Natural Sources and Preparation

Yttrium is found together with other rare earth oxides inmonazite sands (Ce, La, etc.) PO4] and in bastnasite [(Ce, La, etc.)(C03)F] (see Section 1.7.1). Yttrium is extracted together with other rare earth elements in a concentrated solution of sodium hydroxide at 140-150 °C after cooling, the hydroxides of the rare earth elements are separated by filtration. Alternatively, bastnasite may be calcined to drive off CO2 and fluorine, and then leached with hydrochloric acid to dissolve the trivalent rare earth elements. The rare earth hydroxides and chlorides obtained in this way are further processed to produce individual rare earth metal compounds 

The use of yttria ceramics as standalone materials for structural applications is rather limited by their poor mechanical properties. For example, the commercially available 99.9% polycrystalline yttria CeraDoy (Ceradyne Inc.) has a flexural strength of 99 MPa, a hardness of 5.85 GPa, and a fiacture toughness of 1.4MPam. The hardness and fracture toughness of polycrystalline yttria are virtually grain-size 

The Poisson ratio of yttria is 0.31, and the elastic modulus 170 G Pa. The material has a relatively high thermal expansion coefficient (9.1 x 10 K ), which is comparable to that of polycrystalline alumina, and a high thermal conductivity (14 W mK ), which is twice that of the other solid-state laser host material, Y3AI5O12 (YAG). Yttria has also a very high electrical resistivity (lO cm), is refractory by 

Three decades ago, Greskovich and Chernoch created a new field of application of yttria by producing a laser yttria host-based ceramic material [312]. Yttria is not only used as a solid-state laser material as a laser host crystal for trivalent lanthanide activators, such as Yb + and Nd, but also shows significant potential for luminous pipes in high-intensity discharge lamps and heat-resistive windows. 

As an optical ceramic, yttrium oxide transmits well in the IR range, from 1 to 8 pm wavelength. This high IR transmission, together with a good resistance to erosion and thermal shock, means that yttrium oxide would serve as an ideal material for protection domes for IR sensors [313]. 

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Metallurgy is the technology of extracting metals from their natural sources and preparing them for use. 

The amidoxy, or ip[C0-NH-0], link 6 (R1 = H) results from N-acylation of an a-aminoxy acid. a-Aminoxy acids have been isolated from natural sources and some of them present antibacterial properties as such, or when coupled to an a-amino acid, as in malioxamycin H-L-Vali )[C0-NH-0]D-Asp-0Hj115 117l The a-aminoxy acids are homologous to the a-hy-drazino acids with the advantage that there is no problem of regioselectivity for N-acylation. Despite the easy preparation of a-aminoxy acids,1118-1201 the number of (amidoxy) peptides reported in the literature is rather limited, with no examples of solid-phase synthesis. Recently, it has been observed that the a-aminoxy and a-hydrazino acid units induce quite similar local folded structures.1 21-123 ... 

Several benzannelated compounds have been isolated from natural sources and synthesized. However, only one general method, as described below, has been devised for their preparation. 

The importance of salts derived from their uses in daily life, especially in pharmaceutical preparations. Their natural sources and their artificial... 

Toxicological and other biological properties of components other than the flavouring principles in the flavouring. These may be different if a given flavouring is derived from natural sources or prepared synthetically. Special care shall be taken to avoid toxic substances which are known to derive from a specific source of method of synthesis. 

Many oxygenated monocarbocyclic monoterpenes are of use to the flavour and fragrance industry. Some are extracted from natural sources, and others are prepared from the major members of the family, usually by straightforward functional group interconversions. 

Aside from the insulin preparations (see below), the 15 interferons could be considered one of the earliest attempts to use biotechnological means (in all of the definitions of the term) to obtain human proteins for use as drugs. Both recombinant-derived and natural-sourced materials were approved for use within a year of each other, with the recombinant material. Prone being launched in Italy in 1985, closely followed by a natural-sourced material, Wellferon in the UK in 1986. An inspection of Table 4.6 shows that this type of leap-frogging between natural-sourced and recombinant-sourced materials continued over the next few years, with some modified agents being obtained from both sources. 

Cossins, E., Lee, R., and Packer, L., ESR studies of vitamin C regeneration, order of reactivity of natural source phytochemical preparations, Biochem. Mol. Biol. Int, 45, 583-598,1998. 

A diverse array of sources and preparative procedures are available to obtain organic carotenoids. - Lutein may be extracted from the petals of the Mexican marigold (Tagetes erecta) (Fig. 7.32). [69] Since xanthophyils occur in Nature mostly as esters, and while the free alcohol form is advantageous for many applications, the extraction process with a hydrocarbon is often followed by a hydrolytic step. [63]... 

Cellulose thin layers are derived from natural sources and are mainly used in NP partition separation where water is bound to the hydroxyl groups and compounds move between the mobile phase and the water bound to the cellulose surface. The prepared layers can be fibrous or microcrystalline depending on the manufacturer. Recently, ProteoChrom HPTLC cellulose plates, 10 x 10 cm (Merck [HMD] Millipore) have been introduced. Other cellulose prepared plates on the market are standard TLC-sorbent layers. Cellulose sorbents bond extremely well to all supports and no binder is required in their formulations. 

Lithocholic acid was first isolated from a gallstone by Fischer in 1911 (30). It was later isolated from ox bile (1 g from 100 kg of bile) (64) from rabbit bile (0.4 g from 900 ml) (65) and subsequently from monkey, human, pig, and guinea pig bile (2, 66, 67). Lithocholic acid has been identified as one of the bile acids in human blood (68) and as a principal fecal bile acid. Moset-tig et al. (69) isolated lithocholic acid from human stool and estimated its concentration to be 3 g/100 kg of fresh stool. Lithocholic acid is particularly insoluble, is not hydroxylated to an appreciable extent in man (70), and may be the cause of liver disease (4). In early studies lithocholic acid was not available in sufficient amounts from natural sources and was prepared from cholic acid. Lithocholic acid was particularly valuable in establishing the correspondence of the B/C ring structure between bile acids and cholesterol (71). 

Cationic amphipathic CPs, whose nanotubes are oriented parallel to the membrane, have been found to have quite potent and selective antibiotic activity both in vitro and in vivo. The proposed mode of action for this type of peptide is based on a carpet-like mechanism, in which several nanotnbes lie parallel to the membrane surface. Antiviral activity has also been found for this type of CP and the mechanism of action proposed is based on interference of the virus fusion with the cell membrane. Interestingly, mannopeptimycins are very potent antimicrobial agents obtained from natural sources and these contain a hexacyclicpeptide that can be considered as a d,l-o -CP. The mechanism of action for these systems is not known yet, but could be based on the formation of amphipathic nanotnbes. Very recently, D,L-a-glycopeptide analogs have been designed and prepared for their potential applications as antimicrobial agents. ... 

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