OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author -deposited version published in: http://oatao.univ-toulouse.fr/ Eprints ID: 3829

Development of microelectronic devices is driven by a large demand for faster and smaller systems. In the near future, colossal permittivity in nanomaterials will play a key role in the advances of electronic devices. We report on "colossal" permittivity values achieved in dense ceramics displaying ultrafine grain size ranging from 70 nm to 300 nm. Relative permittivity values of ∼ 10 6 at 1 kHz (0.1 < tand < 0.7) were obtained for Ba 0.95 La 0.05 TiO 3-x ceramics. The colossal effective permittivity is related to an interfacial polarization and is achieved in nanomaterials by the activation of a high number of carriers and their trapping at the interfaces. Polarization carriers involving Ti 3+ polaron is proposed to be at the origin of the observed colossal permittivity. These results may have an important technological impact since these ceramics display ultrafine grain size opening a new route to the fabrication of very thin dielectric films.High permittivity values such as e r ∼ 10 000 were reported in polycrystalline BaTiO 3 , a well known ferroelectric material.[1] Decreasing the grain size below 0.7 lm of the BaTiO 3 ferroelectric ceramic was shown to yield unrelieved stresses resulting in smaller permittivity values.[2] Indeed, colossal permittivity values up to 200000 at room temperature were achieved in BaTiO 3 micronic grain size materials in which preparation included incorporation of metallic layers in a complex multi step process. [3,4] The achievement of high permittivity values in this material was ascribed to interfacial polarization phenomena. Recently, giant permittivity values were reported in hexagonal barium titanate (h-BaTiO 3 ) single crystals. [5,6] High permittivity values around 10 5 measured on the oxygen deficient materials were explained by a MaxwellWagner interfacial polarization effect due to the presence of interfacial boundaries consisting of crystal defects such as screw dislocations. Nevertheless, the internal interfaces as well as the nature of the polarization carriers in the h-BaTiO 3 single crystals were not fully identified. Within the last few years, a large class of dielectric materials displaying colossal permittivity was proposed [7][8][9] with colossal dielectric con- [10] The large permittivity values of this material being attributed to the well established IBLC effect.[11] According to the brick layer model, the effective permittivity e eff of the microstructure can be expressed as e eff = e gb t g /t gb where e gb is the grain boundary permittivity and t g and t gb are the thickness of the grain and grain boundary, respectively.[12]Thus the IBLC mechanism is usually associated with enhanced grain size. In this context, a decrease of the average grain size should not favor the IBLC effect and colossal permittivity in ultrafine ceramics, to our knowledge, has not been reported. In this communication, we discuss the "colossal" permittivity values achieved in BaTiO 3-x and Ba 0.95 La 0.05 TiO 3-x materials exhibiting very small grain size (< 300 nm). Dense dielectrics...

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OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.

The synthesis of powders with controlled shape and narrow particle size distributions is still a major challenge for many industries. A continuous Segmented Flow Tubular Reactor (SFTR) has been developed to overcome homogeneity and scale‐up problems encountered when using batch reactors. Supersaturation is created by mixing the co‐reactants in a micromixer inducing precipitation; the suspension is then segmented into identical micro‐volumes by a non‐miscible fluid and sent through a tube. These micro‐volumes are more homogeneous when compared to large batch reactors leading to narrower size distributions, better particle morphology, polymorph selectivity and stoichiometry. All these features have been demonstrated on single tube SFTR for different chemical systems. To increase productivity for commercial application the SFTR is being “scaled‐out” by multiplying the number of tubes running in parallel instead of scaling‐up by increasing their size. The versatility of the multi‐tube unit will allow changes in type of precipitate with a minimum of new investment as new chemistry can be researched, developed and optimised in a single tube SFTR and then transferred to the multi‐tube unit for powder production.

Single-phase spinel manganese cobalt oxides Mn 3Àx Co x O 4 dense ceramics were prepared for the first time and their structural/electrical property relationships characterized. The electrical properties, that is, the resistivity at 25°C, the energetic constant, and the resistance drift at 125°C, were determined and correlated with the cation distribution. Finally, the electrical characteristics of the Mn 3Àx Co x O 4 system were compare'd with other important classes of manganese-based spinel oxides, Mn 3Àx Ni x O 4 and Mn 3Àx Cu x O 4 , already commercialized as negative temperature coefficient (NTC) thermistors. The high values of energetic constant and low resistivities observed in Mn 3Àx Co x O 4 ceramics present a promising interest for such industrial applications. *tenailleau@chimie.ups-tlse.fr

In pursuit of high permittivity materials for electronic application, there has been a considerable interest recently in the dielectric properties of various perovskite oxides like calcium copper titanate or lanthanum doped barium titanate. When processed in a particular way, this later material present at ambient temperature and at f= 1 kHz unusual interesting dielectric properties, a so called "colossal" permittivity value up to several 10 6 with relatively low dielectric losses. Moreover and contrary to what is classically expected and evidenced for this type of materials, no temperature dependence is observed. This behavior is observed in nanopowders based ceramics. An assumption to explain the observed properties is proposed. These results have important technological applications, since these nanoceramics open a new route to the fabrication of very thin dielectric films.

A "soft chemistry" method, the coprecipitation, has been used to synthesize the perovskite CaCu 3 Ti 4 O 12 (CCT). Three main types of materials were obtained for both powders and sintered ceramics: a monophased consisting of the pure CCT phase, a biphased (CCT + CaTiO 3 ), and a three-phased (CCT + CaTiO 3 + copper oxide (CuO or Cu 2 O)). These ceramics, sintered at low temperature, 1050 • C, present original dielectric properties. The relative permittivity determined in the temperature range (−150 < T < 250 • C) is significantly higher than the one reported in the literature. Internal barrier layer capacitor is the probable mechanism to explain the particular behaviour. Moreover, the presence of a copper oxide phase beside the perovksite CCT plays an important role for enhancing the dielectric properties.