Compositions – Inorganic luminescent compositions – Compositions containing halogen; e.g. – halides and oxyhalides
Reexamination Certificate
2000-07-03
2002-07-16
Koslow, C. Melissa (Department: 1755)
Compositions
Inorganic luminescent compositions
Compositions containing halogen; e.g., halides and oxyhalides
C423S263000, C423S311000
Reexamination Certificate
active
06419852
ABSTRACT:
The present invention relates to the use of a compound based on a thulium-containing lanthanum phosphate as a phosphor in a plasma or X-ray system.
Plasma systems (screens and lamps) form part of novel display and illumination techniques which are in the process of undergoing development. One specific example is that of the replacement of current television screens with flat screens which are lighter and larger in size, which replacement is on the point of being solved by the use of plasma panels.
In plasma systems, a gas introduced into an enclosure is ionized due to the effect of an electrical discharge. During this process, high-energy electromagnetic radiation is emitted. The photons are directed onto a luminescent material.
Likewise, in systems involving X-rays, the photons excite a luminescent material.
In order to be effective, this material must be a phosphor absorbing in the plasma or X-ray emission range and emitting in the suitable spectral range with as high a yield as possible.
Now, there is a need for a phosphor that can be used in plasma and X-ray systems, emitting in the blue.
The object of the invention is to provide such a phosphor material.
For this purpose, according to the invention, a compound based on a thulium-containing lanthanum phosphate is used as a phosphor in a plasma or X-ray system.
The invention also relates to a plasma or X-ray system, characterized in that it comprises the aforementioned compound as a phosphor.
The invention also covers a lanthanum phosphate which is characterized in that it contains thulium and in that it consists of particles having a mean size of between 1 and 20 &mgr;m with a dispersion index of less than 0.6.
Finally, the invention relates to a phosphor having the same characteristics as those given above in the case of the phosphate.
Further features, details and advantages of the invention will appear even more fully upon reading the following description, as well as from the various specific but non-limiting examples intended to illustrate it.
The invention is based on the discovery of the luminescence properties of certain phosphates with respect to plasma radiation or X-radiation. The invention therefore firstly relates to the use of the compound described above as a phosphor under conditions which are those of plasma systems. This, for the present description, should be understood to mean all systems using a gas which emits, after ionization, radiation corresponding to at least wavelengths lying between 100 and 200 nm, more particularly between 140 and 200 nm, that is to say the far ultraviolet range.
As systems of this type, mention may be made of plasma screens and lamps.
The phosphor of the invention can also be used in systems using X-rays. X-rays should be understood to mean here, for the present description, photons whose energy is between 10 and 100 kev.
As X-ray systems, mention may be made of imaging systems, especially medical imaging systems.
The phosphor of the invention is a compound comprising a matrix of the LaPO
4
type. This phosphate furthermore contains thulium as a dopant. Thulium is present in the phosphate in a trivalent form.
This phosphate, excited by radiation of the plasma or X-ray type, emits in the blue.
This phosphate may furthermore contain gadolinium as a codopant.
In general, the thulium content, expressed in at % with respect to the lanthanum, is between 0.1 and 10, more particularly between 0.5 and 5.
The gadolinium content, expressed in at % with respect to the lanthanum, may vary between 10 and 40%.
According to a preferred embodiment of the invention, a lanthanum phosphate consisting of particles having a mean size of between 1 and 20 &mgr;m with a dispersion index of less than 0.6 is used.
The particle size may more particularly be between 2 and 6 &mgr;m. The dispersion index may more particularly be at most 0.5.
Throughout the description, the particle size and size distribution analysis characteristics are measured by a sedimentation technique using a particle size analyzer of a Sedigraph type. The measurement is conventionally carried out on an aqueous dispersion of the product which is treated by ultrasonic deagglomeration (5 minutes, 120 watts).
The term “dispersion index” should be understood, to mean the ratio:
&sgr;
/m
=(
d
84
−d
16
)/2
d
50
in which
d
84
is the particle diameter for which 84% of the particles have a diameter of less than d
84
;
d
16
is the particle diameter for which 16% of the particles have a diameter of less than d
16
;
d
50
is the mean diameter of the particles.
The invention also relates to plasma or X-ray systems or devices which comprise a phosphor compound as described above. All of the characteristics that were given above with regard to the phosphor compound and the phosphate also apply here to the description of the systems or devices. These characteristics will therefore not be repeated here.
The invention also relates to the use of the phosphor compound in the manufacture of these systems or devices. This use is accomplished using well-known techniques, for example by screen-printing deposition, electrophoresis or sedimentation.
The invention relates moreover, by way of novel product particularly suitable for use, as described above, as a phosphor, to a specific lanthanum phosphate as well as to its method of preparation.
The method of preparation will firstly be described.
This method consists in producing a direct precipitation at a controlled pH by making a first solution, containing soluble salts of rare earths (lanthanum, thulium and, where appropriate, gadolinium salts), these elements then being present in the required stoichiometricproportions for obtaining the product of the desired formula, react with a second solution containing phosphate ions.
The solution of soluble rare-earth salts is introduced into the solution containing the phosphate ions. In general, the salt solution is introduced into the phosphate-ion solution gradually and continuously.
The initial pH of the solution containing the phosphate ions is less than 3, and preferably between 1 and 3.
The pH of the precipitation medium is then controlled to a pH value of less than 2, and preferably between 1 and 2. If the initial pH of the solution containing the phosphate ions is greater than 3, introduction of the solution of rare-earth salts causes a reduction in the pH value of the reaction mixture formed by mixing this solution with the initial phosphate-ion solution. In this case, the pH value is left to drop to a value of less than 2 and once the desired pH value has been achieved it is this value that is controlled.
The expression “controlled pH” should be understood to mean maintaining the pH of the precipitation medium at a certain constant, or approximately constant, value by the addition of basic compounds or of buffer solutions to the solution containing the phosphate ions, this being done simultaneously with the introduction into the latter of the solution containing the soluble rare-earth salts. The pH of the medium will thus vary by at most 0.5 pH units about the fixed set value, and more preferably by at most 0.1 pH units about this value.
Advantageously, this pH control is achieved by the addition of a basic compound, as will be explained below.
The precipitation is preferably carried out in an aqueous medium at a temperature which is not critical and advantageously lies between room temperature (15-25° C.) and 100° C. This precipitation takes place while the reaction mixture is being stirred.
The concentrations of the rare-earth salts in the first solution may vary over wide limits. Thus, the total concentration of rare earths may be between 0.01 mol/liter and 3 mol/liter.
The rare-earth salts that are suitable are in particular those which are soluble in an aqueous medium, such as, for example, nitrates, chlorides, acetates and carboxylates, or a mixture of these salts. According to the invention, the preferred salts are nitrates.
The phosphate ions intended for reacting with the solution of rare-earth salts may be provided by pure compou
Braconnier Jean-Jacques
Ceintrey Claude
Huguenin Denis
Koslow C. Melissa
Rhodia Chimie
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