Na P-aluminas

Fig. 9 Schematic cross-section of a R thick film electrode on a Na+-p-alumina solid electrolyte of a type III potentiometric COg sensor.

In order to sustain this reaction at the sink side of the PEVD system, a source is required at the other side of the substrate (anode) to supply sodium. Otherwise, depletion of sodium in the Na" -P"-alumina solid electrolyte will lead to an a-alumina phase buildup at the anode that will block the ionic transport path of the PEVD system. The electrolytic properties of the solid electrolyte in this PEVD system will then be lost. Elemental sodium, for instance, could be the source giving the following anodic reaction ... 

Figure 13 is an SEM SE plan-view image of another PEVD sample, where the Pt thick film coverage at the sink side of the sample is not entirely continuous. After PEVD, the product (Na2C03) only formed in area (A), where the Pt thick film of the working electrode is continuous and connected to the external circuit. No PEVD product formed in area (B), where the Pt thick film is discontinuous appearing as individual islands (white spots in the figure) on the Na -P"-alumina substrate. 

Fig. 14 Cross section SE image from a cleaved sample. Area (A) is the Na+-p"-alumina solid electrolyte area (B) is the Pt thick film electrode and area (C) is the PEVD product...

Selective deposition in PEVD is clearly indicated in Figure 13. As discussed previously, this is because of the unique feature of the electrochemical reaction for deposition. As schematically shown in Figure 18, the only area to meet the requirement for nucleation is the three phase boundary (point O) of Na -P"-alumina, Pt and gaseous phase, where Na, e and gaseous phase reactants (CO + O ) are all... 

The X direction is along the surface of the solid electrolyte Na+-P"-alumina, where Na+ and... 

The current in a PEVD process reveals the kinetics of the PEVD cathode reactions which, in turn, indicates the PEVD product growth behavior. Since the electronic transference number in Na+-P -alumina is less than 10" and can be ignored under the current experimental conditions, it is reasonable to assume that the only current passing through the internal circuit of the PEVD system is the sodium ionic current. 

The photoelectron microscopy results described above were necessarily acquired in vacuum. Under such conditions there is no doubt that the Na is present on the Pt surface as sodium metal. However, under reaction conditions, this cannot be the case it is to be expected that the alkali would be present as a submonolayer quantity of surface compound, and indeed this is just what is observed. Furthermore, also in accordance with expectation, the nature of the alkali promoter compound is dependent on the composition of the gas atmosphere. Figure 7 shows postreaction XPS and XANES spectra acquired from Pt/Na P" alumina EP samples after exposure to reaction conditions and without exposure to laboratory atmosphere for the Pt-catalyzed reactions NO-fpropene [Figure 7(a)] and 02-fpropene [Figure 7(b)], respectively. In the first case the promoter phase consists of a mixture of NaN02 and NaNOs, in the second case it consists of Na2C03. This is important... 

Figure 7.1 Arrhenius plots for ionic conductivity. (1) Li2SO4 [2] (2) Agl [2] (3) LisN [22] (4) Na-p-alumina [23], Plots (1) and (2) produced with polycrystals plots (3) and (4) produced with single crystals conductivity perpendicular to the hexagonal axis.

Figure 7.13 Conducting layers in the structures of Na alumina [57] (a) and Mg- or Zn-stabilized Na-P"-alumina [130, 131] (b), viewed alongthe hexagonal axis. White balls —oxygen small black balls = aluminum gray balls = sodium sites (only partially filled).

Trivalent ion-exchange reactions for polycrystalline Na -P"-alumina have rarely been reported (e.g., for Na /Nd -P"-alumina [38]), mainly because ion exchange into Na -P"-alumina polycrystals is much more difficult than for a single crystalline material, especially in the case of trivalent cations. However, no electrical conductivity measurements were conducted on these samples. 

In a similar way, Na2SO4 has been used as an auxiliary electrode with sodium ion conductors (Na P alumina [179] or NASICON [180]). As with carbonates for CO2 sensors, mixed sulfates (e.g., Na2SO4-BaSO4 [181]) can be used for the auxiliary electrode. At low oxygen partial pressures, sulfur maybe present as H2S, rather than as SOx, in which case sulfides can be used as auxiliary electrodes. For example, CaS has been used with a CaF2 electrolyte for measuring H2S gas concentrations [182]. 

Na P-alumina can be replaced by cations such as Li+, K+, Cs+, Rb+, Ag, Tl and H+. However, the conductivities of these materials are lower than that of Na P-alumina the match between the size of the Na ions and the interlayer channels in the host lattice leads to the most efficient cation mobility. The conductivities of Na P-alumina and selected cation and anion conductors exhibiting relatively... 

This is achieved in the electrochemical CO2 sensor to be described now. The gas sensitive electrode in the emf sensors for CO2 detection is based on an alkahne or alkahne earth carbonate.If NazCOs is to be used, Na- p -alumina or Nasicon (Part 1 ) are suitable electrolytes. A thermodynamically well-defined sensor is obtained by using a mixture of a binary and a ternary oxide such as ... 

NaZrOg, ZrOg, Au Na-p-alumina Au, CO2, NagCOj with the cell reaction... 

Systems which work only at elevated temperatures, e.g. Na/P-alumina/S or Li/salt melt/FeS , have a potential for traction and load-levelling purposes. The reader is referred to reviews... 

Fig. 28.2 A schematic representation of part of the structure of Na P-alumina (Na20-llAl203) in which Na ions are mobile between bridged layers of AI2O3 spinel structure. Spinels were introduced in Box 13.6.

At 300°C, the ionic conductivity of Na3Zr2P04(Si04)2 (x = 2) is as good as that of Na-p alumina (Table 12.51). In contrast to the pure phosphate (x = 0), the other end member, Na4Zr2(Si04)3 (x = 3) has both sodium sites filled. While the two end members are rhombohedral, around x = 1.8-2.2, there is a slight lattice distortion to monoclinic and this is associated with the sharp increase in ionic conductivity (Table 12.65). 

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