Loredana Schiavo

Cenospheres can be functionalized by an adequate metallic coating. A universal method to produce a metallic coating is the well-known polyol process. Both noble metals and most of the transition metals can be obtained by chemical reduction of the corresponding metal salts in presence of hot polyols. Hot ethylene glycol is typically used for the reduction of silver nitrate (AgNO₃) to elemental silver, Ag(0). Owing to the capability of ethylene glycol to act as surface steric stabilizer, the metallic phase is obtained in form of spherical nanoparticles. Thus, when cenospheres are present during the chemical reduction of silver nitrate in presence of hot ethylene glycol, silver nanoparticles are precipitated on the cenospheres surface. Here, we have found that such precipitation is strongly influenced by the particle roughness and therefore it takes place only in the case cenospheres have been chemically etched by sodium hydroxide (NaOH) aqueous solution.

Negative temperature coefficient (NTC) materials are usually based on ceramic semiconductors, and electrons are involved in their transport mechanism. A new type of NTC material, adequate for alternating current (AC) applications, is represented by zeolites. Indeed, zeolites are single charge carrier ionic conductors with a temperature-dependent electrical conductivity. In particular, electrical transport in zeolites is due to the monovalent charge-balancing cations, like K⁺, capable of hopping between negatively charged sites in the aluminosilicate framework. Owing to the highly non-linear electrical behavior of the traditional electronic NTC materials, the possibility to have alternative types of materials, showing linearity in their electrical behavior, is very desirable. Among different zeolites, natural clinoptilolite has been selected for investigating NTC behavior since it is characterized by high zeolite content, a convenient Si/Al atomic ratio, good mechanical strength due to its compact microstructure, and low toxicity. Clinoptilolite has shown a rapid and quite reversible impedance change under heating, characterized by a linear dependence on temperature. X-ray diffraction (XRD) has been used to identify the natural zeolite, to establish all types of crystalline phases present in the mineral, and to investigate the thermal stability of these phases up to 150 °C. X-ray photoelectron spectroscopy (XPS) analysis was used for the chemical characterization of this natural clinoptilolite sample, providing important information on the cationic content and framework composition. In addition, since electrical transport takes place in the zeolite free-volume, a Brunauer–Emmett–Teller (BET) analysis of the mineral has also been performed.

Negative temperature coefficient (NTC) materials are usually based on ceramic semiconductors and electrons are involved in their transport mechanism. A new type of NTC material, adequate for alternating current (AC) applications, is represented by zeolites. Indeed, zeolites are single-charge carrier ionic conductors with a temperature-dependent electrical conductivity. In particular, electrical transport in zeolites is due to the monovalent charge-balancing cations, like K⁺, capable of hopping between negatively charged sites in the aluminosilicate framework. Owing to the highly non-linear electrical behavior of the traditional electronic NTC materials, the possibility to have alternative types of materials, showing linearity in the electrical behavior, is very desirable. Among different zeolites, the natural clinoptilolite has been selected for investigating the NTC behavior since it is characterized by high zeolite content, convenient Si/Al atomic ratio, good mechanical strength, due to its compact microstructure, and low toxicity. Clinoptilolite has shown a rapid and quite reversible impedance change under heating, characterized by a linear dependence on temperature. X-ray diffraction (XRD) has been used to identify the natural zeolite, to establish all types of crystalline phases present in the mineral, and to investigate the thermal stability of these phases up to 150°C. X-ray photoelectron spectroscopy (XPS) analysis has been used for the chemical characterization of the natural clinoptilolite sample, providing important information on the cationic content and framework composition. In addition, since electrical transport takes place in the zeolite free-volume, a Brunauer–Emmett–Teller (BET) analysis of the mineral has been also performed.

Geomorphic clinoptilolite, a mechanically and thermally stable mineral substance, has very useful electrical properties due to the presence of extra-framework cations in the crystal structure. Indeed, owing to the electrical transport that alkaline earth metal cations may give, this ceramic material behaves like an electrical insulator at room temperature, while it changes to an electrical conductor with increasing of temperature. Such unusual electrical property of clinoptilolite can be advantageously exploited for a number of functional applications in industrial fields. For example, clinoptilolite-based devices can be used as thermal sensors, electrical/thermal switches, NTC thermistors, etc. Here, the capability of a simple natural clinoptilolite monolith to switch from an electrical insulator to a conductor under fast temperature changes has been investigated via time-resolved a.c. electrical transport measurements.

Molecular traps, based on natural zeolites, are being used to remove harmful gaseous substances (VOCs). These chemical devices must be supported on an adequate substrate (e.g., paper, polymeric films, etc.). To achieve very strong adhesion with the substrate, the planar shape is the most convenient choice for these systems. Clinoptilolite is a nanostructured material made of nanosized lamellae with a thickness of 40 nm. A new technological approach, based on the use of an abrasive drill, was selected to exfoliate a natural clinoptilolite sample, thus leading to a powder made of single lamellar crystals. The obtained lamellae have been characterized by TEM, SEM/EDS, and XPS.