Investigation Of Electrical Properties Of NiO-ZnO-TiO₂System Ceramics

At present, with the rapid development of high-tech, electronic ceramicsand its components are widely used in various fields. BT-based semiconductingceramics are widely utilized as materials with a positive temperature coefficientof resistivity mostly contain lead. Due to various reasons, some oxide containingPb enter into the living environment and the natural environment, therebycausing harm to the human body and the natural environment. Because of thecurrent states of the continuous improvement of environmental protectionrequirements, lead-free PTCR materials have become an inevitable trend. As anew lead-free PTC materials, NiO-ZnO-TiO2system semiconductor PTC heatingelement ceramics attract wide attention of researchers. It has advantages ofsimple manufacturing process, lead-free and is widely used in the temperaturesensors, heating, control, etc. The structure and PTC properties ofNiO-ZnO-TiO2system ceramics prepared by traditional solid state reactionmethod were studied by using X-ray diffraction (XRD), scanning electronmicroscopy (SEM) and ρ-T test methods. Our ultimate aim is to find a newmaterial substitute the lead-contained PTC ceramics.The new PTC ceramics of NiO-ZnO-TiO2system were prepared bytraditional solid state reaction method though two different channels, the resultsshow that ceramic samples prepared by two different channels display the PTCproperties, which illustrate that the new PTC ceramics can be made via the twomethods.Channel1, xZnO-(0.95-x)NiO-0.05TiO2system PTC ceramics wereprepared by traditional solid state reaction method. The test results present that,the best sintering temperature of the samples is1340℃; the obvious PTC effectsappeared with increasing temperature, due to the different expansion coefficientof the three main crystal phases ZnO, NiO and ZnTiO3in the ceramic samples.The resistivity of the samples decrease with the increase of ZnO, and its PTCeffect is best with a higher jump temperature (525℃), and a lower room-temperature resistivity (966Ω.cm) at x=0.65. Ceramics of this system areexpected to become a new lead-free PTC ceramics which have higher jumptemperature and lower room-temperature resistivity.Channel2，xZnOss-(0.875-x)NiOss-0.125SP system ceramics were prepareduse reprepared compounds ZnOss(Zn0.97Ni0.03O), NiOss(Zn0.4Ni0.6O),SP[(Zn0.5Ni0.5)2TiO4] as raw materials. The influence of composition andsintering temperature for the crystal structure, microstructure and PTC propertiesof this system ceramics were investigated. The results of the study are asfollowed, the Three-phase system ceramics exhibit significant PTC effects withlow room-temperature resistivity and the maximum resistivity appeared at thetemperature above340℃, which shows an idear for preparation of lead-free PTCceramic with high jump temperature and low room-temperature resistivity; thebest sintering temperature of the system ceramics is1400℃. The ceramicsamples have low room-temperature resistivity, high jump temperature andsmaller Rmax/Rminat this temperature, due to the PTC effects of the system arebased on thermal expansion. So it is a slow changing PTC ceramics with lowRmax/Rmin.Contrast shows that, the PTC effects of xZnOss-(0.875-x)NiOss-0.125SPsystem ceramics are better than that of the xZnO-(0.95-x)NiO-0.05TiO2systemceramics which have instability PTC properties, poor repeatability. This ismainly due to the former ceramic samples containing solid solution phase, whichcan make the lattice more stable and prevent the transition of certain crystalstructure. At the same time, the strength and hardness of the solid solution areoften higher than the each component. Moreover, the formation of solid solutionalso has a certain influence on the physical properties of materials such aselectrical, thermal, magnetism and so on.The study also find that: the composition and sintering temperature is asignificant impact on the PTC properties of the PTC ceramic samples, obtainingthe optimal composition, and the best sintering temperature. The key forpreparing the system ceramics is the choice of electrode will also produce a greatinfluence on PTC property of ceramics. This is mainly because the contact ofmetal electrode and semiconductor ceramic can produce a non-ohmic contactwhich will affect the resistivity of the ceramic samples. Therefore, PTC ceramic samples prefer to indium-gallium electrodes, if the testing temperature is toohigh, we can also choose aluminum electrodes to ensure the test results close tothe actual value.