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

  1. Hardness 2 - 20GPa
  2. Tensile stresses 500 - 1200 Mpa
  3. Electrical resistivity 1 - 5 10-6 Ohmm
  4. The temperature expansion coefficient close to 0
  5. 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 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.

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 chromel-alumel thermocouple was used to measure the steady state temperature on the ribbon surface.

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 / I, A / V, V / P, W / T, 0C / , W/m*K
1 / 0 / 0 / 0 / 0 / 0
2 / 1.30 / 2.96 / 3.84 / 8 / 9.60
3 / 1.75 / 4.00 / 7.00 / 14 / 10.00
4 / 3.35 / 7.55 / 25.29 / 48 / 10.50
5 / 4.52 / 13.27 / 60.00 / 92 / 11.00
6 / 7.11 / 16.00 / 113.76 / 177 / 12.80

The electrical power has been calculated by the following equation

P = V*I (2)

Heat transfer coefficient has been calculated from:

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/m20C (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

Ribbons width 2.5 10-2 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

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 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 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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