WO1997025276A1

WO1997025276A1 - Compound based on zinc, gallium and oxygen, preparation process and utilization

Info

Publication number: WO1997025276A1
Application number: PCT/FR1996/002101
Authority: WO
Inventors: Denis Huguenin; Pierre Macaudiere
Original assignee: Rhodia Chimie
Priority date: 1996-01-03
Filing date: 1996-12-30
Publication date: 1997-07-17
Also published as: FR2743062A1; JPH11506413A

Abstract

The invention relates to a compound, more particularly a luminescent compound, based on zinc, gallium and oxygen, its preparation process and its utilization as luminophore. The compound is characterized in that it has a spinelle structure and in that it has an atomic ratio Ga/Zn of at least 2.5 and an X ray spectrum within which the position of the spectral lines on the 2 υ axis is situated between the line positions of the compositions Ga2O3 and ZnGa2O4. Said compound is obtained by preparing an homogeneous mixture at the atomic scale of the compounds of zinc, gallium and optionally the dopant; by proceeding to a first calcination with the air of the mixture and by carrying out a second calcination of the product obtained from the first calcination in a reducing atmosphere.

Description

COMPOUND BASED ON ZINC. GALLIUM AND OXYGEN. PROCESS

PREPARATION AND USE

RHONE-POULENC CHEMISTRY

The present invention relates to a compound, more particularly a luminescent compound, based on zinc, gallium and oxygen, its preparation process and its use. It is known that there are display devices in the electronics field using low energy electrons as an excitation source, ie of the order of 200 to 800V. This is the case with screens with field effect or fluorescent screens under vacuum. In this type of material, low energy electrons will excite a phosphor material placed on the screen. This phosphor material must have sufficient luminescence properties, it must emit effectively in the desired color. It must also have an electrical conductivity allowing it to evacuate the charges during its use.

Zinc gailates are already known as materials of this type. However, these products have the drawback of requiring a certain time, which can be several minutes, to reach their maximum emission when they are subjected to electronic excitation. We find this same drawback when the excitation stops because we then note, at this time, the persistence of a residual emission. This is the problem of remanence that must be solved when we want to use these products in display screens.

The object of the invention is therefore to provide a product which is effective as soon as the excitation is applied.

For this purpose, the compound of the invention is based on zinc, gallium and oxygen, and it is characterized in that it has a spinel structure and in that it has an atomic ratio Ga / Zn d 'at least 2.5 and an X-ray spectrum in which the position of the lines on the axis 2 θ is between those of the lines of the compositions Ga2θ3 and ZnGa2θ4.

The invention also relates to a luminescent compound based on zinc, gallium and oxygen and on a doping element, characterized in that it has a spinel structure and in that it has an atomic ratio Ga / Zn d 'at least 2.5 and an X-ray spectrum in which the position of the lines on the axis 2 θ is between those of the lines of the compositions Ga2θ3 and ZnGagθ4.

The invention also relates to a luminescent compound based on zinc, gallium and oxygen and a doping element, with a spinel structure, characterized in that it achieves, under electronic or photonic excitation, a steady state of luminescence in less than a second after the start of excitation.

The invention also relates to a compound based on zinc, gallium and oxygen and optionally a doping element, with a spinel structure, characterized in that it is capable of being obtained by a preparation process comprising the steps following:

• a homogeneous mixture is prepared on the atomic scale of a zinc, gallium compound and optionally the dopant;

- A first calcination is carried out in air of the mixture; - A second calcination of the product resulting from the first calcination is carried out, under a reducing atmosphere.

The invention also relates to the process for the preparation of the above-mentioned compounds, this process being as described above.

Other characteristics, details and advantages of the invention will appear even more completely on reading the description which follows and the appended drawings in which:

- Figure 1 is an RX spectrum of a compound according to the invention;

- Figure 2 is a curve of the emission intensity of a compound of the invention as a function of time when it is subjected to an excitation; - Figure 3 is an emission spectrum of a compound of the invention under photonic excitation.

As indicated above, the compound of the invention is based on zinc, gallium and oxygen and optionally a doping element.

The structure of this compound is of spinel type (AB2O4). The compound of the invention has different characteristics.

First, it has a Ga / Zn atomic ratio of at least 2.5. More particularly, this ratio can be between 2.5 and 4.

The compound of the invention has a specific structure which can be demonstrated by its X-ray spectrum. The specific lines of the compound on the axis 2 θ have a position which is located between the positions of the lines of the compositions Ga £ θ3 and ZnGa2θ4- These latter compositions constitute the two limits marking the ends of the domain of existence of the solid solution of spinel structure. The compound of the invention therefore appears as a solid solution of chemical elements or compounds in a phase of spinel structure. The compound of the invention may further comprise a doping element. The doping elements of this type of material are known. These are those which can confer luminescence properties on the compound or improve these properties. By way of example, mention may be made of lithium, halogens, cadmium, titanium, phosphorus, manganese and rare earths. Manganese is used more particularly. The amount of dopant, expressed as oxide, can generally range up to 10% by weight of the compound. The minimum amount is preferably 0.001%. In the case of the use of manganese, the amount of manganese can more particularly be between 0.1 and 2% by weight.

The compound of the invention can also be characterized in that, when it is subjected to electronic or photonic excitation, it reaches its stationary state of luminescence in less than a second after the start of the excitation. By stationary state of luminescence is meant the fact that the material emits with an essentially constant emission intensity. According to an advantageous embodiment of the invention, this stationary state can be obtained in less than a hundredth of a second.

What has been said above with regard to the structure of the compound and to the Ga / Zn ratio also applies here to the compound as it has just been defined in the preceding paragraph.

The deviation from the stoichiometry ZnGa2θ4 of the Ga / Zn ratio of the compound of the invention can be demonstrated by X-ray diffraction which makes it possible to determine revolution of the parameter of the mesh.

The process for preparing the compound of the invention will now be described. It will be recalled here that the invention also applies to the compound capable of being obtained by the process which will follow. Consequently, any characteristic given in the description of the process can be taken into account for the definition of the compound of the invention.

The first step of the process of the invention consists in producing a homogeneous mixture on the atomic scale of a compound of zinc, gallium and, optionally of dopant. By homogeneous mixture on the atomic scale is meant that there is no difference in the chemical composition of the mixture thus obtained on the atomic scale. This can be demonstrated by measurement by the energy dispersive spectroscopy (EDS) mapping method using a transmission electron microscopy (TEM) microprobe. Thus, in a homogeneous mixture on the atomic scale, there is no difference in the chemical composition of the mixture between the zones of the same surface taken into account by this method of analysis.

The intimate mixture of this type can be obtained by a chemical route, for example by coprecipitation or co-drying of salts of the constituent elements of the compositions of the invention (zinc, gallium and optionally doping). As salts, the salts of inorganic acids such as nitrates, sulfates or chlorides can be chosen, nitrates being the preferred salts. It is also possible to use the salts of organic acids and in particular the salts of saturated aliphatic carboxylic acids or the salts of hydroxycarboxylic acids. By way of examples, mention may be made of formates, acetates, propionates, oxalates or citrates.

In the case of coprecipitation, first of all, in a liquid medium, zinc salts, gallium salts and possibly a dopant are mixed.

Generally the liquid medium in which the mixture is formed is water. Therefore, water-soluble salts are used more particularly.

A base is then added to the medium formed previously.

Hydroxide type products can be used in particular as a base. Mention may be made of alkali or alkaline-earth hydroxides. It is also possible to use ammonia and the secondary, tertiary or quaternary amines.

Preferably, the operation is carried out under conditions such that the pH of the reaction medium is at most 8.

At the end of the reaction, a precipitate or a suspension is obtained which can be separated from the reaction mixture by any known means. The separated precipitate can optionally be washed with water. It is also possible to dry the precipitate at a temperature of about 100 ° C.

In the case of co-drying, the mixture of salts of the constituent elements of the compound is formed in a liquid medium, as described above, then this mixture is dried.

This drying can be done by any known means. However, according to a preferred embodiment of the invention, this drying is carried out by atomization. that is to say by spraying the mixture in a hot atmosphere (spray-drying). This type of drying makes it possible to obtain products with a fine and tight particle size. The atomization can be carried out by means of any sprayer known per se, for example by a spray nozzle of the sprinkler apple type or the like. One can also use so-called turbine atomizers. On the various spraying techniques likely to be used in the present process, reference may be made in particular to the basic work by MASTERS entitled "SPRAY-DRYING" (second edition, 1976, Editions Gerge Godwin - London).

For example, the temperature at the start of gas drying is usually between 200 and 300 ° C, that of the outlet can vary between 110 and 200 ° C. The pressure can be for example between 2 and 3 bars.

The dried product can also be optionally washed with water. One can also consider preparing the mixture of zinc, gallium and optionally dopant, by dry evaporation of the mixture of salts in a liquid medium.

Another method of preparing this mixture is freeze-drying. The process then comprises two stages of calcination. A first calcination is carried out in air of the mixture obtained previously. It will be noted here that it may be advantageous to carry out this first calcination in the presence of a flow. As suitable fluxes, mention may in particular be made of lithium fluoride, lithium chloride, potassium chloride, ammonium chloride, boron oxide and ammonium phosphates. The flux and the precipitate are intimately mixed before calcination, with a proportion of flux of approximately 1% by weight relative to the whole of the powder, for example.

A second calcination of the product resulting from the first calcination is then carried out, under a reducing atmosphere. The reducing atmosphere can be hydrogen or a hydrogen / neutral gas mixture such as argon. Generally, we work at a pressure equivalent to atmospheric pressure.

The conditions under which the calcinations are carried out can vary according to different embodiments of the invention. These modes depend on the value of the Ga / Zn ratio in the mixture which has been prepared for calcination. What has been said above in general about calcinations also applies to the particular embodiments which will be described.

According to a first embodiment, the mixture is prepared in such a way that it has a Ga / Zn atomic ratio of less than 2.5 or less than the desired value in the final compound which it is desired to prepare. In such a case, the calcination is carried out at a temperature which is lower than that which would be necessary, all the other conditions being identical elsewhere, in order to obtain the formation of a crystallized spinel phase. A temperature will be chosen which will allow the appearance of a poorly crystallized or pre-crystallized spinel phase. By poorly crystallized or precrystallized is meant a product whose RX diffraction spectrum presents halos in place of the peaks of a perfectly crystallized product.

Usually, the temperature of this first calcination is at most 500 ° C. It is usually between 300 and 500 * 0.

The second calcination is then carried out at a temperature which is higher than that of the first calcination. The temperature and the duration of this calcination are fixed so as to obtain the spinel phase in a well crystallized form. The maximum temperature is that beyond which there is a risk of demixing and the appearance of parasitic phases. Usually this temperature is between 700 and 1000 * 0. This second calcination makes it possible to remove the zinc from your composition obtained at the end of the first calcination. Depending on the conditions of this second calcination, the Ga / Zn ratio of the final compound can be varied and adjusted. Indeed, the increase in temperature as well as that of the ratio h ^ / Ar, that is to say the increase in the reducing nature of the atmosphere, in particular, makes it possible to vary the Ga / Zn ratio upwards.

The second embodiment of the invention relates to the case where the mixture is prepared in such a way that it has an atomic ratio Ga / Zn equal to the desired value in the final compound.

In this case, the first calcination is carried out at a higher temperature than that given for the previous embodiment and which may be equal to the temperature at which the second calcination is carried out. In this case, the temperature again remains below the demixing temperature. According to a preferred variant, the first calcination is carried out at two distinct temperatures, firstly at a relatively low temperature and secondly at a higher temperature. The first temperature is chosen to be high enough to be able to remove traces of salts, for example nitrates, and it can for example be between 300 and 500 ° C. The second temperature can be between 700 and 900 ° C.

The remainder of the process according to this second mode is ia calcination in a reducing atmosphere. This calcination can take place at a temperature between 700 and 900 ^β C.

The compounds of the invention can be used as luminophores. They can be used in particular in low voltage luminescence, in particular in the manufacture of any device implementing this low voltage luminescence, such as for example screens with field effect. They can also be used in cathode ray tubes.

Finally, the invention also relates to luminescent devices, of the screen type with field effect for example, incorporating the compounds described above or as obtained by the methods described above. In these devices, the compounds are placed on the screens subjected to low energy excitation. This implementation of phosphors in the manufacture of low-voltage luminescence devices is done according to well-known techniques, for example by deposition on screens by screen printing or electrophoresis.

Examples will now be given.

Example 1

Manganese, zinc and gallium nitrates are mixed in proportions corresponding to [Ga] / lZn + Mn] -> 2 and [MnJ / [Zn + Mn} = 0.005. The solution is then atomized on a Buchi device (inlet temperature: 250 ° C, outlet temperature: 115 ° C) to obtain a dry product. The latter is then calcined for 3 hours in air in the presence of LiF (1% by mass of mixture) at 500 ° C. whereby the most of the nitrates. A second calcination is then carried out for 3 hours under a reducing atmosphere (Ar / H2). The reducing gas flow rate is 321 / h, the H2 / Ar ratio is 10% by volume. Depending on the temperature of this second calcination, products with the Ga / Zn ratios given below are obtained.

The RX spectrum of the compounds of this example is given in FIG. 1. The vertical lines designated by arrows are lines characteristic of ZnGa2θ4- The shift observed with respect to the lines of the compounds of the invention highlights the modification of the parameter of mesh. Furthermore, the absence of a peak corresponding to the Ga2O3 phase proves that the products of the invention are in the form of a solid solution.

FIG. 2 is a curve of the emission intensity of the compounds of Example 1, obtained by subjecting them to photon excitation at 254nm. The point E indicates on the time axis the beginning of the excitation. The figure shows that the stationary state is obtained practically immediately.

FIG. 3 gives the emission spectrum of the compounds of Example 1 under photonic excitation at 254nm and at room temperature. Luminescent products in green.

Example 2

Manganese, zinc and gallium nitrates are mixed in proportions corresponding to [Gal / lZn + Mn] = 3 and [Mn] / IZn + Mn] = 0.005. Dried as in Example 1. The product obtained was then calcined in air in the presence of UF at 300 ° C 3 hours and then at 900 ^° C 3 hours. A second calcination is carried out at 875 * 0, the other conditions being the same as those of the second calcination of Example 1.

EXAMPLE 3 Manganese, zinc and gallium nitrates are mixed in proportions corresponding to [Ga] / [Zn + lvln] = 3 and [Mnjy [Zn + MnJ = 0.005. A sodium hydroxide solution is then added with stirring, whereby a mixed hydroxide is precipitated. The addition of sodium hydroxide is stopped as soon as a pH of 8 is obtained. The precipitate is then filtered, washed with water to remove the sodium and then dried at 100 ^* 0. It is then calcined as in Example 2.

The emission intensity curve as a function of time of the product thus obtained is the same as that given in Example 1.

Claims

1- Compound based on zinc, gallium and oxygen, characterized in that it has a spinel structure and in that it has an atomic ratio Ga / Zn of at least 2.5 and a spectrum of rays X in which the position of the lines on the axis 2 θ is between those of the lines of the compositions Ga2θ3 and ZnGa2θ4-

2- Luminescent compound based on zinc, gallium and oxygen and a doping element, characterized in that it has a spinel structure and in that it has an Ga / Zn atomic ratio of at least 2 , 5 and an X-ray spectrum in which the position of the lines on the 2 l'axe axis is between those of the lines of the compositions Ga2θ3 and ZnGa2θ4-

3- Luminescent compound based on zinc, gallium and oxygen and a doping element, with spinel structure, characterized in that it reaches, under electronic or photonic excitation, a steady state of luminescence in less than one second after the start of excitement, more specifically in less than a hundredth of a second.

4- Luminescent compound according to claim 3, characterized in that it has a Ga / Zn atomic ratio of at least 2.5.

5- Compound based on zinc, gallium and oxygen and optionally a doping element, with a spinel structure, characterized in that it is capable of being obtained by a preparation process comprising the following steps: • prepares a homogeneous mixture on the atomic scale of a zinc, gallium compound and optionally the dopant;

- A first calcination is carried out in air of the mixture;

- A second calcination of the product resulting from the first calcination is carried out, under a reducing atmosphere.

6- Compound according to claim 5, characterized in that it is capable of being obtained by a preparation process in which the above-mentioned homogeneous mixture is prepared by mixing zinc, gallium salts and optionally a doping and then drying said mixture, in particular by atomization.

7- A compound according to claim 5, characterized in that it is capable of being obtained by a preparation process in which the homogeneous mixture is prepared above by mixing zinc, gallium and possibly a dopant in a liquid medium, then adding a base to said mixture to precipitate said medium.

8- Compound according to one of claims 5 to 7, characterized in that it is capable of being obtained by a preparation process in which a mixture having a Ga / Zn atomic ratio of less than 2.5 or less is prepared at the desired value in said compound, then the first calcination is carried out in air, at a temperature below the crystallization temperature of the spinel phase and finally, the second calcination is carried out in a reducing atmosphere at a temperature above the temperature of ia first calcination.

9- Compound according to one of claims 7 or 8, characterized in that it is capable of being obtained by a preparation process in which, before the first calcination, the mixture is dried.

10- Compound according to one of claims 5 to 9, characterized in that it is capable of being obtained by a preparation process in which the first calcination is carried out in the presence of a flux.

11- Compound according to one of claims 2 to 10, characterized in that the doping element is ie manganese.

12- Process for the preparation of a compound according to one of claims 1 to 4 or 11, characterized in that it comprises the following steps: • a precursor is prepared by intimate mixing of the zinc, gallium salts and optionally doping;

- A first calcination is carried out in air of the mixture;

• a second calcination of the product resulting from the first calcination is carried out, under a reducing atmosphere.

13- A method according to ia claim 12, characterized in that the above mixture is prepared by mixing in a liquid medium zinc salts, gallium and optionally a dopant and then drying said mixture, in particular by atomization.

14- A method according to claim 12, characterized in that the aforementioned mixture is prepared by mixing in a liquid medium zinc salts, gallium and optionally a dopant and then adding a base to said mixture to precipitate said medium. 15- Method according to one of claims 12 to 14, characterized in that one prepares a mixture having an atomic ratio Ga / Zn less than 2.5 or less than the desired value in the compound which one seeks to prepare , then the first calcination is carried out in air, at a temperature lower than the crystallization temperature of the spinel phase and finally, the second calcination is carried out under a reducing atmosphere at a temperature higher than the temperature of the first calcination.

16- Method according to one of claims 14 or 15, characterized in that before the first calcination, the mixture is dried.

17- Method according to one of claims 12 to 16, characterized in that the first calcination is carried out in the presence of a flow.

18- Use as a phosphor of a compound according to one of claims 1 to 11.

19- Use as a phosphor in the manufacture of devices using low voltage luminescence of a compound according to one of claims 1 to 11.