JP4923242B2 - Manufacturing method of willemite crystal and manufacturing method of fluorescent substance using willemite crystal as mother crystal - Google Patents

Description

  The present invention relates to a method for producing a willemite crystal and a method for producing a fluorescent material using the willemite crystal as a mother crystal.

Willemite (Willemite, Zn ₂ SiO ₄ ) has long been used as a mother crystal of a fluorescent material. For use as a fluorescent material in cathode ray emission (CL) such as a cathode ray tube, electronic illumination (EL) such as a copy machine or a cash payment machine, plasma emission (PL) such as a flat-screen television, etc., Willemite may contain Mn ²⁺ (green), Eu. ^A small amount of ³⁺ (red), Tb ³⁺ (green), etc. is doped. Conventionally, it was synthesized by mechanically mixing zincite (ZnO) and quartz (SiO ₂ ) powder and firing at a high temperature of 1500 ° C. or higher. Therefore, it was difficult to obtain a pure phase due to evaporation of ZnO and the like.

  Then, spray heat denaturation method (spray pyrolysis method) developed, emulsified by carbonizing the solution prepared in advance in a beaker, to obtain gel-like granules by hot spray drying, Furthermore, a method has been developed in which it is heat-treated at a high temperature to obtain crystalline willemite.

There is also a method of using TEOS (Si alkoxide, tetraethylorthosilicate or tetraethoxysilane, <(C ₂ H ₅ O) ₄ Si>) as a SiO ₂ source, whereby the solution in the beaker is heated In the same manner, a gel-like granule is obtained, and is further fired to obtain β-type willemite at 1000 ° C. and α-type willemite at 1200 ° C. (see Non-Patent Document 1). Incidentally, α-type willemite is usually used as the fluorescent substance mother crystal.
YC Kang and SB Park, Zn2SiO4: Mn phosphor particles prepared by spray pyrolysis using a filter expansion aerosol generator, Materials Research Bulletin, 35, 1143-1151, 2000.

However, the method of Non-Patent Document 1 has a high cost because it uses expensive TEOS as the SiO ₂ source.

  Accordingly, the present invention provides a simple and inexpensive method for producing a willemite crystal and a method for producing a fluorescent material using the willemite crystal as a mother crystal.

In the method for producing the willemite crystal of the present invention, the SiO ₂ source solution, the pH adjuster solution and the ZnO source solution have a ZnO / SiO ₂ molar ratio of 1.5 or more, preferably 1.7 or more, particularly 1.7-2. The mixture is mixed so as to be 2 and the precursor gel obtained by drying the produced precipitate is heated to crystallize.

In the above configuration, water glass is used as the SiO ₂ source solution, a base such as alkali or ammonia is used as the pH adjusting solution, preferably caustic soda, and a zinc soluble salt such as zinc chloride or zinc sulfate is used as the ZnO source solution, preferably zinc nitrate. The precursor gel is heated to 800 ° C. to 1400 ° C. for crystallization.

Moreover, the manufacturing method of the fluorescent substance of the present invention is characterized in that the willemite crystal manufactured by the above method is doped with a light emitting metal of Mn ²⁺ , Eu ³⁺ or Tb ³⁺ .

  In the present invention, a willemite crystal can be produced at low cost by treating easily available substances such as water glass, zinc nitrate, and caustic soda in a simple process.

In addition, nano-sized crystals can be easily manufactured. By mixing the luminescent metal source solution with the SiO ₂ source solution, the pH adjusting agent solution and the ZnO source solution to form a gel, the fluorescent material can be easily produced.

One feature of the present invention is that a water-soluble silicate such as an alkali silicate is used as the SiO ₂ source. An alkali silicate, particularly sodium silicate, is obtained by dissolving so-called cullet with caustic soda, and is generally called water glass, and is an inexpensive raw material that is consumed in large quantities as silica gel, an adhesive, a soil hardening agent, and the like.

  It is known that water glass is dehydrated and condensed to gel by adding an acid or a metal salt. When this is dried, it becomes amorphous silicon oxide, but it is also possible to obtain an amorphous silicate in which metal ions of the metal salt to be added are incorporated in an arbitrary ratio between the silicate oxide bonds.

In the present invention, a soluble salt of zinc is added to a SiO ₂ source solution (generally an aqueous solution) such as water glass so that the molar ratio of ZnO / SiO ₂ is 1.5 or more, and when both are mixed. The pH of the supernatant is adjusted to 7 or less, preferably 5.5 to 6.5 by adding a base such as sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, ammonium, etc. And a precursor gel is obtained.

The mixing method is not particularly limited, but it is generally preferable to gradually add a soluble salt solution (generally an aqueous solution) of zinc as a ZnO source while stirring to the SiO ₂ source solution. The zinc soluble salt is generally a water-soluble salt such as zinc chloride, zinc sulfate, zinc oxalate, and zinc acetate, but zinc nitrate is particularly preferred.

A base is used as the pH adjusting agent. For example, sodium hydroxide, potassium hydroxide, sodium carbonate, aqueous ammonia and the like are used. When water glass is used as the SiO ₂ source, sodium hydroxide (caustic soda) is usually preferable. These pH adjusters may be gradually added simultaneously with the addition of the zinc soluble salt, but a predetermined amount of pH adjuster may be mixed in advance in a SiO ₂ source solution such as water glass. The point is that the pH in the supernatant after mixing the SiO ₂ source and the ZnO source is 7 or less, preferably 5.5 to 6.5.

When a ZnO source is added to the SiO ₂ source solution, gel formation starts soon and gelation proceeds slowly. Therefore, it is preferable to ripen with stirring for several hours even after the addition of the ZnO source.

One of the points of the present invention is to add the ZnO source 1.5 mol times or more of the SiO ₂ source. In order to obtain orthosilicate which is the object of the present invention, the molar ratio of ZnO source / SiO ₂ source is 2 as a theoretical amount, but in the present invention, 1.5 molar times or more, preferably 1. Willemite crystals can be obtained at 7 mol times or more, and further at 1.7 to 2.2 mol times. Here, when the amount is less than 1.5 mol times, the generation of amorphous silica increases, and when the amount is 2.2 mol times or more, the zincite tends to increase.

Further, when obtaining a fluorescent substance having willemite as a mother crystal, it is necessary to dope the willemite crystal with other metal ions. In general, a predetermined metal ion may be allowed to coexist during the gelation reaction of the silicate. That is, a soluble salt of the metal, such as a manganese salt (in this case, green fluorescence), a europium salt (in this case, red fluorescence), or a terbium salt (in this case, blue fluorescence), a metal salt such as Zn ₂ SiO ₄ 0.1-5 mass% as a metal with respect to 1 mol, Preferably 0.2-2 mass% should just be added.

Whether or not a metal ion for making a fluorescent substance is added, the gel-like precipitate that is a reaction product of the SiO ₂ source and the ZnO source is filtered or decanted. By collecting and washing with water, impurities such as sodium and unreacted ZnO source are removed and dried. The sodium content can be substantially completely removed by this washing and drying step.

  Next, the gel is heat-treated at 800 ° C. to 1400 ° C., preferably 1000 ° C. to 1300 ° C., to obtain willemite crystals, particularly fine crystals having a particle size of several nanometers.

  In addition, when the heating temperature is relatively low, such as about 800 ° C. to 900 ° C., β-type willemite tends to increase, and at 1000 ° C. to 1300 ° C., α-type willemite is mainly obtained. At this time, a small amount of zincite may be by-produced. Furthermore, if it exceeds 1400 ° C., cristobalite may be produced as a by-product, which is generally not preferred.

  Examples are shown below.

Water glass was used as the SiO ₂ source. In this example, Na ₂ SiO ₃ .9H ₂ O was used and dissolved in ion-exchanged water to obtain a 0.74 M SiO ₂ source solution. As a ZnO source, zinc nitrate Zn (NO ₃ ) ₂ .6H ₂ O was similarly dissolved in ion-exchanged water to obtain a 1.48M ZnO source solution. A 1 M caustic soda solution was used as a pH adjusting solution.

First, the SiO ₂ source solution and the pH adjusting solution were mixed in a beaker, and the ZnO source solution was added dropwise over 2 hours using a burette while stirring with a magnetic stir bar. The mixing ratio is fixed to 1: 1 with 50 mL of the ZnO source solution with respect to 50 mL of the SiO ₂ source solution, and the pH is adjusted by changing the pH adjusting solution in the range of 0 to 100 mL. The pH of the supernatant thus obtained was measured, the precipitate was filtered and washed, and dried in room temperature air for 3 days to prepare a precursor gel.

  Next, the obtained gel was taken in a platinum crucible and heated at a predetermined temperature in an electric furnace for 1 hour to sinter the gel.

  The fired product was identified by powder X-ray diffraction (XRD), the particle size was measured by the Schiller formula, and the form was observed by a scanning electron microscope (SEM). The chemical composition of the gel was determined by ICP analysis after heat-treating at 1000 ° C. for 1 hour in an absolutely dry state.

Table 1 shows the chemical composition of the gel and the composition of the norm mineral. In addition, the volume ratio in Table 1 represents the mixing volume ratio of the SiO ₂ source solution, the ZnO source solution, and the pH adjuster solution, for example, 1: 1: 1 as 1-1-1.

  From the above test, the chemical composition and pH dependence of the gel were examined.

  FIG. 1 is a graph showing the relationship between the molar ratio (Molar ratio) and yield (Yield) of gel and pH.

From FIG. 1, it was found that in the low pH region, the ZnO / SiO ₂ molar ratio was lower than 2, and the added zinc nitrate did not precipitate effectively, and a considerable portion was out of the system by washing. However, as the pH increases, the ZnO / SiO ₂ molar ratio rapidly approaches 2, and the initial molar ratio of 2 is obtained at pH 6.3. As the pH further increases, the molar ratio elongation slows down to a pH of 10. Even at 3, the molar ratio remained at 2.3. In addition, there is little mixing of sodium, and it is 1% or less especially in an acidic region.

From the above, in order to obtain a gel having a ZnO / SiO ₂ molar ratio of 2 or more, as shown in Sample 4 and Sample 5, it is necessary to prepare a pH control solution so that the pH is 6.3 or more.

  Next, the temperature for making the gel into α-type willemite was examined.

Table 2 is a table showing the heating temperature of each sample and the result of the generated crystal phase. 2 to 4 are XRD charts showing changes in gel heating.

  As shown in FIG. 2, when the gel of Sample 2 prepared to pH 5.5 (1-1-0.5) is heated, first, a small amount of zincite (ZnO) (indicated by Z in the figure) is formed, and then Then, β-type willemite (represented by β in the figure) is produced together with a small amount of zincite. Finally, pure phase α-type willemite (not labeled in the figure) that does not involve zincite is formed by heating at 1000 ° C. or higher. However, since there was a little more silica than the stoichiometric composition, a slight amount of cristobalite (denoted by C in the figure) was precipitated by heating at 1400 ° C.

  As shown in FIG. 3, when the gel of Sample 3 prepared to pH 5.7 (1-1-1) is heated, first, a very small amount of zincite (ZnO) is generated, and then β-type willemite is added together with a small amount of zincite. Generate. However, a trace amount of zincite remains at high temperature heating, and finally, pure phase α-type willemite without zincite is generated by heating at 1100 ° C. or higher.

As shown in FIG. 4, when the gel of Sample 4 prepared to pH 6.5 (1-1-1.5) having a ZnO / SiO ₂ molar ratio of 2 or more is heated, a small amount of zincite (ZnO) is first produced, Thereafter, α-type willemite is produced at 800 ° C. to 1400 ° C. Since there is a little more zinc than the stoichiometric composition, a trace amount of zincite always remains even when heated at high temperature, and a pure-phase α-type willemite cannot be obtained, but the symbiosis of zincite is extremely small.

Therefore, it is necessary to crystallize the gel adjusted so that the molar ratio of ZnO / SiO ₂ is 2 or more at 800 ° C to 1400 ° C.

Table 3 shows the crystallite diameter obtained from the Schiller equation for each sample. [113] The size in the plane direction was about 50 nm except for high-temperature firing.

  FIG. 5 is a scanning electron micrograph. In the scanning electron microscope observation, the particles are observed as random particles. In the baking at 900 ° C., generally nano-sized ones of 100 nm or less were obtained. However, the particle diameter increased as the firing temperature increased, and grain growth up to about 1 μm was observed at 1300 ° C. firing.

A phosphor was produced by doping willemite with the light-emitting metal Mn ²⁺ . As a manganese source, manganese nitrate, Mn (NO ₃ ) ₂ .6H ₂ O, 0.15M solution was used. A predetermined amount of it was mixed with a zinc source solution in a beaker in advance. The mixing ratio and pH of the solution were as shown in Table 4.

Next, the mixed solution was separated into a burette and dropped into the beaker containing the silica source over 2 hours together with the zinc source. After measuring the pH of the supernatant, it was washed by filtration and the precipitate was collected. A silicate gel containing Mn ²⁺ was obtained by drying at room temperature for 3 days. The analysis results of the silicate gel are shown in Table 5.

  The gel was wrapped in platinum foil, placed on an alumina boat, inserted into a horizontal tubular furnace in which argon was flowed, and heated at 900 ° C.

  The chemical composition of the silicate gel was determined by fluorescent X-ray analysis except for the sodium content, and by atomic absorption analysis for the sodium content. The sample used was a dehydrated material preheated at 1000 ° C., the fluorescent X-ray analysis was performed by the glass bead method, and the atomic absorption analysis was performed in a Teflon (registered trademark) container by the hydrofluoric acid mixed liquid processing method.

FIG. 6 is an XRD chart (-900 ° C., heated for 1 hour) of a silicate gel obtained by doping with Mn ²⁺ .

  When manganese is doped, sodium contamination is very small and undetected. Further, when manganese is doped, the amount of by-products of zincite and ZnO is reduced, which is almost none.

The uremite doped with Mn ²⁺ was confirmed to emit green light when irradiated with 254 nm black light.

In the same manner as in Example 2, a sample doped with Mn ²⁺ , Eu ³⁺ and Tb ³⁺ was prepared at pH 6.3. The original solution to be doped was prepared by dissolving each nitrate in ion-exchanged water, mixed in a zinc solution and dropped. According to powder X-ray diffraction, only uremite was identified when doped with manganese, but a small amount of zincite was observed when doped with europium or terbium.

The fluorescence spectrum was measured by excitation with ultraviolet light (mercury lamp) of 254 nm. The emission colors are green for Mn: Zn ₂ SiO ₄ [FIG. 7 (a)], red for Eu: Zn ₂ SiO ₄ [FIG. 7 (b)], and blue for Tb: Zn ₂ SiO ₄ [FIG. 7 (c)]. And the spectra are as shown in FIG. The emission intensity of the green phosphor was extremely strong, but the red phosphor was moderate and the blue phosphor was weak.

It is a graph which shows the relationship between the molar ratio and yield of a gel, and pH. It is an XRD chart which shows the heating change of the gel of the sample 2 (alpha without a label). It is an XRD chart which shows the heating change of the gel of the sample 3 (alpha without a label). It is an XRD chart which shows the heating change of the gel of the sample 4 (alpha without a label). It is a scanning electron micrograph. It is an XRD chart of the silicate gel obtained by doping with Mn ²⁺ (α without label). It is a spectrum figure of the luminescent color of Mn2 ⁺ , Eu3 ^+, and Tb3 ⁺ .

Claims (7)

The precursor gel obtained by mixing the SiO ₂ source solution, the pH adjuster solution and the ZnO source solution so that the molar ratio of ZnO / SiO ₂ is 1.5 or more and drying the resulting precipitate is heated. And a method for producing a willemite crystal. Using water glass as SiO _{2 and} a soluble salt of zinc as a ZnO source, using a base as a pH adjuster, adjusting the pH at the end of the gelation reaction to 7 or less to produce a gel was obtained. The method for producing a willemite crystal according to claim 1, wherein the gel is crystallized by heating to 800 ° C to 1400 ° C after drying. The method for producing a willemite crystal according to claim 2, wherein a gel is formed by adding a pH adjusting agent to an aqueous water glass solution and adding an aqueous zinc soluble salt solution with stirring. 4. Water glass as the SiO ₂ source solution, caustic soda as the pH adjusting solution, zinc nitrate as the ZnO source solution, and the precursor gel is heated to 800 ° C. to 1400 ° C. for crystallization. The manufacturing method of the willemite crystal as described in any one of the above.   5. A method for producing a fluorescent material using a willemite crystal as a mother crystal, wherein the willemite crystal produced by the method for producing a willemite crystal according to claim 1 is doped with a luminescent metal. 6. The method for producing a fluorescent material using a willemite crystal as a mother crystal according to claim 5, wherein the luminescent metal is any one of Mn ²⁺ , Eu ^{3+ and} Tb ³⁺ . The method for producing a fluorescent material according to claim 5 or 6, wherein a light-emitting metal ion of Mn ²⁺ , Eu ³⁺ or Tb ³⁺ is mixed with an aqueous solution of zinc soluble salt and added to a water glass aqueous solution.