This article presents an overview of the developments in stainless steels made since the 1990s. Some of the new applications that involve the use of stainless steel are also introduced. A brief introduction to the various classes of stainless steels, their precipitate phases and the status quo of their production around the globe is given first. The advances in a variety of subject areas that have been made recently will then be presented. These recent advances include (1) new findings on the various precipitate phases (the new J phase, new orientation relationships, new phase diagram for the Fe–Cr system, etc.); (2) new suggestions for the prevention/mitigation of the different problems and new methods for their detection/measurement and (3) new techniques for surface/bulk property enhancement (such as laser shot peening, grain boundary engineering and grain refinement). Recent developments in topics like phase prediction, stacking fault energy, superplasticity, metadynamic recrystallisation and the calculation of mechanical properties are introduced, too. In the end of this article, several new applications that involve the use of stainless steels are presented. Some of these are the use of austenitic stainless steels for signature authentication (magnetic recording), the utilisation of the cryogenic magnetic transition of the sigma phase for hot spot detection (the Sigmaplugs), the new Pt-enhanced radiopaque stainless steel (PERSS) coronary stents and stainless steel stents that may be used for magnetic drug targeting. Besides recent developments in conventional stainless steels, those in the high-nitrogen, low-Ni (or Ni-free) varieties are also introduced. These recent developments include new methods for attaining very high nitrogen contents, new guidelines for alloy design, the merits/demerits associated with high nitrogen contents, etc.

Recent developments in stainless steels

High-performance gas sensors prepared using p-type oxide semiconductors such as NiO, CuO, Cr2O3, Co3O4, and Mn3O4 were reviewed. The ionized adsorption of oxygen on p-type oxide semiconductors leads to the formation of hole-accumulation layers (HALs), and conduction occurs mainly along the near-surface HAL. Thus, the chemoresistive variations of undoped p-type oxide semiconductors are lower than those induced at the electron-depletion layers of n-type oxide semiconductors. However, highly sensitive and selective p-type oxide-semiconductor-based gas sensors can be designed either by controlling the carrier concentration through aliovalent doping or by promoting the sensing reaction of a specific gas through doping/loading the sensor material with oxide or noble metal catalysts. The junction between p- and n-type oxide semiconductors fabricated with different contact configurations can provide new strategies for designing gas sensors. p-Type oxide semiconductors with distinctive surface reactivity and oxygen adsorption are also advantageous for enhancing gas selectivity, decreasing the humidity dependence of sensor signals to negligible levels, and improving recovery speed. Accordingly, p-type oxide semiconductors are excellent materials not only for fabricating highly sensitive and selective gas sensors but also valuable additives that provide new functionality in gas sensors, which will enable the development of high-performance gas sensors.

Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview

Metal oxide-based resistive-type gas sensors are solid-state devices which are widely used in a number of applications from health and safety to energy efficiency and emission control. Nanomaterials such as nanowires, nanorods, and nanoparticles have dominated the research focus in this field due to their large number of surface sites facilitating surface reactions. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can have drastic effects on gas sensor performance, especially the selectivity. Recently, these effects have been amplified by designing heterojunctions on the nano-scale. These designs have evolved from mixed commercial powders and bi-layer films to finely-tuned core–shell and hierarchical brush-like nanocomposites. This review details the various morphological classes currently available for nanostructured metal-oxide based heterojunctions and then presents the dominant electronic and chemical mechanisms that influence the performance of these materials as resistive-type gas sensors. Mechanisms explored include p–n and n–n potential barrier manipulation, n–p–n response type inversions, spill-over effects, synergistic catalytic behavior, and microstructure enhancement. Tables are presented summarizing these works specifically for SnO2, ZnO, TiO2, In2O3, Fe2O3, MoO3, Co3O4, and CdO-based nanocomposites. Recent developments are highlighted and likely future trends are explored.

Nanoscale metal oxide-based heterojunctions for gas sensing: A review

Sensor technology has an important effect on many aspects in our society, and has gained much progress, propelled by the development of nanoscience and nanotechnology. Current research efforts are directed toward developing high-performance gas sensors with low operating temperature at low fabrication costs. A gas sensor working at room temperature is very appealing as it provides very low power consumption and does not require a heater for high-temperature operation, and hence simplifies the fabrication of sensor devices and reduces the operating cost. Nanostructured materials are at the core of the development of any room-temperature sensing platform. The most important advances with regard to fundamental research, sensing mechanisms, and application of nanostructured materials for room-temperature conductometric sensor devices are reviewed here. Particular emphasis is given to the relation between the nanostructure and sensor properties in an attempt to address structure-property correlations. Finally, some future research perspectives and new challenges that the field of room-temperature sensors will have to address are also discussed.

Nanostructured Materials for Room-Temperature Gas Sensors

CuO nanostructures: Synthesis, characterization, growth mechanisms, fundamental properties, and applications

Growth and branching of CuO nanowires by thermal oxidation of copper

We have investigated room temperature sensing characteristics of CuO films to various gases i.e. Cl 2 , H 2 S, NH 3 , CH 4 , CO and NO. CuO films were prepared by oxidation of Cu films, which were deposited on polycrystalline alumina substrates by thermal evaporation technique. CuO films have been found to be highly selective towards H 2 S. We demonstrate that the H 2 S response of CuO films can be divided in to three regions: (a) low concentrations (100–400 ppb), (b) intermediate concentrations (500 ppb to 50 ppm), and (c) high concentrations (>50 ppm). For low concentrations (100–400 ppb), the response curves have been found to be highly reversible with very small response (∼60 s) and recovery (∼90 s) times, indicating suitability of CuO films for sub-ppm sensing of H 2 S. Oxidation of H 2 S by adsorbed oxygen is found to be the responsible sensing mechanism. However, at very high H 2 S concentrations (>50 ppm), surface of CuO grains is found to convert into CuS, resulting in an irreversible response curve. For intermediate concentrations (500 ppb to 50 ppm), the response curve is governed by both H 2 S oxidation and CuS formation mechanisms.

Sub-ppm H2S sensing at room temperature using CuO thin films

Hydrogen sulphide (H 2 S) sensors, exhibiting high sensitivity (>10 4 in 50 ppm of H 2 S) and short response time (80 s), have been developed employing tin oxide:copper oxide (SnO 2 :CuO) composite thin films. The films were prepared by thermal evaporation of Sn and Cu metals on recrystallized alumina substrates, followed by oxidation at high temperatures. Results of various studies, such as, scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) reveal that SnO 2 and CuO are mutually non-reactive. The CuO grains, which in turn reside in the inter-granular regions of SnO 2 , inhibit grain growth of SnO 2 as well as form a network of p–n junctions. Results of XPS measurements revealed that the formation of Cu(OH) 2 due interaction of sensor film with ambient moisture causes drifts in sensor characteristics (low sensitivity and slow response) on prolonged operation in the industrial environment. The sensitivity and response time of degraded sensors could be restored either by exposing the sensor to a high H 2 S dose (>100 ppm) or by heating it to an elevated temperature (>400 °C).

Mechanism of drifts in H2S sensing properties of SnO2:CuO composite thin film sensors prepared by thermal evaporation

Pure and copper doped SnO 2 nanowires have been grown by thermal evaporation technique. The results show that Cu may be doped in SnO 2 nanowires in contrast to results reported for thin films and polycrystalline materials. H 2 S gas sensors have been fabricated from single wires and drop cast films prepared using the nanowires. It is seen that pure SnO 2 single wire sensors have better response for H 2 S in comparison to doped films and in contrast drop cast films show improved response on doping. Results show that changes at barrier height between nanowires are responsible for the very high response of drop cast Cu doped nanowire films.

K.P. Muthe

Papers

Growth and branching of CuO nanowires by thermal oxidation of copper

Sub-ppm H2S sensing at room temperature using CuO thin films

Mechanism of drifts in H2S sensing properties of SnO2:CuO composite thin film sensors prepared by thermal evaporation

Copper doped SnO2 nanowires as highly sensitive H2S gas sensor

XPS and AFM investigations of annealing induced surface modifications of MgO single crystals