Imperial College London

DrNaNi

Faculty of EngineeringDepartment of Materials

Honorary Lecturer
 
 
 
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n.ni

 
 
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LM04Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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55 results found

Ni N, Hao W, Liu T, Zhou L, Guo F, Zhao X, Xiao Pet al., 2020, Oxidation/ablation behaviors of hafnium carbide-silicon carbonitride systems at 1500 and 2500 C, CERAMICS INTERNATIONAL, Vol: 46, Pages: 23840-23853, ISSN: 0272-8842

Journal article

Wu B, Ni N, Zhao X, Fan X, Xiao P, Guo Fet al., 2020, Strength retention in scheelite coated SiC fibers: Effect of the gas composition and pre-heat treatment, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, Vol: 40, Pages: 2801-2810, ISSN: 0955-2219

Journal article

Hao W, Ni N, Guo Y, Cai H, Fan X, Yao Y, Zhao X, Xiao Pet al., 2020, Strong and tough HfC-HfB2 solid-solution composites obtained by reactive sintering with a SiB6 additive, CERAMICS INTERNATIONAL, Vol: 46, Pages: 16257-16265, ISSN: 0272-8842

Journal article

Ding Q, Ni D, Ni N, Jiang Y, Chen B, Chen X, Lu J, Niu Y, Zhou H, Dong Set al., 2020, Thermal damage and microstructure evolution mechanisms of C-f/SiBCN composites during plasma ablation, CORROSION SCIENCE, Vol: 169, ISSN: 0010-938X

Journal article

Jiang J, Ni N, Zhao X, Guo F, Fan X, Xiao Pet al., 2020, Flexible and robust YAG-Al2O3 composite nanofibrous membranes enabled by a hybrid nanocrystalline-amorphous structure, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, Vol: 40, Pages: 2463-2469, ISSN: 0955-2219

Journal article

Cai H, Zhang X, Hu L, Guo F, Wang X, Zhao X, Ni N, Xiao Pet al., 2020, Improved stress measurement of YSZ by Raman spectroscopy: Effect of yttrium segregation-dependent tetragonality, INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Vol: 17, Pages: 2416-2423, ISSN: 1546-542X

Journal article

Lu J, Chen Y, Zhang H, Ni N, Li L, He L, Mu R, Zhao X, Guo Fet al., 2020, Y/Hf-doped AlCoCrFeNi high-entropy alloy with ultra oxidation and spallation resistance, CORROSION SCIENCE, Vol: 166, ISSN: 0010-938X

Journal article

Li X, Fan X, Ni N, Zhao X, Li C, Xiao Pet al., 2020, Continuous alumina fiber-reinforced yttria-stabilized zirconia composites with high density and toughness, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, Vol: 40, Pages: 1539-1548, ISSN: 0955-2219

Journal article

Wu B, Ni N, Fan X, Zhao X, Xiao P, Guo Fet al., 2020, Strength degradation of SiC fibers with a porous ZrB2-SiC coating: Role of the coating porous structure, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, Vol: 40, Pages: 961-971, ISSN: 0955-2219

Journal article

Chen W, Shan X, Li J, Guo Y, Guo F, Zhao X, Ni N, Xiao Pet al., 2020, Effects of iron and platinum on the isothermal oxidation of beta-NiAl overlay coatings fabricated by spark plasma sintering, SURFACE & COATINGS TECHNOLOGY, Vol: 382, ISSN: 0257-8972

Journal article

Hao W, Ni N, Guo Y, Li C, Fan X, Xiao W, Zhao X, Xiao Pet al., 2020, Densification, strengthening and toughening in hafnium carbide with the addition of silicon carbonitride, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Vol: 103, Pages: 3286-3298, ISSN: 0002-7820

Journal article

Jiang J, Ni N, Hao W, Zhao X, Guo F, Fan X, Xiao Pet 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

Journal article

Xie X, Liu B, Liu R, Zhao X, Ni N, Xiao Pet 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

Journal article

Xie X, Liu B, Guo Y, Liu R, Zhao X, Ni N, Guo F, Xiao Pet 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

Journal article

Ni N, Wang CC, Jiang SP, Skinner SJ, Skinner S, Ni N, Wang CC, jiang SPet 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.

Journal article

Hao W, Ni N, Guo F, Cao F, Jiang J, Zhao X, Xiao Pet 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

Journal article

Chen W, He L, Guo Y, Shan X, Li J, Guo F, Zhao X, Ni N, Xiao Pet 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

Journal article

Jin D, Ni N, Guo Y, Zou Z, Wang X, Guo F, Zhao X, Xiao Pet 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

Journal article

Ferraro C, Meille S, Réthoré J, Ni N, Chevalier J, Saiz Eet al., 2018, Strong and tough metal/ceramic micro-laminates, Acta Materialia, Vol: 144, Pages: 202-215, ISSN: 1359-6454

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.

Journal article

Reale F, Palczynski P, Amit I, Jones GF, Mehew JD, Bacon A, Ni N, Sherrell PC, Agnoli S, Craciun MF, Russo S, Mattevi Cet al., 2017, High-mobility and high-optical quality atomically thin WS2, Scientific Reports, Vol: 7, Pages: 1-10, 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.

Journal article

Rocha VG, Garcia-Tunon E, Botas C, Markoulidis F, Feilden E, D'Elia E, Ni N, Shaffer M, Saiz E, Rocha VG, García-Tuñón E, Botas C, Markoulidis F, Feilden E, D'Elia E, Ni N, Shaffer M, Saiz Eet 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.

Journal article

Ferraro C, Garcia-Tunon E, Barg S, Miranda M, Ni N, Bell R, Saiz Eet 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.

Journal article

Skinner SJ, li C, ni N, pramana S, kilner Jet al., 2017, 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.

Journal article

Pesci FM, Sokolikova MS, Grotta C, Sherrell PC, Reale F, Sharda K, Ni N, Palczynski P, Mattevi Cet 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.

Journal article

Zapata-Solvas E, Christopoulos SRG, Ni N, Parfitt DC, Horlait D, Fitzpatrick ME, Chroneos A, Lee WEet 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.

Journal article

Saiz Gutierrez E, Picot O, Ferraro C, Garcia Rocha V, Ni N, D'Elia E, Meille S, Chevalier J, Saunders T, Peijs T, Reece MJet 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 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.

Journal article

Feilden E, Giovannini T, Ni N, Ferraro C, Saiz E, Vandeperre L, Giuliani Fet 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.

Journal article

Feilden E, Giovannini T, Ni N, Ferraro C, Saiz E, Vandeperre L, Giuliani Fet al., 2017, 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.

Journal article

Al Nasiri N, Patra N, Ni N, Jayaseelan D, Lee Wet al., 2016, Oxidation behaviour of SiC/SiC ceramic matrix composites in air, Journal of the European Ceramic Society, Vol: 36, Pages: 3293-3302, ISSN: 0955-2219

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.

Journal article

Ni N, Cooper SJ, Williams R, Kemen N, McComb DW, Skinner Set 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.

Journal article

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