Moreover, the self-heating also allows reducing the fabrication complexity, as there is no need of the heater element. Therefore, the sensor could be operated without a heater device with a considerable reduction of its power consumption. If the probing magnitude is increased, the power dissipation through the sensing material, and its temperature, also increases. To obtain the electrical signal of a sensor, the reactive material have to be scanned, usually a current (or voltage) is applied to the sensor, then, the voltage (or current) is read. In addition, another non-common strategy was used to operate the sensor: the so called self-heating effect (or Joule effect). Among these, the use of ultraviolet and visible light sources were tested in order to modulate the sensor properties. The here studied sensors have been characterized with a heater device, but also alternative energy sources and other sensing strategies have been tested in order to reduce the energy cost. Unfortunately, despite the efforts to improve the heater technology, this component is still the most power demanding part of the overall device.
The heater is required in order to stabilize the temperature of operation and to activate a desired chemical reaction. Conductometric gas sensors usually are composed of two main parts: the already mentioned reactive material and the heater device. This material, a specific type of carbon nanofibers (CNFs), shares some suitable properties with other trendy carbon based materials such as carbon nanotubes or graphene.
A carbon based material was chosen to be the reactive compound for the conductometric sensors. Therefore, the sensor material should be compatible with the mentioned properties above. This reaction induces a variation on some electrical property of the material resulting in a change on the electrical signal (conductivity or resistivity of the active material) of the sensor. A chemical reaction between the active material (surface or bulk) and the gas occurs. Conductometric devices base its operating principle on the variation of the electrical conductivity (resistivity) or conductance (resistance) of a reactive (active) material interacting with gas. In this work, conductometric (or resistive) gas sensors are studied. There exist different technologies for gas sensors depending on the transduction mechanism: mass-sensitive, optical, calorimetric, magnetic, electrochemical and conductometric. Gas sensors can be found in many activities ranging from environment protection, risk prevention, agriculture and even in food, chemical, and petrochemical industries.
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