Mario Pelino

NO2 sensitivity of WO3 thin film obtained by high vacuum thermal evaporation

Abstract The gas sensitivity, selectivity and stability properties of WO 3 thin films for the detection of NO 2 gas in the concentration range 0.2–5 ppm, have been evaluated and discussed in the light of the preparation conditions and working temperature. Thin films were obtained by evaporating high purity WO 3 powder by an electrically heated crucible at about 5 × 10 −4 Pa on sapphire substrates provided with Pt interdigital type sputtered electrodes and annealed for 1 h at 400, 500 and 600°C. The film morphology, crystalline phase and chemical composition were characterised through AFM, low angle XRD and XPS. The electrical response was measured by means of DC current mode. The annealed films showed crystallographic orientation belonging to the triclinic structure of WO 3 , while the as-deposited films were found to be amorphous. The binding energies of O 1s and W 4f confirmed the existence of the WO 3 phase, with a stoichiometric ratio close to the theoretical one. All the films showed the highest sensitivity to NO 2 at a working temperature of 200°C. The 500°C annealed film was found to be the most sensitive to NO 2 gas, compared to those annealed at 400 and 600°C. No cross sensitivity effects were found by exposing the sensors to CO, CH 4 . WO 3 films showed strong sensitivity to C 2 H 5 OH and H 2 O. Long term stability test at a working temperature of 350°C, performed by cycling the films in dry air and 5 ppm NO 2 revealed no substantial change in the electrical properties in terms of drift and sensitivity.

Chemical durability of glasses obtained by vitrification of industrial wastes

The vitrification of zinc-hydrometallurgy wastes, electric arc furnace dust (EAFD), drainage mud, and granite mud was shown to immobilize the hazardous components in these wastes. Batch compositions were prepared by mixing the wastes with glass-cullet and sand to force the final glass composition into the glass forming region of the SiO2-Fe2O3-(CaO, MgO) system. The vitrification was carried out in the 1400-1450 degrees C temperature range followed by quenching in water or on stainless steel mold. The United States (US) Environmental Protection Agency (EPA) toxic characterization leaching procedure (TCLP) test was used as a standard method for evaluating the leachability of the elements in the glasses and glass-ceramics samples made with different percentages of wastes. The results for EAFD glasses highlighted that the chemical stability is influenced by the glass structure formed, which, in turn, depends on the Si/O ratio in the glass. The chemical durability of jarosite glasses and glass-ceramics was evaluated by 24 h contact in NaOH, HCl and Na2CO3, at 95 degrees C. Jarosite glass-ceramics containing pyroxene (J40) are more durable than the parent glass in HCl. Jarosite glass-ceramics containing magnetite type spinels (J50) have a durability similar to the parent glass and even lower in HCl because the magnetite is soluble in HCl.

Cross sensitivity and stability of NO2 sensors from WO3 thin film

The H2O, C2H5OH, CO, CH4, NO and SO2 cross sensitivity to NO2 gas, as well as the long term stability of the electrical response of WO3 thin films have been evaluated and discussed in the light of different preparation conditions and working temperatures. Thin films have been obtained by evaporating high purity WO3 powder at 5 × 10−4 Pa on sapphire substrates provided with Pt interdigital sputtered electrodes and annealed at 500°C for 6, 12 and 24 h. The film morphology, crystalline phase and chemical composition have been characterized through AFM, glancing angle XRD and XPS. The as-deposited film is amorphous with WO3 stoichiometry on the surface, after annealing at 500°C the films are well crystallised but with preferential orientation of WO3 along the (200) plane. The increasing of the annealing time shows a positive effect on the crystallite and grain size of the film, while the mean roughness and surface area difference slightly decrease. The binding energies of the annealed films are close to that of WO3 and small downshifts from the characteristic binding energy of W 4f72 reflects the formation of oxygen vacancies on the longer time annealed films. All the films show the highest sensitivity to NO2 gas (0.7–5 ppm concentration range), at 250°C working temperature. At this temperature and 1.7 ppm NO2 the calculated sensitivities yield S = 12, S = 43 and S = 45 for 6, 12 and 24 h annealed films, respectively. No cross sensitivity has been found by exposing the WO3 films to CO and CH4. Negligible H2O cross to NO2 has resulted for the 24 h annealed film in the 40–80% relative humidity range, as well as to 300 ppm SO2 and 10 ppm NO. Only 1000 ppm C2H5OH has resulted in a significant cross to the NO2 measure. The increase in the annealing time had positive effects on the sensitivity, cross sensitivity and long term stability properties. The 45-fold increase in the resistance of the 24 h annealed on exposure to 1.7 ppm of NO2, as well as the good long term stability properties of its electrical response, suggest the possibility of utilising the sensor for air-quality monitoring.

Crystallization phenomena in iron-rich glasses

Abstract In this study, results of the crystallization of iron-rich glasses are summarized. Thermogravimetry (TG)–differential thermal analysis (DTA) were utilized to explain the phase formations and the surface oxidation of FeO to yield Fe2O3. The crystal phases fraction was evaluated utilizing X-ray diffraction analysis (XRD). Low angle XRD technique was used to investigate the distribution of the crystal phases on the surface and in the bulk as a function of the heat treatment. Transmission electron microscopy (TEM) was employed to detect the evolution of the crystalline structure and to determine the variation of the residual glass composition. The crystallization kinetics were investigated in isothermal conditions by measuring the variation of the density. The activation energy of crystal growth was calculated using isothermal and non-isothermal methodologies. The values of 377 and 298 kJ/mol were obtained for the temperature ranges 620–660°C and 720–780°C, respectively. Similar values, 368 and 321 kJ/mol, were estimated for the energy of viscous flow in the same temperature ranges. The results indicate that magnetite and pyroxene are the main crystal phases and that the kinetics of pyroxene formation can be explained as growth on a fixed number of magnetite nuclei. In powder samples, heat-treated in air, the crystallization is inhibited by the surface oxidation of Fe2+ to yield Fe3+ and a layer of haematite is formed on the surface.

Vitrification of electric arc furnace dusts.

Electric arc furnace baghouse dust (EAFD), a waste by-product of the steelmaking process, contains the elements that are volatilized from the charge during the melting (Cr, Pb, Zn, Cu and Cd). The results of leaching tests show that the concentration of these elements exceeds the regulatory limits. Consequently, EAFD cannot be disposed of in ordinary landfill sites without stabilization of the heavy metals. In this work, the vitrification of EAFD, from both carbon and stainless steel productions, were studied. The vitrification process was selected as the inertizing process because it permits the immobilization of the hazardous elements in the glass network and represents an environmentally acceptable method for the stabilization of this waste. Classes of various compositions were obtained by mixing EAFD with glass cullet and sand. The EAFD and the glass products were characterized by DTA, TG, X-ray analysis and by the TCLP test. The results show that the stability of the product is influenced by the glass structure, which mainly depends on the Si/O ratio. Secondary crystallization heat-treatment were carried out on some samples. The results highlighted the formation of spinel phases, which reduced the chemical durability in acid media. The possibility to recover Zn from carbon steel production EAFD was investigated and about 60-70% of metal recovery was obtained. The resulting glass show higher chemical stability than glasses obtained without metal recovery.

Microstructural effect on NO2 sensitivity of WO3 thin film gas sensors Part 1. Thin film devices, sensors and actuators

Abstract Microstructures of thermally evaporated WO3 thin films on sapphire substrates are investigated by wide-angle X-ray diffraction, atomic force microscope and X-ray photoelectron spectrum. The as-deposited film is amorphous and crack-free with WO3 stoichiometry on the surface. After annealing at above 400 °C, the film is crystallized. Compared to the monoclinic phase of the starting WO3 powder, fewer peaks are evident at room temperature for the annealed film. This highlights that the film grown on the sapphire has a preferential orientation of WO3 (200), probably because of the atomic arrangement similar to the sapphire. The crystallite sizes are estimated from the major peak to be 17.5–23.9 nm according to the Scherrer equation. The increasing of annealing temperature exhibits positive effects on the surface roughness or fractal dimension, surface area and grain size of the film. However, abnormal increments in the topographical parameters occur in case of annealing at 600 °C. The binding energy of the annealed film is close to that of WO3, and a small downshift of 0.1 eV reflecs the formation of oxygen vacancies on the surface. The heterogeneity parameter of the film is introduced into the Schottky barrier equation. The highest sensitivity of the 500 °C-annealed sensor is explained in terms of the annealing temperature effect on the geometrical and chemical heterogeneities.

Evaluation of the degree of crystallisation in glass-ceramics by density measurements

A theoretical procedure has been developed with the aim of obtaining an accurate and rapid estimation of the degree of crystallisation in jarosite glasses by measuring the densities of the parent glass and, after the thermal treatment, of the glass-ceramic. Compositions were prepared by mixing jarosite waste with granite scraps and glass cullet, yielding three different glass-batchs, melted at 1450°C, quenched and thermal treated to obtain glass-ceramics with different degrees of crystallisation. Density measurements were carried out by means of a helium displacement pycnometer. The results were compared with the degree of crystallisation as evaluated by XRD and showed a good agreement. The method is applicable if the variation of density connected to the phase transformation is sufficiently large. In the formation of pyroxene and magnetite phases, with large density variation compared to the parent glass, it is precise and easily applicable.

New materials from industrial and mining wastes: glass-ceramics and glass- and rock-wool fibre

Abstract The need to find economically and environmentally more viable methods for the management of industrial wastes has opened the way to the research and development of processes which were still in the laboratory only a few years ago. The examples given concern two very common and acutely-felt problems: the inertization and recycling of hydrometallurgical red muds (RM) and of mine tailings. In the case of red mud, one potential process involves the extraction of metals (mainly Zn, but also Cd, Sn, Ph, Fe), and another involves mixing the RM with granite chippings and cullet to produce glass-ceramics materials via melting and crystallization. The paper also discusses a new flowsheet in which mine tailings are utilized to produce either glass-wool or very strong glass-ceramic products, with good resistance to chemical attack.

Recycling of zinc-hydrometallurgy wastes in glass and glass ceramic materials

Abstract The aim of Brite-Euram project CT94 -1018 has been to recycle jarosite (JW), an iron rich hazardous waste resulting from the hydrometallurgy of zinc ores, to obtain glass and glass-ceramic materials. Granite scraps and mud (GW), generated by the caving, cutting and sawing of the blocks, and glass cullet have been utilized as additives to define the batch composition. The average compositional ranges are 40–70% for JW, 20–40% for GW and 10–40% for glass-cullet. Several compositions were prepared and tailored for the final product destination; melted at the 1400–1450°C temperature range. Glass-ceramic materials have been obtained from the glassy-phase by means of a controlled nucleation and crystallization thermal treatment, yielding a 40–55% crystalline volume fraction. The properties of the glass and glass-ceramics have been tailored to favor the commercial exploitation of the products in the building industry, in the form of paving tiles wall covering panels, glass fibers for insulation and coloring pigments for ceramics. A 1 ton/day vitrification pilot plant, able to work in a continuous or discontinuous way, has been realized and tested using different waste streams.

The effect of Cr2O3 as a nucleating agent in iron-rich glass-ceramics

In this work, the eAect of Cr2O3 as a nucleating agent, in iron rich glasses has been investigated by means of DTA, XRD and density measurements. By Cr2O3 addition, from 0. 4t o 1.0wt%, a lowering of the crystallisation peak temperature resulted in the DTA trace, the maximum eAect corresponding to 0.7wt%. By evaluating the degree of crystallisation of the glass at 0.7wt% Cr2O3, the highest eAciency in the nucleation process also corresponds. The optimum values for the nucleation and crystallisation time and temperature, determined for 0 . 7wt% Cr2O3 addition, have been 70min at 630C and 30 min at 800C. The crystalline phases formed at different thermal treatment temperatures of the parent glass have been investigated by XRD; the spinel is the only phase after the nucleation; pyroxene is the major phase after the crystallisation. The results of this study have highlighted that a small percentage of Cr2O3 strongly aAects the spinel formation thereby reducing the time and temperature of the thermal treatment and enhancing the degree of crystallisation of high iron content glasses. # 1999 Elsevier Science Limited. All rights reserved