FAST graphite tool

Measurement 147 (2019) 106863

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Direct temperature measurement via thermocouples within an SPS/FAST graphite tool M. Radajewski ⇑, S. Decker, L. Krüger TU Bergakademie Freiberg, Institute of Materials Engineering, Gustav-Zeuner-Str. 5, 09599 Freiberg, Germany

a r t i c l e

i n f o

Article history: Received 8 January 2019 Received in revised form 22 July 2019 Accepted 25 July 2019 Available online 27 July 2019 Keywords: SPS/FAST Thermocouple Temperature measurement Graphite

a b s t r a c t Many publications investigate temperature distributions by simulations. Temperature is often not practically measured or only at a few locations inside the sintering tool using thermocouples. However, thermocouple temperature measurement is very sensitive to environmental conditions and can be extremely defective. This study investigates the feasibility of thermocouple temperature measurement at different measuring positions within a graphite tool during SPS/FAST. Three different graphite tool setups and three different thermocouples were used for the temperature measurements. The thermocouples were covered by an Al2O3 tube and placed directly inside a borehole in the setup. Process temperatures, measured by a vertical pyrometer, up to 1200 °C were realized. Experimental data at different temperature plateaus show significant temperature differences depending on the applied thermocouple and tool setup. It is shown that the temperature present is underestimated by the thermocouple and measuring errors vary drastically with the thermocouple length, which is inserted in the sintering tool. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction SPS/FAST (Spark Plasma Sintering/Field Assisted Sintering Technology) is a sintering technology comparable to hot pressing (HT). During the sintering process, the initial material is uniaxially compressed. The heat is generated by Joule heating due to a mostly pulsed direct current (DC) within the (conductive) material and/or the graphite tool. The main advantage of SPS/FAST is the high heating and cooling rate (several hundred K/min), which results in a short total processing time. Thus, undesirable grain growth is inhibited and mechanical properties are improved [1–4]. Usually, the SPS/FAST process is controlled by a temperature measurement via pyrometer or thermocouples [2]. Applying an axial pyrometer (e.g. used in FCT Systeme GmbH SPS/FAST devices), the sintering temperature is measured in a hole of the upper graphite punch few millimeters above the surface of the sample. Radial pyrometers (e.g. used in SPS apparatus of Sumitomo Coal Mining Ltd. or SPS Syntex Ltd.) determine the process temperature at the surface of the graphite die or within a lateral borehole [5]. Typically for temperature measurements via thermocouples, a radial hole inside the graphite die wall is applied, where the thermocouple (TC) is inserted.

⇑ Corresponding author. E-mail address: [email protected] (M. Radajewski). https://doi.org/10.1016/j.measurement.2019.106863 0263-2241/Ó 2019 Elsevier Ltd. All rights reserved.

None of these methods measures the actual temperature distribution of the sample. Furthermore, a wide temperature range is given in literature for sintering of one material, in particular, using different measuring methods for the process temperature [5]. Nevertheless, it is of great research interest to get knowledge about the actual sintering temperature of conductive or non-conductive samples and the temperature distribution in radial and vertical direction of the sample during SPS/FAST [6]. Various studies report that conductive materials exhibit a temperature maximum in their center [6,7], whereas non-conductive samples show this temperature maximum near the sample edges [2,7]. In the last years, several authors attempted to model the temperature distribution within a conductive and/or non-conductive sample during SPS/FAST [7–13]. In contrast, only few studies deal with the direct temperature measurement, for example via thermocouple, especially at temperature above 1000 °C. For instance, Wang et al. [14,15] and Matsugi et al. [16] used TCs at varying measuring points (at least one TC directly within the sample) in their setup to get information about the temperature distribution. Usually, the temperature measurements by TCs, which were used to develop or validate the simulation, were carried out at the edge of the sintering tool or at graphite spacers/adapters, e.g. in [17] or [18]. In addition to radial temperature gradients, temperature gradients can also occur in vertical direction of the sample (or tool). The generation of such temperature gradients is specifically used during the sintering of functionally graded materials (FGMs). It

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the thermal diffusivity, q is the density (according to reference [26]) and cp is the specific heat capacity. Three different tool setups were selected for the measurements. For setup I, a bulk graphite sample with a thickness of 10 mm and a diameter of 20 mm was placed between the graphite punches within the graphite die (Fig. 1(a)). Prior to this, the inner part of the graphite die was covered with a graphite foil (SGL Group: Sigraflex F02510TH, 0.25 mm thick). Furthermore, three layers of the same foil were placed between the sample and each graphite punch. Typically, the graphite foil is used to improve the thermal and electrical contact within the sintering tool and protects the graphite die and plungers from sticking by the sample material. Setup II was produced from one piece without graphite/graphite interfaces (Fig. 1(b)). The height of this tool setup was similar to setup I. Thus, contact resistances, which probably arise in the transition zones between sample surface and graphite die, plunger or foil, were inhibited within the second tool setup. In order to measure the temperature at the interface sample/die (measuring position 1: M1) and in the middle of the sample (measuring position 2: M2), a vertically centered hole with a diameter of 3.5 mm was drilled through the graphite tool of both setups. After finishing the temperature measuring experiments at M1, the hole was drilled to the final depth (M2). Setup III was also produced from one piece (similar to setup II). However, this setup exhibits a clearance hole (diameter: 3.5 mm) instead of a blind hole. The temperature measurements were carried out at the measuring positions 10 mm (M1), 15 mm, 20 mm (M2), 25 mm, 30 mm, and 35 mm (see Fig. 1(c)). To reduce thermal loss due to thermal radiation, the outside surface of the sintering die and the top and bottom side of the die were covered with a graphite felt (SGL Group: Sigratherm GFA10). The SPS/FAST process was controlled by an axial pyrometer measuring the temperature TPyro in a drill hole of the upper punch, approximately 5 mm above the sample. For the TC temperature measurements, different types of sheathed thermocouples (diameter: 1 mm, nominal length: 700 mm) manufactured by

can be achieved by applying an asymmetric setup of the graphite tool [19–21] or by the modification of the heating elements [22– 25], for example by modifying the cross-section of the graphite dies. The actual temperature during the sintering process in different areas of the sample is responsible for the resulting microstructural and mechanical properties. Thus, it is essential to know, which temperature exists in different areas of the sample and which maximum temperature is reached within the sample depending on the set temperature as control variable. In numerous studies where temperature distribution during SPS/FAST is simulated, some verification through temperature measurements is carried out. However, TC measurements are very sensitive and can be affected by many factors. Thus, it is questionable, if the measured temperature values are in accordance to the actual temperature at the respective measuring point. In this work, three different kinds of TCs were used for the temperature measurement and compared to each other. Furthermore, the influence of the inserted length of the TC inside the sintering tool on the measured temperature was investigated. Three different setups were used and several temperature plateaus were taken into account. The whole setup was supposed to resemble an SPS compaction with the common sample diameter of 20 mm and a sample thickness (after sintering) of approximately 10 mm. The results are compared to the available literature concerning temperature modelling and temperature determination during SPS/FAST.

2. Material and methods All parts of the sintering tool and the graphite spacers/adapters with the exception of the graphite foil are manufactured from graphite grade 2333 (Mersen). The thermal and electrical conductivity of the graphite used for the investigation depending on the temperature are given in Table 1. The data concerning the electrical conductivity correspond to personal information of Mersen. The thermal conductivity was determined by k ¼ a  q  cp , where a is

Table 1 Thermal and electrical conductivity of graphite grade 2333 (Mersen) depending on the temperature. Electrical conductivity r [1/(X m)]

Thermal conductivity k [W/(m K)] 20:95 þ 0:29  T  6:043  10 - 10

4

-4

2:32  10  T þ 3:887  10 (T = 293 K . . . 1473 K)

(a)

 T2 þ 5:417  10 - 14

T

-7

 T3

37902:7 þ 105:26  T - 0:063  T2 - 6:55  10 -8

5

4

þ 1:46  10  T - 3:23  10 (T = 293 K . . . 1973 K)

Pyrometer

(b)

- 12

T

-6

 T3

5

40 mm

(c)

(1)

M1

M2

(3)

M1

M2

M1

M2

TC

(4)

10 15 20 25 30 35 [mm]

24 mm

(6)

48 mm

(2)

(5)

Fig. 1. Experimental setup (cross section): (a) graphite tool with graphite sample (setup I), (b) graphite tool without sample (setup II), (c) graphite tool with clearance hole (setup III); (1) graphite die, (2) graphite foil, (3) sample, (4) graphite plunger, (5) alumina tube, (6) thermocouple. The red arrow and red dots mark the measuring position of the pyrometer and of the thermocouples, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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60

Type K TC Type S TC

50 40 30 20 10 0

400

800 1200 1600 Temperature [K]

2000

constant at values of 100 K/min and 5 kN, respectively. To inhibit the influence of the heating rate on the direct temperature measurement during the SPS/FAST process the temperature values are evaluated at several temperature plateaus at a steady state temperature, which is explained below. To compare the set temperature TPyro measured by the pyrometer with the temperature measured by the thermocouples TTC, DT was calculated by

DT ¼ T TC  T Pyro

ð1Þ

The temperature TPyro was the control variable for the temperature measurement during heating cycle I (see Fig. 3(a)). TPyro remains at 900 °C, 1000 °C and 1050 °C or 1100 °C for 6 min for the graphite tool setups I and II. If the temperature measuring took place using type K-I, the final set temperature TPyro was 1000 °C or 1050 °C due to the maximum allowed measuring temperature of TC type K of 1100 °C (limited by the SPS device). Heating cycle I was repeated at least three times for each measuring point within setup I and II and the respective TC. The standard deviation was calculated from the arithmetic average of the temperature measurements with equal test conditions. According to the measuring range of the TCs, the temperature plateaus at the varying measuring positions (duration of 6 min) are 800 °C, 900 °C and 1000 °C (type K-I) and 900 °C, 1000 °C, 1100 °C and 1200 °C (type S-PtRh), for the graphite tool setup III.

Thermal conductivity [W/mK]

Thermocouple voltage [mV]

TMH GmbH, one of type K and two of type S, were used. The wires of the type K TC consist of Chromel (nominal chemical composition: 90 wt% Ni, 10 wt% Cr) and Alumel (nominal chemical composition: 95 wt% Ni, 2 wt% Mn, 2 wt% Al, 1 wt% Si) [27]. For the type S TC, pure Platinum and Platinum-Rhodium (nominal chemical composition: 90 wt% Pt, 10 wt% Rh) are used as wire material [27]. The wires of all used TCs are isolated from each other and their sheathing by MgO powder. Both, the TC of type K (type K-I, wire diameter: 0.18 mm, sheath wall thickness: 0.13 mm) and the TC of type S (type S-I, wire diameter: 0.15 mm, sheath wall thickness: 0.17 mm) exhibit a sheathing consisting of Inconel 600. Furthermore, one TC of type S (type S-PtRh, wire diameter: 0.24 mm, sheath wall thickness: 0.13 mm) was sheathed with Pt/Rh (10 wt% Rh). Fig. 2(a) shows the thermocouple voltage depending on the temperature for TC of type K and TC of type S. To protect the TCs from high current density, carbonization and to avoid sintering of the TC to the surface of the graphite tools, the TC was covered by an alumina tube (outer diameter: 3 mm, inner diameter: 1.6 mm, length: 40 mm, Friatec: Degussit Al23). Fig. 2 (b) shows the thermal conductivity for the temperature measurement materials involved in this study. All experiments were carried out using an SPS device HP D 25 (FCT System GmbH). A direct electric current with a pulse pattern of 10 ms:5 ms (on:off) under vacuum was applied unless specified otherwise. Both, the heating rate as well as the force were kept

100 90 80 70 60 50 40 30 20 10 0

Pt Pt/10Rh

Chromel Alumel

Inconel 600 Al2O3

500

(a)

1000 1500 Temperature [K]

(b)

Fig. 2. (a) Thermocouple voltage for TC of type K and TC of type S according to DIN EN 60584-1, (b) Thermal conductivity of Pt [28], Pt-10wt.%Rh [29], Chromel [30,31], Alumel [30,32], Inconel 600 [33] and Al2O3 [34].

0

TPyro [°C]

TPyro [°C]

1200 1100 1000 900 800 700 600 500 5

10

15 t [min]

(a)

20

25

1200 1100 1000 900 800 700 600 500 0

10 20 30 40 50 60 70 80 t [min]

(b)

Fig. 3. Sintering process for the temperature measurements (a) heating cycle I, (b) heating cycle II. The final temperature plateau depends on the used thermocouple (indicated by dashed lines).

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In addition to heating cycle I, a second heating cycle (see Fig. 3 (b)) starting at 600 °C with temperature plateaus each 50 °C (dwell time: 5 min) was applied for the graphite tool setups I and II using each of the TCs at different measuring positions. Heating cycle II was performed to investigate the progress of DT (see Eq. (1)) over a wide temperature range. The final temperature plateau for the measurements was limited by the maximum allowed measuring temperature of the thermocouples, i.e. 1100 °C for type K-I and 1150 °C for type S-I. In contrast; thermocouple type S-PtRh can be used to measure temperatures up to 1400 °C. For data evaluation of the temperature measured by the TCs and the electrical power, the mean value of the last 30 s at each temperature plateau was used. The precondition was a steady state temperature. This was proved performing calibration tests with a longer dwell time (30 min) at the respective temperature plateau. The criterion for reaching a steady state temperature was to obtain a constant temperature TTC during dwell time, which oscillated maximal 1 K. To find this steady state temperature, the set temperature, measured by the pyrometer, was kept constant for 30 min. These experiments indicated that the steady state is reached already after 5–6 min dwell time, depending on the temperature differences between the single temperature plateaus (heating cycle I: 100 K, heating cycle II: 50 K). To compare the temperature measured by the three different TCs (type K-I, type S-I, type S-PtRh), an internal calibration test was performed. A graphite tool similar to the tool setup in Fig. 1 (b) with four drill holes (hole diameter: 3.5 mm, hole depth: 10 mm) arranged at an angle of 90° to each other was heated from room temperature at 10 K/min to 1050 °C in a muffle furnace. The TCs within the drill holes were protected by alumina tubes. It was observed that all TCs measure a comparable temperature with minor deviation (3 K) after a dwell time of 5 min at the dwell temperature of 1000 °C. For the data recording a 4-channel datalogging thermometer Extech SGL200 was used. Furthermore, the output values of the SPS device for the type S TC and type K TC were verified. Replications with internal data recording (using the SPS device) and external data recording (using the 4-channel datalogging thermometer) were performed. No significant temperature differences were detected between internal and external data recording. 3. Results and discussion Fig. 4 shows the temperature difference between the temperature of the pyrometer and the temperature measured by different TCs DT as a function of the respective temperature plateau. It is obvious that all TCs provide different temperature values for each

setup and measuring position at a given set temperature. The lowest temperature values for M1 and M2 are measured using the type S-PtRh TC whereas the type K-I TC provides the highest temperature values. Smaller differences exist between the type K-I TC and type S-I TC compared to the type S-PtRh TC. It was supposed that the significant differences of the temperature values measured by the respective TCs result from an electromagnetic effect and/or a thermal conduction effect. The pulsed electric current which flows through the graphite tool and the (electrically conductive) sample for heat generation also generates an electro-magnetic field around the sample and the sintering tool [35]. Due to this electro-magnetic field, an alternating voltage is induced, which superimposes the thermocouple voltage. In the case of utilizing different materials for the TC wires and the sheathing of the TCs, changes of the thermocouple voltage could influence the measured temperature values of the several TCs in a different manner. To investigate the influence of the alternating voltage on the thermocouple voltage the pulse pattern was changed from 10 ms:5 ms (on:off) to 255 ms:255 ms (on:off) for a few heating tests. The authors assume that no electro-magnetic field exists during the time period of 255 ms with pulse off. It was observed that the mean value of the thermocouple voltage did not change during the time period of 255 ms pulse on to 255 ms pulse off compared to an on:off ratio of 10 ms:5 ms. Thus, the temperature values measured by the TCs did not vary, too. Hence, a significant influence of the electro-magnetic field on the TC measurement could be excluded. The TC wires and the sheathing are good conductors of heat, which can carry heat along the thermocouple. Thus, the temperature measurement via TC is influenced by the heat, which flows from the tip of the TC to the back end and decreases the TCmeasured temperature [27]. Hence, the effect is intensified by high temperature gradients over a short measuring distance as in the case of SPS/FAST. Further heat loss due to thermal conduction result from the utilization of protection tubes [27]. On the basis of a higher thermal conductivity (see Fig. 2(b)) of the TC protected by Pt/10 wt% Rh (sheathing material in the type S-PtRh TC), compared to Inconel 600 (sheathing material in the type S-I TC), the differences of the TC-temperature values can be explained. In addition, it is supposed that the temperature differences between type K-I TC and type S-I TC arise from the thermal conductivity differences of the wire material used in the respective TC. The effect of thermal conduction loss was investigated by temperature measurements within setup III. Fig. 5 shows the TCmeasured temperature values for the type K-I TC (Fig. 5(a)) and the type S-PtRh TC (Fig. 5(b)) at different set temperatures along the measuring distance. In Fig. 5, higher temperatures at the same set temperatures are measured using the type K-I TC compared to

Fig. 4. Thermocouple temperature measurement (heating cycle II) at M1 and M2 for different temperature plateaus: (a) setup I, (b) setup II.

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M. Radajewski et al. / Measurement 147 (2019) 106863

Fig. 5. Temperature measurement using setup III; (a) type K-I, (b) type S-PtRh.

the type S-PtRh TC. For both TCs, the maximum temperature was not located in the center of the setup (distance from the die surface: 20 mm, corresponds to M2), which is in contrast to the results of numerous reports [36,37]. Only the loss due to thermal conduction can be the reason for this observation. A plateau of the TC-measured temperature was achieved after approximately 25 mm distance from the die surface for the type K-I TC, whereas no temperature plateau arises for the type S-PtRh TC. Based on these results, it is impossible to correctly measure the absolute temperature at a given measuring point as well as the temperature gradient between the middle of the sample (or setup) and the interface sample/die via TC. The TC-measured temperature is the superposition of the actually existing temperature and the loss due to thermal conduction, which varies according to the inserted length of the thermocouple. Thus, the temperature measured by the TC is underestimated. Furthermore, the error of temperature measurement by TC varies with the measuring point. All TCs used in this study (type K-I, type S-I, type S-PtRh) exhibit a specific measurement error depending on the inserted length of the respective TC. Thus, it is not possible to measure the actual temperature. However, qualitative temperature measurements for the comparison between different materials or at a defined measuring point can be carried out, as long as the same TC is used and the inserted length of the TC stays the same (utilization of a similar graphite tool assembly). It is also supposed that the measuring error at a respective measuring point, for example at the interface sample/graphite die or within the sample, can be reduced using a graphite die with higher wall thickness due to the increase of the inserted length of the TC. However, the temperature distribution within the tool and within the sample material is influenced by the change of graphite die wall thickness [6]. In addition, the application of unsheathed TCs could improve the temperature measurement due to the decrease of heat loss by the sheathing. However, applying unsheathed TCs, difficulties considering the exact posi-

tioning of the TC can additionally occur. Furthermore, an electrical contact of the TC wires with each other (displacement of the measuring point) or with the graphite tool (carbonization, sticking, passage of the electrical current from the graphite through the wires) has to be inhibited. In accordance with the results given in Fig. 4, an increased temperature measured by the TC with increasing set temperature at the respective measuring point was observed. Thus, for all TCs an increase of DT with increasing set temperature (DT is taken in an algebraic sense going from negative to positive) was observed (see Figs. 4 and 5). With the exception of the measurements by the type S-PtRh TC, the temperature at M2 (set temperature >650 °C) is higher than the temperature measured by the pyrometer (see Fig. 4). During the SPS/FAST process, heat loss occurs due to heat conduction from the sintering tool to the water-cooled punches of the SPS device and due to radiation heat transport between the hot sintering tool and the process chamber. In reference [11] it is shown that radiation losses become more and more significant at higher temperatures. At lower temperatures, the majority of discharged heat is lost by conduction. As a consequence of the utilization of full isolation by graphite felt, the role of radiation is reduced [18]. To achieve higher temperatures at the measuring point of the pyrometer, an overheating of the center of the sample/setup is required. Thus, an increase of the sample temperature with increasing set temperature results. However, this increase of the sample temperature is superimposed with the measuring error by TC measurement due to the thermal conduction, especially considering the type S-PtRh. For electrically conductive materials like graphite, the increase of the sample temperature is assumed to be intensified with increasing sample height. With increasing sample height, the distance between the center of the sample/setup (symmetrical tool setup) and the measuring point of the pyrometer is increased. Thus, the temperature in the center of the sample/setup is gradually underestimated by the pyrometer.

Table 2 Evaluation of the temperature measurement with different thermocouples for the setups I and II. type K-I

type S-I

type S-PtRh

TPyro [°C]

DTM1 [K]

DTM2 [K]

DT21 [K]

DTM1 [K]

DTM2 [K]

DT21 [K]

DTM1 [K]

DTM2 [K]

DT21 [K]

Setup I

900 1000 1050 1100

6 ± 6 2±2 5±3 –

35 ± 3 41 ± 2 – –

41 ± 7 39 ± 3 – –

21 ± 4 11 ± 3 – 1 ± 4

28 ± 2 35 ± 2 – 43 ± 1

49 ± 4 46 ± 4 – 44 ± 4

91 ± 16 79 ± 12 – 68 ± 9

10 ± 6 1 ± 6 – 14 ± 4

81 ± 17 78 ± 13 – 82 ± 10

Setup II

900 1000 1050 1100

3 ± 6 11 ± 6 18 ± 7 –

28 ± 2 36 ± 3 – –

31 ± 6 25 ± 7 – –

14 ± 8 1±7 – 14 ± 8

26 ± 3 35 ± 4 – 46 ± 4

40 ± 9 34 ± 8 – 32 ± 9

75 ± 14 59 ± 12 – 44 ± 9

4 ± 3 8±2 – 25 ± 3

71 ± 14 67 ± 12 69 ± 9

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M. Radajewski et al. / Measurement 147 (2019) 106863

Electrical power [kW]

4.0 3.5 3.0 2.5 Setup II, M1 Setup II, M2

2.0

900

1000

Setup I, M1 Setup I, M2

1100

TPyro [°C] Fig. 6. Electrical power at different set temperatures.

Table 2 summarizes the results of the temperature measurements by the TCs for setup I and II at M1 and M2 related to the temperature of the pyrometer (DTM1 and DTM2 correspond to DTPyro at the respective measuring point) for different set temperatures. In addition, the temperature difference between M1 and M2, DT 21 ¼ T M2  T M1 , is given. As expected on the basis of literature [7], for both setups, higher temperatures are measured for M2 (in the middle of the setup) compared to M1 (see also Fig. 4). The lowest differences between M1 and M2 are measured using the type K-I TC, which can be explained by the lowest heat loss with decreasing inserted length of the TC. Assuming that the measuring deviation of the TCs due to thermal conduction increases with decreasing inserted length of the TC, an increasing TC-measured temperature gradient from the middle to the interface sample/die is expected. An increasing set temperature influences the temperature difference between M1 and M2 only weakly. For all thermocouples, higher temperature gradients between M1 and M2 are observed for setup I. An explanation is the application of graphite foils and the resulting increase of contact resistances within setup I. According to Ref. [8], an increased electrical resistance and a local decrease of the thermal conductivity arises due to the application of horizontal graphite discs. It is obvious that this effect causes slight differences considering the temperature measurement at M2, however, a stronger drop in temperature in the direction of the interface sample/graphite die apparently occurs. Several authors reported that the temperature of conductive materials is reduced from the middle to the edge of the sample. As an example, according to Wang et al. [38], who sintered BN and TiB2 (slightly lower electrical conductivity compared to their graphite used for the investigation), a sample with 40 mm in diameter exhibits a temperature difference of 345 K from the middle to the interface sample/die, when reaching the final sintering temperature of 1700 °C. These high temperature gradients are attributed to the heating rate used in the SPS process. As it is known from literature, higher heating rates generate higher temperature gradients [7]. To inhibit the influence of the heating rate, temperature values at steady state temperatures during the temperature plateau are evaluated in this report. The recorded electrical power values for the setups I and II within the dwell time at different set temperatures are illustrated in Fig. 6. As expected, with increasing process temperatures TPyro an increased electrical power was used. However, a difference regarding the electrical power DP of approximately 0.95 kW was detected at each set temperature between the setups I and II. For setup I a significantly lower electrical power compared to setup II is required to reach the set temperatures. This can be explained by the use of graphite foil within setup I. Contact resistances arise by the application of graphite foil discs between the graphite spec-

imen and the punches. These contact resistances and the graphite foil discs themselves lead to an increase of the electrical resistance within the graphite tool at the respective set temperature. This effect of graphite foil discs on the electrical resistance of a setup with a graphite sample (thickness: 4.25 mm, diameter: 40 mm) was also shown by Vanmeensel et al. [8]. The authors assumed that the graphite foil discs acted as heating elements so that the given set temperature is reached with a decreased electrical power required for setup I. The electrical power required for setup I and II is independent of the borehole depth (comparison between M1 and M2). Nevertheless, an increasing borehole depth results in a decreased area through which the current passes which potentially affect the temperature distribution within the tool setup. It is important to mention that the data of the electrical power was generated for the same setup with different borehole depth. By applying the same equipment (e.g. graphite felt) as well as the apparently identical setup significant differences considering the electrical power values can occur, possibly due to material inhomogeneity. This was proven by the use of multiple apparently identical setups. However, these differences of the electrical power values during the heating tests seem to have no significant influence on the temperature measurement via TC. 4. Conclusion Direct temperature measurement was carried out using three different kinds of thermocouples and three graphite tool setups to investigate the temperature distribution within the sintering tool. At a given set temperature, different temperature values at the same measuring position and setup were measured depending on the used TC. It was supposed that the significant temperature differences result from a thermal conduction effect as a consequence of the thermal conductivity differences of the wire and sheathing material for the different kinds of TCs. The TCmeasured temperature is a superposition of the actual temperature and the heat loss by thermal conduction through the TC. Thus, the measuring error varied with the inserted length of the thermocouple and is reduced the more the TC is inserted into the sintering tool. Even though the TC-measured temperature is underestimated, a temperature comparison is possible, using the same TC and the same inserted length (for the same sintering tool), e.g. when different kinds of materials are investigated. The temperature gradient is influenced by the graphite tool setup. Nevertheless, a higher temperature in the center of the sample/setup compared to the interface sample/die was measured via TC for a given set temperature. The lowest temperature differences between the center of the sample/setup and interface sample/die independent of the tool setup was determined for the type K TC. This can be explained by the lowest heat loss with decreasing inserted length of the TC. Assuming that the measuring deviation of the TCs increases with decreasing inserted length of the TC, a decreased temperature gradient from the middle to the interface sample/die in comparison to the TC-measured temperature gradient is expected. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to thank the German Research Foundation (DFG) for supporting the investigations, which were part of

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