On Preparation of Crystalline MgO-Ceramics from Amorphous Precursors
- PDF / 216,810 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 16 Downloads / 203 Views
H. JOST*, CH. CARIUS* AND B. PEPLINSKI** *KAI e.V., AG "Hochleistungskeramik", Rudower Chaussee 5, Gebiude 1.1, D-12489 Berlin, Germany **Bundesanstalt flfi Materialforschung und Priifimg (BAM), Rudower Chaussee 5, GebAude 1.5, D-12489 Berlin, Germany
ABSTRACT Amorphous precursors are obtained as a result of the precipitation from magnesium acetate solutions with ammonia in the presence of citric acid. Compared with crystalline precursors they have a higher reactivity and a changed behaviour in calcination process as well as during sintering. Already at 1300 'C sintered pellets are achieved, whereas in technological process 1700 'C are needed.
INTRODUCTION Sintered magnesia has a long-range application as a fireproof material in metallurgical furnaces. In large-scale technical process it is manufactured from seawater and natural magnesite or by precipitation from magnesium chloride solutions. A new wet chemical preparation technique is being used to improve MgO powders for ceramics.
EXPERIMENTAL A 2M Mg(CH 3COO)2 or MgCI2 solution without (way A) or with (way B) an addition of citric acid is dropped into a solution of a 7.5-fold stoichiometric excess of an ammonia solution. The precipitated Mg(OH) 2 is filtrated, washed with water and isopropanol, calcinated at 900 'C for 2 hours, densified to green compacts at a pressure of 35 MPa and sintered at 1300 'C for 2 hours (see Fig. 1).
609 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society
magnesium acetate resp. magnesium chloride solution 0,5 mole 2M Ac A Cl
magnesium acetate solution + citric acid 0,5 mole 2M + 0,035 mole Ac/Cit
conc. ammonia solution 7.5-fold excess
4
precipitation
magnesium hydroxide suspension
Sfiltration, washing with aqua dest and isopropanol, drying (I110 'C, at air)
magnesium hydroxide powder
4
calcining (900 'C,2h)
magnesium oxide powder
Scompacti by dry pressing (35 MPa)
magnesium oxide compacts
4
sintering (1300 -C,2h)
magnesium oxide sintered compacts
Fig. 1. Scheme of MgO preparation technique
610
RESULTS AND DISCUSSION
Resulting from the precipitation with an addition of citric acid the precipitate is an amorphous magnesium hydroxide powder ac/cit [1-3]. Refraining from adding citric acid leads to crystalline precipitates ac. When magnesium hydroxide is precipitated from a magnesium chloride solution with ammonia, so a crystalline material cl is also obtained (see Fig.2). The specific surface area (BET) of Mg(OH)2 depends on the concentration of citric acid (see Fig.3). An addition of 35 mmole citric acid leads to amorphous Mg(OH)2 with surface areas of up to 650 m2/g (mi) and with theoretical particle sizes of 3 nm. The products of ac (m 2) and cl (i 3 ) have only surface areas of 80-100 resp. 60 m2/g. 2-Theta-Scale ac/cit
6420
4289_
ac 2140_
Q.I 9;
_____
ci
2A409
49.20
Fig.2. X-ray diffraction patterns of Mg(OH), precipitates
E
Fig.3.
L1
Specific surface area (BET) of Mg(OH) 2versus addition of citric acid to (CH 3COO)2Mg
re
D9 0.d
Q., W2 M.
0
0.02 0,035
0.06
0,08
0,10
Data Loading...
