The Studies On The Preparation And The Properties Of Ca_0.4Sr_0.6Bi₄Ti₄O₁₅ Ceramic Thick Film

Much attention has been directed in recent years towards the bismuth layered structure compounds for use in sensing devices used at high frequencies and high temperature. Amongst these materials, (Sr_1-xCa_x)Bi₄Ti₄O₁₅ ferroelectric material has these properties of both high Curie temperature of CaBi₄Ti₄O₁₅ and high remanent polarization of SrBi₄Ti₄O₁₅,which can be applied to non-volatile ferroelectric memories(NVFRAM) and piezoelectric devices used under high frequency and high temperature. Meanwhile, ferroelectric thin/thick film has bright future in micro-electrics, opt- electrics and MEMS, because of its particular properties of ferroelectric, piezoelectric, pyroelectric. In the present paper, synthesis of Ca_0.4Sr_0.6Bi₄Ti₄O₁₅ ultra-fine powder by Sol-gel method, preparation of powder-sol slurry and preparation of Ca_0.4Sr_0.6Bi₄Ti₄O₁₅ ferroelectric thick film and investigation of ferroelectric property are discussed,in order to lay solid foundations for the research and development of high performance ferroelectric MEMS.The preparing process of Ca_0.4Sr_0.6Bi₄Ti₄O₁₅ ultra-fine powder and some related issues were studied, which include the crystallizing process, some properties and heat treatment of the gel. The results show that: 1) The crystallizing process of Ca_0.4Sr_0.6Bi₄Ti₄O₁₅ gel includes the following steps: some water and the removal of both water and ethylene glycol groups in the gel. Following this, the decomposition of some organic groups and metal-organic complex., the combustion of residual materials such as carbon and the formation of some middle oxides such as TiO₂, Bi₂O₃, SrO and CaO, the formation of crystalline perovskite phase. 2) Ca_0.4Sr_0.6Bi₄Ti₄O₁₅ ultra-fine powder can be synthesized by Sol-gel method, However, grain agglomeration of powder appears. the particle size distribution of the powder heated at 200℃and 500℃is 0.3-0.5μm, 0.8-1μm, respectively. the grain agglomeration of the powder heated at 600℃is reduced, the average particle size is 0.8μm, whereas the crystalliziation is improved, the powder heated at 800℃has pure bismuth layered structure,the particle size is about 300nm. however ,the particle size of the powder heated at 900℃is 2μm,the grain grows big. 3) Ca_0.4Sr_0.6Bi₄Ti₄O₁₅ nanocrystalline powder with smaller size and good dispersion can be prepared via pre-carbonization of gel assisting sol-gel route, the particles are slightly aggregating, with homogeneous particle size of about 100nm, which are the candidate materials for fabricating ferroelectric thick films.The preparation of the powder-sol slurry was studied, and the technology about how to disperse the slurry was also studied. The results show that, 1) The homogeneous and stable solution is prepared with the addition of Acetyl acetone. 2) When the concentration of the solution between 0.15-0.20mol/l, Ca_0.4Sr_0.6Bi₄Ti₄O₁₅ thin films have the better quality and the bigger thickness. The effect of PH value on the dispersion was characterized by experiment of Zeta potential, it was found that the best PH=6.5. In addition, the effects of dispersants, PVPK30, PEG, ultrasonic and ball milling on sol have been analyzed.Based on the above, Ca_0.4Sr_0.6Bi₄Ti₄O₁₅ ferroelectric thick film was prepared with powder-sol method, and the ferroelectric property was also investigated. The results shows that, 1) Ca_0.4Sr_0.6Bi₄Ti₄O₁₅ thick films more than 1μm are uniform, dense and crack, with the rotating technology that 4000rpm×30s for the first thin film, 3500rpm×25s for the thick film. 2) Bismuth layered structure thick film can be prepared with the following heat processing, 200℃×2min on heat plate, 550℃×5min with RTA for the first film, when the thickness is gotten, 750℃×30min for the whole film. 3) Ca_0.4Sr_0.6Bi₄Ti₄O₁₅ thick films more than 3μm without uniform, dense and crack were prepared. When the thickness is 1,700nm, the remanent polarization is 12.5μC/cm², Ec=50kV/cm.