Microreactors for Thin-Film Calorimetry
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MICROREACTORS FOR THIN-FILM CALORIMETRY J. Rodríguez-Viejo, M.Chacón$, A.F.Lopeandía, M.T. Clavaguera-Mora, Leonel R. Arana+, K.F. Jensen+ Grupo de Física de Materiales I. Dep. Física, Universidad Autónoma de Barcelona, 08193 Bellaterra, Spain. + Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. ABSTRACT We have designed and developed a calorimeter that is capable of measuring the heat released by ultrathin films in the temperature range from 77 to 1050 K. Semiconductor processing techniques are used to fabricate the microreactors. The symmetric design of the Pt heaters in the microreactor channel provide a good temperature profile across the active region of the membrane making it suitable for accurate calorimetric measurements. The low thermal mass of our system allows for a high sensitivity. The effective heat capacity of the microcalorimeters with a 200 nm SixNy membrane is 0.14 µJ/K at room temperature. In high vacuum heating rates of 2x106 K/s have been achieved. Under these conditions the microcalorimeter works as an adiabatic scanning calorimeter and therefore heat capacity is directly obtained from the input power. A thermal characterization of the microcalorimeters in the transient state and calorimetric measurements on indium thin films and films made of CdSe nanocrystals are briefly discussed to show the potentiality of the microreactors. INTRODUCTION Many materials of current fundamental or technological interest are mainly made in thin film form. A critical step towards understanding thin film reaction is to characterize their thermodynamic and kinetic parameters. Pioneering work in this direction was given by Spaepen and Thomson [1] performing calorimetric studies of reactions in thin films and multilayers. Up to recent years, differential scanning calorimetry (DSC) has primarily been used on bulk samples, since in typical thin films, the amount of energy released by a given reaction is too small to be measured and analyzed with accuracy. Thin films can also be measured if they can be grown thick enough and if the film can be removed from the substrate to improved the signal-tonoise ratio. A significant effort during last years is being devoted to build sensitive calorimeters to measure phase transitions in thin film system [2,3]. Microfabrication associated to Silicon technology has paved the way to the development of miniature reactors, through the use of thin film heaters and thermometers and recent advances in membrane technology [4]. Allen et al. $
Permanent address: Universidad del Valle, Departamento de Física, A.A. 25360, Cali, Colombia
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have shown these microsystems can work as adiabatic calorimeters with a very high sensitivity because of their very low thermal mass [5]. The high sensitivity of these calorimeters enables the determination of undetected phenomena in conventional macroscopic systems. In this paper we present our recent efforts to build up a calorimetric setup to explore phase transformations in thi
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