Metal oxide layers

The result is the formation of a dense and uniform metal oxide layer in which the deposition rate is controlled by the diffusion rate of ionic species and the concentration of electronic charge carriers. This procedure is used to fabricate the thin layer of soHd electrolyte (yttria-stabilized 2irconia) and the interconnection (Mg-doped lanthanum chromite). 

Nitric acid reacts with all metals except gold, iridium, platinum, rhodium, tantalum, titanium, and certain alloys. It reacts violentiy with sodium and potassium to produce nitrogen. Most metals are converted iato nitrates arsenic, antimony, and tin form oxides. Chrome, iron, and aluminum readily dissolve ia dilute nitric acid but with concentrated acid form a metal oxide layer that passivates the metal, ie, prevents further reaction. 

Corrosion Control. Sihca in water exposed to various metals leads to the formation of a surface less susceptible to corrosion. A likely explanation is the formation of metahosihcate complexes at the metal—water interface after an initial dismption of the metal oxide layer and formation of an active site. This modified surface is expected to be more resistant to subsequent corrosive action via lowered surface activity or reduced diffusion. 

As a second example, results from a TOP ERDA measurement for a multi-element sample are shown in Fig. 3.65 [3.171]. The sample consists of different metal-metal oxide layers on a boron silicate glass. The projectiles are 120-MeV Kr ions. It can be seen that many different recoil ions can be separated from the most intense line, produced by the scattered projectiles. Figure 3.66 shows the energy spectra for O and Al recoils calculated from the measured TOF spectra, together with simulated spectra using the SIMNRA code. The concentration and thickness of the O and Al layers are obtained from the simulations. 

Since the formation of the Grignard compound takes place at the metal surface, a metal oxide layer deactivates the metal, and prevents the reaction from starting. Such an unreactive metal surface can be activated for instance by the addition of small amounts of iodine or bromine. 

Figure 34. Dissolution of metal through a metal oxide layer with complex formation.

Figure 35. Amplitude factor of the symmetrical fluctuation for anodic dissolution through a metal oxide layer with complex formation. Dm = 1.0 x 10-9 m2 s-1, Jt = 1.0 x 10"5 nr s-1 mol-1, m = 2, m = 2 1.Curves 1,2, and 3 correspond to the surface concentrations of the anion, (C (jr, yt 0)) = 10, 50, and 100 mol m-J, respectively.

Another way to protect a metal uses an impervious metal oxide layer. This process is known as passivation, hi some cases, passivation is a natural process. Aluminum oxidizes readily in air, but the result of oxidation is a thin protective layer of AI2 O3 through which O2 cannot readily penetrate. Aluminum oxide adheres to the surface of unoxidized aluminum, protecting the metal from further reaction with O2. Passivation is not effective for iron, because iron oxide is porous and does not adhere well to the metal. Rust continually flakes off the surface of the metal, exposing fresh iron to the atmosphere. Alloying iron with nickel or chromium, whose oxides adhere well to metal surfaces, can be used to prevent corrosion. For example, stainless steel contains as much as 17% chromium and 10% nickel, whose oxides adhere to the metal surface and prevent corrosion. 

In some cases, the oxide-coating protects the surface from further oxide buildup. One example is that of aluminum where an oxide coating appears almost instantaneously once the pristine surface is exposed to air. Yet, there are many cases where the oxide layer continues to buildup until the metal is totally consumed (One example is that of iron and "rust"). How is this possible Wagner hypothesized that both metal and oxide ions difiosed through the metal oxide layer so as to build up the layer thickness from both sides. The following diagram is one representation of such a mechanism ... 

Y. Cao, Thin metal-oxide layer as stable electron-injecting electrode for light emitting diodes, PCTInt. Appl., WO 2000022683, pp. 37, (2000). 

Fig. 12.1 Main structural models of graphene-metal oxide hybrids, (a) Anchored model oxide particles are anchored to the graphene surface, (b) Encapsulated model oxide particles are encapsulated by graphene, (c) Sandwich-like model graphene is sandwiched between the metal oxide layers, (d) Layered model a structure composed of alternating layers of oxide nanoparticles and graphene, (e) Mixed model graphene and oxide particles are mechanically mixed and graphene sheets form a conductive network among the oxide particles. Red metal oxide Blue graphene. Reprinted with permission from [41]. Copyright 2012, Elsevier B.V.

The heat produced by the reaction of a pyrolant is dependent on various physicochemical properties, such as the chemical nature of the fuel and oxidizer, the fractions in which they are mixed, and their physical shapes and sizes. Metal particles are commonly used as fuel components of pyrolants. When a metal particle is oxidized by gaseous oxidizer fragments, an oxide layer is formed that coats the particle. If the melting point of the oxide layer is higher than that of the metal particle, the metal oxide layer prevents further supply of the oxidizer fragments to the metal, and so the oxidation remains incomplete. If, however, the melting point of the oxide layer is lower than that of the metal particle, the oxide layer is easily removed and the oxidation reaction can continue. 

Upon exposure to oxygen, all metals form surface metal oxide layers which vary in thickness and structure, depending on the identity of the base metal and the oxide formation conditions. Mercury and noble metals generally form very thin oxide films. On the other hand, most metals of primary commercial importance (i.e. aluminum, iron, zinc, etc.), tend to form oxide layers which are thick enough (40-80 A or more), so that the underlying metal atoms do not contribute in an appreciable way to the adhesion forces in metal/polymer systems U). 

Water can reduce adhesion strength by reducing the strength of the metal oxide layer via hydration52,81 . Hydration of the oxide layer is detrimental because the resulting aluminum-, iron-, or other metal-hydrates generally exhibit very poor adhesion to their base metals 52 Therefore, the produced layer of hydrates will effectively act as a weak boundary layer in the system and decrease adhesion strength. Since the hydration reaction has been most heavily studied on aluminum oxides, the authors have chosen to base the discussion of the hydration mechanism on this case. 

Failure can occur in metal/epoxy adhesion systems in any one or more of a number of different regions. The fracture may propagate through the bulk metal or epoxy, the metal oxide layer, the metal oxide/epoxy or metal/metal oxide interfaces, or through weak boundary layers (WBL s) very near the interfaces. Some workers -78,153) be]ieve that most failures that have been claimed to be interfacial have actually... 

Chromate compounds have been considered by many to be the best inhibitors available. They are typically composed of mixtures of sodium bichromate and chromic acid, and art through passivation of the metal surfaces. Passivation involves formation of a tough metal oxide layer or other film on the surfaces. Chromate concentrations of 200 to 1000 ppm in the cooling water are generally required, although for environments where bimetallic influences exist, chromate levels must be much higher. For instance, when steel and copper surfaces are present in the system, chromate levels often exceed 2000 ppm (BETZ 1982, pp. 207, 212). 

If local stresses exceed the forces of cohesion between atoms or lattice molecules, the crystal cracks. Micro- and macrocracks have a pronounced influence on the course of chemical reactions. We mention three different examples of technical importance for illustration. 1) The spallation of metal oxide layers during the high temperature corrosion of metals, 2) hydrogen embrittlement of steel, and 3) transformation hardening of ceramic materials based on energy consuming phase transformations in the dilated zone of an advancing crack tip. 

Figure 1. High-voltage gradient across metal oxide layer.

The results obtained for the solar cell discussed above suggest a strong interaction between the chromophore and the metal oxide layer, a large surface area, thus yielding large absorbances, and an efficient charge-separation upon injection. Several studies have indeed been carried out in an attempt to utilize the potential of nanocrystalline metal oxides as substrates for electrochromic devices. A particularly interesting approach has been reported by Fitzmaurice and co-workers [17]. These authors have constructed an electrochromic device based on the combination of... 

Nitric acid reacts with all metals except the precious metal series and certain alloys. Although chromium, iron and aluminum readily dissolve in dilute nitric acid, the concentrated acid forms a metal oxide layer that protects (passivates) the metal from further oxidation53. 

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