Introduction of materials for thick film electric heating elements
The new concept of thick film circuit electric heating element is the perfect combination of new material technology and traditional thick film circuit preparation technology, and its key technology is the breakthrough of new material research. Compared with the traditional thick film circuit materials, the new materials for electric heating elements must meet different requirements. Materials for thick film electrothermal elements include substrate materials and substrate based electronic pastes, which are described respectively below.
3.1 As all kinds of household and industrial electric heating elements, the substrate material should first have good electric heating function, that is, high power density, high heat conduction efficiency, and at the same time, it should also have high mechanical strength, thermal shock resistance, vibration resistance and other mechanical properties. Therefore, corresponding requirements are put forward for thick film circuit substrate. The physical properties of substrate materials for traditional thick film circuits are shown in Table 1. Although the above substrate materials have good electrical properties and a series of matching electronic pastes have been commercialized for a long time, as an electric heating element, the brittle ceramic substrate can not meet its basic requirements, and the most important problem is that it is fragile during installation and use. Metal has excellent mechanical and physical properties, which can be used as substrate material. However, the expansion coefficient does not match with the commonly used electronic paste, and oxidation caused by thick film sintering process. Porcelain enamelled steel (PES) has been used to make substrates, but it is not an ideal choice because of its high temperature strength, surface oxidation and weak bonding with slurry coating. At present, stainless steel is mostly used as substrate. Stainless steel substrate has good thermal conductivity, good toughness, easy to process, can be made into a larger size and non-planar components, low comprehensive cost other substrate materials can not be compared.
3.2 Electronic slurry
Due to the change of substrate material, electronic paste based on ceramic substrate is no longer applicable. It is necessary to develop a series of electronic materials matching with stainless steel, including medium paste, resistance paste, guide tape paste, etc.
3.2.1 dielectric material
The dielectric material is coated on the substrate surface to provide an insulating layer. The basic composition is glass phase and organic solvent. The basic requirements for dielectric size are as follows: high dielectric strength; No toxic elements; The expansion coefficient is matched with the substrate material; High bonding strength with substrate; Can withstand multiple sintering process and the property is unchanged.
3.2.2 Resistance paste
Resistance paste is used for printing function without controllable resistance circuit, generally used in stainless steel substrate on the medium layer, after screen printing in the oxidizing atmosphere mesh belt furnace into a film, resistance paste is composed of inorganic bond phase, functional phase and organic solvent, the basic requirements for resistance paste are as follows;
Constant resistance temperature coefficient; Sintering for 30 minutes; No lead, tin-nickel; Resistance does not change with the number of heating!