Η κατασκευή συσκευών μικροηλεκτρονικής με οργανικούς ημιαγωγούς όπως τρανζίστορς και φωτοεκπομποί δίοδοι οργανικών (FETs, OLEDs), αναπτύσσεται ταχύτατα τα τελευταία χρόνια. Οι φυσικές και χημικές αλληλεπιδράσεις που συμβαίνουν στις διεπιφάνειες των οργανικών με τα ηλεκτρόδια παίζουν καθοριστικό ρόλο στη λειτουργία τέτοιων συσκευών και επομένως η μελέτη της διεπιφανειακής ηλεκτρονικής δομής είναι σημαντική για την κατανόηση της λειτουργίας αυτών των διατάξεων. Στην παρούσα εργασία η ηλεκτρονική δομή των διεπιφανειών ενός συζυγιακού ολιγομερούς (Ooct-OPV5) με ανόργανα υποστρώματα, συγκεκριμένα το οξείδιο Ινδίου-Κασσιτέρου (ITO), τον πολυκρυσταλλικό Au, την επιφάνεια Si(111) (Si με προσμίξεις τύπου –Ν και -P) και υπέρλεπτα υμένια SiO2(1-5 nm) / Si(111), μελετήθηκε με φασματοσκοπίες φωτοηλεκτρονίων από ακτίνες –Χ και υπεριώδες (XPS, UPS). Το Ooct-OPV5 χρησιμοποιείται ως πρότυπο για το poly(p-phenylenevinylene) (PPV), ένα πολυμερές που έχει ήδη χρησιμοποιηθεί σε συσκευές OLEDs. Το ITO χρησιμοποιείται ως άνοδος στα OLEDs επειδή είναι διαφανές και έχει υψηλή ηλεκτρική αγωγιμότητα. Ο χρυσός είναι αδρανές υπόστρωμα που χρησιμοποιείται ως ηλεκτρόδιο στα FETs. Τέλος, η μελέτη διεπιφανειών του οργανικού με το Si παρουσιάζει ενδιαφέρον, προκειμένου να ενσωματωθούν οι οργανικοί ημιαγωγοί σε μικροηλεκτρονικές συσκευές με βάση το Si. Η μελέτη έγινε σε σύστημα υπερυψηλού κενού (UHV) με τις τεχνικές XPS και UPS. Τα υποστρώματα καθαρίζονταν in-situ με ιοντοβολή με Ar+ και θέρμανση. Ακολούθως γινόταν σταδιακή απόθεση του ολιγομερούς και παρασκευάζονταν υπέρλεπτα υμένια (πάχους ~10 nm) πάνω στα καθαρά υποστρώματα. Σε κάθε στάδιο της απόθεσης λαμβάνονταν τα φάσματα XPS του οργανικού και των υποστρωμάτων. Από την ανάλυση των φασμάτων αυτών προσδιορίζονται οι διεπιφανειακές αλληλεπιδράσεις και η μεταβολή της κάμψης των ζωνών των ημιαγώγιμων υλικών. Με τη φασματοσκοπία UPS μελετάται η ζώνη σθένους της διεπιφάνειας και μετράται η διεπιφανειακή διπολική στοιβάδα. Από το συνδυασμό των πειραματικών αποτελεσμάτων μπορούν να κατασκευάζονται σχηματικά διαγράμματα των ζωνών στις διεπιφάνειες. Με βάση τα πειραματικά αποτελέσματα καταλήγουμε στα εξής συμπεράσματα: Στις διεπιφάνειες του ολιγομερούς με το ITO, τον Au, το Si (τύπου -p) και το SiO2(1-1.8 nm)/Si(111) υπάρχει διεπιφανειακή διπολική στοιβάδα (eD) ενώ στη διεπιφάνεια Ooct-OPV5/ Si (τύπου -n) όχι. Αυτά τα διεπιφανειακά δίπολα σχετίζονται με την ύπαρξη διεπιφανειακών καταστάσεων και εξυπηρετούν τη μεταφορά φορτίου μεταξύ των υλικών που έρχονται σε επαφή στα πρώτα στάδια του σχηματισμού των διεπιφανειών. Κατά το σχηματισμό της διεπιφάνειας Ooct-OPV5/ Si (τύπου - p), το eD σχετίζεται με την αλληλεπίδραση των μορίων του ολιγομερούς με τις επιφανειακές καταστάσεις του Si. Στις διεπιφάνειες Ooct-OPV5/ με Au και Si, η μεταφορά φορτίου ολοκληρώνεται με τη μεταβολή της κάμψης των ζωνών του οργανικού υμενίου κατά ~0.20 eV. Τα φράγματα έγχυσης οπών (eΦbh) ή τα valence band offsets (ΔEV) καθορίστηκαν επίσης σε όλες τις περιπτώσεις. Στη διεπιφάνεια Ooct-OPV5 / Au το eΦbh μετρήθηκε 1.05 eV και επομένως ο Au είναι ακατάλληλο ηλεκτρόδιο για την έγχυση οπών. Το ITO αποδεικνύεται επίσης ακατάλληλο (eΦbh=1.45 eV) και η επιφάνειά του θα πρέπει να υφίσταται κατεργασία προκειμένου να χρησιμοποιείται ως άνοδος σε συσκευές OLEDs. Στην περίπτωση του Si, το valence band offset μεταξύ αυτού και του ολιγομερούς βρέθηκε ~0.4 eV. Η παραπέρα τροποποίηση της επιφάνειας του Si(111) με υπέρλεπτα υμένια SiO2 αυξάνει το ΔEV κατά ~0.2 eV. / The development of organic-based devices, like transistors and light emitting diodes (FETs, OLEDs), is progressing rapidly over the past few years. A great deal of the physics and chemistry that govern the performance of such devices occur at the interfaces between the organic components and the inorganic electrodes, making the study of the interfacial electronic properties essential. In this work, the electronic structure of the interface formed between a conjugated oligomer (Ooct-OPV5) and inorganic substrates, ιn particular indium-tin oxide (ITO), polycrystalline Au, the Si(111) surface (Si n- and p-doped), and ultrathin (1-5 nm) SiO2 films on Si(111), was studied by X-ray and Ultraviolet photoelectron spectroscopies (XPS, UPS). Ooct-OPV5 is a model for poly(p-phenylenevinylene) (PPV), a polymer that has already been used in OLEDs. ITO is the common anode used in OLEDs because of its transparency and high electrical conductivity. Gold was chosen due to its inert nature and because it is used as a source/drain in FETs. Finally, the study of organic/silicon structures is of great importance for the incorporation of organics in Si-based microelectronic systems. All XPS and UPS measurements were carried out in an ultrahigh vacuum (UHV) apparatus. All substrates were cleaned in-situ by Ar+ sputtering and annealing. High purity oligomer films of up to ~10 nm thickness were produced in-situ by stepwise deposition on the clean substrates. The evolution of the oligomer and substrate-related XPS peaks was followed during Ooct-OPV5 deposition on all substrates. Analysis of these spectra clarified the interfacial chemistry and band bending in the semiconducting materials. UPS spectroscopy is used for the determination of the valence band at the interface and the interfacial dipoles. The interfacial energy band diagrams were deduced in all cases from the combination of experimental results. Based on our experimental data we reached the following conclusions: Dipoles are formed at the interfaces of the oligomer with ITO, Au, Si (p-doped) and SiO2(1-1.8 nm)/Si(111), while the Ooct-OPV5/ Si (n-doped) interface is dipole free. These interface dipoles (eD) are related to the existence of interfacial states and serve for the charge transfer between the materials in contact at the initial stages of the interface formation. In the case of the Ooct-OPV5/ Si (p-doped) interface, eD is related to the interaction of the oligomer molecules with Si surface states. At the Ooct-OPV5/Au and Si interfaces, the charge transfer is completed with a band bending of ~0.20 eV in the oligomer film. The hole injection barriers (eΦbh) or valence band offsets (ΔEV) were also determined in all interfaces. This barrier was measured 1.05 eV at the Ooct-OPV5/ Au interface, and thus Au is inappropriate electrode for hole injection. ITO is also proved a poor hole-injecting electrode (eΦbh=1.45 eV), and thus its surface should be modified by treatments when used as an anode in OLEDs. On the other hand, the valence band offset between the Si substrate and the oligomer is measured ~0.4 eV. Modification of the Si(111) surface with ultra-thin SiO2 layers increases ΔEV by ~0.2 eV.

This thesis describes three different design projects that are intellectually connected by the fact that they all involve the development of apparatus to facilitate the precise control of sample temperature in modern microscopes. The first project is a low-temperature sample stage, for a beetle-type scanning tunnelling microscope. The design for this sample stage, and images taken on it with atomic resolution at 114 K are presented. This stage has the capability for variable-temperature sample cooling, which is also discussed. The second project is a set of low- and variable-temperature isothermal radiation shields for a new microscope that is currently being designed and assembled by our research group. These shields provide temperature control between 5 K and room temperature, with measured stability better than +/- 0.1 K. Controlled and stable temperature changes at rates up to 1.5 K per minute have been produced. The shields are modular and can easily accommodate future modifications. The design for the shields, along with their cooling and temperature control capabilities, is presented. The third project is a new stage design for heating, cleaning, and transferring metal and semiconductor samples. Also for use with the new microscope, this stage uses electron bombardment to provide precision temperature control between room temperature to temperatures in excess of 1250 C‰. With this stage, the sample temperature can be determined by measuring the power applied to the sample. The design of this stage, its heating performance, and a method to calculate the sample temperature is presented. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2008-06-03 12:47:18.808

No description available.

No description available.

Crystallization is done in the sugar industry using natural circulation vacuum evaporative crystallizers known as vacuum pans. the fluid which is known as massecuite consists of a suspension of crystals in concentrated molasses. It is highly viscous and slightly non-Newtonian, and laminar conditions prevail in the apparatus. Research on forced convection boiling heat transfer, pressure drop and vapour holdup has been done mostly in turbulent flow under pressures higher than atmospheric, but no studies have been made when boiling viscous fluids under vacuum. This thesis describes a series of experiments which were undertaken with the following objectives: (a) to determine the influence of the pertinent variables on heat transfer, friction losses and vapour holdup while boiling under laminar conditions (b) to produce a method for the calculation of the evaporation and circulation rates in vacuum pans, as this would make possible the optimization of this type of equipment. The apparatus used consisted of a single tube steam heated forced circulation evaporator. The void fraction, pressure and centerline temperature were measured along the tube. The fluids used were syrup, molasses and massecuite covering a thousandfold change in viscosity. The tests were conducted under different conditions of vacuum and steam pressures with varying tube inlet velocities. The experimental results show that the boiling heat transfer coefficient can be correlated as a function of the two phase Reynolds number and dimensionless density ratio and that it is inversely proportional to the tube length to the power of one third. The pressure drop can be estimated using the equation of Oliver and Wright (1964) for bubbly flow. Equations are proposed for calculating the void fraction in the highly subcooled region and point of bubble departure. These equations form the basis of a computer program which by a stepwise and iterative method simulates the boiling process along the tube. Measurements taken on a natural circulation pan with tubes of different length show that this method predicts the effect of the tube length with reasonable accuracy. The limitations of this study are that the experiments were done with a single diameter tube so that the effect of diameter has not been established with certainty. Only sugar products were used in the experiments, and caution is necessary if this method is applied to other fluids. / Thesis (Ph.D.)-University of Natal, Durban, 1985.

An experimental, field replaceable travelling wave tube (TWT) with low profile, TEM transitions has been developed with the aim of combining the cost advantages of field replaceable TWTs with the bandwidth advantages of TEM transitions. Field replaceable TWTs are cheaper because their periodic permanent magnet (PPM) stacks (which are expensive) are easier to produce and can be reused. Standard TEM transitions have a high profile and prevent TWTs from being plugged into PPM stacks. The stacks of packaged TWTs are therefore built onto the tubes, using split ring magnets, before the tubes are potted. Upon failure, such tubes are discarded together with their focusing, coupling and cooling structures. Two types of low profile, TEM transitions were investigated: a ring and a plug transition. In the former, the helix was joined to a metal ring that was sandwiched between two ceramic rings in the vacuum envelope. Due to its high shunt capacitance, this transition was narrowband. In the plug transition, the helix was joined to a small metal plug in the wall of a ceramic tube. Plug transitions with a return loss of better than 10 dB over an octave were made but some technological difficulties were encountered. Metal/ceramic joining with active solder is therefore discussed. The development of graphite attenuators was attempted but graphite of high enough resistivity to achieve the attenuation and matching required was not available. Thin film attenuators were therefore used instead. Some good attenuators were made but the molybdenum films were sensitive to processing conditions. An empirical procedure was developed to magnetize PPM stacks to a wanted profile. The stacks that were designed and built resulted in good beam focusing. TWT tests were made using an automated test station that was built. Maximum RF gains and output powers of 20 dB and 43 dBm respectively were measured. The TWT design, construction and testing was a combination of materials science and microwave work. Some simulations were used to solve electromagnetic problems but much of the work was practical, involving trial and error and many measurements. / Thesis (Ph.D.)-University of Natal, Durban, 1994

Superheated steam (SS) drying of distillers’ spent grain (DSG) is a more energy efficient alternative to conventional hot air drying. SS drying at sub-atmospheric pressure (also referred to as low pressure) can prevent burning and lowering the quality of the food product. The objective of this study was to design, fabricate, and test a SS drying system that could operate at sub-atmospheric pressure for drying DSG. After the custom designed system was constructed, major problems associated with the system were identified. A number of tests were carried out and modifications were made to the system to resolve technical problems. Distillers’ spent grain was then successfully dried using the system under various levels of temperature from 95 to 115°C and pressure of either -25 or -20 kPa, with a SS velocity from 0.100 to 0.289 m/s.

No description available.

In this study, membranes were made from three polyvinylidene fluoride (PVDF) polymers individually and the blend systems of high (H) and low (L) molecular weight PVDF by phase inversion process. After investigating membrane casting solutions’ viscous and thermodynamic properties, the membranes so fabricated were characterized by scanning electron microscopy, gas permeation tests, porosity measurement, contact angle (CA) and liquid entry pressure of water (LEPw) measurement, and further subjected to vacuum membrane distillation (VMD) in a scenario that was applicable for cooling processes, where the feed water temperature was maintained at 27℃. It was found that PVDF solutions’ viscosities and thermodynamic instabilities were determined by the types of PVDF employed in single polymer systems and the mixing ratios of two PVDF polymers in blend systems. Thus the membrane properties and performances were influenced by the aforesaid factors as well. In single polymer systems, it was found that the membrane surface roughness and porosity increased with an increase in molecular weight. Among all the membranes casted in this study, the water vapor flux of VMD was found to be the highest at the intermediate range of H:L ratio, i.e., 4:6, at which the thickness of the sponge-like layer showed a minimum, the finger-like macro-voids formed a more orderly single-layer structure, and the LEPw showed a minimum. A conclusion can be made that blend systems of high molecular weight PVDF polymers and low molecular weight PVDF polymers could be used to optimize membrane performance in vacuum membrane distillation.

Superheated steam (SS) drying of distillers’ spent grain (DSG) is a more energy efficient alternative to conventional hot air drying. SS drying at sub-atmospheric pressure (also referred to as low pressure) can prevent burning and lowering the quality of the food product. The objective of this study was to design, fabricate, and test a SS drying system that could operate at sub-atmospheric pressure for drying DSG. After the custom designed system was constructed, major problems associated with the system were identified. A number of tests were carried out and modifications were made to the system to resolve technical problems. Distillers’ spent grain was then successfully dried using the system under various levels of temperature from 95 to 115°C and pressure of either -25 or -20 kPa, with a SS velocity from 0.100 to 0.289 m/s.