Chemistry of inorganic compounds – Rare earth compound
Reexamination Certificate
2000-03-14
2001-10-02
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Rare earth compound
Reexamination Certificate
active
06296824
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for producing rare earth oxysulfide powder.
2. Description of the Prior Art
Rare earth oxysulfide powders can be utilized as luminescent material powder for the manufacture of X-ray screens.
It is possible to produce translucent ceramics from such powders by means of pressing, preferably hot-pressing, scintillator elements, in turn, can be produced from these translucent ceramics for the X-ray detectors in computed tomography (CT) devices.
It is known to produce gadolinium oxysulfide powder by reducing doped gadolinium sulfite (see German PS 42 24 931). This method is complicated and requires the utilization of toxic sulfur dioxide gas for acquiring gadolinium sulfite from gadolinium oxide. Moreover, excessive sulfur is released in the form of sulfur dioxide and sulfur given the reduction of the gadolinium sulfite to gadolinium oxysulfide; this requires a complicated exhaust gas cleaning or purification. Therefore, it this known technique for the production of rare earth oxysulfide powder is technically complex and expensive.
The same is true for the method known from “Patent Abstracts of Japan”, C-262, Jan. 24, 1985, vol. 9/No. 17, (for Japanese Application 59-164631), wherein rare earth oxysulfide powder is produced by reducing rare earth oxysulfate, with the rare earth oxysulfate being acquired by dissolving a rare earth oxide in sulfuric acid or sulfate, and the corresponding rare earth hydroxide is formed by adding an alkaline substance. The rare earth oxysulfate is acquired from the rare earth hydroxide by heating in an oxidizing atmosphere.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of the above cited species, which method can be carried out with little technical outlay and which can be carried out inexpensively.
This object is inventively achieved in a method for producing rare earth oxysulfide powder, including reduction of rare earth oxysulfate to rare earth oxysulfide powder, with the rare earth oxysulfate being acquired by the method steps of suspending of at least one rare earth oxide in water, to form a slurry adding 1 mol of sulfuric acid or a corresponding dissolved sulfate of at least one of the rare earths per mol of rare earth oxide and thereby producing a powdery precipitate, and calcining the such received powdery precipitation to rare earth oxysulfate *), the at least one of the rare earths, dissolved sulfate of which is added, being one of the rare earths oxides of which are suspended in water
In the inventive method, toxic sulfur dioxide does not have to be handled, since it is not required for the acquisition of rare earth oxysulfate. Moreover, only the amount of sulfur that is absolutely necessary for producing the rare earth oxysulfide is added to the rare earth oxide, so that complicated measures for exhaust gas cleaning are not required. Further, it is possible to control the sulfide content in the rare earth oxysulfide powder via the amount of the added sulfuric acid or the amount of the added sulfate by means allowing only partial conversion of the rare earth oxide into oxysulfate. The known method permits this only to a limited extent. The properties of luminescent material powder, or of scintillator ceramic, which are produced from the rare earth oxysulfide powder, which is acquired according to the inventive method, can be influenced by influencing the sulfide content.
In a version of the invention, the concentration of the suspension can be 0.01 through 2 mol of rare earth oxide per liter of water. The reaction vessel can be used more effectively due to the high concentration of up to 2 mol of rare earth oxide per liter of water. The known methods only allow concentrations of 0.1 mol oxide per liter of solution.
In an embodiment of the invention, the suspension is maintained at an increased temperature before the calcining in order to support the precipitate formation.
In a further version of the invention, at least one dopant of the group praseodymium, europium, terbium, dysprosium, holium, erbium, ytterbium, samarium, and cerium can be added to the suspension as an activator prior to the calcining, the dopant preferably added as an oxide, a hydroxide, a salt and/or a saline solution, with water-soluble salts, such as chlorides, sulfates, nitrates or acetates or aqueous solutions of these salts preferably being utilized. The aforementioned activators are materials which determine the light emission properties of the luminescent material powder or the scintillator ceramic that are produced from the rare earth oxysulfide powder. These light emission properties include not only the wavelength of the light that is emitted in an excited state, but also the duration of the excited state (persistence) given excitation by irradiation with X-radiation. In the case of scintillator ceramics, the activators are embedded in the matrix that is formed by the rare earth oxisulfide.
In a preferred version of the invention the reduction to the rare earth oxysulfide, under protective gas, ensues using a reducing agent of the group hydrogen, carbon monoxide, hydrocarbon gas, such as methane or propane. Thus, exhaust gases that contain sulfur do not arise.
In a further version of the invention, at least one rare earth oxysulfate of the group yttrium oxysulfate, lanthanum oxysulfate, gadolinium oxysulfate, lutetium oxysulfate is utilized as the rare earth oxysulfate, when the rare earth oxysulfide powder is to serve as initial material for producing X-ray screens or scintillator ceramic for X-ray detectors.
In a further version of the invention, ammonium sulfate and/or smoking sulfuric acid can also be added to the suspension before the calcining in order to introduce sulfate ions.
REFERENCES:
patent: 5296163 (1994-03-01), Leppert et al.
patent: 59-013625 (1984-01-01), None
patent: 59-021523 (1984-02-01), None
patent: 59-164631 (1984-09-01), None
Patents Abstracts of Japan, C-262, vol. 9/No. 17, Jan. 24, 1985, for Japanese application No. 58-38257.
Bos Steven
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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