**2. Electrochemical synthesis techniques for ceramic oxides**

For the deposition of ceramic oxides, there are three main methods. These include electropho‐ retic, electrolytic (base generation), and direct electrodeposition. Electrophoretic deposition occurs when a high electric field is applied in a solution that contains suspended particles. The charged particles in solution migrate to and then are deposited on the electrode surface. Typically, much higher voltages or currents are used to drive the ions in solution during this deposition compared to electrodeposition. Electrophoretic deposition tends to give thicker coatings than other techniques.

Electrolytic deposition occurs when cathodic reactions produce colloidal particles in solution next to the electrode surface. This method has an electrogeneration of base or local change of pH at the electrode surface. The solution contains metal salts or metal complexes. This may result in powdery or loosely adherent coatings. The cathodic electrodeposition or base generation method for synthesizing oxides was first described by Switzer [32]. The local increase in pH happens either by consumption of hydronium ions or production of hydroxide ions. Depending on the species in solution, both of these mechanisms may be occurring. The most likely reactions include [33]:

Electrochemical Synthesis of Rare Earth Ceramic Oxide Coatings http://dx.doi.org/10.5772/61056 87

$$\mathbf{H}^{+}\mathbf{+}\mathbf{e}^{\cdot}\to\mathbf{H}\_{\text{ads}}\tag{1}$$

$$\text{H}^+ \text{H}^+ + 2\text{e}^\cdot \rightarrow \text{H}\_2 \tag{2}$$

$$\text{2H}\_2\text{O} + 2\text{e}^\cdot \rightarrow \text{H}\_2\text{+2OH}^\cdot\tag{3}$$

$$\rm O\_2 + 2H\_2O + 4e^- \rightarrow 4OH^- \tag{4}$$

If a nitrate salt is present in the solution then hydronium ions can be consumed or hydroxide ions produced by:

structural ceramics can be used to coat specific components exposed to extreme condi‐ tions, such as high temperature, high stress, or high friction. Of the ceramic oxides, rare earth oxides (REOs) are interesting materials and are a type of ceramic oxide that has many promising properties. Rare earth oxides can be used to color glass, for example, Er2O3 adds a light pink color while Sm2O3 produces a yellow color [4, 5]. REOs are used in the making of phosphors or fluorescent lighting [6, 7]. Rare earth oxides such as cerium oxide have also been important in automotive catalytic converters [8]. The most common stoichiome‐ try for rare earth oxides is R2O3; however other compounds containing Ce, Pr, or Tb can exhibit several oxide phases, ROx (1.5 < x < 2) and compounds like CeO2, Pr6O11, and Tb4O7 are common. Applications for rare earth oxide coatings include gas sensors [9, 10], fuel cells

There is a long list of processing techniques for producing rare earth oxides including spray hydrolysis, pulsed laser deposition, chemical vapor deposition, solid state reactions, sol–gel method, and melt infiltration [19-24]. However, electrochemical synthesis has not been used extensively to deposit rare earth oxide coatings. The electrodeposition mechanism can be complex for many of the reactions and present a formidable challenge. In fact the majority of the electrodeposition work has focused on cerium oxide coatings and powders [25-28]. Typically, the redox potential for the rare earth oxides is not readily accessible in aqueous solutions, making synthesis difficult. But electrochemical deposition offers several advantages including low processing temperature, control of the driving force, and deposition onto various shapes [29-31]. This chapter covers the electrochemical deposition (not electrophoretic

For the deposition of ceramic oxides, there are three main methods. These include electropho‐ retic, electrolytic (base generation), and direct electrodeposition. Electrophoretic deposition occurs when a high electric field is applied in a solution that contains suspended particles. The charged particles in solution migrate to and then are deposited on the electrode surface. Typically, much higher voltages or currents are used to drive the ions in solution during this deposition compared to electrodeposition. Electrophoretic deposition tends to give thicker

Electrolytic deposition occurs when cathodic reactions produce colloidal particles in solution next to the electrode surface. This method has an electrogeneration of base or local change of pH at the electrode surface. The solution contains metal salts or metal complexes. This may result in powdery or loosely adherent coatings. The cathodic electrodeposition or base generation method for synthesizing oxides was first described by Switzer [32]. The local increase in pH happens either by consumption of hydronium ions or production of hydroxide ions. Depending on the species in solution, both of these mechanisms may be occurring. The

[11, 12], catalysis [13, 14], and corrosion protection [1, 15-18].

or soaking methods) of rare earth oxides as films for various applications.

**2. Electrochemical synthesis techniques for ceramic oxides**

coatings than other techniques.

86 Advanced Ceramic Processing

most likely reactions include [33]:

$$\rm NO\_3^+ + 2H^+ + 2e^\cdot \rightarrow NO\_2^\cdot + H\_2O \tag{5}$$

$$\rm{NO}\_3^- + 10\rm{H}^+ + 8e^- \rightarrow \rm{NH}\_4^+ + 3\rm{H}\_2\rm{O} \tag{6}$$

$$\text{NiO}\_3\text{'} + \text{H}\_2\text{O} + 2\text{e'} \rightarrow \text{NO}\_2\text{'} + 2\text{OH'} \tag{7}$$

$$\text{NO}\_3^- + 7\text{H}\_2\text{O} + 8\text{e}^\cdot \rightarrow \text{NH}\_4^+ + 10\text{OH}^\cdot \tag{8}$$

With any of these reactions, the local pH at the electrode surface is increased and can be as high as 11–12 compared to the lower pH in the bulk of the solution.

Direct electrodeposition occurs when there is a direct oxidation or reduction (exchange of electrons) between the metal ion or metal ion complex and electrode to produce the metal oxide on the surface. This method is typical for electrochemical reactions, such as reduction of metal ions in solution to produce pure metal on an electrode surface (plating).

In this chapter, we cover only electrolytic and direct electrodeposition for the production of rare earth oxide coatings. The oxides or hydroxides of lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, and dysprosium have been electro‐ chemically produced. To date, the bulk of the electrochemical literature covers cerium oxide (CeO2), about 40:1 compared to the other rare earth oxides. It must be noted that in aqueous solutions, the lanthanide hydroxides are stable in alkaline solutions but return to their corresponding cations in acid solutions [34]. This is true for all the lanthanide series; however, a few do have a stable oxide that may be accessible during deposition under the correct conditions. These include CeO2, PrO2, NdO2, and TbO2. In practice, this means that most of the rare earths are deposited as hydroxides or hydrated oxide species and post-treatment is needed to produce the desired stoichiometry. We will make note of this in our discussions when applicable for each section.
