A low-temperature plasma sintering method for calcium carbonate white phosphor

Abstract

A low-temperature plasma sintering method for calcium carbonate white phosphor involves co-doping rare earth elements europium and cerium ions into a calcium carbonate matrix, using bismuth ions as the energy resonance transfer center to modulate the red and blue luminescence of the two rare earth elements. This invention employs low-temperature solid-state sintering of calcium carbonate powder in a plasma atmosphere, avoiding the decomposition of the calcium carbonate matrix caused by high temperatures. Furthermore, low-temperature sintering helps overcome the fluorescence quenching phenomenon caused by agglomeration during conventional powder sintering. Plasma also promotes the activity of ions on the powder surface, enabling efficient rare earth co-doping at low temperatures. This invention is simpler and cheaper than existing white phosphor preparation methods, and has a wider range of applications.

Description

Technical Field

[0001] This invention belongs to the field of materials preparation technology, specifically relating to the preparation of white phosphors based on calcium carbonate and its dedicated low-temperature plasma sintering method. Background Technology

[0002] Fluorescent materials are materials that absorb visible and ultraviolet light and then emit visible light of a specific color. Rare-earth-doped alkaline earth aluminates and silicates are the most mainstream fluorescent materials on the market. However, the emission colors of these systems are concentrated in the blue-green wavelength range, and mature red and white phosphors have not yet been developed. Furthermore, aluminates and silicates are unstable in water, and their raw materials are complex and their preparation is energy-intensive. Calcium carbonate-based phosphors offer a solution to these problems.

[0003] Calcium carbonate has advantages such as low price, stable chemical properties, low hardness, and easy control of powder size. Doping calcium carbonate with suitable rare earth elements and using high-temperature solid-state sintering to replace calcium ions with rare earth elements can produce tri-color phosphors (CN114989810A). Traditional white phosphors are essentially mixtures of these tri-color phosphors, not truly pure white phosphors. This brings inconvenience to the practical application of this type of phosphor. Therefore, preparing monomeric white phosphors through co-doping with rare earth ions can promote the application of white light in the field of light emission. According to the traditional sintering mechanism, high-temperature sintering facilitates the transition and substitution of dopant ions, forming sufficient dopant and luminescent centers. Co-doping with rare earth ions increases the difficulty of uniform doping. Increasing the sintering temperature helps improve uniformity, but calcium carbonate is prone to decomposition at high temperatures; therefore, the luminous intensity of calcium carbonate phosphors is generally not high at present. Although phosphors based on calcium oxide can be prepared (Optics Communications, (2014) 311: 266-269.), calcium oxide powder is prone to deliquescence, leading to fluorescence decay, thus limiting its practical value. Therefore, promoting rare earth ion doping while maintaining the calcium carbonate matrix is ​​a challenge in the preparation of calcium carbonate-based phosphors. To promote rare earth element doping and inhibit calcium carbonate decomposition, the team at the Changchun Institute of Applied Chemistry achieved stable phosphor preparation using hot-pressed plasma sintering technology (CN103627399A). Hot-pressed plasma sintering involves applying alternating current to the powder to break down the gas between the powder particles, forming plasma that enhances the powder's activity and promotes the migration and doping of rare earth ions. However, this method requires a mechanical hot-pressing device; to maintain pressure at the MPa level, the cavity cannot be made too large, which affects the efficiency of batch powder preparation. Summary of the Invention

[0004] To address the problems of existing high-efficiency sintering calcium carbonate-based phosphors, this invention uses a tubular low-pressure vacuum chamber to generate plasma through argon and nitrogen glow discharge. This plasma bombards the powder surface, promoting the migration and doping of co-doped rare earth ions, thereby achieving low-temperature sintering of phosphors based on calcium carbonate.

[0005] The technical solution of the present invention is as follows:

[0006] A low-temperature plasma sintering method for calcium carbonate white phosphor is disclosed, which utilizes a plasma atmosphere for low-temperature solid-state sintering of calcium carbonate phosphor powder based on rare earth co-doping effect, without the need for hot pressing or external pressure facilities.

[0007] Furthermore, the phosphor is based on calcium carbonate and sintered with europium and cerium as activators and bismuth as sensitizer. The molar percentage of europium doping is 1-4%, the molar percentage of cerium doping is 1-5%, and the rare earth elements are provided by rare earth oxides.

[0008] Furthermore, the white light powder includes a monomeric phosphor with a cyan emission peak and a red emission peak, wherein europium doping results in a red fluorescence peak and cerium doping results in a cyan fluorescence peak, and the luminescence of the two rare earth elements is modulated by bismuth, which acts as an energy transfer center.

[0009] Powder excitation wavelength: 200-350nm, emission wavelength: 471nm cyan and 615nm red fluorescence peaks, powder particle size: 1-20μm, temperature tolerance: above 400℃.

[0010] Using a low-pressure plasma atmosphere for solid-state sintering, there is no need to apply external mechanical high pressure to the powder during the sintering process. The sintering process uses a tube furnace with a radio frequency plasma chamber. Inside the chamber, a multi-layer sample holder made of alumina ceramic sheets is used to place calcium carbonate doped with rare earth oxides. The powder is loose and maintains a high specific surface area.

[0011] The background pressure of sintering is 10. -1 -10 -3 The working pressure is controlled between 1-500 Pa. The atmosphere used consists of low-activity gases such as argon and carbon dioxide. Argon, as an ionized gas, contains little oxygen and water, and its role in the sintering process is to provide high-energy argon ions. Carbon dioxide plays a role in inhibiting the decomposition of the calcium carbonate matrix during sintering and providing sufficient pressure to maintain the stability of the calcium carbonate matrix.

[0012] The working atmosphere consists of low-activity gases and carbon dioxide. The low-activity gases are argon or nitrogen. During sintering, the flow rate of argon or nitrogen is controlled at 500-1000 sccm, and the flow rate of carbon dioxide is controlled at 1000-1500 sccm.

[0013] In solid-state sintering, the sintering temperature is below 600℃, the sintering time is 2-4 hours, the plasma source frequency is 10-20MHz, and the power is 500-800W.

[0014] The beneficial effects of this invention are as follows: First, white fluorescence of the monomer powder is achieved through co-doping, and the preparation method is simple and easy to implement. Second, low-temperature sintering in a plasma atmosphere can effectively improve the yield of calcium carbonate phosphor, protect the calcium carbonate powder matrix from decomposition, realize industrialization, simplify the post-processing, and is environmentally friendly and efficient, possessing significant advantages in large-scale production. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of a low-temperature plasma sintering furnace. 1 is the radio frequency electrode plate, 2 is the radio frequency power supply, 3 is the plasma atmosphere, 4 is the sintering heating frame (made of alumina ceramic wrapped around heating wires), 5 is the sintering heating power supply and temperature control equipment, 6 is the vacuum pumping pipeline, 7 is the argon and carbon dioxide filling pipeline, and 8 is the powder sample.

[0016] Figure 2 Morphology images of europium-doped phosphors in conventional sintering and low-temperature plasma sintering under CO2 protection, where (a) is conventional sintering and (b) is low-temperature plasma sintering.

[0017] Figure 3 Morphology of europium-doped phosphor sintered by low-temperature plasma without CO2 protection.

[0018] Figure 4 The image shows the XRD pattern of europium-doped phosphors produced by low-temperature plasma sintering.

[0019] Figure 5 PL diagram of europium-doped phosphor sintered under CO2 protection.

[0020] Figure 6 Morphology of europium-cerium co-doped phosphors sintered under CO2 protection at low temperature plasma.

[0021] Figure 7 XRD pattern of europium-cerium co-doped phosphors sintered under CO2 protection in low-temperature plasma.

[0022] Figure 8 Photograph of low-temperature plasma sintered europium and cerium co-doped phosphors protected by CO2.

[0023] Figure 9 Colorimetric image of europium-cerium co-doped phosphors in low-temperature plasma sintering under CO2 protection. Detailed Implementation

[0024] To make the technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. The embodiments described herein are only some embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0025] Reference Figures 1-9 A low-temperature plasma sintering method for calcium carbonate white phosphor is proposed, which utilizes a plasma atmosphere for low-temperature solid-state sintering of calcium carbonate phosphor powder based on rare earth co-doping effect, without the need for hot pressing or external pressure facilities.

[0026] The phosphor is based on calcium carbonate and sintered with europium and cerium as activators and bismuth as sensitizer. The molar percentage of europium doping is 1-4%, the molar percentage of cerium doping is 1-5%, and the rare earth elements are provided by rare earth oxides.

[0027] White light powder includes monomeric phosphors with cyan and red luminescent peaks. Europium doping results in the red fluorescence peak, while cerium doping results in the cyan fluorescence peak. The luminescence of the two rare earth elements is modulated by bismuth, which acts as the energy transfer center.

[0028] Powder excitation wavelength: 200-350nm, emission wavelength: 471nm cyan and 615nm red fluorescence peaks, powder particle size: 1-20μm, temperature tolerance: above 400℃.

[0029] Using a low-pressure plasma atmosphere for solid-state sintering, there is no need to apply external mechanical high pressure to the powder during the sintering process. The sintering process uses a tube furnace with a radio frequency plasma chamber. Inside the chamber, a multi-layer sample holder made of alumina ceramic sheets is used to place calcium carbonate doped with rare earth oxides. The powder is loose and maintains a high specific surface area.

[0030] The background pressure of sintering is 10. -1 -10 -3 The working pressure is controlled between 1-500 Pa. The atmosphere used consists of low-activity gases such as argon and carbon dioxide. Argon, as an ionized gas, contains little oxygen and water, and its role in the sintering process is to provide high-energy argon ions. Carbon dioxide plays a role in inhibiting the decomposition of the calcium carbonate matrix during sintering and providing sufficient pressure to maintain the stability of the calcium carbonate matrix.

[0031] The working atmosphere consists of low-activity gases and carbon dioxide. The low-activity gases are argon or nitrogen. During sintering, the flow rate of argon or nitrogen is controlled at 500-1000 sccm, and the flow rate of carbon dioxide is controlled at 1000-1500 sccm.

[0032] In solid-state sintering, the sintering temperature is below 600℃, the sintering time is 2-4 hours, the plasma source frequency is 10-20MHz, and the power is 500-800W.

[0033] In this embodiment, the physical mechanism of energy resonance transfer between rare earth ions is utilized, and the rare earth particles are co-doped and sintered using rare earth elements that induce red and blue emission. Specifically, europium, the rare earth element causing red emission, constitutes 1-4% molar mass of the matrix calcium carbonate, and cerium, the rare earth element causing blue emission, constitutes 1-5% molar mass of the matrix calcium carbonate. Europium trioxide and cerium trioxide provide the rare earth particles (Eu). 3+ and Ce 3+ Bismuth oxide provides Bi 3+ As an energy resonance transfer center

[0034] K2CO3 or Li2CO3 are used as fluxes and charge compensators.

[0035] Calcium carbonate powder is solid-phase sintered using a low-pressure plasma atmosphere. A radio frequency power supply is used to excite working gases such as argon, nitrogen, or carbon dioxide in a low-pressure environment to generate a plasma atmosphere to activate the powder surface. Carbon dioxide also has the function of inhibiting the decomposition of calcium carbonate to produce carbon dioxide.

[0036] The rare earth raw materials to be doped, europium trioxide and cerium trioxide, are mixed and ground with calcium carbonate powder in a certain proportion, and then placed in a cylindrical sintering cavity. This cavity structure with a certain degree of symmetry helps to achieve uniform plasma distribution. Figure 1 As shown, the sintering furnace is equipped with a vacuum system and a gas filling system. A planar electrode with a spacing of 5-10 cm is installed inside the sintering chamber. The electrode is externally connected to an RF power supply with a voltage of 1-5 kV, a frequency of 10-20 MHz, and a power of 500-800 W. A multi-layer powder rack is also placed inside the chamber. The multi-layered arrangement of powder increases the probability of powder particles being exposed to plasma, utilizing plasma to promote the surface doping activity of the particles.

[0037] The powder is placed on a multi-layer powder rack in the sintering furnace, and the base vacuum of the cavity is set to 10. -1 -10 -3 Pa, the working gas is argon and carbon dioxide, and the molar ratio of carbon dioxide is controlled to be 60%-70% by a flow meter. The working gas pressure is controlled to be 1-500 Pa. After the gas is ignited, the doped calcium carbonate powder is sintered in a low-pressure environment at a low temperature. The sintering temperature is controlled at 500℃-600℃ and the sintering time is 2-4 hours. No grinding and recalcination are required in between.

[0038] The excitation wavelength of the final product, white phosphor powder, is 200-350nm, the emission wavelength is 471nm (cyan) and 615nm (red) with two fluorescence peaks, the powder particle size is 1-20μm, and the temperature tolerance is above 400℃.

[0039] Example 1

[0040] To illustrate the protective effect of low-temperature plasma sintering on the calcium carbonate matrix and the feasibility of low-temperature doping, Example 1 demonstrates the low-temperature plasma sintering of europium-doped red calcium carbonate phosphor.

[0041] In such Figure 1 The sample is placed in the apparatus shown. The sample holder is a multi-layered alumina ceramic sheet structure with an embedded heating wire. The alumina and sample are in direct contact to maintain the sample's purity. This example only prepares europium oxide-doped red calcium carbonate fluorescent powder. A certain amount of CaCO3, Eu2O3, and Li2CO3 are weighed out. The purity requirements for calcium carbonate and lithium carbonate are AR, and the purity requirement for europium oxide is 4N. The ratio of the three is 100:2:3, where Eu... 3+ As an activator, Li₂CO₃ acts as a cosolvent and charge compensator, and Mn can also be added. 3+ As a sensitizer, the mixture was placed in a planetary ball mill and ground thoroughly for 3 hours. After grinding, the powder was placed on an alumina multilayer powder rack and vacuumed to 100 kJ / L. -3 Argon and carbon dioxide gases were introduced at flow rates of 500 sccm and 750 sccm, respectively, with a total pressure of 100 Pa. Argon was used as the ionizing gas. A pulsed power was set; when the pulse voltage reached a certain value, it could break down the argon gas, generating argon ions that bombarded the powder surface, creating a hot zone in the active region of the powder particles. This facilitated high-speed migration and diffusion of dopant ions, promoting the complete migration of dopant elements into the matrix for optimal doping. In this example, the radio frequency used was 12.56 MHz, and the power was 300 W. Carbon dioxide was used as a protective gas to inhibit the decomposition reaction of calcium carbonate powder during sintering. The sintering temperature was 560℃, and the sintering time was 4 hours. This sintering method has high thermal efficiency, and the diffuse distribution of the discharge points allows for uniform heating, thus creating conditions for preparing high-quality phosphors. Figure 2 The morphologies of fluorescent powders prepared by plasma-assisted sintering under carbon dioxide protection and conventional atmospheric pressure sintering at 900°C were compared. It can be seen that plasma sintering, due to its low sintering temperature, prevents powder agglomeration. In contrast, conventional atmospheric pressure high-temperature sintering causes powder agglomeration, which is less conducive to the diffusion and doping of rare earth elements. To compare the protective effect of carbon dioxide on calcium carbonate powder, Figure 3 The product morphology of low-temperature plasma sintering without carbon dioxide protection is shown, revealing a completely different powder morphology. Therefore... Figure 4 By comparing the effects of carbon dioxide protection on the powder lattice, it can be seen that the powder without carbon dioxide protection showed a large number of calcium oxide diffraction peaks, indicating that some calcium carbonate decomposed into calcium oxide, which is not conducive to the application of fluorescent powder. Figure 5By comparing the fluorescence quality of powders sintered by low-temperature plasma sintering and those sintered by conventional high-temperature sintering under the same conditions of carbon dioxide protection, it can be seen that the powder sintered by low-temperature plasma sintering has stronger fluorescence, proving that the sintering method of the present invention has more uniform and in-depth doping.

[0042] Example 2

[0043] To illustrate that a single white phosphor can be realized by using bismuth as the energy resonance transfer center and europium and cerium co-doping, Example 2 demonstrates the low-temperature plasma sintering of europium and cerium co-doped calcium carbonate phosphor.

[0044] Weigh out a certain amount of CaCO3, Eu2O3, Ce2O3, and Li2CO3. The purity requirement for calcium carbonate and lithium carbonate is AR, and the purity requirement for europium oxide and cerium oxide is 4N. The ratio of the three is 100:0.3:0.7:3, where Eu... 3+ and Ce 3+ As a standalone activator, Li₂CO₃ acts as a cosolvent and charge compensator, and Bi can also be added. 3+ As a transfer center, the mixture was placed in a planetary ball mill and ground thoroughly for 3 hours. For co-doped powders, a pre-calcination process was added. After grinding, the powder was placed on an alumina multilayer powder rack, and a calcination temperature of 500°C was set for pretreatment for 120 minutes. The pretreated powder was then placed in a planetary ball mill and ground again for 6 hours. Carbon dioxide and argon were introduced, the pulse power was set to 600W, the radio frequency remained at 12.56MHz, the sintering temperature was 600°C, and the sintering time was 300 minutes. Figure 6 and Figure 7 These are the morphology and structure diffraction patterns of europium-cerium co-doped calcium carbonate phosphor. It can be seen that the powder does not show obvious agglomeration, and the lattice structure of the calcium carbonate matrix remains intact and unchanged. Figure 8 This is the fluorescence spectrum of the phosphor, showing the simultaneous emission of cyan and red light. Compared to phosphors that are purely cerium-doped or purely europium-doped, the red emission peak shape of the europium-cerium co-doped phosphor shows a significant change. For example, the blue emission of pure cerium-doped phosphor has changed to cyan emission, indicating the presence of energy resonance transfer. The two discrete red peaks of pure europium-doped phosphor have become a single peak, indicating that co-doping has an effect on the energy levels of the product.

[0045] This embodiment employs low-temperature plasma solid-state sintering of calcium carbonate powder, avoiding the decomposition of calcium carbonate caused by high temperatures. Furthermore, low-temperature sintering helps overcome the fluorescence quenching phenomenon that occurs during conventional powder sintering due to agglomeration. The white phosphor prepared by this invention is simpler and cheaper than existing methods for preparing three-primary-color phosphors, while also having a wider range of applications and stronger luminescent performance.

[0046] The embodiments described in this specification are merely examples of implementations of the inventive concept and are for illustrative purposes only. The scope of protection of this invention should not be considered limited to the specific forms described in these embodiments; rather, it extends to equivalent technical means conceived by those skilled in the art based on the inventive concept.

Claims

1. A low-temperature plasma sintering method for calcium carbonate white phosphor, characterized in that, A calcium carbonate phosphor based on rare-earth co-doping effect is produced by low-temperature solid-state sintering in a plasma atmosphere, eliminating the need for hot pressing and external pressure facilities. The phosphor uses calcium carbonate as a matrix, with europium and cerium acting as activators and bismuth as a sensitizer. The molar percentage of europium doping is 1-4%, and the molar percentage of cerium doping is 1-5%. Rare earth elements are provided by rare earth oxides. The sintering background pressure is 10... -1 -10 -3 The working pressure is controlled at 1-500 Pa, and the working atmosphere is low-activity gas and carbon dioxide. The low-activity gas is argon or nitrogen. During sintering, the flow rate of argon or nitrogen is controlled at 500-1000 sccm, and the flow rate of carbon dioxide is controlled at 1000-1500 sccm.

2. The low-temperature plasma sintering method for calcium carbonate white phosphor as described in claim 1, characterized in that the white phosphor comprises a monomeric phosphor having a cyan emission peak and a red emission peak, wherein europium doping results in a red fluorescence peak, cerium doping results in a cyan fluorescence peak, and the luminescence of the two rare earth elements is modulated by bismuth as an energy transfer center.

3. The low-temperature plasma sintering method for calcium carbonate white phosphor as described in claim 1, characterized in that, Powder excitation wavelength: 200-350nm, emission wavelength: 471nm cyan and 615nm red fluorescence peaks, powder particle size: 1-20μm, temperature tolerance: above 400℃.

4. The low-temperature plasma sintering method for calcium carbonate white phosphor as described in claim 1, characterized in that, Using a low-pressure plasma atmosphere for solid-state sintering, there is no need to apply external mechanical high pressure to the powder during the sintering process. The sintering process uses a tube furnace with a radio frequency plasma chamber. Inside the chamber, a multi-layer sample holder made of alumina ceramic sheets is used to place calcium carbonate doped with rare earth oxides. The powder is loose and maintains a high specific surface area.

5. The low-temperature plasma sintering method for calcium carbonate white phosphor as described in claim 1, characterized in that, In solid-state sintering, the sintering temperature is below 600℃, the sintering time is 2-4 hours, the plasma source frequency is 10-20MHz, and the power is 500-800W.

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