Multiwalled carbon nanotube (MWCNT) films have been fabricated by using plasma-enhanced chemical vapor deposition system onto Cr–Au patterned alumina substrates, provided with 3nm thick Fe growth catalyst, for NO2 and NH3 gas sensing applications, at sensor temperature in the range of 100–250°C. Nanoclusters of noble metal surface catalysts (Au and Pt) have been sputtered on the surface of MWCNTs to enhance the gas sensitivity with respect to unfunctionalized carbon nanotube films. It was found that the gas sensitivity of Pt- and Au-functionalized MWCNT gas sensors significantly improved by a factor up to an order of magnitude through a spillover effect for NH3 and NO2 gas detections, respectively. The metal-functionalized MWCNT sensors exhibit very high gas sensitivity, fast response, reversibility, good repeatability, and sub-ppm range detection limit with the sensing properties of the MWCNT films tailored by surface catalyst used to functionalize the MWCNT sensors.

Multiwalled carbon nanotube ͑MWCNT͒ films have been deposited by using plasma enhanced chemical vapor deposition ͑PECVD͒ system onto Cr-Au patterned alumina substrates for NO 2 and NH 3 gas sensing applications, at operating temperature of 200°C. Nanoclusters of different MWCNT-growth catalysts ͑Fe and Co͒ have been predeposited onto substrates to promote the growth of the carbon nanotube films with different structural properties. It is demonstrated that the gas sensitivity of the MWCNT films depends on catalyst used for their growth with highest gas sensitivity achieved for Co-grown MWCNT films. The chemiresistor gas response is attributed to the p-type conductivity in semiconducting MWCNTs and the electrical charge transfer is found to be the major sensing mechanism. The results obtained demonstrate that the MWCNT-based sensors exhibit high gas sensitivity, fast response and reversibility, good repeatability and reproducibility, and sub-ppm range detection limit with the gas sensing properties of the MWCNT films tailored by catalyst employed in the PECVD growth process.Carbon nanotubes 1,2 have attracted considerable attention in recent years, most notably because of their potential as active building blocks for novel nanoscale electronic devices, 3,4 expecially for chemical gas nanosensors. 5-10 Sensing devices based on carbon nanotubes are emerging as a powerful class of ultrasensitive and high-performance sensors for detection of chemical 5-12 and biological 13,14 species.Nanostructures 15-17 based on semiconducting metal oxides and noble metals, such as nanobelts, nanowires, nanorods, nanohorns, nanoparticles, nanoclusters, nanocrystals, and nanotubes, have been proposed as interesting nanomaterials for gas sensing applications.The carbon-derived nanostructures, such as carbon nanotubes, are widely perceived as promising nanomaterials for the development of novel chemical nanosensors to generate significant impact in nanoscience and nanotechnology. There are two main types of carbon nanotubes: the single-walled carbon nanotube structure which consists of single graphene sheets seamlessly wrapped into cylindrical tubes with caps normally closed and multiwalled carbon nanotube ͑MWCNT͒ structure which comprises an array of nanotubes that are concentrically nested like rings of a tree trunk. These carbon nanotubes offer many opportunities for the assembly of nanosensors and nanoarrays by the bottom-up paradigm.The rational control in the carbon nanotube growth process of key nanomaterial parameters, including chemical composition, size, structure, morphology, purity, doping, and functionalization, is critical for depositing films of carbon nanotubes with desired and predictable properties to realizing nanosensors with optimal functions. Previously, it has been suggested 18,19 that the type and the size of the growth catalyst used in the CVD growth process can define the diameter of as-grown carbon nanotubes. Consequently, the structural properties of carbon nanotubes can be controlled and thus the possibility ...

Thin films of hafnium oxide have been prepared by using a dual ion beam sputtering system. A study of their physical properties is reported. In particular, structural and compositional characterization was performed by means of x-ray diffraction and x-ray photoelectron spectroscopy techniques, showing a mixture of amorphous and polycrystalline structure and a substoichiometric composition. The atomic force microscopy results have shown a crater-like morphology probably due to the deposition process. In addition the gas sensing characteristics were analyzed in the presence of carbon monoxide. The variations in the electrical resistance have shown the capability of the films to detect CO and then the possibility to use hafnium oxides as a new sensitive material in the field of gas sensors.

A variety of cutting tool materials are used for the contact mode mechanical machining of components under extreme conditions of stress, temperature and/or corrosion, including operations such as drilling, milling turning and so on. These demanding conditions impose a seriously high strain rate (an order of magnitude higher than forming), and this limits the useful life of cutting tools, especially single-point cutting tools. Tungsten carbide is the most popularly used cutting tool material, and unfortunately its main ingredients of W and Co are at high risk in terms of material supply and are listed among critical raw materials (CRMs) for EU, for which sustainable use should be addressed. This paper highlights the evolution and the trend of use of CRMs) in cutting tools for mechanical machining through a timely review. The focus of this review and its motivation was driven by the four following themes: (i) the discussion of newly emerging hybrid machining processes offering performance enhancements and longevity in terms of tool life (laser and cryogenic incorporation); (ii) the development and synthesis of new CRM substitutes to minimise the use of tungsten; (iii) the improvement of the recycling of worn tools; and (iv) the accelerated use of modelling and simulation to design long-lasting tools in the Industry-4.0 framework, circular economy and cyber secure manufacturing. It may be noted that the scope of this paper is not to represent a completely exhaustive document concerning cutting tools for mechanical processing, but to raise awareness and pave the way for innovative thinking on the use of critical materials in mechanical processing tools with the aim of developing smart, timely control strategies and mitigation measures to suppress the use of CRMs.

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