Departamento de Física

URI permanente para esta comunidadhttp://10.0.96.45:4000/handle/11056/14434

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A combined experimental and theoretical study of gas sorption on nanoporous silver triazolato metal-organic frameworks
(2014-01-01) Yang, Guang; Santana, Juan A; Rivera Ramos, Milton E.; García - Ricard, Omar; Saavedra-Arias, José; Ishikawa, Yasuyuki; Hernandez-Maldonado, Arturo; Raptis, Raphael G.
A new silver-triazolate metal-organic framework (MOF), {Ag 3[Ag5(μ3-3,5-tBu 2tz)6](BF4)2}n (2), has been prepared and structurally characterized. The CO2 sorption properties of this new MOF and previously reported isostructural analog {Ag 3[Ag5(μ3-3,5-Ph2tz) 6](NO3)2}n (1), were determined experimentally and probed theoretically by a density functional (DF) method. The structures of 1 and 2 are based on flexible Ag5tz 6-structural building units (SBUs), which allow the expansion/contraction of the structure, depending on either the steric requirements of the pendant groups - tBu, or Ph - of the SBUs, or the pressure of the gaseous sorbate. Both silver-triazolate MOFs sorb CO 2 preferentially to N2, O2, CH4 and H2. DF calculations show CO2 to be bound by van der Waals forces to the organic moieties of the porous framework. This work shows that van der Waals interactions of CO2 and the aromatic and aliphatic hydrocarbons of silver-triazolate MOFs are sufficient to achieve preferential sorption of CO2.
An evaluation of variables affecting the stability and performance of down-flow anaerobic fixed bed reactors treating piggery wastewater
(Taylor & Francis, 2006) Sánchez, E; Borja, R; Travieso, L; Martín, A; Colmenarejo, M; Nikolaeva, S.
The influence of the most important variables on the stability and performance of down-flow anaerobic fixed bed reactors treating piggery wastewater after primary sedimentation was evaluated at HRT in the range of 1-6 d and influent substrate concentration in the range of 2 to 12 g TCOD 1-(1). The effect of HRT was more pronounced compared to that of influent strength. An increase in the HRT increased the process stability and process performance at different influent strengths. TCOD, SCOD, BOD5, TSS, organic nitrogen (N) and Orthophosphate (P) removals increased with the HRT, independently of the initial substrate concentration (S-0). The increase in S-0 brought about an increase in the attached biomass concentration (X) at the end of the experiment. Two empirical models based on the individual effect of HRT, X and S-0 were evaluated and found to be adequate to describe the influence of these variables on the process performance. The first model took all the above-mentioned variables into consideration while the second model was simplified and based on the use of HRT as the only independent variable. The results obtained by using both models were found to be similar. This demonstrated that independently of the characteristics of the operation, the behaviour and performance of the reactors were comparable. The methane yield coefficient was found to be 0.3371 methane g(-1) TCOD removed.
Characteristic atmospheric states during mid‑summer droughts over Central America and Mexico
(Springer, 2020-05-10) Holbrook, Neil; Zhao, Zijie; Oliver, Eric; Ballestero, Daniel; Vargas-Hernández , José Mauro
Annual precipitation over Central America and large areas of Mexico is typically characterised by its bimodal distribution, with a precipitation minimum in July to August that occurs between two separate maxima from May to July and August to October. Several theories have been proposed to explain this phenomenon, which is often termed the mid-summer drought (MSD), but most fail to address the diferent characteristics associated with individual MSD events. Here, a regression-based approach is used to detect and quantify the annual and climatological MSD signature over Central America and Mexico. This approach has been evaluated and shown to be robust for various datasets with diferent spatial resolutions. It was found that in the southeast of the Mexico/Central America region, MSDs start earlier and end later than elsewhere, and are thus longer in duration. However, the coast of the Gulf of Mexico, Cuba, and large areas of Central America, exhibit climatologically stronger MSDs. Changes in precipitation, brought about by the interaction between reversals of the onshore/ofshore winds and orographic forcing associated with the steep mountainous terrain, have also been shown to be signifcant factors in the timing of MSD occurrences, ofering support for a combined theory of large-scale dynamics and regional forcing. Using self-organising maps (SOMs) as an analysis tool, it was found that MSD events over the domain display strong spatial variability. The MSDs over the domain also generate distinct signatures and may be forced by particular mechanisms. We found that El Niño-Southern Oscillation (ENSO) could be a potential classifer for the SOM identifed atmospheric states, based on the correspondence of MSD occurrences with ENSO phases
Coastal currents in the Eastern Gulf of Tehuantepec from coastal altimetry
(Advances in Space Research, 2018-06-02) Salazar- Ceciliano, Juan Pablo
we study the seasonal and interannual variability of the coastal currents along the eastern shelf of the Gulf of Tehuantepec. We use sea level from coastal altimetry distributed by the Centre de Topographie des Oceans et de l’Hydrosphe`re (Toulouse, France). Local validation using tide gauge and hydrographic data indicates that this coastal product measures well the seasonal and interannual scales along this tropical coastline. Long (12 years) time series of altimetry-derived currents, 39–57 km from the coast, reveal new details of the seasonal cycle: currents flow poleward from October to February and equatorward from April to August. Spectral analysis reveals that mesoscale processes are at least as energetic as the seasonal mean. The combination of both determines to a large extent the seasonal and interannual variability of the coastal currents.
Comparative Study Of Conventional And Sbci Cardboard Solar Ovens
(Elsevier, 1992) Nandwani, Shyam S.; Fernández Gómez, Otoniel
Two folding and light solar ovens designed and constructed by Solar Box Cookers International (SBCI), Sacramento, U.S.A., have been studied experimentally. Furthermore, their performance is com pared with standard two glass solar ovens designed by one of us (S.S.N.), in the climate of Costa Rica. According to our measurements, made with and without load, taken during 30 different days, both cardboard ovens are 15-25% less efficient than the conventional oven and, secondly, the improved SBCI oven is slightly better than the previous SBCI oven.
Considerations for functional food design based on starch-protein interactions: a systematic review
(Taylor and Francis Ltd., 2021-03-26) Mejía-Terán, Adriana; Blanco - Lizarazo, Carla María
This systematic review aims to discuss the considerations in the design of starch-based foods from protein interactions, according to their functional, technological, and nutritional effects. Thereof, a systematic search for articles published in English, without excluding material by year, location, or author. Scopus and PubMed were the databases consulted. The results showed a decrease in gelatinisation, gelation, and viscosity of food matrices and a reduction in hydrolysis and starch digestibility rate by adding protein. Furthermore, found effects from other components such as phenolic compounds. In conclusion, in food design, protein interaction can modulate the starch digestibility rate, which also modifies technological, structural, and nutritional properties, depending on the physicochemical nature of ingredients, the percentage of protein incorporation, and processing conditions.
Datos Informe Observatorio 2024: Sensores de presión y temperatura en el Golfo de Nicoya
(Universidad Nacional (Costa Rica), 2025-03-31) Ureña Mora, Pablo; Tisseaux Navarro, Alexandre; Salazar Ceciliano Juan Pablo; Hernández Jiménez, Róger; Salas Cascante, Marcelo; Vargas Hernández, José Mauro
Deforestation processes in the livestock territory of La Vía Láctea, Matagalpa, Nicaragua
(Taylor and Francis Ltd., 2019-05-04) Tobar-López, Diego; Bonin, Muriel; Andrade, Hernán J.; Pulido, Astrid; Ibrahim, Muhammad
Land-use change is considered one of the main causes of environmental degradation. Thus, its analysis will allow stakeholders to make reasonable decisions for land management. The objective of the present study was to understand the patterns of land-use/land-cover change and deforestation in a territory of importance for livestock development in Nicaragua: La Vía Láctea. The methodology was based on the digital processing of satellite images using the geographic information system that allowed the generation of the thematic cartography of land-use/land-cover for 1978, 1986, 1998, and 2011. Between 1978 and 2011, a total of 93% of the forest cover was converted to pasture for livestock development; this change has been influenced by socio-economic and political factors. This study, moreover, reinforces the idea that livestock is the main driver of deforestation. Landscape restoration requires increasing tree cover by adopting silvopastoral systems that improve biodiversity conservation and the provision of ecosystem services.
Design, construction and experimental study of electric cum solar oven-II
(1989) Nandwani, Shyman S.
As in many developing countries,35-401 of the population of Costa Rica still use firewood for domestic cooking.Considering the fact that Costa Rica is blessed with good sunshine,good hydroelectric potential,and there exist good electric network,a hybrid solar oven was thought to be useful.In the present paper the construction and working of new type of Electric Cum Solar Oven(ECSO) has been described.This oven can be used for cooking and baking any type of meal at any time during the year employing solar and/or electric energy but consuming the minimum quantity of electric energy in case it is required.
Design, construction and study of a hybrid solar food processor in the climate of Costa Rica
(Elsevier, 2007) Nandwani, Shyam S.
During the last 25 years, the author has designed, constructed, studied and promoted solar oven, hybrid solar/electric oven, solar oven cum drier, solar cooker cum water heater and solar still. In different parts of the world, solar cookers have been made, studied, patented, however, their real uses are very limited due to many reasons—unstable climate, economic, cultural, social and single use, etc. In order to overcome part of the problems mainly the last one, author has recently designed one hybrid food processor (multi-purposes device) and studied various technical and practical aspects. It has been used for cooking, heating/pasteurizing water (to inactivate microbes) and distillation of small quantity of water (to remove different minerals) and drying domestic products (fruits, vegetables and condiments/herbs, etc.). For more than three years of use, author has found this to be a useful device, mainly from convenience, fuel saving, economic and also from ecological point of view. This device can be used at any time and for different uses but with the reduced consumption of conventional fuel. r 2006 Elsevier Ltd. All rights reserved
Dose-response of Fricke- and PAGAT-dosimetry gels in kilovoltage and megavoltage photon beams: Impact of LET on sensitivity
(Associazione Italiana di Fisica Medica, 2021-04-07) Vedelago, José; Chacón Obando, David M; Romero, Marcelo Ricardo; Venencia, Carlos Daniel; Mattea, Facundo; VALENTE, MAURO
Purpose: Dosimetry of ionizing radiation quantifies the energy deposited by an incident beam to the medium. This study presents the relative response of two types of gel dosimeters describing their differences by estimating radiation chemical yields produced in water radiolysis. Methods: Two types of gel dosimeter were used, namely an acid ferrous ion solution infused with xylenol orange known as Fricke gel and a polymer gel based on acrylamide and N,N’-methylenebis(acrylamide) known as PAGAT. Samples were irradiated using two photon beam energies, one from a conventional X-ray tube operated at 44 kV and the other one from a LINAC operated at 6 MV. The dosimeters were analyzed by optical absorbance and magnetic resonance imaging. Additionally, the linear energy transfer of each beam was calculated using Monte Carlo simulations for further estimation of the radiation chemical yields produced during water radiolysis. Results: Obtained results for both gel dosimeters indicate that their response at 44 kV and 6 MV are different, regardless of the read-out technique. On average, the sensitivity at 44 kV was found to be 65 % of the response at 6 MV. The calculated radiation chemical yields are in agreement with the observed experimental results. Conclusions: The main reason for the difference in the response of the dosimeters may be related to the linear energy transfer of each photon beam, which varies the production of primary chemical species during water radiolysis.
Economic Analysis Of Domestic Solar Still In The Climate Of Costa Rica
(Universidad Nacional (Costa Rica), 1990) Nandwani, Shyam S.
-In many communities of the developing countries, including Costa Rica and especially in coastal areas, there exists a shortage of potable water. Taking into account the fact that transportation/distribution of potable water is expensive for these communities and most of these places are blessed with good sunshine, one domestic solar still per family could help in getting fresh water--at least for drinking purposes (say 10 1 per day). Although the design, construction and technical results of three small-scale solar stills studied by the author have been presented in local technical journals, these results are mentioned briefly and are used for the estimation of total quantity of water distilled per year in three different climates of Costa Rica. In one study, estimation of rain water collected over the glass cover of a solar still has also been made as this can be considered as drinkable water where potable water is not available. In the present work, economic analysis of these stills have been made at these places in Costa Rica--not to show that cost of water produced from these stills is expensive compared to a conventional centralised system (including subsidies), but to see how expensive it is. Comparisons have been made with distilled water produced from a domestic electric still. It has been found that the cost of water produced from the solar still is 15 30 times more expensive than sold presently by the centralised system, however, it is far cheaper than produced by the electric still. Another objective of this study was to produce a basis for the construction of various small stills or one big plant for one community, e.g. Island Chira, lacking potable water. Finally, some social benefits of these types of installations at remote places, even though they are expensive, are mentioned.
Effectiveness of N-waves for predicting morphological changes due to tsunamis
(Elsevier, 2018) Chacon-Barrantes, Silvia
The research on morphological changes caused by tsunamis has increased considerably in the last few years, yet the processes behind this phenomenon are still not fully understood. This paper analyzes and compares numerical simulations of morphological changes caused by the leading elevation and leading depression N-waves and tsunami waves propagating over a channel with a simple bed slope. The simulations were carried out by means of a coupled flow, sediment transport and morphodynamic two-dimensional-vertical numerical model. The modeled channel bed was composed of cohesionless sediments, and range of values for the bed slope and the wave height were employed. Four tsunami waveforms were studied to test the appropriateness of N-waves in modeling the morphological changes caused by tsunamis. On the modeling performed here, runup values were quite well represented by N waves. N-waves also represented qualitatively well the morphological changes caused by tsunamis. Yet, values of flow velocities and suspended sediment concentration showed more severe deviations from the modeled results corresponding with tsunami waves, very likely due to differences on the steepness of the waves. Therefore, even when N-waves can and have been used to represent tsunami runup and inundation distance, our results suggest that they should be considered with caution when intended to predict magnitudes of tsunami flow velocities and consequently morphological changes. The use of N waves to simulate morphological changes caused by tsunamis is not yet converging and must be further investigated.
Experimental And Theoretical Analysis Of A Simple Solar Oven In The Climate Of Costa Rica - I
(Elsevier, 1988) Nandwani, Shyam S.
Inexpensive and easy to fabricate solar ovens have been made with available materials. One reflector has also been mounted to increase the input radiation on the hot absorbing plate which can be adjusted manually according to the need. This has been used by the author for the last 7 years to cook almost all types of vegetables, including rice, pulses, chicken, fish and beef and to bake bread and cake etc. Experimental studies have also been made on the variation of plate temperature, solar intensity, time to cook vegetables. These are given for the various cases of adjustment of the oven, depending on the convenience for the user. Furthermore, the insulating properties of the cooker are also studied. Maximum plate temperature observed was between 130 and 150°C and one cooked meal for a family of four persons can be made in 2–3 h. Thermal efficiency and heat loss coefficient, deduced from these measurements, varies between 30–40% and 4–9 W/m2°C. Simple theoretical analyses have beend used to explain these experimental data, taking into account heat capacity of various materials. The agreement is fairly good. Finally, some practical conclusions have been drawn.
Experimental characterization and Monte Carlo simulations of the dose enhancement on the millimeter scale of PAGAT infused with gadolinium
(Elsevier Ltd, 2021-05-03) Santibañez, M.; Fuentealba, M.; Vedelago, José; Chacón Obando, David M; Mattea, Facundo; VALENTE, MAURO
This study reports on the determination of the K-edge dose enhancement factor achieved in a polymer gel dosimeter based on acrylamide, known as PAGAT, infused with a high concentration of a clinical gadolinium-based compound. PAGAT dosimeters infused with gadolinium concentrations of 10 mg/mL and 20 mg/mL are characterized in terms of dose-response and sensitivity, obtaining the dose enhancement between two beam qualities for these Gd concentrations of clinical interest. Additionally, the dose enhancement ratio between these two concentrations was evaluated for each beam quality. To this aim, Gadodiamide solution, a Gd-based agent commonly used in clinics for magnetic resonance scanning, is used to manufacture Gd-infused dosimeters that are further irradiated with a 50 kVp X-ray beam in order to assess dose enhancement contributions exclusively from absorption from lower edges (L, M, etc.); subsequently, a 160 kVp spectrum optimized by yttrium and indium filtration is used to evaluate K-edge contributions. Gel dosimeter samples are prepared in spectrophotometric vials, and their readout is performed by optical density measurements, which is correlated with the absorbed dose. An appreciable increase was found in the K-edge dose enhancement factor (160kVp/50 kVp) for the samples with concentrations of 20 mg/mL and 10 mg/mL ranging from 1.20 to 1.61 and 1.10–1.41, respectively. These results could be interpreted as the contribution to the absorbed dose due the increment in the secondary radiation emitted from Gd atoms by K-edge interactions. Similarly, a relevant increase was observed in the dose enhancement when the Gd concentration is incremented from 10 mg/mL to 20 mg/mL obtaining factors of around 1.10–1.33 and 1.34–1.47 for the beam qualities 50 kVp and the optimized 160 kVp, respectively. Monte Carlo simulations using the PENELOPE main code are implemented aimed at complementing and comparing with the experimental results, showing differences of less than 4% for the K-edge dose enhancement factor and differences of less than 3% for the dose enhancement ratio for the studied beam qualities. In this regard, the results from the simulations support the reliability of the experimental measurements with this type of dosimeters within the studied concentration range. Finally, it is noticeable that the appreciable dose enhancement reported through this work was obtained infusing mass Gd as part of the Gadodiamide compound, already used as a contrast agent for magnetic resonance imaging, thus ensuring its suitability for clinical practices in humans.
Influence of substrate temperature on phase formation in Al-Cr thin films
(Universidad Nacional, Costa Rica, 2000) Saenz, A.; Chavarri, D.; Hernandez, R.; Castaño, V.M.
Thin films (of the order of nm) of Al-Cr alloys were prepared by successive depositions by an electron gun in a vacuum chamber. Three Al and two Cr layers, of thicknesses as to yield the final composition, were de posited on both hot (350 °C and 440 °C) and cold (70 °C and 108 °C) substrates and the phases formed were char acterized in each case by X-ray diffraction and TEM ob servations, both in bright and in dark field conditions. The results show that on the hot substrates, Bragg peaks that do not correspond to any reported crystalline or qua si-crystalline phase appear. Both the samples on cold substrates and those heated afterwards showed an amor phous structure by X-ray diffractomery but TEM demon strated the presence of tiny faulted crystallites, with the same Bragg reflections than those deposited on hot sub strates. The results show that, for the composition range studied, a new phase not reported before appears and the substrate temperature only produces crystallite growth, not the formation of new phases
Influence of the Madden–Julian oscillation on Costa Rican mid-summer drought timing
(John Wiley and Sons Ltd, 2019) Zhao, Zijie; Oliver, Eric; Ballestero, Daniel; Vargas-Hernández , José Mauro ; Holbrook, Neil J.
The Central American mid-summer drought (MSD) is the decline of precipitation during the middle of the wet season (July and August) over Central America and southern Mexico. It affects agriculture and favours the initiation of bushfires in Costa Rica's national parks, particularly during El Niño years. The MSD is a seasonal phenomenon that varies in intensity and timing inter-annually. The Madden–Julian oscillation (MJO) has been shown to influence Costa Rican rainfall on intra-seasonal time scales, and therefore may be important to the MSD. In this study we use rainfall data from seven stations in Costa Rica to analyse the MJO's influence on the timing of the onset and end of the MSD. We find that the MSD is more likely to start and end in MJO Phases 1 and 8, respectively. Our findings indicate enhanced MSD predictability on intra-seasonal time scales, which could be beneficial to agricultural planning in Costa Rica

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