METHOD FOR PRODUCING CaAl4O7:Mn4+ PHOSPHOR USING NATURAL LIMESTONE

Abstract

FIELD: materials science; chemistry.SUBSTANCE: used in optoelectronics, as well as in the production of decorative and functional building materials. When producing the CaAl4O7:Mn4+ phosphor, natural limestone is used as a source of calcium, which is further crushed to obtain a fine powder. Powdered metallic aluminium, manganese dioxide, and orthoboric acid are added to crushed natural limestone. The resulting mixture is thoroughly mixed in an agate mortar until homogeneity is achieved, the molar ratio of calcium to aluminium is set equal to 1:4. The prepared mixture is loaded into a ceramic crucible and subjected to heat treatment in conditions of free access to atmospheric air according to the following temperature: heating to a temperature of 110 °C at speed 5 °C / min with a holding time of 30 min, then heating to 600 °C at the same speed and holding for 1 hour, after which heating is continued to a temperature of 1400 °C at speed 5 °C / min and holding time at that temperature for 3 hours. Upon completion of the synthesis, the resulting product is cooled to room temperature under conditions of free air circulation, extracted and ground to a fine powder.EFFECT: expansion of the feedstock through the use of available calcium-comprising materials while maintaining the target phase composition and luminescent characteristics of the final product.1 cl, 3 dwg

Description

[0001] The invention relates to the field of materials science and chemical technology of inorganic phosphors and can be used in optoelectronics, as well as in the production of decorative and functional building materials.

[0002] A known method for producing an orange-red glowing phosphor based on calcium aluminate activated by manganese (IV) ions [1] includes solid-phase synthesis from a mixture of calcium carbonate (CaCO3), aluminum oxide (Al2O3) and high-purity manganese carbonate (MnCO3) (99.99%) with the addition of flux (boric acid, H3BO3) in an amount of 5-10 wt.% and heat treatment at a temperature of 1200-1500°C in a nitrogen atmosphere. The phosphor exhibits photoluminescence in the red region of the spectrum with maxima at 644, 656, 666 and 671 nm when excited in the near UV range (320-400 nm), with the most intense emission band observed at 656 nm. Optimal luminescent properties are achieved with Mn content 4+0.3 at.% and using 5-10 wt.% flux.

[0003] The disadvantages of the known method include the use of expensive high-purity reagents (99.99%), which significantly increases the cost of production, and the need to carry out synthesis in a nitrogen atmosphere using specialized furnaces to control the gas environment.

[0004] The closest in technical essence (prototype) is the method for obtaining the phosphor CaAl4O7:Mn 4+ by a solid-phase synthesis method, including mixing aluminum oxide (Al2O3), calcium carbonate (CaCO3) and manganese dioxide (MnO2), reagents of standard analytical purity (AR), with an optimal concentration of Mn 4+0.6 at.% and using 5 wt.% H3BO3 flux, followed by a two-stage heat treatment: preliminary annealing at 300°C for 2 hours and final sintering at 1400°C for 3 hours with the addition of H3BO3 flux, as described in [2]. This method ensures the formation of a phase-pure phosphor with intense red emission in the range of 635-655 nm.

[0005] The disadvantages of the known technical solution are the need for an additional pre-annealing stage, which increases the duration and energy consumption of the technological process, as well as the mandatory use of expensive chemically pure reagents, which significantly increases the cost of production.

[0006] The technical objective of the invention is to develop a method for producing a CaAl4O7:Mn phosphor 4+using available natural and man-made calcium-containing materials, which eliminates the shortcomings of known methods and ensures the expansion of the raw material base.

[0007] To solve the technical problem of the invention, a method for producing a CaAl4O7:Mn phosphor is proposed 4+ Using natural limestone, this process involves pre-crushing the starting components, mixing them uniformly, solid-phase synthesis, cooling, and obtaining a fine powder. Natural limestone is used as the calcium source, which is then further crushed in an agate mortar to obtain a fine powder. Aluminum metal powder, manganese dioxide (MnO2), and orthoboric acid (H3BO3) are added to the crushed natural limestone.

[0008] The resulting mixture of components is thoroughly mixed in an agate mortar until homogeneous. The calcium to aluminum molar ratio is set at 1:4, and the activator content is Mn4+ is 0.40 at.% relative to aluminum in the target phase CaAl4O7, and the content of orthoboric acid is 12 wt.% of the total mass of the batch.

[0009] The prepared mixture is loaded into a ceramic crucible and heat-treated in a muffle furnace under free access to atmospheric air using the following temperature regime: heating to 110°C at a rate of 5°C / min and holding for 30 minutes, then heating to 600°C at the same rate and holding for 1 hour, followed by continued heating to 1400°C at a rate of 5°C / min and holding at this temperature for 3 hours. Upon completion of the synthesis, the furnace is turned off, and the crucible containing the resulting product is left in the furnace until completely cooled to room temperature under free air circulation. The resulting synthesis product is removed from the crucible and ground in an agate mortar to a fine powder.

[0010] As a result of synthesis by solid-phase sintering, a phosphor based on CaAl4O7, activated by manganese (IV) ions, was obtained with a nominal activator content of 0.40 at.% relative to the amount of aluminum in the target phase.

[0011] Examples of the method implementation

[0012] To obtain the reaction mixture in accordance with the proposed method, take 1.0 g of natural limestone (fraction size 5-10 mm), 1.2 g of aluminum metal powder with a particle size in the micron range, 0.014 g of manganese dioxide (MnO2) and 0.3 g of orthoboric acid (H3BO3).

[0013] Manganese dioxide is introduced into the batch as a source of manganese ions in the oxidation state of +4, which provide the luminescent properties of the resulting material.

[0014] Orthoboric acid is introduced into the mixture to lower the melting temperature, increase the degree of sintering of the components and accelerate the solid-phase reactions, which contributes to the formation of the target phase of the phosphor.

[0015] These quantities of components correspond to the molar ratio of Ca:Al = 1:4, the introduction of the activator Mn 4+ in an amount of 0.40 at.% relative to aluminum in the target phase CaAl4O7 and a flux content of 12 wt.% of the total mass of the mixture.

[0016] First, limestone is ground in an agate mortar to a fine powder. Then, specified quantities of aluminum powder, manganese dioxide, and orthoboric acid are sequentially added to the ground limestone. The resulting mixture is thoroughly mixed in an agate mortar until the components are evenly distributed, ensuring a homogeneous reaction mixture.

[0017] The prepared mixture is loaded into a ceramic crucible and subjected to heat treatment in a muffle furnace under conditions of free access to atmospheric air according to the following temperature regime: heating to a temperature of 110°C at a rate of 5°C / min with a holding time of 30 minutes, then heating to 600°C at the same rate and holding for 1 hour, after which heating is continued to a temperature of 1400°C at a rate of 5°C / min and holding at this temperature for 3 hours.

[0018] Holding at 110°C removes adsorbed and crystallized moisture, preventing uneven gas evolution and particle agglomeration. The 600°C step enables controlled decomposition of the natural calcium carbonate contained in limestone, resulting in uniform formation of calcium oxide and reducing the likelihood of incomplete calcination and the formation of amorphous zones. This approach eliminates the need for intermediate cooling and grinding, simplifies the process, improves its reproducibility, and reduces energy consumption.

[0019] Once the synthesis is complete, the furnace is turned off, and the resulting product is cooled to room temperature with free access to atmospheric air. After cooling, the product is removed from the crucible and ground in an agate mortar to a fine powder.

[0020] As a result of synthesis by solid-phase sintering, a phosphor based on CaAl4O7, activated by manganese (IV) ions, was obtained with a nominal activator content of 0.40 at.% relative to the amount of aluminum in the target phase.

[0021] The phase composition of the resulting powder was studied using X-ray diffraction (XRD) analysis on a Diffray 401 benchtop X-ray diffractometer. The X-ray diffraction results (see Figure 1) confirm the formation of the target phase of calcium aluminate CaAl4O7 activated by Mn ions. 4+ , as well as the presence of CaAl phases 12 O 19 and Al2O3, which do not have a significant effect on the luminescent properties of the material. It should be noted that the CaAl phase 12 O 19 is formed as a by-product, even when using high-purity CaCO3 as the feedstock, which indicates its thermodynamic stability in the CaO-Al2O3 system under the given synthesis conditions.

[0022] Also, unlike similar methods that use aluminum oxide (Al2O3) as the aluminum source, the proposed method uses metallic aluminum powder. When heated, it actively interacts with other components of the batch and oxidizes, forming aluminum oxide directly during the synthesis process. Due to the exothermic nature of metallic aluminum oxidation during heating, partial heat release occurs, which helps reduce the phase formation temperature and simplifies the formation of the CaAl4O7 structure.

[0023] The luminescence characteristics of the synthesized material were studied using a Fluorat-02-Panorama spectrofluorimeter and a "Frog" attachment. During the measurements, excitation and luminescence spectra were recorded.

[0024] The excitation spectrum (see Figure 2) was obtained at a recording wavelength of 653 nm, corresponding to the maximum of the emission band of Mn ions 4+in the matrix. The emission spectrum (see Figure 3) was recorded with excitation in the range of 340-525 nm, which corresponds to the absorption bands of both the matrix itself and Mn ions 4+ , confirming their active participation in luminescence processes.

[0025] The luminescence spectrum shows the characteristics of Mn ions 4+ in an octahedral environment, narrow emission bands in the red region of the spectrum with maxima at 641, 653 and 664 nm, which corresponds to the reference spectra of the CaAl4O7:Mn phosphor 4+ , obtained using chemically pure calcium carbonate.

[0026] Thus, the proposed method for obtaining the CaAl4O7:Mn phosphor 4+ The use of natural limestone allows for the expansion of the raw material base through the use of available calcium-containing materials while maintaining the target phase composition and luminescent characteristics of the final product.

[0027] Sources of information

[0028] 1. Liu S.X., Xiong F.B., Lin H.F., Meng X.G., Lian S.Y., Zhu W.Z. A deep red-light-emitting phosphor Mn 4+ :CaAl4O7for warm white LEDs. Optik - International Journal for Light and Electron Optics. 2018. Vol. 170. P. 178-184. DOI: 10.1016 / j.ijleo.2018.05.127.

[0029] 2. Park J., Kim G., Kim Y.J. Luminescent properties of CaAl4O7powders doped with Mn 4+ ions. Ceramics International.2013. Vol. 39. P. S623-S626. DOI: 10.1016 / j.ceramint.2012.10.149.

Claims

Method for producing CaAl4O7:Mn phosphor 4+using natural limestone, cooling and obtaining a fine powder of the material, characterized in that natural limestone is used as a source of calcium, which is additionally crushed to obtain a fine powder; metallic aluminum powder, manganese dioxide, and orthoboric acid are added to the crushed natural limestone, the resulting mixture of components is thoroughly mixed in an agate mortar until homogeneity is achieved, the molar ratio of calcium to aluminum is set equal to 1:4; the prepared mixture is loaded into a ceramic crucible and subjected to heat in conditions of free access to atmospheric air according to the following temperature regime: heating to a temperature of 110 °C at a rate of 5 °C / min and holding for 30 minutes, then heating to 600 °C at the same rate and holding for 1 hour, after which heating is continued to a temperature of 1400 °C at a rate of 5 °C / min and holding at this temperature for 3 hours;After completion of the synthesis, the resulting product is cooled to room temperature under conditions of free air circulation, then extracted and ground to a fine powder.

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