TWI859032B - Near-infrared luminescent chromium-activated forsterite phosphor fibers and producing method thereof - Google Patents

Abstract

A method for preparing near-infrared luminescent chromium-activated forsterite phosphor fibers, including: (1) Preparing a precursor solution containing a starting solution and an activator. The starting solution includes (a) a magnesium source selected from at least one of magnesium alkoxides, (b) silicon source selected from at least one of silicon alkoxides, (c) alcohol solvents, and (d) glacial acetic acid, the activator is a chromium (III)-containing activator; (2) Adding deionized water to the precursor solution for hydrolysis reaction , resulting in a transparent sol; (3) Allowing the transparent sol to undergo a condensation-polymerization reaction to obtain a viscous transparent sol; (4) Dip a glass rod into the viscous transparent sol and directly spin it to obtain a transparent gel fiber; and (5) Drying the transparent gel fibers and subjecting them to annealing treatment at a temperature of above 1200°C to obtain near-infrared luminescent chromium-activated forsterite phosphor fibers.

Description

Preparation method of near-infrared chromium activated magnesium olivine fluorescent fiber

The present invention relates to a method for preparing near-infrared chromium-activated olivine fluorescent fiber, and more particularly to a method for preparing chromium-activated olivine (Mg ₂ SiO ₄ :Cr) fluorescent fiber having broadband near-infrared light emission characteristics by using a sol-gel method and doping with a chromium (III)-containing activator.

According to the definition of the International Commission on Illumination (CIE), near infrared light (NIR) refers to electromagnetic waves with a wavelength between 700 and 1400 nm, which is between visible light and mid-infrared light. The International Organization for Standardization (ISO20473) defines NIR wavelengths as between 780 and 3000 nm. The near-infrared light band has the characteristics of being undetectable to the human eye, having strong penetration ability into biological tissues without damaging them, avoiding interference from spontaneous fluorescence or brightness interference in the living environment. Therefore, NIR light sources are suitable for real-time and non-destructive detection of biological tissues. They can be widely used in biomedical imaging, precision medical diagnosis, food testing, and night vision goggles, etc. However, in applications, a wide radiation spectrum is usually required to achieve effective performance.

Traditional commercial NIR light sources include tungsten halogen lamps, laser diodes, nickel-chromium alloy heaters and spherical light sources, which have the disadvantages of high heat generation, power consumption, poor spectral stability, narrow radiation spectrum and large instrument size. However, the photoluminescence properties of fluorescent powders have the potential to be applied to miniature or portable handheld spectrometers, and have significant advantages such as long life, good spectral stability, on-site measurement and low cost, so they have received much attention in recent years.

However, only a few of them are suitable for practical applications, especially broadband NIR fluorescent powders, and they are mainly prepared by high-temperature solid-state method. Enterprises need to develop new low-cost processes and novel broadband NIR fluorescent materials.

In recent years, there have been literature reports on related NIR phosphors, where the doping activators are rare earth ions (such as Pr ³⁺ , Nd ³⁺ , Sm ³⁺ , Tm ³⁺ , Ho ³⁺ , Er ³⁺ , Eu ²⁺ ) or transition metal ions (such as Cr ³⁺ , Cr ⁴⁺ , Ni ²⁺ , Fe ³⁺ , Mn ²⁺ , Mn ⁴⁺ ). Rare earth ions often emit narrowband radiation due to the shielding and forbidden transition of the 4f orbital domain, and are not suitable for broadband NIR phosphors.

Transition elements Cr ³⁺ or Cr ⁴⁺ can produce narrow or wide spectral distributions depending on the crystallization field strength, so they are of great importance. In the past, chromium-activated NIR fluorescent powders were mainly prepared by high-temperature solid-state reaction, which is basically a high-temperature and energy-consuming process. Enterprises need to develop new low-cost processes.

Traditionally, the research on luminescent materials is mainly focused on fluorescent powders, followed by thin films and single crystals. There have been many studies on the factors affecting the properties of powders on luminescence performance, but there is still a lack of research on fluorescent fibers. The powder properties of fluorescent powders will affect their luminescence characteristics. Powders are usually prepared by solid-state reaction or sol-gel methods. Since they need to be ground again, it is easy to scratch the surface of the powder and reduce the luminescence intensity. It is also easy to cause pollution and reduce purity. If there are segregated heterogeneous phases or agglomerations in the powder, the luminescence intensity and stability will also be reduced.

In recent years, the trend of "light, thin, short and small" light-emitting components has become popular. The most important key component is the fluorescent material. Therefore, if a fluorescent fiber with a homogeneous single phase and no need for grinding can be developed, it can not only improve the intensity and stability of light emission, but also be applied to micro light-emitting components. In addition, the current commercial GaN blue light chip has the best light-emitting efficiency. If it can generate broadband near-infrared light radiation by blue light excitation, it will be more suitable for commercial applications.

The composition of fluorescent materials mainly includes the host lattice and activators. Different activators have specific electronic energy levels and can be used to adjust the luminescent wavelength of the host. Among them, one of the key technologies that has yet to be overcome is to develop a host material with chemical stability, thermal stability and high luminescence efficiency.

Mg ₂ SiO ₄ is an orthosilicate ceramic material belonging to the olivine family. It has a wide band gap (~4.65eV), a high melting point (1890℃), a high resistivity (＞10 ¹⁷ Ω∙cm, 25℃), a low dielectric constant (6‒7), high frequency and low power loss, chemical stability, thermal stability, biocompatibility and good fracture toughness (K _IC ~2.4 MPa∙m ^1/2 ), etc. It is suitable for microwave and high-frequency components, high-temperature insulators, semiconductor substrates and packaging materials, fuel cell gaskets and biomedical bone materials. In the Mg ₂ SiO ₄ structure, Mg ²⁺ ions are located in octahedral coordination and Si ⁴⁺ ions occupy tetrahedral interstitial positions.

Existing magnesium olivine: Cr ³⁺ fluorescent powders are prepared by solid state method. For example, "Japanese Journal of Applied Physics, vol.58 (2019), p.SFFD02-1−SFFD02-4" discloses that Mg ₂ SiO ₄ :Cr ³⁺ powder can be prepared by calcining mixed oxides at 1250℃ for 5 hours. The luminescence spectrum is 650‒1100 nm, with peaks at 810 and 1190 nm, but the powder has the problem of MgO segregation.

There have been reports on the preparation of Mg ₂ SiO ₄ :Cr single crystal materials for use in laser Q switches. For example, "Applied Physics Letters, vol.53 (1988), p. 2593‒2595" disclosed the preparation of Mg ₂ SiO ₄ :0.04 at% Cr single crystal materials by Czochralski method. When excited at a wavelength of 532 nm, it produced a 1200‒1250 nm emission spectrum with a peak at 1235 nm and a half-maximum width (FWHM) of 22 nm. Another example is "Applied Physics Letters, vol.53 (1988), p.2590‒2592" disclosed the preparation of Mg ₂ SiO ₄ : 2 at% Cr single crystal materials by laser heated susceptor growth (LHPG) method. Cr single crystal material, when excited at a wavelength of 1064 nm, produces a 1200‒1260 nm emission spectrum, with a peak at 1221 nm and a half-maximum width (FWHM) of 28 nm. The aforementioned single crystal material still has the problems of high cost and narrow-band near-infrared light.

The sol-gel method has been used to prepare undoped Mg 2 SiO 4 powder and fluorescent powder. For example, "Journal of Sol-Gel Science and Technology, vol.13(1998), p. 359-364" revealed that undoped Mg ₂ SiO ₄ powder can be prepared by the sol-gel method using magnesium silicate as raw material and ethylene glycol monomethyl ether as gelling agent. Crystallization begins after calcination at 750℃, and MgO segregation phase will precipitate at 1200℃. For example, "Chemistry of Materials, vol. 9 (1997), p.2567‒2576" discloses that silanol salt and magnesium methoxide are used as raw materials, hydrolyzed with hydrogen peroxide, and sintered at 800℃ to begin crystallization. At 1000℃, magnesium olivine film can be prepared, but there is MgO segregation phase; for example, "Materials Letters, vol.9(1990), p.405‒409" discloses that Mg ₂ SiO ₄ powder can be synthesized by sol-gel method using silanol salt and magnesium nitrate as raw materials, but MgSiO ₃ will be segregated when the sintering temperature is ≥1200℃; for example, "Journal of Rare Earths，vol.39(2021), p.1181–1186” discloses that Mg ₂ SiO ₄ :Mn ²⁺ powder can be prepared by the sol-gel method using magnesium nitrate and silanol as raw materials. Crystallization begins after calcination at 800°C, and MgSiO ₃ will segregate at 1300°C.

The sol-gel method has also been used to prepare magnesium olivine:Cr powder. For example, "Chemistry of Materials, vol.5(1993), p.518‒524" discloses that magnesium methoxide and silicon ethoxide are used as raw materials and hydrolyzed with hydrogen peroxide. After calcination at 1000℃, Mg ₂ SiO ₄ :Cr ³⁺ powder can be prepared, but MgCr ₂ O ₄ will be segregated. The powder is also grown into single crystal material by Czochralski method. After 514.5 nm excitation, it emits near-infrared light of 800‒1400 nm, with peaks at 900 and 1100 nm, but white silica (SiO ₂ ), pyrophyllite (MgSiO ₃ ), magnesium oxide (MgO) and magnesium chromate (MgCr ₂ O ₄ ) and other heterogeneous phases.

For example, "Chemistry of Materials, vol.12(2000), p.1378‒1385" reveals that magnesium silicon methoxide and chromium chloride (Cr=0.01‒1%) are used as raw materials, hydrochloric acid is used as a catalyst, and crystallization begins after calcination at 850℃ for 6 hours. Mg ₂ SiO ₄ :Cr ³⁺ powder can be prepared after calcination at 1000℃ for 6 hours. When excited at a wavelength of 670nm or 1064nm, it can produce a 1050‒1400nm radiation spectrum with a peak at 1158 nm.

As mentioned above, the preparation of Mg ₂ SiO ₄ :Cr by solid state method or single crystal growth method has the problems of high temperature energy consumption and high cost, and is easy to produce heterogeneous phases such as MgO, MgSiO ₃ , and MgCr ₂ O ₄ ; it is also difficult to prepare homogeneous single-phase fluorescent powder by solution-gel method, and it takes 750-850℃ to start crystallization, especially above 1000℃, which will produce heterogeneous phases. The main reason may be the difference in hydrolysis rate between heterogeneous precursors, which easily leads to local heterogeneity of colloids, and there is a disadvantage of using toxic gelling agents (such as ethylene glycol methyl ether). This polyol has the disadvantages of causing tumors and affecting genetic mutations or other chronic diseases. In addition, the above methods cannot prepare Mg ₂ SiO ₄ ceramic fibers.

So far, there is no literature or report on near-infrared chromium-activated magnesium olivine (Mg ₂ SiO ₄ :Cr) fluorescent fibers.

Traditionally, ceramic fibers are mainly prepared by melting, which has the disadvantages of high temperature energy consumption, high cost and easy segregation of heterogeneous impurities. The process temperature of ceramic fibers prepared by sol-gel method can be much lower than that of melting method, especially when containing high melting point components (such as magnesium oxide). Sol-gel method has the advantage of reducing process temperature and cost.

For example, "Journal of Non-Crystalline Solids, vol.298(2002), p.116‒130" and "Journal of the European Ceramic Society, vol.23(2003), p.1283‒1291" published that metal alkoxides were used as raw materials, and acetic acid was used to control the hydrolysis and polycondensation reactions of the mixed sol, thereby modifying the molecular structure of the colloid to form a linear polymer. Colloidal fibers (length > 30cm) can be directly extracted from the sol. Mg ₂ SiO ₄ crystals begin to form after sintering at 550℃, and homogeneous single-phase Mg ₂ SiO ₄ is formed at 1500℃. For another example, Patent No. I163847 of the Republic of China discloses a method of preparing Mg ₂ SiO ₄ colloidal fiber by a sol-gel method using magnesium methoxide and silicon ethoxide as precursors, and annealing at 1300°C to obtain Mg ₂ SiO ₄ ceramic fiber with high tensile strength.

However, the sol-gel method mentioned above is still not able to prepare Mg ₂ SiO ₄ :Cr fluorescent fibers.

The general sol-gel method is to form a three-dimensional bulk colloidal structure by preparing a sol and then undergoing a condensation polymerization reaction (i.e., gelation). For the preparation of colloidal fibers, the sol and its colloid must have a linear molecular structure in order to be able to spin fibers. Since the sol has rheological properties, it can be transformed from a Newtonian fluid to a non-Newtonian fluid during the condensation polymerization process, which will cause rapid cross-linking between molecules to form a three-dimensional colloidal structure, making it impossible to spin fibers from the sol. In addition, the process conditions of the sol-gel method, such as solvent concentration and acid-base catalyst, will strongly affect the state and microstructure of the sol and its colloid (see Chemical Reviews, vol.90(1990), p.33–72).

Therefore, if appropriate acid and alkali agents can be added during the process, doping treatment can be properly performed, and process conditions such as precursors and hydrolysis can be controlled so that the activator ions are uniformly dispersed in the sol to obtain a viscous transparent sol, colloidal fibers with good homogeneity can be prepared. The prepared colloidal fibers can then be annealed to produce Mg ₂ SiO ₄ :Cr fluorescent fibers.

As mentioned above, the development of Mg ₂ SiO ₄ :Cr fluorescent fiber process can overcome the problem of traditional fluorescent powder being limited by surface damage caused by grinding treatment, which reduces the luminescence intensity, and avoid contamination. If the fluorescent fiber has high homogeneity, it can effectively enhance the intensity and stability of luminescence, and expand its application in miniature near-infrared light components (such as intravenous syringes, etc.).

At the same time, considering the requirements of commercial applicability, Mg ₂ SiO ₄ :Cr fluorescent fiber must be able to be excited by blue light and produce broadband near-infrared light radiation.

Therefore, how to use the sol-gel method with low process temperature and low equipment cost to prepare high homogeneity near-infrared chromium-activated magnesium olivine fluorescent fibers to solve the problems brought about by the above methods has become the current research goal.

Therefore, the purpose of the present invention is to provide a method for preparing chromium-activated magnesium olivine (Mg ₂ SiO ₄ :Cr) fluorescent fiber with broadband near-infrared radiation. The preparation method utilizes a sol-gel method to prepare chromium-activated magnesium olivine (Mg ₂ SiO ₄ :Cr) fluorescent fiber that can generate high near-infrared radiation intensity and broadband near-infrared radioactivity after being excited by blue light.

Therefore, the preparation method of the near-infrared chromium-activated magnesium olivine fluorescent fiber of the present invention comprises the following steps: (1) preparing a precursor solution, the precursor solution comprising a starting solution and an activator; the starting solution comprises (a) a magnesium source selected from at least one of magnesium alkoxides, (b) a silicon source selected from at least one of silicon alkoxides, (c) an alcohol solvent, and (d) glacial acetic acid, the activator is selected from chromium (III)-containing activators, the molar ratio of the glacial acetic acid to the silicon source is in the range of 0.3-1.2, and the molar ratio of the chromium (III)-containing activator to the silicon source is in the range of 0.5-10%; (2) Deionized water is added to the precursor solution to carry out a hydrolysis reaction to obtain a transparent sol, wherein the molar ratio of the deionized water to the silicon source is in the range of 0.5 to 2; (3) the transparent sol is subjected to a condensation polymerization reaction to obtain a viscous transparent sol; (4) a glass rod is immersed in the viscous transparent sol to directly spin a transparent colloidal fiber; and (5) the transparent colloidal fiber is dried and then annealed at a temperature above 1200° C. to obtain a near-infrared chromium-activated magnesium olivine fluorescent fiber, wherein the empirical formula of the near-infrared chromium-activated magnesium olivine fluorescent fiber is Mg ₂ SiO ₄ :xCr, x=[Cr]/[Si]=0.5~10%.

The efficacy of the present invention lies in that: since the preparation method of the present invention uses a specific magnesium source and a silicon source, it is not necessary to add any gelling agent. Only by controlling the hydrolysis conditions (the molar ratio of the deionized water to the silicon source is in the range of 0.5-2) and using glacial acetic acid to control the hydrolysis and polycondensation reaction of the sol, the molecular structure of the colloid is modified to form a linear polymer, and the spinnability of the viscous transparent sol can be significantly increased. Furthermore, by adding a specific amount of glacial acetic acid (the molar ratio of the glacial acetic acid to the silicon source is in the range of 0.3-1.2) which can also serve as a solvent, the dispersion and homogeneity of the generated sol particles can be greatly improved, so that the formed viscous transparent sol can maintain the dense and uniform distribution of the component molecules, and then the chromium-activated magnesium olivine fluorescent fiber prepared after annealing at a temperature of 1300°C has high homogeneity, no segregation phase, nanostructure and high density, and has the infrared chromium-activated magnesium olivine (Mg ₂ SiO ₄ :Cr) fluorescent fiber that can generate broadband near-infrared light radiation characteristics after being excited by blue light.

The present invention is described in detail below: In this article, the term "chromium-activated olivine fluorescent fiber", or "olivine:Cr" or "Mg ₂ SiO ₄ :Cr" refers to chromium-activated olivine fluorescent fiber prepared by doping with a chromium (III)-containing activator.

Step (1)

The step (1) of the preparation method of the present invention is to prepare a precursor solution, which includes a starting solution and an activator. The starting solution includes (a) a magnesium source selected from at least one of magnesium alkoxides, (b) a silicon source selected from at least one of silicon alkoxides, (c) an alcohol solvent, and (d) glacial acetic acid, and the activator is a chromium (III)-containing activator.

The magnesium alkoxide is, for example but not limited to, magnesium methoxide [Mg(OCH ₃ ) ₂ ] or magnesium ethoxide [Mg(OC ₂ H ₅ ) ₂ ], and the above magnesium alkoxides can be used alone or in combination. In a specific embodiment of the present invention, the magnesium source is magnesium methoxide.

The silanol salt may be used alone or in combination, and the silanol salt may be, for example but not limited to, tetramethoxysilane [Si(OCH ₃ ) ₄ ] or tetraethoxysilane [Si(OC ₂ H ₅ ) ₄ ]. In a specific embodiment of the present invention, the silanol salt is tetraethoxysilane.

The alcohol solvent can be used alone or in combination. Preferably, the alcohol solvent is selected from methanol, ethanol or any combination thereof. In a specific embodiment of the present invention, the alcohol solvent is methanol and ethanol.

The chromium (III)-containing activator is at least one selected from the group consisting of chromium (III) chloride, chromium (III) acetate, and chromium (III) nitrate. In a specific embodiment of the present invention, the chromium (III)-containing activator is chromium (III) chloride.

Preferably, in the step (1), the magnesium source and the silicon source are prepared into a magnesium alkoxide-alcohol solvent solution and a silicon alkoxide-alcohol solvent solution respectively at a molar ratio of 2:1, the magnesium source-alcohol solvent and the silicon source-alcohol solvent solution are mixed and refluxed in a constant temperature water tank to form the starting solution, the reflux time range is 1 to 2 hours, and glacial acetic acid and the activator are added, and the precursor solution is formed after stirring and reacting. More preferably, in step (1), the concentration of the magnesium source solution is 20-22wt%, and the concentration of the silicon source solution is 60-65wt%; more preferably, in step (1), the stirring time is 2-6 hours in an environment with a temperature range of 0-5°C and a relative humidity range of 50-85%. In a specific embodiment of the present invention, the magnesium source-alcohol solvent and the silicon source-alcohol solvent solution are magnesium methoxide-methanol solution and tetraethoxysilane-ethanol solution, respectively, the reflux condensation time is 2 hours, and the stirring time is 4 hours.

Preferably, the molar ratio of the acetic acid to the silicon source (A=CH ₃ COOH/Si) is in the range of A=0.3-1.2. If the molar ratio A of the acetic acid to the silicon source is less than 0.3 or greater than 1.2, the prepared sol has low transparency and no spinnability. In a specific embodiment of the present invention, the molar ratio of the acetic acid to the silicon source is 0.5-1.0.

Preferably, the molar ratio of the chromium (III)-containing activator to the silicon source is in the range of 0.5-10%. More preferably, the molar ratio of the chromium (III)-containing activator to the silicon source is in the range of 1-5%. Most preferably, the molar ratio of the chromium (III)-containing activator to the silicon source is 3%, and the chromium-activated magnesium olivine fluorescent fiber prepared has the highest near-infrared light radiation intensity.

Step (2)

The step (2) of the preparation method of the present invention is to add deionized water to the precursor solution to carry out a hydrolysis reaction to obtain the transparent sol.

Preferably, the molar ratio of the deionized water to the silicon source (r=H ₂ O/Si) ranges from r=0.5 to 2. If the molar ratio of the deionized water to the silicon source is higher than 2, the prepared sol will not be spinnable; if the molar ratio of the deionized water to the silicon source is lower than 0.5, the transparency and spinnability of the prepared sol will be reduced.

More preferably, the molar ratio of the deionized water to the silicon source is in the range of r=0.5-1.0.

Preferably, in step (2), the hydrolysis reaction is carried out for 2 to 4 hours in an environment with a temperature range of 0 to 5°C and a relative humidity range of 50 to 85%. In a specific embodiment of the present invention, the hydrolysis reaction is carried out for 2 hours in an environment below 5°C.

Step (3)

Step (3) of the preparation method of the present invention is to subject the transparent sol to a polycondensation reaction to obtain a viscous transparent sol.

Preferably, in step (3), the polycondensation reaction is carried out at a temperature ranging from 25 to 30° C. and a relative humidity ranging from 35 to 80%. In a specific embodiment of the present invention, the polycondensation reaction is carried out at a temperature of 25° C. and a relative humidity of 55%.

More preferably, in step (3), the reaction time of the polycondensation reaction is in the range of 38 to 54 hours.

Preferably, in step (3), the viscosity of the viscous transparent sol is in the range of 10 ² to 10 ⁴ mPa·s. More preferably, the viscosity of the viscous transparent sol is in the range of 10 ³ to 10 ⁴ mPa·s.

Step (4)

The step (4) of the preparation method of the present invention is to immerse a glass rod in the viscous transparent sol and directly spin the transparent colloidal fibers.

Preferably, the diameter of the glass rod is in the range of 1.5-2.0 mm.

Preferably, in step (4), the spinning is performed at a temperature of 25°C and a relative humidity of 55%.

Step (5)

Step (5) of the preparation method of the present invention is to dry the transparent colloidal fiber and then perform annealing treatment at a temperature above 1200°C to obtain near-infrared chromium-activated magnesium olivine fluorescent fiber.

In a specific embodiment of the present invention, the drying temperature is 80° C. and the drying time is 48 hours. Preferably, the length of the dried transparent colloidal fiber is in the range of 3 to 38 cm, and the diameter is in the range of 20 to 200 μm.

Preferably, the annealing treatment in step (5) is performed in an air furnace at a temperature ranging from 1200 to 1300° C. More preferably, the annealing treatment in step (5) is performed in an air furnace at a temperature of 1300° C.

Preferably, in the annealing treatment of step (5), the annealing treatment time ranges from 2 to 6 hours.

Preferably, in the annealing treatment of step (5), the heating rate of the annealing treatment is in the range of 1-5°C/min.

Preferably, the annealing treatment is performed at a temperature of 1300° C. and for a time of 2 hours, and the obtained near-infrared chromium-activated magnesium olivine fluorescent fiber has a nanocrystalline structure.

More preferably, the annealing temperature is 1300°C and the time range is 2-6 hours, and the length of the chromium-activated magnesium olivine fluorescent fiber can reach 30 cm, and the diameter range is 15-150 μm; after being excited by blue light with a wavelength of 470 nm, it can generate significant broadband near-infrared light radiation of 800-1400 nm, with a half-maximum bandwidth (FWHM) of 286 nm and peaks of 956 nm and 998-1018 nm. More preferably, when the concentration of the chromium (III) activator [Cr] = 3% and the annealing time is 6 hours, the chromium-activated magnesium olivine fluorescent fiber has the best near-infrared light emission intensity.

The present invention will be further described with reference to the following embodiments, but it should be understood that these embodiments are only for illustrative purposes and should not be interpreted as limitations on the implementation of the present invention.

The preparation methods of the preparation examples, comparative preparation examples, embodiments and comparative examples in the following property discussion include at least one step as described below.

Preparation examples and comparative preparation examples

The preparation methods of the preparation examples and comparative preparation examples described below include the following steps: Step (1): In an environment of 0-5°C, 0.4 mol of magnesium methoxide and 0.2 mol of tetraethoxysilane are dissolved in methanol (alcohol solvent) and ethanol (alcohol solvent) respectively, and the magnesium methoxide-methanol solution and the tetraethoxysilane-ethanol solution are mixed and refluxed in a constant temperature water tank to form the starting solution, and the reflux condensation time range is 2 hours, and glacial acetic acid and the activator are added, and the precursor solution is formed after stirring and reacting. The concentration of the magnesium methoxide solution is 20-22wt%, and the concentration of the tetraethoxysilane solution is 60-65wt%; the stirring time range is 4 hours. Activator concentration [Cr] = 1% (i.e., the molar ratio of chromium chloride to tetraethoxysilane × 100%), acetic acid concentration (A) A = 0.1~1.3 (i.e., the molar ratio of acetic acid to tetraethoxysilane). Step (2): Add deionized water to the precursor solution for hydrolysis reaction, and perform the hydrolysis reaction at 0~5°C and a relative humidity range of 50%~85% for 2 hours to obtain a transparent sol; hydrolysis concentration (r) r = 0.3~10 (i.e., the molar ratio of deionized water to tetraethoxysilane). Step (3): The transparent sol is subjected to polycondensation reaction at a temperature of 25°C and a relative humidity of 55% to obtain a viscous transparent sol. Step (4): A glass rod is immersed in the viscous transparent sol to directly spin the colloid fiber. Step (5): The transparent colloid fiber is dried at a temperature of 80°C for 48 hours to obtain the dried transparent colloid fiber.

Preparation of Examples and Comparative Examples

The preparation methods of the embodiments and comparative examples described below include the following steps: Step (1): In an environment of 0-5°C, 0.4 mol of magnesium methoxide and 0.2 mol of tetraethoxysilane are dissolved in methanol (alcohol solvent) and ethanol (alcohol solvent) respectively, the magnesium methoxide-methanol solution and the tetraethoxysilane-ethanol solution are mixed and refluxed in a constant temperature water tank for 2 hours to form the starting solution, and glacial acetic acid and the activator are added, and the precursor solution is formed after stirring for 4 hours. The concentration of magnesium methoxide solution is 20-22wt%, the concentration of tetraethoxysilane solution is 60-65wt%; the concentration of activator [Cr] = 0~10% (i.e., the molar ratio of chromium chloride to tetraethoxysilane × 100%), and the concentration of glacial acetic acid A = 0.5~1.0 (i.e., the molar ratio of glacial acetic acid to tetraethoxysilane). Step (2): Add deionized water to the precursor solution for hydrolysis reaction, and perform the hydrolysis reaction at 0~5°C and a relative humidity range of 50%~85% for 2 hours to obtain a transparent sol; the hydrolysis concentration r = 0.5~2.0 (i.e., the molar ratio of deionized water to tetraethoxysilane). Step (3): The transparent sol is subjected to polycondensation reaction at a temperature of 25°C and a relative humidity of 55% to obtain a viscous transparent sol. Step (4): A glass rod is immersed in the viscous transparent sol to directly spin the colloid fiber. Step (5): The transparent colloid fiber is dried at a temperature of 80°C for 48 hours to obtain the dried transparent colloid fiber. Step (6): The dried transparent colloid fiber is annealed at a temperature of 400-1300°C for 2-6 hours and then cooled to room temperature to obtain a fiber product.

Among them, the process treatment and annealing conditions of Examples 1 to 16 and Comparative Examples 1 to 9 are summarized in the following summary table.

Summary Summary Hydrolysis concentration r=H ₂ O/Si Glacial acetic acid concentration A=CH₃COOH/Si Activator concentration [Cr] (%) Annealing temperature(℃) Annealing time (h) Heating rate (℃/min) Embodiment 1 0.5 1 0.5 1300 2 3 Embodiment 2 1 0.5 1 1300 2 3 Embodiment 3 2 0.5 2 1300 2 3 Embodiment 4 2 0.5 3 1300 2 3 Embodiment 5 0.5 1 1 1300 2 3 Embodiment 6 0.5 1 2 1300 2 3 Embodiment 7 0.5 1 3 1300 2 1 Embodiment 8 0.5 1 3 1300 2 3 Embodiment 9 0.5 1 3 1300 6 3 Embodiment 10 0.5 1 4 1300 2 3 Embodiment 11 0.5 1 6 1300 2 3 Embodiment 12 0.5 1 8 1300 2 3 Embodiment 13 0.5 1 10 1300 2 3 Embodiment 14 1 1 3 1300 2 3 Embodiment 15 2 1 3 1300 2 3 Embodiment 16 0.5 1 3 1300 2 5 Comparison Example 1 0.5 1 3 400 2 3 Comparison Example 2 0.5 1 3 500 2 3 Comparison Example 3 0.5 1 3 600 2 3 Comparison Example 4 0.5 1 3 800 2 3 Comparison Example 5 0.5 1 3 1000 2 3 Comparison Example 6 0.5 1 3 1100 2 3 Comparative Example 7 0.5 1 3 1200 2 3 Comparative Example 8 0.5 1 0 1300 2 3 Comparative Example 9 0.5 1 3 ― ― ― Note 1: Hydrolysis concentration r = H ₂ O/Si, which is the molar ratio of deionized water to tetraethoxysilane. Note 2: Glacial acetic acid concentration A = CH₃COOH/Si, which is the molar ratio of glacial acetic acid to tetraethoxysilane. Note 3: Activator concentration [Cr] (%), which is the molar ratio of chromium chloride to tetraethoxysilane × 100%. Note 4: The transparent colloidal fiber of Comparative Example 9 was not annealed, but only dried at 80°C for 48 hours.

Appearance of viscous sol and colloidal fiber (r= _H2O /Si=0.3~10), A=CH₃COOH/Si=0.1~1.3)

Preparation Examples 1 to 8 and Comparative Preparation Examples 1 to 10 were carried out according to the above steps (1) to (5) and the molar ratio of deionized water to tetraethoxysilane (r= _H2O /Si), the molar ratio of glacial acetic acid to tetraethoxysilane (A=CH₃COOH/Si), and the concentration of the activator ([Cr]% = the molar ratio of chromium chloride to magnesium methoxide × 100%) listed in Table 1 below. The appearance of the viscous sols prepared in Preparation Examples 1 to 8 and Comparative Preparation Examples 1 to 10 was observed, and the length and appearance of the prepared colloid fibers after drying at 80°C for 48 hours were recorded. If colloid fibers could not be prepared, the appearance of the colloid was recorded. The results are summarized in Table 1 below.

Table 1 Table 1 Hydrolysis concentration Glacial acetic acid concentration A Activator concentration [Cr] (%) Viscous Sol Appearance Colloidal fiber length (cm) Colloid or colloid fiber appearance Comparative Preparation Example 1 0.3 0.5 1 Turbid ― White opaque colloid Comparative Preparation Example 2 0.4 0.5 1 Turbid ― White opaque colloid Comparative Preparation Example 3 0.5 0.1 1 Turbid ― White opaque colloid Comparative Preparation Example 4 0.5 0.2 1 Turbid ― White opaque colloid Comparative Preparation Example 5 0.5 1.3 1 Turbid ― White opaque colloid Comparative Preparation Example 6 1 0.4 1 Turbid ― Translucent colloid Comparative Preparation Example 7 2 0.4 1 Turbid ― Translucent colloid Comparative Preparation Example 8 3 0.5 1 translucent ― Translucent colloid Comparative Preparation Example 9 5 0.5 1 translucent ― Translucent colloid Comparative Preparation Example 10 10 1 1 translucent ― Translucent colloid Preparation Example 1 0.5 0.3 1 Clear 3 transparent Preparation Example 2 0.5 0.5 1 Clear 18 transparent Preparation Example 3 0.5 1 1 Clear 38 transparent Preparation Example 4 0.5 1.2 1 Clear 10 transparent Preparation Example 5 1 0.5 1 Clear 20 transparent Preparation Example 6 1 1 1 Clear 32 transparent Preparation Example 7 2 0.5 1 Clear 10 transparent Preparation Example 8 2 1 1 Clear twenty two transparent Note 1: Hydrolysis concentration r = H ₂ O/Si, which is the molar ratio of deionized water to tetraethoxysilane. Note 2: Glacial acetic acid concentration A = CH₃COOH/Si, which is the molar ratio of glacial acetic acid to tetraethoxysilane. Note 3: Activator concentration [Cr] (%), which is the molar ratio of chromium chloride to tetraethoxysilane × 100%. Note 4: "-" indicates no spinnability

From the results in Table 1, it can be seen that when the hydrolysis concentration [i.e., the molar ratio of deionized water to tetraethoxysilane (r = H ₂ O/Si)] is r = 0.5~2.0 and the glacial acetic acid concentration [i.e., the molar ratio of glacial acetic acid to tetraethoxysilane (A = When r=0.5~2, the appearance of the colloid fibers prepared was clear and transparent. When r=0.5~1.0 and A=0.5~1.0, the viscous sol prepared had better spinnability and the length of the colloid fibers was longer. The concentration of glacial acetic acid will significantly affect the state and spinnability of the sol. When the concentration of glacial acetic acid A=0.3~1.2, increasing the concentration of glacial acetic acid will slow down the gelation rate, and the viscous sol will be clear and spinnable; but when the concentration of glacial acetic acid A is lower than 0.3 or higher than 1.2, the viscous sol obtained will be turbid and not spinnable; if the hydrolysis concentration r＜0.5, the sol will be turbid and not spinnable due to incomplete hydrolysis. When r＞2.0, the gelation rate will increase, and the viscous sol obtained will not be spinnable and will only be a translucent colloid (r=3~10).

It should be noted that after drying, the cross-section of the prepared colloid fiber can be circular or elliptical, with a length of 3 to 38 cm and a diameter of about 20 to 200 μm. The diameter of the colloid fiber is determined by the speed of spinning the fiber. Increasing the speed of spinning the fiber will make the diameter of the colloid fiber smaller.

Rheology analysis of sol

In Preparation Example 3 and Comparative Preparation Example 9, the sols were prepared according to the above steps (1) to (3), with the hydrolysis concentration (r) being 0.5 and 5, the glacial acetic acid concentration (A) being 1 and 0.5, and the activator concentration being [Cr]=1%. The relationship between the viscosity and the shear rate of the sols of Preparation Example 3 and Comparative Preparation Example 9 at 25°C and their changes with the polymerization reaction time were measured using a viscometer and a rheometer (Brookfield; Model: DVII+; R/S Rheometer). The results are shown in FIG1 (Preparation Example 3, r=0.5, A=1, [Cr]=1%) and FIG2 (Comparative Preparation Example 9, r=5, A=0.5, [Cr]=1%).

As shown in Figure 1, in Preparation Example 3 (r=0.5, A=1), when the polycondensation reaction time is 22-52.0 hours, the viscosity of the prepared sol gradually increases, and the viscosity of the sol is independent of the shear rate, indicating that the sol has Newtonian fluid properties. When the polycondensation reaction time is 53.2 hours, the viscosity of the prepared sol slowly decreases with the increase of the shear rate, and then the sol can recover the original viscosity value as the shear rate decreases, showing a nearly Newtonian fluid property. When the reaction time is 52.0 to 53.2 hours, the prepared sol has good spinnability. When the reaction time reaches 53.8 hours, the viscosity of the prepared sol decreases significantly with the increase of shear rate and is irreversible (that is, the viscosity of the sol cannot be restored by further reducing the shear rate), indicating that the sol behaves as a non-Newtonian fluid, lacks spinnability, and instantly gels into a mass. The above results show that when the sol is a Newtonian or near-Newtonian fluid, it can be directly spun from the sol to produce colloidal fibers. When the glacial acetic acid concentration A=0.3~1.2 and the hydrolysis concentration r=0.5~2, similar results are obtained. It should be noted that the spinnable sol has spinnability when the viscosity is 10 ² ~10 ⁴ mPa.s, and has better spinnability when the viscosity is 10 ³ ~10 ⁴ mPa.s.

As shown in Figure 2, in Comparative Preparation Example 9 (r=5, A=0.5), when the polycondensation reaction time is 6~9.2 hours, the prepared sol is in a low-viscosity fluid state. When the polycondensation reaction time is 9.5~9.7 hours, the prepared sol behaves as a non-Newtonian fluid and gels into a mass instantly, without spinnability. When the hydrolysis concentration is lower than 0.5 (r=0.3~0.4) or higher than 2 (r=3~10) and the glacial acetic acid concentration is lower than 0.3, similar results are obtained. When the glacial acetic acid concentration is higher than 1.2, the prepared sol is turbid and also without spinnability.

Fourier transform infrared spectroscopy (FT-IR) analysis

The fiber or colloid products prepared in Preparation Examples 3 and 7 and Comparative Preparation Example 3 were ground and pressed with KBr, and then the transmittance in the wave number range of 400~2000cm ^-1 was measured using a Fourier transform infrared spectrometer (manufacturer: Varian; model: 2000 FT-IR). The obtained spectrum is shown in FIG3 .

From Figure 3, we can see that there is a significant difference around the wave number of 1500 cm ^-1 . Preparation Example 3 has clear paired absorption peaks at 1595cm ^‒1 , 1536cm ^‒1 and 1442cm ^‒1 , which can be attributed to the formation of a double-base acetate ligand. 1595cm ^‒1 and 1442cm ^‒1 are the antisymmetric and symmetric vibrations of the COO‒ functional group (i.e., ν _asym (COO) and ν _sym (COO)), respectively, with a frequency spacing of Δν = 153cm ^‒1 , indicating that the acetate ion can be used as a bridging ligand. The frequency spacing between 1536cm ^‒1 (ν _asym (COO) and 1442cm ^‒1 (ν _sym (COO)) is Δν = 94 cm ^‒1 indicates the formation of chelate acetate ligands. The absorption peaks at 1686cm ^‒1 and 1348cm ^‒1 may be due to residual acetic acid or the formation of monobasic acetate ligands, and the absorption peak at 1635cm ^‒1 is mainly due to the adsorption of water vapor.

Preparation Example 7 shows weak ν _asym (COO) and ν _sym (COO) characteristic waves at 1578cm ^‒1 and 1438cm ^‒1 , respectively. The frequency interval of this pair of absorption peaks is Δν = 140cm ^‒1 , indicating the formation of acetate bridge ligands. Compared with Preparation Example 3, which lacks the characteristics of the double-base acetate ligand, the absorption peaks at 1484cm ^‒1 , 1423cm ^‒1 and 1386cm ^‒1 are attributed to the C‒H vibration of the alkyl functional group, indicating that the -OR (R is an alkyl) alkoxy group still remains in the colloid; in contrast, the alkoxy groups in Preparation Examples 3 and 7 can be replaced by acetate ligands.

In general, metal alkoxides (M(OR) _n ), where M is a metal, R is an alkyl group ( _{CxH2x +1} ), and n is the metal valence, can undergo a hydrolysis reaction to replace some of the -OR functional groups of the metal alkoxide with -OH groups of water molecules to form partial hydrolysis products, such as M(OH) _x (OR) _nx , and then undergo a condensation polymerization reaction (i.e., gelatinization) to form a bridging bond oxygen structure, such as M-O-M, through the interaction between the -OH and -OR functional groups between the partial hydrolysis products, by utilizing condensation and dehydration and condensation and dealcoholization reactions, thereby forming a colloid with a three-dimensional network structure.

When an appropriate amount of glacial acetic acid is added, the intensity of the residual -OR group decreases, indicating that part of the alkoxy group is replaced by acetate anions (CH ₃ COO ^‒ , or OAc for short), so that soluble molecular substances and dibasic acetate ligands are formed during the hydrolysis and condensation process. In other words, an appropriate amount of glacial acetic acid can react with part of the hydrolysis product to produce a new complex, such as M(OR) _x (OAc) _y , where M is Si or Mg. Due to the interaction between acetate ions and alkoxy groups, the hydrolysis and polycondensation reactions of the sol are delayed, thereby reducing the gelation rate.

Infrared spectra show that acetate ions can play the role of ligands and modify the molecular structure of the colloid to form linear condensation species, such as bridged acetate ligands of M←O-C(CH ₃ )-O-M (M is Si or Mg) (such as Preparation Examples 3 and 7), which is very different from the cross-linked Si-O-Mg network structure (such as Comparative Preparation Example 3). In other words, adding an appropriate amount of glacial acetic acid can inhibit the hydrolysis and condensation reactions of the sol and change its rheological behavior, thereby improving the pumpability of the sol. In addition, it can be observed from FIG3 that the absorption intensity of the diploid acetate ligand in Preparation Example 3 (r=0.5, A=1) is much higher than that in Preparation Example 7 (r=2, A=0.5), indicating that increasing the concentration of glacial acetic acid and reducing the hydrolysis concentration can increase the concentration of diploid acetate ligands, thereby significantly improving the spinnability of the sol and producing longer colloidal fibers. Similar results are obtained when r=0.5~2 and A=0.3~1.2.

When the hydrolysis concentration r is higher than 2 and the acetic acid concentration A is lower than 0.3, the sol has a stronger tendency to hydrolyze and condense, and it is easy to form a cross-linked Si-O-Mg network structure, and the sol is not spinnable. When the acetic acid concentration A is higher than 1.2, the excess acetic acid will precipitate Mg(OH) _2, causing the sol to be turbid and difficult to gel, and not spinnable.

X-ray diffraction (XRD) analysis

The fiber product was subjected to X-ray diffraction analysis using an X-ray diffraction instrument (manufacturer: Bruker; model: D8 Advance). The obtained X-ray diffraction patterns are shown in FIGS. 4 and 5 .

As shown in FIG. 4 , the fiber product obtained when the annealing temperature is lower than 500°C (Comparative Example 1) is an amorphous phase. When the annealing temperature is 500°C (Comparative Example 2), magnesia olivine crystals (Mg ₂ SiO ₄ ; JCPDS Card no. 34-189) begin to form. When the annealing temperature is 600°C to 1300°C (Comparative Examples 3 to 7 and Example 8), single-phase Mg ₂ SiO ₄ crystals are formed, indicating that the preparation method of the present invention can synthesize homogeneous single-phase Mg ₂ SiO ₄ after annealing at 500-1300°C.

As shown in Figure 5, when the hydrolysis concentration r = 0.5 ~ 2, the acetic acid concentration A = 0.5 ~ 1, and the doping activator concentration [Cr] = 0.5 ~ 10%, the fiber products obtained by annealing at 1300 ° C are all homogeneous single-phase magnesian olivine structures, and no other phases exist, indicating that the chromium ions in the activator can be dissolved in the main lattice of Mg ₂ SiO _4. According to the Scherrer equation, the effects of different hydrolysis concentrations, acetic acid concentrations, and activator concentrations on the Mg ₂ SiO ₄ grain size are compared, and the results are summarized in Table 2 below.

Table 2 Table 2 Hydrolysis concentration Glacial acetic acid concentration A Activator concentration [Cr] (%) Average grain size of Mg ₂ SiO ₄ (nm) Annealing temperature(℃) Annealing time (hours) Comparative Example 8 0.5 1 0 64.8 1300 2 Embodiment 1 0.5 1 0.5 64.5 1300 2 Embodiment 2 1 0.5 1 64.0 1300 2 Embodiment 3 2 0.5 2 63.4 1300 2 Embodiment 4 2 0.5 3 63.0 1300 2 Embodiment 8 0.5 1 3 62.8 1300 2 Embodiment 10 0.5 1 4 62.5 1300 2 Embodiment 11 0.5 1 6 61.6 1300 2 Embodiment 12 0.5 1 8 61.4 1300 2 Embodiment 13 0.5 1 10 61.2 1300 2 Embodiment 14 1 1 3 62.8 1300 2 Embodiment 15 2 1 3 63.0 1300 2

According to Table 2, after annealing at 1300°C, when r=0.5~2, A=0.5~1, and activator concentration [Cr]=0~10% (Comparative Example 8 and Examples 1~4, 8, and 10~15), the average grain size of Mg ₂ SiO ₄ is about 64.8~61.2 nm, and the hydrolysis concentration or glacial acetic acid concentration has no significant effect on the grain size of Mg ₂ SiO ₄ , but increasing the activator concentration will significantly reduce the grain size of Mg ₂ SiO _4. The above results show that the transparent colloidal fiber of the present invention can be annealed at 1300°C to obtain a nanocrystalline and homogeneous single-phase Mg ₂ SiO ₄ :Cr fluorescent fiber.

It should be noted that when the annealing temperature is 1300°C and the annealing time is 6 hours (Example 9), the diffraction peak intensity of Mg ₂ SiO ₄ is higher than that of the fiber product obtained when the annealing time is 2 hours (Example 8), indicating that the fiber product obtained by increasing the annealing time has higher crystallinity.

Scanning Electron Microscope (SEM) analysis

The fiber products obtained in Comparative Example 9 (r=0.5, A=1, [Cr]=3%) and Example 8 (r=0.5, A=1, [Cr]=3%) were photographed using a scanning electron microscope (manufacturer: Hitachi; model: S-4800-I). The obtained SEM microstructures are shown in FIGS. 6 to 9 , respectively.

As shown in Figures 6 and 7, the surface and cross section of the fiber product obtained after drying at 80°C for 48 hours (Comparative Example 9) are both dense, and the dried transparent colloidal fibers all have similar shapes.

As shown in Figures 8 and 9, the transparent colloidal fiber obtained by drying at 80°C for 48 hours and then annealing at 1300°C for 2 hours (Example 8) has a dense surface and cross-section, an average grain size of about 68nm, and a nanocrystalline microstructure. It should be noted that similar results can be obtained for fiber products obtained with different r values, A values, or activator concentrations ([Cr]%), and increasing the annealing time will only slightly increase the average grain size of the fiber products, and they all have a nanocrystalline structure, and their surfaces and cross-sections are also dense.

It should be noted that the fiber products obtained after drying and annealing at 1300°C have a fiber length of about 2.5~30cm and a diameter of about 15~150μm, and all have similar microstructures.

X-ray photoelectron spectroscopy (XPS) analysis

The fiber product of Example 8 was measured for the confinement energy and intensity of Cr 2P ³ electrons using an X-ray photoelectron spectrometer (manufacturer: ULVAC-PHI; model: PHI 5000 VersaProbe). The measurement results were first fitted and then plotted using Gaussian simulation. The results are shown in FIG. 10 .

From the Gaussian simulation curve in FIG10 , it can be clearly observed that the characteristic peaks of Cr ³⁺ ions and Cr ⁴⁺ ions at 2P _3/2 and 2P _1/2 are present at the binding energies of 576.2 eV and 585.0 eV and 580.6 eV and 589.3 eV, respectively, indicating that the chromium (III)-containing activator presents a coexistence state of Cr ³⁺ and Cr ⁴⁺ ions in the host lattice.

Fluorescence spectroscopy analysis

The fiber products were analyzed using a fluorescent spectrometer (manufacturer: Edinburgh, model: FLS 980) with a wavelength scanning range of 200~1400nm. The results are shown in Figures 11~12.

Analysis Method 1: Photoluminescence Analysis

The excitation spectrum of the photoluminescence of the fiber product prepared in Example 8 (i.e., r=0.5, A=1, activator concentration [Cr]=3%, annealing at 1300°C for 2 hours) was measured using an FLS 980 fluorescence spectrometer in the wavelength range of 200-800nm ( FIG. 11 ), and the luminescence spectrum of the photoluminescence of the fiber product prepared in Example 8 was measured using an FLS 980 fluorescence spectrometer in the wavelength range of 700-1400nm ( FIG. 12 ).

From the excitation spectrum of FIG11 , it can be found that the fiber product of Example 8 has obvious excitation peaks at wavelengths of 470 nm and 663 nm, which are the transitions of electrons in the ³ A ₂ → ³ T ₁ ( ³ F) and ³ A ₂ → ³ T ₂ ( ³ F) energy levels of Cr ⁴⁺ ions, respectively. The weak absorption near 309 nm is attributed to the transition of ³ A ₂ → ³ T ₁ ( ³ P) of Cr ⁴⁺ ions.

After the fiber product of Example 8 is excited by blue light with a wavelength of 470nm (λ _ex = 470nm), the emission spectrum of Figure 12 will obtain a wideband near-infrared light emission band of 800‒1400nm, with peaks at 956 and 998‒1018nm, which is caused by the transition of ³ T ₂ → ³ A ₂ energy level of Cr ⁴⁺ ions. This is related to the chromium ion lattice coordination and the crystal field strength. According to the crystal field theory, if Cr ³⁺ ions are located in octahedral coordination, a strong crystal field can be formed. The first excited state is ² E ( ² G), the ground state is ⁴ A ₂ ( ⁴ F), and the electronic configuration is t ³ _2g . The emission spectrum will show ² E→ ⁴ A ₂ emission light (i.e., typical R-line) with the same electronic configuration and different spin multiplets. Since it is a spin-forbidden transition, it will show a narrow-band or linear emission spectrum. The peak of the typical R-line is located near 690nm. As mentioned above, the preparation method of the present invention is that Cr ³⁺ and Cr ⁴⁺ ions coexist in the main lattice, so Cr ⁴⁺ ions can replace part of Si ⁴⁺ solid solution in tetrahedral coordination, which basically presents a weak crystal field. The first excited state is ³ T ₂ ( ³ F), the electronic configuration is t _2g e _g , the ground state is ³ A ₂ ( ³ F), the electronic configuration is e ² _g , with different electronic configurations, when the ³ T ₂ → ³ A ₂ energy level transition occurs, it has the same spin multiplets and different electronic configuration characteristics, so it can present 800‒1400 nm wide-band near-infrared light emission characteristics due to spin-allowed transition.

Analysis method 2: Luminescence intensity analysis

The relative emission intensity (normalized emission intensity) of the fiber products prepared in Examples 1, 5-6 and 8-13 (i.e., r=0.5, A=1, activator concentration [Cr]=0.5-10%, annealing at 1300°C for 2-6 hours) at the main emission peak (λ _em ) was measured using the fluorescence spectrometer. The results are shown in Table 3.

Table 3 Table 3 Activator concentration [Cr] (%) Relative intensity (λ _em =956nm) Annealing temperature(℃) Annealing time (hours) Embodiment 1 0.5 0.77 1300 2 Embodiment 5 1 0.83 1300 2 Embodiment 6 2 0.87 1300 2 Embodiment 8 3 0.94 1300 2 Embodiment 9 3 1 1300 6 Embodiment 10 4 0.81 1300 2 Embodiment 11 6 0.67 1300 2 Embodiment 12 8 0.4 1300 2 Embodiment 13 10 0.23 1300 2

From Table 3, it can be found that the fiber product (Example 9) produced when the activator concentration [Cr] = 3% and the annealing time is 6 hours has the best near-infrared light emission intensity. It should be noted that regardless of the activator concentration, the fiber product produced has a broadband near-infrared light emission characteristic of 800‒1400nm.

In summary, since the preparation method of the present invention uses a specific magnesium source and a silicon source, it is not necessary to add any gelling agent. Only by controlling the hydrolysis conditions (the molar ratio of the deionized water to the silicon source is in the range of 0.5-2) and using glacial acetic acid to control the hydrolysis and polycondensation reaction of the sol, the acetate ion can play the role of a ligand and modify the molecular structure of the colloid to generate a new complex, so that it forms a linear polycondensation species, such as a bridged acetate ligand of M←O-C(CH ₃ )-O-M (M is Si or Mg), which significantly increases the spinnability of the viscous transparent sol. Furthermore, by adding a specific amount of glacial acetic acid (the molar ratio of the glacial acetic acid to the silicon source is in the range of 0.3-1.2) which can also serve as a solvent, the dispersion and homogeneity of the generated sol particles can be greatly improved, so that the formed viscous transparent sol can maintain the dense and uniform distribution of the component molecules, and then the chromium-activated magnesium olivine fluorescent fiber prepared after annealing at a temperature of 1300°C has high homogeneity, no segregation phase, nanostructure and high density, and has the infrared chromium-activated magnesium olivine (Mg ₂ SiO ₄ :Cr) fluorescent fiber that can generate broadband near-infrared light radiation characteristics after being excited by blue light.

It is worth mentioning that the fluorescent fiber manufacturing process of the present invention does not require grinding treatment, which can avoid the disadvantage of reducing the luminescence intensity due to surface damage caused by grinding. The preparation method of the near-infrared chromium activated magnesium olivine fluorescent fiber of the present invention produces Mg ₂ SiO ₄ :Cr fluorescent fiber with high homogeneity, nanocrystalline structure and high density. After being excited by blue light, it can generate 800‒1400nm broadband near-infrared light emission band, which can be applied to broadband near-infrared light components and can expand its application in micro-luminescent components. Therefore, the purpose of the present invention can be achieved.

However, the above is only an example of the implementation of the present invention, and it should not be used to limit the scope of the implementation of the present invention. All simple equivalent changes and modifications made according to the scope of the patent application of the present invention and the content of the patent specification are still within the scope of the patent of the present invention.

without

Other features and effects of the present invention will be clearly presented in the embodiments with reference to the drawings, in which:

Figure 1 shows the viscosity and shear rate of a spinnable sol (Preparation Example 3, r=0.5, A=1) at 25°C as a function of different polycondensation reaction times; Figure 2 shows the viscosity and shear rate of a non-spinnable sol (Comparison Preparation Example 9, r=5, A=0.5) at 25°C as a function of different polycondensation reaction times; Figure 3 shows the infrared spectra of the transparent colloid fibers and block colloids obtained in Preparation Examples 3 and 7 and Comparison Preparation Example 3 after drying; Figure 4 is an X-ray diffraction diagram of a transparent colloid fiber obtained by comparing r=0.5, A=1.0 and activator concentration [Cr]=3% after drying at 80°C for 48 hours and then annealing at 400~1300°C (i.e., the fiber products of Comparative Examples 1 to 7 and Example 8); Figure 5 is an X-ray diffraction diagram of a transparent colloid fiber obtained by comparing r=0.5~2, A=0.5~1.0 and activator concentration [Cr]=0.5~10% after drying at 80°C for 48 hours and then annealing at 1300°C (i.e., Examples 1, 2, 4, 11~13); Figures 6 to 7 are SEM microstructures of the transparent colloid fiber obtained with r=0.5 and A=1.0 after drying at 80°C for 48 hours (i.e., the fiber product of Comparative Example 9); Figures 8 to 9 are SEM microstructures of the transparent colloid fiber obtained with r=0.5 and A=1.0 after annealing at 1300°C for 2 hours (i.e., the fiber product of Example 8); Figure 10 is an X-ray photoelectron spectrum of the fiber product obtained in Example 8 (r=0.5, A=1.0, activator concentration [Cr]=3%, annealing at 1300°C for 2 hours); Figure 11 is an X-ray photoelectron spectrum of the fiber product obtained in Example 8 (r=0.5, A=1.0, activator concentration [Cr]=3 % and annealing at 1300℃ for 2 hours); Figure 12 is the radiation spectrum of the fiber product prepared in Example 8 (r=0.5, A=1.0, activator concentration [Cr]=3%, annealing at 1300℃ for 2 hours).

Claims (10)

A method for preparing near-infrared chromium-activated magnesium olivine fluorescent fiber comprises the following steps: (1) preparing a precursor solution, the precursor solution comprising a starting solution and an activator, the starting solution comprising (a) a magnesium source selected from at least one of magnesium alkoxides, (b) a silicon source selected from at least one of silicon alkoxides, (c) an alcohol solvent, and (d) glacial acetic acid, the activator being selected from chromium (III)-containing activators, the molar ratio of the glacial acetic acid to the silicon source being in the range of 0.3-1.2, and the molar ratio of the chromium (III)-containing activator to the silicon source being in the range of 0.5-10%; (2) Deionized water is added to the precursor solution to carry out a hydrolysis reaction to obtain a transparent sol, wherein the molar ratio of the deionized water to the silicon source is in the range of 0.5 to 2; (3) the transparent sol is subjected to a condensation polymerization reaction to obtain a viscous transparent sol; (4) a glass rod is immersed in the viscous transparent sol to directly spin a transparent colloidal fiber; and (5) the transparent colloidal fiber is dried and then annealed at a temperature above 1200° C. to obtain a near-infrared chromium-activated olivine fluorescent fiber, wherein the empirical formula of the olivine fluorescent fiber is Mg ₂ SiO ₄ :xCr, x=[Cr]/[Si]=0.5~10%. A method for preparing near-infrared chromium-activated magnesium olivine fluorescent fiber as described in claim 1, wherein in step (1), the magnesium alkoxide is selected from magnesium methanol, magnesium ethanol or any combination thereof, the silanol salt is selected from tetramethoxysilane, tetraethoxysilane or any combination thereof, the alcohol solvent is selected from methanol, ethanol or any combination thereof, and the chromium (III)-containing activator is selected from at least one of the group consisting of chromium (III) chloride, chromium (III) acetate, and chromium (III) nitrate. The method for preparing near-infrared chromium-activated magnesium olivine fluorescent fiber as described in claim 1, wherein in the step (1), the magnesium alkoxide and the silicon alkoxide are dissolved in methanol (alcohol solvent) or ethanol (alcohol solvent), respectively, wherein the concentration of the magnesium alkoxide solution is 20-22wt%, and the concentration of the silicon alkoxide solution is 60-65wt%. The method for preparing near-infrared chromium-activated magnesium olivine fluorescent fiber as described in claim 1, wherein in step (1), the reaction is carried out at a temperature range of 0-5°C and a relative humidity range of 50-85%. The method for preparing near-infrared chromium-activated magnesium olivine fluorescent fiber as described in claim 1, wherein in the step (2), the hydrolysis reaction is carried out at a temperature range of 0-5°C and a relative humidity range of 50-85%. The method for preparing near-infrared chromium-activated magnesium olivine fluorescent fiber as described in claim 1, wherein in the step (3), the polycondensation reaction is carried out at a temperature range of 25-30°C and a relative humidity range of 35-80%. The method for preparing near-infrared chromium-activated magnesium olivine fluorescent fiber as described in claim 1, wherein in the step (4), the viscosity of the viscous transparent sol is in the range of 10 ² to 10 ⁴ mPa·s. The method for preparing near-infrared chromium-activated magnesium olivine fluorescent fiber as described in claim 1, wherein in the step (5), the length of the dried transparent colloidal fiber ranges from 3 to 38 cm, and the diameter ranges from 20 to 200 μm. The method for preparing near-infrared chromium activated magnesium olivine fluorescent fiber as described in claim 1, wherein the temperature of the annealing treatment in step (5) is 1300°C and the time range is 2-6 hours, and the length of the prepared near-infrared chromium activated magnesium olivine fluorescent fiber can reach 30 cm, the diameter range is 15-150 μm, and it has a nanocrystalline form, high density and a single-phase magnesium olivine structure. The method for preparing near-infrared chromium-activated olivine fluorescent fiber as described in claim 1, wherein the near-infrared chromium-activated olivine fluorescent fiber prepared in step (5) can generate a wideband near-infrared light emission band of 800~1400nm after blue light excitation (λ _ex = 470nm) and the activator concentration [Cr] = 3% has the best luminescence intensity.