Effect of the fluoride additives on the oxidation of AlN

Article abstract of DOI:10.1016/S0025-5408(02)00942-X

The oxidation of AlN powder added by the fluorides at temperatures below 700°C in air was discovered in this study. The obvious onset of oxidation of AlN with cryolite and YF₃ additions is below 700°C with the product of α-Al₂O₃ phase, which usually occurs in single AlN powder above 1100°C. The changes on the weight and the FTIR spectra of the AlN powder fired at temperatures lower than 700°C show that cryolite and YF₃ greatly promote the oxidation of AlN powder at these temperatures. Different from the action of cryolite and YF₃ powder, CaF₂ has no obvious effect on the oxidation of AlN. A possible oxidation process, in part corroborated by FTIR and XRF, was proposed to explain the results in the experiments. The oxidation kinetics of AlN in the presence of cryolite were also discussed at the temperatures ranging from 550 to 700°C from the data of the weight gains in this region. The result shows that the oxidation follows a linear law, which implies a reaction rate-controlled process. The considerably low activation energy of 67 kJ mol^-1, which is associated with the quick oxidation and the formation of α-Al₂O₃ at temperatures below 700°C, was determined from the slope of the line fit.

Full text of DOI:10.1016/S0025-5408(02)00942-X

Materials Research Bulletin 37 (2002) 2427±2435
Effect of the ¯uoride additives on the
oxidation of AlN
Heping Zhoua, Liang Qiaoa,*, Renli Fub
aState Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science
and Engineering, Tsinghua University, Beijing 100084, PR China
bCollege of Mechanical-Electronic and Material Engineering, China University of Mining
and Technology, Jiangsu, Xuzhou 221008, PR China
(Refereed)
Received 19 April 2002; accepted 23 August 2002
Abstract
The oxidation of AlN powder added by the ¯uorides at temperatures below 7008C in air
was discovered in this study. The obvious onset of oxidation of AlN with cryolite and YF3
additions is below 7008C with the product of a-Al2O3 phase, which usually occurs in single
AlN powder above 11008C. The changes on the weight and the FTIR spectra of the AlN
powder ®red at temperatures lower than 7008C show that cryolite and YF3 greatly promote the
oxidation of AlN powder at these temperatures. Different from the action of cryolite and YF3
powder, CaF2 has no obvious effect on the oxidation of AlN. A possible oxidation process, in
part corroborated by FTIR and XRF, was proposed to explain the results in the experiments.
The oxidation kinetics of AlN in the presence of cryolite were also discussed at the
temperatures ranging from 550 to 7008C from the data of the weight gains in this region.
The result shows that the oxidation follows a linear law, which implies a reaction rate-
controlled process. The considerably low activation energy of 67 kJ molÀ1, which is
associated with the quick oxidation and the formation of a-Al2O3 at temperatures below
7008C, was determined from the slope of the line ®t.
# 2002 Elsevier Science Ltd. All rights reserved.
Keywords: A. Fluorides; A. Nitrides; A. Inorganic compounds; D. Surface properties
* Corresponding author. Tel.: ‡86-10-62772550; fax: ‡86-10-62772549.
E-mail address: qiaoliang98@mails.tsinghua.edu.cn (L. Qiao).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 0 9 4 2 - X

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H. Zhouet al. / Materials Research Bulletin 37 (2002) 2427±2435
1. Introduction
Aluminum nitride (AlN) has attracted growing interest as a substrate material in
electronic packaging because of its excellent properties including high thermal
conductivity, low dielectric constant, low loss tangent and coef®cient of thermal
expansion close to that of silicon [1,2]. The thermal conductivity of AlN ceramics, as
one of the main properties for electronic package materials, is always deteriorated by
the oxygen impurity dissolving into AlN lattice from the surface of AlN particles
during the sintering [3,4]. It is important to decrease the oxygen of AlN particles for
achieving a high thermal conductivity.
Binder in the green sheets of AlN substrates is usually burned out in air. Avoiding
the oxidation of AlN during the binder burnout is necessary for decreasing the oxygen
on the surface of AlN particles. In the study of the oxidation of AlN both in powder
form and in ceramic bulk form, some results have been achieved [5±7]. The general
consensus is that the onset of oxidation lies between 700 and 8008C. Also, the
thermodynamically stable rhombohedral a-Al2O3 phase, one of the oxidation pro-
ducts, is always found at temperatures higher than 11008C [8].
Since AlN is a covalent compound, limited atomic mobility prevents complete
densi®cation of pure AlN at reasonable temperatures. For full densi®cation, some
¯uorides such as YF3 and CaF2 are often used as sintering aids in the fabrication of
AlN ceramics. It is not clear whether these additives can in¯uence the oxidation of
AlN powder. When cryolite (1 wt.%) was considered to add as a sintering aid in one
experiment, the obvious oxidation of AlN powder was found during the binder
burnout at 6008C. In the case of YF3 used as an additive, the similar oxidation was
also found. However, no change occurs on the surface of the AlN particles when CaF2
was used. The present study introduces the oxidation of AlN with cryolite and YF3
additions, respectively.
2. Experimental
Commercially available AlN powder (grade US, Toyo Aluminium K.K.), as
described in Table 1, was used as a starting material. Weighed samples of AlN
powder, mixed with 1 and 5 wt.% cryolite powder, respectively, were placed in
alumina crucibles and then ®red in air at temperatures ranging from 550 to 7008C for
different times. After oxidation, the samples were reweighed and the weight change
Table 1
Properties of the starting AlN powder
Chemical composition (wt.%)
Average particle
size (mm)
Specific
surface (m2 gÀ1
)
N
O
C
Si
Fe
33.2
1.11
0.04
0.0059
0.0023
1.46
4.50

H. Zhouet al. / Materials Research Bulletin 37 (2002) 2427±2435
2429
Fig. 1. Schematic diagram of the location of AlN and cryolite.
was determined. The weight change was normalized as a percentage of the unoxidized
sample. The weight measurement was carried out on the balance (Sartorius, BP221S)
with the accuracy of 0.001 g.
YF3 used in this study was made by the reaction in the solution as follows: (i) Y2O3
powder was dissolved into excessive hydrochloric acid; (ii) excessive hydro¯uoric
acid was added into the solution and the white precipitate immediately occurred in it;
(iii) the precipitate was washed using distilled water and ®ltered; and (iv) after being
dried, the white precipitate was ground into powder. Identi®cation of the crystalline
phase of as-received powder was conducted using XRD with Cu Ka radiation
(Rigaku, Automated D/Max 3B). The result shows that the powder consists of a
single YF3.
To identify the phases and observe the surface morphology of oxidized AlN, AlN
pellets contacting AlN, contacting and close to the ¯uoride pellets (including cryolite,
YF3 and CaF2) were ®red at temperatures below 7008C for different times in air. As
shown in Fig. 1, AlNA and AlNB and AlNG refer to the AlN surfaces contacting the
¯uorides, contacting AlN and close to the ¯uorides, respectively. XRD was employed
to identify the phases on different surfaces. SEM (OPTON, CSM950) was used to
observe the morphology of surface AlNA. The changes of chemical bond and
elementary contents in cryolite powder during the ®ring were investigated by FTIR
and XRF, respectively.
3. Results
Fig. 2 shows the percentage changes in weight of AlN with cryolite addition, single
AlN and single cryolite, as a function of time at 7008C. It can be seen that the weight
of AlN powder slowly increases with the increase of time, which implies the very
slight oxidation at this temperature. In the case of cryolite addition, the weight of AlN
powder increases quickly as ®ring time goes on. When ®red for 4 h, the weight gain of
the AlN sample with 5 wt.% cryolite reaches its maximum of 23.1 wt.%, which is
close to the theoretical value (24.4 wt.%) of its weight gain as AlN is completely
oxidized. This shows that cryolite greatly promotes the oxidation of AlN at the
temperature of 7008C.

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H. Zhouet al. / Materials Research Bulletin 37 (2002) 2427±2435
Fig. 2. Plot of percentage increase in weight as a function of time at 7008C.
Fig. 3 shows the phases on surfaces AlNA and AlNB contacting cryolite and AlN
after oxidation at 6008C for 10 h. As shown in Fig. 3(a), a-Al2O3 is identi®ed on
surface AlNA, which indicates that some AlN particles contacting cryolite have been
oxidized into a-Al2O3 at this temperature. Different from the phases on surface AlNA,
no oxidized product is detectable on surface AlNB contacting AlN ®red at the same
temperature (in Fig. 3(b)), which is in good agreement with the results reported by
Katnani and Papathomas [5].
Fig. 4 shows the FTIR spectra of the mixed powders containing AlN (95%) and
YF3 (5 wt.%). For comparison, the spectra of single YF3, AlN and AlN oxidized at
12008C for 2 h are also shown in the same ®gure. The bands, together with some
previous results, are indicated in Table 2. As shown in Fig. 4 and Table 2, at room
temperature YF3 and AlN have the strong bands centered at 418 and 711 cmÀ1,
respectively. However, the mixed powders containing AlN and YF3 ®red at 6508C for
30 min and 10 h, signed by the symbols AlNF30m and AlNF10h, respectively, have
the different spectra. The bands associated with the different aluminas except those
close to 403 and 711 cmÀ1, which indicate the existence of AlN and YF3 powders,
Fig. 3. X-ray diffraction patterns of AlN pellets ®red at 6008C for 10 h with surfaces (a) AlNA and
(b) AlNB.

H. Zhouet al. / Materials Research Bulletin 37 (2002) 2427±2435
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Fig. 4. FTIR spectra of different powders.
Table 2
Comparison of FTIR band position (cmÀ1) for different powders
AlNF30m
AlNF10h
AlN at room
temperature
AlN sample
oxidized at
y-, d- and
a-alumina
mixture [9]
AlN sample
oxidized at
12008C for 2 h
10508C for 1 h [8]
836
791
714
638
583
451
848
701
618
580
546
711
642
609
486
458
845 (s)
778 (w)
643 (s)
589 (v)
440 (s)
817
764
669
618
465
Here w ˆ weak; s ˆ strong; v ˆ very strong.
have a stretch at a broad arrange in samples AlNF10h and AlNF30m since the
different aluminas coexist in the ®red powder. As reported, there are three peaks
centered around 845, 589 and 440 cmÀ1 in the FTIR spectra if a mixture of d-, y-, and
a-Al2O3 is present [9]. Considering the similarities of the bands in the spectra of
samples AlNF10h and AlNF30m, it is concluded that y, d and a phases of aluminas
may be contained in the two ®red powders. In addition, it is noted that the bands near
451, 583 and 836 cmÀ1 increase in intensity with increasing oxidation time from
30 min to 10 h. However, the intensity of the bands near 714 cmÀ1 decreases as the
®ring time goes on. This implies that AlN transfers into a-Al2O3 through the
transition compounds such as y-, d- and a-Al2O3 phases in the ®ring, which is
consistent with the result achieved at the temperature above 10508C by Duchesne et al.
[8]. Such an oxidation sequence of AlN powder ®red at the low temperature of 6508C
has not been reported in the previous paper.
4. Discussions
AlN pellets close to cryolite were ®red at 6008C for 10 h in order to explore the
reason why cryolite and YF3 promote the oxidation of AlN at temperatures lower than

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H. Zhouet al. / Materials Research Bulletin 37 (2002) 2427±2435
Fig. 5. X-ray diffraction patterns of surface AlNG close to cryolite ®red at 6008C for 10 h.
7008C. Fig. 5 shows the oxidation product on surface AlNG identi®ed by XRD. It can
be seen that a-Al2O3 phase also occurs on surface AlNG, resembling that in Fig. 3(a),
even when the AlN pellets has no direct contact with cryolite. This implies the
oxidation is attributed to the reaction between AlN and some gas species formed by
cryolite during the ®ring.
A possible oxidation process is proposed as shown in Fig. 6. During the ®ring,
some oxygen atoms in O2 replace the ¯uorine atoms of cryolite, which results in the
formation of the gaseous species of OxF2x. OxF2x then oxidizes AlN easily since the
oxygen in it has a chemical value higher than that in O2. Following this, the resultant
NyFz oxidizes O2 into OxF2x again. Thus, AlN is oxidized by oxygen and a-Al2O3
phase is formed even at the low temperature less than 7008C using OxF2x as a medium.
The change occurring to cryolite during the ®ring can be revealed by its slight
weight loss shown in Fig. 2 as a result of the replacement of oxygen atoms though the
amounts of the replaced ¯uorine atoms in cryolite are so small that they can not be
identi®ed by XRD, as shown in Fig. 7(a). Further, it is also been indicated indirectly
by detecting the chemical bonds and the elementary contents in the powder using
FTIR and XRF, respectively. It can be seen from the FTIR transmission spectra as
shown in Fig. 8 that single cryolite at room temperature has three bands near 599, 412
Fig. 6. Schematic of the oxidation of AlN with cryolite addition.

H. Zhouet al. / Materials Research Bulletin 37 (2002) 2427±2435
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Fig. 7. X-ray diffraction patterns of (a) cryolite ®red at 6008C for 10 h; (b) YF3 at 6508C for 8 h; and
(c) CaF2 at 6508C for 8 h.
Fig. 8. FTIR spectra of cryolite powder (a) ®red at 7008C for 4 h and (b) at room temperature,
respectively.
and 399 cmÀ1, respectively. However, the additional band in the cryolite ®red at
7008C for 4 h occurs at about 580 cmÀ1, which is probably associated with the
oxygen replacing the ¯uorine atoms. Table 3 shows the contents of different elements
in cryolite at room temperature (RT) and after the ®ring at 7008C for 4 h. It can be
seen that 0.71 wt.% oxygen exists in the ®red powder, which is more than that in the
powder at room temperature where the oxygen content is too small to be detected. On
the contrary, the ¯uorine content decreases from 53.86 to 52.89 wt.% after ®ring. This
implies that some oxygen atoms have replaced ¯uorine atoms in the ®red cryolite.
Table 3
Contents of different elements in cryolite powder
Element
Cryolite (RT, wt.%)
Cryolite (fired at 7008C for 4 h, wt.%)
Na
Al
F
30.29
15.78
53.86
0.05
30.49
15.82
52.89
0.04
Si
S
0.02
0.05
0.71
O

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H. Zhouet al. / Materials Research Bulletin 37 (2002) 2427±2435
Fig. 9. Plot of percentage increase in weight as a function of time at different temperatures.
As shown in Fig. 7(b), YOF phase occurring in the YF3 powder ®red at 6508C for
8 h indicates the action of oxygen on the ¯uoride. However, CaF2 powder is stable in
the same ®ring, which is the reason why no oxidation is observed in AlN powder
contacting CaF2.
Though the mechanism as described earlier explains the oxidation behavior of AlN
with the ¯uorides additions, the gas species OxF2x and NyFz have not been directly
detected due to their very small amounts produced during the ®ring, which remains a
further study in the future.
The activation energy for the oxidation of AlN powder with 1 wt.% cryolite
addition at the temperatures ranging from 550 to 7008C can be determined by using
the isothermal kinetic data on the base of the assumption that the weight loss of
cryolite during the ®ring is neglectable at the level of this addition. Fig. 9 shows the
percentage increase in weight of AlN powder, as a function of time at different
temperatures. The linear ®t plotted in Fig. 10, which is consistent with the Arrhenius
Fig. 10. Arrhenius plot used to calculate the activation energy.

H. Zhouet al. / Materials Research Bulletin 37 (2002) 2427±2435
2435
equation: ln k versus 1/T, implies that the oxidation of AlN powder at this region
yields a reaction rate-controlled mechanism. From the slope of the plot, the activation
energy of 67 kJ molÀ1 is determined, which is much lower than that of the single AlN
as reported before [10]. The lower activation energy is associated with the quick
oxidation and the formation of a-Al2O3 below 7008C.
5. Conclusions
CaF2 has no prominent effect on the oxidation of AlN. However, cryolite and YF3
greatly promote the oxidation of AlN at temperatures below 7008C. The obvious
onset of oxidation occurs near the temperature of 6008C in the case of cryolite, as
indicated by the weight gain of AlN powder during the ®ring. A possible oxidation
process reveals that AlN is oxidized by oxygen in air indirectly through some gaseous
species as media. The oxidation mechanism in the presence of cryolite at the
temperatures ranging from 550 to 7008C is reaction rate-controlled with the low
activation energy of 67 kJ molÀ1, which is associated with the quick oxidation
observed in the experiments.
Acknowledgments
This work is supported by Guangdong Research and Development Center for New
Functional Ceramic Materials.
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