G. Gusmano

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.

The mechanism of MgAl2O4 spinel formation from the thermal decomposition of coprecipitated hydroxides

Abstract The thermal behaviour of coprecipitated hydroxide mixtures leading to the synthesis of MgAl 2 O 4 spinel powders has been investigated. The mixtures of hydroxides have been prepared by coprecipitation reaction from nitrate solutions with a molar ratio for Mg/Al of 1:2. The results have been related to the thermal behaviour of separately precipitated components of the hydroxide mixtures. The different steps in the thermal decomposition have been characterized via X-ray diffractometry (XRD). A mechanism for the spinel formation from the thermal decomposition of the hydroxide mixtures has been hypothesized.

Core–shell Zn-doped TiO2–ZnO nanofibers fabricated via a combination of electrospinning and metal–organic chemical vapour deposition

Zn-doped TiO2 nanofibers shelled with ZnO hierarchical nanoarchitectures have been fabricated combining electrospinning of TiO2 (anatase) nanofibers and metal–organic chemical vapor deposition (MOCVD) of ZnO. The proposed hybrid approach has proven suitable for tailoring both the morphology of the ZnO external shell as well as the crystal structure of the Zn-doped TiO2 core. It has been found that the Zn dopant is incorporated in calcined electrospun nanofibers without any evidence of ZnO aggregates. Effects of different Zn doping levels of Zn-doped TiO2 fibers have been scrutinized and morphological, structural, physico-chemical and optical properties evaluated before and after the hierarchical growth of the external ZnO shell over the electrospun nanofibers. Moreover, doping promotes the incipient transition from the anatase to rutile phase in the core–shell Zn-doped TiO2–ZnO nanostructures at lower temperature than that observed for pure TiO2. Finally, the present core–shell hierarchical nanofibers possess a very large surface to volume ratio and exhibit a marked cathodoluminescence with a strong UV and visible emission.

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

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.

Effect of surface modification on NO2 sensing properties of SnO2 varistor-type sensors

NO2 sensing properties of SnO2-based varistor-type sensors have been investigated in the temperature range of 400-650 °C and in the NO2 concentration range of 15-30 ppm. Pure SnO2 exhibited a weak nonlinear I-V characteristic in air, but clear nonlinearity in NO2 at 450 °C. The breakdown voltage of SnO2 shifted to a high electric field upon exposure to NO2 and the magnitude of the shift was well correlated with NO2 concentration. Thus, SnO2 exhibited some sensitivity to NO2 as a varistor-type sensor. When SnO2 particles coated with a SiO2 thin film were used as a raw material for fabricating a varistor, the breakdown voltage in air was approximately the double that of pure SnO2 and the sensitivity to 15 ppm NO2 was enhanced slightly. However, the sensitivity to 30 ppm NO2 decreased. The Cr2O3-loading on SnO2 also led to an increase in the breakdown voltage in air, but the Cr2O3 addition was not effective for promoting the NO2 sensitivity under the present experimental conditions.

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.

Chemical synthesis and sintering behaviour of highly dispersed W/Cu composite powders

The paper reports the preparation of W/Cu composite powders by a wet process based on the reduction of selected copper precursors in ethylene glycol and in the presence of tungsten powders. Reactions were performed in different conditions of temperature, time and concentration of the copper precursor. Two different Cu compounds, Cu(AcO)2·H2O and CuO (coarse or fine) and two W powders (coarse or fine) were used. The reaction yields ranged from 75% to 98%. Dense bodies (up to 97% fractional density) with highly homogeneous microstructure as well as high electrical conductivity (up to 41% IACS) were obtained by sintering W/Cu powders at 1350 °C.

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.