Sol-gel synthesis of microcrystalline rare earth orthophosphates
- PDF / 483,693 Bytes
- 11 Pages / 612 x 792 pts (letter) Page_size
- 84 Downloads / 223 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Sol-gel synthesis of microcrystalline rare earth orthophosphates Yan Guoa) , Patricia Woznicki, and Aaron Barkattb) Department of Chemistry, The Catholic University of America, Washington DC 20064
Elie E. Saad Miles Corporation, Baltimore, Maryland 21224
Inna G. Talmy Naval Surface Warfare Center, Silver Spring, Maryland 20910 (Received 18 June 1994; accepted 13 November 1995)
Studies of the reactions between rare earth salts and phosphoric acid in aqueous or ethanolic media have shown that in both cases stable gels can be formed. Upon drying, gels prepared in aqueous environments yield macrocrystalline REPO4 products similar to those produced by conventional precipitation and drying. Gels prepared in ethanol, on the other hand, undergo dehydration to form dense microcrystalline products. This observation is based on optical and scanning electron microscopy, as well as on x-ray diffraction studies and infrared spectroscopy. These techniques, as well as differential thermal analysis, indicate that crystal growth of these products takes place around 600–700 ±C. The composition of the dehydrated gels produced in both the aqueous and ethanolic systems corresponds to an orthophosphate structure. Other characteristics of the microcrystalline REPO4 products include high resistance to attack by water, absence of coloration upon exposure to gamma rays, and a high index of refraction.
I. INTRODUCTION
Interest in both crystalline and amorphous phosphate materials has grown in recent years.1 The mechanical and optical properties of crystalline phosphates made by conventional techniques are generally poor due to large grain size and residual porosity, and effective sintering requires very high temperatures and pressures. Phosphate glasses are also of limited use because of the limited range of glass formation in phosphate systems and the poor durability of those glasses that can be prepared. P2 O5 itself is glass-forming, and many P2 O5 -metal oxide combinations include a glass-forming range. However, most of these glasses, such as P2 O5 , alkali phosphates, and amorphous materials based on polyvalent metal oxides with high P2 O5 -to-metal oxide ratio, usually have poor resistance to attack by water. The solubilities of polyvalent phosphates with an orthophosphate stoichiometry are usually low, but few of these phosphates form glasses using conventional melt-derived techniques. Birchall and Kelly2 succeeded in producing AlPO4 glass using a sol-gel technique based on the reaction between the ethanolic complex of AlCl3 and H3 PO4 . Rare earth phosphates have a variety of potentially beneficial properties, including very low solubility in water, high thermal stability, high index of refraction, and, in the cases of Nd, Eu, a) In b)
partial fulfillment of the requirements for a Ph.D. dissertation. The author to whom correspondence should be addressed. J. Mater. Res., Vol. 11, No. 3, Mar 1996
http://journals.cambridge.org
Downloaded: 15 Mar 2015
etc., high concentrations of lasin
Data Loading...
