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Journal articleGarcia-Tunon E, Feilden E, Zheng H, et al., 2017,
Graphene Oxide: An All-in-One Processing Additive for 3D Printing
, ACS Applied Materials and Interfaces, Vol: 9, Pages: 32977-32989, ISSN: 1944-8244Many 3D printing technologies are based on the development of inks and pastes to build objects through droplet or filament deposition (the latter also known as continuous extrusion, robocasting, or direct ink writing). Controlling and tuning rheological behavior is key for successful manufacturing using these techniques. Different formulations have been proposed, but the search continues for approaches that are clean, flexible, robust and that can be adapted to a wide range of materials. Here, we show how graphene oxide (GO) enables the formulation of water-based pastes to print a wide variety of materials (polymers, ceramics, and steel) using robocasting. This work combines flow and oscillatory rheology to provide further insights into the rheological behavior of suspensions combining GO with other materials. Graphene oxide can be used to manipulate the viscoelastic response, enabling the formulation of pastes with excellent printing behavior that combine shear thinning flow and a fast recovery of their elastic properties. These inks do not contain other additives, only GO and the material of interest. As a proof of concept, we demonstrate the 3D printing of additive-free graphene oxide structures as well as polymers, ceramics, and steel. Due to its amphiphilic nature and 2D structure, graphene oxide plays multiple roles, behaving as a dispersant, viscosifier, and binder. It stabilizes suspensions of different powders, modifies the flow and viscoelasticity of materials with different chemistries, particle sizes and shapes, and binds the particles together, providing green strength for manual handling. This approach enables printing complex 3D ceramic structures using robocasting with similar properties to alternative formulations, thus demonstrating the potential of using 2D colloids in materials manufacturing.
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Journal articleArnold M, Zhao S, Ma S, et al., 2017,
Microindentation: a tool for measuring cortical bone stiffness? A systematic review
, Bone & Joint Research, Vol: 6, Pages: 542-549, ISSN: 2046-3758Objectives: Microindentation hasthe potential to measuretheelasticity(stiffness)of individualpatients’bone. Bone elasticity plays a crucial role in the press-fit stability of orthopaedic implants.Arming surgeons with accuratebone elasticityinformation may reduce surgical complicationsincluding peri-prosthetic fractures. The question we address with this systematicreview is whether microindentation can accurately measure cortical bone stiffness.Methods: A systematic review of all English language articles using a keyword search was undertaken in Medline, Embase, PubMed, Scopus and Cochrane databases. Studies thatonly used nanoindentation, cancellous boneoranimal tissue were excluded.Results: 1094abstracts were retrieved and 32papers were included in the analysis, 20 of which used reference point indentation and 12of which used traditional depth sensing indentation.There are a number of factors thatmust be taken into account when using microindentation such as tip size, depth and method of analysis.Only two studies validated microindentation againsttraditional mechanical testing techniques. Bothstudies used reference point indentation(RPI) with one showing that RPI parameters correlate well with mechanical testing, butanother suggestedthatthey do not. Conclusion: Microindentation has been used in various studies to assess bone elasticity but only two studies with conflicting results compared microindentation to traditional mechanical testing techniques. Further research,includingmore studies comparingmicroindentationto other mechanical testing methodsare needed,before microindentation can be reliably used to calculate cortical bone stiffness.
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Journal articleArnold M, Zhao S, Ma S, et al., 2017,
Microindentation - a tool for measuring cortical bone stiffness?
, BONE & JOINT RESEARCH, Vol: 6, Pages: 542-549, ISSN: 2046-3758 -
Conference paperLeong AYL, Mahtar MA, Mattevi C, et al., 2017,
Graphene filled epoxy coatings for enhanced corrosion protection
, 21st International Conference on Composite Materials (ICCM-21)© 2017 International Committee on Composite Materials. All rights reserved. The ability of graphene oxide derived fillers to enhance the moisture barrier performance of epoxy coating has been investigated. Films and coatings of virgin and modified graphene oxide (GO) were produced with different dispersion methods. Water vapour transmission rate (WVTR) was evaluated. Reduction in WVTR of GO/epoxy films were less than predicted and in one case, was higher than unfilled epoxy film. However, moisture barrier and corrosion protection performance were improved by reducing the graphene oxide. Films with reduced GO (rGO) have smaller WVTR while corrosion rate of rGO/epoxy coating was also lowered.
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Journal articleKnowles A, Bhowmik A, Purkayastha S, et al., 2017,
Data on a Laves phase intermetallic matrix composite in situ toughened by ductile precipitates
, Data in Brief, Vol: 14, Pages: 489-493, ISSN: 2352-3409The data presented in this article are related to the research article entitled “Laves phase intermetallic matrix composite in situ toughened by ductile precipitates” (Knowles et al.) [1]. The composite comprised a Fe2(Mo, Ti) matrix with bcc (Mo, Ti) precipitated laths produced in situ by an aging heat treatment, which was shown to confer a toughening effect (Knowles et al.) [1]. Here, details are given on a focused ion beam (FIB) slice and view experiment performed on the composite so as to determine that the 3D morphology of the bcc (Mo, Ti) precipitates were laths rather than needles. Scanning transmission electron microscopy (S(TEM)) micrographs of the microstructure as well as energy dispersive X-ray spectroscopy (EDX) maps are presented that identify the elemental partitioning between the C14 Laves matrix and the bcc laths, with Mo rejected from the matrix into laths. A TEM selected area diffraction pattern (SADP) and key is provided that was used to validate the orientation relation between the matrix and laths identified in (Knowles et al.) [1] along with details of the transformation matrix determined.
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Conference paperHauth M, Lawlor V, Cartellieri P, et al., 2017,
Production and reliability oriented SOFC cell and stack design
, 15th International Symposium on Solid Oxide Fuel Cells (SOFC), Publisher: Electrochemical Society, Pages: 2231-2249, ISSN: 1938-5862The paper presents an innovative development methodology for a production and reliability oriented SOFC cell and stack design aiming at improving the stacks robustness, manufacturability, efficiency and cost. Multi-physics models allowed a probabilistic approach to consider statistical variations in production, material and operating parameters for the optimization phase. A methodology for 3D description of spatial distribution of material properties based on a random field models was developed and validated by experiments. Homogenized material models on multiple levels of the SOFC stack were established. The probabilistic models were related to the experimentally obtained properties of base materials to establish a statistical relationship between the material properties and the most relevant load effects. Software algorithms for meta models that allow the detection of relationships between input and output parameters and to perform a sensitivity analysis were developed and implemented. The capabilities of the methodology is illustrated on two practical cases.
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Journal articleSernicola G, Giovannini T, Patel P, et al., 2017,
In situ stable crack growth at the micron scale
, Nature Communications, Vol: 8, ISSN: 2041-1723Grain boundaries typically dominate fracture toughness, strength, slow crack growth of ceramics. To improve these properties through mechanistically informed grain boundary engineering, precise measurement of the mechanical properties of individual boundaries is essential, although this is rarely achieved due to its complexity. Here we present a new approach to characterise the fracture energy at the lengthscale of individual grain boundaries and demonstrate this capability with measurement of the surface energy of silicon carbide (SiC) single crystals. We perform experiments using an in situscanning electron microscopy based double cantilever beam test, thus enabling viewing and measurement of stable crack growth directly. These experiments correlate well with our density functional theory (DFT) calculations of the surface energy of the same SiC plane. Subsequently, we measure the fracture energy for a bi-crystal of SiC, diffusion bonded with a thin glassy layer. These measurements ultimately promote microstructural engineering of novel and advanced ceramics.
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Journal articleKnowles AJ, Bhowmik, Purkayastha S, et al., 2017,
Laves phase intermetallic matrix composite in situ toughened by ductile precipitates
, Scripta Materialia, Vol: 140, Pages: 59-62, ISSN: 1872-8456Laves phase based materials are of interest for elevated temperature applications for their high melting points and strengths but are critically limited by their low fracture toughness. Here, a Laves phase intermetallic matrix composite toughened by ductile precipitates has been studied. This microstructure was produced in situ by heat treating a Fe2(Mo,Ti) based alloy to precipitate ∼ 12% volume fraction of fine ∼ 250 nm bcc, A2 (Mo,Ti), phase, with an orientation relationship of . The precipitated A2 phase increased the indentation fracture toughness from 1.1 to 2.2 MPa m1/2 while maintaining a high hardness of HV0.5 = 8.9 GPa similar to monolithic Laves phases.
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Journal articleKnowles AJ, Jun T-S, Bhowmik A, et al., 2017,
A new beta titanium alloy system reinforced with superlattice intermetallic precipitates
, Scripta Materialia, Vol: 140, Pages: 71-75, ISSN: 1872-8456Titanium alloys traditionally lack a nm-scale intermetallic precipitate that can be exploited for age-hardening from solid solution. Here such a strengthening concept is developed in the Ti-Fe-Mo system, with it being found that a high temperature β (bcc A2) single-phase field for homogenisation can be obtained, which following ageing (750 °C/80 h) precipitated B2 TiFe <100 nm in size. The orientation relationship was found to be ⟨100⟩A2//⟨100⟩B2, {100}A2//{100}B2, with a misfit of −6.1%. The alloy was found to be very hard (HV0.5 = 6.4 GPa) and strong (σy, 0.2 = 1.9 GPa) with a density of 6.68 g cm−3. TEM observation and micropillar deformation showed that the precipitates resist dislocation cutting.
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Journal articleNi H, Xu F, Tomsia AP, et al., 2017,
Robust Bioinspired Graphene Film via pi-pi Cross-linking
, ACS APPLIED MATERIALS & INTERFACES, Vol: 9, Pages: 24987-24992, ISSN: 1944-8244Graphene composite films inspired by nacre are the subject of ongoing research efforts to optimize their properties for applications in flexible energy devices. Noncovalent interactions do not cause interruption of the delocalized conjugated π-electron system, thus preserving graphene’s excellent properties. Herein, we synthesized a conjugated molecule with pyrene groups on both ends of a long linear chain (AP-DSS) from 1-aminopyrene (AP) and disuccinimidyl suberate (DSS). The AP-DSS molecules are used to cross-link adjacent graphene nanosheets via π–π interfacial interactions to improve properties of graphene films. The tensile strength and toughness of resultant graphene films were 4.1 and 6.4 times higher, respectively, than that of pure rGO film. More remarkably, the electrical conductivity showed a simultaneous improvement, which is rare to be achieved in other kinds of covalent or noncovalent functionalization. Such integration demonstrates the advantage of this work to previously reported noncovalent functionalization of graphene.
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Journal articleMachado GC, García-Tuñón E, Bell RV, et al., 2017,
Calcium phosphate substrates with emulsion-derived roughness: processing, characterisation and interaction with human mesenchymal stem cells
, Journal of the European Ceramic Society, Vol: 38, Pages: 949-961, ISSN: 0955-2219Calcium phosphates (CaP) have been the subject of several studies that often lack a systematic approach to understanding how their properties affect biological response. CaP particles functionalised with a pH-responsive polymer (BCS) were used to prepare microporous substrates (porosity between 70 and 75% and pore sizes of 5–20 μm) through the aggregation of oil-in-water emulsions by controlling solid loading, emulsification energy, pH, drying and sintering conditions. The combined effect of surface roughness (roughness amplitude, Ra between 0.9–1.7 μm) and chemistry (varying Hydroxyapatite/β-Tricalcium phosphate ratio) on human mesenchymal stem cells was evaluated. HA substrates stimulated higher cell adhesion and proliferation (especially with lower Ra), but cell area increased with β-TCP content. The effect of surface roughness depended of chemistry: HA promoted higher mineralising activity when Ra ∼ 1.5 μm, whereas β-TCP substrates stimulated a more osteogenic profile when Ra ∼ 1.7 μm. A novel templating method to fabricate microporous CaP substrates was developed, opening possibilities for bone substitutes with controlled features.
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Journal articleWaheed S, Hao R, Bhowmik A, et al., 2017,
A unifying scaling for the Bauschinger effect in highly confined thin films: a discrete dislocation plasticity study
, Modelling and Simulation in Materials Science and Engineering, Vol: 25, ISSN: 0965-0393In this study, sequential sputter deposition, diffusion bonding and focused ion beam milling are used to fabricate sapphire micropillars encapsulating a thin single crystal niobium film. A distinct Bauschinger effect is observed during the cyclic axial compression of the samples. Plain strain discrete dislocation plasticity is used to interpret the experimental results obtained for the encapsulated film-micropillar geometry. The simulations show that the experimental samples correspond to a saturated source density regime, producing the maximum Bauschinger effect for the chosen mean nucleation strength. Next, the source density and mean nucleation strength are shown to have a coupled effect on the size of the Bauschinger effect, understood in terms of the differing number of pile-ups occurring per source in the film. The coupled effect is found to be represented by the density of dislocations annihilated upon unloading: a consistent linear relationship is observed between the size of the Bauschinger effect and the annihilated dislocation density over the entire source density and nucleation strength parameter space investigated. It is found that different film orientations fulfil the same linear relationship, whereas changing the film thickness causes the slope of the linear trend to vary suggesting a length-scale dependence on reverse plasticity. Finally, all results are found to be unified by a power-law relationship quantifying the Bauschinger effect of the form ${{\rm{\Gamma }}}_{{\rm{B}}}\propto {\rm{\Delta }}{\rho }_{{\rm{ann}}}{l}^{n}$ where it is argued that the number of dislocations undergoing reverse glide in the confined film is represented by ${\rm{\Delta }}{\rho }_{{\rm{ann}}}$, the mean free path of dislocations by l and the effect of hardening processes by the exponent n. The net reverse glide is thus represented by ${\rm{\Delta }}{\rho }_{{\rm{ann}}}{l}^{n}$ which can be used as a measure of the Bauschinger effect.
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Journal articleGoyos-Ball L, Fernandez E, Diaz R, et al., 2017,
Osseous differentiation on freeze casted 10CeTZP-Al2O3 structures
, Journal of the European Ceramic Society, Vol: 37, Pages: 5009-5016, ISSN: 1873-619XThree-dimensional structures with directionally oriented pore networks were fabricated from a 10 mol% ceria-stabilized zirconia and alumina composite (10CeTZP-Al2O3) via freeze casting. Ceramic suspensions of different concentrations (30, 40 and 50 wt% solids) were frozen at various rates (2, 5 and 10 °C/min) to obtain lamellar structures with aligned tubular pores of different characteristics: porosity (75–84%), pore dimensions (small diameter of the elliptical pores: 10–23 μm; large diameter of the elliptical pores: ∼200 ± 70 μm), lamella thickness (2.7–4 μm) and compression strength (1–12 MPa). In vitro assays confirmed the non-cytotoxic nature of the samples. Furthermore, specific osseous differentiation genes were quantified after incubating osteoblasts on different cross sections of the samples during 7 days in supplemented culture medium; results demonstrated that the freeze casted structures induce up to nine times more osseous gene expression than tissue culture polystyrene (TCPS), an advanced surface used for optimized in vitro cell growth.
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Conference paperRounthwaite N, McGilvery CM, Jiang J, et al., 2017,
A chemical and morphological study of diesel injector nozzle deposits - insights into their formation and growth mechanisms
, SAE 2017 World Congress and Exhibition, Publisher: SAE International, Pages: 106-114, ISSN: 1946-3960Modern diesel passenger car technology continues to develop rapidly in response to demanding emissions, performance, refinement, cost and fuel efficiency requirements. This has included the implementation of high pressure common rail fuel systems employing high precision injectors with complex injection strategies, higher hydraulic efficiency injector nozzles and in some cases <100µm nozzle hole diameters. With the trend towards lower diameter diesel injector nozzle holes and reduced cleaning through cavitation with higher hydraulic efficiency nozzles, it is increasingly important to focus on understanding the mechanism of diesel injector nozzle deposit formation and growth. In this study such deposits were analysed by cross-sectioning the diesel injector along the length of the nozzle hole enabling in-depth analysis of deposit morphology and composition change from the inlet to the outlet, using state-of-the-art electron microscopy techniques. Deposits produced in the injector nozzles of the industry standard fouling test (CEC F-98-08 DW10B bench engine) were compared with those formed in a vehicle driven on a chassis dynamometer, using a drive cycle more representative of real world vehicle conditions, to explore the effects of differing drive cycles and engine technologies. Fouling in all tests was accelerated with the addition of 1ppm zinc neodecanoate, as specified in the CEC DW10B test. This in-depth characterisation revealed a complex multi-layered system of deposits inside the diesel injector nozzle. Through analysing these layers the mechanisms enabling the initial deposit formation and growth can be postulated.
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Journal articleGoyos-Ball L, Garcia-Tunon E, Fernandez-Garcia E, et al., 2017,
Mechanical and biological evaluation of 3D printed 10CeTZP-Al2O3 structures
, Journal of the European Ceramic Society, Vol: 37, Pages: 3151-3158, ISSN: 0955-2219Three-dimensional structures were robocasted from a 10 mol% ceria-stabilized zirconia and alumina composite (10CeTZP-Al2O3). A hydrogel-based printable ink was developed using a unique non-ionic copolymer surfactant. Self-supporting and free-standing structures, including round lattices with interconnected pores (200–600 μm pores; 30–50% porosity), rectangular bars (95% density on average) and cones were successfully printed. The round lattices of 200 μm pores and 30% porosity showed compression strengths similar to those of cortical bone, reaching almost 200 MPa. The maximum flexural strength value attained for the rectangular bars was 575 MPa. In vitro biological studies demonstrated that the samples allow for practically 100% cell viability, confirming their non-cytotoxic nature. Cell differentiation tests were performed using osteoblasts incubated for 7 days in supplemented cell culture medium. Quantification of specific osseous differentiation genes showed that the robocasted structures induced a higher degree of osseous differentiation than tissue culture polystyrene.
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Journal articleOlowojoba GB, Kopsidas S, Eslava S, et al., 2017,
A facile way to produce epoxy nanocomposites having excellent thermal conductivity with low contents of reduced graphene oxide
, Journal of Materials Science, Vol: 52, Pages: 7323-7344, ISSN: 1573-4803A well-dispersed phase of exfoliated graphene oxide (GO) nanosheets was initially prepared in water. This was concentrated by centrifugation and was mixed with a liquid epoxy resin. The remaining water was removed by evaporation, leaving a GO dispersion in epoxy resin. A stoichiometric amount of an anhydride curing agent was added to this epoxy-resin mixture containing the GO nanosheets, which was then cured at 90 °C for 1 hour followed by 160 °C for 2 hours. A second thermal treatment step of 200 °C for 30 minutes was then undertaken to reduce further the GO in-situ in the epoxy nanocomposite. An examination of the morphology of such nanocomposites containing reduced graphene oxide (rGO) revealed that a very good dispersion of rGO was achieved throughout the epoxy polymer. Various thermal and mechanical properties of the epoxy nanocomposites were measured and the most noteworthy finding was a remarkable increase in the thermal conductivity when relatively very low contents of rGO were present. For example, a value of 0.25 W/mK was measured at 30 °C for the nanocomposite with merely 0.06 weight percentage (wt%) of rGO present, which represents an increase of ~40% compared with that of the unmodified epoxy polymer. This value represents one of the largest increases in the thermal conductivity per wt% of added rGO yet reported. These observations have been attributed to the excellent dispersion of rGO achieved in these nanocomposites made via this facile production method. The present results show that it is now possible to tune the properties of an epoxy polymer with a simple and viable method of GO addition.
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Journal articleSaiz Gutierrez E, Picot O, Ferraro C, et al., 2017,
Using graphene networks to build bioinspired self-monitoring ceramics
, Nature Communications, Vol: 8, ISSN: 2041-1723The 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.
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Conference paperBoughton OR, Zhao S, Arnold M, et al., 2017,
Measuring bone stiffness using microindentation
, British Orthopaedic Research Society (BORS) 2016 Conference, Publisher: British Editorial Society of Bone and Joint Surgery, Pages: 31-31, ISSN: 2049-4416 -
Journal articleFeilden E, Giovannini T, Ni N, et al., 2017,
Micromechanical strength of individual Al2O3 platelets
, Scripta Materialia, Vol: 131, Pages: 55-58, ISSN: 1359-6462Optimising 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.
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Journal articleGlymond D, Vick M, Giuliani F, et al., 2017,
High Temperature Fracture Toughness of Mullite with Monoclinic Zirconia
, Journal of the American Ceramic Society, Vol: 100, Pages: 1570-1577, ISSN: 1551-2916Reactive sintering of zircon and alumina and zirconia additions to mullite are well establishedmethods for improving the poor fracture toughness of mullite. While it is clear that transformationtoughening is responsible for the improved toughness by addition of partially stabilised zirconia, it isnot clear why adding unstabilised zirconia increases the toughness although microcracking and crackdeflection have been suggested. Therefore the fracture toughness of a mullite composite with 20vol% unstabilised zirconia and a monolithic mullite were investigated at ambient conditions and attemperatures up to 1225 oC. It was found that monoclinic zirconia increases the toughness atambient conditions from the monolithic mullite value of 1.9 MPa m1/2 to 3.9 MPa m1/2. Thetoughness of the composite with zirconia remains relatively constant from ambient to 600 °C butthen decreases rapidly. The mechanism for the toughness enhancement as well as the reason for itsvariation with temperature are explained using changes in residual stress state as deduced using thesphere in shell model from the measured thermal expansion behaviour.
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Journal articleFeilden E, Giovannini T, Ni N, et al.,
Micromechanical strength of Al2O3 platelets
, Scripta Materialia, ISSN: 1359-6462Optimising 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.
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Journal articleFeilden E, Ferraro C, Giuliani F, et al., 2016,
Progress in novel and unexpected areas
, MATERIALS TODAY, Vol: 19, Pages: 544-545, ISSN: 1369-7021 -
Journal articleZhang T, Liang X, Li C, et al., 2016,
Control of drying shrinkage in magnesium silicate hydrate (M-S-H) gel mortars
, Cement and Concrete Research, Vol: 88, Pages: 36-42, ISSN: 0008-8846Magnesium silicate hydrate (M-S-H) gel can be formed by the reaction of MgO with amorphous silica in the presence of sodium hexametaphosphate (Na-HMP). Typical pastes contain 40% MgO and 60% SF and have a w/c ratio of 0.5, but these exhibit shrinkage cracking on drying. The shrinkage characteristics of M-S-H mortar samples containing different additions of sand have been studied using dilatometry. The drying shrinkage was found to decrease with increasing sand addition and to increase with increased water content. Mortars with 60 wt.% sand addition and a w/c ratio of 0.5 had a drying shrinkage of 0.16% and did not show shrinkage cracking. A simple geometrical model based on particle packing is presented that explains the observed changes in drying shrinkage. Based on the geometrical model, the shrinkage of M-S-H mortar system can be reduced to zero when the volume fraction of sand in the mortar is about 0.77.
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Journal articleFeilden E, Garcia Tunon Blanca E, Giuliani F, et al., 2016,
Robocasting of structural ceramic parts with hydrogel inks
, Journal of the European Ceramic Society, Vol: 36, Pages: 2525-2533, ISSN: 0955-2219Robocasting is a 3D printing technique that may be able to achieve the much-coveted goal of reliable, complex ceramic parts with low porosity and high strength. In this work a robust hydrogel formulation was optimised for use as the extrusion paste for robocasting. The paste’s rheological properties were characterised and the printing process was optimised with the aim of attaining dense monolithic ceramic parts. The pastes exhibit a characteristic shear thinning behaviour with yield stresses that can reach values above 1 kPa and depend mostly on their solid content and the particle size distribution. It is possible to formulate printable Al2O3 and SiC inks with solid contents as high as 40 vol% that reached densities up to 95th% for SiC and 97th% for Al2O3 with flexural strengths of 300 MPa and 230 MPa respectively after sintering. The sources of strength limiting defects are identified and related to the printing process.
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Journal articleGarcia Tunon Blanca E, Machado G, Schneider M, et al., 2016,
Complex ceramic architectures by directed assembly of ‘responsive’ particles
, Journal of the European Ceramic Society, Vol: 37, Pages: 199-211, ISSN: 1873-619XSurface functionalization of alumina powders with a responsive surfactant(BCS) leads to particles that react to a chemical switch. These ‘responsive’ building blocksare capable of assembling into macroscopic and complex ceramic structures. The aggregationfollows a bottom up approach and can be easily controlled. The directed assembly ofconcentrated suspensions leads to highly dense (~99%) ceramic components with average4-point bending strength of ~200 MPa. On the other hand, the emulsification of suspensionswith concentrations from 7 to 43 vol% and 50 vol% decane results in emulsions withdifferent properties (stability, droplet size and distribution). The oil droplets provide a softtemplate confining the alumina particles in the continuous phase and at the oil/water interfaces.Aggregation of these emulsions followed by drying and sintering leads tomacroporous (pore sizes ranging from 30 to 4 µm) alumina structures with complex shapesand a wide range of microstructures, from closed cell structures to highly interconnected foams with total porosities up to 83%. Alumina scaffolds with ~55 % porosity can reachcrushing strength values above 300 MPa in compression and ~50 MPa in 4-point bending.
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Journal articleEslava S, Reynal A, Rocha VG, et al., 2016,
Using graphene oxide as a sacrificial support of polyoxotitanium clusters to replicate its two-dimensionality on pure titania photocatalysts
, Journal of Materials Chemistry A, Vol: 4, Pages: 7200-7206, ISSN: 2050-7496The nanostructure optimisation of metal oxides is of crucial importance to exploit their qualities in artificial photosynthesis, photovoltaics and heterogeneous catalysis. Therefore, it is necessary to find viable and simple fabrication methods to tune their nanostructure. Here we reveal that graphene oxide flakes, known for their nano- and two-dimensionality, can be used as a sacrificial support to replicate their nano- and two-dimensionality in photocatalytic titania. This is demonstrated in the calcination of Ti16O16(OEt)32 polyoxotitanium clusters together with graphene oxide flakes, which results in pure titania nanoflakes of <10 nm titania nanoparticles in a two-dimensional arrangement. These titania nanoflakes outperform the titania prepared from only Ti16O16(OEt)32 clusters by a factor of forty in the photocatalytic hydrogen production from aqueous methanol suspensions, as well as the benchmark P25 titania by a factor of five. These outcomes reveal the advantage of using polyoxotitanium clusters with graphene oxide and open a new avenue for the exploitation of the vast variety of polyoxometalate clusters as precursors in catalysis and photovoltaics, as well as the use of graphene oxide as a sacrificial support for nanostructure optimisation.
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Journal articleScatigno GG, Ryan MP, Giuliani F, et al., 2016,
The effect of prior cold work on the chloride stress corrosion cracking of 304L austenitic stainless steel under atmospheric conditions
, Materials Science and Engineering: A, Vol: 668, Pages: 20-29, ISSN: 0921-5093A systematic study of the effect of cold work (CW) on chloride-induced stress corrosion cracking (SCC) in 304L stainless steel was performed. CW between 0% and 40% was applied prior to corrosion of specimens at 75 °C and 70% relative humidity, for 500 h, using MgCl2 (at atmospheric pressure). Samples cracked most readily between 0.5% and 5% CW; at 20% and above no cracks were present. Additionally, above 5% CW, some specific orientation relationships become evident, with cracks primarily aligned along <111> parallel to the transverse direction. The results suggest that at levels of CW >20%, the synergistic effect of micro-mechanisms may hinder SCC in this system.
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Journal articleD'Elia E, Eslava S, Miranda M, et al., 2016,
Autonomous self-healing structural composites with bio-inspired design
, Scientific Reports, Vol: 6, ISSN: 2045-2322Strong and tough natural composites such as bone, silk or nacre are often built from stiff blocks boundtogether using thin interfacial soft layers that can also provide sacrificial bonds for self-repair. Herewe show that it is possible exploit this design in order to create self-healing structural composites byusing thin supramolecular polymer interfaces between ceramic blocks. We have built model brick-andmortarstructures with ceramic contents above 95 vol% that exhibit strengths of the order of MPa(three orders of magnitude higher than the interfacial polymer) and fracture energies that are twoorders of magnitude higher than those of the glass bricks. More importantly, these properties can befully recovered after fracture without using external stimuli or delivering healing agents. This approachdemonstrates a very promising route towards the design of strong, ideal self-healing materials able toself-repair repeatedly without degradation or external stimuli.
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Journal articleBhowmik A, Dolbnya I, Britton TB, et al., 2016,
Using coupled micropillar compression and micro-Laue diffraction to investigate deformation mechanisms in a complex metallic alloy Al13Co4
, Applied Physics Letters, Vol: 108, ISSN: 0003-6951In this study, we have used in-situ micro-Laue diffraction combined with micropillar compression of focused ion beam milled Al13Co4 complex metallic alloy to investigate the evolution of deformation in Al13Co4. Streaking of the Laue spots shows that the onset of plastic flow occurs at stresses as low as 0.8 GPa, although macroscopic yield only becomes apparent at 2 GPa. The measured misorientations, obtained from peak splitting, enable the geometrically necessary dislocation density to be estimated as 1.1 1013 m-2.
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Journal articleNasiri NA, Saiz E, Giuliani F, et al., 2016,
Grain bridging locations of monolithic silicon carbide by means of focused ion beam milling technique
, Materials Letters, Vol: 173, Pages: 214-218, ISSN: 1873-4979A slice and view approach using a focused ion beam (FIB) milling technique was employed to investigate grain bridging near the tip of cracks in four silicon carbide (SiC) based materials with different grain boundary chemistries and grain morphologies. Using traditional observations intergranular fracture behaviour and hence clear evidence of grain bridging was found for SiC based materials sintered with oxide additives. More surprisingly, in large grain materials, the FIB technique reveals evidence of grain bridging irrespective of the grain boundary chemistry, i.e. also in materials which macroscopically fail by transgranular failure. This helps to explain why the toughness of large grained materials is higher even if failure is transgranular.
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