**3. Porous ceramic materials for sensors. Operational principles for gases detection**

The complex operational mechanism of porous ceramic sensors for hydrocarbon gases is affected by factors, such as chemical composition, humidity, temperature, morphology, and so on. This mechanism is determined by chemical and electronic interactions between the porous ceramic and the specific gas resulting in a resistance change. The main operational principles of sensing devices are shown in **Table 1** [36].

There are three principal reasons for monitoring hydrocarbon gases: (1) combustible/flammable gas, (2) toxic/irritant gases and (3) oxygen levels control. As is well known, any hydrocarbon leaks is a potential explosive hazard, where to avoid an explosion, atmospheric levels must be maintained below the lower explosive limit (LEL) for each gas, or purged of oxygen. In addition, for a flame to exist, three conditions must be met: (1) a source of fuel (any hydrocarbon source, e.g., methane or gasoline vapors), (2) enough oxygen (greater than 10–15%) to oxidize or burn the fuel, and (3) a source of heat (ignition) to start the process [36]. Moreover, combustion can occur at both extreme low-end and high-end gas concentrations. These extremes are called the lower explosive limit (LEL) also known as lower flammability limit (LFL), and the upper explosive limit (UEL) or upper flammability limit (UFL). Any gas or vapor concentration that falls between these two limits is in the flammable (explosive) range [36]. Therefore, the control and knowledge about the sensors' operational conditions


are the master key to understand the hydrocarbon gases monitoring process in order to get alarms before a potential explosive condition occurs. On the other hand, the developments of low cost and power efficient devices that can selectively detect hazardous analytes with very high sensitivity have evolved constantly. Two main operating principles have been intensely investigated: catalytic sensors and metal oxide semiconductor (MOS) have been investigated

**Gas detection technology Operational description Gas type detected**

a Wheatstone Bridge. The detecting sensor is exposed to the gas of interest; the compensating sensor is enclosed in a sealed compartment filled with clean air. Exposure to the gas sample causes the detecting sensor to cool, changing the electrical resistance. This change is proportional to the gas concentration. The compensating sensor is used to verify that the temperature change

is caused by the gas of interest and not by other factors

the compound of interest. Ions are collected on a "getter," a current is produced and the concentration of the compound is displayed in

Thermal conductivity Two sensors (detecting and compensating sensors) are built into

Photoionization A photoionization detector (PID) uses an ultraviolet lamp to ionize

parts per million on the instrument meter

**Table 1.** Types of gas detection technologies and their operational descriptions [36].

*C*atalytic sensors are very common to detect combustible gases. These consist of two elements: a detector element which contains the sensitive material and an inert compensator element. The operating principle is based on the oxidation reaction of the combustible gas with the detector element. The heat released by this exothermic reaction changes the electrical resistance of the detector element. Combustible gas does not burn on the compensator element, so its temperature and resistance are maintained unchanged. When sensor is located at combustible gas-free atmosphere, a balance of the bridge circuit is maintained. On the contrary, when combustible gases are present, the resistance of the detector element increases and causes an imbalance in the bridge circuit producing an output voltage signal, which is proportional to the combustible gas concentration. This kind of sensors is very sensitive to environmental conditions such as temperature, humidity, and pressure. On the other hand, basic electrochemical-type sensors consist on a working electrode, a counter electrode, and an ion conductor. An electrical signal is produced by the chemical reaction (oxidation or reduction) of the analyte (gas) with the working electrode, giving a current proportional to the gas concentration that flows between electrodes [38]. This kind of sensor depends on establishment of an electrochemical potential which is not affected by surface morphology. Electrochemical sensors are minimally affected by pressure, but they are very sensitive to

In MOS sensors, a metal oxide is used as sensing material. In a typical MOS sensor, oxygen is

oxygen molecules attract free electrons from the metal oxide which forming a potential barrier to prevent electron flow. If sensor is exposed to certain gas atmosphere, the gas reacts with the

, ZnO, In<sup>2</sup>

O3 , TiO<sup>2</sup>

, and WO<sup>3</sup>

Combustible gas; toxic gases

57

Porous Ceramic Sensors: Hydrocarbon Gas Leaks Detection

http://dx.doi.org/10.5772/intechopen.72315

Toxic (organic compounds)

). These

in sensors for wide range of chemical analytes [37].

adsorbed onto the surface of the sensing material (i.e., SnO<sup>2</sup>

temperature.


**Table 1.** Types of gas detection technologies and their operational descriptions [36].

**Gas detection technology Operational description Gas type detected**

material, and is electrically heated to a temperature that allows it to burn (catalyze) the gas being monitored, releasing heat, and increasing the temperature of the wire. As the temperature of the wire increases, so does its electrical resistance. This resistance is measured by a Wheatstone Bridge circuit and the resulting measurement is converted to an electrical signal used by gas detectors. A second sensor, the compensator, is used to compensate

A semiconducting material (metal oxide) is applied to a nonconducting substrate between two electrodes. The substrate is heated to a temperature at which the presence of the gas can cause a reversible change in the conductivity of the semi-conducting material. When no gas is present, oxygen is ionized onto the surface and the sensor becomes semi-conductive; when molecules of the gas of interest are present, they replace the oxygen ions, decreasing the resistance between the electrodes. This change is measured electrically and is proportional to the concentration of the gas being

Uses an electrically modulated source of IR energy and two detectors that convert the IR energy into electrical signals. Each detector is sensitive to a different range of wavelengths in the IR portion of the spectrum. The source emission is directed through a window in the main enclosure into an open volume. A mirror may be used at the end of this volume to direct the energy back through the window and onto the detectors. The presence of a combustible gas will reduce the intensity of the source emission reaching the analytical detector, but not the intensity of emission reaching the reference detector. The microprocessor monitors the ratio of these

two signals and correlates this to a %LEL reading

cloud width, and readings are given in %LEL/meter.

its molecules generate a pressure pulse. The magnitude of the pressure pulse indicates the gas concentration present

Sensor is a chamber containing a gel or electrolyte and two active electrodes—the measuring (sensing/working) electrode (anode) and the counter electrode (cathode). A third electrode (reference) is used to build up a constant voltage between the anode and the cathode. The gas sample enters the casing through a membrane; oxidation occurs at the anode and reduction takes place at the cathode. When the positive ions flow to the cathode and the negative ions flow to the anode, a current proportional to the gas

to a gas sampling path of up to 100 m. Like point IR monitors, they utilize a dual beam concept. The "sample" beam is in the infrared wavelength which absorbs hydrocarbons, while the second "reference" beam is outside this gas absorbing wavelength. The ratio of the two beams is continuously compared. When no gas is present, the signal ratio is constant; when a gas cloud crosses the beam, the sample signal is absorbed or reduced in proportion to the amount of gas present while the reference beam is not. System calculates the product of the average gas concentration and the gas

Open (long path) infrared Open-path IR monitors expand the concepts of point IR detection

Photoacoustic infrared The gas sample is exposed to infrared light; as it absorbs light,

concentration is generated

Combustible gas

Combustible gas; toxic gas

Combustible gas

Combustible gas

Combustible gas; toxic gas

Toxic gas or oxygen deficiency/ enrichment

Catalytic bead A wire coil is coated with a catalyst-coated glass or ceramic

for temperature, pressure and humidity

measured

Metal oxide semiconductor (also known as "solid

56 Recent Advances in Porous Ceramics

Point infrared (IR) short

Electrochemical for toxic gas or/and oxygen

detection

state")

path

are the master key to understand the hydrocarbon gases monitoring process in order to get alarms before a potential explosive condition occurs. On the other hand, the developments of low cost and power efficient devices that can selectively detect hazardous analytes with very high sensitivity have evolved constantly. Two main operating principles have been intensely investigated: catalytic sensors and metal oxide semiconductor (MOS) have been investigated in sensors for wide range of chemical analytes [37].

*C*atalytic sensors are very common to detect combustible gases. These consist of two elements: a detector element which contains the sensitive material and an inert compensator element. The operating principle is based on the oxidation reaction of the combustible gas with the detector element. The heat released by this exothermic reaction changes the electrical resistance of the detector element. Combustible gas does not burn on the compensator element, so its temperature and resistance are maintained unchanged. When sensor is located at combustible gas-free atmosphere, a balance of the bridge circuit is maintained. On the contrary, when combustible gases are present, the resistance of the detector element increases and causes an imbalance in the bridge circuit producing an output voltage signal, which is proportional to the combustible gas concentration. This kind of sensors is very sensitive to environmental conditions such as temperature, humidity, and pressure. On the other hand, basic electrochemical-type sensors consist on a working electrode, a counter electrode, and an ion conductor. An electrical signal is produced by the chemical reaction (oxidation or reduction) of the analyte (gas) with the working electrode, giving a current proportional to the gas concentration that flows between electrodes [38]. This kind of sensor depends on establishment of an electrochemical potential which is not affected by surface morphology. Electrochemical sensors are minimally affected by pressure, but they are very sensitive to temperature.

In MOS sensors, a metal oxide is used as sensing material. In a typical MOS sensor, oxygen is adsorbed onto the surface of the sensing material (i.e., SnO<sup>2</sup> , ZnO, In<sup>2</sup> O3 , TiO<sup>2</sup> , and WO<sup>3</sup> ). These oxygen molecules attract free electrons from the metal oxide which forming a potential barrier to prevent electron flow. If sensor is exposed to certain gas atmosphere, the gas reacts with the oxygen molecules through a reducing reaction which causes the release of electrons and allows that a current flows freely through the sensor. Consequently, the gas concentration is detected by the resistance change of MOS [4]. This kind of sensor seems to be the most important because the huge amount of investigation regarding to them. Pt nanoparticles-modified Al-doped ZnO (AZO) porous macro/mesoporous nanosheets prepared by solution combustion method were assessed for butane gas sensor at low temperature. The large surface area of 50.17 m<sup>2</sup> /g and the broad pore size distribution between 3 and 110 nm calculated by BJH method provided a good contacting interface between sensing material and gas molecules allowing a maximum response of 56–3000 ppm of butane. The gas sensitivity is related to the electron flow through the interface from AZO to Pt. An oxidation-reduction reaction happened when Pt nanoparticles-modified AZO nanosheets were exposed to reducing gas. A large amount of electrons are released which back to the conduction band of AZO leading a decrease of resistance [39]. Hierarchical flower-like structure composite formed by combination of metal oxides with high surface area and porous structure have shown improved gas sensing performance in comparison with pure metal oxide. NiO:CuO nanocomposites (molar ratio 1:1) showed 2 s response time to 100 ppm NO<sup>2</sup> at room temperature and relative humidity of 42%. The heterojunction formed at the interface between NiO and CuO could accelerate the speed response. O<sup>2</sup> molecules are absorbed on the surface of sensor when it is exposed to air. These O<sup>2</sup> molecules capture electrons from conduction band of sensing material forming ions O<sup>2</sup> − . At NO<sup>2</sup> atmosphere, these gas molecules are adsorbed onto the surface by extracting electron from conduction band. Because electrons are transferred to NO<sup>2</sup> , the resistance is decreased [40].

O2

(ads) + e<sup>−</sup>

dots treated with ZnCl<sup>2</sup>

H2

ZnCl<sup>2</sup>

tive NH<sup>3</sup>

(surface) ↔ O<sup>2</sup>

70.2–100 ppm was reported by using porous SnO<sup>2</sup>

S. Nearly, no response (1.07–50 ppm H<sup>2</sup>

Changes in resistance of porous BiNbO<sup>4</sup>

attributed to the large surface area (41.27 m<sup>2</sup>

sites for adsorption of gas molecules [47].

**thermomechanics principles**

time. The synergistic effect of 2D porous structure of SnO<sup>2</sup>

sensing. The complete oxidization of NH<sup>3</sup>

**4. Specific application: hydrocarbon leak sensors. The** 

extra needs for specially design sensors or sensing technology.

which means a response three times higher than those obtained with SnO<sup>2</sup>

(ads) [43]. The Au nanoparticles act as the catalyst leading oxygen

and annealing at 200–300°C were used to fabricate gas sensor toward

S) was obtained with untreated ZnO sensor whereas

nanostructures-based sensors were reported for selec-

onto the sensor surface makes changes on

/g) and high porosity that increase the accessible

nanosheets loaded with Au nanoparticles,

Porous Ceramic Sensors: Hydrocarbon Gas Leaks Detection

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which allowed facile gas diffusion


59

dissociation. In this way, the koxy in chemical reaction allows an improved sensor response. The resistance values showed a reduction when sensor was exposed to ethanol vapor. If the ethanol exposition is stopped, the sensor resistance returned to the initial state. Moreover, the resistance response is improved when UV illumination is included in tests. This behavior is associated to the large density of active photoelectrons due to the UV excitation [10]. Novel approaches toward ethanol sensor enhancement using decorated metal oxides have recently studied [44]. Improved ethanol sensing at operating temperature of 160°C and response of

response curves increased with increasing ethanol concentration and returned to baseline when ethanol supply was stopped inside test chamber, indicating quick response and recovery

and the catalytic activity of Ag particles allowed superior ethanol-detection properties [45]. The fabrication of NiO/ZnO nanoplates by solvothermal method and thermal treatment is also reported. The ratio of the electrical resistance in air Ra, and electrical resistance in ethanol-air mixed gas was used to determine the sensor performance. The response to ethanol in NiO/ ZnO sensor is better than those in NiO gas sensor. A short response of 2.1 s and time recovery of 4 s promised good sensor performance for ethanol sensing [8]. Colloidal ZnO quantum


the conductivity sensor. The maximum response of 16 s and recovery time less than 17 s were

Diverse fluids used in the petrochemicals processes such as propane, liquid petroleum gas (LPG), butane, propylene; and other organics and inorganics fluids are widely stored, transported, or used in a pressure-liquefied state. Therefore, these fluids are very important in the industry when leak sensing technology is needed. Specifically, pressure-liquefied gas (PLG have a high probability of ending in a fatal accidental leak due to its own fluid properties and behavior [48, 49]. So, sensing technology needed must cover a range of fluids; however, there are some clear examples where the widely and frequently used of some fluids highlight some

The combination of p-type Sb<sup>2</sup> O5 with n-type SnO<sup>2</sup> in composite sensor for NO<sup>2</sup> have also evaluated thought output voltage measurements and the sensing response was defined by the ratio of sensor resistance in NO<sup>2</sup> and air atmospheres. Relevant results indicate low operation temperature (100°C) and high response (800–5 ppm NO<sup>2</sup> ) at short time (5 s) of composite sensor. The existence of p-n junction in composite sensor which induced a new potential barrier when electrons transfer between SnO<sup>2</sup> particles, large surface area and porous structure of Sb<sup>2</sup> O5 and SnO<sup>2</sup> particles gave benefits to exhibit superior gas sensing performance. The reduction of operating temperature was attributed to lower energy required for electron transition derived from the decrease of band gap of p- and n-type semiconductor [41]. In another work, porous hollow balls formed by self-assembly of α-Fe<sup>2</sup> O3 nanoparticles were synthesized by hydrothermal method and were evaluated for sensing ethanol, CO, and NH<sup>3</sup> in a temperature range from 250 to 450°C. Sensor response (S) was determined following the equation S = Rair/Rgas, where Rair and Rgas are the sensor resistances recorded in presence of test gas and dry air, respectively. The initial resistance of α-Fe<sup>2</sup> O3 hollow balls sensor decreasing when operating temperature increasing, leading 85 kΩ at 250°C and 18 kΩ at 450°C in ethanol sensing test. In addition, the sensor response increased from 1.77 to 3.29 with increasing ethanol concentration from 50 to 500 ppm at 400°C. Low sensor response to CO and NH<sup>3</sup> was detected, thus, this sensor is suggested only for ethanol detection [42]. In order to improve its ethanol sensing capability at the temperature range between 25 until 125°C, other researchers have been introduced Au nanoparticles into ZnO nanostructures by sputtering technique. The sensing mechanism is based on the surface electron density changes of the semiconductor. The oxygen molecules in air react to the surface electrons of ZnO forming oxygen species determined by the constant reaction koxy as follows: O2 (ads) + e<sup>−</sup> (surface) ↔ O<sup>2</sup> (ads) [43]. The Au nanoparticles act as the catalyst leading oxygen dissociation. In this way, the koxy in chemical reaction allows an improved sensor response. The resistance values showed a reduction when sensor was exposed to ethanol vapor. If the ethanol exposition is stopped, the sensor resistance returned to the initial state. Moreover, the resistance response is improved when UV illumination is included in tests. This behavior is associated to the large density of active photoelectrons due to the UV excitation [10]. Novel approaches toward ethanol sensor enhancement using decorated metal oxides have recently studied [44]. Improved ethanol sensing at operating temperature of 160°C and response of 70.2–100 ppm was reported by using porous SnO<sup>2</sup> nanosheets loaded with Au nanoparticles, which means a response three times higher than those obtained with SnO<sup>2</sup> -based sensor. The response curves increased with increasing ethanol concentration and returned to baseline when ethanol supply was stopped inside test chamber, indicating quick response and recovery time. The synergistic effect of 2D porous structure of SnO<sup>2</sup> which allowed facile gas diffusion and the catalytic activity of Ag particles allowed superior ethanol-detection properties [45]. The fabrication of NiO/ZnO nanoplates by solvothermal method and thermal treatment is also reported. The ratio of the electrical resistance in air Ra, and electrical resistance in ethanol-air mixed gas was used to determine the sensor performance. The response to ethanol in NiO/ ZnO sensor is better than those in NiO gas sensor. A short response of 2.1 s and time recovery of 4 s promised good sensor performance for ethanol sensing [8]. Colloidal ZnO quantum dots treated with ZnCl<sup>2</sup> and annealing at 200–300°C were used to fabricate gas sensor toward H2 S. Nearly, no response (1.07–50 ppm H<sup>2</sup> S) was obtained with untreated ZnO sensor whereas ZnCl<sup>2</sup> -treatment ZnO sensor showed a response up to 5 with very slow recovery. Recovery properties were improved by annealing at 300°C for 1 h. Sensor under that conditions at room temperature reached a response of 113.5 and recovery time of 820 s. Based on results, the fast and sensitive response makes this sensor very attractive in comparison with others [46]. Changes in resistance of porous BiNbO<sup>4</sup> nanostructures-based sensors were reported for selective NH<sup>3</sup> sensing. The complete oxidization of NH<sup>3</sup> onto the sensor surface makes changes on the conductivity sensor. The maximum response of 16 s and recovery time less than 17 s were attributed to the large surface area (41.27 m<sup>2</sup> /g) and high porosity that increase the accessible sites for adsorption of gas molecules [47].
