Development of a Continuous Segmented Flow Tubular Reactor and the “Scale‐out” Concept – In Search of Perfect Powders

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.     

Structural properties of nickel manganite Ni x Mn3−x O4 with 0.5 ⩽x ⩽ 1

Single-phase nickel manganite spinels, Ni _x Mn_3–x O₄, with 0.5 x 1, were prepared by a careful thermal processing of nickel-manganese coprecipitated oxalate precursors. Powder X-ray diffraction analysis of the spinel revealed the presence of cubic single spinel phase with parametera which decreases with nickel content. The lattice parameter variation can be explained in terms of the distribution of Ni²⁺ ions on the octahedral sites. Therefore, a fine analysis of data shows that some Ni²⁺ ions (forx>0.56) are located in tetrahedral sites. The percentage of nickel in A-sites increases with nickel content (x) following the relation % Ni²⁺ in A sites =P = – 82.1x ²+192.4x–81.5 and thus the general formula for cation distribution is

Kinetics and mechanism of ferrous spinel oxidation studied by electrical conductivity and thermogravimetry

A mechanism of valence transfer between Fe²⁺ and Fe³⁺ ions in the octahedral, as well as the tetrahedral position of the spinel lattice is suggested to account for the electrical behaviour with the number of Fe²⁺-Fe³⁺ pairs in either position for oxidation reactions in finely-grained ferrous spinels. From the =f(t) curves which are identified with those obtained in thermogravimetry, it was established for inverse spinels such as Fe₃O₄ or for normal spinels such as FeCr₂O₄ that the kinetics is governed by a diffusion law under variable working conditions. For more complex spinels such as chromium-or titanium-substituted magnetites which exhibit both Fe²⁺ ions on octahedral and tetrahedral sites, the results are discussed only qualitatively although the profile of the =f(t) curves can also be related to the electronic exchange between the Fe²⁺ and Fe³⁺ ions from the two cation sites.     

Optimization of a predictive controller for closed-loop adaptive optics

For closed-loop adaptive optics systems limited by time delay and measurement noise, we demonstrate that the ideal rejection transfer function is proportional to the frequency signal-to-noise ratio of the wave-front input. We describe a new modal linear predictive controller that approaches this ideal transfer function. Its parameters are optimized by minimization of the residual wave-front error with a modified recursive least-squares algorithm. The optimization can be performed with closed-loop data in the case of evolving turbulent conditions. We present numerical simulations to show the significant improvements brought by the predictor.     

Noise propagation in wave-front sensing with phase diversity

The phase diversity technique is studied as a wave-front sensor to be implemented with widely extended sources. The wave-front phase expanded on the Zernike polynomials is estimated from a pair of images (in focus and out of focus) by use of a maximum-likelihood approach. The propagation of the photon noise in the images on the estimated phase is derived from a theoretical analysis. The covariance matrix of the phase estimator is calculated, and the optimal distance between the observation planes that minimizes the noise propagation is determined. The phase error is inversely proportional to the number of photons in the images. The noise variance on the Zernike polynomials increases with the order of the polynomial. These results are confirmed with both numerical and experimental validations. The influence of the spectral bandwidth on the phase estimator is also studied with simulations.     

Study of the oxidation kinetics of finely-divided magnetites. II - Influence of chromium substitution

The oxidation kinetics of magnetites substituted by chromium (Fe²⁺Fe³⁺_2?xCr³⁺_x)O^2?₄ (0 < x < 2) into γ(Fe³⁺_1?yCr³⁺_y)₂O^2?₃, (x = 3y), which is a metastable phase, was found out to be ruled by the law of diffusion, under variable working conditions, of vacancies generated at the surface. The chemical diffusion coefficient is a function of the substitution ratio, crystallite size and the number of vacancies in the spinel lattice. Contrary to magnetites substituted by aluminum, the activation energy varies irregularly with the substitution ratio.     

Preparation and properties of active submicronic solids and new metastable phases obtained via a metal-organic precursor method

The mixing of constituent ions on an atomic scale accomplished in single phase metal-organic precursors facilitates the occurrence and completion of the reactions such as decomposition, oxidation and reduction at reduced temperatures and in shorter times. This has been particularly shown for the first series of transition metals and for the lanthanide elements for which simple, and mixed oxides or their oxide solid-solutions have been prepared via decomposition and treatment of their respective organic salt precursors. The method gives compounds with increased homogeneity, purity and reactivity and allows isolation of submicronic solids and sometimes of new metastable phases, which could not have been obtained by high-temperature reactions. The morphological and other application properties of these submicronic solids and of new metastable phases can be widely modified and regulated by varying the composition and treatment given to the metal-organic precursors.