pirani gauge, penning ionization gauge and the quadrupole mass spectrometer (analyzer). The working principle of vacuum pumps and vacuum gauges is explained with 3D animations in the video lecture above.