Study Of Low-temperature Preparation And Performance Of Multilayer BaTiO₃ Based PTCR Ceramics

Multilayer BaTiO₃-based ceramics having positive temperature coefficient of resistivity?PTCR?are widely utilized in circuits for overcurrent and overheat protection because of its excellent R-T characteristics.Recently integrated and miniaturized electronic components are needed to meet demands of rapid development of microelectronics,however,further miniaturization of PTCR ceramics can result in high room-temperature?RT?resistance,which is not suitable for practical application.Besides lower sintering temperature help to further lower fabrication costs of PTCR ceramics,so developing materials with low sintering temperature becomes a research with great value in practice.In this paper,PTCR ceramics with low sintering temperature and excellent performance were developed aiming at low cost,miniaturization and low RT resistance.The main research contents as follows:Since the electrical properties of PTCR ceramics are sensitive to sintering atmosphere,the material composition will not include sintering aids with severe volatilization.Hence the effects of particle size on the densification and semiconducting of La-doped BaTiO₃ceramics were studied firstly.Powders with formula of Ba_1.027La_0.003TiO₃ were synthesized by sol-gel and solid-reaction method.It was found that dense and semiconducting ceramics were obtained at a low sintering temperature as ceramics were prepared by sol-gel method.For example,when ceramics were sintered at 1075oC,the relative density of ceramics prepared by sol-gel was about 76%.The RT resistivity was about 753?cm and PTC jump was 10^4.1 after ceramics were reoxidized at 800oC.On the other hand,the relative density of ceramics prepared by solid-reaction was only 65%and they all exhibited insulating characteristic.Since the two samples have different particle size and second phase,the effects of above two factors on the densifcation and semiconducting of La³⁺-doped BaTiO₃were studied.It was found that BaCO₃ helps to promote densification and semiconducting of the ceramics.The mechanism of low-temperature sintering was studied in detail.Due to the simple process and cheap raw materials,solid-reaction method was widely used in industrial production,so solid-reaction method was used to synthesize La-doped BaTiO₃ powders instead of sol-gel method.The powders were synthesized by two steps to avoid the decomposition of BaCO₃:first,synthesizing powders with composition of Ba_0.997La_0.003TiO₃;second,adding BaCO₃ into the as-synthesized powders.It was found that ceramics prepared by above method had a lower sintering temperature?<1100oC?than current ones?¹200oC?.Moreover,the effects of reoxidation temperature on the electrical properties of BaTiO₃-based ceramics were studied in detail.As ceramics were reoxidized at650-850oC,it was found that the RT resistivity vs.reoxidation temperature abnormally presented an inverted“U”curve.The resistivity firstly increased to a maximum value at750oC and then abnormally decreased with the reoxidation temperature.The results of impedance spectroscopy,potential barrier and XRD indicated that the reduction of second phase,Ba2TiO4,at grain-boundary area help to reestablishment of conducting pathway and then lead to reduction of RT resistivity.On the other hand,it was also found that the annihilation of oxygen vacancy and formation of adsorbed oxygen can lead to an increase of RT resistivity.Based on the above results,a probable interpretation was proposed for the abnormal reoxidation phenomenon.When ceramics were reoxidized at low temperature from 650-750oC,oxygen vacancies were annihilated and acceptor states formed,resulting in an increase of RT resistivity.When further increasing the reoxidation temperature up to800oC,the semiconducting grains would be oxidized and then Ba₆Ti₁₇O₄₀ precipitated.Ba₆Ti₁₇O₄₀ would immediately react with Ba₂TiO₄ so that Ba₂TiO₄ was gradually consumed resulting in a reduction of RT resistivity.The second phase,Ba₂TiO₄,at grain boundary can destruct conducting pathway leading to a high resistivity,so that decreasing BaCO₃ content helps to preparation of PTCR ceramics with low resistivity because of the reduction of Ba₂TiO₄.It was found that trace of BaCO₃ addition could promote the grain growth,meanwhile the closure of pores could be hindered due to the decomposition of BaCO₃ to expel generated CO₂ gas.Grain growth helps to decrease RT resistivity and pores help the reoxidation of grain boundary.As a result,lowering RT resistivity and improving PTC jump were simultaneously achieved as suitable amount of BaCO₃ was added.An optimal addition of 0.3 mol%BaCO₃ led to a low RT resistivity of about 28?cm and a PTC jump up to 10^3.7.Besides,it is also found that suitably increasing La-doping level help to promote grain growth,lower RT resistivity and improve oxygen resistance.The tape casting method was employed to prepare multilayer ceramics with one pair of Ni internal electrodes.The thickness of sheets between Ni internal electrodes was 10-50?m.SEM images suggested that excellent contact between Ni internal electrodes and BaTiO₃ceramics was obtained after ceramics were sintered at 1100oC.The results of chemical element distribution and EDS indicated that the content of Ni diffused into BaTiO₃ceramics was quite low,so that the RT resistance exhibited a positive dependence with the thickness of the sheet.Subsequently,multilayer samples laminated with 20 layers of ceramics with a thickness of 30?m were prepared.When sintered at 1100oC and then reoxidized at 700oC,the RT resistance was about 0.25?and PTC jump was about 10^3.1.