Solid Electrolyte Zirconia Ion Flow Detection Technology
Introduction to Zirconia Analyzer Detection Technology and Ion Flow Sensors
With the development and maturation of zirconia sensor technology, the applications of zirconia sensors have expanded from automotive exhaust emission testing to industries and fields such as heating boiler control, industrial process control, combustion systems, oxygen/nitrogen generation systems, agricultural composting, and flue gas emissions. The analytical objects of zirconia sensors have also extended from the analysis of simple oxygen concentration to that of nitrogen oxides concentration, water vapor concentration, sulfur dioxide concentration and more. Nowadays, zirconia sensors have become one of the most important and widely used sensors in the field of gas analysis.
>> Zirconia Analyzer Detection Technology
The material used in zirconia sensors is a zirconia solid electrolyte. It is fabricated by doping pure zirconia with a certain proportion of low-valent metal such as yttria oxide (Y2O3) or calcium oxide (CaO) as stabilizers, followed by high-temperature sintering to form stabilized zirconia. At temperatures above 700 ℃, zirconia is an excellent conductor for oxygen ions.
Porous platinum (Pt) electrodes are sintered respectively on both sides of the zirconia electrolyte (ZrO2 tube). At a certain temperature, when the oxygen concentrations on the two sides of the electrolyte are different, oxygen molecules on the high-concentration side (air) are adsorbed onto the platinum electrode and combine with electrons (4e) to form oxygen ions O2−, making this electrode positively charged. O2− ions migrate through the oxygen ion vacancies in the electrolyte to the platinum electrode on the low oxygen concentration side, release electrons, and convert back into oxygen molecules, causing that electrode to become negatively charged. The reaction equations for the two electrodes are as follows:
Reference side: O₂+4e→2O²¯
Measuring side: 2O²¯ - 4e→O2₂
This generates a certain electromotive force between the two electrodes. The zirconia electrolyte, platinum electrodes, and gases with different oxygen concentrations on both sides together form the oxygen probe, namely the so-called zirconia concentration cell. The electromotive force E between the two electrodes is calculated by the Nernst equation: that is,
In the equation:
E―Output of concentration cell, mV;
R―Ideal gas constant, 8.314 W·s/mol;
T―Absolute temperature (K);
n―Number of electron transfers (4 in this equation);
F―Faraday constant, 96,500 C;
P0―Oxygen concentration percentage of the reference gas;
P1―Oxygen concentration percentage of the gas under test.
It is the basis of zirconia oxygen measurement. When the temperature at the zirconia tube is heated to 600～1400°C, the gas on the high-concentration side uses a gas with a known oxygen concentration as the reference gas; if air is used, P0=20.6%. By combining this value with the constant terms in the formula, and considering that the actual zirconia cell exhibits thermoelectric potential, contact potential, reference potential and polarization potential, a local potential C (mV) is generated. The actual calculation formula is:
As can be seen, if the output electromotive force E of the oxygen probe and the absolute temperature T of the measured gas can be determined, the oxygen partial pressure (concentration) P1 of the measured gas can be calculated. This is the basic oxygen measurement principle of the zirconia analyzer.
Note: The content of Detection Technology of Zirconia Analyzer is excerpted from: Mei Bo, Jin Haifeng. Principle, Maintenance and Application of Zirconia Oxygen Analyzer. Ethylene Industry (in Chinese), 2009, 21(3): 28-31.
>> Introduction to Ion Flow Sensor
Ion flow sensors are all based on the zirconia principle, and their oxygen measurement principle is referred to the Section 11.1.2. Foreign manufacturers such as Fujikura in Japan and Sensore in Austria, as well as early domestic manufacturers including Chengdu Kangda, all adopt single limiting orifices. With technological advancements and drawing on the extensive field application experience summarized by Shanghai Chang Ai, companies such as Shanghai Aici have developed porous layer oxygen sensors. This design adopts the porous ceramic substrate as the diffusion layer to control the oxygen supplied to the sensor cathode (replacing the mechanical restriction of a single hole). Due to the special material properties, uniformly distributed mesh holes are naturally formed during sintering, which are resistant to clogging.
A comparison of typical ion current sensors is shown in Table 1.
Table 1: Comparison of Common Ion Flow Sensors
| Comparison Item | Sensore/Fujikura | AICI |
| Principle | Ion flow | 3D ion flow |
| Thermal Effect | Glass glaze bonding technology. The glaze and zirconia substrate are different materials with different thermal expansion coefficients, making them highly susceptible to thermal stress. Cold and thermal shock easily leads to cracking at the bonded interface. | Tape-casting lamination and co-firing technology, enabling uniform heating and immunity to the impact of cold and thermal shock. |
| Limiting current hole aperture: 10 μm | Laser drilling, a form of photothermal ablation method. When a high-energy beam irradiates the surface of a material, the material rapidly heats up and vaporizes as it absorbs the light energy. Irregular spatter deposits are formed around the hole and on the inner wall, which directly affect the performance and consistency of the sensor. | Porous ceramics are used; due to the special properties of the material, sintering naturally forms a uniformly distributed mesh holes. |
| Number of holes | Single hole is prone to clogging. | Naturally formed reticular porous structure, resistant to clogging. |
| Sensitivity | T90< 60s | T90< 45s |
| Quality guarantee | 15000 hours | More than 50, 000 hours |
| Physical object | ||
>> Development Trend of Ion Flow Sensors (Zirconia)
Ion flow sensors (zirconia) have a wide range of applications and play a crucial role in oxygen concentration detection across various industries and fields, including air separation, SMT, medical treatment, automotive exhaust, flue gas emissions, gas-fired boilers, iron & steel, chemicals, papermaking, and printing & dyeing. In the field of gas analysis, with the development of optical technology, there will be an increasing number of optical gas analyzers; nevertheless, ion flow sensors will still maintain their important application value. The trends in the development of ion flow sensors are as follows:
● Extend the service life of the sensor. The service life and environmental adaptability of the sensor can be enhanced by optimizing its structure, catalytic electrodes and manufacturing processes.
● Miniaturization and low power consumption. The miniaturization of gas analyzers is an inevitable trend of analyzers; high power consumption and high heat generation of sensors can restrict the miniaturization and high integration of gas analyzers.
Oxygen Detection Technology of Ion Flow Sensors (Zirconia)
In an alkaline electrolyte solution, the reduction of oxygen to OH- at the silver cathode can be expressed by the following formula.
Introduction to the oxygen measurement principle of ion flow sensors; the working principle of ion flow sensors for oxygen measurement is shown in Figure 2.
Stabilized zirconia (doped with a certain proportion of low-valent metal oxides such as yttria as a stabilizer) is coated with platinum (Pt) electrodes on both sides. The cathode side is connected to a cover with gas diffusion holes, forming a cathode cavity. At a certain temperature, oxygen molecules within the cavity gain electrons at the cathode under the catalytic action of the platinum electrode and form oxygen ions (O2-). When a certain voltage is applied across the zirconia electrolyte, O2− migrates to the anode through oxygen ion vacancies in the zirconia under the external electric field, undergoes an oxidation reaction, releases electrons, and recombines into oxygen molecules to be discharged. This phenomenon is defined as an electrochemical pump.
Under the action of the electrochemical pump, oxygen in the cathode cavity is continuously pumped out of the cavity by the zirconia electrolyte, generating a current in the circuit. When the mole fraction of oxygen is constant, the current intensity increases with rising voltage. When the voltage exceeds a certain value, the current intensity reaches saturation; this is attributed to the limitation of oxygen diffusion into the cathode cavity through the tiny holes. This saturated current is defined as the limiting current. The diffusion mechanism of gas through tiny holes determines the characteristics of the sensor. The diffusion of tiny holes generally involves two types of limiting case: molecular diffusion and Knudsen diffusion. When the diameter of tiny holes is larger than the average diameter of the gas molecules, the limiting current value IL in the molecular diffusion region is expressed as:
In the formula
F—Faraday constant;
D—Diffusion coefficient of oxygen molecules in free space;
S —Cross-sectional area of the diffusion hole;
L—Length of the diffusion hole;
C—Mole fraction of oxygen around the sensor;
CT—Total mole fraction of all gas substances.
When C/CT＜1, it can be seen from Formula (3) that the limiting current value is in direct proportion to the mole fraction of oxygen, and the limiting current value IL is:
As can be seen from Formula (4), there is an almost linear relationship between the ionic current and the molar fraction of oxygen. The molar fraction of oxygen in the measured gas can be determined based on the magnitude of the output current.
The above-mentioned ion flow sensors adopted a single limiting orifice structure by foreign manufacturers such as Fujikura in Japan and Sensore in Austria, as well as domestic manufacturers including Chengdu Kangda. With technological advancements and drawing on the extensive field application experience summarized by Shanghai Chang Ai, companies such as Shanghai Aici have developed porous layer oxygen sensors. This design adopts the porous ceramic substrate as the diffusion layer to control the oxygen supplied to the sensor cathode (replacing the mechanical restriction of a single hole). The structure is illustrated in Figure 3.
The limiting current of this porous-layer oxygen sensor is identical to that in Formula (4), namely:
In the formula
F—Faraday constant;
Deff—the effective diffusion coefficient of oxygen within the porous layer;
S—the cathode area;
L—the thickness of the porous layer substrate;
C—the molar fraction of oxygen surrounding the sensor.
As shown in Formula (5), the limiting current of a porous-layer oxygen sensor has a linear relationship with the oxygen mole fraction.
Chang Ai's Ion Flow Oxygen Analyzer
Based on porous-layer ion flow sensors, Chang Ai has developed a series of oxygen analyzers, including the CI-PC84, CI-PC89, CI-PC832, CI2000-CY, GNL-2100L, and SP-980L. Compared with other oxygen analyzers adopting conventional ion flow sensors, these models offer advantages such as resistance to cold and thermal shock, reduced susceptibility to clogging, and long service life. Their application fields cover oxygen concentration monitoring of flue gas during combustion processes, oxygen purity detection of products in the air separation industry, and oxygen concentration measurement in medical and health scenarios such as hyperbaric oxygen chambers, infant incubators, ventilators and anesthesia machines as well as the oxygen concentration determination in households, naval vessels, tunnels, deep wells, civil air defense projects and urban tunnels.
Chang Ai ion flow oxygen analyzers can be categorized by measuring range as follows:
>> Nitrogen/Oxygen analyzer
Table 2 Measuring Range of Nitrogen/Oxygen Analyzer
| Display Mode | Model | Measuring Range |
| Oxygen (O₂) Display | 3O | 1000ppm～21.0% O₂ |
| 4O | 100ppm～21.00% O₂ | |
| 5O | 10ppm～21.000% O₂ | |
| Nitrogen (N₂) Display | 3N | 79.0%～99.9% N₂ |
| 4N | 79.00%～99.99% N₂ | |
| 5N | 79.000%～99.999% N₂ |
>> Typical products: CI-PC841
| Technical Specifications | |
| Linearity error | 0～100ppm O2 ≤±5%FS |
| 0～2% O2 ≤±2%FS | |
| 0～21% O2 ≤±1.5%FS | |
| Repeatability | 0～100ppm O2 ≤±2.5%FS |
| 0～2% O2 ≤±1%FS | |
| 0～21% O2 ≤±0.75%FS | |
| Stability | 0～100ppm O2 ≤±2.5%FS/7d |
| 0～2% O2 ≤±1%FS/7d | |
| 0～21% O2 ≤±0.75%FS/7d | |
| Response time | T90＜15s |
| Operating ambient temperature | -10～+45℃ |
| Operating ambient humidity | ＜80%RH |
| Gas inlet pressure | Atmospheric pressure ±10% |
| Gas inlet flow | 400～600ml/min |
| Typical application | Detection of crude oxygen / nitrogen in air separation |
Trace oxygen (ppm grade) analyzer
>> Typical Product: GNL-120
| Technical Specifications | |
| Measurement Range | 0～10/100/1000 ppm O2 |
| Linearity error | ±2%FS |
| Repeatability | ±1%FS |
| Stability | ±1%FS/7d |
| Response time | T90＜30s |
| Operating ambient temperature | -10～+50℃ |
| Operating ambient humidity | ＜90%RH |
| Gas inlet pressure | Atmospheric pressure ±10% |
| Typical applications | Glove box, air separation |
High-content oxygen analyzer
>> Typical Product: CI-PC832
| Technical Specifications | |
| Measurement Range | 10～97.00%/ 99.99% O2 |
| 97%～99.99% O2 | |
| Linearity error | ±1%FS |
| Repeatability | ±0.5%FS |
| Stability | ±0.5%FS/7d |
| Response time | T90＜20s |
| Operating ambient temperature | -10～+45℃ |
| Operating ambient humidity | ＜80%RH |
| Gas inlet pressure | Atmospheric pressure±10% |
| Typical applications | Oxygen purity measuring of products in air separation |
>> Detection Technology for High-temperature Humidity Measurement Based on Ion Flow (Zirconia) Sensors
Ion flow sensors enable accurate humidity measurement. By adjusting the voltage applied to the anode and cathode of the zirconia sensor, humidity can be measured, thereby addressing the issue that conventional humidity sensors fail to work properly in high-temperature environments (such as above 100 °C).
A working voltage is applied across the anode and cathode of zirconia to form an electric field, which drives oxygen ions to migrate from the cathode to the anode through the zirconia and generate oxygen ion current. When the oxygen concentration in the measured atmosphere is fixed, the output current of the zirconia sensor stops rising with the increase of applied voltage, reaching a certain constant value. This constant current value is referred to as the limiting current value for that oxygen concentration, which we call the first limiting current value.
The reactions at the sensor's cathode and anode are as follows:
Cathode side: O₂+4e¯→2O²¯
H₂O+2e¯ →H₂+O²¯
Anode side: O²- → 1/2O₂+2e¯
In accordance with the Fick's law restricted by gas diffusion holes of the sensor, assuming that the diffusion coefficient of oxygen is equal to that of water vapor, the first limiting current I1 and the second limiting current I2 are expressed by the following formulas respectively:
In the formula: F is Faraday constant, D is the diffusion coefficient of mixed gas molecules, S is the area of diffusion holes, P is the total pressure of the gas mixture, PO2 is the partial pressure of oxygen, PH2O is the partial pressure of water vapor, R is the gas constant, T is the absolute temperature, L is the length of gas diffusion holes, and 0.21 is the oxygen content in air.
Detection Technology for Nitrogen Oxides Measurement by Ion Flow Zirconia Sensors
The principle of zirconia applied to NOₓ monitoring is shown in Figure 6. The core component of the sensor is a thick-film zirconia, which acts as a solid-state electrolyte, with porous platinum (Pt) electrodes sintered onto both sides. The NOx sensor based on the zirconia principle essentially measures oxygen. In the figure below, the wall of the second measurement cell is coated with a catalytic electrode made of rhodium. At a certain temperature, under the catalytic action of the rhodium electrode, NO molecules in the gas are catalytically decomposed into nitrogen ions and oxygen ions. The concentration of NOx is measured by detecting the current generated by the oxygen ions. The working process consists of two stages, which are completed sequentially in two measuring cells.
The measured gas diffuses into the first measuring cell of the sensor. Oxygen in the first cell is pumped out by the oxygen pump and generates a limiting current. The oxygen concentration in the measured gas is determined by detecting the limiting current. Meanwhile, NO₂ in the first measuring cell undergoes decomposition: NO₂→NO+1/2O₂, completing the conversion from NO₂ to NO.
The measured gas further diffuses into the second measuring cell of the sensor, where NO undergoes decomposition: NO→1/2N₂+1/2O₂. The oxygen generated by decomposition is pumped out by the oxygen pump and produces a limiting current. The NOX concentration in the measured gas is measured by detecting this limiting current.
Figure 6 Working Principle Diagram of NOₓ Sensor
Nitrogen Oxide Analyzer
With the microprocessor as the core and the dual-cell thick-film zirconia sensor as the measuring unit, the CI-XT682 nitrogen oxide gas analysis system is specially designed for NOₓ detection in gas-fired boilers. It primarily analyzes NO and NO₂, or the total NOx concentration of both. Different from conventional analysis systems that only detect NO, this system avoids significant analytical errors caused by overlooking the loss resulting from the conversion of NO₂ to NO. The CI-XT682 analysis system is a reliable choice for monitoring NOx emissions from gas-fired boilers.
>>Technical Specifications
| Measurement range | 0～50/ 100 mg/m3 NOX |
| 0～25.00% O2 | |
| Linearity error | NOX ±2%FS |
| O₂ ±2%FS | |
| Repeatability | NOX ±1%FS |
| O₂ ±1%FS | |
| Stability | NOX ±1%FS/7d |
| O₂ ±1%FS/7d | |
| Response time | NOX T90<120s (After sufficient preheating) |
| O₂ T90<60s (After sufficient preheating) | |
| Mounting method | DN50-65 flange (Compatible with DN50 and DN65 flanges) |
| Sampling method | Ejection sampling |
>>Main Features
● The probe is inserted directly into high-temperature flue gas at 0–600°C for in-situ measurement of NOx and O2;
● No complicated pretreatment unit is required. It features low maintenance and is unaffected by other interfering gases. The unique sensor working principle prevents interference from exhaust gases such as CO2, H2O and SO2.
● Equipped with an automatic calibration function to complete periodic calibration automatically, improving measurement accuracy.
● Support automatic back-blowing function to keep the probe clean, reduce maintenance workload and extend probe service life.
● The probe features pressure monitoring to verify whether sample gas collection is normal;
● The probe Adopts a cast aluminum heater with automatic temperature control from 100 to 180°C, preventing condensate water formation on the probe.
>>Application Fields
● NOₓ detection for combustion equipment such as gas-fired boilers;
● NOₓ detection in denitrification systems and desulfurization units;
● Exhaust gas analysis for diesel engines and marine engines;
● Combustion exhaust gas analysis in industries such as steel, cement, and power generation.
