Group 3 metal clusters are synthesized by laser vaporization in a pulsed cluster beam source and identified with laser ionization time-of-flight mass spectrometry. The adiabatic ionization energies and vibrational frequencies of these clusters are measured using mass-analyzed threshold ionization (MATI) spectroscopy. Their structures and electronic states are determined by combining the MATI spectra with quantum chemical calculations and spectral simulations. This dissertation focuses on the study of several small molecules, which include LaO2, La2, M2O2, M3O4, M3C2, and La3C2O, where M = Sc, Y, and La. Except for La2, these molecules exhibit strong ionic characters between the metal and oxygen or carbon atoms and can be described as [O-][La2+][O-], [M2+]2[O2-]2, [M8/3+]3[O2-]4, [M2+]3[C3-]2, and [La8/3+]3[C3-]2[O2-]. The interactions between the metal atoms form covalent bonds, which can be described by a triple bond in La2, a two-center two electron bond in M2O2, a three-center one electron bond in M3O4, and a three-center three electron bond in M3C2. In addition, the electron in the non-bonding highest occupied molecular orbital (HOMO) is localized in the La 6s orbital in LaO2 and La3C2O. The ground states of these molecules are all in low electron-spin states with the spin multiplicities of 1 or 2. Although the ground electronic state of LaO2 is a linear structure, the excited quartet state of the molecule is determined to be a bent structure. M2O2 and M3O4 have the planar rhombic and cage-like structures, respectively; whereas M3C2 has a trigonal bipyramid structure. La3C2O is formed by oxygen binding with two La atoms of La3C2. Ionization removes a metal-based (n+1)s electron in all neutral molecules, and the resultant ions have similar geometries to those of the corresponding neutral states. In the case of La2, additional ionization of a La 5d electron is also observed.

As our world continues to develop and contend with the impacts of climate change, the scale up renewable energy technologies has never been more urgent. Alternative fuels derived from biomass-derived oxygenates and water splitting offer promising solutions for the transition towards sustainable chemical feedstocks and integration of clean renewable energy sources. However, this technology continues to be hampered by the need for scarce and costly precious metal catalysts. The work done in this thesis explored the facet-dependence of glycerol electrooxidation and studied the application of earth-abundant transition metal carbides (TMCs) and nitrides (TMNs) for reducing precious metal catalyst loadings in water electrolysis and electrooxidation of methanol and glycerol. Glycerol valorization has drawn significant interest in recent years due to the growth in biodiesel production leading to the market saturation of glycerol. While this molecule can be converted into a variety of value-added products, the possibilities have been limited by poor selectivity for C-C bond scission. The breaking of the C-C bonds in glycerol allows for complete extraction of energy from the molecule via complete glycerol oxidation, thereby opening the door for utilizing glycerol as an electrochemical fuel. While platinum (Pt) has been among the most popular catalysts, its tendency for poisoning due to adsorbed CO has hindered its activity. Previously demonstrated to enhance the catalytic activity of platinum (Pt) by reducing CO binding energy and increasing C-C bond scission selectivity in ethanol electrooxidation, TMCs were employed as catalyst supports for the glycerol electrooxidation reaction. This work used electrochemical techniques and in-situ IRRAS to study various loadings of Pt/TaC and Pt/WC to find enhanced C-C bond scission activity at reduced Pt loading because of the synergistic effects between Pt and TMCs. While Pt has remained the benchmark catalyst for glycerol electrooxidation due to its high C-C scission activity, gold (Au) has also found popularity with its high catalytic activity attributed to greater resistance to CO poisoning, despite its favorability for partial glycerol oxidation. Previous studies have hinted at the significance of Au surface facets on glycerol oxidation activity and product selectivity, but none had used nanoparticles with controlled surface facets. This thesis sought to bridge the knowledge gap using precisely-synthesized Au nanocrystals with well-characterized {100}, {110}, and {111} surface facets to provide insight into glycerol electrooxidation on Au. Electrochemical techniques were used in parallel with in-situ IRRAS analysis to uncover the differences in product selectivity and oxidation activity between the three Au surfaces, with Au {111} exhibiting the greatest activity for C-C bond scission, while Au {110} showed the lowest onset potential due to facile AuOH- formation. Hydrogen (H₂) fulfills a critical role in modern society, not only as a renewable fuel, but also as a key chemical feedstock. Production of H₂ from water electrolysis creates opportunities for storing excess energy from renewable sources as an energy-dense fuel and reducing the environmental footprint of chemical processes requiring H₂. However, efforts have been hampered by the dependence on scarce Pt-group catalyst materials. This thesis explores the application of TMNs as an earth-abundant material for enhancing the activity of Pt in the hydrogen evolution reaction (HER). Combined with DFT calculations, the HER activity of monolayer Pt- and Au-modified TMN thin films was correlated with the ΔGH* values in a volcano-type relationship. Electrocatalytic experiments in acidic electrolyte showed that TMN-supported monolayer Pt exhibited similar HER activity to the Pt foil, correlating with intermediate hydrogen adsorption strength. TiN-supported Pt and Au powders were studied to extend the correlations from thin films. Furthermore, the electrochemical stability of TMNs was studied across a wide range of potentials and pH values to generate pseudo-Pourbaix diagrams and identify TMN candidates for HER, alcohol oxidation, ORR and OER applications. Using the pseudo-Pourbaix findings, Pt/TMN catalysts were selected for studying methanol electrooxidation activity. Methanol electrooxidation has drawn significant attention particularly due to interest in direct alcohol fuel cells. Much like the case for glycerol oxidation, while Pt has been the benchmark catalyst, it has been hindered by strong adsorption of CO. As the modification of Pt with other materials, such as ruthenium, has shown promising enhancements to methanol electrooxidation activity, the synergistic effects of Pt modification with TMNs were studied in this work. In the resulting electrochemical experiments, Pt/Mo₂N was found to exhibit negligible activity likely because of its oxidative instability. In contrast, Pt/TiN showed enhanced activity, and in-situ IRRAS experiments suggest that Pt/TiN enhanced the COads-free pathway leading to increased formic acid selectivity. This thesis demonstrated avenues for developing more optimized catalysts with reduced loadings of Pt and other precious metals for applications in alternative fuel production and utilization. The influence of Au surface facets on glycerol oxidation was examined and the synergistic effects between Pt and earth-abundant TMC and TMN materials were used to enhance the electrooxidation of biomass-derived oxygenates and H₂ production from water electrolysis. These electrochemical stability and activity trends can guide future catalyst design for other critical reactions such as oxygen evolution and challenging applications like glycerol electroreduction.

<p>In view of the important applications of carbides and carbo-nitrides of transition metals in the heat-resistant and hard materials industries, the thermodynamic activities of Cr and Mn in the Cr-C, Fe-Cr-C, Mn-Ni-C and Mn-Ni-C-N systems have been studied in the present work by the use of the galvanic cell technique. CaF₂single crystals were used as the solid electrolyte. The phase relationships in selected regions of the systems in question were investigated by the use of the equilibration technique. The phase compositions and microstructures of the alloys were analysed by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM).</p><p>In the Cr-C system, the Gibbs energy of formation of Cr3C2 were obtained from ElectroMotive Force (EMF) measurements conducted in the temperature range 950-1150 K. The values of the enthalpy of formation of Cr₃C₂ were evaluated by the third-law method. The ground-state energy of the hypothetic end-member compound CrC3, in the bcc structure at 0 K, was calculated by use of the Ab-initio method. Based on the obtained results the Cr-C system was reassessed by use of the CALPHAD approach.</p><p>In the Fe-Cr-C system, 16 different alloys were quenched at 1223 K and their equilibrium phases identified by XRD. The experimental results show that the substitution of Cr by Fe in the (Cr,Fe)₇C₃ carbide changes the lattice parameters of the phase. A slight decrease of the lattice parameters with an increase in the Fe content was established. The lattice parameters of the γ-phase in the Fe-Cr solid solution did also show a decrease with an increase of the Fe content. The activities of chromium in Fe-Cr-C alloys were investigated in the temperature range 940-1155 K. The activity of chromium decreases with an increase in the Fe content when the ratio of C/(Cr+C) was constant. It was also established that the activity of chromium decreases with an increase of the carbon content when the iron content was constant. The experimental results obtained were compared with the data calculated by use of the Thermo-Calc software. </p><p>In the Mn-Ni-C system the phase relationships were investigated at 1073 K as well as at 1223 K. The experimental results obtained showed that the site fraction of Ni in the metallic sublattice of the carbides M₂₃C₆, M₇C₃ and M₅C₂ (M=Mn and Ni) was quite low (approximately 2~3 percent). The activities of manganese in Mn-Ni-C alloys were investigated in the temperature range 940-1165 K. The three-phase region γ/M₇C₃/graphite was partly constructed at 1073 K. </p><p>In the Mn-Ni-C-N system, nitrogen was introduced into Mn-Ni-C alloys by equilibrating with N2 gas. It was established that the solubility of nitrogen in the investigated alloys was effected by the carbon content, and that a (Mn,Ni)₄(N,C) compound was formed in the nitrided alloys. EMF measurements were performed on Mn-Ni-C-N alloys in the temperature interval 940-1127 K. The addition of nitrogen to Mn-Ni-C alloys was found to decrease the activity of manganese. The negative effect of nitrogen on the activity of manganese was found to decrease as the carbon content increased.</p><p><b>Keywords:</b> Thermodynamic activity; Galvanic cell technique; Transition metal carbides; Transition metal nitrides; Phase equilibrium; Thermodynamics; Differential thermal analysis; Scanning electron microscopy; Transmission electron microscopy; Ab-initio calculations; CALPHAD approach;</p>

<p>The subject of the present thesis is theoretical first principles electronic structure calculations on designer hard materials such as the transition metal carbides and oxides. The theoretical investigations have been made in close collaboration with experimental research and have addressed both bulk electronic properties and surface electronic properties of the materials.</p><p>Among the bulk studies are investigations on the effects of substoichiometry on the relative phase stabilities and the electronic structure of several phases of MoC and the nature of the resulting vacancy peaks. The changes in phase stabilities and homo-geneity ranges in the group IV to VI transition metal carbides have been studied and explained, from calculations of the T=0 energies of formation and cohesive energies. The anomalous volume behavior and phase stabilities in substoichiometric TiC was studied including effects of local relaxations around the vacancy sites. The vacancy ordering problem in this compound was also studied by a combination of electronic structure calculations and statistical physics.</p><p>The studies of the surface electronic properties include research on the surface energies and work functions of the transition metal carbides and an investigation on the segregation of transition metal impurities on the TiC (100) surface.</p><p>Theoretical studies with the aim to facilitate the realization of novel designer hard materials were made, among these a survey of means of stabilizing potentially super-hard cubic RuO₂, studying the effects of alloying, substoichiometry and lattice strains. A mechanism for enhancing hardness in the industrially important hard transition metal carbides and nitrides, from the discovery of multi-phase/polytypic alloys, has also been predicted from theoretical calculations.</p>

In view of the important applications of carbides and carbo-nitrides of transition metals in the heat-resistant and hard materials industries, the thermodynamic activities of Cr and Mn in the Cr-C, Fe-Cr-C, Mn-Ni-C and Mn-Ni-C-N systems have been studied in the present work by the use of the galvanic cell technique. CaF2single crystals were used as the solid electrolyte. The phase relationships in selected regions of the systems in question were investigated by the use of the equilibration technique. The phase compositions and microstructures of the alloys were analysed by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). In the Cr-C system, the Gibbs energy of formation of Cr3C2 were obtained from ElectroMotive Force (EMF) measurements conducted in the temperature range 950-1150 K. The values of the enthalpy of formation of Cr3C2 were evaluated by the third-law method. The ground-state energy of the hypothetic end-member compound CrC3, in the bcc structure at 0 K, was calculated by use of the Ab-initio method. Based on the obtained results the Cr-C system was reassessed by use of the CALPHAD approach. In the Fe-Cr-C system, 16 different alloys were quenched at 1223 K and their equilibrium phases identified by XRD. The experimental results show that the substitution of Cr by Fe in the (Cr,Fe)7C3 carbide changes the lattice parameters of the phase. A slight decrease of the lattice parameters with an increase in the Fe content was established. The lattice parameters of the γ-phase in the Fe-Cr solid solution did also show a decrease with an increase of the Fe content. The activities of chromium in Fe-Cr-C alloys were investigated in the temperature range 940-1155 K. The activity of chromium decreases with an increase in the Fe content when the ratio of C/(Cr+C) was constant. It was also established that the activity of chromium decreases with an increase of the carbon content when the iron content was constant. The experimental results obtained were compared with the data calculated by use of the Thermo-Calc software. In the Mn-Ni-C system the phase relationships were investigated at 1073 K as well as at 1223 K. The experimental results obtained showed that the site fraction of Ni in the metallic sublattice of the carbides M23C6, M7C3 and M5C2 (M=Mn and Ni) was quite low (approximately 2~3 percent). The activities of manganese in Mn-Ni-C alloys were investigated in the temperature range 940-1165 K. The three-phase region γ/M7C3/graphite was partly constructed at 1073 K. In the Mn-Ni-C-N system, nitrogen was introduced into Mn-Ni-C alloys by equilibrating with N2 gas. It was established that the solubility of nitrogen in the investigated alloys was effected by the carbon content, and that a (Mn,Ni)4(N,C) compound was formed in the nitrided alloys. EMF measurements were performed on Mn-Ni-C-N alloys in the temperature interval 940-1127 K. The addition of nitrogen to Mn-Ni-C alloys was found to decrease the activity of manganese. The negative effect of nitrogen on the activity of manganese was found to decrease as the carbon content increased. Keywords: Thermodynamic activity; Galvanic cell technique; Transition metal carbides; Transition metal nitrides; Phase equilibrium; Thermodynamics; Differential thermal analysis; Scanning electron microscopy; Transmission electron microscopy; Ab-initio calculations; CALPHAD approach;

The subject of the present thesis is theoretical first principles electronic structure calculations on designer hard materials such as the transition metal carbides and oxides. The theoretical investigations have been made in close collaboration with experimental research and have addressed both bulk electronic properties and surface electronic properties of the materials. Among the bulk studies are investigations on the effects of substoichiometry on the relative phase stabilities and the electronic structure of several phases of MoC and the nature of the resulting vacancy peaks. The changes in phase stabilities and homo-geneity ranges in the group IV to VI transition metal carbides have been studied and explained, from calculations of the T=0 energies of formation and cohesive energies. The anomalous volume behavior and phase stabilities in substoichiometric TiC was studied including effects of local relaxations around the vacancy sites. The vacancy ordering problem in this compound was also studied by a combination of electronic structure calculations and statistical physics. The studies of the surface electronic properties include research on the surface energies and work functions of the transition metal carbides and an investigation on the segregation of transition metal impurities on the TiC (100) surface. Theoretical studies with the aim to facilitate the realization of novel designer hard materials were made, among these a survey of means of stabilizing potentially super-hard cubic RuO2, studying the effects of alloying, substoichiometry and lattice strains. A mechanism for enhancing hardness in the industrially important hard transition metal carbides and nitrides, from the discovery of multi-phase/polytypic alloys, has also been predicted from theoretical calculations.

The present thesis deals with a theoretical description of issues regarding chemical bonding, structure and stability of transition metal carbides and multilayered structures. First principles density functional theory has been used extensively to investigate the properties of alloyed solutions of transition metal carbides. Joint theoretical and experimental investigations have shown that there is a driving force for carbon to be released from these ternary carbide systems as a response to the alloying. This release of carbon was shown to yield favorable lubricating properties in the case of alloyed solutions of Ti-Al-C, that were not present in the case of pure TiC, a property that can be used to design new materials that combine high hardness with favorable tribological properties. From calculations of the activation energy of C diffusion in the vicinity of substitutional transition metal impurities (M) in TiC, it is found that the mobility of C atoms is increased due to the presence of the impurities. The lowering of the activation energy barriers suggests that the mobility of C in alloyed solutions of Ti-M-C is increased and will be more pronounced at lower temperature than for C diffusion in TiC. The magnetic properties of alloyed solutions of Ti-Fe-C has been investigated using both theory and experiment. Theoretical calculations reveal that the magnetic moment and the critical temperature increase when increasing the Fe content as well as when lowering the C content in the system. Furthermore, the magnetic exchange parameters between Fe atoms were found to clearly reflect changes in the chemical bonding when varying the C content. Experimentally the magnetic properties were found to be rather substantial. Furthermore, the magnetic properties changes upon annealing due to the formation of Fe-rich and Fe-poor regions in the system. After long enough annealing times precipitates of α-Fe are formed which is consistent with theoretical predictions. The interaction between TiC(111) surfaces and C in the form of graphite has also been investigated. For these systems it was found that graphite was rather strongly bonded to the carbide surface and that the atomic as well as electronic structure at the interface depend on the termination of the carbide surface. This research was motivated by the recent interest in graphene, but also to investigate how carbide grains interacts with C when dispersed in a carbon matrix. A model for the calculation of structural parameters in multilayer structures has been presented and evaluated. The model is based on classical elasticity theory and uses the elastic constants of the materials constituting the multilayer as the only input.

Solar selective coatings based on carbon transition metal carbide nanocomposite absorber layers were designed. Pulsed filtered cathodic arc was used for depositing amorphous carbon: metal carbide (a-C:MeC, Me = V, Mo) thin films. Composition and structure of the samples were characterized by ion beam analysis, X-ray diffraction, Raman spectroscopy, and transmission electron microscopy. The optical properties were determined by ellipsometry and spectrophotometry. Three effective medium approximations (EMA), namely Maxwell-Garnett, Bruggeman, and Bergman, were applied to simulate the optical behaviour of the nanocomposite thin films. Excellent agreement was achieved between simulated and measured reflectance spectra in the entire wavelength range by using the Bergman approach, where in-depth knowledge of the nanocomposite thin film microstructure is included. The reflectance is shown to be a function of the metal carbide volume fraction and its degree of percolation, but not dependent on whether the nanocomposite microstructure is homogeneous or a self-organized multilayer. Solar selective coatings based on an optimized a-C:MeC absorber layer were designed exhibiting a maximum solar absorptance of 96% and a low thermal emittance of ~5 and 15% at 25 and 600ºC, respectively. The results of this study can be considered as predictive design tool for nanomaterial-based optical coatings in general.

The present work was initiated to investigate the stability, structural and thermodynamic properties of transition metal carbides, nitrides and carbo-nitrides by atomistic simulations and experimentations. The interatomic pair potentials of Cr-Cr, Mn-Mn, Fe-Fe, C-C, Cr-C, Mn-C, Fe-C, Cr-Fe, Cr-N and Mn-N were inverted by the lattice inversion method and ab initio cohesive energies, and then employed to investigate the properties of Cr-, Mn- and Fe-carbides by atomistic simulations in this work. For the binary M7C3 carbide, the structural properties of M7C3 (M = Cr, Mn, Fe) were investigated by atomistic simulations. The results show that the stable structure for these compounds is hexagonal structure with P63mc space group. The cohesive energy of M7C3 calculated in this work indicates that the stability of carbides decreases with the increasing in metal atomic number. Further, the vibrational entropy of Cr7C3 was calculated at different temperatures and compared with the entropy obtained by experimentations. The comparison demonstrates that the main contribution to the entropy is made by the vibrational entropy. For the binary τ-carbides, the structural properties of Cr23C6 and Mn23C6, as well as the vibrational entropy of Cr23C6 were computed. Further, the site preference of ternary element Fe among 4a, 8c, 32f and 48h symmetry sites in Cr23-xFexC6 was studied. It has been seen that Fe atoms would firstly occupy 4a sites and then 8c sites. The lattice constant and stability of Cr23-xFexC6 were also computed with different Fe content. In order to understand the relative stability of the transition metal carbides and nitrides, the standard formation Gibbs energies of carbides and nitrides for Cr, Mn and Fe were compared. The order of carbon and nitrogen affinities for Cr, Mn and Fe was further clarified by the comparison of the interatomic pair potentials among Cr-C, Mn-C, Fe-C, Cr-N and Mn-N. It was found that Cr-N interaction was very strong in comparison with other binary interactions above and consequently, nitrogen addition would lead to a strong decrease in the thermodynamic activity of chromium in Cr-containing alloys. This was confirmed by the investigations of thermodynamic activities of Cr in the Fe-Cr-N and Fe-Cr-C-N alloys. The activities were measured in the temperature range 973-1173 K by solid-state galvanic cell method involving CaF2 solid electrolyte under the purified N2 gas. In addition, the analysis of nitrogen content and phase relationships in the Fe-Cr-N and Fe-Cr-C-N alloys equilibrated at 1173 K were carried out by inert-gas fusion thermal conductivity method, X-ray diffraction and scanning electron microscopy technique. The experimental results show that the solubility of nitrogen in the alloys decreases with the decreasing chromium content, as well as the increasing temperature. The addition of nitrogen to the alloys was found to have a strong negative impact on the Cr activity in Fe-Cr-N and Fe-Cr-C-N systems. / QC 20100929

Group IV transition metal carbides are promising materials for high temperature structural application, due to their unique sets of properties such as high melting temperature, high temperature strength, hardness, elastic modulus, wear and corrosion resistance, metal-like thermal and electrical conductivity and thermal shock resistance. This group includes zir-conium carbide, which, along with its composites, are potential candidates for applications such as nose cones for re-entry vehicle, engines, wear resistant parts and in nuclear fuel cladding. Such structural applications demand high strength material with minimal flaws, in order to achieve the required reliability. Attainment of high strength calls for fully dense material with as small a grain size as possible. Producing fully dense zirconium carbide requires very high temperature, which is a direct consequence of its high melting point. Higher processing temperatures increase grain size, thereby also causing a loss in strength, along with the increased cost. Therefore, there is always a driving force to produce such a material in fully densified form at as low a temperature as possible. There have been a number of studies on processing and densification of zirconium carbide. Pressureless sintering of zirconium carbide requires temperature of 2400oC-3000oC to reach reasonably high density. At such high temperatures, abnormal grain growth limits the final density, as pores get entrapped inside the grains. Hot pressing of zirconium carbide also requires upwards of 2000oC to reach high density and is the primary route to produce densified zirconium carbide product. Reactive hot pressing (RHP), is a relatively new processing approach. Here, the reaction between zirconium and carbon to produce zirconium carbide and the densification of the porous mass, occurs simultaneously. Study on reactive hot pressing of zirconium carbide have shown that, it is possible to achieve very high density at much lower temperatures 1600oC. Clearly, reactive processing is an exciting new technique to process zirconium carbide. However, there has been a lack of studies to understand why it provides better densification than conventional hot pressing. Such understanding is of paramount importance, as it can lead to better optimization of RHP and perhaps even lower the process temperature further. The objective of the present study is to understand the densification process in RHP of zirconium carbide through systematic and carefully designed experiments. A model of reactive hot pressing is also constructed to get more insight into the phenomenon. 0.1 Pressureless Reaction Sintering of Zirconium Car-bide Pressureless reaction sintering (RS) of zirconium carbide is studied to understand the role of stoichiometry and zirconium metal in densification. ZrC of four different stoichiometries are chosen for these sets of experiments which are conducted in vacuum at 1200oC and 1600oC for 1 hour to understand the role of stoichiometry. One sample of pure Zr is also sintered to elucidate the role of zirconium in densification. After reaction sintering, all the samples are characterized by density measurement, x-ray diffraction and microstructure, using scanning electron microscopy. After pressureless sintering at 1600oC, zirconium metal reaches the highest relative density of ~ 95%. Densification decreases monotonically with increasing stoichiometry. Zr+0.5C composition reaches the next best relative density (of 90%), while Zr+0.67C composition shows much lower densification. The other two compositions, Zr+0.8C and Zr+C, in contrast, display de-densification rather than densification. Since the pure zirconium sample reaches high density, it can, in principle, help in densification of the mixed powders before getting fully reacted. Non-stoichiometric carbides also exhibit higher diffusivity of carbon, which aids the densification and the greater the deviation from stoichiometry, the smaller the deleterious effects of de-densification from reaction. This troika of factors is responsible for the substantially better densification in non-stoichiometric carbide, compared to stoichiometric carbide. 0.2 Reactive Hot Pressing of Zirconium and Carbon Reactive hot pressing of zirconium carbide is explored with the emphasis on finding the underlying densification mechanism. The earlier proposed densification mechanism for RHP is the plastic flow of transient non-stoichiometric carbide. To differentiate the effect of transient phases from that of zirconium, RHP is carried out at 800oC. At this low temperature, transient phases cannot take part in plastic flow and subsequent densification. Thus, any densi cation at this temperature can be totally attributed to zirconium and the role of zirconium thus can be separated from that of transient phases. A combination of RHP and RS experiments are carried out at 1200oC to better understand the phenomenon. Again, ZrC carbide of four different stoichiometries are investigated in this RHP study. After RHP at 800oC, all the four different ZrC compositions reached more than 90% RD through plastic flow of the Zr leading to a continuous matrix with embedded graphite particles. Since the reaction remains incomplete at this temperature, it is clear that Zirconium alone is responsible for enabling densification at such a low temperature. It is therefore argued that any unreacted Zr would, at higher temperature, be able to drive densification even more. Thus, zirconium does not only participate in densification; it is a dominant factor enabling low temperature densification. Pressureless reaction sintering at 1200oC following the RHP at 800oC, results in de-densi fication, as the reaction between zirconium and carbon occurs with significant volume shrinkage. Since such shrinkage increases with stoichiometry of the carbide, the higher stoichiometry carbides are more susceptible to de-densification. RHP at 1200oC, mostly completes the reaction, but only ZrC0:5 reaches near theoretical density. Thus, the final density of the fully reacted mixture is arrived at through a combination of processes in which the more stoichiometric carbides suffer from not only the smaller metal content but also a greater volume shrinkage during reaction. Thus, ZrC0:5 reaches 99% RD whereas ZrC reaches only 85% RD. The interplay between these two processes may be controlled by a two step RHP begin-ning at 800oC followed by a ramp up to 1200oC. The higher RD achieved at 800 C results in a higher final density for all the four compositions. Thus, two step RHP is a novel way to get better densification in RHP of zirconium carbide. 0.3 Hot Pressing of Zirconium Carbide Powders of Different Stoichiometry In the literature, densification in RHP is mostly attributed to the presence of transient non-stoichiometric carbides. To examine this hypothesis, ZrC of three different stoichiometries are prepared and then subjected to hot pressing at the same temperature and pressure as the previous RHP experiments (i.e. 1200oC and 40MPa for 30 min). After the hot pressing experiments, ZrC0:5 composition shows significant densification (95% RD), whereas ZrC0:67 composition shows very limited densification (70% RD) and ZrC composition shows little or no densification (50% RD). Evidently, the transient phase formed with stoichiometry close to ZrC0:5 can certainly contribute substantially to densification. But for the more carbon-rich compositions, the transient phases do not appear to play a significant role and the benefit of RHP, wherein ZrC can reach 90% RD, must come from the contribution of metal plasticity. 0.4 Reactive Hot Pressing of Zirconium and Zirconium Carbide Two limiting factors for densification during RHP are, de-densification (courtesy of the reaction) and the gradual increase in volume fraction of a rigid, non-sintering phase. To investigate the role of these factors further, two compositions of mixed metal and carbide powders, namely Zr+ZrC and 0.5Zr+ZrC, are subjected to RHP. When reaction is complete, the compositions after RHP will correspond to ZrC0:5 and ZrC0:67, respectively, but with the following difference with respect to the metal-carbon mixtures investigated earlier: these new compositions do not experience de-densification due to reaction and they contain significantly more amount of hard phase (53 and 69%) in the starting composition than their zirconium and carbon mixture counterparts i.e. Zr+0.5C and Zr+0.67C (16 and 20%). These two compositions are subjected to the same process schedules, i.e., RHP at 800oC, pressureless reaction sintering at 1200oC following RHP at 800oC and two step 800oC and 1200oC RHP. After 800oC RHP, Zr+ZrC and 0.5Zr+ZrC compositions reach much lower density than Zr+0.5C and Zr+0.67C compositions as a direct consequence of the larger amount of hard phase hindering densification at the lower temperature. After the 1200oC pressureless sintering following the RHP at 1200oC, the RD of Zr+ZrC and 0.5Zr+ZrC compositions increase (which is opposite to the behaviour of Zr+0.5C and Zr+0.67C com-positions) as they do not su er from reaction derived de-densification. After two step RHP, Zr+ZrC and 0.5Zr+ZrC compositions reach a final RD that is higher than the Zr+0.5C and Zr+0.67C compositions, even though after the first RHP at 800oC, they were much less densified. Thus, the absence of de-densification during reaction is able to more than compensate for the increase in hard phase content. 0.5 Reactive Hot Pressing: Low temperature process-ing route Based on the major factors of densification identified earlier, it was investigated whether RHP temperatures could be brought down further while being supplemented by a free sintering step to complete the reaction without de-densification. From a practical standpoint, such a process would allow dense products to be made by hot pressing with low temperature dies and fixtures while carrying out a more economical pressureless sintering at higher temperatures Therefore, Metal-carbide mixtures, Zr+ZrC and Ti+ZrC, are chosen, along with a temperature of 900 C which is above the allotropic phase transformation temperature for Zr around 880oC, thereby utilizing a zirconium phase that is softer than the hexagonal Zr. For completion of reaction, pressureless reaction sintering is done at 1300oC and 1400oC. It is found that after 1400oC reaction sintering, both the compositions reach almost full density and the Ti+ZrC composition also shows a higher hardness (13 vs 10 GPa) than the Zr+ZrC composition, due to the formation of a binary carbide with consequent solid solution hardening. 0.6 Effect of Particle Size on Reactive Hot Pressing During RHP, premature exhaustion of zirconium by reaction can limit densification. One way to have better densification is to slow down the reaction, so that significant amount of densification takes place before the metal zirconium is exhausted. One way to reduce reaction rates is to increase particle size. Larger particles are expected to slow down the reaction without affecting sintering, as densification is controlled by power law creep of Zr which is grain size independent. Because of lack of availability of Zr with different particle sizes, two different graphite particle sizes, i.e. 7-10 m and 50-60 m, were studied and it was shown that after 1200oC RHP, indeed the larger particle size improves densification. 0.7 Modelling of Reactive Hot Pressing Reactive hot pressing is a complicated phenomenon, and to get an insight and also to optimize the parameters, the availability of a computational model is of paramount importance. Keeping that in mind, a model of RHP has been constructed based on four different parts, namely: 1. Densification of zirconium under pressure 2. Reaction of zirconium and carbon 3. The constraint on sintering from a rigid phase and, finally, 4. The volume contraction during reaction. The model uses published data for the 4 steps and shows reasonable qualitative and quantitative agreement with the experimental results. Further experiments are done with the model to optimize the processing parameters. Results from the virtual experiments consolidates our earlier conviction gained from experimental results, by showing zirconium is the principal factor in densification and exhaustion of zirconium coupled with reaction derived de-densification prevent the higher stoichiometric carbide from achieving full densification. It also shows, RHP gives best densification when reaction is 70-80% complete. So two step RHP where the first RHP will only complete the reaction 70-80%, and a final RHP at temperature which will complete the reaction, will possibly be the way to achieve best densification. 0.8 Conclusions The study on RHP of zirconium carbide led to the following conclusions: • Zirconium plays the most crucial role in densification. • Transient phases only play a role when the final stoichiometry of RHPed carbide is close to that of ZrC0:5. • De-densification from reaction prevents higher stoichiometric carbide from reaching full densification. • Two step RHP, with one RHP at lower temperature at which reaction will remain incomplete, and the other at higher temperature to complete the reaction, yields best densification. • For lower stoichiometric carbide (ZrC0:5,ZrC0:67), full densification can be achieved at 1200oC. For higher stoichiometric carbide, even though large amount of densification upward of 90% RD is achieved at 1200oC, full densification will be out of reach. • RHP shows better densification than conventional hot pressing for all stoichiometries.