A strong yellow light Dy 3+ :NaGdGeO4 phosphor and its preparation method and application

Abstract

The invention discloses a strong yellow light Dy 3+ :NaGdGeO4 phosphor and its preparation method and application. The present invention is to mix Na2CO3, Gd2O3, GeO2 and Dy2O3 and grind them thoroughly, sinter the ground powder at 1100±2℃ for 5-6 hours, grind the sintered product into powder, and obtain Dy 3+ : NaGdGeO4 phosphor, which has a relatively stable phase structure and luminescence performance, and can be used in products such as yellow light LEDs.

Description

Technical Field

[0001] The invention belongs to the technical field of new fluorescent material development, and particularly relates to a strong yellow light Dy 3+ :NaGdGeO4 phosphor and its preparation method and application. Background Art

[0002] Rare earth (RE) element doped phosphors can be used in light emitting diode (LED) devices. They are gaining more and more attention due to their low power consumption, excellent thermal stability, long life, and high efficiency. Yellow phosphors are widely used in white light illumination and ophthalmic treatment due to their high brightness and good safety. Generally, UV LED chips coated with yellow phosphors can emit yellow light. 3+ ) Because it is in the 576nm yellow light region 4 F 9 / 2 → 6 H 13 / 2 Typical energy level transition properties of some doped Dy 3+ The phosphor of the ion can emit yellow light under the excitation of the ultraviolet chip. 3+ Ions also show another typical 4 F 9 / 2 → 6 H 15 / 2 The radiation transition of the yellow phosphor is conducive to the direct emission of white light by mixing the appropriate yellow light. At present, the yellow YAG material is widely used, which has the problems of easy phase change and high color temperature. Therefore, the research of new LED yellow phosphor has become an urgent need in the current market. Summary of the invention

[0003] In order to overcome the shortcomings and deficiencies of the prior art, the present invention aims to provide a strong yellow light Dy 3+ :NaGdGeO4 (ie Dy:NGGO) phosphor and its preparation method and application.

[0004] The present invention is achieved by: a Dy 3+ : A method for preparing NaGdGeO4 phosphor, the method comprising the following steps:

[0005] (1) Mix and grind Na2CO3, Gd2O3, GeO2 and Dy2O3 thoroughly, and sinter the ground powder at 1100±2℃ for 5-6 hours;

[0006] (2) Grind the sintered product into powder to obtain Dy 3+ :NaGdGeO4 phosphor.

[0007] Preferably, the Dy 3+ :Dy in NaGdGeO4 phosphor3+ The doping amount is 0.5 at.% to 5 at.%.

[0008] Preferably, the Dy 3+ in the Dy:NaGdGeO4 phosphor is 3+ with a doping amount of 2 at.%.

[0009] Preferably, in step (1), the purities of Na2CO3, Gd2O3, GeO2 and Dy2O3 are all above 99.99%.

[0010] The present invention further discloses the Dy 3+ :NaGdGeO4 phosphor prepared by the above method.

[0011] The present invention further discloses the application of the above Dy 3+ :NaGdGeO4 phosphor in yellow light LEDs.

[0012] The present invention overcomes the deficiencies of the prior art and provides a strong yellow light Dy 3+ :NaGdGeO4 (i.e., Dy:NGGO) phosphor and its preparation method and application. The NGGO matrix can reduce its phonon energy emission due to its olivine structure. Research shows that Na + and Gd 3+ ions show a long-range ordered distribution at octahedral positions in the structure of NGGO. When Dy 3+ ions replace Gd 3+ ions as the luminescent positions mainly located at non-centrosymmetric centers, the typical transition of Dy 3+ ions 4 F 9 / 2 → 6 H 13 / 2 dominates, and the phosphor emits strong yellow light, and white light can be achieved by mixing a certain proportion of blue light and yellow light. In the present invention, the doped NGGO phosphor with Dy 3+ is used to obtain a yellow phosphor material.

[0013] Compared with the disadvantages and deficiencies of the prior art, the present invention has the following beneficial effects: The present invention synthesizes the Dy 3+ :NaGdGeO4 phosphor by the high-temperature solid-phase method. At different temperatures, the phosphor has a relatively stable phase structure and luminescent properties and can be applied to products such as yellow light LEDs. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 are the analysis results of the electronic structure and properties of NGGO; wherein, Figure 1 a is the calculated energy band structure of NGGO, Figure 1b is the electronic DOS of NGGO;

[0015] Figure 2 is the diffraction pattern of Dy:NGGO phosphor (i.e., Dy 3+ :NaGdGeO4 phosphor, the same below); among them, Figure 2 a is the diffraction pattern of Dy:NGGO phosphor at room temperature; Figure 2 b is the diffraction pattern of 2at% Dy:NGGO phosphor in the temperature range of 303K to 453K, Figure 2 c is the enlarged view of the diffraction peak of Dy:NGGO phosphor in the range of 31.5 - 32.0 °C;

[0016] Figure 3 is the scanning electron microscope image of 2at% Dy:NGGO and the elemental surface distribution image of 2at% Dy:NGGO;

[0017] Figure 4 is the excitation spectrum of Dy:NGGO phosphor;

[0018] Figure 5 is the emission spectrum of Dy:NGGO phosphor; among them, Figure 5 a is the emission spectrum of Dy:NGGO phosphor excited at 274nm, Figure 5 b is the emission spectrum of Dy:NGGO phosphor excited at 347nm, Figure 5 c is the luminescence intensity of Dy:NGGO phosphor excited at 347nm;

[0019] Figure 6 is the response spectrum of Dy:NGGO phosphor; among them, Figure 6 a is the temperature-dependent PL spectrum of Dy:NGGO phosphor; Figure 6 b is the change of normalized light intensity with temperature;

[0020] Figure 7 is the PL spectrum of Dy:NGGO phosphor and blank sample at 2at.% Dy:NGGO;

[0021] Figure 8 is the fluorescence decay curve of Dy:NGGO phosphor;

[0022] Figure 9 is the chromaticity diagram of Dy:NGGO phosphor. Detailed implementation manners

[0023] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0024] 1. Experimental process

[0025] In the embodiments of the present invention, preferably, 99.99% of Na2CO3, 99.999% of Gd2O3, 99.999% of GeO2, and 99.99% of Dy2O3 are used as raw materials. Dy-doped Dy:NGGO samples with different mass ratios of Na2CO3, Gd2O3, GeO2, and Dy2O3 are prepared by a simple high-temperature solid-phase method, with the doping levels of Dy being 0.5 at.%, 1 at.%, 2 at.%, 3 at.%, 4 at.%, and 5 at.%, respectively. Among them, 3+ During the preparation of the 0.5 at.% Dy:NGGO sample, the masses of Na2CO3, Gd2O3, GeO2, and Dy2O3 are 0.7818 g, 2.6606 g, 1.5438 g, and 0.0138 g, respectively;

[0026] During the preparation of the 1 at.% Dy:NGGO sample, the masses of Na2CO3, Gd2O3, GeO2, and Dy2O3 are 0.7818 g, 2.6471 g, 1.5436 g, and 0.0275 g, respectively;

[0027] During the preparation of the 2 at.% Dy:NGGO sample, the masses of Na2CO3, Gd2O3, GeO2, and Dy2O3 are 0.7817 g, 2.6199 g, 1.5434 g, and 0.055 g, respectively;

[0028] During the preparation of the 3 at.% Dy:NGGO sample, the masses of Na2CO3, Gd2O3, GeO2, and Dy2O3 are 0.7815 g, 2.5928 g, 1.5432 g, and 0.0825 g, respectively;

[0029] During the preparation of the 4 at.% Dy:NGGO sample, the masses of Na2CO3, Gd2O3, GeO2, and Dy2O3 are 0.7814 g, 2.5657 g, 1.5429 g, and 0.1099 g, respectively;

[0030] During the preparation of the 5 at.% Dy:NGGO sample, the masses of Na2CO3, Gd2O3, GeO2, and Dy2O3 are 0.7813 g, 2.5385 g, 1.5427 g, and 0.1375 g, respectively.

[0031] The raw materials used are accurately weighed according to the stoichiometric coefficients, fully mixed in an agate mortar, and then transferred to a corundum crucible and sintered at 1100 ± 2 °C for 5 hours. After sintering, the obtained Dy:NGGO samples are ground into powder form for performance testing.

[0032] ​

[0033] 2. Performance testing and analysis

[0034] To understand the electronic structure and properties of existing NGGO, theoretical calculations were carried out using density functional theory. The band structure calculated along the high-symmetry points of the first Brillouin zone is shown as Figure 1 shown in a. It can be seen that NGGO has a direct bandgap of 3.74 eV. It has high luminous efficiency, short emission wavelength, and high color purity, which is beneficial to the fabrication of light-emitting devices. Using Figure 1 the total density and partial density of the density of states diagram shown in b to analyze the composition of each band. The results show that the conduction band mainly originates from Gd-s, Gd-d, O-p, and Ge-s states, while the valence band from -5.0 to the Fermi level (0 eV) is mainly composed of O-p and Ge-p states. Therefore, the optical transition is mainly attributed to the GeO4 group and Gd 3+ .

[0035] To study the effect of Dy 3+ ion doping on NGGO, XRD tests were performed on phosphors with different doping concentrations in the range of 10° to 80°. The results are shown as Figure 2 (a). The results show that the diffraction peaks of the Dy-doped samples are all in line with the standard card (PDF#88-1176), indicating that the doped Dy 3+ ions were synthesized into the matrix without phase transformation. Since the ionic radius of Dy 3+ ions is similar to that of Gd 3+ ions, the Dy 3+ ions are most likely to replace Gd 3+ ions. Generally, heat is generated during the working process of the phosphor, which may lead to phase transformation. Therefore, the thermal stability of the structure is also an important indicator for judging the performance of the phosphor. To study the thermal stability of the samples, variable-temperature XRD tests were carried out. The results are shown as Figure 2 (b). It can be seen from the enlarged view of Figure 2 (b) that in the temperature range from 303K to 453K, the highest peak gradually moves to the left. This is because the material is a material with a positive expansion coefficient and it expands as the temperature increases. This phenomenon can be explained by Bragg's formula.

[0036] Bragg's formula is as follows:

[0037] 2dsinθ = nλ (1)

[0038] Among them, d and θ are variables representing the crystal plane spacing and diffraction angle, respectively, which vary with the change of materials. n and λ are constants representing fixed values. According to Bragg's equation, an increase in the d value at a constant wavelength necessarily results in a smaller sinθ. In the diffraction range, when θ becomes smaller, the peak position shifts to the left. Within the temperature range, the sample maintains a stable phase structure.

[0039] To observe the particle morphology and elemental distribution on the sample surface, 2 at.% Dy:NGGO was selected for SEM testing. The scanning electron micrograph is as Figure 3 shown. Figure 3 (a~e) shows the NGGO detected by measuring the particle sizes with different scales. To observe the elemental distribution in the sample, the elemental mapping was tested. Figure 3 (f~j) shows the elemental distribution under a 5-μm scale. It can be seen that the elements O, Gd, Na, Dy, and Ge are non-aggregated and uniformly distributed in the sample particles.

[0040] To study the luminescence properties of the phosphor, the excitation spectrum of the sample was tested, as Figure 4 shown. For Dy:NGGO with different doping concentrations, at a monitoring wavelength of 576 nm, the wavelength range for measuring the phosphor excitation spectrum data is from 240 to 400 nm. As shown in the figure, the corresponding transitions of Dy 3+ ions at 322, 347, 361, and 383 nm are respectively 6 H 15 / 2 → 6 P 3 / 2 , 6 H 15 / 2 → 6 P 7 / 2 , 6 H 15 / 2 → 6 P 5 / 2 , 6 H 15 / 2 → 4 F 7 / 2 . Among them, the strongest peak is located at 347 nm. The results show that the Dy:NGGO phosphor at a wavelength of 347 nm can be effectively excited. At 274 nm, as the Dy 3+ concentration increases, the peak intensity gradually decreases. This is because the peak change at 274 nm belongs to the absorption of the matrix itself, and the peak change at 347 nm belongs to the absorption of Dy 3+ ions.

[0041] Figure 5 (a) gives the emission spectrum of the Dy:NGGO phosphor excited at 274 nm. It can be seen that the peak intensity at 576 nm gradually decreases as the Dy 3+ ion concentration increases.Figure 5 (b) shows the emission spectrum of the Dy:NGGO phosphor excited at 347 nm. As can be seen from this figure, the peak intensity at 576 nm is the strongest, corresponding to the 3+ ions of 4 F 9 / 2 → 6 H 13 / 2 transition. The spectrum also shows that the yellow emission intensity of the Dy 3+ ions is much greater than the blue emission intensity, mainly because the Dy 3+ ions are in a lower non-inversion symmetric center position. Figure 5 (c) gives the relationship between the fluorescence intensity at 576 nm and the doping concentration of the Dy 3+ ions. As the concentration of Dy 3+ increases, the luminescence intensity first increases and then decreases, and the highest intensity belongs to 2 at.%. This is because as the number of Dy 3+ ions increases, more and more Dy 3+ occupy the luminescence center, thus enhancing the luminescence intensity. However, the higher the doping concentration of the Dy 3+ ions, the shorter the distance between the Dy 3+ ions, resulting in the transfer of non-radiative energy through the cross-relaxation process and concentration quenching occurring.

[0042] The critical spacing calculation formula is as follows:

[0043]

[0044] where \(R_C\) and \(V\) represent the critical distance and the unit cell volume respectively, \(x\) c represents the concentration of Dy 3+ ions, and \(N\) represents the number of NGGO unit cells where the ions are located.

[0045] The thermal stability of luminescence is an important index of the phosphor performance. In order to better study the luminescence performance of the phosphor in a high-temperature environment. The present invention studied the emission spectrum of the 2 at.% Dy:NGGO phosphor under 347 nm excitation light in the temperature range from 293 K to 416 K, and the results are as Figure 6 (a) shown. Figure 6 (b) shows the comparison of the normalized luminescence peak intensity with the initial intensity at a temperature of 293 K. Like most fluorescent materials, as the temperature increases, the luminescence intensity gradually decreases due to thermal quenching. However, fortunately, when the sample is heated to 416 K, the luminescence peak intensity still remains 77%. The results show that the Dy:NGGO phosphor has good thermal stability within the LED operating temperature range. Therefore, this phosphor is expected to be applied to LEDs with stable luminescence performance.

[0046] The internal quantum efficiency (IQE) is an important indicator of the performance parameters of phosphors, and its value determines the energy conversion performance of phosphors. In this invention, the IQE of 2 at.% Dy:NGGO was studied. As Figure 7 shown, the peak at 347 nm represents the excitation spectra of the sample and the blank sample, Figure 6 and the peak in the enlarged area represents the absorption peak of the sample.

[0047] The calculation formula for IQE is as follows:

[0048]

[0049] where ηint represents the internal absorption efficiency of the Dy:NGGO phosphor, LS represents the emission spectrum of the Dy:NGGO phosphor, and ER and ES represent the excitation spectra of the blank sample and the Dy:NGGO phosphor, respectively. Calculated by the above formula, the internal quantum efficiency of 2 at.% Dy:NGGO is 40.86%.

[0050] The fluorescence lifetime is another important parameter of phosphors. Figure 8 shows the fluorescence decay curve of the Dy 3+ ion 4 F 9 / 2 energy level, with an excitation wavelength of 347 nm and a monitoring wavelength of 576 nm. From Figure 8 it can be observed that the slope of the lifetime curve gradually increases with the increase of the reference Dy 3+ ion concentration. This indicates that with the increase of the Dy 3+ ion concentration, the lifetime gradually decreases. The fluorescence lifetime curves are respectively fitted with the following formula:

[0051]

[0052] where I(t) represents the luminescence intensity at time t, τ1 and τ2 represent the fluorescence decay lifetimes, and A1 and A2 represent constants.

[0053] The average expected lifetime is calculated according to the following formula:

[0054]

[0055] The results of the average lifetime fitting of the Dy:NGGO sample are shown in Table 1. When the Dy 3+ concentration increases from 00.5 at.% to 5 at.%, the fluorescence lifetime decreases from 410 μs to 224 μs.

[0056] Table 1 Fluorescence lifetime of Dy:NGGO phosphor

[0057]

[0058] Figure 9Shows the color coordinates of the Dy:NGGO phosphor. It can be seen that all the samples are located in the yellow emission region. Table 2 shows the color coordinate values and the related color temperature (CCT) of the samples. The related color temperature can be calculated by the following formula:

[0059] CCT = -449n 3 +3525n 2 -6823.3n + 5520.33 (6)

[0060] Where xe is 0.3320 and ye is 0.1858, then n = (x - xe) / (y - ye). The color coordinates and CCT of the Dy:NGGO phosphor are shown in Table 2.

[0061] Table 2 Color coordinates and CCT values of the Dy:NGGO phosphor

[0062]

[0063]

[0064] 3. Conclusions

[0065] A series of Dy:NGGO phosphors with different Dy doping amounts were successfully synthesized by a simple high-temperature solid-state method. The electronic structure of NaGdGeO4 was studied by first-principles based on density functional theory. It shows a direct band gap of 3.74 eV. The variable-temperature XRD results indicate that the ion doping of Dy in the NGGO matrix does not change its phase structure at room temperature and high temperature. By observing the emission spectra of the samples, it can be seen that 3+ the F 3+ → 4 F 9 / 2 → 6 H 13 / 2 transition dominates and can emit strong yellow light. By comparing the luminescence intensity and the concentration of the activator ion Dy 3+ , it can be concluded that with the increase of the Dy 3+ ion concentration, the luminescence intensity first increases and then decreases. The optimal doping concentration of the Dy 3+ ion is 2 at.%. Based on the variable-temperature fluorescence spectra of the 2 at.% Dy:NGGO phosphor, it can be found that when the temperature reaches 416 K, the luminescence intensity still has 77% of the original. The IQE of the 2 at.% Dy:NGGO phosphor is calculated to be 40.86%, and the lifetime is 338 μs. In addition, the color coordinates of the Dy:NGGO samples are all located in the yellow region, and the CCT is as high as 3000 K. The above results indicate that the Dy:NGGO phosphor has broad prospects in the application of yellow LEDs and can meet the lighting and decoration needs of different scenarios.

[0066] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing Dy 3+ :NaGdGeO4 phosphor, characterized in that The method comprises the following steps: (1) Mix and grind Na2CO3, Gd2O3, GeO2 and Dy2O3 thoroughly, and sinter the ground powder at 1100±2℃ for 5-6 hours; (2) Grind the sintered product into powder to obtain Dy 3+ :NaGdGeO4 phosphor; the Dy 3+ :NaGdGeO4 phosphor has a doping amount of Dy 3+ ranging from 0.5 at.% to 5 at.%.

2. The method according to claim 1, characterized in that, The Dy 3+ : doping amount of Dy in NaGdGeO4 phosphor 3+ is 2 at.%.

3. The method according to claim 1, wherein In step (1), the purity of Na2CO3, Gd2O3, GeO2 and Dy2O3 is above 99.99%.

4. Dy prepared by the method according to any one of claims 1 to 3 3+ : NaGdGeO4 phosphor 5. The Dy as claimed in claim 4 3+ : Application of NaGdGeO4 phosphor in yellow LED.

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