Frédéric Boschini

Importance of soft solution processing for advanced BaZrO3 materials

Abstract Barium zirconate is an interesting material for refractory applications as well as a good substrate for the manufacturing of high temperature superconductors. However, its solid state synthesis requires high temperature and provides inhomogeneous powder with a broad particle size distribution. In order to avoid these disadvantages, soft solution routes are of growing importance in the ceramic powder synthesis. Precipitation, decomposition of precursors, combustion techniques offer alternative ways to the solid state method. Advantages of these are a lower calcination temperature, production of homogeneous and fine monodisperse powders. The obtained powders have been characterised by XRD, IR, SEM and DTA/TG analysis. The influence of the synthesis conditions on the properties of BaZrO 3 has been studied.

Effect of an electric field on an intermittent granular flow.

Granular gravity driven flows of glass beads have been observed in a silo with a flat bottom. A dc high electric field has been applied perpendicularly to the silo to tune the cohesion. The outlet mass flow has been measured. An image subtraction technique has been applied to visualize the flow geometry and a spatiotemporal analysis of the flow dynamics has been performed. The outlet mass flow is independent of voltage, but a transition from funnel flow to rathole flow is observed. This transition is of probabilistic nature and an intermediate situation exists between the funnel and the rathole situations. At a given voltage, two kinds of flow dynamics can occur: a continuous flow or an intermittent flow. The electric field increases the probability to observe an intermittent flow.

Preparation of BaZrO3 powders by a spray-drying process

The potential use of barium zirconate for the manufacture of corrosion-resistant substrates emphasizes the need for a simple, inexpensive, and easily scalable process to produce high-quality powders with well-controlled composition and properties. However, the classical solid-state preparation of barium zirconate leads to an inhomogeneous powder unsuitable for applications in highly corrosive environment. For this paper, the possibility to use the spray-drying technique for the preparation of BaZrO 3 powders with a controlled size distribution and morphology was investigated. The influence of the nature and concentration of the precursor solution and the influence of the spray-drying step are discussed on the basis of x-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and dilatometric measurements.

Linking flowability and granulometry of lactose powders

The flowing properties of 10 lactose powders commonly used in pharmaceutical industries have been analyzed with three recently improved measurement methods. The first method is based on the heap shape measurement. This straightforward measurement method provides two physical parameters (angle of repose αr and static cohesive index σr) allowing to make a first screening of the powder properties. The second method allows to estimate the rheological properties of a powder by analyzing the powder flow in a rotating drum. This more advanced method gives a large set of physical parameters (flowing angle αf, dynamic cohesive index σf, angle of first avalanche αa and powder aeration %ae) leading to deeper interpretations. The third method is an improvement of the classical bulk and tapped density measurements. In addition to the improvement of the measurement precision, the densification dynamics of the powder bulk submitted to taps is analyzed. The link between the macroscopic physical parameters obtained with these methods and the powder granulometry is analyzed. Moreover, the correlations between the different flowability indexes are discussed. Finally, the link between grain shape and flowability is discussed qualitatively.

Preparation of Nanosized Barium Zirconate Powder by Precipitation in Aqueous Solution

Several ways were explored to synthesize barium zirconate by soft chemistry methods in aqueous solution. In the first method the synthesis of barium zirconate was initiated by urea decomposition, through an homogeneous precipitation of barium and zirconium salts followed by a “low temperature” thermal treatment. The kinetic of the reaction and the optimum urea/cation ratio have been determined by means of X-ray diffraction and Inductive Coupled Plasma analyses. It has been demonstrated that an amorphous zirconium hydrated oxide starts to precipitate followed by the precipitation of barium carbonate[1]. A calcination at 1200°C during 2 hours gives rise to the formation of a pure barium zirconate phase. In the other methods, barium zirconate was synthesized, in one step without any thermal treatments, by precipitation in highly basic aqueous solutions containing barium and zirconium salts. The effect of the hydroxide concentration was discussed in relation to the barium zirconate phase formation, the particles size and the particles size distribution. For each powder, microstructural characterisations have been performed on sintered bodies in order to evaluate the influence of the thermal treatment on the final density. Dilatometric measurements have been also performed in order to quantify the densification process. Important informations were obtained by these techniques, as for example the existence of an internal porosity which severely limits the final density of the material, even if sintering was performed at high temperature. Thus a careful control of the heating profile seems to be necessary in order to produce dense materials. 1.Preparation of nanosized barium zirconate powder by thermal decomposition of urea in aqueous solution containing barium and zirconium, and calcination of the precipitate. It is well know that the pH of a salt solution can be increased homogeneously by using the thermal decomposition of urea at about 90°C. The decomposition of urea gives rise to a controlled release of ammonia and carbon dioxide into the solution. OH and CO3 ions induce the precipitation of metal hydroxides and/or hydroxycarbonates [2] which are the precursors for the perovskite compounds. In order to produce a precursor with a barium/zirconium ratio equal to 1 from solutions containing barium and zirconium chloride, ICP measures were performed on the solid phase for different molar ratios urea/cations and different decomposition time. Quantitative precipitation is observed for a urea/cations molar ratio equals to 30 and a decomposition time of 24 hours. The precursor is a mixture of crystalline barium carbonate and an amorphous hydrated zirconia phase. This reactive mixture was calcined to produce barium zirconate and the phase evolution was followed as a function of the temperature by XRD measurements as shown on figure 1. Pure barium zirconate phase is formed at about 1100°C . The particles are spherical and their size lies in the range of 50 to 120 nm with some bigger aggregates characterized by a diameter close to 300 nm. A lot of neck-formation between particles can be seen on figure 2. Sintering of the powder. As shown by the dilatometric studies, the onset of shrinkage starts between 1250 and 1300°C. SEM photomicrographs of the pellets surface sintered at 1200°C (A), 1300°C (B), 1400°C (C) and 1500°C (D), for 2 hours with a heating rate of 3°C/min (figure 3) were taken to study the temperature effect on the densification of the material. Shrinkage at 1300 °C are confirmed by the photomicrographs. No open porosity can be observed after a sintering temperature at 1500°C for 2 hours. Although, Archimedes measurements on the pellets indicated an important open porosity equal to 26 % at 1200°C which decreases to 3.6 % by rising the sintering temperature at 1500°C (Figure 4). Moreover, BaZrO3 sintered at 1400 and 1500°C shows a strong enlargement in grain size (figures 3). Figure 3: SEM photomicrographs of the pellets surface sintered at 1200°C (A),1300°C (B), 1400°C (C) and 1500°C (D), for 2 hours. Figure 4: SEM photomicrographs of polished cross section of BaZrO3 pellets sintered at 1200°C (A), 1300°C (B), 1400°C (C), and 1500°C(D) for 2 hours. Magnification rate 2500X. Figure 1: Evolution of the reacting mixture calcined between 600°C and 1100°C Figure 2: photomicrographs of calcined BaZrO3 powder at 1200°C for 2 hours. Magnification rate 50000X.

Spray-Drying of Electrode Materials for Lithium- and Sodium-Ion Batteries

The performance of electrode materials in lithium-ion (Li-ion), sodium-ion (Na-ion) and related batteries depends not only on their chemical composition but also on their microstructure. The choice of a synthesis method is therefore of paramount importance. Amongst the wide variety of synthesis or shaping routes reported for an ever-increasing panel of compositions, spray-drying stands out as a versatile tool offering demonstrated potential for up-scaling to industrial quantities. In this review, we provide an overview of the rapidly increasing literature including both spray-drying of solutions and spray-drying of suspensions. We focus, in particular, on the chemical aspects of the formulation of the solution/suspension to be spray-dried. We also consider the post-processing of the spray-dried precursors and the resulting morphologies of granules. The review references more than 300 publications in tables where entries are listed based on final compound composition, starting materials, sources of carbon etc.

Li4Ti5O12 powders by spray-drying: influence of the solution concentration and particle size on the electrochemical properties

The lithium battery electrode compound Li4Ti5O12 was synthesized by calcination of precursor powders obtained through spray-drying of solutions prepared with titanium isopropoxide and lithium nitrate. X-ray diffraction and thermal analysis coupled to mass spectrometry show that single phase crystalline Li4Ti5O12 particles can be obtained after calcination at 800 °C for 2 hours. Decreasing the solution concentration leads to smaller particle sizes but also to an unexpected decrease of the electrochemical capacity, probably related to the presence of residual Li2TiO3. On the contrary, the capacity of the Li4Ti5O12 powder prepared with the high concentration solution can be increased from 150 mAh/g to 165 mAh/g (C/4 rate) by grinding. These results highlight the fact that smaller particles do not systematically display better performances for Li+ intercalation/desintercalation and confirm the need for a comprehensive approach including parameters such as crystallinity, phase purity or agglomeration.

Nuclear probes for battery materials investigations: Mössbauer spectroscopy, nuclear scattering, and neutron scattering

Selected examples of application of Mössbauer spectroscopy, nuclear resonance scattering of synchrotron radiation, and neutron scattering to battery and related materials research are presented. The charms of Mössbauer spectroscopy as a technique for screening materials, for detailed structure investigations, and for in situ measurements are illustrated. New developments of nuclear resonance scattering for isotopes with 30-90 keV resonant energy are presented. Structural, diffusive and dynamic studies utilizing neutron scattering are exemplified.

Preparation of Spherical Submicronic Barium Zirconate particles in Highly Basic Solution below 100?C

In this study, a new method has been developed to produce pure crystalline BaZrO3 powders from Ba+Zr solution or weakly soluble reactants by using precipitation route in highly basic aqueous solution. The influence of several synthesis parameters is studied. At high OH?concentration ([NaOH] = 20 mol/l), it is possible to obtain the well-crystallized stoichiometric perovskite phase at relatively low temperature (~80?C), after a short reaction time (15 minutes) and without requiring any precaution to avoid the presence of CO2. This synthesis method yields spherical particles, whose size can be controlled by changing the concentration of the Ba+Zr solution. No calcination treatment is necessary since the precipitate is crystalline. Suitable choice of the synthesis parameters ([NaOH] = 20 mol/l, [Ba+Zr] = 1 mol/l, reaction time = 15 minutes) yields a sub-micron precipitate.