M. Pollet

CaZrO3, a Ni-co-sinterable dielectric material for base metal-multilayer ceramic capacitor applications

Abstract The aim of this work is to obtain CaZrO 3 powders that are co-sinterable with nickel electrodes. Since CaZrO 3 is often sintered at temperatures higher than 1550xa0°C to achieve high sample densities, obtaining a powder co-sinterable with Ni would require lower sintering temperatures. The co-sintering also requires that the reaction takes place in a reducing atmosphere. After optimizing the thermal cycle for the phase synthesis, several approaches were investigated to decrease the CaZrO 3 sintering temperature. First, the grain size effect on the densification was studied to enhance the calcined powders reactivity before sintering. In addition, the Ca/Zr ratio was investigated, and the effect of doping with lithium salts was examined. Similarly, in order to allow co-sintering with base-metal inner electrodes, the sinterability and the properties of the dielectric were examined in a reducing atmosphere. Finally, a co-sintered material was synthesized, resulting in good dielectric properties. The samples were analyzed in terms of structure and microstructure. The electric and dielectric properties were also measured on sintered samples.

Experimental evidence for an intermediate phase in the multiferroic YMnO3

We have studied YMnO3 by high-temperature synchrotron x-ray powder diffraction, and have carried out differential thermal analysis and dilatometry on a single crystal sample. These experiments show two phase transitions at about 1100 K and 1350 K, respectively. This demonstrates the existence of an intermediate phase between the room temperature ferroelectric and the high-temperature centrosymmetric phase. This study identifies for the first time the different high-temperature phase transitions in YMnO3.

Low temperature sintering of CaZrO3 using lithium fluoride addition

Abstract The effects of lithium fluoride on the sinterability, the microstructure and the low frequency dielectric properties of acceptor/donor-doped CaZrO3 ceramics were investigated. The acceptor(Mn)/donor(W) doping was used to avoid CaZrO3 reduction. Lithium fluoride was selected as a liquid phase sintering aid to lower the sintering temperature. The dielectric properties of CaZrO3 ceramics with LiF additions are strongly dependent on the densification, the microstructure, and the reaction with LiF. CaZrO3 ceramics without LiF addition sintered at high temperature display a high permittivity, low losses and a good behavior under an electrical field as a function of temperature. Using LiF, this dielectric can be sintered at 1000xa0°C to achieve theoretical densities of 91%, er values of 31 and losses close to 0.4%. This low sintering temperature allows co-sintering with base metal, nickel and copper, for which scanning electron microscopy and energy dispersive spectroscopy results are presented.

Low temperature sintering of B2O3/LiNO3 added BaMg1/3Ta2/3O3 ceramics

Abstract The lowering of the sintering temperature of BaMg1/3Ta2/3O3 (BMT) ceramic is investigated using sintering aids. First is considered the addition of B2O3 and it is shown that such addition allows to sinter BMT material at around 1100xa0°C, whereas the usual sintering temperature is higher than 1500xa0°C in air atmosphere. Sinterability of B2O3 added BMT material in reductive atmosphere (RA) is also discussed. To enhance the sintering temperature lowering, the combination of LiNO3 and B2O3 is secondly investigated. Using both sintering aids with appropriate amount, it is shown that BMT material can be sintered at 1050xa0°C in a reductive atmosphere as well as in air atmosphere while maintaining dielectric properties (e=24, |τe| 1000). These results are promising for the manufacturing of base metal electrodes (BME)–multilayers capacitor (MLCC).

Vibrational spectroscopy study of the lattice defects in CaZrO3 ceramics

Abstract CaZrO 3 materials were prepared by a solid state reaction. The samples were pre-calcined and sintered at 1673 K for 6 h. They were then analysed using Fourier Transform-Infrared spectroscopy. Microwave dielectric function ( e = e ′+i e ″) were determined by Kramers–Kronig analysis. The reflectance spectra were then adjusted using the classical dispersion equations taking into account the LO/TO (longitudinal/transverse optic) mode splitting. The most unfavourable mode in terms of dissipation was found to be the first one ( ω T =106.9 cm −1 ). A simple adjustment of this mode transverse optic damping constant was then carried out to fit the reflectance spectra of slightly non-stoichiometric CaZrO 3 materials. This procedure was conclusive except for excessive Zr non-stoichiometry (extrapolated to >2.78%).

Lowering of CaZrO3 sintering temperature using lithium-calcium fluoride flux addition

Abstract The effects of a lithium–calcium fluoride flux on the sintering and the low frequency dielectric properties of acceptor/donor-doped CaZrO 3 ceramics were investigated. The acceptor(Mn)/donor(W) doping was used to avoid reduction of Zr in the CaZrO 3 ceramics. The lithium–calcium fluoride flux was selected since liquid phase sintering lower the CaZrO 3 sintering temperature. With such additions, several low-temperature mechanisms emerge and the sintering temperatures can be effectively lowered. A simple model was used to analyse these different mechanisms in terms of their origin and efficiency. The dielectric properties of CaZrO 3 with flux additions are strongly dependent on the densification. CaZrO 3 ceramics sintered at high temperature without any addition display a high permittivity, low losses and a good behaviour under electrical field and temperature. Using this flux, CaZrO 3 can be sintered below the nickel’s melting point (1450xa0°C) and 92% of the theoretical density can be achieved. The dielectric properties at 1xa0MHz are then e r ≈34 and a loss factor close to 0.1%.

Effect of lithium salts addition on sintering temperature, cationic ordering and dielectric properties of Ba(Mg1/3Ta2/3)O3 ceramic

The effect of different lithium salts (LiF, Li2CO3, LiNO3) addition on sintering temperature, cationic ordering and dielectric properties of Ba(Mg1/3Ta2/3)O3 ceramic has been investigated. This perovskite was synthesised by solid state reaction from BaCO3, MgO and Ta2O5. It was shown that, by addition of LiNO3, the sintering temperature was decreased to 1300°C (versus 1550°C–1600°C for the pure Ba(Mg1/3Ta2/3)O3 ceramic) without altering dielectric properties at 1 MHz. A cationic ordering was also observed. Sintering with co-fired nickel electrodes in Ar/H2 atmosphere was also successfully performed for Multi Layer Ceramic Capacitors applications.

Copper electrodes multilayer ceramic capacitors Part II Chips fabrication, optimisation and characterisation

In a companion paper, a ceramic formulation based on CaZrO3 was developed to decrease the sintering temperature of this oxide. We achieved in obtaining a low sintering temperature ceramic fireable below the copper melting point while maintaining attractive properties whatever the atmosphere used (oxidising or reductive). The present paper deals with the fabrication of the copper electrodes multilayer ceramic capacitors (MLCC) and their optimisation. Several points have been tested to enhance the characteristics of the components. They are based on the precursors origin and the manufacturing process optimisation. These studies result in NPO components with good properties (C ∼ 30 pF/5 active layers; τε = −45 ppm/K; ESR ∼ 20 mΩ at 30 MHz; tgδ < 10 × 10−4; ρi > 1012 Ω · cm).

Vibrational spectroscopy study of doped-CaZrO3 ceramics

Abstract Doped-CaZrO 3 materials were prepared by solid-state reaction. The substitution was either on the A site of the perovskite (Mg, Ca, Ba) or on the B site (Ti, Hf). The samples were pre-calcined and sintered at 1673 K for 6 h. They were analysed using Fourier transform-infrared spectroscopy. The microwave dielectric function was determined by a Kramers–Kronig analysis. The reflectance spectra were then adjusted using the classical dispersion equations taking into account the longitudinal/transverse optic (LO/TO) modes splitting. The calculated values of the Q factor match very well with the measured ones and follow a linear tendency depending on the reduced mass of the oscillator composed of Ca- and Zr-like virtual-ions. The first phonon mode is (as in many perovskite) the most incident mode on the dielectric losses. Numerous modes’ parameters are dependent on both the mass of the dopant and its concentration.