Five applications of the hottest ceramic sensors i

2022-08-12
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Five applications of ceramic sensors in automobiles

1. Introduction to ceramic sensors

automotive sensors, as the information source of automotive electronic control system, are the key components of automotive electronic control system and one of the core contents of automotive electronic technology research. The higher the degree of automotive electronization and automation, the greater the dependence on sensors. Therefore, vehicle sensor technology is listed as a key high-tech development at home and abroad. At present, electronic components account for 30% of the average cost of components for each high-end car, and there are hundreds to hundreds of automotive sensors. Advanced sensors previously installed in luxury, high-end cars or special vehicles are now settling in medium and low-end cars, and ceramic sensors are one of their products

ceramics is a multiphase system containing three phases (single crystal phase, glass phase and gas phase). The ceramic sensor adopts standard operators. First of all, they should understand the production of universal experimental machine ceramic process or one of its variants (sol-gel, etc.). After completing appropriate preparatory operations, the formed components are sintered at high temperature. Its working process is mainly the change of capacitance value between capacitor plates caused by the differential pressure acting on the ceramic substrate and the measuring diaphragm, which is detected by the electrode located on the ceramic substrate

II. Five applications of ceramic sensors

(I) detecting vehicle temperature

a vehicle generally needs more than 10 ceramic temperature sensors to detect temperature. For example, the engine EFI system needs sensors that continuously and accurately measure the cooling water temperature, intake air temperature and exhaust temperature, so as to correct or compensate the fuel injection volume according to the temperature change, change the target value of idle speed control, etc., and obtain the best air-fuel ratio; The temperature characteristic of NTC thermistor with negative temperature coefficient is an exponential function. With the increase of temperature, the resistance value decreases, showing the characteristics of negative temperature characteristic, high sensitivity, low price and so on. It is often used as a sensor to detect the temperature of cooling water, intake air and engine oil; NTC thermistor is made of transition metal oxides such as Mn, Cu, Ni and Fe through ceramic sintering process. According to different formulas, it is mainly divided into binary system, ternary system, quaternary system and other materials. NTC with the working temperature range of -200 ℃ ~ 130 ℃ is used to detect the inlet temperature of water temperature. Its structure is to assemble the NTC resistance in the bolted metal shell and connect it in series with the resistance of the electronic control unit. Another kind of PTC thermistor with positive temperature coefficient, which is mainly made of BaTiO3 and sintered with metal oxide, is used as liquid level sensor or low-temperature starting heating element of automobile

(II) detection of automobile exhaust

using solid electrolyte gas sensitive ceramic materials, an oxygen sensor for automobile exhaust monitoring is developed to measure the oxygen concentration in exhaust emissions to detect the air-fuel ratio of the engine, which can not only save fuel, but also reduce the emission of harmful gases such as CO, NO2. Because of its high sensitivity and good reliability, ZrO2 oxygen sensors are mostly of this type in practical applications of automobiles. Its main structure is composed of U-shaped ZrO2 electrolyte sensitive tubes that generate electromotive force, bushings that act as electrodes, resistance heaters, protective housings with exhaust gas inlets, and porous ceramic caps. The inner and outer surfaces of the ZrO2 tube are coated with a thin layer of R, Pt is used as electrode and EMF amplification, and R is coated on the ZrO2 tube for catalysis. The outer electrode is the measuring electrode and the inner electrode is the reference electrode. The amount of gas passing through the porous ceramic cap (diffusion barrier) is small. When the exhaust temperature is low, the electric control unit will energize and heat the resistance heater to ensure the normal operation of the oxygen sensor. ZrO oxygen sensor is installed in the manifold or front exhaust pipe. At a high temperature of 400 ℃, when there is an oxygen concentration difference inside and outside the sensitive pipe, it will generate electric heat, providing a feedback signal of 0 ~ 1V. By detecting the oxygen pressure ratio in the exhaust gas, it is very suitable for the control of air-fuel ratio. According to the working principle, it can be divided into concentration battery type and electrochemical pump limit current type oxygen sensors. These two kinds of oxygen sensors have similar structures and manufacturing processes, which are suitable for the control of theoretical air-fuel ratio and lean combustion system air-fuel ratio respectively. In addition, the research and development of oxide ceramic oxygen sensors such as TiO2, Nb2O5 and CeO2, thin-film and thick film oxygen sensors and their applications in automobiles are also actively carried out

when the vehicle is powered by diesel engine, it is also very important to further improve the combustion state and exhaust gas reprocessing with NOx sensor in addition to oxygen sensor. ZnO and SnO2 thin films with a thickness of about 100 m are formed on the alumina substrate by sputtering, then electrodes are added, and a heater is installed on the inner side of the substrate to form a NOx sensor. NOx adsorbs negative charge on the film surface. When the NOx concentration increases, the film resistance increases. The NOx concentration in the exhaust gas can be detected within 3 ~ 15s, and the sensitivity is 5 ~ 800ppm

(III) detect the working state of the cylinder

piezoelectric ceramics based on piezoresistive effect can monitor the working state of the cylinder. Piezoelectric ceramic knock sensor is composed of piezoelectric ceramic vibrator, metal sheet, gasket, metal shell, etc. The charge generated by the piezoelectric vibrator is in direct proportion to the vibration of the engine cylinder. The generated voltage enters the electronic control unit through the shielding wire, so as to detect the voltage generated by the vibration of about 7kHz. The electronic control unit judges the knock intensity according to the magnitude of this voltage, and timely corrects or correspondingly postpones the ignition advance angle to eliminate the knock, so that the engine works at the ignition time close to the knock, with the highest thermal efficiency and the lowest fuel consumption, Realize the knock free working state and ensure that the engine operates with the maximum possible power and economic indicators

(IV) guide the safe driving of vehicles

the ultrasonic sensor based on piezoresistive effect is used as the anti-collision alarm device for vehicle reversing, also known as the ultrasonic reversing radar or reversing sonar system, which is especially suitable for large vehicles such as long loading vehicles, heavy-duty trucks, mining vehicles, etc. Ultrasonic sensors are usually composed of aluminum alloy shells, piezoelectric ceramic transducers, sound-absorbing materials, and lead electrodes. They have wide horizontal characteristics and limited vertical directivity. In principle, they use the positive and negative piezoelectric effects of lead cuoqinate PZT piezoelectric ceramics in the conversion between electrical energy and mechanical energy, that is, adding an electrical signal to the piezoelectric ceramics will produce mechanical vibration and emit ultrasonic waves, When ultrasonic waves encounter obstacles in the process of air transmission and are immediately reflected back. When they act on piezoelectric ceramics, electrical signals will be output. Through data processing, time difference ranging, the calculation can display the distance between the car and obstacles, and give an alarm in time when it collides with the danger. It can accurately detect small obstacles in the rear of the car and the blind area of the driver's visual angle, which is quite practical. In order to obtain high transmitting efficiency and receiving sensitivity, the ultrasonic sensor that transmits and receives the spontaneous disk bending vibration transducer is the mainstream product in the market at present, which has high transmitting efficiency, receiving sensitivity and sharp directivity. Ultrasonic has a certain detection angle and range. To cover the whole rear area of the car, two ultrasonic sensors need to be installed in narrow body vehicles, while four or more ultrasonic sensors need to be installed in wide body vehicles

ceramic acceleration sensor can be used in automobile airbag system. The inertial force formed by collision inertia will produce shear force in the piezoelectric ceramic body, resulting in charge and voltage proportional to the acceleration. It is high-precision, reliable and can quickly distinguish the actual collision. Two piezoelectric ceramic sheets are bonded in series through the internal common electrode to form a two pole structure, which is installed in the moving direction and forms a cantilever beam, and is integrated with the thick film of the peripheral circuit and fabricated in a shell. It is used to detect the instantaneous low-speed or high-speed collision intensity of the car, convert it into electrical signal output, and meet the requirements of diagnosis and control. 2. Accessories method, so as to ensure that when the collision intensity is high, the airbag can be opened accurately and timely, so as to improve the safety performance of the car

(V) detection of automobile humidity

humidity sensitive ceramics are characterized by wide humidity measurement range, fast response time and simple production process. They are the main materials of automobile humidity sensors. It is applicable to the detection of frost and condensation on the window glass and the air humidity of the intake part of the engine carburetor. The interior of the humidity sensor is equipped with a porous sintered body made of metal oxide series ceramic materials, which is sensitive to humidity by using the adsorption of water molecules on the surface of the sintered body. Its sensitivity depends on the porosity and pore diameter of the material. The characteristic quantity of humidity sensing is resistance, showing negative humidity sensitivity. When the sintered body adsorbs water molecules, its resistance value will change. In the environmental experiment, the plated electrode must ensure that the surrounding of the tested product is output under the same temperature environmental conditions. When the humidity increases, the resistance value decreases. When the relative humidity changes from 0 to 100% RH, the electrical resistance value of the sensor will change thousands of times, so the humidity change can be detected

III. Development Trend of ceramic sensors

ceramic sensors have a history of more than 40 years. Ceramics is a recognized material with high elasticity, corrosion resistance, wear resistance, impact resistance and vibration resistance. The thermal stability and thick film resistance of ceramics can make its working temperature range as high as -40 ℃ ~ 125 ℃, and it has high accuracy and stability of measurement. The electrical insulation degree is greater than 2KV, the output signal is strong, and the long-term stability is good

Nearly all household packaging is reusable

ceramic sensors with high characteristics and low price will be the development direction of pressure sensors. Ceramic sensors have a trend to comprehensively replace other types of sensors in European and American countries, and more and more users in China use alternative diffused silicon pressure sensors. In short, with the development of electronic technology and the increasingly extensive application of automotive electronic control system, the market demand for automotive sensors will maintain a high-speed growth, and the sensors with high stability, high precision, long life, wireless, integrated and networked will gradually replace the traditional sensors and become the mainstream of automotive sensors. (end)

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