Different types of commercially available papers: Munken; and Matt; a Bulgyeong; a Daerye; merit; Aqua satin; woolen paper; and a new craft board that has different properties is used to produce TIM graphene paper with a simple and cost-effective ink coating process: dip coating; bar cover; and slot cover. The in-plane and through-plane thermal conductivity of TIM graphene paper is measured using the laser flash technique.
Introduction
The thermal conductivity of the TIMs and the bond line thickness (BLT, hereafter) mean the thickness of the deposited TIMs. RC – contact thermal resistance occurs between the substrate and the TIMs due to thermal contact.
Fabrication
- Types of paper
- Dip coating process
- Use of co-polymer
- Bar coating process
- Slot die coating process
The rod coating and slot coating process are also used in conjunction with the dip coating process. SEM images of the fabricated TIM via dip coating process of (a) Munken, (b) Matt, and (c) cross section of Matt. This co-polymer is often used for the adhesive layer in the coating process and has its own hardness [43].
Therefore, in the rod coating process, it is possible to produce thin and scaled graphene paper TIMs for uniform coating surface and thickness. SEM images of TIMs fabricated via strip coating process of (a) Munken, (b) Matt and (c) Cross section of Matt. As shown in Figure 2.12., the slot coating process for fabricating graphene paper TIMs is as follows.
Accordingly, the graphene paper TIMs are fabricated using a slot die coating process. SEM images of fabricated TIM via slot die coating process of (a) Munken, (b) mat, (c) cross section of Matt. SEM images of fabricated TIM via slot die coating process of (a) Bulgyeong, (b) Daerye, (c) Merite, (d) Aqua satin, (e) Wool paper, and (f) New craft board.
In this way, graphene paper TIMs are fabricated an easy, simple and scalable process using the dip-coating, bar-coating and slot-die coating process.
Experimental method
Laser Flash Analysis
A laser pulse on the front surface (x=0) of the sample, the laser pulse is used to ensure a uniform heat flux. And the absorbed heat energy is transferred to the back side of the test object at this time. The top and bottom surfaces of the sample are coated with graphite powder to ensure uniform heat flow and thermal diffusion.
The temperature distribution at the center of the back surface of the sample can be expressed as a function of the thickness L of the sample and the time t axis. Where Tmax is the maximum temperature of the center of the back surface, which corresponds to the stable temperature caused by the laser pulse. Finally, the thermal diffusion alpha is determined by the following estimated correlation through the sample thickness and the time required to reach half of the maximum temperature.
The density of the sample is calculated by estimating the mass, calculating the volume, and then dividing the mass into volumes. The accuracy of the laser flash technique measurement using LINSEIS XFA 300 Xenon flash instrument is approximately ~5.
Four-points probes measurement method
Both surfaces of the tested sample are coated with the material of the comparison sample before the test to set the emissivity of the tested sample to the same value with the reference sample, thus ε is equal to ε'. Apply power to the external two probes and operate by measuring the voltage drop between the two internal probes. For the bulk sample whose thickness is much larger than the distance between the probes, a spherical current ring from the outer probe tips can be assumed as follows.
In this regard, for the determination of the resistance between the voltage measurement points, one integrates between x1 and x2 is as. For a thin film sample whose thickness is less than probe spacing, it is assumed that the case of current rings. As before, superposition of current at the outer probe two points leads to R=V/2I, so that the sheet resistance for a thin film sample as, does not depend on the probe spacing.
Here, the constant is a geometric factor, and in the case of the thin film, the taht for a semi-infinity is 4.53. This equation is valid for the material being tested is not thicker than 40% of the gap between the probes, the thickness of the very thin layer t << probe spacing s, and it should be noted that the width of the sample is quite large.
Results and discussion
Thermal conductivity
The track nozzle coating process resulted in the greatest in-plane thermal conductivity ~ 8 W/m-K, while the dip coating showed the worst performance. It can be confirmed that in-plane thermal conductivities are much higher than through-plane thermal conductivities. The slot matrix coating showed the greatest through-plane thermal conductivity among the three coating processes.
In particular, for in-plane thermal conductivity, the differences are larger than the through-plane thermal conductivity. It also shows that the in-plane thermal conductivity is much higher than the through-plane thermal conductivity. Regardless of the type of paper, it can be confirmed that the thermal conductivity increases in line with the coating thickness, both in the thermal conductivity in the plane and in the through-plane.
This figure also shows that the in-plane thermal conductivity also increases linearly with the measured coating thickness. Furthermore, the increase of the in-plane thermal conductivity is much greater than the trend of the through-plane thermal conductivity.
Effect of paper types
Thermal conductivities are also investigated in relation to the thickness of the coating, which is an important factor of film-typed TIM [35]. It was shown that the WDG content or coating thickness does not significantly affect the through-plane thermal conductivity. The anisotropic thermal conductivity of TIM graphene paper considered in this study is due to the crystalline structure of graphene acting as conductive fillers.
Therefore, it is necessary to select suitable TIMs with high thermal conductivity that meet the thickness requirements for thermal management applications. It can be confirmed that the thermal conductivities are different depending on the type of paper, despite the same coating thickness of 3 mm [38]. As in the previous results, it also shows that in-plane thermal conductivities are relatively higher than through-plane thermal conductivities.
As a result, it can be confirmed that the porosity which is the characteristic of the paper and the thermal conductivity are somewhat proportional. From the measured thermal conductivities, it can be confirmed that the in-plane thermal conductivity is significantly greater than the through-plane thermal conductivity.
Hybrid filler
In this regard, graphene has a strong bond that stretches between carbon in the planar direction and can transfer vibrational energy faster in the planar direction, as shown in Figure 4.5. Graphene used in practice exists in a laminated form, but the van der Waals forces between graphene atoms in the vertical direction are weak and the through-plane thermal conductivity is relatively low. SWNTs were mixed with water-dispersible graphene at 70oC for 24 hours to combine the hybrid fillers as shown in Figure 4.6.
In this regard, conventional low aspect ratio micro-particulate fillers were studied as shown in figure 4.7. The effects of hybrid filler of one-dimensional SWNTs or conventional low aspect ratio micro-particles of aluminum with two-dimensional graphene on the thermal performance of TIMs were investigated, shown in figure 4.8. However, in the case of the hybrid filler of SWNTs and graphene, the increasing rate of the through-plane thermal conductivity decreases over a mass fraction of 25 %.
It is also expected to have percolation behavior in the continuous heat transfer path [46]. In the case of hybrid fillers, additional channels are provided for the heat transport passing the cellulose matrix.
Electrical and mechanical properties
Mechanical properties can be another important factor in the application of the thermal management system for using graphene paper TIMs. First, graphene paper TIMs has flexibility and has the advantage that it can be applied by deforming it into a desired shape depending on the application of thermal management, as shown in Figure 4.10. This implies that graphene paper with high flexibility can restore the overall flexibility of the cellulose paper.
The TIM graphene paper, which was expected to be weak to moisture due to the use of a paper matrix, has graphene embedded in the porous structure of the paper and surface coated, resulting in an acceptable moisture resistance result. Finally, the hardness of commercially available graphite sheet and TIM graphene paper was compared. Also, TIM graphene paper is said to have a slightly higher value due to the embedded graphene in the porous structure of the paper.
The compression ratio of pure paper and TIM graphene paper according to pressure was measured using digital multimeter. It can be confirmed that the compression ratio of pure paper and TIM graphene paper is 30~40% which is comparable to that of graphite sheet.
Comparison in the previous literatures
Conclusion
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