1. No category

Low Curie temperature material for induction heating self-temperature
controlling system
Abstract
This paper presents low Curie temperature magnetic materials for induction heating. These materials are alloys based on
SUS430. The magnetization decreases with increasing amount of additives. In order to increase the saturation magnetization, while
keeping the Curie temperature constant, rare earth materials were added in this study. The compositions of alloys were investigated to
realize a suitable Curie temperature. The temperature self-controlling function was also investigated in measurement of temperature
distribution by using thermograph apparatus. Furthermore, thermoelectromagnetic field analysis was carried out as a numerical
examination.
PACS: 74.25.Ha; 75.50.Gg; 71.35.Ji
Keywords: Low Curie temperature; Temperature self controlling; Induction heating
1. Introduction
The Curie point is the magnetic transformation temperature of a ferromagnetic material between its ferromagnetic and paramagnetic phase. The saturation
magnetization decreases gradually with temperature and
disappears above the Curie point. The suitable critical
point is about 100–300 1C for induction heating, when this
function is used in household appliances.
Also the material needs to have high corrosion resistance
and melting point. The principle of the self-temperature
moderator function is as follows:
(2) Since the Curie point of the developed material is
about 100–300 1C, the saturation magnetization and
the magnetic permeability decrease gradually with
increasing temperature and the ferromagnetic property
(1) In electromagnetic induction heating, the magnetic flux
generated by the high-frequency work coil passes
through the heating material in a ferromagnetic phase
at room temperature. Thus, eddy currents flow in the
material. The material is heated by the eddy current
losses.
Fig. 1. Principle of the temperature self-controlling function.
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T. Todaka et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e702–e707
Fig. 2. Effect of additional materials on the Curie temperature of SUS430.
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vanished in the range 100–300 1C. At the same time, the
linkage flux and the eddy currents decrease, and the
heating energy is also reduced due to the lower
inductance. The exciting current increases inversely;
however, it is usually controlled to remain below the
limitation current of the inverter. As a result, the
temperature of the material decreases slowly, as shown
in Fig. 1.
(3) Next, as the temperature falls below the 100–300 1C
range, the ferromagnetic property recovers and the
eddy currents increase. Therefore it is possible to keep
the material temperature constant under a safe value
depending on the Curie point without control. This
sensorless control gives high reliability operation and
safe use of the heating device.
This material is an alloy based on SUS430. Fig. 2 shows
the effect of additives to SUS430 on the Curie temperature.
Table 1
Selected material for self-temperature controlling
Fig. 3. Effect of Al addition on Curie temperature and saturation
magnetization.
Composition (wt%)
Tc (1C)
Ms (emu/g)
SUS430—Al11.7–Dy0.5
SUS430—Al11.7–Gd0.3
SUS430—Al11.7–Sm0.3
SUS430—Al12.8–Gd0.3
SUS430—Al12.8–Sm0.1
SUS430—Al12.8–Y0.3
SUS430—Al13.5–Gd0.3
SUS430—Al13.5–Sm0.1
SUS430—Al13.5–Y0.3
301
300
300
194
195
198
106
116
109
93
99
97
77
73
75
54
55
53
Fig. 4. Effect of addition of rare earth metals on Tc and Ms: (a) SUS430—11.7 wt% Al–Dy. (b) SUS430—11.7 wt% Al–Gd. (c) SUS430—11.7 wt%
Al–Sm. (d) SUS430—12.8 wt% Al–Sm. (e) SUS430—12.8 wt% Al–Y. (f) SUS430—13.5wt% Al–Sm. (g) SUS430—13.5 wt% Al–Y.
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Fig. 4. (Continued)
Fig. 5. Rolled sample and measured point.
Fig. 6. Investigation of temperature self-control: (a) Tc ¼ 300 1C. (b) Tc ¼ about 200 1C. (c) Tc ¼ about 100 1C.
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Fig. 7. Transition of temperature distribution.
2. Low Curie temperature materials
Fig. 8. Analytical model and conditions.
Al was effective in reducing the Curie temperature.
Therefore, as a basic alloy, SUS430 including 11.7 wt%
Al was selected for further studies.
Fig. 3 shows the effect of Al addition to SUS430 on its
Curie temperature Tc and saturation magnetization Ms. In
this investigation a vibration magnetometer was used. The
Tc and Ms decrease with increasing Al content. As a result,
we can realize the selected low Curie temperature; for
example, Tc was 300 1C for 11.7 wt% Al, 200 1C for
12.8 wt% Al, and 100 1C for 13.5 wt% Al, respectively.
We also added rare earth elements in order to increase the
saturation magnetization.
Figs. 4(a)–(g) show the effect of the addition of rare
earth elements, Dy, Sm, Y, and Gd on Tc and Ms. Table 1
shows the suitable components for the temperature
self-control during induction heating. These materials
can be hot-rolled as shown in Fig. 5. Figs. 6(a)–(c) show
the results of the temperature self-controlling function.
Fig. 7 shows the results observed by using a thermograph.
In these results, the autoregulation of temperature is
confirmed.
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T. Todaka et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e702–e707
Fig. 9. Material properties: (a) Relative permeability vs. temperature. (b) Conductivity vs. temperature. (c) Thermal conductivity vs. temperature. (d)
Specific heat vs. temperature.
3. Numerical analysis
A finite element analysis was also carried out. The
governing equations are written as follows [1]:
q vz qðrAy Þ
q
qAy
qAy
þ Jy s
þ grad f ¼ 0
þ
vr
qz
qt
qr r qr
qz
(1)
l q
qT
q
qT
qT
r
r
(2)
þ
þ Q ¼ rc
r qr
qr
qz
qz
qt
2
J 2e
qA
þ grad f
Q¼
¼s
qt
s
(3)
where A is the vector potential, n is the magnetic reluctivity,
s is the electric conductivity, l is the thermal conductivity,
c is the specific heat, T is the temperature, and f is the
electric scalar potential, respectively. Fig. 8 shows
the analyzed model and conditions used in the thermoelectromagnetic field analysis with the simultaneous
equations of electromagnetic and thermal conductive fields.
Figs. 9(a)–(c) show the dependence of material properties
on temperature. These material properties were used in the
numerical simulation.
Fig. 10 shows the analyzed transition of the temperature distribution. The temperature increased with time.
It was found that the plate could be heated quickly
and the temperature was around the Curie point as shown
in Fig. 9.
Fig. 10. Simulated transition of the temperature distribution.
4. Conclusions
(1) By adding small amount of rare earth element (Sm, Gd,
Dy, Y) and Al in SUS430, materials which have the
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T. Todaka et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e702–e707
Curie point in the range 100–300 1C were developed for
self-temperature control.
(2) The developed materials could be hot-rolled to a thin
plate of around 0.5 mm thickness.
(3) The temperature characteristic of the produced thin
plates was measured, and the temperature controlling
function was confirmed.
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References
[1] M. Enokizono, T. Todaka, Modeling induction heating processes for
numerical simulation (invited paper), in: 17th ASM Heat Treating
Society Conference Proceedings including the First International
Induction Heat Treating Symposium, 1997, pp. 601–608.