Aeronautical Engineer with a Master of Science in Applied Physics and a PhD in Materials Science. I am currently doing research on the phenomenon of thermal energy transport in complex fluids, nanofluids and condensed matter.
The ZnS thin films have been deposited by radio frequency magnetron sputtering at room temperatur... more The ZnS thin films have been deposited by radio frequency magnetron sputtering at room temperature. Post-deposition rapid thermal annealing treatment was done for the films deposited at different powers ranging from 70 to 100 W. One peak is observed for as-deposited and annealed thin films at around 28.48° corresponding to the (111) reflection plane indicating a zincblende structure. The overall intensity of the peaks and the FWHM values of as-deposited films increased after annealing corresponding to the increase in crystallinity. The optical energy bandgap is found in the range of 3.24-3.32 eV. With increasing annealing temperature, the decrease in the Urbach energy values indicating a decrease in localized states which is in good agreement with the XRD results where the crystallinity increased. The surface morphology of the films seems to be composed of Nano-granules with a compact arrangement. Apparently, the grain size increases in the deposited films as annealing temperature increases. The compositional ratio attained close to the stoichiometric ratio of 1:1 after annealing. From the Hall effect measurement, the carrier concentration and mobility are found to increase after annealing. The high carrier concentration and mobility also comply with structural and optical analysis. Best results are found for the film annealed at 400 °C deposited at 90 W.
Magneto-optic phenomena in ferrofluids have been shown to be related to the formation of chain st... more Magneto-optic phenomena in ferrofluids have been shown to be related to the formation of chain structures, due to the arrangement of the ferromagnetic particles, induced by an applied magnetic field. In this work, the effects on transmission of polarized light due to anisotropic effects induced by an external magnetic field in ferrofluids with carbon nanotubes are studied. The time response of the system presents two well defined stages, in the first one, which is very short, the fluid behaves as a polarizer. In contrast in the second stage, the effects of light transmission dominate. In this stage the transmitted light intensity grows with time and after a long time reaches a constant stable value. It is shown that these phenomena depend on the carbon nanotubes concentration as well as on the strength of the applied magnetic field. Using a simple model that considers a chain-like structure formation, it is possible to determine the rate of agglomeration of the formed structures and the attenuation coefficient of the transmitted light. The formation of nanostructures leads to variation in the transmitted light, depending on the polarization of the incident light. These magnetic nanostructures can find numerous applications in nanotechnology, optical devices and medicine.
In this work, the thermal enhancement in silicone grease-based compounds as a function of carbon ... more In this work, the thermal enhancement in silicone grease-based compounds as a function of carbon nanofiber (CNF) volume fraction was investigated. The thermal diffusivity of the samples was determined by a photopyroelectrical technique with a sample thickness scan. The results show that heat transport on these compounds strongly depends on the CNF volume fraction, due to the high thermal conductivity of CNF compared to the matrix; hence, a low loading percentage of the fibers produce a significant growth in the thermal diffusivity of the composite. The results show that the thermal diffusivity values of the CNF-silicone composite are comparable with commercial thermal compounds based on diamond and Ag microparticle fillers. The thermal conductivity of the samples was calculated, and its enhancement was analyzed using a modified Lewis-Nielsen model, taking into account the dependence of the maximum packing fraction and the form factor with the aspect ratio of the CNF. The influence of the Kapitza thermal resistance was discussed. These materials might find practical applications in systems in which the CNF improves the ebbing of heat away from semiconductor devices or in any other application in which heat dissipation is needed.
The thermal conductivity and viscosity of a magnetorheological suspension composed of carbonyl ir... more The thermal conductivity and viscosity of a magnetorheological suspension composed of carbonyl iron particles immerse in silicone oil were studied. Thermal wave resonant cavity was employed to measure the thermal diffusivity of the magnetorheological fluid as a function of an externally applied magnetic field. The dynamic viscosity was also measured and its relationship with the concentration of the particles and the magnetic field strength was investigated. The results show that higher concentrations of carbonyl iron particles as well as higher magnetic field intensities lead to a significant increase in thermal conductivity. The relationship between the thermal conductivity and the dynamic viscosity was explored. Our measurements were examined using an analytical relation between the thermal conductivity and the dynamic viscosity. The results show that by using highly viscous materials, the order induced in the micro particles can be kept for a relatively long time and therefore the increase in thermal conductivity can also be maintained.
In this work, the development of a new crystallization technique is reported, using nitrogen plas... more In this work, the development of a new crystallization technique is reported, using nitrogen plasma (AC) to obtain nanostructured anatase and rutile from amorphous titanium oxide (TiO 2). This methodology increases throughput and minimizes thermal effects. Nanostructured amorphous TiO 2 was obtained by the sol-gel method and subsequently subjected to AC treatment, at a controlled pressure, applying different powers and treatment times in order to obtain phase changes. The obtained samples were characterized using X-ray diffraction (XRD), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). The results show the crystallization in parallel with anatase and rutile phases with a proportion that is directly related to the applied power in the plasma and the treatment time. This technique allows us to obtain smaller crystals in comparison with those of classic thermal methodologies. It is also demonstrated that the application of plasma represents a novel and innovative method to obtain phase polymorphic changes in titanium oxide without needing to apply prolonged heat treatments at high temperatures and can therefore be taken into consideration as a technique with low energy costs, in comparison with conventional heat treatments.
Influence of the thickness of CdS layer on the texture characteristics (morphology and crystallin... more Influence of the thickness of CdS layer on the texture characteristics (morphology and crystalline structure) of the photovoltaic active layer CdS/CdTe was studied. CdS and CdTe films were grown by Close Space Sublimation technique combined with Substrate Rotation (CSSSR). In order to growth the CdTe films, three glass substrates with previously deposited CdS thin film with thicknesses of 106.3, 125.1 and 221.0 nm respectively were used. The conditions of CdTe films deposition were, vacuum pressure of 1x10-6 mTorr, sublimation source temperature 650 o C, substrate temperature 280 o C, source-substrate distance 5.0 mm, substrate rotation 1025 rpm and time deposition 10 min. The CdTe films obtained were with uniform thickness and dimensions of 4.73 m and compacted grain with sizes of 112.5, 108.2 and 140.3 nm respectively. The samples shown high quality crystalline with a preferential orientation in the plane (111), and the CdTe unit cell volume was less than the standard pattern as...
In this work, the study of the heat transfer enhancement induced by aligning iron nanoparticles i... more In this work, the study of the heat transfer enhancement induced by aligning iron nanoparticles in an ethylenglicol fluid matrix is presented. In particular the effect of the interface is studied by analyzing the cases in which the nanoparticles are coated with carbon and comparing with uncoated ones. Results indicate that the coating acts as a thermal barrier making thermal conductivity to decrease. Moreover, the magnetic field creates aligned columns that enhance the heat transfer. Effective models are used to determine the role of the coating as well as of the aligning of the nanoparticles.
We study the thermal conductivity of five kinds of Magnetic Fluids (MFs) by varying the magnetic ... more We study the thermal conductivity of five kinds of Magnetic Fluids (MFs) by varying the magnetic material volume fraction and the direction and intensity of an homogeneous magnetic field: The studied MFs are: Magnetorheological fluids (MRF), carbon coated and uncoated Fe nanofluids (CcFeNF, FeNF), ferrofluis (FF); and two kind of composed fluids; ferrofluids loaded with carbon nanotubes (FFCNTs) and ferrofluids
Thermal properties of solids are obtained by fitting the exact complex photothermal model to the ... more Thermal properties of solids are obtained by fitting the exact complex photothermal model to the normalized photoacoustic (PA) signal in the front configuration. Simple closed-form expressions for the amplitude and phase are presented in all frequency ranges. In photoacoustic it has been common practice to assume that all the absorptions of radiation take place in the sample. However, in order to obtain the accurate thermal properties it is necessary to consider the PA signal contributions produced at the cell walls. Such contributions were considered in our study. To demonstrate the usefulness of the proposed methodology, commercial stainless steel layers AISI 302 were analyzed. It is shown that using our approach the obtained thermal diffusivity and effusivity were in good agreement with those reported in the literature. Also, a detailed procedure for the calculation of the standard error in the thermal properties is discussed.
The effects of the interactions of hydrogen (H) atoms on graphene (G) with potassium (K) pre-adso... more The effects of the interactions of hydrogen (H) atoms on graphene (G) with potassium (K) pre-adsorbed, were predicted by means of first-principles calculations. The results were obtained with the pseudopotentials method and the generalized gradient approximation for the ...
Real time measurement of thermal diffusivity during the evolution of the light curing process in ... more Real time measurement of thermal diffusivity during the evolution of the light curing process in dental resins is reported using photothermal radiometry. The curing is induced by a non-modulated blue light beam, and at the same time, a modulated red laser beam is sent onto the sample, generating a train of thermal waves that produce modulated infrared radiation. The monitoring
The photoactivation time of sol-gel nanostructured titania, in an ethanol suspension, was studied... more The photoactivation time of sol-gel nanostructured titania, in an ethanol suspension, was studied by optical methods in the UV-Vis region. Titania activation was induced using a Hg lamp and the process was analyzed using light transmittance. The Hg lamp light beam was transmitted through the sample and collected with a spectrophotometer as a function of time. The titania powder was thermally treated to observe the effect in the photoactivation. When the photoactivation of the titania suspension occurs, the transmittance falls and afterwards, a constant value is reached. It was found that the activation time depends on the crystalline structure and the crystallite size.
ABSTRACT The thermal wave field in a sample of thickness L and optical absorption coefficient β i... more ABSTRACT The thermal wave field in a sample of thickness L and optical absorption coefficient β is derived and analyzed, by considering the effects of the multiple reflections of a modulated light beam within the sample. By comparing the corresponding amplitude and phase spectra, in presence and absence of those reflections, it is shown that their effects are strongly determined by the product βL. When the thermal wave field is measured at the illuminated surface, those effects appear remarkably on the amplitude and phase in the thermally thin and thermally thick regimes, respectively, and for βL≲2. The deviation of the amplitude spectrum due to the multiple reflections of incident light beam can be as large as 25%. In contrast, when the thermal wave field is measured at the non-illuminated surface, the contribution of the multiple reflections shows up on both the amplitude and phase in the thermally thick regimes, and for βL≲5. These spectra exhibit deviations of about 20% from their corresponding values without the effects of the light reflections. The obtained results show that the accurate measurement of the thermal and/or optical properties of the sample material, based on the amplitude and phase spectra, requires the consideration of those multiple reflections, especially for the cases where βL ≈ 1. The predictions of the proposed approach for the optical absorption coefficient are in reasonable agreement with the results obtained through an independent technique based on a spectrometer.
Combining the well-known Bruggeman theory and Nan et al. results, formulas for predicting the eff... more Combining the well-known Bruggeman theory and Nan et al. results, formulas for predicting the effective thermal conductivity of anisotropic particulate composites with an interface layer are derived. These formulas are valid for a composite material containing arbitrarily oriented ellipsoidal particles with any aspect ratio, and they can be expected to be suitable mainly for large volume fractions, when the thermal interaction between neighboring particles needs to be considered. Results of the present approach are reduced to simpler formulas for some limiting cases in the particle shape. Theoretical analysis of the effective thermal conductivity as a function of volume fraction and shape of the particles is performed. Comparison of the obtained formulas with previously reported experimental data for the effective thermal conductivity is also presented.
Aqueous solutions of agar show a sol-gel transition temperature in the range from 32 • C to 47 • ... more Aqueous solutions of agar show a sol-gel transition temperature in the range from 32 • C to 47 • C depending on colloid concentration; however, the width of the transition and the subsequent process occurring after it, are not well understood. In this work the sol-gel transition of agar is studied by a simple optical method. It consists of the illumination of the sample using a non-coherent light source and the monitoring of transmitted light using a photodiode. It is shown that during the sol-gel transition, the transmitted light decays over a broad range of temperature. Simultaneously, it is possible to observe that fluctuations in the transmitted light decrease along the sol-gel transition, and at a specific temperature, they become very small. Based on these observations, the sol-gel transition temperature for three different concentrations of agar (0.5, 1.5, and 2.5% w/v) is determined. These results are compared with the sol-gel transition temperature values provided by the conventional rheological method.
In this work the formation of thin films by evaporation of liquid mixtures of agglomerated amorph... more In this work the formation of thin films by evaporation of liquid mixtures of agglomerated amorphous untreated fumed silicon dioxide nanoparticles is analyzed. Fumed silica is a material that has been used as a thickening agent due to the fact that it has chain-like particle morphology. When mixed with a liquid, the chains bond together and increase the viscosity. The experiments were performed on samples deposited on a metallic substrate. Thermal waves were generated by sending a modulated laser beam at constant frequency onto the substrate. These waves are propagated to the evaporating solution allowing the monitoring of the film formation. It is shown that the last stage of the film formation is strongly affected by the formation of cracks and migration of the material to the lateral surface of the container. The experimental data are compared with predictions of effective thermal property models. Keywords Evaporation process • Fumed silica • Photoacoustic technique • Time evolution 1 Introduction During recent years photothermal (PT) techniques have been demonstrated to be useful tools in materials characterization and in the study of diverse phenomena [1-3]. The versatility of these techniques is based on the fact that they look directly at the
The ZnS thin films have been deposited by radio frequency magnetron sputtering at room temperatur... more The ZnS thin films have been deposited by radio frequency magnetron sputtering at room temperature. Post-deposition rapid thermal annealing treatment was done for the films deposited at different powers ranging from 70 to 100 W. One peak is observed for as-deposited and annealed thin films at around 28.48° corresponding to the (111) reflection plane indicating a zincblende structure. The overall intensity of the peaks and the FWHM values of as-deposited films increased after annealing corresponding to the increase in crystallinity. The optical energy bandgap is found in the range of 3.24-3.32 eV. With increasing annealing temperature, the decrease in the Urbach energy values indicating a decrease in localized states which is in good agreement with the XRD results where the crystallinity increased. The surface morphology of the films seems to be composed of Nano-granules with a compact arrangement. Apparently, the grain size increases in the deposited films as annealing temperature increases. The compositional ratio attained close to the stoichiometric ratio of 1:1 after annealing. From the Hall effect measurement, the carrier concentration and mobility are found to increase after annealing. The high carrier concentration and mobility also comply with structural and optical analysis. Best results are found for the film annealed at 400 °C deposited at 90 W.
Magneto-optic phenomena in ferrofluids have been shown to be related to the formation of chain st... more Magneto-optic phenomena in ferrofluids have been shown to be related to the formation of chain structures, due to the arrangement of the ferromagnetic particles, induced by an applied magnetic field. In this work, the effects on transmission of polarized light due to anisotropic effects induced by an external magnetic field in ferrofluids with carbon nanotubes are studied. The time response of the system presents two well defined stages, in the first one, which is very short, the fluid behaves as a polarizer. In contrast in the second stage, the effects of light transmission dominate. In this stage the transmitted light intensity grows with time and after a long time reaches a constant stable value. It is shown that these phenomena depend on the carbon nanotubes concentration as well as on the strength of the applied magnetic field. Using a simple model that considers a chain-like structure formation, it is possible to determine the rate of agglomeration of the formed structures and the attenuation coefficient of the transmitted light. The formation of nanostructures leads to variation in the transmitted light, depending on the polarization of the incident light. These magnetic nanostructures can find numerous applications in nanotechnology, optical devices and medicine.
In this work, the thermal enhancement in silicone grease-based compounds as a function of carbon ... more In this work, the thermal enhancement in silicone grease-based compounds as a function of carbon nanofiber (CNF) volume fraction was investigated. The thermal diffusivity of the samples was determined by a photopyroelectrical technique with a sample thickness scan. The results show that heat transport on these compounds strongly depends on the CNF volume fraction, due to the high thermal conductivity of CNF compared to the matrix; hence, a low loading percentage of the fibers produce a significant growth in the thermal diffusivity of the composite. The results show that the thermal diffusivity values of the CNF-silicone composite are comparable with commercial thermal compounds based on diamond and Ag microparticle fillers. The thermal conductivity of the samples was calculated, and its enhancement was analyzed using a modified Lewis-Nielsen model, taking into account the dependence of the maximum packing fraction and the form factor with the aspect ratio of the CNF. The influence of the Kapitza thermal resistance was discussed. These materials might find practical applications in systems in which the CNF improves the ebbing of heat away from semiconductor devices or in any other application in which heat dissipation is needed.
The thermal conductivity and viscosity of a magnetorheological suspension composed of carbonyl ir... more The thermal conductivity and viscosity of a magnetorheological suspension composed of carbonyl iron particles immerse in silicone oil were studied. Thermal wave resonant cavity was employed to measure the thermal diffusivity of the magnetorheological fluid as a function of an externally applied magnetic field. The dynamic viscosity was also measured and its relationship with the concentration of the particles and the magnetic field strength was investigated. The results show that higher concentrations of carbonyl iron particles as well as higher magnetic field intensities lead to a significant increase in thermal conductivity. The relationship between the thermal conductivity and the dynamic viscosity was explored. Our measurements were examined using an analytical relation between the thermal conductivity and the dynamic viscosity. The results show that by using highly viscous materials, the order induced in the micro particles can be kept for a relatively long time and therefore the increase in thermal conductivity can also be maintained.
In this work, the development of a new crystallization technique is reported, using nitrogen plas... more In this work, the development of a new crystallization technique is reported, using nitrogen plasma (AC) to obtain nanostructured anatase and rutile from amorphous titanium oxide (TiO 2). This methodology increases throughput and minimizes thermal effects. Nanostructured amorphous TiO 2 was obtained by the sol-gel method and subsequently subjected to AC treatment, at a controlled pressure, applying different powers and treatment times in order to obtain phase changes. The obtained samples were characterized using X-ray diffraction (XRD), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). The results show the crystallization in parallel with anatase and rutile phases with a proportion that is directly related to the applied power in the plasma and the treatment time. This technique allows us to obtain smaller crystals in comparison with those of classic thermal methodologies. It is also demonstrated that the application of plasma represents a novel and innovative method to obtain phase polymorphic changes in titanium oxide without needing to apply prolonged heat treatments at high temperatures and can therefore be taken into consideration as a technique with low energy costs, in comparison with conventional heat treatments.
Influence of the thickness of CdS layer on the texture characteristics (morphology and crystallin... more Influence of the thickness of CdS layer on the texture characteristics (morphology and crystalline structure) of the photovoltaic active layer CdS/CdTe was studied. CdS and CdTe films were grown by Close Space Sublimation technique combined with Substrate Rotation (CSSSR). In order to growth the CdTe films, three glass substrates with previously deposited CdS thin film with thicknesses of 106.3, 125.1 and 221.0 nm respectively were used. The conditions of CdTe films deposition were, vacuum pressure of 1x10-6 mTorr, sublimation source temperature 650 o C, substrate temperature 280 o C, source-substrate distance 5.0 mm, substrate rotation 1025 rpm and time deposition 10 min. The CdTe films obtained were with uniform thickness and dimensions of 4.73 m and compacted grain with sizes of 112.5, 108.2 and 140.3 nm respectively. The samples shown high quality crystalline with a preferential orientation in the plane (111), and the CdTe unit cell volume was less than the standard pattern as...
In this work, the study of the heat transfer enhancement induced by aligning iron nanoparticles i... more In this work, the study of the heat transfer enhancement induced by aligning iron nanoparticles in an ethylenglicol fluid matrix is presented. In particular the effect of the interface is studied by analyzing the cases in which the nanoparticles are coated with carbon and comparing with uncoated ones. Results indicate that the coating acts as a thermal barrier making thermal conductivity to decrease. Moreover, the magnetic field creates aligned columns that enhance the heat transfer. Effective models are used to determine the role of the coating as well as of the aligning of the nanoparticles.
We study the thermal conductivity of five kinds of Magnetic Fluids (MFs) by varying the magnetic ... more We study the thermal conductivity of five kinds of Magnetic Fluids (MFs) by varying the magnetic material volume fraction and the direction and intensity of an homogeneous magnetic field: The studied MFs are: Magnetorheological fluids (MRF), carbon coated and uncoated Fe nanofluids (CcFeNF, FeNF), ferrofluis (FF); and two kind of composed fluids; ferrofluids loaded with carbon nanotubes (FFCNTs) and ferrofluids
Thermal properties of solids are obtained by fitting the exact complex photothermal model to the ... more Thermal properties of solids are obtained by fitting the exact complex photothermal model to the normalized photoacoustic (PA) signal in the front configuration. Simple closed-form expressions for the amplitude and phase are presented in all frequency ranges. In photoacoustic it has been common practice to assume that all the absorptions of radiation take place in the sample. However, in order to obtain the accurate thermal properties it is necessary to consider the PA signal contributions produced at the cell walls. Such contributions were considered in our study. To demonstrate the usefulness of the proposed methodology, commercial stainless steel layers AISI 302 were analyzed. It is shown that using our approach the obtained thermal diffusivity and effusivity were in good agreement with those reported in the literature. Also, a detailed procedure for the calculation of the standard error in the thermal properties is discussed.
The effects of the interactions of hydrogen (H) atoms on graphene (G) with potassium (K) pre-adso... more The effects of the interactions of hydrogen (H) atoms on graphene (G) with potassium (K) pre-adsorbed, were predicted by means of first-principles calculations. The results were obtained with the pseudopotentials method and the generalized gradient approximation for the ...
Real time measurement of thermal diffusivity during the evolution of the light curing process in ... more Real time measurement of thermal diffusivity during the evolution of the light curing process in dental resins is reported using photothermal radiometry. The curing is induced by a non-modulated blue light beam, and at the same time, a modulated red laser beam is sent onto the sample, generating a train of thermal waves that produce modulated infrared radiation. The monitoring
The photoactivation time of sol-gel nanostructured titania, in an ethanol suspension, was studied... more The photoactivation time of sol-gel nanostructured titania, in an ethanol suspension, was studied by optical methods in the UV-Vis region. Titania activation was induced using a Hg lamp and the process was analyzed using light transmittance. The Hg lamp light beam was transmitted through the sample and collected with a spectrophotometer as a function of time. The titania powder was thermally treated to observe the effect in the photoactivation. When the photoactivation of the titania suspension occurs, the transmittance falls and afterwards, a constant value is reached. It was found that the activation time depends on the crystalline structure and the crystallite size.
ABSTRACT The thermal wave field in a sample of thickness L and optical absorption coefficient β i... more ABSTRACT The thermal wave field in a sample of thickness L and optical absorption coefficient β is derived and analyzed, by considering the effects of the multiple reflections of a modulated light beam within the sample. By comparing the corresponding amplitude and phase spectra, in presence and absence of those reflections, it is shown that their effects are strongly determined by the product βL. When the thermal wave field is measured at the illuminated surface, those effects appear remarkably on the amplitude and phase in the thermally thin and thermally thick regimes, respectively, and for βL≲2. The deviation of the amplitude spectrum due to the multiple reflections of incident light beam can be as large as 25%. In contrast, when the thermal wave field is measured at the non-illuminated surface, the contribution of the multiple reflections shows up on both the amplitude and phase in the thermally thick regimes, and for βL≲5. These spectra exhibit deviations of about 20% from their corresponding values without the effects of the light reflections. The obtained results show that the accurate measurement of the thermal and/or optical properties of the sample material, based on the amplitude and phase spectra, requires the consideration of those multiple reflections, especially for the cases where βL ≈ 1. The predictions of the proposed approach for the optical absorption coefficient are in reasonable agreement with the results obtained through an independent technique based on a spectrometer.
Combining the well-known Bruggeman theory and Nan et al. results, formulas for predicting the eff... more Combining the well-known Bruggeman theory and Nan et al. results, formulas for predicting the effective thermal conductivity of anisotropic particulate composites with an interface layer are derived. These formulas are valid for a composite material containing arbitrarily oriented ellipsoidal particles with any aspect ratio, and they can be expected to be suitable mainly for large volume fractions, when the thermal interaction between neighboring particles needs to be considered. Results of the present approach are reduced to simpler formulas for some limiting cases in the particle shape. Theoretical analysis of the effective thermal conductivity as a function of volume fraction and shape of the particles is performed. Comparison of the obtained formulas with previously reported experimental data for the effective thermal conductivity is also presented.
Aqueous solutions of agar show a sol-gel transition temperature in the range from 32 • C to 47 • ... more Aqueous solutions of agar show a sol-gel transition temperature in the range from 32 • C to 47 • C depending on colloid concentration; however, the width of the transition and the subsequent process occurring after it, are not well understood. In this work the sol-gel transition of agar is studied by a simple optical method. It consists of the illumination of the sample using a non-coherent light source and the monitoring of transmitted light using a photodiode. It is shown that during the sol-gel transition, the transmitted light decays over a broad range of temperature. Simultaneously, it is possible to observe that fluctuations in the transmitted light decrease along the sol-gel transition, and at a specific temperature, they become very small. Based on these observations, the sol-gel transition temperature for three different concentrations of agar (0.5, 1.5, and 2.5% w/v) is determined. These results are compared with the sol-gel transition temperature values provided by the conventional rheological method.
In this work the formation of thin films by evaporation of liquid mixtures of agglomerated amorph... more In this work the formation of thin films by evaporation of liquid mixtures of agglomerated amorphous untreated fumed silicon dioxide nanoparticles is analyzed. Fumed silica is a material that has been used as a thickening agent due to the fact that it has chain-like particle morphology. When mixed with a liquid, the chains bond together and increase the viscosity. The experiments were performed on samples deposited on a metallic substrate. Thermal waves were generated by sending a modulated laser beam at constant frequency onto the substrate. These waves are propagated to the evaporating solution allowing the monitoring of the film formation. It is shown that the last stage of the film formation is strongly affected by the formation of cracks and migration of the material to the lateral surface of the container. The experimental data are compared with predictions of effective thermal property models. Keywords Evaporation process • Fumed silica • Photoacoustic technique • Time evolution 1 Introduction During recent years photothermal (PT) techniques have been demonstrated to be useful tools in materials characterization and in the study of diverse phenomena [1-3]. The versatility of these techniques is based on the fact that they look directly at the
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Papers by Ruben Medina