The present review describes mainly the history of SnO2-based voltage-dependent resistors, discusses the main characteristics of these polycrystalline semiconductor systems and includes a direct comparison with traditional ZnO-based voltage-dependent resistor systems to establish the differences and similarities, giving details of the basic physical principles involved with the non-ohmic properties in both polycrystalline systems. As an overview, the text also undertakes the main difficulties involved in processing SnO2- and ZnO-based non-ohmic systems, with an evaluation of the contribution of the dopants to the electronic properties and to the final microstructure and consequently to the system’s non-ohmic behavior. However, since there are at least two review texts regarding ZnO-based systems [Levinson, L. M., and Philipp, H. R. Ceramic Bulletin 1985;64:639; Clarke, D. R. Journal of American Ceramic Society 1999;82:485], the main focus of the present text is dedicated to the SnO2-based varistor systems, although the basic physical principles described in the text are universally useful in the context of dense polycrystalline devices. However, the readers must be careful of how the microstructure heterogeneity and grain-boundary chemistry are capable to interfere in the global electrical response for particular systems. New perspectives for applications, commercialization and degradation studies involving SnO2-based polycrystalline non-ohmic systems are also outlined, including recent technological developments. Finally, at the end of this review a brief section is particularly dedicated to the presentation and discussions about others emerging non-ohmic polycrystalline ceramic devices (particularly based on perovskite ceramics) which must be deeply studied in the years to come, specially because some of these systems present combined high dielectric and non-ohmic properties. From both scientific and technological point of view these perovskite systems are quite interesting.

SnO2, ZnO and related polycrystalline compound semiconductors: An overview and review on the voltage-dependent resistance (non-ohmic) feature

A versatile and highly efficient method was developed to fabricate durable, superhydrophobic surfaces on various substrates, including wood, lignocellulosic fibers, cotton textiles, glass, and metals. This method employs various nanoparticles (i.e., SiO2 and TiO2) in combination with vinyltriethoxysilane (VTES) modification under gravity and pressure. The obtained superhydrophobic surfaces exhibited water contact angles greater than 150° and water sliding angles less than 4.5°. The superhydrophobicity of the coated surfaces was created by the random distribution of oligomer-wrapped nanoparticles having different sizes and shapes, and the oligomer originated from the hydrolyzed VTES. All superhydrophobic surfaces were able to sustain different types of abrasion, including finger wiping, brushing, and knife scratching. The mechanical durability of these superhydrophobic surfaces, with the exception of cloth and textiles, maintained the original superhydrophobicity even after intense sandpaper-induced abrasion. The excellent tolerance against vigorous abrasions was attributed to the hierarchical structure and strong interfacial adhesion of the coatings. Furthermore, all surfaces survived under the environmental conditions of 35 °C and 90% RH for 96 h. It was confirmed that these surfaces exhibited excellent self-cleaning and oil/water separation properties. The modification method developed in this work provides a facile, efficient route to fabricate large-scale, mechanically stable, and durable superhydrophobic surfaces for a wide range of potential applications.

A versatile and efficient method to fabricate durable superhydrophobic surfaces on wood, lignocellulosic fiber, glass, and metal substrates

The effect of sintering processes, such as open sintering, sintering inside a closed crucible, and sintering within a powder bed, on the microstructure and V–I characteristics of ZnO–Bi2O3-based varistor ceramics was investigated at sintering temperatures in the range 1000–1200 °C. The results from the experiments showed that the microstructure and electrical properties of the samples varied according to the sintering method and temperature. Optimal values for the electrical characteristics of the varistor ceramics by different sintering processes were obtained when the sintering was conducted at 1100 °C. At the same sintering temperature, the different processes affected the properties differently. At 1000 °C, the samples sintered within a powdered bed showed better electrical properties than those subjected to the other two processes, while at 1100 or 1200 °C, the samples sintered in an open crucible exhibited the best electrical properties.

Microstructure and electrical properties of ZnO–Bi2O3-based varistor ceramics by different sintering processes

The materials currently used in proton-exchange membrane fuel cells (PEMFCs) require complex control of operating conditions to make them sufficiently durable to permit commercial deployment. One of the major materials challenges to allow simplification of fuel cell operating strategies is the discovery of catalyst supports that are much more stable to oxidative decomposition than currently used carbon blacks. Here we report the synthesis and characterization of Ti0.7W0.3O2 nanoparticles (approximately 50 nm diameter), a promising doped metal oxide that is a candidate for such a durable catalyst support. The synthesized nanoparticles were platinized, characterized by electrochemical testing, and evaluated for stability under PEMFC and other oxidizing acidic conditions. Ti0.7W0.3O2 nanoparticles show no evidence of decomposition when heated in a Nafion solution for 3 weeks at 80 °C. In contrast, when heated in sulfuric, nitric, perchloric, or hydrochloric acid, the oxide reacts to form salts such as titany...

Sol−Gel Synthesis, Electrochemical Characterization, and Stability Testing of Ti0.7W0.3O2 Nanoparticles for Catalyst Support Applications in Proton-Exchange Membrane Fuel Cells

Hydrophobic titanium dioxide (TiO2) was successfully grown on a wood surface using a hydrothermal method. Energy dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and water contact angle (WCA) were employed to characterize the features of grown TiO2 and its hydrophobicity. EDS, XRD, and FTIR proved that anatase TiO2 chemically bonded to the wood surface through the combination of hydrogen groups during the hydrothermal process. The values of WCAs manifested that the hydrophobicity of the treated wood was mainly dependent on specific reaction conditions, especially on reaction pH value and hydrothermal temperature. The highest WCA reached 154° when the hydrothermal temperature was 130 °C. The treated wood thus possessed a superhydrophobic surface.

Growth of hydrophobic TiO2 on wood surface using a hydrothermal method

Bi 2 O 3 -doped ZnO ceramic varistors are usually sintered at temperatures near to 1200 °C in the presence of a Bi-rich liquid phase, which is partially vaporized during the sintering process Volatilization of bismuth oxide depends on the total surface area in direct contact to the reaction atmosphere and this in turn is related to the area/volume ratio of the ceramic compact This loss of Bi 2 O 3 has a significant role on the development of the varistor microstructure and more specifically ZnO grain growth, which is strongly enhanced by the presence of the liquid phase, should be particularly affected In the present paper, X-ray fluorescence analysis is performed to describe the Bi 2 O 3 vaporization profile as a function of distance to the outer surface, taking into account its influence on microstructural evolution

Bi2O3 vaporization from ZnO-based varistors

Materials in the TiO 2 –RuO 2 system, in different shapes, as thin films, coatings, fine powders, xerogels and aerogels are of great interest for many applications, such as energy storage and production systems, solar cells, chemical sensors, electronics, gas separation processes, and industrial electrochemistry. However, a serious drawback in the processing of materials based on RuO 2 is the well-established chemical reactivity above ∼700 °C where RuO 2 becomes volatile and oxidizes to RuO 3 /RuO 4 gases (in air) or reduces to Ru metal (in a vacuum). For this reason, it is not obvious to attain dense materials in the systems containing Ru. In this study, a novel method has been used to obtain rutile-type Ti 1− x Ru x O 2 ( x  = 0.05, 0.08 and 0.10) as dense ceramics without the use of the hot-pressing for the sintering step. The relative densities of the materials prepared were higher than 99.0 ρ th %). This method combines the sol–gel synthesis, a pressureless fast-firing method (heating and cooling rate of 20 °C min −1 ) at 1350 °C for 10 min and the use of a buffer during the sintering that provides a RuO 3  + RuO 4 rich atmosphere.

M.J. Velasco

Papers

Bi2O3 vaporization from ZnO-based varistors

Rutile-type dense ceramics fabricated by pressureless sintering of Ti1−xRuxO2 powders prepared by sol–gel