G. Montesperelli

Sol—gel processed TiO2-based thin films as innovative humidity sensors

Abstract Active elements for humidity sensors based on alkali-doped and undoped TiO2 films have been prepared by the sol-—gel technique. TiO2-based films are deposited onto Al2O3 substrates with comb-type Au electrodes by dip-coating, from the appropriate solutions, to obtain four different compositions of 1, 3, 6, and 10 at.% Li and K, with respect to the alkali metal/Ti system. The humidity-sensing electrical properties of the films are evaluated using d.c. and a.c. measurements. The electrical characterization of TiO2 films doped with 10 at.% K shows that this material has some unique properties. Resistence versus relative humidity(rh) values show variations as high as seven orders of magnitude in the humidity range tested. Very high humidity sensitivity is also observed at the lowest rh values (4−10% rh), at 40°C, when the resistence is measured at frequencies lower than 1 Hz. The results also show great stability over time. The possibility of modifying the rh sensitivity of TiO2, films doped with 10 at.% K by varying the frequency at which the resistance is measured is a characteristic that allows this material to be described as smart.

Electrochemical Noise Resistance as a Tool for Corrosion Rate Prediction

Abstract Current and potential fluctuations (electrochemical noise [EN]) between two nominally identical carbon steel electrodes were recorded in a solution of sodium phosphate (Na3PO4) at differen...

An EIS study of the humidity-sensitive electrical conduction of alkali-doped TiO2 films

This paper reports the study of the humidity-sensitive electrical properties of 10 at% Li- and K-doped TiO2 films, which have been proposed as active materials for integrated humidity sensors, in comparison with pure TiO2 films. Prototype sensors have been prepared by depositing with a sol-gel technique dense TiO2-based films onto Al2O3 substrates with comb-type Au electrodes. The humidity-sensitive electrical behaviour of the films has been investigated using electrochemical impedance spectroscopy (EIS) at relative humidity (RH) values ranging from 4–87%. The variation of dielectric parameters as a function of frequency and RH has been evaluated to confirm the conduction mechanism. The pure TiO2 films show a moderate variation of the impedance with RH, while the alkali-doped films show a very large decrease in resistance and a nearly constant capacitance with increasing RH. The humidity sensing mechanism for pure TiO2 films is due to the proton hopping between water molecules adsorbed on oxide surfaces. A novel sensing mechanism is proposed for alkali-doped TiO2 films. Alkali ions directly participate in their RH-sensitive conduction, with a different mechanism for K+ and Li+ ions.

Humidity-sensitive electrical properties of MgAl2O4 thin films

humidity control in domestic environments and for industrial purposes is the main cause of the increasing need for electrical humidity sensors [l, 21. The development of humidity sensing elements in thin-film form is dictated by on-chip in- tegration technology, which is the emerging technology in the area of sensor fabrication [3]. In a previous paper, the authors have proposed the use of MgAl,O, thin films as humidity sensors, with promising results [4]. This paper discusses the comparison between ax. and d.c. measurements of the humidity-sensitive elec- trical properties of MgA&O, thin Glms in order to obtain more detailed informations on their conduction mechanism. Experimental procednre The simple resistors used as the basic device for the sensors were prepared depositing on silica substrates thin tihn wires of chromium (200 nm thick). The elec- trodes, 100 pm wide and spaced 40 pm apart, were defined by a standard photolithographic process. Sen- sitive MgAl,O, thin films, about SO nm thick, were deposited on the electrodes by radiofrequency (r.f.) sputtering, using a sintered MgAl,O, target. The electrical response of thin Ghns was analysed by dc. and a.c. measurements at different relative humidity (r.h.) values. Electrical measurements were carried out in a suitable cell where both temperature and humidity were monitored. Monitor@ of r-h. was performed using a commercial sensor (Phys-them, mod. PCRC-II), which gave results accurate to within f2%. 1 V d.c. was applied to the thin tims by using a Keithley quasistatic CV-meter 595, for the period of time nec- essary to reach a steady-state value of the current, at room temperature and r.h. ranging from approximately 0 to 85%, in order to measure the charging current

Humidity sensitivity of sputtered TiO2 thin films

Abstract The humidity sensitivity of sputtered TiO 2 thin films was studied by d.c. analysis techniques. three different sets of specimens were prepared by sputtering in different conditions. The films prepared by reaction sputtering from a Ti target showed a current versus relative humidity (r.h.) sensitivity of 4 orders of magnitude in the r.h. range from 2 to 90%. For these films the current-voltage ( I-V ) characteristics were linear, while films prepared by sputtering from a TiO 2 target and those prepared by the thermal oxidation of sputtered Ti layers showed a back-to-back diode behaviour. The formation of Schottky barriers at the metal/oxide interfaces indicates that these films are semiconductors. Using the charging and discharging processes it was determined that for films prepared by reactive sputtering intrinsic electronic conduction was negligible, while for the other films at low r.h. the conduction carriers were mainly electrons, and protons and electrons at high r.h., protons being the dominant carriers. The films prepared by reactive sputtering showed a slow response time during water adsorption, and a faster response time during water desorption.

The effect of pretreatments with siloxanes on the corrosion resistance of aluminium in NaCl solution

Abstract The inhibitory action of different pretreatments with metacryloxypropylmethoxysilane (MAOS) on the corrosion of aluminium in NaCl aqueous 3.5% solutions has been examined. Free corrosion and electrochemical tests (potentiodynamic polarisation and electrochemical impedance spectroscopy) were performed to characterise the resistance to general and localised corrosion of pretreated specimens. The surfaces of uncorroded pretreated samples were characterised by SEM and AFM. Their surfaces were also examined with XPS technique before and after corrosion attack. It was found that the inhibitory action of MAOS against general and localised corrosion depends on the nature of the solvent in which it is dispersed. The siloxane polymeric film formed by MAOS aqueous dispersion acts as a good corrosion inhibitor in the whole exposure time range, whereas the inhibitory power of the polymeric film formed by MAOS methanol solutions decreases with increasing immersion time.

Study of the conduction mechanism of MgAl2O4 at different environmental humidities

Abstract In this paper, the electrical properties of MgAl 2 O 4 , which has been proposed as a sensitive material for humidity detection devices, are discussed in relation to its microstructure. MgAl 2 O 4 was studied both in bulk and in thin-film forms. Different pellets of MgAl 2 O 4 were obtained by sintering powders prepared by different processes, in order to produce a different microstructure 80 nm thick thin-films were prepared by radio-frequency sputtering on a Si/SiO 2 substrate. Surface area, density and pore-size distribution were measured for the pellets. The microstructure of all the specimens was observed using SEM. The electrical behaviour of the specimens was studied using electrochemical impedance spectroscopy in the frequency range from 10 −2 to 10 5 Hz at relative humidity (RH) values ranging from 5 to 85%. The variation of dielectric parameters, calculated at different frequencies as a function of RH, was investigated to study the conduction mechanism. The humidity-sensitive electrical properties of MgAl 2 O 4 were correllated with its microstructure. In order to determine the conduction carriers, the samples were polarized at 1 V dc and the charging and discharging currents were measured.

Thick films of MgFe2O4 for humidity sensors

In this paper the humidity-sensitive electrical properties of MgFe2O4 thick-films are studied. Thick films were prepared by screen printing on alumina substrates and subsequent firing. Pastes for screen printing were obtained by adding an organic vehicle and Bi2O3, as a sintering aid, to MgFe2O4 powders. The microstructure of the films was varied using MgFe2O4 powders with different grain sizes and by changing the firing temperatures. The microstructure of the films was studied by scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), and Hg porosimetry. The humidity-sensitive electrical properties of thick films were tested by electrochemical impedance spectroscopy (EIS). D.C. measurements were made to test reproducibility and time response, and to study the conduction mechanism. The electrical response of the films was correlated with their microstructure and the distribution of Bi2O3 in the films.

Humidity-sensitive electrical response of sintered MgFe2O4

Pellets of MgFe2O4 were prepared by sintering, at different temperatures, powders prepared either by solid-state reaction between MgO and Fe2O3, or by the thermal decomposition of hydroxide mixtures, co-precipitated from magnesium and iron nitrate solutions with an Mg/Fe ratio of 1:2. Mercury porosimetry, specific surface area measurements and scanning electron microscopy were used in order to determine the main microstructural characteristics of the pellets. Electrochemical impedance spectroscopy (EIS) was used to correlate the humidity-sensitive electrical response of the pellets with their microstructure, in particular with total open porosity and pore-size distribution. EIS measurements showed a close correlation between their relative humidity dependence of the electrical resistance and the microstructure of the sintered bodies. Rather good reproducibility and a fast response time to humidity variations, evaluated from d.c. measurements, were also observed. These properties are also strictly dependent on the microstructure.

Li passivation in different electrolytes during storage and cycling — An impedance spectroscopy study

Abstract A comparison of the Li passivation was made by impedance spectroscopy in four solutions of interest in Li cells, i.e., LiClO4PC, LiClO4PC/DME, LiAsF6EC/2MeTHF and LiAsF6THF/2MeTHF. after some days of storage, usually three semicircles were observed in the impendance plots. They have been assigned to the geometrical characteristics of the passivating film, to the Rct/Cdl parallel, and to the diffusional impedance (Zd), respectively. The main contribution to the interphase impedance comes by the charge transfer process. Higher Rct were observed in the two solutions where Li can be cycled more efficiently, i.e. LiAsF6EC/2MeTHF and LiAsF6THF/2MeTHF. During an extended cycling in a Li//LiClO4PC/DME//Li1+xV3O8 cell, it was noticed that both Rct and Zd markedly increased with cycle number at the Li electrode. The latter was therefore the one mainly responsible for capacity decline and cell failure.