IE903320A1 - Macrocrystals of ó-alumina in the form of platelets and¹process for obtaining them - Google Patents

Macrocrystals of ó-alumina in the form of platelets and¹process for obtaining them

Info

Publication number
IE903320A1
IE903320A1 IE332090A IE332090A IE903320A1 IE 903320 A1 IE903320 A1 IE 903320A1 IE 332090 A IE332090 A IE 332090A IE 332090 A IE332090 A IE 332090A IE 903320 A1 IE903320 A1 IE 903320A1
Authority
IE
Ireland
Prior art keywords
alumina
flux
macrocrystals
process according
diameter
Prior art date
Application number
IE332090A
Original Assignee
Atochem
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Atochem filed Critical Atochem
Publication of IE903320A1 publication Critical patent/IE903320A1/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00—Compounds of aluminium
    • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/10—Inorganic compounds or compositions
    • C30B29/16—Oxides
    • C30B29/20—Aluminium oxides
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B9/00—Single-crystal growth from melt solutions using molten solvents
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/20—Particle morphology extending in two dimensions, e.g. plate-like
    • C01P2004/22—Particle morphology extending in two dimensions, e.g. plate-like with a polygonal circumferential shape
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/54—Particles characterised by their aspect ratio, i.e. the ratio of sizes in the longest to the shortest dimension
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/60—Particles characterised by their size
    • C01P2004/61—Micrometer sized, i.e. from 1-100 micrometer

Abstract

Macrocrystals of alpha alumina in the form of monocrystalline hexagonal platelets. The invention also relates to a process for the manufacture of these macrocrystals by calcination of alumina, for example transition alumina, and in the presence of a fluorine-containing flux, at a relatively low temperature. The macrocrystals of alpha alumina can be employed as reinforcing materials.

Description

The invention relates to macrocrystals of aalumina or aluminium oxide or corundum, having the form of hexagonal platelets and to a process for the preparation of the said crystals from aluminium oxide or aluminium hydroxide.
The preparation of macrocrystals of alumina from aluminium hydroxide has already been described. Thus, in French Patent 2,441,594 macrocrystals of a-alumina are proposed which are in the form of hexagonal platelets having a particle size of from 16 to 250 μία and a diameter/thickness ratio of between 3/1 and 7/1, these macrocrystals being obtained by calcination of aluminium hydroxide at a temperature of between 1200"C and 1450°C in the presence of a mineralizing agent containing 0.001 to 0.5 % by weight of fluorinated compounds.
In German Patent Application DE-OS 2,623,482 it has been proposed to prepare primary crystals having an average size of between 16 and 25 μνα by calcination of aluminium hydroxide in the presence of at least one fluorinated salt and a vanadium salt. According to the abovementioned French patent (page 2, lines 19-22), raising the temperature or the use of other mineralizing agents has no or virtually no influence on the size and the shape of the crystals.
Moreover, Keiji DAIMON and Etsun KATO have proposed (Yogyo Kvokai Shi 94 [3] 1986 pp. 380-382 [7880]) the production of hexagonal platelets of o-alumina having a diameter of between 1.5 and 40 μία from hydrated aluminium sulphate in the presence of aluminium trifluoride.
In Japanese Patent Application No. 60/54916 it has been proposed to prepare platelets having an average diameter of 1.7 μτα. and a thickness of 0.18 μία from a mixture of hydrated aluminium sulphate and an alkali metal carbonate.
In Japanese Patent Application 60/38486 it has been proposed to prepare platelets having a thickness of less than 5 μία and a diameter (from the diameter/thickness ratio) of between 25 and 150 gm.
In Russian Patent 416313 it has been proposed to prepare hexagonal platelets of alumina which have a diameter of between 4 and 12 gm by thermal dissociation of an aluminium compound in the presence of hexagonal alumina seed at 1170°C.
Finally, in INTERCERAM No. 3, 1981 B.P. LOCSEL has proposed the preparation of α-alumina crystals having a diameter/thickness ratio of between 6 and 10 from bauxite having a low ferric oxide content, in the presence of aluminium trifluoride.
The invention relates to new macrocrystals of α-alumina having essentially the form of monocrystalline hexagonal platelets, the said platelets being characterized in that they have a diameter of between 2 and 20 μία., a thickness of between 0.1 and 2 μία and a diameter/thickness ratio of between 5 and 40.
Amongst these macrocrystals, the invention relates more particularly to macrocrystals of α-alumina in the form of hexagonal platelets having a diameter of between 2 and 18 μΐη, a thickness of between 0.1 and 1 μπι and a diameter/thickness ratio of between 5 and 40.
The invention also relates to a process for the 10 production of α-alumina in the form of essentially hexagonal platelets as defined above, this process being characterized in that it consists in carrying out the calcination of transition alumina or hydrated alumina in the presence of a flux which has a melting point of at most 800°C, contains chemically bonded fluorine and, in the molten state, dissolves the transition alumina or hydrated alumina.
The term transition alumina or hydrated alumina is used to indicate that this process applies to all types of aluminium oxide or aluminium hydroxide with the exception of a-alumina.
The flux, also termed mineralizing agent, having the characteristics defined above, essentially consists of one or more non-hydrolysable fluorinated compounds or a system comprising a phase consisting of the said non-hydrolysable, fluorinated compound or compounds and a phase consisting of a hydrolysable fluorinated compound, one of the said phases being dissolved in the other phase.
The systems comprising aluminium trifluoride and one or more alkali metal fluorides or alkaline earth metal fluorides, and in particular lithium fluoride, sodium fluoride, potassium fluoride or calcium fluoride, will be mentioned very particularly by way of illustration of the abovementioned fluxes. More specifically, the system AlF3-LiF in the form Li3AlF6 (lithium cryolite) or Li3Na3(AlFs)3 (cryolithionite) or 3AlFj,5LiF (lithium chiolite).
The fluxes used in the invention are in the form of powder, the particle size of which is preferably less than 1 mm (for at least 50 % by weight of the particles).
When carrying out the process according to the invention, at least 2 % and preferably 4 to 20 % by weight of flux are used relative to the weight of transition alumina or hydrated alumina employed.
The transition alumina or hydrated alumina can be chosen from a wide range of products and powders of various diameters and specific surface areas. The aluminas in which at least 50 % by weight of the particles have a diameter of less than 50 lm and preferably of less than 25 lm and yet more preferably of 1.5 lm will be used very particularly. Amongst these aluminas, preference will be given to the aluminas having a specific surface area equal to or larger than 100 m2/g (measured by the BET method) and preferably of κ u ti η ω ο .
MU - UtJ.u:*. cji OOCUJi PQuc between 100 and 400 m2/g.
Although dried aluminas may be used, preference will be given to the types which are non-dried or hydrated (AljOj, H20) , it being possible for the water content to reach 15 % of the weight of the alumina.
The calcination temperature can reach 1200’C or even higher. However, and this is a very particular valuable feature of the process according to the invention, the calcination can be carried out at a very much lower temperature, for example of between 900 and 1100°C.
In general, the mixture of transition alumina or hydrated alumina/flux, prepared by the customary techniques for powder mixtures, can be placed at ambient temperature in a heating chamber, the temperature being progressively raised to the abovementioned values. The temperature is advantageously raised in a rapid manner, that is to say of the order of lQOQ’C in 1 hour, this indication in no case limiting the field of the invention.
The calcination is advantageously carried out in an inert atmosphere, for example under a gentle stream of nitrogen. Once the calcination temperature has been reached, which temperature, as has been specified, can be between 900 and 1100C and more precisely around 1000*C, it is advantageous to maintain this temperature for a period which can range from half an hour to a few hours (for example up to 5 hours).
At the end of the calcination operation, the platelets can be chilled, or they can be allowed to cool naturally or in a refrigerated chamber, as desired.
The platelets collected essentially (> 95 % by weight) consist of α-alumina, which may be associated with the flux or with a product derived from the flux, such as AltLiOgF in the case of Li2AlF3. The flux or its derivative can be removed, for example by the action of a concentrated acid (H2SO* or HCl) under hot conditions (for example around 100’C).
The hexagonal platelets of α-alumina, which are the subject of the present invention, are suitable for numerous applications. They can be used, in particular, in the production of polishing products and as reinforcing materials for diverse materials, in particular ceramic matrices and metal and polymer matrices, on their own or in combination with fibres, particles or whiskers.
The following examples will enable the invention to be illustrated.
EXAMPLES 1 TQ 14 The equipment used consists of a furnace heated by a quartz tube in which a stream of nitrogen can be circulated and into which sintered aluminium crucibles containing the powder to be calcined are introduced.
The following aluminas are used (Dx - Υ pm indicates that X % of the particles (by weight) have a diameter smaller than Y lm): . P3s gamma-Al2O3 Dso = 1.1 im specific surface area = 172 m2/g . P2: gamma-Al2O3 Dso = 4.6 im specific surface area = 100 mJ/g . P,: Al2O3.H2O (boehmite) Dso = 25 lm D95 = 45 im specific surface area = 196m2/g . P4: gamma-Al2O3 D55 = 25 lm D37 = 45 lm specific surface area = 244 m2/g . P5: gamma-Al2O3 Di0 = 3.5 lm The following fluxes are used (D50 = lm having the meaning given above and D corresponding to all of the particles) in an amount of 5 % by weight relative to the weight of alumina (Px to P5): . FXJ Li3AlF6 (m.p. : 776‘C) D50 = 0.9 lm . F2: Li3AlF6 400 . F3: Li3AlF6 D > 630 lm . F4: Li3AlF6 160 < D <400 lm . Fs: LijNa3(AlF6)2 (m.p. : 710°C) . Fs: 3AlF3,5LiF The mixtures of aluminas and flux are prepared at ambient temperature and then placed in crucibles and the latter are introduced into the furnace, under a stream of nitrogen (10 1/h).
The temperature of the furnace is raised to X"C in the course of one hour (gradient) and then kept at this temperature for Z hours (plateau).
Cooling is carried out in the ambient air.
The product obtained is a white block which releases from the mould and deagglomerates easily. The average diameter d (in gm) and the average thickness e (in gm) of the platelets formed are measured.
The conditions and results are collated in the table which follows:

Claims (16)

1. Macrocrystals of α-alumina having essentially the form of monocrystalline hexagonal platelets, characterized in that the said platelets have a diameter of between 2 and 20 pm, a thickness of between 0·1 and 2 pm and a diameter/thickness ratio of between 5 and 40.
2. Macrocrystals according to Claim 1, characterized in that the hexagonal platelets have a diameter of between 2 and 18 pm and a thickness of between 0,1 and 1 pm.
3. Process for the production of macrocrystals according to either of Claims 1 and 2, from transition alumina or hydrated alumina and a flux, characterized in that the flux used has a melting point of at most 800’C, contains chemically bonded fluorine and, in the molten state, dissolves the transition alumina or hydrated alumina.
4. Process according to Claim 3, characterized in that the flux essentially consists of one or more nonhydrolysable fluorinated compounds or a system comprising a phase consisting of the said fluorinated compound or compounds and a second phase consisting of a hydrolysable fluorinated compound, one of the said phases being dissolved in the other phase.
5. Process according to Claim 4, characterized in that the flux consists of a system formed from aluminium trifluoride on the one hand and one or more alkali metal fluorides or alkaline earth metal fluorides on the other hand.
6. Process according to Claim 5, characterized in that the alkali metal fluoride or alkaline earth metal fluoride is chosen from lithium fluoride, sodium fluoride, potassium fluoride and calcium fluoride.
7. Process according to any one of Claims 4 to 6, characterized in that the flux consists of A1F 2 -LIF in the form Li 3 AlF s (lithium cryolite), Li 2 Na 2 (AlF s ) 2 (cryolithionite) or 3AlF 3 ,5Lif (lithium chiolite) .
8. Process according to any one of Claims 4 to 7, characterized in that the flux is used in an amount of at least 2 % and preferably of 4 to 20 % of flux relative to tha weight of transition alumina or hydrated alumina subjected to calcination.
9. Process according to any one of Claims 4 to 8, characterized in that the transition alumina or hydrated alumina consists of a powder in which at least 50 % (by weight) of the particles have a diameter of less than 50 μΐη, the said alumina having a specific surface area larger than 100 m 2 /g.
10. Process according to Claim 8, characterized in that the alumina contains up to 15 % by weight of water.
11. Process according to any one of Claims 4 to 10, characterized in that the calcination temperature is between 900 and 1100“C.
12. Use of the macrocrystals of «-alumina according to either of Claims 1 or 2 for making up polishing products or as reinforcing materials for matrices, in particular ceramic matrices, or metal or polymer matrices .
13. A macrocrystal of «-alumina according to Claim 1, substantially as hereinbefore described and exemplified.
14. A process for the production of a macrocrystal of «-alumina according to Claim 1, substantially as hereinbefore described and exemolified.
15. A macrocrystal of α-alumina according to Claim 1, whenever produced by a process claimed in a preceding claim.
16. Use according to Claim 12, substantially as hereinbefore described.
IE332090A 1989-09-21 1990-09-12 Macrocrystals of ó-alumina in the form of platelets and¹process for obtaining them IE903320A1 (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
FR8912402A FR2652075B1 (en) 1989-09-21 1989-09-21 ALPHA ALUMINA MACROCRYSTALS IN THE FORM OF PLATES AND PROCESS FOR OBTAINING SAME.

Publications (1)

Publication Number Publication Date
IE903320A1 true IE903320A1 (en) 1991-04-10

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EP (1) EP0425325B1 (en)
JP (1) JP2564205B2 (en)
KR (1) KR950003419B1 (en)
CN (1) CN1047576C (en)
AT (1) ATE98304T1 (en)
AU (1) AU627933B2 (en)
CA (1) CA2025037A1 (en)
DD (1) DD295885A5 (en)
DE (1) DE69005077T2 (en)
DK (1) DK0425325T3 (en)
ES (1) ES2060108T3 (en)
FR (1) FR2652075B1 (en)
IE (1) IE903320A1 (en)
IL (1) IL95719A0 (en)
NO (1) NO903690L (en)

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CN108560052B (en) * 2018-04-26 2020-05-01 闽江学院 Preparation method and application of monocrystal hexagonal alumina
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JP6708281B2 (en) * 2019-03-27 2020-06-10 Dic株式会社 Plate-shaped alumina particles
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AU6269790A (en) 1991-04-11
NO903690L (en) 1991-03-22
CN1050366A (en) 1991-04-03
DE69005077T2 (en) 1994-05-26
JPH03131517A (en) 1991-06-05
EP0425325A1 (en) 1991-05-02
CA2025037A1 (en) 1991-03-22
ES2060108T3 (en) 1994-11-16
CN1047576C (en) 1999-12-22
FR2652075B1 (en) 1991-12-06
FR2652075A1 (en) 1991-03-22
IL95719A0 (en) 1991-06-30
ATE98304T1 (en) 1993-12-15
DD295885A5 (en) 1991-11-14
KR910006145A (en) 1991-04-27
DK0425325T3 (en) 1994-02-14
DE69005077D1 (en) 1994-01-20
JP2564205B2 (en) 1996-12-18
AU627933B2 (en) 1992-09-03
NO903690D0 (en) 1990-08-22
EP0425325B1 (en) 1993-12-08
KR950003419B1 (en) 1995-04-12

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