Gas sensors are the core of gas detection systems and are typically installed inside the probe head. Essentially, a gas sensor is a converter that transforms the volume fraction of a gas into a corresponding electrical signal. The probe head conditions the gas sample through the gas sensor, usually including filtering out impurities and interfering gases, drying or cooling, sample aspiration, and even chemically treating the sample to enable faster measurements by the chemical sensor.
Gases are diverse in type and properties, resulting in a wide variety of gas sensors. Based on the properties of the gas being detected, they can be categorized as follows: sensors for detecting flammable and explosive gases, such as hydrogen, carbon monoxide, methane, and gasoline fumes; sensors for detecting toxic gases, such as chlorine, hydrogen sulfide, and arsine; sensors for detecting gases used in industrial processes, such as oxygen in steel furnaces and carbon dioxide in heat treatment furnaces; and sensors for detecting air pollution, such as NOx, CH4, and O3 (which form acid rain) and formaldehyde (a household pollutant). Based on their structure, gas sensors can be classified as dry or wet types; based on their output, they can be classified as resistive or non-resistive types; and based on the method of detection, they can be categorized as electrochemical, electrical, optical, and chemical methods.
Semiconductor gas sensor
Semiconductor gas sensors can be divided into resistive and non-resistive types (junction type, MOSFET type, and capacitive type). The principle of resistive gas sensors is that gas molecules cause changes in the resistance of the sensitive material; non-resistive gas sensors mainly include MOSFET diodes, junction diodes, and field-effect transistors (MOSFETs). They utilize the principle that the sensitive gas changes the turn-on voltage of the MOSFET, and their principle and structure are the same as those of ISFET ion sensors.
Resistive semiconductor gas sensor
Working principle
It has been discovered that materials such as SnO2, ZnO, Fe2O3, Cr2O3, MgO, and NiO2 all exhibit gas-sensing effects. Gas-sensitive thin films made from these metal oxides are impedance devices; gas molecules can exchange ions with the sensitive film, undergoing a reduction reaction that causes a change in the film's resistance. As a sensor, this reaction must be reversible, meaning that an oxidation reaction must occur to eliminate gas molecules. A heater within the sensor facilitates the oxidation process. SnO2 thin-film gas-sensitive devices are currently the mainstream product due to their good stability, ability to operate at relatively low temperatures, wide range of detectable gases, and mature manufacturing processes. In addition, Fe2O3 is also a widely used and researched material. Besides the traditional three major categories of SnO, SnO2, and Fe2O3, a number of new materials have been researched and developed, including single metal oxide materials, composite metal oxide materials, and mixed metal oxide materials. The research and development of these new materials has greatly improved the characteristics and application range of gas sensors.
Selectivity is a key performance characteristic of gas sensors. Since SnO2 thin films are sensitive to a variety of gases, improving the selectivity and sensitivity of SnO2 gas-sensitive devices has been a major research focus. Key measures include: adding different noble metals or metal oxide catalysts to the matrix material, setting appropriate operating temperatures, and using filtration equipment or external filtration of the sensitive gas using a permeable membrane. Doping within the SnO2 material is a primary method for improving sensor selectivity. Adding noble metals such as Pt, Pd, and Ir not only effectively improves the sensitivity and response time of the element, but also, different catalysts lead to different adsorption tendencies, thereby improving selectivity. For example, doping SnO2 gas-sensitive materials with noble metals such as Pt, Pd, and Au can increase sensitivity to CH4, doping with Ir can decrease sensitivity to CH4, doping with Pt and Au can increase sensitivity to H2, and doping with Pd can decrease sensitivity to H2.
Operating temperature affects the sensitivity of the sensor. The left figure below shows the resistance characteristic curves of the SnO2 gas sensor to various gases at different temperatures. As can be seen from the figure, the device's sensitivity to various gases differs at different temperatures, and this characteristic can be used to identify the type of gas.
The preparation process also significantly affects the gas-sensitive properties of SnO2. For example, adding ThO2 to SnO2 and changing the sintering and heating temperatures can produce different gas-sensitive effects. By mass, adding 3-5% ThO2 and 5% Sm2 to SnO2, and sintering it in a H2 atmosphere at 600℃ to create a thick-film device with an operating temperature of 400℃, can be used as a CO detection device. The right figure above shows the characteristics of the gas-sensitive device at a sintering temperature of 600℃. It can be seen that within the operating temperature range of 170-200℃, the sensitivity curve for H2 is parabolic, while changing the operating temperature has little effect on CO. Therefore, this characteristic of the device can be used to detect H2. The device prepared at a sintering temperature of 400℃, with an operating temperature of 200℃, shows sensitivity curves for both H2 and CO that are approximately linear, but the sensitivity for CO is much higher, making it suitable for fabricating a CO-sensitive gas sensor.
Structure and parameters
SnO2 resistive gas-sensitive devices are typically manufactured using a sintering process. Porous SnO2 ceramic is used as the substrate material, with various other substances added, and then sintered using a ceramic-making process. During sintering, heating resistance wires and measuring electrodes are embedded. In addition, thin-film and multilayer film devices are also available, fabricated using processes such as evaporation and sputtering. These devices offer high sensitivity and good dynamic characteristics. Thick-film and hybrid-film devices are also available, fabricated using screen printing processes. These devices offer advantages such as high integration, easy assembly, convenient use, and suitability for mass production.
The image below shows a typical structure of a resistive gas sensor, which mainly consists of a SnO2 sensing element, a heater, electrode leads, a base, and a stainless steel mesh cover. This sensor has a simple structure, is easy to use, and can detect reducing gases, flammable gases, vapors, etc.
The main characteristic parameters of resistive gas sensors are:
1. Inherent resistance R0 and operating resistance Rs
The inherent resistance Ro, also known as the normal resistance, represents the resistance of the gas sensor under normal air conditions. The operating resistance Rs represents the resistance of the gas sensor in a specific concentration of the gas being measured.
2. Sensitivity S
It is usually expressed as S=Rs/R0, and sometimes it is also expressed as the ratio of the resistance of the element in the detection gas of two different concentrations (C1, C2): S=Rs(C2)/R0(C1).
3. Response time T1
Reflecting the dynamic characteristics of a sensor, it is defined as the time required for the sensor's resistance to rise from a certain concentration of gas to a stable value at that concentration. It is also commonly expressed as the time required to reach 63% of the rate of change of resistance at that concentration.
4. Recover time T2
Also known as desorption time. It reflects the dynamic characteristics of the sensor and is defined as the time from when the sensor is removed from the detected gas until the sensor resistance value recovers to the value under normal air conditions.
5. Heating resistance RH and heating power PH
RH is the resistance of the heating wire that provides the operating temperature for the sensor, and PH is the heating power required to maintain the normal operating temperature.
Resistive gas sensors have advantages such as low cost, simple manufacturing, high sensitivity, fast response speed, long life, low sensitivity to humidity, and simple circuitry. However, they also have disadvantages, including the need to operate at high temperatures, poor gas selectivity, dispersed component parameters, less than ideal stability, high power requirements, and susceptibility to poisoning when the detected gas contains sulfides.
Non-resistive semiconductor gas sensor
Non-resistive gas sensors are also a common type of semiconductor gas sensor. These devices are easy to use, require no temperature setting, and are easy to integrate, leading to their widespread application. They mainly come in two types: junction type and MOSFET type.
Junction gas-sensitive devices
Junction gas sensors, also known as gas-sensitive diodes, operate by utilizing the alteration of diode rectification characteristics by a gas. Their structure is shown in the left figure below. The principle is as follows: the noble metal Pd exhibits selectivity for hydrogen, forming a contact barrier with the semiconductor. When a forward bias is applied to the diode, the number of electrons flowing from the semiconductor to the metal increases, thus enabling forward conduction. When a negative bias is applied, the number of charge carriers remains essentially unchanged; this is the rectification characteristic of a Schottky diode. In the detection atmosphere, due to the adsorption of hydrogen, the work function of the noble metal changes, weakening the contact barrier and leading to an increase in charge carriers and forward current. This causes the diode's rectification characteristic curve to shift to the left. The right figure below shows the characteristic curves of a Pd-TiO2 gas-sensitive diode in air with different concentrations of H2. Therefore, the hydrogen concentration can be detected by measuring the diode's forward current.
MOSFET type gas sensor
The leftward shift of the characteristic curve of a gas-sensitive diode can be seen as a change in the diode's forward voltage. If this characteristic occurs at the gate of a field-effect transistor (FET), it will change the FET's threshold voltage UT. This principle can be used to fabricate MOSFET-type gas-sensitive devices.
The hydrogen-sensitive MOSFET is a typical gas-sensitive device, using a palladium (Pd) gate. In a hydrogen atmosphere, due to the catalytic effect of palladium, hydrogen molecules decompose into hydrogen atoms that diffuse to the palladium-silicon dioxide interface, ultimately causing a change in the MOSFET's threshold voltage UT. In practice, the gate and drain are often short-circuited to ensure the MOSFET operates in the saturation region, where the drain current ID = β(UGS - UT)². This circuit can be used to measure the hydrogen concentration.
The characteristics of hydrogen-sensitive MOSFETs include:
1. Sensitivity
When the hydrogen concentration is low, the hydrogen-sensitive MOSFET is highly sensitive; a 1 ppm change in hydrogen concentration can result in a ΔUT value of 10 mV. However, when the hydrogen concentration is high, the sensor's sensitivity decreases.
2. Gas selectivity
The "gaps" between palladium atoms are just enough to allow hydrogen atoms to pass through, so the palladium gate only allows hydrogen to pass through, which is very selective.
3. Response time
The response time of this device is affected by temperature and hydrogen concentration. Generally, the higher the temperature and the higher the hydrogen concentration, the faster the response. The response time at room temperature is tens of seconds.
4. Stability
In practical applications, the UT exhibits a drift characteristic over time. To address this, growing a SiO2 insulating layer in an HCl atmosphere can significantly improve the drift of the UT.
Except for hydrogen, other gases cannot pass through the palladium gate. Therefore, certain measures are required to fabricate Pd-MOSFET gas sensors for other gases. For example, when fabricating a CO-sensitive MOSFET, a small hole of about 20 nm is made in the palladium gate to allow CO gas to pass through. Furthermore, since Pd-MOSFETs have high sensitivity to hydrogen but low sensitivity to CO, a protective layer of aluminum about 20 nm thick can be evaporated onto the palladium gate to prevent hydrogen from passing through. Palladium has a weak catalytic effect on the decomposition reaction of ammonia; therefore, an active metal layer, such as Pt, Ir, or La, must first be deposited on the SiO2 insulating layer before fabricating the palladium gate to create an ammonia-sensitive MOSFET.
Solid electrolyte gas sensor
Solid electrolytes are solid materials with the same ionic conductivity as aqueous electrolyte solutions. When used as gas sensors, they function as batteries. They eliminate the need for the gas to pass through a permeable membrane and dissolve in the electrolyte, avoiding problems such as solution evaporation and electrode consumption. Due to their high conductivity, sensitivity, and selectivity, these sensors are widely used in almost every field, including petrochemicals, environmental protection, mining, and food processing. Their importance is second only to metal-oxide-semiconductor gas sensors.
Solid electrolyte oxygen sensor principle
Bulk electrolytes only exhibit significant conductivity at high temperatures. Zirconia (ZrO2) is a typical material for gas sensors. Pure zirconium oxide has a monoclinic crystal structure at room temperature. When the temperature rises to around 1000℃, it undergoes an allotropic transformation, changing from a monoclinic to a polycrystalline structure, accompanied by volume shrinkage and an endothermic reaction, thus making it an unstable structure. Adding stabilizers such as alkaline earth calcium oxide (CaO) or rare earth yttrium oxide (Y2O3) to ZrO2 makes it a stable cubic fluorite crystal; the degree of stability depends on the concentration of the stabilizer. After adding stabilizers, ZrO2 is sintered at 1800℃ in an atmosphere, during which some zirconium ions are replaced by calcium ions, forming (ZrO·CaO). Since Ca²⁺ is a divalent ion and Zr⁴⁺ is a tetravalent ion, oxygen ion O²⁻ vacancies are generated within the crystal to maintain electroneutrality. This is why (ZrO·CaO) can conduct oxygen ions at high temperatures, resulting in (ZrO·CaO) becoming a conductor of oxygen ions at 300–800℃. However, for it to truly conduct oxygen ions, there must be different oxygen partial pressures (oxygen potential difference) on both sides of the solid electrolyte, forming what is known as a concentration cell. Its structural principle is shown in the figure, with porous noble metal electrodes on both sides forming a sandwich structure with a dense ZrO·CaO material in the middle.
Suppose the oxygen partial pressures on both sides of the electrode are PO2(1) and PO2(2), respectively. The following reaction occurs at the two electrodes:
(+) pole: PO2(2), 2O2-→O2+4e
(-) pole: PO1(1), O2 + 4e → 2O2-
The electromotive force of the above reaction is expressed by the Nernst equation:
It can be seen that, at a certain temperature, with PO2(1) fixed, the concentration of oxygen to be measured at the sensor (+) electrode can be calculated using the above formula.
Fixed PO2(1) is actually an electrode with a fixed potential formed at the (-) electrode, i.e., a reference electrode. There are two types: gas reference electrodes and coexisting phase reference electrodes. Gas reference electrodes can be air or other mixed gases, such as H2-H2O, CO-CO2, which can also form fixed PO2(1). Coexisting phase reference electrodes refer to mixed powders (solid phase) of metal-metal oxides and low-valence metal oxides-high-valence metal oxides. These mixtures can form a fixed oxygen pressure by undergoing an oxidation reaction when mixed with oxygen (gas phase), and therefore can also be used as reference electrodes.
Besides oxygen measurement, solid electrolyte sensors such as β-Al₂O₃, carbonates, and NASICON can also be used to measure gases such as CO, SO₂, and NH₄. In recent years, gas sensors using antimony acid and La₃F, which can be used at low temperatures, have also emerged and can be used to detect positive ions.
Infrared gas sensor
Working principle
Molecules composed of different atoms have unique vibrational and rotational frequencies. When they are irradiated with infrared light of the same frequency, infrared absorption occurs, causing a change in infrared light intensity. Gas concentration can be determined by measuring this change in infrared intensity. It's important to note that vibration and rotation are two different modes of motion, each corresponding to a different infrared absorption peak. Vibration and rotation themselves also exhibit diversity; therefore, a gas molecule typically has multiple infrared absorption peaks. The position of a single infrared absorption peak can only determine the functional groups within the gas molecule. Accurate gas identification requires examining the positions of all absorption peaks in the mid-infrared region—the gas's infrared absorption fingerprint. However, under known environmental conditions, the position of a single infrared absorption peak can roughly determine the type of gas. Since all substances above absolute zero (minus 273 degrees Celsius) emit infrared radiation, and infrared radiation is positively correlated with temperature, infrared gas sensors, similar to catalytic elements, consist of a pair of infrared detectors to eliminate changes in infrared radiation caused by variations in ambient temperature.
A complete infrared gas sensor consists of an infrared light source, an optical cavity, an infrared detector, and a signal conditioning circuit.
Why can't infrared gas sensors measure gas molecules such as oxygen, hydrogen, and nitrogen, which are composed of the same atoms?
The Moon and Earth, and Earth and the Sun, are connected by gravity, while atoms within molecules are connected by chemical bonds. If both were ideal spheres without other gravitational interference, Earth's orbit would be circular. However, neither of these conditions holds true; therefore, its orbit is elliptical. This means the distance between Earth and the Sun constantly shifts between the minor and major radii, i.e., vibrating, with a period lasting a year. During this process, the gravitational force between Earth and the Sun differs at the minor and major radius points, indicating different energy levels. Similarly, within molecules, atoms are connected by chemical bonds. The spatial distance, angle, and direction between atoms constantly change due to uneven electron distribution, i.e., vibration and rotation. Different molecules have unique vibrational and rotational frequencies. When exposed to infrared radiation of the same frequency, resonance occurs, changing the interatomic distance and electron distribution, i.e., the dipole moment. This is how infrared absorption occurs (the same applies to ultraviolet absorption).
The above content includes two fundamental conditions for infrared absorption: resonance and dipole moment change. Both conditions must be met simultaneously for infrared absorption to occur.
Why do molecules composed of the same type of atoms, such as oxygen, hydrogen, and nitrogen, not have infrared absorption peaks? There are two fundamental conditions: first, the vibrational frequency of the gas molecules must be the same as the frequency of the irradiated infrared radiation; second, the dipole moment must change. It's easy to understand that the first condition is readily met, while the second is impossible.
Molecules composed of identical atoms have completely overlapping centers of positive and negative charge, meaning their dipole moment is zero. As a result, the distribution of electrons within the molecule is balanced. Given the low energy density of infrared light, irradiation will not alter this balance, nor will it ionize the molecule, thus causing no energy change. However, molecules composed of different atoms—for example, water (vapor) molecules—have electrons distributed towards the oxygen end. Microscopically, the hydrogen end of a water molecule is positively charged, and the oxygen end is negatively charged. The centers of positive and negative charge do not overlap, meaning their dipole moment is not zero. This is because oxygen attracts electrons more strongly than hydrogen.
When water molecules are irradiated with infrared radiation at the same vibrational and rotational frequencies as water molecules, the distribution of electrons within the water molecule becomes more biased towards the oxygen end. This results in a shorter average distance between hydrogen and oxygen atoms, i.e., a shorter dipole moment and higher energy. Consequently, water molecules exposed to infrared radiation transition from lower energy levels to higher energy levels, which is how infrared absorption occurs. This can be simply understood as follows: when infrared radiation encounters molecules composed of identical atoms, the interaction is a perfectly elastic collision because these molecules are ideal elastic spheres; there is only energy exchange, no energy transfer. However, when molecules composed of different atoms interact with infrared radiation, energy transfer occurs. Therefore, the principle of infrared absorption cannot measure molecules composed of identical atoms.
Nondispersive infrared absorption gas sensor
Non-dispersive: White light passing through a prism will be separated into seven colors: red, orange, yellow, green, cyan, blue, and violet. This prism is a beam-splitting system that can separate these seven colors. Optical systems with beam-splitting systems are called dispersive optical systems, while those without are called non-dispersive optical systems. Non-dispersive systems are simple, reliable, compact, and inexpensive. The white light, ultraviolet light, and infrared light we perceive are all mixtures of different frequencies and wavelengths; while light of a single frequency and wavelength is monochromatic light. As mentioned earlier, infrared absorption only occurs when the frequency of infrared radiation matches the vibrational and rotational frequency of gas molecules. Theoretically, when designing gas sensors, we want to use monochromatic light to illuminate the gas or obtain monochromatic light by setting up a grating (filter) after illumination.
Nondispersive infrared gas sensors typically consist of a light source, an optical cavity, a filter (grating), a detector, and a signal conditioning circuit. In the sensor, the filter and the detector are integrated.
Advantages of infrared gas sensors:
1. All gases, except those composed of the same atoms, can be measured.
2. Full range.
3. The sensing process itself does not interfere with sensing.
shortcoming:
1. Expensive. Infrared gas sensors are essentially temperature sensors where infrared radiation causes changes in the detector's temperature, which in turn affects its electrical properties. The sensing process is complex. The system requires the following characteristics: a stable infrared radiation source; stable physical and chemical properties of the optical cavity; and stable filters and infrared detectors. While these issues can be largely resolved with appropriate manufacturing techniques, the high production costs result in an expensive price.
2. In a typical design using a broadband infrared light source, filter, and detector, the filter itself cannot achieve ideal selective filtering; therefore, interference, especially from water, is always present. The underlying reason for this selectivity issue is that many different gas molecules share the same chemical bonds, resulting in similar or even overlapping infrared absorption.
3. Dust, background radiation, strong adsorption, and substances that easily undergo gas-liquid-solid transformation can all affect the test results.
Catalytic combustion gas sensor
Working principle
Generally, a high-purity platinum coil with a wire diameter of 15um, 20um, or 30um is wrapped with a sphere in the form of a catalyst carrier. Under certain temperature conditions, when a combustible gas comes into contact with the sphere, it will undergo a violent flameless combustion reaction with the adsorbed oxygen on its surface. The heat released by the reaction causes a change in the temperature of the platinum coil, which in turn causes a change in the resistance of the platinum coil. The gas concentration can be measured by measuring the change in resistance.
Therefore, it is more accurate to say that the catalytic element is a temperature sensor rather than a gas sensor. To overcome the interference caused by changes in ambient temperature, the catalytic elements are paired to form a complete element. One of the elements reacts to the gas, while the other does not react to the gas but only to the ambient temperature. In this way, the two elements counteract each other and can eliminate the interference caused by changes in ambient temperature.
Unlike semiconductor elements, the sensing process of catalytic elements is more complex. In the former, the chemical reaction that occurs after the gas comes into contact with the sensor directly leads to a change in the sensor's resistance, i.e., the electrical signal. In the latter, the chemical reaction that occurs on the catalytic element first results in a change in the temperature of the sensor carrier surface and interior. The temperature change of the carrier, through heat transfer, ultimately leads to a change in the resistance of the platinum coil, thus completing the entire sensing process.
Existing problems
The complexity of the sensing process increases the likelihood of problems.
1. For long-chain organic compounds and unsaturated hydrocarbons, incomplete reactions leading to carbon buildup in semiconductors only affect the reaction process and do not significantly impact electron transport. However, in catalysis, the presence of carbon not only affects the reaction process but also drastically impacts heat transfer. As a result, the efficiency of heat transfer from the reaction to the sensor becomes lower, and most of the heat is lost. Ultimately, for the same gas concentration and the same amount of heat released, the presence of carbon results in only a small change in sensor temperature, meaning the sensitivity becomes very low.
2. Because heat transfer is required, the reaction must be completed instantaneously to ensure thermal efficiency, which requires extremely high reaction efficiency. This necessitates a large number of nanoscale catalysts and nanoscale pores. Such characteristics are beneficial for both sensing and poisoning.
3. The linearity of the catalytic element is determined by two factors: a) The resistance-temperature characteristic of the temperature sensing material, the platinum coil, is linear. b) The exothermic reaction and gas concentration are linear within the lower explosive limit. Therefore, a change in either factor will cause a linear change in the sensor. In reality, the platinum coil will continuously sublimate and become thinner, i.e., the conduction resistance will increase; the linear relationship between the heat released by the reaction and the concentration only holds true when the gas concentration is within the lower explosive limit.
Future Development
The future of catalytic elements depends primarily on advancements in process technology.
1. Structural improvements address the issue of drift caused by vibration.
2. The improved filter layer solves the problem of poisoning.
3. Develop new materials to improve carbon buildup.
4. The manufacturing process ensures the realization of the design, such as avoiding deformation.
5. MEMS Integration. It's important to note that improvements in device structure, packaging, and manufacturing processes not only enhance the overall performance of the device but also lead to new applications. Compared to semiconductors, the challenge of MEMS integration for catalytic elements lies in achieving higher catalytic and thermal efficiency within a small surface area.
6. The application and positioning of catalytic elements will be more precise and specific.
7. Catalytic elements will not be phased out.
Electrochemical sensors
Electrochemistry is the study of the relationship between electrical and chemical behavior. Its most important applications are the efficient conversion between electrical and chemical energy and high-power-density energy storage technologies. We know that a sensor is essentially an energy conversion device; for example, a pressure sensor converts mechanical energy into electrical energy. Therefore, it's easy to understand that an electrochemical gas sensor is essentially a battery, called a gas fuel cell.
The most common type of battery consists of a group of conductive chemicals, with two electrodes made of different materials inserted. Connecting them with a wire generates electricity. Taking a lead-acid battery as an example, sulfuric acid solution is the conductive chemical. When lead is placed in it, electricity is generated at the interface where lead and sulfuric acid meet. When lead oxide is added, electricity also forms at the interface. The difference in charge between the two interfaces creates a voltage. Connecting them with a wire allows electrons to flow from lead to lead oxide, transforming lead into lead oxide, and then lead oxide into lead sulfite. The amount of electricity is related to the chemical quantities and the reaction process.
The most important concepts here are: First, inserting a conductor into a conductive chemical substance creates an interface potential; inserting different conductors into the same substance produces different potentials. Second, when different potentials are connected, a reaction occurs at the interface. Third, a conductive circuit consists of two parts: the battery and the external wires. Outside the battery, within the connecting wires, are electrons; inside the battery, are ions. That is, the conduction process is accomplished by both the movement of electrons and the movement of ions.
An electrochemical CO gas sensor is a chemical battery, specifically a CO fuel cell. CO is the electron-donating electrode (working electrode), oxygen is the electron-gaining electrode, and sulfuric acid solution is the electrolyte. The biggest difference between this sensor and a lead-acid battery lies in the electrode materials: the electrochemical CO gas sensor uses a gaseous electrode, while lead-acid batteries use a solid electrode. The electrodes of an electrochemical gas sensor are called gas electrodes. In an electrochemical CO gas sensor, the working electrode CO acts as the electron-donating electrode. Electron release, collection, and conduction cannot occur simply because CO and sulfuric acid solution are in contact. Firstly, CO needs certain conditions to complete the electron-donating process, namely, reducing the difficulty of CO donating electrons under electrocatalytic conditions. In practice, this condition is provided by a porous platinum electrode (or other electrocatalytic conductive electrode). Secondly, the electrons donated by CO need to be collected and conducted by a conductor, which is also accomplished by a porous platinum electrode.
Similarly, the oxygen electrode, acting as the counter electrode, also requires a porous platinum electrode to assist in acquiring electrons. The platinum electrode is essentially a reaction platform. While the sensing principle of electrochemical sensors is simple, achieving reliable and accurate sensing is difficult: Firstly, the platinum electrode needs a stable porous structure with a sufficient number of pores to allow sulfuric acid aqueous solution and CO (or oxygen) to enter the pores. Electron provision occurs at the three-phase interface, where gas (CO), solid (Pt), and liquid (water in the sulfuric acid aqueous solution) come into contact. Therefore, maintaining the stability of the three-phase interface under long-term sulfuric acid immersion, electrochemical reaction impact, and electrophoretic drive is crucial for reliable and accurate sensing. Secondly, the sulfuric acid aqueous solution must be stable, non-volatile, non-hygroscopic, and non-leaking. Any change in the mass of the sulfuric acid aqueous solution will lead to a change in the internal pressure of the sensor, which in turn will cause changes in the three-phase interface. Thirdly, the contact stress between the electrode and the sulfuric acid aqueous solution, determined by the packaging and material physical properties, must remain stable.
The main problems with electrochemical sensors currently stem primarily from the factors mentioned above. One of the core technologies and processes of electrochemical sensors is constructing electrodes with a rationally designed, stable, and reliable physical structure, which is closely related to sensitivity, response recovery, lifetime, and temperature characteristics. The second issue is packaging. Problems with electrochemical sensors include deactivation due to water loss under dry conditions, leakage due to water absorption under high humidity conditions, poisoning deactivation due to prolonged contact with the analyte gas, and deactivation due to the disintegration of the electrode pore structure. Performance-wise, this manifests as leakage, short lifetime (compared to other principles), and large size. Manufacturing-wise, it manifests as complex design and processes, and high manufacturing costs.
The future of electrochemical sensors: The clear direction is the room-temperature solidification of electrolytes and the subsequent MEMS integration based on this. Achieving solid-state and MEMS-based electrochemical sensors will not only overcome most challenges, including manufacturing difficulties, but also inspire new applications and drive new growth for businesses. These electrochemical sensors will be highly integrated, easily integrable, and compact electronic systems. However, such results still cannot overcome the performance changes caused by high concentrations of the analyte gas or prolonged contact with the sensor.
PID—Photoionization Detector
A PID (Programmable Detector) consists of an ultraviolet (UV) light source and a gas chamber. The principle of UV emission is the same as that of a fluorescent lamp, only with a higher frequency and greater energy. When the gas to be measured reaches the gas chamber, it is photoionized by the UV light emitted by the UV lamp, generating a charge current. The gas concentration is positively correlated with the magnitude of the charge current; by measuring the charge current, the gas concentration can be determined.
Special gases: These gases exhibit diverse physical forms and complex chemical processes and reaction products. They include inorganic gases such as ammonia and organic gases such as toluene.
The various gas sensors introduced above face significant challenges in detecting complex gases. For example, the infrared absorption principle for detecting organic vapors faces insurmountable difficulties: a) Due to their large molecular weight, organic vapors have long characteristic absorption wavelengths, resulting in small energy changes after infrared absorption and typically low sensitivity. b) Long-chain organic vapors are easily adsorbed and adhere to the detector, disrupting light transmission. c) They cannot detect total VOC levels. To achieve total VOC assessment, infrared systems require full-spectrum response filters, detectors, and full-spectrum infrared light sources. Such requirements are not only difficult to meet, but even if achieved, interference from inorganic gases and water across the full spectrum is inevitable. While chemical sensors, particularly semiconductor sensors, are susceptible to interference and drift from inorganic gases, temperature, and humidity, resulting in low concentration resolution, their wide detection range and coverage of various gases limit their suitability for low-end applications. In this context, PLDs (Plug-in Devices) are a better choice for VOC detection in industrial settings.
The most significant characteristic of PLDs compared to other sensors is their sensitivity to only a few inorganic gases, such as ammonia and phosphine. This is because most inorganic gases have very high ionization energies (greater than 11.7 eV). Currently, the highest ultraviolet radiation energy of a PLD lamp is only 11.7 eV. Therefore, in petrochemical industrial parks, the response of a PLD can be considered a VOC response.
PID working principle
1. Fill the vacuum glass cavity with high-purity rare gases such as argon or krypton.
2. Seal the glass cavity with a magnesium fluoride single crystal ultraviolet-transmitting sheet, in which the magnesium fluoride crystal is transparent to ultraviolet light.
3. Install electrodes on the outer wall of the glass cavity.
4. By adding electrodes and an electric field to the magnesium fluoride window to serve as the gas chamber for the measured gas, a complete ultraviolet lamp capable of ionizing VOCs is created. During operation, a high-frequency electric field is applied outside the glass cavity. The rare gas inside the ultraviolet lamp is ionized by the applied electric field, producing electrons and ions. When these electrons and ions recombine, energy is radiated outwards in the form of ultraviolet light. The ultraviolet light passes through the magnesium fluoride window and reaches the gas chamber, where the measured gas is photoionized by the ultraviolet light, producing electrons and ions. These charges generate a current under the influence of the electric field, which can then be measured.
PLD stable operation requires:
1. A PID controller must radiate sufficient energy to ionize the gas being measured;
2. The high-frequency electric field that generates ultraviolet light must be stable.
3. There must be no impurity gases inside the glass cavity, as impurity gases can cause additional ionization and affect the efficiency of ultraviolet luminescence.
4. The ultraviolet spectrum is stable and uniform.
5. The transmission of ultraviolet light to the gas chamber is stable and uniform and does not interact with the metal electrode material constituting the gas chamber to produce heavy metal deposition. Heavy metal deposition in the ultraviolet radiation window will block ultraviolet light from reaching the gas chamber.
This necessitates that the luminescent material filling the UV lamp must be a gas to ensure uniform light emission and transmission. The cavity must be free of impurities to prevent additional ionization. These requirements limit the choice of luminescent gas to rare gases. The window material must be transparent to UV light and possess stable physicochemical properties; in fact, the selection of UV window materials is extremely limited. These constraints ultimately determine the limitations of PID applications.
Why can't current PID controllers measure propane, ethane, methane, and most inorganic substances?
The essence of a PID (Programmable Analyzer) is to measure charge current after ionizing the analyte; ionization requires energy. Currently, the most common ultraviolet radiation energies for PIDs are 8.3 eV, 9.8 eV, and 10.6 eV. Ionizing methane requires 12.6 eV, ethane 11.56 eV, propane 10.95 eV, and carbon dioxide 13 eV, among others. In fact, there is a strong desire to develop PIDs with higher energy, but the limitations are: the variety of rare gases is extremely limited, and the ultraviolet wavelength (energy) is determined by the electronic energy levels of the rare gases themselves, which cannot be changed by humans; another limitation is the availability of specific wavelengths of ultraviolet light transmission window materials. The wavelengths of ultraviolet light that can be transmitted depend on the lattice constant of the window material, and the choices within the current material system are extremely limited. Although 11.7 eV emitters have been developed, the only suitable window material is lithium fluoride (LiF), which is highly hygroscopic, resulting in a PID lifetime of only two months for the 11.7 eV PID. In other words, current ultraviolet lamps, due to limitations in output energy, are still unable to detect substances with high ionization energy, such as methane.
Why does PID control lack selectivity?
If we choose a PID with an ultraviolet radiation energy of 10.6 eV, it means that all gas molecules in the measured environment with an ionization energy less than 10.6 eV will be ionized. The charge flow we measure is the sum of the charge flows of all ionized gases, not the charge flow of a single gas. This is why PID is non-selective.
When a PID lamp is in operation, ionized substances in the gas chamber recombine and reduce upon contact, causing long-chain molecules and dust to deposit on the window surface. Furthermore, the ion beam generated during sensor operation bombards the gas chamber electrodes, leading to heavy metal deposition on the window surface. This obviously affects ultraviolet light transmission, resulting in zero-point drift, reduced sensitivity, and ultimately, compromised detection results. In fact, besides the fabrication technology and gas chamber design of the PID lamp, the cleaning technology of the PID lamp's ultraviolet transmission window is also a core technology.
The Future of PID
1. PiD, as an ideal non-radioactive ion source, will exist forever;
2. Improve the vacuum level and purity of the filling gas before filling the PID lamp to enhance luminous efficiency and luminous stability;
3. Develop new window materials and processing precision to improve light transmittance, emitted light uniformity, encapsulation quality, stability, and lifespan;
4. Prevents heavy metal deposition on the window due to dispersion, extending its lifespan;
5. Window cleaning technology to prevent the deposition of large organic molecules and small particulate matter;
6. Development of long-life PID lamps with higher output energy;
7. Small size.
Development direction of gas sensors
气体传感器的研究涉及面广、难度大，属于多学科交叉的研究内容。要切实提高传感器各方面的性能指标需要多学科、多领域研究工作者的协同合作。气敏材料的开发和根据不同原理进行传感器结构的合理设计一直受到研究人员的关注。未来气体传感器的发展也将围绕这两方面展开工作。具体表现如下:
气敏材料的进一步开发一方面寻找新的添加剂对已开发的气敏材料性能进行进一步提高；另一方面充分利用纳米、薄膜等新材料制备技术寻找性能更加优越的气敏材料。
新型气体传感器的开发和设计根据气体与气敏材料可能产生的不同效应设计出新型气体传感器。近年来表面声波气体传感器、光学式气体传感器、石英振子式气体传感器等新型传感器的开发成功进一步开阔了设计者的视野。目前仿生气体传感器也在研究中。
气体传感器传感机理的进一步研究新的气敏材料和新型传感器层出不穷，很有必要在理论上对它们的传感机理进行深度的研究。只有机理明确了，下一步的工作才会少走弯路。
气体传感器的智能化生产和生活日新月异的发展对气体传感器提出了更高的要求，气体传感器智能化是其发展的必由之路。智能气体传感器将在充分利用微机械与微电子技术、计算机技术、信号处理技术、电路与系统、传感技术、神经网络技术、模糊理论等多学科综合技术的基础上得到发展。
仿生气体传感器的迅速发展警犬的鼻子就是一种灵敏度和选择性都非常好的理想气敏传感器，结合仿生学和传感器技术研究类似狗鼻子的＂电子鼻＂将是气体传感器发展的重要方向之一。
