Lanthanide oxides, deposition

J. Paivasaari, M. Putkonen, L. Niinisto, A comparative study on lanthanide oxide thin films grown by atomic layer deposition, Thin Solid Films 472 (2005) 275-281. 

In this section various existing lanthanide and actinide metal-organic enolate precursors for rare earth metal oxide deposition are discussed and the rationale of their selection is addressed. CVD, ALD and ultrasonic spray pyrolysis (USP) of the lanthanide or actinide enolate starting materials has been carried out under a variety of conditions as can be seen from Table 7. 

Current literature on long-term (20 year) occupational exposure to high levels of lanthanide dust suggests that the lanthanide oxides may cause benign pneumoconiosis (the deposition of material in the lung, visible on X-ray, without any impairment of lung function). This conclusion is further supported by animal studies. 

In a wear situation (in presence of an electrolyte), the aqueous medium could act as a coolant, reducing the temperature at the point of triboeontact. The amount of oxygen available is limited to its concentration in the solution, so it does not seem that the formation of the layer of oxide controlled by the mechanism of oxygen diffusion could be responsible for these improvements. On the contrary, the electrochemical activity associated with local deformation could promote the surface deposition of the lanthanide oxides in the cathodic areas. These oxides could be eliminated and/or redueed in thickness during any further wearing action. 

D Olieslaeger, M., Van Bael, M.K., and Mullens, J. (2007) Aqueous chemical solution deposition of ultrathin lanthanide oxide dielectric films. /. Mater. Res., 22, 3484-3493. 

Bastnasite has been identified in various locations on several continents. The largest recognized deposit occurs mixed with monazite and iron ores in a complex mineralization at Baiyunebo in Inner MongoHa, China. The mineral is obtained as a by-product of the iron ore mining. The other commercially viable bastnasite source is the Mountain Pass, California deposit where the average Ln oxide content of the ore is ca 9%. This U.S. deposit is the only resource in the world that is minded solely for its content of cerium and other lanthanides. 

Clearance to pulmonary lymph nodes will occur at a fractional rate of 0.0001 per day. Dissolution of the deposited particles and absorption of cerium into the systemic circulation will occur at rates that are between the extremes represented by CeCh in CsCl particles and Ce oxide or Ce in fused aluminosilicate particles as given by the functions included in Figure 9. These rates should not be expected to be constant over the entire clearance period and will depend upon the overall composition of the bulk aerosol particles, which indude particle size, amount of stable lanthanide present, acidity, and the solubility of other components of the particles. The accuracy of predicting respiratory tract clearance and internal organ uptake of radiocerium will depend heavily upon adequate determination of the particle solubility characteristics. 

Figure 4.42. Molecular structures of commonly used CVD precursor classes. Shown are (a) metal p-diketonate (acetylacetonate, acac) complex to grow a metal oxide film (H2 as the coreactant gas yields a metal film) (b) a heteroleptic (more than one type of ligand bound to the metal) p-diketonate complex to yield a Cu film the ancillary ligand helps prevent oligomerization, enhancing volatility (c) various types of complexes to deposit metallic, oxide, nitride, or oxynitride films (depending on coreactant gas(es) used - respective ligands are p-ketoiminato, p-diketiminato, amidinato, and guanidinato (d) a metal azolato complex commonly used to deposit lanthanide metal thin films.

Creating metal oxide based advanced materials using lanthanide alkoxide complexes as molecular precursors is another area where the lanthanide alkoxide chemistry has found significant applications [5, 7-10]. A technique heavily used in the semiconductor industry for the growth of metal oxides materials is the process of metal-organic vapor chemical vapor deposition... 

Excellent overviews on the deposition methods of lanthanide and actinide oxides were published in several reviews by Niinisto" Zhang and Yan", Lee" and Yagi". ... 

TABLE 7. Deposition studies of lanthanide and actinide oxide films other than cerium oxide... 

Many alkoxides in particular have been known since the 1960s, but interest in them has been stimulated recently by their potential use as precursors for deposition of metal oxides using the sol-gel or MOCVD process. A review covering the literature to 1990 has appeared. " Traditionally, alkoxides are made by salt-elimination reactions of lanthanide chlorides with alkali metal alkoxides (or aryloxides) which sometimes causes chloride retention... 

Reactions of carboxylates containing the more electropositive cations yield product carbonates, or sometimes the basic carbonates. Some of these salts, e.g., those of the alkali metals, melt before decomposition. The oxide products from decomposition of the lanthanide compounds may contain carbon deposited as a result of carbon monoxide disproportionation. Kinetic measurements must include due consideration of the possible retention of carbon dioxide by the product (as COj ) and the secondary reactions involved in carbon deposition. 

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