Process engineering

cfi/Ber. DKG 94 (2017) No. 6-7 E 23

1 Motivation for characterization at high temperature

The study of material behaviour at high temperatures and their characterization at these respective temperatures is the driving force behind the generation of a market for high temperature furnaces. Owing to a very high demand and emphasis on the develop-ment of novel resistant material for varied applications, from aerospace to fireproof materials for buildings, there is a constant growth in the demand for thermal systems to test and qualify such materials. Other applications include test systems to validate and aid the ever expanding field of multi-physics simulation, quality control of materials, and high temperature material fabrication and machining. In the recent years, high temperature debinding and sin-tering furnaces have also found their way in to the field of 3D-printing, where they are used to heat treat metal printed parts to im-prove compactness and reduce fragility [1].

2 Heating methods

The type of heating used for the furnace is the fundamental criteria used for the classi-fication of furnaces. Based on this the three fundamental types of furnaces are radiative, inductive, and resistive furnaces.

2.1 Radiation heating

This involves the use of halogen lamps to heat up the test sample radiatively, and en-joys the benefit of having a very high heat density i.e. heat available per square meter of test space. However, such systems suffer from the drawback of being extremely sen-sitive to contamination, and large ex pend-iture associated with cooling circuits for the cold parts of the furnace, besides having a maximum temperature of 2000 °C (Fig. 1). An overview of the reachable temperatures with radiation heating has been given in Tab. 1.

2.2 Inductive heating

Inductive heating is an extremely efficient method of heating up electrically con duct-ive materials. Besides being capable of pro-viding heating directly at the sample, it also offers the unique advantage that one can change the penetration depth of heating in-side the sample by changing the frequency of the electric field, and is calculated with the help of the eq. 1:

Measurements of Thermophysical and Thermochemical Properties of Materials at High and Ultra-High Temperatures

U. Lohse, S. Tiwari

Uwe Lohse

XERION Group

09599 Freiberg, Germany

Siddharth Tiwari

XERION Berlin Laboratories GmbH

10587 Berlin, Germany

Corresponding author: S. Tiwari

E-mail: s.tiwari@xerion.de

www.xerion.de

Keywords: furnace, vacuum, induction,

radiation, high temperature

A generalized discussion about the design of high and ultra-high temperature furnaces for material characterization has been made in the article below. Depending upon the desired application, a care-ful selection of the furnace type and material is extremely critical in order to optimise both cost and performance, while simultaneously en suring an acceptable lifetime for the complete system. With this in mind, a comparison of the heating methods and heater materials has been made followed by examples of some interesting thermal systems and their applications built by the XERION Group/DE.

Fig. 1 Example of radiation furnace

whereδ = penetration depth [m]ρ = resistivity [Ω·m]μ = magnetic permeability [H·m–1]f = frequency [Hz].

As evident from the above formula, the penetration depth is inversely proportional to the magnetic permeability of the ma-

Measurements of Thermophysical and Thermochemical Properties of Material at High and Ultra-High Temperatures

Uwe Lohse XERION GROUP Freiberg, Germany u.lohse@xerion.de

Siddharth Tiwari XERION BERLIN LABORATORIES GmbH

Berlin, Germany s.tiwari@xerion.de

Abstract—A generalized discussion about the design of high and ultra-high temperature furnaces for material characterization has been made in the article below. Depending upon the desired application, a careful selection of the furnace type and material is extremely critical in order to optimize both cost and performance, while simultaneously ensuring an acceptable lifetime for the complete system. With this in mind, a comparison of the heating methods and heater materials has been made followed by examples of some interesting thermal systems and their applications built by the XERION Group.

Keywords—Furnace; Vacuum; Induction; Radiation; High Temperature; XERION.

I. MOTIVATION FOR CHARACTERIZATION AT HIGH TEMPERATURE

The Study of material behavior at high temperatures and their characterization at these respective temperatures is the driving force behind the generation of a market for high temperature furnaces. Owing to a very high demand and emphasis on the development of novel resistant material for varied applications, from Aerospace to fireproof materials for buildings, there is a constant growth in the demand for thermal systems to test and qualify such materials. Other applications include test systems to validate and aid the ever expanding field of multi-physics simulation, quality control of materials, and high temperature material fabrication and machining. In the recent years, high temperature debinding and sintering furnaces have also found their way in to the field of 3D printing, where they are used to heat treat metal printed parts to improve compactness and reduce fragility [1].

II. HEATING METHODS The type of heating used for the furnace is the fundamental criteria used for the classification of furnaces. Based on this the three fundamental types of furnaces are Radiative, Inductive, and Resistive furnaces.

A. Radiation heating This involves the use of halogen lamps to heat up the test sample radiatively, and enjoys the benefit of having a very high heat density i.e. heat available per square meter of test space. However, such systems suffer from the drawback of being extremely sensitive to contamination, and large expenditure associated with cooling circuits for the cold parts of the furnace, besides having a maximum temperature of 2000°C.

An overview of the reachable temperatures with radiation heating has been given below:

TABLE I.

Maximum achievable temperature in various Radiation configurations

Configuration Temperature

Array configuration 1400°C

Test machine (Planar) 1300°C

Test machine (Punctiform) 1800°C

Highly focused with arc lamps 2000°C

Fig. 1. Example of Radiatiation Furnace

B. Inductive heating Inductive heating is an extremely efficient method of heating up electrically conductive materials. Besides being capable of providing heating directly at the sample, it also offers the unique advantage that one can change the penetration depth of heating inside the sample by changing the frequency of the electric field, and is calculated with the help of the equation given below:

𝛿𝛿 =𝜌𝜌

𝜋𝜋. 𝜇𝜇. 𝑓𝑓

𝛿𝛿: Penetration depth [m] 𝜌𝜌: Resistivity [Ω-m] 𝜇𝜇: Magnetic permeability [H-m] 𝑓𝑓: Frequency [Hz]

(eq. 1)

E 24 cfi/Ber. DKG 94 (2017) No. 6-7

Process engineering

other physical and environmental forms of loading to study its behaviour in a very spe-cific thermodynamic state [4]. This includes testing in vacuum, high pres-sure, inert gas atmosphere, reactive gas environment (mainly used for material fab-rication), electromagnetic environment etc. (Fig. 5–6).A summary of various forms of loading and their respective measurement has been given in Tab. 4.In some cases, possibility to embed a hot press as a part of the furnace is also con-ceivable along with special inspection stations, besides the traditional ones com-monly used in association with furnaces. All XERION systems are also accompanied

SiC and MoSi2 are generally used for ma-terials to be treated in air, while Mo, W, and C are used for the design of commonly christened cold wall furnaces i.e. furnaces requiring cooling water for the outer vessel due to the very high internal temperature (Fig. 4). Incidentally, the Swedish-German Chemist Carl Wilhelm Scheele was one of the first to identify the presence of isolated stable states for all these three elements in the 18th century.Tab. 3 gives a summary of the heating methods discussed in this article.

3 Environment for testing

Besides thermal loading inside the furnace, the test material can be subjected to several

ter ial, which implies that ferromagnetic materials showcase much more skin effect (lower penetration depth) as compared to non-magnetic materials for a given fre-quency (Fig. 2). Fig. 3 shows an example of an inductive furnace.

2.3 Resistance heating

Resistance heating, although afflicted by the demerit of having low heat density, of-fers great flexibility in terms of geometries and materials to be tested, and environ-mental conditions and temperature for the tests. The most commonly used heater materials for such systems are silicon carbide (SiC) and molybdenum disilicide (MoSi2) of the ceramic family, and molybdenum (Mo), tungsten (W), and graphite (C) of the non-ceramic family. W is also the element with the highest melting point i.e. 2526,85 °C (2800 °K) at atmospheric pressure in the periodic table; graphite changes state at a temperature more than 2726,85 °C (3000 °K), however it sublimates instead of melting [2]. Tab. 2 gives the maximum reachable temperature with these materials when used as resistance heaters.

Fig. 2 Comparision of penetration depth at 1000 °C (copper and iron)

Fig. 3 Example of an induction furnace Fig. 4 Example of a resistance furnace

Tab. 2 Maximum reachable temperatures with resistance heaters

Heater Type Maximum Temperature Environment

SiC 1600 °C Air, restricted use with inert gas and vacuum

MoSi2 1800 °C Air, restricted use with inert gas and vacuum

Mo 1700 °C High vacuum, inert gas

W 2500 °C High vacuum, inert gas

C 3000 °C Inert gas, restricted use in vacuum

Tab. 3 Summary for heating methods

Heating Method Merits Demerits

Radiation • High heat density• Clean heating

• Dependent on absorption coeff.• High costs for water and air cooling• Sensitive to contamination

Inductive • Direct heating of probe• High heat density• Adjustable penetration depth

• Only for electrically conductive samples

• High cost for frequency generator• Not suitable for complex

geometries

Resistive

• Independent of conductivity of material

• Independent of complicated and changing geometries

• Less expenditure on electricity

• Low heat density• For temperatures >1800 °C inert gas

or vacuum required

Tab. 1 Maximum achievable temperature in various radiation configurations

Configuration Temperature

Array configuration 1400 °C

Test machine (planar) 1300 °C

Test machine (punctiform) 1800 °C

Highly focused with arc lamps 2000 °C

Process engineering

cfi/Ber. DKG 94 (2017) No. 6-7 E 25

German-Japanese Joint Symposium, Freiberg,

July 2015

[3] Lohse, U.: A method for controlling the tem-

perature of a halogen lamp, tempering and the

use thereof, Registration No. DE1998253059

C2. German Patent and Trade Mark Office

(DPMA), 1998

[4] Pesnecker, W.; Gros, U.; Lohse, U.: Method

and apparatus for determining the specific

heat capacity, thermal conductivity and/or

temperature conductivity. Registration No.

DE19943076 C2. German Patent and Trade

Mark Office (DPMA), 1999

[5] Lohse, U.: Thermal equipment for PV produc-

tion & research. Tech talk made at the World

Future Energy Summit 2014, Abu Dhabi

[6] Kanthal AB: Kanthal Super Handbuch für Elek-

trische Hochtemperaturheizelemente, 1997

[7] Waitz, R.; Wübben, P.: Elektrisch beheizte

Öfen für Schutzgas-und Vakuumbetrieb. Elek-

trowärme Int. (2010) 227

Such systems will have mathematical mod-els of the furnace stored in the “cloud” and will also aid in the offline study of the sensitivity of design parameters associated with the furnace, thus further reducing the design and troubleshooting time associated with the construction of such complex sys-tems. To conclude, giant leaps made in the field of automation and robotics will eventu-ally lead to the successful realization of the factories of the future allowing large scale integration for manufacturing processes.

References

[1] Yamazaki: Development of a hybrid multi-

tasking machine tool: Integration of additive

manufacturing technology with CNC machin-

ing. Procedia CIRP 42 (2016) 81

[2] Uhlig, V.; Richter, H.; Lohse, U.: High tempera-

ture furnaces made from graphite materials.

with a user friendly control interface (HMI), allowing an operator with the correct cre-dentials to manually control each valve and sensor of the system or run the complete system autonomously for batch processes (Fig. 7).

4 Next generation of furnaces

Adhering to the Industry 4.0 trend for manu facturing technologies, the next gen-eration of furnaces will have a more stable and faster access digitally, with remote control and supervision of systems possible with minimal infrastructural requirements and preparation. The developments in Internet of Things (IoT) will further facilitate real time cloud based intensive computations to assist in a far better thermal control of the furnace than currently achievable with the current state-of-the-art controller technology.

Tab. 4 Type of loading and measurement

Property Device

Temperature Thermoelement, pyrometer

Tension and compression Force sensor, displacement sensor, load cell

Atmosphere/vacuumPressure sensor, gas sensor, mass spectrometer, chemical analysis

Chemically reactive fluidDifferential scanning calorimeter, mass spectrometers, rest gas analyser

Magnetic field Hall sensor

Fig. 5 Rest gas analysis to study the behaviour of test materials

Fig. 6 Automised test stand (Tmax 1500 °C, inert gas, press force 5 kN, 20 samples)

Fig. 7 Sample control interface to the furnace