A Novel Approach to Sintering Nanocrystalline Barium Titanate Ceramics Anton Polotai, Kristen Breece, Elizabeth Dickey, and Clive Randall w Materials Science and Engineering, State College, The Pennsylvania State University, University Park, Pennsylvania 16802 Andrey Ragulya Institute for Problems in Materials Science, National Academy of Science of Ukraine, Kiev, Ukraine A novel approach to pressureless sintering based on the combi- nation of rapid-rate sintering, rate-controlled sintering, and two- step sintering under a controlled atmosphere is proposed. This combined sintering method facilitates control of grain and pore morphology. The application of this sintering approach for pure nanocrystalline barium titanate powder enables the suppression of grain growth during the intermediate and final stages of sin- tering and the production of fully dense ceramics with 108 nm grain size. The grain growth factor is 3.5, which is three and 17 times smaller than rate-controlled and conventional sintering, respectively. I. Introduction T HE continuous development of new, smaller electronic de- vices with enhanced properties requires miniaturization of internal components. The multilayer ceramic capacitor (MLCC) is one of the key components in electronic devices. Further mini- aturization of MLCCs is possible by decreasing the width of the dielectric layer less than 1 mm while simultaneously increasing the number of dielectric layers. Because of its extremely high dielectric constant, barium titanate is the most widely used di- electric constituent of commercial MLCCs. In order to produce micrometer dielectric layers, the BaTiO 3 grain size should be nanoscale. Furthermore, as grain boundaries can retard oxygen vacancy migration, it is necessary to have multiple layers of grains within each dielectric layer. 1 The production of dense nanocrystalline barium titanate ceramics is a difficult task. The nanocrystalline powder has a tendency to form agglomerates, which cause differential densi- fication, intra-agglomerate coalescence, and grain growth during sintering. 2,3 The activation energy of grain boundary migration in barium titanate is quite small, resulting in increased grain growth during the final stage of sintering. 4 Some success with the manufacture of dense barium titanate nanocrystalline ce- ramics is obtainable by high-pressure sintering and spark-plas- ma sintering. 5–8 Although dense nanoceramics are possible, these techniques may not be practical commercial methods because of their lack of cost-effectiveness. Moreover, these techniques could not be implemented in the current MLCC production technologies. For this reason, conventional furnace sintering is still desirable for commercial production. Three different approaches can be used to control micro- structure evolution in BaTiO 3 ceramics during sintering. The first one utilizes segregation of dopants to grain boundaries and suppression of grain boundary mobility. It is well known that a small amount of Y, Nb, Ca, or Dy acts as a grain growth in- hibitor in BaTiO 3 . 9–12 Another method uses a controlled atmos- phere to prevent abnormal grain growth and to retain a small final grain size. Grain boundary migration is suppressed most likely because of an increasing quantity of Ti 31 ions as well as a grain boundary structural transition from a faceted to a rough mode. There is some evidence that a reduction atmosphere can affect grain growth in barium titanate. 13–15 A final approach utilizes flexible ‘‘smart’’ sintering regimes, which take into ac- count competition between densification and grain growth kinetics in the system of nanoparticles. Among them are non- isothermal rapid-rate sintering, 16 rate-controlled sintering (RCS), 17,18 and two-step sintering. 19 Each of these methods has its own advantages and drawbacks. Non-isothermal rapid-rate sintering allows quick passage through the temperature range where surface diffusion-control- led coalescence prevails over other sintering mechanisms. Ulti- mately, fast heating rates result in decreased final density and significant dedensification compared with slower heating rates. 16 RCS enables control of microstructure evolution during sinte- ring because of a feedback existing between densification and heating rate. The first stage of heating during RCS follows the methodology of rapid rate sintering. It is well known that open porosity effectively impedes grain boundary migration. During the final stage of sintering (around 90% theoretical density), the open porosity network loses stability, forming closed spherical or ellipsoidal pores, which are located in triple junctions. Split- ting of cylinder-shaped pores and localization of spherical pores are accompanied by extended grain growth. Flexible control of densification during RCS maintains an open network of pores at higher densities (92%–96%) than in conventional sintering, re- sulting in a finer-grained microstructure. 17,19 Two-step sintering, first introduced by Chen and Wang, 19 exploits the competition between the driving forces of grain boundary-controlled densification and grain boundary-control- led grain growth to achieve densification without grain growth during the final stage of sintering. This problem is identical to the one in the final stage of RCS. The success of two-step sin- tering is determined by the pore morphology of the material prior to low-temperature second-stage sintering. Initially, the authors of this method proposed using rapid-rate sintering to obtain a high starting density (above 70%), at which all pores become subcritical and unstable against shrinkage. However, as previously mentioned, rapid-rate sintering is not always an op- timal technique for producing a narrow, uniform pore size dis- tribution within a material, which is a necessary condition for successful two-step sintering. The goal of the first stage of two- step sintering is to produce a uniform pore microstructure while J ournal J. Am. Ceram. Soc., 88 [11] 3008–3012 (2005) DOI: 10.1111/j.1551-2916.2005.00552.x r 2005 The American Ceramic Society 3008 E. Slamovich—contributing editor This work was supported by the Center for Dielectric Studies, The Pennsylvania State University, and the National Science Foundation under Grant No. 0312196. w Author to whom correspondence should be addressed. e-mail: car4@psu.edu Manuscript No. 20077. Received January 26, 2005; approved April 4, 2005.