Thermal oxidation kinetics of Ni100-xPx powders

Article abstract of DOI:10.1023/A:1017956430616

The oxidation of Ni_100-xP_x (7.3 at%

Full text of DOI:10.1023/A:1017956430616

Journal of Thermal Analysis and Calorimetry, Vol. 65 (2001) 367–372
THERMAL OXIDATION KINETICS OF Ni100–xPx
POWDERS
A. Ma³ecki and M. Wierzbicka
University of Mining and Metallurgy, Faculty of Materials Science and Ceramics,
Al. Mickiewicza 30, 30 059 Cracow, Poland
Abstract
x
The oxidation of Ni100–xP (7.3 at%<x<25.0 at%) powders in air in the temperature range 350–450°C
was determined by kinetics and X-ray diffraction. The isothermal kinetics was modeled using the
Ginstling–Brounstein equations. The oxidation process was found to be thermally activated with ac-
–
1
–1
tivation energy 127.8 kJ mol for x=7.3 at% to 157.7 kJ mol for x=25.0 at% It was found that the
rate constants for x=7.3 at% were approximately 100 times lower than those for x=25.0 at%.
Keywords: kinetic models, nickel phosphides, Ni–P alloy, oxidation
Introduction
For an amorphous alloy system the investigation of oxidation kinetics is very impor-
tant in order to understand the nature of the oxidation processes more deeply. The ox-
idation process of Ni–P system is very complex, and controversial results have been
reported. To our knowledge, there have been no recent or detailed studies of the ki-
netic of the thermal oxidation of Ni–P powders. In addition, because previous investi-
gators have been primarily concerned with the oxidation kinetics of electroless Ni–P
coatings at temperatures >600° [1–3], there is a need for data below this temperature.
The kinetics of thermal oxidation of Ni100–xPx (7.3 at%<x<25.0 at%) in 0.14 atm
of oxygen in the temperature range 350–450°C using the isothermal thermogravi-
metric analysis. X-ray diffraction was used to identify the phases formed during oxi-
dation. In this investigation, nano-size amorphous powders were used. Such a fine
particle size makes its easier to study the early stages of oxidation.
Experimental
Sample characterization
Ni100–xP (7.3 at%<x<25.0 at%) ultrafine amorphous alloy particles have been pre-
−
x
pared by chemical reduction of Ni(II) ions by phosphate(I) ions (H PO ). This auto-
catalytic reaction goes on, when a small amount of PdCl is added to the solution. The
powders were prepared at 80–90°C. The black slurry obtained was washed by
2
2
2
1
418–2874/2001/ $ 5.00
2001 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest
©
Kluwer Academic Publishers, Dordrecht

3
68
x
MA£ECKI, WIERZBICKA: THERMAL OXIDATION KINETICS OF Ni100–xP POWDERS
deionized water and collected in a filter, then dried in a vacuum dryer at 90°C. A sta-
ble powder sample was obtained. The composition of the powders was determined by
chemical analysis. Seven samples of Ni100–xPx of phosphorus content equal to: 7.3,
1
0.6, 15.2, 19.8, 22.4 and 25.0 at% were obtained. They have an almost spherical
morphology with diameters decreasing with the increase phosphorous contents. The
preparation method, the fundamental properties, and the crystallization behavior of
Ni100–xPx ultrafine amorphous particles were described in our earlier papers [4–6].
Oxidation kinetics
The powder specimens (~34 mg each) were suspended in the thermal gravimetric an-
alyzer (TGA) (SDT 2960 TA Instruments) in platinum crucibles. During isothermal
–
studies, specimens were heated at a rate of 50°C min . At 250°C they were held for
1
3
min to thermally equilibrate the system, at the desired temperature 350 to 450°C –
for 60 to 240 min, and finally at 550°C for 300 min to reach the oxidation process
completed. Mass change data were collected every 10 s during all TG experiments.
3 –1
The flow rates of synthetic air were held constant at 100 cm min .
Results
Figures 1 and 2 show the representative isothermal TG curves for Ni100–xPx (x=25.0 at%)
and Ni100–xPx (x=7.3 at%) powders, respectively. The same oxidation behavior was ob-
served for all studied alloys compositions. The high phosphorus samples exhibited less
mass gain. As the powders were heated, there was some mass loss (about 0.5 mass%) ob-
served due to the loss of adsorbed water. At temperature 550°C oxidation process is com-
pleted for all samples and no significant mass changes took place.
x
Fig. 1 Isothermal TG curves for for Ni100–xP (x=25.0 at%)
The mass gain is due to the formation of Ni P and NiO [6]:
2
J. Therm. Anal. Cal., 65, 2001

MA£ECKI, WIERZBICKA: THERMAL OXIDATION KINETICS OF Ni100–x
P
x
POWDERS
369
1
00−3x
Ni100–xPx(s) +
O (g) = (100–3x)NiO(s) + xNi P(s) (<600°C)
2
(1)
2
2
To find out the underlying thermodynamic basis for the Ni100–xPx oxidation, we
have made some calculations using thermodynamic data from Barin [7]. The oxida-
tion of Ni100–xP (x=25.0 at%), i.e. Ni P to Ni P and NiO can be expressed as:
x
3
2
Ni P(s) + 0.5O (g) = NiO(s) + Ni P(s) (<600°C)
3
(2)
2
2
The Gibbs free energies for above reaction are –1482 and –1318 kJ for the reac-
tion temperatures of 350 and 550°C, respectively. The formation of Ni P and NiO is
2
confirmed by X-ray analysis (Fig. 3). Ni100–xPx (x=7.3 at%) powder has the largest
mass gain (22.6 mass%) among all seven studied powders, it is very close to
x
Fig. 2 Isothermal TG curves for for Ni100–xP (x=7.3 at%)
Fig. 3 X-ray diffraction patterns of Ni100–x
50°C
x
P (x=22.4 at%) powder after oxidation at
5
J. Therm. Anal. Cal., 65, 2001

3
70
x
MA£ECKI, WIERZBICKA: THERMAL OXIDATION KINETICS OF Ni100–xP POWDERS
22.1 mass%, as predicted by Eq. (1). Comparison of the mass gains calculated from
Eq. (1) with the experimental results is shown in Fig. 4.
Fig. 4 Comparison of the mass gain calculated from Eq. (1) with the experimental re-
sults
Attempts were made to fit the isothermal data to a variety of kinetic models
based on mechanisms, including diffusion control. All kinetics could be modeled
closely at temperatures 350–450°C using diffusion-rate-limited model for reactions
involving spherical particles, i.e., the Ginstling–Brounstein equation modified by
Ma³ecki [8–10]:
2
2
/3
n
1
− α −(1−α ) =kt
(3)
3
where k, like n are constant parameters at constant temperature, α is the fraction of pow-
der that has reacted. The modification of the Ginstling–Brounstein model extended the
range of its application to the cases where, for instance, diffusion processes, determining
the rate of reaction, proceeded simultaneously in different ways. A suitable coordinate
system enabled linearization of the obtained α(t) functions and calculation of the slopes
and lnk. The values of n, found for all the prepared Ni100–xPx samples did not significantly
differ from 1, which was confirmed by non-parametric statistical tests. This indicates
clearly that the oxidation kinetics of the investigated alloys is consistent with
Ginstling–Brounstein model. Moreover, the fact that no temperature dependence of pa-
rameter n was observed allows for the conclusion that the same diffusion process controls
the oxidation rate independently of the reaction progress.
The parabolic rate constants obtained through simple linear regression on
Eq. (3) are summarized in Fig. 5. From Arrhenius dependence activation energy of
–
Ni100–xP powders oxidation was determined and was found to be 127.8 kJ mol for
1
x
–
x=7.3 at% to 157.7 kJ mol for x=25.0 at% (Fig. 6).
1
J. Therm. Anal. Cal., 65, 2001

MA£ECKI, WIERZBICKA: THERMAL OXIDATION KINETICS OF Ni100–x
P
x
POWDERS
371
x
Fig. 5 Parabolic rate constants for the oxidation of Ni100–xP powders
x
Fig. 6 Activation energy for the oxidation Ni100–xP powders
Conclusions
The present study has given the following results:
•
•
the obtained results allow to propose an equation for the oxidation reaction,
below 600°C the oxidation products are crystalline Ni P and NiO; no gaseous
2
or amorphous phases are formed,
high phosphorus samples oxidize about 100 times faster than low phosphorus
ones,
diffusion-rate-limited Ginstling–Brounstein model satisfactorily describes
the oxidation kinetics.
•
•
J. Therm. Anal. Cal., 65, 2001

3
72
x
MA£ECKI, WIERZBICKA: THERMAL OXIDATION KINETICS OF Ni100–xP POWDERS
References
1
2
3
W. J. Tomlinson and S. C. Randall, Corrosion Science, 18 (1978) 573.
W. J. Tomlinson, J. Materials Science, 21 (1986) 97.
B. Gillot, K. El Amri, P. Pouderoux, J. P. Bonino and A. Rousset, J. Alloys and Compounds,
89 (1992) 151.
R. Gajerski, A. Ma³ecki, B. Prochowska-Klisch and M. Wierzbicka, J. Thermal Anal., 45
1995) 979.
1
4
(
5
6
7
8
9
M. Wierzbicka and A. Ma³ecki, J. Therm. Anal. Cal., 55 (1999) 981.
M. Wierzbicka, Thesis at the University of Mining and Metallurgy, 1999.
I. S. Barin, Thermochemical data of Pure Substances, VCH, New York 1993.
A. Ma³ecki, Thesis no. 163 L’Université de Bordeaux I, 1985.
A. Ma³ecki, J. A. Tareen, J. P. Doumerc, L. Rabardel and J. C. Launay, J. Solid State Chem.,
5
0 (1989) 49.
1
0 A. Ma³ecki and J. P. Doumerc, J. Thermal Anal., 36 (1990) 215.
J. Therm. Anal. Cal., 65, 2001