Vanadium-pentoxide sol

Rheopectic behavior is the opposite of thixotropy. Shear stress increases with time at constant shear rate. Rheopeclic behavior has been obsei ved in bentonite sols, vanadium pentoxide sols, and gypsum suspensions in water (Bauer and Colhns, ibid.) as well as in some... 

Vanadium pentoxide sols can be employed to bring about coagulation of positively charged colloids for example, ferric hydroxide and aluminium hydroxide. The amount necessary for the coagulation of a given quantity of the positive colloid is very small in comparison with the required quantities of arsenic trisulphide, antimony trisulphide, and other negative colloids. It appears, therefore, that the colloidal... 

Rheopectic fluids have apparent viscosities that increase with time, particularly at high rates of shear as shown on Figure 6.3. Figure 6.2(f) indicates typical hysteresis effects for such materials. Some examples are suspensions of gypsum in water, bentonite sols, vanadium pentoxide sols, and the polyester of Figure 6.3. 

The diameters of atoms and molecules of classical chemistry lie below one half mfjL. The region of colloids has usually been chosen to begin at a dimension of 1 mp. and to end upwards at about 1 (jl, where the region of emulsions and suspensions begins. In the mean time there are objections of various kinds against the choice of these limits. In the first place there is as the basis of this old subdivision the supposition that one is always dealing with practically spherical particles, which has proved frequently not to be the case. Particles which have these small sizes in one dimension (e.g. discshaped particles) or in two dimensions (e.g. needle-shaped particles) but which arc otherwise much larger, behave as colloids both when the small sizes correspond with those of small molecules (e.g. in linear macromolecules) and when the latter He within the colloid limits (e.g. in vanadium pentoxide sol). 

A colloidal suspension of conductive vanadium pentoxide [130] can be used to perform intercalation, adsorption or encapsulation of electroactive molecules or biomolecules for electrodes or biosensor realization [131]. Encapsulation of glucose oxidase in nanocomposite films made with polyvinyl alcohol and V205 sol-gel matrix or in ferrocene intercalated V2Os sol-gel [132] were envisaged to prepare glucose biosensors. 

Vanadium pentoxide reacts with caustic soda to form a series of water-sol-... 

The preparation of lyophilic sols is easy and most of the time a mixture of the dispersion medium and the substance to be dispersed need only be stirred. Gelatine, for example, disperses almost spontaneously in water. The hydroxides of iron, aluminium, chromium and zirconium as well as vanadium pentoxide and silicic acids all belong to the group of hydrophilic colloids. 

The vanadium pentoxide cataKtic membrane reactor was prepared by coating its sol inside the Vycor tube membrane. After heat treatment of the prepared membrane, the [010] planes of vanadium pentoxide layer were grown largely which contributes to partial oxidation reaction of 1-butene to maleic anhydride. The partial oxidation of 1-butene to maleic anhydride was carried out in the catalytic membrane reactor. The maximum selectivity of 95% was obtained at 350 °C when the surface velocity was 500cm/h. And at this condition, oxygen permeability was almost four times higher than the reaction had not occured. 

Furthermore it appears that the thixotropic state can best (or perhaps only) be obtained with sols containing disc-shaped or rod-shaped particles, such as bentonite (a colloidal clay), ferric oxide, vanadium pentoxide, etc. 

The second class of sol-gels contains redox-active metal oxides, such as tungsten oxide, vanadium pentoxide, manganese oxide, and other transition metal oxides. Moreover, many n-type semiconductors such as zinc oxide, barium titanate, and titanium dioxide can be used in this class (113). The structures of these gels are sensitive to the pH and oxidation state of the precursors. Many redox-active sol-gels exhibit electrochromism (different oxidation states exhibit different colors, allowing spectroscopic determination of redox states). These gels can also accommodate the reductive insertion of lithium and other moieties. 

The V2O5 System. Vanadium pentoxide films obtained by the sol-gel process have recently received a strong interest for their applications as reversible cathodes for Li batteries (Livage, 1991). Layered structures ofVaOs-l.bHaO xerogel have been shown to be efficient in reversible electrochemical storage of Li+ ions (Araki, 1983), via the reaction ... 

Vinod M.P., Bahnemann D. Materials for all-solid-state thin-film rechargeable lithium batteries by sol-gel processing. J. Solid State Electrochem. 2002 6 498-501 Vivier V., Farcy J., Pereira-Ramos J.P. Electrochemical lithium insertion in sol-gel crystalline vanadium pentoxide thin films. Electrochim. Acta 1998 44 831-839 Wang J., Bell J.M., Skryabin I.L. Kinetics ofcharge injection in sol-gel deposited WO3. Solar Ener. Mater. Solar Cells 1999 56 465-475... 

If the ageing is not only based upon the influence of the free surface energy, but is promoted by an allotropic phase change, ageing phenomena may be still more pronounced A good example is the sol of vanadium pentoxide which, when first formed, contains particles with a true solubility of the order of 1 g/1, whereas on standing, these particles disappear, and are replaced by the characteristic rod-shaped particles, while the solubility decreases to about 0 1 g/1. 

In most cases hydrophobic sols are NEWTONian liquids with a viscosity only slightly deviating from that of the dispersion medium. In the flocculated state more or less clear indications of non-NEWTONian behaviour, especially the presence of a yield value, may be detected. This is most evident for sols containing blade-shaped or rodshaped particles like iron oxide or vanadium pentoxide. When these sols are not too dilute and electrolyte is added to them in quantities insufficient to obtain flocculationp gel formation is often observed. The who.le mass becomes solid, probably becauste by partial flocculation the particles of the sol form a loose network, in the meshes of which the intermiccllar liquid is retained. 

Several colloidal systems containing anisodimensional particles may under certain circumstances show a reversible separation into two phases. The oldest example is the iron hydroxide sol described by Cotton and Mouton The same phenomenon has been studied more extensively on iron hydroxide, vanadium pentoxide, tungsten trioxi-de, benzopurpurin and other dyes by Zocher % Szegvari Heller and recently on tobacco mosaic virus by BerNal and Fankuchen and cn cucumber virus by Bawden and Pterie ... 

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