Lead oxide scale

Another important class of titanates that can be produced by hydrothermal synthesis processes are those in the lead zirconate—lead titanate (PZT) family. These piezoelectric materials are widely used in manufacture of ultrasonic transducers, sensors, and minia ture actuators. The electrical properties of these materials are derived from the formation of a homogeneous soHd solution of the oxide end members. The process consists of preparing a coprecipitated titanium—zirconium hydroxide gel. The gel reacts with lead oxide in water to form crystalline PZT particles having an average size of about 1 ]lni (Eig. 3b). A process has been developed at BatteUe (Columbus, Ohio) to the pilot-scale level (5-kg/h). 

In some circumstances, the reaction rates may not be exactly parabolic, and even initially parabolic rates may be influenced by changes within the oxide scale with time. As an oxide scale grows, the build-up of inherent growth stresses, externally applied strains and chemical changes to either oxide scale or metal may all compromise the initial protection offered by the scale, leading to scale breakdown and ultimately partial or complete loss of protection paralinear, or linear kinetics may ensue. In other circumstances, as will be seen later in this chapter, very small additions of contaminants to... 

The tensile failure strain of oxides grown on EN2 steel between 600°C and 900°C lies in the range 1 x 10 " to 2.5 x 10""" . Components in service may be stressed beyond these failure strains, leading to scale cracking. 

An interesting case of ash attack is encountered with valves in engines powered by high octane fuels containing lead compounds. These compounds are deposited from the gases as mixtures of lead oxide, sulphate and bromide, and can cause serious scale-fluxing effects with high-alloy valve steels. 

Whilst chlorine-based processes are well understood from a mechanistic viewpoint, there are differences between these and the permonosulphuric acid processes. Understanding of the mechanism of permonosulphuric acid treatment has improved in recent years but there are still aspects requiring elucidation [300]. An important difference between these two types of oxidative treatment is that chlorine-based processes lead to scale modification or... 

Traditional alloy design emphasizes surface and structural stability, but not the electrical conductivity of the scale formed during oxidation. In SOFC interconnect applications, the oxidation scale is part of the electrical circuit, so its conductivity is important. Thus, alloying practices used in the past may not be fully compatible with high-scale electrical conductivity. For example, Si, often a residual element in alloy substrates, leads to formation of a silica sublayer between scale and metal substrate. Immiscible with chromia and electrically insulating [112], the silica sublayer would increase electrical resistance, in particular if the subscale is continuous. 

Lead Oxide, Yellow, or Litharge (Lead Monoxide, Lead Protoxide or Plumbous Oxide). PbO, mw 223.21, yellow to yellowish-red, heavy, odorless pdr or minute, cryst scales mp 888°, bp (vol at red heat) d 9.53g/cc insol in w, ale sol in acet ac, dil nitric acid, in warm solns of fixed alkali hydroxides. It may be prepd in the lab by heating Pb nitrate, carbonate or hydroxide commercially, it is made by heating Pb to a temp considerably above its mp and continually skimming off the litharge produced. It is used in some primer compns... 

Production. In large-scale production, lead or lead oxide is reacted with nitric acid to give lead nitrate solutions, which are then mixed with sodium dichromate solution. If the precipitation solutions contain sulfate, lead sulfochromate is formed as a mixed-phase pigment. After stabilization the pigment is filtered off, washed until free of electrolyte, dried, and ground. 

The aforementioned requirements on surface stability are typical for all exposed areas of the metallic interconnect, as well as other metallic components in a SOFC stack (e.g., some designs use metallic frames to support the ceramic cell). In addition, the protection layer for the interconnect, or in particular the active areas that interface with electrodes and are in the path of electric current, must be electrically conductive. This conductivity requirement differentiates the interconnect protection layer from many traditional surface modifications as well as nonactive areas of interconnects and other components in SOFC stacks, where only surface stability is emphasized. While the electrical conductivity is usually dominated by their electronic conductivity, conductive oxides for protection layer applications often demonstrate a nonnegligible oxygen ion conductivity as well, which leads to scale growth beneath the protection layer. With this in mind, a high electrical conductivity is always desirable for the protection layers, along with low chromium cation and oxygen anion diffusivity. 

Fig. 6. Schematic diagrams demonstrating the fluxes of AI and Ni caused by oxidation of NiAl, leading to vacancy formation and condensation as voids beneath the oxide scale.

The intensity curves iz(t) for a-Al203 from Fig. 15 correspond to the mass gain curves for the total oxide scale in Fig. 8. Both thermogravimetry and in situ X-ray diffraction lead to the result that 0-NiAl + Ce forms a thicker oxide scale than the other... 

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