J.F. Bocquet

A semi-continuous process for the synthesis of nanosize TiO2 powders and their use as photocatalysts

Abstract A semi-continuous process has been developed for the synthesis of TiO 2 powders: a sol-gel reaction, using titanium isopropoxide as precursor and acetylacetone as chelating agent, followed by supercritical drying is performed. The obtained powders have been characterized by X-ray diffraction, SEM, TEM, BET method and OH surface groups measurements. The recovered solids are always crystallized in the anatase structure with particle size ranging from 8 to 13 nm. Surface area and OH surface groups density are around 100 m 2 ·g −1 and 0.6 × 10 −3 mol·g −1 respectively. The photocatalytic activity of the elaborated powders has been studied following the degradation of phenol and compared to that of two commercial powders (P25 Degussa and Acros).

Syntheses of submicron TiO2 powders in vapor, liquid and supercritical phases, a comparative study

Abstract Submicron TiO2 powders have been prepared from titanium isopropoxide by the sol-gel process and thermal decomposition of the pure vapor phase or dissolved in supercritical ethanol. The microstructure and phase transformations in the obtained solids are compared. Differences have been observed in particle size and agglomeration state as well as on the crystallinity of just-prepared powders and its evolution under thermal treatment. The new supercritical route allows to obtain narrow sized submicron particles (20–60 nm), partly crystallized and softly associated into spherical agglomerates (0.5–2 μm). Such powders are expected to have as low a sintering temperature as sol-gel samples subsequently treated in an autoclave. The supercritical synthesis then appears as a simpler method, avoiding the various powder recovery steps of wet chemistry processes and easily adaptable to continuous production.

Submicronic MgAl2O4 powder synthesis in supercritical ethanol

Submicronic MgAl2O4 powders have been synthesized by decomposition of the double alkoxide Mg[Al(O-secBu)4]2 in supercritical ethanol. The influence was studied of various experimental parameters (reaction time and temperature, reactional medium concentration and density) on the main characteristics of the obtained materials. Particularly, sample crystallinity and chemical composition, particle size distribution and mean diameter were investigated by X-ray diffraction, IR spectroscopy, laser scattering and electronic microscopy.

Barium titanate powders synthesis from solvothermal reaction and supercritical treatment

Abstract A new semi-continuous process is reported for BaTiO 3 powders synthesis under high pressure (10xa0MPa). The first step is the hydrolysis of the alkoxide BaTi(O– i C 3 H 7 ) 6 in isopropanol at temperatures between 100°C and 200°C in tubular flow reactor. The second one is a thermal treatment of the formed solids, under the supercritical state of the solvent. Particle size of the dried crystalline recovered powders is around 10xa0nm. Powder sintering and ceramic dielectric properties have been tested.

A new TiO2 film deposition process in a supercritical fluid

Abstract A new process for TiO 2 film formation with a high deposition rate is proposed. The thermal decomposition of titanium isopropoxide dissolved in a supercritical CO 2 -isopropanol mixture is carried out below 300°C on alumina substrates. High solute concentrations, compared with chemical vapor deposition processes, allow one to obtain homogeneous anatase film with around 5 μm thickness in less than 10 min.

Heat capacity and thermodynamic behaviour of Mn3O4 and ZnMn2O4 at low temperatures

Abstract Heat-capacity measurements on Mn3O4 allow us to be precise about the Neel temperature at 43.15 K; the entropy change associated with the transition between the ferrimagnetic and paramagnetic phases shows that residual disorder must remain in the low-temperature phase as previously reported from magnetic measurements. In the study of the thermal behaviour of ZnMn2O4, important metastability phenomena have been encountered. When the low-temperature phase is formed, two successive phase transitions can be observed. The first one (weak and broad) between 45 and 80 K can be related to the disappearance of the spiral magnetic spin vector arrangement; the second one (very violent) takes place at 230 or 271 K according to the series. Although this last transition is found to be first-order, no structural changes were observed from X-ray diffraction.

Reactions in supercritical fluids, a new route for oxide ceramic powder elaboration, synthesis of the spinel MgAl2O4

Abstract We have performed the decomposition of the alkoxide Mg(Al(OR) 4 ) 2 , (R ≡ sec-butyl group), in supercritical fluid ethanol. The partly crystallized submicronic powder formed around 350°C and 15MPa leads to the spinel MgAl 2 O 4 after heating at 1200°C. The dried solid particle size ranges from about 0.04 to 0.2μm. The main characteristics of these powders have been compared with those of samples elaborated by sol-gel process. From IR spectroscopic study, a first attempt has been made in order to determine the nature of reactions taking place in the supercritical fluid medium.

Modeling of a continuous reactor for TiO2 powder synthesis in a supercritical fluid — experimental validation

The use of supercritical fluids as a reaction medium for the synthesis of metal oxide powders allows many advantages. The powders obtained are dry, fine, and crystallized. TiO2 production, from decomposition of Ti(OC3H7)4 used as a precursor, has been developed in a continuous reactor at a laboratory scale. The present work is concerned with the modeling of such a reactor, based on experimental studies including (1) a kinetic study of the Ti(OC3H7)4 decomposition in supercritical alcohol and (2) an hydrodynamic study. The first one allows us to define the experimental working conditions for the continuous system producing TiO2 powder. The second one shows the existence of a hotter zone at the reactor inlet leading to a nonisothermal operation inside the reactor. These two investigations show that temperature and pressure values in the investigated range appear to have no significant influence on the primary particle size (around 30 nm) obtained.

Syntheses of submicron magnesium oxide powders

Abstract Submicron MgO powders have been synthesized from thermal decomposition of magnesium chelates or acetate at around 200 °C in liquid alcohol or supercritical alcohol-CO 2 mixtures. Because of a similar coordination of the magnesium atom in both types of precursors, a common amorphous alkoxy-hydroxy derivative is first obtained. When the reaction occurs in a supercritical medium with carbon dioxide as cosolvent, the microstructure of the solid is quite different and a CO 2 group is incorporated into the alkoxy bond. In all cases, the intermediate compound crystallizes into cubic magnesium oxide under thermal treatment between 400 and 600 °C; the elementary particle size in the powders obtained is less than 100 nm.

Low-temperature adiabatic calorimeter with an automatic data-acquisition system for the temperature range 10 to 310 K the molar heat capacity of NH3(CH2)3NH3MnCl4

A calorimeter of the adiabatic type has been constructed for measurements of the heat capacity between 10 and 310 K. An on-line microcomputer combined with an automatic data acquisition system controlled the calorimeter heating rate and computed the temperature and the heat capacity. The system allowed unattended operation for 15 h. The calorimeter with sample cell of 7 cm3 internal volume, used two miniature germanium and platinum sensors in the temperature ranges 10 to 30 K and 30 to 310 K, respectively. The resistance was converted into temperature (IPTS-68) by calibration tables stored in the microcomputer. The performance of the calorimeter assembly was tested by measurements on the heat capacity of NBS benzoic acid. The deviations of the smoothed values from those previously reported are better than 0.5 per cent between 100 and 300 K, increase to 1 per cent at 40 K, and are about 5 per cent below 20 K. The heat capacity of NH3(CH2)3NH3MnCl4 has been measured from 10 to 310 K. As previously reported by other methods, a structural phase transition occurs at 307.61 K associated with a molar enthalpy change of (735±20) J·mol−1. In addition a magnetic transition was observed in the temperature range 45 to 65 K. Finally, d.t.a. results between 100 to 350 K are reported and compared with those previously claimed by other authors.