Gellan gels, in the presence of potassium and calcium ions and their mixtures, were tested using large deformation compression experiments. Characterisation was based on stress and strain at failure along with Young’s modulus. Samples prior to compression were refrigerated at 5 °C for 1, 2 and 24 h. In the case of potassium, the strongest gel was obtained for a cation concentration of 80 mm . Increasing concentrations of calcium resulted in decreased gel strength. Brittleness values for potassium and calcium gels fluctuated around ~0.5 and ~0.3 respectively. The strength for both, potassium and calcium, gels did not increase significantly over time. For each cation, Young’s modulus and stress at failure showed the same variation. In the case of mixtures, strength was determined by calcium as its progressive replacement by potassium was actually similar to reducing its concentration. Potassium determined the remaining properties. 相似文献
In this study, we examined relations between outside school computer experiences, perceived social support for using computers, and self-efficacy and value beliefs about computer learning for 340 Greek elementary school boys and girls. Participants responded to a questionnaire about their access to computer use outside school (e.g. frequency of use and nature of activities), perceived parental and peer support, and computer self-efficacy and value beliefs. Although almost all students used computers outside school, there were significant gender differences in frequency and type of computer use. Also, boys reported more perceived support from their parents and peers to use computers and more positive computer self-efficacy and value beliefs than girls. Parental support and, to a lesser extent, peer support were the factors more strongly associated with boys’ and girls’ computer self-efficacy and value beliefs, while home computer access was not related to students’ motivation. Our findings highlight the role of socialization in the gender gap in computing and the need for research and educational interventions that focus on the social practices that communicate gendered expectations to young boys and girls. 相似文献
Tissue P systems (tPS) represent a class of P systems in which cells are arranged in a graph rather than a hierarchical structure.
On the other hand, communicating X-machines (XMs) are state-based machines, extended with a memory structure and transition
functions instead of simple inputs, which communicate via message passing. One could use communicating XMs to create models
built out of components in a rather intuitive way. There are investigations showing how various classes of P systems can be
modelled as communicating XMs. In this paper, we define a set of principles to transform communicating XMs into tPS. We describe
the rules that govern such transformations, present an example to demonstrate the feasibility of this approach and discuss
ways to extend it to more general models, such as population P systems, which involve dynamic structures. 相似文献
In recent years, significant changes in precipitation regimes have been observed and these manifest in socio economic and
ecological problems especially in regions with increased vulnerability such as the Mediterranean region. For this reason,
it is necessary to estimate the future projected precipitation on short and long-term basis by analyzing long time series
of observed station data. In this study, an effort was made in order to forecast the monthly maximum, minimum, mean and cumulative
precipitation totals within a period of the next four consecutive months, using Artificial Neural Networks (ANNs). The precipitation
datasets concern monthly totals recorded at four meteorological stations (Alexandroupolis, Thessaloniki, Athens, and Patras),
in Greece. For the evaluation of the results and the ability of the developed prognostic models, appropriate statistical indexes
such as the coefficient of determination (R2), the index of agreement (IA) and the root mean square error (RMSE) were used. The findings from this analysis showed that
the ANN’s methodology provides satisfactory precipitation totals in four consecutive months and these results are better results,
than those obtained using classical statistical methods. A fairly good consistency between the observed and the predicted
precipitation totals at a statistical significance level of p < 0.01 for the most of the examined cases has been revealed. More specifically, the Index of Agreement (IA) ranges between
0.523 and 0.867 and the coefficient of determination (R2) ranges between 0.141 and 0.603. The most accurate forecasts concern the mean monthly and the cumulative precipitation for
the next four consecutive months. 相似文献
During the last years photovoltaic (PV) generators comprise a promising option for satisfying the electrification needs of both grid connected and isolated systems worldwide. At the same time, the mobile telecommunication (T/C) sector presents a vast growth that leads to the expansion of the respective networks even at the most remote of areas. In this context, there are numerous T/C stations that cannot appreciate connection to an electricity grid and thus cover their energy needs usually on the basis of oil-consuming diesel generators. On the other hand, replacement of the oil-based generation with PV-battery stand-alone configurations implies system over-sizing so as to obtain zero load rejections throughout the year. In an attempt to both reduce the amounts of oil consumed and downsize the PV-battery system, a hybrid PV-based stand-alone system, employing also a diesel engine and a battery bank, is currently proposed. System sizing is first undertaken for various scenarios of annual fuel consumption and panel tilt angle under the restriction of zero load rejection, while following, the main directions for obtaining optimum solutions are given. Finally, an extensive energy balance analysis of typical configurations also provided reflects the fact that such hybrid systems may support better utilization of the PV energy production and also reduce considerably the annual amounts of oil required for the diesel-only solution. 相似文献
Aiming to evaluate the efficacy of constructed floating wetlands (CFW) in removing agrochemicals (nutrients and pesticides), a series of experiments were run continuously for a 16-week period in pilot-scale CFW systems to study the effect of two aquatic plant species (duckweed and water hyacinth) and climatic parameters. The CFW systems were loaded daily with agricultural polluted water containing a fertilizer and five pesticides, whose concentrations and removal efficiencies were measured in the experiments. Average nutrient and pesticide reductions varied from 27.4% to 83.6% and from 12.4% to 42.7%, respectively. The two plants performed almost equally well. High temperatures and increased solar radiation significantly contributed to increased removal performance. The results suggest the use of CFW systems as effective and low-cost agricultural pollution control technologies.