An Experimental Study on Heat Conduction and Thermal Contact Resistance for the AlN Flake

The electrical technology has been a fast development over the past decades. Moreover, the tendency of microelements and dense division multiplex is significantly for the electrical industries. Therefore, the high thermal conductible and electrical insulating device will be popular and important. It is well known that AlN still maintains stablility in the high temperature. This is quite attractive for the research and development department. Moreover, the thermal conduct coefficient of AlN is several times larger than the others.Therefore, it has been thought to play an important role for the radiator of heat source in the future.Therefore, this paper is focused on the studies of heat conduction and thermal contact resistance between the AlN flake and the copper specimens. The heating temperatures and the contact pressures were selected as the experimental parameters. According to the experimental results, the materials are soft and the real contact areas between the interfaces significantly increase under higher temperatures. As a result, the thermal contact resistance significantly decreases and the heat transfer rate increases with increasing the heating temperature or the contact pressures.


Introduction
The electrical technology has been a fast development during the first decade of the 21st century.Moreover, the tendency of microelements and dense division multiplex is significantly attractive for the research and development department.To achieve the above goals, the high heat dissipative and electrical insulating device will be popular and important in the future.
It is well known that AlN still maintains stablility under the high temperature conditions.Moreover, the thermal conduct coefficient of AlN material is several times larger than the other nonmetallic solids [1][2][3].Hence, it is widely used for the heat dissipation.Besides, AlN film has good electrical insulating properties to avoid the risk of a short circuit of electronic components [4][5][6][7].These advantages are quite attractive for the research and development department of the electrical corporations [8].
The method of using continuous variations of triboelectrification and friction coefficient to monitor the dynamic tribological properties between the metal films has been applied successfully by the authors [9][10][11][12].The related results also showed that the thermal contact resistance of the metal films is significantly influenced by the real contact areas between the interfaces.Moreover, the major factors for the real contact areas are the normal loads and the surface hardness of the materials.On the other hand, the defects of the material crystal always play an important role for the heat transfer of AlN [1][2][3][4][5][6][7].Therefore, the relationships between the microstructures of AlN film and the efficiency of the heat transfer are the key points.Based on the above statements, this paper is focused on the studies of heat conduction and thermal contact resistance between the AlN flake and the copper specimen by referring the previous studies of thermal contact resistance between the metal films.extractor.There are ten thermocouples in the systems.The temperatures at the measuring point can be obtained by the calibration.Then, the thermal contact resistance and the heat transfer rate can be obtained by the three mathematical formulas as shown in Section 2.3.

Experimental Apparatus and Procedures
Figure 2 shows the main parts of the experimental apparatus.The loading systems contain the following: one-way pneumatic valve manual control, with a model DJA25N75 pneumatic cylinder, and 1HP air compressor.
The insulation equipment is made of ceramic glass fiber cotton ( = 0.036 W/m ∘ C).The heating equipment includes the three heating rods (CIR-2034/240 V).Several counterweights (1 kgw × 3, 5 kgw × 2, and 10 kgw × 1) are used for maintaining the equilibrium of the contact pressure.Moreover, the water circulating pump and the ice water maker are disposed for the temperature control.

Test Specimens.
AlN thin flakes were used as the main experimental specimens.Material properties are shown in Table 1.The thickness of AlN slices is only 0.7 mm.The size and shape of the specimens are shown in Figure 3

Experimental Procedures.
There are two types of thermal contact resistance in this study.The upper measuring point of the heat transfer rod is  1 and the lower is  2 .Several parameters influence  1 and  2 , such as temperature, pressure, material, and surface roughness.Therefore, it is necessary to clarify and calculate them step by step.
The formula of the heat transfer () is The formula of the thermal contact resistance () is Moreover, the related temperatures are obtained by the following: The heating temperatures were set at 50 ∘ C, 65 ∘ C, 80 ∘ C, and 100 ∘ C. The temperature of the ice water circulating system was fixed at 10 ∘ C. Before each test, the preparatory time was set at 4 hours.The normal loads were set at 0.5 kgw and 1.5 kgw, and the corresponding contact pressures were 16 kPa and 48 kPa.The average room temperature for the test was 25 ± 2 ∘ C, and the average relative humidity was 65 ± 5%.

Results and Discussions
Figure 4 shows the effects of the heating temperature on the thermal contact resistance for Cu/AlN/Cu under 16 kPa.It is seen from this figure that  1 and  2 are independent of the heating temperatures and the experimental time for the temperatures is larger than 50 ∘ C.  1 maintains about 1 ∘ C/W and  2 maintains about 2.2 ∘ C/W for the higher temperatures.Moreover,  1 increases from 0.7 ∘ C/W to 0.91 ∘ C/W and  2 increases from 1.8 ∘ C/W to 2.2 ∘ C/W with the experimental time for the heating temperature of 50 ∘ C.
The effects of the heating temperature on the heat transfer rate for Cu/AlN/Cu under 16 kPa are shown in Figure 5.It is seen from this figure that the heat transfer rate increases from 10 W to 19 W with increasing the heating temperature from 50 ∘ C to 100 ∘ C.
Figure 6 shows the effects of the heating temperature on the thermal contact resistance for Cu/AlN/Cu under 48 kPa.It is seen from Figure 6(a) that the increasing trends are similar to the cases of 16 kPa except 100 ∘ C. The thermal contact resistance especially decreases with the experimental time under 100 ∘ C and 48 kPa.This indicates that the materials are soft under the higher temperatures.As a result, the real contact areas between the interfaces significantly increase.Hence, the thermal contact resistance significantly decreases.
The effects of the heating temperature on the heat transfer rate for Cu/AlN/Cu under 48 kPa are shown in Figure 7.It is seen from this figure that the heat transfer rate increases from 10 W to 17 W with increasing the heating temperature from 50 ∘ C to 80 ∘ C.Moreover, the heat transfer rate especially increases to 22∼25 W under 100 ∘ C and 48 kPa.
Comparing to the cases of Cu/Cu, the thermal contact resistance doubles and the heat transfer rate increases 33% with AlN flake under 50 ∘ C and 16 kPa.When the heating temperature increases to 100 ∘ C, the thermal contact resistance decreases slightly and the heat transfer rate decreases about 40% with AlN flake.
According to the SEM of AlN-flake surface shown in Figure 8, they are rough and uneven.Moreover, the aspects are in the range of 1∼10 m.Hence, many gaps exist between the interfaces.That is, when the copper is soft under the higher temperatures and contact pressures, it will get into the gaps and the real contact areas between the interfaces significantly increase.Therefore, the thermal contact resistance especially decreases under the higher temperatures and contact pressures.The corresponding results can be observed in Figures 4 and 6.

Conclusions
The thermal contact resistance and the heat transfer rate between Cu/AlN/Cu were experimentally investigated in

Figure 3 :
Figure 3: (a) The size and shape of the specimens, (b) AlN flake, and (c) copper specimen and heat transfer rod.