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A ceramic thick film humidity sensor was produced from an emulsion of titania powders by a spin coating technique using a low speed. Electrical measurements were taken between interdigital electrodes obtained by depositing silver paste on the oxide, then cured at 500 • C for 15 min. Different relative humidities of a dynamic atmosphere were obtained by mixing dry and 23 • C saturated synthetic air in convenient proportions. Complex impedance spectra of the titania sensor at various relative humidities (RH) and different temperatures were measured and compared. The humidity sensing behaviour is due to surface water molecules adsorption and capillary condensation. The proposed sensing mechanisms, explaining the registered impedance spectra, are a combination of proton hopping, hydronium electrical drift and diffusion, and electrolytic conduction. In the frequency range 1-400 Hz, resistance and capacitive reactance show variations of three to four-order magnitude over the RH range 10-100%. The curves representing the variations of resistance and capacitive reactance versus RH show clearly the existence of two dominant electrical charge transport mechanisms. A parameter called characteristic humidity is defined to represent the sensitive response of the sensor. It was found that the sensitivity was highly dependent on the frequency. This work also shows that, for the same RH, both resistance and capacitive reactance vary with the atmosphere temperature.

The variation of the electrical signal with humidity in ceramic sensors is originated by the chemical and physical sorptions of water molecules existing in the atmosphere. The aim of the work described in the present paper is to establish an equivalent electrical circuit for the case of two titania thick-film samples. It is shown, at least for the temperature of 23• C, that the same type of circuit represents adequately these two samples for various relative humidities. Chemisorption and physisorption are responsible for the different charge transport mechanisms -ion hopping, ion diffusion and electrolytic conduction. Complex impedance data were obtained at the temperature of 23• C and various relative humidities, in the frequency range 0.1 Hz-40 MHz. The best and simpler circuit representation we found, which gives the best fitting for the Cole-Cole and Bode plots, consists of two RC parallel circuits in series with two constant-phase elements (CPEs). The values of the electrical components are tabled and, as an example, the Cole-Cole and Bode plots fitting obtained for one of our samples, the sample B, for 87.5% RH, in the frequency range 0.1 Hz-40 MHz is shown.

The recently emergent field of Nanotechnology involves the production and use of structures at the nanoscale. Research at atomic, molecular or macromolecular levels, has led to new materials, systems and structures on a scale consisting of particles less than 100 nm and showing unique and unusual physical, chemical and biological properties, which has enabled new applications in diverse fields, creating a multimillion-dollar high-tech industry. Nanotechnologies have a wide variety of uses from nanomedicine, consumer goods, electronics, communications and computing to environmental applications, efficient energy sources, agriculture, water purification, textiles, and aerospace industry, among many others. The different characteristics of nanoparticles such as size, shape, surface charge, chemical properties, solubility and degree of agglomeration will determine their effects on biological systems and human health, and the likelihood of respiratory hazards. There are a number of new studies about the potential occupational and environmental effects of nanoparticles and general precautionary measures are now fully justified. Adverse respiratory effects include multifocal granulomas, peribronchial inflammation, progressive interstitial fibrosis, chronic inflammatory responses, collagen deposition and oxidative stress. The authors present an overview of the most important studies about respiratory nanotoxicology and the effects of nanoparticles and engineered nanomaterials on the respiratory system.