In order to increase agricultural productivity, it is necessary to manage the nutrients that the plants receive. In this sense, nitrogen is one of the most important nutrients for the adequate development and growth of plants and whose control contributes to the conservation of the environment. Traditionally, the quantification of nitrates is performed employing methods such as ion chromatography conducted in a laboratory, which leads to problems such as processing time, complexity in the analysis process itself and the need to have people trained to handle such analysis equipment. To alleviate these difficulties, some sensors use electrochemical principles; however, they present several limitations such as manufacturing complexity, cost, and difficulties in conducting real-time measurements. On the other hand, capacitive sensors have also been developed whose operating principle is based on analyzing the changes in the electrical characteristics of these devices produced by variations in the environment surrounding the sensor. For example, solutions with different concentrations of nitrates will produce an alteration of the electric field generated by the electrodes of the capacitor, producing changes in parameters such as its electrical impedance. However, the materials used to manufacture these sensors are metals, being unattractive for agricultural applications due to the corrosion they may suffer. To counteract this problem, it is possible to use other types of materials, among which graphene emerge as it has been shown to have excellent properties such as being a mechanically strong material and presenting good electrical conductivity. For this reason, in this work, the study of the sensitivity of a graphene-based interdigital capacitive sensor applied to the measurements of nitrate concentrations is performed. Different analytical methods were used. In general, the interdigital capacitive sensor shows repeatability in the measurements, especially for concentrations greater than 10 ppm with variability of . Also, parameters such as the detection limit of the sensor were calculated, the result of which was 1.71 ppm. Also, measurements were made on agricultural soil samples showing differences in readings with respect to the ion chromatography method. Differences that are attributable to the different methodologies used in the calculation of the nitrate concentration and the fertilization process that was applied to the crop field a few weeks before. Despite the difference between the sensor measurements and the results obtained by ion chromatography, both procedures showed a high amount of nitrate concentration.

La combinación de diferentes calidades de residuos de cuero con diferentes polímeros y las propiedades mecánicas resultantes son potencialmente importantes en la obtención de nuevos materiales. En este estudio, experimentos combinando residuos de viruta de cuero, resina vinnapas ®400 y agua destilada produjeron un nuevo material compuesto con propiedades mecánicas interesantes. En la elaboración se controló el pH de la viruta de cuero, tamaño de viruta, temperatura de la resina, temperatura de ablandamiento, la presión y el tiempo de presionado ejercido sobre la plancha de cuero reconstituido. Empleando diseño factorial fraccionado se determinó la influencia de las variables independientes seleccionadas: tamaño de la viruta, relación viruta/resina, porcentaje de agua, temperatura de la resina, la presión y el tiempo de presionado sobre las propiedades mecánicas del material compuesto. Los resultados experimentales mostraron que había un efecto principal de la variable relación Viruta/Resina en su valor mínimo sobre el esfuerzo máximo de tracción, llegándose a obtener un valor de 2.657 MPa. Adicionalmente, en el ensayo mecánico de fluencia se encontró que el material compuesto denominado cuero reconstituido muestra un comportamiento viscoelástico, lo que lo asemeja al comportamiento de un cuero curtido.

Biomorphic SiC/Si compounds were fabricated from copaiba wood (Copaifera officinalis, natural wood native to Peru), by reactive infiltration of molten silicon in a porous carbon preform obtained by a controlled pyrolysis process of wood. Structural and microstructural characterization tests by X-ray diffraction and scanning electron microscopy, respectively, revealed, on the one hand, the presence of crystalline phases of SiC, Si and C, and on the other, the typical morphology of this type of material, which it consists of a continuous SiC scaffold with elongated channels in the direction of tree growth and the presence of residual Si and C located mainly in the porosities of the material. The mechanical behavior in uniaxial compression was also studied at a constant compression rate of 0.05 mm/min and as a function of temperature (from ambient to 1400 ºC) and test atmosphere (ambient air, humid air, dry air, Ar, N2 and reducing mixture (95% Ar + 5% H2). The mechanical results were evaluated based on values of maximum stress and modulus of elasticity (stiffness), finding a clear reduction in the values of maximum stress and stiffness of the material when the samples passed of ambient test temperatures at 1400 ºC. On the other hand, mechanical tests in a controlled atmosphere were carried out at a constant temperature of 1100 ºC and the results showed that the mechanical behavior of the studied compounds is slightly influenced by the working atmosphere. Mechanical data found in the various test conditions will be an important support for the definition of the maximum allowable stress (considering the safety factor applied for a particular case) in the industrial application of the materials studied in this work.

This work presents the results of the thermomechanical evaluation of geopolymeric concrete fabricated from mining tailings, rice husk ash and fine sand. Ten types of geopolymeric concrete were studied and the relationship between the initial volumetric concentrations of the components in the mixtures and the maximum resistance in uniaxial compression under conditions of variable temperature (between ambient and 600 °C) was analyzed. The results revealed that increases in the concentration of mining tailings and fine sand lead to an increase in the value of the maximum mechanical resistance, in contrast, the increase in the concentration of rice husk ash led to a reduction in the value of the maximum mechanical resistance. Furthermore, increases in test temperature, up to 500 °C, led to systematic increases in maximum mechanical strength. Finally, the geopolymeric concretes presented a brittle-ductile transition between 500 and 600 °C showing only a ductile behavior when tested at 600 °C and only brittle up to test temperatures of 500 °C.

Reinforced geopolymeric mortars were obtained by mixing mine tailing, fine sand, alpaca wool fibers ( in variable amounts) sodium hydroxide and potable water, it was possible to verify the effect of the addition of alpaca wool on the mechanical behavior in uniaxial compression of the mortars studied. The mechanical data found revealed a systematic decrease in the maximum stress as the volume of wool added in the mortar mixtures manufactured increased. On the other hand, a higher degree of deformation was verified in mixtures with a greater volume of added fibers, reaching deformation values of up to 5%. The maximum strength values were in the range of 4 to 21 MPa for samples with 8 and 0 Vol. % of added fibers, respectively. Among the microstructural characteristics of the mortars studied, a continuous binder phase corresponding to the geopolymer could be appreciated, with sand particles and wool fibers dispersed within the binder phase. The real density and average porosity of the reinforced mortars were 2.65 g/cm3 and 32%, respectively.

The traditional method of manufacturing SiC compounds is associated with a serious environmental problem, mainly due to the need for large amounts of energy (generally derived from oil) to reach processing temperatures (typically above 2500 ºC). In addition, the chemical reaction that gives rise to the formation of SiC has CO and CO2 as by-products. Therefore, in this work an alternative method to manufacture SiC/Si composites using waste from the wood industry as the main raw material was developed. SiC/Si composites were fabricated by infiltration of molten silicon into carbon preforms at 1500 °C. The carbon preforms were obtained by pyrolysis (in an inert Ar atmosphere) of four types of resin-carbon mixtures. The carbon used in the mixtures was obtained by pyrolysis of sawdust powder. The mechanical and thermomechanical behavior in uniaxial compression was studied at a constant compression rate of 0.05 mm/min at different temperatures (ambient, 1100 °C and 1400 °C). The maximum resistance values found were in the range of 58 and 384 MPa, while the Young's modulus values were between 40 and 120 GPa. The porosity found in the materials was between 1 and 4%. Finally, the fabricated compounds presented a homogeneous microstructure of interconnected silicon carbide in gray contrast and dispersed and unconnected whitish phases of uniformly distributed silicon.

Geopolymeric mortars with volumetric fractions of 0.6:1:0.3 for a binder powder, fine sand and sodium hydroxide solution (12M), respectively; have been fabricated by mixing the solid materials and the subsequent addition of sodium hydroxide solution 12M to form a workable paste, to later be cured for 28 days at room temperature. The microstructures of the fabricated materials reveal the existence of two phases with notable difference, one continuous to the geopolymer binder phase and another discontinuous of fine sand particles agglutinated by the binder phase. Mechanical compression tests are performed at a constant compression rate of 0.05 mm/min and at temperatures ranged from room temperature to 500°C. The mechanical results are ranged from 19 and 69 MPa for all the materials studied. On the other hand, there was an increase in mechanical resistance up to test temperatures of 200°C and the progressive reduction of resistance at temperatures above 200°C, with a fragile-ductile transition zone between 400 and 500°C and completely ductile behavior from test temperatures of 500°C.