Carbon formation at high temperatures (550–1400 ◦c): Kinetics, alternative mechanisms and growth modes

UIDB/50006/2020This Note aims at clarifying the alternative mechanisms of carbon formation from gases at temperatures above 550 ◦C. Both the growth of carbon nanotubes (CNTs) by a hybrid route, and of graphene layers deposition by a pyrolytic route are analyzed: The transition had no influence in apparent kinetics, but the carbon structure was totally different. The transition temperature from hybrid to pyrolytic growth varies with the gas pressure: Higher temperature transition was possible using lower active gas pressures. The rate-determining step concept is essential to understanding the behavior. In catalytic and hybrid carbon formation, the slower step controls and determines kinetics. In the pyrolytic region, the faster step dominates, and carbon bulk diffusion is blocked: Layers of graphene cover the external catalyst surface. It is easier to optimize CNTs growth (rate, shape, properties) knowing the details of the alternative mechanisms operating.publishersversionpublishe

Kinetics of carbon nanotubes and graphene growth on iron and steel: Evidencing the mechanisms of carbon formation

This work was supported by national funds through FCT-Fundacao para a Ciencia e a Tecnologia, I.P., under the Scientific Employment Stimulus-Institutional Call Partially supported by the Associate Laboratory for Green Chemistry-LAQV, financed by national funds from FCT/MCTES (UIDB/50006/2020).Carbon formation on steel has recently become an active research area with several important applications, using either carbon nanotubes (CNTs) or graphene structures. The production of vertically aligned CNT (VACNT) forests with combined metals has been explored with important results. Detailed kinetics is the best approach to understand a mechanism. The growth behavior seems complex but can be simplified through the knowledge of the three more common alternative reaction mechanisms/routes. The time required to optimize the production and properties might be reduced. The mechanistic proposal reported in 1971 was better explained recently. The volcano shape Arrhenius plot reported is observed only when Fe, Co, and Ni are used as reaction catalysts. Other metals are catalytically active at higher temperatures, following a different route, which does not require surface catalysis decomposition of the reactive gas. C2H2 and low olefins react well, but CH4 is not reactive via this surface catalysis route. Optimizing production of CNTs, research work is usually based on previous experience, but solid-state science-based studies are available.publishersversionpublishe

Safety evaluation of timber structures through probabilistic analysis

This work presents a procedure for the use of probabilistic modelling techniques in the assessment of safety parameters of existing timber structures, when influenced by two types of actions. The parameters of the models have been defined as probabilistic variables, and Monte Carlo simulation technique was taken into account for safety evaluation. The case study here examined concerns four king post trusses on which a safety evaluation method was conducted to determine a target reliability index. Since the structures presented a high level of deterioration, a model considering the variation of the residual cross-section and the influence of environment in resistance was taken into account. Time dependent deterioration models have also been considered. With the results given by the different used models, it was possible to obtain probabilities of failure and respective reliability indices, as well as time evolution deterioration curves for a deteriorated historical structure.The financial support of the Portuguese Science Foundation (Fundacao de Ciencia e Tecnologia, FCT), through project PTDC/66527-2006, is gratefully acknowledged. The third author acknowledges the support of FCT through research centre UNIC

Psychosocial and economic impacts of a charge in lightweight plastic carrier bags in Portugal: Keep calm and carry on?

Reducing plastic waste has become an urgent global challenge. To help fight this problem, European countries have undertaken the mission to reduce lightweight plastic bag consumption. In 2014, Portugal implemented a charge on lightweight plastic bags, calling for an adjustment from both consumers and firms. The present re- search aims to study the psychosocial and economic impacts of this tax measure, namely in terms of acceptability of the measure, potential attitude and behaviour changes, and impact on plastic bag markets. The psychosocial impacts were studied based on an online survey (N = 198) and on available datasets illustrating societal trends. Survey results showed that participants agreed with the charge and with widening it to all types of plastic bags. They attributed environmental motives to its implementation, developed reuse habits and mentioned a de- creased consumption of disposable plastics. In addition, individuals appear more concerned with the impacts of everyday plastic products in the environment, and more willing to pay higher taxes and prices to protect the environment. The economic impacts were studied using available data on the charge revenue and on the production and sales of plastic bags, among others. We illustrate that the implementation of the charge led to a sharp fall in the use of lightweight bags, generating a decrease in the production and sale of plastic bags overall, while avoiding significant impacts on the relevant economic sectors.info:eu-repo/semantics/acceptedVersio

Inference on stiffness and strength of existing chestnut timber elements using Hierarchical Bayesian Probability Networks

The assessment of the mechanical properties of existing timber elements could benefit from the use of probabilistic information gathered at different scales. In this work, Bayesian Probabilistic Networks are used to hierarchically model the results of a multiscale experimental campaign, using different sources of information (visual and mechanical grading) and different sample size scales to infer on the strength and modulus of elasticity in bending of structural timber elements. Bayesian networks are proposed for different properties and calibrated using a large set of experimental tests carried out on old chestnut (Castanea sativa Mill.) timber elements, recovered from an early 20th century building. The obtained results show the significant impact of visual grading and stiffness evaluation at different scales on the prediction of timber members’ properties. These results are used in the reliability analysis of a simple timber structure, clearly showing the advantages of a systematic approach that involves the combination of different sources of information on the safety assessment of existing timber structures

Reliability-based design of interventions in deteriorated timber structures

When accommodating new uses or mitigating the consequences of deterioration, the strength increase of existing structures is significantly more onerous than a similar increase at the design stage of new structures. The safety methods prescribed in current standards were defined for the design of new structures and are frequently conservative for the assessment and repair of existing structures. This work will introduce the fundamental aspects of structural reliability and their application in the context of existing timber structures regarding the use of target reliability indices. First, the fundamental methods of structural reliability are introduced. The use of reliability methods requires the use of more detailed information in respect with material properties, loads, and model uncertainty. The main sources of such information are described. After overviewing the fundamental methods of structural reliability, methods to introduce additional information, namely results of non-destructive tests in the structural assessment are discussed. Finally, the intervention on a timber structure will be analyzed, within a case study, by considering different repair scenarios that lead to discussions on different suitable safety thresholds for existing and repaired structures

Modulation of fatty acid profile in nannochloropsis oculata adaptation to temperature stress

info:eu-repo/semantics/publishedVersio