Oxide films solution-derived

ZnO films for use as buffer layers in photovoltaic cells (see Chap. 9) have been chemically deposited from aqueous solutions of ZnS04 and ammonia [57]. The solution was heated to 65°C, and adherent, compact Zn(OH)2 + ZnO films were formed after one hour. Low-temperature annealing converted the hydroxide to oxide. The solution composition will be important in this deposition. On one hand, increased ammonia concentration will increase the pH and therefore the homogeneous Zn(OH)2 precipitation in solution. However, further increase in ammonia concentration will redissolve the hydroxide as the ammine complex. There will clearly be an optimum ammonia (and zinc) concentration where Zn(OH)2 does form, but slowly enough to prevent massive homogeneous precipitation. The use of ammonia in (hydr)oxide deposition derives, in part at least, from its gradual loss by evaporation if the system is not closed [58], Any open solution of an ammonia-complexed metal ion (which forms an insoluble hydroxide or hydrated oxide) should eventually precipitate the (hydr)oxide for this reason alone. 

The parameters in these equations are as follows x> a> P, and n are the oxidation state of the cation in the barrier layer the polarizability of the film/solution interface (i.e., the dependence of the potential drop across the film/solution interface on the applied voltage) the dependence of the potential drop across the film/solution interface on the pH and the kinetic order of the film dissolution reaction with respect to hydrogen ion concentration, respectively. Note that, in deriving Eqs (12) and (13), the oxidation state of the cation in the barrier layer (x) is set equal to the oxidation state of the same cation in the solution/outer layer. The standard rate constants, k , and a, correspond to the reaction shown in Fig. 4, e is the electric field strength, Y = F/RT, and K = sy. The three terms on the right side of Eq. (12) arise from the transmission of cation interstitials, the transmission of cation vacancies, and the transmission of oxygen vacancies (or dissolution of the film), respectively. Values for these parameters are readily obtained by optimizing the PDM on... 

Crystallization and reduction of sol-gel prepared zinc oxide films derived from zinc acetate by irradiation with an UV lamp (185 and 254 nm) was studied (Asakuma et al. 2003). UV irradiation induced the formation of hexagonal ZnO crystals from amorphous ZnO films preheated at 100 C, while irradiation of porous ZnO films preheated at 60 C led also to formation of metallic zinc. Composite ZnO/Cu and ZnO/Ag/Cu nanostructures were prepared via the photocatalytic reduction (wavelength 310-390 nm) of cuprous chloride and silver nitrate over the chemically prepared ZnO nanoparticles in aqueous solution (Shvalagin et al. 2004). Amorphous ZnO thin films were prepared... 

In some cases, the solubility of the oxides and hydroxides derived from Mg and/or its alloying elements in an aqueous solution can also affect the composition of the surface film. The lower solubility of A1 oxides and hydroxides compared with Mg oxides and hydroxides should contribute to the Al/Mg ratios within the film, too. 

Bhuiyan, M.S., Paranthaman, M., and Salama, K. (2006) Solution-derived textured oxide thin films a review. Supercond, Sci. TechnoL, 19 (2), R1-R21. 

Under exposure to heavy-particle radiation, the protective oxide film forms on the metal as it does out-of-radiatioii, and the kinetics of the protective oxide formation are about the same in and out of radiation. Under irradiation, however, the film does not continue to increase in thickness. Radiation produces defects of unspecified nature in the protective oxide, and in the pre.sence of these defects, the oxide breaks up and/or reacts with the solution to form a nonprotective scale. The rate at which the protectiv c oxide breaks up is proportional to the concentration of defects in the oxide at the oxide-solution interface. Under these conditions, a steady state is established in which oxide is removed at a rate equal to the rate of formation and in which a steady-state thi( kness of film results. The corrosion rate is determined by the rate of transfer of reagents across this film. The defects are produced at a rate proportional to the intensity of radiation and are removed by thermal annealing at a rate proportional to the concentration of defects. In the derivation of the general equation, it was assumed that the rate of oxidation of the metal, R, at a given protective film thickness, X, is given by... 

Polyaniline (PANI) can be formed by electrochemical oxidation of aniline in aqueous acid, or by polymerization of aniline using an aqueous solution of ammonium thiosulfate and hydrochloric acid. This polymer is finding increasing use as a "transparent electrode" in semiconducting devices. To improve processibiHty, a large number of substituted polyanilines have been prepared. The sulfonated form of PANI is water soluble, and can be prepared by treatment of PANI with fuming sulfuric acid (31). A variety of other soluble substituted AJ-alkylsulfonic acid self-doped derivatives have been synthesized that possess moderate conductivity and allow facile preparation of spincoated thin films (32). 

Figure 13.5 Potential-step electro-oxidation of formic acid on a Pt/Vulcan thin-film electrode (7 p,gptcm, geometric area 0.28 cm ) in 0.5 M H2SO4 solution containing 0.1 M HCOOH upon stepping the potential from 0.16 to 0.6 V (electrol)Te flow rate 5 p,L s at room temperature). (a) Solid line, faradaic current transients dashed line, partial current for HCOOH oxidation to CO2. (b) Solid line, m/z = 44 ion current transients gray line, potential-step oxidation of pre-adsorbed CO derived upon HCOOH adsorption at 0.16 V, in HCOOH-ftee H2SO4 solution.

Desizing by chemical decomposition is applicable to starch-based sizes. Since starch and its hydrophilic derivatives are soluble in water, it might be assumed that a simple alkaline rinse with surfactant would be sufficient to effect removal from the fibre. As is also the case with some other size polymers, however, once the starch solution has dried to a film on the fibre surface it is much more difficult to effect rehydration and dissolution. Thus controlled chemical degradation is required to disintegrate and solubilise the size film without damaging the cellulosic fibre. Enzymatic, oxidative and hydrolytic degradation methods can be used. 

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