Abstract Theoretical investigations on the ZrxTa1-xO1+xN1-x system predict that some of its phases are likely to possess relatively high hardness values.(1) Such materials may be suitable for industrial application as cutting tools. The motive of the project was to investigate the best synthesis route and a method for obtaining well sintered, dense oxynitride phases with a nominal composition TaON (x=0), Ta0.8Zr0.2O1.2N0.8 (x=0.2) and Ta0.3Zr0.7O1.7N0.3 (x=0.7). This was achieved through three main steps, i.e. synthesis of the oxynitride powders, high pressure sintering and evaluation of mechanical properties. A sol gel method was used to obtain the precursor oxide powders. TaCl5 and 70wt% zirconium propoxide were used as the starting materials. Oxide gels were formed by dissolving precursor materials in absolute ethanol for 15minutes with continuous stirring, followed by subsequent hydrolysis to form gels which were aged for 24hrs at 800C. The gels were dried in air at 1000C for 12hrs in a drying oven followed by calcinations in a muffle furnace at 6000C for 6hrs to remove the alkyls and chloride ions. High surface area amorphous powders were obtained (~6.60 ± 0.02 m2/g in the case of Ta2O5) after milling with 4mm steel balls for 4hrs in a planetary mill. The respective oxynitrides were obtained by thermal nitridation using an ammonia (99.99%) flow method. A temperature of 9000C maintained for 4hrs in the presence of water vapour at an ammonia flow rate of 50cm3/min were found to be the optimum nitridation conditions. The water vapour pressure was realised by bubbling the ammonia through a water bath at room temperature prior to supply to the furnace. The water vapour pressure of such a set up was approximated to be ~3.1*103Pa. This nitridation process was carried out in a tube furnace using a silica tube of length 1200mm and external diameter of 40mm and an alumina boat as the holding vessel. Approximately 2g of oxide powder were used for each run. The dependency of nitridation on temperature and ammonia flow rate iii was investigated for the formation of TaON. Pure TaON formation was found to be more favoured by temperatures of 9000C with a heating rate of 200C/min and by an ammonia flow rate range of 40-50cm3/min. These conditions were also used for the mixed Ta-Zr oxynitrides. Ta0.3Zr0.7O1.7N0.3 formation was found to be dependent on the heating rate with ZrO2 forming beside the oxynitride solid solution above a heating rate of 100C/min. In the present work the phenomenon has been found to be dependent on the kinetics of the crystallisation reactions. At higher heating rates crystallisation of the separate phases is favoured leading to the formation of separate phases. On the other hand with an optimum heating rate the solid solution is maintained to the final nitridation temperature. The powders were found to be thermally stable in air above 6000C with TaON being the most stable with a weight change occurring at a temperature of ~6900C. The powders were stable in pure nitrogen well above 10000C. Sintering in a hot press in the temperature range of 900-14000C at a heating rate of 500C/min and a pressure range of 50-85MPa using previously heat treated h-BN crucibles in argon resulted in porous, partially densified materials. A maximum % theoretical density of 81.6% was obtained for TaON at 10000C and 85MPa pressure applied for 1hr. TaON oxidised to Ta2O5 above 10000C with an oxide phase transition being observed above 13000C. High pressure sintering was carried out in the temperature and pressure regime of 920- 12000C and 3-5.5GPa respectively in the case of TaON. The mixed Ta-Zr oxynitrides were sintered at 3GPa at a temperature of 11000C. No phase transitions were observed in all cases. An average hardness value of ~16.8GPa and fracture toughness of ~3.4MPam1/2 were obtained for the TaON phase. Ta0.3Zr0.7O1.7N0.3 and Ta0.8Zr0.2O1.2N0.8 were found to possess hardness values of 13.4GPa and 13.02GPa respectively under the same sintering conditions. It was observed that the hardness values obtained for TaON are higher than those for ZrO2 or HfO2 ceramics, due to the stronger covalent bonding in nitrogen present in TaON. On the other hand the fracture toughness values are as low as those of fully stabilised ZrO2 materials due to lack of phase transformation toughening.

No description available.

¡@¡@In this thesis, the formation of m-plane (10-10) Gallium nitride (GaN) on the surface of a-plane (100) lithium gallate (LiGaO2, LGO) substrates via nitridation with ammonia (NH3) at high temperature. The parameters in this research were mainly focus on temperature, ammonia flow rate, reaction pressure, and growth time. ¡@¡@Specimens were analyzed with various instruments. X-ray Diffraction patterns showed that the nitridation process on LGO substrate resulted in the formation of the GaN single crystalline films. The crystalline quality of the GaN film could be improved by changing parameters of nitridation process. Scanning electron microscope image showed that the structure of GaN films was nanoporous. A red shift in the E2(high) phonon peak of GaN from micro-Raman indicates a compressive stress in the porous GaN with respect to the single crystalline epitaxial GaN. PL intensity ratio (INBE/IYL) of the porous GaN was found to be increased as changing parameters of nitridation process, namely the optical and crystalline quality of porous GaN was improved. Hall measurement showed that the porous GaN was p-type, and it had high hole concentration, good mobility, and low resistivity. Analyses of the elements depth profile by Auger electron spectroscopy. Transmission electron microscopy was used to observe the high resolution cross-section of porous GaN. From the selected area electron diffraction patterns, the orientation relationship between porous and LGO was determined as [100]LGO//[10-10]GaN and [0-10]LGO//[11-20]GaN when zone axis was [0001].

In this work, Nitridated b-LiGaO2 substrates have been used to grow nonpolar a-plane ZnO epitaxial films by a homemade thermal chemical vapor deposition. We control the quality of the films by adjusting the deposition position, growth temperature and oxygen partial pressure. The properties of the ZnO films was investigated by x-ray diffraction(XRD), scanning electron microscopy(SEM), atomic force microscopy(AFM), photoluminescence spectra(PL), Raman spectra(Raman) and transmission electron microscopy(TEM). The results show that the crystal quality, surface morphology and optical properties of a-plane ZnO films are strongly related to the deposition position and growth temperature. It is also found that the surface roughness of the ZnO films gets more smoothly with increasing the oxygen partial pressure. Up to now, ZnO films grown at 700¢J diplay the best crystal quality, the full width at half maximum values of (11-20)ZnO omega scan rocking curve is 0.5¢X. In comparison with above growth condition, it demonstrate better optical properties by shortening deposition position and increasing growth temperature to 750¢J. Room temperature PL spectra exhibit a strong near band edge emission and a negligible green band. Furthermore, low temperature PL spectra is dominated by neutral donor-bound excitons and free electron-to-bound emission. From the TEM selected area diffraction patterns and the XRD phi angle scan, the epitaxy relationship between ZnO and LGO is determined as [001]LGO//[0001]ZnO and [100]LGO//[10-10]ZnO. XRD omega-two theta scan and Raman spectra analysis are carried out to characterize the status of the strain in ZnO film is compressive, the reason is illustrate in detail.

Doctor of Philosophy / Department of Chemical Engineering / Peter H. Pfromm / Since its inception, the Haber-Bosch (HB) process for ammonia (NH3) synthesis has allowed for a significant increase in global food production as well as a simultaneous decrease in global hunger and malnutrition. The HB process is estimated to be responsible for the subsistence of 40% of the world population as approximately 85% of the over 182 metric tons of NH3 produced in 2017 was used as fertilizer for crop production. The natural gas consumed (mostly to generate H2) represents approximately 2% of the global energy budget, while the CO2 produced is about 2.5% of all global fossil CO2 emissions. Approximately 40% of food consumed is essentially natural gas transformed by the HB process into agricultural products. However global food production will need to double due to expected increase in world population to 9.6 billion by 2050 and rising demand for protein among developing nations. A novel thermochemical reaction cycle for sustainable NH3 synthesis at atmospheric pressure is explored herein. Both thermochemical and kinetic rationales are discussed regarding choice of Mn as the cycled reactant. The energetic driving force for these reactions is conceptually derived from concentrated solar energy. Mn was reacted with N2 forming Mn-nitride, corrosion of Mn-nitride with steam at 500 °C formed MnO and NH3, and lastly MnO was reduced at 1150 °C in a 4 vol % CH4 – 96 vol % N2 stream to Mn-nitride closing the cycle. Optimum nitridation at 800 °C and 120 min produced a Mn6N2.58-rich Mn-nitride mixture containing 8.7 ± 0.9 wt. % nitrogen. NH3 yield was limited to 0.04 after 120 min during nitride corrosion but addition of a NaOH promotor improved NH3 yield to 0.54. Mn6N2.58 yield was 0.381 ± 0.083 after MnO reduction for 30 min with CO and H2 but no CO2 detected in the product. Mn-nitridation kinetics were investigated at temperatures between 600 and 900 °C for 10 and 44 μm reactant powder particle sizes. That equilibrium conversion decreased with increasing temperature was confirmed. Jander’s rate law, which assumes gaseous reactant diffusion through a solid product layer, described the experimental data reasonably well. The rate constants and initial rates were as much as an order of magnitude greater for the 10 μm Mn reactant particle size. Additionally the activation energy was found to be 44.1 kJ mol-1 less for the 10 μm reactant particle size. Reducing the particle size had a small but positive effect on Mn-nitridation kinetics. Further reducing particle size will likely have a greater impact. A review of relevant classical thermodynamics is discussed with special attention paid to open systems. Confidence issues regarding over-reliance on x-ray diffraction are considered with options suggested for mitigation. Opportunities for future work are assessed.

Veeco Applied EPI 930 molecular beam epitaxy system equipped with a radio frequency plasma assisted nitrogen source has been introduced and the growth procedure and some specialized measurements are also described. The GaN thin films grown by RF-MBE have been talked about nitridation, low temperature GaN buffer layer, and GaN epilayer. The nitridation is a necessary for grown GaN on c-sapphire. From the result of the HRXRD symmetric (002) rocking curve, the magnitude of the FWHM of the GaN films without nitridation was larger than the films with nitridation. The growth temperature of the LT GaN buffer layer was the major factor on the quality of GaN epilayer which grown on the almost without nitridated sapphire substrate. The growth condition of high growth temperature, thin, low growth rate, and low N/Ga ratio of the LT GaN buffer layer can improve the sequent GaN epilayer quality. On the other hand, in the N/Ga flux ratio of GaN epitaxy layer experiment, we have found three interesting results. First, the narrowest peak width of PL spectrum appeared in a slight Ga-rich condition. Second, the smallest of HRXRD FWHM appeared in the nearly stoichiometry condition. Third, the highest electron mobility and less overall dislocations appeared in a slight N-rich condition. Finally, we report the results about AlGaN/GaN heterostructure grown by metalorganic chemical vapor deposition. The piezoelectric effect on the Alx-£_In£_Ga1-xN/GaN heterostructures was investigated and we found that a little In atom in the spacer (£_< 0.01 %) will substantially reduce the strain at interface due to the much larger size of the In atom in comparison to Al and Ga atoms. The electric field at the interface is reduced one order of magnitude smaller than that of the heterostructure without In atom. Two SdH oscillations beat each other due to the population of the lowest two subbands was been seen. Another two SdH oscillations beating have been observed in modulation-doped AlxGa1-xN/GaN heterostructures caused by the finite zero-filed spin splitting due to the inversion-asymmetry-induced bulk k3 term.

Presented thesis is focused on the study of properties of Ga and GaN nanostructures on graphene. In the theoretical part of the thesis a problematics of graphene and GaN fabrication is discussed with a focus on the relation of Ga and GaN to graphene. The experimental part of the thesis deals with the depositions of Ga on transferred CVD-graphene on SiO2. The samples are analyzed by various methods (XPS, AFM, SEM, Raman spectroscopy, EDX). The properties of Ga on graphene are discussed with a focus on the surface enhanced Raman scattering effect. Furthermore, a deposition of Ga on exfoliated graphene and on graphene on a copper foil is described. GaN is fabricated by nitridation of the Ga structures on graphene. This process is illustrated by the XPS measurements of a distinct Ga peak and the graphene valence band during the process of nitridation.

No description available.

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