Final Report Summary - FUNEA (FUnctional Nitrides for Energy Applications) Summary of the Scientific Progress of the Work PackagesWP 1 (TUD) – Synthesis of Multifunctional Perovskite Nitrides: A model that predicts the formability of perovskite oxynitride was developed. Moreover, the thermal conversion of scheelite ABO4 into perovskite AB(O,N)3 was investigated. Rapid sintering of dense SrM(O,N)3 (M=Mo, W) oxynitride ceramics was performed by Sparking Plasma Sintering (SPS), in collaboration with SU. Additionally, perovskite-structure type solid-solution SrMo1-xWx(O,N)3 oxynitrides were synthesized and their magnetic and electrical properties were investigated.WP 2 (SU) - Processing of multifunctional nitrides: Fundamental studies were performed in order to assess the behavior of nanoceramics (i.e. nitrides, oxynitrides) during rapid sintering. Within this context, the SITR (Sintering by intense thermal radiation) process was developed. Thus, SITR allows for obtaining near-dense oxynitrides with reasonable high purity, both parameters being significantly better than that of any counterparts prepared by conventional sintering. Also Si3N4 foams were processed via SITR, thus highly porous (~80 vol%) Si3N4 foams possessing dense cell walls decorated with SiC NWs which might be used as industrial filters for airborne pollutions.WP 3 (CNRS) - Modeling of functional nitrides: Our work aimed at understanding some structural and physical properties of various transition metal nitrides using quantum chemical calculations. The study of the electronic structure of the layered ternary nitrides AMN2 (A: alkaline-earth, M: group 4 transition metal) was firstly tackled. Our results show that the KCoO2 structure type (tetragonal, P4/nmm) is more favorable than that of α-NaFeO2 (hexagonal, R3 ̅) Band structures of the former suggest semiconducting properties whereas that of the latter suggest semi metallic or semiconducting behavior depending on the nature of the metals. Investigations on the structural properties of some oxynitride compounds of the formula AMO2N (A = Rb, Sr, Y; M = Cr, Mo, W) with a perovskite-type structure were performed and metallic properties were envisioned for most of these compounds; some of them may exhibit magnetic properties. Finally, some work has been devoted to the study of ternary nickel nitrides of the general formula ANiN (A = alkaline or an alkaline-earth metal). DFT calculations carried out for CaNiN using hybrid functional show a magnetic behavior, as measured experimentally.WP 4 (UM2) - Hydrogen storage nitrides: The general objective of WP4 concerned the development of functional (carbo)nitrides for hydrogen generation and storage (material design, elaboration, properties and applications). The PDC route was applied to produce functional (carbo)nitride-based materials. Firstly, porous binary systems such as AlN and BN were prepared by replicating the structure of CMK-3 and that of activated carbon; their potential for the nano confinement of two chemical hydrides, NaAlH4 and ammonia borane, was assessed and revealed that in both cases, the nano confinement destabilized the network of the hydride and favored the release of H2 at low temperature. Secondly, we prepared porous quaternary systems through the association of AlN/BN with Si-based ceramics. In particular, we investigated the preparation of SiAlCN with tailored porosity and investigated the preparation of ordered mesoporous materials to be used as catalytic supports for hydrolysis of alkaline solution of NaBH4. We succeeded in generating high amounts of H2 with attractive kinetics. Concerning the latter approach, the work was focused on the investigation of the chemistry of SiAlCN and SiBCN materials with a particular focus on the elaboration of SiAlCN microcellular foams by a sacrificial processing route.WP5 (TUD): Gas separation membranes: A facile and general synthesis strategy has been developed to tune the chemical composition and pore size as well as the surface area of microporous SiCNO ceramics. This method is based on modifying the structure of preceramic polymers under an NH3 atmosphere. Under these synthesis conditions, the materials derived from a polysiloxane and polysilazane transform to microporous ceramics, while the materials derived from the polycarbosilane remain nonporous. The microporous silicon oxycarbonitrides (SiCNO) ceramics synthesized from the polysilazane and polysiloxane possess surface area and micropore volume as high as 250-300 m2 g-1 and 0.16 cm3 g-1, respectively. CO2 adsorption isotherm analysis reveals the presence of ultra microporosity in polysilazane-derived samples which has been tested for CO2 and H2 capture. The latter samples possess a CO2 uptake of 2.35 mmol g-1 at 273 K and 1 bar and a H2 uptake of 56 cm3 g-1 at 0.66 bar and 77 K. Polycarbosilane-derived ceramics have been tested as membranes by coating the material on macroporous Al2O3 or ZrO2 substrates. The gas permeance measurements (H2, CO2, N2, CH4, C3H6, C3H8, C4H10, C4H8, SF6) performed on the synthesized membranes reveals that gas transport occurs mainly by Knudsen and surface diffusion. The results achieved within this work indicate that the ammonolysis of preceramic polymers provides an excellent preparative access to microporous materials with high CO2 and H2 capture capability as well as gas separation properties.WP 6 (UT) - Electrical/N-doped SiCO sensors for harsh environment: Different N-doped SiOC precursors were successfully synthesized via the hydrosilylation reaction of poly(methylhydrosiloxane) with nitrogen-containing organic compounds in the presence of Pt-based catalyst. The concentration and chemical bonding nature of the N atoms in the final SiOCN ceramics is related to the bonding of N in the starting precursors. In addition, the presence of C-N bonds was found in both of the ceramic samples derived from with and without C-N pre-existing precursors. The structure of the free-carbon phase plays an important role in determining the electrical conductivity of the samples. Fluorescence of the SiOCN samples treated at low temperatures, namely 400 and 600 °C, shows emission in the visible range. For the electrical gas sensing, SiOCN ceramics pyrolyzed at 1400 °C are sensitive to NO2 at temperatures below 400 and to H2 at temperatures above 400 °C. In terms of optical gas sensing, SiOCN samples heated at 400 °C are sensitive towards organic vapors such as acetone and hexane which quench their fluorescence.WP 7 (MDA) - Gas filters: The aim of the project was to produce silicon nitride interconnected macroporous foams with large pores sizes up to the mm rage to be used in particle filtration application, particularly in diesel particulate filters. Direct foaming method was adopted to produce porous silicon nitride. The foams sintered at 1750 °C had a relative porosity of 90 vol%, 40% of which was due to the macroporosity and the remaining pores were due to pores located within Si3N4 grains. The size of the macropores was 800-1000 μm and these pores were connected to each other via 100-400 μm cell windows. The compressive strength of the foam was measured to be around 0.9 MPa. The project provides a basis for understanding the fabrication of macroporous ceramics, which would enable better filtration efficiency of particulate matters and thus to improve public health. WP 7 (UPD) - Gas filters: The goals of the proposed research were reached, as highly porous Si3N4 foams were obtained using different foaming methods. In addition to that, the foaming of other ceramic systems (Ti2AlC, Ti3AlC2 and ZrB2-C) was explored, thereby extending the range of materials that can successfully be shaped into complex highly porous parts using the processing methods developed in this work. The characterization of the samples demonstrated that, by using either emulsions (containing high to moderate amounts of apolar volatile phases or other kinds of oil, such as vegetable oils) or gel casting, it was possible to fabricate silicon nitride and other advanced ceramic components with a porosity ranging from ~70 to ~85 vol%, with cells in the range of 10 to 850 μm, possessing high mechanical strength (e.g. ~33 MPa for samples showing 74 vol% of porosity, which is significantly higher than that of commercially available ceramic foams of similar porosity) and good permeability, to be used for filtration applications at high temperature and/or in harsh environments. These ceramic components can successfully contribute to the abatement of particulates from industrial or automotive gaseous emissions, thereby improving the environment.WP 8 (UBT ) - Thermal barrier coatings: This project aimed to develop a new Thermal Barrier Coating (TBC) system based on polymer-derived ceramics (PDCs) for application at temperatures up to 1000 °C. The proposed coating system is composed of a bond coat and a thick insulate layer on a metallic substrate. The studies started with the development of a coating system, composed by the precursor (HTT1800, supplied by AZ-EM), active (B4C) and passive (YSZ) fillers, using the steels 1.4509 and 1.4828 (supplied by Faurecia) as substrate. The steel 1.4828 has proven not to be suitable due to the high CTE and the achieved coatings showed thermal stability at temperatures up to 1000 °C but still with only average insulating performance. By changing the active filler to ZrSi2 and adjusting the system, thicker coatings with better thermal properties were obtained. Thermal conductivity measurements provided evidence of the achievement of a coating with exceptionally low thermal conductivity (0.44 W/mK). Tests at Faurecia confirmed the insulating effect of the coatings. Adhesion tests were also carried out and resulted in values, which are in accordance to conventional TBCs. Additionally, a new device was designed and build to enable the application of the coatings on the inner wall of pipes by spraying. The parameters were then adjusted to the new processing method, which favors the transfer of the technology to the industry. The novel TBC based on the PDC technology has good potential to be transferred to the industry in the next years. This new technology might contribute to the reduction of air pollution generated by the exhaust gases of automobiles in the near future, bringing benefits not only to the automotive industry but also to the society as a whole.WP 8 (AZ-EM) - Thermal barrier coatings: The aim of this project was the synthesis of silazane based polymers and their characterization according to the requirements of its application as thermal barrier coating. This required the set-up of a high pressure lab-scale plant for the production of polysilazane R&D samples with a similar process to that currently performed on industrial scale. The screening of the synthesis conditions such as temperature and time as well as type of monomer was oriented to obtain polysilazanes with adequate rheological properties, solubility, molar volume, molar thermal expansivity and latent reactivity suitable to be used in thermal barrier coating applications. Parallel to this, the prediction of these properties was carried our using a semi-empirical method based on the group contribution method that speeded up the selection of the most promising types and ratios of monomers.WP 9 (IIC SAS) - Phosphors for LEDs: By the optimization of the nitridation conditions and high-temperature annealing of metal silicide, silicon, silicon nitride and rare earth oxides, efficient MgSiN2 and LaSi3N5-based phosphors were prepared. The photoluminescence measurements showed that by appropriate combination of host lattice (MgSiN2 or LaSi3N5) and rare earth dopant (Eu, Ce, Sm) (oxy)nitride-based phosphors can be prepared, which cover the whole visible light region: LaSi3N5:Ce phosphor – violet-blue light, MgSiN2:Ce – blue-green, LaSi3N5:Eu – green-yellow, MgSiN2:Eu – red and LaSi3N5:Sm – red light. By the combination of these phosphors white LEDs can be prepared. Moreover, the high intensity red and green phosphors allow the construction of highly efficient "warm" white LED with a yellowish daylight tinge. These LEDs can be widely used for indoor lighting and can significantly contribute to energy saving.First principle density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP) to enhance the understanding of the electronic structure of stoichiometric LaSi3N5 and RE3+- and RE2+-doped LaSi3N5 (RE = Ce, Pr, Nd, Pm, Sm, and Eu). Besides of the different RE dopant, the band gaps were calculated also for different N/O substitution and for different position of oxygen in the solid state structure. With increasing oxygen content and increasing RE-O distance the band gap decreased. The calculated electron structure gave an information about the possible electron transitions (4f→5d, p→4f transition). Finally, the energy level schemes were constructed from the ab initio calculated electronic structures and agreed well with the experimental energy level diagram. This agreement shows the reliability of hybrid functional HSE06 to describe correctly the bands of nonbonding RE 4f electrons.
