White light-emitting diodes (LEDs) have gained wide attention due to their low energy consumption, low heat generation, small size, long life and fast response [1~3]. At present, the industrialized white LED mainly achieves white light emission through "blue GaN die + YAG: Ce yellow luminescent powder", but the main peak of YAG:Ce is located near 550 nm, and the red luminescent component is relatively lacking, which makes the color reproduction of such white LED. Poor sex . On the one hand, the light quality of white LEDs can be improved by adding suitable red compensation powders ; on the other hand, a new type of blue-excited yellow phosphors is sought to obtain white LEDs with better color reproduction, as reported. Materials such as Eu2+, Ce3+, etc. activated silicates, nitrides, and aluminates [6-8]. However, the phase structure of the above luminescent system is complicated, the product is prone to heterogeneous phase, and the sintering temperature is high, resulting in a large particle size and uneven distribution. In view of this, exploring new yellow phosphors for white LEDs is still a frontier topic in the field of luminescence [9,10].
Chloroborate has the characteristics of low synthesis temperature, easy control of phase structure and good absorption in ultraviolet-near ultraviolet and blue light regions [11-13]; yellow fluorescence for white LEDs prepared with chloroborate as matrix Powder, and the use of appropriate methods to improve material properties, etc., has become a research hotspot in this direction [14 ~ 18]. In this paper, a high-temperature solid-phase reaction method was used, Ca2BO3Cl was used as the matrix, and Eu2+ was used as the activator to prepare blue-light-exciting yellow luminescent powder. The effects of Ce3+, CaCl2 and H3BO3 on the luminescence properties of the materials were studied. The results will be fluorescent for white LEDs. The development of powder provides help.
2.1 Sample preparation
The sample was synthesized by a high temperature solid phase method. The reagents used were Ca-CO3 (AR), H3BO3 (AR), CaCl2 (AR), Eu2O3 (99.99%) and CeO2 (99.99%). The above materials were weighed according to the stoichiometric ratio given by the formula Ca2-xBO3Cl:xEu2+. The raw materials were ground uniformly in an agate mortar, placed in a corundum crucible, placed in a muffle furnace, and sintered at 900 Â° C for 2.5 h to obtain a Ca2BO3Cl:Eu2+ series sample.
2.2 Sample characterization
American XRD6000 X-ray diffraction (XRD) instrument (radiation source Cu target KÎ±, 40kV, 40mA, Î»=0.154 06nm, scanning speed 8Â°/min, step size 0.02Â°, scanning range 10Â°ï½ž60Â° Determination of the powder diffraction pattern of the sample; Japan Shimadzu F-4600 fluorescence spectrophotometer to measure the excitation and emission spectra of the material (excitation source is 450 W xenon lamp), scanning range 200 ~ 700nm; PMS-80UV-VIS-NEAR IR spectral color The meter measures the color coordinates of the material. All measurements were taken at room temperature.
3 Results and discussion
3.1 Crystal structure of the material
The melting point of CaCl2 is 782 Â°C, and the melting point of H3BO3 is 169 Â°C, which is lower than the synthesis temperature (900 Â°C) of Ca2BO3Cl:Eu2+ material. Therefore, in the preparation of Ca2BO3Cl:Eu2+, excess CaCl2 and H3BO3 must be appropriately added to make up for the volatile CaCl2 and H3BO3. Figure 1 is an XRD pattern of Ca1.98BO3Cl: 2% Eu2+. By comparing with the standard powder diffraction card, the XRD diffraction peak data of the sample is consistent with the JCPDS 29-0302 card data, indicating that a small amount of doped Eu2+ or Ce3+ partially replaces Ca2+ into the matrix lattice without changing the crystal structure of the sample. In the experiment, the excess CaCl2 and H3BO3 added did not have a significant effect on the matrix structure, and no new phase was formed. Ca2BO3Cl belongs to orthorhombic system, P21/c(14) space group, and the lattice parameters are a=0.394 8nm,
b = 0.886 2 nm and c = 1.240 2 nm.
3.2 Luminous properties of the material
3.2.1 Effect of Ce3+ on the luminescence properties of materials
Figure 2 shows the excitation and emission spectra of Ca2BO3Cl: 2% Eu2+ and Ca2BO3Cl: 3% Ce3+ materials. Fig. 2(a) shows that the emission spectrum of Ca2BO3Cl:Eu2+ material is a broad spectrum with a peak at 573 nm, which belongs to the 4f65d1â†’4f7 characteristic transition emission of Eu2+, which is the allowable electric dipole transition. Since the 5d energy level is exposed, it is greatly affected by the crystal field environment. The 5d energy levels in different crystal field environments are different in degree of splitting, so that Eu2+ can emit light from different wavelengths from ultraviolet to visible in different substrates . Compared with YAG:Ce, the spectral coverage of Ca2BO3Cl:Eu2+ is closer to the long-wave direction and has a broad absorption band in the blue region, so it can be used as a good blue-light-emitting yellow luminescent powder for white LEDs. The 573 nm emission peak was monitored, and its excitation spectrum covered 300-500 nm. The main peaks were at 413 and 475 nm, which belonged to the 4f-5d transition characteristic excitation band of Eu2+, and matched the emission peaks of the blue and ultraviolet LED dies. Therefore, the material can be used as a yellow luminescent powder in the "blue + yellow" mode, or as an ultraviolet luminescent yellow luminescent powder, and mixed with other luminescent powders to obtain white light emission.