High Accuracy Photopyroelectric Calorimetry for Magnetic Nanofluids

           The main purpose of this project is the use of a high accuracy photopyroelectric (PPE) calorimetry to study thermal properties of magnetic nanofluids and their changes as a function of the relevant parameters of the nanofluid (carrier liquid, surfactant, type, size and concentration of nanoparticles). The research is focused on the water-based nanofluids with Fe3O4, gamma-Fe2O3 and CoFe2O4 type of nanoparticles, covered with polymer as stabilizer. Through the PPE calorimetry one can obtain with high accuracy all static and dynamic thermal parameters (by directly measuring two, the fundamental ones: thermal diffusivity and effusivity) of a liquid sample. The main originality of this project consists in the fact that, due to the increase of the sensitivity of the PPE investigations, it is possible to study, through calorimetric techniques, intimate structural processes, associated with small variations of the thermal parameters, as it is specific to magnetic nanofluids too. Water-based magnetic nanofluids are currently investigated in order to develop new magnetically controlled drug delivery systems and magnetic separation methods of biomaterials. The project is organized in two main stages: (i) the increase of the performances of the PPE calorimetry (through theoretical and experimental optimizations) up to an accuracy and reproducibility of 96% and (ii) the study of the thermal properties of the magnetic nanofluids as a function of the relevant parameters of the nanofluid.

           Photothermal Methods. Principle and Resulting Information. The photothermal (PT) experiments can be performed both in time or frequency domain regime. The main information obtained with PT techniques are: - thermal properties (specific heat, thermal conductivity, diffusivity and effusivity, coefficients of thermal exchange, etc); - optical properties (surface and volume optical absorption coefficient, reflexion and diffusion coefficients, etc); - quantum properties (fluorescence and radiation to heat conversion efficiency). In the photopyroelectric (PPE) technique, the temperature variation of a sample is directly measured by using a pyroelectric sensor situated in thermal contact with the sample. The values and the dependence of the thermal parameters as a function of temperature, time and composition are connected with associated processes (ex: phase transitions), or can lead to structural information.

           The Performances of PPE Method.The performances of the method depend on: the sensor's detectivity, the precision in monitoring the main experimental parameters (chopping frequency, sample's thickness control, temperature, etc), the quality of the sensor-sample thermal contact, the way of performing the acquisition and processing of experimental data. Some typical parameters are: the detectivity of the pyroelectric LiTaO3 sensors is higher than 108 cm Hz1/2/W; the precision in monitoring the main experimental parameters: minimum detectable temperature variation: 1 μK; minimum controllable temperature variation rate: 100 mK; signal processing: performed with digital lock-in nanovoltmeters (S/N better than 102-103); sensor-sample thermal contact: perfect in the case of  liquid samples (as nanofluids are); data acquisition and processing: performed with optimized software. All these performances lead to an accuracy and reproducibility of the method about 80-90%; we hope in an improvement through the present project up to 95- 96%. Other particularities of the method: - the necessary quantity of sample: 0.2-0.3 ml; no preliminary processing of the sample required (the method is non-destructive); the pyroelectric sensors react only to the temperature variation, and the signal processing is selective, consequently, no thermostatic precautions are required.

           Magnetic Nanofluids. Ferrofluids represents a special category of smart nanomaterials, consisting of stable dispersions of magnetic nanoparticles in different liquid carriers. Stabilization of ferrofluids implies various procedures depending on the nature of the liquid. The most stable ferrofluids are known among those based on organic non-polar solvents, in which case the presence of a single layer of surfactant on the surface of magnetic nanoparticles is enough to avoid the particles aggregation. Stabilization of ferrofluids based on polar carriers requires a double layer stabilization method, involving a primary layer chemisorbed on the particle surface and a secondary surfactant of the same or another type, physically adsorbed on the previous layer.  We point out that biological applications of ferrofluids require stable polar ferrofluids especially based on water.
The surface properties of the magnetic nanoparticles, the magnetic component of magnetic nanofluids, may be tailored by modifying their surface coating in order to meet the requirements of magnetic separation and recovery of target bio-molecules. Water based magnetic nanofluids are currently investigated to develop new magnetically controlled drug delivery systems and magnetic separation methods of biomaterials. Various separation processes and drug delivery systems require magnetic nanofluids with organic liquid or especially water as carrier liquid. Their magnetic properties, as well as their structural and flow behaviour have to be tailored to fulfil the requirements of the process envisaged.
           If the magnetic properties of the magnetic nanofluids are the subject of an intensive study, there are few data in literature concerning their thermal properties. Normally, the thermal properties of the magnetic nanofluids must depend on the composition of the nanofluid: type size and concentration of nanoparticles, type of surfactant and carrier fluid. Even more, the values of the thermal parameters and their temperature behaviour are correlated with structural changes and with the dynamics of the processes (drug delivery, for example) occurring inside the nanofluid.
On the other side, the few calorimetric data from literature indicate sometimes drastic changes (as a function of nanoparticles’ size) of the thermal conductivity of some types of nanofluids (Cu nanoparticles or carbon nanotubes immersed in ethylene glycol, for example). The lack of thermal data concerning magnetic nanofluids can be due in principle to the fact that the classical calorimetric techniques don’t fit properly to nanofluids: they are time consuming, not always very precise, usually they need calibration and a large quantity of sample. Even more, any classical calorimetric technique is able to measure only one thermal parameter, usually the static one (specific heat), or the thermal conductivity. Through the PPE calorimetric one can obtain all static and dynamic thermal parameters (by directly measuring two). PPE is able to directly measure the “fundamental“ thermal parameters (contained in the thermal diffusion equation), the thermal diffusivity and effusivity, and not the “derived” ones, thermal conductivity and specific heat. The main originality of this project consists in the fact that, due to the increase of the sensitivity of the PPE investigations, it is now possible to study, through calorimetric techniques, intimate structural processes, associated with small variations of the thermal parameters, as it is probably specific to magnetic nanofluids too.

           International Context. The development of the photothermal (PT) methods started in the mid '80s, due to the facilities offered by laser and radiation detection techniques. The discussion forum of the international scientific community in the field is the "Photothermal and Photoacoustic Phenomena Conference" (IPPPC), where our group participated regularly since 1987. Since 1993 a Gordon type conference is organized each year, on the same topic. There are several tens of groups presently dealing with photothermal techniques. Concerning the development of this field of research, after passing through the stages of experimental demonstrations and fundamental theory, the PT techniques are presently looking for the optimization of the experimental configurations, with the purpose of quantitative analysis of the thermo-optical properties of materials. The compatibility of the PT techniques with a large class of materials has been proved. It includes homogeneous materials with well defined properties, as well as non-homogeneous products with complex structure as composite materials, agricultural and biological products, drugs, etc.

           National Context. There is no exhaustive theoretical and experimental approach of the PT methods. The interest in the field started at National Institute for R&D of Isotopic and Molecular Technologies - INCDTIM Cluj-Napoca in 1984, when some PPE spectroscopic experiments were set up for the first time, simultaneously and independently from other groups from IBM, USA and University of Toronto, Canada. At my knowledge, there is only one other group in Romania (IFTAR Bucharest) dealing with PT techniques. The group from INCDTIM developed a general theoretical model and applied the method to optical and thermal characterization of various solid and liquid samples. Among the most important original contributions of the group, we mention:  the general theory of the PPE effect; PPE spectroscopy of transparent and absorbent liquids; PPE detection of structural, magnetic, ferroelectric and glassy transitions; PPE measurement of static (specific heat) and dynamic (thermal conductivity, diffusivity and effusivity) thermal parameters of condensed matter. Our group kept a continuous contact with the groups from abroad, active in the field. We have to mention some research studies performed through projects financially supported by EU (see the CV of the director of project) and bilateral cooperation with universities as Wageningen Agricultural University, Holland, University of Reims and Dunkerque, France, 2-nd University Rome, Italy, Catholic University Leuven, Belgium, Campinas University, Brazil.         

           Method and approach. We estimate a research developed in 4 stages (see the working plan), and following 3 main aspects:The first aspect will be a theoretical one, when the theoretical concepts will be elaborated. The most suitable detection configurations for investigating the thermal diffusivity and effusivity of nanofluids will be selected. The theoretical analysis will be focused on the possibilities offered by the two PPE detection configurations (front and back). We will select only those particular PPE cases that can be experimentally adapted to nanofluids and, additionally, allow for optimization.
- The second aspect is mainly experimental when, based on the theoretical criteria of optimization, we will set up the measurement lines; this means in fact to adapt the existing calorimetric setups to the requirements imposed by nanofluids investigation and to re-design the detection cells. Among the estimated optimizations, some concern  “fundamental” changes in the PPE detection practice: the use of the phase of the PPE signal (instead of the amplitude) for collecting the information (in this way, the “noise” associated with the fluctuations of the radiation source intensity will be eliminated), the use of the sample’s thickness (instead of chopping frequency) as a scanning parameter, a control of the thickness variation with a 30 nm step, “interactive” software for data acquisition, optimization of the fitting programs.  Consequently we estimate an increase of the accuracy of the measurements up to 95-96%. Both thermal diffusivity and effusivity will be analyzed; the theoretical and experimental optimizations will be correlated, the research being extended over one year period, for each parameter. The remaining thermal parameters (thermal conductivity and specific heat) can then be easily deduced with similar precision. In the same period, the first measurements on magnetic nanofluids will be started.
- During the last stage of the research, the study of thermal properties of the magnetic nanofluids and their changes as a function of the relevant parameters of the nanofluid (carrier liquid, type of surfactant, type, size and concentration of nanoparticles) will be performed. The research will be focussed on the water-based nanofluids with Fe3O4, γ-Fe2O3 and CoFe2O4 type of nanoparticles, covered with polymer as stabilizer of the colloid. If the temperature dependence of the thermal parameters will indicate some phase transitions, they will be also studied.