Plastic and nonmetallic article shaping or treating: processes – Shaping or treating luminescent material
Reexamination Certificate
2000-03-21
2001-08-07
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Shaping or treating luminescent material
C264S102000, C264S667000
Reexamination Certificate
active
06270702
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for the manufacture of scintillator ceramic by hot pressing a powder containing at least one rare earth oxisulfide (rare earth oxisulfide powder).
2. Description of the Prior Art
Scintillator elements of the detector elements in the radiation detector system are for computed tomography (CT) are manufactured as scintillator ceramic from rare earth oxisulfides. When irradiated by x-rays, such scintillator elements produce light that is coupled out from the translucent ceramic and is converted by a photodiode into an electrical signal.
Since the intensity of the light produced by scintillator element is a measure for the intensity of the incident radiation, the output current of the photodiode represents a measure for the intensity of the radiation incident on a scintillator element.
Because of its limited by the oxygen sensitivity of the rare earth oxisulfide at elevated temperatures, the compression of the rare earth oxisulfide powder into ceramic by means of hot pressing can take place only under protective gas or in a vacuum since, temperatures of up to 1,500° C. are required.
In the manufacture of rare earth oxisulfide powder and in the handling thereof, contamination by dust and abrasion particles is unavoidable. These particles are then carbonized as a result of the high temperatures during hot pressing and remain as black specks in the finished ceramic. If the scintillator element contains such imperfections, the light propagation in the scintillator element is disturbed, and such a ceramic is unsuitable as a detector for x-ray in so far as the imperfection exceeds a specific level.
In scintillator ceramic manufactured according to known methods, reject rates of a not inconsiderable level occur due to these reasons.
German OS 37 02 357 discloses supporting discoloration of a rare earth oxisulfide ceramic article pressed from rare earth oxisulfide particles by a heat treatment between 400 to 800° C. in air prior to an isostatic hot pressing. The surfaces of the rare earth oxisulfide powder particles are thereby oxidized by the formation of a limited amount of rare earth oxisulfate, and this the rare earth oxisulfide is not oxidized to rare earth oxisulfate in the interior of the rare earth oxisulfide powder particles. A discoloration of rare earth oxisulfides is therefore suppressed during hot pressing by means of the formation of rare earth oxisulfate on the surfaces of the rare earth oxisulfide powder particles.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of the above type that permits the manufacture of scintillator ceramic having a reduced rejection rate.
The above object is achieved in accordance with the invention in a method for manufacturing of scintillator ceramic by hot pressing a powder containing at least one rare earth oxisulfide, with sulfur oxyacid anions being added to the rare earth oxisulfide powder prior to hot pressing .
The added sulfur oxyacid anions form oxisulfates with the rare earth ions that are present in the rare earth oxisulfide powder during the heating of the rare earth oxisulfide powder which take place in the hot pressing in a protective gas or in a vacuum. These decay at 800° C. or higher, into the corresponding oxide and sulfur trioxide. Sulfur trioxide is an oxidizing agent that oxidizes carboniferous particles to carbon dioxide or carbon monoxide, and is itself reduced into sulfur dioxide or sulfur. Carbonized particles that might be present are therefore reduced in size or disappear completely as a result of the oxidation. Since gas is thereby released in the rare earth oxisulfide powder, the method is particularly well-suited for uniaxial hot pressing, since the gases can then escape unhindered. In the case of isostatic hot pressing—which is also possible—alkaline substances must be introduced into the envelope which can bind the gases being released.
The sulfur oxyacid anions are added in the form of sulfate ions and/or sulfite ions, the sulfate ions being added, for example, in the form of sulfates, hydrogen sulfates or thiosulfates, and the sulfite ions being added, for example, in the form of sulfites or hydrosulfites. It is preferred to add sulfates, since sulfates of many elements can be purchased and need not be specifically synthesized for the purposes of the present invention.
The harmful effect of calcium ions in scintillator ceramic disclosed in U.S. Pat. No. 3,502,590 is avoided by adding the sulfate ions and/or sulfite ions in the form of at least one combination of R
2
A or RA, whereby R is from the group H, Li, Na, K, Cs, NH
4
, and A is from the group SO
4
(sulfate), SO
3
(sulfite), HSO
4
(hydrogen sulfate), HSO
3
(hydrogen sulfite), S
2
O
3
(thiosulfate). Alternatively, these harmful effects can be avoided by adding the sulfate ions and/or sulfite ions in the form of at least one sulfate and/or sulfite of at least one of the rare earths contained in the rare earth oxisulfide powder.
The generation of foreign phase occlusions from compression additives disclosed in German PS 42 24 931 is avoided by adding 0.0001 to 0.05 mole, preferably 0.0025 to 0.01 mole, of sulfur oxyacid anions to the rare earth oxisulfide powder per mole of sulfide ions of the rare earth oxisulfide powder.
REFERENCES:
patent: 3502590 (1970-03-01), Royce et al.
patent: 4863882 (1989-09-01), Matsuda et al.
patent: 5296163 (1994-03-01), Leppert et al.
patent: 5518659 (1996-05-01), Rossner et al.
patent: 5676891 (1997-10-01), Boedinger
Fiorilla Christopher A.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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