1. NTC negative temperature coefficient thermal ceramics
NTC thermistor ceramics are a kind of ceramic materials whose resistivity decreases exponentially as the temperature increases. Most of them are oxide ceramics with spinel structure or other structures, and most of them contain one or more excessive Metal oxides are mainly composed of CoO, NiO, MnO, CuO, ZnO, MgO, Fe2O3, Cr2O3, Al2O3, ZrO2, TiO2, etc.
The development of NTC thermistors has gone through a long stage: in 1834, scientists first discovered that silver sulfide has a negative temperature coefficient. In 1930, scientists discovered that cuprous oxide-copper oxide also has negative temperature coefficient properties, and they It was successfully used in the temperature compensation circuit of aeronautical instruments. Subsequently, due to the continuous development of transistor technology, the thermistor research made significant progress. In 1960, the NTC thermistor was developed.
NTC thermistor ceramics are mainly used in temperature measurement and temperature compensation, etc. Its advantage is that the resistance value is not greatly affected by oxygen, and it is stable in the air, high in sensitivity and low in price. When used for temperature measurement, the larger the thermistor constant B, the higher the sensitivity. When used for temperature compensation, the value of B must be appropriately selected according to the object of compensation.
2. Classification of NTC thermistor ceramic resistors
1. According to the resistance temperature characteristics, it can be divided into three types:
①Slowly changing thermistor, this type is the most used type in actual use;
②The thermistor with critical temperature coefficient (CRT) with VO2 as the basic component, its structure drops sharply within a specific temperature. Using this thermistor can be made into a solid state contactless switch, which is widely used for automatic temperature control , In overheating protection and refrigeration equipment;
③Ceramic thermal elements with linear resistance temperature characteristics, generally for CdO-Sb2O3-WO3 series and CdO-SnO2-WO3 series ceramic materials, the resistivity is proportional to temperature in a wide temperature range (-100~300℃) The linear relationship, due to the linearization of the resistance temperature characteristic, makes the measurement convenient and easy to digitize.
2. According to the scope of use, it can be divided into three types:
① Low temperature type: The temperature range of low temperature NTC thermistor ceramics is generally 4.2~300K. Generally, two or more transition metal oxides such as Fe, Co, Ni, Mn, and Cu are used as the main components to form a spinel structure. In order to reduce the B value, a small amount of rare earth elements such as La, Nd, Yb, etc. can be doped. Mainly used in aerospace and a series of cryogenic engineering and superconducting research fields;
②Medium temperature type: The temperature range of medium temperature NTC thermistor ceramics is generally from room temperature to 300℃, and most of them are spinel type Mn oxide ceramics;
③High temperature type: The use temperature range of high temperature NTC thermistor ceramics is generally 300~1000℃, and it is generally composed of materials with ZrO2-Y2O3 series and ZrO2-CaO series fluorite structure. The following table shows the main components and applications of typical NTC thermistor ceramics under this category.
3. The production process and characteristics of NTC thermistor ceramics
The manufacturing process of NTC thermistor ceramics is similar to that of general ceramics, and the process is slightly different depending on the type of material, performance requirements, shape, and size. The following figure shows the production process of chip and rod NTC ceramic thermistors.
The following mainly describes the production process characteristics of NTC ceramic thermistors:
1. Electrode preparation
NTC thermistor ceramics are generally ρ-type semiconducting ceramics, which can form a reliable ohmic contact with silver. Therefore, most NTC thermistors use silver paste as electrode material.
2. Resistance adjustment
In mass production, in order to obtain standard resistance values of various specifications, it is not enough to simply change the composition ratio and sintering conditions. Measures must be taken to adjust resistance. The common method is heat treatment to adjust resistance.
3. Sensitization treatment
After the resistance value of the NTC thermistor body is adjusted, although its electrical parameters have basically met the requirements for use, its performance is not stable enough. The new crystal will absorb or absorb oxygen in the air and change the resistance value. In the manufacturing process, the heat treatment is usually carried out in the range of 200~600℃ for 50~100h to eliminate the instability in use. This heat treatment process only changes the oxygen adsorption or absorption of the material, but does not change its original crystal structure, which is called sensitization.
4. Sophisticated
After the assembly is completed, the performance of the thermistor product is not stable enough and must be treated with sophisticated treatment. Sophistication is to place the product in a constant temperature box equal to or higher than the working temperature for 100 to 500 hours, or to perform multiple positive and negative temperature cycles in a range slightly higher than the working temperature. This is equivalent to accelerating the aging of the product under artificial conditions. In the first week of practice, the resistance value changed greatly, and then gradually stabilized.
Fourth, the application of NTC thermistor
NTC thermistors are widely used in temperature sensing and control of household appliances, automobiles, and industrial production equipment due to their high sensitivity, high reliability and low price. From the functional point of view of the components, the main functions are as follows: ①Temperature detection and control ②Temperature compensation ③Suppression of surge current
It can be roughly divided into the following situations:
1. High-power NTC thermistor
Application scope: suitable for the protection of conversion power supply, switching power supply, UPS power supply, various electric heaters, electronic energy-saving lamps, electronic ballasts, power supply circuits of various electronic devices, and color display tubes, incandescent lamps and other lighting fixtures Filament protection.
2. Compensating NTC thermistor
Application range: Suitable for general precision temperature measurement and temperature compensation in metering equipment and transistor circuits.
3. Temperature measurement NTC thermistor
Application range: Mainly used in cooling systems of automobiles, diesel locomotives, large motors, oil-immersed transformers, etc. as temperature sensing elements for fixed-point temperature measurement, and can also be used for temperature measurement in other occasions.
4. Glass-encapsulated temperature measurement NTC thermistor
Applications: air conditioning, heating equipment, electronic thermometers, liquid level sensors, automotive electronics, electronic desk calendars and other fields.
5.CMF chip NTC thermistor
Applications: Temperature compensation of semiconductor integrated circuits, liquid crystal displays, transistors and quartz oscillators for mobile communication equipment; it can be used as temperature detection of batteries, temperature detection of computer microprocessors and various circuits that require temperature compensation.