Advanced glasses, Composites And Ceramics for High growth Industries

CoACH (Advanced glasses, Composites and Ceramics for High Growth Industries, www.coach-etn.eu) is a 4 year project coordinated by Politecnico di Torino and supported by the European Commission through the Marie Sklodowska-Curie Action. It provides an innovative and inter-sectorial doctoral training for young researchers in prestigious academic institutions as well as private companies. CoACH, which includes five academic partners and ten private companies from seven different European countries, promotes international excellence in glass, ceramic and composite science and technology, modelling, design, characterization and commercialization of advanced glass, ceramic and composite based products. In the CoACH project, 15 recruited PhD researchers are currently trained in creative, independent problem solving under time and resource constraints. This is typical of a scientific and technical working environment in continuous contact with the industrial world, through inter-sectorial and interdisciplinary secondment and mobility activities between academia and industry. CoACH provides training-through-research in: • Glasses and composites for HEALTH care industries. • Glasses, ceramics and composites for the ENERGY production and ICT industries. • ENVIRONMENTALLY-friendly, low cost glass, ceramic and composite materials. Its scientific goals, based on specific needs expressed by industries, have strong potential for excellent research and technological development and for disseminating and converting the results into social and economic benefits

Flash joining of graphite with polymer derived ceramic interlayer

High-temperature ceramics for structural applications are often characterized by poor machinability and very high melting temperature. Therefore, it is difficult to manufacture them into net-shape large components with complex geometry, with manufacturing technologies such as casting, plastic forming and machining not being viable. Therefore the development of novel joining technologies for high temperature ceramics is of considerable interest. However, ceramic joining is a challenging task because of their high melting temperature; their resistance to plastic deformation; and their brittle behavior (which can cause failure when thermal stresses are developed). Please click Additional Files below to see the full abstract

High entropy transition metal carbides

Since their discovery in 2004, High Entropy Alloys (HEAs) have become a major research area in the field of metallurgy. These materials are typically single-phase mixtures of several (\u3e4) different alloying elements in equi- or near-equiatomic proportions. The result is a material which has structural order, yet chemical disorder; an arrangement which has been reported to lead to enhanced mechanical, physical and chemical properties. Where previously it was believed that the mixing of elements in this way would lead to impractical multi-phase and brittle intermetallic materials, the discovery that single phase solid solutions can be stabilised by their high configurational entropy has opened up a wide new range of useful compositional space to be explored 1. The ‘entropy-stabilised materials’ concept has recently been successfully applied to metal oxide2 and transition metal diboride3 systems, sparking significant interest in the ceramics, and particularly the Ultrahigh Temperature Ceramics (UHTCs), community. These single-phase materials consist of a chemically ordered anion sublattice (O or B) and a chemically disordered metal cation sublattice; and initial testing suggests that these materials may possess enhanced hardness and oxidation resistance. We report on the fabrication of entropy-stabilised UHTC refractory metal carbides. It is shown that it is possible to produce bulk homogeneous high entropy carbides. Our findings include densification trials, multi-scale microstructural investigations, and mechanical and physical properties characterisation. The significance of the work will be discussed in relation to the opportunities created for the development of new UHTCs. References: [1] Brian Cantor (2014) Multicomponent and High Entropy Alloys: Review. Entropy, 16, 4749-4768; doi:10.3390/e16094749 [2] Rost et al. (2015) Entropy-stabilised oxides. Nature Communications, 6, 8485; doi:10.1038/ncomms9485 [3] Gild et al. (2016) High-entropy metal diborides: a new class of high-entropy materials and a new type of ultrahigh temperature ceramics. Scientific Reports, 6, 37946; doi:10.1038/srep3794

Nanoindentation, micropillar compression and nanoscratch testing of ZrB2 grains

The mechanical response under nanoindentation, micropillar compression and nanoscratch tests of ultra-high temperature ZrB2 ceramic grains were investigated. The tests were carried on selected surface areas where the grain orientations were mapped by electron backscatter diffraction (EBSD) prior to the measurements (Fig.1a). Instrumented indentation were applied for compression using flat punch tip for nanoindentation and nanoscratch tests Berkovich tips were used. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and additional EBSD were performed to study the surface morphology and to characterize the deformations. Strong influence of crystal orientation was observed during micropillar compression while nanoindentation and nanoscratch tests revealed smaller anisotropy. Considerable plastic deformation is revealed under pillar compression, as it is shown in Fig. 1b,c, but indentation and scratch tests showed detectable plasticity, as well. Uniformly, basal oriented grains exhibited higher hardness, yield stress and rupture stress values compared to the prismatic orientation. The elastic anisotropy showed reversed tendency with lower indentation and Young’s modulus values corresponding to the basal orientation in comparison with the prismatic. To describe the elastic anisotropy, the Vlassak-Nix model and finite element model (FEM) calculations were performed based on the single crystal elastic constants of ZrB2. To explain the obtained hardness anisotropy, a theoretical model was proposed in which the critical force for slip activation is determined as a function of crystal orientation, based on the possible slip systems of materials. The calculated results shows similar tendency as the experimental values

Field Assisted Material Engineering (FAME)

In order to further improve the energy saving of Spark Plasma Sintering we have developed a very rapid sintering technique called Flash SPS (FSPS) with heating rates in the order of 104-105 ˚C/minute[1]. Unlike the Flash Sintering based on high voltage (≈100V), FSPS is based on low voltage (≈10V) and it can be up-scaled to samples volumes of several tens of cubic centimetres. Flash SPS allows densification of metallic conductors like ZrB2 and HfB2, under a discharge time as short as 20-30 seconds. FSPS of semiconductors like silicon carbide and boron carbide was also demonstrated. Highly customized and versatile equipment with ultrafast responsive controls and programmable bipolar power supplies (up to 20 kHz, 1 MA, 500V) has been built. The developed methodology has been applied to produce FSPSed samples even larger than 6 cm in diameter of ultra refractory materials. Understanding the intrinsic electrical field role in the triangle properties-microstructure-processing remains one our primary scientific goal and the main open question. We tried to give some answers by approaching the problem at different length scales (see figure 1) by developing dedicated equipment/controls, simulations (FEM and ab-initio), thermo-kinetic analysis, in situ observations and accurate temperature measurements/calibrations. Please click Additional Files below to see the full abstract

Ferroelectricity in Dion–Jacobson ABiNb2O7(A = Rb, Cs) compounds

The ferroelectric properties of 2-layer Dion–Jacobson compounds ABiNb2O7 (A = Rb and Cs) were studied. Ferroelectricity and piezoelectricity of CsBiNb2O7 were demonstrated for the first time. The ferroelectric domain structure of Dion–Jacobson compounds were imaged using PFM. The Curie points of RbBiNb2O7 and CsBiNb2O7 are 1098 ± 5 and 1033 ± 5 °C, respectively. The piezoelectric constant of RbBiNb2O7 and CsBiNb2O7 are approximately 5 and 8 pC N−1. Thermal depoling was also studied to confirm the Curie temperature and the stability of the piezoelectricit

Electrochemical, optical and thermal effects during flash sintering of 8YSZ

We report on the electrochemical effects occurring during the flash sintering of 8YSZ. In-situ observations for both polycrystalline and single crystal specimens confirm electrochemical blackening/darkening during the incubation period prior to flash sintering (Figure1), even though chromatic alterations are usually visible only after the samples are cooled down in a protective atmosphere rather than in air. The phenomenon is induced by cathodic partial reduction under a DC field. When using a low frequency AC (square 0.1 – 10 Hz) field, the blackening becomes reversible and it follows the imposed polarity switching. Thermal imaging combined with sample color changes (transparent single crystals) and electrical conductivity mapping give a complete picture of the multi-physical phenomena occurring during each stage of the flash sintering event. Please click Additional Files below to see the full abstract

Effect of spark plasma sintering on the structure and properties of Ti1-xZrxNiSn half-heusler alloys

XNiSn (X = Ti, Zr and Hf) half-Heusler alloys have promising thermoelectric properties and are attracting enormous interest for use in waste heat recovery. In particular, multiphase behaviour has been linked to reduced lattice thermal conductivities, which enables improved energy conversion efficiencies. This manuscript describes the impact of spark plasma sintering (SPS) on the phase distributions and thermoelectric properties of Ti0.5Zr0.5NiSn based half-Heuslers. Rietveld analysis reveals small changes in composition, while measurement of the Seebeck coefficient and electrical resistivities reveals that all SPS treated samples are electron doped compared to the as-prepared samples. The lattice thermal conductivities fall between 4 W·m−1·K−1 at 350 K and 3 W·m−1·K−1 at 740 K. A maximum ZT = 0.7 at 740 K is observed in a sample with nominal Ti0.5Zr0.5NiSn composition

Mechanical and magnetic properties of spark plasma sintered soft magnetic FeCo alloy reinforced by carbon nanotubes

Different volume fractions (0.5 vol. % to 4.5 vol. %) of CNTs were used to reinforce a binary Fe50Co soft magnetic alloy. The first method for dispersion was involved dry mixing and ball milling of the powder, while the second was included wet mixing in dimethylformamide under ultrasonic agitation, drying and then dry ball milling. The powders were consolidated using spark plasma sintering. Tensile test and SEM analyses were performed to characterize the mechanical properties and the fracture surface of the sintered materials. The best magnetic and mechanical properties were achieved using the first method. A maximum enhancement in tensile strength of around 20% was observed in the 0.5 vol. % CNT composite with improved elongation compared to the monolithic Fe50Co alloy. In addition, the magnetic properties were enhanced by adding CNTs up to 1 vol. %, and an improvement in densification was observed in composites up to 1.5 vol. % CNT with respect to monolithic Fe50Co alloy

Creep of HfB2-based UHTCs up to 2000oC

Ultra-high temperature ceramics (UHTCs) are promising candidates for hypersonic applications as a consequence of their high melting points, in excess of 3000 ºC for ZrB2 and HfB2 UHTCs. The UHTCs community has traditionally focused on development of more oxidation-resistant UHTC composites as a consequence of poor oxidation resistance of monolithic UHTCs, which has led to the choice of SiC-reinforced MeB2 (where Me is Zr or Hf) as the baseline material for extreme environments. An overview of current understanding of high temperature creep of MeB2–based UHTCs will be described, discussing the following points: • Poor creep resistance of SiC-reinforced HfB2 and their structural instabilities. • Plastic behavior of HfB2 which deforms like an hcp-metal. • Plastic behavior of HfB2/2 wt.% La2O3 or how to maintain the creep resistance while improving the oxidation resistance. • New approaches to increase the creep resistance of HfB