Optical Properties of Ar Ions Irradiated Nanocrystalline ZrC and ZrN Thin Films

Thin nanocrystalline ZrC and ZrN films (less than 400 nanometers), grown on (100) Si substrates at a substrate temperature of 500 degrees Centigrade by the pulsed laser deposition (PLD) technique, were irradiated by 800 kiloelectronvolts Ar ion irradiation with fluences from 1 times 10(sup 14) atoms per square centimeter up to 2 times 10(sup 15) atoms per square centimeter. Optical reflectance data, acquired from as-deposited and irradiated films, in the range of 500-50000 per centimeter (0.066 electronvolts), was used to assess the effect of irradiation on the optical and electronic properties. Both in ZrC and ZrN films we observed that irradiation affects the optical properties of the films mostly at low frequencies, which is dominated by the free carriers response. In both materials, we found a significant reduction in the free carriers scattering rate, i.e. possible increase in mobility, at higher irradiation flux. This is consistent with our previous findings that irradiation affects the crystallite size and the micro-strain, but it does not induce major structural changes

Enhanced Radiation Tolerance of Ceramic Thin Films by Nano-structural Design

Thin films techniques are widely used in microelectronic devices, optical coatings, batteries and solar cells, which would be applied under harsh environment spanning from outer space to nuclear plant where defects are easily generated, accumulate and eventually degrade the materials properties. Oxide and nitride thin films are widely used and mostly studied in such applications. Moreover, MgO and ZrN have been considered as promising candidates of Inert Matrix materials to reduce hazardous nuclear waste. Therefore, studies on the defects development, and more importantly, the enhanced damage tolerance by microstructural design are of great importance. However, only limited study has been conducted up-to-date. In this dissertation, interface mitigation effects on radiation damage have been explored in TiN/MgO epitaxial thin films. After ion implantation with He+ ions, no hardness variation is observed in the epitaxial multilayers, and high resolution TEM indicates no obvious ion damage in the MgO layers within the epitaxial multilayer samples. However, single layer MgO film shows a significant hardness increase of ∼20% and high density point defect clusters are clearly identified. The results suggest that, in this system, epitaxial interfaces could act as effective point defect sinks in reducing the defect density and suppressing the ion-implantation induced hardening in MgO, and thus are responsible for the enhanced radiation tolerance properties. The grain size dependent response in nanocrystalline (nc) ZrN under high dose heavy ion implantation has been studied with Fe2+ ion, and it is found that the ZrN film with the average grain size of 9 nm shows prominently self-healing effects as evidenced by suppressed grain growth, alleviated radiation softening, as well as reduced variation in electrical resistivity. In contrast, ZrN with the larger average grain size of 31 nm shows prominent softening and resistivity increase after implantation, attributed to the high density of vacancy like defect clusters formed inside the grains. The distinct implantation effects on microstructure, residual stress, grain growth, and electric resistivity of thin films with different grain sizes were discussed, and the influence of grain boundaries on enhanced tolerance to implantation damage in nc-ZrN is demonstrated. In order to further study the real-time response of the designed microstructures, and their kinetic interactions with defects. In-situ irradiation on MgO/ZrN multilayer systems with non-epitaxial interfaces as well as grain boundaries has been conducted, which shows clearly the cyclic process of the defects removal by high angle grain boundaries and effectively absorbed by interfaces. Another In-situ study on the MgO/TiN epitaxial films has demonstrated that the implantation induced defects migrate to the interfaces, and annihilate there, that improve the MgO tolerance against amorphization. The comparison has shown that the non-epitaxial interfaces are more effective in absorbing defects manifested by the higher mobility of defects migration towards the MgO/ZrN interfaces. The research findings could provide guidance for microstructural design of functional ceramic thin films for advanced technological applications under extreme conditions from outer space exploration to nuclear energy generation

Microstructural evolution of zirconium carbide (ZrCₓ) ceramics under irradiation conditions

A comprehensive understanding of the microstructural evolution of Zirconium Carbide (ZrC2) ceramics under irradiation conditions is required for their successful implementation in advanced Gen-IV gas-cooled nuclear reactors. The research presented in this dissertation focusses on elucidating the ion and electron irradiation response of ZrC2 ceramics. In the first part of the research, the microstructural evolution was characterized for ZrC2 ceramics irradiated with 10 MeV Au3+ ions up to doses of 30 displacement per atom (dpa) at 800 ºC. Coarsening of the defective microstructure, as a function of dose, was revealed by transmission electron microscopy analysis. The lack of change in the irradiated microstructure, at doses above 5 dpa, indicated that a balance between irradiation damage accumulation and dynamic annealing of defects was reached. It was also found that concurrent oxidation occurred during the ion irradiation. The effects of irradiation on the morphology and microstructure of the initial oxide formed on the surface of ZrC2 were investigated. The concomitant reduction in size and surface coverage of the oxide nodules at high doses, indicated that oxide dissolution was the predominant mechanism under irradiation conditions. In the second part of the research, Zirconium Carbide (ZrC2) was irradiated with 10 MeV Au3+ ions to a dose of 10 dpa and subsequently with 300 keV electrons in a transmission electron microscope (TEM). It was found that high-energy electron irradiation of pre-damaged ZrC2 foils induce atomic mixing via radiation enhanced diffusion (RED), producing surface oxidation of the TEM foil

Rapid microwave-assisted synthesis and characterization of transition metal carbides and nitrides

The aim of this thesis is to describe the rapid microwave synthesis of a number of transition metal carbides and nitrides as well as their structural characterization and develop reproducible procedures that can cut processing times and, hence, reduce the energy consumption. Specifically, 4 binary systems are investigated: V–C, Zr–C, Hf–C and Zr–N. Carbide syntheses were conducted using either elemental or oxide precursors under argon, whereas the nitride system was investigated from zirconium powder under either nitrogen or ammonia gas. Microwave syntheses were conducted using both multi-mode cavity (MMC) and single- mode cavity (SMC) microwave reactors at a power of 800 W and 1 kW, respectively, with an operating microwave frequency of 2.45 GHz. Vanadium carbide production from both oxide and elemental precursors was achieved in 6 minutes for MMC experiments and 2 minutes for SMC experiments. Zirconium carbide was obtained from zirconium powder and graphite in 20 minutes in a MMC reactor and 6 minutes in a SMC reactor. Unfortunately, the carbothermal reduction of ZrO2 to ZrC was not successful as the starting materials did not react with each other and no product formation was observed. Similar results were obtained for the carburization of HfO2. However, hafnium carbide was synthesized combining graphite with hafnium metal in 20 minutes in a MMC reactor and 6 minutes in a SMC reactor but the formation of additional oxide phases (i.e. HfO2) was also observed. Finally, zirconium nitride production was investigated in a MMC reactor and prepared in 20 minutes from zirconium metal under either N2 or NH3 gas. Generally, oxygen inclusion was observed in all experiments either in the form of oxycarbide or additional oxide phase(s). Once a reproducible experimental technique was established, products were characterized by several analytical techniques. Powder X-ray diffraction (PXRD) was used to identify product phases, study the phase evolution of the microwave processes and refine the MW-synthesized structures by Rietveld method. Powder neutron diffraction (PND) was used on the V-C and Zr-C samples to evaluate product purity and the C and O occupancies of the final products. Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX) provided information about product morphology, particle size and purity. EDX supported the evidence for oxygen inclusion across samples. Supporting information in favour of this was additionally offered by Raman spectroscopy. X-ray photoelectron spectroscopy (XPS) was used to analyze the surface of products together with the chemical state of the elements present in it

Laser Ablation Applied for Synthesis of Thin Films: Insights into Laser Deposition Methods

This chapter will focus on laser ablation applied for thin film deposition. The first thin films deposition method based upon laser ablation was pulsed laser deposition (PLD), that could produce thin films out of metals, ceramics and even temperature resistant organics. The need of depositing increasingly complex and delicate materials, lead to radical modifications of PLD and allowed other laser ablation methods to develop. If complex libraries are to be synthesized two or more plasmas will be mixed and the thin films will have a variable composition over surface. This technique is called Combinatorial PLD (CPLD)

Impact of ion irradiation damage on SiC and ZrN mechanical properties

Key to the safe operation of nuclear reactors is the understanding of materials degradation due to neutron damage. Ion implantation is often used as a surrogate for nutron damage when screening nuclear candidate materials. Ion implantation results in a thin damage layer, the mechanical properties of which are often difficult to determine. In this study a micromechanical test regime is developed in a model material, 6H single crystal silicon carbide (SiC). This test technique is then applied to gold ion irradiated zirconium nitride (ZrN). Micromechanical test samples are often prepared using a focused ion beam. However, ion beam milling has the potential to damage the crystal structure of a material and introduce residual stress. Therefore, a range of cutting strategies were used to assess the effects of focused ion beam cutting on the modulus and strength of SiC cantilevers. The effects of sample size were also explored. Gallium ion milling resulted in amorphisation of the surface of the SiC crystal micro cantilevers. The thickness of the amorphous zone was then reduced using low voltage cleaning. Low voltage cleaning did not, however, result in increased mechanical performance as other unintended consequences such as cantilever edge rounding occurred. SiC exhibited a plastic deformation threshold of around 0.3 × 0.3 µm but did not exhibit a significant size effect. Nanoindentation was used as a benchmark test to compare to the mechanical properties gathered during micro bend testing. Under indentation conditions, a size effect was identified in hardness and modulus but not in fracture toughness. Modulus results from indentation, and micro bend testing was comparable when ion damage was accounted for.Hot pressed ZrN samples were ion implanted with gold ions. Microstructural characterisation, nanoindentation and micromechanical tests were performed in the ion implanted zone. Microstructural characterisation identified a dual phase microstructure consisting of ZrN and Zr2ON2. The implanted layer consisted of implanted gold ions followed by a network of dislocations centred around a depth of 1.20 µm. High-resolution electron backscatter diffraction (HR-EBSD) identified that tensile surface stresses and compressive subsurface stress had been introduced. Nanoindentation linked ion implantation to increased hardness and no modification in modulus. Micromechanical testing indicated a reduction in modulus and strength. This work highlighted the need to understand sample size effect and ion damage on micro mechanical tests if they are to be used for screening nuclear materials.</div

Processing and Characterisation of ZrCxNy Ceramics as a Function of Stoichiometry via Carbothermic Reduction-Nitridation

Carbothermal reduction-nitridation of ZrO2 has been studied in the context of application of non-oxide zirconium ceramics as fuel components in advanced nuclear fuels. Varying processing parameters of nitridation of ZrCx (where 0.7 x 1) powders revealed the rate increased with dwell time, dwell temperature and higher carbon content of the starting ZrCx powders. A novel mechanism is reported whereby nucleation of small ( 500 nm) ZrN containing crystals occurs on the surface of the ZrCx powder particles, growing separate to the carbide particle and resulting in mixed phases. Sintering of the ZrCxNy powders by hot pressing resulted in higher densities than commercially-available ZrC powders suggesting nitrogen content improves the sinterability of ZrC containing ceramics. Thermal and electrical conductivity of the ZrCxNy ceramics were all higher than the ceramics produced from commercially-available ZrC and ZrN powders. Room temperature thermal conductivities of the ZrCxNy ceramics were found to be 35 and 43 Wm−1K−1 for the lowest and highest N-containing ZrCxNy ceramics and increased with temperature to 45 and 55Wm−1K−1 respectively at 2073 K. Electrical conductivities were in the range 250-450 × 104 −1m−1 for the ZrCxNy ceramics (at 298 K) and again increased with increasing nitrogen content. The increase in thermal conductivity of ZrCxNy with nitrogen content is due to the increase in electrical conductivity. Oxidation studies of ZrN revealed oxidation begins at around 773 K with an initial destabilisation of ZrN occurring at around 673 K. A decrease in oxidation rate was observed between lower (973-1073 K) and higher temperatures (1173-1273 K). This is attributed to dense protective oxide scales forming at higher temperature (1173-1273 K) compared to porous oxide scales forming at lower temperature ( 1073 K). However, this protective layer fails at higher temperature (1373 K), attributed to increased oxygen diffusion through the oxide layer.Open Acces

Phase formation and thermal conductivity of zirconium carbide

“This research focused on the synthesis and phase formation of zirconium carbide with different carbon contents, and lattice thermal conductivity of zirconium carbide with different carbon vacancy, hafnium, and oxygen contents. Nominally pure phase ZrCx was synthesized by solid-state reaction of zirconium hydride (ZrHs) and carbon black at a temperature as low as 1300°C. The powder synthesized at 1300C was carbon deficient ZrCx . Carbon stoichiometry of the as- synthesized powders increased as the synthesis temperature increased. As the synthesis temperature increase, the oxygen content of ZrCx decreased due to the increasing C site occupancy. Low stoichiometry ZrC0.6 powders were synthesized at 1300C and 2000C, and the formed phases were investigated. Carbon vacancy ordered phases were detected by neutron diffraction and selected area electron diffraction. Lattice thermal conductivities of ZrCx with different carbon contents (x = 1.0, 0.75, 0.5) and different hafnium contents (3.125 at% and 6.25 at% were studied theoretically. A combination of first-principles calculations and the Debye-Callaway model was employed to predict the lattice thermal conductivities. Lattice thermal conductivities of all the compositions decreased as temperature increased. Increasing carbon vacancy content reduced the lattice thermal conductivity while increasing the grain size increased the lattice thermal conductivity. Lattice thermal conductivities of ZrCx also decreased as the content of Hf increased. Carbon vacancies and Hf impurities decreased the phonon transport, thus the lattice thermal conductivity decreased”--Abstract, page iv