What Parameters Affect the Selectivity and the Sensitivity of the Gas Response?

The two main problems with most semiconducting oxide gas resistors are their poor sensitivity at low gas concentration and mainly their lack o f selectivity. Thus, most metal oxides appear to respond to a wide spectrum o f reducing gases, thereby making difficult to assign the signal to a particular gas o f interest in a mixture o f several. The question is, therefore, whether by choice o f oxide materials or some other means, selectivity can be enhanced. Many methods have been tried to solve this problem such as the use of:

• filters where only the gases o f interest can reach the sensor while the others are adsorbed [2 0].

• thermal cycling o f the sensor element [2 1].

• promoters and catalysts such as Pt, Pd [22] and Pd/Au [23].

In this work the effects o f microstructure and stoichiometry, combustion gradients, surface segregation on gas response have been explored as well as systematic cation substitution in solid solution compounds as a route to understanding the surface chemistry mediating the selectivity and the sensitivity o f the gas response.

1.6.1 Microstructure

As previously mentioned, a gas to be detected is adsorbed on the surface or may react with the reactive surface oxygen o f the semiconducting oxide. This induces a change in the surface space layer, which is then converted into a change in electrical resistance o f the polycrystalline elements. The gas recognition is carried out through the surface chemical processes, or, more specifically the adsorption sites or reaction sites play the role o f receptor for gases.

On the other hand, the physical processes associated with the transport o f electrons are responsible for the conversion o f such an interaction to the electrical resistance change. This propriety is controlled not only by the semiconductive properties o f the oxide but also by the microstructure o f the element such as the grain size o f the

particles relative to the Debye length and the geometry o f the connection between particle [24].

Hence, so far the surface chemical processes have been investigated thoroughly, but the processes concerned with transducer function (by opposition with receptor function associated with gas recognition) have not been very well studied due to the complexity o f practical polycrystalline elements. A characteristic change in behavior o f electrical resistance at critical crystallite size, has been observed and interpreted [25] as the formation o f an electron-depleted charge layer on the particles. The proportion o f space charge region in each particle decreases with increasing D, since the thickness {L) o f the space charge layer is kept constant. The critical value o f D

has been assumed to be related to a stage where D = 2L. For D > 2L, the depletion is not so much dependent on the particle size and takes place on the surface only. On the other hand, for D < 2L where the whole region o f the particles is depleted o f electrons, the degree o f depletion will be mitigated sharply as D increases.

To try to understand these phenomena, one must look into the microstructure o f polycrystalline elements. The geometry o f contacts between particles is important as the phenomenon is related to electron transport. Hence, it has been showed that necks form between particles. A model described in Fig 1.9. consisting o f a large number o f necks and a small number o f direct grain boundary contacts has been widely accepted.

Three cases have been differentiated:

• D < 2L (grain control). Each particle is included in the space charge region. Electron transport inside the particle becomes dependent on the surface effect.

• D = 2L (neck control). Each channel is narrow enough to be resistive to the electron conduction. The neck size-dependent sensitivity is due to fact that the resistance o f the materials is controlled by the resistance o f the necks, which are in excess compared to the grain boundary contacts.

D » 2L (grain boundary control). The resistance at grain boundary contacts determines the whole resistance which leads to a gas sensitivity independent o f D.

Grain boundary control (open neck)

Neck control (closed neck)

0

D » 2 L

^

D>2L

Grain control

D<2L

Fig 1.9. M odel fo r conductance accounting fo r the grain size effects. Unshaded part shows core region (low resistivity), while the shaded region indicates space charge

region (high resistivity)

1.6.2. Temperature

The temperature is an important factor because some gas (e.g., alcohols or CO) are easier to oxidise than others (e.g., CH4 and alkanes). Thus, a low temperature would induce selectivity toward alcohols or CO, and a high selectivity towards CH4.

1.6.3. Combustion Gradients

Regardless o f the mechanism proposed for a gas response o f the conductance, it is clear that, at the elevated temperature necessary to observe such a response, a surface- catalyzed combustion of the gas being measured occurs. The sensor response, determined by the gas composition in the vicinity of the sensor surface, is dependent