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κ materials: a theoretical perspective
Published in Michel Houssa, κ Gate Dielectrics, 2003
Marshall Stoneham, Alexander Shluger, Adam Foster, Marek Szymanski
Oxides can be non-stoichiometric in many ways [44]. Whereas SiO2 composition is usually not far from stoichiometric with two oxygens per silicon (alternatively, O is bonded to two Si and each Si bonded to four O), other systems can deviate a lot from their nominal composition. Examples are systems for which cations can easily exist in more than one charge state (such as TiO2−x) or where interstitial oxygen is readily formed (like SnO2). Why might non-stoichiometry matter? First, the defects, which enable nonstoichiometry, often have charge carriers associated with them. These can give rise to charge transport (in some cases by activated small polaron transport) or to dielectric loss. Conduction along grain boundaries or dislocations may be especially worrying [45]. Second, these defects are involved in degradation processes, such as resistance degradation. This degradation may become more important for very thin films, since the dielectric will have statistical variations in composition, and some regions will be more vulnerable than others. In certain cases, doping can help: for SrTiO3, for instance, doping with Er apparently suppresses O vacancies and reduces the rate of resistance degradation [46]. Third, there are likely to be sample-to-sample variations. These will arise both from the nature of the material as created and from changes during subsequent process steps.
Activation of Combustion Process of Anthracite by Metal Nitrates
Published in Combustion Science and Technology, 2022
K.B. Larionov, A.Z. Kaltaev, N.I. Berezikov, A.S. Gorshkov, A.V. Zenkov, K.V. Slyusarskiy
According to Eqs. (2)–(3), the first stage of nitrates decomposition during their heating is associated with the formation of NO (4–5) and NO2 nitrogen oxides (8). The latter has strong oxidizing properties during combustion activation of organic compounds (6–7) (Wang et al. 2015b, 2015b). It is important to note that carbon in this system can act as a reducing agent, which contributes to an earlier start of the nitrates decomposition, which is also confirmed by the data presented in Figures 4, 5. Further interaction of anthracite (carbon residue) with nitrogen dioxide is accompanied by the NO formation, which is further oxidized by air oxygen in accordance with the reaction (8). After the nitrates decomposition is completed, dispersed non-stoichiometric metal oxides (copper and iron) are formed, which may contain a set of different phases. Copper and iron oxides formed during the oxidation reaction are known as very active catalysts of heterogeneous reactions of complete oxidation (9) (Chen, Li, Li 2008).
Weathering induced morphological modification on the thermal diffusivity of natural pyrrhotite: a thermal lens study
Published in Philosophical Magazine, 2021
M. S. Swapna, V. Gokul, Vimal Raj, R. Manu Raj, S. N. Kumar, S. Sankararaman
The Earth’s crust contains an immense collection of various kinds of minerals. Among the 4660 species of minerals approved by the International Mineralogical Association (IMA), sulphides belong to one of the most important categories of ore minerals. A sulphide mineral is composed of Sulphur as an anion with a metal/semi-metal cation(s). Even though there exist hundreds of sulphide minerals in nature, only five among them are common and abundant. They are galena, pyrrhotite, chalcopyrite, pyrite, and sphalerite [1]. The pyrite and pyrrhotite are the most dominant iron sulphides in nature seen in many rocks and ores, such as in pegmatites, intrusions of mafic igneous rocks, and contact metamorphic zones [2–4]. The naturally occurring pyrrhotite, possessing NiAs crystal structure, occurs in several polytypes with a non-stoichiometric composition of Fe(1-x) S (x =0.0–0.2). The presence of ordered vacancies in the iron (Fe) lattices is the reason for the non-stoichiometry. The Fe is filled in the octahedral site, and the sulphide groups are filled in the trigonal prismatic sites. When the Fe-rich structures, having hexagonal and orthorhombic symmetry shows antiferromagnetic behaviour, the Fe-deficient forms having monoclinic symmetry exhibits ferromagnetic behaviour [5]. A higher concentration of Fe vacancies adds to the enhanced ferromagnetic property shown by Fe-deficient, defect-induced pyrrhotites. These iron sulphides possess various limitations as well as advantages [6–9].
SO2 Emission Characteristics of Bituminous Coal, Lignite, and Its Blends with Cedar Nut Shells under O2/N2 and O2/CO2 Combustion Environments in a Bubbling Fluidized Bed
Published in Combustion Science and Technology, 2020
Wojciech Jerzak, Zofia Kalicka, Elżbieta Kawecka-Cebula, Monika Kuźnia
Understanding the impact of these compounds on the emission of SO2 during coal combustion is helped by pyrolysis test results. They show that mineral compounds present in coal decompose at temperatures lower than in their pure state. According to Chen et al. (2000), pyrite – FeS2– starts to decompose in coal in the presence of nitrogen atmosphere at 420°C, that is an approx. 100°C lower temperature than in its pure state. The decomposition product is a non-stoichiometric sulfide – pyrrhotite FeSx (1 < x < 2) – and sulfur. According to Zhang and Yani (2011), iron(II) sulfate(VI) present in coal starts to decompose at 230°C (approx. 300°C lower than in pure state), while calcium sulfate(VI) decomposes from 350°. These temperatures are approx. 100–200°C lower than those for pyrite. This may constitute the reason why SO2 emissions during lignite combustion commence at a lower temperature than for bituminous coal combustion. As a general rule, sulfates decompose into SO3; thus, they should not impact upon SO2 emission, but in the presence of organic matter reduction may occur and SO2 may be formed. Oxygen does not participate in sulfate decomposition reactions; thus, % O2 in OEA should not have a substantial influence on SO2 emissions. However, according to Duan et al. (2011), sulfate decomposition under conditions of fluidized bed combustion is not possible. In such a case, SO2 emission from lignite would mostly originate from organic sulfur, and only marginally from pyrite.