Two-dimensional graphene, a single layer of graphite, has emerged as an excellent candidate for future electronic material due to its unique electronic structure and remarkably high carrier mobility. Even higher carrier mobility has been demonstrated in graphene devices using hexagonal boron nitride as an underlying dielectric support instead of silicon oxide. Interestingly, both graphene and boron nitride exhibit superior thermal properties, therefore may potentially offer a solution to the increasingly severe heat dissipation problem in nanoelectronics caused by increased power density. In this thesis, we focus on the investigation of the thermal properties of graphene and hexagonal boron nitride. First, scanning thermal microscopy based on a sub-micrometer thermocouple at the apex of a microfabricated tip was employed to image the temperature profiles in electrically biased graphene devices with ~ 100 nm scale spatial resolution. Non-uniform temperature distribution in the devices was observed, and the "hot spot" locations were correlated with the charge concentrations in the channel, which could be controlled by both gate and drain-source biases. Hybrid contact and lift mode scanning has enabled us to obtain the quantitative temperature profiles, which were compared with the profiles obtained from Raman-based thermometry. The temperature rise in the channel provided an important insight into the heat dissipation mechanism in Joule-heated graphene devices. Next, thermal conductivity of suspended single and few-layer graphene was measured using a micro-bridge device with built-in resistance thermometers. Polymer-assisted transfer technique was developed to suspend graphene layers on the pre-fabricated device. The room temperature thermal conductivity values of 1-7 layer graphene were measured to be lower than that of bulk graphite, and the value appeared to increase with increasing sample thickness. These observations can be explained by the impact of the phonon scattering by polymer residue remaining on the sample surfaces. Lastly, thermal conductivity of few-layer hexagonal boron nitride sample was measured by using the same device and technique used for suspended graphene. Measurements on samples with different suspended lengths but similar thickness allowed us to extract the intrinsic thermal conductivity of the samples as well as the contribution of contact thermal resistance to the overall thermal measurement. The room temperature thermal conductivity of 11 layer sample approaches the basal-plane value reported in the bulk sample. Lower thermal conductivity was measured in a 5 layer sample than an 11 layer sample, which again supports the polymer effect on the thermal transport in few-layer hexagonal boron nitride. / text

Two-dimensional (2D) layered hexagonal boron nitride (h-BN) has become a very popular material in nanoelectronics in recent years because of its extraordinary chemical stability and thermal conductivity [1]. Recently, h-BN is also commonly used as a dielectric material [2], and research in this area is still in its early stages. The commonly used methods for fabricating h-BN include mechanical exfoliation and chemical vapor deposition (CVD). CVD is a recognized industry-compatible method for producing large-area h-BN. However, studies have shown that multilayer h-BN grown by CVD is polycrystalline and contains multiple local defects [3]. These defects and inhomogeneity cannot be avoided and lead to small amounts of atom-wide amorphous regions that have weak dielectric strength [3]. Although the general characteristics of h-BN prepared by these two fabrication methods can be learned from different works in the literature, it is difficult to study the quality of h-BN without systematically comparing the differences between the two growth methods under the same experimental conditions and with large number of samples. This also makes it difficult for researchers to choose the best-quality h-BN. In this work, the morphological characteristics and electrical properties of mechanically exfoliated h-BN and CVD-grown h-BN from different sources have been compared under different conditions. Commercially available h-BN flakes mechanically exfoliated from NIMS h-BN bulk crystal show no leakage current at electrical fields up to 25.9 MV/cm, and above this applied electrical force, the size of the conductive spots is extremely small (1.99 ± 1.81 nm2). On the contrary, “monolayer” CVD-grown h-BN samples from Graphene Supermarket were shown to be amorphous in ~20% of their area, which makes them appear discontinuous from an electrical point of view, plus they contain large thickness fluctuations up to 6 layers. Moreover, in nanoelectronic measurements collected with a conductive atomic force microscope (CAFM) working in vacuum, mechanically exfoliated h-BN showed better electrical homogeneity and presented later dielectric breakdown compared to the h-BN samples fabricated by the CVD method.

Master of Science / Department of Chemical Engineering / James H. Edgar / Boron nitride is a purely synthetic material that has been known for over 150 years but only recently has sparked interest as a semiconductor material due to its potential in ultraviolet lasing and neutron detection. Thin-layer hexagonal boron nitride (hBN) is probably most attractive as a complementary material to graphene during its intense research endeavors. But for hBN to be successful in the realm of semiconductor technology, methods for growing large single crystals are critical, and its properties need to be accurately determined. In this study, hBN crystals were grown from metal solvents. The effects of soak temperature, soak time, source materials and their proportions on hBN crystal size and properties were investigated. The largest crystals of hBN measured five millimeters across and about 30 micrometers thick by precipitation from BN powder dissolved in a nickel-chromium solvent at 1700°C. High temperatures promoted outward growth of the crystal along the a-axis, whereas low temperatures promoted growth along the c-axis. Crystal growth at high temperatures also caused bulk hBN to adopt a triangular habit rather than a hexagonal one. A previously unreported method of synthesizing hBN was proven successful by substituting BN powder with elemental boron and a nitrogen ambient. XRD and Raman spectroscopy confirmed hBN from solution growth to be highly crystalline, with an 8.0 cm[superscript]-1 FWHM of the Raman peak being the narrowest reported. Photoluminescence spectra exhibited peaks mid-gap and near the band edge, suggesting impurities and defects in the hBN samples. However, high-purity reactants and post-growth annealing showed promise for synthesizing semiconductor-grade hBN. Several etchants were explored for defect-selective etching of hBN. A molten eutectic mixture of KOH/NaOH was the most effective defect-selective etchant of hBN at temperatures of 430-450°C for about one minute. The two prevalent hexagonal etch pit morphologies observed were deep, pointed-bottom pits and shallow, flat-bottom pits. TEM and SAED confirmed basal plane twists and dislocations in hBN crystals, but due to the highly anisotropic nature of hBN, their existence may be inevitable no matter the growth technique.

Hexagonal boron nitride (h-BN) formation by carbothermic reduction of B2O3 under nitrogen atmosphere at 1500&deg / C and effect of CaO addition into the initial B2O3 &amp / #8211 / active C mixture were investigated during this study. Reaction products were characterized by powder X-ray diffraction, scanning electron microscopy (SEM) and quantitative chemical analysis. Main aim of this study was to investigate the presence of a second reaction mechanism that catalytically affects h-BN formation during CaO or CaCO3 addition into the initial mixture. It was found that similar to CaCO3 addition, CaO addition has a catalytic effect on carbothermic formation h-BN. In order to investigate the reaction mechanism experiments with B2O3 &amp / #8211 / CaO mixtures without active carbon addition into the mixture were conducted. Furthermore nucleation of h-BN from calcium borate melts had been investigated and experiments were conducted with h-BN addition into CaO &amp / #8211 / B2O3 mixtures. It was concluded that nucleation of h-BN in calcium borate slags under experimental conditions is not possible. Hexagonal BN should be present in the system in order to activate the second nitrogen dissolution followed by h-BN precipitation mechanism. Highest efficiency was achieved in the experiment conducted with CaCO3 addition and largest particle size was observed during the experiment conducted to investigate the effect of nucleation.

Effect of Na2CO3 on formation of hexagonal boron nitride (h-BN) by carbothermic method has been studied by subjecting B2O3-C and Na2CO3-added B2O3-C mixtures to N2 (g) atmosphere. Na2CO3 amount in the mixtures was changed in the range of 0-40 wt. %. Time and temperature were used as experimental variables. Reaction products were analyzed by XRD and scanning electron microscope. Na2CO3 was found to increase both the amount and the particle size of h-BN similar to CaCO3 [1]. Na2CO3 was found to be less effective than CaCO3 in increasing the amount while it was more effective than CaCO3 in increasing the particle size of h-BN forming.

Carbothermic production of hexagonal BN by using boric acid and ground colemanite mined from Bigadi&ccedil / Region in Turkey was investigated by subjecting pellets prepared from B2O3, activated carbon and colemanite mixtures to nitrogen gas at 1500&deg / C. Similar to CaCO3 addition, colemanite addition to the B2O3-C mixtures resulted in higher amounts of h-BN in the final products. As a result of the experiments conducted with colemanite and CaCO3 additions providing the same quantity of CaO to the initial mixtures, similar amounts of hexagonal BN in the reaction products were observed. As a result of the experiments conducted with different compositions of colemanite- B2O3- C mixtures, 5 wt % colemanite addition was determined to be the optimum composition giving the highest amount of hexagonal BN in the reaction products. Increasing duration of the experiments increased the amount and particle size of h-BN formed in the products. Optimum amount of colemanite addition resulted in higher amounts and coarser particles of h-BN in the products than the optimum amounts of CaCO3 addition.

Doctor of Philosophy / Department of Chemical Engineering / James H. Edgar / Hexagonal boron nitride (hBN) has recently been envisioned for electronic, optoelectronic, and nanophotonic applications due to its strong anisotropy and unique properties. To realize these applications, the ability to synthesize single crystals with large size and low defect density is required. Furthermore, a detailed mechanistic understanding of hBN growth process is helpful for understanding and optimizing the synthesis technique for high quality crystals. In this dissertation, the production of large-scale, high-quality hBN single crystals via precipitation from metal solvents, including Ni-Cr and Fe-Cr, was demonstrated. The use of Fe-Cr mixture provides a lower cost alternative to the more common Ni-Cr solvent for growing comparable crystals. The clear and colorless crystals have a maximum domain size of around 2 mm and a thickness of around 200 μm. Detailed characterizations demonstrated that the crystals produced are pure hBN phase, with low defect and residual impurity concentrations. The temperature-dependent optical response of excitons showed that the exciton-phonon interaction in bulk hBN is in the strong-coupling regime. A new growth method for monoisotopic hBN single crystals, i.e. h¹⁰BN and h¹¹BN, was developed, by which hBN single crystals were grown using a Ni-Cr solvent and pure boron and nitrogen sources at atmospheric pressure. The chemical bonding analysis revealed that the B-N bond in h¹¹BN is slightly stronger than that in h¹⁰BN. The polariton lifetime in our monoisotopic hBN samples increases threefold over the naturally abundant hBN, and the isotopic substitution changes the electron density distribution and the energy bandgap of hBN. The ability to produce crystals in this manner opens the door to isotopically engineering the properties and performance of hBN devices. Atomistic-scale insights into the growth of hBN were obtained from multiscale modeling combining density functional theory (DFT) and reactive molecular dynamics (rMD). The energetics and kinetics of BN species on Ni(111) and Ni(211) surfaces were calculated by DFT. These DFT calculations data were subsequently used to generate a classical description of the Ni-B and Ni-N pair interactions within the formulation of the reactive force field, i.e., ReaxFF. MD simulations under the newly developed potential helped reveal the elementary nucleation and growth process of an hBN monolayer - nucleation initiates from the growth of linear BN chains, which further evolve into branched and then hexagonal lattices. In the end, molecular dynamics simulations demonstrated that the thermodynamic preference of hBN geometries varying from triangle to hexagonal can be tuned by B to N molar ratios, and gas phase N₂ partial pressure, which is also supported by quantum mechanics calculations. The modeling confirms that the nitrogen species indeed plays an important role in dictating sizes and edge terminations of hBN sheets.

Master of Science / Department of Chemical Engineering / James H. Edgar / Hexagonal boron nitride has been used as an inert, refractory material with excellent resistance to thermal decomposition and oxidation for more than fifty years. In the past few years, hBN has been targeted for potential electrical and optical devices such as neutron detectors, ultraviolet light emitters, deep ultraviolet light detectors, and substrates for graphene and other atomically-thin two-dimensional materials. All of these potential applications benefit from high quality, single crystals, with thicknesses varying from nanometers to microns. This research was undertaken to investigate four aspects of hBN crystal growth and recovery. (1) In an effort to optimize hBN crystal growth from a nickel-chromium flux, a series of stepped cooling experiments were undertaken. The temperature profile was stepped in a way as to promote growth in both the a and c directions, at their optimal growth conditions. Crystals were found to be typically 100-500 µm across and thickness of approximately 20-30 µm with a pyramid-like crystal habit. (2) A method for the removal of hBN crystals prior to freezing of the metal flux was demonstrated using a specialized hot pressed boron nitride crucible capable of removing hBN crystals from the flux in situ. (3) Growth of isotopically pure hBN crystals was undertaken. By modifying the crucible material for solution growth, enrichment of hBN crystals over 90% was accomplished. (4) Exfoliation of hBN has many potential applications, specifically as graphene-hBN heterostructures where layers approaching thicknesses of single atoms are most effective surface to interact with graphene as an electronic device. Several methods were tested toward exfoliating a single crystal resulting in few-layered hexagonal boron nitride nanosheets. As a result of these investigations a greater understanding of hBN bulk growth, its isotopic enrichment, its recovery, and its exfoliation was obtained.

The SRV test rig was used to evaluate the friction and wear properties of a lubricant in a laboratory setup. Normally, the coefficient of friction and the amount of wear that occurred are measured while the wear scar surface is also evaluated. Special attention was paid to factors that affect the repeatability. The test fluid was subjected to a friction and wear test on the SRV test rig in order to determine what factors affect the repeatability of the coefficient of friction, the amount of wear that occurred and the wear scar appearance. The test fluid used was based on rapeseed oil and white mineral oil. The fluid also contained an extreme pressure additive in the form of sulphurised ester. This was also compared for the same test fluid with dispersed hexagonal-boron nitride (h-BN) nanoparticles. The standard test method as described by ASTM D 6425, was used as test method. Instead of the standard temperature, the block temperature was increased to 100 °C in order to simulate harsher operating environments. The load was set at 200 N It was found that:  The rapid load increase from 50 to 200 N at the end of the running-in period (as described in the standard test method) caused poor repeatability. The test was modified with a more gradual load application for the duration of the running-in period (30 N/min), which resulted in improvement in the repeatability of the tests conducted.  The moisture content in the atmosphere also affected the repeatability of the friction and wear tests. This was most likely due to the formation of a corrosion layer that involves water and by keeping the relative humidity constant, a further improvement in the repeatability was observed. The addition of the h-BN nanoparticles resulted in an improvement of the repeatability of the coefficient of friction (COF), wear scar surface (WSS) and wear scar volume (WSV), since the wear scar surfaces indicated that the particles remove the corrosion layers. This could have led to more consistent wear surfaces for the duration of the test.  The particles also influenced the corrosion layer formation. For both fluids, Raman spectroscopy indicated that greigite (Fe3S4) and goethite (α-FeOOH) were found on the surface, while additional corrosion products were found on the wear scar surface for the test fluid with dispersed particles. These compounds were melanterite (FeSO4.7H2O) and rozenite (FeSO4.4H2O). All these corrosion products were most likely formed due to the reaction of iron from the specimens with sulphurised esters in the test fluid. / Dissertation (MEng)--University of Pretoria, 2015. / tm2015 / Chemical Engineering / MEng / Unrestricted

The addition of hexagonal boron nitride (h-BN) has introduced a comprehensive reinforcing effect to the mechanical and electrochemical properties of commercial concrete, including fiber reinforced concrete (FRC) and steel fiber reinforced concrete (SFRC). Although this has been proven effective and applicable, further investigation and study is still required to optimize the strengthen result which will involve the exfoliation of h-BN into single-layered nano sheet, improving the degree of dispersion and dispersion uniformity of h-BN into concrete matrix. There is currently no direct method to test the degree of dispersion of non-conductive particles, including h-BN, in concrete matrix, therefore it is necessary to obtain an analogous quantification method like SEM, etc. The reinforcing mechanism on concrete, including FRC and SFRC is now attracting a great number of interest thanks to the huge potential of application and vast demand across the world. This study briefly describes the reinforcing mechanism brought by h-BN. In this study, different samples under varied conditions were prepared according to the addition of h-BN and dispersant to build a parallel comparison. Characterization is mainly focused on their mechanical properties, corrosive performance and SEM analysis of the cross-section of post-failure samples.