1,721,124 research outputs found
Cobalt based layered perovskites as cathode material for intermediate temperature Solid Oxide Fuel Cells: A brief review
Full text linkNowadays, the cathode is the most studied component in Intermediate Temperature-Solid Oxide Fuel Cells (IT-SOFCs). Decreasing SOFCs operating temperature implies slow oxygen reduction kinetics and large polarization losses. Double perovskites with general formula REBaCo2O5+δ are promising mixed ionic-electronic conductors, offering a remarkable enhancement of the oxygen diffusivity and surface exchange respect to disordered perovskites. In this review, more than 250 compositions investigated in the literature were analyzed. The evaluation was performed in terms of electrical conductivity, Area Specific Resistance (ASR), chemical compatibility with electrolytes and Thermal Expansion Coefficient (TEC). The most promising materials have been identified as those bearing the mid-sized rare earths (Pr, Nd, Sm, Gd). Doping strategies have been analyzed: Sr doping on A site promotes higher electrical conductivity, but worsen ASR and TECs; B-site doping (Fe, Ni, Mn) helps lowering TECs, but is detrimental for the electrochemical properties. A promising boost of the electrochemical activity is obtained by simply introducing a slight Ba under-stoichiometry. Still, the high sensitivity of the electrochemical properties against slight changes in the stoichiometry hamper a conclusive comparison of all the investigated compounds. Opportunities for an improvement of double perovskite cathodes performance is tentatively foreseen in combining together the diverse effective doping strategies
Properties of cement-lime mortars vs. cement mortars containing recycled concrete aggregates
No full textCement based materials have a very special presence amongst building and construction materials, and therefore their recycling is of capital importance. In this research, concrete wastes were used as replacement of natural aggregates in cement and cement-lime mortars. The mortars were cured for 28-days with an increasing substitution of fine natural aggregates with recycled aggregates and have been prepared as well as characterized. The results show that, as expected, cement mortars always have better mechanical properties than the corresponding cement-lime mortars. A worsening of mechanical properties is observed upon increasing the amount of recycled aggregates in the cement mortars; on the contrary, cement-lime mortars show an improvement in mechanical properties of up to 60% when increasing the amount of recycled aggregates. The experimental results suggest that this improvement could arise from a synergic effect of lime hydraulicity and the filler effect due to the fine fraction of recycled aggregates within the mix, that lead to better densification of the lime mortars by blocking the capillary pores
Synthesis of nanostructured perovskite powders via simple carbonate co-precipitation
No full textA very simple, cost-effective, chloride- and alkali-free, carbonate co-precipitation synthesis in aqueous medium was applied in the preparation of perovskite-type lanthanum manganese oxide-based powders, i.e. La0.70Sr0.30MnO3-δ(LSM) and La0.75Sr0.25Cr0.5Mn0.5O3-δ(LSCrM). The precursors so obtained yielded nano-structured perovskite oxides when treated at 900 C and 800 C, respectively. The measured BET surface areas were in the low-end range for high temperature oxides (4 m2g-1and 10 m2g-1) but the X-ray crystallite size was as low as 50 nm for LSCrM and 90 nm for LSM. © 2013 Institute of Chemistry, Slovak Academy of Sciences
Life cycle assessment of natural building materials: the role of carbonation, mixture components and transport in the environmental impacts of hempcrete blocks
Full text linkHempcrete is a natural building material that, in recent years, has known an increased popularity in a number of European countries. Hempcrete-based construction materials are used in non-bearing walls, as finishing plasters and floor/roof insulators. In the present work, the environmental performances of a non-load-bearing wall made of hempcrete blocks were assessed via Life Cycle Assessment (LCA). The analysis encompassed the whole life cycle but the end of life, due to the lack of reliable data for this stage. The production phase of the raw materials was identified as the main source of environmental impacts, but the transport distance of raw materials, as well as the amount and composition of the binder mixture, can considerably affect the results. An experimental assessment (via X-ray Powder Diffraction analysis) of the carbonation process taking place within the binder during the use phase of the wall showed that the carbonation rate may be smaller than assumed in previous works: after 240 d, only the outermost layers of the blocks showed significant levels of carbonation, while the innermost layers experienced only a negligible increase in the amount of carbonates. Nevertheless, the overall emission balance is very favourable: thanks to biogenic CO2uptake during hemp growth and to CO2uptake by carbonation, hempcrete blocks have a negative carbon footprint and act therefore as effective carbon sinks
Weathering's beneficial effect on waste-stabilised rammed earth: a chemical and microstructural investigation
No full textThe most common method to improve the mechanical properties of a rammed earth mix is to add chemical stabilisers like cement and/or lime. Varying the stabiliser type will affect strength gain but also the environmental impact. In this paper, the effect of wetting-drying cycles on the long-term unconfined compressive strength of stabilised rammed earth (SRE) mixes was investigated through the chemical characterisation of the soil components and microstructural analyses. The mixes were stabilised using different agents characterised by distinct environmental impacts, such as cement, calcium carbide residue and fly ash. These last two are considered waste materials, significantly affecting their use's associated environmental implications. The results of this experimental campaign support others in this series of work and showed an improvement of the mechanical properties after cyclic wetting-drying due to the formation of new hydration products which bound particles together. The use of waste materials proved to be an effective solution to stabilise RE
Rammed Earth incorporating Recycled Concrete Aggregate: a sustainable, resistant and breathable construction solution
Full text linkConstruction and demolition debris, mainly concrete and masonry rubble, represent a significant share of municipal waste. Recycling crushed concrete aggregates and using them as substitutes for natural ones might therefore be determinant in reducing landfilling and mineral resource depletion. An innovative way to give new value to Recycled Concrete Aggregates (RCAs) is to ram them in layers to form load-bearing walls for stabilised Rammed Earth (RE) applications. However, the success of those few existing RE projects using RCA is mainly due to the knowledge and experience of the contractors rather than official standards or guidelines or scientific literature. The objective of this study was to further the knowledge of this building technique by determining the effect of different RCA replacements on the material's mechanical resistance, sustainability and hygroscopic properties: indicative of the structure's structural, environmental and hygrothermal performance. Mechanical resistance was assessed by means of the Unconfined Compressive Strength (UCS, commonly used for rammed earth-like materials), hygroscopic properties via Moisture Buffer Value (MBV) and sorption isotherms while the sustainability was assessed via consequential Life Cycle Assessment (LCA). Microstructural investigations via mercury intrusion porosimetry, nitrogen adsorption-desorption isotherms, scanning electron microscopy and X-ray diffraction were performed to understand and explain material mechanical and hygroscopic behaviour. The building technique, already proven to be durable, was demonstrated to be resistant (from 4 to 12 MPa at 28 days depending on the RCA replacement and cement content), sustainable (down to 25 kg CO2-eq. of embodied carbon per square meter of load-bearing wall) and to have good moisture buffering abilities (0.88 g/(m2%RH) for mixtures containing only RCA). Strength appeared to be more related to the particle size distribution of the mix rather than to the percentage of RCA added. The amount and type of stabiliser added to the mix and the distance covered by the RCA during its lifetime strongly affected the environmental sustainability of the mixture; to maximise the potential of this building technique, reducing the amount of cement in the mixture by using alternative stabilisers should be the main priority
Use of electrochemical impedance spectroscopy for the evaluation of performance of PEM fuel cells based on carbon cloth gas diffusion electrodes
Full text linkPolymer electrolyte membrane fuel cells (PEMFCs) have attracted great attention in the last two decades as valuable alternative energy generators because of their high efficiencies and low or null pollutant emissions. In the present work, two gas diffusion electrodes (GDEs) for PEMFCs were prepared by using an ink containing carbon-supported platinum in the catalytic phase which was sprayed onto a carbon cloth substrate. Two aerograph nozzles, with different sizes, were used. The prepared GDEs were assembled into a fuel cell lab prototype with commercial electrolyte and bipolar plates and tested alternately as anode and cathode. Polarization measurements and electrochemical impedance spectroscopy (EIS) were performed on the running hydrogen-fed PEMFC from open circuit voltage to high current density. Experimental impedance spectra were fitted with an equivalent circuit model by using ZView software which allowed to get crucial parameters for the evaluation of fuel cell performance, such as ohmic resistance, charge transfer, and mass transfer resistance, whose trends have been studied as a function of the applied current density
Chemistry and microstructure of cement pastes admixed with organic liquids
No full textThe influence of organic admixtures on hydration of cement pastes have been investigated to assess the possibility of immobilizing toxic organic wastes in cement-based materials. The model organic waste used is a mixture of water, methanol and 2-chloroaniline: 100/76/4 volume ratios, respectively. Four pastes were obtained admixing this solution with two different cements (ordinary and white Portland), with weight ratios equal to 0.40 and 0.65, respectively; in addition, cement pastes without organic wastes were prepared (water to cement ratio 0.40). The progress of hydration process of these six series of samples was monitored for the first 8 months of ageing by thermogravimetric analysis; moreover, a detailed microstructural investigation on mature pastes was performed by quantitative X-ray diffraction and scanning electron microscopy. Dynamic leach tests were carried out on samples cured for 28 days, according to the international standards
Reduction of rammed earth's hygroscopic performance under stabilisation: an experimental investigation
Full text linkOne of the acknowledged qualities of rammed earth (RE) is its moisture buffering capacity. Recently, stabilisation of RE has become a common practice to improve the mechanical resistance but very little is known about the effect that stabilisation has on hygroscopic properties. The present study aims to fill this gap by understanding the role that stabilisation plays in the buffering and sorption capacity of RE. The use of alternative stabilisers such as fly ash and calcium carbide residue and a comparison with traditional unstabilised RE (URE) have also been investigated. Moreover, the effect of weathering, simulated by cyclic wetting-drying, on hygroscopic performance has been analysed. Moisture Buffer Value (MBV) testing, moisture and nitrogen adsorption-desorption isotherms and mercury intrusion porosimetry were performed on stabilised samples to examine microstructural phenomena responsible for behavioural changes. URE was confirmed to be a good-to-excellent passive air conditioner according to the MBV scale but its performance seemed to be highly influenced by the soil particle size distribution and mineralogy. Based on the experimental outcomes of the mixtures investigated, stabilisation had a detrimental effect on the moisture buffer capacity of rammed earth, likely due to the inhibition of the physico-chemical interactivity between moisture and clays. Weathering had a variable effect on the buffering capacity, depending on the availability of unreacted particles in the matrix
LA-ICP-MS and EDS characterization of electrode/electrolyte interfaces in IT-SOFC materials
No full textLaser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in combination with scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) is used for the determination of elemental spatial distribution in ceramic multi-layer systems such as those found in intermediate-temperature solid oxide fuel cells (IT-SOFCs). Because layer sintering occurs at high temperature (usually well over 1000 °C), there may be mutual diffusion of ions from one layer to another, with dramatic consequences on cell performances. In this work, two model materials have been used to test LA-ICP-MS: La0.83Sr0.17Ga0.83Mg0.17O2.83 (LSGM), one of the most promising electrolytes for IT-SOFCs, and La0.8Sr0.2MnO3 (LSM), a highly representative perovskite material, which are amply used to design electrode materials. A two-layer system screen printed onto an LSM pellet (LSM–LSGM–LSM pellet) was successively sintered at a typical processing temperature, i.e. 1300 °C, for a short time (1 h). Elemental spatial distribution was determined by line profile analyses carried out on fracture surfaces; for comparison SEM-EDS line profiles were tested on the same surface.
LA-ICP-MS line profile analysis evidenced that, notwithstanding the relatively low sintering temperature and short firing time (1 h per sintering), manganese cation diffusion into LSGM is relatively abundant, in agreement with previous literature reports and present EDS results. While line scan EDS analyses are not as conclusive for Ga and Mg diffusion, LA-ICP-MS shows that both ions diffuse across both interfaces, and Ga diffuses even over very long distances into the LSM pellet; on the contrary, only trace amounts of Mg can be found far from the LSGM/LSM interface
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