Preparation Of Mg: LN Polycrystalline Powders By Wet Chemical-Spray Drying Method And Investigation Of Crystal Growth

As a prominent multifunctional material, lithium niobate (LiNbO3, LN) possesses well ferroelectric, piezoelectric, nonlinear optical, electro-optical, photorefractive properties. LN crystal is widely used in the field of electric and optical, which has stable chemical and physical properties and bargain price. On the basis of above advantages, LN is applied to surface acoustic wave (SAW) filter, optical communication, optical waveguide and so on. With the development of demond of communication, the number of cordless telephone become large, and the quantity demanded of LN craystal increases greatly. As so far LN is a very popular artificial material which any other crystal can be instead of.Commercial LiNbO3 is a typical non-stoichiometry crystal (Congruent LN, abbreviate CLN) with [Li]/[Nb]=48.6/51.4, which is extensively grown by Czochralski method. Although CLN possesses high consistency of composition, LN crystal has many intrisic defects because of lack of lithium atoms, which weakens lots of physical properties of LN especially the ability of resistant of optical damage. MgO is usually doped in LN crystal for increasing the threshhold value of optical damage.The superior quality of polycrystalline powders is one of the key factors for growth of high quality LN crystal. The polycrystalline powders are usually prepared via solid state reaction at about 1200℃by using mixed MgO, Li2CO3 and Nb2O5 as raw materials. During the calcinaton the composition of the powders deviate due to the volatilization of Li+, what is more, the defects of inclusion and scattering particles are turn up. Last, the distribution of Mg+is not homogeneous in LN crystal in solid state reaction, because the molten point of MgO is higher than 2500℃.In this paper, Mg:LN was synthesized by using Nb(OH)5, Li2CO3, MgO, CA(citric acid), HC1, NH3H2O as the raw materials. Spherical Mg:LN polycrystalline powders with homogeneous distribution of Mg element are obtained by calcining precursor which derives from the solution of precursor via spray drying. The synthesis process is as follows:the Nb(OH)5 is dissovled in HCl under water bath; the Nb5+ions are coordinated with critic acid and form stable transparent solutions by adding CA in the system; Measuring the content of Nb5+in the solution by thermogravimetric analysis exactly; Weighting the amount of Li2CO3 and MgO in accordance with the content of Nb5+, preparing Li-CA, Mg-CA solutions individually, and mixing the above three solution to get precursor solution; The precursor powders are obtained by spray drying using SD-BASIC SPRAY DRYER; Lastly, the Mg:LN polycrystalline powders was obtained via calcination of the precursor at 800℃. Precursor and final product was characterazed to investigate the reaction mechanism. The main contents are as follows:(1) The reaction mechanism is discussed;(2) The coordination reaction and complex compound are verified by IR analysis of the product before and after coordination;(3) The TG-DTA is performed on the precursor to study the thermal behavior during calcination;(4) By analysis of IR spectra of the products derived from different calcination temperatures, the final temperature is determined to be 800℃.(5) XRD analysis on LN and Mg:LN, indicates that the Mg2+ions enter LN lattice, and changes the cell parameters of LN;(6) Morphology of precursor and polycrystalline powders derived from different concentrations of solution was observed on scanning electron microscope (SEM).A high quality Mg:LN crystal is grown from melt by traditional Czochralski method in air. A new growth system is developed according to characteristics of crystal. the main factor influenced the crystal growth is discussed, including:thermal field, seed, and growth parameters, etc. Several properties of as-growth crystal is studied:(1) Composition of Li2O on different sites on the crystal was measured by a ultraviolet absorption adge method, which indicates high homogeneity of Mg:LN crystal grown by using above spherical Mg:LN powder. (2) The conoscope image on centre and edge of crystal testifies that the as-growth crystal possesses high optical homogeneity;(3) The refractive index of Mg:LN crystal is smaller than that of CLN crystal;(4) The optical homogeneity of Mg:LN crystal is 2.02×10-5 by optical homogeneity test.This synthesis method will have great potential in growth of high quality Mg:LN crystals, which will have great applications in Laser machine, nonlinear, laser monitor, electro-optical, optical wave-guide, and other fields.