Nanofluid as Advanced Cooling Technology. Success Stories

Nanofluids are defined as heat transfer fluids with enhanced heat transfer properties by the addition of nanoparticles. Nanofluid’s stability, nanoparticles’ type and their chemical compatibility with the base fluid are essential not only to increase the nanofluid’s thermophysical properties but also to ensure a long-lasting and thermal efficient use of the equipment in which it is used. Some of these aspects are discussed in this chapter. Likewise, the improvement in terms of the heat transfer capacity (thermal resistance) that the use of nanofluids has on the heat pipes-thermosyphons is shown. On the other hand, the improvement in energy efficiency that nanofluid causes in a vapor compression system is also presented

Los diseños factoriales: otra forma de planificar la investigación

Peer reviewe

Effects of aging and drying conditions on the structural and textural properties of silica gels.

9 figures, 2 tables.Sol–gel derived materials are widely used as porous matrices for preparing optical chemical sensors or biosensors. The porosity of the support matrix is a key variable that affects the sensitivity and response time of the sensors. The aim of this research is to study the effect of different aging and drying conditions on the structure and porosity of silica gels to obtain materials of tailored porosity. The gels were aged in ethanol or NH3(aq) (0.5 M and 2.0 M), and dried under atmospheric (xerogels) or supercritical conditions for ethanol (aerogels). Aging and drying conditions strongly affected the porous texture of silica gels. The surface area and micropore volume was higher in xerogels than in aerogels. Xerogel aged in ethanol was mainly microporous, while xerogels aged in ammonia were mesoporous due to cross-linking reactions in alkaline media. The maximum from the BJH distribution appeared at 4.0 nm for the xerogel aged in 0.5 M NH3(aq) and at 5.4 nm for the xerogel aged in 2 M NH3(aq). Aerogels were macroporous materials, and macropore volume constituted over 92% of the total pore volume, independently of the aging media. Esterification reactions inside the reactor promoted cross-linking, which resulted in a higher skeletal density and an increase in the absorbance of Si–O related IR bands. Cross-linking strengthens the gel network and reduces the amount of shrinkage under atmospheric conditions. SEM and TEM micrographs confirmed the textural properties of xero- and aerogels deduced by gas adsorption, Hg porosimetry and density measurements.This work was supported by Ministerio de Ciencia y Tecnología (BQU2002-04090-CO2-02 and CTQ2005-08099-CO3-02-BQU). Juncal Estella Redín is thankful to Departamento de Industria y Tecnología, Comercio y Trabajo del Gobierno de Navarra for the fellowship. The authors are grateful to two anonymous reviewers for their fruitful comments.Peer reviewe

Silica xerogels of tailored porosity as support matrix for optical chemical sensors. Simultaneous effect of pH, ethanol:TEOS and water:TEOS molar ratios, and synthesis temperature on gelation time, and textural and structural properties

9 pages, 6 figures, 4 tables.The sensing of a chemical environment is achieved mainly in the surface by interactions of the sensor material with its chemical surroundings. Therefore, porous structure control is key in developing good chemical sensors. The aim of this work was to obtain materials of tailored porosity to be used as support matrix for optical chemical sensors. We studied the simultaneous effect of pH, temperature, ethanol:TEOS, and water:TEOS molar ratios on gelation time, and textural and structural properties. We used a 24 factorial design that evaluates the effect of each independent variable and their interactions. Samples were characterized by XRD, SEM, FTIR, and gas adsorption (N2 at 77 K and CO2 at 273 K). The gelation time decreased with increasing temperature, water:TEOS molar ratio, and pH; and with decreasing ethanol:TEOS molar ratios. Synthesis conditions also affected the xerogels porous textures. Xerogels obtained at pH 2.5 were ultramicroporous. In general, samples synthesized at pH 4.5 and ethanol:TEOS molar ratio of 2.25:1 were mesoporous, but the material is not appropriate for use as support in fiber optical sensors.This work was supported by Ministerio de Ciencia y Tecnología (DGI BQU2002-04090-C002-02). Juncal Estella Redín is thankful to the Departamento de Industria y Tecnología, Comercio y Trabajo del Gobierno de Navarra for the fellowship.Peer reviewe

Effects of aging and drying conditions on the structural and textural properties of silica gels.

9 figures, 2 tables.Sol–gel derived materials are widely used as porous matrices for preparing optical chemical sensors or biosensors. The porosity of the support matrix is a key variable that affects the sensitivity and response time of the sensors. The aim of this research is to study the effect of different aging and drying conditions on the structure and porosity of silica gels to obtain materials of tailored porosity. The gels were aged in ethanol or NH3(aq) (0.5 M and 2.0 M), and dried under atmospheric (xerogels) or supercritical conditions for ethanol (aerogels). Aging and drying conditions strongly affected the porous texture of silica gels. The surface area and micropore volume was higher in xerogels than in aerogels. Xerogel aged in ethanol was mainly microporous, while xerogels aged in ammonia were mesoporous due to cross-linking reactions in alkaline media. The maximum from the BJH distribution appeared at 4.0 nm for the xerogel aged in 0.5 M NH3(aq) and at 5.4 nm for the xerogel aged in 2 M NH3(aq). Aerogels were macroporous materials, and macropore volume constituted over 92% of the total pore volume, independently of the aging media. Esterification reactions inside the reactor promoted cross-linking, which resulted in a higher skeletal density and an increase in the absorbance of Si–O related IR bands. Cross-linking strengthens the gel network and reduces the amount of shrinkage under atmospheric conditions. SEM and TEM micrographs confirmed the textural properties of xero- and aerogels deduced by gas adsorption, Hg porosimetry and density measurements.This work was supported by Ministerio de Ciencia y Tecnología (BQU2002-04090-CO2-02 and CTQ2005-08099-CO3-02-BQU). Juncal Estella Redín is thankful to Departamento de Industria y Tecnología, Comercio y Trabajo del Gobierno de Navarra for the fellowship. The authors are grateful to two anonymous reviewers for their fruitful comments.Peer reviewe

Application of gold complexes in the development of sensors for volatile organic compounds

Two different kinds of sensors have been developed by using the same kind of vapochromic complexes. The vapochromic materials [Au2Ag2(C6F5)(4)L-2](n) have different colours depending on the ligand L. These materials change, reversibly, their optical properties, colour and fluorescence, in the presence of the vapours of volatile organic compounds (VOCs). For practical applications, two different ways of fixing the vapochromic material to the optical fibre have been used: the sol-gel technique and the electrostatic self-assembly method (ESA). With the first technique the sensors can even be used to detect VOCs in aqueous solutions, and using the second method it has been possible to develop nanosensors.The authors thank the Ministerio de Ciencia y Tecnología Spain for financial support (CTQ2005-08099-C03-01 and CTQ2005-08099-C03-02