Study On Low Temperature Sintering Of Barium Titanate Thermosensitive Ceramics With High Curie Point

Donor doped barium titanate（BaTiO₃） based thermosensitive ceramic elements are widely used in computer,household appliances,automobiles and other fields.Barium titanate based PTCR elements developed by using the Positive Temperature Coefficient effect of barium titanate have the function of constant temperature heating,and have the advantages of low cost,energy saving,fast heating rate and good stability.They have been widely used in electric vehicle air conditioning and battery heating.In industry,high Curie point barium titanate thermosensitive ceramics are generally used as heating elements,and Pb is often used as Curie temperature shifting agent to improve the Curie temperature of components.However,the preparation of barium titanate thermosensitive ceramics containing Pb generally requires a higher sintering temperature,above 1300℃,which will cause serious volatilization of Pb,deteriorate the performance of thermosensitive ceramics and also cause environmental pollution.Therefore,a series of research work was carried out on low temperature sintering of high Curie point barium titanate thermosensitive ceramics in this paper.In this paper,barium titanate thermosensitive ceramics containing Pb were prepared by using nano.scale hydrothermal powder barium titanate as raw material.XRD results show that the samples sintered at 1250～1350℃,using 60 nm and 100 nm BaTiO₃ powder as raw materials,Pb as Curie moving agent,Nb and Mn as acceptant and main doping respectively are pure phases,indicating that the acceptor can completely solution into nano barium titanate.The relative density of sintered samples with 60 nm and 100 nm initial powders at 1250℃can reach more than 90%,and the semi-conductivity is realized at 1270℃.The sintering temperature is lower than that of the traditional solid-phase method.In the selected temperature range,combining sintering temperature and electrical properties,the samples sintered at 1270℃with initial powders of 60 nm and 100 nm have better performance.The resistivity at room temperature is 886Ω·cm and 843Ω·cm,and the PTC jump is 3.3 and 2.8orders of magnitude,respectively.In order to further reduce the sintering temperature,on the basis of using hydrothermal powder barium titanate as raw material,adding SiO₂ as combustion aid,the sintering and electrical properties of the samples were studied.The results show that SiO₂ can promote the growth of grain,and the sample sintered at 1250℃with 0.5 mol%SiO₂ reaches 94%relative density,which is equivalent to the sample sintered at 1350℃without SiO₂ additives.The addition of SiO₂ also plays an optimization role in electrical performance,which can reduce the room temperature resistance and improve the PTC jump.Combining sintering temperature and electrical properties,the sample doped with 1 mol%SiO₂ has a good performance at1250℃,with a resistivity at room temperature of 18.8Ω·cm and a PTC jump of 3.5 orders of magnitude.In addition,due to the limited role of SiO₂ in reducing sintering temperature,we continue to study the influence of BN as a sintering agent on samples.BN doping can promote grain growth,which is conducive to the density of ceramics,but due to the volatilization of gas,the formation of porous structure will reduce the maximum density of ceramics.High doped BN samples can reach the maximum approximate density of 90%at 1100℃,while the relative density of undoped BN samples is only 65%at this temperature,indicating that BN can effectively promote sintering at low temperature.However,the maximum density of highly doped BN is only 90%,which is difficult to reach the maximum density of undoped sample of 95%.BN also optimizes electrical properties by reducing room temperature resistance and increasing the PTC jump.Combining sintering temperature and electrical properties,the sample doped with 3 mol%BN has a good performance at 1150℃,with a resistivity of 44Ω·cm at room temperature and a PTC jump of 4.5 orders of magnitude.