Tel Aviv UniversityThe following project topics are available in the Electrical Discharge and Plasma Laboratory.
The following projects are suitable for 4th year undergraduates in engineering, in fulfillment of their senior project requirement. For further details, contact Prof. R.L. Boxman at 972-3-640-7364, or by e-mail at boxman@eng.tau.ac.il, or visit the laboratory in the Wolfson Mechanical Engineering Building, Room 451.
The Electrical Discharge and Plasma Laboratory constructed a high power metal vapor plasma source based on a high current vacuum arc discharge. The arc is sustained between water-cooled electrodes, and other elements are water-cooled as well. It is desired to determine the heat-flux to these elements, to be used both as scientific data which will help characterize and understand the physics of the vacuum arc, and as engineering data that will be helpful in optimizing the operation of the present source and to plan future plasma sources. The objective of the proposed project is to build a system to measure the heat flux to various elements of the plasma source, and then use the system to make these measurements.
1. Recommend appropriate measurement components and number of simultaneous measurements, considering budgetary constraints and convenience.
2. Order or build components, assemble system.
3. Devise data collection and evaluation stategy, and implement with software.
4. Conduct measurements, to determine heat flow to each element as a function of arc current and magnetic fields.
5. Summarize results.
The measurement system can be based on the assumption that almost all of the heat flux to an element is removed by the coolant. Hence the system may be based on a flow meter and thermocouples, to measure the water flow and temperature difference between the input and output, electronics to input the data to an existing data collection system, and a program to analyze the data.
Carbon nano-tubes (CNTs) are hollow cylindrical structures composed of a one or a few layers of carbon atoms. They typically have an inside diameter of 7 nm, and lengths up to a few mm’s. They are of technological interest because of their extreme mechanical strength, aspect ratio, stability, and electrical conductivity as a component in composite materials, as scanning microscope tips and electron emitters for flat panel displays and electron microscopes. Generally they are produced in a controlled atmosphere at high temperature.
Recently it was discovered in the Electrical Discharge and Plasma Lab that CNTs can be produced in the open air on room temperature substrates with a single ms duration pulsed arc. Examples are shown below. The objective of the proposed project is to build a device to facilitate microscopic and spectroscopic study of the arc, and the plasma it produces.
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Transmission electron micrograph of a pulsed arc produced CNT. |
High resolution transmission electron micrograph of a CNT. Distance between atomic carbon walls is 0.35 nm. |
The project will comprise some of the following activities:
(1) design and construction of an electrode holder assembly permitting accurate setting of the inter-electrode gap, which can be mounted on the stage of an optical microscope;
(2) design and construction of a circuit to supply the arc pulse; and
(3) characterization of the system, including initial microphotographs of the optical radiation from the arc.
(1) The critical element is providing a repeatably adjustable interelectrode gap in the range of 10-300 mm. Various design alternatives including a micrometer screw, and a differential micrometer screw will be evaluated. The design will also provide the means for holding a graphite electrode (e.g. mechanical pensil), and for protecting the microscope lens from arc damage (e.g. glass cover slip for microscope slide).
(2) The power supply should be simple and inexpensive, but provide repeatable results. The supply may be based on a d.c. power supply charging a switch-selected capacitor, and discharge to the electrodes via an SCR.
(3) The device will be mounted on an optical microscope stage. Repeatability of the gap setting will be tested by measuring the gap optically. The device will be repeatedly arced, and the electrode configuration and interelectrode gap will be photographed before and after each arc to determine electrode erosion, and damage rate to the glass cover slip. The arc will also be photographed using its own radiation. Repeatability of the arc radiation disbribution will be determined from analysis of the digital photograph.
The project is ideally suited for an inter-disciplinary team consisting of one student from mechanical engineering, and one student from the combined electrical engineering/physics program. However the scope of the project can be adjusted for any single EE or ME student.
Pulsed air arc treatment is a method of plasma treating metal surfaces with short duration plasma pulses. A system was developed in the Electrical Discharage and Plasma Laboratory for treating Aluminum coupons, in order to enhance adhesive bonding, as part of a collaborative project with the Israel Aircraft Industries. A close-up photograph of the arc in action is shown below in Fig. 1.
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Fig. 1. Pulsed Air Arc Treatment of an Aluminum surface. |
Fig. 2. Pulsed arc engraving. |
A B.Sc. project student developed a computer control program, which could take a bitmap file and translate it into a sequence of substrate motion and arc pulse commands, such that a graphical image was engraved on the metal surface – see Fig. 2 above. Only limited control of the “dot size” was possible with the existing pulsed power supply, however, and thus the image quality was limited.
In the proposed project, the system will be improved with the goal of being able to engrave a recognizable portrait of a person onto a metal surface.
The project will involve some or all of the following activities:
1. Modifying the existing pulsed power supply or building a new supply so that the pulse duration can be controlled by from a personal computer.
2. Engineering a metal surface layer to improve engraved image contrast.
3. Devising an objective measurement of engraved image quality (e.g. resolution and contrast).
4. Improving the algorithm controlling the pulsed arc sequence.
1. The pulsed power supply currently used discharges a capacitor through a pulse transformer, and the only control is of the charging voltage over a very limited range. Two alternative strategies will be analyzed, and the most suitable chosen: (1) Controlling pulse duration, using a GTO (gate turn-off) device. (2) Controlling pulse duration by charging several capacitors with different capacitances simultaneously, and discharging the capacitors selectively by gating one or more selected SCRs, with a separate SCR controlling the discharge of each capacitor.
2. The engraved metal surface shown in Fig. 2 presents an image using texture contrast, i.e. the light is mostly speculatively reflected from untreated areas, but scattered from the discharge craters in treated areas. Several techniques will be evaluated to improve the image quality, including: (1) improving the “untreated” surface finish by better polishing – possibly choosing materials which maintain their polish longer: (2) applying a coating to substrate (using filtered vacuum arc deposition) having a contrasting color or reflectivity (e.g. black copper oxide, gold titanium nitride). Arc craters will expose the substrate so that the image dots will having contrasting color and/or reflectivity, as well as scattering.
3. For example: (1) Engrave a standard test patter (e.g. bar pattern with increasingly small period) on a sample substrate. (2) Photograph the sample under standardized conditions with a digital camera. (3) Transfer the image to a p.c., and evaluate the modulation transfer function (MTF).
4. Write program in LabView, enabling the improvements outlined in 1 above. Test various schemes for translating image to engraving, against performance test established in 3 above.
The project is suitable for a single student, or a multi-disciplinary team. The project scope and definition will be adjusted to the team composition. Students are sought with backgrounds in the following areas:
a. power electronics
b. materials
c. image processing/programming
The following graduate research topics are suitable for either a M.Sc. and Ph.D. thesis. There is a possibility for financial support on some of the topics listed. For additional details, please contact:
Prof. Reuven Boxman
Faculty of Engineering
tel: 972-3-640-7364
e-mail: boxman@eng.tau.ac.il
or visit the laboratory in the Wolfson Mechanical Engineering Building, Room 451.
1. Diamond growth in hot anode vacuum arcs
In 1990, researchers in the Electrical Discharge and Plasma Laboratory were studying evaporation and deposition of metals from a graphite crucible in a Hot Anode Vacuum Arc (HAVA). After the metal in the crucible was completely evaporated, they were surprised to find a residue in the form of small, transparent hard particles. Some of these particles reached macroscopic dimension (about 0.25 mm), and could be picked up with tweezers. X-ray diffraction (XRD) and electron diffraction tests conducted within a few weeks after production indicated that these particles had the structure of diamond. However Raman tests conducted on old samples, and XRD tests conducted after several months on samples that initially had a diamond structure did not detect any crystalline structure. These results, together with an examination of C-Al phase diagram and theoretical studies of the HAVA, suggest that (1) the crucible was heated to sufficiently high temperatures so that the molten Al dissolved some of the carbon from the inner surface of the crucible, (2) as the Al was evaporated, the solution became supersaturated, (3) for reasons unknown, the carbon precipitated into a diamond-like form, and (4) for reasons unknown, this diamond-like form is unstable, and in a time period of a few weeks to a few months, transformed into an amorphous material.
In the intervening period, little follow-up research was pursued to establish or disprove the above conjecture, mostly due to the lack of requisite facilities. Since then, however, there have been two developments which allow re-opening this research topic: (1) Research on the HAVA has led to a better understanding of its operation, and in particular, techniques have been developed for measuring the crucible temperature, thus facilitating controlled experimentation. (2) The Wolfson Materials Research Center has been established at Tel Aviv University, which will allow characterization of this mysterious material, and tracking any changes of its characteristics with time.
A graduate student in the Materials Science and Engineering Program is sought to conduct research on diamond (?) growth in the hot anode vacuum arc. The research will involve operation of the HAVA apparatus, measurement of the crucible temperature, coordinating diagnostic measurements, and seeking optimum production conditions, possibly leading to the formation of stable diamonds. Interested students should contact Prof. Boxman.
Last modified: Feb. 2005
