Preparation method and application of zirconia oxygen sensor measuring electrode

Abstract

The invention relates to a preparation method and application of a zirconia oxygen sensor measuring electrode, and the preparation method comprises the following steps: preparing a solid electrolyte, selecting industrial zirconia and a calcium oxide reagent which are stable in work and have the purity of more than 99% as stabilizers, and carrying out fine grinding and pickling, pulping, forming, finishing, high-temperature sintering and the like on raw materials to complete the preparation of a zirconia measuring tube, a measuring probe used on site comprises a zirconium oxide measuring tube, a ceramic electric heater, a ceramic filter, a temperature thermocouple and the like, the measuring tube is of a U-shaped tubular structure, a reference electrode and a measuring electrode are arranged at the corresponding positions of the inner wall and the outer wall of the measuring tube, the measuring tube is arranged in a heating cavity of the electric heater, and the ceramic filter is arranged in the heating cavity of the electric heater. The ceramic filter has the functions of dust prevention, damping and electrode protection. The measuring method is innovative, the structure is novel, and the practicability is high.

Description

Technical Field

[0001] The present invention relates to a preparation method and application of a measurement electrode of a zirconia oxygen sensor, and more particularly to a zirconia oxygen sensor. Background Art

[0002] The zirconia oxygen measurement electrode is a measurement probe designed based on the phenomenon of ionic conduction of zirconia at high temperatures discovered by Nernst. By using a stable zirconia ceramic as a solid electrolyte that can only conduct oxygen ions in an environment above 600°C, porous metal materials such as platinum or silver are attached to both sides of the electrolyte. When different oxygen-concentration gases pass through both sides of the electrolyte, oxygen ions migrate from the side with a higher concentration to the side with a lower concentration, and charge accumulation occurs on both surfaces, generating a concentration cell electromotive force. After being converted by a transmitter, the oxygen content in the flue gas is directly displayed. Through optimization and adjustment, harmful gas emissions such as hydrocarbons, carbon monoxide, and nitrogen oxides can be reduced, which is of great significance for saving energy, improving efficiency, and protecting the environment. It has been widely used in various coal-fired, oil-fired, gas-fired, and other industrial furnaces in industries such as iron and steel metallurgy, petrochemical fiber, power generation and heating, chemical engineering, and printing and textile.

[0003] Currently, the structural forms of zirconia oxygen measurement devices are divided into three types.

[0004] 1. Constant-temperature convection type: The zirconia oxygen measurement probe is installed on the flue gas expander of the boiler bypass flue. When the working temperature is 750 ± 10°C, the flow of the measured gas and the reference gas in the measurement probe is mainly achieved by the thermal convection of the gas.

[0005] 2. Constant-temperature extraction type: The structural form of the zirconia measurement probe is basically the same as that of the constant-temperature convection type. The flow mode of the reference gas is the same as in 1, and the flow of the measured gas is achieved by extracting gas under negative pressure in the flue.

[0006] 3. Isothermal direct insertion type: For the installation point of the zirconia oxygen probe, the flue gas temperature should meet the requirements between 400 - 600 °C. It should be installed at a location with good flue gas flow, and it is necessary to avoid dead corners with poor flue gas flow. The measurement method is basically the same as that in 1. For the zirconia oxygen measurement device, regardless of the measurement method and structural form, the core component for measurement is the zirconia measurement tube. When zirconia is at a high temperature, due to the lattice transformation process, it is prone to cause cracking of the product. Calcium oxide with a similar structure is added to zirconia. Because the oxygen ions of calcium oxide have an ionic radius similar to that of zirconia, under certain conditions, calcium oxide can also become a stable solid electrolyte that can only conduct oxygen ions. Porous metal materials such as platinum or silver are attached to the inner and outer sides of the zirconia measurement tube as measurement electrodes. When gases with different oxygen concentrations pass through both sides, oxygen ions migrate from the side with a higher concentration to the side with a lower concentration through the electrolyte. Charge accumulates on the two surfaces of zirconia, generating a certain potential difference, which is called the electromotive force of the concentration cell. At this time, the magnitude of the battery electromotive force is only related to the oxygen content in the gases on both sides. The quality of the measurement electrode plays a crucial role in determining the accuracy and reliability of the measurement result. There are the following problems with the measurement electrodes in the commonly used zirconia oxygen measurement devices during use.

[0007] 1. At high temperatures, the measurement electrode is scoured and corroded by dust and harmful substances in the measured gas, causing the electrode layer to peel off.

[0008] 2. For the inner electrode layer of the measurement electrode, the coating process is not smooth, and the surface of the coating is uneven, causing the internal resistance of the oxygen cell to change and resulting in inaccurate measurement results.

[0009] 3. For the inner electrode layer of the measurement electrode, the coating is difficult, the coating is not standardized, resulting in a smaller coating area, insufficient oxygen cell capacity, increased background potential, unstable measurement results, and inaccurate measured values.

[0010] 4. Due to improper installation method of the zirconia oxygen measurement device, the measured and reference gases are blocked and do not flow smoothly inside the measurement tube, which is also one of the reasons for inaccurate measurement results. Summary of the Invention

[0011] To overcome the deficiencies in the above technologies, the purpose of the present invention is to provide a preparation method and application of the measurement electrode in a zirconia oxygen measurement probe, making the working performance of the measurement electrode more stable and the measurement results more accurate and reliable.

[0012] To solve the existing technical problems, the technical solution adopted by the present invention is: The preparation method and application of the measurement electrode in the zirconia oxygen measurement probe include: I. The manufacturing process of the zirconia solid electrolyte.

[0013] 1. The raw materials selected are industrial zirconia with a purity of over 99% and reagent calcium oxide as a stabilizer. The amount of calcium oxide added has a direct impact on the lattice transformation degree of zirconia, the thermal shock resistance of the product, and the oxygen ion migration rate at high temperatures. To ensure a large ion migration rate of zirconia products at high temperatures and meet the requirements of thermal shock performance, the fully stabilized method is adopted to transform the zirconia measuring tube into a stable cubic lattice. The two raw materials, industrial zirconia and calcium oxide, are loaded into a rubber-lined mill, mixed evenly, added with a binder and stirred evenly, and then pressed into blocks and calcined at high temperatures to achieve stabilization.

[0014] 2. Fine grinding and pickling: The calcined blocks are crushed into particles, then loaded into a vibration mill, and the grinding particle size is below 5μm. After that, they can be taken out of the mill, then soaked in hydrochloric acid for three days for pickling, and then dewatered and dried for standby.

[0015] 3. Pulp making: The dried blocks are mixed in a ball mill for 4 hours according to different ratios of material:ball:water, with a suspending agent added externally, and the pulp making work is completed.

[0016] 4. Molding: The plaster mold is used for casting molding. The mold suction time of the green body depends on the dry and wet temperature of the plaster mold. The semi-finished product taken out of the mold is dried naturally or in a drying oven below 60°C. After the green body is dried, it is rested for high-temperature sintering.

[0017] 5. High-temperature oil kiln sintering is adopted. The products are placed on a sagger or brick plate padded with zirconia sand. When using a brick plate to load the products, the surrounding and above should also be covered with brick plates to avoid direct impact of the flame. Sintering can also be carried out by the hanging sintering method.

[0018] II. Connection of the measuring electrode output leads: Two planar wires are respectively bonded to the inner and outer walls of the zirconia tube with low-temperature ceramic materials. After drying, they are sintered in a high-temperature furnace at a maximum temperature of 1150°C. They can also be bonded with fast-drying inorganic glue.

[0019] III. Coating method and standard of the inner and outer electrodes of the zirconia tube: 1. In order to extract the concentration potential from the zirconia tube, a measuring electrode capable of reading data must be attached to the surface of the zirconia measuring tube. The electrode material should be selected as a material that is relatively stable in an oxygen atmosphere. Platinum or silver metal is selected. Both are porous metal materials. Attached to the surface of the zirconia measuring tube, they become a measuring electrode capable of measuring data. As shown in Figure 1 - a, the platinum or silver powder electrode material is adjusted into a paste-like material, and the coating is applied to the inner and outer tube walls of the zirconia measuring tube, and then sintered (the electrode can also be attached by evaporation or sputtering). If the coating is too thick and the sintering temperature is too high, the electrode will melt and densify, making it difficult for oxygen to contact the zirconia ceramic surface, increasing the surface resistance of the electrode. If the sintering temperature is too low, the performance of the selected metal material will be unstable, and the performance of the measuring electrode will deteriorate.

[0020] 2. The coating of the measuring electrode of the zirconia tube is composed of two porous metal materials, platinum or silver. It is a relatively stable substance when measuring the oxygen content atmosphere. The measuring electrode coating has two slurries, platinum or silver. The silver slurry is composed of silver oxide, aluminum borate, rosin, turpentine and other materials. Method: After fully stirring the above materials, add silver oxide and stir evenly to form a silver slurry coating. The platinum slurry is composed of ethyl cellulose, turpentine, oxyplatinic acid and other materials. After fully stirring the above materials, it can become a platinum slurry electrode coating. Cleaning of zirconia tube: Use chromium trichloride (chromic acid) to clean the inside and outside of the zirconia tube (detergent and water can also be used), clean up oil and dust After that, rinse it with clean water, heat and dry the zirconia tube for standby use. The coating method of the measuring electrode is as follows: fix the zirconia tube on a slowly rotating device, use a special brush to apply the slurry on the inner and outer walls of the middle section of the zirconia measuring tube, and after it is dry, place it in an electric heating furnace, heat it to 500℃, turn off the power, cool it, repeat the above steps 4-5 times, use a multimeter to measure the resistance value of any two points on the electrode surface should be ≤0.5Ω, that is, qualified, raise the electric heating furnace to 850℃ and 1100℃ respectively for half an hour, heat treat the silver or platinum measuring electrodes respectively, the measured electrodes have more stable and durable performance, and can complete the production of dozens or even hundreds of measuring components at one time.

[0021] Advantages 1. The amount of calcium oxide added to the zirconia is 6.9%, which has a direct impact on the lattice transformation, thermal shock resistance and oxygen ion mobility at high temperatures of the zirconia. In order to ensure that the zirconia measuring element has a large ion mobility at high temperatures and meets the thermal shock performance requirements, a full stabilization method is used to make the zirconia a good oxygen ion conductor.

[0022] Advantage 2. The zirconia measuring electrode uses porous metal materials such as platinum or silver as electrode coating. Platinum and silver are both precious metals with a market price difference of about ten times. If silver is used as the measuring electrode coating material, the production cost can be reduced, and the purpose of accurate measurement and fast response speed can be achieved.

[0023] Advantage 3. The electrode coating on the inner and outer walls of the zirconia measuring tube has a simple coating process and can be completed manually without complicated operating procedures. After the completion, the output oxygen concentration difference potential value of the zirconia measuring tube has an error of no more than 2% compared with the theoretical calculated value, and the performance is stable and the complex linearity is good.

[0024] Advantage 4. The provided platinum or silver coating slurry formula of the zirconium oxide measuring electrode, the coating method of the measuring electrode, and the identification method and standard after heat treatment make the measuring electrode more stable and durable, and the measurement results more accurate and reliable.

[0025] The present invention has the following effects: The measurement method of the present invention is novel, with a reasonable structure and strong practicability. The ceramic filter can isolate the dust in the flue gas, preventing the erosion of the dust on the electrode surface and the corrosion of harmful substances, which may cause the surface of the measurement electrode to fall off or fail. The ceramic filter can also have a damping effect on the rapidly flowing flue gas in the flue. The pressure of the flue gas passing through the filter is greatly reduced, making the flow rates of the measured gas and the reference gas relatively consistent, meeting the specifications and requirements of the zirconia measurement technology for gas flow rates, and being conducive to improving the accuracy of the measurement results.

[0026] When the zirconia measurement probe is installed, the front end of the probe is tilted upward by 10 - 15° from the horizontal position, which can change the contact surface between the ceramic filter and the flue gas flow direction. The rapidly flowing flue gas in the flue passes through the lower end of the ceramic filter and enters the oxygen probe, forming a micro-pressure and contacting the measurement electrode. Due to the negative pressure effect of the flue, the measured gas entering the oxygen measurement probe is extracted through the upper end of the ceramic filter. A micro-pressure difference is formed between the upper and lower ends of the filter, enabling the measured gas to flow. Since the zirconia measurement probe is tilted upward, it also causes the reference gas inside the measurement tube (due to the action of thermal convection in the high-temperature zone, the air continuously flows and contacts the reference electrode), as shown by the dotted arrow in Figure 1 - b indicating the flow direction of the reference gas. Description of the Drawings

[0027] Figure 1 - a Schematic structural diagram of the embodiment of the present invention In the figure: 1. Zirconia measurement tube, 2. Measurement outer electrode, 3. Measurement inner electrode, 4. Measurement outer electrode lead hole Figure 1 - b Schematic structural diagram of the embodiment of the present invention In the figure: 1. Ceramic electric heater, 2. Measurement electrode, 3. Measurement electrode lead hole, 4. Ceramic filter, 5. Reference electrode, 6. Temperature measurement thermocouple, 7. Zirconia measurement tube. Figure 2 It is the relationship between the oxygen concentration difference potential and the oxygen concentration at different temperatures. Detailed Embodiment

[0028] The embodiments of the present invention will be further described in conjunction with the accompanying drawings. Embodiment

[0029] As shown in Figure 1-a, the preparation method and application of the zirconia oxygen sensor measurement electrode include a zirconia measurement tube 1, an outer electrode of the measurement tube 2, an inner electrode of the measurement tube 3, an outer electrode lead hole 4. The zirconia measurement tube is a U-shaped tubular structure. The production raw materials are of a grade above 99%. 6.9% calcium oxide is added to industrial zirconia and sintered at high temperature through multiple production processes. The zirconia measurement tube is provided with inner and outer electrodes 2 and 3. The inner and outer electrodes use porous metallic silver as the electrode coating. Through coating, heating, drying, high-temperature heat treatment, etc., the electrode performance becomes more stable. The width of the electrode coating is 22 mm to increase the oxygen battery capacity. The outer electrode lead hole 4 can make the measurement structure more reasonable.

[0030] Example 2 As shown in Figure 1-b, the zirconia measurement probe includes: a ceramic electric heater 1, a measurement electrode 2, a measurement electrode lead hole 3, a ceramic filter 4, a reference electrode 5, a temperature-measuring thermocouple 6, and a zirconia measurement tube 7. The zirconia measurement tube is a U-shaped tubular structure. At corresponding positions on its inner and outer walls, a reference electrode 5, a measurement electrode 2, and a temperature-measuring thermocouple 6 are provided. The temperature-measuring thermocouple 6 is arranged at the middle position of the inner side of the zirconia measurement tube 7 and the reference electrode 5. On the outer side of the zirconia measurement tube 7, a ceramic electric heater 1 is provided. The ceramic filter 4 is arranged at the front end of the zirconia measurement tube 7. The ceramic filter 4 can isolate dust in the flue gas, prevent the dust from scouring the measurement electrode, and prevent the aging and falling off of the measurement electrode and the change of the internal resistance of the measurement electrode, which may cause abnormal measurement results. The ceramic filter 4 also has a damping effect on the flue gas flow in the flue, making the flow rates of the measured gas and the reference gas relatively consistent, meeting the specifications and requirements of the zirconia oxygen measurement technology for gas flow. When the zirconia measurement probe is installed, the front end of the probe is tilted upward by 10-15° from the horizontal position, changing the contact surface between the front end of the ceramic filter 4 and the flue gas flow direction, so that the fast-flowing flue gas in the flue passes through the lower end of the ceramic filter 4 and enters the measurement probe, forming a micro-pressure that directly contacts the measurement electrode. During operation, due to the negative pressure in the flue, the measured gas in the measurement probe is extracted from the upper end of the ceramic filter 4, forming a micro-pressure difference between the upper and lower ends of the front filter surface of the filter 4, enabling the measured gas to flow. According to the gas thermal convection effect at high temperature, it can also assist the flow of the measured gas. Embodiment

[0031] The zirconia measuring tube is a U-shaped tubular structure. The raw materials used for production are industrial zirconia of over 99% grade. 6.9% calcium oxide is added to zirconia as a stabilizer. The two raw materials are mixed evenly, and then a binder is added and stirred evenly to form a briquette. After high-temperature calcination, the material properties are stable. It is completed through processes such as fine grinding, pickling, pulping, forming, and high-temperature sintering. Porous metal material silver measuring electrodes are attached to the inner and outer sides of the zirconia measuring tube. Through processes such as coating, drying, and high-temperature heat treatment, the electrode performance becomes more stable. The working temperature is 750 ± 10 °C. It is heated by the ceramic electric heater 1 and the temperature-measuring thermocouple 6, and the measurement data is transmitted to the control unit with measurement, display, and adjustment functions to provide a stable working temperature field. During operation, the strong-flow gas in the flue passes through the lower end of the ceramic filter 4 and enters the zirconia probe at high temperature to form a micro-pressure, causing the measured gas to contact the surface of the measuring electrode 2 at high temperature. Due to the negative pressure in the flue, the measured gas flows out through the upper end of the ceramic filter 4. For the zirconia measuring tube at high temperature, since the high-temperature heating end is tilted upward, the reference gas (air) in the zirconia measuring tube forms a natural thermal convection working state from normal temperature to high temperature, causing the reference gas to flow naturally. After contacting the surface of the reference electrode 5, it is discharged along the upper part of the inner wall of the zirconia measuring tube 7. Measuring the oxygen content in the flue gas is mainly achieved by the oxygen concentration difference cell formed by zirconia plus calcium oxide solid electrolyte. When the flue gas contacts the surface of the zirconia measuring tube electrode, an oxygen concentration difference cell is formed due to the different oxygen partial pressures on both sides of the electrode, thus generating an electromotive force between the two electrodes. The magnitude of the electromotive force is related to the magnitude of the oxygen partial pressures on both sides of the electrode. The measurement result should conform to the theoretical potential output value (mv) at a working temperature of 750 °C as shown in Figure 2 .

Claims

1. Preparation method and application of a measurement electrode for a zirconia oxygen sensor. The measurement electrode includes a zirconia measurement tube 1, an outer electrode of the measurement tube 2, an inner electrode of the measurement tube 3, and an outer electrode lead hole 4. The zirconia measurement tube is a U-shaped tubular structure, with one end closed and the other end open. The zirconia material is stable, and industrial-grade zirconia with a purity of over 99% and 6.9% calcium oxide are selected as stabilizers. The material is completed through processes such as fine grinding, pickling, pulping, forming, and high-temperature sintering. Porous metal material silver is used as the measurement electrode coating, which is completed by manual operation mode. The test measurement probe includes: a ceramic electric heater 1, a measurement electrode 2, a measurement electrode lead hole 3, a ceramic filter 4, a reference electrode 5, a temperature-measuring thermocouple 6, a zirconia measurement tube 7, inside and outside the measurement tube. The reference electrode 5, the measurement electrode 2, and the temperature-measuring thermocouple 6 are arranged in sequence. The temperature-measuring thermocouple 6 is arranged at the middle position of the reference electrode on the inner side of the measurement tube. A ceramic electric heater 1 and a ceramic filter 4 are arranged on the outside of the measurement tube 7 at the front end of the measurement tube 7, which can isolate the dust in the measured gas and has a damping effect on the flue gas flow. When installing the measurement probe, it should be tilted upward by 10 - 15° from the horizontal position, so that the contact surface between the filter 4 and the flue gas flow direction changes, and the measured gas can flow in the measurement probe.

2. The preparation method and application of a zirconia oxygen sensor measuring electrode according to the claim, characterized in that The stabilizer added to the zirconia is 6.9%.

3. The preparation method and application of a zirconia oxygen sensor measuring electrode according to claim 2, characterized in that Porous metal silver is used as the measurement electrode coating slurry.

4. The preparation method and application of a zirconia sensor measuring electrode according to claim 1 or 2, characterized in that The provided measurement electrode, platinum, and silver electrode coating slurry formulation, operation procedures, and methods.

5. The preparation method and application of a zirconia sensor measurement electrode according to claim 1 or 2, characterized in that The front end of the probe should be tilted upward by 10 - 15° from the horizontal position.

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