FR2692277A1 - Zn2SiO4: Mn phosphor powder and production methods. - Google Patents

Abstract

A phosphor powder based on manganese-doped zinc silicate (Zn2SiO4:Mn), with an average particle size of less than 20 micrometres and a decay time of less than 7 ms. A production method is also provided and comprises mixing zinc oxide, silica and manganese sulphate in an aqueous medium, drying the mixture and curing it at 1200-1400 DEG C. Said phosphor is suitable for use in cathode ray tubes.

Description

LUMINOPHORE POWDER OF Zn2SiO4: Mn
AND METHODS OF MAKING
The invention relates to a phosphor powder of zinc silicate doped with manganese (Zn2SiO4: Mn) and methods of making this powder.

 Zn2SiO4: Mn zinc silicate with a wiliemite structure is a phosphor known commercially under the name P1. Excited by photons or electrons, it emits green light. This phosphor is nowadays widely used, in particular, in plasma display devices.

In cathode ray tube display application, the green emission phosphor commonly used is zinc sulfide doped with copper and aluminum (ZnS
Cu, A1). Its decline time and its average grain diameter are adapted to television. However, this phosphor emits a little saturated green (green-yellow emission).

 Manganese-doped zinc silicate has the advantage of providing an emission in highly saturated green, which makes it possible to increase the number of color shades.

 However, the zinc silicate powders produced by the known methods have mean grain diameters greater than 30 μm and have high decay times (greater than 20 ms) which make them unsuitable for television applications.

 In general, the quality of display screens which require the use of phosphor powders (color plasma screens, cathode ray tubes, etc.) is very dependent on the particle size of the phosphors used. Powders with a high average particle size, for example greater than 30 films, do not make it possible to obtain sufficiently dense phosphor deposits. Indeed, the phosphor layer on the support (generally a glass slab), being an arrangement of grains, it is easy to imagine that its compactness will be all the greater as the "holes" in the layer, which correspond to grains missing, will be smaller.

 The invention relates to a luminophore powder which overcomes these drawbacks and which in particular makes it possible to be used in visualization applications.

 The invention therefore relates to a phosphor powder of zinc silicate doped with manganese (Zn2SiO4: Mn), characterized in that it has an average grain size of less than 20 micrometers and a decline time of less than 7 ms.

The invention also relates to a method for producing a phosphor powder according to claim 1, characterized in that it comprises the following steps
- mixing in an aqueous medium of zinc oxide (ZnO) of silica (SiO2) and of manganese sulphate (MnSO4)
- drying of this mixture
- cooking of the dried product obtained.

Another production method according to the invention comprises the following steps
- mixing in an aqueous medium of zinc oxide (ZnO) of silica (SiO2), of manganese carbonate (MnCO3) and of an alkaline salt
- drying of the mixture obtained
- cooking of the product obtained.

Finally, another production method according to the invention comprises the following steps
- preparation in aqueous medium of a zinc sulphate solution (ZnS04) and a manganese sulphate solution (MnSO4)
- mixing in a neutral gas atmosphere of these two solutions
- coprecipitation of zinc and manganese hydroxides from the two solutions mixed under an atmosphere of neutral gas
- preparation of a "sol" of silica
- mixing under a neutral gas atmosphere of this "sol" of silica and the two coprecipitated solutions
- transformation of the "soil" from silica to gel
- drying of the product obtained
- elimination by washing of sodium sulphate (Na2S04)
- drying of the product obtained and obtaining a powder
- cooking of the powder obtained.

 The various objects and characteristics of the invention will appear more clearly in the description which follows and in the appended figure which represents the particle size curves of phosphor materials.

 A phosphor powder material based on manganese-doped zinc silicate (Zn2SiO4: Mn) is known in the art. Its formula can also be written ZnO ~ Zn xMnxSiO4 and x can take different values such as for example x = 0.08.

 The phosphor powder based on zinc silicate according to the invention has the characteristic of having an average grain size of less than 20 micrometers while having a decay time of less than 10 ms and more precisely between 1 and 5 ms.

A method for producing the phosphor powder according to the invention comprises the following steps
Step n01: mixing in an aqueous medium of zinc oxide ZnO, of silica SiO2 and of manganese sulphate MnS04 in determined proportions. For example, to obtain Zn2 xMnxSiO4 with x = 0.08 the proportions can be as follows
ZnO: 19.5311 g
MnS04, 1120: 1.6901 g diluted in 50 cm3 of H2O
SiO2: 7.5106 g
The whole is suspended in 60 cm3
- Step nO 2: the aqueous solution obtained is dried. The zinc sulfate being soluble in water, when the suspension dries, it is adsorbed on the surface of the grains of the mixture (Si02, ZnO). Its distribution is thus very homogeneous.

 This drying can be done under infrared radiation at a temperature between 50 and 1000C for a few hours.

 - Step 3: Cooking the dried material obtained.

The manganese sulfate being distributed in a very homogeneous way, its decomposition during the heat treatment, will lead to an excellent distribution of the manganese dopant.

 This cooking is done in an atmosphere of neutral gas such as nitrogen or argon at a temperature between 12000 and 14000C for 1 to 10 hours. For example, a temperature of around 13000C for 4 hours is perfectly suitable.

 - Step nO 4: it is possible, although this is not essential depending on the applications, to provide a sieving step intended to destroy the aggregates that may have formed.

 Likewise before the cooking step (step No. 3) it is also possible to provide a sieving step.

 This process has the particularity of using manganese sulfate as the starting product. Manganese sulphate is easily soluble in water whereas, in known methods, manganese carbonate is used which is not soluble in water. Due to the solubility of manganese sulphate, during steps nb 1 and nO 2, a film of MnSO 4 is therefore formed around the grains of SiO 2 and ZnO.

In this way, a homogeneous distribution of manganese is obtained.

 The particle size distribution, measured with a device of the Coulter Multisizer type, is represented on the curve nO 1 of the appended figure. An average grain size of less than 20 µm and commonly 17 clam is obtained. The measured decay time is less than 7 ms (approximately 6.7 ms).

We will now describe a second method according to the invention making it possible to obtain a zinc silicate doped with manganese having at least the characteristics indicated above. This process includes the following steps
Step 1
Mixture in an aqueous medium of zinc oxide ZnO, of silica SiO2 of manganese carbonate MnCO3 and of an alkaline salt serving as flux and this in determined proportions. The alkaline salt can be for example KC1, Nazi, K2CO3 or
Na2CO3. KCl has given good results.

For example, to obtain Zna Mn Si04 with x = 0.08 the following proportions of the different products can be adopted
ZnO: 19.5311 g
MnCO3: 1.1495 g
SiO2: 7.5106 g
KCl: 1.1649 g
Step 2
Drying of the aqueous solution obtained, for example under infrared radiation at a temperature between 50 and 900C for a few hours.

Step 3
Optionally sieving the dried product to destroy the agglomerates that may exist.

Step 4
Baking the powder obtained after drying at a temperature between 1200 and 14000C for 1 to 10 hours, for example at 12750C for 4 hours.

 This cooking takes place in an atmosphere of neutral gas such as nitrogen so that the manganese, which must be divalent in the composition obtained, does not oxidize and does not become trivalent.

The advantage of this process lies in the use of an alkaline salt (KC1 for example) during the first step. Thus during cooking, during the melting of KCl, there will be creation of a flux and this will result in a mobility of the ZnO grains, of
SiO2 and MnCO3 and, therefore, a homogeneous distribution of the different elements.

Step 5
Possible sieving of the product obtained to eliminate aggregates that may exist.

 The particle size distribution of this powder, measured with the Coulter Multisizer, is shown in the figure and gives an average size of 7 EUM. The decay time is less than 7 ms.

 We will now describe a third process for obtaining zinc sulfate doped manganese according to the invention.

This process includes the following steps
Step 1
Preparation of two separate solutions
- a solution of zinc sulfate ZnSO4;
- a solution of manganese sulfate MnSO4,
By way of example, these solutions will have the following compositions ZnSO4> 7H20: 27.6047 g of ZnSO4 in 30 cm3 of H20 MnSO4, H2O: 0.6761 g of MnSO4 in 10 cm3 of 1120
Step 2
These two solutions are then mixed in a sealed enclosure placed under vacuum and then filled with a neutral gas such as nitrogen to avoid oxidation of manganese. The solutions are mixed by stirring for approximately 20 minutes.

Step 3
Still under an atmosphere of neutral gas, an aqueous solution of a base such as sodium hydroxide NaOH is added to the mixture obtained, having the effect of coprecipitating the two hydroxides Zn (OH) 2 and Mn (OH) 2 in the mixture.

Step 4
A silica "sol" is prepared by mixing, in predetermined proportions, TEOS tetraethoxysilane of formula
If (C2H5O) 4, water and acid (HCl for example). The whole is carefully mixed by stirring.

 For the proportions of products indicated in step n01 above, the amount of TEOS is 10.4167 g in 2 cm3 of H2O and the amount of acid (HCl 25 w) is for example 10 drops.

Step 5:
This "sol" is added to the coprecipitate mixture obtained in step No. 3. The whole is always stirred under an atmosphere of neutral gas.

Step 6
Then an ammonia solution is added to the mixture obtained in order to transform the "sol" of silica into gel. According to the proportions indicated above the amount of ammonia
NH40H required is approximately 1.5 to 2 cm3.

Step 7
We then proceed to drying the whole in an oven or under infrared at a temperature between 50 and 1300C for example 1200C for about 15 hours.

Step 8
The dried powder is washed with hot water in order to remove the sodium sulphate Na2 SO4 formed during the coprecipitation of the hydroxides Zn (OH) 2 and Mn (OH) 2.

Step 9
The washed product is dried in an oven under conditions similar to the previous drying.

Step 10
An optional sieving of the powder is carried out to remove the aggregates if there are any.

Step # 11
The sieved powder thus obtained is baked, under an atmosphere of neutral gas, at a temperature between 800 and 12000C and for 1 to 10 hours. For example, this cooking is done under an atmosphere of nitrogen at 9500C for 8 hours.

 The purpose of the neutral gas used in the various preceding steps is to avoid manganese oxidation. By way of example, this gas can be nitrogen.

 The particle size distribution of the powder obtained measured, with a device of the Coulter Multisizer type, is represented by the curve nO 3 of the appended figure. The average grain size is about 4.7 jirri. The decay time is less than 7 ms (about 6.5 ms).

 The appended figure represents particle size distribution curves of Zn1.92Mn0.08SiO4 powders.

The curve nO 0 represents a particle size distribution of a powder obtained by a known process; it is a process providing for the production of a mixture of silica SiO2, ZnO and MnCO3 followed by heating to around 13000C. This process provides agglomerates of large dimensions (about 30 µm on average).

 The curve nO 1 corresponds to the particle size distribution of the powder obtained by the first process described above.

 The curve nO 2 corresponds to the particle size distribution of the powder obtained by the second process.

 The curve nO 3 corresponds to the particle size distribution of the powder obtained by the third process described above.

 As can be seen the third method provides better results.

 In the foregoing, provision has been made, by way of example, for producing a Zn2 powder xMnxSiO4 with x = 0.08. We then obtained a decay time of about 6.7 ms.

However, by providing a different percentage of manganese, we obtain different decay times. For example for
x = 12 we obtain a decay time of about 4.3 ms
x = 0.15, a decay time of approximately 3.1 ms is obtained.

It can therefore be seen that the invention provides a phosphor powder of Zn2SiO4: Mn combining the three qualities necessary for display screens:
- average particle size less than 20 pm
- decay time less than 7 ms
- homogeneity of the distribution of the different elements (manganese in particular) in the powder.

Claims (16)

 1. A phosphor powder of manganese-doped zinc silicate (Zn2SiO4: Mn), characterized in that it has an average grain size of less than 20 micrometers and a decline time of less than 7 ms.  2. Method for producing a luminophore powder according to claim 1, characterized in that it comprises the following steps  - mixing in an aqueous medium of zinc oxide (ZnO) of silica (SiO2) and of manganese sulphate (MnSO4)  - drying of this mixture  - cooking of the dried product obtained.  3. Method according to claim 2, characterized in that the cooking is done at a temperature between 1200 and 1400 OC for 1 to 10 hours.  4. Method for producing a phosphor powder according to claim 1, characterized in that it comprises the following steps  - mixing in an aqueous medium of zinc oxide (ZnO) of silica (SiO2), of manganese carbonate (MnCO3) and of an alkaline salt  - drying of the mixture obtained  - cooking of the product obtained.  5. Method according to any one of claims 2 or 4, characterized in that the cooking takes place in an atmosphere of neutral gas.  6. Method according to claim 5, characterized in that the neutral gas is nitrogen.  7. Method according to claim 4, characterized in that the cooking is done at a temperature between 1200 and 14000C for a time between 1 and 10 hours.  8. A method of producing a phosphor powder according to claim 1, characterized in that it comprises the following steps  - preparation in aqueous medium of a solution of zinc sulphate (ZnSO4) and of a solution of manganese sulphate (MnSO4)  - mixing in a neutral gas atmosphere of these two solutions  - coprecipitation of the zinc and manganese hydroxides from the two solutions mixed under an atmosphere of neutral gas  - preparation of a "sol" of silica  - mixing under a neutral gas atmosphere of this "sol" of silica and the two coprecipitated solutions  - transformation of the "soil" from silica to gel  - drying of the product obtained  - elimination by washing of sodium sulphate (Na2 SO4)  - drying of the product obtained and obtaining a powder  - cooking of the powder obtained.  9. Method according to claim 8, characterized in that the coprecipitation is done by adding an aqueous solution of a base to the two mixed solutions.  10. Method according to claim 9, characterized in that the aqueous solution of a base is NaOH.  11. Method according to claim 8, characterized in that the "sol" of silica is made by mixing tetraethoxysilane (TEOS), water and acid.  12. Method according to claim 8, characterized in that the transformation of the "sol" from silica to gel is done by addition under an atmosphere of nitrogen of an ammonia solution.  13. Method according to claim 8, characterized in that the different steps from the mixture of the two solutions is carried out in an atmosphere of neutral gas.  14. Method according to claim 13, characterized in that the neutral gas is nitrogen.  15. Method according to any one of claims 2, 4 or 8, characterized in that the cooking step is preceded by a step of sieving the phosphor powder.  16. Method according to any one of claims 2, 4 or 8, characterized in that the cooking step is followed by a step of sieving the phosphor powder obtained.