44 results found
Jiang J, Ni N, Hao W, et al., 2019, Effects of sintering atmosphere on the densification and microstructure of yttrium aluminum garnet fibers prepared by sol-gel process, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, Vol: 39, Pages: 5332-5337, ISSN: 0955-2219
Xie X, Liu B, Liu R, et al., 2019, Comparison of hydrothermal corrosion behavior of SiC with Al2O3 and Al2O3 + Y2O3 sintering additives, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Vol: 103, Pages: 2024-2034, ISSN: 0002-7820
Xie X, Liu B, Guo Y, et al., 2019, Effect of hydrothermal corrosion on the fracture strength of SiC layer in tristructural-isotropic fuel particles, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Vol: 102, Pages: 5555-5564, ISSN: 0002-7820
Ni N, Wang CC, Jiang SP, et al., 2019, Synergistic effects of temperature and polarization on Cr poisoning of La <inf>0.6</inf> Sr <inf>0.4</inf> Co <inf>0.2</inf> Fe <inf>0.8</inf> O <inf>3-: δ</inf> solid oxide fuel cell cathodes, Journal of Materials Chemistry A, Vol: 7, Pages: 9253-9262, ISSN: 2050-7496
La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) solid oxide fuel cell cathodes were poisoned by Cr at different temperatures and polarization conditions with a Cr-Fe alloy as the interconnect. Cr induced degradation was analysed by electrochemical impedance spectroscopy (EIS) focusing on the electrochemical resistance (R chem ) that reflects the cathode electrochemical properties. It was found that R chem increased more with increasing temperatures. However cathodic polarization exhibited a synergistic effect with the temperature, which accelerated the LSCF cathode degradation at 800 °C while lowering the degree of degradation at 900 °C. By correlating complementary micro- and nano-scale microstructure characterization with the impedance analysis, the degradation mechanisms were investigated. A new Cr incorporation mechanism involving preferential formation of nanometre size Fe-Co-Cr-O spinel particles within the cathode up to the cathode/electrolyte interface was found to be responsible for the reduced degradation at 900 °C combined with cathodic polarization. The new mechanism reveals that the activity of B site elements in LSCF and possibly other perovskite cathodes plays an important role under certain combined temperature and polarization conditions, therefore future research in designing Cr resistant perovskite cathode materials may consider strategies that utilize the exsolution of B site elements for the formation of beneficial spinel phases.
Hao W, Ni N, Guo F, et al., 2019, High fracture toughness of HfC through nano-scale templating and novel sintering aids, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Vol: 102, Pages: 997-1009, ISSN: 0002-7820
Chen W, He L, Guo Y, et al., 2019, Effects of reactive element oxides on the isothermal oxidation of beta-NiAl coatings fabricated by spark plasma sintering, SURFACE & COATINGS TECHNOLOGY, Vol: 357, Pages: 322-331, ISSN: 0257-8972
Jin D, Ni N, Guo Y, et al., 2018, Corrosion of the bonding at FeCrAl/Zr alloy interfaces in steam, JOURNAL OF NUCLEAR MATERIALS, Vol: 508, Pages: 411-422, ISSN: 0022-3115
Reale F, Palczynski P, Amit I, et al., 2017, High-Mobility and High-Optical Quality Atomically Thin WS2, Scientific Reports, Vol: 7, ISSN: 2045-2322
The rise of atomically thin materials has the potential to enable a paradigm shift in modern technologies by introducing multi-functional materials in the semiconductor industry. To date the growth of high quality atomically thin semiconductors (e.g. WS2) is one of the most pressing challenges to unleash the potential of these materials and the growth of mono- or bi-layers with high crystal quality is yet to see its full realization. Here, we show that the novel use of molecular precursors in the controlled synthesis of mono- and bi-layer WS2 leads to superior material quality compared to the widely used direct sulfidization of WO3-based precursors. Record high room temperature charge carrier mobility up to 52 cm2/Vs and ultra-sharp photoluminescence linewidth of just 36 meV over submillimeter areas demonstrate that the quality of this material supersedes also that of naturally occurring materials. By exploiting surface diffusion kinetics of W and S species adsorbed onto a substrate, a deterministic layer thickness control has also been achieved promoting the design of scalable synthesis routes.
There is a growing interest in the development of composites with complex structures designed to generate enhanced mechanical properties. The challenge is how to implement these structures in practical materials with the required degree of control. Here we show how freeze casting of ceramic preforms combined with metal infiltration can be used to fabricate Al2O3/Al-4wt% Mg micro-laminated composites. By manipulating the solid content of the suspension and the morphology of the ceramic particles (from platelets to round particles) it is possible to access a range of structures with layer thickness varying between 1 and 30 μm and metallic contents between 66 and 86 vol%. The mechanical response of the materials is characterized by combining bending tests with observation of crack propagation in two and three dimensions using different imaging techniques. These composites are able to combine high strength and toughness. They exhibit a rising R-curve behaviour although different structures generate different toughening mechanisms. Composites fabricated with Al2O3 particles exhibit the highest fracture resistance approaching 60 MPa m1/2, while laminates prepared from Al2O3 platelets exhibit higher strengths (above 700 MPa) while retaining fracture resistance up to ∼40 MPa m1/2. The results provide new insights on the effect of structure on the mechanical properties in metal-ceramic composites as well as on the design of appropriate testing procedures.
Rocha VG, Garcia-Tunon E, Botas C, et al., 2017, Multimaterial 3D Printing of Graphene-Based Electrodes for Electrochemical Energy Storage Using Thermoresponsive Inks, ACS APPLIED MATERIALS & INTERFACES, Vol: 9, Pages: 37136-37145, ISSN: 1944-8244
The current lifestyles, increasing population, and limited resources result in energy research being at the forefront of worldwide grand challenges, increasing the demand for sustainable and more efficient energy devices. In this context, additive manufacturing brings the possibility of making electrodes and electrical energy storage devices in any desired three-dimensional (3D) shape and dimensions, while preserving the multifunctional properties of the active materials in terms of surface area and conductivity. This paves the way to optimized and more efficient designs for energy devices. Here, we describe how three-dimensional (3D) printing will allow the fabrication of bespoke devices, with complex geometries, tailored to fit specific requirements and applications, by designing water-based thermoresponsive inks to 3D-print different materials in one step, for example, printing the active material precursor (reduced chemically modified graphene (rCMG)) and the current collector (copper) for supercapacitors or anodes for lithium-ion batteries. The formulation of thermoresponsive inks using Pluronic F127 provides an aqueous-based, robust, flexible, and easily upscalable approach. The devices are designed to provide low resistance interface, enhanced electrical properties, mechanical performance, packing of rCMG, and low active material density while facilitating the postprocessing of the multicomponent 3D-printed structures. The electrode materials are selected to match postprocessing conditions. The reduction of the active material (rCMG) and sintering of the current collector (Cu) take place simultaneously. The electrochemical performance of the rCMG-based self-standing binder-free electrode and the two materials coupled rCMG/Cu printed electrode prove the potential of multimaterial printing in energy applications.
Ferraro C, Garcia-Tunon E, Barg S, et al., 2017, SiC porous structures obtained with innovative shaping technologies, Journal of the European Ceramic Society, Vol: 38, Pages: 823-835, ISSN: 0955-2219
SiC structures with porosities ranging between 20–60% have been fabricated using two methods emulsification and freeze casting. While emulsification results in foam-like isotropic materials with interconnected pores, freeze casting can be used to fabricate highly anisotropic materials with characteristic layered architectures. The parameters that control the pore size and final porosity have been identified (solid content in the initial suspensions, emulsification times or speed of the freezing front). We have found that liquid state sintering (suing Al2O3 and Y2O3 as additives) at 1800 °C on a powder (SiC/Al2O3) bed provides optimum consolidation for the porous structures. The mechanical strength of the materials depends on their density. Freeze casted materials fabricated with bimodal particle size distributions (a controlled mixture of micro and nanoparticles) exhibit higher compressive strengths that can reach values of up to 280 MPa for materials with densities of 0.47.
Skinner SJ, li C, ni N, et al., Surface chemistry of La<sub>0.99</sub>Sr<sub>0.01</sub>NbO<sub>4-d</sub> and its implication for proton conduction, ACS Applied Materials and Interfaces, Vol: 9, Pages: 29633-29642, ISSN: 1944-8244
Acceptor-doped LaNbO4 is a promising electrolyte material for proton-conducting fuel cell (PCFC) applications. As charge transfer processes govern device performance, the outermost surface of acceptor-doped LaNbO4 will play an important role in determining the overall cell performance. However, the surface composition is poorly characterized, and the understanding of its impact on the proton exchange process is rudimentary. In this work, the surface chemistry of 1 atom % Sr-doped LaNbO4 (La0.99Sr0.01NbO4-d, denoted as LSNO) proton conductor is characterized using LEIS and SIMS. The implication of a surface layer on proton transport is studied using the isotopic exchange technique. It has shown that a Sr-enriched but La-deficient surface layer of about 6–7 nm thick forms after annealing the sample under static air at 1000 °C for 10 h. The onset of segregation is found to be between 600 and 800 °C, and an equilibrium surface layer forms after 10 h annealing. A phase separation mechanism, due to the low solubility of Sr in LaNbO4, has been proposed to explain the observed segregation behavior. The surface layer was concluded to impede the water incorporation process, leading to a reduced isotopic fraction after the D216O wet exchange process, highlighting the impact of surface chemistry on the proton exchange process.
Pesci FM, Sokolikova MS, Grotta C, et al., 2017, MoS2/WS2 heterojunction for photoelectrochemical water oxidation, ACS Catalysis, Vol: 7, Pages: 4990-4998, ISSN: 2155-5435
The solar-assisted oxidation of water is an essential half reaction for achieving a complete cycle of water splitting. The search of efficient photoanodes that can absorb light in the visible range is of paramount importance to enable cost-effective solar energy-conversion systems. Here, we demonstrate that atomically thin layers of MoS2 and WS2 can oxidize water to O2 under incident light. Thin films of solution-processed MoS2 and WS2 nanosheets display n-type positive photocurrent densities of 0.45 mA cm–2 and O2 evolution under simulated solar irradiation. WS2 is significantly more efficient than MoS2; however, bulk heterojunctions (B-HJs) of MoS2 and WS2 nanosheets results in a 10-fold increase in incident-photon-to-current-efficiency, compared to the individual constituents. This proves that charge carrier lifetime is tailorable in atomically thin crystals by creating heterojunctions of different compositions and architectures. Our results suggest that the MoS2 and WS2 nanosheets and their B-HJ blend are interesting photocatalytic systems for water oxidation, which can be coupled with different reduction processes for solar-fuel production.
Zapata-Solvas E, Christopoulos SRG, Ni N, et al., 2017, Experimental synthesis and density functional theory investigation of radiation tolerance of Zr₃(Al₁–ₓ,Siₓ)C₂ MAX phases, Journal of the American Ceramic Society, Vol: 100, Pages: 1377-1387, ISSN: 1551-2916
Synthesis, characterisation and density functional theory calculations have been combined to examine the formation of the Zr3(Al1–xSix)C2 quaternary MAX phases and the intrinsic defect processes in Zr3AlC2 and Zr3SiC2. The MAX phase family is extended by demonstrating that Zr3(Al1–xSix)C2, and particularly compositions with x ≈ 0.1, can be formed leading here to a yield of 59 wt.%. It has been found that Zr3AlC2 – and by extension Zr3(Al1–xSix)C2 – formation rates benefit from the presence of traces of Si in the reactant mix, presumably through the in situ formation of ZrySiz phase(s) acting as a nucleation substrate for the MAX phase. To investigate the radiation tolerance of Zr3(Al1–xSix)C2 we have also considered the intrinsic defect properties of the end members. Aelement Frenkel reaction for both Zr3AlC2 (1.71 eV) and Zr3SiC2 (1.41 eV) phases are the lowest energy defect reactions. For comparison we consider the defect processes in Ti3AlC2 and Ti3SiC2 phases. It is concluded that Zr3AlC2 and Ti3AlC2 MAX phases are more radiation tolerant than Zr3SiC2 and Ti3SiC2 respectively. Their applicability as cladding materials for nuclear fuel is discussed.
Saiz Gutierrez E, Picot O, Ferraro C, et al., 2017, Using graphene networks to build bioinspired self-monitoring ceramics, Nature Communications, Vol: 8, ISSN: 2041-1723
The properties of graphene open new opportunities for the fabrication of composites exhibiting unique structural and functional capabilities. However, to achieve this goal we should design and build materials with carefully designed architectures. Here, we describe the fabrication of ceramic-graphene composites by combining graphene foams with pre-ceramic polymers and spark plasma sintering. The result is a material containing an interconnected, microscopic network of very thin (20-30 nm), electrically conductive, carbon interfaces. This network generates electrical conductivities up to two orders of magnitude higher than those of other ceramics with similar graphene or carbon nanotube contents and can be used to monitor “in situ” structural integrity. In addition, it directs crack propagation, promoting stable crack growth and increasing the fracture resistance by an order of magnitude. These results demonstrate that the rational integration of nanomaterials could be a fruitful path towards building composites combining unique mechanical and functional performances.
Feilden E, Giovannini T, Ni N, et al., 2017, Micromechanical strength of individual Al2O3 platelets, Scripta Materialia, Vol: 131, Pages: 55-58, ISSN: 1359-6462
Optimising the properties of platelet reinforced composites requires the strength of the reinforcing phase to be known, however strength measurements at such small scales are difficult and therefore data is sparse. In this work the flexural strength and Weibull modulus of microscopic, alumina platelets has been measured as 5.3 ± 1.3 GPa and 3.7 respectively, using an in-situ micro 3-point bend test. A general approach to correct for the effect of variation in sample size on the Weibull modulus is presented, and the internal structure of the platelets is revealed by TEM.
Feilden E, Giovannini T, Ni N, et al., Micromechanical strength of Al2O3 platelets, Scripta Materialia, ISSN: 1359-6462
Optimising the properties of platelet reinforced composites requires the strength of the reinforcing phase to be known, however strength measurements at such small scales are difficult and therefore data is sparse. In this work the flexural strength and Weibull modulus of microscopic, alumina platelets has been measured as 5.3±1.3 GPa and 3.7 respectively, using an in-situ micro 3-point bend test. A general approach to correct for the effect of variation in sample size on the Weibull modulus is presented, and the internal structure of the platelets is revealed by TEM.
Ni N, Cooper SJ, Williams R, et al., 2016, Degradation of (La0.6Sr0.4)0.95(Co0.2Fe0.8)O3-δ Solid Oxide Fuel Cell Cathodes at the Nanometre Scale and Below, ACS Applied Materials & Interfaces, Vol: 8, Pages: 17360-17370, ISSN: 1944-8244
The degradation of intermediate temperature solid oxide fuel cell (ITSOFC) cathodes has been identified as a major issue limiting the development of ITSOFCs as high efficiency energy conversion devices. In this work, the effect of Cr poisoning on (La0.6Sr0.4)0.95(Co0.2Fe0.8)O3-δ (LSCF6428), a particularly promising ITSOFC cathode material, was investigated on symmetrical cells using electrochemical impedance spectroscopy and multi-scale structural/chemical analysis by advanced electron and ion microscopy. The systematic combination of bulk and high-resolution analysis on the same cells allows, for the first time, to directly correlate Cr induced performance degradation with subtle and localized structural/chemical changes of the cathode down to the atomic scale. Up to two orders of magnitude reduction in conductivity, oxygen surface exchange rate and diffusivity were observed in Cr poisoned LSCF6428 samples. These effects are associated with the formation of nanometer size SrCrO4; grain boundary segregation of Cr; enhanced B-site element exsolution (both Fe and Co); and reduction in the Fe valence, the latter two being related to Cr substitution in LSCF. The finding that significant degradation of the cathode happens before obvious microscale change points to new critical SOFC degradation mechanisms effective at the nanometer scale and below.
Al Nasiri N, Patra N, Ni N, et al., 2016, Oxidation behaviour of SiC/SiC ceramic matrix composite in air, Journal of the European Ceramic Society, Vol: 36, Pages: 3293-3302, ISSN: 1873-619X
Oxidation of silicon melt infiltrated SiC/SiC ceramic matrix composites (CMC) was studied in air at 1200–1400 °C for 1, 5, 24 and 48 h. Weight gain and oxide layer thickness measurements revealed the oxidation follows parabolic reaction kinetics with increase in temperature and time. XRD showed the extent of oxide layer (SiO2) formation was greatest after 48 h at 1400 °C: an observation confirmed by X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) analyses. Oxide layer thickness varied from 1 μm after 48 h at 1200 °C to 8 μm after 48 h at 1400 °C. Oxidation of SiC/SiC composites is both temperature and time dependent with an activation energy of 619 kJ mol−1. BN coatings around SiC fibres showed good resistance to oxidation even after 48 h at 1400 °C.
Skinner SJ, Ni N, 2016, Combined Cr and Mo Poisoning of (La,Sr)(Co,Fe)O3-δ Solid Oxide Fuel Cell Cathodes at the Nanoscale, Solid State Ionics, Vol: 288, Pages: 28-31, ISSN: 1872-7689
Cr poisoning has been identified as a critical issue for solid oxide fuel cell (SOFC) cathodes degradation when metallic alloys are used for intermediate temperature SOFCs. In addition, Mo is also a common alloying element in ferritic stainless steel or Ni based alloys considered as interconnect materials and molybdenum trioxide is very volatile, raising concerns on Mo poisoning to the cathodes. In this work, the intrinsic reactivity of Cr with a porous (La0.6Sr0.4)0.95(Co0.2Fe0.8)O3 − δ cathode (LSCF6428) with and without the presence of Mo was investigated by transmission electron microscopy (TEM) in order to reveal the nanoscale incorporation and evolution behavior of Cr and/or Mo in LSCF. Cr incorporation was identified with formation of Cr containing phases including chromium oxide and SrCrOx with sizes of as small as ~ 100 nm, suggesting that the poisoning can take effect with subtle changes at the very small scale. Co-poisoning of Mo with Cr significantly changes the pattern of Cr behavior. CrCo2Fe3Ox spinel becomes the main reaction product in addition to chromium oxide as a result of Sr reacting preferentially with Mo to form SrMoO4. The LSCF grain boundary was found to be rich in Cr and deficient in Co. These results suggest that potential Mo poisoning effects should be considered when developing metallic interconnects containing Mo.
Boix M, Eslava S, Machado GC, et al., 2015, ATR-FTIR measurements of albumin and fibrinogen adsorption: Inert versus calcium phosphate ceramics, JOURNAL OF BIOMEDICAL MATERIALS RESEARCH PART A, Vol: 103, Pages: 3493-3502, ISSN: 1549-3296
Taub S, Neuhaus K, Wiemhoefer H-D, et al., 2015, The effects of Co and Cr on the electrical conductivity of cerium gadolinium oxide, Solid State Ionics, Vol: 282, Pages: 54-62, ISSN: 1872-7689
Ni N, Barg S, Garcia-Tunon E, et al., 2015, Understanding Mechanical Response of Elastomeric Graphene Networks, Scientific Reports, Vol: 5, ISSN: 2045-2322
Ultra-light porous networks based on nano-carbon materials (such as graphene or carbon nanotubes) have attracted increasing interest owing to their applications in wide fields from bioengineering to electrochemical devices. However, it is often difficult to translate the properties of nanomaterials to bulk three-dimensional networks with a control of their mechanical properties. In this work, we constructed elastomeric graphene porous networks with well-defined structures by freeze casting and thermal reduction, and investigated systematically the effect of key microstructural features. The porous networks made of large reduced graphene oxide flakes (>20 μm) are superelastic and exhibit high energy absorption, showing much enhanced mechanical properties than those with small flakes (<2 μm). A better restoration of the graphitic nature also has a considerable effect. In comparison, microstructural differences, such as the foam architecture or the cell size have smaller or negligible effect on the mechanical response. The recoverability and energy adsorption depend on density with the latter exhibiting a minimum due to the interplay between wall fracture and friction during deformation. These findings suggest that an improvement in the mechanical properties of porous graphene networks significantly depend on the engineering of the graphene flake that controls the property of the cell walls.
D'Elia E, Barg S, Ni N, et al., 2015, Self-healing graphene-based composites with sensing capabilities., Advanced Materials, Vol: 32, Pages: 4788-4794, ISSN: 1521-4095
A self-healing composite is fabricated by confining a supramolecular polymer in a graphene network. The network provides electrical conductivity. Upon damage, the polymer is released and flows to reform the material. Healing is repeatable and autonomous. The composite is sensitive to pressure and flexion and recovers its mechanical and electrical properties even when rejoining cut surfaces after long exposure times.
Al Nasiri N, Ni N, Saiz E, et al., 2015, Effect of microstructure and grain boundary chemistry on slow crack growth in silicon carbide at ambient conditions, Journal of the European Ceramic Society, Vol: 35, Pages: 2253-2260, ISSN: 0955-2219
Silicon carbide (SiC) is being used increasingly as a room temperature structural material in environments where moisture cannot always be excluded. Unfortunately, there have been almost no reports on slow crack growth (SCG) in SiC at room temperature. To address this gap, SCG in SiC was studied using constant stress rate and double torsion tests in water. SiC based materials were produced with a wide range of grain boundary chemistries and microstructures, which may affect their slow crack growth behaviour. To clarify the role of chemistry and microstructure respectively, solid state (SS) sintering with carbon and boron along with liquid phase (LP) sintering using oxides additives were used to produce materials with fine and coarse grains. The LP-SiC was three times more sensitive to SCG than SS-SiC materials. Moreover, the larger grained material with a higher toughness was less sensitive to SCG than the materials with fine grains.
Hu J, Garner A, Ni N, et al., 2015, Identifying suboxide grains at the metal-oxide interface of a corroded Zr-1.0%Nb alloy using (S)TEM, transmission-EBSD and EELS, MICRON, Vol: 69, Pages: 35-42, ISSN: 0968-4328
Frankel PG, Wei J, Francis EM, et al., 2015, Effect of Sn on Corrosion Mechanisms in Advanced Zr-Cladding for Pressurised Water Reactors, 17th International Symposium on Zirconium in the Nuclear Industry, Publisher: ASTM INTERNATIONAL, Pages: 404-437, ISSN: 0066-0558
Barg S, Perez FM, Ni N, et al., 2014, Mesoscale assembly of chemically modified graphene into complex cellular networks, Nature Communications
Nicholls RJ, Ni N, Lozano-Perez S, et al., 2014, Crystal Structure of the ZrO Phase at Zirconium/Zirconium Oxide Interfaces, Advanced Engineering Materials, Vol: 17, Pages: 211-215, ISSN: 1527-2648
Zirconium-based alloys are used in water-cooled nuclear reactors for both nuclear fuel cladding and structural components. Under this harsh environment, the main factor limiting the service life of zirconium cladding, and hence fuel burn-up efficiency, is water corrosion. This oxidation process has recently been linked to the presence of a sub-oxide phase with well-defined composition but unknown structure at the metal–oxide interface. In this paper, the combination of first-principles materials modeling and high-resolution electron microscopy is used to identify the structure of this sub-oxide phase, bringing us a step closer to developing strategies to mitigate aqueous oxidation in Zr alloys and prolong the operational lifetime of commercial fuel cladding alloys.
Ni N, Kaufmann Y, Kaplan WD, et al., 2014, Interfacial energies and mass transport in the Ni(Al)–Al2O3 system: The implication of very low oxygen activities, Acta Materialia, Vol: 64, Pages: 282-296, ISSN: 1359-6454
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