Estudo de dióxido de estanho nanoestuturado aplicado em nanosensores de gás

Trabalho de Conclusão de Curso (graduação)—Universidade de Brasília, Faculdade UnB Gama, Curso de Engenharia de Energia, 2015.Sensores de gases têm grande influência em muitas áreas importantes, como o monitoramento ambiental, segurança social e pública. O monitoramento de gases e a detecção de sua composição, têm recebido especial atenção. O dióxido de estanho (SnO2), dopado ou não com elementos de transição, vem sendo preparado por diferentes técnicas e empregado em células solares, dispositivos óptico-eletrônicos, displays de cristal líquido, catalisadores e sensores de gases. Neste trabalho nanosensores de SnO2, dopados com Pr e Tb (terras raras), foram eletricamente testados na presença dos gases GNV, GLP e CO2. Para tanto, nanopartículas de SnO2 foram sintetizadas pelo método dos precursores poliméricos e dopados com sais de Pr e Tb nas concentrações de 3, 5 e 10 mol%. As amostras sintetizadas foram caracterizadas por Difração de Raios X (DRX), Espectroscopia de Infravermelho com Transformada de Fourier (FTIR), Espetroscopia Raman e área superficial específica analisada pelo método método de Brunauer, Emmett e Teller (BET). Diante dos resultados foi possível verificar que o uso das terras raras como dopantes aumentou a superfície de contato do SnO2, fazendo as partículas diminuírem de 34 nm para 8 nm com o aumento da concentração dos dopantes. Os gases utilizados foram coletados e submetidos a FTIR. Por fim, as nanopartículas foram depositadas sobre substrato de alumina usando o método painting coating e em sequência caracterizadas eletricamente, pelo equipamento B1500A Semiconductor Device Analyzer da Agilent, com medições das curvas IxV, para determinar o tipo de sensor e Rxt para verificar o comportamento resistivo dos sensores à temperatura ambiente na presença dos gases de teste. Melhoras significativas nas propriedades dos sensores foram observadas nas amostras dopadas. Podem-se destacar melhorias nas propriedades de seletividade, sensibilidade, tempo de resposta e de recuperação para os testes de variação de condutância realizados com os gases de interesse. Contudo, as mudanças não foram significativas para que os sensores possam operar à temperatura ambiente já que a condutividade foi baixa e a resistência elevada.Gas sensors have great influence in many important areas, such as, environmental monitoring, social and public security. The gases monitoring and the detection of its composition have received a special attention. The tin dioxide (SnO2), doped or not with transition elements has been prepared by different methods and employed in solar cells, optical-electronic devices, liquid crystal displays, catalysts and gas sensors. In this work, nanoparticles SnO2, doped with Pr or Tb (rare earth) were electrically tested for gases CNG, LPG and CO2. For this, nanoparticles of SnO2 were synthesized by Pechini method and doped with salts of Pr or Tb at concentrations of 3, 5 e 10 mol%. The synthesized samples were characterized by X-Ray Diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and specific surface area analyzed by Brunauer, Emmett e Teller (BET) method. From the results it was possible to verify that the use of rare earths as doping increased the contact surface of SnO2, causing the particles to decrease from 34 nm to 8 nm with increasing doped concentration. The used gases were collected and analyzed by FTIR. Finally, the nanoparticles were deposited on alumina substrate using the method painting coating and in sequence electrically characterized by the equipment B1500A Semiconductor Device Analyzer of Agilent to curve measurements IxV, to determine the type of sensor and Rxt to check the resistive behavior of the sensors at room temperature in the presence of the test gases. Significant improvements were observed on sensors made with doped samples. It may be noted improvements on selectivity and sensitivity properties, response and recovery time for conductance variation tests with the gases of interest. However, the changes were not significant so that the sensors could operate at room temperature, as the conductivity was low and the resistance high

Characterisation of microbial attack on archaeological bone

As part of an EU funded project to investigate the factors influencing bone preservation in the archaeological record, more than 250 bones from 41 archaeological sites in five countries spanning four climatic regions were studied for diagenetic alteration. Sites were selected to cover a range of environmental conditions and archaeological contexts. Microscopic and physical (mercury intrusion porosimetry) analyses of these bones revealed that the majority (68%) had suffered microbial attack. Furthermore, significant differences were found between animal and human bone in both the state of preservation and the type of microbial attack present. These differences in preservation might result from differences in early taphonomy of the bones. © 2003 Elsevier Science Ltd. All rights reserved