AMORPHOUS METALLIC ALLOY RIBBONS
HEATING ELEMENT
M.A Geller, E.T.Brook-Levinson
Advance Heating Technology Ltd., Israel
1. Introduction
Various types of heaters are widely used in different
technological processes and domestic applications. Metals and their
alloys are mainly used as materials for heating element construction.
The most usable form of such elements is wire. High service properties
and relatively low price makes them an attractive material for heating
applications. At the same time, scientists and engineers are looking for
new materials, which can serve as heating elements. For example,
recently heaters became known using electrical conductive ceramics and
even conductive plastics.
The amorphous metallic ribbons known for many years and
widely used as soft magnetic materials, were never used in the past in
the heating applications although the amorphous ribbons reveal high
mechanical and electrical properties. The study of the amorphous ribbon
as heating elements has been carrying out. The results of these studies
are presented in the present article.
2. The method for amorphous ribbon production and their
physical properties
The amorphous metallic alloy ribbons are usually produced by
the method of rapid quenching by pouring the melted alloy on a rotating
massive drum. Linear velocity at the drum edge is about 30 m/s.
Normally the drum is made from copper or bronze to provide high
cooling rate through the high thermal conductivity of the drum material.
The cooling rate providing amorphous state of the solidified
metal alloy is about 105-106 0C/s depending on the alloy composition.
The ribbons with the 20 - 30 m thickness and 5 - 100 mm width can be
fabricated using rapid quenching procedure.
2.1 The typical properties of amorphous ribbons
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1.
2.
3.
4.
5.
Hardness 2 - 20GPa
Tensile stresses 500 - 1200 Mpa
Electrical resistivity 1 - 5 10-6 Ohmm
The temperature expansion coefficient close to 0
The crystallization temperature 250 - 5000C
2.2 Comparison between crystalline and amorphous metallic
materials use as heating elements
2.3 Crystalline materials
Metals and their alloys are the mostly used materials for heating
element fabrication. They expose the following properties:
1. High electrical resistivity
2. High operation temperature
3. High anticorrosion resistance
Canthal and nickel-chromium alloys are mostly used for high
temperature heaters. They have high electrical resistivity of 1.4-1.6*10-6
Ohmm and high operation temperature ranging 8000C to 15000C. The
manganin, constantan and reotan are used for middle temperature
heaters with operation temperature of up to 3000C - 5000C. The heaters
are produced mainly in the form of wire spirals. The wires usually work
under high temperature due to their relatively low heat transfer area. All
electrical power is transferred to the ambient in steady state heater
operation mode. The transferred electric power in steady state is defined
by the well-known equation [1]:
P = ST
(1)
Here P is the electrical power of the heater,  is the coefficient
of heat transfer between the heating element and ambient, S is the heat
transfer area of the heating element, T is the temperature difference
between the heating element and ambient.
One can readily can see from Eq. (1) that the same power
transfer can be provided with high temperature difference and
small area of the heating element or, vice versa, with low temperature
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and large heat transfer area. The wires as heating elements have
minimum area per unit volume. This results in high temperature of the
heating element. For example, if we compare two heaters, one of them
an wire element 0.5 mm by diameter and another an amorphous ribbon
10 mm by width, proving the same electrical power, we can conclude
that their operating temperatures are essentially different. The
temperature of the ribbon element is by 12 times lower then the
temperature of the wire element.
This explains why the amorphous ribbons are of great interest as
heating element. Nevertheless, even the crystalline ribbons are
practically not used in the heating element applications. We can assume,
that the reason is in conventional approach of the designers using the
high temperature wires as heating elements for all temperature ranges of
the heating applications although the wire possesses minimum surface
area per unit volume, which results in high working temperature of the
heating element. Because the temperature of the ribbon elements is
essentially lower such low temperature heaters have advantages in
comparison with wires with the same electrical power. Among them:
1. Low operation temperature reduces requirements to
the heat and electrical insulation
2. Better environmental friendliness (the dust is burnt
on the surface of the high temperature heating elements)
3. Amorphous metallic alloy ribbon as heating element
Why amorphous metallic alloy ribbon is good heating element?
The following set of parameters gives the answer to this question.
Ribbons thickness
20 to 30 m;
Width range
2 to 100 mm;
Resistivity
>10-6 ohmm;
Operation temperature 4000C
High corrosion resistance (cobalt and chromium based alloys);
High ribbon ductility and flexibility;
High heat transfer area due to the large ribbon width;
Short warm-up time to steady state due to the low ribbon mass.
Saving of electrical energy in unsteady of the heaters operation modes.
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The heat design includes the following steps:
1. Calculation of the heater power
2. Calculation of the heater temperature
3. Checking of the insulation
4. Design of the heating element
5. Testing of the heating element in accordance with standards
for heating applications
As the amorphous ribbons have never been used as heating
element they must be tested as heating elements at all stages of the heat
element fabrication. Two serious restrictive factors exist when using the
amorphous metallic alloy ribbon:
1) The operation temperature of the ribbon cannot exceed the
crystallization temperature of the amorphous material. At the
temperature higher than the crystallization temperature the material
becomes crystalline and the electrical resistance sharply drops down.
2) The operation temperature of the ribbon cannot exceed the
embrittlement temperature of the material. Below this temperature, the
mechanical properties deteriorate and the material can be even
destroyed.
Both factors should be taken into account during the design of
the heating element based on amorphous ribbons.
3.1 Investigation of the heat transfer between the amorphous
ribbon and the ambient
To calculate the temperature of the ribbon element the heat
transfer coefficient must be known. The measurement scheme is
presented in Fig. 1.
The amorphous ribbon 1 m by lengths; 25 mm by width and 27
m by thickness has been plugged to an electrical source. The electrical
current and voltage at the ribbon has been measured. The chromelalumel thermocouple was used to measure the steady state temperature
on the ribbon surface.
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Fig 1. Measurement scheme for heat transfer coefficient
1 - amorphous ribbon; 2 – voltmeter; 3 – ampermeter; 4 - temperature sensor; 5
– electric source
The results of the measurements are presented in Table 1
Table 1
Current, voltage, power and temperature difference
between the ribbons surface and ambient air
No
1
2
3
4
5
6
I, A
0
1.30
1.75
3.35
4.52
7.11
V, V
0
2.96
4.00
7.55
13.27
16.00
P, W
0
3.84
7.00
25.29
60.00
113.76
T, 0C
0
8
14
48
92
177
, W/m*K
0
9.60
10.00
10.50
11.00
12.80
The electrical power has been calculated by the following
equation
P = V*I
Heat transfer coefficient has been calculated from:
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(2)
P = 2ST
(3)
Heat transfer coefficient  has been calculated from (3) as
 = P/2ST
(4)
The average value of the heat transfer coefficient has been
calculated from the experimental data using the EXCEL program. The
average value of the heat transfer coefficient was found to be
 = 13 W/m2 0C
(5)
4. Design of the electric heater with amorphous metallic
alloy ribbon
4.1 Calculation of the heater
A 500 W power heater was designed and tested at the voltage of
24 V. All steps of the calculations are presented below.
Step 1. Calculation of the electric resistance.
R = V2/P = 242/500 = 1.15 Ohm
Step 2. The case of the heater had the dimensions of 33 cm x 23
cm x 40 cm. For such dimensions the optimum design is the radiator
comprise 12 plates connected in series. Each plate consist of 7 ribbons
connected in parallel. The electric resistance of a plate was 0.095 Ohm.
The amorphous ribbon had the following dimesions:
Ribbons thickness
2.5 10-5 m
-2
Ribbons width
2.5 10 m
The specific electric resistance is 1.4 10-6 Ohm m
The length of one ribbon was 30 cm that allows the plate
resistance of 0.095 Ohm. The electric resistance of the one ribbon 30 cm
length is 0.672 Ohm.
The electric resistance of the plate consisting of seven ribbons
connected in parallel is 0.692/7 = 0.095 Ohm.
Step 3. Ribbon temperature calculation. The consumed electric
power equals
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P1 = P/12 = 500/12 = 41.66 W
The temperature difference can be calculated from the Newton
equation (3)
T = (P1/2S) = 41.66/(2*13*2.1*2.510-2) = 300C
At the ambient temperature of 200C the ribbon temperature
reached 500C. It must be underlined that the embrittlement temperature
for the FeBCr ribbon composition is in the temperature range of 3500C
to 4500C. As one can see, the ribbon operation temperature is very far
from ribbon embrittlement temperature.
Step 4. Heating element design. The view of the one heating
plate is presented in the Fig.2.
Fig 2. The view of the heating plate
1 – amorphous ribbon; 2 – ribbons width; 3 – electric connected copper strip;
4 – the distance between ribbons
One plate consists of 7 ribbons of 25 mm by width and 30 cm by
length. The distance between ribbons is 6 mm. The ribbons are
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connected by copper strips.
4.2 Testing of the heater
The heater was tested in the operation conditions. The ribbon
temperature as a function of the heating and cooling time is presented in
the Fig 3.
Fig 3. Ribbons temperature as the function of the heating and cooling time.
The ribbons temperature has been measured in the unsteady
mode using the Data Logger. The measuring accuracy was 10C. As one
can be see, the heater achieves the steady state in a short time
(approximately in 80 s). The ribbon has very low mass which results in
a very low thermal inertia and short time to achieve steady state. Actual
ribbon temperature is 800C (Fig 3) which is 25% higher than the
calculated value. The measurement showed approximately the same
temperature of all ribbons in 12 heating plates.
4.3 Comparison between amorphous metallic alloy ribbon and
crystalline wires heating elements
The theoretical analysis has been carried out for the heater
power of 500 W. The crystalline wire with NiCr composition was selected because the
specific electric resistance of the NiCr is close to the specific electric resistance of
amorphous metallic alloy ribbon with Fe-B-Cr composition. To provide the same
electric power both elements had the same electric resistance. Therefore
the calculated diameter of the NiCr wire was 0.89 10-3 m. The heat
transfer coefficient of the wire under free convection heat transfer
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conditions was taken 5.6 W/m2*K [1]. The calculated wire temperature
in this case was approximately 12000C that more then by 20 times larger
that the calculated ribbon temperature. If we take into account, that heat
transfer coefficient is 13 W/m2*K (the average value for the ribbon) the
wire calculated temperature is approximately 520C. In both cases the
ribbon temperature is essentially lower then that of wire.
5. Conclusions
1. Amorphous metallic alloy ribbon can be used as low
temperature heating element
2. Low temperature of the heating element provides that
amorphous ribbon heaters are much more environmentally friendly
3. The amorphous ribbon element possesses very low heat
inertia and achieves steady state in a short time.
4. Low operation temperature of the heating element allows
significant reduction of its production cost (as low temperature electric
insulation can be used)
References
1. J.P Holman, ‘Heat transfer’, McGraw-Hill Book Company,
1989, 459 p.
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