Jean-Pierre Bonnet

Relation between solution chemistry and morphology of SnO2-based thin films deposited by a pyrosol process

Abstract The chemical nature of precursors and morphology (surface aspect and preferred orientations) for SnO2 films deposited by a pyrosol process are examined for starting solutions containing either SnCl4 or SnCl2 · 2H2O dissolved in methanol. A study by comparison shows that molecules formed from SnCl4 deposit more easily than polymer molecules produced from SnCl2 · 2H2O. The addition of HC1 in this last solution modifies significantly the chemical nature of the molecule participating in film development as well as the growth rate. Electrical measurements of the solution resistivity as a function of HC1 concentration show a minimum in the equivalent conductivity which is directly correlated with a maximum in the growth rate. This is explained by the association of SnCl2 and HC1 to form neutral HSnCl3 molecules. A geometrical approach is then developed to describe the growth form of the film crystallites. For thin films deposited from SnCl2 · 2H2O, the crystallite faces consist of (101) corresponding to (110) preferred orientation. For SnCl4, the crystallites consist of 111 faces corresponding to a (200) preferred orientation. The presence of (101) and (301) twin planes is used to explain the observation of (301) (211) and (101) preferred orientations respectively. Lastly, the electrical and optical characteristics of the corresponding films are also reported.

Identification of solvated species present in concentrated and dilute sodium silicate solutions by combined 29Si NMR and SAXS studies.

Both concentrated and diluted sodium silicate solutions have been investigated by combining (29)Si NMR spectroscopy and SAXS experiments. The chemical nature of the entities responsible for the high siliceous species solubility observed in such alkaline concentrated sodium silicate solutions and their evolution according to dilution have been identified. For the most concentrated solution ([Si]=7 mol/l; pH=11.56; Si/Na atomic ratio=1.71), the results evidence the preponderant presence of neutral Si(7)O(18)H(4)Na(4) complexes, which behave like colloids of about 0.6-0.8 nm able to form very small aggregates with an average size lower than 3 nm. Addition of distilled water to this initial concentrated solution leads, on one hand, to a doubling of the colloid size, i.e. 1.2-1.5 nm, and, on the other hand, to a progressive decrease of the aggregate size until their total disappearance. Such a behavior could be explained by considering, first, the dissociation of the neutral Si(7)O(18)H(4)Na(4) complexes present in the concentrated solution into Na(+) ions and charged (Si(7)O(18)H(4)Na(4-n))(n-) complexes (with 1 ≤ n ≤ 4) and, second, the condensation of these siliceous charged species in order to form larger (Si(7y)O(18y-z)H(4y-2z)Na((4-n)y))(ny-) colloids. The mean size of these colloids suggests that the condensation occurs between 2 and 8 (Si(7)O(18)H(4)Na(4-n))(n-) groups.

MINERALOGICAL AND PHYSICOCHEMICAL INVESTIGATION OF Mg-SMECTITE FROM JBEL GHASSOUL, MOROCCO

Abstract‘Ghassoul’ clay is a Mg-rich clay from Morocco which is of great industrial use and interest, but its characterization is still incomplete. The purpose of this study was to provide further details regarding the structure and characteristics of this important commercial clay mineral. Mineralogical and physicochemical characterizations of the raw form of ‘Ghassoul’ clay from Jbel Ghassoul in Morocco, and of its <2 µm size fraction, purified and Na+-saturated, were carried out using X-ray diffraction (XRD), X-ray fluorescence (XRF), infrared spectroscopy (IR), and differential and thermogravimetric analysis (DTA/TG). The XRD data revealed that the raw Ghassoul clay consists mainly of a Mg-rich trioctahedral smectite, stevensite, together with quartz and dolomite. The IR spectra consisted of vibrations typical of trioctahedral smectites. The DTA/TG curves were also similar to those of trioctahedral smectites. Chemical analysis confirms that this Mg-rich trioctahedral smectite is a stevensite characterized by a limited isomorphic substitution of Si4+ by Al3+. The thermal transformations examined by in situ XRD as a function of the firing temperature from 100 to 1200°C indicated that stevensite was transformed to enstatite (MgSiO3) at temperatures >800°C and that quartz was transformed to cristobalite when the temperature exceeded 1100°C. These transformations were irreversible. The specific surface area and cation exchange capacity (CEC) of the ‘Ghassoul’ clay are 133 m2/g and 75 meq/100 g, respectively. The main exchangeable cation is Mg2+ (53 meq/100 g).

Morphology and physical properties of SnO2-based thin films deposited by the pyrosol process from dibutyltindiacetate

Abstract Tin oxide based thin films were deposited by the pyrosol process from methanol solutions of dibutyltindiacetate (DBTDA) and variable quantities of ammonium fluoride (NH4F). The optical and electrical characteristics of these films are closely linked to the film morphology (surface aspect and preferred orientation), which is strongly influenced by the chemical nature of the tin precursors. DBTDA precursor was chosen because of easy handling. The film crystallites do not present sharp edges which is an advantage for application as a transparent and conducting electrode in a device. When the ratio F/Sn is equal to 25at.% in the starting solution, the optical transmittance is between 78 and 95%, even for the thicker deposits (1.45μm) which exhibit a corresponding sheet resistance of 8.3 Ω/□.