Investigation of the systems KAlF4-M3AlF6 (M=Rb, Cs)

Article abstract of DOI:10.1016/S0040-6031(00)00452-4

Phase relations in the systems KAlF₄-Rb₃AlF₆ and KAlF₄-Cs₃AlF₆ were investigated by DTA and XRD methods for the purpose of finding a low melting, non-corrosive aluminum flux. In each system, congruent compounds Rb₃AlF₆·2KAlF₄ and 2Cs₃AlF₆·3KAlF₄ were observed, respectively. Eutectics located at 46 mol% Rb₃AlF₆ and 511°C, 25 mol% Rb₃AlF₆ and 512°C for the former system, and 48 mol% Cs₃AlF₆ and 550°C, 25 mol% Cs₃AlF₆ and 480°C in the latter were confirmed. The phase α-2Cs₃AlF₆·3KAlF₄ was XRD indexed as having an orthorhombic cell a=10.401±0.003, b=10.078±0.006, c=8.792±0.006 A?. (C) 2000 Elsevier Science B.V.

Full text of DOI:10.1016/S0040-6031(00)00452-4

Thermochimica Acta 354 (2000) 117±120
Investigation of the systems KAlF4±M3AlF6 (MˆRb, Cs)
Rong Chen*, Qiyun Zhang
Department of Chemistry, Peking University, Beijing 100871, China
Received 27 August 1999; accepted 10 February 2000
Abstract
Phase relations in the systems KAlF4±Rb3AlF6 and KAlF4±Cs3AlF6 were investigated by DTA and XRD methods for the
purpose of ®nding a low melting, non-corrosive aluminum ¯ux. In each system, congruent compounds Rb3AlF6Á2KAlF4 and
2Cs3AlF6Á3KAlF4 were observed, respectively. Eutectics located at 46 mol% Rb3AlF6 and 5118C, 25 mol% Rb3AlF6 and
5128C for the former system, and 48 mol% Cs3AlF6 and 5508C, 25 mol% Cs3AlF6 and 4808C in the latter were con®rmed.
The phase a-2Cs3AlF6Á3KAlF4 was XRD indexed as having an orthorhombic cell aˆ10.401Æ0.003, bˆ10.078Æ0.006,
Ê
cˆ8.792Æ0.006 A. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: KAlF4; Rb3AlF6; Cs3AlF6; Phase diagram
1. Introduction
2. Experimental
The eutectic of KAlF4±K3AlF6 occurring at 5608C
is well known as Nocolok ¯ux, which is widely used
in the aluminum brazing technique. The temperature
of the eutectic appears to be too high for brazing
the most of other aluminum alloys. For the purpose
of lowering the melting temperature of the ¯ux, the
ternary systems AlF3±KF±CsF and AlF3±KF±RbF
should be taken into account. The pseudo-binary
systems KAlF4±Rb3AlF6 and KAlF4±Cs3AlF6 are
the two most important in these ternary systems.
This research can be considered as a continuing
study of the systems AlF3±KF [1], AlF3±RbF [2]
and AlF3±CsF [3], and also for the basis of patents
[4,5].
2.1. Preparation of ¯uorides
CsF (purity >99.5%, Beijing Chem. Work) was
dehydrated at 4008C for 3 h; AlF3Á3.5H2O (A.R.,
Tianjin Chem. Work) was heated in N2 plus HF
atmospheres at 6008C for 2 h, the product was identi-
®ed as anhydrous AlF3 by XRD. All ¯uorides were
kept in a desiccator.
2.2. Preparation of samples
Samples were prepared by reacting mixtures of
anhydrous AlF3 with certain solutions of CsF in
HF. Products were placed in Pt crucibles and heated
until dry at 2008C, then annealed for 48 h at a higher
temperature at which no melting of any phase would
occur. During the annealing process, grinding and
mixing of the samples were carried out repeatedly
* Corresponding author. Fax: ‡86-10-6275-1496.
E-mail address: qyzh@chemms.chem.pku.edu.cn (R. Chen)
0040-6031/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ( 0 0 ) 0 0 4 5 2 - 4

118
R. Chen, Q. Zhang / Thermochimica Acta 354 (2000) 117±120
Table 1
in order to obtain homogeneous and equilibrium
samples.
DTA data of the system Rb3AlF6±KAlF4
KAlF4
(mol%)
Eutectic 1
temperature
(8C)
Eutectic 2
temperature
(8C)
Liquidus
temperature
(8C)
2.3. Method for determination
A CR-G type high-temperature DTA equipment
(Beijing Optical Instrument) was employed and cali-
brated with standard substances with known melting
point (calibrating both in heating and in cooling).
Al2O3 was used as the reference. The heating rate
was 158C min 1. The liquidus temperature was deter-
mined from on cooling, and the other temperatures
were determined by extended extrapolating initial
temperature of the peaks on heating. Experiments
were carried out in static dry air (relative humidity
<30%). The error in measuring temperature was
Æ38C.
0
12.5
25
878
855
770
690
595
573
39
513
513
512
511
512
513
513
50
52
54
56
570
620
650
652
640
60
63
66.7
70
75
512
511
513
512
513
78
80
605
594
85
2.4. X-ray powder diffraction analysis
90
530
533
575
95
100
Intermediates were determined by Rigaku
Dmax 2400 X-ray diffractometery (Radiation
Table 2
DTA data of the system Cs3AlF6±KAlF4
KAlF4
Polymorphic
Eutectic 1
Eutectic 2
Solidus
Liquidus
(mol%)
transformation (8C)
temperature (8C)
temperature (8C)
temperature (8C)
temperature (8C)
0
13.5
27
790
765
725
700
590
540
585
574
565
540
530
512
485
485
485
504
523
540
560
575
550
548
39
50
427
52
56
60
433
432
474
476
476
63
66.7
69
70
427
427
75
78
80
418
475
478
490
481
465
85
491
513
502
525
90
93
97
100

R. Chen, Q. Zhang / Thermochimica Acta 354 (2000) 117±120
119
Fig. 2. Phase diagram of the pseudo-system KAlF4±Cs3AlF6.
Fig. 1. Phase diagram of the pseudo-system KAlF4±Rb3AlF6.
CuKa-lˆ1.5409, Filter Ni). Si powder was added
as a cross-reference for more precise determina-
tion of the absorbance lines in the diffraction
patterns.
From these results, the binary systems KAlF4±
Rb3AlF6 and KAlF4±Cs3AlF6 both form congruent
melting
compounds
Rb3AlF6Á2KAlF4
and
2Cs3AlF6Á3KAlF4. If this is compared with the system
KAlF4±K3AlF6, a simple eutectic at ca. 5608C [1] is
observed. As K ion is changed with the larger radius
‡
‡
3. Results and discussion
ion Rb ,
a
congruent melting compound
‡
Rb3AlF6Á2KAlF4 is formed. The larger ion Cs pro-
duces more complex compound 2Cs3AlF6Á3KAlF4.
Using cations with the larger radii lowers the melting
point and produces more complex composition for the
intermediate compounds.
Phase diagrams of KAlF4±Rb3AlF6 and KAlF4±
Cs3AlF6 based on the results of DTA data listed in
Tables 1 and 2 are given in Figs. 1 and 2.
Fig. 1 reveals the presence of an intermediate
compound Rb3AlF4Á2KAlF4 melting at 6528C and
the system KAlF4±Rb3AlF6 is divided into two eutec-
tic systems. The e1 and e2 for the system were located
at 46 mol% Rb3AlF6 at 5118C and 25 mol% Rb3AlF6
at 5128C, respectively.
In both systems the eutectic temperatures which
were at 511 and 5128C for systems Rb3AlF6±KAlF4;
550 and 4808C for Cs3AlF6±KAlF4 were all lower
than in K3AlF6±KAlF4 (5608C). It is also important to
enhance this with the eutectic e2 in Cs3AlF6±KAlF4.
The temperature is as low as 4808C, the molten salt in
this composition is expected to be a non-corrosive
brazing ¯ux for most aluminum alloys which have
lower collapse or over burn temperatures.
In the pseudo-binary system KAlF4±Cs3AlF6 which
was also comprised of two sub-eutectic systems, a
limited solid solution formed along with KAlF4. The
e1 is at 48 mol% Cs3AlF6 and 5508C; e2 at 25 mol%
Cs3AlF6 and 4808C. The congruent melting com-
pound 2Cs3AlF6Á3KAlF4 has a polymorphic transfor-
mation at 4308C. The XRD indexed data for a-
2Cs3AlF6Á3KAlF4 is listed in Table 3. The compound
is orthorhombic, aˆ10.401Æ0.003, bˆ10.078Æ0.006,
Acknowledgements
The authors wish to acknowledge the support from
Beijing Science Fund.
Ê
cˆ8.792Æ0.006 A.

120
R. Chen, Q. Zhang / Thermochimica Acta 354 (2000) 117±120
Table 3
a
Indexed data of 2Cs3AlF6Á3KAlF4
Ê
d (A) Ð observed
Ê
d (A) Ð calculated
I/I0
h
k
l
5.035
4.405
5.035
4.392
4.371
4.092
3.751
3.616
3.195
3.184
3.140
3.076
2.554
2.519
2.516
2.360
2.210
2.083
2.046
2.045
1.980
1.978
1.813
1.809
1.806
1.569
1.567
1.476
1.403
1.400
1.399
1.337
1.335
1.280
1.186
1.185
1.185
1.184
1.183
6
25
0
0
0
2
1
2
1
2
0
3
2
0
4
1
0
5
3
4
1
1
1
4
1
3
4
6
2
4
4
4
0
7
0
7
5
3
5
2
0
2
1
1
2
3
1
3
1
0
4
1
4
3
0
2
2
5
2
3
4
5
0
5
1
6
5
2
3
6
4
5
5
4
8
7
0
2
1
1
2
0
0
2
1
1
3
0
0
1
3
0
3
2
0
4
4
0
2
5
1
3
3
3
5
5
4
0
6
1
5
0
0
4.081
3.748
3.619
3.195
11
10
10
17
3.123
3.062
2.552
100
14
36
2.516
2.361
2.212
2.082
2.046
16
7
32
6
7
1.978
1.806
14
24
1.566
1.476
1.402
13
8
13
1.335
1.280
6
5
1.184
10
a
Ê
Orthorhombic cell aˆ10.401Æ0.003, bˆ10.078Æ0.006, cˆ8.792Æ0.006 A.
[3] Rong Chen, Jie Cao, Qiyun Zhang, Thermochim. Acta 303
(1997) 145.
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[5] Japanese patent Application Kokai (Laid-Open) No. 62-
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