No description available.

This thesis presents investigations in to the properties of bulk graphite, graphitised silicon carbide and exfoliated graphene. The background physics and theory relevant to the investigations is detailed. This is followed by descriptions of the equipment and methods used during experiments presented within this thesis. Millikelvin de Haas-van Alphen and Shubnikov-de Haas experiments were performed for several types of graphite. These types included forms of natural, kish and highly orientated pyrolitic graphite. Oscillations for two sets of carriers (holes and electrons) were observed with varying strengths between the types. This suggests that the mobility of the charge carriers in graphite can vary significantly depending on the formation of the graphite bulk. Hall measurement results support this statement, as the electron to hole mobility ratio appears much greater in natural graphite than in highly orientated pyrolitic graphite. Analysis of the oscillations for each sample indicates that the electrons have a higher mobility than the holes, and that the effective mass of the holes is lower than that of the electrons. Depending on the sample, the mobility of the holes was found to vary between 1.07->1.42 m^2/Vs and the mobility of the electrons between 1.64->16.0 m^2/Vs. The effective masses of the charge carriers were found to be 0.031+-0.007 m0 and 0.046+-0.003 m0 for holes and electrons respectively. The nature of the carriers for the different types of graphite was determined. It was found in the de Haas-van Alphen experiments that the phase of the carriers varied between samples. However, in the majority of cases, the electrons were shown to be Dirac fermions. In Shubnikov-de Haas experiments, the electrons demonstrated an indeterminate nature. In both types of measurement, the holes were found to have an indeterminate nature. De Haas-van Alphen experiments were also performed on two graphitised silicon carbide samples. The sample fabricated at The University of Leeds exhibited a complicated background magnetisation. This is not characteristic of a carbon system and so indicates a lack of a carbon film. The sample fabricated by the Georgia Institute of Technology exhibited a magnetisation akin to ZYB grade highly orientated pyrolitic graphite. Sheet magnetoresistance measurements were also performed on a more recent graphitised silicon carbide sample, fabricated at The University of Leeds. Weak localisation was observed as well as a large number of non-oscillatory features. These features were attributed to the absence of a defined Hall-bar geometry and the presence of multiple graphene domains between surface contacts. Magnetotransport and activation energies of exfoliated graphene flakes were also investigated. The charge carrier mobilities at 240K for a particular sample were found to be 20,670+-30 cm^2/Vs for holes and 22,770+-40 cm^2/Vs for electrons. These mobilities rose to 25,600+-200 cm^2/Vs for holes and 25,900+-200 cm^2/Vs for electrons by 2.5K. This observation implies that the holes experience stronger phonon scattering than the electrons. The activation energies for filling factors nu=+-2 and nu=+-6 were found at several magnetic fields for the sample. The activation energies allowed for determination of the broadening of the Landau levels. The broadening of the nu=+-6$ levels were found to be constant as a function of magnetic field, with Gamma_{+-6}=260+-40 K. The nu=-2 level also showed a constant value for the broadening, Gamma_{-2}=620+-40 K. However, for nu=+2, the activation energy approached the bare Landau level separation at high magnetic fields. This implies a zero-energy Landau level that is narrower than the higher levels. Further to the mobility asymmetry at high temperatures, the difference in broadening between the nu=-2 and nu=+2 sides of the zeroth Landau level suggests that there is a form of scattering which affects the holes more than the electrons near the Dirac point.

The aim of this project was to produce expanded graphite (EO) and modified EO-filled electrically conductive polymer composites and to investigate the effects of different additive modifications and preparation conditions on the microstructure and electrical properties of these composites. Modifications included the use of dry blending and ultrasound to reduce their size, use of various suspension media and surfactants to stabilize particle suspensions. To compare the effectiveness of different filler modification processes on electrical conductivity, unmodified and treated EO were incorporated into polypropylene (PP) by melt mixing and EO based dispersions were used to make polyvinyl alcohol (PV A) composites by solution casting. The PP composites were made using various processing methods and conditions at filler concentrations up to 12 wt%, while the polyvinyl alcohol samples contained graphite concentrations up to 8 wt%. To analyse the crystalline morphology of sample and the dispersion of the filler in the composites samples were analysed by light and electron microscopy, DSC and X-ray diffraction. TOA was also used to investigate the thermal stability of the composites. It was found that the presence of graphite, significantly changed the crystal morphology of PP. Solution mixed PVA samples showed improved dispersion and the particle size was effectively reduced.

No description available.

Graphite has high in-plane thermal conductivity and is a potential candidate to combat the thermal management problems in high density electronic devices. However, the properties of graphite are not fully exploited and this study was carried out with the aim of fabricating and analysing high thermally conducting graphite based composites. Model composites were prepared with three different average flake sizes: 180 μm, 300 μm and 600 μm. The constituents( flakes and binders) were characterised in terms of density, carbon yield and crystallographic perfection using combination of techniques. The electrical resistivity of a single flake was found to be 0.6 μS2 m. A fabrication route was developed using 75% starting volume fraction (67% - 72%) estimated volume fraction in the composite) of small flake graphite. A high volume fraction was used so as to obtain composites with properties dominated by the flakes. The selection of the optimum route of fabrication was based on achieving the lowest electrical resistivity of the composite after carbonisation (1000 °C). An extensive study was carried out on composites prepared using 75% (starting volume fraction) flakes. The mis-alignment of the 002 plane was found to decrease with an increase in the flake size. The optical texture of small flake composites showed that the binder was inhomogeneously distributed and was observed to shrink away from the flakes whereas in the case of composites with large flakes, the binder was found to wet the flakes. The layers of the binder were also found to align along the basal planes of the flakes in the vicinity of the flakes. The composites with small flakes were found to be `brittle' whereas the large flake composites showed a more `ductile' behaviour. The Young's modulus and work-of-fracture were estimated from load versus extension curves. Raman studies showed an increase in the a-direction coherence length in the binder with an increase in heat-treatment temperature and the electrical resistivity of the composites was found to decrease with increase in flake size and heat-treatment temperature. The thermal conductivities were determined and samples were imaged in a thermal microscope. Comparison between quantitative thermal conductivity and that predicted using Lavin's relationship showed that Lavin's relationship is not applicable to these composites. A thermal conductivity of 655 W/m K (160% of that of copper) was achieved in graphitised large flake composites. The volume fraction of graphite was varied (estimated volume fraction 45%-75% in the composite) and graphitised large flake composites were studied. The density of the composites increased with an increase in the volume fraction whereas the mis-alignment of the 002 plane was found to decrease with an increase in the estimated volume fraction of graphite up to -70%. When the estimated volume fraction of graphite was further increased (-75%), the mis-alignment was found to increase. This was attributed to the microstructure which showed high packing density giving rise to twisting and distortion of flakes in the composite. Existing composite-type models failed to correctly predict the transport properties and hence, a first approach to developing a model specifically for these composites was attempted. However, due to their complex nature and insufficient information to define the modelling parameters its validity is uncertain. A thermal conductivity of 750 W/m K (190% that of copper) and density -1.8 g/cm3 (nearly one fourth of that of copper) and all this at relatively lower cost was achieved in graphitised large flake composites with -75% estimated volume fraction of graphite. This material is particularly attractive to combat thermal management problems.

Graphene and monoatomic boron nitride as members of the new class of two dimensional materials are discussed in this thesis. Since the discovery of graphene in 2004, various aspects of this one atom thick material have been studied with previously unexpected results. Out of many outstanding amazing properties of graphene, its elastic properties are remarkable as graphene can bear strain up to 20% of its actual size without breaking. This is the record value amongst all known materials. In this work experiments were conducted to study the mechanical behaviour of graphene under compression and tension. For this purpose graphene monolayers were prepared on top of polymer (PMMA) substrates. They were then successfully subjected to uniaxial deformation (tension- compression) using a micromechanical technique known as cantilever beam analysis. The mechanical response of graphene was monitored by Raman spectroscopy. A nonlinear behaviour of the graphene G and 2D Raman bands was observed under uniaxial deformation of the graphene monolayers. Furthermore the buckling strength of graphene monolayers embedded in the Polymer was determined. The critical buckling strain as the moment of the final failure of the graphene was found to be dependent on the size and the geometry of the graphene monolayer flakes. Classical Euler analysis show that graphene monolayers embedded in the polymer provide higher values of the critical buckling strain as compared to the suspended graphene monolayers. From these studies we find that the lateral support provided by the polymer substrate enhances the buckling strain more than 6 orders of magnitude as compared to the suspended graphene. This property of bearing stress more than any other material can be utilized in different applications including graphene polymer nanocomposites and strain engineering on graphene based devices. The second part of the thesis focuses on a two dimensional insulator, single layer boron nitride. These novel two dimensional crystals have been successfully isolated and thoroughly characterized. Large area boron nitride layers were prepared by mechanical exfoliation from bulk boron nitride onto an oxidized silicon wafer. For their detection, it is described that how varying the thickness of SiO2 and using optical filters improves the low optical contrast of ultrathin boron nitride layers. Raman spectroscopy studies are presented showing how this technique allows to identify the number of boron nitride layers. The Raman frequency shift and intensity of the characteristic Raman peak of boron nitride layers of different thickness was analyzed for this purpose. Monolayer boron nitride shows an upward shift as compared to the other thicknesses up to bulk boron nitride. The Raman intensity decreases as the number of boron nitride layers decreases. Complementary studies have been carried out using atomic force microscopy. With the achieved results it is now possible to successfully employ ultrathin boron nitride crystals for precise fabrication of artificial heterostrutures such as graphene-boron nitride heterostrutures.

Μία από τις σημαντικότερες αποθέσεις λιγνίτη της Δυτικής Τουρκίας εντοπίζεται στην λεκάνη της Mugla. Η λεκάνη της Mugla αποτελείται από δύο τάφρους ΒΔ-ΝΑ διεύθυνσης, οι οποίες χωρίζονται από μία πλατιά έξαρση του υποβάθρου, δημιουργώντας τις υπολεκάνες του Yatağan και του Milas. Η εμφάνιση του υποβάθρου αποτελείται στα νότια από τα Λυκκιακά καλύμματα, τα οποία περιλαμβάνουν ένα σύμπλεγμα πετρωμάτων ωκεάνιας και ηπειρωτικής προέλευσης και στα βόρεια από μεταμορφωμένα πετρώματα, όπως σχιστόλιθοι, γνεύσιοι, αμφιβολίτες, και μάρμαρα της Μάζας Menderes. Η παρούσα εργασία εστιάζει στο λιγνίτη της περιοχής Milas και τα στερεά παραπροϊόντα καύσης του (ιπτάμενη τέφρα και τέφρα εστίας) από τις περιοχές Yeniköy, Yatağan και Kemerköy της Τουρκίας. Σκοπός της μελέτης είναι η εκτίμηση της κινητικότητας των ιχνοστοιχείων κατά την εκμετάλλευση του λιγνίτη για ηλεκτροπαραγωγή και κατά την αποθήκευση και διάθεση των στερεών παραπροϊόντων της καύσης του στην περιοχή Mugla της Τουρκίας. Αντικείμενο της εργασίας αποτέλεσε η γεωχημική μελέτη του λιγνίτη και των παραπροϊόντων του, καθώς και η χημική μελέτη των διαλυμάτων που προέκυψαν κατά την απόπλυσή τους σε συνθήκες pH 5 και 8,5. Επιπρόσθετα, πραγματοποιήθηκε μία σειρά εργαστηριακών αναλύσεων, που περιλαμβάνουν την προσεγγιστική και στοιχειακή ανάλυση, την ανθρακοπετρογραφική εξέταση, καθώς και τις ορυκτολογικές αναλύσεις όλων των δειγμάτων. Στα πλαίσια των εργαστηριακών αναλύσεων, προσδιορίστηκε η υγρασία, η τέφρα, η περιεκτικότητα σε πτητικά συστατικά, καθώς και η περιεκτικότητα των C, H, N και S του λιγνίτη και των παραπροϊόντων του, ο μόνιμος άνθρακας, η θερμαντική ικανότητα και η ανακλαστικότητα του λιγνίτη. Οι συγκεντρώσεις των ιχνοστοιχείων στον λιγνίτη, την ιπτάμενη τέφρα και την τέφρα εστίας προσδιορίστηκαν με τη χρήση του οργάνου ELAN 6100 Perkin Elmer®. Τα στοιχεία, που προσδιορίστηκαν είναι τα Αg, As, B, Ba, Be, Co, Cr, Cu, Fe, Ga, Hf, Hg, Li, Mn, Mo, Ni, Pb, Se, Sr, U, V και Zn. Σύμφωνα με τις περιεκτικότητες των ιχνοστοιχείων του λιγνίτη και των παραπροϊόντων της καύσης του εκτιμήθηκε η κινητικότητά τους κατά την καύση με τον υπολογισμό του συντελεστή εμπλουτισμού (R.E.). Πολύ μικρό συντελεστή εμπλουτισμού και κατά συνέπεια έντονη πτητικότητα παρουσιάζουν τα στοιχεία As, Cd, Mo, Pb και Sr. Μέτρια πτητικότητα εμφανίζουν τα Ag, B, Cr, Ni, U, V, Zn, ενώ όλα τα υπόλοιπα έχουν μικρή πτητικότητα, καθώς συμμετέχουν στο τμήμα των παραπροϊόντων που παραμένει στην τέφρα των ατμοηλεκτρικών σταθμών. Ακολούθως, έλαβαν χώρα πειράματα έκπλυσης των ιχνοστοιχείων από το λιγνίτη και τα παραπροϊόντα του σε συνθήκες pH 5 και 8,5, ούτως ώστε να προσδιοριστεί η κινητικότητα τους, με τη βοήθεια του Ποσοστού Σχετικής Έκπλυσης (RML) και της Έντασης έκπλυσης (Ιl). Τα αποτελέσματα των παραπάνω έδειξαν ως πιο κινητικά ιχνοστοιχεία σε συνθήκες pH 5 και 8,5, στο λιγνίτη τα As, B, Mn, Sr, στην ιπτάμενη τέφρα τα Cr, Li, Mo και Sr, και στην τέφρα εστίας τα Li, Mo, Sr. Προκειμένου να προσδιοριστεί η γεωχημική συγγένεια των ιχνοστοιχείων στα «ως έχει» δείγματα του λιγνίτη και των παραπροϊόντων καύσης του, αλλά και να περιγραφεί η συμπεριφορά τους κατά τη διάρκεια της έκπλυσής τους σε διαφορετικές τιμές pH, πραγματοποιήθηκε στατιστική επεξεργασία των αποτελεσμάτων της στοιχειακής ανάλυσης. Ειδικότερα, στα αποτελέσματα των στοιχειακών αναλύσεων και των τιμών έκπλυσης των ιχνοστοιχείων όλων των δειγμάτων ξεχωριστά για pH 5 και pH 8,5, καθώς και των ορυκτών της ιπτάμενης τέφρας και της τέφρας εστίας εφαρμόστηκε η μέθοδος της παραγοντικής ανάλυσης R-τύπου. / One of the most important lignite deposits of Western Turkey is located in the basin of Mugla. The Mugla basin consists of two NW-SE direction grabens, which are separated by a wide rise of the basement that creates the sub-basins of Yatağan and Milas. Southern, the basement is consisted of the Lyccian nappes, which contain a cluster of ocean and continental origin rocks, while northern is consisted of metamorphic rocks, such as schists, gneiss, amphibolites and marbles of the Menderes Mass. The present study focuses on the lignite of Milas region and its combustion solid by-products (fly ash and bottom ash) from Yeniköy, Yatağan and Kemerköy areas in Turkey. The purpose of this study is to estimate the mobility of elements and trace elements in the exploitation of the lignite deposits for generating power in the region of Mugla in Turkey. The object of the present study lies in the geochemical study of lignite and its combustion products and in the determination of the mobility of trace elements during the combustion of lignite and the leaching of lignite and its by-products, under pH 5 and 8.5 conditions. In addition, several laboratory analyses are performed, including the proximate and the elemental analysis, the coal-petrographical examination and the mineralogical analysis of all samples. During these laboratory analyses are determined the moisture, the ash, the content of volatile components and the content of C, H, N and S of lignite and lignite combustion products the permanent carbon, the calorific value and the reflectivity of lignite. The contents of trace elements in lignite, in fly ash and in bottom ash are determined using an ELAN 6100 Perkin Elmer ® instrument. The elements, which were identified, are Ag, As, B, Ba, Be, Co, Cr, Cu, Fe, Ga, Hf, Hg, Li, Mn, Mo, Ni, Pb, Sr, U, V and Zn. According to the trace elements contents of lignite and its combustion products, the mobility during combustion was determined using the Enrichment Factor (RE). Very low rate of enrichment and thus, strong volatility, have the elements As, Cd, Mo, Pb and Sr. Medium volatility is being observed in the elements Ag, B, Cr, Ni, U, V, Zn, while all the rest have low volatility due to their existence in the part of the lignite co-products that remain in the ash of the Thermal Power Plants. Furthermore, leaching experiments under pH 5 and 8.5 conditions took place to estimate the mobility of trace elements from lignite and its by-products using the Relative Mass Leached (RML) and Leaching Intensity (Ιl). The above results, under pH 5 and 8.5 conditions, indicate as more mobile trace elements in the lignite the elements As, B, Mn, Sr, in the fly ash the elements Cr, Li, Mo and Sr, and in the bottom ash the elements Li, Mo, Sr. In order to determine the geochemical affinity and the mode of occurrence of trace elements in the samples of lignite and its combustion by-products and to describe their behavior during the leaching at different pH conditions, a statistical process of the elemental analysis results is performed. In particular, the methodology of factor analysis R-type is used in the elemental analysis results, in the leaching values of trace elements of all samples, separately for pH 5 and pH 8,5 and in the fly ash and the bottom ash minerals.