Ceramics nonstoichiometric

The record for the highest superconducting temperature in the year 2004 is 138 K, held by a nonstoichiometric ceramic oxide, HgQ g Tig 2 Ba2 Ca2 C1I3 Og 33. This is still far below room temperature, but research continues. 

Because these materials are the first examples of highly oxidized nonstoichiometric ceramic oxide superconductors, the determination and optimization of the physical properties has been a major technical and scientific challenge. The observed properties, and the impact of chemistry on them are reviewed here. 

For nonstoichiometric compounds, the general rule is that when there is an excess of cations or a deficiency of anions, the compound is an n-type semiconductor. Conversely, an excess of anions or deficiency of cations creates a / -type semiconductor. There are some compounds that may exhibit either p- or n-type behavior, depending on what kind of ions are in excess. Lead sulfide, PbS, is an example. An excess of Pb + ions creates an n-type semiconductor, whereas an excess of ion creates a /7-type semiconductor. Similarly, many binary oxide ceramics owe their electronic conductivity to deviations from stoichiometric compositions. For example, CU2O is a well-known / -type semiconductor, whereas ZnO with an excess of cations as interstitial atoms is an n-type semiconductor. A partial list of some impurity-controlled compound semiconductors is given in Table 6.9. 

Specific examples in which results from ALCHEMI are compared with X-ray or neutron diffraction studies on the same sample have been presented for olivine [51], clinopyroxene [57] and feldspar [54] To date, all published planar ALCHEMI studies on mineral samples other than oxides containing high-Z elements (e.g. ceramic nuclear wasteforms, [58]) have been internally consistent and tractable. For example, a combined X-ray diffraction and ALCHEMI study on a complex, nonstoichiometric clinopyroxene [52] has been used to determine the location of vacancies in the structure. In... 

Mixed oxides have a widespread application as magnets, catalysts, and ceramics. Often, nonstoichiometric mixtures with unusual properties can be prepared for example, Fe203 and ZnO have been milled for the production of zinc ferrite [40], while mixed oxides of Ca(OH)2 and Si02 were described by Kosova et al. [77]. Piezoceramic material such as BaTi03 from BaO and anatase Ti02 has been prepared [78], while ZnO and Cr203 have been treated by Marinkovic et al. [79] and calcium silicate hydrates from calcium hydroxide and silica gel by Saito et al. [80]. The thermal dehy-droxylation of Ni(OH)2 to NiO or NiO-Ni(OH)2 nanocomposites has also been investigated [81]. 

Ceramics are primarily compounds. Ceramics other than glasses generally have a crystalline structure, while silica-based glasses, a subclass of ceramic materials, are noncrystalline. In crystalline ceramic compounds, stoichiometry dictates the ratio of one element to another. Nonstoichiometric ceramic compounds, however, occur frequently. Some important ceramic materials are listed in Table... 

Yellow lead(II) oxide, known as litharge, is widely used to glaze ceramic ware. Lead(IV) oxide does not exist in nature, but a substance with the formula PbOj.9 can be produced in the laboratory by oxidation of lead(II) compounds in basic solution. The nonstoichiometric nature of this compound is caused by defects in the crystal structure. The crystal has some vacancies in positions where there should be oxide ions. These imperfections in the crystal (called lattice defects) make lead(IV) oxide an electrical conductor, since the oxide ions jump from hole to hole. This makes possible the use of lead(IV) oxide as an electrode (the cathode) in the lead storage battery. 

With the notable exception of transition metal oxides that generally exhibit wide deviations from stoichiometry, the concentration of intrinsic or nonstoichiometric defects in most ceramic compounds is so low that their defect concentrations are usually dominated by the presence of impurities. 

The present review will cover the phase equilibria of these ceramic nuclear fuels at high temperatures, and a summary will be given on their nonstoichiometric region, defect structure and thermodynamic data. In addition, diffusion and vaporization processes will be taken as representative phenomena eharacteristic of these materials at high temperatures, and these phenomena also will be reviewed in their relation to nonstoichiometry. 

Descriptions will be given below of the methods available for determining the defect structure in nonstoichiometric compositions of nuclear ceramic fuels, but the discussions on the defect structure will be limited mainly to binary system. 

There are several ways that we can create point defects in ceramics. We have seen already that point defects can be produced in nonstoichiometric oxides, such as ZnO,... 

Now you can compare diffusion in stoichiometric oxides with diffusion in nonstoichiometric oxides. The first question concerns the diffusing species when two species are present. For many ceramics AE is large. For MgO, AI2O3, and B2O3, AHs is 600kJ/mol ( 6eV/formula unit). Thus... 

I 1 Combustion Synthesis of Nitrides for Development of Ceramic Materials of New Generation of a lower composition and nonstoichiometric nitrides. 

Calcium phosphate ceramics are ceramics with varying calcium-to-phosphate ratios. Among them, the apatite ceramics, defined by the chemical formula M,o(X04)6Z2, have been studied most. The apatites form a range of solid solutions as a result of ionic substitution at the XO , or Z sites. In general, apatites are nonstoichiometric and contain less than 10 mol of ions, less than 2 mol of Z" ions, and exactly 6 mol of XOJ" ions (Van Raemdonck et al., 1984). The species is typically a bivalent metallic cation, such as Ca, Sr +, Ba ", Pb ", or Cd2+. The XO species is typically one of the following trivalent anions AsO , VO, CrO , or MnO. The monovalent 7r ions are usually F", OH , Br , or Cj (Van Raemdonck et al., 1984). 

The oxygen partial pressure affects the defect concentrations in an oxidic solid and therefore also the diffusivity in nonstoichiometric oxide ceramic compounds. The oxygen gas pressure then affects the apparent reaction rate through the solid state diffusion rates even when there is no mass transport through the gas phase during sintering. 

Non-stoichiometric. A chemical compound is said to be nonstoichiometric if the ratio of its constituents differs from that demanded by the chemical formula. This may happen with oxides that are readily reducible, or with compounds containing an element of variable valency, or when interstitial atoms are present in the lattice. Some nonstoichiometric ceramics are of interest as being semi-conducting. 

Ralstonia eutropha Electrochemical reaction CO2 + IT + e HCOO Microbial reaction (nonstoichiometric) HCOO —> C5H12O + C4H10O Platinum mesh anode separated by ceramic membrane from the indium foil cathode. 1.2 mM isobutanol and 0.6 mM 3-methyl-l-butanol after 1(X) h m... 

Findings from the theories of nucleation have also contributed to the optimization of these comprehensive experimental investigations. As a result, glass-ceramics with improved properties have been produced, specifically glass-ceramics with specific solid-solution limits like j3-spodumene and stuffed P-quartz or with stoichiometric compositions like cordierite and lithium disilicate. Initial results for the successfU application of the theory of nucleation to multicomponent glass-ceramics with a nonstoichiometric composition were achieved with mica and anosovite glass-ceramics. 

It must be noted that a significant improvement of the chemical durability of lithium disilicate glass-ceramics was achieved later in the development of glass-ceramics with nonstoichiometric compositions. 

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