Surface modification of Strontium Aluminates luminescent powder

Abstract. The inorganic Al2O3 encapsulation on SrAl2O4: Eu2+, Dy3+ luminescent powder for reform in its water resistance was firstly studied. Water-resistance of the coated and uncoated samples was evaluated by measuring the pH of water in which the samples were dispersed. The microstructure and the luminescent properties of the luminescent powder were investigated by SEM, fluorescence(FL) spectra and FT-IR. The experiment results showed that the encapsulation film was compose of Al2O3. The water resistance properties were better than that of the uncoated samples. The SrAl2O4 phosphor encapsulated with Al2O3 still keeps good persistent luminescent properties, and the emission spectrum peak value drops by about 5.2% and the peal value of the excitation spectrum drops by about 7.6%.

Key words: Al2O3; luminescent powder;SrAl2O4: Eu2+, Dy3+; encapsulation

Divalent europium-activated alkaline earth aluminates are known to be their high quantum efficiency in the visible regions. These are essentially interesting, as they do not involve any radioaction isotope [1]. Particularly the Eu2+ and Dy3+co-activated SrAl2O4 phase with tridymite-like structure was reported by murayama et al [2], and the composite exhibits very bright and long-lasting phosphorescence (>50h) with emission wavelength of 530-540nm [3], longer persistent and brighter bluish-green phosphorescence and greater chemical stability than traditional sulphide-based ZnS:Cu,Co phosphors.

The synthesis of these materials for display applications with considerably good initial brightness and long afterglow has been a major goal of many research groups [4-6] all over the world both in industry and academia for well over a decade.Which resulted in an unexpectedly large field of applications, such as luminous paints in highway, airport, buildings and ceramics products[7].In addition, it can also be applied in textile, the dial plates of glow watch, warning signs, escape routine and some instruments,etc.[8].

But SrAl2O4: Eu2+, Dy3+ luminescent powder in its water resistance was not well, it was very easy to take place hydrolyze reaction, crystal fabric of material was exposed to destroy, So SrAl2O4: Eu2+, Dy3+ luminescent powder in the application of the ability in water system was greaterencapsulation of SrAl2O4: Eu2+, Dy3+ luminescent powder was to for reform in its water resistance. Following this reason, the objective of this work to minimize the SrAl2O4: Eu2+, Dy3+ luminescent powder to hydrolyze that allow in obtaining the monoclinic polymorph. For this purpose, a further encapsulation method has been developed, and the results are compared with the uncoated samples.

As one of the crucial points in the characterization of our results is to determine whether the monoclinic polymorph of SrAl2O4: Eu2+, Dy3+ luminescent powder is encapsulated or not , and because the hexagonal and monoclinic diffractograms exhibit overlapped peaks, Digital Acidity Meter and fluorospectrophotometer and scanning electron microscope(SEM) and FTIR are analyzed in detail. The validity for the encapsulation of phase-content evaluation of criteria based on relative intensities of particular diffraction peaks is disscussed, as compared with full profile fitting. In the same line of determining the coexistence of monoclinic diffractograms and hexagonal polymorphs,and complementary to Digital Acidity Meter and fluorospectrophotometer and scanning electron microscope(SEM) and FTIR analysis, excitation and emission and afterglow decay curves spectroscopy have been used, and appearance pattern has been disscussed. This technique is particularly useful for phase identification, as SrAl2O4: Eu2+, Dy3+ luminescent powde limitatively[9]. The r different phases with very similar diffractograms can be present in the sample. As far as we know, the characterization by excitation and emission and afterglow decay curves spectroscopy and appearance pattern of the monoclinic and hexagonal polymorphs has not been reported well yet.

1. Experimental

1.1 Experimental principle

Add SrAl2O4 and A12(SO4)3 to NH4OH with stirring uniformly, and regulate the solution with lye to 5～6 of PH and then the reaction takes place as follows:

A12(SO4)3+ 6NH4OH=2Al(OH)3+3 ( NH4)2SO4

Al(OH)3 exists in the form of colloid, and keeps stability under the action of double electrolysis layer. The double electrolysis layer is destroyed after the alkali is added to the solution. Under the reaction of Van Der Waals Force, Al(OH)3 begins to grow up and separate out. The hydroxyl of the new generated Al(OH)3 is very active. The hydrate of Al2O3 aggraded at the surface of Al(OH)3 and the hydroxyl at the surface of Al(OH)3 dehydrate and generate a bond of Al-O-Al and a link. As the hydrolyzing progressing, Al(OH)3 is adsorbed continuatively at the link, and dehydrates and takes place condensation polymerization, and generates encapsulation layer of Al2O3 gradually, and then SrAl2O4 is micro capsulated.

1. 2 Experiment Course

Add right volume of phosphor to glycol and disperse for 30 minutes with ultrasonic. At the same time of stirring, add the solution just made to the solution of A12(SO4)3 and glycol. Add the ammonia of 5% to the solution of A12(SO4)3 slowly. When the PH of the solution is up to 5, transfer the solution to water case with constant temperature and heat it with the water of (80±1) ℃. After the reaction accomplished, take away the product. After placid placement for 12 hours, clean the product with pure NH4OH for some times till it is pure. Then put the product into the torrefaction box of 100℃ for 2 hours, then transfer it to the muffle of 550℃ for 2 hours. And then the samples are produced out.

1.3 Analysis Method

Check and measure the water-resistance of the encapsulated product with PHS-10B Digital Acidity Meter; Analyze the luminescence property of the encapsulated product with F-4500 Fluorometry; Check and measure the surface shape of phosphor granule with SEM; Test the status of the bond forming with Fourier Infrared Meter.

2.Experiment Results and Discussion

2.1 Water-resistance

Disperse the sample in the ion water, then check and measure the PH of the serosity with Digital Acidity Meter. As the hydrolyzing of SrAl2O4, the reaction takes place as follows:

SrAl2O4+4H2O Sr2++2OH-+2Al (OH) 3

Because of producing of plentiful OH-, the alkalescency of the serosity becomes to show gradually. The hydrolyzing degree and speed of the sample can be seen through checking and measuring the PH of the serosity qualitatively. Then the water-resistance of the sample can be confirmed. Al (OH) 3 deposition generates at the same time. This reaction not only destroys the crystal structure of the material, but affects the acid and alkali balance of the system, and limits the application of luminous materials in the water systems greatly. To improve the water-resistance of the material and enhance the compatibility between the material and the dope film-forming material, Al2(SO4)3 is adopted as the material of encapsulation to change the surface encapsulation property of SrAl2O4:Eu2+,Dy3+ luminous powder. When analyze the water-resistance of the sample with Digital Acidity Meter, disperse 0.3g samples in 3g de-ion water with 6.50 of pH, disperse 0.3g original samples in 3g de-ion water with 6.50 of pH as contrast experiment at the same time. Check and measure the pH of the serosity every 30 minutes. The results as follows:

Fig.1 The relationship curves between pH and time of uncoated(A) and coated(B) phosphors after soaking in water

As shown in Fig.1, the product encapsulated doesn’t hydrolyze in 2 hours. The PH of it is 7. This status remains after one week. But the original samples hydrolyze completely in 90 minutes and the pH of the solution is up to 12, and then the PH remains. So the excellent water-resistance of the encapsulated product is illuminated, and the integrated and compact film layer on the surface of phosphor is formed.

2.2 Fluorescence Property

To illustrate the impact on the luminescence intensity after the luminous powder is encapsulated with Al2O3, the fluorescence property of the luminous powder before and after encapsulating is measured. As shown in the following figure:

Fig.2EM（above） and EX（below） spectra of uncoated(A) and coated(B) phosphors

From Fig.2, the emission spectrum peak value of SrAl2O4 phosphor encapsulated with Al2O3 drops by about 5.2%, from 9700 to 9200. The peal value of the excitation spectrum drops by 7.6%, from 9200 to 8500. Compared with silicon dioxide, the euphotic property of Al2O3 is poorer. Encapsulating product with it has more impact on the fluorescence property. The reason is that the existing of the encapsulation layer, some fluorescence is reflected or absorbed by the film and the fluorescence energy absorbed by the phosphor decreases and then the fluorescence eradiated from it weakens, and accordingly, the peak value of the eradiation spectrum and the excited spectrum drops, and peak area decrease.

2.3 SEM

To illustrate the status of encapsulation of luminous powder with Al2O3clearly, SEM is carried out to the luminous powder before and after encapsulating on the condition of magnifying with 1K and 10um. As the following figure shown：

Fig.3 SEM photos of encapsulated phosphors(A) and non-encapsulated phosphors(B)

Fig.3 is the SEM photos of encapsulated products and original samples. As shown in the pictures, the surface of the original samples is rough. The smallest granule diameter of it is 1.3um. But the granule surface of the encapsulated products is slippery. There is the encapsulation layer on the surface of it obviously. The smallest granule diameter of the original samples is 1.73um. However, there are some granules on the surface. This is probably because it’s too fast to add the solution of luminous powder in the solution of A12(SO4)3 in the process of the experiment, and then some granules of self-forming core is formed on the surface of the luminous powder.

2.4 Infrared Ray Measurement

To illustrate whether the luminous powder is encapsulated by Al2O3 or not, it is measured before and after it is encapsulated with Fourier Infrared Meter. The results as follows:

Fig.4 FTIR photos of encapsulated phosphors(A) and non-encapsulated phosphors(B)

Fig.4 shows the measured results of the encapsulated products and the original samples. As shown in the figure, the peak shapes of them are different obviously from 750 to 1250. It’s the flexing tremor of Al-0-Al in Al2O3 at 735cm-1. However the flexing tremor blue shift to 1050cm-1 after encapsulating. The absorbing peak of Al-0-Al in Al2O3 appeared at 1140cm-1. All this shows that a compact Al2O3 film is formed on the surface of the luminous powder. The film separates the granule from outside, and then the encapsulated luminous powder has excellent property of water-resistance.

3.Conclusion

（1）The encapsulated products have excellent property of water-resistance. A comparatively full and compact film layer is formed on the surface of the luminous powder.

（2） When pH

（3）Because of existing of the encapsulation layer, some fluorescence is reflected or absorbed by the film layer. So luminous energy absorbed by the phosphor drops, and then the fluorescence it eradiated decreases. Accordingly, the peak values of the eradiation spectrum and the excited spectrum drop, so does the peak area.

（4）When adjusting the pH of the solution with ammonia, strong stir is needed to disperse SrAl2O4 and ammonia sufficiently. Otherwise, local alkalescency is too high and vast self-forming core granules generated deposit, not encapsulate the phosphor.

4.Acknowledgements

Funded by Natural Science Foundation of China（No.51174085 ).

5.References

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[3]I-Cherng Chen, Teng-Ming Chen, J. Mater. Res. 3 (16)(2001) 644.

[4]T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Eelectrochem. Soc. 143 (1996) 2670.

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[7]Y. Murayama, N. Takeuchi, Y. Aoki, T. Matsuzawa, Phosphorescent Phosphor,US Patent 5424006 (1995).

[8]H. Yamamoto, T. Matsuzawa, J. Lumin. 72-74 (1997) 287.

[9]Yong-ping Huang. The inorganic Al2O3 encapsulation on SrAl2O4: Eu2+, Dy3+ luminescent powder [C], Patent journal (2005).