Carbon Fiber Thermal Insulation Material: High Temperature Thermal Conductivity Test Technology Under Vacuum and Inert Gas Environment

 

 

Hard Carbon Fiber Thermal Insulation Material

 

1. Carbon fiber thermal insulation material and its thermal conductivity test characteristics

Carbon fiber thermal insulation material is a soft felt material made by molding carbon fiber and a certain proportion of binder. On the basis of the soft felt material, it is made into hard carbon fiber thermal insulation material through carbonization, graphitization and machining. An important indicator for evaluating the thermal insulation performance of this type of material is thermal conductivity, and there are different characteristics from other types of thermal insulation materials in the thermal conductivity test:

1. High test temperature: up to 1000-2000℃;
2. Inert gas environment; vacuum, nitrogen, argon, helium, etc.;
3. Two temperature distribution forms: uniform temperature and large temperature gradient;
4. Two types of material forms: flexible and rigid;
5. Material conductivity: conductive material.

 

Commonly used test methods for high-temperature thermal conductivity of thermal insulation materials at home and abroad

 

For thermal insulation materials with low thermal conductivity, the commonly used thermal conductivity test methods are mainly divided into the following three categories:

 

 

Common Test Methods for Three Types of Thermal Conductivity

From the above list, it can be seen that the only commercial equipment that can meet the thermal conductivity test of carbon fiber thermal insulation materials at home and abroad is the steady-state protected hot plate method thermal conductivity instrument of Germany’s NETZSCH and the steady-state heat flow meter method thermal conductivity instrument of Shanghai Yiyang Industrial Co., Ltd., which can test the thermal conductivity of carbon fiber thermal insulation materials under vacuum and inert gas environment, while the steady-state heat flow meter method thermal conductivity instrument of NASA in the United States is a non-standard self-made non-commercial test instrument.

 

2.1 Steady-sState Protected Hot Plate Method

Based on the standards: ASTM C177 and GB/T 10294, the measurement principle is shown in Figure 1.

 

 

Figure 1 Schematic Diagram Of The Measurement Principle Of The Single Sample Steady-State Guarded Hot Plate Method

 

For the steady-state guarded hot plate method thermal conductivity test instrument, the only equipment capable of performing thermal conductivity tests under high temperature and vacuum conditions at home and abroad is the commercial equipment produced by Germany’s NETZSCH and the standardized test equipment made by the US NIST, as shown in Figures 2 and 3.

 

 

Figure 2 Steady-state guarded hot plate thermal conductivity meter from NETZSCH, Germany

 

 

Figure 3 US NIST Steady-State Guarded Hot Plate Thermal Conductivity Instrument

 

2.2 Steady-State Heat Flow Meter Method

 

The standards are: ASTM C201, GB/T 10295 and YBT 4130-2005. Among them, YBT 4130-2005 completely copied ASTM C201, which is a water calorimeter method for heat flux density measurement, and is also a heat flow meter method. The basic principle of the steady-state heat flow meter method is shown in Figure 4.

 

 

Figure 4 Schematic Diagram of Steady-State Heat Flow Meter Method Measurement

 

For the steady-state heat flow meter method thermal conductivity test instrument, the following four institutions currently have equipment capable of performing thermal conductivity tests under high temperature conditions, as shown in Figures 5 and 6, but only NASA and Shanghai Yiyang Industrial Co., Ltd. have self-made standardized test equipment, as shown in Figures 7 and 8.

 

 

Figure 5 Domestic Water Calorimeter Method High Temperature Thermal Conductivity Instrument

 

 

Figure 6 Water Calorimeter High Temperature Thermal Conductivity Instrument From Orton, USA

 

 

Figure 7 NASA Steady-state Heat Flow Meter High-temperature Thermal Conductivity Test System

 

Figure 8 Steady-State Heat Flow Meter Method High-Temperature Thermal Conductivity Test System Of Shanghai Yiyang Industrial Co., Ltd.

 

2.3 Transient Hot Wire Method

Based on the standards: ASTM C1133 and GB/T 5990. The basic principle of the transient hot wire method is shown in Figure 9.

 

 

Figure 9 Schematic diagram of transient hot wire method thermal conductivity instrument (parallel line method)

 

For transient hot wire method thermal conductivity test instruments, the equipment capable of performing thermal conductivity tests under high temperature conditions at home and abroad are the following two companies’ equipment, as shown in Figures 10 and 11.

 

 

Figure 10 Hot Wire Method High Temperature Thermal Conductivity Instrument from TA, USA

 

 

Figure 11 High-Temperature Thermal Conductivity Instrument Using Hot Wire Method from German Netzsch Company

 

3. Current Status Of Testing Technology for Carbon Fiber Thermal Insulation Materials

 

From the above three types of thermal insulation material testing methods and related thermal conductivity testing equipment, it can be seen that only the German Netzsch protective hot plate method and Shanghai Yiyang’s heat flow meter method equipment can meet the testing requirements of carbon fiber thermal insulation materials under inert gas environment.

Foreign countries mostly use non-standard self-made equipment for thermal conductivity testing of carbon fiber thermal insulation materials, and all the literature and reports from thermal insulation material manufacturers are heat flow meter method and hot wire method equipment. This is mainly because only these two methods can achieve high temperatures.

Except for Shanghai Yiyang Industrial Co., Ltd., no other domestic institutions have thermal conductivity testing equipment for carbon fiber thermal insulation materials, nor have they seen corresponding test results reported in literature.

 

4. Two Main Testing Technologies For Thermal Conductivity Of Carbon Fiber Thermal Insulation Materials

 

From the above introduction, it can be seen that for the thermal conductivity test of carbon fiber thermal insulation materials, only the steady-state heat flow meter method and the transient hot wire method can achieve testing under high temperature conditions at home and abroad. The following introduces the characteristics of these two methods in specific thermal conductivity testing.

 

4.1 Steady-State Heat Flow Meter Method For High-Temperature Thermal Conductivity Test

This is a mainstream method for high-temperature thermal conductivity test of thermal insulation materials at home and abroad. In addition to achieving high temperature, its main feature is to simulate the large temperature difference environment during actual insulation, measure composite components, and test thermal conductivity in different directions.

The thermal conductivity test can be carried out in vacuum and inert gas controlled pressure environment. NASA has a large number of literature reports. The technology is very mature and almost all aerospace thermal insulation materials have been tested and evaluated. Shanghai Yiyang also uses this technology to meet the domestic aerospace high-temperature thermal insulation material thermal conductivity test needs.

The product information of flexible and rigid thermal insulation felts of foreign carbon fiber thermal insulation material manufacturers can also be seen that the steady-state heat flow meter method is used.

 

4.2 Transient Hot Wire Method For High-Temperature Thermal Conductivity Test

Before the steady-state heat flow meter method appeared, it was the mainstream test method for thermal insulation materials and carbon fiber thermal insulation materials. It was previously used in the thermal conductivity test of refractory materials.

The hot wire method thermal conductivity test equipment has a simple structure and is easier to achieve high-temperature testing.

One of the characteristics of the hot wire method thermal conductivity test equipment is the uniform temperature test, which obtains the true thermal conductivity, rather than the effective thermal conductivity where radiation heat transfer plays a leading role when there is a large temperature difference at high temperature.

However, for conductive materials such as carbon fiber insulation materials, it is necessary to try to solve the problem of high-temperature insulation of hot wires. At the same time, the entire test process is very long, and the temperature of the entire sample needs to be constant.

 

4.3 The Difference Between The Measurement Results Of The Steady-State Heat Flow Meter Method And The Transient Hot Wire Method

 

During the thermal conductivity test of the steady-state heat flow meter method, there is a large temperature difference in the thickness direction of the sample. At high temperatures, there will be multiple heat transfer forms such as conduction, convection and radiation. At this time, the thermal conductivity obtained by the test corresponds to the equivalent thermal conductivity.

During the thermal conductivity test of the transient hot wire method, the temperature of the sample under test is uniform and there is no temperature difference. During the test, there are only solid and gas heat conduction forms. At this time, the thermal conductivity obtained by the test corresponds to the true thermal conductivity.

FIG12 shows the corresponding thermal conductivity curves of two different low-density insulation materials during conduction, convection and radiation heat transfer versus temperature. It can be clearly seen from the curves that the overall thermal conductivity will increase significantly due to the influence of radiation heat transfer.

 

 

Figure 12 Curves of thermal conductivity components corresponding to solid, gas and radiation heat transfer changing with temperature

In addition, the equivalent thermal conductivity obtained by the heat flow meter method for the same sample is greater than the true thermal conductivity obtained by the transient method and hot wire method, as shown in Figure 13.

 

 

Figure 13 Comparison of effective thermal conductivity and true thermal conductivity

 

 

5.1  NASA Langley Research Center

 

5. Typical Reports on Foreign Carbon Fiber Insulation Material Testing

 

5.1 Work at NASA Langley Research Center in the United States

The technical indicators of the high-temperature thermal conductivity test system using the heat flow meter method developed by NASA Langley Research Center in the United States are as follows:

 

5.1 NASA Langley Research Center Work

The technical indicators of the heat flow meter method high-temperature thermal conductivity test system developed by NASA Langley Research Center are as follows:

1. Test object: rigid and flexible sheet materials;
2. Maximum sample hot surface temperature: 1800℉;
3. Air pressure control range: 0.0001 ~ 760 torr.

The structure of the heat flow meter method high-temperature thermal conductivity test system developed by NASA Langley Research Center is shown in Figure 14.

 

 

Figure 14 Schematic diagram of the structure of the high-temperature thermal conductivity test system of NASA and Shanghai Yiyang Steady-state Heat Flow Meter Method

 

①Daryabeigi, Kamran. “Effective thermal conductivity of high temperature insulations for reusable launch vehicles.” NASA/TM-1999-208972 (1999).

②Daryabeigi, Kamran, George R. Cunnington, and Jeffrey R. Knutson. “Combined heat transfer in high-porosity high-temperature fibrous insulation: Theory and experimental validation.” Journal of thermophysics and heat transfer 25, no. 4 (2011): 536-546.

 

For Related Reports, Please Refer To The Following Literature:

①Daryabeigi, Kamran. “Effective thermal conductivity of high temperature insulations for reusable launch vehicles.” NASA/TM-1999-208972 (1999).

②Daryabeigi, Kamran, George R. Cunnington, and Jeffrey R. Knutson. “Combined heat transfer in high-porosity high-temperature fibrous insulation: Theory and experimental validation.” Journal of thermophysics and heat transfer 25, no. 4 (2011): 536-546.

 

5.2 Product Performance of Japan NIPPON CARBON Company

The carbon fiber thermal insulation materials of Japan NIPPON CARBON Company mainly include GF-F soft felt series and FGL multi-layer composite hard felt series, as shown in Figures 15 and 16.

 

Soft Felt GF-F Series

 

Felt Laminated FGL Series

For these two types of carbon fiber thermal insulation materials, Japan’s NIPPON CARBON company gave the high-temperature thermal conductivity test results on its official website, as shown in Figures 17 and 18.

 

 

Figure 17 High temperature thermal conductivity test results of Japan Carbon Soft Felt

 

Figure 18 High temperature thermal conductivity test results of multilayer hard felt of Japan Carbon Co., Ltd.

 

From the high-temperature thermal conductivity test results of soft felt and hard felt given by NIPPON CARBON above, it can be seen that the thermal conductivity test was carried out in a vacuum environment of 20Pa, and it is stated that the test is perpendicular to the surface of the sample, which represents the steady-state heat flow meter method used in the high-temperature thermal conductivity test, because only the steady-state heat flow meter method has a clear directionality.

 

5.3 Performance of KRECA FR Graphite Hard Felt Products of Kureha Co., Ltd., Japan

 

The carbon fiber thermal insulation materials of Kureha Co., Ltd., Japan mainly include the KRECA FR Graphite Hard Felt series, as shown in Figure 19.

Figure 19 KRECA FR graphite felt series from Japan’s Kureha Corporation

 

For the KRECA FR graphite felt series, the high-temperature thermal conductivity test results published by Japan’s Kureha Corporation on its Chinese official website are shown in Figure 20.

Figure 20. Test results of high-temperature thermal conductivity of hard felt of Japan’s Kureha Co., Ltd.

As can be seen from Figure 20, the high-temperature thermal conductivity test was conducted in a vacuum environment of 1.33Pa, and the sample thickness was 50mm. Although Japan’s Kureha Co., Ltd. did not mark the thermal conductivity test method, judging from the sample thickness, it should be the steady-state heat flow meter method, because the sample thickness is relatively large in the hot wire method thermal conductivity test.

 

5.4 Thermal Conductivity Of Products of American Carbon Composites Company

 

American Carbon Composites Company announced on its official website the measurement results of the high-temperature thermal conductivity of its carbon fiber thermal insulation material products in argon and vacuum environments, as shown in Figures 21 and 22.

 

Figure 21 Comparison of thermal conductivity of carbon fiber thermal insulation materials from CCI, USA – Argon atmosphere

 

Figure 22 Comparison of thermal conductivity of carbon fiber thermal insulation materials from CCI, USA – Vacuum environment

 

In addition, from the product technical index document on the official website of CCI, USA, it can be seen that the above thermal conductivity measurement results have obvious thermal conductivity directional identification. Although there is no clear directional identification, as long as there is a directional identification, it represents the steady-state heat flow meter method used.

 

5.5 Literature Report On The Test Of High-Temperature Thermal Conductivity Of Traphite Felt By Transient Hot Wire Method

In 2005, Chahine et al. in Australia reported the measurement of high-temperature thermal conductivity of graphite felt by transient hot wire method:

Chahine, Khaled, Mark Ballico, John Reizes, and Jafar Madadnia. “Thermal Conductivity of Graphite Felt at High Temperatures.” In Australasian Heat & Mass Transfer Conference. Curtin University of Technology, 2005.

The article reported the test of thermal conductivity of WDF grade graphite felt by hot wire method. The density of graphite felt was 80 kg/m^3, the diameter of graphite fiber was in the range of 7.0 ~12.5 μm, and the average diameter was 10.5 ± 3.2 μm. The test was carried out under vacuum and argon conditions, and the measurement results are shown in Figure 23.

 

Figure 23 Transient hot wire method to measure the high temperature thermal conductivity of graphite felt under different atmospheres

 

1. Work Done by Shanghai Yiyang Industrial Co., Ltd.

 

6.1 Thermal Conductivity Test Instrument

For carbon fiber thermal insulation materials, Shanghai Yiyang Industrial Co., Ltd. used a self-made commercialized heat flow meter method high temperature thermal conductivity instrument (model TC-HFM-1000) and a transient plane heat source method thermal conductivity instrument (model TC-TPS 1010) to test the thermal conductivity at room temperature and high temperature respectively. For the first time in China, the thermal conductivity test results of carbon fiber thermal insulation materials in the range of room temperature to 1000℃ under different vacuum degrees were obtained. The transient plane heat source method thermal conductivity instrument (model TC-TPS 1010) and sample installation are shown in Figures 24 and 25, and the heat flow meter method high temperature thermal conductivity instrument (model TC-HFM-1000) and sample installation are shown in Figures 26 and 27.

 

 

Figure 24 Shanghai Yiyang Company transient plane heat source method thermal conductivity instrument

 

 

Figure 25 Transient plane heat source method thermal conductivity instrument test sample installation

 

Figure 26 Shanghai Yiyang Company vacuum heat flow meter method high temperature thermal conductivity instrument

 

Figure 27 Sample installation of high-temperature thermal conductivity instrument using heat flow meter method

 

6.2 Technical Indicators Of High-Temperature Thermal Conductivity Instrument Using Vacuum Type Heat Flow Meter Method

 

1. Test Object: rigid and flexible sheet materials;
2. Temperature Range: 100℃～1000℃ (maximum 1500℃);
3. Air Pressure Range: 10 Pa ~ 1 atm;
4. Thermal Conductivity Test Range: <5 W/mK;
5. Sample Size: square 300 × 300 mm;
6. Sample Thickness Range: 10 ~ 100 mm;
7. Temperature Measurement Accuracy: ±1%;
8. Air Pressure Measurement Accuracy: ±1%;
9. Thermal Conductivity Measurement Accuracy: ±5%.

 

6.3 Carbon Fiber Thermal Insulation Material Samples (Graphite Hard Felt)

 

The thermal conductivity of carbon fiber thermal insulation material samples (graphite hard felt) provided by domestic manufacturers was tested. The manufacturer provided two sizes of graphite hard felt samples of the same material for transient plane heat source method and steady-state heat flow meter method tests, respectively. The material density is 156 kg/m^3. One sample size is 50mm×50mm×40mm, as shown in Figure 28; the other sample size is 310mm×310mm×44.5mm, as shown in Figure 29.

 

50mm×50mm×40mm

 

310mm×310mm×44.5mm

 

6.4 Test Results Of Thermal Conductivity Of Transient Plane Heat Source Method Under Normal Temperature and Pressure Atmosphere

 

The thermal conductivity of graphite hard felt sample was repeatedly measured 15 times under normal temperature and pressure atmosphere using transient plane heat source method thermal conductivity instrument. The test results are shown in Figure 30. The average value of thermal conductivity measurement is 0.112±0.002 W/mK.

 

 

Figure 30 Multiple measurements of thermal conductivity of graphite felt at room temperature and pressure using transient plane heat source method

 

6.5 Results Of High-Temperature Thermal Conductivity Of Graphite Felt Measured Using Heat Flow Meter Method Thermal Conductivity Instrument Under Normal Pressure Nitrogen Environment

 

For the high-temperature thermal conductivity measurement of carbon fiber thermal insulation materials, the high-temperature thermal conductivity was first measured at different temperature points under normal pressure inert gas (nitrogen) environment. The measured values ​​of thermal conductivity at different temperatures are shown in Figure 31, and the graphical representation with the horizontal axis being the sample hot surface temperature and the vertical axis being the effective thermal conductivity is shown in Figure 32.

 

Figure 31 Test parameters and result values ​​of graphite hard felt sample

 

 

Figure 32 Measurement results and fitting curve of effective thermal conductivity of graphite felt as the temperature of the sample hot surface changes

 

From the measurement results shown in Figure 31, it can be seen that the fitting curve is a cubic polynomial formula. As the temperature of the hot surface increases, the curve bends upward, which means that as the temperature increases, the effect of radiation heat transfer becomes more obvious.

 

6.6 Results Of Measuring High-Temperature Thermal Conductivity Of Graphite Felt Using Heat Flow Meter Thermal conductivity instrument at different nitrogen pressures (vacuum degrees)

 

In order to measure the high-temperature thermal conductivity of graphite felt samples at different nitrogen pressures (vacuum degrees), the sample hot surface temperature was controlled at 200, 600 and 1000℃, as shown in Figure 33. In each hot surface temperature constant control process, the change of nitrogen pressure (vacuum degree) was controlled, and the vacuum setting values ​​were 10, 100, 1000, 5000 and 10000Pa, respectively. The effective thermal conductivity at different temperatures and different vacuum degrees was measured, and the values ​​of the effective thermal conductivity measurement results are shown in Figure 34.

 

Figure 33. Sample hot surface temperature change curve during variable vacuum test

 

Figure 34 The effective thermal conductivity measurement results of graphite hard felt at different temperatures and different vacuum degrees

 

The effective thermal conductivity measurement results obtained in Figure 34 are plotted into a graph, as shown in Figure 35. It can be seen from the figure that at each constant temperature, the effective thermal conductivity increases with the increase of air pressure, and the change of thermal conductivity tends to be stable when approaching normal pressure, which is completely consistent with the change law of thermal conductivity of low-density insulation materials with the increase of air pressure.

 

Figure 35 Measurement results of thermal conductivity of graphite felt under different temperatures with vacuum degree

 

Through the above test of graphite felt under high temperature and vacuum conditions using the thermal conductivity test equipment of Shanghai Yiyang Industrial Co., Ltd., the complete thermal insulation performance test and evaluation results of graphite felt were obtained for the first time in China, which will be helpful for the research, production, quality control and performance evaluation of carbon fiber thermal insulation materials.

 

VII. Steady-State Heat Flow Meter Method Thermal Conductivity Test Higher Temperature (1500℃) Test System Solution

 

The existing test equipment of Shanghai Yiyang Industrial Co., Ltd. has been proven to be fully capable of meeting the thermal conductivity test of carbon fiber insulation materials below 1000℃. If the test temperature needs to be increased to 1500℃, the following changes need to be made, but there is no technical difficulty.

1. Change the heating method, replace the metal heating element with graphite or carbon/carbon material heating element, and use a higher power low-voltage and high-current DC power supply;
2. The thermal conductivity of carbon fiber insulation materials is generally high, and the sample cold surface temperature control needs to be replaced with a high-precision cooling circulation system with greater cooling power.
3. Temperature measurement uses S-type thermocouples with higher operating temperatures;
4. Thickened high-temperature thermal protection devices to ensure safety at the highest operating temperature;
5. Vacuum extraction is equipped with corresponding vacuum systems according to vacuum requirements.