Study On The Multilayer Chip BaTiO₃Based Thermosensitive Ceramic Materials

In recent years, with the rapid development of microelectronics and surface-mountingtechnique （SMT）, integration, miniaturization and chip type of electronic component havebecome the trend of today’s microelectronics technology. Accordingly micromation, chiptype and low room-temperature resistance are the developmental trend of the positivetemperature coefficient of resistivity （PTCR） ceramics. It is difficult to further reduce theinherent resistance of the BaTiO₃-based PTCR ceramics by the traditional method. However,the multilayer chip type PTCR ceramics not only have the lower room-temperatureresistance and the higher resistivity jump, but also have small size, well stability, goodenduring large current rushing, which are used to the protection component of the overloadcurrent in integrated circuit, therefore become a research subject with large market prospectand practical value.The multilayer chip BaTiO₃-based PTCR ceramics are prepared by tape casting andreduction reoxidation method. The influence of Ba/Ti rate, donor dopants of A site, donordopants of B site, sintering temperature and reoxidation heat treatment on the electricalproperties of the multilayer chip Ba(Ti_1-xNb_x)O₃ceramics has been studied. The ceramicswith the room-temperature resistance of0.3and a resistance jump （Lg（Rmax/Rmin）） of3.3were successfully prepared. moreover, the multilayer chip Ba1.001（Ti1-xNbx）O3ceramics withroom-temperature resistance of0.13and a resistance jump of3.2. The detailed research isas follows:The chip type BaTiO₃-based ceramics are prepared by reduction reoxidation method.The effect of Ba/Ti rate on the electrical properties, microstructure, and PTCR effect of thesamples is studied. The results indicated that the mean grain size of the excess-Ti sampleshas a little change; its room-temperature resistivity is very small and rarely affected by thereoxidation time and Ba/Ti rate, while the characteristics of the excess-Ba samples is justcontrary to the excess-Ti samples. The room-temperature resistivity and the resistivity jumpof the (Ba_m-0.007Sm_0.007)TiO₃ceramics with Ba/Ti rate （m） of1.006and1.026are80.8·cm,3and108.5·cm,3.8, respectively.The effect of the sintering temperature and the cooling mode on microstructure, electrical properties and PTCR characteristics of the chip type Ba_1.022-xSm_xTiO₃ceramicswith high Ba/Ti rate has been investigated. The results indicated that the grain growth isinhibited by the high donor-doped concentration, the room-temperature resistivity and theresistivity jump of the samples decrease with an increase of the sintering temperature. Thelow room-temperature resistivity of the samples can be obtained by the higher sinteringtemperature. The higher breakdown voltage of the samples is obtained based on the lowersintering temperature. In addition, it is indicated that the lower cooling rate, the porosity isless. The room-temperature resistivity of the samples increases with increase in the coolingrate, and lower room-temperature resistivity is obtained by prolonging the cooling duration;moreover, the corresponding dopant concentration of the minimum resistivity is shifted tolower values with increasing cooling rate. In addition, samples that have been cooled at acooling rate of4°C/min after sintering in a reducing atmosphere show pronounced PTCRcharacteristics, with a resistance-jumping ratio greater by3.16orders of magnitude, inaddition to achieving a low room-temperature resisitivity of157.4·cm. Furthermore, theacceptor-state density （Ns） and height of the potential barrier （） for these samples are6.9110¹³cm^–2and0.59eV, respectively. A higher cooling rate leads to a larger number ofoxygen losses in the grain-boundary region.On the basis of the reduction reoxidation method, the influence of the donor-dopedconcentration, the sintering temperature and reoxidation temperature on the electricalproperties and PTCR characteristics of the multilayer chip type Ba(Ti_1-xNb_x)O₃ceramicshave been investigated. The ceramics with a size of3.6mm×1.8mm×1.4mm obtained ata low reoxidized temperature of600°C after sintering at1160°C show a remarkable PTCReffect, with a resistance jump larger by3.3orders of magnitude, along with a low RTresistance of0.3. Furthermore, the acceptor-state density of the samples sintered at thesintering temperature of1100–1220°C is in the range from2.47×1013to7.19×1013cm–2.Finally, the influence of the annealing treatment on the electrical properties and PTCRbehavior of mutilayer Ba(Ti_1-xNb_x)O₃-based ceramics with Ni internal electrode has beeninvestigated. The results indicate that the room-temperature resistance and grain-boundaryresistance of the ceramics both increase with the increasing reoxidation temperature or time.In addition, the0.35mol%Nb⁵⁺-doped Ba(Ti_1-xNb_x)O₃ceramics with a size of3.6mm×1.8mm×1.4mm reoxidized under a low temperature of600°C for6h after sintering1160°C for2h showed a remarkable PTCR behavior, with a resistance jump of3.5orders ofmagnitude, along with a low RT resistance of0.38. Meanwhile, the multilayer chip typeBa_1.001(Ti_1-xNb_x)O₃-based ceramics with a size of3.6mm×1.8mm×1.4mm, a resistanceof0.13, a resistance jump of3.2, and a temperature coefficient of6%/C was successfullyprepared. In addition,700C for the grains is the critical reoxidation temperature. Theperfect reoxidation temperature should be controlled as not longer than850C, moreover,the suitable reoxidation time should be controlled within6h. In particularly, it is consideredthat Ti site in BaTiO₃should be replaced by Nb⁵⁺in order to compensate the acceptor actionsof Ni diffused from inner electrodes for the interface layer of Ni Ba_1.001(Ti_1-xNb_x)O₃, this isthe advantages of the B-site donor dopant to the multilayer chip type BaTiO₃-based PTCRceramics.