**10.5. Sensors**

Compacted sinters of Ln9.33+x/3Si6−xAlxO26 (0 ≤ *x* ≤ 2.0, Ln = La, Nd and Sm) are composed of an apatite-like phase with a hexagonal structure. Compacted sinters were used as potentiomet‐ ric oxygen sensors (**Fig. 11**). The concentration dependence of EMF was well expressed by the Nernst equation:

$$\mathbf{E}\_{\rm obs} = \frac{\mathbf{R}\mathbf{T}}{\mathbf{n}\mathbf{F}} \text{ } \ln \frac{\mathbf{p}\_{\rm O\_2}\left(\mathbf{I}\right)}{\mathbf{p}\_{\rm O\_2}\left(\mathbf{II}\right)}\tag{11}$$

Furthermore, the electron number *n* is comparable to the theoretical value of 4. The sensing characteristics of the sinters are comparable to those of the sensors with 3 and 8 mol.% YSZ [86].

**Fig. 11.** Schematic picture of the O2 concentration cell with the electrolyte of Ln9.83Si4.5Al1.5O26 where Ln = La, Nd and Sm [86].

Electrical properties and humidity sensor characteristics of lead hydroxyapatite material were reported by TUDORACHE et al [87]. The electrical characteristics of lead hydroxyapatite materi‐ al treated at different temperatures made us focus on the analysis of the influence of water vapors upon the electrical characteristics. Thus, the electrical response to humidity adsorp‐ tive processes of lead hydroxyapatite material suggested that we analyze the material characteristics in terms of its use as a humidity sensor.

The humidity-sensitivity of yttrium-substituted calcium oxyhydroxyapatites was studied by OWADA et al [88]. The logarithm of electricalresistance of the present sensors decreased linearly with increasing relative humidity (RH) from 30 to 65%. The resistance of [Ca9.0Y1.0] (PO4)6[O1.5□0.5] with the highest OH vacancy content was about one order of magnitude lower than that of calcium hydroxyapatite. It was found that the larger ratio of surface hydroxyl groups per unit surface area in the sample, the lower the resistance and the higher the amount of OH vacancies.

Using the hydroxylapatite ceramics as CO2 sensor, which is based on electrical conductivity changes, was investigated by NAGAI et al [89]. Starting powders prepared by usual wet process were cast in film with an organic vehicle and fired on alumina substrates after the electrodes had been arranged. It was necessary to soak the samples in a CaCl2 solution in order to make them reactive with CO2. Both D.C. and A.C. measurements were carried out in various atmospheres including air, CO2 and air containing different amounts of CO2.
