The different types of interactions and chemical reactions that occur during any physicochemical processes inevitably involve an exchange of energy in the form of heat. Those changes offer a great amount of information about the internal behaviour of the systems under study, reaction mechanisms, feasibility of a given chemical reaction, etc. Therefore, it is important to design devices that allow a rapid and accurate detection of this energy, using as little sample as possible.
In this project we evaluated the suitability of the Nernst effect for rapid detection of the heat power released during physicochemical processes, for applications in microcalorimetry. We use a permalloy Ni80Fe20 sensor (28 micron X 4 micron X 20 nm) sandwiched between a top/bottom heater, to mimic the effect of an exothermic/endothermic process on the sensor. The tested devices show a sensitivity in the order of micro-Watt and response times in the order of miliseconds. These values can be further optimized by improving the geometry and the thermal contacts of the sensor.
Currently, calorimetric detection of physicochemical processes is done by Isothermal titration calorimetry, ITC. This technique allows the quantitative determination of the thermodynamic association constants and their corresponding energies (enthalpy and entropy).
The ITC temperature sensors are based on classic Peltier modules, made up of semiconductor p/n pairs, connected electrically in series for increasing the output voltage under the presence of a perpendicular thermal gradient.
The idea explored in this project is to use the large Nernst effect observed in ferromagnetic conductors to detect the temperature gradient produced by such processes with a high spatial and temporal resolution. Unlike the Peltier modules, the sensor in a Nernst module is made of only one material, which offer great advantages with regard to the miniaturization capabilities. This further allows the scalability during manufacturing processes using standard microfabrication methods, widely used in the industry of microelectronic components.
The main results of the project are:
i) We have shown that Nernst-based devices fabricated from ferromagnetic metallic alloys, show a sensitivity of the order of micro-Watts and time response of the order of millisecond.
ii) The ductility of the sensors makes them appropriate for deposition on irregular or curved surfaces, as well as on plastic substrates.
iii) The signal/noise ratio can be improved by increasing the area of the device, while keeping the micron^2 size.
iv) Their small thermal mass decreases the time-response to a thermal pulse, making them suitable for monitoring of fast reactions.
v) The possible metallic magnetic alloys that show ANE and that could be studied for an improved response is enormous.
vi) The ANE is a thermoelectric voltage, and therefore these devices can substitute a Peltier module in a conventional ITC calorimeter without important changes of design.
