Lead dioxide PbO

Structure. Lead dioxide exhibits two polymorphic forms (i) scrutinyite (a-PbOj) with orthorhombic crystals (a = 497.1 pm, b = 595.6 pm, and c = 543.8 pm) with a density of 9867 kg.m and (ii) plattnerite (P-PbO ) with tetragonal crystals (a = 495.25 pm and c = 338.63 pm) having a rutile-type structure and a density of 9564 kg.m 

Properties. Only plattnerite has attractive features for electrochemical applications such as a low electrical resistivity (40 to 50 p 2.cm), a good chemical and electrochemical corrosion resistance in sulfates media even at low pH, and a high overvoltage for the evolution of oxygen in suhuric- and nitric-acid-containing electrolytes while it withstands chlorine evolution in hydrochloric acid. In fact, the more electrochemically active phase consists 

Failure modes. For lead dioxide supported on lead, the mismatch of strength and thermal expansion between the lead metal substrate and its lead-dioxide ceramic coating leads to flaking and spalling with loss of coating. As mentioned previously, a thin platinum underlayer can 

Thangappan, R. Nachippan, S. Sampath, S. (1970) Lead dioxide-graphite electrode. Ind. Eng. Chem. Prod. Res. Dev., 9(4), 563-567. 

Richert, H. (1980) In Trasatti, S. (ed.) Electrodes of Conductive Metallic Oxides, Part A. Elsevier, Amsterdam, Chap. 4, pp. 183-220. 

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Lead dioxide (PbO,) forms the charged state of the active material in the positive electrode. 

To a certain extent, the formation of the tetrabasic variant is desired, because 4PbO PbS04 forms fairly large crystals when transformed into lead dioxide (PbO,). This results in a mechanically stable active material, but there are disadvantages, because it is more difficult to transform this material into lead dioxide, i.e., the formation process (see below) is more expensive (and takes longer) and the initial capacity is slightly reduced (cf., e.g., Ref. [19]. For "long-life batteries" (Bell systems cell), a special process has been developed to produce pure tetrabasic material [20]. 

Another model assumes that gel zones are formed by hydrated lead dioxide (PbO(OH)2) and act as bridging elements between the crystallite particles. Electrons can move along the polymer chains of this gel and so cause electronic conductivity between the crystalline zones 137],... 

Other oxidizers, including barium chromate (BaCrO,), lead chromate (PbCrO 4), sodium nitrate (NaNO 3), lead dioxide (PbO 2), and barium peroxide (BaO 2) will also be encountered in subsequent chapters. Bear in mind that reactivity and ease of ignition are often related to the melting point of the oxidizer, and the volatility of the reaction products determines the amount of gas that will be formed from a given oxidizer /fuel combination. Table 3.2 contains the physical and chemical properties of the common oxidizers, and Table 5.8 lists the melting and boiling points of some of the common reaction products. 

Phosphorus is element number 15 and the 11th most abundant element in Earths crust. Usually found in the form of phosphates, it exists in two allotropic forms white phosphorus and red phosphorus. Because white phosphorus is poisonous, it is too hazardous for common use. A large amount of red phosphorus, in the form of P Sj, is combined with lead dioxide (PbO ) to coat the tips of matches. When struck, the lead dioxide rapidly oxidizes the phosphorus and the match bursts into flames. With safety matches, the phosphorus and an abrasive coat the side of the box the matches will not ignite unless they are struck on the box in which they came. 

The lead-acid automobile battery is the most familiar secondary battery. As this battery is discharged, metallic lead is oxidized to lead sulfate at the anode, and the Pb + in lead dioxide (PbO ) is reduced at the cathode to the Pb in lead sulfate (PbSO ). The reaction takes place in the presence of sulfuric acid (battery acid), and the equations for the reactions are... 

Lead forms two types of chemical compounds lead (II), and lead (IV) compounds based on Pb " and Pb ions, where those based on Pb ions are the more stable. The metal is oxidized even at room temperature to lead oxide (PbO) and also by water that contains oxygen and forms lead hydroxide (Pb(OH),). In the lead-acid battery, the (less stable) lead (IV) oxide (lead dioxide, PbO ), is of greatest importance. Beside these two, a number of oxides are observed in the battery that are mostly mixtures. A brief survey will now be given of those compounds that are of interest for lead-acid batteries. 

Fig. 9. Indirect electrosynthetic process for the oxidation of anthracene to anthraquinone [10]. The charcoal column serves to remove organics which lead to corrosion and poisoning of the lead dioxide (PbO ) anode...

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