K. Chhor

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.

Batch and semi-continuous synthesis of magnesium oxide powders from hydrolysis and supercritical treatment of Mg(OCH3)2

Abstract A new process for submicronic MgO powder synthesis starting with Mg(OCH 3 ) 2 is studied. The influence of various experimental parameters (water to alkoxide ratio, temperature, reaction time) is investigated. Aerogels and crystallized MgO powders with surface area up to 280 and 210 m 2 · g −1 have been obtained in batch and semi-continuous reactors, respectively.

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.

New low‐temperature crystalline phase of ferrocene: Isomorphous to orthorhombic ruthenocene

A new low‐temperature phase of ferrocene Fe (C5H5)2, stable below 250 K, has been characterized by means of x‐ray diffraction on powder and differential scanning calorimetry experiments. Lattice of this phase is orthorhombic and its molecular packing is isomorphous to ruthenocence one, so configuration of ferrocene molecules is eclipsed (D5h symmetry). Experimental conditions to obtain this orthorhombic phase are discussed in connection with crystallite size.

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.

Thermodynamic studies on the successive phase transitions in LiKSO4 crystals at low temperatures

The thermal behaviour of LiKSO4 has been investigated in the temperature range 10 to 300 K by heat-capacity measurements and d.s.c. studies. The known phase transitions around 190 and 250 K have been studied and are not found to be first-order as previously reported. Three more transitions have been found to occur at 38, 65, and 83 K. We emphasized the influence of the nature of the sample on the transitions, particularly that at 250 K. The results are discussed and compared with the extensive literature on this compound published during recent years.

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.