United States Patent [191
`Schuil
`
`4,024,070
`[1 1]
`[45] May 17, 1977
`
`[54] METHOD OF MANUFACTURING A CERIUM
`ACTIVATED LUMINESCENT RARE-EARTH
`ALUMINATE
`[75] Inventor: Roelof Egbert Schuil, Emmasingel,
`Netherlands
`[73] Assignee: U.S. Philips Corporation, New York,
`NY.
`May 16, 1975
`[22] Filed:
`[21] Appl. No.: 578,165
`[30]
`Foreign Application Priority Data
`May 24, 1974 Netherlands .................... .. 7406960
`
`[52] US. Cl. ...................................... .. 252/3014 R
`[51] Int. Cl.2 ...................................... .. C09K 11/46
`[58] Field of Search ............ .. 252/3014 R; 423/263
`[56]
`References Cited
`UNITED STATES PATENTS
`
`3,564,322
`3,577,350
`
`2/1971
`5/1971
`
`Blasse ................... .. 252/301.4 R X
`Amster ..................... .. 252/301.4 R
`
`Royce et al. .............. .. 252/301.4 R
`3,623,994 11/1971
`3,657,140 4/1972 Gibbons et a1. .......... .. 252/3014 R
`FOREIGN PATENTS OR APPLICATIONS
`
`Japan ..................... .. 252/301.4 R
`9/1971
`4,633,002
`267,784 7/1970 U,S.S.R.
`........ .. 257/301.4 R
`236,441
`6/1969 U.S.S.R.
`............... .. 423/263
`Primary Examiner-Jack Cooper
`Attorney, Agent, orFirm-Frank R. Trifari; Norman N.
`Spain
`‘
`
`ABSTRACT
`[5 7]
`A method of manufacturing a rare-earth aluminate,
`particularly a luminescent rare-earth aluminate, in
`which a starting mixture of the composite oxides or of
`compounds producing these oxides at an elevated tem
`perature together with a ?ux is heated at a high temper
`ature. At least one of the compounds rubidium ?uo
`ride, cesium ?uoride and potassium ?uoride is used as
`a ?ux.
`
`6 Claims, No Drawings
`
`VIZIO 1015
`
`

`
`1
`
`METHOD OF MANUFACTURING A CERIUM
`ACTIVATED LUMINESCENT RARE-EARTH
`ALUMINATE
`
`5
`
`20
`
`25
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`35
`
`4,024,070
`2
`replaced by aluminum ?uoride. A drawback of the use
`of AIF3 as a ?ux is that this material is very hygroscopic
`and is thus not stable in the atmosphere. It has been
`found that the use of an AlFa hydrate for the manufac
`ture of rare-earth aluminates yields very poor results.
`Netherlands Pat. Application 7213860 describes the
`use of ammonium chloride as a ?ux in the manufacture
`of a luminescent cerium-activated yttrium aluminate.
`After a preheating period in this method, a very long
`heat treatment has to be carried out, for example, for
`16 hours at l250°-1280° C. A major drawback of this
`known method is that the luminescent material ob
`tained is found to have a persistence period which is
`considerably longer than the persistence period of alu
`minates prepared without a ?ux, so that the material is
`less suitable for practical purposes.
`It is an object of the invention to provide a method of
`manufacturing rare-earth aluminates in which the
`drawbacks of the known methods are overcome.
`According to the invention a method of manufactur
`ing a rare-earth aluminate, particularly a luminescent
`rare-earth aluminate in which a starting mixture of the
`composite oxides or of compounds producing these
`oxides upon elevation of the temperature, together
`with a ?ux is heated at a high temperature is character
`ized in that at least one of the compounds rubidium
`?uoride, cesium ?uoride and potassium ?uoride is used
`as a ?ux.
`It has been found that the use of rubidium ?uoride
`and/or cesium ?uoride and/or potassium ?uoride as a
`?ux in the manufacture of rare-earth aluminates en
`hances the reaction speed in such a manner that a
`starting mixture of aluminium oxide and rare-earth
`oxides (or of compounds producing these oxides at an
`elevated temperature) can be used, so that mixtures of
`very reactive hydroxides or carbonates are not neces
`sary. In addition it has been found that there is a com
`plete reaction at relatively low reaction temperatures.
`Finally, a method according to the invention is found to
`yield a very satisfactorily crystallized powder. An ad
`vantage of the method according to the invention is
`that the ?uorides used as a ?ux are stable in air.
`Comparatively large quantities of rare-earth alumi
`nates having very satisfactory properties can be ob
`tained in an economical manner with the aid of the
`method according to the invention. A great advantage
`of the method according to the invention is that lumi
`nescent rare-earth aluminates having very satisfactory
`luminescence properties can be obtained. Notably, the
`persistence period of these materials (approximately
`0.1 usec) is practically not affected.
`In a method according to the invention it is advanta
`geous to use rubidium and/or cesium and/or potassium
`?uoride in a quantity of from 0.1 to 15% by weight,
`calculated with respect to the starting mixture. With
`quantities of less than 0.1% by weight the resultant
`effect is poor and for quantities of more than 15% by
`weight materials having a lower brightness are obtained
`in the case of manufacture of luminescent aluminates.
`The ?uoride is preferably used in quantities of from 1
`to 10% by weight, because then optimum results are
`achieved.
`The use of rubidium ?uoride as a ?ux is preferred. In
`fact, when manufacturing luminescent aluminates, the
`highest brightnesses are obtained with this material. It
`is to be noted that the use of other alkali?uorides (such
`as sodium, lithium or ammonium ?uoride) as a ?ux
`
`The invention relates to a method of manufacturing a
`rare-earth aluminate, particularly a luminescent rare
`earth aluminate. Furthermore, the invention relates to
`the aluminate manufactured by this method.
`The aluminates of one or more of the rare earths are
`used in different important ?elds of technical applica
`tions. In this description and in the claims the rare
`earths to be denoted by the general symbol Ln are
`understood to include the elements having atomic
`numbers 57 to 71 and the element yttrium. An impor
`tant group of rare-earth aluminates may be de?ned by
`the formula LnaAlsOlz. These materials have the garnet
`crystal structure and are used, for example, as laser
`material or as a luminescent material. Generally, part
`of the element chosen for Ln is replaced by another
`rare-earth element serving as an activator. Known laser
`materials are neodymium-activated yttrium aluminate
`and thulium-activated erbium aluminate. A known,
`very efficient luminescent material having a very short
`persistence is cerium-activated yttrium aluminate
`whose most important use is in cathode ray tubes for
`?ying spot scanners. Another group of rare-earth alu
`minates is de?ned by the formula LnAlO3. These alu
`minates generally have the perovskite crystal structure.
`These materials are also used as luminescent materials
`in which again a generally small part of Ln serves as an
`activator, for example, cerium-activated YAIO3.
`It is generally desirable to have very pure, satisfacto
`rily crystallized aluminates. So far this has only been
`possible if very cumbrous methods of manufacture are
`used. In order to obtain a luminescent cerium-activated
`yttrium aluminate having the garnet structure in the
`form of a satisfactorily crystallized powder, the starting
`material is, for example, a mixture of reactive hydrox
`ides or carbonates of aluminum and the rare earths.
`This mixture is prepared by precipitation from a solu
`tion comprising Y3", Al3+and Ce“, for example, in
`nitrate form, with the aid of ammonium hydroxide,
`ammonium carbonate or ammonium hydrocarbonate.
`The precipitate thus obtained is very voluminous and
`jelly-like so that ?ltering and subsequent washing with
`water is very dif?cult and time-consuming. After drying
`of the mixture of hydroxides and/or carbonates thus
`obtained, it is preheated at a temperature of, for exam
`ple, 300° C so as to remove the ammonium nitrate
`remainders left. To obtain a satisfactorily luminescent
`material, the mixture must subsequently be heated for
`2 hours, for example, at a relatively high temperature,
`for example, 1400° C. The known method thus de
`scribed has great drawbacks, particularly when com
`paratively large quantities of the luminescent material
`must be manufactured.
`The rare-earth aluminates may alternatively be man
`ufactured by starting from aluminum oxide and the
`rare-earth oxides. However, to obtain the desired reac
`tion, a heat treatment at a very high temperature, for
`example, l400°—l600° C has to be carried out. Another
`drawback of this method is that the resultant product is
`less satisfactorily crystallized. To decrease the reaction
`temperature and obtain a better crystallization of the
`product, it is known to add a so-called ?ux or meltig
`salt to the starting mixture of oxides. For example, part
`of the aluminum oxide in the starting mixture may be
`
`40
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`45
`
`50
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`55
`
`60
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`65
`
`VIZIO 1015
`
`

`
`Table '
`
`example
`
`% by weight RbF
`
`l
`II
`III
`IV
`
`2.5
`5
`10
`20
`
`H
`
`110
`120
`I25 '
`92
`
`4,024,070
`4
`3
`to the above-mentioned standard) are stated in column
`yields materials having low or even very low bright
`H the Table below.
`nesses, which is very surprising.
`The reaction circumstances in a method according to
`the invention, such as the duration of the heat treat
`ment and the temperature, may be chosen to be within
`wide limits, on the understanding that shorter heating
`periods may generally suf?ce when using compara
`tively high temperatures. A heat treatment of from 0.5
`to 4 hours at a temperature of from 1200° to 1400° C is
`preferred. The atmosphere in which the heat treatment
`is carried out is of little in?uence in a method accord
`ing to the invention and may be neutral, weakly reduc
`ing or oxidizing.
`The ?uoride used as a ?ux largely disappears by
`‘evaporation during the heattreatment. In many cases it
`is, however, preferred to remove ?ux residues, if any,
`from the resultant reaction product after cooling, by
`washing with water.
`A method according to the invention is preferred in
`which a luminescent yttrium aluminate having the gar
`net structure is obtained. These materials may very
`advantageously be used in cathode-ray tubes, discharge
`‘lamps and the like. Particularly preferred is the manu
`facture, according to the invention, of cerium-activated
`aluminates de?ned by the formula Y3_,,Ce,Al5O,2,
`where 0.005 s x s 0.5. These cerium-activated alumi
`nates are often used in cathode-ray tubes for ?ying spot
`scanners.
`The invention will now be described in greater detail
`with reference to the following examples.
`
`EXAMPLE V
`The method according to example I is repeated, with
`the difference that 10% by weight of CsF instead of
`RbF is used in the starting mixture. The brightness of
`the obtained product upon electron excitation is 121%
`relative to the said standard.
`
`15
`
`20
`
`EXAMPLE VI
`The use of 10% by weight of KF instead of RbF in a
`method according to example I yields a luminescent
`aluminate having a brightness of 109% relative to the
`standard.
`It is to be noted that the use of other alkali-?uorides
`as a ?ux leads to poor results in the manufacture of the
`rare-earth aluminates. This is apparent from the bright
`ness measurements of cerium-activated yttrium alumi
`nate obtained when using the optimum quantity of
`NaF, NH4F and LiF which are: 92, 72 and 44%, respec
`tively.
`What is claimed is:
`1. In the method of manufacturing a luminescent rare
`earth aluminate selected from the group consisting of
`the cerium activated rare earth aluminates of the for
`mula Ln3Al5Om and LnAlOs wherein Ln is at least one
`element selected from those having atomic numbers 57
`to 71 inclusive and yttrium by heating a starting mix
`ture of the oxides required to produce said aluminate
`or of compounds capable of producing such oxides
`upon being heated, together with a ?ux at a tempera
`ture of from about 1200° to 1400° C for about 0.5 to 4
`hours the improvement wherein the ?ux is at least one
`compound selected from the group consisting of rubid
`ium ?uoride, cesium ?uoride and potassium ?uoride
`employed in a quantity of from 0.1 to 15% by weight
`based on the starting mixture.
`2. The method of claim 1 wherein the ?ux is em
`ployed in a quantity of from about 1% to 10% by
`weight.
`3. The method of claim 2 wherein, after cooling, the
`resultant product is washed with water.
`4. A method as claimed in claim 2, characterized in
`that rubidium ?uoride is used as the ?ux.
`5. A method as claimed in claim 2, characterized in
`that a luminescent yttrium aluminate having the garnet
`crystal structure is manufactured.
`6. A method as claimed in claim 5, characterized in
`that the aluminate is defined by the formula Y3_,Ce,.
`A150... wherein 0.005 s x $ 0.5.
`*****
`
`30
`
`EXAMPLE I
`A mixture is made of 53.12 g Y2Oa and 62.4 g A1
`(OI-D3. This mixture is rubbed to a paste with 48 ml of
`a 0.1 M Ceaisolution and 2.5% by weight of RbF (ap
`proximately 2.90'g). After drying at 120° C the mixture
`is homogenized and subsequently heated for 2 hours in
`a closed quartz crucible at 1400° C. After cooling, the
`' product obtained is washed with water and dried. The
`product is a luminescent cerium-activated aluminate
`de?ned by the formula YzmCeoymAl?o12 and has the
`garnet crystal structure as shown by means of an X-ray
`diffraction analysis. The aluminate has the shape of a
`satisfactorily crystallized powder and when excited by
`electrons it has a brightness of 110% relative to the
`brightness of a standard. As a standard a material de
`fined by the same formula was used, but it was obtained
`by the known method starting from very reactive hy
`droxides.
`
`35
`
`4 O
`
`4 LII
`
`50
`
`EXAMPLES II to IV
`The method according to example I is repeated. sev
`eral times, each time with a different quantity of RbF so
`as to determine the in?uence of the quantity of ?ux on
`the brightness of the obtained luminescent material.
`The measuring results of the brightness (in % relative
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`55
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`65
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`VIZIO 1015