Preparation And Properties Of M: Al₂O₃ (M=Cr, Ti) Nano-Powders And Transparent Ceramics

Laser media with Al₂O₃ as host materials have characteristics of high hardness, good transmittance and optical uniformity, as well as high thermal conductivity, stable chemical composition and structures. There are mainly two types of laser materials using Al₂O₃ as host materials, that is ruby (Cr³⁺:Al₂O₃) and sapphire (Ti:Al₂O₃). Ruby laser is the earliest and most important solid laser. It is still used widely in high power laser of visible wavelength. Sapphire is the optimal wide-band tunable laser crystal, and has characteristics of wide-gain, high-saturation flux and high laser damage threshold, so it is widely used as tunable laser materials. But single crystal has some restrictions, such as high-cost, low doping concentration and small size etc. Comparing with single crystal, transparent ceramics become a hot research field because of its characteristics of short producing cycle, low cost, high productivity, and transparent ceramics also possesses uniform optical property and high doping rate.In this paper, preparation method and properties of Cr³⁺:Al₂O₃ and Ti:Al₂O₃ nano-powders were researched, the optimum parameters and the influences of fabrication parameters and doping rate of Cr³⁺ on Cr³⁺:Al₂O₃ ceramic properties were studied. The properties of precursor, nano-powders and transparent ceramics were characterized by TG/DTA, IR, XRD, TEM, SEM, EDS methods and fluorescence spectrum test.1. Preparation of Cr³⁺:Al₂O₃ nano-powders of ceramicsCr³⁺:Al₂O₃nano-powders was firstly prepared by co-precipitation method using NH₄HCO₃ as the precipitant. During the process, NH₄MO(OH)HCO₃ (M= Al³⁺,Cr³⁺) precursor was obtained , the precursor was transformed into pureα- Al₂O₃ after calcined at 1200℃for 1h. The influences of dispersant, beginning concentration of reactant, pH value, doping concentration of Cr³⁺ on the morphology and luminescence properties of powders were studied. The adding of (NH₄)₂SO₄ improved the dispersion property of the powders, the particles distributed uniformly with diameter of 58nm. The excitation peaks of the samples located at 429nm and 588nm respectively, corresponding to the transition of ⁴A₂→⁴T₂ and ⁴A₂→⁴T₁, the emission peaks are at 693nm, 668nm and 693nm respectively, corresponding to the R spectrum line of ²E_g→⁴A_2g transition of ⁴A₂ level. Emission peak at 668nm is the superimposition of blue-shifted R line and T(T₁,T₂,T₃) spectrum line of ²T_1g→⁴A_2g transition. The optimum doping concentration of Cr³⁺ is 1.0wt%.Cr³⁺:Al₂O₃ nano-powder was firstly synthesized by low temperature combustion method. Properties of powders by different reductive agents (urea and citric acid) were studied. Pureα-Al₂O₃ prepared by using urea as the reductive agent has severe aggregation. When using citric acid as the reductive agent, precursors can burn at 500℃and can transform into pureα- Al₂O₃ when calcined at 1000℃for 1h. The adding of polyethylene glycol as the disperser can alleviate the aggregation of the sample. The emission peak of nano-powders using former two kind of fuel are at 429nm and 588nm respectively, emission peaks are at 693nm and 668nm respectively, the optimum doping concentration of Cr³⁺ is 1.0wt%.2. Preparation of Ti:Al₂O₃ nano-powders of ceramicsTi:Al₂O₃ nano-powders was synthesized by co-precipitation and sol-gel low combustion methods. For co-precipitation process, when calcined at 1200℃for lh, the particle size is less than 50nm, and morphology is spherical, and there is no intermediate phase when Ti doping is 2 wt%. For sol-gel low combustion process, pureα-Al₂O₃ phase can be obtained when calcined at 1000℃for 1h.3. Fabrication of Cr³⁺:Al₂O₃ transparent ceramicsIt can be concluded that the property of samples made by co-precipitation method is better than sol-gel low temperature combustion method. The effect of addictives to the properties of ceramics was studied, the optimum conditions of the amount of addictives was analyzed. The concentration of MgO is 0.1wt%, La₂O₃ is 0.15wt%, Y₂O₃ is 0.05wt%. There are great influences of sintering temperature and time on the properties of the sample. The optimum sintering parameters is 1700℃, 10¹5h. The transmittance of Cr³⁺:Al₂O₃ ceramics samples sintered at 1700℃for 10h under hydrogen atmosphere is about 40%, the sample has good luminescence property at 688nm and 693nm when the doping concentration of Cr³⁺ is 1.25wt%, and this result provides the possibility of laser oscillation and high rate of doping. The functions of Cr₂O₃ in Cr³⁺:Al₂O₃ ceramic are to provide luminescent center and improve sintering of ceramics.