| BaTiO3-based?BTO-based?thermistor is a kind of basic electronic component,which is very sensitive to temperature.When the temperature rised to the Curie temperature of the materials,the valve of its resistance jumps to 103108 times of the original value.After decades of research and development,they have been used as current limiters,overheated protection,current stabilizers,etc in the electronics industry,and creat billions of yuan of output value each year.Although the domestic thermal ceramics industry has developed rapidly over the years,the performance of the thermal ceramics prepared based on the existing production processes and materials are unsatisfactory,and there is still a huge gap between the development and production of high-performance thermal ceramic materials.Especially in the production and research and development of high-performance chip-type thermal elements,the problem of low comprehensive performance?including high room temperature resistivity and low temperature coefficient?needs to be solved by researchers.The main research goal of this paper is to produce thermal ceramic materials and chip components with high-performance.In order to achieve this goal,the dry pressing method was firstly used to systematically and thoroughly study the effect of Sodium bismuth titanate?BNT?doping on BTO-based thermosensitive ceramic materials and their thermal properties?especially the temperature coefficient and room temperature resistivity?and its mechanism.It provides a theoretical basis for the research and development of the preparation technology of chip-type BNT-BTO-based thermal ceramics.Based on this research,the reduction-reoxidation and casting-lamination preparation technology of chip BNT-BTO-based thermosensitive ceramic materials is studied and explored.Finally,through further research on the physical and chemical mechanisms that affect the main performance indicators,the BNT-BTO-based thermal ceramic material,thermal ceramic materials and components with a very low room temperature resistivity?about 6?·cm?,a high PTCR jump(103.4)and a high temperature coefficient?7%?were prepared by optimizing the preparation technology and material formulation.By far,there is not any published papers have reported the preparation and study on thermosensitive ceramic materials and components with such excellent properties.In addition,the research results of this paper have important guiding significance for further improving the performance of thermal ceramic materials and chip components.The specific findings of this article are as follows:The effects of BNT doping on the properties and properties of BTO-based thermosensitive ceramic materials and their mechanisms are studied and discussed.BNT-doped barium titanate-based thermal ceramic materials were prepared by dry pressing preparation process.The influence of preparation process and material formulation?including BNT doping and micro-application main doping,etc.?on the electrical properties,PTCR characteristics and micromorphology were studied systematically.A representative sample has an excellent performance of a temperature coefficient(?10/25)of>35%,a PTCR jump of 106.5,and a switching temperature of 105±5°C.Thermal ceramic materials have a temperature coefficient(?10/25)of up to 54%,more than twice the current highest reported value?25%?.The results shown that the room temperature resistivity of the sample decreases firstly and then increases with the increase of BNT doping amount.By studying and discussing the donor effect of BNT,it is clear that the donner effect of BNT is caused by the segregation of sodium on grain boundaries.The results also shown that BNT doping can significantly increase the temperature coefficient of the material,and its adverse effect on the switching temperature can be suppressed by increasing the doping amount of Sr.The dielectric temperature measurement shown that the BNT doping change the phase change characteristic value?of the material,and the temperature coefficient is increased by shortening the phase change temperature region of the material.By analyzing the components of a large number of sampling points,it is found that BNT reduces the phase change characteristic?of the material by suppressing the fluctuation of the microscopic composition of Sr.Research on effect of micro-doping of donner and accepter shown that the curical doping amount of donor decreases with the increase of BNT doping amount,the optimum acceptor doping amount is almost unchanged.The doping of the donner and acceptor affect the temperature coefficient and the PTCR jump by affecting the acceptor concentration.The temperature coefficient and the PTCR are always consistent changed here.At the end of this section,we summarize three phenomena related to temperature coefficient changes in BNT-doped barium titanate-based thermal ceramic materials,and a more complete explanation is obtained through our research.Based on the previous research,explore the preparation technology of chip BNT-BTO-based thermosensitive ceramics.BNT-BTO-based thermal ceramic materials and components were prepared by casting and lamination processes.The preparation technology route was established and the material formula was studied.The influnce of material formulation,including BNT doping and Ba/Ti ratio,etc.,on electrical performance and micro-structure was studied.Representative samples have a room temperature resistivity of less than 18?·cm,a temperature coefficient??T?greater than 9%,and a PTCR jump higher than 103.6.The results show that the introduction of trace amounts of barium carbonate after calcination and before sintering can form a low-temperature co-melt with Ti and O during sintering,which promotes the formation of microstructures with large grains and more pores in ceramics,which helps to improve its performance.BNT participates in pre-firing help achieve semi-conductivity of BNT-doped barium titanate-based thermal ceramics.The results of XPS analysis of the adsorption oxygen(Oadd")on the grain surface show that the doping of sodium strontium titanate can increase the oxygen adsorption efficiency of the grain boundary and increase the effective acceptor concentration and the grain boundary barrier height.The PTCR jump also increases significantly with increasing acceptor concentration.During the sintering process,BNT doping has a complex dual effect on grain growth.In addition to suppressing grain growth by producing a sodium-rich grain shell,BNT doping can also produce a sodium-rich liquid phase under certain conditions to promote grain growth.The presence of sodium-rich liquid phase in the grain boundary is detected by backscattered SEM and EDS.Continuing the previous research,further studying and optimizing the chip preparation process and formula of the thermosensitive ceramic material,the physical and chemical mechanisms which affecting the main performance indicators were analyzed by experiment.The results show that one of the most critical factors in preparing a thermally sensitive ceramic material with extremely low room temperature resistivity is controlling the thickness and quality of the grain boundary depletion layer.By controlling the thickness of the grain boundary depletion layer and optimizing its continuity,thermal ceramic materials with higher performance were prepared by adjusting the reduction-reoxidation process and optimizing the main doping amount.Representative chip thermal ceramics have a very low room temperature resistivity of 6?·cm,high PTCR jump of 103.4,and temperature coefficient??T?of 7%.The room temperature resistivity is far less than previously reported the preparation of high-performance thermal ceramics at room temperature resistivity??28?·cm?,other electrical parameters and includes the PTCR jump and temperature coefficient were hold the same level.The donor La inhibited the grain growth of BNT-BTO-based thermal ceramics,affected the adsorption of grain boundary oxygen by affecting the density of ceramics,thus increasing the PTCR jump and temperature coefficient.However,excessive lanthanum doping will reduce the thickness of the depletion layer and destroy the continuity of the grain boundary depletion layer.The analysis of the valence state of manganese by electron spin resonance shown that the chemical valence of manganese change from+3 valence to+2 valence during phase transition.This phenomenon contributes to the preparation of chip thermal ceramics with low resistivity and high PTCR jump.At the same time,manganese doping can also increase the thickness of the grain boundary barrier and improve the continuity of the depletion layer,which also contributes to the improvement of material performance.Finally,through the research of the reoxidation temperature,we reveal the creation of new low-resistance channel on the grain boundary and its effect on the performance of room temperature resistivity. |
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