RU2807809C1 - Red phosphor for composite white light leds based on fluoride borate polycrystals and method for its preparation - Google Patents

Abstract

FIELD: chemical industry; lighting engineering.

SUBSTANCE: used in the manufacture of composite white light LEDs, consisting of a chip emitting in the near ultraviolet and three phosphors: red, green and blue. To obtain a phosphor based on polycrystals of doped Eu³⁺ fluoride borate with the general formula LiBa₁₂(BO₃)₇F₄, a mixture of BaCO₃, BaF₂, Li₂CO₃, HBO₂, taken in stoichiometric proportions, and Eu₂O₃ is added in such an amount as to ensure a Eu content of 0.5-2.0 wt.% in the composition of the final product. Then solid-phase synthesis is carried out by three-stage annealing of the resulting mixture in air in the temperature range of 465-825°C, while the duration of the first stage, carried out in the temperature range of 465-500°C, at least 0.5 hours, and two subsequent ones, carried out at 700-750°C and 815-825°C, respectively, for at least 48 hours. The resulting phosphor is characterized by the above chemical formula and isomorphic replacement of barium ions with trivalent europium ions.

EFFECT: when excited in the near ultraviolet region at a wavelength of 395 nm, a red glow with a quantum yield of 60-63% and photoluminescence with CIE chromaticity coordinates and correlated color temperature (0.630; 0.374), 1947 K - (0.647; 0.374), 2072 K are provided.

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Description

The invention relates to materials for use as a red phosphor of composite LEDs, consisting of a chip emitting in the near ultraviolet and three phosphors, red, green and blue (NUV chip + RGB phosphors), based on polycrystals of fluoride borates.

Currently, there are several approaches to creating white LEDs. There are three-component LEDs, consisting of three semiconductor emitters (chips) of red, green and blue colors combined in one volume [Taki, T., Strassburg, M. “Review - Visible Leds: More Than Efficient Light” - ECS J. Solid State Sci . Technol. 2020, 9, 015017]. The main disadvantages of such LEDs include uneven angular color characteristics and a line spectrum, determined by the spectrum of the semiconductor emitters used. These LEDs are not used for indoor lighting; their main area of application is decorative lighting.

Another approach used to produce white light, suitable for use including indoor lighting, is the combination of a semiconductor chip emitting in the near ultraviolet region and one or more phosphors that convert part of the chip’s radiation into visible radiation as a result of photoluminescence (PL). . Currently, phosphors based on yttrium-aluminum garnet doped with cerium ions Y ₃ Al ₅ O ₁₂ : Ce ³⁺ are widely used. The combination of chip emission and phosphor produces white light [Ye S. et al. “Phosphors in Phosphor-Converted White Light-Emitting Diodes: Recent Advances in Materials, Techniques and Properties” - Mater. Sci. Eng., R 2010, 71, P. 1-34; Smet, P. F. et al "Selecting Conversion Phosphors for White Light-emitting Diodes" - J. Electrochem. Soc., 2011, 158, R37-R54].

The disadvantage of LEDs based on cerium-doped yttrium aluminum garnet crystals is the low color rendering index and high color temperature correlation due to the absence of the red component [Jang H.S. et al. "Improvement of Electroluminescent Property of Blue LED Coated with Highly Luminescent Yellow-Emitting Phosphors." - Appl. Phys. B: Lasers Opt, 2009, V. 95, P. 715-720; Huang X. Y. “Red phosphor converts white LEDs” - Nature Photonics, 8, 2014, P. 748-749].

The low color rendering index and high correlation of color temperature can be overcome by combining a chip emitting in the near ultraviolet with red, green and blue phosphors, the combination of which makes it possible to obtain white color. For this purpose, research is being conducted aimed at obtaining effective red ones [Ding X., Wang Y. "Commendable Eu²⁺-Doped Oxide-Matrix-Based LiBa₂₊(BO₃)₇F₄ Red Broad Emission Phosphor Excited by NUV Light: Electronic and Crystal Structures, Luminescence Properties" - ACS Appl. Mater. Interfaces, 2017, V. 9 (28), P. 23983-23994], green [Li L. et al. "Structural and optical properties of (Sr, Ba)₂SiO₄: EU²⁺ thin films grown by magnetron sputtering". - J. Lumin. 2014, V.152, P. 234-237] and blue phosphors [Singh V. et al. "EPR and optical properties of green emitting Mn-doped BaMgAl₁₀O₁₇ nano-phosphors prepared by a combustion reaction". - J. Mater. Sci.: Mater. Electron. 2016, V. 27, P. 3697-3703].

In [Ding X., Wang Y. "Commendable Eu²⁺-Doped Oxide-Matrix-Based LiBa₂₊(BO₃)₇F₄ Red Broad Emission Phosphor Excited by NUV Light: Electronic and Crystal Structures, Luminescence Properties" - ACS Appl. Mater. Interfaces, 2017, V. 9 (28), P. 23983-23994] describes a red phosphor - polycrystal LiBa₁₂(BO₃)₇F₄, doped with divalent europium ions Eu²⁺ (0.1% ≤ C(Eu²⁺) ≤ 4%). Eu was used in the solid-phase synthesis of this material.₂O₃, which was reduced to Eu²⁺ heating at a temperature of 940°C in a mixture with graphite in a tube furnace in an atmosphere of nitrogen and hydrogen, taken in the ratio N₂:H₂ = 6:45. In the photoluminescence spectrum of the obtained LiBa samples₁₂(BO₃)₇F₄:Eu²⁺ upon excitation at a wavelength of 405 nm, a broad peak with a maximum near 650 nm, characteristic of 4f, was observed⁶5d¹ → 4f⁷ transitions of Eu ions²⁺. Maximum photoluminescence intensity with CIE (Commision Internationale de. I'Eclairage) chromaticity coordinates (0.6350; 0.3586) and maximum PL quantum yield = 26.6% were achieved at Eu concentration²⁺- 1%.

Recently, red phosphors based on matrices activated by Eu ³⁺ have been widely developed due to intense narrow-band radiation in the region of 610-630 nm, LaSc ₃ (BO ₃ ) ₄ :Eu ³⁺ [Wang et al. "A red phosphor LaSc ₃ (BO ₃ ) ₄ :Eu ³⁺ with zero-thermal quenching and high quantum efficiency for LEDs." - Chem. Eng. J., 2021, V.404, 125912], Na ₂ Ln ₂ Ti ₃ O ₁₀ :Eu ³⁺ , Ln=Gd, Y [Zhang et al. “Red emitting phosphors of Eu ³⁺ doped Na ₂ Ln ₂ Ti ₃ O ₁₀ (Ln=Gd, Y) for white light emitting diodes” - J. Alloys Compd., 2015, 635, P. 66-72], Ca ₂ GdTaO ₆ :Eu ³⁺ [Wang et al., “Novel highly efficient and thermally stable Ca ₂ GdTaO ₆ :Eu ³⁺ red-emitting phosphors with high color purity for UV/blue-excited WLEDs,” J. Alloys Compd. 2019, 804, P. 93-99].

The closest analogue is the invention described in patent RU 2784929 (class C 30 V 29/12, 12/01/2022) “Crystalline material for phosphors for white light LEDs.” The crystalline material is a phosphor based on fluoridoborate with an “antzeolite” structure of the general formula Ba12(BO3)6[BO3][LiF4], in the framework [Ba12(BO3)6]6+ of which barium ions are isomorphically replaced by ions of rare earth elements: europium, terbium and cerium. The specified phosphor was obtained by reducing the temperature from a high-temperature melt solution of the initial components of the composition, mol.%: BaO:BaF ₂ :B ₂ O ₃ :Li ₂ O=24:22:34:20 containing oxides of rare earth elements, wt. %: 0.13-0.30 Eu ₂ O ₃ ; 0.10-0.30 Tb ₂ O ₃ ; 0.13-0.50 Ce ₂ O ₃ . When the resulting single-phase phosphor is excited by radiation in the ultraviolet region of 300-370 nm at a temperature of 77-300 K, multiband luminescence close to white light is provided. By changing the content of rare earth elements, you can vary the shades of emitted white light.

The objective of the present invention was to obtain a phosphor for composite white light LEDs based on polycrystals of LiBa ₁₂ (BO ₃ ) ₇ F ₄ fluoride borate (hereinafter referred to as LBBF), doped with Eu ³⁺ , which, when excited by near-ultraviolet radiation, provides a red glow with a high quantum yield . The use of these polycrystals as red phosphors has not been identified from the prior art.

In fig. Figure 1 shows the photoluminescence spectra of LBBF samples with Eu ³ contents of 0.5, 2 and 4 wt.% ⁺ when excited at a wavelength of 395 nm at 300 K. Measurements were carried out using a Fluorolog 3 spectrofluorimeter (Horiba Jobin Yvon). The above PL spectra consist of weak emission corresponding to the transition ⁵ D ₀ → ⁷ F ₀ (579 nm), and relatively strong broad bands corresponding to the transitions ⁵ D ₀ → ⁷ F ₁ , ⁵ D ₀ → ⁷ F ₂ , ⁵ D ₀ → ⁷ F ₃ (Fig. 1). Among the transitions listed above, ⁵ D ₀ → ⁷ F ₀ in the region of 595 nm and ⁵ D ₀ → ⁷ F ₂ in the region of 617 nm are the most sensitive to positional symmetry [Santosh, KG “Eu ³⁺ local site analysis and emission characteristics of novel Nd ₂ Zr ₂ O ₇ :Eu phosphor: insight into the effect of europium concentration on its photoluminescence properties" - RSC Adv., 2016, 6, 53614]. The observed broadening of the peaks for these transitions is due to different coordination of Eu ³⁺ ions in the structure. There are 4 crystallographically nonequivalent barium positions in the structure: Ba1, Ba2, Ba3 and Ba4, coordinated by 9, 10, 11 and 8 ligand atoms (oxygen and fluorine), respectively. Relative intensity of the most intense transition ⁵ D ₀ → ⁷ F ₂ of Eu ³⁺ ions for LiBa ₁₂ (BO ₃ ) ₇ F ₄ :Eu ³⁺ samples with Eu ³⁺ content of 0.1, 0.5, 2 and 4 wt.% is 3.5·10 ^-4 , 0.09, 0.29 and 1, respectively, differing in intensity by almost four orders of magnitude.

Depending on the concentration of Eu ³⁺ in LBBF samples, the chromaticity coordinates measured at 300 K change (Table 1), but do not go beyond the red region.

Table 1. Eu ³⁺ content in LBBF, wt.% 0.1 0.5 2.0 4.0 Chromaticity coordinates (0.533; 0.354) (0.630; 0.374) (0.647; 0.374) (0.650; 0.345)

In fig. Figure 2 shows X-ray diffraction patterns of an undoped LiBa ₁₂ (BO ₃ ) ₇ F ₄ sample and LBBF samples with Eu ³⁺ contents of 0.1, 0.5, 2 and 4 wt.%. The measurements were carried out on a DRON 8 diffractometer, CuKα radiation (1.5418 Å), Mythen2 R1 detector. It can be seen that the homogeneity region of the LiBa ₁₂ (BO ₃ ) ₇ F ₄ :Eu ³⁺ solid solution corresponds to no less than 2 wt.% Eu ³⁺ . The X-ray diffraction pattern of the LiBa ₁₂ (BO ₃ ) ₇ F ₄ :Eu ³⁺ sample with a Eu ³⁺ content of 4 wt.% contains reflections of additional phases.

The quantum yield was measured using a G8 spectral coated integrating sphere (GMP SA, Switzerland) connected to a Fluorolog 3 spectrofluorimeter. The PL quantum yield values for LBBF samples containing 0.5, 2 and 4 wt% Eu ³⁺ are shown in Figs. 3 and are 63, 60 and 4%, respectively. For a sample with a Eu ³⁺ content of 0.1 wt.%, the quantum yield could not be detected.

Thus, the isomorphic replacement of barium ions in the crystal structure of fluoride borate LiBa ₁₂ (BO ₃ ) ₇ F ₄ with trivalent europium ions in concentrations of 0.1-2.0 wt.% provides, upon excitation at a wavelength of 395 nm, a red glow with a quantum yield of 60 -63% and photoluminescence with CIE chromaticity coordinates and correlated color temperature from (0.630; 0.374), 1947 K to (0.647; 0.374), 2072 K.

Fluoroborates LiBa ₁₂ (BO ₃ ) ₇ F ₄ :Eu ³⁺ were obtained by solid-phase synthesis from a mixture of compounds BaCO ₃ , BaF ₂ , Li ₂ CO ₃ , HBO ₂ taken in stoichiometric proportions, and Eu ₂ O ₃ added in such quantities to ensure a Eu content of 0.5-2.0 wt.% in the composition of the final product. Solid-phase synthesis was carried out by three-stage heating of the mixture in an air atmosphere in the temperature range 465-825°C and holding at the final temperature for at least 48 hours.

In contrast to the method for producing a phosphor containing Eu ions²⁺, according to the method proposed in [Ding X., Wang Y. "Commendable Eu²⁺-Doped Oxide-Matrix-Based LiBa₂₊(BO₃)₇F₄ Red Broad Emission Phosphor Excited by NUV Light: Electronic and Crystal Structures, Luminescence Properties" - ACS Appl. Mater. Interfaces, 2017, V.9(28), P. 23983-23994], doping of LiBa crystals₁₂(BO₃)₇F₄ Eu ions³⁺ does not require a reducing atmosphere when performing solid-phase synthesis. The proposed technical solution involves the synthesis of red phosphor based on LiBa₁₂(BO₃)₇F₄ can be carried out in milder conditions and in an air atmosphere. In addition, the LiBa phosphor₁₂(BO₃)₇F₄:Eu³⁺ with a europium content of 0.5-2.0 wt.% is characterized by a higher quantum yield, more than twice the quantum yield of the most effective sample doped with divalent europium ions, described in [Ding X., Wang Y. "Commendable Eu²⁺-Doped Oxide-Matrix-Based LiBa₂₊(BO₃)₇F₄ Red Broad Emission Phosphor Excited by NUV Light: Electronic and Crystal Structures, Luminescence Properties" - ACS Appl. Mater. Interfaces, 2017, V.9(28), P. 23983-23994].

The following are examples of the preparation of fluoride borate polycrystals LiBa ₁₂ (BO ₃ ) ₇ F ₄ :Eu ³⁺ with different europium contents.

Example 1.

LiBa polycrystals₁₂(BO₃)₇F₄:Eu³⁺with europium content 0.5 wt.% prepared from a mixture of BaCO reagents₃, BaF₂, Li₂CO₃, HBO₂, taken in stoichiometric proportions, and Eu₂O₃. The initial sample contained: BaCO₃- 2.30 g, BaF₂- 0.41 g, HBO₂- 0.45 g, Li₂CO₃- 0.04 g, Eu₂O₃- 0.028 g. Solid-phase synthesis was carried out in a platinum crucible in a standard electric resistance furnace. The sample was annealed in air in three stages: 465 °C for 0.5 hours, 700 °C for 48 hours, 815 °C for 50 hours.

Isomorphic occurrence of europium in the LBBF structure at a concentration of 0.5 wt.% is confirmed by the single-phase nature of the resulting sample (Fig. 2). The transitions observed in the photoluminescence spectrum (Fig. 1) are typical for matrices doped with europium ions in the trivalent state [Lemański K. et al “Luminescent properties of Eu³⁺ ions in CaB₆O₁₀ polycrystals" - J. Lumin. 2015, 159, 219-222]. The broadening of the peaks is associated with the entry of europium into barium positions with different coordination. The CIE coordinates correspond to (0.630; 0.374). The PL quantum yield was 63%.

Example 2.

Polycrystals of LiBa ₁₂ (BO ₃ ) ₇ F ₄ :Eu ³⁺ with a europium content of 2.0 wt.% were obtained from a mixture of reagents BaCO ₃ , BaF ₂ , Li ₂ CO ₃ , HBO ₂ taken in stoichiometric proportions, and Eu ₂ O ₃ . The initial sample contained: BaCO ₃ - 2.30 g, BaF ₂ - 0.41 g, HBO ₂ - 0.45 g, Li ₂ CO ₃ - 0.04 g, Eu ₂ O ₃ - 0.11 g. Solid-phase synthesis carried out in a platinum crucible in a standard electric resistance furnace. The sample was annealed in air in three stages: 500 °C for 0.5 h, 750 °C for 48 h, 825 °C for 48 h.

Isomorphic occurrence of europium in the LBBF structure at a concentration of 2, they were annealed in air in three stages: this is confirmed by the single-phase nature of the resulting sample (Fig. 2). The transitions observed in the photoluminescence spectrum (Figure 1) are typical for matrices doped with europium ions in the trivalent state [Lemański, K. et al. Luminescent properties of Eu³⁺ ions in CaB₆O₁₀ polycrystals. J. Lumin. 2015, 159, 219-222]. The broadening of the peaks is associated with the inclusion of europium in barium positions with different coordination. The CIE coordinates correspond to (0.647; 0.374). The PL quantum yield was 60%.

Claims (2)

1. A phosphor based on fluoride borate polycrystals with the general formula LiBa ₁₂ (BO ₃ ) ₇ F ₄ for composite white light LEDs, characterized by isomorphic replacement of barium ions with trivalent europium ions, characterized in that it contains trivalent europium in a concentration of 0.5-2, 0 wt.%, providing, upon excitation at a wavelength of 395 nm, red glow with a quantum yield of 60-63% and photoluminescence with CIE chromaticity coordinates and correlated color temperature (0.630; 0.374), 1947 K - (0.647; 0.374), 2072 K. 2. A method for producing a phosphor based on polycrystals of doped Eu ³⁺ fluoride borate with the general formula LiBa ₁₂ (BO ₃ ) ₇ F ₄ , characterized in paragraph 1, characterized in that solid-phase synthesis is carried out, including three-stage annealing in air of a mixture of BaCO ₃ , BaF ₂ , Li ₂ CO ₃ , HBO ₂ , taken in stoichiometric proportions, and Eu ₂ O ₃ added in such an amount to ensure a Eu content of 0.5-2.0 wt.% in the composition of the final product, in the temperature range 465- 825°C, while the duration of the first stage, carried out in the temperature range 465-500°C, is not less than 0.5 hours, and the next two, carried out at 700-750°C and 815-825°C, respectively, is not less than 48 h.