Estudo da difusividade térmica de materiais em pó utilizando a célula fotoacústica aberta (OPC) e seu comportamento na liga magnetocalórica Gd5,09Si2,03Ge1,88

• Ano de defesa: 2011
• Autor(a) principal: Mariucci, Vinícius Vaulei Gonçalves
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Estadual de Maringá; Brasil; Programa de Pós-Graduação em Física; UEM; Maringá, PR; Departamento de Física
• Palavras-chave em Português: Fotoacústica; Magnetocalórica; OPC; Material em pó; Difusividade térmica.; Ciências Exatas e da Terra; Física
• Link de acesso: http://repositorio.uem.br:8080/jspui/handle/1/2686

This dissertation proposes the use of the technique of open photoacoustic cell (OPC) to study the thermal diffusivity of powder materials. The samples were stored in small sample holders (racks) in the form of a hard cast, especially made for powder materials. Mating a Peltier device to OPC, it was possible to control the temperature of the samples. The determination of the diffusivity as a function of temperature allowed the detection of phase transitions of first and second order for the magnetocaloric alloy Gd5, 09Si2, 03Ge1,88, with grain sizes between 45 and 63 micrometers. By heating the powder from -11ºC to -2ºC, the first order transition occurred around -7°C and - 6.5 °C, and by cooling from 50°C to 0ºC, the second order transition was detected at 27ºC, both reported in the literature. Still, for the macerated Gd5,09Si2,03Ge1,88 alloy were measured diffusivity as a function of particle size for a series of four racks at a constant temperature of 25°C. Each rack was filled with a different size, ranging from smaller grains sizes up to 25 micrometers between 45 and 63 micrometers. With the attainment of thermal diffusivity as a function of grain sizes, it was found that the diffusivity increases linearly with increasing particle size. Besides the experimental techniques, is presented a theoretical model for OPC using a sample of three layers. The purpose of the model is to describe the variation of photoacoustic signal as a function of optical frequency modulation as for a few samples such as: 316L stainless steel, germanium, silicon, and silica glass. Theoretical considerations of this initial model did not take into account properties such as thermal resistance at the interfaces between the contacts of the sample layers, or characteristics existing within the volume of powder, such as porosity, grain shape, amount of fluid phase and solid phase, and other characteristics that may influence the propagation of heat. Such considerations are left to be discussed in future works. Investigations on the thermal properties of powder materials and the magnetocaloric alloy showed that the OPC technique is not limited only to homogeneous samples (such as thin films and laminate) and could be adapted and applied at low cost to different types of materials, including porous samples.