The research of adhesive force between copper fiber and micro-groove wall of heat column wick structure

Su-lian TAO, Zhen-ping WAN, Zhen-yu LU, Bi-yun HE, Jia-yuan LI

(1Department of Mechanical Engineering，Guangdong Technical College of Water Resources and Electric Engineering， Guangzhou 510925， China)(2South China University of Technology, Guangzhou 510640， China )

Abstract: Understanding the adhensiveforce between the copper fibre fiber and microgroove of composite wick structure is very important in designing the heat column. In this study, a composite wick structure of copper fiber sintered on microgrooves was fabricated by multi cutter milling and high-temperature solid-state sintering. An ultrasonic vibration experiment was conducted to investigate the effect of sintering parameters and copper fiber parameters on the adhesive force of composite wick structure. Results indicated that sintering parameters such as sintering time, sintering temperature, temperature rising rate, sintering pressure and copper fiber parameters have an important effect on the adhesive force between copper fiber and micro-groove of heat column wick structure. When the composite wick structure was fabricated by copper fiber with sintering layer's thickness of 1mm, copper fiber diameter of 50 μm, sintering at 900 ℃ for 60 min under 0.45 MPa at 5 ℃/s,the best adhesive effect can be obtained for the wick structure of heat column.

Key words: Heat column,Micro-groove, Copper fiber, Adhesive force

1 Introduction

Recently with the rapid development of microphotoelectricity technology, microphotoelectricity device and chip have become progressively thinner, have a higher performance, and are more compact[1-2]. At the same time, the highly integrated photoelectric product brings about its limited heat dissipation space,traditional micro heat pipes cannot meet the heat transfer requirement. A thick and short heat pipe has been proposed and has become the research focus of modern heat pipe technology[3]. In actual production applications, a thick and short heat pipe with a diameter greater than16mm and a ratio of length to a diameter less than 5 is defined as heat column, which is particularly suitable for the heat dissipation of highly integrated electronic devices because its evaporator bottom is flat and it can fit closely on the surface of the electronic components.

Heat column is a column copper airproof vacuum body with the wick structure on its inner wall. Fig.1 is the schematic diagram of the heat column and its associated flow mechanism. The bottom rotundity plane contacting heat source serves as an evaporator; the outside circle surface of heat column serves as a condenser which plays a role of condensing. External heat transfers to working fluid through evaporator flat and capillary wick structure, it prompts the working fluid boiled and vaporized; and the vapor subsequently moves to the condenser through vapor core. The vapor phase of working fluid is condensed to the liquid phase by releasing heat on the condenser, and the liquid subsequently returns to the evaporator through wick structure. Then a work circulation is completed and then repeats. The wick structure is a core element of the heat transfer devices, which provides the capillary force to drive the closed circulation of the working fluid and an interface for liquid-vapor phase and plays a key role in its capillary performance [4]. To date, the most basic wick structure is classified into the following four groups: micro-grooves wick structure, sintered metal power wick structure, fiber wick structure, and metal silk screen wick structure. The relation between capillary pressure and liquid permeability for different wick structure is shown in Fig.2 [5]. The groove wick structure has the highest liquid permeability and smallest capillary pressure. The sintered metal power wick structure has the most excellent capillary pressure and lowest liquid permeability. The silk screen wick structure has lower liquid permeability than the groove wick structure and better capillary pressure than fiber wick structure and the sintered metal power wick structure. The fibre wick structure has lower liquid permeability than the groove wick structure and the silk screen wick structure, and has better capillary pressure than the sintered metal power wick structure.

Fig.1 the structure of heat column

Fig.2 Comparison of capillary pressure and permeability between different wicks

A single wick structure can't satisfy permeability and capillary pressure requirement; so more complex or composite wick structure began to appear in both scientific and industrial areas. Five biporous wicks with average cluster (μm) to powder (μm) ratios and wick thickness (mm) 600/60/4，300/60/4，600/40/1，300/60/1，and300/40/1 were tested with degassed with distilled water by Tadej Semenic et al.[6]. G.S.Hwang et al.[7]studied modulation of evaporator wick thickness provided an extra cross-sectional area for enhanced axial capillary liquid flow and extra evaporation surface area, with only a moderate increase in wick superheat (conduction resistance). This modulated wick (periodic stacks and grooves over a thin, uniform wick) is analyzed and optimized with a prescribed, empirical wick super heat limit. Composite wicks with a combination of sintered metal powders and mesh screens were proposed and tested by Canti et al.[8]. Hybrid wick with a flat heat pipes combing rectangular groove with meshed layers were proposed for cooling LED lighting module by Hsieh et al.[9], Chan Byon et al.[10]fabricated bi-porous sintered metal wick samples by sintering glass particles to visualize the capillary flow in the wick. Theoretical and experimental studies were conducted by Li et al.[11] on three kinds of micro—diameter heat pipes with composite wick．The composite wicks consist of copper powder mesh，copper foam-mesh and mesh-mesh．

As stated above, there is no report on the wick structure of copper fiber sintered on grooves. The copper fiber has more unique advantages than metal silk screen and powder; the diameter of copper fiber can be ranged from 10 μm to 1 mm; its porosity can be ranged from 0 to 0.95. We can optimize the pore radius and permeability of copper fiber sintered layer and get small pore radius and large porosity. The copper fiber sintered on groove wick structure has the advantage of both groove wick structure and copper fiber wick structure, so it has excellent capillary pressure and high permeability, which has gigantic potential in heat column. However, the compactness of copper fiber adhering on micro-groove affects heat transfer performance of heat column. It is necessary for us to study the adhesive force between copper fiber and perpendicular microgrooves of composite wick structure for heat column.

In the present study, the wick structure of copper fiber sintered on microgrooves is manufactured through multi cutter milling and sintering at different sintering parameters, the adhesive force between copper fibers and microgrooves is tested by experiment method.

2 Expriment process and method

2.1 Manufacturing process of test sample

The test samples were produced by sintering copper fibers on perpendicular micro grooves of copper plates. The thickness of the copper plate was only 1mm, so the micro grooves were fabricated on milling machining. A composite tool were used to fabricate simultaneously multiple microgrooves in this work. The composite tool consists of a tool handle, a locking nut, six slice saws and five gaskets, as shown in Fig.3 (a). The slice saws and gaskets were stacked together and fixed by the locking gasket and locking nut for easy replacement. The composite tool is assembled inside the spindle by the handle. The slice saw had 72 triangular teeth and each with 5° rake angle and 16° back angle; its tooth depth is 1 mm. The thickness of slice saw is determined by the width of orthogonal micro groove. The thickness of test slice saw is 0.3 mm. The slice saw measured 40 mm in the diameter. High speed steel W18Cr4V was chosen as the slice saw material.

Fig.3 The composite tool stacked by multiple slotting cutters and slice saw

The experiment was carried out on the X5032 vertical milling machine as shown in Fig.4(a), the work piece material was T2 copper plate of 90 mm×90 mm×1 mm. Clamp (Fig.4(b)) was made by an interference fit between a flat plate and a cylindrical metallic rod. The work piece was fixed on the fixture plate with four fixed bolts;a cylindrical metal rod of clamp was fixed on the dividing head. Similarly, microgrooves were processed by the main motion of milling cutter and the feed movement of worktable. When milling was completed, the dividing head was rotated 90°; orthogonal micro groove could be gotten. The copper plate with 20×60×1 mm was gotten by wire-electrode cutting, and then copper fibers were covered on micro-grooves of copper plate at high temperature sintering. The micro-groove cooper fiber plate was obtained as shown in Fig.5.

Fig.4 Fabrication of The micro-groove cooper fiber plate with a single slotting saw

Fig.5 Test samples

2.2 Experiment method

The micro-groove cooper fiber plate is vibrated by an ultrasonic cleaning machine (JP-C50) for 3 minutes, where its vibration frequency is set as 40 kHz. Later, the sample is dried and weighed. Meanwhile, compared with its weight before and after vibrating. If there is a small change in weight, it means that the fiber combines the orthogonal grooves compactly. Otherwise it shows the copper fiber and the orthogonal grooves do not combine compactly.

3 Results and discussion

3.1 The effect of sintering temperature on the adhesive force

Fig.6 shows the combination status of copper fibers with the same diameter and micro-grooves under different temperatures. It can be seen that the fallen copper fiber quality percentage decreases first and then increases with increasing sintering temperature, which means the adhesive strength increases first and then decreases. Furthermore, it can be found that the largest adhesive strength between copper fiber and the micro-grooves happens when the sintering temperature is 900 ℃. There are many accidented microcosmic granules not only on the surface of copper fiber by turning but also the micro-groove surface of the vertical grooves by multi cutter milling. These particles move acutely with the increasing sintering temperature, which results that the seal point joint compactly. Next, the adhesive strength decreases with continually increasing temperature because the melting point of copper is 1 083 ℃, which is too high temperature makes the copper fibers and micro-grooves over- melt; it means that there is a disfigurement on the surface of fibers and micro-grooves, which leads to that the adhesive strength decreases.

Fig.6 The influence of sintering temperature on the adhesive force

3.2 The effect of temperature rising rate on the adhesive force

As can be seen from Fig.7 that the fallen copper fiber quality percentage decreases to a certain value and then increases with the increasing temperature rising rate, namely the adhesive force between copper fibers and micro-grooves increases to a certain value and then decreases. The less temperature rising rate is, the longer sintering time is, which causes the fibers and grooves surface very smooth, it is not conducive to the formation of welding points. Otherwise, the temperatuer rise rate is too large, the sintering proces is too fast, the vice will appear inside copper fibers, it is not conducive for jointing between fibers and micro-grooves. So the temperature rising rate should be moderate.

Fig.7 The influence of temperature rising velocity on the adhensive force

3.3 The effect of sintering time on adhesive force

Fig.8 shows the adhesive case of sintering copper fibers and micro-grooves for different sintering time. As it can be seen from the chart, fallen copper fiber quality decreases first and then increases. It means that the adhesive force between copper fibers and micro-grooves increases first and then decreases with the increasing sintering time. When sintering time is 60 minutes, the adhesive force is maximum, because the density of sintering becomes larger and larger. With the increas ing sintering time,the combining between particles on fiber surface and groove surface is compacter and solider. But with the further extension of sintering time, futher growth will happen on fiber grain, which results in a lot of defects, it is unfavorable for the combining of particles on fiber surface and fine particles on groove surface.

Fig.8 The influence of sintering time on the adhesive force

3.4 The effect of sintering pressure on the adhesive force

Fig.9 shows the relation of fallen copper fiber quality and sintering pressure. As can be seen from the chart the adhesive force of copper fiber and microgrooves sintering at sintering pressure of 0.4 MPa is larger than that sintering at sintering pressure of 0.3 MPa. The higher sintering pressure is, the greater diffusion and dissolve speed of atom on the fiber and groove interface are, which cause the particle atoms on the surface of fibers and microgrooves to penetrate each other more quickly, and more infiltration capacity, and then results in larger sintering necks formed between fibers and grooves.

Fig.9 The influence of sintering pressure on the adhesive force

3.5 The effect of fiber diameter on the adhesive force

Fig.10 shows the change of fallen copper fiber quality with fiber diameter. What we can see from the chart is that the fallen fiber quality increases with the increase of fiber diameter. It means that the adhesive force between fibers and microgrooves decreases with the increasing fiber diameter. The finer the fiber, the less micro particle on its surface. The more number fiber included by the same quality copper fiber, the more micro particle. and the larger surface energy. This is beneficial for sintering, and then results in more sintering necks formed between copper fibers and microgrooves, and the adhesive force is greater. Otherwise, the thicker copper fiber, the smaller adhesive force.

Fig.10 The influence of the fiber diameter on the adhesive force

3.6 The effect of sinter layer's thickness on adhesive force

Fig.11 shows the change of fallen copper fiber quality with the thickness of the fiber sintering layer. What we can see from the chart is that the fallen copper fiber quality decreases with the increasing of copper fiber layer's thickness. The adhesive force increases with the increase of the thickness for fiber sintering layer. Because the thicker fiber sintering layer is the greater mold press is, this induces to the penetrating of copper fibers and microgroove, brings about the greater adhesive force between fibers and microgrooves. When the sintering layer is up to 1 mm，the adhesive force is the greatest, then the adhesive force decreases when the sintering layer increases.

Fig.11 The influence of the thickness of fiber sintering layer on the adhesive force

4 Conclusions

The adhesive force between copper fibers and micro-grooves of heat column wick structure increases first and then decreases with the increasing sintering temperature, temperature rising rate and sintering time. It increases with the increasing sintering pressure, the increasing fiber diameter. and the increasing of thickness for fiber sintering layer, the adhesive force between fibers and microgroovesl decreases with the increasing fiber diameter. When the composite wick structure of heat column is fabricated by copper fiber sintering layer with thickness of 1mm, the copper fiber diameter of 50 μm, sintering at 900 ℃ for 60 minutes under 0.45 MPa at 5 ℃/s, the best adhensive effect can be obtained.