Mixed-conducting LSC/CGO and Ag/CGO composites for passive oxygen separation membranes

Dense ceramic oxygen separation membranes can pass oxygen perm-selectively at elevated temperature and have potential for improving the performance and reducing the cost of several industrial processes: such as the conversion of natural gas to syngas, or to separate oxygen from air for oxy-fuel combustion in electricity generation (to reduce NOx emissions and facilitate CO2 sequestration). These pressure-driven solid state membranes are based on fast oxygen-ion conducting ceramics, but also need a compensating flow of electrons. Dual-phase composites are attractive since they provide an extra degree of freedom, compared with single phase membranes, for optimising the overall membrane performance. In this study, composites containing gadolinia doped ceria (CGO, Ce0.9Gd0.1O2- ) and either strontium-doped lanthanum cobaltite (LSC, La0.9Sr0.1CoO3- or La0.6Sr0.4CoO3- ) or silver (Ag) were investigated for possible application as oxygen separation membranes in oxy-fuel combustion system. These should combine the high oxygen ion conductivity of CGO with the high electronic conductivity and fast oxygen surface exchange of LSC or silver. Dense mixed-conducting composite materials of LSC/CGO (prepared by powder mixing and sintering) and Ag/CGO composites (prepared by silver plus copper oxide infiltration method) showed high relative density (above 95%), low background gas leakage and also good electrical conduction. The percolation threshold of the electronic conducting component was determined to be approximately 20 vol.% for both LSC compositions and 14 vol.% for Ag. Isotopic exchange and depth profiling by secondary ion mass spectrometry was used to investigated the oxygen tracer diffusion (D*) and surface exchange coefficient (k*) of the composites. Composites just above the electronic percolation threshold exhibited high solid state oxygen diffusivity, fast surface exchange activity moderate thermal expansion and sufficient mechanical strength thus combining the benefits of their constituent materials. The preliminary work on oxygen permeation measurement showed that the reasonable magnitude of oxygen fluxes is possible to be achieved. This indicates that the composites of LSC/CGO and Ag/CGO are promising for further development as passive oxygen separation membranes

Probing polymer chain constraint and synergistic effects in nylon 6-clay nanocomposites and nylon 6-silica flake sub-micro composites with nanomechanics

In this study, we report that a synergistic effect exists in the surface mechanical properties of nylon 6–clay nanocomposites (NC) that can be shown by nanomechanical testing. The hardness, elastic modulus, and nanoindentation creep behavior of nylon 6 and its nanocomposites with different filler loading produced by melt compounding were contrasted to those of model nylon 6 sub-microcomposites (SMC) reinforced by sub-micro-thick silica flakes in which constraint cannot occur due to the difference in filler geometry. Polymer chain constraint was assessed by the analysis of nanoindentation creep data. Time-dependent creep decreased with increasing the filler loading in the NC consistent with the clay platelets exerting a constraint effect on the polymer chains which increases with filler loading. In contrast, there was no evidence of any reduced time-dependent creep for the SMC samples, consistent with a lack of constraint expected due to much lower aspect ratio of the silica flake

The influence of oxidative-thermal degradation of polypropylene on measured interface strength of glass fibre-polypropylene

It was previously found that thermal-oxidative degradation of the polypropylene could significantly affect the measured interfacial strength of glass fibre reinforced polypropylene (GF-PP) micro-composite. In this work, different approaches have been employed to justify this influence. Hot-stage microscopy was used to establish a degradation profile of PP microdroplets that had different initial dimensions and results revealed that the reduction in droplet dimensions was affected by not only its initial droplet size and but also the presence of the fibre in the droplet. The Young's moduli of PP microdroplets with different heat treatments were examined by using nanoindentation technique and the results showed that there was significant stiffness deterioration in degraded samples and the severity is also related to the droplet size for a given heat treatment. Comparison of adhesion for GF-PP was also made between degraded and non-degraded samples. It shows that non-degraded samples give much higher values for interface strength of GF-PP than degraded ones

Transverse cracking in metal/ceramic composites with lamellar microstructure

Open Access funded by European Structural Integrity SocietyPeer reviewedPublisher PD

Restorative Dentistry: Dental composite depth of cure with halogen and blue light emitting diode technology

Objectives To test the hypothesis that a blue light emitting diode (LED) light curing unit (LCU) can produce an equal dental composite depth of cure to a halogen LCU adjusted to give an irradiance of 300 mWcm–2 and to characterise the LCU's light outputs. Materials and methods Depth of cure for three popular composites was determined using a penetrometer. The Student's t test was used to analyse the depth of cure results. A power meter and a spectrometer measured the light output. Results The spectral distribution of the LCUs differed strongly. The irradiance for the LED and halogen LCUs were 290 mWcm–2 and 455 mWcm–2, when calculated from the scientific power meter measurements. The LED LCU cured all three dental composites to a significantly greater (P < 0.05) depth than the halogen LCU. Conclusions An LED LCU with an irradiance 64% of a halogen LCU achieved a significantly greater depth of cure. The LCU's spectral distribution of emitted light should be considered in addition to irradiance as a performance indicator. LED LCUs may have a potential for use in dental practice because their performance does not significantly reduce with time as do conventional halogen LCUs

Quantification of flexural fatigue life and 3D damage in carbon fibre reinforced polymer laminates

Carbon fibre reinforced polymer (CFRP) laminated composites have become attractive in the application of wind turbine blade structures. The cyclic load in the blades necessitates the investigation on the flexural fatigue behaviour of CFRP laminates. In this study, the flexural fatigue life of the [+45/−45/0]2s CFRP laminates was determined and then analysed statistically. X-ray microtomography was conducted to quantitatively characterise the 3D fatigue damage. It was found that the fatigue life data can be well represented by the two-parameter Weibull distribution; the life can be reliably predicted as a function of applied deflections by the combined Weibull and Sigmodal models. The delamination at the interfaces in the 1st ply group is the major failure mode for the flexural fatigue damage in the CFRP laminate. The calculated delamination area is larger at the interfaces adjacent to the 0 ply. The delamination propagation mechanism is primarily matrix/fibre debonding and secondarily matrix cracking

Studies on mechanical behavior of glass epoxy composites with induced defects and correlations with NDT characterization parameters

Degradation in mechanical properties (compression, flexural and ILS) of glass epoxy composite laminates with induced defects (simulating delaminations) was studied. The defects were characterized by using the A-scan ultrasonic pulse-echo technique. The two A-scan parameters, viz. the back wall echo amplitude and time of flight, were followed respectively, as functions of the defect size (diameter) and its location in the laminate thickness. The mechanical properties of laminates were evaluated destructively for different defect sizes and locations. An attempt has been made to express the mechanical properties in terms of the two NDT scan parameters characterizing the defects and empirical equations presente

Lubricated sliding wear behaviour of aluminium alloy composites

Interest in aluminium alloy (Al-alloy) composites as wear resistant materials continues to grow. However, the use of the popular Al-alloy-SiC composite can be limited by the abrasive nature of the SiC, leading to increased counterface wear rates. This study reports new Al-alloy composites that offer high wear resistance, to a level similar to Al-alloy-SiC. Aluminium alloy (2124, 5056) matrix composites reinforced by nominally 15 vol.% of Cr3Si, MoSi2, Ni3Al and SiC particles were prepared by a powder metallurgy route. The aluminium alloy matrix was produced by gas atomisation, and the Cr3Si, MoSi2 and Ni3Al were prepared by self-propagating high temperature synthesis (SHS), while the SiC was from a standard commercial supply. Following blending, the particulates were consolidated by extrusion, producing a homogenous distribution of the reinforcement in the matrix. Wear testing was undertaken using a pin-on-ring configuration against an M2 steel counterface, with a commercial synthetic oil lubricant, at 0.94 m/s and a normal load of 630 N, corresponding to initial Hertzian contact pressures of 750–890 MPa (the exact value depending on the material properties). Specific wear rates at sliding distances exceeding 400 km were in the range 4.5–12.7 × 10?10 mm3/Nm. The monolithic alloys gave the highest specific wear rates, while the MoSi2 and Cr3Si reinforced alloys exhibited the lowest. The worn surface has been analysed in detail using focused ion beam (FIB) microscopy to determine the sub-surface structural evolution and by tomographic reconstruction of tilted scanning electron microscopy (SEM) images, to determine the local worn surface topography. Consequently, the wear mechanisms as a function of alloy composition and reinforcement type are discussed.<br/