Nanocalorimetric studies of several kinetic phenomena over a wide range of heating rates
- Autores: Alfonso Sepúlveda Márquez
- Directores de la Tesis: Javier Rodríguez Viejo (dir. tes.)
- Lectura: En la Universitat Autònoma de Barcelona ( España ) en 2010
- Idioma: inglés
- Tribunal Calificador de la Tesis: Josep Lluís Tamarit Mur (presid.), Maria Lourdes Vega Fernández (secret.), Juan José Suñol Martínez (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: DDD
- Resumen
To understand essential properties of materials it is necessary to dispose of an experimental countenance with a high accurate thermodynamic measurement system. Custom-made and commercial types of nanocalorimeters are available for different kind of applications. Membrane-based nanocalorimetry is an important tool for thermo-physical analysis since this technique can achieve pJ/K resolution and characterize the presence of possible unknown nano-phases in modern materials. Some new class materials may be complicated to produce in large size samples and can just be available in small quantities or thin films. Examples of attractive applications of novel calorimetric techniques include high-throughput synthesis and screening of thin films and metal hydrides for hydrogen storage, the study of quantum effects and phase transitions in magnetic systems, in nano-scale systems to investigate synthetic and/or organic glasses above room temperature and bellow. Adiabatic conditions become more complicated to be satisfied when the temperature and sample dimension decreases. Accurate measurements of low temperature heat capacities become thus more difficult because small heat fluxes from the surroundings can lead to significant errors. Power compensated calorimetry: Slow heating regime With the goal of measuring the heat released or absorbed during phase transitions occurring in small samples, we have developed a power compensated membrane-based calorimeter that can maintain linear heating rates spanning 1 to 1000 K/s under non-adiabatic conditions. The device works in the intermediate range of heating rates between conventional, ß < 5 K/s, and thin film, ß >10^3 K/s, scanning calorimeters. Active control in real time during heating/cooling experiments is achieved using the NI-7833 FPGA card, which includes a 3M field programmable array with a control loop timer of 20 µs. An improvement of the existing methodology achieved by minimizing control action through the use of predefined temperature profiles is also demonstrated. With ~1µJ energy sensitivity, this is a very sensitive scanning calorimeters working in power-compensation mode. In addition, we highlight the suitability of the thin film membrane-based calorimetric technique to measure kinetically limited phase transitions such as the dehydrogenation of metallic hydrides and provide a first insight into the development of a multiparallel high-throughput screening technique. We analyzed the dehydrogenation reaction in several metallic hydrides (pure Mg, Mg/Al and MgTi) thin films. We determine the influence of the alloy composition on the onset dehydrogenation temperature. Adiabatic Calorimetry: Rapid heating regime Glassy films of organic molecules grown by physical vapor deposition at temperatures slightly below the glass transition temperature behave as stable glasses compared to those cooled directly from the liquid. Higher stability is achieved when depositing at 0.8Tg. By means of nanocalorimetry we can monitor the behavior of the samples and determine their kinetic and thermodynamic stability. Glassy toluene, ethylbezene and water films were directly deposited using a home-made evaporator design in an ultra high vacuum chamber onto the SiNx membrane of the microcalorimetric chips at temperatures above liquid-nitrogen. Calorimetric scans are done in situ with a high sensitivity calorimetric setup with scanning rates up to 10^5 K/s. In quasi-adiabatic treatment ultrafast heating rates are applied for samples from 5 to 100 nm thickness. Thinner films dispose of a less kinetic stability while increasing their thermodynamical stability. Aging a glass will modify its physical properties and approach the sample to more stable configurations. Physical aging effects are erased by heating the glass above its glass transition. The optimal aging temperature will correspond to fictive temperature (Tf) of the corresponding AD film.
