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High pressure synthetic form

A high pressure synthetic form ofFeOOH can be prepared by hydrothermal conversion of a-Fe203 in NaOH at 500 °C at a pressure of 80-90 kb for one hour (Chenavas et al., 1973). The crystal structure is that of In-OOH. It consists of single chains of octahedra rumiing parallel to the c axis and linked by hydrogen bonds. [Pg.10]

Diamond is the perfect example of an atomic crystal or giant molecule, where there is complete electron pair covalent bonding, which links all atoms in all directions in space. Diamond can occur in several crystal forms and these are classified using the crystallographic notation for the simple planes of a cubic crystal (Figure 2.5). Diamond has three major crystal forms cubic (100 plane), dodecahedral (110 plane) and the octahedral form (111 plane), which are shown in Figure 2.6. Both cubic and octahedral forms occur in high pressure synthetic diamond and CVD diamond. [Pg.20]

These simple planes correspond to the faces of the three major crystal forms of diamond the 100 cubic, the 110 dodecahedral and the 111 octahedral (Fig. 11.5). Both cubic and octahedral surfaces predominate in high-pressure synthetic diamond where they are found alone or in combination to form blocky crystals. ... [Pg.250]

Phenohc resins are the oldest form of synthetic stmctural adhesives. Usage ranges from bonding automobile and other types of brake linings to aerospace apphcations. These adhesives have a reputation for providing the most durable stmctural bonds to aluminum. Because of volatiles, however, and the need for high pressures, the phenohc resins are used less as adhesives than the epoxy resins. [Pg.233]

Boron and carbon form one compound, boron carbide [12069-32-8] B C, although excess boron may dissolve ia boron carbide, and a small amount of boron may dissolve ia graphite (5). Usually excess carbon appears as graphite, except for the special case of boron diffused iato diamonds at high pressures and temperatures, eg, 5 GPa (50 kbar) and 1500°C, where boron may occupy both iaterstitial and substitutional positions ia the diamond lattice, a property utilized ia synthetic diamonds (see Carbon, diamond, synthetic). [Pg.219]

Noble gas hydrates are formed similarly when water is frozen under a high pressure of gas (p. 626). They have the ideal composition, [Gg(H20)46], and again are formed by Ar, Kr and Xe but not by He or Ne. A comparable phenomenon occurs when synthetic zeolites (molecular sieves) are cooled under a high pressure of gas, and Ar and Kr have been encapsulated in this way (p. 358). Samples containing up to 20% by weight of Ar have been obtained. [Pg.893]

All Diels-Alder reactions of tropones 51 as dienes with different types of dienophiles shown in Scheme 11 are accelerated by pressure, so that in some cases the desired cycloadducts are only formed at high pressure. An interesting synthetic equivalent of the unreactive acetylene in Diels-Alder syntheses is the oxanorbomadiene derivative 52 (Scheme 11 entry 2). 52 reacts with tropones forming the adducts 53, 54 and 55, which undergo a retro-Diels-Alder reaction leading to 56 and 57, the formal [4+2] cycloadducts of tropones to acetylene. [Pg.575]

The procedure described illustrates a new general synthetic method for the preparation of (E)-3-allyloxyacrylic acids and their conversion to a-unsubstituted y,6-unsaturated aldehydes by subsequent Claisen rearrangement-decarboxylation. Such aldehydes are traditionally prepared by Claisen rearrangements of allyl vinyl ethers. Allyl vinyl ethers are typically prepared by either mercury-catalyzed vinyl ether exchange with allylic alcohols or acid-catalyzed vinylation of allylic alcohols with acetals. The basic conditions required for alkoxide addition to the betaine to produce carboxyvinyl allyl ethers, as described in this report, nicely complements these two methods. In addition, this Claisen rearrangement is an experimentally very simple procedure, since sealed tube and other high pressure vessels are not required. The allyloxyacrylic acids are heated neat (in most cases a small amount of hydroquinone is added) and, by adjusting the pressure at which the reaction is performed, the aldehyde products distill from the reaction mixture in analytically pure form. [Pg.33]

Hydrothermal Growth. Hydrothermal growth is used in the form of solution transport for the growth of synthetic quartz. Cmshed natural quartz is placed into the lower part of a high pressure steel vessel, called a bomb, and thin seed plates are located in the upper region, as seen in Figure 5. The vessel is filled, for example, to 80% capacity with a 4% NaOH [1310-73-2] solution the NaOH acts as a mineralizer to increase the solubility of Si02. [Pg.216]


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