JP7640132B2 - Method for producing stress-induced luminescent material - Google Patents

Description

The present invention relates to a stress-stimulated luminescent material and a method for producing the same.

To date, the inventors have invented the world's first stress-induced luminescent material and are developing its industrial applications.

Stimuli-luminescent materials are materials that emit luminescence that correlates with the energy of an external mechanical stimulus, and are expected to be used in a wide variety of applications, including sensors, non-destructive testing, visualization of stress distribution, stress sensing, and detection of abnormalities and dangers in structures.

Therefore, stress-luminescent materials have been developed that can emit light in response to stress at various wavelengths, from ultraviolet to near-infrared wavelengths.

In particular, the present inventors have been conducting intensive research in this technical field day and night and have proposed a wide variety of mechanoluminescent materials, such as an SAO-based mechanoluminescent material that emits green light with high luminance (see, for example, Patent Document 1 and Patent Document 2), a _LiNbO3 -based mechanoluminescent material that is sensitive to even minute stresses and emits light in the visible light range (see, for example, Patent Document 3), and an SSN-based mechanoluminescent material that emits mechanoluminescence in the near-infrared range (see, for example, Patent Document 4).

JP 2001-049251 A International Publication No. 2005/097946 International Publication No. 2018/070072 JP 2015-086327 A

ZnO (zinc oxide) has a wide range of applications, from pigments and paints to catalytic materials and pharmaceuticals, and is used in a variety of fields as a relatively common material in modern times.

ZnO has also been the subject of extensive research in various fields for a long time, and in terms of its physical properties, for example, it is known that it can have a variety of particle shapes and optical properties depending on the material design and synthesis process.

However, no stress-stimulated luminescent materials using ZnO as a host material have been reported to date, and they were thought to be impossible to realize.

The present invention has been made in consideration of the above circumstances, and provides a stress-induced luminescent material using ZnO as a base material.

The present invention also provides a method for producing a stress-stimulated luminescent material using ZnO as a base material, and a method for converting ZnO into a stress-stimulated luminescent material.

In order to solve the above-mentioned problems, the method for producing a mechanoluminescent material according to the present invention comprises the steps of:
( 1 ) A method for producing a stress-stimulated luminescent material using ZnO as a base material, comprising mixing a zinc compound, a luminescence center supplying compound which supplies a luminescence center ion , and a predetermined co-dopant ion supplying compound which supplies a co-dopant ion, and firing the mixture (excluding a mixture containing a solid protective agent which is melted by heating to isolate a reaction system from an oxidizing atmosphere) under an oxidizing atmosphere, wherein the luminescence center ion is an ion of Nd or an ion of Nd and an ion of any one element selected from Cr, Mn, Cu, Ag, Ce, Pr, Sm, Eu, Tb, and Yb, the co-dopant ion is an ion of any one element selected from Li, Na, K, Rb, and Cs, or an ion of Li and Ba, and the stress-stimulated luminescent material has a composition represented by a general formula ZnO:E _x M _y (where E is the co-dopant ion and M is the luminescence center ion), where x=0.0001 to 0.7 and y=0.001 to 0.2 .

In addition, in the method for producing a stress-stimulated luminescent material according to the present invention,
( 2 ) A method for producing a stress-stimulated luminescent material using ZnO as a base material, comprising the steps of: making an aqueous solution containing zinc ions, in which a luminescent center ion and a predetermined co-doped ion are present, alkaline, or reacting the aqueous solution with a sulfide, coprecipitating the co-doped ion and the luminescent center ion with zinc hydroxide or zinc sulfide, and firing the coprecipitate (excluding those containing a solid protective agent which melts by heating to isolate the reaction system from an oxidizing atmosphere) in an oxidizing atmosphere, wherein the luminescent center ion is an ion of Nd or an ion of Nd and any one element selected from Cr, Mn, Cu, Ag, Ce, Pr, Sm, Eu, Tb, and Yb, the co-doped ion is an ion of any one element selected from Li, Na, K, Rb, and Cs, or an ion of Li and Ba, and the stress-stimulated luminescent material has a general formula of ZnO:E _x M _y (wherein E is the co-doped ion and M is the luminescent center ion), x=0.0001 to 0.7, and y=0.001 to 0.2 .

According to a method for producing a stress-stimulated luminescent material of the present invention, a zinc compound, a luminescence center supplying compound which supplies a luminescence center ion , and a predetermined co-doped ion supplying compound which supplies a co-doped ion are mixed , and the mixture (excluding a mixture containing a solid protective agent which is melted by heating to isolate the reaction system from an oxidizing atmosphere) is fired in an oxidizing atmosphere. The luminescence center ion is an ion of Nd or an ion of Nd and any one element selected from Cr, Mn, Cu, Ag, Ce, Pr, Sm, Eu, Tb, and Yb, the co-doped ion is an ion of any one element selected from Li, Na, K, Rb, and Cs, or an ion of Li and Ba, and the stress-stimulated luminescent material has a general formula of ZnO:E _x M _y (where E is the co-doped ion, and M is the luminescence center ion), where x=0.0001-0.7, and y=0.001-0.2, it is possible to provide a method for producing a stress-stimulated luminescent material using ZnO as a host material. In particular, if the predetermined co-doped ion supplying compound is a silver ion supplying compound, an aluminum ion supplying compound, or an alkali metal compound, it is possible to more reliably produce a stress-stimulated luminescent material using ZnO as a host material.

According to the method for producing a stress-stimulated luminescent material of the present invention, the method for producing a stress-stimulated luminescent material using ZnO as a base material includes making an aqueous solution containing zinc ions, in which a luminescent center ion and a predetermined co-doped ion are present, basic or reacting the aqueous solution with a sulfide to coprecipitate the co-doped ion and the luminescent center ion with zinc hydroxide or zinc sulfide, and firing the coprecipitate (excluding those containing a solid protective agent which melts by heating to isolate the reaction system from an oxidizing atmosphere) in an oxidizing atmosphere, in which the luminescent center ion is an Nd ion or an Nd ion and an ion of any one element selected from Cr, Mn, Cu, Ag, Ce, Pr, Sm, Eu, Tb, and Yb, the co-doped ion is an ion of any one element selected from Li, Na, K, Rb, and Cs, or an ion of Li and Ba, and the stress-stimulated luminescent material has a general formula of ZnO:E _x M _y (where E is the co-doped ion, and M is the luminescent center ion), where x=0.0001-0.7, and y=0.001-0.2, it is possible to provide a method for producing a stress-stimulated luminescent material using ZnO as a host material. In particular, if the specified co-doped ion is a silver ion, an aluminum ion, or an alkali metal ion, it is possible to more reliably produce a stress-stimulated luminescent material using ZnO as a host material.

FIG. 2 is an explanatory diagram showing the crystal structure of ZnO. FIG. 2 is an explanatory diagram showing an XRD diffraction pattern of ZnO:Nd _0.006 ,Li _0.006 . FIG. 2 is an explanatory diagram showing an SEM image of ZnO:Nd _0.006 ,Li _0.006 . FIG. 2 is an explanatory diagram showing the fluorescence and excitation spectra of ZnO:Nd _0.006 ,Li _0.006 . FIG. 13 is an explanatory diagram showing stress-luminescence characteristics against a compressive load. FIG. 13 is an explanatory diagram showing a state of frictional luminescence. FIG. 13 is an explanatory diagram showing the stress-luminescence intensity when the measurement is repeated. FIG. 1 is an explanatory diagram showing stress-luminescence intensity under various conditions. FIG. 1 is an explanatory diagram showing the PL and ML intensities of ZnO:Nd _y ,Li _0.01 versus the amount of Nd added. FIG. 1 is an explanatory diagram showing the PL and ML intensities of ZnO:Nd _0.006 ,Li _x versus the amount of Li added. FIG. 1 is an explanatory diagram showing the PL and ML intensities of ZnO:Nd _0.006 ,Li _x versus the amount of Li added. FIG. 2 is an explanatory diagram showing an XRD spectrum. FIG. 1 is an explanatory diagram showing the effect of additives on ML and PL intensities. FIG. 2 is an explanatory diagram showing an XRD spectrum of a stress-stimulated luminescent material obtained by a urea liquid phase method. FIG. 2 is an explanatory diagram showing an SEM image of a stress-stimulated luminescent material obtained by a urea liquid phase method. FIG. 1 is an explanatory diagram showing the PL and ML intensities versus the Zn concentration of a stress-stimulated luminescent material obtained by a urea liquid phase method. FIG. 2 is an explanatory diagram showing an XRD spectrum of a stress-stimulated luminescent material obtained by a thioacetamide liquid phase method. FIG. 2 is an explanatory diagram showing an SEM image of a stress-stimulated luminescent material obtained by a thioacetamide liquid phase method. FIG. 2 is an explanatory diagram showing PL spectra in the presence or absence of alkali metal ions and in both liquid phase methods. FIG. 1 is an explanatory diagram showing the structures of zinc hydroxide and zinc sulfide. FIG. 2 is an explanatory diagram showing XRD diffraction patterns (30-40°) of ZnO:Nd _0.006 ,Li _0.006 at each firing temperature. FIG. 1 is an explanatory diagram showing the firing temperature dependence of ML and PL intensities. FIG. 2 is an explanatory diagram showing the firing temperature dependence of ML center luminescence of alkali metal elements. FIG. 13 is an explanatory diagram showing the presence or absence of stress-stimulated luminescence when the Li ion concentration is changed in ZnO:Li _x ,Nd _0.006 . FIG. 1 is an explanatory diagram showing the presence or absence of stress-induced luminescence when the Li concentration and Nd concentration are changed in ZnO:Li _x ,Nd _y . FIG. 1 is an explanatory diagram showing the presence or absence of stress-stimulated luminescence when Li and Ba ions are added as co-doped ions. FIG. 13 is an explanatory diagram showing the presence or absence of stress-stimulated luminescence when Na ions or K ions are used as co-doped ions. FIG. 1 is an explanatory diagram showing the presence or absence of stress-induced luminescence when other element ions are added together with Nd ions as luminescence center ions. FIG. 1 is an explanatory diagram showing the presence or absence of stress-stimulated luminescence when the concentration of added Nd ions, Yb ions, or Tb ions is changed.

The present invention relates to a stress-stimulated luminescent material that emits luminescence correlated to the energy of an external mechanical stimulus, and relates to a stress-stimulated luminescent material whose base material is ZnO (zinc oxide).

In particular, a feature of the stress-stimulated luminescent material according to this embodiment is that it is made up of a solid solution of luminescent center ions and specified co-doped ions in ZnO.

The solid solution of a specified co-doped ion or luminescent center ion in ZnO may be a substitutional solid solution or an interstitial solid solution.

The predetermined co-doped ions to be dissolved in ZnO are not particularly limited, but may be, for example, silver ions, aluminum ions, or alkali metal ions. The alkali metal ions may be ions of one or more metals selected from alkali metals, that is, Li (lithium), Na (sodium), K (potassium), Rb (rubidium), Cs (cesium), and Fr (francium). In particular, if the alkali metal ions are ions of at least one element selected from Li, Na, K, Rb, and Cs, and Nd ions are selected as the luminescence center ions, a stress-stimulated luminescent material with more reliable luminescence can be obtained.

In addition, the luminescence center ion to be dissolved in ZnO is not particularly limited as long as it is an ion that can be dissolved in ZnO together with the above-mentioned co-doped ion and function as a luminescence center, but can be, for example, an ion of at least one metal selected from transition metals and rare earths.

Here, transition metal ions can be understood as ions of transition metals that exist between group 3 elements and group 11 elements.

Rare earth (rare earth metal) ions can be understood as, for example, ions of Sc (scandium), Y (yttrium), La (lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium), Pm (promethium), Sm (samarium), Eu (europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), and Lu (lutetium).

Among transition metal and rare earth metal ions, if Nd ions are used as the luminescence center ions to be dissolved in ZnO, then by adding co-doped ions as described above, and using ions of at least one element selected from Ag, Al, Li, Na, K, Rb, and Cs as the co-doped ions, a mechanoluminescent material with more consistent luminescence can be obtained.

The stress-stimulated luminescent material according to this embodiment has such a configuration, and can be a stress-stimulated luminescent material having ZnO as a base material. In addition, the stress-stimulated luminescent material according to this embodiment can be said to be a stress-stimulated luminescent material containing a ZnO crystal in which a co-doped ion and a luminescence center ion are dissolved.

The stress-stimulated luminescent material according to this embodiment may have a composition represented by the general formula ZnO:E _x M _y (wherein E is a predetermined co-doped ion, and M is a luminescent center ion which is at least one metal ion selected from the group consisting of transition metal ions and rare earth ions), in which x=0.0001 to 0.7 and y=0.001 to 0.2.

The compound may also have a composition represented by the general formula Zn1 _{-xExO : My} (wherein E is a predetermined co-doped ion and M is a luminescent center ion which is at least one metal ion selected from a transition metal ion and a rare earth ion), where x = 0.0001 to 0.7 and y = 0.001 to 0.2; or the compound may have a composition represented by the general formula Zn1 _{-yMyO : Ex} (wherein E is a predetermined co-doped ion and M is a luminescent center ion which is at least one metal ion selected from a transition metal ion and a rare earth ion), where x = 0.0001 to 0.7 and y = 0.001 to 0.2.

Here, adding co-doped ions in an amount x exceeding 0.7 is not preferred because almost no further effect can be expected. In addition, while a lower limit of x of, for example, 0.0001 or more can provide solid mechanoluminescence characteristics, the inventor's experience has shown that mechanoluminescence is possible even if the value is below this limit. In other words, the range of x should be 0<x≦0.7, and by setting the range to 0.0001≦x≦0.7, light emission according to stress can be more reliably generated.

Similarly, for y, if y is less than 0.001, the effect of adding the luminescence center ion is hardly obtained, which is not preferable. By making y 0.001≦y≦0.2, luminescence in response to stress can be more reliably generated.

In addition, in the stress-stimulated luminescent material of this embodiment represented by the above general formula, the co-doped ions are dissolved in a concentration of 0.01 to 70 mol % and the luminescent center ions are dissolved in a concentration of 0.1 to 20 mol %, preferably 0.1 to 12 mol %, and more preferably 0.1 to 1.2 mol %, relative to the Zn contained in the ZnO base material.

The stress-luminescent material according to this embodiment may be dispersed in a predetermined matrix material to form a stress-luminescent body. For example, by using a curable resin as the matrix material and dispersing powdered stress-luminescent material in the resin before hardening and hardening it, a stress-luminescent body of a desired shape that emits light when stress is applied can be easily formed. The matrix material used is one that is at least capable of transmitting excitation light for exciting the stress-luminescent material mixed in the matrix material and fluorescence emitted from the stress-luminescent material.

In addition, the stress-stimulated luminescent material is not limited to a solid, but may be a liquid having fluidity. Specifically, paint containing the stress-stimulated luminescent material according to this embodiment is also included in the concept of a stress-stimulated luminescent material.

This application also provides a method for producing a mechanoluminescent material using ZnO as a base material. There are two types of production methods: a solid-phase reaction method and a liquid-phase reaction method.

In the method for producing a mechanoluminescent material using ZnO as a base material according to the present embodiment, which is a solid-state reaction method, a zinc compound, a compound of at least one metal selected from transition metals and rare earths, and a predetermined co-doped ion supply compound are mixed and fired. In the following description, the method for producing a mechanoluminescent material using ZnO as a base material according to the present embodiment by the solid-state reaction method is also referred to as the "solid-state method according to the present embodiment."

Here, the zinc compound is not particularly limited as long as it is a compound that can form ZnO crystals, i.e., a hexagonal wurtzite crystal structure as shown in Figure 1, through mixing and firing processes, and can be, for example, ZnO, ZnS, etc.

The co-additive ion supply compound may be any compound capable of dissolving the co-additive ions described above in the ZnO crystal. The co-additive ion supply compound is not particularly limited, but if one dares to give an example, it may be a silver ion supply compound capable of dissolving silver ions in the ZnO crystal, an aluminum ion supply compound capable of dissolving aluminum ions, or an alkali metal compound. The alkali metal compound may be any compound capable of dissolving alkali metal ions in the ZnO crystal, and may be, for example, an alkali metal carbonate, an alkali metal nitrate, or an alkali metal chloride. More specifically, for example, when Li is selected as the alkali metal, it may be Li ₂ CO ₃ , LiNO ₃ , LiCl, etc., when Na is selected, it may be Na ₂ CO ₃ , NaNO ₃ , NaCl, etc., and when K is selected, it may be K ₂ CO ₃ , KNO ₃ , KCl, etc.

In addition, the compound of at least one metal selected from transition metals and rare earths is not particularly limited as long as it is a compound capable of dissolving at least one metal ion selected from transition metal ions and rare earth ions as luminescence center ions in ZnO crystals, and oxides and salts (chlorides, nitrates, carbonates, etc.) of the same metals can be used. More specifically, for example, when Nd is selected, Nd ₂ O ₃ , NdCl ₃ , Nd(NO ₃ ) _{3.6H 2} O, etc. can be used. In the following description, the compound that supplies the luminescence center ion, that is, the compound of at least one metal selected from transition metals and rare earths, is also referred to as the "luminescence center supplying compound".

It should be noted that it is not necessary to use separate compounds for each of the above-mentioned compounds; if there is a compound that contains any two of the elements, it is also possible to use such a compound.

Then, these compounds are weighed out in predetermined amounts, thoroughly mixed in a mortar or the like, and then fired, and the fired product is crushed as necessary to obtain the stress-stimulated luminescent material of this embodiment.

The weighing of each compound can be in accordance with the composition ratio of the stress-stimulated luminescent material according to this embodiment, and it is sufficient to weigh and mix the amount of ZnO required to form the base material, the amount of co-additive ion supply compound capable of dissolving co-additive ions equivalent to 0.01 to 70 mol % of the Zn contained in the ZnO, and the amount of luminescent center supply compound capable of dissolving luminescent center ions equivalent to 0.1 to 20 mol %, preferably 0.1 to 12 mol %, and more preferably 0.1 to 1.2 mol % of Zn.

In addition, in the method for producing a stress-stimulated luminescent material using ZnO as a base material according to this embodiment, it is preferable to perform the firing in an oxidizing atmosphere. Generally, firing when preparing a stress-stimulated luminescent material is performed under conditions that do not contain oxygen, such as in a nitrogen gas atmosphere or an inert gas atmosphere. In this respect, the solid-phase method according to this embodiment is characterized in that a stress-stimulated luminescent material with good luminescence properties can be obtained by performing the firing in an oxidizing atmosphere, for example, in air (atmospheric air).

In particular, while known near-infrared stress-luminescent materials, such as Sr ₃ Sn ₂ O ₇ :Nd ³⁺ (SSN), require firing at about 1500°C in a nitrogen atmosphere, the stress-luminescent material according to the present embodiment, which uses neodymium ions as the luminescent center ion, is extremely useful in that it can be manufactured by firing in air, even though it is the same near-infrared stress-luminescent material. Note that, as will be explained later with reference to data, the stress-luminescent properties are observed even when firing is performed in a deoxidized environment, so firing in an oxidizing atmosphere is not an essential condition. However, this does not prevent the applicant from limiting the firing conditions when granting the patent of this application.

Furthermore, according to the solid-phase method of this embodiment, a method for producing a stress-stimulated luminescent material using ZnO as a base material by a solid-phase reaction method can be provided.

This application also provides a manufacturing method by a liquid phase reaction method. That is, in the manufacturing method of the stress-stimulated luminescent material using ZnO as a base material according to this embodiment, the liquid property of a zinc ion-containing aqueous solution in which a luminescent center ion and a predetermined co-doped ion are present is made basic, or a sulfide is reacted with the aqueous solution, and the co-doped ion and the luminescent center ion are coprecipitated with zinc hydroxide or zinc sulfide, and the coprecipitate is fired. In the following description, the manufacturing method of the stress-stimulated luminescent material using ZnO as a base material according to this embodiment by the liquid phase reaction method is also referred to as the "liquid phase method according to this embodiment."

The aqueous solution used in the liquid phase method according to this embodiment contains zinc ions to form the base material, and further contains co-doped ions and luminescent center ions.

The preparation of the aqueous solution requires a zinc compound which is a source of zinc ions, a co-additive ion supply compound which is a source of co-additive ions, and a luminescence center supply compound which is a source of luminescence center ions, and is not particularly limited as long as these compounds are water-soluble and can be coprecipitated, and further, a mechanoluminescent material in which the co-additive ions and luminescence center ions are solid-dissolved in the ZnO crystals can be prepared by firing the coprecipitated product.

As the zinc compound, a compound that is water-soluble and capable of liberating zinc ions is suitable, and for example, zinc salts, more specifically _ZnCl2 , etc. can be used.

As the co-additive ion supplying compound, a compound that is water-soluble and can liberate silver ions, aluminum ions, alkali metal ions, etc. is suitable. As the alkali metal compound, a compound that is water-soluble and can liberate alkali metal ions, such as lithium ions, sodium ions, potassium ions, etc., is suitable. For example, alkali metal salts, more specifically LiCl, NaCl, KCl, etc. can be used.

In addition, as the luminescent center supplying compound, a compound that is water-soluble and can liberate luminescent center ions, such as neodymium ions, is suitable, and for example, a salt of at least one metal selected from transition metal ions and _{rare earth ions, more specifically Nd(NO3)3.6H2O , can} be used.

The concentration of each ion in the aqueous solution derived from these compounds can be in accordance with the composition ratio of the stress-stimulated luminescent material according to this embodiment. That is, the aqueous solution may contain the zinc ions necessary to form the base material, the co-additive ions equivalent to 0.01 to 70 mol % of the zinc ions, and the luminescence center ions equivalent to 0.1 to 20 mol %, preferably 0.1 to 12 mol %, and more preferably 0.1 to 1.2 mol % of the zinc ions.

Another feature of the liquid phase method according to this embodiment is that it produces a precipitated zinc compound and precipitates (co-precipitates) it while incorporating the co-doped ions and luminescent center ions.

There is no particular limitation on the method for producing the precipitated zinc compound, but it may be produced, for example, by making the aqueous solution basic or by reacting the aqueous solution with a sulfide. When the aqueous solution is made basic, zinc hydroxide is produced and precipitated, and when the aqueous solution is reacted with a sulfide, zinc sulfide is produced and precipitated.

One example of a method for precipitating zinc hydroxide is a method using urea as a precipitant. More specifically, a predetermined amount of urea is dissolved in an aqueous solution, and the aqueous solution is heated to about 70 to 90°C to decompose the urea, i.e.,
CO(NH ₂ ) ₂ + H ₂ O → 2NH ₃ + CO ₂
This is a way to encourage this.

In the presence of zinc chloride,
CO(NH ₂ ) ₂ + ZnCl ₂ + 3H ₂ O → Zn(OH) ₂ ↓ + 2NH ₄ Cl + CO ₂
The reaction takes place, and zinc hydroxide precipitates. At this time, the co-additive ions and luminescence center ions present in the aqueous solution can be caught in the precipitation (co-precipitation).

Another example of a method for precipitating zinc hydroxide is to use thioacetamide as a precipitant. More specifically, a certain amount of thioacetamide is dissolved in an aqueous solution, and the aqueous solution containing zinc chloride is heated to 70°C or higher to promote decomposition of the thioacetamide.
C ₂ H ₅ NS + ZnCl ₂ + H ₂ O → ZnS↓ + C ₂ H ₅ NO + 2HCl
The reaction takes place, and zinc sulfide precipitates. The co-doped ions and luminescent center ions present in the aqueous solution can be precipitated (co-precipitated).

In the liquid phase method, the concentration of Zn ions in the aqueous solution can be in the range of 0.1 to 0.5 mol/L.

In addition, the concentration of alkali metal ions in the aqueous solution can be in the range of 0.01 to 70 mol% of the Zn ions contained in a unit volume, i.e., 0.00001 to 0.35 mol/L.

In addition, the concentration of the luminescent center ion in the aqueous solution can be an amount equivalent to 0.1 to 20 mol%, preferably 0.1 to 12 mol%, and more preferably 0.1 to 1.2 mol% of the Zn ions contained in a unit volume, for example, 0.0001 to 0.006 mol/L for 0.1 to 1.2 mol%.

The amount of precipitant in the aqueous solution can be adjusted as appropriate, but for example, it can be an amount that can react adequately with Zn ions to precipitate them. For example, when urea (CO(NH ₂ ) ₂ ) or thioacetamide (C ₂ H ₅ NS) is used as the precipitant, the amount can be about 1 mol, preferably a little more than 1 mol (e.g., about 1.1 to 1.5 mol), per mol of Zn ions.

The obtained coprecipitate is calcined, and if necessary, the calcined product is pulverized to obtain the stress-stimulated luminescent material of this embodiment.

In the liquid phase method according to this embodiment, firing can be performed in an oxidizing atmosphere, for example, in air (atmosphere), in the same manner as in the solid phase method according to this embodiment described above, to obtain a mechanoluminescent material with good luminescence properties. Note that firing in an oxidizing atmosphere is not essential in the liquid phase method according to this embodiment, and the applicant is free to limit the firing conditions.

The present application also provides a method for making ZnO into a stress-stimulated luminescent material by dissolving co-doped ions and luminescent center ions in ZnO. In other words, the present application provides a method for making ZnO, which has been unable to function as a stress-stimulated luminescent material until now, into a stress-stimulated luminescent material, and is characterized by dissolving co-doped ions and luminescent center ions in ZnO.

The following describes in detail the stress-stimulated luminescent material and stress-stimulated luminescent body according to the present embodiment, the method for manufacturing the stress-stimulated luminescent material, and the method for making ZnO into a stress-stimulated luminescent material, with reference to actual manufacturing examples and test results. Note that although the present application uses several representative examples to clarify the contents of the present invention, based on the inventor's many years of experience, stress-stimulated luminescence is possible within the range of elements and values shown in the general formula, and that the present invention is not limited to the elements and values of these representative examples.

1. Preparation of mechanoluminescent materials by solid-phase method
By the solid-state reaction method according to this embodiment, ZnO:Nd ³⁺ _0.006 ,Li ⁺ _0.006 which is the stress-stimulated luminescent material according to this embodiment was prepared.

First, ZnO (Sakai Chemical, 99.7%) as the zinc compound, Li ₂ CO ₃ (Kojundo Chemical Laboratory, 99.99%) as the alkali metal compound (co-additive ion supply compound), and Nd ₂ O ₃ (Kojundo Chemical Laboratory, 99.9%) as the luminescent center supply compound were weighed out based on the stoichiometric ratio, and then thoroughly mixed and ground in an agate mortar.

Next, the ground mixture was sintered by firing in an air atmosphere at temperatures of 600°C, 700°C, 800°C, 900°C, 1000°C, 1100°C, and 1200°C for a predetermined time (e.g., 3 to 10 hours, more specifically, 4 to 6 hours) using an electric furnace. The resulting fired product was ground in an agate mortar to obtain ZnO:Nd ³⁺ _0.006 ,Li ⁺ _0.006 as the stress-stimulated luminescent material according to this embodiment. In the following description, the above-mentioned manufacturing method is also referred to as the solid-phase based manufacturing method.

Here, we also prepared samples in which the amount of co-added ion supply compound added was changed compared to the solid-phase standard method, and samples in which the amount of luminescence center supply compound added was changed.

The samples with different amounts of alkali metal compounds added as co-doped ion supplying compounds were prepared by changing the amount of Li added from 0 to 70 mol%. Specifically, the sample was ZnO:Nd ³⁺ _0.006 ,Li ⁺ _x (x=0 to 0.7), and more specifically, samples were prepared for x=0, 0.001, 0.002, 0.003, 0.004, 0.006, 0.012, 0.018, 0.05, 0.1, 0.2, 0.4, 0.5, and 0.7.

The samples with different amounts of the luminescence center supplying compound were prepared by varying the amount of Nd added from 0 to 1.2 mol%. Specifically, the sample was ZnO:Nd ³⁺ _y ,Li ⁺ _0.01 (y=0 to 0.12), and more specifically, samples were prepared with y=0, 0.003, 0.006, and 0.012.

Here, we also prepared samples using ZnS (Wako Pure Chemical Industries, 99.999%) instead of ZnO, which was used as the zinc compound in the solid-phase standard method. In the following explanation, this method is also referred to as the solid-phase ZnS method.

In addition, samples were also prepared using LiCl (CERAC 99.8%), Na ₂ CO ₃ (Kojundo Chemical Laboratory 99.8%), K ₂ CO 3 (Kojundo Chemical Laboratory 99%), MgCO ₃ (Kojundo Chemical Laboratory 99.9%), _{CaCO 3} (Kojundo Chemical Laboratory 99.99%), SrCO ₃ (Kojundo Chemical Laboratory 99.99%), BaCO ₃ (Kojundo Chemical Laboratory 99.95%) and H ₃ BO ₃ (ALDRICH 99.99%) as replacements for Li ₂ CO ₃ , an alkali metal compound used as a co-doped ion supply compound in the solid-phase standard method. Note that the amount of each target element added in the prepared samples was adjusted to 1 mol%.

In addition, with regard to sintering in an air atmosphere in the solid-phase standard manufacturing method, samples were also prepared in which the sintering atmosphere was a _N2 atmosphere or a flowing Ar atmosphere.

[2. XRD analysis of mechanoluminescent materials]
The crystal phases of the samples obtained by the solid-phase standard method and the solid-phase ZnS method were identified using a powder X-ray diffractometer (XRD). Figure 2 shows the XRD diffraction pattern of ZnO:Nd ³⁺ _0.006 ,Li ⁺ _0.006 .

As shown in Figure 2, XRD measurement showed that the sample obtained by the solid-phase standard method was a single-phase hexagonal zinc oxide (ZnO) (PDF_01-078-3322).

Furthermore, analysis of samples obtained using the solid-phase ZnS manufacturing method confirmed that when ZnS was used as the raw material for synthesis, the crystallinity improved and the (101) plane was oriented. This was thought to be due to the fact that ZnS has a lower melting point than ZnO, as well as the influence of changes in the crystal structure during oxidation.

As shown in Figure 1, ZnO has a structure without a center of symmetry, classified into the space group P63mc. The tetrahedron with Zn at the center forms a plate-like structure. This easily distorted structure is thought to be advantageous for mechanoluminescence.

[3. Microscopy of Stimuli-Luminescent Materials]
Next, the samples obtained by the solid-phase reference method and the solid-phase ZnS method were observed using a scanning electron microscope. Figure 3(a) shows an SEM image of the sample obtained by the solid-phase reference method, and Figure 3(b) shows an SEM image of the sample obtained by the solid-phase ZnS method.

As can be seen from the SEM image shown in Figure 3 (a), the ZnO particles obtained by the solid-phase method (synthesized based on ZnO) were amorphous and had a shape resembling an aggregate of particles of about 1 μm in size, with some of them having a layered structure.

In contrast, the ZnO particles obtained by the solid-phase ZnS method (synthesized based on ZnS) had an irregular but smooth surface, as shown in Figure 3 (b), and a striped pattern was observed.

4. Measurement of Fluorescence Spectrum and Excitation Spectrum
Next, the fluorescence spectrum and excitation spectrum of the sample obtained by the solid-phase reference method were measured. The fluorescence spectrum was measured using a spectrometer. The measurement result of the fluorescence spectrum is shown in Figure 4(a), and the measurement result of the excitation spectrum is shown in Figure 4(b).

As can be seen from Figure 4 (a), ZnO:Nd ³⁺ _0.006 ,Li ⁺ _0.006 obtained by the solid-phase standard method was confirmed to show near-infrared photoluminescence (PL) based on ⁴ F _3/2 → ⁴ I _9/2 at 898 nm and ⁴ F _3/2 → ⁴ I _11/2 at 1086 nm. Moreover, this PL was very strong compared to the fluorescence in the visible region.

Furthermore, as shown in Figure 4(b), it was shown that their PL can be excited by ultraviolet and visible light.

5. Study of mechanoluminescence properties
Next, the mechanoluminescence (ML) properties of ZnO:Nd ³⁺ _0.006 ,Li ⁺ _0.006 obtained by the solid-phase standard method were investigated.

The evaluation of mechanoluminescence properties was carried out on a cylindrical test specimen. This test specimen was made of a composite resin pellet (diameter 25 mm, thickness 10 mm) made by mixing 0.5 g of powder sample obtained by the solid-phase standard manufacturing method with 4.5 g of epoxy resin, and corresponds to the mechanoluminescent material according to this embodiment.

The specimen was pre-irradiated with excitation light for 5 minutes, waited for 10 seconds, and then mechanically compressed for evaluation. The light source used for pre-irradiation was an LED (MDRL-CUV31, MORITEX, Saitama, Japan) with a central wavelength of 365 nm (λ1/215 nm).

The specimens were compressed at 3 mm/min with a pressure of 0 to 1000 N using a compression machine (Tensilon, RTC-1310A, Orintec Co., Tokyo, Japan). The ML strength of the upper and central parts of the specimens was measured using a camera equipped with an InGaAs sensor (C14041-10U, HAMAMATSU, Shizuoka, Japan). The results are shown in Figure 5.

As can be seen from Figure 5, ZnO:Nd ³⁺ _0.006 ,Li ⁺ _0.006 obtained by the solid-phase standard method exhibits near-infrared stress-stimulated luminescence, and it was confirmed that in the center of the test specimen (ROI 2), the stress-stimulated luminescence intensity increased in response to the applied force from 0 to 500 N.

On the other hand, the top end of the test specimen (ROI 1) shows a sudden increase in stress-induced luminescence up to 200 N, after which the amount of luminescence tends to decrease. This indicates that the top end of the cylindrical test specimen is subjected to extremely strong stress, and therefore has a strong effect of destructive luminescence.

Furthermore, as shown in Figure 6, when letters were written on the flat surface of the test specimen using a glass rod, near-infrared triboluminescence was observed along the trace.

Next, the same test was repeated 20 times in succession on the test specimen, and the mechanoluminescence intensity was checked for each test. The results are shown in Figure 7.

As can be seen in Fig. 7, the intensity of mechanoluminescence at the center of the specimen (ROI 2) did not change significantly between the 1st and 20th times, and stable mechanoluminescence was observed. This is evidence that ZnO:Nd ³⁺ _0.006 ,Li ⁺ _0.006 exhibits piezoluminescence, which is luminescence in the elastic region.

On the other hand, the upper end of the pellet (ROI 1) contained destructive luminescence, indicating that quantitation was not highly reliable.

In this way, it has been shown that the mechanoluminescent material according to this embodiment is a mechanoluminescent material that uses ZnO as a host material. In particular, this is the first time that mechanoluminescence has been discovered that uses ZnO as a host material, and in this respect, the present invention is of great significance.

6. Comparison of mechanoluminescence intensity between samples obtained by the solid-phase standard method and other samples
Next, the mechanoluminescence intensity of the sample obtained by the solid-phase reference method was compared with that of other samples. The five samples used for comparison were: [A] a sample obtained by the solid-phase reference method, [B] a sample obtained by the solid-phase ZnS method, [C] a sample prepared in the same manner as the solid-phase reference method without adding an alkali metal compound (a co-additive ion supply compound), [D] a sample prepared in the same manner as the solid-phase reference method except that it was fired under an argon atmosphere, and [E] a sample prepared in the same manner as the solid-phase reference method except that it was fired under a nitrogen atmosphere. All samples fired at 1100°C were used here.

Figure 8 shows a graph of the mechanoluminescence intensity of each sample. In the graph in Figure 8, [F] shows the mechanoluminescence intensity in the visible region of the sample obtained by the solid-phase reference method.

First, it was confirmed that the mechanoluminescence intensity was improved by about 1.5 times in the case of the solid-phase ZnS manufacturing method, as shown by [B] in Figure 8. This is thought to be because, as shown by the XRD spectrum in Figure 2, the crystallinity is improved when ZnS is used as the zinc compound.

Furthermore, as shown by [C], when no alkali metal compound (co-doped ion supply compound) was added, very little mechanoluminescence was observed, but the mechanoluminescence intensity was significantly reduced. This shows that the addition of a co-doped ion supply compound is important in endowing ZnO with mechanoluminescent properties. In addition, it can be said that the addition of a co-doped ion supply compound is essential to realize a ZnO-based mechanoluminescent material with practical utility.

In addition, as shown in [D] and [E], when the sintering was performed in an Ar or _N2 atmosphere, a significant decrease in the mechanoluminescence intensity was observed, although not as much as in [C]. This is thought to be due to the fact that oxygen defects in ZnO are deeply related to the loss of the mechanoluminescence properties. In other words, although not essential, it is clear that sintering in an atmosphere containing oxygen, such as in the air, is important in realizing a ZnO-based mechanoluminescent material with good mechanoluminescence intensity.

As shown by [F], no mechanoluminescence was observed in the visible region (Wavelength 400-800 nm) when the sample obtained by the solid-phase standard manufacturing method [A] was observed. However, this result is for a sample fired at 1100°C, and the inventor's experience has shown that luminescence will occur in the visible light region if fired at other temperatures. Although the data is omitted, in fact, mechanoluminescence in the visible light region has been observed in a sample fired at 900°C using a method similar to the solid-phase standard manufacturing method.

[7. Consideration of the amount of luminescence center ions]
Next, in order to study the optimization of the manufacturing conditions for the stress-stimulated luminescent material, the amount of the luminescence center ion was first examined.

Specifically, the mechanoluminescence intensity was measured for ZnO:Nd ³⁺ _y ,Li ⁺ _0.01 (y=0, 0.003, 0.006, 0.012) prepared in [1. Preparation of mechanoluminescent material by solid phase method]. The results are shown in Figure 9.

As can be seen from Fig. 9, both photoluminescence and mechanoluminescence are due to the emission of the luminescence center ion (here, Nd ³⁺ ), so no emission was observed when the Nd addition amount was 0 mol%. In contrast, mechanoluminescence was observed for ZnO:Nd ³⁺ _y ,Li ⁺ _0.01 (y = 0.003, 0.006, 0.012), but no clear correlation with the amount of addition was confirmed. Although the data is not shown in figures, no significant effect was observed on either photoluminescence or mechanoluminescence when the addition was within the range of y = 0.001 to 0.12 (0.1 to 12 mol%). Mechanoluminescence was also observed in the range of 0 < y < 0.001.

8. Consideration of the amount of co-added ions
Next, the amount of co-doped ions was examined. Specifically, the mechanoluminescence intensity was measured for ZnO:Nd ³⁺ _0.006 ,Li ⁺ _x (x=0, 0.001, 0.002, 0.003, 0.004, 0.006, 0.012, 0.018, 0.05, 0.1, 0.2, 0.4, 0.5, 0.7) prepared in [1. Preparation of mechanoluminescent material by solid-state method]. The results are shown in Figure 10.

First, for x=0, 0.001, 0.002, 0.003, 0.004, 0.006, 0.012, 0.018 (0 to 1.8 mol%), as can be seen from Figure 10, when the amount of co-doped ions (here, Li ions, which are alkali metal ions) added was 0 mol%, no mechanoluminescence characteristics were observed. On the other hand, for subsequent additions, mechanoluminescence was observed, but no correlation was observed with the amount of co-doped ions added. This suggests that the addition of co-doped ions creates traps, which is a major factor in the mechanoluminescence characteristics.

In addition, for x = 0.05, 0.1, 0.2, 0.4, 0.5, and 0.7 (5 to 70 mol%), as shown in Figure 11, stress-induced luminescence was observed in response to the addition of co-doped ions, but no correlation with the amount added was observed.

From these results, it was found that the addition of co-doped ions is an essential element for the mechanoluminescent properties of the mechanoluminescent material according to this embodiment. In addition, for ZnO:Nd ³⁺ _0.006 ,Li ⁺ _x , the results of XRD measurements when the amount of co-doped ions (here, Li ions, which are alkali metal ions) added is 0 to 1.8 mol% are shown in Fig. 12(a), and the results of XRD measurements when the amount is 5 to 70 mol% are shown in Fig. 12(b). From these XRD measurements, no change in the zinc oxide crystal phase due to the addition of co-doped ions was confirmed. However, according to further research by the present inventors, although detailed data will not be presented here, a change in the lattice constant due to the addition of Li was observed, and it was found that Li replaced the Zn site.

Next, in order to consider the significance of adding co-additive ions, several alkali metal compounds, alkaline earth metal compounds, and boric acid were added and compared.

[9. Consideration of the type of co-doped ions]
Here, we investigated the types of co-doped ions. Specifically, we investigated the photoluminescence and mechanoluminescence properties of samples prepared using various compounds in [1. Preparation of mechanoluminescent material by solid-state method], that is, samples using LiCl, _Na2CO3 , _K2CO3 , _MgCO3 , _CaCO3 , _SrCO3 , _BaCO3 , and _H3BO3 instead of _Li2CO3 , _an alkali metal compound used as _{a co} -doped _ion supply compound in the solid-state standard method. The results are shown in Figure 13.

As can be seen from FIG. 13, the highest luminescence intensity for both photoluminescence and stress-stimulated luminescence was obtained when, among the compounds used this time, the alkali metal ion was lithium ion and Li ₂ CO ₃ was used as the alkali metal compound.

In contrast, when the co-doped ion was the same lithium ion but the alkali metal compound used was a chloride, or when the co-doped ion was a sodium ion or potassium ion, a slight decrease in ML/PL intensity was observed.

Also, it is worth noting that the samples using other compounds, namely MgCO ₃ , CaCO ₃ , SrCO ₃ , BaCO ₃ and H ₃ BO _{3 ,} did not show ML/PL characteristics.

Considering this point, one of the reasons why alkali metal elements have luminescence properties is thought to be that their charge is 1+. In other words, since Nd is thought to substitute for Zn ²⁺ in the 3+ state, alkali metal elements are convenient for playing the role of charge compensation. However, this effect cannot be obtained when other compounds are used, so Nd ³⁺ may not have been substituted well.

From this, it can be said that in order for ZnO to have mechanoluminescent properties, the addition of a co-doping element that plays a role in charge compensation is essential, and that the effect of this is extremely large.

10. Preparation of mechanoluminescent material by liquid phase method
Next, the mechanoluminescent material according to the present embodiment was prepared by the liquid phase reaction method according to the present embodiment.

Aqueous solutions of arbitrary concentrations were prepared using ion-exchanged water by adding ZnCl ₂ (Wako Pure Chemicals, 98%) as the zinc compound raw material, Nd(NO ₃ ) ₃ ·6H ₂ O (Wako Pure Chemicals, 99.5%) as the luminescence center supply compound, and LiCl (CERAC, 99.8%), an alkali metal compound, as the co-additive ion supply compound. In this section, the Zn ion concentration in the aqueous solution was varied from 0.025 to 0.1 mol/L, the Nd ion concentration was varied from 0.5 mol% relative to the Zn ions, and the Li ion concentration was varied from 0 to 2.4 mol% relative to the Zn ions.

In addition, CO(NH ₂ ) ₂ (Wako Pure Chemical Industries, Ltd., 99.9%) and CH ₃ CSNH ₂ (Wako Pure Chemical Industries, Ltd., 99.8%) were selected as precipitants and added in the same amount as the Zn concentration.

Synthesis was carried out using an autoclave, and hydrothermal synthesis was carried out at a temperature of 95°C for 24 hours. The resulting precipitate was filtered by suction and dried at 70°C for 3 hours. Next, the crushed mixture was sintered in an electric furnace by firing it in air at temperatures of 600°C, 700°C, 800°C, 900°C, 1000°C, 1100°C, and 1200°C for a specified time (for example, 3 to 10 hours, more specifically 4 to 6 hours). The resulting fired product was crushed in an agate mortar and sample evaluation was carried out.

Next, we will discuss a method for producing a mechanoluminescent material using urea (CO(NH ₂ ) ₂ ) as a precipitant. First, regarding the reaction in the liquid phase method according to this embodiment, urea in an aqueous solution starts to hydrolyze at 70-80°C, and decomposition progresses rapidly at 90°C or higher. Ammonia is also highly soluble in water and is alkaline.

On the other hand, _ZnCl2 in an aqueous solution exists in the liquid phase as Zn2 ⁺ , which precipitates as Zn(OH) ₂ under basic conditions. When Zn(OH) ₂ precipitates, it often co-precipitates with surrounding ions such as Nd3 ⁺ and Li ⁺ .

Therefore, in the liquid phase to which urea has been added as a precipitant, it can be extracted as a precipitate of Zn(OH) ₂ by heating to 70°C or higher. In addition, if the entire liquid phase is at a uniform temperature, it is thought that ions such as urea and zinc are also uniformly dispersed. Therefore, the system present in the obtained precipitate becomes a uniform solid phase.

The obtained Zn(OH) ₂ is oxidized by baking at 800°C or higher to become ZnO. At this time, if the temperature is low enough, ZnO can be obtained while maintaining its particle shape, so ZnO of any particle shape can be obtained. In the following description, the liquid phase method according to this embodiment using urea as a precipitant is also called the "urea liquid phase method according to this embodiment."

Next, a method for producing a stress-stimulated luminescent material using thioacetamide (CH ₃ CSNH ₂ ) as a precipitating agent will be described.

The aqueous solution of thioacetamide is hydrolyzed at 70°C or higher, releasing hydrogen sulfide. Here, the Zn ²⁺ ions present in the aqueous solution react with hydrogen sulfide and precipitate as ZnS. At this time, like the above-mentioned urea, it precipitates in a uniform state by involving surrounding ions. ZnS becomes ZnO by baking at 800°C or higher. Therefore, ZnO particles that maintain any shape can be obtained. In the following description, the liquid phase method according to this embodiment using thioacetamide as a precipitant is also referred to as the "thioacetamide liquid phase method according to this embodiment."

[11. XRD analysis of mechanoluminescent materials obtained by urea liquid phase method]
The crystal phase of the sample obtained by the urea liquid phase method according to this embodiment was identified by a powder X-ray diffractometer (XRD). Figure 14 shows the XRD diffraction patterns of the samples obtained by the liquid phase method using urea as a precipitant, with and without the presence of co-additive ions (here, Li ions, which are alkali metal ions) in the aqueous solution.

XRD measurements showed that all the synthesized materials were ZnO (PDF_01-078-3322) single-phase hexagonal zinc oxide. In addition, ZnO doped with co-doped ions (alkali metal ions) showed strong C-axis orientation.

[12. Microscopic examination of mechanoluminescent material obtained by urea liquid phase method]
Next, in performing the liquid phase method using urea as a precipitant, the samples obtained with and without the presence of co-additive ions (here, Li ions, which are alkali metal ions) in the aqueous solution were observed using a scanning electron microscope. Figure 15(a) is an SEM image of a sample without co-additive ions, and Figure 15(b) is an SEM image of a sample with co-additive ions added.

As can be seen from Fig. 15(a), ZnO without co-doped ions was amorphous, with particle sizes ranging from 1 to 5 μm. In contrast, as shown in Fig. 15(b), ZnO with co-doped ions and C-axis orientation was layered particles, and this easily distorted shape is thought to be advantageous for mechanoluminescence.

[13. Examination of ML/PL intensity as a function of Zn concentration]
Next, the influence of changing the Zn concentration in the aqueous solution on the ML/PL intensity of the stress-stimulated luminescent material obtained by the urea liquid phase method according to the present embodiment was examined. Fig. 16 is a graph showing the intensities of stress-stimulated luminescence and photoluminescence of the stress-stimulated luminescent material obtained by the urea liquid phase method when the Zn concentration in the aqueous solution is 0.1, 0.125, 0.3, and 0.5 mol/L. Note that the PL/ML intensity of the sample obtained by the solid-phase reference method described above is taken as 100 in this graph.

The inventors' experience shows that in the liquid phase method, if the Zn concentration is too high, precipitation occurs all at once in a short period of time, which tends to have a negative effect on particle shape control. For this reason, the Zn concentration was set to a maximum of 0.5M in this study, but the results showed that the photoluminescence properties were weaker at higher Zn concentrations.

In contrast, although no clear correlation with Zn concentration could be confirmed, it was confirmed that mechanoluminescence was observed at Zn concentrations exceeding 0.1 M.

[14. XRD analysis of mechanoluminescent materials obtained by the thioacetamide liquid phase method]
The crystal phase of the sample obtained by the thioacetamide liquid phase method according to this embodiment was identified by a powder X-ray diffractometer (XRD). Figure 17 shows the XRD diffraction patterns of the samples obtained by the liquid phase method using thioacetamide as a precipitant, with and without the presence of co-additive ions (here, Li ions, which are alkali metal ions) in the aqueous solution.

As shown in Figure 17, when thioacetamide was used as the precipitant, a large change in the crystal structure of ZnO without co-added ions was observed compared to the urea liquid phase method. When no co-added ions were added, the crystallinity improved, and strong orientation was observed in the (100) and (002) planes.

In contrast, when co-doped ions were added, the XRD spectrum showed a hexagonal crystal structure similar to that obtained with solid-phase synthesis.

[15. Microscopic examination of mechanoluminescent material obtained by thioacetamide liquid phase method]
Next, in performing the liquid phase method using thioacetamide as a precipitant, the samples obtained with and without the presence of co-additive ions (here, Li ions, which are alkali metal ions) in the aqueous solution were observed using a scanning electron microscope. Figure 18(a) is an SEM image of a sample without co-additive ions, and Figure 18(b) is an SEM image of a sample with co-additive ions added.

As can be seen from Figure 18 (a), SEM measurements showed that the particles without co-added ions had beautiful hexagonal columnar particles.

In contrast, ZnO particles synthesized in the liquid phase and containing co-doped ions have a shape that resembles a collapsed hexagonal columnar particle.

[16. Considerations regarding the two-phase method]
The photoluminescence spectra of the mechanoluminescent materials obtained by both liquid phase methods are shown in Figure 19. As can be seen from Figure 19, when no co-doped ions were added, Nd ³⁺ could not be substituted well, and almost no PL characteristics were observed.

In addition, ZnO synthesized by liquid phase synthesis using thioacetamide and co-doped ions showed slight PL properties, but the intensity was weak.

Since the PL intensity is stronger when urea is used, it is expected that the precipitation probability of coexisting ions will differ depending on the precipitant. Urea precipitates as zinc hydroxide, and thioacetamide precipitates as zinc sulfide, but it is thought that zinc hydroxide has a greater ability to precipitate by involving surrounding ions structurally. The structure of zinc hydroxide is shown on the left in Figure 20, and zinc sulfide on the right.

[17. Effect of the baking temperature in the solid-phase method on mechanoluminescence intensity]
We investigated the effect of the firing temperature of the solid-state method on mechanoluminescence intensity. Specifically, the samples were prepared according to the standard solid-state method, except that the sample was ZnO _0.994 Nd _0.006 Li _0.006. For firing, an Al ₂ O ₃ crucible was used, and the samples were sintered in air at temperatures between 900°C and 1300°C, or even higher temperatures up to 1500°C if necessary.

The XRD diffraction patterns of ZnO _0.994 Nd _0.006 Li _0.006 at each firing temperature are shown in Figure 21. A single phase of ZnO (PDF_01-078-3322) was obtained at every firing temperature. The XRD diffraction pattern showed a clear peak shift due to the change in firing temperature. The shift to the lower angle side is thought to be due to an increase in the lattice.

The dependence of ML and PL intensities on the firing temperature was also recorded. As shown in Figure 22, both ML and PL were low at 900℃ and 1000℃, but both showed significantly higher values at higher temperatures from 1100℃ onwards. Note that the ML (stimulated luminescence intensity) at a firing temperature of 1000℃ was not 0, and light was emitted, albeit slightly.

In addition, similar experiments were conducted on the sintering temperature dependence of other alkali metal elements K and Na at temperatures of 900°C, 1000°C, 1020°C, 1040°C, 1060°C, 1080°C, 1100°C, 1200°C, 1300°C, 1400°C, and 1500°C. As shown in Figure 23, a comparison of ML center emission showed that the results were basically the same as those of Li. Note that the emission intensity of Li at a sintering temperature of 1000°C is not 0, and it emits light, albeit slightly. Note that Figure 22 above is a graph normalized to the maximum value of 100, while Figure 23 shows an unnormalized graph.

In addition, although an explanation showing the data is omitted, in [1. Preparation of Stress-Stimulated Luminescent Material by Solid-Phase Method], for the stress-stimulated luminescent material ZnO:Nd ³⁺ _0.006 ,Li ⁺ _0.006 sintered at temperatures of 600°C, 700°C, 800°C, 900°C, 1000°C, 1100°C, and 1200°C, stress-stimulated luminescence was not observed at 600°C or 650°C, but was observed at sintering temperatures of 700°C or higher, and at temperatures between 900°C and 1500°C, the results were roughly similar to those of the stress-stimulated luminescent material described above.

These results show that the ML intensity of ZnO:Nd ³⁺ is significantly improved by firing at 1100°C or higher and decreases above 1400°C. In addition, the lower limit of the firing temperature is 700°C or higher, more preferably 900°C or higher, and even more preferably 1020°C or higher, and the upper limit of the firing temperature is 1500°C or lower, preferably 1400°C or lower, and more preferably 1300°C or lower. It is possible to produce a stress-stimulated luminescent material that exhibits stable stress-stimulated luminescence by firing at a temperature range that is any combination of these upper and lower limits.

[18. Presence or absence of mechanoluminescence when changing the Li ion concentration in ZnO:Li _x ,Nd _0.006]
Next, in ZnO:Li _x ,Nd _0.006 , the presence or absence of mechanoluminescence was confirmed when the concentration of Li ions, which are co-doped ions, was changed from 0 to 0.7. The results are shown in FIG.

As can be seen from Figure 24, when the value of x was set to 0, no stress-induced luminescence was observed in the near-infrared region (>750 nm); however, when Li ions were added, stress-induced luminescence was observed in the near-infrared region (>750 nm) up to a maximum value of x of 0.7.

[19. Presence or absence of mechanoluminescence when changing the Li concentration or Nd concentration in ZnO:Li _x ,Nd _y]
Next, in ZnO:Li _x ,Nd _y , the presence or absence of mechanoluminescence was confirmed when the concentration of the co-doped Li ions and the concentration of the luminescent center Nd ions were variously changed. The results are shown in FIG.

As can be seen from Figure 25, stress-induced luminescence was observed in the near-infrared range (>750 nm) for all materials.

[20. Presence or absence of mechanoluminescence when Li and Ba ions are added as co-doped ions]
Next, we investigated whether mechanoluminescence occurs when multiple co-doped ions are present. In particular, in this study, as a representative example, we used two types of co-doped ions, Li ion and Ba ion, and the luminescence center ion was Nd ion, and we fixed the concentrations of Li ion and Nd ion at 0.6 mol% and changed the concentration of Ba ion.

In the above, when there is one type of co-doped ion or one type of luminescence center ion, the formula is expressed as ZnO:E _x M _y , but when there are multiple types of co-doped ions, for example, two types, as in this study, the formula is the same and E _x = E1 _x1 , E2 _x2 , x = x1 + x2, and when there are three types of luminescence center ions, M _y = M1 _y1 , M2 _y2, M3 _y3 , y = y1 + y2 + y3. In other words, in this study, E1 _x1 = Li _0.006 , E2 _x2 = Ba _x2 , M _y = Nd _0.006 , and the presence or absence of mechanoluminescence was confirmed by changing the value of x2 in the range of 0 to 0.05.

As a result, as shown in Figure 26, stress-induced luminescence was confirmed in the near-infrared range (>750 nm) in all materials in which the Ba ion concentration was changed in the range of 0 to 0.05.

[21. Whether or not mechanoluminescence occurs when co-doped ions are alkali metal ions other than Li]
Next, we investigated whether mechanoluminescence occurs when the co-doped ions are alkali metal ions other than Li, such as Na, K, Rb, or Cs ions. In particular, we investigated the cases where the luminescent center ion is 0.6 mol% Nd ion and the co-doped ion is 0.2 mol% Na, K, Rb, or Cs ion, as a representative example.

As a result, as shown in Figure 27, stress-induced luminescence was confirmed in the near-infrared region (>750 nm) regardless of whether the co-doped ions were Na ions, K ions, Rb ions, or Cs ions.

[22. Presence or absence of mechanoluminescence when other element ions are added together with Nd ions as the luminescence center ions]
Next, we investigated whether mechanoluminescence occurs when multiple luminescence center ions are present. In particular, in this study, as a representative example, we confirmed the presence or absence of mechanoluminescence in the case where the co-doped ion is 10 mol% Li ion, the first luminescence center ion is 0.4 mol% Nd ion, and the second luminescence center ion is 0.4 mol% of any one of ions selected from Cr (chromium), Mn (manganese), Cu (copper), Ag (silver), Ce (cerium), Pr (praseodymium), Sm (samarium), Eu (europium), Tb (terbium), and Yb (ytterbium).

As a result, as shown in Figure 28, regardless of the second luminescence center ion, mechanoluminescence was confirmed in the visible region (≦750 nm). In addition, for each of the Mn, Ag, Ce, Pr, Sm, Eu, Tb, and Yb ions, mechanoluminescence was also confirmed in the near-infrared region (>750 nm).

23. Whether or not mechanoluminescence occurs when the concentration of Nd, Yb, or Tb ions is changed
Next, for Yb or Tb examined in the above section [22. Presence or absence of mechanoluminescence when other element ions are added together with Nd ions as luminescence center ions], the presence or absence of mechanoluminescence in the near infrared region was confirmed when the concentration of the first luminescence center ion (Nd ion) and the concentration of the second luminescence center ion (Yb ion or Tb ion) were changed.

In particular, in this study, as a representative example, the case where the co-doped ion is 10 mol% Li ion, the first luminescent center ion is 0.2 mol% Nd ion, and the second luminescent center ion is 0.6 mol% Yb ion was confirmed. In addition, the case where the co-doped ion is 10 mol% Li ion, the first luminescent center ion is 0.6 mol% Nd ion, and the second luminescent center ion is 0.2 mol% Tb ion was also confirmed.

As a result, as shown in Figure 29, stress-induced luminescence was confirmed even in the near-infrared range (>750 nm) regardless of whether the second luminescent center ion was a Yb ion or a Tb ion.

As described above, the stress-stimulated luminescent material of the present invention is formed by dissolving alkali metal ions and luminescent center ions in ZnO, so that it is possible to provide a stress-stimulated luminescent material using ZnO as the base material.

Finally, the above-mentioned embodiments are merely examples of the present invention, and the present invention is not limited to the above-mentioned embodiments. Therefore, even if the embodiments are different from those described above, various modifications can be made depending on the design, etc., as long as they do not deviate from the technical concept of the present invention.

Claims (2)

A method for producing a stress- stimulated luminescent material using ZnO as a base material, comprising mixing a zinc compound, a luminescence center supplying compound which supplies luminescence center ions , and a predetermined co-doped ion supplying compound which supplies co-doped ions, and firing the mixture (excluding a mixture containing a solid protective agent which is melted by heating to isolate a reaction system from an oxidizing atmosphere) in an oxidizing atmosphere,
the luminescence center ion is a Nd ion or a Nd ion and an ion of any one element selected from the group consisting of Cr, Mn, Cu, Ag, Ce, Pr, Sm, Eu, Tb, and Yb;
The co-doped ion is an ion of any one element selected from Li, Na, K, Rb, and Cs, or an ion of Li and Ba,
The method for producing a stress-stimulated luminescent material using ZnO as a base material is characterized in that the stress-stimulated luminescent material has a composition represented by the general formula ZnO:E _x M _y (wherein E is the co-doped ion and M is the luminescent center ion), where x = 0.0001 to 0.7 and y = 0.001 to 0.2. A method for producing a stress-stimulated luminescent material using ZnO as a base material, comprising the steps of: making an aqueous solution containing zinc ions, in which a luminescent center ion and a predetermined co-doped ion are present, basic , or reacting said aqueous solution with a sulfide, coprecipitating said co-doped ion and luminescent center ion with zinc hydroxide or zinc sulfide, and firing said coprecipitate (excluding those containing a solid protective agent which is melted by heating and isolates a reaction system from an oxidizing atmosphere) in an oxidizing atmosphere,
the luminescence center ion is a Nd ion or a Nd ion and an ion of any one element selected from the group consisting of Cr, Mn, Cu, Ag, Ce, Pr, Sm, Eu, Tb, and Yb;
The co-doped ion is an ion of any one element selected from Li, Na, K, Rb, and Cs, or an ion of Li and Ba,
The method for producing a stress-stimulated luminescent material using ZnO as a base material is characterized in that the stress-stimulated luminescent material has a composition represented by the general formula ZnO:E _x M _y (wherein E is the co-doped ion and M is the luminescent center ion), where x = 0.0001 to 0.7 and y = 0.001 to 0.2.

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