Metal oxides volumes

Pilling-Bedworfli ratio (P-B ratio). The ratio of metal oxide volume to metal volume used to predict whether a scale that forms will protect a metal from further oxidation, pitting. A form of very localized corrosion in which small pits or holes form, usually in a vertical direction, plain carbon steel. A ferrous alloy in which carbon is the prime alloying element. 

The manufacture of metal in powder form is a complex and highly engineered operation. It is dominated by the variables of the powder, namely those that are closely connected with an individual powder particle, those that refer to the mass of particles which form the powder, and those that refer to the voids in the particles themselves. In a mass of loosely piled powder, >60% of the volume consists of voids. The primary methods for the manufacture of metal powders are atomization, the reduction of metal oxides, and electrolytic deposition (15,16). Typical metal powder particle shapes are shown in Figure 5. 

Approximately 5% of the U.S. consumption of is in agriculture. Boron is a necessary trace nutrient for plants and is added in small quantities to a number of fertilizers. Borates are also used in crop sprays for fast rehef of boron deficiency. Borates, when apphed at relatively high concentration, act as nonselective herbicides. Small quantities of borates are used in the manufacture of alloys and refractories (qv). Molten borates readily dissolve other metal oxides usage as a flux in metallurgy is an important apphcation. Other important small volume apphcations for borates are in fire retardants for both plastics and ceUulosic materials, in hydrocarbon fuels for fungus control, and in automotive antifreeze for corrosion control (see Corrosion and corrosion inhibitors). Borates are used as neutron absorbers in nuclear reactors. Several borates, which are registered with the Environmental Protection Agency (EPA) can be used for insecticidal purposes, eg, TIM-BOR. 

Sohd rocket propellants represent a very special case of a particulate composite ia which inorganic propellant particles, about 75% by volume, are bound ia an organic matrix such as polyurethane. An essential requirement is that the composite be uniform to promote a steady burning reaction (1). Further examples of particulate composites are those with metal matrices and iaclude cermets, which consist of ceramic particles ia a metal matrix, and dispersion hardened alloys, ia which the particles may be metal oxides or intermetallic compounds with smaller diameters and lower volume fractions than those ia cermets (1). The general nature of particulate reinforcement is such that the resulting composite material is macroscopicaHy isotropic. 

Design nd Operation. The destruction efficiency of a catalytic oxidation system is determined by the system design. It is impossible to predict a priori the temperature and residence time needed to obtain a given level of conversion of a mixture in a catalytic oxidation system. Control efficiency is determined by process characteristics such as concentration of VOCs emitted, flow rate, process fluctuations that may occur in flow rate, temperature, concentrations of other materials in the process stream, and the governing permit regulation, such as the mass-emission limit. Design and operational characteristics that can affect the destmction efficiency include inlet temperature to the catalyst bed, volume of catalyst, and quantity and type of noble metal or metal oxide used. 

Such behavior is observed with alkah and alkahne earth oxidations where the oxide volume is less than the metal volume and cracks develop in the product coat, permitting ready access for further reaction. 

Volume 45 Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung... 

Metal Oxide/metal volume ratio (r) Metal Oxide/metal volume ratio (r)... 

The oxygen vacancies then diffuse to the gas interface where they are annihilated by reaction with adsorbed oxygen. The important point, however, is that metal is consumed and oxide formed in the same reaction zone. The oxide drift has thus only to accommodate the net volume difference between the metal and its equivalent amount of oxide. In theory this net volume change could represent an increase or a decrease in the volume of the system, but in practice all metal oxides in which anionic diffusion predominates have a lower metal density than that of the original metal. There is thus a net expansion and the oxide drift is away from the metal. 

Table 21.18 Structures, thermal data, and molar volumes of metal oxides and hydroxides, and of some double oxides ...

Perhaps the first practical application of carbonaceous materials in batteries was demonstrated in 1868 by Georges Le-clanche in cells that bear his name [20]. Coarsely ground MnO, was mixed with an equal volume of retort carbon to form the positive electrode. Carbonaceous powdered materials such as acetylene black and graphite are commonly used to enhance the conductivity of electrodes in alkaline batteries. The particle morphology plays a significant role, particularly when carbon blacks are used in batteries as an electrode additive to enhance the electronic conductivity. One of the most common carbon blacks which is used as an additive to enhance the electronic conductivity of electrodes that contain metal oxides is acetylene black. A detailed discussion on the desirable properties of acetylene black in Leclanche cells is provided by Bregazzi [21], A suitable carbon for this application should have characteristics that include (i) low resistivity in the presence of the electrolyte and active electrode material, (ii) absorption and retention of a significant... 

The characteristics of a metal which contribute to a good incendiary capability are not yet fully understood nor have they been completely defined. Hillstrom (Ref 64) found that pyrophoricity is related to the ratio of the oxide volume to the metal volume, a ratio above one giving rise to the popcoming effect which is... 

From the coverage made thus far, it may be of interest to record in one place the different factors which influence the rate of chemical reactions. The rate of chemical reaction depends essentially on four factors. The nature of reactants and products is one. For example, certain physical properties of the reactants and products govern the rate. As a specific example in this context mention may be of oxidation of metals. The volume ratio of metallic oxide to metal may indicate that a given oxidation reaction will be fast when the oxide is porous, or slow when the oxide is nonporous, thus presenting a diffusion barrier to the metal or to oxygen. The other two factors are concentration and temperature effects, which are detailed in Sections. The fourth factor is the presence of catalysts. 

In the reaction shown above, the volume of the reaction products (2 mol CO) is seen to be much greater than that of the reactants (2 mol of solid carbon plus 1 mol of oxygen). The effect of pressure on the free energy of formation of an oxide associated with an increase in the number of gas molecules which is representative of the type of reaction in the present illustration is shown in Figure 4.2 (A). Applying the criterion of volume increase per mole accompanying reaction at standard state to the case of metal oxidation such as... 

Table 4. Reduction of cell volumes of ternary transition metal oxides (M Y OJ relative to the volume of ternary Mg-containing oxides...

Crucibles must be suitable for the required experimental conditions with respect to their material, capacity and shape. As in the techniques of the chemical laboratory, crucible materials are selected to avoid the possibility of reaction between crucible and sample material. The main materials used are the precious metals, oxide ceramics, quartz and graphite. The size of the crucible is determined by the volume to be weighed. 

Measured surface-to-volume ratio of metal oxides were too low to explain observed hydrogen uptakes. 

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