Preparation Of Ceramics For Laminated PTCR With Sol-gel Derived Powders

For the increasing trends of miniaturization, integration and low power consumption of the current electronic systems, the development of PTCR thermistors gradually turned to a chip and laminated type. What are different from the conventional PTCR are that laminated PTCR have special requirements for its ceramics and its preparation processes are much more complicated. This thesis mainly focuses on the study on the preparations of ceramics for laminated PTCR.In this thesis donor Y doped barium titanate nanocrystalline powder was prepared by sol-gel technique with barium acetate (Ba(CH₃COO)₂), tetrabutyl titanate (Ti (OC₄H₉)₄) and nitric acid yttrium (Y (NO₃)₃·6H₂O) as raw materials. And then, the powder was used to prepare the ceramics for laminated PTCR. Boron nitride (BN) and Barium Carbonate were used as the glass phase and raw material for excess Ba(Ba/Ti>1) respectively. Tape-casting method was used to form the green sheets. The green sheets were sintered at a reducing atmosphere of 97%N₂-3%H₂. Finally, the reoxidation of the ceramics was carried out in air.Through changing the amount of Y, the sintering temperature, the reoxidation temperature and time and the pre-sintering temperature the influence of these preparation parameters on the properties of the ceramics for laminated PTCR was studied. The results are as follows: the grain size increases with the donor concentration; the grain size of the reduced samples is larger than that of air sintered samples; for reduced ceramics, the minimum point of the curve of room temperature resistivity versus Y concentration shifts to a lower value of Y concentration as the sintering temperature elevates and the PTC effect increases with Y concentration; with the reoxidation temperature increases the room temperature resistivity first increases and then decreases and the PTC effect gradually increase; in effect of the reoxidation time on the properties of PTC ceramics is not conspicuous; when the pre-sintering of the sol-gel derived BaTiO₃ powders is 850℃, the PTC ceramics can achieve the best overall properties. Herein barium acetate is first used as the Ba source material for preparation of nanocrystalline powder of BaTiO₃ for laminated PTCR. When sintered under a reduced atmosphere at 1200℃for 10 min and reoxidized at 850℃for 1.5 h, a ceramic with grain size of 1.59μm, room temperature resistivity of 131Ω·cm and PTC effect of 3.1 orders was obtained which is applicable for the laminated PTCR.