Desenvolvimento de termoelétricos por laser para conversão de energia a altas temperaturas

Laser-processed donor-substituted calcium manganite Ca1-xPrxMnO3 (x=0.03, 0.06, 0.1) was accessed as a promising n-type thermoelectric material for high-temperature applications. Rod-like fibres were grown by laser floating zone technique (LFZ) at pulling rates of 200, 100 and 25 mm h-1 in various gas atmospheres, including air, vacuum and argon. Laser processing conditions were firstly optimized by preliminary experiments and corresponding structural and microstructural characterization. Strongly non-equilibrium conditions imposed by LFZ in all cases resulted in non-single phase (Ca,Pr)MnO3- ceramic fibres, in several cases with possessing dendritic structures specific for LFZ. Slow pulling rate of 25 mm h-1 favoured perovskite single-phase composition but resulted in very fragile fibres. Thermal post-treatment was attempted to improve the composition regarding content of the thermoelectric perovskite-type phase and mechanical strength of the samples. This approach was successful, leading to homogeneous phase composition, fibre morphology and significantly higher thermoelectric performance. Electrical conductivity was enhanced with the praseodymium content, in contrast to lowering the Seebeck coefficient. A maximum power factor value of 303 μW m-1 K-2 at 1120 K was observed for the fibre with x=0.06 grown at 100 mm h-1 and thermally treated in air, being comparable or higher compared to those so far reported in the literature, processed by conventional routes. Isothermal tests of electrical conductivity and Seebeck coefficient under elevated temperatures were performed and revealed an increase in thermoelectric performance for as-grown fibres. The results have demonstrated that LFZ is a suitable technique for processing of thermoelectric perovskite-type manganites, if optimized control over growth parameters and re-equilibration conditions is imposedNeste trabalho foi processada por fusão de zona por laser (LFZ – Laser Floating Zone) a manganite de cálcio, em que o cálcio foi substituído por um dador: Ca1-xPrxMnO3 (x=0.03, 0.06, 0.1). Este sistema é um promissor semicondutor do tipo n para aplicação termoelétrica a altas temperaturas. Foram crescidas fibras com velocidades de 200, 100 e 25 mm h-1 em atmosferas de ar, vácuo e Árgon. As condições de processamento foram primeiro otimizadas com testes preliminares e sua caracterização estrutural e microestrutural. As condições de equilíbrio impostas pelo processamento da LFZ induziram amostras polifásicas nas fibras cerâmicas de (Ca,Pr)MnO3-, em alguns casos acompanhadas de um pronunciado crescimento de dendrites, muito típicos desta técnica. Nas baixas velocidades de crescimento (25 mm h-1) verificou-se que favoreceram o aparecimento da fase do tipo perovskite, contudo as fibras eram muito frágeis. Foi estudado o efeito do tratamento térmico às amostras crescidas a 100 e 25 mm h-1 para a alteração para as fases desejadas neste material. Foi observado que estes tratamentos térmicos originavam maior homogeneidade na composição das fases, e morfologia das fibras melhorando o comportamento termoelétrico. A condutividade elétrica foi melhorada com o aumento da incorporação de praseodímio, por consequência observou-se o decréscimo no coeficiente de Seebeck. No fator de potência foi obtido um valor máximo de 303 μW m-1 k-2 a 1120 K para amostra com x = 0.06 crescida a 100 mm h-1, e tratada termicamente em ar, sendo os valores comparáveis ou mesmo superior aos até agora reportados na literatura, processados pela via convencional. Testes isotérmicos da condutividade elétrica e coeficiente de Seebeck em temperaturas elevadas mostraram aumento da capacidade termoelétrica com o tempo em fibras crescidas sem tratamento térmico. Os resultados demonstraram que a técnica de LFZ pode ser usada como processamento de materiais termoelétricos, se otimizados os parâmetros de crescimento e condições de reequilíbrio das fasesMestrado em Engenharia de Materiai

Projecto de fundações e estruturas do edifício administrativo do parque de materiais e oficinas de Ceira

O presente trabalho refere-se às fases de projecto da estrutura do Edifício Administrativo do Parque de Material e Oficinas, destinado ao apoio à frota circulante para a rede de transportes do Metro do Mondego. As instalações serão construídas junto à Estação de Ceira. Na escolha das soluções a adoptar foram considerados todos os condicionalismos existentes, em particular os inerentes à implantação geral da obra e sua utilização. Foram igualmente tidas em conta as acções passíveis de actuar na estrutura, bem como as propriedades dos materiais utilizados e os processos construtivos que venham a ser utilizados

Algebraic thinking of grade 8 students in solving word problems with a spreadsheet

This paper describes and discusses the activity of grade 8 students on two word problems, using a spreadsheet. We look at particular uses of the spreadsheet, namely at the students’ representations, as ways of eliciting forms of algebraic thinking involved in solving the problems. We aim to see how the spreadsheet allows the solution of formally impracticable problems at students’ level of algebra knowledge, by making them treatable through the computational logic that is intrinsic to the operating modes of the spreadsheet. The protocols of the problem solving sessions provided ways to describe and interpret the relationships that students established between the variables in the problems and their representations in the spreadsheet

The academic performance of student-workers in higher education – increasing rapidly regions’ intellectual capital

Although in recent years access to higher education in Portugal has expanded, there are still some people who interrupt their academic progression and enter the labour market not because of lack of skills but because of monetary constraints. Thus, returning to school is considered of great importance to mitigate this discrimination and, at the same time, to reinforce regions’ intellectual capital and workers’ qualification, promoting labour productivity and firms’ competitiveness in the short run – being therefore important for both dimensions of equity and efficiency. Even though the determinants of academic performance, as a proxy for the effective acquisition of skills and growth of intellectual capital, have been deeply studied for the traditional students, the different characteristics, the lack of time and the multiple motivations of adult student-workers justify them to receive an independent treatment. In particular, it is expected for adult student-workers that job characteristics play an important role determining their academic success. Therefore, based on an extended set of observed attributes, which allow us to control for individual and degree characteristics, we develop two regression models to find out how job characteristics affect the academic performance (measured by the final grade point average – GPA, and by the completion time) of adult student-workers in higher education. We use a longitudinal dataset constituted by 332 student-workers that have enrolled in an undergraduate program at Leiria Polytechnic Institute (IPLeiria) in 2008 or 2009 and have completed it until 2015. The data was obtained by matching an internal dataset of IPLeiria with data from the Ministry of Education and Science, Portugal. The results show that student-workers who finish their degrees behave similarly to the non-worker students in their academic performance, but with different determinants explaining it, especially in the case of final GPA, where job characteristics play a more important role than individual and degree characteristics. In detail, we found that self-employment and enrolling in a degree from a scientific field unrelated with the professional activity have a positive effect in the final GPA of male student-workers, and that exercising a qualified job or being employed in the private sector may limit academic performance as these workers tend to choose more demanding degrees. In addition, flexible professional schedules, peer effects, higher average grades within-degree and improved academic integration all seem to contribute positively to the academic performance of student-workers. We expect that our work contributes to develop policies that improve the academic success of student-workers and increase the participation of adult workers in higher education, thus enhancing regions’ intellectual capital, labour productivity and firms’ competitiveness in the short run.info:eu-repo/semantics/publishedVersio

Mature vs Young Working Students: Similarities, Differences, and Drivers of Graduation and Dropout

The high dropout rate among working students in higher education in Portugal, including both those who are older and at a mature stage in their professional careers and those who are young and recent workers, is one of the main concerns of educational policy. Identifying the drivers of the academic pathway for these students is essential to promote successful education and increase labour productivity and business competitiveness. Using an extensive longitudinal database of personal, course, and employment variables of 1,561 working students from a Portuguese higher education institution, we perform a duration analysis to determine and compare the factors that drive the dropout and graduation risks of mature and young working students in higher education. The results show that, in general, young working students are more positively influenced by financial aid, their motivation for the course, and the desire to find a new qualified professional career, while mature working students depend more on social and academic integration. The accumulation of knowledge/skills in the 'out of class' experience seems to contribute positively to academic performance.info:eu-repo/semantics/publishedVersio

Determinants of academic performance of student-workers in higher education

Although in recent years access to higher education in Portugal has expanded, there are still some people who interrupt their academic progression and enter the labour market not because of lack of skills but because of monetary constraints. Thus, returning to school is considered of great importance to mitigate this discrimination and reinforce workers’ qualification. Even though the determinants of academic performance have been deeply studied, the different characteristics, the lack of time and the multiple motivations of student-workers justify them to receive an independent treatment. Therefore, based on an extended set of observed attributes, which account for individual, degree and, for the first time to our knowledge, job characteristics, we develop two regression models to find the main determinants of the academic performance (measured by the final grade point average and by the completion time) of student-workers in higher education. We use a longitudinal dataset constituted by 332 student-workers that have enrolled in an undergraduate program at Leiria Polytechnic Institute (IPLeiria) in 2008 or 2009 and have completed it until 2015. The data was obtained by matching an internal dataset of IPLeiria with data from the Ministry of Education and Science, Portugal. The results show that student-workers who finish their degrees behave similarly to the non-worker students in their academic performance, but with different determinants explaining it. Parents’ education has a negative effect on final grade whereas higher previous qualifications, peer effects, better integration and higher average grades withindegree increase academic performance. The access regime and the field of study are also relevant, as well as self-employment, job-degree relation, private/public nature of the employer, and whether or not the job is qualified. We expect that our work contributes to develop policies that improve the success of student-workers and therefore increase the participation of workers in higher education.info:eu-repo/semantics/publishedVersio

Pathways of adult student-workers in higher education: explaining the risks of early dropout, late dropout and graduation

In Portugal, student-workers represent 8.5% of higher education students. They are mainly adults with a less favourable socioeconomic and professional background that return to school while working and after experiencing some years in the labour market so as to find a new profession or a job promotion. However, due to their particular characteristics and time restrictions, adult student-workers observe high dropout rates, justifying the need of identifying their determinants of dropout and graduation risks in a separate way. With this purpose, we match five sources to obtain a unique five-year longitudinal dataset with extensive information on individual, degree and employment variables of 976 student-workers from Leiria Polytechnic Institute, Portugal. We then use event history analysis, with competing risks, to investigate if and when the event of dropping out or graduation occurs and how a set of covariates affects the risk of each event. In addition, we distinguish between those that drop out with few accumulated credits (labelled as early dropouts) and those that drop out despite having a significant number of credits completed (late dropouts). We found that early dropouts depend more on academic failure, school-residence distance, personal motivation and employment variables, while late dropouts are more influenced by other factors such as marital status and degree characteristics. Among policy recommendations, beyond the frequently referred actions to reduce academic failure, we highlight the adoption of measures to avoid stopout behaviour, the adequate definition of the schedule and composition of classes (daytime classes and higher proportions of student-workers in the classroom, and with distinct academic performance, seem to reduce dropout risk), and the curriculum appreciation at the admission moment (some employment variables seem to contribute to increase the probability of graduation while others seem to contribute to enhance the risk of dropout), including criteria for regional preference and personal motivation.info:eu-repo/semantics/publishedVersio

Policy recommendations for improving academic achievement among IPLeiria student-workers

In Portugal, student-workers represent 8.5% of higher-education students and observe higher dropout rates, partially because of a higher education system that is more fitted to the traditional students. Using an extensive longitudinal database with original information on individual, degree and employment characteristics of 1566 undergraduate studentworkers from IPLeiria, we determine the factors that drive the dropout and graduation risks of adult workers in higher education. We concluded that academic failure explains only a fraction of dropout behaviour among adult student-workers. Indeed, academic achievement is also affected by school-residence distance, financial difficulties, marital status, motivation, field of study, academic integration and professional background. We found that men are more likely to drop out and more influenced by employment variables (self-employment, job qualification and job-degree relation) while, for women, marriage (associated with household responsibilities) seems to increase the risk of dropout. Some policy recommendations are suggested for the higher education system to adapt better to the particular characteristics of adult workers, namely by: adjusting the schedule and composition of classes; appreciating the curriculum and orienting candidates, stopouts and poor performance students; providing scholarships; distributing the vacancies across admission regimes; and introducing shorter/simplified versions of the degrees.info:eu-repo/semantics/publishedVersio

The development of wind energy in Galicia: public policies, effects on the economy and international comparison

Nowadays energy constitutes a very important concern of policy and public debate. It is justified by the effects of climate change and the national strategies to be self-sufficient in the supply of energy. Renewable energies are the alternative and public policies could play a key role in their development. Furthermore, those energies have potential to contribute to the endogenous development. In this paper we study the development of wind energy in Galicia (the most important renewable energy in terms of contribution to the energy balance) and the role of public policy in promotion of this type of energy. Its development is a recent phenomena but with a growing relevance, because of their impact in the regional economy. For this reason, we analyse the effects of this sector in the economy (employment, manufacturing production, exports, imports...) Moreover, we also compare the Galician model with the Denmark one, because it has a truly known successful integrated framework, in which, all the economical agents collaborate in order to build a competitive and innovated sector with several spill-over effects.