Preparation Of The Dielectric/glass Phase Composite Thick Films And Their Dielectric And Pyroelectric Properties

The performance of uncooled pyroelectric detector mainly depends on the properties of pyroelectrics. The present research mainly focused on improving the pyroelectric coefficient at room temperature by adjusting the Curie temperature of the materials close to the R.T. Unfortunately, this method limit the operating temperature range to a large extent because that the depolarization happens to pyroelectrics if the operating temperature exceed the Curie temperature. Furthermore, the in-used uncooled pyroelectric detectors almost manufactured from the bulk ceramic and the commercialization of the thin films pyroelelctric detector encounters lots of barriers. So, it is very important and meaningful to develop a new kind of pyroelelctric composite thick films, which contains several pyroelectrics and gives good pyroelectric response in a wide temperature range.The aim of our work is to fabricate a kind of composite thick films, which performs high pyroelectric Figure-of-Merit F_d in a wide temperature. By forming the particles encapsulated by glass phase structures, the interdiffusion among different crystal phases in composite thick films have been avoided. Composite thick films with stable-coexisted BST and PLT phase have been successfully fabricated. After detailed study on the diffusion mechanism between the BST or PLT phase and the glass phase, and its influence on dielectric and pyroelectric properties, the relationship between the dielectric and pyroelectric properties and the structures have been understood.(1) The fabrication parameters of the BST thick films, PLT thick films and BST/PLT composite thick films have been optimized. BST(Ba_0.8Sr_0.2TiO₃)powder and PLT(Pb_0.82La_0.18TiO₃) powder with good particles size, narrow size distribution and pure phase features have been synthesized by using Sol-gel process. The xPbO-(1-x)B₂O₃ glass powder with PbO concentration x= 0.2, 0.35, 0.5, 0.65 and 0.8 have been prepared by using quenching method, respectively. The as-synthesized BST, PLT and xPbO-(1-x)B₂O₃ glass powder have been used to prepare the printing paste, and BST thick films, PLT thick films and BST/PLT composite thick films have been fabricated by screen-printing the paste onto the Al₂O₃ electronic ceramic substrate with Ag bottom electrode. After analyzing the structure, microstructure, dielectric and pyroelectric properties of the thick films fabricated at different conditions, the best fabrication parameter have been determined: the adding amount of glass power is 12mol%, the PbO concentration of glass is x=0.5, the sintering temperature is 750℃ and the hold time is30min.(2) The formation of the thick films has been investigated in detail. The small-scale particles system consisting of glass powder and BST/ PLT power in thick film have very high surface energy, which is the major driving force to forming the thick films. The good wetting ability between the BST/PLT particles and the glass phase is the key factor to the formation process of the thick films, and it also lead to the glass phase uniformly spreading upon the particles, therefore the particles encapsulated by glass phase structure come into exist in the thick films. The glass phase is uniformly distributed among the BST/PLT particles, which can prevent the agglomeration of the particles and bond the BST/PLT particles, and then bring the uniformly porous microstructure into the thick films. The encapsulation structure provide the possibility for interdiffusion between the BST/PLT phase and glass phase, which lead to the extinction of domain structure in shell especially corner phase of the BST/PLT particles contrasting with the survival of domain structure in the core phase, which means the core-shell structure formed in the thick films. Connection ways between the BST and PLT have two choices: direct-connection and indirect-connection. A few of BST/PLT particles is direct-connection caused by agglomeration, and most of BST/PLT particles are indirect connection via glass phase.(3) The effects of fabrication process on the phase structure of thick films have been studied. The thermal stress occurs in BST phase, which is derived from the different CTE(coefficient of thermal expansion)between glass and has a value about 0.3Gpa. The thermal stress in BST phase is a factor to the reduction of c-axis of BST phase. The PLT and glass have almost same CTE and the thermal stress can be ignored. The B3+ ions can diffuse into the BST and PLT phase during the sintering process, which result in theformation of BTi defects in both BST and PLT phase. To balance the charge generatedi tt ?? tr ??byBTi defects, the FBaand Vo defects appear in BST phase, and Vpbd DO O *â– invariable, and the BST phase will transit into Ba2TiO4 phase while the concentration of BTl,Vpb andFo" defects is high. Because the Pb2+ ions from PLT phase and the diffusion ofB3+ ions into BST/PLT phase increased the PbO concentration of glass phase, the and BaTi(BO3)2 formed in the BST thick films with low-PbO glass addition by the reaction of BST and B2O3, but disappeared in the BST/PLT composite thick films with the same glass addition.(4) Effects of the interdiffusion between the BST/PLT phase and the glass phaseon dielectric properties have been researched. The existence of BTl,, VBa and Vo defectsgenerated by interdiffuison in BST phase give rise to the redistribution of micro-regions in BST phase, and therefore the dielectric properties of BST phase have been modified. The extent of redistribution of the micro-regions depends on the defects concentration and distribution condition in BST phase. The Pb/La ration in the shell phase of PLT particles have been changed by the interdiffusion between PLT phase and glass phase. The shell PLT phase with new Pb/La ratio lead to the broadening of the PLT dielectric peak, and decreasing of the original PLT lower the maximum relative dielectric constant. The foreign ions concentration and their distribution is controlled by the fabrication parameter of the thick films, therefore the dielectric properties of BST andPLT thick films can be adjusted. A formula (--------)ocv/exp(----)can be drawn tos ï¿¡?? Tmdescribe the relationship between the relative dielectric constant and the sintering temperature T hold time t of the sintering process in both BST and PLT thick films, the relationship between the dielectric constant of the composite thick films and components, having volume fraction m and relative dielectric constant ej , basically matched the natural logarithm mixing rules: kie = n}lnï¿¡l +n2lns2. The stable coexistence of BSTand PLT improve the dielectric temperature stability of the composite thick films. Indirect-connection BST/PLT particles and Direct-connection BST/PLT particles play a very different role on contributing to the dielectric response of composite thick films. Indirect-connected BST and PLT particles have the same dielectric properties as BST thick films and PLT thick films. Interdiffusion happens to the direct-connected BST and PLT particles. The interdifrusion lead to the formation of (Ba,Pb)TiO3 and (Pb,Sr)TiO3 phase with >300°C Curie temperature, and (Ba, La)TiO3 phase with itsCurie temperature much lower than BST, which mean the dielectric constant of composite thick films at temperature 250-300°C and 0-50°C have been enlarged. And the amount of the original BST and PLT phase is reduced by the interdiffusion. The cooperation of these factors improve the dielectric temperature stability of the composite thick films, and the dielectric properties of composite thick films in 0-300 °C isï¿¡2vc >580 and tan6>(0³00°C) (5) Effects of the interdiffusion between the BST/PLT phase and the glass phase on pyroelectric properties have been researched. The interdiffusion between the BST or PLT phase and glass phase affected the behavior and distribution of the polarization unit in BST or PLT phase, and the maximum pyroelectric coefficient at -70 °C or ¹70°C is therefore reduced, respectively. And the decease in pyroelectric coefficient dp at -70 °C is related to the sintering temperature T and hold time t, which is ruled by the formuladpcc-Jt exp(—). BST thick films sintered at 750°C for 30min show good pyroelectricproperties, it R.T pyroelectric coefficient /?=564uC/m2K and Figure-of-Merit Fo=19.4uPa"1/2 . The high Figure-of-Merit Fq is originated the low volume capacity, dielectric constant and loss of the BST thick films. In temperature range 0-90 °C, BST thick films show good pyroelectric properties as p >350uC/m2K and Fd > 14uPa'1/2. hi temperature range 90-200 °C, PLT thick films show good pyroelectric properties as p>300|j,C/m2K, Fz)=5.7uPa"1/2. The relationship on pyroelectric properties between the composite thick films and its component, having volume fraction ni and pyroelectric coefficient pi, has been drawn as the formula: p = nxpx + n2p2. It is can be concludedthat the coexistence of BST and PLT in composite thick films show good pyroelectric response in a wide temperature, and in temperature range 0-200°C, the pyroelectric coefficient is/?>250uC/m2K and the Figure-of-Merit is FD>5\iPa¹/2. At the end of the paper, we put forward the future works based on the present works.